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

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