Calixarenes

Calixarenes a group of compounds first observed many years ago (ref. 90) have been studied intensively only in the last few years. Their chemistry has been reviewed (ref.91,92). Calix[4]arene in dichloromethane was acetylated with acetyl chloride in the presence of aluminium chloride to give a 4,4 -diacetyl derivative in 63% yield without the formation of any 0-acetyl isomers (ref.93). 

Calixarenes, the cyclic oligomers formed from condensation reactions between para-substituted phenols and formaldehyde, are inexpensive compounds that are stable to both basic and acidic media.  

Their ability to cormlex both neutral and ionic species has driven their employment as complexing agents, extractants, in chemical sensing (detection) devices, and as catalysts. 

The ability of calixarenes to bind large metal ions with high kinetic stability is important in the search for complexants for radionuclides such as Cs (ti/2 = 30.2 yr) and Sr (ti/2 = 65 d) from the reprocessing of exhausted nuclear fuel. There has been considerable interest in caesium-complexed calix[4]-bis-crowns as selective Cs-carriers. Transport isotherms of trace level Cs through supported liquid membranes containing calix[4]-bis-crowns have been determined as a function of the ionic concentration of the aqueous feeder solutions, and l,3-calix[4]-bis-o-benzo-crown-6 appears to be much more efficient in decontamination than mixtures of crown ethers and acidic exchangers, especially in highly acidic media.  

Cation-TT interactions, which are frequently encountered in calixarenes complexes, are observed in three related potassium complexes of calix[6]arenes, [K2(MeOH)5] /)-H-calix[6]arene-2H, [K2(MeOH)4] / -t-butylcalix[6]arene-2H and [K2(H20)5] p-H-calix[6]arene-2H. The crystal 

Much interest has centred on the branch of cyclophanes known as calixarenes. They are polyphenol systems that can act as hosts in the formation of inclusion compounds, where a small guest molecule resides completely in a cavity within a single host they are cone-shaped cavitands . Several accounts have appeared of their history. The discovery by Baeyer of a formaldehyde/phenol resin led to Bakelite and to the work of A. Zincke and E. Ziegler, who gave to the first oligomer a tetrameric structure of a calix[4]arene. Later syntheses by Gutsche (1978) led to calixarenes with 4, 6 or 8 phenol residues.107-109 

Selective complexation has been demonstrated (130) by crystallizing p-tert-butylcalix[4]arene (128) from 50 50 mixtures of two guest molecules such as benzene and p-xylene. Anisole and p-xylene are complexed in preference to most other simple aromatic hydrocarbons. 

The isolation and characterization of solid state complexes does not necessarily indicate that similar complexes exist in solution. Little evidence in 

Modified calixarenes that are conformationally immobile, fixed in either the cone or partial cone conformation, have been successfully synthesized by 

Solution complexation studies of these rigid calixarenes with neutral organic guests have as yet not been reported. 

Obviously both the molecular dynamics and Monte Carlo methods can be used separately to provide more than structural information about the system. Molecular dynamics can be used to study the movement of the guest within the host while the Monte Carlo method can be used to provide thermodynamic data about the system. 

The method described above for the searching of conformational space has been applied [52] to a wide range of guest molecules in calixarenes. 

It has been found that this cone stabilisation is a prerequisite for interaction with solvent in the cavity. It has been established that inclusion is not at all significant in the other 

---

Fig. 15. Prototype examples of (a) cyclodextrins and (b) calixarenes, showing conformational stmctures and dimensions.

Calixarenes (from the Latin ca/ x) may be understood as artificial receptor analogues of the natural cyclodextrins (96,97). In its prototypical form they feature a macrocycHc metacyclophane framework bearing protonizable hydroxy groups made from condensation of -substituted phenols with formaldehyde (Fig. 15b). Dependent on the ring size, benzene derivatives are the substrates most commonly included into the calix cavity (98), but other interesting substrates such as C q have also been accommodated (Fig. 8c) (45). 

C. D. Gutsche, Calixarenes, Monographs in Supramolecular Chemisty, Vol 1, The Royal Society of Chemistry, Cambridge, 1989. 

J. Vicens and V. Bnhmer, eds., Calixarenes—-A Versatile Class ofMacroyclic Compounds, Kluwer, Dordrecht, 1991. 

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. 

These molecules are significant in the field of research devoted to host—guest complexation. Synthetic routes to a number of calixarenes have been developed (11). 

The use of arachidic acid and different amphiphilic calixarenes for modifying of field effect transistor sensors and determination of some volatile organic contaminants will be considered. 

Stmctural and chemical modification of urethane containing polymer matri-ces with macrocycles - calixarenes having reactive hydrazide groups have been carried out and stmcture, physico chemical and sensor properties of polyure-thanesemicarbazides (PUS) synthesised have been studied. The polymers obtained (on the base of polypropylene glycol MM 1000 and polysiloxane diol MM 860, hexamethylene diisocyanate and calixarene dihydrazide) are identified by IR-spectroscopy, size exclusion chromatography (SEC), DSC, WAXS and SAXS methods. 

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

ZINKE ZIEGLER Synthesiso1 Cahxarenes Synthesis of calixarenes (a basket-llke macrocyclic compound)... 

CHCl3-MeOH m 410-412°. Stability in KOH-xylene is same as for the 4-/ert-butylcalix[4]arene. [J Am Chem Soc 103 3782 7957 see also J.Vicens and V.Bohner ds,Calixarenes, Kluawer Academic Publ., Boston, 1991.]... 

Because of the need for basic initiators, cyanoacrylate adhesives do not perform well on acidic surfaces, such as wood. However, the addition of sequestering agents, such as crown ethers [30], 10, or calixarenes [31], 11, and others [32] to the adhesive improves the reactivity of the adhesive on less active surfaces. 

Progress in molecular recognition of functionalized calixarenes with hetero-atomic bridges as synthetic receptors 99MI43. 

Calixarenes including heterocyclic fragments as a rich source for molecular receptors 97G637. 

Solution State of Metal Complex Calixarenes and Polymeric Calixarenes... 

McKervey [7] and Chang and Cho [9] carried out similar studies. They prepared various ester derivatives of calixarene and tested their ion carrying capacities (Scheme 1). 

Chang and coworkers [10] have synthesized amide derivatives of calixarenes and examined their ion binding properties with Group I and Group II cations. They observed that although the amides are much less effective than the esters for the complexation of Group I cations they are more effective for Group II cations. 

Kimura and coworkers [17], Diamond [18], and Damien et al. [19] have described that the polymeric calix-[4]arenes have been used as ionophores in ion selective electrodes for Na (based on calixarene esters and amides) and for Na and Cs (based on p-alkylcalixarene acetates). The electrodes are stated to function as poten-tiometric sensors as well, having good selectivity for primary ion, virtually no response to divalent cations, and being stable over a wide pH range. 

The majority of the literature reports deal with the reaction of calixarenes with Group I and II cations. Polymeric calixarenes have been the subject of a more recent innovation. Harris et al. [23] have prepared a calix[4]ar-ene methacrylate, its polymerization, and Na complex-ation (Scheme 3). They concluded that both monomers and polymers form stable complexes with sodium thiocyanate. 

Harrowfield et al. [37-39] have described the structures of several dimethyl sulfoxide adducts of homo bimetallic complexes of rare earth metal cations with p-/e rt-butyl calix[8]arene and i /i-ferrocene derivatives of bridged calix[4]arenes. Ludwing et al. [40] described the solvent extraction behavior of three calixarene-type cyclophanes toward trivalent lanthanides La (Ln = La, Nd, Eu, Er, and Yb). By using p-tert-huty ca-lix[6Jarene hexacarboxylic acid, the lanthanides were extracted from the aqueous phase at pH 2-3.5. The ex-tractability is Nb, Eu > La > Er > Yb. 

The titanium complexes of calixarene were obtained by Olmstead et al. [44] and Bott et al. [45], who examined their x-ray characteristics. Recent research in that field has been conducted by Rudkevich et al, [46]. They prepared calix[4]arene-triacids as receptors for lan-tanides. 

Based on the above results they have concluded that the ligand groups circularly arranged on the lower rim of the calixarene cavity construct a novel cyclic metal receptor for selective extraction of transition metal cations. Results suggest that the fine tuning in molecular... 

---