Proton di-ionizable p-tert-butylcalix[4]arene-crown-6 compounds in cone, partial-cone and 1,3-alternate conformations: synthesis and alkaline earth metal cation extraction

Novel proton di-ionizable p-tert-butylcalix[4]arene-crown-6 compounds in cone, partial-cone and 1,3-alternate conformations are synthesized to compare the efficiency and selectivity with which they extract alkaline earth metal ions. In these ligands, a crown-6 polyether unit links alternate aromatic rings of the calix[4]arene framework. To the remaining lower-rim positions are attached oxyacetic acid or N-(X)sulfonyl oxyacetamide groups. Changing the conformation varies the spatial relationship between a polyether-complexed divalent metal ion and the ionized side arms of the ligands. This is found to markedly affect the efficiency and selectivity in competitive solvent extraction of alkaline earth metal ions from aqueous solutions into chloroform by the di-ionizable calix[4]arene-crown-6 ligands.     

Comparison of upper and lower rim-functionalized,di-ionizable calix[4]arene-crown-6 structural isomers in divalent metal ion extraction

Upper rim-functionalized, di-ionizable calix[4]arene-crown-6 ligands are synthesized and compared with structural isomers having the two acidic side arms attached to the lower rim. Solvent extractions of selected divalent metal cations (alkaline earth metal cations, Pb²⁺, and Hg²⁺) from aqueous solutions into chloroform by the upper and lower rim-functionalized, di-ionizable calix[4]arene-crown-6 isomers are utilized to assess the effects of this structural modification on metal ion complexation abilities of the ligands. The observed effects are compared with those reported for analogous di-ionizable calix[4]arene-crown-5 structural isomers.     

Di-ionizable p-tert-butylcalix[4]arene-1,2-crown-3 ligands in cone and 1,2-alternate conformations: synthesis and metal ion extraction

Novel di-ionizable p-tert-butylcalix[4]arene-1,2-crown-3 ligands in cone and 1,2-alternate conformations were prepared as potential metal ion extractants. Selective bridging of proximal hydroxyl groups of the calix[4]arene platform by a crown-3 polyether unit was achieved under Mitsunobu reaction conditions. In addition to the carboxylic acid group, the acidity tunable N-(X)sulfonyl oxyacetamide functions [OCH(2)C(O)NHSO(2)X] with X group variation from methyl to phenyl to p-nitrophenyl to trifluomethyl were used as the proton-ionizable groups. Conformations and regioselectivities of the new ligands were established by (1)H and (13)C NMR spectroscopy. Competitive solvent extractions of alkali metal cations and of alkaline earth metal cations from aqueous solutions into chloroform were performed, as were single species extractions of lead(ii) and mercury(ii).     

Di-ionizable p-tert-butylcalix[4]arene-1,3-crown-4 ligands: synthesis and alkaline earth metal cation extraction

A series of di-ionizable calix[4]arene-1,3-crown-4 compounds has been synthesized. The ionizable groups are oxyacetic acid and N-(X)sulfonyl oxyacetamide groups with X=methyl, phenyl, 4-nitrophenyl and trifluoromethyl, which ‘tunes’ the acidity of the latter. The efficiency and selectivity of these novel ligands are assessed with competitive solvent extractions of alkaline earth metal cations from aqueous solutions into chloroform. The results are compared with those reported previously for calix[4]arene-crown analogues with crown-6 and crown-5 rings.     

Di-ionizable calix[4]arene-1,2-crown-5 and -crown-6 ethers in cone conformations: synthesis and divalent metal ion extraction

Two series of di-ionizable calix[4]arene-1,2-crown-5 and -crown-6 ethers in cone conformations are synthesized. The ionizable groups are oxyacetic acid moieties and N-(X)sulfonyl oxyacetamide units with X=methyl, phenyl, 4-nitrophenyl, and trifluoromethyl, which ‘tunes’ their acidity. For competitive solvent extraction of alkaline earth metal cations from aqueous solutions into chloroform, the new ligands with N-(X)sulfonyl carbamoyl groups are efficient extractants with Ba²⁺ selectivity. On the other hand, the dicarboxylic acid analogues exhibit little selectivity in extraction of alkaline earth metal cations. For single species extractions of Pb²⁺, the ligands with both types of ionizable groups show very good extractions abilities. In single species extractions of Hg²⁺, the N-(X)sulfonyl carboxamide ligands are highly efficient, in contrast to the dicarboxylic acid compounds. Influences of the ionizable group identity, the crown ether ring size, and the presence of upper-rim p-tert-butyl groups on divalent metal ion extraction are explored.     

Lower-rim versus upper-rim functionalization in di-ionizable calix[4]arene-crown-5 isomers. Synthesis and divalent metal ion extraction

Two series of di-ionizable calix[4]arene-crown-5 isomers in the cone conformation are synthesized to probe the effect of the pendant acidic group location on their metal ion extraction properties. In one series, the ionizable groups are attached to the lower rim of the calix[4]arene scaffold, which orients them near the crown ether cavity. In the second series, the ionizable groups are connected to the upper rim positioning them away from the crown ether ring, but close to the hydrophobic pocket of the calix[4]arene unit. The metal ion extraction behaviors of the two series of ligands are compared in extractions of alkaline earth metal cations, Hg²⁺, and Pb²⁺.     

Di-ionizable p-tert-butylcalix[4]arene-1,2-monothiacrown-3 ligands in the cone conformation: synthesis and metal ion extraction

A series of di-ionizable calix[4]arene-1,2-crown-3 compounds with a sulfur-containing unit bridging the proximate phenolic oxygens have been synthesized. The ionizable groups are oxyacetic acid and N-(X)sulfonyl oxyacetamide groups with X=methyl, phenyl, 4-nitrophenyl, and trifluoromethyl, which ‘tunes’ the acidity of the latter. The efficiency and selectivity of these novel ligands are assessed for competitive solvent extractions of alkali metal cations and of alkaline earth metal cations from aqueous solutions into chloroform. Also the efficiencies for single species extractions of Pb²⁺ and of Hg²⁺ are determined. The results are compared with those reported previously for related ligands in which only oxygen heteroatoms are present in the crown ether unit.     

Synthesis and alkaline earth metal cation extraction by proton di-ionizable p-tert-butylcalix[4]arene-crown-5 compounds in cone, partial-cone and 1,3-alternate conformations

Synthetic strategies for novel, proton di-ionizable p-tert-butylcalix[4]arene-crown-5 compounds in cone, partial-cone and 1,3-alternate conformations are reported. Selective linkage of the two diametrical phenolic oxygens in p-tert-butylcalix[4]arene with tetraethylene glycol ditosylate gave 1,3-bridged p-tert-butylcalix[4]arene-crown-5. The two remaining phenolic units were alkylated using NaH and KH as the bases to give the cone and partial-cone conformers, respectively. Preparation of the 1,3-alternate conformers utilized a different sequence in which O-alkylation was followed by crown ether ring formation. Structures of these new ligands were elucidated by (1)H and (13)C NMR spectroscopy. These proton-ionizable ligands were tested for their solvent extraction properties toward alkaline earth metal cations. Surprising differences in their extraction behaviors are noted compared to those reported previously for di-ionizable p-tert-butylcalix[4]arenecrown-6 analogues.     

Discrimination of methylammonium from organic ammonium ions using ion-selective electrodes based on calix[4]arene-crown-6 conjugates.

Calix[4]-bis-2,3-naphtho-crown-6 can be used to discriminate between methylammonium and other organic ammonium ions. An electrode based on this ionophore, potassium tetrakis(p-chlorophenyl)borate (20 mol% relative to the ionophore) as an ionic additive and bis(2-ethylhexyl) sebacate as a solvent mediator in a poly(vinyl chloride) membrane matrix, displayed higher selectivity for methylammonium than for various other organic ammonium ions. However, there was considerable interference by inorganic cations, especially Cs+. Similar calix[4]arene-crown-6 conjugates, such as calix[4]-bis-1,2-benzo-crown-6, calix[4]-bis-crown-6, 1,3-dioctyloxy-calix[4]arene-crown-6, 1,3-diisopropoxy-calix[4]arene-crown-6 and 1,3-dimethoxy-calix[4]arene-crown-6 were less effective in discriminating methylammonium.     

Synthesis of cone, partial-cone, and 1,3-alternate 25, 27-Bis

The preparation of 25,27-bis[1-(2-ethyl)hexyl]- and 25, 27-bis[1-(2-tert-butoxy)ethyl]calix[4]arene-crown-6 combining one polyether crown-6 and one alkylchain O-attached on each side of a calix[4]arene in the cone, partial-cone, and 1,3-alternate conformations are reported. The control over 25, 27-bisalkylcalix[4]arene-crown-6 conformation via varying specific reaction conditions was studied. The series of calix[4]arenes have been prepared by two routes, which differ in the order in which the alkyl or polyether groups were introduced. Moreover, methods have been developed to selectively prepare the cone and partial-cone conformers by using an appropriate base in the alkylation reactions. The conformations of these new derivatives have been probed by (1)H NMR analysis and X-ray crystallography. The (1)H and (13)C NMR spectra of 25,27-bis[1-(2-ethyl)hexyl]calix[4]arene-crown-6, 1, 3-alternate 1, cone 2, and partial-cone 3 are also discussed.     

Synthesis and lead(II) sorption of silica gel-immobilized, di-ionizable calix[4]arenes

Di-ionizable calix[4]arenes with two lower-rim N-(X)sulfonyl carboxamide groups were covalently linked to silica gel via a single attachment on the upper rim. Both the acidity of the ionizable groups (X group variation) and the length of the spacer that joins the ligands to the silica gel matrix were varied. Preliminary evaluation of these new ion-exchange resins for sorption of lead(II) from aqueous solutions was conducted.     

Di-ionizable calix[4]arene-1,3-crown-4 ligands in 1,3-alternate,cone, and partial-cone conformations: synthesis and metal ion extractions

Di-ionizable calix[4]arene-1,3-crown-4 compounds locked in 1,3-alternate, cone, and partial-cone conformations are synthesized for evaluation in metal ion separations. The ionizable functions include carboxylic acid and N-(X)sulfonyl carboxamide groups in which the acidity is tuned by variation of the electron-withdrawing ability of X. Similar synthetic routes were employed for preparation of the cone and 1,3-alternate ligand series. A different preparative route utilizing protection and deprotection was required to obtain the partial-cone analogues. Ligand conformations were confirmed by their proton and/or carbon NMR spectra. X-ray diffraction verified an unusual 1,2-alternate conformation in the solid-state for one synthetic intermediate. Effects of ligand conformation and ionizable group variations on competitive solvent extractions of alkali and alkaline earth metal cations from aqueous solutions into chloroform were assessed. Single species solvent extractions of Hg²⁺ and Pb²⁺ were also performed.     

Mono-ionizable calix[4]arene-benzocrown-6 ligands in 1,3-alternate conformations: synthesis, structure and silver(I) extraction

Two series of novel mono-ionizable calix[4]arene-benzocrown-6 ligands in 1,3-alternate conformations are synthesized. In one series, the proton-ionizable group (PIG) is attached to the para position of one aromatic ring in the calixarene framework, thereby positioning it over the polyether ring cavity. In the other series, the PIG is a substituent on the benzo group in the polyether ring. This orients the PIG away from the crown ether cavity. In addition to carboxylic acid functions, the PIGs include N-(X)sulfonyl carboxamide groups. With X group variation from methyl to phenyl to 4-nitrophenyl to trifluoromethyl, the acidity of the PIG is ‘tuned’. Solvent extraction of Ag⁺ from aqueous solutions into chloroform is used to probe the influence of structural variation within the mono-ionizable calixcrown ligand on metal ion extraction efficiency, including the identity and acidity of the PIG and its orientation with respect to the polyether ring.     

Electrochemical investigation of nano-baskets of calix[4]-1,3-crowns-5,6 complexes

Four nano-baskets of calixarene including cone 25,27-di(carboxymethoxy)calix[4]arene-crown-5, 1,3-alternate 25,27-di[carboxymethoxy]-calix[4]arene-crown-5, cone 25,27-bis[carboxymethoxy]calix[4]arene-crown-6 and 1,3-alternate 25,27-di[carboxymethoxy]-calix[4]arene-crown-6 were synthesized and their binding abilities towards alkali and alkaline earth metals as well as some lanthanides were studied using differential pulse voltammetry. The novelty of this study was investigation of those macrocyclic complexes by voltammetric behaviors of two acidic moieties in each scaffold during complexation of crown ether ring. Their voltammetric behaviors were closely related to the complex formation by entrapment of cation into crown ether cavity and ion–dipole interaction between cation and acidic moieties in calixcrowns. The results revealed the selective changes in voltammetric behavior of synthesized scaffolds toward the cations. Moreover, the position of crown ether in 1,3-alternate instead of cone enhanced the domain of binding ability to more cations. Furthermore, it was shown that those carboxylic acid moieties, which were far from the crown ether ring in the 1,3-alternate, did not affected by encapsulated cations in the coordination space of crown ether and showed no voltammetric behavior.     

Di-ionizable p-tert-butylcalix[4]arene-1,2-crown-5 and -crown-6 compounds in the cone conformation: synthesis and alkaline earth metal cation extraction

Di-ionizable p-tert-butylcalix[4]arene-1,2-crown-5 and -crown-6 ethers in the cone conformation were prepared and their conformations and regioselectivities were verified by NMR spectroscopy. The metal ion-complexing properties of these ligands were evaluated by competitive solvent extractions of alkaline earth metal cations from water into chloroform. The ligands were found to be efficient extractants with selectivity for Ba(2+). The maximal loadings were 95-100% as calculated for formation of 1 : 1 ionized ligand-metal ion complexes. With the variation of proton-ionizable groups, which were oxyacetic acid moieties and N-(X)sulfonyl oxyacetamide units with X = methyl, phenyl, 4-nitrophenyl, and trifluoromethyl, "tunable" acidity was obtained.     

Self-assembled monolayers of organosulphur molecules bearing calix[4]arene moieties

We investigate the self-assembly of modified calix[4]arene on gold surfaces. Calix[4]arene was modified through a reaction sequence which led to assembling of the crown-5 moiety and to the insertion of two thioether groups into the starting molecule. The so-obtained calix[4]arene-crown-5 bis(7-thiatridecyloxy) (hereafter called calix[4]arene) was in the stable 1,3-alternate conformation. The calix[4]arene/gold interface was investigated by means of spectroscopic ellipsometry (SE), scanning tunneling microscopy (STM) and cyclic voltammetry (CV). SE data indicate a layer thickness compatible with the formation of a monomolecular layer. This result is confirmed by STM imaging which shows the formation of a high density of small pits, one gold layer deep, a typical feature of self-assembled organosulphur monolayers on gold. CV measurements performed in presence of the [Ru(NH(3))(6)(2+/3+)] redox couple indicate a passivation of the metal electrode, resulting in a reduction of the redox current, after the layer deposition. CV has also been used to investigate the selectivity properties of calix[4]arene-covered gold electrodes by measuring the redox current decrease in the presence of different salt solutions. It is found that calix[4]arene-covered electrodes are able to complex K(+) and Ba(2+), while no complexation is observed in the case of Li(+), Na(+), Cs(+), Mg(2+) and Ca(2+).     

Selectivity of calix[4]arene-bis(benzocrown-6) in the complexation and transport of francium ion

It is shown for the first time that a representative member of the "cesium-selective" calix[4]arene-crown-6 family exhibits a high affinity for Fr+ ion. In the investigation, the transport of 221Fr+ and Cs+ ions by calix[4]arene-bis(benzocrown-6) from an aqueous sodium nitrate solution into the water-immiscible diluent 1,2-dichloroethane was measured and compared to address the question of selectivity of the calix-crown-6 cavity toward alkali metal ions of increasing size. Selective separation of 221Fr+ from its parent 225Ac and from the matrix Na+ ions was demonstrated. Higher distribution ratios were obtained than those for the Cs+ ion. The extraction equilibria were determined for the case of the Cs+ ion, and the same equilibria were shown to be applicable to the case of Fr+ with inclusion of additional competitive effects.     

Protonation of 25,27-bis(1-octyloxy)calix[4]arene-crown-6 in the 1,3-alternate conformation

From extraction experiments in the two-phase water-nitrobenzene system and γ-activity measurements, the stability constant of protonated 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6 in nitrobenzene saturated with water was determined. By using DFT calculations, the most probable structure of the 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6 · H₃O⁺ complex species was derived. Correspondence: Emanuel Makrlík, Faculty of Applied Sciences, University of West Bohemia, Pilsen, Czech Republic.     

Synthesis of calix[4]arene-crown-6 with alkoxysilyl-containing substituents

An efficient procedure has been proposed for the synthesis of calix[4]arene-crown-6 conjugate having triethoxysilyl fragments in the para positions. The calixarene fragment adopts a 1,3-alternate conformation. Unlike known methods of synthesis of analogous structures, the proposed procedure ensures introduction of two ethoxysilyl groups into the oppositely located benzene rings in the macrocycle.     

香豆素冠醚双功能化杯[4]芳烃的合成

以对叔丁基杯[4]芳烃为原料,经碱催化合成了26,28-双(4,7-双甲氧基香豆素)-5,11,17,23-四叔丁基杯[4]-冠-6(2)。其结构经1H NMR,IR和元素分析表征,2为锥形结构。