Unidentate and Bidentate Binding of Nickel(II) Complexes to an Fe4S4 Cluster via Bridging Thiolates: Synthesis, Crystal Structures, and Electrochemical Properties of Model Compounds for the Active Sites of Nickel Containing CO Dehydrogenase/Acetyl-CoA Synthase

Abstract

[Ni(EtN2S2)] (1) (EtN2S2 = N,N‘-diethyl-3,7-diazanonane-1,9-dithiolate) and [Ni(S4)] (2) (S4 = 3,7-dithianonane-1,9-dithiolate) react with [Fe4S4I4]2- to afford the neutral clusters [{Ni(EtN2S2)}2Fe4S4I2] (3) and [{Ni(S4)}2Fe4S4I2] (4), respectively. Metathesis with potassium arenethiolates yields the clusters [{Ni(EtN2S2)}2Fe4S4(Stip)2] (5) and [{Ni(S4)}2Fe4S4(Stip)2] (6) (tip = 2,4,6-triisopropylbenzene). Alternatively, the latter compounds can be synthesized by action of the corresponding Ni complexes on [Fe4S4(Stip)2L2] (L = 2,3-dimethyl-1-phenyl-3-pyrazoline-5-thione), where the neutral ligands are displaced by the Ni complexes. As elucidated by single-crystal X-ray crystallography, 3, 5, and 6 are composed of an [Fe4S4]2+ unit and two Ni(II) complexes and the Ni and Fe centers are bridged via one and two μ2-sulfur atoms from either the EtN2S2 or the S4 ligands. In 3 and 5 two N and two S (thiolate) atoms serve as donors for a square planar coordinated Ni(II) ion, whereas 6 contains two Ni(II) ions in a square planar S2 (thioether) S2 (thiolate) environment. The structures of the NiFe heterometallic clusters are discussed and related to the structures of the active sites of Ni containing CO dehydrogenases/acetyl-CoA synthases. As demonstrated by 1H-NMR spectroscopy, 3, 5, and 6 retain their bridged structures in solution. However, the spectra of 5 and 6 can be interpreted in terms of a symmetrical bidentate coordination of both Fe4S4-bound Ni complex fragments, which would be in contrast to the unidentate (5) or unsymmetrically bidentate (6) binding observed in the solid state. The redox properties of clusters 3, 5, and 6 were determined by cyclic voltammetry. In dichloromethane solution 5 exhibits two quasi-reversible oxidation processes at 0 and +485 mV and one quasi-reversible reduction wave at −1050 mV (vs SCE).