Reduction of CO2 Directed toward Carbon–Carbon Bond Formation

Abstract This paper describes electrochemical reduction of CO2 directed toward carbon–carbon bond formation via metal–CO2 adducts. An electrophilic attack of CO2 to penta-coordinated low valent polypyridyl Ru complexes affords a Ru–η1-CO2 adduct, which is easily converted to Ru–CO species either by an acid-base equilibrium in protic media and oxide transfer to CO2 under aprotic conditions. Two-electron reduction of resultant Ru–CO in protic solutions competitively causes a cleavage of the Ru–CO bond (CO evolution) and formation of a thermally labile Ru–CHO bond. Besides further reduction of the latter to Ru–CH2OH as precursors to CH3OH and HOOCCH2OH, Ru–CHO reacts with CO2 to afford HCOOH with regenerating Ru–CO as the precursor to CO. Thus, the difficulty of multi-electron reduction of CO2 in protic solutions is ascribed to the thermal lability and strong hydride donor character of Ru–CHO. On the other hand, two-electron reduction of Ru–CO in the presence of (CH3)4N+ or CH3I under aprotic conditions produces thermally stable R–C(O)CH3, which works as a precursor to CH3C(O)CH3. Two-electron reduction of M3(μ3-S)2 clusters (M = Co, Rh, Ir) causes an M–M bond cleavage and the nucleophilicity of the μ3-S ligand is also enhanced. As a result, two CO2 molecules reductively activated probably on μ3-S and metal sites undergo the coupling reaction to give oxalate selectively.