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 Fe2 (SR)2 (CO)2 (dppv)2 for R = Me ([1]0 ), Ph ([2]0 ), CH2 Ph ([3]0 ). Whereas related derivatives adopt C2 -symmetric Fe2 (CO)2 P4 cores, [1]0 -[3]0 have Cs symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [1]0 -[3]0 undergo 1e- oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe2 (SR)2 (CO)2 (dppv)2 ]+ . As established crystallographically, [3]+ adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [1]+ and [2]+ and many other [Fe2 (SR)2 L6 ]+ derivatives, [3]+ undergoes C-S bond homolysis, affording the diferrous sulfido-thiolate [Fe2 (SCH2 Ph)(S)(CO)2 (dppv)2 ]+ ([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 × 10-6 s-1 (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-CH2 Ph. DFT calculations predict that the homolysis of a C-S bond is exergonic for [Fe2 (SCH2 Ph)2 (CO)2 (PR3 )4 ]+ 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 [Fe2 (SCH2 Ph)(S)(CO)4 (dppv)2 ]+ ([5]+ ), which adopts a bioctahedral structure.