Initial synthesis and structure of an all-ferrous analogue of the fully reduced [Fe4S4]0 cluster of the nitrogenase iron protein.

The synthetic cubane-type iron-sulfur clusters [Fe(4)S(4)(SR)(4)](z) form a four-member electron transfer series (z = 3-, 2-, 1-, and 0), all members of which except that with z = 0 have been isolated and characterized. They serve as accurate analogues of protein-bound [Fe(4)S(4)(SCys)(4)](z) redox centers, which, in terms of core oxidation states, exhibit the redox couples [Fe(4)S(4)](3+/2+) and [Fe(4)S(4)](2+/1+). Clusters with the all-ferrous core [Fe(4)S(4)](0) have never been isolated because of their oxidative sensitivity. Recent work on the Fe protein of Azotobacter vinelandii nitrogenase has demonstrated the formation of the all-ferrous state upon reaction with a strong reductant. Treatment of the cyanide cluster [Fe(4)S(4)(CN)(4)](3-) with K[Ph(2)CO] in acetonitrile/tetrahydrofuran affords the all-ferrous cluster [Fe(4)S(4)(CN)(4)](4-), isolated as the Bu(4)N(+) salt. The x-ray structure demonstrates retention of a cubane-type structure with idealized D(2)(d) symmetry. The Mössbauer spectrum unambiguously demonstrates the [Fe(4)S(4)](0) oxidation state. Bond distances, core volumes, (57)Fe isomer shifts, and visible absorption spectra make evident the high degree of structural and electronic similarity with the fully reduced Fe protein. The attribute of cyanide ligation causes positive [Fe(4)S(4)](2+/1+) and [Fe(4)S(4)](1+/0) redox potential shifts, facilitating the initial isolation of an analogue of the [Fe(4)S(4)](0) protein site.