Hybrid Organic-Inorganic Materials:
Synthesis, solid state characterisation and
solution studies of organoarsonate
and -phosphonate functionalised coordination
clusters and networks
Camelia Ioana Onet
A thesis submitted to the University of Dublin for the degree of
Doctor of Philosophy
School of Chemistry
University of Dublin
2013
Declaration
I declare that this thesis has not been submitted as an exercise for a degree at this or
any other university and it is entirely my own work.
I agree to deposit this thesis in the University’s open access institutional repository
or allow the Library to do so on my behalf, subject to Irish Copyright Legislation and
Trinity College Library conditions of use and acknowledgement.
____________________________
Camelia Ioana Onet
I
Acknowledgments
First and foremost, I would like to thank my supervisor Prof. Dr. Wolfgang Schmitt
for offering me the possibility of joining his research group, for his guidance, and
encouragement. I am really grateful for his understanding and support throughout my time
here in Trinity College Dublin.
I am especially grateful to Dr. Tom McCabe, Dr. Lei Zhang and Dr.
Nianyong Zhu for their invaluable assistance with the X-ray diffraction measurements and
crystal structure determinations. I also would like to express my gratitude to Prof.
Rodolphe Clérac and Mathieu Rouzieres for the tireless effort they have put into the
magnetic studies. I would sincerely like to thank Dr. J. Bernard Jean-Denis and Dr. Martin
Feeney for their help with the ESI-MS measurements and to Dr. John O’Brien and Dr.
Manuel Rüther for NMR measurements. Many thanks to all the academic, technical and
administrative staff within the School of Chemistry for their help over the years.
Special thanks must go to my laboratory colleagues Lei, Ian, John, David,
Giuseppe, Bartosz, Gerard, Adam, Anne Marie, Lukas, Theresa, Nianyong, Mariyatra,
Anil and Jian Di for all their support on both a professional and personal level. Thanks as
well to all the project students who have joined us over the years, especially to Raphaele
Clement and Pierre Heijboer. Also, I would like to thank all the members in Professor
Draper`s group.
I am very grateful to Mrs. Corinne Harrison for all her support during my years of
study in Dublin and for treating me like her daughter while living at her house.
I would also like to thank SFI for their financial support and Trinity College Dublin
for making my training here possible.
Finally, I would like to thank my family and friends for their support and
encouragement.
II
Summary
The research presented in this thesis focuses on the development of novel hybrid
organic-inorganic materials with potential applications in the areas of catalysis, gas
sorption and separation, magnetism. The project involves the synthesis and structural
characterisation of coordination complexes and networks stabilised by organophosphonate
and -arsonate ligands. Solution studies were performed using electrospray ionisation mass
spectrometry (ESI-MS). ESI-MS proved to be a very powerful analytical tool that allowed
us to monitor the synthetic approach to hybrid organic-inorganic oxo-clusters and
investigate their stability in solution. The physicochemical properties of the synthesised
compounds have also been studied.
The first chapter of this work introduces the reader to the field of research,
highlights main achievements of the field and puts the work into context.
The second chapter, “Hybrid Organic-Inorganic Polyoxomolybdates”, details the
synthesis and characterisation of a series of hybrid polyoxomolybdates stabilised by
organoarsonate
and –phosphonate ligands: (NH4)2H2[MoV4O8(O3AsC6H5)4]·5H2O (1),
(NH4)2H2[MoV4O8(O3AsC6H4NH2)4]·DMF·4H2O
(O3AsC6H4OH)5]·9H2O
(3),
(6),
(NH4)5[MoVI2MoV3O11
(NH4)4[MoVI4O10(O3AsC6H3NO2OH)4]·2H2O
(NH4)4H4{Mn[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O
(O3PC6H5)4]2} 8H2O
(2),
(5),
(NH4)4H4{Fe[MoV6O12(OH)3
(NH4)4H4{Co[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O
(NH4)4H4{Ni[MoV6O12(OH)3(O3PC6H5)4]2}
8H2O (8) and
(4),
(7),
(NH4)4H4{Mg[MoV6O12(OH)3
(O3PC6H5)4]2} 8H2O (9). It has been demonstrated that slight perturbations of the ligand
functionalities can be exploited to stabilise unprecedented core structures. Additionally, the
electrospray ionisation mass spectrometry (ESI-MS) technique has been exploited to
investigate the self-assembly process of hybrid organic-inorganic polyoxomolybdates that
form upon partial reduction of (NH4)6Mo7O24·4H2O in the presence of aromatic
organoarsonates. Functionalised POMs were also obtained by incorporating d-block
hetero-atoms (Mn, Fe, Co, Ni) and a main group element (Mg) into defined POM
structures. We intended to explore how different heteroatoms would influence the structure
and magnetic properties of the compounds.
The third chapter, “Polynuclear Manganese Coordination Complexes”, presents the
synthesis and characterisation of a series of manganese cluster compounds stabilised by
III
organophosphonate
ligands:
·8H2O
13( 4-
(10),
(11), [ n
[ n
13( 4-
[ n
13( 4-
[ n
15( 2-H2
)2(C
)2( 3- )4( 2- H)2( 2-C
)2( 3- )4( 2- H)2( 2-C
)2( 3- )4( 2- H)2( 2-C
3
3
)4(C6
)4(C6
5C
)4(C6
3
5C
2P
)16(C6
3
2P
5P
5P
3)10(C5
3)10(C5
3)10(C6
3)20]Cl5·22CH3OH
5-C3
5
)5Cl]·3H2O
5
)6]Cl·5H2O (12),
6-C5
4
)6]Cl·5H2O
K(H2O)4[MnIII12( 3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Cl2·3CH3OH
(13),
·4H2O
(14),
K(H2O)4[MnIII12( 3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Br2
·2CH3OH·2H2O (15) and (H3O)4[MnIII2MnII4( 4-O)2(H2O)2(CH3CN)2{(C6H5)3CPO3}6]Cl2
·2CH3CN·4H2O (16). It has been shown that small changes to the reaction conditions
result in drastic changes of the cluster nuclearity and topology. The ESI-MS technique was
again used to screen the reaction mixtures of 10-16 in order to identify new polynuclear
manganese species that form in solution. Magnetic studies performed for these compounds
revealed that compounds 11-13 exhibit single molecule magnet (SMM) behaviour.
Chapter 4, describes the synthesis and characterisation of two coordination
networks
employing
mononuclear
(H3O){ n(H2 )(C
3
)2[C6H3(C6
(H3O){Cu(H2 )2(C
3
)[C6H3(C6
4P
4P
SBUs
3
3
and
triphosphonate
)3]}·xCH3OH·yH2O
(17)
linkers:
and
)3]}·xCH3OH·yH2O (18). The compounds
exhibit a 2D layered architecture stabilised in the crystal structure by week hydrogen bonds
and - interactions. The materials did not show any permanent porosity.
Chapter 5 provides a description of the applied materials and methods, and
provides the experimental details for the synthesis of the compounds.
IV
Table of contents
Acknowledgements……………………………………………………………………….II
Summary…………………………………………………………………………………III
List of compounds……………………………………………………………………..VIII
Abbreviations………………………………………………………………………..…..IX
1.
Introduction……………………………………………………………………………………………………………………....1
1.1
Polyoxometalates……………………………………………………………………………………………………………..2
1.1.1 Historical background of POMs………………………………………………………………….…………………..3
1.1.2 Condensation reactions as synthetic procedure.......................................................................................5
1.1.3 Polyoxomolybdates……………………………………………………………………………………………………….6
1.1.4 Functionalisation of POMs……………………………………………………………………………………………..8
1.1.4.1 Transition metal substituted POMs (TMSPs)…………………………………………………………….8
1.1.4.2 Incorporation of main group elements into POMs……………………………………………………..9
1.1.4.3 Organic and organometallic derivatives of POMs…………………………………………………….10
1.1.4.4 Organophosphonate and –arsonate functionalised polyoxomolybdates……………………11
1.2
Metal-organic frameworks..................................................................................................................................14
1.3
Mass spectrometry in coordination chemistry…………………………………………………………………18
1.4
Molecular magnetism……………………………………………………………………………………………………..24
1.5
Aims and objectives………………………………………………………………………………………………………..29
References…………………………………………………………………………………………………………………………………32
2.
Hybrid Organic-Inorganic Polyoxomolybdates…………………………………………………………………37
2.1
Introduction and motivation…………………………………………………………………………………………...38
2.2
Functionalisation of polyoxomolybdates using organo-arsonates and –phosphonates……….40
2.2.1 Synthesis and characterisation of organoarsonate functionalised polyoxomolybdate
clusters………………………………………………………………………………………………………………………………….40
2.2.1.1 (NH4)2H2[MoV4O8(O3AsC6H5)4]·5H2O (1)………………………………………………………………….40
2.2.1.2 (NH4)2H2[MoV4O8(O3AsC6H4NH2)4]·DMF·4H2O (2)…………………………………………………...50
2.2.1.3 (NH4)5[MoVI2MoV3O11(O3AsC6H4OH)5]·9H2O (3)……………………………………………………….59
2.2.1.4 (NH4)4[MoVI4O10(O3AsC6H3NO2OH)4]·2H2O (4)………………………………………………………...71
2.2.2 Synthesis and characterisation of organophosphonate functionalised
heteropolyoxomolybdate clusters…………………………………………………………………………………………..78
2.2.2.1 (NH4)4H4{Mn[MoV6O12(OH)3(O3PC6H5)4]2}·8H2O (5)………………………………………………...78
2.3
ESI-MS studies of complex reaction mixtures to investigate the formation of hybrid organic-
inorganic polyoxomolybdates…………………………………………………………………………………………………….88
2.3.1 Investigation of the reaction system that led to the formation of the cubane structures in 1
and 2……………………………………………………………………………………………………………………………………88
V
2.3.2 Investigation of the reaction system that led to the formation of the {Mo5} complex in 3...94
2.3.3 Investigation of the reaction system that led to the formation of the {Mo4} complex in 4...97
2.4
Conclusion and future work……………….…………………………………………………………………………...99
References……………………………………………………………………………………………………………………………….102
3.
Polynuclear Manganese Coordination Complexes…………………………………………………………..105
3.1
Introduction and motivation…………………………………………………………………………………………106
3.2
Phosphonate ligands……………………………………………………………………………………………………108
3.3
Polynuclear manganese complexes stabilised by organophosphonates…………………………...109
3.3.1 Synthesis and characterisation of a pentadecanuclear manganese complex………………….109
3.3.1.1 [ΜnΙΙΙ15(μ2-H2Ο)2(CΗ3ΟΗ)16(C6Η5PΟ3)20]Cl5·22CH3OH·8H2O (10)……………………………109
3.3.2 Synthesis and characterisation of tridecanuclear manganese complexes………………………123
3.3.2.1 [ΜnΙΙΙ13(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5PΟ3)10(C5Η5Ν)5Cl]·3H2O (11) ............... 123
3.3.2.2 [ΜnΙΙΙ13(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5CΗ2PΟ3)10(C5Η5Ν)6]Cl·5H2O (12) ....... 143
3.3.2.3 [ΜnΙΙΙ13(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5CΗ2PΟ3)10(C6Η5-C3Η6-C5Η4Ν)6]Cl·5H2O
(13) ................................................................................................................................................................................... 159
3.3.3 Synthesis and characterisation of dodecanuclear manganese complexes ................................ 172
3.3.3.1 K(H2O)4[MnIII12(μ3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Cl2·3CH3OH·4H2O (14)
................................................................................................................................................................................... 172
3.3.3.2 K(H2O)4[MnIII12(μ3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Br2·2CH3OH·2H2O
(15) ................................................................................................................................................................................... 187
3.3.4 Synthesis and characterisation of a hexanuclear manganese complex ...................................... 196
3.3.4.1 (H3O)4[MnIII2MnII4(μ4-O)2(H2O)2(CH3CN)2{(C6H5)3CPO3}6]Cl2·2CH3CN·4H2O (16) ....... 196
3.4
ESI-MS studies of complex reaction mixtures to investigate the formation of polynuclear
manganese coordination complexes………………………………………………………………………………………….206
3.4.1 Investigation of the reaction system that led to the formation of the {Mn15} complex in 10 ...
........................................................................................................................................................................................ 206
3.4.2 Investigation of the reaction system that led to the formation of {Mn13} complexes
observed in 11-13 ........................................................................................................................................................... 211
3.4.3 Investigation of the reaction system that led to the formation of {Mn12} complexes
observed in 14-15 ........................................................................................................................................................... 215
3.4.4 Investigation of the reaction system that led to the formation of the {Mn6} complex in 16.....
........................................................................................................................................................................................ 220
3.5
Conclusion and future work…………………….…………………………………………………………………....222
References……………………………………………………………………………………………………………………………….226
4.
Supramolecular Coordination Networks Employing Triphosphonate Linkers……………….229
4.1
Introduction…………………………………………………………………………………………………………………230
4.2
Extended triphosphonate ligands………………………………………………………………………………….231
4.2.1 Synthesis of 1,3,5-Tris(4-phosphonophenyl)benzene (P-TPB)……………………………………..233
4.2.2 Synthesis of 1,3,5-Tris(4`-phosphonobiphenyl-4-yl)benzene (P-TBB)………………………….235
VI
4.3
Synthesis and characterisation of coordination networks employing mononuclear inorganic
SBUs and triphosphonate linkers……………………………………………………………………………………………...240
4.3.1 (H3O){ΜnII(H2Ο)(CΗ3ΟΗ)2[C6H3(C6Η4PΟ3Η)3]}·xCH3OH·yH2O (17)………………………………240
4.3.2 (H3O){Cu(H2Ο)2(CΗ3ΟΗ)[C6H3(C6Η4PΟ3Η)3]}·xCH3OH·yH2O (18)…………………………………247
4.4
Conclusion and future work…………………..………………………………………………………………………253
References……………………………………………………………………………………………………………………………….254
5.
Experimental…………………………………………………………………………………………………………………..256
5.1
Materials and methods………………………………………………………………………………………………….257
5.2
Ligand synthesis…………………………………………………………………………………………………………...260
5.3
Synthesis of the Metal Complexes………………………………………………………………………………….265
References……………………………………………………………………………………………………………………………….272
Appendix…………………………………………………………………………..…….273
VII
List of compounds
1
(NH4)2H2[MoV4O8(O3AsC6H5)4]∙5H2O
2
(NH4)2H2[MoV4O8(O3AsC6H4NH2)4]∙DMF∙4H2O
3
(NH4)5∙[MoVI2MoV3O11(O3AsC6H4OH)5]∙9H2O
4
(NH4)4∙[MoVI4O10(O3AsC6H3NO2OH)4]∙2H2O
5
(NH4)4∙H4{Mn[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O
6
(NH4)4∙H4{Fe[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O
7
(NH4)4∙H4{Co[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O
8
(NH4)4∙H4{Ni[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O
9
(NH4)4∙H4{Mg[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O
10
[ΜnΙΙΙ15(μ2-H2Ο)2(CΗ3ΟΗ)16(C6Η5PΟ3)20]Cl5·22CH3OH·8H2O
11
[ΜnΙΙΙ13(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5PΟ3)10(C5Η5Ν)5Cl]·3H2O
12
[ΜnΙΙΙ13(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5CΗ2PΟ3)10(C5Η5Ν)6]Cl·5H2O
13
[ΜnΙΙΙ13(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5CΗ2PΟ3)10(C6Η5-C3Η6-C5Η4Ν)6]Cl·5H2O
14
K(H2O)4[MnIII12(μ3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Cl2·3CH3OH·4H2O
15
K(H2O)4[MnIII12(μ3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Br2·2CH3OH·2H2O
16
(H3O)4[MnIII2MnII4(μ4-O)2(H2O)2(CH3CN)2{(C6H5)3CPO3}6]Cl2·2CH3CN·4H2O
17
(H3O){Μn(H2Ο)(CΗ3ΟΗ)2[C6H3(C6Η4PΟ3Η)3]}·xCH3OH·yH2O
18
(H3O){Cu(H2Ο)2(CΗ3ΟΗ)[C6H3(C6Η4PΟ3Η)3]}·xCH3OH·yH2O
VIII
Abbreviations
3D
three dimensional
ac
alternating current
a.m.u.
atomic mass units
ATR
attenuated total reflectance
BVS
bond valence sum analysis
C
Currie constant
CHN
carbon hydrogen nitrogen elemental analysis
D
zero-field splitting
dc
direct current
DMF
N,N`-dimethylformamide
DMSO
dimethyl sulfoxide
EDX
energy dispersive X-ray analysis
ESI-MS
electrospray ionization mass spectrometry
FTIR
Fourier-transform infrared
g
Landé g-factor
IR
infrared
J
exchange coupling constant
M
magnetisation
MALDI
matrix-assisted laser desorption ionisation
min
minutes
MOF
metal-organic framework
m/z
mass to charge ratio
NMR
nuclear magnetic resonance
Oe
Oersted
POM
polyoxometalate
P-MOF
polyoxometalate metal organic framework
S
ground spin state
SBU
secondary building unit
SMM
single molecule magnet
SQUID
superconducting quantum interference device
T
temperature
TGA
thermogravimetric analysis
IX
TOF-MS
Time-of-flight mass spectrometer
U
spin reversal barrier
UV-vis
ultraviolet visible
XRD
X-ray diffraction
chemical shift
magnetic susceptibility
wavelength
QTM
quantum tunnelling of the magnetisation
X
Chapter 1 – Introduction
1.
INTRODUCTION
1
Chapter 1 – Introduction
1.1 POLYOXOMETALATES
Polyoxometalates (POMs) are anionic metal–oxide clusters of the early transition
metals (V, Nb, Ta, Mo, W) in their highest oxidation state, and represent a class of
compounds whose preparation is a very active area in chemical and materials research.1-5
The interest in these compounds arises not only from their extraordinary structural
characteristics but also from their unique intrinsic electronic, optical and chemical
attributes (including redox and photochemical activity, charge distribution and band
structures) which promote applications in many diverse disciplines such as catalysis, the
development of sensors, photocatalysts, electrochromics, magnetic materials, energy
storage and conversion devices.1, 4-16
The field of polyoxometalate chemistry has evolved very rapidly in the last two
decades and research developments are governed by newly developed synthetic
methodologies that led to the synthesis and characterisation of ultra-large, molecular
clusters that contain up to 368 metal atoms.17 Much of the interest in these molecules has
arisen because such clusters represent molecular entities whose structures can be
rationalised by consecutive condensation reactions of well defined sub-units. Variations of
the reaction conditions can initiate aggregation reactions of specific molecular species
producing oligomeric complexes and large nanoscale clusters.1, 17 The versatile nature of
this class of compound in terms of structure, size, redox chemistry, photochemistry, and
charge distribution, means that the chemistry of polyoxometalates is arguably one of the
many areas in inorganic chemistry that is developing very rapidly.1
While many of these systems can be easily prepared, the ability to functionalise
polyoxometalates is an ongoing challenge.3, 18-20 The early transition metal ions are able to
polarise terminal M-O bonds stabilising large molecular cage structures and other
aggregates. These bonds that usually point radial to the outside of the 0-D cluster species
are
characterised
by
d-
contributions
and
double-bond
character
impeding
functionalisation. Consequently many advanced developments of polyoxometalatecontaining materials are hindered by the limited availability of functionalised POMs.21
Currently many research groups are developing synthetic approaches to functionalised
POMs, whereby the preparation of hybrid POMs appears to be more challenging than
ligand–stabilised compounds containing late transition metal ions.
2
Chapter 1 – Introduction
1.1.1 Historical background of POMs
The history of polyoxometalate chemistry dates to the discovery of ammonium 12molybdophosphate [PMo12O40]3- by Berzelius in 1826. Many other heteropoly compounds
were subsequently reported and analysed throughout the following decades, but the
structure of most polyoxometalates remained not fully understood.22 The development of
X-ray crystallographic techniques provided a breakthrough for the subject area allowing
the detailed elucidation of POM structures and providing access to structure-property
relationships.
In 1933 employing a powder X-ray diffraction study, J. F. Keggin solved the
structure of H3[PW12O40]·5H2O, a nowadays well-known 12:1 type heteropolyanion.23
Named after its discoverer, the Keggin structure (Figure 1.1, a), contains 12 {WO6}
octahedra linked by edge and corner sharing motifs, with the heteroatom occupying a
tetrahedral coordination site in the centre of the oxo cluster. A further 6:1 type, the
Anderson`s heteropolyanion [TeVIMo6O24]6-, was studied by single-crystal X-ray
diffraction by Evans in 1948. This structure is now commonly called the Anderson-Evans
structure24 (Figure 1.1, b). It consists of six edge sharing {MoO6} octahedra adopting a
hexagonal arrangement around a central octahedrally coordinated tellurium atom. Closely
related to the Keggin structure, a 18:2 heteropolyanion [P2W18O62]6- was reported in 1953
by Dawson25, referred to as the Wells-Dawson`s structure (Figure 1.1, c). This
heteropolyanion consists of two identical subunits in which a central {PO4} tetrahedron is
surrounded by 9 {WO6} octahedra linked by edge and corner sharing connectivity motifs.
a)
b)
c)
Figure 1.1 – Polyhedral representation of: (a) Keggin structure,23 (WO6 unit – blue polyhedra; P –
yellow sphere); (b) Anderson-Evans structure,24 (MoO6 unit – blue polyhedra; Te – red polyhedron); (c)
Wells-Dawson structure,25 (WO6 unit – blue polyhedra; P – purple spheres).
3
Chapter 1 – Introduction
Another important POM cluster is the isopolyanion [M6O19]n-, (M = Mo, W, Nb,
Ta, and V) discovered in 1952 by Lindqvist and known as the Lindqvist structure26 (Figure
1.2).
a)
b)
Figure 1.2 – The Lindqvist Structure: (a) polyhedral representation; (b) ball-and-stick representation.26
Colour code: M blue, O red.
Lindqvist POMs have an overall octahedral geometry and consist of one central
oxygen atom that is octahedrally surrounded by six metal ions. Each of these six metal
atoms is
2-bridged
to other metal atoms by four different oxygen ligands and has one
terminal oxygen atom.26 One particular Lindqvist POM containing Mo metal ions,
hexamolybdate [Mo6O19]2-, is of importance, because it can be directly functionalised to
incorporate organoimido ligands. This unique feature has made [Mo6O19]2- a very popular
POM cluster that has been studied extensively in recent years.27, 28
Recent exciting new developments in the field of polyoxometalates include:
the discovery of large, highly symmetric polyoxomolybdates such as the
wheel-shaped [(MoO3)176(H2O)80H32]
spherical
clusters
cluster29 (Figure 1.4, c), giant
[{(Mo)Mo5O21(H2O)6}12{MoV2O4(CH3COOH)}30]42-
(Figure 1.4, a)30 and Keplerates Na48[H16Mo368O1032(H2O)240(SO4)48] ca.
1000H2O (Figure 1.4, d)17;
hybrid organic-inorganic materials that contain POM cores;31,
32
new
potential applications based on unusual magnetic7 and optical33 properties
of some POMs, and potential medical applications such as anti-tumor and
anti-viral uses.34
4
Chapter 1 – Introduction
1.1.2 Condensation reactions as synthetic procedure
The main synthetic approaches used to produce oligomeric complexes or large
nanoscale clusters of the early transition metal ions are simple and involve usually the
acidification of aqueous solutions containing the relevant metal–oxide anions (molybdates,
tungstates and vanadates).
However, the specific reaction variables and parameters that manipulate the
synthesis of POMs are manifold and depend on the type of metal–oxide anion, the
concentration, the pH and type of acid, the heteroatom concentration, the ligand types, the
reducing agent, temperature and the solvent type. The synthesis of POM clusters is based
on self assembly processes involving covalent linking of transferable building blocks,
under "one-pot" conditions (Figure 1.3). These structural building blocks can assemble into
pre-defined architectures, and consist of aggregates of metal–oxygen units that adopt a
defined polyhedral arrangement with the metal ions in the centre and the oxygen ligands as
vertices.1, 35-37
a)
b)
Figure 1.3 – (a) Fundamental units and building blocks;38 (b) Self-assembly of polyoxometalate clusters
from {Mo12} to the protein-sized {Mo368}. The Mo centres are shown as green polyhedra.35
5
Chapter 1 – Introduction
The overall structure of the POM cluster can be visualised as a combination of a set
of polyhedra that adopt corner and/or edge sharing modes. The formation reactions that
often occur through a set of consecutive condensation reactions are mechanistically not yet
fully understood. Therefore, the concept that regards metal-centered polyhedra as structural
building blocks became extremely useful to rationalise the observed POM structures and
their formation processes.35, 36, 38
1.1.3 Polyoxomolybdates
Polyoxomolybdates represent a very important subclass of POM clusters. Research
developments and synthetic approaches in this fascinating area of POM chemistry take
advantage of a unique library of molybdenum-oxide based building blocks to generate a
large variety of molecular architectures.36 The fundamental units and building blocks of
molybdenum-based POMs include {Mo1}, {Mo2}, {(Mo)Mo5}, {Mo8} and {Mo17} =
[{Mo8}2{Mo1}] units, as seen in Figure 1.3, a.38 A huge diversity of structures, from small
to large nanoscale clusters, can be produced upon combination of these building units. The
broad range of polyoxomolybdate structures presently known include {Mo6},24 {Mo7},39
{Mo8},40 {Mo10},41 {Mo12},42 {Mo18},43 {Mo36},44 {Mo37},45 {Mo132},30 {Mo154},46
{Mo176},29 {Mo248}47 and {Mo368}17 oxo-clusters.
One of the most exciting developments in POM chemistry is represented by the
discovery of a high-nuclearity cluster commonly known as the “big-wheel”, the
[Mo154O462H14(H2O)70]14- ≡ {Mo154} cluster, reported by Müller et al. in 1995.46,
48
The
{Mo154} cluster anion has a ring topology and consists of 14 {Mo8} building units linked
by 14 {Mo2} and 14 {Mo1} units, respectively (Figure 1.4, b). A structurally related
wheel-shaped metal oxide cluster, the [(MoO3)176(H2O)80H32] ≡ {Mo176} cluster,29 can be
obtained by linking 16 instead of 14 sets of the above mentioned {Mo8}, {Mo2} and {Mo1}
building units, respectively (Figure 1.4, c). The {Mo154} cluster has an external diameter of
3.4 nm, while that of the {Mo176} cluster is 4.1 nm. The {Mo8} building block is itself
built-up by a densely packed pentagonal unit {(Mo)Mo5} and two more weakly bonded
{MoO6} octahedra. The pentagonal unit {(Mo)Mo5} consists of a central bipyramidal
{MoO7} unit sharing edges with five {MoO6} octahedra. These pentagonal units are also
key
for
the
formation
of
giant
spherical
clusters
or
Keplerates,
like
the
6
Chapter 1 – Introduction
[{(Mo)Mo5O21(H2O)6}12{MoV2O4(CH3COOH)}30]42-
{Mo132}30
≡
and
the
[H16Mo368O1032(H2O)240(SO4)48]48- ≡ {Mo368}17 clusters. The {Mo368} cluster anion is the
largest cluster known to date. It has an approximate D4 symmetry and consists of 368
molybdenum atoms. It can be regarded as composed of 40 pentagonal {(Mo)Mo5} units,
32 dinuclear {Mo2} units and 64 mononuclear {Mo1} units (Figure 1.4, d).1, 48
a)
b)
c)
d)
Figure 1.4 – Giant sphere- and wheel-shaped polyoxomolybdates: (a) The spherical {Mo132} cluster;
(b,c) The wheel-shaped {Mo154} and {Mo176} clusters; (d) The {Mo368} cluster, highlighting the related
pentagonal {(Mo)Mo5} (cyan and blue), dinuclear {Mo2} (red) and mononuclear {Mo1} (yellow)
building units.48
7
Chapter 1 – Introduction
Despite the fact that the high nuclearity molybdenum clusters, {Mo132},
{Mo154},{Mo176}, {Mo248} and {Mo368}, have complex nanoscale structures, they are quite
easy to prepare and handle.1, 48 The versatile nature of polyoxomolybdate chemistry is due
to the flexibility of the Mo-O-Mo bridges that allow “split and link” type processes,48 the
number of variable redox states of the molybdenum centeres, different coordination
numbers, strong hydration stabilisation and the presence of terminal Mo=O groups that
‘protect’ the entities to the outside stabilising discrete molecular clusters.48
1.1.4 Functionalisation of POMs
The functionalisation of POMs may provide a tool to fine tune the electronic
properties of the parent anions. This can be achieved by replacing the metal-oxo
functionality of a POM by an alternate one, formal replacement of some oxo ligands, or
grafting of a functional group at the surface of the polyanion.49 The ability to functionalise
polyoxometalate clusters is of great importance, as it aims to extend their utility.
1.1.4.1 Transition metal substituted POMs (TMSPs)
A simple approach to achieve POM functionalisation is to incorporate transition
metal ions into the cluster. This allows us to influence their properties, such as magnetic
behaviour50, 51 or catalytic activity4; examples are illustrated in Figure 1.5.
All first row and most of the second row transition metals have been incorporated
into various POM clusters. Lacunary POM fragments are ideal for the incorporation of
transition metals. The most stable lacunary POMs derive from Keggin and Wells-Dawson
polyoxotungstates, which were intensively used for the preparation of TMSPs. On the
other hand, TMSPs incorporating paramagnetic 3d metals Mn, Fe, Co, Ni and Cu are of
particular interest due to their electronic and magnetic properties.51 For instance, the two
Fe
substituted
polyoxotungstate
[(FeIII4W9O34(H2O))2(FeIIIW6O26)]19-
clusters,
[FeIII4(H2O)2(FeIIIW9O34)2]10-
and
(Figure 1.5, a, b) exhibit fascinating single molecule
magnet (SMM) behaviour.52
8
Chapter 1 – Introduction
a)
b)
c)
Figure 1.5 – Structural representations of: (a) [Fe4(H2O)2(FeW9O34)2]10- and (b) [(Fe4W9O34(H2O))2
(FeW6O26)2]19- POMs52 (displaying SMM behaviour) and (c) [WZn3(H2O)2(ZnW9O34)2]12- oxidation
catalyst.4 Colour code: W red polyhedra, Fe blue spheres, O red spheres, Zn black.
1.1.4.2 Incorporation of main group elements into POMs
Polyoxometalates can also incorporate main group elements within their cluster
structure. Examples of such main group derivatives are the halogenated POMs, which are
useful precursors for further functionalisation.20 This type of functionalised POMs have the
halide ions occupying a terminal position, e.g. [Mo4O10(OMe)4Cl2]2-,53 [W6O14Cl10]2-,54
and [PW9O28Br6]3-.55 However, there are also numerous examples of functionalised POMs
in which the halide ions are incorporated within the cluster anion, e.g. [H2NaW18O56F6]7-,
or where the halide ion acts as a template for the self assembly of a larger cluster shell, e.g.
[V18O42Cl]13-.20 Some examples are illustrated in Figure 1.6. Other examples of main group
derivatives include Si, Sn or P containing POMs: [SiW11O39{O(SiR)2}]4-, (R = C2H5, C6H5,
C3H5), [( -A-XW9O34)(SiR2)3](n-6)- (R = CH3, C6H5; X = Si, n = 10; X = P or As, n = 9),
[SiW9O34(SiEt)3(O3SiEt)]4-, [SiM11O39(SnR)]5- (M = W, R = CH3, C2H5, C6H5, C3H5,
C4H9; M = Mo, R = C2H5), [PW10O38(SnR)2]5- (R = CH3, C6H5), [PW11O39(SnCl)]3-,
[P2W16O60(SnPh)2]8-, [XW11O39Sn]n- (X= P, Si, Ge, B, Ga),
a)
b)
2-[P2W17O61Sn]
8- 20, 56, 57
.
c)
Figure 1.6 – Structural representations of: (a) [W6O14Cl10]2-; (b) [H2NaW18O56F6]7- and (c) [V18O42Cl]13cluster anions.20
9
Chapter 1 – Introduction
1.1.4.3 Organic and organometallic derivatives of POMs
Numerous experimental studies have been devoted to the development of strategies
to functionalise POM species using organic or organometallic precursors as functional
groups. Main-group element, organic, and organometallic derivatives of polyoxometalates
have been reviewed by A. Proust et al.,3, 20 highlighting several synthetic approaches to
obtain such systems. One synthetic approach intensively exploited was the replacement of
some terminal oxo ligands with various nitrogen-containing molecules such as nitrido,
organoimido, diazenido ligands. The first nitrido POM derivative was reported by Zubieta
and was a Lindqvist type POM [Mo6O18N]3-. Later on, Maatta and Proust showed that Reand Os-nitrido species can be incorporated in both Keggin and Dawson type POMs,
[PW11O39(MN)]x- and [P2W17O61(MN)]x- (M = Re, Os) (Figure 1.7, a).3,
58, 59
Another
exciting feature of the Lindqvist POM is the tendency to undergo multiple
functionalisation steps upon reaction with organoimido ligands. Maatta et al. demonstrated
that it is possible to replace successively all six terminal oxo groups of the [Mo6O19]2anion by 2,6-(diisopropyl)phenylimido moieties (Figure 1.7, b, c).3, 60, 61
a)
b)
c)
Figure 1.7 – Structural representations of: (a) the nitrido-Keggin ion [PW11O39(MN)]x- (W blue
polyhedra, M = Re, Os cian, P purple, N blue spheres, O red); (b) [Mo6O18(NC6H3-2,6-Me2)]2- and (c)
[HMo6O13(NC6H3-2,6-Me2)6]- Lindqvist cluster anions (Mo blue, O red, N green, C black).20
Most recent studies exploit the direct grafting of organic substrates and organic
ligands at the nucleophilic oxygen atoms of the POM core. Examples of aromatic
phosphonate and arsonate stabilised POMs are illustrated in Figure 1.8.
18, 21, 62-66
In these
compounds the aromatic ligands stabilise rather symmetrical cluster cores. The ligands
10
Chapter 1 – Introduction
point into defined directions of space suggesting that these or related structures could be
suitable building units for metal-organic frameworks.61
a)
c)
b)
d)
Figure 1.8 – Aromatic phosphonate/arsonate stabilised POMs with regular topology as potential SBUs
for MOFs. (a) [Cl2⊂V14O22(OH)4(H2O)2(C6H5PO3)8]6-;62 (b) {Na4(H2O)10}[V12O12(OH)4(H2O)2
(O3AsC6H4NH2)10];21 (c) [Mo12O34(O3AsC6H4NH2)4]4-;63 (d) [Mo12O30(BPO4)2(O3PC6H5)6]5-.64 Colour
code: V green, Mo cyan, O red, P purple, N blue, Cl light green, Na yellow, C black, H white.
1.1.4.4 Organophosphonate and –arsonate functionalised polyoxomolybdates
Metal organophosphonates and -arsonates have attracted considerable research
interest due to their interesting coordination chemistry and potential applications as
catalysts and ion exchange materials.67 The structural diversity of these hybrid organicinorganic materials spans over a range of structures from molecular clusters to threedimensional (3D) frameworks.68 A very important subclass of these materials is
represented by oganophosphonate and -arsonate functionalised polyoxometalates. The
11
Chapter 1 – Introduction
tetrahedral functional group of the organophosphonate and -arsonate ligands displays
geometrical and electronic similarities to metal-oxygen units in polyoxometalates
suggesting that these ligands could be good candidates for POM functionalisation.
The V/O/RPO32- system proved to be quite fruitful in yielding a range of
structurally unique clusters and extended compounds, while the corresponding
Mo/O/RPO32- system remain relatively undeveloped, being mostly limited to some
molecular species. The most common structural type of the Mo/O/RPO32- system is
represented by the cyclic pentanuclear core structure, with the general formula
[Mo5O15(RPO3)2]4-.3, 61 These pentamolybdobisphosphonates were isolated using different
phosphonate ligands (R = CH3, C2H5, C6H5, CH2C6H5, C2H4NH3+, p-CH2C6H4NH3+). The
{Mo5} clusters have a ring topology with two phosphonate ligands capping both sides of
the ring (Figure 1.9, a).3, 20, 69
Tetra-, hexa- and dodecanuclear architectures can also be isolated. Examples
[Mo4O10(PhPO3)4]4-,
include
[Mo6O18(ButPO3)2]4-,
[{(C6H5P)Mo6O21(H2O)3}2]4-,
[RPMo6O21(O2CCH2NH3)3]2- (R = OH, CH3, C2H5), [(RPO3)4Mo12O34]4- (R = CH3, C2H5),
[Na{Mo6O12(OH)3(PhPO3)4}2]9-, [Mo12O30(BPO4)2(O3PC6H5)6]5-.20, 61, 64, 69-71
The tetranuclear complex [Mo4O10(PhPO3)4]4- consists of two {Mo2O10} dinuclear
units connected by two phosphonate ligands to form a six membered {Mo4P2} ring. The
remaining two phosphonate ligands cap both sides of the central cavity of the ring (Figure
1.9, b).20 The core structure of the hexanuclear complexes, [{(C6H5P)Mo6O21(H2O)3}2]4-,
[RPMo6O21(O2CCH2NH3)3]2-, adopts a similar ring motif capped by only one
organophosphonate ligand, while the [Mo6O18(ButPO3)2]4- species adopts a non-planar
conformation as seen in Figure 1.9, c.69, 70 The dodecanuclear complex [(RPO3)4Mo12O34]4is stabilised by methyl- or ethyl- phosphonates and adopts a so-called inverted-Keggin
structure
(Figure
1.9,
d).71
The
sandwich
type
dodecanuclear
compounds
[Na{Mo6O12(OH)3(PhPO3)4}2]9- consist of two hexanuclear MoV moieties linked by a Na
ion (Figure 1.9, e), while the dodecanuclear polyanion, [Mo12O30(BPO4)2(O3PC6H5)6]5(Figure 1.8, d), is part of a class of POMs that resemble Dawson anions.20, 64, 72, 73
The incorporation of diphosphonate ligands into POM structures was also
investigated. Methylenediphosphonate (O3PCH2PO3)4- was the first to be explored for this
purpose. Dolbecq and Mialane reported a series of cyclic compounds with the general
formula {(MoV2O4)(O3PCH2PO3)}n] (n = 3, 4, 10) (Figure 1.9, f).74,
75
The compounds
12
Chapter 1 – Introduction
consist of alternating {MoV2O4} units and diphosphonate ligands, and their geometry
depends on the nature of the co-ligand used and the nature of the counterions (NH4+, Na+,
Li+).61 The [(Mo2O4)10(O3PCH2PO3)10(CH3COO)8(H2O)4]28-
anion differentiates itself
from conventional cyclic, adopting a double wheel architecture built from interconnected
octa- and dodecanuclear wheels that are linked via two sodium ions (Figure 1.9, g).75
Other diphosphonate functionalised POMs include {Mo5O15(O3P(CH2)4PO3)},
[MoV6MoVIO16(O3PCH2PO3)]8- and [MoV7MoVIO16(O3PPhPO3H)]3- (Figure 1.9, h).65, 76, 77
a)
b)
c)
d)
e)
f)
g)
h)
Figure 1.9 – (a) [Mo5O15(CH3PO3)2]4-;20 (b) [Mo4O10(REO3)4]4- R = Ph, E = P, As;69 (c)
72, 73
[Mo6O18(ButPO3)2]4-;69 (d) [(EtPO3)4Mo12O34]4-;71 (e) [Na{Mo6O12(OH)3(PhPO3)4}2]9-;
(f)
[(Mo2O4)4(O3PCH2PO3)4(CO3)2]12-,74 (g) [(Mo2O4)10(O3PCH2PO3)10(CH3COO)8(H2O)4]28-,75 ({MoO6}
orange octahedra, {PO3C} green tetrahedra, Na blue/gray spheres, C black spheres); (h)
[MoV7MoVIO16(O3PPhPO3H)]3-.65
13
Chapter 1 – Introduction
Even though organoarsonates display reactivities and structural characteristics
close to that of organophosphonates, organoarsonate-stabilised POM derivatives have far
less been isolated.61 Examples of organoarsonate-functionalised polyoxomolybdates
include
[Mo4O10(RAsO3)4]4-,
[Mo5O15(RAsO3)2]4-,
[Mo6O18(RAsO3)2]4-,
[Mo12O34(RAsO3)4]4- (R = CH3, C3H7, C6H5, p-C6H4NH3+, C2H4OH, C6H4COOH).3, 20, 78, 79
The [Mo12O34(RAsO3)4]4- (Figure 1.8, c) species are analogous to the organophosphonate
derivatives that display an inverted-Keggin core with a tetrahedral arrangement of the
organic ligands. These type of clusters may be suitable to be used as building units for
metal-organic frameworks.61
1.2 METAL-ORGANIC FRAMEWORKS
Metal-organic frameworks (MOFs) represent a class of porous materials consisting
of clusters or metal ions linked through rigid organic ligands. MOFs can be prepared using
a fundamental construction principle, based on building block approaches where
molecules, metal ions or clusters are considered as nodes, and coordination bonds represent
node connectors. Choosing the desired combination of nodes and connectors, and using
suitable building blocks, microporous networks with unprecedented surface areas can be
obtained. The extraordinary low densities (1.00 to 0.20 gcm-3) and high surface areas (500
to 7000 m2 g-1) make MOFs ideal candidates for the storage and separation of gases (N2,
Ar, CO2, CH4, and H2).80-82
The conceptual approach used to synthesise materials designed to have
predetermined structures, compositions and properties is known as ‘the reticular synthetic
approach’. This concept was described by O. M. Yaghi as “the process of assembling
judiciously designed rigid molecular building blocks into predetermined ordered structures
(networks), which are held together by strong bonding.”83-85 This approach requires the use
of secondary building units (SBU). The term ‘secondary building unit’ refers to the
geometry of the units (molecules, metal ions or clusters) defined by the points of extension
(ligand coordination modes and metal coordination environments). These SBUs have
intrinsic geometric properties that are used to direct the assembly of ordered frameworks.
New materials with pre-desired structure, composition and properties can be obtainable by
14
Chapter 1 – Introduction
linking these SBUs through coordination bonds. A few examples of inorganic and organic
SBUs with varying geometries are presented in Figure 1.10. 81, 83-87
Inorganic units
SBUs
Organic units
SBUs
Figure 1.10 – Examples of SBUs from carboxylate MOFs. Colour code: O red, N green, C black. In
inorganic units metal-oxygen polyhedra are blue.83
The design of rigid frameworks based on such SBUs has been highly successful as
demonstrated by Yaghi and co-workers. Their strategy was to use {Zn4O(CO2)6}
octahedral units and polyaromatic carboxylate ligands (Figure 1.10) to assemble extended
15
Chapter 1 – Introduction
network structures with high surface areas and permanent porosity (Figure 1.11).83 For
instance, by combining octahedral {Zn4O(CO2)6} units and large triangular tricarboxylate
ligands, MOFs exhibiting exceptional porosity were generated. Examples include
[Zn4O(BTB)2] (MOF-177, BTB3- = 4,4 ,4 -benzene-1,3,5-triyl-tribenzoate) (Figure 1.11),
[Zn4O(BBC)2]
(MOF-200,
BBC3-
=
4,4 ,4 -[benzene-1,3,5-triyl-tris(benzene-4,1-
diyl)]tribenzoate) and [Zn4O(BTE)(BPDC)] (MOF-210, BTE3- = 4,4 ,4 -[benzene-1,3,5triyl-tris(ethyne-2,1-diyl)]tribenzoate and BPDC2- = biphenyl-4,4 -dicarboxylate).85 MOF177 shows a surface area of 4500 m2 g-1 and has a hydrogen gas uptake capacity of 7.5 wt
% at 77 K.85
COO-
COO-
MOF-5
COO-
+
MOF-10
COO-
COO-
-OOC
COO-
MOF-177
Figure 1.11 – Representation of the reticular synthesis concept which can be applied to prepare metalorganic frameworks.83-85
16
Chapter 1 – Introduction
Current developments have led to the formation of metal-organic frameworks with
ultrahigh surface areas. For instance, [Cu3(L6-(109))(H2O)3]n (NU-109, LH6(109) = 1,3,5tris[(1,3-carboxylic acid-5-(4-(ethynyl)phenyl))butadiynyl]-benzene, Figure 1.12) and
[Cu3(L6-(110))(H2O)3]n
(NU-110,
LH6(110)
=
1,3,5-tris[((1,3-carboxylic
acid-5-(4-
(ethynyl)phenyl))ethynyl)phenyl]-benzene) display the highest surface areas reported to
date (∼ 7000 m2 g-1).82
Figure 1.12 – NU-109 showing the presence of different types of cages within the structure and the
packing arrangement viewed in the direction of the crystallographic a-axis.82
The striking increase of the surface areas reported for MOFs is primarily due to the
development of new SBUs with extended structures and multiple binding sites. However, it
is to a certain extend surprising that the vast number of existing POM clusters have not yet
been systematically exploited as building blocks for MOFs. Only a very limited number of
polyoxometalate metal–organic frameworks (P-MOFs) have been reported.80, 88, 89 P-MOFs
are hybrid organic-inorganic compounds that might be obtainable by functionalisation of
POMs with rigid organic ligands. They may be regarded as a new generation of solid-state
materials with promising attributes for advanced applications in the areas of gas storage
materials and catalysis.80 In contrast to traditional POMs, P-MOFs might provide
additional functionalities arising from their organic components. Highly ordered and
amendable cavities could give rise to shape-selectivity and may impact directly on
important catalytic acid-base and redox processes.
17
Chapter 1 – Introduction
1.3 MASS SPECTROMETRY IN COORDINATION CHEMISTRY
Mass spectrometry (MS) is a very powerful analytical technique used for both
quantitative and qualitative investigations of a wide range of compounds ranging from
small inorganic and organic molecules to biological macromolecules. MS technologies
have faced huge progress in the past 20 years, leading to the development of highly
sensitive mass spectrometers.90-92
The ability of MS to be used to analyse complex systems and labile solution species
is due to the advances in the development of soft ionisation techniques such as electrospray
ionisation (ESI) and matrix-assisted laser desorption ionisation (MALDI) processes.
Furthermore, the sensitivity of a mass spectrometer is correlated to the mass analyser used,
which separates the ions according to their mass-to-charge ratio (m/z). The most commonly
used analysers are quadrupole and time of flight mass analysers. Different ionisation
techniques and mass analysers have different advantages and disadvantages that one has to
be aware of when performing MS analyses. For a specific class of compounds or for
particular applications it is a requirement to choose the optimal ionisation technique and
mass analyser.90, 91
For instance, electrospray ionisation mass spectrometry (ESI-MS) has been
successfully applied in the study of coordination compounds. The growing interest in the
use of ESI-MS to characterise metal complexes arises from the simplicity of the technique
which is capable of transferring non-volatile solution phase ions to the gas phase.91-94
Furthermore, the soft character of ESI permits the transfer of the ions in the gas phase with
minimum fragmentation, reflecting the actual species present in the solution. ESI-MS is a
sensitive and direct method of detection that enables the analysis of highly diluted
solutions and the examination of several species in solution simultaneously.91, 94, 95 Another
attractive feature of ESI-MS is the flexibility with regard to the sample medium, as a wide
range of solvents can be used and a wide pH range can be tolerated.93
Mass spectrometry investigations of complex coordination clusters is furthermore
facilitated by the large number of natural isotopes that some of the elements possess. As a
result, the mass spectrum of metal complexes displays characteristic wide distributions of
isotopic envelopes that can be used for the interpretation of the spectra. Thus, the isotopic
distribution gives the possibility to identify the presence and the number of polyisotopic
18
Chapter 1 – Introduction
metals by comparison of experimental spectra with theoretically calculated isotopic
patterns.93, 95, 96 The shape of the isotopic envelope is determined by the type and number
of polyisotopic elements present. A high number of polyisotopic elements, e.g.
molybdenum (92Mo, 14.8%;
94
Mo, 9.3%;
95
Mo, 15.9%;
96
Mo, 16.7%;
97
Mo, 9.6%;
98
Mo,
24.1%; 100Mo, 9.6%) and tungsten isotopes (180W, 0.1%; 182W, 26.5%; 183W, 14.3%; 184W,
186
W, 28.4%) even can give rise to a Gaussian type distribution of the resulting
30.6%;
isotopic envelope (see Figure 1.13). Some other non-metallic elements have distinct
isotopic distributions that can be used as “fingerprints” for their identification, i.e. chlorine
(35Cl, 75.8%; 37Cl, 24.2%) and bromine atoms (79Br, 50.7%; 81Br, 49.3%).95, 97
An early example that demonstrated that a polynuclear cluster can be examined by
ESI-MS
was
conducted
by
Colton
and
co-workers
in
1992
using
the
heteropolyoxomolybdate (NEt4)4[S2Mo18O62] dissolved in acetonitrile solution. The study
demonstrated that the intact [S2Mo18O62]4- cluster ion can be observed in the mass
spectrum and its isotopic pattern was compared with the calculated spectrum (Figure
1.13).97
Figure 1.13 – Comparison of calculated and experimental negative-ion ESI mass spectral isotope
pattern for [S2Mo18O62]4-.97
In addition to the here discussed main advantages of ESI-MS, a number of potential
limitations must also be considered for data interpretation. Limitations when investigating
19
Chapter 1 – Introduction
solution processes may include artefacts or signals in the mass spectrum due to ionisation
processes or gas-phase reactions such as adduction, fragmentation or polymerisation, as
well as redox processes.92, 93
Until very recently, mass spectrometry in coordination chemistry was mainly used
as a complementary tool for the characterisation of novel compounds. Such studies
involved structural characterisations of metal complexes dissolved in various solvent
systems.92, 93, 95 In these studies, MS was assisted by other techniques such as NMR, X-ray
crystallography, electrochemistry, etc.92,
96
On the other hand, recent directions of MS
demonstrate that this technique is also capable of monitoring reaction mixtures in order to
investigate the formation processes and reactivity of complex coordination clusters in
solution.98-100 Only a few studies of this kind were carried out to date. An early example of
real-time monitoring of reacting polyoxometalate species in solution by ESI-MS is
represented by the work performed by Howarth et al. in 1997.101 Aqueous solutions of two
separate tungsten and molybdenum complexes were mixed and the ESI-MS spectra of the
reaction mixture were recorded over a time period of 45 min (Figure 1.14). The
experimental results showed that the two anions [HPW12O40]2- and [HPMo12O40]2interchange metal ions with one another. The most intense signal in the mass spectrum
recorded after 2 min after the solutions were mixed is due to the [HPW9Mo3O40]2- ion.
During a time period of 45 min the relative distribution of the metal species for the anions
reaches an equilibrium and the most intense signal in the mass spectrum corresponds to the
[HPW6Mo6O40]2- ion.101
Figure 1.14 – The ESI mass spectra of mixtures of phosphododecatungstate and
phosphododecamolybdate acquired (a) 2, (b) 17, (c) 32 and (d) 47 min after the two solutions were
initially mixed. Labels a-m correspond, in order, to the formulae [HPWnMo12-nO40]2- with n = 0 to 12.101
20
Chapter 1 – Introduction
These experiments indeed demonstrated that it is possible to monitor formation and
possible rearrangement processes of polyoxometalate species that occur in solution.
The power of MS technique in the polyoxometalate field has recently been
highlighted by Cronin et al., who used it to study the self-assembly processes of
polyoxometalate systems that emerge in solution. For example, cryospray ionisation mass
spectrometry (CSI-MS) was used to analyse the reaction solution of the compound ((nC4H9)4N)2n)[Ag2Mo8O26]n.98
a)
b)
c)
Figure 1.15 – (a) CSI-MS data collected of the reaction solution of ((n-C4H9)4N)2n)[Ag2Mo8O26]n; (b)
Representation of the [AgMo2O7]- and [AgMo4O13]- species as building blocks of the (Ag{Mo8}Ag)
synthon showing the isotopic envelopes for their corresponding mass peaks at 410.7 m/z and 700.5 m/z,
respectively; (c) Structural representation of the higher mass fragments identified within the CSI-MS
analysis of the reaction solution of ((n-C4H9)4N)2n)[Ag2Mo8O26]n.98
21
Chapter 1 – Introduction
CSI-MS allowed access to direct observation of the rearrangement of Lindqvist
anions into the (Ag{Mo8}Ag) synthon units and subsequent addition of the organic cations.
The mass spectrum of the reaction mixture is presented in Figure 1.15 and reveals the
presence of six monoanionic series: [MomO3m]- (m = 2, 3 or 5); [HMomO3m+1]- (m = 2 to 6);
[H7MomO3m+2]- (m = 2 to 6); [H7MomO3m+3]- (m = 2 to 5); [H9MomO3m+4]- (m = 2 to 6);
[AgMomO3m+1]- (m = 2 to 4). The detection of the [AgMo2O7]- (signal at m/z = 410.7
a.m.u.) and [AgMo4O13]- (signal at m/z = 700.5 a.m.u.) fragments of the (Ag{Mo8}Ag)
synthon units are particularly important in order to understand the formation of the
compound. Also the detection of the species [(AgMo8O26)TBA2]- (signal at m/z = 1776.6
a.m.u.),
[(Ag2Mo8O26)(Mo4O13)TBA3]- (signal
at
m/z
=
2718.3
a.m.u.),
and
[(Ag2Mo8O26)(Mo8O26)TBA5]- (signal at m/z = 3796.5 a.m.u.) (Figure 1.15), containing an
increasing number of organic cations is of great importance. The analysis demonstrates
that the nuclearity of the chain-type compound increases with the number of organic
cations present highlighting that the formation process involves “monomeric” units that
assemble
into
larger
fragments
which
eventually
lead
to
crystals
of
((n-
C4H9)4N)2n)[Ag2Mo8O26]n. The experimental results also demonstrate how organic cations
can impose structure-directing effects on POM structures that assemble in solution.98
A related study, reported in 2011, employs electrospray ionisation mass
spectrometry (ESI-MS) to monitor in real-time the self-assembly of an organic-inorganic
POM cluster. The reaction system involves the rearrangement of [ -Mo8O26]4- into ((nC4H9)4N)3[MnMo6O18((OCH2)3CNH2)2], a Mn-Anderson type cluster (Figure 1.16).99 ESIMS spectra of the reaction mixture containing ((n-C4H9)4N)3[MnMo6O18((OCH2)3CNH2)2]
were recorded at different time intervals over the course of the reaction (∼ 30 h), in order to
identify different fragment ions present in the reaction mixture. A very important
observation of the study is that the intensity of the signal attributed to the reactant ions
decreases exponentially, whilst the intensity of the product signal increases at a lower rate
over the course of the reaction. This suggests that the mechanism of formation of
[MnIIIMo6O18((OCH2)3CNH2)2TBA2]- proceeds through further intermediates. ESI-MS
studies allowed the identification of intermediate fragment ions involved in the
rearrangement of [ -Mo8O26]4- into ((n-C4H9)4N)3[MnMo6O18((OCH2)3CNH2)2]. It was
proposed that the rearrangement initially results in the formation of the [Mo4O13]2- cluster
species (i.e. [Mo4O13Na]-, m/z = 614.6 a.m.u. and [Mo4O13TBA]-, m/z = 833.8 a.m.u.)
representing half-fragments of the {Mo8} clusters. It is believed that further decomposition
to smaller fragment ions (i.e. [Mo2O7H]-, m/z = 304.8 a.m.u. and [Mo3O10TBA]-, m/z =
22
Chapter 1 – Introduction
690.0 a.m.u.) which subsequently bind to tris(hydroxymethyl)aminomethane ligands
(TRIS)
([Mo2O5((OCH2)3CNH2)]-
m/z
=
389.8
a.m.u.),
manganese
ions
([MnIIIMo3O8((OCH2)3CNH2)2- m/z = 706.7 a.m.u.), and anionic molybdate units, leads to
the
formation
of
the
final
Mn-Anderson-TRIS
cluster,
[MnIIIMo6O18((OCH2)3CNH2)2TBA2]- (m/z = 1640.0 a.m.u.) (Figure 1.16).99
a)
b)
Figure 1.16 – (a) ESI mass spectrum recorded of the reaction mixture of ((nC4H9)4N)3[MnMo6O18((OCH2)3CNH2)2]; spectrum was recorded after it has been heated at reflux (80°C)
for approximately 30 h; (b) Illustration to visualize the prominent, intermediate fragment ions identified
in this study (labelled b-f), which are involved in the rearrangement of the [ -Mo8O26]4- anion (labelled
a), into the symmetrical Mn-Anderson anion [MnMo6O18((OCH2)3CNH2)2]3- (labelled g). Structures (bf) represent the following fragment ions identified in the ESI-MS investigations: b) [Mo4O13TBA]-, c)
[Mo2O7H]-, d) [Mo3O10TBA]-, e) [Mo2O5((OCH2)3CNH2)]-, f) [MnIIIMo3O8((OCH2)3CNH2)2-. Colour
scheme: Mo green polyhedra, Mn orange polyhedra, O red, N blue, C gray spheres.99
Mass spectrometry proves to be an indispensable tool in coordination chemistry
which when used in combination with other techniques (i.e. X-ray crystallography, NMR
23
Chapter 1 – Introduction
spectroscopy, electrochemistry, etc.) provides important information about the analyte (e.g.
structure, purity and composition). Most remarkably the analytical technique provides the
possibility of exploring real-time growth reactions of complex coordination clusters.91, 92,
100
1.4 MOLECULAR MAGNETISM
Magnetic materials are considered to be indispensable in our daily lives finding
applications in electronics, acoustic and telecommunication devices, data storage and
readout devices, diagnostic equipment and medical therapy, etc.102
Molecular magnetism is an interdisciplinary area that deals with the design,
synthesis, and characterisation of molecular-based magnetic materials.103
Single molecule magnets (SMMs) are isolated molecules with a finite number of
interacting spin centres. SMMs represent a class of compounds that combine a defined spin
ground state (S) and a large and negative magnetic anisotropy (D). This type of compound
shows slow relaxation of the magnetisation at low temperatures, due to the large spin
reversal barrier (U).102-104
SMMs are fascinating materials as they show magnetic hysteresis effects as
classical magnets do, but at the same time their nanoscopic nature gives rise to quantum
effects. The combination of classical and quantum mechanical phenomena makes SMMs
highly attractive for theoretical studies.102,
105
Furthermore, the well defined spin or
quantum states of SMMs may lead to future prospective applications in the IT sector
whereby information could be stored at high density using a single molecule. This
conceptional approach could satisfy the need for the miniaturisation of electronic devices
and increase the data storage capacity.102, 105, 106
The most representative class of SMMs is based on manganese clusters.105, 107-109
Other large molecular clusters such as iron-oxo clusters110-112, polyoxovanadates113-116 or
lanthanide based compounds117-119 have also shown to give rise to SMM behaviour (Table
1.1).
24
Chapter 1 – Introduction
Table 1.1 Examples of single molecule magnets.120
Molecule
S
D (cm-1)
Ref.
51/2
- 0.02
[109]
33/2
- 0.04
[111]
3
- 1.5
[113]
11
- 0.4
[119]
[Mn25O18(OH)2(N3)12(pdm)6(pdmH)6](Cl)2
(pdmH2 = pyridine-2,6-dimethanol)
{Mn25}
[Fe19O6(OH)14(metheidi)10(H2O)12]+
(metheidi = N-(1-hydroxymethylethyl)iminodiacetic acid)
{Fe19}
[V4O2(EtCO2)7(bpy)2]+
(bpy = 2,2`-bipyridine)
{V4}
[Mn21DyO20(OH)2(ButCO2)20(HCO2)4(NO3)3(H2O)7]
{Mn21Dy}
25
Chapter 1 – Introduction
The first reported example of a single-molecule magnet is a dodecanuclear
manganese cluster, [MnIV4MnIII8O12(CH3COO)16(H2O)4]⋅2CH3COOH⋅4H2O (Figure 1.17),
synthesised by Lis in 1980121 and intensively studied in the 1990s.106, 122, 123 In 1993 Novak
et al. reported that the magnetisation of the {Mn12} cluster is highly anisotropic and that
the relaxation time of the magnetisation becomes very long below 4K, giving rise to
pronounced hysteresis effects (Figure 1.17).123 The compound possesses a large spin
ground state which arises from antiferromagnetic interactions between the spins of the four
MnIV ions (S = 3/2) and the spins of the eight MnIII ions (S = 2) to give a total spin of S =
[(4 x 3/2) – (8 x 2)] = 10. The magnetisation relaxation half-life of the {Mn12} cluster is
more than 2 months at 2 K. Unfortunately the relaxation time of the magnetisation
decreases dramatically with increasing temperature and the hysteresis effects are no longer
observed above 4 K.103,
104
The hysteresis loop observed for a single crystal at low
temperature (Figure 1.17) shows step-like features that occur at values of the applied field
where the energies of different spin states coincide. At these particular values of the field
the relaxation from one spin state to another is enhanced, due to quantum tunnelling of the
magnetisation (QTM) through the energy barrier U.104, 106
Since the discovery of the first SMM, significant research work has been devoted to
the development of molecule-based magnets that display spin reversal barriers (U) large
enough to be able to show magnetic hysteresis effects at higher temperature.102, 104
[Mn12O12(CH3COO)16(H2O)4]⋅2CH3COOH⋅4H2O and its
magnetisation hysteresis loops measured at low temperature between magnetic fields of 5
Figure 1.17 – The crystal structure of
106, 121
26
Chapter 1 – Introduction
Even though an increasing number of metal complexes have been demonstrated to
behave as SMMs, {Mn12} still remains the SMM with the most striking features.
The
largest
SMM
yet
reported
is
a
{Mn84}
cluster,
[Mn84O72(O2CMe)78(OMe)24(MeOH)12(H2O)42(OH)6], synthesised by Christou et al. in
2004 (Figure 1.18).105 The cluster adopts a circular torus motif with a diameter of ∼ 4.2
nm, a height of ∼ 1.2 nm and an inner free space with a diameter of ∼ 1.9 nm. It has a
ground state spin of S = 6 and exhibits both magnetic hysteresis and QTM.103, 105
Figure 1.18 – The crystal structure of the {Mn84} torus. The rectangle shows the repeating {Mn14} unit
that represents the contents of the asymmetric unit. Magnetisation hysteresis loops measured on
single crystals of {Mn84} at low temperature showing SMM behaviour.105
Another
interesting
SMM
is
a
{Mn6}
complex,
[MnIII6O2(Et-
sao)6(O2CPh(Me)2)2(EtOH)6] (Et-saoH2 = 2-hydroxyphenylpropane oxime), that exhibits
the highest energy barrier for the reversal of the magnetisation Ueff = 86.4 K and a blocking
temperature of ∼ 4.5 K. This complex breaks the record held by the {Mn12} family which
was known to show the highest blocking temperatures (∼ 3.5 K). {Mn6} has a ground state
spin of S = 12 and exhibits both magnetic hysteresis and QTM (Figure 1.19).124
The greatest spin ground state of S = 83/2 was achieved in a {Mn19} complex,
[MnIII12MnII7(µ4-O)8(µ3,η1-N3)8(HL)12-(MeCN)6]Cl2·10MeOH·MeCN
(H3L
=
2,6-
bis(hydroxymethyl)-4-methylphenol), which comprises twelve MnIII (S = 2) and seven
MnII (S = 5/2) ions. Despite this large spin, {Mn19} does not exhibit SMM behaviour due
to a very small positive D.125 Anisotropy can be introduced in the {Mn19} complex by
27
Chapter 1 – Introduction
replacing the central MnII with a DyIII ion to generate a {Mn18Dy} complex,
[MnIII12MnII6DyIII(µ4-O)8(µ3-Cl)6.5(µ3-N3)1.5(HL)12(MeOH)6]Cl3·25MeOH, with the same
core topology but showing SMM behaviour (Figure 1.20).117
Figure 1.19 – The crystal structure of the {Mn6} complex and its hysteresis loops measured at a
constant field sweep rate of 1 mT s-1 at low temperature.124
and its hysteresis loops measured
at a field scan rate of 2 mT s in the temperature range 0.5-0.04 K.117
Figure 1.20 – The crystal structure of the {Mn18Dy} complex
-1
28
Chapter 1 – Introduction
1.5 AIMS AND OBJECTIVES
The main aim of this research project is to synthesise hybrid organic-inorganic
materials using a series of mono- and trifunctional organophosphonate and -arsonate
ligands. These products could possess advantageous properties which could lend
themselves to applications in the areas of catalysis, gas sorption and separation,
magnetism. The research developed in the course of this project is focused on the solution
behaviour, synthesis and structure elucidation of novel hybrid organic-inorganic materials.
In the first part of the work we set out to explore the formation of mixed-valent
molybdates in the presence of organoarsonates and -phosphonates. As already discussed in
the previous sections of the introduction, early transition metal ions in their higher
oxidation states have the ability to polarise terminal O2– ligands efficiently, often resulting
in stable purely ‘inorganic’ oxo-clusters.21 The terminal metal-oxygen bonds limit the
functionalisation of these clusters and advanced applications, for instance in catalysis are
hampered by the limited availability of hybrid polyoxometalates that contain organic
molecules. Organophosphonates and -arsonates appear to be good candidates for POM
functionalisation as they have tetrahedral binding sites which display geometrical and
electronic similarities to metal-oxygen units in polyoxometalates. Functionalisation of
spherical clusters with rigid polyphosphonate and –arsonate ligands that point in defined
directions of space might link the clusters into polyoxometalate metal-organic frameworks
(P-MOFs) that are highly desired for catalysis, gas sorption and separation studies.
The second part of this project is focused on the preparation of novel polynuclear
manganese clusters stabilised by organophosphonates. The resulting coordination clusters
may display interesting magnetic properties. The chemistry of manganese coordination
clusters has drawn much attention due to their potential to exhibit properties of single
molecule magnets. Carboxylate ligands have previously been intensively exploited to
stabilise
high-nuclearity
molecular
compounds
of
manganese,
whereas
organophosphonates have rarely been employed.126 However, organophosphonates appear
to be good candidates to stabilise such species as they are versatile ligands that show high
binding affinities towards metal ions.67, 127-129
A third aspect of the project explores the use of rigid extended triphosphonate
ligands for the formation of supramolecular coordination networks. Porous hybrid organic29
Chapter 1 – Introduction
inorganic materials are of particular interest for potential applications in catalysis, gas
storage and separation. Several classes of MOFs have been explored, of which the
carboxylate-based compounds are the most studied. This class of MOFs may possess high
surface areas and uniform pore size distribution, but unfortunately the structural stability of
many MOFs can be quite limited. This prompted us to explore the preparation of network
structures using organic triphosphonate ligands. Organophosphonates are known to form
stronger bonds with metal ions than carboxylates and therefore have a greater potential to
form robust porous solids.67 Furthermore, the multidentate binding mode of the
triphosphonate ligands makes them ideal for the preparation of hybrid materials with
unprecedented topologies or enhanced porosity.
Another aspect of the work aims to investigate the stability, formation processes
and the reactivity of complex polynuclear clusters in solution. For this study we decided to
take advantage of electrospray ionisation mass spectrometry (ESI-MS) which is a very
powerful analytical tool. ESI-MS proved to be very efficient in monitoring complex
reaction mixtures in order to gain a better understanding of how polynuclear clusters form
and how we can influence the formation processes towards desired materials. Solution
studies of polyoxometalates that involve ESI-MS screening of reaction mixtures is still at
its infancy, while ESI-MS studies that investigate complex Mn coordination cluster
systems have not been reported in the literature. This analytical technique was also applied
to investigate the stability of the synthesised compounds in solution, an essential
requirement for potential applications, e.g. as homogeneous catalysts.
The organophosphonate and -arsonate ligands used for the preparation of these
materials are presented in Figure 1.21.
In order to accomplish the main aims of the research project, we initially set out to
attain the following specific objectives:
To synthesise and characterise rigid polyphosphonate and -arsonate ligands;
To stabilise supramolecular coordination complexes and networks using
organophosphonates and -arsonates;
To monitor the formation of these compounds via mass spectrometry and to
investigate the stability of these compounds in solution;
To structurally characterise the compounds using single crystal and powder
X-ray diffraction;
30
Chapter 1 – Introduction
To investigate the magnetic properties of all relevant compounds;
To determine other supplemental physicochemical properties (optical
properties, thermal stability, surface areas etc.) of the compounds.
AsO3H2
AsO3H2
AsO3H2
AsO3H2
NO2
NH2
PAA
OH
APAA
HPAA
OH
HNPAA
PO3H2
PO3H2
PO3H2
PPA
BPA
P-TPM
PO3H2
PO3H2
H2O3P
P-TPB
PO3H2
H2O3P
P-TBB
PO3H2
Figure 1.21 – Phosphonate and arsonate ligands used to stabilise supramolecular coordination
complexes and networks: PAA – phenylarsonic acid; APAA – (4-aminophenyl)arsonic acid (p-arsanilic
acid); HPAA – (4-hydroxyphenyl)arsonic acid; HNPAA – (4-hydroxy-3-nitrophenyl)arsonic acid; PPA
- phenylphosphonic acid; BPA – benzylphosphonic acid; P-TPM – triphenylmethylphosphonic acid; PTPB – 1,3,5-tris(4-phosphonophenyl)benzene; P-TBB – 1,3,5-tris(4`-phosphonobiphenyl-4-yl)benzene.
31
Chapter 1 – Introduction
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
D.-L. Long, E. Burkholder and L. Cronin, Chemical Society Reviews, 2007, 36,
105-121.
B. Hasenknopf, K. Micoine, E. Lacôte, S. Thorimbert, M. Malacria and R.
Thouvenot, European Journal of Inorganic Chemistry, 2008, 2008, 5001-5013.
A. Proust, R. Thouvenot and P. Gouzerh, Chemical Communications, 2008, 18371852.
C. L. Hill and C. M. Prosser-McCartha, Coordination Chemistry Reviews, 1995,
143, 407-455.
T. Yamase, Chemical Reviews, 1998, 98, 307-326.
I. Kozhevnikov and S. M. Roberts, Chichester: Wiley, 2002, 2.
A. Müller, M. Luban, C. Schröder, R. Modler, P. Kögerler, M. Axenovich, J.
Schnack, P. Canfield, S. Bud'ko and N. Harrison, A European Journal of Chemical
Physics and Physical Chemistry, 2001, 2, 517-521.
P. Wang, X. Wang and G. Zhu, Electrochimica Acta, 2000, 46, 637-641.
M.-S. Wang, G. Xu, Z.-J. Zhang and G.-C. Guo, Chemical Communications, 2010,
46, 361-376.
W. Qi, Y. Wang, W. Li and L. Wu, Chemistry – A European Journal, 2010, 16,
1068-1078.
K. C. Dey and V. Sharma, ChemInform, 2010, 41, 368-375.
Z. Wang, R. Zhang, Y. Ma, L. Zheng, A. Peng, H. Fu and J. Yao, Journal of
Materials Chemistry, 2010, 20, 1107-1111.
Z. Wang, R. Zhang, Y. Ma, A. Peng, H. Fu and J. Yao, Journal of Materials
Chemistry, 2010, 20, 271-277.
U. Kortz, A. Müller, J. van Slageren, J. Schnack, N. S. Dalal and M. Dressel,
Coordination Chemistry Reviews, 2009, 253, 2315-2327.
D. E. Katsoulis, Chemical Reviews, 1998, 98, 359-388.
N. Mizuno and M. Misono, Chemical Reviews, 1998, 98, 199-218.
A. Müller, E. Beckmann, H. Bögge, M. Schmidtmann and A. Dress, Angewandte
Chemie International Edition, 2002, 41, 1162-1167.
P. Mialane, A. Dolbecq and F. Secheresse, Chemical Communications, 2006, 34773485.
D.-L. Long and L. Cronin, Chemistry – A European Journal, 2006, 12, 3698-3706.
P. Gouzerh and A. Proust, Chemical Reviews, 1998, 98, 77-112.
J. M. Breen and W. Schmitt, Angewandte Chemie International Edition, 2008, 47,
6904-6908.
L. C. W. Baker and D. C. Glick, Chemical Reviews, 1998, 98, 3-50.
J. W. Illingworth and J. F. Keggin, Journal of the Chemical Society, 1935, 575-580.
H. T. Evans, Journal of the American Chemical Society, 1948, 70, 1291-1292.
B. Dawson, Acta Crystallographica, 1953, 6, 113-126.
W. A. LaRue, A. T. Liu and J. San Filippo, Inorganic Chemistry, 1980, 19, 315320.
Y. Du, A. Rheingold and E. A. Maatta, Journal of the American Chemical Society,
1992, 114, 345-346.
J. Strong, R. Ostrander, A. Rheingold and E. A. Maatta, Journal of the American
Chemical Society, 1994, 116, 3601-3602.
A. Müller, E. Krickemeyer, H. Bögge, M. Schmidtmann, C. Beugholt, P. Kögerler
and C. Lu, Angewandte Chemie International Edition, 1998, 37, 1220-1223.
32
Chapter 1 – Introduction
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
A. Müller, P. Kögerler and A. W. M. Dress, Coordination Chemistry Reviews,
2001, 222, 193-218.
Y.-F. Song, D.-L. Long and L. Cronin, Angewandte Chemie International Edition,
2007, 46, 3900-3904.
H.-X. Guo and S.-X. Liu, Inorganic Chemistry Communications, 2004, 7, 12171220.
J. Schnack, M. Brüger, M. Luban, P. Kögerler, E. Morosan, R. Fuchs, R. Modler,
H. Nojiri, R. C. Rai, J. Cao, J. L. Musfeldt and X. Wei, Physical Review B, 2006,
73, 094401.
R. Prudent, V. Moucadel, B. Laudet, C. Barette, L. Lafanechère, B. Hasenknopf, J.
Li, S. Bareyt, E. Lacôte, S. Thorimbert, M. Malacria, P. Gouzerh and C. Cochet,
Chemistry and Biology, 2008, 15, 683-692.
H. N. Miras, E. F. Wilson and L. Cronin, Chemical Communications, 2009, 12971311.
L. Cronin, Comprehensive Coordination Chemistry II, 2003, 7, 1-56.
D.-L. Long, R. Tsunashima and L. Cronin, Angewandte Chemie International
Edition, 2010, 49, 1736-1758.
A. Müller, P. Kögerler and C. Kuhlmann, Chemical Communications, 1999, 13471358.
H. T. Evans, B. M. Gatehouse and P. Leverett, Journal of the Chemical Society,
Dalton Transactions, 1975, 505-514.
A. J. Wilson, V. McKee, B. R. Penfold and C. J. Wilkins, Acta Crystallographica
Section C, 1984, 40, 2027-2030.
J. L. Garin and J. A. Costamagna, Acta Crystallographica, 1988, 44, 779-782.
J. Fielden, K. Quasdorf, L. Cronin and P. Kogerler, Dalton Transactions, 2012, 41,
9876-9878.
H. Yu, X. Zhang, L. Kong and J. Xu, Acta Crystallographica, 2009, 65, 1698-1699.
Z. Han, W. Pei, J. Xie, Y. Zou and X.-M. Ren, Inorganic Chemistry
Communications, 2012, 16, 61-64.
A. Müller, J. Meyer, E. Krickemeyer, C. Beugholt, H. Bögge, F. Peters, M.
Schmidtmann, P. Kögerler and M. J. Koop, Chemistry – A European Journal,
1998, 4, 1000-1006.
A. Müller, E. Krickemeyer, J. Meyer, H. Bögge, F. Peters, W. Plass, E. Diemann,
S. Dillinger, F. Nonnenbruch, M. Randerath and C. Menke, Angewandte Chemie
International Edition in English, 1995, 34, 2122-2124.
A. Muller, S. Q. N. Shah, H. Bogge and M. Schmidtmann, Nature, 1999, 397, 4850.
A. Muller and P. Gouzerh, Chemical Society Reviews, 2012.
J. J. Borrás-Almenar, E. Coronado, A. Müller and M. Pope, Polyoxometalate
Molecular Science, NATO Science Series, Kluwer Academic Publishers, 2003, 98.
E. Coronado and C. J. Gómez-García, Chemical Reviews, 1998, 98, 273-296.
S.-T. Zheng and G.-Y. Yang, Chemical Society Reviews, 2012.
J.-D. Compain, P. Mialane, A. Dolbecq, I. M. Mbomekallé, J. Marrot, F.
Sécheresse, E. Rivière, G. Rogez and W. Wernsdorfer, Angewandte Chemie
International Edition, 2009, 48, 3077-3081.
H. Kang, S. Lui, S. Shaikh, T. Nicholson and J. Zubieta, Inorganic Chemistry,
1989, 28, 920-933.
W. Clegg, R. Errington, D. Hockless and C. Radshaw, Polyhedron, 1989, 8, 17881789.
R. Errington, R. Wingad, W. Clegg and M. Elsegood, Angewandte Chemie
International Edition in English, 2000, 39, 3889.
33
Chapter 1 – Introduction
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
A. Mazeaud, N. Ammari, F. Robert and R. Thouvenot, Angewandte Chemie
International Edition in English, 1996, 35, 1961-1964.
F. Xin, M. T. Pope, G. J. Long and U. Russo, Inorganic Chemistry, 1996, 35, 12071213.
H. Kwen, S. Tomlinson, E. A. Maatta, C. Dablemont, R. Thouvenot, A. Proust and
P. Gouzerh, Chemical Communications, 2002, 2970-2971.
C. Dablemont, C. G. Hamaker, R. Thouvenot, Z. Sojka, M. Che, E. A. Maatta and
A. Proust, Chemistry – A European Journal, 2006, 12, 9150-9160.
J. B. Strong, B. S. Haggerty, A. L. Rheingold and E. A. Maatta, Chemical
Communications, 1997.
A. Dolbecq, E. Dumas, C. d. R. Mayer and P. Mialane, Chemical Reviews, 2010,
110, 6009-6048.
A. Müller, K. Hovemeier, E. Krickemeyer and H. Bögge, Angewandte Chemie
International Edition in English, 1995, 34, 779-781.
B. J. S. Johnson, S. A. Geers, W. W. Brennessel, J. V. G. Young and A. Stein,
Dalton Transactions, 2003, 4678-4681.
C. Sassoye, K. Norton and S. C. Sevov, Inorganic Chemistry, 2003, 42, 1652-1655.
N. Calin and S. C. Sevov, Inorganic Chemistry, 2003, 42, 7304-7308.
A. Müller, K. Hovemeier and R. Rohlfing, Angewandte Chemie, 1992, 104, 12141216.
A. Clearfield and K. Demadis, Royal Society of Chemistry, 2011.
P. DeBurgomaster, A. Aldous, H. Liu, C. J. O’Connor and J. Zubieta, Crystal
Growth & Design, 2010, 10, 2209-2218.
Y.-D. Chang and J. Zubieta, Inorganica Chimica Acta, 1996, 245, 177-198.
D.-G. Lyxell, D. Bostrom, M. Hashimoto and L. Pettersson, Acta
Crystallographica Section B, 1998, 54, 424-430.
T. Ueda, T. Yonemura, M. Shiro, M. Fukudome and M. Hojo, Inorganic Chemistry
Communications, 2007, 10, 1301-1303.
A. Gammersbach, P. Angenault, W. F. Hölderich, W. Clegg and M. R. J. Elsegood,
Polyhedron, 1998, 17, 2535-2541.
M. I. Khan, Q. Chen and J. Zubieta, Inorganica Chimica Acta, 1995, 235, 135-145.
A. Dolbecq, L. Lisnard, P. Mialane, J. Marrot, M. Bénard, M.-M. Rohmer and F.
Sécheresse, Inorganic Chemistry, 2006, 45, 5898-5910.
C. d. Peloux, A. Dolbecq, P. Mialane, J. Marrot and F. Secheresse, Dalton
Transactions, 2004, 1259-1263.
R. C. Finn, E. Burkholder and J. Zubieta, Chemical Communications, 2001, 18521853.
E. Dumas, C. Sassoye, K. D. Smith and S. C. Sevov, Inorganic Chemistry, 2002,
41, 4029-4032.
W. Kwak, L. M. Rajkovic, J. K. Stalick, M. T. Pope and C. O. Quicksall, Inorganic
Chemistry, 1976, 15, 2778-2783.
K. M. Barkigia, L. M. Rajkovic-Blazer, M. T. Pope and C. O. Quicksall, Inorganic
Chemistry, 1981, 20, 3318-3323.
Y.-Q. Lan, S.-L. Li, X.-L. Wang, K.-Z. Shao, D.-Y. Du, H.-Y. Zang and Z.-M. Su,
Inorganic Chemistry, 2008, 47, 8179-8187.
J. L. C. Rowsell, E. C. Spencer, J. Eckert, J. A. K. Howard and O. M. Yaghi,
Science, 2005, 309, 1350-1354.
O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser, C. E. Wilmer, A. A. Sarjeant,
R. Q. Snurr, S. T. Nguyen, A. Ö. Yazaydın and J. T. Hupp, Journal of the
American Chemical Society, 2012, 134, 15016-15021.
34
Chapter 1 – Introduction
83.
84.
85.
86.
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
O. M. Yaghi, M. O'Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim,
Nature, 2003, 423, 705.
M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O'Keeffe and O. M.
Yaghi, Science, 2002, 295, 469-472.
H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, A. Ö. Yazaydin, R.
Q. Snurr, M. O’Keeffe, J. Kim and O. M. Yaghi, Science, 2010, 329, 424-428.
D. J. Tranchemontagne, J. L. Mendoza-Cortes, M. O'Keeffe and O. M. Yaghi,
Chemical Society Reviews, 2009, 38, 1257-1283.
A. R. Millward and O. M. Yaghi, Journal of the American Chemical Society, 2005,
127, 17998-17999.
L. Yuan, C. Qin, X. Wang, Y. Li and E. Wang, Dalton Transactions, 2009, 41694175.
Xiang, Wu, ZhangZhang, Fu, Hu and ZhangZhang, Journal of the American
Chemical Society, 2005, 127, 16352-16353.
A. El-Aneed, A. Cohen and J. Banoub, Applied Spectroscopy Reviews, 2009, 44,
210-230.
R. Colton, A. D'Agostino and J. C. Traeger, Mass Spectrometry Reviews, 1995, 14,
79-106.
D. Schaumlöffel and A. Tholey, Analytical and Bioanalytical Chemistry, 2011,
400, 1645-1652.
M. J. Keith-Roach, Analytica Chimica Acta, 2010, 678, 140-148.
I. I. Stewart, Spectrochimica Acta Part B: Atomic Spectroscopy, 1999, 54, 16491695.
R. Jirásko and M. Hol apek, Mass Spectrometry Reviews, 2011, 30, 1013-1036.
B. F. G. Johnson J and S. McIndoe, Coordination Chemistry Reviews, 2000, 200–
202, 901-932.
R. Colton and J. C. Traeger, Inorganica Chimica Acta, 1992, 201, 153-155.
E. F. Wilson, H. Abbas, B. J. Duncombe, C. Streb, D.-L. Long and L. Cronin,
Journal of the American Chemical Society, 2008, 130, 13876-13884.
E. F. Wilson, H. N. Miras, M. H. Rosnes and L. Cronin, Angewandte Chemie
International Edition, 2011, 50, 3720-3724.
H. N. Miras, M. Sorus, J. Hawkett, D. O. Sells, E. J. L. McInnes and L. Cronin,
Journal of the American Chemical Society, 2012, 134, 6980-6983.
M. J. Deery, O. W. Howarth and K. R. Jennings, Journal of the Chemical Society,
Dalton Transactions, 1997, 4783-4788.
I.-R. Jeon and R. Clerac, Dalton Transactions, 2012, 41, 9569-9586.
T. Glaser, Chemical Communications, 2011, 47, 116-130.
J. R. Long, in The Chemistry Of Nanostructured Materials, World Scientific
Publishing: Hong Kong, Editon edn., 2003, pp. 291-315.
A. J. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K. A. Abboud and G. Christou,
Angewandte Chemie International Edition, 2004, 43, 2117-2121.
L. Thomas, F. Lionti, R. Ballou, D. Gatteschi, R. Sessoli and B. Barbara, Nature,
1996, 383, 145-147.
J. T. Brockman, J. C. Huffman and G. Christou, Angewandte Chemie International
Edition, 2002, 41, 2506-2508.
M. A. Bolcar, S. M. J. Aubin, K. Folting, D. N. Hendrickson and G. Christou,
Chemical Communications, 1997, 1485-1486.
M. Murugesu, M. Habrych, W. Wernsdorfer, K. A. Abboud and G. Christou,
Journal of the American Chemical Society, 2004, 126, 4766-4767.
35
Chapter 1 – Introduction
110.
111.
112.
113.
114.
115.
116.
117.
118.
119.
120.
121.
122.
123.
124.
125.
126.
127.
128.
129.
A. K. Powell, S. L. Heath, D. Gatteschi, L. Pardi, R. Sessoli, G. Spina, F. Del
Giallo and F. Pieralli, Journal of the American Chemical Society, 1995, 117, 24912502.
J. C. Goodwin, R. Sessoli, D. Gatteschi, W. Wernsdorfer, A. K. Powell and S. L.
Heath, Journal of the Chemical Society, Dalton Transactions, 2000, 1835-1840.
C. Sangregorio, T. Ohm, C. Paulsen, R. Sessoli and D. Gatteschi, Physical Review
Letters, 1997, 78, 4645-4648.
Z. Sun, D. N. Hendrickson, Z. Sun, C. M. Grant, S. L. Castro and G. Christou,
Chemical Communications, 1998, 721-722.
A. Barra, D. Gatteschi, L. Pardi, A. Müller and J. Döring, Journal of the American
Chemical Society, 1992, 114, 8509-8514.
A. Müller, R. Sessoli, E. Krickemeyer, H. Bögge, J. Meyer, D. Gatteschi, L. Pardi,
J. Westphal, K. Hovemeier, R. Rohlfing, J. Döring, F. Hellweg, C. Beugholt and
M. Schmidtmann, Inorganic Chemistry, 1997, 36, 5239-5250.
D. Gatteschi, L. Pardi, A. L. Barra, A. Muller and J. Doring, Nature, 1991, 354,
463-465.
A. M. Ako, V. Mereacre, R. Clerac, W. Wernsdorfer, I. J. Hewitt, C. E. Anson and
A. K. Powell, Chemical Communications, 2009, 544-546.
R. Sessoli and A. K. Powell, Coordination Chemistry Reviews, 2009, 253, 23282341.
C. Papatriantafyllopoulou, W. Wernsdorfer, K. A. Abboud and G. Christou,
Inorganic Chemistry, 2011, 50, 421-423.
J. R. Long, Chemistry of Nanostructured Materials, 2003, 291-315.
T. Lis, Acta Crystallographica Section B, 1980, 36, 2042-2046.
J. R. Friedman, M. P. Sarachik, J. Tejada and R. Ziolo, Physical Review Letters,
1996, 76, 3830-3833.
R. Sessoli, D. Gatteschi, A. Caneschi and M. A. Novak, Nature, 1993, 365, 141143.
C. J. Milios, A. Vinslava, W. Wernsdorfer, S. Moggach, S. Parsons, S. P. Perlepes,
G. Christou and E. K. Brechin, Journal of the American Chemical Society, 2007,
129, 2754-2755.
A. M. Ako, I. J. Hewitt, V. Mereacre, R. Clérac, W. Wernsdorfer, C. E. Anson and
A. K. Powell, Angewandte Chemie International Edition, 2006, 45, 4926-4929.
G. E. Kostakis, A. M. Ako and A. K. Powell, Chemical Society Reviews, 2010, 39,
2238-2271.
Y.-S. Ma, Y. Song, Y.-Z. Li and L.-M. Zheng, Inorganic Chemistry, 2007, 46,
5459-5461.
M. Shanmugam, M. Shanmugam, G. Chastanet, R. Sessoli, T. Mallah, W.
Wernsdorfer and R. E. P. Winpenny, Journal of Materials Chemistry, 2006, 16,
2576-2578.
M. Wang, C. Ma, H. Wen and C. Chen, Dalton Transactions, 2009, 994-1003.
36
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.
HYBRID ORGANIC-INORGANIC
POLYOXOMOLYBDATES
37
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.1 INTRODUCTION AND MOTIVATION
The chemistry of the polyoxometalates (POMs) is one of the most active and
rapidly advancing areas of inorganic chemistry.1-4 Functionalisation approaches to generate
hybrid polyoxometalate structures or advanced inorganic materials remain an ongoing
challenge and new synthetic and analytical approaches are pivotal to the progress of the
field.2-5
We are interested in the chemistry of hybrid organic-inorganic materials, and we
decided to explore the formation of mixed-valent molybdates in the presence of
organoarsonates and phosphonates. Hybrid organic-inorganic polyoxomolybdates would
combine the characteristics of both the inorganic oxo clusters and the organic moieties
resulting in new products with potentially new or even enhanced properties.
Organophosphonate and -arsonate stabilised oxo clusters that contain redox-active metal
centres are of interest for applications in catalysis.2, 6, 7 A limited number of organoarsonate
and -phosphonate stabilised molybdates have previously been reported in the literature.8-19
Despite an ever increasing interest in POM complexes, it is surprising that their
solution behaviour, i.e. formation reactions that often prevail through a set of consecutive
condensation reactions, are often fairly poorly understood. However, speciation within
POM solutions is not only important to devise rational synthetic approaches to desired
products but it is moreover a pre-requisite to improve catalytic processes that draw on the
active sites or electronic characteristics of POMs. Recent accounts demonstrate that
electrospray ionisation mass spectrometry (ESI-MS) provides a very powerful analytical
tool to characterise the formation and the reactivity of complex polyoxometalate clusters in
solution.20-33
We decided to use electrospray ionisation mass spectrometry (ESI-MS) to
investigate the self-assembly process of hybrid polyoxomolybdates that form upon partial
reduction of (NH4)6Mo7O24·4H2O in the presence of aromatic organoarsonates. We were
interested in exploring how perturbations of the ligand functionality influence the
formation of unprecedented species, and we used ESI-MS to screen the reaction mixtures
prior to crystallisation attempts. Although, several arsonate stabilised molybdates have
previously been reported,9, 11, 34-39 this mass spectrometry-guided approach allowed us to
38
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
selectively identify species with unprecedented core structures. The structure and
composition of these species were later confirmed by single crystal XRD studies.
The first part of this chapter presents the synthesis and characterisation of four
unprecedented hybrid cluster compounds stabilised by organoarsonate ligands:
(NH4)2H2[MoV4O8(O3AsC6H5)4]·5H2O (1),
(NH4)2H2[MoV4O8(O3AsC6H4NH2)4]·DMF·4H2O (2),
(NH4)5[MoVI2MoV3O11(O3AsC6H4OH)5]·9H2O (3),
(NH4)4[MoVI4O10(O3AsC6H3NO2OH)4]·2H2O (4).
Stimulated by the interest in transition metal functionalised POMs and in so-called
“sandwiched atoms” involving POMs,40 we additionally decided to explore how d-block
hetero-atoms can be incorporated into defined POM structures. A known molybdenum
phosphonate is (NH4)5Na4{Na[Mo6O12(OH)3(O3PC6H5)4)]2} 6H2O, in which two {Mo6}
fragments are linked through a Na ion.13 Considering this, we sought to investigate if
transition metal ions could be used to link the hexanuclear subunits to produce sandwichtype compounds. We intended to explore how different heteroatoms would influence the
structure and magnetic properties of the compounds. This part of the chapter presents the
synthetic approach, the structure characterisation and properties of five isostructural
compounds (4 transition metal derivatives and 1 main group compound) that self-assemble
under closely related reaction conditions:
(NH4)4H4{Mn[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (5),
(NH4)4H4{Fe[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (6),
(NH4)4H4{Co[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (7),
(NH4)4H4{Ni[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (8),
(NH4)4H4{Mg[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (9).
39
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.2 FUNCTIONALISATION OF POLYOXOMOLYBDATES USING ORGANOARSONATES AND -PHOSPHONATES
2.2.1
Synthesis and characterisation of organoarsonate functionalised
polyoxomolybdate clusters
2.2.1.1 The Phenylarsonic Acid – MoVI/MoV Reaction System
(NH4)2H2[MoV4O8(O3AsC6H5)4]·5H2O (1)
Partial reduction of (NH4)6Mo7O24 4H2O in the presence of phenylarsonic acid and
acetic acid using N2H4 H2SO4 results in a deep blue solution from which dark red crystals
of (NH4)2H2[MoV4O8(O3AsC6H5)4]·5H2O (1) separate within a time period of three weeks.
Single crystal X-ray analysis reveals that 1 crystallises in the triclinic crystal system
in the space group P and contains a tetranuclear MoV complex in which the Mo atoms and
bridging O donors adopt a typical [Mo4(µ 3-O)4]12+ cubane structure. The structure is shown
in Figure 2.1. In this structure, the four Mo atoms are situated diagonally across from each
other, occupying four corners of a cube. Four O2- oxo ligands occupy the remaining four
corners of the cube, and each one binds to three Mo atoms. The structure is further
stabilised by four fully deprotonated phenylarsonic acid ligands that each bind in a η1:η1:μ2
bridging mode to two Mo ions, resulting in an overall octanuclear compound (the As atoms
are also considered to be core atoms). The slightly distorted octahedral coordination
environment of each of the four Mo ions in 1 comprises of three µ 3-O oxo ligands, two O
donors originating from two different arsonate ligands and is completed by a terminal
Mo=O bond. The distorted nature of the octahedra arises from the geometrical restrictions
of the binding ligands and can be observed from the bond angles and bond lengths of the
MoV metal centres. The bond lengths of the Mo=O bonds range between 1.660(4) Å –
1.679(4) Å and are, as expected, significantly shorter than the Mo–(µ 3-O) distances of
1.969(4) Å – 2.424(4) Å and the Mo-Oarsonate bond distances of 2.043(4) Å – 2.090(4) Å,
(Table 2.1). The following bond angles: O(12)-Mo(1)-O(6), O(7)-Mo(1)-O(10) and O(8)Mo(1)-O(11) of 170.50(17)°, 152.42(14)° and 151.57(14)°, respectively, deviate from the
ideal octahedral angle of 180°, while the bond angles O(12)-Mo(1)-O(7), O(12)-Mo(1)O(11), O(8)-Mo(1)-O(7), O(7)-Mo(1)-O(11), O(8)-Mo(1)-O(10), O(7)-Mo(1)-O(6) and
40
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
O(8)-Mo(1)-O(6) of 110.67(16)°, 97.79(19)°, 88.68(17)°, 86.42(17)°, 85.21(16)°,
76.37(14)° and 75.74(15)°, respectively, deviate from the ideal angle of 90°. Similar values
can be found for the bond angles of the Mo(2), Mo(3) and Mo(4) ions, which are
summarized in Table 2.2.
a)
b)
Figure 2.1 – Crystal structure of the tetranuclear Mo complex in 1: (a) ball-and-stick representation,
(b) polyhedral presentation. Colour code: MoV blue, As orange, O red, C grey, H white (hydrogen
atoms have been omitted for clarity in b).
The [Mo4(µ 3-O)4]12+ core structure deviates slightly from the geometry of an ideal
cube. The Mo–O–Mo and O–Mo–O angles range between 83.82(3)º – 104.04(1)º and
76.18(1)º – 89.72(3)º, respectively, differing from the ideal angle of 90º. Bond valence sum
analysis41 confirms that all four Mo atoms in 1 adopt the oxidation state +V. Short Mo(1)Mo(2) and Mo(3)-Mo(4) contacts of 2.647(1) Å and 2.642(1) Å, respectively, are in
agreement with the assigned +V oxidation states. The cluster is further stabilised by weak
intramolecular - interactions42 with an interplanar separation distance that range between
3.604(21) Å – 4.091(19) Å.
The packing arrangement of the [MoV4O8(O3AsC6H5)4]4- clusters in the crystal
structure is stabilised by weak hydrogen bonds between the cluster anions and constitution
water molecules (O-O distances: 2.769(48) Å – 2.915(57) Å) and also between the cluster
anions and NH4+ counterions (N-O distances: 2.698(36) Å – 3.045(30) Å). The solvent
molecules are situated in small channels that run in the direction of the crystallographic aaxis (Figure 2.3). Very weak intermolecular - interactions (shortest contact 3.975(28) Å)
of the aromatic rings of the arsonate ligands also contribute to the stability of 1 in the solid
state.
41
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Figure 2.2 – The core structure of the tetranuclear Mo complex in 1. Colour code: MoV blue, As
orange, O red, C grey.
a)
b)
Figure 2.3 – Packing arrangement of 1 in the crystal structure viewed in the direction of the
crystallographic: (a) a-axis and (b) b-axis. Colour code: MoV blue, As orange, O red, C grey
(crystallization water molecules, NH4+ counterions and hydrogen atoms have been omitted for clarity).
42
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.1 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 1.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mo(1)
Mo(1)-O(12)
Mo(1)-O(7)
Mo(1)-O(8)
Mo(1)-O(11)
Mo(1)-O(10)
Mo(1)-O(6)
1.660(4)
1.989(4)
1.983(4)
2.055(4)
2.065(4)
2.405(4)
5.159
+5
Mo(2)
Mo(2)-O(19)
Mo(2)-O(7)
Mo(2)-O(8)
Mo(2)-O(15)
Mo(2)-O(16)
Mo(2)-O(5)
1.673(4)
1.974(4)
1.986(4)
2.043(4)
2.046(4)
2.424(4)
5.161
+5
Mo(3)
Mo(3)-O(2)
Mo(3)-O(5)
Mo(3)-O(6)
Mo(3)-O(14)
Mo(3)-O(9)
Mo(3)-O(8)
1.667(4)
1.970(4)
1.983(4)
2.047(4)
2.090(4)
2.364(4)
5.165
+5
Mo(4)
Mo(4)-O(1)
Mo(4)-O(5)
Mo(4)-O(6)
Mo(4)-O(3)
Mo(4)-O(4)
Mo(4)-O(7)
1.679(4)
1.969(4)
1.972(4)
2.066(4)
2.087(4)
2.386(4)
5.085
+5
Mo(1) ··· Mo(2)
Mo(3) ··· Mo(4)
2.647(1)
2.642(1)
Structurally related cubane structures were previously isolated using squaric acid
((ColMe)4[Mo4O8(C4O4)4]·2MeOH·2Col, (ColH)4(PyEt)[Mo4O8(C4O4)4]Br; Col = 2,4,6collidine), diphenylphosphinic acid (Mo4(µ 3-O)4(µ-O2PPh2)4O4) and dimethylphosphinic
acid (Mo4(µ 3-O)4(µ-O2PMe2)4O4) as ligands.43-45 However, it is noteworthy that the cubane
arrangement of MoV atoms is far less frequently observed than the structurally related
rhombic planar arrangement.43
43
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.2 − Selected bond angles [º] for compound 1.
Bond Angle (º)
Bond Angle (º)
O(12)-Mo(1)-O(6)
O(7)-Mo(1)-O(10)
O(8)-Mo(1)-O(11)
O(12)-Mo(1)-O(7)
O(12)-Mo(1)-O(11)
O(8)-Mo(1)-O(7)
O(7)-Mo(1)-O(11)
O(8)-Mo(1)-O(10)
O(7)-Mo(1)-O(6)
O(8)-Mo(1)-O(6)
170.50(17)
152.42(14)
151.57(14)
110.67(16)
97.79(19)
88.68(17)
86.42(17)
85.21(16)
76.37(14)
75.74(15)
O(19)-Mo(2)-O(5)
O(7)-Mo(2)-O(16)
O(8)-Mo(2)-O(15)
O(19)-Mo(2)-O(7)
O(19)-Mo(2)-O(15)
O(19)-Mo(2)-O(16)
O(7)-Mo(2)-O(8)
O(8)-Mo(2)-O(16)
O(16)-Mo(2)-O(5)
O(8)-Mo(2)-O(5)
171.66(17)
154.38(15)
152.70(14)
109.20(17)
98.17(17)
96.04(18)
89.02(16)
86.95(16)
77.54(15)
76.61(14)
O(2)-Mo(3)-O(8)
O(5)-Mo(3)-O(9)
O(6)-Mo(3)-O(14)
O(2)-Mo(3)-O(6)
O(2)-Mo(3)-O(9)
O(5)-Mo(3)-O(6)
O(6)-Mo(3)-O(9)
O(14)-Mo(3)-O(9)
O(5)-Mo(3)-O(8)
O(14)-Mo(3)-O(8)
170.03(19)
154.78(15)
154.28(16)
109.62(19)
95.60(18)
89.45(16)
88.05(17)
84.41(17)
78.38(15)
77.62(14)
O(1)-Mo(4)-O(7)
O(5)-Mo(4)-O(4)
O(6)-Mo(4)-O(3)
O(1)-Mo(4)-O(6)
O(1)-Mo(4)-O(3)
O(5)-Mo(4)-O(6)
O(5)-Mo(4)-O(3)
O(3)-Mo(4)-O(4)
O(5)-Mo(4)-O(7)
O(3)-Mo(4)-O(7)
170.26(17)
154.73(14)
152.73(14)
109.66(17)
96.97(18)
89.81(18)
87.12(16)
82.92(16)
77.92(15)
75.73(15)
44
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
Infrared spectroscopy was used as a basic tool for preliminary characterisation of
compound 1. The IR spectrum of 1 (Figure 2.4) reveals some characteristic stretches of the
organoarsonate ligands. These include signals at 1093 cm-1 attributed to As–C vibrations
and 1439 cm-1 associated with C–C skeletal vibrations of the phenyl rings. Additionally
observed bands arising from the molybdenum-oxygen and arsenic-oxygen stretching
vibrations appear in the 1000 – 650 cm-1 region.8, 15, 37, 46-48
Figure 2.4 – Infrared spectrum of 1.
-
Thermogravimetric analysis
The thermal stability of compound 1 was investigated by thermogravimetric
analysis (TGA) using a freshly prepared crystalline sample. The analysis was carried out in
the temperature range between 30 and 900 °C, in an N2 atmosphere. The TGA of 1 reveals
a weight loss of 6.4 % in the temperature range between 30 – 90 °C. This
45
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
thermogravimetric step can be attributed to the loss of five water molecules (calcd: 6.2 %).
A further increase in the temperature up to 500 °C produces a weight loss that occurs in a
two step process. This weight loss can be associated with the decomposition of the organic
ligands of the cluster. The weight loss centered at ca. 770 °C is most likely being
associated with the formation of oxides.
100
Weight % (%)
90
80
70
60
50
40
30
0
200
400
600
800
1000
Temperature (°C)
Figure 2.5 – Thermogravimetric analysis of 1.
-
Mass spectrometry
The stability of compound 1 in solution was investigated by electrospray ionization
mass spectrometry (ESI-MS). The negative mode ESI-MS spectrum of compound 1
dissolved in DMSO is presented in Figure 2.6. The mass spectrum displays only one major
isotopic envelope in the high molecular mass region, centered at m/z = 1315.3 a.m.u. The
signal was assigned to the {H3[MoV4O8(O3AsC6H5)4]}- species. The expected signal was
modelled (Figure 2.6, Inset) and good fits between the experimental and simulated isotopic
envelopes confirm the assignment.
46
Relative ionic abundance
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Figure 2.6 – Negative-mode ESI-MS spectra for crystals of 1 dissolved in DMSO. Inset:
Comparison of the experimental isotopic envelopes (black spectrum) with simulated patterns (red
spectrum) for {H3[MoV4O8(O3AsC6H5)4]}- centered at m/z = 1315.3 a.m.u. (cone voltage: 30 V).
-
NMR spectroscopy
The 1H NMR spectrum of 1 recorded in deuterated DMSO further confirms the
stability of the compound in DMSO environment. The spectrum of 1 exhibits resonance
shifts at 7.97 ppm (d, 8H) and 7.67 ppm (m, 12H) corresponding to aromatic H-atoms of
the arsonate ligands. A section of the 1H NMR spectrum of 1 is presented in Figure 2.7.
1
2
Figure 2.7 – A section of the 1H NMR (400MHz, DMSO) spectrum showing the assignment for 1.
47
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 1 was recorded in DMF (Figure 2.8). The
spectrum displays an absorption band at ca. 457 nm ( = 1000 L mol-1 cm-1) due to d – d
transitions involving the MoV metal centeres. These d – d transitions can be assigned to a
Eg
2
T2g transition for the octahedral MoV ions within 1.38
0.12
Absorbance (a.u.)
2
0.10
0.08
0.06
0.04
0.02
400
450
500
550
600
Wavelength (nm)
Figure 2.8 –A section of the UV-Vis spectrum of a 10-4 M solution of 1 in DMF.
48
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.3 − Crystal data and structural refinement parameters for 1.
Compound 1
Empirical formula
Molecular mass/g mol
C24H40As4Mo4N2O25
-1
Crystal colour/shape
3
1440.02
Red / triangular plate
Crystal size/mm
0.20×0.20×0.05
Crystal system
Triclinic
Space group
P
a/ Å
10.009(4)
b/ Å
15.767(5)
c/ Å
17.287(5)
/º
117.09(2)
/º
96.03(2)
/º
98.454(18)
3
V/ Å
2356.1(14)
Z
2
Temperature (K)
118(2)
-3
Density/Mg m
2.001
-1
Absorp. coef./mm
3.906
F(000)
1356
2
50
max/º
Reflections collected
36989
Independent reflections
8303 [R(int) = 0.1174]
Data / restraints / parameters
8303 / 2128 / 577
2
S on F
1.036
R1, wR2 [I>2 (I)]
0.0985, 0.2684
R1, wR2 (all data)
0.1171, 0.2869
Largest diff. peak and hole/e.Å-3
2.217 and -2.503
49
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.2.1.2 The p- Aminophenylarsonic Acid – MoVI/MoV Reaction System
(NH4)2H2[MoV4O8(O3AsC6H4NH2)4]·DMF·4H2O (2)
Encouraged by these results we decided to alter the ligand functionality and
introduced an amino group in the para position to the arsonate functionality. When this
ligand is used to control the self-assembly process upon reduction of (NH4)6Mo7O24 4H2O,
a blue solution is obtained, as in the previous case. During a time period of one week, rodshaped red crystals of (NH4)2H2[MoV4O8(O3AsC6H4NH2)4] ·DMF·4H2O (2) were obtained.
2 was characterised by single crystal X-ray diffraction measurements.
2 crystallises in the triclinic crystal system in the space group P . The core
structure within 2 (Figure 2.10) is almost isostructural to the cluster core found in 1, having
only slightly different structural and geometrical parameters. The bond valence sums and
underlying M−O and Mo−Mo distances agree very well with those observed for 1.
Selected interatomic distances and angles for compound 2 are given in Table 2.4 and Table
2.5, respectively. Similar to 1 the core structure within 2, [Mo4(µ 3-O)4]12+,
deviates
slightly from the geometry of an ideal cube. The Mo–O–Mo and O–Mo–O angles range
between 83.19(29)º – 104.78(30)º and 75.16(27)º – 90.33(30)º, respectively, differing from
the ideal angle of 90º.
a)
b)
Figure 2.9 – Crystal structure of the tetranuclear Mo complex in 2: (a) ball-and-stick representation,
(b) polyhedral presentation. Colour code: MoV blue, As orange, O red, N light blue, C grey, H white
(hydrogen atoms have been omitted for clarity in b).
50
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Figure 2.10 – The core structure of the tetranuclear Mo complex in 2. Colour code: MoV blue, As
orange, O red, C grey.
a)
b)
Figure 2.11 – Packing arrangement of 2 in the crystal structure viewed in the direction of the
crystallographic: (a) a-axis and (b) b-axis. Colour code: MoV blue, As orange, O red, N light blue, C
grey (crystallisation solvent molecules, NH4+ counterions and hydrogen atoms have been omitted for
clarity).
The amine functionalities of the organic ligands only impose an influence on the
packing arrangement of the clusters in the crystal structure, resulting in a grid-like packing
arrangement, with small intercluster cavities (filled with solvent molecules and NH4+
counterions) running in the direction of the crystallographic a-axis (Figure 2.11). The
51
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
packing arrangement of the clusters in the crystal structure is stabilised by weak hydrogen
bonds between the cluster anions and constitution solvent molecules (O-O distances:
2.847(21) Å – 3.060(24) Å) and also between the cluster anions and NH4+ counterions (NO distances: 2.861(19) Å – 3.083(23) Å). Very weak offset π-π interactions42 between two
adjacent clusters with interplanar separation distances that range between 3.475(29) Å –
4.145(32) Å also contribute to the stability of 2 in the solid state.
The protonation of the tetranuclear Mo clusters in 1 and 2, respectively, are in
agreement with the elemental analysis and the constutional representation in the form of
Hx[clustercore] is commonly applied in this field of research.9
Table 2.4 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 2.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mo(1)
Mo(1)-O(17)
Mo(1)-O(18)
Mo(1)-O(13)
Mo(1)-O(7)
Mo(1)-O(6)
Mo(1)-O(14)
1.679(9)
1.978(8)
1.976(7)
2.024(8)
2.079(8)
2.420(8)
5.122
+5
Mo(2)
Mo(2)-O(19)
Mo(2)-O(15)
Mo(2)-O(14)
Mo(2)-O(2)
Mo(2)-O(1)
Mo(2)-O(13)
1.663(9)
1.966(7)
1.967(8)
2.060(8)
2.084(8)
2.353(8)
5.227
+5
Mo(3)
Mo(3)-O(20)
Mo(3)-O(14)
Mo(3)-O(15)
Mo(3)-O(8)
Mo(3)-O(12)
Mo(3)-O(18)
1.678(8)
1.978(8)
1.985(8)
2.036(8)
2.067(8)
2.411(8)
5.112
+5
Mo(4)
Mo(4)-O(16)
Mo(4)-O(13)
Mo(4)-O(18)
Mo(4)-O(11)
Mo(4)-O(3)
Mo(4)-O(15)
1.674(9)
1.984(7)
1.983(8)
2.044(7)
2.074(8)
2.393(8)
5.110
+5
Mo(1) ··· Mo(4)
Mo(2) ··· Mo(3)
2.630(1)
2.615(1)
52
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.5 − Selected bond angles [º] for compound 2.
Bond Angle (º)
Bond Angle (º)
O(17)-Mo(1)-O(14)
O(18)-Mo(1)-O(6)
O(13)-Mo(1)-O(7)
O(17)-Mo(1)-O(13)
O(17)-Mo(1)-O(7)
O(17)-Mo(1)-O(6)
O(13)-Mo(1)-O(18)
O(7)-Mo(1)-O(6)
O(13)-Mo(1)-O(6)
O(6)-Mo(1)-O(14)
171.1(3)
155.5(3)
152.0(3)
109.6(4)
97.9(3)
95.0(3)
89.5(3)
87.4(3)
85.4(3)
78.0(3)
O(19)-Mo(2)-O(13)
O(14)-Mo(2)-O(2)
O(15)-Mo(2)-O(1)
O(19)-Mo(2)-O(14)
O(19)-Mo(2)-O(1)
O(15)-Mo(2)-O(14)
O(2)-Mo(2)-O(1)
O(15)-Mo(2)-O(2)
O(2)-Mo(2)-O(13)
O(15)-Mo(2)-O(13)
170.0(3)
155.9(3)
154.7(3)
109.4(4)
95.7(4)
90.6(3)
88.1(3)
85.7(3)
78.7(3)
77.5(3)
O(20)-Mo(3)-O(18)
O(15)-Mo(3)-O(8)
O(14)-Mo(3)-O(12)
O(20)-Mo(3)-O(15)
O(20)-Mo(3)-O(12)
O(20)-Mo(3)-O(8)
O(14)-Mo(3)-O(15)
O(15)-Mo(3)-O(12)
O(14)-Mo(3)-O(8)
O(8)-Mo(3)-O(18)
171.4(3)
155.2(3)
153.9(3)
108.9(4)
97.0(4)
95.6(4)
89.7(3)
86.8(3)
85.8(3)
78.8(3)
O(16)-Mo(4)-O(15)
O(18)-Mo(4)-O(3)
O(13)-Mo(4)-O(11)
O(16)-Mo(4)-O(13)
O(16)-Mo(4)-O(11)
O(16)-Mo(4)-O(3)
O(18)-Mo(4)-O(13)
O(3)-Mo(4)-O(15)
O(18)-Mo(4)-O(15)
O(11)-Mo(4)-O(15)
171.5(3)
155.9(3)
152.0(3)
109.3(4)
98.2(3)
94.7(4)
89.1(3)
79.0(3)
76.9(3)
75.8(3)
53
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 2 is presented in Figure 2.12 and reveals some characteristic
stretches of the organic ligands. The set of bands observed between 1600 – 1400 cm-1 can
be attributed to C–C skeletal vibrations of the phenyl rings while the signal at 1095 cm-1
can be associated with As–C vibrations. Other characteristic stretches observed in the IR
spectrum of 2 appear as strong bands in the 1000 – 650 cm-1 region. Some of these bands
can be attributed to the molybdenum-oxygen and arsenic-oxygen stretching vibrations. The
O–H stretching vibrations and H–O–H bending vibrations of the crystallisation water
molecules engaged in H-bonds appear as broad bands centered at ca. 3200 cm-1 and ca.
1630 cm-1, respectively. Stretches relating to NH2 group of the organic ligands are
expected to appear in the same region as the O–H stretches of the crystallisation water
molecules.8, 15, 37, 46-48
Figure 2.12 – Infrared spectrum of 2.
54
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
Thermogravimetric analysis
The thermal stability of 2 was investigated by thermogravimetric analysis using a
freshly prepared crystalline sample (Figure 2.13). Compound 2 reveals a weight loss of
about 9.7 % below 150 °C corresponding to the loss of four water and one DMF
molecules (calcd: 9.3 %). From 150 to 600 °C a gradual weight loss is associated with the
decomposition of organic moieties finally producing oxide materials.
100
Weight % (%)
80
60
40
20
0
0
100
200
300
400
500
600
700
Temperature (°C)
Figure 2.13 – Thermogravimetric analysis of 2.
-
Mass spectrometry
ESI-MS studies on pristine crystals of 2 dissolved in DMSO were performed in
order to investigate the stability of the molybdenum cluster in solution. Similar to
compound 1, the mass spectrum of 2 (Figure 2.14) reveals only one major isotopic
envelope in the high molecular mass region of the spectrum. The signal is centered at m/z =
1374.4 a.m.u. and corresponds to the {H3[MoV4O8(O3AsC6H4NH2)4]}- species. Again, the
signal of the species was modelled and a comparison of the experimental isotopic
envelopes with the simulated envelope is presented in Figure 2.14, Inset. It is noteworthy
that both compounds 1 and 2 are insoluble in H2O.
55
Relative ionic abundance
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Figure 2.14 – Negative-mode ESI-MS spectra for crystals of 2 dissolved in DMSO. Inset: Comparison
of the experimental isotopic envelopes (black spectrum) with simulated patterns (red spectrum) for
{H3[MoV4O8(O3AsC6H4NH2)4]}- centered at m/z = 1374.4 a.m.u. (cone voltage: 30 V).
-
NMR spectroscopy
Similar to 1, the cluster of 2 appears to be stable in a DMSO environment. The 1H
NMR spectrum of 2 (Figure 2.15) exhibits signals at 7.56 ppm (d, 8H), 6.73 ppm (d, 8H)
and 5.80 ppm (s, 8H). The two most downfield signals were assigned to the aromatic Hatoms of the (4-aminophenyl)arsonate ligands, while the signal at 5.80 ppm was assigned
to the amine H-atoms of the organic ligands.
1
2
3
Figure 2.15 – A section of the 1H NMR (400MHz, DMSO) spectrum showing the assignment for 2.
56
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 2 recorded in DMF is similar to that of 1
displaying an absorption band at ca. 460 nm ( = 1000 L mol-1 cm-1). This singnal most
likely originates from d – d transitions that can be assigned to 2Eg
2
T2g transitions
involving the octahedrally coordinated MoV ions within 2.38
Absorbance (a.u.)
0.10
0.08
0.06
0.04
0.02
0.00
400
450
500
550
600
Wavelength (nm)
Figure 2.16 –A section of the UV-Vis spectrum of a 10-4 M solution of 2 in DMF.
57
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.6 − Crystal data and structural refinement parameters for 2.
Compound 2
Empirical formula
Molecular mass/g mol
C27H49As4Mo4N7O25
-1
Crystal colour/shape
3
1555.16
Red / rod-shaped
Crystal size/mm
0.20×0.15×0.10
Crystal system
Triclinic
Space group
P
a/ Å
10.207(4)
b/ Å
13.740(5)
c/ Å
19.700(8)
/º
87.426(13)
/º
82.265(10)
/º
71.005(11)
3
V/ Å
2588.6(17)
Z
2
Temperature (K)
118(2)
-3
Density/Mg m
1.978
-1
Absorp. coef./mm
3.567
F(000)
1494
2
50
max/º
Reflections collected
39817
Independent reflections
9114 [R(int) = 0.1044]
Data / restraints / parameters
9114 / 13 / 604
2
S on F
1.006
R1, wR2 [I>2 (I)]
0.0836, 0.1961
R1, wR2 (all data)
0.0892, 0.2006
Largest diff. peak and hole/e.Å-3
2.238 and -1.076
58
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.2.1.3 The p-Hydroxyphenylarsonic Acid – MoVI/MoV Reaction System
(NH4)5[MoVI2MoV3O11(O3AsC6H4OH)5]·9H2O (3)
Surprisingly when p-hydroxyphenylarsonic acid is used as a stabilising ligand
under the same reaction conditions as described above, we obtain a reaction mixture from
which blue crystals of (NH4)5[MoVI2MoV3O11(O3AsC6H4OH)5]·9H2O (3), separate within a
time period of one week. The structure of 3 is significantly different to that of 1 and 2.
Single crystal X-ray diffraction measurements reveal that compound 3 crystallises
in the triclinic crystal system in the space group P , and contains a pentanuclear, mixedvalent cluster stabilised by five p-hydroxyphenyl arsonate ligands. The structure of the
cluster anion is shown in Figure 2.17. All Mo ions in the structure are distorted
octahedrally surrounded by O donors. The coordination spheres of Mo(1) and Mo(2), each
consist of three bridging
2-
2-
oxo ligands, two O donors that derive from two
deprotonated organoarsonate ligands and are completed by a terminal Mo=O bonds. The
bond distances between these Mo ions and the bridging
2-
2-
oxo ligands range between
1.930(5) Å – 2.060(5) Å. The bond distances between the Mo ions and the O donors
originating from the organic arsonate ligands range between 2.083(5) Å – 2.299(5) Å,
while the Mo=O bond distances between Mo(1)-O(9) and Mo(2)-O(10) are 1.701(5) Å and
1.685(5) Å, respectively (Table 2.7). The remaining Mo ions in 3, Mo(3), Mo(4) and
Mo(5), also show distorted octahedral geometries each consisting of two bridging
2-
2-
oxo ligands, three Oarsonate donors and a terminal Mo=O bond. The Mo – ( 2- ) bond
distances range between 1.817(5) Å – 1.998(5) Å; the Mo – Oarsonate bond distances range
between 2.013(5) Å – 2.312(5) Å, while the Mo=O bond distances between Mo(3)-O(11),
Mo(4)-O(12) and Mo(5)-O(13) are 1.701(5) Å, 1.671(5) Å and 1.682(5) Å, respectively.
The distorted nature of the octahedral geometries of the coordinated metal centres can also
be observed from their bond angles that deviate from the ideal octahedral geometry. The
bond angle O(2)-Mo(1)-O(7) of 155.3(2)° shows the greatest deviation from the ideal
octahedral angle of 180°, while the bond angle O(9)-Mo(1)-O(2) of 106.5(2)° shows the
greatest deviation from the ideal octahedral angle of 90°. Selected bond lengths and bond
angles for the Mo ions in 3 are listed in Table 2.7 and Table 2.8.
59
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
a)
b)
Figure 2.17 – Crystal structure of the pentanuclear Mo complex in 3: (a) ball-and-stick representation,
(b) polyhedral presentation. Colour code: MoVI lavender blue, MoV blue, As orange, O red, C grey
(hydrogen atoms have been omitted for clarity).
Figure 2.18 – The core structure of the pentanuclear Mo complex in 3. Colour code: MoVI lavender
blue, MoV blue, As orange, O red, C grey.
60
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
The coordination polyhedra of Mo(1), Mo(2), Mo(3) and Mo(5) share common
edges. The polyhedron of Mo(4), shares common vertices with the polyhedra of Mo(3)
and Mo(5) and closes the resulting circular entity in 3. Four organic ligands are situated on
the outer side of the ring, and each binds via its deprotonated arsonate functionalities in a
η1:η1:μ2 bridging mode to two Mo ions. The organic ligands bridge between Mo(2) and
Mo(3), Mo(3) and Mo(4), Mo(4) and Mo(5) and Mo(5) and Mo(1), respectively. The fifth
ligand is situated above the centre of the {Mo5} ring and binds with its three O atoms to all
Mo ions (O(26) binds to Mo(4), and O(3) and O(8) bridge between Mo(2) and Mo(3), and
Mo(5) and Mo(1), respectively). The aromatic ring systems of the ligands all point in one
direction and project perpendicularly to the virtual plane of the {Mo5} ring. The phenolic
OH functionalities of the organic ligands remain protonated and are engaged in hydrogen
bonding within the crystal structure.
a)
b)
Figure 2.19 – Packing arrangement of 3 in the crystal structure viewed in the direction of the
crystallographic: (a) a-axis and (b) c-axis. Colour code: Mo blue, As orange, O red, C grey
(crystallisation water molecules, NH4+ counterions and hydrogen atoms have been omitted for clarity).
The Mo−O bond distances of Mo(3) and Mo(5) are significantly shorter than the
corresponding distances of the other Mo ions in the structure. Further bond valence sum
analyses confirm that these two Mo ions adopt the oxidation state +VI, while Mo(1),
Mo(2) and Mo(4) are in the oxidation state +V. The assignment of the oxidation states is
further substantiated by the observed Mo−Mo distances within 3. The short interatomic
distance of 2.574(1) Å between Mo(1) and Mo(2) is indicative for Mo−Mo contacts and in
agreement with the +V oxidation states. Comparable MoV−MoVI distances are expectedly
61
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
significantly longer (Mo(1)-Mo(5) 3.359(1) Å, Mo(2)-Mo(3) 3.353(3) Å, Mo(3)-Mo(4)
3.680(2) Å and Mo(4)-Mo(5) 3.671(5) Å). Its mixed valence nature distinguishes 3 from
many other organoarsonate-stabilised polyoxomolybdates, which tend to exist, with few
exceptions,38, 39 predominantly in their fully oxidised forms.
Table 2.7 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 3.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mo(1)
Mo(1)-O(9)
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(7)
Mo(1)-O(15)
Mo(1)-O(8)
1.701(5)
1.930(5)
1.954(5)
2.060(5)
2.083(5)
2.252(5)
5.251
+5
Mo(2)
Mo(2)-O(10)
Mo(2)-O(2)
Mo(2)-O(1)
Mo(2)-O(4)
Mo(2)-O(17)
Mo(2)-O(3)
1.685(5)
1.938(5)
1.940(5)
2.053(5)
2.101(6)
2.299(5)
5.277
+5
Mo(3)
Mo(3)-O(11)
Mo(3)-O(5)
Mo(3)-O(4)
Mo(3)-O(18)
Mo(3)-O(20)
Mo(3)-O(3)
1.701(5)
1.826(5)
1.828(5)
2.033(6)
2.069(5)
2.312(5)
5.919
+6
Mo(4)
Mo(4)-O(12)
Mo(4)-O(5)
Mo(4)-O(6)
Mo(4)-O(21)
Mo(4)-O(23)
Mo(4)-O(26)
1.671(5)
1.980(5)
1.998(5)
2.013(5)
2.022(5)
2.183(5)
5.462
+5
Mo(5)
Mo(5)-O(13)
Mo(5)-O(6)
Mo(5)-O(7)
Mo(5)-O(24)
Mo(5)-O(14)
Mo(5)-O(8)
1.682(5)
1.817(5)
1.843(5)
2.045(5)
2.085(5)
2.272(5)
5.981
+6
Mo(1) ··· Mo(2)
2.574(1)
62
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.8 − Selected bond angles [º] for compound 3.
Bond Angle (º)
Bond Angle (º)
O(9)-Mo(1)-O(8)
O(1)-Mo(1)-O(15)
O(2)-Mo(1)-O(7)
O(9)-Mo(1)-O(2)
O(9)-Mo(1)-O(7)
O(1)-Mo(1)-O(2)
O(7)-Mo(1)-O(15)
O(1)-Mo(1)-O(7)
O(2)-Mo(1)-O(15)
O(15)-Mo(1)-O(8)
165.6(2)
161.5(2)
155.3(2)
106.5(2)
97.6(2)
94.7(2)
87.3(2)
84.5(2)
85.9(2)
77.56(19)
O(10)-Mo(2)-O(3)
O(1)-Mo(2)-O(17)
O(2)-Mo(2)-O(4)
O(10)-Mo(2)-O(2)
O(10)-Mo(2)-O(4)
O(2)-Mo(2)-O(1)
O(2)-Mo(2)-O(17)
O(1)-Mo(2)-O(3)
O(2)-Mo(2)-O(3)
O(17)-Mo(2)-O(3)
166.9(2)
161.0(2)
153.6(2)
106.4(2)
99.4(2)
94.9(2)
86.7(2)
85.1(2)
81.8(2)
76.3(2)
O(11)-Mo(3)-O(3)
O(5)-Mo(3)-O(18)
O(4)-Mo(3)-O(20)
O(11)-Mo(3)-O(5)
O(11)-Mo(3)-O(20)
O(11)-Mo(3)-O(18)
O(4)-Mo(3)-O(18)
O(20)-Mo(3)-O(3)
O(18)-Mo(3)-O(3)
O(4)-Mo(3)-O(3)
175.1(2)
158.2(2)
156.3(2)
101.1(2)
99.0(2)
96.4(2)
88.3(2)
83.64(18)
80.1(2)
75.4(2)
O(12)-Mo(4)-O(26)
O(5)-Mo(4)-O(23)
O(6)-Mo(4)-O(21)
O(12)-Mo(4)-O(21)
O(12)-Mo(4)-O(5)
O(5)-Mo(4)-O(6)
O(5)-Mo(4)-O(21)
O(6)-Mo(4)-O(23)
O(5)-Mo(4)-O(26)
O(21)-Mo(4)-O(26)
178.8(2)
166.1(2)
165.6(2)
97.1(2)
96.8(2)
95.4(2)
88.8(2)
86.6(2)
83.72(19)
81.7(2)
O(13)-Mo(5)-O(8)
O(6)-Mo(5)-O(14)
O(7)-Mo(5)-O(24)
O(6)-Mo(5)-O(7)
O(13)-Mo(5)-O(7)
O(13)-Mo(5)-O(14)
O(6)-Mo(5)-O(24)
O(6)-Mo(5)-O(8)
O(14)-Mo(5)-O(8)
O(7)-Mo(5)-O(8)
172.6(2)
157.4(2)
155.9(2)
103.7(2)
99.3(2)
97.2(2)
87.8(2)
84.2(2)
78.76(19)
74.3(2)
The pentanuclear polyoxomolybdate cluster in 3 carries an overall charge of -5,
which is compensated for by NH4+ counterions. In the crystal structure, the cluster packs to
form a layered lamellar structure with alternating inorganic and organic areas. This
structure, viewed in the direction of the crystallographic c-axis, is shown in Figure 2.19, b.
The counterions and the crystallisation water molecules are engaged in hydrogen bonds
(O-O distances: 2.497(67) Å – 3.142(95) Å, N-O distances: 2.589(74) Å – 3.122(66) Å)
and prevail in the hydrophilic part of the structure.
63
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 3 (Figure 2.20) is similar to that of 1 and 2. Characteristic C–C
skeletal vibrations of the phenyl rings can be observed in the range 1600 – 1400 cm-1. The
signal at 1093 cm-1 can be associated with As–C vibrations, while some bands in the 1000
– 650 cm-1 region can be attributed to the molybdenum-oxygen and arsenic-oxygen
stretching vibrations.8, 15, 37, 46-48
Figure 2.20 – Infrared spectrum of 3.
64
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
Thermogravimetric analysis
The thermal stability of compound 3 was investigated by thermogravimetric
analysis in the temperature range between 30 and 900 °C, in a N2 atmosphere (Figure
2.21). Upon thermolysis, 3 decomposes in a similar sequence as 2. A weight loss of about
8.3 % occurs below 150 °C. This weight loss can be attributed to the loss of nine water
molecules (calcd: 8.1 %). Decomposition of the organic ligands occurs between 200 – 400
°C and a step at 700 °C indicates the transformation into oxides.
100
Weight % (%)
90
80
70
60
50
40
0
200
400
600
800
1000
Temperature (°C)
Figure 2.21 – Thermogravimetric analysis of 3.
65
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
Magnetism
The assignments of the oxidation states of the Mo atoms in 3 are in agreement with
the observed magnetic properties. In the {Mo5} units, three S = ½ MoV (light pink) and two
diamagnetic MoVI (purple) metal ions are present (Figure 2.22 a). Considering the topology
of the complex, two different magnetic exchange pathways could possibly be envisaged for
the {Mo5} core. These magnetic pathways are highlighted in Figure 2.22. Moreover, it is
worth noting that the interaction denoted with J' will likely be very strong as two oxo
bridges mediate the magnetic interactions between MoV centres.
a)
b)
χT /cm3 K mol-1
0.5
0.4
0.3
0.2
0.1
0
1000 Oe
10000 Oe
0
50
100
150
200
250
300
T /K
Figure 2.22 – (a) {Mo5} units of compound 3 showing: the S = ½ Mo(V) (light pink), the diamagnetic
Mo(VI) (purple) metal ions, and the J and J` magnetic interactions; (b) Temperature dependence of the
χT product (with χ defined as the magnetic susceptibility and equal to M/H after diamagnetic and
experimental corrections) at 0.1 and 1 T.
The temperature dependence of the χT product of 3 is shown in Figure 2.22, b. At
room temperature, the χT product is 0.37 cm3 K mol-1 which is in good agreement with the
presence of one S = ½ MoV metal ion (expected value: 0.375 cm3 K mol-1 with g = 2). The
two other MoV ions appear to be magnetically silent as expected for two spin centres that
are strongly antiferromagnetically coupled (|J`| >> 500 K) to give a diamagnetic dinuclear
unit. Such strong antiferomagnetic exchange is common in polyoxomolybdates that feature
edge- and face-sharing dinuclear molybdenum octahedra.49,
50
When the temperature is
lowered, the χT product at 1000 Oe remains constant at 0.37 cm3 K mol-1 down to 1.8 K
indicating a Curie type paramagnetism. From the Curie constant (0.37 cm3 K mol-1), the g
value is estimated to be very close to 2.
66
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
a)
b)
1.2
1.2
1.8 K
3K
5K
8K
1
1
0.8
M / µB
M / µB
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
1.8 K
3K
5K
8K
0
0
20000
40000
H / Oe
60000
b
Chisq
R
0
y = bri1d(1,2,2)
Value
Error
2.0503
0.00084494
0.0010978
NA
0.99996
NA
20000
40000
H T-1 / Oe K -1
Figure 2.23 – (a) M vs H and (b) M vs H/T data below 8 K. The solid lines are guides for eyes on the
left figure while the blue line on the right figure is the best fit obtained with an S = ½ Brillouin
function and g = 2.05.
The magnetisation as a function of field at low temperatures has been measured up
to 7 T between 1.8 and 8 K (Figure 2.23). It is worth noting that no hysteresis effect were
detected even at 1.8 K and that the magnetisation plotted as a function of H/T is perfectly
superimposed on a single master curve, as expected for isotropic systems. At 7 T and 1.8
K, the magnetic moment saturates at 1.01 µB in perfect agreement with the presence of an
S = ½ spin. The S = ½ spin ground state of the {Mo5} unit is confirmed by the fit of the
magnetisation to an S = ½ Brillouin function that reproduces very well the experimental
data with g = 2.05.
67
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
Mass spectrometry
Negative mode ESI-MS spectra of compound 3 dissolved in a suitable solvent (H2O
and DMSO, respectively), were recorded. ESI-MS studies using the crystalline material
reveal that compound 3 undergoes structural transformations when dissolved in distilled
water but is stable in a DMSO environment. The mass spectrum of 3 dissolved in DMSO
(Figure 2.24) reveals the presence of a signal centered at m/z = 1739.1 a.m.u. which
corresponds to the {H3[MoVI3MoV2O11(O3AsC6H4OH)5]}- species. A comparison of the
experimental isotopic envelopes with simulated patterns is presented in Figure 2.24 and
Relative ionic abundance
confirms our assignment.
Figure 2.24 – Negative-mode ESI-MS spectra for crystals of 3 dissolved in DMSO. Inset:
Comparison of the experimental isotopic envelopes (black spectrum) with simulated envelope (red
spectrum) for {H3[MoVI3MoV2O11(O3AsC6H4OH)5]}- centered at m/z = 1739.1 a.m.u. (cone
voltage: 30 V).
68
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 3 recorded in DMF displays an absorption
band at ca. 531 nm ( = 2700 L mol-1 cm-1). Compared to 1 and 2, the absorption band
observed for 3 is much broader and shifted to higher wavelengths. Thus, this band is a
result of two different types of absorptions: d – d transitions involving the MoV metal
centeres and intervalence charge transfer between MoV and MoVI via an oxo bridge. The d
– d transitions can be assigned to a 2Eg
2
T2g transition of the octahedrally coordinated
MoV ions.38, 51, 52
0.13
Absorbance (a.u.)
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.05
400
450
500
550
600
650
700
Wavelength (nm)
Figure 2.25 – A section of the UV-Vis spectrum of a 10-4 M solution of 3 in DMF.
69
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.9 − Crystal data and structural refinement parameters for 3.
Compound 3
Empirical formula
Molecular mass/g mol
C30H63As5Mo5N5O40
-1
Crystal colour/shape
3
1988.13
Blue / rectangular plate
Crystal size/mm
0.50×0.40×0.40
Crystal system
Triclinic
Space group
P
a/ Å
19.417(4)
b/ Å
21.040(4)
c/ Å
21.562(4)
/º
113.09(3)
/º
112.90(3)
/º
99.16(3)
3
V/ Å
6941(2)
Z
2
Temperature (K)
108(2)
-3
Density/Mg m
1.864
-1
Absorp. coef./mm
3.334
F(000)
3732
2
50
max/º
Reflections collected
103006
Independent reflections
24429 [R(int) = 0.0648]
Data / restraints / parameters
24429 / 15280 / 1587
2
S on F
1.059
R1, wR2 [I>2 (I)]
0.0704, 0.1844
R1, wR2 (all data)
0.0789, 0.1930
Largest diff. peak and hole/e.Å-3
2.729 and -2.598
70
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.2.1.4 The (4-Hydroxy-3-Nitrophenyl)arsonic Acid – MoVI/MoVI Reaction System
(NH4)4[MoVI4O10(O3AsC6H3NO2OH)4]·2H2O (4)
It was decided to investigate the influence of a disubstituted arsonate ligand on the
self-assembly process of hybrid polyoxomolybdates under the same outlined reaction
conditions that led to the formation of 1−3. Using a substituted arsonate ligand with a
hydroxyl group in the para position and a nitro group in the meta position, a dark-green
solution is obtained, which undergoes a colour change to red-orange within approximately
ten days. Orange crystals of (NH4)4[MoVI4O10(O3AsC6H3NO2OH)4]·2H2O (4) were
obtained from the reaction mixture within a time period of three weeks, and single crystal
X-ray studies were performed.
4 crystallises in the triclinic crystal system in the space group P . The anionic
cluster [MoVI4O10(O3AsC6H3NO2OH)4]4– in 4 (Figure 2.26) contains two {Mo2O10}8subunits in which octahedrally coordinated Mo atoms share a common edge (Mo(1) with
Mo(2) and Mo(1`) with Mo(2`)). The dimeric units are connected by four deprotonated
arsonate ligands. Two of the ligands have their As atoms (As(3) and As(3`)) arranged
nearly coplanar to the four Mo atoms. Each of the ligands is bridging between two Mo ions
in a µ 2-syn, syn bridging mode to form a six membered {As2Mo4} ring. The remaining two
arsonate ligands cap both sides of the central cavity of the ring and each provides one μ2bridging O-donor (O(4) and O(4`)) of the common edges of the {Mo2O10}8- dimer
moieties. The other two O atoms of the tetrahedral arsonate functionality of each of the
ligands (O(7), O(13) and O(7`), O(13`) respectively) link to the adjacent {Mo2O10}8moiety and interconnect the dimeric subunits. The involved O donors bridge in a µ 2-syn,
syn bridging mode and occupy two apical positions of the octahedrally coordinated Mo
atoms within the dimer units. All four Mo atoms show distorted octahedral coordination
environments. Each Mo ion in 4 is involved in two short Mo=O bonds (range between
1.692(3) - 1.720(3) Å), three elongated Mo–Oarsonate bonds that range between 2.009(3) 2.427(3) Å and one Mo–(µ 2-O) bond with a bond length of 1.915(3) Å for Mo(1)-O(1) and
1.894(3) Å for Mo(2)-O(1) (Table 2.10). The distorted nature of the octahedral
coordination environments of the Mo ions can be exemplified by examining the bond
lengths and bond angles of the metal centres. The bond angles O(1)-Mo(1)-O(5) and O(1)Mo(1)-O(4) of 156.65(11)° and 75.20(10)°, respectively, show the greatest deviation from
the ideal octahedral geometry (Table 2.11).
71
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
a)
b)
Figure 2.26 – Crystal structure of the tetranuclear Mo complex in 4: (a) ball-and-stick
representation, (b) polyhedral presentation. Colour code: MoVI sky blue, As orange, O red, N light
blue, C grey (hydrogen atoms have been omitted for clarity).
Figure 2.27 – The core structure of the tetranuclear Mo complex in 4. Colour code: MoVI sky blue,
As orange, O red, C grey.
The
core
structure
of
(Et3NH)4[Mo4O10(C6H5PO3)4]·2CH3CN
4
is
and
related
to
those
observed
in
(Et3NH)4[Mo4O10(C6H5AsO3)4]·4H2O,
72
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
respectively.11 In these compounds the anionic clusters share a common structural motif
but are stabilised by different organic ligands.
Bond valence sum analyses confirm that all four Mo atoms in 4 adopt the oxidation
state +VI (Table 2.10).
Table 2.10 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 4.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mo(1)
Mo(1)-O(3)
Mo(1)-O(2)
Mo(1)-O(1)
Mo(1)-O(5)
Mo(1)-O(13)
Mo(1)-O(4)
1.692(3)
1.720(3)
1.915(3)
2.009(3)
2.223(3)
2.276(3)
5.979
+6
Mo(2)
Mo(2)-O(8)
Mo(2)-O(9)
Mo(2)-O(1)
Mo(2)-O(6)
Mo(2)-O(7)
Mo(2)-O(4)
1.707(3)
1.714(3)
1.894(3)
2.049(3)
2.113(3)
2.427(3)
5.937
+6
Table 2.11 − Selected bond angles [º] for compound 4.
Bond Angle (º)
O(2)-Mo(1)-O(13)
O(3)-Mo(1)-O(4)
O(1)-Mo(1)-O(5)
O(3)-Mo(1)-O(2)
O(3)-Mo(1)-O(5)
O(2)-Mo(1)-O(1)
O(2)-Mo(1)-O(4)
O(3)-Mo(1)-O(13)
O(5)-Mo(1)-O(4)
O(1)-Mo(1)-O(4)
167.73(12)
162.67(12)
156.65(11)
104.73(14)
98.60(13)
97.13(13)
92.55(12)
86.93(12)
82.40(11)
75.20(10)
Bond Angle (º)
O(8)-Mo(2)-O(4)
O(9)-Mo(2)-O(7)
O(1)-Mo(2)-O(6)
O(8)-Mo(2)-O(9)
O(8)-Mo(2)-O(1)
O(8)-Mo(2)-O(6)
O(6)-Mo(2)-O(4)
O(9)-Mo(2)-O(6)
O(6)-Mo(2)-O(7)
O(7)-Mo(2)-O(4)
169.07(12)
154.73(13)
158.36(12)
104.19(14)
99.20(13)
96.61(13)
90.51(10)
88.55(13)
77.79(11)
74.87(10)
As expected, the di-substituted arsonate ligand imposes an influence on the selfassembly process under the investigated conditions that produce 1 and 2. In fact, the
composition of the cluster anion 4 is closely related to that of 1 and 2. However, it seems
73
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
that the -OH and -NO2 functionalities of the organic ligands enforce a rearrangement of the
cubane core structure. The rearrangement can formally be explained by an intramolecular
condensation reaction under acidic conditions (Figure 2.28).
+2H2O
+4H++4e-
Figure 2.28 – Formal rearrangement of the {Mo4} core structure in 1 or 2 to give 4.
The packing arrangement of the [MoVI4O10(O3AsC6H3NO2OH)4]4– clusters in the
crystal structure is characterised by small intercluster channels (filled with constitution
water molecules and NH4+ counterions) that run in the direction of the crystallographic aaxis (Figure 2.29). The counterions and the crystallisation water molecules are engaged in
H bonds whose N-O and O-O contacts range between 2.697(10) Å – 3.112(34) Å.
a)
b)
Figure 2.29 – Packing arrangement of 4 in the crystal structure viewed in the direction of the
crystallographic: (a) a-axis and (b) b-axis. Colour code: MoVI sky blue, As orange, O red, N light blue,
C grey (crystallisation water molecules, NH4+ counterions and hydrogen atoms have been omitted for
clarity).
74
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 4 shown in Figure 2.30 is comparable to that of 1 – 3. Notably,
the 1620 – 1050 cm-1 region of the spectrum is quite crowded displaying a set of sharp
bands of medium intensity. These bands arise from C–C skeletal vibrations of the phenyl
rings, As–C vibrations and N–O stretches of the organoarsonate ligands.8, 15, 37, 46-48
Figure 2.30 – Infrared spectrum of 4.
75
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
Thermogravimetric analysis
Compound 4 was also analysed by TGA (Figure 2.31). A gradual weight loss of 2.3
% observed below 200 °C which corresponds to the loss of two water molecules (calcd:
2.1 %). A further weight loss between 200 – 450 °C indicates the decomposition of the
organic moieties, while the thermogravimetric step between 450 – 550 °C can be
associated with the decomposition into oxides.
100
Weight % (%)
80
60
40
20
0
0
200
400
600
800
1000
Temperature (°C)
Figure 2.31 – Thermogravimetric analysis of 4.
ESI-MS studies reveal that 4 is unstable in solution. The compound most likely
decomposes into insoluble inorganic or polymeric materials when dissolved in H2O or
DMSO.
76
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.12 − Crystal data and structural refinement parameters for 4.
Compound 4
Empirical formula
Molecular mass/g mol
C24H36As4Mo4N8O36
-1
Crystal colour/shape
3
1696.02
Orange / rectangular plate
Crystal size/mm
0.20×0.20×0.20
Crystal system
Triclinic
Space group
P
a/ Å
10.7255(19)
b/ Å
10.937(3)
c/ Å
12.000(3)
/º
89.474(13)
/º
66.129(10)
/º
63.926(7)
3
V/ Å
1131.6(5)
Z
1
Temperature (K)
108(2)
-3
Density/Mg m
2.465
-1
Absorp. coef./mm
4.109
F(000)
808
2
50.04
max/º
Reflections collected
14710
Independent reflections
3987 [R(int) = 0.0313]
Data / restraints / parameters
3987 / 3 / 343
2
S on F
1.036
R1, wR2 [I>2 (I)]
0.0282, 0.0817
R1, wR2 (all data)
0.0303, 0.0831
Largest diff. peak and hole/e.Å-3
1.502 and -0.739
77
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.2.2
Synthesis and characterisation of organophosphonate functionalised
heteropolyoxomolybdate clusters
2.2.2.1
The Phenylphosphonic Acid – MoVI/MoV Reaction System
(NH4)4H4{Mn[MoV6O12(OH)3(O3PC6H5)4]2}·8H2O (5)
Partial reduction of molybdic acid using N2H4 H2O in the presence of
phenylphosphonic acid and MnCl2 4H2O results in a red-brown solution from which
orange octahedral crystals of
(NH4)4H4{Mn[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (5)
separate within a time period of three days.
Single crystal X-ray analysis reveals that 5 crystallises in the orthorhombic crystal
system
in
the
space
[MoV6O12(OH)3(O3PC6H5)4]5-
group
Pbca.
The
structure
consists
of
two
II
fragments linked through a Mn ion (Figure 2.32). Within
each fragment, six Mo ions form distorted octahedral coordination polyhedra, each sharing
two common edges with adjacent polyhedra to form a six membered ring. Three
organophosphonate ligands are situated on the periphery of the resulting ring, each
bridging two Mo ions in a η1:η1:μ2 bridging mode. The O donors of a fourth organic ligand
cap the central cavity of the ring and provide µ2 bridging O donors (O(2), O(4) and O(12))
to three, dimeric edges-sharing subunits (Mo(2) and Mo(3), Mo(4) and Mo(5), and Mo(6)
and Mo(1), respectively). The aromatic moieties of the four organic ligands are arranged
approximately perpendicular to the mean plane of the six membered ring. The six
molybdenum atoms within the ring display distorted octahedral coordination environments.
This distortion mainly results from the presence of short Mo=O bonds (range between
1.678(7) - 1.699(6) Å) situated in trans positions to significantly longer Mo–(µ 2-O) bonds
(range between 2.302(6) - 2.333(6) Å). The µ 2 bridging O-donors (O(2), O(4) and O(12))
engaged in these elongated bonds originate from the phosphonate functionality of the
central ligand. The remaining coordination sites of the Mo ions are occupied by one O
donor originating from a peripheral organic ligand, two
two Mo ions and a
3-
2-
2-
2-
ions that bridge between
ligand bridging between two Mo ions and the central Mn ion.
The bond distances between the Mo ions and the O donors of these peripheral phosphonate
ligands range between 2.066(6) Å – 2.093(7) Å, the Mo-( 2between 1.941(7) Å – 2.135(6) Å, while the Mo-( 3-
2-
) bond distances range
2-
) bond distances range between
1.972(6) Å – 1.989(6) Å. The distorted octahedral coordination environment of the Mo
78
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
ions in 5 can also be exemplified by examining the bond angles that deviate from the ideal
octahedral geometry. Selected bond lengths and bond angles for the Mo ions in 5 are listed
in Table 2.13, Table 2.14.
a)
b)
Figure 2.32 – Crystal structure of the dodecanuclear Mo complex in 5. Colour code: MoV blue, P
purple, O red, Mn cyan, C grey (hydrogen atoms have been omitted for clarity).
Figure 2.33 – The {Mn[MoV6O12(OH)3(O3PC6H5)4]}3- fragment in 5. Colour code: MoV blue, P
purple, O red, Mn cyan, C grey.
79
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.13 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 5.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mo(1)
Mo(1)-O(21)
Mo(1)-O(25)
Mo(1)-O(3)
Mo(1)-O(16)
Mo(1)-O(11)
Mo(1)-O(12)
1.699(6)
1.951(7)
1.987(6)
2.084(7)
2.127(6)
2.326(6)
5.174
+5
Mo(2)
Mo(2)-O(20)
Mo(2)-O(25)
Mo(2)-O(3)
Mo(2)-O(15)
Mo(2)-O(13)
Mo(2)-O(2)
1.689(7)
1.945(7)
1.987(6)
2.093(7)
2.109(6)
2.302(6)
5.047
+5
Mo(3)
Mo(3)-O(19)
Mo(3)-O(5)
Mo(3)-O(1)
Mo(3)-O(14)
Mo(3)-O(13)
Mo(3)-O(2)
1.678(7)
1.943(7)
1.972(6)
2.079(7)
2.118(6)
2.318(6)
5.134
+5
Mo(4)
Mo(4)-O(18)
Mo(4)-O(5)
Mo(4)-O(1)
Mo(4)-O(8)
Mo(4)-O(6)
Mo(4)-O(4)
1.689(7)
1.941(7)
1.989(6)
2.092(7)
2.127(6)
2.333(6)
5.001
+5
Mo(5)
Mo(5)-O(23)
Mo(5)-O(9)
Mo(5)-O(10)
Mo(5)-O(7)
Mo(5)-O(6)
Mo(5)-O(4)
1.692(7)
1.943(6)
1.978(6)
2.068(6)
2.135(6)
2.320(6)
5.044
+5
Mo(6)
Mo(6)-O(22)
Mo(6)-O(9)
Mo(6)-O(10)
Mo(6)-O(17)
Mo(6)-O(11)
Mo(6)-O(12)
1.687(7)
1.945(6)
1.978(6)
2.066(6)
2.117(6)
2.325(6)
5.091
+5
Mo(1) ··· Mo(2)
Mo(3) ··· Mo(4)
Mo(5) ··· Mo(6)
2.6058(14)
2.6027(14)
2.5934(15)
Bond valence sum analysis confirms that all six Mo atoms in 5 adopt the oxidation
state +V. In the crystal structure of 5, the molecular entities pack to form a layered lamellar
structure with alternating hydrophobic organic and hydrophilic inorganic regions (Figure
2.34). The NH4+ counterions and the crystallisation water molecules are situated in the
hydrophilic part of the structure being engaged in hydrogen bonds (O-O and O-N
distances: 2.901(26) Å – 3.084(21) Å). Weak offset π-π interactions between two adjacent
80
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
clusters with an interplanar separation distance of about 3.9 Å also contribute to the
stability of 5 in the solid state.
Table 2.14 − Selected bond angles [º] for compound 5.
Bond Angle (º)
a)
Bond Angle (º)
O(21)-Mo(1)-O(12)
O(3)-Mo(1)-O(16)
O(25)-Mo(1)-O(11)
O(21)-Mo(1)-O(25)
O(21)-Mo(1)-O(11)
O(25)-Mo(1)-O(3)
O(25)-Mo(1)-O(16)
O(16)-Mo(1)-O(11)
O(3)-Mo(1)-O(12)
O(11)-Mo(1)-O(12)
169.8(3)
160.2(3)
155.6(3)
106.2(3)
97.3(3)
95.4(3)
86.5(3)
84.1(3)
81.3(2)
73.0(2)
O(20)-Mo(2)-O(2)
O(3)-Mo(2)-O(15)
O(25)-Mo(2)-O(13)
O(20)-Mo(2)-O(25)
O(20)-Mo(2)-O(15)
O(25)-Mo(2)-O(3)
O(3)-Mo(2)-O(13)
O(15)-Mo(2)-O(13)
O(3)-Mo(2)-O(2)
O(13)-Mo(2)-O(2)
168.7(3)
159.3(3)
155.9(3)
105.9(3)
97.5(3)
95.6(3)
86.5(3)
84.6(3)
80.9(2)
72.1(2)
O(19)-Mo(3)-O(2)
O(1)-Mo(3)-O(14)
O(5)-Mo(3)-O(13)
O(19)-Mo(3)-O(5)
O(19)-Mo(3)-O(13)
O(5)-Mo(3)-O(1)
O(1)-Mo(3)-O(13)
O(5)-Mo(3)-O(2)
O(1)-Mo(3)-O(2)
O(13)-Mo(3)-O(2)
168.1(3)
159.5(3)
155.1(3)
106.7(3)
96.9(3)
95.4(3)
86.6(3)
84.2(2)
80.7(2)
71.7(2)
O(1)-Mo(4)-O(8)
O(18)-Mo(4)-O(4)
O(5)-Mo(4)-O(6)
O(18)-Mo(4)-O(5)
O(18)-Mo(4)-O(6)
O(5)-Mo(4)-O(1)
O(5)-Mo(4)-O(8)
O(8)-Mo(4)-O(6)
O(1)-Mo(4)-O(4)
O(6)-Mo(4)-O(4)
160.6(3)
169.8(3)
155.8(3)
106.1(3)
97.3(3)
94.9(3)
87.2(3)
84.0(3)
81.5(2)
73.0(2)
O(23)-Mo(5)-O(4)
O(10)-Mo(5)-O(7)
O(9)-Mo(5)-O(6)
O(23)-Mo(5)-O(9)
O(23)-Mo(5)-O(7)
O(9)-Mo(5)-O(10)
O(9)-Mo(5)-O(7)
O(7)-Mo(5)-O(6)
O(10)-Mo(5)-O(4)
O(6)-Mo(5)-O(4)
169.5(3)
158.6(3)
156.9(3)
105.5(3)
97.6(3)
95.5(2)
86.6(3)
84.6(3)
80.5(2)
73.2(2)
O(22)-Mo(6)-O(12)
O(10)-Mo(6)-O(17)
O(9)-Mo(6)-O(11)
O(22)-Mo(6)-O(9)
O(22)-Mo(6)-O(17)
O(9)-Mo(6)-O(10)
O(9)-Mo(6)-O(17)
O(17)-Mo(6)-O(11)
O(10)-Mo(6)-O(12)
O(11)-Mo(6)-O(12)
169.0(3)
158.9(3)
156.8(3)
105.9(3)
97.1(3)
95.4(3)
86.4(3)
84.3(3)
80.4(2)
73.2(2)
b)
Figure 2.34 – Packing arrangement of 5 in the crystal structure viewed in the direction of the
crystallographic: (a) a-axis and (b) c-axis. Colour code: MoV blue, P purple, O red, Mn cyan, C grey
(crystallisation water molecules, NH4+ counterions and hydrogen atoms have been omitted for clarity).
81
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Several of these sandwich-type compounds were successfully synthesised using
similar synthetic procedure as for 5, but replacing the MnII ion with FeIII, CoII, NiII, and
MgII metal ions, respectively, in order to link the two {Mo6} fragments:
(NH4)4H4{Fe[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (6),
(NH4)4H4{Co[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (7),
(NH4)4H4{Ni[MoV6O12(OH)3(O3PC6H5)4]2} 8H2O (8),
(NH4)4H4{Mg[MoV6O12(OH)3 (O3PC6H5)4]2} 8H2O (9).
The resulting compounds are isostructural to 5 and have very similar structural and
geometrical parameters. A single crystal X-ray diffraction study was carried out on the FeIII
complex, confirming the expected sandwich structure (Table 2.16). XRD powder
diffraction analyses were performed on the CoII, NiII, and MgII complexes to certify that
the compounds are isostructural. The measured powder pattern of each compound fits to
the simulated pattern which is based on the single crystal X-ray diffraction data of 5
(Figure 2.35).
4000
2000
Measured
Intensity/a.u
0
Calculated
-2000
-4000
-6000
5
7
8
9
-8000
-10000
5
10
15
20
25
2 Theta/°
Figure 2.35 – X-ray powder diffraction analysis, comparing the experimental patterns of 5 (black), 7
(pink), 8 (purple) and 9 (blue) complexes with simulated pattern (red) based on the single crystal X-ray
diffraction data of 5.
EDX analyses further confirm the presence of the transition metal ions within the
corresponding compounds.
82
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 5 is presented in Figure 2.36 and reveals some characteristic
stretches of the organophosphonate ligands. The C–C skeletal vibrations of the phenyl
rings can be identified in the range 1600 – 1400 cm-1, while some of the bands in the 800 –
650 cm-1 region arise from C–H out-of-plane bending vibrations of the aromatic rings.
Different P–O stretching vibrations of the phosphonate groups can be observed in the
range 1200 – 1000 cm-1. The very strong band at ca. 950 cm-1 is characteristic of Mo=O
stretching vibrations. Finally, the broad bands at ca. 3200 cm-1 and ca. 1600 cm-1 arise
from O–H stretching vibrations and H–O–H bending vibrations of the crystallisation water
molecules engaged in H-bonds.13, 48, 53, 54
Figure 2.36 – Infrared spectrum of 5.
83
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
Thermogravimetric analysis
The TGA curve of compound 5 is presented in Figure 2.37. It reveals a weight loss
of 4.6 % below 100 °C, which was attributed to the loss of eight water molecules (calcd:
4.5 %). The decomposition of the organic ligands occurs between 300 – 600 °C and further
cluster degradation processes result in the formation of metal oxides. The thermal stability
of compounds 6, 7, 8 and 9 was also investigated. As expected, all five isostructural
compounds reveal similar decomposition behaviour.
105
100
Weight % (%)
95
90
85
80
75
70
65
60
0
200
400
600
800
1000
Temperature (°C)
Figure 2.37 – Thermogravimetric analysis of 5.
The CHN analysis of the compounds is also in good agreement with the
composition of the compounds.
84
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
-
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 5 recorded in CH3CN is presented in Figure
2.38. The strong absorption band observed at ca. 305 nm ( = 11000 L mol-1 cm-1) is due to
charge transfer transitions involving either the phosphonate ligands or the {Mo-O}
moieties. A very weak absorption band can also be observed at ca. 443 nm ( = 1050 L
mol-1 cm-1) involving a d – d transition that can be assigned to 2Eg
2
T2g transitions of the
octahedrally coordinated MoV ions within 5. Compounds 6, 7, 8 and 9 display similar
absorption spectra as 5 showing characteristic charge transfer absorptions around 300 nm
and very weak absorption bands due to d – d transitions at ca. 450 nm.38, 55
0.18
0.12
0.16
0.10
Absorbance (a.u.)
0.14
0.08
0.12
0.06
0.10
0.04
0.08
400
450
500
550
600
0.06
0.04
0.02
0.00
-0.02
300
400
500
600
700
800
Wavelength (nm)
Figure 2.38 – UV-Vis spectrum of a 10-5 M solution of 5 in CH3CN. Inset: A section
of the UV-Vis spectrum of a 10-4 M solution of 5 in CH3CN.
85
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.15 − Crystal data and structural refinement parameters for 5.
Compound 5
Empirical formula
Molecular mass/g mol
C48H82MnMo12N4O62P8
-1
Crystal colour/shape
3
3161.16
Orange / octahedral
Crystal size/mm
0.25×0.25×0.10
Crystal system
Orthorhombic
Space group
Pbca
a/ Å
19.757(7)
b/ Å
17.463(6)
c/ Å
30.649(10)
/º
90
/º
90
/º
90
3
V/ Å
10574(6)
Z
1
Temperature (K)
108(2)
-3
Density/Mg m
2.085
-1
Absorp. coef./mm
1.718
F(000)
6404
2
50
max/º
Reflections collected
41398
Independent reflections
9259 [R(int) = 0.0438]
Data / restraints / parameters
9259 / 0 / 664
2
S on F
1.073
R1, wR2 [I>2 (I)]
0.0763, 0.2084
R1, wR2 (all data)
0.0863, 0.2185
Largest diff. peak and hole/e.Å-3
1.859 and -1.627
86
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.16 − Crystal data and structural refinement parameters for 6.
Compound 6
Empirical formula
Molecular mass/g mol
C48H82FeMo12N4O62P8
-1
Crystal colour/shape
3
3162.07
Red brown / polygonal
Crystal size/mm
0.15×0.15×0.25
Crystal system
Orthorhombic
Space group
Pbca
a/ Å
19.601(8)
b/ Å
17.357(6)
c/ Å
30.430(12)
/º
90
/º
90
/º
90
3
V/ Å
10353(7)
Z
1
Temperature (K)
108(2)
-3
Density/Mg m
2.071
-1
Absorp. coef./mm
1.767
F(000)
6170
2
50
max/º
Reflections collected
58886
Independent reflections
9089 [R(int) = 0.0648]
Data / restraints / parameters
9089 / 0 / 646
2
S on F
1.049
R1, wR2 [I>2 (I)]
0.0964, 0.2517
R1, wR2 (all data)
0.1057, 0.2613
Largest diff. peak and hole/e.Å-3
1.807 and -1.966
87
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.3 ESI-MS STUDIES OF COMPLEX REACTION MIXTURES TO
INVESTIGATE THE FORMATION OF HYBRID ORGANIC-INORGANIC
POLYOXOMOLYBDATES
2.3.1
Investigation of the reaction system that led to the formation of the
cubane structures in 1 and 2
Partial reduction of (NH4)6Mo7O24 4H2O in the presence of phenylarsonic acid and
acetic acid using N2H4 H2SO4 resulted in a deep blue solution. When the initial reaction
mixture is examined by ESI-MS, the spectrum reveals isotopic envelopes in the high
molecular mass region centered at m/z = 1315.3 a.m.u. and m/z = 1659.2 a.m.u. (Figure
2.41).
Within
a
time
period
of
three
weeks,
dark
red
crystals
1,
of
(NH4)2H2[MoV4O8(O3AsC6H5)4]·5H2O separate from this reaction mixture (Figure 2.39).
Figure 2.39 – The core structure of the tetranuclear Mo complexes in 1 and 2. Colour code: MoV
blue, As orange, O red, C grey.
The signal at m/z = 1315.3 a.m.u. in the mass spectrum of the reaction mixture that
produced 1 is in agreement with the crystallographically determined formula of the cluster
anion in 1 (Table 2.17). The signal can be reproduced by recording a spectrum of pristine
crystals of 1 dissolved in dimethyl sulfoxide (DMSO), further confirming that the cluster
core of 1 is stable in this polar solution as seen in section 2.2.1 of this chapter. Lower
88
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
molecular mass signals in the ESI-MS spectrum centered at m/z = 816.6 a.m.u. and m/z =
646.6 a.m.u. originate from the cubane cluster and can be assigned to the species
{H9[MoV4O14(O3AsC6H5)]}- and [MoVI4O16H7]-, respectively. The ESI-MS spectrum of the
reaction mixture further contains a high molecular mass signal centered at m/z = 1659.2
a.m.u. We were able to assign this signal to a {Mo6} species with the formula
{(NH4)2H4[MoV4MoIV2O12(OH)3(O3AsC6H5)4]}-. A closely related core structure that is
stabilised
by
phosphonates
has
previously
been
isolated
in
a
compound
{Na[MoV6O12(OH)3(O3PC6H5)4]2}9-.13 It can be crystallised from comparable reaction
mixtures that contain alkali metal ions, producing a solid state structure in which two
[MoV6O12(OH)3(O3PC6H5)4]5- fragments are linked through monovalent ions. We have
successfully isolated the hexanuclear molybdenum fragments in compounds 5-9 and a
description of the {Mo6} core structure was presented in section 2.2.2.
Table 2.17 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 1.
m/z
Species attributed
646.6
[MoVI4O16H7]-
→ {Mo4}
Reaction
816.6
{H9[MoV4O14(O3AsC6H5)]}-
→ {Mo4}
mixture of 1
1315.3
{H3[MoV4O8(O3AsC6H5)4]}-
→ {Mo4}
1659.2
{(NH4)2H4[MoV4MoIV2O12(OH)3(O3AsC6H5)4]}-
→ {Mo6}
{Mo6}
Figure 2.40 – Representation of a structural motif that agrees with the constitutional assignment for
the {Mo6} species identified in the mass spectrum of the reaction mixture that led to the
crystallisation of 1. Colour code: Mo blue, As orange, C grey, O light grey.
We successfully simulated the isotopic envelopes for all of the identified species
and decomposition products of 1 that originate from the reaction mixture (Figure 2.41),
89
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
and good fits between experimental and theoretical data further confirm our structural and
constitutional assignments. The ESI-MS analysis further supports the assignment of the
oxidation states and, in addition, suggests the presence of other relatively labile species in
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
the solution.
Figure 2.41 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of 1
(24 h after preparation). Comparison of the experimental isotopic envelopes (black spectrum) with
simulated patterns (red spectrum) for [MoVI4O16H7]- centered at m/z = 646.6 a.m.u.,
{H9[MoV4O14(O3AsC6H5)]}- centered at m/z = 816.6 a.m.u., {H3[MoV4O8(O3AsC6H5)4]}- centered at m/z
= 1315.3 a.m.u. and {(NH4)2H4[MoV4MoIV2O12(OH)3(O3AsC6H5)4]}- centered at m/z = 1659.2 a.m.u.
(cone voltage: 30 V).
90
−
−
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Figure 2.42 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of 1
at different cone voltages: 10V, 25V, 30V, 50V and 75V, respectively.
Cone voltage (CV) variation experiments were performed in order to investigate its
effect on the ESI-MS spectra (Figure 2.42). According to literature observations, through
(CV) variations one can distinguish between the signals originating from the parent ions
that form in solution and the fragment ions produced within the spraying chamber. Upon
increasing the CV values during the ESI-MS experiments, the relative ionic abundance of
the parent species that form in solution decreases, while the abundance of the fragment
ions that form in the gas phase, increases. We and others observe that higher cone voltages
result in the defragmentation of the coordination clusters within the spraying chamber.56, 57
91
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
As seen in Figure 2.42, the relative ionic abundances of {Mo4} and {Mo6} species
decrease with increasing CV values. These experimental observations suggest that {Mo4}
and {Mo6} species form in the same reaction mixture. The {Mo6} species appears to be
structurally related to the hexanuclear molybdenum units in 5-9, while the {Mo4} species
is structurally related to the cubane structure in 1 which crystallises out from this reaction
mixture. The most probable structural arrangement for {Mo6} species is represented in
Figure 2.40.
When p-aminophenylarsonic acid was used as a stabilising ligand under the same
reaction conditions that produced 1, we obtained a blue solution. The ESI-MS spectrum of
this reaction mixture (Figure 2.43) only contains one isotopic envelope in the high mass
region of the spectrum. The signal is centered at m/z = 1374.4 a.m.u. and corresponds to
the cubane structure in 2, (NH4)2H2[MoV4O8(O3AsC6H4NH2)4]·DMF·4H2O, which
crystallises from this solution as red rod-shaped crystals.
Table 2.18 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 2.
Species attributed
1374.4
{H3[MoV4O8(O3AsC6H4NH2)4]}-
→ {Mo4}
Relative ionic abundance
Reaction
mixture of 2
m/z
Figure 2.43 – Negative-mode ESI-MS spectra for the reaction mixture that led to the
crystallisation of 2 (24 h after preparation). Inset: Comparison of the experimental isotopic
envelopes
(black
spectrum)
with
simulated
patterns
(red
spectrum)
for
{H3[MoV4O8(O3AsC6H4NH2)4]}- centered at m/z = 1374.4 a.m.u. (cone voltage: 30 V).
92
−
−
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Figure 2.44 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of 2 at
different cone voltages: 10V, 25V, 30V, 50V and 75V, respectively.
ESI-MS and NMR analyses of the isolated compound in DMSO, presented in
section 2.2.1, further confirm our assignments.
ESI-MS cone voltage variation experiments performed on the reaction mixture that
led to the crystallisation of 2, show that the signal attributed to the {Mo4} species is
independent on CV variations up to 75 V (Figure 2.44), and the relative ionic abundance
of this species start to decrease in intensity at CV greater than 100 V. These observations
further confirm that the {Mo4} species form in solution and is structurally related to the
cubane structure in 2.
93
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.3.2
Investigation of the reaction system that led to the formation of the
{Mo5} complex in 3
Compound 3, (NH4)5[MoVI2MoV3O11(O3AsC6H4OH)5]·9H2O crystallises under the
same reaction conditions that led to the formation of 1 and 2, but using phydroxyphenylarsonic acid as a stabilising ligand. The ESI-MS spectrum of this reaction
mixture is complex and is characterised by four major isotopic envelopes centred at m/z =
1377.3 a.m.u., m/z = 1522.2 a.m.u., m/z = 1704.1 a.m.u. and m/z = 1740.2 a.m.u. in the
higher mass region of the spectrum (Figure 2.46).
Figure 2.45 – Crystal structure of the pentanuclear Mo complex in 3. Colour code: MoVI lavender
blue, MoV blue, As orange, O red, C grey (hydrogen atoms have been omitted for clarity).
The spectrum proved to be very interesting, as we were able to identify three
different species, a {Mo4}, a {Mo5} and a {Mo6} species, in the high molecular mass
region of the spectrum (Table 2.19). The signal at m/z = 1377.3 a.m.u. can be assigned to
the cubane structure, also observed in the previously examined reaction mixtures, whilst
the high molecular mass signal centered at m/z = 1740.2 a.m.u. originates from a new
pentanuclear mixed valent Mo cluster in 3. The signal at m/z = 1522.2 a.m.u. can be
attributed to a defragmentation product of 3, in which one organic ligand is abstracted from
the cluster anion. The ESI-MS spectrum of the reaction mixture of 3 displays a fourth
signal at m/z = 1704.1 a.m.u. which can be assigned to a hexanuclear compound
{(NH4)H3[MoV6O12(OH)3(O3AsC6H4OH)4]}–. A similar compound was also observed
during analysis of the previously described reaction systems.
94
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.19 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 3.
Species attributed
1377.3
{H3[MoV4O8(O3AsC6H4OH)4]}-
→ {Mo4}
Reaction
1522.2
{H2[MoVI2MoV3O11(O3AsC6H4OH)4]}-
→ {Mo5}
mixture of 3
1704.1
{(NH4)H3[MoV6O12(OH)3(O3AsC6H4OH)4]}-
→ {Mo6}
1740.2
{H4[MoVI2MoV3O11(O3AsC6H4OH)5]}-
→ {Mo5}
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
m/z
Figure 2.46 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation
of 3 (24 h after preparation). Comparison of the experimental isotopic envelopes (black spectrum)
with simulated patterns (red spectrum) for {H3[MoV4O8(O3AsC6H4OH)4]}- centered at m/z = 1377.3
a.m.u.,
{H2[MoVI2MoV3O11(O3AsC6H4OH)4]}centered
at
m/z
=
1522.2
a.m.u.,
{(NH4)H3[MoV6O12(OH)3(O3AsC6H4OH)4]}centered at m/z = 1704.1 a.m.u. and
{H4[MoVI2MoV3O11(O3AsC6H4OH)5]}- centered at m/z = 1740.2 a.m.u. (cone voltage: 30 V).
95
−
−
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Figure 2.47 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of 3 at
different cone voltages: 10V, 25V, 30V, 50V and 75V, respectively.
ESI-MS analyses including cone voltage variations clearly confirm that the
tetranuclear cubane structure and the pentanuclear complex 3 form in solution in the same
reaction mixture.
We successfully simulated the isotopic envelopes for all of the identified species
and decomposition products of the pentanuclear cluster, and good fits between
experimental and theoretical data further substantiate our assignments (Figure 2.46).
96
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.3.3
Investigation of the reaction system that led to the formation of the
{Mo4} complex in 4
The same reaction conditions that led to the formation of 1-3 but using a
disubstituted arsonate ligand to control the self-assembly process, gave a dark green
solution from which compound 4, (NH4)4[MoVI4O10(O3AsC6H3NO2OH)4]·2H2O, was
obtained. The ESI-MS spectrum of this reaction mixture is complex, showing the presence
of multiple species in the higher mass region of the spectrum. We were able to identify
unambiguously the composition of one of these species. Its isotopic envelope is centered at
m/z = 1615.4 a.m.u. and corresponds to the formula of a tetranuclear molybdenum complex
with the composition {H9[MoV2MoIV2O10(O3AsC6H3NO2OH)4] ·H2O}– (Figure 2.49, Table
2.20). Orange crystals of a closely related compound 4 were obtained from the reaction
mixture within a time period of three weeks, and single crystal X-ray studies were
performed.
Figure 2.48 – Crystal structure of the tetranuclear Mo complex in 4. Colour code: MoVI sky blue, As
orange, O red, N light blue, C grey (hydrogen atoms have been omitted for clarity).
97
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Table 2.20 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 4.
Species attributed
1615.4
{H9[MoV2MoIV2O10(O3AsC6H3NO2OH)4]·H2O}–
→ {Mo4}
Relative ionic abundance
Relative ionic abundance
Reaction
mixture of 4
m/z
Figure 2.49 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation
of 4 (24 h after preparation). Comparison of the experimental isotopic envelopes (black spectrum)
with simulated patterns (red spectrum) for {H9[MoV2MoIV2O10(O3AsC6H3NO2OH)4] H2O}- centered
at m/z = 1615.4 a.m.u. (cone voltage: 30 V).
98
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
2.4 CONCLUSION AND FUTURE WORK
In this chapter we present a facile synthetic approach to functionalise
polyoxomolybdate clusters. The approach involves the partial reduction of MoVI salts in
the presence of organoarsonate and phosphonate ligands.
We demonstrate how slight perturbations of the ligand functionalities can be
exploited to stabilise unprecedented core structures. Our investigations underline the
stability of 1-3 in solution, an essential requirement for potential applications as catalysts.
Redox-active transition metals that adopt cubane as observed in 1 and 2 or related
structures are of particular interest to scientists because of their resemblance to active sites
of enzymes, and it has recently been suggested that such oxo clusters might hold the key to
catalysing the splitting of water,58-62 a process that might have an impact on future energy
requirements providing a conceptional solution to climate issues. We demonstrate that the
cubane structures tolerate amine functional groups in the para position to the arsonate
group, a characteristic that might allow the tethering of clusters to surfaces, for instance,
electrodes. We isolated an unprecedented pentanuclear ring structure in 3 when phydroxyphenyl arsenic acid is employed as a ligand. The mixed-valent oxo-cluster is
characterised by strong antiferromagnetic interactions between the MoV centres. The use of
(4-hydroxy-3-nitrophenyl)arsonic acid as a ligand promotes the formation of a rhombic
planar arrangement in a tetranuclear complex in 4 which can formally be related to the
rearrangement of the cube-type structures in 1 and 2.
Another aspect of the work focused on the synthesis of hybrid organic-inorganic
heteropolyoxomolybdates that incorporate additional d-block elements (Mn, Fe, Co, Ni).
The secondary transition metal ions serve as a bridging linker between two
polyoxomolybdate moieties leading to sandwich structures. We synthesised and isolated a
series of isostructural transition metal compounds and a main group derivative (Mg2+
compound). Our investigations include the structure determination and some
physicochemical characterisations of the compounds. Preliminary studies of the magnetic
properties of compounds 5-9 suggest that the S = ½ MoV centres in these compounds are
strongly antiferromagnetically coupled at room temperature leading to magnetically silent
hexanuclear subunits. The temperature dependence of the susceptibility of the compounds
is mainly determined by the presence of the transition metal ions. Based on the results of
our synthetic efforts it appears feasible that f-block elements such as lanthanide ions (Tb,
99
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Dy, Ho) could also be utilised to link the two {Mo6} fragments. The resulting
heteropolyoxomolybdates might exhibit interesting photophysical and magnetic properties.
Future
work
will
involve
the
synthesis
of
hybrid
organic-inorganic
heteropolyoxomolybdates that incorporate f-block elements (Tb, Dy, Ho) in order to
expand the family of clusters 5-9. We also intend to use a series of rigid extended polyphosphonate and –arsonate ligands which could give rise to the formation of porous
polyoxometalate metal−organic frameworks (P-MOFs). The electrochemical and catalytic
properties of the compounds will also be investigated.
The ESI-MS study of complex reaction mixtures presented in the last section of this
chapter aims to investigate the formation processes of hybrid organic-inorganic
polyoxomolybdates. We have demonstrated that ESI-MS, in combination with X-ray
crystallography, provides an extremely powerful tool to identify and characterise new
species that form in solution. Our approach allowed us to use simple, common, wellinvestigated, and commercially available ligands and to screen their involvement in
condensation reactions, providing us with a time-effective protocol to selectively isolate
and structurally characterise a series of novel species (structures described in section
2.2.1). The effectiveness of mass spectrometry in this field of science has only recently
been highlighted, and our efforts extend accomplishments to complex hybrid structures.
Our results underline the possibility of exploring real-time growth reactions of
polyoxometales that emerge in solution, transforming from small or oligonuclear species
and aggregating into larger molecular clusters. A summary of the ESI-MS assignment for
the reaction mixtures that led to the formation of the cubane structures in 1 and 2, and also
to the {Mo5} complex in 3 highlighting related structural motifs is presented in Figure
2.50. We observe the {Mo4} cubane species in the ESI-MS spectra of the reaction mixture
that led to the crystallisation of 1, 2 and 3. A {Mo6} species is observed in the ESI-MS
spectra of the reaction mixture that led to the crystallisation of 1 and 3 and seems to be
structurally related to the hexanuclear molybdenum units in 5-9. The third species
observed in the ESI-MS spectrum of the reaction mixture that led to the crystallisation of 3,
{Mo5}, forms in solution and structurally relates to the pentanuclear molybdenum cluster
that crystallises from this reaction mixture.
Our efforts focus on the application of analytical techniques that allow us to
monitor and direct complex condensation reactions to produce novel functionalised hybrid
materials.
100
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
Reaction mixture that led to
the crystallisation of the
following species; X-Ray
crystal structure
ESI-MS assigned
species
Structural motifs associated with the ESIMS assignment
{Mo4}
{Mo6}
{Mo4}
{Mo4}
{Mo4}
{Mo4}
{Mo5}
{Mo6}
{Mo5}
Figure 2.50 – Summary of the ESI-MS assignment for the reaction mixtures that led to the formation
of the cubane structures in 1 and 2, and also, to the {Mo5} complex in 3 highlighting related structural
motifs.
In conclusion, we demonstrated that the topological features of the described hybrid
molybdenum phosphonates and –arsonates are strongly dependent on the nature of the
employed organic ligand. The described approach provides an efficient protocol to prepare
hybrid polyoxomolybdates. Our results provide a solid foundation for research objectives
whereby rigid extended poly-phosphonate and –arsonate ligands could give rise to the
formation of porous polyoxometalate metal−organic frameworks (P-MOFs). These
systems would be interesting for catalysis, gas sorption and separation applications.80
101
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
D.-L. Long, R. Tsunashima and L. Cronin, Angewandte Chemie International
Edition, 2010, 49, 1736-1758.
A. Proust, R. Thouvenot and P. Gouzerh, Chemical Communications, 2008, 18371852.
D.-L. Long and L. Cronin, Chemistry – A European Journal, 2006, 12, 3698-3706.
P. Gouzerh and A. Proust, Chemical Reviews, 1998, 98, 77-112.
P. Mialane, A. Dolbecq and F. Secheresse, Chemical Communications, 2006, 34773485.
W. Qi, Y. Wang, W. Li and L. Wu, Chemistry – A European Journal, 2010, 16,
1068-1078.
N. Mizuno and M. Misono, Chemical Reviews, 1998, 98, 199-218.
B. J. S. Johnson, S. A. Geers, W. W. Brennessel, J. V. G. Young and A. Stein,
Dalton Transactions, 2003, 4678-4681.
B. J. S. Johnson, R. C. Schroden, C. Zhu and A. Stein, Inorganic Chemistry, 2001,
40, 5972-5978.
C. Sassoye, K. Norton and S. C. Sevov, Inorganic Chemistry, 2003, 42, 1652-1655.
Y.-D. Chang and J. Zubieta, Inorganica Chimica Acta, 1996, 245, 177-198.
A. Gammersbach, P. Angenault, W. F. Hölderich, W. Clegg and M. R. J. Elsegood,
Polyhedron, 1998, 17, 2535-2541.
M. I. Khan, Q. Chen and J. Zubieta, Inorganica Chimica Acta, 1995, 235, 135-145.
N. Calin and S. C. Sevov, Inorganic Chemistry, 2003, 42, 7304-7308.
E. Burkholder and J. Zubieta, Inorganica Chimica Acta, 2004, 357, 301-304.
D.-G. Lyxell, D. Bostrom, M. Hashimoto and L. Pettersson, Acta
Crystallographica Section B, 1998, 54, 424-430.
B. Liu, Z.-T. Yu, J. Yang, W. Hua, Y.-Y. Liu and J.-F. Ma, Inorganic Chemistry,
2011, 50, 8967-8972.
J. Hao, J. Zhang, P. Yin, Z. Xiao, F. Xiao and Y. Wei, Chemistry – A European
Journal, 2012, 18, 2503-2506.
P. DeBurgomaster, A. Aldous, H. Liu, C. J. O’Connor and J. Zubieta, Crystal
Growth & Design, 2010, 10, 2209-2218.
J. L. Q. Tuoi and E. Muller, Rapid Communications in Mass Spectrometry, 1994, 8,
692-694.
M. J. Deery, O. W. Howarth and K. R. Jennings, Journal of the Chemical Society,
Dalton Transactions, 1997, 4783-4788.
D. K. Walanda, R. C. Burns, G. A. Lawrance and E. I. von Nagy-Felsobuki,
Journal of the Chemical Society, Dalton Transactions, 1999, 311-322.
M. Bonchio, O. Bortolini, V. Conte and A. Sartorel, European Journal of Inorganic
Chemistry, 2003, 699-704.
F. Sahureka, R. C. Burns and E. I. von Nagy-Felsobuki, Inorganica Chimica Acta,
2003, 351, 69-78.
M. T. Ma, T. Waters, K. Beyer, R. Palamarczuk, P. J. S. Richardt, R. A. J. O’Hair
and A. G. Wedd, Inorganic Chemistry, 2008, 48, 598-606.
Y.-F. Song, D.-L. Long, S. E. Kelly and L. Cronin, Inorganic Chemistry, 2008, 47,
9137-9139.
E. F. Wilson, H. Abbas, B. J. Duncombe, C. Streb, D.-L. Long and L. Cronin,
Journal of the American Chemical Society, 2008, 130, 13876-13884.
R. Llusar, I. Sorribes and C. Vicent, Journal of Cluster Science, 2009, 20, 177-192.
102
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
H. N. Miras, E. F. Wilson and L. Cronin, Chemical Communications, 2009, 12971311.
R. Tsunashima, D.-L. Long, H. N. Miras, D. Gabb, C. P. Pradeep and L. Cronin,
Angewandte Chemie International Edition, 2010, 49, 113-116.
L. Vila-Nadal, S. G. Mitchell, A. Rodriguez-Fortea, H. N. Miras, L. Cronin and J.
M. Poblet, Physical Chemistry Chemical Physics, 2011, 13, 20136-20145.
E. F. Wilson, H. N. Miras, M. H. Rosnes and L. Cronin, Angewandte Chemie
International Edition, 2011, 50, 3720-3724.
H. N. Miras, M. Sorus, J. Hawkett, D. O. Sells, E. J. L. McInnes and L. Cronin,
Journal of the American Chemical Society, 2012, 134, 6980-6983.
K. Y. Matsumoto, Bulletin of the Chemical Society of Japan, 1978, 51, 492-498.
X.-Z. You, H.-L. Li, K.-B. Yu and L. Li, Acta Crystallographica Section C, 1993,
49, 1300-1303.
B. J. S. Johnson, C. E. Buss, V. G. Young, Jnr and A. Stein, Acta
Crystallographica Section C, 1999, 55, 549-551.
E. Burkholder, S. Wright, V. Golub, C. J. O'Connor and J. Zubieta, Inorganic
Chemistry, 2003, 42, 7460-7471.
S. Tan, M. Hobday, J. Gorman, G. Amiet and C. Rix, Journal of Materials
Chemistry, 2003, 13, 1180-1185.
A. Dolbecq, J.-D. Compain, P. Mialane, J. Marrot, F. Sécheresse, B. Keita, L. R. B.
Holzle, F. Miserque and L. Nadjo, Chemistry – A European Journal, 2009, 15, 733741.
H.-X. Guo and S.-X. Liu, Inorganic Chemistry Communications, 2004, 7, 12171220.
D. I. Brown, Oxford University Press, 2002, 26.
C. A. Hunter and J. K. M. Sanders, Journal of the American Chemical Society,
1990, 112, 5525-5534.
A. Jimtaisong, L. Feng, S. Sreehari, C. Bayse and R. Luck, Journal of Cluster
Science, 2008, 19, 181-195.
B. Modec, J. V. Brencic, E. M. Burkholder and J. Zubieta, Dalton Transactions,
2003, 4618-4625.
W. Schirmer, U. Flörke and H. J. Haupt, Zeitschrift für anorganische und
allgemeine Chemie, 1989, 574, 239-255.
W. Kwak, L. M. Rajkovic, J. K. Stalick, M. T. Pope and C. O. Quicksall, Inorganic
Chemistry, 1976, 15, 2778-2783.
K. M. Barkigia, L. M. Rajkovic-Blazer, M. T. Pope and C. O. Quicksall, Inorganic
Chemistry, 1981, 20, 3318-3323.
K. Nakamoto, Infrared and Raman spectra of Inorganic and coordination
compounds. Part B, Applications in coordination, organometallic, and
bioinorganic chemistry, 5th ed., John Wiley & Sons, New York; Chichester, 1997.
X. Zhang, J.-Q. Xu, J.-H. Yu, J. Lu, Y. Xu, Y. Chen, T.-G. Wang, X.-Y. Yu, Q.-F.
Yang and Q. Hou, Journal of Solid State Chemistry, 2007, 180, 1949-1956.
Y. Ma, Y. Li, E. Wang, Y. Lu, X. Wang, D. Xiao and X. Xu, Inorganica Chimica
Acta, 2007, 360, 421-430.
A. Hashikawa, M. Fujimoto, Y. Hayashi and H. Miyasaka, Chemical
Communications, 2011, 47, 12361-12363.
B. Botar, P. Kogerler, A. Muller, R. Garcia-Serres and C. L. Hill, Chemical
Communications, 2005, 5621-5623.
D. M. Poojary, Y. Zhang, B. Zhang and A. Clearfield, Chemistry of Materials,
1995, 7, 822-827.
103
Chapter 2 – Hybrid Organic-Inorganic Polyoxomolybdates
54.
55.
56.
57.
58.
59.
60.
61.
62.
G. Cao, R. C. Haushalter and K. G. Strohmaier, Inorganic Chemistry, 1993, 32,
127-128.
S. Gupta, S. Roy, T. N. Mandal, K. Das, S. Ray, R. J. Butcher and S. K. Kar,
Journal of Chemical Sciences, 2010, 122, 239-245.
W. Henderson and J. S. McIndoe, John Wiley & Sons, 2005, 292.
V. A. Pashynska, M. V. Kosevich, H. V. d. Heuvel and M. Claeys, Rapid
Communications in Mass Spectrometry, 2006, 20, 755-763.
M. W. Kanan, Y. Surendranath and D. G. Nocera, Chemical Society Reviews, 2009,
38, 109-114.
R. Brimblecombe, D. R. J. Kolling, A. M. Bond, G. C. Dismukes, G. F. Swiegers
and L. Spiccia, Inorganic Chemistry, 2009, 48, 7269-7279.
M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072-1075.
J. Barber, Inorganic Chemistry, 2008, 47, 1700-1710.
T. G. Carrell, E. Bourles, M. Lin and G. C. Dismukes, Inorganic Chemistry, 2003,
42, 2849-2858.
104
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.
POLYNUCLEAR MANGANESE
COORDINATION COMPLEXES
105
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.1 INTRODUCTION AND MOTIVATION
The chemistry of Mn coordination clusters has gained considerable attention during
the last decades due to their relevance to bioinorganic chemistry and potential applications
as molecular magnetic materials. Manganese is an essential redox active element in
biology, playing an important role in the active sites of many proteins and enzymes
responsible for carrying out vital chemical transformations. For example, the
photosynthetic water oxidation is catalyzed within photosystem II (PSII) by a {Mn4O4Ca}
cluster species.1-5
Mn complexes with various nuclearities and topologies have been intensively
studied for their fascinating physical properties and, also for the intrinsic architectural
beauty they often possess.6 Depending on the oxidation states, the nuclearity, topology and
bridging modes, these coordination clusters can reveal properties of single molecule
magnets (SMMs) that exhibit superparamagnetic behaviour below a certain blocking
temperature. This type of materials show potential future applications in high-density
magnetic storage devices and molecular electronics.6-11
A large number of Mn complexes have been prepared using carboxylate ligands,
whereas phosphonates have only rarely been employed.6 However, the versatile nature of
these potential ligands and their high binding affinity towards metal ions renders this class
of organic molecules to be highly suitable to stablise low-dimenesional molecular
species.12-15
Based on these considerations we set out to investigate the influence of different
phosphonate ligands on the Mn coordination cluster formation and to explore the
properties of the resulting materials. Our aim was to develop new Mn coordination clusters
that may display interesting magnetic properties. Thus, we selected a set of three different
ligands with different characteristics: phenylphosphonic acid, benzylphosphonic acid and
triphenylmethylphosphonic acid. Benzylphosphonic acid contains, in comparison to
pheylphosphonic acid, a flexible −CH2− group that can freely rotate to meet the steric
requirements imposed by the coordination geometries of aggregating Mn ions. In contrast,
triphenylmethylphosphonic acid represents a more bulky ligand, which might restrict
oligomerisation and lead to the formation of discrete cages.
106
Chapter 3 – Polynuclear Manganese Coordination Complexes
The preparation of new Mn coordination clusters and SMMs is predominantly
based on serendipitous synthetic approaches. In order to produce novel materials with
interesting magnetic properties, one needs to understand the formation processes of such
complex molecular entities and to be able to control the reaction systems to generate
targeted species. There is a need to develop rational synthetic approaches for the formation
of single molecule magnets.11, 16 Additionally, suitable characterisation tools, that would
allow the identification of new and interesting species in solution, or would give the
possibility to identify building units and monitor their aggregations, are required.17
NMR spectroscopy is one of the main techniques which can provide detailed
information about the structure, reaction state, chemical environment of molecules, etc.,
but, is of little use when a large number of paramagnetic metal centres are involved. Other
techniques must be employed in such cases. Generally, a combination of techniques
provides the most complete picture of the chemistry of the reaction system under study.18,
19
Mass spectrometry in combination with X-ray crystallography proved to be an extremely
powerful approach to identify and characterize new species that form in solution. It was
previously used to study the self-assembly processes of polyoxometalate systems that
emerge in solution.20-25 Based on these considerations, we decided to exploit the ESI-MS
technique and apply it to characterise manganese-phosphonate coodination clusters system.
Surprisingly ESI-MS studies that investigate complex Mn coordination cluster systems
have not been reported in the literature.
Our aim was to investigate the formation processes of polynuclear manganese
clusters. The characterisation of complex reaction mixtures might provide insights into
underlying condensation reactions and might lead the development of more rational
approaches for the design of single molecule magnets.
In this chapter, we demonstrate how electrospray ionisation mass spectrometry can
be used to screen complex Mn reaction mixtures in order to identify new polynuclear
manganese species that form in solution. Some of these species were isolated and their
structures and other physicochemical properties were characterised.
107
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.2 PHOSPHONATE LIGANDS
A set of different aromatic phosphonates were used to prepare a series of
polynuclear
manganese
coordination
complexes.
Phenylphosphonic
acid
and
benzylphosphonic acid, which were commercially available, and triphenylmethylphosphonic acid that was prepared according to literature procedures.26-28 For the synthesis
of triphenylmethylphosphonic acid, triphenylcarbinol was treated with phosphorus
trichloride to give the corresponding phosphorous acid dichloride according to the equation
(i) in Scheme 3.1. Then, the desired phosphonic acid was produced by treating the
triphenylmethoxyphosphorous dichloride with an alcoholic solution of potassium
hydroxide (Scheme 3.1 (ii)).
(i)
(ii)
OH
+ PCl3
OPCl2
2KOH + EtOH
OPCl2
+ HCl
PO3H2
-2KCl
Scheme 3.1 – Reaction scheme for the synthesis of triphenylmethylphosphonic acid.
These phosphonate ligands were then reacted with manganese salts in
comproportionation reactions with the aim of generating manganese phosphonate
coordination clusters. The geometrical characteristics of the functional group of the
organophosphonates and the strong coordination capability of these ligands towards metal
ions were expected to promote the formation of unprecedented core structures with novel
properties.
108
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.3 POLYNUCLEAR
MANGANESE
COMPLEXES
STABILISED
BY
ORGANOPHOSPHONATES
3.3.1
Synthesis and characterisation of a pentadecanuclear manganese
complex
3.3.1.1 [Μn
ΙΙΙ
15
(μ2-H2Ο)2(CΗ3ΟΗ)16(C6Η5PΟ3)20]Cl5·22CH3OH·8H2O (10)
Compound 10, [ nΙΙΙ15(μ2-H2 )2(C
3
)16(C6
5P
3)20]Cl5·22CH3OH·8H2O,
was prepared by a comproportionation reaction between MnCl2·4H2O and KMnO4, in
MeOH, in the presence of phenylphosphonic acid. The reaction mixture was stirred for five
hours, filtered and left undisturbed for four days at room temperature. Then, the reaction
mixture was transferred into a refrigerator. Rectangular, red-brown crystals of 10 were
obtained after keeping the reaction mixture at about 2 °C for another four days. The crystal
structure of 10 was determined by single crystal X-ray diffraction measurements.
10 crystallises in the triclinic crystal system in the space group P and contains a
pentadecanuclear MnIII complex (Figure 3.1). The manganese cluster core in 10 is
centrosymmetric, consisting of two symmetry equivalent {MnIII5O25} units, (Figure 3.1,
A), linked by a central {MnIII5O28} unit (Figure 3.1, B) to give the {Mn15} core structure.
Within the {MnIII5O25} unit four Mn ions (Mn(1), Mn(3), Mn(4) and Mn(6)) are connected
through eight phosphonate ligands to form a twelve membered ring (As ions included).
The fifth Mn ion (Mn(2)) caps one side of the ring (Figure 3.2). The “basketlike”
{MnIII5O25} unit is further stabilized by five CH3OH ligands. Each Mn ion in A displays a
square pyramidal coordination environment in which four O donors are provided by four
different phosphonate ligands whilst the remaining O donor arises from a CH3OH
molecule. All Mn ions in A adopt the oxidation state +III as calculated using bond valence
sum analysis29 based on the bond distances between the Mn centers and the surrounding O
donors. The bond distances between the Mn ions and the O donors originating from the
organic phosphonate ligands vary between 1.875(7) Å – 1.934(7) Å, whilst the remaining
Mn-Omethanol bond lengths are 2.171(7) Å, 2.211(7) Å, 2.139(7) Å, 2.116(8) Å and 2.156(8)
109
Chapter 3 – Polynuclear Manganese Coordination Complexes
Å, respectively for Mn(1)-O(33), Mn(2)-O(36), Mn(3)-O(38), Mn(4)-O(34) and Mn(6)O(35) (Table 3.1).
(A)
(B)
(A)
Figure 3.1 − Crystal structure of the pentadecanuclear MnIII complex in 10 formally constructed of two
{MnIII5O25} units A and a connecting {MnIII5O28} unit B; (schematic representation {MnIII5O25} – blue
sphere, {MnIII5O28} – blue rod). Colour code: MnIII blue, P purple, O red, C grey, Cl green (hydrogen
atoms have been omitted for clarity).
110
Chapter 3 – Polynuclear Manganese Coordination Complexes
a)
b)
(A)
Figure 3.2 − Different views of the {MnIII5O25} unit A. (a) Ball-and-stick and (b) polyhedral
representation. Colour code: MnIII blue, P purple, O red, C grey, Cl green (hydrogen atoms have been
omitted for clarity).
111
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.1 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 10.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(1)
Mn(1)-O(19)
Mn(1)-O(9)
Mn(1)-O(20)
Mn(1)-O(5)
Mn(1)-O(33)
1.888(7)
1.890(7)
1.894(7)
1.928(7)
2.171(7)
3.066
+3
Mn(2)
Mn(2)-O(28)
Mn(2)-O(22)
Mn(2)-O(11)
Mn(2)-O(21)
Mn(2)-O(36)
1.898(7)
1.904(7)
1.905(7)
1.934(7)
2.211(7)
2.956
+3
Mn(3)
Mn(3)-O(30)
Mn(3)-O(4)
Mn(3)-O(12)
Mn(3)-O(6)
Mn(3)-O(38)
1.876(7)
1.876(7)
1.892(7)
1.909(7)
2.139(7)
3.182
+3
Mn(4)
Mn(4)-O(24)
Mn(4)-O(29)
Mn(4)-O(23)
Mn(4)-O(7)
Mn(4)-O(34)
1.875(7)
1.882(7)
1.885(7)
1.902(8)
2.116(8)
3.222
+3
Mn(5)
Mn(5)-O(31)
Mn(5)-O(10)
Mn(5)-O(27)
Mn(5)-O(26)
Mn(5)-O(37)
Mn(5)-O(39)
1.884(7)
1.889(7)
1.893(8)
1.903(7)
2.191(9)
2.364(8)
3.299
+3
Mn(6)
Mn(6)-O(25)
Mn(6)-O(14)
Mn(6)-O(13)
Mn(6)-O(8)
Mn(6)-O(35)
1.890(7)
1.886(8)
1.896(7)
1.929(7)
2.156(8)
3.077
+3
Mn(7)
Mn(7)-O(2)
Mn(7)-O(15)
Mn(7)-O(3)
Mn(7)-O(16)
Mn(7)-O(32)
Mn(7)-O(39)
1.892(7)
1.900(8)
1.902(7)
1.919(8)
2.210(8)
2.345(7)
3.212
+3
Mn(8)
Mn(8)-O(1)
Mn(8)-O(1`)
Mn(8)-O(17`)
Mn(8)-O(17)
Mn(8)-O(18`)
Mn(8)-O(18)
1.877(7)
1.877(7)
1.912(8)
1.912(7)
2.243(8)
2.243(8)
3.320
+3
Mn(1) ··· Cl(1)
Mn(2) ··· Cl(1)
Mn(6) ··· Cl(1)
2.644(3)
2.651(3)
2.640(3)
The {MnIII5O28} unit B (Figure 3.3) consists of two {MnIII2O11} symmetry
equivalent subunits connected to each other through a central {MnO6} unit involving four
112
Chapter 3 – Polynuclear Manganese Coordination Complexes
bridging phosphonate ligands. The {MnIII5O28} unit is further stabilized by six CH3OH
ligands. The position of the central Mn(8) atom coincides with the position of an inversion
symmetry centre.
(B)
Figure 3.3 − Polyhedral representation of the {MnIII5O28} unit B. Colour code: MnIII blue, P purple, O
red, C grey (hydrogen atoms have been omitted for clarity).
Within the {MnIII2O11} subunit two Mn ions (Mn(5) and Mn(7)) share a common
vertex (O(39)) of their distorted octahedral coordination environment. The O(39) donor
arises from a bridging H2O ligand, whilst the remaining coordination sites are occupied by
one Omethanol and four O donors originating from four distinct phosphonate ligands. The
distorted nature of the octahedra can be observed from the bond angles and bond lengths of
the MnIII metal centres. The following bond angles: O(37)-Mn(5)-O(39), O(31)-Mn(5)O(10) and O(27)-Mn(5)-O(26) of 178.8(3)°, 178.0(3)° and 176.6(3)° respectively, deviate
from the ideal octahedral angle of 180°, whilst the bond angles O(31)-Mn(5)-O(39), O(26)Mn(5)-O(39), O(10)-Mn(5)-O(26), O(10)-Mn(5)-O(39), O(10)-Mn(5)-O(27) and O(27)Mn(5)-O(39) of 91.6(3)°, 91.1(3)°, 91.0(3)°, 90.3(3)°, 87.8(3)° and 85.7(3)° respectively,
deviate from the ideal angle of 90° (Table 3.2). Similar values can be found for the bond
angles of the Mn(7) ion, which are summarized in Table 3.2. The four Mn-O bond
distances between Mn ions and O donors originating from the organic ligands, vary
between 1.884(7) Å – 1.919(8) Å, whilst the remaining two bond lengths involving trans
located MeOH and H2O ligands of the octahedral coordination environment are slightly
elongated. Thus, the Mn(5) and Mn(7) ions display Jahn-Teller (JT) axial elongation
113
Chapter 3 – Polynuclear Manganese Coordination Complexes
typical for MnIII ions.30 Mn-Omethanol bond distances, Mn(5)-O(37) and Mn(7)-O(32) are
2.191(9) Å and 2.210(8) Å, respectively, and the bond lengths between Mn(5), Mn(7) and
the μ2-H2O ligand, O(39), are 2.364(8) Å and 2.345(7) Å (Table 3.1). The central Mn(8)
ion located on an inversion center displays a more regular octahedral coordination
polyhedron compared to those of Mn(5) and Mn(7) ions. The bond angles of the Mn(8) ion
are very close to the ideal octahedral angle of 180° and 90°, as seen in Table 3.2. However,
substantial distortion can be observed due to the elongated Mn-Omethanol bonds of 2.243(8)
Å occupying the apical positions of the Jahn-Teller distorted MnIII ion.
Table 3.2 − Selected bond angles [º] for compound 10.
Bond Angle (º)
O(37)-Mn(5)-O(39)
O(31)-Mn(5)-O(10)
O(27)-Mn(5)-O(26)
O(31)-Mn(5)-O(39)
O(26)-Mn(5)-O(39)
O(10)-Mn(5)-O(26)
O(10)-Mn(5)-O(39)
O(10)-Mn(5)-O(27)
O(27)-Mn(5)-O(39)
178.8(3)
178.0(3)
176.6(3)
91.6(3)
91.1(3)
91.0(3)
90.3(3)
87.8(3)
85.7(3)
O(17)`-Mn(8)-O(17)
O(1)-Mn(8)-O(1`)
O(18`)-Mn(8)-O(18)
O(1)-Mn(8)-O(17)
O(17`)-Mn(8)-O(18)
O(1)-Mn(8)-O(18)
O(1)-Mn(8)-O(18`)
O(17)-Mn(8)-O(18)
O(1)-Mn(8)-O(17`)
180.0(2)
179.999(4)
179.997(1)
93.9(3)
93.4(3)
92.2(3)
87.8(3)
86.6(3)
86.1(3)
Bond Angle (º)
O(2)-Mn(7)-O(3)
O(15)-Mn(7)-O(16)
O(32)-Mn(7)-O(39)
O(3)-Mn(7)-O(39)
O(15)-Mn(7)-O(3)
O(2)-Mn(7)-O(39)
O(15)-Mn(7)-O(39)
O(3)-Mn(7)-O(16)
O(16)-Mn(7)-O(39)
175.8(3)
175.5(3)
174.3(3)
93.8(3)
91.3(3)
90.4(3)
89.5(3)
88.3(3)
86.1(3)
Each of the twenty PhPO32- moieties in 10 bridge between three Mn atoms in a
η1:η1:η1:μ3 bridging mode. The oxidation states of the Mn ions and the protonation of the
bridging donor species, i.e. H2O ligands were established by Mn and O bond valence sum
calculations (Table 3.1), close examination of the geometric parameters and detection of
MnIII Jahn-Teller (JT) elongation axes. The Mn – (μ2-H2O) distances in 10 are comparable
to those observed in other reported aqua-bridged dimanganese complexes (2.25 Å – 2.18
Å).31-34 These distances are found to be significantly longer than the Mn – (μ2-OH-)
distances of 2.05 Å – 2.09 Å and the Mn – (μ2-O2-) distances of 1.78 Å – 1.81 Å. The
overall charge of the cluster is compensated by five chlorine ions. Two of the chlorine ions
are encapsulated within the {MnIII5O25} units and may serve as templates promoting the
114
Chapter 3 – Polynuclear Manganese Coordination Complexes
cluster formation. The distances between the chlorine ion Cl(1) and Mn(1), Mn(2) and
Mn(6) are 2.644(3) Å, 2.651(3) Å and 2.640(3) Å, respectively (Table 3.1).
In the solid state, the packing arrangement of the clusters is stabilized by weak
hydrogen bonding interactions between the coordinated methanol molecules and
constitutional solvent molecules. A detailed analysis of these interactions was not possible
due to the large number of disordered solvent molecules and the complexity of the cluster
entity. Weak offset π-π interactions35 between two adjacent clusters can also be observed.
These involve the phenyl rings of the phosphonate ligands from the coordination sphere of
Mn(2) and its symmetry related partners, resulting in an interplanar separation distance of
4.144(2) Å. The clusters are further linked through halogen bonding interactions between
Cl(3) and two hydrogen atoms of two phosphonate rings of two adjacent coordination
clusters (2.763(5) Å and 2.805(5) Å, respectively). In addition some weak interaction
between Cl(3), Cl(4), Cl(5) and hydrogen atoms of some constitutional solvent molecules
preveil. The crystal structure displays small intercluster cavities which are filled with
solvent molecules. These channels extend in the direction of the crystallographic b-axis as
can be seen in Figure 3.4 (b).
a)
b)
Figure 3.4 – Packing arrangement of the pentadecanuclear MnIII clusters in 10, viewed in the direction
of the crystallographic: (a) a-axis and (b) b-axis. Colour code: MnIII blue, P purple, O red, C grey, Cl
green (crystallization solvent molecules and hydrogen atoms have been omitted for clarity).
115
Chapter 3 – Polynuclear Manganese Coordination Complexes
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
Compound 10 was characterized by infrared spectroscopy. The IR spectrum of 10
is shown in Figure 3.5 and reveals some characteristic stretches of the organophosphonate
ligands. The set of bands observed between 3000 – 2800 cm-1 can be attributed to C–H
stretching vibrations, while the bands between 1500 – 1430 cm-1 are most likely due to C–
C skeletal vibrations of the phenyl rings. Typical C–H out-of-plane bending vibrations of
the aromatic rings can be found between 800 – 650 cm-1 as sharp bands. The set of bands
between 1200 – 900 cm-1 are due to the different P–O stretching vibrations of the
phosphonate groups. Finally, the corresponding O–H stretching vibrations and H–O–H
bending vibrations of the crystallization water molecules engaged in H-bonds appear as
broad bands centered at ca. 3266 cm-1 and ca. 1628 cm-1, respectively.36-40
95
90
85
1628
Transmittance (%)
80
1487
2835
2944
75
70
3266
65
1438
60
55
50
45
40
750
722
35
30
25
693
1124
1032
20
15
10
4000
983
3600
3200
2800
2400
2000
1800
Wavenumber
1600
1400
1200
1000
800 650
(cm-1)
Figure 3.5 – Infrared spectrum of 10.
116
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Thermogravimetric analysis
The thermal stability of compound 10 was investigated by thermogravimetric
analysis (TGA) using a freshly prepared crystalline sample. The analysis was carried out in
the temperature range between 30 and 900 °C, in an N2 atmosphere. The TGA curve of 10
shown in Figure 3.6 exhibits an initial weight loss of 10.9 % between 30 – 200 °C that can
be attributed to the removal of sixteen coordination CH3OH molecules (calcd: 11.0 %).
The other crystallization solvent molecules of 10 may have been lost prior to the TGA
during the storage of the sample at room temperature. A further increase of the temperature
affected a gradual weight loss between 200 – 500 °C corresponding to the decomposition
of the organic ligands, followed by further cluster degradation above 500 °C.
100
Weight % (%)
90
80
70
60
50
0
200
400
600
800
1000
Temperature (°C)
Figure 3.6 – Thermogravimetric analysis of 10.
117
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Magnetism
The magnetic properties for compound 10 were studied on a powdered
microcrystalline sample. The temperature dependence of the magnetic susceptibility of 10
was measured between 298 and 1.8 K (Figure 3.7). The χT value of 22.4 cm3 K mol-1 at
room temperature is significantly lower than the expected value of 45 cm3 K mol-1 for the
presence of fifteen S = 2 MnIII carriers (C =
S(S+1) with
= 0.12505 cm3 K mol-1
and g = 2).41 This result is characteristic for predominatly strong antiferromagnetic (AF)
exchange parameters between spins carriers. Upon lowering the temperature, the χT
product decreases down to a minimum of ca. 0.8 cm3 K mol-1 at 1.8 K. This thermal
behavior confirms the presence of dominant AF interactions between spin carriers and the
low temperature χT value suggests an ST = 0 ground state. This result does not exclude the
presence of ferromagnetic interactions but only suggests that the AF interactions are
dominant.
a)
b)
Figure 3.7 – (a) Temperature dependence of the χT product of 10 at 0.1 and 1 T. (b) A magnified view
of the χT product in (a) between 1 and 100 K.
Unfortunately the experimental data can not be fitted to a Curie-Weiss law
probably because the magnetic exchange parameters within the cluster core are so large
and thus a mean-field model can not be used for this molecular system.
The field dependence of the magnetisation for this compound has been measured at
low temperatures between 1.8 and 8 K (Figure 3.8). The M vs H plot is typical of a
complex with diamagnetic ground state chacterised by weak magnetisation values, almost
linear field dependence and no saturation even at high field.
118
Chapter 3 – Polynuclear Manganese Coordination Complexes
8
7
6
5
4
1.8 K
3K
5K
8K
3
2
1
0
0
20000
40000
60000
H / Oe
Figure 3.8 – Field dependence of the magnetisation at and below 8 K.
It is worth noting that no hysteresis effects upon measureing the field-dependence
of the magnetisation (M vs H, 100 – 200 Oe/min) and no ac susceptibility in zero dc field
(at 1000 Hz) have been observed above 1.8 K.
119
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Mass spectrometry
The stability of compound 10 in solution was investigated by electrospray
ionization mass spectrometry which provides a very powerful analytical tool for the
characterization of coordination compounds in solution.42-48 Negative mode ESI-MS
spectra of compound 10 dissolved in a suitable solvent (CH3CN, DMF and DMSO,
respectively), were recorded (Table 3.3, Figure 3.9). The mass spectra of these solutions
reveal only one major isotopic envelope in the high molecular mass region, centered at m/z
= 1684.3 a.m.u. This signal can be assigned to the {H17[MnIV6MnIIIO15(C6H5PO3)6]Cl3}species, which was modelled (Figure 3.9, inset) and good fits between the experimental
and simulated patterns could be found. The entire {Mn15} complex could not be identified
in the ESI-MS spectrum, presumably due to a facile dissociation involving the partially
solvated, central Mn(8) ion (Figure 3.3) .
Table 3.3 – ESI-MS assignment for compound 10.
Crystals of
m/z
Species attributed
CH3CN
1684.3
{H17[MnIV6MnIIIO15(C6H5PO3)6]Cl3}-
→ {Mn7}
DMF
1684.3
{H17[MnIV6MnIIIO15(C6H5PO3)6]Cl3}-
→ {Mn7}
DMSO
1684.3
{H17[MnIV6MnIIIO15(C6H5PO3)6]Cl3}-
→ {Mn7}
Relative ionic abundance
10 in:
Solvent
Figure 3.9 – Negative-mode ESI-MS spectra for crystals of 10 dissolved in DMF. Inset: Comparison
of the experimental isotopic envelopes (black spectrum) with simulated patterns (red spectrum) for
{H17[MnIV6MnIIIO15(C6H5PO3)6]Cl3}- centered at m/z = 1684.3 a.m.u. (cone voltage: 30 V).
120
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 10 recorded in CH3CN can be seen in Figure
3.10. The shoulder observed at ca. 280 nm ( = 34000 L mol-1 cm-1) can be attributed to π −
π* transitions of the phosphonate ligands, while the weak band at ca. 465 nm ( = 6400 L
mol-1 cm-1) involve a d – d transition that can be assigned to a 5T2g
III
octahedral Mn ions within 10.
5
Eg transition for the
49-52
0.7
Absorbance (a.u.)
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
400
450
500
550
600
0.2
0.1
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 3.10 – UV-Vis spectrum of a 10-5 M solution of 10 in CH3CN. Inset: A
section of the UV-Vis spectrum of a 10-4 M solution of 10 in CH3CN.
121
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.4 − Crystal data and structural refinement parameters for 10.
Compound 10
Empirical formula a
Molecular mass/g mol
C136H168Cl5Mn15O78P20
-1 a
Crystal colour/shape
3
Red brown / rectangular block
Crystal size/mm
0.50×0.20×0.20
Crystal system
Triclinic
Space group
P
a/ Å
14.5661(5)
b/ Å
16.5025(5)
c/ Å
51.7573(16)
/º
95.1300(10)
/º
97.2120(10)
/º
102.500(2)
3
V/ Å
11962.6(7)
Z
1
Temperature (K)
150(2)
-3
Density/Mg m
1.379
-1
Absorp. coef./mm
0.974
F(000)
5044
2
50
max/º
Reflections collected
93851
Independent reflections
41381 [R(int) = 0.0329]
Data / restraints / parameters
41381 / 0 / 2782
2
a
4671.55
S on F
1.079
R1, wR2 [I>2 (I)]
0.1236, 0.2785
R1, wR2 (all data)
0.1286, 0.2813
Largest diff. peak and hole/e.Å-3
2.823 and -2.275
Excluding solvate molecules
122
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.3.2
Synthesis and characterisation of tridecanuclear manganese complexes
3.3.2.1 [Μn
ΙΙΙ
13
(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5PΟ3)10(C5Η5Ν)5Cl]·3H2O (11)
Compound 11 was prepared using a similar synthetic procedure that led to the
formation of 10. MnCl2·4H2O and KMnO4 were reacted with phenylphosphonic acid in a
MeOH/CH3CN solution and pyridine was added as an organic base. After stirring the
reaction mixture for five hours, it was filtered and left at room temperature for slow
evaporation of the solvent. During a time period of one week small rhombic red-brown
crystals of [ n
13( 4-
)2( 3- )4( 2- H)2( 2-C
3
)4(C6
5P
3)10(C5
5
)5Cl]·3H2O (11)
were obtained and characterized by single crystal X-ray diffraction measurements.
11 crystallises in the orthorhombic crystal system in the space group P212121. It
contains a tridecanuclear MnIII complex that is shown in Figure 3.11. The core structure of
11 features a three layered metal-centered distorted cuboctahedron (Figure 3.11, Figure
3.12). The triangular units A and A` are staggered in a trigonal antiprismatic fashion. They
consist of three MnIII ions (Mn(9), Mn(10) and Mn(11), and Mn(4), Mn(8) and Mn(12),
respectively) connected through a
2-
H- moiety and three phosphonate ligands (Figure
3.13). Unit A is further stabilised by two pyridine molecules, one chloride ion and two
methanol molecules, while unit A` is further stabilised by three pyridine and two methanol
molecules. Two of the three Mn ions in A and A` (Mn(9) and Mn(10), Mn(8) and Mn(12),
respectively) adopt distorted square pyramidal geometries that share a common vertex
O(33) and O(10), respectively. The coordination spheres of Mn(9), Mn(10), Mn(8) and
Mn(12), each consist of one
2-
H- group, three O donors from three distinct phosphonate
ligands and a N atom from a pyridine molecule.
The bond distances between Mn ions and the
2-
H- are 1.899(13) Å, 1.960(13) Å,
1.897(13) Å, and 1.953(13) Å for Mn(9)-O(33), Mn(10)-O(33), Mn(8)-O(10) and Mn(12)O(10), respectively. The bond distances between these latter Mn ions and the O donors
originating from the organic phosphonate ligands range between 1.868(11) Å – 1.934(12)
Å, while the bond distances between Mn(9)-N(2), Mn(10)-N(3), Mn(8)-N(5) and Mn(12)N(6) are 2.195(14) Å, 2.243(17) Å, 2.255(17) Å and 2.231(15) Å, respectively (Table 3.5).
123
Chapter 3 – Polynuclear Manganese Coordination Complexes
(A)
(B)
(A`)
(A)
(B)
(A`)
Figure 3.11 − Crystal structure of the tridecanuclear manganese complex in 11 showing the
triangular units A and the hexagonal unit B. Colour code: MnIII blue, P purple, O red, Cl green, C
grey (hydrogen atoms have been omitted for clarity).
124
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.12 – The core structure of the tridecanuclear manganese complex in 11. Colour code: MnIII
blue, P purple, O red, Cl green, C grey.
The third MnIII ion in the A and A` moieties, Mn(11) and Mn(4) respectively,
displays a distorted octahedral coordination environment. The distortion arises from the
geometrical restrictions of the binding ligands and the nature of the MnIII Jahn-Teller ion,
and can be exemplified by examining the bond angles and bond lengths that deviate from
the ideal octahedral geometry. Selected bond angles for Mn(11) and Mn(4) ions are listed
in Table 3.7. The distorted octahedral geometry of Mn(11) and Mn(4) ions consists of two
Ophosphonate donors forming Mn-O bond lengths that range between 1.905(11) Å – 2.092(13)
Å, two Omethanol donors which engage in Mn-O bonds that range between 1.943(10) Å –
2.141(13) Å, and a
4-
2-
ion forming Mn-O bond lengths of 1.959(10) Å and 1.928(10) Å,
for Mn(11)-O(34) and Mn(4)-O(24), respectively. The coordination environment of
Mn(11) is completed by a Cl ion that forms a Mn(11)-Cl(1) bond of 2.281(7) Å, while the
coordination environment of Mn(4) is completed by a N atom from a pyridine molecule
that forms a Mn(4)-N(1) bond of 2.208(11) Å (Table 3.5). The Mn(11)-Cl(1) and Mn(4)N(1) bonds are about 0.1 – 0.3 Å longer than the other bonds of the coordination
environments of Mn(11) and Mn(4), respectively. In addition it is worth noting that the
bond angles O(6)-Mn(11)-O(17), O(16)-Mn(11)-O(6), O(22)-Mn(4)-O(8) and O(22)125
Chapter 3 – Polynuclear Manganese Coordination Complexes
Mn(4)-N(1) of 168.6(5)°, 95.5(5)°, 167.2(5)° and 97.9(4)°, respectively, show the greatest
deviation from the ideal octahedral geometry.
(A)
(A`)
Figure 3.13 − Polyhedral representation of the triangular units A and A` in 11. Colour code: MnIII blue,
P purple, O red, Cl green, C grey.
The hexagonal unit B (Figure 3.14) consists of six Mn atoms (Mn(1), Mn(2),
Mn(3), Mn(5), Mn(6), Mn(7)) and one Mn atom located in the centre (Mn(13)). All Mn
ions in B adopt a near-octahedral geometry, with an obvious Jahn−Teller (JT) distortion
(axial elongation) further supporting the assignment of the +III oxidation state of the Mn
ions.30 The distorted octahedral geometry of Mn(1), Mn(2), Mn(5), and Mn(7) consists of
three Ophosphonate donors, one Omethanol donor, one
4-
2-
ion and a
3-
2-
ion. The Mn-
Ophosphonate bond distances range between 1.913(10) Å – 2.253(11) Å, the Mn-Omethanol bond
distances range between 1.871(12) Å – 1.932(11) Å, the Mn-( 4between 1.901(9) Å – 1.946(9) Å, while the Mn-( 3-
2-
) bond distances range
2-
) bond distances range between
1.908(10) Å – 1.971(12) Å. The distorted octahedral geometry of Mn(3) and Mn(6)
consists of four Ophosphonate donors and two
3-
2-
ions. The Mn-Ophosphonate bond distances
range between 1.906(10) Å – 2.240(11) Å, while the Mn-( 3-
2-
) bond distances range
between 1.904(11) Å – 1.937(9) Å. Finally, the octahedral coordination sphere of Mn(13)
126
Chapter 3 – Polynuclear Manganese Coordination Complexes
4-
compress two
2-
ions and four
3-
2-
ions. The Mn-( 4-
2-
) bond distances are
2.369(10) Å and 2.423(10) Å for Mn(13)-O(34) and Mn(13)-O(24), respectively, while
the Mn-( 3-
2-
) bond distances range between 1.935(9) Å – 1.967(9) Å. The distorted
nature of the octahedral geometry of the metal ions within the hexagonal sub-unit can be
observed from the bond lengths and the bond angles of the MnIII metal centres that deviate
from the ideal octahedral geometry. Selected bond lengths and bond angles for the Mn ions
in 11 are listed in Table 3.5 and Table 3.7.
(B)
Figure 3.14 – Ball-and-stick representation of the hexagonal unit B in 11. Colour code: MnIII blue,
P purple, O red, C grey.
The hexagonal unit B is stabilized to the outside by six phosphonate ligands and
four methanol molecules. The units A and B are connected to each other by a combination
of O2-, CH3O- and phosphonate ligands to give the {Mn13} core structure. All four
methanol molecules are deprotonated and bridge between two Mn ions (Mn(1) and Mn(4),
Mn(2) and Mn(4), Mn(5) and Mn(11), Mn(7) and Mn(11), respectively).
127
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.5 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 11.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(1)
Mn(1)-O(37)
Mn(1)-O(39)
Mn(1)-O(28)
Mn(1)-O(24)
Mn(1)-O(36)
Mn(1)-O(23)
1.908(10)
1.909(12)
1.921(11)
1.946(9)
2.156(10)
2.162(11)
3.271
+3
Mn(2)
Mn(2)-O(22)
Mn(2)-O(24)
Mn(2)-O(21)
Mn(2)-O(25)
Mn(2)-O(13)
Mn(2)-O(23)
1.882(12)
1.901(9)
1.913(10)
1.926(11)
2.209(9)
2.229(10)
3.281
+3
Mn(3)
Mn(3)-O(26)
Mn(3)-O(4)
Mn(3)-O(30)
Mn(3)-O(37)
Mn(3)-O(7)
Mn(3)-O(36)
1.904(11)
1.906(10)
1.909(12)
1.928(10)
2.194(11)
2.240(11)
3.238
+3
Mn(4)
Mn(4)-O(24)
Mn(4)-O(14)
Mn(4)-O(22)
Mn(4)-O(39)
Mn(4)-O(8)
Mn(4)-N(1)
1.928(10)
2.020(12)
2.072(11)
2.073(11)
2.092(13)
2.208(11)
2.799
+3
Mn(5)
Mn(5)-O(34)
Mn(5)-O(3)
Mn(5)-O(43)
Mn(5)-O(26)
Mn(5)-O(11)
Mn(5)-O(7)
1.913(9)
1.926(9)
1.932(11)
1.971(12)
2.154(11)
2.220(10)
3.127
+3
Mn(6)
Mn(6)-O(27)
Mn(6)-O(20)
Mn(6)-O(19)
Mn(6)-O(25)
Mn(6)-O(13)
Mn(6)-O(35)
1.920(10)
1.924(9)
1.926(11)
1.937(9)
2.226(10)
2.226(12)
3.117
+3
Mn(7)
Mn(7)-O(17)
Mn(7)-O(18)
Mn(7)-O(34)
Mn(7)-O(27)
Mn(7)-O(11)
Mn(7)-O(35)
1.871(12)
1.930(12)
1.934(10)
1.967(11)
2.181(10)
2.253(11)
3.153
+3
Mn(8)
Mn(8)-O(12)
Mn(8)-O(10)
Mn(8)-O(2)
Mn(8)-O(38)
Mn(8)-N(5)
1.868(11)
1.897(13)
1.910(15)
1.911(12)
2.255(17)
3.122
+3
Mn(9)
Mn(9)-O(1)
Mn(9)-O(32)
Mn(9)-O(33)
Mn(9)-O(40)
Mn(9)-N(2)
1.880(13)
1.887(11)
1.899(13)
1.911(12)
2.195(14)
3.200
+3
128
Chapter 3 – Polynuclear Manganese Coordination Complexes
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(10)
Mn(10)-O(29)
Mn(10)-O(31)
Mn(10)-O(41)
Mn(10)-O(33)
Mn(10)-N(3)
1.881(13)
1.884(10)
1.893(13)
1.960(13)
2.243(17)
3.082
+3
Mn(11)
Mn(11)-O(16)
Mn(11)-O(43)
Mn(11)-O(34)
Mn(11)-O(6)
Mn(11)-O(17)
Mn(11)-Cl(1)
1.905(11)
1.943(10)
1.959(10)
2.057(12)
2.141(13)
2.281(7)
3.340
+3
Mn(12)
Mn(12)-O(9)
Mn(12)-O(42)
Mn(12)-O(5)
Mn(12)-O(10)
Mn(12)-N(6)
1.871(12)
1.901(11)
1.934(12)
1.953(13)
2.231(15)
3.019
+3
Mn(13)
Mn(13)-O(26)
Mn(13)-O(25)
Mn(13)-O(27)
Mn(13)-O(37)
Mn(13)-O(34)
Mn(13)-O(24)
1.935(9)
1.945(9)
1.946(11)
1.967(9)
2.369(10)
2.423(10)
2.766
+3
Mn(1) ··· Mn(4)
Mn(1) ··· Mn(3)
Mn(1) ··· Mn(2)
Mn(1) ··· Mn(13)
Mn(2) ··· Mn(4)
Mn(2) ··· Mn(6)
Mn(2) ··· Mn(13)
Mn(3) ··· Mn(13)
2.940(4)
3.023(4)
3.099(3)
3.119(4)
2.944(4)
3.023(4)
3.147(4)
2.905(3)
Mn(3) ··· Mn(5)
Mn(5) ··· Mn(11)
Mn(5) ··· Mn(7)
Mn(5) ··· Mn(13)
Mn(6) ··· Mn(13)
Mn(6) ··· Mn(7)
Mn(7) ··· Mn(11)
Mn(7) ··· Mn(13)
3.031(4)
2.929(4)
3.072(3)
3.087(4)
2.895(4)
3.033(4)
2.993(4)
3.106(4)
Table 3.6 − Bond valence sum (BVS) calculations for some O atoms in 11.
Atom
O(24)
O(34)
O(26)
O(37)
O(27)
O(25)
O(10)
O(33)
BVS
2.090
2.063
1.866
1.877
1.825
1.865
1.284
1.269
Assignment a
μ4-O2μ4-O2μ3-O2μ3-O2μ3-O2μ3-O2μ2-OHμ2-OH-
An oxygen BVS in the ∼1.8-2.0, ∼1.0-1.2, and ∼0.2-0.4
ranges is indicative of non-, single- and double protonation,
respectively.7, 53
a
129
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.7 − Selected bond angles [º] for compound 11.
Bond Angle (º)
Bond Angle (º)
O(28)-Mn(1)-O(24)
O(39)-Mn(1)-O(37)
O(23)-Mn(1)-O(36)
O(28)-Mn(1)-O(23)
O(39)-Mn(1)-O(28)
O(39)-Mn(1)-O(23)
O(39)-Mn(1)-O(36)
O(37)-Mn(1)-O(23)
O(39)-Mn(1)-O(24)
O(24)-Mn(1)-O(23)
178.1(5)
171.2(5)
169.1(4)
97.5(4)
94.0(5)
93.5(5)
91.8(5)
89.5(4)
84.3(5)
81.8(4)
O(4)-Mn(3)-O(37)
O(30)-Mn(3)-O(26)
O(7)-Mn(3)-O(36)
O(4)-Mn(3)-O(36)
O(30)-Mn(3)-O(7)
O(30)-Mn(3)-O(37)
O(26)-Mn(3)-O(36)
O(30)-Mn(3)-O(4)
O(30)-Mn(3)-O(36)
O(37)-Mn(3)-O(36)
178.2(4)
176.8(4)
170.8(4)
98.3(5)
97.2(4)
93.3(4)
90.2(4)
88.4(4)
89.2(4)
81.2(4)
O(14)-Mn(4)-O(39)
O(24)-Mn(4)-N(1)
O(22)-Mn(4)-O(8)
O(22)-Mn(4)-N(1)
O(24)-Mn(4)-O(14)
O(8)-Mn(4)-N(1)
O(39)-Mn(4)-N(1)
O(14)-Mn(4)-O(8)
O(14)-Mn(4)-N(1)
O(14)-Mn(4)-O(22)
175.3(5)
171.4(4)
167.2(5)
97.9(4)
96.6(4)
94.9(4)
92.0(4)
93.1(5)
90.6(4)
87.4(5)
O(16)-Mn(11)-O(43)
O(34)-Mn(11)-Cl(1)
O(6)-Mn(11)-O(17)
O(16)-Mn(11)-O(6)
O(17)-Mn(11)-Cl(1)
O(43)-Mn(11)-Cl(1)
O(6)-Mn(11)-Cl(1)
O(16)-Mn(11)-O(34)
O(16)-Mn(11)-Cl(1)
O(16)-Mn(11)-O(17)
172.4(5)
171.3(4)
168.6(5)
95.5(5)
95.4(4)
95.0(4)
95.0(4)
92.4(4)
91.8(4)
88.8(5)
O(34)-Mn(13)-O(24)
O(26)-Mn(13)-O(25)
O(37)-Mn(13)-O(27)
O(26)-Mn(13)-O(24)
O(27)-Mn(13)-O(24)
O(26)-Mn(13)-O(27)
O(26)-Mn(13)-O(37)
O(26)-Mn(13)-O(34)
O(37)-Mn(13)-O(24)
O(25)-Mn(13)-O(24)
178.0 (4)
177.4(6)
176.6(6)
105.2(4)
102.4(4)
98.9(4)
81.6(4)
76.4(4)
74.2(4)
72.5(4)
130
Chapter 3 – Polynuclear Manganese Coordination Complexes
The phosphonate ligands bridge between the hexagonal unit B and the triangular
units A in η1:η1:η1:μ3 (P(4), P(5), P(7) and P(8)), and η1:η1:η2:μ4 (P(1), P(2), P(3), P(6),
P(9), and P(10)) bridging modes (Figure 3.15). The oxidation states of the Mn ions and the
protonation states of O ions were assigned by bond valence sum analysis (Table 3.5, Table
3.6).
In the solid state the tridecanuclear clusters are intermolecularly linked by weak
hydrogen bonds involving constitution solvent molecules resulting in a relatively densely
packed structure that can be seen in Figure 3.16.
Figure 3.15 – Ball-and-stick representation of the {Mn13} core structure in 11, showing the
phosphonates’ bridging modes. Colour code: MnIII blue, P purple, O red, Cl green, C grey.
131
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.16 – Packing arrangement of the tridecanuclear manganese clusters in 11 viewed in the
direction of the crystallographic a- and b-axis. Colour code: Mn blue, P purple, O red, C grey,
(crystallization solvent molecules and hydrogen atoms have been omitted for clarity).
The metal-centered cuboctahedral core structure of 11 is unprecedented but can be
regarded as a member of a family of {Mn13} complexes stabilised by carboxylate and
phosphonate ligands, such as: [ nΙV nΙΙΙ6 nΙΙ6O8(O2CPh)12(OEt)6], [ nΙV nΙΙΙ6 nΙΙ6O8
(OH)6(ndc)6], [ nΙV nΙΙΙ6 nΙΙ6O8(OEt)5(OH)(ndc)6], [ nΙV nΙΙΙ6 nΙΙ6O8(OMe)6(ndc)6],
(ndcH2
=
1,8-naphthalenedicarboxylic
[ nΙΙ nΙΙΙ12O6( H)6(
Cl2, [ nIII13O6(t-BuP
3PC6
3)10(
11)10(py)6],
acid),
[ n13O8( Et)6(O2CC6H4OPh)12],
[ nΙΙ nΙΙΙ12O8Cl6(t-BuP
H)2(N3)6(MeCOOH)2(H2O)2]-.12,
{Mn13} complexes the neutral [ nΙΙ nΙΙΙ12O6( H)6(
48,
3PC6
3)8][
53-55
nΙΙ(CH3CN)6]
Amongst these
11)10(py)6]
compound
reported by Zheng et al. reveals the closest similarity to 11.12 Zheng used cyclohexyl
phosphonic acid as a stabilising ligand and obtained a mixed-valent {Mn13} cluster in
which the central Mn ion adopts the oxidation state +II. There are a number of other
elements that differentiate Zheng`s compound from compound 11. The triangular units in
Zheng`s compound consist of one square pyramidally and two octahedrally coordinated
132
Chapter 3 – Polynuclear Manganese Coordination Complexes
MnIII ions. In contrast to 11 in the reported compound four
4-
2-
and two
bridge between the triangular and hexagonal units. In addition the four
11 are replaced with four
2-
The [ nIII13O6(t-BuP
2-
3-
2-CH3
ligands
ligands in
H- ligands in Zheng`s compound.
H)2(N3)6(MeCOOH)2(H2O)2]- coordination cluster
3)10(
reported by Chen et al. also shows similarities to the here presented complex. It comprises
of thirteen octahedrally coordinated MnIII ions connected through six
4-
2-
oxo bridges,
ten t-BuPO32-, two OH-, six N3-, two CH3COOH and two H2O ligands.48,
55
Another
{Mn13} complex with metal centred cuboctahedral cluster-cores stabilised by tertbutylphosphonates, [ nΙΙ nΙΙΙ12( 4-O8)( 4-Cl6)(t-BuP
3)8],
was reported by Schmitt et al.
In this compound the central Mn ion adopts the oxidation state +II and is coordinated by
eight
4-
2-
oxo ligands. The remaining twelve MnIII atoms adopt tetragonally distorted
octahedral coordination environments, and the peripheral ligation is provided by six Clligands and eight t-BuPO32- ligands.48 Structurally related compounds were also reported
by Christou et al.53 and Murrie et al.54 in which the metal centred cuboctahedral atom
arrangement is stabilised by carboxylate ligands. The core structures of these reported
mixed valent species comprise of one central MnIV ion, six MnIII ions forming the
triangular units and six MnII ions forming the hexagonal unit. In his paper, Christou reports
the
structure
and
properties
[ nΙV nΙΙΙ6 nΙΙ6O8(OH)6(ndc)6],
[ nΙV nΙΙΙ6 nΙΙ6O8(OMe)6(ndc)6]
of
the
following
{Mn13}
complexes:
[ nΙV nΙΙΙ6 nΙΙ6O8(OEt)5(OH)(ndc)6],
and
[Mn13O8(O2CPh)12(OEt)6]
(ndcH2
=
1,8-
naphthalenedicarboxylic acid). All the Mn atoms in these compounds adopt a near5-
octahedral coordination geometry and are connected by six
Additional bridging and peripheral ligation is provided by
2-
3-
and two
H-,
3-Et
3-
2-
or
ligands.
3-Me
-
ions, and ndc2- or benzoate ligands, respectively. The {Mn13} core in Murrie`s compound
[ n13O8( Et)6(O2CC6H4OPh)12] is stabilised by six
4-
2-
, two
3-
2-
, six
3-Et
-
and
twelve 2-phenoxybenzoate ligands. The central MnIV and the six MnIII ions in Murrie`s
compound display a distorted octahedral coordination environment, whilst the remaining
six MnII ions adopt a distorted square pyramidal geometry.
133
Chapter 3 – Polynuclear Manganese Coordination Complexes
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The infrared spectrum of 11 presented in Figure 3.17 is very similar to that of 10.
Characteristic C–C skeletal vibrations of the aromatic rings appear in the 1600 – 1430 cm-1
region, while the C–H out-of-plane bending vibrations can be observed in the 800 – 650
cm-1 region. Compared with 10 the IR spectrum of 11 displays some extra bands in the
1200 – 900 cm-1 region due to C–C stretching vibrations of the pyridine rings, along with
the different P–O stretching vibrations of the phosphonate groups present in this region.
Some O–H stretching vibrations appear as a broad band at ca. 3500 cm-1.36-40, 56, 57
Figure 3.17 – Infrared spectrum of 11.
134
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Thermogravimetric analysis
The thermogravimetric analysis for compound 11 was performed using a freshly
prepared crystalline sample, in the temperature range 30 – 900 °C, in a N2 atmosphere
(Figure 3.18). Upon thermolysis 11 undergoes a weight loss of 6.8 % in the temperature
range 30 – 200 °C which may be assigned
to the loss of three crystallization H2O
molecules and four coordination CH3OH molecules (calcd: 6.1 %). This assignment is in
agremment with composition determined by elemental analysis. The next step in the TGA
curve that corresponds to a weight loss of 12.7 % between 200 – 300 °C can be attributed
to the loss of five pyridine molecules (calcd: 13.1 %). A further weight loss that occurs in
two distinct steps can be observed above 300 °C. This can be associated with the
degradation of the organic ligands within 11 and cluster degradation processes resulting in
the probable formation of metal oxide materials.
Weight % (%)
100
90
80
70
60
50
0
200
400
600
800
1000
Temperature (ºC)
Figure 3.18 – Thermogravimetric analysis of 11.
135
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Magnetism
The temperature dependence of the magnetic susceptibility of 11 was measured
between 298 and 1.8 K (Figure 3.19). The χT value of 30.7 cm3 K mol-1 at room
temperature is significantly lower than the expected value of 39 cm3 K mol-1 for the
presence of thirteen S = 2 MnIII carriers (C =
S(S+1) with
= 0.12505 cm3 K mol-1
and g = 2).41 This result highlights dominant antiferromagnetic (AF) couplings between
spin carriers. Upon lowering the temperature, the χT product decreases down to a
minimum of ca. 18.3 cm3 K mol-1 at about 20 K. This thermal behavior confirms the
presence of dominant AF interactions between spin carriers. The result does not exclude
the presence of ferromagnetic interactions but only suggests that the AF interactions are
dominant. Experimental data can be fitted to a Curie-Weiss law for a temperature above
120 K, with C = 38.3(5) cm3 K mol-1 and
= -68(2) K. The Curie constant is in good
agreement with the expected value (39 cm3 K/mol) arising from thirteen MnIII S = 2 spins.
Below 20 K, at 1000 Oe the χT product increases to reach 19.3 cm3 K mol-1 at 1.8 K. This
value suggests an ST = 6 ground state. At 1 T, below 10 K, the χT product decreases to
reach 7.9 cm3 K mol-1 at 1.8 K due to field saturation effects of the magnetisation.
a)
b)
Figure 3.19 – (a) Temperature dependence of the χT product of 11 at 0.1 and 1 T. (b) A magnified view
of the χT product in (a) between 1 and 100 K. The green solid line corresponds to the best fit of the
experimental data with the Curie-Weiss law (C = 38.3(5) cm3 K mol-1 and = -68(2) K).
The field dependence of the magnetisation for this compound has been measured at
low temperatures between 1.8 and 8 K (Figure 3.20, a). The magnetisation at low field
displays a rapid increase without inflexion point confirming the absence of weak
antiferromagnetic interactions and the presence of a well-defined ground state. The high
136
Chapter 3 – Polynuclear Manganese Coordination Complexes
field behavior that displays a non-linear increase without clear saturation even at 1.8 K at 7
T, suggests the presence of magnetic anisotropy. It is worth noting that the presence of
low-lying excited states and also inter-complex magnetic interactions could contribute to
this M vs H data even if these two effects are not reflected in χT vs T data at 0.1 T. The
data represented in M vs H/T plots at different temperatures (Figure 3.20, b) confirm the
presence of anisotropy intrinsic to MnIII metal ions as the data are not superposed on a
single master-curve as expected for an isotropic system with a well defined spin ground
state. At 1.8 K, the magnetisation reaches 12.2
B
at 7 T which is in agreement with an ST
= 6 ground state.
a)
b)
Figure 3.20 – (a) M vs H and (b) M vs H/T data at and below 8 K.
Both measurements, the field dependence of the magnetisation at 1.8 K and the
temperature dependence of the χT product at 1000 Oe, are coherent with an ST = 6 ground
state. This experimental value can be explained with a configuration of eight MnIII centres
whose spin vectors are oriented in an opposite direction to those of the five remaining Mn
centres, as a possible result of competing interactions between spin carriers.
The M vs H data at 1.8 K do not show any sign of slow relaxation i.e. hysteresis
effects. Nevertheless, the ac susceptibility in zero dc field has been measured to probe
possible slow dynamics of the magnetisation for this compound. Clearly at temperatures
below 4 K (for frequency around 10000 Hz), slow relaxation of the magnetisation is
observed based on the appearance of an out-of-phase signal.
137
Chapter 3 – Polynuclear Manganese Coordination Complexes
b)
10 Hz
30 Hz
60 Hz
100 Hz
150 Hz
200 Hz
300 Hz
400 Hz
600 Hz
800 Hz
10
χ' / cm3 mol-1
8
6
1000 Hz
1200 Hz
1500 Hz
2000 Hz
3000 Hz
4000 Hz
5000 Hz
6000 Hz
8000 Hz
10000 Hz
4
10 Hz
30 Hz
60 Hz
100 Hz
150 Hz
200 Hz
300 Hz
400 Hz
600 Hz
800 Hz
1.6
χ"/ cm3 mol-1
a)
1.2
0.8
0.4
2
Hdc = 0 Oe
Hdc = 0 Oe
0
1000 Hz
1200 Hz
1500 Hz
2000 Hz
3000 Hz
4000 Hz
5000 Hz
6000 Hz
8000 Hz
10000 Hz
0
1
2
3
4
5
6
7
8
1
1.5
2
2.5
3
3.5
4
4.5
5
T/K
T/K
Figure 3.21 – Temperature dependence of the (a) in-phase (χ') and (b) out-of-phase (χ") ac
susceptibility under zero dc field for 11.
In consecutive experiments we measured the ac susceptibility as a function of the
frequency at different temperatures. This measurement was performed in zero dc field with
the objective to more accurately estimate the relaxation time. The observed frequency
dependence and resulting shape of the curve almost by itself demonstrates that this
compound is consistent with an SMM.7
b)
12
χ' / cm3 mol-1
10
8
6
4
2
0
10
2
Hdc = 0 Oe
1.8 K
1.9 K
2K
2.1 K
2.2
2.3
2.4
2.5
K
K
K
K
100
2.6
2.7
2.8
2.9
K
K
K
K
1000
3K
3.2 K
3.4 K
10000
χ'' / cm3 mol-1
a)
1.5
1.8 K
1.9 K
2K
2.1 K
2.2 K
2.3 K
2.4 K
2.5 K
2.6 K
2.7 K
2.8 K
2.9 K
3K
3.2 K
3.4 K
1
Hdc = 0 Oe
0.5
0
10
100
1000
10000
ν / Hz
ν / Hz
Figure 3.22 – Frequency dependence of the (a) in-phase (χ') and (b) out-of-phase (χ") ac susceptibility
from 1.8 to 3.4 K under zero dc field for 11.
Using a scaling technique, the relaxation time of the compound was determined up
to 3.4 K.
138
Chapter 3 – Polynuclear Manganese Coordination Complexes
a)
b)
0.2
10-4
τ = τ0 exp(
χ"/χ'0
0.15
0.1
Hdc = 0 Oe
0.05
0
10
100
1000
10
τ /s
1.8 K
1.9 K
2K
2.1 K
2.2 K
2.3 K
2.4 K
2.5 K
2.6 K
2.7 K
2.8 K
2.9 K
3K
3.2 K
3.4 K
eff
/kBT)
-5
10-6
Hdc = 0 Oe
10
-7
10
-8
10000
τ0 = 8.8 10-10 s
0
0.1
0.2
= 19.2 K
0.4
0.5
0.3
T-1
αν / Hz
eff/kB
0.6
/K-1
Figure 3.23 – (a) χ"/χ' vs from 1.8 to 3.4 K under zero dc field; (b) Magnetisation relaxation time ( )
vs T-1plot for 11 under zero dc field (the solid line corresponds to the Arrhenius law).
From these data, the relaxation time can be deduced between 1.9 and 3 K and fitted
to an Arrhenius law. The exponential increase of the relaxation time allowed us to
determine the energy barrier of 19.2 K of the thermally activated regime while
8.8 × 10
-10
0
is about
s.
The ac susceptibility has been measured at dc fields at 1.8 K in order to induce
possible quantum relaxation pathways in zero-field and to probe the quantum contribution
to the observed relaxation above 1.8 K.
a)
0 Oe
200 Oe
400 Oe
600 Oe
1000 Oe
1500 Oe
15
Oe
Oe
Oe
Oe
Oe
Oe
b)
2.5
2
χ' / cm3 mol-1
χ' / cm3 mol-1
T = 1.8 K
2000
2500
3000
4000
5000
6000
10
5
1.5
1
0 Oe
200 Oe
400 Oe
600 Oe
1000 Oe
1500 Oe
2000 Oe
2500 Oe
3000 Oe
4000 Oe
5000 Oe
6000 Oe
T = 1.8 K
0.5
0
10
100
1000
10000
0
10
100
1000
10000
ν / Hz
ν / Hz
Figure 3.24 – Frequency dependence of the (a) in-phase (χ') and (b) out-of-phase (χ") ac susceptibility
with dc fields at 1.8 K for 11.
With a small dc field, the characteristic frequency increases which demonstrate that
the quantum pathway of magnetisation relaxation is only negligible above 1.8 K.
Therefore, the ac susceptibility measurements under dc field have not been performed.
139
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Mass spectrometry
Negative mode ESI-MS spectra of compound 11 dissolved in a suitable solvent
have been recorded (Table 3.8, Figure 3.25) in order to investigate the stability of the
tridecanuclear manganese cluster in solution.42-48 The mass spectra of 11 dissolved in DMF
and DMSO reveal the presence of two isotopic envelopes centered at m/z = 1279.1 a.m.u
and m/z = 2559.1 a.m.u. These signals were attributed to a -2 charged and a -1 charged
{Mn13} species as shown in Table 3.8. The mass spectrum of 11 in CH3CN exhibits a
major signal centered at m/z = 1278.5 a.m.u which corresponds to the doubly charged
species {H[MnIV2MnIII11
10(C
3
)4(C6
5P
3)10]}
2-
.
The ESI-MS studies using the
crystalline material confirmed the stability of the {Mn13} cluster in solution. The signals of
the identified species were modelled and good fits between the experimental and simulated
isotopic envelopes confirm the assignments.
Table 3.8 – ESI-MS assignment for compound 11.
Crystals of
Solvent
m/z
CH3CN
1278.5
{H[MnIV2MnIII11
10(C
3
)4(C6
5P
23)10]}
→ {Mn13}
1279.1
{H2[MnIVMnIII12
10(C
3
)4(C6
5P
23)10]}
→ {Mn13}
2559.1
{H[MnIV3MnIII10
10(C
3
)4(C6
5P
3)10]}
→ {Mn13}
IV
2-
DMF
11 in:
1279.1
{H2[Mn
MnIII12
10(C
3
)4(C6
5P
3)10]}
→ {Mn13}
2559.1
{H[MnIV3MnIII10
10(C
3
)4(C6
5P
3)10]}
→ {Mn13}
Relative ionic abundance
DMSO
Species attributed
Figure 3.25 – Negative-mode ESI-MS spectra for crystals of 11 dissolved in DMF. Inset:
Comparison of the experimental isotopic envelopes (black spectrum) with simulated patterns (red
spectrum) for {H2[MnIVMnIII12 10(C 3 )4(C6 5P 3)10]}2- and {H[MnIV3MnIII10 10(C 3 )4
(C6 5P 3)10]}- centered at m/z = 1279.1 a.m.u. and m/z = 2559.1 a.m.u., respectively (cone
voltage: 30 V).
140
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 11 recorded in CH3CN is presented in Figure
3.26. The π − π* transitions of the organic ligands can be observed by a band centred at ca.
270 nm ( = 162000 L mol-1 cm-1), while the d – d transition of the octahedrally
coordinated MnIII ions is displayed as a weaker band at ca. 446 nm ( = 3400 L mol-1 cm-1).
The signal corresponds to a 5T2g
5
Eg transition for the octahedrally coordinated MnIII
ions within 11.49-52
Absorbance (a.u.)
1.8
1.6
0.35
1.4
0.30
1.2
0.25
0.20
1.0
0.15
0.8
0.10
400
0.6
450
500
550
600
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 3.26 – UV-Vis spectrum of a 10-5 M solution of 11 in CH3CN. Inset: A
section of the UV-Vis spectrum of a 10-4 M solution of 11 in CH3CN.
141
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.9 − Crystal data and structural refinement parameters for 11.
Compound 11
Empirical formula a
Molecular mass/g mol
C89H89ClMn13N5O42P10
-1 a
Crystal colour/shape
3
Red brown / rhombic
Crystal size/mm
0.30×0.30×0.15
Crystal system
Orthorhombic
Space group
P212121
a/ Å
18.8873(6)
b/ Å
23.1480(7)
c/ Å
25.3591(7)
/º
90
/º
90
/º
90
3
V/ Å
11087(6)
Z
4
Temperature (K)
100(2)
-3
Density/Mg m
1.778
-1
Absorp. coef./mm
13.959
F(000)
5920
2
100
max/º
Reflections collected
42875
Independent reflections
11337 [R(int)=0.1006]
Data / restraints / parameters
11337 / 0 / 1075
2
a
2960.05
S on F
1.028
R1, wR2 [I>2 (I)]
0.0753, 0.1869
R1, wR2 (all data)
0.0989, 0.2047
Largest diff. peak and hole/e.Å-3
1.353 and -0.712
Excluding solvate molecules
142
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.3.2.2 [Μn
Compound
(C5
5
ΙΙΙ
13
(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5CΗ2PΟ3)10(C5Η5Ν)6]Cl·5H2O (12)
12,
[ n
13( 4-
)2( 3- )4( 2- H)2( 2-C
3
)4(C6
5C
2P
3)10
)6]Cl·5H2O was obtained by the replacement of the phenylphosphonic acid ligand
with benzylphosphonic acid during the preparation procedure that led to the formation of
11. Rectangular brown crystals of 12 separate from the reaction mixture in a time period of
about four days and were characterized by single crystal X-ray diffraction measurements.
12 crystallises in the triclinic crystal system in the space group P
and is
structurally related with 11. The core structure within 12 is centrosymetric, revealing a
similar metal-centred, distorted cuboctahedral arrangement as observed in 11. However,
here the atom positions are slightly displaced leading to a lower symmetry compared to 11
(Figure 3.27). The triangular units A in 12 appear slightly moved and are not centrally
located above and below the hexagonal plane.
(A)
(B)
(A)
Figure 3.27 − Crystal structure of the tridecanuclear manganese complex in 12. Colour code: MnIII blue,
P purple, O red, C grey (hydrogen atoms have been omitted for clarity).
143
Chapter 3 – Polynuclear Manganese Coordination Complexes
Compound 11
Compound 12
a)
7.785(9) Å
7.639(4) Å
b)
83.61(1)ᵒ
79.43(3)⁰
~ 0.6 Å
1.227(1) Å
6.352(8) Å
89.13(1)ᵒ
6.461(4) Å
6.359(1) Å
6.377(1) Å
c)
89.61(3)ᵒ
1.228(2) Å
Figure 3.28 – (a) Comparison of the metal-centred distorted cuboctahedral arrangement of the {Mn13}
clusters in 11 and 12; (b) The oblique triangular antiprisms constructed using the two triangular units
A within 11 and 12, respectively; (c) Schematic representation showing the position of the centroid of
one base relative to the other base in the triangular antiprism representation of units A within 11 and
12, respectively.
144
Chapter 3 – Polynuclear Manganese Coordination Complexes
Compound 11
Compound 12
a)
5.482(4) Å
5.375(4) Å
5.408(4) Å
5.565(6) Å
3.640(4) Å
3.656(1) Å
b)
3.031(4) Å
2.988(5) Å
3.023(4) Å
3.072(4) Å
3.158(4) Å
3.091(1) Å
Figure 3.29 – Comparison of the structural parameters of the triangular units A and hexagonal unit
B within 11 and 12. (a) Top view of the oblique triangular antiprisms constructed using the two
triangular units A within 11 and 12, respectively; (b) Hexagonal unit B within 11 and 12,
respectively.
A close examination of complexes 11 and 12 reveals the structural differences
between the compounds (Figure 3.28, Figure 3.29). The first one is represented by the
peripheral ligation which is accomplished by ten phenylphosphonates, five pyridine
molecules and a chloride ion for 11 and ten benzylphosphonates and six pyridine
molecules for 12. Then, the two triangular units in 11 and 12 form oblique triangular
antiprisms that have edges of 7.639(4) Å and 7.785(9) Å (distances measured between
Mn(8) ··· Mn(9) in 11 and Mn(5) ··· Mn(7) in 12), respectively and show an angle of
83.61(1)⁰ and 79.43(3)⁰, respectively between one base and the centroid of the other
triangular base (Figure 3.28). Thus, the triangular units A in 12 appear moved aside with
about 0.6 Å compared to those in 11. Also, the structural parameters within the triangular
and hexagonal units in 12 differ slightly from those in 11. The geometry of the units in 12
are characterised by interatomic Mn-Mn distances of 3.656(1) Å for Mn(5) ··· Mn(7`),
5.408(4) Å for Mn(5) ··· Mn(6) and 5.565(6) Å for Mn(6) ··· Mn(7`), compared with
3.640(4) Å for Mn(9) ··· Mn(10), 5.375(4) Å for Mn(9) ··· Mn(11) and 5.482(4) Å for
Mn(10) ··· Mn(11) in 11 (Figure 3.29).
145
Chapter 3 – Polynuclear Manganese Coordination Complexes
(A)
Figure 3.30 − Polyhedral representation of the triangular unit A in 12. Colour code: MnIII blue, P
purple, O red, C grey.
(B)
Figure 3.31 – Ball-and-stick representation of the hexagonal unit B in 12. Colour code: MnIII blue,
P purple, O red, C grey.
146
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.10 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 12.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(1)
Mn(1)-O(19)
Mn(1)-O(21)
Mn(1)-O(17)
Mn(1)-O(9)
Mn(1)-O(3)
Mn(1)-O(13)
1.947(4)
1.962(4)
1.961(5)
1.963(4)
2.174(4)
2.239(4)
2.939
+3
Mn(2)
Mn(2)-O(5)
Mn(2)-O(12)
Mn(2)-O(21)
Mn(2)-O(18)
Mn(2)-O(3)
Mn(2)-O(14)
1.898(4)
1.918(5)
1.911(4)
1.960(4)
2.273(4)
2.299(4)
3.069
+3
Mn(3)
Mn(3)-O(19`)
Mn(3)-O(18)
Mn(3)-O(6)
Mn(3)-O(7`)
Mn(3)-O(14)
Mn(3)-O(13`)
1.948(4)
1.965(4)
1.964(5)
1.964(4)
2.237(4)
2.276(4)
2.849
+3
Mn(4)
Mn(4)-O(18)
Mn(4)-O(18`)
Mn(4)-O(19)
Mn(4)-O(19`)
Mn(4)-O(21`)
Mn(4)-O(21)
2.008(5)
2.008(5)
2.017(4)
2.017(4)
2.292(5)
2.292(5)
2.494
+3
Mn(5)
Mn(5)-O(8`)
Mn(5)-O(16)
Mn(5)-O(15)
Mn(5)-O(2`)
Mn(5)-N(1)
1.819(5)
1.890(5)
1.942(5)
1.949(4)
2.303(6)
3.067
+3
Mn(6)
Mn(6)-O(11)
Mn(6)-O(10)
Mn(6)-O(12)
Mn(6)-O(17)
Mn(6)-O(21)
Mn(6)-N(3)
1.975(5)
1.985(6)
2.032(5)
2.023(5)
2.107(5)
2.419(4)
2.685
+3
Mn(7)
Mn(7)-O(20`)
Mn(7)-O(1)
Mn(7)-O(4)
Mn(7)-O(16`)
Mn(7)-N(2)
1.852(5)
1.873(4)
1.970(6)
1.988(5)
2.253(6)
2.967
+3
Mn(2) ··· Mn(3)
Mn(2) ··· Mn(6)
Mn(4) ··· Mn(2)
Mn(4) ··· Mn(3)
2.9877(16)
2.8701(18)
3.2364(13)
2.9675(11)
Mn(1) ··· Mn(2)
Mn(1) ··· Mn(3`)
Mn(1) ··· Mn(4)
Mn(1) ··· Mn(6)
3.0913(14)
3.1575(16)
3.0260(13)
3.1309(15)
The angles within each triangular unit in 12 measure 38.9(1)° for Mn(5)-Mn(6)Mn(7`), 68.20(1)° for Mn(5)-Mn(7`)-Mn(6) and 72.90(1)° for Mn(6)-Mn(5)-Mn(7`), while
for 11 the corresponding angles are 39.64(5)° for Mn(8)-Mn(4)-Mn(12), 66.73(7)° for
147
Chapter 3 – Polynuclear Manganese Coordination Complexes
Mn(4)-Mn(12)-Mn(8) and 73.63(8)° for Mn(4)-Mn(8)-Mn(12). The interatomic distances
within the hexagonal brucite plane in 12, Mn(1) ··· Mn(2), Mn(2) ··· Mn(3) and Mn(3) ···
Mn(1`) are 3.091(1) Å, 2.988(5) Å and 3.158(4) Å, respectively, whereas in 11 the
corresponding interatomic distances Mn(1) ··· Mn(3), Mn(3) ··· Mn(5) and Mn(5) ···
Mn(7) are 3.023(4) Å, 3.031(4) Å and 3.072(4) Å, respectively. The hexagonal unit in 12
displays angles of 113.83(1)°, 121.19(1)° and 124.98(1)° for Mn(1)-Mn(2)-Mn(3), Mn(2)Mn(1)-Mn(3`) and Mn(1`)-Mn(3)-Mn(2), respectively, whereas the hexagonal unit in 11
displays coressponding angles of 116.16(11)°, 117.22(11)° and 126.07(12)° for Mn(5)Mn(7)-Mn(6), Mn(3)-Mn(5)-Mn(7) and Mn(1)-Mn(3)-Mn(5), respectively.
However, despite the slight differences, the bond distances and bond angles within
12 are comparable with those found in 11. Selected bond lengths and bond angles for
compound 12 are listed in Table 3.10 and Table 3.11.
Table 3.11 − Selected bond angles [º] for compound 12.
Bond Angle (º)
Bond Angle (º)
O(5)-Mn(2)-O(21)
O(12)-Mn(2)-O(18)
O(3)-Mn(2)-O(14)
O(5)-Mn(2)-O(3)
O(5)-Mn(2)-O(12)
O(12)-Mn(2)-O(3)
O(21)-Mn(2)-O(12)
O(18)-Mn(2)-O(3)
O(12)-Mn(2)-O(14)
O(21)-Mn(2)-O(3)
178.5(2)
170.3(2)
168.03(15)
97.89(18)
89.9(2)
93.5(2)
91.6(2)
89.91(17)
88.44(19)
82.27(16)
O(18)-Mn(3)-O(7`)
O(19`)-Mn(3)-O(6)
O(14)-Mn(3)-O(13`)
O(6)-Mn(3)-O(13`)
O(7`)-Mn(3)-O(14)
O(19`)-Mn(3)-O(14)
O(6)-Mn(3)-O(18)
O(6)-Mn(3)-O(7`)
O(18)-Mn(3)-O(14)
O(6)-Mn(3)-O(14)
176.10(19)
175.94(18)
169.83(15)
104.22(18)
95.01(18)
93.92(17)
92.93(19)
89.93(19)
87.99(16)
82.72(18)
O(18)-Mn(4)-O(18`)
O(19)-Mn(4)-O(19`)
O(21`)-Mn(4)-O(21)
O(18`)-Mn(4)-O(21)
O(19`)-Mn(4)-O(21)
O(18`)-Mn(4)-O(19`)
O(18)-Mn(4)-O(19`)
O(19`)-Mn(4)-O(21`)
O(19)-Mn(4)-O(21)
O(18)-Mn(4)-O(21)
180.000(1)
180.000(1)
180.000(1)
109.66(15)
100.23(16)
98.19(17)
81.81(17)
79.77(16)
79.77(16)
70.34(15)
N(3)-Mn(6)-O(21)
O(10)-Mn(6)-O(12)
O(11)-Mn(6)-O(17)
N(3)-Mn(6)-O(17)
N(3)-Mn(6)-O(10)
O(11)-Mn(6)-O(10)
N(3)-Mn(6)-O(12)
O(10)-Mn(6)-O(21)
N(3)-Mn(6)-O(11)
O(10)-Mn(6)-O(17)
172.65(15)
171.8(2)
170.41(19)
102.21(16)
97.8(2)
93.6(3)
90.18(17)
88.9(2)
86.92(19)
88.1(2)
The phosphonate ligands within 12 adopt two different bridging modes η1:η1:η1:μ3
and η1:η1:η2:μ4, similar as in 11. The oxidation states of the Mn and O ions in 12 were
established by bond valence sum analysis and charge considerations (Table 3.10 and Table
3.12).
148
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.12 − Bond valence sum (BVS) calculations for some O atoms in 12.
Atom
O(21)
O(19)
O(18)
O(16)
BVS
1.871
1.677
1.667
1.242
Assignment a
μ4-O2μ3-O2μ3-O2μ2-OH-
An oxygen BVS in the ∼1.8-2.0, ∼1.0-1.2, and ∼0.2-0.4
ranges is indicative of non-, single- and double protonation,
respectively.7, 53
a
The packing arrangement of the {Mn13} clusters in 12 displays small channels that
extend in the direction of the crystallographic b-axis (Figure 3.32). These channels are
filed with constitutional solvent molecules.
Figure 3.32 – Packing arrangement of the tridecanuclear manganese clusters in 12 viewed in the
direction of the crystallographic a- and b-axis. Colour code: Mn blue, P purple, O red, C grey,
(crystallization solvent molecules and hydrogen atoms have been omitted for clarity).
149
Chapter 3 – Polynuclear Manganese Coordination Complexes
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
In comparison to 11, the IR spectrum of 12 presented in Figure 3.33 displays some
extra bands in the 1500 – 1150 cm-1 and 850 – 690 cm-1 region due to the C–H bending
and rocking vibrations of the –CH2 groups of the benzylphosphonic acid ligands. The
bands located in the 3040 – 2800 cm-1 region are more intense due to the overlapping C–H
stretching vibrations of the aromatic rings and aliphatic –CH2 groups.36-40, 56, 57
Figure 3.33 – Infrared spectrum of 12.
150
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Thermogravimetric analysis
Thermogravimetric analysis was performed using a freshly prepared crystalline
sample of 12, in the temperature range 30 – 900 °C, in an N2 atmosphere. The compound
reveals a similar decomposition behaviour as 11. The first thermogravimetric step of about
6.5 % occurs in a temperature range between 30 – 195 °C as a result of the proposed loss
of five crystallization H2O molecules and four coordinated CH3OH molecules (calcd: 6.7
%). This constitutional assignemnet is in agreement with the elemental analysis of 12. The
next step in the TGA curve corresponds to a weight loss of 14.3 % between 195 – 270 °C,
which is associated with the loss of six pyridine molecules (calcd: 14.5 %). Further cluster
degradation and decomposition of the organic ligands can be observed above 270 °C.
Weight % (%)
100
90
80
70
60
50
0
200
400
600
800
1000
Temperature (ºC)
Figure 3.34 – Thermogravimetric analysis of 12.
151
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Magnetism
The temperature dependence of the magnetic susceptibility of 12 was measured
between 298 and 1.8 K (Figure 3.35). At room temperature, the experimental χT product
reaches 38.1 cm3 K mol-1. That is approximately in agreement with the expected value of
39 cm3 K mol-1 for the presence of thirteen S = 2 MnIII metal ions (C = 3 cm3 K mol-1 with
g = 2). Upon lowering the temperature, the χT product decreases down to 34.7 at 100 K,
then increases to 35.1 cm3 K mol-1 at 50 K and then finally decreases down to 32.6 cm3 K
mol-1 at 1.8 K. This thermal behavior suggests the presence of dominant antiferromagnetic
(AF) interactions between the spin carriers. This result does not exclude the presence of
ferromagnetic interactions but only suggests that the AF exchange pathways are dominant.
It is worth mentioning that this magnetic behavior is significantly different from that
observed for 11 showing that the intra-complex magnetic interactions must be different.
Despite the fact that the spin ground state is very difficult to establish only based on the χT
vs T data. The value of about 35 cm3 K mol-1 at low temperature suggests a spin ground
state of 8.
a)
b)
Figure 3.35 – (a) Temperature dependence of the χT product of 12 at 0.1 T. (b) A magnified view of
the χT product in (a) between 1 and 100 K.
The field dependence of magnetisation for this compound has been measured at
low temperatures between 1.8 and 8 K (Figure 3.36, a). The magnetisation at low field
displays a rapid increase without inflexion point confirming the absence of weak
antiferromagnetic interactions. The high field behavior displays a non-linear and nonmonotone increase without clear saturation even at 1.8 K at 7 T, suggesting the presence of
magnetic anisotropy and also low lying excited states. The data shown as a M vs H/T plot
(Figure 3.36, b) confirms the presence of both magnetic anisotropy intrinsic to MnIII metal
ions and low-lying excited states as the data are not superposed on a single master-curve as
152
Chapter 3 – Polynuclear Manganese Coordination Complexes
expected for an isotropic system with a well defined spin ground state. At 1.8 K, the
magnetisation reaches 18.9
B
at 7 T which is above the 16
B
expected for an ST = 8
ground state confirming the field induce population of low lying excited states.
a)
b)
Figure 3.36 – (a) M vs H and (b) M vs H/T data at and below 8 K.
Both measurements, the field dependence of the magnetisation at 1.8 K and the
temperature dependence of the χT product at 1000 Oe, are coherent with an ST = 8 ground
state even if this conclusion is not as clear as for 11.
The M vs H data at 1.8 K do not show any sign of slow relaxation i.e. hysteresis
effects. Nevertheless, the ac susceptibility in zero dc field has been measured to probe
possible slow dynamics of the magnetisation in this compound. Clearly at temperatures
below 4 K (for frequency around 10000 Hz), slow relaxation of the magnetisation is
observed in agreement with the appearance of an out-of-phase signal.
a)
b)
Figure 3.37 – Temperature dependence of the (a) in-phase (χ') and (b) out-of-phase (χ") ac
susceptibility under zero dc field for 12.
153
Chapter 3 – Polynuclear Manganese Coordination Complexes
Cosecutively, we measured the ac susceptibility as a function of the frequency at
different temperatures in zero dc field in order to estimate the relaxation time. The
observed frequency dependence and shape of the curve idicates that this compound is
consistent with an SMM.7
a)
b)
Figure 3.38 – Frequency dependence of the (a) in-phase (χ') and (b) out-of-phase (χ") ac susceptibility
from 1.8 to 2.8 K under zero dc field for 12.
Using a scaling technique, the relaxation time of the compound was determined up
to 2.8 K.
a)
b)
Figure 3.39 – (a) χ"/χ' vs from 1.8 to 2.8 K under zero dc field; (b) Magnetisation relaxation time ( )
vs T-1plot for 12 under zero dc field (the solid line correspond to the Arrhenius law).
From these data, the relaxation time can be deduced between 1.9 and 2.8 K and
fitted to an Arrhenius law. The exponential increase of the relaxation allowed us to
determine the energy barrier of 20.8 K of the thermally activated regime while
0
is about
8.5 × 10-10 s.
154
Chapter 3 – Polynuclear Manganese Coordination Complexes
The ac susceptibility has been measured in dc fields at 1.8 K in order to investigate
possible quantum relaxation pathways in zero-field and to probe the quantum contribution
to the observed relaxation above 1.8 K.
a)
b)
Figure 3.40 – Frequency dependence of the (a) in-phase (χ') and (b) out-of-phase (χ") ac susceptibility
with dc fields at 1.8 K for 12.
Figure 3.41 – vs H plot at 1.8 K.
The shift of the characteristic frequency (with an optimum field of 300 Oe) is very
small highlighting that the quantum pathway only provides a minor contribution to the
relaxation of the magnetisation at 1.8 K. Therefore, the ac susceptibility measurements at
300 Oe have not been performed as the resulting data would be extremely similar to that
obtained at zero-dc field.
155
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Mass spectrometry
The stability of the {Mn13} cluster in solution was investigated by electrospray
ionisation mass spectrometry. Negative mode ESI-MS spectra of 12 in CH3CN, DMF
(Figure 3.42) and DMSO reveal the presence of an isotopic envelope centred at m/z =
2699.1
{H[
a.m.u.
nIV3MnIII10
(Table
10(CH3
3.13).
)4(C6
This
5CH2P
signal
3)10]}
-
can
be
attributed
species, confirming that
to
the
the cluster is
stable in CH3CN, DMF or DMSO environments.
Table 3.13 – ESI-MS assignment for compound 12.
Crystals of
m/z
Species attributed
CH3CN
2699.1
{H[MnIV3MnIII10
10(C
3
)4(C6
5C
2P
3)10]}
→ {Mn13}
DMF
2699.1
{H[MnIV3MnIII10
10(C
3
)4(C6
5C
2P
3)10]}
→ {Mn13}
DMSO
2699.1
{H[MnIV3MnIII10
10(C
3
)4(C6
5C
2P
3)10]}
→ {Mn13}
Relative ionic abundance
12 in:
Solvent
Figure 3.42 – Negative-mode ESI-MS spectra for crystals of 12 dissolved in DMF. Inset: Comparison
of the experimental isotopic envelopes (black spectrum) with simulated patterns (red spectrum) for
{H[ nIV3MnIII10 10(CH3 )4(C6 5CH2P 3)10]}- centered at m/z = 2699.1 a.m.u. (cone voltage: 30 V).
156
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 12 recorded in CH3CN (Figure 3.43) displays a
shoulder at ca. 257 nm ( = 61000 L mol-1 cm-1) attributed to π − π* transitions of the
organic ligands. A very weak absorption band is also displayed at ca. 443 nm ( = 4500 L
mol-1 cm-1) which corresponds to a 5T2g
III
coordinated Mn ions.
5
Eg transition within the octahedrally
49-52
Absorbance (a.u.)
0.8
0.7
0.5
0.6
0.4
0.5
0.3
0.2
0.4
0.1
0.3
400
450
500
550
600
0.2
0.1
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 3.43 – UV-Vis spectrum of a 10-5 M solution of 12 in CH3CN. Inset: A
section of the UV-Vis spectrum of a 10-4 M solution of 12 in CH3CN.
157
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.14 − Crystal data and structural refinement parameters for 12.
Compound 12
Empirical formula a
Molecular mass/g mol
C104H114ClMn13N6O42P10
-1 a
Crystal colour/shape
3
Brown / rectangular block
Crystal size/mm
0.50×0.15×0.10
Crystal system
Triclinic
Space group
P
a/ Å
14.990(3)
b/ Å
15.833(3)
c/ Å
16.410(3)
/º
73.19(3)
/º
83.00(3)
/º
64.78(3)
3
V/ Å
3372.9(11)
Z
1
Temperature (K)
150(2)
-3
Density/Mg m
1.530
-1
Absorp. coef./mm
1.366
F(000)
1555
2
60
max/º
Reflections collected
44311
Independent reflections
15941 [R(int)=0.0527]
Data / restraints / parameters
15941 / 6 / 751
2
a
3179.41
S on F
1.035
R1, wR2 [I>2 (I)]
0.0814, 0.2364
R1, wR2 (all data)
0.1200, 0.2619
Largest diff. peak and hole/e.Å-3
3.320 and -0.925
Excluding solvate molecules
158
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.3.2.3 [Μn
ΙΙΙ
13
(μ4-Ο)2(μ3-Ο)4(μ2-ΟH)2(μ2-CΗ3Ο)4(C6Η5CΗ2PΟ3)10(C6Η5-C3Η6-C5Η4Ν)6]Cl
·5H2O (13)
Compound 13, [ n
C3
6-C5
4
13( 4-
)2( 3- )4( 2- H)2( 2-C
3
)4(C6
5C
2P
3)10(C6
5-
)6]Cl·5H2O was the result of replacing the pyridine ligands in the reaction
mixture of 12 with 4-(3-phenylpropyl)pyridine ligands. The crystallisation process that
occurs over a week affords rectangular brown crystals of 13 which were characterized by
single crystal X-ray diffraction measurements.
13 crystallises in the triclinic crystal system in the space group P
and is
structurally related to 11 and 12. The metal-centered, distorted cuboctahedral arrangement
of the {Mn13} cluster in 13 is very similar to the highly symmetric arrangement in 11
(Figure 3.44). The bond distances and bond angles within 13 are comparable with those
found in 11 and 12 as can be seen in Table 3.15 and Table 3.16.
(A)
(B)
(A)
Figure 3.44 − Crystal structure of the tridecanuclear manganese complex in 13. Colour code: MnIII
blue, P purple, O red, C grey (hydrogen atoms have been omitted for clarity).
159
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.15 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 13.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(1)
Mn(1)-O(17)
Mn(1)-O(17`)
Mn(1)-O(16)
Mn(1)-O(16`)
Mn(1)-O(10)
Mn(1)-O(10`)
1.947(4)
1.947(4)
1.954(5)
1.954(5)
2.404(5)
2.404(5)
2.738
+3
Mn(2)
Mn(2)-O(21)
Mn(2)-O(10`)
Mn(2)-O(19)
Mn(2)-O(17)
Mn(2)-O(8`)
Mn(2)-O(20)
1.909(5)
1.926(5)
1.926(5)
1.944(5)
2.185(5)
2.220(5)
3.156
+3
Mn(3)
Mn(3)-O(11)
Mn(3)-O(18)
Mn(3)-O(16)
Mn(3)-O(17)
Mn(3)-O(3)
Mn(3)-O(20)
1.925(5)
1.926(5)
1.929(5)
1.929(5)
2.228(5)
2.228(5)
3.105
+3
Mn(4)
Mn(4)-O(9)
Mn(4)-O(5)
Mn(4)-O(16)
Mn(4)-O(10)
Mn(4)-O(8)
Mn(4)-O(3)
1.878(6)
1.915(5)
1.946(5)
1.957(5)
2.195(5)
2.229(5)
3.162
+3
Mn(5)
Mn(5)-O(10)
Mn(5)-O(2)
Mn(5)-O(19)
Mn(5)-N(1)
Mn(5)-O(1)
Mn(5)-O(9)
1.902(5)
1.958(5)
1.992(5)
2.033(6)
2.079(6)
2.132(6)
3.215
+3
Mn(6)
Mn(6)-O(23)
Mn(6)-O(7)
Mn(6)-O(6)
Mn(6)-O(12)
Mn(6)-N(3)
1.873(6)
1.887(5)
1.898(6)
1.928(5)
2.229(7)
3.138
+3
Mn(7)
Mn(7)-O(22)
Mn(7)-O(4)
Mn(7)-O(14)
Mn(7)-O(12)
Mn(7)-N(2)
1.871(6)
1.871(6)
1.891(5)
1.908(6)
2.216(8)
3.234
+3
Mn(1)-Mn(3)
Mn(1)-Mn(2)
Mn(1)-Mn(4)
Mn(2)-Mn(5`)
2.9125(11)
3.1145(11)
3.1551(11)
2.8865(16)
Mn(2)-Mn(3)
Mn(2)-Mn(4`)
Mn(3)-Mn(4)
Mn(4)-Mn(5)
3.0407(15)
3.1115(15)
3.0415(15)
2.9452(17)
160
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.16 − Selected bond angles [º] for compound 13.
Bond Angle (º)
Bond Angle (º)
O(17`)-Mn(1)-O(17)
O(16`)-Mn(1)-O(16)
O(10`)-Mn(1)-O(10)
O(16`)-Mn(1)-O(10)
O(17)-Mn(1)-O(10)
O(17)-Mn(1)-O(16`)
O(17)-Mn(1)-O(16)
O(17)-Mn(1)-O(10`)
O(17`)-Mn(1)-O(10)
O(16)-Mn(1)-O(10)
180.000(1)
180.000(1)
180.000(1)
105.82(18)
105.04(17)
97.87(19)
82.13(19)
74.96(17)
74.95(17)
74.18(18)
O(21)-Mn(2)-O(10`)
O(19)-Mn(2)-O(17)
O(8`)-Mn(2)-O(20)
O(21)-Mn(2)-O(8`)
O(21)-Mn(2)-O(19)
O(21)-Mn(2)-O(20)
O(21)-Mn(2)-O(17)
O(19)-Mn(2)-O(20)
O(19)-Mn(2)-O(8`)
O(10`)-Mn(2)-O(19)
179.9(2)
169.5(2)
167.82(19)
98.1(2)
96.2(2)
93.6(2)
92.8(2)
91.3(2)
90.6(2)
83.7(2)
O(11)-Mn(3)-O(17)
O(18)-Mn(3)-O(16)
O(3)-Mn(3)-O(20)
O(18)-Mn(3)-O(3)
O(18)-Mn(3)-O(17)
O(17)-Mn(3)-O(3)
O(11)-Mn(3)-O(3)
O(16)-Mn(3)-O(17)
O(17)-Mn(3)-O(20)
O(16)-Mn(3)-O(3)
177.7(2)
177.5(2)
170.45(18)
98.5(2)
94.5(2)
89.8(2)
89.7(2)
83.3(2)
82.87(19)
82.59(19)
O(10)-Mn(5)-N(1)
O(2)-Mn(5)-O(19)
O(1)-Mn(5)-O(9)
N(1)-Mn(5)-O(9)
O(10)-Mn(5)-O(1)
O(19)-Mn(5)-N(1)
O(2)-Mn(5)-O(1)
O(2)-Mn(5)-N(1)
O(19)-Mn(5)-O(1)
N(1)-Mn(5)-O(1)
175.7(2)
175.2(2)
174.9(2)
97.4(2)
95.0(2)
94.2(2)
94.5(2)
90.3(2)
87.1(2)
87.6(2)
A close inspection of the three complexes 11, 12 and 13 reveals only a few
structural differences, the most important ones being the peripheral ligation and the
symmetry of the core structure. The structural parameters within 13 are closer to that of 11
than 12. In accordance to this observation the triangular units in 13 display lateral Mn-Mn
distances of 3.616(3) Å for Mn(6) ··· Mn(7), 5.387(3) Å for Mn(5) ··· Mn(6) and 5.408(4)
Å for Mn(5) ··· Mn(7), and angles of 39.15(2)° for Mn(6)-Mn(5)-Mn(7), 70.11(3)° for
Mn(5)-Mn(7)-Mn(6) and 70.75(3)° for Mn(5)-Mn(6)-Mn(7). Further, the interatomic
distances in the hexagonal unit in 13 are 3.041(3) Å, 3.042(3) Å and 3.111(3) Å for Mn(2)
··· Mn(3), Mn(3) ··· Mn(4) and Mn(4) ··· Mn(2), respectively. Also, the angles within the
hexagonal unit in 13 are closer to those in 11. The Mn(2)-Mn(4)-Mn(3), Mn(3)-Mn(2)Mn(4), and Mn(2)-Mn(3)-Mn(4) angles measure 115.63(5)°, 117.36(5)° and 127.00(5)°,
respectively.
161
Chapter 3 – Polynuclear Manganese Coordination Complexes
(A)
Figure 3.45 − Polyhedral representation of the triangular unit A in 13. Colour code: MnIII blue, P
purple, O red, C grey.
(B)
Figure 3.46 – Ball-and-stick representation of the hexagonal unit B in 13. Colour code: MnIII blue, P
purple, O red, C grey.
162
Chapter 3 – Polynuclear Manganese Coordination Complexes
All phosphonate ligands within 13 bridge between three MnIII ions in a η1:η1:η1:μ3
and η1:η1:η2:μ4 bridging modes. The oxidation states of the Mn and O ions in 13 were
established by bond valence sum analysis and charge considerations (Table 3.15 and Table
3.17).
Table 3.17 − Bond valence sum (BVS) calculations for some O atoms in 13.
Atom
O(10)
O(16)
O(17)
O(12)
BVS
2.079
1.827
1.842
1.270
Assignment a
μ4-O2μ3-O2μ3-O2μ2-OH-
An oxygen BVS in the ∼1.8-2.0, ∼1.0-1.2, and ∼0.2-0.4
ranges is indicative of non-, single- and double protonation,
respectively.7, 53
a
In the solid state the tridecanuclear manganese clusters are linked by weak
hydrogen bonds between the Mn clusters and constitutional solvent molecules resulting in
a lamellar arragement with alternating organic hydrophobic and polar inorganic areas
(Figure 3.47). In addition dispersion forces between the peripheral, hydrophobic 4-(3phenylpropyl)pyridine ligands stabilise the observed packing arrangement. The
coordination clusters arrange in the ac-plane and the latter forces involving the
hydrophobic moieties stabilise the packing in [010].
Figure 3.47 – Packing arrangement of the tridecanuclear manganese clusters in 13 viewed in the
direction of the crystallographic c-axis. Colour code: Mn blue, P purple, O red, C grey, (crystallization
solvent molecules and hydrogen atoms have been omitted for clarity).
163
Chapter 3 – Polynuclear Manganese Coordination Complexes
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 13 (Figure 3.48) is comparable with that of 12. Some
dissimilarities can be observed due to the contribution of the C–H vibrations of the –CH2
groups of the 4-(3-phenylpropyl)pyridine ligands. This has an effect on the C–H vibration
bands which appear in the spectrum as more intense and better resolved bands compared
with those of 12.36-40, 56, 57
Figure 3.48 – Infrared spectrum of 13.
164
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Thermogravimetric analysis
13 displays a similar thermal decomposition behaviour in N2 as 11 and 12 (Figure
3.49). An initial weight loss of 5.6 % occurs between 30 – 150 °C as a possible result of
the loss of five crystallization H2O molecules and four coordinated CH3OH molecules
(calcd: 5.3 %). This assignment is also in agreement with the elemental analysis of the 13.
The following step in the TGA curve corresponds to a weight loss of 27.3 % between 150
– 550 °C, which is associated with the loss of six 4-(3-phenylpropyl)pyridine molecules
(calcd: 29.7 %). Further degradation of the organic ligands occours above 550 °C.
Weight % (%)
100
90
80
70
60
0
200
400
600
800
1000
Temperature (ºC)
Figure 3.49 – Thermogravimetric analysis of 13.
165
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Magnetism
Compound 13 shows similar magnetic behaviour as 11. The room temperature χT
value of 30.3 cm3 K mol-1 (Figure 3.50) is significantly lower than the expected value of 39
cm3 K mol-1 for the presence of thirteen S = 2 MnIII carriers (C = 3 cm3 K mol-1 with g =
2). This result indicates dominant antiferromagnetic (AF) exchange couplings between
spins carriers. Upon lowering the temperature, the χT product decreases down to a
minimum of ca. 16.5 cm3 K mol-1 at about 20 K. The experimental data can be fitted to a
Curie-Weiss law for temperatures above 70 K, with C = 39.3(9) cm3 K mol-1 and
=-
84(5) K. The Curie constant is in good agreement with the expected value (39 cm3 K/mol)
agreeing well with the presence of the thirteen MnIII S = 2 spin centres. Below 20 K, at
1000 Oe the χT product increases to reach 17.8 cm3 K mol-1 at 1.8 K. This value suggests
an ST = 6 ground state as for 11. At 1 T, below 10 K, the χT product decreases to reach 7.2
cm3 K mol-1 at 1.8 K due to field saturation effects.
a)
b)
Figure 3.50 – (a) Temperature dependence of the χT product of 13 at 0.1 and 1 T. (b) A magnified view
of the χT product in (a) between 1 and 100 K. The green solid line corresponds to the best fit of the
experimental data with the Curie-Weiss law (C = 39.3(9) cm3 K mol-1 and = - 84(5) K).
The field dependence of magnetisation for this compound has also been measured
below 8 K (Figure 3.51, a). The magnetisation at low field displays a rapid increase
without inflexion point confirming the absence of weak antiferromagnetic interactions and
the presence of a well-defined ground state. The high field behavior that displays no clear
saturation even at 1.8 K at 7 T, suggests the presence of magnetic anisotropy. It is worth
noting that the presence of low-lying excited states and inter-complex magnetic
interactions could contribute to this M vs H data even if these two effects have not been
166
Chapter 3 – Polynuclear Manganese Coordination Complexes
observed for χT vs T data at 0.1 T. The data presented as a M vs H/T plot (Figure 3.51, b)
confirms the presence of magnetic anisotropy intrinsic to MnIII metal ions and the cluster
as the data are not superposed on a single master-curve as expected for an isotropic system
with a well defined spin ground state. At 1.8 K, the magnetisation reaches 11.3
B
at 7 T in
agreement with an ST = 6 ground state.
a)
b)
Figure 3.51 – (a) M vs H and (b) M vs H/T data at and below 8 K.
Both measurements, the field dependence of the magnetisation at 1.8 K and the
temperature dependence of the χT product at 1000 Oe, are coherent with a ground state ST
= 6. This experimental value can be explained by a configuration in which the spin vectors
of eight MnIII centres are oriented in opposite direction than those of the five remaing
centre, due to probable competing interactions between spin carriers.
The M vs H data at 1.8 K do not show any sign of slow relaxation i.e. hysteresis
effects. Nevertheless as for 11 and 12, the ac susceptibility in zero dc field has been
measured to probe possible slow dynamics of the magnetisation. Clearly below 4 K (for
frequency around 10000 Hz), slow relaxation of the magnetisation is observed in
agreement with the appearance of an out-of-phase signal.
167
Chapter 3 – Polynuclear Manganese Coordination Complexes
10 Hz
30 Hz
60 Hz
100 Hz
150 Hz
a)
200
300
400
600
800
Hz
Hz
Hz
Hz
Hz
1000
1200
1500
2000
3000
Hz
Hz
Hz
Hz
Hz
4000 Hz
5000 Hz
6000 Hz
8000 Hz
10000 Hz
10 Hz
30 Hz
60 Hz
100 Hz
150 Hz
b)
1000
1200
1500
2000
3000
Hz
Hz
Hz
Hz
Hz
4000 Hz
5000 Hz
6000 Hz
8000 Hz
10000 Hz
Hdc = 0 Oe
Hdc = 0 Oe
10
2
χ" / cm3 mol-1
χ' / cm3 mol-1
Hz
Hz
Hz
Hz
Hz
2.5
12
8
6
4
1.5
1
0.5
2
0
200
300
400
600
800
1
2
3
4
5
6
7
T/K
8
0
1
1.5
2
2.5
3
3.5
4
T/K
Figure 3.52 – Temperature dependence of the (a) in-phase (χ') and (b) out-of-phase (χ") ac
susceptibility under zero dc field for 13.
Frequency-dependent out-of-phase (χ") ac susceptibility signals were also observed
at temperatures below 3.2 K, suggesting that 13 is consistent with an SMM.7
For this compound the relaxation time can be deduced between 1.9 and 3.2 K and
fitted to an Arrhenius law. The fit of the thermally activated region gives the energy barrier
of 17.8 K while
0
is about 2.3 × 10-9 s.
168
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Mass spectrometry
The {Mn13} cluster in 13 was also found to be stable in CH3CN, DMF and DMSO
environments. The {H[ nIV3MnIII10
10(CH3
)4(C6
5CH2P
3)10]}
-
species could be
identified in the ESI-MS spectrum of pristine crystals dissolved in CH3CN, DMF or
DMSO (Table 3.18, Figure 3.53) as a signal centered at m/z = 2699.1 a.m.u. Furthermore,
the ESI-MS spectrum of 13 dissolved in CH3CN reveals the presence of a signal centered
at
(C6
m/z
=
5CH2P
2753.4
3)10]}
a.m.u.
that
was
assigned
to
{H7[ nIV3MnIII10
13(CH3
)4
-
. This latter species is related to the former identified species but has
three additional H2O molecules associated with the coordination cluster. Both species were
modelled and good fits between the experimental and simulated isotopic envelopes could
be found.
Table 3.18 – ESI-MS assignment for compound 13.
Solvent
Crystals of
Species attributed
2699.1
{H[MnIV3MnIII10
IV
10(C
3
)4(C6
5C
2P
3)10]}
→ {Mn13}
-
2753.4
{H7[Mn
III
3Mn 10
13(C
3
)4(C6
5C
2P
3)10]}
→ {Mn13}
DMF
2699.1
{H[MnIV3MnIII10
10(C
3
)4(C6
5C
2P
3)10]}
→ {Mn13}
DMSO
2699.1
{H[MnIV3MnIII10
10(C
3
)4(C6
5C
2P
3)10]}
→ {Mn13}
Relative ionic abundance
13 in:
CH3CN
m/z
Figure 3.53 – Negative-mode ESI-MS spectra for crystals of 13 dissolved in DMF. Inset: Comparison
of the experimental isotopic envelopes (black spectrum) with simulated patterns (red spectrum) for
{H[ nIV3MnIII10 10(CH3 )4(C6 5CH2P 3)10]}- centered at m/z = 2699.1 a.m.u. (cone voltage: 30 V).
169
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
UV-Vis spectroscopy
Similar to the previous three compounds the UV-Vis absorption spectrum of 13
recorded in CH3CN (Figure 3.54) displays two absorption bands, an intense band at ca.
256 nm ( = 41000 L mol-1 cm-1) and a weak band at ca. 446 nm ( = 3600 L mol-1 cm-1).
These corresponds to a π − π* transition of the organic ligands and a 5T2g
III
within the octahedrally coordinated Mn ions.
5
Eg transition
49-52
Absorbance (a.u.)
0.45
0.40
0.4
0.35
0.3
0.30
0.2
0.25
0.1
0.20
400
450
500
550
600
0.15
0.10
0.05
0.00
300
400
500
600
700
800
Wavelength (nm)
Figure 3.54 – UV-Vis spectrum of a 10-5 M solution of 13 in CH3CN. Inset: A
section of the UV-Vis spectrum of a 10-4 M solution of 13 in CH3CN.
170
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.19 − Crystal data and structural refinement parameters for 13.
Compound 13
Empirical formula a
Molecular mass/g mol
C158H174ClMn13N6O42P10
-1 a
Crystal colour/shape
3
Brown / rectangular block
Crystal size/mm
0.11×0.06×0.05
Crystal system
Triclinic
Space group
P
a/ Å
14.8581(5)
b/ Å
16.0216(6)
c/ Å
19.3531(7)
/º
102.721(2)
/º
97.439(2)
/º
90.847(2)
3
V/ Å
4451.6(3)
Z
1
Temperature (K)
100(2)
-3
Density/Mg m
1.498
-1
Absorp. coef./mm
8.863
F(000)
2064
2
60
max/º
Reflections collected
25996
Independent reflections
12843 [R(int)=0.0571]
Data / restraints / parameters
12843 / 2151 / 1012
2
a
3888.47
S on F
0.981
R1, wR2 [I>2 (I)]
0.0960, 0.2609
R1, wR2 (all data)
0.1120, 0.2775
Largest diff. peak and hole/e.Å-3
2.005 and -1.021
Excluding solvate molecules
171
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.3.3
Synthesis and characterisation of dodecanuclear manganese complexes
3.3.3.1 K(H2O)4[MnIII12(μ3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Cl2·3CH3OH
·4H2O (14)
The labile nature of the pyridine ligands in Jahn-Teller sites in 12 allowed the
replacement of these ligands with 4-(3-phenylpropyl)pyridine in 13. This observation
prompted us to investigate if we could use a bipyridine ligand to connect the polynuclear
manganese complexes into polymers. Therefore, 4,4'-trimethylenedipyridine and 4,4'bipyridine were used as organic bases in similar reaction mixtures that led to the formation
of the tridecanuclear manganese complexes. Unfortunately, no crystalline material was
obtained when 4,4'-trimethylenedipyridine was used. However, when 4,4'-bipyridine was
employed,
rectangular
brown
crystals
of
K(H2O)4[MnIII12( 3-O)6(CH3OH)6
(C6H5CH2PO3H)7(C6H5CH2PO3)8]Cl2·3CH3OH·4H2O (14) separated from the reaction
mixture within a time period of a week.
Single crystal X-ray diffraction measurements revealed that 14 contains a
dodecanuclear manganese complex. 14 crystallises in the orthorhombic crystal system in
the space group Pbca. The exceptional ring-shaped cluster core in 14 possesses some
structural features that resemble those of the complexes in 11-13. The cluster core can be
visualised as constructed of two triangular units A and three dinuclear units B (Figure
3.55). However, in this case the triangular units A are structurally different to those in 1113. They adopt an eclipsed arrangement, are directly linked through O-donar atoms, and
are further stabilised by three dimer moijeties B. The two triangular units A in 14 each
consist of three MnIII ions (Mn(2), Mn(3) and Mn(5), and Mn(1), Mn(4) and Mn(6),
respectively) connected through three deprotonated, bridging phosphonate ligands to form
a ring motif (Figure 3.56). Each Mn ion in the each triangular unit adopts a square
pyramidal geometry that shares a common edge with its partner from the second triangular
unit to generate a {Mn2O8}10− subunit (Figure 3.56). The coordination environments of the
MnIII ions in A comprise of two bridging
3-
2-
oxo ligands involved in the {Mn2O8}10−
subunits and three phosphonate O donors that derive from three deprotonated
organophosphonate ligands. The short Mn ··· Mn contact in the {Mn2O8}10− subunit
(2.8043(14) Å – 2.8281(14) Å) is a result of the short Mn – ( 3-
2-
) distances (1.871(4) Å
172
Chapter 3 – Polynuclear Manganese Coordination Complexes
– 1.891(4) Å). The other three Mn–O bond lengths involving the phosphonate ligands vary
between 1.873(4) Å – 2.114(4) Å (Table 3.20).
(A)
(A)
(B)
(A)
Figure 3.55 − Crystal structure of the dodecanuclear manganese complex in 14 showing the triangular
units A and the dinuclear units B. Colour code: MnIII blue, P purple, O red, C grey, Cl green, K yellow
(hydrogen atoms have been omitted for clarity).
173
Chapter 3 – Polynuclear Manganese Coordination Complexes
(A)
Figure 3.56 − Polyhedral representation of the triangular units A in 14. Colour code: MnIII blue, P
purple, O red, C grey, Cl green.
The remaining six MnIII ions in 14 (Mn(7) – Mn(12)) are organised into three
{Mn2O10}14− dimer units (B) (Figure 3.57). In these dinuclear units the Mn ions display
slightly distorted octahedral coordination environments. The polyhedra in the dimeric
subunit share a common edge that involves two phosphonate O donors. The remaining
coordination sites of the polyhedra are occupied by two other phosphonate O donors (Mn –
Ophosphonate distances 1.885(4) Å – 2.374(4) Å), one terminal methanol O-donor (Mn –
Omethanol distances 2.185(4) Å – 2.220(5) Å) and a
1.857(4) Å – 1.870(4) Å). The
3-
2-
3-
2-
oxo ligand (Mn – ( 3- ) distances
oxo ligands bridge between the three {Mn2O10}14−
174
Chapter 3 – Polynuclear Manganese Coordination Complexes
dimer units B and the triangular units A to give the {Mn12} core structure. The distorted
nature of the octahedral coordination environment of the Mn ions in B arises from the JT
axial elongation and sterical restraints. The distorsion is reflected in various bond lenths
and angles that deviate from the ideal octahedral geometrical parameters. Thus, the atoms
in the elongated JT axes show bond lengths to the Mn centres that range between 2.185(4)
Å – 2.374(4) Å and employ one terminal methanol O-donor and one phosphonate O donor.
The bond angle O(38)-Mn(11)-O(41) of 165.74(16)° shows the greatest deviation from the
ideal octahedral angle of 180°, while the bond angle O(43)-Mn(9)-O(49) of 96.44(18)°
shows the greatest deviation from the ideal octahedral angle of 90°. Selected bond angles
for the Mn ions in B are listed in Table 3.21.
The cluster is stabilized by altogether fifteen phosphonate ligands that bridge
between the A and B units. Nine ligands are situated on the periphery of the {Mn12} ring,
with six of them bridging between an octahedron and a square pyramidal polyhedron in a
η1:η1:μ2 mode (P(8), P(9), P(10), P(12), P(14), P(15)). The other three perifical organic
ligands bridge between two octahedra: one phosphonate moijety adopts a η1:η1:μ2 mode
(P(11)) and the two other phosphonate groups act as η1:η1:η1:μ3 ligands, (P(6), P(7)). The
remaining six stabilising organophosphonate ligands can be visualised as pairs of three,
situated on opposite sides of the ring moijety. Each of these ligands bridge between two
square pyramidal polyhedra and a common vertex of two octahedra in a η1:η1:η1:μ3 mode
(P(1) – P(5) and P(13)) (Figure 3.58). The {Mn12} cluster in 14 is further stabilised by
intramolecular hydrogen bonding between the protonated phosphonate ligands, giving rise
to O-O distances of 2.447(0) Å – 2.564(0) Å (Figure 3.58).
All Mn atoms in 14 adopt the oxidation state +III. The assignment of the oxidation
states was confirmed by BVS calculations (Table 3.20). The cluster is surrounded by two
Cl− counterions, one situated in the central cavity of the ring. The ion is located 2.835(2)
Å, 2.8124(19) Å and 2.830(2) Å from Mn(2), Mn(3) and Mn(5), respectively. The other
Cl− counterion, Cl(2), resides in the vicinity of the cluster core, being situated at 3.612(0)
Å from the Cl(1) atom of a neighbouring cluster. The charge of the cluster and the Cl- ions
is further compensated by partially hydrated potassium ions {K(H2O)4}+ that link the
cluster entities into 1D zigzag chains. The resulting packing arrangement with views in the
direction of the crystallographic a- and c- axes is shown in Figure 3.59.
Intermolecular hydrogen bonding between the crystallisation solvent molecules and
the core structure within 14, together with weak - interactions (shortest contact 3.6 Å)
175
Chapter 3 – Polynuclear Manganese Coordination Complexes
invoving the aromatic ring moieties of phosphonate ligands further contribute to the
stability of 14 in the solid state.
Table 3.20 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 14.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(1)
Mn(1)-O(37)
Mn(1)-O(26)
Mn(1)-O(3)
Mn(1)-O(12)
Mn(1)-O(32)
1.873(4)
1.883(4)
1.887(4)
1.916(4)
2.074(4)
3.244
+3
Mn(2)
Mn(2)-O(3)
Mn(2)-O(26)
Mn(2)-O(18)
Mn(2)-O(24)
Mn(2)-O(50)
1.879(4)
1.879(4)
1.919(4)
1.936(4)
2.103(4)
3.114
+3
Mn(3)
Mn(3)-O(39)
Mn(3)-O(6)
Mn(3)-O(22)
Mn(3)-O(19)
Mn(3)-O(42)
1.871(4)
1.885(4)
1.921(4)
1.931(4)
2.101(4)
3.125
+3
Mn(4)
Mn(4)-O(6)
Mn(4)-O(39)
Mn(4)-O(27)
Mn(4)-O(46)
Mn(4)-O(14)
1.875(4)
1.888(4)
1.900(4)
1.919(4)
2.060(4)
3.044
+3
Mn(5)
Mn(5)-O(61)
Mn(5)-O(17)
Mn(5)-O(25)
Mn(5)-O(35)
Mn(5)-O(21)
1.882(4)
1.891(4)
1.915(4)
1.921(4)
2.114(4)
3.106
+3
Mn(6)
Mn(6)-O(17)
Mn(6)-O(61)
Mn(6)-O(40)
Mn(6)-O(53)
Mn(6)-O(48)
1.877(4)
1.884(4)
1.891(4)
1.922(4)
2.078(4)
3.434
+3
Mn(7)
Mn(7)-O(17)
Mn(7)-O(51)
Mn(7)-O(28)
Mn(7)-O(7)
Mn(7)-O(45)
Mn(7)-O(15)
1.858(4)
1.895(4)
1.903(4)
1.963(4)
2.211(5)
2.324(4)
2.886
+3
Mn(8)
Mn(8)-O(3)
Mn(8)-O(60)
Mn(8)-O(31)
Mn(8)-O(15)
Mn(8)-O(30)
Mn(8)-O(7)
1.862(4)
1.893(4)
1.921(4)
1.986(4)
2.186(4)
2.293(4)
2.670
+3
Mn(9)
Mn(9)-O(26)
Mn(9)-O(52)
Mn(9)-O(43)
Mn(9)-O(13)
Mn(9)-O(49)
Mn(9)-O(20)
1.858(4)
1.894(4)
1.899(4)
1.975(4)
2.202(5)
2.313(4)
3.232
+3
176
Chapter 3 – Polynuclear Manganese Coordination Complexes
Atom
Bond
Mn(10)
Mn(10)-O(39)
Mn(10)-O(16)
Mn(10)-O(29)
Mn(10)-O(20)
Mn(10)-O(11)
Mn(10)-O(13)
1.870(4)
1.913(4)
1.914(4)
1.970(4)
2.210(4)
2.313(4)
3.148
+3
Mn(11)
Mn(11)-O(6)
Mn(11)-O(33)
Mn(11)-O(59)
Mn(11)-O(44)
Mn(11)-O(38)
Mn(11)-O(41)
1.857(4)
1.885(4)
1.919(5)
1.984(4)
2.220(5)
2.374(4)
3.155
+3
Mn(12)
Mn(12)-O(61)
Mn(12)-O(34)
Mn(12)-O(47)
Mn(12)-O(41)
Mn(12)-O(54)
Mn(12)-O(44)
1.861(4)
1.906(4)
1.916(4)
1.965(4)
2.185(4)
2.298(4)
3.212
+3
Mn(1) ··· Mn(2)
Mn(3) ··· Mn(4)
Mn(5) ··· Mn(6)
2.8135(14)
2.8281(14)
2.8043(14)
Bond distances
(Å)
BVS
Mn(2) ··· Cl(1)
Mn(3) ··· Cl(1)
Mn(5) ··· Cl(1)
Cl(1) ··· Cl(2)
Assigned
oxidation state
2.835(2)
2.8124(19)
2.830(2)
3.612(0)
Figure 3.57 – Polyhedral representation of the {Mn2O10}14− units in 14. Colour code: MnIII blue, P
purple, O red, C grey, Cl green.
177
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.58 – Polyhedral representation of the {Mn12} core structure in 14 showing the intramolecular
hydrogen bonding. Colour code: MnIII blue, P purple, O red, C grey, Cl green, K yellow.
178
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.21 − Selected bond angles [º] for compound 14.
Bond Angle (º)
Bond Angle (º)
O(17)-Mn(7)-O(28)
O(51)-Mn(7)-O(7)
O(45)-Mn(7)-O(15)
O(51)-Mn(7)-O(45)
O(17)-Mn(7)-O(51)
O(17)-Mn(7)-O(45)
O(7)-Mn(7)-O(45)
O(28)-Mn(7)-O(45)
O(17)-Mn(7)-O(15)
O(17)-Mn(7)-O(7)
177.8(2)
174.97(19)
169.03(17)
94.40(19)
92.81(18)
91.61(17)
90.41(17)
88.96(17)
88.93(16)
88.52(17)
O(3)-Mn(8)-O(31)
O(60)-Mn(8)-O(15)
O(30)-Mn(8)-O(7)
O(60)-Mn(8)-O(30)
O(3)-Mn(8)-O(30)
O(3)-Mn(8)-O(60)
O(15)-Mn(8)-O(30)
O(31)-Mn(8)-O(30)
O(3)-Mn(8)-O(15)
O(3)-Mn(8)-O(7)
176.06(19)
172.88(19)
170.59(16)
95.09(19)
93.78(17)
92.01(18)
91.86(17)
89.69(17)
89.08(17)
88.19(16)
O(26)-Mn(9)-O(52)
O(43)-Mn(9)-O(13)
O(49)-Mn(9)-O(20)
O(43)-Mn(9)-O(49)
O(26)-Mn(9)-O(43)
O(26)-Mn(9)-O(49)
O(52)-Mn(9)-O(49)
O(13)-Mn(9)-O(49)
O(26)-Mn(9)-O(13)
O(26)-Mn(9)-O(20)
177.48(19)
174.67(18)
166.69(16)
96.44(18)
92.75(18)
91.90(17)
89.15(18)
88.65(17)
88.64(17)
88.77(16)
O(39)-Mn(10)-O(16)
O(29)-Mn(10)-O(20)
O(11)-Mn(10)-O(13)
O(39)-Mn(10)-O(11)
O(20)-Mn(10)-O(11)
O(29)-Mn(10)-O(11)
O(39)-Mn(10)-O(29)
O(39)-Mn(10)-O(20)
O(16)-Mn(10)-O(11)
O(39)-Mn(10)-O(13)
176.30(18)
172.45(19)
171.53(16)
94.00(17)
93.77(17)
93.51(18)
92.34(18)
89.16(17)
88.87(18)
88.50(16)
O(33)-Mn(11)-O(44)
O(6)-Mn(11)-O(59)
O(38)-Mn(11)-O(41)
O(33)-Mn(11)-O(38)
O(6)-Mn(11)-O(38)
O(6)-Mn(11)-O(33)
O(44)-Mn(11)-O(38)
O(59)-Mn(11)-O(38)
O(6)-Mn(11)-O(44)
O(6)-Mn(11)-O(41)
176.0(2)
175.65(19)
165.74(16)
94.84(19)
93.76(17)
93.07(18)
88.92(17)
88.91(19)
88.14(18)
86.79(16)
O(61)-Mn(12)-O(47)
O(34)-Mn(12)-O(41)
O(54)-Mn(12)-O(44)
O(34)-Mn(12)-O(54)
O(61)-Mn(12)-O(54)
O(61)-Mn(12)-O(34)
O(41)-Mn(12)-O(54)
O(47)-Mn(12)-O(54)
O(61)-Mn(12)-O(41)
O(61)-Mn(12)-O(44)
176.62(19)
174.30(19)
169.67(17)
94.58(19)
92.97(18)
92.57(19)
90.64(18)
90.04(18)
89.46(18)
87.53(17)
179
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.59 – Packing arrangement of the dodecanuclear manganese clusters in 14 viewed in the
direction of the crystallographic a- and c-axis. Colour code: Mn blue, P purple, O red, C grey, Cl green,
(crystallization solvent molecules and hydrogen atoms have been omitted for clarity).
180
Chapter 3 – Polynuclear Manganese Coordination Complexes
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 14 is presented in Figure 3.60. Characteristic C–H out-of-plane
bending vibrations of the benzylphosphonic acid ligands can be found in the range 800 –
650 cm-1. C–C skeletal vibrations of the phenyl rings and C–H bending and rocking
vibrations of the –CH2 groups of the benzylphosphonic acid ligands appear in the 1500 –
1200 cm-1 region. Different P–O stretching vibrations of the phosphonate groups can be
observed in the range 1200 – 900 cm-1. In addition, the corresponding O–H stretching
vibrations and H–O–H bending vibrations of the crystallization water molecules engaged
in H-bonds appear as broad bands at ca. 3200 cm-1 and ca. 1600 cm-1, respectively.36-40
Figure 3.60 – Infrared spectrum of 14.
181
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Thermogravimetric analysis
The thermal stability of a freshly prepared crystalline sample of 14 was
investigated, in an N2 atmosphere, in the temperature range 30 – 900 °C (Figure 3.61). The
first thermogravimetric step observed in the TGA curve of 14 corresponds to a weight loss
of 4.7 % and occurs in the temperature range between 30 – 100 °C. This event corresponds
to the loss of three crystallization CH3OH molecules and four crystallization H2O
molecules (calcd: 4.4 %). The following step in the TGA curve corresponds to a weight
loss of 6.9 % between 100 – 210 °C, which is associated with the loss of six coordinated
CH3OH molecules and four coordinated H2O molecules (calcd: 6.8 %). The compound
then undergoes a gradual decomposition between 210 – 500 °C involving the organic
ligands. The final thermogravimetric step is centered at 550 °C .
Weight % (%)
100
90
80
70
60
50
0
200
400
600
800
1000
Temperature (ºC)
Figure 3.61 – Thermogravimetric analysis of 14.
182
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Magnetism
The temperature dependence of the magnetic susceptibility of 14 was measured
between 298 and 1.8 K (Figure 3.62). The χT value of 29.5 cm3 K mol-1 at room
temperature is significantly lower than the expected value of 36 cm3 K mol-1 of twelve S =
2 MnIII spin carriers (C =
S(S+1) with
= 0.12505 cm3 K mol-1 and g = 2).41 This
result indicates that antiferromagnetic interactions dominate between the spin centres. The
gradually decrease of the χT product down to a minimum of ca. 18.5 cm3 K mol-1 at ca. 40
K further confirm the presence of AF interactions. Below 40 K, at 1000 Oe the χT product
decreases rapidely down to 6 cm3 K mol-1 at 1.8 K suggesting an ST = 3 spin ground state.
a)
b)
Figure 3.62 – (a) Temperature dependence of the χT product of 14 at 0.1 and 1 T. (b) A magnified view
of the χT product in (a) between 1 and 100 K.
The field dependence of the magnetisation measured at temperatures between 1.84
and 8 K is shown in Figure 3.63, a. The M vs H plot reveals a continuous increase of the
magnetisation without clear saturation even at 1.84 K at 7 T. This observation suggests the
presence of magnetic anisotropy. The data represented as a M vs H/T plot (Figure 3.63, b)
confirms the presence of anisotropy intrinsic to MnIII metal ions and the cluster as the data
are not superposed on a single master-curve as expected for isotropic systems. However,
the complex is not expected to have a significant anisotropy as the MnIII Jahn-Teller axes
are approximately aligned with the plane of the {Mn12} ring (see crystal structure in Figure
3.58).
183
Chapter 3 – Polynuclear Manganese Coordination Complexes
a)
b)
Figure 3.63 – (a) M vs H and (b) M vs H/T data at and below 8 K.
-
Mass spectrometry
ESI-MS studies on the crystalline material revealed that the {Mn12} cluster in 14 is
stable in CH3CN, DMF and DMSO environments (Table 3.22). The mass spectrum of 14
dissolved in DMSO is presented in Figure 3.64 and reveals the presence of one major
isotopic envelop in the high molecular mass region. This signal is centered at m/z = 1673.4
a.m.u. and was assigned to a doubly charged species {H7[ nIII10MnII2
6(C6
5CH2P
3)15]
Cl}2-. A summary of the ESI-MS assignment is given in Table 3.22. The envelopes of all
Relative ionic abundance
assigned species were modelled.
Figure 3.64 – Negative-mode ESI-MS spectra for crystals of 14 dissolved in DMSO. Inset:
Comparison of the experimental isotopic envelopes (black spectrum) with simulated patterns (red
spectrum) for {H7[ nIII10MnII2 6(C6 5CH2P 3)15]Cl}2- centered at m/z = 1673.4 a.m.u. (cone
voltage: 30 V).
184
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.22 – ESI-MS assignment for compound 14.
Crystals of
Solvent
m/z
Species attributed
CH3CN
1673.4
{H7[MnIII10MnII2O6(C6H5CH2PO3)15]Cl}2-
→ {Mn12}
DMF
1673.4
{H7[MnIII10MnII2O6(C6H5CH2PO3)15]Cl}2-
→ {Mn12}
1673.4
{H7[MnIII10MnII2O6(C6H5CH2PO3)15]Cl}2-
→ {Mn12}
14 in:
-
DMSO
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 14 recorded in CH3CN (Figure 3.65) displays
an intense absorption band at ca. 257 nm ( = 36000 L mol-1 cm-1) which can be attributed
to π − π* transitions of the aromatic phosphonate ligands. The weak band observed at ca.
488 nm ( = 3200 L mol-1 cm-1) may arise from d – d transitions that can be assigned to
T2g
5
Eg transitions of octahedrally coordinatied MnIII ions within 14.49-52
0.8
0.35
0.30
0.6
Absorbance (a.u.)
5
0.25
0.20
0.15
0.4
0.10
400
450
500
550
600
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 3.65 – UV-Vis spectrum of a 10-5 M solution of 14 in CH3CN. Inset: A
section of the UV-Vis spectrum of a 10-4 M solution of 14 in CH3CN.
185
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.23 − Crystal data and structural refinement parameters for 14.
Compound 14
Empirical formula a
Molecular mass/g mol
C111H144Cl2KMn12O61P15
-1 a
Crystal colour/shape
3
Dark brown / rectangular block
Crystal size/mm
0.50×0.20×0.15
Crystal system
Orthorhombic
Space group
Pbca
a/ Å
27.2527(17)
b/ Å
29.947(2)
c/ Å
41.980(3)
/º
90
/º
90
/º
90
3
V/ Å
34261(4)
Z
1
Temperature (K)
150(2)
-3
Density/Mg m
1.402
-1
Absorp. coef./mm
1.125
F(000)
14144
2
50
max/º
Reflections collected
198099
Independent reflections
30136 [R(int)=0.1519]
Data / restraints / parameters
30136 / 0 / 1880
2
a
3688.16
S on F
1.024
R1, wR2 [I>2 (I)]
0.0602, 0.1412
R1, wR2 (all data)
0.1146, 0.1514
Largest diff. peak and hole/e.Å-3
2.299 and -1.597
Excluding solvate molecules
186
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.3.3.2 K(H2O)4[MnIII12(μ3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5CH2PO3)8]Br2·2CH3OH·2H2O (15)
Compound
15,
K(H2O)4[MnIII12( 3-O)6(CH3OH)6(C6H5CH2PO3H)7
(C6H5CH2PO3)8]Br2·2CH3OH·2H2O was prepared in a similar manner to 14 using
MnBr2·4H2O instead of MnCl2·4H2O, in order to investigate the role of the halide ion as a
potential templating agent. X-ray structural analysis revealed that 15 (Figure 3.66) is
almost isostructural to 14, having only slightly different structural and geometrical
parameters. Selected interatomic distances and angles for compound 15 are given in Table
3.24 and Table 3.25, and agree very well with those observed for 14. Bond valence sum
analysis confirms again that all the Mn ions in 15 are in the oxidation state +III; the overall
charge of the cluster is compensated by one K+ ion and two Br− ions. As before, one Br−
ion is situated in the central cavity of the ring, at 2.954(1) Å, 2.973(1) Å and 2.981(1) Å to
Mn(1), Mn(3) and Mn(5) respectively. The other Br− ion, Br(2) is situated just above the
cavity of the ring, residing 3.879(2) Å from Br(1). As for 14, the potassium ions link the
clster entities into 1D zigzag chains.
Figure 3.66 − Crystal structure of the dodecanuclear manganese complex in 15. Colour code: MnIII
blue, P purple, O red, C grey, Br dark green, K yellow (hydrogen atoms have been omitted for clarity).
187
Chapter 3 – Polynuclear Manganese Coordination Complexes
(A)
Figure 3.67 − Polyhedral representation of the triangular units A found in 15. Colour code: MnIII
blue, P purple, O red, C grey, Br dark green.
(B)
Figure 3.68 – Polyhedral representation of the {Mn2O10}14− units in 15. Colour code: MnIII blue, P
purple, O red, C grey, Br dark green.
188
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.24 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 15.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(1)
Mn(1)-O(46)
Mn(1)-O(47)
Mn(1)-O(39)
Mn(1)-O(45)
Mn(1)-O(8)
1.871(5)
1.877(5)
1.916(5)
1.938(5)
2.104(5)
3.135
+3
Mn(2)
Mn(2)-O(46)
Mn(2)-O(47)
Mn(2)-O(23)
Mn(2)-O(41)
Mn(2)-O(48)
1.874(5)
1.891(5)
1.899(5)
1.914(5)
2.084(5)
3.198
+3
Mn(3)
Mn(3)-O(33)
Mn(3)-O(32)
Mn(3)-O(38)
Mn(3)-O(60)
Mn(3)-O(31)
1.873(5)
1.877(5)
1.911(5)
1.943(5)
2.077(5)
3.161
+3
Mn(4)
Mn(4)-O(32)
Mn(4)-O(16)
Mn(4)-O(33)
Mn(4)-O(24)
Mn(4)-O(26)
1.873(5)
1.884(5)
1.891(4)
1.920(5)
2.076(5)
3.224
+3
Mn(5)
Mn(5)-O(51)
Mn(5)-O(61)
Mn(5)-O(59)
Mn(5)-O(43)
Mn(5)-O(63)
1.879(5)
1.893(5)
1.909(5)
1.930(5)
2.094(5)
3.125
+3
Mn(6)
Mn(6)-O(61)
Mn(6)-O(51)
Mn(6)-O(40)
Mn(6)-O(14)
Mn(6)-O(22)
1.873(5)
1.882(5)
1.897(5)
1.925(5)
2.080(5)
3.204
+3
Mn(7)
Mn(7)-O(51)
Mn(7)-O(52)
Mn(7)-O(53)
Mn(7)-O(42)
Mn(7)-O(56)
Mn(7)-O(44)
1.863(4)
1.911(5)
1.939(5)
1.974(5)
2.188(5)
2.324(5)
3.127
+3
Mn(8)
Mn(8)-O(46)
Mn(8)-O(50)
Mn(8)-O(54)
Mn(8)-O(44)
Mn(8)-O(49)
Mn(8)-O(42)
1.865(4)
1.892(5)
1.912(5)
1.989(5)
2.218(5)
2.366(5)
3.135
+3
Mn(9)
Mn(9)-O(47)
Mn(9)-O(9)
Mn(9)-O(7)
Mn(9)-O(25)
Mn(9)-O(36)
Mn(9)-O(37)
1.862(4)
1.903(5)
1.923(5)
1.968(5)
2.214(5)
2.313(5)
3.166
+3
189
Chapter 3 – Polynuclear Manganese Coordination Complexes
Atom
Bond
Mn(10)
Mn(10)-O(32)
Mn(10)-O(34)
Mn(10)-O(28)
Mn(10)-O(37)
Mn(10)-O(35)
Mn(10)-O(25)
1.866(4)
1.890(5)
1.902(5)
1.992(5)
2.217(5)
2.316(5)
3.178
+3
Mn(11)
Mn(11)-O(33)
Mn(11)-O(29)
Mn(11)-O(1)
Mn(11)-O(12)
Mn(11)-O(2W)
Mn(11)-O(58)
1.863(4)
1.903(5)
1.917(4)
1.991(5)
2.197(5)
2.303(5)
3.161
+3
Mn(12)
Mn(12)-O(61)
Mn(12)-O(18)
Mn(12)-O(5)
Mn(12)-O(58)
Mn(12)-O(4W)
Mn(12)-O(12)
1.861(5)
1.895(5)
1.907(5)
1.978(5)
2.212(5)
2.332(5)
3.185
+3
Mn(1) ··· Br(1)
Mn(3) ··· Brl(1)
Mn(5) ··· Br(1)
Br(1) ··· Br(2)
2.954(1)
2.973(1)
2.981(1)
3.879(2)
Mn(1) ··· Mn(2)
Mn(3) ··· Mn(4)
Mn(5) ··· Mn(6)
2.8232(15)
2.8098(15)
2.8029(16)
Bond distances
(Å)
BVS
Assigned
oxidation state
190
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.25 − Selected bond angles [º] for compound 15.
Bond Angle (º)
Bond Angle (º)
O(51)-Mn(7)-O(53)
O(52)-Mn(7)-O(42)
O(56)-Mn(7)-O(44)
O(52)-Mn(7)-O(56)
O(51)-Mn(7)-O(56)
O(51)-Mn(7)-O(52)
O(42)-Mn(7)-O(56)
O(53)-Mn(7)-O(56)
O(51)-Mn(7)-O(42)
O(51)-Mn(7)-O(44)
177.3(2)
173.5(2)
169.23(19)
94.9(2)
91.9(2)
91.8(2)
91.4(2)
90.4(2)
89.8(2)
88.27(19)
O(46)-Mn(8)-O(54)
O(50)-Mn(8)-O(44)
O(49)-Mn(8)-O(42)
O(50)-Mn(8)-O(49)
O(46)-Mn(8)-O(50)
O(46)-Mn(8)-O(49)
O(44)-Mn(8)-O(49)
O(54)-Mn(8)-O(49)
O(46)-Mn(8)-O(44)
O(46)-Mn(8)-O(42)
176.6(2)
175.8(2)
165.46(19)
95.0(2)
92.6(2)
92.5(2)
88.9(2)
88.7(2)
88.6(2)
87.76(19)
O(47)-Mn(9)-O(9)
O(7)-Mn(9)-O(25)
O(36)-Mn(9)-O(37)
O(7)-Mn(9)-O(36)
O(25)-Mn(9)-O(36)
O(47)-Mn(9)-O(36)
O(47)-Mn(9)-O(7)
O(9)-Mn(9)-O(36)
O(47)-Mn(9)-O(25)
O(47)-Mn(9)-O(37)
177.2(2)
171.8(2)
171.41(19)
94.1(2)
94.0(2)
92.8(2)
91.8(2)
89.3(2)
89.05(19)
88.64(18)
O(32)-Mn(10)-O(34)
O(28)-Mn(10)-O(37)
O(35)-Mn(10)-O(25)
O(28)-Mn(10)-O(35)
O(32)-Mn(10)-O(28)
O(32)-Mn(10)-O(35)
O(34)-Mn(10)-O(35)
O(32)-Mn(10)-O(25)
O(32)-Mn(10)-O(37)
O(37)-Mn(10)-O(35)
178.0(2)
174.5(2)
165.67(18)
96.6(2)
92.7(2)
91.51(19)
89.3(2)
88.90(18)
88.82(19)
88.63(19)
O(33)-Mn(11)-O(1)
O(29)-Mn(11)-O(12)
O(2)-Mn(11)-O(58)
O(29)-Mn(11)-O(2)
O(33)-Mn(11)-O(2)
O(12)-Mn(11)-O(2)
O(33)-Mn(11)-O(29)
O(1)-Mn(11)-O(2)
O(33)-Mn(11)-O(12)
O(33)-Mn(11)-O(58)
176.7(2)
172.1(2)
170.60(19)
95.8(2)
92.83(19)
91.95(19)
91.7(2)
90.09(19)
89.46(19)
88.39(18)
O(61)-Mn(12)-O(5)
O(18)-Mn(12)-O(58)
O(4)-Mn(12)-O(12)
O(18)-Mn(12)-O(4)
O(61)-Mn(12)-O(18)
O(61)-Mn(12)-O(4)
O(58)-Mn(12)-O(4)
O(61)-Mn(12)-O(12)
O(5)-Mn(12)-O(4)
O(61)-Mn(12)-O(58)
178.8(2)
175.0(2)
168.66(19)
94.4(2)
92.3(2)
91.13(19)
90.4(2)
89.52(18)
89.37(19)
88.9(2)
191
Chapter 3 – Polynuclear Manganese Coordination Complexes
- PHYSICOCHEMICAL CHARACTERISATION
The IR spectrum of compound 15 exhibits the same general characteristics than that
of 14. The UV-Vis absorption spectrum of 15 in CH3CN also displays idential features to
the spectrum of 14, showing an intense absorption band at ca. 258 nm ( = 52000 L mol-1
cm-1) (π − π* transitions of the phosphonate ligands), and a weak absorption band at ca. 489
nm ( = 4000 L mol-1 cm-1) (5T2g
5
Eg most likely resulting from d-d transtions of
octahedrally coordinated MnIII centers).
-
Thermogravimetric analysis
Weight % (%)
100
90
80
70
60
50
0
200
400
600
800
1000
Temperature (ºC)
Figure 3.69 – Thermogravimetric analysis of 15.
The TGA analysis in an N2 atmosphere reveals that 15 also undergoes a similar
decomposition behaviour as 14. The first termogravimetric step observed in the TGA curve
of 15 (Figure 3.69) corresponds to a weight loss of 2.6 % between 30 – 100 °C. This step
was attributed to the loss of two crystallization CH3OH molecules and two crystallization
H2O molecules (calcd: 2.6 %). The next termogravimetric step observed in the TGA occurs
in the temperature range between 100 – 200 °C and corresponds to a weight loss of 6.3 %.
192
Chapter 3 – Polynuclear Manganese Coordination Complexes
This weight loss can be associated with the loss of six coordinated CH3OH molecules and
four coordinated H2O molecules (calcd: 6.8 %). Decompositions of the organic ligands and
thermolysis processes between 200 – 600 °C destroy the cluster entities.
-
Magnetism
Magnetisation data for compound 15 were collected at high magnetic field
strengths and low temperatures. The magnetic properties of 15 are almost identical with
those of 14, beeing caracterised by dominant antiferromagnetic interactions between the
spin centres and an ST = 3 ground state. As expected, exchange of the halide ions has no
significant effect on the structure and thus on magnetic properties of the cluster core.
15
14
Figure 3.70 – Comparison of the χT vs T plot of 14 and 15 at 0.1 T.
193
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Mass spectrometry
The solution stability of the {Mn12} cluster in 15 was again investigated by
electrospray ionisation mass spectrometry. The ESI-MS spectrum of pristine crystals of 15
dissolved in CH3CN is presented in Figure 3.71 and reveals the presence of three major
isotopic envelopes in the high mass region of the spectrum. Two of the signals were
assigned to {H6[ nIII10MnII2
{H6[ nIII11MnII
6(C6
5CH2P
6(C6
5CH2P
3)15]Br}
2-
3)15]}
2-
centered at m/z = 1655.0 a.m.u. and
centered at m/z = 1695.4 a.m.u., while the third
signal could not be assigned, most likely due to overlapping species. ESI-MS studies
revealed that the {Mn12} cluster in 15 is stable in CH3CN, DMF and DMSO environments.
A summary of the ESI-MS assignment is presented in Table 3.26. The isotopic envelopes
of all assigned species were modelled.
Table 3.26 – ESI-MS assignment for compound 15.
Solvent
CH3CN
Crystals of
15 in:
DMF
Species attributed
1655.0
{H6[MnIII10MnII2O6(C6H5CH2PO3)15]}2-
→ {Mn12}
1695.4
{H6[MnIII11MnIIO6(C6H5CH2PO3)15]Br}2-
→ {Mn12}
1655.0
{H6[MnIII10MnII2O6(C6H5CH2PO3)15]}2-
→ {Mn12}
1655.0
{H6[MnIII10MnII2O6(C6H5CH2PO3)15]}2-
→ {Mn12}
1695.4
{H6[MnIII11MnIIO6(C6H5CH2PO3)15]Br}2-
→ {Mn12}
Relative ionic abundance
DMSO
m/z
Figure 3.71 – Negative-mode ESI-MS spectra for crystals of 15 dissolved in CH3CN. Inset:
Comparison of the experimental isotopic envelopes (black spectrum) with simulated patterns (red
spectrum) for {H6[ nIII10MnII2 6(C6 5CH2P 3)15]}2- centered at m/z = 1655.0 a.m.u. and
{H6[ nIII11MnII 6(C6 5CH2P 3)15]Br}2- centered at m/z = 1695.4 a.m.u. (cone voltage: 30 V).
194
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.27 − Crystal data and structural refinement parameters for 15.
Compound 15
Empirical formula a
Molecular mass/g mol
C111H144Br2KMn12O61P15
-1 a
Crystal colour/shape
3
Brown / rectangular block
Crystal size/mm
0.50×0.10×0.10
Crystal system
Orthorhombic
Space group
Pbca
a/ Å
27.112(2)
b/ Å
29.957(2)
c/ Å
41.972(3)
/º
90
/º
90
/º
90
3
V/ Å
34089(4)
Z
1
Temperature (K)
150(2)
-3
Density/Mg m
1.458
-1
Absorp. coef./mm
1.569
F(000)
14584
2
50
max/º
Reflections collected
194198
Independent reflections
29987 [R(int)=0.1221]
Data / restraints / parameters
29987 / 0 / 1859
2
a
3777.06
S on F
0.965
R1, wR2 [I>2 (I)]
0.0708, 0.2030
R1, wR2 (all data)
0.1095, 0.2196
Largest diff. peak and hole/e.Å-3
2.661 and -3.278
Excluding solvate molecules
195
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.3.4
Synthesis and characterisation of a hexanuclear manganese complex
3.3.4.1 (H3O)4[MnIII2MnII4(μ4-O)2(H2O)2(CH3CN)2{(C6H5)3CPO3}6]Cl2·2CH3CN·4H2O (16)
Compound
16,
(H3O)4[MnIII2MnII4( 4-O)2(H2O)2(CH3CN)2{(C6H5)3CPO3}6]Cl2
·2CH3CN·4H2O, (Figure 3.72) was prepared by a comproportionation reaction between
MnCl2·4H2O and KMnO4, using triphenylmethylphosphonic acid as a stabilising ligand
and triethylamine as an organic base. The reaction mixture was stirred for five hours,
filtered and kept undisturbed at 25 °C for slow evaporation of the solvent. Rectangular redbrown crystals of 16 separated from the reaction mixture within a time period of five days.
These were characterised by single crystal X-ray diffraction measurements.
16 crystallises in the monoclinic crystal system in the space group C2/c. This
compound contains a mixed-valent, hexanuclear manganese cluster in which the six Mn
ions are arranged to form two edge-sharing tetrahedra, with a μ4-O2- ion at the center of
each tetrahedron (Figure 3.73). The symmetry of the cluster core is characterised by an
inversion centre located at the mid-point of the common edge of the two terahedra.
Figure 3.72 − Crystal structure of the hexanuclear manganese complex in 16. Colour code: MnIII blue,
MnII cyan, P purple, O red, C grey, N dark blue, Cl green (hydrogen atoms have been omitted for clarity).
The resultant {MnIII2MnII4O2}10+ core structure is related to previously reported
coordination clusters that are stabilised by carboxylate ligands.58-64 However, in 16, the
196
Chapter 3 – Polynuclear Manganese Coordination Complexes
two edge-sharing tetrahedra are further connected by six fully deprotonated phosphonate
ligands, that bridge between three Mn atoms in a η1:η1:η1:μ3 bridging mode. The cluster is
further stabilised by two CH3CN and two H2O molecules that complete the trigonal
bipyramidal environments of the four peripheral MnII ions (Mn(3) – O(6) bond length
2.250(4) Å and Mn(2) – N(1) bond length 2.312(5) Å). The μ4-O2- oxo ligand and its
symmetry equivalent are situated in trans positions to the H2O/CH3CN ligands giving rise
to Mn(2) – O(3) and Mn(3) – O(3) distances of 2.347(3) Å and 2.319(4) Å, respectively.
The remaining coordination sites of the trigonal bipyramidal coordination environments of
these outer Mn centres, Mn(2), Mn(3) and symmetry equivalents, are occupied by three
phosphonate O donors. The resulting Mn – O distances range between 2.034(4) Å –
2.073(4) Å.
The two central Mn ions, Mn(1) and Mn(1`), adopt square pyramidal coordination
environments, with the polyhedra facing in opposite directions with respect to each other.
The square pyramidal polyhedra consist of two μ4-O2- oxo ligands (Mn(1) – O(3) and
Mn(1) – O(3`) distances 1.896(3) Å and 1.880(3) Å, respectively), and three phosphonate
O donors (Mn – O distances range between 1.952(3) Å – 2.143(4) Å).
Bond valence sum analysis (Table 3.28) confirms that the two Mn ions Mn(1) an
Mn(1’) are in the oxidation state +III. The remaning four Mn ions are in the oxidation state
+II. The short Mn(1) ··· Mn(1`) contact of 2.7950(16) Å is in agreement with the assigned
+III oxidation state of the Mn ions, and compares well with bond distances observed in
structurally related compounds [Mn6O2(O2CPh)10(py)2(MeCN)2]·2MeCN (2.820(3) Å),
[Mn6O2(O2CPh)10(4hmpH)3 (MeCN)] (2.817(19) Å) and
[Mn6O2(O2CPh)10(pym)2
(MeCN)2] (2.823(3) Å) previously reported in the literature.58, 59
16 is further stabilised by two symmetry equivalent chloride counterions that reside
in the vicinity of the cluster core. Four H3O+ ions were assigned in order to balance the
overall charge of the cluster. Hydronium counterions have previously been reported to
stabilise related Mn coordination clusters, e.g. in
{(H3O)2[Mn4-(4-Haba)2(4-
aba)6(SCN)4(H2O)2]} (4-Haba = 4-aminobenzoic acid), (H3O)[Na2MnIII6MnII2(μ4-O)2(μ1,1N3)7(μ1,3-N3)(H2L)6Cl]·3H2O,
(H3O)[Na4MnIII12MnII4(μ4-O)4(μ1,1-N3)9(μ1,1,3-N3)2(H2L)12
(CH3O)4(H2O)4]·2ClO4·2(CH3OH)2·4H2O,
(H4L =
2-{[(2-hydroxy-3-methoxyphenyl)
methylene]amino}-2-(hydroxymet-hyl)-1,3-propanediol).65, 66
197
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.73 – Polyhedral representation of the {Mn6} core structure in 16. Colour code: MnIII blue,
MnII cyan, P purple, O red, C grey.
Table 3.28 − Selected bond lengths [Å] and bond valence sum (BVS) for compound 16.
Atom
Bond
Bond distances
(Å)
BVS
Assigned
oxidation state
Mn(1)
Mn(1)-O(3`)
Mn(1)-O(3)
Mn(1)-O(8)
Mn(1)-O(7)
Mn(1)-O(9)
1.880(3)
1.896(3)
1.952(4)
1.954(4)
2.143(4)
2.955
+3
Mn(2)
Mn(2)-O(2`)
Mn(2)-O(10)
Mn(2)-O(1`)
Mn(2)-N(1)
Mn(2)-O(3)
2.034(4)
2.037(4)
2.064(4)
2.312(5)
2.347(3)
1.971
+2
Mn(3)
Mn(3)-O(11`)
Mn(3)-O(4)
Mn(3)-O(5)
Mn(3)-O(6)
Mn(3)-O(3)
2.048(4)
2.055(4)
2.073(4)
2.250(4)
2.319(4)
1.977
+2
Mn(1) ··· Mn(1`)
2.7950(16)
198
Chapter 3 – Polynuclear Manganese Coordination Complexes
The {MnIII2MnII4O2}10+ core structure in 16 can alternatively be described as
consisting of two triangular units with Mn(1), Mn(2) and Mn(3), and Mn(1`), Mn(2`) and
Mn(3`) as vertices (Mn(1) ··· Mn(2), Mn(2) ··· Mn(3) and Mn(3) ··· Mn(1) are 3.499(55)
Å, 3.664(3) Å and 3.598(2) Å, respectively). These triangular units are connected to each
other by four phosphonate ligands and two μ4-O2- ions.
The crystal structure of 16 is stabilized by weak intermolecular hydrogen bonds and
π-π interactions, resulting in a grid-like packing arrangement that can be seen in the
direction of the crystallographic b-axis. A number of hydrogen bonds involve the two
coordinated H2O molecules of the Mn cluster and the N atoms of the CH3CN solvent
molecules (O(6) – N(2) distance 2.764(31) Å).
Figure 3.74 – Packing arrangement of the hexanuclear manganese clusters in 16 viewed in the
direction of the crystallographic a- and b-axis. Colour code: Mn blue, P purple, O red, C grey, Cl green,
(crystallization solvent molecules and hydrogen atoms have been omitted for clarity).
199
Chapter 3 – Polynuclear Manganese Coordination Complexes
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 16 presented in Figure 3.75 displays some characteristic bands
of the aromatic rings of the phosphoate ligands, such as C–C skeletal vibrations (1600 –
1400 cm-1) and C–H out-of-plane bending vibrations (900 – 650 cm-1). Different P–O
stretching vibrations of the phosphonate groups can be observed in the range 1200 – 900
cm-1, while the corresponding O–H stretching vibrations and H–O–H bending vibrations of
the crystallization water molecules engaged in H-bonds appear as broad bands at ca. 3200
cm-1 and ca. 1600 cm-1, respectively.36-40
Figure 3.75 – Infrared spectrum of 16.
200
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Thermogravimetric analysis
The thermal stability of the compound in an N2 atmosphere in the temperature
range 30 – 900 °C was investigated using a freshly prepared crystalline sample of 16
(Figure 3.76). The compound exhibits a weight loss of 6.3 % in the temperature range
between 30 – 150 °C that can be attributed to the loss of two crystallization CH3CN
molecules and four crystallization H2O molecules (calcd: 5.7 %). A further temperature
increase results in a gradual weight loss of 4.8 % between 150 – 280 °C, which could be
associated with the loss of two coordinated CH3CN molecules and two coordinated H2O
molecules (calcd: 4.4 %). Between 280 – 600 °C the oxidative degradation of the organic
ligands takes place and further thermolysis processes result in the cluster degradation.
100
Weight % (%)
90
80
70
60
50
40
0
200
400
600
800
1000
Temperature (ºC)
Figure 3.76 – Thermogravimetric analysis of 16.
201
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Magnetism
The magnetic studies for compound 16 were carried out on a polycrystalline sample
at high magnetic field strengths and low temperatures. The temperature dependence of the
χT product (Figure 3.77) reveals dominant AF interactions between the spin centres; a
Curie-Weiss fit was applied to analyse the data. The Curie constant of 23.2 cm3 K mol-1 is
comparable to the expected value of 23.5 cm3 K mol-1 of two S = 2 MnIII and four S =
MnII spin carriers (C =
S(S+1) with
= 0.12505 cm3 K mol-1 and g = 2).41 The
extrapolation of the χT vs T data at low temperature implies that the ground state of 16 is
ST = 0.
Figure 3.77 – Temperature dependence of the χT product of 16 at 0.1 T. The red solid line corresponds
to the best fit of the experimental data with the Curie-Weiss law (C = 23.2 cm3 K mol-1 and = 34 K).
202
Chapter 3 – Polynuclear Manganese Coordination Complexes
-
Mass spectrometry
The stability of the {Mn6} cluster in solution was investigated by electrospray
ionisation mass spectrometry. A major isotopic envelop centered at m/z = 2296.1 a.m.u is
observed in the ESI-MS spectrum of the pristine crystals dissolved in CH3CN and DMF
and it can be assigned to a {H nIII2MnII4
2[(C6
5)3CP
3]6}
-
species. The corresponding
doubly charged species could also be identified in the ESI-MS spectrum of the pristine
crystals dissolved in CH3CN (m/z = 1147.4, {H2 nII6
2[(C6
5)3CP
23]6} ).
A summary of
the ESI-MS signal assignment for compound 16 is presented in Table 3.29. The mass
spectrum of 16 dissolved in DMSO does not show any signals in the high molecular mass
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
region, suggesting that 16 is not stable in DMSO.
Figure 3.78 – Negative-mode ESI-MS spectra for crystals of 16 dissolved in CH3CN. Comparison of
the experimental isotopic envelopes (black spectrum) with simulated patterns (red spectrum) for
{H2MnII6 2[(C6 5)3CP 3]6]}2- centered at m/z = 1147.4 a.m.u., {H nIII2MnII4 2[(C6 5)3CP 3]6]}centered at m/z = 2296.1 a.m.u. and {H7MnII6 4[(C6 5)3CP 3]6]}- centered at m/z = 2334.0 a.m.u.,
(cone voltage: 30 V).
203
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.29 – ESI-MS assignment for compound 16.
Solvent
Crystals of
CH3CN
16 in:
DMF
-
m/z
Species attributed
1147.4
{H2MnII6O2[(C6H5)3CPO3]6]}2-
→ {Mn6}
2296.1
{HMnIII2MnII4O2[(C6H5)3CPO3]6]}-
→ {Mn6}
2334.0
{H7MnII6O4[(C6H5)3CPO3]6]}-
→ {Mn6}
2296.1
{HMnIII2MnII4O2[(C6H5)3CPO3]6]}-
→ {Mn6}
UV-Vis spectroscopy
The UV-Vis absorption spectrum of 16 recorded in CH3CN is presented in Figure
3.79. The intense absorption band observed at ca. 263 nm ( = 87000 L mol-1 cm-1) can be
attributed to π − π* transitions of the phosphonate ligands, while the weak band observed at
ca. 487 nm ( = 900 L mol-1 cm-1) involves d – d transitions.49-52
Absorbance (a.u.)
0.90
0.85
0.12
0.80
0.10
0.08
0.75
0.06
0.70
0.04
0.65
400
450
500
550
600
0.60
0.55
0.50
300
400
500
600
700
800
Wavelength (nm)
Figure 3.79 – UV-Vis spectrum of a 10-5 M solution of 16 in CH3CN. Inset: A
section of the UV-Vis spectrum of a 10-4 M solution of 16 in CH3CN.
204
Chapter 3 – Polynuclear Manganese Coordination Complexes
Table 3.30 − Crystal data and structural refinement parameters for 16.
Compound 16
Empirical formula a
Molecular mass/g mol
C118H112Cl2Mn6N2O26P6
-1 a
Crystal colour/shape
3
Red brown / rectangular block
Crystal size/mm
0.60×0.20×0.10
Crystal system
Monoclinic
Space group
C 2/c
a/ Å
31.374(6)
b/ Å
14.709(3)
c/ Å
28.425(6)
/º
90
/º
93.46(3)
/º
90
3
V/ Å
13094(5)
Z
1
Temperature (K)
150(2)
-3
Density/Mg m
1.336
-1
Absorp. coef./mm
0.743
F(000)
5385
2
50
max/º
Reflections collected
49746
Independent reflections
11477 [R(int)=0.0473]
Data / restraints / parameters
11477/ 0 / 743
2
a
2560.52
S on F
1.081
R1, wR2 [I>2 (I)]
0.0804, 0.2205
R1, wR2 (all data)
0.0832, 0.2233
Largest diff. peak and hole/e.Å-3
1.834 and -0.714
Excluding solvate molecules
205
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.4 ESI-MS STUDIES OF COMPLEX REACTION MIXTURES TO
INVESTIGATE THE FORMATION OF POLYNUCLEAR MANGANESE
COORDINATION COMPLEXES
3.4.1
Investigation of the reaction system that led to the formation of the
{Mn15} complex in 10
The comproportionation reaction between MnCl2·4H2O and KMnO4, in MeOH, in
the presence of phenylphosphonic acid resulted in a deep brown solution, from which redbrown crystals of 10, [ nΙΙΙ15(μ2-H2 )2(C
3
)16(C6
5P
3)20]Cl5·22CH3OH·8H2O,
separate within a time period of approximately one week (Figure 3.80).
Figure 3.80 – Crystal structure of the pentadecanuclear manganese complex in 10. Colour code:
MnIII blue, P purple, O red, C grey, Cl green (hydrogen atoms have been omitted for clarity).
The initial reaction mixture was examined by ESI-MS and a rather complex mass
spectrum was obtained (Figure 3.82). The spectrum revealed to be very interesting as three
different species, {Mn7}, {Mn8} and {Mn13}, could be identified in the high molecular
mass region of the spectrum (Table 3.31).
The most intense signal in the high molecular mass region of the spectrum is
centered
at
m/z
=
1684.3
a.m.u.
and
corresponds
to
a
{Mn7}
species,
{H17[MnIV6MnIIIO15(C6H5PO3)6]Cl3}-. The signals centered at m/z = 1016.7 a.m.u. and m/z
= 2034.3 a.m.u. can be attributed to a {Mn8} species, the {H5[MnIII8O5(C6H5PO3)9]Cl3}2and {H4[MnIV2MnIII6O5(C6H5PO3)9]Cl3}-, respectively. The fourth isotopic envelope that
206
Chapter 3 – Polynuclear Manganese Coordination Complexes
can be assigned in the mass spectrum of the reaction mixture of 10 is centered at m/z =
2122.0 a.m.u. and corresponds to the formula of a tridecanuclear manganese complex,
{H[MnIV8MnIII5O15(CH3O)4(C6H5PO3)6]Cl3}-. The {Mn15} complex itself could not be
identified in the mass spectrum of the reaction mixture that led to the crystallisation of 10
as there is no signal present in the spectrum above 3000 a.m.u. The species could also not
be identified in the low molecular mass region of the spectrum as this part of the spectrum
contains crowded large number of signals due to fragmentation and arising from multiple
charged species. All species identified in the mass spectrum were modelled and good fits
between the experimental and simulated isotopic envelopes confirm the assignments
(Figure 3.82).
Table 3.31 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 10.
m/z
Species attributed
1016.7
{H5[MnIII8O5(C6H5PO3)9]Cl3}2IV
III
→ {Mn8}
-
Reaction
1684.3
{H17[Mn 6Mn O15(C6H5PO3)6]Cl3}
→ {Mn7}
mixture of 10
2034.3
{H4[MnIV2MnIII6O5(C6H5PO3)9]Cl3}-
→ {Mn8}
2122.0
{H[MnIV8MnIII5O15(CH3O)4(C6H5PO3)6]Cl3}-
→ {Mn13}
a)
b)
{Mn7}
{Mn8}
Figure 3.81 – Representation of structural motifs that agree with the constitutional assignment: (a)
{Mn7} and (b) {Mn8} species identified in the mass spectrum of the reaction mixture that led to the
crystallisation of 10. Colour code: Mn blue, P purple, C grey, O and Cl light grey.
207
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.82 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation
of 10 (24 h after preparation). Comparison of the experimental isotopic envelopes (black spectrum)
with simulated patterns (red spectrum) for {H5[MnIII8O5(C6H5PO3)9]Cl3}2- centered at m/z = 1016.7
a.m.u.,
{H17[MnIV6MnIIIO15(C6H5PO3)6]Cl3}centered
at
m/z
=
1684.3
a.m.u.,
IV
{H4[Mn 2MnIII6O5(C6H5PO3)9]Cl3}centered
at
m/z
=
2034.3
a.m.u.
and
{H[MnIV8MnIII5O15(CH3O)4(C6H5PO3)6]Cl3}- centered at m/z = 2122.0 a.m.u. (cone voltage: 30 V).
208
−
−
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.83 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of
10 at different cone voltages: 10V, 20V, 30V, 50V and 75V, respectively.
In an effort to confirm that the assigned species coexist in solution and that they are
not a result of a primary processes occurring in the spraying chamber, we set out to
investigate the effect of cone voltage (CV) variations on the obtained ESI-MS spectrum
(Figure 3.83). Usually the species observed in the gas phase are closely related to the
actual species in solution, and lower CV values favour preservation of these species. In
addition, it was observed in the literature that the ionic abundances of the fragment ions
209
Chapter 3 – Polynuclear Manganese Coordination Complexes
produced in the gas phase grow with increasing CV values, while the abundance of the
parent ions decreases with increasing CV values.67, 68
The signal attributed to the {Mn7} species in the mass spectrum of the reaction
mixture that led to the crystallisation of 10 is independent on CV variations up to 75 V
(Figure 3.83), and the relative ionic abundance of the species start to decrease in intensity
at CV greater than 100 V. The signal attributed to the {Mn8} species shows similar
behaviour and the relative ionic abundance of this species starts to decrease in intensity at
75 V, due to the induced defragmentation processes. These suggests that {Mn7}, and
{Mn8} species exist in solution, and we believe that these species represent building units
of the pentadecanuclear manganese complex which crystallises out from this reaction
mixture. The most probable structural arrangement for {Mn7} and {Mn8} species are
represented in Figure 3.81. These structural motifs that agree with the constitutional
assignment correspond to subunits of the {Mn15} cluster, and are further supported by the
experimental observation of the same {Mn7} species in the mass spectra of pristine crystals
of 10 dissolved in CH3CN, DMF and DMSO. As earlier mentioned in section 3.3.1 of this
chapter the {Mn15} cluster is not stable in solution and break into two {Mn7} units as a
consequence of the labile methanol groups from the JT sites of the central Mn(8) ion, (see
section 3.3.1, Figure 3.3).
Another remarkable feature observed in the mass spectrum of the reaction mixture
of 10 are signals that are undoubtably attributable to a {Mn13} species. Surprisingly CV
variations experiments show that the ionic abundance of this species increases when CV
values were increased from 10 to 75 V. This behaviour suggests that this species could
indeed form in the spaying chamber and may not be structurally related to the {Mn15}
structure in 10. Alternatively, it could be related to the {Mn13} clusters that crystallise form
other reaction mixtures, such those of compounds 11, 12 and 13. Considerable
fragmentation can be observed at higher cone voltages. The {Mn7}, {Mn8} and {Mn13}
species cannot be identified at CV greater than 150 V, suggesting that the species no longer
retain their integrity under these harsh ionisation conditions.
210
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.4.2
Investigation of the reaction system that led to the formation of {Mn13}
complexes observed in 11-13
Using a similar synthetic procedure as for 10 and adding pyridine as an organic
base, a dark brown solution was obtained. The ESI-MS spectrum of this reaction mixture is
much simpler (Figure 3.85) than the original mixture that led to the crystallisation of 10. It
reveals the presence of only one major isotopic envelope in the high mass region centered
at m/z = 2594.0 a.m.u. This signal was assigned to the phenylphosphonic acid-stabilised
{H2[MnIV4MnIII9O11(C6H5PO3)10]Cl4}- species, which corresponds to the tridecanuclear
manganese
C
3
)4(C6
complex
5P
3)10(C5
characterised
5
in
11,
[ n
13( 4-
)2( 3- )4( 2- H)2( 2-
)5Cl]·3H2O, that indeed crystallizes out from this reaction
mixture (Table 3.32).
Figure 3.84 – Crystal structure of the tridecanuclear manganese complex in 11. Colour code: MnIII
blue, P purple, O red, Cl green, C grey (hydrogen atoms have been omitted for clarity).
Table 3.32 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 11.
Reaction
mixture of 11
m/z
Species attributed
2594.0
{H2[MnIV4MnIII9O11(C6H5PO3)10]Cl4}-
→ {Mn13}
211
Relative ionic abundance
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.85 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation
of 11 (24 h after preparation). Inset: Comparison of the experimental isotopic envelopes (black
spectrum) with simulated patterns (red spectrum) for {H2[MnIV4MnIII9O11(C6H5PO3)10]Cl4}- centered
at m/z = 2594.0 a.m.u. (cone voltage: 30 V).
The ESI-MS technique proved to be very efficient in identifying new polynuclear
manganese species that form under certain conditions in the reaction mixture, and therefore
we decided to exploit the analytic screening technique further and investigate the influence
of different ligands on the formation of manganese coordination clusters.
When benzylphosphonic acid was used instead of phenylphosphonic acid under the
same reaction conditions that led to 11, we obtained a reaction mixture from which
compound
12,
[ n
13( 4-
)2( 3- )4( 2- H)2( 2-C
3
)4(C6
5C
2P
3)10(C5
5
)6]Cl
·5H2O separates over a time period of four days.
Table 3.33 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 12.
m/z
1086.6
Reaction
mixture of 12
1348.6
2138.3
2212.3
2699.1
Species attributed
{H2[MnIV2MnIII11
{H[MnIV2MnIII11
13(C
10(C
)4(C6
3
3
)4(C6
5C
5C
2P
2P
23)6]Cl3}
→ {Mn13}
23)10]}
→ {Mn13}
{H9[MnIV2MnIII11O18(C6H5CH2PO3)6]Cl3}{H7[MnIV2MnIII11
{H[MnIV3MnIII10
15(C
10(C
3
3
)4(C6
)4(C6
5C
5C
2P
2P
→ {Mn13}
3)6]Cl3}
→ {Mn13}
3)10]}
→ {Mn13}
212
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.86 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of
12 (24 h after preparation). Comparison of the experimental isotopic envelopes (black spectrum) with
simulated patterns (red spectrum) for {H2[ nIV2MnIII11 13(CH3 )4(C6 5CH2P 3)6]Cl3}2- centered at
m/z = 1086.6 a.m.u., {H[ nIV2MnIII11 10(CH3 )4(C6 5CH2P 3)10]}2- centered at m/z = 1348.6 a.m.u.,
{H9[MnIV2MnIII11O18(C6H5CH2PO3)6]Cl3}centered
at
m/z
=
2138.3
a.m.u.,
{H7[ nIV2MnIII11 15(CH3 )4(C6 5CH2P 3)6]Cl3}- centered at m/z = 2212.3 a.m.u. and
{H[ nIV3MnIII10 10(CH3 )4(C6 5CH2P 3)10]}- centered at m/z = 2699.1 a.m.u. (cone voltage: 30 V).
The ESI-MS spectrum of this reaction mixture is presented in Figure 3.86 showing
an intense peak centered at m/z = 2138.3 a.m.u. which corresponds to the parent
{H9[MnIV2MnIII11O18(C6H5CH2PO3)6]Cl3}- species. Two other singly charged and two
other doubly charged {Mn13} species stabilized by a different number of phosphonate
ligands, water molecules and chloride ions could be identified in the high mass region of
the spectrum (Table 3.33). We successfully simulated the isotopic envelopes for these
213
Chapter 3 – Polynuclear Manganese Coordination Complexes
identified species and good fits between experimental and theoretical data further
substantiate our assignments (Figure 3.86).
The same reaction conditions as for 12 but replacing the pyridine ligands with 4-(3phenylpropyl)pyridine gave a dark brown solution from which compound 13, [ n
)2( 3- )4( 2- H)2( 2-C
3
)4(C6
5C
2P
3)10(C6
5-C3
6-C5
4
13( 4-
)6]Cl·5H2O,
was
obtained. The mass spectrum of this reaction mixture recorded in negative mode is simple
(Figure 3.87). It reveals the presence of an intense signal corresponding to a singly charged
{Mn13} species and a much lower intensity signal arising from a doubly charged species.
The ESI-MS peak assignment for the reaction mixture of 13 is presented in Table 3.34.
Table 3.34 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 13.
m/z
1086.6
2138.3
{H2[MnIV2MnIII11
13(C
3
)4(C6
5C
2P
23)6]Cl3}
{H9[MnIV2MnIII11O18(C6H5CH2PO3)6]Cl3}-
→ {Mn13}
→ {Mn13}
Relative ionic abundance
Reaction
mixture of 13
Species attributed
Figure 3.87 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation
of 13 (24 h after preparation). Inset: Comparison of the experimental isotopic envelopes (black
spectrum) with simulated patterns (red spectrum) for {H2[ nIV2MnIII11 13(CH3 )4(C6 5CH2P 3)6]
Cl3}2- centered at m/z = 1086.6 a.m.u. and {H9[MnIV2MnIII11O18(C6H5CH2PO3)6]Cl3}- centered at m/z
= 2138.3 a.m.u. (cone voltage: 30 V).
214
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.4.3
Investigation of the reaction system that led to the formation of {Mn12}
complexes observed in 14-15
Preliminary studies on the coordination clusters 11 – 13 demonstrate that the
ligands in apical position of the MnIII Jahn Teller sites are labile and can be successively
replaced. This observation prompted us to explore if the tridecanuclear manganese clusters
could be connected using bifunctional pyridine ligands. Therefore, 4,4`-bipyridine was
employed as an organic base in a similar reaction to that used for the synthesis of the
tridecanuclear manganese complexes. However, this approach did not result in the linkage
of the tridecanuclear manganese clusters. However, under these conditions a {Mn12}
coordination cluster was obtained instead. The ESI-MS spectrum of the reaction mixture
that led to the crystallisation of 14, K(H2O)4[MnIII12( 3-O)6(CH3OH)6(C6H5CH2PO3H)7
(C6H5CH2PO3)8]Cl2·3CH3OH·4H2O (Figure 3.88), is relatively simple. Nevertheless, the
resulting spectrum is interesting, as we were able to identify three different species that
exist in the same reaction mixture: a {Mn7}, a {Mn12} and a {Mn13} species (Table 3.35,
Figure 3.89).
Figure 3.88 – Crystal structure of the dodecanuclear manganese complex in 14. Colour code: MnIII
blue, P purple, O red, C grey, Cl green, K yellow (hydrogen atoms have been omitted for clarity).
The most intense isotopic envelope in the high molecular mass region of the
spectrum is centered at m/z = 1796.4 a.m.u. and corresponds to the {Mn7} species
{H29[MnIII3MnII4O16(C6H5CH2PO3)6]Cl3}-. Two other significantly less intensive signals
centered at m/z = 915.7 a.m.u. and m/z = 1920.1 a.m.u. could be observed in the high
molecular mass region of the spectrum. These signals were attributed to a {Mn12} species,
215
Chapter 3 – Polynuclear Manganese Coordination Complexes
{H6[MnIII4MnII8O11(C6H5CH2PO3)5]Cl4}2-
and
a
{Mn13}
species,
{H6[MnIII7MnII6O13(C6H5CH2PO3)5]Cl4}-, respectively. The former species relates to the
cluster of 14 that crystallises under these conditions, the latter is expected to be related to
the cluster core structures of 11-13.
Table 3.35 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 14.
Reaction
Species attributed
915.7
{H6[MnIII4MnII8O11(C6H5CH2PO3)5]Cl4}2-
→ {Mn12}
1796.4
{H29[MnIII3MnII4O16(C6H5CH2PO3)6]Cl3}-
→ {Mn7}
1920.1
{H6[MnIII7MnII6O13(C6H5CH2PO3)5]Cl4}-
→ {Mn13}
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
mixture of 14
m/z
Figure 3.89 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation
of 14 (24 h after preparation). Comparison of the experimental isotopic envelopes (black spectrum)
with simulated patterns (red spectrum) for {H6[MnIII4MnII8O11(C6H5CH2PO3)5]Cl4}2- centered at m/z
= 915.7 a.m.u., {H29[MnIII3MnII4O16(C6H5CH2PO3)6]Cl3}- centered at m/z = 1796.4 a.m.u. and
{H6[MnIII7MnII6O13(C6H5CH2PO3)5]Cl4}- centered at m/z = 1920.1 a.m.u. (cone voltage: 30 V).
A summary of the ESI-MS assignment for the reaction mixture that led to the
crystallisation of 14 is presented in Table 3.35, and a comparison of the experimental
isotopic envelopes together with the simulated patterns for all identified species can be
found in Figure 3.89.
216
−
−
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.90 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of
14 at different cone voltages: 10V, 20V, 30V, 50V and 75V, respectively.
Cone voltage variations experiments show that the relative ionic abundance of
{Mn7}, {Mn12} and {Mn13} species decreases with increasing CV values. These
experimental observations indicating that the assigned species form in solution in the same
reaction mixture (Figure 3.90). Furthermore, we believe that the {Mn7} species is
structurally related to the {Mn13} core structure, adopting a hexagonal brucite arrangement
as shown in Figure 3.91.
217
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.91 – Representation of a structural motif that agrees with the constitutional assignment for the
{Mn7} species identified in the mass spectrum of the reaction mixture that led to the crystallisation of
14. Colour code: Mn blue, P purple, C grey, O light grey.
15,
Compound
K(H2O)4[MnIII12( 3-O)6(CH3OH)6(C6H5CH2PO3H)7
(C6H5CH2PO3)8]Br2·2CH3OH·2H2O was obtained by replacing the MnCl2·4H2O with
MnBr2·4H2O in the reaction mixture of 14 in order to investigate the role of the halide ions
as a potential templating agent.
The ESI-MS study on the reaction mixture that led to the crystallisation of 15 is
essentially analogous to that of 14. The mass spectrum displays characteristic signals at m/z
= 740.0 a.m.u. and m/z = 1930.3 a.m.u. which correspond to {Mn3} and (Mn7} species,
{HMnIV3O4(C6H5CH2PO3)3]}- and {H29[MnIII3MnII4O16(C6H5CH2PO3)6]Br3}-, respectively.
Two other significantly less intense isotopic envelopes centered at m/z = 2032.3 a.m.u. and
m/z
= 2062.9
a.m.u.
were
attributed
to
the {Mn12}
and
{Mn13}
species,
{H[MnIII12O12(C6H5CH2PO3)6]Br2}- and {H3[MnIII6MnII7O11(C6H5CH2PO3)5]Br4}- (Table
3.36, Figure 3.92).
Table 3.36 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 15.
m/z
Species attributed
740.0
{H[MnIV3O4(C6H5CH2PO3)3]}-
→ {Mn3}
Reaction
1930.3
{H29[MnIII3MnII4O16(C6H5CH2PO3)6]Br3}-
→ {Mn7}
mixture of 15
2032.3
{H[MnIII12O12(C6H5CH2PO3)6]Br2}-
→ {Mn12}
2062.9
{H3[MnIII6MnII7O11(C6H5CH2PO3)5]Br4}-
→ {Mn13}
218
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Relative ionic abundance
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.92 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation
of 15 (24 h after preparation). Comparison of the experimental isotopic envelopes (black spectrum)
with simulated patterns (red spectrum) for {H[MnIV3O4(C6H5CH2PO3)3]}- centered at m/z = 740.0
a.m.u., {H29[MnIII3MnII4O16(C6H5CH2PO3)6]Br3}- centered at m/z = 1930.3 a.m.u.,
{H[MnIII12O12(C6H5CH2PO3)6]Br2}centered
at
m/z
=
2032.3
a.m.u.
and
{H3[MnIII6MnII7O11(C6H5CH2PO3)5]Br4}- centered at m/z = 2062.9 a.m.u. (cone voltage: 30 V).
219
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.4.4
Investigation of the reaction system that led to the formation of the
{Mn6} complex in 16
When the reaction mixture of compound 16, (H3O)4{MnIII2MnII4( 4-O)2
(H2O)2(CH3CN)2[(C6H5)3CPO3]6}Cl2·2CH3CN·4H2O, was examined by ESI-MS we
obtained a spectrum in which one major isotopic isotope could be observed. Unfortunately,
we were not able to assign this signal, but we succeeded to identify a {Mn6} species which
relates to a considerably less intense signal centered at m/z = 2296.1 a.m.u. The signal
corresponds to the hexanuclear complex in 16 which crystallizes from this reaction mixture
(Table 3.37, Figure 3.94).
Figure 3.93 – Crystal structure of the hexanuclear manganese complex in 16. Colour code: MnIII blue,
MnII cyan, P purple, O red, C grey, N dark blue, Cl green (hydrogen atoms have been omitted for clarity).
Table 3.37 – ESI-MS assignment for the reaction mixture that led to the crystallisation of 16.
Reaction
mixture of 16
m/z
Species attributed
2296.1
{HMnIII2MnII4O2[(C6H5) 3CPO3]6}-
→ {Mn6}
220
Relative ionic abundance
Chapter 3 – Polynuclear Manganese Coordination Complexes
Figure 3.94 – Negative-mode ESI-MS spectra for the reaction mixture that led to the crystallisation of
16 (24 h after preparation). Inset: Comparison of the experimental isotopic envelopes (black spectrum)
with simulated patterns (red spectrum) for {HMnIII2MnII4O2[(C6H5)3CPO3]6}- centered at m/z = 2296.1
a.m.u. (cone voltage: 30 V).
221
Chapter 3 – Polynuclear Manganese Coordination Complexes
3.5 CONCLUSION AND FUTURE WORK
A facile synthetic approach to polynuclear manganese coordination complexes that
involves comproportionation reactions between MnII and MnVII salts in the presence of
organophosphonate ligands was developed. This synthetic approach led to the formation of
three types of unprecedented manganese cluster compounds in 10, 14 and 15, three
structurally related compounds 11, 12 and 13 that belong to a family of {Mn13} complexes
containing a metal-centered cuboctahedral core structures and a hexanuclear, mixed valent
coordination complex (16).
We found that small changes to the reaction conditions result in drastic changes of
the cluster nuclearity and topology. The nature of the phosphonate ligand and organic base
are most influencial and determine the product formation. Our approach allowed us to
utilize organophosphonate as organic, stabilizing ligands. In comparison to carboxylate
ligands, there are only a few polynuclear manganese complexes stabilized by
phenylphosphonic acid ligands reported in the literature, which makes the here reported
class of compounds interesting. 10 contains a remarkable {Mn15} oxo-cluster with
characteristic dunbell-type arrangement of its sub-units. Compound 11 was the result of
adding pyridine to the reaction mixture of 10, while compound 12 was obtained by
replacing the phenylphosphonic acid ligands in 11 with benzylphosphonic acid ligands.
We noticed that the {Mn13} clusters contain active sites provided by ligands that
reside in tetragonally elongated positions of the Jahn-Teller distorted, octahedral
coordination environments. The kinetic lability of these sites gives rise to ligand exchange
reactions and functionalisation of the compounds (e.g. introduction of organic moieties in
these positions). In consecutive experiments, we further aimed to exploit this feature and
connect the {Mn13} clusters using bi-functional ligands, e.g. bipyridine-type ligands,
however, our attempts were unsuccessful.
Studies of the magnetic properties reveal dominant antiferromagnetic interactions
between the spin centres and a ground state spin values of ST = 0 for 10 and 16, ST = 3 for
14 and 15, ST = 6 for 11 and 13 and ST = 8 for 12. The ac susceptibility studies in zero dc
field for compounds 11-13 reveal slow relaxation of the magnetisation highlighted by
appearance of out-of-phase signals below 4 K (for frequency around 10000 Hz), indicating
that these compounds are SMMs.
222
Chapter 3 – Polynuclear Manganese Coordination Complexes
The thermal stability of the compounds was investigated by thermogravimeric
analysis. Solution stability studies for 10 – 16 were performed using ESI-MS and UV-Vis
spectroscopy. In particular, the ESI-MS technique proved to be highly valuble to
charactterise the stability of the complexes. It is surprising that this technique has not
previously been utilized to investigate the solution behavior of larger, polynuclear Mn
complexes. Indeed we also applied ESI-MS to screen reaction mixtures prior crystallisation
attempts of 10-16. These approaches aimed to investigate the real-time growth reactions of
polynuclear Mn complexes and analyses of the magnetic properties of 10-16.
The {Mn7} and the {Mn13} species were observed in the ESI-MS spectra of the
reaction mixture that led to the crystallisation of 10, 14 and 15. The {Mn7} species may be
a brucite-type, disk-like building unit which appears in several clusters reported in the
literature and in tridecanuclear clusters in 11 - 13.69-72 The {Mn13} species identified by
ESI-MS relates to the latter family of tridecanuclear manganese complexes which contain a
metal-centered cuboctahedral topology. Another noticeable feature observed in the mass
spectra of the reaction mixture that led to the crystallisation of 10, 14 and 15 is that the
signals attributed to the {Mn7} species appear as the most intense signals in the high mass
region of the spectra. This observation in combination with cone voltage variations suggest
that this {Mn7} species is the predominant high molecular mass species present in these
reaction mixtures. Consequently, one expects several cluster topologies to incorporate this
building unit and forming larger assemblies involving for instance hydroxo- and oxogroups or donor atoms of multidentate ligands. This observation is substantiated by the
crystal structures of 11-13 and several structures reported in the literature that contain the
said brucite-type core.12,
48, 53-55
In addition the identification of trinuclear Mn species
support this proposal. The {Mn12} species identified in the reaction mixture that led to the
crystallisation of 14 and 15 seems to be in equilibrium with the {Mn13} species. This
equilibrium can be manipulated in order to crystallise either the {Mn12} or the {Mn13}
species by slight changes of the reaction conditions. In the case of the pentadecanuclear
manganese complex in 10, we found that the {Mn15} cluster is not stable in solution since
this species could not be identified by ESI-MS studies. Moreover, we were able to identify
building units of the {Mn15} cluster as {Mn7} and {Mn8} species present in the reaction
mixture that led to the crystallisation of 10. The fact that the {Mn7} species was also
present in the mass spectra of pristine crystals of 10 dissolved in CH3CN, DMF and DMSO
suggest that the {Mn15} cluster break into two {Mn7} units, most likely due to the labile
methanol groups from the JT sites of the central Mn ion.
223
Chapter 3 – Polynuclear Manganese Coordination Complexes
A summary of the ESI-MS assignment for the reaction mixtures of the {Mn15},
{Mn13} and {Mn12} systems, highlighting related structural motifs and possible building
units, is presented in Figure 3.95.
Future work aims to exploit the kinetic lability of the Jahn-Teller sites of the
{Mn13} clusters for further functionalisation of these compounds. It could also be possible
to connect the {Mn13} clusters using bi-functional ligands, which could lead to the
formation of porous structures. Additionally, ESI-MS technique will be further exploited to
characterise new polynuclear manganese coordination complexes.
The extensive research in the field of molecular magnetism is not only concerned
with the understanding of the magnetic properties of these materials, and providing
structure-property relationships but also with the real need for the development of rational
synthetic approaches to complex polynuclear species.
Currently it is rather challenging to predict what kind of cluster topologies will be
obtained and which would be desired to achieve exciting magnetic properties. Under this
prevue mass spectrometry analyses might provide a step towards more rational approaches
to more predictable, structurally related materials that will help to understand and improve
the properties of molecule magnetic materials.
224
Chapter 3 – Polynuclear Manganese Coordination Complexes
Reaction mixture that led to
the crystallisation of the
following species; X-Ray
crystal structure
ESI-MS assigned
species
Structural motifs associated with the ESIMS assignment
{Mn7}
{Mn8}
{Mn15}
{Mn13}
{Mn13}
{Mn13}
{Mn3}
{Mn7}
{Mn12}
{Mn13}
{Mn12}
Figure 3.95 – Summary of the ESI-MS assignment for the reaction mixture of the {Mn15}, {Mn13} and
{Mn12} systems, showing some related structures and possible building units.
225
Chapter 3 – Polynuclear Manganese Coordination Complexes
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
Y. Umena, K. Kawakami, J.-R. Shen and N. Kamiya, Nature, 2011, 473, 55-60.
J. S. Kanady, E. Y. Tsui, M. W. Day and T. Agapie, Science, 2011, 333, 733-736.
A. W. Rutherford and A. Boussac, Science, 2004, 303, 1782-1784.
R. K. Hocking, R. Brimblecombe, L.-Y. S. Chang, Archana, M. H. Cheah, C.
Glover, W. H. Casey and L. Spiccia, Nature Chemistry, 2011, 6, 461-466.
K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber and S. Iwata, Science, 2004,
303, 1831-1838.
G. E. Kostakis, A. M. Ako and A. K. Powell, Chemical Society Reviews, 2010, 39,
2238-2271.
T. C. Stamatatos, V. Nastopoulos, A. J. Tasiopoulos, E. E. Moushi, W.
Wernsdorfer, G. Christou and S. P. Perlepes, Inorganic Chemistry, 2008, 47,
10081-10089.
T. C. Stamatatos, K. A. Abboud, W. Wernsdorfer and G. Christou, Angewandte
Chemie International Edition, 2007, 46, 884-888.
M. Murugesu, M. Habrych, W. Wernsdorfer, K. A. Abboud and G. Christou,
Journal of the American Chemical Society, 2004, 126, 4766-4767.
J. R. Long, Chemistry of Nanostructured Materials, 2003, 291-315.
G. Christou, Polyhedron, 2005, 24, 2065-2075.
Y.-S. Ma, Y. Song, Y.-Z. Li and L.-M. Zheng, Inorganic Chemistry, 2007, 46,
5459-5461.
A. Clearfield and K. Demadis, Royal Society of Chemistry, 2011.
M. Shanmugam, M. Shanmugam, G. Chastanet, R. Sessoli, T. Mallah, W.
Wernsdorfer and R. E. P. Winpenny, Journal of Materials Chemistry, 2006, 16,
2576-2578.
M. Wang, C. Ma, H. Wen and C. Chen, Dalton Transactions, 2009, 994-1003.
T. Glaser, Chemical Communications, 2011, 47, 116-130.
R. Jirásko and M. Hol apek, Mass Spectrometry Reviews, 2011, 30, 1013-1036.
J. R. Johansson and B. Norden, Proceedings of the National Academy of Sciences
of the United States of America, 2012, 109, 2186-2187.
D. Schaumlöffel and A. Tholey, Analytical and Bioanalytical Chemistry, 2011,
400, 1645-1652.
C. I. Onet, L. Zhang, R. Clérac, J. B. Jean-Denis, M. Feeney, T. McCabe and W.
Schmitt, Inorganic Chemistry, 2010, 50, 604-613.
E. F. Wilson, H. Abbas, B. J. Duncombe, C. Streb, D.-L. Long and L. Cronin,
Journal of the American Chemical Society, 2008, 130, 13876-13884.
E. F. Wilson, H. N. Miras, M. H. Rosnes and L. Cronin, Angewandte Chemie
International Edition, 2011, 50, 3720-3724.
L. Vila-Nadal, S. G. Mitchell, A. Rodriguez-Fortea, H. N. Miras, L. Cronin and J.
M. Poblet, Physical Chemistry Chemical Physics, 2011, 13, 20136-20145.
L. Vilà-Nadal, A. Rodríguez-Fortea, L.-K. Yan, E. F. Wilson, L. Cronin and J. M.
Poblet, Angewandte Chemie International Edition, 2009, 48, 5452-5456.
C. Ritchie, E. M. Burkholder, D.-L. Long, D. Adam, P. Kogerler and L. Cronin,
Chemical Communications, 2007, 468-470.
D. R. Boyd and G. Chignell, Journal of the Chemical Society, 1923, 123, 813-817.
D. R. Boyd and F. J. Smith, Journal of the Chemical Society, Transactions, 1924,
125, 1477-1480.
V. Chandrasekhar and P. Sasikumar, Dalton Transactions, 2008, 6475-6480.
226
Chapter 3 – Polynuclear Manganese Coordination Complexes
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
D. I. Brown, Oxford University Press, 2002, 26.
I. B. Bersuker, Chambridge University Press, 2006, 616.
B.-H. Ye, I. D. Williams and X.-Y. Li, Journal of Inorganic Biochemistry, 2002,
92, 128-136.
S. B. Yu, S. J. Lippard, I. Shweky and A. Bino, Inorganic Chemistry, 1992, 31,
3502-3504.
D. Coucouvanis, R. A. Reynolds and W. R. Dunham, Journal of the American
Chemical Society, 1995, 117, 7570-7571.
B.-H. Ye, T. Mak, I. D. Williams and X.-y. Li, Chemical Communications, 1997,
1813-1814.
C. A. Hunter and J. K. M. Sanders, Journal of the American Chemical Society,
1990, 112, 5525-5534.
R. Lei, X. Chai, H. Mei, H. Zhang, Y. Chen and Y. Sun, Journal of Solid State
Chemistry, 2010, 183, 1510-1520.
M. M. Gómez-Alcántara, A. Cabeza, M. A. G. Aranda, A. Guagliardi, J. G. Mao
and A. Clearfield, Solid State Sciences, 2004, 6, 479-487.
N. Stock and T. Bein, Journal of Solid State Chemistry, 2002, 167, 330-336.
A. Cabeza, M. A. G. Aranda, S. Bruque, D. M. Poojary and A. Clearfield,
Materials Research Bulletin, 1998, 33, 1265-1274.
N. Stock and T. Bein, Journal of Materials Chemistry, 2005, 15.
O. Kahn, VCH: New York, 1993, 380.
R. K. Seidler-Egdal, A. Nielsen, A. D. Bond, M. J. Bjerrum and C. J. McKenzie,
Dalton Transactions, 2011, 40.
J. Yoon, M. S. Seo, Y. Kim, S.-J. Kim, S. Yoon, H. G. Jang and W. Nam, Bulletin
of the Korean Chemical Society, 2009, 30, 679-682.
J. A. Lessa, A. Horn Jr, C. B. Pinheiro, L. L. Farah, M. N. Eberlin, M. Benassi, R.
R. Catharino and C. Fernandes, Inorganic Chemistry Communications, 2007, 10,
863-866.
W.-Y. Hsieh and S. Liu, Inorganic Chemistry, 2006, 45, 5034-5043.
D. Ruiz-Molina, P. Gerbier, E. Rumberger, D. B. Amabilino, I. A. Guzei, K.
Folting, J. C. Huffman, A. Rheingold, G. Christou, J. Veciana and D. N.
Hendrickson, Journal of Materials Chemistry, 2002, 12.
V. Pavlishchuk, F. Birkelbach, T. Weyhermüller, K. Wieghardt and P. Chaudhuri,
Inorganic Chemistry, 2002, 41, 4405-4416.
L. Zhang, R. Clérac, P. Heijboer and W. Schmitt, Angewandte Chemie
International Edition, 2012, 51, 3007-3011.
A. Eichhöfer, O. Hampe, S. Lebedkin and F. Weigend, Inorganic Chemistry, 2010,
49, 7331-7339.
T. G. Carrell, E. Bourles, M. Lin and G. C. Dismukes, Inorganic Chemistry, 2003,
42, 2849-2858.
W. S. Kijlstra, E. K. Poels, A. Bliek, B. M. Weckhuysen and R. A. Schoonheydt,
The Journal of Physical Chemistry B, 1997, 101, 309-316.
S. Kunju and G. Krishnan, PHI Learning Private Limited, 2010.
C. Lampropoulos, C. Koo, S. O. Hill, K. Abboud and G. Christou, Inorganic
Chemistry, 2008, 47, 11180-11190.
A. Ferguson, K. Thomson, A. Parkin, P. Cooper, C. J. Milios, E. K. Brechin and M.
Murrie, Dalton Transactions, 2007.
M. Wang, C. Ma and C. Chen, Dalton Transactions, 2008.
P. J. Larkin, Elsevier Inc, 2011.
S. Bayari and S. Yurdakul, Spectroscopy Letters, 2000, 33, 475-483.
227
Chapter 3 – Polynuclear Manganese Coordination Complexes
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
T. C. Stamatatos, D. Foguet-Albiol, S. P. Perlepes, C. P. Raptopoulou, A. Terzis, C.
S. Patrickios, G. Christou and A. J. Tasiopoulos, Polyhedron, 2006, 25, 1737-1746.
A. R. Schake, J. B. Vincent, Q. Li, P. D. W. Boyd, K. Folting, J. C. Huffman, D. N.
Hendrickson and G. Christou, Inorganic Chemistry, 1989, 28, 1915-1923.
V. Ovcharenko, E. Fursova, G. Romanenko and V. Ikorskii, Inorganic Chemistry,
2004, 43, 3332-3334.
M. A. Kiskin, I. G. Fomina, G. G. Aleksandrov, A. A. Sidorov, V. M. Novotortsev,
Y. V. Rakitin, Z. V. Dobrokhotova, V. N. Ikorskii, Y. G. Shvedenkov, I. L.
Eremenko and I. I. Moiseev, Inorganic Chemistry Communications, 2005, 8, 89-93.
P. Karsten and J. Strahle, Acta Crystallographica Section C, 1998, 54, 1403-1406.
M. A. Halcrow, W. E. Streib, K. Folting and G. Christou, Acta Crystallographica
Section C, 1995, 51, 1263-1267.
A. R. E. Baikie, A. J. Howes, M. B. Hursthouse, A. B. Quick and P. Thornton,
Journal of the Chemical Society, Chemical Communications, 1986, 1587-1587.
R. Wang, D. Yuan, F. Jiang, L. Han, S. Gao and M. Hong, European Journal of
Inorganic Chemistry, 2006, 8, 1649-1656.
D. Liu, Q. Zhou, Y. Chen, F. Yang, Y. Yu, Z. Shi and S. Feng, Dalton
Transactions, 2010, 39, 5504-5508.
W. Henderson and J. S. McIndoe, John Wiley & Sons, 2005, 292.
V. A. Pashynska, M. V. Kosevich, H. V. d. Heuvel and M. Claeys, Rapid
Communications in Mass Spectrometry, 2006, 20, 755-763.
M. A. Bolcar, S. M. J. Aubin, K. Folting, D. N. Hendrickson and G. Christou,
Chemical Communications, 1997, 1485 - 1486.
T. Liu, B.-W. Wang, Y.-H. Chen, Z.-M. Wang and S. Gao, Zeitschrift für
anorganische und allgemeine Chemie, 2008, 634, 778-783.
T. C. Stamatatos, K. M. Poole, D. Foguet-Albiol, K. A. Abboud, T. A. O’Brien and
G. Christou, Inorganic Chemistry, 2008, 47, 6593-6595.
S.-Y. Chen, C. C. Beedle, P.-R. Gan, G.-H. Lee, S. Hill and E.-C. Yang, Inorganic
Chemistry, 2012, 51, 4448-4457.
228
Chapter 4 – Supramolecular Coordination Networks
4.
SUPRAMOLECULAR COORDINATION
NETWORKS EMPLOYING
TRIPHOSPHONATE LINKERS
229
Chapter 4 – Supramolecular Coordination Networks
4.1 INTRODUCTION
The massive increase in energy demand, the depletion of the world`s fossil fuels
combined with the undesired products of burning fossil fuels, such as carbon dioxide, has
led to a need for alternative energy sources. A promising alternative fuel which has
attracted much attention in the last few years is hydrogen, which is a clean energy carrier
having the highest energy content per unit of weight of any known fuel.1, 2 Thus, hydrogen
is shown to be the fuel of the future. But, before being used at large scale there are a few
problems that need to be solved, such as finding a safe and cost effective storage medium
for hydrogen.3-5
Considerable research efforts have been devoted to the development of innovative
materials including high surface area adsorbents such as metal-organic frameworks for
reversible hydrogen storage and carbon dioxide capture.6-8
Metal-organic frameworks (MOFs) represent a new class of porous materials with
high surface areas and tunable pore sizes, which can result in the selective uptake of small
molecules. These unique properties make MOFs interesting materials for potential
applications in catalysis, gas storage and separation.7, 9-11 The most studied class of MOFs
is represented by the carboxylate-based compounds, while the use of phosphonte ligands
for the formation of framework structures is less common. The reason for this is that many
metal phosphonates tend to form densely packed layered structures that are not porous;
growth of single crystals with phosphonates represents a real challenge as they often
precipitate rapidly as less ordered, insoluble phases; phosphonates have more possible
ligating modes compared to carboxylates, making the coordination chemistry of
phosphonates less predictable. Even though, carboxylate-based MOFs have high surface
area and uniform pore size distribution, the large scale applications are hampered by their
lack of stability. Phosphonates have shown to form stronger bonds with metal ions than
carboxylates and therefore have great potential to form robust porous solids.12-14
In this chapter we describe the synthesis and characterisation of coordination
networks employing mononuclear SBUs and triphosphonate linkers.
230
Chapter 4 – Supramolecular Coordination Networks
4.2 EXTENDED TRIPHOSPHONATE LIGANDS
The triphosphonate ligands were synthesised by a modified Michaelis-Arbuzov
reaction (Scheme 4.1, b).15,
16
The synthetic procedure involves a catalyzed
phosphonylation of aryl halides by trialkyl phosphites in the presence of 1,3diisopropylbenzene as a solvent and at high temperature. Nickel(II) salts are usually used
as catalysts.17
a)
(RO)3P
b)
(RO)3P:
+
R'
R1
(RO)2(R')P+ - O - R
+
X-
P O
NiBr2
(RO)3P+R' + X-
X
OR
RO
X
R1
NiL2
(RO)2(R')PO + RX
c)
OR
OH
OR
HO
RO
RO
P
P O
P O
Ni
R1
Ester Hydrolysis
RO
OR
P O
OR
Ni
X
R1
L
L
H+
R1
RX
R1
RO
OR
P
X = Cl, Br, I
L = P(OEt)3
L
Ni
R1
L
OR
X
L
Scheme 4.1 – a) Mechanism of the Michaelis – Arbuzov reaction for the preparation of
alkylphosphonates (R` = alkyl); b) Modified Michaelis – Arbuzov reaction for the preparation of
arylphosphonates; c) Ester hydrolysis under acidic conditions.15-17
The reaction mechanism involves in situ reduction of the nickel(II) halide to a
nickel(0) phosphite complex, followed by the formation of an aryl nickel(II) intermediate.
This intermediate is formed by an oxidative insertion of the nickel(0) phosphite complex
into the carbon-halogen bond of the aryl halide. Then, an aryl(alkoxy)phosphonium salt is
formed which rearranges to give the desired phosphonate ester. The major by-product of
the reaction is an alkyl halide. It was observed that in order to obtain better yields one
needs to remove the generated alkyl halide continuously from the reaction mixture until the
reaction is complete. The phosphonate ester is then converted to the corresponding
phosphonic acid by ester hydrolysis under acidic conditions.15, 16 The synthetic approach
used to obtain phosphonic acids is presented in Scheme 4.1, a - c.
231
Chapter 4 – Supramolecular Coordination Networks
The halogenated triarylbenzenes, used as starting materials for the preparation of
the triphosphonate ligands, were synthesised through a triple condensation reaction
according to literature procedures.18,
19
A reaction mechanism for the formation of
triarylbenzenes was proposed by A. Goel et al.20 As shown in Scheme 4.2, the protonation
of acetophenone results in the formation of intermediates (a) and (b). Then subsequent
reactions between the intermediates, and further dehydration and 6 -electrocyclisation
processes afford the desired triarylbenzene ligand.
(A)
(a)
(b)
(a)
(b)
(a)
(A)
(A)
Scheme 4.2 – Plausible reaction mechanism for the formation of triarylbenzene.20
232
Chapter 4 – Supramolecular Coordination Networks
4.2.1 Synthesis of 1,3,5-Tris(4-phosphonophenyl)benzene (P-TPB)
1,3,5-Tris(4-phosphonophenyl)benzene (P-TPB) was prepared by a modified
Arbuzov reaction using the aryl halide tris(4-bromophenyl)benzene (Br-TPB) as an
intermediate.17 The synthetic route to Br-TPB involves a triple condensation reaction using
4-bromoacetophenone as the starting material, in the presence of H2SO4 and K2S2O7
(Scheme 4.3).18
Br
Br
H2SO4, K2S2O7
3
- 3H2O
O
Br
Br
- 3EtBr 3P(OEt)3
NiBr2
PO3Et2
PO3H2
6HCl
- 6EtCl
H2O3P
PO3H2
Et2O3P
PO3Et2
Scheme 4.3 – Schematic representation for the formation of P-TPB.
Br-TPB formed in high yields (80%), while the P-TPB ligand formed in a lower
yield of 58%. The ligands were characterised by FT-IR and NMR spectroscopy, and mass
spectrometry. The 1H NMR spectra of the Br-TPB and P-TPB ligands, including the
assignment of the aromatic H-atoms of the triphenyl-benzene backbone moieties, can be
seen in Figure 4.1 and Figure 4.2. The conversion of Br-TPB into P-TPB was confirmed by
the presence of an intense absorption band at 1139 cm-1 in the IR spectrum, due to P-O
233
Chapter 4 – Supramolecular Coordination Networks
stretching vibrations. In addition the signal at 13.7 ppm in the
31
P NMR spectrum is
characteristic for the desired phosphonate ligand. A detailed description of the preparation
procedures together with the analytical data of the ligands is presented in the experimental
section of chapter 8.
3
1
Br
2
1
2
Br
3
Br
Figure 4.1 – A section of the 1H NMR (400MHz, CDCl3) spectrum showing the assignment for Br-TPB.
1
H2O
DMSO
PO3H2
2
1
3
H2O3P
2
3
PO3H2
Figure 4.2 – The 1H NMR (400MHz, CDCl3) spectrum and assignment for P-TPB.
234
Chapter 4 – Supramolecular Coordination Networks
4.2.2 Synthesis of 1,3,5-Tris(4`-phosphonobiphenyl-4-yl)benzene (PTBB)
1,3,5-Tris(4`-phosphonobiphenyl-4-yl)benzene (P-TBB) is a novel triphosphonate
ligand which was prepared by a similar method to that that led to P-TPB. In this case,
1,3,5-tris(4`-bromobiphenyl-4-yl)benzene (Br-TBB) was used as aryl halide reactant. It
was synthesised from commercially available 4-acetyl-4`-bromobiphenyl via a triple
condensation reaction.19 Br-TBB formed in about 50% yield, while in the next synthetic
step P-TBB ligand formed in a yield of 66%. The synthetic route used for the preparation
of the P-TBB ligand is presented in Scheme 4.4.
Br
Br
CF3SO3H
3
- 3H2O
O
Br
Br
- 3EtBr 3P(OEt)3
NiBr2
PO3Et2
PO3H2
6HCl
- 6EtCl
H2O3P
PO3H2
Et2O3P
PO3Et2
Scheme 4.4 – Schematic representation for the formation of P-TBB.
The Br-TBB and P-TBB ligands were characterised by FT-IR and NMR
spectroscopy, and mass spectrometry. The conversion of Br-TBB into P-TBB was
235
Chapter 4 – Supramolecular Coordination Networks
confirmed by FT-IR and
31
P NMR spectroscopy. The IR spectrum of the P-TBB ligand
displays an intense absorption band at 1137 cm-1 due to P-O stretching vibrations, while
the
31
P NMR spectrum reveals a characteristic signal at 12.6 ppm. The identity and
structure of the P-TBB ligand was further characterised by X-ray diffraction
measurements. Colourless crystals were obtained after re-crystallisation, upon dissolving
the P-TBB ligand in DMF and layering the resulting solution with CH3CN (43% yield).
The ligand crystallises in the monoclinic crystal system in the space group C 2/c.
The crystal structure of the ligand is presented in Figure 4.3. The benzene rings of the PTBB molecule are tilted with respect to each other. The tilt angles of the aryl rings denoted
as 2, 3, and 4 to the central benzene ring 1 (Figure 4.3) are 37.21(99)°, 42.59(99)° and
28.76(87)°, respectively, while the tilt angles of the aryl rings 5, 6, and 7 to the central
benzene ring 1 are 86.85(94)°, 6.51(78)°, 57.86(77)°, respectively.
In the solid state the P-TBB molecules are linked by weak - stacking interactions
and hydrogen bonding to generate a honeycomb-like supramolecular structure that
supports voids of about 2.5 nm in diameter. The solvent molecules within these voids were
highly disordered and could not be modelled properly, thus the SQUEEZE routine of the
PLATON program was applied to remove the contributions to the scattering from the
solvent molecules. The honeycomb-like structure can be seen in the direction of the
crystallographic c-axis and is presented in Figure 4.4. In the crystal structure the unit cell
comprises of eight P-TBB molecules organised into dimers. The molecules in these dimer
assemblies exhibit a face-to-face (A) and a head-to-tail (B) arrangement, creating an
interdigitating packing arrangement which can be seen in the direction of the
crystallographic a-axis (Figure 4.5). The dimers are stabilised by hydrogen bonding and interactions. Strong intermolecular H-bonding between O(1) and O(2) of two adjacent
molecules of the dimer unit A are characterised by an interatomic distance of 2.466(16)
Å.21 Face-to-face - interactions between rings 1, 3, 6 of two adjacent molecules in dimer
A range from 3.722(12) Å to 3.891(12) Å and off-set - interactions of 3.663(10) Å and
3.828(13) Å can also be observed in dimer A.22 Dimer B is stabilised by off-set interactions of 3.958(12) Å and 3.977(13) Å.
236
Chapter 4 – Supramolecular Coordination Networks
Figure 4.3 – The crystal structure of the P-TBB ligand. Colour code: P purple, O red, C black (H atoms
have been removed for clarity).
~25 Å
Figure 4.4 – The packing arrangement of the P-TBB ligand viewed in the direction of the
crystallographic c-axis. Colour code: P purple, O red, C black, H grey.
237
Chapter 4 – Supramolecular Coordination Networks
a)
B
B
A
A
b)
Figure 4.5 – (a) The unit cell content showing the face-to-face (A) and head-to-tail (B) arrangement of the
molecules; (b) The packing arrangement of the P-TBB ligand viewed in the direction of the
crystallographic a-axis. Colour code: P purple, O red, C black, H grey.
238
Chapter 4 – Supramolecular Coordination Networks
Table 4.1 − Crystal data and structural refinement parameters for P-TBB.
P-TBB Ligand
Empirical formula a
Molecular mass/g mol
C42H33O9P3
-1 a
Crystal colour/shape
3
Colourless / needle
Crystal size/mm
0.30×0.15×0.15
Crystal system
Monoclinic
Space group
C 2/c
a/ Å
24.378(14)
b/ Å
50.24(3)
c/ Å
12.132(7)
/º
90
/º
90.761(10)
/º
90
3
V/ Å
14856(14)
Z
8
Temperature (K)
150(2)
-3
Density/Mg m
0.693
-1
Absorp. coef./mm
0.109
F(000)
3216
2
40
max/º
Reflections collected
17967
Independent reflections
6933 [R(int) = 0.1149]
Data / restraints / parameters
6933 / 27 / 328
2
a
774.62
S on F
0.809
R1, wR2 [I>2 (I)]
0.1303, 0.3031
R1, wR2 (all data)
0.2345, 0.3528
Largest diff. peak and hole/e.Å-3
0.590 and -0.431
Excluding solvate molecules
239
Chapter 4 – Supramolecular Coordination Networks
4.3 SYNTHESIS
AND
CHARACTERISATION
OF
COORDINATION
NETWORKS EMPLOYING MONONUCLEAR INORGANIC
SBUS AND
TRIPHOSPHONATE LINKERS
4.3.1 (H3O){ΜnII(H2Ο)(CΗ3ΟΗ)2[C6H3(C6Η4PΟ3Η)3]}·xCH3OH·yH2O (17)
Compound 17, (H3O){ n(H2 )(C
3
)2[C6H3(C6
4P
3
)3]}·xCH3OH· yH2O
was prepared by the reaction of 1,3,5-tris(4-phosphonophenyl)benzene (P-TPB) with
MnCl2·4H2O in MeOH. Rectangular yellow crystals of 17 were obtained during a time
period of four days and characterised by single crystal X-ray diffraction measurements.
17 crystallises in the triclinic crystal system in the space group P , and consists of a
coordination network containing a mononuclear MnII SBU. The Mn ion in 17 displays a
distorted octahedral coordination environment formed by one oxygen atom from a
coordinating water molecule, two oxygen donors arising from two CH3OH ligands and the
remaining coordination sites are occupied by oxygen donors originating from three distinct
phosphonate ligands (Figure 4.6). The bond distances between the Mn ion and the O
donors originating from the organic phosphonate ligands vary between 2.105(4) Å –
2.185(4) Å, the Mn-Omethanol bond lengths for Mn(1)-O(2) and Mn(1)-O(6) are 2.216(4) Å
and 2.225(4) Å, respectively, while the bond distance between the Mn ion and the O atom
of the water molecule, Mn(1)-O(1), is 2.222(4) Å.
The distorted nature of the octahedral coordination geometry of the MnII ion can be
observed from the bond angles involving the metal centre. Indeed, the bond angles, O(3)Mn(1)-O(6), O(5)-Mn(1)-O(2) and O(4)-Mn(1)-O(1) of 174.53(16)°, 170.51(16)° and
165.38(16)°, respectively, deviate significantly from the ideal octahedral angle of 180°,
whilst the bond angles O(3)-Mn(1)-O(1) and O(4)-Mn(1)-O(6), of 98.07(16)° and
81.57(16)°, respectively, show the greatest deviation from the ideal angle of 90°.
Selected bond distances and bond angles for compound 17 are summarised in Table
4.2 and Table 4.3, respectively.
240
Chapter 4 – Supramolecular Coordination Networks
a)
b)
~19 Å
c)
d)
e)
f)
Figure 4.6 – (a) The hexagonal packing arrangement of the mononuclear MnII SBUs in 17 as seen in the
direction of the crystallographic a-axis; (b) The coordination environment of the MnII ion; (c) Packing
arrangement showing two layers with hexagonal motif viewed in the direction of the crystallographic aaxis; (d) - stacking of the organic moieties; (e) and (f) Packing arrangement of the 2D networks in 17
viewed in the direction of the crystallographic a- and c-axis, respectively. Colour code: Mn blue, P
purple, O red, C dark grey, H light grey (crystallization solvent molecules have been omitted for clarity).
241
Chapter 4 – Supramolecular Coordination Networks
Table 4.2 − Selected bond lengths [Å] for compound 17.
Bond
Bond distances (Å)
Mn(1)-O(3)
Mn(1)-O(4)
Mn(1)-O(5)
Mn(1)-O(2)
Mn(1)-O(1)
Mn(1)-O(6)
P(2)-O(3)
P(2)-O(9)
P(2)-O(10)
P(3)-O(4)
P(3)-O(11)
P(3)-O(12)
P(4)-O(8)
P(4)-O(5)
P(4)-O(7)
2.105(4)
2.136(4)
2.185(4)
2.216(4)
2.222(4)
2.225(4)
1.490(4)
1.522(4)
1.575(4)
1.488(4)
1.542(4)
1.575(4)
1.516(5)
1.519(4)
1.578(5)
Table 4.3 − Selected bond angles [º] for compound 17.
Bond Angle (º)
O(3)-Mn(1)-O(6)
O(5)-Mn(1)-O(2)
O(4)-Mn(1)-O(1)
O(3)-Mn(1)-O(1)
O(3)-Mn(1)-O(4)
O(4)-Mn(1)-O(5)
O(5)-Mn(1)-O(6)
O(4)-Mn(1)-O(2)
174.53(16)
170.51(16)
165.38(16)
98.07(16)
96.52(16)
95.82(15)
94.57(17)
93.26(16)
Bond Angle (º)
O(3)-Mn(1)-O(5)
O(2)-Mn(1)-O(6)
O(2)-Mn(1)-O(1)
O(3)-Mn(1)-O(2)
O(5)-Mn(1)-O(1)
O(1)-Mn(1)-O(6)
O(4)-Mn(1)-O(6)
90.72(15)
89.50(18)
86.95(16)
85.48(18)
84.97(15)
83.82(16)
81.57(16)
All three phosphonate groups of the P-TPB ligand are monodentate and singly
protonated. The protonation state of the phosphonate groups was established by examining
the P-O bond lengths (Table 4.2). Each phosphonate group of the organic ligands display
an elongated P-O bond that range between 1.575(4) Å – 1.578(5) Å, and which is
consistent with the values found in the literature for P-OH bonds.23-25 Thus each anionic
ligand carries three negative charges which are compensated by one MnII ion and a H3O+
ion. Several coordination networks stabilised by hydronium counterions have previously
been reported, e.g. in [MII6(C4O7)(C3HO5)3(H2O)6(H3O)] 8H2O (M = Zn or Mn),
{(H3O)2[Mn4(4-Haba)2(4-aba)6(SCN)4(H2O)2]}
(4-Haba
=
4-aminobenzoic
acid),
242
Chapter 4 – Supramolecular Coordination Networks
[H3O]2x[M(pyzdc)2]x (M = Mn, Cd; pyzdcH2 = 2,3-pyrazinedicarboxilic acid),
(H3O)2[Cu3(tbip)3.5(H2O)2] H2O (H2tbip = 5-tert-butyl isophtalic acid).26-29
The connectivity of the organic ligands and the mononuclear MnII SBUs in 17
results a layered architecture, whereby the hexagonal topology extends parallel to the (011)
plane (Figure 4.6, a). The layers stack in an ABAB fashion and can be described using the
Schläfli symbols as having a honeycomb {6,3} topology.30 The two-dimensional
honeycomb-like net results voids of almost 2 nm in diameter and is stabilised in the crystal
structure by week hydrogen bonding and - interactions between different layers that
stack off-set in the direction of the crystallographic a-axis. The hydrogen bonding
interactions involve the coordinating and constitutional solvent molecules, but a detailed
analysis of these interactions was not possible due to the disordered nature of the
constitutional solvent molecules and the low quality of the obtained data-set of the crystal
structure analysis. The - interactions observed in the crystal structure of 17 involve the
organic moieties of different layers and range between 3.795(25) Å – 4.095(54) Å. The
packing arrangement of the mononuclear MnII SBUs in 17 and the network topology
viewed in the direction of the crystallographic a- and c-axis are shown in Figure 4.6.
243
Chapter 4 – Supramolecular Coordination Networks
- PHYSICOCHEMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 17 shown in Figure 4.7 reveals some characteristic stretches of
the organophosphonate ligands. The set of bands in the region 1600 – 1350 cm-1 arise from
C–C skeletal vibrations of the phenyl rings, while the bands in the region 1200 – 900 cm-1
are attributed to different P–O stretching vibrations of the phosphonate groups. The two
sharp bands observed between 820 – 650 cm-1 arise from C–H out-of-plane bending
vibrations of the aromatic rings. Then, the corresponding O–H stretching vibrations and H–
O–H bending vibrations of the crystallization water molecules engaged in H-bonds appear
as broad bands at ca. 3200 cm-1 and ca. 1650 cm-1, respectively.31-35
Figure 4.7 – Infrared spectrum of 17.
244
Chapter 4 – Supramolecular Coordination Networks
-
Thermogravimetric analysis
The thermal stability of compound 17 was investigated by thermogravimetric
analysis (TGA) using a freshly prepared crystalline sample. The analysis was carried out in
the temperature range between 30 and 900 °C, in an N2 atmosphere. Upon heating 17
undergoes a weight loss of 25.1 % below 200 °C. This weight loss can be attributed to the
loss of two crystallization CH3OH molecules, three crystallization H2O molecules, two
coordinating CH3OH molecules and one coordinating H2O molecule (calcd: 24.5 %). Other
crystallization solvent molecules of 17 may have been lost prior to the TGA. The
compound then undergoes a gradual thermal decomposition between 200 – 500 °C that can
be associated with the degradation of the organic ligands. The thermogravimetric step
centered at 550 °C is, most likely, being associated with the formation of a metal oxide
material. In addition it is expected that under the applied N2 atmosphere, amorphous
carbon is formed.
100
Weight % (%)
90
80
70
60
50
40
30
0
200
400
600
800
1000
Temperature (ºC)
Figure 4.8 – Thermogravimetric analysis of 17.
245
Chapter 4 – Supramolecular Coordination Networks
Table 4.4 − Crystal data and structural refinement parameters for 17.
Compound 17
Empirical formula a
Molecular mass/g mol
C26H31MnO13P3
-1 a
Crystal colour/shape
Yellow / rectangular
Crystal size/mm3
0.50×0.20×0.10
Crystal system
Triclinic
Space group
P
a/ Å
7.7458(15)
b/ Å
16.352(3)
c/ Å
16.975(5)
/º
112.58(3)
/º
100.65(3)
/º
100.90(3)
3
V/ Å
1869.2(7)
Z
1
Temperature (K)
150(2)
-3
Density/Mg m
1.358
-1
Absorp. coef./mm
0.547
F(000)
756
2
50
max/º
Reflections collected
28825
Independent reflections
6578 [R(int) = 0.2102]
Data / restraints / parameters
6578 / 0 / 464
2
a
699.37
S on F
1.027
R1, wR2 [I>2 (I)]
0.0802, 0.1962
R1, wR2 (all data)
0.1260, 0.2678
Largest diff. peak and hole/e.Å-3
0.948 and -1.329
Excluding solvate molecules
246
Chapter 4 – Supramolecular Coordination Networks
4.3.2 (H3O){Cu(H2Ο)2(CΗ3ΟΗ)[C6H3(C6Η4PΟ3Η)3]}·xCH3OH·yH2O (18)
Compound 18, (H3O){Cu(H2 )2(C
3
)[C6H3(C6
4P
3
)3]}·xCH3OH· yH2O
was prepared by the reaction of P-TPB with CuCl2·4H2O in MeOH. Needle-like green
crystals of 18 were obtained during a time period of five days and characterised by single
crystal X-ray diffraction measurements.
18 crystallises in the triclinic crystal system in the space group P , and is
structurally closely related to 17. It consists of a coordination network containing
mononuclear CuII SBUs, in which the CuII ion displays a distorted octahedral coordination
environment. The coordination sphere of the Cu ion consists of two oxygen atoms
originating from two coordinating water molecules, one oxygen donor arising from a
CH3OH ligand and the remaining coordination sites are occupied by monodentate oxygen
donors originating from three distinct phosphonate ligands (Figure 4.9). The deviation of
the CuII ion from the ideal octahedral coordination geometry can be visualised by
examining the bond angles and bond lengths of the Cu(II) centre. The bond distances
between the Cu ion and the O donors originating from the organic phosphonate ligands
vary between 1.897(6) Å – 1.962(5) Å. The three O donors originating from the organic
ligands are located in the equatorial plane of the octahedra, along with a forth O donor of a
water molecule (Cu(1)-O(1`) of 1.961(6) Å). The apical binding sites of the octahedral
coordination polyhedron are occupied by one O atom from a water molecule and one O
atom from a methanol molecule. The bond distance between the Cu ion and the O atom of
the water molecule Cu(1)-O(8`) is 2.201(15) Å, and the Cu-Omethanol bond length, Cu(1)O(7`) is 2.209(11) Å. These two bonds involving the O atoms located in the apical position
of the octahedra are slightly elongated when compared to those in the equatorial plane and
which might be explained by the Jahn-Teller (JT) effect of d9 ion in octahedral
coordination environment. The bond angles of the Cu ion also deviate slightly from those
expected of an ideal octahedral geometry. The bond angles O(8`)-Cu(1)-O(7`), O(1`)Cu(1)-O(4) and O(1)-Cu(1)-O(7) of 178.1(7)°, 171.5(3)° and 168.5(3)°, respectively,
deviate from the ideal octahedral angle of 180°, whilst the bond angles O(4)-Cu(1)-O(8`)
and O(4)-Cu(1)-O(7`) of 101.9(5)° and 79.3(3)°, respectively, show the greatest deviation
from the ideal angle of 90°. Selected bond distances and bond angles for compound 18 are
summarised in Table 4.5 and Table 4.6, respectively.
247
Chapter 4 – Supramolecular Coordination Networks
a)
b)
~19 Å
c)
d)
Figure 4.9 – (a) The hexagonal packing arrangement of the mononuclear CuII SBUs in 18 as seen in
the direction of the crystallographic a-axis; (b) The coordination environment of the CuII ion; (c) and
(d) Packing arrangement of the 2D networks in 18 viewed in the direction of the crystallographic aand b-axis, respectively. Colour code: Mn blue, P purple, O red, C dark grey, H light grey
(crystallization solvent molecules have been omitted for clarity).
Table 4.5 − Selected bond lengths [Å] for compound 18.
Bond
Cu(1)-O(1)
Cu(1)-O(7)
Cu(1)-O(1`)
Cu(1)-O(4)
Cu(1)-O(8`)
Cu(1)-O(7`)
P(1)-O(7)
P(1)-O(9)
P(1)-O(8)
Bond distances (Å)
1.897(6)
1.926(6)
1.961(6)
1.962(5)
2.201(15)
2.209(11)
1.505(7)
1.510(9)
1.559(11)
248
Chapter 4 – Supramolecular Coordination Networks
Table 4.6 − Selected bond angles [º] for compound 18.
Bond Angle (º)
O(8`)-Cu(1)-O(7`)
O(1`)-Cu(1)-O(4)
O(1)-Cu(1)-O(7)
O(4)-Cu(1)-O(8`)
O(7)-Cu(1)-O(4)
O(7)-Cu(1)-O(7`)
O(1)-Cu(1)-O(7`)
O(1)-Cu(1)-O(7`)
178.1(7)
171.5(3)
168.5(3)
101.9(5)
94.8(3)
94.5(7)
94.4(7)
93.9(2)
Bond Angle (º)
O(1`)-Cu(1)-O(7`)
O(1)-Cu(1)-O(8`)
O(1`)-Cu(1)-O(8`)
O(7)-Cu(1)-O(1`)
O(1)-Cu(1)-O(1`)
O(7)-Cu(1)-O(8`)
O(4)-Cu(1)-O(7`)
92.2(4)
87.0(6)
86.6(6)
86.3(3)
86.2(3)
83.9(7)
79.3(3)
Similar with 17, all three phosphonate groups of the organic ligands in 18 are
monodentate and singly protonated, and the overall charge of the network is compensated
by one H3O+ ion. In the solid state 18 displays a similar packing arrangement with 17, and
builds up into a two-dimensional honeycomb-like net that adopts a {6,3} topology, to form
a layered structure. The packing arrangement of the mononuclear CuII complexes in 18
viewed in the direction of the crystallographic a- and b-axis is shown in Figure 4.9.
249
Chapter 4 – Supramolecular Coordination Networks
- PHYSICOCHMICAL CHARACTERISATION
-
FT-IR spectroscopy
The IR spectrum of 18 shown in Figure 4.10 is almost identical with that of 17.
Characteristic C–C skeletal vibrations and C–H out-of-plane bending vibrations of the
phenyl rings can be observed in the IR spectrum between 1600 – 1350 cm-1 and 850 – 650
cm-1, respectively. Different P–O stretching vibrations of the phosphonate groups can be
observed in the range 1200 – 900 cm-1, while the O–H stretching vibrations and H–O–H
bending vibrations of the crystallization water molecules engaged in H-bonds appear as
broad bands at ca. 3200 cm-1 and ca. 1650 cm-1, respectively.31-35
Figure 4.10 – Infrared spectrum of 18.
250
Chapter 4 – Supramolecular Coordination Networks
-
Thermogravimetric analysis
The thermal stability of a freshly prepared crystalline sample of 18 was investigated
in an N2 atmosphere, in the temperature range 30 – 900 °C (Figure 4.11). TGA reveals that
18 undergoes a similar decomposition pathway as 17. The first thermogravimetric step
corresponding to 18.9 % is observed in the temperature range between 30 – 200 °C. This
weight loss can be attributed to the loss of four crystallization H2O molecules, one
coordinating CH3OH molecule and two coordinating H2O molecules (calcd: 18.3 %).
Other crystallization solvent molecules of 18 may have been lost prior to the TGA. The
oxidation of the organic ligands can be observed above 200 °C resulting, most likely, in the
formation of a metal oxide material and amorphous carbon.
100
Weight % (%)
90
80
70
60
50
40
0
200
400
600
800
1000
Temperature (ºC)
Figure 4.11 – Thermogravimetric analysis of 18.
251
Chapter 4 – Supramolecular Coordination Networks
Table 4.7 − Crystal data and structural refinement parameters for 18.
Compound 18
Empirical formula a
Molecular mass/g mol
C25H29CuO13P3
-1 a
Crystal colour/shape
Light green / needle
Crystal size/mm3
0.60×0.20×0.20
Crystal system
Triclinic
Space group
P
a/ Å
7.4116(15)
b/ Å
15.240(3)
c/ Å
15.942(3)
/º
105.45(3)
/º
98.15(3)
/º
95.68(3)
3
V/ Å
1700.4(6)
Z
2
Temperature (K)
150(2)
-3
Density/Mg m
1.443
-1
Absorp. coef./mm
0.849
F(000)
748
2
50
max/º
Reflections collected
18080
Independent reflections
5964 [R(int) = 0.0287]
Data / restraints / parameters
5964 / 0 / 409
2
a
693.95
S on F
1.029
R1, wR2 [I>2 (I)]
0.1171, 0.4288
R1, wR2 (all data)
0.1237, 0.4555
Largest diff. peak and hole/e.Å-3
1.749 and -1.129
Excluding solvate molecules
252
Chapter 4 – Supramolecular Coordination Networks
4.4 CONCLUSION AND FUTURE WORK
In this chapter we describe that room temperature synthetic methods can be used to
produce coordination networks that employ mononuclear, partially solvated SBUs and
triphosphonate ligands as linkers.
One of the aims of this project was to synthesise organophosphonate-based openframeworks. To accomplish this, extended triphosphonate ligands, 1,3,5-tris(4phosphonophenyl)benzene (P-TPB) and 1,3,5-tris(4`-phosphonobiphenyl-4-yl)benzene (PTBB) were employed.
Metal organic frameworks in 17 and 18, containing mononuclear Mn(II) and Cu(II)
SBUs assemble in the presence of the P-TPB ligand. The two structurally related
compounds exhibit a 2D layered architecture with hexagonal topologies. These 2D nets
pack in the solid state in an ABAB fashion. Due to the fact that the metal complexes are
partially solvated one expects that the stability of the two-dimensional honeycomb-like net
is limited. This is substantiated by the thermogravimetric analysis of the compounds. The
packing structure of 17 and 18 is further characterised by week hydrogen bonding and interactions. BET analyses of 17 and 18 revealed almost negligible surface areas of
samples that were treated under vacuum. The materials did not show any permanent
porosity.
During the course of the project, we designed an even more extended tri-functional
phosphonate ligand, P-TBB. The ligand crystallises to generate a honeycomb-like
supramolecular structure that has cross-sectional diameters of about 2.5 nm. We believe
that this ligand has the potential to generate MOFs. However, up to now we were not able
to unambiguously characterise new coordination compounds using this ligand. The low
solubility of this ligand severely hampered the isolation of crystalline materials. In
addition, synthetic approaches often resulted in precipitations.
Future research efforts will be devoted to the construction of rigid threedimensional networks using this rather unique ligand and solvothermal synthesis
approaches in less polar solutions might allow the crystallisation of the MOFs.
253
Chapter 4 – Supramolecular Coordination Networks
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
J. R. Bartels, M. B. Pate and N. K. Olson, International Journal of Hydrogen
Energy, 2010, 35, 8371-8384.
L. Peraldo Bicelli, International Journal of Hydrogen Energy, 1986, 11, 555-562.
L. Zhou, Renewable and Sustainable Energy Reviews, 2005, 9, 395-408.
http://www.hydrogenenergycenter.org/content.aspx?page_id=22&club_id=108367
&module_id=8616.
A. Zuttel, Naturwissenschaften, 2004, 91, 157-172.
J. Yang, A. Sudik, C. Wolverton and D. J. Siegel, Chemical Society Reviews, 2010,
39, 656-675.
H.-C. Zhou, J. R. Long and O. M. Yaghi, Chemical Reviews, 2012, 112, 673-674.
T. Y. Ma and Z. Y. Yuan, CORD Conference Proceedings, 2011, 4, 1407-1419.
J. L. C. Rowsell and O. M. Yaghi, Microporous and Mesoporous Materials, 2004,
73, 3-14.
S. T. Meek, J. A. Greathouse and M. D. Allendorf, Advanced Materials, 2011, 23,
249-267.
R. J. Kuppler, D. J. Timmons, Q.-R. Fang, J.-R. Li, T. A. Makal, M. D. Young, D.
Yuan, D. Zhao, W. Zhuang and H.-C. Zhou, Coordination Chemistry Reviews,
2009, 253, 3042-3066.
S. Natarajan and P. Mahata, Current Opinion in Solid State and Materials Science,
2009, 13, 46-53.
K. J. Gagnon, H. P. Perry and A. Clearfield, Chemical Reviews, 2012, 112, 10341054.
V. Chandrasekhar, T. Senapati, A. Dey and S. Hossain, Dalton Transactions, 2011,
40, 5394-5418.
Q. Yao and S. Levchik, Tetrahedron Letters, 2006, 47, 277-281.
A. K. Bhattacharya and G. Thyagarajan, Chemical Reviews, 1981, 81, 415-430.
Z. Wang, J. M. Heising and A. Clearfield, Journal of the American Chemical
Society, 2003, 125, 10375-10383.
J. Palomero, J. A. Mata, F. Gonzalez and E. Peris, New Journal of Chemistry, 2002,
26, 291-297.
J. Lu, Y. Tao, M. D'Iorio, Y. Li, J. Ding and M. Day, Macromolecules, 2004, 37,
2442-2449.
A. Kumar, M. Dixit, S. P. Singh, R. Raghunandan, P. R. Maulik and A. Goel,
Tetrahedron Letters, 2009, 50, 4335-4339.
T. Steiner, Angewandte Chemie International Edition, 2002, 41, 48-76.
C. A. Hunter and J. K. M. Sanders, Journal of the American Chemical Society,
1990, 112, 5525-5534.
J.-L. Song and J.-G. Mao, Journal of Solid State Chemistry, 2005, 178, 3514-3521.
H. Li, L. Zhang, G. Li, Y. Yu, Q. Huo and Y. Liu, Microporous and Mesoporous
Materials, 2010, 131, 186-191.
J.-L. Song and J.-G. Mao, Journal of Molecular Structure, 2005, 740, 181-186.
B. F. Abrahams, T. A. Hudson and R. Robson, Journal of Molecular Structure,
2006, 796, 2-8.
R. Wang, D. Yuan, F. Jiang, L. Han, S. Gao and M. Hong, European Journal of
Inorganic Chemistry, 2006, 2006, 1649-1656.
254
Chapter 4 – Supramolecular Coordination Networks
28.
29.
30.
31.
32.
33.
34.
35.
L. Mao, S. J. Rettig, R. C. Thompson, J. Trotter and S. Xia, Canadian Journal of
Chemistry, 1996, 74, 2413-2423.
X.-F. Wang, L. Li, Y.-M. Kong and Y. Liu, Inorganic Chemistry Communications,
2012, 21, 72-75.
V. A. Blatov, M. O'Keeffe and D. M. Proserpio, CrystEngComm, 2010, 12, 44-48.
N. Stock and T. Bein, Journal of Solid State Chemistry, 2002, 167, 330-336.
N. Stock and T. Bein, Journal of Materials Chemistry, 2005, 15.
R. Lei, X. Chai, H. Mei, H. Zhang, Y. Chen and Y. Sun, Journal of Solid State
Chemistry, 2010, 183, 1510-1520.
M. M. Gómez-Alcántara, A. Cabeza, M. A. G. Aranda, A. Guagliardi, J. G. Mao
and A. Clearfield, Solid State Sciences, 2004, 6, 479-487.
A. Cabeza, M. A. G. Aranda, S. Bruque, D. M. Poojary and A. Clearfield,
Materials Research Bulletin, 1998, 33, 1265-1274.
255
Chapter 5 – Experimental
5.
EXPERIMENTAL
256
Chapter 5 – Experimental
5.1 MATERIALS AND METHODS
5.1.1
Reagents
All chemicals and solvents were of reagent grade and purchased from SigmaAldrich Ltd., Fluka Chemica-Biochemica (U.K.), ABCR GmbH & Co. KG (Germany) or
local solvent suppliers and used as received, unless otherwise stated. Water was deionised
before use.
5.1.2
Elemental Analysis
Elemental analyses were performed using an Exeter Analytical CE 440 housed at
the analytical laboratory, UCD Belfield.
5.1.3
Infrared Spectroscopy
Infrared spectra were recorded on a PerkinElmer Spectrum One FT-IR
spectrometer using a universal Attenuated Total Reflectance (ATR) sampling accessory.
Data were collected and processed using Spectrum v5.0.1 (2002 PerkinElmer Instrument
LLC) software. The scan rate was 16 scans per minute with a resolution of 4 scans in the
range 4000-600 cm-1. The following abbreviations were used to describe the intensities: vs,
very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder; br, broad and
vbr, very broad.
5.1.4
1
Nuclear Magnetic Resonance Spectroscopy
H NMR,
13
C NMR and
31
P NMR spectra were recorded on a Bruker DPX 400
spectrometer operating at 400.13 MHz, 100.14 MHz and 161.98 MHz, respectively, by
either Dr. John O’Brien or Dr. Manuel Rüther. Samples were carried out in deuterated
solvents and are listed for each spectrum. Chemical shifts are reported in ppm and coupling
constants in Hertz. Standard abbreviations for spectra: s, singlet; d, doublet; dd, doublet of
doublets; t, triplet; qt quartet; q, quaternary; m, multiplet; br broad, J, coupling constant.
257
Chapter 5 – Experimental
5.1.5
Ultraviolet – Visible Spectroscopy
UV/vis spectra were recorded in the range 200-800 nm on a Cary 300 Scan
spectrophotometer at 20 ºC using quartz cells of 1 cm path length.
5.1.6
Thermogravimetric Analysis
Thermogravimetric analysis was carried out using a Perkin Elmer Pyriss 1TGA, in
an N2 atmosphere, using a ceramic crucible (ca. 2 mg sample; heating rate of 10 ºC/min;
range 25-900 ºC). The instrument was calibrated to Ni and Fe standards in N2 atmosphere.
5.1.7
Single Crystal X-ray Diffraction
X-ray analyses for crystals described in this report were performed by Dr. Tom
McCabe, Dr. Lei Zhang, Dr. Nianyong Zhu or Dr. Wolfgang Schmitt with a Bruker
SMART APEX CCD diffractometer, a Rigaku Saturn-724 diffractometer or a Bruker
APEX2 Duo diffractometer.
The Bruker SMART APEX CCD and the Rigaku Saturn-724 diffractometers utilise
a graphite-monochromated Mo-K radiation ( = 0.71073
) source, while the Bruker
APEX2 Duo utilises two radiation sources, a Mo-K source and a high intensity Cu-K
source ( = 1.5418
) generated from a microfocus anode. This Cu source was especially
useful for the analysis of small crystal samples. The omega and phi scans method was used
to collect either a full sphere or hemisphere of data for each crystal with a detector to
crystal distance of either 5 or 6 cm. The data sets collected from the Bruker SMART
APEX CCD diffractometer were processed and corrected for Lorentz and polarisation
effects using SMART1 and SAINT-PLUS2 software. The structures were solved using
direct methods with the SHELXTL3 program package. The data sets from the Rigaku
Saturn-724 diffractometer were collected using Crystalclear-SM 1.4.0 software. Data
integration, reduction and correction for absorption and polarisation effects were all
performed using the Crystalclear-SM 1.4.0 software.4 Space group determination, structure
solution and refinement were obtained using the Crystalstructure ver.3.8 and the Bruker
SHELXTL3 software. Datasets collected on the Bruker APEX2 Duo were processed and
the structures solved using Bruker APEX v2011.8-0 software.5
258
Chapter 5 – Experimental
All non-hydrogen atoms were refined anisotropically. Hydrogen atoms (excluding
water) were assigned to calculated positions using a riding model with appropriately fixed
isotropic thermal parameters. The SQUEEZE/PLATON program was used in the structural
refinement in the case where there were a large number of solvent molecules disordered
within the structure.6
5.1.8
X-Ray Powder Diffraction
X-ray powder diffraction was performed using a Siemens D500 diffractometer with
Cu-K
1 radiation
5.1.9
with a wavelength of = 1.54056 Å.
Magnetic Measurements
All magnetic measurements were performed and modelled by Prof. Rodolphe
Clérac at the Centre de Recherche Paul Pascal (CRPP), Pessac, France. The magnetic
susceptibility measurements were obtained with the use of a Quantum Design SQUID
magnetometer MPMS-XL and susceptometer PPMS-9. The MPMS-XL magnetometer
operates between 1.8 and 400 K for dc applied fields ranging from -7 to 7 T.
Measurements were performed on polycrystalline samples. ac-Susceptibility measurements
were performed with an oscillating ac field of 3 Oe with a frequency between 1 to 1500 Hz
or 1 Oe with a frequency between 10 to 10000 Hz (PPMS-9). The magnetic data were
corrected for the sample holder and the diamagnetic contributions.
5.1.10 Mass Spectrometry
Electrospray ionization (ESI) mass spectra were collected by Dr. J. Bernard JeanDenis and Dr. Martin Feeney, using a TOF-MS (Time-of-Flight – Mass Spectrometer,
LCT Premier) instrument supplied by Waters Corp. All data, including simulated isotope
patterns and molecular weight calculations, were processed using Masslynx v 4.0 (Waters
Corp.) data analysis software.7 An injection of a standard sodium formate (Na[CHO2Na]n,
10% formic acid/0.1M NaOH/acetonitrile, 1:1:8, v/v/v) solution was used, for a mass
calibration between m/z 100-3000. Samples were introduced into the MS via a waters
alliance 2690 HPLC at a solvent flow rate of 200 L min-1, whilst a Leu-Enk solution (10
g mL-1 in acetonitrile/0.1 TFA in water, 50:50, v/v) was co-injected via a micropump at 2
259
Chapter 5 – Experimental
L min-1 and used as an internal lock mass. The ESI settings were set with the nebuliser
gas and desolvatation gas at 60 and 500 L h-1, respectively. The ESI gas used was nitrogen.
Source and desolvatation gas temperature were set at 100 and 200 °C, respectively. The ion
polarity for all MS scans were recorded in negative mode with voltage of the capillary tip
set between 2.5-3 kV, sample cone at 30 V, extraction cone at 3 V, RF value set between
100-1000, m/z range set between 100-3000, scan time at 0.9 sec and inter-scan delay set at
0.1 sec.
5.2 LIGAND SYNTHESIS
5.2.1
Synthesis of Triphenylmethylphosphonic acid (P-TPM)
Triphenylmethylphosphonic acid was prepared according to
literature procedures.8-10 Triphenylcarbinol (10 g, 38 mmol) and
PO3H2
phosphorus trichloride (10 g, 74 mmol) were added to an ice-cooled
flask and kept in the ice bath for 30 min. After removal from the ice
bath the reaction mixture was refluxed for one hour. A small quantity
of diethyl ether was added to the resulting pale-yellow solid, followed by the addition of
some ice-cooled deionised H2O. The formed product was collected by filtration, washed
with aqueous ammonia and water and dried in a vacuum desiccator. In order to remove the
triphenylchloromethane, which is produced along with triphenylmethoxyphosphorus
dichloride,
the
crude
product
was
boiled
for
2
min
in
ethanol.
The
triphenylmethoxyphosphorus dichloride was further purified by recrystallizing from
chloroform to which acetone was added (2:1 v:v mixture).
Triphenylmethoxyphosphorus dichloride (4 g, 11 mmol) and 60 mL of 8%
potassium hydroxide solution in ethanol was heated under reflux for 45 min. The reaction
mixture was treated with 30 mL of water and the mixture was evaporated to dryness on a
water bath. The solid residue was treated with 1 L of water and the mixture was made
alkaline by the addition of a small quantity of potassium hydroxide. After this, the mixture
was boiled, cooled and filtered. Then, some hydrochloric acid was added to the clear
filtrate (till the solution became slightly acidic) and a white precipitate of
260
Chapter 5 – Experimental
triphenylmethylphosphonic acid formed. After standing for 2 h, the precipitate was filtered,
washed with 100 mL of water containing 1 mL of HCl and dried in a vacuum desiccator.
The product was recrystallized from ethanol. Yield: ~ 55%. Mp: 235 °C. FT-IR (cm–1)
νmax: 1597(w), 1492(m), 1445(m), 1178(m), 1158(m), 1086(w), 1031(s), 988(s), 968(s),
930(sh), 880(w), 758(s), 743(s), 698(s). 1H NMR (400 MHz, DMSO):
15H, C6H5).
13
C NMR (100 MHz, DMSO):
(ppm) = 7.28 (m,
(ppm) = 142.5 (3Cq, C6H5), 130.1 (6C,
C6H5), 127.6 (6C, C6H5), 126.5 (3C, C6H5), 61.8 (Cq, C-Ph3).
31
P NMR (162 MHz,
DMSO): (ppm) = 25.1. ESI-MS (DMSO): 347.0 m/z [C19H17O3PNa]+.
5.2.2
Synthesis of Tris(4-bromophenyl)benzene (Br-TPB)
The
Br
Br-TPB
ligand
was
prepared
by
a
condensation reaction, according to literature methods.11
4-Bromoacetophenone (20 g, 10.050 mmol), 1 mL of
H2SO4(c) and K2S2O7 (30 g, 118 mmol) were added to a
round-bottom flask and heated at 180 °C for 14 h. The
Br
Br
resulting crude solid was cooled to room temperature and
refluxed in 100 mL of EtOH for 1 h. Then, the yellow
solid was collected by filtration and refluxed again in 100 mL of H2O. The resulting
product was dried under vacuum and purified by recrystallizing from chloroform. Yield: ~
80%. Mp: 260 °C. FT-IR (cm–1) νmax: 3090(w), 1626(m), 1584(m), 1509(m), 1488(s),
1440(m), 1378(m), 1244(m), 1072(s), 1004(s), 885(w), 809(s), 698(m). 1H-NMR (400
MHz, CDCl3):
(ppm) = 7.72 (s, 3H, C6H3), 7.63 (d, 6H, 3JHH = 8.5 Hz, C6H4), 7.55 (d,
6H, 3JHH = 8.5 Hz, C6H4).
13
C-NMR (100 MHz, CDCl3):
(ppm) = 141.0 (3Cq, C6H3),
139.1 (3Cq, C6H4), 131.6 (6C, C6H4), 128.4 (6C, C6H4), 124.5 (3C, C6H3), 121.6 (3Cq,
C6H4–Br).
261
Chapter 5 – Experimental
5.2.3
Synthesis of 1,3,5-Tris(4-phosphonophenyl)benzene (P-TPB)
A slightly modified Arbuzov reaction was
PO3H2
used
for
the
diethylester
of
synthesis
the
of
desired
the
phosphonate
acid.12
Tris(4-
bromophenyl)benzene (10.860 g, 20 mmol) in 50
mL of 1,3-diisopropylbenzene was heated to 185 °C
H2O3P
PO3H2
in a 250-mL three-necked round-bottom flask, fitted
with a Dean Stark apparatus used in combination
with a reflux condenser, N2 inlet, and an additional funnel. The flask was purged with N2,
and 0.50 g of NiBr2 was added as a catalyst. Then, 10 mL triethyl phosphite was added
over a 6 h period under a gentle stream of N2. The generated ethyl bromide was
continuously removed from the reaction mixture until the reaction was complete. The
mixture was further refluxed for 24 h followed by the addition of 0.25 g of NiBr2 and 5 mL
of triethyl phosphite. After refluxing for another 24 h, the cooled black solution was
distilled under vacuum to remove the solvent and unreacted triethyl phosphite. The
resulting black solid was dissolved in CHCl3, and the product was extracted with
petroleum ether to afford a viscous yellow oil after removing the solvent. This phosphonate
diethyl ester was then converted to the corresponding triphosphonic acid by refluxing in
HCl. The viscous residue resulted from removing the solvent was dried at high vacuum to
obtain the product as an off-white, foam-like solid. Yield: ~ 58%. Mp: 300 °C. FT-IR (cm–
1
) νmax: 2976(vbr), 1600(m), 1558(vw), 1387(w), 1139(s), 1042(s), 1016(s), 923(s), 818(s),
755(m), 680(s). 1H NMR (400 MHz, DMSO):
3
(ppm) = 8.02 (s, 3H, C6H3), 7.99 (m, 6H,
JHH = 8.0 Hz, 4JPH = 2.5 Hz, C6H4), 7.80 (dd, 6H, 3JHH = 8.0 Hz, 3JPH = 12.5 Hz, C6H4-
PO3H2). 13C NMR (100 MHz, DMSO):
(ppm) = 142.0 (3Cq, C6H4), 141.0 (3Cq, C6H3),
134.2 (3Cq, C6H4-PO3H2), 131.1 (6C, C6H4), 127.0 (6C, C6H4), 125.1 (3C, C6H3).
31
P
NMR (162 MHz, DMSO): (ppm) = 13.7. ESI-MS (H2O): 545.0 m/z [C24H20O9P3]-.
262
Chapter 5 – Experimental
5.2.4
Synthesis of 1,3,5-Tris(4`-bromobiphenyl-4-yl)benzene (Br-TBB)
The Br-TBB ligand was prepared by a
Br
condensation
procedure.
13
reaction,
according
to
literature
4-Acetyl-4`-bromobiphenyl (4.400 g,
0.016 mol), trifluoromethanesulfonic acid (0.8 mL),
and toluene (35 mL) were added to a 100 mL threenecked round-bottom flask fitted with a magnetic
Br
Br
stirrer, a condenser, and a nitrogen inlet. The reaction
solution was refluxed under nitrogen for 15 h at 120
°C. After cooling, the crude product was collected by filtration, washed with methanol, and
purified by recrystallizing twice from chloroform. The product was washed again with
plenty of methanol and the off white (slightly orange) powder was dried in the oven at 60
°C. Yield: ~ 48%. Mp: 280 °C. FT-IR (cm–1) νmax: 3033(w), 1650(m), 1584(m), 1479(m),
1441(w), 1386(m), 1274(m), 1219(m), 1195(m), 1075(m), 999(m), 952(w), 848(w),
807(s), 700(w), 669(w). 1H NMR (400 MHz, CDCl3):
(ppm) = 7.89 (s, 3H, C6H3), 7.82
(d, 6H, 3JHH = 8.0 Hz, C6H4), 7.72 (d, 6H, 3JHH = 8.0 Hz, C6H4), 7.62 (d, 6H, 3JHH = 8.0 Hz,
C6H4), 7.56 (d, 6H, 3JHH = 8.0 Hz, C6H4). 13C NMR (100 MHz, CDCl3):
(ppm) = 141.4
(3Cq, C6H3), 138.8 (9Cq, C6H4), 131.5 (6C, C6H4-Br), 128.2 (6C, C6H4), 127.4 (6C, C6H4),
126.9 (6C, C6H4), 124.6 (3C, C6H3), 121.3 (3Cq, C6H4-Br). MALDI-TOF MS (DMSO):
767.9 m/z [C42H27Br3].
5.2.5
Synthesis of 1,3,5-Tris(4`-phosphonobiphenyl-4-yl)benzene (P-TBB)
The phosphonate diethylester of the desired
PO3H2
acid was prepared using a slightly modified Arbusov
reaction, and then converted to the corresponding
triphosphonic acid by ester hydrolysis under acidic
conditions.12
H2O3P
PO3H2
1,3,5-Tris(4`-bromobiphenyl-4-
yl)benzene (7.827 g, 10.104 mmol) and 35 mL of 1,3diisopropylbenzene were added to a 250 mL three263
Chapter 5 – Experimental
necked round-bottom flask, fitted with a Dean-Stark apparatus used in combination with a
reflux condenser, N2 inlet and an addition funnel. The solution was heated to 185°C, and
0.25 g of NiBr2 was added as a catalyst. The flask was purged with N2, and 7 mL triethyl
phosphite was added over a 4 h period under a gentle stream of N2. The generated ethyl
bromide was continuously removed from the reaction mixture until the reaction was
complete. The mixture was further refluxed for 24 h followed by the addition of 0.125 g of
NiBr2 and 3 mL of triethyl phosphite. After refluxing for another 24 h, the cooled black
solution was distilled under vacuum to remove the solvent and unreacted triethyl
phosphite. The resulted grey solid was then refluxed in HCl to convert the ester into the
corresponding triphosphonic acid. CH3OH was added to aid dissolution. The solvent was
then removed under vacuum, and the viscous residue was dried at high vacuum, which
afforded the product as a brown solid. Yield: ~ 66%. Mp: 310 °C. FT-IR (cm–1) νmax:
3334(br), 2247(w), 1599(m), 1492(w), 1441(w), 1387(w), 1139(m), 989(s), 922(s), 810(s),
757(s), 659(s). 1H NMR (400 MHz, DMSO):
NMR (100 MHz, DMSO):
(ppm) = 8.05 (m, 9H), 7.87 (m, 18H). 13C
(ppm) = 141.6 (3Cq, C6H4), 140.9 (3Cq, C6H3), 139.5 (6Cq,
C6H4), 138.5 (3Cq, C6H4-PO3H2), 131.2 (6C, C6H4), 127.7 (12C, C6H4), 127.2 (6C, C6H4),
126.2 (3C, C6H3). 31P NMR (162 MHz, DMSO): (ppm) = 12.6. ESI-MS (CH3CN): 773.1
m/z [C42H32O9P3]-.
1,3,5-Tris(4`-phosphonobiphenyl-4-yl)benzene (P-TBB) (0.116 g, 0.150 mmol)
was dissolved under stirring in DMF (5 mL), at room temperature, and 2 mL of CH3CN
were layered to obtain needle-like colourless crystals overnight. Yield: ~ 43%. Anal. Calc.
for C50H65N4O19P3, Expected: C% 53.67, H% 5.85, N% 5.01. Found: C% 53.40, H% 5.55,
N% 5.15. FTIR (cm–1) νmax: 2929(w), 2321(br), 1655(s), 1596(m), 1493(m), 1437(m),
1410(m), 1386(s), 1253(m), 1137(s), 1092(s), 1000(s), 918(s), 814(s), 757(s), 658(s).
264
Chapter 5 – Experimental
5.3 SYNTHESIS OF THE METAL COMPLEXES
5.3.1
Synthesis of (NH4)2H2[MoV4O8(O3AsC6H5)4]∙5H2O (1)
(NH4)6Mo7O24 4H2O (0.224 g, 0.180 mmol) and CH3COONH4 (0.500 g, 6.488
mmol) were dissolved under stirring in H2O (10 mL) at room temperature and N2H4 H2SO4
(0.032 g, 0.244 mmol) was added as a reducing agent. Then phenylarsonic acid (0.612 g,
3.030 mmol) was added and the resulting reaction mixture was stirred for 10 minutes.
Subsequently, the pH was adjusted to pH = 3.9 (20°C) through the addition of 3.32 mL of
aqueous CH3COOH solution (50%, v/v). After three weeks red-brown crystals were
collected from a dark blue solution, and washed with cold water. Yield: ~ 57%. Anal. Calc.
for As4C24H40Mo4N2O25, Expected: C% 20.01, H% 2.79, N% 1.94. Found: C% 19.65, H%
2.15, N% 1.69. FT-IR (cm–1) νmax: 2981(b), 1641(w), 1439(m), 1092(m), 966(vs), 743(vs).
1
H NMR (400 MHz, DMSO):
C6H5). UV/Vis (DMF):
5.3.2
max
(ppm) = 7.67 (m, 12H, C6H5), 7.97 (d, 3JHH = 7.0 Hz, 8H,
( ) = 457 nm (1000 L mol-1 cm-1).
Synthesis of (NH4)2H2[MoV4O8(O3AsC6H4NH2)4]∙DMF∙4H2O (2)
N2H4 H2SO4 (0.032 g, 0.244 mmol) was added to a mixture of (NH4)6Mo7O24 4H2O
(0.224 g, 0.180 mmol), CH3COONH4 (0.500 g, 6.488 mmol), H2O (5 mL) and DMF (5
mL). The solution was vigorously stirred for 5 min and (4-aminophenyl)arsonic acid
(0.659 g, 3.030 mmol) was added. The resulting reaction mixture was further stirred for 10
min and subsequently the pH was adjusted to pH = 4.9 (20°C) through addition of an
aqueous CH3COOH solution (50%, v/v). Red-brown crystals were obtained after 5-7 days.
Yield: ~ 53%. Anal. Calc. for As4C27H49Mo4N7O25, Expected: C% 20.85, H% 3.17, N%
6.30. Found: C% 21.14, H% 2.61, N% 6.23. FT-IR (cm–1) νmax: 3329(m), 3209(m),
1714(w), 1628(m), 1594(s), 1502(m), 1425(m), 1295(w), 1182(w), 1095(m), 958(s),
768(vs), 692(s). 1H NMR (400 MHz, DMSO): (ppm) = 5.80 (s, 8H, NH2), 6.73 (d, 3JHH =
8.5 Hz, 8H, C6H5), 7.56 (d, 3JHH = 8.5 Hz, 8H, C6H5). UV/Vis (DMF):
max
( ) = 460 nm
(1000 L mol-1 cm-1).
265
Chapter 5 – Experimental
5.3.3
Synthesis of (NH4)5∙[MoVI2MoV3O11(O3AsC6H4OH)5]∙9H2O (3)
(NH4)6Mo7O24 4H2O (0.224 g, 0.180 mmol) and CH3COONH4 (0.500 g, 6.488
mmol) were dissolved under stirring in H2O (10 mL) at room temperature and N2H4 H2SO4
(0.032 g, 0.244 mmol) was added as a reducing agent. Then (4-hydroxyphenyl)arsonic acid
(0.660 g, 3.030 mmol) was added and the resulting reaction mixture was stirred for 10
minutes. Subsequently the pH was adjusted to pH = 4.0 (20°C) through addition of 3.32
mL of an aqueous CH3COOH solution (50%, v/v). Dark blue crystals were collected after a
week, and washed with cold water. Yield: ~ 53%. Anal. Calc. for As5C30H63Mo5N5O40,
Expected: C% 18.12, H% 3.19, N% 3.52. Found: C% 19.03, H% 2.70, N% 3.20. FT-IR
(cm–1) νmax: 2998(vbr), 1582(m), 1500(w), 1425(s), 1253(m), 1171(w), 1092(s), 944(s),
729(s). UV/Vis (DMF):
5.3.4
max (
) = 531 nm (2700 L mol-1 cm-1).
Synthesis of (NH4)4∙[MoVI4O10(O3AsC6H3NO2OH)4]∙2H2O (4)
Synthetic procedure as for 1, using (4-hydroxy-3-nitrophenyl) arsonic acid (0.797
g, 3.030 mmol) as ligand. The pH was adjusted to pH = 3.9 (20°C) through addition of
3.32 mL of an aqueous CH3COOH solution (50%, v/v) and orange crystals of
(NH4)4[Mo4O10(O3AsC6H3NO2OH)4] 2H2O were obtained within three weeks. Yield: ~
75%. Anal. Calc. for As4C24H36Mo4N8O36, Expected: C% 17.00, H% 2.14, N% 6.61.
Found: C% 16.52, H% 2.06, N% 5.61. FT-IR (cm–1) νmax: 3189(br), 1614(m), 1570(w),
1532(w), 1407(m), 1355(w), 1327(m), 1247(w), 1158(m), 1099(m), 940(w), 810 (vs),
762(s).
5.3.5
Synthesis of (NH4)4∙H4{Mn [MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O (5)
MoO3 (0.135 g, 0.930 mmol), H3BO3 (0.096 g, 1.550 mmol) and H2O (10 mL)
were heated to 90ºC and N2H4 H2O (0.075 mL) were added to obtain a blue clear solution.
A second solution of phenylphosphonic acid (0.700 g, 4.420 mmol) in H2O (10 mL) was
heated to 90 ºC and added to the first solution. The mixture was then stirred at 70 ºC for 10
min, and MnCl2 4H2O (0.140 g, 0.700 mmol) was added. After stirring for another 30 min
at 70 ºC the red brown solution was kept at 45ºC and orange crystals were obtained within
a time period of three days. Yield: ~ 30%. Anal. Calc. for C48H82MnMo12N4O62P8,
Expected: C% 18.23, H% 2.61, N% 1.77. Found: C% 19.14, H% 2.23, N% 1.50. FT-IR
266
Chapter 5 – Experimental
(cm–1) νmax: 3211(vbr), 1594(m), 1438(m), 1142(m), 1024(s), 937(vs), 741(s), 721(s),
691(s). UV/Vis (CH3CN):
max
( ) = 305 nm (11000 L mol-1 cm-1), 443 nm (1050 L mol-1
cm-1).
5.3.6
Synthesis of (NH4)4∙H4{Fe[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O (6)
Synthetic procedure as for 5, using FeCl3 6H2O (0.189 g, 0.700 mmol). Yield: ~
40%. Anal. Calc. for C48H82FeMo12N4O62P8, Expected: C% 18.23, H% 2.61, N% 1.77.
Found: C% 18.42, H% 2.02, N% 1.36. FT-IR (cm–1) νmax: 3216(vbr), 1595(w), 1438(m),
1142(s), 1022(s), 960(vs), 744(s), 720(s), 689(vs). UV/Vis (CH3CN):
-1
-1
-1
max
( ) = 274 nm
-1
(88000 L mol cm ), 440 nm (1300 L mol cm ).
5.3.7
Synthesis of (NH4)4∙H4{Co[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O (7)
Synthetic procedure as for 5, using CoCl2 6H2O (0.166 g, 0.700 mmol). Yield: ~
20%. Anal. Calc. for C48H82CoMo12N4O62P8, Expected: C% 18.21, H% 2.61, N% 1.77.
Found: C% 18.42, H% 2.13, N% 1.69. FT-IR (cm–1) νmax: 3209(vbr), 1617(m), 1441(w),
1142(m), 1093(w), 1039(s), 930(vs), 761(w), 739(m), 721 (s), 701 (w), 686 (m). UV/Vis
(H2O):
max
( ) = 252 nm (30000 L mol-1 cm-1), 450 nm (2900 L mol-1 cm-1).
5.3.8
Synthesis of (NH4)4∙H4{Ni[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O (8)
Synthetic procedure as for 5, using NiCl2 6H2O (0.166 g, 0.700 mmol). Yield: ~
20%. Anal. Calc. for C48H82NiMo12N4O62P8, Expected: C% 18.21, H% 2.61, N% 1.77.
Found: C% 17.83, H% 1.99, N% 1.52. FT-IR (cm–1) νmax: 3215(vbr), 1618(m), 1439(m),
1142(m), 1093(w), 1038(s), 935(ws), 739(s), 721(s), 700 (w), 687 (s). UV/Vis (H2O):
max
( ) = 301 nm (14000 L mol-1 cm-1), 440 nm (3900 L mol-1 cm-1).
5.3.9
Synthesis of (NH4)4∙H4{Mg[MoV6O12(OH)3(O3PC6H5)4]2}∙8H2O (9)
Synthetic procedure as for 5, using MgCl2 (0.028 g, 0.300 mmol). Yield: ~ 22%.
Anal. Calc. for C48H82MgMo12N4O62P8, Expected: C% 18.41, H% 2.64, N% 1.78. Found:
C% 17.71, H% 1.99, N% 1.55. FT-IR (cm–1) νmax: 2981(w), 1617(w), 1438(m), 1142(m),
267
Chapter 5 – Experimental
1021(s), 940(vs), 745(s), 718(s), 691(vs). UV/Vis (CH3CN):
-1
-1
-1
max
( ) = 297 nm (12000 L
-1
mol cm ), 450 nm (800 L mol cm ).
5.3.10 Synthesis of [
nΙΙΙ15(μ2-H2 )2(C
3
)16(C6
5
P
) ]Cl5·22CH3OH·8H2O (10)
3 20
MnCl2·4H2O (0.198 g, 1.000 mmol), KMnO4 (0.039 g, 0.250 mmol) and
phenylphosphonic acid (0.158 g, 1.000 mmol) were dissolved in CH3OH (10 mL), stirred
for 5h, filtered and left undisturbed for four days at room temperature. Then the reaction
mixture was kept at ca. 2°C in a refrigerator for another four days, to obtain rectangular
red-brown crystals. Yield: ~ 30%. Anal. Calc. for a dried sample with the expected formula
C120H136Cl5Mn15O78P20, Expected: C% 32.41, H% 3.08. Found: C% 32.04, H% 2.68. FTIR (cm–1) νmax: 3266(vbr), 2944(w), 2835(w), 1628(br), 1487(w), 1438(m), 1124(s),
1032(s), 983(s), 750(s), 722(s), 693(s). UV/Vis (CH3CN):
1
max
( ) = 280 nm (34000 L mol-
cm-1), 465 nm (6400 L mol-1 cm-1).
5.3.11 Synthesis
(C5
5
of
[ n
13( 4-
)2( 3- )4( 2- H)2( 2-C
3
)4(C6
5P
3)10
)5 Cl]·3H2O (11)
MnCl2·4H2O (0.198 g, 1.000 mmol), KMnO4 (0.039 g, 0.250 mmol) and
phenylphosphonic acid (0.094 g, 0.600 mmol) were dissolved under stirring in a mixture of
CH3OH/CH3CN (5/5 mL), at room temperature, and 1.2 mL of pyridine were added. The
reaction mixture was stirred for 5h, filtered and left undisturbed for slow evaporation.
Rhombic, red-brown crystals were obtained during a time period of one week. Yield: ~
20%. Anal. Calc. for a dried sample with the expected formula C85H87ClMn13N5O45P10,
Expected: C% 34.51, H% 2.96, N% 2.37, Cl% 1.20. Found: C% 34.74, H% 2.49, N% 2.17,
Cl% 0.94. FT-IR (cm–1) νmax: 3503(w), 3055 (w), 1601(m), 1485(s), 1438(m), 1120(s),
1084(s), 1032 (sh), 1003 (sh), 971(s), 753(s), 721(m), 695(s). UV/Vis (CH3CN):
max
( )=
270 nm (162000 L mol-1 cm-1), 446 nm (3400 L mol-1 cm-1).
268
Chapter 5 – Experimental
5.3.12 Synthesis of [
(C5
5
n
13( 4-
)2( 3- )4( 2- H)2( 2-C
3
)4(C6
5C
2P
3)10
)6]Cl·5H2O (12)
MnCl2·4H2O (0.198 g, 1.000 mmol), KMnO4 (0.039 g, 0.250 mmol) and
benzylphosphonic acid (0.102 g, 0.600 mmol) were dissolved under stirring in a mixture of
CH3OH/CH3CN (5/5 mL), at room temperature, and 0.5 mL of pyridine were added. The
reaction mixture was stirred for 5h, filtered and left undisturbed for slow evaporation.
Rectangular, brown crystals were obtained within four days. Yield: ~ 23%. Anal. Calc. for
a dried sample with the expected formula C100H116ClMn13N6O47P10, Expected: C% 37.38,
H% 3.64, N% 2.62. Found: C% 36.95, H% 3.13, N% 2.17. FT-IR (cm–1) νmax: 3423(br),
3028(w), 2918(w), 2815(w), 1601(m), 1495(m), 1446(m), 1409(w), 1240(m), 1192(w),
1133(sh), 1116(sh), 1066(sh), 982(s), 830(m), 788(m), 730(m), 693(s). UV/Vis (CH3CN):
max
( ) = 257 nm (61000 L mol-1 cm-1), 443 nm (4500 L mol-1 cm-1).
5.3.13 Synthesis of [
(C6
5-C3
6-C5
4
n 13( 4- )2( 3- )4( 2- H)2( 2-C
)6]Cl·5H2O (13)
3
)4(C6
5C
2P
3)10
MnCl2·4H2O (0.198 g, 1.000 mmol), KMnO4 (0.039 g, 0.250 mmol) and
benzylphosphonic acid (0.102 g, 0.600 mmol) were dissolved under stirring in CH3OH (20
mL), at room temperature, and 1 mL of 4-(3-phenylpropyl)-pyridine was added. The
reaction mixture was stirred for 5h, filtered and left undisturbed for slow evaporation.
Rectangular, brown crystals were obtained within one week. Yield: ~ 37%. Anal. Calc. for
a dried sample with the expected formula C154H176ClMn13N6O47P10, Expected: C% 47.16,
H% 4.52, N% 2.14, Cl% 0.90. Found: C% 47.19, H% 4.16, N% 2.03, Cl% 1.08. FT-IR
(cm–1) νmax: 3471(br), 3026(w), 2921(w), 1952(w), 1615(m), 1558(w), 1495(m), 1453(m),
1426(m), 1240(w), 1193(w), 1118(sh), 1067(s), 984(s), 828(m), 789(m), 730(m), 695(s).
UV/Vis (CH3CN):
max
( ) = 256 nm (41000 L mol-1 cm-1), 446 nm (3600 L mol-1 cm-1).
5.3.14 Synthesis of K(H2O)4[MnIII12( 3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5
CH2PO3)8]Cl2·3CH3OH·4H2O (14)
MnCl2·4H2O (0.198 g, 1.000 mmol), KMnO4 (0.019 g, 0.125 mmol) and
benzylphosphonic acid (0.079 g, 0.450 mmol) were dissolved under stirring in CH3OH (20
mL), at room temperature, and 4,4`-bipyridine (0.015 g, 0.096 mmol) was added. The
269
Chapter 5 – Experimental
reaction mixture was stirred for 5h, filtered and left undisturbed for slow evaporation.
Rectangular, brown crystals were obtained within one week. Yield: ~ 38%. Anal. Calc. for
a dried sample with the expected formula C105H140Cl2KMn12O65P15, Expected: C% 34.31,
H% 3.84. Found: C% 34.29, H% 3.49. FT-IR (cm–1) νmax: 3029(vbr), 1602(w), 1495(m),
1454(m), 1406(w), 1245(w), 951(s), 781(m), 727(m), 675(s). UV/Vis (CH3CN):
-1
-1
-1
max
( )=
-1
257 nm (36000 L mol cm ), 488 nm (3200 L mol cm ).
5.3.15 Synthesis of K(H2O)4[Mn
III
12( 3-O)6(CH3OH)6(C6H5CH2PO3H)7(C6H5
CH2PO3)8]Br2·2CH3OH·2H2O (15)
MnBr2·4H2O (0.143 g, 0.500 mmol), KMnO4 (0.019 g, 0.125 mmol) and
benzylphosphonic acid (0.158 g, 0.918 mmol) were dissolved in CH3OH (20 mL), at room
temperature, and stirred for 5h. The resulting orange - brown solution was filtered and left
undisturbed for slow evaporation. Small, brown crystals were obtained during a time
period of ten days. Yield: ~ 26%. Anal. Calc. for a dried sample with the expected formula
C105H136Br2KMn12O63P15, Expected: C% 33.82, H% 3.68. Found: C% 33.74, H% 3.32. FTIR (cm–1) νmax: 3417(br), 3028(w), 1602(m), 1495(m), 1453(m), 1407(w), 1256(w),
1071(s), 960(s), 831(m), 786(s), 730(s), 679(s). UV/Vis (CH3CN):
max
( ) = 258 nm
(52000 L mol-1 cm-1), 489 nm (4000 L mol-1 cm-1).
(H3O)4[MnIII2MnII4( 4-O)2(H2O)2(CH3CN)2{(C6H5)3
CPO3}6]Cl2·2CH3CN·4H2O (16)
5.3.16 Synthesis
of
MnCl2·4H2O (0.099 g, 0.500 mmol), KMnO4 (0.039 g, 0.250 mmol) and
triphenylmethylphosphonic acid (0.324 g, 1.000 mmol) were dissolved under stirring in a
mixture of CH3CN/CH2Cl2 (10/10 mL), at room temperature. 1.2 mL of triethylamine was
added and the reaction mixture was further stirred for 5h, filtered and left undisturbed for
slow evaporation. Rectangular red-brown crystals were obtained within five days. Yield: ~
16%. Anal. Calc. for a dried sample with the expected formula C116H113Cl2Mn6NO28P6,
Expected: C% 54.52, H% 4.46, N% 0.55. Found: C% 54.14, H% 3.98, N% 0.25. FT-IR
(cm–1) νmax: 3590(w), 3054(w), 1594(m), 1491(m), 1444(m), 1188(w), 1121(s), 1076(s),
1037(s), 1005(s), 959(s), 931(s), 894(w), 851(w), 745(s), 699(s). UV/Vis (CH3CN):
max
( ) = 263 nm (87000 L mol-1 cm-1), 487 nm (900 L mol-1 cm-1).
270
Chapter 5 – Experimental
5.3.17 Synthesis of (H3O){
n(H2 )(C
3
)2[C6H3(C6
4P
3
)3]}·xCH3OH·yH2O
(17)
1,3,5-Tris(4-phosphonophenyl)benzene (P-TPB) (0.081 g, 0.150 mmol) was
reacted with MnCl2·4H2O (0.029 g, 0.150 mmol) in CH3OH (10 mL). The resultant pale
orange solution was stirred at room temperature for 1 hour, and then it was left undisturbed
for slow evaporation. During a time period of four days rectangular yellow crystals of 17
were separated from the solution. Yield: ~ 33%. Anal. Calc. for a dried sample with the
expected formula C32H63MnO23P3, Expected: C% 39.88, H% 6.59. Found: C% 40.19, H%
6.94. FT-IR (cm–1) νmax: 3189(br), 2278(w), 1647(w), 1597(m), 1385(m), 1135(s), 1036(s),
908(s), 818(s), 689(s).
5.3.18 Synthesis of (H3O){Cu(H2 )2(C
3
)[C6H3(C6
4P
3
)3]}·xCH3OH·yH2O
(18)
1,3,5-Tris(4-phosphonophenyl)benzene (P-TPB) (0.081 g, 0.150 mmol) was
reacted with CuCl2·2H2O (0.050 g, 0.300 mmol) in CH3OH (10 mL). The resultant pale
green solution was stirred at room temperature for 1 hour, and then it was left undisturbed
for slow evaporation. During a time period of five days pale green crystals were separated
from the solution. Yield: ~ 34%. Anal. Calc. for a dried sample with the expected formula
C24H28CuO13.5P3, Expected: C% 41.84, H% 4.10. Found: C% 41.86, H% 3.62. FT-IR (cm–
1
) νmax: 2943(vbr), 2285(w), 1597(m), 1557(w), 1505(w), 1385(w), 1135(s), 1003(s),
914(s), 817(s), 688(s).
271
Chapter 5 – Experimental
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Bruker SMART, Version 5.629, 1997-2003, Bruker-Axs Inc.
Bruker Saint-Plus, Version 6.22, 1997-2003, Bruker-Axs Inc.
G. M. Sheldrick, Version 5.1, 1999, Bruker Axs-Inc.
Rigaku Americas Corporation, Crystalclear-SM 1.4.0 software, 9009 New Trails
Drive, The Woodlands, TX 77381, United States.
Software Reference Manual, version 5.625; Bruker Analytical X-Ray Systems Inc.:
Madison, WI, 2001.
A. L. Spek, Journal of Applied Crystallography (Wiley-Blackwell), 2003, 36, 7.
Masslynx, version 4.0, 2002, Waters Corp.
D. R. Boyd and G. Chignell, J. Chem. Soc., 1923, 123, 813-817.
D. R. Boyd and F. J. Smith, J. Chem. Soc., Trans., 1924, 125, 1477-1480.
V. Chandrasekhar and P. Sasikumar, Dalton Transactions, 2008, 6475-6480.
J. Palomero, J. A. Mata, F. Gonzalez and E. Peris, New Journal of Chemistry, 2002,
26, 291-297.
Z. Wang, J. M. Heising and A. Clearfield, Journal of the American Chemical
Society, 2003, 125, 10375-10383.
J. Lu, Y. Tao, M. D'Iorio, Y. Li, J. Ding and M. Day, Macromolecules, 2004, 37,
2442-2449.
272
Appendix
APPENDIX
273
Attached CD-ROM
The attached CD-ROM contains electronic versions of various files for the
structures presented in this thesis.
The CIF folder contains crystallographic information files (*.cif) for the
compounds. The file names correspond to the name of the compounds as presented in the
thesis (e.g. file 1 corresponds to compound 1). The CIF files can be viewed using the
Mercury program. The Mercury program is available to download for free from the CCDC
website (www.ccdc.cam.ac.uk).
The PDF folder contains a PDF file of the entire thesis. PDF files can be viewed
using the Adobe Acrobat Reader program. The Adobe Acrobat Reader program is
available
to
download
for
free
from
the
Adobe
website
(www.adobe.com/products/acrobat/readermain.html).
