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 amino-diphosphine [(Ph2P)(2)NR; denoted as PNP in this study]-chelated diiron N-phenyl-aza- and ethanedithioate complexes Fe-2(mu-xdt)(CO)(4){kappa(2)-(P(h)2P)(2)NR} (1a-1e) and (2a-2e), respectively, were successfully synthesized by the carbonyl substitution reactions of all-carbonyl diiron complexes Fe-2(mu-xdt)(CO)(6) (xdt = SCH2N(Ph)CH2S (adt(NPh)) and SCH2CH2S (edt)) with PNP (PNP = (Ph2P)(2)NR, R = CMe3, CH2CHMe2, (CH2)(3)Me, (CH2)(3)Si(OEt)(3), and (CH2)(3)NMe2) in the presence of Me3NO center dot 2H(2)O 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, P-31{H-1} 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.