Gas-Phase Reactions of Molecular Oxygen with Uranyl(V)
Anionic ComplexesSynthesis and Characterization of New Superoxides
of Uranyl(VI)
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Abstract
Gas-phase
complexes of uranyl(V) ligated to anions X<sup>–</sup> (X =
F, Cl, Br, I, OH, NO<sub>3</sub>, ClO<sub>4</sub>, HCO<sub>2</sub>, CH<sub>3</sub>CO<sub>2</sub>, CF<sub>3</sub>CO<sub>2</sub>, CH<sub>3</sub>COS, NCS, N<sub>3</sub>), [UO<sub>2</sub>X<sub>2</sub>]<sup>−</sup>, were produced by electrospray ionization and
reacted with O<sub>2</sub> in a quadrupole ion trap mass spectrometer
to form uranyl(VI) anionic complexes, [UO<sub>2</sub>X<sub>2</sub>(O<sub>2</sub>)]<sup>−</sup>, comprising a superoxo ligand.
The comparative rates for the oxidation reactions were measured, ranging
from relatively fast [UO<sub>2</sub>(OH)<sub>2</sub>]<sup>−</sup> to slow [UO<sub>2</sub>I<sub>2</sub>]<sup>−</sup>. The reaction
rates of [UO<sub>2</sub>X<sub>2</sub>]<sup>−</sup> ions containing
polyatomic ligands were significantly faster than those containing
the monatomic halogens, which can be attributed to the greater number
of vibrational degrees of freedom in the polyatomic ligands to dissipate
the energy of the initial O<sub>2</sub>-association complexes. The
effect of the basicity of the X<sup>–</sup> ligands was also
apparent in the relative rates for O<sub>2</sub> addition, with a
general correlation between increasing ligand basicity and O<sub>2</sub>-addition efficiency for polyatomic ligands. Collision-induced dissociation
of the superoxo complexes showed in all cases loss of O<sub>2</sub> to form the [UO<sub>2</sub>X<sub>2</sub>]<sup>−</sup> anions,
indicating weaker binding of the O<sub>2</sub><sup>–</sup> ligand
compared to the X<sup>–</sup> ligands. Density functional theory
computations of the structures and energetics of selected species
are in accord with the experimental observations