Approach to multi-electron reduction beyond two-electron reduction of CO2

Abstract

Dicationic [Ru(bpy)2(CO)2](2+) (bpy = 2,2'-bipyridyl) exists as an equilibrium mixture of [Ru(bpy)2(CO)(COOH)](+) and [Ru(bpy)2(CO)(CO2)] in H2O. Photo- and electrochemical reduction of [Ru(bpy)2(CO)2](2+) in the presence of proton sources under CO2 produced CO and HCOOH with generation of [Ru(bpy)2(CO)(CO2)], which spontaneously comes to an equilibrium of [Ru(bpy)2(CO)(COOH)](+) and [Ru(bpy)2(CO)2](2+). Two-electron reduction of CO2, giving CO and/or HCOOH depending on the proton concentration, is ascribed to reductive cleavage of M-COOH and M-CO bonds derived from M-CO2 complexes. Catalytic DMF production in the electrochemical reduction of [Ru(bpy)2(CO)2](2+) in the presence of Me2NH under CO2 is also explained by reductive cleavage of the Ru-C(O)NMe2 bond. On the other hand, the treatment of [Ru(bpy)2(CO)2](2+) with BH4(-) produced CH3OH via [Ru(bpy)2(CO)(CHO)](+) and [Ru(bpy)2(CO)(CH2OH)](+), indicating that reduction of M-CO complexes with renewable hydride donors is a feasible pathway to realize catalytic six-electron reduction of CO2 affording CH3OH. Along these lines, [Ru(bpy)2(pbn)](2+) (pbn = 2-(2-pyridyl)benzo[b]-1,5-naphthyridine) was prepared to simulate the biological role of the NAD/NADH redox couple. Irradiation of visible light on[Ru(bpy)2(pbn)](2+) in the presence of a sacrificial electron donor produced smoothly the two-electron reduced form, [Ru(bpy)2(pbnHH)](2+) in a quantum yield of 20%. Furthermore, hydride transfer from [Ru(bpy)2(pbnHH)](2+) to CO2 took place in the presence of base to regenerate [Ru(bpy)2(pbn)](2+) accompanied by HCOO(-) formation.