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, LI (propylene spacer) and LII (ethylene spacer) differ only in the spacer length connecting the three diglycolamide units with the tripodal N platform. While LII – LIV have the same spacer length (ethylene spacer), but LII has a hydrogen atom, LIII has methyl group, and LIV 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 LI to an ethylene group in LII – LIV. While a cation-exchange mechanism was observed in case of LI, a solvation extraction mechanism was seen for the other three ligands with the extracted species being Np(NO3)2(L)2+ for LI, and [Np(NO3)4·L] for LII – LIV. The extraction efficiencies of Np4+ with LII – LIV were significantly affected by the nature of the alkyl substituents and follow the order: LII > LIII > LIV. 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.