Cation-cation interactions between NpO2(+) and UO2(2+) at different temperatures and ionic strengths

Ion association with tetra-n-alkylammonium cations stabilizes higher-oxidation-state neptunium dioxocations

Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water. Herein, we report the stabilization of higher-oxidation-state Np dioxocations in aqueous chloride solutions by hydrophobic tetra-n-alkylammonium (TAA⁺) cations—an effect not exerted by fully hydrated Li⁺ cations under similar conditions. Experimental and molecular dynamics simulation results indicate that TAA⁺ cations not only drive enhanced coordination of anionic Cl^– ligands to Np^V/VI but also associate with the resulting Np complexes via non-covalent interactions, which together decrease the electrode potential of the Np^VI/Np^V couple by up to 220 mV (ΔΔG = −22.2 kJ mol⁻¹). Understanding the solvation-dependent interplay between electrolyte cations and metal–oxo species opens an avenue for controlling the formation and redox properties of metal complexes in solution. It also provides valuable mechanistic insights into actinide separation processes that widely use quaternary ammonium cations as extractants or in room temperature ionic liquids.

The electrochemical behaviour of redox-active metal cations foremost depends on the metal centre’s inner-sphere coordination environment. Here the authors show that electrolyte cations unexpectedly stabilize higher-oxidation-state neptunium dioxocations in water through extended-coordination sphere interactions.

Insight of solvent extraction process: Reassessment of trace level determinations

Solvent extraction is hoary yet modern technique with great scope of research due to the various intriguing phenomena in the system. Tri-n-butyl phosphate (TBP) is a well known extractant which has been extensively used for separation of uranium matrix prior to elemental profiling. In this paper, one of the impurities namely Fe is being considered as it posed a challenge to the separation due to its co-extraction with TBP along with uranium. In these studies, for the first time, the existence of cation-cation inner sphere complexes between the UO2(2+)and Fe(3+) ions in both aqueous and organic phases have been establisted in addition to the selective separation of iron from uranium sample matrix using only TBP. The data from both spectrophotometric and thermophysical studies corroborated one another confirming the presence of cation-cation interactions (CCIs). The developed solvent extraction with only TBP showed almost no interferences on the iron extraction from matrix uranium and other co-ions like aluminum and copper. This has been the first time application of pure TBP for selective removal of iron from uranium samples. The procedure possessed excellent reproducibility and robustness.

Three-Dimensional Network of Cation-Cation-Bound Neptunyl(V) Squares: Synthesis and in Situ Raman Spectroscopy Studies

Cation-cation interactions (CCIs) are an essential feature of actinyl chemistry, particularly neptunyl(V). To better understand the formation mechanisms of CCIs, the crystallization process of Np(V) CCI compounds has been explored during the evaporation of acidic Np(V) stock solutions using X-ray diffraction and both ex situ and in situ Raman spectroscopy. At least four Np solid products have been isolated from evaporation of the same Np(V) acidic solution. In situ evaporation using a continuous wave laser (532 nm) as a local heat source produced similar solid products to ex situ experiments with matching Raman signatures. The formation of these products is highly dependent on the evaporation conditions. Slower evaporation appears to favor the formation of a new neptunyl(V) compound, (NpO2)Cl(H2O)2 (1), over other solid products. The structure of 1 features a three-dimensional network of NpO2(+) cations, where neighboring Np(V) ions are only connected to each other through CCIs in a square arrangement. The O═Np═O stretching region shows similar Raman bands in both the solids and solution suggesting that CCIs between Np(V) cations exist prior to crystallization. These results provide new insight into the formation mechanism of Np(V) CCI compounds from solutions.