Effect of geometrical isomerism on the reactivity of 3-hexene on clean Ru(0 0 1)

The chemical behaviour of E-3-hexene on clean Ru(0 0 1) was studied by RAIRS, under UHV conditions, and compared to that of its geometrical isomer, Z-3-hexene. It is shown that, above 140 K, E-3-hexene dehydrogenates to 3-hexyne di-σ/π complex, which, by thermal activation above 170 K, undergoes C2–C3 and C4–C5 bond breaking, yielding ethylidyne (CCH3) and ethyne di-σ/π complex. However, while above ∼120 K the Z-isomer is completely decomposed, E-3-hexene is remarkably stable, since a significant fraction of physically adsorbed molecules remain on the Ru(0 0 1) surface up to 240 K. By direct adsorption at high temperatures (e.g. 220 K), the decomposition product detected by RAIRS is ethylidyne and, less clearly, the 3-hexyne di-σ/π complex, co-adsorbed with unreacted E-3-hexene. On the contrary, in the same adsorbing conditions, Z-3-hexene follows a second decomposition path involving CC bond scission, with formation of propylidyne and ethylidyne. These results highlight the importance of geometrical isomerism on the stability and decomposition paths of C6 alkenes on Ru(0 0 1), since it may modify the accessibility of the double bond.