Iron Complexes for the Electrocatalytic Oxidation of Hydrogen: Tuning Primary and Secondary Coordination Spheres

A series of iron hydride complexes featuring PRNR′PR (PRNR′PR = R2PCH2N(R′)CH2PR2 where R = Ph, R′ = Me; R = Et, R′ = Ph, Bn, Me, tBu) and cyclopentadienide (CpX = C5H4X 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(PEtNMePEt)(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 FeIII hydride by the pendant amine and the su...