Abstract
The photocatalytic reduction of carbon dioxide (CO2) to value-added chemicals is an attractive strategy to utilize CO2 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 CO2 into value-added chemicals. Our previous findings have provided insights for the development of photocatalytic organic hydride reduction reactions for CO2, 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)](PF6)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)](PF6)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 CO2 reduction using the NAD+/NADH redox function has been constructed.
Highlights
The design and development of novel visible-light photoredox catalysts for carbon dioxide (CO2) reduction are considered to be crucial challenges
The use of transition-metal coordination compounds as photocatalysts for CO2 reduction has drawn significant attention because some of these compounds exhibit significant photocatalytic activity for CO2 reduction (Morris et al, 2009; Berardi et al, 2014), and they play an essential role in artificial photosynthesis (Fukuzumi et al, 2018), as well as natural photosynthesis (Silva et al, 2015)
We have focused on Ru complexes having NAD+/NADH-functionalized ligands because the biological function of the NAD+/NADH redox couple is as a generator and reservoir of hydride ions (H−), which are equivalent to two electrons and one proton (Eisner and Kuthan, 1972; Walsh, 1980; Stout and Meyers, 1982; Gebicki et al, 2004; Bilan et al, 2015), and is of great interest for the development of photorenewable hydride reagents
Summary
The design and development of novel visible-light photoredox catalysts for carbon dioxide (CO2) reduction are considered to be crucial challenges. There are several problems facing photochemical CO2 conversion, related to the side reactions and low selectivity toward specific reduction products, as well as poor energy efficiency (Leitner, 1995; Jones et al, 2014) To solve these problems, the use of transition-metal coordination compounds as photocatalysts for CO2 reduction has drawn significant attention because some of these compounds exhibit significant photocatalytic activity for CO2 reduction (Morris et al, 2009; Berardi et al, 2014), and they play an essential role in artificial photosynthesis (Fukuzumi et al, 2018), as well as natural photosynthesis (Silva et al, 2015). We have focused on Ru complexes having NAD+/NADH-functionalized ligands because the biological function of the NAD+/NADH redox couple is as a generator and reservoir of hydride ions (H−), which are equivalent to two electrons and one proton (Eisner and Kuthan, 1972; Walsh, 1980; Stout and Meyers, 1982; Gebicki et al, 2004; Bilan et al, 2015), and is of great interest for the development of photorenewable hydride reagents
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