Calix receptors

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). 

Covalent and noncovalent combination of porphyrins as well as calix[4]arenes, resorcin[4]arenes including macroheterocyclic fragments, and cyclodextrins by construction of supramolecular artificial receptors 98EJ02689. 

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. 

Macrocycles containing isoxazoline or isoxazole ring systems, potential receptors in host—guest chemistry, have been prepared by multiple (double, triple or quadruple) 1,3-dipolar cycloadditions of nitrile oxides, (prepared in situ from hydroxamoyl chlorides) to bifunctional calixarenes, ethylene glycols, or silanes containing unsaturated ester or alkene moieties (453). This one-pot synthetic method has been readily extended to the preparation of different types of macrocycles such as cyclophanes, bis-calix[4]arenes and sila-macrocycles. The ring size of macrocycles can be controlled by appropriate choices of the nitrile oxide precursors and the bifunctional dipolarophiles. Multiple cycloadditive macrocy-clization is a potentially useful method for the synthesis of macrocycles. 

Bodenant B, Weil T, Businelli-Pourcel M et al (1999) Synthesis and solution structure analysis of a bispyrenyl bishydroxamate calix[4]arene-based receptor, a fluorescent chemosensor for Cu2+ and Ni2+ metal ions. J Org Chem 64 7034-7039... 

Beer PD, Timoshenko V, Maestri M et al (1999) Anion recognition and luminescent sensing by new ruthenium(II) and rhenium(I) bipyridyl calix[4]diquinone receptors. Chem Commun... 

In an effort to impart selectivity and enhance complex stability for this class of anion receptor novel ditopic biscobaltocenium receptor molecules (Fig. 42) have been synthesized and their coordination and electrochemical properties have been studied (Beer et al., 1993f, 1995h). The two positively charged metallocene centres linked by various alkyl, aryl and calix[4]arene spacers... 

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

Anion sensing by complexes involving macrocyclic and calix[4]arene domains has been reviewed. The receptors [Ru(bpy)2(252)] + (in (252), R = Ph, 2-HOCeH4, 3-HOCeH4, 4-HOCeH4, Bu, or 4- BuC6H4) bind Cl, Br, and I hydrogen-bonded interactions that contribute to this process are discussed. All the complexes show the same anion selectivity CC > Br > The crystal... 

Shinkai s 38 is also a PET system whose fluorescence is controlled by Na" binding to a coordinatively active spacer which is a calix[4]aiene tetraester in this case. However, the through-space distance between the photoactive termini is expanded by Na complexation, thus reducing the PET efficiency. Kuhn s 39 is not dissimilar in that a PET-type quencher (a nitroaromatic unit) is held away from the lumophore by Ca binding. However a conventional lumophore-spacer-receptor is also contained within 39 as found in 24. At this point it would not be out of place to mention several important studies on the control of PET/EET by ion binding to a coordinatively active spacer between photoactive terminii." " System 36 is structurally related to Verhoewen s 40 since they both contain an aromatic lumophore and an aromatic amine with one or more interposed aliphatic amines. System 40 also displays the functional similarities that PET processes were... 

The SF is given as SF = DCs syn//)Cs B, the ratio of DCs obtained under synergistic conditions versus Da obtained for the calix-crown alone. The variable O is the organic-to-aqueous phase volume ratio. The absence of an anion receptor is indicated by subscript/ = 0, and the subscripts init, aq, and org refer to initial, aqueous, and organic. 

Cobaltocenium calix[4]arene receptors, characteristics, 12,475 Cobaltocenium-metallacarborane salts, preparation, 3, 23 Cobaltocenium receptors, characteristics, 12, 474 Cobalt phosphines, as supports, 12, 683 Cobalt-platinum nanoparticles, preparation, 12, 74 Cobalt-ruthenium clusters, as heterogeneous catalyst precursors, 12, 768... 

Diamond and coworkers attribute an increase in fluorescence from the anthracene reporter sites of 44 to the increased rigidity induced by complexation of Li+, Na+, and K+ to the calixarene s tetraester cleft. The tetraamide derivative, 45, shows an especially selective response to Na+ ion [383], Restricted motion of the calix[4]arene is believed to lead to the enhanced luminescence response. This contention is supported by H NMR studies, which show metal ions to confer significant order on the calix[4]arene receptor. 

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