Second-Coordination Sphere Effect on the Reactivity of Vanadium-Peroxo Complexes: A Computational Study.

Abstract

Vanadium-oxo and vanadium-peroxo complexes are common intermediates in biology and are, for instance, found in the catalytic cycle of vanadium haloperoxidases. In biomimetic chemistry synthetic models have been created that mimic the structural features of the coordination environment of these vanadium-oxo and vanadium-peroxo species. Recently, two novel vanadium-oxo complexes were trapped and characterized with a trigonal bipyramidal ligand design with either a solvent exposed vanadium center or the vanadium inside a cage, designated as the bowl-shaped configuration and the dome-shaped structure, respectively. Density functional theory calculations are reported here on these bowl- and dome-shaped structures where we study the reaction with t-butylhydroperoxide to form the vanadium-peroxo species and its reaction with thioanisole. Although the structural features of the vanadate core are close for both structures, the calculations display a strong second-coordination sphere effect of the ligand architecture on the barrier heights of the reaction with a terminal oxidant even though the rate-determining transition states show little structural differences. A similar observation is seen for the reaction of the two vanadium-peroxo species with thioanisole. Overall, the calculations implicate that vanadium-peroxo is an efficient oxidant of sulfoxidation reactions, although it is not as efficient as analogous iron(IV)-oxo heme and nonheme oxidants that react with substantially lower barriers. The reactivity differences are analyzed with thermochemical cycles and valence bond patterns that explain the differences in chemical properties and identify how the ligands affect the chemical reactivity with substrates.