Redox-Noninnocence of the S,S‘-Coordinated Ligands in Bis(benzene-1,2-dithiolato)iron Complexes

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Abstract
The electronic structures of complexes of iron containing two S,S‘-coordinated benzene-1,2-dithiolate, (L)2-, or 3,5-di-tert-butyl-1,2-benzenedithiolate, (LBu)2-, ligands have been elucidated in depth by electronic absorption, infrared, X-band EPR, and Mössbauer spectroscopies. It is conclusively shown that, in contrast to earlier reports, high-valent iron(IV) (d4, S = 1) is not accessible in this chemistry. Instead, the S,S‘-coordinated radical monoanions (L•)1- and/or (LBu•)1- prevail. Thus, five-coordinate [Fe(L)2(PMe3)] has an electronic structure which is best described as [FeIII(L)(L•)(PMe3)] where the observed triplet ground state of the molecule is attained via intramolecular, strong antiferromagnetic spin coupling between an intermediate spin ferric ion (SFe = 3/2) and a ligand radical (L•)1- (Srad = 1/2). The following complexes containing only benzene-1,2-dithiolate(2−) ligands have been synthesized, and their electronic structures have been studied in detail: [NH(C2H5)3]2[FeII(L)2] (1), [N(n-Bu)4]2[FeIII2(L)4] (2), [N(n-Bu)4]2[FeIII2(LBu)4] (3); [P(CH3)Ph3][FeIII(L)2(t-Bu-py)] (4) where t-Bu-py is 4-tert-butylpyridine. Complexes containing an FeIII(L•)(L)- or FeIII(LBu)(LBu•)- moiety are [N(n-Bu)4][FeIII2(LBu)3(LBu•)] (3 ox), [FeIII(L)(L•)(t-Bu-py)] (4 ox), [FeIII(LBu)(LBu•)(PMe3)] (7), [FeIII(LBu)(LBu•)(PMe3)2] (8), and [FeIII(LBu)(LBu•)(PPr3)] (9), where Pr represents the n-propyl substituent. Complexes 2, 3 ox, 4, [FeIII(L)(L•)(PMe3)2] (6), and 9 have been structurally characterized by X-ray crystallography.

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