Copper immobilized onto polymer-coated magnetic nanoparticles as recoverable catalyst for ‘click’ reaction

Article abstract of DOI:10.1002/aoc.3604

Copper supported on polymer-coated magnetic nanoparticles was designed and synthesized as a separable heterogeneous catalyst. The catalyst was fully characterized using several techniques such as Fourier transform infrared and energy-dispersive X-ray spec

Full text of DOI:10.1002/aoc.3604

Received: 9 June 2016
DOI 10.1002/aoc.3604
Revised: 30 July 2016
Accepted: 7 August 2016
F U L L PA P E R
Copper immobilized onto polymer‐coated magnetic nanoparticles
as recoverable catalyst for ‘click’ reaction
1
2
1
*
Alireza Banan | Ahmad Bayat | Hassan Valizadeh
1 Department of Chemistry, Faculty of Sciences,
Copper supported on polymer‐coated magnetic nanoparticles was designed and syn-
thesized as a separable heterogeneous catalyst. The catalyst was fully characterized
using several techniques such as Fourier transform infrared and energy‐dispersive
X‐ray spectroscopies, scanning and transmission electron microscopies, X‐ray dif-
fraction, vibrating sample magnetometry, thermogravimetric analysis and induc-
tively coupled plasma atomic emission spectrometry. All results showed that
copper was successfully supported on the polymer‐coated magnetic nanoparticles.
Also, results showed that the synthesized material can be used as an efficient cata-
lyst for the preparation of a series of 1,4‐disubstituted 1,2,3‐triazoles from corre-
sponding halides, alkynes and sodium azide. The catalyst can be easily isolated
from the reaction solution by applying an external magnet and reused for nine runs
without any significant loss of catalytic activity.
Azarbaijan Shahid Madani University, 53714‐161
Tabriz, Iran
2 Department of Chemistry, Sharif University of
Technology, PO Box 11365‐9516, Tehran, Iran
Correspondence
Hassan Valizadeh, Department of Chemistry,
Sharif University of Technology, PO Box
11365‐9516, Tehran, Iran.
Email: h-valizadeh@azaruniv.edu
KEYWORDS
click reaction, heterogeneous catalyst, magnetic nanoparticles, polymer, triazole
1
|
INTRODUCTION
these salts have been less used in the 1,3‐dipolar Husingen
cycloaddition reactions.
1,2,3‐Triazoles are an very important class of heterocyclic
compounds which are used in agrochemical and industrial
applications.[1–3] Because of their importance, various syn-
thetic methods have been developed recently for their synthe-
sis. The 1,3‐dipolar Huisgen cycloaddition reaction of
terminal alkynes and azides is a popular and well‐studied
method for building of the 1,2,3‐triazole framework. This
method is one of the most powerful ‘click’ reactions.[4–7] This
click reaction provides 1,4‐disubtituted 1,2,3‐triazoles as
unique products with high regioselectively when terminal
alkynes are used as reactant. This reaction at low temperature
proceeds very slowly in the absence of any catalyst.
It has been confirmed that copper(I) can promote this
reaction. For example, Sharpless and co‐workers used
copper(I) salts to accelerate the reaction between azide and
terminal alkynes to afford 1,4‐disubstituted products.[8]
Copper(I) salts have low thermodynamic stability, causing
the formation of alkyne–alkyne coupling products in reac-
tions between azide and terminal alkynes.[9,10] Therefore,
Reduction of copper(II) salts in the presence of sodium
ascorbate as reducing agent can generate in situ active
copper(I) catalytic species.[11–14] In this regard, several
sources of copper(II) have been used for this reaction such
as halide salts and coordination complexes in homogeneous
and heterogeneous form.[15–17]
Separation of copper source from final product is one
important drawback when a homogeneous form of copper is
used for click reactions, because the final products should
be completely pure without any copper contamination.
Design of heterogeneous catalysts is a useful way to realize
separation and improve the recovery and reuse of copper.
Therefore, developing an effective heterogeneous copper cat-
alyst with high stability, low leaching, high loading and clean
catalytic system is still required.
Immobilization of copper(II) onto polymer is one of the
best ways for the construction of heterogeneous catalysts with
high stability, high loading and high activity. In the last few
decades, several polymers have been designed and used as
Appl Organometal Chem 2016; 1–7
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

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BANAN ET AL.
supports for immobilization of copper(II).[18–24] Most of
these polymer‐based catalyst systems need a filtration or cen-
trifugation step to recover the catalyst. One of the best for
recovering a catalyst is the immobilization of the catalytic
system on magnetic nanoparticles making a magnetic hetero-
geneous catalytic system. Magnetic nanoparticles can be eas-
ily dispersed in a reaction mixture with intrinsically high
surface area providing efficient accessibility of substrates to
the surface.[25–27] Moreover, the most important advantage
of magnetic nanoparticles is the magnetic property that
enables their separation from a reaction mixture using an
external magnet and allowing reuse of the catalyst itself. This
method for separation from a reaction mixture offers a prom-
ising option that can meet the requirements of high accessi-
bility with improved reusability without the need for
filtration or centrifugation step.
Taking into account of the importance of triazoles, we
decided to design a new efficient polymer‐coated magnetic
catalytic system for the synthesis of 1,2,3‐trazole derivatives.
In the present paper, we report magnetic nanoparticles func-
tionalized with poly(4‐vinylpyridine) and with copper con-
tent as a recoverable heterogeneous catalyst for the
synthesis of 1,2,3‐triazoles.
SCHEME 1 Preparation of Mag‐Cu catalyst.
magnetization curves for both materials exhibit no hystere-
sis loop which demonstrates the superparamagnetic charac-
teristics. These results show that all magnetic materials can
be separated from a reaction mixture under an applied
magnetic field. Therefore, the catalyst can be easily recov-
ered and reused.
2
| RESULTS AND DISCUSSION
Figure 2 shows the X‐ray diffraction (XRD) pattern of the
Mag‐Cu catalyst. Diffraction peaks at 2θ = 30.5°, 35.8°,
43.4°, 53.9°, 57.4° and 63.1° correspond to (220), (311),
(400), (422), (551) and (440) diffraction planes of Fe3O4
magnetic nanoparticles, respectively. These peaks are in
accordance with the standard XRD data (JCPDS card num-
bers 89–43191, 19–0629, 79–0419).[28] The XRD result for
the Mag‐Cu catalyst indicates that the crystal structure of
the Fe3O4 core does not change during the functionalization
process.
In the first step, Fe3O4 nanoparticles were synthesized via
conventional co‐precipitation. For protecting the bare Fe3O4
from oxidation and agglomeration, SiO2 shell was used as
an inert material. (3‐Mercaptopropyl)trimethoxysilane was
employed to modify the surface of Fe3O4@SiO2 (SMNP) to
yield thiol‐modified silica‐coated magnetic nanoparticles
(SMNP‐SH). The presence of SH groups on surface of
SMNP makes SMNP active in polymerization reaction. In
order to immobilize poly(4‐vinylpyridine) on the magnetic
nanoparticles, copolymerization of monomer and SMNP‐
SH was performed in the presence of N,N′‐
methylenebisacrylamide (MBA) as cross‐linker and
ammonium persulfate (APS) as initiator (Scheme 1).
Finally, Cu(II) was loaded on the poly(4‐vinylpyridine)‐
functionalized SMNP by dispersion in a solution of CuCl2.
It was confirmed that nitrogen ligands greatly improve the
catalytic performance. Therefore, 4‐vinylpyridine was used
for polymerization to generate nitrogen ligands to stabilize
the copper and improve the catalytic performance. Also,
due to high loading of pyridine rings in the polymer, a
high absorption of copper ion was observed in comparison
with traditional heterogeneous catalysts.
Thermal analysis of the catalyst gives information about
its thermal stability. The thermogravimetric analysis (TGA)
After synthesis of the catalyst (Mag‐Cu), various tech-
niques were used for its characterization. Easy separation
of the catalyst depends on the magnetic property of the
nanoparticles. This is revealed from the enlarged vibrating
sample magnetometry curves shown in Figure 1 for
Fe3O4 and Mag‐Cu catalyst. As seen in Figure 1, the
FIGURE 1 Magnetization curves of (a) Fe3O4 and (b) Mag‐Cu catalyst.

BANAN ET AL.
3
Fourier transform infrared (FT‐IR) spectroscopy was uti-
lized to confirm the existence of organic groups on the sur-
face of the magnetic nanoparticles. FT‐IR spectra of SMNP‐
SH and Mag‐Cu catalyst are shown in Figure 4. Several new
sharp bands appear in the range 1200–1700 cm−1 in the
spectrum of the catalyst which are not present in the spec-
trum of SMNP‐SH. These peaks are attributed to C¼N,
C¼C, C¼O and CꢀC bonds in the polymer structure. The
peaks appearing at 556 and 1093 cm−1 are attributed to
FeꢀO and SiꢀO stretching vibrations of Fe3O4 and SiO2
in the catalyst, respectively. The peaks at 2929 and
2853 cm−1 are attributed to CꢀH of alkyl chains.
FIGURE 2 XRD pattern of Mag‐Cu catalyst.
The scanning electron microscopy (SEM) image
(Figure 5a) shows that the encapsulated nanoparticles are
present as almost uniform particles. The transmission elec-
tron microscopy (TEM) image of the prepared catalyst
(Figure 5b) shows that nanoparticles are dispersed in the
polymeric matrix. Also, TEM images show that the
nanoparticle size is less than 50 nm. Energy‐dispersive
X‐ray (EDX) analysis (Figure 5c) obviously shows the copper
peak that confirms the presence of copper ion in the cata-
lyst structure. In addition, the amount of copper in the
Mag‐Cu catalyst was measured using inductively coupled
plasma atomic emission spectrometry (ICP‐AES) to be
about 0.46 mmol g−1
.
FIGURE 3 TGA and differential thermogravimetry curves of Mag‐Cu
catalyst.
2.1 | Optimization of reaction
To investigate the catalytic activity of the Mag‐Cu catalyst,
we chose the synthesis of various 1,2,3‐triazole derivatives
catalysed by Mag‐Cu. In order to get an insight into the
optimum catalytic conditions, the reaction of sodium azide,
phenylacetylene, benzyl bromide and sodium ascorbate was
used as a model reaction. The effect of solvent, temperature
and amount of Mag‐Cu catalyst on the reaction was
curve (Figure 3) of the catalyst shows two weight loss steps.
The first step is below 150°C, attributed to residual
physisorbed water and/or organic solvents, which were
applied during the catalyst preparation. The second weight
loss is attributed to the decomposition of organic groups on
the magnetic nanoparticles. This decomposition starts at
250°C and complete degradation occurs up to 400°C which
corresponds to 56 wt% of the initial sample weight. This high
content of organic groups shows that large amounts of polymer
have been immobilized on the magnetic nanoparticles.
FIGURE 5 (a) SEM image, (b) TEM image and (c) EDX analysis of Mag‐
FIGURE 4 FT‐IR spectra of (a) SMNP‐SH and (b) Mag‐Cu catalyst.
Cu catalyst.

4
BANAN ET AL.
studied. In the first step, various solvents were applied in
this reaction in the presence of 1.0 mol% of Mag‐Cu cata-
lyst (Table 1, entries 1–11). The yield of the reaction
conducted in H2O–t‐BuOH (4:1) was higher in comparison
with other solvents and mixtures of solvents.
yield in convenient reaction time. Aliphatic alkynes such as
1‐heptyne also participate well in the reaction and produce
the corresponding triazoles in high yields, although the time
of reaction is longer than for other derivatives (Table 2,
entries 9–11). Various alkyl and benzyl halides were
subjected to the same reaction conditions as benzyl bromide
to produce the corresponding 1,2,3‐triazole derivatives.
According to the results summarized in Table 2, the
regioselectivity of the reaction is high in all cases, with only
the 1,4‐regioisomers being produced.
The amount of Mag‐Cu catalyst and the effect of temper-
ature were also investigated after choosing H2O–t‐BuOH as
the solvent. As evident from Table 1, no significant amount
of 1‐benzyl‐4‐phenyl‐1H‐1,2,3‐triazole is produced in the
absence of Mag‐Cu catalyst (entry 13). The yield of 1‐benzyl‐
4‐phenyl‐1H‐1,2,3‐triazole is 85% when the catalyst
amount is 0.5 mol%. As evident from Table 1, on increasing
the amount of catalyst to 2 mol%, the yield reaches 98%.
With an increase of the Mag‐Cu catalyst amount from 2 to
3 mol%, the yield of 1‐benzyl‐4‐phenyl‐1H‐1,2,3‐triazole
does not change markedly. Thus, we performed the reaction
in the presence of 2 mol% catalyst as the optimum amount.
At room temperature, the obtained yield of product is 79%
(entry 18). With an increase of temperature to 55°C, the yield
of product increases. Therefore, 2 mol% catalyst at 55°C in
H2O–t‐BuOH (4:1) were chosen as optimum conditions for
the synthesis of 1,2,3‐triazole derivatives.
2.2 | Catalyst recycling
The recyclability and reusability of a catalyst are important
aspects from an economic point of view. In this regard, a
recovery and reusability study of the catalyst was
performed for the production of 1‐benzyl‐4‐phenyl‐1H‐
1,2,3‐triazole. After the completion of each cycle, the cata-
lyst was separated using an external magnet and washed
with water and ethanol, dried, and without any purification
used in the subsequent reaction. According to Scheme 2,
only a slight reduction in the yield of 1‐benzyl‐4‐phenyl‐
1H‐1,2,3‐triazole is observed after nine consecutive runs.
The catalyst leaching was studied as follows. The
reaction was terminated by removal of the catalyst from
the reaction mixture at half the reaction time. Residual mix-
ture was kept back for reaction under the same conditions.
Our results indicate that the yield of product does not
increase. To determine the amount of copper leaching from
To investigate the generality and versatility of this
method, the optimized reaction conditions were subsequently
applied to the reaction of a variety of organic halides with
different aliphatic and aromatic terminal alkynes (Table 2).
It is observed that all reactions are undergone efficiently
and provide the corresponding products in good to excellent
TABLE 1 Effect of solvent, temperature and amount of catalyst on 1,3‐dipolar Huisgen cycloaddition reaction of benzyl bromide, phenylacetylene and sodium
azide
Entry
Catalyst
Amount of catalyst (mol%)
Solvent
Neat
Time (h)
T (°C)
Yield (%)b
1
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
CuCl
1
1
5
5
65
65
65
65
65
65
65
65
65
65
65
65
80
55
55
55
55
r.t.
55c
<5
52
2
DMF
3
1
THF
5
48
4
1
CH3CN
5
55
5
1
H2O
5
71
6
1
Toluene
5
39
7
1
Ethanol (96%)
t‐BuOH
5
64
8
1
5
59
9
1
H2O–t‐BuOH
H2O–THF
H2O–DMF
H2O–t‐BuOH
H2O–t‐BuOH
H2O–t‐BuOH
H2O–t‐BuOH
H2O–t‐BuOH
H2O–t‐BuOH
H2O–t‐BuOH
H2O–t‐BuOH
5
92
10
11
12
13
14
15
16
17
18
19
1
5
52
1
5
57
1
8
35
—
—
0.5
1
24
4
<4
85
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
Mag‐Cu
4
92
2
3
98
3
3
98
2
3
79
2
5
Trace
aReaction conditions: phenylacetylene (1 mmol), benzyl bromide (1 mmol), sodium azide (1.1 mmol), sodium ascorbate (10 mol%), solvent (5 ml).
bIsolated yield.
cWithout sodium ascorbate.

BANAN ET AL.
5
TABLE 2 One‐pot synthesis of 1,2,3‐triazoles from alkyl halides, sodium azide and terminal alkynesa
Entry
R
R′
Product
Time (h)
Yield (%)b
1
PhCH2–
PhCH2–
PhCH2–
4‐BrPhCH2–
C3H7–
Ph–
3a
3a
3a
3b
3c
3 (X = Br)
4.5 (X = Cl)
4 (X = OTs)
3 (X = Br)
3.5 (X = Br)
4 (X = Br)
7 (X = Cl)
6 (X = Br)
5.5 (X = Br)
3 (X = Br)
5 (X = Br)
5 (X = Br)
7 (X = Br)
4 (X = Br)
6 (X = Br)
5 (X = Br)
6 (X = Br)
6 (X = Br)
7 (X = Br)
98
96
97
95
90
93
93
88
95
93
94
92
89
97
93
98
95
94
73
2
Ph–
3
Ph–
4
Ph–
5
Ph–
6
Allyl–
Ph–
3d
3e
7
1‐Naphthyl–CH2–
(CH3)2C¼CHCH2–
PhCH2–
Ph–
8
Ph–
3f
9
C5H11
C5H11
C5H11
–
3 g
3 h
3i
10
11
12
13
14
15
16
17
18
19
Allyl–
–
–
4‐BrPhCH2–
PhCH2–
4‐MeOPh–
4‐NO2Ph–
Ph–
3j
PhCH2–
3 l
3 m
3n
3o
3p
3q
3r
NH2COCH2–
PhCOCH2–
C2H5OCOCH2–
PhCH2–
Ph–
Ph–
2‐NO2PhOCH2–
Ph–
C7H15
–
Azacrown ether–NCOCH2–
Ph–
aReaction conditions: phenylacetylene (1 mmol), benzyl bromide (1 mmol), sodium azide (1.1 mmol), sodium ascorbate (10 mol%), Mag‐Cu catalyst (2 mol% Cu), solvent
(H2O–t‐BuOH 4:1, 5 ml) at 55 °C.
bIsolated yield.
1,2,3‐triazole derivatives. Under the optimized reaction con-
ditions, a wide range of 1,2,3‐triazole derivatives were syn-
thesized from various benzyl halides and alkyl/aryl
acetylenes using the prepared catalyst. This catalyst can be
considered as a heterogeneous version of Cu(II) and easily
prepared from starting materials. The properties of high
activity, high stability, easy recoverability and reusability of
the catalyst mean that it is potentially valuable in industrial
applications. Also, the magnetic nature of catalyst eliminates
the need of catalyst centrifugation or filtration from the reac-
tion mixture.
In addition, complete removal of the prepared catalyst
causes total removal of copper ions from the reaction media
that makes it possible to use the procedure for pharmaceuti-
SCHEME 2 Recycling experiment of catalyst in the Huisgen reaction of
cals and drug discovery and other sensitive synthetic proce-
phenylacetylene and benzyl bromide.
dures. The catalyst was separated and reused for at least
nine cycles with high activity.
the catalyst, ICP‐AES analysis of the liquid phase was
conducted. The results of ICP‐AES analysis show no detect-
able amount of copper in the liquid phase. This result indi-
cates that copper ions are not leached from the catalyst
surface during the reaction.
4
| EXPERIMENTAL
4.1 | General
All materials used are commercially available and were pur-
chased from Merck and used without any additional purifica-
3
| CONCLUSIONS
1
tion. H NMR and 13C NMR spectra were recorded with a
In summary, we have developed a new magnetically separa-
ble nanocatalyst of copper immobilized on polymer‐coated
magnetic nanoparticles, which can be readily prepared from
simple materials. This catalyst was used for the synthesis of
Bruker (Avance DRX‐400) spectrometer using CDCl3 as sol-
vent at room temperature. Chemical shifts were measured in
ppm relative to tetramethylsilane as an internal standard.
FT‐IR spectra of samples were obtained using an ABB

6
BANAN ET AL.
Bomem MB‐100 spectrophotometer using potassium
bromide (KBr) pellets. The morphology of the catalyst was
observed using a KYKYEM3200 SEM instrument. An
EDX facility coupled to the microscope was used to identify
chemical elements of the prepared catalyst. TGA was
conducted under a nitrogen atmosphere with a TGA Q 50
thermogravimetric analyser. XRD patterns were recorded
with an APD 2000, using Cu Kα radiation (50 kV,
150 mA) in the range 2θ = 10–120°. Room‐temperature mag-
netization under an applied magnetic field was measured
10 min and then APS (0.1 g) was added. The flask was
equipped with a condenser and placed into an oil bath at
60°C. After 4 h, the solid product was magnetically collected
using an external magnet and washed three times with etha-
nol and water and dried in vacuum at 50°C. For immobiliza-
tion of copper ions, the synthesized product was dispersed
into tetrahydrofuran (20 ml) with sonication for 20 min and
then CuCl2 was added to the mixture and stirred for 6 h.
Finally, the product was separated with a magnet and dried
in vacuum at 60°C.
using
a
homemade vibrating sample magnetometer
(Meghnatis Daghigh Kavir Company, Iran) from −10 000
to +10 000 Oe. ICP‐AES was performed with a PerkinElmer
Optima 3300 DV.
4.6 | General procedure for synthesis of triazoles
To a glass tube was added sodium ascorbate (10 mol%)
followed by phenylacetylene (1 mmol), benzyl bromide
(1 mmol), sodium azide (1.1 mmol), catalyst (2 mol% Cu,
43 mg) and H2O–t‐BuOH (5 ml) which were dispersed using
ultrasound for 10 min. The mixture was left stirring at 55°C
for an appropriate time (Table 2). The completion of the reac-
tion was monitored by TLC. After the reaction was complete
the catalyst was separated from the reaction mixture using an
external magnet. The residue of the mixture was diluted with
water (30 ml) and extracted with ethyl acetate (3 × 10 ml).
The organic layer was dried over MgSO4 and then the solvent
was removed by rotary evaporation under vacuum to give the
corresponding triazole. For further purification, the product
was recrystallized from ethyl acetate–n‐hexane.
4.2 | Synthesis of magnetic nanoparticles
The synthesis of Fe3O4 was achieved using a procedure
reported in the literature.[25,29] FeCl2 (2.7 g) and FeCl3
(1 g) were dissolved in 25 ml of 1 M hydrochloric acid at
room temperature under mechanical stirring at room temper-
ature. After the salts dissolved completely, 20 ml of aqueous
ammonia (25%) was added dropwise over 20 min under
nitrogen atmosphere. With the addition of aqueous ammonia,
a black precipitate of magnetic nanoparticles was formed
immediately. After continuous mechanical stirring for 1 h,
the synthesized solid was separated using an external magnet,
washed with double‐distilled water (3 × 30 ml) and then vac-
uum‐dried at 60°C overnight.
For examining the recoverability of the catalyst, it was
separated by applying an external magnet and then washed
successively with ethanol and water. The catalyst was then
used directly for subsequent reaction without further
purification.
4.3 | Preparation of silica‐coated Fe3O4 (SMNP)
Typically, 1 g of the prepared magnetic nanoparticles was
suspended with ultrasonication in a mixture of ethanol
(80 ml) and water (20 ml), and the pH of the solution was
adjusted to 10 by adding aqueous ammonia.[29] Then,
tetraethoxysilane (TEOS; 2 ml) was added to the mixture
and stirred at 50°C for 12 h. The solid material was separated
and washed several times with ethanol and water and dried at
50°C overnight.
ACKNOWLEDGMENTS
We gratefully acknowledge the funding support received
from the Research Council of Azarbaijan Shahid Madani
University.
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