Reaction mechanisms of catalytic photochemical CO2 reduction using Re(I) and Ru(II) complexes

Abstract

Reduction of CO2 is one of the most important reactions that might solve the problems of global warming and shortage of fossil-fuel resources simultaneously. Metal complex catalysts are sometimes called molecular catalysts because they can be designed and synthesized on the molecular level. Some metal complexes are quite efficient and selective for CO2 reduction. Recently, such complexes have been applied as semiconductor photocatalysts to yield hybrid metal complex/semiconductor systems. Compared to heterogeneous catalysts, metal complexes are more advantageous for elucidating reaction mechanisms. This review summarizes the reaction mechanisms that have been proposed for the photochemical CO2 reduction reaction catalyzed by rhenium and ruthenium complexes. Rhenium complexes efficiently reduce CO2 to selectively produce CO under various reaction conditions. On the other hand, ruthenium complexes yield CO and HCOOH, and the product selectivities are strongly dependent on the reaction conditions. Numerous reaction mechanisms have been proposed; however, no universal mechanism that can completely explain the activities and product selectivities of these catalysts exists. Why are two important intermediates, the η1-CO2 adduct and the hydride complex, proposed? How does the η1-CO2 adduct produce HCOOH? Does the hydride complex yield CO via the formate complex? What is the second electron source for the intermediate that produces CO and HCOOH? This review highlights what is already known about photocatalytic CO2 reduction reaction mechanisms and what remains to be clarified.