Effect of Counteranions on the Extraction and Complexation of Trivalent Lanthanides with Tetradentate Phenanthroline-Derived Phosphonate Ligands.

Soft-hard-donor-combined ligands are a type of promising extractant for actinide and lanthanide separation. In this work, the effects of counteranions (Cl, NO, and ClO) 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 ClO and at low acidity. NMR titration studies demonstrated that both 1:1 and 1:2 Lu(III)/C4-POPhen complexes were formed in a CDOD solution with all three counteranions and the 1:2 species was easier to form in a complexation of C4-POPhen with Lu(ClO) under the same conditions. Furthermore, the stability constants of Nd(III) complexation with C4-POPhen in the counteranions of Cl, NO, and ClO systems were determined through UV-vis titration, and a much larger value of log β of complexes was found in the ClO system, which was in good agreement with the results of solvent extraction. In addition, the structures of C2-POPhen complexation with Ln(NO)/Ln(ClO) 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.