Sequential Oxidation of the Cubane [4Fe−4S] Cluster from [4Fe−4S]- to [4Fe−4S]3+ in Fe4S4Ln- Complexes

Gaseous Fe4Sn- (n = 4-6) clusters and synthetic analog complexes, Fe4S4Ln- (L = Cl, Br, I; n = 1-4), were produced by laser vaporization of a solid Fe/S target and electrospray from solution samples, respectively, and their electronic structures were probed by photoelectron spectroscopy. Low binding energy features derived from minority-spin Fe 3d electrons were clearly distinguished from S-derived bands. We showed that the electronic structure of the simplest Fe4 S4- cubane cluster can be described by the two-layer spin-coupling model previously developed for the [4Fe] cubane analogs. The photoelectron data revealed that each extra S atom in Fe4S5- and Fe4S6- removes two minority-spin Fe 3d electrons from the [4Fe-4S] cubane core and each halogen ligand removes one Fe 3d electron from the cubane core in the Fe4S4Ln- complexes, clearly revealing a behavior of sequential oxidation of the cubane over five formal oxidation states: [4Fe-4S]- ? [4Fe-4S]0 ? [4Fe-4S]+ ? [4Fe-4S]2+ ? [4Fe-4S]3+. The current work shows the electron-storage capability of the [4Fe-4S] cubane, contributes to the understanding of its electronic structure, and further demonstrates the robustness of the cubane as a structural unit and electron transfer center.