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.