Re(bipy)(CO)&I (bipy = 2,2'-bipyridine) is an efficient homogeneous catalyst for the selective and sustained electrochemical reduction of C 0 2 to CO a t low potential.

Wac-Re(bpy) (CO),Cl] (bpy = 2,2'-bipyridine) is an efficient homogeneous catalyst for the selective and sustained photochemical or electrochemical reduction of CO, to CO. A quantum yield of 14% and a faradic efficiency of 98 % were measured in the presence of excess C1-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 ofexcess CI-ions. Substitution by CI-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 x lo7M-'S-'. The 19e-complex [Re(bpy) (CO),X]-produced either photochemically or electrochemically appears to be the active precursor in the CO-generation process. Detailed spectroscopic studies on I3C-enriched carbonyl-rhenium and formato-rhenium complexes derived from I3C-enriched C 0 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 C 0 2 photoreduction process is presented; it involves separate pathways for CO and formate generation, in which the [Re(bpy) (CO),X] complex plays the role of boih rhephotoactivr and fhe catalytic center.

Efficient photogeneration of either (CO + H2) simultaneously, or CO selectively, occurs on visible light irradiation of COz solutions in (HOCH,CH,) ,N-dimethylformamide containing, respectively, Ru(bipy) 32 +-Co2 + combinations or the single component Re(bipy) (CO)3X (X = CI, Br) as homogeneous catalysts.Carbon dioxide fixation is the basic reaction by which natural photosynthesis produces organic matter. A major goal of research in artificial photosynthesis is to discover abiotic photocatalytic systems capable of producing reduced species by photochemical reduction of CO,. The development of such systems is of interest from three points of view, viz., finding catalysts for CO, activation, devising means for conversion and storage of solar energy by production of fuels and organic raw materials, and setting up models of natural photosynthesis.Compared to water photolysisl little research has been devoted to the photoreduction of CO, (refs. 2-4 and references therein), which may yield a variety of products by reactions proceeding most easily via polyelectronic p r o c e s s e ~. ~~~ It requires the photochemical generation of species of sufficient reducing power and the discovery of suitable catalysts for these reactions.Extending our earlier work on the photoreduction of water,K we have recently shown3 that visible light irradiation of CO, solutions in aqueous acetonitrile, containing Ru(bipy),,+ as photosensitizer, Co" ions as catalyst, and a tertiary amine as electron donor, yields a mixture of CO and H, (synthesis gas) by simultaneous photoreduction of CO, and H20. We now report results on (i) marked improvements of this Ru(bipyyi+ / CoII system and (ii) a 'single component' system using Re-(bipy)(CO),X both as photosensitizer and as catalyst for the CO, -+ CO photoconversion.

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