Extraction and Coordination Chemistry Study of Methyl-branched Asymmetric Diglycoamide with Ln(Ⅲ)

The Purex process, the sole commercial method for recovering uranium and plutonium, generates large volumes of high-level liquid waste (HLLW) containing recyclable elements, making processing of HLLW critical. Diglycolamide-based ligands, owing to their excellent selectivity for An(Ⅲ) and Ln(Ⅲ), have emerged as a research focus in HLLW treatment. In this study, two asymmetric diglycolamide ligands with branched alkyl chains were synthesized via atwo-step method: N,N'-dimethyl-N,N'-di(1-methylheptyl) diglycolamide (DMDMHDGA, L^Ⅰ) and N,N'-dimethyl-N,N'-di(2-methylheptyl) diglycolamide (DMD2MHDGA, L^Ⅱ). In the extraction experiments, a 40%/60% (v/v) n-octanol/kerosene mixture was used as the diluent. Key variables, including extraction time, initial aqueous phase acidity, extractant concentration, and temperature, were systematically investigated in a nitric acid system. The extraction performance and coordination behavior of these ligands toward Ln(Ⅲ) were compared. The results indicate that the salting-out effect and competitive extraction influence the distribution ratio (D) as the initial aqueous acidity increases. The L^Ⅰ ligand exhibits superior extraction performance compared to L^Ⅱ. Furthermore, L^Ⅰ forms a 1∶3 complex with heavy lanthanides, whereas L^Ⅱ only forms a 1∶2 complex with Ln(Ⅲ). The extraction process for both ligands is spontaneous and exothermic. Density functional theory (DFT) was employed to analyze the structural and electronic properties of the ligands and their Ln(Ⅲ) complexes. Geometric optimization was performed at the B3LYP/6-311G(d)/RECP level in the gas phase. For Ln(Ⅲ), the 28-core pseudopotential basis set (ECP28MWB_SEG) was applied. The ligand properties were analyzed using van der Waals surface electrostatic potential, Mulliken charge population analysis, and the hard-soft acid-base (HSAB) theory. The results reveal that L^Ⅰ has greater hardness, leading to stronger coordination with hard-acid Ln(Ⅲ) ions. Additionally, L^Ⅰ exhibits a more uniform Mulliken charge distribution, enhancing its coordination ability compared to L^Ⅱ. Mayer bond order analysis and QTAIM (quantum theory of atoms in molecules) were used to compare the differences between the two ligands and their Ln(Ⅲ) complexes. The results suggest that coordination primarily involves electrostatic interactions with partial covalent character. Notably, L^Ⅰ (with an α-branched alkyl chain) shows higher selectivity for heavy lanthanides, whereas L^Ⅱ (with a β-branched alkyl chain) exhibits a slight advantage in light lanthanide extraction. This study combines experimental extraction data and theoretical calculations to evaluate the extraction and coordination properties of branched asymmetric diglycolamide ligands in a nitric acid system, and can provide a theoretical foundation for the future design and synthesis of advanced ligands.