Calixarenes calixcrowns

Amalgamation of stmctural units typical of crowns and calixarenes has led to the development of calixpodands, calixcrowns, and calixspherands (55). Naturally they behave as cation complexants rather than iaclusion hosts for uncharged molecules. 

Figure 1. Calixcrown extractant adopted for CSSX, as complexed with Cs+ ion. Left chemical drawing of the complex. Right a space-filling view of part of a crystal structure of the model complex Cs2Calix[4]arene-bis(benzo-crown-6)(N03)2 3CHCl3 [69] showing the good fit of the Cs+ ion inside the calixarene cavity the crown ether atoms have been removed for clarity.

Largest quantum yields reported for Eu(III) and Tb(III) complexes with calixcrowns and calixcryptands. From data reported by N. Sabbatini et al in Calixarenes 2001, Z. Asfari, V. BBhmer, J.M. Harrowfield, J. Vicens, eds, Dordrecht Kluwer Academic Publishers, 2001, Ch. 31. 

During nearly four decades of study, it has become clear that extremely large heteromacrocyclic rings can be prepared and that nearly any combination of O, N, and/or S can be included within the cycle. Numerous subcyclic units including hydrocarbons (cyclohexane, naphthalene, etc.) and heterocycles (furan, pyridine, imidazole, quinoline, etc.) have also been incorporated. Heteromacrocycles have also been fused with other receptors such as calixarenes to make calixcrowns. In recent years, the fusion of receptors and variations in the attached sidearms has been a dominant theme. 

Finally, both upper and lower rim substitution have been used to produce a class of macrocycles referred to as calixarene-crown ethers, or calixcrowns. One simple example is shown here (103). These receptors combine characteristics of the crown ethers and calixarenes, and have been intensively studied for metal ion extraction, in particular, for the removal of cesium from nuclear waste. ... 

Recently, a new type of calixarene ligands, the calixcrowns, were examined for complexation of lanthanide ions. The calixcrowns 75 and 76 were synthesized and ligand 77 was obtained as a secondary product in the... 

Wlakowiak and Kozlowski " recently reviewed the application of macrocycle carriers including crown ethers, calixarenes, calixcrowns, and CDs in liquid membrane processes. The role of macrocyclic ligands as ion carriers for cations such as alkali and alkaline metals, and heavy metals such as Zn ", Cd +, Hg + and Pb " " was summarized. Mutihac reviewed the calixarenes as membrane... 

Calixcrown A calixarene with two lower rim phenolic rings linked by a polyether. 

Although all calixcrown-5 (10 and 12a,p,c) are selective for potassium, and all calixcrown-6 (11 and 13a,p,c) for cesium, the efficiency and selectivity of complexation are strongly dependent on the calixarene stereochemistry. 

Abstract. The synthesis of 1,2- and l,3-calix[4]-Z w-crowns, double calix[4]arenes and double calixcrowns have been shown to depend on the reaction conditions (nature of the base, structure of the ditosylates, and the stoichiometry of the reactants). The 1,3-altemate conformation of the 1,3-calix[4]- w-crowns was shown to be favourable to the selective complexation of cesium cation. The observed Na /Cs selectivity was exploited in separation processes using them as carriers in transport through supported liquid membranes (SLMs). The best Na "/Cs selectivity (1/45 000) was observed for the naphthyl derivative 7. Calix(aza)crowns and 1,3-calix[4]-/ w-(aza)-crowns were also produced through the preliminary formation of the Schiff base-calixarenes, which were further hydrogenated. The syntheses consisted of the 1,3-selective alkylation of calixarenes followed by cyclization into a 1,3-bridged calixarene or by the direct 1,3-capping of the calixarene with appropriate ditosylates. Soft metal complexation by these ligands is also presented. 

Calixcrowns refer to the family of macropolycyclic or cage molecules in which the monocyclic structures of calixarenes and crown ethers are combined through the bridging of phenolic oxygens of a calixarene by a polyether chain. The first member of this family was produced by Alfieri etal. [11], who reacted pentaethylene glycol ditosylate with p-terr-butylcalix[4]arene under basic conditions to produce 1,3-p-tert-butylcalix[4]crown-6, see Figure 1. 

Subsequent to these results, we discovered a striking example of the formation of a double calixcrown by changing the stoichiometry of the reactants, p-tert-Butylcalix[4]arene was treated with a 15 equivalent excess of tetraethylene glycol ditosylate to afford double p-terr-butylcalix[4]-few-crown-5 15 in which each calixarene unit is in the 1,3-altemate conformation and 1,3-capped by a tetraethylene glycolic chain [21]. 

Both families of calixcrown compounds allowed selective removal of i37caesium from sodium containing solutions less than 100 mg of sodium is transported within 24 h for compound 7, whereas more than 95% of trace level caesium is concentrated in the stripping solution. Nitric acid transport, due to the basicity of both the organic diluent and the calixarene, could not be limited to less than 5% (0.05 mol L ) within 24 h, thus leading to concentration factors (ratio of initial waste concentration to final waste concentration) greater than 100 for a single step process. 

In this chapter selected examples of metal ion separations with polymeric macrocycles such as crown ethers, calixarenes, resorcinarenes, calixcrowns and cyclodextrins, reported in recent literature, are presented. Particularly, the use of those polymers in separation processes such as ion flotation, solvent extraction as well as transport across liquid and polymer membranes is shown. First, selected examples of crown ether polymers variety cross linked as metal ion carriers are described, then selectivity species by donor sites bonding and coordination are characterized. 

Selected calixarene and calixcrown polymers as ligands of metal ions are described showing their possible applications, especially in the aspect of the separation of toxic metals ions. 

Macrocyclic ligands such as crown ethers, calixarenes, resorcinarenes and calixcrowns are interested organic compounds, which are important in fundamental and applied chemistry [1,2], These compounds can be used as ion receptors for selective removal of metal ions from aqueous solutions in ion exchange processes, such as solvent extraction, ion flotation and transport through liquid membranes [3]. 

On the other way, to obtain higher selectivity and efficiency of metal ions removal in ion exchange processes those macrocycles are polimerized. In this way macrocycles obtained are more hydrophobic. They can be used as ion carriers for separation of metal ions from dilute aqueous solutions, especially in transport across plasticizer and liquid membranes [4]. For example, calixcrown oligomers used as ion carriers in the transport across liquid membranes exhibit the high selectivity for alkali metal cations. Also the application of crown ethers and calixarenes as the liquid membrane carriers is presented. The structures of monomers and polymers determines recognition and selectivity for metal ions. 

Calixcrowns are a family of cahxarenes in which the phenoUc oxygen atoms are linked by poly(oxyethylene) chains intramolecularly. Since calixcrowns possess well-preorganized structures in comparison with crown ethers and calixarenes, they exhibit high recognition abiUty, specially toward cations [13]. Recently more attentions are paid for polymers based on cahxcrowns. Yang et al. [13] has synthesized a series of novel calix[6]-l,4-crown-4 based polymers 15a-c (Fig. 15). 

The well known complexing properties of macrocyclic compounds towards metal ions have led to their incorporation into polymeric matrices. Polymer-supported crown ethers have many advantages, such as easy handling and recoverability when used for the removal of the toxic metal ions from the environment. Crown ether-, calixarene-, calixcrown- and cyclodextrin- based polymers have been recently receiving attention as the new polymers and may be processed into materials suitable as the extractant (solvent extraction), collector (ion flotation) or the ion carrier (transport across liquid membranes or ion selective electrodes). 

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