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

摘要

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), CH2Ph ([>3]⁰). Whereas related derivatives adopt C2-symmetric Fe2(CO)2P4 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)2L₆]⁺ derivatives, [>3]⁺ undergoes C–S bond homolysis, affording the diferrous sulfido-thiolate [Fe₂(SCH₂Ph)(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-CH₂Ph. DFT calculations predict that the homolysis of a C–S bond is exergonic for [Fe₂(SCH₂Ph)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(SCH₂Ph)(S)(CO)₄(dppv)₂]⁺ ([>5]⁺), which adopts a bioctahedral structure.