INTRODUCTION:

The alkali metals, due to their large size and low charge density, were known to be the most unwilling to form complexes. The macrocyclic polyethers show a remarkable range of specificity for a wide variety of cations and a thorough understanding of the crown-cation interaction may provide a basis for the rational design of new ligands, which could be used in many areas. These polyethers could be used in the prevention of environmental pollution, the separation of radionuclides from nuclear waste and in the treatment of hazardous waste in storage.¹ Fluoroionophores, consisting of a fluorophore linked to an ionophore (e.g. crown ether), represent another interesting use for crown ethers and related macrocyclic ligands.² From a medicinal point of view, crown ethers and cryptands have been studied for potential applications as chemosensors for potassium and other ions in biological matrices such as blood.³ There is also a growing interest in the use of crown ethers, cryptands and other ligands for radio-immunotherapy treatment of carcinomas.⁴The potassium ion, crown ethers and lariat ethers have plenty of physiological and industrial importance. Keeping this as a view point we have studied interaction of organic salts containing potassium ion with 1,4,7,10,13,16,-hexaoxacyclooctadecane (18-crown-6 ether).

ATERIALS AND METHODS:

The nitrophenols (o-nitrophenol, 2,4-dinitrophenol, 2,4,6- trinitrophenol) and 8-hydroxyquinoline used were of S. Aldrich / E. Merck, A.R. grade. Other chemicals used were also of AR grade. The metal contents were estimated by flame photometric method. Result of elemental analysis of synthesized compounds tallied with required value within experimental error. The melting point of the synthesized compounds were determined on electrical tempo T-1150 melting point 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 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 organic salts

i. Preparation of potassium salt of nitrophenols; K(ONP), K(DNP) and K(TNP) :

About 0.02 mol of appropriate nitrophenol was taken in a conical flask and dissolved in 25 ml of dry ethanol with constant stirring with the help of glass rod. Further 0.02 mol 1.12 gm (0.02 mol) of KOH was dissolved in ethanol and was slowly added to the alcoholic solution of nitrophenol with constant stirring. The mixture was continuously refluxed on hot plate equipped with magnetic stirrer for 35 minutes and the temperature was maintained at 80⁰C. The solution in conical flask was corked and kept standed. On cooling this solution solid crystalline product began to precipitate slowly. Product was filtered, washed with absolute ethanol and dried in an electric oven at 80⁰C.

ii Preparation of sodium salt of 8-hydroxyquinoline; K(8HQ) :

About 2.90 gm (0.02 mol) of 8-hydroxyquinoline was taken in a conical flask and dissolved in 25 ml of ethanol with constant stirring with the help of glass rod. 1.12 gm (0.02 mol) of KOH was dissolved in ethanol and was slowly added to the alcoholic solution of 8-hydroxyquinoline with constant stirring. The mixture was continuously refluxed on hot plate fitted with magnetic stirrer for 45 minutes at 80⁰C. The solution in conical flask was corked and kept standed for overnight. Cream coloured product was obtained. It was filtered, washed with absolute ethanol and dried over KOH desiccator. Physical properties of synthesized strontium salts are given in table -1.1.

Preparation of 18-crown-6 ether :

Preparation of crown ether which may work as a strong complexing host molecule was one of the important part of this research work. 18-crown-6 ether, which is also known as 1,4,7,10,13,16-hexaoxacyclooctadecane was prepared by the synthetic route as reported in literature.^5,6

Table 1.1 Physical properties of potassium salt

Compound	Colour	Melting point (⁰C)	% Nitrogen Found
K(ONP)	Red	280 d	7.90
K(DNP)	Brownish yellow	280 d	12.45
K(TNP)	Yellow	260 e	15.39
K(HQ)	Cream colour	222	7.60

d – decomposition temp, e – explosion temp

Preparation of adducts of 18-crown-6 with potassium salt of o-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenol.

[1] 18C6.K(ONP), [2] 18C6.K(DNP) and [3] 18C6.K(TNP) :

The dried potassium salt (0.002 mol) was suspended in 25 ml dry methanol in a conical flask and heated it with constant stirring to get a clear solution. Stoichiometric proportion of 18-crown-6 ether (0.002 mol, 0.528 gm) was added in this solution. This reaction mixture was refluxed on a hot plate equipped with magnetic stirrer at 50-55 ⁰C. A clear solution was formed. On cooling this solution solid crystalline product began to precipitate slowly. The product separated was allowed to stand overnight and filtered on a buckner funnel. The compound was washed with a little cold dry methanol and dried over KOH desiccator.

18C6.K(ONP) : C₁₈H₂₈NO₉K Yield: 43 %

18C6.K(DNP) : C₁₈H₂₇N₂O₁₁K Yield: 46 %

18C6.K(TNP) : C₁₈H₂₆N₃O₁₃K Yield: 44 %

Preparation of adduct of 18-crown-6 with potassium salt of 8-hydroxyquinoline.

[4] 18C6.K(8HQ) :

About 0.002 mol of potassium salt of 8-HQH was dissolved in 25 ml of dry methanol. 0.002 mol (0.528 gm) of 18-crown-6 ether was added to this solution with constant stirring using hot plate equipped with magnetic stirrer at temperature 50-55 ⁰C. On stirring and refluxing there was slight change in colour of the solution. The refluxed solution was evaporated at reduced pressure to a syrupy mass. The residue was crystallized with hot dichloromethane to yield crystalline solid.

18C6.K(8HQ) : C₂₁H₃₀NO₇K Yield: 42 %

Table -1.2 Physical properties of complexes

Compound	Colour	Ω^-1 cm²mol^-1
18C6.K(ONP)	Light Red	8.6
18C6.K(DNP)	Brown	10.1
18C6.K(TNP)	Light Yellow	10.3
18C6.K(8HQ)	Light Brown	11.4

Figure 1.1 : IR of 18C6.K(ONP)

Figure 1.2 : IR of 18C6.K(TNP)

Figure 1.3 : ¹H−NMR of 18C6.K(TNP)

RESULTS AND DISCUSSION:

Most of the adducts formed by alkali metal ions and crown ethers are consequence of host-guest relationship among interacting cation entering into appropriate hole of encapsulating host molecule. The interactions between metal cations and coordinating molecule are ionic or Vanderwaal forces. The size factor among interacting host-guest is vital factor.⁷ The study of non-covalent binding, extraction reactivity of macrocyclic donor and cationic species has become the area of remarkable importance.⁸ The most commonly used methods of cationic complexation are the picrate extraction, which has been studied by selective electrode method,IR, UV and NMR spectral results.^9,10

Here, it has been found that size factor of crown ether hole and potassium ion diameter largely favour the interaction and formation of potassium ion adducts. In some adducts the parent anionic ligand also contributes remarkably in complexation. The nitro groups present will reduces the electron cloud population of benzene ring creating electron withdrawing capacity and that is provided by centralized electron cloud density of six donor oxygen atoms of 18-crown-6 ether.

The formation of these crown ether complexes with potassium ion is in accordance with previously reported potassium crown ether complexes which suitably fits in size of host-guest encapsulating donor crown ether. The present compounds have composition 18C6.(KL), where KL = K(ONP), K(DNP), K(TNP) and K(8HQ). Analytical results of compounds agreed with calculated elemental analysis within experimental error. Physical properties of synthesized complexes/adducts are given in table-1.2. The methanol solution of compounds shows negligible electrical conductance value (6 – 12 Ω^-1 cm²mol^-1) indicating interaction of potassium salts with 18-crown-6 ether.¹¹ All the compounds are diamagnetic.

Spectral studies and structure of complexes:

The complex compounds isolated and studied in present investigation are of potassium metal. As expected the magnetic and electronic spectral studies are of little significance. All the complexes are soluble in acetone, methanol and dimethyl sulphoxide and sparingly soluble in diethyl ether and benzene.

[1] UV−VIS study :

The electronic spectral studies of complexes with potassium ion will provide only some deviation of p - p*, n - p*, s - p* as well as s - s* transitions. Electronic absorption peaks of complexes are shown in table-1.4. Their band positions shifted due to shifting of electron density from donor atoms towards cationic species. The slight change in spectral band positions are usually taken as either solvent effect or interaction of electron cloud of donor atom of ligand with cationic charge of metal ions. Thus, studies of electronic absorption spectra of compounds will provide a positive evidence of bonding in synthesized complex compounds.

[2] IR study :

The free nitro groups of nitrophenols display -NO_2, n_as and n_s stretching around 1620±15 cm^-1, 1260±10 cm^-1. The -NO₂ bending band is located at 840±10 cm^-1 which has been found to be effected and shifted to lower frequency by 10–15 cm^-1 on bond formation. The phenyl group in all nitrophenols, 8-hydroxyquinoline and dibenzo18-crown-6 display phenyl group (C=C) and (C–H) skeletal vibration at four positions in finger print region. The first phenyl group skeletal vibration is observed at 1590–1620 cm^-1 and second at about 1510±15 cm^-1. The third and fourth band is observed near 1280±10 cm^-1. The IR band observed near 740–780 cm^-1 is attributed to phenyl ring (C−H) out of plane bending band. The absorption at about 1110 cm^-1 is attributed to phenolic (C–O) stretching band. In present study all nitrophenols display n(O-H) freuquency as broad band in the region 3140–3320 cm^-1 and n(C–O) near 1110±10 cm^-1.¹² The n(O–H) disappears in potassium salts and n(C–O) band shifted to higher frequency due to acquiring higher (C–O) bond order on deprotonation. This increase is attributed to bonding of phenolic oxygens (C–O) in all complexes.¹³

The stretching bands of -NO₂ vibrations shifted to lower frequency in complexes. The crown ethers display n(CH₂) stretching vibrations at 2925±10 cm^-1 and these are little effected on bonding with metal ions. The crown ethers in uncoordinated state display n(C–O–C) stretching vibration band near 1140 cm^-1. This n(C–O–C) vibration band shifted to lower frequency by 10 to 15 cm^-1in almost all complexes suggesting involvement of crown ether oxygen in bond formation with potassium ion.¹⁴ Some complexes are hygroscopic in nature and thus their IR spectrum display a broad band of water molecules around 3350–3420 cm^-1, with maxima near 3405 ±10 cm^-1. In the far-IR region new bands, absent in the spectrum of the free ligands, are found in the 425–599 cm^-1region, which may be assigned to the n(M–O_crown) stretching frequency.^15-17 Prominent IR bands of complexes are shown in table-1.3. Thus IR studies of complexes unambigously suggests bonding of potassium salts of 8-hydroxyquinoline and nitrophenols with crown ether oxygen atoms. Some IR-spectrum of complexes were shown in figure 1.1–1.2. Since nature of IR peaks are almost similar thus graph of only 18C6.K(ONP) and 18C6.K(TNP) are given.

[3] ¹H−NMR study :

The study of absorption of radio frequency radiation by a magnetic nucleus in presence of applied magnetic field (NMR) provides effective information about structure of a number of organic and inorganic compounds. Noticeable changes were observed in the chemical shift of proton in 18C6,¹⁸ which moved downfield upon complexation. The chemical shift variation indicates a possible change in the structure and/or electronic environment of proton in these system on complexation. Possible reason for this downfield shift are the conformational change in the macrocyclic skeleton during complexation which could change the position of the aliphatic protons with respect to the phenyl ring, the electric field effect of the metal cation in the complexes or change in bonding to adjacent atoms, which could effect the electron density on the hydrogens through the Fermi contact term.¹⁹ After formation of the [Metal-Crown ether]Ligand complex the ethereal proton chemical shift, d(–CH₂–O–) (3.45-3.65 ppm, 24H, m), shows significant downfield shifting [∆d(–CH₂–O–)=0.12-0.25 ppm], indicating metal-ligand bond formation.^20,21The shift of –CH₂– signals in complexes from free crown ether suggested the coordination of crown ether oxygen with metal ions. Since nature of ¹H–NMR peaks are almost similar thus graph of only 18C6.K(TNP) is given (figure 1.3). On the basis of elemental analysis, conductivity measurement, UV, IR and ¹H-NMR spectral analysis, the bonding pattern and structure of complexes were suggested and shown below.

Proposed structure of complexes of 18-crown-6 ether

[ 18C6.ML ]

(M = K and L = OX = ONP, DNP, TNP or 8HQ)

Figure : 1.4

The coordination of 18-crown-6 ether molecule with potassium ion provides much information regarding bonding owing to change of electron shielding effect, spin-spin coupling and spin-spin splitting pattern. These values provides significant information about the change of magnetic environment of protons on bonding.

ACKNOWLEDGEMENT:

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

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Received on 08.01.2012 Modified on 20.02.2012

Asian J. Research Chem. 5(4): April 2012; Page 547-551

Compound	n(C–H)_bending n(–CH₂)_bending	n(N=O)_{str in}C–NO₂	n_as(C-O-C)	n₁(-NO₂), n₃(-NO₂)	n(C-H)_Phenolic _{out of Plane}	n(M–O_crown)
18C6.K(ONP)	1482, 1330	1218	1133	1607,843	771	505, 599
18C6.K(DNP)	1472, 1375	1247	1121	1605,836	762	452, 528,
18C6.K(TNP)	1482, 1317	1220	1109	1633,838	770	500, 536
18C6.K(8HQ)	1451, 1354	1251	1137	1633,831	741	462, 505,

Compound	Absorption peaks (nm)
18C6.K(ONP)	225, 264, 350
18C6.K(DNP)	232, 360
18C6.K(TNP)	234
18C6.K(8HQ)	242