Toward more efficient photochemical CO2 reduction: Use of scCO2 or photogenerated hydrides

Rhenium(I) and ruthenium(II) complexes have been successfully used for photochemical CO2 reduction to CO or formate. However, a typical turnover frequency for such reactions is <20 h−1 and the formation of reduced species beyond CO or formate is very limited. In the case of the rhenium(I) bipyridyl tricarbonyl system, the key intermediate has been shown to decay with a first-order dependence on [CO2] to produce CO, which is the rate-determining step. The limited concentration of dissolved CO2 in organic solvents results in extremely slow CO2 reduction. To improve the reaction rate, we prepared new CO2-soluble rhenium(I) bipyridine complexes bearing fluorinated alkyl ligands and investigated their photophysical properties in CH3CN and supercritical CO2. We also investigated the properties of a metal complex with an NAD+ model ligand, [Ru(bpy)2(pbn)]2+ (bpy = 2,2′-bipyridine, pbn = 2-(2-pyridyl)-benzo[b]-1,5-naphthyridine), and prepared the corresponding NADH-like complex [Ru(bpy)2(pbnHH)]2+ upon MLCT excitation followed by reductive quenching. This species can be used as a renewable hydride donor. The electrochemical and photochemical properties, and the reactivity of these species toward CO2 reduction were investigated.