Design, preparation and characterization of urea-functionalized Fe3O4/SiO2 magnetic nanocatalyst and application for the one-pot multicomponent synthesis of substituted imidazole derivatives

Article abstract of DOI:10.1016/j.catcom.2015.05.014

Novel urea-functionalized silica-based magnetite hybrid core-shell nanoparticles were prepared and characterized by Fourier transform infrared spectroscopy (FT-IR), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), X-ray diff

Full text of DOI:10.1016/j.catcom.2015.05.014

Design, preparation and characterization of urea-functionalized Fe 3O4/SiO2
magnetic nanocatalyst and application for the one-pot multicomponent
synthesis of substituted imidazole derivatives
Ali Maleki, Zahra Alirezvani, Saeid Maleki
PII:
DOI:
Reference:
S1566-7367(15)00204-6
doi: 10.1016/j.catcom.2015.05.014
CATCOM 4334
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Catalysis Communications
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Revised date:
Accepted date:
8 April 2015
21 May 2015
22 May 2015
Please cite this article as: Ali Maleki, Zahra Alirezvani, Saeid Maleki, Design, preparation
and characterization of urea-functionalized Fe3O4/SiO2 magnetic nanocatalyst and ap-
plication for the one-pot multicomponent synthesis of substituted imidazole derivatives,
Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.05.014
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ACCEPTED MANUSCRIPT
Design, preparation and characterization of urea-functionalized
Fe3O4/SiO2 magnetic nanocatalyst and application for the one-pot
multicomponent synthesis of substituted imidazole derivatives
Ali Maleki*, Zahra Alirezvani, Saeid Maleki
Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science
and Technology, Tehran 16846-13114, Iran
Tel: +98-21-77240640-50; fax: +98-21-73021584; e-mail: maleki@iust.ac.ir
Abstract
Novel urea-functionalized silica-based magnetite hybrid core-shell nanoparticles were
prepared and characterized by Fourier transforms infrared spectroscopy (FT-IR), energy-
dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), X-ray
diffraction (XRD) and vibrating sample magnetometer (VSM) analyses. It was used as a
highly efficient and recoverable heterogeneous nanocatalyst for the one-pot three-component
condensation reaction between benzil or benzoin with various substituted aldehydes and
ammonium acetate to afford the corresponding imidazoles under mild reaction conditions in
high-to-quantitative yields. The catalyst was recycled at least five times without significant
decrease in the catalytic activity.
Keywords: Heterogeneous nanocatalyst, Fe3O4/SiO2, Imidazole, Benzil, Magnetite
nanoparticles.
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1. Introduction
The use of environmentally benign, maintainable and efficiently reusable catalysts
provides both economic and ecological advantages [1]. Recycling of homogeneous catalysts
is thus a main issue in large-scale production of chemicals [2]. Recently, nanoparticles have
appeared as efficient catalysts and supports for the immobilization of homogeneous catalysts
[3-6]. High specific surface area of metallic nanoparticles provides more catalytically active
sites are guaranteed.
Nanoparticle Fe3O4-supported catalysts can be simply separated from the reaction mixture
by an external permanent magnet. This strategy is typically more effective than filtration or
centrifugation that become annoying for such small nanoparticles [2, 7-9]. Pristine iron oxide
magnetite nanoparticles (MNPs) will aggregate quickly into large bunch and therefor will
lose their unique properties. It is needed to cover nanoparticles with organic component to
prevent aggregation and thus retain their nano-scale properties [10]. The coating of iron
oxides with silica to form core–shell structures is rather simple because of the presence of
surface FeOH groups. Here, we report a versatile and useful preparation of recoverable
Fe3O4/SiO2-supported urea nanocatalyst through a covalent bond linkage.
Developing of new multicomponent reactions (MCRs) are an area of considerable interest
due to the fact that the products are formed in a single step and also the variety could be
achieved only by changing the reaction components [11-13]. Multi-substituted imidazoles are
an important class of pharmaceutical compounds and play important roles in biochemical
processes [14, 15]. Among these, the imidazole derivatives are well known to anti-
inflammatory [16], anti-parasitic [17], antifungal [18], antiviral [19] and high cytotoxicity,
which have indicated them as new candidates in cancer therapy [20].
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Recently, the synthesis of 2,4,5-trisubstituted imidazoles has been performed by
condensation of benzil and benzoin, aldehydes and ammonium acetate in the presence of
ZrCl4 [21], TBAB [22], DABCO [23], Al2O3 [24] and nano-crystalline magnesium oxide
[25]. But, some of these synthetic methods are associated with one or more disadvantages
such as using expensive reagents, long reaction times, complex work-up and purifications and
generation of large amount of toxic wastes. Therefore, the development of simple, effective,
high-yielding, and environmentally friendly approaches using new catalysts for the synthesis
of highly substituted imidazoles is an important task for organic chemists.
In continuation of our research on the introduction of recoverable catalysts in organic
synthesis [26-29], in current study, we disclose that MNPs-urea can be used as a novel
magnetic nanocatalyst for the synthesis of substituted imidazoles.
To the best of our knowledge, this is the first report of design, preparation, and
characterization Fe3O4/SiO2/urea nanocomposite and its application as a heterogeneous
catalyst in organic reactions.
2. Experimental section
2.1. General
Analytical thin layer chromatography (TLC) for monitoring reactions was performed
using Merck 0.2 mm silica gel 60 F-254 Al-plates using ethyl acetate and n-hexane as
eluents. Melting points were determined in open capillaries using an Electrothermal 9100
instrument. Infrared (IR) spectra were acquired on a Shimadzu FT-IR-8400S spectrometer.
1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded on a Bruker DRX-500
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Avance spectrometers with CDCl3 as solvent at ambient temperature and tetramethylsilane
(TMS) as the internal standard. All chemical shifts are given relative to TMS. All yields refer
to the isolated products. The X-ray powder diffraction (XRD) data were collected on an
X’Pert MPD.Philips diffractometer with Cu radiation source (λ = 1.54050˚A) at 40 Kv
voltage and 40 mA current. Magnetic susceptibility measurements were carried out using a
vibrating sample magnetometer (BHV-55, Riken, Japan) at room temperature. EDX spectra
were recorded on Numerix DXP–X10P. SEM images were obtained on a KYKY EM-3200.
2.2. Preparation of the magnetic Fe3O4 nanoparticles (MNPs)
Typically, iron (III) chloride hexahydrate (4.6 g, 0.017 mol) and iron (II) chloride
tetrahydrate (2.2 g, 0.011 mol) were added to 100 mL deionized water and sonicated until the
salts dissolved completely. Then, 10 mL of 25% aqueous ammonia was quickly added into
the reaction mixture under N2 atmosphere at room temperature followed by stirring about 30
min with mechanical stirrer. The precipitates were collected using an external magnet and
washed distilled water.
2.3. Preparation of MNPs coated by silica (MNPs-SiO2)
Following the Stöber method with some modifications [30], the surface adapted MNPs (1
g) were dispersed in 50 mL of distilled water by the ultrasonic treatment for 20 min to form a
Ferro fluid. Then, the consequence dispersion was centrifuged for 30 min and adjusted to 2.0
wt%. 2 mL of the Ferro fluid was first diluted with water (40 mL), the consequence
suspension and 5 mL of NH3.H2O were poured into 140 mL of ethanol with stirring at 40 °C.
In conclusion, under continuous mechanical stirring, 1 mL of TEOS diluted in ethanol (20
mL) was drop wise added to this dispersion. The resulting dispersion was kept stirred
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mechanically for 14h at room temperature. The magnetic Fe3O4/SiO2 nanoparticles were
collected by magnetic separation and washed with ethanol and deionized water [31].
2.3. Preparation of MNPs-SiO2 coated by (3-chloropropyl)-trimethoxysilane (MNPs-SiO2-Cl)
Chloropropyl-modified silica-coated magnetic nanoparticles were prepared following the
modified procedure of Zeng et al. [32]. In a typical route 1 mL (5 mmol) of (3-chloropropyl)-
trimethoxysilane (CPTS) was dissolved in 100 mL of dried toluene, this mixture was added
to 1 g of Fe3O4/SiO2 and the solution was stirred for 18 h at 60 °C. The chloropropyl-
functionalized solid (Fe3O4/SiO2-Cl) was washed with toluene, separated by a magnet, and
dried in vacuum.
2.4. Preparation of urea-functionalized magnetic Fe3O4 nanoparticles (MNPs-SiO2-urea)
The prepared Fe3O4/SiO2-Cl (1 g) and KI (1.66 g, 10 mmol) were added to a solution of
urea (0.6 g, 10 mmol) and K2CO3 (1.38 g, 10 mmol) in acetonitrile (50 mL) in a round-
bottom flask and the mixture was stirred under reflux condition for 8 h. The obtained solid
was then magnetically collected from the solution and washed abundantly with water/ethanol
followed by drying at 80 °C for 12 h.
2.5. General procedure for the preparation of 2,4,5-trisubstituted imidazole derivatives
MNPs-SiO2-urea (0.0088 g) was added to a mixture of benzil or benzoin (1.0 mmol),
various substituted aldehydes (1.0 mmol) and NH4OAc (4.0 mmol) in refluxing ethanol and
stirred for appropriate times. The progress was monitored by TLC. After completion of the
reaction the catalyst was separated by an external magnet and the simple product was isolated
by filtration of the reaction mixture. Pure products were obtained via re-crystallization by
EtOH.
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3. Results and discussions
3.1. Characterization of MNPs-SiO2-urea
The process of the preparation urea-functionalized Fe3O4/SiO2 nanoparticles is shown in
Scheme 1.
< Scheme 1>
MNPs-urea was characterized using a variety of different techniques. The XRD pattern of
MNPs-urea is shown in Fig. 1. The position and relative intensities of all peaks are in a good
agreement with standard XRD pattern of Fe3O4 (JCPDS card No. 19-0629). The XRD
patterns of the particles show six characteristic peaks, revealing a cubic iron oxide phrase
(2θ= 18.01°, 30.26°, 37.31°, 43.29°, 55.52°, 57.32°). It is implied that the resultant
nanoparticles are pure Fe3O4 with a spinel structure and that the grafting process did not
induce any phase change of Fe3O4.
< Fig. 1>
As illustrated in Fig. 2, the particle size was studied by scanning electron microscopy
(SEM) and the identification of Fe3O4/SiO2/urea was based on the analysis of SEM images. It
was confirmed that the catalyst was made up of nanometer-sized particles less than 70 nm.
Fig. 3 shows that the Fe3O4/SiO2/urea nanoparticles have a core–shell structure and spherical
morphology.
<Fig. 2>
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To identify the elemental composition of MNPs-SiO2/urea, an energy-dispersive X-ray
spectroscopy (EDX) was obtained. As can be seen in Fig. 3, EDX pattern obviously approves
the well-dispersion of Fe3O4 nanoparticles. Chemical characterization of the typical sample
showed that it was composed of iron, silicon, oxygen and carbon elements.
<Fig. 3>
Fourier transform infrared (FT-IR) spectra of MNPs, urea and MNPs-SiO2/urea were
prepared (Please see the Supplementary data file). The peaks at 635 and 577 cm−1 are
assigned to the Fe3+ O Fe3+ and Fe2+ O Fe2+ symmetrical stretching vibrations, respectively.
The covalent bond between silane and magnetite surface is underlined by the appearance of
the band at 1097 cm-1characteristic for Si–O groups. Absorption bands at 960 and 810 cm-1 is
associated to vibrating bands for Si–O–Si groups. These results indicate that SiO2 layer is
formed on the surfaces of Fe3O4. The presence of anchored propyl groups were confirmed by
stretching vibrations appeared at about 2935, 2986 cm−1 (C–H stretching vibrations) in the
FT-IR spectrum. The signals appeared at 1530-1560 cm−1 region can be assigned to the N-H
bending and 1645 cm−1 is attributed to C=O stretching of urea.
Magnetic measurements were registered by using vibrating sample magnetometer (VSM)
at 300 K. The magnetization curves measured for Fe3O4 nanoparticles, Fe3O4/SiO2 and
Fe3O4/SiO2/urea are compared in Fig. 4. As it was seen in Fig. 4 the values of the saturation
magnetization were 50.5, 38.2 and 24.7 emu/g for Fe3O4 nanoparticles, Fe3O4/SiO2 core-shell
and Fe3O4/SiO2/urea respectively in +8000 Oe. The reduction of the saturation magnetization
proposes the formation of Fe3O4/SiO2/urea. Even with this decreasing in the saturation
magnetization, the catalyst still can be expertly separated simply from the solution by
applying of an external magnetic force.
<Fig. 4>
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3.2. Application of MNPs-urea for the synthesis of imidazole derivatives
MNPs-urea was tested as basic magnetically separable heterogeneous nanocatalyst for the
synthesis of the imidazole (5a-t) from reaction of benzil 1 or benzoin 2, wide range of
aromatic aldehyde 3 and ammonium acetate 4 under reflux ethanol at 80 °C (Scheme 2).
After completion of the reaction, the catalyst was easily separated by a magnet and the solid
product was purified by recrystallization from ethanol.
<Scheme 2>
To find the optimal reaction conditions, the one-pot three-component condensation
reaction of benzoin (1 mmol), 4-chlorobenzaldehyde (1 mmol) and ammonium acetate (4
mmol) as an ammonia source was carried out under various reaction conditions. In the
absence of any catalyst, the desired product 5b was not obtained after 420 min in various
solvents and conditions (Table 1, entries 1-7). Interestingly, the yield of the desired product
5b was improved when amounts of catalyst were added to the reaction mixture (entries 8-14).
The next optimization experiments were carried out for obtained the amount of catalyst
required for this condensation. It was found that using 0.0088 g Fe3O4/SiO2/urea in EtOH
under reflux conditions is sufficient to push the reaction forward (entries 15-18). To illustrate
the need of nano-Fe3O4/SiO2/urea for these reactions, a model reaction was conducted with
Fe3O4, urea and Fe3O4/SiO2 (entries 19-22). These results suggest that Fe3O4/SiO2/urea is
more suitable for this reaction.
<Table 1>
In order to generalize the optimum conditions, different derivatives of imidazoles (5a-t)
were prepared from the one-pot reaction mixture of aromatic aldehyde (3a-t), benzil 1 or
benzoin 2 and ammonium acetate 4 under refluxing condition. The results are summarized in
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Table 1S (Please see the Supplementary data file). It is obvious from Table 1S that when
benzoin 2 is used instead of benzil 1, the reaction time was increased. Aromatic and
heteroaromatic aldehydes carrying both different electron-donating and electron-withdrawing
groups were subjected to the condensation and in all cases the desired product was
synthesized in high yields.
Also, the reusability of Fe3O4/SiO2/urea for the MCR to afford 5b was also investigated for at
least five runs. The catalyst was then washed, air-dried and used directly without a
considerable loss of catalytic activity. The results are summarized in Fig. 5.
<Fig. 5>
4. Conclusion
In summary, we have introduced a novel urea-functionalized heterogeneous magnetic
nanocatalyst through a facile and simple procedure starting from commercially available raw
materials. It was found that the magnetic nanocatalyst can be utilized as an efficient
heterogeneous catalyst for the one-pot three-component condensation reaction of benzil or
benzoin with various substituted aldehydes and ammonium acetate in refluxing ethanol in
short reaction times and excellent yields under mild reaction conditions. The present reaction
can be classified as a new protocol for the preparation of synthetically, biologically and
pharmaceutically relevant imidazole derivatives. This procedure includes further important
aspects like the easy workup procedure, reusability of the catalyst, high atom economy,
excellent yields, and environmentally benign reaction conditions.
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Acknowledgements
The authors gratefully acknowledge the financial support from the Iran National Science
Foundation (INSF) and partial support from the Research Council of the Iran University of
Science and Technology.
Supplementary data
Supplementary data associated with this article can be found, in the online version, at
www.elsevier.com.
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Scheme 1. Preparation of Fe3O4/SiO2-urea nanoparticles.
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Fig. 1. XRD pattern the Fe3O4/SiO2/urea.
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Fig. 2. SEM images of the Fe3O4/SiO2/urea.
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Fig. 3. EDX pattern of the Fe3O4/SiO2/urea.
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Fig. 4. VSM curve of the a) Fe3O4, b) Fe3O4/SiO2, c) Fe3O4/SiO2/urea.
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Scheme 2. Three-component reaction of benzil or benzoin with aromatic aldehydes and
ammonium acetate by Fe3O4/SiO2/urea under refluxing conditions.
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Table 1. Optimization of the three-component reaction of benzoin (1), p-chlorobenzaldehyde
(3b) and ammonium acetate (4) under various conditionsa
Catalyst
Temperature
Time
(min)
Entry
Catalyst
Condition
Yield (%)b
loading (g)
(°C)
r.t.
1
2
3
4
5
6
7
8
-
-
-
-
Solvent-free
H2O
420
-
-
-
r.t.
420
420
420
420
420
420
420
240
360
120
120
120
120
150
70
-
EtOH
r.t.
Trace
Trace
20
-
-
EtOH
50
-
-
EtOH
Reflux
Reflux
50
-
-
-
-
H2O
Trace
-
Solvent-free
EtOH
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4/SiO2/urea
Fe3O4
0.0500
r.t.
32
9
0.0500
0.0500
0.0500
0.0500
0.0500
0.0500
0.0017
0.0035
0.0088
0.0176
0.0500
0.0500
0.0500
0.0088
EtOH
Solvent-free
EtOH
50
54
48
80
28
42
29
70
87
95
86
55
36
72
46
10
11
12
13
14
15
16
17
18
19
20
21
22
50
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
H2O
CH3CN
THF
EtOH
EtOH
EtOH
50
EtOH
50
EtOH
120
120
120
50
urea
EtOH
Fe3O4/SiO2
EtOH
Fe3O4/SiO2
EtOH
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a
Reaction conditions: p-chlorobenzaldehyde (1 mmol), benzoin (1 mmol), ammonium acetate (4 mmol),
solvent (2 mL) and required amount of the catalysts. b The yields refer to the isolated product 5b.
Fig. 5. Reusability of Fe3O4/SiO2/urea for the synthesis of 5b.
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Graphical Abstract
Design, preparation and characterization of urea-functionalized
Fe3O4/SiO2 magnetic nanocatalyst and application for the one-pot
multicomponent synthesis of substituted imidazole derivatives
Ali Maleki*, Zahra Alirezvani, Saeid Maleki
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Highlights
 A new urea-functionalized silica-based magnetite hybrid core-shell structure was
prepared.
 It showed excellent catalytic activity for the synthesis of substituted imidazoles.
 The heterogeneous nanocatalyst was simply separated from the reaction mixture.

The catalyst was recycled several times without significant loss in catalytic activity.
22