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

The photochemical reduction of CO 2 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 = OCH 3, CH 3, H, COOC 2 H 5, CF 3, NH 2, or COOH) was studied to assess how modifications in the electronic structure of the catalyst affect its selectivity, defined as the HCOOH:H 2 product ratio. A direct molecular-level influence of the functional group on the initial reaction rate for CO 2 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 CO 2 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.