Cobalt manganese oxide nanoparticles as recyclable catalyst for efficient synthesis of 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles under solvent-free conditions

Article abstract of DOI:10.1002/aoc.3283

Cobalt manganese oxide nanocatalyst was synthesized and it was found that it is a highly efficient green catalyst for the synthesis of 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles under solvent-free conditions. The marked advantages of this method are the si

Full text of DOI:10.1002/aoc.3283

Full Paper
Received: 7 December 2014
Revised: 26 December 2014
Accepted: 27 December 2014
Published online in Wiley Online Library: 17 February 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3283
Cobalt manganese oxide nanoparticles as
recyclable catalyst for efficient synthesis of
2-aryl-1-arylmethyl-1H-1,3-benzimidazoles
under solvent-free conditions
Hossein Ahmadiana, Hojat Veisia*, Changiz Karamib, Alireza Sedrpoushanc,
Maryam Nourid, Fariba Jamshidid and Iman Alavioone
Cobalt manganese oxide nanocatalyst was synthesized and it was found that it is a highly efficient green catalyst for the synthesis
of 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles under solvent-free conditions. The marked advantages of this method are the sim-
ple experimental procedures, shorter reaction times, high yields of product, reusable and non-toxic catalyst and solvent-free con-
ditions. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: cobalt manganese oxide; nanocatalyst; solvent-free; benzimidazoles
from Nanostructured & Amorphous Materials Inc. (http://www.
Introduction
nanoamor.com). Infrared spectra were obtained with
a
Recently, supported nanoparticles have been extensively studied as
catalysts for synthesis of organic compounds. As the particle size
decreases, the relative number of surface atoms increases, and thus
the activity increases.[1] Nanoparticle catalysts can also be easily
separated and recycled with more retention of catalytic activity
than their bulk counterparts.[2]
PerkinElmer GX FT-IR spectrometer. Melting points were deter-
mined using a Barnstead Electrothermal 9300 melting point
apparatus. 1H NMR spectra were recorded with Bruker 200
MHz NMR spectrometer.
Benzimidazoles and their derivatives constitute an important
class of heterocyclic compounds which possess a wide range of
therapeutic and pharmacological properties. The derivatives possess
various biological activities such as antioxidant,[3] antimicrobial,[4]
5-lipoxygenase inhibitory,[5] anticancer,[6] antihypertensive,[7] anti-
inflammatory,[8] antihelmintic[9] and antiprotozoal activities.[10] The
traditional synthesis of benzimidazoles involves the reaction be-
tween an o-phenylenediamine and a carboxylic acid or its
derivatives (nitriles, amidates, orthoesters) under harsh dehydrating
conditions.[11] Very recently, a literature survey revealed several
methods for synthesis of benzimidazole and its derivatives
using L-proline,[12] glyoxalic acid,[13] SiO2/ZnCl2,[14] Fe(ClO4)3,[15]
trimethylsilyl chloride,[16] silica sulfuric acid,[17] oxalic acid,[18] mesopo-
rous metal oxide nanocrystals,[19] Indion 190 resin,[20] sodium
dodecylsulfate–water,[21] nano-In2O3[22] and alumina–sulfuric acid.[23]
Generally, in the past we have used a new nano-zinc catalyst with
Co and Mn in the synthesis of bis(indolyl)methanes.[24] In continua-
tion of our works[25–28] and in the application of this catalyst, we
showed that the zinc-supported nanocatalyst is usable, generaliz-
able and also efficient for the synthesis of benzimidazole derivatives.
Preparation of Catalyst
Aqueous solutions of Co(NO3)2Á6H2O (0.5 mol lÀ1) and Mn(NO3)
2Á6H2O (0.5 mol lÀ1) with 1:6 molar ratios [Mn/Co] and nano-ZnO
(15 wt%) were pre-mixed in a round-bottomed flask fitted with a
condenser and the resulting solution heated. Aqueous Na2CO3
(0.5 mol lÀ1) was added slowly to the mixed nitrate solution, which
was continuously stirred whilst the temperature was maintained
isothermally in the range 35–85°C. The final pH achieved was 10.
This procedure took approximately 15 min to complete and was
refluxed for 5–6 h. The precipitate was first filtered and then
washed several times with warm distilled water until no further
*
Correspondence to: Hojat Veisi, Department of Chemistry, Payame Noor
University, Tehran, Iran. E-mail: hojatveisi@yahoo.com
a Department of Chemistry, Payame Noor University, Tehran, Iran
b Department of Chemistry, Faculty of Science, Islamic Azad University,
Kermanshah Branch, Kermanshah, Iran
c
Institute of Industrial Chemistry, Iranian Research Organization for Science and
Technology, Tehran, Iran
Experimental
Materials and Instrumentation
d Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University,
Kermanshah, Iran
All reagents and solvents were commercially available and were
used as such. Zinc oxide (ZnO, 99.5%, 20 nm) was purchased
e School of Chemistry, College of Science, University of Tehran, Tehran, Iran
Appl. Organometal. Chem. 2015, 29, 266–269
Copyright © 2015 John Wiley & Sons, Ltd.

Synthesis of benzimidazoles using cobalt manganese oxide nanoparticles
Results and Discussion
Nanocrystals of cobalt manganese oxide nanocatalyst were
characterized. The catalyst mainly consists of oxides Mn2O3 (cubic)
and Co3O4 (cubic). Transmission electron microscopy (TEM) and
scanning electronic microscopy (SEM) investigations were carried
out to observe the morphology of the prepared catalyst. The TEM
image in Fig. 1(a) reveals the product consists of spherical particles
with an average size of 20–25 nm. SEM was used to obtain
information on the microstructural and metal dispersion properties
(Fig. 1(b)).
To choose optimum conditions, first we prepared 1-(4-
chlorobenzyl)-2-(4-chlorophenyl)-1H-1,3-benzimidazole from the
reaction of 4-chlorobenzaldehyde (2 mmol) and o-phenylene-
diamine (1 mmol) as a model reaction in the absence and presence
of the cobalt manganese oxide nanocatalyst under thermal
solvent-free conditions. As evident from Table 1, this transforma-
tion requires 0.1 g of cobalt manganese oxide nanocatalyst at
80°C for 10 min for the preparation of 2-aryl-1-arylmethyl-1H-1,3-
benzimidazole derivatives.
Scheme 1. Synthesis of 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles (NPs,
nanoparticles).
Na+ was observed in the washings tested by flame atomic absorp-
tion. The precipitate was then dried at 150°C for 16 h.
General Procedure for Synthesis of 2-Aryl-1-arylmethyl-1H-1,
3-benzimidazoles
A mixture of o-phenylenediamine (1 mmol) and aryl aldehyde (2
mmol) was stirred with cobalt manganese oxide nanoparticles
(0.1 g) at 80°C for the required period of time (Scheme 1). The
reaction was monitored using TLC (n-hexane–acetone, 7:3). After
completion of the reaction, H2O (10 ml) was added. The mixture
was extracted with CH2Cl2 (4 × 10 ml) and dried over anhydrous
MgSO4. Evaporation of the solvent under reduced pressure gave
the crude products. Finally, the crude product was recrystallized
from ethanol (90%).
The range of [Co]/[Mn] solution ratios varied from 100% Mn to
100% Co and the catalytic performance for the synthesis of 2-aryl-
1-arylmethyl-1H-1,3-benzimidazoles was studied. Reaction time
decreases with increasing concentration of cobalt. However, the
shortest reaction time is related to the ratio [Co]/[Mn] = 6:1. To find
Table 1. Optimum conditions for the preparation of 2-aryl-1-
arylmethyl-1H-1,3-benzimidazoles under solvent-free thermal condi-
tions at 80°Ca
Entry
Solvent
Catalyst
weight
(g)
Catalystb
Temp. Time Yield
(°C)
(h)
(%)c
Effect of solvent
1
2
3
4
5
EtOH
0.1
0.1
0.1
0.1
0.1
Co/MnNPs
Co/Mn NPs
Co/Mn NPs
Co/Mn NPs
Co/Mn NPs
80
80
80
80
80
5
70
50
40
50
96
H2O
5
n-Hexane
CH3CN
5
5
Solvent-free
0.25
Effect of catalyst weight
9
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
0.04
0.06
0.08
0.1
Co/Mn NPs
Co/Mn NPs
Co/Mn NPs
Co/Mn NPs
Co/Mn NPs
80
80
80
80
80
0.25
2
27
60
80
97
97
10
11
12
13
4
2
0.12
5
Effect of temperature
15
16
17
Solvent-free
Solvent-free
Solvent-free
0.1
0.1
0.1
Co/Mn NPs
Co/Mn NPs
Co/Mn NPs
80
60
40
0.25
0.25
0.25
96
80
30
Effect of catalyst
18
19
20
21
22
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
—
None
80
80
80
80
80
0.16
0.16
0.16
0.16
0.16
—
20
30
25
97
0.1
0.1
0.1
0.1
ZnO
MnCl2
Co(NO3)2
Co/Mn NPs
aReaction conditions: o-phenylenediamine (1 mmol) and aryl aldehyde
(2 mmol) were stirred with cobalt manganese oxide nanoparticles
(0.1g) at 80°C under solvent-free conditions.
bNP, nanoparticle.
cIsolated yield.
Figure 1. (a) TEM and (b) SEM images of the catalyst with optimum Co:Mn
= 6:1 molar ratio.
Appl. Organometal. Chem. 2015, 29, 266–269
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

H. Ahmadian et al.
the optimized amount of cobalt manganese oxide nanocatalyst as
shown in Table 1, the reaction was carried out by varying the
amount of the catalyst in the synthesis of 1-(4-chlorobenzyl)-2-(4-
chlorophenyl)-1H-1,3-benzimidazole. The yield of product increases
linearly with catalyst weight up to 100 mg and then becomes
constant.
The effect of temperature on the rate of reaction was studied for
the preparation of 1-(4-chlorobenzyl)-2-(4-chlorophenyl)-1H-1,3-
benzimidazole (Table 1). It is observed that the reaction does not
proceed at room temperature. Further increase in temperature to
40, 60 and 80°C increases the rate of reaction. Therefore, we kept
the reaction temperature at 80°C (giving short reaction time and
high yield).
Scheme 2. Proposed mechanism for synthesis of 2-aryl-1-arylmethyl-1H-
1,3-benzimidazoles.
We then studied various solvents to find the best one for the
reaction of o-phenylenediamine (1 mmol) and 4-cholorobenzal-
dehyde (2 mmol) in the presence of 0.1 g of cobalt manganese
oxide nanocatalyst (Table 1). Under solvent-free conditions, the
reaction completes within 15 min and at 80°C to give 1-(4-
chlorobenzyl)-2-(4-chlorophenyl)-1H-1,3-benzimidazole in 96%
yield, because the solvent-free condition increases the effective
collisions of molecules in the reaction. The reactions in other
solvents require longer reaction times and lead to low yield.
The optimized conditions were further extended for the
synthesis of substituted 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles
(Table 2). As evident from Table 2, both electron-rich and
electron-deficient aldehydes react without any significant differ-
ence to give the corresponding benzimidazoles in good yield.
There use of nano-zinc supported with cobalt and manganese
was investigated in the reaction between 4-chlorobenzaldehyde
and o-phenylenediamine. After the completion of the reaction,
the products together with the catalyst precipitated out. The cata-
lyst was separated from the precipitate by adding dichloromethane
which dissolved the organic compound. The catalyst was dried at
110°C and subjected to fresh reaction. The catalyst was found to
be reusable for four cycles without significant loss of activity (Fig. 2).
The proposed mechanism for the synthesis of the 1,2-
disubstituted benzimidazoles may involve the iminium-catalysed
formation of N,N′-dibenzylidene-o-phenylenediamine, activation
by catalyst and ring closure giving a five-member ring either in a
sequential or a concerted manner (Scheme 2).[30]
Table 2. Synthesis of 1,2-disubstituted benzimidazoles
Producta
R
Time (min) Yield (%) M.p. (°C) Reported (°C)
a
b
c
4-Me
4-NO2
4-OMe
4-Cl
10
20
10
15
15
5
97
85
95
96
95
97
91
89
123–125
184–186
129–130
135–137
130–132
152–154
170–172
204–206
126–128[29]
192–194[29]
130–131[29]
137–139[29]
132–134[29]
151–153[29]
169–170[11]
205–207[29]
Conclusions
d
e
f
H
In summary, we have developed a new method for the synthesis of
2-aryl-1-arylmethyl-1H-1,3-benzimidazoles using nano-zinc sup-
ported with cobalt and manganese under solvent-free conditions.
Our method has several advantages including short reaction times,
mild conditions, excellent yields, inexpensive and non-toxic cata-
lyst, and simple operation and work-up.
2-OMe
2-NO2
3-NO2
g
h
15
20
aProducts were characterized from their physical properties,
comparison with authentic samples.
Acknowledgements
We are grateful to Payame Noor University (PNU) for partial support
of this work.
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