Modulating Trivalent Lanthanide Coordination and Extraction Performance via the N-Donor Basicity of Tetradentate N,O-Hybrid Ligands

N,O-Hybrid ligands featuring N-heterocyclic backbones are promising extractants for lanthanide separation, yet the governing role of N-donor basicity in their complexation and extraction behavior toward trivalent lanthanides remains less understood. This work investigates the effect of N-donor basicity of two bipyridine- and bipyrimidine-based tetradentate diamides, i.e., Et-Tol-BPDA and Et-Tol-BPymDA, on the coordination and extraction of lanthanides. Results from solvent extraction experiments indicate that both ligands extract light lanthanides via a cation-exchange mechanism, while Et-Tol-BPymDA exhibits markedly higher selectivity and efficiency with 1–6 M HNO 3, achieving a La/Sm separation factor of 90 at 3 M HNO 3 . The superior extraction performance of Et-Tol-BPymDA over Et-Tol-BPDA stems from its enhanced protonation resistance under acidic conditions, where the electron-withdrawing effect of the extra bipyrimidine nitrogen atoms lowers the N-donor basicity, allowing the ligand to retain its metal-binding capacity even in highly acidic media, as validated by combined spectroscopic, crystallographic, and theoretical analyses. The bonding analysis of crystallographic and optimized complex structures confirms that amide oxygen atoms serve as the primary coordination sites and produce shorter metal–ligand bonds and stronger coordination interactions for Et-Tol-BPDA. These findings unravel how the subtle variations in the basicity of N-heterocycle modulate the coordination and extraction properties of N,O-hybrid ligands toward trivalent lanthanides.