Reusable functionalized polysiloxane-supported palladium catalyst for Suzuki–Miyaura cross-coupling

Article abstract of DOI:10.1016/j.jcat.2011.06.023

Palladium catalyst, obtained in the reaction of PdCl₂(cod) with poly[(3-N-midazolopropyl)methylsiloxane-co-bisdimethylsiloxane], was used in the Suzuki–Miyaura cross-coupling of aryl bromides with phenylboronic acid. Catalytic reactions, performed at 40–80 °C in water or 2-propanol/water mixture, led to high yields of 2-methylbiphenyl with TOF up to 25,000 h^?1. In recycling experiments, excellent results were obtained in eight subsequent runs. With application of microwave heating, more than 95% of product was formed under mild conditions and short time. XRD, TEM, and XPS methods evidenced the presence of Pd(0) nanoparticles bonded to the polysiloxane support. Their important role in the catalytic process was indicated by the results of mercury poisoning test.

Full text of DOI:10.1016/j.jcat.2011.06.023

Journal of Catalysis 282 (2011) 270–277
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Reusable functionalized polysiloxane-supported palladium catalyst
for Suzuki–Miyaura cross-coupling
b
c
T. Borkowski a, W. Zawartka a, P. Pospiech b, U. Mizerska b, A.M. Trzeciak a, , M. Cypryk , W. Tylus
⇑
a Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
b Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Engineering of Polymer Materials, 112 Sienkiewicza, 90-363 Lodz, Poland
c
_
´
Wrocław University of Technology, Institute of Inorganic Technology, 27 Wybrzeze Wyspianskiego, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium catalyst, obtained in the reaction of PdCl2(cod) with poly[(3-N-imidazolopropyl)methylsilox-
ane-co-dimethylsiloxane], was used in the Suzuki–Miyaura cross-coupling of aryl bromides with phen-
ylboronic acid. Catalytic reactions, performed at 40–80 °C in water or 2-propanol/water mixture, led to
high yields of 2-methylbiphenyl with TOF up to 25,000 hÀ1. In recycling experiments, excellent results
were obtained in eight subsequent runs. With application of microwave heating, more than 95% of prod-
uct was formed under mild conditions and short time. XRD, TEM, and XPS methods evidenced the pres-
ence of Pd(0) nanoparticles bonded to the polysiloxane support. Their important role in the catalytic
process was indicated by the results of mercury poisoning test.
Received 14 March 2011
Revised 22 June 2011
Accepted 26 June 2011
Available online 29 July 2011
Keywords:
Palladium
Suzuki–Miyaura reaction
Polysiloxane support
Immobilized catalyst
Palladium nanoparticles
Microwave heating
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
Suzuki–Miyaura reaction [14]. Pd(0) stabilized on polyvinylpyrrol-
idone [15], polyvinylpyridine [16] or other polymers have been
Heterogenized catalysts supported on polymers or porous oxi-
des have been attracting more and more attention because of the
increasing environmental requirements [1–11].
widely applied [17]. However, very few reports have presented
the application of imidazole-functionalized polymers [18] or silicas
[19] as catalyst supports.
In this context, the expected advantage of immobilization con-
sists in efficient separation of the catalyst and its recycling in the
next run. This diminishes the cost of the catalytic process and, in
addition, makes it possible to minimize the metal content in the fi-
nal organic product. This aspect is very important when pharma-
ceutical applications of products are considered.
One of the strategies used for the preparation of active catalysts
containing metal ions covalently attached to the support is the
functionalization of the surface with organic ligands such as phos-
phines, pyridines, amines, or mercapto groups [1–3,12].
It can be expected that the covalent attachment of the metal to
the support offers some advantages, mainly higher stability under
catalytic conditions, with regard to metals supported on porous
materials. Moreover, functionalization of the support with coordi-
nating groups provides better control over the distribution of the
metal on the surface, which is important for catalytic activity
and selectivity.
Recently, it has been shown that Pd(II) complexes with coordi-
nated imidazole ligands exhibit high catalytic activity in the
Suzuki–Miyaura reaction with 100% of the cross-coupling product
formed at 60 °C in 1 h [20]. At the same time, polysiloxane-
supported palladium has recently been successfully used in the
Heck reaction [21,22]. Therefore, we decided to examine the cata-
lytic activity of palladium supported on imidazole-functionalized
polysiloxane in the Suzuki–Miyaura reaction with the expectation
to obtain a highly active and recyclable catalyst. It is also worth
noting that polysiloxanes are chemically resistant, non-toxic mate-
rials and imidazole functionality plays an important role in many
biological systems. Thus, imidazole-substituted polysiloxanes
could be considered as potentially biodegradable materials
although their high antibacterial potency should be also men-
tioned in this context [23].
2. Results and discussion
Palladium complexes anchored on differently functiona-
lized polymers have been used in catalysis [13], including
2.1. Structural studies
Coordination of palladium to poly[(3-N-imidazolopropyl)meth-
⇑
Corresponding author.
E-mail addresses: ania@wchuwr.chem.uni.wroc.pl, anna.trzeciak@chem.uni.
ylsiloxane-co-dimethylsiloxane] (P1) resulted in
a change of
wroc.pl (A.M. Trzeciak).
composition and decrease in N, C, and H amounts in comparison
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.06.023

T. Borkowski et al. / Journal of Catalysis 282 (2011) 270–277
271
Table 1
XPS data Pd 3d.
with P1. The Pd/N ratio, calculated from the N and Pd contents,
indicated that there were ca. 3.5 imidazole groups per one palla-
dium ion. The most probable structure of C1, containing palladium
bonded to two imidazole groups and two chloride ions, is shown in
Fig. 1.
Amount
(%)
BE Pd
3d3/2 (eV)
BE Pd
3d5/2 (eV)
C1
N2–Pd–Cl2
PdO
82.8
17.2
343.6
341.5
338.3
336.2
In is worth noting that palladium acted as a cross-linking agent,
which was observed during the synthesis of C1, when solid C1 pre-
cipitated after the addition of the solution of PdCl2(cod) to the li-
quid polymer. Because of the very low solubility of C1 in
common solvents, CP MAS 13C and 29Si NMR spectra were mea-
sured in the solid state. The obtained parameters of 13C were sim-
ilar to those determined in a CDCl3 solution of polymer P1;
however, only one broad signal was found in the 29Si CP MAS spec-
trum of C1 instead of two signals found for P1 in solution.
The XPS spectrum of catalyst C1 exhibited two dominating
peaks (ca. 83%) of Pd 3d at BE 338.3 eV (Pd 3d5/2) and 342.6 eV
(Pd 3d3/2), typical for Pd(II), and assigned to palladium bonded to
two imidazole nitrogen atoms and two chlorides. Such interpreta-
[(mim)3PdCl]Cl
N3–Pd–Cl
PdO
89.1
10.9
343.3
341.5
338.0
336.2
C1 after Suzuki–Miyaura
reaction
Pd(0)
61.5
340.3
335.0
N2–Pd–Cl2
Pd–Br
22.9
15.6
343.6
342.1
338.3
336.8
tion was confirmed by comparison with the spectrum of the com-
plex [(mim)3PdCl]Cl (mim = 1-methylimidazole), which showed
two peaks at BE 338.0 eV (Pd 3d5/2) and 343.3 eV (Pd 3d3/2). The
small difference in the BE values (0.3 eV) between the two spectra
can be related to the fact that in C1 two nitrogen atoms are bonded
to palladium, whereas in [(mim)3PdCl]Cl three nitrogen atoms are
present in the first coordination sphere (Fig. 2 and Table 1).
The second form of palladium observed in the XPS spectra of C1
and [(mim)3PdCl]Cl in ca. 17% and 11%, respectively, was PdO char-
acterized by BE 336.2 and 341.5 eV. The presence of some amounts
of PdO can be explained by air oxidation of palladium at the
surface.
XPS spectra of catalyst C1 and the complex [(mim)3PdCl]Cl in
the N 1s region show two lines indicating the presence of differ-
ently surrounded nitrogen atoms, namely imidazole nitrogen
bonded to palladium (Pd–N, BE 399.4 eV) and free imidazole nitro-
gen (N, BE 400.8 eV). In the XPS spectrum of [(mim)3PdCl]Cl, the
ratio of the surface areas of these peaks, Pd–N/N, was 5:5, as ex-
pected if all imidazole ligands were bonded to palladium. In case
of catalyst C1, the Pd–N/N ratio was 4:6, in agreement with earlier
supposition that not all imidazole groups are involved in the for-
mation of bondings with palladium.
Fig. 1. The proposed structure of catalyst C1.
3. Studies of the Suzuki–Miyaura reaction
3.1. Catalytic activity of catalyst C1
The Suzuki–Miyaura reaction of 2-bromotoluene with phenyl-
boronic acid was selected as a model catalytic reaction for catalyst
C1 (Scheme 1).
The first tests of the activity of catalyst C1 were performed at
40 °C over 1 h using 2-propanol as solvent. Under these conditions,
43% of 2-methylbiphenyl was formed. Next, the catalyst was sepa-
rated, washed, and reused in four subsequent runs. The yield of the
product ranged from 56% to 76% in the fifth reaction, which indi-
cates the high stability of the catalyst under reaction conditions.
In the next experiment, the reaction time was prolonged to 3 h,
which resulted in the yield increasing to 82%. When microwave
heating was applied instead of conventional heating, even better
results were obtained and yields of 90–100% were found after 1 h
in three subsequent runs with the same sample of C1 (Table 2).
In further investigations, a screening of solvents was performed
in the temperature range 60–110 °C. At 60 °C, the Suzuki–Miyaura
reaction in 2-propanol proceeded, as expected, with a higher yield
than at 40 °C, and in the second run, the highest yield, 100%, was
achieved. In subsequent reactions performed with the recycled
C1 catalyst, the yield of 2-methylbiphenyl decreased to 43% in
the fifth reuse (Table 2). Very good results were obtained at
80 °C; however, like before, the yield was lower in the fifth run
(63%). Following the rules of green chemistry, we attempted to
use water as an inexpensive, non-toxic, and non-flammable sol-
Fig. 2. XPS spectra Pd 3d of: (a) C1, (b) [(mim)3PdCl]Cl, (c) C1 isolated after Suzuki–
Miyaura reaction.

272
T. Borkowski et al. / Journal of Catalysis 282 (2011) 270–277
Scheme 1. Suzuki–Miyaura reaction under studies.
Table 2
performed at 60 °C in 2-propanol/water (1:1) mixture. After 1 h,
Results of Suzuki–Miyaura reaction with catalyst C1.a,b
catalyst was separated by filtration and used for the second run.
2-Methylbiphenyl was extracted from a residue with hexane,
which was next removed in vacuo. The amount of palladium found
in product, using ICP method, was as low as 0.6 ppm in the first and
0.2 ppm in the second run.
Further experiments confirmed a high activity and stability of
catalyst C1 also under microwave conditions. The results obtained
in a series of experiments performed in a 2-propanol/water (1:1)
mixture at 60 °C over 1 h were comparable with those obtained
with conventional heating over 3 h (Fig. 3). In both cases, it was
possible to perform eight successive runs with high yield of the
product. The decrease in product yield observed after the fifth
run can be related to some decomposition of the catalyst or the loss
of material during separation procedure, but alternatively palla-
dium leaching should also be considered.
Solvent
Reaction conditions
Run
1
2
3
4
5
2-Propanol
2-Propanol
40 °C, 1 h
40 °C, 3 h
40 °C, 1 h, MW
60 °C, 3 h
80 °C, 3 h
110 °C, 3 h
60 °C, 3 h
40 °C, 1 h, MW
60 °C, 3 h
43
68
90
56
89
76
82
80
56
82
90
100
92
88
58
86
93
92
72
63
76
100
79
98
60
32
92
93
99
35
83
73
16
82
97
98
43
63
Water
2-Propanol/water 4/1
2-Propanol/water 1/1
2-Propanol/water 1/1
2-Propanol/water 1/1
68
96
98
100
96
60 °C, 1 h, MW
a
The yield of 2-methylbiphenyl obtained in subsequent runs;
The amount of biphenyl (homocoupling product) decreased from ca. 4% in the
b
first run to maximum 1% in the subsequent runs.
Only slightly lower yields were obtained when catalyst C1 was
used at 40 °C with microwaves in 2-propanol/water (1:1) mixture.
In this case, the yield of 2-methylbiphenyl obtained after 1 h was
higher than in 2-propanol only, and five efficient recycling runs
were successfully carried out (Table 2). The maximum yield, 92%,
was obtained in the third run. An increase in the reaction yield
in subsequent runs can be explained by the transformation of C1
to the more active form, namely Pd(0) nanoparticles supported
on the polymer.
vent in the Suzuki–Miyaura reaction [24]. The results were quite
good: At 60 °C, the cross-coupling product was formed with 40%
yield, whereas at 110 °C the yield increased to 88% in the second
run (Table 2).
The application of a 4:1 2-propanol/water mixture produced
poorer results than with neat 2-propanol, whereas a 1:1 mixture
of 2-propanol/water turned out to be the best solvent in the stud-
ied system. In addition, the reaction was successfully performed at
a relatively low temperature, 60 °C. It was possible to obtain a high
yield of 2-methylbiphenyl in eight successive Suzuki–Miyaura
reactions using the same sample of catalyst C1 (Fig. 3). High yields,
over 90%, were seen in seven runs, while in the eighth one the yield
decreased to 70.5%.
3.2. The effect of C1 amount on catalytic activity
To follow the influence of the amount of palladium on the yield
of the Suzuki–Miyaura reaction, a series of experiments were per-
formed in 2-propanol over 15 min in 80 °C. The [Pd]/[2-bromotol-
uene] ratio was changed by changing the amount of C1 or by
applying different amounts of organic substrates. In order to
compare the obtained results, the TOF (hÀ1) values were calculated
An important criterion to assess the efficiency of the catalytic
process is the amount of palladium in the organic product. To
estimate palladium content, the Suzuki–Miyaura reaction was
Fig. 3. Results of Suzuki–Miyaura reaction with catalyst C1 in recycling experiments.

T. Borkowski et al. / Journal of Catalysis 282 (2011) 270–277
273
(Table 3). It is worth noting that the 15-min reaction time was, in
most cases, suitable to obtain over 90% yield of 2-methylbiphenyl.
It was found that the optimal amount of catalyst C1 was
6 Â 10À7 mol, enabling the formation of 98.6% of the product, the
corresponding TOF value being 6573 hÀ1. Application of smaller
or larger amounts of catalyst C1 led to poorer results. According
to the literature, such non-linear dependence of the reaction yield
on the catalyst amount was frequently observed, also in reactions
catalyzed by palladium nanoparticles [25,26].
finished, some black precipitate was seen in the solution. It was,
however, impossible to judge whether there was palladium black
formed from soluble palladium species or palladium bonded to
polymer P1 that was not separated from the solution during the
first filtration.
In the second experiment, a StratoSpheres SPE resin (Varian)
designed for removing of metal species from the solution was used
to separate palladium from the reaction mixture after 5 min. The
reaction then continued with the remaining solution, and the final
yield of 2-methylbiphenyl was 50% after 60 min, close to that
noted after 5 min (56%). The small decrease in the product yield
was probably caused by fact that some amount of 2-methylbiphe-
nyl remained on the resin after filtration. Thus, in this case, the
reaction stopped because probably all palladium was removed,
both supported on the polymer and soluble.
Using the same optimal amount of catalyst C1 (6 Â 10À7 mol),
the reactions were performed with higher excesses of the sub-
strates to demonstrate that TOF as high as 25,173 can be reached.
It is important to point out that in a blank experiment performed
under the same conditions but without C1, 2-methylbiphenyl
was not formed.
It is also worth to note that palladium content found in the C1
catalyst recovered after Suzuki–Miyaura reaction was always
higher than in the as-prepared sample, and consequently, palla-
dium analysis could not be used for estimation of the palladium
leaching.
3.3. The Suzuki–Miyaura reaction with other substrates
The scope of the Suzuki–Miyaura reaction catalyzed by catalyst
C1 was evaluated by reacting phenylboronic acid with different
aryl bromides as substrates. High conversion to the cross-coupling
product was found for 4-bromotoluene and 2-bromoanisole. Lower
yield was obtained when 2-bromobenzaldehyde or 2-bromoaceto-
phenone was used. Application of microwave conditions made it
possible to react the less reactive 4-chlorotoluene and 1-chloro-
4-nitrobenzene (Table 4).
To obtain more information about the nature of the catalytically
active species, the mercury test was performed [6,27].
3.5. Mercury test
Two experiments were performed using 500-fold excess of
Hg(0) relative to palladium in the Suzuki–Miyaura cross-coupling
in a 2-propanol/water mixture. In the first case, Hg(0) was intro-
duced at the beginning, together with other reactants, whereas in
the second experiment, Hg(0) was added after 5 min of reaction.
Analysis of the products showed 100% and 98% yield of the product
in both cases. Thus, the reaction was not inhibited by Hg(0), indi-
cating small, if any, influence of free Pd(0) nanoparticles on the
reaction course. Consequently, immobilized palladium nanoparti-
cles can be considered as the main catalytically active form.
3.4. Filtration test
In order to check whether immobilized or leached palladium
was responsible for the observed catalytic activity, the hot filtra-
tion test was performed [6]. The Suzuki–Miyaura reaction in 2-
propanol/water (1:1) at 60 °C was stopped after 5 min, when con-
version of 2-bromotoluene reached 56%. Next, the solution was
separated from the catalyst by filtration, and the reaction contin-
ued for 60 min. After that time, the yield of the product increased
to 100%, indicating that the solution had contained some catalyti-
cally active palladium. In fact, when the catalytic reaction was
3.6. Catalytic activity of catalyst C2
In reactions catalyzed by C1, performed in 2-propanol at 40 °C
and at 60 °C, an increase in reaction yield was noted in the second
run, compared with the first reaction, which produced 56–68% of
the cross-coupling product. In the second reaction, a higher yield,
82% or 100%, was obtained. These observations suggest that the
catalytically active species, presumably a Pd(0) catalyst, was
slowly formed under the conditions applied. In fact, when pre-
reduced catalyst C2, containing supported Pd(0) nanoparticles,
was used, a high yield, 90%, was seen already in the first reaction
performed at 60 °C in 2-propanol. This result, indicating, as ex-
pected, the higher catalytic activity of Pd(0) compared with Pd(II),
prompted us to perform structural studies of C1 isolated after the
catalytic reaction.
Table 3
Results of Suzuki–Miyaura reaction with different amounts of catalyst C1.
Amount of Pd (mol)
[Pd]/[2-bromotoluene]
Yielda (%)
TOF (hÀ1
)
1 Â 10À5
5 Â 10À6
6 Â 10À7
6 Â 10À7
6 Â 10À7
3 Â 10À7
0.01
0.005
0.0006
0.0003
0.00015
0.0003
83.5
93.1
98.6
95.7
94.4
83.2
334
745
6573
12,760
25,173
11,093
a
GC determined as conversion of 2-bromotoluene.
3.7. Structural studies of catalyst C1 recovered after the Suzuki–
Miyaura reaction
Table 4
Results of Suzuki–Miyaura reaction with
catalyst C1 and different aryl halides.
Analysis of the composition of catalyst C1a, namely C1 sepa-
rated from the reaction mixture after one run of Suzuki–Miyaura,
showed some changes in the amounts of N, C, and H and a remark-
able increase in palladium content from 7.6% to 12.0%. When C1
was reduced with hydrazine to form C2, an increase in palladium
content even up to 31.5% was observed. This can be explained by
the reformation of the liquid polymer P1 during formation of
Pd(0) nanoparticles.
Substrate
Yielda (%)
2-Bromotoluene
4-Bromotoluene
2-Bromoanisole
2-Bromobenzaldehyde
2-Bromoacetophenone
4-Chlorotoluene
100
100
97
45
13
10b
16b
1-Chloro-4-nitrobenzene
a
The XRD diffractograms of C1 before and after reduction are
shown in Fig. 4. In the spectrum of C1, containing Pd(II), there is
a broad line at ca. 2h = 20°, originating from polymer P1 (Fig. 4a).
GC determined as conversion of aryl
halide.
b
1 h, 60 °C, with microwaves.

274
T. Borkowski et al. / Journal of Catalysis 282 (2011) 270–277
The sample of C1 separated after the Suzuki–Miyaura reaction per-
formed in 2-propanol (Fig. 4b) and in 2-propanol/water (Fig. 4c)
showed a new signal at 2h = 40.1°, characteristic for crystallites
of Pd(0). The average diameter of Pd(0) nanoparticles was 4.7 nm
and 6.2 nm, respectively. Slightly bigger Pd(0) nanoparticles,
6.7 nm, were found in the sample of C2a, obtained by reduction
in C1 with hydrazine in methanol (Fig. 4d).
The application of the TEM method allowed us to collect more
structural data for the catalysts under study. The samples of C1
separated after the Suzuki–Miyaura reaction in 2-propanol and in
2-propanol/water (1:1) contained uniformly distributed Pd(0)
nanoparticles of ca. 4 nm in diameter. Fig. 5 shows typical TEM pic-
tures and size analyses of both materials. A similar distribution and
size of Pd(0) nanoparticles were also found for catalyst C2 (Fig. 5c)
with the maximum at 4.1 nm. TEM analysis of C2a, obtained by the
reduction in C1 in methanol using a higher concentration of hydra-
zine, revealed aggregates ranging from tens to hundreds of nano-
meters, formed by individual Pd(0) nanoparticles.
The XPS Pd 3d spectrum of catalyst C1a, separated after the
Suzuki–Miyaura reaction, differs from that measured for the
freshly prepared C1 and contains three forms of palladium species.
The dominating lines, observed at BE = 335.0 eV (Pd 3d5/2) and
340.3 eV (Pd 3d3/2), are typical for Pd(0). The relatively low BE val-
ues can indicate a high dispersion of palladium nanoparticles on
polymer P1. The content of Pd(0) was estimated as 61%. The pres-
ence of lines at 338.3 and 343.6 eV confirmed that reduction in pal-
ladium was not complete, and 23% of the starting C1 remained
unaffected. The third form of palladium, with BE = 336.8 eV and
342.1 eV, was identified as Pd–Br containing form (16%) [28]. The
formation of such species in the Suzuki–Miyaura reaction with 2-
bromotoluene as a substrate is expected (Fig. 2 and Table 1).
The SEM method was used to obtain more information about
structural changes in the catalyst during its reduction. The sample
for analysis was prepared by treating C1 with KOH in 2-propanol un-
der reflux. SEM micrographs showed that the as-prepared C1 con-
tained spongy and more regular fragments, whereas after
reduction only spongy-type structures remained. Table 5 sets out
the collected EDS data determined for both samples as average val-
ues from five measurements. In line with the conclusions from ICP
analysis, the amount of palladium increased during reduction. At
the same time, the contents of C, O, and Si, the main components
of the polymer, decreased. This can be explained by some solubiliza-
tion of polymer P1 during palladium reduction and the formation of
Pd(0) nanoparticles attached to the polymer. The Cl/Pd atomic ratio
determined by EDS spectroscopy decreased remarkably, from 2.8 in
the as-prepared C1 to 0.7 as a result of C1 reduction with KOH in 2-
propanol. As expected, during this process, Pd(0) nanoparticles were
formed with simultaneous removal of chloride ions.
4. Experimental
4.1. General
TEM measurements were performed using a FEI Tecnai G2 20
X-TWIN electron microscope with LaB6 catode providing 0.25 nm
resolution. To the small sample of catalyst, 2 cm3 of methanol
was added and the resulted mixture was ultrasonically treated
Fig. 4. XRD pictures of: (a) C1 as prepared, (b) C1 after Suzuki–Miyaura reaction in 2-propanol, (c) C1 after Suzuki–Miyaura reaction in 2-propanol/water 1/1, (d) C2a (C1
reduced with hydrazine in methanol).

T. Borkowski et al. / Journal of Catalysis 282 (2011) 270–277
275
Fig. 5. TEM pictures and Pd(0) nanoparticles size distribution of: (a) C1 after Suzuki–Miyaura reaction in 2-propanol, (b) C1 after Suzuki–Miyaura reaction in 2-propanol/
water 1/1, (c) C2 (C1 reduced with hydrazine in 2-propanol), (d) C2a (C1 reduced with hydrazine in methanol).
for 5 min. Specimens for TEM studies were prepared by putting a
droplet of a colloidal suspension on a copper microscope grid
followed by evaporating the solvent under IR lamp for 15 min.
The mean particle diameter and size distributions were calcu-
lated by counting at least 400 particles from the enlarged
micrographs.

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T. Borkowski et al. / Journal of Catalysis 282 (2011) 270–277
13C CP/MAS NMR d 1.4 (m, 3C, SiCH3); 14.1 (s, 1C, SiCH2); 24.9
(s, 1C, SiCH2CH2); 50.9 (s, 1C, CH2N); 120.3 (s, 1C, NCH); 128.9 (s,
1C, NCHCH); 139.3 (s, 1C, NCHN) ppm.
Table 5
EDS data (%) for catalyst C1 (as prepared) and C1 after reaction with KOH in 2-
propanol under reflux.
29Si CP/MAS NMR d À19.3 (b.m., 2Si, (CH3)2SiO, CH3SiCH2) ppm.
Element
C1
C1 after reaction with KOH in 2-propanol
C
O
Pd
Cl
Si
46.4
16.9
4.1
11.6
4.0
13.8
8.3
25.0
17.4
0
4.4. Synthesis of catalyst C2
Catalyst C1 (0.1 g) and 2-propanol (2 mL) were added to a
Schlenk tube, which was then introduced into an oil bath preheated
to 60 °C. Next, 1 mL of hydrazine solution in 2-propanol (containing
0.2 mL of 80% hydrazine hydrate in 10 mL of 2-propanol) was
added. The resulting solution was heated at 60 °C for 30 min, the
color of the solution changing from yellow to black. Next, the solu-
tion was cooled down and evaporated to dryness in vacuo.
Found: C 17.5; N 14.7; H 4.8; Pd 31.5.
X-ray powder diffraction (XRD) measurements were performed
using BRUKER D8 Advance diffractometer.
XPS spectra were obtained with a SPECS XPS System equipped
with a hemispherical PHOIBOS analyzer and Mg-K
a radiation
source operated at 250 W under a residual pressure better then
5 Â 10À10 mbar. The analyses were carried out under fixed ana-
lyzer transmission (FAT) with a constant pass energy of 10 eV in
a high-resolution scans. For charge compensation, a flood gun
was used. Additionally, the raw data were corrected for substrate
charging with the binding energy of the Si 2p (101.9 eV) on silicate
support as a reference. The samples were analyzed as pressed pow-
ders mounted on a double-sided adhesive tape. The measured
spectra were fit by a least-squares procedure to a product of Gauss-
ian–Lorentzian functions after subtraction of background noise by
Shirley method. The concentration of each element was calculated
using SpecLab software.
GC-FID and GC/MS spectra of organic products were obtained
using HP 5890 (Hewlett Packard) instrument with mass detector
5971 A. Capillary column HP 5 was used with non-polar liquid
phase containing 95% of dimethyl- and 5% of diphenyl-
polysiloxane.
4.5. Synthesis of catalyst C2a
Catalyst C1 (0.1 g) and methanol (2 mL) were added to a
Schlenk tube, which was then introduced into an oil bath pre-
heated to 60 °C. Next, 0.5 cm3 of hydrazine solution in methanol
(containing 1 mL of 80% hydrazine hydrate in 10 mL of methanol)
was added. The color of the solution changed from yellow to black.
The resulting solution was heated at 60 °C for 30 min, cooled down,
and evaporated to dryness in vacuo.
4.6. The Suzuki–Miyaura reaction procedure
Suzuki–Miyaura reactions were performed in a Schlenk tube.
Weighed amounts of the solid reactants: phenylboronic acid
(1.1 mmol; 0.135 g), KOH (1.95 mmol; 0.112 g), catalyst (1.064
mg Pd), and liquids: 2-bromotoluene (1 mmol; 0.118 mL) and
5 mL of solvent (2-propanol, 2-propanol/water mixture, or water)
were introduced to the Schlenk tube. Next, the Schlenk tube was
sealed with a rubber tap and introduced into an oil bath preheated
to 40 °C, 60 °C, or 80 °C. The reaction mixture was magnetically
stirred at a given temperature for 1 h or 3 h and after this time
cooled down. Next, organic products were separated by extraction
with hexane (4 mL, 3 mL, and 3 mL). In the first extraction, 3 mL of
water was added in order to facilitate the separation of phases. The
combined extracts (10 mL) were GC-FID analyzed with dodecane
(0.076 mL) as an internal standard to determine the conversion
of 2-bromotoluene.
ICP measurements of palladium content were performed using
spectrometer ARL model 3410. Before analysis, weighted samples
of palladium catalyst were mineralized with 2 cm3 of aqua regia,
left for 7 days and diluted next to 10 cm3 with distilled water.
4.2. Synthesis of palladium catalysts
Palladium complexes [(mim)3PdCl]Cl [29] and PdCl2(cod) [30]
were obtained according to literature methods.
Poly[(3-N-imidazolopropyl)methylsiloxane-co-dimethylsiloxane],
P1, was obtained according to literature method [23].
1H NMR (CDCl3) d 0.05 (m, 3H, SiCH3); 0.43 (b.m., 2H, SiCH2);
1.75 (b.m., 2H, SiCH2CH2); 3.86 (t, 2H, CH2N); 6.86 (s, 1H, NCHCH);
7.05 (s, 1H, NCHCH); 7.46 (s, 1H, NCHN) ppm.
13C NMR (CDCl3) d 0.54 (s, 1C, SiCH3); 1.11 (s, 2C, CH3SiCH3);
14.18 (s, 1C, SiCH2); 25.00 (s, 1C, SiCH2CH2); 49.51 (s, 1C, CH2N);
118.64 (s, 1C, NCH); 129.39 (s, 1C, NCHCH); 137.03 (s, 1C, NCHN)
ppm.
4.7. Separation of the catalyst and recycling
When the Suzuki–Miyaura reaction was finished, 1 mL of water
was added to the reaction mixture in order to facilitate solubiliza-
tion of side products. Next, all liquid components were transferred
to another Schlenk tube using a U-shaped glass bridge equipped
with a glass fit. The solid catalyst left in the first Schlenk tube
was washed twice with 2-propanol and used in the next catalytic
run with new portions of the reactants.
29Si NMR (CDCl3) d À20.95 (b.m., 1Si, (CH3)2SiO); À23.05 (b.m.,
1Si, CH3SiCH2) ppm.
Found: C, 41.2; N 8.4; H 7.9.
M = 23,600 g/mol.
4.8. Application of StratoSpheres SPE resin for palladium removal
4.3. Synthesis of catalyst C1
Palladium was removed from the post-reaction mixture using
StratoSpheres™ resin (PL3582-CM89, Varian) with thiol groups.
Siloxane polymer P1 (0.10405 g) was dissolved in 1 mL of CHCl3,
and in another flask, a solution containing 0.02683 g of PdCl2(cod)
in 2 mL CHCl3 was prepared. The solution of the palladium com-
plex was slowly added dropwise to the stirred solution of P1. When
the entire amount of the solution was added, the product started to
precipitate. Stirring continued for 30 min, after which time the
product was filtered off and dried in vacuo.
5. Conclusions
It was demonstrated that palladium supported on siloxane
polymer functionalized with imidazole groups efficiently catalyzes
Suzuki–Miyaura reaction in environmentally friendly solvents
such as water or a 2-propanol/water mixture. Very good results
Found: C 34.9; N 6.9; H 5.5; Pd 7.6.

T. Borkowski et al. / Journal of Catalysis 282 (2011) 270–277
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crease in catalytic activity in the last one. Another advantage of the
catalyst under study is its high activity at low temperatures, 40 or
60 °C. Excellent activity was demonstrated by the achievement of a
high yield of the cross-coupling product and by high TOF values,
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In contrast, application of a strong reducing agent, hydrazine, re-
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On the basis of the experimental data, we propose that Pd(0)
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tionally corroborated by an experiment involving the application
of pre-formed supported Pd(0) nanoparticles, in which high yield
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Acknowledgments
Financial support of the Polish Ministry of Science and Higher
Education (N164/COST/2008) and COST D40 is gratefully
acknowledged.
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Authors are grateful to Ms. Magdalena Słomiany (Faculty of
Chemistry, University of Wrocław) for performing of catalytic tests
and to Mr. Marek Hojniak (Faculty of Chemistry, University of Wro-
cław) for GC and GC/MS analyses.
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