Percorrer por autor "Cragg, Peter J."
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- Complexation and DFT studies of lanthanide ions by (2-pyridylmethoxy)homooxacalixarene derivativesPublication . Marcos, Paula M.; Teixeira, Filipa A.; Segurado, Manuel A.P.; Ascenso, José R.; Bernardino, Raul; Cragg, Peter J.; Michel, Sylvia; Hubscher-Bruder, Véronique; Arnaud-Neu, FrançoiseThe binding of lanthanide cations by 2-pyridylmethoxy derivatives of p-tert-butyldihomooxacalix[4]arene (1b), in the cone conformation, and p-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. The p-tert-butylcalix[4]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.
- Synthesis, binding properties and theoretical studies of p-tert-butylhexahomotrioxacalix[3]arene tri(adamantyl)ketone with alkali, alkaline earth, transition, heavy metal and lanthanide cationsPublication . Marcos, Paula M.; Ascenso, José R.; Segurado, Manuel A.P.; Bernardino, Raul J.; Cragg, Peter J.; Bernardino, Raulp-tert-Butylhexahomotrioxacalix[3]arene tri(adamantyl)ketone (1b) was synthesized for the first time. Compound 1b was obtained in a cone conformation in solution at room temperature, as established by NMR spectroscopy (1H and 13C). The binding properties of ligand 1b for alkali, alkaline earth, transition, heavy metal and lanthanide cations have been assessed by phase transfer and proton NMR titration experiments. Molecular mechanics and ab initio techniques were also employed to complement the NMR data. The results are compared to those obtained with other closely related homooxacalixarene derivatives. Although triketone 1b is a weak extractant, it shows a strong peak selectivity for Na+ and also some preference for Ag+. Proton NMR titrations indicate the formation of 1:1 complexes between 1b and the cations studied, and also that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. Although the molecular mechanics results show little correlation with the NMR data, a good agreement was obtained with the ab initio models.
- Synthesis, NMR and DFT conformational studies of homooxacalixarene (cyanopropyl)oxy derivatives, precursors to urea-terminated compoundsPublication . Marcos, Paula M.; Proença, Carla S.; Teixeira, Filipa A.; Ascenso, José R.; Bernardino, Raul; Cragg, Peter J.Direct O-alkylation of the parent compounds p-tert-butyldihomooxacalix[4]arene (1) and p-tertbutylhexahomotrioxacalix[3]arene (3) with 4-bromobutyronitrile and K2CO3 in acetonitrile afforded tetra- and tri-[(cyanopropyl)oxy] derivatives 2 and 4, respectively, as a mixture of conformers. These conformers were isolated and their conformational features studied by NMR spectroscopy (1H, 13C, COSY and NOESY) and DFT methods. Dihomooxacalix[4]arene tetra[(cyanobutyl)oxy] derivative 5 was also obtained and studied for comparison purposes. In general, good agreement was obtained between theoretical calculations and the NMR experimental data. For compounds 2 and 4 the partial cone conformation was the most stable, while the cone conformation was the most stable for derivative 5.