Synthesis and characterization of CdS and Mn doped CdS nanoparticles and their catalytic application for chemoselective synthesis of benzimidazoles and benzothiazoles in aqueous medium

Article abstract of DOI:10.1016/j.catcom.2012.08.020

CdS and Manganese doped CdS nanoparticles of ~ 2 nm have been prepared at room temperature by a wet chemical technique. The heterogeneous catalysts were fully characterized by XRD, TEM, EDAX, ICP-AES and UV/VIS. These nanoparticles were exploited to study

Full text of DOI:10.1016/j.catcom.2012.08.020

Catalysis Communications 28 (2012) 90–94
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis and characterization of CdS and Mn doped CdS nanoparticles and their
catalytic application for chemoselective synthesis of benzimidazoles and
benzothiazoles in aqueous medium
a,
a
b
Anshu Dandia ⁎, Vijay Parewa , Kuldeep S. Rathore
a
Centre of Advanced Studies, Department of Chemistry, University of Rajasthan, Jaipur-302004, India
Semiconductor and Polymer Science Laboratory. Department of Physics, University of Rajasthan, Jaipur, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2012
Received in revised form 14 August 2012
Accepted 15 August 2012
Available online 23 August 2012
CdS and Manganese doped CdS nanoparticles of ~2 nm have been prepared at room temperature by a wet
chemical technique. The heterogeneous catalysts were fully characterized by XRD, TEM, EDAX, ICP-AES and
UV/VIS. These nanoparticles were exploited to study their catalytic activities towards the chemoselective
aqueous mediated synthesis of 2-aryl benzimidazoles and benzothiazoles. Doping of Mn promotes the activ-
ity and selectivity of CdS nanoparticles indicated by high TOF value, providing the products in good to excel-
lent yields, and with good chemoselectivity. The catalyst could also be recovered easily and used repetitively
four times without significantly affecting the catalytic activity.
Keywords:
Aqueous medium
CdS and Mn doped CdS nanoparticles
Heterogeneous catalyst
© 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Exciting new opportunities are emerging in the field of catalysis
there is still a need to search for better catalysts that could be superior to
the existing ones regarding toxicity, handling, and operational simplic-
ity. To meet the requirement of recyclability, nanoparticles as a catalyst
is opted which has unique properties such as high stability, ease of recy-
clability, easy preparation, possible processability. Mn based catalysts
has attracted a great deal of attention in the past as oxidation catalysts.
Particular for oxidation reactions, redox active transition metals such as
Co, Fe and Mn have been investigated as promoter elements [13]. None-
theless, the outcome of the promoting effect of the Mn depends on its
concentration and its location within the catalyst [14]. It has also been
reported that MnO acts as a structural promoter, increasing the stability
of the catalyst [15]. As a part of our continuing activities to explore the
novel protocols for heterocyclic frameworks [16–18] and nanoparticles
synthesis by wet chemical method [16,19], we report herein the synthe-
sis, characterization and catalytic application of CdS and Mn doped CdS
nanoparticles for selective oxidative synthesis of 2-aryl benzimidazoles
and benzothiazoles by the cyclocondensation reaction of aldehydes and
o- phenylenediamine or o-aminothiophenols in aqueous medium re-
spectively (Scheme 1).
based on nanotechnology approaches. Nanoparticles have attracted
considerable attention in catalysis: their unique properties, intermedi-
ate between those of the bulk and the single particles, combine the ad-
vantages of heterogeneous catalysis (recovery and recyclability) with
those of homogeneous catalysis (low loadings and good selectivity) [1].
To speak of bioactive molecules, 2-aryl benzimidazoles and
benzothiazoles behold an arena in diverse array of such com-
pounds. Various pharmacophores bearing these heterocyclic units
have revealed broad spectrum of biological activities in past [2,3]
as well as recent medicinal chemistry applications [4,5]. The poten-
tialities of these compounds have led to them being encountered
through the development of various synthetic protocols, several
approaches for the synthesis of these compounds were developed
[
6–9]. Most widely used protocol involves the direct condensation
of o- phenylenediamine/o-aminothiophenols with aryl aldehydes
using variety of oxidative and catalytic reagents [10–12] e.g. sulfamic
3 2 3 3 3 4
acid, DDQ, Oxone, FeCl ·6H O, In(OTf) , Yb(OTf) , Sc(OTf) , KHSO .
However, all of these procedures suffer from one or more of the
following disadvantages such as long reaction times, low yields of
the products, harsh reaction conditions, use of excess amounts of re-
agents, tedious workup procedures, and co-occurrence of several side
reactions with less selectivity of the process. In addition, some of the
catalysts and reagents are expensive, toxic, and air sensitive. Therefore,
2. Results and discussion
2.1. Synthesis and characterisation of catalyst
CdS NPs and Mn doped CdS nanoparticles were synthesized
through the wet chemical precipitation method, which is a promis-
ing technique for synthesis of these NPs with excellent catalytic
properties and high stability. The advantages of this method lie in
its simplicity, cost effectiveness, environment friendliness, easier
⁎
Corresponding author.
E-mail address: dranshudandia@yahoo.co.in (A. Dandia).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.08.020

A. Dandia et al. / Catalysis Communications 28 (2012) 90–94
91
with 4-chlorobenzaldehyde (1 mmol) and o-phenylenediamine/
aminothiophenol (1 mmol) as the model substrate. With 4-
chlorobenzaldehyde (1 mmol), o-phenylenediamine/aminothiophenol
(
1 mmol) and CdS nanoparticles (10 mol%) the condensation gave a
yield of 79% at 90 °C for 2.5 h. Further the study of catalytic ability of
Mn doped CdS nanoparticles showed that doping increase the product
yield to 95% and reducing the reaction time (1.5 h) with decrease the
catalyst loading(5 mol%). These nanoparticles were used in water as
obtained by wet chemical method. Additionally, water served as a suit-
able solvent for the currently probed transformation as well, based on
the solubility difference of the product from the starting materials, lead-
ing to separation of product from the reaction mixture upon comple-
tion, thereby facilitating easy isolation of solid product from the
reaction mixture.
Literature reveals that in the oxidative cyclization of aldehyde
and o-phenylenediamine the formation of the required 2-aryl-1H-
benzimidazole is accompanied by the occurrence of 1-benzylated
Scheme 1. Chemoselective synthesis of 2-aryl benzimidazoles and benzothiazoles.
2
-aryl-1H-benzimidazoles as side products [23], and sometimes
these disubstituted derivatives have been isolated as the main product
[24,25]. The present protocol gives 2-aryl-1H-benzimidazole selectively.
Further we extended our studies on oxidative reactions of aldehydes
with o-aminothiophenol. Although thiols are good nucleophiles and SET
agents [26], in present studies, no substitution of the halogen atom or
the nitro group, dealkylation/debenzoylation took place as reported
earlier [27–29]. Further the dithioacetal formation is a common reaction
of aldehydes with thiols [30], no competitive dithioacetal formation was
observed under the present conditions.
Under these reaction conditions, we performed a series of experi-
ments for reactions of o-phenylenediamine/aminothiophenol with
different aldehydes. The results are summarized in Table-1, aldehydes
bearing both electron-donating and electron-withdrawing substitu-
ents afforded desired products in excellent yields. Heterocyclic alde-
hydes were well tolerated under these conditions. It was found that
only 43% yield was observed under similar conditions in the absence
of catalyst, even after 4 h. It was proved that the catalyst did play an
important role in this reaction. The effect of catalyst concentrations
on the reaction rate and yield of the product was also investigated. It
was found that the optimum reaction rate and yield could be achieved
at the catalyst concentration of 5 mol%. Elemental analysis (by ICP-AES)
of the product after completion of the reaction showed that there was
no nanocatalyst present in the final product.
scaling up for large scale synthesis while avoiding the use of high
pressure, temperature and toxic chemicals as required in earlier
method [20]. The nanostructure of the nanoparticles has been char-
acterized by XRD, TEM, EDAX, ICP-AES and UV/VIS (See Supporting
Information).The diffraction peaks appearing for all the samples at
2
θ values of about 26.55, 44.03 and 52.08 correspond to the (111),
(
220) and (311) planes of cubic phase of CdS and the addition of
Mn to CdS nanoparticles does not create any change in the CdS ma-
trix (Fig. 1). The average particle size of Mn doped CdS nanoparticles
and pure CdS is ~2 nm, which is confirmed from TEM image. It shows
abundance of spherical particles (Fig. 2).
EDAX has been employed to confirm the Mn doping in the so-
prepared samples of nanoparticles (Fig. 3). In UV a slight blue shift is ob-
served with doping of Mn (Fig. 4). This slight blue shift in the absorption
edge is the outcome of the quantum confinement effect produced due to
increased nucleation rate with increase in doping concentration [21].
The surface Lewis acidity of this material was confirmed through
the adsorption of pyridine vapour [22] on the surface of the NPs.
FT-IR spectra of pyridine adsorbed on the catalysts are shown in Fig. 5
(
See Supporting Information).
2
.2. Catalytic performance in chemoselective synthesis of benzimidazoles
and benzothiazoles
2.3. Catalysis and comparison of catalytic activity of Mn doped CdS
nanoparticles with other catalysts
To examine the feasibility of CdS NPs catalysed reaction
in aqueous medium, an initial experiment was performed
A comparison of efficiency of catalytic activity of Mn doped CdS NPs
with CdS and several other catalysts is presented in Table 2. It was found
-
1
that the best result in terms of turnover frequency (602 h ) could be
achieved by using Mn doped CdS NPs catalyst (5 mol% loading), as com-
−
pared to undoped CdS NPs (TOF=142 h ).This result is in agreement
with our working hypothesis that a higher concentration of acidic
sites gives more products in the reaction. Therefore, lower catalyst load-
ing required for this transformation as compared to many other catalyt-
ic systems. These results show that this method is superior to the other
methods in terms of yield and reaction time.
Based on the above observations, a plausible mechanism for the for-
mation of 3 is depicted in Scheme 2. The Lewis acid sites of NPs were co-
ordinated to the oxygen of carbonyl groups. The coordination of NPs
with the carbonyl oxygen of 2 induces electrophilic activation of alde-
hyde, which benefits the initial condensation of 1 with 2 to give an
imine A, the subsequent intramolecular nucleophilic cyclization of A
afforded B, which subsequently undergoes an oxidation promoted by
Mn(IV) to give 3 as the final product (Mn (IV) species formed in situ
by oxidation of Mn (II) with air). At the same time, the in situ formed
Mn (IV) could be reduced to regenerate Mn(II) for the next catalytic
cycle.
Fig. 1. XRD patterns of nanoparticles.

92
A. Dandia et al. / Catalysis Communications 28 (2012) 90–94
Fig. 2. TEM image of nanoparticles.
2
.4. Recyclability
After the completion of the reaction, the nanoparticles were recycled
3. Experimental
3.1. Catalyst preparation
by separating them from the reaction mixture by sonication and used as
a catalyst for the same reaction again (Table 3). Powder XRD of reused
nanoparticles is also recorded which shows that the structural integrity
remains unaltered after the reaction and thus proves the efficiency of
nanoparticles as a recyclable catalyst.
Nanoparticles of CdS were prepared at room temperature by dropping
simultaneously 50 ml of 1 M solution of CdSO
of Na S into 200 ml of distilled water containing 50 ml of 0.1 M solution
of EDTA, which was vigorously stirred using a magnetic stirrer under Ar
4
and 50 ml of 1 M solution
2
Fig. 3. EDX spectrum of the nanoparticles.

A. Dandia et al. / Catalysis Communications 28 (2012) 90–94
Table 1 (continued)
93
°
S.
No.
R
X
Time Product
(h)
Yield Mp ( C)
(%)
⁎
1
3. 4-BrC
6
H
5
S
1.5
S
N
93
129–131
Br
14. 2-OMeC
H
6 5
S
2.0
OMe
96
107–109
S
N
1
1
1
1
5. 4-NO
2
C
6
H
5
S
S
S
S
2.5
3.0
2.0
1.5
S
91
91
97
95
231–233
97–99
O
2
N
N
6. 2-Thienyl
S
N
S
7. 4-MeC
6
H
5
S
82–84
Me
Cl
N
8. 4-ClC
6
H
5
S
111–113
Fig. 4. UV spectrum of the nanoparticles.
N
⁎
Isolated yield.
Table 1
Synthesis of benzimidazoles and benzothiazoles.
°
S.
No.
R
X
Time Product
(h)
Yield Mp ( C)
(%)
⁎
atmosphere. The role of EDTA was to stabilize the particles against aggre-
gation which may lead to an increase in the particle size. The doping of
1
.
C
6
H
5
NH 1.5
H
N
93
292–294
203–205
Mn has been done by adding 2 wt.% of metal sulphate (MnSO
4
) to
CdSO (at the starting of the reaction) for the formation of CdS:Mn
4
N
NPs. The precipitate was separated from the reaction mixture and
was dried at room temperature. After sufficient drying, the precipi-
tate was crushed to fine powder with the help of mortar and pestle.
2
.
3-NO
2
C
6
H
5
NH 2.5
O N
92
2
H
N
N
3
4
.
.
3-ClC
2-ClC
6
6
H
5
NH 1.5
NH 1.5
Cl
97
94
235–237
231–233
3.2. Determination of surface acidity
H
N
To check the nature of acid sites present in NPs, the material was
kept in contact with pyridine vapor in a closed container for 24 h.
Pyridine adsorbed material was analyzed with FT-IR spectra.
N
H
5
Cl
H
N
3.3. Catalyst activity measurement:
N
5
6
7
8
9
.
.
.
.
.
4-NO
2
C
6
H
5
NH 2.0
NH 1.5
NH 1.5
NH 1.5
NH 2.5
H
N
91
98
95
95
87
313–315
274–276
287–289
224–227
330–331
In a 50 ml round bottom flask, o-phenylenediamine/aminothiophenol
1 mmol) and aromatic aldehydes (1 mmol) in water (4 ml) were mixed
2
O N
(
N
4-MeC
6
H
5
H
and stirred at 90 °C. To this, catalyst was added. The progress of the reac-
tion was checked on TLC. After completion, the reaction mixture was
cooled at room temperature. Then, it was extracted with ethyl acetate;
the organic layer was dried over sodium sulphate and concentrated in a
vacuum to afford the crude products. The pure products were obtained
by recrystallization from ethanol. The structure of the products was
N
Me
Cl
N
4-ClC
6
H
5
H
N
N
4-OMeC
6
H
5
H
N
MeO
N
2- Thienyl
H
N
Table 2
Comparison of catalytic activity of catalyst.
S
N
Yield (%)⁎
TOF(h-1
)
Entry
Condition
MeSO H (10 mol%), H
Time (h)
1
1
0.
C
6
H
5
S
S
1.5
1.5
S
N
94
96
110–111
124–125
1.
2.
3.
4.
5.
6.
7.
3
2
O,
3.5
3.5
3.0
3
2.5
2.5
1.5
74
80
77
65
67
79
95
95
103
116
98
118
142
602
PTSA (10 mol%), H
CAN (10 mol %), H
2
O,
O,
1. 4-OMeC
H
6 5
S
2
ZnS powder (10 mol%), H
CdS powder (10 mol%), H
2
O ,
O ,
MeO
2
N
CdS NPs (10 mol%), H
2
O ,
O,
12. 2-OHC
6
H
5
S
1.5
OH
98
225–227
Mn:CdS NPs(5 mol%), H
2
S
N
Reaction conditions: 4-chlorobenzaldehyde (1 mmol), o-phenylenediamine
aminothiophenol (1 mmol) in water as a solvent at 90 °C.
/
⁎
Isolated yield.

94
A. Dandia et al. / Catalysis Communications 28 (2012) 90–94
Scheme 2. Plausible mechanism.
Table 3
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Isolated yield.
[
[
1
unambiguously established on the basis of their spectral analysis (IR, H
1
NMR, 13C NMR and GC–MS mass spectral data).
[
[
4
. Conclusions
[
[
In this study, CdS and Mn doped CdS nanoparticles have been pre-
[
pared, characterized and used as efficient and chemoselective cata-
lysts for the synthesis of 2-aryl benzimidazoles and benzothiazoles.
The enhanced catalytic activity and product yield by Mn doping
could be attributed to the increase of surface acidity. Overall, this
methodology offers competitive advantages such as recyclability of
the catalyst without further purification or without using additives
or cofactors, low catalyst loading, broad substrate applicability, and
high yields. The application of this new nanocatalyst in organic syn-
thesis will provide a novel pathway for the synthesis of pharmaceuti-
cally pertinent compounds.
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4
[
(
[
[
[
[
[
[
[
Appendix A. Supplementary data
4
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.08.020.
4
6
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