Aqueous Complexes for Efficient Size-based Separation of Americium from Curium

Complexation of the adjacent actinide ions americium(III) and curium(III) by the ligand N,N′-bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H2bp18c6) in aqueous solution was studied to quantify and characterize its americium/curium selectivity. Liquid–liquid extraction and spectrophotometric titration indicated the presence of both fully deprotonated and monoprotonated complexes, An(bp18c6)+ and An(Hbp18c6)2+ (An = Am or Cm), at the acidities that would be encountered when treating nuclear wastes. The stability constants of the complexes in 1 M NaNO3 determined using competitive complexation were log β101 = 15.49 ± 0.06 for Am and 14.88 ± 0.03 for Cm, indicating a reversal of the usual order of complex stability, where ligands bind the smaller CmIII ion more tightly than AmIII. The Am/Cm selectivity of bp18c62– that is defined by the ratio of the Am and Cm stability constants (β101 Am/β101 Cm = 4.1) is the largest reported so far for binary AnIII–ligand complexes. Theoretical density functional theory calculations using the B3LYP functional suggest that the ligand’s size-selectivity for larger 4f- and 5f-element cations arises from steric constraints in the crown ether ring. Enhanced 5f character in molecular orbitals involving actinide–nitrogen interactions is predicted to favor actinide(III) complexation by bp18c62– over the complexation of similarly sized lanthanide(III) cations.