(560ij) Elucidating Reaction Pathways through Combined Insights from Experimental and Computational Hammett Analysis

Density functional theory (DFT) has been extraordinarily successful in advancing mechanistic studies and catalysis. However, application of DFT to transition metal chemistry can be challenging owing to high sensitivity to the choice of theory coupled with limitations associated with describing multireference characteristics, spin-state energetics, and relativistic effects. This issue is illustrated in the case of bio-inspired oxo-complexes of copper, for which prior DFT studies have led to conflicting conclusions regarding the long-standing problem of identifying the preferred mechanism of CH activation. Our work goes beyond the traditional approach of contrasting proposed mechanisms through the calculation of potential energy surfaces for a single catalyst-substrate system. Instead, we systematically vary the identity of N-donors bound to the active center to achieve twin goals of (a) examining the barrier response of the two proposed mechanisms – oxo-insertion and radical recombination – and contrasting them with experiment, and (b) identifying characteristics or descriptors that enable the design of active candidates. Our approach combines DFT with energy decomposition analysis (EDA) and the activation-strain model. By varying the electrophilicity of amine- and imidazole-based N-donors bound to a dioxo-dicopper center and contrasting the resulting (inverted) Hammett plots with experimental Hammett studies, we demonstrate that methane activation most likely occurs via the single-step oxo-insertion pathway. Radical barriers are insensitive to catalyst variations by virtue of a very early transition state. Further, oxo-insertion barriers vary linearly with EDA-based charge transfer contributions, which are in turn dependent on electrophilicity of the ligand motif. We are now combining these insights with a DFT+EDA investigation into the role of strain and steric factors on catalytic activity, through systematic variation of the N-Cu-N bite angle in the dioxo-dicopper complex.