Manuel A. P. Segurado

p-tert-Butyldihomooxacalix[4]arene/p-tert-Butylcalix[4]arene: Transition and Heavy Metal Cation Extraction and Transport Studies by Ketone and Ester Derivatives

The binding properties of two phenylketones (2a and 3a) and two ethylesters (2b and {3b) derived from p-tert-butyldihomooxacalix[4]arene or from p-tert-butylcalix[4]arene, in the cone conformation, towards transition (Ag+, Ni2+, Cu2+, Co2+, Zn2+, Fe2+ and Mn2+) and heavy (Cd2+, Hg2+ and Pb2+) metal cations have been determined by extraction studies with metal picrates and liquid membrane transport experiments with the same salts. The affinity of these ligands for Ag+ has also been investigated by 1H NMR spectroscopy. Both ketones are better extractants than the esters, and show a strong preference for Ag+, while Cu2+ is the most extracted cation with the esters. 1H NMR titrations with AgSO3CF3 indicate 1 : 1 complexes for all ligands, those with ketones are more stable, on the NMR time scale, than those with esters. Both esters are good carriers for Ag+, and 2b exhibits the highest transport rate (4.7 μmol h-1) found until now with dihomooxacalix[4]arene derivatives.

Complexation and transport of alkali, alkaline earth, transition and heavy metal cations by p-tert-butyldihomooxacalix[4]arene tetra(diethyl)amide

The binding properties of the tetra(diethyl)amide (2) derived from p-tert-butyldihomooxacalix[4]arene, in the cone conformation, towards alkali, alkaline earth, transition (Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) and heavy (Ag+, Cd2+, Hg2+ and Pb2+) metal cations have been established by extraction studies of metal picrates from an aqueous solution into dichloromethane, transport experiments with the same salts through a dichloromethane membrane and stability constant measurements in methanol. Results concerning the calorimetric study of Na+ and K+ complexes in methanol are presented. The affinity of 2 for some cations (Na+, K+, Ba2+, Ag+ and Zn2+) has been investigated by 1H NMR spectrometry, as well. The results are compared to those obtained with the analogous calix[4]arene tetraamide derivative 3. Amide 2 displays a preference for the alkali cations (mainly Na+ and K+) in contrast to amide 3 that prefers the alkaline earth cations. Ag+ and Cd2+ soft Lewis acids are also strongly bound by both amides. 1H NMR titrations confirm the formation of 1∶1 complexes between 2 and all cations studied, also suggesting that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. 2 shows transport rates that do not follow the same trends as the stability constants or extraction percentages; it can therefore be characterized as a selective receptor.

Alkali and alkaline-earth metal cation complexation and transport properties by tetraphenyl ketone ofp-tert-butyldihomooxacalix[4]arene

The tetraphenyl ketone of p-tert-butyldihomooxacalix[4]arene (2) was synthesized for the first time. This derivative possesses the cone conformation in solution at room temperature, which was confirmed by NMR measurements (1H, 13C and NOE 1D). Extraction studies with metal picrates from neutral aqueous solution into dichloromethane, transport experiments with metal picrates through a dichloromethane membrane and stability constant measurements by UV absorption spectroscopy in methanol and acetonitrile were performed to evaluate the binding ability of ketone 2 towards alkali and alkaline earth metal cations. The tetraphenylketone of p-tert-butylcalix[4]arene (3) was also studied, and the results for the two derivatives were compared. Compound 2 shows a preference for K+ and Ba2+ cations, being selective only in the alkaline earth metal cation series. Compound 3 is a stronger binder than 2, but is a weaker extractant. Both show transport rates that are not proportional to complex stability or to extraction efficiency. Ketone 2 can be characterized as a selective receptor. Copyright

Synthesis, NMR conformational analysis, complexation and transport studies of an inherently chiral dihomooxacalix[4]arene triester

Abstract The synthesis of the inherently chiral triethyl ester monomethyl ether of p-tert-butyldihomooxacalix[4]arene (3) is reported. A distorted cone conformation in solution at room temperature has been established for triester 3 by NMR measurements (1H, 13C, NOE 1D and 2D). The extracting and complexing properties of 3 towards the entire alkali and alkaline earth cation series have been assessed by picrate extraction and stability constant determinations. Transport experiments with metal picrates through a CH2Cl2 membrane have also been performed. Comparison is made with tetraethyl ester of p-tert-butylcalix[4]arene (4) described in literature and also studied by us. 3 is a poor phase transfer agent and binder. In contrast, it is a good carrier, mainly for the alkali cations and displays a selective transport for Ba2+.

Bidentate Urea Derivatives of p-tert-Butyldihomooxacalix[4]arene: Neutral Receptors for Anion Complexation

Three new bidentate ureidodihomooxacalix[4]arene derivatives (phenyl 5a, n-propyl 5b, and tert-butyl 5c) were synthesized in four steps from the parent compound p-tert-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 (1)H 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 K(assoc) = 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 transport of transition and heavy metal cations by p-tert-butyldihomooxacalix[4]arene tetraketones and X-ray crystal structure of the tert-butyl ketone derivative

The binding properties of three p-tert-butyldihomooxacalix[4]arene tetraketone derivatives (tert-butyl 2b, adamantyl 2c and phenyl 2d) in the cone conformation and one derivative (methyl 2a) in a partial cone conformation, towards transition (Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) and heavy (Ag+, Cd2+, Hg2+ and Pb2+) metal cations have been established by extraction studies of metal picrates from water into dichloromethane, transport experiments with the same salts through a dichloromethane membrane and stability constant measurements in methanol and acetonitrile. Results concerning the microcalorimetric study of Cu2+, Ag+ and Pb2+ complexes in methanol are presented. The affinity of the ligands for some cations (Zn2+, Ag+, Pb2+ and Hg2+) has also been investigated by proton NMR spectrometry. The X-ray crystal structure of tert-butylketone 2b was determined. Towards transition metal cations, ketones 2b, 2c and 2d are reasonable binders, showing in general preference for Cu2+. Methylketone 2a is a poor binder for these cations, due to its partial cone conformation. The three ligands in the cone conformation present a high affinity for the heavy metal cations, with selectivities for Ag+ and Pb2+. This series of cations is even fairly well complexed by methylketone 2a, which presents high stability constants for Hg2+ and Pb2+. The cases studied by 1H NMR titrations confirm the formation of 1 : 1 complexes between the ketones and the cations, also indicating that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. Ketones 2b, 2c and 2d show transport rate sequences that follow, in general, the same trends observed in extraction and complexation for transition cations, but they are reversed for the heavy metal cations.

Synthesis, conformational behaviour, alkali and alkaline-earth metal cation extraction and transport studies of p-tert-butyldihomooxacalix[4]crowns

Abstract 1,3-Dimethoxy- p - tert -butyldihomooxacalix[4]arene-crown-6 ( 4 ) was synthesized for the first time. 4 was isolated in a partial cone A conformation in solution at room temperature, as established by NMR measurements ( 1 H, 13 C, COSY and NOESY). Conformational behaviour of 4 and p - tert -butyldihomooxacalix[4]-crown-5 ( 2 ) and crown-6 ( 3 ) was studied by dynamic 1 H NMR and by MD/MM calculations. Two-phase (water/CH 2 Cl 2 ) picrate extraction studies and transport experiments with the same salts through a CH 2 Cl 2 membrane were performed to evaluate the binding properties of these ligands towards alkali and alkaline earth metal cations. 2 and 3 are poor phase transfer agents for all cations, while 4 is a reasonable extractant for the alkali cations, showing preference for Cs + . However, all three calixcrowns are good carriers for the alkali cations and display a very selective transport for Ba 2+ .

Lanthanide cation binding properties of homooxacalixarene diethylamide derivatives

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 β = 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.

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

The 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 Transport of Alkali and Alkaline Earth Metal Cations by p-tert-Butyldihomooxacalix[4]arene Tetraketone Derivatives

The binding properties of three p-tert-butyldihomooxacalix[4]arene tetraketone derivatives (tert-butyl 2b, adamantyl 2c and phenyl 2d) in the cone conformation and one derivative (methyl 2a) in a partial cone conformation, towards alkali and alkaline earth metal cations have been established by extraction studies of metal picrates from water into dichloromethane, stability constant measurements in methanol and acetonitrile, and by 1H NMR spectrometry. Transport experiments of metal picrates through a dichloromethane membrane were also performed. The results are compared to those obtained with closely-related calix[n]arene derivatives (n = 4 and 5) and discussed in terms of the substituents, size and conformational effects. Methylketone 2a is a poor binder for all the cations studied, due to its partial cone conformation. Ketones 2b, 2c and 2d show high extraction and complexation levels for the alkali cations, with similar profiles and preference for K+ and Na+ (plateau selectivity). Towards alkaline earth cations, these ketones show a strong peak selectivity for Ba2+ in extraction, but a plateau selectivity for Ca2+, Sr2+ and Ba2+ in complexation. The nature of the substituent attached to the ketone function has some influence on their binding properties, with phenylketone 2d being a slightly weaker binder than ketones 2b and 2c. 1H NMR titrations confirm the formation of 1:1 complexes between the ketones and the cations studied, also indicating that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. Ketones 2b, 2c and 2d show transport rates that do not follow, in general, the same trends observed in extraction and complexation.