Chelation of Group-6 Metal Tetracarbonyl Fragments by Dicobalt Thioalkyne Complexes

Reactions of [Co2(mu-RSCH2C=CCH2SR)(mu-dppm)(CO)4] (dppm = bis(diphenylphosphino)methane) with [M(CO)4nbd] (nbd = norbornadiene) afford [Co2{mu-C2(CH2SR)2M(CO)4}(muA-dPPM)(CO)4] (M = Mo, W; R = Me, Et, (n)Bu, (t)Bu, Bz and M = Cr; R = Me, Et). The molecular structure of [Co2{mu-C2(CH2SMe)2Mo(CO)4}(mu-dppm)(CO)4] has been established by X-ray diffraction, and comprises a Co2(mu-dppm)(CO)4 unit transversely bridged by 2,7-dithio-4-octyne co-ordinated through the thioether sulphur atoms to a molybdenum tetracarbonyl moiety. The chelation of [Co2(mu-MeSCH2C=CCH2SMe)(mu-dppm)(CO)4] to the molybdenum centre produces a seven-membered ring which adopts a chair conformation, and the thiomethyl groups adopt a cis configuration with respect to the plane defined by the molybdenum and the two sulphur atoms. The mass spectra and NMR spectra of the new compounds are reported. The compounds display dynamic NMR behaviour which can be interpreted in terms of alkyne rocking and sulphur inversion.