Complexation ability, crown

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. 

Diederich F, Jonas U, Gramlich V, Herrmann A, Ringsdorf H and Thilgen C 1993 Synthesis of a fullerene derivative of benzo[18]crown-6 by Diels-Alder reaction complexation ability, amphiphilic properties, and x-ray crystal structure of a dimethoxy-1,9-(methano[1, 2]benzomethano)fullerene[60] benzene clathrate Helv. Chim. Acta 76 2445-53... 

Quite recently, Okahara and his coworkers have extended their method to the formation of long chain N-alkylmonoazacrowns. It was expected that such compounds as N-decylmonoaza-18-crown-6 may be useful as new surfactants with complexing ability with metal salts, phase transfer catalysts and selective ion carriers . ... 

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... 

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... 

Drastic changes in complexing ability are observed upon replacing a CH2CH2 unit in 18-crown-6 by other groups. The binding constants for K+ in methanol were 1 x 104 and 2.7 x 104 times lower for compounds [25] and [26]... 

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... 

The complexation ability of crown-ethers has been improved by the introduction of secondary donor sites covalently bonded to the macrocyclic ring through a flexible arm, e.g. "lariat ethers" (27). It is also known that in particular conditions crown-ethers can make 2 1 sandwich complexes with the cation (8). 

Surface-active crown ethers are distinctly differ from usual type of nonionics in salt effect on the aqueous properties, due to the selective complexing ability with cations depending on the ring size of the crown. As shown in Figure 3 (22), the cloud point of the crowns is selectively raised by the added salts. This indicates that the degree of cloud point increase is a measure of the crown-complex stability in water (23). 

Lariat ethers of structure 8 were found to be selective toward Li ion and the lariat crown ether-Li+ complexes are more stable than the corresponding complexes with Na or K+, in methanol. Nevertheless, experiments conducted in aqueous solution showed that Na+ had a better complexation ability than the other two alkali metal cations. Hence, selective complexation of lariat crown ethers with cations changes with the solvent system this may be due in part to the difference in solvation between solvent and cation (Figure 9 f. ... 

Similarly, a lipophilic crown ether is partitioned into the organic phase. Its complexing ability serves to transfer salts with alkali cations into the organic phase. The anions in the organic phase are poorly solvated and highly reactive. The overall reactivity in phase-transfer-catalyzed nucleophilic displacement reactions thus is a function of both the partition coefficient for extraction of the reactive anion into the organic... 

Changes in the electron density of crown ether oxygen atoms greatly affect the stability constants for cation complexation. The electron density on phenol ether oxygens is less than that on alkyl ether oxygens consequently dibenzo[18]crown-6 (2) shows weaker complexing ability than [18]crown-6 (1) and hexabenzo[18]crown-6 does not complex cations at all (80APO(17)279). 

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