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By using a water-soluble calix[4]arene derivative and a p-nitro-substituted calix[4]arene the pKa values of the OH groups were determined for the first time. The first dissociation occurred at pH < 1 while the dissociation of the last proton occurred at pH > 11, indicating that these molecules have a super-acidic proton as well as a super-basic oxy-anion within a molecule. We also found that the calixarene conformation changes as a function of medium pH. These novel findings are rationalized in terms of strong intramolecular hydrogen bonding among the OH groups.

A new hexacarboxylate uranophile has been synthesised from calix [6]arene: log K (stability constant for the complex with U022+) was estimated to be 18.7.The selective extraction of uranium from sea water has attracted extensive attention from chemists because of its importance in relation to energy problems. In order to design a ligand that can selectively extract the uranyl ion (U022+), one has to overcome a difficult problem: the ligand must strictly discriminate between U022+ and other metal ions present in great excess in sea water. It has been established that U022+ adopts a pseudoplanar six-co-ordinate structure,lJ which is quite different from the co-ordination structures of other metal ions. This suggests that a macrocyclic host having a nearly coplanar arrangement of six ligand groups would serve as a specific ligand for U022+. Tabushi et a l . s 5 have currently been interested in this approach and reported the macrocyclic host molecule (1) having six carboxylate groups in the ring.4 Although the stability constant for U022+ and (1) is large (log K 16.4 at pH 10.4 and 25 OC),4 compound (1) is not easy to synthesise.Recently, we have been interested in the functionalisation of calixarenes (cyclic oligomers made up of benzene units in a similar way to that in which cyclodextrins are made up of glucose units); they are expected to be useful in the design of enzyme mimics in totally synthetic systems.6.7 In the course of

Water-soluble calix[6]arenes bearing both acidic protons and sulfonate groups were synthesized. They catalyzed a hydration reaction of 1-benzyl-1,4-dihydronicotinamide (BNAH) according to Michaelis-Menten kineticts and the rate constants were greater by 426–1220 fold than those for noncyclic analogues. Fluorescence data supported the complexation of BNAH with these water-soluble calixarenes. These findings indicate that calixarenes serve as a new starting material to design functionalized host molecules.