Fiber-palladium complex as highly active and recyclable catalyst for heck reaction in air

Article abstract of DOI:10.14233/ajchem.2013.oh90

A new amidoxime fibers-palladium catalyst was synthesized by the 'coordination-reduction' reaction between the amidoxime groups in fibers and Pd(II) ions. The morphology, structure and composition of the as-obtained complexes were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The results indicated that the Pd(II) ions were reduced to Pd(0) nanospheres in situ with the diameter of about 100 nm and there were coordination interactions between the Pd nanoparticles and the amidoxime fibers. The prepared catalyst demonstrated high activities for Heck coupling reaction of aryl iodine. During the reaction, the fibers serve as both the ligand and the support. In all cases, the coupling reactions proceeded efficiently without inert atmosphere. The catalyst can be easily recycled for at least eight times.

Full text of DOI:10.14233/ajchem.2013.oh90

Asian Journal of Chemistry; Vol. 25, No. 10 (2013), 5783-5786
http://dx.doi.org/10.14233/ajchem.2013.OH90
Fiber-Palladium Complex as Highly Active and Recyclable Catalyst for Heck Reaction in Air†
*
ZHICHUAN WU, QI WU, MENG CHEN and TINGXIAN TAO
College of Biochemical Engineering, Anhui Polytechnic University, Wuhu 241000, P.R. China
*
Corresponding author: Tel: +86 553 2871346; E-mail: tingxiantao@yahoo.cn
AJC-13334
A new amidoxime fibers-palladium catalyst was synthesized by the "coordination-reduction" reaction between the amidoxime groups in
fibers and Pd(II) ions. The morphology, structure and composition of the as-obtained complexes were characterized by X-ray diffraction
and X-ray photoelectron spectroscopy. The results indicated that the Pd(II) ions were reduced to Pd(0) nanospheres in situ with the
diameter of about 100 nm and there were coordination interactions between the Pd nanoparticles and the amidoxime fibers. The prepared
catalyst demonstrated high activities for Heck coupling reaction of aryl iodine. During the reaction, the fibers serve as both the ligand and
the support. In all cases, the coupling reactions proceeded efficiently without inert atmosphere. The catalyst can be easily recycled for at
least eight times.
Key Words: Amidoxime fibers, Fibers-palladium complex, Palladium nanoparticles, Heterogeneous catalyst, Heck reaction.
All these make this protocol an efficient and meaningful
alternative for palladium-catalyzed Heck reactions.
INTRODUCTION
Palladium-catalyzed coupling of olefins with aryl or
vinyl halides, known as Heck reaction, is an important and
powerful reaction for the formation of C-C bond in organic
C N PAN
1
-3
synthesis . Many documents have reported that carboxylic
NH
2
OH
4
5
6
7
8
acids , esters , amides , aldehydes and ketones can be
prepared by Heck reaction. In general, these reactions were
NH
2
NH
2
NH
2
PdCl
2
2 2
NH NH
C
C
Pd(II)
OH
C
Pd(0)
N OH
homogeneously catalyzed by Pd-catalysts such as PdCl
2
,
N OH
N
Pd(OAc) etc. But they suffer from drawbacks of difficult separa-
2
AOFs
(Amidoxime fibers)
AOFs-Pd (II)
AOFs-Pd (0)
9,10
tion and limited lifetime of catalysts . In addition, phosphine
or amine ligands are often necessary and the reactions are often
sensitive to oxygen in these cases. To overcome these problems,
Scheme-I: Synthesis route of the amidoxime fibers supported palladium
complex
1
1,12
heterogeneous palladium catalysts supported on polymers
and inorganic solids
EXPERIMENTAL
13-15
were tentatively employed. Herein we
report a new amidoxime fibers-palladium complex (Pd/AOFs)
catalyst for Heck reaction. The amidoxime fibers can be easily
obtained by the reaction of polyacrylonitrile (PAN) and
PdCl
2
, NH
2
OH, HCl, 50 wt. % NH
2
NH
2
aqueous solution
and Na CO
2
3
were purchased from the Beijing Chemical
Reagents Co. (Beijing, China). The PAN fibers were provided
by the Jilin Carbon Co. Ltd., (Jilin, China).
NH
2
OH. The -NH and -OH in amidoxime fibers have high
2
1
6,17
coordinating ability with metal cations
.
Preparation of the catalysts: The PAN fibers (1.0 g) were
Scheme-I shows the synthesis route of Pd/AOFs. The
results of the Pd/AOFs catalyzed Heck reaction showed that
the reaction could be carried out under mild conditions without
protection of inert atmosphere. In addition, the catalyst is environ-
ment-friendly due to no use of phosphine or amine ligands
and the catalyst can be easily recycled for at least eight times.
first immersed in 100 mL 1 mol/L NH
2
OH aqueous solution
at 70 ºC for 1.5 h to introduce -NH and -OH groups onto the
2
18
fiber surface . The obtained fibers were termed as amidoxime
fibers (AOFs). Subsequently, the amidoxime fibers were rinsed
with distilled water and then immersed in 100 mL 7.05 mmol/
L PdCl aqueous solution at about 25 ºC for 1 h to allow
2
†
Presented to the 6th China-Korea International Conference on Multi-functional Materials and Application, 22-24 November 2012, Daejeon, Korea

2θ (º)
5
784 Wu et al.
Asian J. Chem.
2
+
amidoxime fibers to coordinate with Pd ions and the colour
of fibers turned from colourless into yellow/orange at this stage.
After being rinsed with distilled water again, these yellow/
orange fibers were further immersed in 50 mL 0.344 mol/L
TABLE-1
DATA OF XPS FOR Pd, PdCl , AOFs and Pd /AOFs
2
Samples
Pd
Pd3d5/2/eV
335.4
N1s/eV
O1s/eV
PdCl2
338.0
aqueous solution of NH
2
NH at room temperature for 0.5 h.
2
AOFs
Pd/AOFs
399.3/400.5
399.5/401.2
532.4
532.2/534.0
The products were rinsed with distilled water and dried, then
black fibers of Pd/AOFs were obtained. Pd content of Pd/AOFs
was measured by ICP, it is 0.6 mmol/g.
Typical procedure for the Heck reaction: In a typical
reaction, 9 mmol aryl halides, 15 mmol acrylic acid, 30 mmol
tributylamine and 6 mL DMF were added into a round-
bottomed flask with 0.8 mol % Pd/AOFs as catalyst. The
reaction was initiated at a certain temperature controlled by
silicon oil bath. After completion of the reaction, the reaction
mixture was cooled to room temperature. Then 50 mL of 0.38
337.8
Fig. 1 showed the XRD diffraction peaks of amidoxime
fibers and Pd/AOFs. Fig. 1(a) showed the diffraction peaks
centered at 18.6º of amidoxime fibers. Fig. 1(b) revealed the
XRD pattern of Pd/AOFs. Only the characteristic peaks of
Pd(0) centered at the 2θ angles of 40.1º, 46.7º and 67.8º were
observed except amidoxime fibers, which indicating that Pd(II)
was reduced to palladium particles on the surface of amidoxime
fibers by hydrazine hydrate and the product has a high degree
of crystallinity.
mol/L Na
2
CO aqueous solution was added into the mixture
3
under stirring for 15 min and Pd/AOFs was separated from
the mixture by filtration. 15 mL 2 mol/L HCl was added in the
filtrate to obtain the white precipitate, which was filtered and
washed with distilled water (3 × 10 mL) and then dried to
obtain the desired products.
Characterizations: Elements on the surface of samples
were analyzed by X-ray photoelectron spectroscopy using a
Thermo Escalab 250 X-ray photoelectron spectroscopy
operating at the AlK radiation of 1486.6 eV. A Hitachi
α
S-4800 field-emission scanning electron microscope was
employed to examine the morphologies of samples at an
accelerating voltage of 15 kV. X-ray diffraction patterns were
acquired from a Bruker AXS-D8X X-ray diffractometer. A
rotating X-ray generator (40 kW, 50 mA)with CuK
λ = 1.54 Å )was used in the XRD experiments; and the XRD
profiles were recorded from 10 to 80º at the scanning speed of
α
radiation
1
0
20
30
40
50
60
(
-1
6
º min . The Pd(II) content of Pd-AOFs surface was obtained
Fig. 1. XRD patterns of amidoxime fibers (a) and AOFs-Pd(0) (b)
by IRIS INTREPID II XSP. IR measurements were performed
on a WQF-300 Fourier transform infrared spectrometer
Fig. 2 showed the SEM images of Pd/AOFs under different
magnifications. It could be seen clearly that Pd particles had
uniform distribution on the surface of amidoxime fibers and
the average size of the Pd particles was about 100 nm.
(
Beijing Ruili Analytical Instruments, Beijing, China). The
FTIR spectra were acquired by scanning the specimens with a
-1
resolution of 4 cm for 64 times in the wavenumber range of
-1 1
4
000-400 cm . H NMR spectra were recorded on a Bruker
AC-300 NMR spectrometer in CDCl
as internal standards.
3
with tetramethylsilane
RESULTS AND DISCUSSION
Characterization of the catalyst: The X-ray photoelectron
spectroscopy spectrum of Pd(0)/AOFs had the spin-orbital
splitting peaks of Pd3d5/2 at the binding energy of 337.8 eV
(
Table-1). Compared to the standard value of Pd and PdCl
at 335.4 eV and 338.3, the binding energy of Pd3d5/2 in Pd(0)/
AOFs was higher than Pd and lower than PdCl , indicating that
2
3d5/2
Fig. 2. SEM images of Pd/AOFs catalyst under different magnifications:
(
a) Low magnification; (b)high magnification.
2
there is electronic interaction between Pd(0) and the amidoxime
groups. Additionally, the binding energy values of N 1s in
amidoxime fibers were 399.3 eV and 400.5 eV, corresponding
to the two different nitrogen atoms in amidoxime groups .
However, the two values in Pd(0)/AOFs were 399.5 eV and
Catalytic performances of the catalyst: Most of the
Heck reactions were carried out in inert atmosphere at rela-
2
1
19
tively high temperature . Herein, Pd/AOFs catalyst was
employed in Heck reaction efficiently in the air.
Table-2 (entry 1-6) showed the Heck reaction of
iodobenzene with acrylic acid at different temperature. It can
be seen that the reaction temperature has some influences on
the catalytic performance. Using DMF as solvent and
tributylamine as base. The Heck reaction could be carried out
4
01.3 eV. The binding energy values of O 1s in amidoxime
fibers was 532.4 eV, while the corresponding values were 532.2
eV and 534.0 eV in Pd(0)/AOFs. These distinguishable diffe-
rences suggested that there existed coordinations between
20
amidoxime groups and palladium nanoparticles .

Vol. 25, No. 10 (2013)
Fiber-Palladium Complex as Highly Active and Recyclable Catalyst for Heck Reaction in Air 5785
at low temperature (80 ºC) with the yield of 74.9 %. With the
temperature increased from 80 to 110 ºC , the yield increased
and when the temperature was above 110 ºC , the yield had
hardly any change. So 110 ºC was selected as the optimum
reaction temperature.
had been employed. Results of these reactions were presented in
Table-3. The identifications of compounds 3a-f were fully estab-
1
lished by the m.p., FT-IR and H NMR. 3a: m.p.: 133-134 ºC.
-1
FT-IR (KBr, νmax, cm ): 3064, 3027, 1681, 1623, 1573, 1495,
1
1453, 978, 772, 708. H NMR(300 MHz, CDCl
3
): δ = 6.50 (d,
1
H, J = 15.9 Hz), 7.84 (d, 1H, J = 15.9 Hz), 7.26-7.58 (m,
-1
TABLE-2
5H). 3b: m.p.: 124-126 ºC. FT-IR (KBr, νmax, cm ): 3076, 3057,
EFFECTS OF THE REACTION TEMPERATURE, CATALYST
AMOUNT AND BASE ON CATALYTIC PERFORMANCE OF
HECK REACTION OF IODOBENZENE WITH ACRYLIC ACID
1
3019, 1654, 1598, 1577, 1496, 963, 764, 691. H NMR (300
MHz, CDCl ): δ = 7.13 (s, 2H), 7.28-7.55(m, 10H). 3c: m.p.:
289-290 ºC. FT-IR (KBr, νmax, cm ): 3110, 3050, 1688, 1630,
1604, 1595, 1525, 987, 840, 757. H NMR (300 MHz, DMSO):
3
-1
COOH
I
1
COOH
AOFs-Pd(0), base (30 mmol)
δ = 6.71 (d, 1H, J = 16.0 Hz), 7.66 (d, 1H, J = 16.0 Hz), 7.93
DMF(6 ml), temperature, time, in air
(
d, 2H, J = 7.08 Hz), 8.19 (d, 2H, J = 7.98 Hz), 12.64 (s, 1H).
9
mmol
15 mmol
-1
a
3d: m.p.: 157-159 ºC. FT-IR(KBr, νmax, cm ): 3077, 3025,
Temperature AOFs-Pd(0)
Time
(h)
7
5
5
5
5
5
5
5
3
3
3
3
3
3
3
Yield
(%)
Entry
Base
1
1
678, 1632, 1598, 1515, 971, 767, 694. H NMR (300 MHz,
CDCl ): δ = 7.18 (d, 1H, J = 16.2 Hz), 7.31 (d, 1H, J = 15.8
Hz), 7.34-8.38 (m, 9H). 3e: m.p.: 172-174 ºC. FT-IR (KBr,
(ºC)
(mol /%)
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.2
0.4
0.8
1.6
3.4
0.8
0.8
0.8
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
80
90
Bu N
74.9
78.4
79.4
82.1
82.0
82.1
57.8
82.1
85.5
90.3
90.2
90.3
95.5
92.5
2.3
3
3
Bu N
3
100
110
120
130
110
110
110
110
110
110
110
110
110
110
110
Bu N
-1
3
ν
max, cm ): 2973, 2938, 1682, 1624, 1598, 1510, 971, 827,
Bu N
3
1
7
73. H NMR(300 MHz, CDCl ): δ = 3.86 (s, 3H), 6.36 (d,
3
Bu N
3
3
Bu N
1H, J = 15.9 Hz), 7.78 (d, 1H, J = 15.9 Hz), 6.94 (d, 2H, J =
.25 Hz), 7.51 (d, 2H, J = 8.22Hz). 3f: m.p.: 138-139 ºC. FT-
Bu N
3
8
Bu N
3
-1
IR (KBr, νmax, cm ): 2962, 2935, 1686, 1598, 1566, 1515,
969, 815, 751. H NMR (300 MHz, CDCl ): δ = 3.84 (s, 3H),
Bu N
3
1
0
1
2
3
4
5
6
7
Bu N
3
3
Bu N
6.90-7.52 (m, 11H). It can be seen from the table that the Heck
reaction of acrylic acid or styrene with aryl iodides could be
carried out efficiently with relatively higher yield than aryl
bromides or aryl chlorides even at lower temperature. Neither
electron-donating group nor electron-withdrawing group on
aryl iodides has effect on the yield of the Heck reaction of
aryl iodides. While the group on aryl bromides or aryl chlorides
has much effect on the yield. By the way, all the reactions
3
Bu N
3
Et N
3
Bu N
3
NaHCO3
KOH
Na CO3
2
0.8
0.8
3
3
1.8
2.5
a
Isolated yield based on the iodobenzene used
It is easy to understand that the amount of catalyst can play
1
afforded exclusively trans products based on H NMR analysis.
an important role in the catalytic performances. So we explored
the reaction of iodobenzene with acrylic acid catalyzed with
different amount of Pd/AOFs ranging from 0.1 to 3.4 mol % at
TABLE-3
HECK REACTION OF ARYL HALIDES WITH CONJUGATED
1
10 ºC. The results are listed in Table-2 (entry 7-12). It could be
ALKENES CATALYZED BY AOFs-Pd(0)
R2
obviously seen that the employed amount of catalyst has
significant effect on this transformation. The reaction could
proceed efficiently even with trace of the catalyst (entry 7, 0.1
mol %, yield 57.8 %). Increasing the amount of catalyst led to
higher yield and shorter reaction time. The yield had almost no
changes when the employed amount of Pd catalyst was increased
to a certain value (> 0.8 mol %).
I
R2
AOFs-Pd(0)
DMF,Et N
3
R1
R1
c
Open to air
1
2
3
3
a
b
d
e
f
R1=H, NO2, OCH3 R2=COOH, Ph
R1
H
H
NO2 NO2 OCH3 OCH3
R2 COOH Ph COOH Ph COOH Ph
From the classical catalytic mechanism for Heck reaction
of aryl halides with conjugated alkenes, it can be seen that the
a
b
o
c
Entry
X
I
I
I
I
R1
H
H
NO
NO2
R2
Product T ( C) t (h) Yield (%)
1
2
3
4
5
6
CO H
3a
3b
3c
3d
3e
3f
110
90
110
90
110
90
3
3
7
3
7
3
95.5
89
93.7
89.3
95.7
89.9
2
22
active site of palladium catalyst is Pd(0) . During the reaction
process, acid (HX) may be produced from the formal exchange
Ph
CO H
2
2
23
Ph
2
of a hydrogen atom with an aryl or vinyl group , so alkaline
reagents were desired to eliminate HX in time. We also investi-
gated the influence of different alkaline reagents on Heck
reaction, including tributylamine, triethylamine, sodium
bicarbonate, potassium hydroxide and sodium carbonate. The
corresponding yield was 95.5 %, 92.5 %, 2.3 %, 1.8 %, 2.5 %,
respectively (Table-2, entry 13-17). It could be seen that organic
bases is preferred, since inorganic bases (sodium bicarbonate,
potassium hydroxide, sodium carbonate) are insoluble and thus
can not participate in the reaction very well.
I
I
OCH3 CO H
OCH3
Ph
a
All reactions were carried out with aryl halides (9 mmol), conjugated
alkenes (15 mmol), Et N (30 mmol), AOFs-Pd(0) (0.8 mol %) and
DMF (6 mL) in the air. All products were characterized by m.p., FT-
IR and H NMR. Isolated yield based on the iodobenzene used
3
b
1
c
Recycling of the catalyst: The catalyst can be easily sepa-
rated from the reaction mixture just by filtration during the
work-up procedure. Eight recycles were carried out and the
results were summarized in Table-4. It could be seen that the
catalytic activities of AOFs-Pd(0) were almost unchanged
In order to experiment the catalytic activity of the AOFs-
Pd(0) catalyst to other substrates, some interrelated experiments

5
786 Wu et al.
Asian J. Chem.
TABLE-4
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95.5
4
3
3
3
3
5
5
5
5
5
3
4
5
.
.
.
th
1
2
3
4
5
6
7
8
recycle
recycle
recycle
recycle
recycle
recycle
recycle
recycle
95.5
95.4
89.5
88.7
86.3
84.1
82.8
80.3
3
th
th
th
th
th
th
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a
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b
(
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(
1
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1
1
1
1
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In summary, AOFs-Pd(0) catalysts have been prepared
and exhibit high activity for the Heck reaction of aryl iodides
with conjugated alkenes without inert atmosphere. Low amount
usage of palladium catalyst is required to perform the Heck
reaction of aryl iodides. The catalyst can be easily separated
from the reaction mixtures just by filtration and can be reused
for at least eight times. It could be considered as an effective
alternative heterogeneous catalyst for Heck reaction.
2
2
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ACKNOWLEDGEMENTS
The authors are thankful to financial support from the
National Nature Science Foundation of China (21171003,
2
0771002).