Complexes of sodium and caesium perrhenates with calix[4]arene bis(crown-6):: a model for pertechnetate ion extraction

The ability of calix[4]arene bis(crown-6) in the 1,3-alternate conformation (BC6) to co-extract caesium and pertechnetate ions from acidic or basic aqueous to organic solutions, which is of interest for nuclear waste reprocessing, has previously been demonstrated. We report the crystal structures of the complexes formed by this ligand with sodium and caesium ions with perrhenate (isoelectronic to pertechnetate) counter-ions, in an acidic medium. The sodium complex [(Na+. H2O)(2)BC6](ReO4-)(2) (1) differs from the previously reported sodium nitrate complex mainly by the dissociation of the anion-cation pair. Three forms of the caesium complex are described, [(Cs+)(2)(ReO4-)BC6](ReO4-). H2O (2), [(Cs+)(2)(ReO4-)(H2O)BC6](ReO4-)(H2O)(0.5) (3) and [(Cs+)(1.5)(ReO4-)BC6](ReO4-)(0.5) (4), which differ one from the other by the coordination mode of the perrhenate ions (monodentate, bidentate, bridging) and the position of the caesium ion in the crown. (C) 2000 Elsevier Science Ltd. All rights reserved.