Low-Spin Pseudotetrahedral Iron(I) Sites in Fe2(μ-S) Complexes

Fe-I centers in iron-sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron-sulfur clusters. A series of three diiron [(L3Fe)(2)(mu-S)] complexes that were isolated and characterized in the low-valent oxidation states Fe-II-S-Fe-II, Fe-II-S-Fe-I, and Fe-I-S-Fe-I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe-2(mu-S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low-spin S = 1/2 states upon reduction from Fe-II to Fe-I. The possibility of accessing low-spin, pseudotetrahedral Fe-I sites compatible with S2- as a ligand was previously unknown.