Chemisorbed methylacetylide intermediates were generated on clean Ag(111) to study the mechanism and kinetics of C-C σ-bond formation between sp-hybridized carbon centers. Following the adsorption of propynyl iodide (C3H3I) at 110 K, surface heating led to the cleavage of the C-I bond at 200 K, and the resulting surface Cα≡Cβ-CH3 species were tilted and coordinated to the surface via a metal-carbon σ-bond with weak π-interactions, confirmed by reflection absorption infrared spectroscopy and theoretical calculations. Temperature programmed reaction (TPR) showed that a Csp-Csp coupling reaction occurred at ∼500 K to afford 2,4-hexadiyne through head-to-head dimerization involving Cα carbons. We attribute the relatively slow acetylide-acetylide coupling rate against those from vinyl-vinyl (Csp2-Csp2) and alkyl-alkyl (Csp3-Csp3) couplings to the stronger metal-carbon (M-Cα) bond strength as well as the anionic character bestowed upon the adsorbed acetylides, which all correlate with the increased s-character in the sp-hybridized α-carbon. Copyright ? 2007 American Chemical Society.
Relation:
Journal of the American Chemical Society Volume 129, Issue 5, 7 February 2007, Pages 1028-1029
