Immobilized palladium nanoparticles on silica-starch substrate (PNP-SSS): As a stable and efficient heterogeneous catalyst for synthesis of p-teraryls using Suzuki reaction

Article abstract of DOI:10.1016/j.jorganchem.2012.07.039

Immobilized palladium nanoparticles on silica-starch substrate (PNP-SSS) were found to be an efficient reusable catalyst for excellent synthesis of teraryls using Suzuki reaction. The catalytic reactivity of PNP-SSS was examined over a set of substrates, demonstrating that it is reactive toward a variety of functionalities. In this process, the PNP-SSS catalyst can be reused more than six times with almost consistent efficiency and can be recovered by simple filtration.

Full text of DOI:10.1016/j.jorganchem.2012.07.039

Journal of Organometallic Chemistry 717 (2012) 141e146
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Journal of Organometallic Chemistry
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Immobilized palladium nanoparticles on silicaestarch substrate (PNPeSSS): As
a stable and efficient heterogeneous catalyst for synthesis of p-teraryls using
Suzuki reaction
*
*
Ali Khalafi-Nezhad , Farhad Panahi
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:
Immobilized palladium nanoparticles on silicaestarch substrate (PNPeSSS) were found to be an efficient
reusable catalyst for excellent synthesis of teraryls using Suzuki reaction. The catalytic reactivity of
PNPeSSS was examined over a set of substrates, demonstrating that it is reactive toward a variety of
functionalities. In this process, the PNPeSSS catalyst can be reused more than six times with almost
consistent efficiency and can be recovered by simple filtration.
Received 12 May 2012
Received in revised form
9 July 2012
Accepted 26 July 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Palladium nanoparticles
Suzuki reaction
Heterogeneous catalyst
Silicaestarch substrate
p-Teraryls
1. Introduction
and boronic acids [8]. In fact, among arylearyl coupling protocols,
Suzuki reaction has been widely used as a synthetic strategy in the
There are many natural products, pharmaceuticals, and
advanced materials with biaryl structural units which possess
interesting pharmacological or physical properties [1]. p-Ter-
phenyls have received considerable attention, due to their presence
as a structural motif in natural products, and their applicability in
biological and materials science [2]. Some of the p-terphenyl
derivatives such as terphenyllin, terferol, and terprenin have been
isolated from the nature. These materials show versatile biological
activities and widely used as therapeutic agents [3]. Since, p-ter-
phenyls also possesses unique photophysical characteristics, they
widespread applied in material researches including molecular
electronics, photonics and chemical sensing [4,5]. For synthesis of
p-teraryls there are two general methods in the literature. One is
the transition metal-catalyzed arylearyl coupling reactions and
other is cycloaddition reactions using diaryl-substituted open chain
precursors [6]. The latter method is limited to specific class of ter-
phenyls and not has widespread structural diversity [7]. Suzuki
reaction is the most important and efficient strategy for the
construction of polyaryls (biaryls, teraryls, etc.) from aryl halides
synthesis of polyaryls, because organoboranes are stable and less
toxic in comparison with other aryl nucleophiles such as aryl-
magnesium bromides, and arylstannyl halides [9].
On the other hand, in the field of palladium reactions, prepa-
ration of high performance Pd catalysts and the use of green
reaction conditions are two important strategies for the prepara-
tion of new Pd catalyst systems, for excellent performing of Pd-
catalyzed processes [10]. High performance Pd catalysts are
considered, because they allowed the use of aryl chlorides, which
are less expensive than aryl bromides or iodides for coupling
reactions [11]. This issue is improved not only a cost factor but also
is provided a more efficient reaction route for Pd-catalyzed reac-
tions. Moreover, with green chemistry approaches such as using
recyclable catalysts and use of less toxic materials as solvents and
reagents the synthetic usefulness of these valuable reactions are
improved [12].
Pd nanoparticles (Pd-nano) catalyst systems are considered as
efficient and green catalysts in Pd-catalyzed process [13]. In Pd-
nano systems, stability, reactivity and reusability of catalyst are
highly depended to type of selected substrate for immobilization of
nanoparticles [14]. Silica is a stable substrate for immobilization of
Pd nanoparticles, but leaching of metal is unavoidable for this
material, so the reactivity of catalyst is decreased after one or two
runs [15]. Silica substrate can be modified by organic moieties using
* Corresponding authors.
E-mail addresses:Khalafi@chem.susc.ac.ir (A. Khalafi-Nezhad), panahi@shirazu.ac.ir
(F. Panahi).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.07.039

142
A. Khalafi-Nezhad, F. Panahi / Journal of Organometallic Chemistry 717 (2012) 141e146
a chemical process to improve its applicability as support in prep-
aration of Pd-nano catalysts. One of the important classes of
materials which have been used for stabilization of metal nano-
particles is natural poly hydroxyl compounds. Cellulose, chitosan,
gelatin, collagen and starch contain free hydroxyl groups on their
backbones, which have the potential for the chelation with tran-
sition metals [16]. They also acts as a highly hydroxyl functionalized
substrate to catch up and stabilize the Pd nanoparticles by their
ligation with hydroxyl groups. Considering to this characteristics
they have been used as substrate for preparation of Pd nano catalyst
systems. Unfortunately, leaching of Pd, less stability and tedious
workup process are main problems with these catalytic systems.
However, when these poly hydroxyl substrates are grafted to
a stable backbone such as silica, some of the problems can be
resolved [17]. Considering to this strategy, we have introduced an
efficient heterogeneous catalyst for Heck and copper-free Sonoga-
shira reactions based on immobilization of palladium nanoparticle
on silicaestarch substrate [18]. In present work we would like to
develop the synthetic usefulness of PNPeSSS catalyst by synthesis
of some new p-teraryls using Suzuki reaction under green
conditions.
water (2 Â 10 mL). After drying in the oven, silicaestarch substrate
was obtained as a white powder.
2.3. Pd nanoparticles on silicaestarch substrate (PNPeSSS)
To a mixture of silicaestarch substrate (5 g) in absolute ethanol
(30 mL), palladium acetate (0.3 g, 1.3 mmol) was added and stirred
24 h at room temperature. Then, the mixture was filtered and
washed with ethanol (3 Â 10 mL) and diethyl ether (2 Â 10). After
drying in a vacuum oven, PNPeSSS catalyst was obtained as
a dark solid.
2.4. General procedure for the synthesis of p-teraryls using
PNPeSSS catalyst
To a mixture of diarylhalide (1 mmol), aryl boronic acid
(2.1 mmol), and NaOH (3 mmol) in 2 mL water, PNPeSSS catalyst
(0.05 g) was added and heated in an oil bath at the refluxing
temperature of water. The reaction was followed by TLC. After
completion of the reaction, the mixture was cooled down to room
temperature and filtered and the remaining solid was washed with
dichloromethane (3 Â 5 mL) in order to separate catalyst. After the
extraction of dichloromethane from water, the organic extract was
dried over Na2SO4. The products were purified by column chro-
matography (hexane or hexane/ethyl acetate) to obtain the desired
purity.
2. Experimental
Chemicals were purchased from Fluka, Merck and Aldrich
chemical companies and used without further purification. The
known products were characterized by comparison of their spectral
and physical data with those reported in the literature. TEM anal-
yses of the catalyst were performed on a Philips model CM 10
instrument. Inductively Coupled Plasma (ICP) technique (Varian,
Vista-pro) was employed for the determination of the amount of
immobilized palladium nanoparticles on support. 1H NMR spectra
were recorded on a Bruker Avance 250 MHz spectrometer in CDCl3
solution with tetramethylsilane (TMS) as an internal standard. FT-
IR spectroscopy (Shimadzu FT-IR 8300 spectrophotometer) was
employed for characterization of the compounds using KBr pellets.
Elemental analyses (CHNS) were performed on a PerkineElmer
240-B micro-analyze. Melting points were determined in open
capillary tubes in a Barnstead Electrothermal 9100 BZ circulating oil
melting point apparatus. The reaction monitoring was accom-
plished by TLC on silica gel PolyGram SILG/UV254 plates. Column
chromatography was carried out on columns of silica gel 60
(70e230 mesh).
2.5. General procedure for recycling of the PNPeSSS catalyst
Upon completion of the reaction between 1,4-dichlorobenzene
and phenylboronic acid under optimized condition, the reaction
mixture was cooled down to room temperature and filtered; the
remaining solid was washed with water (2 Â 10) and dichloro-
methane (3 Â 5 mL) in order to separate catalyst. The reused
catalyst was dried in oven at 100 ꢀC and used for next run.
3. Results and discussion
3.1. Catalyst preparation
As shown in Scheme 1, the heterogeneous PNPeSSS catalyst is
prepared in three steps. First, activated silica [19] is inverted to
silica chloride (SC) based on Firouzabadi et al. producer [20].
Second, SC is reacted with starch in chloroform to obtain
silicaestarch substrate (SSS). It is noteworthy that, SC is not soluble
in all organic solvents and due to this property; it can accept
different nucleophiles particularly hydroxyl groups (due to gener-
ation of strong SieO bond) [21]. Considering to this issue, SC is
reacted with hydroxyl groups of starch conveniently. However, SC is
a moisture capture reagent and its SieCl groups can convert to
SieOH groups in the presence of water molecules. So for fine
chemical synthesis of SSS we have to use from the fresh SC. Finally,
palladium acetate is reduced to Pd nanoparticles with ethanol as
reducing agent on SSS to obtain PNPeSSS catalyst. The PNPeSSS
catalyst has been fully characterized using some different micro-
scopic and spectroscopic techniques [18].
It is noteworthy that the SSS is a very stable substrate (based on
thermal gravimetric analysis it is stable to above 250 ꢀC), because
the starch species is connected to silica covalently. SSS is a poly-
hydroxyl ligand and so it can stabilize the Pd nanoparticles effec-
tively with its hydroxyl groups. When starch reacts with SC it forms
a platform on the surface of silica and these organic chains prevents
the aggregation of nanoparticles and their separation from the
substrate surface. This organiceinorganic hybrid material also
provides suitable catalytic sites for reactions in aqueous media. We
2.1. Synthesis of silica chloride (SC)
In a three-necked flask (100 mL) that was equipped with
a dropping funnel containing thionyl chloride (SOCl2, 40 mL),
condenser and a gas inlet tube for conducting HCl gas over an
adsorbing solution (10% NaOH), 10 g of silica was charged. Thionyl
chloride was added drop-wise over a period of 30 min at room
temperature. When the addition was completed, the mixture was
stirred for 24 h at the refluxing temperature of SOCl2. Then, the
untreated SOCl2 was removed by distillation. The silica chloride was
dried in a vacuum at 90 ꢀC and the resulting grayish powder was
stored in a desiccator under vacuum. Based on previous study the
amount of chlorosilyl groups on the silica surface was determined
about w0.88 mmol gÀ1 [18].
2.2. Preparation of silicaestarch substrate (SSS)
To a magnetically stirred mixture of silica chloride (5 g) in CHCl3
(30 mL), potato starch (purchased from Aldrich company) (2 g) and
triethyl amine (0.5 mL) were added and refluxed for 12 h. Then, the
mixture was filtered and washed with chloroform (2 Â 10 mL) and

A. Khalafi-Nezhad, F. Panahi / Journal of Organometallic Chemistry 717 (2012) 141e146
143
terphenyl (3a) was obtained with 61% isolated yield. We observed
that the yield of p-terphenyl was enhanced to 71% when some
water (Ratio 5:1 of DMF:H2O) was added to the reaction media
(Table 1, entry 2). Additionally, increasing the amount of water
(Ratio 1:5 of DMF:H2O) also increased the yield of product (Table 1,
entry 3). Regarding this point, we chose water as solvent for the
reaction and interestingly in this condition p-terphenyl product
was obtained with 88% isolated yield after only 5 h (Table 1, entry
4). It is noteworthy that, in aqueous media not only the yield of
product did not deceased but it also increased to some extent,
which this may be resulted from organiceinorganic nature of the
silicaestarch substrate.
When the type of base was changed, it was observed that in the
presence of NaOH a good yield of product was obtained (Table 1,
entry 6). It is also observed that, the reaction between 1,4-
dichlorobenzene and phenylboronic acid did not accomplished at
room temperature using PNPeSSS catalyst (Table 1, entry 7). The
results showed that, with increasing the amount of catalyst (1.9 mol
% per 1 mmol of 1,4-dichlorobenzene), the yield of product did not
change much (Table 1, entry 8). While, reducing the amount of
catalyst (0.7 per 1 mmol of 1,4-dichlorobenzene) decreased the
yield of product (Table 1, entry 9). In according to Table 1, reducing
the quantity of base was also affected the reaction yield (entry 11).
Thus, as optimum condition, the reaction was carried out under
aerobic conditions in refluxing water as green solvent, using NaOH
as a cheep base, without the addition of free ligand or any
promoting additives.
Scheme 1. Synthetic route for preparation of PNPeSSS catalyst.
need to point out that, PNPeSSS is a stable, cheap, green and easy
handling catalyst for application in Pd-catalyzed reactions.
The results were shown that, under optimized conditions 1,4-
diiodobenzene react faster than chloride and bromide analogs
and gave higher yield of product in more short reaction time
(Table 2).
3.2. PNPeSSS catalyzed Suzuki reaction of phenylboronic acid and
1,4-dihalobenzene (optimization, generality and limitations)
Any product from the reaction of 1,4-diarylfluoride did not
observe. Although, the PNPeSSS catalyst is suitable for construction
of p-teraryls using aryl chlorides, but by use of aryl iodides and aryl
bromides the desired p-teraryl compound can be synthesized in
relatively short reaction time with high efficiency.
After optimization studies, a range of p-teraryls were synthe-
sized using PNPeSSS catalyst under optimum conditions (Fig. 1 and
Table 3).
The PNPeSSS catalyzed Suzuki reaction between phenylboronic
acid (1a) and 1,4-dichlorobenzene (2a) was chosen as a model
reaction to evaluate the effects of solvent, base, temperature and
amount of catalyst. Optimization conditions studies are summa-
rized in Table 1.
In our initial selection we used DMF as solvent and K2CO3 as
base for the reaction between 1,4-dichlorobenzene and phenyl-
boronic acid in the presence of PNPeSSS catalyst, where p-
Considering to the Table 3 and Fig. 1, this is an efficient approach
for one-pot synthesis of p-teraryl derivatives based on the Pd-
catalyzed Suzuki reaction of aryl boronic acids and diarylhalides.
This facile protocol also can be applied to the synthesis of other
polyaryls and synthetic compounds with biaryl structural unit. As
can be seen in Table 3, by selecting the appropriate boronic acids
and diarylhalide a wide range of p-teraryls can be synthesized. The
reaction between 1,4-dichlorobenzene and phenylboronic acid is
resulted the formation of p-terphenyl with 91% isolated yield after
5 h (Table 3, entry 1). A similar process allowed the combination of
4-ethylphenylboronic acid with 1,4-dichlorobenzene to provide an
excellent yield of the corresponding p-terphenyl product (Table 3,
entry 2).
Table 1
Effect of solvent, base, temperature and amount of catalyst on PNPeSSS catalyzed
Suzuki reaction of 1,4-chlorobenzene with phenylboronic acid.a
Entry
1
Solvent
DMF
Base (mmol)
K2CO3 (3)
Temp (ꢀC)
100
Time (h)
Yield (%)b
5
12
5
12
5
61
73
71
78
79
81
88
91
67
91
0
2
3
4
DMF:H2O (5:1)
DMF:H2O (1:5)
H2O
K2CO3 (3)
K2CO3 (3)
K2CO3 (3)
100
100
The PNPeSSS system is also shown to be most effective in the
coupling of diarylhalides with functionalized boronic acids for
12
5
Reflux
12
12
12
24
12
24
12
12
5
6
7
8
9
10
11
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Et3N (4)
Reflux
Reflux
r.t.
Reflux
Reflux
Reflux
Reflux
NaOH (3)
NaOH (3)
NaOH (3)
NaOH (3)
NaOH (4)
NaOH (2)
Table 2
Effect of type of halogen on reaction time and yield of product.a
93c
65d
91
85
Entry
X
Time/h
Yield (%)b
1
2
3
4
I
1
5
12
24
97
92
91
0
Br
Cl
F
a
Reaction conditions: 1,4-dichlorobenzene (1 mmol), phenylboronic acid
(2.1 mmol), solvent (2 mL).
b
a
Isolated yield.
0.08 g of catalyst used.
0.03 g of catalyst used.
Reaction condition: biarylhalide (1 mmol), phenylboronic acid (2.1 mmol),
c
PNPeSSS (0.05 g), NaOH (3 mmol) and water (2 mL).
d
b
Isolated yield.

144
A. Khalafi-Nezhad, F. Panahi / Journal of Organometallic Chemistry 717 (2012) 141e146
Table 4
Comparison of the results of the synthesis of terphenyl, using SBSAN catalyst with
those obtained by the reported catalysts.
Entry
1
Catalyst & conditions
X
Time (h)
Yield (%)
Ref.
PNPeSSS, NaOH,
H2O, reflux
Cl
Br
I
12
5
1
90
92
97
90
This
work
2
3
4
Starch-PNP, NaOAc,
DMF/H2O, 80 ꢀC
Pd Cl2(PPh3)2, K2CO3, N2,
TBAB, H2O, MW
Silica-supported
palladium, K2CO3,
dodecane, o-xylene,
N2, 110 ꢀC
I
1
[16c]
[29]
[9]
Br
Br
0.17
20
92
65
5
PalladiumeNHC-containing
polymer,
Cl
8
94
[30]
NaOtBu, IPA, 80 ꢀC
Pd(OAc)2, Na2CO3,
H2O/DMF, 60 ꢀC
LDH-DS-Pd0, K2CO3,
H2O/DMF, 60 ꢀC
Silica supported
palladium-phosphine
complex, K2CO3,
MeOH/H2O, r.t.
6
7
8
Br
Br
I
12
3
96
94
77
[31]
[32]
[33]
Fig. 1. The chemical structure of synthesized p-teraryls.
synthesis of symmetrical functionalized p-terphenyls (Table 3,
entries 3, 4 and 8). These compounds are needed for the applica-
tions in synthesis of new polymers and molecules with terphenyl
structural units [22]. Thiophenes are found as structural units in
a variety of materials such as conductive polymers, organic field
effect transistors and organic light emitting diodes [23]. Although,
polyaryl compounds incorporating a thiophene moiety are often
prepared via a transition metal catalyzed coupling reaction, but
there are several limitation for the synthesis of these compounds
by thiophene boronic acids in Suzuki reaction [24]. Considering to
these points, we decided to synthesis two new p-teraryls based on
thiophene ring using thiophen-2-yl-2-boronic acid and results
were shown that PNPeSSS catalyst also provides efficient and clean
condition for synthesis of these compounds (Table 3, entries 9 and
10). Since, naphthalene, phenanthrene and anthracene structural
units widely used in design of advanced material for application in
organic electronic devices [25], we also synthesized three p-teraryls
3
Fig. 2. Synthesis of o, m and p-terphenyl in the presence of PNPeSSS catalyst under
optimized conditions.
containing these fragment (Table 3, entries 11, 12 and 13). Thus, our
environmentally beginning catalyst system can also provide effi-
cient procedure for the preparation of these materials.
When a fluoro moiety incorporated into an organic molecule its
physical, chemical, and biological properties often are dramatically
improved [26]. For example, fluorinated aromatics are useful as
excellent n-type organic materials, because they possess a low
barrier to electron injection and a high propensity for electron
Table 3
The p-teraryl products of reactions between diarylhalides and boronic acids in the
presence of PNPeSSS catalyst under optimized conditions.a
,b
Entry
Ar
R
X
Product
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
Ph
4-EtePh
4eCOCH3ePh
4eCHOePh
Ph
H
H
H
H
CH3
C6H13
CH3
C6H13
CH3
C6H13
CH3
CH3
H
Cl
Cl
Br
Br
I
I
I
I
I
I
I
I
I
3a
3b
3c
3d
3e
3f
3g
3h
3i
8
8
5
5
1
2
2
2
3
3
5
5
5
91
92
92
91
96
94
96
93
89
86
89
91c
Ph
3,4-FePh
4eCOCH3ePh
Thiophenyl
Thiophenyl
Naphthyl
Anthracenyl
Phenanthrenyl
9
10
11
12
13
3j
3k
3l
,d
3m
86c
a
Reaction condition: diarylhalide (1 mmol), phenylboronic acid (2.1 mmol),
PNPeSSS (0.05 g), NaOH (3 mmol) and water (2 mL).
b
Isolated yield.
Solvent used: DMF:H2O (1:5) (2 mL).
Amount of boronic acid (1 mmol) and aryl halide (2 mmol).
c
Fig. 3. Recyclability of PNPeSSS catalyst in the reaction of 1,4-dichlorobenzene with
phenylboronic acid under optimized condition; reaction time is 8 h.
d

A. Khalafi-Nezhad, F. Panahi / Journal of Organometallic Chemistry 717 (2012) 141e146
145
Fig. 4. The TEM image of a) fresh PNPeSSS catalyst and b) reused PNPeSSS catalyst after 6 times.
transporting, so they widely used in organic electronic materials
[27]. For this purpose, the development of efficient methods for the
synthesis of fluorinated compounds has been considered [28]. One
of the efficient protocols for synthesis of this class of compounds is
Suzuki reaction. Herein, we also synthesized a new fluorinated p-
terphenyl using PNPeSSS catalyst (Table 3, entry 7).
In order to show the merit and the reactivity of PNPeSSS cata-
lyst, a comparison with some other reported homogeneous and
heterogeneous palladium catalysts for preparation of p-terphenyl
are presented in Table 4.
and aryl boronic acids using a heterogeneously catalyzed Suzuki
reaction. By use of PNPeSSS catalyst, teraryls were synthesized
with high isolated yields under green conditions. Reusability and
easy workup were two other advantages of PNPeSSS catalyst in this
process. We anticipate that the application of this catalyst can be
extended for synthesis of new material using other palladium
catalyzed reactions.
Appendix A. Supplementary material
As shown in Table 4, our catalyst is superior to some of the
previously reported catalysts in terms of reaction condition, reac-
tion time and yield.
Supplementary material related to this article can be found
online at http://dx.doi.org/10.1016/j.jorganchem.2012.07.039.
We also checked the catalytic activity of PNPeSSS for synthesis
of o-, m- and p-terphenyl under optimized conditions. As shown in
Fig. 2, for all three dichlorobenzenes, an excellent yield of corre-
sponding products were obtained, indicating that PNPeSSS is
efficient for synthesis of three isomers of terphenyls.
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These experiments and results are confirmed that the PNPeSSS
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