What singles out the FeO~(2+) Moiety? A Density-Functional Theory Study of the Methane-to-Methanol Reaction Catalyzed by the First Row Transition-Metal Oxide Dications MO(H_2O)_p~(2+), M = V—Cu

摘要

Metaloxo species are often postulated as key active species in oxidative catalysis. Among all, the quintet FeO~(2+) moiety is particularly widespread and active: aliphatic C-H bonds undergo hydroxylation easily through a H-abstraction/0-rebound mechanism. The high electrophilicity of quintet FeO~(2+) originates from its electronic structure: a low lying vacant σ~* can accept electronic density from the aliphatic C—H bond. What singles out this quintet FeO_(2+)? Its lowest vacant acceptor orbital energy? its shape (σ~* vs π~*)? or has its biological importance more simply arisen from the high iron abundance? To answer those questions, we have performed density-functional theory calculations to study systematically the methane-to-methanol reaction catalyzed by MO(H-2O)_P/~(2+) complexes (M = V, Cr, Mn, Fe, Co, p = 5 and M = Ni, Cu, p = 4) in gas phase. We show here that the lower the Mo~(2+) acceptor orbital lies in energy, the lower the H-abstraction barrier is in general. However, a σ~* acceptor orbital is much more efficient than a π~* acceptor orbital for a given energy. Finally, we found that indeed, the FeO~(2+) moiety is particularly efficient but also CoO~(2+) and MnO~(2+) could be good candidates to perform C-H hydroxylation.