Evidence for Low-Valent Electronic Configurations in Iron-Sulfur Clusters.

Although biological iron-sulfur (Fe-S) clusters perform some of the most difficult redox reactions in nature, they are thought to be composed exclusively of Fe and Fe ions, as well as mixed-valent pairs with average oxidation states of Fe. We herein show that Fe-S clusters formally composed of these valences can access a wider range of electronic configurations─in particular, those featuring low-valent Fe centers. We demonstrate that CO binding to a synthetic [FeS] cluster supported by -heterocyclic carbene ligands induces the generation of Fe centers via intracluster electron transfer, wherein a neighboring pair of Fe sites reduces the CO-bound site to a low-valent Fe state. Similarly, CO binding to an [FeS] cluster induces electron delocalization with a neighboring Fe site to form a mixed-valent FeFe pair in which the CO-bound site adopts partial low-valent character. These low-valent configurations engender remarkable C-O bond activation without having to traverse highly negative and physiologically inaccessible [FeS]/[FeS] redox couples.