Synthesis, characterization and catalytic properties of tetrachlorocuprate(II) immobilized on layered double hydroxide

Article abstract of DOI:10.1002/aoc.3710

Readily prepared copper(II) immobilized on layered double hydroxide has been found to effectively catalyse the 1,3-dipolar cycloaddition (CuAAC) of a variety terminal alkynes and benzyl azides generated in situ from sodium azide and benzyl halides furnishing the corresponding 1,2,3-triazoles in excellent yields. The advantages of the protocol are short reaction time, mild reaction conditions, reusability of the catalyst and applicability to a wide range of substrates.

Full text of DOI:10.1002/aoc.3710

Received: 14 October 2016
DOI 10.1002/aoc.3710
Revised: 17 November 2016
Accepted: 18 November 2016
F U L L PA P E R
Synthesis, characterization and catalytic properties of
tetrachlorocuprate(II) immobilized on layered double hydroxide
Mojtaba Amini1 | Mohammad Nikkhoo1,2 | S. Morteza F. Farnia2
1 Department of Chemistry, Faculty of Science,
Readily prepared copper(II) immobilized on layered double hydroxide has been
University of Maragheh, Maragheh, Iran
2 Department of Chemistry, University of Tehran,
found to effectively catalyse the 1,3‐dipolar cycloaddition (CuAAC) of a variety
terminal alkynes and benzyl azides generated in situ from sodium azide and benzyl
Tehran, Iran
Correspondence
Mojtaba Amini, Department of Chemistry, Faculty
halides furnishing the corresponding 1,2,3‐triazoles in excellent yields. The
advantages of the protocol are short reaction time, mild reaction conditions,
reusability of the catalyst and applicability to a wide range of substrates.
of Science, University of Maragheh, Maragheh,
Iran.
Email: mamini@maragheh.ac.ir;
mfarnia@khayam.ut.ac.ir
KEYWORDS
azide–alkyne cycloaddition, copper, layered double hydroxide
1
|
INTRODUCTION
2 | EXPERIMENTAL
1,3‐Cycloaddition reactions of azides with olefins provide
direct access to awide range of 1,2,3‐triazoles as useful hetero-
cyclic systems with remarkably stable aromatic structures,
which can serve, among other roles, as structural analogues of
apeptidelinkage.[1–10] Coppercatalystsacceleratethecycload-
dition reactions ofazides with such terminal alkynes bya factor
of at least 106 relative to the uncatalysed case[11] and copper‐
catalysed azide–alkyne cycloaddition (CuAAC) is a well‐
established methodology in modern organic synthesis.[12–20]
Layered double hydroxides (LDHs), also known as a class
of synthetic two‐dimensional nanostructured anionic clays,
are a family of materials that have attracted increasing attention
in recent years.[21–25] LDHs have anionic exchange capacity
with the ability to capture organic and inorganic anions that
makes them almost unique as inorganic materials for practical
applications in catalysis,[26] adsorption,[27] pharmaceutics,[28]
photochemistry[29] and other areas.[30] LDHs provide inexpen-
siveandpotentiallyrecyclablecatalystsupportsduetotheir rel-
ative ease of synthesis, high versatility and readily tailored
properties.[31–33]
2.1 | Materials
Chemicals and solvents were purchased from Merck and
Fluka and were used without further purification. LDH of
composition Mg1−xAlx(OH)2(Cl)x⋅zH2O and Na2CuCl4 were
prepared according to previously reported procedures.[34,35]
2−
2.2 | Preparation of LDH‐CuCl4
LDH (0.25 g) was suspended in a freshly prepared aqueous
solution (15 ml) of Na2CuCl4 (0.063 g, 0.25 mmol) and
stirred at 25 °C for 24 h under air. The solid catalyst was
filtered, washed thoroughly with water and air‐dried.
2.3 | Preparation of LDH‐Cu0
2−
LDH‐CuCl4 (0.3 g) was reduced with hydrazine hydrate
(0.3 g, 6 mmol) in ethanol (10 ml) for 3 h at room tempera-
ture. The product was filtered and washed with ethanol to
afford an air‐unstable brown‐black powder (0.21 g).
We were inspired by an earlier report of Choudary
3
|
RESULTS AND DISCUSSION
et al.[34] who synthesized a LDH‐supported Pd(0) catalyst
2−
via an exchange of PdCl4 followed by reduction with
2−
3.1 | Preparation of LDH‐CuCl4
hydrazine hydrate. Herein, we report the synthesis of LDH‐
supported nanocopper(0) via the same method and its
catalytic application in azide–alkyne cycloaddition in the
presence of water and air leading to excellent yields.
LDH was suspended in a freshly prepared aqueous solution
of Na2CuCl4 and tetrachlorocuprate (CuCl4
exchanged onto chloride‐saturated (Mg/Al) LDH to obtain
2−
)
was
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DOI 10.1002/aoc.3710

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AMINI ET AL.
green‐coloured LDH‐CuCl42−. The Cu(II) immobilized on
LDH was then reduced with hydrazine hydrate, giving an
air‐ unstable brown‐black powder of LDH‐Cu0.[36,37] On
exposure to air, LDH‐Cu0 is again oxidized to the LDH‐CuII
form as a green powder.
3.2 | Catalyst Characterization
The powder X‐ray diffraction (XRD) patterns of the stable
2−
synthesized materials LDH and LDH‐CuCl4 show peaks
at 11.2°, 22.6° and 32.7° which fit well to LDH with basal
reflections of (003), (006) and (009) without any crystalline
impurity phases (Figure 1).[38] No peaks attributed to copper
are observed in the XRD pattern of LDH‐CuCl42−. Elemental
analysis confirms the presence of 9.6% copper in the
2−
LDH‐CuCl4 sample.
The surface structure information of LDH and supported
copper particles was obtained using scanning electron
microscopy (SEM) as shown in Figure 2. The SEM images
2−
of LDH‐CuCl4 reveal the presence of many copper fine
particles with an average diameter of 50–70 nm covering
the surface of LDH. These results clearly show that CuCl42−
is successfully loaded on the surface of the LDH support,
which is expected to improve the catalytic properties.
The transmission electron microscopy (TEM) images
2−
presented in Figure 3 show that the LDH‐CuCl4 particles
2−
FIGURE
2
SEM images of (a, b) LDH and (c, d) LDH‐CuCl4
2−
nanoparticles
FIGURE 1 XRD patterns of LDH and LDH‐CuCl4 nanoparticles

AMINI ET AL.
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are largely spherical in shape with diameters ranging from
100 to 200 nm.
and interlayer water. The main bands related to intercalated
and adsorbed anions and the metal–oxygen ν(M─O─M)
stretching modes are, respectively, observed at 1000–1800
The Fourier transform infrared (FT‐IR) spectra of the
prepared samples of LDH and LDH‐CuCl4 are shown in
and 400–1000 cm−1 [39]
. The positions of all bands in the
2−
Figure 4. A broad band at 3000–3600 cm−1 in both spectra
is characteristic of O─H stretching of hydroxide basal layer
spectrum of LDH‐CuCl4 are almost coincident with those
in the spectrum of pure LDH, due to the fact that most bands
of CuCl42− almost coincide with the bands of LDH.
2−
3.3 | Catalytic Effects
Synthetic utility of prepared materials LDH and LDH‐
CuCl42− was further explored as catalysts in the azide–alkyne
cycloaddition reaction with the purpose of optimizing
the reaction conditions for dipolar cycloaddition of
phenylacetylene, benzyl chloride and NaN3 as model sub-
strates (Table 1). The investigation was initiated by compar-
ing the activity of LDH and LDH‐CuCl42−. The control
experiments in the absence of catalyst or pure LDH give no
conversion at 70 °C in water, suggesting the role of copper‐
catalysing click reactions (entries 1 and 3). With increasing
2−
LDH‐CuCl4 catalyst concentration from 0.001 to 0.004 g
we observe 97% isolated yield of 1‐benzyl‐4‐phenyl‐1,2,3‐
triazole after 12 h (entries 4–6). The yield of reaction
2−
decreases to 70% when LDH‐CuCl4
is replaced by
Na2CuCl4 (entry 2). As indicated in Table 1, at lower temper-
atures a modest yield is achieved, but a significant increase in
the product yield is observed at higher temperatures (entries
6–8). To determine the most effective solvent for 1,3‐dipolar
cycloaddition using LDH‐CuCl42− as catalyst, the reaction of
phenylacetylene, benzyl chloride and NaN3 was carried out
in various solvents such as acetone, chloroform, ethyl acetate,
ethanol, methanol and acetonitrile (entries 9–14). Water is
found to be the best solvent system for the azide–alkyne
2−
cycloaddition reaction in the presence of LDH‐CuCl4
.
Also, according to Table 1 (entry 16), the reaction is com-
pleted within 6 h.
CuAAC reactions were also carried out under optimized
conditions with various aromatic and aliphatic benzyl halides
and alkynes (Table 2). The presence of electron‐donating and
electron‐withdrawing groups on the phenyl ring of the benzyl
2−
FIGURE 3 TEM images of LDH‐CuCl4 nanoparticles
FIGURE 4 FT‐IR spectra of LDH and LDH‐CuCl42− nanoparticles

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AMINI ET AL.
TABLE 1 Effect of reaction conditions on azide–alkyne cycloaddition
Entry
Catalyst
Catalyst amount (g)
Solvent
Temperature (°C)
Time (h)
Yield (%)a
1
—
—
H2O
70
70
70
70
70
70
50
r.t.
70
70
70
70
70
70
70
70
12
12
12
12
12
12
12
12
12
12
12
12
12
12
4
0
70
2
Na2CuCl4
0.004
0.004
0.001
0.003
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
H2O
3
LDH
H2O
0
4
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
LDH‐CuII
H2O
58
5
H2O
68
6
H2O
97
7
H2O
82
8
H2O
31
9
Acetone
CH3Cl
EtOAc
C2H5OH
CH3OH
CH3CN
H2O
90
10
11
12
13
14
15
16
Trace
Trace
26
22
Trace
86
H2O
6
97
aIsolated yield.
2−
TABLE 2 Cycloaddition of alkyl halides with terminal alkynes in the presence of LDH‐CuCl4 catalyst
Entry
1
R1
X
R2
Yield (%)a
Ph
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Br
Ph
96
91
80
96
94
95
62
66
96
95
95
71
85
2
4‐CH3Ph
Ph
3
2‐CH3Ph
Ph
4
4‐NO2Ph
Ph
Ph
5
4‐CH3OPh
4‐CH3Ph
HOC(CH3)2
HOCH2
Ph
6
Ph
7
Ph
8
Ph
9
Ph
10
11
12
13
Ph
4‐CH3OPh
4‐CH3Ph
HOC(CH3)2
HOCH2
Ph
Ph
Ph
aIsolated yield.
halides and the phenylacetylenes does not greatly affect the
yield of cycloaddition products. When aliphatic alkynes are
coupled with benzyl halides (entries 7 and 8), a decrease in
the yield of cycloaddition products is observed, but the
reactions have good yields when the reaction time is extended
to 12 h. Replacement of benzyl chloride with benzyl bromide
does not have a decisive effect on the catalytic activity, as ben-
zyl bromide reacts to give the corresponding 1,2,3‐triazole
product in almost quantitative yields (entries 9–11), except
for aliphatic alkynes (entries 12 and 13).
a heterogeneous system for organic reactions. Indeed the
reusability of the LDH‐CuCl4 catalyst was investigated
2−
for the cycloaddition of benzyl chloride, NaN3 and
phenylacetylene (Figure 5). The concentration of both catalyst
and reactants were increased fivefold, and the experiment was
performed under the previously optimized reaction condi-
tions. After the first run, the product was extracted with ethyl
acetate, and the residual catalyst was reused for a new cycle of
the reaction under the same conditions. The results indicate
that it is possible to recycle and reuse the catalytic system
for a minimum of four cycles with relatively high catalytic
activity. The small decrease in the activity of the catalyst from
The lifetime of a catalyst and its level of reusability are
highly significant factors for the useful applications of such

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We have developed an efficient and simple protocol for the
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the synthesis of 1,2,3‐triazoles by 1,3‐dipolar cycloaddition
of terminal alkynes with benzyl azides generated in situ from
sodium azide and various benzyl halides. This protocol has
been performed efficiently to provide the desired products
in excellent yields. Simple operation, easy separation, short
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ACKNOWLEDGMENTS
Partial funding support was provided by the Research Coun-
cil of the University of Maragheh (M.A.) and Tehran Univer-
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How to cite this article: Amini M, Nikkhoo M,
F. Farnia S M. Synthesis, characterization and catalytic
properties of tetrachlorocuprate(II) immobilized on
layered double hydroxide. Appl Organometal Chem.
2017;e3710. doi: 10.1002/aoc.3710
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