New metal cation-selective ionophores derived from calixarenes: their syntheses and ion-binding properties

Abstract

Various derivatives (ethoxycarbonylmethyl, carboxymethyl, and ethoxyethyl, etc.) of calix[4]arene, calix[6]arene, and calix[8]arene have been synthesized and their ion-binding properties investigated by solvent extraction, ion-transport through liquid membranes, and u.v. spectroscopy. The extraction of various metal ions (as picrate salts) from the aqueous phase into dichloromethane by the calixarene derivatives showed that the ethoxycarbonylmethyl derivatives exhibited markedly increased extraction efficiency compared with their unsubstituted phenol analogues, and the efficiency was in general comparable to those of crown ethers. The most notable feature was the selectivities exhibited by their ethoxycarbonylmethyl derivatives; calix[4]arene for Na⁺, calix[6]arene for Cs⁺, and calix[8]arene for K⁺. A liquid membrane study with a water–chloroform–water system also indicated that the ion-transport efficiencies exhibited by calixarenes were found to be in accordance with their extraction abilities, confirming the strong participation of the ester carbonyl oxygen in this function. However, the ester derivative of calix[4]arene was an exception in that inefficient carrier activity was observed in spite of high extraction efficiency. The pronounced Cs⁺ selectivity exhibited by the calix[6]arene ester was further confirmed by the changes in the u.v. spectrum of the ester from 277 and 270 nm to 275 and 267 nm upon interaction with Cs⁺ ion in methanol.