Modelling on a Biomimetic [Cu-O-Cu]2+-mediated Methane-to-Methanol Conversion Unveils the Site for Methane Activation.

Abstract

The Cu-O-Cu core exhibits methane-to-methanol conversion, mirroring the reactivity of the copper-containing enzyme pMMO. Herein, we computationally examined the reactivity of a biomimetic Cu-O-Cu core towards methane-to-methanol conversion. The oxygen atom of the Cu-O-Cu core abstracts hydrogen present in the C-H bond of methane. The spin density at the bridging oxygen helps to abstract hydrogen from the C-H bond. We modulated the spin density of the bridging oxygen by substituting only a single copper atom of the Cu-O-Cu core by metals (M) such as Fe, Co, and Ag. These substitutions result in bimetallic [Cu-O-M]2+ models. We observed that the energy barriers for the C-H activation step and the subsequent rebound step vary with the metal M. [Cu-O-Ag]2+ exhibits the highest reactivity for M2M conversion, while [Cu-O-Fe]2+ displays the lowest reactivity. To understand the different reactivity of these models towards M2M conversion, we employed distortion-interaction analysis, orbital analysis, spin density analysis, and quantum theory of atoms in molecules analysis. Orbital analysis reveals that all four adducts follow a hydrogen atom transfer mechanism for C-H activation. Further, spin density analysis reveals that a higher spin density on the bridging oxygen leads to a lower C-H activation barrier.