A facile and rapid route for the synthesis of Cu/Cu2O nanoparticles and their application in the Sonogashira coupling reaction of acyl chlorides with terminal alkynes

Article abstract of DOI:10.1039/c4cy00868e

We have demonstrated a facile one-step synthetic strategy for the preparation of Cu/Cu₂O nanoparticles (NPs) via a microwave method. The microwave energy acts as a driving force for the synthesis of Cu/Cu₂O NPs, which makes the proce

Full text of DOI:10.1039/c4cy00868e

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A facile and rapid route for the synthesis of
Cu/Cu2O nanoparticles and their application
in the Sonogashira coupling reaction of acyl
chlorides with terminal alkynes†
Cite this: DOI: 10.1039/c4cy00868e
a
b
a
*
Manohar A. Bhosale, Takehiko Sasaki and Bhalchandra M. Bhanage
We have demonstrated a facile one-step synthetic strategy for the preparation of Cu/Cu2O nanoparticles
(NPs) via a microwave method. The microwave energy acts as a driving force for the synthesis of Cu/Cu2O
NPs, which makes the process economical. In the current methodology, Cu/Cu2O NPs were synthesized
within three minutes using copper(II) acetate as a precursor and 1,3-propanediol as the solvent. The
1,3-propanediol plays multiple roles in the reaction including solvent, reactant, promoter and capping
agent, and there is no need for any other additives. A structural and morphological study of the Cu/Cu2O
NPs was carried out using XRD, FEG-SEM, EDS, TEM, FT-IR, DSC-TGA and TPR techniques. This is one of
the simpler, faster, less expensive and greener approaches for the synthesis of nanocrystalline Cu/Cu2O.
Furthermore, the nanocrystalline Cu/Cu2O showed excellent catalytic activity in the Sonogashira coupling
reaction of alkynes with acyl chlorides.
Received 4th July 2014,
Accepted 27th July 2014
DOI: 10.1039/c4cy00868e
www.rsc.org/catalysis
multistep synthesis, the need for capping agents and reducing
agents, long reaction times, and the requirement of external
Introduction
Recently, there has been increasing interest in the synthesis
of size- and shape-controllable nanoparticles because of their
selective size and morphology, along with monodispersity in
their crystal structure which increases the catalytic activity.1
Microwave-assisted synthesis of colloidal inorganic nano-
materials is an important area of advanced nanoscience and
materials chemistry.2 Cu/Cu2O nanoparticles (NPs) are attrac-
tive materials due to their physical as well as chemical proper-
ties and their numerous applications. Cu/Cu2O NPs have
shown extensive applications in assorted fields such as cata-
lytic degradation of dyes,3 glucose sensing,4 catalysis,5 and
generation of hydrogen using ammonia-borane.6 Researchers
have reported various methods such as solvothermal methods,4
thermal decomposition7 and reduction methods8 for the
synthesis of Cu/Cu2O nano- and microparticles with different
sizes and morphologies. However, these methods have numer-
ous disadvantages, such as the use of high temperatures, the
utilization of toxic and costly reagents (NaBH4 and PVP),
additives including bases, stabilizers and promoters during
the reaction for the synthesis of Cu/Cu2O nanoparticles.
Recently, Liu and co-workers9 demonstrated the two-step
synthesis of Cu2O/Cu nanocomposites via a solvent-thermal
route using N,N-dimethylformamide as solvent in a Teflon-
lined stainless steel autoclave; however, the methodology has
one or more limitations, such as the high temperature (180 °C),
long reaction time (26 h) and multistep synthesis. Salavati-
Niasari and co-workers10 reported the synthesis of copper
and copper(I) oxide NPs by thermal decomposition. Zhang
and co-workers demonstrated the interfacial hydrothermal
synthesis of Cu@Cu2O core–shell microspheres.11 Even
though they achieved particles in the nano region, the
method has one or more of the same drawbacks mentioned
above. Therefore, there is a need to develop an alternative
method which is simple, facile, one-step, economical and
additive-free, without the use of templates or capping agents
and using a greener protocol, for the synthesis of nanoparticles.12
Nowadays, the synthesis of NPs by using microwave-
assisted routes is receiving significant attention owing to the
multiple advantages offered by microwave irradiation.2,13 In
our recent work, we showed the advantages of the microwave
method and the importance of microwave-assisted synthesis
of NPs.14 Microwave irradiation is a simpler, faster, one-step
and greener method for synthesis of NPs. Along with this, it
has several advantages such as providing volumetric heating,
energy efficiency, rapid reaction kinetics, homogeneity,
a Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai
400019, India. E-mail: bm.bhanage@ictmumbai.edu.in, bm.bhanage@gmail.com;
Fax: +91 22 3361 1020; Tel: +91 22 3361 2601
b Department of Complexity Science and Engineering, Graduate School of Frontier
Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561,
Japan. E-mail: takehiko@k.u-tokyo.ac.jp
† Electronic supplementary information (ESI) available: General information,
experimental procedure for the reaction, characterization of nanocatalyst,
analysis of product by GC-MS. See DOI: 10.1039/c4cy00868e
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selectivity, time economy, convenience, compactness of
equipment and better product yields.13a,14d Overall, consider-
ing all these advantages, microwave irradiation is a simpler,
less expensive and more economical method for the synthesis
of NPs compared to other conventional methods. The effi-
ciency of microwave heating is given by the following
equation:
in Fig. 1). The synthesized NPs were collected by decanting
the reaction solvent. The separated product was then washed
with distilled water and absolute ethanol several times and
dried in an oven at 70 °C for 1 h. The obtained NPs were
characterized by different analytical techniques.
Method of characterization of Cu/Cu2O NPs
The as-prepared Cu/Cu2O NPs were characterised using
an X-ray diffractometer (Shimadzu XRD-6100 using CuKα
radiation = 1.5405 Å) with a scanning rate of 2° per min and
2 theta (θ) angle ranging from 10° to 80° at current 30.0 mA
and voltage 40.0 kV. The morphology of the samples was
examined using field emission gun-scanning electron micros-
copy (FEG-SEM) analysis (Tescan MIRA 3 model) and trans-
mission electron microscopy (TEM) using a Philips CM-200,
operating at voltage 200 kV. The energy dispersive X-ray spec-
trum (EDS) was recorded by using an INCA x-act Oxford
instrument (Model 51-ADD0007). X-ray photoelectron spectra
(XPS) were monitored by using a PHI 5000 versaprobe scan-
ning ESCA microprobe. Fourier transform infrared (FT-IR)
spectra were measured on a Brucker Perkin Elmer-100 spec-
trometer. Temperature programmed reduction (TPR) was
recorded on a Thermo scientific TPDRO 1100 and differential
scanning calorimetry-thermogravimetric analysis (DSC-TGA)
was carried out using a Perkin Elmer STA 6000.
P = cE2fε″
where P is the microwave power dissipation per unit volume
in solvent, c is the radiation velocity, E is the electric field in
the material, f is the radiation frequency and ε″ is the dielec-
tric loss constant. ε″ is the most significant parameter that
determines the ability of a material to heat in the microwave
field. 1,3-Propanediol has a high ε″ value and a high boiling
point (217 °C).
In the present work, we have developed an effective, facile,
rapid, template-free, capping agent-free and additive-free
method for the synthesis of Cu/Cu2O NPs via a microwave-
assisted route under mild reaction conditions, using only
two reagents: Cu(CH3COO)2 as the starting material and
1,3-propanediol as the solvent. 1,3-Propanediol plays a funda-
mental role in the reaction by controlling the size and shape
of NPs. This is a facile technique for the synthesis of Cu/Cu2O
NPs via a microwave method, and it may also be further
extended to the synthesis of other metal oxides. We have also
shown the application of Cu/Cu2O NPs in catalysis, and they
exhibited good catalytic activity for the coupling of terminal
alkynes and acyl chlorides (benzoyl chlorides).
Results and discussion
Characterization of prepared Cu/Cu2O NPs
The general representation for the synthesis of Cu/Cu2O NPs
by microwave irradiation is shown in Scheme 1. The phase
identification of Cu/Cu2O NPs was carried out using an X-ray
diffractometer (Fig. 1), and shows Bragg's reflections at 43.3°,
Experimental section
Materials
A.R. grade copper acetate [Cu(CH3COO)2·H2O] and 1,3-propanediol
were procured from S. D. Fine Chemicals Pvt. Ltd. India. All
chemicals were highly pure and were used without further
purification.
Synthesis of Cu/Cu2O NPs
Cu/Cu2O NPs were synthesized by taking a mixture of 0.4 g of
Cu(CH3COO)2·H2O in 10 mL 1,3-propanediol in a 100 mL
glass beaker and keeping this inside a domestic microwave
oven (LG intellowave, operating at 100% power of 800 watts (W)
and a frequency of 2.45 GHz) for 3 min at electric power 600 W
with the on/off mode having a time interval of 30 s (Scheme 1).
The reaction progress was observed by the change in colour
of the reaction mixture, and finally it converted from blue to
brick-red, signifying the formation of Cu/Cu2O (inset image
Fig. 1 XRD pattern of Cu/Cu2O NPs with the reaction progress observed
Scheme 1 Synthesis of Cu/Cu2O nanoparticles by microwave irradiation.
by colour change (inset).
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50.4°, 74.1° which correspond to the (111), (200), (220) planes
of Cu, whereas diffraction peaks at 29.5°, 36.4°, 42.3°, 61.3°,
73.5°, 77.3° correspond to the (110), (111), (200), (220), (311),
(222) planes of Cu2O, respectively. The reaction progress was
observed by the gradual change in colour of the reaction mix-
ture during the reaction (inset Fig. 1). The obtained results
are in good agreement with JCPDS cards no. 04-0836 for Cu
and JCPDS cards no. 05-0667 for Cu2O.4b,8b The FEG-SEM
images (Fig. 2) clearly indicate the size and shape of the
NPs and show spherical as well as tubular morphology of the
Cu/Cu2O NPs. The morphology of the Cu/Cu2O NPs seems to
be slightly irregular and it was observed that the particles are
in the nano region, with particle size ranging from 70 nm to
110 nm, which leads to the availability of a high surface area
for catalytic activity (see ESI† Fig. S1, S2 and S3). To check
the changes in morphology, the microwave irradiation time
for the reaction was extended to 4 min and 6 min. However,
in both cases similar morphology and nearly the same parti-
cle sizes were observed (see ESI† Fig. S4 and S5). The particle
structure and size can also be determined by TEM analysis.
The TEM analysis was carried out by dispersing a Cu/Cu2O
powder sample in ethanol, which was then exposed to an
ultrasonication bath for 10 min. A drop from the resulting
slurry was deposited on a copper grid. The TEM images
Fig. 3 show the Cu/Cu2O NPs are in the nano region and
exhibit spherical as well as tubular morphology, along with
some aggregation. The morphology and particle size of the
Cu/Cu2O NPs from the TEM micrograph displays good simi-
larity to the result obtained by FEG-SEM (see ESI† Fig. S2).
The energy-dispersive X-ray spectrum (EDS) shows peaks only
for copper and oxygen, which indicates the prepared nano-
material is pure and free from impurities (Fig. 4a). To check
the complete reduction of the precursor into Cu NPs, the
Fig. 3 TEM images of Cu/Cu2O NPs.
microwave irradiation time for the reaction was extended to
4 min and 6 min; however, in both cases the presence of
copper and oxygen was observed in the EDS analysis which
indicated the presence of Cu2O (see ESI† Fig. S4 and S5). The
selected area electron diffraction (SAED) pattern of Cu/Cu2O
NPs points to the product being fine crystalline in nature
(Fig. 4b). Fig. 5a shows the typical survey spectrum of Cu/Cu2O
NPs, which shows the copper photoelectron peaks (Cu3p Cu3s,
Cu2p and its Cu LMM Auger), the oxygen peak (O1s) and the
photoelectron peak of the carbon (C1s). The major peaks in the
XPS survey spectrum show that the main elements present on
the surface of the Cu/Cu2O NPs were Cu, O and C. The carbon
peak results from the carbon tape used for XPS analysis. The
corresponding high resolution XPS spectrum of the O1s region
shown in Fig. 5b and the high resolution XPS spectrum of the
Cu2p region shown in Fig. 5c indicate the presence of Cu2O.
The peak at 931.8 eV is assigned to Cu2p3/2 and the peak at
951.35 eV is assigned to Cu2p1/2. The additional peaks attrib-
uted to Cu2p3/2 and Cu2p1/2 are the two shake-up satellite
peaks appearing at binding energies of 943.5 eV and 963.7 eV.
These peaks are diagnostic of an open 3d9 shell, correspond-
ing to the Cu+1 state. The thermal analysis was carried out by
DSC/TGA measurement of the synthesized Cu/Cu2O NPs in
an inert N2 atmosphere (20 mL min−1) for the range from
30 °C to 800 °C with a ramp rate of 10 °C min−1 (Fig. 6a). The
Fig. 2 FEG-ESM images of Cu/Cu2O NPs.
Fig. 4 a) EDS spectrum and b) SAED pattern of Cu/Cu2O NPs.
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Fig. 5 XPS spectra of Cu/Cu2O NPs: (a) survey spectrum, (b) O1s
region and (c) Cu2p region.
TGA curve (Fig. 6a, blue) shows a mass loss in the range of
80 °C to 120 °C which is due to the evaporation of moisture
absorbed by Cu/Cu2O nanopowder. The DSC curve (Fig. 6a,
green) shows the thermal stability of the Cu/Cu2O catalyst.
The DSC curve of the Cu/Cu2O exhibits an endothermic peak
at 217 °C which is due to the thermal decomposition of
1,3-propanediol (boiling point 217 °C), which was used as
the reaction solvent for the synthesis of Cu/Cu2O NPs. The
exothermic curve shows a mass gain in DSC at 285 °C due to
oxidation of the catalyst.15 According to Duval16 the oxidation
of Cu2O starts at about 285 °C and oxygen comes from the
decomposition gases. In Fig. 6b the sharp peak arising at
630 cm−1 in the FT-IR spectrum for Cu/Cu2O NPs is attributed
to the Cu–O stretching band. The H2-temperature programmed
reduction (H2-TPR) analysis of the as-synthesized Cu/Cu2O NPs
is shown in Fig. 7. In the H2-TPR analysis, the sample was
heated under a temperature ramp in a hydrogen environment,
and it was observed that the reduction of Cu2O to Cu(0) takes
place at 280 °C.
Fig. 7 H2-TPR curve of Cu/Cu2O NPs.
Scheme 2 Synthesis of ynones catalyzed by Cu/Cu2O NPs.
organic compounds.17 Recently, Sun et al. reported the
synthesis of ynones using silica gel and γ-Al2O3-supported
Cu NPs.18 From a survey of the literature, the synthesis of
ynones has been carried out using various catalysts such as
palladacycle complexes,19 FeBr2,20 Pd/C,21 Pd(PPh3)Cl2/CuI,22
PdNPs-PPS,23 PdCl2(PPh3)2,24 Pd(OAc)2,25 ZnBr2,26 palladium(II)
acyclic diaminocarbene complex,27 CuI/cryptand-22 complex,28
Pd(PPh3)4/ZnCl2,29 and polystyrene-supported palladium(0)
complex.30 However, some of the above protocols have several
disadvantages such as the use of various air and moisture
sensitive ligands (phosphine ligands), the necessity of addi-
tives and surfactants, high reaction temperature, the require-
ment of a co-catalyst and high catalyst loading. To overcome
these drawbacks, we have explored the catalytic activity
of Cu/Cu2O NPs for the synthesis of ynones, and it was
found that Cu/Cu2O NPs are an efficient catalyst for the
Sonogashira coupling reaction of acyl chlorides and alkynes
(Scheme 2). Notably, the reaction does not require any ligand
source and has a lower catalyst loading than other methods;
low reaction temperature and good to excellent yields of
the desired products make the protocol attractive. The litera-
ture reveals that both Cu(I) and Cu(0) act as catalysts for the
reaction.31 This is one of the more efficient, greener and
ligand-free protocols for the synthesis of ynones.
Evaluation of the catalytic activity of Cu/Cu2O NPs
The catalytic activity of the synthesized Cu/Cu2O NPs was
investigated for the synthesis of ynones using terminal
alkynes and acyl chlorides (Scheme 2). The ynone products
have great applications for the development of valuable
Initially, the reaction of phenylacetylene with benzoyl chlo-
ride was chosen as a model reaction, and the effects of
assorted parameters such as the catalyst loading, solvent,
base, temperature and time were studied. In the beginning,
Fig. 6 a) DSC/TGA curve and b) FT-IR spectrum of Cu/Cu2O NPs.
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Table 1 Optimization of reaction conditionsa
Entry
Catalyst (mol%)
Solvent
Base
Temp. (°C)
Time (h)
Yieldb (%)
Effect of catalyst loading
1
2
3
4
—
5
10
15
Toluene
Toluene
Toluene
Toluene
NEt3
NEt3
NEt3
NEt3
80
80
80
80
24
24
24
24
n.r.
60
92
94
Effect of solvent
5
6
7
8
10
DMSO
DMF
Acetonitrile
Dichloroethane
1,4-Dioxane
NEt3
NEt3
NEt3
NEt3
NEt3
80
80
80
80
80
24
24
24
24
24
24
Trace
40
Trace
32
10
10
10
10
9
Effect of base
10
11
12
13
14
15
10
10
10
10
10
10
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
—
80
80
80
80
80
80
24
24
24
24
24
24
n.r.
45
22
Trace
n.r.
20
N-Methyl morpholine
K2CO3
Na2CO3
DBU
KOH
Effect of temperature
16
17
18
19
10
10
10
10
Toluene
Toluene
Toluene
Toluene
NEt3
NEt3
NEt3
NEt3
70
90
100
90
24
24
24
24
66
95
84
62c
Effect of time
20
21
22
23
24
10
10
10
10
10
Toluene
Toluene
Toluene
Toluene
Toluene
NEt3
NEt3
NEt3
NEt3
NEt3
90
90
90
90
90
10
15
20
24
30
60
67
72
95
68
a
Reaction conditions: phenylacetylene (1 mmol), benzoyl chloride (1.2 mmol), nano-Cu/Cu2O (mol%), Et3N (2 mmol), toluene (2 mL),
N2 atmosphere. b GC yield. c Without N2 atmosphere; n.r. = no reaction.
the reaction was carried out in the absence of catalyst but the
reaction did not proceed (Table 1, entry 1). Subsequently,
we screened the catalyst loading using the model reaction
(Table 1, entries 2–4) and it was found that 10 mol% of catalyst
gave an excellent yield (92%) of the desired product (Table 1,
entry 3). Next, we screened various solvents such as toluene,
DMSO, DMF, acetonitrile, dichloromethane and 1,4-dioxane to
improve the reaction outcome (Table 1, entries 3, 5–9). Among
these solvents, the catalyst shows good activity in toluene
(Table 1, entry 3).
Furthermore, we have studied the reaction without using
base and it was observed that the reaction did not proceed
(Table 1, entry 10). After this, the effect of various bases such
as NEt3, K2CO3, Na2CO3, N-methyl morpholine, DBU and KOH
was studied (Table 1, entries 3, 11–15) and it was observed that
NEt3 gives an excellent yield of the desired product (Table 1,
entry 3). Subsequently, we investigated the effect of reaction
time and temperature (Table 1, entries 3, 16–24), and it was
found that the highest yield of the desired product was
obtained at 90 °C within 24 hours (Table 1, entry 17). It was
observed that the use of an inert atmosphere gives a good yield
of the desired product. However, the yield decreases in the
absence of the N2 atmosphere (Table 1, entry 19).
under an N2 atmosphere.32 With these optimized reaction
conditions, the scope of the developed protocol was extended
for the synthesis using various derivatives of alkyne and acyl
chloride (Table 2). The reaction with various derivatives of
phenylacetylene with substituents at the ortho-, meta- and
para-positions proceeded well (Table 2, entries 1–7). Cyclic
alkyne derivatives also gave good yields of the corresponding
products (Table 2, entries 8 and 9). Furthermore, benzoyl
chloride substituted with –Me, –OMe, –Cl and 2-naphthoyl
chloride also furnished good yields of the corresponding
products (Table 2, entries 10–14). However, substrates with a
strong electron-withdrawing group such as ortho-nitro-
benzoyl chloride gave a trace yield under the optimized reac-
tion conditions. Cyclic acyl chloride derivatives also smoothly
undergo the coupling reaction, providing a good yield of the
desired product (Table 2, entry 15).
Conclusions
In summary, a simple and facile protocol for the synthesis of
Cu/Cu2O NPs was developed with 1,3-propanediol as the
solvent using microwave irradiation. 1,3-Propanediol itself
has a versatile role as solvent, reactant, promoter and cap-
ping agent in the reaction. The reaction time is considerably
shorter compared to other existing methods and there is no
requirement for other extraneous chemicals for the synthesis
Thus, the optimized reaction conditions were phenylacetylene
(1 mmol), benzoyl chloride (1.2 mmol), nano-Cu/Cu2O
(10 mol%), Et3N (2 mmol) in toluene (2 mL) at 90 °C for 24 h
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Table 2 Substrate study for the coupling reaction of alkynes and acyl
chloridesa
Acknowledgements
Author M. A. Bhosale is grateful to the Council of Scientific
and Industrial Research (CSIR), India for financial support.
We thank the Department of Science and Technology (DST),
India for financial support for this work under Nano Mission
project no. SR/NM/NS-1097/2011. XPS measurements were
conducted at the Research Hub for Advanced Nano Character-
ization, the University of Tokyo, supported by the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan. This work is supported by JSPS and DST under the
Japan–India Science Cooperative Program.
Entry Acyl chloride Alkyne
1
Product
Yieldb (%)
95
2
3
4
5
6
7
86
90
99
95
98
93
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sealed tube containing a magnetic stir bar was charged with
phenylacetylene (1 mmol), benzoyl chloride (1.2 mmol),
nano-Cu/Cu2O (0.1 mmol, 10 mol%) as the catalyst, Et3N
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GC analysis (Perkin-Elmer, Clarus 400). After completion of
the reaction, the reaction mixture was filtered and the
filtrate was washed with sat. NaHCO3 solution (1 × 15 mL),
and the product was extracted with ethyl acetate (3 × 10 mL),
dried over Na2SO4 and evaporated under vacuum. All the
obtained products are well known in the literature and were
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comparison with the literature data.
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