Electron-Rich, Diiron Bis(monothiolato) Carbonyls: C-S Bond Homolysis in a Mixed Valence Diiron Dithiolate.

The synthesis and redox properties are presented for the electron-rich bis(monothiolate)s Fe(SR)(CO)(dppv) for R = Me ([1]), Ph ([2]), CHPh ([3]). Whereas related derivatives adopt C-symmetric Fe(CO)P cores, [1]-[3] have C symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [1]-[3] undergo 1e oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe(SR)(CO)(dppv)]. As established crystallographically, [3] adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [1] and [2] and many other [Fe(SR)L] derivatives, [3] undergoes C-S bond homolysis, affording the diferrous sulfido-thiolate [Fe(SCHPh)(S)(CO)(dppv)] ([4]). According to X-ray crystallography, the first coordination spheres of [3] and [4] are similar, but the Fe-sulfido bonds are short in [4]. The conversion of [3] to [4] follows first-order kinetics, with k = 2.3 × 10 s (30 °C). When the conversion is conducted in THF, the organic products are toluene and dibenzyl. In the presence of TEMPO, the conversion of [3] to [4] is accelerated about 10×, the main organic product being TEMPO-CHPh. DFT calculations predict that the homolysis of a C-S bond is exergonic for [Fe(SCHPh)(CO)(PR)] but endergonic for the neutral complex as well as less substituted cations. The unsaturated character of [4] is indicated by its double carbonylation to give [Fe(SCHPh)(S)(CO)(dppv)] ([5]), which adopts a bioctahedral structure.