In-situ synthesis of a palladium-polyaniline hybrid catalyst for a Suzuki coupling reaction

Article abstract of DOI:10.1016/j.jorganchem.2010.11.039

Palladium-catalyzed cross-coupling reactions are one of the most frequently used synthetic tools for the construction of new carbon-carbon bonds in organic synthesis. The present study describes the use of palladium-polyaniline composite material as a catalyst for the Suzuki coupling of aryl halides. Palladium-polyaniline nanocomposite material was synthesized using an in-situ technique in which palladium acetate and aniline hydrochloride were used as the precursors of the composite. Electron microscopy imaging showed that the palladium particles were well-dispersed within the polymer matrix and were typically 3-5 nm in diameter. The metal-polymer composite material was used as a catalyst for the coupling of phenylboronic acid with aryl halides in the presence of an inorganic base and showed excellent yield with high TOF values.

Full text of DOI:10.1016/j.jorganchem.2010.11.039

Journal of Organometallic Chemistry 696 (2011) 2206e2210
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
In-situ synthesis of a palladium-polyaniline hybrid catalyst for a Suzuki coupling
reaction
Rafique Ul Islam a, Michael J. Witcomb b, Elma van der Lingen c, Michael S. Scurrell d,
,
Willem Van Otterlo d, Kaushik Mallick c
*
a Department of Applied Chemistry, Birla Institute of Technology, Mesra, Ranchi-835215, India
b Microscopy and Microanalysis Unit, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
c Advanced Materials Division, Mintek, Private Bag X3015, Randburg 2125, South Africa
d Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium-catalyzed cross-coupling reactions are one of the most frequently used synthetic tools for the
construction of new carbonecarbon bonds in organic synthesis. The present study describes the use of
palladium-polyaniline composite material as a catalyst for the Suzuki coupling of aryl halides. Palladium-
polyaniline nanocomposite material was synthesized using an in-situ technique in which palladium
acetate and aniline hydrochloride were used as the precursors of the composite. Electron microscopy
imaging showed that the palladium particles were well-dispersed within the polymer matrix and were
typically 3e5 nm in diameter. The metal-polymer composite material was used as a catalyst for the
coupling of phenylboronic acid with aryl halides in the presence of an inorganic base and showed
excellent yield with high TOF values.
Received 1 October 2010
Received in revised form
15 November 2010
Accepted 23 November 2010
Keywords:
Pd-polymer composite
Suzuki coupling reaction
TOF
Ó 2010 Elsevier B.V. All rights reserved.
TEM
1. Introduction
natural products and functional materials. The classical conditions
for performing the SuzukieMiyaura coupling reaction involve the
Palladium based catalysts particularly nanoscale palladium
particles have recently drawn enormous attention due to their
versatile role in organic synthesis [1e3]. The use of palladium
nanoparticles in catalysis is not only industrially important [4e6],
but also scientifically interesting as a result of the sensitive rela-
tionship between catalytic activity, nanoparticle size and shape as
well as the nature of the surrounding media [7]. Unfortunately,
aggregation of bare nanoparticles often prohibits the tailoring of
particle size [8]. To overcome this problem, catalytic nanoparticles
have been immobilized on solid supports, such as carbon [9], titania
[10], zeolites [11], alumina [12], silica spheres [13] and polymers
[14]. Over recent years, especially following the rapid development
of combinatorial chemistry, the use of polymeric supports in
organic synthesis has become common practice in constructing
small organic molecules [15,16]. One of the well-known applica-
tions of palladium nanoparticles as catalysts for the coupling of
carbonecarbon molecules is the Suzuki-Miyaura reaction [17,18].
The SuzukieMiyaura coupling reaction is one of the most useful
methods for selective CeC bond formation during the construction
of biaryl-skeletons, which are often used in pharmaceuticals,
use of a palladium complex or palladium salt and a phosphine
ligand [19]. Palladium-catalyzed reactions under phosphine-free
conditions are a topic of considerable interest as a result of
economic and environmental reasons.
We describe here the fabrication of a metal-polymer hybrid
material in which palladium nanoparticles are stabilized in
a macromolecular matrix and its application in Suzuki coupling
reactions. The coupling of arylboronic acids with aryl halides, the
Suzuki reaction, is perhaps the most powerful and versatile method
for the formation of new biaryls. Our approach is to use an ‘In situ
polymerization and composite formation (IPCF)’ [20] method for
the synthesis of palladium-polyaniline hybrid material and to use
that composite as a catalyst for Suzuki coupling reaction. We found
that the hybrid material was very active as a catalyst for the Suzuki
reaction and that the reaction could be carried out in the absence of
a phosphine ligand.
2. Experimental
2.1. Materials
Reagent grade aniline hydrochloride was purchased from
* Corresponding author.
E-mail address: kaushikm@mintek.co.za (K. Mallick).
Merck. Pd-acetate from Next Chimica was dissolved in toluene
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.11.039

R.U. Islam et al. / Journal of Organometallic Chemistry 696 (2011) 2206e2210
2207
Fig. 3. IR spectra obtained from the composite material. The main peaks at 1510 and
1600 cmꢁ1 correspond respectively to the stretching deformation of the quinine and
the benzene rings.
Fig. 1. UVevisible spectrum of the polyaniline compound. A shoulder-like appearance
at 430 nm (indicated by an arrow) is due to the polaron/bipolaron transition.
added to toluene (5 ml) in a small round bottom flask with
a magnetic stirring bar. The reaction mixture was placed on an oil
bath at 80e90 ꢂC and stirred for 6e8 h depending on the aryl halides
used. The reaction was monitored by a thin layer chromatography
(TLC) technique. Subsequently, the mixture was extracted with ethyl
acetate three times. Subsequently, the reaction mixture was cooled,
diluted with Et2O, filtered through a pad of silica gel with copious
washings and purified by flash chromatography on silica gel.
(Merck). All other chemicals were purchased from Aldrich and used
as received.
2.2. In-situ synthesis of palladium-polymer composite
In a typical experiment, aniline hydrochloride (0.442 g) was
dissolved in methanol under continuous stirring conditions. Pd-
acetate (0.4 ꢀ 10ꢁ2 moldmꢁ3) was added drop by drop to aniline
hydrochloride in methanol solution. The solution turned a light
green colour and the precipitation occurred after the addition of all
2.4. Characterization techniques
of the Pd-acetate. TEM specimens were prepared by pipetting 2 mL
For UVevis spectra analysis, a small portion of the solid sample
was dissolved in methanol and scanned within the range
300e800 nm using a Varian, CARY, 1E, digital spectrophotometer.
Raman spectra were acquired using the green (514.5 nm) line of an
argon ion laser as the excitation source. Light dispersion was
undertaken via the single spectrograph stage of a Jobin-Yvon
T64000 Raman spectrometer. Power at the sample was kept low
(0.73 mW), while the laser beam diameter at the sample was
of the colloid solution onto lacey carbon coated copper grids.
Subsequently after viewing, the TEM grids were sputter coated
with a 10 nm thick conducting layer of Au-Pd and viewed in a SEM.
A small portion of the colloidal sample was used for optical char-
acterization. The rest of the solution was filtered and the solid mass
was dried under vacuum at 60 ꢂC. The solid sample was used as
a catalyst for Suzuki coupling reaction. The amount of Pd present in
the catalyst was quantified by ICPMS technique, which revealed the
Pd loading in the sample to be 1.7 wt %.
w1 mm. A PerkineElmer 2000 FT-IR spectrometer, operating within
the range 800e1700 cmꢁ1, with a resolution 4 cmꢁ1, was used for
the infrared spectra analyses. For this study, the sample was
deposited in the form of a thin film on a KBr disk. Transmission
electron microscopy studies of the composite were carried out at an
accelerating voltage of 197 kV using a Gatan GIF Tridiem on a Phi-
lips CM200 TEM equipped with a LaB6 source. SEM studies were
undertaken in a FEI Quanta 400F ESEM at 2 kV. The X-ray
2.3. Procedure for the Suzuki coupling reactions catalyzed by
palladium-polymer composite
In typical experiment aryl halide (1.0 mmol), phenylboronic acid
(1.5 mol), K2CO3 (1.5 mmol) and the catalyst (0.036 mol% Pd) were
Fig. 2. Raman spectra recorded for the green precipitate. The characteristic bands from 1100 to 1700 cmꢁ1 indicate the formation of polyaniline. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this article).

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R.U. Islam et al. / Journal of Organometallic Chemistry 696 (2011) 2206e2210
Fig. 4. SEM image of polyaniline nanofibers with a three dimensional morphology (A). TEM images of the polyaniline (B and C). The high-density distribution of dark spots on the
polymer corresponds to the palladium nanoparticles (B). The higher magnification TEM image (C) clearly indicates the 2e4 nm diameter palladium nanoparticles encapsulated and
dispersed within the polyaniline matrix (some of them are within the circle).
photoelectron spectra (XPS) were collected in a UHV chamber
attached to a Physical Electronics 560 ESCA/SAM instrument.
benzenoid-to-quinoid excitonic transition. In our previous work
[22,23] when the starting material was aniline and no protonic acid
was used as a doping agent, the benzenoidequinoid excitonic tran-
sition showed the absorption peak to be at 620 nm. In that case, the
polyaniline was formed under either neutral or basic conditions
resulting in the absorptionpeak for thebenzenoidequinoidexcitonic
transition appearing at a lower wavelength. The position of the
excitonic peak can shift from 800 to 560 nm depending on various
factors such as nature of the counter ions, the type of solvent, the pH
value and the chemical structure of the polymer [24].
3. Results and discussion
The electronic absorption spectrum of the polyaniline salt has
been documented in the literature [21]. The polyaniline salt shows
three absorption peaks at 310-360, 400-440 and 700e825 nm. In the
present study, the UVevisible spectrum of the resultant material,
Fig.1, shows one sharp and prominent absorption band with its peak
at325nm,whichcorrespondstothe
p
-
p
*transitionofthebenzenoid
In the Raman spectrum (Fig. 2A), the characteristic bands from
1100 to 1700 cmꢁ1 are sensitive to the oxidation state of the pol-
yaniline. The Raman spectrum of polyaniline obtained during the
reaction between palladium acetate and aniline hydrochloride
rings. A shoulder-like appearance at 430 nm (indicated by an arrow)
is due to the polaron/bipolaron transition. A broad absorption band
starting from 500 to 750 nm is also seen which results from the

R.U. Islam et al. / Journal of Organometallic Chemistry 696 (2011) 2206e2210
2209
Table 1
Suzuki reaction of benzene bromide and phenylboronic acid with different bases.
Base
K2CO3
93
Na2CO3
85
K3PO4
91
Cs2CO3
87
KF
Yield(%)
73
Reaction condition: Benzene bromide (1.0 mmol), phenylboronic acid (1.5 mmol),
base (1.5 mmol), toluene (5 ml) and catalyst (polymer supported Pd 0.02 mol%),
80 ꢂC, 8 h.
and dispersed within the polymer matrix (some of them are within
the circle). Selected area diffraction (SAD) patterns indicated that
the Pd nanoparticles were amorphous. To identify the chemical
state of the polymer stabilized palladium particles, X-ray photo-
electron spectroscopy (XPS) measurements were carried out. The
Pd 3d region of the XPS spectrum of the Pd-polyaniline composite
is illustrated in Fig. 5, which reveals the presence of Pd 3d5/2 and
3d3/2 peaks at binding energies of 335.5 and 340.85 eV respectively.
These binding energy values are in accordance with those reported
for metallic palladium [31].
A wide variety of methods have been applied to the preparation
of polyaniline or substituted polyaniline-type compounds by the
oxidative polymerization of their respective monomers [32].
According to the most widely accepted mechanism, the first step
during the polymerization involves the formation of a radical cation
accompanied by the release of an electron. This step is the initiation
process of the polymerization reaction. The released electron [32] is
then used to reduce the Pd2þ ions to form palladium atoms. The
coalescence of these palladium atoms ultimately forms palladium
nanoparticles, which are encapsulated by the polyaniline.
Fig. 5. Palladium 3d XPS spectrum of the palladium-polyaniline composite. The peaks
at binding energies at 335.55 eV for 3d5/2 and 340.85 eV for 3d3/2 are the indicative of
metallic palladium.
reveals CeC deformation bands of the benzenoid ring at 1615 and
1585 cmꢁ1, which are characteristic for semiquinone rings [25,26].
The band at 1520 cmꢁ1 corresponds to the NeH bending defor-
mation. Intense overlapping features between 1300 and 1400 cmꢁ1
with two prominent bands at 1316 and 1350 cmꢁ1 along with
a clearly visible shoulder at 1370 cmꢁ1 (indicated with an arrow)
correspond to semiquinone radical structures. The band at
1414 cmꢁ1 is due to the CeC stretching vibration of the quinoid
units. Applying a Gaussian fit to the 1100e1300 cmꢁ1 region
(Fig. 2B), two peaks at 1182 and 1220 cmꢁ1 are clearly visible. The
position of the benzene CeH bending deformation band at
1182 cmꢁ1 is characteristic of the reduced and semiquinone
structures while the 1220 cmꢁ1 band corresponds to the CeN
stretching mode of single bonds.
The IR spectroscopic technique was used to further investigate
the optical behaviour of the solid sample. The IR spectrum of the
resultant compound is presented in Fig. 3. The bands at 1605 and
1500 cmꢁ1 can be assigned to the C ¼ C stretching vibration of the
quinoid (N ¼ Q ¼ N, Q represents the quinoid ring) and benzenoid
(NeBeN, B represents the benzenoid ring) rings respectively
[27,28]. The material shows a band at 1240 cmꢁ1, which corre-
sponds to the CeN stretching vibration [29]. The high intensity
peaks in the region 1150e1050 cmꢁ1 correspond to the aromatic
CeH in-plane bending vibration [30]. Aromatic out-of-plane CeH
deformation vibration corresponds to the band at 830 cmꢁ1. This
band is characteristic of the presence of 1e4 substituted benzene
rings [30]. The Raman and IR spectrum indicate the formation of
the polymeric product of aniline hydrochloride with quinoid and
benzenoid type of ring structure in the polyaniline chain.
The polyaniline-supported palladium nanoparticle based cata-
lyst was used in the Suzuki coupling reaction, which is a versatile
method for carbonecarbon bond formation, especially when
applied to the synthesis of biaryls in organic synthesis. Initially,
a solvent-base optimization study was performed for the coupling
of phenylboronic acid with bromobenzene using toluene and 1, 4-
dioxane as solvents and K2CO3, Na2CO3, K3PO4, Cs2CO3 and KF as
bases. Among them, K2CO3 and K3PO4 in combination with toluene
Table 2
Suzuki reaction between derivatives of benzene bromide and phenylboronic acid.
The SEM image (Fig. 4A) shows that the product consists of
regular straight nanofibers up to 20
mm in length and of rectangular
cross section 0.4e1.7 m by about 0.08
m
mm thick. The fibers were
almost identical in morphology and very straight suggesting a high
level of rigidity. Fig. 4B displays TEM images of the composite. The
high-density distribution of dark spots on the polymer corresponds
to the location of the palladium nanoparticles. Fig. 4C is a higher
magnification image and this and stereo images (not shown) clearly
indicate that the 2e4 nm diameter nanoparticles are encapsulated
Reaction conditions: Derivatives of benzene bromide (1.0 mmol)(1.0 mmol), phe-
nylboronic acid (1.5 mmol), K2CO3 (1.5 mmol), toluene (5 ml) and catalyst (polymer
supported Pd 0.02 mol%), 80 ꢂC, 8 h. Entry 6, 0.07 mol % Pd as catalyst was used at
80 ꢂC, 10 h to achieve a higher yield.
Scheme 1. Suzuki coupling of benzene bromide and phenylboronic acid.

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R.U. Islam et al. / Journal of Organometallic Chemistry 696 (2011) 2206e2210
(R1X) and Pd(0) to form the aryl-palladium halide complex [R1Pd
(II)X], which then couples with arylboronic acid, R2B(OH)2, in the
presence of a base to produce the [R1-(Pd2þ)-R2] intermediate,
finally providing the biaryl product (R1-R2) via the reductive
elimination of Pd2þ to Pd(0) as outlined in Scheme 1. It was also
found that the reaction did not occur in the absence of Pd nano-
particles [33]. The recovery of the catalyst for the recyclability study
was practically not feasible in this study due to the nature of the
support and minute amount of the material used for the reaction as
a catalyst.Please note that scheme 2 was not cited in the text.
Table 3
Suzuki reaction between derivatives of benzene iodide and phenylboronic acid.
4. Conclusions
The metal-polymer composite material in which the palladium
nanoparticles are stabilized in the polyaniline matrix showed
excellent activity towards a Suzuki coupling reaction. Such high
catalytic activities presumably result from the small size and narrow
size distribution of the palladium nanoparticles. When aniline was
used as a precursor no composite formationwas observed, but when
utilizing aniline hydrochloride an instant reaction took place with
Pd-acetate resulting in the formation of the metal-polymer hybrid
material. This hybrid material acts as a catalyst and shows good
performance for the Suzuki coupling reaction under phosphine-free
conditions which is a topic of considerable interest from both
economic and environmental reasons. The combination of easy
synthesis route, resistance to air, and a long expiry period could
make the hybrid material very attractive for industrial applications.
Reaction conditions: Derivatives of benzene iodide (1.0 mmol)(1.0 mmol), phenyl-
boronic acid (1.5 mmol), K2CO3 (1.5 mmol), toluene (5 ml) and catalyst (polymer
supported Pd 0.02 mol%), 70 ꢂC, 6 h.
as a solvent turned out to be highly efficient under the given
conditions (Scheme 1 and Table 1). The reaction conditions were
systematically optimized and, based upon the results, a series of
different bromobenzene based derivatives were tested in the
presence of the Pd-polymer composite and the toluene-K2CO3
combination under the reaction conditions at 80 ꢂC for 8 h (Table 2).
In order to investigate the potential of aryl halides in coupling
with phenylboronic acid, different aryl bromides were employed in
the reaction. Electron-rich and electron-deficient aryl bromides
were readily coupled in the presence of the metal-polymer hybrid
material. However, aryl bromides with an electron donating group
(entries 1-3) showed a slight drop of reactivity in comparison with
the aryl bromides with an electron withdrawing group (entries 4
and 5). In the case of 4-bromoaniline a higher palladium loading
(0.07 mol %) was required to obtain a good yield of the desired
biaryl product (Table 2, entries 1 and 6). This may be due to the fact
that the amine functionality in the 4-bromoaniline could coordi-
nate to the support source from the catalyst used in the reaction.
Furthermore, the polyaniline-supported palladium nano-
particles can also be applied as a very active catalyst for the
synthesis of biaryls starting from aryl iodides and phenylboronic
acid under comparatively mild conditions and shorter reaction
times (Table 3, entry 1e6) than for aryl bromide. The reaction
between chlorobenzene and phenylboronic acid necessitated
a higher temperature, higher catalyst concentration (0.35 mol% Pd)
and longer reaction time to afford a biphenyl coupling product. A
moderate yield of 37% was achieved at 120 ꢂC for 16 h for the
coupled product. The metal-polymer composite catalyst was found
to be very stable, insensitive to air and showed no performance loss
even after one year of manufacture.
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Scheme 2. Possible mechanism of Suzuki coupling reaction.