title: Aqueous complexes for efficient size-based separation of americium from curium.
abstract: 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 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.
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory , 9700 S. Cass Ave., Argonne, Illinois 60439, United States.
authors.forename: Mark P
authors.initials: MP
authors.lastname: Jensen
authors.forename: Renato
authors.initials: R
authors.lastname: Chiarizia
authors.forename: Ilya A
authors.initials: IA
authors.lastname: Shkrob
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Ulicki
authors.forename: Brian D
authors.initials: BD
authors.lastname: Spindler
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Murphy
authors.forename: Mahmun
authors.initials: M
authors.lastname: Hossain
authors.forename: Adrián
authors.initials: A
authors.lastname: Roca-Sabio
authors.forename: Carlos
authors.initials: C
authors.lastname: Platas-Iglesias
authors.forename: Andrés
authors.initials: A
authors.lastname: de Blas
authors.forename: Teresa
authors.initials: T
authors.lastname: Rodríguez-Blas