What Singles out the FeO2+ Moiety? A Density-Functional Theory Study of the Methane-to-Methanol Reaction Catalyzed by the First Row Transition-Metal Oxide Dications MO(H2O)p2+, M = V-Cu

Metaloxo species are often postulated as key active species in oxidative catalysis. Among all, the quintet FeO2+ moiety is particularly widespread and active: aliphatic C-H bonds undergo hydroxylation easily through a H-abstraction/O-rebound mechanism. The high electrophilicity of quintet FeO2+ originates from its electronic structure: a low lying vacant sigma* can accept electronic density from the aliphatic C-H bond. What singles out this quintet FeO2+? Its lowest vacant acceptor orbital energy? its shape (sigma* vs pi*)? 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(H2O)(p)(2+) complexes P (M = V, Cr, Mn, Fe, Co, p = 5 and M = Ni, Cu, p = 4) in gas phase. We show here that the lower the MO2+ acceptor orbital lies in energy, the lower the H-abstraction barrier is in general. However, a sigma* acceptor orbital is much more efficient than a pi* acceptor orbital for a given energy. Finally, we found that indeed, the FeO2+ moiety is particularly efficient but also CoO2+ and MnO2+ could be good candidates to perform C-H hydroxylation.