Crown ethers chromogenic complexes

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... 

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]. 

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. 

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. 

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