Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO₄^–

Abstract

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, AnVO2(C2O4)−, where An = Pa or U, are essentially actinyl ions, AnVO2 +, coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, AnVO­(OH)2(C2O4)−. The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas PaVI is inaccessible, UVI is very stable. The UVO2(C2O4)− complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O2 to yield UO4 – and two CO2 molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, UVIO2 2+, coordinated in the equatorial plane by two equivalent O atoms each having formal charges of −1.5 and U–O bond orders intermediate between single and double. The unreactive nature of PaVO2(C2O4)− toward O2 is a manifestation of the resistance toward oxidation of PaV, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO4 –, reacts with water to yield UO5H2 –. Infrared spectra obtained for UO5H2 – confirm the computed lowest-energy structure, UO3(OH)2 –.

Hydrolysis of Small Oxo/Hydroxo Molecules Containing High Oxidation State Actinides (Th, Pa, U, Np, Pu): A Computational Study

“…30 As another example, Gibson and co-workers studied gasphase reactions of actinyl-oxolate complexes with H 2 O and O 2 in a quadrupole ion trap. 31 Their results showed that water addition to UO 4 was spontaneous, forming the hydrate, UO 4 (H 2 O) -, and the hydroxide, UO 3 (OH) 2 -.…”

“…Tsushima predicted that two Th­(OH) 4 (OH 2 ) 2 0 molecules can be connected via hydrogen bonds to form the OH-bridged dimer complex Th 2 (OH) 8 0 . As another example, Gibson and co-workers studied gas-phase reactions of actinyl-oxolate complexes with H 2 O and O 2 in a quadrupole ion trap . Their results showed that a water addition to UO 4 – was spontaneous, forming the hydrate UO 4 (H 2 O) − and the hydroxide UO 3 (OH) 2 – .…”

Collision‐induced dissociation of [UO2(NO3)(O2)]− and reactions of product ions with H2O and O2

“…The intra‐complex proton transfer to the dihydroxides in the present study is consistent with our previous study 15 and several intermediates in mechanisms proposed by Michelini and coworkers 38 for the reactions of gas‐phase uranates [i.e., UO x − ] with methanol. In addition, isomerization of H 2 O adducts to generate dihydroxides was investigated in detail by Vasiliu et al 39 and shown to be favorable for small actinyl species such as ThO 2 , PaO 2 + and UO 2 2+ .…”

Collision‐induced dissociation of [UVIO2(ClO4)]+ revisited: Production of [UVIO2(Cl)]+ and subsequent hydrolysis to create [UVIO2(OH)]+

“…Since then, ESI has been used to create gas‐phase uranyl species for a range of tandem mass spectrometry (MS/MS), anion photoelectron spectroscopy, and ion mobility studies . ESI and MS/MS have also been used to study and compare the intrinsic dissociation and ion‐molecule reactions of transuranic species …”