Synthesis of BIS-calix[4]arene-calix[4]pyrrole as ion pair receptor

Abstract

Bis-calix[4]arene-calix[4]pyrrole (5), calix[4]arene-calix[4]pyrrole (9) and p-tert-butylcalix[4]arene-calix[4]pyrrole (10) and 1,3-alternate-calix[4]arene-calix[4]pyrrole-crown-5 (11) were synthesized. Crystal structure of 9 was analyzed by X-ray crystallography which showed that calix[4]arene unit was in cone conformation while calix[4]pyrrole moiety was in 1,3-alternate form. Complexation studies of ligands 9 and 11 were carried out with tetrabutylammonium and potassium salts by [superscript 1]H-NMR spectroscopy. Ligand 9 formed complexes with high affinity to fluoride following by chloride ion, but could not bind potassium ion. Fluoride ion was bound to calix[4]pyrrole by using hydrogen bond. Upon complexation, the conformation of calix[4]pyrrole changed from 1,3-alternate to cone. Complexation studies of ligand 11 with potassium ion and fluoride ion was carried out which revealed that the potassium ion was bound to ligand 11 by the crown ether moiety whereas fluoride ion was complexed by calix[4]pyrrole. Thus, ligand 11 could act as an ion pair receptor. The stability constants of 11 with anions were determined by using [superscript 1]H-NMR titrations. It was found that the association constants of ligand 11 with fluoride and chloride ion were 1721 and 457 M[superscript -1], respectively. The influence of potassium ion upon complexation of fluoride ion by ligand 11 was also studied. It was found that the fluoride ion was accommodated in an exo-fashion, whereas in the case of KF salt, fluoride was encapsulated inside cavity nearby calix[4]pyrrole unit (endo-fashion)