A series of cone conformational p-t-octylcalix[4]arene derivatives with propyl and/or acetic acid groups have been synthesized and investigated for alkali metal extraction in individual and competitive metal systems. All ion-exchangeable derivatives exhibited extraction ability. Tripropyl—monoacetic acid derivative exhibited lithium selectivity, whereas other derivatives with multiple acetic acid groups exhibited sodium selectivity over lithium in an individual metal system. The significant metal selectivity was observed in a competitive system, because only the preferential metal ion occupied the derivative. A coordinatively-inert lipophilic propyl group or a few introduced on the calix[4]arene contributed to the size-discriminating property not only for the first metal ion but also for the second ion. The metal selectivity is discussed based on structural change before and after metal loading by using 1H-NMR spectra and an effect of lipophilic propyl groups.

A phosphonic acid derivative of calix[4]arene in a cone configuration with two different alkylphosphonic acid substituents positioned at distal positions has been synthesized and used to investigate the extraction behavior towards rare earth metals. The new reagent exhibited both heavy rare earth metal extraction selectivity and very high extraction ability that is comparable to the separation efficiency of the commercial phosphonic acid extraction reagent, 2-ethylhexyl hydrogen 2-ethylhexylphosphonate, towards rare earth metals. The latter reagent is currently used as the most effective means for separating the rare earths. The high extraction and separation efficiency are largely attributed to the convergence of the functional groups in the above calix[4]arene derivative providing a preorganized metal binding site that reflects both the cyclic structure and the cone configuration of the calix[4]arene backbone. Any loss of “binding complementarity” due to the presence of different spacer groups connecting the two phosphonic acid groups to the calix[4]arene backbone is proposed to have been compensated by the operation of a chelate effect giving rise to high extraction ability being maintained. Extraction equilibrium constants and separation factors were estimated based on the extraction data. The stripping of the loaded rare earth metal ions with hydrochloric acid was also investigated.

A p-tert-octylcalix[4]arene derivative with four phenyl ketonic side arms has been synthesized as a solvent extraction reagent for precious metal ions in order to compare the extraction behavior with that of the corresponding methyl ketonic derivative of p-tert-octylcalix[4]arene in nitric acid media. The extraction efficiency of the phenyl ketonic derivative for palladium was superior to that of the methyl ketonic one over the whole nitric acid concentration range. Silver and platinum were also moderately extracted by the phenyl ketonic derivative in low nitric acid concentration. The results demonstrated that the extraction behavior of the ligands towards precious metals is related to several structural factors, such as the flexibility of the side arms, the electric charge on the carbonyl oxygen atoms and the lipophilic properties. This work also determined the extraction mechanism of palladium with the phenyl ketonic derivative of p-tert-octylcalix[4]arene based on slope analysis and the peak shift for the FT-IR spectra of the ligand before and after the metal loading. Stripping from the organic phase after forward extraction was performed to separate palladium and silver.

A well organized calix[4]arene derivative in the cone configuration with carboxylic and phosphonic acid groups positioned at distal pairs has been prepared in order to investigate the extraction behavior of trivalent rare earths together with selected divalent metals. This product exhibited high extraction ability for the rare earths over the divalent metals investigated due to the complementarity of these trivalent ions with the above two functional groups without chelation to the phenoxy oxygen atoms. The extraction of the rare earths takes place by one of ion-exchange, where three protons are replaced by one Ln(III) cation. The stoichiometry of rare earth complexation was also determined by slope analysis, loading test and the continuous variation procedures. The extraction ability of the above reagent towards the respective rare earth ions was constant and their group separation over other metal ions proved possible. It was concluded that the presence of the two different functional groups on the calixarene structure results in a favorable synergistic interaction with the lanthanides. The results of stripping experiments for the loaded metals are also reported.

Solvent extraction behavior of p-tert-butylcalix[6]arene hexacarboxylic acid towards Pb2+ and other divalent transition metal ions has been studied to investigate the selectivity, extraction mechanism, stoichiometry of complexation and composition of the complex in the organic phase. Extractability and selectivity of the cyclic ligand were analyzed in comparison with the acyclic analog. The macrocyclic effect was genuinely operative and the size-match effect was the governing factor for cation selectivity. Accordingly, the cyclic ligand selectively extracted the Pb2+ ion in a pH range fairly more acidic than the acyclic ligand. The cations were extracted by ion exchange mechanism, and the electroneutral complexes were resulted by the exchange of two protons for a divalent cation. A molecule of p-tert-butyl calix[6]arene hexacarboxylic acid ligand extracted two Pb2+ ions. One Pb2+ was extracted due to the size-fit effect inside the hydrophilic cavity of calix[6]arene composed by phenoxy and carbonyl oxygen atoms. The macrocyclic ring inversion was obstructed due to guest encapsulation, and the encapsulated Pb2+ ion acted as a template for aggregation of the carboxyl groups. This binding event caused a positive allosteric effect and led to the extraction of the second Pb2+ ion at the aggregated functional group site.

Crossed carboxylic acid types of calix[4]arene derivatives with two longer carboxylic acids and two acetic acids at the distal position have been prepared to investigate the solvent extraction of three alkali metal ions in individual and competitive systems. These extractants selectively extracted sodium ions among other alkali ions at low pH, and the first extracted sodium ion acted as a “trigger” causing a change in extraction ability and metal selectivity. Spacer groups with different lengths induced significant differences in the extraction behavior. The extraction equilibrium constants, Kex1 and Kex2, between the present cyclic tetramers and the extracted two alkali metal ions were estimated in order to obtain a numerical evaluation of the allosteric effect.Highlights► New preparation of crossed carboxylic acid types of calix[4]arene derivatives with two longer carboxylic acids and two acetic acids at the distal position. ► Solvent extraction of alkali metal ions in individual and competitive systems. ► First numerical evaluation for extraction constants as the allosteric effect on alkali metal extraction. ► Elucidation of the allosteric effect by using extraction data and 1H NMR spectra.

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−3hydrochloric 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.

Solvent extraction of three alkali metal ions with p-t-octylcalix[6]arene hexacarboxylic acid, p-t-octylcalix[4]arene tetracarboxylic acid, corresponding linear trimeric and monomeric analogs was investigated. Cyclic tetramer selectively extracts sodium ion among alkali ions at extremely low pH, while the corresponding cyclic hexamer, the trimer, and the monomer exhibited only poor extraction ability for all alkali metals examined. The detailed extractive investigation of sodium ions with the cyclic tetramer was carried out. It was found that two sodium ions are simultaneously extracted by a single molecule of calix[4]arene derivative and that the second sodium extraction is facilitated by the uptake of the first sodium. The self-coextraction mechanism of sodium ions proposed in the present paper also strongly supports allosteric coextraction of sodium and other metal ions. In the competitive extraction of four alkali metal ions, potassium ion was slightly extracted as the second ion at low pH region, whereas it was hardly extracted in the individual extraction system. The result also supports the coextraction mechanism and role of the first-extracted sodium ion as an allosteric trigger. The extraction equilibrium constants of the cyclic tetramer and two sodium ions, Kex1 and Kex2, were estimated.

Three different resins namely 1:5 MC-resin, 1:3 MC-resin and 1:1 MC-resin have been synthesized by methylene crosslinking of 2-pyridylcalix[4]arene. Adsorption behavior of these resins towards the metal ions existing in photographic waste was investigated. The resins show absolute efficiency for adsorption of silver ion with no affinity for other coexisting ions. First two resins form 1:1 complex whereas the third one forms 2:1 complex with silver ion. Maximum loading capacity of silver ion on the present resins was found to be 1.15, 1.29 and 0.69 mol kg−1, respectively. Column chromatographic separation of silver ion in presence of excess of sodium ions was also carried out with 1:5 MC-resin. Selective adsorption of silver ions over excess of sodium ions was achieved.