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 Fe2+ and Fe3+ ions, as well as mixed-valent pairs with average oxidation states of Fe2.5+. 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 Fe1+ centers. We demonstrate that CO binding to a synthetic [Fe4S4]0 cluster supported by N-heterocyclic carbene ligands induces generation of Fe1+ centers via intracluster electron transfer, wherein a neighboring pair of Fe2+ sites reduces the CO-bound site to a low-valent Fe1+ state. Similarly, CO binding to an [Fe4S4]+ cluster induces electron delocalization with a neighboring Fe site to form a mixed-valent Fe1.5+Fe2.5+ 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 Fe2+/Fe1+ redox couples.