Exploring the complexation behaviour of NpO2+ with diglycolamide in ionic liquid vs. aqueous solution

Abstract

The complexation of NpO₂⁺ with tetramethyl diglycolamide (TMDGA) was investigated in an ionic liquid (IL: 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide, Bumim.Tf₂N), with the objective of understanding its complexation behaviour and optical properties in both IL and aqueous media. We successfully prepared the NpO₂(Tf₂N) salt, and its complexation with TMDGA was studied using absorption spectroscopy. Similar to the aqueous medium, TMDGA formed a perfectly centrosymmetric [NpO₂L₂]⁺ complex, as evidenced by the disappearance of the 980 nm absorption band of the neptunyl cation. The formation constants (log β) were determined to be 3.82 ± 0.11 and 7.13 ± 0.24 for the [NpO₂L]⁺ and [NpO₂L₂]⁺ complexes, respectively. The log β values for these complexes were several times higher in the IL medium than in the aqueous medium (1.37 ± 0.05 and 2.47 ± 0.71). Our results indicated that although the NpO₂⁺ cation formed a stronger complex with TMDGA in the IL medium than in the aqueous medium, the nature of the bonding and the structural conformations in both media were identical.