Unveiling the Mechanisms of Catalytic Oxidation Reactions Mediated by Oxo-Molybdenum Complexes: A Computational Overview

Mo(VI) complexes MoO2X2L2 (X=halide or Me, L neutral ligand) behave as catalysts for olefin epoxidation in the presence of t-butylhydroperoxide (TBHP). The active species results from OH activation of TBHP, which protonates one oxo group and leads to a seven coordinate complex, with a new OOR ligand. It was found that several Mo(II) complexes Cp'Mo(CO)3X (Cp'=C5R5, Cp* or C5H5, Cp) acted as precursors for the same reactions and the resulting Cp'MoO2X could also oxidize sulfides and sulfoxides, both with TBHP and H2O2 as oxidants. A review of the reaction mechanisms proposed for these reactions, by us and some other authors, and based on computational studies is given in this work. More than one active species can be found for the CpMoO(OH)( η1-OOR)X intermediate, opening several competitive pathways. They differ by the O-H···O hydrogen bond formed between OH and one oxygen of the OOR ligand. This complex can also further react with oxidant to afford a peroxide complex CpMoO(η2-O2)X, which can also promote oxidation reactions. The activation energies depend on hydrogen bond assistance, so that the Cl and Me derivatives of CpMoO2X behave differently (the peroxide complex has only been found active with Me), and on the steric constraints, more obvious when comparing Cp with Cp*. The preferred mechanism will thus depend on the specific substituents, but energy barriers are comparable.