Activation of methane by Ti+: a cluster assisted mechanism for sigma-bond activation, experiment, and theory

Reactions of Ti+ with methane were studied by both temperature-dependent eq uilibrium measurements and density functional theory. Experimentally, we ob served Ti(CH4)(n)(+) clusters (n = 1-5) and the H-2 elimination products (C H4)Ti(CH3)(2)(+), Ti(CH3)(2)(+), and (CH4)(2)Ti(C2H4)(+). The binding energ ies for the Ti(CH4)(n)(+) clusters were measured to be 16.8 +/- 0.6, 17.4 /- 0.6, 6.6 +/- 1.5, 9.8 +/- 0.8, 5.1 +/- 0.7 kcal/mol for n = 1-5, respect ively. From analysis of the association entropies it was clear that the fir st solvation shell was completed at n = 4 and the fifth CH4 ligand began th e second shell. For the addition of the third methane ligand to Ti+, we obs erved a-bond activation to be competitive with adduct formation and dehydro genation of the cluster produced (CH4)Ti(CH3)(2)(+). Theoretically we chara cterized the Ti(CH4)(n)(+) clusters (n = 1-3) and reproduced the trend in b inding energies observed experimentally. We also calculated many local mini ma and several transition states on the potential energy surfaces for dehyd rogenation for n = 1-3. In agreement with experiment, we found dehydrogenat ion of the first methane to be highly unfavorable, dehydrogenation of the s econd to be slightly unfavorable, and dehydrogenation of the third to be sl ightly favorable under the given conditions. Moreover, addition of a fourth methane resulted in further dehydrogenation and formation of an ethylene l igand bound to the metal center, (CH4)(2)Ti(C2H4)(+). Hence, it appears tha t methane can be converted to ethylene in a cluster mediated sigma-bond act ivation mechanism using first row transition metal centers at thermal energ ies. (Int J Mass Spectrom 185/186/187 (1999) 989-1001) (C) 1999 Elsevier Sc ience B.V.