Cesium Selective Calix[6]arene Bisbridged Receptor

Abstract

Calixcrowns are macromolecular hybrid structures composed of calix[n]arenes and crown ethers which have been demonstrated to be very effective complexing agents for alkali and other metal ions. In particular, calix[4]arene crown-6 hosts have been extensively investigated in the sequestration and removal of radioactive Cs ions from aqueous waste mixtures. Cesium-137 is a relatively abundant nuclear fission product and constitutes a major source of heat in nuclear waters along with strontium-90. Much effort has been made to the development of improved processes for the removal of cesium-137 from nuclear wastes. For the efficient removal of cesium, several crown ethers have been prepared and their binding properties were investigated. Calixcrowns have been constructed from all four conformational isomers of calix[4]arenes (cone, partial cone, 1,2-, and 1,3-alternate) and structure-property correlations have been developed to a level where application of these molecules has become common. Also the bisbridged calix[4]crowns has been developed since the introduction of a second bridging unit generally increases the rigidity of the calix[4]arene framework. For the development of a selective ligand for cesium ion, calix[6]arenes have been utilized as a receptor. But, selectivity over potassium and rubidium is not quite high. The lack of selectivity has been attributed into the flexibility of the parent calix[6]arene moieties. The most effective approach to make them immobile is to build a bridge at the lower. Since Gutsche’s lower rim-bridged calix[6]arene in 1993, several lower rim-multibridged calix[6]arenes and their ion binding properties were reported. For the purpose of developing cesium selective ion receptors, we prepared three new calix[6]arene bisbridged receptor and investigated their structural and alkali metal ion binding properties by UV, H NMR and solvent extraction. The bridging of the calix[6]arene was accomplished by the reaction of 1,4-dialkyl ethers 1 and 1,2-bis(bromomethyl)benzene in the presence of Cs2CO3 in dilute solution as shown in Scheme 1. H NMR spectrum of 2c showed two pairs of doublets at δ 4.4-3.0 for the bridged methylene protons and a pair of doublets of eight diastereotopic bridged methylene protons (ArCH2O-) at δ 5.0-4.4. But, H NMR spectrum of 2a and 2b did not show clear two pairs of doublets for the bridged methylene protons. The 1,2-bridged calix[6]arene ligands 1,2,3 could be defined as cone or 1,2,3alternate conformer based on H NMR. The crystal structure confirms that 2a exists as the 1,2,3alternate conformation as shown in Figure 1. Two methoxymethyl substituents were tilted inward to fill the empty calixarene cavity. Table 1 shows the detail data for X-ray structure refinement for 2a. The alkali metal binding properties were investigated from