| Abstract |
The binding properties of two homooxacalixarene diethylamides (1b and 2b) derived from p-tert-butyldihomooxacalix[4]arene and p-tert-butylhexahomotrioxacalix[3]arene, respectively, in the cone conformation, for lanthanide cations, were investigated. These properties were assessed by extraction studies of the metal picrates from water into dichloromethane and stability constant measurements in methanol, using spectrophotometric and potentiometric techniques. Microcalorimetric studies of La3+, Pr3+, Eu3+ and Gd3+ complexes in the same solvent were also performed. Proton NMR titrations with the representative lanthanides La3+, Eu3+ and Yb3+ were also carried out to establish the sites of interaction of the ligands with the cations. The analogous derivative (3b) of p-tert-butylcalix[4]arene was also studied and the results of the three compounds are compared. Diethylamide 1b is the strongest binder, showing some preference for the light lanthanides in extraction, but exhibiting practically no selectivity in complexation. Compound 1b displays the highest stability constant values ever found with this ligand (log =8.6-9.2). In contrast, 2b shows lower extraction and stability constant values, but it is a more selective ligand, showing a clearer preference for the light lanthanides. Diethylamide 3b exhibits a similar behaviour to that of its analogue 1b. Proton NMR titrations confirm the formation of 1:1 complexes between the amides and the cations studied, also indicating that they should be located inside the cavity defined by the phenoxy and the carbonyl oxygen atoms. |
