Catalyst Protonation Changes the Mechanism of Electrochemical Hydride Transfer to CO₂

It is well-known that addition of a cationic functional group to a molecule lowers the necessary applied potential for an electron transfer (ET) event. This report studies the effect of a proton (a cation) on the mechanism of electrochemically driven hydride transfer (HT) catalysis. Protonated, air-stable [HFe₄N(triethyl phosphine (PEt₃))₄(CO)₈] (H4) was synthesized by reaction of PEt₃ with [Fe₄N(CO)₁₂]^- (A ^-) in tetrahydrofuran, with addition of benzoic acid to the reaction mixture. The reduction potential of H4 is -1.70 V vs SCE which is 350 mV anodic of the reduction potential for 4 ^-. Reactivity studies are consistent with HT to CO₂ or to H⁺ (carbonic acid), as the chemical event following ET, when the electrocatalysis is performed under 1 atm of CO₂ or N₂, respectively. Taken together, the chemical and electrochemical studies of mechanism suggest an ECEC mechanism for the reduction of CO₂ to formate or H⁺ to H₂, promoted by H4. This stands in contrast to an ET, two chemical steps, followed by an ET (ECCE) mechanism that is promoted by the less electron rich catalyst A ^-, since A ^- must be reduced to A ^2- before HA ^- can be accessed.