Synthesis and study of some crown ether complexes of Ba(II) ion by  IR and ¹H−NMR analysis

 

Rajeev Ranjan

PG Department of Chemistry, Ranchi College, Ranchi-834008

*Corresponding Author E-mail: rajeevran7@yahoo.com

The affinity of crown ether for an alkali and alkaline earth metal ion is dependent on the size of the ring of the crown ether with some exceptions. The crown ether-metal ion binding is enhanced in the absence of non-coplanar donor oxygen atoms and electron withdrawing substituents in the crown ether skeleton. The study of absorption of radio frequency radiation by a magnetic nucleus (I¹0), in the presence of applied magnetic field provides useful information regarding structure of a number of organic and inorganic compounds. The present paper describes preparation and characterization of some Ba(II) complexes with 1,4,7,10,13,-pentaoxacyclopentadecane (15C5), 1,4,7,10,13,16,-hexaoxacyclooctadecane (18C6) and 2,3,11,12-dibenzo-1,4,7,10,13,16,-hexaoxacyclooctadeca-2,11-diene (dibenzo18C6), having five and six donar oxygen atoms respectively. Small but noticeable changes were observed in the chemical shift of 1-H in 15C5, 18C6 and, 1-H as well as 2-H, in dibenzo18C6, which moved downfield upon complexation with barium ion. The metal salts used for complexation are salts of nitrophenols. Products were isolated from barium salts of all the three monoionic ligands, 2-nitrophenol (ONPH), 2,4-dinitrophenol (DNPH), 2,4,6-trinitrophenol (TNPH) and 8-hydroxyquinoline (8HQH). The bonding pattern and structure of complexes were suggested from the studies of elemental analysis, molar conductivity, IR, UV-Vis and ¹H-NMR spectral analysis.

 

 

 

INTRODUCTION:

The alkali and alkaline earth 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, 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 alkali metal 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.⁴ The alkaline earth metal ions, crown ethers and lariat ethers have plenty of physiological and industrial importance. Keeping this as a view point I have studied interaction of organic salts containing barium ion with selected synthesized crown ethers.

 

MATERIALS AND METHODS:

The nitrophenols (o-nitrophenol, 2,4-dinitrophenol, 2,4,6- trinitrophenol) and 8-hydroxyquinoline used were of S. Aldrich / E. Merck,  AR 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 points 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 barium salt of nitrophenols; Ba(ONP)₂, Ba(DNP)₂ and Ba(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 mol of barium hydroxide was dissolved in ethanol and itwas 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 barium salt of 8-hydroxyquinoline; Ba(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. Further 0.01 mol of barium hydroxide was dissolved in ethanol and it 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 barium salts are given in table -1.1.

 

(iii)    Preparation of crown ethers:

Preparation of crown ether which may work as a strong complexing host molecule was one of the important part of this research work. 1,4,7,10,13,-pentaoxacyclopentadecane, 1,4,7,10,13,16-hexaoxacyclooctadecane and 2,3,11,12-dibenzo-1,4,7,10,13,16,-hexaoxacyclooctadeca-2,11-diene was prepared by the synthetic route as reported in literature.^5,6

 

Table 1.1: Physical properties of barium salts

Compound	Colour	Melting point (0C)
Ba(ONP)2	Light yellow	285 d
Ba(DNP)2	Deep yellow	275 d
Ba(TNP)2	Deep orange	260 e
Ba(HQ)2	Yellow	245 d

d–decomposition temp, e–explosion temp

 

Preparation of adducts of crown ethers with barium salt of nitrophenols.

The barium salt (0.002 mol) was suspended in 50 ml dry methanol in a conical flask and mixed with stoichiometric proportion of appropriate crown ether (0.002 mol). This reaction mixture was refluxed on a hot plate equipped with magnetic stirrer at 50-55 ⁰C. A clear solution was formed. It was filtered and concentrated to half of its bulk. On cooling this solution, solid crystalline product began to precipitate slowly. The product separated was allowed to stand overnight and filtered on a buchner funnel. The compound was washed with a little cold dry methanol and dried over KOH desiccator.

 

Preparation of adduct of crown ethers with barium salt of 8-hydroxyquinoline.

About 0.002 mol of barium salt was dissolved in 50 ml of dry methanol. Further 0.002 mol of appropriate crown ether was added to this solution and refluxed on a hot plate equipped with magnetic stirrer at temperature 50-55 ⁰C. On stirring and refluxing light brown coloured solution formed. The refluxed solution was evaporated at reduced pressure to a syrupy mass. The residue was crystallized with hot dichloromethane to yield crystalline solid.

 

Table-1.2:  Some physical properties of complexes

Compound	Colour	Temperature in oC Melting/ Decomposition	Conductance Ω-1 cm2 mol-1
15C5.Ba(ONP)2	Light Pink	216	11.2
15C5.Ba(DNP)2	Dark Yellow	228	8.4
15C5.Ba(TNP)2	Dark Brown	241	8.2
18C6.Ba(ONP)2	Red	228	8.5
18C6.Ba(DNP)2	Dark Yellow	256	8.7
18C6.Ba(TNP)2	Chocolate	218	6.9
18C6.Ba(8HQ)2	Light Brown	236	7.8
DB18C6.Ba(DNP)2	Yellow	243	7.7
DB18C6.Ba(TNP)2	Chocolate	219	8.7
DB18C6.Ba(8HQ)2	Light Grey	207	8.6

 

Table-1.3: Prominent IR bands of complexes ( in cm^-1)

Compound	n1 (-NO2), n3 (-NO2)	n (C-H) Phenolic out of plane
15C5.Ba(ONP)2	1609, 819	753
15C5.Ba(DNP)2	1593, 830	772
15C5.Ba(TNP)2	1624, 866	770
18C6.Ba(ONP)2	1604, 837	765
18C6.Ba(DNP)2	1604, 810	770
18C6.Ba(TNP)2	1590, 868	771
18C6.Ba(8HQ)2	1636, 833	759
DB18C6.Ba(DNP)2	1594, 860	769
DB18C6.Ba(TNP)2	1660, 842	769
DB18C6.Ba(8HQ)2	1587, 870	769

 

Table-1.4: Prominent IR bands of complexes ( in cm^-1)

Compound	ns (C–O–C)	n(M–O) / n(M–Ocrown)	n (C–H) str
15C5.Ba(ONP)2	1055	535	2928
15C5.Ba(DNP)2	1050	490, 530, 575	2930
15C5.Ba(TNP)2	1087	508, 525	2926
18C6.Ba(ONP)2	1098	532, 563	2949
18C6.Ba(DNP)2	1100	510, 567	2925
18C6.Ba(TNP)2	1095	525, 595	2866
18C6.Ba(8HQ)2	1108	468, 503, 575	2927
DB18C6.Ba(DNP)2	1055	523, 578	2922
DB18C6.Ba(TNP)2	1056	502, 590	2922
DB18C6.Ba(8HQ)2	1057	523, 592	2922

 

Table-1.5: Prominent IR bands of complexes ( in cm^-1)

Compound	ns (C–H)bending, nas(–CH2–)bending	n(N=O)str  in C–NO2
15C5.Ba(ONP)2	1491, 1330	1223
15C5.Ba(DNP)2	1476, 1325	1219
15C5.Ba(TNP)2	1430, 1327	1218
18C6.Ba(ONP)2	1427, 1354	1251
18C6.Ba(DNP)2	1478, 1330	1257
18C6.Ba(TNP)2	1425, 1350	1219
18C6.Ba(8HQ)2	1468, 1353	1255
DB18C6.Ba(DNP)2	1453, 1366	1248
DB18C6.Ba(TNP)2	1451, 1325	1251
DB18C6.Ba(8HQ)2	1455, 1379	1252

 

 

Table -1.6: Electronic absorption peaks of complexes

 

Compound	Absorption peaks  (in nm)
15C5.Ba(ONP)2	289, 347
15C5.Ba(DNP)2	360, 392
15C5.Ba(TNP)2	232, 347
18C6.Ba(ONP)2	255
18C6.Ba(DNP)2	238, 361
18C6.Ba(TNP)2	235
18C6.Ba(8HQ)2	245
DB18C6.Ba(DNP)2	235
DB18C6.Ba(TNP)2	236
DB18C6.Ba(8HQ)2	239

 

Figure 1.1 : ¹H−NMR of DB18C6.Ba(DNP)₂

 

Figure 1.2 : ¹H−NMR of DB18C6.Ba(HQ)₂

 

Figure 1.3 : ¹H−NMR of DB18C6.Ba(HQ)₂ (Enlarged figure)

 

Figure 1.4 : ¹H−NMR of 18C6.Ba(DNP)₂ (Enlarged figure)

 

RESULTS AND DISCUSSION:

Most of the adducts formed by alkaline earth 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 barium ion diameter favour the interaction and formation of barium ion adducts. In some adducts the parent anionic ligand also contributes remarkably in complexation. The nitro group present will reduces the electron cloud population of benzene ring creating electron withdrawing effect and that is provided by centralized electron cloud density of six donor oxygen atoms of crown ether. 

 

Formation of these crown ether complexes with barium ion is in accordance with previously reported barium crown ether complexes, which suitably fits in size of host-guest encapsulating donor crown ether. The present compounds have composition C.KL₂, where C=Crown ether and KL₂=Ba(ONP)₂, Ba(DNP)₂, Ba(TNP)₂ and Ba(8HQ)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 (6.9–11.2 Ω^-1 cm² mol^-1) indicating interaction of barium salts with crown ether.¹¹

 

Spectral studies and structure of complexes :

The complex compounds isolated and studied in present investigation are of barium ion. 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 barium 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 -NO₂ bending band is located at 842±30 cm^-1 which has been found to be affected and shifted to lower frequency by 10–15 cm^-1 on bond formation. 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 753–772 cm^-1 is attributed to phenyl ring (C−H) out of plane bending band. The absorption at about 1055 cm^-1 is attributed to phenolic (C–O) stretching band. In present study all nitrophenols display n(O-H) frequency as broad band in the region 3140–3320 cm^-1 and n(C–O) near 1080±10 cm^-1.¹² The n(O–H) disappears in barium 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 oxygen (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 affected 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 30 to 45 cm^-1 in almost all complexes suggesting involvement of crown ether oxygen in bond formation with barium 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 468–595 cm^-1 region, which may be assigned to the n(M–O_crown) stretching frequency.^15-17 Prominent IR bands of complexes are shown in table 1.3-1.5. Thus IR studies of complexes suggest bonding of barium salts of 8-hydroxyquinoline and nitrophenols with crown ether oxygen atoms.

 

[3]  ¹H−NMR study :

The study of absorption of radio frequency radiation by a magnetic nucleus in presence of applied magnetic field 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 systems 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 or change in bonding to adjacent atoms, which could affect the electron density on the hydrogen through the Fermi contact term.¹⁹

 

 

                                                            15-crown-5                                                                                                   18-crown-6

 

Figure : 1.5                                                                                                                      Figure : 1.6

 

DB18-crown-6

Figure : 1.7

 

Noticeable changes were observed in the chemical shift of 1-H in 15C5, 18C6 and, 1-H as well as 2-H, in dibenzo18C6, which moved downfield upon complexation with barium ion. After formation of the [Metal-Crown ether]Ligand complex the ethereal proton chemical shift, d(–CH₂–O–) (24H, 3.85-4.15 ppm), shows significant downfield shifting [∆d(–CH₂–O–)= 0.08-0.25 ppm], indicating metal-ligand bond formation. The degree of downfield shift shows the relative strength of the complexes. The ¹H-NMR spectrum of DB18C6 shows peaks at, d₁=3.9–4.2 ppm, (16H, 8 –CH₂O–), d₂=6.8–7.0 ppm, (8H, aryl –CH–), in CDCl₃.^20,21 The 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 DB18C6.Ba(DNP)₂ and DB18C6.Ba(HQ)₂ are given (figure 1.1-1.4) . The coordination of crown ether molecule with barium ion provides significant information about the change of magnetic environment of protons on bonding.

 

ACKNOWLEDGEMENT:

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

 

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Received on 20.05.2013       Modified on 25.04.2013

Accepted on 30.05.2013      © AJRC All right reserved