Synthesis of 2-substituted benzimidazoles in the presence of polyaniline nanoparticles doped with 12-tungstophosphoric acid as reusable heterogeneous catalyst

Article abstract of DOI:10.1080/15533174.2012.761233

Polyaniline nanoparticles doped with 12-tungstophosphoric acid was prepared with different loading amounts of 12-tungstophosphoric acid. Doped polyaniline was characterized by XRD, SEM, and FT-IR techniques and the Keggin anion of heteropoly acid was dete

Full text of DOI:10.1080/15533174.2012.761233

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Synthesis of 2-Substituted Benzimidazoles in the
Presence of Polyaniline Nanoparticles Doped With 12-
Tungstophosphoric Acid as Reusable Heterogeneous
Catalyst
Mohammad Abdollahi-Alibeik a , Marzieh Moosavifard a & Safoora Poorirani a
a Department of Chemistry , Yazd University , Yazd , I. R. Iran
Accepted author version posted online: 16 May 2013.
To cite this article: Mohammad Abdollahi-Alibeik , Marzieh Moosavifard & Safoora Poorirani (2013) Synthesis of 2-
Substituted Benzimidazoles in the Presence of Polyaniline Nanoparticles Doped With 12-Tungstophosphoric Acid as Reusable
Heterogeneous Catalyst, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:10, 1365-1371,
DOI: 10.1080/15533174.2012.761233
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:1365–1371, 2013
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2012.761233
Synthesis of 2-Substituted Benzimidazoles in the Presence
of Polyaniline Nanoparticles Doped With
12-Tungstophosphoric Acid as Reusable
Heterogeneous Catalyst
Mohammad Abdollahi-Alibeik, Marzieh Moosavifard, and Safoora Poorirani
Department of Chemistry, Yazd University, Yazd, I. R. Iran
carboxylic acids,[11] Mannich reaction,[12] and acetalization of
carbonyl compounds.[13]
Polyaniline nanoparticles doped with 12-tungstophosphoric
acid was prepared with different loading amounts of 12-
tungstophosphoric acid. Doped polyaniline was characterized by
XRD, SEM, and FT-IR techniques and the Keggin anion of het-
eropoly acid was detected on the polymer. Doped polymer was
used as heterogeneous solid acid catalyst for the synthesis of 2-
substituted benzimidazoles by the reaction of o-phenylenediamine
with various types of aldehydes. The activity and reusability of the
catalyst were found to be good.
In this study we aimed to focus on the preparation of polyani-
line doped with 12-tungstophosphoric acid (TPA/PANI) as solid
acid catalyst for the synthesis of 2-substituted benzimidazoles.
Many synthetic methodologies have been developed for the syn-
thesis of benzimidazoles by the reaction of o-phenylenediamine
with aldehydes,[14] with carboxylic acids,[15,16] and with or-
thoesters[17,18] in the presence of various acid catalysts.
However, many of these methods suffer from one or more
disadvantages such as prolonged reaction times, low yields,
harsh reaction conditions, requirement of excess amount of
the catalysts, and lack of easy availability of the starting
materials. Direct condensation reaction of aldehydes with o-
phenylenediamine has also some disadvantages including for-
mation of 1,2-disubstituted benzimidazoles, the corresponding
bis-anils, and dihydrobenzimidazoles as by-products.[19]
In continuation of our studies on the application of solid-
supported catalysts in organic transformations,[20–24] herein we
report the efficient synthesis of 2-substituted benzimidazoles in
the presence of catalytic amount of TPA/PANI as hetrogeneous
and reusable solid acid catalyst.
Keywords 12-tungstophosphoric acid, benzimidazole, doped poly-
mer, nanoparticles, o-phenylenediamine, polyaniline
INTRODUCTION
In view of environmental and economical concerns, there is
a global effort to replace the conventional catalysts with eco-
friendly solid acid catalysts. This is mainly due to advantages of
solid acid catalysts such as non-corrosiveness, ease of handling
and reusability.[1,2]
During the past few years much attention has been focused
on the application of the polymer supported catalysts in or-
ganic synthesis as heterogeneous and regenerable solid acid cat-
alysts.[3–6] The main advantages of polymer supported catalysts
are ease of preparation and simple separation of the catalyst.
Among the various types of polymers, polyaniline (PANI) has
attracted considerable attention because of its environmental
stability, controllable electrical conductivity, redox properties
and ease of preparation.[7–10] Several acid doped polyanilines
have been used as solid acid catalysts for some organic trans-
formations such as synthesis of quinoxalines,[9] esterification of
SCH. 1.
EXPERIMENTAL
Received 2 June 2012; accepted 17 December 2012.
The authors are thankful to the Yazd University Research Council
for partial support of this work.
Materials and Methods
All of the chemicals were commercial products. Aniline from
E. Merck (Germany) was distilled prior to use. All of the other
Address correspondence to Mohammad Abdollahi-Alibeik, De-
partment of Chemistry, Yazd University, Yazd 89195-741, I. R. Iran. compounds were used without purification. All reactions were
monitored by TLC and all yields refer to isolated products. 1H
E-mail: abdollahi@yazduni.ac.ir.
1365

1366
M. ABDOLLAHI-ALBEIK ET AL.
and 13C NMR spectra were recorded in DMSO-d6 on a Bruker
DRX-500 AVANCE spectrometer (Germany). Infrared spectra
of the catalysts and reaction products were recorded on a Bruker
FT-IR Equinax-55 spectrophotometer (Germany) in KBr with
absorption in cm–1. XRD patterns were recorded on a Bruker
D8 ADVANCE X-ray diffractometer (Germany) using nickel
filtered Cu Kα radiation. The morphology was studied using a
Philips XL30 scanning electron microscopy (Netherlands).
cipitate was filtered and washed with water (5 mL). After drying
of the solid, the corresponding 2-substituted benzimidazole was
obtained with high purity. Further purification was achieved by
column chromatography on silica gel (eluent; EtOAc:n-hexane,
1:1).
Physical and Spectroscopic Data for Selected Compounds
2-(3-Chlorophenyl)benzimidazole, mp: 230–232◦C (Lit.[26]
232–233◦C); IR (KBr): υmax (cm–1) 1572, 1442 (C C aro-
matic); 1H NMR (500 MHz, DMSO-d6): δ 7.20–7.27 (dd, 3J =
14.9 Hz, 3J = 8.3 Hz, 2 H, ArH), 7.55–7.61 (m, 3 H, ArH), 7.7
(t, 3J = 7.7 Hz, 1 H, ArH), 8.16 (dd, 3J = 7.3 Hz, 4J = 1.3 Hz,
Synthesis of the Catalyst
Synthesis of the polyaniline
Polyaniline salt of hydrochloric acid was prepared by aque-
ous polymerization and then was converted to polyaniline base
according to our previously reported method.[25] Polyaniline salt
was then undoped to obtain polyaniline base. In a typical exper-
iment, freshly distilled aniline (6 mL) was added to 1.7 M HCl
(35 mL) in a 500 mL round bottom flask. The solution was kept
under constant stirring at 0–5◦C. To this solution ammonium
persulfate (13.68 g) in water (250 mL) was added dropwise for
60 min. The solution was allowed to warm up to room temper-
ature and the reaction was allowed to continue for 24 h. The
precipitated polyaniline hydrochloride was filtered and washed
with distilled water (250 mL) followed by methanol (50 mL).
The polyaniline salt powder was dried at 120◦C for 1 h.
Polyaniline salt of hydrochloric acid (5 g) was stirred in
25 wt% aqueous ammonia solution (100 mL) for 24 h at room
temperature. Polyaniline base powder was filtered, washed with
water (100 mL) followed by acetone (50 mL), and dried at 120◦C
for 1 h.
4
1 H, ArH), 8.23 (t, J = 1.6 Hz, 1 H, ArH), 13.03 (s, 1 H,
NH); 13C NMR (125 MHz, DMSO-d6): δ 112.4, 119.9, 122.8,
123.8, 125.9, 126.9, 130.4, 131.8, 133.1, 134.6, 135.9, 144.5,
150.6.
2-(2-Furyl)benzimidazole, mp: 300◦C (Lit.[27] 288◦C); IR
(KBr): υmax (cm–1) 1620 (C N), 3400 (NH); 1H NMR
(500 MHz, DMSO-d6): δ = 6.73–6.72 (m, 1 H); 7.22–7.18
3
3
(m, 3 H), 7.56 (d, J = 3.0 Hz, 2 H), 7.94 (d, J = 1.0 Hz, 1
H), 12.97 (s, 1 H), 13C NMR (125.7 MHz, 1H-decoupled): δ =
146.48, 145.45, 144.53, 123.03, 113.15, 111.35.
RESULTS AND DISCUSSION
The Catalyst Characterization
In recent years, polyaniline salts of various Lewis and
Brønsted acids have attracted considerable attention as a
mild solid acid catalyst in organic transformation.[9,11–13]
Although, heteropoly acids have been supported on the
various supports, the use of heteropoly acids as doping
agent for polyaniline has received less attention and there
is only a few reports about preparation and characteriza-
tion of polyaniline doped heteropoly acids (12-tungstosilicic
acid and 12-molybdophosphoric acid).[28–30] In this work,
polyaniline nanoparticles doped with 12-tungstophosphoric acid
(TPA/PANI) were prepared with different loading ammount of
TPA and was applied as solid acid catalyst for the synthesis of
2-substituted benzimidazoles.
Polyaniline doped with 12-tungstophosphoric acid (TPA/PANI)
To a suspension of polyaniline (500 mg) in CH3CN (50 mL)
in a 100 mL round bottom flask, 12-tungstophosphoric acid
(250 mg) was added with stirring. The mixture was stirred for
24 h. The solid was filtered, washed with water (200 mL) fol-
lowed by acetone (50 mL), and dried at 120◦C for 1 h. The
TPA/PANI with 18% loading amount of TPA was obtained.
General Experimental Procedure for the Synthesis
of 2-Substituted Benzimidazoles
Initially, TPA/PANI catalysts with different loading amounts
Synthesis of 2-substituted benzimidazoles were carried of TPA (13%, 18%, and 28%) were prepared with appropriate
out according to our previous reported procedure.[25] In portions of heteropoly acid relative to polyaniline. The doped
a typical experimental procedure, a mixture containing o- polymer samples were analyzed by FT-IR in order to confirm
phenylenediamine (1 mmol), ethanol (3 mL), and 18 wt% the presence of Keggin anion of heteropoly acid on the polymer.
TPA/PANI (150 mg) were taken in a 10 mL round bottom flask.
The FT-IR spectra of PANI, TPA, and TPA/PANI with dif-
To this suspension, a solution containing aldehyde (1 mmol) in ferent loading amounts of TPA are shown in Figure 1. The vi-
ethanol (2 mL) was added dropwise at room temperature for brational bands observed for the polyaniline base (Figure 1) are
30 min under vigorous stirring. After completion of the reaction reasonably explained on the basis of the normal modes of aniline
(monitored by TLC, eluent; EtOAC:n-hexane, 1:1), the reaction and benzene. The characteristic peak at 1597 cm–1 arise from
mixture was filtered, washed with EtOH (3 × 5 mL) and 50% the stretching vibration of N Q N ring. The peak at 1496 cm–1
of the solvent was evaporated.
is attributed to the stretching vibration of N–B–N ring (where
To the obtained solution, water (10 mL) was added and the pH B refers to the benzenoid ring and Q refers to the quinoid ring).
of the solution was adjusted to 9 by adding ammonia. The pre- The peaks near 1294 cm–1 and 820 cm–1 can be assigned to

POLYANILINE DOPED WITH 12-TUNGSTOPHOSPHORIC ACID
1367
FIG. 2. SEM image of polyaniline.
Keggin ions.[22,23] In the FT-IR spectra of 13–28 wt% TPA/PANI
(Figure 1), apart from the main peaks of PANI, additional peaks
corresponding to TPA appeared in the patterns. In these spec-
tra, three peaks of TPA are appeared around 1080, 983, and
888 cm–1, while the characteristic peak of TPA at 806 cm–1 may
overlap with the C-N stretching of PANI at 820 cm–1, resulting
increase of intensity and broadening of this peak of PANI with a
weak shoulder around 806 cm–1. Apparently, with the increase
of TPA loading in samples, the intensity of bands assigned TPA
increases remarkably (Figure 1). These results confirm that TPA
in samples still retains the Keggin structure.
A scanning electron micrograph (SEM) of PANI base is
shown in Figure 2. The SEM image shows that PANI was formed
as some individual nanoparticles and also some agglomerated
nanoparticles.
The X-ray diffraction patterns of the PANI, TPA, and
TPA/PANI with different loading of TPA are shown in Figure 3.
As shown in Figure 3, XRD of PANI base indicates amorphous
nature. The peaks of PANI doped with TPA in 13 wt% and
18 wt% loading differ from PANI base, TPA/PANI with 28 wt%
loading and pure TPA. As shown in Figure 2, absence of any
peaks of crystalline TPA, indicates that TPA are inserted into the
polymer matrix. The peak at 2θ = 7.5◦ in the pattern of 13 wt%
TPA/PANI was assigned at distance between the dopant and
N atom on adjacent chains. This sharp peak indicates a short
range ordering between counter anion and the polymer chain.
The peak at 2θ = 14◦ in the pattern of 18 wt% TPA/PANI,
partially overlapped with the peak at 2θ = 7.5◦, may be due
to higher doping was take placed at N atoms of quinoid rings.
These results well confirm that TPA is amorphous in loading up
to 18 wt% and molecularly dispersed to the surface of the poly-
mer. The peaks of TPA/PANI with 28 wt% loading are the same
as pure TPA. This indicates formation of a separate crystalline
phase of TPA on the polymer.
FIG. 1. FT-IR spectra of (a) PANI, (b) 13 wt% TPA/PANI, (c) 18 wt%
TPA/PANI, (d) 28 wt% TPA/PANI, and (e) TPA.
C–N stretching of the secondary aromatic amine and an aro-
matic C–H out-of-plane bending vibration, respectively.[25] The
characteristic peaks of polyaniline base is N Q N stretch-
ing bond at 1166 cm–1. As shown in Figure 1, the IR spectra
have been changed after doping of polyaniline with heteropoly
acid.
When comparing with the PANI base, the band at 1597 cm–1
(arising from quinoid diimine unit) and the band at 1496 cm–1
(arising from C-C aromatic ring stretching of the benzenoid di-
imine unit) in TPA/PANI are shifted to lower wavenumbers 1583
and 1492 cm–1, respectively (Figure 1). These results confirm the
doping of PANI and are in agreement with other reports.[13,25,31]
Moreover, in doped polyaniline, characteristic peaks of TPA
confirm presence of Keggin anion on the polymer (Figure 1).
As shown in Figure 1, pure TPA, exhibited the typical
The Catalyst Activity
To show the catalytic activity of TPA/PANI in the syn-
IR bands at 1080, 983, 888, and 806 cm–1 associated to the thesis of 2-substituted benzimidazoles, the reaction of o-

1368
M. ABDOLLAHI-ALBEIK ET AL.
TABLE 2
Synthesis of 2-phenylbenzimidazole by the reaction of
o-phenylenediamine and benzaldehyde in the presence of
various TPA/PANI catalysts in EtOH at room temperature
Catalyst
amount
(mg)
TPA loading
amount
Time
(min)
Yield
(%)
Entry
(wt%)
1
2
3
4
5
6
150
150
150
150
100
27∗
13
18
24
28
18
240
120
300
300
270
75
86
70
72
70
30
100
24 (h)
∗Pure TPA
amounts of TPA (Table 2). As shown in Table 2 (entries 1–4),
the catalytic performance was increased with an increase in
TPA loading up to 18 wt% and decreased with further increase
loading. In order to use the lowest amount of the catalyst, the
reaction was also carried out in the presence of 100 mg 18 wt%
TPA/PANI and result show that 150 mg of 18 wt% TPA/PANI
has the best activity in terms of reaction time and yield (Table 2,
entries 2 and 5).
To show efficiency of the support on the catalytic activity of
TPA, the model reaction in the presence of the pure TPA (27 mg,
1 mol%, the same as loading amount of TPA on the 150 mg of
18 wt% TPA/PANI) was carried out under the same reaction
conditions and low yield of the product was obtained after 24 h
(Table 2, entry 6).
In spite of these results, the reaction of o-phenylenediamine
(1 mmol) with benzaldehyde (1 mmol) in the presence of 150 mg
of 18 wt% TPA/PANI at room temperature in EtOH was selected
as the best condition for the synthesis of 2-phenylbenzimidazole.
To show generality of the catalyst, the reaction of various
types of aromatic aldehydes with electron donating and elec-
tron withdrawing substituents were carried out and the cor-
responding 2-arylbenzimidazoles were obtained in 70–90%
yields (Table 3). The reaction of heteroaryaldehydes with o-
phenylenediamine was also preformed in the same reaction
conditions and the corresponding 2-heteroaryl substituted benz-
imidazoles were obtained in 70–75% yields (Table 3, entries 15
and 16).
Reusability of the catalyst was investigated in the reaction
of benzaldehyde with o-phenylenediamine. The TPA/PANI was
recovered and washed with EtOH and dried in an oven at 120◦C
for 2 h. The recovered catalyst was reused for a further four times
and the reaction proceeded smoothly with a yield of 86–76%
(Table 4). These results indicate that the catalyst is reusable with
moderate declining in its activity.
FIG. 3. XRD Pattern (a) PANI, (b) 13 wt% TPA/PANI, (c) 18 wt% TPA/PANI,
(d) 28 wt% TPA/PANI, and (e) TPA.
phenylendiamine (1 mmol) with benzaldehyde (1 mmol) was
carried out as model reaction in various conditions. To investi-
gate the effect of the solvent on the catalytic reaction, the model
reaction in the presence of 150 mg of 18 wt% TPA/PANI was
carried out in various solvents (Table 1). The results show that
100% conversion was achieved in MeOH and EtOH for 1 h and
2 h respectively (Table 1, entries 1 and 2). However, due to en-
vironmental considerations, EtOH was selected as the solvent
for the reaction.
To investigate the effect of the loading amount of TPA on the
catalytic activity of TPA/PANI, the model reaction was carried
out in the presence of 150 mg of catalysts with different loading
TABLE 1
Synthesis of 2-phenylbenzimidazole by the reaction of
o-phenylenediamine and benzaldehyde in the presence of
18 wt% TPA/PANI in different solvents at room temperature
Entry
Solvent
MeOH
Time (h)
Conversion (%)
1
2
3
4
5
6
1
2
7
9
5
3
100
100
100
50
50
40
EtOH
CH3CN
CH2Cl2
CH2ClCH2Cl
-

POLYANILINE DOPED WITH 12-TUNGSTOPHOSPHORIC ACID
1369
TABLE 3
Synthesis of benzimidazoles in the presence of 18 wt% TPA/PANI in EtOH at room temperature
Entry
1
Aldehyde
CHO
Benzimidazole
Yield (%)
86
Time (min)
120
H
N
N
2
3
80
70
90
CHO
Cl
H
N
N
Cl
120
H
N
CHO
N
Cl
Cl
7
4
H
N
70
70
50
105
CHO
Cl
Cl
N
5
H
N
210
210
CHO
OH
HO
N
6∗
H
N
CHO
N
NO2
NO2
7∗
73
72
82
210
165
165
CHO
CHO
H
N
NO2
CH3
O2N
H3C
N
8
H
N
N
9
H
N
CHO
N
OCH3
OCH3
10
83
140
CHO
H
N
OCH3
H3CO
N
(Continued on next page)

1370
M. ABDOLLAHI-ALBEIK ET AL.
TABLE 3
Synthesis of benzimidazoles in the presence of 18 wt% TPA/PANI in EtOH at room temperature (Continued)
Entry
11
Aldehyde
Benzimidazole
Yield (%)
90
Time (min)
190
OCH3
OCH3
H3CO
CHO
H
N
N
OCH3
12
13
H3CO
80
90
40
H3CO
CHO
H
N
OCH3
N
OCH3
H
N
180
H3CO
H3CO
CHO
OCH3
OCH3
N
14
15
O
75
70
150
360
H
N
O
CHO
N
S
H
N
S
CHO
N
∗The reactions were performed under reflux condition.
TABLE 4
Reusability of 18 wt% TPA/PANI
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