A Novel Photo-Driven Hydrogenation Reaction of an NAD(+)-Type Complex Toward Artificial Photosynthesis

The photocatalytic reduction of carbon dioxide (CO(2)) to value-added chemicals is an attractive strategy to utilize CO(2) as a feedstock for storing renewable energy, such as solar energy, in chemical bonds. Inspired by the biological function of the nicotinamide adenine dinucleotide redox couple (NAD(+)/NADH), we have been developing transition-metal complexes containing NAD(+)/NADH-functionalized ligands to create electro- and/or photochemically renewable hydride donors for the conversion of CO(2) into value-added chemicals. Our previous findings have provided insights for the development of photocatalytic organic hydride reduction reactions for CO(2), however, further examples, as well as investigation, of these photo-driven NAD(+)/NADH-type hydrogenation and organic hydride transfer reactions are required not only to explore the mechanism in detail but also to develop a highly efficient catalyst for artificial photosynthesis. In this paper, we report the synthesis, characterization, and photo-induced NAD(+)/NADH conversion properties of a new ruthenium(II) complex, [Ru(bpy)(2)(Me-pn)](PF(6))(2) (1), which contains a new NAD(+)-type ligand, Me-pn (2-methyl-6-(pyridin-2-yl)-1,5-naphthyridine). In addition, we have succeeded in the isolation of the corresponding two-electron reduced ruthenium(II) complex containing the NADH-type ligand Me-pnHH (2-methyl-6-(pyridin-2-yl)-1,4-dihydro-1,5-naphthyridine), i.e., [Ru(bpy)(2)(Me-pnHH)](PF(6))(2) (1HH), by the photo-induced hydrogenation reaction of 1. Thus, in this study, a new photo-driven NAD(+)/NADH-type hydrogenation reaction for possible CO(2) reduction using the NAD(+)/NADH redox function has been constructed.