GAS-PHASE REACTIVITY OF LANTHANIDE CATIONS WITH HYDROCARBONS

The gas-phase reactions of all lanthanide cations Ln(+) (Ln = La-Lu, w ith the exception of Pm+) with several Linear, branched, and cyclic al kanes, cyclopropane, and alkenes have been examined by Fourier transfo rm ion cyclotron resonance mass spectrometry. This series of substrate s allows to evaluate estimates for the relative reactivities of Ln(+) cations with respect to C-H and C-C bond activation of hydrocarbons. N one of the Ln+ cations was found to react with methane, in accord with the unfavorable thermochemical situation for formation of a cationic carbene complex LnCH(2)(+) from Ln(+) and CH4. Very slow single dehydr ogenation of ethane is observed for La+ and Ce+. Ah acyclic alkanes la rger than ethane, as well as cyclopropane and cyclohexane, are only ac tivated by La+, Ce+, and Gd+, and the reaction rates approach the coll isional Limit with increasing polarizability. The nonreactivity of all other lanthanide cations toward alkanes provides experimental support for Schilling and Beauchamp's suggestion that a minimum of two non-f valence electrons is required for the activation of C-H or C-C bonds. In addition to La+, Ce+, and Gd+, Pr+ and Tb+, the two of the 6s(1)4f( n) configurated lanthanide cations with the lowest excitation energies to states with at least two non-f valence electrons, also activate pr opene but are unreactive with cyclopropane. The occurrence of C-H bond activation of propene by Pr+ and Tb+ is described in terms of a curve -crossing model in which an electronically excited asymptote of a stat e with two non-f electrons is involved en route to the products. With l-butene also Nd+, Dy+, Ho+, and Er+ cations mediate dehydrogenation, and only Sm+, Eu+, Tm+, and Yb+ are unreactive with this substrate; th ese are precisely those lanthanide cations which exhibit the largest e xcitation energies to states with at least two non-f electrons. Furthe rmore, the relative rates for the ion/molecule reactions are in qualit ative agreement with the curve-crossing model proposed for the reactio n of Pr+ and Tb+ with propene. Finally, with 1,4-cyclohexadiene as sub strate even Sm+, Eu+, and Tm+ mediate C-H activation to yield the corr esponding benzene/Ln(+) complexes.