Synthesis and Characterization of Positively Charged tris-Imidazolium Calix[6]arene Hosts for Anion Recognition

摘要

The syntheses of new tripodal ligands with effective C3 symmetry and anion recognition properties,are reported herein.The ligands have a 1,3,5-tris-methoxy-2,4,6-tris-(α,ω-bromoalkoxy)calix-[6]arene as a backbone,and were prepared from 1,3,5-tris-methoxy-p-tert-butyl-calix[6]arene by alkylation with a series of α,ω-dibromo-alkanes [Br(CH2)nBr] of different chain lengths(n=2,4 or 6).Further substitution of the bromo groups,via the use of 1-methyl-,1-mesityl-or 2,6-diisopropylphenyl-1H-imidazole,led to alkylation by imidazole units and afforded tripodal and tricationic 1,3,5-tris-methoxy-2,4,6-trisimidazolium-p-tert-butyl-calix[6]arenes.These scaffolds provide a framework for H-bonded and electrostatic interactions with various anions and their anion binding properties were investigated by ~1HNMR and UV-visible spectroscopy.A sizeable selectivity for HSO_4~-–(present as the n-Bu4N+ salt)was demonstrated for most of the tripodal ligands considered in this work.Other anions such BF_4~-,ClO_4~-,PF_6~-,NO_3~-,Cl~-exhibited weak to medium associations with the investigated ligands.A single crystal X-ray study,unfortunately of poor quality,corroborated the geometry of the calix[6]arenes that was previously established by 1HNMR spectroscopy.The bindingsite geometry was more definitively probed by DFT calculations.