Influence of Dithiolate Bridges on the Structures and Electrocatalytic Performance of Small Bite-Angle PNP-Chelated Diiron Complexes Fe2(μ-xdt)(CO)42‑(Ph2P)2NR} Related to [FeFe]-Hydrogenases

As a further exploration of the asymmetrically substituted diiron models for the active site of [FeFe]-hydrogenases, two new types of small bite-angle aminodiphosphine [(Ph2P)2NR; denoted as PNP in this study]-chelated diiron N-phenyl-aza- and ethanedithioate complexes Fe2(μ-xdt)­(CO)42-(Ph2P)2NR} (1a1e) and (2a2e), respectively, were successfully synthesized by the carbonyl substitution reactions of all-carbonyl diiron complexes Fe2(μ-xdt)­(CO)6 (xdt = SCH2N­(Ph)­CH2S (adtNPh) and SCH2CH2S (edt)) with PNP (PNP = (Ph2P)2NR, R = CMe3, CH2CHMe2, (CH2)3Me, (CH2)3Si­(OEt)3, and (CH2)3NMe2) in the presence of Me3NO·2H2O or UV irradiation. All the new complexes obtained above have been well characterized by elemental analysis, FT-IR, NMR spectroscopy, and particularly for 1a, 1b, 2a, and 2d by single-crystal X-ray diffraction analysis. By comparison, 31P­{1H} NMR and X-ray crystallographic studies have clearly revealed that the change of the dithiolate bridge from adtNPh to edt has a significant influence on the coordination geometry of the chelating PNP ligands in Fe2S2 complexes, in which the basal–basal configuration in the adtNPh complexes 1a1e is favorable whereas the apical–basal conformation in the edt complexes 2a2e is main. In addition, the electrochemical properties of complexes 1b and 2b as a pair of representative counterparts are evaluated and compared by cyclic voltammetry in the absence and presence of HOAc as a proton source, indicating that they are found to be electrocatalytically active.