GAS-PHASE REACTIVITY OF LANTHANIDE CATIONS WITH HYDROCARBONS

The gas-phase reactions of all lanthanide cations Ln(+) (Ln = La-Lu, with the exception of Pm+) with several Linear, branched, and cyclic alkanes, cyclopropane, and alkenes have been examined by Fourier transform ion cyclotron resonance mass spectrometry. This series of substrates allows to evaluate estimates for the relative reactivities of Ln(+) cations with respect to C-H and C-C bond activation of hydrocarbons. None 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 dehydrogenation of ethane is observed for La+ and Ce+. Ah acyclic alkanes larger than ethane, as well as cyclopropane and cyclohexane, are only activated by La+, Ce+, and Gd+, and the reaction rates approach the collisional 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 propene 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 state 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; these are precisely those lanthanide cations which exhibit the largest excitation energies to states with at least two non-f electrons. Furthermore, the relative rates for the ion/molecule reactions are in qualitative agreement with the curve-crossing model proposed for the reaction of Pr+ and Tb+ with propene. Finally, with 1,4-cyclohexadiene as substrate even Sm+, Eu+, and Tm+ mediate C-H activation to yield the corresponding benzene/Ln(+) complexes.