The unusual stability of [NpO2Cl4]2−: Synthesis and characterisation of [NpO2(DPPMO2)2Cl]2[NpO2Cl4] and [Ph3PNH2]2[NpO2Cl4]

Three-Dimensional Noncovalent Interaction Network within [NpO2Cl4]2- Coordination Compounds: Influence on Thermochemical and Vibrational Properties

Noncovalent interactions (NCIs) can influence the stability and chemical properties of pentavalent and hexavalent actinyl (AnO2+/2+) compounds. In this work, the impact of NCIs (actinyl-hydrogen and actinyl-cation interactions) on the enthalpy of formation (ΔHf) and vibrational features was evaluated using Np(VI) tetrachloro compounds as the model system. We calculated the ΔHf values of these solid-state compounds through density functional theory+ thermodynamics (DFT+ T) and validated the results against experimental ΔHf values obtained through isothermal acid calorimetry. Three structural descriptors were evaluated to develop predictors for ΔHf, finding a strong link between ΔHf and hydrogen bond energy (EHtotal) for neptunyl-hydrogen interactions and total electrostatic attraction energy (Eelectrostatictotal) for neptunyl-cation interactions. Finally, we used Raman spectroscopy together with bond order analysis to probe Np=O bond perturbation due to NCIs. Our results showed a strong correlation between the degree of NCIs by axial oxygen and red-shifting of Np=O symmetrical stretch (ν1) wavenumbers and quantitatively demonstrated that NCIs can weaken the Np=O bond. These properties were then compared to those of related U(VI) and Np(V) phases to evaluate the effects of subtle differences in the NCIs and overall properties. In general, the outcomes of our study demonstrated the role of NCIs in stabilizing actinyl solid materials, which consequently governs their thermochemical behaviors and vibrational signatures.

Bounding [AnO2]2+ (An = U, Np) covalency by simulated O K-edge and An M-edge X-ray absorption near-edge spectroscopy

Restricted active space simulations are shown to accurately reproduce and characterise both O K-edge and U M4,5-edge spectra of uranyl in excellent agreement with experimental peak positions and are extended to the Np analogue. Analysis of bonding orbital composition in the ground and O K-edge core-excited states demonstrates that metal contribution is underestimated in the latter. In contrast, An M4/5-edge core-excited states produce bonding orbital compositions significantly more representative of those in the ground state. Quantum Theory of Atoms in Molecules analysis is employed to explain the discrepancy between K- and M-edge data and demonstrates that the location of the core-hole impacts the pattern of electron localisation in core-excited states. An apparent contradiction to this behaviour in neptunyl is rationalised in terms interelectronic repulsion between the unpaired 5f electron and the excited core-electron.

Synthesis, Structure, and Theoretical Calculations on NpO2Br42

The actinide-halogen complexes (AnO2X42-, X = Cl, Br, and I) are the simplest and most representative compounds for studying the bonding nature of actinides with ligands. In this work, we attempted to synthesize the crystals of NpO2X42- (X = Cl, Br, and I). The crystals of NpO2Cl42- and NpO2Br42- were successfully synthesized, in which the structure of NpO2Br42- was obtained for the first time. The crystal of NpO2I42- could not be obtained due to the rapid reduction of Np(VI) to Np(V) by I-. The molecular structures of NpO2Cl42- and NpO2Br42- were characterized by single-crystal X-ray diffraction and infrared, Raman, and UV-Vis-NIR absorption spectroscopy. The complexes of NpO2X42- (X = Cl, Br, and I) were also investigated by density functional theory calculations, and the calculated vibration frequencies and absorption features were comparable to the experimental results. Both the experimental results and theoretical calculations demonstrate the strengthened Np-O bonds and the weakened Np-X bonds across the NpO2X42- series; however, the population analysis on the frontier molecular orbitals (MOs) of NpO2X42- indicates a slight reduction in the Np-O bonding covalency and an enhancement in the Np-X bonding covalency from NpO2Cl42- to NpO2I42-. Results in this work have enriched the crystal database of the AnO2X42- family and provided insights into the bonding nature in the actinide complexes with soft- and hard-donor ligands.

Impacts of hydrogen bonding interactions with Np(v/vi)O2Cl4 complexes: vibrational spectroscopy, redox behavior, and computational analysis

The neptunyl (Np(v)O₂⁺/Np(vi)O₂²⁺) cation is the dominant form of ²³⁷Np in acidic aqueous solutions and the stability of the Np(v) and Np(vi) species is driven by the specific chemical constituents present in the system. Hydrogen bonding with the oxo group may impact the stability of these species, but there is limited understanding of how these intermolecular interactions influence the behavior of both solution and solid-state species. In the current study, we systematically evaluate the interactions between the neptunyl tetrachloride species and hydrogen donors in coordination complexes and in the related aqueous solutions. Both Np(v) compounds (N₂C₄H₁₂)₂[Np(v)O₂Cl₄]Cl (Np(V)pipz) and (NOC₄H₁₀)₃[Np(v)O₂Cl₄] (Np(V)morph) exhibit directional hydrogen bonding to the neptunyl oxo group while Np(vi) compounds (NC₅H₆)₂[Np(vi)O₂Cl₄] (Np(VI)pyr) and (NOC₄H₁₀)₄[Np(vi)O₂Cl₄]·2Cl (Np(VI)morph) assemble via halogen interactions. The Raman spectra of the solid-state phases indicate the activation of vibrational bands when there is asymmetry of the neptunyl bond, while these spectral features are not observed within the related solution phase spectra. Density functional theory calculations of the Np(V)pipz system suggest that activation of the ν₃ asymmetric stretch and other combination modes lead to additional complexity within the solid-state spectra. Electrochemical analyses of complexes in the solution phases are consistent with the results of the crystallization experiments as the voltammetric potentials of Np(v)/Np(vi) complexes in the presence of protonated heterocycles differ from the potentials of pure Np(v) and may correlate with the hydrogen bonding interactions.

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.

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

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)O₂]⁺ 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)O₂]^+,2+, compounds: Np1a ([NpO₂(18-crown-6)]ClO₄), Np1b ([NpO₂(18-crown-6)]AuCl₄), Na-Np ([Np(V)O₂(18-crown-6)(Na(H₂O)(18-crown-6)][Np(VI)O₂Cl₄], Np-Np ([NpO₂(18-crown-6)](NpO₂Cl₂NO₃)], and Np-Cl (NpO₂Cl(H₂O)_1.75). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO₃) 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 ν₃ 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.

Neptunyl(vi) centred visible LMCT emission directly observable in the presence of uranyl(vi)

The room temperature detection of neptunyl(vi) LMCT emission in a coordination compound and in the presence of uranyl(vi) is reported for the first time. Differences in the excitation profile of each complex enables spectral editing of the neptunyl(vi) and uranyl(vi) emission.