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

[Ni(EtN 2 S 2 )] ( 1 ) (EtN 2 S 2 = N, N ‘-diethyl-3,7-diazanonane-1,9-dithiolate) and [Ni(S 4 )] ( 2 ) (S 4 = 3,7-dithianonane-1,9-dithiolate) react with [Fe 4 S 4 I 4 ] 2 - to afford the neutral clusters [{Ni(EtN 2 S 2 )} 2 Fe 4 S 4 I 2 ] ( 3 ) and [{Ni(S 4 )} 2 Fe 4 S 4 I 2 ] ( 4 ), respectively. Metathesis with potassium arenethiolates yields the clusters [{Ni(EtN 2 S 2 )} 2 Fe 4 S 4 (Stip) 2 ] ( 5 ) and [{Ni(S 4 )} 2 Fe 4 S 4 (Stip) 2 ] ( 6 ) (tip = 2,4,6-triisopropylbenzene). Alternatively, the latter compounds can be synthesized by action of the corresponding Ni complexes on [Fe 4 S 4 (Stip) 2 L 2 ] (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 [Fe 4 S 4 ] 2+ unit and two Ni(II) complexes and the Ni and Fe centers are bridged via one and two μ 2 -sulfur atoms from either the EtN 2 S 2 or the S 4 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 S 2 (thioether) S 2 (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 1 H-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 Fe 4 S 4 -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).