Syntheses of and metal cation oscillation in ionophoric biscalix [4] arenes

Abstract

A series of biscalix[4]arenes (2_n) doubly-bridged with oxyethylene chains at the lower rim have been synthesized. As in conventional ionophoric monocalix[4]arene derivatives, these compounds showed ion selectivity towards Na⁺ . The ¹H NMR spectroscopic studies established that Na⁺ and K⁺ ions complexed with 2_n oscillate intramolecularly between two calix[4]arene ionophoric sites. Two coalescence temperatures were found in the temperature-dependent ¹H NMR spectra in the presence of these metal cations: the coalescence for the intermolecular metal exchange appears at a high temperature region whereas the coalescence for the intramolecular metal oscillation appears at a low temperature region. These two processes could be discriminated by the difference in the concentration dependence: the intermolecular process is concentration-dependent whereas the intramolecular process is concentration-independent. This is a unique dynamic system in which metal cations jump from one ionophoric site to another ionophoric site in biscalix[4]arenes in the NMR time-scale.