Sequential dehydrogenation of unsaturated cyclic C-5 and C-6 hydrocarbons on Pt(111)

The structures and reactivities of various cyclic C-5 and C-6 hydrocarbons (cyclopentene, cyclopentadiene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cy clohexadiene) adsorbed on Pt(111) have been examined by means of reflection -absorption infrared (RAIR) spectroscopy. At temperatures below 200 K, thes e molecules bind intact to the Pt(lll) surface by means of strong interacti ons with C=C double bonds. Steric interactions between the surface and cert ain CH2 groups on the ring systems figure prominently in determining the co nformation adopted in the bound states of several of the molecular adsorbat es (e.g., cyclopentene and cyclohexene). These bonding habits are identifie d 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 temperature s above 200 K, the Cs species are dehydrogenated in high yield to a planar, surface-bound pentahaptocyclopentadienyl species (eta(5)-C5H5) while the C -6 cyclic hydrocarbons react to give benzene; these surface-bound products, which are stable to temperatures >400 K, have been identified in earlier s tudies as well. The present work adds to the understanding of the nature an d energetics of the sequential C-H bend activation processes involved in th eir formation. In addition, several intermediates lying along the reaction pathways to the respective planar intermediates have been identified and sp ectroscopically characterized for the first time. Most notably, we observe that 1,3-cyclohexadiene loses one hydrogen between 200 and 250 K to give a stable eta(5)-cyclohexadienyl intermediate. An efficient partial dehydrogen ation of cyclopentene at 250 K to give the corresponding diene is also obse rved. Our data also demonstrate the importance of heretofore unappreciated hyperconjugation effects in the vibrational spectroscopy of the C-H stretch ing modes of metal-surface-bound pi systems. The insights developed in this study regarding such electronic interactions are used to develop an unders tanding of the binding sites and conformational states adopted by the vario us adsorbates and intermediates formed during their decomposition.