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

The binding of lanthanide cations by 2-pyridylmethoxy derivatives of p-tert-butyldihomooxacalix4]arene (1b), in the cone conformation, and p-tert-butylhexahomotrioxacalix3]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. The p-tert-butylcalix4]arene analogue (3b) was also studied, and the results of the four ligands were compared. Partial cone-2b is 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, log β ≥ 7). Besides the formation of ML complexes, ML₂ species were also obtained. In most cases, these species were corroborated by the proton NMR studies. For partial cone-2b with Pr³⁺ the complexation process is enthalpically driven, whereas for 3b the formation of the ML₂ species with this cation is due to a favourable entropy term. DFT studies indicate that ligand 3b forms the most stable complex with La³⁺, followed by partial cone-2b.