A complete biomimetic iron-sulfur cubane redox series.

Abstract

Synthetic iron-sulfur cubanes are models for biological cofactors, which are essential to delineate oxidation states in the more complex enzymatic systems. However, a complete series of [Fe₄S₄]<i>ⁿ</i> complexes spanning all redox states accessible by 1-electron transformations of the individual iron atoms (<i>n</i> = 0-4+) has never been prepared, deterring the methodical comparison of structure and spectroscopic signature. Here, we demonstrate that the use of a bulky arylthiolate ligand promoting the encapsulation of alkali-metal cations in the vicinity of the cubane enables the synthesis of such a series. Characterization by EPR, ⁵⁷Fe Mössbauer spectroscopy, UV-visible electronic absorption, variable-temperature X-ray diffraction analysis, and cyclic voltammetry reveals key trends for the geometry of the Fe₄S₄ core as well as for the Mössbauer isomer shift, which both correlate systematically with oxidation state. Furthermore, we confirm the <i>S</i> = 4 electronic ground state of the most reduced member of the series, [Fe₄S₄]⁰, and provide electrochemical evidence that it is accessible within 0.82 V from the [Fe₄S₄]²⁺ state, highlighting its relevance as a mimic of the nitrogenase iron protein cluster.