Influence of Dithiolate Bridges on the Structures and Electrocatalytic Performance of Small Bite-Angle PNP-Chelated Diiron Complexes Fe₂(μ-xdt)(CO)₄{κ²‑(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)₄{κ²-(Ph₂P)₂NR} (1a–1e) and (2a–2e), respectively, were successfully synthesized by the carbonyl substitution reactions of all-carbonyl diiron complexes Fe₂(μ-xdt)­(CO)₆ (xdt = SCH₂N­(Ph)­CH₂S (adt^NPh) 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 adt^NPh 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 adt^NPh complexes 1a–1e is favorable whereas the apical–basal conformation in the edt complexes 2a–2e 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.