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

Abstract

The synthetic cubane-type iron–sulfur clusters [Fe₄S₄(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 [Fe₄S₄(SCys)₄]^z redox centers, which, in terms of core oxidation states, exhibit the redox couples [Fe₄S₄]^3+/2+ and [Fe₄S₄]^2+/1+. Clusters with the all-ferrous core [Fe₄S₄]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 [Fe₄S₄(CN)₄]^3– with K[Ph₂CO] in acetonitrile/tetrahydrofuran affords the all-ferrous cluster [Fe₄S₄(CN)₄]^4–, isolated as the Bu₄N⁺ salt. The x-ray structure demonstrates retention of a cubane-type structure with idealized D _{2 d} symmetry. The Mössbauer spectrum unambiguously demonstrates the [Fe₄S₄]⁰ oxidation state. Bond distances, core volumes, ⁵⁷Fe 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 [Fe₄S₄]^2+/1+ and [Fe₄S₄]^1+/0 redox potential shifts, facilitating the initial isolation of an analogue of the [Fe₄S₄]⁰ protein site.

• iron–sulfur clusters
• synthetic analogue
• all-ferrous cluster