Chromogenic crown ethers

Two new chromogenic crown ether derivatives based on an identical design principle have been synthesized. The lipophilic KBC-002 shown in Figure 23 is a useful new chromo- ionophor for the highly selective calcium determination with cation exchange type optode53. 

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]. 

A chromogenic crown ether (AA -bis-(2-hydroxy-5-nitrobenzyl)4,13-diazadibenzo-18-crown-6) in a mixed micellar medium was proposed for the determination of Cd in water (the detection limit was 6 ppb) [1]. The preconcentration of Cd on a chelating resin prepared by coupling Pyrocatechol Violet to Amberlite XAD-2 was studied [2]. Quantitative adsorption was achieved at pH 5-7. Nitric acid (4 M) was used as elluent. The effects of various anions (fluoride, chloride, nitrate, sulfate and phosphate) on the adsorption of Cd (Zn, Pb(Il) and Ni) were investigated. 

Y. P. Wu and G. E. Pacey, Spectrophotometric Determination of Lithium Ion with the Chromogenic Crown Ether, 2",4"-Dinitro-6"-Trifluoromethyl-phenyl-4 -Aminobenzo-14-Crown-4. Anal. Chim. Acta, 162 (1984) 285. 

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... 

Tucker and co-workers have recently reported a ferrocene ion-pair receptor which acts as a chromogenic molecular switch [24]. Appended to one cyclo-pentadienyl ring of the ferrocene of molecule 28 is a phenyl urea unit for anion binding and to the other a crown ether for cation binding. On addition... 

Figure 2. Chromogenic chiral crown ethers 16 and guests 17 for which the enantiomers can be distinguished by the color.

Macrocyclic ionophores are molecules whose atoms are organized to form a cavity into which metal ions fit and bind with high affinity. Such compounds are also called polycyclic ethers, crown ethers, cryptands, or cryptahemispherands. Different macrocyclics can be made with cavities tailored to fit the ionic radii of different elements. When chromogenic properties are imparted to these ionophores, spectral shifts... 

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]. 

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 cationic complex of silver and 1,10-phenanthroline (phen) has been found to react with Bromopyrogallol Red to yield an ion-associate which can be extracted into nitrobenzene (e = 3.2T0" at 590 nm) [31,32]. The cationic complex, Ag(phen)2 gives ion-associates also with acid dyes, such as Rose Bengal (formula 4.35) and eosin (formula 4.34) (nitrobenzene) [33]. Extractable ion-associates of cationic silver complex with 1,4,8,11-tetrathiacyclotetradecane (crown ether), and various chromogenic anions [34] should also be mentioned. 

The complexing affinity of crown ethers opens up the prospect of the molecular design of chromogenic reagents sensitive to alkali metal/alkaline-earth metal ions [266-268], A simple design for such a reagent molecule would be the introduction of a monobasic and chromophor into a crown ether, such as in... 

In the acetonitrile solution a compound 2c has in the absorption spectrum an intensive long-wave band with a maximum at X=435 nm. On the addition of alkaline Na, K and alkaline-earth metals Mg, Ca, Ba into the solution of perchlorates a hypsochromic shift of the long-wave band is observed [13]. With the concentration of perchlorate Cm> 10 M the absorption band maximum is at X=392 nm (e,nax=3.69 10 1/molxcm), at A,=396 nm = 3.62 10 1/molxcm) and at k=407 nm (6, 3x=3.66 10 1/molxcm) in case of Mg. Ca and Ba ", respectively. Similar spectral changes were earlier observed when studying complexation of metal ions with the crown ethers not containing styryl chromogene [20]. Thus we attribute the observed spectral changes to the complexation. 

Gromov, Sergei P., Fedorova, Olga A., Alfimov, Michail V., Tkachev, Valery V. and Atovmyan, Lev O. 1991 Crown ether styryl dyes. 3. Synthesis and X-ray crystal analysis of chromogenic 15-crown-5-ether , Dokl. Acad. Nauk SSSR 319,1141-1144. 

Alfimov, M.V., Vedernikov, A.I., Gromov, S.P., Fedorov, Yu.V., Fedorova, O.A., Churakov, AV., Kuz mina, L.G., Howard, J.A.K., Bossmann, S., Braun, A, Woemer, M., Sears, D.F., Saltiel, J. (1999) Synthesis, structure and ion selective complexation of trans and cis isomers of photochromic dithia-18-crown-6 ethers, J. Am. Chem. Soc., 121, 4992-5000 b) Stanislavskii, O.B., Ushakov, E.N., Gromov, S.P., Fedorova, O.A, Alfimov, M.V. (1996) Crown-containing styryl dyes. 14. The influence of N-substitute length on the complex formation of betainic chromogenic 15-crown-5-ether with alkaline earth metal cations, Russ. Chem. Bull, 45, 564-572. 

Gromov, S.P., Fedorova, O.A., Ushakov, E.N., Buevich, AV., Alfimov, M.V. (1995) Crown-containing styryl dyes. 15. Synthesis and two pathways of the regio- and stereospecific cation-dependent [2+2]-autophotocycloaddition of chromogenic 15-crown-5 ether betaines of quinoline series, Russ. Chem. Bull, 44, 2131-2136. 

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