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 ideal adsorption solution theory (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 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 × 10⁶ Hz cf. 10¹¹-10¹³ Hz). Upon introduction of CO₂ or C₂H₂ 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 CO₂ and C₂H₂ around the triazine core are entirely consistent with the ²H NMR spectroscopic observations.