Ion Extraction of Alkali Metal Cations by a Calix[4]arene Bearing Two Ester Groups

Abstract

The lipophilic calix[4]arene 1,3-alternate conformer possessing two (ethoxycarbonyl)methoxy groups 1 exhibits novel extraction behaviors towards alkali metal cations. This compound shows the peak selectivity for K⁺ as the highest value (95.1%), however, the selectivity among Na⁺, K⁺, Rb⁺, Cs⁺ is not so large (extraction values: ca. 85±10%) because of the flexible cavity formed by two ester groups. Hence the extraction values would be better explained by “induced-fit-geometry” than by “hole-size selectivity”. The ion-selectivity by the lipophilic calix[4]arene cone conformer possessing four (ethoxycarbonyl)methoxy groups 2 was strongly dependent on ionic size as for the size selectivity exhibited by typical crown ether type ionophores..