Photochemical reduction of carbon dioxide to formate catalyzed by 2,2t́-bipyridine- or 1,10-phenanthroline-ruthenium(II) complexes

Formate is photogenerated by visible light irradiation of systems containing either the [Ru(bpy)3]2+ 2Cl− (bpy = 2,2′-bipyridine) complex alone, or a mixture of two ruthenium(II) complexes. The former system produces the active catalytic species by photolabilisation of a bpy ligand. The latter system consists of a mixture of [Ru(L)3]2+ (L = bpy derivatives or 1,10-phenanthroline (phen)) as photosensitizer and cis-[Ru(bpy)2(CO)(X)]n+ (X = Cl, H, n = 1 or X = CO, n = 2) or cis-Ru(bpy)(CO)2(Cl)2 as homogeneous catalysts which mediate carbon dioxide reduction to formate. The efficiency of formate production is dependent on the presence of water and excess ligand but is independent of CO2 pressure. A maximum quantum yield of 15% was measured for the mixed [Ru(bpy)3]2+ / cis-[Ru(bpy)2(CO)(H)]+ system. The photochemical process consumed triethanolamine as electron donor and was studied by 13C NMR using labelled carbon dioxide in order to determine the origin of the formate. This CO2 photoreduction system consists of two catalytic cycles: a photochemical one for the ruthenium-trischelate and a dark-reaction pathway for the ruthenium bis- or mono-bpy complex. The reaction involves reductive quenching of the [Ru(L)3]2+ excited state by the tertiary amine to the corresponding ruthenium(I) complex which reduces the carbon dioxide activation catalyst to RuI and further to Ru0.