A one-step method for preparation of Cu@Cu2O nanoparticles on reduced graphene oxide and their catalytic activities in N-arylation of N-heterocycles

Article abstract of DOI:10.1016/j.apcata.2014.04.023

Core-shell Cu@Cu₂O nanoparticles on reduced graphene oxide (Cu@Cu₂O NPs-RGO) nanocomposite were prepared at room temperature by in situ reduction of graphene oxide (GO) and copper sulfate using l-ascorbic acid as reducing agent and t

Full text of DOI:10.1016/j.apcata.2014.04.023

Accepted Manuscript
Title: A one-step method for preparation of Cu@Cu2O
nanoparticles on reduced graphene oxide and their catalytic
activities in N-arylation of N-heterocycles
Author: Siyavash Kazemi Movahed Minoo Dabiri Ayoob
Bazgir
PII:
S0926-860X(14)00264-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.04.023
APCATA 14796
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
21-9-2013
9-4-2014
12-4-2014
Please cite this article as: S.K. Movahed, M. Dabiri, A. Bazgir, A one-step method
for preparation of Cu@Cu2O nanoparticles on reduced graphene oxide and their
catalytic activities in N-arylation of N-heterocycles, Applied Catalysis A, General
(2014), http://dx.doi.org/10.1016/j.apcata.2014.04.023
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A one-step method for preparation of Cu@Cu2O nanoparticles on reduced
graphene oxide and their catalytic activities in N-arylation of N-heterocycles
Siyavash Kazemi Movahed, Minoo Dabiri,* Ayoob Bazgir*
Department of Chemistry, Shahid Beheshti University, G.C. Tehran, Iran.
[Phone: +98-21-2990-3104; Fax: +98-21-2243-1661; e-mail: a_bazgir@sbu.ac.ir]
ABSTRACT
Core-shell Cu@Cu2O nanoparticles on reduced graphene oxide (Cu@Cu2O NPs-RGO)
nanocomposite were prepared at room temperature by in situ reduction of graphene oxide (GO)
and copper sulfate using L-ascorbic acid as reducing agent and then the air oxidation of copper
nanoparticles’ surface. The Cu@Cu2O NPs–RGO nanocomposite was characterized by infrared
spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy,
transmission electron microscopy, and energy dispersive X-ray spectroscopy. Finally, Cu@Cu2O
NPs-RGO nanocomposite was used as an efficient and reusable heterogeneous catalyst in N-
Arylation of N-heterocycles.
Keyword: Graphene; Copper; N- Arylation; Nanoparticle; L-Ascorbic acid
1- Introduction
The ability for efficient synthesis of N-arylated heterocycles is currently an active area in
organic synthesis due to the prevalence of this structural motif in numerous bioactive important
natural bioactive products and pharmaceutically interesting compounds [1,2]. Traditionally, N-
arylated heterocycles have been prepared by Ullmann-type coupling of N–H containing
heterocycles with aryl halides [3]. The classic Ullmann reaction normally requires harsh
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conditions, such as high temperature (typically above 140 oC), stoichiometric amounts of copper
and selective halide substrates, which is problematic for industrial use because of high cost and
waste disposal. This protocol had other limitations such as low yields, limited scope, and lack of
selectivity [4,5]. To overcome these drawbacks, a number of efforts have been made recently to
develop a new catalytic system. A recent notable achievement has been the use of various
copper-chelating ligands [1,6]. However, most of the successful copper-catalyzed N-arylation
protocols suffer from serious limitations such as non-reusability of the catalysts, possible
contamination of the product with metal, and usage of toxic and/or expensive ligands [7].
Ligand-free N-arylation reactions are rare, and generally they involve chelating substrates or they
require (over)stoichiometric quantities of copper salts [8]. In principle, the use of heterogeneous
catalysts could overcome some of these drawbacks mentioned above. Heterogeneous metal
nanoparticles (MNPs) including CuO and Cu2O have recently gained considerable attention
because they exhibit very good activity and reusability [9,10]. In spite of their advantages,
drawbacks remain in these catalysts too, as the MNPs easily agglomerate during the reaction and
also less stability under severe reaction conditions, which lead to low catalytic activity as well as
poor reusability of the catalyst [11]. In addition, higher stoichiometric amounts of Cu catalyst are
often required. To overcome these disadvantages, anchoring the Cu catalyst onto a suitable
support that would allow for the catalyst to be easily separated and recycled with minimal
contamination of the product with metal is used. Recently, several different solid supports with
copper anchored to their surfaces provide both the advantages of simplified isolation of the
product, and easy recovery and recyclability of the catalysts [12-16]. Therefore, it is important to
develop an efficient and recyclable catalyst for the N-arylation of heterocycles with the use of
lower amounts of Cu.
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Graphene has attracted great interest for its excellent mechanical, electrical, thermal and
optical properties [17]. Graphene with the large theoretical specific surface area [18], high
Young’s modulus [19], and excellent thermal conductivity [20] and has great potential in the
development of new composite materials [21], especially as a substrate to host metal and metal
oxide nanoparticles [22]. Chemical reduction is the most popular strategy for the synthesis of
graphene–metal nanocomposites from GO and metal salts. Hydrazine hydrate, hydroxyl-amine,
and sodium borohydride are frequently used as reductants [22]. However, precautions must be
taken when large quantities of these highly toxic and potentially explosive reductants are used
[23,24]. Due to significant advantages of graphene, a few reports have involved the application
of graphene based nanocomposites as heterogeneous catalysts in organic transformations [25].
In this work, a simple, environmentally benign, and cost effective method for the preparation
of Cu@Cu2O NPs–RGO nanocomposite by in situ reduction of GO and copper sulfate using L-
ascorbic acid as an inexpensive and nontoxic reducing agent [26] with subsequent surface
oxidation of Cu NPs by air is reported (Scheme 1). L-ascorbic acid is a very effective reducing
reagent for converting copper salts to copper nanoparticles [27,28] as well as efficiently
converting GO to reduced graphene oxide [26]. The Cu@Cu2O NPs-RGO nanocomposite was
used as an efficient and reusable heterogeneous catalyst for N-Arylation of N-heterocycles.
Additionally, the effects of the solvent polarity, base, and temperature on yield, and recycling
potential of the catalyst have all been assessed.
Scheme 1. Illustration of the synthesis procedure of Cu@Cu2O NPs-RGO nanocomposite.
2. Experimental
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2.1. Characterization
All chemicals were purchased from commercial suppliers and all solvents were purified
and dried using standard procedures. IR spectra were recorded on a Bomem MB-Series FT-IR
spectrophotometer. Diffraction data were collected on a STOE STADI P with scintillation
detector, secondary monochromator and Cu Kα radiation (λ =1. 5406 Å). XPS analysis was
performed using a Gammadata-scienta ESCA 200 hemispherical analyzer equipped with an Al
Ka (1486.6 eV) X-ray source. Raman spectra were recorded on a Bruker SENTERR (2009) with
an excitation beam wavelength at 785 nm. Transmission Electron Microscopy characterization of
GO, Cu@Cu2O NPs-RGO and Cu@Cu2O NPs-RGO reused composite were performed using a
1
transmission microscope Philips CM-30 with accelerating voltages of 150 and 250 kV. HNMR
spectra were recorded on a BRUKERDRX-300AVANCEspectrometer at 300.13 MHz,
1
respectively. HNMR spectra were obtained in DMSO-d6 using TMS as the internal standard.
Melting points of products were measured on an Elecrtothermal 9100 apparatus and are
uncorrected. The concentration of Copper was estimated using Shimadzu AA-680 flame atomic
absorption spectrophotometer. Gas chromatography was performed on a Trace GC ultra from the
Thermo Company equipped with FID detector and Rtx®-1 capillary column.
2.2. Synthesis
2.2.1. Preparation of Graphene Oxide (GO)
The graphite powder (2.5 g) was first treated with a mixture of 12.5 ml of concentrated
H2SO4 with 2.5 g K2S2O8 and 2.5 g P2O5. The mixture was kept at 80 ºC for 6 h. Subsequently,
the mixture was cooled to room temperature and diluted with 500 mL de-ionized (DI) water and
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left overnight. The mixture was then filtered and washed with DI water to remove the residual
acid. The product was dried under ambient conditions overnight. The pre-oxidized graphite was
then subjected to oxidation by Hummers’s method. The pretreated graphite powder was put into
cold (0 ºC) concentrated H2SO4 (125 ml). Then KMnO4 (15 g) was added gradually under
stirring, and the temperature of the mixture was kept below 20 ºC by cooling. The mixture was
then stirred at 35 ºC for 4 h and then diluted with DI water (250 ml). Because adding water to
concentrated sulfuric acid medium releases a large amount of heat, the dilution was carried out in
an ice bath to keep the temperature below 50 ºC. After adding all of the 250 mL DI water, the
mixture was stirred for 2 h, and then an additional 750 mL DI water was added. Shortly
thereafter, 20 ml 30% H2O2 was added to the mixture and the color of the mixture changed into
brilliant yellow and began bubbling. The mixture was filtered and washed with 0.1 M HCl to
remove metal ions, followed by 500 mL DI water to remove the acid, and then was dialyzed
against DI water. The resulting GO solid was dried in air. [29,30]
2.2.2. Preparation of Reduced Graphene Oxide (RGO)
The GO (100 mg) was ultrasonically dissolved in 20 mL of water. Then L-ascorbic acid
(15 mmol) was gradually added into the solution and stirred for 24 h. The resulted precipitation
was collected, washed with water (2 × 10 ml) and ethanol (5 × 10 ml) and dried at RT.
2.2.3. Preparation of Cu@Cu2O NPs -RGO nanocomposite
The GO (100 mg) was ultrasonically dissolved in 20 mL of water. Then NaOH (1 mmol)
was dissolved in the solution under stirring to form a uniform dispersion. After that, CuSO4 (0.5
mmol) was added into the solution, the mixture was ultrasonically treated for 30 min. L-ascorbic
acid (15 mmol) was gradually added into the solution and stirred for 24 h. The resulted
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precipitation was collected, washed with water (5 × 10 ml) and Ethanol (3 ×10), and was
exposed to air for 4h. The atomic absorption spectroscopy (AAS) analysis gave the actual Cu
contents as 26.2 wt.% for Cu@Cu2O NPs-RGO nanocomposite.
2.2.4. General procedure for N-Arylation of N-heterocycles
A mixture of Cu@Cu2O NPs nanocomposite (5 mol % of Cu), Cs2CO3 (1.5 mmol), N-
heterocycle (1.0 mmol), aryl halide (1.0 mmol), and DMSO (2 mL) under air was stirred for 1 h
at 110 ºC. After completion of the reaction as indicated by TLC, the heterogeneous mixture was
cooled to room temperature and diluted with Ethyl acetate (10 mL). The mixture was filtered
through a pad of celite. The filtrate was concentrated and then residue was purified by column
chromatography (SiO2, Ethyl acetate and n-Hexane) to yield pure product. The catalysts were
recovered by simple filtration and washed extensively with acetone and deionized water and then
drying in the air.
3. Results and discussion
3.1. Characterization of the Cu@Cu2O NPs-RGO nanocomposite
The FT-IR spectra of GO and Cu@Cu2O NPs-RGO are shown in Fig. 1. In the FT-IR
spectroscopy of GO, the characteristic bands at 1036, 1232, 1402, 1623 and 1735 cm−1
correspond to the C–O–C stretching vibrations, the C–OH stretching peak, the O–H deformation
of the C–OH groups, the C=C stretching mode and the C=O stretching vibrations of the –COOH
group, respectively [31]. The FT-IR spectroscopy of Cu@Cu2O NPs-RGO nanocomposite
indicate that the peak at 1735 corresponding to the stretching vibration peak of C=O disappear
[32]
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Fig. 1. IR spectra of GO and Cu@Cu2O NPs-RGO nanocomposite
In the Raman spectra of GO and Cu@Cu2O NPs-RGO nanocomposite two bands were
observed around 1318 and 1592 cm-1, which correspond to the disorder-induced D band and the
in-phase vibration of the graphene lattice (G band) of graphene oxide, respectively (Fig. 2). The
intensity ratio of D and G peaks, I(D)/I(G), is used as a measure of the disorder or restoration of
the graphene lattice and expressing the sp2/sp3 carbon ratio. The I(D)/I(G) value variation is
inversely proportional to the average crystallite size in graphite materials. Accordingly, while the
G mode occurs at all sp2 sites, the intensity of D mode is strictly connected to the presence of
six-fold aromatic rings [33]. The I(D)/I(G) intensity ratio of Cu@Cu2O NPs-RGO composite
(1.49) is slightly higher than the GO (1.40) suggests a decrease in the average size of sp2
domains and a high concentration of defects, possibly caused by the reduction process [34-36].
Additionally, the D band enhancement suggests interaction between the metal nanoparticles and
graphene [37-41] and occurs due to the preferential growth of nanoparticles at graphene
vacancies [42]. Recently, the results of atomic force microscopy (AFM) based dynamic force
measurements and density functional theory based calculations shows that the interaction of the
graphene surface with Cu cluster indicates the formation of charge-transfer complexes [43].
Fig. 2. Raman spectra of GO and Cu@Cu2O NPs-RGO nanocomposite.
X-ray Diffraction (XRD) patterns of GO and Cu@Cu2O NPs-RGO are displayed in Fig.
3a-b. It can be seen that GO showed a strong diffraction peak centered at 2θ=10.9o
corresponding to the (001) interlayer spacing of 0.80 nm (Fig. 3a). In the XRD pattern of
Cu@Cu2O NPs-RGO where the major peaks at 2θ= 43.2º, 50.4º, 74.1º can be assigned to the
(111), (200), (220) lattice planes of Cu (0) that are consistent with the presence of face-centred
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cubic copper (JCPDS No: 4-0836) and also three minor diffraction peaks 2θ= 36.4º, 42.4o and
61.5 can be assigned to the (111), (200) and (220) lattice planes of Cu2O (PDF No: 01-078–
2076) (Fig. 3b). An additional broad diffraction peak appears at 2θ=19.8–28.7o which can be
indexed to the disordered RGO sheets, with the disappearance of 2θ=10.9o, indicating the
reduction of GO by L-ascorbic acid.
Fig. 3. XRD patterns a) GO and b) Cu@Cu2O NPs-RGO nanocomposite.
In order to investigate the chemical composition of the Cu@Cu2O NPs-RGO
nanocomposite, XPS spectra were recorded for GO, RGO (prepared under the same method) and
Cu@Cu2O NPs-RGO nanocomposite (Fig. 3). As is expected, all the three samples show a C 1s
peak and O 1s peak at 284.6 and 531.3 eV, respectively (Fig. 4). The XPS survey spectrum of
the Cu@Cu2O NPs-RGO nanocomposite exhibits the Cu 2p (935.0 eV) peak with its
corresponding binding energy, additionally.
Fig. 4. Full range XPS spectrum of GO, RGO and Cu@Cu2O NPs-RGO nanocomposite.
The XPS spectrum of Cu 2p core level region for Cu@Cu2O NPs-RGO nanocomposite
displays main peaks at 934.4 and 954.0 eV which are attributed to the binding energy of Cu 2p3/2
and Cu 2p1/2, respectively (Fig. 5). The broad Cu 2p3/2 peak has been deconvoluted into two paks
which are marked as peaks at 933.6 and 935.6 eV, and assigned to Cu2O/Cu (Cu1+/Cu0) and
CuO/Cu(OH)2 (Cu2+), respectively. Actually, it is difficult to differentiate Cu2O (Cu1+) and Cu0
by the XPS feature of Cu 2p3/2 as the binding energies of Cu and Cu2O are very close and
different by only 0.1-0.2 eV. One can distinguish them from the position of their LMM auger
transition in XPS spectra which are 568 eV and 570 eV for Cu and Cu2O, respectively [44].
There is a peak at about 570.2 eV in the XPS spectra of Cu@Cu2O NPs-RGO nanocomposite.
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According to above mentioned fact, one can conclude that peak at 933.6 eV is related to Cu2O.
Also, a shake-up satellite at 943.5 eV was observed in XPS spectrum. This peak is typically
associated with copper in the bivalent oxidation state, which is assigned to Cu2+ in CuO (or
probably Cu(OH)2 species). From these results it is inferred that the surface of copper
nanoparticles is oxidized. Based on the difference between the two XRD and XPS spectra it can
be reasoned that XPS only detects signals within the upper 5 nm thickness on the surface, while
the XRD can detects signals in micro-order thickness [45].
Fig. 5. XPS spectrum of the Cu 2p core level region of Cu@Cu2O NPs-RGO nanocomposite.
The presence of O-containing surface functional groups was also identified by XPS
measurements (Fig. 6). The broad O1s peak of GO was deconvoluted into three peaks: C=O at
530.4 eV, C(O)OH at 532.1 eV and C–OH at 533.1 eV [46]. After chemical reduction of GO by
L-ascorbic acid, the peaks at 530.4 and 532.1 eV disappear or become weak that shows a nearly
complete reduction of the GO [47]. As can be seen from XPS spectrum of the O 1s core level
region of Cu@Cu2O NPs-RGO nanocomposite (Fig. 6), the presence of dominant peak at 531.3
eV, which corresponds to oxygen in metal oxides such as Cu2O and CuO, demonstrates that the
surface of copper nanoparticles is oxidized [45].
Fig. 6. XPS spectrum of the O 1s core level region of GO, RGO and Cu@Cu2O NPs-RGO
nanocomposite.
A transmission electron microscopy (TEM) image of the GO displays a crumpled, layer-
like structure with the size of tens of micrometers (Fig. 7a). TEM of Cu@Cu2O NPs-RGO
reveals the polycrystalline nature of the nanoparticle indicating that the oxidation surface of
copper nanoparticles is dispersed on the RGO sheet (Fig. 7b-d). The TEM image (Fig. 7d) of a
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signal particle reveals a core-shell structure consisting of Cu2O shell that is of ca. 1-2 nm
thickness covering a Cu core with a 4-5 nm thickness. Additionally, Cu@Cu2O nanoparticles are
not found outside of the RGO sheets. The energy dispersive spectroscopy (EDS) of Cu@Cu2O
NPs-RGO nanocomposite in Fig. 7e clearly demonstrates the presence of Cu in the
nanocomposite.
Fig. 7. (a) TEM image of GO (b-d) TEM images of Cu@Cu2O NPs-RGO nanocomposite (e)
EDS result for Cu@Cu2O NPs-RGO nanocomposite
3.2. Catalytic activity toward N-Arylation of N-heterocycles
3.2.1. Effect of solvent polarity, base, and temperature
The catalytic activity of the Cu@Cu2O NPs-RGO nanocomposite was then tested in
Ullmann C-N cross-coupling of the N-heterocycles with aryl halides. To optimize the reaction
conditions, 1-bromo-4-nitrobenzene 1g and imidazole 2a were selected as model substrates in
the presence of various bases and solvents (Table 1). As shown in Table 1, the desired product
was scarcely obtained in the presence of Cs2CO3 in the commonly used solvents such as CH3CN,
PhCH3, and DMF (Table 1, entries 1-3). It was found that DMSO is the optimal solvent (Table 1,
entry 4). By using DMSO as the solvent, the reaction was explored employing other bases,
including K2CO3, K3PO4, and KOH (Table 1, entries 5-7). A superior yield was obtained when
Cs2CO3 was used as the base (Table 1, entry 4). To study the effect of amount of the catalyst, the
reactions were carried out at different amounts of Cu@Cu2O NPs-RGO nanocomposite ranging
from 3.5-5.0 mol% of Cu (Table 1, entries 4 and 8). It was found that when the amount of the Cu
NPs-RGO increases from 3.5 to 5.0 mol %, the yields increase from 73 to 98%, respectively. It
was found that 5.0 mol% Cu@Cu2O NPs-RGO is sufficient to push forward this reaction
forward (Table 1, entry 4). To optimize the reaction temperature, we also performed several
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experiments at 90 oC, 100 oC, 110 oC, and 120 oC in the presence of Cs2CO3 in DMSO using 5.0
mol % Cu@Cu2O NPs-RGO Table 1, entries 4 and 10-12). As can be seen from Table 1, the
o
optimal reaction temperature is 110 C (Table 1, entry 4). It is also noteworthy that, when this
reaction was carried out with GO or RGO, we failed to isolate any coupled product (entries 13
and 14). Additionally, Cu@Cu2O-Activated Carbon was tested and exhibited middle catalytic
activity.
Table 1. Screening of the reaction conditionsa
Br
N
N
+
N
HN
O2N
O2N
1g
3d
2a
Entry
Solvent
Base
Temp. (oC) Time (h) Conversion (%)b Yield (%)c
1
2
CH3CN
PhCH3
DMF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
K3PO4
Reflux
Reflux
110
10
24
10
1
31
Trace
57
27
Trace
52
3
4
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
110
100
69
98
5
110
2
62
6
110
2
92
84
7
KOH
110
2
60
53
8d
9
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
110
1
81
73
90
1
76
71
10
11
100
1
91
87
120
1
100
98
11
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12e
13f
15g
DMSO
DMSO
DMSO
Cs2CO3
Cs2CO3
Cs2CO3
110
110
110
1
1
1
n. r.
n. r.
65
n. r.
n. r.
57
a 1-bromo-4-nitrobenzene (0.5 mmol), imidazole (0.5 mmol), Base (0.75 mmol), and Cu@Cu2O
NPs-RGO nanocomposite (5.0 mol% of Cu).
b
Conversion determined by gas chromatography based on arylhalide and n-dodecane was used
as an internal standard
c Isolated yield
d Cu@Cu2O NPs-RGO nanocomposite (3.5 mol% of Cu).
e GO (100 mg)
f RGO (100 mg)
g Cu@Cu2O-Activated Carbon (5.0 mol% of Cu).
After determining the optimized conditions, we then investigated the scope of the
Cu@Cu2O NPs-RGO nanocomposite for the C–N cross-coupling reaction of a variety of aryl
halides with imidazole (Table 2). The N-arylation of imidazole with aryl iodides and aryl
bromides bearing electron-donating and electron-withdrawing groups was done effectively and
gave good yields (Table 2, entries 1-4 and 6-5). Under these conditions, chlorobenzene show less
reactivity as compared to bromobenzene and iodobenzene (entry 8). The reactivity of different
aryl halides is ArI> ArBr > ArCl. Other N-heterocycles such as benzimidazole, benzotriazole and
indole reacted with aryl bromides under standard experimental conditions and provided products
with 83-94% yields (Table 3, entries 9-13).
Table 2. Cu@Cu2O NPs-RGO nanocomposie catalyzed N-arylation of N-heterocycles
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X
Cu@Cu O -RGO (5 mol%), Cs2CO3
2
N
NH
+
DMSO, 110 °C, 1h
Y
Y
13
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Entry
1
Aryl Halide
N-Heterocycle
Product
Conversion (%)a
100
Yield (%)b
96
I
N
N
N
NH
2a
1a
3a
I
N
N
2
3
2a
2a
100
100
96
98
1b
3b
O
I
O
1c
N
N
3c
O2N
I
N
N
4
2a
100
98
1d
O2N
3d
Br
1e
5
6
7
2a
2a
2a
3a
95
92
94
98
Br
3b
3d
100
100
1f
Br
1g
O2N
Cl
8
9
2a
3a
64
93
93
56
90
87
1h
1e
1g
N
N
H
3e
2b
O2N
N
10
2b
3f
N
O2N
N
11
12
1g
1f
88
83
94
N
N
H
N
3g
N
2c
CN
CN
100
N
N
3h
H
2d
14
O2N
N
N
N
Page 14 of 43
N
H
13
1g
89
85
2e
3i

c Conversion determined by gas chromatography based on arylhalide, n-dodecane was used as an
internal standard
b Isolated yield
Table 3, compares efficiency of Cu@Cu2O NPs-RGO nanocomposite (reaction
conditions, yield and time) with efficiency of other reported heterogeneous copper catalysts in
Ullmann C-N cross-coupling. It is clear from Table 3 that our method is simpler, and less time
consuming for N-arylation of N-heterocycles with aryl bromides.
Table 3. Comparison of efficiency of various heterogeneous copper catalysts in N-arylation of
N-heterocycles with aryl bromides
Catalyst
Condition
Yield (%) Time (h)
Ref.
This
work
Cu@Cu2O NP-RGO (5 mol%)
Cs2CO3, DMSO, 110 oC
83-98
85-98
96-47
1
CuFAP (12.5 mol%)
K2CO3, DMSO, 110 ºC
K2CO3, DMAc, 120 oC
4-15
24
48
13
CuO/MWCNT (0.98 mol%)
CuI/MNP-3 (10 mol%)
Activated-Cu (10 mol%)
Cs2CO3, DMF, 110 oC
30-98
35-81
24
24
49
50
LiOH, TBAB, H2O, 120 oC
3.2.2. Recycling the Cu@Cu2O NPs-RGO composite
The X-ray Diffraction (XRD) pattern Cu@Cu2O NPs-RGO after five runs is shown in
Fig. 9. The peaks of Cu and Cu2O phases can be easily observed.
Fig. 8. XRD pattern of Cu@Cu2O NPs-RGO nanocomposite after five runs used
TEM image of the Cu@Cu2O NPs-RGO composite after five runs is showed in Fig. 10
indicating the more oxidation surface of copper nanoparticles.
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Fig. 9. TEM image of Cu@Cu2O NPs-RGO nanocomposite after five runs used.
The XPS spectrum of Cu 2p core level region for Cu@Cu2O NPs-RGO nanocomposite
after five runs used displays main peaks at 933.9 and 953.8 eV which are attributed to the
binding energy of Cu 2p3/2 and Cu 2p1/2, respectively (Fig. 11). The broad Cu 2p3/2 peak has been
deconvoluted into two peaks which are marked as peaks at 933.6 and 934.5 eV, and assigned to
Cu2O/Cu (Cu1+/Cu0) and CuO/Cu(OH)2 (Cu2+), respectively. By integrating the area in the XPS
has been found the ratio of Cu2O/Cu (Cu1+/Cu0): CuO/Cu(OH)2 (Cu2+) to be 3:7 (than this ratio
for fresh nanocomposite 6:4) that indicated more oxidation surface of Cu@Cu2O nanoparticles.
Binding energy of Cu 2p3/2 of the used nanocomposite is 933.9 eV (Fig. 10), which has −0.5 eV
difference from Cu@Cu2O species of the fresh catalyst (Fig. 5). This illustrates that the
Cu@Cu2O nanoparticles surfaces have adsorbed or chelated with some anions such as Br−,
CO32− in reaction mixture [16].
Fig. 10. XPS spectrum of the Cu 2p core level region of Cu@Cu2O NPs-RGO
nanocomposite after five runs used.
3.2.3. Hot filtration test and AAS analysis
The heterogeneous nature of the catalysis was proved using a hot filtration test and AAS
analysis. To determine whether the catalyst is actually functioning in a heterogeneous manner or
whether it is merely a reservoir for more active soluble copper species, a hot filtration test was
performed in the C-N cross-coupling of bromobenzene with imidazole after ~50% of the
coupling reaction is completed. The hot filtrates were then transferred to another flask containing
Cs2CO3 (1.5 equiv.) in DMSO (2 mL) at 110 °C. Upon the further heating of catalyst-free
solution for 2 h, no considerable progress (~7% by GC analysis) was observed. Then, the
possibility of Cu leakage from Cu@Cu2O NPs-RGO nanocomposite to the medium during the
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reaction was investigated (Table 4). AAS results of the used Cu@Cu2O NPs-RGO
nanocomposite catalyst indicate leaching of 0.9% of copper in the N-arylation reaction of
bromobenzene with imidazole after the first cycle and 6.1% after the fifth cycle.
Table 4. Reusability of Cu@Cu2O NPs-RGO nanocomposite and leaching of Cu in
multi-cycle N-arylation reactions
Cycle
Product yielda (%)
Leaching of Cu (%)
1
2
3
4
5
92
91
87
83
75
0.9
1.3
1.2
1.4
1.3
a Reaction conditions: bromobenzene (0.5 mmol), imidazole (0.5 mmol), Cs2CO3 (0.75
mmol), and Cu@Cu2O NPs-RGO nanocomposite (5.0 mol% of Cu) (for cycle 1 and the
remaining recovered amount of the catalyst was used for subsequent cycles), DMSO (2 mL), 1 h,
110 oC.
To determine the reproducibility of the catalyst, we applied the same batch of catalyst
and also two different batches of catalyst for replicate experiments, this was checked by
repeating the runs at least three times on the same batch of catalyst and for another three times of
two different batches of catalyst for N-arylation of imidazole by bromobenzene. These results
were found to be within acceptable limits (± 1% for the same batch of the catalyst and ± 3% for
different batches of the catalyst).
4. Conclusions
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In conclusion, we have demonstrated for the first time that Cu@Cu2O NPs-RGO
nanocomposite is an effective and reusable heterogeneous catalyst for the Ullmann C-N cross-
coupling reaction with good yields under aerobic conditions for a short reaction times without
using any external ligands or an additives as promoters. The catalyst was recovered by simple
filtration and reused for several cycles.
Acknowledgements
We gratefully acknowledge financial support from the Research Council of Shahid
Beheshti University.
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Figure captions
Fig. 1. IR spectra of GO and Cu@Cu2O NPs-RGO nanocomposite
Fig. 2. Raman spectra of GO and Cu@Cu2O NPs-RGO nanocomposite.
Fig. 3. XRD patterns a) GO and b) Cu@Cu2O NPs-RGO nanocomposite.
Fig. 4. Full range XPS spectrum of GO, RGO and Cu@Cu2O NPs-RGO nanocomposite.
Fig. 5. XPS spectrum of the Cu 2p core level region of Cu@Cu2O NPs-RGO nanocomposite.
Fig. 6. XPS spectrum of the O 1s core level region of GO, RGO and Cu@Cu2O NPs-RGO
nanocomposite.
Fig. 7. (a) TEM image of GO (b-d) TEM images of Cu@Cu2O NPs-RGO nanocomposite (e)
EDS result for Cu@Cu2O NPs-RGO nanocomposite.
Fig. 8. XRD pattern of Cu@Cu2O NPs-RGO nanocomposite after five runs used.
Fig. 9. TEM image of Cu@Cu2O NPs-RGO nanocomposite after five runs used.
Fig. 10. XPS spectrum of the Cu 2p core level region of Cu@Cu2O NPs-RGO nanocomposite
after five runs used.
Table captions
Table 1. Screening of the reaction conditions
Table 2. Cu@Cu2O NPs-RGO catalyzed N-arylation of N-heterocycles
Table 3. Comparison of efficiency of various heterogeneous copper catalysts in N-arylation of
N-heterocycles with aryl bromides
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Table 4. Reusability of Cu@Cu2O NPs-RGO nanocomposite and leaching of Cu in multi-cycle
N-arylation reactions
Scheme caption
Scheme 1. Illustration of the synthesis procedure of Cu@Cu2O NPs-RGO nanocomposite.
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Fig. 1. IR spectra of GO and Cu@Cu2O NPs-RGO nanocomposite
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Fig. 2. Raman spectra of GO and Cu@Cu2O NPs-RGO nanocomposite.
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Fig. 3. XRD patterns a) GO and b) Cu@Cu2O NPs-RGO nanocomposite.
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Fig. 4. Full range XPS spectrum of GO, RGO and Cu@Cu2O NPs-RGO nanocomposite.
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Fig. 5. XPS spectrum of the Cu 2p core level region of Cu@Cu2O NPs-RGO nanocomposite.
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Fig. 6. XPS spectrum of the O 1s core level region of GO, RGO and Cu@Cu2O NPs-RGO
nanocomposite.
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