Reduced graphene oxide supported Cu2O nanoparticles as an efficient catalyst for Sonogashira coupling reaction

Article abstract of DOI:10.1016/j.catcom.2017.07.020

Reduced graphene oxide supported Cu₂O nanoparticles were prepared by a liquid-phase reduction method and employed as an efficient heterogeneous catalyst for Sonogashira cross-coupling reaction. The catalyst exhibits high activity and selectivit

Full text of DOI:10.1016/j.catcom.2017.07.020

Accepted Manuscript
Reduced graphene oxide supported Cu2O nanoparticles as an
efficient catalyst for Sonogashira coupling reaction
Bing Wang, Yingyong Wang, Xiaoning Guo, Zhifeng Jiao,
Guoqiang Jin, Xiangyun Guo
PII:
DOI:
Reference:
S1566-7367(17)30321-7
doi: 10.1016/j.catcom.2017.07.020
CATCOM 5139
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Catalysis Communications
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Accepted date:
20 April 2017
7 July 2017
25 July 2017
Please cite this article as: Bing Wang, Yingyong Wang, Xiaoning Guo, Zhifeng Jiao,
Guoqiang Jin, Xiangyun Guo , Reduced graphene oxide supported Cu2O nanoparticles as
an efficient catalyst for Sonogashira coupling reaction. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2017), doi: 10.1016/j.catcom.2017.07.020
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ACCEPTED MANUSCRIPT
Reduced graphene oxide supported Cu2O nanoparticles as an
efficient catalyst for Sonogashira coupling reaction
Bing Wanga,b, Yingyong Wanga,*, Xiaoning Guoa, Zhifeng Jiaoa,b, Guoqiang Jina and Xiangyun
Guoa,*
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of
Sciences, Taiyuan 030001, China.
b University of the Chinese Academy of Sciences, Beijing 100039, China.
* E-mail: wangyy79@sxicc.ac.cn; xyguo@sxicc.ac.cn (X. Y. G).

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Abstract
Reduced graphene oxide supported Cu2O nanoparticles were prepared by a
liquid-phase reduction method and employed as an efficient heterogeneous
catalyst for Sonogashira cross-coupling reaction. The catalyst exhibits high
activity and selectivity towards the reaction of aryl halides with phenylacetylene
under palladium-free and ligand-free conditions. Cu2O nanoparticles could
activate phenylacetylene to produce copper(I) acetylide compounds, which
subsequently react with aryl halides to form the corresponding disubstituted
alkynes. The catalyst also shows a high stability and there is only slight decrease
of activity after five cycles.
Keywords: Cu2O nanoparticles, Reduced graphene oxide, Heterogeneous
catalysis, Sonogashira cross-coupling reaction

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1. Introduction
Sonogashira cross-coupling reaction between aryl halides and aryl alkynes is an
important way to construct C(sp2)-C(sp) bonds, which are very popular in
pharmaceuticals 1-5. Traditionally, palladium phosphine complexes with CuI as
co-catalyst possess high activity and selectivity for this cross-coupling reaction under
homogeneous condition 6-7. However, this system has a few drawbacks, such as
costliness of ligands, high Pd loading, homocoupling of reactants 6,8. Recently,
Cu(I)-catalyst as sole active component has been used for Sonogashira cross-coupling
reaction due to the low cost of copper salts 9-12. Zou et al. found that a variety of
copper catalysts could catalyze Sonogashira coupling reaction and the structure of
13
ligands had great effects on the catalytic performance of copper catalysts
.
Nevertheless, the use of ligands is harmful to the environment and copper catalysts
cannot be recycled due to the homogeneous reaction conditions. The copper-based
heterogeneous catalysts have also been applied in Sonogashira cross-coupling
reaction 14-15. Bhosale et al. reported that Cu/Cu2O nanoparticles prepared by a
microwave method showed excellent catalytic activity towards Sonogashira coupling
reaction of alkynes with acyl chlorides 14. However, the recyclability of the catalysts
was not studied. It is desirable to prepare effective and stable heterogeneous copper
catalysts for Sonogashira coupling reaction.
Graphene has attracted much attention due to its large specific surface area, good
chemical stability and excellent electroconductivity 16-17. Based on these excellent

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properties, graphene has been widely studied as catalyst support 18-19. Graphene can
disperse and stabilize nanoparticles and the two dimensional configuration of
graphene provides favorable accessibility of reactants to active centers 20. In our
previous work, graphene was found to be able to prevent the oxidation of Cu2O and
Cu nanoparticles due to the highly delocalized electrons over the grapphene sheets
21-22. Reduced graphene oxide supported Cu2O (Cu2O/RGO) catalyst showed excellent
23
activity and recyclability for Ullmann coupling reaction of phenols with aryl halides
,
indicating great potentials of this catalyst for more organic transformations. In this
work, the Cu2O/RGO catalyst was employed for Sonogashira cross-coupling reaction
between aryl halides and phenylacetylene. The catalyst exhibits not only excellent
activity and selectivity but also good stability in the cross-coupling reaction under
palladium-free and ligand-free conditions.
2. Experimental
2.1 Catalyst preparation. Cu2O/RGO was prepared by a liquid-phase reduction
method [21]. Firstly, 95 mg graphene oxide (GO) and 14 mg copper acetate were added
into 50 mL absolute ethanol. After sonicating for 2 h, the mixture was heated at 60 ᵒC
for 12 h. Secondly, the mixture was moved into 80 mL diethylene glycol in a flask
and heated to 140 ᵒC in oil bath. Then 2.5 mL deionized water was added into the
flask. After stirring for 30 min, the mixture was heated to 180 ᵒC and reacted for 2h.
Later, the mixture was cooled down to room temperature and washed with ethanol to
get the Cu2O/RGO catalyst. The theoretical loading of Cu2O was 5 wt%. Pure RGO
was also prepared by the reduction of GO using above-mentioned method.

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2.2 Sonogashira cross-coupling reaction. The cross-coupling reaction was
performed in a 25 mL stainless steel reactor. 1.1 mmol phenylacetylene, 1 mmol
iodobenzene or one of its derivatives, 2 mmol KOH, 10 mL N,N-Dimethylformamide
(DMF) and 50 mg Cu2O/RGO were added into the reactor. After purging the reactor
with Ar gas, the reaction was carried out at 150 ᵒC for 12 h. After that, the reaction
mixture was collected and centrifuged to separate the catalyst and the supernatant.
The supernatant was analyzed by BRUKER SCION SQ 456 GC−MS to measure the
concentration changes of reactants and products. The quantitative analysis of product
was detected by single ion monitoring mode in GC−MS.
2.3 Catalyst characterization. X-ray diffraction analysis (XRD) was charecterized
by a Rigaku D-Max/RB X-ray diffractometer using Cu Kα radiation. X-ray
photoelectron spectroscopy (XPS) was recorded on a AXIS ULTRA DLD
spectrometer using Al Kα radiation. Transmission electron microscopy (TEM) images
were observed on JEOL-2100 trasmission electron microscope at accelerating voltage
of 200 kV. The loading of Cu2O was measured by inductively coupled plasma-atomic
emission spectrometry (ICP-AES).
3. Results and discussion
Figure 1A shows the XRD patterns of GO, RGO and Cu2O/RGO. It can be seen
that the precursor GO has two sharp diffraction peaks at 11.7° and 42.5°
corresponding to (002) and (100) planes, respectively 19, 24. The (002) plane represents
the existence of various oxygen functional groups in GO 25. The small peak at 26.5°
indicates a little residual graphite in GO 26. Typical wrinkled morphology of GO can

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be seen in TEM images (Supplemental material, Figure S1A). When GO is reduced to
RGO, the peaks of GO disappear and two broad peaks of RGO at around 24° and 43°
arise 17,27. The broad peak at around 24° means that RGO has disordered sheet
structures and is comprised of single or a few layers 27. This is in accord with TEM
analysis of RGO (Supplemental material, Figure S1B). For Cu2O/RGO, the
diffraction peaks at 36.4°, 42.3° and 61.3° correspond to the (111), (200) and (220)
planes of Cu2O 14, respectively. Notably, a new pronounced peak appears at 23° in
Cu2O/RGO catalyst. It results from that the GO support is not completely reduced to
RGO during the Cu2O/RGO preparation process and there are still some oxygen
functional groups or structural defects in RGO 28. Figure 1 B and C show the TEM
images of Cu2O/RGO catalyst. It can be seen that Cu2O/RGO is thinner and more
transparent than RGO and Cu2O nanoparticles are homogeneously dispersed on the
surface of RGO sheets. The agglomeration of Cu2O nanoparticles and restack of RGO
layers are effectively controlled. The inset of HRTEM shows an interplanar spacing of
0.25 nm corresponding to the (111) plane of Cu2O nanoparticle 23. The X-ray
photoelectron spectrum is shown in Figure 1D. The peak at 932.7 eV is assigned to
Cu(I) 2p3/2 and the peak at 952.4eV corresponds to Cu(I) 2p1/2 29. The peak at 943.5
eV is shake-up satellite peak of Cu(I) 2p3/2 14. ICP-AES analysis demonstrates that the
loading of Cu2O nanoparticles is 4.73 wt%. These characterizations distinctly identify
that Cu2O/RGO has been successfully synthesized.
The reaction of iodobenzene with phenylacetylene was chosen as model reaction
to optimize the reaction conditions. The conversion of iodobenzene was used to

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measure the activity of Cu2O/RGO catalyst (Supplemental material, Table S1). First,
the effect of reaction time was investigated (Table S1, entries 1-3). The conversion of
iodobenzene was 38%, 59% and 84% at 8h, 10h and 12h, respectively. Lowering the
reaction temperature and the amount of catalyst resulted in a sharp drop of
iodobenzene conversion (Table S1, entries 3-8). The base used also had an obvious
effect on the cross-coupling reaction (Table S1, entries 9-12). The conversion of
iodobenzene was dramatically increased to 95% by using KOH as base. K2CO3 had
similar effect on the iodobenzene conversion as Cs2CO3. The weak bases, K3PO4 and
CH3COONa showed lower iodobenzene conversion of 79% and 48%, respectively.
Besides, we also checked the influence of solvent and found N,N-Dimethylformamide
(DMF) was the most suitable solvent for the Sonogashira reaction (Table S1,
entries13-15). Notably, the selectivities of diphenylacetylene in these reactions were
all above 97% and there was only tiny side product of diphenyldiacetylene, indicating
that Cu2O/RGO catalyst efficiently impeded the homocoupling of iodobenzene and
phenylacetylene. Thus, the optimum reaction conditions are 50 mg Cu2O/RGO
catalyst, 2 mmol KOH in 10 mL DMF at 150 ᵒC for 12 h (Table S1, entry 9).
A series of reactions between aryl halides with various functional groups and
phenylacetylene were performed over the Cu2O/RGO catalyst under the optimum
reaction conditions. As shown in Table 1, the catalyst exhibits high activity for the
reaction of substituted aryl iodides with phenylacetylene (Table 1, entries 1-10).
Different substituent groups have negligible effect on the reaction activity. However,
the catalytic activities of Cu2O/RGO for bromobenzene and chlorobenzene are still

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poor. The conversion is 11% and 0% for bromobenzene and chlorobenzene,
respectively (Table 1, entries 11-12).
Several controlled experiments were conducted for exploring the reaction
mechanism. No any product was detected when the cross-couping reaction was
performed in the absence of base or Cu2O/RGO. When the reactant was only
iodobenzene, it was converted to biphenyl with a conversion of 12%. For
phenylacetylene alone, no homocoupling reaction occurred. According to literature
30-32, Cu2O nanoparticles could react with phenylacetylene to form copper(I)
phenylaceylide, which exists in a polymeric form instead of homocoupling. Based on
the experimental data and existing references 9, 33-34, a plausible mechanism is
proposed for the Sonogashira reaction catalyzed by Cu2O/RGO as shown in Figure 2.
Cu2O nanoparticles could activate phenylacetylene to form copper(I) phenylaceylide
compounds in the presence of base. Then, the copper(I) phenylaceylide compounds
react with aryl halides after which the target product is produced.
The excellent recycle stability is important for heterogeneous catalysts. To
investigate the cycling stability of Cu2O/RGO, the obtained mixture after reaction was
centrifugated to get precipitate. After washed with deionized water and ethanol, the
precipitate was dried at 60 ᵒC in vacuum oven overnight. Then it is used in the next
run. From the result shown in Figure S2 (Supplemental material), it can be seen that
there is a slight loss of activity from 95% to 81% after five cycles. The XPS spectrum
of used catalyst (Supplemental material, Figure S3A) shows that there are two new
peaks at 934 eV and 953.8 eV appear, which correspond to Cu(II) 2p3/2 and Cu(II)

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2p1/2 of CuO, respectively 29. It means Cu2O was partly oxidized to CuO. No obvious
change in morphology is observed for Cu2O/RGO catalyst after five cycles by TEM
analysis (Supplemental material, Figure S3B). Besides, ICP analysis shows the
amount of Cu2O after five reaction cycles is reduced to 4.08 wt% compared to the
original amount of 4.73 wt%. Therefore, the loss in activity may be caused by both
partial oxidation of Cu2O nanoparticles and leaching of Cu2O in the strong basic
conditions.
4. Conclusions
In this paper, we demonstrate that Cu2O nanoparticles supported on reduced
graphene oxide can act as an efficient heterogeneous catalyst for Sonogashira
cross-coupling reaction. Under palladium-free and ligand-free conditions, the
Cu2O/RGO catalyst exhibits excellent catalytic performance for the reaction of aryl
iodides with phenylacetylene. Graphene can effectively stabilize Cu2O nanoparticles
during the Sonogashira cross-coupling reaction. In the catalytic processes, Cu2O
nanoparticles could activate phenylacetylene to form copper(I) acetylide compounds,
which subsequently react with aryl halides and produce diphenylacetylene in the
presence of base. There is a slight decrease of activity after five reaction cycles due to
the partial oxidation and leaching of Cu2O under the strong basic conditions. The
Cu2O/RGO heterogeneous catalyst provides a new direction to replace noble metal
palladium catalyst for the Sonogashira cross-coupling reaction.
Acknowledgements
The work was financially supported by NSFC (21473232 and 21403270), Shanxi

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Province (MC2014-01) and CAS Youth Innovation Promotion Association (2013115).
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Figure Captions:
Figure 1 (A) XRD patterns of GO, RGO and Cu2O/RGO catalyst, (B, C) TEM
images of Cu2O/RGO catalyst and (D) XPS spectra of Cu2O/RGO. The inset in (C) is
a HRTEM of Cu2O nanoparticles.
Figure 2 Proposed mechanism for the Sonogashira reaction of aryl halides and
phenylacetylene over Cu2O/RGO.
Table 1 Sonogashira reactions of various aryl halides with phenylacetylene over
Cu2O/RGO.

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Figure 1
Figure 2

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Table 1 Sonogashira reactions of various aryl halides with phenylacetylene over
[a]
Cu2O/RGO.
Entry
1
Aryl halides
Products
Conv. /% Select. /%
95
88
99
96
2
3
4
97
98
98
94
5
6
7
8
9
92
90
97
99
92
99
98
95
99
97

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10
11
12
86
11
0
95
98
0
[a] Reaction conditions: 1 mmol aryl halide, 1.1 mmol phenylacetylene, 2 mmol KOH,
50mg Cu2O/RGO catalyst, 10 mL DMF at 150 ᵒC for 12 h.

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Graphical Abstract

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Highlights:
 Cu2O/Reduced graphene oxide catalyst was prepared by a liquid-phase
reduction method.
 The catalyst exhibited high activity and excellent selectivity for Sonogashira
reaction.
 The catalyst showed good recycle stability.