Structural and dynamic studies of substrate binding in porous metal–organic frameworks

Porous metal–organic frameworks (MOFs) are the subject of considerable research interest because of their high porosity and capability of specific binding to small molecules, thus underpinning a wide range of materials functions such as gas adsorption, separation, drug delivery, catalysis, and sensing. MOFs, constructed by the designed assembly of metal ions and functional organic linkers, are an emerging class of porous materials with extended porous structures containing periodic binding sites. MOFs thus provide a new platform for the study of the chemistry and reactivity of small molecules in confined pores using advanced diffraction and spectroscopic techniques. In this review, we focus on recent progress in experimental investigations on the crystallographic, dynamic and kinetic aspects of substrate binding within porous MOFs. In particular, we focus on studies on host–guest interactions involving open metal sites or pendant functional groups in the pore as the primary binding sites for guest molecules.


Introduction
Porous metal-organic frameworks (MOFs) are assembled from metal ions or clusters which are bridged by functional organic molecules. These materials often adopt three-dimensional extended microporous or mesoporous framework structures incorporating nano-and meso-sized pores, yielding large and accessible internal surface areas. Over the past decade, these hybrid materials have received considerable attention due to their potential for gas storage and separation, catalysis, optical properties, sensing, substrate recognition and drug delivery. 1,2 These important properties primarily rely on the selective recognition and binding of guest molecules by these functional materials.
Within the field of gas storage, separation and purification, there is particular emphasis on optimising the interactions between MOF hosts and the adsorbed substrate molecules, leading to the discovery of new functional materials with higher storage capacities and stronger binding energies. For this reason, visualisation of the guest-host binding interactions involved in the gas adsorption applications is crucial to the understanding of how these materials function, and to identify important potential mechanisms for the sensing and discrimination of guest molecules. The experimental investigations reviewed herein are threefold. (i) The determination of the locations of guest molecules within porous MOFs by static crystallographic studies, typically by in situ diffraction experiments as a function of gas loading, gives key insights into the preferred binding sites within pores. (ii) The direct observation of the dynamics of these adsorbed/ trapped/captured guest molecules within the host together with the change of the molecular motion of the MOF materials, typically by spectroscopic experiments, reveals the nature of these binding interactions. (iii) Studies on the diffusion of guest molecules within the pores and channels of porous MOFs by pulsed field gradient (PFG) NMR or quasi-elastic neutron scattering (QENS) experiments can afford important knowledge on the kinetics of gas loading, and such information is fundamental to optimise gas adsorption processes in industry. Computational modelling, including density function theory (DFT) and molecular dynamics (MD), linked to experimental data has played an important role in visualising molecular motions and diffusion within pores. A combination of these static, dynamic and kinetic approaches offers a comprehensive understanding of the guest-host binding processes that ultimately govern these materials properties.
However, gaining such insight by experiment is highly challenging for a number of reasons. Firstly, MOF materials with excellent gas sorption properties usually crystallise with large unit cells and inevitably incorporate huge structural voids and framework disorder, resulting in weak, broad and/or heavilyoverlapped diffraction peaks, thereby raising the complexity of structure solution and refinement. To overcome this, synchrotron X-ray radiation is often used instead of conventional in-house instrumentation, because diffraction patterns obtained from a synchrotron radiation source have much improved resolution and data quality.
Secondly, these materials and adsorbed gas molecules typically contain many light atoms (e.g., H, D, Li, C, N, O) which have relatively small scattering lengths for X-rays and may be poorly located in X-ray diffraction experiments. Unlike X-ray experiments in which the scattering lengths are generally proportional to Z 2 of the elements (i.e., heavy atoms with more electrons can scatter X-rays more effectively than light elements), neutrons interact with the nuclei of elements and the cross-section of each element is isotope-dependent and does not follow any particular rules. In general, light elements can have large cross-sections in terms of neutron scattering and therefore are more readily ''seen'' by neutrons (Table 1). 3 Thus, neutron diffraction offers unique possibilities for determining key features of both the structural details of MOF materials and the precise locations of gas molecules within the pore cavities. Indeed, neutron diffraction is an excellent experimental method to observe D 2 positions.
Thirdly, the nature of these host-guest interactions is often based upon weak supramolecular mechanisms (e.g., hydrogen bonds, pÁ Á Áp interactions, van der Waals, electrostatic or dipole interactions). Such supramolecular interactions often involve hydrogen atoms and undergo dynamic processes. The dynamics of these interactions are difficult to probe directly by conventional spectroscopic experiments such as infrared (IR) or Raman spectroscopy, which are the most common methods for the study of molecular

Wide spectral range
INS spectrometers cover the whole molecular vibrational range of interest (16-4000 cm À1 ). The lower energy range (below 400 cm À1 ) is readily accessible, a region that is traditionally more difficult experimentally for infrared and Raman spectroscopies. This low energy region often carries key information on the lattice dynamics and translational/rotational motions of adsorbed gas molecules. With modern instrumentation, the quality of INS spectra approaches that of infrared and Raman spectra obtained from the same system under the same conditions. These features make INS an ideal probe to study the MOF-gas binding dynamics, in particular for those involving hydrogen atoms. QENS, a sub-class of INS in which the energy transfer between the neutron and sample is very small (i.e. almost elastic collision), is another powerful neutron spectroscopic technique to study the slow motion and dynamics of gas-loaded MOFs owing to the large cross-section for light atoms, especially for H. In particular, QENS has played an important role in probing the diffusion rate of the gas molecules along the pore structure of MOFs.
Recent studies on porous metal-organic frameworks (MOFs) for H 2 storage, selective carbon capture, and hydrocarbon separations have shown, in exceptional cases, location of guest molecules within the host via advanced crystallography studies (i.e., both X-ray and neutron diffractions), providing invaluable structural rationale for their function and properties. Most of these successes have been achieved within host systems that display strong confinement effects on the guest molecules and/or have specific binding sites such as open metal centres. More recently the applications of INS and QENS to study gasloaded MOFs have enabled the interrogation of the dynamics and kinetics of adsorbed gas molecules within porous MOFs, including the host-guest systems involving primarily soft supramolecular interactions. In this critical review we discuss recent progress using crystallographic, dynamic and kinetic studies on host-guest binding within a range of functional MOF materials, primarily for those containing open metal centres or pendant functional groups for specific guest binding within the pore.

Cu(II)-based systems
Cu(II)-based MOFs, which show a high level of structural diversity and high porosity, have been intensively studied for In situ neutron powder diffraction (NPD) experiments on D 2 -loaded [Cu 3 (BTC) 2 ] at 25 K up to a saturated D 2 dosage of 6.5 D 2 :Cu revealed a total of nine different D 2 adsorption sites. 11 The strongest binding sites were found at the coordinatively unsaturated Cu(II) sites with a Cu-D 2 (centroid) distance of 2.39(1) Å (Fig. 1). The adsorption of D 2 in this trimodal pore system follows a complex pore filling mechanism, with the D 2 site occupancies changing with dosage. This is attributed to the concentration-dependant optimal D 2 arrangements that result from the D 2 -D 2 and framework-D 2 interactions. At lower loadings, the adsorption sites D 2 (2), D 2 (3), and D 2 (5) were found within the smallest cage, while D 2 (4) and D 2 (6) reside in the intermediate sized pore. At 5.5 D 2 :Cu coverage, D 2 starts to populate the largest cage (Fig. 2). A dynamic rearrangement of the D 2 in the pores at loadings close to saturation reveals 4 D 2 molecules placed tetrahedrally within the smallest pore, 32 D 2 as a truncated octahedron in the intermediate sized cage, and 48 D 2 forming a rhombic dodecahedron in the largest pore.
An INS study of H 2 adsorbed in [Cu 3 (BTC) 2 ] has been carried out by Liu et al. 12 Three distinct spectral features from the INS spectra at H 2 loadings of less than 2.0 H 2 :Cu were assigned to the same first three binding sites identified previously 11 ( Fig. 3 and 4). These three binding sites are progressively populated at low coverage, consistent with the results obtained from the NPD study. It is widely accepted that MOFs incorporating ultra-micropores (below 10 Å) are optimal for H 2 binding due to enhanced overlapping potentials, consistent with the observed order of pore-filling. This has been further supported in a series of Cu(II)-tetracarboxylate MOFs in which lower H 2 uptake was observed with increasing pore size. [13][14][15] A recent study on CH 4 storage in porous MOFs revealed that [Cu 3 (BTC) 2 ] shows record high volumetric CH 4 uptakes of 230 cc(STP) cc À1 at 35 bar and 270 cc(STP) cc À1 at 65 bar at room temperature. 16 A detailed structural analysis of [Cu 3 (BTC) 2 ] with various CD 4 loadings has been performed using NPD, and reveals four distinct CD 4 binding sites (Fig. 5). 17 The first two favoured adsorption sites are located on the open Cu(II)   sites and in the centre of the triangular window formed by three [Cu 2 (OOCR) 4 ] paddlewheels and three BTC 3À units. The small window pocket shows an excellent complementarity with the geometry of a CH 4 molecule, thus creating an enhanced van der Waals binding interaction to CD 4 . Two secondary adsorption sites are located at the centre of the smallest octahedral cage and in the corner of the intermediate-sized cage.
In addition to its excellent storage capacities for H 2 and CH 4 , [Cu 3 (BTC) 2 ] shows very high acetylene storage capacity with an uptake of 201 cm 3 g À1 at 295 K and 1 atm. NPD studies on C 2 D 2 -loaded [Cu 3 (BTC) 2 ] reveal the significant contribution of open Cu(II) sites for acetylene binding (Fig. 6). 18 Structural refinement on the NPD data at 0.62 C 2 D 2 per Cu loading in [Cu 3 (BTC) 2 ] indicates that C 2 D 2 is adsorbed exclusively at the open Cu(II) sites with a distance between the Cu(II) ion and a C 2 D 2 molecule of 2.62 Å. The C 2 D 2 molecule lies parallel to the O-Cu-O axes with two half-occupied orientations. The second strongest C 2 D 2 binding site was identified at a loading of 1.5 C 2 D 2 per Cu, and resides at the entrance window of the smallest cage and reminiscent of the second binding site observed with CH 4 (above). First-principle total-energy calculations confirm the two preferential adsorption sites from the NPD results, and also suggest a slightly distorted geometry of the adsorbed C 2 H 2 on Cu(II) sites. It was postulated that the distorted C 2 H 2 has an induced dipole moment which creates an enhanced Coulombic interaction between the C 2 H 2 molecule and the open Cu(II) charge density.
[Cu 3 (BTC) 2 ] has been investigated for its applications in the capture of I 2 as a nuclear fission product and the adsorption of I 2 vapour in the presence of moisture has been assessed. 19 A gas stream containing I 2 and water was used for the adsorption experiment as a model of the mixed gas streams in nuclear energy industrial processes. Synchrotron-based powder X-ray diffraction (PXRD) data on [Cu 3 (BTC) 2 ] loaded with different amounts of I 2 were collected. Due to the strong scattering nature of I 2 , structural analysis clearly identified two significant I 2 adsorption sites in this porous host (Fig. 7). The wet gas stream results in binding of water to the apical sites of the Cu(II) paddlewheels such that the primary binding sites of I 2 were located within the smallest cage, close to the [Cu 2 (OOCR) 4   linkers with various functionalities, and these materials incorporating [Cu 2 (OOCR) 4 ] paddlewheels exhibit high H 2 storage capacities, up to an uptake of 77.8 mg g À1 at 77 K, 60 bar.
NPD was used to determine the H 2 adsorption sites in MFM-101 (MFM = Manchester Framework Material, replacing the NOTT designation) and the structural refinement identified three distinct binding sites. 21 The first adsorption site was located at the axial position of the [Cu 2 (OOCR) 4 ] paddlewheel with a Cu-D 2 (centroid) distance of 2.50(3) Å, slightly longer than that observed in [Cu 3 (BTC) 2 ] (2.40 Å). This type of D 2 -Cu interaction is not of the ''Kubas'' type s-bond binding. 22 Two other adsorption sites were located at the middle of the triangular [(Cu 2 ) 3 (isophthalate) 3 ] window and in the cusp of three phenyl rings (sites II and III, Fig. 9), respectively, with both residing on the same 3-fold symmetry axis. Site occupancy analysis   indicates that the open Cu(II) site does not show significantly higher binding affinity to D 2 than the other two identified sites.
Tri-branched hexacarboxylate linkers with three isophthalate units connected through a central core in a coplanar fashion have been widely developed for the construction of highly porous (3,24)-connected networks. 23 These MOFs show exceptionally high porosity and gas sorption capacities. One archetype of this series is MFM-112, assembled from a C 3 -symmetric aromatic hexacarboxylate linker and [Cu 2 (OOCR) 4 ] paddlewheels. 24 MFM-112 is constructed by three different cages: cage A, a cuboctahedron containing of 24 isophthalate units and 12 [Cu 2 ] paddlewheels; cage B, a truncated tetrahedron comprising four hexagonal linker faces and four triangular windows; and cage C, a truncated octahedron formed by eight linker faces and six square windows. MFM-112 shows both high H 2 adsorption at low and high pressures at 77 K: 23 mg g À1 at 1 bar and 111 mg g À1 at 77 bar. NPD studies revealed an interesting difference between the two Cu(II) ions of the same [Cu 2 (OOCR) 4 ] moiety in terms of their interaction with D 2 molecules. 15 The Cu(II) ions inside cage A (CuA) show a higher binding energy towards D 2 molecules than the Cu(II) centres at the exterior of the cage (CuB) [CuA-D 2 (centroid) 2.23(1) Å vs. CuB-D 2 (centroid) 2.41(1) Å] (Fig. 10). This study provides the first direct structural evidence demonstrating that a specific geometrical arrangement of open Cu(II) sites within the [Cu 24 (isophthalate) 24 ] cuboctahedral cage strengthens the interactions between D 2 molecules and the coordinatively unsaturated metal sites. Three other adsorption sites were identified at higher loadings, with two residing in the triangular opening and one located within cage B close to the 3-fold axis of the window.
A tetrazolate linker has been used to synthesize the robust sodalite-type framework HCu[(Cu 4 Cl) 3 (BTT) 8 ]Á3.5HCl (Cu-BTT, BTT 3À = 1,3,5-benzenetristetrazolate). 25 The NPD study on the D 2 -loaded sample demonstrated that D 2 molecules directly bind to unsaturated Cu(II) ions, revealing a Cu-D 2 (centroid) distance of 2.47 Å (Fig. 11). The observed Cu-D 2 distance is slightly elongated compared to the Mn-D 2 distance of [2.21 Å] and the Fe-D 2 distance [2.17(5) Å] found in the structural analogues Mn-BTT 26 and Fe-BTT, 27 respectively, probably due to the Jahn-Teller effect of the Cu(II) ions. Three additional D 2 binding sites were identified in Cu-BTT. Site II resides in the  pocket of four tetrazolate units with distance of 3.46 Å from the framework Cl À ion. Site III and IV, occupied at higher loadings, are located within van der Waals contact of two tetrazolate rings (site III) or benzene rings (site IV).
One of the small gas molecules that has attracted recent and growing attention is CO, a key chemical feedstock. One of the primary issues in utilising the massive quantities of CO produced as a bi-product of industrial processes, or as syngas, is the separation of CO from other gases (e.g. N 2 , or H 2 in the syngas case). Long et al. have recently described further developments in this area in which they have demonstrated that the binding energy of CO with the open metal sites of MOF-74-M (M = Mg, Mn, Fe, Co, Ni, Zn) follows the Irving-Williams order of stability. 28 A further notable example of CO binding with promising separation capability in a dynamic MOF material is described in this section.
In 2014, Kitagawa et al. described a remarkable dynamic MOF, [Cu(aip)] (aip 2À = azidoisophthalate), that exhibits excellent separation of CO from N 2 by an unusual mechanism in which the interaction of CO with the framework causes a macroscopic structural change. 29 This comes about because the isolated [Cu 2 (OOCR) 4 ] paddlewheels within the as-synthesised material respond to removal of solvent by twisting to form infinite chains of paddlewheels coordinated axially via one of the carboxylate oxygens of the nearest paddlewheel, reducing the solvent-accessible volume from 38% to 25% of the unit cell. The adsorption of N 2 and CO by the desolvated material show markedly different behaviour: the N 2 uptake is unremarkable but the adsorption isotherm of CO shows both stepped and strongly hysteretic behaviour (Fig. 12).
In situ PXRD and IR studies demonstrated that above a threshold pressure of B1 bar, the framework undergoes a significant phase change to a more open form that closely resembles the as-synthesised material, with CO bound to the axial positions of the [Cu 2 (OOCR) 4 ] paddlewheels, and located in the newly re-opened channels of the material (Fig. 13). This was confirmed by Rietveld refinement of the synchrotron PXRD data for the CO-loaded material. Notably, on testing CO and N 2 uptake from a range of mixtures of the two gases, it was concluded that even in the open form, N 2 is not efficiently absorbed. This was rationalised by the presence of three weakly coordinated CO molecules surrounding the channel that sterically block access to the N 2 molecules. This study, therefore, provides a detailed analysis and explanation at the molecular level of the observed highly selective adsorption of CO at 120 K.
In addition to the numerous other Cu(II)-based MOFs, we note that a number of key studies on the location and dynamics of guest molecules have also been reported on Zn(II)-based MOFs, in some cases as analogues of Cu(II) frameworks (e.g., with the BTC 3À linker). [30][31][32] The majority of these experiments have already been reviewed extensively.  2 ] N moieties that form a 3D honeycomb structure containing 1D hexagonal pore channels bounded by dobdc 4À ligands which bridge M(II) ions (Fig. 14). 37 All the ligand oxygen atoms are involved in coordination to the M(II) centres and the resulting MO 5 units commonly have the sixth coordination site occupied by a solvent (water or DMF) molecule which points into the channels.  The M-O bond trans to coordinated solvent is typically elongated, as is the M-O bond to the solvent, affording a distorted octahedral coordination geometry. One of the carboxylates coordinates via both oxygen atoms, while the other carboxylate bridges two metal atoms, as do the hydroxy oxygens. These M(II) octahedra are linked by alternating adjacent and opposite edge-sharing resulting in a helical arrangement of metal ions down the c-axis, parallel to the channels.
2.2.1 Structural studies of host-guest binding. These materials show high stability even in their desolvated forms upon removal of coordinated and uncoordinated solvent. The desolvated MOF contains ca. 60% of void space and a high density of vacant metal sites that are readily accessible for guest binding. The M-O bond which was trans to the solvent molecule typically shortens significantly on desolvation, and the M(II) retains a square-pyramidal coordination of ligand oxygen donors (Fig. 14).
A number of studies have investigated the binding of small molecules to the vacant metal sites in MOF-74 and a systematic review of techniques, molecules and approaches to this family of MOFs was published by Valenzano et al. in 2012. 37 Since 2006, water and CO binding have been thoroughly investigated, but by far the most studied small molecules are H 2 and CO 2 , particularly in the Mg, Ni and Zn-based materials. In recent years hydrocarbon binding has become an area of intense investigation, with reports of binding and/or separation experiments with all the MOF-74 series. Here we focus the discussion on the most recent examples of guest binding in MOF-74 materials.
The binding of NO to MOF-74 was first described by Bordiga et al. in 2008. 38 In a study which employed structural (EXAFS, XRD), vibrational (IR, Raman) and electronic (UV/vis, XANES, emission) techniques they investigated the formation of the 1 : 1 adduct of NO with open metal sites in MOF-74-Ni. Adduct formation was found to cause a large perturbation of the vibrational and electronic properties of the framework compared to the dehydrated state, and the bound NO was slowly displaced by water, leading to a potential application for this material as an NO delivery agent in biological tissues.
The separation of O 2 and N 2 is a major industrial process that is currently undertaken using energy-intensive cryogenic  distillation processes. 39 The use of zeolites which selectively adsorb N 2 over O 2 in medical devices is one approach, but this is limited by poor selectivity. 39 The use of MOF-74-Fe as an O 2 selective adsorbent was reported in 2011 and at 298 K the material has an O 2 capacity of 9.3 wt% corresponding to one oxygen molecule per two Fe(II) centres. 40 This capacity doubles at 211 K, with both Mössbauer and infrared spectra consistent with partial charge transfer from Fe(II) to O 2 at low temperatures. At room temperature the spectra show complete charge transfer to form Fe(III) and O 2

2À
. This interpretation was confirmed by NPD data analysis which revealed a symmetric side-on binding mode for O 2 at low temperature and a 'slipped' side-on binding mode in the room temperature structure. In contrast N 2 was found to adopt an end-on coordination mode to the M(II) centre (Fig. 15). Experimentally, the selectivity factor for adsorption of O 2 over N 2 at 298 K was calculated to be 7.5 in MOF-74-Fe, but while this is among the highest reported for MOF materials, [41][42][43][44][45][46][47] the binding of O 2 at this temperature was irreversible and attempts to remove it via heating under vacuum resulted only in decomposition of the framework lattice. In contrast, reversible O 2 binding with retention of the framework structure was achieved at lower temperatures (201-226 K) and hence breakthrough O 2 /N 2 measurements were simulated at 211 and 226 K, with the results suggesting that MOF-74-Fe could make an excellent O 2 selective adsorbent in a modified vacuum-swing adsorption process.
Binding of N 2 O in MOFs has not been widely studied. The oxidation of Fe(II) in MOF-74-Fe to Fe(III) and or Fe(IV) species by N 2 O has been reported, with the latter Fe(IV) complex catalysing oxygenation of ethane to ethanol. 48  , with PXRD confirming the presence of a terminal hydroxide bound to Fe(III) in the latter species. Because this oxidation reaction employs a two-electron oxidant, it is likely that the an intermediate Fe(IV)-oxo species is produced; however, the Fe(IV) species could not be identified or isolated from the reaction. It is this Fe(IV)-oxo species that is believed to catalyse C-H activation and oxidizes ethane to ethanol (Fig. 16). Although the catalytic yield is too low (1%) for practical applications, the study demonstrates that the system is catalytic if competing substrates are excluded.
The binding and selective separation of hydrocarbons including ethane/ethylene/acetylene and propylene/propane is an area in which the use of MOFs has grown rapidly in the last few years [49][50][51] 56 The binding of light hydrocarbons in MOF-74-Fe was addressed 59 by in situ NPD experiments in which desolvated MOF-74-Fe was dosed with the pure deuterated gas (i.e., acetylene, ethylene, ethane, propane, and propylene) at 300 K and then cooled to 4 K for data collection. In all cases only one adsorption site was identified, at the open metal site of the material, shown for each hydrocarbon in Fig. 17. The study revealed binding between MOF-74-Fe and unsaturated hydrocarbons via the specific p-interactions which underpin the excellent separation between alkanes and alkenes, and the much poorer separation between alkynes and alkenes. Continuing research in this area has focussed primarily  2 Dynamic studies of host-guest binding. The above studies have been based on the use of static techniques (e.g., PXRD, NPD) to understand dynamic processes in gas breakthrough experiments by trapping species at low temperatures. This approach has yielded results with small guest molecules described above, and has been applied widely to the studies discussed below on H 2 , CO 2 , CO, CH 4 , H 2 O and hydrocarbons, for which more detailed investigations of the structural dynamics of substrate binding have been reported.

Hydrogen.
Several studies have investigated the vibrational properties of adsorbed H 2 in MOF-74 materials by IR spectroscopy. [62][63][64][65][66][67] The internal stretch modes of free ortho-H 2 (4155 cm À1 ) and para-H 2 (4161 cm À1 ) are IR inactive, but binding of H-H causes a perturbation that induces a dipole making them IR active. The magnitude of the shift of the IR stretch can be often correlated to the binding energy, while the integrated area of the IR bands corresponds to the loading of H 2 within the framework. 63,68,69 However, Chabal et al. demonstrated that in MOFs containing fully saturated metal centres the H-H stretch is dominated by the environment (ligands, metal, structure/geometry). 70 In 2010, their variable temperature IR (VTIR) spectroscopic and computational study examined MOF-74-M (M = Mg, Co, Ni, Zn) to probe the effect of the unsaturated metal site on the vibrational modes of H 2 . 65 The IR spectra recorded at a low loading of H 2 (i.e. H 2 adsorbed at the metal site) appear to show a shift of B30 cm À1 to lower energy over a wide temperature range (20-300 K), a finding which initially appears to disagree with previous studies that reported a shift of B70 cm À1 on H 2 adsorption. 63,64,67 A shift of B70 cm À1 to lower energy was observed by Chabal et al. but only on increasing the loading such that the second oxygen site was occupied. This behaviour was ascribed to a H 2 -H 2 'pairing' interaction between the occupied sites 1 and 2. This perspective was further supported in the observations of Ahn 71 and Rowsell 72 that there is an unusual behaviour in the binding energies of H 2 in MOF-74, as indicated by the H 2 isotherms measured at 77 K and low loading, which was tentatively assigned to H 2 -H 2 interactions. Further studies of the IR spectra of MOF-74-Zn at 40 K at a range of H 2 loadings, 67 confirmed the first binding site to be the open metal centre, with an IR shift of B70 cm À1 . Once the second binding site starts to be occupied (at a loading of B0.85 H 2 per Zn), another IR band with a shift of B30 cm À1 begins to appear, in line with the traditional assignment of these features. A small but measureable shift (B2 cm À1 ) of the initially formed bands on appearance of the second set of bands was observed, which was tentatively ascribed to an H 2 -H 2 'pairing' interaction by comparison with the B6 cm À1 red shift of the H 2 stretch in the free solid state.
However, strong evidence against the H 2 pair formation has been observed by INS experiments, which suggests that the filling of MOF-74 with D 2 is sequential without the formation of pairs. 71   and correspondingly the highest perturbation of H 2 vibrational frequency.
A neutron scattering study of mixtures of D 2 and para-H 2 in MOF-74-Fe was used to directly investigate the possibility of pair formation. 74 The NPD study confirmed sequential site binding of D 2 (Fig. 18). The INS spectra of H 2 -loaded MOF-74-M (M = Fe) at low loadings (o0.5 H 2 per metal) are entirely consistent with INS spectra recorded for MOF-74-M (M = Mg, Co, Ni and Zn), 71 but show the largest splitting of the J = 1 rotational state reported for this series. It was noted that there appears to be no correlation between the binding strength and the peak position in the INS spectra. This study further confirmed the absence of formation of strong H 2 pairs in this system.
Quasi-elastic neutron scattering (QENS) has been used to probe the diffusion of H 2 through MOF-74-Mg. 75 At a low loading of 0.3 H 2 per Mg, the QENS spectrum indicated no diffusion of H 2 on the picosecond timescale. On increasing the H 2 loading, the spectrum progressively broadens. This broadening was fitted to three components corresponding to static H 2 , H 2 diffusing along the pore surface via the adsorption sites, and bulk-like H 2 diffusion through the pores. The component of diffusion along the adsorption sites was indicative of liquid-like jump-diffusion without distinct directionality. The diffusion coefficient was of a similar magnitude to H 2 diffusing on a carbon surface but an order of magnitude lower than the one-dimensional diffusion of H 2 reported in MIL-53(Cr) and MIL-47(V) under similar conditions. 76 A variation on the synthesis of MOF-74-M has been developed by employing the regioisomer 4,6-dioxido-1,3-benzenedicarboxylate (m-dobdc 4À ) in place of the linker 2,4-dioxido-1,3-benzenedicarboxylate (dobdc 4À ). A family of structural isomers of the MOF-74-M (M = Mg, Mn, Fe, Co and Ni) were prepared 77 and these are structurally analogous to MOF-74-M, sharing the same topology and high density of open metal sites, but with different local geometry that subtly alters the electronic properties of the binding sites. By using both X-ray and neutron diffraction techniques combined with INS and IR spectroscopic analyses, three binding sites were identified for H 2 within [Co 2 (m-dobdc)] in a similar arrangement to that of MOF-74 (Fig. 19), but with slightly increased binding enthalpies by 0.4-1.5 kJ mol À1 in comparison to MOF-74-M.

Carbon dioxide.
The series MOF-74-M has been studied extensively in terms of CO 2 uptake up to 42 bar. 33 Of the MOF-74-M (M = Mg, Ni, Zn) series, MOF-74-Mg shows the highest CO 2 uptake (23.6 wt% at 0.1 atm, 35.2 wt% at 1 atm, both at 296 K). This was ascribed at the time to the increased ionic character of the Mg-O bond increasing the adsorption enthalpy at the first binding site. 78 Since these first reports, studies exploiting vibrational spectroscopies 53,79-81 EXAFS, 79 81 CO and N 2 form nearly linear Mg(II)Á Á ÁCO and Mg(II)Á Á ÁN 2 complexes, respectively, but CO 2 binds at an angle with a MgÁ Á ÁO-CO angle of 1291 and with a relatively short Mg-O distance (2.310 Å). This angular orientation is ascribed to a lateral electrostatic interaction between the CO 2 molecule and the carboxylate oxygen of the framework ligand, which also contributes to the marked increase in the adsorption enthalpy of CO 2 in the framework (À47 kJ mol À1 ) compared with N 2 (À21 kJ mol À1 ) or CO (À29 kJ mol À1 ), calculated from VTIR measurements.
X-ray and neutron diffraction studies by Blom 103   bond bending. The primary CO 2 to metal interaction was therefore still regarded as physisorptive. 82 In 2011, a second adsorption site for CO 2 in MOF-74-Mg was identified using variable temperature NPD measurements (Fig. 21). 87 This study also speculated that further filling of the MOF with CO 2 would occur via formation of a second layer within the pores. In both binding sites identified in this NPD study, a O-C-O bond angle of 1701 was modelled from the 20 K data, less distorted than observed in previous studies. 82,103 The use of NMR spectroscopy to probe the dynamics, rotation and diffusion of CO 2 within MOF-74 has been illustrated via several studies. 90,91 For example, 13 C NMR line shape and spin-lattice relaxation were used to investigate the motional dynamics of adsorbed CO 2 in MOF-74-Mg. 90 Uniaxial rotation of 13 C-enriched CO 2 at the metal-bound site (Fig. 22) was invoked in order to model the variable temperature NMR spectra recorded between 12 and 400 K. The rotational axis was assumed to lie along the Mg-O(CO 2 ) vector, with the simulated angle of rotation of CO 2 varying between 561 and 691 from this axis, in line with the observations derived from PXRD and NPD studies of a significant deviation of M-O-C angle from 1801. Significant slowing of molecular rotation was observed at temperatures below 150 K, with rotation essentially ceasing below 100 K. Interestingly, Arrhenius plots of the 13 C spin-lattice (T 1 ) relaxation data appear to show two different activation processes operating at different temperatures with the higher energy process dominating at higher temperatures. The onset temperature of the higher activation energy process for 0.3 CO 2 /Mg (10.0 vs. 4.0 kJ mol À1 ) coincides with the onset of gaseous CO 2 release (300 K), while that for 0.5 CO 2 /Mg (6.0 vs. 3.5 kJ mol À1 ) appears to correlate with an NMR line shape change attributed to the change from a single angle rotation of the CO 2 to a rotation with 'slight angle variations'. The lower activation energy for rotation in the higher loading experiment hints at a reduction in Mg(II)-CO 2 binding strength as loading increases. 90 A number of issues left unresolved by the above study 90 were subsequently addressed. 91 These problems include the lack of detail as to why two different activation processes should be occurring in CO 2 -loaded MOF-74-Mg, along with the invocation of a symmetrical uniaxial rotation when the chemical environment is not perfectly symmetric. To address this, the free-energy landscape of CO 2 molecules in MOF-74-Mg was investigated from 100 K to 375 K using canonical Monte Carlo (MC) simulations 104 at loadings of infinite dilution, 0.3 CO 2 /Mg and 0.5 CO 2 /Mg. In this MC simulation, two types of motion (i) a 'fluctuation' of the CO 2 molecule near the minimum energy bound configuration and (ii) a hopping motion of the CO 2 molecule between binding sites were identified (Fig. 23). Comparisons of the MC simulations and NMR data reveal that the former process dominates at lower temperatures (o150 K) and  the latter at higher temperatures (4200 K). Importantly, the symmetry of the crystal environment means that the hopping motion of CO 2 molecules from site to adjacent site gives an identical NMR profile to that predicted by a uniaxial rotation of 801 about the Mg-O(CO 2 ) vector. Both the calculated and experimental equivalent rotational angles decrease with increasing temperature, an outcome of a thermally activated increase in site-to-site motion along the channels. This demonstrates the complexity of CO 2 diffusion dynamics with the hopping motion described herein corresponding well to the first diffusion mechanism proposed by Thonhauser et al. 83 for H 2 , CO 2 and H 2 O, as outlined in Fig. 23.
There are many studies investigating the high selectivity of MOF-74 (and its derivatives) for CO 2 by examining mixed gas separation experiments. 86,93,94,97,[99][100][101][102]105 Alkylamines have been bound to the metal centres of an extended MOF-74-Mg analogue to enhance selectivity for CO 2 . 97 Flue gases from coal-fired power plants are typically released at B1 bar at temperatures from 40-60 1C and contain a mixture of N 2 (70-75%), CO 2 (15-16%), H 2 O (5-7%), and O 2 (3-4%). 106,107 Given the limitations of gas diffusion imposed by the relatively narrow channels of MOF-74-M (B11 Å) and the target to   incorporate amine functionality, 108 the dobdc 4À linker was replaced by the longer dobpdc 4À (4,4 0 -dioxido-3,3 0 -biphenyldicarboxylate). 97 The overall topology of the subsequent MOF (Fig. 24) remains unchanged but the pores are considerably larger (B18.4 Å) than that of MOF-74. Subsequent functionalisation of the vacant metal site with N,N 0 -dimethylethylenediamine (mmen) affords [mmen-Mg 2 (dobpdc)]. 97 Activated [mmen-Mg 2 (dobpdc)] takes up 2.0 mmol g À1 (8.1 wt%) of CO 2 at 0.39 mbar and 25 1C and 3.14 mmol g À1 (12.1 wt%) at 0.15 bar and 40 1C, the latter being relevant to flue gas conditions. The purity of the CO 2 removed from simulated flue gases was estimated to be B98% and the performance as an adsorbent under these conditions was comparable to aminegrafted silicas and aqueous amine solutions. Interestingly, the [mmen-Mg 2 (dobpdc)] shows unusual stepwise adsorption of CO 2 at low pressure, and this step shifts to higher pressures at higher temperatures. 97 The mechanism of stepwise CO 2 adsorption in the isostructural and more crystalline Mn(II) analogue [mmen-Mn 2 (dobpdc)] has been studied by combined spectroscopic and diffraction methods. 109 Diffraction data were collected at 100 K before and after exposure of a sample to 5 mbar of CO 2 . Structural determination revealed that the mmen molecules were bound through one N-donor to the Mn(II) centre, Mn-N = 2.29(6) Å, with the other N-centre exposed on the surface of the wall of the pore (Fig. 25). On exposure to CO 2 , CO 2 inserts into the M-N bond forming a carbamate with an O-centre bound to Mn(II), Mn-O = 2.10(2) Å (Fig. 25). The second O atom of the carbamate has a close interaction of 2.69(1) Å with the N-centre of a neighboring mmen, resulting in chains of ammonium carbamate running along the channel of the framework. This well-ordered chain structure was maintained at 295 K, as determined from a full Rietveld refinement of PXRD data collected at 295 K. Thus, the adsorption of CO 2 by [mmen-M 2 (dobpdc)] at ambient temperatures is associated with a structural transition to form an extended chain structure held together by ion pairing between the metal-bound carbamate units and the outstretched ammonium group of a neighboring mmen molecule (Fig. 26). These studies demonstrated an excellent example of utilising the advanced diffraction technique to gain molecular understanding on the unusual properties in MOFs, in this case stepwise CO 2 adsorption.    [110][111][112][113] The major characteristic of these materials is their ability to ''breathe'', i.e. to expand or contract drastically their crystalline structure adapting to the nature and amount of adsorbed guest molecules (Fig. 27).
The structural flexibility of these MOFs in response to guesthost interactions has encouraged the investigation of adsorption mechanisms for various guests of interest by in situ characterisation techniques. Moreover, in the isostructural V(IV) material, MIL-47, 114 the m 2 -OH are replaced by m 2 -O groups and this MOF is much more rigid, which offers interesting opportunities for comparison. The adsorption isotherm of CO 2 in MIL-53(Cr) up to 30 bar at room temperature shows a two-step behaviour (Fig. 28a). After an initial rapid uptake of 3 mmol g À1 below 1 bar, a plateau is observed, and then a second uptake to 9 mmol g À1 occurs for pressures higher than 5 bar. 115 This unusual behaviour was studied by means of high resolution in situ PXRD coupled with IR spectroscopy and DFT calculations. 116 It was confirmed that the two-step adsorption of CO 2 corresponds to a breathing phenomenon (Fig. 28b): starting from the activated open form MIL-53HT (HT = high temperature), the initial adsorption of CO 2 causes a shrinkage of the structure leading to a new low pressure (LP) MIL-53LP phase, the structure of which has been refined from PXRD data at 195 K under 1 bar of CO 2 . In MIL-53LP, the CO 2 molecules could be located and their occupancy refined to 0.54, in good agreement with the amount of CO 2 adsorbed at the first plateau. The adsorbed CO 2 molecules were found to interact with two m 2 -OH groups from the two closest opposite chains of the pores (Fig. 29) and are aligned along the pore direction with intermolecular distances of 3.4 Å, indicating CO 2 -CO 2 interactions. With increasing pressures of CO 2 , the second gas uptake corresponds to a full opening of the structure, leading to the MIL-53HP (HP = High Pressure) phase in which the gas molecules are disordered.
Diffusion of CO 2 molecules and their interactions with the MIL-53(Cr) framework during adsorption have been investigated by a combination of QENS and MD. 117 At low CO 2 loadings, in the closed MIL-53LP phase, the minimum free energy regions calculated for the MOF correspond to the OH groups of the octahedra chains. This explains the slow diffusion of CO 2 because of its strong interaction with the hydroxy groups and is consistent with the in situ PXRD study. It was demonstrated that with increasing pressures, the transport diffusivity of CO 2 is two orders of magnitude faster in the HP form than in the LP form, but surprisingly, the diffusion mechanism remains strictly one dimensional, suggesting that the CO 2 molecules, which still interact strongly with OH groups, restrict the motions of additional adsorbates perpendicularly to the channels. Interestingly, a similar study has been reported by Salles et al. on the diffusion of CO 2 in the rigid MIL-47 system. 118 Although the authors could not elucidate unambiguously the microscopic diffusion mechanism of CO 2 from the QENS experiment in this case, their analysis of the MD trajectories suggests a 3D mechanism for CO 2 diffusion in MIL-47. This difference has been attributed to the absence of the hydroxy groups as preferential adsorption sites in the pores of MIL-47 in contrast with the case of MIL-53(Cr).

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In both MIL-53(Cr) and MIL-47, adsorption of CH 4 occurs in a single step without framework breathing. 115 A dynamic QENS/ MD study on the diffusion of CH 4 in these materials reveals a 1D diffusion mechanism of the adsorbate in the fully open pores in both cases. 119 The significantly higher diffusivity of CH 4 in MIL-47 than in MIL-53(Cr) and the higher activation energy for the latter are again related to the m 2 -OH groups which act as attractive sites and steric barriers. In both systems, the diffusivity of adsorbates is higher for CH 4 than for CO 2 , consistent with higher activation energies calculated for CO 2 .
Recently, QENS/MD studies of the diffusion of CO 2 /CH 4 mixtures in the MIL-53(Cr) and MIL-47 have sought to evaluate the impact of the CO 2 concentration on the diffusivity of CH 4 . 120 In both cases, the diffusivity for CH 4 was found to decrease with increasing CO 2 loading, although CO 2 has little or no influence on the diffusivity for CH 4 as a function of total gas loading. MD trajectories of the gases show that in both materials CO 2 and CH 4 follow a 1D type diffusion mechanism. CH 4 is distributed in the central zone of the pore while CO 2 molecules mainly diffuse in the vicinity of the polar m 2 -OH groups for MIL-53(Cr) and close to the pore wall for MIL-47.
A similar methodology has been used to describe the dynamic properties of H 2 in MIL-53(Cr) and MIL-47 at 77 K. 76 In the case of H 2 , no framework breathing phenomenon was observed and the pores stay fully open throughout the experiment. QENS experiments linked to MD simulations fitted a 1D diffusion mechanism in the case of MIL-53(Cr), revealing that the m 2 -OH groups act as steric barriers rather than specific binding sites governing the diffusion process. A 3D model was established for H 2 -loading in MIL-47 since there is no specific steric barrier for adsorbed H 2 molecules (Fig. 30). The most striking phenomenon is the supermobility of adsorbed H 2 in both materials as evidenced by a sudden increase of the diffusivity at low loadings, not previously observed in nanoporous materials.
In situ NPD was used to characterise the H 2 adsorption sites in MIL-53(Cr) at 10 K at various loadings. 121 With increasing pressures, four different adsorption sites are sequentially occupied (Fig. 31), the first site D1 being located in the acute corner of the lozenge-shaped pore (Fig. 32). This site is closer to the C-C bond of the carboxylates (3.3 Å) and to the H from the opposite benzene ring (2.5 Å) than to the O from the m 2 -OH group (4.3 Å), further confirming their steric role of the latter in the  adsorption mechanism. The second and third adsorption sites are found in the obtuse corner of the pore, and the only specific interactions are with the carboxylate functions for D2 and with benzene rings of the ligands for D3. The D4 site, closer to the centre of the pore, only involves interactions between adsorbate molecules. The adsorption mechanism of H 2 in MIL-53(Cr) is thus governed by the steric effects of binding guest species to the m 2 -OH groups that determine the position of the initial and hence subsequent binding sites, leading to a 1D diffusion pathway for the guest molecules.  30 Binding sites were assigned to the Zn(II) cluster and to the benzene ring of the ligand. The first primary adsorption site was assigned 32,123 to a position equidistant to three carboxylates from the cluster above a face of the inorganic cluster, and the second adsorption site to an edge of the cluster, followed by sites around the organic linker. These assignments resulted from analysis of INS experiments for a series of MOFs built around from the same [Zn 4 O(O 2 CR) 6 ] clusters and different organic linkers, as well as results from in situ X-ray diffraction studies of Ar-and N 2 -loaded single crystals of MOF-5. 32,123 Single crystal neutron diffraction was also used to determine the adsorption sites for H 2 in MOF-5. 124 The diffraction experiment was carried out on a single crystal loaded at room temperature with H 2 gas at a pressure of 1 atm. Two primary adsorption sites were located from the neutron diffraction data collected at 5 K (Fig. 33). The first fully occupied site (a-site) is situated at 3.61(5) Å from a Zn(II) centre, 3.75 (6)

Zeolitic imidazolate framework ZIF-8
Zeolitic imidazolate frameworks (ZIFs) are porous materials assembled by tetrahedrally coordinated M(II) (M = typically Zn, Co) ions and imidazolate linkers to give zeolite-like structures. 125 ZIFs show exceptional chemical stability and are versatile structures via incorporation of various functionalised organic linkers, and have been widely explored for applications in gas sorption, 126 small molecule separation, 127 and catalysis. 128 The H 2 adsorption properties of the archetypal ZIF-8 129 have been investigated in detail using NPD. 130 ZIF-8 is comprised of tetrahedral ZnN 4 units bridged by 2-methylimidazolate (MeIM) to give a sodalite zeolite-type structure. The framework can be viewed as body-centred cubic packing of truncated octahedral cages, each with internal diameter of 11 Å, in which four Zn(II) ions and four MeIM rings form a 4 Å square aperture and six ZnN 4 clusters and six MeIM rings generate a hexagonal face. NPD data were collected at different D 2 loadings at 3.5 K, and six distinct D 2 binding sites were identified and sequentially occupied with increasing loadings (Fig. 34). The strongest adsorption sites are directly associated with the organic moieties, with site D1 residing on top of the MeIM ring close to the CQC bond in the MeIM unit, with site D2 located at the centre of the hexagonal face. The third site (D3) is also located is at the centre of the hexagonal opening but on the other side of the aperture. The fourth site (D4) was found in the face centre of the D 2 nanocage formed by the first three binding sites. With further loading of 28 D 2 per 6 Zn at 3.5 K, two additional sites D5 and D6 sited close to the centre of the 11 Å cage were progressively occupied.

Selective CO 2 binding in amine-functionalised MOFs
It is widely accepted that MOFs containing pendant amine functional groups can have specific binding to CO 2 molecules, thus improving the selectivity and uptake capacity of CO 2 in the host. However, experimental evidence on the direct visualisation of NH 2 Á Á ÁCO 2 binding in MOFs has only been reported in exceptional cases. 131 For example, Shimizu et al. have reported the location of CO 2 molecules in [Zn 2 (Atz) 2 (ox)] N (Atz 2À = 3-amino-1,2,4-triazole; ox 2À = oxalate) via in situ single crystal diffraction and confirmed the strong interaction of amines with CO 2 with a heat of adsorption of ca. 40 kJ mol À1 . 131,132 Two binding sites for CO 2 were located: site I is close to the free amine group and forms a strong dipole interaction and site II is in the middle of the pore forming a ''T-shape'' interaction with the CO 2 residing on site I (Fig. 35).
[Zn 2 (Atz) 2 (ox)] N was compared to the phosphonate analogue, [Zn 3 (Atz) 3 (PO 4 )] N , with the hypothesis that the latter might give better CO 2 capture properties. 133 Fewer trianionic phosphonate  groups per metal are required to provide charge-balance than dianionic oxalates, and larger amine-lined pores were anticipated and indeed observed. However, CO 2 uptake did not exceed that of [Zn 2 (Atz) 2 (ox)] N and the adsorbed CO 2 molecules could not be readily located in [Zn 3 (Atz) 3 (PO 4 )] by diffraction experiments. The porous 3D networks in both cases are made up of cationic Zn-Atz layers pillared by the anions (Fig. 36). Only the triazole N-centres of the aminotriazole ligand are coordinated to Zn(II), with the amine N-centres remaining uncoordinated. The structure of [Zn 3 (Atz) 3 (PO 4 )] N does indeed have larger pores, but the ZnAtz layers of the structure adopt a buckled or staggered conformation which results in the amines not pointing directly into the pores. Classical GCMC and MD simulations and periodic DFT calculations were used to model the CO 2 adsorption isotherms, heats of adsorption and locations of CO 2 molecules (Fig. 37) in both MOFs. The isotherms were modelled with excellent agreement in both cases, with the exception of a slight overestimate of the CO 2 adsorption of [Zn 3 (Atz) 3 (PO 4 )] N at low pressure.
The GCMC and DFT calculations located CO 2 molecules in two regions in [Zn 3 (Atz) 3 (PO 4 )] N , denoted as a and b in Fig. 37. It is noteworthy that the electrostatic interactions of CO 2 at the a 1 binding site with the amine groups were calculated to be weakly attractive for two adjacent amines but repulsive for the third; in the oxalate framework all these interactions were found to be attractive. Interestingly, occupation of the a 1 /a 2 /b triad of binding sites in [Zn 3 (Atz) 3 (PO 4 )] N by three CO 2 molecules was calculated to have an average binding energy of 31.3 kJ mol À1 per CO 2 molecule, a full 7.4 kJ mol À1 higher than the mean average of the individual binding energies of the sites in an empty framework (30.6, 26.9 and 29.0 kJ mol À1 respectively). However, immediately adjacent a 1 and a 2 sites are mutually exclusive (red and blue sites in Fig. 37c) and so a dynamic 'slipping' of CO 2 between these positions was proposed as a mechanism to benefit from favourable site-site cooperative binding effects. Therefore, ab initio MD simulations on four CO 2 molecules in a single unit cell were performed to investigate the mobility of CO 2 . The results are shown in Fig. 37d: two of the four calculated regions resulting from a 35 ps MD simulation are depicted and broadly the same regions are identified as by the GCMC simulations. Slipping of the CO 2 molecules between a and b sites was observed in this short simulation and snapshots show the triad formation arises when this occurs.
In conclusion, this study neatly demonstrates that simulation of parameters for the individual binding site of CO 2 in a framework needs to be combined with experimental and computational analysis of the molecular dynamics of the guest and  the more subtle intermolecular interactions between CO 2 molecules in different binding sites. It also demonstrates that excessive clustering of amine groups actually hinders CO 2 binding rather than enhancing it. Selective capture and removal of harmful flue gases (e.g., CO 2 , SO 2 ) is a major challenge for power stations and coal-fired industry. Materials functionalised with amine-groups dominate this area, primarily because of their potential to form carbamates via H 2 N(d À )Á Á ÁC(d + )O 2 interactions, thereby trapping CO 2 covalently. 134,135 The use of these materials, however, is energyintensive, with significant environmental impact. The development of alternative materials without incorporating toxic amine-groups can potentially reduce the energy and environmental penalties. Recently, a non-amine-containing material MFM-300(Al) (MFM = Manchester Framework Material, replacing the previous NOTT designation) in which hydroxy groups within pores bind CO 2 and SO 2 was reported. 136 In situ synchrotron PXRD and INS studies, combined with modelling have demonstrated the power in revealing the preferred binding sites and dynamics for adsorbed gas molecules.
MFM-300(Al) is comprised of infinite chains of [AlO 4 (OH) 2 ] moieties bridged by biphenyl-3,3 0 ,5,5 0 -tetracarboxylate ligands to afford a porous extended framework structure with squareshaped 1D channels with hydroxy groups protruding into them (Fig. 38a). The diameter of the channel window, taking into account the van der Waals radii of the surface atoms, is approximately 6.5 Å. Desolvated MFM-300(Al) has a pore volume of 0.38 cc g À1 and a BET surface area of 1370 m 2 g À1 and so the general porosity of MFM-300(Al) is moderate within the family of MOF complexes. 136,137 Desolvated MFM-300(Al) shows highly selective uptake for CO 2 (7.0 mmol g À1 ) and SO 2 (8.1 mmol g À1 ) at 273 K and 1.0 bar, among the highest values observed so far under these conditions. In contrast, under the same conditions the isotherms for CH 4 , CO, N 2 , H 2 , O 2 , and Ar show only surface adsorption by MFM-300(Al) with very low uptake of gas, thus giving ultra-high selectivities for CO 2 and SO 2 .
Static studies using NPD have been previously been employed to locate CO 2 in porous MOFs, 82,88 but although INS has been used widely to investigate the H 2 binding interactions within various MOFs, this technique cannot directly detect the CO 2 binding interaction within a porous system because the scattering cross-sections for carbon and oxygen are too small to obtain reasonable signals (Table 1). A combination of INS and DFT can however indirectly visualise binding of CO 2 molecules within MFM-300(Al) by investigating the change in the dynamics of the hydrogen atoms in the local structure, including those of the hydroxy groups and benzene rings of the ligand. Two major changes on the INS spectra were observed upon CO 2 loading into MFM-300(Al), indicating the presence of two distinct types of interactions involved in binding CO 2 in the pore (Fig. 38b). DFT calculations based on INS results confirm that the adsorbed CO 2 molecules interact end-on to the hydroxy groups via the formation of a moderate-to-weak hydrogen bond (OÁ Á ÁH = 2.335 Å). In addition, each adsorbed CO 2 molecule is surrounded by four aromatic C-H groups, forming weak cooperative supramolecular interactions between O(d À ) of CO 2 and H(d + ) from -CH [OÁ Á ÁH = 3.029, 3.190 Å] (Fig. 38c). This calculation is in excellent agreement with the experimental INS data and confirms the presence of both moderate-to-weak hydrogen bonds and supramolecular interactions.
The preferred binding sites for CO 2 molecules within MFM-300(Al) have also been determined by static in situ PXRD analysis which also confirms end-on binding of CO 2 to both the hydroxy group and the surrounding C-H groups. This PXRD analysis is in excellent agreement with the INS/DFT model. Additionally, a second CO 2 (II) site has been identified from this PXRD analysis, and this interacts principally with the first CO 2 via dipole interaction, similar to that observed in solid CO 2 .
The corresponding INS/DFT and PXRD studies on SO 2 -loaded MFM-300(Al) lead to similar observations revealing the presence of three different types of binding interactions: moderate-toweak hydrogen bonds to hydroxy group (SO 2 Á Á ÁHO-), supramolecular interactions to aromatic hydrogen atoms (SO 2 Á Á ÁHC-) and intermolecular dipole interactions between adsorbed SO 2 molecules. The binding mechanism in this case is very similar to that observed in CO 2 -loaded MFM-300(Al), albeit SO 2 has a bend structure with oOQSQO angle of B1101 (Fig. 38d). Significantly, in situ PXRD of SO 2 -loaded sample confirms retention of the structure of MFM-300(Al) upon inclusion and subsequent removal of SO 2 , thereby confirming the high stability of MFM-300(Al) in the presence of corrosive SO 2 . This is highly unusual for porous MOF materials that often react irreversibly with SO 2 . Notwithstanding the high uptake of CO 2 and SO 2 observed for MFM-300(Al), it shows surprisingly low uptake for H 2 (0.22 wt% at saturation at 77 K). This is almost ten-fold lower than the expected H 2 uptake of 2.7 wt% based upon the pore volume and BET surface area of MFM-300(Al). As discussed above for the series of Cu(II)-tetracarboxylate MOFs in which lower H 2 uptake was observed with increasing pore size, [13][14][15] computational modelling widely predicts that MOFs incorporating ultra-micropores (below 10 Å) are optimal for H 2 binding due to enhanced overlapping potentials. The exceptionally low H 2 uptake for MFM-300(Al) is thus highly unusual. INS has been used to study the H 2 binding interaction owing to its large incoherent cross-section (Table 1) and therefore high detection sensitivity. INS spectra of H 2 -loaded MFM-300(Al) display a broad hump centred at B20 meV with only one small energy transfer peak at 8.8 meV (Fig. 39). 138 This result suggests that the majority of adsorbed H 2 in the pore has a recoil motion at 5 K (below its melting point), reminiscent of the behaviour of liquid H 2 . This observation is distinct from previous studies on adsorbed H 2 displaying binding to open metal sites, which induce clear host-guest interactions to H 2 . Thus, the INS spectra suggests that adsorbed H 2 molecules have very weak interactions with the MFM-300(Al) host and, therefore, can rotate freely in the channel to give recoil rotational motion. This study represents a unique example of surprisingly low H 2 uptake within an ultramicroporous (B6 Å) MOF material, and complements the wide range of studies on systems showing higher uptake capacities and binding interactions, several of which are highlighted in this review.
The excellent performance of MFM-300(Al) in the capture of CO 2 and SO 2 motivated a study on the separation of small molecule hydrocarbons. 139 At 293 K, the total adsorption uptakes for C 2 H 2 , C 2 H 4 , C 2 H 6 , and CH 4 in MFM-300(Al) were measured as 6.34, 4.28, 0.85, 0.29 mmol g À1 , respectively at 1.0 bar. Analysis of the pure-component isotherms at 293 K via ideal adsorbed solution theory (IAST) 140 was carried out to estimate selectivity between these hydrocarbons. For equimolar mixtures at 1.0 bar, the C 2 H 2 /C 2 H 4 selectivity of 2.30 for MFM-300(Al) by IAST is higher than that observed for [Fe 2 (dobdc)] (1.87), 59 but is lower than for M 0 MOF-3a (5.23), 141 although the latter system exhibits a relatively low total capacity owing to its narrow pores. The selectivity for C 2 H 4 /C 2 H 6 in MFM-300(Al) was calculated to be 48.7. Significantly, this value is higher than that observed for the current state-of-the-art ethylene/ethane separating materials. 50,59,[142][143][144] The C 2 H 2 /CH 4 , C 2 H 4 /CH 4 , and C 2 H 6 /CH 4 selectivities in MFM-300(Al) were estimated by IAST analysis as 41000, B380, and 5, respectively. The above IAST selectivity data were also validated via the measurement of dual-component adsorption isotherms for equimolar mixtures of C 2 H 2 /C 2 H 4 , C 2 H 4 /C 2 H 6 , C 2 H 2 /CH 4 , and C 2 H 4 /CH 4 at 293 K under flow mode, indicating the preferred binding of unsaturated hydrocarbons to MFM-300(Al) host. The binding sites for C 2 H 2 , C 2 H 4 , and C 2 H 6 were studied by in situ synchrotron PXRD and NPD experiments. Two independent binding sites (I and II) were observed in each case: the unsaturated C 2 -molecules at site I exhibit a side-on interaction to the HO-Al group via formation of weak hydrogen bonds that are supplemented by additional supramolecular contacts to the aromatic hydrogen atoms and pÁ Á Áp interactions to the phenyl rings; C 2 -molecules at site II are located in the middle of the pore and interact primarily with the molecules at site I via intermolecular dipole interactions. The bond distances obtained from independent analysis of synchrotron X-ray diffraction, neutron diffraction, and DFT calculations are compared in Fig. 40 with very good agreement observed between the three methods. In addition to the static crystallographic study, a combined INS and DFT study was undertaken to visualise the binding dynamics for adsorbed C 2 H 2 , C 2 H 4 , and C 2 H 6 molecules in MFM-300(Al). The study revealed a novel Al-OH to p(CRC) hydrogen-bond interaction, which has not been observed previously, and thus represents a new type of supramolecular contact in host-guest systems. This study reveals the involvement of simultaneous and cooperative hydrogen-bonding, pÁ Á Áp interactions and inter-molecular dipole interactions in the binding of acetylene and ethylene to give up to twelve individual weak supramolecular interactions aligned within the host to form an optimal geometry for intelligent selective binding of hydrocarbons.
The separation of C 2 H 2 and C 2 H 4 is a technologically important goal, but is highly challenging due to their similar molecular sizes, volatilities, and electronic structures based upon unsaturated carbon-carbon bonds. This leads to very similar binding interactions for these substrates to open metal sites, with low preferential adsorption. 50,59,142 The dynamics of MFM-300(Al) loaded with an equimolar mixture of C 2 H 2 /C 2 H 4 were analysed to determine the relative binding of these two guests, and confirmed that in this binding competition, C 2 H 2 has a stronger interaction to the host than C 2 H 4 and directly supports the optimal selectivity and uptake capacity observed in the isotherm experiments. This INS study represents the first dynamic study of competing C 2 -hydrocarbons binding in a functional host material.
This work represents a classic example of applying in situ INS, NPD, PXRD and DFT techniques to analyse dynamic and structural properties of gas-loaded MOF materials to reveal that a combination of multiple weak supramolecular binding interactions is sufficient enough to bind CO 2 , SO 2 , C 2 H 2 , C 2 H 4 , and C 2 H 6 molecules with both high selectivity and capacity. This offers the potential for application of new ''easy-on/easy-off'' capture systems for selective gas binding and separations that carry fewer economic and environmental penalties (Fig. 41). MOFs are usually synthesised using a single type of metal ion, and those containing mixtures of different metal ions are of great interest and represent a methodology to enhance and tune materials properties. MFM-300(Ga 2 ) is isostructural 145 with the Al(III) analogue MFM-300(Al) 136 with pores decorated with -OH groups bridging Ga(III) centres. The isostructural Fe-doped material [Ga 1.87 Fe 0.13 (OH) 2 (L)], MFM-300(Ga 1.87 Fe 0.13 ) can be prepared under similar conditions to MFM-300(Ga 2 ) via reaction of a homogeneous mixture of Fe(NO 3 ) 3 and Ga(NO 3 ) 3 with biphenyl-3,3 0 ,5,5 0 -tetracarboxylic acid. 145 Thus, Fe-doping can be used to monitor the effects of the heteroatom centre within a parent Ga(III) framework without the requirement of synthesising the isostructural Fe(III) analogue MFM-300(Fe 2 ). MFM-300(Ga 2 ) shows the highest CO 2 uptake (2.86 mmol g À1 at 273 K at 1 bar) for a Ga-based MOF. Fe-doping of MFM-300(Ga 2 ) affords positive effects on gas adsorption capacities, particularly  145 Upon loading of CO 2 into the hosts, significant residual electron density was found in the pore by difference Fourier map analysis, and was sequentially assigned as two independent CO 2 molecules populating at sites I (O 1s QC 1s QO 2s ) and II (O 3s QC 2s QO 4s ). In MFM-300(Ga 2 ), the occupancy for these CO 2 molecules at sites I and II refined to values of 0.43(3) and 0.74(3), respectively, yielding the overall formula [Ga 2 (OH) 2 (C 16 H 6 O 8 )]Á2.35CO 2 for the CO 2 -loaded material. The CO 2 at site I is ordered and binds to the -OH group in an end-on fashion via a moderate hydrogen bonding interaction [HÁ Á ÁO 1s = 1.883(10) Å; oO-HÁ Á ÁO 1s = 1801]. Interestingly, this hydrogen bond distance is much shorter than that observed in MFM-300(Al 2 )Á3.2CO 2 system [HÁ Á ÁO = 2.376(13) Å; oO-HÁ Á ÁO = 1801] studied by in situ PXRD at 273 K, 136 indicating the formation of a stronger hydrogen bonding interaction in MFM-300(Ga)Á2.35CO 2 . Given that MFM-300(Ga 2 ) and MFM-300(Al 2 ) have the same framework structure and pore surface chemistry, this difference in hydrogen bond length is most likely due to the different metal centre (Al or Ga) affecting the relative acidity of the M-OH (M = Al, Ga) group. The CO 2 at site II is disordered over two equally-occupied positions. In contrast to CO I 2 , CO II 2 does not interact directly with framework atoms instead forming two weak electrostatic dipole interactions with CO I 2 , between the electropositive C and electronegative O centres [C 1s Á Á ÁO 3s = 3.85(4) Å; C 2s Á Á ÁO 2s = 4.39(4) Å]. Significantly, this pattern of intermolecular interactions is distinct from the traditional ''T-shape'' dipole interaction observed in solid CO 2 , in [Zn 2 (Atz) 2 (ox)]Á1.3CO 2 (Atz 2À = 3-amino-1,2,4-triazole; ox 2À = oxalate) 131 and in MFM-300(Al 2 )Á3.2CO 2 . 136 In contrast, the occupancies of the CO 2 molecules at sites I and II in MFM-300(Ga 1.87 Fe 0.13 ) both refined to values of 0.50 (1), yielding a formula of [Ga 1.87 Fe 0.13 (OH) 2 (C 16 H 6 O 8 )]Á2.0CO 2 for the CO 2 -loaded material. The CO 2 at site I is ordered and binds to the -OH group in an end-on fashion via a weak hydrogen bonding interaction [HÁ Á ÁO 1s = 2.259(12) Å, oO-HÁ Á ÁO 1s = 1801]. Interestingly, this hydrogen bond distance is longer than that observed in the MFM-300(Ga 2 )Á2.35CO 2 system [HÁ Á ÁO 1s = 1.883(10) Å] indicating the formation of a weaker hydrogen bonding interaction in MFM-300(Ga 1.87 Fe 0.13 )Á2.0CO 2 . This is consistent with the observed heat of adsorption which also decreases. The CO 2 at site II is disordered over two equally-occupied positions. However, CO II 2 adopts a different orientation to that observed in MFM-300(Ga 2 )Á2.35CO 2 , forming a typical ''T-shape'' dipole interaction with CO I 2 between the electropositive C centre and electronegative O centre [C 1s Á Á ÁO 3s = 3.207(6) Å, occurring twice]. Thus, the small percentage of Fe-doping into this solid solution has significant effect on the CO 2 binding details, including the formation of both adsorbate-adsorbent hydrogen bond and adsorbate-adsorbate intermolecular dipole interactions.

CH 4 and D 2 binding in MFM-300(In)
The microporous hydroxy-decorated material, MFM-300(In) displays a high volumetric uptake of 202 v/v at 298 K and 35 bar for CH 4 and 488 v/v at 77 K and 20 bar for H 2 . 146 Direct observation and quantification of adsorbed H 2 and CH 4 molecules within MFM-300(In) have been achieved using a combination of NPD and INS techniques coupled with computational investigations. These complementary results suggest that the adsorbed CH 4 molecules form specific interactions with metal-bound hydroxy groups within the pore, supplemented by intermolecular dipole interactions between adsorbed CH 4 molecules and the phenyl-ring lined walls of the pore (Fig. 44). These cooperative supramolecular interactions allow packing of CH 4 to very high density upon saturation (0.37 g cm À3 ) reminiscent of liquid CH 4 (0.42 g cm À3 at 111 K) and 71% of that of the solid (0.52 g cm À3 at 20.4 K). Interestingly, the H 4 C guest Á Á ÁH-O host interaction represents the first example of such molecular contact in MOFs, and contrasts with the structures observed in CH 4 /water clathrate materials in which the CH 4 molecule is orientationally disordered and not observed to interact with the HÁ Á ÁOH cage-forming linkage (Fig. 45). 147 Similar binding has also been observed in H 2 -loaded MFM-300(In).

Studies of H 2 binding in anionic MOFs with extra-framework cations
The worldwide research on H 2 storage in MOFs over the past decade has produced several MOF systems with exceptionally   high gravimetric uptake of H 2 with a record of B16 wt% total uptake capacity observed in NU-100 148 and MOF-200 149 (albeit at high pressure and low temperature, typically 77 K). However, these high uptakes drop dramatically with increasing temperature as a result of the weak binding interaction between H 2 molecules and MOF hosts, and thus no MOF system is able to act as suitable H 2 stores for on-board application to date. Increasing the interaction between H 2 molecules and the host framework, and hence the associated isosteric heat of adsorption, represents a major challenge if these systems are to find practical use at more ambient temperatures.
One particularly promising approach to significantly enhancing the H 2 -MOF interaction is to load active alkali and alkaline earth metal centres M (e.g., Li, Na, Mg) into MOF structures and to exploit the potential of strong binding of molecular H 2 at free metal sites. This has been supported by computational and theoretical studies on the modelling of H 2 adsorbed into M-doped MOF hosts. [150][151][152][153][154][155][156] However, doping of active M(0) into a MOF material is intrinsically problematic by experiment due to the high reactivity of M(0) with the cationic metal clusters and organic ligands that constitute the MOF structures. Thus, doping with M +/2+ ions appears to be a more controllable and feasible route to interrogate and understand H 2 binding in a doped MOF host. Porous anionic MOFs can provide an excellent platform to introduce M +/2+ ions within a framework structure via balancing the overall charge of the framework.
Zeolite-like MOFs (ZMOFs) are anionic and as such extra framework cations (i.e., M +/2+ ions) can readily be introduced. One representative example from this ZMOF family, rho-ZMOF, was reported to exhibit facile ion-exchange capability in a similar way as zeolites. 157 The original In(III)-imidazoledicarboxylate rho-ZMOF has a formula of [In 48 (HImDC) 96 ] 48À and can incorporate 48 mono-cations or 24 double cations within the cage. Three different cations (dimethylammonium: DMA + , Li + , Mg 2+ ) were introduced into the cage to give iso-structural samples of DMArho-ZMOF, Li-rho-ZMOF and Mg-rho-ZMOF. 157 The rotational transitions of adsorbed H 2 molecules in these three materials were studied by INS to compare the effect of extra-cation on the H 2 binding.  The adsorption isotherms for H 2 in Li-rho-ZMOF and Mgrho-ZMOF below 20 mbar display a steeper rise in uptake than the parent DMA-rho-ZMOF, indicating the presence of stronger H 2 binding interactions in the Li/Mg-exchanged MOFs. This has also been supported by calculation of isosteric heats of adsorption of 8.0, 9.1, and 9.0 kJ mol À1 for DMA-rho-ZMOF, Li-rho-ZMOF and Mg-rho-ZMOF, respectively. Synchrotron single crystal diffraction on Mg-rho-ZMOF has successfully determined the position and coordination of the Mg 2+ ion which is bound to six water molecules to give a hexaaqua complex [Mg(OH 2 ) 6 ] 2+ which forms hydrogen bonds to carboxylate oxygen centres of the framework (Fig. 46). Unfortunately, TGA data shows that it is not possible to remove the coordinated water molecules on the Mg 2+ site before the MOF host starts to decompose. The position of the Li + ion was not located due to the disorder of the cationic site. However, it is reasonable to estimate that Li + in Li-rho-ZMOF is also fully coordinated to four water molecules to give a tetraaqua complex [Li(OH 2 ) 4 ] + which form hydrogen bonds to the MOF host. Therefore, neither Mg 2+ nor Li + site is directly accessible to the adsorbed H 2 molecules from a crystallographic view.
INS spectra for H 2 -loaded DMA-rho-ZMOF, Li-rho-ZMOF and Mg-rho-ZMOF are similar. At low loading, each has four reasonably well-defined binding sites for H 2 with very small energy barrier for H 2 rotation on each site. In comparison to a similar study carried out on Li + -exchanged zeolite FAU-LiX which presents two strong energy transfer peaks at B1 and 7.5 meV, 158 Li-rho-ZMOF does not show such low energy transfer peaks, indicating the lack of strong binding to the Li + site. This is consistent with the formation of [Li(OH 2 ) 4 ] + in the pore with Li + cation not directly accessible. A similar conclusion has been drawn for H 2 -loaded Mg-rho-ZMOF. A direct comparison of the INS spectra for H 2 -loaded Li-rho-ZMOF and the parent DMA-rho-ZMOF shows that in the former there is an additional increase in intensity in the region between B6 and 10 meV. This can be assigned to be the interaction between H 2 molecules and the Li + -water complex, though the contribution is not significant. Based upon these results, the relatively small observed enhancement of H 2 adsorption properties most likely originates from the increased electrostatic field in the cavity due to the extra framework cations rather than due to any direct binding interaction between these sites to H 2 .
One alternative approach to synthesise anionic MOFs is based upon the linking of tetrahedral nodes derived from In(III) centres bound to isophthalate tetracarboxylates to form 4-connected (4-c) structures. The discrepancy between the charge on In(III) and the negative organic linker L 4À leads to the formation of anionic complexes [In(L)] À in which the net charge has to be balanced by counter-cation(s). A series of anionic MOFs (MFM-200, 204, 206, 208; MFM = Manchester Framework Material, replacing the NOTT designation) were designed and synthesised via this route. [159][160][161][162] In the as-synthesised sample, organic cations (DMA + or dihydropiperazinium 2+ ) act as the counter cations and partially block the pore entrance owing to their bulky nature. These organic counter-cations within the as-synthesized materials are exchangeable to smaller Li + cations while still maintaining the porosity of the framework structure. In this way, a number of corresponding Li + -loaded iso-structural MOFs (MFM-201, 205, 207, 209) were generated to provide the platform to study the H 2 binding interaction to the framework cation Li + sites. [159][160][161][162] One of significant improvements of this system is the successful location of the position of Li + ion within the channels of MFM-201, 207, and 209.
The H 2 binding interaction in MFM-209 was studied with INS and is discussed here. 159 MFM-209-solv is doubly interpenetrated and has three different types of channels (A, B and C) due to the overlapping of the two networks. The three channels have different sizes and surface chemistry ( Fig. 47a and b). 162 X-ray crystallography confirms that the Li + cation sits in the smallest channel C, tetrahedrally coordinated to four O-centres, two from carboxylate groups and two from coordinated H 2 O molecules. Thus, the [In(O 2 CR) 4 ] moiety is able to anchor Li + via two chelate carboxylate groups, leaving potentially accessible and exposed Li + sites after desolvation. Indeed, an in situ IR experiment has confirmed the removal of these two coordinated water molecules on Li + sites at around 150 1C, giving open Li + sites in the activated MOF. This observation is also consistent with the 7 Li-NMR experiment which suggests that in the desolvated MOF the coordination environment of Li + ion has lower symmetry. 162 Desolvated MFM-209 shows an enhancement of H 2 uptake capacity of 31% in comparison to the parent MOF that contains organic counter-cations. This increase in uptake capacity is also accompanied by an increase of 38% in isosteric heat of H 2 adsorption. Significantly, MFM-209 displays a very high heat of adsorption of 12 kJ mol À1 , which is higher than most reported MOF materials (4-7 kJ mol À1 typically).   (Fig. 47c). With increasing H 2 loadings, these peaks increase linearly in intensity until saturation indicating that for all loadings, the adsorbed H 2 molecules are interacting with the same sites within the framework of MFM-209 and these sites do not reach saturation at low pressure. The peaks in the INS spectra correspond to the rotational J = 0 to J = 1 transition of the H 2 molecule, J(1 ' 0). In principle, three lines are present due to the contribution from the initial state (where m = 0) and the final state (where m = 1, 0, +1). The lowest energy transfer peak at 10 meV in the spectra suggests the adsorbed H 2 molecules in MFM-209 interact with [In(O 2 CR) 4 ] centres, rather than via direct binding to open Li + sites. This is because, in the latter case, the INS peak will stop increasing when the site reaches saturation for adsorbed H 2 molecules. Thus, the enhancement of the enthalpy of adsorption of H 2 upon loading Li + cations must originate from a change in the charge distribution in the [In(O 2 CR) 4 ] moiety by incorporation of Li + ions. This deduction is also consistent with the observation that Li + doping enhances the adsorption enthalpy at all H 2 loadings, rather than only at low H 2 loadings. The nature of the extra-framework cations introduced from cation-exchange reactions here is not strictly comparable with those of M(0)-doping [150][151][152][153][154][155][156] since the latter also involves reduction of the framework host. It, therefore, still remains a practical challenge to introduce more active M(0) sites into MOFs to improve the H 2 binding at more ambient conditions.

Trapping of unusual guests and reactive intermediates in porous hosts
In 2002, Fujita et al. reported the synthesis of the doubly interpenetrated 3D coordination polymer [(ZnI 2 ) 3 (TPT) 2 ]Á 6(C 6 H 5 NO 2 ) [TPT = 2,4,6-tris(4-pyridyl)triazine]. 163 Upon removal of the enclathrated solvent molecules under vacuum, a compression of the two nets occurs with the material retaining its crystallinity. This process is not only reversible, but the guest molecules can also be exchanged for a variety of solvents in a single-crystal to single-crystal manner. In the case of benzene, the guest molecules were reported to be ordered in the pores, although no specific interactions with the frameworks could be identified. Large aromatic molecules such as triphenylene (Fig. 48), anthracene and perylene have also been inserted in the pores of [(ZnI 2 ) 3 (TPT) 2 ] which remains crystalline while the guests are ordered due to p-p interactions with the electron deficient ligand. 164 Further investigations on the insertion of functionalized triphenylenes 165 led to the observation, by X-ray crystallography, of a transient hemiaminal in a porous coordination network, a demonstration of the kinetic-trapping of a normally short-lived reaction intermediate. 166 The [[(ZnI 2 ) 3 (2) 2 (3)]Áx(G)] n network 1 (2 = TPT = 2,4,6-tris(4-pyridyl)triazine; 3 = 1-aminotriphenylene; G = ethyl acetate, x B 4) contains two types of pores of different shapes (A and B) with the amino groups of 3 facing directly into type-A pores (Fig. 49). A single crystal of [[(ZnI 2 ) 3 (2) 2 (3)]Áx(G)] n 1 was cooled to 215 K on a diffractometer and immersed for 10 minutes in a cooled flow of an ethyl acetate solution of acetaldehyde applied via a capillary. The crystal was then further cooled to 90 K to stop the reaction and the structure collected to reveal a combination of unreacted amine (64%) and the transient hemiaminal (36%). Notably, the percentage conversion is consistent with the disordering of the amine over three sites (a-c) in the unreacted crystal (distributed a: 44%, b: 33% and c: 23%), with reaction occurring at only site b. The lack of reaction at a and c sites was attributed to steric hindrance. The hemiaminal intermediate was kinetically trapped at 90 K, but warming to 270 K and standing for 30 minutes led to completion of the condensation reaction. Collecting a final structure at 90 K confirmed complete conversion of the hemiaminal to the imine product.
The exchange properties of [(ZnI 2 ) 3 (TPT) 2 ] are not limited to large, simple aromatic molecules. The flexibility and strong binding ability of this coordination framework were used to develop a methodology for direct crystallographic characterisation of trace amounts of a large variety of molecules, including absolute configuration of chiral compounds such as santonin (Fig. 50) by loading them in the pores of a single crystal of the coordination polymer. [167][168][169][170] While this approach has some significant challenges, both in reproducibility and in substrate scope, a few other examples that exploit the concept have been reported. 171,172 7. Adsorbate superlattice formation in MOFs revealed by SAXS The applicability of a laboratory-designed small-angle X-ray scattering (SAXS) set-up for studying the adsorption behavior of guest molecules in the mesoporous IRMOF-74 series has been reported. 173 Specifically, high resolution in situ SAXS data on Ar adsorption in IRMOF-74-V-hex enabled the accurate mapping of Ar distribution in the pore structure at different pressures (Fig. 51). The adsorption of Ar was defined by five different adsorption stages during the entire studied pressure range (0-100 kPa). The adsorbate electron-distribution maps from SAXS analysis revealed that Ar interacts with the open metal sites at stages 1 and 2 (low pressure region o27 kPa). When the pressure was increased to stage 3 (27-33 kPa), Ar molecules begin to fill the pores, leading to a steep increase in gas uptake. The corresponding hk = 10 reflection intensity decreased sharply during this stage, while a new broad peak at q = 0.10 Å À1 emerged. The emergence of this broad peak unambiguously represents the aggregation point in the initiation of formation of extra adsorption domains, whereby adsorbate atoms gather in certain pore regions in higher numbers than the average. As confirmed by the appearance of superlattice reflections in the SAXS patterns, these adsorption domains form the adsorbate superlattice at stage 4 (33-50 kPa). Upon further increases in pressure (stage 5, 450 kPa), the extra adsorption domains and superlattice reflections disappear, and the electron density in the centre region of the pores gradually increases, indicating argon fills among the pores in a uniform manner. This study indicates that adsorbate-adsorbate interactions within and across adjacent pores play a major role in gas uptake and release in porous MOFs.

Conclusions
Significant advances have been made in the field of porous MOFs, particularly in the field of high capacity gas storage and selective recognition, binding and separation of molecular substrates. Understanding the molecular mechanism by which these MOFs bind gas molecules from both crystallographic and dynamic perspectives provides key insight into the design and discovery of materials with improved properties. This article has reviewed recent advances and significant achievements on the successful characterisation of guest-loaded MOF materials via synchrotron PXRD, single crystal diffraction, NPD, INS,  QENS, SAXS, IR, and NMR techniques. The combination of these experimental approaches, particularly with both diffraction and spectroscopic methods, has yielded important rationalisation of materials property and function in terms of the mechanism of substrate binding at a molecular level. The role of open metal centres and pendant functional groups in the pore of MOFs has been discussed with the former providing specific binding to guest molecules, often with high selectivity. However, materials incorporating such sites usually suffer from stability and reversibility issues, while the binding of guest molecules via a combination of moderate-to-weak supramolecular based interactions via ligand functionalization affords fewer energy penalties with improved materials stability. The studies described herein on the elucidation of binding sites and host-guest interactions with porous MOFs enables and assists the design and optimisation of future ''smart materials'' in which high adsorption capacity, selectivity and stability may be achieved simultaneously.