The Interplay of Steric hindrance and Electronic Effects in Asymmetric diglycolamides, Revealed through Their Ln(III) Coordination and Extraction Behavior

Abstract

The diglycolamide ligands show good extraction performance for Ln(Ⅲ) and An(Ⅲ), and has good application prospects in the field of HLLW separation. In this study, two asymmetric diglycolamides with ethyl branches at the α and β positions of long alkyl chains were synthesized through a microwave-assisted silane-mediated improved amidation method: N,N'-dimethyl-N,N'-bis(1-ethylhexyl) diglycolamide (DMDEHDGA, L1) and N,N'-dimethyl-N,N'-bis(2-ethylhexyl) diglycolamide (DMD2EHDGA, L2). The ligands’ extraction and coordination behaviors were studied by extraction and spectroscopic titration, and the electronic structures and bonding properties of the ligands and complexes were analyzed using density functional theory (DFT). The results show that both ligands have better extraction performance for heavy Ln(Ⅲ), and the extraction performance of L1 is stronger than that of L2. The complexes exhibit a non-centrosymmetric structure, and the branched chain of L1 directly affects the coordination. The complexes are formed through electrostatic interactions with covalent bond characteristics between the ligands and Ln(Ⅲ). Combined with relevant conclusions regarding DMDODGA, an isomer of the ligand with a linear substituent, it was found that the addition of a branched chain provides varying degrees of steric hindrance and electron-donating effects, leading to differences in extraction and coordination properties among ligands with different structures.