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(μ-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 Fe2(μ-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 S(2-) as a ligand was previously unknown.