A Biomimetic Superreduced Iron-Sulfur Cubane and its Oxidized Analogues
In nature, cystein-ligated iron-sulfur (FeS) cubanes of Ferredoxins (Fds) and High-Potential Iron-Sulfur Proteins (HiPIPs) mediate electron transfer and are known to exist in four different oxidation states, namely [Fe4S4]0/1+/2+/3+.[1,2] Among these, the elusive “superreduced” all-ferrous state, [Fe4S4]0, is only retained by the Iron Protein (FeP) of the Nitrogenase enzyme, which belongs to the Fd family.[3–5] Pioneered by R. Holm in the early 70s, bioinorganic chemists have isolated and characterized a variety of FeS cubanes. However, a systematically comparable redox series, which bears the same thiolate ligands and counter-cations and covers all biologically relevant oxidation states, including [Fe4S4]0, is yet lacking.[6–8]
Herein we report the synthesis of [Fe4S4(SR)4]4- via twofold chemical reduction of a novel cubane, [Fe4S4(SR)4]2-, in its traditional oxidation state. To the best of our knowledge, [Fe4S4(SR)4]4- is the first example of an all-ferrous FeS cubane supported by thiolate ligands, that is isolable in substance and characterized by X-ray diffraction analysis. Its oxidation state assignment is supported by extensive spectroscopic characterization including UV-Vis, EPR, XAS, EXAFS as well as variable-temperature 57Fe Mössbauer spectroscopy. Further, for the first time, using straightforward redox-chemistry, all biologically pertinent oxidation states of this new FeS cubane system, [Fe4S4(SR)4]n (n = 1-, 2-, 3-, 4-), are accessible. X-ray diffraction analysis of all members of this biomimetic electron-transfer series reveals that reduction of the Fe4S4-core is accompanied by structural compression of the Fe4-tetrahedron and a concomitant expansion of the S4-tetrahedron in a linear relationship. Thereby, because the S4-expansion supersedes the Fe4-contraction, the overall FeS cubane-volume as well as its surface area are being maximized, which confirms the long-standing hypotheses of biological studies.
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