Catalyst Protonation\nChanges the Mechanism of Electrochemical\nHydride Transfer to CO<sub>2</sub>

It is well-known that addition of a cationic functional\ngroup to\na molecule lowers the necessary applied potential for an electron\ntransfer (ET) event. This report studies the effect of a proton (a\ncation) on the mechanism of electrochemically driven hydride transfer\n(HT) catalysis. Protonated, air-stable [HFe<sub>4</sub>N(triethyl\nphosphine (PEt<sub>3</sub>))<sub>4</sub>(CO)<sub>8</sub>] (H<b>4</b>) was synthesized by reaction of PEt<sub>3</sub> with [Fe<sub>4</sub>N(CO)<sub>12</sub>]<sup>−</sup> (<b>A</b><sup>–</sup>) in tetrahydrofuran, with addition of benzoic acid\nto the reaction mixture. The reduction potential of H4 is −1.70\nV vs SCE which is 350 mV anodic of the reduction potential for <b>4</b><sup>–</sup>. Reactivity studies are consistent with\nHT to CO<sub>2</sub> or to H<sup>+</sup> (carbonic acid), as the chemical\nevent following ET, when the electrocatalysis is performed under 1\natm of CO<sub>2</sub> or N<sub>2</sub>, respectively. Taken together,\nthe chemical and electrochemical studies of mechanism suggest an ECEC\nmechanism for the reduction of CO<sub>2</sub> to formate or H<sup>+</sup> to H<sub>2</sub>, promoted by H<b>4</b>. This stands\nin contrast to an ET, two chemical steps, followed by an ET (ECCE)\nmechanism that is promoted by the less electron rich catalyst <b>A</b><sup>–</sup>, since <b>A</b><sup>–</sup> must be reduced to <b>A</b><sup>2–</sup> before H<b>A</b><sup>–</sup> can be accessed.