Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO2 to Formate by Ruthenium Complexes

A series of 4,4ʹ-dimethyl-2,2'-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride transfer step to CO2 from a singly-reduced metal hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor, time-resolved infrared (TRIR) measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron-transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a Zwitterionic alkylcarbonate adduct, and was actively engaged in key ca...