Selective Extraction of Americium(III) over Europium(III) with the Pyridylpyrazole Based Tetradentate Ligands: Experimental and Theoretical Study

1,3-Bis­[3-(2-pyridyl)­pyrazol-1-yl]­propane
(Bippp) and 1,2-bis­[3-(2-pyridyl)­pyrazyl-1-methyl]­benzene (Dbnpp),
the pyridylpyrazole based tetradentate ligands, were synthesized and
characterized by MS, NMR, and FT-IR. The solvent extraction and complexation
behaviors of Am­(III) and Eu­(III) with the ligands were investigated
experimentally and theoretically. In the presence of 2-bromohexanoic
acid, the two ligands can effectively extract Am­(III) over Eu­(III)
and other rare earth­(III) metals (RE­(III)) in HNO3 solution
with the separation factors (SFAm/RE)
ranging from 15 to 60. Slope analyses showed that both Am­(III) and
Eu­(III) were extracted as monosolvated species, which agrees well
with the results observed from X-ray crystallography and MS analyses.
The stability constants (log K) obtained from UV–vis
titration for Eu­(III) complexes with Bippp and Dbnpp are 4.75 ±
0.03 and 4.45 ± 0.04, respectively. Both UV–vis titration
and solvent extraction studies indicated that Bippp had stronger affinity
for Eu­(III) than Dbnpp, which is confirmed by density functional theory
(DFT) calculations. DFT calculations revealed that the AmL­(NO3)3 (L = Bippp and Dbnpp) complexes are thermodynamically
more stable in water than their Eu­(III) analogues, which is caused
by greater covalency of the Am–N than Eu–N bonds. Theoretical
studies gave an insight into the nature of the M­(III)–ligand
bonding interactions.