Structures and Reactivities of Cycloheptane, Cycloheptene, 1,3-Cycloheptadiene, and Cycloheptatriene on Pt(111)

The structures and reactivities of various cyclic C₇ hydrocarbons (cycloheptane, cycloheptene, 1,3-cycloheptadiene, and cycloheptatriene) adsorbed on Pt(111) have been examined by means of temperature-programmed reaction, reflection-absorption infrared, and Auger electron spectroscopies. At temperatures below 200 K, cycloheptane binds weakly to the surface and exhibits vibrational mode softening consistent with the presence of agostic C-H⋯M interactions. In contrast, the cyclic alkenes cycloheptene, 1,3-cycloheptadiene, and cycloheptatriene all bind strongly (and largely irreversibly) to the surface principally by means of interactions with their π-systems. The chemistry of these adsorbates at higher temperatures has been mapped out in considerable detail. For cycloheptane, most simply desorbs but about 30% is dehydrogenated; interestingly, cycloheptene is not an intermediate in this process. Cycloheptene first loses two hydrogen atoms to give what we propose is an η³,η¹-C₇H₁₀ product. 1,3-Cycloheptadiene first loses two hydrogen atoms to afford an η⁵-cycloheptadienyl product, which dehydrogenates further to give surface-bound cycloheptatriene in (apparently) high yield. Between 325 and 375 K, all of the cyclic C₇ hydrocarbons dehydrogenate further and eventually yield a planar η⁷-cycloheptatrienyl (C₇H₇) species on the surface, although for cycloheptene this conversion is inefficient. The thermolytic decomposition of the η⁷-C₇H₇ species begins at about 425 K and affords surface-bound hydrocarbons that are largely undetectable by RAIR spectroscopy. At higher temperatures (>800 K), these hydrocarbons are completely dehydrogenated to form a carbonaceous overlayer. Ring contraction products, such as surface-bound benzene, are not observed as intermediates in any of the thermolytic reactions. The RAIR spectra of the η⁵-cycloheptadienyl and cycloheptatriene adsorbates both contain an unusual low-frequency vibrational band near 2770 cm^-1. This band is not indicative of an agostic C-H⋯M interaction, but is instead the signature of a methylene C-H bond that has been perturbed by hyperconjugative drain of electron density into the adjacent π-system.