C-S BOND-CLEAVAGE IN REACTIONS OF THIOLATE NUCLEOPHILES WITH BRIDGING THIOETHERS IN ANIONIC DITUNGSTEN(III) COMPLEXES

The complexes [PPh4][Cl3W(mu-Cl)(mu-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(mu-SR2)(mu-SR)(mu-Cl)WCl3]2-. The structure of the complex [PPh4]2[Cl3W(mu-SEt2)(mu-SEt)(mu-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) angstrom] 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 mu-SEt group is axial with reference to the mu-SEt2 ligand. This geometry is retained upon dissolution in dichloromethane or acetonitrile for sufficient time to record the H-1 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 x 10(-4) s-1. An absolute assignment of all the proton resonances for both isomers was achieved.