Controlling Hydrogen Evolution during Photoreduction of CO 2 to Formic Acid Using [Rh(R-bpy)(Cp*)Cl] + Catalysts: A Structure–Activity Study

International audienceThe photochemical CO 2 reduction 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, R = OCH 3 , CH 3 , H, COOC 2 H 5 , CF 3 , NH 2 and COOH) was studied in order 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 vs. proton reduction reactions was established. Density functional theory computations elucidated for the first time the respective role of the [RhH] vs [Cp*H] tautomers, recognizing the 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 lowering the hydricity of the Rh-H bond, resulting in lower hydride transfer barrier towards formic acid production as compared to substituents with electron-withdrawing nature that favor more strongly the proton reduction to hydrogen