Gas-phase reactions of americium ion, Am+, with alkenes

The investigation of gas-phase activation of alkene C-H bonds by f-element metal ions has been extended to americium (Am+) using the technique of laser ablation with prompt reaction and detection; reactions were carried out with ethene, l-butene, 2-butene, cyclohexene, benzene, 1,5-cyclooctadiene (COD), and cyclooctatetraene (COT). The product (Am-L-*)(+) compositions and abundances were compared with those for lighter An(+) and Ln(+). For the linear and cyclic Cs alkene reactants, Am+ was found to be essentially inert-only a minuscule amount of the (Am+-benzene) condensation product was detected. Reactions with COD and COT produced the dehydrogenation complexes, {Am-C8H8}(+) and (Am-C8H6)(+), though in lower yields than for lighter An(+) Overall, the C-H activation efficacy of Am+ was smaller than that for the preceding An(+) but was closer to that of the relatively unreactive lanthanide ion Tm+. This result is interpreted to indicate that the 5f electrons of Am+, like the 4f electrons of the Ln(+), do not substantially participate in C-H (or C-C) activation. Apparently, ground-state Am+ with its [Rn]5f(7)7s(1) orbital occupancy must be excited to the [Rn]5f(6)6d(1)7s(1)-state, 245 kJ mol-l above ground, which comprises two chemically active 6d/7s electrons and is capable of forming a C-Am+-H dehydrogenation intermediate. The greatly enhanced reactivity of Am+ with COD and COT compared with the smaller alkenes can be rationalized in the context of a "curve-crossing" model for the C-H activation process.