Tunable Zn2+ and Cu2+ calixarene complexes as polytopic building blocks for guest recognition

Two calix[4]arene receptors containing two or four 3-pyridylmethyl pendants at the lower rim and fixed in the cone conformation have been synthesised. Their complexes with Cu2+ and Zn2+ have been studied via UV – vis absorption spectroscopy in acetonitrile; 1H NMR spectra provide further insights into the binding sites and show that the macrocycles suitably arrange their structure to meet the coordination features of the metal ion. Both ligands form with both Cu2+ and Zn2+ multiple complex species with large stability. The large binding constant values allow to drive the system to the formation of the desired species by appropriately selecting the metal/ligand ratio. The host – guest binding properties of N-methylpyridinium (NMPy) with both the free ligands and their metal complexes have been investigated by 1H NMR spectroscopy in CDCl3/CD3CN (9:1, v/v). The metal ions do not negatively affect the affinity of the calixarenes for NMPy and also allow for the formation of systems with multiple cavities available for guest inclusion, thus paving the way to new heteroditopic receptors for molecular recognition in solution.