Polypyrrole-palladium nanoparticles composite as efficient catalyst for Suzuki-Miyaura coupling

Article abstract of DOI:10.1016/j.molcata.2011.11.004

Synthesis of a new hybrid material (Pd/PPy) composed of polypyrrole globules with uniformly incorporated Pd nanoparticles via direct redox reaction between pyrrole and Pd(NH₃)₄Cl₂ in water has been recently reported (V.A. Zinovyeva, M.A. Vorotyntsev, I. Bezverkhyy, D. Chaumont, J.-C. Hierso, Adv. Funct. Mater. 21 (2011) 1064-1075). In the actual study, this procedure has been extended to synthesize a series of Pd/PPy powders with variable palladium content and morphological parameters. Depending on the monomer-to-oxidant ratio in reaction mixture, average diameters of Pd and PPy particles may change in the ranges of 1.25-1.45 and 27-62 nm, respectively, the Pd concentration being within 33.5-42.0 wt.%. In general terms, decrease of the monomer-to-oxidant ratio led to formation of the Pd/PPy hybrid material with smaller diameters of both components and a higher Pd loading. The Pd/PPy composites have been studied in Suzuki-Miyaura coupling and showed high catalytic efficiency. Aryl iodides, bromides and chlorides are active. The reaction can be performed using arylboronic acids or tetraarylborates, both in organic solvents and in water, thus making the process ecologically friendly. The recycling of the catalyst is possible if its particles are immobilized on the graphite support. The comparison of two samples revealed that Pd/PPy nanocomposite with the diameter of PPy spheres of about 30 nm is more efficient in catalysis, as compared to the sample with bigger PPy spheres (about 60 nm), due to transport limitations for reagents inside the polypyrrole sphere in the latter case. For palladium/polypyrrole nanocomposites with the small diameter of PPy sphere, which are easily penetrable for the reagents and in which all Pd nanoparticles are active, Pd content in polypyrrole spheres does not influence the yield of biaryls: the more is the Pd content in polypyrrole spheres, the less amount of catalyst is necessary to obtain the same yield of biaryl.

Full text of DOI:10.1016/j.molcata.2011.11.004

Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Polypyrrole–palladium nanoparticles composite as efficient catalyst for
Suzuki–Miyaura coupling
Tatiana V. Magdesievaa,∗, Oleg M. Nikitina,d, Oleg A. Levitskya, Veronika A. Zinovyevab,
Igor Bezverkhyyc, Ekaterina V. Zolotukhinab, Mikhail A. Vorotyntseva,b,∗∗
a Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory, 1/3, Moscow 119991, Russia
b Institut de Chimie Moléculaire de l’Université de Bourgogne (ICMUB), UMR-CNRS 5260, Université de Bourgogne, 9 Av. A. Savary, BP 47870, 21078 Dijon Cedex, France
c Laboratoire Interdisciplinaire Carnot de Bourgogne (ICB), UMR-CNRS 5209, Université de Bourgogne, 9 Av. A. Savary, BP 47870, 21078 Dijon Cedex, France
d A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Science, Vavilova Str. 28, 119991, Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2011
Received in revised form 18 October 2011
Accepted 1 November 2011
Available online 10 November 2011
Synthesis of a new hybrid material (Pd/PPy) composed of polypyrrole globules with uniformly incor-
porated Pd nanoparticles via direct redox reaction between pyrrole and Pd(NH3)4Cl2 in water has been
recently reported (V.A. Zinovyeva, M.A. Vorotyntsev, I. Bezverkhyy, D. Chaumont, J.-C. Hierso, Adv. Funct.
Mater. 21 (2011) 1064–1075). In the actual study, this procedure has been extended to synthesize a series
of Pd/PPy powders with variable palladium content and morphological parameters. Depending on the
monomer-to-oxidant ratio in reaction mixture, average diameters of Pd and PPy particles may change in
the ranges of 1.25–1.45 and 27–62 nm, respectively, the Pd concentration being within 33.5–42.0 wt.%. In
general terms, decrease of the monomer-to-oxidant ratio led to formation of the Pd/PPy hybrid material
with smaller diameters of both components and a higher Pd loading. The Pd/PPy composites have been
studied in Suzuki–Miyaura coupling and showed high catalytic efficiency. Aryl iodides, bromides and
chlorides are active. The reaction can be performed using arylboronic acids or tetraarylborates, both in
organic solvents and in water, thus making the process ecologically friendly. The recycling of the cata-
lyst is possible if its particles are immobilized on the graphite support. The comparison of two samples
revealed that Pd/PPy nanocomposite with the diameter of PPy spheres of about 30 nm is more efficient in
catalysis, as compared to the sample with bigger PPy spheres (about 60 nm), due to transport limitations
for reagents inside the polypyrrole sphere in the latter case. For palladium/polypyrrole nanocomposites
with the small diameter of PPy sphere, which are easily penetrable for the reagents and in which all Pd
nanoparticles are active, Pd content in polypyrrole spheres does not influence the yield of biaryls: the
more is the Pd content in polypyrrole spheres, the less amount of catalyst is necessary to obtain the same
yield of biaryl.
Keywords:
Nanocomposite
Polypyrrole
Pd nanoparticles
Catalysis
Suzuki–Miyaura coupling
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
both homogeneous and heterogeneous [1,3]. Each of them has
advantages and drawbacks, as well as limitations in application. The
Palladium-catalyzed cross-coupling of aryl halides with aryl-
boronic derivatives is a very convenient and widely used approach
to C(sp2)–C(sp2) bond construction and is the most popular for
accessing biaryls [1]. Biaryl motif is often present in natural prod-
ucts, pharmaceuticals, herbicides and other useful molecules [2].
Plenty of catalysts for this reaction have been already developed,
rapid development of the pharmaceutical and fine chemical indus-
tries dealing with complex multifunctional compounds brings to
a focus new additional requirements. Namely, design and syn-
thesis of new types of efficient and selective catalysts satisfying
the requirements of sustainable chemistry, such as a possibility of
recycling, small palladium loadings and replacement of hazardous
organic solvents by water is still of primary importance. Most of
the catalysts used in Suzuki–Miyaura coupling are based on pal-
ladium complexes with phosphine ligands [1,3–6] which are very
efficient but economically and environmentally undesirable due to
the difficulties with their recover and the formation of toxic phos-
phines as byproducts in the reaction. The latter can be overcome
by using new types of ligands such as carbenes [7,8], pincer lig-
ands [9] or palladacycles [10,11] which demonstrated excellent
∗
Corresponding author. Tel.: +7 495 939 3065; fax: +7 495 939 0290.
Corresponding author at: Institut de Chimie Moléculaire de l’Université de Bour-
∗∗
gogne (ICMUB), UMR-CNRS 5260, Université de Bourgogne, 9 Av. A. Savary, BP 47870,
21078 Dijon Cedex, France. Tel.: +33 3 8039 6064; fax: +33 3 8039 6064.
E-mail addresses: tvm@org.chem.msu.ru (T.V. Magdesieva),
mv@u-bourgogne.fr (M.A. Vorotyntsev).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.11.004

T.V. Magdesieva et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
51
catalytic results in Suzuki–Miyaura coupling. However, the recy-
cling of the homogeneous catalyst is still an open question in the
majority of cases. Besides, contamination of the products by pal-
ladium residues and losses of expensive palladium are common
drawbacks of homogeneous catalysis.
common approaches. A few examples of direct chemical reduc-
tion of palladium ions during pyrrole oxidation have been also
reported, namely the syntheses of a Pd/PPy composite in an inverse
microemulsion medium [56] and in hydrochloric acid [57]; and
bimetallic Cu/Pd/PPy composite formation in water [58].
Thus, one of the key goals in the design of a new catalyst is
to combine high efficiency of the catalyst with its easy separation
from the reaction products and the possibility of recycling; and in
this aspect nanosize metallic catalysts in solid supports create a
perspective type of catalytic materials. Its preparation is usually
performed in ligand-free conditions and rather cheap inorganic
precursors are used [12]. Catalysis is one of the principle appli-
cations of nanoscience and nanotechnology [13–16]. The use of
nanoparticles provides a bridge between homogeneous and het-
erogeneous catalysis integrating the advantages of each form and
this is the main reason for increasing interest in catalysis by metal
nanoparticles.
Recently, we described a simple non-template one-step method
for the synthesis of palladium nanoparticles encapsulated into
polypyrrole globules (Pd/PPy) via direct redox reaction between
various palladium(II) salts and pyrrole in aqueous and organic
media [59,60]. This Pd/PPy composite has shown a high catalytic
activity for direct arylation of heteroaromatics with bromoarenes
[60]. The same synthetic procedure based on mixed aqueous solu-
tions of pyrrole and PdCl2 in the presence of KCl has also been
used to deposit the Pd/PPy nanocomposite as a layer around
micrometer-sized polystyrene (PS) latex spheres, with demonstra-
tion of their catalytic activity [61]. The formation of nano catalysts
in redox-polymerization of heterocyclic precursor oxidized by
metal ions is one of the approaches promising for catalytic applica-
tion. It allows avoiding addition of surface stabilizers (as it is often
the case in the preparation of stable nanosize systems) since the
simultaneously formed conducting polymer matrix itself stabilizes
nanoparticles and prevents their aggregation. Pd/PPy nanocompos-
ites on the surface of PS latex particles are efficient catalysts for
Suzuki–Miyaura-type coupling reactions in aqueous media [61].
Pd component for this 26 wt.% PPy–Pd nanocomposite system was
represented by metal nanoparticles with a bimodal size distribu-
tion: 6.0 nm Pd nanoparticles forming about 130 nm Pd aggregates
on the surface of the PPy shell and 1.8 nm Pd nanoparticles dis-
persed in the shell [61].
The influence of a size and morphology of nanocomposite on
its catalytic properties is an important question. Consequently, the
systems with tunable parameters, allowing to create an optimal
catalyst, are of special interest. Recently we described synthetic
protocol with the use of Pd(NH3)4Cl2 as oxidant in water resulted
in formation of small Pd nanoparticles (about 2.4 nm) uniformly
distributed inside PPy spheres [60]. In a continuation of this inves-
tigation, we found that sizes of both Pd nanoparticles and PPy
globules may be reliably controlled by the choice of the synthe-
sis conditions, both geometrical parameters being uniform, with
a low dispersion. In this context, we have extended the prepara-
tion method of Pd/PPy composites with the use of Pd(NH3)4Cl2 as
oxidant in aqueous medium by varying of initial reagent concentra-
tions. In the present paper several types of Pd/PPy nanocomposites
with different size of PPy globules and Pd content were synthesized,
to establish the influence of polymerization conditions on chemical
composition, morphological and then catalytic properties of com-
posites. As a model catalytic reaction, Suzuki–Miyaura coupling of
aryl halides with arylboronic acids or sodium tetraphenylborate
in organic and aqueous media was chosen and it clearly demon-
strated that the size of PPy globules should be considered as an
important parameter determining the catalytic efficiency of the
nanocomposite.
However, the application of palladium nanocatalysts in
Suzuki–Miyaura coupling is still not as wide as can be expected.
In the majority of publications aryl bromides and aryl iodides
were used [12,17–31], the examples of the efficient couplings
of aryl chlorides are rarer [32–36]. The reactions can be per-
formed in organic solvents, organic solvent/H2O mixtures, or in
pure water [19,23,24,26–31,37], which is of special interest in the
context of green chemistry. Microwave irradiation can be used
for enhancement of catalytic activity [35] though it often makes
impossible a recycling of the catalyst. Various types of polymeric
support were used for incorporation of nanosized Pd particles, such
as polyvinylpyrrolidone [27,38,39], hypercrosslinked polystyrene
matrix [32], zeolite [20,40], polyurea [21], epoxide-containing
copolymers [36,41], copolymers of styrene with divinylbenzene
[22], 4-vinylpyridine [26], vinylpyrrolidone [28] or methacrylic acid
[28,31] and some others [23,29]. In [28] a strategy to facilitate
reuse of palladium catalyst stabilized by block copolymer micelles
was proposed. Following this strategy, the polystyrene-poly(4-
vinyl pyridine) micelles were firstly anchored on the surface of
polystyrene-co-poly(methyl acrylic acid) core–shell microspheres
to form raspberry-like colloid. Then it was used as scaffold to sup-
port colloidal palladium catalyst and showed similar activity to the
catalyst stabilized by block copolymer micelles but much improved
recoverability [28]. Nanosized palladium on polyvinylpyrrolidone
proposed in [27] also showed excellent recoverability. It was recy-
cled for eight consecutive trials without significant loss of its
activity in Suzuki–Miyaura coupling of aryl bromides and iodides.
Dendrimers also can behave as “molecular boxes” that can
entrap and stabilize Pd nanoparticles especially if there are
heteroatoms in the dendrimer interiors. The effect of the
poly(amidoamine) dendrimer generation on the catalytic activity in
Suzuki–Miyaura coupling was investigated in [42–44] and the 3rd
and 4th generations turned out to be the most efficient. However,
the form of Pd nanoparticles within the dendrimer is still not clear
despite TEM studies [45], as well as the question if the nanospecies
are really completely inside the dendrimer.
The high activity of Pd/PANI composites prepared by inter-
action of Pd(II) ions with reduced polyaniline (PANI) matrix in
Suzuki–Miyaura coupling has been shown in several reports, that
makes this class of composites, metal component/conducting poly-
mer, perspective for catalytic studies [46–48].
2. Experimental
2.1. Materials
Polypyrrole is also used for preparation of Pd nanocompos-
ites. Electrochemical incorporation of palladium nanoparticles into
electroactive polypyrrole matrices during electrochemical synthe-
sis of PPy films [49]; electrodeposition of metal nanoparticles
on preformed electrodes (surface of electrodes was coated with
thin polymer films) [50–53]; absorption of metal ions by poly-
mer film/particles from the solution of transition metal salts,
with their subsequent chemical reduction [54,55] – these are
Pd(NH3)4Cl2 monohydrate (99%, Alfa Aesar) and ammonium
carbonate (for analysis, Carl Roth) were used as received. Pyr-
role (98+%, Alfa Aesar) was distilled under argon atmosphere
before use. Aryl halides and sodium tetraphenyl borate were com-
mercially available from Aldrich. Substituted arylboronic acids
were synthesized via standard protocol [62]. N-methylpyrrolidone
(NMP, Aldrich, spectroscopic quality) was stirred over CaH2 for
3 h and distilled under reduced pressure. Acetonitrile (AN, Aldrich

52
T.V. Magdesieva et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
spectroscopic quality, <0.02% water content) was refluxed over
CaH2 for 12 h and distilled. Water was distilled using aquadistillator
BE-4 (Russia) for catalytic experiments.
acetonitrile, the solvent was evaporated under reduced pressure
and the products were extracted from the residue with the diethyl
ether. The ether extract was washed with water, dried over MgSO4
and analyzed using GC–MS “Pegasus 4D” (LECO) with naphthalene
as an internal standard.
2.2. Preparation of Pd/PPy nanocomposites
Pd/PPy nanocomposites were obtained via direct redox reac-
tion between pyrrole and Pd(NH3)4Cl2 in water (purified by means
of GFL-2008 distiller) with the use of the synthetic procedure
described in [60]. In typical synthesis equal volumes (500 ml)
of aqueous solutions of Pd(NH3)4Cl2 (2 mM) and pyrrole (varied
concentration) were mixed under vigorous stirring to obtain the
following molar ratios of oxidant to monomer: 1:10 (Pd/PPy-10),
1:100 (Pd/PPy-100) and 1:300 (Pd/PPy-300). The reaction mixture
was treated in ultrasonic bath (48 kHz, output power 50 W; Bran-
sonic 220, Germany) at T = 40–50◦ C for 5 h per day during 5 days.
The sedimentation of colloid was either spontaneous at the end
of the 7th day, or provoked by addition of about 2 g of (NH4)2CO3
to standardize the reaction time. The supernatant was decanted,
and the dark precipitate was rinsed several times by water and
acetonitrile, then dried under vacuum at 60 ◦C for 4 h.
2.4.2. Pd/PPy on the graphite support
To obtain an immobilized catalyst, 1 mg of Pd/PPy-100 was
dispersed in 2 ml of NMP (or acetonitrile) using ultrasonication.
300 ␮l2 (0.3 mol.% of Pd respectively to aryl halide) of suspension
was carefully deposited on a graphite tissue (2 cm × 1 cm) with
specific surface area of 12 m2/g (“URAL”, Russia). Afterwards the
graphite tissue was dried in a Schlenk tube under reduced pressure
(10 Torr) at 80 ◦C for 1 h.
Thus prepared immobilized Pd/PPy samples were used in the
Suzuki–Miyaura coupling in NMP or in water according to the gen-
eral procedure. A modified graphite tissue was placed in a reaction
mixture containing 2 ml of a solvent (NMP or H2O), 0.2 mmol of aryl
halide, 0.05 mmol of NaBPh4, and 0.5 mmol of a base – Na2CO3.
The mixture was stirred with magnetic stirrer for several hours
at 100 ◦C. Afterwards, the modified graphite tissue was taken off,
washed with 5 ml of a solvent. When the reaction was performed in
water, the joint solution was saturated with NaCl and the products
were extracted with the diethyl ether (3 × 10 ml). When the sol-
vent was NMP, before ether extraction of the products (3 × 15 ml),
30 ml of water saturated with NaCl was added to the reaction mix-
ture. In both cases ether extracts were washed with water, dried
over MgSO4 and analyzed using GC–MS “Pegasus 4D” (LECO) with
naphthalene as an internal standard.
2.3. Characterization
The chemical composition of Pd/PPy powders was deter-
mined by means of elemental analysis (detection of C, H, N, S;
Flash EA 1112 Thermo Electron analyzer), inductively coupled
plasma atomic emission spectroscopy (ICP-AES; analysis of Pd
concentration in Pd/PPy composites) and energy-dispersive X-ray
spectroscopy (EDX; Oxford Instruments EDX analyzer attached to
the scanning electron microscope).
Morphology of the synthesized Pd/PPy nanocomposites was
studied by scanning and transmission electron microscopies (SEM
and TEM, respectively). SEM micrographs of dried powders were
registered on a JEOL JSM-6400F device at 15 kV accelerating volt-
age. Due to the technical limitations of the accurate detection of
small particles (less than 50 nm) by SEM method, the detailed anal-
ysis of the obtained SEM images of Pd/PPy samples is not provided.
TEM images were obtained on a Jeol JEM-2100 TEM instrument
with LaB6 source operating at accelerating voltage of 200 kV.
The particle size distribution (PSD) of Pd/PPy and Pd globules
was determined by analysis of at least 100 particles using TEM
images. On basis of these data, the average diameters of PPy and Pd
components with corresponding confidence intervals (at the 95%
confidence level) were calculated.
For recycling experiments, after the first reaction was finished,
the graphite tissue with the immobilized catalyst was taken off,
washed with NMP and dried in a Schlenk tube under reduced pres-
sure (10 Torr) at 80 ◦C for 1 h. Afterwards, the catalyst was used in
the subsequent reaction runs with new portions of reagents with-
out any additional treatment.
Mass-spectra for selected compounds:
Mes-Ph:
EI-MS m/z (I, %): 196 [M]+ (62), 181 [M−CH3]+ (100), 165
[M−2CH3]+ (32), 119 [M−Ph]+ (20), 76 [M−MesH]+ (12)
4-NO2C6H4Ph
EI-MS m/z (I, %): 199 [M]+ (85), 153 [M−NO2]+ (100), 76
[M−NO2–PhH]+ (13)
4-CH3OC6H4Ph
EI-MS m/z (I, %): 184 [M]+ (100), 169 [M−CH3]+ (35), 153
[M−CH3O]+ (6), 76 [M−CH3O–PhH]+ (2)
The description of Pd/PPy sample preparation for analyses is
given in [60].
2-CH3C6H4Ph
EI-MS m/z (I, %): 168 [M] + (100), 153 [M−CH3]+ (32)
4-CH3C6H4Ph
2.4. Suzuki–Miyaura cross-coupling reaction
EI-MS m/z (I, %): 168 [M]+ (100), 153 [M−CH3]+ (24)
2-Py-Ph
2.4.1. Pd/PPy in solution
EI-MS m/z (I, %): 155 [M]+ (100), 77 [M−Ph–H]+ (9), 76 [M−Py–H]+
Pd/PPy (0.01 mmol of Pd) was suspended in 2 ml of NMP or
CH3CN using ultrasonication. A mixture of aryl halide (1 mmol),1
NaBPh4 (0.25 mmol) or arylboronic acid (1 mmol) and 2.5 mmol of
Na2CO3 was added to the suspension. The reaction mixture was
vigorously stirred under Ar atmosphere at 100 ◦C (NMP) or 82 ◦C
(CH3CN) for several hours. After cooling to room temperature,
the reaction mixture was separated from Pd/PPy particles using
centrifuge (centrifuge 5424 Eppendorf AG, Hamburg, 15,000 rpm, g-
force = 20,238 rcf). 30 ml of H2O saturated with NaCl was added to
the reaction mixture in NMP and organic products were extracted
with the diethyl ether (3 × 15 ml). In case of reaction performed in
(5)
4-BrC6H4Ph
EI-MS m/z (I, %): 232 [M]+ (100), 153 [M−HBr]+ (59), 76
[M−HBr–Ph]+ (25)
4-ClC6H4Ph
EI-MS m/z (I, %): 188 [M]+ (100), 153 [M−HCl]+ (39), 76
[M−HCl–Ph]+ (15)
2
The efficiency of the immobilized catalyst was tested in a model reaction
(PhBr + NaBPh4, NMP, 100 ◦C, 4 h) with three samples of graphite tissue with
0.5 mol.% (500 ␮l), 0.3 mol.% (300 ␮l) and 0.2 mol.% (200 ␮l) of the catalyst. The yields
of Ph2 were >99% in the first two cases and 88% in the third case. That is why in all
consequent experiments 0.3 mol.% of Pd was used.
1
To carry out the test reactions one fifth amount of the reactants were used.

T.V. Magdesieva et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
53
•
4-Diphenylbenzene
Palladium content in the PPy globule (33.5–35.0 wt.% for Pd/PPy-
300 and Pd/PPy-100 and 42.0 wt.% for Pd/PPy-10).
EI-MS m/z (I, %): 230 [M]+ (100)
All these spectra are in complete agreement with the corre-
sponding data in Wiley database.
Suzuki–Miyaura coupling (an interaction of aryl halides with
boronic acid or tetraphenyl borate) was chosen as a test-reaction:
NaBAr4 or ArB(OH)2
X
Hal
Ar
3. Results and discussion
X
Pd/PPy
The yields of biaryls were determined using GC/MS. We started
3.1. Pd/PPy nanocomposites
with the easier case and used organic solvents. Pd/PPy nanocom-
posites were suspended in acetonitrile or N-methylpyrrolidone
(NMP) using ultrasonication. The initial screening of catalytic effi-
ciency was performed using a Pd/PPy-300 sample. The comparison
of two solvents (CH3CN or NMP) and the screening of the reaction
parameters are given in Table 1. For the optimization of the reaction
conditions, a model reaction of PhBr with NaBPh4 in acetonitrile in
the presence of Na2CO3 as a base was chosen. The reaction time
and amount of catalyst were varied. An increase in the reaction
time from 2 h to 8 h at 82 ◦C increases the yield of biphenyl from
56% to 84%, but still 14% of the starting PhBr was detected in the
reaction mixture (see entries 2–4 in Table 2). A gradual increase
in the amount of the catalyst suspended in CH3CN from 0.5 to
4 mol.% (as calculated to the starting aryl halide) does not influence
the yields of biphenyl substantially (62% and 66%, respectively, see
entries 1, 3, 5, 6 in Table 2). The reason for this is, probably, the
low stability of the suspensions of the nanocomposite in acetoni-
trile. The significant part of the catalyst is precipitated soon after
the ultrasonication is switched off. Contrary to CH3CN, the suspen-
sions of Pd/PPy in NMP are much more stable and, consequently,
more efficient. Almost quantitative yield of biphenyl was obtained
in 4 h in NMP with 4 mol.% of the catalyst (Table 2, entry 10). The
investigation of the other model compound – mesitylboronic acid
(MesB(OH)2) revealed the same tendencies: an increase in the reac-
tion time increases the yield of biaryl and NMP is much better
solvent for the reaction, as compared to CH3CN (compare entries
7, 8 and 7, 11 in Table 2). Evidently, an increase in temperature
(when using NMP), in addition to the stability of dispersed catalyst,
also contributes to the enhancement of the yield of the coupling
product.
For estimation of the influence of geometrical parameters of
the Pd/PPy nanocomposite on its catalytic efficiency, two sam-
ples were prepared, Pd/PPy-100 and Pd/PPy-300. They had similar
Pd nanoparticles size distributions (see below on the absence of
effects dependent on the Pd particle sizes), the same Pd content
in the composite material and differ only in the diameter of the
polypyrrole spheres. The comparison of the PPy nanocomposites
with diameters of polymer spheres of 62 and 27–28 nm demon-
strated a moderate but systematic increase in catalytic activity for
smaller particles (Fig. 2). The possible reason might be transport
limitations: in larger polymer spheres not all Pd works since it takes
more time for the reagent to penetrate into the inner part of the
sphere.
Elemental composition of the synthesized Pd/PPy materials was
studied by means of a set of methods. The presence of four ele-
ments (carbon, nitrogen, oxygen and palladium) was revealed by
EDX chemical analysis of Pd/PPy powders, while chlorine was not
detected in our systems. These data point on the low oxidation
state of polypyrrole in Pd/PPy composites, and they are in full con-
formity with our previous report [60] on the synthesis of the Pd/PPy
material with the use of Pd(NH3)4Cl2 as oxidant.
The results of CHNS elemental and ICP-AES analyses of Pd/PPy
powders are summarized in Table 1. We have found that the
decrease of the monomer to oxidant ratio in the reaction mixture
leads to augmentation of the metal content in such materials: from
33.5 to 35.0 wt.% of Pd for Pd/PPy-100 and Pd/PPy-300 samples to
42.0 wt.% for Pd/PPy-10 composites.
The studies of morphology of Pd/PPy composites prepared with
low initial oxidant concentration (1 mM) and various pyrrole con-
tents (10, 100, 300 mM) in solution were carried out by TEM method
after the reaction completing (Fig. 1 and Table 1). Similar morpholo-
gies were observed for samples Pd/PPy-10 and Pd/PPy-100: the
polypyrrole matrix in such composite materials is represented by
spherical globules, their average diameters ( confidence interval)
being close to 27 1 and 28 1 nm, respectively (see Section 2 for
details). The use of the high initial pyrrole concentration, 300 mM,
resulted in increase of average diameter of polymer globules up to
62 nm (Fig. 1c). Inside all polymer globules, we have observed uni-
formly distributed dark spots of high contrast. These small particles
are attributed to Pd component in agreement with our previous
reports [59,60]. The observed diameters of all Pd particles in TEM
images are very small, below 2.20 nm in synthesized Pd/PPy com-
posites, and their PSD distributions are very narrow (Fig. 1d–f).
One may note a slight increase of the average Pd particle size in
Pd/PPy composites with increase of the monomer to oxidant ratio
in the reaction mixture: 1.25 0.05 nm (Pd/PPy-10), 1.40 0.04 nm
(Pd/PPy-100) and 1.45 0.05 nm (Pd/PPy-300).
3.2. Catalytic Suzuki–Miyaura coupling of aryl halides
To be successfully used in catalysis, the composite materials
must meet several key requirements:
•
nano-size metal particles should be stable towards aggregation,
theyshouldbeuniformlydistributedinsidethestabilizingmatrix,
the stabilizing matrix should be inert and penetrative for the
reagents.
•
In order to explore the scope of the reactions catalyzed by
this material, various boron derivatives and various aryl halides
(Table 3) have been tested with the use of the catalyst with smaller
diameter of PPy globules (Pd/PPy-100) which showed the best
results in the preliminary screening. A variety of substituted aryl
halides featuring an electron-releasing or an electron-withdrawing
group were examined, the former are less active, as usual, but still
the yields are not bad. Aryl chlorides are also active. 2-bromo-
and 2-chloropyridines can be involved in the reaction leading
to 2-phenylpyridine with high yield. Both arylboronic acids and
tetraaryl borates can be used in the cross-coupling. In the latter
case the addition of a base allows utilization of all four aryl groups
which is in line with atom-economy strategy. Sterically hindered
mesitylboronic acid gives smaller yields of biaryls, as it is usually
•
Our Pd/PPy system meets all these requirements. Moreover, as
it was demonstrated in Section 3.1, it is possible to obtain particles
with narrow size distributions of both PPy globules and Pd nanopar-
ticles by varying monomer to oxidant ratio. For testing in catalytic
applications, several Pd/PPy samples were prepared. The diame-
ter of Pd nanoparticles in each sample falls within the interval of
1.2–1.4 nm. The following parameters were varied:
•
The diameter of PPy spheres (27–28 nm for Pd/PPy-10 and
Pd/PPy-100 and 62 nm for Pd/PPy-300).

54
T.V. Magdesieva et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
Table 1
Chemical composition (in wt.%) and morphological parameters (average diameters confidence interval, in nm) of Pd/PPy composites prepared with the use of Pd(NH3)4Cl2
as oxidant.
System
C
N
H
Pd
Asha
d(PPy)
d(Pd)
Pd/PPy-10
Pd/PPy-100
Pd/PPy-300
30.7
33.0
37.6
12.4
13.3
14.5
2.6
3.4
2.9
42.0 2.1
33.5 1.7
35.0 1.8
12.3
16.8
10.0
27
28
62
1
1
2
1.25 0.05
1.40 0.04
1.45 0.05
a
The ash of 10–17 wt.% is attributed to oxygen component in Pd/PPy composite.
observed. Besides biaryl, only certain amounts of the starting aryl
halide were detected in the reaction mixtures. This means that no
side reactions occur and the increase in the reaction time should
increase the conversion of the aryl halide. The data given in the
entries 4 and 5 (Table 3) and 2–4 (Table 2) confirm this conclusion.
Among possible concurrent reactions homo-coupling is of spe-
cial importance since it often occurs in the experimental conditions
discussed above. Investigation of substituted aryl halides allowed
us not only to determine the activity of the catalyst but also to esti-
mate the contribution of homo-coupling in the products formation.
GC–MS analysis of the reaction mixtures obtained (using naphtha-
lene as internal standard) revealed the presence of only traces of
homo-coupling product (Ph2) in the resulting solutions (Table 4);
no Ar2 was detected in any reaction mixture. This means that the
Fig. 1. TEM images of Pd/PPy composites obtained with the use of Pd(NH3)4Cl2 as oxidant and the corresponding PSD histograms: (a, d, g) – Pd/PPy-10; (b, e, h) – Pd/PPy-100;
(c, f, i) – Pd/PPy-300.

T.V. Magdesieva et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
55
Table 2
Screening of the reaction parameters for Suzuki–Miyaura coupling (Pd/PPy-300, 0.1 M PhBr, 0.025 M NaBPh4 or 0.1 M ArB(OH)2, 0.5 M Na2CO3).
No.
Solvent, T (◦C)
Reagent
Amount of catalyst (mol.%)
Time (h)
The yield of ArPh (%)
Starting PhBr (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
CH3CN, 82 ◦
CH3CN, 82 ◦
CH3CN, 82 ◦
CH3CN, 82 ◦
CH3CN, 82 ◦
CH3CN, 82 ◦
CH3CN, 82 ◦
CH3CN, 82 ◦
C
C
C
C
C
C
C
C
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
MesB(OH)2
MesB(OH)2
NaBPh4
0.5
1
1
1
2
4
1
1
1
4
1
1
1
4
2
4
8
4
4
4
8
4
4
4
4
6
62
56
64
84
65
66
32
64
82
97
69a
68
90
41
47
38
14
36
33
66
38
19
3
NMP, 100 ◦
NMP, 100 ◦
NMP, 100 ◦
C
C
C
NaBPh4
MesB(OH)2
NaBPh4
32
31
9
NMP, 100 ◦Cb
NMP, 100 ◦Cb
NaBPh4
a
1.5% of Ph2 was detected.
b
NO2C6H4Cl was used as a starting aryl halide and Pd/PPy-100 as a catalyst; no homo-coupling products were detected in the reaction mixture.
catalytic process is highly selective in favor of the cross-coupling
route.
The investigation of two dihalobenzenes revealed that in case of
p-IC6H4Cl it is possible to substitute one of halogen atoms (iodine)
for phenyl group selectively (only 4% of PhC6H4Ph is formed when
p-IC6H4Hal: NaBPh4 molar ratio is 4:1). When the halogen activi-
ties differ not so dramatically (in case of p-IC6H4Br) the dominant
product in the applied reaction conditions was PhC6H4Ph, in spite
of deficiency of NaBPh4 which was taken in ¼ molar ratio respec-
tively to aryl dihalide, almost one half of which stayed unreactive.
The yield of p-PhC6H4Br was only 11%.
The next step of our investigation was a comparison of catalytic
activity of two samples (Pd/PPy-10 and Pd/PPy-100) having almost
identical diameters of PPy spheres (27–28 nm), with close sizes of
Pd nanoparticles (the size distribution graphs are given in Fig. 1d
and e) but with different Pd contents (42 and 35 wt.%). Two different
substrates were examined in the test reaction with NaBPh4: phenyl
bromide and p-tolyl bromide. It turned out that the yields of biaryls
are almost equal (compare entries 2, 3 and 9, 10 in Table 3) if the
total amount of the Pd catalyst used in the reaction is the same
(1 mol.% in both cases). In other words, the higher is the Pd content
inside polypyrrole spheres, the less amount of catalyst is necessary
to obtain the same yield of the biaryl. This observation supports the
expectation that small PPy spheres are completely penetrable for
the reagents and all Pd nanoparticles are active.
Fig. 2. Influence of the size of Pd/PPy spheres (Pd/PPy-100 and Pd/PPy-300) on the
catalytic activity in Suzuki coupling (NMP, 100 ◦C, 4 h, 0.1 M ArHal, 0.025 M NaBPh4
or 0.1 M ArB(OH)2, 0.5 M Na2CO3, 1 mol.% of Pd).
When dealing with Pd-containing catalyst, an estimation of
Pd leaching is very important, both from a mechanistic point of
view and to prevent losses of precious metal. We performed a
special investigation to clarify this aspect. For this aim a set of
leaching tests was performed as described in [63]. The reaction
NO2C6H4Br + NaBPh4 was selected as a model for leaching tests.
The reaction was performed in NMP as described in Experimen-
tal but it was interrupted after 30 min of heating; the catalyst
was centrifuged (15,000 rpm, g-force = 20,238 rcf), the solution
was decanted from the solid precipitate and divided in two por-
tions. One was analyzed using the standard catalytic procedure,
Table 3
Various aryl halides and boron derivatives in Suzuki–Miyaura coupling (0.1 M ArHal,
0.025 M NaBPh4 or 0.1 M ArB(OH)2, 0.5 M Na2CO3, NMP, Pd/PPy-100, 1 mol.% Pd).
ArHal
Reagent
t (h) Arꢀ–Ar yield (%)
ArHal (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
PhI
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
4
4
4
4
6
4
6
4
4
4
4
4
6
4
4
4
6
>99
98a
95
63
84
>99
90
71
84
83
39
96
81
98
77
–
1
2
39
16
–
PhBr
PhBrb
PhCl
PhCl
4-NO2C6H4Br
4-NO2C6H4Cl
4-CH3OC6H4Br
4-CH3C6H4Br
4-CH3C6H4Brb
2-CH3C6H4Br
2-Bromopyridine NaBPh4
2-Chloropyridine
PhBr
9
31
15
15
63
6
22
2
22
Table 4
The estimation of relative contribution of homo-coupling products (0.1 M ArHal,
0.025 M NaBPh4, 0.5 M Na2CO3, NMP, Pd/PPy-100, 1 mol.% Pd).
NaBPh4
ArHal
ArPh yield (%)
ArHal (%)
31
15
63
–
9
41
7
Ph2 (%)
PhB(OH)2
MesB(OH)2
NaBPh4
4-CH3OC6H4Br
4-CH3C6H4Br
2-CH3C6H4Br
4-NO2C6H4Br
4-NO2C6H4Cl
4-IC6H4Br
71
84
39
>99
2
1
2
–
–
PhBr
4-IC6H4Br
4-IC6H4Cl
11 (44% PhC6H4Ph) 41
91 (4% PhC6H4Ph)
NaBPh4
7
a
The isolated yield was 90%.
Pd/PPy-10 was used as a catalyst.
90
b
11 (44 PhC6H4Br)
91 (4-PhC6H4Cl)
–
<0.5
4-IC6H4Cl

56
T.V. Magdesieva et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
Table 5
Table 6
The recycling of the Pd/PPy-100 catalyst (NMP, 100 ◦C, 0.1 M PhBr, 0.025 M NaBPh4,
0.5 M mol Na2CO3, 0.3 mol.% Pd).
The yields of ArPh in water (Pd/PPy-100, 4 h, 100 ◦C, 0.1 M ArHal, 0.025 M NaBPh4,
0.5 M Na2CO3, 0.3 mol.% Pd).
Immobilization conditions
The yield of Ph2, %
I run
ArHal
Time (h)
ArPh (%)
>99
92
>99
72
96
79
74
ArHal (%)
PhI
PhI
PhBr
PhCl
4-NO2C6H4Br
4-CH3C6H4Br
4-CH3OC6H4Br
4
4a
4
6
5
6
7
–
7
–
26
2
19
22
II run
III run
NMP, usa
>99 (4 h)
>99 (4 h)
74 (4 h)
72 (4 h)
71 (6 h)
70 (6 h)
CH3CN, usa
a
Ultrasonication.
additional portion of the base (0.5 mmol Na2CO3) was added to the
other and it was treated further for 3 h under the standard reac-
tion conditions as described in the Experimental and then analyzed.
GC–MS data obtained for the first (treated) and the second (non-
treated) portions of solution were almost identical: NO2C6H4Ph
(53%), NO2C6H4Br (46%) for the portion corresponding to 30 min
conversion and NO2C6H4Ph (52%), NO2C6H4Br (46%) after addi-
tional treatment of the filtrate.
This observation is radically different from literature results
[63,64] where a significant amount of Pd was found in the solu-
tion fraction after ultrafiltration or centrifugation experiments. In
[63] it has been demonstrated that the catalytic activity was mainly
due to leached Pd species. In [64] the solution turned out to be
catalytically much more active than the deposited fraction, with
respect to the subsequent reaction run. On the contrary, the above
experimental data for our system do not show any noticeable cat-
alytic activity of the solution fraction. Thus, one can conclude that
the amount of solute Pd in the centrifuged solution for our sys-
tem is probably too small to observe its catalytic activity, i.e. the
presence of Pd/PPy composite is necessary for catalysis of these
reactions. This specific feature of our system might be related to
a stronger structure-forming properties of chemically synthesized
PPy globules compared e.g. to polymeric micelles.
a
Pd/PPy-300 was used as a catalyst.
catalytic reactions, especially involving precious metals as media-
tors, in aqueous medium.
Suspensions of Pd/PPy nanocomposites in water (which passed
through the sedimentation stage, contrary to the relatively stable
initial colloid solution) exhibit fast precipitation. However, Pd/PPy
nanocomposites immobilized on the graphite support turned out to
work very efficiently in water, thus making the process ecologically
friendly. The reactions were performed at 100 ◦C in argon atmo-
sphere, the reaction mixture was intensively stirred. The results
obtained are given in Table 6. Comparison of Pd/PPy activity in
water and organic solvents reveals (Tables 3 and 6) that the yields
of biaryls are similar. In some cases a small decrease in the yields
in water is observed but in the context of green chemistry it seems
not very significant.
4. Conclusion
Palladium/polypyrrole nanocomposites synthesized via redox
reaction of a Pd salt and pyrrole, have been tested as catalysts
in Suzuki–Miyaura coupling and turned out to be very efficient.
Aryl iodides, bromides and chlorides are active. The reaction can
be performed both in organic solvents and in water, thus making
the process ecologically friendly. The recycling of the catalyst is
possible if its particles are immobilized at graphite tissue.
Our operationally simple synthesis procedure results in the
Pd/PPy nanocomposite in the form of spherical polypyrrole glob-
ules with relatively uniform sizes. Another component, Pd, is
distributed inside each globule as nanoparticles, also with close
sizes. Both these geometrical parameters may be varied by means
of synthesis conditions, the average polymer size being from 27 to
62 nm while the size of metal nanoparticles may be from 1.25 to
1.45 nm.
The comparison of two different samples revealed that Pd/PPy
nanocomposite with the diameter of PPy spheres of about 30 nm is
more efficient in catalysis, as compared to the sample with big-
ger PPy spheres (about 60 nm), due to transport limitations for
reagents inside the polypyrrole sphere in the latter case. For palla-
dium/polypyrrole nanocomposites with the small diameter of PPy
sphere, which are easily penetrable for the reagents and in which
all Pd nanoparticles are active, Pd content in polypyrrole spheres
does not influence the yield of biaryls. As a consequence, the more
is the Pd content in polypyrrole spheres, the less amount of catalyst
is necessary to obtain the same yield of biaryl.
3.3. The recycling of Pd/PPy catalyst
The recycling of the catalyst is a very important aspect for its
application prospects. Unfortunately, reuse of Pd/PPy catalyst in
solution turned out to be less efficient (the yield of biaryl decreases
in the second run from 80% to 17% for coupling of PhBr with
NaBPh4). The reason is not clear yet and further investigations are
in progress. On the other hand, experiments with immobilized cat-
alyst turned out to be rather promising. We immobilized Pd/PPy
nanocomposite (Pd/PPy-100) on the graphite support by the depo-
sition of Pd/PPy suspension in NMP or acetonitrile. Thus modified
graphite support was used in a test Suzuki–Miyaura coupling of
PhBr with NaBPh4 (NMP, 100 ◦C, 4 h). As follows from Table 4, at
least three consequent reactions with new portions of reagents
are possible, with only small decrease in the catalytic activity.
After each run the graphite support with the immobilized cata-
lyst was washed with NMP and used for the next catalytic reaction
without any additional treatment. Comparison of the yields of the
coupling product in three subsequent runs for the catalysts immo-
bilized using freshly prepared suspensions in NMP or in acetonitrile
(Table 5) shows that the nature of the solvent used for the immobi-
lization of the catalyst is not important, contrary to the solvent in
which the reaction is performed (in both cases it was NMP). If the
reaction with the immobilized catalyst is performed in acetonitrile,
the yields of the coupling product are lower.
3.4. Suzuki–Miyaura coupling in water
Acknowledgements
Organic synthesis is routinely performed in non-aqueous sol-
vents. In contrast, the nature, in its unique way, uses water
for enzymatic transformations of all organic substrates. Current
emphasis of green chemistry underscores the need for developing
Support provided by the “Region Bourgogne”, CNRS, University
of Burgundy and the Russian Foundation for Basic Research (project
09-03-01172a) is gratefully acknowledged.

T.V. Magdesieva et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 50–57
57
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