Understanding the Role of Ligand Structure on Extraction of Neptunium by N-Pivot Tripodal Diglycolamides in an Ionic Liquid

The extraction of Np⁴⁺ was studied by a series of N-pivot tripodal diglycolamides, with varying substituents (alkyl groups with varying branching and chain length) and varying spacer length, dissolved in an ionic liquid, i.e. [C₄mim][Tf₂N]. Out of a series of four ligands, L_I (propylene spacer) and L_II (ethylene spacer) differ only in the spacer length connecting the three diglycolamide units with the tripodal N platform. While L_II–L_IV have the same spacer length (ethylene spacer), but L_II has a hydrogen atom, L_III has methyl group, and L_IV has isopropyl group as a substituent at the amidic N atom close to the central N atom. A reversal extraction mechanism of Np⁴⁺ was observed when the spacer chain length reduced from a propylene group in L_I to an ethylene group in L_II–L_IV. While a cation-exchange mechanism was observed in case of L_I, a solvation extraction mechanism was seen for the other three ligands with the extracted species being Np(NO₃)₂(L)²⁺ for L_I, and [Np(NO₃)₄·L] for L_II–L_IV. The extraction efficiencies of Np⁴⁺ with L_II–L_IV were significantly affected by the nature of the alkyl substituents and follow the order: L_II > L_III > L_IV. This trend in the extraction pattern affirms that the alkyl substitution at the amidic N atom creates steric strain in the ligand while complexing with the metal ion.