Direct observation of supramolecular binding of light hydrocarbons in vanadium(III) and (iv) metal-organic framework materials

Fine tuning of host-guest supramolecular interactions in porous systems enables direct control over the properties of functional materials. We report here a modification of hydrogen bonding and its effect on guest binding in a pair of redox-active metal-organic frameworks (MOFs). Oxidation of MFM-300(V^III) {[VIII2(OH)₂(L)], LH₄ = biphenyl-3,3′,5,5′-tetracarboxylic acid} is accompanied by deprotonation of the bridging hydroxyl groups to afford isostructural MFM-300(V^IV), [VIV2O₂(L)]. The precise role of the hydroxyl groups, O-carboxylate centres and π-π interactions in the supramolecular binding of C₂ hydrocarbons in these materials has been determined using neutron diffraction and inelastic neutron scattering, coupled with DFT modelling. The hydroxyl protons are observed to bind to adsorbed unsaturated hydrocarbons preferentially in MFM-300(V^III), particularly to C₂H₂, which is in a sharp contrast to MFM-300(V^IV) where interactions with O-carboxylate centres and π-π interactions predominate. This variation in structure and redox leads to notably higher separation selectivity for C₂H₂/CH₄ and C₂H₄/CH₄ in MFM-300(V^III) than in MFM-300(V^IV). Significantly, owing to the specific host-guest interactions, MFM-300(V^III) shows a record packing density for adsorbed C₂H₂ at 303 K and 1 bar, demonstrating its potential for use in portable acetylene stores.