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 the 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 [Fe4S4]0/[Fe4S4]− redox couples.