Crown ethers metal extractants

The common disadvantage of crown ethers as extractants of amino compounds is the interference from hard metal ions, such as alkali and alkaline earth cations. This may cause problems in extraction-based analytical and technological applications, as these metals, particularly sodium and potassium, are often present in (bio)media of interest. For illustration. Fig. 1 shows the influence of alkali metals on the extraction-photometric determination of benzylamine with DC18C6 and picrate [56,70]. 

The concept of accomplishing lanthanide/actinide separations by exploiting the differences in the reaction kinetics of the metal ions is another innovative concept with intriguing potential. Separations based on the use of alkaline solutions fall into this category, as does the use of crown-ether caboxylate extractants. 

There are fewer reports of linear, acyclic, ion-binding polymers. It has been reported that poly(oxyethylene) improves the solubility of alkali metals in ethers such as tetrahydrofuran, dime thoxy ethane, and diglyme, stabilizes fluorenyl alkali metal compounds, accelerates Williamson reactions and accelerates several other nucleophilic reactions.All of these effects were attributed to the ability of poly(oxyethylene) to complex with cations in solution. Yanagida and coworkers studied the alkali metal cation complexation of poly(oxyethylene), using a picrate salt extraction technique similar to the one used by Pedersen and Frensdorff. Polymers with more than 23 oxyethylene units were effective iono-phores for potassium, with degrees of extraction (percent extracted) comparable to crown ethers. The extractability per oxyethylene unit was nearly constant, and the complex stability increased linearly with increasing numbers of repeating oxyethylene units. Seven oxyethylenes were the minimum number of repeat units necessary to bind potassium ion effectively in the aqueous phase. The less efficient extraction of short-chain poly(oxyethylene) is apparently caused by its hydrophilic character. 

Cesium isotopes can be recovered from fission products by digestion in nitric acid, and after filtration of waste the radioactive cesium phosphotungstate is precipitated using phosphotungstic acid. This technique can be used to prepare radioactive cesium metal or compounds. Various processes for removal of Cs isotopes from radioactive waste have been developed including solvent extraction using macrocycHc polyethers (62) or crown ethers (63) and coprecipitation with sodium tetraphenylboron (64). 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

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

Macrocyclic ligands such as crown ethers have been widely used for metal ion extraction, the basis for metal ion selectivity being the structure and cavity size of the crown ether. The hydrophobicity of the ligand can be adjusted by attachment of alkyl or aromatic ligands to the crown. Impressive results have been obtained with dicyclohexano-18-crown-6 as an extractant for Sr in [RMIM][(CF3S02)2N] IL/aque-... 

On the other hand, Bartsch et al. have studied cation transports using crown ether carboxylic acids, which are ascertained to be effective and selective extractants for alkali metal and alkaline earth metal cations 33-42>. In a proton-driven passive transport system (HC1) using a chloroform liquid membrane, ionophore 31 selectively transports Li+, whereas 32-36 and 37 are effective for selective transport of Na+ and K+, respectively, corresponding to the compatible sizes of the ring cavity and the cation. By increasing the lipophilicity from 33 to 36, the transport rate is gradually... 

A similar study was undertaken on the related crown ether systems 201 <2001PS29>. They all showed moderate extraction of both Ag(l) and Hg(ll) ions and so were less selective than compounds 184a and 184b from the previous study. However, the presence of the benzo-15-crown-5 substituent offered the simultaneous complexation of the hard alkali cation Na(l) as well as the thiophilic metals Ag(l) and Hg(n) by the thieno sulfur. Interestingly, this second extraction was not influenced by the presence of the other metal. 

Photoresponsive systems incorporating an azobenzene moiety. The capped crown ether (196), shown as the (E) isomer, was synthesized initially by a high-dilution condensation between diaza-18-crown-6 and 3,3 -bis(chlorocarbonyl)azobenzene (Shinkai et al., 1980). Extraction patterns for the alkali metals differed between the (E) and (Z) isomers giving a clear example of photochemical control of the complexation behaviour. Subsequently, the analogue (197) was synthesized in which 2,2 -azopyridine was used for the cap (Shinkai Manabe, 1984). Photo-... 

Sr2+ so well that it was selectively extracted from a bulk sample of a barium salt (Helgeson et al., 1973a). Binding constants for metal-cation complexes of 1,3-xylyl-crown ethers [66]—[69] carrying an additional carboxylate binding... 

A different concept of chiral recognition was used by Lehn et al. (1978) for the differentiation between pairs of enantiomeric anions. Following the terminology used for metallo-enzymes, the chiral crown ether [309] acts as an apo-receptor, complexing a metal cation and thus becoming a chiral metal receptor that may discriminate between enantiomeric anions (cascade-type complexation). Extraction experiments with racemic mandelic acid dissolved in... 

Crown ethers are not chromogenic unless they contain a pendant chromogen able to dissociate a proton in a basic medium. The resulting anion interacts strongly with the crown-complexed cation compensating the electric charge. The formation of a zwitterion leads to a hydrophobic extractable species with a considerably shifted absorption maximum compared with the protonated species. This allows the same spectrophotometric determination to be used for a large number of metal ions, provided the appropriate crown compound is used in each case. Another method involves... 

Crowns with Stilbene Fluorophores. Stilbenes undergo cis-trans isomerism on irradiation and this feature is exploited in (3.82). In the case of the frany-isomer, large ionic radius ions such as K+, and Cs, give fluorescent complexes whilst the smaller Lb and Na " cause quenching. UV irradiation to the dy-isomer causes a considerable change in the conformation of the crown ether substructure and increases its ability to extract alkali metals, including Lb and Na+, from water into benzene. 

Chun, S., Dzyuba, S. V., Bartsch, R. A., Influence of structural variation in room-temperature ionic liquids on the selectivity and efficiency of competitive alkali metal salt extraction by a crown ether. Anal. Chem., 73,3737-3741,2001. 

Although the thermodynamic data reported for the solvent extraction equilibria are limited, we obtained a well-correlated AH-TAS compensation plot (Figure 22) for the solvent extraction of aqueous metal picrates with crown ethers in organic solvents. Quite interestingly, the slope (a 0.73) and the intercept (TASq 2.6) are very... 

For this puq)ose, the photoswitchable bis(crown ether)s 88 and 89 as well as the reference compound 90 have been synthesized. Compounds 88 and 89 are highly lipophilic derivatives of azobis(benzo-15-crown-5). The parent azobis crown ether was originally developed by Shinkai and its photoresponsive changes in complexation, extraction, and transport properties thoroughly examined. Compared to 87, more distinct structural difference between the cis and trans isomers can be expected for 88 and 89 because in the latter compounds the 15-crown-5 rings are directly attached to the azobenzene group. The photoequilibrium concentrations of the cis and trans forms and the photoinduced changes in the complexation constants for alkali metal ions are summarized in Table 7. 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

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