Cysteine residue-bridged dinuclear Ni–Fe complexes related to [NiFe]-H2ases

Abstract

To develop the biomimetic chemistry of [NiFe]-H₂ases, we have synthesized cysteine residue-containing [NiFe]-H₂ase mimics (diphosphine)Ni[SCH₂CH(CO₂Me)NHCOC₅H₄-η⁵]₂Fe (1, diphosphine = Ph₂P(CH₂)₂PPh₂; 2, Ph₂PCHCHPPh₂; 3, 1,2-(Ph₂P)₂C₆H₄; 4, (Ph₂P)₂NCH₂C₆H₄Me-p; 5, (Ph₂P)₂NCH₂CO₂Et; and 6, (Ph₂P)₂N-2-CH₂C₅H₄N) for the first time by a simple and convenient method. The full spectroscopic characterization of 1–6 along with X-ray crystallographic characterization of 1, 2 and 4 has proved that complexes 1–6 consist of two cysteine residue moieties that are bridged to their diphosphine-chelated Ni(II) centers and two η⁵-cyclopentadienyl-coordinated Fe(II) centers. On the basis of electrochemical and electrocatalytic studies, complexes 1 and 4 have been found to be catalysts for H₂ production from HOAc with moderate i_cat/i_p and overpotential values under CV conditions. In addition, a possible 2E2C (E = electrochemical, C = chemical) mechanism is briefly suggested for this type of H₂ evolution reaction (HER).