Gas-phase reactions of bare and oxo-ligated actinide and lanthanide cations with pentamethylcyclopentadiene studied in a quadrupole ion trap mass spectrometer

Abstract

Reactions of bare and oxo-ligated monopositive ions of uranium and thorium with 1,2,3,4,5-pentamethylcyclopentadiene, C 10H 16, (HCp ∗) were examined in a quadrupole ion trap (QIT) mass spectrometer. Representative lanthanide ions, Ln + and LnO +, and tantalum ions, Ta + and TaO +, were studied for comparison. The product branching ratios for both primary and secondary reactions of the actinide ions demonstrated gas-phase organoactinide chemistry that is quite disparate from organolanthanide chemistry under comparable conditions for this neutral reactant. Particularly revealing were product distributions for ThO + and UO +, which indicated chemical behavior similar to that of bare Sm +. We conclude that at least one valence electron at the metal center of the actinide oxide ions must remain chemically active. In the case of UO +, this provides evidence for the chemical engagement of the quasi-valence 5f electrons, which is in distinct contrast to the inert character of the 4f electrons of the lanthanides in both Ln + and LnO +. Mass-selective chemistry of two primary products, UC 10H 10 + and UC 9H 8 +, also showed behavior similar to that of Sm + and UO +, implying that there are two covalent organouranium bonds in these complex ions. In comparing the QIT results for the lanthanides with those from a low-pressure ion cyclotron resonance (ICR) mass spectrometry study [Organometallics 16 (1997) 3845], qualitative agreement was found, but significant quantitative differences were apparent. Based on results from collision-induced dissociation and effects of variations in bath gas pressure in the QIT, we conclude that the discrepancies arise from the very different pressure regimes in the ICR and QIT. Evidently, the QIT can be operated over a range of pressures that manifest effects of collisional cooling for some reactions. For the lowest pressure QIT experiments, the high degree of fragmentation is reminiscent of the ICR results. We propose that the QIT bath gas can essentially act as an inert “solvent,” which serves to mediate high-energy processes due to energy transfer from nascent hot intermediate products via energy-dissipating collisions.