Comparative studies of lewis acidity of alkyl-tin chlorides in multicomponent biginelli condensation using grindstone chemistry technique

Article abstract of DOI:10.4067/S0717-97072012000100013

A simple and efficient procedure for the one pot Biginelli Condensation Reaction of aldehydes, β-ketoester and urea employing SnCl ₄.5H₂O/mono/di/tri-butyltin chloride as a novel catalyst is described. Compared to classical Biginelli

Full text of DOI:10.4067/S0717-97072012000100013

J. Chil. Chem. Soc., 57, Nº 1 (2012)
COMPARATIVE STUDIES OF LEWIS ACIDITY OF ALKYL-TIN CHLORIDES IN MULTICOMPONENT
BIGINELLI CONDENSATION USING GRINDSTONE CHEMISTRY TECHNIQUE
HARSHITA SACHDEVA *, REKHA SAROJ, SARITA KHATURIA AND HAR LAL SINGH
Department of Chemistry, Faculty of Engineering and Technology,
Mody Institute of Technology and Science, Lakshmangarh- 332311(Sikar), Rajasthan, India
(Received: June 28, 2011 - Accepted: September 14, 2011)
ABSTRACT
A simple and efficient procedure for the one pot Biginelli Condensation Reaction of aldehydes, β-ketoester and urea employing SnCl .5H2O /mono/di/tri-
butyl- tin chloride as a novel catalyst is described. Compared to classical Biginelli reaction conditions, the present method has the advantages 4of good yields, short
reaction times and experimental simplicity. Further, comparative efficiency of alkyl- tin chlorides in multicomponent Biginelli Condensation Reaction is also
studied under solvent free conditions.
Keywords: Lewis Acids, Dihydropyrimidinones, Biginelli condensation, Grindstone technique, SnCl4.5H2O.
or to dienophile, lowering energy in Diels-Alder reaction. To the best of our
knowledge, neither tin chloride nor alkyl-tin chlorides have been explored as
a catalyst for Biginelli condensation reaction using Grindstone Technology.
INTRODUCTION
The multicomponent reactions (MCR1s) are one of the most
important protocols in organic synthesis and medicinal chemistry1. The
3,4-dihydropyrimidin-2- (1H)-ones (DHPM’s) have recently emerged as
important target molecules due to their therapeutic and pharmacological
properties2 such as antiviral3, antimitotic4, anticarcinogenic5, antihypertensive6
and noteworthy, as calcium channel modulators.7 Owing to the immense
therapeutic and medicinal significance of DHPM’s, exploring convenient and
efficient methods for their synthesis with readily available reagents is of prime
importance.
The development of new strategies for the preparation of complex
molecules in neat conditions is a challenging area of organic synthesis. For
instance, a large number of organic reactions are typically carried out under
anhydrous conditions, using volatile organic solvents like benzene, which are
the cause of environmental problems and are also potentially carcinogenic.
Hence, it is required to develop safe, practical and environmentally friendly
processes.
RESULTS AND DISCUSSION
As per our ongoing efforts to synthesize privileged class of compounds24and
exploiting the inherent capacity of Biginelli reaction to be promoted by
acids, in this communication a novel, simple and effective modification of
Biginelli reaction is reported. The procedure using the grindstone technique is
characterized by high yields of DHPM’s using catalytic amount of SnCl4.5H O
while preserving the original ‘one- pot’ protocol of Biginelli condensati2on
and it also favors environmentally benign reaction conditions. Literature
survey revealed that there are few reports25 on its use in the synthesis of
dihydropyrimidinones. This procedure is superior to the existing methods,
since grinding does not require solvents leading to safe and environmentally
friendly synthesis. Furthermore, the proposed technique does not require
external heating or cooling at any stage, leading to energy efficient synthesis
providing high yields of products.
Therefore, it was thought worthwhile to optimize and compare the
efficiency of substituted tin chlorides as promoters in multi-component
reaction in the absence of any solvent. All tin chlorides have demonstrated
their ability to act as promoter in multi-component Biginelli condensation
reaction (Scheme-I). It was found that tin chlorides employed differed in their
efficiency in terms of yields and purity (Table-1).
The pioneering work of Toda et al8 has shown that many exothermic
reactions can be accomplished in high yield by just grinding solids together
using mortar and pestle, a technique known as ‘Grindstone Chemistry’ which is
one of the ‘Green Chemistry Techniques’. Reactions are initiated by grinding,
with the transfer of very small amounts of energy through friction9. In addition
to being energy efficient Grindstone Chemistry also results in high reactivity
and less waste products. Such reactions are simple to handle, reduce pollution,
comparatively cheaper to operate and may be regarded as more economical and
ecologically favourable procedure in chemistry10. Solid-state reactions occur
more efficiently and more selectively than does the solution reaction, since
molecules in the crystals are arranged tightly and regularly11.
This work focuses on the synthesis of Biginelli compounds, or (DHPM’s)
using SnCl .5H O /mono/di/tri-butyl-tin chloride under the framework of
‘Grindstone4Tec2hnique’.
Biginelli reaction is an important multicomponent reaction for the
condensation of DHPM’s. The classical Biginelli reactions were conducted
under strongly acidic conditions, which suffer from poor yields, long reaction
times, and sensitive functional groups are lost during the reaction conditions.
This has lead to the development of several new methodologies, which improve
the yields compared to the original procedure. These new strategies involve
the combinations of Lewis acids and/or transition metal salts e.g. BF3•OEt2,
montmorillonite (KSF), polyphosphate esters and reagents like InCl312, LiBr13,
TMSCl/NaI14, LaCl3 .7H2O15, CeCl3 .7H2O16, Mn(OAc)3 .2H O17 , InBr18 , FeCl3
and HCl19 , ytterbium triflate20 , Iodine21, ZnCl222, CoCl223 et2c. Although, many
Lewis acids and transition metal salts have been found to catalyze this reaction,
they still have limitations like high cost, prolonged reaction time, and the use
of strong acids. The combination of solvents and long reaction time, costly
chemicals/catalysts makes this method environmentally hazardous. Therefore,
search for a milder and more efficient protocol for the synthesis of dihydro
pyrimidinone continues to draw the attention of researchers.
SnCl4.5H2O is a strong Lewis acid. It is monomeric, highly soluble in
organic solvents as well as in water, easy to handle and therefore attractive
alternative to many Lewis acids. It is extensively used as a catalyst in conjugate
additions. It can bind to the electron-withdrawing group in Michael acceptors
e-mail: drhmsachdevaster@gmail.com
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J. Chil. Chem. Soc., 57, Nº 1 (2012)
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J. Chil. Chem. Soc., 57, Nº 1 (2012)
To check promoter efficiency of catalyst and reproducibility of the reaction,
different aldehydes were reacted with urea to give 13 different compounds. A
mixture of aromatic aldehyde (1), ethyl acetoacetate (2), urea (3), SnCl4.5H2O
/mono/di/tri-butyl-tin chloride are ground together in a mortar using pestle for
nearly 5-10 minutes to give the desired products 4 (Table 1; 4a-m). A catalytic
amount of SnCl4.5H O /mono/di/tri-butyl-tin chloride and friction created by
grinding was suffici2ent to push the Biginelli reaction forward. (Scheme-I).
As indicated in Table-1, catalyzing the reaction by tin chloride gave superior
results over the other three mono, di and tri butyl tin chlorides both in terms
of yields and purity. Monobutyltin chloride had displayed satisfactory results
as compared to di and tributyl tin chloride. We have also observed that Lewis
acid ability of the tin derivative to catalyze-Biginelli condensation decreases on
increasing electron-donating groups. Reaction proceeds faster with mono butyl
tin chloride as compared to di and tributyl tin chloride (Table-1). From this
observation we conclude that Lewis acid character of tin chlorides to catalyze
condensation reaction follows the order:
5-10 min using a mortar and pestle of appropriate size. The initial syrupy
reaction mixture solidifies within 15-20 minutes. The solid mass was left
overnight, then washed with cold water and purified by recrystallization.
Traces of impurities associated with the catalytic modification were removed
either by recrystallization from ethyl acetate and pet ether (1:3) or by column
chromatography of resulting crude material over silica gel using ethyl acetate
and pet ether (1.5:8.5) as the mobile phase. The obtained products were
identified by comparison with authentic samples (synthesized by conventional
process) and from their spectral (1H NMR and IR) data and their melting points.
Method B:
A
mixture of an aromatic aldehyde (10mmol),
ethylacetoacetate(10mmol), urea (20 mmol), tin chloride (5 mmol) were mixed
in R.B. flask and the mixture was magnetically stirred at 70ºC for the time
needed to complete the reaction (as monitored by TLC). The initial syrupy
reaction mixture solidifies within 25-30 minutes. The solid mass was poured
onto crushed ice, filtered and recrystallized by using either ethanol or ethyl
acetate and pet ether (1:3).
All the compounds (4a-m) were synthesized by both the methods and
it was observed that yield of the compound obtained by Method B is high
as compared to that obtained by Method A (Table 1). The spectroscopic
characterization data of DHPM’s (4a-m) are given below:
Tin chloride>Monobutyl-tin chloride > di butyl-tin chloride > tributyl-
tin chloride
This process worked well with aromatic aldehydes possessing either
an electron-donating or electron-withdrawing group. Apparently, for the
employed reaction conditions the nature of the substituents does not affect
significantly the yield of the reactions. The product 4b, which has an electron
donating substituent attached at C-4 position of the aromatic ring was produced
in 85% yields while the dihydropyrimidinone 4d with an electron withdrawing
group was also obtained in 85% yield.
Ethyl-6-methyl-2-oxo-4-phenyl-1, 2, 3, 4-tetrahydropyrimidin-5-
carboxylate (4a) m.p. 201-203 ºC; IR (KBr): 3242, 3117, 2980, 1722, 1645,
1600,1462,1388,1091,781 cm-1; 1H NMR (DMSO-d6): 1.12 (t, 3H, OCH2CH3),
2.27 (s, 3H, 6-CH ), 4.02 (q, 2H, OCH2CH3), 5.11 (s, 1H, CH), 7.18-7.28
(m, 5H, aromatic), 37.31 (s, 1H, N-H), 9.37 (s, 1H, N-H) ppm. Anal.calcd for
C14H16N2O3: C, 64.60; H, 6.20; N, 10.76. Found: C, 64.78; H, 6.22; N, 10.79.
It should be noted that SnCl4.5H2O was used as the sole promoter agent
in neutral media while for others previously reported23 hydrates of metal
halides such as Fe (III), Ni (II) and Co (II) a catalytic amount of conc. HCl
was needed as a Bronsted acid co-catalyst. Reaction proceeded without using
acids as additional proton source. All compounds were obtained in good to
excellent yields. Melting points of all compounds were found to be much closer
to reported substances indicating high purity of the compounds. The structure
E t h y l - 6 - m e t h y l - 2 - o x o - 4 - ( 4 - m e t h o x y p h e n y l ) - 1 , 2 , 3 , 4 -
tetrahydropyrimidin-5-carboxylate (4b) m.p. 194-196 ºC; IR (KBr): 3234,
3110, 2933, 2833, 1703, 1649, 1511,1455,1276,1175,791 cm-1; 1H NMR
(DMSO-d6): 1.12 (t, 3H, OCH2CH ), 2.27 (s, 3H, 6-CH3), 3.72 (s, 3H, OCH3),
4.02 (q, 2H, OCH CH3), 5.23 (s, 13H, CH), 6.75-7.25 (m, 4H, aromatic), 7.22
(s, 1H, N-H), 9.472(s, 1H, N-H) ppm. Anal.calcd for C15H18N2O4: C, 62.06; H,
6.25; N, 9.65. Found: C, 62.23; H, 6.28; N, 9.66.
1
of all the dihydropyrimidinones prepared is characterized by IR and HNMR
and are well correlated with the available literature data.
In the plausible mechanism catalyzed by tin chloride, the initial step is
the formation of imine. The Sn ion co-ordinates with the nitrogen atom of
imine to give an intermediate complex which activates the C=N bond towards
nucleophile. Further, complexation of ß-ketoester with Sn ion increases the
nucleophilicity of a-carbon of enolate, facilitating the attack on imine carbon.
Attack of free amidic group to the carbonyl carbon which is on the β-position
of ester, results in the formation of a six-membered heterocyclic intermediate
which on dehydration gives the desired DHPM’s. This is in harmony with the
mechanism proposed by Kappe et al.26
E t h y l - 6 - m e t h y l - 2 - o x o - 4 - ( 4 - h y d r o x y p h e n y l ) - 1 , 2 , 3 , 4 -
tetrahydropyrimidin-5-carboxylate (4c) m.p. 215-216 ºC; IR (KBr): 3348,
3244, 3082 2989, 2845, 1686, 1638, 1515,1462,1232,1087,759 cm-1; 1H
NMR (DMSO-d ): 1.12 (t, 3H, OCH2CH ), 2.27 (s, 3H, 6-CH ), 4.02 (q, 2H,
OCH2CH3), 5.116(s, 1H, CH), 6.61-7.05 (3m, 4H, aromatic), 7.631 (s, 1H, N-H),
9.45 (s, 1H, N-H), 9.80 (s, 1H, OH) ppm. Anal.calcd for C14H16N2O4: C, 60.86;
H, 5.84; N, 10.14. Found: C, 60.68; H, 5.81; N, 10.10.
E t h y l - 6 - m e t h y l - 2 - o x o - 4 - ( 4 - c h l o r o p h e n y l ) - 1 , 2 , 3 , 4 -
tetrahydropyrimidin-5-carboxylate (4d) m.p. 205-207 ºC; IR (KBr):
3420, 3242, 2985, 2845, 1708, 1645, 1515,1462,1232,1087,759 cm-1; 1H
NMR (DMSO-d ): 1.12 (t, 3H, OCH2CH ), 2.27 (s, 3H, 6-CH ), 4.02 (q, 2H,
OCH2CH3), 5.116(s, 1H, CH), 7.29-7.34 (3m, 4H, aromatic), 7.631 (s, 1H, N-H),
9.47 (s, 1H, N-H) ppm. Anal.calcd for C14H15ClN2O3: C, 57.05; H, 5.13; N,
9.50. Found: C, 57.24; H, 5.16; N, 9.53.
CONCLUSIONS
In the present investigation, tin chloride is found to be superior catalyst over
other three mono/di /tri-butyl-tin chlorides both in terms of yields and purity of
Biginelli compounds. It is found that SnCl4.5H2O works as an excellent catalyst
for the one pot three component and solvent free synthesis of DHPM’s. This
procedure is simpler (preserving the one pot synthesis), economical, milder,
faster, and is also consistent with the green chemistry theme since no solvent is
needed and affords excellent yields.
E t h y l - 6 - m e t h y l - 2 - o x o - 4 - ( 3 - m e t h o x y p h e n y l ) - 1 , 2 , 3 , 4 -
tetrahydropyrimidin-5-carboxylate (4e) m.p. 210-212ºC; IR (KBr): 3234,
3110, 2933, 2833, 1703, 1649, 1511,1455,1276,1175,791 cm-1; 1H NMR
(DMSO-d6): 1.12 (t, 3H, OCH2CH ), 2.27 (s, 3H, 6-CH3), 3.73 (s, 3H, OCH3),
4.02 (q, 2H, OCH CH3), 5.11 (s, 13H, CH), 6.58-7.03 (m, 4H, aromatic), 7.61
(s, 1H, N-H), 9.472(s, 1H, N-H) ppm. Anal.calcd for C15H18N2O4: C, 62.06; H,
6.25; N, 9.65. Found: C, 62.24; H, 6.23; N, 9.62.
Experimental Section
General. Reagents and solvents were obtained from commercial sources
and used without further purification. Melting points were determined on a
Toshniwal apparatus. The spectral and elemental analyses of synthesized
compounds have been carried out at SAIF, Punjab University, Chandigarh.
The purity of compounds was checked on thin layers of silica gel in various
non-aqueous solvent systems, e.g. benzene: ethyl acetate (9:1), benzene:
ethyl acetate: Methanol (8.5:1.4:0.1). IR spectra were recorded in KBr on a
Perkin Elmer Infrared RXI FTIR spectrophotometer and 1H NMR spectra were
recorded on Bruker Avance II 400 NMR Spectrometer using DMSO-d and
CDCl3 as solvent and tetramethylsilane (TMS) as internal reference standa6 rd.
Ethyl-6-methyl-2-oxo-4-(2,4-dimethylphenyl)-1,2,3,4-
tetrahydropyrimidin-5-carboxylate (4f) m.p. 198-200 ºC; IR (KBr): 3234,
3110, 2933, 2833, 1703, 1649, 1511,1455,1276,1175,791 cm-1; 1H NMR
(DMSO-d ): 1.12 (t, 3H, OCH CH ), 2.27 (s, 3H, 6-CH3), 2.35 (s, 3H, 2-CH3),
2.35 (s, 36H, 4-CH ), 4.02 (q,2 2H3, OCH2CH3), 5.11 (s, 1H, CH), 6.74-6.82
(m, 3H, aromatic), 37.61 (s, 1H, N-H), 9.47 (s, 1H, N-H) ppm. Anal.calcd for
C16H20N2O3: C, 66.65; H, 6.99; N, 9.72. Found: C, 66.46; H, 6.97; N, 9.75.
Ethyl-6-methyl-2-oxo-4-(3-hydroxy-4-methoxyphenyl)-1,2,3,4-
tetrahydropyrimidin-5-carboxylate (4g) m.p. 221-223 ºC; IR (KBr): 3234,
3244, 3110, 2933, 2833, 1703, 1649, 1511,1455,1276,1175,791 cm-1; 1H NMR
(DMSO-d6): 1.12 (t, 3H, OCH2CH3), 2.27 (s, 3H, 6-CH ), 3.73 (s, 3H, OCH3),
4.02 (q, 2H, OCH2CH3), 5.23 (s, 1H, CH), 6.42-6.80 (m,33H, aromatic), 7.22 (s,
General procedure for the synthesis of dihydropyrimidinones:
Compounds were synthesized by two different methods:
Method A:Amixture ofanaromaticaldehyde (10mmol), ethylacetoacetate
(10mmol), urea (20 mmol), tin chloride (5 mmol) was ground together for
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J. Chil. Chem. Soc., 57, Nº 1 (2012)
1H, N-H), 9.47 (s, 1H, N-H), 9.83 (s, 1H, OH) ppm. Anal.calcd for C15H18N2O5:
(1991).
C, 58.82; H, 5.92; N, 9.15. Found: C, 59.01; H, 5.94; N, 9.18.
8. Toda, F.; Tanaka, K. and Sekikawa, A., J. Chem. Soc. Chem. Commun.
279, (1987).
Ethyl-6-methyl-2-oxo-4-(4-hydroxy-3-methoxyphenyl)-1,2,3,4-
tetrahydropyrimidin-5-carboxylate (4h) m.p. 230-232 ºC; IR (KBr): 3234,
3244, 3110, 2933, 2833, 1703, 1649, 1511,1455,1276,1175,791 cm-1; 1H NMR
(DMSO-d6): 1.12 (t, 3H, OCH2CH3), 2.27 (s, 3H, 6-CH ), 3.77 (s, 3H, OCH3),
4.02 (q, 2H, OCH2CH3), 5.23 (s, 1H, CH), 6.50-6.67 (m,33H, aromatic), 7.22 (s,
1H, N-H), 9.47 (s, 1H, N-H), 9.83 (s, 1H, OH) ppm. Anal.calcd for C15H18N2O5:
C, 58.82; H, 5.92; N, 9.15. Found: C, 59.01; H, 5.94; N, 9.18.
9. Bose, A. K.; Pednekar, S.; Ganguly, S. N.; Chakraborty, G. and Manhas,
S. M., Tetrahedron Lett. 45, 8351, (2004)
10. Nagendrappa, G., Resonance, 59, (2002).
11. Rothenberg, G.; Dowine, A.P.; Raston, C. L. and Scott, J. L., J. Am.
Chem. Soc. 123, 8701, (2001).
12. Ranu, B. C.; Dey S. S. and Samanta, S., Arkivoc, (iii), 44-50, (2005).
13. Baruah, P. P.; Gadhwal, S.; Prajapati, D. and Sandhu, J. S., Chem. Lett.
31(10), 1038, (2002).
Ethyl-6-methyl-2-oxo-4-(3,4-dimethoxyphenyl)-1,2,3,4-
tetrahydropyrimidin-5-carboxylate (4i) m.p. 146-148 ºC; IR (KBr): 3234,
3110, 2933, 2833, 1703, 1649, 1511,1455,1276,1175,791 cm-1; 1H NMR
(DMSO-d ): 1.12 (t, 3H, OCH CH3), 2.27 (s, 3H, 6-CH3), 3.75 (s, 3H, OCH3),
3.83 (s, 36H, OCH ), 4.02 (q,2 2H, OCH2CH3), 5.23 (s, 1H, CH), 6.73-6.86
(m, 3H, aromatic),37.22 (s, 1H, N-H), 9.47 (s, 1H, N-H) ppm. Anal.calcd for
C16H20N2O5: C, 59.99; H, 6.29; N, 8.74. Found: C, 60.16; H, 6.27; N, 8.79
14. Zhu, Y.; Pan, Y. and Huang, S., Synth .Comm. 34(17), 3167, (2004).
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E t h y l - 6 - m e t h y l - 2 - o x o - 4 - ( 2 - h y d r o x y p h e n y l ) - 1 , 2 , 3 , 4 -
tetrahydropyrimidin-5-carboxylate (4j) m.p. 198-199 ºC; IR (KBr): 3348,
3244, 3082 2989, 2845, 1686, 1638, 1515,1462,1232,1087,759 cm-1; 1H
NMR (DMSO-d ): 1.12 (t, 3H, OCH2CH ), 2.27 (s, 3H, 6-CH ), 4.02 (q, 2H,
OCH2CH3), 5.116(s, 1H, CH), 6.61-7.04 (3m, 4H, aromatic), 7.631 (s, 1H, N-H),
9.45 (s, 1H, N-H), 9.83 (s, 1H, OH) ppm. Anal.calcd for C14H16N2O4: C, 60.86;
H, 5.84; N, 10.14. Found: C, 60.68; H, 5.86; N, 10.17.
E t h y l - 6 - m e t h y l - 2 - o x o - 4 - ( 2 - c h l o r o p h e n y l ) - 1 , 2 , 3 , 4 -
tetrahydropyrimidin-5-carboxylate (4k) m.p. 215-216ºC; IR (KBr): 3420,
3242, 2985, 2845, 1708, 1645, 1515,1462,1232,1087,759 cm-1; 1H NMR
(DMSO-d ): 1.12 (t, 3H, OCH2CH3), 2.27 (s, 3H, 6-CH ), 4.02 (q, 2H,
OCH2CH36), 5.11 (s, 1H, CH), 7.28-7.35 (m, 4H, aromatic), 73.61 (s, 1H, N-H),
9.47 (s, 1H, N-H) ppm. Anal.calcd for C14H15ClN2O3: C, 57.05; H, 5.13; N,
9.50. Found: C, 57.23; H, 5.11; N, 9.53
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E t h y l - 6 - m e t h y l - 4 - ( 3 - n i t r o p h e n y l ) - 2 - o x o - 1 , 2 , 3 , 4 -
tetrahydropyrimidine-5-carboxylate (4l). m.p 217-219°C IR (KBr): IR
(KBr) : 3348, 3244, 3082, 2989, 2845, 1686, 1638, 1515,1462,1232,1087,759
cm-1; 1H NMR (DMSO-d ):1.12 (t, 3H, OCH2CH ), 2.27 (s, 3H, 6-CH3), 4.02
(q, 2H, OCH CH3), 5.11 6(s, 1H, CH), 7.45-8.16 3 (m, 4H, aromatic), 7.61 (s,
1H, N-H), 9.42 7 (s, 1H, N-H) ppm. Anal.calcd for C14H15N3O5: C, 55.08; H,
4.95; N, 13.76. Found: C, 55.26; H, 4.93; N, 13.79.
Ethyl-6-methyl-4-(3,4,5-trimethoxyphenyl)-2-oxo-1,2,3,4-
tetrahydropyrimidin-5-carboxylate (4m) m.p. 213-215 ºC; IR (KBr): 3234,
3110, 3085, 2933, 2833, 1703, 1649, 1511,1455,1276,1175,791 cm-1; 1H NMR
(DMSO-d6): 1.12 (t, 3H, OCH2CH3), 2.27 (s, 3H, 6-CH3), 3.71 (s, 3H, OCH3),
3.72 (s, 3H, OCH ), 3.71 (s, 3H, OCH3), 4.02 (q, 2H, OCH2CH ), 5.11 (s, 1H,
CH), 6.29 (d, 2H,3aromatic), 7.61 (s, 1H, N-H), 9.47 (s, 1H, N-3H) ppm. Anal.
calcd for C17H22N2O6: C, 58.28; H, 6.33; N, 8.00. Found: C, 58.10; H, 6.35;
N, 8.03
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ACKNOWLEDGEMENTS
The authors are thankful to the Dean, Prof. P. K. Das and to the Head of
the Department, Prof. K. Singh (Science and Humanities), FET, MITS, for
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from FET, MITS is gratefully acknowledged. We are also thankful to SAIF
Punjab University, Chandigarh for the spectral and elemental analyses.
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