Thiolate-bridged dinuclear iron(tris-carbonyl)-nickel complexes relevant to the active site of [NiFe] hydrogenase

The reaction of NiBr₂(EtOH)₄ with a 1:2-3 mixture of FeBr₂(CO)₄ and Na(SPh) generated a linear trinuclear Fe-Ni-Fe cluster (CO)₃Fe(μ-SPh)₃Ni(μ-SPh)₃Fe(CO)₃, 1, whereas the analogous reaction system FeBr₂(CO)₄/Na(StBu)/NiBr₂(EtOH)₄ (1:2-3:1) gave rise to a linear tetranuclear Fe-Ni-Ni-Fe cluster [(CO)₃Fe(μ-StBu)₃Ni(μ-Br)]₂, 2. By using this tetranuclear cluster 2 as the precursor, we have developed a new synthetic route to a series of thiolate-bridged dinuclear Fe(CO)₃-Ni complexes, the structures of which mimic [NiFe] hydrogenase active sites. The reactions of 2 with SC(NMe₂)₂ (tmtu), Na{S(CH₂)₂SMe} and ortho-NaS(C₆H₄)SR (R = Me, tBu) led to isolation of (CO)₃Fe(μ-StBu)₃NiBr(tmtu), 3, (CO)₃Fe(StBu)(μ-StBu)₂Ni{S(CH₂)₂SMe}, 4, and (CO)₃Fe(StBu)(μ-StBu)₂Ni{S(C₆H₄)SR}, 5a (R = Me) and 5b (R = tBu), respectively. On the other hand, treatment of 2 with 2-methylthio-phenolate (ortho-O(C₆H₄)SMe) in methanol resulted in (CO)₃Fe(μ-StBu)₃Ni(MeOH){O(C₆H₄)SMe}, 6a. The methanol molecule bound to Ni is labile and is readily released under reduced pressure to afford (CO)₃Fe(StBu)(μ-StBu)₂Ni{O(C₆H₄)SMe}, 6b, and the coordination geometry of nickel changes from octahedral to square planar. Likewise, the reaction of 2 with NaOAc in methanol followed by crystallization from THF gave (CO)₃Fe(μ-StBu)₃Ni(THF)(OAc), 7. The dinuclear complexes, 3-7, are thermally unstable, and a key to their successful isolation is to carry out the reactions and manipulations at -40°C.