Sequestration of Np⁴⁺ and NpO₂²⁺ ions by using diglycolamide-functionalized azacrown ethers in C₈mim·NTf₂ ionic liquid: Extraction, spectroscopic, electrochemical and DFT studies

Complexation of Np⁴⁺ and NpO₂²⁺ ions was studied in a room temperature ionic liquid (C₈mim•NTf₂) using two multiple diglycolamide (DGA) ligands with three and four DGA arms tethered to the nitrogen atoms of the aza-9-crown-3 and -12-crown-4 scaffolds termed as L_I and L_II, respectively. The studies include solvent extraction, UV–visible spectrophotometry, cyclic voltammetry, and density functional theory (DFT). While the extraction of NpO₂²⁺ ion, as a function of the nitric acid concentration, resembled that of the ‘solvation’ mechanism, seen in case of neutral donor ligands in molecular diluent, an opposite trend was observed for the Np⁴⁺ ion suggesting a ‘cation-exchange’ mechanism often operating in the case of ionic liquid-based solvent systems. Slope analysis suggested formation of Np(NO₃)]³⁺•L and NpO₂(NO₃)₂•L extracted species for the extraction of Np⁴⁺ and NpO₂²⁺, respectively, in both the ligand systems. The complexation of the metal ions was supported by peak shifts in the UV–visible spectrophotometric as well as cyclic voltammetric studies. DFT studies were carried out to get structural information of the complexes.