Carbon nanofiber supported copper nanoparticles catalyzed Ullmann-type coupling reactions under ligand-free conditions

Article abstract of DOI:10.1039/c4ra07184k

A carbon nanofiber supported copper nanoparticle (CuNP/CNF) catalyst was prepared and used for Ullmann-type couplings under ligand-free conditions. The catalyst exhibited high catalytic activity in C-O and C-N bond formation couplings. This journal is

Full text of DOI:10.1039/c4ra07184k

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Carbon nanofiber supported copper nanoparticles
catalyzed Ullmann-type coupling reactions under
ligand-free conditions†
Cite this: RSC Adv., 2014, 4, 48362
Received 16th July 2014
Accepted 27th August 2014
*
Hongqiang Li, Chunping Li, Jie Bai, Chenglin Zhang and Weiyan Sun
DOI: 10.1039/c4ra07184k
www.rsc.org/advances
A carbon nanofiber supported copper nanoparticle (CuNP/CNF) bases,5b,8 copper species,5a,5e,9 alternatives to aryl halides5c,10
catalyst was prepared and used for Ullmann-type couplings under and new experimental techniques.11 However, almost all of
ligand-free conditions. The catalyst exhibited high catalytic activity in these methods involve specic ligands or a well-dened
C–O and C–N bond formation couplings.
catalyst, which may increase the cost and limit the scope of
applications. Therefore, mild, simple and low-cost methods
are still highly desirable.
The high surface areas and reactive morphologies of the
copper nanoparticles or composite nanomaterials made them
successful catalysts in Ullmann cross-coupling reactions. Since
Cu NPs easily agglomerate and oxidize due to high surface
energy in aqueous solutions, hence, it's crucial to nd a method
to solve the problem. In many cases, metal nanoparticles are
immobilized or synthesized in situ on various supports, such as
polymeric and inorganic materials, including silica,12 zeolites,13
metal oxide,14 microspheres,15 activated carbon16 and carbon
nanotube.17 Among these supports, carbon nanobers (CNFs)
have attracted more and more researchers' attention due to
their one-dimensional properties, high specic surface area and
high mechanical strength. What's more, carbon nanobers with
low density, low weight and superb thermal stability could be
easily recycled due to their one-dimensional structure. Alter-
natively, electrospinning is a simple electrostatic method for
the preparation of polymer nanobers and composite nano-
bers.18 Electrospinning followed by calcination has also been
widely applied to prepare carbon ber materials.19 Here we
report an easy method for Cu NPs/carbon nanobers matrix
synthesis. The resulting composite materials can avoid the
agglomeration of Cu NPs.
In this paper, we fabricated the b-CDs (b-cyclodextrins)/PAN
(poly (acrylonitrile)) composite bers by electrospinning tech-
nology. Copper nanoparticles were synthesized on the surface
by adsorbing copper ions from copper nitrate solution and
subsequent chemical reduction. Then, the composite nano-
bers were treated by high temperature calcination processing.
By this method, the CuNPs/CNFs composite nanobers were
fabricated. The as-prepared CuNPs/CNFs composite nanobers
were applied in catalytic Ullmann-type couplings. We report
here what is, to the best of our knowledge, the rst use of CNFs-
Introduction
Supported metallic nanoparticles and their catalytic appli-
cations in heterogeneous organic reactions have attracted
extensive attention in recent years.1 Among these metallic
nanoparticles, Cu NPs are expected to be one of the next-
generation materials because of their wide application and
low cost.2 There have been many synthesis methods such as
thermal reduction metal vapor synthesis and chemical
reduction.3 Among these methods, chemical reduction is
widely used because the morphology and size of Cu NPs are
easily controlled by varying the copper salts, solvents,
reducing agents, reduction temperature, concentrations and
other parameters.
Ullmann-type coupling reactions are very important and
powerful tools for preparing numerous biologically active
natural products, pharmaceutical compounds and polymers
in the material science industries.4 Cu-catalysed arylation
reactions devoted to the formation of C–C, C–O, and C-
heteroatom bonds (Ullmann-type couplings) have acquired
great importance over the past decade.5 However, the classic
Ullmann-type coupling requires harsh reaction conditions,
stoichiometric use of copper compounds,6 strong bases, and
bipyridine or phosphine-type ligands. Great efforts have been
made recently to develop a new catalytic system and marked
progress has been achieved, mainly focusing on ligands,7
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot,
010051, People's Republic of China. E-mail: hgcp_li@126.com; Fax: +86 471
6575722; Tel: +86 471 6575722
† Electronic supplementary information (ESI) available: Synthesis of CuNPs/CNFs
catalyst and GC-MS spectra. See DOI: 10.1039/c4ra07184k
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Fig. 1 FESEM image of the CuNPs/CNFs catalyst.
Fig. 4 X-ray photoelectron spectra of the recycled CuNPs/CNFs
catalyst after use five times.
Fig. 1 showed the FESEM image of the CuNPs/CNFs catalyst.
The diameter of the carbon bers ranges from 250 nm to
300 nm, the nanoparticles were uniformly distributed across
the surface of each ber without aggregation, offering high level
exposure of the nanoparticles surface. The particle size of
spherical copper nanoparticle is about 25 nm. The chemical
composition of the copper nanoparticles was conrmed by
high-resolution X-ray photoelectron spectroscopy on a dried
sample. As shown in Fig. 2, the major contribution of the Cu
2p3/2 peak (932.6 eV) is that associated with zerovalent copper,
consistent with the electron diffraction data. The X-ray photo-
electron spectrometer was accomplished with pass energy
200 eV for survey, 30 eV for high resolution scans. The X-ray
source was Monochromated Al Kalph 150 W. The binding
energy for Cu was referenced to the C 1s line (284.8 eV) of the
carbon bers.
Fig. 2 X-ray photoelectron spectra of the CuNPs/CNFs catalyst.
supported copper nanoparticles catalyst for ligand free
Ullmann-type couplings. Moreover, the novel catalyst exhibited
high catalytic activity and selectivity, and can be recyclable for
this process.
X-ray diffraction patterns were recorded to conrm the
crystalline phase of the copper particles. The sample of X-ray
diffraction (XRD, Rigaku Ultima IV, Japan) was prepared
by grinding into powder. XRD was carried out by using Cu Ka
(40 kV) radiation. The diffraction angle ranged from 20ꢀ to 80ꢀ.
Fig. 5 FESEM image of the recycled CuNPs/CNFs catalyst after use
five times.
Fig. 3 XRD patterns of the CuNPs/CNFs catalyst.
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As shown in Fig. 3, the strong diffraction peaks at 43.24ꢀ, 50.38ꢀ, 1, entries 1, 2 and 4–6). Different bases were also tested in this
and 74.03ꢀ can be assigned to the (111), (200), and (220) planes reaction system (Table 1, entries 3, 7 and 8). Cs2CO3 found to be
of face centered cubic (fcc) Cu crystals, respectively, which a suitable base, whereas other bases such as K2CO3 and tBuOK
showed that the Cu crystals were formed. In addition, nitrogen largely reduced the catalytic activity in this reaction under these
adsorption/desorption studies showed that the catalyst's BET conditions. From entries 3 and 9–11 (Table 1), when the reac-
surface area was 14.963 m2 gÀ1
.
tion time is decreased from 24 h to 4 h, 8 h and 12 h, reactants
The Fig. 4 showed the XPS of the recycled CuNPs/CNFs 1a conversion also decreased. From entries 3, 12 and 13
catalyst aer use ve times, the major contribution of the Cu (Table 1), it is observed that the reactant 1a conversion and
2p3/2 peak (932 eV) is associated with zerovalent copper, this product 3a selectivity are improved by adding CuNPs/CNFs
suggested that the copper nanoparticles have not been oxidized catalyst 0.03–0.09 g. As expected, no reaction was induced in
in the catalytic reaction process. The binding energy for Cu was the absence of CuNPs/CNFs catalyst (Table 1, entry 14). In the
referenced to the C 1s line (284.2 eV) of the carbon bers. Fig. 5 following studies, we choose DMAc as the solvent and Cs2CO3 as
showed the FESEM image of the recycled CuNPs/CNFs catalyst the base for further investigation of the catalytic properties of
aer use ve times, part of the copper nanoparticles leached CuNPs/CNFs catalyst.
from the surface of nanobers and aggregated.
To demonstrate the general applicability of the CuNPs/CNFs
A set of experiments was carried out to optimize the reaction catalyst for Ullmann coupling and the scope of the process,
conditions. As a model reaction, the C–O coupling reaction various substituted iodobenzenes and phenols were investi-
between aryl halide and phenol was studied with iodobenzene gated, as shown in Table 2. In the reaction of iodobenzene with
(1a, 0.50 mmol) and phenol (2a, 0.75 mmol). The results of various substituted phenols, gave the high iodobenzene
these experiments are summarized in Table 1. From entries 1 to conversion and diaryl ethers high selectivity (Table 2, entries 1
6, it is apparent that the solvent has a signicant role on the and 3–5). The electron-donating groups like methyl and
reaction; DMAc was found to be quite successful for the trans- methoxy groups in the meta or para position of the phenols gave
formation. Reactant 1a was 99.12% conversion and product 3a the aryl halide in high conversion (Table 2, entries 3–5 and
was 95.26% selectivity (Table 1, entry 3). When acetonitrile, 10–12), but when the electron donating group is in ortho posi-
DMF, DMSO, toluene and NMP were used as solvents, the tion, the conversion became lower because of steric hindrance
reactants 1a were only 49.60%, 98.09%, 66.28%, 51.89% and (Table 2, entries 2 and 9). To further explore the scope of the
70.74% conversion, what's more, the products 3a were only reaction, several aryl halides were used (Table 2, entries 6 to 15).
84.81%, 73.39%, 33.43%, 75.68% and 86.05% selectivity (Table It is observed that the aryl bromides (Table 2, entries 8 to 14)
Table 1 CuNPs/CNFs catalyzed Ullmann O-arylation of iodobenzene (1a) with phenol (2a)a
Entry
Base
Time (h)
Solvent
Conversion 1ab (%)
Selectivity 3a (%)
1
2
3
4
5
6
7
8
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
24
24
24
24
24
24
24
24
4
MeCN
DMF
DMAc
DMSO
Toluene
NMP
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
49.60
98.09
99.12
66.28
51.89
70.74
67.60
60.23
7.75
31.22
43.15
61.56
63.18
n.d.f
84.81
73.39
95.26
33.43
75.68
86.05
86.74
86.10
66.19
85.42
91.31
92.85
81.17
n.d.f
tBuOK
9
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
10
11
12c
13d
14e
8
12
24
24
24
a
Conditions: 1a (0.5 mmol) was allowed to react with 2a (0.75 mmol) in the presence of 0.09 g CuNPs/CNFs catalyst and base (1.0 mmol) in solvent
(10 mL) at 140 ꢀC for a certain time (h) under an nitrogen atmosphere. b GC conversion based on 1a used. c 0.03 g CNFs/CuNPs catalyst used. d 0.06 g
CuNPs/CNFs catalyst used. e Without CuNPs/CNFs catalyst. f Not detected.
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Table 2 CuNPs/CNFs catalyzed Ullmann O-arylation of aryl halides (1) with various phenols (2)a
Entry
1
Aryl halide
Phenol
Product
Conversion (%)
99.12
Selectivity (%)
95.26
2
3
4
5
90.85
100
94.34
97.49
84.73
95.79
100
100
6
100
88.10
7
100
90.52
95.01
99.70
93.70
96.14
98.36
59.07
66.72
8
86.98
41.42
60.77
60.40
76.99
51.11
57.23
9
10
11
12
13
14
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Table 2 (Contd.)
Entry
Aryl halide
Phenol
Product
Conversion (%)
5.96
Selectivity (%)
30.24
15
a
All reactions were carried out using 0.50 mmol of aryl halide, 0.75 mmol of phenol, 0.09 g of CuNPs/CNFs catalyst, 1 mmol of Cs2CO3 in 10 mL
DMAc at 140 ꢀC for 24 h.
Table 4 Recycling of CuNPs/CNFs catalyst for the coupling reaction
of iodobenzene with phenol
show less reactivity in comparison to aryl iodides (Table 2,
entries 1 to 7), and the aryl chloride shows lower reactivity
(Table 2, entry 15). We have also carried out the preliminary
Entry
Recycle
Conversion (%)
Selectivity (%)
study of C–N bond formation reaction (Table 3). The N-arylation
of imidazole (Table 3, entry 1), pyrrole (Table 3, entry 2) and
pyrazole (Table 3, entry 3) with iodobenzene has been exam-
ined. They all afforded the high iodobenzene conversion and
high products selectivity.
1
2
3
4
5
1st
99.12
88.16
71.29
65.89
63.72
95.26
96.18
90.91
73.09
86.33
2nd
3rd
4th
5th
The recyclability of CuNPs/CNFs was also examined by the
Ullmann coupling of iodobenzene with phenol as a test model.
Aer completion of the reaction, the catalyst was recovered by
ltering, washing with distilled water and anhydrous ethanol,
and was dried in a hot air oven at 100 ꢀC for 2 h. The recovered
catalyst was reused under similar conditions for the next run,
and it was observed that the conversion of the iodobenzene
gradually dropped with each reaction cycle (Table 4). This may
due to the copper particles leaching from the surface of the
catalyst. In spite of this, the CuNPs/CNFs could be reused as a
catalyst for the coupling reaction at least ve times.
Conclusion
In conclusion, for the rst time, we have demonstrated the
application of copper nanoparticles stabilized on carbon
nanobers is an efficient heterogeneous catalytic system for the
Ullmann-type couplings. This catalytic system is particularly
advantageous for easy, quick, ligand-free and reusable. Further
Table 3 CuNPs/CNFs catalyzed N-Arylation of N-heterocycles with iodobenzene
Entry
Aryl halide
N-heterocyclic
Product
Conversion (%)
100
Selectivity (%)
98.09
1
2
3
95.78
100
98.52
84.09
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investigation of this catalytic system for C–C, C–N and C–S bond
formation is underway in our laboratory.
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Acknowledgements
This work is nancially support by the National Natural Science
Foundation of China (21166016), and the Inner Mongolia
Natural Science Foundation (2010ZD02).
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