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

Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State

Neptunium can exist in multiple oxidation states, including the rare and poorly understood heptavalent form. In this work, we monitored the formation of heptavalent neptunium [Np(VII)O4(OH)2]3- during ozonolysis of aqueous MOH (M=Li, Na, K) solutions using a combined experimental and theoretical approach. All experimental reactions were closely monitored via absorption and vibrational spectroscopy to follow both the oxidation state and the speciation of neptunium guided by the calculated vibrational frequencies for various neptunium species. The mechanism of the reaction partly involves oxidative dissolution of transient Np(VI) oxide/hydroxide solid phases, the identity of which are dependent on the co-precipitating counter-cation Li+/Na+/K+. Additional calculations suggest that the most favorable energetic pathway occurs through the reaction of a [Np(V)O2(OH)4]3- with the hydroxide radical to form [Np(VI)O2(OH)4]2-, followed by an additional oxidation with HO⋅ to create [Np(VII)O4(OH)2]3-.

Synthesis, Isolation, and Study of Heterobimetallic Uranyl Crown Ether Complexes

Although crown ethers can selectively bind many metal cations, little is known regarding the solution properties of crown ether complexes of the uranyl dication, UO22+. Here, the synthesis and characterization of isolable complexes in which the uranyl dication is bound in an 18-crown-6-like moiety are reported. A tailored macrocyclic ligand, templated with a Pt(II) center, captures UO22+ in the crown moiety, as demonstrated by results from single-crystal X-ray diffraction analysis. The U(V) oxidation state becomes accessible at a quite positive potential (E1/2) of -0.18 V vs Fc+/0 upon complexation, representing the most positive UVI/UV potential yet reported for the UO2n+ core. Isolation and characterization of the U(V) form of the crown complex are also reported here; there are no prior reports of reduced uranyl crown ether complexes, but U(V) is clearly stabilized by crown chelation. Joint computational studies show that the electronic structure of the U(V) form results in significant weakening of U-Ooxo bonding despite the quite positive reduction potential at which this species can be accessed, underscoring that crown-ligated uranyl species could demonstrate unique reactivity under only modestly reducing conditions.

Periodic Trends within Actinyl(VI) Nitrates and Their Structures, Vibrational Spectra, and Electronic Properties

A series of actinyl(VI) nitrate salts of the form MAnO₂(NO₃)₃, where M = NH₄⁺ K⁺, Rb⁺, Cs⁺, and Me₄N⁺ and AnO₂²⁺ = U, Np, Pu, and AnO₂(NO₃)₂(H₂O)₂·H₂O, and the uranyl tetranitrates M₂UO₂(NO₃)₄ have been synthesized from aqueous solution and their structures determined using single-crystal X-ray diffraction. Together, these complexes represent an isostructural series of actinide complexes among the salts crystallized with the same charge-compensating cation and have been studied using vibrational spectroscopy including Raman and Fourier-transform infrared. Periodic trends in both the structural properties of these complexes and their vibrational spectra are presented and discussed, in particular the invariant nature of the O≡An≡O asymmetric stretching frequencies observed across the actinyl series. Electronic structure calculations were performed at a variety of levels of theory to aid in the interpretation of the vibrational data and to correlate trends in the data with the underlying electronic properties of these molecules.

Use of vibrational spectroscopy to identify the formation of neptunyl-neptunyl interactions: A paired Density Functional Theory and Raman spectroscopy study

Abstract: Actinyl-Actinyl interactions (AAIs) occur in pentavalent actinide systems, particularly for Np(V), and lead to complex vibrational signals that are challenging to analyze and interpret. Previous studies have focused on...

Reactions of Neptunium(V) in Alkali-Metal Hydroxides

The preparation of two new neptunium hydroxide compounds synthesized in concentrated potassium and rubidium hydroxide is reported. The phases K₄[(NpO₂)₂(OH)₆]·4H₂O and Rb₄[(NpO₂)₄(OH)₈]·2H₂O were prepared and their chemical structures determined using single-crystal X-ray diffraction. Raman spectra of the compounds are also presented. The newly synthesized phases are structurally related to Np₂O₅ and Na[NpO₂(OH)₂]. The potassium-containing phase reported here consists of infinite chains of edge-sharing neptunium hydroxide polyhedra but lacking the cation-cation interactions (CCIs) observed in Np₂O₅ and Na[NpO₂(OH)₂]. Rb₄[(NpO₂)₄(OH)₈]·2H₂O is a an expanded three-dimensional framework based on NpO₂⁺ CCIs like those observed in Np₂O₅ and Na[NpO₂(OH)₂]. Together these complexes begin to develop a structural series of neptunium(V) oxides and hydroxides of varying dimensionalities within the alkali-metal series. The potential roles of the alkali-metal cations and neptunyl(V) CCIs in directing the resulting structures are discussed.

Unusual Metal-Organic Framework Topology and Radiation Resistance through Neptunyl Coordination Chemistry

A Np(V) neptunyl metal-organic framework (MOF) with rod-shaped secondary building units was synthesized, characterized, and irradiated with γ rays. Single-crystal X-ray diffraction data revealed an anionic framework containing infinite helical chains of actinyl-actinyl interaction (AAI)-connected neptunyl ions linked together through tetratopic tetrahedral organic ligands (NSM). NSM exhibits an unprecedented net, demonstrating that AAIs may be exploited to give new MOFs and new topologies. To probe its radiation stability, we undertook the first irradiation study of a transuranic MOF and its organic linker building block using high doses of γ rays. Diffraction and spectroscopic data demonstrated that the radiation resistance of NSM is greater than that of its linker building block alone. Approximately 6 MGy of irradiation begins to induce notable changes in the long- and short-range order of the framework, whereas 3 MGy of irradiation induces total X-ray amorphization and changes in the local vibrational bands of the linker building block.

Exploring crown-ether functionalization on the stabilization of hexavalent neptunium

Crown-ether molecules are used in radiochemical separations due to their high selectivity for a range of metal cations. Previous investigations regarding the interactions of 18-crown-6 (18C6) with ²³⁷Np suggested the formation of a Np(v) inclusion complex, but also reported rapid reduction of Np(vi) to Np(v) in the presence of the ether molecule. Herein, we investigate the impact of crown ether functionalization by exploring the Np(v) and Np(vi) dicyclohexano-18-crown-6 (DCH-18C6) systems. Two [X(DCH-18C6)]₂[Np(vi)O₂Cl₄] compounds (X = K (1) and Na (2)) were crystallized and characterized by single crystal X-ray diffraction and Raman spectroscopy. Additional studies of Np(vi), Np(v), and Np(v)/Np(vi) in solution indicated redox stability in the presence of functionalized crowns and preferential crystallization of Np(vi) DCH-18C6 solids. These results indicate that functionalization of the crown can lead to higher resistance to radiolysis and increased stability of the Np(vi) oxidation state in solution.

Actinyl-cation interactions: experimental and theoretical assessment of [Np(vi)O2Cl4]2- and [U(vi)O2Cl4]2- systems

The interaction of the actinyl (AnO₂²⁺) oxo group with low-valent cations influences the chemical and physical properties of hexavalent actinides, but the impact of these intermolecular interactions on the actinyl bond and their occurrence in solution and solid state phases remain unclear. In this study, we explore the coordination of alkali cations (Li⁺, Na⁺, K⁺) with the [NpO₂Cl₄]^2- coordination complexes using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory (DFT) calculations and compare to the related uranyl system. Three solid-state coordination compounds ([Li(12-crown-4)]₂[NpO₂Cl₄] (LiNp), [Na(18-crown-6)H₂O]₂[NpO₂Cl₄] (NaNp), and [K(18-crown-6)]₂[NpO₂Cl₄] (KNp) have been synthesized and characterized using single-crystal X-ray diffraction and Raman spectroscopy. Only Li⁺ cations interact with the neptunyl oxo in the solid-state compounds and this results in a red-shift of the NpO₂²⁺ symmetric stretch (ν₁). Raman spectra of Np(vi) solutions containing lower Li⁺ concentrations display a single peak at ∼854 cm^-1 and increasing the amount of Li⁺ results in the ingrowth of a second band at 807 cm^-1. DFT calculations and vibrational analysis indicate the lower frequency vibrational band is the result of interactions between the Li⁺ cation and the neptunyl oxo. Comparison to the related uranyl system shows similar interactions occur in the solid state, but subtle differences in the actinyl-cation modes in solution phase.

Elucidating cation–cation interactions in neptunyl dications using multi-reference ab initio theory

Understanding the binding mechanism in neptunyl clusters formed due to cation–cation interactions is of crucial importance in nuclear waste reprocessing and related areas of research.

Complex Uranyl Dichromates Templated by Aza-Crowns

Three new uranyl dichromate compounds templated by aza-crown templates were obtained at room temperature by evaporation from aqueous solutions: (H2diaza-18-crown-6)2[(UO2)2(Cr2O7)4(H2O)2](H2O)3 (1), (H4[15]aneN4)[(UO2)2(CrO4)2(Cr2O7)2(H2O)] (H2O)3.5 (2) and (H4Cyclam)(H4[15]aneN4)2[(UO2)6(CrO4)8(Cr2O7)4](H2O)4 (3). The use of aza-crown templates made it possible to isolate unprecedented and complex one-dimensional units in 2 and 3, whereas the structure of 1 is based on simple uranyl-dichromate chains. It is very likely that the presence of relatively large organic molecules of aza-crown ethers does not allow uranyl chromate chain complexes to condense into the units of higher dimensionality (layers or frameworks). In general, the formation of 1, 2, and 3 is in agreement with the general principles elaborated for organically templated uranyl compounds. The negative charge of the [(UO2)(Cr2O7)2(H2O)]2−, [(UO2)2(CrO4)2(Cr2O7)2(H2O)]4− and [(UO2)3(CrO4)4(Cr2O7)2]6− one-dimensional inorganic motifs is compensated by the protonation of all nitrogen atoms in the molecules of aza-crowns.