Investigation of cyclooctatetraene on reduced TiO2(001) as a possible intermediate in alkyne cyclotrimerization

Cyclotrimerization of alkynes to form aromatics can be carried out catalyti cally with low valent transition metal complexes in solution, with supporte d and single-crystal metal catalysts, and with partially reduced TiO2 surfa ces. In each of these cases the reaction has been proposed to proceed via f ormation of a metallacyclopentadiene intermediate, i.e., by association of a pair of acetylene molecules to form a C-4 ligand bound to the catalytic s ite. At least one group of authors has proposed that on late-transition met al surfaces a pair of C-4 ligands combine to form the C-8 ligand, cycloocta tetraene (COT). In this alternative mechanism, benzene is then formed by th e elimination of a C-2 fragment from the C-8 intermediate. This reaction ch annel was previously examined on Pd(lll) and Cu(110), and was found to be a minor or nonexistent channel on each, reinforcing the case for direct addi tion of a third acetylene molecule to the metallacyclopentadiene intermedia te to form the aromatic product. We have examined the reactions of 1,3,5,7- cyclooctatetraene on reduced TiO2 surfaces to determine whether this specie s can be converted to benzene, and if so, whether the kinetics of this reac tion is compatible with a COT intermediate in alkyne cyclotrimerization. It was found that the reduced surface of TiO2(001) does convert COT into benz ene, although at a higher temperature (550 K) than that at which acetylene cyclotrimerizes on similarly reduced surfaces (400 K),This suggests that th e COT channel is, at best, a minor route to benzene in the course of acetyl ene cyclooligomerization on this oxide.