Electronic Tuning of Copper(II) Imidazole-Benzimidazole Complexes for Efficient and Selective Oxygen Reduction Reaction

Abstract

A series of nature-inspired copper(II) complexes with electronically tunable imidazole-benzimidazole (ImBenz) ligands, including Cu(ImBenz-H)Cl<inf>2</inf>, Cu(ImBenz-NO<inf>2</inf>)Cl<inf>2</inf>, and Cu(ImBenz-OCH<inf>3</inf>)Cl<inf>2</inf>, was designed to probe the cooperative influence of redox-active ligands and metal centers on oxygen reduction reaction (ORR) activity. Single-crystal X-ray diffraction results for all three complexes confirm a distorted square planar geometry (τ<inf>4</inf> = 0.11–0.20). Electrochemical studies and rotating ring-disk electrode (RRDE) experiments in neutral aqueous electrolyte revealed that Cu(ImBenz-NO<inf>2</inf>)Cl<inf>2</inf> bearing an electron-withdrawing group on the benzimidazole ring exhibited high selectivity for the 4-electron reduction of O<inf>2</inf> to H<inf>2</inf>O, with the number of electrons transferred (n) of 3.8, yielding ∼14% H<inf>2</inf>O<inf>2</inf>. Cu(ImBenz-H)Cl<inf>2</inf> favored the 2-electron reduction of O<inf>2</inf> to H<inf>2</inf>O<inf>2</inf>, producing the highest H<inf>2</inf>O<inf>2</inf> yield (∼37%). In contrast, Cu(ImBenz-OCH<inf>3</inf>)Cl<inf>2</inf> showed the mixed 2- and 4-electron ORR pathways. Foot-of-the-wave analysis (FOWA) revealed turnover frequencies of 10² s⁻¹ for the catalysts with catalytic rates and product selectivity strongly influenced by the redox-active ligand structure. These results demonstrate that the systematic electronic modification of the ligand with Cu(II) metal center is a powerful strategy for tuning ORR activity and selectivity, underscoring the potential of earth-abundant copper complexes for future clean energy applications.