Computational and Experimental Investigations of the Fe2(μ-S2)/Fe2(μ-S)2 Equilibrium.

Density functional theory (DFT) calculations on Fe2S2(CO)6-2n(PMe3)2n for n = 0, 1, and 2 reveal that the most electron-rich derivatives (n = 2) exist as diferrous disulfides lacking an S-S bond. The thermal interconversion of the FeII2(S)2 and FeI2(S2) valence isomers is symmetry-forbidden. Related electron-rich diiron complexes [Fe2S2(CN)2(CO)4]2- of an uncertain structure are implicated in the biosynthesis of [FeFe]-hydrogenases. Several efforts to synthesize electron-rich derivatives of Fe2(μ-S2)(CO)6 (1) are described. First, salts of iron persulfido cyanides [Fe2(μ-S2)(CO)5(CN)]- and [Fe2(μ-S2)(CN)(CO)4(PPh3)]- were prepared by the reactions of NaN(tms)2 with 1 and Fe2(μ-S2)(CO)5(PPh3), respectively. Alternative approaches to electron-rich diiron disulfides targeted Fe2(μ-S2)(CO)4(diphosphine). Whereas the preparation of Fe2(μ-S2)(CO)4(dppbz) was straightforward, that of Fe2(μ-S2)(CO)4(dppv) required an indirect route involving the oxidation of Fe2(μ-SH)2(CO)4(dppv) (dppbz = C6H4-1,2-(PPh2)2, dppv = cis-C2H2(PPh2)2). DFT calculations indicate that the oxidation of Fe2(μ-SH)2(CO)4(dppv) produces singlet diferrous disulfide Fe2(μ-S)2(CO)4(dppv), which is sufficiently long-lived as to be trapped by ethylene. The reaction of 1 and dppv mainly afforded Fe2(μ-SCH=CHPPh2)(μ-SPPh2)(CO)5, implicating a S-centered reaction.