Synthesis and recognition properties of calix[4]arene semitubes as ditopic hosts for N-alkylpyridinium ion pairs

Abstract

A series of calix[4]arene semitubes (4a–c) having alkyl bridging spacers of different lengths and flexibilities on their lower rim was synthesised and characterised in solution and in the solid state by ¹H NMR spectroscopy and X-ray crystallography. The binding properties of these novel ditopic hosts toward a series of N-alkylpyridinium ion pairs were investigated in low polar media by ¹H NMR and UV-vis spectroscopy and compared with those of the corresponding calix[4]arene monotopic host 5. These studies showed that in low polar solvents, the calix[4]arene semitubes behave as homoditopic hosts giving rise with N-alkylpyridinium ion pairs to supramolecular adducts of 1 : 2 host–guest stoichiometry.