INTRODUCTION:

Crown ethers are macrocyclic polyethers whose structure possess hole capable of trapping cations by coordinating with lone pair of electrons on oxygen atoms forming stable complexes. In crown ethers, the hetero atom binding sites face inwards toward the cavity while the hydrocarbon backbone faces outward. This creates an electron rich center which is ideal for cation complexation. This causes exceptional stability of complexes. The size of the cation and its fit into the macrocyclic internal cavity of the ligand is a dominant factor. The number of donor atoms in the ring of the macrocycle does not affect the binding strength. The macrocyclic polyethers show a remarkable range of specificity for a wide variety of cations. The cryptands, podands and lariat ethers are excellent host for accommodating alkali and alkaline earth metal ions. The strontium 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 strontium ion with 18-crown-6 ether.

MATERIALS AND METHOD:

The nitrophenols (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 agreed with required value within experimental error. The melting point of the synthesized compounds was 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. Infrared spectral results were obtained from SAIF, CDRI, Lucknow. IR spectra were recorded by Perkin Elmer Spectrum RX 1 (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 obtained from SAIF, CDRI, Lucknow.

Experimental:

Preparation of organic salts:

(i) Preparation of strontium salt of nitrophenols; Sr(DNP)₂ and Sr(TNP)₂ :

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 0.01 mole of strontium hydroxide 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 45 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 strontium salt of 8-hydroxyquinoline; Sr(8HQ)₂ :

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. 0.01 mole of strontium hydroxide 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 stand 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 method as reported in literature.^1,2

Table -1.1:Physical properties of strontium salts

Compound	Colour	Melting point (⁰C)	% Nitrogen Found
Sr(DNP)₂	Deep yellow	270 d	12.28
Sr(TNP)₂	Deep orange	260 e	15.34
Sr(HQ)₂	Cream Yellow	248	7.45

d - decomposition temp, e - explosion temp

Preparation of adduct of 18-crown-6 with strontium salts of 2,4-dinitrophenol and 2,4,6-trinitrophenol.

[1] 18C6.Sr(DNP)₂and [2] 18C6.Sr(TNP)₂ :

The appropriate dried strontium salt (0.002 mol) was suspended in 25 ml of 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.Sr(DNP)₂ : C₂₄H₃₀N₄O₁₈Sr Yield: 44 %

18C6.Sr(TNP)₂ : C₂₄H₂₈N₆O₂₀Sr Yield: 48 %

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

[4] 18C6.Sr(HQ)₂ :

About 0.002 mol of strontium salt of 8-hydroxyquinoline 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 product.

18C6.Sr(HQ)₂: C₃₀H₃₆N₂O₈Sr Yield: 42 %

Table -1.2: physical properties of complexes

Compound	Colour	Ω^-1 cm²mol^-1
18C6.Sr(DNP)₂	Deep Yellow	10.2
18C6.Sr(TNP)₂	Yellow	11.3
18C6.Sr(8HQ)₂	Cream Colour	7.6

Figure 1.1 : IR of 18C6.Sr(TNP)₂

Figure 1.2 : ¹H−NMR of 18C6.Sr(TNP)₂

Table -1.3:Prominent IR bands of complexes

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.Sr(DNP)₂	1467, 1352	1249	955	1639, 838	754	456,524
18C6.Sr(TNP)₂	1434, 1334	1268	963	1625, 848	772	510,560
18C6.Sr(8HQ)₂	1466, 1353	1249	952	1638, 838	756	475,520

Table -1.4: Electronic absorption peaks of complexes

compound	Absorption peaks (nm)
18C6.Sr(DNP)₂	240, 326, 361
18C6.Sr(TNP)₂	235, 347
18C6.Sr(8HQ)₂	245

RESULTS AND DISCUSSION:

In present study, the adducts formed by strontium ion and crown ether are consequence of host-guest relationship among interacting cation entering into appropriate hole of encapsulating host molecule.³ The study of non-covalent binding, extraction reactivity of macrocyclic donor and cationic species has become the area of remarkable importance.⁴ The most commonly used method of cationic complexation is the picrate extraction, studied by elemental analysis, IR, UV and NMR spectral results.^5,6

The crown ether interaction with strontium salts have been tried for isolation of solid complexes of strontium ion. Here, it has been found that size factor of crown ether hole and strontium ion diameter is not much favorable for formation of strontium ion adducts but, the parent anionic ligand contributes remarkably in complexation. The isolation of the products is due to the presence of electron withdrawing groups in ligands, which reduces the electron density of benzene ring creating electron deficiency and that is provided by centralized electron cloud density of six donor oxygen atoms of 18-crown-6 ether. The synthesized compounds have composition 18C6.Sr(L)₂ where Sr(L)₂= Sr(DNP)₂, Sr(TNP)₂ and Sr(HQ)₂. 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 (7–12 Ω^-1 cm²mol^-1) indicating interaction of strontium ion with 18-crown-6.⁷ As expected all the compounds are diamagnetic.

Spectral studies and structure of complexes:

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 strontium ion will show 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. 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 ion. Thus, study of electronic absorption spectra provide a positive evidence of bonding in synthesized complex compounds.

[2] IR study:

The phenyl groups in nitrophenols and 8-hydroxyquinoline display phenyl (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 free nitro groups of nitrophenols display -NO_2, n_as and n_s stretching around 1625±15 cm^-1, 1260±12 cm^-1. The -NO₂ bending band is located at 840±20 cm^-1 which has been found to be affected and shifted to lower frequency by 10–15 cm^-1 on bond formation. The IR band observed near 750–780 cm^-1 is attributed to phenyl ring (C–H) out of plane bending band. In present study all nitrophenols display n(C–O) near 1120 ± 10 cm^-1.⁸ The n(O–H) disappears in sodium salts and n(C–O) band shifted to higher frequency. This increase is attributed to bonding of phenolic oxygens (C–O) in all complexes.

The stretching bands of -NO₂ in Sr(DNP)₂, Sr(TNP)₂ and Sr(HQ)₂ is at around 1605 cm^-1, 1620±5 cm^-1 and 1640–1645 cm^-1. These vibrations shifted to lower frequency in complexes. The n(NO₂) located near 840±15 cm^-1 also shifted to lower vibrational 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 1120±10 cm^-1. This n(C–O–C) vibration band shifted to lower frequency by 10 to 60 cm^-1in almost all compounds suggesting involvement of crown ether oxygen in bond formation with strontium ion.⁹ 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 425–560 cm^-1region, which may be assigned to the n(M–O_crown) stretching frequency.^10-12 Prominent IR bands of complexes are shown in table-1.3. Thus IR studies of complexes suggests bonding of strontium salts of 8-hydroxyquinoline and nitrophenols with crown ether oxygen atoms. Since nature of IR peaks are almost similar thus graph of only 18C6.Sr(TNP)₂ (fig-1.1) is given.

[3] ¹H−NMR study:

The study of absorption of radio frequency radiation by a magnetic nucleus (I¹O) in presence of applied magnetic field (NMR) provides effective information regarding structure of a number of organic and inorganic compounds. Small but noticeable changes were observed in the proton chemical shift in 18C6,¹³ 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 is the conformational change in the macrocyclic skeleton during complexation which could change the position of the aliphatic protons with respect to the phenyl ring, which effect the electron density on the hydrogens.¹⁴

After formation of the [Metal–Crown ether]Ligand complex the proton chemical shift of d(–CH₂–O–) (3.90–3.96 ppm, 24H, m) shows significant downfield shifts [∆d(–CH₂–O–)=0.12–0.25 ppm], indicating metal–ligand bond formation.¹⁵The shift of –CH₂– signals in complexes from free crown ether unambiguously suggests the coordination of crown ether oxygen with metal ion. Since nature of ¹H–NMR peaks are almost similar thus graph of only 18C6.Sr(TNP)₂ (fig-1.2) is given. On the basis of elemental analysis, IR, UV 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.ML₂]

(M = Sr and L = OX = DNP, TNP and 8HQ)

Figure : 1.3

Thus coordination of 18-crown-6 molecule with strontium ion provides much information regarding bonding pattern and extent of coupling constant. These values provide significant information about the change of magnetic environment of protons on bonding, and change in orientation of the molecules on the formation of new bond.

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.

REFERENCES:

1. C. J. Pedersen, J. Am. Chem. Soc, 89, 7017, 1967

2. C. J. Pedersen, J. Am. Chem. Soc, 92, 386, 1970

3. P. D. Beer, J. Chem. Soc., Chem. Commun, 689, 696, 1996

4. L. Fabbrizzi and A. Pogg, Chem. Soc. Review, 24, 174-202, 1995

5. K. Wang and G.W. Gokel, Pure and Appl. Chem, 68, 6, 1267, 1996

6. P. D. Beer and J. Cadman, Coord. Chem. Review, 205, 131, 2000

7. W. J. Geary, Coord. Chem. Review, 7, 81, 1971

8. J. C. Bailer Jr. and B. D. Sharma, J. Am. Chem. Soc, 77, 5476, 1955

9. R. A. Condrate and K. Nakamoto, J. Chem. Phys, 42, 2590, 1965

10. T. Lu, X. Gan, N. Tang and M. Tan, Polyhedron, 9, 2371, 1990

11. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3^rd, Ed, John Wiley, New York, 1978

12. D. J. Olszanski and G. A. Melson, Inorg. Chim. Acta, 26, 263, 1978

13. M. J. Wilson, R. A. Pethrick, D. Pugh and M. S. Islam, J. Chem. Soc, Faraday Trans, 93, 11, 2097-2104, 1997

14. G. R. Fulmer, J. M. A. Miller and N. H. Sherden, Organometallics, 29, 2176, 2010

15. M. J. Wilson, R. A. Pethrick, D. Pugh, M. S. Islam, J. Chem. Soc, Faraday Trans, 93, 387, 1997.

Received on 03.12.2011 Modified on 10.12.2011

Asian J. Research Chem. 4(12): Dec., 2011; Page 1941-1944