1,3-calix[4]-bis-crown-6. Cesium complexation studies by Cs-133- and H-1-NMR spectroscopy. Evidence for mononuclear and binuclear complexes from X-ray diffraction. Modeling of the metal complexation

The complexing properties of 1,3-calix[4]-bis-crown-6 or Bis-C6 towards cesium ions have been investigated by Cs-133- and H-1-NMR spectroscopy. Mononuclear and binuclear complexes have been observed that exhibit cesium-ligand exchange. The crystal structures of free Bis-C6 and three of its mononuclear and binuclear cesium complexes have been determined by X-ray diffraction on single-crystals, showing that the cations are bound in the polyether loops with pi-cation interactions. These structures show that the ligand is preorganized for cesium ion complexation, which may explain the high selectivity previously observed for this cation. Molecular modeling shows that mononuclear species, associated with one molecule of water, are the most stable.