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Synthetic iron-sulfur clusters of general formulation [FeSL] with core atoms Fe and S and terminal ligands L constitute a family of molecular clusters with remarkably diverse geometrical and electronic structures. Several structure types are also found in proteins. The large majority of research on these clusters has involved elucidation of physical properties. Here, we direct attention to reactivity in the form of cluster conversions in which the FeS cores of reactants are transformed to new structures, usually of different nuclearity, in overall reactions such as self-assembly and fragment condensation and dissociation. An extensive body of core conversions, many of which have not been recognized as such, are presented including those in biological systems. All structural core types are depicted, and all core conversions are diagrammatically summarized. Clusters containing the cubane-type FeS core play a central role in conversion chemistry. The core conversion concept tends to reinforce the description of iron-sulfur cores as modular units subject to various covalent bond interactions that lead to different structures.

An extensive series of 3:1 site-differentiated cubane-type clusters [Fe4S4(PPri3)3 L] (L = Cl−, Br−, I−, RO−, RS−, RSe−) has been prepared in 40–80% yield by two methods: ligand substitution of [Fe4S4(PPri3)4]1+ in THF/acetonitrile by reaction with monoanions, and reductive cleavage of ligand substrates (RSSR, RSeSeR, I2) by the all-ferrous clusters [Fe8S8(PPri3)6]/ [Fe16S16(PPri3)8] in THF. These neutral clusters are stable and do not undergo ligand redistribution reactions involving charged species in benzene and THF solutions. X-ray structural studies confirm the cubane stereochemistry but with substantial and variable distortions of the [Fe4S4]1+ core from idealized cubic core geometry. Based on Fe-S bond lengths, seven clusters were found to have compressed tetragonal distortions (4 short and 8 long bonds) and the remaining seven display other types of distortions with different combinations of long, short and intermediate bond lengths. These results further emphasize the facile deformabililty of this core oxidation state previously observed in [Fe4S4(SR)4]3− clusters. The Fe2.25+ mean oxidation state was demonstrated from 57Fe isomer shifts and the appearance of two quadrupole doublets arises from the spin-coupled | 9/2,4,1/2〉 state. The S = 1/2 ground state was further supported by EPR spectra and magnetic susceptibility data.

The stability of cubane-type [Fe4S4(SR)4]2− clusters in mixed organic/aqueous solvents was examined as an initial step in the development of stable water-soluble cluster compounds possibly suitable for reconstitution of scaffold proteins in protein biosynthesis. The research involves primarily spectrophotometric assessment of stability in 20–80% Me2SO/aqueous media (v/v), from which it was found that conventional clusters tend to be stable for up to 12 hours in 60% Me2SO but are much less stable at higher aqueous content. α-Cyclodextrin mono- and dithioesters and thiols were prepared as ligand precursors for cluster binding, which was demonstrated by spectroscopic methods. A potentially bidentate cyclodextrin dithiolate was found to be relatively effective for cluster stabilization in 40% Me2SO, suggesting (together with earlier results) that other exceptionally large thiolate ligands may promote cluster stability in aqueous media.