Investigation of cyclooctatetraene on reduced TiO2(001) as a possible intermediate in alkyne cyclotrimerization
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abstract
Cyclotrimerization of alkynes to form aromatics can be carried out catalytically with low valent transition metal complexes in solution, with supported and single-crystal metal catalysts, and with partially reduced TiO2 surfaces. In each of these cases the reaction has been proposed to proceed via formation of a metallacyclopentadiene intermediate, i.e., by association of a pair of acetylene molecules to form a C4 ligand bound to the catalytic site. At least one group of authors has proposed that on late-transition metal surfaces a pair of C4 ligands combine to form the C8 ligand, cyclooctatetraene (COT). In this alternative mechanism, benzene is then formed by the elimination of a C2 fragment from the C8 intermediate. This reaction channel was previously examined on Pd(111) and Cu(110), and was found to be a minor or nonexistent channel on each, reinforcing the case for direct addition of a third acetylene molecule to the metallacyclopentadiene intermediate to form the aromatic product. We have examined the reactions of 1,3,5,7-cyclooctatetraene on reduced TiO2 surfaces to determine whether this species can be converted to benzene, and if so, whether the kinetics of this reaction is compatible with a COT intermediate in alkyne cyclotrimerization. It was found that the reduced surface of TiO2(001) does convert COT into benzene, although at a higher temperature (550 K) than that at which acetylene cyclotrimerizes on similarly reduced surfaces (400 K). This suggests that the COT channel is, at best, a minor route to benzene in the course of acetylene cyclooligomerization on this oxide.
