Deciphering the active species and reaction mechanism in water oxidation catalyzed by a copper complex with redox-active ligands

Abstract

Homogeneous water oxidation catalysts play a crucial role in the efficient utilization of hydrogen energy. The exploration of cost-effective metal catalysts based on redox-active ligands represents a promising approach in this field. Non-precious metal catalysts, especially copper-based complexes, have emerged as viable alternatives, addressing the challenges associated with precious metals. In this study, theoretical calculations were employed to deeply investigate the catalytic mechanism of electrochemical water oxidation reactions mediated by a copper complex with redox-active ligands. Our theoretical research reveals the reaction sequence of proton-coupled electron transfer (PCET) oxidation, where the ligand undergoes PCET oxidation first, followed by the coordination of water to the copper center. The calculated redox potentials are in close agreement with experimental values. We considered two possible active species, Cu^II–OH˙ and Cu^II–O˙˙, and the calculation results indicate that the reaction pathway of Cu–O˙˙ has a lower activation energy barrier. For the critical O–O bond formation process, the catalyst guides the reaction through a unique single-electron transfer-water nucleophilic attack (SET-WNA) mechanism. It is noteworthy that the copper center of all the intermediates remains at the +2 oxidation state, highlighting the redox inertness of copper. These findings provide theoretical guidance for optimizing copper-based water oxidation catalysts.