Template-less synthesis of polymer hollow spheres: An efficient catalyst for Suzuki coupling reaction

Article abstract of DOI:10.1002/aoc.3024

We report here a method for the preparation of polymer hollow spheres in which 3-aminoquinoline (3-AQ) and palladium acetate were used as precursors. During the reaction 3-AQ was oxidized and formed poly(3-AQ). IR and Raman spectra provided information on the chemical structure of the polymer. The oxidation state of palladium was confirmed by X-ray photoelectron spectroscopic analysis. Transmission and scanning electron microscopy were used to determine the size and morphology of the polymer. The palladium-poly(3-AQ) supramolecular system was used as an effective catalyst for the Suzuki coupling reaction of aryl halides in the absence of a phosphine ligand. Copyright

Full text of DOI:10.1002/aoc.3024

Full Paper
Received: 15 April 2013
Revised: 21 May 2013
Accepted: 29 May 2013
Published online in Wiley Online Library: 23 July 2013
(wileyonlinelibrary.com) DOI 10.1002/aoc.3024
Template-less synthesis of polymer hollow
spheres: an efficient catalyst for Suzuki
coupling reaction
Meenakshi Choudharya, Rafique Ul Islama, Michael J Witcombb,
Motlatsi Phalia and Kaushik Mallicka*
We report here a method for the preparation of polymer hollow spheres in which 3-aminoquinoline (3-AQ) and palladium
acetate were used as precursors. During the reaction 3-AQ was oxidized and formed poly(3-AQ). IR and Raman spectra
provided information on the chemical structure of the polymer. The oxidation state of palladium was confirmed by X-ray
photoelectron spectroscopic analysis. Transmission and scanning electron microscopy were used to determine the size and
morphology of the polymer. The palladium–poly(3-AQ) supramolecular system was used as an effective catalyst for the Suzuki
coupling reaction of aryl halides in the absence of a phosphine ligand. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: polymer hollow sphere; poly(3-aminoquinoline); Suzuki coupling reaction; IPCF; phenyl boronic acid
poly(o-methoxyaniline) composite material has been reported in
which Pd-acetate and o-methoxyaniline were used as the precur-
sor. During the reaction, o-methoxyaniline undergoes oxidation
Introduction
Conjugated polymers with nano- and micro-structures, such as
and forms poly(o-methoxyaniline) while, at the same time, palla-
tubes,[1] belts[2] and fibers,[3] have received growing interest
dium ions form nanoparticles which, under the reaction conditions
used, were stabilized in the polymer matrix.[18] The fabrication of a
because of their unique physical properties and potential
applications in polymeric conducting molecular wires,[4] chemical
poly(3,5-dimethylaniline) nanofiber and palladium nanoparticle
sensors or actuators[5] and gas separation membranes.[6] In the
(polymer–metal) composite material has also been reported,[17]
family of conjugated polymers, polyaniline and related structures
are fascinating because of their environmental stability, controlla-
where 3,5-dimethylaniline and Pd-acetate were used as precursors
of polymer fibers and Pd particles, respectively. A single-step, in situ
chemical synthetic route has been described for the preparation of
a Pd–[poly(o-aminophenol)] composite material in which highly
ble electrical conductivity, interesting redox properties and ease
of synthesis in various sizes and shapes.[7–13] Conjugated polymer
can be prepared either by chemical[14] or electrochemical[15]
dispersed Pd nanoparticles of the order of ~2 nm diameter were
encapsulated within the polymer matrix.[23] A similar kind of IPCF
polymerization technique. In situ polymerization and composite
formation (IPCF) types of reactions[16–21] for the synthesis of
type of reaction has been reported with polyaniline and cerium am-
monium nitrate (Ce4+), in which the product formed a polyaniline–
‘metal–polymer composite material’ have potential advantages
cerium(III) supramolecular complex.[24] A similar approach has also
in the area of ‘synthetic material science’ owing to both the
polymer and the nanoparticles being produced simultaneously,
been reported for the synthesis of a platinum(II)–phenylenediamine
hybrid material with a spherical morphology.[25]
which facilitates an intimate contact between the particles and
the polymer through functionalization.[22] IPCF is a kind of
In the present communication we report on an in situ technique
template-less synthesis route where the nanoparticles are mainly
catalyze the direction and orientation of the growth of the
polymer chain.
for the preparation of
a palladium–poly(3-aminoquinoline)
supramolecular material by applying an IPCF route using palladium
acetate as an oxidizing agent for polymerizing 3-aminoquinoline
(3-AQ). During the reaction 3-AQ forms poly(3-AQ) and palladium
(II) reduced to palladium(I). The ion–polymer composite material
served as an effective and versatile catalyst for Suzuki coupling
We have reported[16] on a gold–polyaniline composite material
using a phase transfer catalyst, in which polyaniline spheres of
several hundred nanometers in diameter were decorated with gold
nanoparticles (10–50 nm). An in situ protocol has been developed
by our group for the synthesis of a copper–poly(o-toluidine)
composite material using cupric sulfate as an oxidizing agent.[12]
The copper nanoparticles produced had an average size of 5 nm
and were highly dispersed within the polymer matrix. A single step,
in situ chemical synthetic route for the preparation of a copper–
poly(3,5-dimethylaniline) composite material has also been
reported in which 7–10 nm diameter copper nanoparticles are
highly dispersed within the fiber-shaped polymer.[20] An IPCF
synthetic route for the preparation of a palladium nanoparticle–
*
Correspondence to: K. Mallick, Department of Chemistry, University of
Johannesburg, Auckland Park 2006, South Africa. Email: kaushikm@uj.ac.za
a Department of Chemistry, University of Johannesburg, Auckland Park 2006,
South Africa
b DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand,
WITS 2050, South Africa
Appl. Organometal. Chem. 2013, 27, 523–528
Copyright © 2013 John Wiley & Sons, Ltd.

M. Choudhary et al.
reaction both for aryl iodide and bromide systems in a very efficient
way under phosphine-free conditions, which is very important from
environmental and economic points of view. The Suzuki reaction is
the coupling of an aryl or vinyl boronic acid with an aryl or vinyl
halide in the presence of a palladium catalyst. It is a powerful
cross-coupling method that allows for the synthesis of conjugated
olefins, styrenes and biphenyls. The palladium(I)–poly(3-AQ) supra-
molecular material has been characterized using various micro-
scopic and optical techniques. X-ray photoelectron spectroscopy
(XPS) showed the presence of Pd(I)-like species. We have found that
the hybrid material is very active as a catalyst for Suzuki reactions
both for aryl iodide and bromide systems.
Results and Discussion
Chemical Structure of the Polymer
IR spectrum study
Figure 2. Raman spectrum of the product in the range 1700–1100 cmÀ1
indicates the formation of polymeric product of 3-AQ with the presence
of both the quinoid and benzenoid ring structure.
In the IR spectra (Fig. 1), the groups N-B-N, where B represents a
benzenoid ring, and N=Q=N, where Q represents a quinoid ring, ab-
sorb at 1510 and 1600 cmÀ1, respectively. A low-intensity peak at
1340 cmÀ1 corresponds to the C-N stretching mode of the quinoid
ring and confirms the presence of this form in the polymer. The
presence of a C-N stretching vibration can be confirmed due to
the presence of the band at 1275 cmÀ1. The aromatic C-H in-plane
of the polaronic units. The 1221 cmÀ1 band, corresponds to C-N
stretching mode due to the single bond. The positions of the
C-H benzene deformation modes fall at 1175 cmÀ1, indicating
the presence of quinoid rings.
The Raman and IR spectra indicate the formation of polymeric
product of 3-AQ with the presence of both the quinoid and the
benzenoid ring structure.
bending mode was observed in the region of 1215–1040 cmÀ1
.
The out-of-plane deformations of the 1,4 disubstituted rings
are located at 880 cmÀ1. This study indicates that the polymer
is neither in the fully oxidized nor the fully reduced state because
of the presence of both the quinoid and the benzenoid structure
in the resultant material.
UV-visible spectra
The electronic absorption spectrum of poly(3-AQ) has been
recorded and Fig. 3 shows two prominent absorption bands.
The absorption peak at 320 nm is due to the π–π* transition of
the benzenoid rings.
An absorption peak at 600 nm has been observed which is due
to the transition from a localized benzenoid highest occupied mo-
lecular orbital to a quinoid lowest unoccupied molecular orbital, i.e.
a benzenoid to quinoid excitonic transition.[27] The position of exci-
tonic peak shifts from 560 to 630 nm. It is well known that the po-
sition of this peak is sensitive to the nature of the counter-ions and
solvent and to the chemical structure of the polymer.[28]
Raman spectrum study
In the range 1100–1700 cmÀ1, the Raman spectrum bands of the
product are sensitive to many conformation-dependent features
(Fig. 2). The C-C deformation bands of benzenoid ring at 1620
and 1590 cmÀ1, which are characteristic for semiquinone
rings.[26,27] A strong band at 1463 cmÀ1 corresponds to the C=N
stretching mode of the quinoid units. The band at 1376 cmÀ1
correspond to the C-N•+ stretching mode of the delocalized
polaronic charge carriers and the high intensity of the band
confirms the high concentration of these forms in the product.
The band at 1278 cmÀ1 can be assigned to C-N stretching mode
Morphology of the Polymer
A scanning electron microscopy (SEM) image (Fig. 4A) reveals
that the product, poly(3-AQ), is composed of microspheres. A
Figure 3. UV–visible spectrum of the product shows two prominent ab-
sorption bands at 320 and 600 nm, indicating π–π* transition of the ben-
zenoid rings and benzenoid to quinoid excitonic transition respectively.
Figure 1. IR spectrum indicates the presence of both benzenoid and
quinoid rings in the product at 1510 and 1600 cmÀ1 respectively.
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Appl. Organometal. Chem. 2013, 27, 523–528

Polymer hollow spheres for Suzuki coupling reaction
higher magnification SEM image (Fig. 4B) indicates spheres with
a wide range of size distribution. In addition, the figure also
reveals that the surface of the spheres is not smooth.
monomer.[29] According to the most widely accepted mecha-
nism, the first step during 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.
In the present work, during the addition of palladium acetate to
3-AQ a greenish-yellow precipitation formed. Spectroscopic
analysis confirmed that the product only has a head-to-tail-like
arrangement rather than a head-to-head arrangement. The
head-to-head coupling occurs only under neutral or basic pH
conditions. On the other hand, 3-AQ oxidation products obtained
in an acidic medium (presence of palladium acetate) have a
predominantly head-to-tail-like arrangement.[29]
During the reaction between Pd(OAc)2 and 3-AQ, the metal salt
acts as an oxidizing agent[16,30,31] that can oxidize AQ and form
poly(3-AQ). During polymerization, each step involves the release
of an electron[29] and that free electron is then used to reduce the
Pd(II) ion. In general, the released electron reduces the Pd(II) to
Pd(0), which ultimately forms Pd nanoparticles[17,18,21] but in this
reaction we found evidence of formation of Pd(I) rather than Pd
(0) state. The most probable explanation of this event is that
A transmission electron microscopy (TEM) image shows the
more in situ structure of the microspheres (Fig. 5). Figure 5(A)
shows a two-dimensional image with a dark shell and light core,
indicating the hollow-spherical structure of the polymer. Figure 5
(B) is a single-polymer hollow sphere. A typical energy-dispersive
X-ray (EDX) analysis obtained from an electron beam focused
onto the various parts of a single sphere yielded Pd X-ray peaks
with almost identical intensity. This was found to be the case
for all the spheres analyzed. Figure 6 is a representative EDX
spectrum from a composite sphere.
Mechanism of Formation of the Polymer
3-AQ has structural similarity with aniline or aniline derivatives. A
wide variety of methods have been applied for the preparation of
polyaniline-type compounds by oxidative polymerization of their
Figure 4. (A) SEM image showing the formation of the poly(3-AQ) microspheres. (B) Higher-magnification SEM image indicates spheres with a wide
range of size distribution.
Figure 5. (A) TEM image of the poly(3-AQ) microspheres, focus being at the top of the spheres. (B) Higher-magnification image of a single-polymer
hollow sphere, clearly showing the dark shell and the light core, focus being on the background.
Appl. Organometal. Chem. 2013, 27, 523–528
Copyright © 2013 John Wiley & Sons, Ltd.
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M. Choudhary et al.
Figure 7. X-ray photoelectron spectrum of the Pd–poly(3-AQ) composite
with peak position at 336.50 eV indicates the presence of Pd(I).
Figure 6. EDX analysis resulting from the electron beam being focused
onto a spot on a composite sphere.
the coupling of phenylboronic acid with bromobenzene. Among
the different bases, such as NaOAc, Na2CO3, K2CO3, Cs2CO3, Et3N
and KOH, we found that K2CO3 delivered the product with high
yields when combined with toluene as a solvent.
the Pd(II) ionic state first formed Pd(I) species, which then
coordinated with the nitrogen of quinoline to form an N→Pd(I)
type of bond. XPS analysis confirmed the formation of Pd(I)-like
species. There is no evidence for formation Pd nanoparticles from
the TEM image.
We have investigated the coupling of aryl bromide and iodide
with phenylboronic acid in the presence of a Pd–poly(3-AQ)
catalyst (Table 1). High catalytic activity of Pd–poly(3-AQ) for both
the deactivated and activated aryl iodide and bromides was
observed, with formation of the corresponding biphenyl
compounds with excellent yields. Activity of the aryl halides
decreases in the order I > Br > Cl and the electron-deficient aryl
halide is generally more active than the electron-rich one.[35] Aryl
iodide with electron-withdrawing groups (Table 1, entries 1 and 2)
showed slightly higher reactivity than those possessing electron-
donating groups (Table 1, entries 3 and 4). A similar trend was also
observed in the case of aryl bromide (Table 1, entries 5–8).
It was found that the turnover frequency (TOF) was as high as
2340 hÀ1 for the coupling of 4-nitroiodobenzene with
phenylboronic acid (entry 1) when 0.048 mol% of Pd used as cata-
lyst. A slight fall of TOF value (2240 h-1) was noted when a compar-
atively weak electron-withdrawing (-COOH) group was attached at
the para position of iodobenzene (entry 2) under a similar amount
of catalytic influence. Again, in the presence of electron-donating
groups (-CH3 and -NH2) in the iodobenzene ring (entries 3 and 4)
the TOF value dropped to 1908 and 1730 hÀ1 with a yield of 75%
and 68%, respectively, under same catalytic conditions to achieve
the coupled products. For aryl bromides, a comparable yield could
be achieved, like aryl iodides, when 0.073 mol% of Pd was used as
catalyst (entries 5–8). We found that 80°C was the ideal tempera-
ture for achieving optimum yield. A small improvement in yield
was achieved by adding higher amounts of the catalyst, which,
whoever, would adversely affect the overall turnover frequency
value of the reaction.
In the case of 4-bromoaniline a higher catalyst loading (0.097 mol%
Pd) was required to obtain a higher yield of the desired biaryl product
(Table 1, entry 9) along with a longer reaction time, but the TOF value
further decreased to 954 hÀ1 (entry 9). This may be due to the fact that
the amine functionality in 4-bromoaniline could coordinate to the
support source from the catalyst used in the reaction.
The supramolecular material, poly(aminophenol)-supported
palladium (I), can also be applied as a catalyst for the synthesis of
biaryls from 4-chloronitrobenzene and phenylboronic acid at
higher temperature, higher catalyst concentration (0.22 mol% Pd)
and longer reaction time. A moderate yield of 42% with a TOF value
195 hÀ1 was achieved at 100°C for 10 h to obtain the coupled
product (entry 10).
To identify the chemical state of the polymer-stabilized palla-
dium particles, XPS measurement was carried out. The peak
corresponding to 3d5/2 state from spin-orbital splitting appearing
at 335.7 eV indicates the presence of zero-valent Pd,[32] whereas
the peak around 337.75 eV was assigned to Pd(II) state.[33] The char-
acteristic peaks corresponding to Pd 3d3/2 and 3d5/2 states from the
spin-orbital splitting are shown in Fig. 7. In this work we used only
the binding energy value of the Pd 3d5/2 line to find the oxidation
state of palladium. The Pd-poly(3-AQ) sample shows (Fig. 7) a broad
spectrum within the range 334.5–339.0 eV. After deconvolution
two separate peaks appeared at 336.47 and 337.37 eV. As stated
before, the peak at 337.37 eV is due to the presence of unreacted
Pd(II), whereas the new peak that appeared at 336.47 could be
due to the presence of Pd(I) in the sample, as supported by other
studies.[33,34] XPS analysis also confirmed that 1.64 wt% Pd was
present on the surface of the polymer matrix. TEM (Fig. 5) also con-
firmed that there was no evidence of formation of metallic palla-
dium in the sample.
Catalytic Performance of Pd–poly(3-AQ) for
Suzuki coupling reaction
The reaction reported first by Miyaura and Suzuki[35] is the cross-
coupling reaction between aromatic (or vinyl) halides and boranes,
boronic acids or esters to form biaryls. The original recommenda-
tion for the Suzuki reaction involved a phosphine-based palladium
catalyst, it being commonly believed that one role of the
phosphine ligand was the formation of catalytically active zero-
valent palladium species.[36–38] Recently, pre-formed palladium
nanoparticles were also used as a catalyst for the Suzuki
coupling.[36–40] Pd anchored with amine and pyridine-based
ligands showed remarkable catalytic performance for Suzuki
coupling reactions.[41,42] The Pd–poly(3-AQ) composite material
was used as a catalyst for the Suzuki coupling reaction, which is a
versatile method for carbon–carbon bond formation, especially
when applied to the synthesis of biaryls in organic synthesis.
Initially, a solvent-based optimization study was performed for
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 523–528

Polymer hollow spheres for Suzuki coupling reaction
Table 1. Suzuki coupling reaction between aryl halides and phenylboronic acid
Entry
X
R
Yield (%)
TOF (hÀ1
)
1
2
-I
-I
-I
-I
-NO2
92
88
75
68
89
84
74
58
75
42
2340
2240
1908
1730
1509
1425
1255
983
-COOH
-CH3
3
4
-NH2
-NO2
-COOH
-CH3
5
-Br
-Br
-Br
-Br
-Br
-Cl
6
7
8
-NH2
-NH2
-NO2
9
954
10
195
Entries1–4: iodobenzene and derivatives (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (1.5 mmol), toluene (5 ml) and catalyst concentration
0.048 mol% Pd. Entries 5–8: bromobenzene and derivatives (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (1.5 mmol), toluene (5 ml) and
catalyst concentration 0.073 mol% Pd. Entry 9: catalyst concentration 0.097 mol% Pd. Entry 10: 4-chloronitrobenzene (1.0 mmol), phenylboronic
acid (1.5 mmol), K2CO3 (1.5 mmol), toluene (5 ml) and catalyst concentration 0.22 mol% Pd at 100°C for 10 h.
XPS indicates the presence of Pd(I) and unreacted Pd(II) species
in the composite material. Reduction of Pd(II) to Pd(0) by
phenylboronic acid has been reported,[43,44] so reduction of Pd
(I) to Pd(0) by phenylboronic acid is also possible. It is believed
that for the Suzuki coupling reaction Pd(0) is the catalytic
species.[45,46] The interaction of aryl halide (R1X) and Pd(0) forms
the aryl–palladium halide complex [R1(Pd2+)X], which then
couples with phenylboronic acid [R2B(OH)2] in the presence of a
base to produce the [R1–(Pd2+)–R2] intermediate and finally
produces the biaryl product (R1–R2) via the reductive
elimination of Pd2+ to Pd(0), as outlined in Scheme 1.
The recyclability study was performed for the coupling of 4-
nitroiodobenzene with phenylboronic acid using 0.048 mol% of
Pd catalyst. A 12% drop in product yield was measured after
three cycles of the reaction. This fall in catalytic performance of
the ‘Pd–polymer’ composite may be due to agglomeration of
the palladium particles. It is also important to mention that the
study employing inductively coupled plasma mass spectrometry
did not support any kind of leaching of palladium during the re-
covery process of the catalyst. The composite of metal ion and
conjugated polymer is not well known as a catalyst from an appli-
cation point of view. In most cases the reaction between metal
salt and monomer ended with the formation of polymer-encap-
sulated metal nanoparticles.[18,23,47] In this present study, how-
ever, Pd ion-incorporated poly(aminoquinoline) showed it to
have excellent catalytic activity and high stability for a longer
time period without noticeable deactivation.
Scaling up of the catalyst, working with different derivatives as
well as in heterocyclic systems, the effect of solvent, temperature
and the reaction kinetic studies are all currently under investiga-
tion in our laboratory. The material described here also has
potential as a catalyst for other carbon–carbon bond formation
reactions and this will be reported elsewhere.
Conclusion
The present article reports on the synthesis of a functional poly-
mer with a unique hollow sphere-like morphology using palla-
dium acetate as an oxidizing agent. During the reaction,
palladium acetate reduced to Pd(I) species and formed a complex
with poly(aminoquinoline). The ion–polymer composite material
served as an effective and versatile catalyst for Suzuki coupling
reaction both for aryl iodide and bromide systems in a very effi-
cient way under phosphine-free conditions, which is very impor-
tant from environmental and economic points of view. The
composite catalyst was found to be very stable and could be kept
for several years without noticeable deactivation.
Experimental
Materials
3-AQ, palladium acetate and all the other chemicals were
purchased from Sigma-Aldrich and used without further purification.
All the solvents used in this work, including toluene, were supplied
by Merck.
Characterization Techniques
Instrumentation
TEM was carried out at an accelerated voltage of 200 kV using a
Philips CM200 transmission electron microscope equipped with an
LaB6 source. An ultrathin windowed energy-dispersive X-ray
Scheme 1. Recommended mechanism of Suzuki coupling reaction.
Appl. Organometal. Chem. 2013, 27, 523–528
Copyright © 2013 John Wiley & Sons, Ltd.
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M. Choudhary et al.
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SEM studies were undertaken in an FEI FEG Nova 600 Nanolab at 5 kV
instrument. As a precaution to prevent possible charging, the samples
were sputter coated with a thin uniform layer of Au–Pd prior to viewing.
For UV–visible spectra analysis, a small portion of the solid
sample was dissolved in methanol and scanned within the range
300–700 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
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In a typical experiment 0.35 g 3-AQ was added to 15 ml tolu-
ene in a 50 ml conical flask. The flask was then shaking manu-
ally with appropriate precautions. Palladium acetate (Pd(OAc)2)
in toluene (10 mL) having a concentration of 2.0 × 10À3
M
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system. A greenish-yellow color developed at the bottom of
the conical flask. The solution was kept under static conditions
for a further 10 min. A precipitation slowly formed at the bot-
tom of the flask. The material was then allowed to settle for an
additional 15 min. The whole process was carried out at room
temperature (~25°C). The amount of product obtained after the
reaction between 3-AQ and Pd(OAc)2 was 0.31 g. Subsequently,
the colloidal precipitation was taken from the bottom of the
flask and pipetted onto lacey, carbon-coated, copper TEM grids
for SEM and TEM analysis. The rest of the solution was filtered,
washed with distilled water and kept under vacuum overnight.
A small portion of the dried powder was used for Raman, IR
and UV–visible analysis. The remaining portion was used for
the study of the catalytic properties of the materials.
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Appl. Organometal. Chem. 2013, 27, 523–528