Reactions of actinide ions with pentamethylcyclopentadiene: atypical hydrocarbon activation

Gas-phase reactions of 1,2,3,4,5-pentamethylcyclopentadiene (HCp*) with thr ee actinide ions, Np+, Pu+, and Am+, were studied by laser ablation mass sp ectrometry. The primary goal was to determine if an unusual, non-insertion mode of C-H activation reported for lanthanide ions, Ln(+), with this subst rate obtains for the homologous actinide ions; the Cp* ligand is central in organoactinide chemistry and the interaction of the quasivalence 5f electr ons with HCp* is of special interest. It was found that Np+ and Pu+ behave similarly to the "reactive" Ln(+), and that Am+ is similar to the "inert" g roup of Ln(+). For bath Ln(+) and An(+), reactivity appears to reflect the energy necessary to excite the ground state metal ion to a configuration wi th two non-f valence electrons. Evidently Am+, which exhibits a large excit ation energy, comparable to those of the inert Ln(+) induces C-H activation and single-H-2-elimination from HCp* by a non-insertion process not seen w ith other alkenes; this lanthanide-like behavior suggests no distinctive ro le of the 5f electrons in the Am+-HCp* interaction. Activation by Np+ and P u+ of the methyl groups of a tetramethylfulvene intermediate results in dou ble- and triple-dehydrogenation which evidently proceeds by a conventional insertion process involving a C-An(+)-H complex, in accord with the relativ ely small Np+ and Pu+ excitation energies. Results for reactions of AnO(+) with HCp* revealed an enhanced dehydrogenation activity of NpO+ compared wi th LnO(+), which may be a result of the greater accessibility of high oxida tion states at the Np metal center. (Int J Mass Spectrom 202 (2000) 19-29) (C) 2000 Elsevier Science B.V.