Ionizable crown ethers

We do not discuss in detail the cases of tautomerism of heterocycles embedded in supramolecular structures, such as crown ethers, cryptands, and heterophanes, because such tautomerism is similar in most aspects to that displayed by the analogous monocyclic heterocycles. We concentrate here on modifications that can be induced by the macrocyclic cavity. Tire so-called proton-ionizable crown ethers have been discussed in several comprehensive reviews by Bradshaw et al. [90H665 96CSC(1)35 97ACR338, 97JIP221J. Tire compounds considered include tautomerizable compounds such as 4(5)-substituted imidazoles 1///4//-1,2,4-triazoles 3-hydroxy-pyridines and 4-pyridones. 

Dozol, J.F., Bohmer, V., McKervey, A. et al. 1997. New macrocyclic extractants for radioactive waste treatment Ionizable crown ethers and functionalized calixarenes. Report EUR-17615. 

J.-F. Dozol, V. Bohmer, M. A. McKervey, F. Lopez-Calahorra, D. N. Reinhoudt, M. J. Schwing, R. Ungaro and G. Wipff, New macrocyclic extractants for radioactive waste treatment Ionizable crown ethers and functionalized calixarenes, EUR-OP Reference CG-NA-17615-EN-C (EUR-17615), European Commission, Nuclear Science and Technology, Luxembourg, 1997. 

Wai, C. M. (1997) Separation of lanthanides and actinides with ionizable crown ethers, Recent Progress in Actinides Separation Chemistry, Proceedings of the Workshop on Actinides Solution Chemistry, WASC 94, Tokai, Japan, Sept. 1-2, 1994, 81-94. 

The use of ionizable crown ethers to improve SFE applications has so far been restricted to the extraction of mercury ions spiked to uncomplicated matrices such as sand, cellulose filter paper and liquids in any case, the results obtained under mild SFE conditions (viz. 200 bar and 60°C) have been quite acceptable. In addition, the extraction is made highly selective such ions as Cd(Il), Co(II), Mn(Il), Ni(Il), Pb(II) and Zn(ll) are virtually unextractable under these conditions [94]. 

Bartsch, R.A. Charewicz, W.A. Kang, S.I. Walkowiak, W. Proton-coupled transport of alkali metal cations across liquid membranes by ionizable crown ethers. In Liquid Membranes Theory and Applications Noble, R.D., Way, J.D., Eds. ACS Symp. Ser. No. 347 American Chemical Society Washington, D.C., 1987 86-97. 

The ionizable crown ethers which were utilized in the metal ion transport studies include a series of dibenzo crown ether carboxylic acids 1-6 and a series of crown ether phosphonic acid monoethyl esters"2, (Figure 2). Within the first series, the crown ether ring sizeTs systematically varied from 14-crown-4 to 16-crown-5 to 19-crown-6 in compounds 1, 2 and 2 respectively. For compounds 2, 4 and 5, the crown etfier ring size is held constant but tTie attachment site of the lipophilic alkyl group is altered. Finally, for compounds 2 and 6, the crown ether ring size and lipophilic... 

For the jair of ionizable crown ethers Z and 6, the structural modification is to replace four alkyl aryl Fther oxygens with more el ctr n-rjch ialjcyl ether oxygens. The transport selectivity of Na K >L1, Rb, Cs was the same for both carriers which indicates that the variation of oxygen basicity has little Influence upon bulk liquid membrane transport behavior for this compound series. 

The results provided above demonstrate that considerable selectivity can be achieved in alkali metal transport across bulk chloroform membranes by appropriately-structured ionizable crown ether carrier molecules. Although such bulk liquid membrane... 

Results from these experiments demonstrate that alkali metal cation separations can be achieved on an attractive time scale using ionizable crown ethers in liquid surfactant membrane systems. 

Metal ion extraction with crown ethers containing linear lateral groups capable of acid ionization 98PAC2393. 

Another material based on the crown ether extractant 4,4 (5 )-bis(t-butyl-cyclohexano)-18 crown-6, marketed under the name Sr-Spec, is useful for separations involving divalent cations including Pb, Ba, and Ra (Horwitz et al. 1991). For Ra analysis by TIMS, Ra-Ba separations are required because the presence of Ba drastically decreases the ionization efficiency of fg Ra samples from 10% to <1%. This material has been widely used for separations of Ra from Ba (e.g., Chabaux et al. 1994 Lundstrom et al. 1998 Rihs et al. 2000 Joannon and Pin 2001 Pietruszka et al. 2002) and is a complement or alternative to cation exchange separations for EDTA complexes of these elements (Volpe et al. 1991 Cohen and O Nions 1991). Sr-Spec material would also be useful for °Pb analysis, since Pb has a greater distribution coefficient than Sr with this extractant. 

In principle, mass spectrometry is not suitable to differentiate enantiomers. However, mass spectrometry is able to distinguish between diastereomers and has been applied to stereochemical problems in different areas of chemistry. In the field of chiral cluster chemistry, mass spectrometry, sometimes in combination with chiral chromatography, has been extensively applied to studies of proton- and metal-bound clusters, self-recognition processes, cyclodextrin and crown ethers inclusion complexes, carbohydrate complexes, and others. Several excellent reviews on this topic are nowadays available. A survey of the most relevant examples will be given in this section. Most of the studies was based on ion abundance analysis, often coupled with MIKE and CID ion fragmentation on MS " and FT-ICR mass spectrometric instruments, using Cl, MALDI, FAB, and ESI, and atmospheric pressure ionization (API) methods. 

Jankowski, C.K., Dozol, J.F., Attain, F. et at. 2002. Nitration of calixcrown 6 influence on extracting abilities. Use of cesium salts for detection of crown ether macrocycles with the electrospray ionization mass spectrometry technique. Polish J. Chem. 76 701-709. 

The current-voltage curve was interpreted on the basis of the mechanism illustrated in Figure 17.15a, which is derived from the behavior of the same catenane 134+ in solution.116,117 Conformation I is the switch open state and conformation IV the switch closed state of the device. When 134+ is oxidized (+2 V), the TTF unit is ionized in state II and experiences a Coulombic repulsion inside the tetra-cationic cyclophane component, resulting in circumrotation of the crown ether and formation of conformation III (note that in solution at +2 V TTF undergoes two-electron oxidation and the dioxynaphthalene unit is also oxidized).116 When the voltage is reduced to near-zero bias, a metastable conformation IV is obtained... 

Mass spectrometry of (4A,8A)-2,2-bi s-(4-m ethoxy phony l)M,8-dimethyl-1,3,6-dioxathiocane 14 by electrospray ionization mass spectrometry (ESI-MS) method indicates [M+K]+, [M+Na]+, and [M+MeOH+Na]+ as the most important peaks <2001HCA3319>. This phenomena was explained by formation of crown ether-type complexes 24 and 25 (Scheme 1). 

Coordination to a crown ether or a crypfand can also indnce ionization. For example, an X-ray crystal structure of the complex between Np2Mg and 2,1,1 -cryptand showed it to consist of RsMg and cryptand-complexed RMg+ ions. ... 

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