Ditopic calix arenes

Finally, the complementary molecular affinity between a [60]fullerene component and a complementary host in organic solution has been used to construct supramolecular polymeric nano-arrays of [60]fullerene. Haino and coworkers have developed a supramolecular fullerene polymer through the iterative com-plexation of ditopic calix[5]arene 30 and dumbbell-shaped [60]fullerene 31 (Figure 9.33) [110]. Although pulsed-field gradient NMR studies indicate that the... 

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. ... 

A ditopic receptor 71, comprising a semitubular calix[4]arene system [91] for cation recognition and urea moieties for anion recognition, was reported very recently. The complexation of sodium or potassium cations into the central ethylene glycol cavity invokes substantial enhancement of the binding strength (more than one order of magnitude) towards selected anions (halides, acetate). 

An interesting example of a self-assembled ditopic receptor (55) has been developed by Fukazawa and coworkers starting from two calix[5]arenes with bipyridine units at the upper rim (Figure 7.23) [59]. In a previous article, the same authors... 

Calix[8]arenes are also found to act as a ditopic receptor in most cases. The calixarenes assume various pinched conformations in these complexes, resembhng two calix[4]arene cone units hnked together. Two 1 1 complexes, of Ca° and Eu, have been characterized where a roughly planar circular conformation is observed, with only two of the phenolic O atoms interacting with the metal atom. It is interesting to note that the free jo-t-butylcalix[8]arene has been found to crystallize as a pyridine solvate with a planar, circular conformation in one case, and a pinched conformation in a subsequent report. 

A rather sophisticated ditopic ligand, based on a calix[4]arene, suitable for acidity controlled metal translocation has been recently described by Shinkai [23]. The calix-derivative 10 exhibits an 1, 3-alternate conformation and possesses two distinct coordinating compartments an N04-crown-like moiety on one side, and a bis(ethoxyethoxy) chelating subunit on the opposite... 

Beer. P.D. Hesek, D. Kingston. J.E. Smith, D.K. Stokes, S.E. Drew, M.G.B. Anion recognition by redox-responsive ditopic bis-cobaltocenium receptor molecules including a noVel calix[4]arene derivative that binds a dicarboxylate dianion. Organometallics 1995. 14. 3288. 

Calixarenes may also be linked through covalent interactions to form ditopic receptors, such as in the symmetric linking of two p-rerr-butylcalix[4] arene molecules via ethylene linkages to form a calix[4]tube. The calix[4]tube shows remarkable selectivity toward cations, with bound within the central ethylether cage of the calix[4]tube. Molecular mechanics calculations suggest that the uptake of cations requires an... 

Another family of calixarene-based ditopic receptors are the l,3-calix[4]arene-Z w(crown ethers), where the calixarene is locked into the 1,3-alternate conformation, and substrate binding normally occurs at the crown ether loops. These may form mononuclear or dinuclear complexes with alkali inetal cations, or may bind neutral guest molecules. The unsymmetrical 1.3-alternate calix[4]ar-ene-l,3-crown-6 2,4-(l,2-phenylene)-crown-6 forms a 1 1 complex with Cs. The crystal structure shows that the cation is bound only within the adapted phenylene-crown-6 loop, and a molecule of acetonitrile is bound within the unmodified crown-ether loop. ... 

In an effort to produce selective anion spectroelectrochemical reagents the Lewis acidic redox-active cobalticinium unit and the redox- and photo-active ruthenium(II) bipyridyl moiety have recently been incorporated into calixarene host structural frameworks to produce novel calix[4]arene anion receptors which are extremely rare. The calix[4]arene ditopic anion receptor (22) containing two cobalticinium moieties was prepared as shown in Scheme 4 and its crystal structure is illustrated in (Fig 7). Proton NMR titration experiments suggested (22) acts as a ditopic anion receptor forming (22) 2X"(X = Cl",Br",N03, HS04") stoichiometric solution complexes. Analogous titration... 

Exotic calix[4]arene monotopic (23) and ditopic (24) anion receptors containing one and two ruthenium(II) bipyridyl moieties have been prepared (Schemes 5 and 6) and shown by NMR and cyclic voltammetry to bind and electrochemically recognise halide, dihydrogen phosphate and hydrogen sulphate anions. 

Not only calix[4] but also calix[5]arene and calix[6]arene have been used as scaffolds for the construction of ditopic receptors for ion pairs. 

A ditopic receptor consisting of two calix[4]arene units linked by two ethylene chains at the lower rim for cation recognition and urea-based hydrogen-bonding recognition sites for anions, 40, has been developed by the group of Beer in the United Kingdom." ... 

The synthesis of functionalized calix[4]pyrroles able to accommodate ion pairs within their cavities was first approached by using crown ether-like polyether straps to link two of the four meso-csxhon atoms of the calix[4]pyrrole framework. The first of such ditopic systems, receptor 7, reported by Kim, Sessler, and coworkers, was designed to contain a the calix[4]arene crown-6-moiety for cesium cation recognition, while retaining a calix[4]pyrrole subunit for anion recognition (cf. Fig. 12.6) [22]. In the solid state, the individual ions within the CsF ion pair complex, which are bound to 7, are separated from one another by a single methanol molecule that was presumably captured during crystallization. Proton NMR spectroscopic analyses, carried out in deuterated methanol/chloroform (1 9 vA ),... 

Functionalized calixpyrroles able to accommodate ion pairs within their cavities were synthesized by Kim, Sessler, Lee, Moyer, and coworkers using crown etherlike polyether straps to link two of the four wso-carbon atoms of the calix[4] pyrrole core. One example is the cahx[4]arene crown-5-strapped calix[4]pyrrole 27 (cf. Fig. 12.17) [56]. This ditopic receptor was designed to control cation binding and release via cation methathesis. Receptor 27 was found to bind selectively rel-... 

A remarkable example of the use of the calixarene cavity for substrate recognition is provided by calix[6]arenes 42 developed by de Mendoza et al. [37] These ditopic receptors, featuring a guanidinium unit at the catalytic site, have been reported as effective turnover catalysts of the basic methanolysis of p-nitrophenylcholine carbonate (PNPCC), (Eq. 26.7). 

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