Sluggish σ-Bond Formation between sp-Hybridized Carbons:  Acetylide Coupling on Ag(111)

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 (Csp 2 −Csp 2 ) and alkyl−alkyl (Csp 3 −Csp 3 ) 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.