Observation of Weak Driving-Force Dependence of Hydride Formation with Iron Carbonyl Clusters

Abstract

Metal-hydride formation reactions are common elementary reactions in catalytic systems, including fuel formation and organic transformations. Here, metal-hydride formation rates with metal carbonyl clusters [Fe4N(CO)12]− and [Fe5N(CO)15]− were determined in MeCN and aqueous media using peak-shift analysis (PSA) and foot-of-the-wave analysis (FOWA) performed on cyclic voltammetry data. In MeCN, hydride formation exhibits exceptionally small dependence on the pKa of the proton source, as inferred from Brønsted slopes (α = 0.06 and 0.03) that are determined from plots of PT rate vs pKa of the proton substrate. This indicates a minimal dependence of the PT rate on the driving force for the PT reaction. These values contrast with the larger slopes commonly observed for mononuclear transition-metal hydrides (α ≈ 0.3–0.7). In aqueous solution, larger Brønsted slopes were observed (α = 0.54 and 0.23). Overall, these results demonstrate that multinuclear metal clusters exhibit hydride-formation behavior distinct from conventional single-site complexes. The ability to maintain fast rates for metal-hydride formation under low driving-force conditions may be a key feature that enables the previously reported selective hydride transfer to CO2 by [Fe4N(CO)12]−.