Selective Gas Uptake and Rotational Dynamics in a (3,24)-Connected Metal-Organic Framework Material

: The desolvated (3,24)-connected metal-organic framework (MOF) material, MFM-160a, [Cu 3 (L)(H 2 O) 3 ] [H 6 L = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid)] exhibits excellent high pressure uptake of CO 2 (110 wt% at 20 bar, 298 K) and highly selective separation of C 2 hydrocarbons from CH 4 at 1 bar pressure. Henry’s law selectivities of 79:1 for C 2 H 2 :CH 4 and 70:1 for C 2 H 4 :CH 4 at 298 K are observed, consistent with IAST predictions. Significantly, MFM-160a shows a selectivity of 16:1 for C 2 H 2 :CO 2 . Solid state 2 H NMR spectroscopic studies on partially deuterated MFM-160-d 12 shows an ultra-low barrier to rotation of the phenyl group in the activated MOF (~2 kJ mol -1 ), and a rotation rate five orders of magnitude slower than usually observed for solid state materials (1.4 x 10 6 Hz cf. 10 11 - 10 13 Hz). Upon introduction of CO 2 and C 2 H 2 into desolvated MFM-160a, this rate of rotation was found to increase with increasing gas pressure, a phenomenon attributed to the weakening of an intramolecular hydrogen bond in the triazine-containing linker upon gas binding. DFT calculations of binding energies and interactions of CO 2 and C 2 H 2 around the triazine-core are entirely consistent with the 2 H NMR spectroscopic observations.


INTRODUCTION
Over the past two decades there has been a great deal of interest in metal-organic framework (MOF) materials capable of selectively storing and separating gases.0][11][12][13][14][15] CO2 is well-established as a leading contributor to climate change and over 85% of the world's energy demand involves the burning of fossil fuels. 16,17Materials capable of storing CO2 from flue gas and automobiles are therefore in high demand.][20][21] Natural gas is a vitally important fuel and is also a feedstock for production of a variety of chemicals. 22Comprising mainly of CH4, it is also increasingly being utilized in vehicles as its high H:C ratio results in lowered emissions of CO and CO2 compared to normal hydrocarbon fuels. 22The main sources of impurity in natural gas are the C2 hydrocarbons, C2H2, C2H4 and C2H6. 11Removing these hydrocarbons increases the purity of natural gas and provides a source of C2 hydrocarbons for further use. 23,24Purification and separation of C2 hydrocarbons are usually achieved by cryogenic distillation, a very energy intensive process, and the use of selective adsorbents to separate different components at room temperature is a potential alternative. 23,24e report herein the synthesis of the hexacarboxylate linker H6L which has been used to prepare a (3,24)-connected Cu(II) material (denoted MFM-160; MFM = Manchester Framework Material) designed for both high CO2 uptake and selective sorption of C2 hydrocarbons over CH4.With this in mind, our ligand design was based around a triazine core, 11,[23][24][25][26] coupled with a triangular arrangement of three pendant isophthalic acid moieties, which can combine with a Cu(II) paddlewheel motif to give a framework of rht topology, used by us [27][28][29][30][31][32] and others [33][34][35] to prepare MOFs with exceptional gas storage capacity.We show here that MFM-160 has the potential for extremely high CO2 uptake at higher pressures, whilst at lower pressures it is capable of highly selective sorption of C2 hydrocarbons over CH4 and CO2.

EXPERIMENTAL
The preparation of H6L was achieved via microwave-assisted nucleophilic substitution between 4-bromoaniline and cyanuric chloride to give the tribromo intermediate (Scheme 1). 36A microwave-assisted Pd(II) catalyzed Suzuki cross-coupling of this intermediate with 3,5-di(ethoxycarbonyl)phenylboronic acid gave the hexa-ester product, which was then hydrolyzed in aqueous NaOH followed by exceptionally rapid, facile and energy-efficient route to this linker.Solvothermal reaction of H6L and Cu(NO3)2 .3H2O in acidified DMF at 80 °C for 24 h yielded green octahedral single crystals of MFM-160, [Cu3(L)(H2O)3] .13DMF .7H2O.Density functional theory (DFT) calculations were performed, as implemented in the Q-Chem quantum chemistry package. 37The strength of adsorption sites was analyzed using a fragment of the linker formed by the central triazine ring and three phenyl rings (Figure S33).The binding energies (BE) between the guest molecule and the linker were calculated in addition to their relative positions corresponding to the strongest binding.These calculations were performed in two stages.Geometry optimization was carried out using dispersion-corrected DFT calculations at the B3LYP/6-31+G** level of theory and the binding energies were subsequently calculated at the higher B3LYP/6-311+G** level with BE = Eopt(complex) -Eopt(linker) -Eopt(guest molecule).Binding energies were corrected for basis set superposition error (BSSE).
RESULTS AND DISCUSSIONS MFM-160 crystallizes in the tetragonal space group I4/m with unit cell dimensions a = 32.6838(3)Å, c = 48.0624(7)  Å and V = 51341(1) Å 3 and shows a (3,24)-connected framework structure of rht topology containing three distinct cages, A, B and C (Figure 1).Cage A is a truncated cuboctahedron formed from 12 [Cu2(O2CR)4] paddlewheel units connected to 24 different linkers.With the spacing between cuboctahedra determined by the distance between isophthalate moieties on the arms of the linker.This results in the formation of larger cages B (truncated tetrahedron) and C (truncated cube).With the aqua ligands removed from the axial positions of the paddlewheel, the accessible internal diameter of cage A is 13.0 Å, and of cage B it is 13.8 Å with a cage length of 22.7 Å with the accessible windows between these cages measuring 6.5-8.2Å.The largest cage, C, has an accessible diameter of 19.6 Å and a cage length of 35.2 Å, with windows of 6.4-8.4Å.All internal diameters were calculated by fitting a sphere from the center of the cage to its walls, taking into account van der Waals radii.Window diameters were calculated from the largest sphere able to fit through the aperture.The potential accessible volume of desolvated MFM-160a ("a" denotes the desolvated form) is 77% as calculated by the VOID algorithm within the software PLATON 38 (after removal of all guest solvates and coordinated water molecules), with a calculated crystal density of 0.540 g cm -3 .Thermogravimetric analysis of the as-synthesized sample of MFM-160 in air showed loss of DMF and H2O solvent below 170 °C, with the material stable up to 290 °C, above which a mass loss corresponding to framework decomposition is observed (Figure S2).To prepare activated MFM-160a for gas sorption experiments, the DMF of the as-synthesized sample was exchanged with analytical grade acetone by decanting the mother liquor and replacing with analytical grade acetone.This process was repeated twice daily for 5 days.The sample was then activated by heating at 100 °C under reduced pressure for 20 h, whereupon a color change from dark green to purple was observed; this is a typical indicator of the removal of coordinated solvent from the Cu(II) paddlewheel node.The permanent porosity of the framework was confirmed via an N2 sorption isotherm measured volumetrically at 77 K which showed reversible type-I behavior and an uptake of 983 cm 3 g -1 (123 wt%) at 1 bar (Figure S5).The slight changes in gradient in the range 30-120 mbar are assigned to the sequential filling of the micropores and mesopores.The estimated Braun-Emmett-Teller (BET) surface area of MFM-160a as calculated from this isotherm was 3847 m 2 g -1 , with a total pore volume of 1.52 cm 3 g -1 .

Cage A Cage B Cage C
In order to probe the effectiveness of MFM-160a to adsorb different gases, we performed gas adsorption isotherm experiments between 0-20 bar using CO2 and CH4 (298 K and 273 K, Figure 2), and H2 (77 K and 87 K, Figure S6).Low pressure (0-1 bar) isotherms of C2 hydrocarbons were run at 273 K and 298 K for comparison with the data obtained for CH4 (Figure 3).MFM-160a shows exceptional capacity for CO2 storage at 20 bar with uptakes of 558 cm 3 g -1 (110 wt%) and 719 cm 3 g -1 (141 wt%) at 298 K and 273 K, respectively.At 195 K and 1 bar MFM-160a nears saturation at 881 cm 3 g -1 (173 wt%).This uptake of CO2 up to 20 bar at 298 K is one of the highest reported to date: to the best of our knowledge the current record uptake under these conditions is 627 cm 3 g -1 (123 wt%, 19.8 bar) in MOF-177. 5,20The CH4 uptake of MFM-160a at 20 bar was 199 cm 3 g -1 (14.2 wt%) at 298 K and 274 cm 3 g -1 (19.6 wt%) at 273 K.The H2 uptake at 20 bar was 6.4 wt% at 77 K and 5.3 wt % at 87 K, whereas the uptake at 1 bar was 2.14 wt% at 77 K and 1.40 wt% at 87 K (Figure S6).As shown in Figure 3, the uptakes of C2H2, C2H4 and C2H6 at 273 K are 212 cm 3 g -1 (24.7 wt%), 175 cm 3 g -1 (21.9 wt%) and 201 cm 3 g -1 (27.0 wt%) at 1 bar, respectively.At 298 K these values become 128 cm 3 g -1 (14.9 wt%), 115 cm 3 g -1 (14.4 wt%) and 110 cm 3 g -1 (14.8 wt%).This comparatively extremely low uptake of CH4 at 1 bar (15.3 cm 3 g -1 at 298 K, 27.1 cm 3 g -1 at 273 K) in MFM-160a makes the material of great interest for selective separation of C2 hydrocarbons over CH4.To estimate the selectivity of each hydrocarbon over CH4 we employed Henry's law, in which constants (KH) were determined using a virial fit of the measured isotherm data (Figures S7-S16).Using these constants, the selectivity (S) of one gas, i, over the second gas, j, was determined from the equation: Sij = KHi/KHj The selectivities for C2H2:CH4, C2H4:CH4 and C2H6:CH4 at 273 K were calculated to be 104:1, 130:1 and 10:1 respectively.While lower than current benchmark materials such as NKMOF-1-Ni 39 and ZJU-74a 40 , the C2H2:CH4 separation of 79:1 at 298 K is comparable to other Cu(II) triazine-based MOFs such as [Cu3(TDPAT)] 11 (TDPAT 6-= 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine) and [Cu3(TDPAH)] 23 (TDPAH 6-= 2,5,8-tris(3,5-dicarboxylphenylamino)-s-heptazine) which have selectivities of 127:1 and 81:1, respectively.However, the calculated selectivities are higher than those reported for smaller-pore Cu(II) MOFs such as UTSA-50a 13 and UTSA-15a 41 (68:1 and 56:1 respectively).The selectivity for C2H2 over CO2 (15:1 at 298 K; 16:1 at 273 K) demonstrates the potential of MFM-160a for the purification of acetylene, in which CO2 is a common impurity.A study by Li et al. 42 strongly suggested that the presence of N-centers within pores does not improve selectivity for C2H2 over CO2.Thus, in the case of MFM-160a, we tentatively attributed this selectivity to the greater van der Waals interactions between the framework and C2H2 molecules, as described by Samsonenko and co-workers for selective uptake of C2H2 over CO2 in porous formates. 43The separation of CO2 from C2H2 is notoriously difficult as a result of their similar size and sublimation points, 44 and the current high selectivity for C2H2 over CO2 is a promising and represents one of the highest selectivities for a metal-organic framework for this separation. 45,46It should be noted that repeating the desorption-adsorption cycles of these isotherms gives identical results, confirming the stability of MFM-160 to repeated activation and re-use.In addition to Henry's law, the gas selectivities for C2H2:CH4, C2H4:CH4, C2H6:CH4 and C2H2:CO2 in binary mixtures were analyzed using the IAST model. 47Each isotherm was fitted using a dual-site or single-site Langmuir-Freundlich model with the chosen model based on the accuracy of the resulting fitting (Figures S17-26).The selectivities for each mixture (Figure 3c and S27) are in agreement with the Henry's law calculations at 273 K, as the order of selectivity over CH4 is shown to be C2H4 > C2H2 >> C2H6.At very low pressures (~20 mbar) the C2H4:CH4 selectivity is predicted to be 110:1, which decreases rapidly with increasing pressure before reaching 26:1 at 1 bar.The C2H2:CH4 selectivity is 97:1 below 30 mbar and decreases to 28:1 at 1 bar.At 298 K the IAST predictions show that the selectivity for C2H4 over CH4 is much greater than that of C2H2 between 20 and 200 mbar (111:1 versus 51:1 at 20 mbar), although the selectivities are approximately equal at 1 bar.
Solid state 2 H NMR spectroscopic studies of activated and gas-loaded MFM-160 In order to gain a greater understanding of the C2H2:CO2 separation and of the behavior of both gases within the porous material, 2 H NMR spectroscopic studies were carried out on a partially deuterated analogue of MFM-160, denoted MFM-160-d12 (Scheme 2).The dynamics of the phenyl ring can affect the adsorption properties in two ways: firstly, the phenyl rings themselves provide an accessible adsorption site as a result of the aromatic π-system; secondly, the phenyl rotation directly affects the effective size and geometry of the pore.Probing the framework dynamics is also of interest for comparison with other known MOFs, thus improving understanding of structure-property relationships.The rotational dynamics in MFM-160-d12 were probed in the activated, desolvated material (Figures 4 and S28), in the presence of CO2 at 1 bar and 5 bar loadings (Figure S29 and S30, respectively) and in the presence of C2H2 at 0.32 bar and 1 bar (Figure S31 and 32, respectively).As the low pressure uptake of C2H2 is significantly higher than that of CO2, respective pressures of 0.32 and 1 bar were chosen for comparison of the two gases at the same concentrations.The loading of 0.32 bar C2H2 and 1 bar of CO2 each correspond to ~3.1 gas molecules per linker, where the empirical formula of MFM-160-d12 gives a density of 1043.43 g mol -1 .The higher loadings of 1 bar C2H2 and 5 bar CO2 correspond to 5.9 and 10.8 molecules per linker, respectively.In the activated, desolvated MFM-160-d12 the temperaturedependent behavior of the line shapes in the 2 H NMR spectra confirms that the deuterated phenyl rings are mobile, with a dynamic process gradually developing between ~ 120 K and 300 K (Figures 4 and 5).Below 123 K the line shape is mainly composed of a Pake-powder pattern with quadrupolar coupling parameters (Q0 = 176 kHz, η0 ~ 0) typical for a static C-D group on the 2 H NMR spectroscopic timescale.At ~ 300 K, the spectrum is dominated by the narrowed line-shape expected for an axial rotation around the phenyl C2 axis (Q1 ~20 kHz ~ Q0/8). 48At the same time, over the whole temperature range studied, these two components co-exist.Such a phenomenon has been reported for UiO-66 49 and indicates that the phenyl motion at each given temperature is characterized by a distribution of rotation rates, which suggests that the rotational potential fluctuates to a certain degree across the framework.In common with work by Schmidt et al., 50 the rate constant is assumed to obey a log-normal distribution with k1 being the mean value of the rotation rate at each give temperature and σ being the width of the distribution.By considering the C2 symmetry of the phenyl fragment, the uniaxial rotation around the C2 axis can be modeled either by a 4-site or a 6-site jump rotation (b) (c) model (Figure 6).Notably, in most cases the 6-site jump model can efficiently describe also more complex axial rotations. 51Numerical fitting of the experimentally observed 2 H NMR spectroscopic patterns shows that only the 6-site jump rotation gives an agreement with our data.Although the distribution of the jump sites has to obey C2 symmetry, the initial position of the phenyl orientation might be distorted from a fully symmetric case.This distortion is readily introduced into the re-orientation model as an angular parameter φi (Figure 6).For a 6-site jump rotation mechanism the jump positions are given as follows: φ1 = φi, φ2 = π/2 , φ3 = π -φ1, φ4 = π + φ1, φ5 = π/2, φ6 = π + φ2.Within such a model we assume that all positions have equal probability.In our model, the angle φi = 45 °, where φi is the angle between the plane of the mobile phenyl ring and the plane of the aromatic ring fixed to the Cu(II) node.Thus, it follows that the value Δφ1 = 2φi can be directly compared with the crystal structure.Despite the model being able to give distinct rate constants for each elementary jump, we have found that a simple kinetic matrix defined by one rate constant is sufficient to provide good agreement with the experimental observations.Thus, this 6-site jump rotation model accurately simulates the experimental temperature dependence of the 2 H NMR spectral line shapes.The model Δφ1 = 2φi ~ 90 ○ is consistent with the crystallographic data and underscores that the orientation of the phenyl rings is not homogeneously distributed.The log-normal distribution width is almost constant over the temperature range and is σ ~ 1.3-1.4.Thus, the present inhomogeneity is considerably lower than that observed in UiO-66 and may be attributed to the flexible nature of MFM-160.The most striking features of the 2 H NMR spectra for MFM-160-d12 are the kinetic parameters of the deuterated phenyl motion: the Arrhenius plot (Figure 7a) is linear and shows an activation barrier, E0 = 2.1 kJ mol -1 and collision factor, k00/2π = 1.4 x 10 6 Hz.This activation barrier is so low that it is comparable to the torsional barrier for methyl groups in xylenes or 2-methylimidazole, i.e. aromatic systems where the internal steric interactions are minimized.The torsional barrier for methyl groups in such compounds has been measured experimentally by neutron scattering techniques and 2 H NMR spectroscopy which confirm in both cases that E0 does not exceed 1.5 kJ mol -1 . 52,53Such low values have not been reported previously for solid state materials based upon carboxylate ligands and have been confirmed for many different samples of MFM-160-d12.However, similar low barriers have been reported recently for MOFs incorporating metal-amine linkers such as 2-methylimidazole in ZIF-8 52 and dabco in [Zn2(bdc)2(dabco)]. 54t is interesting to consider the possible sources of steric interaction hindering phenyl rotation in MFM-160-d12.The electrostatic interactions are maximized when all aromatic rings are in the same plane, in which case the closest interatomic distances can be given as d1 ~ 2.2 Å, d2 ~ 3.1 Å and d3 ~ 2.0 Å (Figure 8).The main interaction is likely governed by sites III and I, but these distances do not offer an obvious explanation for the ultra-low value of k00 (usually ~ 10 [11][12][13] Hz in MOF materials) However, this may be explained by the short intramolecular hydrogen bond (1.91 Å) between the phenyl protons and the triazine N atoms observed in the crystal structure, thus restricting the rotation of the ring by affecting its pre-exponential factor.Although the effect of gas-loading is not very strong in terms of absolute values, it is an unexpected one (Figure 5).With CO2 as the guest species, the concentration dependence at 123 K is as expected with a higher gas loading resulting in a slower rate of phenyl rotation.At the higher loading of ~5 bar (at 298 K), the decrease in rotation rate is more pronounced and evident even without detailed numerical analysis of the observed line shape.However, at 223 K this effect is inverted: with increased loading of CO2, the rate of phenyl rotation actually increases, a phenomenon previously unobserved in a MOF or any other solid state material. 55On a quantitative level the effect is clearly seen on an Arrhenius plot (Figure 7b) for the corresponding rotational rate constants: the slope for the CO2-loaded material is notably steeper in comparison to that of the guest-free material.While not very pronounced for the intermediate loading (1 bar) with E1 = 2.6 kJ mol -1 , k10/2π = 2.3 x 10 6 Hz, the effect becomes more evident at higher concentration (5 bar) with E2 = 4.2 kJ mol -1 , k20/2π = 7.6 x 10 6 Hz.In the latter case the barrier is approximately doubled compared with the desolvated material, but the collision factor is increased by six-fold, so even though the barrier has increased the collision factor overrides this suggesting that CO2 blocks or breaks the internal H-bonding, thus allowing more rapid rotation of the phenyl groups.It is possible to readily observe the collision factor behavior because of the extremely low starting value for the guest-free material, i.e. k00/2π =1.4 x 10 6 Hz, while normal pre-exponential factors for an elementary rotation are typically in the range of 10 11-13 Hz.For the C2H2-loaded material, the situation is different: at the intermediate concentration (~0.32 bar at RT), conditions in which the C2H2 concentration is equimolar to that of CO2 at 1 bar, no effect on the phenyl dynamics is observed (Figure 7c) and the rotation is characterized by the same parameters as for the activated, desolvated MFM-160-d12 with E3 = 1.9 kJ mol -1 and k30 =1.2 x 10 6 Hz.Upon increasing C2H2 concentration to 1 bar, the rate of phenyl ring rotation clearly increases, however the activation barrier remains the same E4 = 1.9 kJ mol -1 but the collision factor rises to k40 =1.8 x 10 6 Hz.This is shown on the corresponding Arrhenius plots (Figure 7c).This behavior can be interpreted in terms of host-guest interactions.At the lower loading the phenyl rings do not interact with C2H2 and only at higher loadings is there a subtle increase of the collision factor.This shows that occupancy of C2H2 sites around the mobile phenyl rings is low, even at higher concentrations.Thus, C2H2 coordinates predominantly to the Cu(II) paddlewheel sites and interacts with the phenyl rings only through random collisions, most likely when most of the metal sites are occupied.At 0.32 bar C2H2 (3.1 gas molecules per formula unit) the Cu(II) sites are very likely saturated with C2H2 bound strongly and not able to influence phenyl rotation, as shown by 2 H NMR spectroscopy.

Computational modelling of gas binding sites in MFM-160
To understand further the observations from 2 H NMR spectroscopy and to help explain the hydrocarbon selectivity demonstrated by MFM-160, DFT calculations were performed using the Q-Chem quantum chemistry package. 37n extensive search for binding sites between all guest molecules and the linker fragment was performed, with configurations showing the strongest binding and binding energy ranges summarized in Tables 1; Table S2 shows the full range of binding configurations.The strongest binding of CO2 to the linker fragment was found to be at site A1 (Table 1), in which the CO2 molecule interacts above the linker with a binding energy of -19.5 kJ mol -1 .The interaction is dominated by a strong electrostatic interaction between the carbon of CO2 and the N-center of the central ring.This interaction is enhanced by two weak hydrogen bonds between the other oxygen of CO2 and nearby protons of the neighboring phenyl ring and the bridging -NH group.These interactions have a significant cooperative effect on the binding of CO2 to the linker fragment.The slow rotation of the phenyl ring as observed by 2 H NMR spectroscopy was attributed to a hydrogen bond between a phenyl proton and an N atom of triazine core.Therefore, the increased rotation upon CO2 loading may be explained by the weakening or removal of this hydrogen bond upon CO2 binding at site A1.Increased occupancy of gas at this site at higher CO2 loading would also explain the increased rate of rotation from the 1 bar to the 5 bar-loaded sample.Overall, CO2 interacts more strongly with the linker fragment than C2H2, giving a range of binding energies from -19.5 to -12.4 kJ mol -1 .The strongest binding site for C2H2 is over the bridging amine nitrogen between the triazine ring and the outer phenyl rings with a binding energy of -18.2 kJ mol -1 (site C1, Table 1).There is a cooperative binding effect with the hydrogens of the C2H2 interacting with the triazine ring and the neighboring phenyl ring, but the binding is dominated by an interaction between the π-bonds of the C2H2 and the bridging nitrogen.As with CO2, this interaction is likely to affect the hydrogen bond between the phenyl proton and the triazine core, leading to the increased phenyl rotation rate observed by 2 H NMR spectroscopy upon addition of 1 bar C2H2.At lower pressures (0 to 0.32 bar), this site is unoccupied, as confirmed by the values of E0 and k00 at 0.32 bar remaining the same as those observed in the guest-free material.Between 0.32 and 1 bar, where the Cu(II) paddlewheel sites are expected to be saturated, the increased occupancy of this site disrupts the intramolecular hydrogen bond of the linker and the phenyl rotation increases.It would have been interesting to observe the effect of a higher loading of C2H2 on the rotational dynamics, but there are well-established safety concerns with the use of C2H2 above 1 bar.The π-bond interaction of C2H2 with the linker leads to stronger interactions overall compared to other hydrocarbons, and as a result the C2H2 molecule interacts with a similar range of binding energies as CO2 (-13.3 to -18.2 kJ mol - 1 ).These comparable binding energies strongly suggest that the triazine functionality is not responsible for the selectivity for C2H2 over CO2.As stated earlier, it is likely that increased van der Waals interactions between C2H2 and the framework in comparison to those with CO2, as well as the higher affinity of the former for the open Cu(II) sites, are responsible for the selective uptake.The range of interactions between the hydrocarbons C2H4 and C2H6 and the linker were also investigated and found to be very similar; -11.7 to -16.2 kJ mol -1 and -10.2 to -16.3 kJ mol -1 respectively.The strongest interaction between C2H4 and the linker and also C2H6 and the linker occurs over the central triazine ring.This is an interaction between the central carbon-carbon bond of the two hydrocarbons, C2H4 and C2H6 and the delocalized π system of the central ring.On average, the binding of C2H4 is stronger than C2H6 due to the strength of the interaction between the alkene double bond and the linker.To complete the computational investigation, we found that the strongest binding of CH4 to the linker was found to be in complex B (Table 1) where the CH4 molecule interacts above the central triazine ring to give a binding energy of -10.3 kJ mol -1 .There is a weak hydrogen bond between a CH4 hydrogen atom and a nitrogen atom of the triazine ring.There are also additional weak electrostatic interactions between other CH4 hydrogen atoms and the central ring.The binding energy interactions between CH4 and the linker are the weakest of all those studied, consistent with the experimental isotherm data.

CONCLUSIONS
In conclusion, we have demonstrated an energy and costefficient route to a large hexacarboxylate linker through microwave-assisted synthesis.This led to the synthesis of a new, highly porous (3,24)-connected Cu(II) MOF, designated MFM-160.We have shown that its activated form MFM-160a exhibits an exceptional capacity for uptake of CO2 at 20 bar and 298K (110 wt%).We have also shown that the relatively poor uptake of CO2 and CH4 at lower pressures (≤ 1 bar) makes MFM-160a a material of interest for the purification of natural gas and of acetylene, as confirmed by Henry's Law and IAST calculations.A 2 H NMR spectroscopic study of phenyl rotation dynamics confirmed an ultra-low rotation barrier, but very slow phenyl rotation.The rotation was shown to increase upon gas loading, a unique phenomenon not previously observed in a solid state material and is attributed to an intramolecular hydrogen bond, which is weakened by gas binding, leading to an increased rate of rotation.The results of DFT calculations are consistent with this, as both gases show clear binding interactions near this hydrogen bond.

ASSOCIATED CONTENT
Supporting Information: Experimental details and supporting cif files, additional figures, graphs, and tables.This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 1 .
Figure 1.View of cages A, B and C in MFM-160; the large coloured spheres in the centre of the cages represent the solvent accessible volume.Cu: cyan, O: red, N: blue, C: grey.Hydrogen atoms are omitted for clarity.

Figure 4 .
Figure 4. Temperature dependence of the line shape of resonances due to the phenyl groups in guest-free MFM-160-d12 as measured by 2 H NMR spectroscopy (experimental, black; simulation, red).

Figure 5 .
Figure 5.Comparison of the experimental line shapes and rotation rates for phenyl groups in guest-loaded MFM-160-d12 at 123 K and 223 K.

Figure 6 .
Figure 6.View of stable orientation sites for the phenyl rings depending on the jump rotation model: (a) the starting position of the plane of the mobile phenyl ring is tilted by an angle φi.The axial jump-rotation about the C2 symmetry axis of the phenyl group is then realised by either (b), an n = 4 site exchange motion, or by (c) an n = 6 site exchange.The distribution of the orientation sites is governed only by the value of the first position φ1 = φi.All elementary jumps are assumed to be governed by the same rate constant k = k1/n.Even in the case of the same value of φi in the intermediate jump rate the two models give qualitatively distinct patterns.

Figure 7 .
Figure 7. Arrhenius plots of the rotation rate constants k1.a) Arrhenius plot of all gas-loaded samples b) Arrhenius plot of CO2-loaded MFM-160 c) Arrhenius plot of C2H2-loaded MFM-160 (■) guest-free MFM-160-d12; (blue-○) 1 bar CO2 loading; (red-Δ) 5 bar CO2 loading; (pink-•) 0.32 bar C2H2 loading; (dark green-▼) 1 bar C2H2 loading.All gas loadings were performed at 298 K.However, with an equimolar loading of CO2, where the Cu(II) sites were also expected to be saturated, the phenyl ring rotation is much more affected by the guest CO2 molecules than for C2H2, strongly suggesting that CO2 occupies binding sites on the linker, while open Cu(II) sites are still available.These observations, therefore, provide a good insight into the selectivity for C2H2 over CO2, as the stronger binding of C2H2 at the open Cu(II) sites very likely accounts for the greater uptake of C2H2 at low pressures.Interestingly, the C2H2:CO2 selectivity in [Cu2(pzdc)2(pyz)] (pzdc 2-= pyrazine-2,3-dicarboxylate; pyz = pyrazine), a MOF without open Cu(II) sites, was assigned to the binding of C2H2 with free carboxylate oxygen atoms lining the pores.56

Figure 8 .
Figure 8.View of the deuterated (green) mobile phenyl ring in MFM-160-d12.The scheme shows the possible hydrogen bonding sites (I, II, and III) that will influence the rotational potential for rotation of the phenyl group.