Theoretical Study of the Oxidation of Methane to Methanol by the [CuIICuII(μ-O)2CuIII(7-N-Etppaz)]1+ Complex

The reactivity patterns of a series of trivalent copper complexes have been studied to gain a better understanding of the chemical reactions occurring at the active site of particulate methane monooxygenase (pMMO). In this study, hybrid density functional theory is used to study the oxidation of methane to methanol mediated by the [(CuCuII)-Cu-II(mu-O)(2)Cu-III(7-N-Etppz)](1+) complex. Reaction mechanisms in different spin states were explored. Based on the calculated free-energy profile, a mechanism is suggested for the reaction of the oxidation of methane to methanol. The first step (1 -> 2) is a hydrogen transfer to the bridged oxygen in the Cu2O2 core from the methane to form a methyl radical. The second step (2 -> 3) is the radical recombination, in which the bridged hydroxyl rotates upward and exposes the oxygen moiety toward the methyl radical to form methanol. The radical recombination step is rate-limiting, with a calculated free-energy barrier of 19.6 kcal mol(-1), which is in good agreement with the experimental value of 18.4 kcal mol(-1). The mixed valent bis(mu-oxo)(CuCuIII)-Cu-II species in the Cu3O4 core is directly responsible for the C-H activation of methane.