Structure—Reactivity Correlations in the Reactions of Hydrocarbons on Transition Metal Surfaces. 2. Hydrogenation of Norbomene and Bicyclo[2.2.2]octene on Platinum(111)

The reactivity of two bicyclic alkenes, bicyclo[2.2.2]octene (BOE) and norbornene (NBE), has been studied on Pt(111) in both the absence and the presence of co-adsorbed hydrogen. The inability of these alkenes to rearrange to alkylidyne species on the surface considerably alters their reaction chemistry. At 130 K, the alkenes are bound molecularly to the surface via two interactions: (1) a π di-σ interaction with a C=C double bond, and (2) an apparent agostic interaction with a C-H bond. The geometries of these bicyclic alkenes strongly suggest that they are interacting with three mutually-adjacent surface Pt atoms, but it is not clear from the data whether the alkene group bridges between two Pt atoms and the agostic interaction involves one Pt atom or vice versa. Several reactions ensue upon thermolysis. At ∼250 K, the agostic C-H bond is cleaved and a surface-bound alkyl intermediate is formed. The resulting surface-bound hydrogen atoms do not immediately desorb, but some of them transfer to unreacted BOE and NBE molecules to form the alkanes bicyclo[2.2.2]octane (BOA) and norbornane (NBA), respectively. The rate-determining step for this self-hydrogenation reaction is the dehydrogenation of BOE or NBE; these processes appear to follow first-order rate laws with activation energies of ∼16 kcal/mol. If the Pt(111) surface is first treated with hydrogen and then dosed with the bicyclic alkene, alkane is formed at lower temperatures (as low as 190 K) and in significantly greater amounts. In the presence of co-adsorbed D2, BOE and NBE are hydrogenated to a distribution of alkane isotopomers with up to four deuterium atoms per molecule; these observations suggest that the surface-bound alkyl intermediates can α-eliminate and reversibly form alkylidenes. Surface carbon atoms, when present at sufficiently high coverages, inhibit the hydrogenation and self-hydrogenation of these bicyclic alkenes due to the reduced ability of the carbonaceous Pt(111) surface to activate H-H or C-H bonds. At higher temperatures (470-520 K), both BOE and NBE eventually decompose to give benzene (part of which desorbs) and surface C_xH_y fragments. The latter decompose by ∼620 K to give a partial carbonaceous overlayer and H₂ gas.