H3O2 bridging ligand in a metal-organic framework. Insight into the Aqua-Hydroxo↔hydroxyl equilibrium: A combined experimental and theoretical study

Abstract

A metal-organic framework (MOF) bearing the aqua-hydroxo species (O 2H3)- in the framework, as well as the processes that govern the equilibrium aqua-hydroxo (O2H 3)-↔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, (O2H 3)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, [Sc 3(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. © 2013 American Chemical Society.