Energetics of Proton-Coupled Electron Transfer in Hydroxy-Bridged Dicopper Complexes with N-Heterocyclic Carbene (NHC) Ligands.

Abstract

A series of dinuclear μ-hydroxo bridged copper(II) complexes bearing N-heterocyclic carbene (NHC) ligands were designed to probe the ligand effect governing the activity patterns of particulate methane monooxygenase (pMMO). By systematically correlating experimental parameters (redox potentials E1/2 and acid dissociation constants pKa), we constructed stepwise thermodynamic frameworks that reveal a continuous O─H bond dissociation free energy (BDFEO-H) range spanning 68.5 to 106.7 kcal/mol in these copper-oxygen cores. Notably, while ligand substitution elicited only marginal variations in BDFEO-H when maintaining a fixed overall charge of Cu─O─Cu structure, modulation of the Cu─O─Cu core charge demonstrated superior efficacy over conventional steric/electronic ligand modifications for activating inert C─H bonds. Mechanistic studies revealed an efficient concerted electron-proton transfer (CEPT) pathway between the CuII─O─CuII and TEMPOH, achieving a remarkable second-order rate constant of 1745.4 M-1 s-1. This quantitative correlation between thermodynamic parameters and reaction kinetics provides fundamental principles for rational catalyst engineering.