Initial Synthesis and Structure of an All-Ferrous Analogue of the Fully Reduced [ Fe4 S4]0 Cluster of the Nitrogenase Iron Protein

The synthetic cubane-type iron-sulfur clusters [ Fe4 S4( 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 [ Fe4 S4( SCys)4]z redox centers, which, in terms of core oxidation states, exhibit the redox couples [ Fe4 S4]3+/2+ and [ Fe4 S4]2+/1+. Clusters with the all-ferrous core [ Fe4 S4]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 [ Fe4 S4( CN)4]3- with K[ Ph2 CO] in acetonitrile/tetrahydrofuran affords the all-ferrous cluster [ Fe4 S4( CN)4]4-, isolated as the Bu4 N+ salt. The x-ray structure demonstrates retention of a cubane-type structure with idealized D2d symmetry. The Mössbauer spectrum unambiguously demonstrates the [ Fe4 S4]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 [ Fe4 S4]2+/1+ and [ Fe4 S4]1+/0 redox potential shifts, facilitating the initial isolation of an analogue of the [ Fe4 S4]0 protein site.