Soft–hard-donor-combined ligands are a type of promising extractant for actinide and lanthanide separation. In this work, the effects of counteranions (Cl–, NO3 –, and ClO4 –) on the extraction and complexation behaviors of a recently reported tetradentate phenanthroline-derived phosphonate (POPhen) ligand toward lanthanides were thoroughly investigated using solvent extraction, NMR titration, UV–vis titration, and single-crystal X-ray diffraction measurements. It is found that C4-POPhen showed excellent extraction and selectivity toward heavy lanthanides [Lu­(III)] compared to light lanthanides, particularly with the counterion of ClO4 – and at low acidity. NMR titration studies demonstrated that both 1:1 and 1:2 Lu­(III)/C4-POPhen complexes were formed in a CD3OD solution with all three counteranions and the 1:2 species was easier to form in a complexation of C4-POPhen with Lu­(ClO4)3 under the same conditions. Furthermore, the stability constants of Nd­(III) complexation with C4-POPhen in the counteranions of Cl–, NO3 –, and ClO4 – systems were determined through UV–vis titration, and a much larger value of log β of complexes was found in the ClO4 – system, which was in good agreement with the results of solvent extraction. In addition, the structures of C2-POPhen complexation with Ln­(NO3)3/Ln­(ClO4)3 in the solid state were clearly unraveled by the single-crystal X-ray diffraction technique. This work demonstrated that the solvent extraction and complexation mechanisms of POPhen ligands with Ln­(III) were significantly affected by the counteranions from both the solution and solid-state aspects, which might shed light on the lanthanide/actinide separation.

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N,O-hybrid diamide ligands with N-heterocyclic skeletons are one of the promising extractants for the selective separation of actinides over lanthanides in a highly acidic HNO3 solution. In this work, three hard–soft donor mixed diamide ligands, pyridine-2,6-diylbis­(pyrrolidin-1-ylmethanone) (Pyr-PyDA), 2,2′-bipyridine-6,6′-diylbis­(pyr-rolidine-1-ylmethanone) (Pyr-BPyDA), and (1,10-phenanthroline-2,9-diyl)­bis­(pyrrolidin-1-ylmethanone) (Pyr-DAPhen), were synthesized and used to probe the influence of N-heterocyclic cores on the complexation and extraction behaviors with trivalent lanthanides and actinides. 1H NMR titration experiments demonstrated that 1:1 metal-to-ligand complexes were mainly formed between the three ligands and lanthanides, but 1:2 type complexes were also formed between tridentate Pyr-PyDA and Lu­(III). The stability constants (log β) of these three ligands with two typical lanthanides, Nd­(III) and Eu­(III), were determined through spectrophotometric titration. It is found that Pyr-DAPhen formed the most stable complexes, while Pyr-PyDA formed the most unstable complexes with lanthanides, which coincided well with the following solvent extraction results. The solid-state structures of 1:1 type complexes of these three ligands with La­(III), Nd­(III), and Er­(III) in nitrate media were identified by a single-crystal X-ray diffraction technique. Nd­(III) and Er­(III) were 10-coordinated with Pyr-PyDA, Pyr-BPyDA, and Pyr-DAPhen via one ligand molecule and three nitrate ions. La­(III), because of its larger ionic radius, was 11-coordinated with Pyr-DAPhen through one ligand molecule, three nitrate ions, and one methanol molecule. Solvent extraction experiments showed that the preorganized phenanthroline-derived Pyr-DAPhen had the best extraction performance for trivalent actinide among the three ligands tested. This work provides some experimental insights into the design of more efficient ligands for trivalent actinide separation by adjusting the N-heterocyclic cores.