Efficacy of Redox-Active Cu(II) Dipyrrin Complexes toward Electrochemical Reduction of CO2

Abstract

New D-A-type catalysts based on Cu (II) complexes (C1 and C2) including dipyrrin ligands with phenothiazine/carbazole as the meso-substituent have been described. The complexes have been thoroughly characterized by various methods (1H, 13C, ESI-MS, EPR, and UV-vis studies), and structures of both C1 and C2 unequivocally determined by X-ray single crystal analyses. The catalysts C1 and C2 are stable at room temperature and exhibit Faradaic efficiency values of ∼56% (C1) and ∼46% (C2) toward homogeneous reduction of CO<inf>2</inf> to CO. The release of CO has been validated by gas chromatographic (GC) studies. Electron-rich phenothiazine and carbazole included in the catalysts facilitate proton transfer, enabling rapid and selective formation of CO over H<inf>2</inf> with FE<inf>H2</inf> values of ∼22% for C1 and ∼7% for C2 and turnover numbers (TON) of ∼46 for C1 and ∼21 for C2. Furthermore, the formation of formate ions has been affirmed by ion chromatography (C1, ∼16%; C2, ∼18%). Detailed electrochemical studies and product analyses suggested that C1 displays superior catalytic activity relative to C2 which has been further supported by theoretical studies. © 2025 American Chemical Society.