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p-tert-Butylcalixarene hexacarboxylic acid initially binds with low symmetry, to later adopt a highly symmetric up-down alternating conformation in the presence of Pb, Sr or Ba. The conformational dynamics for the three ions are distinct, from 15 hours, to 20 days, to 38 days, respectively.

Calixarene-based cation exchange resin has been developed by methylene crosslinking of calix[6]arene hexacarboxylic acid derivative and the resin has been exploited for solid phase extraction of some toxic heavy metal ions. The selectivity order of the resin towards some metal ions follows the order Pb(II) > Cu(II)> Zn(II), Ni(II), Co(II). The maximum lead ion binding capacity of the resin was found to be 1.30 mmol g(-1) resin. The loaded lead was quantitatively eluted with dilute acid solution regenerating the resin. Mutual separation of Pb(II), Cu(II) and Zn(II) was achieved by using the column packed with the resin., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Calix[6]arene hexacarboxylic acid binds instantly and with low symmetry to Pb, Sr and Ba. Later a highly symmetric up-down alternating conformation emerges. The solution structures are identical to their p-tert-butylcalix[6]arene hexacarboxylic acid counterparts. With either receptor, an octahedral cage is formed around the metal. The transformation from low to high symmetry however proceeds at significantly faster rates for the de-t-butylated host. [ABSTRACT FROM AUTHOR]

Lower rim carboxylic acid calix[n]arenes and upper rim phosphonic acid functionalized calix[4]arenes effect selective transport of distinct molecular payloads through a liquid membrane. The secret to this success lies in the attachment of a receptor-complementary handle. We find that the trimethylammonium ethylene group present in choline is a general handle for the transport of drug and drug-like species. Furthermore, neurotransmitters possessing ionizable amine termini are also transported. Some limitations to this strategy have been uncovered as payloads become increasingly lipophilic. These developments reveal new approaches to synthetic receptor-mediated selective small molecule transport in vesicular and cellular systems. [ABSTRACT FROM AUTHOR]

p-tert-Butylcalixarene hexacarboxylic acid initially binds with low symmetry, to later adopt a highly symmetric up-down alternating conformation in the presence of Pb, Sr or Ba. The conformational dynamics for the three ions are distinct, from 15 hours, to 20 days, to 38 days, respectively. [ABSTRACT FROM AUTHOR]

A series of supramolecular calixarenes efficiently transport distinct molecular species through a liquid membrane when attached to a receptor-complementary choline handle. Calix[6]arene hexacarboxylic acid was highly effective at transporting different target molecules against a pH gradient. Both carboxylic- and phosphonic-acid-functionalized calix[4]arenes effect transport without requiring a pH or ion gradient. NMR binding studies, two-phase solvent extraction, and three-phase transport experiments reveal the necessary and subtle parameters to effect the transport of molecules attached to a choline 'handle'. On the other hand, rescorin[4]arene cavitands, which have similar guest recognition profiles, did not transport guest molecules. These developments reveal new approaches towards attempting synthetic-receptor-mediated selective small-molecule transport in vesicular and cellular systems. [ABSTRACT FROM AUTHOR]

Studies on extraction equilibrium constants at different temperatures and thermodynamic parameters of solvent extraction of Pb2+ion with carboxylic acid derivatives of different ring size calixarenes and structure related monomeric compound have been carried out. The extraction equilibrium constants corresponding to calix[n]arene (n = 4, 5, 6) derivatives decrease in the order [5]arene > [6]arene > [4]arene. In all cases, the complexation process is primarily enthalpy driven. The favorable enthalpic contribution for extraction of Pb2+is in the order hexamer ≈ monomer > tetramer > pentamer. However, the unfavorable entropic loss follows the order: monomer > hexamer > tetramer > pentamer. Overall stability of the host-guest complex is the function of entropy-enthalpy compensation and the free energy of complexation is minimum for the pentamer, followed by tetramer ≈ hexamer and monomer. Although the carboxylic acid derivative of calix[4]arene is more preorganized than the calix[5]arene derivative, extraction of Pb2+ion with the tetramer passes through greater entropic loss than that with the pentamer and the degree of preorganization of calix[4]arene derivative is far from perfect for the complexation and extraction of Pb2+ion. As compared to tetrameric and hexameric counterparts, the structural features of the carboxylic acid derivative of calix[5]arene prior to complexation contribute much to interact with the Pb2+ion and form a thermodynamically stable complex. Supplementary materials are available for this article. Go to the publisher's online edition ofSolvent Extraction and Ion Exchangeto view the supplemental file. [ABSTRACT FROM AUTHOR]

Abstract: Novel polyvinylcalix[4]arene resin has been synthesized by direct polymerization of calix[4]arene through vinylic groups at upper rim and an ion-exchange resin has been developed by appending carboxylic acid groups onto the resin. Adsorption behavior of the polyvinylcalix[4]arene carboxylic acid resin towards some divalent heavy metal ions has been investigated, together with its corresponding monomeric analogue as a control, in terms of effect of pH on selectivity, equilibrium time of adsorption and maximum uptake capacity. The calixarene resin was found to be superior to the monomeric analogue for adsorption ability and separation efficiency. Pb(II) ions were highly preferred over Cu(II), Zn(II), Co(II) and Ni(II) ions by calixarene resin. Size fit complementarity effect of the host cavity and the guest species was found to be the prevailing factor for specific lead ion selectivity of the resin. Surface reactive adsorption of cation on the resin due to ion exchange with carboxyl groups was sufficiently fast and significant adsorption of Pb(II) ions on the resin was achieved within 15min. However, the equilibrium of adsorption was achieved within 4h. Maximum lead loading capacity of calix[4]arene resin was found to be 1.82mmolg−1 while that of the monomeric analogue was 0.064mmolg−1 resin. Because of the resin''s sensitivity to change in pH, the loaded lead was easily eluted with dilute acid solution and the resin was regenerated without any physical and chemical damage. Separation of trace amount of lead in presence of excess of zinc ions was achieved in column chromatographic experiment. [Copyright &y& Elsevier]

Abstract: A calix[5]arene-based solvent extraction reagent displaying very high selectivity for lead has been prepared, one molecule of which extracts two lead ions in a stepwise manner. The first Pb2+ ion is bound to five phenoxy oxygen atoms inside the calixarene cavity due to which the ligand undergoes conformational freezing in stable cone conformer. This causes positive allosteric effect for co-extraction of the second lead ion due to the aggregation of functional groups. [Copyright &y& Elsevier]

p-tert-Butylcalix[6]arene hexaacetic acid is in a symmetric cone conformation in CHCl3, but it becomes conformationally flexible in CHCl3/CH3CN (1 : 1). In this mixture the host has a strong binding affinity towards Pb(ii) and instantly forms a complex of low symmetry – shortly thereafter structural reorganization occurs resulting in a high symmetry complex of Pb(ii) in an octahedral cage of carboxylates. Sr(ii) and Ba(ii) display similar behavior over a longer period of time. [ABSTRACT FROM AUTHOR]