Copper nanoparticles supported on CeO2 as an efficient catalyst for click reactions of azides with alkynes

Article abstract of DOI:10.1016/j.catcom.2016.07.006

Readily prepared copper nanoparticles supported on CeO₂ have been found to effectively catalyse the 1,3-dipolar cycloaddition (CuAAC) of a variety terminal alkynes and organic azides generated in situ from sodium azide and different organic halides furnishing the corresponding 1,2,3-triazoles in excellent yields. Cu nanoparticles supported on CeO₂ have been characterized by X-ray diffraction analysis, energy dispersive X-ray analysis, scanning electron microscope and transmission electron microscope. The salient features of the present protocol are shorter reaction time, mild reaction conditions, reusability of the catalyst, and applicability to a wide range of substrates.

Full text of DOI:10.1016/j.catcom.2016.07.006

Catalysis Communications 85 (2016) 13–16
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Copper nanoparticles supported on CeO2 as an efficient catalyst for click
reactions of azides with alkynes
a
b
c
d,
Mojtaba Amini a, , Ramin Hassandoost , Mojtaba Bagherzadeh , Sanjeev Gautam , Keun Hwa Chae
⁎
⁎
a
Department of Chemistry, Faculty of Science, University of Maragheh, Maragheh, Iran
Chemistry Department, Sharif University of Technology, Tehran, P.O. Box 11155-3615, Iran
University Institute of Chemical Engineering & Technology (SSB UICET), Panjab University Chandigarh, 160-014, India
Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Readily prepared copper nanoparticles supported on CeO2 have been found to effectively catalyse the 1,3-dipolar
cycloaddition (CuAAC) of a variety terminal alkynes and organic azides generated in situ from sodium azide and
different organic halides furnishing the corresponding 1,2,3-triazoles in excellent yields. Cu nanoparticles sup-
ported on CeO2 have been characterized by X-ray diffraction analysis, energy dispersive X-ray analysis, scanning
electron microscope and transmission electron microscope. The salient features of the present protocol are
shorter reaction time, mild reaction conditions, reusability of the catalyst, and applicability to a wide range of
substrates.
Received 18 March 2016
Received in revised form 27 June 2016
Accepted 9 July 2016
Available online 12 July 2016
Keywords:
Nanoparticles
Copper
© 2016 Elsevier B.V. All rights reserved.
CeO2
1,3-dipolar cycloaddition
1. Introduction
With the aim to expand and improve catalytic applications of
CuNPs-based catalysts, we want to present herein preparation of copper
The 1,3-dipolar cycloaddition of organic azides and terminal alkynes
is one of the most attractive synthetic tools for convenient and reliable
construction of 1,2,3-triazole derivatives, whose subunits are often
found in a rich source of various biologically and pharmacologically ac-
tive molecules [1–5]. The most catalytically active metal for these reac-
tions is copper. Different copper sources have been described for the
1,3-dipolar azide-alkyne cycloaddition, including simple salts and tran-
sition metal complexes in homogeneous or heterogeneous form [6–13].
Many of these homogeneous catalysis processes, which often are the
first choice of chemists because of their high activity and selectivity,
however are not easy to handle, difficult to separate, and have limited
reuse potential often due to contamination with final products or for-
mation of metal-complexes [14–20]. Catalyst separation and reusability
of the catalyst is a critically important issue for the sustainable develop-
ment of any catalytic process [21–23].
To overcome this drawback, in recent years, due to higher stability
and dispersibility of nanoparticles as well as the demand for higher ac-
tivity and recyclability of catalysts, copper nanoparticles are supported
over a variety of organic and inorganic supports. Due to high reaction
rate and high turnover numbers (TON) of this type of catalysts, hetero-
geneous copper nano-catalysis has emerged as a sustainable and com-
petitive alternative to conventional catalysis [24–29].
nanoparticles supported on CeO2, and the study of its catalytic perfor-
mance in the 1,3-dipolar cycloaddition of organic azides and alkynes
(CuAAC).
2. Experimental
2.1. Materials
Chemicals and solvents were purchased from Merck and Fluka and
were used without further purification.
2.2. Synthesis of CeO2 nanoparticles
An aqueous solution (10 cm3) of Ce(NO3)3. 6H2O (1.0 g) was basified
with a NaOH solution (1.0 M; 10 mL), and a white precipitate of
Ce(OH)4 was obtained. The solid of Ce(OH)4 was washed with water
for several times and dried at 50 °C. 1.0 g of urea (as a fuel) was
mixed and pulverized with Ce(OH)4. Then the residual solid was ground
and calcinated at 400 °C for 5 h.
2.3. Synthesis of copper nanoparticles supported on CeO2 (Cu/CeO2)
Copper nanoparticles supported on CeO2 was prepared by an im-
pregnation method using Cu(NO3)2 as a source for Cu. The required
amount of Cu(NO3)2, to give 10 wt% Cu loading, was mixed and was
⁎
Corresponding authors.
E-mail address: mamini@maragheh.ac.ir (M. Amini).
http://dx.doi.org/10.1016/j.catcom.2016.07.006
1566-7367/© 2016 Elsevier B.V. All rights reserved.

14
M. Amini et al. / Catalysis Communications 85 (2016) 13–16
Fig. 1. a) XRD pattern of CeO2 and Cu/CeO2 nanoparticles; b) EDXS analysis of the final catalyst.
pulverized with CeO2. Methanol (20 mL) was added to the mixture of
Cu(NO3)2 and CeO2. This mixture was mechanically stirred to ensure
the homogeneous distribution of Cu(II) over the CeO2. Then, CH3OH
was evaporated using a rotary evaporator. After impregnation, Cu(0)
nanoparticles were obtained by reduction of the Cu(II)/CeO2 with hy-
drazine hydrate.
3. Results and discussion
3.1. Catalyst characterization
The XRD technique was used in order to identify the phases present
in the samples. In Fig. 1 XRD profiles of the CeO2 and Cu supported on
CeO2 samples are shown. XRD features allow to detect only the CeO2
Fm3m cubic fluorite phase while peaks related to Cu were not detected
(JCPDS file no. 34–0394) [30,31]. It can be known that the copper spe-
cies are well dispersed on the surface of CeO2. Also for CeO2 and Cu/
CeO2 no characteristic peaks from other impurities are detected in the
patterns demonstrating that the samples have high phase purity. Ener-
gy dispersive X-ray spectroscopy (EDXS) confirms the presence of 9.3%
Cu in Cu/CeO2 sample.
The Cu L-edge XANES spectra for Cu/CeO2 nanoparticles were shown
in Fig. S1. The peaks at 936 and 956 eV indicate L3 2p3/2–4 s and L2 2p1/
2–4 s transition, respectively. There is no experimental evidence of 2p3/
2 → 3d transition, clearly indicates that copper is mainly 0 oxidation
state on the CeO2 with an [Ar]3d104s1 like electronic configuration for
the Cu atoms the ground state.
2.4. General procedure for azide–alkyne cycloaddition
Alkyne (0.5 mmol), the organic halide (0.55 mmol), NaN3
(0.55 mmol) were added to a suspension of Cu/CeO2 in H2O (2 mL).
The reaction mixture was warmed to 70 °C and stirred for 4 h. The reac-
tion is monitored by TLC. After total conversion of the starting materials,
water (5 mL) was added to the resulting mixture and catalyst was re-
covered by filtration. Then the product was extracted with EtOAc
(2 × 10 mL). The collected organic phases were dried with anhydrous
Na2SO4 and the solvent was removed under reduced pressure to give
the corresponding triazoles, which did not require any further purifica-
tion. The catalyst must be washed with ethanol, and then dried in vacu-
um in order to use the next run without addition of any fresh catalyst.
Fig. 2. a) SEM images; b) TEM images of CeO2 and Cu/CeO2 nanoparticles.

M. Amini et al. / Catalysis Communications 85 (2016) 13–16
15
Table 1
to 1.8 mmol%, the yield of product increased dramatically from 68 to
95% (entries 2–7). With a further increase of the catalyst amount to
2.1 mmol%, no significant change in yield was observed (entry 8). Inter-
estingly, from the different solvents tested, water was found to be the
most effective for this reaction (entries 6 and 9–14). As indicated in
Table S1, a significant improvement in the product yield was observed
at higher temperatures than room temperature (entries 15–17). The re-
action is completed at 4 h and thus, using 6 mg of Cu/CeO2 catalyst
(1.8 mmol% Cu) and heating at 70 °C in water, the reaction of
phenylacetylene with benzyl chloride and sodium azide, gave 1-ben-
zyl-4-phenyl-1H-1,2,3-triazole almost quantitatively in 4 h of reaction
time (entry 19).
Cycloaddition of alkyl halides with terminal alkynes in the presence of Cu/CeO2.
Entry
1
Aliphatic halide
Alkyne
Yield (%)a
96
2
91
The same methodology was successfully applied to a wide range of
diversely substituted phenylacetylenes with a mixture of benzyl bro-
mides/chlorides and sodium azide to produce the corresponding 1,4-di-
substituted-1,2,3-triazoles. The results are summarized in Table 1. The
substitution of electron withdrawing and electrondonating groups on
the phenyl ring of the benzylhalides and the phenyl acetylenes did not
have any appreciable influence on the outcome of the reaction. Further-
more, minor steric hindrance was observed for ortho substituents on
the benzyl chloride as compared to the para derivative (entries 2 and
3). Aliphatic alkynes required longer reaction times to give high yields
compared with aromatic alkynes, but the reactions also gave good
yields when the reaction time was extended to 12 h (entries 7 and 8).
Replacement of benzyl chloride with benzyl bromide seem not to
have a decisive effect on the catalytic activity, as benzyl bromide reacted
to give the corresponding 1,2,3-triazole products in essentially quantita-
tive yields (entries 7–9), except for aliphatic alkynes (entries 12 and
13).
3
4
80
96
5
6
7
94
95
62
8
66
96
95
95
71
It is noteworthy that the degree of copper leaching during these ex-
periments was below the atomic absorption spectrometry detection
limit, since no presence of copper was detected in the aqueous phase
of the residue mixture.
9
The reusability of Cu/CeO2 nanoparticles was also examined. After
the reaction, the catalyst was separated by centrifugation, washed
three times with water and methanol, dried at 50 °C, and subjected to
the subsequent run. The catalyst can be used for three times without
dramatic yield loss. The comparison of efficiency of catalyst on repeated
use is reported in Fig. S2. Small losses of catalyst mass during washing
procedure may be caused in the low diminution in the activity from
the first to the third cycle. It is important to note that the total mass of
catalyst employed is very low (6 mg), thus the loss of only 1 mg of cat-
alyst when handling the sample, means loss of 17% of the total mass of
catalyst.
10
11
12
13
85
In order to show the merit and efficiency of the present catalytic sys-
tem, the catalytic behavior of Cu/CeO2 catalyst was compared with pre-
viously reported copper nanoparticles catalysts. As shown in Table 2,
the catalytic reactivity of Cu/CeO2 is superior to some of the previously
reported catalysts in terms of reaction conditions. In contrast to previ-
ously reported systems, the present catalytic system does not suffer
from the harsh reaction conditions, such as much amount of catalyst
(entries 2–5), addition of an additive agent (entries 3, 5) and using of
hazardous solvents (entries 3–5). Also other advantages of our catalytic
system are simple catalyst preparation and in situ generation of organic
azides from sodium azide and different organic halides.
a
Isolated yield.
Due to the smaller size of copper nanoparticles, the specific surface
area of the as-obtained Cu/CeO2 sample calculated from the BET curve
reaches about 8.13 m2 g−1
, which is bigger than that of
CeO2nanoparticles (6.40 m2 g−1).
SEM and TEM observations of Cu/CeO2 were carried out and shown
in Fig. 2. From the TEM image it can be seen that the particles are nearly
spherical with relatively uniform diameters and the particle size is
found to be 8–15 nm.
4. Conclusions
3.2. Catalytic effects
In conclusion, we have developed a simple and efficient protocol for
the synthesis of the corresponding 1,2,3-triazoles through 1,3-dipolar
cycloaddition of terminal alkynes with organic azides generated in situ
from sodium azide and different organic halides using copper nanopar-
ticles supported on CeO2 as an efficient and recyclable catalyst. The
above protocol has been performed efficiently to provide the desired
products in excellent yields. Simple operation, easy separation, wide
in scope, short reaction times, readily available starting materials and
The Cu/CeO2 catalyst demonstrated to be very efficient in the multi-
component 1,3-dipolar cycloaddition of terminal alkynes and organic
azides yielded in situ from sodium azide and different organic halides.
As shown in Table S1, phenylactelylene and benzyl chloride were cho-
sen as model starting compounds for the optimization of the reaction
conditions. Without catalyst, no product was obtained after 12 h
(entry 1). When the amount of the Cu was increased from 0.3 mmol%

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M. Amini et al. / Catalysis Communications 85 (2016) 13–16
Table 2
Recently reported catalytic systems for azide-alkyne cycloaddition by copper nanoparticles catalysts.
Entry
Catalyst
Conditions
Yield (%)
Ref.
1
2
3
4
5
Cu/CeO2
6 mg catalyst/H2O/4 h/70 °C
40 mg catalyst/H2O/2 h/70 °C
40 mg catalyst/PhNHNH2/THF/2 h/25 °C
60 mg catalyst/EtOH/5 h/25 °C
60 mg catalyst/Et3N/THF/30 min/65 °C
96
83
38
95
95
Present work
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Cu NPs/aminopropyl-silica
Cu NPs
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