Search

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]

Effective recovery of gold from aqueous acidic chloride media was achieved with different kinds of polysaccharides with different structures by treating with concentrated sulfuric acid. The gels were found to be highly selective for Au(iii) over other precious and base metals at varying hydrochloric acid solution. The adsorption isotherms of Au(iii) followed the Langmuir type of adsorption and the maximum adsorption capacities for Au(iii) was evaluated as high as 7.57, 7.20, 5.64 and 4.80 mmol g−1 for the gels of cellulose, dextran, alginic acid and pectic acid, respectively. Analysis of kinetic data shows that the uptake of gold(iii) on celulose gel at the initial stage follows pseudo-first-order kinetics. Crystalline structure of the cellulose before and after the treatment with concentrated sulfuric acid was measured by XRD analysis. Furthermore, Au(iii) was found to be reduced to elemental form by all types of polysaccharides gels, which was confirmed by the formation of clearly visible elemental gold particles and by means of the XRD-spectra of the adsorbents after adsorption. Infrared spectrum studies provided supporting evidence for the reduction of Au(iii) to Au(0) with a suitable mechanism of Au(iii) adsorption followed by reduction using various polysaccharide gels. [ABSTRACT FROM AUTHOR]

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]

The solvent extraction behavior of multiple proton ionizable p-tert-butylcalix[4]arene and [6]arene carboxylic acid derivatives towards indium has been investigated along with an acyclic monomeric analogue from weakly acidic media into chloroform. The extraction mechanism is ion exchange and carboxylic acid groups are adequate ligating sites for extraction. The cyclic structure of calixarene ligands to accommodate the potential guest species and the cooperativity effect of multifunctional groups significantly affect the complexation behavior and calixarene derivatives are found to be excellent extractants over the monomeric analogue. The composition of the extracted complex depends on the solution pH and attempts to determine the composition of the extracted complex for the extraction of indium have been stymied by complications arising from the formation of polynuclear species of indium and bridged polymeric species of calixarene carboxylic acid derivatives. One mole of calix[4]arene derivative extracts 2.5 moles of indium whereas the calix[6]arene derivative tends to extract 4.0 moles of indium. The loaded indium is back extracted with 1 mol dm(-3) hydrochloric acid solution. Though quantitative back extraction of indium was achieved from the fully loaded calix[6]arene derivative, it was only achieved up to 85% in the case of the calix[4]arene derivative.

A calix[5]arene based solvent extraction reagent 3, appending carboxylic acid groups at the lower rim, has been developed and its complexation behavior towards some transition metal ions has been studied. The host 3 can selectively and quantitatively extract Pb(II) ions above pH 1.8 while other divalent ions such as Cu(II), Zn(II), Co(II), Ni(II) are extracted quantitatively only above pH 3.0. The outstanding Pb(II) selectivity of 3 comes from the size fit complementarity effect of the Pb(II) ion in the calix[5]arene cavity. One molecule of 3 extracts two Pb(II) ions in a stepwise manner. The first Pb(II) ion is extracted into the deep cavity of the calix[5]arene defined by phenoxy oxygen atoms. The first complexed Pb(II) ion acts as a template to bring the host into a cone conformation and induces a positive allosteric effect for the extraction of the second Pb(II) ion at an oxygen rich coordinating site composed of carboxyl groups. Both the Pb(II) ions are extracted through an ion exchange mechanism and the electroneutral complex in the organic phase is formed by the release of an equivalent number of hydrogen ions into aqueous solution. The loaded Pb(II) is easily back-extracted from Pb(II)-complexed 3 using dilute acid solution.