Simultaneous Formation of cis- and trans-CH₃OCu(OH) Intermediates in Methane Activation by Cu in Solid Ar

Understanding the mechanisms of methane activation is an important and active research area of the contemporary catalyzed conversion of hydrocarbons to shippable, valuable feedstock and has invoked close collaborations between experimentalists and theorists. This article describes the trapping of reaction intermediates in a copper-catalyzed direct methane-to-methanol conversion. Specifically, two hydroxy(methoxy)copper(I) [CH₃OCu(OH)] isomeric intermediates were distinguished and characterized by matrix isolation infrared spectroscopy and ¹⁸O₂, CD₄, and ¹³CH₄ isotopic substitution experiments combined with quantum chemical calculations. Initially, laser-evaporated copper reacted with oxygen to form CuO₂. Upon successive broadband UV irradiation, methane activation via the insertion of CuO₂ into one of the C-H bonds produced both cis- and trans-CH₃OCu(OH). All possible structures of the reactants, intermediates, transition states, minimum-energy crossing points, and products were optimized. The results indicated that the two CH₃OCu(OH) isomers are not thermodynamically distinguishable, although both have different frequencies, which agrees with experimental observation. The proposed reaction mechanism involved (i) O-O bond cleavage, (ii) C-H bond activation, and (iii) H and CH₃ competitive transfer. The small energy barrier for cis to trans conversion accounts for the simultaneous formation of cis- and trans-CH₃OCu(OH). These findings indicate that the CH₃OCu(OH) species would be a potent precursor in other families of copper-cored oxidants that can trigger methanol formation.