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 [Os-II(NN)(CO)(2)Cl-2] complexes where NN is a 2,2'-bipyridine ligand substituted in the 4,4' positions by H (C1), CH3 (C-2), 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 Os-0-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 Os-I dimer precatalyst if the reduction is performed by an electrode and an Os-I mononuclear species in case of a photoreduction process.