Structures and reactivities of cycloheptane, cycloheptene, 1,3-cycloheptadiene, and cycloheptatriene on Pt(111)

The structures and reactivities of various cyclic C-7 hydrocarbons (cyclohe ptane, cycloheptene, 1,3-cycloheptadrene, and cycloheptatriene) adsorbed on Pt(lll) have been examined by means of temperature-programmed reaction, re flection-absorption infrared, and Auger electron spectroscopies. At tempera tures below 200 K, cycloheptane binds weakly to the surface and exhibits vi brational mode softening consistent with the presence of agostic C-H ... M interactions. In contrast, the cyclic alkenes cycloheptene, 1,3-cycloheptad iene, and cycloheptatriene all bind strongly (and largely irreversibly) to the surface principally by means of interactions with their pi-systems. The chemistry of these adsorbates at higher temperatures has been mapped out i n considerable detail. For cycloheptane, most simply desorbs but about 30% is dehydrogenated; interestingly, cycloheptene is not an intermediate in th is process. Cycloheptene first loses two hydrogen atoms to give what we pro pose is an eta(3),eta(1)-C7H10 product. 1,3-Cycloheptadiene first loses two hydrogen atoms to afford an eta(5)-cycloheptadienyl product, which dehydro genates further to give surface-bound cycloheptatriene in (apparently) high yield. Between 325 and 375 K, all of the cyclic C-7 hydrocarbons dehydroge nate further and eventually yield a planar eta(7)-cycloheptatrienyl (C7H7) species on the surface, although for cycloheptene this conversion is ineffi cient. The thermolytic decomposition of the eta(7)-C7H7 species begins at a bout 425 K and affords surface-bound hydrocarbons that are largely undetect able by RAIR spectroscopy. At higher temperatures (>800K), these hydrocarbo ns are completely dehydrogenated to form a carbonaceous overlayer. Ring con traction products, such as surface-bound benzene, are not observed as inter mediates in any of the thermolytic reactions. The RAIR spectra of the eta(5 )-cycloheptadienyl and cycIoheptatriene adsorbates both contain an unusual low-frequency vibrational band near 2770 cm(-1). This band is not indicativ e of an agostic C-H .. M interaction, bur is instead the signature of a met hylene C-H bond that has been perturbed by hyperconjugative drain of electr on density into the adjacent pi-system.