Polyethylene glycol-supported recyclable NC palladacycle catalyst for Heck cross-coupling reactions

Article abstract of DOI:10.1016/j.catcom.2013.08.027

Palladium nanoparticles with narrow size distribution were easily prepared by applying polyethylene glycol (PEG 1000 and 15000) and NC palladacycle in the absence of other chemical agents. The Pd-PEG catalysts were fully characterized by a variety of techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis and Fourier transform infrared (FT-IR) spectroscopy. TEM micrographs of Pd/PEG 15000 show that palladium nanoparticles have mostly very well-defined geometrical shapes. Also Pd/PEG 15000 was relatively an efficient catalyst system for the Heck reaction, affording a diverse range of products in moderate to good yields.

Full text of DOI:10.1016/j.catcom.2013.08.027

Catalysis Communications 43 (2014) 25–28
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Polyethylene glycol-supported recyclable NC palladacycle catalyst for
Heck cross-coupling reactions
⁎
Kazem Karami , Zohreh Karami Moghadam, Mahboubeh Hosseini-Kharat
Department of Chemistry, Isfahan University of Technology, Isfahan 84156/83111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 June 2013
Received in revised form 20 August 2013
Accepted 31 August 2013
Available online 6 September 2013
Palladium nanoparticles with narrow size distribution were easily prepared by applying polyethylene glycol
(PEG 1000 and 15000) and NC palladacycle in the absence of other chemical agents. The Pd-PEG catalysts
were fully characterized by a variety of techniques including X-ray diffraction (XRD), transmission electron
microscopy (TEM), UV–vis and Fourier transform infrared (FT-IR) spectroscopy. TEM micrographs of Pd/PEG
15000 show that palladium nanoparticles have mostly very well-defined geometrical shapes. Also Pd/PEG
15000 was relatively an efficient catalyst system for the Heck reaction, affording a diverse range of products in
moderate to good yields.
Keywords:
Palladium nanoparticles
Heck reaction
© 2013 Elsevier B.V. All rights reserved.
Polyethylene glycol
Palladacycle
Heterogeneous catalysis
1. Introduction
supported soluble palladacycle as an excellent and recyclable catalyst
for Heck, Suzuki and Sonogashira reactions. The activity and stability of
The palladium-catalyzed Heck reaction developed in the early 1970s
was a milestone in modern organic chemistry [1,2]. Heck reaction is nor-
mally carried out in the presence of phosphine ligands and base under
an inert atmosphere. However, the relatively high price of the palladium
complex has greatly limited the industrial application of homogeneous
Heck reaction, and some of the phosphine ligands are sensitive to air
and moisture [3,4]. Most of the problems related to homogeneous cata-
lysts can be solved by immobilizing the catalyst or catalyst precursor on
polymer supports with good solvation attributes [5].
Palladium (Pd) nanoparticles are a unique class of heterogeneous
catalysts that is widely used and effective for a variety of organic reac-
tions due to its high surface-to-volume ratio. These catalysts can be
based on either supported palladium nanoparticles or supported palladi-
um complexes on various supports, such as, carbon [6], silica [7], resin
[8], chitosan [9], alumina [10], zeolite [11], and organic polymer supports
[12,13].
the PEG-anchored oxime palladacycle in coupling reactions have been
also reported by Corma et al. [20].
Among polymer supported nanoparticles, polyethylene glycol (PEG)
is a promising candidate which is inexpensive, nontoxic and its proper-
ties can be tuned by simply changing molecular weight [21]. Herein, in
continuation of our studies in developing new supported catalysts
[22,23], we report a simple and environmentally friendly route for the
preparation of PEG supported NC palladacycle (Scheme 1). To the best
of our knowledge, the present work is the first example of the heteroge-
neous nanocatalyst based on amine NC palladacycle supported on PEG.
We also investigated the application of the prepared catalyst in the Heck
cross-coupling reaction which was carried out in the absence of phos-
phine ligands under air.
2. Experimental
On the other hand, palladacycles have been known as well-defined
catalysts or precatalysts in cross-coupling reactions [14–16]. They can
be easily prepared, show high stability under atmospheric conditions
and can be anchored to different solid supports. Supported palladacycles
are increasingly investigated by several research groups and being used
in various organic transformations. Najera and Alacid [17,18] performed
Heck and Suzuki cross-couplings using polystyrene supported oxime
palladacycle in water as solvent. Luo et al. [19], used polystyrene-
2.1. Instruments and reagents
All chemicals and solvents were purchased from Merck and Aldrich
and used without further purification or drying. Conversions were mon-
itored using an Agilent 6890N gas chromatograph equipped with a cap-
illary HP-5+ column, based on aryl halides. Electronic absorption
spectra were recorded on a JASCO 7580 UV–vis–NIR spectrophotometer.
Infrared spectra were recorded on a FT-IR JASCO 680 spectrophotometer
in the spectral range 4000–400 cm−1 using the KBr pellets technique.
X-ray powder diffraction data were collected on an XD-3A diffractome-
ter using Cu Kα radiation. Transmission electron microscopy analyses
were performed by PHILIPS (model CM120) electron microscope.
⁎
Corresponding author. Tel.: +98 3113913239; fax: +98 3113912350.
E-mail address: karami@cc.iut.ac.ir (K. Karami).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.08.027

26
K. Karami et al. / Catalysis Communications 43 (2014) 25–28
Table 2
Heck coupling of aryl halides with styrene and methyl acrylate catalyzed by Pd/PEG 15000.
1) PEG 1000 or 15000
85 °C, 3h
H2N
Pd
Cl
Cl
Pd
NH2
PdNPs/PEG
2) Cooling
Scheme 1. Preparation of supported palladium nanoparticles (PdNPs) using different
PEGs.
Entry
Ar
X
R
Yielda (%)
1
2
3
4
5
6
7
8
9
10
C6H5
I
I
I
I
Br
Br
I
I
I
C6H5
C6H5
C6H5
C6H5
80
40
53
68
61
27
40
19
27
67
m-CH3C6H4
p-CH3C6H4
p-CH3OC6H4
C6H5
C6H5
C6H5
m-CH3C6H4
p-CH3C6H4
p-CH3OC6H4
Palladium content of the catalyst was measured by Inductively Coupled
Plasma (ICP-OES) analyzer (PerkinElmer 7300DV spectrometer). Di-
meric NC palladacycle was obtained using procedure described earlier
[24].
C6H5
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
I
2.2. Preparation of Pd-PEG catalyst
Reaction conditions: aryl halide 1 mmol, olefin 1.5 mmol, Et3N 1.5 mmol, toluene 9 mL,
ethanol 1 mL, catalyst (4.5 mmol% relative to the amount of aryl halides) at 100 °C, 16 h.
Dimeric NC palladacycle (0.22 mmol, 0.136 g) was added into PEG
(2 mmol) with different molecular weights (1000 or 15000) in a 25-mL
round-bottomed flask. The mixture was heated in a water bath at 85 °C
under vigorous, continuous stirring for 3 h. During this stage, the red-
brown color of the solution turned to dark gray, indicating the formation
of Pd nanoparticles. The mixture of Pd nanoparticles in PEG solidified
upon cooling at room temperature.
a
Isolated yields determined by GC, based on aryl halide.
a rotavapor and the residue was extracted with cold ether (5 × 15 mL).
The combined ether solution was taken for GC analyses.
3. Results and discussion
2.3. General procedure for Heck reaction
3.1. Catalysts characterization
A mixture of aryl halide (1.0 mmol), olefin (1.5 mmol), Et3N
(1.5 mmol), toluene (9 mL), ethanol (1 mL) and catalyst (4.5 mmol% re-
lated to aryl halides) was stirred at 80–120 °C for the time given in
Tables 1 and 2. After the reaction, toluene and ethanol were removed in
The crystalline structure of supported Pd-catalyst was obtained
using powder XRD. The typical patterns of catalysts prepared using NC
palladacycle in PEG (1000 and 15000) are presented in Fig. S1
(Supporting information). As shown in the XRD patterns, two peaks at
about 19.2° and 23.4° indicated the presence of pure PEG polymer
[12]. Also, broad weak peak near 2θ = 40° is detected, which can be
indexed to the characteristic reflection (111) plane for face-centered-
cubic Pd(0) with another weak peak around 2θ = 43°, consistent
with the (200) crystalline plane (JCPDS card no. 87-641). The broaden-
ing of the diffraction peaks as compared to that of bulk Pd indicates the
formation of the palladium nanoparticles. The intensity of palladium
peaks slightly increases in the case of using PEG 15000, which related
to the different degrees of chemical reduction of NC palladacycle in
PEGs. The crystallite size was calculated using Scherer's equation, and
the values corresponding to the refection planes (111) were found to
be 14.8 nm and 13.2 nm for Pd/PEG 1000 and 15000, respectively. The
same result was also obtained for the XRD pattern of the Pd/PEG
15000 after its use in the Heck reaction for several cycles. This reveals
the excellent stability and recovery of the catalyst.
Fig. S2 (Supporting information) displayed the UV–vis spectra of di-
meric NC palladacycle before and after the reduction with different mo-
lecular weight PEGs as a reducing agent. As can be seen, the UV–vis
spectrum of dimer shows absorption maximum at around 330 nm.
After the reaction of Pd(II) ions with different molecular weight PEGs,
the peak observed at 330 nm has strongly decreased, indicating the
conversion of Pd(II) to Pd(0) nanoparticles. Also, the intensity of this
peak further decreased when PEG with larger chain length was applied.
Fig. 1 presents the TEM images of palladium nanoparticles prepared
at 85 °C using PEG 15000. The average dimension of Pd nanoparticles is
around 2–12 nm, in good agreement with the crystallite size calculated
from XRD data. As shown in Fig. 2(a), palladium nanoparticles derived
from PEG 15000 have interestingly very well-defined geometrical
shapes including triangle, rhombohedral, pentagonal and unidentified.
Comparing to the previously reported catalysts (from various starting
materials of palladium) [13,20], this typical catalyst exhibits such a dis-
tinguished shapes.
Table 1
Heck coupling of iodobenzene with styrene and methyl acrylate: Reaction conditions
study.
Entry
Olefin
Base
Temp (°C)/time (h)
Yielda (%)
1
2
3
4
5
6
7
8
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Methyl acrylate
Na2CO3
NaOAc
KOH
K3PO4·3H2O
Et3N
Na2CO3
NaOAc
KOH
K3PO4·3H2O
Et3N
Et3N
Et3N
Na2CO3
Et3N
Et3N
80/8
80/8
80/8
80/8
80/8
Trace
Trace
Trace
Trace
7
Trace
Trace
Trace
Trace
17
21
40
4
12
43
80
9
50
100/8
100/8
100/8
100/8
100/8
120/8
100/16
100/8
80/8
100/8
100/16
100/16
100/16
100/24
9
10
11
12
13
14
15
16
17
18b
19
Et3N
Me3N
Et3N
Et3N
48
Reaction conditions: iodobenzene 1 mmol, olefin 1.5 mmol, base 1.5 mmol, Toluene 9 mL,
ethanol 1 mL, catalyst (4.5 mmol% relative to the amount of iodobenzene).
a
Isolated yields determined by GC, based on iodobenzene.
Using Pd/PEG 1000.
b

K. Karami et al. / Catalysis Communications 43 (2014) 25–28
27
50 nm
Fig. 1. TEM images of PEG 15000 stabilized palladium nanoparticles.
The formation of palladium nanoparticles by heating the NC
palladacycle in PEG in the absence of any other reducing agent may pro-
ceed through the oxidation of the hydroxyl group in PEG. The analysis of
PEG after the reaction by FT-IR revealed that a new absorption peak
appeared at around 1739 cm−1, which could be attributed to the forma-
tion of the aldehyde during the reduction of Pd(II). This is actually in
agreement with a similar analysis done by 1H NMR and confirmed the
formation of aldehyde group for a similar reaction system [13].
temperatures were used. We use toluene and ethanol (9:1) as solvent,
because this combination remarkably reduced the palladium leaching
[13,25].
It was found that PEG 15000 showed the best reactivity to the Heck
reaction. When PEG 1000 was applied as the reaction medium, the yield
was decreased (entry 18, Table 1), which could be attributed to the less
stabilizing effect to nano-Pd and lower reducing reactivity of short chain
length PEG to Pd(II) [13]. The optimized conditions were applied to
Heck reactions between styrene and methyl acrylate with various aryl
iodides and bromides (Table 2). As shown in Table 2, styrene was
more reactive than methyl acrylate when using the same aryl halide.
The results obtained demonstrate moderate efficiency of Pd/PEG
15000 as a catalyst in the Heck reaction. In another study, Pd-PEG (pre-
pared by loading Pd(OAc)2 into the PEG matrix) [13] was tested in the
Heck reactions of aryl halides with olefins and gave high yields of prod-
ucts, which is comparable with our results employing Pd/PEG 15000 in
the same reactions.
3.2. Catalytic activity
The catalytic reactivity of Pd-PEG to Heck reaction involving the
cross-coupling of aryl halides with styrene and methyl acrylate was
tested. The reactions were performed in Pd-PEG by using 4.5 mmol%
of the palladium related to aryl halide. Initially, the Heck reactions of
iodobenzene with methyl acrylate and styrene were chosen as model
reactions. Various parameters including bases, temperature and time
were screened to optimize the reaction conditions (Table 1). As
shown in Table 1, when the organic base Et3N was used, good yield
was obtained but longer reaction time was required (Table 1, entry
16). However, other bases gave trace yields, even when higher
The recovery and reusability of the catalyst were investigated using
the reaction of iodobenzene with styrene as a model system. After the
reaction was completed, the solvent was removed and the product
was extracted into diethyl ether and separated by a simple decantation.
The Pd/PEG 15000 catalyst is immiscible in ether and could be easily
50 nm
50 nm
50 nm
692 nm
Fig. 2. a) (top) TEM micrographs showing typical shapes of Pd nanoparticles in the Pd/PEG 15000: triangular and rhombohedral (left), pentagonal and unidentified (right). b) (bottom)
TEM images of Pd/PEG 15000 after its use as catalyst in the Heck coupling reaction.

28
K. Karami et al. / Catalysis Communications 43 (2014) 25–28
Table 3
80
70
60
50
40
30
20
10
0
Evaluation of palladium leaching in the recycling experiments.
Recycle number
1
2
3
4
Concentration (μg/mL)
Pd leaching (%)
0.16
0.034
0.77
0.16
0.21
0.043
0.25
0.052
80
73
76
78
The coupling of iodobenzene with styrene was taken as the model reaction. The amount of
nano-Pd was 4.5 mmol% relative to the amount of iodobenzene.
Acknowledgments
We are sincerely grateful for the financial support from the Iranian
Nano Technology Initiative Council. Funding of our research from the
Isfahan University of Technology (IUT) is also acknowledged.
1
2
3
4
Run
Fig. 3.
A recyclability test for Pd/PEG 15000 in Heck cross-coupling reaction of
Appendix A. Supplementary data
iodobenzene with styrene (reaction conditions are the same as given in Table 2).
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.08.027.
recovered and used for the next reaction cycle. The recycled catalyst was
then used in the model reaction under the conditions mentioned in
Table 2, giving 80, 78, 73 and 76% yields (Fig. 3).
The leaching of palladium in the recycling of the Pd/PEG was detect-
ed by ICP-OES analysis and the result was presented in Table 3. These
data indicated that the extent of leaching of the palladium species was
extremely low. This result demonstrates that the Pd/PEG 15000 catalyst
may have practical utility.
Fig. 2(b) shows the TEM image of the PEG stabilized Pd(0)
nanoparticles after their use as catalyst in the Heck coupling reactions.
It was found that the size of the palladium (0) nanoparticles had in-
creased to 20–30 nm after the reaction, most probably, due to the ag-
glomeration of the particles during the catalysis.
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4. Conclusion
We have successfully prepared PEG supported NC palladacycle
catalyst with no other stabilizer and reducing agent. PEG appeared
to act as both reducing agent and stabilizer. The nano-Pd preparation
process is very simple and “green”. The results of UV show that the
PEG molecular weight has significant effect on reducing reactivity
of polymeric matrix. Both XRD and TEM reveal that nanosized palla-
dium was not only fabricated by reducing of PEG, but was stabilized
suitably by PEG. Also Pd/PEG 15000 was relatively an efficient cata-
lyst system for the Heck reaction. Furthermore, the catalyst is stable
to the reaction conditions, oxygen-insensitive, and can be reusable
with negligible leaching of palladium.