Impacts of Oxo Interactions on Np(V) Crown Ether Complexes

Abstract

Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation-cation interactions) are dependent on the strength of the actinyl bond. These cation-cation interactions are prominently observed for the neptunyl cation [Np(V)O] and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np(V,VI)O], compounds: Np1a ([NpO(18-crown-6)]ClO), Np1b ([NpO(18-crown-6)]AuCl), Na-Np ([Np(V)O(18-crown-6)(Na(HO)(18-crown-6)][Np(VI)OCl], Np-Np ([NpO(18-crown-6)](NpOClNO)], and Np-Cl (NpOCl(HO)). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (Np═O) bond lengths are not perturbed upon interaction with the Na cation (Na-Np), but elongation is observed upon formation of a neptunyl-neptunyl interaction (Np-Np). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl-neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl-neptunyl motif, which also results in activation of the ν asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations.

Use of vibrational spectroscopy to identify the formation of neptunyl–neptunyl interactions: a paired density functional theory and Raman spectroscopy study

“…The presence of Na + has previously been observed in the solution due to the use of NaOH for neptunium reprocessing which would lead to the formation of the salt upon significant evaporation. 51 This also supports our assignment of the two bands at 716 cm −1 and 721 cm −1 to two unique nitrate modes associated with NaNO 3 and a neptunyl nitrate species. The other major phase that was identified within the diffractogram is (NpO 2 ) 2 (NO 3 ) 2 (H 2 O) 5 , which was previously characterized by Grigor'ev et al 52 The system contains two crystallographically unique Np(V) cations that link through AAIs to form a 1-D chain structure (Fig.…”

Continuous Shape Measures Study of the Coordination Spheres of Actinide Complexes – Part 1: Low Coordination Numbers

“…The rest of the structures all have actinyl groups for which the rest of the coordination sphere has not been attributed. For instance, a neptunyl complex [23] found as having coordination number two is in fact coordinated by a hexadentate macrocyclic ligand that completes a hexagonal bipyramidal structure, and the same happens for the uranyl compound (NH 2 Et 2 ) 2 (UO 2 ) 4 (MoO 4 ) 5 • 2H 2 O with the local UO 2 l(μ-MoO 4 ) 6 coordination sphere. Two further examples of ill-attributed coordination number are a phosphonate uranyl compound [24] and a neptunyl benzenetetracarboxylate, [25] both with a pentagonal bipyramidal coordination geometry.…”

Elucidating cation–cation interactions in neptunyl dications using multi-referenceab initiotheory

“…10,39,40 Specifically, such inter-cationic interactions promote the formation of crystalline structures. 21,[41][42][43][44][45][46][47][48][49][50] CCIs are a technical difficulty for nuclear waste reprocessing systems as they facilitate the stabilization of actinide complexes in spent nuclear fuel.…”