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)Cl2, Cu(ImBenz-NO2)Cl2, and Cu(ImBenz-OCH3)Cl2, 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 (τ4 = 0.11–0.20). Electrochemical studies and rotating ring-disk electrode (RRDE) experiments in neutral aqueous electrolyte revealed that Cu(ImBenz-NO2)Cl2 bearing an electron-withdrawing group on the benzimidazole ring exhibited high selectivity for the 4-electron reduction of O2 to H2O, with the number of electrons transferred (n) of 3.8, yielding ∼14% H2O2. Cu(ImBenz-H)Cl2 favored the 2-electron reduction of O2 to H2O2, producing the highest H2O2 yield (∼37%). In contrast, Cu(ImBenz-OCH3)Cl2 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. © 2025 The Author(s). ChemCatChem published by Wiley-VCH GmbH.