Interactions between Methyl Branched-Chain Asymmetric Diglycolamides and Ln ³⁺ : Extraction and Spectroscopic and Theoretical Investigations

Diglycolamides show good selectivity for trivalent lanthanides and actinides and have good application prospects in the treatment of high level liquid waste (HLLW). In this study, two asymmetric diglycolamides with methyl groups at the α-position and β-position of the branched chains were synthesized through a microwave-assisted silane-mediated method: <i>N</i>,<i>N</i>'-dimethyl-<i>N</i>,<i>N</i>'-di(1-methylheptyl) diglycolamide (DMDMHDGA, L₁) and <i>N</i>,<i>N</i>'-dimethyl-<i>N</i>,<i>N</i>'-di(2-methylheptyl) diglycolamide (DMD2MHDGA, L₂). The extraction performance of these two ligands for Ln³⁺ was tested in the HNO₃ system. The coordination properties of the ligands with Ln³⁺ were studied by spectroscopy, and the coordination properties of the complexes were analyzed by density functional theory (DFT). Both ligands have better extraction performance for heavy Ln³⁺ over light Ln³⁺, while the extraction ability of L₂ to light Ln³⁺ is relatively stronger than that of L₁. The ligands form 2:1 complexes with Ln³⁺, which has a noncentrosymmetric octahedral structure. The ligands primarily form complexes with Ln³⁺ through electrostatic interactions with covalent characteristics. Differences in the positions of the branched chains lead to steric hindrance and electron-donating effects, thereby influencing their extraction and coordination behaviors toward Ln³⁺.