Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′‐Bipyridine)tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts

[ fac ‐Re(bpy) (CO) 3 Cl] (bpy = 2,2′‐bipyridine) is an efficient homogeneous catalyst for the selective and sustained photochemical or electrochemical reduction of CO 2 to CO. A quantum yield of 14% and a faradic efficiency of 98% were measured in the presence of excess Cl − ions. The photochemical process took place under visible‐light irradiation and consumed a tertiary amine as electron donor. A formato‐rhenium complex was isolated in the absence of excess Cl − ions. Substitution by Cl − ion generated free formate, but no CO was detected. Luminescence measurements showed that the tertiary amine quenches the metal‐to‐ligand charge‐transfer excited state of the rhenium complex via a reductive mechanism, with a rate constant of 3.4 × 10 7 M −1 S −1 . The 19e‐complex [Re(bpy) (CO) 3 X] − produced either photochemically or electrochemically appears to be the active precursor in the CO‐generation process. Detailed spectroscopic studies on 13 C‐enriched carbonyl‐rhenium and formato‐rhenium complexes derived from 13 C‐enriched CO 2 were performed in order to confirm the origin of the products and to study the exchange of the ligands. A mechanism for the present CO 2 photoreduction process is presented; it involves separate pathways for CO and formate generation, in which the [Re(bpy) (CO) 3 X] complex plays the role of both the photoactive and the catalytic center .