Palladium NPs supported on novel imino-pyridine-functionalized MWCNTs: Efficient and highly reusable catalysts for the Suzuki-Miyaura and Sonogashira coupling reactions

Article abstract of DOI:10.1039/c5nj02842f

In this article a new heterogeneous nanocatalyst based on palladium supported on functionalized multi-walled carbon nanotubes (MWCNTs) has been introduced. The synthetic process of the mentioned nanocatalyst, MWCNT-imino-pyridine/Pd, has been described. The characterization of the MWCNT-imino-pyridine/Pd was afforded by SEM, EDX, TEM, FTIR, ICP, and XRD. The surface structure of the materials was confirmed using Fourier transform infrared (FTIR) spectroscopy. The catalytic activity of MWCNT-imino-pyridine/Pd was tested in Sonogashira and Suzuki-Miyaura cross-coupling reactions affording various derivatives of both aryl alkynes and biaryls. The catalyst can be readily recovered and recycled at least six times without significant loss of catalytic activity.

Full text of DOI:10.1039/c5nj02842f

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Palladium NPs Supported on Novel Imino-Pyridine-
Functionalized MWCNTs: Efficient and Highly Reusable
Catalyst for the Suzuki-Miyaura and Sonogashira Coupling
Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mehdi Adib, a* Rahman KarimiꢀNami,a Hojat Veisib
www.rsc.org/
In this article the new heterogeneous nanocatalyst based on palladium supported on functionalized multiꢀwalled carbon
nanotubes (MWCNTs) has been introduced. The synthetic process of preparation of mentioned nanocatalyst, MWCNTꢀ
iminoꢀpyridine/Pd, has been described. The characterization of the MWCNTsꢀiminoꢀpyridine/Pd was afforded by the SEM,
EDX, TEM, FTIR, ICP, and XRD. The surface structure of the materials was confirmed using Fourier transform infrared
(FTIR) spectroscopy. The catalytic activity of MWCNTꢀiminoꢀpyridine/Pd was tested in Sonogashira and SuzukiꢀMiyaura
crossꢀcoupling reactions affording various derivatives of both aryl alkynes and biaryls. The catalyst can be readily recovered
and
recycled
at
least
six
times
without
significant
loss
of
catalytic
activity.
biguanideꢀfunctionalized singleꢀwalled carbon nanotubes
(SWCNTs) hybrid materials14 and functionalization of fullerene
(C60) with metformine to immobilized palladium15 as two
novel heterogeneous and reusable nanocatalysts in the Suzukiꢀ
Miyaura coupling reaction at room temperature. Biaryls are
important skeletons in the structures of biologically active
compounds,16 agrochemicals, pharmaceuticals,17 ligands,18 and
functional materials.19 There have been reported many
approaches using heterogeneous catalysts for the synthesis of
biaryls. In addition to the two above described methods,14,15 we
can mention ligand free palladium supported on solid supports
too, such as activated carbon,20 zeolites and molecular sieves,21
metal oxides,22 and clays.23 Also, various palladium catalysts
are applied to form sp2ꢀsp bonds between aryl or alkenyl
halides or triflates and terminal alkynes. This coupling reaction
has become the most important method for the preparation of
arylalkynes and conjugated enynes, which are precursors for
natural products, pharmaceuticals, and molecular organic
materials.24 However, the catalytic activity of Pd2+ in most
cases decreased gradually when the catalyst was used
repeatedly. This could be ascribed to the weak interaction
between palladium and the nonꢀfunctionalized support material,
as well as the agglomeration and accumulation of Pd
nanoparticles on the surface of the material.25,26 Accordingly,
development of novel ligands in order to increase of stability,
recyclability, and applicability of heterogeneous catalysts is one
of the most important criteria in the field of heterogeneous
catalysts. Unfortunately the ligands that have been used until
now for the functionalization of solid beds suffer from high
cost. Here, we imagined that Schiff bases could offer a solution
as simple and inexpensive alternatives for functionalization of
supports and preparation of Pd Schiff base complexes.27ꢀ29
Herein, we attempted to use ethylenediamine and pyridine
Introduction
Heterogeneous catalysts have attracted significant attention in
organic synthesis as a result of numerous advantages over
related homogenous catalysts , such as avoidance of formation
of inorganic salts, recyclability, nonꢀtoxicity, ease of handling,
safety, shelf life, ease of removal via
filtration or
centrifugation, and ease and safety of disposal.1ꢀ7 Among
heterogeneous catalysts, nano transition metals supported on
functionalized carbon nanotubes (CNTs) and fullerenes (C60)
have been applied in a wide range of organic reactions for
carbonꢀcarbon bond formation. Here, the functionalization of
the carbon nanotube surface with a ligand for immobilization of
the transition metal is a very important step. Complexation of
the ligand to the transition metal prevents leaching and
agglomeration of nanoparticles and improves reusability of the
heterogeneous catalyst.8ꢀ12 Among the fundamental transition
metal supported on functionalized CNTs and fullerene,
palladium nanoparticles have occupied a very particular place.
Specifically, these catalysts have been applied in Suzukiꢀ
Miyaura coupling reactions for the synthesis of biaryls and
heteroaryls from aryl halides with organoboron compounds.13
as well as the sonogashira coupling reaction between a terminal
alkyne and an aryl or vinyl halide. We can mention synthesis of
a.School of Chemistry, University College of Science, University of Tehran, P. O. Box
14155 – 6455, Tehran, Iran. E -mail: madib@khayam.ut.ac.ir
b.Department of Chemistry, Payame Noor University, Tehran, Iran
c. Address here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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The overall synthesis of MWCNTꢀiminoꢀpyridine/Pd2+
nanocatalyst is schematically demonstrated in Scheme 1.
carbaldehyde to prepare iminoꢀpyridine ligands for the
synthesis of palladium nanocatlyst supported on iminoꢀ
The reduction of MWCNTꢀiminoꢀpyridine/Pd2+ by hydrazine
hydrate was performed as follows: 30 mg of MWCNTꢀiminoꢀ
pyridine/Pd2+ was dispersed in 60 mL of water, and then 100
ꢁL of hydrazine hydrate (80%) was added. The pH of the
mixture was adjusted to 10 with 25% ammonium hydroxide
and the reaction was carried out at 95 °C for 2 h. The final
product MWCNTꢀiminoꢀpyridine/Pd0 was washed with water
and dried in vacuum at 50 °C. Scheme 2 depicted the synthetic
procedure of MWCNTꢀiminoꢀpyridine/Pd2+ and MWCNTꢀ
iminoꢀpyridine/Pd0. The concentration of palladium in
pyridineꢀfunctionalized multiꢀwalled carbon
(MWCNTs) hybrid materials. This immobilized palladium
represents new recyclable heterogeneous nanocatalyst
nanotubes
a
(Scheme 1) for SuzukiꢀMiyaura (Scheme 3) and Sonogashira
crossꢀcouplings (Scheme 2).
MWCNTꢀiminoꢀpyridine/Pd2+
and
MWCNTꢀiminoꢀ
pyridine/Pd0 were 19 and 17 wt%, respectively, which were
determined by ICPꢀAES and TGA.
Experimental
2.1 Materials
All the reagents were purchased from Aldrich and Merck and
were used without any purification. The pure MWCNTs
without functional groups were purchased from Petrol Co.
(Tehran, Iran). HCl, H2SO4, HNO3, deionized water, NaH
(80%), ethylenediamine, 2ꢀpyridine carbaldehyde, acetonitrile,
2.5 General procedure for synthesis of biphenyls
A
mixture of MWCNTsꢀiminoꢀpyridine/Pd0 (2 mg),
bromobenzene (1 mmol), phenylbronic acide (1.2 mmol, 145
mg), and K2CO3 (2 mmol, 276 mg) in EtOH/ H2O (1:1, 2 mL)
was stirred at room temperature for the appropriate time as
indicate in Table 4. After completion of the reaction, as
indicated by TLC, the catalyst was separated from the reaction
mixture by centrifugation. Solvent was then evaporated and
pure reaction product was obtained by column chromatography
using hexane: ethyl acetate as the eluent and product was
purified by recrystallization with the mixture of EtOH/ H2O
(1:1). The catalyst was washed with ethanol, dried and
preserved for next run.
PdCl2, and
N,Nꢀdicyclohexylcarbodiimide (DCC) were
obtained from Sigma Aldrich and Merck.
2.2 Synthesis of carboxylic derivative of multi walled carbon
nanotubes (MWCNTs-COOH)
First MWCNTs were added to 50 mL HCl (10 %) then, after
sonication for 15 min, the solution was stirred for 24 h to
purification of MWCNTs by remove the metal ions and other
impurities adsorbed on it. Then the purified MWCNTs were
precipitated and separated using 9000 rpm centrifuging. Then
1000 mg of purified MWCNTs was suspended in 400 mL of
concentrated HNO3 and refluxed for 12 h with vigorous
stirring. The product (MWCNTsꢀCOOH) was separated by
9000 rpm centrifuging and washed with deionized water to
2.6 General procedure for synthesis of 1,2-diphenylethylene
A mixture of ethynylbenzene (102 mg, 1 mmol), bromobenzene
(157 mg, 1 mmol), K2CO3 (138 mg, 1 mmol) and MWCNTsꢀ
iminoꢀpyridine/ Pd0 (2 mg) added to round bottom flask under
aerobic atmosphere at DMF (2mL). The resulting mixture was
placed in an oil bath preheated to 120 °C and stirred for 40 min
(as indicated by TLC) After cooling down the reaction to the
room temperature, the catalyst was separated from the reaction
mixture by centrifugation and the reaction mixture was diluted
with EtOAc (20 mL) and H2O (20 mL). The combined organic
layers were washed with 3 mL brine and dried (MgSO4). The
solvent was removed in vacuum, and the product was purified
by column chromatography on silica gel using a mixture of
EtOAc/hexanes as the eluent. Finally, the white solid (169 mg,
95%) was obtained. The catalyst was washed and preserved for
the next run in the same manner as described in the general
neutral
and
dried
under
vacuum
at
80 ºC for 12 h (Scheme 1).
2.3 Synthesis of MWCNTs-EDA
In the next step a 1000 mg amount of MWCNTsꢀCOOH was
added to 50 mL of ethylenediamine (EDA). After heating and
stirring for 5 min, the DCC was added to the solution and
refluxed for 48 h. Then the prepared MWCNTSꢀEDA was
separated by centrifuge, washed with ethanol and dried in an
oven at 80 ºC for 8 h. The synthesis route of MWCNTsꢀEDA is
illustrated in scheme 1.2.4. Synthesis of iminoꢀpyridine
functionalized MWCNTs
procedure for synthesis of Suzukiꢀ Miyaura product
.
For the synthesis of MWCNTsꢀiminoꢀpyridine, the prepared
MWCNTsꢀEDA was suspended in 25 mL ethanol, and 4 mmol
of pyridine 2ꢀcarbaldhyde was added to the solution. After 12 h
stirring at room temperature the pure product was separated by
centrifugation and washed with ethanol (3× 10 mL).
2.4 Synthesis of MWCNT-imino-pyridine/Pd2+ and MWCNT-
imino-pyridine/Pd0
The iminoꢀpyridine functionalized MWCNTs (1000 mg) and
PdCl2 (70 mg, 0.4 mmol) were dispersed separately in
acetonitrile (30 ml) until soluble at 40 ºC. The two solutions
were mixed together and sonicated for 10 min. The mixture was
stirred for 24 h at room temperature to complete attainment of
coordination. Then yielded MWCNTꢀiminoꢀpyridine/Pd2+
nanocatalyst was subjected to centrifugation, washed with
acetonitrile and DI water and dried in vacuum at 50 ºC for 12 h
2 | J. Name., 2012, 00, 1-3
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HO
OH
O
O
O
OH
HNO3
Reflux
EDA, DCC
Reflux
HO
O
NH2
NH
H2N
HN
O
O
H2N
O
NH2
HN
N
CHO
N
H
O
N
N
N
N
N
NH
O
HN
O
N
N
N
O
HN
N
H
O
Cl
Cl
Pd
N
N
PdCl2
Pd
Cl
N
Cl
N
Cl
N
Pd
NH
O
HN
O
Cl
N
N
N
O
HN
Fig. 1 FTꢀIR spectra of (1) MWCNTs, (2) MWCNTsꢀCOOH,
(3) MWCNTsꢀCOꢀEDA, (4) MWCNTsꢀCOꢀiminoꢀpyridine, (5)
MWCNTsꢀCOꢀiminoꢀpyridine/Pd(II), (6) MWCNTsꢀCOꢀ
iminoꢀpyridine/Pd(0).
Cl
N
H
O
Cl
NH2NH2
N
N
Pd
Pd
N
N
Pd
N
NH
O
HN
O
Pd
N
N
N
Pd
O
HN
Pd
N
O
H
Pd
Pd
Scheme. 1 Synthesis of MWCNTꢀiminoꢀpyridine/Pd2+ and
MWCNTꢀiminoꢀpyridine/Pd0.
Results and discussion
The characterization of the MWCNTsꢀiminoꢀpyridine/Pd0 was
afforded by the SEM, EDX, TEM, FTIR, ICP, and XRD The
surface structure of the materials was confirmed using Fourier
transform infrared (FTIR) spectroscopy. Figure 1 shows the
FTꢀIR spectra obtained for (1) MWCNTs, (2) MWCNTsꢀ
COOH, (3) MWCNTsꢀCOꢀEDA, (4) MWCNTsꢀCOꢀiminoꢀ
pyridine, (5) MWCNTsꢀCOꢀiminoꢀpyridine/Pd(II), (6)
MWCNTsꢀCOꢀiminoꢀpyridine/Pd0. Comparison of the FTꢀIR
spectra of MWCNTsꢀCOOH with MWCNTs, a new band 1723
cmꢀ1, the band at 1723 cmꢀ1 corresponds to the carbonyl
stretch of the carboxylic acid group. Also The FTꢀIR spectrum
of MWCNTsꢀCOꢀEDA exhibits several signals originating
from amino ethyl groups, which are related to CꢀH stretching
modes of the ethyl. These signals appear in the area of 1461ꢀ
1563 cmꢀ1 and 2858ꢀ2931 cmꢀ1. In curve 3 the peak at 1680
cmꢀ1 is attributed to the carbonyl stretching of the amide
groups (ꢀCONHꢀ). The doublet peak at 3300 cmꢀ1 corresponds
to the amine group. Also, in curve 4 the band in the spectral
region of 1645 cmꢀ1 can be assigned to the imine (C=NH) bond
of the attached pyridineꢀ2ꢀcarbaldehyde. These results indicate
that the pyridineꢀ2ꢀcarbaldehyde is bound to the surface of
MWCNTs through amidation reaction. The C=NH bands of
complex 5 were shifted to a lower frequency in the IR spectrum
(1625 cmꢀ1) compared to that of 4. The lowering in frequency
of the C=NH peak is indicative of the formation of a metal–
ligand bond. Also, scanning electronic microscopy (SEM)
analysis of MWCNTs before (a) and after (b) functionalization
with pyridineꢀ2ꢀcarbaldehyde (Figure 2) confirmed the
Fig. 2 SEM images of MWCNT before (a) and after (b)
functionalization by iminoꢀpyridine.
successful functionalization of multiꢀwalled carbon nanotubes
.
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Transmission electron microscopy (TEM) investigations were
carried out to observe the morphology and distribution of
palladium particles supported on MWCNTsꢀiminoꢀpyridine/Pd.
The existence of Pd nanoparticles deposited on fꢀMWCNTs is
clearly distinguishable as dark spots in Fig. 3. From Fig. 3 the
Pd can be observed to be well dispersed on the surface of
iminoꢀpyridine modified MWCNTs. The results indicate that
iminoꢀpyridine plays an important role in improving the
dispersibility of Pd.
Counts
I93-4756-001-PdO...CAF
1000
500
0
10
20
30
40
50
60
70
80
90
Position[°2Theta]
Fig. 4 The XRD patterns for MWCNTsꢀiminoꢀpyridine/Pd
Fig. 3 TEM image of MWCNTꢀiminoꢀpyridine/Pd.
Fig. 5 EDS of MWCNTsꢀiminoꢀpyridine/Pd.
To check the thermal stability of the catalyst (MWCNTsꢀiminoꢀ
pyridine/Pd), a thermoꢀgravimetric analysis (TGA) was done which
showed a slight weight loss up to 130 ºC followed by continuous
weight loss up to 500 ºC. The analysis indicated the stability of
catalyst up to a range of 0ꢀ130 ºC and hence is safe to be used in
carrying out the reaction under chosen conditions (room
temperature, and 120 ºC) (Fig. 6). The actual weight loss
corresponding to the combustion of supported ligand was determined
by thermogravimetry as the weight loss between 150 and 600 ºC,
i.e., excluding the weight loss due to desorption of water. The first
mass loss occurs at temperature range of 100 ºC that is related to the
loss of solvent or water trap in the composite. Another mass loss
occurs at temperature range of 130ꢀ600 ºC that is related to the loss
of ligand (iminoꢀpyridine) the surface of carbon nano tubes. The
TGA exhibited a weight loss of 24% in the temperature range from
150–600 ºC due to degradation of iminoꢀpyridines grafted onto
CNTs, which indicates that the amount of iminoꢀpyridines grafted to
CNTs was 24 wt% (Fig. 6).
Xꢀray diffraction spectra (XRD) patterns of MWCNTsꢀiminoꢀ
pyridine/Pd are displayed in Fig. 4. The wide diffraction peak at
2θ = 26° can be assigned to disorderedly stacked hexagonal
graphite structure [15]. The wellꢀdefined peaks around 39° and
47° can be assigned to (111) and (200) crystal planes of Pd0.14
Thus the XRD results indicate efficient immobilization of fcc
structured Pd nanoparticles on fꢀMWCNTs.
The existence of metallic Pd in iminoꢀpyridineꢀfunctionalized
multiꢀwalled carbon nanotubes was also confirmed by the EDX
detector coupled to the SEM which showed the presence of C,
O and N in Figure 5. The peaks derived from Cu were from the
copper grid used in SEM measurements.
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Fig. 7. XPS spectra of MWCNTꢀiminoꢀpyridine/Pd for Pd 3p.
The reaction of ethynylbenzene and bromobenzene was
selected as a model reaction for optimization of Sonogashira
coupling conditions (Table 1).
Fig. 6. TGA of MWCNTꢀiminoꢀpyridine/Pd.
Scheme.
2 Synthesis of 1,2ꢀdiphenylethylene derivatives
through the Sonogashira reaction.
The XPS spectroscopic analysis of the heterogeneous catalyst is a
quantitative technique to indicate the electron properties of the
species immobilized on the surface, such as oxidation state, the
electron environment and the binding of the core electron (E
binding) of the metal. Fig. 4 displays the Pd binding energy of
MWCNTꢀiminoꢀpyridine/Pd. The study of the MWCNTꢀiminoꢀ
pyridine/Pd at the Pd 3p level shows peaks at 531.8 and 553.9 eV for
Pd 3p3/2, which clearly indicates that the Pd nanoparticles are stable
as metallic state in the nanocomposite structure. In comparison to the
standard binding energy of Pd0, with Pd 3p3/2 of about 532.4 eV and
Pd 3p1/2 of about 560.2 eV, it can be concluded that the Pd peaks in
the MWCNTꢀiminoꢀpyridine/Pd shifted to lower binding energy
than Pd0 standard binding energy. The previous studies30,31 indicated
that the position of Pd 3p peak is usually influenced by the local
chemical/physical environment around Pd species besides the formal
oxidation state, and shifts to lower binding energy when the charge
density around it increases. Therefore, the peaks at 553.9 and 531.8
could be due to Pd0 species bound directly to iminoꢀpyridine groups
in the MWCNTꢀiminoꢀpyridine/Pd.
The effect of different solvents such as DMF, CH3CN, and H2O
along with effect of various bases such as Et3N and K2CO3 at
different temperatures was studied. All experiments were
performed with catalyst loadings of 2 mg of MWCNTsꢀiminoꢀ
pyridine/Pd. Optimization studies identified DMF as the most
effective solvent and K2CO3 as the most effective base. After
40 min at 120 °C the desired product was formed in 95 % yield.
When solvent was excluded from the reaction yields suffered
(~40%)
Table 1 The effect of various solvents, bases, and temperatures
on product yield.a
Entry
Solvent
Base
Temperature Yieldb Time
(˚C)
(%)
(min)
1
2
DMF
DMF
Et3N
Et3N
80
Reflux
120
Reflux
Reflux
80
Reflux
90
110
70
80
95
50
65
10
20
ꢀ
60
40
40
90
90
120
120
150
150
150
3
DMF
K2CO3
4
5
6
7
CH3CN
CH3CN
EtOH/H2Oc
Et3N
K2CO3
Et3N
EtOH/H2Oc K2CO3
8
9
10
Free
Free
Free
K2CO3
K2CO3
Et3N
10
40
90
aReaction conditions: Ethynyl benzene (1 mmol),
bromobenzene (1 mmol), MWCNTsꢀiminoꢀpyridine/Pd (2 mg,
1 mol%), solvent (4 ml).
bisolated yield
cEtOH/ H2O= 1:1
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In an effort to establish the scope of this Sonogashira coupling Table
3. The optimization of SuzukiꢀMiyaura coupling
we applied these conditions for the crossꢀcoupling of a variety reaction.a
of aryl halides. These results are summarized in Table 2. Steric
and electronic variation of the aryl halide did not significantly
influence yields and a number of useful functional groups were
tolerated. Aryl bromides and iodides were demonstrated to be
equally effective coupling partners. In all cases, excellent yields
(80ꢀ95%) were obtained. These results attest to the reliability of
this catalyst for Sonogashira coupling reactions.
Entry
Solvent
Catalyst
(mg)
Base
Time
(min)
120
120
25
120
120
60
Yield
(%)b
60
30
98
65
80
95
98
1
2
3
4
5
6
7
EtOH
H2O
2
2
2
2
2
1
3
K2CO3
K2CO3
K2CO3
NaOAc
Et3N
EtOH/H2Oc
EtOH/H2Oc
EtOH/H2Oc
EtOH/H2Oc
EtOH/H2Oc
K2CO3
K2CO3
25
aReaction conditions: Bromobenzene (1 mmol), PhB(OH)2 (1.1 mmol),
MWCNTsꢀiminoꢀpyridine/Pd (2 mg, 1 mol%), rome temperature,
solvent (4 mL, bisolated yield, cEtOH/ H2O= 1:1
Table 2. Synthesis of 1,2ꢀdiphenylethylene derivatives by use
of various aryl halides.
After optimization of the reaction conditions, the generality of
the reaction was confirmed by testing the various aryl halides
and phenylboronic acids (Table 2, entries 1ꢀ23). In all cases
biaryl products were afforded in good to nearly quantitative
yields. It is noteworthy that the MWCNTsꢀiminoꢀpyridine/Pd is
effective in the coupling of conventionally challenging aryl
chlorides in addition to aryl bromides and iodides (Table 4,
entries 3, 5, 6, 16, 17).
Entry
1
R
H
X
Time (min) Yield (%)
Br
Br
Br
Br
Br
Br
Br
Br
I
40
45
45
40
40
60
45
60
30
30
30
60
60
95
93
89
95
91
89
92
88
98
96
95
89
88
2
3
4
5
6
7
1ꢀNaphthyl
2ꢀCH3
4ꢀCH3O
4ꢀCH3
3ꢀNO2
4ꢀCl
Table 4. Suzuki crossꢀcoupling reaction of aryl halides with
phenylboronic acids.
B(OH)2
+
X
8
4ꢀCN
9
H
R1
R2
R1
R2
10
11
12
13
4ꢀCH3
4ꢀCH3O
5ꢀPyrimidinyl Br
3ꢀPyridinyl Br
I
I
Entry
R1
R2
X
Time Yield
(min)
10
30
180
30
150
150
45
30
30
30
30
240
60
180
60
360
360
120
20
120
120
45
(%)
98
98
89
98
85
86
90
92
95
95
90
72
87
60
90
55
30
65
98
95
90
96
96
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
I
The efficiency of MWCNTsꢀiminoꢀpyridine/Pd0 as a catalyst
for the SuzukiꢀMiyaura crossꢀcoupling was next examined. For
this purpose the reaction between bromobenzene and
phenylboronic acid was chosen as a model system. For
optimization of the reaction various solvents, bases, and
temperature were tested and the products yield were compare to
each other. In all case 2 mg of MWCNTsꢀiminoꢀpyridine/ Pd
was used as catalyst. These experiments identified that the
reaction in EtOH/H2O (1:1) as solvent and K2CO3 as base
afforded the highest yield. Results are shown in Table 3.
Br
Cl
Br
Cl
Cl
Br
Br
Br
Br
Br
Br
Br
Br
I
Cl
Cl
Br
I
Br
Br
Br
Br
4ꢀCl
4ꢀCH3O
4ꢀCH3
4ꢀCN
4ꢀNO2
4ꢀCHO
4ꢀCOCH3
3ꢀNO2
2ꢀCHO
3ꢀPyridinyl
2ꢀThienyl
2ꢀThienyl
4ꢀPyridinyl
3ꢀPyridinyl
2ꢀNO2
4ꢀCH3O
H
B(OH)2
+
X
MWCNTsꢀiminoꢀpyridine/Pd
H2Oꢀ EtOH, K2CO3, r.t.
R1
R2
R1
R2
X = Cl, Br, I
Scheme 3. Synthesis of biaryles through the Suzuki crossꢀ
coupling reaction.
4ꢀNO2
4ꢀNO2
4ꢀCH3
4ꢀCH3
4ꢀCOCH3
H
4ꢀCOCH3
60
Also, in order to confirm reusability of MWCNTsꢀiminoꢀ
pyridine/Pd in the SuzukiꢀMiyaura crossꢀcoupling reaction
between aryl halides and phenylboronic acid, the reaction
between iodobenzene and phenylboronic acid was chosen as a
model reaction. After each reaction, the catalyst was washed
with a mixture of ethanol and ethyl acetate (1: 1) and dried at
60 °C for three hours. The results showed that the catalyst can
6 | J. Name., 2012, 00, 1-3
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be reused six times without significant loss of its catalytic
activity (Fig. 8).
3
4
5
6
7
8
9
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Fig. 8 The recycling of MWCNTsꢀiminoꢀpyridine/Pd in the
SuzukiꢀMiyaura reaction.
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Several studies have successfully determined the amount of
metal leaching using a hotꢀfiltration technique. A reaction
mixture of bromobenzene with phenylbronic acid in the aboveꢀ
described catalytic system was stirred at 40 °C for 10 min,
resulting in a yield of 45%. The hot reaction mixture was then
filtered through a dried Celite pad under nitrogen to remove the
MWCNTsꢀiminoꢀpyridine/Pd catalyst and any insoluble
species, and the clear filtrate was introduced to another Schlenk
tube at 40 °C. Further detection by GC demonstrated
improvement of the yield to only 49% after 2 h. This result
shows that no active species were dissolved in the solution to
catalyze the coupling reaction. We further determined the Pdꢀ
content in the filtrate by ICPꢀAES, and only 0.6 ppm of
palladium was found in the solution, which indicated that the
catalytic activity may mainly result from the grafted palladium
complex.
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Briefly, in the present research we demonstrated the catalytic
activity of a new reusable Pd catalyst. The most significant
properties of this catalyst are commercially inexpensive and
available starting materials, convenient experimental procedure,
high catalytic performance and reusability with minimal
palladium leaching. The MWCNTsꢀiminoꢀpyridine/Pd was
demonstrated to promote SuzukiꢀMiyaura and Sonogashira
coupling reactions under mild and environmental friendly
conditions. Also, reaction set up and purification was
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catalysts. We anticipate that this approach will be beneficial for
the advancement of heterogeneous nanocatalysis.
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