Alkyl Coupling on Copper, Silver, and Gold: Correlation Between the Coupling Rate and the Metal-Alkyl Bond Strength

Recent studies have shown that high coverages of alkyl groups can be generated and isolated on metal surfaces under ultrahigh vacuum conditions by the thermal [Zaera, F., Acc. Chem. Res. 25, 260 (1992)] and photochemical [Zhou, X. -L., Zhu, X. -Y., and White, J. M., Surf. Sci. Rep. 13, 74 (1991)] dissociation of alkyl halides. This paper describes studies in which these approaches have been employed to study the coupling of methyl groups generated on a Au(111) surface. The coupling rate, as measured by temperature-programmed reaction experiments, is found to be sensitive to the structure of the surface. For a well-annealed surface, coupling occurs at ∼ 270 K, but for a sputtered and incompletely annealed surface, coupling occurs at ∼ 350 K. An interesting and unexpected finding is that coadsorbing P(CH3)3 with the methyl groups on a Au(100) surface inhibits the coupling reaction so that methyl radical desorption is observed. Similarities between the rates of methyl coupling and methyl radical desorption have also been observed previously for a Cu(111) surface, suggesting a correlation between the methyl coupling rate and the metal-alkyl bond strength. Such a correlation can account for the trends in the coupling rates reported for alkyl groups on Ag(111), Au(100), Cu(111), and Cu(110). The coupling rate is fastest on surfaces and for alkyl groups that are expected to form the weakest carbon-metal bonds.