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, [Cu3(L)(H2O)3] [H6L = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid)] exhibits excellent high pressure uptake of CO2 (110 wt% at 20 bar, 298 K) and highly selective separation of C2 hydrocarbons from CH4 at 1 bar pressure. Henry’s law selectivities of 79:1 for C2H2:CH4 and 70:1 for C2H4:CH4 at 298 K are observed, consistent with IAST predictions. Significantly, MFM-160a shows a selectivity of 16:1 for C2H2:CO2. Solid state 2H NMR spectroscopic studies on partially deuterated MFM-160-d12 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 106 Hz cf. 1011 - 1013 Hz). Upon introduction of CO2 and C2H2 into desolvated MFM-160a, this rate of rotation was found to increase with increas-ing 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 CO2 and C2H2 around the triazine-core are entirely consistent with the 2H NMR spectroscopic observations.