Quantum Chemical Studies of Methane Oxidation to Methanol on a Biomimetic Tricopper Complex: Mechanistic Insights

Particulate methane monooxygenase (pMMO) is known to be the most efficient oxidizer for the conversion of methane (CH4) into methanol (CH3OH) at room temperature. Recently, the tricopper cluster complex [Cu-3(7-N-Etppz)](1+) has been shown to mediate facile CH4 oxidation upon activation by dioxygen (O-2) just like the pMMO enzyme. In this work, we investigate by unrestricted density functional theory (DFT) calculations the oxidation of CH4 on the biomimetic tricopper complex. We find that CH4 can interact with the activated tricopper complex to form a C-H center dot center dot center dot O hydrogen bond between one of the C-H bonds of CH4 and the O-2 molecule activating the tricopper cluster complex. We consider both the direct "oxene insertion" mechanism as well as the "hydrogen-atom abstraction geminol radical rebound" mechanism for the process. Our calculations indicate low kinetic barriers for both reaction pathways, accounting for why the tricopper cluster of the biomimetic complex Ku(3)(7-N-Etppz)(1+) is a good functional model of the active site in pMMO.