A Uranyl Peroxide Dimer in the Gas Phase

Abstract

The gas-phase uranyl peroxide dimer, [(UO)(O)(L)] where L = 2,2'-trifluoroethylazanediyl)bis(N,N'-dimethylacetamide), was synthesized by electrospray ionization of a solution of UO and L. Collision-induced dissociation of this dimer resulted in endothermic O atom elimination to give [(UO)(O)(L)], which was found to spontaneously react with water via exothermic hydrolytic chemisorption to yield [(UO)(OH)(L)]. Density functional theory computations of the energies for the gas-phase reactions are in accord with observations. The structures of the observed uranyl dimer were computed, with that of the peroxide of particular interest, as a basis to evaluate the formation of condensed phase uranyl peroxides with bent structures. The computed dihedral angle in [(UO)(O)(L)] is 145°, indicating a substantial deviation from the planar structure with a dihedral angle of 180°. Energies needed to induce bending in the most elementary gas-phase uranyl peroxide complex, [(UO)(O)], were computed. It was found that bending from the lowest-energy planar structure to dihedral angles up to 140° required energies of <10 kJ/mol. The gas-phase results demonstrate the inherent stability of the uranyl peroxide moiety and support the notion that the uranyl-peroxide-uranyl structural unit is intrinsically planar, with only minor energy perturbations needed to form the bent structures found in studtite and uranyl peroxide nanostructures.

Destruction of the Uranyl Moiety in a U(V) “Cation–Cation” Interaction

“…The U=Ot distances in C, 1.80 Å at the B3LYP level (1.827 Å at PBE), are consistent with typical U=Oyl distances, and the U-Ob distances, 2.11 Å, are similar to typical U-O single bond distances, such as terminal U-OH 41 and bridging U-Ob-U. 71 The basis for the semi-empirical bond-valence approach to estimating oxidation states was described 90 years ago by Pauling, 82 and subsequently elaborated by Brown. [83][84][85] Like other such sound and persistent models, bond-valence is necessarily somewhat qualitative but has proved sufficiently valid that it remains relevant.…”

“…A particularly remarkable result is CID elimination of O2 from bridging-peroxide dimer B to yield 71 Our calculations indicate that conversion of B gives C occurs with reduction of both U(VI) to U(V). Reaction (2) can be formulated as (2') in which the bridging O2 is eliminated while retaining the +2 charge and dimeric structure, by creating a direct unmediated CCI interaction between two U(V) moieties.…”

“…We initially presumed that A consists of two uranyl moieties linked by a bridging peroxide, in analogy with previously reported [(DMA)-(UO2)-(O2)-(UO2)-(DMA)] 2+ where terminal donor ligands DMA are 2,2'-trifluoroethylazanediyl-bis-N,N'-dimethylactamide. 71 We tentatively postulate that…”

Structural and computational characterization of a bridging zwitterionic-amidoxime uranyl complex

“…The complex is puckered ( Figure 2B), with uranyl oxygens tipped away from the tautomeric oxime nitrogen atoms, resembling a curvature evocative of uranyl-peroxide dimers and clusters. [25][26][27][28][29] Convergent and divergent uranyl oxygens are separated by 3.256(5) Å and 4.807(5) Å, respectively, while the uranium centers are 4.0513(3) Å apart. Unremarkably, the uranyl group is linear and symmetrical with U-O oxo bond lengths of 1.757(3) Å and 1.764(3) Å and a O=U=O angle of 178.83(12)° (Table S1).…”

Performance of group additivity methods for predicting the stability of uranyl complexes

“…Actually, the size of such clusters is greatly influenced by the pH, ranging from 20 up to 120 uranium atoms . Further studies have proved that the bending of the U─O 2 ─U dihedral angle encourages curved shapes and thus cage cluster formation . In addition, classical molecular dynamic simulations have been developed so as to address the role of counter cations in the solubility and size of such systems .…”