Crown ethers complexation ability

The physical state of the silver salt has an influence on the reaction. Our data are obtained with the same batch of finely powdered AgNO. The selectivity was determined from NMR analysis. The results are coherent with the crown ether complexing ability. Crown ethers and are better ligands for silver ion than crown ethers and They form cryptand separated ion pairs which favor nitrate derivative formation. 

Crown ethers1 are perhaps the most widely used family of host compounds in supramolecular chemistry2—the chemistry of the non-covalent bond. The fortuitous discovery3 of the macrocyclic polyethers by Pedersen in 1967 laid the foundation for an exhaustive study of their preparation and complexing abilities, primarily with metal cations4 but also with neutral and even anionic species.5 Moreover, the metal ions which are complexed by crown ethers can also be utilised as templates6 for their formation. 

C60 and Cyo fullerenes are able to form complexes with resorcinarene [80]. In this case, the host molecule can guest only one fullerene molecule in its inclusion complex structure. The n-n, CH-n and n-n interactions were observed by CPMAS and FTIR analysis data. Chattopadyay and coworkers [81] investigated the ability of C o and Cyo fullerenes to form ICs with a series of crown ethers. They showed that fullerenes can form 1 1 molecular complexes with crown ethers, but two of them, dibenzeno-24-crown-8 and dibenzeno-30-crown-10, are able to form complexes only with C6o fullerene, not with Cjq. 

Gas-phase binding energies calculated for the penta aquo complexes and crown ether complexes of the actinides studied show that there is no intrinsic preference, or better fit, for actinyl(V) cations as compared to actinyl(Vt) ones. Rather, the ability of Np02" (Np-V) to form in-cavity 18-crown-6 complexes in water is traced to solvation effects in polar solvents. Thus, the effective screening of the charge provided by the macrocycle leads to destabilization of the An(VI) crown complexes relative to their An(V) counterparts." ... 

Ions of diameter significantly smaller than that of the corresponding macrocyclic cavity are not effective for construction of the compounds of interest [12, 17J. However, these products can be synthesised in reasonable yield by using cations larger than the corresponding cavity size [12, 18-20]. The effectiveness of cations in assembling macrocycles is in one to one correspondence with their ability to form complexes with crown ethers, including those with a cavity size smaller than the metal ion diameter. 

The most remarkable property of crown ethers is their ability to form stable complexes with alkali and alkaline-earth metal ions and with ammonium ions. Numerous complexes of crown ethers with nonionic organic molecules are also known [187]. 

Since 1950 a number of polyether antibiotics have been discovered using fermentation technol ogy They are characterized by the presence of sev eral cyclic ether structural units as illustrated for the case of monensm in Figure 16 3a Monensin and other naturally occurring polyethers are similar to crown ethers in their ability to form stable complexes... 

The second ligand type consists of a large group of cyclic compounds incorporating numbers of ether functions as donors. Structure (22) illustrates a typical example. Such crown polyethers usually show strong complexing ability towards alkali and alkaline earth ions but their tendency to coordinate to transition metal ions is less than for the above... 

Polyether complexation. The solution of the above problem is to add a suitable crown ether or cryptand to the alkali metal solution. This results in complexation of the alkali cation and apparently engenders sufficient stabilization of the M+ cation for alkalide salts of type M+L.M" (L = crown or cryptand) to form as solids. Thus the existence of such compounds appears to reflect, in part, the ability of the polyether ligands to isolate the positively charged cation from the remainder of the ion pair. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

The complexing ability of crown ethers in solvents of low polarity has been studied using two-phase partition experiments (Frensdorf, 1971b). The equilibrium between an aqueous solution of the salt (MX) and an organic solution containing the crown ether (Cr) is given by (2). Further dissociation of... 

The influence of additional functional groups on the complexing abilities of 1,3-xyly 1-crown ethers [28] and [50]—[53] and binaphthyl-crown ethers... 

The effect of remote substitutents on the complexing ability of crown ethers [256] towards t-butylammonium salts has been studied by Moore et al. (1977). The results (Table 52) show that electron-attracting substituents decrease the stability of the complex. The t-BuNHJ cation is even more sensitive to remote substituent effects than alkali-metal cations. This fact was attributed to the non-polar character of the alkyl group and to the different structure of the... 

FABMS has been used as a semiquantitative indication of the selectivity of receptors for particular guest metal cations (Johnstone and Rose, 1983). The FABMS competition experiment on [7] with equimolar amounts of the nitrates of sodium, potassium, rubidium and caesium gave gas-phase complex ions of ([7] + K)+ ion (m/z 809) and a minor peak ([7] + Rb)+ ion (m/z 855) exclusively. The relative peak intensities therefore suggested a selectivity order of K+ Rb+ Na+, Cs+, indicative of the bis-crown effect, the ability of bis-crown ether ligands to complex a metal cation of size larger than the cavity of a single crown ether unit, forming a sandwich structure. 

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