We propose a computational framework that analyzes barrier responses to system perturbations in order to resolve the long-standing question of the mechanism of methane hydroxylation in enzyme-mimics like dioxo-dicopper complexes. Rather than contrasting the density functional theory (DFT)-based potential energy surfaces of the two proposed pathways, oxo-insertion and radical recombination, we vary the electrophilicity of amine and imidazole-based N-donors in the dioxo-dicopper complex. The response of oxo-insertion and radical recombination barriers are then compared with prior experiments with substituted aromatic substrates by means of Hammett plots. Radical barriers are insensitive to catalyst perturbations by virtue of a very early transition state possessing a smaller CH stretch (1.291Å) relative to oxo-insertion (1.552Å). On the other hand, Hammett slopes for the oxo-insertion pathway for imidazole-substituted catalysts (ρ = -2.29) are in remarkable agreement with the electrophilic substitution mechanism proposed in experiments (ρ = -2.2). Energy decomposition analysis (EDA) between the methane substrate and dioxo-dicopper complex shows that electron-withdrawing ligands stabilize the oxo-insertion transition state via charge transfer interactions leading to lower barriers. By invoking the activation strain model along with EDA, we also show that the oxo-insertion barriers vary linearly with total substrate-catalyst interaction energies. Taken together, we have preliminary evidence to suggest that one-step oxo-insertion is the preferred pathway for alkyl CH oxidation with dioxo-dicopper complexes.

Barrier response analysis framework to probe CH activation mechanisms

2019 Spring National Meeting

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