Synthesis and characterization of Cu(II) containing nanosilica triazine dendrimer: A recyclable nanocomposite material for the synthesis of benzimidazoles, benzothiazoles, bis-benzimidazoles and bis-benzothiazoles

Article abstract of DOI:10.1016/j.molcata.2013.08.009

The present study describes the synthesis, characterization, and catalytic activity of Cu(II) containing nanosilica triazine dendrimer (Cu(II)-TD@nSiO ₂). The prepared catalyst was characterized by FT-IR, TGA, elemental analysis, UV-vis, FE-SEM

Full text of DOI:10.1016/j.molcata.2013.08.009

Accepted Manuscript
Title: Synthesis and characterization of Cu(II) containing
nanosilica triazine dendrimer: A recyclable nanocomposite
material for the synthesis of benzimidazoles, benzothiazoles,
bis-benzimidazoles and bis-benzothiazoles
Author: Mahboobeh Nasr-Esfahani Iraj
Mohammadpoor-Baltork Ahmad Reza Khosropour Majid
Moghadam Valliolah Mirkhani Shahram Tangestaninejad
PII:
S1381-1169(13)00292-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.08.009
MOLCAA 8830
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
2-1-2013
3-8-2013
8-8-2013
Please cite this article as: M. Nasr-Esfahani, I. Mohammadpoor-Baltork, A.R.
Khosropour, V. Mirkhani, S. Tangestaninejad, Synthesis and characterization of
Cu(II) containing nanosilica triazine dendrimer: A recyclable nanocomposite
material for the synthesis of benzimidazoles, benzothiazoles, bis-benzimidazoles
and bis-benzothiazoles, Journal of Molecular Catalysis A: Chemical (2013),
http://dx.doi.org/10.1016/j.molcata.2013.08.009
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*Graphical Abstract (for review)
Mahboobeh Nasr-Esfahani, Iraj Mohammadpoor-Baltork,* Ahmad Reza Khosropour,* Majid Moghadam,
Valliolah Mirkhani, Shahram Tangestaninejad
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
Synthesis and characterization of Cu(II) containing nanosilica triazine dendrimers: A recyclable nanocomposite
material for the synthesis of benzimidazoles, benzothiazoles, bis-benzimidazoles and bis-benzothiazoles
Page 1 of 64

*Research Highlights
Highlights
A new Cu(II) containing nanosilica triazine dendrimer was prepared and characterized ► Cu(II)-
TD@nSiO was applied for the synthesis of benzimidazoles and benzothiazoles ► Also mixed
2
benzimidazole-benzothiazole compounds was prepared using Cu(II)-TD@nSiO ► The catalyst
2
was reused several times without significant loss of its activity.
Page 2 of 64

*Manuscript
Synthesis and characterization of Cu(II) containing nanosilica triazine
dendrimer: A recyclable nanocomposite material for the synthesis of
benzimidazoles, benzothiazoles, bis-benzimidazoles and bis-
benzothiazoles


Mahboobeh Nasr-Esfahani, Iraj Mohammadpoor-Baltork , Ahmad Reza Khosropour , Majid
Moghadam, Valliolah Mirkhani, Shahram Tangestaninejad
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
Abstract: The present study describes the synthesis, characterization, and catalytic activity of Cu(II)
containing nanosilica triazine dendrimer (Cu(II)-TD@nSiO ). The prepared catalyst was characterized
2
by FT-IR, TGA, elemental analysis, UV-vis, FE-SEM, TEM, XPS and ICP-OES techniques. The
Cu(II)-TD@nSiO has been used as an efficient catalyst for the preparation of various benzimidazoles,
2
benzothiazoles, symmetrical bis-benzimidazoles and bis-benzothiazoles under mild conditions. For the
first time, we report the synthesis of an unsymmetrical bis-benzimidazole and also a compound
containing both benzimidazole and benzothiazole moieties, indicating the efficiency of this catalytic
system. The present method offers advantages of excellent yields, short reaction times and simple
work-up. Also, the catalyst can be easily recycled and reused several times, which makes this method
attractive, economic and environmentally-benign.
Keywords: Cu(II) supported catalyst; Triazine dendrimer; Benzimidazoles; Benzothiazoles.
1
. Introduction
Dendrimers are a unique class of polymers with tree-like molecular structures, internal cavities and
chemical functionalities which make them useful building blocks and carriers for molecules and ions at
nanoscales [1-4]. The encapsulation of transition metals into dendrimers has become an exciting
*
Corresponding authors: Tel.: +98-311-7932705; fax: +98-311-6689732. E-mail addresses: imbaltork@sci.ui.ac.ir (I.
Mohammadpoor-Baltork), khosropour@chem.ui.ac.ir (A. R. Khosropour).
1
Page 3 of 64

approach in the field of catalysis and nanoscience [5,6]. In addition, the metal ions are stabilized by
encapsulation within the dendrimers, and therefore, do not agglomerate [7,8]. Moreover, dendrimer-
encapsulated metallic particles (DEMs), due to more activation of the surface of metallic particles, can
exhibit better catalytic activities compared to the other types of ligand-stabilized particles [9-11].
Although the synthesis and catalytic activities of various DEMs have been successfully demonstrated,
their separation from the reaction mixture and recycling are tedious and very costly which limit their
use for practical applications. To overcome these difficulties, heterogenizing DEMs on solid supports
has received increased attention. By immobilizing homogeneous catalysts on insoluble supports, the
advantages of both homogeneous as well as heterogeneous catalysts are achieved, so that they can be
separated easily from the desired products by a simple filtration. During the past years, several
dendrimers covalently-bonded to the inorganic or polymeric supports have been efficiently prepared
[
12-16].
The use of transition metal complexes in the field of catalysis has rapidly increased in recent years
because of their ability to catalyze a range of chemical reactions [17]. Among them, Cu(II) complexes
have long been found as effective catalysts in a variety of organic synthesis [18]. Due to the high
activity and selectivity, these complexes have also found numerous industrial applications in
dehydrogenation of alcohols [19], methanol synthesis [20] and production of hydrogen in fuel cells
[
21]. Despite of much anticipation, however, Cu(II)-dendrimer complexes have been only investigated
for structure studies [22], water treatment [23] and preparation of dendrimer-encapsulated Cu(II)
nanoparticles [24], and a few reports are available for their catalytic applications [25-27].
Recently, the synthesis and characterization of dendrimers containing 1,3,5-triazine units have received
considerable attention due to their ease of synthesis, as well as low cost and chemoselective reactivity
of the starting material, 2,4,6-trichlorotriazine [28-31]. The stepwise substitution of the chlorine atoms
in 2,4,6-trichlorotriazine allows the preparation of unsymmetrically substituted dendrimers for
2
Page 4 of 64

different catalytic applications. Due to the importance of these compounds, several triazine-based
dendrimers on a variety of supports have been synthesized during the past decades [16,32-37] .
Benzimidazoles and benzothiazoles are of great significance because of their wide spectrum of useful
physiological and biological properties [38]. These compounds have been also used as valuable
intermediates in various organic syntheses [39]. In addition, the bis-derivatives of these compounds act
as ligands for transition metals in modeling biological systems [40,41]. Accordingly, many synthetic
methods have been devised for the preparation of these important heterocyclic compounds. Generally,
benzimidazoles and benzothiazoles are prepared via condensation of 1,2-phenylenediamines and 2-
aminothiophenol with carboxylic acids or their derivatives in the presence of strong Brønsted and
Lewis acids [42-44]. The oxidative cyclization of 1,2-phenylenediamines and 2-aminothiophenol with
aldehydes have been also reported for the synthesis of these heterocycles [45-55]. Although some of
the reported methods are quite satisfactory, from the synthetic efficiency point of view, many of these
methods suffer from disadvantages such as long reaction times, low yields, harsh reaction conditions,
tedious experimental procedures and formation of side products. Another disadvantage of some of the
existing methods is that the catalysts are destroyed during the work-up procedure and cannot be
recovered and reused. Consequently, the introduction of efficient and green approaches using
recoverable and reusable catalysts for the synthesis of these compounds is highly desirable.
As a part of our ongoing research on the development of useful synthetic protocols [56,57], herein
we report an efficient and environmentally-friendly method for the synthesis of benzimidazole,
benzothiazole, bis-benzimidazole, bis-benzothiazole and mixed benzimidazole-benzothiazole
compounds using Cu(II)-TD@nSiO as a recoverable and reusable catalyst under mild conditions
2
(
Scheme 1).
3
Page 5 of 64

O
NH2
ZH
Cu(II)-TD@nSiO2
rt or 50 °C, air
N
Z
Ar
H
Ar
R
R
R
R
O
Z
NH2
H Cu(II)-TD@nSiO2
N
50 °C, air
N
ZH
H
Z = NH, S
Z
O
R
Scheme 1. Synthesis of benzimidazoles, benzothiazoles, bis-benzimidazoles and bis-benzothiazoles
catalyzed by Cu(II)-TD@nSiO2.
2
. Experimental
Melting points were determined using a Stuart Scientific SMP2 apparatus. FT-IR spectra were
-1
1
13
recorded on a Nicolet-Impact 400D instrument in the range of 400-4000 cm . H and C NMR (400
and 100 MHz) spectra were recorded in a CDCl or DMSO-d solution on a Bruker-Avance 400
3
6
spectrometer. Mass spectra were recorded on a Platform II spectrometer from Micromass; EI mode at
0 eV. Elemental analysis was done on LECO, CHNS-932 analyzer. Thermogravimetric analysis
TGA) was carried out on a Mettler TG50 instrument under air flow at a uniform heating rate of 5
7
(
o
-1
o
Cmin in the range of 30-600 C. The UV-vis diffuse reflectance spectra of the samples were recorded
in a JASCO V-670 spectrophotometer. The X-ray photoelectron spectroscopy (XPS) measurements
were performed using a Gammadata-scienta ESCA200 hemispherical analyzer equipped with an Al
(Kα = 1486.6 eV) X-ray source. The Cu content of the catalyst was measured by an inductively
coupled plasma optical emission spectrometry (ICP-OES), using a Jarrell-Ash 1100 ICP analyzer.
Scanning electron microscopy measurements were performed on a Hitachi S-4700 field emission-
scanning electron microscope (FE-SEM). Transmission electron microscopy (TEM) measurements
were carried out on a Philips CM10 analyzer operating at 100 kV.
2
.1. Preparation of aminopropylated nano-silica (AP-nSiO2)
4
Page 6 of 64

For the preparation of AP-nSiO , the nano-silica was first activated according to literature procedure
2
[
58]. Then, a mixture of activated nano-silica (3 g) and 8 mL 3-aminopropyltrimethoxysilane (APTS)
in 50 mL anhydrous toluene was stirred under reflux conditions for 8 h. The reaction mixture was
filtered and the solid material was extracted with toluene using a Soxhlet apparatus and dried in a
o
vacuum oven at 110 C [59].
2
.2. Preparation of triazine-based dendrimer supported on nano-silica (G2 or TD@nSiO2)
A mixture of AP-nSiO (2 g, 0.99 mmol/g), cyanuric chloride (CC) (1.85 g, 10 mmol) and
2
diisopropylethylamine (DIPEA) (10 mmol, 1.7 mL) in THF (10 mL) was shaken overnight at room
temperature. The solid material (CC1-nSiO ) was separated by filtration, washed with hot THF in a
2
o
continuous extraction apparatus (Soxhlet) for 12 h and dried in a vacuum oven at 50 C [57]. Then,
To a slurry of CC1-nSiO (1 g) in DMF (12 mL) was added bis(3-aminopropyl)amine (8.11 mmol,
2
o
1
mL) and DIPEA (8.11 mmol, 1.4 mL). The reaction mixture was stirred at 80 C for 16 h. The
resulting first generation (G1) was filtered, washed with hot ethanol in Soxhlet for 12 h and then
o
dried in a vacuum oven at 50 C.
The second generation TD@nSiO (G2) was prepared following the above procedure for the
2
preparation of G1. A mixture of G1 (1 g, 0.45 mmol), cyanuric chloride (CC) (1.66 g, 9 mmol) and
DIPEA (9 mmol, 1.56 mL) in THF (20 mL) was shaken at room temperature for 16 h. The reaction
mixture was filtered and the resulting solid material (CC2-nSiO ) was washed with hot THF in
2
o
Soxhlet for 16 h and dried in a vacuum oven at 50 C. Then, to a slurry of CC2-nSiO (1 g, 0.36
2
mmol) in DMF (20 mL) was added bis(3-aminopropyl)amine (9.36 mmol, 1.14 mL) and DIPEA
o
(
9.36 mmol, 1.61 mL). The reaction mixture was stirred at 80 C for 16 h and then filtered. The
resulting second generation of nano-silica supported dendrimer (G2 or TD@nSiO ) was washed
2
o
with hot ethanol in Soxhlet for 24 h and dried in a vacuum oven at 50 C.
5
Page 7 of 64

2
.3. Preparation of Cu(II) containing nanosilica triazine dendrimer (Cu(II)-TD@nSiO2)
To a mixture of nano-silica supported dendrimer (G2) (0.5 g) in DMF (10 mL) was added
CuCl ·2H O (0.4 mmol). The mixture was stirred at room temperature for 10 h. The reaction
2
2
mixture was filtered; the solid material was washed sequentially with 25 mL of acetone and then
dried in a vacuum oven at 40 °C. Finally, Cu(II)-TD@nSiO catalyst was obtained as a blue-green
2
solid.
2
.4. General procedure for synthesis of benzimidazoles in the presence of Cu(II)-TD@nSiO catalyst
2
In a round-bottomed flask equipped with a condenser and a magnetic stirrer, a mixture of aldehyde
(
1 mmol), 1,2-phenylenediamine (1 mmol) and Cu(II)-TD@nSiO (10 mg, containing 0.0034
2
o
mmol Cu(II)) in EtOAc (2 mL) was stirred at 50 C. The progress of the reaction was monitored by
TLC (eluent: n-hexane/EtOAc, 2:1). After completion of the reaction, the mixture was cooled to
room temperature and EtOAc/EtOH (3:2, 15 mL) was added. The catalyst was separated by
filtration and washed with EtOAc (10 mL). The filtrate was evaporated and the crude product was
purified by recrystallization from EtOAc or EtOH to afford the pure product.
2
.5. General procedure for synthesis of benzothiazoles in the presence of Cu(II)-TD@nSiO Catalyst
2
A mixture of aldehyde (1 mmol), 2-aminothiophenol (1 mmol) and Cu(II)-TD@nSiO (10 mg,
2
containing 0.0034 mmol Cu(II)) was stirred at room temperature. The progress of the reaction was
monitored by TLC (eluent: n-hexane/EtOAc, 5:1). At the end of the reaction, EtOAc (15 mL) was
added; the catalyst was filtered and washed with EtOAc (10 mL). Evaporation of the filtrate and
purification of the crude product by recrystallization from n-hexane/EtOAc (10:1) afforded the
pure product.
6
Page 8 of 64

2
.6. General procedure for synthesis of bis-benzimidazoles and bis-benzothiazoles in the presence of
Cu(II)-TD@nSiO catalyst
2
2
.6.1. Synthesis of bis-benzimidazoles
In a round-bottomed flask equipped with a condenser and a magnetic stirrer, a mixture of
terephthaldialdehyde (1 mmol), 1,2-phenylenediamine (2 mmol) and Cu(II)-TD@nSiO (20 mg,
2
o
containing 0.0068 mmol Cu(II)) in EtOAc (2 mL) was stirred at 50 C (for synthesis of
unsymmetrical bis-benzimidazole, terephthaldialdehyde (1 mmol) reacted with 1,2-
phenylenediamine (1 mmol) and 4-methyl-1,2-phenylenediamine (1 mmol) in the presence of the
catalyst). The work-up was performed as described for synthesis of benzimidazoles and the pure
product was obtained in 87-92% yields.
2
.6.2. Synthesis of bis-benzothiazole
A mixture of terephthaldialdehyde (1 mmol), 2-aminothiophenol (2 mmol) and Cu(II)-TD@nSiO2
20 mg, containing 0.0068 mmol Cu(II)) was stirred at room temperature for 150 min. The work-
(
up was performed as mentioned for synthesis of benzothiazoles and the pure product was obtained
in 87% yield.
2
.7. Synthesis of 2-(4-(1H-benzimidazol-2-yl)phenyl)benzothiazole in the presence of Cu(II)-
TD@nSiO catalyst
2
In a round-bottomed flask equipped with a condenser and a magnetic stirrer, a mixture of
terephthaldialdehyde (1 mmol), 1,2-phenylenediamine (1 mmol), 2-aminothiophenol (1mmol) and
Cu(II)-TD@nSiO (20 mg, 0.0068 mmol Cu(II)) EtOAc (2 mL) and stirred at 50 °C for 160 min.
2
The reaction progress was monitored by TLC (eluent: n-hexane/EtOAc, 3:1). After completion of
the reaction, EtOAc (15 mL) was added; the catalyst was filtered and washed with EtOAc (10 mL).
The filtrate was evaporated and the resulting crude product was purified by recrystallization from
EtOAc to afford the pure product in 84% yield.
7
Page 9 of 64

3
. Results and discussion
3
.1. Synthesis and characterization of Cu(II) containing nanosilica triazine dendrimer (Cu(II)-
TD@nSiO2)
For the synthesis of Cu(II)-TD@nSiO , activated nano-silica was reacted with 3-
2
aminopropyltrimethoxysilane (APTS) according to the literature [59] to produce the AP-nSiO2
needed for growing dendritic molecules. Reaction of cyanuric chloride (CC) with Ap-nSiO was
2
carried out at room temperature to substitute one chlorine atom of cyanuric chloride to afford CC1-
nSiO which upon reaction with bis(3-aminopropyl)amine gave G1. The same procedure was
2
applied to increase the dendrimer generation, CC2-nSiO and G2 (Scheme 2). The G2-dendrimer
2
with NH groups on its outer periphery is suitable for encapsulation of Cu(II). These processes
2
were monitored by thermogravimetric analysis (TGA), FT-IR and diffuse reflectance UV-vis
spectroscopic techniques, as well as elemental analysis.
H2N
NH2
H2N
NH2
N
N
N
Cl
N
Cl
N
N
N
N
NH2
Si
NH
NH
Cl
N
NH2
NH2
N
N
OH OH OH
nSiO2
EtO
NH2
Si
Si
Cl
Si
nSiO2
CC1-nSiO2
Et O
Cl
N
H
O Et
nSiO2
AP-nSiO2
nSiO2
G1
THF, rt, 12 h
Toluene, reflux, 8 h
DMF, 80 °C, 16 h
D
I
P
E
Cl
A
1
6
,
T
h
N
N
H
NH2
Cu2+
NH2
N
F
NH2
H2N
Cu
H2N
NH2
H2N
H2N
,
NH2
Cu2+
NH2
r
Cl
N
Cl
Cl
2+
t
H2N
Cu2+
H2N
H2N
N
N
N
N
NH2
N
NH2
Cl
N
N
N
Cl
NH2
NH2
N
N
N
N
H2N
H2N
N
N
Cl
N
N
N
N
N
N
N
Cu2+
N
N
N
N
NH
Cu2+
N
NH
N
N
HN
Cl
H2N
HN
HN
Cl
NH2
NH2
NH
Cu2+
N
N
N
N
N
H
N
N
Cu2+
H
N
H
N
N
N
N
H2N
N
H2N
N
N
N
N
H
N
N
N
H
N
N
N
N
N
N
Cl
N
N
N
Cu2+
N
N
N
N
Cl
N
N
+NH2
Cu2
N
N
NH2
N
N
N
N
N
N
Cu2+ NH
NH
NH
H2N
Cu2+
N
H2N
NH2
NH2
NH2
NH2
H2N
H2N
Si
Si
N
H
Si
CuCl2. 2 H20
DMF, rt, 10 h
nSiO2
nSiO2
G2
DMF, 80 °C, 16 h
nSiO2
Cu(II)-TD@nSiO2
CC1-nSiO2
Scheme 2. Preparation route for Cu(II)-TD@nSiO2.
8
Page 10 of 64

The FT-IR spectrum of AP-nSiO (Fig. 1; a) showed a broad O-H stretching band at 3200-3400
2
-
1
-1
cm and a strong Si-O-Si stretching band about 1000-1100 cm [57]. Also, the characteristic
-
1
bands for C-Haliph bond appear at 1480 and 2990 cm . In the FT-IR spectra of CC1-nSiO , G1 and
2
-
1
G2 (Fig. 1; b, c and d), in addition to the above mentioned vibrations, the bands at 1580-1603 cm
(
C=N) is a good indication for the presence of triazine fragment on the nano-silica.
Fig. 1. FT-IR spectrum of: (a) AP-nSiO , (b) CC1-nSiO , (c) G1, and (d) G2.
2
2
The nitrogen content of AP-nSiO was determined by elemental analysis, showing a value of about
2
0
.99 mmol/g of nano-silica.
The amount of organic moieties attached to nSiO was measured by using TGA and elemental
2
o
analysis. The weight loss of G2 between 30 to 600 C as a function of temperature shows an
irreversible process [57]. The TGA plots of AP-nSiO , CC1-nSiO , CC2-nSiO , G1 and G2 (Fig.
2
2
2
o
2
) depict a two-step thermal decomposition. The first step weight loss up to 200-250 C
corresponds to removal of physically adsorbed water, whereas, at temperature higher than about
o
2
50 C, the main weight loss (second step) is due to removal of organic moieties on the surface.
The TGA results are summarized in Table 1. The observed total weight loss for AP-nSiO , G1 and
2
9
Page 11 of 64

G2 are 5.757%, 17.77% and 36.64%, respectively. On the basis of these values, the theoretical
conversions are 77%, 60%, 57% and 80% for AP-nSiO CC1-nSiO , CC1-nSiO G1,
2
2
2
G1CC2-nSiO and CC2-nSiO G2, respectively. A summary of the elemental analysis and
2
2
TGA data is presented in Table 2, indicating a good agreement between the two methods.
Fig. 2. TGA curve of: (a) AP-nSiO , (b) CC1-nSiO , (c) G1, (d) CC2-nSiO , and (e) G2.
2
2
2
In addition, the cyanuric chloride functionalized AP-nSiO (CC1-nSiO ) was characterized by
2
2
TGA and elemental analysis for determination of the amount of organic moieties which have been
attached to nano-silica. These methods showed that the total amount of organic moieties on nSiO2
was about 0.76 mmol/g. The amount of released chloride ion was also determined by titration with
AgNO , which is in accordance with the results obtained by TGA and elemental analysis.
3
The diffuse reflectance UV-vis spectra of the compounds are shown in Fig. 3. The UV-vis
spectrum of AP-nSiO (Fig. 3a) showed a very week absorption peak at ~297 nm. Upon reaction
2
with CC (Fig. 3b), a strong and sharp absorption peak at ~235 nm and a weak peak at ~361 nm
appeared, relating to the π→π* transition of C=N in triazine and n→π* transition, resulting from
the attachment of CC to AP-nSiO [60]. In the next step in which CC1-nSiO intermediate was
2
2
reacted with bis(3-aminopropyl)amine (Fig. 3c), the intensity of the π→π* and n→π* peaks were
1
0
Page 12 of 64

reduced, whereas, a strong peak at ~218 nm and a very weak peak at ~326 nm appeared. The same
trend was observed for the second generation, CC2-nSiO and G2 (Fig. 3d and 3e). The diffuse
2
reflectance UV-vis spectrum of Cu(II)-TD@nSiO (Fig. 3f) showed a new broad band of moderate
2
intensity centered at ~854 nm which is usually assigned to a d-d transitions, indicating that the
solid is a copper(II) compound, Cu(II)-TD@nSiO [61]. Also an intense peak at ~325 nm is
2
assigned to the strong ligand-to-metal charge transfer transition from p orbitals of the donor atoms
π
to d orbitals of Cu(II) [60].
Fig. 3. Diffuse reflectance UV-vis spectrum of: (a) AP-nSiO , (b) CC1-nSiO , (c) G1, (d) CC2-nSiO ,
2
2
2
(e) G2, and (f) Cu(II)-TD@nSiO2.
The morphology of the surface of TD-nSiO and Cu(II)-TD@nSiO was studied by field emission
2
2
scanning electron microscopy (FE-SEM) (Fig. 4a and 4b). The size and morphology of TD-nSiO2
and Cu(II)-TD@nSiO can be directly observed from SEM images. As can be seen, the Cu(II)-
2
TD@nSiO particles are spherical. The energy dispersive X-ray (EDX) results, obtained from SEM
2
analysis of TD-nSiO and Cu(II)-TD@nSiO , are shown in Fig. 4c and 4d. Comparison of these
2
2
spectra clearly shows that Cu(II) particles have been introduced into the TD-nSiO (Fig. 4d).
2
1
1
Page 13 of 64

(
a)
(b)
(
c)
(d)
Fig. 4. FE-SEM images of: (a) TD@nSiO and (b) Cu(II)-TD@nSiO . SEM-EDX spectrum of: (c)
2
2
TD@nSiO and (d) Cu(II)-TD@nSiO .
2
2
The Cu(II)-TD@nSiO catalyst was also characterized by TEM (Fig. 5a). The dark colored regions or
2
black spots in the photograph correspond to the copper species while the colorless parts being that of
silica, this is due to the higher electron density of copper compared to nanosilica [62]. Furthermore, the
dendrimers agglomerate and local higher densities of Cu salts are observed. The hystogram of size
distribution showed that the matrix has an average diameter of about 20-30 nm (Fig. 5b).
1
2
Page 14 of 64

(
a)
(b)
Fig. 5. (a) TEM image of Cu(II)-TD@nSiO and (b) Particle size distribution histogram of Cu(II)-
2
TD@nSiO2.
X-ray photoelectron spectroscopy (XPS) is a powerful tool for assessing the chemical composition
of macromolecules, ligands and mixed valent complexes [63]. The XPS spectrum of Cu(II)-
TD@nSiO from 0.0 to 1200.0 eV is shown in Fig. 6a. In this spectrum, the characteristic binding
2
energies of C 1s, Si 2p, O 1s, N 1s and Cu 2p are identified. In general, the binding energy at
1
02.40 eV is due to Si 2p in the nSiO and the binding energy at 532.1 eV is attributed to O 1s in
2
Si-O network [64]. Furthermore, the presence of Cu(II) ions was also confirmed by X-ray
photoelectron spectroscopy. The XPS analysis of Cu(II) in the catalyst shows two intense peaks at
9
32–935 eV and 952–955 eV, corresponding to Cu 2p3/2 and Cu 2p1/2 peaks [63]. Each of the Cu 2p
peaks in the spectrum of the catalyst was fitted using two Gaussian–Lorentzian peaks with energies of
9
9
34.78 and 933.16 eV for Cu 2p , and 954.67 and 952.92 eV for Cu 2p (Fig. 6b). The peak at
3/2 1/2
34.78 eV (83.37%, Table 3) corresponds to Cu(II) ions [65] while the lower peak at 933.16 eV
(16.62%, Table 3) corresponds either to Cu(I) or Cu(0), though the chemical states of Cu(I) and Cu(0)
are not distinguishable by XPS analysis [65,66].The presence of reduced forms of Cu(II) has been
described by Hwang et al. [67] who have reported that Cu(II) is reduced to the lower oxidation states
under X-ray irradiation in Ultra-high vacuum (by calcination followed by heating in He). The N 1s
1
3
Page 15 of 64

spectrum was fitted using two Gaussian–Lorentzian peaks with energies of 399.18 and 400.79 eV,
respectively (Fig. 6c and Table 3). The peak at 399.18 nm was assigned to the nitrogen coordinated
to Cu(II) ion in dendritic structure [68]. Comparing the N 1s spectrum of the primary amine [64]
with the spectrum of Cu(II)-TD@nSiO , a significant shift of the binding energy of the 398.1 eV
2
(
398.1→399.18 eV) was observed. This phenomenon can be attributed to the formation of RNH2-
Cu(II) complexes, in which a lone pair of electrons of the nitrogen atom are coordinated with
Cu(II) ions, and as a consequence, the electron cloud density of the nitrogen atom is reduced,
resulting in a higher binding energies. In addition, the peak at 400.79 eV, related to N 1s of
triazine, showed an increase in the binding energy, due to the coordination with Cu(II) ions [64].
All these observations suggested that these nitrogen atoms are in a different chemical environment.
Table 3 shows the XPS binding energies and the proportion of different types of atoms for N and
Cu.
(
a)
(b)
(
c)
1
4
Page 16 of 64

Fig. 6. XPS spectra of Cu(II)-TD@nSiO : (a) survey spectrum, (b) the close-up Cu 2p, and (c) the
2
close-up N 1s in the complex.
The copper content of Cu(II)-TD@nSiO , measured by ICP analysis, was obtained to be 0.34
2
mmol per gram of Cu(II)-TD@nSiO catalyst.
2
3
.2. Synthesis of benzimidazoles, benzothiazoles, bis-benzimidazoles and bis-benzothiazoles
catalyzed by Cu(II)-TD@nSiO2
Initially, In order to optimize the conditions for the synthesis of benzimidazoles, the reaction of 4-
chlorobenzaldehyde (1 mmol) with 1,2-phenylenediamine (1 mmol) was chosen as a model
reaction. The model reaction was first performed in the absence of the catalyst. No product was
obtained under this condition (Table 4, entry 1). Then, the reaction was carried out in the presence
of nano-SiO , TD@nSiO , Cu(II)@nSiO [59] and Cu(II)-TD@nSiO (Table 4, entries 2-5). The
2
2
2
2
results showed that Cu(II)-TD@nSiO was the most effective catalyst for the reaction. Next, the
2
effects of reaction time, solvent, amount of Cu(II)-TD@nSiO catalyst and temperature on the
2
yield of the desired product was examined (Table 4, entries 6-16); the highest yield was obtained
using 10 mg catalyst (containing 0.0034 mmol Cu(II)) in EtOAc at 50 °C within 20 min (Table 4,
entry 5).
To find the optimized conditions for the synthesis of benzothiazoles, 2-aminothiophenol (1 mmol)
was treated with 4-chlorobenzaldehyde (1 mmol) under conventional conditions. After some
preliminary experiments (Table 4, entries 17-20), it was found that the best yield of the desired
benzothiazole can be obtained in the presence of 10 mg catalyst at room temperature within 15 min
under solvent-free conditions.
The optimized reaction conditions were then applied for the synthesis of a series of benzimidazoles
and benzothiazoles to explore its scope and generality. As shown in Table 5, aromatic aldehydes
containing electron-donating and electron-withdrawing groups reacted efficiently with 1,2-
1
5
Page 17 of 64

phenylenediamines in the presence of catalytic amount of Cu(II)-TD@nSiO at 50 °C to afford the
2
corresponding benzimidazoles in high yields (89–97%) (Table 5, entries 1-5). Under the same
reaction conditions, polycyclic aldehydes such as 1-naphthaldehyde and anthracene-9-
carbaldehyde afforded the corresponding benzimidazoles in 88-93% yields (Table 5, entries 6-8). It
is worth mentioning that heterocyclic aldehydes such as pyridine-3-carbaldehyde, indole-3-
carbaldehyde and thiophene-2-carbaldehyde took part smoothly in the reaction to give the desired
products in 91-92% yields (Table 5, entries 9-11).
In order to show the efficiency of this catalytic system further, the synthesis of benzothiazoles was
investigated in the presence of Cu(II)-TD@ nSiO . As shown in Table 6, treatment of a variety of
2
aldehydes with 2-aminothiophenol in the presence of catalytic amount of Cu(II)-TD@nSiO at
2
room temperature afforded the corresponding benzothiazoles in 87-98% yields.
Due to the extensive applications of bis-benzimidazoles and bis-benzothiazoles as ligands in
chemistry [37,38] and to further widen the applicability of the present methodology, the catalytic
activity of Cu(II)-TD@nSiO was also evaluated in the preparation of bis-benzimidazoles and bis-
2
benzothiazoles. In this manner, dialdehyde such as terephthaldialdehyde was reacted with 2 mmol
of 1,2-phenylenediamine or 4-chloro-1,2-phenylenediamine and/or 2-aminothiophenol in the
presence of a catalytic amount of Cu(II)-TD@nSiO under conventional conditions to afford the
2
corresponding symmetrical bis-benzimidazoles and bis-benzothiazoles in high yields (Scheme 3).
This catalytic system also showed high activity in the synthesis of unsymmetrical bis-
benzimidazole, so the reaction of terephthaldialdehyde with a mixture containing 1 mmol 1,2-
phenylenediamine and 1 mmol 4-methyl-1,2-phenylenediamine proceeded efficiently in the
presence of Cu(II)-TD@nSiO to furnish the corresponding unsymmetrical bis-benzimidazole in
2
9
0% yield (Scheme 4). It is noteworthy that, when 1 mmol 1,2-phenylenediamine and 1 mmol 2-
aminothiophenol are used in the reaction with terephthaldialdehyde in the presence of this catalyst,
1
6
Page 18 of 64

the desired product which contains both benzimidazole and benzothiazole species was produced in
4% yield (Scheme 4). To the best of our knowledge, this is the first report on the synthesis of an
8
unsymmetrical bis-benzimidazole and a compound containing both benzimidazole and
benzothiazole moieties. Therefore, such a syntheses can be considered as a useful practical
achievement in the preparation of benzimidazole and benzothiazole derivatives.
O
H
H
N
H N
X
X
NH2
2
X
N
Cu(II)-TD@nSiO2
EtOAc, 50 C, air
N
H
N
H N
NH2
2
X
X = H
9
2% ( 120 min)
H
O
3
a
1
mmol
1 mmol
H
1 mmol
X = Cl
3b
8
7% (240 min)
O
H N
NH2
2
N
S
Cu(II)-TD@nSiO2
EtOAc, rt, air
S
HS
N
SH
H
O
3
c
87% ( 150 min)
1
mmol
1 mmol
1 mmol
Scheme 3. Synthesis of symmetrical bis-benzimidazoles and bis-benzothiazole catalyzed by Cu(II)-
TD@nSiO2.
H
O
H N
2
H
N
NH2
N
Cu(II)-TD@nSiO2
EtOAc, 50 C, air
H N
2
N
H
N
NH2
O
H
4
a
90% ( 200 min)
1
mmol
1 mmol
H
1 mmol
O
H N
H
N
NH2
2
N
S
Cu(II)-TD@nSiO2
EtOAc, 50 C, air
H N
2
N
SH
O
H
1
mmol
1 mmol
1 mmol
4b
84% ( 160 min)
Scheme 4. Synthesis of unsymmetrical bis-benzimidazole and compound containing both
benzimidazole and benzothiazole moieties catalyzed by Cu(II)-TD@nSiO2.
1
7
Page 19 of 64

In addition, we also performed a scale-up experiment with 20 mmol each of 1,2-phenylenediamine
and 2-bromobenzaldehyde in the presence of Cu(II)-TD@nSiO catalyst at 50 °C in EtOAc. Under
2
these conditions, the corresponding benzimidazole was obtained in 95% yield which is comparable
to the yield obtained by the small scale experiment.
A plausible mechanism for the synthesis of benzimidazoles and benzothiazoles is illustrated in
Scheme 5. First, activation of aldehyde by the catalyst followed by condensation with amino group
of 1,2-phenylenediamine or 2-aminothiophenol results in imine A, which upon intramolecular
nucleophilic attack of the remaining NH or SH group in the presence of catalyst affords
2
intermediate B. Finally, oxidative aromatization of intermediate B under ambient atmosphere in
the presence of the catalyst afforded the desired product C, releasing the catalyst for the next
catalytic cycle. To confirm the suggested mechanism, the model reaction was performed under
argon atmosphere; no desired product was obtained under these conditions. On the other hand,
bubbling of oxygen into the reaction mixture accelerated the reaction considerably, indicating that
the presence of oxygen is essential for the reaction. These observations revealed reasonability of
the proposed mechanism.
3
.3. Catalyst recovery and reuse
The recovery and reusability of the catalyst are very important for commercial and industrial
applications as well as green process considerations. In this manner, the reusability of Cu(II)-
TD@nSiO2 catalyst was investigated in the reaction of 4-chlorobenzaldehyde with 2-
aminothiophenol. After completion of the reaction, ethyl acetate was added, the catalyst was
o
separated by filtration; dried in a vacuum oven at 40 C and then reused. As seen in Fig. 7, the
catalyst showed no substantial reduction in the activity even after eight runs. Furthermore, the
leaching of Cu(II) ion in Cu(II)-TD@nSiO catalyst was determined by ICP-OES analysis. It was
2
found that the leaching level is negligible (less than 1% in the two first runs). These observations
1
8
Page 20 of 64

indicated that the catalyst is stable under the reaction conditions, and can be recovered and reused.
To show the stability and efficiency of the Cu(II)-TD@nSiO catalyst in comparison with that of
2
Cu(II)@nSiO [69], the reusability of Cu(II)@nSiO was also investigated under the same reaction
2
2
conditions. As shown in Fig. 7, the desired product was obtained in 80%, 25% and 12% yields in
the first, second and third runs, respectively, indicating that significant leaching of the active
copper species from the nSiO support occurred during the catalytic process. These results clearly
2
revealed the crucial role of the triazine-based dendrimer in stabilizing of the copper containing
catalyst.
NH2
Cu(II)
O
O
ZH
Ar
H
Ar
H
H O
2
Cu(II)
Ar
N
H
Cu(II)
.
.
ZH
A
-
H O
2
N
H
N
Ar
Ar
z
H
C
z
Cu(II), air
B
Scheme 5. Proposed mechanism.
1
9
Page 21 of 64

Fig. 7. Comparison of reusability results of Cu(II)-TD@nSiO and Cu(II)@nSiO catalysts in the
2
2
synthesis of 2-(4-chlorophenyl)benzothiazole
4
. Conclusions
In conclusion, the synthesis and characterization of a new stable and heterogeneous catalyst, Cu(II)
containing nanosilica triazine dendrimer (Cu(II)-TD@nSiO ), is reported. This catalyst has been
2
successfully applied for the synthesis of benzimidazoles and benzothiazoles via the condensation
of 1,2-phenylenediamines and 2-aminothiophenol, respectively, with a wide variety of aromatic,
polycyclic and heteroaromatic aldehydes at ambient atmosphere under conventional conditions.
Efficient synthesis of bis-benzimidazoles, bis-benzothiazoles and a compound containing both
benzimidazole and benzothiazole moieties from terephthaldialdehyde has been also achieved using
this catalytic system. Furthermore, the catalyst is stable under the reaction conditions and can be
recycled and reused several times without a significant loss in activity.
Acknowledgements
Financial support of this work by Center of Excellence of Chemistry and Research Council of
University of Isfahan is gratefully acknowledged
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2
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Page 22 of 64

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4
Page 26 of 64

Fig. 1. FT-IR spectrum of: (a) AP-nSiO , (b) CC1-nSiO , (c) G1, and (d) G2.
2
2
Fig. 2. TGA curve of: (a) AP-nSiO , (b) CC1-nSiO , (c) G1, (d) CC2-nSiO , and (e) G2.
2
2
2
Fig. 3. Diffuse reflectance UV-vis spectrum of: (a) AP-nSiO , (b) CC1-nSiO , (c) G1, (d) CC2-nSiO ,
2
2
2
(
e) G2, and (f) Cu(II)-TD@nSiO2.
Fig. 4. FE-SEM images of: (a) TD@nSiO and (b) Cu(II)-TD@nSiO . SEM-EDX spectrum of:
2
2
(
c)TD@nSiO and (d) Cu(II)-TD@nSiO
2 2.
Fig. 5. (a) TEM image of Cu(II)-TD@nSiO and (b) Particle size distribution histogram of Cu(II)-
2
TD@nSiO2.
Fig. 6. XPS spectra of Cu(II)-TD@nSiO : (a) survey spectrum, (b) the close-up Cu 2p, and (c) the
2
close-up N 1s in the complex.
Fig. 7. Comparison of reusability results of Cu(II)-TD@nSiO and Cu(II)@nSiO catalysts in the
2
2
synthesis of 2-(4-chlorophenyl)benzothiazole
2
5
Page 27 of 64

Table 1. Thermogravimetric analysis (TGA) results.
Sample
AP-nSiO2
CC1-nSiO2
G1
Organic (wt%) Organic (mmol/g nSiO2) Yield (%)
0.99
0.76
0.45
0.26
0.21
5.757
15.58
17.77
25.301
36.64
-
77
60
57
80
CC2-nSiO2
G2
2
6
Page 28 of 64

Table 2. The TGA and elemental analysis (EA) results for synthesis of nano-silica triazine dendrimer
AP-nSiO2
CC1-nSiO2
G1
CC2-nSiO2
G2
TGA (wt%) EA (wt%) TGA (wt%) EA (wt%) TGA(wt%) EA (wt%) TGA (wt%) EA (wt%)
TG (wt%) EA (wt%)
C
H
3.63
0.70
1.41
5.74
3.71
1.02
1.55
6.28
5.47
0.53
5.53
0.83
9.72
1.75
10.07
2.10
12.96
1.26
13.01
1.92
20.41
3.59
20.41
4.20
N
4.25
4.04
6.31
6.35
11.08
25.30
11.22
26.15
14.1
14.18
38.79
Total
10.25
10.40
17.78
18.52
38.11
2
7
Page 29 of 64