Formation of Solvent-Separated Ion Pairs in Calix[4]arene Possessing Two Ester Groups: Molecular Tweezers

Abstract

Binding constants (K_a) of calix[4]arene 1,3-alternate conformer 2 bearing two ester groups for alkali picrates have been determined in tetrahydrofuran. It has been established that (i) 2 forms the 1:1 complexes with alkali metal cations, (ii) the M⁺-2 complexes are not considerably solvent-separated because of the “weak encapsulation effect” for two ester groups of 2, however the K_a values for 2 are generally as large as those for 1 having four ester groups. Interestingly, the largest K_a was attained for K⁺ and 2 complexes. This is the first comparison for the recognition capacity between two ester groups and four ester groups in calix[4]arene ester complexes.