FeCl3-doped polyaniline nanoparticles as reusable heterogeneous catalyst for the synthesis of 2-substituted benzimidazoles

Article abstract of DOI:10.1080/00397910903318658

FeCl₃-doped polyaniline nanoparticles efficiently catalyze the synthesis of 2-substituted benzimidazoles by the reaction of aldehydes with o-phenylenediamine. Synthesis and characterization of the catalyst are described, and the obtained results confirms good dispersion of the FeCl3 on the polyaniline. Simple workup, mild reaction conditions, low cost, easy separation, and reusability of the catalyst are some advantages of this method.

Full text of DOI:10.1080/00397910903318658

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FeCl3-Doped Polyaniline Nanoparticles
as Reusable Heterogeneous Catalyst
for the Synthesis of 2-Substituted
Benzimidazoles
Mohammad Abdollahi-Alibeik a & Marzieh Moosavifard a
a Department of Chemistry, Yazd University, Yazd, Iran
Published online: 09 Aug 2010.
To cite this article: Mohammad Abdollahi-Alibeik & Marzieh Moosavifard (2010): FeCl3-Doped
Polyaniline Nanoparticles as Reusable Heterogeneous Catalyst for the Synthesis of 2-Substituted
Benzimidazoles, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 40:18, 2686-2695
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Synthetic Communications1, 40: 2686–2695, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903318658
FeCl3-DOPED POLYANILINE NANOPARTICLES AS
REUSABLE HETEROGENEOUS CATALYST FOR THE
SYNTHESIS OF 2-SUBSTITUTED BENZIMIDAZOLES
Mohammad Abdollahi-Alibeik and Marzieh Moosavifard
Department of Chemistry, Yazd University, Yazd, Iran
FeCl3-doped polyaniline nanoparticles efficiently catalyze the synthesis of 2-substituted
benzimidazoles by the reaction of aldehydes with o-phenylenediamine. Synthesis and
characterization of the catalyst are described, and the obtained results confirms good
dispersion of the FeCl3 on the polyaniline. Simple workup, mild reaction conditions,
low cost, easy separation, and reusability of the catalyst are some advantages of this
method.
Keywords: Benzimidazoles; FeCl3; nanoparticle; o-phenylenediamine; polyaniline
INTRODUCTION
In recent years, the use of solid acid catalysts instead of conventional acids has
attracted considerable attention in organic transformations. This is mainly due to
advantages of solid acids such as nontoxicity, ease of handling, enhanced reaction
rates, ease of recovery, and reusability of the catalysts.[1] Among the various types
of solid-supported catalysts, polymer-supported catalysts have advantages of low
cost, ease of preparation, and recyclability.[2]
Polyaniline (PANI) is one of the conductive polymers and has received
considerable attention because of its excellent thermal and chemical stability,
inexpensive monomer, and high molecular weight.[3] PANI doped with various
types of Lewis and Brønsted acids has been used as the catalyst in many organic
transformations such as synthesis of quinoxalines,[4] synthesis of bisindoles,[5]
esterification of carboxylic acids,[6] Mannich reaction,[7] and acetalization of
carbonyl compounds.[8]
Benzimidazoles have recently attracted considerable attention as an important
class of heterocyclic compounds because of their biological activities. These com-
pounds are widely used as antitumor,[9] antiparasitic,[10] anti-inflammatory,[11] and
anti-HIV[12] agents. These compounds can also act as ligands to transition metals
for modeling biological systems.[13]
Received July 13, 2009.
Address correspondence to Mohammad Abdollahi-Alibeik, Department of Chemistry, Yazd
University, Yazd 89195-741, Iran. E-mail: abdollahi@yazduni.ac.ir
2686

FeCl3/PANI NANOCATALYST IN BENZIMIDAZOLE SYNTHESIS
2687
Scheme 1. Reaction of aldehydes and o-phenylenediamine in the presence of FeCl3=PANI in EtOH at
room temperature.
Despite their wide range of pharmacological activities, the synthesis of 2-
substituted benzimidazoles has received little attention, and few methods for their
preparation are reported in literature.
Of these methods, the acid-catalyzed condensation of o-phenylenediamines
with aldehydes is one of the most simple and straightforward approaches for the
synthesis of 2-substituted benzimidazoles. A variety of acid catalysts such as
zirconyl(IV) chloride,[14] Bi(III) salts,[15] ytterbium triflate,[16] sulfamic acid,[17]
Brønsted and Lewis acids,[18] polyphosphoric acid,[19] alumina–methanesulfonic acid,[20]
and heteropoly acid[21] have been employed to accomplish this transformation.
However, many of these methodologies are associated with several shortcomings,
such as the use of strong acids, high-temperature conditions, occurrence of several
side products, nonreusability, and difficulty in recovery of the catalysts.
In continuation of our studies on the application of solid-supported reagents in
organic transformations,[22–24] in the present investigation we report the preparation,
characterization, and catalytic application of FeCl3-doped PANI nanoparticles for
the synthesis of 2-substituted benzimidazoles by the reaction of aldehydes with
o-phenylenediamine (Scheme 1).
RESULTS AND DISCUSSION
Initially, PANI and PANI doped with FeCl3 were prepared by the method
described in the literature.[8] The polymerization of hydrochloric acid salt of freshly
distilled PANI was carried out in the presence of ammonium persulfate as initiator
for oxidative polymerization in the appropriate portion of acetone=water. The
obtained HCl-doped PANI was undoped by an aqueous solution of ammonia,
and the obtained PANI base was doped with various amounts of FeCl3 in acetone
as solvent. Amounts of FeCl3 present in the PANI chain in doped samples were
found to be 5, 13, 16, and 19 wt%.
To study the morphology of the PANI and to confirm the presence of FeCl3 as
dopant on the polymer, the scanning electron microscopy (SEM), x-ray diffraction
(XRD), and Fourier transform-infrared (FT-IR) spectra of PANI and FeCl3=PANI
were studied.

2688
M. ABDOLLAHI-ALIBEIK AND M. MOOSAVIFARD
Figure 1. FT-IR spectra of (a) PANI and (b) 19 wt% FeCl3=PANI.
The FT-IR spectra of PANI and FeCl3=PANI are shown in Fig. 1. The
vibrational bands of PANI base [Fig. 1(a)] are closed to that of the PANI base
systems in the literature.[8] These bands are reasonably explained on the basis of nor-
mal modes of aniline (quinoid ring, Q) and benzene (benzenoid ring, B). A broad
band at 3400 cmꢀ1 is assigned to the N-H stretching vibration, the bands at 1585
and 1492 cmꢀ1 are assigned to the quinoid and benzenoid rings, respectively, the
bands at 1379 and 1307 cmꢀ1 are assigned to C-N stretching vibrations in QBB,
QBQ, and BBQ, and a 1215 cmꢀ1 band is assigned to C-N stretching vibration of
aromatic amine. The band at 1159 cmꢀ1 can be assigned to C-H in-plane-bending
mode of aromatic rings. The C-H out-of-plane bending mode was observed as a sin-
gle band at 829 cmꢀ1, which fell in the range 800–860 cmꢀ1 reported for 1,4-substi-
tuted benzene.
As shown in Fig. 1(b), the spectrum of FeCl3=PANI is similar to that of PANI
base. When comparing with the PANI base, the band at 1570 cmꢀ1 (arising from
=
=
both C N and C C stretching of the quinoid diimine unit) and the band at
1487 cmꢀ1 (arising from C-C aromatic ring stretching of the benzenoid diimine unit)
in FeCl3=PANI are shifted to lower wavenumbers. These results confirm the doping
of PANI with FeCl3 and are in agreement with other reports.[8]
The XRD pattern of PANI base and FeCl3=PANI are shown in Fig. 2. As
shown in pattern a, XRD of PANI base indicates the amorphous nature with a
peak at 2h ¼ 19.5ꢁ. In the XRD pattern of FeCl3=PANI, broad peaks around
2h ¼ 13.5ꢁ and 2h ¼ 30ꢁ were observed. The peaks at 2h ¼ 13.5ꢁ and 2h ¼ 30ꢁ
are strong peaks which are similar to that of highly doped PANI salt as reported
elsewhere.[8]

FeCl3/PANI NANOCATALYST IN BENZIMIDAZOLE SYNTHESIS
2689
Figure 2. XRD pattern of (a) PANI and (b) 19 wt% FeCl3=PANI.
A scanning electron micrograph (SEM) of PANI base is shown in Fig. 3. The
SEM image shows that PANI was formed as some individual nanoparticles and also
some agglomerated nanoparticles.
To optimize the reaction conditions, the condensation reaction of benzal-
dehyde and o-phenylenediamine in the presence of a catalytic amount of FeCl3=
PANI was selected as the model reaction, and results are shown in Table 1.
To investigate the effect of different loading amounts of FeCl3 (5–19 wt%) on
the reactivity of doped PANI, the model reaction was carried out with equimolar
amounts of o-phenylenediamine and benzaldehyde (1 mmol) in the presence of
FeCl3=PANI with 5–19 wt% FeCl3 (Table 1, entries 1–4).
As shown in Table 1, the conversion increased with an increase in FeCl3 load-
ing up to 19 wt%. According to this result, the 19 wt% FeCl3=PANI was selected as
the best catalyst. The efficiency of the support was also investigated by performing
Figure 3. SEM image of PANI base.

2690
M. ABDOLLAHI-ALIBEIK AND M. MOOSAVIFARD
Table 1. Reaction of o-phenylenediamine with benzaldehyde in the presence of
FeCl3=PANI with various loading amounts of FeCl3 at room temperature
Entry
FeCl3 (wt.%)
Time (min)
Yielda (%)
1
2
3
4
5
5
13
90
60
50
30
60
50
50
70
90
60
16
19
100
aIsolated yields.
the reaction in the presence of the pure FeCl3 as catalyst. The model reaction was
carried out in the presence of 19 mg FeCl3, and a poorer yield was achieved
(Table 1, entry 5).
In a typical experimental procedure, the reaction of benzaldehyde (1 mmol)
and o-phenylenediamine (1 mmol) in the presence of 19 wt% FeCl3=PANI (100 mg)
was carried out in EtOH as solvent at room temperature. After completion of the
reaction [monitored by thin-layer chromatography (TLC), eluent: EtOAC–n-hexane,
1:1], the reaction mixture was filtered and washed with EtOH. Ammonia was added
to the obtained solution, and the solution was diluted with water. The precipitate
was filtered, washed with water, and dried to obtained 2-phenyl benzimidazole with
high purity. Further purification was achieved by column chromatography on silica
gel using EtOAc–n-hexane as eluent. The pure 2-phenyl benzimidazole was obtained
in 90% yield (Table 2, entry 1).
To explore the generality of this method, reactions of various types of alde-
hydes with electron-withdrawing and electron-donating groups were performed in
the same reaction conditions, and the corresponding 2-aryl benzimidazoles were
obtained in excellent yields (Table 2, entries 2–12).
Heteroaryl aldehydes and aldehydes with a reactive functional group were also
subjected to the same reaction, and the corresponding 2-substituted benzimidazoles
were obtained in good yields (Table 2, entries 13–15). The same reaction was
carried out with 1,4-benzene dicarbaldehyde using a 1:2 molar ratio of aldehyde–
o-phenylenediamine, and the corresponding bis-benzimidazole was obtained in
97% yield (Table 2, entry 16).
The reusability of the catalyst is one of the most important features of
solid acid catalysts. To investigate reusability of the FeCl3=PANI, after the reaction
of benzaldehyde and o-phenylenediamine the recovered catalyst was reused three
times in the same reaction (Table 3). After each run, the catalyst was filtered, washed
with distilled water (100 mL) and acetone (50 mL), and dried at 120 ꢁC for 1 h. As
shown in Table 3, the catalyst can be reused without considerable decrease in
activity.
In summary, we introduced FeCl3=PANI as an efficient and ecofriendly
solid acid catalyst for the synthesis of 2-substituted benzimidazoles. Excellent yields
of the products, short reaction times, mild reaction conditions, reusability of the
catalyst, and simple experimental procedure make this method complementary to
the existing method.

FeCl3/PANI NANOCATALYST IN BENZIMIDAZOLE SYNTHESIS
2691
Table 2. Synthesis of 2-substituted benzimidazoles by the reaction of o-phenylenediamine with aldehydes
in the presence of a catalytic amount of FeCl3=PANI
Yielda Time
(%) (min)
Entry
1
Aldehyde (2)
Benzimidazole (3)
90
84
30
45
2
3
4
5
6
7
83
86
78
91
90
45
30
240
60
135
8
70
180
(Continued )

2692
M. ABDOLLAHI-ALIBEIK AND M. MOOSAVIFARD
Table 2. Continued
Yielda Time
(%) (min)
Entry
9
Aldehyde (2)
Benzimidazole (3)
92
83
45
30
10
11
83
45
12
91
120
13
14
86
80
45
80
15
16
86
97
180
145
aIsolated yields.

FeCl3/PANI NANOCATALYST IN BENZIMIDAZOLE SYNTHESIS
2693
Table 3. Reusability of FeCl3=PANI in the reac-
tion of o-phenylenediamine with benzaldehyde at
room temperature
Run
Yielda (%)
1
2
3
4
90
83
82
83
aIsolated yields.
EXPERIMENTAL
All of the chemicals were commercial products. Anilline from E. Merck was
distilled prior to use. Reagent-grade solvent and sodium persulfate (E. Merck) were
used without purification. o-Phenylenediamine and benzaldehyde were distilled and
freshly used, and other aldehydes (E. Marck and Aldrich) were used as received. All
melting points were obtained on a Buchi B-540 apparatus and are uncorrected. All
1
reactions were monitored by TLC, and all yields refer to isolated products. H and
13C NMR spectra were recorded in CDCl3 and dimethylsulfoxide (DMSO-d6) on a
Bruker 500-MHz spectrometer. Infrared (IR) spectra were recorded on a Bruker
FT-IR Equinax 55 spectrophotometer in KBr with absorption in centimetersꢀ1
.
XRD patterns of PANI base and doped PANI were recorded on a Bruker D8
Avance x-ray diffractometer using nickel-filtered CuKa radiation. The morphology
of the catalyst was investigated by scanning electron microscopy (SEM) using a Phi-
lips XL30 instrument. The amount of FeCl3 present in the PANI was measured
based on the weight of FeCl3-doped PANI and the weight of PANI base used.
Synthesis of the PANI
The PANI salt of hydrochloric acid was prepared by an aqueous polymeriza-
tion pathway. The PANI base was obtained by undoping the obtained salt. In a typi-
cal experiment, freshly distilled aniline (6 mL) was added to 1.7 M HCl (75 mL) in a
500-mL round-bottom flask. The solution was stirred constantly at (ꢀ5 to 0 ꢁC). To
this solution, ammonium persulfate (13.68 g) in water (250 mL) was added dropwise
over 60 min. The solution was allowed to warm up to room temperature, and the
reaction was allowed to continue for 4 h. The precipitated PANI hydrocholoride salt
was filtered and washed with distilled water (250 mL) followed by methanol (50 mL).
The PANI powder was dried at 120 ꢁC for 1 h.
The PANI salt of hydrochloric acid (5 g), synthesized previously, was stirred
in 25 wt% aqueous ammonia solution (100 mL) for 24 h at room temperature.
PANI base powder was filtered, washed with water (100 mL) followed by acetone
(50 mL), and dried at 120 ꢁC for 1 h.
FeCl3-Doped PANI
PANI (250 mg) was added to a 0.05 M solution of FeCl3 in acetone (25 mL) in
a 50-mL round-bottom flask under vigorous stirring. The mixture was stirred for 1 h.

2694
M. ABDOLLAHI-ALIBEIK AND M. MOOSAVIFARD
The solid was filtered, washed with water (100 mL) followed by acetone (50 mL), and
dried at 120 ꢁC for 1 h. Doped PANI with 19 wt% FeCl3 was obtained.
General Experimental Procedure for the Synthesis of 2-Substituted
Benzimidazoles
A mixture containing o-phenylenediamine (1 mmol), ethanol (3 mL), and
FeCl3=PANI (100 mg) were taken in a 10-mL round-bottom flask. To this mixture,
a solution of aldehyde (1 mmol) in ethanol (2 mL) was added dropwise at room
temperature for 20 min. After completion of the reaction (monitored by TLC, eluent
EtOAc–n-hexane, 50:50), the reaction mixture was filtered and washed with ethanol
(3 ꢂ 5 mL). To the obtained solution, 5% ammonia solution (2 mL) was added, then
water (5 mL) was added, and the precipitate 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 chroma-
tography on silica gel (eluent; EtOAc–n-hexane; 50:50). The corresponding
2-substituted benzimidazoles were obtained in 70–97% yields.
2-(2-Furyl)benzimidazole (Table 2, entry 13)
1
Mp 300 ꢁC (lit. mp 288 ꢁC[25]); IR (KBr): t (cmꢀ1) 1620 (C N), 3400 (NH); H
=
NMR (500 MHz, DMSO-d6): d ¼ 12.97 (s, 1 H), 7.94 (d, J ¼ 1 Hz, 1 H), 7.56 (d,
J ¼ 3 Hz, 2 H), 7.22–7.18 (m, 3 H), 6.73–6.72 (m, 1 H); 13C NMR (125.7 MHz,
1H-decoupled): d ¼ 146.48, 145.45, 144.53, 123.03, 113.15, 111.35.
ACKNOWLEDGMENT
We are thankful to the Yazd University Research Council for partial support
of this work.
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