Complexation and DFT studies of lanthanide ions by (2-pyridylmethoxy)homooxacalixarene derivatives

The binding of lanthanide cations by 2-pyridylmethoxy derivatives ofp-tert-butyldihomooxacalix[4]arene (1b), in the cone conformation, andp-tert-butylhexahomotrioxacalix[3]arene (2b), in both cone and partial cone conformations, was studied. These properties were assessed by extraction studies of the metal picrates from water into dichloromethane and stability constant measurements in methanol and acetonitrile, using spectrophotometric and microcalorimetric techniques. Proton NMR titrations with La3+and Yb3+cations were done in order to get information on the binding sites. Computational methods (density functional theory (DFT) calculations) were also used to complement the NMR data. Thep-tert-butylcalix[4]arene analogue (3b)was also studied, and the results of the four ligands were compared. Partial cone-2bis the best extractant for lanthanide ions, showing some preference for the heavy lanthanides. In complexation, all four ligands show the same trend and a high selectivity for Yb3+(ML, logβ≥7). Besides the formation of ML complexes, ML2species were also obtained. In most cases, these species were corroborated by the proton NMR studies. For partial cone-2bwith Pr3+the complexation process is enthalpically driven, whereas for3bthe formation of the ML2species with this cation is due to a favourable entropy term. DFT studies indicate that ligand3bforms the most stable complex with La3+, followed by partial cone-2b.