Transition metal complexes consisting of a dicopper core bound by oxygen that mimic the active site in methanophilic bacterial enzymes are promising catalyst candidates for the direct hydroxylation of methane to methanol. However, the development of synthetic homogeneous catalysts for direct conversion has been challenging owing to poor understanding of the CH activation mechanism. We are utilizing quantum chemistry methods to not only probe whether methane hydroxylation proceeds via a concerted oxo-insertion pathway or a stepwise rebound mechanism, but also examine the influence of the identity and denticity of N-coordinated ligands on the activity of the [Cu2O2]2+ core. The energetics of the two possible pathways are determined for various spin configurations using range-separated, dispersion-corrected density functional theory and automated reaction path search methods. The study demonstrates that, for the smallest model catalyst consisting of two ammonia ligands coordinated to each Cu atom, the activation energy for the triplet state is significantly lower than the singlet even though the singlet configuration is more stable for the bare catalyst. Therefore, the possibility of spin-crossing in these systems will be discussed. The substrate-catalyst interactions at the transition state, in terms of physically meaningful contributions such as electrostatics, dispersion, polarization, and so on, will also be described. A comparative energy decomposition analysis study of singlet and triplet transition states across systematically substituted N-coordinated ligands is employed to determine these contributions, which, going forward, will lead to quantitative descriptors for catalyst activity.

Mechanistic approach to probe ligand-dependence of CH activation kinetics in bioinspired dicopper complexes

255th National Meeting (2018)

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