Synthetic Models for Nickel-Iron Hydrogenase Featuring Redox-Active Ligands

The nickel-iron hydrogenase enzymes efficiently and reversibly interconvert protons, electrons, and dihydrogen. These redox proteins feature iron-sulfur clusters that relay electrons to and from their active sites. Reported here are synthetic models for nickel-iron hydrogenase featuring redox-active auxiliaries that mimic the iron-sulfur cofactors. The complexes prepared are Ni^II(μ-H)Fe^IIFe species of formula [(diphosphine)Ni(dithiolate)(μ-H)Fe(CO)₂(ferrocenylphosphine)]⁺ or Ni^IIFe^IFe complexes [(diphosphine)Ni(dithiolate)Fe(CO)₂(ferrocenylphosphine)]⁺ (diphosphine = Ph₂P(CH₂)₂PPh₂ or Cy₂P(CH₂)₂PCy₂; dithiolate = ^-S(CH₂)₃S^-; ferrocenylphosphine = diphenylphosphinoferrocene, diphenylphosphinomethyl(nonamethylferrocene) or 1,1'-bis(diphenylphosphino)ferrocene). The hydride species is a catalyst for hydrogen evolution, while the latter hydride-free complexes can exist in four redox states - a feature made possible by the incorporation of the ferrocenyl groups. Mixed-valent complexes of 1,1'-bis(diphenylphosphino)ferrocene have one of the phosphine groups unbound, with these species representing advanced structural models with both a redox-active moiety (the ferrocene group) and a potential proton relay (the free phosphine) proximal to a nickel-iron dithiolate.