1,3-CALIX[4]-BIS-CROWN-6 - CESIUM COMPLEXATION STUDIES BY CS-133-NMR 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 toward s cesium ions have been investigated by Cs-133- and H-1-NMR spectrosco py. Mononuclear and binuclear complexes have been observed that exhibi t cesium-ligand exchange. The crystal structures of free Bis-C6 and th ree of its mononuclear and binuclear cesium complexes have been determ ined by X-ray diffraction on single-crystals, showing that the cations are bound in the polyether loops with pi-cation interactions. These s tructures show that the ligand is preorganized for cesium ion complexa tion, which may explain the high selectivity previously observed for t his cation. Molecular modeling shows that mononuclear species, associa ted with one molecule of water, are the most stable.