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

Reactions of Ti+ with methane were studied by both temperature-dependent equilibrium measurements and density functional theory. Experimentally, we observed Ti(CH4)(n)(+) clusters (n = 1-5) and the H-2 elimination products (CH4)Ti(CH3)(2)(+), Ti(CH3)(2)(+), and (CH4)(2)Ti(C2H4)(+). The binding energies 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, respectively. From analysis of the association entropies it was clear that the first solvation shell was completed at n = 4 and the fifth CH4 ligand began the second shell. For the addition of the third methane ligand to Ti+, we observed a-bond activation to be competitive with adduct formation and dehydrogenation of the cluster produced (CH4)Ti(CH3)(2)(+). Theoretically we characterized the Ti(CH4)(n)(+) clusters (n = 1-3) and reproduced the trend in binding energies observed experimentally. We also calculated many local minima and several transition states on the potential energy surfaces for dehydrogenation for n = 1-3. In agreement with experiment, we found dehydrogenation of the first methane to be highly unfavorable, dehydrogenation of the second to be slightly unfavorable, and dehydrogenation of the third to be slightly favorable under the given conditions. Moreover, addition of a fourth methane resulted in further dehydrogenation and formation of an ethylene ligand bound to the metal center, (CH4)(2)Ti(C2H4)(+). Hence, it appears that methane can be converted to ethylene in a cluster mediated sigma-bond activation mechanism using first row transition metal centers at thermal energies. (Int J Mass Spectrom 185/186/187 (1999) 989-1001) (C) 1999 Elsevier Science B.V.