Formation of Bare UO₂²⁺and NUO⁺by Fragmentation of Gas-Phase Uranyl–Acetonitrile Complexes

Abstract

In a prior study [Van Stipdonk; et al. J. Phys. Chem. A 2006, 110, 959-970], electrospray ionization (ESI) was used to generate doubly charged complex ions composed of the uranyl ion and acetonitrile (acn) ligands. The complexes, general formula [UO2(acn)n](2+), n = 0-5, were isolated in an 3-D quadrupole ion-trap mass spectrometer to probe intrinsic reactions with H2O. Two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-exchange reactions. For the former, the intrinsic tendency to add H2O was dependent on the number and type of nitrile ligand. For the latter, charge exchange involved primarily the formation of uranyl hydroxide, [UO2OH](+), presumably via a collision with gas-phase H2O and the elimination of a protonated nitrile ligand. Examination of general ion fragmentation patterns by collision-induced dissociation, however, was hindered by the pronounced tendency to generate hydrated species. In an update to this story, we have revisited the fragmentation of uranyl-acetonitrile complexes in a linear ion-trap (LIT) mass spectrometer. Lower partial pressures of adventitious H2O in the LIT (compared to the 3-D ion trap used in our previous study) minimized adduct formation and allowed access to lower uranyl coordination numbers than previously possible. We have now been able to investigate the fragmentation behavior of these complex ions completely, with a focus on tendency to undergo ligand elimination versus charge reduction reactions. CID can be used to drive ligand elimination to completion to furnish the bare uranyl dication, UO2(2+). In addition, fragmentation of [UO2(acn)](2+) generated [UO2(NC)](+), which subsequently fragmented to furnish NUO(+). Formation of the nitrido by transfer of N from cyanide was confirmed using precursors labeled with (15)N. The observed formation of [UO2(NC)](+) and NUO(+) was modeled by density functional theory.

Gas Phase Reactions of Ions Derived from Anionic Uranyl Formate and Uranyl Acetate Complexes

“…However, in a recent investigation of the fragmentation behavior of UO 2 2+ complexes containing acn [27], we showed that the amount of adventitious H 2 O in the LIT is significantly lower than in the 3-D ion trap employed in our previous studies. CID was used to drive complete acn ligand elimination to generate UO 2 2+ , and this was the first reported generation of the bare di-cation by multiple-stage CID.…”

Collision‐induced dissociation of uranyl‐methoxide and uranyl‐ethoxide cations: Formation of UO 2 H + and uranyl‐alkyl product ions

“…Formation of [MCH 3 ] + (M = metal) product ions by heterolytic C–C bond cleavage has been reported by Wu et al following CID of transition‐metal complexes of composition [M 2 + (acetone) 2 ] 2+ . Heterolytic cleavage to liberate CH 3 has also been observed following CID of metal–acetonitrile complexes, including those that contain UO 2 2 + . However, in this case the CN − fragment remains coordinated to the metal ion.…”

“…Recently, we began to revisit the CID of uranyl complexes using a linear quadrupole ion trap (LIT). In an investigation of the fragmentation behavior of uranyl complexes containing acetonitrile (acn), we showed that the amount of adventitious H 2 O in the LIT is significantly lower than in the 3‐D ion trap employed in our previous studies. The low partial pressure of the H 2 O allowed the fragmentation chemistry of the species to be studied more completely.…”

“…[48,49] [50] Heterolytic cleavage to liberate CH 3 has also been observed following CID of metal-acetonitrile complexes, including those that contain UO 2 2+ . [38,51] However, in this case the CN À fragment remains coordinated to the metal ion.…”

Speciation of uranium‐mandelic acid complexes using electrospray ionization mass spectrometry and density functional theory

“…[11] The gentle ionization process involved in this technique is suitable for transferring the species from solution to the gas phase with the retention of stoichiometry. In recent years, the formation of uranyl complexes has been studied in great detail using ESI-MS. [12][13][14][15][16][17] For example, direct infusion of solution into the mass spectrometer has helped to ascertain the stoichiometry and stability constants of uranyl complexes with 1hydroxyethane-1,1'-diphosphonic acid (HEDP). The mass spectrometric data were found to be in close agreement with those derived from conventional techniques such as potentiometry and spectroscopy.…”