Catalyst Protonation Changes the Mechanism of Electrochemical Hydride Transfer to CO.

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 [HFeN(triethyl phosphine (PEt))(CO)] (H) was synthesized by reaction of PEt with [FeN(CO)] ( ) 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 . Reactivity studies are consistent with HT to CO or to H (carbonic acid), as the chemical event following ET, when the electrocatalysis is performed under 1 atm of CO or N, respectively. Taken together, the chemical and electrochemical studies of mechanism suggest an ECEC mechanism for the reduction of CO to formate or H to H, promoted by H. 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 , since must be reduced to before H can be accessed.