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

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 similar to 56% (C1) and similar to 46% (C2) toward homogeneous reduction of CO2 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 H2 with FEH2 values of similar to 22% for C1 and similar to 7% for C2 and turnover numbers (TON) of similar to 46 for C1 and similar to 21 for C2. Furthermore, the formation of formate ions has been affirmed by ion chromatography (C1, similar to 16%; C2, similar to 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.