Alkyne adducts of [W2(OCH2tBu)8]n (M = M): comparisons of bridging and terminal addition products.

Abstract

Alkynes are found to react with [W2(OCH2tBu)8] (M = M) in hydrocarbon solvents at room temperature or 45 degrees C to give 1:1 adducts. These are shown to be either bridged (mu-PhCCH and mu-MeCCMe) or terminal-bound (eta2-PhCCMe) in the solid state by single-crystal X-ray crystallography. In solution NMR spectroscopy reveals that bridged and terminal species exist in equilibrium for MeCCH, MeCCMe, and PhCCMe. By NMR spectroscopy the PhCCH and Me3SiCCH adducts are present in solution in bridging and terminally bonded species, respectively. The interconversion of bridged and terminal-bound adducts is chemically rapid but slow on the NMR time scale even though each type of adduct shows fluxional behavior. Calculations employing density functional theory have been carried out on alkyne adducts of the model template W2(OCH3)8 and reveal very small differences in energy between a mu-skewed structure and one having a terminal eta2-alkyne.