Increasing the rate of hydrogen oxidation without increasing the overpotential: a bio-inspired iron molecular electrocatalyst with an outer coordination sphere proton relay

Abstract

Oxidation of hydrogen (H₂) to protons and electrons for energy production in fuel cells is currently catalyzed by platinum, but its low abundance and high cost present drawbacks to widespread adoption. Precisely controlled proton removal from the active site is critical in hydrogenase enzymes in nature that catalyze H₂ oxidation using earth-abundant metals (iron and nickel). Here we report a synthetic iron complex, (Cp^C₅F₄N)Fe(P^EtN^{(CH₂)₃NMe₂}P^Et)(Cl), that serves as a precatalyst for the oxidation of H₂, with turnover frequencies of 290 s⁻¹ in fluorobenzene, under 1 atm of H₂ using 1,4-diazabicyclo[2.2.2]octane (DABCO) as the exogenous base. The inclusion of a properly tuned outer coordination sphere proton relay results in a cooperative effect between the primary, secondary and outer coordination spheres for moving protons, increasing the rate of H₂ oxidation without increasing the overpotential when compared with the analogous complex featuring a single pendant base. This finding emphasizes the key role of pendant amines in mimicking the functionality of the proton pathway in the hydrogenase enzymes.