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 <i>N</i>,<i>N</i>′-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 Cm^III ion more tightly than Am^III. The Am/Cm selectivity of bp18c6^2– 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 An^III–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 bp18c6^2– over the complexation of similarly sized lanthanide­(III) cations