H₃O₂ Bridging Ligand in a Metal–Organic Framework. Insight into the Aqua-Hydroxo↔Hydroxyl Equilibrium: A Combined Experimental and Theoretical Study

A metal–organic framework (MOF) bearing the aqua-hydroxo species (O2H3)− in the framework, as well as the processes that govern the equilibrium aqua-hydroxo (O2H3)−↔hydroxyl (OH) in Sc-MOFs, are studied experimentally and theoretically. Computational studies were employed to determine the relative energies for the two compounds that coexist under certain hydrothermal conditions at pH < 2.8. The thermodynamically more stable [Sc3(3,5-DSB)2(μ-O2H3)­(μ-OH)2(H2O)2] (from now on, (O2H3)­Sc-MOF; 3,5-DSB = 3,5-disulfobenzoic acid) was obtained as a pure and stable phase. It was impossible to isolate [Sc3(3,5-DSB)2(μ-OH)3(H2O)4] as a pure phase, as it turned out to be the precursor of (O2H3)­Sc-MOF. Additionally, a third compound that appears at pH between 3.5 and 4, [Sc3(3,5-DSB)­(μ-OH)6(H2O)] and a fourth, [Sc­(3,5-DSB)­(Phen)­(H2O)]­(H2O), in whose formula neither OH groups nor H3O2– anions appear, are reported for comparative purposes. A study of the (O2H3)­Sc-MOF electronic structure, and some heterogeneous catalytic tests in cyanosilylation of aldehydes reactions, are also reported.