C–S Bond cleavage in reactions of thiolate nucleophiles with bridging thioethers in anionic ditungsten(III) complexes

Abstract

The complexes [PPh₄][Cl₃W(µ-Cl)(µ-SR₂)₂WCl₃](R = Me, Et, or SR₂= tetrahydrothiophene) react with thiolate anions, SR^′–(R′= C₆H₄Me-4), via a nucleophilic displacement of the R group to yield the appropriate mixed thioether, RSR′, and the dianion [Cl₃W(µ-SR₂)(µ-SR)(µCl)WCl₃]^2–. The structure of the complex [PPh₄]₂[Cl₃W(µ-SEt₂)(µ-SEt)(µ-Cl)WCl₃]·2CH₂Cl₂ was determined by single-crystal X-ray diffraction, which shows that the anion possesses a confacial bioctahedral framework. A very short W–W interatomic distance [2.4418(6)Å] is indicative of a metal–metal triple bond. The unique combination of bridging ligands in this complex, all of which are trans to terminal chlorides, allows a comparison to be made between the W–S_thioether W–S_thiolate and W–Cl bond lengths. The configuration of the ethyl group on the µ-SEt group is axial with reference to the µ-SEt₂ ligand. This geometry is retained upon dissolution in dichloromethane or acetonitrile for sufficient time to record the ¹H NMR spectra, but equilibration with the equatorial isomer occurs over a period of ca. 2 or 6 h respectively. A kinetic study of this process revealed that it is first order, with a forward rate constant of 1.32 × 10^–4 s^–1. An absolute assignment of all the proton resonances for both isomers was achieved.