Biomimics of [FeFe]-hydrogenases incorporating redox-active ligands: synthesis, redox properties and spectroelectrochemistry of diiron-dithiolate complexes with ferrocenyl-diphosphines as Fe 4 S 4 surrogates.

[FeFe]-Ase biomimics containing a redox-active ferrocenyl diphosphine have been prepared and their ability to reduce protons and oxidise H 2 studied, including 1,1'-bis(diphenylphosphino)ferrocene (dppf) complexes Fe 2 (CO) 4 (μ-dppf)(μ-S(CH 2 ) n S) ( n = 2, edt; n = 3, pdt) and Fe 2 (CO) 4 (μ-dppf)(μ-SAr) 2 (Ar = Ph, p -tolyl, p -C 6 H 4 NH 2 ), together with the more electron-rich 1,1'-bis(dicyclohexylphosphino)ferrocene (dcpf) complex Fe 2 (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 Fe 2 ⋯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, Fe 2 (CO) 4 (μ-dppf)(μ-pdt) shows two chemically reversible one-electron oxidations occurring sequentially at Fe 2 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 Fe 2 centre. In CH 2 Cl 2 , Fe 2 (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 HBF 4 ·Et 2 O to afford bridging-hydride cations, [Fe 2 (CO) 4 (μ-H)(μ-dcpf)(μ-pdt)] + , species which catalytically reduce protons to generate H 2 . In the presence of pyridine, [Fe 2 (CO) 4 (μ-dppf)(μ-pdt)] 2+ catalytically oxidises H 2 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 H 2 oxidation have been studied by DFT allowing speculative reaction schemes to be developed.