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

Abstract

AbstractThe 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‐tris‐imidazolium‐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 1H NMR and UV‐visible spectroscopy. A sizeable selectivity for HSO4– (present as the n‐Bu4N+ salt) was demonstrated for most of the tripodal ligands considered in this work. Other anions such BF4–, ClO4–, PF6–, NO3–, 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 1H NMR spectroscopy. The binding‐site geometry was more definitively probed by DFT calculations.Introduction