Water-Soluble Calixarenes as Synthetic Receptors. Remarkable Influence of Stereochemistry on the Coordination Properties of Two New Conformational Isomers of a Calix[4]arene Tetracarboxylate

Two new water-soluble calix[4]arene tetracarboxylic acids in the cone and partial cone blocked conformations were synthesized. Conformational assignment was made on the basis of their Ar-CH2-Ar H-1 and C-13 NMR patterns. Their proton and Cu(II) coordination properties were studied by several techniques to evaluate the role played by the different stereochemistry of the two isomers. The combination of potentiometric, calorimetric and spectroscopic (NMR, ESR and UV-visible) results gives a detailed and coherent description of the complex formation processes. The NMR analysis of the partial-cone isomer highlights the role played by solvophobic forces on the degree of inclusion of the carboxylic group inside the apolar calixarene cavity. The results show that the protonation of the cone isomer significantly differs from that of the partial cone isomer, indicating that hydrogen bonding, involving oxygen atoms of two adjacent carboxylate groups, occurs only in the cone isomer. Also the complex formation with copper(II) is strongly affected by the different conformations of the two calixarenes which produce not only species with different stabilities but also different species.