Macrocyclic extractants crown ethers

CEA Cadarache and seven European universities were involved in a research programme to synthesize and test new macrocyclic extractants (crown-ethers and calixarenes). The main aim of this study was to selectively remove caesium, strontium and actinides from medium level liquid waste (MLLW) to decontaminate them to the extent that they can be disposed of in a near surface site. A new calixarene has been synthesized by the University of Parma, which has a cesium/sodium selectivity 100 times higher than that of the best current extractant for cesium. The large selectivity of this molecule has been explained theoretically. The results obtained for cesium extraction from simulated MLLW have been confirmed with real HLLW. Finally, new functionalized calixarenes have been also synthesized, which are more selective to actinides and lanthanides than the best extractant available on the market. 

This review groups the information published on degradation of the main families of extractants studied in the frame of long-lived minor-actinide and fission-product recovery (1-4) (see Chapter 1) alkyl-phosphorus compounds (phosphates, phosphonic acids, bifunctional compounds like CMPO), amide compounds (dialkyl-amides, malonamides, and diglycolamides), N-donor compounds, and macrocycles like crown ethers and calixarenes (Table 8.1). The multicomponent systems based on the chlorinated cobalt dicarbollide process have not been considered. 

Neutral-Species Extractants. Ketones, ethers, linear poly-ethers, and more recently, macrocyclic or "crown ethers all have useful properties as extractants if they are sufficiently water insoluble. Sulfoxides, the trialkyl phosphates, the trialkylphosphine oxides, and the carbamoylmethylphosphonates are among the stronger coordinators classified as neutral species extractants. These compounds usually coordinate directly with the metal ion and supply coordination that would be suppli by water in the aqueous phase. 

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-... 

A series of N-alkyl aza-18-crown-6 efhers were synfhesized and characterized. These monoaza-substituted crown ethers in ILs were investigated as recyclable extractants for separation of Sr " and Cs from aqueous solutions. The pH-sensitive complexation capability of these ligands allows for a facile stripping process to be developed so that both macrocyclic ligands and ILs can be reused. 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

Dozol, J.F., Dozol, M., Macias, R.M. 2000. Extraction of strontium and cesium by dicarbollides, crown ethers and functionalized calixarenes. J. Incl. Phenom. Macrocycl. Chem. 38 (1-4) 1-22. 

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. 

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. 

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. 

Finally, both upper and lower rim substitution have been used to produce a class of macrocycles referred to as calixarene-crown ethers, or calixcrowns. One simple example is shown here (103). These receptors combine characteristics of the crown ethers and calixarenes, and have been intensively studied for metal ion extraction, in particular, for the removal of cesium from nuclear waste. ... 

Crown ether complexes of the -block metals number in the many hundreds, " and reviews focused on them, including their use in separation chemistry and selective ion extractions, are extensive. Growing interest has been expressed in the use of macrocyclic ethers in the design of electroactive polymers. ... 

An example of crown ether applied in selective separation of alkali metals is dibenzo-18-crown-6 (formula 1.15). Extractive separations of metal ions are also performed with macrocyclic ligands containing nitrogen or oxygen atoms, as well as macrocycles with combinations of oxygen, nitrogen, and sulphur atoms (N-0, S-0, N-S) [45,48]. A macrocyclic compound with only nitrogen hetero-atoms (formula 1.16) is selective for copper. 

Macrocycles (crown ethers, cryptands) with chromogenic groups combine the natural selectivity of macrocycles with the possibility of direct spectrophotometric determination of some metals (e.g., K, Ca) in an organic phase after extraction [127-129]. 4-Picrylamlnobenzo-15-crown-5 crown ether (formula 4.41) is applied in the extraction and spectrophotometric determination of potassium. The determinations are based on extractable ion-associates of metals (e.g., Li, Na, K, Pb) with crown ethers and xanthene or sulphophthalein dyes [130]. 

Macrocycllc compounds (some crown ethers and cryptands) are selective reagents for extractive separation of alkali metals [22-27]. These ligands form cationic complexes with alkali metal ions, and these can be extracted as ion-pairs with suitable counter-ions e.g., picrate) [28], most often into chloroform. For potassium, p-nitrophenoxide was used as counter-ion [29]. In cases, where a coloured anionic complex is a counter-ion [30], the extract may serve as a basis for determining the alkali metal. The effect of the structure of the dibenzo-crown ether rings upon the selectivity and effectiveness of isolation of alkali metals has been studied in detail [31]. Chromogenic macrocyclic reagents applied for the isolation and separation of alkali metals have been discussed [32]. 

Macrocyclic reagents, such as chromogenic crown ethers of the type 14-crown-4 (extraction to 1,2-dichloroethane in the presence of picrate) were used for determination of Li [53]. The 14-crown-4 type derivatives have been applied for determination of Li in blood by a continuous FIA method [54,55]. The use of chromogenic reagents aza-12 (-13 or -14)-crown-4 has also been proposed [56,57]. The effect of substituents on the selectivity of separation of Li (and Na) by means of benzo-14-crown-4 and 13-crown-4 ethers was studied [58]. A review of chromogenic macrocyclic reagents used for determination of lithium (and other alkali metals) has been published [32]. 

Sodium can be determined directly by selective methods involving chromogenic macrocyclic reagents, e.g., crown ethers of the type 15-crown-5 (e= 1.4 10 at 422 nm) [60], of the type 12-crown-4 (with picrate counter-ion, in 1,2-dichloroethane, e = 1.8 10" at 375 nm) [61], 18-crown-6 [62,63], crown ether with azo group [64], cryptand-(2,l,l) (picrate, toluene, 350-fold excess of K does not interfere) [65], and (2.2.2)-cryptand [63]. The crown ether, benzo-18-crown-6, has been applied in the FIA technique [62], These reagents have been used for the determination of Na in blood. The co-extraction of Cs and Na was studied with the use of various crown ethers [66]. A review of the reagents has been published [32]. 

A very sensitive method for determining calcium is based on the complex of Ca with the chromogenic macrocyclic reagent (formula 14.4) (1,2-dichloroethane, e = 5.5-10 at 406 nm) [65]. Other diaza-crown ethers have been also used in determinations of Ca (and Mg) [66]. Calcium has been determined after extraction (CHCI3 + benzene) with the crown ether and association with Propyl Orange [67]. 

The use of macrocyclic compounds for extraction of various analyte cations in the presence of suitable anions is still growing. Extraction-spectrophotometric methods for the determination of alkali metals (with picrate counter ion) using 18-crown-6 and its derivatives [12-15] and calix[4]arene crown ethers [16] have recently been developed. The application of hexa-acetatocalix[6]arene for extraction of Fe [17] and Pb [18] have been described. 

---