Tailoring Electrocatalysts for Selective CO₂ or H⁺ Reduction: Iron Carbonyl Clusters as a Case Study

The design of electrocatalysts that will selectively transfer hydride equivalents to either H(+) or CO2 to afford H2 or formate is a long-standing goal in molecular electrocatalysis. In this Forum Article, we use experimentally determined thermochemical parameters, hydricity and pKa values, to rationalize our observations that the carbide-containing iron carbonyl cluster [Fe4C(CO)12](2-) reduces H(+) to H2 in the presence of CO2 in either acetonitrile (MeCN), MeCN with 5% water, or buffered water (pH 5-13), with no traces of formate or other carbon-containing products observed. Our previous work has shown that the closely related nitride-containing clusters [Fe4N(CO)12](-) and [Fe4N(CO)11(PPh3)](-) will also reduce H(+) to H2 in either MeCN with 5% water or buffered water (pH 5-13), but upon the addition of CO2, they selectively generate formate. The thermochemical measurements on [Fe4C(CO)12](2-) predict that the free energy for transfer of hydride, in MeCN, from the intermediate [HFe4C(CO)12](2-) to CO2 is thermoneutral and to H(+) is -32 kcal mol(-1). In water, these values are less than -19.2 and -8.6 kcal mol(-1), respectively. These results suggest that generation of both H2 and formate should be favorable in aqueous solution and that kinetic effects, such as a fast rate of H2 evolution, must influence the observed selectivity for generation of H2. The hydride-donating ability of [HFe4N(CO)12](-) is lower than that of [HFe4C(CO)12](2-) by 5 kcal mol(-1) in MeCN and by at least 3 kcal mol(-1) in water, and we speculate that this more modest reactivity provides the needed selectivity to obtain formate. We also discuss predictions that may guide future catalyst design.