Electro- and Photo-driven Reduction of CO2 by a trans-(Cl)-[Os(diimine)(CO)2Cl2] Precursor Catalyst: Influence of the Diimine Substituent and Activation Mode on CO/HCOO− Selectivity

A series of [OsII(NN)(CO)2Cl2] complexes where NN is a 2,2′-bipyridine ligand substituted in the 4,4′ positions by H (C1), CH3 (C2), C(CH3)3 (C3), or C(O)OCH(CH3)2 (C4) has been studied as catalysts for the reduction of CO2. Electrocatalysis shows that the selectivity of the reaction can be switched toward the production of CO or HCOO− with an electron-donating (C2, C3) or -withdrawing (C4) substituent, respectively. The electrocatalytic process is a result of the formation of an Os0-bonded polymer, which was characterized by electrochemistry, UV/Visible and EPR spectroscopies. Photolysis of the complexes under CO2 in DMF+TEOA produces CO as a major product with a remarkably stable turnover frequency during 14 h of irradiation. Our results suggest that electrocatalysis and photocatalysis occur through two distinct processes, starting mainly from an OsI dimer precatalyst if the reduction is performed by an electrode and an OsI mononuclear species in case of a photoreduction process.