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 ideal adsorption solution theory (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 confirm an ultra-low barrier (∼2 kJ mol –1 ) to rotation of the phenyl group in the activated MOF and a rotation rate 5 orders of magnitude slower than usually observed for solid-state materials (1.4 × 10 6 Hz cf. 10 11 –10 13 Hz). Upon introduction of CO 2 or 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.