Sequential Dehydrogenation of Unsaturated Cyclic C5 and C6 Hydrocarbons on Pt(111)

The structures and reactivities of various cyclic C5 and C6 hydrocarbons (cyclopentene, cyclopentadiene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene) adsorbed on Pt(111) have been examined by means of reflection-absorption infrared (RAIR) spectroscopy. At temperatures below 200 K, these molecules bind intact to the Pt(111) surface by means of strong interactions with CdC double bonds. Steric interactions between the surface and certain CH2 groups on the ring systems figure prominently in determining the conformation adopted in the bound states of several of the molecular adsorbates (e.g., cyclopentene and cyclohexene). These bonding habits are identified both by observing significant electronic interactions that weaken certain C-H bonds (so-called mode softening) and also by developing analogies with trends seen with similar ring systems and complexes. At higher temperatures above 200 K, the C5 species are dehydrogenated in high yield to a planar, surface-bound pentahaptocyclopentadienyl species (η-C5H5) while the C6 cyclic hydrocarbons react to give benzene; these surface-bound products, which are stable to temperatures >400 K, have been identified in earlier studies as well. The present work adds to the understanding of the nature and energetics of the sequential C-H bond activation processes involved in their formation. In addition, several intermediates lying along the reaction pathways to the respective planar intermediates have been identified and spectroscopically characterized for the first time. Most notably, we observe that 1,3cyclohexadiene loses one hydrogen between 200 and 250 K to give a stable η5-cyclohexadienyl intermediate. An efficient partial dehydrogenation of cyclopentene at 250 K to give the corresponding diene is also observed. Our data also demonstrate the importance of heretofore unappreciated hyperconjugation effects in the vibrational spectroscopy of the C-H stretching modes of metal-surface-bound π systems. The insights developed in this study regarding such electronic interactions are used to develop an understanding of the binding sites and conformational states adopted by the various adsorbates and intermediates formed during their decomposition.