Novel silica tungstic acid (STA): Preparation, characterization and its first catalytic application in synthesis of new benzimidazoles

Article abstract of DOI:10.1016/j.catcom.2012.01.012

A novel silica tungstic acid (STA) as a highly efficient catalyst has been synthesized and employed for the cyclocondensation of 1,2-phenylenediamines with orthoesters under solvent-free conditions. Catalyst loadings can be as low as 1 mol% to give high yields of the corresponding benzimidazoles at 80 °C. To make the catalyst, sodium tungstate reacted with silica chloride under refluxing n-hexane. The STA as a novel solid acid was characterized by X-ray fluorescence, X-ray diffraction, and Fourier transform infrared spectroscopy.

Full text of DOI:10.1016/j.catcom.2012.01.012

Catalysis Communications 20 (2012) 71–75
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Novel silica tungstic acid (STA): Preparation, characterization and its first catalytic
application in synthesis of new benzimidazoles
Bahador Karami ⁎, Vahideh Ghashghaee, Saeed Khodabakhshi
Department of Chemistry, Yasouj University, Yasouj, 75918–74831 P.O. Box 353, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel silica tungstic acid (STA) as a highly efficient catalyst has been synthesized and employed for the
cyclocondensation of 1,2-phenylenediamines with orthoesters under solvent-free conditions. Catalyst load-
ings can be as low as 1 mol% to give high yields of the corresponding benzimidazoles at 80 °C. To make the
catalyst, sodium tungstate reacted with silica chloride under refluxing n-hexane. The STA as a novel solid
acid was characterized by X-ray fluorescence, X-ray diffraction, and Fourier transform infrared spectroscopy.
Received 20 November 2011
Received in revised form 7 January 2012
Accepted 12 January 2012
Available online 20 January 2012
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Silica tungstic acid
X-ray diffraction
X-ray fluorescence
Solvent-free
Benzimidazole
1
. Introduction
[10,11]. Esters, lactones, and anhydrides can also produce benzimid-
azoles via the cyclization of amide [12]. However, some of the reported
methods suffer from drawbacks, such as the requirement for toxic and
unrecyclable catalysts and solvents [13], harsh reaction conditions [14],
prolonged reaction time [15], and difficult work-up procedures [16].
In recent years, preparation of new catalysts which will improve ef-
ficiency or produce a greater yield has been the subject of interests. Cat-
alytic technology plays a major role in many of the chemical reactions,
therefore, preparing and employing a green catalyst are extremely valu-
able [1]. For example, 90%, over a trillion dollars' worth, of manufac-
tured items are produced with the help of catalysts every year.
Besides, pressure from environmentalists has led to a search for more
eco-friendly forms of catalysis. Recently, synthesis of supported cata-
lysts as clean materials has attracted considerable attention [2]. Al-
though supported catalysts are available on different supports
including charcoal, alumina, silica, and polymer, silica has many other
advantages such as no swelling, good mechanical and thermal stabili-
ties, and ease of scalability. In fact, these heterogeneous catalysts de-
crease reactor and plant corrosion problems and are more
environmentally safe. Among the wide variety of nitrogen heterocycles
that have been explored as pharmaceutically important compounds,
benzimidazoles exhibit relatively high biological activities. They have
exhibited activity against cancer [3], HIV [4], herpes (HSV-1) [5], influ-
enza [6], certain fungi [7], and raf kinase [8]. The most prominent benz-
imidazole compound in nature is N-ribosyldimethylbenzimidazole,
which serves as an axial ligand for cobalt in vitamin B12 [9].
2. Experimental
2.1. General
The chemicals were purchased from Merck and Aldrich chemical
companies. The silica chloride 1 was synthesized according to the pub-
lished procedure [17]. The reactions were monitored by TLC (silica–gel
60 F254, hexane: EtOAc). Fourier transform infrared (FT-IR) spectrosco-
py spectra were recorded on a Shimadzu-470 spectrometer, using KBr
pellets and the melting points were determined on a KRUSS model in-
strument. 1H NMR spectra were recorded on a Bruker Avance II 400
3
NMR spectrometer at 400 MHz, in which CDCl was used as solvent
and TMS as the internal standard. X-ray diffraction (XRD) pattern was
obtained by Philips X Pert Pro X diffractometer operated with a Ni-
filtered Cu Kα radiation source. X-ray fluorescence (XRF) spectroscopy
4
was recorded by X-Ray Fluorescence Analyzer, Bruker, S Pioneer, Ger-
many. All products (except for novel compounds) were characterized
through comparison with those reported in Ref. [18]. Novel compounds
were also characterized by IR, MS, and NMR spectroscopy.
Until now, several methods for the synthesis of benzimidazoles and
derivatives thereof have been reported. The most common method is
the condensation of an arylenediamine with a carbonyl equivalent
2.2. Preparation of silica chloride 1
To an oven-dried (125 °C, vacuum) sample of silica–gel 60 (10 g)
⁎
Corresponding author. Tel.: +98 7412223048; fax: +98 7412242167.
E-mail address: karami@mail.yu.ac.ir (B. Karami).
in a round bottomed flask (250 mL) equipped with a condenser and
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.01.012

72
B. Karami et al. / Catalysis Communications 20 (2012) 71–75
Scheme 1. Preparatio of silica chloride and tungstic acid.
(t, 2H, J=8 Hz); 13C-NMR (CDC1 , 100 MHz) δ (ppm): 197.31,
a drying tube, thionyl chloride (40 mL) was added and the mixture in
the presence of CaCl as a drying agent was refluxed for 48 h. The
resulting white-grayish powder was filtered and stored in a tightly
3
2
1
1
5
3
57.80, 143.20, 138.11, 132.46, 132.34, 130.05, 128.32, 125.29,
+
15.12; MS, m/z: 222 (M
)
capped bottle [17].
-Benzoyl-2-methyl-benzimidazole 5k: IR (KBr) νmax (cm−1):
200, 2350, 1665, 1620, 1510, 1320, 1020, 820, 720; 1H-NMR
2
.3. Preparation of silica tungstic acid 2
(
3
CDC1 , 400 MHz) δ (ppm): 8.04 (s, 1H), 7.81 (d, 2H, J=7.6 Hz),
7
.75 (d, 1H, J=8 Hz), 7.57 (d, 2H, J=7.2 Hz), 7.47 (t, 2H,
To a mixture of silica chloride (6.00 g) and sodium tungstate (7.03
13
J=7.2 Hz), 2.67 (s, 3H); C-NMR (CDC1
3
, 100 MHz) δ (ppm):
gr) n-hexane (10 ml) was added. The reaction mixture was stirred
under refluxing conditions (70 °C) for 4 h. After completion of the re-
action, the reaction mixture was filtered and washed with distilled
water, and dried and then stirred in the presence of 0.1 N HCl
1
97.51, 154.42, 138.28, 132.24, 131.64, 130.02, 128.27, 124.94,
+
15.17; MS, m/z: 236 (M
5-Benzoyl-2-ethyl-benzimidazole 5l: IR (KBr) νmax (cm−1):
)
(
40 mL) for an hour. Finally, the mixture was filtered, washed with
3300–2300, 1650, 1620, 1540, 1475, 1400, 1100, 960, 870,700;
H-NMR (CDC1 , 400 MHz) δ (ppm): 10.70 (s, 1H), 8.06 (s, 1H),
3
distilled water, and dried to afford STA 2.
1
7
.78 (dd, 3H, J=8.1 Hz), 7.58 (t, 2H, J=6.6 Hz), 7.47 (t, 2H,
2
.4. Preparation of benzimidazole derivatives 5 using STA 2
13
J=7.6 Hz), 3.02 (q, 2H, J=7.6 Hz), 1.47 (t, 3H, J=7.6 Hz); C-
NMR (CDC1
3
, 100 MHz) δ (ppm): 197.62. 159.75, 138.30, 132.21,
A mixture of o-phenylenediamine 3 (1 mmol), orthoester 4
1.1 mmol) and STA 2 (0.01 mmol) was stirred with small Teflon sick-
1
31.51, 129.97, 128.25, 124.86, 118.16, 114.27, 22.81, 12.32; MS,
(
+
m/z: 250 (M ).
le blades and heated at 80 °C in a preheated oil bath for an appropri-
ate time. After completion of the reaction as indicated by TLC (EtOAc/
n-hexan, 1:2), the reaction mixture was dissolved in hot EtOAc and
catalyst was separated. The solvent was evaporated and the crude
product 5 was purified by recrystallization in EtOAc. Finally, the cata-
lyst was washed with EtOH, dried at 120 °C for 1 h, and reused in an-
other reaction.
3
. Results and discussion
In previous papers, we reported several organic transformation
using green solid acids, such as silica supported zirconium oxychlor-
ide (ZrOCl ·8H O/SiO ), molybdate sulfuric acid (MSA) and tungstate
2
2
2
sulfuric acid (TSA) [19]. Also, Nafion-H®, silica sulfuric acid [20] and
silica chloride [21] have been used by other researcher groups for sev-
eral organic reactions. As can be seen in Scheme 1, from the reaction
1
13
2
.5. H-NMR, C-NMR, and MS data of novel compounds
5
1
4
-Benzoyl-benzimidazole 5j: IR (KBr) νmax (cm−1): 3250–2300,
650, 1615, 1450, 1100, 950, 870, 690; 1H-NMR (CDC1
00 MHz) δ (ppm): 8.22 (s, 1H), 8.18 (s, 1H), 7.84 (t, 3H,
3
,
J=9.4 Hz), 7.72 (d, 1H, J=8.4 Hz), 7.60 (t, 1H, J=7.2 Hz), 7.49
Table 1
XRF data of STA 2.
Entry
Compound
SiO
Concentration (% W/W)
1
2
3
4
5
6
7
8
9
2
81.250
1.660
0.320
0.023
0.560
0.030
0.028
0.024
0.021
0.017
0.009
0.023
16.29
99.96
WO
4
2 5
P O
CaO
SO
Fe
ZnO
Al
TiO
SrO
3
2
O
3
2
O
3
2
10
11
12
13
14
2
2 3
Y O
CuO
a
LOI
Total
a
Loss on ignition.
Fig. 1. XRD pattern for the STA 2.

B. Karami et al. / Catalysis Communications 20 (2012) 71–75
73
Table 2
Effect of various regents and optimization of STA 2 in the synthesis of 5a at 80 °C under
solvent-free conditions.
Entry
Reagent/amount
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
9
–
360
360
360
360
8
5
5
5
10
10b
25
15
80
SiO
Na
SiO
STA/1 mol%
STA/2 mol%
STA/3 mol%
STA/5 mol%
STA/10 mol%
2
/1 mol%
WO /1 mol%
2
–Cl/1 mol%
2
4
b
90
90
b
90
85
85
a
Isolated yields.
Not completed.
b
synthesis of organic compounds [25], we found that STA 2 can be
used as a powerful, safe and recyclable catalyst for the condensation
of o-phenylenediamines 3 and orthoesters 4 under solvent-free con-
ditions to afford benzimidazol derivatives 5 in good to excellent
yields (Scheme 2).
Generally, in this protocol, the STA 2 has valuable and special fea-
tures. For example, high thermal stability, and iii) a small amount of
catalyst was used, and therefore this catalyst has made this reaction
economic and eco-friendly.
2 2 4
Fig. 2. FT-IR spectra for the comparison of SiO , STA, and Na WO .
of readily available materials such as silica gel and thionyl chloride,
silica chloride 1 has been prepared [22]. Accordingly, we found that
anhydrous sodium tungstate can react with 1 to give silica tungstic
acid (STA) 2. This reaction is clean and easy. From the synthetic
point of view, the nucleophilic substitution at silicon is also attractive.
STA 2 was characterized by X-ray fluorescence (XRF), X-ray dif-
fraction (XRD), and FT-IR spectra. As can be seen in Table 1, XRF
data for the STA 2 show the composition of the catalyst as 81.25
A solvent-free or solid state reaction obviously reduces pollution,
and helps to decrease costs due to the simplification of experimental
procedure, work up technique and saving in labor. Interest in the en-
vironmental control of chemical processes has increased remarkably
during three decades ago as a response to public concern about the
use of hazardous chemical materials. Therefore, to improve the effec-
tiveness of this method in preventing chemical waste, it is important
to investigate optimal reaction conditions. To determine the simple
and suitable conditions for the synthesis of benzimidazole derivatives
5 using STA 2 as a solid acid catalyst, the treatment of triethyl ortho-
formate with o-phenylenediamine was chosen as a model reaction. At
first, catalytic efficiency of STA 2 (different amounts) among the other
reagents was investigated for the model reaction and comparative
data are shown in Table 2. We found that in the absence of the cata-
lyst, the reaction did not complete in long reaction time even at a
high temperature. However, according to the results (Table 2), it
can be concluded that the STA (1 mol%) is better than the other ones.
In another experiment, the optimization of the reaction tempera-
ture was evaluated (Table 3). After examination of various amounts
of STA 2 and a wide range of temperatures, it was found that this con-
densation reaction can be efficiently carried out by adding 1 mol% of
catalyst at 80 °C under solvent-free conditions in a short time span
of 8 min. The use of excess amounts of the catalyst did not have a
marked influence on the product yield or reaction rate. The probable
reason for this is the coordination of excessive catalyst to the nitrogen
of amine groups.
(
2 4
%W/W) SiO (Entry 1) and 1.66 (%W/W) WO (Entry 2).
Fig. 1 shows the XRD patterns for the silica tungstic acid 2 which
exhibits the presence of tungstic acid crystalline phase supported on
amorphous silica as a broad peak around 22° (2 θ) (θ is the Bragg's
angle). The three peaks in the 23–25° region of the XRD spectrum
could be attributed to the presence and linking of WO
gel [23].
3
to the silica
The FT-IR spectra for the anhydrous sodium tungstate, silicachlor-
ide, and silica tungstic acid 2 are shown in Fig. 2. This spectrum shows
the characteristic bonds of anhydrous sodium tungstate and silica
chloride.
The adsorption in 3459, 1636, 1096, 970, 1620, and 799 cm−1 in
the catalyst spectrum reveals both bonds in SiO
2
–Cl and WO
4
group.
using
We evaluated the amounts of tungstic acid supported on SiO
2
two methods, including (a) titration with 0.1 N NaOH (neutralization
reaction) and (b) calculating the weight difference between primary
solid acid loosed chloride and new silica tungstic acid 2. After these
experiments, we found that 1 g catalyst includes 0.05 g –OWO
3
H. Re-
4
garding to the molecular weight of WO H (249 g), therefore, 1 g of
catalyst is equal to 0.2 mmol.
In regard of the importance of novel catalyst applications, after
characterization of the catalyst, we decided to use the STA 2 for the
synthesis of some benzimidazoles. In fact, the broad applicability of
some benzimidazole derivatives [24] attracted the interest of various
organic researchers with the aim of finding improved methods for the
synthesis of these compounds. In connection with our studies on
In order to prove the versatility of this method, after optimization
of the reaction conditions, different orthoesters with o-
phenylenediamines were treated. The results are summarized in
Table 3
Optimization of the temperature for the synthesis of 5a using STA 2 (1 mol%) under
solvent-free conditions.
Entry
Temperature (°C)
Time (min)
Yield (%)a
1
2
3
4
5
25
40
60
80
120
240
240
30
8
15
25
72
90
82
5
Scheme 2. Synthesis of benzimidazole derivatives using STA 2 under solvent-free
conditions.
a
Isolated yields.

74
B. Karami et al. / Catalysis Communications 20 (2012) 71–75
Table 4
The design and synthesis of recoverable catalysts is a highly chal-
lenging interdisciplinary field combining chemistry, materials, sci-
ence, and engineering from economic and environmental point of
view. It should be noted that the main disadvantage of many of the
reported methods for the synthesis of benzimidazoles is that the cat-
alysts are destroyed in the work-up procedure and cannot be recov-
ered or reused. In this process, as indicated in Fig. 3, the recycled
catalyst was used for four cycles during which a little appreciable
loss was observed in the catalytic activities.
As shown in Scheme 3, a plausible mechanism for the synthesis of
benzimidazoles 5 and recyclability of the STA 2 was proposed. STA 2
acts as a Brønsted acid so that it can release a proton and activate
the orthoester, thus, will decrease the energy of the transition state
and increase the rate of nucleophilic displacement.
Synthesis of benzimidazole derivatives using STA 2 (1 mol%) at 80 °C under solvent-
free conditions.
Producta
R1
R2
G
Time (min)
Yield (%)b
Mp ( C)
ο
c
5
5
5
5
5
5
5
5
5
5
5
5
a
b
c
d
e
f
g
h
i
H
Me
Et
H
Me
Et
H
Me
Et
H
Me
Et
Et
Me
Et
Et
Me
Et
Et
Me
Et
Et
Me
Et
H
H
H
8
5
20
12
9
25
5
5
25
20
18
32
90
92
90
80
82
78
86
80
80
80
92
90
170–172
176–178
166–168
108–110
198–200
165–167
208–210
225–227
177–179
136–138
168–170
140–142
Me
Me
Me
NO
NO
NO
PhCO
PhCO
PhCO
2
2
2
d
j
d
k
d
l
a
b
c
Identified by comparison with authentic samples.
Refers to isolated yields.
Ref. [13].
4. Conclusion
d
Novel compounds.
To conclude, in this study, we presented a simple and clean meth-
od for the synthesis of new silica tungstic acid and its catalytic activ-
ities in the synthesis of novel and known benzimidazole derivatives.
We believed that by the use of this heterogeneous solid acid catalyst,
in most cases the desired level of activity can be achieved and the cat-
alyst can be removed easily from the reaction mixture and reused.
The simple experimental procedure, solvent-free reaction conditions,
use of inexpensive and recyclable catalyst, low catalyst loading, short
period of reactions and good to excellent yields make this method a
valid contribution to the existing methodologies for the synthesis of
benzimidazoles.
Acknowledgments
The authors gratefully acknowledge partial support of this work
by the Yasouj University, Yasouj, Iran.
Fig. 3. Recyclability of the STA 2 as catalyst in the synthesis of 5a at 80 °C under
solvent-free conditions (reaction time=8–15 min).
Appendix A. Supplementary data
Table 4. Encouraged by the above results, we next focused on reaction
of 4-benzoyl-1,2-phenylenediamine with orthoesters to prepare the
new benzimidazoles 5j, 5k, and 5l.
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2012.01.012.
Scheme 3. Proposed mechanism for the condensation of o-pehenylenediamines 3 with orthoesters 4 using STA 2.

B. Karami et al. / Catalysis Communications 20 (2012) 71–75
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