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Activation of double and triple bonds in C 6 unsaturated hydrocarbons by the Ru(001) surface: an overview
Author(s) -
Garcia Ana R.,
Barros Ricardo B.,
Ilharco Laura M.
Publication year - 2008
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1366
Subject(s) - chemistry , degree of unsaturation , double bond , steric effects , alkyne , thermal decomposition , hexene , photochemistry , triple bond , ruthenium , dehydrogenation , infrared spectroscopy , medicinal chemistry , stereochemistry , crystallography , polymer chemistry , catalysis , organic chemistry , ethylene
The decomposition patterns of hexene and hexyne isomers on clean Ru(001), under ultra‐high vacuum, are overviewed and correlated with functionality and isomerism. The key surface intermediates and products were identified by reflection absorption infrared spectroscopy (RAIRS). The results evidence the relevant role of the unsaturation position over its nature. The effects of steric hindrance and geometrical isomerism are apparent only in the inhibition of some decomposition pathways. The RAIRS data show that 1‐hexene chemisorbs at low temperature (∼90 K) and coverage as a di‐σ complex, whereas 1‐hexyne forms a di‐σ/π complex. By thermal activation, both these species dehydrogenate (in C1), yielding hexylidyne [ µ 3 ‐ η 1 ‐C(CH 2 ) 4 CH 3 ], which further decomposes (at 280–290 K) into surface metallocycles, [Ru 3 C(CH 2 ) 4 CH 2 ‐Ru] and [Ru 3 C(CH 2 ) 4 CRu 3 ]. Eventually, at 300 K, complete CC bond breaking occurs, yielding just adsorbed methylidyne [ µ 3 ‐ η 1 ‐CH]. The hexene and hexyne isomers with the unsaturation between secondary carbons may follow two surface‐assisted decomposition mechanisms. At low temperatures, they adsorb as the corresponding alkyne di‐σ/π complex, which implies a rehybridization of the sp 2 (or sp) carbons with reduction of the bond order, plus, for alkenes, dehydrogenation at the same carbons. These complexes decompose by breaking the CC bonds adjacent to the surface anchors: C1C2 and C3C4 in the case of the 2‐isomer, yielding methylidyne, ethyne di‐σ/π complex [ µ 3 ‐ η 2 ‐CHCH] and propylidyne [ µ 3 ‐ η 1 ‐CCH 2 CH 3 ], and C2C3 and C4C5 in the 3‐isomer, with the formation of the ethyne di‐σ/π complex and ethylidyne [ µ 3 ‐ η 1 ‐CCH 3 ]. The second decomposition path occurs upon direct adsorption at the reaction temperatures. It involves the scission of the multiple bond, with the formation of shorter chain alkylidynes: propylidyne (for 3‐hexyne and Z‐3‐hexene), ethylidyne and butylidyne [ µ 3 ‐ η 1 ‐C(CH 2 ) 2 CH 3 ] (for 2‐hexyne). The reactivity of Z‐2‐hexene revealed to be different, since no evidence was found for the second decomposition path. This was ascribed to a reduced accessibility of the double bond to the surface, due to a steric hindrance effect of the alkyl chain. The influence of geometrical isomerism was particularly clear in the decomposition of E‐3‐hexene, which has a remarkable stability. Copyright © 2008 John Wiley & Sons, Ltd.