Thermodynamics of host-guest interactions in lower rim functionalized calix[4]arenes and metal cations: The medium effect

A new calix[4]arene derivative containing mixed pendant arms in its lower rim, 5,11,17,2 3-tetra-tert-butyl[25,27-bis(ethylethanoate)oxy-26,28-bis(ethylthioethoxy)]-calix[4]arene, 1, has been synthesized and characterized by H-1 and C-13 NMR. H-1 NMR data carried out in CDCl3, CD3CN, CD3OD, and C3D7NO suggest that as far as acetonitrile is concerned, the hydrophobic cavity is likely to embrace a solvent molecule. It is shown that the hosting capacity of I toward metal cations is greater in acetonitrile than in N,N-dimethylformamide and in methanol. Thus, in the former solvent, complexation with various cations (Li+, Na+, Ag+, Ca2+, Cu2+, Hg2+, and Pb2+) occurs while in the latter media, 1 interacts only with Ag+ and Hg2+. This statement is corroborated by H-1 NMR, conductance, calorimetric and potentiometric measurements. It is concluded that through molecular inclusion of acetonitrile in the hydrophobic cavity of 1, the hydrophilic cavity of the resulting adduct becomes more receptive to host metal cations than that of the free ligand. In propylene carbonate, the results show that the ligand loses its ability to interact with metal cations. Thus in acetonitrile, selective recognition of 1 for Hg2+ is demonstrated to an extent that the selectivity for this cation is greater by factors of 1.8 x 10(3), 1.9 x 10(3), 6.9 x 10(3), 1.8 x 10(4), 4.1 x 10(4) and 4.5 x 10(4), relative to Pb2+, Na+, Li+, Cu2+, Ag+, and Ca2+, respectively. This statement is supported by the thermodynamic characterization of the complexation process involving these systems in acetonitrile, N,N-dimethylformamide and in methanol. Thus, the medium effect on the binding process is carefully assessed. The results show that replacement of two ester groups in two alternate pendant arms of the tetraester calix[4]arene derivative by thioethyl moieties has altered significantly the binding capacity and the selective behavior of the latter relative to the former. Final conclusions are given.