Carbon−Hydrogen Bond Activation and Cyclodehydrogenation Reactions of Cyclic C₈ Hydrocarbons on Pt(111)

Abstract

The mechanisms of the thermolytic decomposition of a series of unsaturated cyclic C8 hydrocarbons adsorbed on a platinum(111) single crystal surface are described. This study both confirms and extends the results reported by Frei and Campbell which corrected errors in an earlier report from our laboratory on the mechanisms of thermal decomposition seen in this adsorbate system. We find that the dehydrocyclization of cyclooctene, cyclooctadiene (1,3 and 1,5 isomers), and cyclooctatetraene on Pt(111) proceeds through a related set of intermediates and ultimately yields a novel surface-bound bicyclic ring system. As noted by Frei et al., our earlier finding that benzene is produced in high yield is incorrect. The spectroscopic evidence reported here demonstrates that all four of the cyclic C8 hydrocarbons form a bicyclo[3.3.0]octenyl (or, as more commonly named, a pentalenyl) intermediate of stoichiometry C8H6. This bicyclic ring system forms via a dehydrocyclization of cyclooctatetraene, which itself is formed in varying yields from each of the less-unsaturated c-C8 adsorbates. The formation of the pentalenyl species proceeds at low temperatures (<375 K). Data from reflection−absorption infrared (RAIRS) and temperature-programmed reaction spectroscopies show that the pentalenyl intermediate is remarkably stable, persisting on the surface up to at least 450 K. At temperatures above 450 K, this intermediate fragments, liberating hydrogen, and leaving a carbonaceous overlayer. The formation of cyclooctatetraene, and thus the pentalenyl intermediate, proceeds with the highest efficiency for the two dienes and lowest for cyclooctane and cis-cyclooctene. RAIRS data further serve to identify several intermediates formed along the reaction pathway leading to the platinum-bound C8H6 species.

Unprecedented triphosphinine iron interactions: Intramolecular electron transfer, reactivity round a corner, and a low-activated ring element exchange reaction

“…A quantitative dehydrogenation reaction of 1,5-COD to form pentalene is catalyzed at Pt-surfaces. As for the observations of this study, the process requires no heating [20]. Dehydrogenation of a cyclooctenyl ligand to form the same h 5 -trihydropentalenyl ligand of 13 was reported for [(h 3 -cyclooctenyl)(h 4 -1,5-COD)Co] in the presence of free 1,5-COD as an intermolecular hydrogen acceptor [21].…”

[(1,5‐Cyclooctadiene)(toluene)ruthenium(0)]: A Novel Precursor for the MOCVD of Thin Ruthenium Films

“…The transforma- tion of pentalene into carbon and hydrogen is a known process. [24] When the CVD temperature was increased to 350 C, additional gas phase species became observable, probably related to the C±C bond activation of the free ligands, and the effect became even more apparent at a deposition temperature of 450 C. Figure 8b illustrates the consequences on the composition of organic compounds in the exhaust gas. When compared to Figure 8a, a dramatic drop of the 1,5-COD concentration has to be stated, whereas novel cyclic C 8 species such as styrene and ethylbenzene are growing in.…”

Decomposition of Saturated Hydrocarbons Adsorbed on Ni(755): Comparison of Decomposition Starting Temperatures among Cyclic and Straight-Chain Hydrocarbons

“…The ratio of desorption-limited H 2 peak area ( X ) to the reaction-limited one ( Y ) in TPD spectra for hydrogen nearly equals one (i.e., X / Y ≈ 1; the sum of X and Y is normalized to the total number of hydrogen atoms in a hydrocarbon), indicating that each carbon atom of cyclic hydrocarbons releases one hydrogen atom on average when decomposition to dehydrogenerated intermediates is completed. Although the error bars for X and Y are ±1 or 2 due to the poor separation of the peaks, dehydrogenated intermediate species which have stoichiometries similar to those in Table are reported for Pt(111). ,, For the dehydrogenation of the cyclic C 5 and C 7 hydrocarbons, stable planar intermediates have been identified by IRAS to be η 5 -cyclopentadienyl (C 5 H 5 ) 18 and η 7 -cycloheptatrienyl (C 7 H 7 ) species, respectively. The dehydrogenation of cyclic C 8 hydrocarbons proceeds through sequential C−H bond cleavage steps to transient formation of cyclooctatetraene (C 8 H 8 ), which is converted via a ring-closure process to the stable bicyclic ring species of stoichiometry C 8 H 6 …”

“…Although the error bars for X and Y are ±1 or 2 due to the poor separation of the peaks, dehydrogenated intermediate species which have stoichiometries similar to those in Table are reported for Pt(111). ,, For the dehydrogenation of the cyclic C 5 and C 7 hydrocarbons, stable planar intermediates have been identified by IRAS to be η 5 -cyclopentadienyl (C 5 H 5 ) 18 and η 7 -cycloheptatrienyl (C 7 H 7 ) species, respectively. The dehydrogenation of cyclic C 8 hydrocarbons proceeds through sequential C−H bond cleavage steps to transient formation of cyclooctatetraene (C 8 H 8 ), which is converted via a ring-closure process to the stable bicyclic ring species of stoichiometry C 8 H 6 …”