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.