Water-Nucleophilic Attack Mechanism for the Cu^II(pyalk)₂ Water-Oxidation Catalyst

We investigate the mechanism of water oxidation catalyzed by the Cu^II(pyalk)₂ complex, combining density functional theory with experimental measurements of turnover frequencies, UV-visible spectra, H/D kinetic isotope effects (KIEs), electrochemical analysis, and synthesis of a derivative complex. We find that only in the cis form does Cu^II(pyalk)₂ convert water to dioxygen. In a series of alternating chemical and electrochemical steps, the catalyst is activated to form a metal oxyl radical species that undergoes a water-nucleophilic attack defining the rate-limiting step of the reaction. The experimental H/D KIE (3.4) is in agreement with the calculated value (3.7), shown to be determined by deprotonation of the substrate nucleophile upon O-O bond formation. The reported mechanistic findings are particularly valuable for rational design of complexes inspired by Cu^II(pyalk)₂.