Use of the 2,3-bis(diphenylphosphino)-N-phenylmaleimide ligand as a redox [Fe4S4] cubane surrogate in di-iron complexes related to [FeFe]-hydrogenases: influence of the dithiolate bridge on the redox behaviour.

The redox behaviours of two complexes with the 2,3-bis(diphenylphosphino)-N-phenylmaleimide (bmi) ligand related to the active site of [FeFe]-hydrogenases, [Fe2(CO)4(κ2-bmi)(μ-pdt)] (1) and [Fe2(CO)4(κ2-bmi)(μ-adtBn)] (2) [pdt = propane-1,3-dithiolate (μ-S(CH2)3S) and adtBn = N-benzyl-azapropane-1,3-dithiolate (μ-SCH2N{CH2(C6H5)}CH2S)], were studied. The complexes were synthesized by photolysis and they were characterized by IR, NMR (1H, 13C, 31P) spectroscopy and elemental analyses. The structures of 1 and 2 in the solid state were solved by X-ray diffraction analysis. Their electrochemical behaviours were also studied by cyclic voltammetry in the presence and in the absence of protons. Two electron transfers are centred on the bmi ligand. Upon addition of an acid (CH3CO2H or CH3SO3H), electrocatalytic events of proton reduction are detected. DFT studies were performed to rationalize the electron and proton transfer processes involving compounds 1 and 2 and they confirmed the role of bmi as a proton-sensitive redox ligand. The diphosphine is the most favorable site for the transfer of electrons and protons but no direct involvement of the NPh group was observed. These findings support that upon electron and proton transfers, the catalytic species are the doubly reduced and doubly protonated species at the bmi ligand and not their hydrido isomers. In 2, the amine of the dithiolate bridge plays the expected role in proton relay. Proton transfer to the bmi ligand is triggered upon reduction.