Biomimics of [FeFe]-hydrogenases incorporating redox-active ligands: synthesis, redox properties and spectroelectrochemistry of diiron-dithiolate complexes with ferrocenyl-diphosphines as Fe4S4 surrogates

Abstract

[FeFe]-Ase biomimics containing a redox-active ferrocenyl diphosphine have been prepared and their ability to reduce protons and oxidise H₂ studied, including 1,1′-bis(diphenylphosphino)ferrocene (dppf) complexes Fe₂(CO)₄(μ-dppf)(μ-S(CH₂)_nS) (n = 2, edt; n = 3, pdt) and Fe₂(CO)₄(μ-dppf)(μ-SAr)₂ (Ar = Ph, p-tolyl, p-C₆H₄NH₂), together with the more electron-rich 1,1′-bis(dicyclohexylphosphino)ferrocene (dcpf) complex Fe₂(CO)₄(μ-dcpf)(μ-pdt). Crystallographic characterisation of four of these show similar overall structures, the diphosphine spanning an elongated Fe–Fe bond (ca. 2.6 Å), lying trans to one sulfur and cis to the second. In solution the diphosphine is flexible, as shown by VT NMR studies, suggesting that Fe₂⋯Fe distances of ca. 4.5–4.7 Å in the solid state vary in solution. Cyclic voltammetry, IR spectroelectrochemistry and DFT calculations have been used to develop a detailed picture of electronic and structural changes occurring upon oxidation. In MeCN, Fe₂(CO)₄(μ-dppf)(μ-pdt) shows two chemically reversible one-electron oxidations occurring sequentially at Fe₂ and Fc sites respectively. For other dppf complexes, reversibility of the first oxidation is poor, consistent with an irreversible structural change upon removal of an electron from the Fe₂ centre. In CH₂Cl₂, Fe₂(CO)₄(μ-dcpf)(μ-pdt) shows a quasi-reversible first oxidation together with subsequent oxidations suggesting that the generated cation has some stability but slowly rearranges. Both pdt complexes readily protonate upon addition of HBF₄·Et₂O to afford bridging-hydride cations, [Fe₂(CO)₄(μ-H)(μ-dcpf)(μ-pdt)]⁺, species which catalytically reduce protons to generate H₂. In the presence of pyridine, [Fe₂(CO)₄(μ-dppf)(μ-pdt)]²⁺ catalytically oxidises H₂ but none of the other complexes do this, probably resulting from the irreversible nature of their first oxidation. Mechanistic details of both proton reduction and H₂ oxidation have been studied by DFT allowing speculative reaction schemes to be developed.