Evidence of metallocycle formation by decomposition of 1-hexyne on Ru(0 0 0 1): a RAIRS study

The adsorption and decomposition of 1-hexyne on Ru(0 0 0 1) was studied by reflection–absorption infrared spectroscopy (RAIRS). Hexylidyne, formed at 120 K by direct adsorption of 1-hexyne or by annealing the di-σ/π surface complex formed at 92 K, is stable up to 230 K. At 280 K, it is clear that the alkylidyne decomposes, yielding adsorbed methylidyne (RuCH) and a new species whose RAIR spectrum is consistent with a partially dehydrogenated metallocycle, Ru3C–(CH2)5–Ru. At 290 K, residual hexylidyne is still present on the surface, co-adsorbed with a new symmetric metallocycle, Ru3C–(CH2)4–CRu3, and additional methylidyne. The decomposition of hexylidyne is complete at 300 K, methylidyne being the only product detectable by RAIRS. In spite of yielding the same final product (methylidyne) as 1-hexene and 3-hexyne on Ru(0 0 0 1), 1-hexyne follows a similar decomposition pathway to 1-hexene, which is rather different from 3-hexyne, allowing to conclude that the reaction mechanism is strongly dependent on the position of the functionality.