Use of 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 behaviour 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. Complexes were synthesized by photolysis and they were characterized by IR, NMR (1H, 13C, 31P) spectroscopies and elemental analyses. The structures of 1 and 2 in 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 proton. Two electron transfers are centred on the bmi ligand. Upon addition of acid (CH3CO2H or CH3SO3H), electrocatalytic events of proton reduction are detected. DFT studies were performed to rationalize the electron and proton transfers processes involving compounds 1 and 2 and have 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 appeared. They support that upon successive 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 of proton relay. Proton transfer to the bmi ligand is triggered upon reduction.