Electro‐ and Photo‐driven Reduction of CO 2 by a trans ‐(Cl)‐[Os(diimine)(CO) 2 Cl 2 ] 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 ), CH 3 ( C2 ), C(CH 3 ) 3 ( C3 ), or C(O)OCH(CH 3 ) 2 ( C4 ) has been studied as catalysts for the reduction of CO 2 . 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 CO 2 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.