Percorrer por autor "Marcos, Paula M."
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- Bidentate Urea Derivatives of p-tert-Butyldihomooxacalix[4]arene: Neutral Receptors for Anion ComplexationPublication . Marcos, Paula M.; Teixeira, Filipa A.; Segurado, Manuel A. P.; Ascenso, José R.; Bernardino, Raul J.; Michel, Sylvia; Hubscher-Bruder, VéroniqueThree new bidentate ureidodihomooxacalix[4]- arene derivatives (phenyl 5a, n-propyl 5b, and tert-butyl 5c) were synthesized in four steps from the parent compound ptert- butyldihomooxacalix[4]arene and obtained in the cone conformation, as shown by NMR studies. The binding ability of these neutral receptors toward spherical, linear, trigonal planar, and tetrahedrical anions was assessed by 1H NMR and UV-vis titrations. The structures and complexation energies of some complexes were also studied by DFT methods. The data showed that the association constants are strongly dependent on the nature of the substituent (aryl/alkyl) at the urea moiety. In general, for all the receptors, the association constants decrease with decrease of anion basicity. Ph-urea 5a is the best anion receptor, showing the strongest complexation for F- (log Kassoc = 3.10 in CDCl3) and also high binding affinity for the carboxylates AcO- and BzO-. Similar results were obtained by UV-vis studies and were also corroborated by DFT calculations.
- 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.
- Complexation and DFT studies of lower rim hexahomotrioxacalix[3]arene derivatives bearing pyridyl groups with transition and heavy metal cations. Cone versus partial cone conformationPublication . Marcos, Paula M.; Teixeira, Filipa A.; Segurado, Manuel A. P.; Ascenso, José R.; Bernardino, Raul; Peter J. Cragg; Sylvia Michel; Véronique Hubscher-Bruder; Françoise Arnaud-NeuThe binding of representative alkali, alkaline earth, transition and heavymetal cations by 2-pyridylmethoxy derivatives (1b, in cone and partial cone conformations) of p-tert-butylhexahomotrioxacalix[3]arene was studied. Binding was assessed by extraction studies of the metal picrates from water into dichloromethane and by stability constant measurements in acetonitrile and methanol, using spectrophotometric and potentiometric techniques. Microcalorimetric studies of some selected complexes in acetonitrile were performed, as well as proton NMR titrations. Computational methods (density functional theory calculations) were also employed to complement the NMR data. The results are compared with those obtained with the dihomooxacalix[4]arene 2b and the calix[4]arene 3b derivative analogues. Partial cone-1b is the best extractant for transition and heavy metal cations. Both conformers of 1b exhibit very high stability constants for soft and intermediate cations Pb2+, Cd2+, Hg2+, Zn2+ and Ni2+, with cone-1b the strongest binder (ML, log b ≥7) and partial cone-1b the most selective. Both derivatives show a slight preference for Na+. Besides the formation of ML complexes, ML2 and M2L species were also observed. The former complexes were, in general, formed with the transition and heavy metal cations, whereas the latter were obtained with Ag+ and Hg2+ and partial cone-1b. In most cases, these species were corroborated by the proton NMR and density functional theory studies.
- Experimental and computational studies of the binding properties of lower rim tetra- and di-substituted calix[4]arene amide derivatives with metal ionsPublication . Marcos, Paula M.; Fonseca, Joel D.; Proença, Carla S.; Ascenso, José R.; Bernardino, Raul; Kulesza, Joanna; Bochenska, MariaExperimental and theoretical binding studies of representative alkali, alkaline earth, transition, heavy metal and lanthanide cations by tetra- and di-substituted calix[4]arene amide derivatives (diethyl amide 1a–c and morpholide amide 2a–c) in the cone conformation were carried out. Binding was assessed by extraction experiments of the metal picrates from water to dichloromethane and proton NMR titrations. Density functional theory calculations were also performed to determine the binding energy of the complexes formed and to analyse the host–guest interaction modes. In the cases of ligands 1b and 2c with Na+ and Ag+ picrates, the extraction energy was also determined using the polarisable continuum model. The results are discussed in terms of the nature of the amide residue and the substitution pattern (1,3 vs. 1,2). Both tetra-amide derivatives are good extractants, showing preference for Na+, Ca2+, Ag+ and Pb2+ cations, mainly di-ethylamide 1a. Concerning di-amide derivatives, the relative position of the substituents seems to be more important than the nature of the amide group in the extraction process. Proton NMR studies indicate the formation of 1:1 complexes between the amides and the cations studied, and DFT data show that all ligands form the most stable complexes with La3+.
- Synthesis and anion binding properties of new dihomooxacalix[4]arene diurea and dithiourea receptorsPublication . Marcos, Paula M.; Teixeira, Filipa A.; Segurado, Manuel A. P.; Ascenso, José R.; Bernardino, Raul J.; Brancatelli, Giovanna; Geremia, SilvanoFunctionalization of the lower rim of p-tert-butyldihomooxacalix[4]arene with two (thio)ureido moieties provided new diurea (n-propyl 5a, tert-butyl 5b and phenyl 5c) and dithiourea (phenyl 5d) derivatives, all in the cone conformation, as shown by NMR studies. The X-ray crystal structure of 5c is reported. The binding ability of these neutral receptors towards a large variety of anions was assessed by 1H NMR titrations. The structures and complexation energies of some complexes were also studied using DFT methods. The data showed that, in general, the association constants decrease with decrease of anion basicity and they are strongly dependent on the nature of the substituent at the urea moiety. Phenyl-(thio)urea derivatives 5c and 5d are the best anion receptors, showing the strongest complexation for F− (log Kass=2.70 and 2.75, respectively) and also for the oxoanions AcO−, BzO− and H2 PO4. These results were corroborated by DFT calculations.
- 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.