INTRODUCTION:

The macrocyclic polyethers having lipophilic hosts show a remarkable range of specificity for a wide variety of cations.^1-4 Solvent extraction is extensively used as an indirect method for determining the complexing behavior of various polyether-salt systems.⁵ The cation selectivity of macrocyclic ligands can be determined by different types of experiments among which, solvent extraction,^6-8 stability constant determination and permeabilities of cations through macrocycle-containing liquid membranes⁹ are important.Sandwiching complexation, substitution effect, ring size effect, uncommon complex stoichiometry of a series of crown ethers by solvent extraction experiments have been reported.^10-12 Present paper reports cation binding behavior of synthesized benzocrown ether derivatives (fig-1.1) with some bivalent metal ions by using solvent extraction method.

MATERIALS AND METHODS:

All chemicals used were of AR grade. The commercially available regents were used without further purification. The metal contents were estimated by flame photometric method. Results of elemental analysis of synthesized compounds agreed with required value within experimental error. The melting points of synthesized compounds were determined on electrical tempo T-1150 apparatus. Molar conductivities of the compounds were measured using Systronic conductivity meter-306. The conductivities of the compounds were measured at the concentration 10^-3 M in methanol solvent at 30(±0.5)⁰C. IR spectra were recorded by Perkin Elmer spectrometer RX1 (4000-450 cm^-1). UV-visible spectral data were recorded through Systronic double beam spectrophotometer-2203 (600-200 nm). The ¹H−NMR spectra of ligand and crown ether complexes were recorded in CDCl₃ by Bruker DRX-300.

EXPERIMENTAL:

Preparation of salt of 2,4,6-trinitrophenol, M(TNP) :

About 4.58 gm (0.02 mol) of 2,4,6-trinitrophenol was taken in a conical flask and dissolved in 25 ml of ethanol with constant stirring with the help of a glass rod. Further 0.01 mol of appropriate alkaline earth metal hydroxide was dissolved in ethanol and was slowly added to the alcoholic solution of 2,4,6-trinitrophenol with constant stirring. The mixture was continuously refluxed on hot plate equipped with magnetic stirrer for 50 minutes and the temperature was maintained at 78⁰C. The solution in conical flask was corked and kept standed. Then coloured crystalline product was obtained. It was filtered, washed with absolute ethanol and dried in an electric oven at 80⁰C. Some physical properties of synthesized alkaline earth metal salts are given in table-1.1.

Table – 1.1

Physical properties of alkaline earth metal salts

Compound	Colour	Melting point (⁰C)	% Nitrogen Found
Mg(TNP)₂	Dull orange	270 e	17.36
Ca(TNP)₂	Dull orange	260 e	16.86
Sr(TNP)₂	Deep orange	260 e	15.34
Ba(TNP)₂	Deep orange	260 e	13.80

e – explosion temp

Preparation of crown ethers:

Preparation of crown ether which may work as a host molecule was one of the important part of this research work. Crown ethers, were prepared by the known synthetic methods as reported in literature. Benzo-15-crown-5 (1)^13-17, 4′-iodo-benzo-15-crown-5 (2), 4′-amino-benzo-15-crown-5 (3),¹⁸dibenzo-18-crown-6 (5),^13-17 2,14-diamino-6,7,9,10,17,18,20,21-octahydro-5,8,11,16,19,22-hexaoxadibenzo[b,k]cyclooctadecene(6),¹⁹ 2,13-diamino-6,7,9,10,17,18,20,21-octahydro-5,8,11,16,19,22-hexaoxadibenzo[b,k]cyclooctadecene (7)¹⁹ and benzocrown ether derivative 4,²⁰ were prepared according to the reported procedure.

Figure : 1.1

In solvent extraction method, metal cations were extracted by crown ether from a water phase into an organic phase.^21-24 It is a method of separation based on the transfer of a solute from one immiscible solvent into another. The solvents, CH₂Cl₂and H₂O were saturated with each other prior to use for preventing volume changes of both phases during extraction. Equal volumes of CH₂Cl₂solution (10 ml) of the respective crown ether (0.3 mmol/l) and an aqueous solution of alkaline earth metal picrate (0.03 mmol/l) were introduced into a stoppered Erlenmeyer flask and the mixture was shaken for 30 minutes in an incubater thermostated at (28.0±0.2)^oC. This mixture was then allowed to stand for 6 hour at that temperature for complete phase separation. The concentration of alkaline earth metal picrates in the aqueous phase was determined through measuring the absorbance at 354 nm by UV-vis spectrometer. The extractability of bivalent metal ions (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) by synthesized crown ethers is listed in table-1.2.

Table – 1.2

Solvent extraction of aqueous bivalent metal salt with benzocrown ether derivatives^a

Ligand	Extractability^b %
Ligand	Mg²⁺	Ca²⁺	Sr²⁺	Ba²⁺
1	0.46	0.28	0.83	0.52
2	0.11	0.26	0.27	0.12
3	0.46	0.62	0.69	0.29
4	0.10	0.26	0.35	0.10
5	0.46	0.64	0.10	0.16
6	2.42	4.45	1.65	2.86
7	3.14	7.24	4.12	1.64

^aTemperature (28.0±0.2) ^oC; aqueous phase (10 ml), [picrate] = 0.03 mmol/l; organic phase (CH₂Cl₂, 10 ml), [Ligand] = 0.3 mmol/l. ^b Percent of picrate extracted into the organic phase. Average of two independent runs.

RESULTS AND DISCUSSION:

The obtained results, when compared with the relevant data for compounds 1–3 and 5–7, have furnished further understanding of the complexation behavior of the benzocrown ether derivatives with alkaline earth metal ions.^25-27 In the solvent extraction of bivalent metal picrates, crown ethers 1-7 generally show low extractability as expected. Modified single armed 4, shows hardly any extraction effect, indicating very weak complexation between 4 and bivalent cations. Crown ether 3 shows high extractability with Ca²⁺and Sr²⁺. The crown ether 7 gives high extractability for Sr²⁺and Ca²⁺and also about 4.5 times Ca²⁺/Ba²⁺ selectivity. It indicates strong complexation behavior of crown ether 7.

ACKNOWLEDGEMENT:

The author thank to the Chairman, UGC, New Delhi, for providing financial assistance to this research programme under UGC-Minor Research Programme. The author further extend his sincere thank to the Head, SAIF, CDRI, Lukhnow, for providing IR-spectra, ¹H-NMR spectra and necessary facilities.

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Received on 12.11.2012 Modified on 19.12.2012

Asian J. Research Chem. 5(12): Dec., 2012; Page 1479-1481