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

Abstract

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 CO 2 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 CO 2 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 Ru I and further to Ru 0.