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)₃Cl] (bpy = 2,2′‐bipyridine) is an efficient homogeneous catalyst for the selective and sustained photochemical or electrochemical reduction of CO₂ 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 × 10⁷M⁻¹S⁻¹. The 19e‐complex [Re(bpy) (CO)₃X]⁻ produced either photochemically or electrochemically appears to be the active precursor in the CO‐generation process. Detailed spectroscopic studies on ¹³C‐enriched carbonyl‐rhenium and formato‐rhenium complexes derived from ¹³C‐enriched CO₂ were performed in order to confirm the origin of the products and to study the exchange of the ligands. A mechanism for the present CO₂ photoreduction process is presented; it involves separate pathways for CO and formate generation, in which the [Re(bpy) (CO)₃X] complex plays the role of both the photoactive and the catalytic center. Copyright © 1986 Verlag GmbH & Co. KGaA, Weinheim