Controlling Hydrogen Evolution during Photoreduction of CO₂ to Formic Acid Using [Rh(R-bpy)(Cp*)Cl]⁺ Catalysts: A...

The photochemical reduction of CO₂ to formic acid catalyzed by a series of [Rh(4,4'-R-bpy)(Cp*)Cl]⁺ and [Rh(5,5'-COOH-bpy)(Cp*)Cl]⁺ complexes (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, and R = OCH₃, CH₃, H, COOC₂H₅, CF₃, NH₂, or COOH) was studied to assess how modifications in the electronic structure of the catalyst affect its selectivity, defined as the HCOOH:H₂ product ratio. A direct molecular-level influence of the functional group on the initial reaction rate for CO₂ versus proton reduction reactions was established. Density functional theory computations elucidated for the first time the respective role of the [RhH] and [Cp*H] tautomers, recognizing rhodium hydride as the key player for both reactions. In particular, our calculations explain the observed tendency of electron-donating substituents to favor CO₂ reduction by means of decreasing the hydricity of the Rh-H bond, resulting in a lower hydride transfer barrier toward formic acid production as compared to substituents with an electron-withdrawing nature that favor more strongly the reduction of protons to hydrogen.