DFT modeling of a methane-to-methanol catalytic cycle via Group 6 organometallics: The role of metal in determining the mode of C–H activation

Computational modeling of Group 6 complexes of the form [LnM(Y)O]n (Ln: Tp = hydrido-tris(pyrazolyl)borate, Ep = 1,1,1-tris(pyrazolyl)ethane; M = CrIV, MoIV, WIV; Y = CO, pyridine, n = 1+ or 2+) with respect to an oxy-insertion catalytic cycle for the conversion of methane to methanol is reported. Through a DFT study of the reaction mechanism – involving C–H activation and oxygen-atom transfer – competing transition state pathways, molecular geometries, and Gibbs free energies were analyzed. The results indicate that the [EpCr(CO)O]2+ catalyst is the most promising among the twelve Group 6 systems modeled as the transition state for C–H bond activation requires ∼10 kcal mol−1 for [2 + 2] addition with a reasonably flat potential energy surface for the complete catalytic cycle. Calculations indicate that most Cr-oxo and all Mo-oxo complexes favor a hydrogen atom abstraction (HAA) pathway, whereas the majority of the W-oxo complexes and one of the Cr complexes favors a [2 + 2] mechanism for methane C–H bond activation. With regards to the catalyst components, the following orders, M (Cr > Mo > W) > Ln (Ep > Tp) > Y (carbonyl > pyridine), expresses the importance of the individual constituents with respect to their impact on the calculated catalytic cycle.