Iron complexes for the electrocatalytic oxidation of hydrogen: Tuning primary and secondary coordination spheres

A series of iron hydride complexes featuring P^RN ^R′^OP^R (P^RN^R′ P^R = R₂PCH₂N(R′)CH₂PR ₂ where R = Ph, R′ = Me; R = Et, R′ = Ph, Bn, Me, ^tBu) and cyclopentadienide (Cp^X = C₅H ₄X where X = H, C₅F₄N) ligands has been synthesized; characterized by NMR spectroscopy, X-ray diffraction, and cyclic voltammetry; and examined by quantum chemistry calculations. Each compound was tested for the electrocatalytic oxidation of H₂, and the most active complex, (CpC₅F₄N)Fe(P^EtN^MeP ^Et)(H), exhibited a turnover frequency of 8.6 s^-1 at 1 atm of H₂ with an overpotential of 0.41 V, as measured at the potential at half of the catalytic current and using N-methylpyrrolidine as the exogenous base to remove protons. Control complexes that do not contain pendant amine groups were also prepared and characterized, but no catalysis was observed. The rate-limiting steps during catalysis are identified through combined experimental and computational studies as the intramolecular deprotonation of the Fe^III hydride by the pendant amine and the subsequent deprotonation by an exogenous base.