Palladium nanoparticles supported on poly (N-vinylpyrrolidone)-grafted silica as new recyclable catalyst for Heck cross-coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2010.09.028

Novel catalytic system based on palladium nanoparticles supported on poly (N-vinylpyrrolidone) (PVP) grafted silica was prepared. Aminopropylsilica was reacted with acryloyl chloride to form acrylamidopropylsilica, and onto this functionalized silica vinylpyrrolidone monomer was polymerized by free-radical polymerization. The complexation of PVP-grafted silica with PdCl₂ was carried out to obtain the heterogeneous catalytic system. X-ray diffraction (XRD) technique and transmission electron microscopy (TEM) image showed that palladium dispersed through the support in nanometer size. This catalytic system exhibited excellent activity in cross-coupling reactions of aryl iodides, bromides and also chlorides with olefinic compounds in Heck-Mizoraki reactions in short reaction time and high yields. Elemental analysis of Pd by inductively coupled plasma (ICP) technique and hot filtration test showed low leaching of the metal into solution from the supported catalyst. The catalyst can be reused several times in repeating Heck reaction cycles without considerable loss in its activity.

Full text of DOI:10.1016/j.jorganchem.2010.09.028

Journal of Organometallic Chemistry 696 (2011) 594e599
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Palladium nanoparticles supported on poly (N-vinylpyrrolidone)-grafted silica
as new recyclable catalyst for Heck cross-coupling reactions
*
Bahman Tamami , Hamed Allahyari, Soheila Ghasemi, Fatemeh Farjadian
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel catalytic system based on palladium nanoparticles supported on poly (N-vinylpyrrolidone) (PVP)
grafted silica was prepared. Aminopropylsilica was reacted with acryloyl chloride to form acryl-
amidopropylsilica, and onto this functionalized silica vinylpyrrolidone monomer was polymerized by
free-radical polymerization. The complexation of PVP-grafted silica with PdCl2 was carried out to obtain
the heterogeneous catalytic system. X-ray diffraction (XRD) technique and transmission electron
microscopy (TEM) image showed that palladium dispersed through the support in nanometer size. This
catalytic system exhibited excellent activity in cross-coupling reactions of aryl iodides, bromides and also
chlorides with olefinic compounds in HeckeMizoraki reactions in short reaction time and high yields.
Elemental analysis of Pd by inductively coupled plasma (ICP) technique and hot filtration test showed
low leaching of the metal into solution from the supported catalyst. The catalyst can be reused several
times in repeating Heck reaction cycles without considerable loss in its activity.
Received 9 August 2010
Received in revised form
8 September 2010
Accepted 10 September 2010
Available online 8 October 2010
Keywords:
Palladium nanoparticles
Polyvinylpyrrolidone-grafted silica
Heck reaction
Ó 2010 Published by Elsevier B.V.
1. Introduction
groups of these modified silica act as surface active sites for metal
complexes with high suitable catalytic properties.
Palladium-catalyzed cross-coupling reactions is a versatile tool
for the generation of CeC bonds [1e5]. Among different methods,
the reaction of aryl halides with olefinic compounds, Heck reaction,
is an excellent method in the synthesis of organic compounds [6e8].
These reactions are carried out in the presence of Pd catalysts
involving ligands such as phoshines, amines, carbenes, dibenzyli-
dineacetone (dba), etc [1]. The use of such homogenous systems is
accompanied with the loss of expensive metal complexes. Many
chemists have tried to solve this problem by designing heteroge-
neous catalytic systems [5]. The immobilization of homogenous
catalysts on polymeric organic [9e13] or inorganic [14e19] supports
is an expanding research area offering the advantages of easy
product separation, catalyst recovery, recycling and inhibiting metal
losing. In this regard, several methods include supporting Pd
complexes on polymers [9e13] and anchoring of Pd complexes to
various solids such as silica or modified silica [8,15] has been
successfully applied. The supported catalysts on silica or modified
silica are good alternatives to traditional homogenous catalysts and
are environmentally friendly and display some advantageous
properties, such as excellent stability (chemical and thermal), good
accessibility and good dispersion of catalytic sites. The functional
Most of the cross-coupling reactions have been carried out in
the presence of phosphine ligands, but these ligands are usually
poisonous, air sensitive, unrecoverable and may degrade at
elevated temperature, so researchers tried to test phosphine-free
catalytic systems [5]. Nitrogen chelating ligands are good candidate
in replacement of phosphine ligands in Pd catalyzed cross-coupling
reactions since they are air stable, less expensive and nontoxic.
Several types of ligands containing nitrogen such as: Schiff bases,
N-heterocyclic carbene, imidazole and pyridine supported onto
silica have been used in Heck reactions [8]. Furthermore, in recent
years, there has been growing interest in grafting polymer chains to
the surface of silica for practical application and fundamental
studies of interfacial phenomena [20e23]. This technique has
provided an opportunity to take the advantage of a solid linear
functional polymer that can be completely compatible with
solvents and substrates and at the same time having high
mechanical stability and good dispersion [24]. Chemical bindings of
polymer chains onto silica have been studied with various mono-
mers, such as; styrene [25], vinyl acetate [26], acrylic acid [23],
methyl acrylate [27], vinylpyrrolidone [20,24], vinylimidazole [28]
and vinylpyridine [29]. In 2008 Gao et al. reported the prepara-
tion of poly (4-vinylpyridine)eCu complex grafted silica and its use
as a heterogeneous catalyst in oxidation reactions [29]. In 2009 Han
et al. used poly (N-vinylpyrrolidone)eRu complex grafted silica as
a catalyst for hydrogenation of aromatic compounds [30].
* Corresponding author. Tel.: þ98 711 2284822; fax: þ98 711 2280926.
E-mail address: tamami@chem.susc.ac.ir (B. Tamami).
0022-328X/$ e see front matter Ó 2010 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2010.09.028

B. Tamami et al. / Journal of Organometallic Chemistry 696 (2011) 594e599
595
O
C Cl
O
O
O
O
O
THF/Et3N
+
H2C
C
H
O
O
Si
NH C C CH2
Si
NH2
O
0 o
C
H
(I)
N
O
O
O
O
PdCl2
Si
NH C CH CH2 CH
n
CH2I
N
Benzoyl peroxide
O
DMF,
°
80 C
(II)
,
°
90 C
Sealed tube
I= initiator fragment
O
Cl O
Pd
Si-PVP-Pd Complex
N
N
Cl
(III)
A possible complex formed from PVP molecule and PdCl2
Scheme 1. Synthetic strategy for the preparation of poly (N-vinylpyrrolidone)-grafted silica loaded palladium catalyst.
There are few reports in the literature on the application of PVP
as stabilizer of Pd [31,32] and colloidal solution of PVP-Pd [33e35]
in cross-coupling reactions. However, as far as we know there is no
report in the literature on poly (N-vinylpyrrolidone)ePd complex
grafted onto silica to be used in cross-coupling reactions. In
continuation of our previous studies on heterogeneous Pd catalyst
based on polymeric support [36] and polymer grafted silica [37],
herein, we report the synthesis and characterization of poly (N-
vinylpyrrolidone)-grafted silica palladium complex and its appli-
cation as heterogeneous catalyst in Heck reactions with various
substrates.
rate of temperature change of 20 ꢀC/min was used. The Pd analysis
and leaching test were carried out by inductively coupled plasma
(ICP) analyzer (Varian, Vista-Pro). X-ray diffraction data obtained
with XRD, D8, Advance, Bruker, axs. Transmission electron micros-
copy (TEM) analyses were performed on a Philips model CM 10
instrument. Scanning electron microscopy (SEM) were obtained by
SEM, XL-30 FEG SEM, Philips, at 20 kV.
2.2. Preparation of the catalyst
2.2.1. Preparation of acrylamidopropylsilica
Acrylamidopropylsilica was prepared by the reaction between
aminopropylsilica (AMPS) and acryloyl chloride according to
a previous procedure [38]. The capacity of AMPS was first deter-
mined by reverse acid-base titration method, and was found to be
0.95 mmol of amino group per gram of AMPS. The amino-
propylsilica (10 g, 9.5 mmol amino groups) was suspended in dry
THF (200 mL) and the suspension cooled down to 0 ꢀC. Triethyl-
amine (1.51 g, 0.015 mol) was added, followed by addition of
acryloyl chloride (1.09 g, 0.012 mol) over a period of 1 h. Temper-
ature of the reaction reached 5 ꢀC at the end of the addition. The
thick slurry was then stirred at 0 ꢀC for a further 4 h and the
modified silica was isolated by filtration and washing with THF
(100 mL), water (2 ꢁ 100 mL) and acetone (100 mL). The solid
obtained was then dried in an oven at 110 ꢀC for 24 h.
2. Experimental
2.1. General information
Substrates were purchased from Fluka, Merck or Aldrich
Companies. Aminopropylsilica gel with an average particle size of
0.015e0.035 mm (>400 mesh ASTA) was supplied by Fluka. All
products were characterized by comparison of their FT-IR and NMR
spectra and physical data with those reported in the literature. All
yields refer to the isolated products. Progress of the reactions was
followed by TLC on silica-gel Polygram SIL/UV 254 plates or by GC on
Shimadzu GC 14-A instrument with hydrogen flame ionization
detector. FT-IR spectra were run on a Shimadzu FT-IR-8300 spec-
trophotometer. 1H and 13C NMR spectra were recorded on a Bruker
Avance DPX instrument (250 MHz). TGA thermograms were recor-
ded on an instrument of Perkin Elmer with N2 carrier gas and the
Fig. 1. FT-IR spectra of (A) aminopropylsilica (B) acrylamidopropylsilica (C) poly (N-
vinylpyrolidone)-grafted silica.
Fig. 2. Thermogravimetric analysis (TGA) of poly (N-vinylpyrrolidone)-grafted silica.

596
B. Tamami et al. / Journal of Organometallic Chemistry 696 (2011) 594e599
Fig. 3. (A) Scanning electron microscopy (SEM) image of palladium supported on poly (N-vinylpyrrolidone)-grafted silica (B) SEM close-up of image A.
2.2.2. Preparation of poly (N-vinylpyrrolidone)-grafted silica
To a suspension of acrylamidopropylsilica (2.0 g) in fresh 1-vinyl-
2-pyrrolidone (4 mL), recrystallized benzoyl peroxide (0.05 g) was
added in a 10 mL sealed tube. The mixture was heated at 90 ꢀC in an
oven for 15 h. The product was soxhlet-extracted with CHC13
(200 mL) for 24 h, followed by washing with methanol (2 ꢁ 200 mL)
and water (2 ꢁ 100 mL) and dried for 12 h under vacuum.
reaction was monitored by TLC (or GC if necessary). On completion
of the reaction, the mixture was filtered and the filtrate poured into
water (50 mL) and extracted with CH2Cl2 (3 3 15 mL). The combined
organic phases were dried over Na2SO4, filtered and evaporated in
vacuum. The mixture was then purified by column chromatography
over silica-gel or recrystallization to afford a product with high
purity. Characterization of the products was performed by com-
parison of their FT-IR,1H NMR,13C NMR and physical data with those
of the authentic samples.
2.2.3. Preparation of palladium catalyst
Poly (N-vinylpyrrolidone)-grafted silica (1.0 g) was added to
a solution of PdCl2 (0.177 g, 1.0 mmol) in DMF (30 mL) and stirred at
80 ꢀC for 5 h under nitrogen atmosphere. The solid catalyst was
filtered, washed thoroughly with DMF and water and dried under
reduced pressure. To determine the Pd content of the catalyst, it was
treated with 30 mL mixture of concentrated H2SO4 and HCl (1/1),
and filtered. The filtrate was diluted to 50 mL with distilled water
and subjected to ICP determination using calibration curve method.
2.4. General procedure for recycling reactions
When the Heck reaction according to the above procedure was
finished, the suspension was cooled down to room temperature and
filtered off. The polymer was washed with DMF, water and acetone.
It was dried under vacuum and then used in the next reaction cycle
with a new portion of reagents without any pretreatment.
2.3. General procedure for the MizorakieHeck reaction
3. Results and discussion
To a suspension of aryl halide (1 mmol), K2CO3 (2.0 mmol) and Pd
complex (0.5 mol%) in DMF (30 mL) was added n-butyl acrylate or
styrene (1.2 mmol). The reaction mixture was stirred at 120 ꢀC. The
3.1. Synthesis and characterization of supported catalyst
The palladium catalyst was designed by the sequence of reactions
given in Scheme 1. Acrylamidopropylsilica (I) was prepared by
reaction between aminopropylsilica (AMPS) and acryloyl chloride
according to a previous procedure [38]. The FT-IR spectrum of AMPS
showed a broad band at 1050e1200 cmꢂ1 due to SieO and at
3250 cmꢂ1 corresponding to NH2 (Fig. 1). In FT-IR spectrum of
acrylamidopropylsilica absorption frequencies of amide group
appeared at 1558, and 1662 cmꢂ1 and double bond at 1627 cmꢂ1
(Fig. 1). The appearance of these bands suggests that the reaction
between amine groups on the surface of the AMPS and acryloyl
chloride have occurred successfully. Poly (N-vinylpyrrolidone)-graf-
ted silica (II) was obtained by free-radical copolymerization between
acrylamidopropylsilica (I) and vinylpyrrolidone monomer in the
X
R2
Cat. 0.5 mol%/ 2eq K2CO3
R2
+
DMF, 120 o
C
R1
R1
X= Cl, Br, I
R1= H, Me, OMe, COMe, NO2
R2= CO2Bun, Ph
Fig. 4. Transmission electron microscopy (TEM) image of palladium supported on poly
(N-vinylpyrrolidone)-grafted silica.
Scheme 2. MizorakieHeck cross-coupling reactions.

B. Tamami et al. / Journal of Organometallic Chemistry 696 (2011) 594e599
597
Table 1
Effect of different bases on the reaction of bromobenzene and n-butyl acrylate.a
Table 3
Heck reaction of styrene and n-butyl acrylate with different aryl halides.a
Entry
Base
Time (h)
Yield (%)b
X
+
R2
1
2
3
4
5
none
K2CO3
NaOAc
Et3N
K3PO4.12H2O
24
4
10
10
10
trace
80
70
50
65
Cat. 0.5 mol%/ K2CO3
R2
DMF, 120 o
C
R1
R1
a
Reaction conditions: bromobenzene (1 mmol), n-butyl acrylate (1.2 mmol), base
Entry
1
Ar-X
R2
Time (h)
Yield (%)b
95
(2 mmol), Pd catalyst (0.5 mol%) in DMF (3 mL) at 120 ꢀC.
b
Isolated yield.
I
I
CO2Bun
0.5
1
presence of benzoyl peroxide as an initiator. FT-IR spectrum of (II)
shows the characteristic band of NeC]O at 1670 cmꢂ1 (Fig. 1). The
amount of grafted poly (N-vinylpyrro1idone) onto silica was calcu-
lated by thermogravimetric analysis (TGA) upon heating the grafted
silicainnitrogenatmosphereattherateof20ꢀC/minfrom0to700ꢀC.
It was found to be 1.9 mmol/g (5.8% water and 20.1% PVP) (Fig. 2).
Palladium complex (III) was obtained by mixing poly (N-vinyl-
pyrro1idone)-grafted silica with PdCl2 in DMF at 80 ꢀC. Determina-
tion of Pd content was carried out on catalyst digestion followed by
ICP analysis. The Pd content of the catalyst was 0.25 mmol/g.
To obtain the morphology and particle size of the catalyst,
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) images of the catalyst were carried out. The SEM
image shows shiny Pd particles on the crushed ice like of the
grafted PVP which dispersed with a well degree of homogeneity
(Fig. 3). According to TEM image, the palladium particles are well
dispersed through the catalyst surface in nanometer size between
20e25 nm (Fig. 4).
2
3
Ph
93
95
CO2Bun
1
4
5
6
7
8
9
Ph
2.5
4
95
80
80
85
90
95
CO2Bun
Ph
6.5
4
CO2Bun
3.2. Catalytic activity of the catalyst in Heck cross-coupling
reactions
Ph
7
The activity of the supported catalyst was first examined in the
Heck coupling reaction of different aryl halides with styrene and n-
butyl acrylate (Scheme 2). In a model reaction, the coupling of
bromobenzene with n-butyl acrylate was initially studied. In the
process of coupling reaction the choice of solvent and base is
crucial, so these parameters were optimized and the results are
summarized in Tables 1 and 2. It was found that the best system
was DMF as solvent and K2CO3 as base using 0.5 mmol% of catalyst
at 120 ꢀC. The generality of this reaction system was shown with
other coupling reactions using n-butyl acrylate and styrene as
olefinic substrates with different aryl halides including iodo, bromo
and even chloroarens. The results are presented in Table 3.
The electron-neutral, electron-rich and electron-poor aryl halides
reacted with n-butyl acrylate and styrene efficiently to produce the
corresponding cross-coupling products in good to excellent yields.
The more easily accessible and cheaper aryl chlorides have not
been employed much, in palladium-catalyzed coupling reactions,
primarily because the oxidative addition of CeCl bond to Pd (0)
species is usually difficult. Few heterogeneous Pd catalysts were
CO2Bun
2
10
Ph
3.5
1
92
93
95
60
60
40
50
80
11
CO2Bun
12
Ph
2
13c
14c
15c
16c
17c
CO2Bun
15
Ph
18
24
24
8
Table 2
Effect of different solvents on the reaction of bromobenzene and n-butyl acrylate.a
CO2Bun
Entry
Base
Time (h)
Yield (%)b
1c
2
H2O
DMF
CH3CN
THF
MeOH
24
4
24
24
4
55
80
60
20
40
Ph
3c
4c
5c
CO2Bun
a
Reaction conditions: bromobenzene (1 mmol), n-butyl acrylate (1.2 mmol),
K2CO3 (2 mmol), Pd catalyst (0.5 mol%), and 3 mL of solvent at 120 ꢀC.
b
Isolated yield.
reflux temperature of solvents.
c
(continued on next page)

598
B. Tamami et al. / Journal of Organometallic Chemistry 696 (2011) 594e599
Table 3 (continued )
Table 4
Coupling of iodobenzene with n-butyl acrylate using recycled catalysts.a
Entry
Ar-X
R2
Time (h)
10
Yield (%)b
70
Cycle
Conversion (%)b
Time (min)
1
2
3
4
5
6
7
100
100
100
100
100
100
100
30
30
45
45
45
60
60
18c
19c
20c
21c
Ph
CO2Bun
4
6
85
85
a
Reaction conditions: iodobenzene (1 mmol), n-butyl acrylate (1.2 mmol), K2CO3
(2 mmol), Pd catalyst (0.5 mol%) and DMF (3 mL) at 120 ꢀC.
Ph
b
conversion based on iodobenzene by GC.
investigated the recovery and reusability of the supported catalyst
using iodobenzene with n-butyl acrylate as model substrates.
Seven consecutive cycles of the reaction showed that the catalyst
did not lose its activity and could be completely recycled (Table 4).
There is no change in the size and morphology of the palladium
nanoparticles of the used catalyst. The estimated turn over number
(TON ¼ mole of product/mole of catalyst) is calculated to be 1400
for the whole recycling processes.
CO2Bun
Ph
1
2
90
85
22c
a
Molar ratio of the reagents ArX:n-butyl acrylate or styrene:K2CO3:palladium
catalyst ¼ 1.0:1.2:2.0:0.005. Reaction conditions: DMF, 120 ꢀC.
b
Isolated yields.
c
With additional tetrabutylammonium bromide (0.05 mmol).
Low contamination of residual metal in the isolated products is
an important characteristic for supported metal catalyst. To probe
the issue of palladium leaching in our system, the filtrate of the
reaction between bromobenzene and n-butyl acrylate was analyzed
by ICP. Analysis of the crude reaction mixture indicated a low Pd
leaching of 0.62%. In addition, by adding new portions of reactants
(bromobenzene, n-butyl acrylate and K2CO3) to the clear filtrate, the
reaction did not progress and bromobenzene remained almost
intact in the reaction mixture.
Hitherto, our catalytic system may be a reservoir for more active
soluble form of Pd that redeposit to the support after reaction, or
behaving in a truly heterogeneous manner. In order to investigate
this issue about working mechanism, hot filtration test was per-
formed. In a typical experiment, Pd compelx (0.5 mol %), bromo-
benzene (1 mmol), styrene (1.2 mmol), K2CO3 (2.0 mmol), and DMF
(5 mL) were taken in a round-bottomed flask and stirred at 120 ꢀC
for 75 min under a nitrogen atmosphere. At this stage (25%
conversion), the catalyst was filtered off and the experiment was
continued with the filtrate for another 24 h. There was no detect-
able increase in the product concentration, as is evident from the
GC analysis. It is confirmed the heterogeneous character of the
catalytically active species in this reaction.
found to convert activated aryl chlorides at high temperature
[39e42]. In our catalytic system, reactions of chloroarens required to
increase the temperature up to 140 ꢀC in long reaction times, but the
addition of tetrabutylammonium bromide (TBAB) (Jeffery Catalyst)
as an additive [43,44] permitted us to decrease the temperature to
120 ꢀC and reactions performed in acceptable time (entries 13e22).
The powder X-ray diffraction (XRD) patterns for the catalyst
after a successful Heck reaction showed the expected crystallinity
of Pd (0) nanoparticles and amorphous silica (Fig. 5). The diffraction
rings can be ascribed to the (111), (200), (220), (311), crystallo-
graphic planes of the Pd (0) nanoparticles. The Pd nanoparticle size
was estimated by Scherrer equation from XRD pattern data, to be
19.6 nm [45]. The calculated size matches approximately to the size
observed from TEM image. The same result was also obtained for
the XRD pattern of the catalyst after its use in the Heck reaction for
several cycles. This reveals the excellent stability and recovery of
the catalyst.
3.3. Heterogeneity tests (experimental evidence for leaching and
redeposition of palladium) and catalyst reuse
4. Conclusion
The possibility of recycling the catalyst is very important theme
and makes them useful for commercial applications. Thus, we
In conclusion, a novel catalytic system based on palladium
nanoparticles supported on poly (N-vinylpyrrolidone)-grafted silica
was designed and applied in MizorakieHeck cross-coupling reac-
tions with a variety aryl halides. Different aryl halides including
chloroarens showed good reactivity to generate the corresponding
products in good to excellent yields. Through TEM and XRD we can
observe high Pd dispersion and small particle sizes in nano scale.
Short reaction times, high yields, easy purification, recyclability and
very low Pd leaching are main characteristic of the process.
Acknowledgment
The authors gratefully acknowledge the partial support of this
study by Shiraz University Research Council.
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[45] Scherrer Equation:
¼ 1.54A ¼ 0.154 nm (X-ray wavelength); B ¼ 0.281 ¼ 0.0049 rad (FWHM
which is line broadening at half the maximum intensity);
¼ 20.0235 and
Cos 0939 (Bragg angle); thickness of particle 0.9 0.154/
0.0075 ꢁ 0.94 ¼ 19.6 nm.
t
¼
K
l/B cos
q;
K
¼
0.9 (shape factor);
l
q
q
¼
t
¼
¼
ꢁ