Reduction of CO2 Directed toward Carbon-Carbon Bond Formation

Article abstract of DOI:10.1246/bcsj.71.17

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.