Neutral calix arenes

Calixarene crown-6 compounds, which are neutral extractants like crown ethers, are able to coextract technetium with cesium. Tests carried out with several calix-arene-crown ethers (MC7, MC8, MC14, BC2, BC5, BC8, and BC10) show that the extraction of technetium, present in the aqueous phase at a concentration 10 5 M, is enhanced as the cesium concentration in the aqueous phase increases from 10 5 to 10-2 M. As expected, an increase of nitrate concentration prevents pertechnetate extraction in competition with nitrate anion. The extraction of technetium is only appreciable when the nitric acid does not exceed 1 M. Distribution ratios DCs (close to 8) are comparable for the various calixarenes. However, a decrease of extraction is observed for naphtho derivatives.88 89... 

R. Arnecke, V. Bohmer, R. Cacciapaglia, A. DaUa Cort, L. Mandolini, cation-TT Interactions between neutral calix[5]arene hosts and cationic organic guests. Tetrahedron, 1997, 53, 4901 908. 

Scheme 30.9 Directional shuttling motion in cationic/ neutral calix[6]arene-based [2]rotaxanes 21bV22...

The highly dispersible calix[4]arene neutral carriers can also improve the durability for neutral-carrier-type ion sensors. Time-course changes in both sensitivity (slope for Na+ calibration graph) and selectivity (selectivity coefficient for Na" " with respect to K+) were followed in the Na -ISFETs based on ion-sensing membranes of silicone rubber-(l), plasticized PVC-(l), and plasticized PVC-(2) (Fig. 2). Deterioration proceeded quite quickly in the Na -ISFETs of plasticized PVC-(2) both the Na+ sensitivity and selectivity... 

As neutral carriers for the chemical modification, 16-crown-5 and calix[4]arene derivatives possessing a triethoxysilyl group (7) and (8) were designed for Na sensors. Triethoxysilylethyl-16-crown-5(7) was then mixed with a silicone-rubber precursor for the membrane fabrication accompanying covalent bonding of the neutral carrier. Comparison of IR spectra before and after extraction of the nonbonded neutral carrier... 

The Na -selective electrodes based on silicone-rubber membranes modified chemically by (8) and (9), were also investigated for Na assay in control serum and urine [22]. The found values for the Na concentrations in both of the serum and urine samples are in good agreement with their corresponding actual values with a relative standard deviation of about 1%. These results suggest that the Na -selective electrodes based on silicone-rubber membranes modified chemically by calix[4]arene neutral carrier (8) are reliable on assay in human body fluid. 

Regnouf-de-Vains, J. B. Dalbavie, J. O. Lamartine, R. Fenet, B. Quantitative solvent extraction from neutral aqueous nitrate media of silver(I) against lead(II) with a new calix-4-arene-based bipyridine podand. Tetrahedron Lett. 2001, 42, 2681-2684. 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

Reinhoudt et al. have coupled 3-CD with a probe-modified calix[4]arene to detect neutral analytes in aqueous solutions [160,161], The fluorescence capabilities of the multireceptor chemosensor arise from dansyl (26) and 2-naphthylamine (27) fluorophores attached to the rim of the calixarene. Like... 

Liu, S. Y., He, Y. B., Wu, J. L., et al., Calix[4]arenes containing thiourea and amide moieties neutral receptors towards alpha,omega-dicarboxylate anions. Org. Biomol. Chem. 2004, 2, 1582-1586. 

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

Aluminium-based catalysts with tetraphenylporphinato [32,35,38,81], Schiff s base [37-40] and calix[4]arene ligands [41] are characterised by the appearance of a non-associated, isolated pentacoordinate metal atom forming an active bond Mt X with nucleophilic substituent X. As stated above, epoxide polymerisation with these catalysts involves the rearward attack of the nucleophilic substituent on the coordinating epoxide molecule [scheme (1)]. In order to explain this, a mechanism involving the simultaneous participation of two catalyst molecules in the initiation and propagation reaction has been proposed [40,41,62,82]. According to this mechanism, the rearward attack of the nucleophilic substituent on the coordinating monomer is carried out by the six-coordinate aluminium species. These species can appear as neutral epoxide... 

The doubly functionalized calix[4]arene 9, bearing two chiral arms , was prepared recently [48]. This neutral system proved not only to be a good receptor for a-phenylglycine anions, it was also found to display good enantioselectivity towards the L-isomer fCa(L)//< l(D) = 4.76. It was thus proposed by the authors, that this presumably preorganized receptor could function as a chromogenic sensor for nonracemic a-phenylglycine anions. 

Senfeque O et al (2000) Calix[6]arenes and zinc biomimetic receptors for neutral molecules. J Am Chem Soc 122 6183-6189... 

Because cavitands and calix[n]arenes with various functional groups (G) can be synthesized, a range of reactions can be applied to obtain substituted derivatives. Some of these molecules can be used as receptors for neutral molecules and ions, for the synthesis of stationary phases for chromatography and as catalysts for... 

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