Activation of double and triple bond in C-6 unsaturated hydrocarbons by the Ru(001) surface: an overview

摘要

The decomposition patterns of hexene and hexyne isomers on clean Ru(001), under ultra-high vaccum, are over-viewed and correlated with functionality and isomerism. The key surface intermediates and products were identified by reflection absorbtion infrared spectroscopy (RAIRS). The results evidence the relavant role of the unsatuaration position over its nature. The effects of steric hindrane and geometrical isomersim are apparent only in the inhibition of some decomposition pathways. The RAIRS data show that 1-hexene chemisorbs at low temperature (similar to 90K) and coverage as a di-sigma complex, whereas 1-hexyne forms a di-sigma/pi complex. By thermal activation, both these species dehydrogenate (in C1), yielding hexylidyne [mu(3) -eta(1)-C(CH2)(4)CH3], which further decomposes (at 280-290 K) into surface metallocycles, [Ru3 C(CH2)(4)CH2-Ru] and [Ru-3 C(CH2)(4)C Ru-3]. Eventually, at 300 K, complete C-C bond breaking occurs, yielding just adsorbed methylidyne [mu(3)-eta(1)-CH]. The hexen and hexyne isomers with the unsatuation between secondary carbons may follow two surface-assisted decomposition mechanisms. At low temperatures they adsrob as the corresponding alkyne di-sigma/pi complex, which implies a rehybridization of the sp(2) (or sp) carbons with reduction of the bond order, plus, for alkense, dehydrogenation at the same carbons. These complexes decomposes by breaking the CC bonds ajacent to the surface anchors: C1-C2 and C3-C4 in the case of the 2-isomer, yielding methylidyne, ethyne di-sigma/pi cmplexp [mu(3)-eta(2)-CHCH] and propylidyne [mu(3)-eta(1)-CCH2CH3], and C2-C3 and C4-C5 in the methylidyne, ethyne di-sigma/pi complex [mu(3)-eta(2)-CHCH] and propylidyne [mu(3)-eta(1)-CCH2CH3], and C2-C3 and C4-C5 in the 3-isomer, with the formation of the ethyne di-sigma/pi complex and ethylidyne [mu(3)-eta(1)-CCH3]. The second decomposition path occurs upon direct adsroption at the reaction temperatures. It involvves the scission of the multiple bond, with the formation of shorter chain alkylidnes: propylidney (for 3-hexyne and Z-3-hexene), ethylidyne and butylidyne [mu(3)-eta(1)-C(CH2)(2)CH3] (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 gemetrical isomersim was particularly clear in the decomposion of E-3-hexen, which has a remarkable stability.