Ionic liquid-immobilized hybrid nanomaterial: an efficient catalyst in the synthesis of benzimidazoles and benzothiazoles via anomeric-based oxidation

Article abstract of DOI:10.1007/s13738-020-02055-1

Abstract: In this study, a novel ionic liquid immobilized on silica-coated cobalt-ferrite magnetic nanoparticles. This novel hybrid nanostructure (CoFe₂O₄@SiO₂@PAF-IL) was characterized by various microscopic and spectroscopic techniques including Fourier transformation infrared spectroscopy (FT–IR), X-ray powder diffraction (XRD), field emission scanning electron microscopy (SEM), the electron-dispersive X-ray spectroscopy (EDS), vibrating sample magnetometer (VSM), and thermogravimetric analysis (TGA/DTG). The catalytic activity of prepared nanomaterial was considered in the synthesis of the benzothiazole and benzimidazole derivatives. This method has several advantages such as good to excellent yields, short reaction times, solvent-free and environmentally-benign conditions, and simple work-up. Besides, nanocatalyst can be easily separated from the reaction mixture with the external magnetic field and reused several times without any loss of its catalytic activity. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-020-02055-1

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-02055-1
ORIGINAL PAPER
Ionic liquid‑immobilized hybrid nanomaterial: an efcient
catalyst in the synthesis of benzimidazoles and benzothiazoles
via anomeric‑based oxidation
Mohammad Ali Bodaghifard1,2 · Saeideh Shaf1
Received: 8 April 2020 / Accepted: 1 September 2020
© Iranian Chemical Society 2020
Abstract
In this study, a novel ionic liquid immobilized on silica-coated cobalt-ferrite magnetic nanoparticles. This novel hybrid
nanostructure (CoFe2O4@SiO2@PAF-IL) was characterized by various microscopic and spectroscopic techniques includ-
ing Fourier transformation infrared spectroscopy (FT–IR), X-ray powder difraction (XRD), feld emission scanning elec-
tron microscopy (SEM), the electron-dispersive X-ray spectroscopy (EDS), vibrating sample magnetometer (VSM), and
thermogravimetric analysis (TGA/DTG). The catalytic activity of prepared nanomaterial was considered in the synthesis
of the benzothiazole and benzimidazole derivatives. This method has several advantages such as good to excellent yields,
short reaction times, solvent-free and environmentally-benign conditions, and simple work-up. Besides, nanocatalyst can
be easily separated from the reaction mixture with the external magnetic feld and reused several times without any loss of
its catalytic activity.
Graphic abstract
Keywords Benzimidazoles · Benzothiazoles · Heterogeneous catalysis · Hybrid nanostructure · Magnetic nanoparticles
Introduction
* Mohammad Ali Bodaghifard
m-bodaghifard@araku.ac.ir
From the initial use of ionic liquids (ILs), which are made up
1
2
of cationic and anionic components, they have found appli-
cations in various felds such as environmentally friendly
solvents and catalysis because they have the specifc features
such as undetectable vapor pressure, good thermal stability,
Department of Chemistry, Faculty of Science, Arak
University, 38156-88138 Arak, Iran
Institute of Nanosciences and Nanotechnology, Arak
University, 38156-88138 Arak, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
and high solubility [1–5]. Ionic liquids have successfully
been used in various catalytic areas afording high catalytic
performance [2–5]. Unfortunately, the difculties in the
recovery and unendurable viscosity of ILs have restrained
their wide applications in catalytic reactions [2–5]. So,
the heterogenization of ILs on appropriate porous carriers
or suitable magnetic nanoparticles would be a viable and
appealing approach to fabricate the efcient solid catalyst
with superior activity and stability [6–9]. Furthermore, the
heterogenized ILs on magnetic nanoparticles have found
wonderful applications such as adsorbent for magnetic solid-
phase extraction (SPE) of sulfonylurea herbicides, adsorbent
for magnetic dispersive SPE of Cephalosporins, and removal
of anionic dye from water [10–12].
anticonvulsant activity [33]. Therefore, these compounds
are attractive targets for synthesis. There are diferent
methods for the synthesis of benzimidazoles and benzo-
thiazoles that involve multistep synthesis, use of toxic
reagents and solvent, long reaction times, and low yields.
Therefore, several modifed processes have been reported
in the literature using diferent catalysts such as Fe(III)-
Schif base/SBA-15 [34], [Dsim]Cl/FeCl3 [35], hybrid
crystal NH3(CH2)4NH3SiF6 [36], NiFe2–xEuxO4 nano-
structures [37], Co-doped NiFe2O4 [38], 1,4-Diazabicyclo
[2.2.2] octanium diacetate [39], and NH3(CH2)5NH3BiCl5
[40].
As part of our continuous efort to develop efcient het-
erogeneous magnetic nanocatalysts and green organic reac-
tions [3a, 41–48], herein we have reported the preparation
of a novel heterogeneous ionic liquid-supported hybrid
nanostructure (CoFe2O4@SiO2@PAF-IL) as an impressive
catalyst for the synthesis of benzimidazole, benzothiazole
derivatives (Scheme 2). The presented method has consider-
able efciency of the catalyst and environmental compatibil-
ity. Also, the catalyst was inexpensive and highly efcient,
and it could easily be recovered and reused.
The magnetic nanoparticles (MNPs) not only have high
surface areas but also they can be easily separated by using
an external magnetic feld and do not have tedious separation
procedures which resulted from the small sizes of nanopar-
ticles and have other advantages including easy synthesis,
easy surface modifcation and functionalization, low tox-
icity, and low cost [13–15]. Magnetic nanoparticles have
many important applications such as drug delivery [16, 17],
food industry [18], agriculture [19], textile industry [20],
water treatment [21], and catalysis [6–9]. However, mag-
netic nanoparticles can easily aggregate into larger clusters
because of their anisotropic dipolar attraction. Besides, they
have other defciencies such as leaching under acidic condi-
tions and being susceptible to auto-oxidation and toxicity.
Therefore, it is necessary to protect the surface of MNPs to
reduce these undesirable features. For this purpose, MNPs
are usually coated with a polymeric or inorganic matrix
[6–9]. Among inorganic compounds, SiO2 can be a suit-
able candidate for protecting the surface of MNPs due to its
high chemical and thermal stability and most importantly
its easy modifcation by a wide range of functional groups,
which increase the chemical and colloidal stability of these
compounds [6–9]. The cobalt-ferrite nanoparticles (CoFe2O4
NPs) have appealed much attention due to their high chemi-
cal stability, thermal and corrosive stability, and moderate
saturation magnetization [22–24, 38].
Experimental
All reagents were purchased from reputable chemical
companies (Merck, Across, Fluka) and were used with-
out further purifcation. Melting points were measured by
using electrothermal digital apparatus and are uncorrected.
Obtained products were identifed with a comparison of
their melting points and spectral data reported in the lit-
erature. The FT–IR spectra were recorded by Unicom Gal-
axy Series. The 1H, 13CNMR spectra for obtained products
were reported on Brucker DRX-300 spectrometer operating
at 300 and 75 MHz, respectively, in DMSO-d6 with TMS
as an internal standard. The crystal structure was carried
out by Philips Xpert X-ray powder difraction (XRD) dif-
fractometer (Cu-Kα radiation and λ=0.15406) in the range
of Bragg angle 10–80 using 0.05° as the step length. The
thermal stability of CoFe2O4@SiO2@PAF-IL was investi-
gated by a thermogravimetric analyzer with a model Met-
tler TA4000 System under an N2 atmosphere at a heating
rate of 10 °C min−1. The surface morphology and elemental
content of CoFe2O4@SiO2@PAF-IL were investigated on a
Hitachi S-4160. The elemental chemical analysis [energy-
dispersive X-ray spectroscopy (EDS)] coupled with SEM
was reported for the characterization of chemical elements
of the CoFe2O4@SiO2@PAF-IL catalyst. The magnetization
and hysteresis loop for the synthesized magnetic nanoparti-
cles were measured at room temperature using a 7300 VSM
system with a maximum feld of 10 kOe.
Green or sustainable chemistry is the designing of chemi-
cal processes with the reduction or elimination of harmful
substances. Each green reaction should have three compo-
nents: solvent, reagent or catalyst, and energy consumption.
In recent years, a wide spectrum of the organic syntheses is
designed to reduce byproduct or waste generation, reduce
the use of hazardous reagents and solvents, and improve
energy efciency [25, 26].
Benzimidazoles and benzothiazoles are bicyclic het-
erocyclic compounds containing imidazole and thiazole
scafold with great properties [27]. They have biological
activity such as anti-infammatory [28], antimicrobial [29]
anticancer [30], antiviral [31], antihypertensive [32], and
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Journal of the Iranian Chemical Society
Preparation of the cobalt ferrite nanoparticles
(CoFe2O4)
Stabilization of ionic liquid on magnetic
nanoparticles (CoFe2O4@SiO2@PAF‑IL)
Magnetic cobalt ferrite nanoparticles were prepared by the
co-preparation method according to the procedure reported
in the literature [49]. Firstly, COCl2∙6H2O (5 mmol, 1.8 g),
FeCl3∙6H2O (10 mmol, 2.7 g) were added to 50 mL of
deionized water in a round bottom fask (100 mL), and the
mixture was stirred at room temperature for 10 min. The pH
was adjusted to 11–12 using NaOH (0.3 M) solution. The
reaction mixture was stirred at 80 °C for 1 h. After cooling
to room temperature, the precipitate was separated of using
a magnet and washed with deionized water several times and
fnally washed with ethanol and dried in an oven at 80 °C.
To prepare the CoFe2O4@SiO2@PAF-IL hybrid nanostruc-
ture, CoFe2O4@SiO2@PAF (1 g) was mixed with K2CO3
(0.5 g) and MeI (2 mL) in DMF (25 mL) under sonication
for 10 min. The resulted mixture was stirred at 100 °C for
24 h. Finally, the precipitate was separated using a magnet
and washed with deionized water several times and dried in
an oven at 80 °C.
General procedure for the synthesis
of benzimidazole and benzothiazole derivatives
CoFe2O4@SiO2@PAF-IL (20 mg) was added to the mix-
ture of aromatic aldehyde (1 mmol), o-phenylenediamine
(1 mmol, 0.11 g), or 2-aminobenzenethiol (1 mmol, 0.125 g)
and heated in solvent-free conditions at 70 °C for an appro-
priate time. After completion, the resulting mixture was
diluted with hot ethanol (10 mL), and the catalyst was eas-
ily separated using an external magnet. Finally, the gained
precipitate was recrystallized in ethanol to aford the pure
product.
Synthesis of silica‑coated MNPs (CoFe2O4@SiO2)
The silica shell was coated onto CoFe2O4 nanoparticles via
hydrolysis of TEOS in basic solution [45]. Magnetic cobalt
ferrite nanoparticles (1 g), ethanol (40 mL), deionized water
(6 mL), and ammonia 25% (1.5 mL) were introduced in a
100-mL fask and sonicated for 20 min. Then, Tetraethyl-
orthosilicate (TEOS) (1.4 mL) was added to the mixture.
The reaction mixture was stirred at room temperature for
12 h. Finally, the precipitate was separated by a magnet and
washed with deionized water several times and ethanol and
dried in an oven at 80 °C.
Selected spectroscopic data
2-(p-tolyl)-1H-benzo[d]imidazole (4b): IR (KBr, νmax):
3342, 3257, 3035, 2948, 2871, 1678, 1652, 1580, 1523,
1
1335, 832 cm−1. H NMR (300 MHz, DMSO-d6): 12.25
(1H, s, NH), 7.15–8.05 (8H, m, HAr), 2.19 (3H, s, CH3) ppm.
Anal. Calcd. for C14H12N2: C, 80.74; H, 5.81; N, 13.45.
Found: C, 80.67; H, 5.79; N, 13.59.
Synthesis of 3‑aminopropyl‑functionalized
magnetic nanoparticles (CoFe2O4@SiO2‑PrNH2)
4-(1H-benzo[d]imidazol-2-yl)-2-methoxyphenol (4k):
IR (KBr, νmax): 3338, 3059, 2935, 2841, 1602, 1578, 1506,
1273, 1031 cm−1. 1H NMR (300 MHz, DMSO-d6): δ=12.45
(1H, s, NH), 9.35 (1H, s, OH), 6.17–7.55 (7H, m, HAr), 3.44
(3H, s, CH3) ppm. Anal. Calcd. for C14H12N2O2: C, 69.99;
H, 5.03; N, 11.66. Found: C, 67.11; H, 5.09; N, 11.76.
4-(1H-benzo[d]imidazol-2-yl)-N,N-dimethylaniline (4l):
IR (KBr, νmax): 3338, 3020, 2918, 2800, 1608, 1500, 1470,
CoFe2O4@SiO2-PrNH2 nanoparticles were synthesized
using a reported procedure [47]. CoFe2O4@SiO2 nanopar-
ticles (1 g) were dispersed in 50 ml toluene for 10 min under
sonication. Then, (3-aminopropyl)triethoxysilane (APTES)
(2 g) was added and refuxed for 24 h at 110 °C. Finally, the
precipitate was separated using a magnet and washed with
toluene several times and dried in an oven at 80 °C.
1
1250, 837, 746 cm−1. H NMR (300 MHz, DMSO-d6):
δ=6.35–7.79 (8H, m, HAr), 3.16 (6H, s, 2CH3) ppm. Anal.
Calcd. for C15H15N3: C, 75.92; H, 6.37; N, 17.71. Found: C,
75.81; H, 6.45; N, 17.83.
Preparation of poly(aniline‑formaldehyde)
silica‑coated MNPs (CoFe2O4@SiO2@PAF)
2-(4-methoxyphenyl)benzo[d]thiazole (5c): IR (KBr,
νmax): 3380, 3057, 2995, 1602, 1578, 1500, 1257, 1028 cm−1.
1H NMR (300 MHz, DMSO-d6): δ = 6.91–7.92 (8H, m,
HAr), 3.66 (3H, s, CH3) ppm. Anal. Calcd. for C14H11NOS:
C, 69.68; H, 4.59; N, 5.80; S, 13.29. Found: C, 69.51; H,
4.67; N, 5.69; S, 13.41.
To synthesize the CoFe2O4@SiO2@PAF nanocompos-
ite, a mixture of aniline (0.5 g), formaldehyde (1 g), and
CoFe2O4@SiO2-PrNH2 (1 g) in 25 mL DMF at 100 ml
round-bottomed fask was stirred under sonication. Then,
the mixture was stirred with a magnetic stirrer for 48 h at
100 °C. After the reaction was completed, the precipitate
was washed several times with hot ethanol and dried in an
oven at 80 °C.
4-(benzo[d]thiazol-2-yl)-N,N-dimethylaniline (5i): IR
(KBr, νmax): 3382, 3053, 2963, 2899, 1608, 1500, 1367,
1
1188, 817, 752 cm−1. H NMR (300 MHz, DMSO-d6):
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Journal of the Iranian Chemical Society
δ=6.62–7.85 (8H, m, HAr), 3.14 (6H, s, 2CH3) ppm. Anal.
Calcd. for C15H14N2S: C, 70.83; H, 5.55; N, 11.01; S, 12.60.
Found: C, 70.69; H, 5.63; N, 10.97; S, 12.73.
(FE-SEM), energy-dispersive X-ray spectroscopy (EDS),
thermogravimetric analysis (TGA), and vibrating sample
magnetometry (VSM) were employed to identify and char-
acterize the prepared hybrid nanostructure.
The FT–IR spectra of CoFe2O4 (a), CoFe2O4@SiO2
(b), CoFe2O4@SiO2@Pr-NH2 (c), CoFe2O4@SiO2@PAF
(d) and CoFe2O4@SiO2@PAF-IL (e) are shown in Fig. 1.
The FT–IR spectrum of magnetic CoFe2O4 nanoparticles
shows the characteristic Fe–O and Co–O absorption bands
around 590 and 390 cm−1 (Fig. 1a) [45]. Also, a broadband
around 3450 cm−1 refers to the stretching vibration vibra-
tions of H–O–H groups on the surface of nanoparticles.
CoFe2O4@SiO2 shows characteristic FT–IR bands at around
1089 cm−1, 947 cm−1, 860 cm−1, and 457 cm−1 which are
attributed to the asymmetric stretching, symmetric stretch-
ing, in-plane bending, and rocking mode of the Si–O–Si
group, respectively, that confrms the formation of SiO2 shell
(Fig. 1b) [48]. The absorption band at 1640 cm−1 is referred
to stretching vibration mode of Si–OH groups. The weak
absorptions at 2920 and 2935 cm−1 are attributed to symmet-
ric and asymmetric stretching modes of the attached alkyl
groups (Fig. 1c–e) [47]. Thus, the results confrm that the
functional groups were successfully grafted on the surface
of CoFe2O4 nanoparticles.
Result and discussion
Preparation and characterization of the hybrid
nanostructure
The magnetic hybrid nanoparticles were prepared sequen-
tially, as illustrated in scheme 1. Firstly, CoFe2O4 was pre-
pared easily by the co-preparation method from the chemical
reaction of Co2 + and Fe3 + ions in the base conditions [49].
Then, CoFe2O4 nanoparticles were coated with a silica layer
using the stöber method [45]. The aminopropyl-modifed sil-
ica-coated CoFe2O4 nanoparticles were prepared by the reac-
tion of CoFe2O4@SiO2 with (3-aminopropyl)triethoxisilane
(APTES) which can bind covalently to the free-OH groups
at the surface of the particles and aforded the CoFe2O4@
SiO2-PrNH2. The reaction of formaldehyde and aniline with
CoFe2O4@SiO2-PrNH2 nanoparticles resulted in the con-
struction of CoFe2O4@SiO2@PAF nanocomposite [50].
Finally, the hybrid nanomaterial CoFe2O4@SiO2@PAF-
IL was fabricated by the reaction of excess methyl iodide
with free amine groups on the surface of CoFe2O4@SiO2@
PAF nanocomposite. The resulting precipitate was washed
with ethanol and dried in the vacuum oven to gain the fnal
hybrid nanomaterial (CoFe2O4@SiO2@PAF-IL). The Fou-
rier transform infrared (FT–IR) spectroscopy, X-ray difrac-
tion (XRD), feld emission scanning electron microscopy
The X-ray powder difraction (XRD) patterns were used
to study the crystallinity of prepared nanostructure. The
XRD patterns of CoFe2O4 (a) and CoFe2O4@SiO2@PAF-
IL (b) show characteristic peaks at 74.5, 62.8, 57.2, 53.9,
43.2, 35.6, and 30°, which can be attributed to the 533, 440,
511, 422, 400, 311, and 220 planes of CoFe2O4, respectively
(Fig. 2a). These data are according to the standard CoFe2O4
Scheme 1 Schematic route for the synthesis of CoFe2O4@SiO2@PAF-IL hybrid nanostructure
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Journal of the Iranian Chemical Society
Fig. 1 The FT–IR spectra of
CoFe2O4 (a), CoFe2O4@SiO2
(b), CoFe2O4@SiO2@Pr-NH2
(c), CoFe2O4@SiO2@PAF (d),
and CoFe2O4@SiO2@PAF-IL
(e)
Fig. 2 XRD patterns of
CoFe2O4 (a) and CoFe2O4@
SiO2@PAF-IL (b)
sample (standard JCPDS no. 22-1086) and confrm a cubic
spinel structure for CoFe2O4 nanoparticles. The broad peak
at 2θ = 23–28° can be attributed to the amorphous SiO2
coated on the surface of CoFe2O4 core in CoFe2O4@SiO2@
PAF-IL nanostructure (Fig. 2b) [48]. The average crystallite
sizes estimated using Scherrer’s equation (D=0.9 λ/βcosθ).
The crystallite size of CoFe2O4@SiO2@PAF-IL calculated
from the width of the peak at 2θ = 35.6° (311) is 20 nm,
which is in the range determined using FE-SEM analysis
(Fig. 4a).
The thermal stability of CoFe2O4@SiO2@PAF-IL
was determined by thermal analysis of TGA and DTG at
50–950 °C range (Fig. 3). The magnetic nanomaterial shows
three weight loss steps, and the total weight loss of MNPs
in three steps was estimated at 17 wt%. The frst stage,
including a low amount (2.5%) of weight loss at T<200 °C,
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Journal of the Iranian Chemical Society
Fig. 3 TGA and DTG of
CoFe2O4@SiO2@PAF-IL
Fig. 4 FE-SEM image (a), and
EDX analysis (b) of CoFe2O4@
SiO2@PAF-IL
was due to the removal of the physically adsorbed solvent,
water, and surface hydroxyl groups. The second step at about
200 °C to nearly 580 °C is attributed to the decomposition
of the organic layer in the nanocomposite. Therefore, the
weight loss between 200 and 580 °C gives the organic moi-
ety ratios grafted on the prepared nanomaterial. The organic
moiety grafted on the CoFe2O4@SiO2@PAF-IL was approx-
imately 10% wt. The third step shows weight loss of 4.5%
at T>580 °C attributed to the destruction and deformation
of the silica layer.
The size and morphology of the CoFe2O4@SiO2@
PAF-IL particles were investigated by the feld emission
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Journal of the Iranian Chemical Society
scanning electron microscopy (Fig. 4a). The SEM image
shows CoFe2O4@SiO2@PAF-IL particles are nearly spheri-
cal shapes with a mean diameter of 25 nm. The electron-
dispersive X-ray spectrum (EDS) confrms the presence of
carbon (C), silicon (Si), nitrogen (N), oxygen (O), cobalt
(Co), iron (Fe), and iodine(I) elements in CoFe2O4@SiO2@
PAF-IL nanostructure (Fig. 4b). The higher intensity of
the Si peak compared to the Fe and Co peaks exhibits the
CoFe2O4 nanoparticles have been trapped by the SiO2 layer.
So, based on the results, the CoFe2O4@SiO2@PAF-IL nano-
composite has been fabricated successfully.
Synthesis of benzimidazoles and benzothiazoles
using CoFe2O4@SiO2@PAF‑IL as a catalyst
After the successful preparation and characterization of
CoFe2O4@SiO2@PAF-IL, its catalytic activity was evalu-
ated in the synthesis of biologically important benzimidazole
and benzothiazole derivatives. The reaction of benzaldehyde
and o-phenylenediamine was selected as model substrates,
and the reaction was carried out in various conditions to
obtain the optimized condition. (Scheme 2). The diferent
solvents, as well as solvent-free medium, temperatures, and
catalyst amounts, were examined on model reaction and the
results are shown in Table 1. Under the solvent-free con-
dition, the use of 20 mg of CoFe2O4@SiO2@PAF-IL as a
catalyst at 70 °C serves as the best condition concerning
the green nature and clean work-up procedure for this syn-
thesis (Table 1, entry 7). To defne the role of CoFe2O4@
SiO2@PAF-IL as a catalyst for the preparation of benzimi-
dazoles, the model reaction was performed under the same
conditions with CoFe2O4, CoFe2O4@SiO2, CoFe2O4@SiO2-
PrNH2, CoFe2O4@SiO2@PAF, and without any catalyst.
The magnetic properties for CoFe2O4 (a) and CoFe2O4@
SiO2@PAF-IL (b) nanoparticles are measured by vibrat-
ing sample magnetometer (VSM) in −12,000 to 12,000 Oe
range at room temperature (Fig. 5). The hysteresis loops
show superparamagnetic behavior in two samples, and no
hysteresis phenomenon was observed. The obtained satu-
ration magnetizations (Ms) for CoFe2O4 and CoFe2O4@
SiO2@PAF-IL were observed at 47 and 25 Oe, respectively.
These results show that functionalization led to a decrease
in the magnetic properties of the nanostructure.
Fig. 5 VSM analysis of
CoFe2O4 (a) and CoFe2O4@
SiO2@PAF-IL (b)
Scheme 2 The synthesis of 2-arylbenzimidazole and 2-arylbenzothiazole derivatives in the presence of CoFe2O4@SiO2@PAF-IL as a catalyst
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Table 1 The optimization of reaction conditions for synthesis of benzimidazoles and benzothiazoles
Entry
Amount of catalyst (mg)
Temperature (°C)
Solvent
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
CoFe2O4@SiO2@PAF-IL (10)
CoFe2O4@SiO2@PAF-IL (10)
CoFe2O4@SiO2@PAF-IL (10)
CoFe2O4@SiO2@PAF-IL (10)
CoFe2O4@SiO2@PAF-IL (5)
CoFe2O4@SiO2@PAF-IL (20)
CoFe2O4@SiO2@PAF-IL (20)
CoFe2O4@SiO2@PAF-IL (20)
CoFe2O4@SiO2@PAF-IL (20)
CoFe2O4 (20)
Refux
Refux
Refux
50 °C
50 °C
50 °C
70 °C
r.t
100 °C
70 °C
70 °C
70 °C
70 °C
EtOH
EtOH: H2O (1:1)
DMF
75
75
75
15
25
15
10
60
8
8
8
8
8
70
65
57
85
77
85
93
53
83
80
75
55
50
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
9
10
11
12
13
CoFe2O4@SiO2 (20)
CoFe2O4@SiO2-PrNH2 (20)
CoFe2O4@SiO2@PAF (20)
Concerning the reaction time and yield of the product, the
best results were achieved using Fe3O4@SiO2@PAF-IL as
the catalyst (Table 1, entry 7, 10–13).
the recycled catalyst could be reused six times without any
additional treatment or appreciable reduction in catalytic
activity. In Fig. 6B and 6C, the FT–IR spectra and XRD pat-
terns of the fresh (a) and reused (b) catalyst are compared,
respectively. The consistent structure and activity of recov-
ered and reused CoFe2O4@SiO2@PAF-IL catalyst confrm
its stability, recyclability, and performance for the synthesis
of desired heterocycles.
After optimization of the reaction condition, to determine
the generality and efcacy of the hybrid nanocatalyst, vari-
ous aldehydes carrying either electron-donating or electron-
withdrawing groups were reacted under the optimized reac-
tion condition (Scheme 2). It was found that the reactions for
all of the various substrates proceed efciently to produce
corresponding 2-arylbenzimidazole and 2-arylbenzothiazole
derivatives in good to excellent yields without the formation
of side products (Table 2). The aryl aldehydes with electron-
withdrawing substituents produced higher yields of desired
products compared to aryl aldehydes bearing electron-donat-
ing substituents (Table 2). The comparison of CoFe2O4@
SiO2@PAF-IL catalyst with some previously reported cata-
lysts for the synthesis of benzimidazole derivatives is shown
in Table 3. The presented data confrm that the CoFe2O4@
SiO2@PAF-IL is a suitable and efcient catalyst for the syn-
thesis of desired benzimidazole derivatives.
Based on the literature survey, two plausible mecha-
nisms for the synthesis of 2-arylbenzimidazole derivatives
in the presence of CoFe2O4@SiO2@PAF-IL are depicted
in Scheme 3. It is supposed that the reaction proceeds via
carbonyl group activation by the hybrid catalyst and subse-
quent condensation with benzene-1,2-diamine (1), leading
to the imine intermediate (I). In route A, the intermediate I
undergoes ring closure by nucleophilic attack of the second
amino group on the imine double bond to gain the intermedi-
ate II. In the last step, previous studies have proposed aero-
bic auto-oxidation and aromatization for the conversion of
intermediate (II) to its corresponding 2-aryl benzimidazoles
in the presence of a hybrid catalyst [51, 52]. In route B, the
intermediate (II) is produced via intramolecular cyclization
and proton exchange of imine intermediate (I). Finally, inter-
mediate (II) was transformed into 2-aryl benzimidazoles (3)
via feasible anomeric-based oxidation (ABO) [40, 53–56].
The major reason for ABO is the driving force of aromati-
zation which will be supported via stereoelectronic and/or
anomeric efect [54, 56]. As described, in the ABO process,
the C–H bond is so weakened via electron donation from
the nitrogen’s lone pairs into the C–H anti-bonding orbital
The recovery and reusability of the catalyst are very
important for commercial and industrial applications as
well as green process aspects. Thus, the recovery and reus-
ability of CoFe2O4@SiO2@PAF-IL was investigated in the
model reaction (Fig. 6). After completion of the reaction,
the resulting solidifed mixture was diluted with hot EtOH
(15 mL). Then, the catalyst was easily separated, washed
with hot EtOH, dried under vacuum, and reused in a subse-
quent reaction. Nearly, quantitative recovery of catalyst (up
to 96%) could be obtained from each run. As seen in Fig. 6,
1 3

Journal of the Iranian Chemical Society
Table 2 Synthesis of
benzimidazole and
Entry
Ar
1 or 2
Product
Yield (%)
M.p. (°C)
Found
benzothiazole derivatives using
CoFe2O4@SiO2@PAF-IL as a
catalysta
Reported [34–40]
1
2
3
4
5
5
7
8
C6H5
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
4a
4b
4c
4d
4e
4f
4g
4h
4i
93
90
89
87
80
87
94
93
88
90
87
87
90
85
83
90
90
91
83
83
290–293
269–271
226–227
273–275
274–275
223–224
>300
291–292
265–267
>300
274–277
105–107
84–85
120–121
269–271
230–232
253–255
140–142
221–223
284–286
290–292
275–276
224–226
271–273
274–276
221–224
318–320
288–290
264–265
–
275–278
113–115
88–89
120–121
275–278
231–233
261–262
144–146
225–227
284–286
4-CH3-C6H4
4-OCH3-C6H4
2-pyridyl
2-thiophine
4-F-C6H4
4-NO2-C6H4
4-Cl-C6H4
4-OH-C6H4
3-OCH3-4-OH-C6H3
4-(N(CH3)2)-C6H4
C6H5
4-CH3-C6H4
4-OCH3-C6H4
2-pyridyl
4-F-C6H4
4-NO2-C6H4
4-Cl-C6H4
9
10
11
12
13
14
15
16
17
18
19
20
4k
4l
5a
5b
5c
5d
5e
5f
5g
5h
5i
4-OH-C6H4
4-(N(CH3)2)-C6H4
aReaction condition: benzene-1,2-diamine (1, 1 mmol) or 2-aminobenzenethiol (2, 1 mmol), Aryl aldehyde
(3, 1 mmol), CoFe2O4@SiO2@PAF-IL (20 mg), solvent-free, 70 °C, 10 min
bIsolated yield
cThe melting points are not corrected
Table 3 The comparison of
CoFe2O4@SiO2@PAF-IL as
a catalyst in the synthesis of
benzimidazoles with some other
reported catalysts
Entry Catalyst
Condition
Time (min) Yield (%)
1
2
3
4
5
6
7
8
9
10
[Dsim]Cl/FeCl3
Co-doped NiFe2O4
NiEuFe2O4
NH3(CH2)5NH3BiCl5
MnFe2O4
Cu-complex-phen/bipy-MCM-41 60 °C, ultrasonic irradiation 90
[2,6-DMPy-NO2]C(NO2)3
H2O2, HCl
H2O2, CAN
EtOAc, 60 °C
13
55
50
6
93 [35]
97 [38]
84 [37]
90 [40]
Solvent-free, 70 °C
H2O, ultrasonic irradiation
Solvent-free, r.t.
CH3OH, r.t.
240
92 [51]
93 [42]
95 [54]
97 [57]
95 [58]
Solvent-free, r.t.
CH3CN, r.t.
Solvent-free, 50 °C
Solvent-free, 70 °C
15
35
35
10
CoFe2O4@SiO2@PAF-IL
93 [This work]
(σ*C–H) which can be abstracted via reaction at the surface
(CoFe2O4@SiO2@PAF-IL) was characterized successfully,
and its catalytic activity was evaluated in the synthesis of
2-arylbenzimidazole and 2-arylbenzothiazole derivatives.
The desired products were synthesized in high yields via
feasible anomeric-based oxidation within short reaction
times. This protocol has some advantages in terms of sol-
vent-free condition, high yield and purity, easy work-up and
eco-friendly process, simple separation of catalyst as well as
the recyclability of it without a notable decrease in catalytic
performance.
of the catalyst, to aford molecular hydrogen [54].
Conclusion
In this work, a layer of poly aniline-formaldehyde was fab-
ricated on magnetic nanoparticles surfaces and function-
alized by ionic liquid tags successfully. The novel ionic
liquid-supported inorganic-polymer hybrid nanostructure
1 3

Journal of the Iranian Chemical Society
Fig. 6 a The recyclability of hybrid nanocatalyst in the model reaction during six runs, b FT–IR spectrum, and c XRD patterns for fresh and
reused catalyst
Scheme 3 The proposed paths
for the synthesis of 2-arylben-
zimidazole derivatives using
CoFe2O4@SiO2@PAF-IL as a
hybrid catalyst
1 3

Journal of the Iranian Chemical Society
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Acknowledgments The authors acknowledge the fnancial support
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