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

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.