Synthesis, X-ray Structure and Binding Properties of Calix[6]arene-1,4-2,5-Biscrowns

Abstract

Calixcrowns1 are a family of macropolycyclic molecules in which the subunits of calixarene and crown ethers are combined through the bridging of phenolic oxygen atoms of the calixarene by polyoxyethylene chains. The first member of this family was reported by Alfieri.2 As calixcrowns possess well preorganized structures and more rigid binding sites in comparison with calixarenes and crown ethers, they exhibited superior recognition ability toward alkali metal cations. Much attention has been paid to more sophisticated molecules: calix-biscrowns. Cesium-137 is a relatively abundant nuclear fission product and constitutes a major source of heat in nuclear wastes 7 along with strontium-90. Much effort has been made to the development of improved processes 8 for the removal of cesium-137 from nuclear wastes. For the purpose of developing cesium selective ion receptors, we prepared two new calix [6]arene biscrowns: calix[6]arene-1,4-2,5-biscrown-4 (5) and calix [6]arene-1,4-crown-4-2,5-crown-5 (6) and investigated their X-ray structure and alkali metal ion binding properties.By treating 1 and 2 5,6 with 2 equivalent of triethylene glycol ditosylate ((TsOCH2CH2OCH2)2), it was expected to produce the trisbridged calix[6]crowns 3 and 4 from the previous results in our lab.9 But, two bisbridged calix[6]-crowns 5 and 6 10 were obtained instead in 20% yield. It was believed that the long chain of poly ether unit did not allowed to react 2,3-position, rather to bridge 2,5-position of calixarene. Due to the complexity of 1 H NMR spectrum, the conformation of calix[6]arene biscrowns 5 and 6 could not be deduced. But the X-ray crystallography analysis provide * Corresponding Author. e-mail: kcnam@chonnam.ac.kr 958 Bull. Korean Chem. Soc. 2004, Vol. 25, No. 7 Communications to the Editor the alternate conformation. Figure 1 showed a X-ray structure of alternate calix[6]-arene biscrown 5. Two crown units stayed at the opposite side of calixarene circle. It is believed that this has been the first example of the X-ray structure of alternate conformer of calix[6]arene thus far. Table 1 showed the detail data for Xray structure 5. A similar X-ray structure of alternate calix[6]arene biscrown 6 was seen in Figure 1. Also two crown units stayed at the opposite side of calixarene circle. Table 2 showed the detail data for X-ray structure 6.The alkali metal binding properties were investigated from two phases extraction experiment where aqueous solutions of the picrate salts are shaken with methylene chloride solutions of ligands. The amount of metal ion was determined from picrate absorption in the UV spectrum. Table 3 showed the percent extraction of metal picrate salts by 5 and 6. The ligand 5 extracted 93% of the cesium picrate from aqueous solution into the methylene chloride solution of the host, but also 87% of the rubidium, 61% of the potassium, 37% of the sodium and 6% lithium was extracted. Extraction percentage of cesium salt slightly decreased to 89% for 6, potassium and sodium extraction was reduced remarkably as compar...