O₂ Activation and Catalytic Alcohol Oxidation by Re Complexes with Redox-Active Ligands: A DFT Study of Mechanism

Abstract

As a contribution to understanding catalysis by transition metal complexes with redox-active ligands (here: catecholate – cat), we report a computational study on the mechanism of a catalytic cycle where (i) O₂ is activated at the metal center of the catecholate complex [Re^V(O)­(cat)₂]⁻ to yield [Re^VII(O)₂(cat)₂]⁻, which (ii) subsequently is applied to oxidize alcohols. We were able to identify the steps where the redox-active ligands played a crucial role as e^– buffer. For O₂ homolysis, a series of sequential 1e^– steps leads to superoxo and bimetallic intermediates, followed by facile cleavage of the bimetallic peroxo O–O linkage. The <i>trans–cis</i> isomerization of <i>trans</i>-[Re^V(O)­(cat)₂]⁻ is the crucial step of O₂ activation, with an absolute free energy barrier of 16.8 kcal mol^–1 in methanol. Due to the ionic character of intermediates, all reaction barriers of O₂ activation are significantly lowered in a polar solvent, thus rendering O₂ homolysis kinetically accessible. With computational results for the activation barriers of all elementary steps as well as the calculated solvent effects, we are able to rationalize all pertinent experimental findings. For catalytic alcohol oxidation, we propose a novel cooperative mechanism that involves two units of the metal complexes, ruling out the reaction via a seven-coordinated active oxidant, as previously hypothesized. We present in detail calculated energies and barriers for the reaction steps of the oxidation of methanol as model alcohol as well as the energetics of crucial steps of the experimentally studied oxidation of benzyl alcohol, both transformations for methanol as solvent