Carbon coated copper nanostructures as a green and ligand free nanocatalyst for Suzuki cross-coupling reaction

Article abstract of DOI:10.1016/j.catcom.2016.12.028

In this work, carbon coated copper nanoparticles and nanowires were synthesized as a ligand free nanocatalyst that naturally contains ppm levels of Pd with no post-modification via a two-steps reduction-hydrothermal process. Transmittance electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction, inductively coupled plasma optical emission spectrometry (ICP-OES), and Raman spectroscopy were employed for the characterization of the nano-catalyst. The utilization of the synthesized Cu/C nano-catalyst in Suzuki cross coupling reaction showed a high performance in the synthesis of various biaryls in water. Moreover, this catalyst reused successfully with no significant yield decrease even after four subsequently runs.

Full text of DOI:10.1016/j.catcom.2016.12.028

Accepted Manuscript
Carbon coated copper nanostructures as a green and ligand free
nanocatalyst for Suzuki cross-coupling reaction
Kamran Lamei, Hossein Eshghi, Mehdi Bakavoli, Seyyed Amin
Rounaghi, Elaheh Esmaeili
PII:
S1566-7367(16)30481-2
doi: 10.1016/j.catcom.2016.12.028
CATCOM 4897
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
30 May 2016
23 December 2016
24 December 2016
Please cite this article as: Kamran Lamei, Hossein Eshghi, Mehdi Bakavoli, Seyyed Amin
Rounaghi, Elaheh Esmaeili , Carbon coated copper nanostructures as a green and ligand
free nanocatalyst for Suzuki cross-coupling reaction. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2016), doi: 10.1016/j.catcom.2016.12.028
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ACCEPTED MANUSCRIPT
Carbon coated copper nanostructures as a green and ligand free
nanocatalyst for Suzuki cross-coupling reaction
a
a,
a
b
Kamran Lamei , Hossein Eshghi *, Mehdi Bakavoli , Seyyed Amin Rounaghi , Elaheh
c
Esmaeili
ª
Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, 91775-1436, Iran. E-mail: heshghi@um.ac.ir Tel.:
+98 761 6670242; fax: +98 761 6670242 (F.M.)
b
Department of Materials Engineering, Birjand University of Technology, Birjand, 9719866981-236, Iran.
cDepartment of Chemical Engineering, Birjand University of Technology, Birjand, 9719866981-212, Iran.
Abstract
In this work, carbon coated copper nanoparticles and nanowires were synthesized as a ligand free
nanocatalyst that naturally contains ppm levels of Pd with no post-modification via a two-steps
reduction-hydrothermal process. Transmittance electron microscope (TEM), Scanning electron
microscope (SEM), X-ray diffraction, inductively coupled plasma optical emission spectrometry
(
ICP-OES), and Raman spectroscopy were employed for the characterization of the nano-
catalyst. The utilization of the synthesized Cu-C nano-catalyst in Suzuki cross coupling reaction
showed a high performance in the synthesis of various biaryls in water. Moreover, this catalyst
reused successfully with no significant yield decrease even after four subsequently runs.
Keywords: Copper nanoparticles; Copper nanowires; Suzuki reaction; Carbon Coated Copper
nanostructures.
1
. Introduction
The Suzuki-Miyaura cross-coupling reaction of Arylboronic acid with Aryl halides (C -X) has
sp2
become a popular tool for selective construction of carbon-carbon bonds and mainstay of modern
synthetic organic chemistry for the preparation of biaryl compounds [1, 2]. These compounds are
important structural substructures in numerous natural products, polymers, agrochemicals,
pharmaceutical intermediates [3-5], and other materials [6-8]. During the past decades, numerous
efforts have been made to develop efficient and selective catalytic systems for the Suzuki-
1

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Miyaura cross-coupling reaction [9] involves the use of palladium (0) complex together with
phosphorous ligands as catalysts [10, 11]. The high price and toxicity of Pd renders commercial
processes based on Pd less attractive. Also, for industrial scale applications the use of less values
of catalyst is of great importance. Recently, the new catalytic systems with extremely low
loadings of the expensive metals (ppm or ppb) were developed [12]. But, when very low
concentrations of metal can efficiently catalyze the reactions, impurity artifacts can be an
important issue. The effectiveness of Pd at ppm or ppb loadings directly leads to the topic of
impurity artifacts. Sachin and co-workers reported the Fe-ppm Pd nanoparticles catalyzed Suzuki
reaction formed from FeCl that naturally contains ppm levels of Pd [13]. Also, they showed that
3
successful couplings require the presence of both Fe and Pd within these nanocomposites.
Leadbeater and co-workers found that 50 ppb of Pd existed in the reactants was sufficient to
initiate the microwave activation of Suzuki cross-coupling reaction [14]. The use of copper-
based alternatives in the cross-coupling reactions are taken into consideration, due to their orders
of magnitude lower cost and harmfulness to the environment than other noble metals.
Surprisingly, only a few copper-catalyzed Suzuki cross-coupling procedures exist [15-17].
Dimerization of arylboronic acids was first reported with copper acetate as a catalyst under
o
oxygen at 100 C [18]. Also, it was found that CuCl(OH)(phen) complex was much more
o
efficient as a catalyst to afford dimer in high yields at 28 C [19]. Also, the effective copper-
catalyzed cross-coupling reactions of aryl halides with phenylboronic acid using mixed
nanocluster catalysts was reported [17]. Core-shell structured Cu/Cu O nanowires were
2
demonstrated as efficient catalysts for C–S cross coupling reactions [20]. The synthesis
methodologies of various metal nanowires such as Gold, Copper, Nickel, and Silver nanowires
[
21-24] have been reviewed in the last years, with a wide range of magnetic, electronic, and
2

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optical properties and many potential applications in the various fields such as biomedicine [25],
catalysis science [26, 27], and electronic devices [28, 29]. Generally, according to the nanowires
preparation conditions two hard and soft synthesis strategies, especially in case of copper
nanowires (Cu NWs), have been developed, that suffer from many drawbacks such as foreign
hard templates, harsh reaction conditions, high vacuum, high temperature, high cost and energy
consumption. In contrast, the soft approaches, offers the low crystal growth temperature and an
inexpensive route to synthesize nanowires under mild conditions [30].
As part of our ongoing program in novel heterogeneous catalysts designing and development of
environmentally benign green methods in organic chemistry and catalysis [31-32], our aim has
been to investigate application of the efficient, inexpensive, and ligand free copper based nano-
catalyst with high activity for Suzuki cross-coupling reaction with minimum by-products in
green media for the first time. Based on ICP-OES results Cu/C nanocatalyst naturally contains
ppm levels of Pd resulted from inexpensive CuCl2 precursor. For this, a two-steps
reduction/hydrothermal approach was exploited for synthesis of copper nanostructures
comprised of nanoparticle (NPs) and nanowires (NWs) embedded in a carbonaceous matrix. This
green nontoxic catalytic system reused at least four times in Suzuki reaction without significant
loss of catalytic activity.
2
. Experimental
2
.1 Materials and methods
3

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All compounds were obtained from Sigma-Aldrich in analytical grade and used without further
purification (see supporting information, S-1). For catalyst preparation procedure see supporting
information (S-1 and S-2).
2
.2 Synthesis of biaryls 3.
In a test tube, 1.0 mmol of aryl halides 1, 1.2 mmol of phenylboronic acid 2 were mixed together
and then 2.0 mmol of K CO , and the Cu-C nano-catalyst (0.1 mol% Cu) in 2 mL of H O, were
2
3
2
o
added in air. The reaction mixture was then stirred at 50 C for appropriate time. After
completion of the reaction (monitored by TLC), the catalyst was removed by simple filtration.
o
The recycled catalyst was washed with ethanol and water (3×5 mL) and dried at 60 C in oven
for further use. The aqueous layer was extracted with ethyl acetate, and organic layer dried over
anhydrous MgSO . The solvent was evaporated under reduced pressure to give the
4
corresponding biaryl compounds. All the products were previously reported [9-11, 33, 34] and
1
13
were confirmed by the spectroscopic method using H and C NMR (see supporting
information, S-3 to S-35). The spectral data for selected product:
o
1
4
-methoxy-1,1'-biphenyl: White solid, mp: 88-89 C. H-NMR (CDCl , 300 MHz): δ = 3.88 (s,
3
1
3
3
H), 7.01 (d, J= 8.7 Hz, 2H), 7.29-7.36 (m, 1H), 7.46 (d, J= 8.7 Hz, 2H), 7.54-7.60 (m, 4H). C-
NMR (CDCl , 75 MHz): δ = 55.36, 114.22, 126.66, 126.75, 128.16, 128.72, 133.81, 140.85,
3
1
59.16.
3
. Results and Discussion
Figure 1 shows the XRD pattern of the synthesized catalyst. The peaks are mainly indexed to the
crystalline Cu with a trace of Cu O which is resulted by surface oxidation of particles during
2
handling and sample preparation. The pattern also reveals that the copper particles are
4

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nanostructured and a value of 38 nm was calculated for the Cu crystallite size using Scherrer’s
equation.
Fig. 1. XRD pattern of synthesized nano-catalyst.
Raman spectrum of the catalyst clearly shows the presence of two distinct peaks corresponding
-1
to D and G bands of a carbonaceous material (Fig. 2). The D band centroid at about 1321 cm
indicates the presence of the defected and irregular structure of carbon and the G peak at ca.
-1
2
1
580 cm is related to the graphitic carbon (sp ). The overall shape of Raman spectrum showing
an intense D peak accompanied with a broad valley between D and G bands indicates that the
synthesized carbon has an amorphous structure consisted of few fused aromatic rings. This
carbonaceous phase has been originated by the polymerization of glucose during the
hydrothermal step and was not detected by XRD analysis due to the amorphous structure (Fig.
1
).
5

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Fig. 2. Raman spectrum of synthesized nano-catalyst.
The ICP-OES analysis of Cu/C nano-catalyst for Pd content resulted ina value of about 12 ppm
(0.001 wt. %) (See supporting information, S-36). The morphology of the catalyst particles was
investigated by SEM and TEM techniques as shown in Fig. 3. The typical SEM image (Fig. 3a)
shows the structure is consisted of two distinct morphology including very fine particles and
wires, which most of them are in the nanometer size range. These constituents are highly
aggregated and form agglomerated particles. Figure 3b provide higher resolution images from
Cu nanoparticles and a typical Cu wire. Although, the presence of carbon in the structure cannot
be recognized by SEM analysis but it is realized from the TEM images. Figure 3b reveals that
the Cu nanoparticles with regular shape are embedded in the carbonaceous phase.
6

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a
b
Fig. 3. SEM (a), and TEM (b) images of synthesized nano-catalyst.
The efficiency of the prepared carbon coated Cu nano-catalyst was investigated for the synthesis
of biaryls in water as a green solvent. Probably, actual catalysis operated by traces of Pd
presented in Cu/C nano-catalyst. Cross-coupling can initiate with ppm levels of Pd which
afterwards can continue as bimetallic Pd/Cu reaction [35].
7

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The Suzuki reaction was investigated with both Cu salts and carbon coated Cu nanostructures
(Table 1). 4-Bromoacetophenone and phenylboronic acid were chosen as substrates in the
coupling model reaction to obtain the product 3e. As shown in Table 1, the use of 0.1 mol% of
o
CuCl and Cu(OAc) led to product 3e with 10% yield at 80 C (entries 1 and 2). By increasing
2
2
o
the catalyst amount up to 0.5 mol%, yield of product 3e were increased up to 15% at 80 C
entries 1 and 2). Also, the reaction was carried out without any catalyst (entry 3). Product 3e
(
was not produced even after 24 h. Performing the reaction in 0.1 mol% of Cu-C nano-catalyst led
o
to the product 3e in 99 and 96% yields at 80 and 50 C respectively, without any side products,
but in high temperature leaching of Cu species was increased. These results show the high
activity of the Cu nanostructures in presented Suzuki reaction condition. Based on these results
o
0
.1 mol% of Cu-C nano-catalyst at 50 C were chosen as optimized condition for catalyst
amount.
a
Table 1. Different catalysts in the model reaction of Suzuki coupling .
o
entry
1
[Cu]
Temp. ( C) Mol% Cu
Cu-leaching (%)
Yield 3e (%)
10/5/0
8
0/50/r.t
0.1
0.25
0.5
-
-
-
CuCl2
80/50/r.t
15/5/0
8
8
0/50/r.t
0/50/r.t
15/10/0
0.1
0.25
0.5
-
-
-
10/10/0
10/5/0
2
3
Cu(OAc)2
80/50/r.t
0/50/r.t
8
15/15/0
b
Cu/C Nano-
80/50/r.t
0
-
0/0/0
8

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catalyst
80/50/r.t
0.1
0.25
0.5
0.05/0.02/0.0
0.09/0.06/0.0
99/96/20
99/97/15
96/92/10
8
8
0/50/r.t
0/50/r.t
0.15/0.09/0.0
a
b
1
mmol of substrates. K CO (2 equiv.), H O (2 ml), 5 h. after 24 h.
2
3
2
The effect of various parameters, such as different solvents, and bases in model reaction in the
presence of Cu-C nano-catalyst, was investigated. Results in Table 2 show a strong influence of
the used solvent and basic medium on the catalyst activity during coupling reaction. In model
reaction results showed that with 2 equiv. of K CO as base and H O as a solvent product 3e
2
3
2
o
could access in 96 % yield after 3 h with 0.1 mol% of catalyst at 50 C (Table 2, entry 1).
Despite the increase of Cu leaching by the substitution of water with DMF in the same
o
conditions, this results in the yield values of 30% and 91% at r.t. and 50 C, respectively (Table
2
, entry 1). For comparison, the model reaction was tested under the same conditions with
Cs CO as base, the reaction was completed in 4.5 h with 95% yield of 3e, without any side
2
3
products (Table 2, entry 3), that because of availability and price, K CO was selected for future
2
3
o
studies. For BaCO , K PO , KOH, and Ba(OH) in DMF and H O at r.t. and 50 C with 0.1
3
3
4
2
2
mol% of Cu the same results were achieved for model reaction (Table 2, entries 2, 4, 5, and 6).
o
Hence, the K CO , H O, Cu-C nano-catalyst and 50 C were chosen as optimized conditions for
2
3
2
the base, solvent, catalyst, and reaction temperature, respectively.
a
Table 2. Optimization of model reaction over the Cu-C nano-catalyst .
o
b
entry Base Solvent Mol% Cu Time (h) Temp ( C) Yield (%)
DMF
H O
2
0.1
0.1
3
3
r.t./50
r.t./50
30/91
20/96
1
2
3
K CO
2 3
DMF
H O
2
0.1
0.1
5
6
r.t./50
r.t./50
5/65
20/60
BaCO3
Cs CO
DMF
H O
2
0.1
0.1
4.5
4.5
r.t./50
r.t./50
5/95
10/95
2
3
9

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DMF
H O
2
0.1
0.1
8
7
r.t./50
r.t./50
5/50
10/30
4
5
K PO
3
4
c
c
KOH
H O
2
0.1
0.1
8
9
r.t./50
r.t./50
25/60
30/65
6
Ba(OH)2
H O
2
a
Reaction conditions: 1 mmol 4-Methoxy bromobenzene, 1.2 mmol phenylboronic acid, 2
mmol base, 2 mL solvent. Isolated yield. leached Cu was observed.
b
c
Versatility of the optimized condition was demonstrated by coupling of phenylboronic acid (2)
with iodo/bromo/chloro benzene. For this preliminary screening, halobenzenes (1ai-iii),
halobenzenes bearing an electron withdrawing group (p-nitrohalobenzenes, 1b ), and electron
i-iii
donating- (p-methoxyhalobenzenes, 1c ) were also selected (Table 3, see supporting
i-iii
information, S-2). These studies showed that, the catalytic system is efficient with various
substituents on the halobenzene. The formation of the nitro biaryl 3b (Table 3, entries 4-6) is
faster than the methoxy analog 3c (Table 3, entries 7-9). The Cu-C nano-catalyst promotes the
coupling of iodinated and brominated aryl substrates. In case of chlorinated aryl substrates, often
homo coupling product was observed (Table 3, entries 3, 6, and 9).
The catalytic activity of Cu-C nano-catalyst in the coupling of various aryl halide 1 and
phenylboronic acid 2 under the optimized conditions was also studied (Table 4). Electron
withdrawing substituted halobenzene derivatives (1c, entry 3), (1f, entry 6), (1g, entry 7), (1i,
entry 9), 4- 1j, entry 10), and (1k, entry 11) were converted in excellent yields to the
corresponding biphenyl products 3c (93/94%), 3f (94/95%), 3g (93%), 3i (91/93%), 3j (96%),
and 3k (96%) (Table 4). The simultaneous double-coupling of phenylboronic acid on 1-iodo-4-
bromobenzene (1h, entry 8) and 2-iodo-5-bromopyridine (1l, entry 12) provided triaryl
compounds 3h and 3l in 96% and 92% yield respectively (Table 4, entries 8 and 12). Electron
withdrawing and electron donating substituted halobenzene derivatives with 3-
nitrophenylboronic acid in Suzuki cross-coupling were also converted in high yields to the
1
0

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corresponding biaryl products 3 (Table 4, entries 13-17). Electron donating substituted
phenylboronic with 3-nitrohalobenzene and halobenzene were also converted in excellent yields
to the corresponding biphenyl products (Entries 18 and 19).
Table 4. Scope of the Cu-C nano-catalyst promoted Suzuki coupling of various substrates.
Time
h)
Yield
(%)
Entry
1
1
2
X
3
b
(
I/Br
3/3
96/95
95/95
93/94
3
a
2
3
I/Br 3/3.5
I/Br 2/2.5
3
b
3
c
4
5
6
I
2
96
3
d
I/Br
3/3
96/93
94/95
3
e
I/Br 2/2.5
3
f
7
I
2
93
3
g
1
1

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8
9
Br
2.5
96
3
h
I/Br 3/3.5
91/93
3
i
1
0
Br
Br
3
96
3
j
1
1
1
2
3.5
96
92
3
k
Br 3.5/4
I/Br 3.5/4
3
l
1
1
1
3
4
5
90/87
92/87
89/91
3
m
I/Br
3/4
3
n
I/Br 3/3.5
3
o
1
1
6
7
I
2.5
93
3
p
q
I/Br 3/3.5
95/94
3
1
2

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1
1
8
9
I/Br
3/3
96/94
93/89
3
3
b
n
I/Br 3/3.5
c
2
2
2
0
1
2
Cl
Cl
Cl
4
4
15
3
a
c
20
3
3
c
e
c
4.5
15
c
2
2
3
4
Cl
Cl
4
4
20
3
j
c
20
3
k
a
Conditions: 1 (1 mmol), 2 (1.2 mmol), K CO (2 equiv.), Cu-C nano-catalyst 0.1 mol%, H O (2 ml),
2
3
2
o
b
c
5
0 C. Isolated yield. 0.2 mol% catalyst.
More challenging Suzuki coupling of chlorinated substrates was showed that higher catalyst
loading (0.2 mol%) and reaction time (4-4.5 h) were required. The reaction led to the products 3
in low yield (15-20%) with electron deficient and electron rich substrates (Table 4 entries 20-24).
To evaluate the effect of Pd impurities at ppm level on the kinetic of the Cu/C nanostructures
catalyzed Suzuki reaction, the model reaction was carried out with Cu/C nanocatalyst and
different amount of aqueous solution of Pd. To study the differences in the rate of the reaction 5,
1
0, 15, and 20 ppm of Pd aqueous solution were added to the model reaction in the optimized
reaction conditions, separately. As shown in Table 5 (see supporting information), reaction
1
3

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progress were studied after 5, 10, 30, 90, 140, and 180 min which no significant change was
observed based on isolated yields. Based on these results the Pd at ppm level is not involved in
the rate-determining step and major contribution is made by Cu-catalyzed reaction.
Also, the recyclability of our proposed catalyst was studied in the model reaction. Results
exhibite that the catalyst has good recyclability with its easy recovery. In this study, Cu-C nano-
catalyst has ability to be recycled at least for 4 runs with 96-78% yield (Fig. 4, see supporting
information, S-3).
4
. Conclusion
In summary, carbon coated copper nanostructure comprised of nanoparticles and nanowires was
easily synthesized through a two steps reduction and carbonization process. The material was
employed as a ligand free, low cost, and efficient nano-catalyst for the promotion of Suzuki
cross-coupling of halo benzenes with phenylboronic acids in the presence of 12 ppm of Pd as
impurity. Higher temperatures caused to a significant decrease in the product yield due to
increase of leached Cu species. The nanocatalyst was recycled and reused at least for four
successful runs. The efficient Suzuki reaction yield was achieved when the reaction parameters
such as base, solvent, catalyst, and reaction temperature were chosen K CO , water, Cu-C nano-
2
3
o
catalyst and 50 C, respectively.
5
. Reference
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ACCEPTED MANUSCRIPT
Highlights
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Inexpensive copper/carbon nanocomposite for the cross-coupling reaction is proposed.
The Suzuki reactions of various halobenzenes are performed in aqueous media.
High catalytic activity of the copper-based heterogeneous catalyst.
The catalyst could be recycled up to four times without loss in activity.
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