Effect of alkyl chain architecture on diglycolamide complexation with uranyl ion: Insights from spectroscopy, calorimetry, and DFT

Diglycolamides (DGAs) are recognized as versatile ligands for actinide ions and facilitate the separation processes relevant to nuclear waste management. While DGAs with linear alkyl chains have been extensively characterized, very little is known about how the alkyl chain branching influences their coordination properties. In this study, the complexation of uranyl ions (UO₂²⁺) with three tetraalkyl DGA derivatives, namely, tetra-n-butyl (TnBDGA), tetra-sec-butyl (TsBDGA), and tetra-isobutyl (TiBDGA), was examined using UV–vis spectrophotometry, isothermal titration calorimetry, and single-crystal X-ray diffraction. The thermodynamic and spectroscopic results demonstrate that complex formation is endothermic and mainly driven by favorable entropy changes, attributed to solvent reorganization and ligand conformational effects. The order of the complex stability (TnBDGA > TsBDGA) indicates that increasing alkyl branching introduces steric constraints that weaken the complexation reaction. Structural analysis of the UO₂²⁺/TiBDGA complex reveals a tridentate coordination through the three oxygen donor atoms. These findings highlight the significant role of ligand structural variations in determining both the energetics and geometry of the uranyl/DGA interactions, providing valuable guidance for designing more efficient extractants for nuclear fuel cycle applications.