Selective Extraction of Americium(III) over Europium(III) Ions with Pyridylpyrazole Ligands: Structure-Property Relationships

To clarify the structure-property relationships of pyridylpyrazole ligands and provide guidance for the design of new and more efficient ligands for the selective extraction of actinides over lanthanides, a series of alkyl-substituted pyridylpyrazole ligands with different branched chains at different positions of the pyrazole ring were synthesized. Extraction experiments showed that the pyridylpyrazole ligands exhibited good selective extraction abilities for Am-III ions, and the steric effects of the branched chain had a significant impact on the distribution ratios of Am-III and Eu-III ions as well as the separation factor. Moreover, both slope analyses and UV/Vis spectrometry titrations indicated the formation of a 1:1 complex of 2-(1-octyl-1H- pyrazol-3-yl)pyridine (C8-PypzH) with Eu-III ions. The stability constant (log K) for this complex obtained from the UV/Vis titration was 4.45 +/- 0.04. Single crystals of the complexes of 3-(2- pyridyl) pyrazole (PypzH) with Eu(NO3)(3) and Sm(NO3)(3) were obtained; PypzH acts as a bidentate ligand in the crystal structures, and the N atom with a bound H atom did not participate in the coordination. In general, this study revealed some interesting findings on the effects of the alkyl-chain structure and the special complexation between pyridylpyrazole ligands and Ln(III) ions.