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.