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

Abstract

[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.