An ammonium molybdate deposited amorphous silica coated iron oxide magnetic core-shell nanocomposite for the efficient synthesis of 2-benzimidazoles using hydrogen peroxide

Article abstract of DOI:10.1039/c3gc42551g

An ammonium molybdate deposited amorphous silica coated iron oxide (Fe ₃O₄@SiO₂) magnetic core-shell nanocomposite was prepared and tested in the application to the synthesis of 2-benzimidazoles from aromatic aldehydes and o-phenylenediamines using hydrogen peroxide as an oxidant at room temperature. It was found that the yields were fundamentally influenced by the nature and position of the substituents on the phenyl ring. Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS) confirmed that the active ammonium molybdate species are highly dispersed on the surface of the Fe₃O ₄@SiO₂ magnetic core-shell nanocomposite, accounting for its good activity. A method was eventually developed for the synthesis of 2-benzimidazoles highlighting the good activity and easily recyclable nature of this magnetic catalyst. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42551g

Green Chemistry
View Article Online
View Journal | View Issue
PAPER
An ammonium molybdate deposited amorphous
silica coated iron oxide magnetic core–shell
nanocomposite for the efficient synthesis of
Cite this: Green Chem., 2014, 16,
160
3
2-benzimidazoles using hydrogen peroxide†
Guoyi Bai,* Xingwang Lan, Xiaofang Liu, Chen Liu, Lingjuan Shi, Qingzhi Chen and
Guofeng Chen
3 4 2
An ammonium molybdate deposited amorphous silica coated iron oxide (Fe O @SiO ) magnetic core–
shell nanocomposite was prepared and tested in the application to the synthesis of 2-benzimidazoles
from aromatic aldehydes and o-phenylenediamines using hydrogen peroxide as an oxidant at room
temperature. It was found that the yields were fundamentally influenced by the nature and position of the
substituents on the phenyl ring. Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction (XRD), trans-
mission electron microscopy (TEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy
(XPS) confirmed that the active ammonium molybdate species are highly dispersed on the surface of the
Received 16th December 2013,
Accepted 17th March 2014
3 4 2
Fe O @SiO magnetic core–shell nanocomposite, accounting for its good activity. A method was even-
DOI: 10.1039/c3gc42551g
tually developed for the synthesis of 2-benzimidazoles highlighting the good activity and easily recyclable
nature of this magnetic catalyst.
www.rsc.org/greenchem
1
0
11
12
13
14
15
21
include In(OTf)
3
,
FeCl
3
,
DDQ,
I
2
,
4
NH OAc, SDS,
Introduction
1
6
17
18
19
20
IBD, DMP, NaHSO3, Na S O , Scolecite, Dowex 50W,
-Benzimidazole and its derivatives are important aromatic H O –HCl, H O –Bu NI, and H O –CAN. The application
2 2 2 2 4 2 2
2
2 5
2
2
23
24
2
nitrogen heterocycles due to their ubiquitous presence and of these reagents has resulted in the efficient synthesis of benz-
wide applications in many pharmaceutical and biological imidazoles, but some procedures still have problems such as
areas as antiulcer agents, antihypertensives, antivirals, anti- the use of toxic or precious metal catalysts, and the difficulties
fungals, anticancer agents, and antihistamines, as well as in in catalyst recycling. These problems have limited the practical
1
–6
organic synthesis.
The widespread interest in 2-benzimida- applications of this approach and there has been a great
zoles has prompted extensive studies on their synthesis and a demand for the application of a green oxidant and a recyclable
series of methods has been subsequently developed in recent catalyst for the synthesis of 2-benzimidazoles.
decades. The coupling of o-phenylenediamines with carboxylic
acids or their derivatives is an established general method to as homogeneous or heterogeneous catalysts in organic syn-
prepare benzimidazoles, but the harsh reaction conditions thesis due to their excellent catalytic activity, good solubility,
On the other hand, polyoxometalates have been widely used
7
8
2
5–29
28
and relatively low yields limit its applications. Alternatively, and non-toxicity.
For instance, Iraj et al. have reported
the oxidative cyclodehydrogenation of aniline Schiff bases, the synthesis of various oxazolines and imidazolines over tung-
generated in situ from the condensation of o-phenylenedi- stophosphoric acid, and excellent yields were achieved in a
9
29
amines with aldehydes, over various oxidants or catalysts has short time. Autino et al. have obtained good results under
become popular due to the high efficiency and green nature of solvent-free conditions in the synthesis of substituted cou-
this approach. The generally employed oxidants or catalysts marins from phenols and β-ketoesters over Wells–Dawson
polyoxometalates. However, the difficulty in efficient recycling
of the used polyoxometalates has limited their further appli-
cation, mainly ascribed to their high solubility in water. Thus,
preparation and application of supported polyoxometalates
Key Laboratory of Chemical Biology of Hebei Province, College of Chemistry and
Environmental Science, Hebei University, Baoding 071002, PR China.
E-mail: baiguoyi@hotmail.com; Fax: +86-312-5937102; Tel: +86-312-5079359
3
0,31
have attracted much attention in recent years.
For
example, García-Gutierrez et al. have developed an efficient
γ-Al O supported ammonium molybdate catalyst for the ultra-
†
Electronic supplementary information (ESI) available: Segmental details of the
31
experimental procedures, the catalyst characterization results (SEM, TG, DTG,
and TEM), and the spectral data of the obtained compounds (IR, NMR, and
HRMS) are shown in this file. See DOI: 10.1039/c3gc42551g
2
3
deep oxidative desulfurization of diesel fuel using hydrogen
3160 | Green Chem., 2014, 16, 3160–3168
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
peroxide as an oxidant. Recently, magnetic core–shell nano- lenediamine, was carried out over hydrogen peroxide and
composites have become popular as novel supported catalysts different polyoxometalates or surfactant combinations, and
due to their chemically modifiable surface, magnetically separ- the results are listed in Table 1. As can be seen, the selectivity
3
2–34
33
able nature and good stability.
For instance, Wu et al.
for 2-phenylbenzimidazole was only 48.3%, together with for-
have reported a magnetically separable Fe O @SiO @Pd-Au mation of appreciable quantities of Schiff base intermediates,
3
4
2
nanocomposite for the liquid-phase hydrodechlorination of in the absence of a catalyst. Gratifyingly, among the catalysts
-chlorophenol under mild conditions, exhibiting an excellent studied, ammonium molybdate was found to markedly
4
3
4
catalytic performance. An et al. have prepared a novel improve the selectivity of 2-phenylbenzimidazole, achieving
Fe @SiO @AgCl:Ag nanocomposite via the visible-light 97.7% in 180 min, much higher than other catalysts. On the
3
O
4
2
reduction method and tested it in the decomposition of other hand, 2-phenylbenzimidazole was obtained with 48.7%
organic pollutants. This catalyst can be effectively recycled selectivity in the presence of air without hydrogen peroxide,
many times without the loss of its activity. However, to the whereas it decreased to 14.4% in the absence of both hydrogen
best of our knowledge, no work on the synthesis of 2-benzimi- peroxide and air (Table 1, entries 8 and 9), indicating that
dazoles over core–shell structured nanocomposites has been ammonium molybdate itself has poor catalytic activity.
reported.
Therefore, we speculated that peroxomolybdate intermediates
3
5
In continuation of our previous work, we herein report or other peroxides, generated from ammonium molybdate
our recent research into the green and efficient synthesis of with hydrogen peroxide and air, account for the high activity
2
-benzimidazoles from aromatic aldehydes and o-phenylene- of this catalytic system. Furthermore, we also examined the
diamines via a hydrogen peroxide oxidation system involving a effect of reaction time. It was found that, although the conver-
novel ammonium molybdate deposited Fe O @SiO magnetic sion still remained at a high level, the selectivity for 2-phenyl-
3
4
2
core–shell nanocomposite (Fe
catalyst exhibits good catalytic performance in the synthesis of when the reaction time was decreased from 180 min to
-benzimidazoles and can be easily recycled. The mechanism 60 min, whereas it was nearly unchanged when the reaction
3 4 2 4 6 7
O @SiO @(NH ) Mo O24). This benzimidazole decreased dramatically from 97.7% to 17.4%
2
of this transformation will be discussed in this paper and time was prolonged from 180 min to 240 min. Thus, 180 min
the structure of this core–shell nanocomposite has been is regarded as the appropriate reaction time for this catalytic
confirmed.
system.
To solve the difficulty of efficient recycling of the used
ammonium molybdate and shorten the reaction time, we pro-
posed to prepare a new supported ammonium molybdate cata-
lyst. Considering that magnetic separation is more attractive
Results and discussion
Initially, the synthesis of 2-phenylbenzimidazole as a represen- and efficient than traditional filtration or centrifugation,
tative of 2-benzimidazole, from benzaldehyde and o-pheny- we decided to immobilize ammonium molybdate on a
a
Table 1 Effect of different catalytic systems on the synthesis of 2-phenylbenzimidazole
b
b
Entry
Oxidant
Catalyst
Time (min)
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
2
2
2
2
2
2
2
O
O
O
O
O
O
O
2
2
2
2
2
2
2
—
180
180
180
180
180
180
180
180
180
120
60
93.2
95.4
96.0
98.0
94.1
92.4
98.4
98.3
96.2
96.7
97.9
93.5
48.3
64.9
20.3
71.6
97.7
38.4
29.2
48.7
14.4
62.1
17.4
94.4
3 40
H PW12O
Na
Na
(NH
TBAB
CTAB
(NH
(NH
(NH
(NH
(NH
2
WO
4
2
MoO
4
4 6
) Mo
7
O
24
—
—
4 6
) Mo
4 6
) Mo
4 6
) Mo
4 6
) Mo
4 6
) Mo
O
7 24
O
7 24
O
7 24
O
7 24
O
7 24
c
10
11
12
H
H
H
2
O
2
O
2
O
2
2
2
240
a
b
Reaction conditions: benzaldehyde (1.0 mmol), o-phenylenediamine (1.1 mmol), EtOH (5.0 mL), catalyst (0.04 mmol), H
Determined by HPLC based on the starting material. Operated under a nitrogen atmosphere. TBAB = tetrabutyl ammonium bromide. CTAB =
2
O
2
(0.2 mL).
c
cetyl trimethyl ammonium bromide.
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3160–3168 | 3161

View Article Online
Paper
Green Chemistry
Fig. 1 Stepwise preparation of Fe
core–shell nanocomposites.
3 4 2 4 6 7
O @SiO @(NH ) Mo O24 magnetic
Fig. 2 Reusability studies on Fe
thesis of 2-phenylbenzimidazole. Reaction conditions: benzaldehyde
3 4 2 4 6 7
O @SiO @(NH ) Mo O24 for the syn-
(
(
1.0 mmol), o-phenylenediamine (1.1 mmol), EtOH (5 mL), catalyst
0.22 g, including 0.04 mmol (NH Mo 24), H (0.2 mL) at room
4
)
6
7
O
2 2
O
temperature.
Table 2 Effect of ammonium molybdate supported on Fe
the synthesis of 2-phenylbenzimidazole
3
O
4
@SiO
2
on
a
c
c
Furthermore, the effect of solvents was also investigated
under the same reaction conditions (Table 2). It became appar-
ent that the conversion of benzaldehyde decreased in other
solvents compared to ethanol. Considering that application of
Conversion Selectivity
Entry Catalyst
Solvent
(%)
(%)
1
2
3
4
Catalyst-free
(NH Mo
EtOH
EtOH
EtOH
EtOH
95.2
96.2
87.3
92.0
11.6
11.9
18.3
94.0
)
4 6
7
O
24
3
5
Fe
Fe
3
O
O
4
@SiO
@SiO
2
ultrasound often can reduce the reaction time dramatically,
3
4
2
@
this reaction was also carried out under ultrasound but we
only observed a slight increase in the conversion and selecti-
vity. Thus, Fe O @SiO @(NH ) Mo O was finally chosen as
(
Fe
Fe
NH
4
)
6
Mo
@(NH
@SiO
Mo
@SiO
Mo
@SiO
Mo
7 24
O
5
6
3
O
O
4
4
)
6
Mo
7
O
24
EtOH
Solvent-
free
81.0
71.8
97.1
94.7
3
4
2
@
24
@
24
3
4
2
4 6
7 24
(
NH
Fe
NH
Fe
NH
4
O
)
4
6
7
O
2
the catalyst of choice for the synthesis of 2-phenylbenzimida-
zoles in ethanol as the solvent of choice.
7
8
3
2
H O
47.1
92.6
86.2
96.8
(
)
4 6
7
O
O
4
2
@
EtOH
In addition, the stability of Fe O @SiO @(NH ) Mo O
3
4
2
4 6
7
24
24
3
b
(
)
4 6
7 24
O
was investigated. The recovered Fe
3
O
4
@SiO
2
@(NH
4
)
6
Mo
7
O
a
was easily separated with an external magnet after reaction
Reaction conditions: benzaldehyde (1.0 mmol), o-phenylenediamine
(
1.1 mmol), solvent (5.0 mL), catalyst (0.22 g, including 0.04 mmol and then recycled under the same reaction conditions.
NH Mo (0.2 mL), 30 min at room temperature.
Ultrasound heating for 30 min at room temperature. Determined by
HPLC based on the starting material.
(
)
4 6
7 2 2
O24), H O
The results depicted in Fig. 2 show that Fe
3 4 2
O @SiO @
b
c
(
NH ) Mo O can be recycled five times without an appreci-
4 6
7 24
able loss of its catalytic activity, demonstrating its good stabi-
lity as a catalyst for the synthesis of 2-phenylbenzimidazole.
Fe O @SiO magnetic core–shell nanocomposite by combin- Furthermore inductively coupled plasma (ICP) analysis
3
4
2
ing the Stöber and the equilibrium adsorption methodology, revealed that the amount of Mo in the supernatant after the
as illustrated in Fig. 1. Thus,
a
novel Fe O @SiO @ reaction is very low and nearly negligible, indicating that there
3
4
2
(NH ) Mo O catalyst was first prepared and applied to the is almost no loss of the ammonium molybdate active species
4 6 7 24
synthesis of 2-benzimidazoles (Table 2).
As envisaged, the Fe O @SiO @(NH ) Mo O
7 24
during the recycle, in agreement with the stability results.
catalyst
3
4
2
4 6
could be effectively separated from the reaction mixture by an
external magnetic field after the completion of reaction. Of
Catalyst characterization
note, the selectivity for 2-phenylbenzimidazole increased The physicochemical properties of ammonium molybdate and
markedly to 94.0% in 30 min, whereas it was only 11.9% over the magnetic core–shell nanocomposites were measured and
the unsupported ammonium molybdate, 11.6% in the absence selected results are summarized in Table 3. ICP results
of any catalyst and 18.3% over Fe O @SiO . On the other indicated that the Mo content of Fe O @SiO @(NH ) Mo O
3
4
2
3 4 2 4 6 7 24
hand, the conversion of benzaldehyde was only 81.0% when
catalyzed by ammonium molybdate directly immobilized onto
Fe O (Fe O @(NH ) Mo O ) by the same method, although
Table 3 Selected physicochemical properties of ammonium molybdate
and the magnetic core–shell nanocomposites
3
4
3
4
4 6
7 24
the selectivity for 2-phenylbenzimidazole was similar. Thus, it
appears that ammonium molybdate supported on Fe @SiO
not only solves the problem of separation of ammonium
molybdate from the reaction mixture but also improves the (NH ) Mo O
a
Mo loading Surface area Pore volume
3
O
4
2
2
−1
(10 cm g−1)
−2
3
Sample
(wt%)
(m g
)
4
6
7
24
—
—
3.4
27.5
8.3
1.1
5.8
2.4
Fe
Fe
3
O
O
4
@SiO
@SiO
2
dispersion of the active ammonium molybdate on the surface
of SiO , accounting for the high activity of Fe O @SiO @
3
4
2
4 6 7
@(NH ) Mo O24 10.0
2
3
4
2
a
(NH ) Mo O .
Based on ICP results.
4
6
7 24
3162 | Green Chem., 2014, 16, 3160–3168
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
Fig. 4 Nitrogen adsorption–desorption isotherms and the pore size
distributions (the inset) of the core–shell nanocomposites (a)
Fe O @SiO and (b) Fe O @SiO @(NH ) Mo O24.
Fig. 3 XRD patterns of (NH
4
)
6
Mo
@SiO
The inset shows the SAED image of Fe
7
O
24 and the core–shell nanocompo-
, (c) Fe @SiO @(NH Mo
@SiO @(NH Mo
sites. (a) (NH
4
)
6
Mo
7
O
24, (b) Fe
3
O
4
2
3
O
4
2
4
)
6
7 24
O .
3
4
2
3
4
2
4 6
7
3
O
4
2
4
)
6
7 24
O .
from capillary condensation. The pore size distribution curve
was 10.0%, in agreement with the actual loading amount of of Fe O @SiO showed a sharp peak at 3.9 nm and fluctu-
3
4
2
ammonium molybdate. The surface area and pore volume ations between 5 and 40 nm (inset), which might be due to the
of Fe O @SiO @(NH ) Mo O were higher than those of micropore nature and interstices between the primary par-
3
4
2
4 6
7 24
ammonium molybdate, but much lower than those ticles, improving the adsorption ability of Fe O @SiO ,
3
4
2
3 4 2
3 4 2
of Fe O @SiO , probably owing to the active ammonium whereas the pore size distribution curve of Fe O @SiO @
molybdate species present in the pores and assembled on its (NH ) Mo O merely fluctuated over a wide range, indicating
4
6
7 24
3
8
surface.
only interstices between the primary particles. Thus, we
Fig. 3 shows X-ray diffraction (XRD) patterns of ammonium speculate that the active ammonium molybdate species are
molybdate and the corresponding magnetic core–shell nano- deposited on the Fe O @SiO magnetic core–shell nanocom-
3
4
2
composites. The characteristic diffraction peaks of ammonium posite, obstructing the micropores and some interstices, in
molybdate appear around 12.5°, 25.1°, 37.3° and 41.4°, con- agreement with its smaller BET surface area and pore volume.
3
6
firming the presence of crystalline ammonium molybdate.
Fe @SiO shows characteristic diffraction peaks at about the corresponding magnetic core–shell nanocomposites were
0.4°, 35.8°, 43.5°, 53.5°, 57.6° and 63.3°, just like those of recorded by transmission electron microscopy (TEM) and high
pure Fe O (JCPDS: 00-003-0862), indicating that the resolution transmission electron microscopy (HRTEM). From
The surface morphology and the particle distributions of
3
O
4
2
3
3
4
embedded Fe
coating, while the SiO
the diffraction peaks of Fe O @SiO @(NH ) Mo O
3
O
4
retains its crystalline structure after SiO
2
Fig. 5a and b, it can be seen that each nanocomposite
2
exists in an amorphous state. However, contains several uniform spherical shaped Fe O nanoparticles
3
4
are (approximately 6 nm in diameter) in one SiO shell. The exist-
3
4
2
4 6
7
24
2
similar to those of Fe
3 4 2
O @SiO with no characteristic diffrac- ence of the ammonium molybdate species on the surface of
tion peak corresponding to ammonium molybdate being Fe O @SiO nanocomposites was difficult to confirm from the
3
4
2
observed, indicating that the active ammonium molybdate TEM and HRTEM investigations (Fig. 5c and d), indicating
species either exists in an amorphous state or is highly dis- that the ammonium molybdate species were highly dispersed
persed on the surface and in the pores of Fe
Selected-area electron diffraction (SAED) patterns obtained state. Further analysis of particle distributions showed that
from the shell edges of Fe @SiO @(NH Mo 24 are Fe O @SiO @(NH ) Mo O has a little larger average particle
characterized by successive diffraction rings (inset in Fig. 3), size (approximately 62 nm) than Fe O @SiO (approximately
3
O
4
@SiO
2
.
on the surface or in pores and were not in a crystalline
3
O
4
2
4
)
6
7
O
3
4
2
4 6
7 24
3
4
2
demonstrating the crystalline structure of Fe O
in 58 nm) (Fig. 5, insets in a and c), although both of them
3
4
Fe
results.
3
O
4
@SiO
2
@(NH
4
)
6
Mo
7
O
24, in accordance with the XRD intend to form larger aggregates due to the magnetic nature of
Fe O . This suggests that the active ammonium molybdate
3
4
The nitrogen adsorption–desorption isotherms and the species might assemble on the surface of Fe O @SiO to form
3
4
2
pore size distribution curves of Fe
Fe @SiO @(NH Mo 24 are shown in Fig. 4. The nitrogen microscopy (SEM) and energy dispersive X-ray (EDX)
sorption isotherms of both magnetic core–shell nanocompo- spectroscopy results (Fig. S1 and S2†).
3
O
4
@SiO
2
and a monolayer, in agreement with the scanning electron
3
O
4
2
4
)
6
7
O
3
7
sites resembled typical type IV isotherms together with a hys-
teresis loop of type H3 in the P/P
Scanning transmission electron microscopy (STEM) images
0
range of 0.4–1.0, arising of Fe O @SiO @(NH ) Mo O are depicted in Fig. 6, together
3 4 2 4 6 7 24
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3160–3168 | 3163

View Article Online
Paper
Green Chemistry
3 4 2
Fig. 5 TEM and HRTEM images of (a and b) Fe O @SiO and (c and d)
Fe
3
O
4
@SiO
2
@(NH
4
)
6
Mo
7
O
24 nanocomposites. (Insets a and c: the par-
@SiO and Fe @SiO @(NH Mo 24.)
ticle distributions of Fe
3
O
4
2
3
O
4
2
4
)
6
7
O
with a representative high-angle annular dark-field (HAADF)
image and elemental mapping of the nanocomposite. The
results demonstrate the presence of the expected molybdenum
in the structure of this nanocomposite, as confirmed by the
mapping of the related Si, Fe and O elements (Fig. 6c–g). The
distribution of the molybdenum supports our previous
assumption that the ammonium molybdate species were
Fig. 6 (a) STEM image of Fe
image of Fe @SiO @(NH
3
O
4
@SiO
2
@(NH
4
)
6
Mo
7
O
24. (b) HAADF-STEM
3
O
4
2
4
)
6
Mo O24, with elemental mapping of (c) Si,
7
highly dispersed on the surface and in the pores of (d) Fe, (e) O, (f) Mo, and (g) their overlap shown in the bottom row.
Fe O @SiO nanocomposites.
3
4
2
X-ray photoelectron spectra (XPS) of Fe
@SiO @(NH Mo 24 are shown in Fig. 7. It was found
that no Fe element was detected on the surface of either nano-
composites, demonstrating that the Fe nanoparticles were
well coated with the pyknotic SiO shell. Furthermore, two
peaks related to Mo 3d5/2 and Mo 3d3/2 (inset) could be
observed on the surface of Fe @SiO @(NH Mo 24. Their
binding energies are 232.7 and 235.8 eV, respectively,
3 4 2
O @SiO and
(
NH
4 6 7 3 4 2
) Mo O24 on the surface of Fe O @SiO . Moreover, the
Fe
3
O
4
2
4
)
6
7
O
magnetization curves showed no remanence or coercivity in
both samples, suggesting their superparamagnetic character.
Thus, the Fe @SiO @(NH Mo 24 nanocomposites can be
easily separated from the reaction mixture by an external mag-
netic field (inset in Fig. 8), but pure ammonium molybdate
cannot be separated.
4
0
3 4
O
3
O
4
2
4
)
6
7
O
2
3
O
4
2
4
)
6
7
O
6
−
suggesting the presence of the (Mo O ) structure, as also
7
24
3
9
Speculation of the reaction mechanism
reported in previous literature. Further thermogravimetric
(
(
TG) and differential thermogravimetric (DTG) measurements A plausible mechanism of this reaction is proposed based on
Fig. S3†) confirmed that Fe O @SiO @(NH ) Mo O is rela- the experimental results and previous literature obser-
3
4
2
4 6
7 24
2
3,24,31
tively stable within 473 K, whereas (NH
4
)
6
Mo
7
O
24·4H
2
O will vations,
as shown in Scheme 1. Intermediate 2 is
only lose its crystalline water during the preparation process, initially formed via cyclization of Schiff base intermediate 1,
in agreement with the above XPS results.
which is formed by rapid condensation from benzaldehyde
The room temperature hysteresis loops of Fe
O @SiO @(NH ) Mo O24 nanocomposites are shown in Zhu and Bahrami have described. Then, the intermediate 2
3 4 2 4 6 7
3
O
4
@SiO
2
and and o-phenylenediamine in the absence of the catalyst, as
2
3
24
Fe
Fig. 8. As can be seen, the saturation magnetization (M ) value reacts with the in situ generated peroxomolybdate intermedi-
s
−
1
31
of Fe
Oe, whereas the M
decreases to 28.0 emu g , ascribed to the deposition of 3, together with the regeneration of 5 and irreversible loss of
3
O
4
@SiO
2
is 37.6 emu g at an applied field of 20 000 ates 6, from the oxidation of the supported molybdate
s
value of Fe @SiO @(NH Mo
3
O
4
2
4
)
6
7
O
24 species 5 by hydrogen peroxide, to afford the key intermediate
−
1
3164 | Green Chem., 2014, 16, 3160–3168
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
Fig. 8 Room temperature hysteresis loops of the core–shell nanocom-
posites. (a) Fe
3
O
4
@SiO
2
and (b) Fe
3
O
4
@SiO
2
@(NH Mo
4
)
6
7
O24. The inset
shows photographs of Fe
3
O
4
@SiO
2
@(NH
4
)
6
Mo O24 before (left) and
7
after (right) magnetic separation by an external magnetic field.
water. Finally, the rapid oxidative dehydrogenation of inter-
41
mediate 3 generates the desired 2-phenylbenzimidazole.
Synthesis of derivatives
To evaluate the versatility of this method, a series of aromatic
aldehydes was then examined with o-phenylenediamines
under the optimum reaction conditions, as shown in Table 4.
In all cases, the reaction was carried out over periods of
1
3
0 min to 65 min. All products were characterized by FT-IR, H
1
3
NMR, C NMR and HRMS spectroscopy. As expected, all aro-
matic aldehydes reacted smoothly with o-phenylenediamine to
provide the corresponding 2-benzimidazoles in good to excel-
lent isolated yields. Furthermore, reactions of the p-substituted
aldehydes possessing electron-withdrawing groups, such as
–
Cl and –NO , underwent much faster reaction than those
2
possessing electron-donating groups, such as –OCH and –OH,
3
suggesting that the electron-withdrawing groups are favorable
for this transformation. In contrast, comparing the reaction
results of substituted benzaldehydes with the –Cl group in
Fig. 7 XPS spectra of (a) Fe
3
O
4
@SiO
2
3 4 2
and (b) Fe O @SiO @
(
NH
4
)
6
Mo
7
O24 nanocomposites. The inset in (b) shows the high resolu-
tion XPS spectra of the Mo element in Fe
3
O
4
@SiO @(NH Mo
2
4
)
6
7 24
O .
Scheme 1 A plausible mechanism for the synthesis of 2-phenylbenzimidazole.
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3160–3168 | 3165

View Article Online
Paper
Green Chemistry
a
Table 4 Synthesis of 2-benzimidazoles from aldehydes and diamines
b
Entry
1
Diamine
Aldehyde
Product
Time (min)
30
Yield (%)
90.3
94.6
93.0
2
3
30
45
4
35
90.4
5
6
7
8
9
35
65
40
65
60
90.9
87.3
89.7
88.9
91.7
1
1
0
1
2
35
35
45
95.2
93.7
91.0
1
a
4 6 7 2 2
Reaction conditions: aldehyde (1.0 mmol), diamine (1.1 mmol), EtOH (5.0 mL), catalyst (0.22 g, including 0.04 mmol (NH ) Mo O24), H O
b
(0.2 mL). Yields of isolated products.
various positions, the steric factors of the substituents are
proven to have less influence on the reaction rate. Thus, we
Conclusions
can conclude that electronic factors are the main controlling In summary, we have developed a green and efficient one-pot
factors in this transformation rather than steric factors. synthetic route to prepare 2-benzimidazoles from aldehydes
Moreover, the reaction of benzaldehyde with some substituted and o-phenylenediamines using hydrogen peroxide as an
o-phenylenediamines also gave the corresponding 2-benzimi- oxidant at room temperature. A novel Fe O @SiO @(NH ) -
3
4
2
4 6
7
dazoles smoothly in excellent yields, indicating the wide versa- Mo O24 magnetic core–shell nanocomposite has been pre-
tility of this method.
pared and demonstrated to be a highly efficient catalyst for
3166 | Green Chem., 2014, 16, 3160–3168
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
this transformation. It can be recycled and reused several (BE) values were calibrated by the C 1s peak of contaminant
times without any appreciable loss of its initial catalytic carbon at 284.6 eV. Magnetization curves were obtained on a
activity. The noteworthy advantages of this method are its high LDJ9600-1 Superconducting quantum interference device at
efficiency, rapid and easy isolation of the used catalyst, and room temperature.
green and mild reaction conditions, which make it an attrac-
tive and useful contribution to present methodologies.
General procedure for the synthesis of 2-benzimidazoles
A mixture of aromatic aldehydes (1.0 mmol) and o-phenylene-
diamines (1.1 mmol) was first stirred in ethanol (5.0 mL) at
room temperature. The catalyst (0.22 g, including 0.04 mmol
Experimental section
Catalyst preparation
(
4 6 7 2 2
NH ) Mo O24) and 30.0% H O (0.2 mL) were then added to
the above mixture and reacted at room temperature for the
requisite period of time while monitoring the reaction pro-
gress by TLC or HPLC. The reaction mixture was analyzed by
HPLC using a BaseLine C18 column (150 mm × 4.6 mm)
3 4 2 4 6 7
The Fe O @SiO @(NH ) Mo O24 magnetic core–shell nano-
composite was prepared by a combination of the Stöber and
equilibrium adsorption methodology, described as follows.
The core–shell Fe
pared by the hydrolysis of tetraethyl orthosilicate (TEOS) on
the surface of Fe nanoparticles in a similar manner to
3 4 2
O @SiO nanocomposite was first pre-
with a mobile phase of methanol–water (7 : 3 by volume) at a
−1
0
.6 mL min constant flow.
3 4
O
After completion of the reaction, the catalyst was first separ-
4
2
the protocol reported in previous literature. In a typical pro-
cedure, 1.4 g Fe O powder was first dispersed in 70 mL de-
ated using an external magnet for recycling and the residual
reaction mixture was diluted with ethyl acetate (30.0 mL).
Then, the solution was concentrated and the products were
finally isolated by column chromatography on 200–
3
4
ionized water. An absolute ethanol solution containing 5.0 mL
5 wt% aqueous ammonia was then added to the magnetite
suspension with stirring at 303 K over 15 min. After that,
.0 mL TEOS was added slowly and the mixture was vigorously
stirred for 12 h. Finally, the Fe @SiO was washed with
2
300 mesh silica gel using petroleum ether–ethyl acetate = 6 : 1
4
∼
4 : 1 as an eluent. Analytical and spectroscopic data of the
3
O
4
2
series of 2-benzimidazoles are given in the ESI.†
absolute ethanol several times, collected using a magnet, dried
and then kept under vacuum for further use.
Fe
3 4 2 4 6 7
O @SiO @(NH ) Mo O24 was then prepared by the fol-
Acknowledgements
lowing run: the prepared Fe O @SiO nanocomposite (1.0 g)
3
4
2
was first impregnated with an aqueous solution of 0.25 g
ammonium molybdate at room temperature. The wet samples
were dried at room temperature overnight and then dried at
The authors thank Professor Laurence M. Harwood for his
kind assistance. Financial support by the National Natural
Science Foundation of China (21376060 and 20806018)
and the Natural Science Foundation of Hebei Province
4
3 4 2 4 6 7 24
03 K in air for 2 h. Finally, the Fe O @SiO @(NH ) Mo O
nanocomposite thus obtained was kept under vacuum for
further use.
(
B2014201024) is gratefully acknowledged.
Instruments and methods
Notes and references
The amount of Mo in the supernatant after the reaction was
tested by ICP on an Agilent7500a spectrometer. The Mo
content of the Fe O @SiO @(NH ) Mo O24 magnetic core–
3 4 2 4 6 7
shell nanocomposite was determined by ICP on a Varian Vista-
MPX spectrometer. BET surface areas and pore volumes of the
catalysts were tested by nitrogen physisorption at 77 K on
Micromeritics Tristar II 3020 apparatus. Before measurements,
the samples were dried at 393 K for 2 h. Powder XRD patterns
were obtained with a Bruker D8 diffractometer using a Cu Kα
radiation source at 40 kV and 100 mA with a step size of 0.01°
1 A. R. Porcari, R. V. Devivar, L. S. Kucera, J. C. Drach and
L. B. Townsend, J. Med. Chem., 1998, 41, 1252.
2 W. A. Craigo, B. W. LeSueur and E. B. Skibo, J. Med. Chem.,
1999, 42, 3324.
3 W. Guba, L. G. Green, R. E. Martin, O. Roche,
N. Kratochwil, H. Mauser, C. Bissantz, A. Christ and
M. Stahl, J. Med. Chem., 2007, 50, 6295.
4 L. J. Goossen and T. Knauber, J. Org. Chem., 2008, 73,
8631.
2
θ over the range 10–80°. TEM images were acquired with a
5 S. Gangula, C. R. Elati, A. Neredla, S. R. Baddam,
U. K. Neelam, R. Bandichhor and A. Dongamanti, Org.
Process Res. Dev., 2010, 14, 229.
6 M. H. Pourgholami, L. M. Khachigian, R. G. Fahmy,
S. Badar, L. Wang, S. Wai, L. Chu and D. L. Morris,
Biochem. Biophys. Res. Commun., 2010, 397, 729.
7 C. Zhang, L. R. Zhang and N. Jiao, Green Chem., 2012, 14,
3273.
FEI Tecnai G2 F20 X-Twin microscope at an extraction voltage
of 3950 V, gun lens 1 and spot size 1. STEM/EDX maps were
obtained at a spot size of 6 and the diffraction pattern was
acquired with a camera length of 300 mm. The average particle
sizes were estimated on the basis of 50 particles. XPS analysis
was carried out on a PHI 1600 spectrometer using mono-
chromatic Mg Kα as the excitation source. The catalyst was
mounted on the sample holder, where the background
pressure was lower than 2 × 10−8 Torr. All the binding energy
8 (a) J. S. Peng, M. Ye, C. J. Zong, F. Y. Hu, L. T. Feng,
X. Y. Wang, Y. F. Wang and C. X. Chen, J. Org. Chem., 2011,
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3160–3168 | 3167

View Article Online
Paper
Green Chemistry
7
6, 716; (b) X. Q. Diao, Y. J. Wang, Y. W. Jiang and
V. S. Kamble, Appl. Catal., A, 2003, 240, 53; (c) C. Castro,
A. Corma and J. Primo, J. Mol. Catal. A: Chem., 2002, 177,
273.
D. W. Ma, J. Org. Chem., 2009, 74, 7974; (c) M. H. Shen and
T. G. Driver, Org. Lett., 2008, 10, 3367.
9
(a) S. N. Lin and L. H. Yang, Tetrahedron Lett., 2005, 46, 28 M. B. Iraj, M. Majid, T. Shahram, M. Valiollah and
4
315; (b) Y. Kawashita, N. Nakamichi, H. Kawabata and
M. Hayashi, Org. Lett., 2003, 5, 3713.
0 R. Trivedi, S. K. De and R. A. Gibbs, J. Mol. Catal. A: Chem.,
006, 245, 8.
S. F. Hojati, Catal. Commun., 2008, 9, 1153.
29 G. P. Romanelli, D. Bennardi, D. M. Ruiz, G. Baronetti,
H. J. Thomas and J. C. Autino, Tetrahedron Lett., 2004, 45,
8935.
1
1
1
2
1 M. P. Singh, S. Sasmal, W. Lu and M. N. Chatterjee, 30 G. D. Yadav, N. S. Asthana and V. S. Kamble, J. Catal., 2003,
Synthesis, 2000, 1380. 217, 88.
2 J. J. V. Eynde, F. Delfosse, P. Lor and Y. V. Haverbeke, Tetra- 31 J. L. García-Gutierrez, G. A. Fuentes, M. E. Hernández-
hedron, 1999, 51, 5813.
Terán, P. García, F. Murrieta-Guevara and F. Jiménez-Cruz,
1
3 P. Gogoi and D. Konwar, Tetrahedron Lett., 2006, 47, 79.
Appl. Catal., A, 2008, 334, 366.
1
4 H. Sharghi, O. Asemani and R. Khalifeh, Synth. Commun., 32 (a) A. H. Lu, E. L. Salabas and F. Schth, Angew. Chem., Int.
2
008, 38, 1128.
5 K. Bahrami, M. M. Khodaei and A. Nejati, Green Chem.,
010, 12, 1237.
Ed., 2007, 46, 1222; (b) X. X. Ma, J. T. Wang, M. Sun,
W. N. Wang, Q. H. Wu, C. Wang and Z. Wang, Anal.
Methods, 2013, 5, 2809; (c) F. H. Chen, Q. Gao and J. Z. Ni,
Nanotechnology, 2008, 19, 165103; (d) H. L. Ding,
Y. X. Zhang, S. Wang, J. M. Xu, S. C. Xu and G. H. Li, Chem.
Mater., 2012, 24, 4572.
1
2
1
1
6 L. H. Du and X. P. Luo, Synth. Commun., 2010, 40, 2880.
7 S. K. Dabhade, R. O. Bora, M. Farooqui and C. H. Gill,
Chin. Chem. Lett., 2009, 20, 893.
1
8 T. Yamashita, S. Yamada, Y. Yamazaki and H. Tanaka, 33 Z. J. Wu, C. X. Sun, Y. Chai and M. H. Zhang, RSC Adv.,
Synth. Commun., 2009, 39, 2982. 2011, 1, 1179.
9 N. V. Gabriel, M. D. Hermenegilda, A. C. Francisco, 34 C. H. An, X. J. Ming, J. Z. Wang and S. T. Wang, J. Mater.
1
L. R. Ismael, V. M. Rafael, M. M. Omar and E. S. Samuel,
Bioorg. Med. Chem. Lett., 2006, 16, 4169.
0 L. S. Gadekar, B. R. Arbad and M. K. Lande, Chin. Chem.
Lett., 2010, 21, 1053.
1 C. Mukhopadhyay and P. K. Tapaswi, Catal. Commun.,
Chem., 2012, 22, 5171.
35 G. Y. Bai, X. W. Lan, G. F. Chen, X. F. Liu, T. Y. Li and
L. J. Shi, Ultrason. Sonochem., 2014, 21, 520.
36 L. X. Song, M. Wang, Z. Dang and F. Y. Du, J. Phys. Chem.
B, 2010, 114, 3404.
2
2
2
2
008, 9, 2392.
2 K. Bahrami, M. M. Khodaei and I. Kavianinia, Synthesis,
007, 0547.
37 Y. H. Deng, Y. Cai, Z. K. Sun, J. Liu, C. Liu, J. Wei, W. Li,
C. Liu, Y. Wang and D. Y. Zhao, J. Am. Chem. Soc., 2010,
132, 8466.
2
2
2
3 C. J. Zhu and Y. Y. Wei, ChemSusChem, 2011, 4, 1082.
4 K. Bahrami, M. M. Khodaei and F. Naali, J. Org. Chem.,
38 X. J. Du, D. S. Zhang, R. H. Gao, L. Huang, L. Y. Shi and
J. P. Zhang, Chem. Commun., 2013, 49, 6770.
39 G. Ilangovan and K. C. Pillai, Langmuir, 1997, 13, 566.
2
008, 73, 6835.
2
2
2
5 (a) M. Misono, Catal. Today, 2005, 100, 95; (b) M. B. Iraj, 40 (a) K. J. Yu, X. B. Zhang, H. W. Tong, X. Y. Yan and
M. Majid, T. Shahram, M. Valiollah and S. F. Hojati, Catal.
Commun., 2008, 9, 1153.
6 (a) T. D. Phan, M. A. Kinch, J. E. Barker and T. Ren, Tetra-
S. M. Liu, Mater. Lett., 2013, 106, 151; (b) X. Q. Liu,
Z. Y. Ma, J. M. Xing and H. Z. Liu, J. Magn. Magn. Mater.,
2004, 270, 1.
hedron Lett., 2005, 46, 397; (b) J. Zhang, Y. Tang, G. Y. Li 41 H. Sharghi, M. Aberi and M. M. Doroodmand, Adv. Synth.
and C. W. Hu, Appl. Catal., A, 2005, 278, 251. Catal., 2008, 350, 2380.
7 (a) J. Kaur, K. Griffin, B. Harrison and I. V. Kozhevnikov, 42 G. Y. Bai, L. J. Shi, Z. Zhao, Y. L. Wang, M. D. Qiu and
J. Catal., 2002, 208, 448; (b) G. D. Yadav, N. S. Asthana and H. X. Dong, Mater. Lett., 2013, 96, 93.
3168 | Green Chem., 2014, 16, 3160–3168
This journal is © The Royal Society of Chemistry 2014