H₂ Oxidation Electrocatalysis Enabled by Metal-to-Metal Hydrogen Atom Transfer: A Homolytic Approach to a Heterolytic Reaction

Oxidation of H₂ in a fuel cell converts the chemical energy of the H−H bond into electricity. Electrocatalytic oxidation of H₂ by molecular catalysts typically requires one metal to perform multiple chemical steps: bind H₂, heterolytically cleave H₂, and then undergo two oxidation and two deprotonation steps. The electrocatalytic oxidation of H₂ by a cooperative system using Cp*Cr(CO)₃H and [Fe(diphosphine)(CO)₃]⁺ has now been invetigated. A key step of the proposed mechanism is a rarely observed metal-to-metal hydrogen atom transfer from the Cr−H complex to the Fe, forming an Fe−H complex that is deprotonated and then oxidized electrochemically. This “division of chemical labor” features Cr interacting with H₂ to cleave the H−H bond, while Fe interfaces with the electrode. Neither metal is required to heterolytically cleave H₂, so this system provides a very unusual example of a homolytic reaction being a key step in a molecular electrocatalytic process.