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 ([<b>1</b>]⁰), Ph ([<b>2</b>]⁰), CH₂Ph ([<b>3</b>]⁰). Whereas related derivatives adopt <i>C</i>₂-symmetric Fe₂(CO)₂P₄ cores, [<b>1</b>]⁰–[<b>3</b>]⁰ have <i>C</i>_s symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [<b>1</b>]⁰–[<b>3</b>]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, [<b>3</b>]⁺ adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [<b>1</b>]⁺ and [<b>2</b>]⁺ and many other [Fe₂­(SR)₂L₆]⁺ derivatives, [<b>3</b>]⁺ undergoes C–S bond homolysis, affording the diferrous sulfido-thiolate [Fe₂­(SCH₂Ph)­(S)­(CO)₂­(dppv)₂]⁺ ([<b>4</b>]⁺). According to X-ray crystallography, the first coordination spheres of [<b>3</b>]⁺ and [<b>4</b>]⁺ are similar, but the Fe–sulfido bonds are short in [<b>4</b>]⁺. The conversion of [<b>3</b>]⁺ to [<b>4</b>]⁺ follows first-order kinetics, with <i>k</i> = 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 [<b>3</b>]⁺ to [<b>4</b>]⁺ 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 [<b>4</b>]⁺ is indicated by its double carbonylation to give [Fe₂­(SCH₂Ph)­(S)­(CO)₄­(dppv)₂]⁺ ([<b>5</b>]⁺), which adopts a bioctahedral structure