Review
Flexibility in Metal–Organic Frameworks:
A Basic Understanding
Noor Aljammal 1,†, Christia Jabbour 1,†, Somboon Chaemchuen2,3, Tatjána Juzsakova 4
and Francis Verpoort 1,2,3,*
Center for Environmental and Energy Research (CEER), Ghent University Global Campus,
119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon 406-840, Korea; noor_aljmmal_1986@hotmail.com (N.A.);
christia_jabbour@hotmail.com (C.J.)
2 Laboratory of Organometallics, Catalysis and Ordered Materials, State Key Laboratory of Advanced
Technology for Materials Synthesis and Processing, Wuhan University of Technology,
Wuhan 430070, China; sama_che@hotmail.com
3 Research School of Chemistry & Applied Biomedical Sciences, National Research Tomsk Polytechnic
University, Lenin Avenue 30, Tomsk 634050, Russian Federation.
4 Institute of Environmental Engineering, University of Pannonia, Veszprem, 10 Egyetem St.,
8200 Veszprém, Hungary; yuzhakova@almos.uni-pannon.hu
* Correspondence: francis.verpoort@ghent.ac.kr
† These authors contributed equally to this work.
1
Received: 15 May 2019; Accepted: 31 May 2019; Published: 6 June 2019
Abstract: Much has been written about the fundamental aspects of the metal–organic frameworks
(MOFs). Still, details concerning the MOFs with structural flexibility are not comprehensively
understood. However, a dramatic increase in research activities concerning rigid MOFs over the
years has brought deeper levels of understanding for their properties and applications.
Nonetheless, robustness and flexibility of such smart frameworks are intriguing for different
research areas such as catalysis, adsorption, etc. This manuscript overviews the different aspects of
framework flexibility. The review has touched lightly on several ideas and proposals, which have
been demonstrated within the selected examples to provide a logical basis to obtain a fundamental
understanding of their synthesis and behavior to external stimuli.
Keywords: metal–organic frameworks; flexibility; mechanical properties; secondary building unit;
characterization
1. Introduction about Metal–Organic Frameworks (MOFs) and Their Structure
Coordinated polymeric materials were first discovered by Kinoshita in the late 1950s [1,2], and
Berlin [3], then Block [4] followed his footsteps in the 1960s. In the last century, many great
scientists focused their research on understanding the behavior of these porous materials [5]. The
term MOF dates to the late 1990s in which Kitagawa [6], Yaghi [7] and Férey [8] primed a quantum
leap in understanding, advancing the synthesis of such hybrid materials. The pioneering work of
these scientists and their outstanding discoveries in MOFs is highly appreciated. Their studies and
descriptions of the science (or art) of their synthesis and characterization have engendered vast
literature, as numerous books and review articles attest. Moreover, recent research affords access to
a much wider range of investigational and synthesis techniques.
Structural formulas of various degrees of an infinite number of combinations and
permutations between metal centers with either a tetrahedral or an octahedral array of organic
rod-like connecting units resulted in successfully synthesizing thousands of MOFs. From these
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newly discovered scaffolding-like materials, flexible frameworks with dynamic properties are
rather limited, and only quite a few have been thoroughly investigated [6].
The flexible features of some of these porous materials are intriguing for so-called “smart”
materials in that they could structurally respond to an external stimulus. The flexibility of the
MOF(s) might play a crucial role in the MOF(s) interfacial structure, and thus in their compatibility.
The fundamental understanding of MOFs flexibility introduces a desire to interpret the
flexibility phenomena by studying the properties of individual organic linkers and metal nodes of
specific MOFs. To this end, literature has devised a variety of ways to classify and envision their
composition, size, and shape.
Full characterization should be reserved for MOFs with interesting flexible behavior, and
adequate explanation should be devoted to this. The goal of this review is to be able to cite as much
detail and as many needed examples to reach a fundamental and comprehensive understanding of
MOFs’ flexibility, its origin, and control methods.
In this review, three main themes will be discussed: (i) the flexibility behavior and the
dependence of the chemical identity and physical state of the framework on its organic linkers and
its metal nodes, (ii) phenomena of flexible MOFs, how to control it and its application; this theme is
predicated on the forms of adsorbed species on the flexible framework (guest–host, guest–guest,
and host–host interaction), and (iii) the observations on which these themes will be expressed in a
real application and not merely as theory statements.
To explore the flexibility of MOFs, help in adjacent areas of science such as reticular chemistry
and joint experimental–modeling is needed. This review will give a literature analysis of the main
advances in flexible MOFs up until 2018, where synthetic strategies, characterization, and
application are thoroughly discussed.
1.1. Origin of MOFs Flexibility
The design and synthesis of flexible scaffold-like material depend on the choice of the
scaffolding elements as metal nodes and organic linkers. It is worth mentioning that the choice of
metal centers would depend partially on the framework building process to be used [5]. However,
organic linkers bearing functional groups are gaining more attention, and play a major role in
flexibility, e.g., rigid MOFs with flexible ligands Thus, any of the three possible sources contribute to
the flexibility of MOFs. Two speculations can be offered in this respect: (i) the source of the flexibility
and (ii) the way the flexible MOF behaves toward external stimuli. The structure and functionality
depend on the deliberate way of construction and assembly on either the two components or the
nature and type of connection linking them.
The rational and intelligent design of the coordinating ligand and metal nodes have been the
subject of much interest and attention to synthesize a wide range of scaffolding-like solids with
unusual and useful properties. Essentially, most of the research has been adequately focusing on the
conceivable linking of the scaffold-like compounds using the supramolecular building blocks (SBBs)
or the secondary building units (SBUs) and the molecular linkers [9–12]. In principle, MOFs with a
high degree of rigidity are synthesized using fixed organic building blocks (based on phenyl rings or
multiple bonds, rigid benzene di-, tri-, and tetra-carboxylic acids, terephthalic acid, azolate-based
ligands, as well as their derivatives). Such frameworks usually show relatively high thermal and
mechanical stability and are capable of retention of porosity upon guest solvents removal. It should
not come as a surprise that the properties of MOFs with flexible ligands cannot determine the whole
framework if it is flexible or not. As the nature of the resulting frameworks cannot only be
determined by the linkers’ flexibility or rigidity since in some cases, flexible ligands can be used to
build both robust and flexible MOFs. However, in view of the above remarks concerning the nature
of flexibility, attention is needed when choosing the source of flexibility. In general, there is an
obvious difference between flexible MOFs and MOFs with flexible ligands (FL-MOFs). The latter
corresponds to MOFs with one flexible component in the framework. The organic linker’s rigidity or
flexibility is not directly linked to the nature of the framework.
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What is not generally recognized is the probable role of the connection of metal nodes and
organic linker and the functionality of an organic ligand, as a connector to design and tune flexible
structures [6].
MOFs that are capable of responding to external stimuli, their entire framework is dynamic,
and their response represents in conformational mobility combined with coordination preferences,
can be classified as flexible MOFs [13]. Such response depends on the choice of a metal compound,
or on whether the linkers can rotate, twist or bend. This unique feature significantly improves the
performance of the MOF in several applications such as storage, separation, sensing, and others. In
contrast with the discovery of rigid MOFs, at present, misconceived thinking still appears in other
areas of design, synthesis, and applications of flexible MOFs. This review will provide insight into
seminal examples from different laboratories to get a profound understanding of the structure and
bonding of individual elements and their properties controlling characterizations and applications.
1.2. The Flexibility of Linkers/Functionalized Linkers
Linkers have a large impact on the final nature of this kind of porous material. Linkers or
ligands are defined as bridging organic elements, which connect the aggregates or metal centers
into prolonged framework structures. The geometry, length, functionality, and connectivity of a
linker will direct the resulting structure of the framework [14–16]. For example, changing the linker
connectivity and fixing the cornerstone geometry can result in a very different framework with
different properties. Many studies have found that such linkers may allow conformational diversity
owing to their potential energies, which can be expressed by their binding strengths [17]. Their
response to external stimuli can occur through a variety of mechanisms such as rotation of the host
framework between a metal ion and an organic ligand, rotation around single bonds within an
organic ligand or displacement of sub-networks.
The following types of linkers provide a flexible structure with dynamic behavior:
1.
2.
3.
4.
Aliphatic carbon chain; intrigue significant interest in flexible MOFs synthesis because of the
ability to reorient themselves in response to external stimuli [18,19]. The greater the length of
the chain, the greater is the variety of possible reorientation.
Aromatic rings with the capability of rotation or movement of dangling side chains of the
organic ligands [20].
Ligands with an open structure. This type of ligand depends on the formation of coordination
bonds with the central metal atom M. The decrease of the dentation gives multiple degrees of
rotational freedom of the ligand around the inorganic moiety. The ratio of the metal/linker
(M/L) and linker/linker (L/L) and the synergistic effect of metal nodes and organic linker on
framework flexibility are major-league factors that play a vital role in determining the degree of
flexibility [21,22].
Linkers’ surfaces can also be used as anchoring points for introducing additional functionalities
to tailor the framework flexibility by the substituent effects at the linker [23]. Such
functionalization results in multivariate MOFs (MTV-MOFs).
The methods that are used to modify the backbones’ flexibility of the framework may be put
into two general classes: (a) application of specific organic synthetic strategies with a particular
selection of the electron withdrawing or donating nature of the “contributing” functional linkers,
and (b) incorporation of extended conjugated aromatic systems to the linker(s) itself. Furthermore,
the shape, the size, and the shared angle between the functional group added onto the main linker,
play a role in identifying the structure of the MOF and its overall network topology [15,24,25].
Asymmetric linkers also contribute to tuning the topological flexibility of the network [26,27].
In view of the almost endless ways that a linker(s) can manifest itself, a benchmark class of
MOFs based on carboxylate linkers or functionalized 1,4-benzenedicarboxylate (BDC) linkers have
been extensively explored so far. Recently these types of linkers have been dominant in the field of
flexibility recognition of MOFs.
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1.2.1. Carboxylate Linkers and Their Isomerism Features
Férey and Serre proposed several empirical rules concerning the impact of the carboxylate
linkers on the flexibility of MOFs [28]: (i) The first rule states that ditopic carboxylate ligands, which
are linked to two metal clusters or SBUs, are favorable for the design of flexible MOFs. Hence, the
inorganic cornerstones, which own a mirror plane with the carboxylate linkers, have the ability to
permit framework swelling; MIL-88 is a prized example to study this rule. (ii) The second rule is
about the ratio of the number of carbons of the carboxylate surrounding the cluster to the number of
metallic atoms within the cluster (C/M), several studies demonstrated that if this ratio is higher than
or equal 2, then the brick possibly allows swelling. Mill-88 is demonstrative once again of this theory
with a C/M = 2. However, this rule needs to be delineated by other MOFs. (iii) The third rule states
that on the contrary to ditopic carboxylate ligands, tri- or tetra-topic carboxylate linkers prohibit the
breathing in MOFs. Presumably, the forgoing rules represent the main structural requirements for
the flexibility phenomenon.
Kitagawa and his group [29] investigated the linker rotation of pyridinedicarboxylic acid linker
(PDC) in {[Cd2(pzdc)2L(H2O)2] 5(HO) (CH3CH2OH)}n. The PDC linker, which bears ethylene glycol
side chains, acts as a molecular gate with locking/unlocking interactions when this coordination
polymer is triggered by guest inclusion, see Figure 1.
Figure 1. Schematic representation of the structural transformations triggered by water adsorption
[29].
Particular ligand functionalization can modify conformational flexibility [30]. The extra
decoration on the pores using functionalized linker molecules can add some complexity to their
properties [31,32]. For example, adding a group which can interact through their hydrogen bonding
such as –NH2, –COOH and OH, will lead to a higher degree of deprotonation and higher density of
open metal sites, and hence the ratio of M/L will increase [22,33]. Fischer’s group created a library of
functionalized BDC-type linkers, with minimal additional dangling side groups and diverse
functionalities by varying chain length at different positions of the benzene core [30].
MIL-53 and MIL-88 are two types of flexible MOFs that belong to the third-generation porous
materials. Their fascinating properties attracted attention for experimental and theoretical linker
functionalization investigations [34,35]. Ahnfeldt et al. investigated the insertion of amine-bearing
organic linkers in MIL-53. The structure contained linkages of AlO4(OH)2-octahedra with
1,4-benzenedicarboxylic acid; such inclusion resulted in a slight expansion of the unit cell volume
[36]. The observation of this process has provided fundamental clues to understand the importance
of functionalization, making it possible to point out that the type of functionalization will strongly
depend on the type of desired application.
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1.2.2. Mixed-Linker
Emphasis is being placed on improving the properties of linkers by mixing various organic
linkers. Such a combination can have a higher degree of flexibility. Yaghi and coworkers followed this
strategy in order to synthesize 18 one-phase MTV of MOF-5 starting with eight types of functionalized
1,4-BDC organic linkers. The resulting series of MOFs contained distinct functionalities [37].
Another well-studied example concerning mixed-linker structures was presented by Seo and
coworkers [29], where they investigated the pillared-layer MOFs. This type of MOFs is featured by
their structural elasticity, the interpenetration of their layers, and flexibility. A simple modification
of the dipyridyl linker can change the channel size and functionality as well as adjust the degree of
flexibility and hydrophobicity of the framework while maintaining its pillared layer structure. Sen et
al. [38] have also demonstrated the effectiveness of using a mixed ligand system to enhance the
functional property of aromatic carboxylate linkers. As the resulting MOFs gave rise to sustained
flexibility, i.e., using a semi-flexible tricarboxylic acid ligand H3L and pyridyl-based co-ligand under
solvothermal/hydrothermal conditions to synthesize Cd(II) coordination polymers. The aromatic
polycarboxylate co-ligands have an important effect on the features and the overall structures of
Cdbmb polymers (bmb= (1,4-bis(2-methylbenzimidazol-1-ylmethyl) benzene)), concerning their
construction, flexibility, coordination modes, etc. [39,40]. Reflection on some of these examples leads
to the conclusion that the flexibility of MOFs has mainly been accredited to a conformation change of
the flexible organic linkers [13,41].
1.3. Flexibility of the Metal Nodes
The inorganic brick or the so-called metal nodes provides the local structure of the overall
framework. These inorganic nodes, which can either be metal clusters or metal ions [42], are the
place where flexibility and transformation could start as many of metals clusters/ions are not
permanently quiescent but in a state of temporary stability until exposure to external stimuli, and
then it takes different shapes.
In many cases, the metal nodes of the framework do not follow the symmetry of the overall
structure. Several studies have pointed this issue out, and the changes that occur when external
stimuli induced those structures [43–46]. Thus, the structure at the metal node will also play a
critical factor in controlling the flexibility of the framework. One criterion for distinguishing flexible
MOFs from rigid MOFs is (i) the changed coordination environment of metal ions [47,48]. Other
criteria have been proposed such as (ii) the deformed configuration/connectivity of secondary
building units (SBUs) containing metal ions in response to the removal or binding of coordinative
molecules [49]. Furthermore, observational studies demonstrated that the framework flexibility have
a tendency to scale inversely with metal–ligand interaction strength [50,51].
The presence of some unpaired electrons in the chains of the metal centers can also delimit the
dynamic behavior by puckering and slightly modifying the overall contraction or elongation
distances between adjacent metal centers [52]. MIL-53(M) can be taken as a prototypical example to
explain the role of the metal nodes in the overall framework flexibility. Several structures can be
composed by varying the metal M as such [M(OH) (BDC)2]n. (M = Cr [53], Sc [54], In [55], Ga [56], Al
[57], Fe [58]). The beneficial interplay with the choice of the metal center will result in different
flexible behavior, MIL-53(Cr) exhibits an improved ability for pore opening upon dehydration [53].
A transition from large pores (LP) to narrow pores (NP) was detected upon applying a mercury
intrusion–extrusion test using a mechanical pressure below 500 MPa on MIL-53(Cr) and MIL-53(Al),
mercury penetration took place within pores larger than 3 nm. The result of this test demonstrated
that MIL-53(Al) solid was shown to exhibit an irreversible contraction, while MIL-53(Cr) undergoes
a reversible structure change under similar conditions, see Figure 2 [59]. Such impressive behavior
can be explained only on the basis of corner-sharing chains of AlO4(OH)2 octahedra, which lead to
stiffness of structure upon inducing a pressure [60].
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Figure 2. Large pores (LP) to narrow pores (NP) transition in MIL-53(Cr) sample upon intrusion–
extrusion of mercury: cumulative intruded volume versus applied pressure. The arrows show the
steps that correspond to the LP to NP transition. Squares: first intrusion–extrusion cycle. Triangles:
second intrusion–extrusion cycle [59].
In summary, to design flexible MOFs, it is not enough to introduce specific types of metal
nods/ions, linkers or their functionalities, but a smart functionalization and gentle interplay must
exist between them concerning concentration, location, and distance of individual elements of the
framework. Therefore, it is expected to see some parallelism between the effects of metals nodes
and organic linkers and how the overall framework can respond to any changes in temperature,
pressures or guest molecules.
2. General Aspects of Framework Flexibility
Several phenomena have been addressed with regards to framework flexibility. For each
phenomenon, detailed and profound insight into the structure and the connectivity of individual
MOFs’ element is needed. Moreover, a look over their structural transition during exposure to
external provocations to discern which factors may be improved during an application with
particular reference to apparently anomalous behaviors. Flexible MOFs perform in an extraordinary
stimuli-responsive fashion. When a researcher discusses the dynamic behavior of flexible MOFs
upon exposure to external stimuli; three different conceptual levels of response should be borne in
mind; (i) host coordination network structure (cage), (ii) the crystal structure, and (iii) the overall
particle structure [20]. In order to simulate the stereo-dynamic behavior of third generation MOFs,
the action of certain external stimuli such as light, temperature or pressure is needed. The type of the
stimuli has a sole behavior on the obtained response from the targeted framework. Several
important consequences then follow. The degree of responsive will depend on the potential of an
architectural framework to restructure itself according to stimuli-dependence behaviors. Gate
opening, phase change, and change in the cell parameters could present such responses. Several
natural stimuli used to irritate the structural transition such as:
•
Exposure to temperature. When MOFs are exposed to different temperatures; a change in the
linker’s conformation can be observed (e.g., rotation of aromatic rings or the movement of
dangling side chains of the organic ligands) [35,61–63].
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•
•
•
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Exposure to external pressure. Mechanical stability of the framework can be affected upon
exposure to pressure [61,64,65].
Insertion/removal of guest molecules [66,67], to study guest-dependent dynamic behavior since
the framework can expand or contract upon guest exchange.
Light can also be used as a stimulus for the dynamic movement of MOFs [68,69]. Switching
between cis–trans/LP–NP transition upon interaction with light [68–70].
2.1. Breathing, Swelling, and Linker Rotation
2.1.1. Breathing
In the early stage of research, most dynamic behaviors were related to the presence or removal
of guest molecules, known as the host–guest interaction [6,71]. At the outset, breathing effects were
considered in many laboratories; despite the variety of MOFs, only several classes exhibit the unique
breathing phenomena in which the frameworks can easily change and adapt its structure upon host–
guest interaction. Breathing phenomenon is caused by the existence of “weak points” in the
framework at which substantial geometrical changes can occur. Figure 3 displays the three principle
breathing cases of MIL-53 [72].
Figure 3. Types of breathing: (I) flexibility of the inorganic building units (IBUs), (II) structural
changes of the linker molecules, and (III) shift of interpenetrated or interwoven frameworks [72].
•
•
•
The dynamic behavior of the inorganic building units (IBU) e.g., Figure 3 (I), is subject to the
ordered rotation of the ligand molecules or a hinge motion of the linkers [28,73,74]. These
dynamic behaviors result in opening or closing of pores and accordingly affect the loss/uptake
of guest molecules. Carboxylate ligands are considered an example for weak IBU since they are
capable of switching their binding mode in the so-called kneecap mechanism in which the
ligand rotates around the O–O-axis of the carboxylate group.
When organic linkers with reversible structural and low energy are considered, one must keep
in mind that guests’ exposure will lead to many isomerized linkers; see Figure 3 (II). The
reversible photoisomerization of azobenzene molecules with alternating UV/visible light irradiation
is a good example of such type of flexible transformation [75].
The free pore volume of interlocked linkers and interpenetrated networks can also act as a weak
point; see Figure 3 (III). In this case, breathing arises by shifting the individual frameworks
against each other by van der Waals interactions [76,77].
After evoking the weak points in a framework, it is worth recalling types of MOFs that exhibit
flexibility under the inclusion/evacuation of guests within the coordination framework, according to
the structural dimensionality, see Figure 4 [71].
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Figure 4. Illustration of dynamic behaviors of the metal-organic frameworks’ (MOFs) structure
under the incorporation/removal of the guest molecule [71].
The first category corresponds to MOFs with 1D channels with diamond or square
cross-sections. This type are known as pillared MOFs in which rod-like linkers are connecting the
metals. The pillaring linkers of the frameworks are flexible as they can expand or shrink (elongated
or shortened) upon probe molecule inclusion [78,79].
The second category presented are rigid MOFs with 2D layers covalently connected with
flexible pillars in an overlaid or shifted manner [80]. Generally, a strong framework–guest
interaction can result in a significant perturbation of the cell volume of the so-called sponge-like
MOFs. e.g., elongation of pillars results in the framework expansion during absorption of guest
molecules while the frameworks shrink upon the removal of guest molecules. Moreover, the rotation
of organic moieties resulting from strong guest–host interactions, persuades the volume change
[80,81]. An interesting and potentially very useful property of this category is that the topology of
the framework preserves its single crystallinity upon the adsorption of guest molecules. A salutary
example of this case can be presented by a bilayer framework synthesized from bismacrocyclic Ni(II)
and BTC (BTC = 1,3,5-benzenetricarboxylate) ligand. This framework presents a sponge-like
behavior, shrinking and swelling according to the number of guest solvent molecules contained
within the networks [82].
The last category is presented by interpenetrated 3D grids that contain layers connected
covalently with flexible pillars. The introduction of guest molecules causes the sliding of the adjacent
interpenetrated net and hence cause the pores to open or close upon the adsorption of guest
molecules whilst keeping the same topology [78,83]. Some of the 3D frameworks are densely packed
in the deficiency of guests and introducing of molecules engenders a sliding of one network [28].
This classification provides confidence that a wide range of scaffolding-like solids should provide
accessible structures regardless of their dimension.
A particularly well-studied example concerning the breathing phenomena in MOFs is given by
flexible chromium terephthalate. This MOF consist of 3D metal (III) terephthalates with 1D-pore
channels of disordered BDC linkers. Llewellyn and coworkers investigated the CO2 separation over
MIL-53. This MOF shows a selective flexible by means of breathing during CO2 adsorption. The
breathing behavior is attributed to the disordered BDC linker since it switched from NP to LP upon
hydration and dehydration respectively, with a 50% volume increase without any change in the
topology (Figure 5) [84]. The large breathing effect is ascribed to the deficiency of the hydrogen-bond
interactions between the hydrogen atoms of water molecules and oxygen atoms of the carboxylic
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group and the μ2-hydroxo group. As exposure is increased, the framework structures are ever more
densely packed.
Figure 5. Breathing behavior of MIL-53(Al, Cr) [84].
2.1.2. Swelling
In contrary to the well-studied MOFs that display such distinct transitions such are those with a
pillared structure, there have however been several studies investigating MOFs that exhibit large
flexibility and continuous breathing behavior without defined transitions. Such behaviors are erratic
and are not fully understood but highly sought after [34,85]. The common name of such behavior is
often referred to as “swelling behavior.” The swelling mode is featured by gradual enlargement of
the MOF unit cell volume without a change in the shape of the unit cell shape or the space group.
MIL-88 is a prime example of swelling behavior as it exhibits a large breathing behavior
resulted from pore size change during solvation and desolvation [79]. It could swell upon immersion
in liquids with variations in accessible free cell volumes from 85% to 230% depending on the
nature/length of the organic spacer (see Figure 6) [35,85].
Figure 6. Illustration for swelling phenomena of the structure and the flexibility of the MIL-88B and
MIL-88D modified solids as a function of the number of functional groups (X) per spacer (on the top)
[35].
Another study was done on MIL-88, by Ramsahye and his group. They studied the impact of
n-hexane adsorption over MIL-88 and modified MIL-88. During adsorption, the external surface of
the structure opens the pore sufficiently for the first molecules to be able to diffuse into the
framework and thus initiating a pore swelling. However, the modified MIL-88 shows only a small
cell volume contraction of about 20% upon drying and thus bears a pore size in its dried state over 5
Å [86].
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2.1.3. Linker Rotation
Linker rotation or gate opening behavior is described as a (continuous) transition where the
fragments of a linker can rotate around a certain axis. It is important to note that linker rotation is
not essentially accompanied by crystallographic phase transitions [52,87]. That kind of dynamic
behavior results in a significant change in the inner geometry of the pore, as well as the positions of
possible gas uptake [88]. Figure 7 shows the rotational behavior of the aromatic ring within the
linker structure around a certain axis within the void.
Figure 7. Illustration of aromatic ring linker rotation for isostructural (3,24)-connected porous
materials, MFM-112a, MFM-115a, and MFM-132a, with different linker backbone functionalization
[88].
Fairen-Jimenez and his group [89] presented an experimental–theoretical study on the gate
opening of ZIF-8, in which a reversible linker reorientation was observed on the organic linker of the
MOF (methylimidazole linker) upon the application of high pressures (14–700 bar). The size of the
pore window increased as the implemented pressure increased, see Figure 8.
Figure 8. Linker reorientation can be observed on the linker of ZIF-8 viewing from the axis direction
(a), and the diagonal direction (c), the corresponding structure of ZIF-8 loaded with N2, (b,d) [90].
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The distinctive characteristic between linker rotation/gate opening behavior and breathing
phenomena is that the rotational movement by the linker results in the expansion of the pore
windows while a change in the unit cell of the crystal occurs during breathing.
2.2. Thermo-responsivity
Temperature plays a distinct part in characterizing the conformational change of flexible MOFs
[51,52,91]. Some of the flexible MOFs show a drastic change in their transition state from narrow
pore (NP) state at low temperatures, to large pore (LP) structures upon reaching the threshold
temperature.
It is worth mentioning in such responsive behavior, increasing the side chain length and angles
results in a decrease in the transition temperature. On the contrary, the more polar the linker, the
higher the transition temperature, as the pillars rotation eventually ends up in the opening of the
pore space [92,93].
Going back to the prominent example, when inducing a high temperature to MIL-53(Cr) an
increase in the rotational motion of the phenyl rings occurs and consequently a transition to the open
form. Therefore, changing the nature of the aromatic spacer has a strong impact on the flexible
character of these solids [54]. Figure 9 displays thermo-responsive behavior of the Sc–OH–Sc chain
in MIL-53(Sc) upon heating to 623 K, as it leads unit cell expansion.
Figure 9. Effect of heating on Sc–OH–Sc chain in MIL-53(Sc). The dashed line represents the plane of
the infinite Sc–O–Sc chains [54].
2.3. Mechanical Properties, Elasticity
Pressure plays a major role in several practical applications. Thus studying its effect has
triggered the interest of many researchers as applying pressure on specific framework imparts the
flexibility to allow drastic perturbations on their structural characteristics.
Until now, some pressure-induced behaviors of flexible MOFs have been investigated [94–97].
However, the number of MOFs that have been exposed to mechanical stress is trivial in comparison
to the immense number of MOFs that have been produced [61]. Chapman and Moggach groups
have borne out the significance of applying small pressures upon single crystals with porous
frameworks to study the structural changes upon the framework response [51,98,99]. Their unique
studies demonstrated that pressure offers a unique meaning to thoroughly explore the
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often-complex structure–function relationships of flexible MOFs since it was evident that their
porous nature gives an additional dimension upon exposure to high pressure. Notable flexible
behavior was noticed when frameworks with metal polyhedra were exposed to high pressure.
Polyhedral metal is considered as a vital sign of highly flexible and high elastic MOFs, as it can relax,
rotate and change the unit cell volume when external pressure is induced [100].
Figure 10 provides a basis for understanding the possible unit cell transformations of MOFs
upon pressure, as illustrated exposure [101]:
•
•
•
•
Amorphization, e.g., pressure-induced amorphization of ZIF-4 [102].
Compression, e.g., the linear compressibility of MIL-53(Al) [102].
Reversible phase transition, e.g., ZIF-8 [98].
Phase formation (bond rearrangement), e.g., a pressure-induced bond rearrangement in
[tmenH2][Er(HCOO)4]2 [64].
This classification does not, however, point uniquely to any of the reversible behavior, as a truly
flexible material should show reversible behavior under pressure relief, e.g., many compression
cases are fully reversible, such as ZIF-8, after the pressure has changed back to the initial conditions
[65,99,101,103]. However, some structural transformations resulting from pressure stimuli are not
necessarily reversible.
Figure 10. Shows the illustration of possible transformations that PCN-250 can undergo upon
applying mechanical pressure such as the inclination of the unit cell and flipping of linkers, (A–D)
Structure of (A) Fe3-μ3-oxo cluster; (B) ABTC linkers with different configurations; (C) comparison of
the crystal structures of PCN-250, PCN-2500, and PCN-25000; (D) scheme representation of the
structural transformation including the inclination of unit cell and flipping of linkers. [101].
2.4. Photo-responsiveness
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A number of proof-of-concept research studies support unique behavior upon interaction of
MOFs with light, which provide the basis for identifying structure–function relationships of
photo-switches phenomena. The photo-responsive frameworks classified into three main
generations [104]. The first generation contains MOFs with thermally or optically functional pendant
groups to the overall framework. These groups acts as responsive soldiers within these frameworks,
and they alter their conformation upon light exposure. Commonly, this kind of MOFs are
synthesized using azobenzene and its derivatives due to their photochromic molecules that can
undergo clean, efficient, and extrinsic reversible photoisomerization [105]. Stock and his group have
pioneered this fact as they studied porous twofold interpenetrated MOF which contains an
azo-functionality that protrudes into the pores. Exposure of this MOF to UV-light of a wavelength of
365 nm triggers the cis–trans transformation of the linker, while back switching has been noticed
when irradiated with a wavelength of 440 nm (See Figure 11) [106].
Figure 11. Photo-switchable azobenzene side groups. (a) Azo-linker as part of the backbone of the
MOF, (b) azo-group covalently attached to the inner pore wall and extended into the pore [106].
It is evident that the cis–trans transition and guest’s photo isomerization could lead to drastic
changes in the gas adsorption properties. Most attention is usually paid to the MOFs with of
azobenzene-functionalized linker. Brown and co-workers examined a photo-switched characteristic
of azobenzene-functionalized linker hosted by IRMOF-74-III framework [Mg2(C26H16O6N2)]n. The
authors observed a switching from the trans-conformer to the cis-conformer upon arousing this
MOF with light. Furthermore, a distance increment between the para-carbon atoms in azobenzene
molecule from 8.3 Å to 10.3 Å was also remarked [106]. In a more recent study, Park and his group
examined the reversible alteration of CO2 uptake upon light exposure or thermal treatment. It was
observed that the reversible alteration of the azobenzene has a significant effect on the framework’s
capacity of CO2 uptake. Particularly, when the azobenzene is in the cis form, the framework has a
significantly lower CO2 uptake in contrast to the material framework when the linker is in the
trans-form [70]. This concept was studied by other research groups and further exploited with a
different host framework, e.g., that some host frameworks forbid and block the cis–trans
transformation as in case of azobenzene in MIL-53(Al) [107]. One drawback for this generation is
that switching is strongly hindered due to the fact that the azo groups are an integral part of the
linker molecules. Thus, this type has not been revealed to date.
In contrary to the first generation, the second generation includes MOFs which are synthesized
by protruding azo groups into the pores of the linker, in another word the photo-responsive unit is
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directly incorporated to the “backbone” of the linker as a side chain. It was ascertained that the
photo-switching properties of azobenzene side-chain fully retained its photoisomerization ability,
with a change in pore dimensions upon light exposure [106,108–110].
In analogy to the preceding examples, Modrow and his group [108] introduced the azo
functional group to the Cr-MIL-101-NH2 mesoporous cages. Photo-responsive behavior of this MOF
was tested by CH4 adsorption experiments (Figure 12). Upon interaction of the prepared MOFs with
sporadic irradiation of UV-Vis light, a variation in the CH4 sorption behavior was perceived, e.g., an
increase of the cis-isomer concentration. The rise in the CH4 adsorption capacity is attributed to the
change of the polarity, dipole moment and free space for each conform [70,108].
Figure 12. The product of Cr-MIL-101-NH2 with p-phenylazobenzoylchloride
4-(phenylazo)phenylisocyanate (2). Blue amide and green urea [108].
(1) and
The third generation includes frameworks that change themselves, upon exposure to light. This
kind of dynamic photo-switching MOFs is difficult to synthesize, and it is considered as the most
affected light-induced transformation. Lyndon and his group performed a very interesting study to
employ MOFs with photo-switching behaviors in low-energy CO2 capture and release. They
conducted their study on a triple interpenetrated framework (Zn(AzDC)(4,4′-(BPE)0.5) using UV
light. The results revealed that an immediate discharge of up to 64% of the adsorbed CO2 using
broadband radiation, similar to concentrated solar sources. Moreover, a fully reversible response
has been noticed [105]. In summary, different modes of attachment lead to different
photo-switching behavior.
This brief survey must suffice to introduce a pervasive phenomenon, which must often affect
the structure of MOFs with flexible nature, and hence their responsive behaviors: this will be a
recurring item in the application section.
3. How to Control MOFs Flexibility
Either modifying the organic linker or the metal ion can easily tune the structure of a MOF.
Reactions such as Friedel–Craft acylation or Lewis acid catalyzed alkylation and reactions involving
the amide formation and nucleophilic substitution, can be applied in order to introduce flexibility
into the backbone of the MOF [111,112].
3.1. Ligand Control
Alternative techniques are adopted, when active species fail to be incorporated within a
framework. Solvent assisted ligand incorporation (SALI), is a technique that allows the addition of
new active species into zirconium based MOFs (Zr-MOF), through simple acid-base chemistry [113].
These catalytic functions are either added at the linker, or the Zr oxide node, or through pores
encapsulation [114]. Deria and co-workers were able to successfully incorporate perfluoro alkane
carboxylates of various chain lengths on the Zr6 nodes of NU-1000 through the SALI technique.
Catalytic functionality is introduced through the ionic bonding of the OH groups on the NU-1000
node and the carboxylate group of the perfluorinated chain (Figure 13) [115–117].
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Figure 13. Solvent-assisted ligand incorporation onto Zr-nodes of NU-1000. Perfluoro alkane
carboxylates were successfully incorporated on the Zr6 nodes of NU-1000 [114].
3.1.1. Linker Substitution
The choice of organic linker highly affects the flexibility behavior of the MOF. Adsorption
properties, as well as network porosity and flexibility, will be highly influenced by the selected
linker within a MOF structure.
The adsorption behavior of a series of isoreticular Cr (III) or Fe(III) dicarboxylates labeled
MIL-88A–D was studied by Serre and co-worker [34]. The difference between the four MIL-88 is the
choice of organic ligand which is as follows: dicarbox = fumarate (88A); terephthalate (1,4-BDC)
(88B); 2,6-naphthalene dicarboxylate 2,6 NDC) (88C); and 4-4′-biphenyl dicarboxylate (4-4′-BPDC)
(88D). Upon adsorption, the interaction between guest molecules and MIL-88 caused noticeable
breathing motion for the MIL with the linker with the longest chain length. The longer the length of
the linker, the higher the ratio of the cell volume of the open form to that of the dried solid (1.85 for
MIL-88A to 3.3 for MIL-88D). Water stable isoreticular phosphonate-based Cu-MOFs, having both
phosphonate and N-heterocyclic linkers, were first synthesized by Taddei and co-workers. The
flexibility of this specific catalyst was studied by varying the choice of the linker. When
N,N,N′,N′-tetrakis
(phosphonomethyl)
hexamethylenediamine
(denoted
as
L)
and
1,2-bis-(4-pyridyl)ethane (denoted as etbipy) were used as organic ligands for the construction of
Cu3(L)(etbipy)2 framework, a reversible breathing effected was noticed. On the contrary, when
replacing one of the mentioned linkers with a more rigid linker such as etbipy with 4,4′-bipyridyl,
the breathing effect during dehydration was irreversible [118].
Vermoortele and coworkers studied the effect of linker substitution on a series of functionalized
UiO-66 (Zr). They were able to prove, through molecular modeling calculations, that the activity of
UIO-66 with coordinatively unsaturated sites was drastically increased when functionalized linkers
were added to the framework. Therefore, when the nitro substitution was added, the activation free
energy of the reaction was lowered due to low adsorption capacity [119].
3.1.2. Linker Rotation
A breathing MOF can be formed by applying the concept of linker rotation. In order to obtain a
“gate” effect, allowing the pores to open and close freely, the linker must rotate between fixed
positions (see Figure 14) [74,120].
Figure 14. Ligands can rotate to open and close the pore, upon adsorption and desorption
respectively. Green spheres represent metal centers or secondary building units (SBUs), gray
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represents rigid portions of the ligand, and blue components represent flexible portions of the ligand
[74].
Functional groups that are added into the linker have a major effect on the rotational behavior
of the ligand. The introduction of four CH3 groups in the aromatic linker of MIL-53(Cr), causes the
phenyl rings to rotate by 90°, preventing the adsorption of guest molecules [121,122]. The host–guest
interaction of some gases and liquids during adsorption/desorption, tend to also control the
flexibility of some MOFs [74]. In the case of MIL-53, the adsorption of carbon dioxide, water, and
some hydrocarbons tend to control the size and the structure of the pores from large rectangular to
narrow trapezoids [123].
In the case of MIL-88, the absence of any sort of linkage between its “equatorial” trimers, causes
the framework to swell upon hydration, leading to the adsorption of large molecules. The free rotation
of the COO groups around the single carbon–carbon bond causes this type of swelling in addition to
the change in the dihedral angle between the COO plane and the trimers plane (Fe–Fe–Fe) [85].
3.1.3. Post-Synthetic Modification (PSM)
The steric hindrance of the functional groups that are incorporated within the MOF framework
tends to reduce the flexibility of the ligands. Rigid phenyl rings or heterocycles are such an example.
Incorporation of such groups can be predefined, either during synthesis or through post-synthetic
modifications (PSM) [124]. The secondary building units (SBU) approach can also be used in order to
incorporate some functional groups within the framework [34,125]. Monocarboxylic acids with
different backbones were added to the framework of JLU-Liu4 (JLU—JiLin University) during
synthesis. The inclusion of small carboxylic acids (formic acid and acetic acid) resulted in a gate
opening behavior during adsorption. On the contrary, when bulkier carboxylates such as benzoic
acid and cinnamic acid were added a different structural transformation was observed. For MOF
containing benzoic acid, classical breathing behavior was attained, while a step-wise increase in the
unit cell was observed for cinnamic acid MOF [126].
Horcajada and coworkers analyzed the flexibility of MIL-88 after a few ligand modifications
through the introduction of functional groups onto the aromatic rings. Different flexibility behaviors
were observed for iron (III) 4,40-biphenyl dicarboxylate and iron(III) terephthalate MIL-88, due to
different steric and intermolecular interactions between the phenyl rings. The bigger the size of the
functional group that was introduced, the smaller the breathing amplitude [35].
3.2. Metal Ion Modification
Since a MOF is a combination of organic linkers and metal nodes, the flexibility will depend on
the type of metal node and how it is connected to the linker. For cobalt-based MOF, Co(BDP) (BDP =
1,4-benzene dipyrazolate), the framework adapts five structural transitions during the adsorption of
nitrogen at 77 K [61,127]. Upon adsorption, a breathing effect is noticeable, and the geometry of the
framework adopts a tetrahedral form, which is caused by the long 0.26 nm Co–N bond [127]. Even
though Cr3+ and Fe3+ possess similar electronegativities and ionic radii, MIL-53(Fe) and MIL-53(Cr)
show different behavior towards the sorption of CO2. The interaction of guest molecules with
MIL-53 upon adsorption is highly affected by the choice of the metal ion within the framework
[127,128].
Greathouse and Allendorf studied the intramolecular interaction behavior of Zn ions and water
oxygen atoms upon the adsorption of water on MOF-5. MOF-5 is composed of Zn4O clusters bridged
with BDP organic ligands; each Zn ion is coordinated by one inorganic O atom (O1) and three BDC
O atoms (O2). Changes in Zn coordination with increasing water content were observed [129].
Molecular dynamics simulations show that there are three possible interaction routes between water
and MOF-5. The first is a direct attack on the ZnO4 tetrahedron, where a water oxygen atom replaces
an O atom of the coordinating MOF. The second is a possible interaction between the water
hydrogen atom and O2, and the third interaction is the interaction of H atom to one or more ZnO4
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tetrahedra (Figure 15). Greathouse and Allendorf suggest a possible modification of the metal–linker
interaction so that the framework can withstand stronger metal–water interaction.
Figure 15. Zn coordination behavior upon water adsorption. Zn retains tetrahedral coordination, but
the relative amounts of BDC oxygen (O2) and inorganic oxygen (O1) decrease as they are replaced by
water oxygen atoms [129].
4. Characterization Techniques to Detect Flexibility
Understanding the dynamic and structural changes in flexible MOFs is key in order to be able
to explain potential behavior and possible framework modification [52]. The mechanical properties
of flexible porous MOFs can be studied through different methods such as mercury porosimetry.
This method relies on the intrusion of mercury molecules within the pores at a specific pressure. The
transition between LP to NP can be analyzed by applying mechanical pressure [59]. Other
spectroscopy techniques that are used to study the flexibility behavior of MOFs are mentioned in
this section.
4.1. Nuclear Magnetic Resonance (NMR)
The detection of flexibility through NMR allows monitoring of the structural changes of the
host during adsorption. For instance, 129Xe NMR spectroscopy relies on the adsorption of xenon gas
to study the Xe chemical shift as an indicator of the extended transition from LP to NP.
Springuel-Huet et al. were able to perform in-situ 129Xe adsorption NMR experiments on the flexible
MIL-53(Al) material at different pressures and temperatures [130]. They based their findings on the
chemical shift of Xe to study the local Xe amount and the transformation from LP to NP form.
The transformation of an anisotropic framework was studied by Murdock and coworkers using
a 13C cross polarization magic-angle-spinning (CP-MAS) NMR. They were able to directly monitor
phenyl rotation as a function of solvation [74,131]. Results show that this spectroscopic method is a
powerful tool to monitor the breathing effect in MOFs.
2H NMR spectroscopy can also be used to characterize the dynamic behavior of many catalysts,
like porous coordination polymers (PCPs). Horike et al. synthesized a new class of PCPs with
rotational organic groups and regulated their dynamic motion by guest sorption [132]. 2H NMR
spectra were recorded for the same sample at different temperatures in order to demonstrate the
change in the topology of the framework with temperature as well.
4.2. Powder X-ray Diffraction (PXRD)
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Powder X-ray diffraction (PXRD) is another powerful analytical technique that can be used to
determine structural transformation in flexible MOFs [60]. The freedom of motion of polar linker
groups in the framework of IRMOF-2 was investigated by Winston et al. The framework of IRMOF
consists of octahedrally coordinated zinc oxide clusters linked by the bromoterephthalate group.
These groups contain a rotatable bromo-p-phenylene moiety [133]. The XRD for IRMOF-2 yielded
good agreement with the published data for the atomic positions showing eight-fold disorder of the
bromine.
Devic and co-workers used a combination of techniques in order to evaluate the effect of the
functional group X (X = –Cl, Br, CH3, NH2, (CO2H)2), of a series of functionalized flexible MIL-53(Fe)–
X during adsorption of CO2. Different diffraction patterns were obtained, indicating the effect of the
functional group on the pore opening during adsorption from NP to LP [134]. An example is given
in Figure 16, showing the difference between spectra of MIL-53(Fe) with and without a functional
group.
Figure 16. CO2 adsorption on MIL-53(Fe) –X (X = –CH3) at 230 K followed by PXRD. Spectra were
recorded at different pressures. Black: anhydrous closed pores form; red: intermediate form; blue:
NP form and green: LP form [134].
4.3. Infrared Spectroscopy (IR)
Infrared spectroscopy is a vibrational method often used to determine the chemical and
structural changes of the host framework caused by the interaction of the skeleton with the guest
molecule. Salles and coworkers, relied on IR spectroscopy to study the pyrazolate ring stretch
vibrations in Co(BDP), during N2 adsorption at 100 K. As the pressure is increased, the C–N
stretching vibrations of the pyrazolate rings undergo different transition states. In situ infrared
spectra were recorded to show the transition from NP to intermediate phase ip1 and ip2 (at 0.01 and
0.023 bar, respectively), see Figure 17 [127].
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Figure 17. In situ infrared spectra of Co(BDP), during N2 adsorption at 100 K. Phase transitions from
the dry form (gray) to ip1 (green) and ip2 (blue). Reprinted [127].
4.4. Raman Spectroscopy
The structural flexibility of a MOF plays an important role in the separation of CO2 and CH4.
Hamon et al., studied the effect of flexibility of MIL-53(Cr) on the breakthrough curves, during the
adsorption of a mixture of CO2 and CH4 [135]. They examined the NP/LP fraction through Raman
spectroscopy, by varying the ratio of CO2 to CH4. As the concentration of CO2 increases, the narrow
pores open up and CH4 molecules are pushed out of the NP to accommodate CO2 molecules.
Figure 18 shows the consistency of integrated Raman band intensities of adsorbed CO2 and CH4
molecules with the adsorption isotherms. These results show that Raman spectroscopy is a useful
technique to track the concentration of the adsorbed species in situ.
Figure 18. Adsorption isotherms of CO2 (♦) and CH4 (●) on MIL-53(Cr) at 303 K compared to the
relative Raman intensity of CO2 (◊) and CH4 (○). Gas composition CO2:CH4, left: 0–100 and right: 50–
50. The simulation of CO2 is represented in full line and in dashed for CH4 [135].
5. Application
There is already a very considerable literature on the synthesis and application of rigid MOFs,
and no doubt, its growth is set to continue. However, interest has been shown during the last years
in the remarkable properties of the third generation MOFs such as flexibility, large surface area, and
high porosity, giving those properties particular attention in numerous applications, especially in
the area of storage and separation, sensing, and guest capture processes.
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The value of good theoretical studies or explanation of the flexibility of MOFs lies in its ability
to correlate a wide range of observed phenomena through useful applications, such as selective gas
separation [136–138], molecular recognition [139], catalytic process [140], sensing [141] , and
biomedical application (i.e., controlled drug release) [142]. Each application has its own set of
requirements when it comes to choosing the correct type of MOF, depending on the intended use;
this section will focus on what is desired for the efficient application of flexible MOFs. BASF is the
only chief company known to be working towards the industrial-scale synthesis of MOFs [143,144],
they offered six MOFs only two with flexible nature, namely ZIF-8 and MIL-53(Al) (corresponding
to Basolites A100 and Basolites Z1200 respectively). A general industrial application for different
flexible MOFs is still awaited.
5.1. Catalytic Application
Given the importance of MOFs during the past two decades, it is surprising that so little
research has been devoted to the industrialization of flexible MOFs in catalytic reactions. The
researchers and licensors are concerned with the industrial application of flexible MOFs to the
particular problems that led to the development of those catalysts. This neglect is perhaps explained
by observing that flexible MOFs are not stable under different operating conditions and different
reactions.
Das and coworkers [140] pioneered the utilization of the pores of a flexible MOF as a catalyst to
monitor the cyanosilylation and Knoevenagel reaction. During the reaction changes in the space and
shape of the voids were detected. Those changes were induced by the introduction of guest
molecules inside the cavity. The dynamic change of the pore space triggered the interest in intense
research in investigating other chemical and biological reactions inside the pores of flexible MOFs.
Since flexible MOFs are still only of limited interest as catalysts for several reactions, a more
expectant statement cannot be determined, because the breadth of these studies is still not extensive.
5.2. Separation
By choosing the appropriate physical form of the framework, MOFs can be used in various
types of selective gas separation applications. The framework is considered a vital element in
determining remarkable separation. Stavitski and coworkers investigated the separation of CO2
from a mixture of gases through NH2-MIL-53(Al) [145]. Figure 19 shows the structures and relative
energies of NH2-MIL-53 materials and their complexes with CO2. The confinement of the CO2
molecule within the NP and LP NH2-MIL-53(Al) structures is also given. The outstanding high
efficiency in capturing CO2 molecules is attributed to the flexibility of the framework and its capacity
to modify the structure upon gas adsorption.
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Figure 19. Structures and relative energies of NH2-MIL-53 materials and their complexes with CO2
[145].
Pressure swing separation is an essential application for flexible MOFs[145].
Kim and coworkers [138] investigated the textural properties of NH2 MIL-53(Al) upon CO2/N2
separation. This MOF shows high selectivity towards CO2 adsorption and remarkable structural
flexibility during six cycles of pressure swing adsorption–desorption. In this respect, it is worth
mentioning that the adsorption isotherms of flexible MOF follow different isotherms. Therefore,
their behavior cannot be classified according to the IUPAC scheme because such MOFs can undergo
structural changes during adsorption [146].
Li and coworkers [147] also studied the challenging separation of a mixture with similar
molecular size “propyne/propylene” over rigid–flexible MOF. This type of MOF is of particular
interest with regards to its unique properties since it can function at the low-pressure range as well
as high-pressure range, where it transforms into a flexible MOF. In their study, ELM-12 was studied
as it has a rigid square-grid copper bipyridine scaffold with dynamic dangling OTf− (OTf− =
trifluoromethanesulfonate) groups. It was noticed that the strong binding affinity of ELM-12
towards C3H4 increased with increasing pressure, contrary to ELM-12 adsorption capacity towards
C3H6. Therefore this type of MOF could be applied to separate different types of gas mixtures with
similar molecular size. Their results expanded the dictionary of application of third generation
porous MOFs for combined separation/purification applications. Much attention has been given to
investigate the ability of flexible MOF in liquid phase separation due to their capability of fine
control guest uptake compared to conventional inorganic structures. For example, flexible porous
MIL-53(Fe) was used as a stationary phase during the application of HPLC, to separate xylene
isomers from mixtures of BTEX (benzene, toluene, ethylbenzene, and the three xylene isomers) [148].
Figure 20 shows that the xylene isomers are well separated.
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Figure 20. Chromatograms of the BTEX (benzene, toluene, ethylbenzene, and the three xylene
isomers) on MIL-53(Fe) at 293 K (left a) and 323 K (right b) [149].
5.3. Guest Capture
For fundamental studies, there is much to be said about using flexible MOFs in controllable
capture and release, particularly in gas capturing via host−guest interactions or the pore structure
state. This intriguing aspect deserves investigating due to its importance in some applications such
as drug delivery application.
A seminal work based on the controllable gas capturing and releasing was reported by
Matranga and co-workers [149]. They investigated 3D and 1D porous coordination polymers (PCP),
with different degrees of flexibility and similar gate opening pressure response towards CO2. Their
results revealed a change in the pore structure of both PCPs upon gas sorption, regulated domain
growth, and ligand reorientation accordingly. In a more precise description, the length of the linker
spacing and consequent size of the void spaces changes in a controlled manner.
In an exciting study to investigate the metal-ion-capture properties, Thallapally and co-workers
[48] studied the potential of the trinuclear cluster with a flexible ligand MOF [Mn3 (L)2]−2·2[NH2
(CH3)2]. 9DMF to capture harmful divalent transition-metal cations. This MOF sequesters itself by
deformation of its trinuclear cluster and the coordination of additional metal ions such as Cu2+, and
Ni2+. The results revealed that the MOF displayed a high capture selectivity initiating from the
coordination interaction of targeted ions and the framework via cooperative breakage/formation of a
metal−carboxylate, see Figure 21.
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Figure 21. Scheme showing the incorporation of transition metal (TM2+) cations into the anionic
networks to afford neutral and heterobimetallic systems via ligand directing
single-crystal-to-single-crystal structural rearrangement [48].
5.4. Sensing
Flexible MOFs with a reversible structural flexibility accompanied with (i) distinct
color-changing and/or (ii) crystal size deformation or change the framework structures from closed
to open forms upon thermal stimuli or gas uptake demonstrated their ability in the smart sensing
application. Such composites would selectively adsorb gases for which interactions with the
composites were strong enough to change the framework structures. This has been the subject of
some recent research. Yanai and his group investigated the role of host−guest interactions of the
fluorescent guest molecules and flexible structure and their effect on sensing mechanism. Their
study tested the selective detection of CO2 and C2H2 over DSB@[Zn2(BDC)2dabco]n
(DSB—distyrylbenzene). This MOF showed different fluorescence responses towards CO2 and C2H2
[150]. It is now possible to dive deeper into the versatile sensing and switching fluorescent MOFs.
For example, Dong and his group adopting the concepts of this material to study the potential and
behavior of a new type of flexible emissive silver-chalcogenolate cluster-based MOF (SCC-MOF) for
the adsorption of the volatile organic compound “VOCs.” This MOF showed unparalleled distinctly
different visible luminescence colors upon chloromethanes adsorption (CH2Cl2, CHCl3, CCl4) [151].
In summary, the general desire to engage flexible MOFs that have detectable relevance to industrial
applications, triggers scientists to invest in academic laboratories in order to remove the barriers
towards large-scale applications.
6. Conclusions
The purpose of this review is to try to construct a map to guide the reader through the literature
on the types, nature, control, and characterization of flexible MOFs. In order to evolve some general
principles, this review delineated the main features and outlined the strategy to be adopted in their
design in order to find the desired route for any application. Many excellent reviews and books shed
light on the advances in both theoretical thinking and skillful experimental on the rigid MOFs.
However, theoretical and experimental studies that deal with flexible MOFs are lacking, and any
attempt to rationalize the changes in their structure through the external stimuli to the industrial
application is still missing. In this review, a short but informative account of the literature on flexible
MOFs has been investigated and guidance provided to the reader to see wherein the complications
lie in their design and accordingly their responsive behavior. In conclusion, smart manipulation of
metal−ligand coordination along with their general responsive stimuli can lead to outstanding third
generation porous material that covers a wide variety of applications.
Funding: This research was funded by National Natural Science Foundation of China grant number
21850410449 and by Tomsk Polytechnic University Competitiveness Enhancement Program grant number
(VIU-2019).
Acknowledgments: The authors would like to express their deep accolade to the “State Key Laboratory of
Advanced Technology for Materials Synthesis and Processing” for financial support.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
ABTC
BDC
BDP
BHE-bpb
bmb
BPE
BTC
Azobenzenetetra carboxylate
Benzene dicarboxylate
1,4-benzene dipyrazolate
2,5-bis(2-hydroxyethoxy)-1,4-bis(4-pyridyl)benzene
1,4-bis(2-methylbenzimidazol-1-ylmethyl) benzene
Bis-pyridyl ethylene
Benzene tricarboxylate
Catalysts 2019, 9, 512
CP
SCC
FL-MOFs
H3L
HKUST-1
IBU
IR
JLU-Liu4
L
LP
M
MIL
MTV
NMR
NP
PCP
PDC
PSM
PXRD
Pz
Pzdc
SALI
SBBs
SBUs
TM
VOC
ZIF
ZIF-8
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Closed Pores
Chalcogenolate cluster
Flexible ligand metal organic framework
5-(2-carboxybenzyloxy)
Hong Kong University of Science and Technology
Inorganic building unit
Infrared spectroscopy
JiLin University
Linker
Large Pores
Metal
Materials of Institute Lavoisier
multivariable
Nuclear Magnetic Resonance
Narrow pores
Porous coordination polymers
Pyridine dicarboxylic acid
Post-synthetic modifications
Powder X-ray diffraction
pyrazolate
2,3-pyrazinedicarboxylate
Solvent assisted ligand incorporation
Supramolecular building blocks
Secondary building units
Transition metal
Volatile Organic Compound
Zeolitic Imidazolate Framework
[Zn(mIm)2]n (mIm, also Im) = 2-methylimidazole
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