Pd nanoparticles immobilized on PNIPAM-halloysite: Highly active and reusable catalyst for Suzuki-Miyaura coupling reactions in water

Article abstract of DOI:10.1002/aoc.3103

Poly(N-isopropylacrylamide)-halloysite (PNIPAM-HNT) nanocomposites exhibited inverse temperature solubility with a lower critical solution temperature (LCST) in water. Palladium (Pd) nanoparticles were anchored on PNIPAM-HNT nanocomposites with various amounts of HNT from 5 to 30 wt%. These Pd catalysts exhibited excellent reactivities for Suzuki-Miyaura coupling reactions at 50-70 °C in water. In particular, Pd anchored PNIPAM/HNT (95:5 w/w ratio) nanocomposites showed excellent recyclability up to 10 times in 96% average yield by simple filtration. Copyright

Full text of DOI:10.1002/aoc.3103

Full Paper
Received: 30 August 2013
Revised: 1 November 2013
Accepted: 1 November 2013
Published online in Wiley Online Library: 20 January 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3103
Pd nanoparticles immobilized on PNIPAM–
halloysite: highly active and reusable catalyst
for Suzuki–Miyaura coupling reactions in water
Myeng Chan Honga, Hyunseok Ahnb, Myung Chan Choic, Yongwoo Leea,
Jongsik Kimd* and Hakjune Rheea,b*
Poly(N-isopropylacrylamide)–halloysite (PNIPAM-HNT) nanocomposites exhibited inverse temperature solubility with a lower
critical solution temperature (LCST) in water. Palladium (Pd) nanoparticles were anchored on PNIPAM-HNT nanocomposites
with various amounts of HNT from 5 to 30 wt%. These Pd catalysts exhibited excellent reactivities for Suzuki–Miyaura coupling
reactions at 50–70 °C in water. In particular, Pd anchored PNIPAM/HNT (95:5 w/w ratio) nanocomposites showed excellent
recyclability up to 10 times in 96% average yield by simple filtration. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: Suzuki–Miyaura reactions; Pd nanoparticles; PNIPAM-halloysite nanocomposite; reusable catalysts; water
by simple filtration. These interesting properties have made
Introduction
PNIPAM a promising candidate as a support for recyclable Pd
nanoparticle catalysts in SM reactions.[55–61] However, Pd
The Suzuki–Miyaura (SM) reaction is one of the most widely used
synthetic protocols for synthesizing biaryl compounds through a
palladium (Pd)-catalyzed coupling reaction between an aryl
halide and an arylboronic acid.[1–5] The Suzuki reaction is typically
leaching of Pd-anchored PNIPAM during reactions has limited
their wide application in SM reactions.
Halloysite nanotube (HNT) is a two-layered aluminosilicate clay
with a hollow tubular structure. It possesses Al―OH groups on
the internal surface and Si―OH groups on the external
surfaces.[62–75] Due to these surface functional groups, HNT has
been used as a solid support for catalytic particles such as Ru,
TiO2, Ag and Ni.[71–75] In our previous work, we used PNIPAM-
performed with
a homogeneous Pd catalyst, phosphorus
derivatives as ligands and an inorganic bases (i.e. carbonate,
bicarbonate and hydroxide) in aqueous organic solvents.[5,6] How-
ever, a homogeneous Pd-catalyzed SM reaction has several draw-
backs if it is to be utilized in industrial applications, such as (i) the
difficulty in reusing Pd catalysts, (ii) the need for toxic and
expensive ligands and (iii) the difficulty in removing the residual
Pd and ligand after the reaction is completed. Recently, enormous
efforts have been focused on developing recyclable Pd systems
with catalytic activities comparable to those of their homogeneous
counterparts. For example, Pd nanoparticles were immobilized on
ionic liquids,[7–9] functionalized polymers,[10–22] graphene oxide
and its derivatives[23–25] and inorganic substrates such as silica and
zeolites.[26–34] However, there still remains the need to improve their
catalytic activities and reaction conditions such as a low reaction
temperature and the sole use of water as a solvent system.[35–45]
It is well known that poly(N-isopropylacrylamide) (PNIPAM)
microgels exhibit temperature-responsive phase separation (i.e.
coil-to-globule transition) from an aqueous solution at the lower
critical solution temperature (LCST), typically about 32 °C.[46–52]
The LCST of PNIAPM can also be controlled by employing a clay
mineral such as montmorillonite at around 20–46 °C.[53,54] This
indicates that PNIPAM can selectively provide hydrophilic or
hydrophobic nano-environments by simply changing the
temperature of the reaction medium. Therefore, the SM reaction
can likely be carried out with hydrophobic substrates in water
without any surfactants or organic solvents if PNIPAM is utilized
as a solid support of Pd nanoparticles. Furthermore, PNIPAM
can easily be recovered from aqueous solution above its LCST
co-4-vinylpyridine (PNIPAM-co-4-VP) as
a support for Pd
nanoparticles in order to decrease Pd leaching during SM
reactions.[61] As an extension of this study, herein we utilized
temperature-responsive PNIPAM/HNT (halloysite nanotube)
hydrogel nanocomposites as a support for Pd nanoparticle.
PNIPAM/HNT hydrogel nanocomposites were expected to
provide both strong anchoring sites for Pd nanoparticles due to
*
Correspondence to: Hakjune Rhee, Hanyang University, Department of
Chemistry and Applied Chemistry, 1271 Sa-3-Dong, Sangrok-gu, Ansan,
Kyunggi-do 426–791, Korea. Email: hrhee@hanyang.ac.kr
Jongsik Kim, Dong-A University, Department of Chemistry, 550 Hadan-Dong,
Saha-gu, Busan 604-714, Korea. Email: jskimm@dau.ac.kr
a Department of Chemistry and Applied Chemistry, Hanyang University, 1271
Sa-3-Dong, Sangrok-gu, Ansan, Kyunggi-do, 426-791, Korea
b Department of Bionanotechnology, Hanyang University, 1271 Sa-3-Dong,
Sangrok-gu, Ansan, Kyunggi-do, 426-791, Korea
c
Department of Chemical Engineering, Hanyang University, 1271 Sa-3-Dong,
Sangrok-gu, Ansan, Kyunggi-do, 426-791, Korea
d Department of Chemistry, Dong-A University, 550 Hadan-Dong, Saha-gu,
Busan 604-714, Korea
Appl. Organometal. Chem. 2014, 28, 156–161
Copyright © 2014 John Wiley & Sons, Ltd.

Pd nanoparticles immobilized on PNIPAM–halloysite
Scheme 1. Preparation of Pd(0) nanoparticles supported on PNIPAM/HNT hydrogel nanocomposites.
Synthesis of PNIPAM/HNT hydrogel nanocomposites
the existence of HNT and temperature-responsive behavior due
to the PNIPAM moiety during SM reactions in an aqueous system.
In this study, PNIPAM/HNT (halloysite nanotube) hydrogels
were synthesized in ratios of 95:5, 85:15 and 70:30 (w/w) and
then Pd nanoparticles were impregnated on them through Pd2+
adsorption and its reduction to Pd(0) species. These catalysts
are designated as C1, C2 and C3, respectively. The optimal
catalytic system and reaction conditions for SM reactions were
thoroughly investigated. The optimized catalytic system was also
applied in various substrates. Furthermore, the reusability of the
catalyst was examined up to 10 times in recycling tests with
two model reactions: (i) bromoacetophenone and phenylboronic
acid; and (ii) bromobenzene and 4-methylphenylboronic acid.
Pd(0) nanoparticles supported on PNIPAM/HNT hydrogel com-
posites were prepared as shown in Scheme 1. PNIPAM/HNT
hydrogel nanocomposites were first synthesized by free radical
solution polymerization according to the standard procedure,
except for the existence of HNT.[61,76] In this polymerization,
HNT content was varied to 5, 10 and 15 wt%. In a typical run for
the synthesis of a cross-linked PNIPAM/HNT (95:5, w/w) hydrogel
nanocomposites, 1.00 g NIPAM (8.57 mmol, 94 mol%) dissolved in
2 ml methanol, 0.085 g MBAAm (0.55 mmol, 6 mol%) dissolved in
2 ml methanol and 0.05 g HNT in 16 ml toluene were placed in a
100 ml flask. To homogeneously disperse the HNT in the solution,
the suspension was stirred for 12 h at room temperature.
Polymerization was initiated by adding 3 mol% AIBN (0.045 g,
0.27 mmol) and then the reaction mixture was heated at 80 °C
for 4 h. The formed solid polymer was filtered off, washed with
methanol, and dried at 20 °C under vacuum for 24 h.
Experimental
Materials
Pd(0) nanoparticles anchored on the nanocomposites
All reagents were purchased from commercial sources (Aldrich,
TCI). Palladium acetate (Pd(OAc)2), methanol (MeOH), tetrahydrofu-
ran (THF), ethyl acetate (EA), N-isopropylacrylamide (NIPAM;
Aldrich, 97%), and N,N′-methylenebisacrylamide (MBAAm; Aldrich,
99%) were used as received. 2,2′-Azobisisobutyronitrile (AIBN;
Junsei Chemicals, 99%), which was used as an initiator, was
recrystallized from methanol. Halloysite nanotubes (HNTs; Aldrich)
were used without any chemical purification. The HNT has a
Brunauer–Emmet–Teller (BET) specific surface area of about
64 m2 gÀ1, cation exchange capacity of 8 mequiv. gÀ1 and pore
As-synthesized PNIPAM/HNT (95:5, w/w) hydrogel nanocom-
posite (100 mg) was dispersed in MeOH (20 ml) under mechanical
shaking. Palladium acetate (Pd(OAc)2) (10 mg, 0.0445 mmol) was
then gradually added to the suspension, followed by sonication
for 1 min to thoroughly dissolve the Pd source. The Pd mixture
was shaken for 12 h under argon. As the hydrogel reacted with
Pd(OAc)2, the hydrogel color changed to yellow. The mixture
was filtered using a 0.20 μm Millipore Nylon membrane filter
and washed three times with 20 ml MeOH. The filtered Pd2+
-
volume of 1.25 mlgÀ1
.
adsorbed copolymer was then dried in air for 12 h. Black Pd(0)
nanoparticles were generated by adding 10 mg (0.264 mmol)
NaBH4 as a reducing reagent to a suspension of the dried copol-
ymers in THF (20 mL) under mechanical shaking for 12 h. 10 ml
MeOH was added to remove the residual NaBH4 and then the
Pd(0)-grafted copolymer was separated by filtration. The filtered
copolymer was thoroughly washed with 20 ml MeOH and 20 ml
THF and then dried in air for 12 h. The other Pd-grafted
PNIAPM/HNT nanocomposites, C2 and C3, were also prepared
by the same method but with different content of HNT (~15
and 30 wt%), respectively. The loading amounts of Pd(0)
nanoparticles in C1, C2, and C3 were estimated by ICP measure-
ments to be 0.494, 4.84 and 1.63 mmol Pd gÀ1, respectively.
Characterization
Transmission electron microscopic (TEM) and energy dispersive
X-ray analysis (EDAX) measurements were performed on an
HRTEM JEOL transmission electron microscope at an acceleration
of 300 kV. X-ray diffraction (XRD) patterns were collected on a
Rigaku D/MAX-2500 X-ray diffractometer (Cu-Kα radiation,
λ = 1.5418 Å). The diffraction pattern obtained was compared
with that at the International Centre for Diffraction Data (ICDD).
The amount of Pd in the catalysts was estimated by inductively
coupled plasma (ICP) analysis with a JY Ultima2C. The lower
critical solution temperatures (LCSTs) of the synthesized
nanocomposites were characterized by differential scanning
calorimetry (DSC) measurements (TA Instrument, DSC 2010). All
samples were immersed in pH 10 buffer solution at room temper-
ature for 24 h to bring them to equilibrium state before DSC
measurements. DSC analyses of the swollen hydrogels were
performed from 0 to 50 °C at a heating rate of 2 °C minÀ1 under
nitrogen atmosphere at a flow rate of 40 ml minÀ1. 1H NMR spec-
tra were obtained in CDCl3 with Bruker and Varian spectrometers
operating at 400 MHz and 500 MHz, with tetramethylsilane as an
internal standard.
General procedure for the SM reaction
A 10 ml round-bottom flask was charged with 4-bromoaceto-
phenone (1 h, 1 mmol, 1 equiv.), phenylboronic acid (2a, 1.5mmol,
1.5 equiv.), K2CO3 (414 mg, 3 mmol, 3 equiv.), H2O (2ml) and Pd
catalyst (1mol%). The flask was stirred at 70 °C in air. The reaction
was monitored by thin-layer chromatography (TLC). After the
reaction was complete, the reaction mixture was cooled to room
temperature and then simply filtered to recover the catalyst. It
was then washed with 10 ml H2O and ethyl acetate (EtOAc). The
organic phase was separated from the aqueous phase, which
Appl. Organometal. Chem. 2014, 28, 156–161
Copyright © 2014 John Wiley & Sons, Ltd.
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M. C. Hong et al.
was extracted three times with 30 ml EtOAc. The organic phases
were collected together, dried over MgSO4, and filtered. The
solvent was then evaporated under reduced pressure. The pure
product was obtained via silica gel column chromatography with
an eluent of EtOAc and hexane. The resulting product was
analyzed by 1H NMR spectroscopy.
Results and Discussion
Differential scanning calorimetry (DSC) experiments show the
obvious LCST of the PNIPAM at about 31 °C, which is consistent
with the previously reported LCST of PNIPAM.[46–51] The synthe-
sized PNIPAM/HNT (95:5, w/w) nanocomposites, C1, shows a shift
of the endothermic peak to about 34 °C (Fig. 1). However, no
reproducible thermal responsive behavior was observed for C2
and C3, probably due to the aggregation of HNT inside the matrix
of PNIPAM. The temperature ranges of SM reactions were
determined on the basis of the DSC measurements.
The Pd nanoparticles were grafted on to the PNIPAM/HNT
nanocomposites by dispersing Pd(OAc)2 in the solution of the
nanocomposites and then reducing adsorbed Pd2+ to Pd(0) with
NaBH4. The formation of Pd nanoparticles was visually observed
with the mixture’s change in color to black, and it was character-
ized by TEM, EDAX and XRD measurements. Figure 2(a)-(b) show
TEM images of the Pd nanoparticles on PNIPAM/HNT (95:5 wt/wt
ratio) as a representative sample (Note that TEM images of the
other Pd catalysts (C2 and C3) are not shown here because
they did not show satisfactory activities in the SM reactions
(see Table 1).). According to the TEM images, C1 had rod-shaped
HNT particles with a diameter of about 0.15 μm and a length of
about 0.6 μm. The Pd nanoparticles were mainly anchored on
the surface and inside the pores of the HNT particles, having
spherical morphologies with diameters of less than about
15 nm inside the pores. The particle sizes were increased to
approximately 50–70 nm on the surface probably due to the
aggregations of the smaller Pd nanoparticles. The synthesized
C1 sample was further characterized by EDAX and XRD measure-
ments as shown in Figs S1 and S2, respectively (supporting
information). The EDAX spectrum of C1 confirms the presence
Recycling the Pd catalyst
Two kinds of SM reactions were carried out. The first five runs
were with bromoacetophenone 1 h and phenylboronic acid 2a,
and the second five runs were with bromobenzene 1a and
4-methylphenylboronic acid 2b. The SM reactions were
performed as described above. After the first reaction was
complete, C1 was recovered by simple filtration and air-dried.
The recovered catalyst was successively subjected to the next five
runs of the coupling reaction under the same reaction conditions
(e.g. concentration of reactants) as the first run. The filtrate (H2O)
was analyzed with ICP measurements to estimate the amount of
Pd leaching after each run.
Figure 1. Differential scanning calorimetry (DSC) curves of PNIPAM and
PNIPAM/HNT (95:5, w/w).
Figure 2. Transmission electron microscope (TEM) images of Pd nanoparticles grafted on to PNIPAM/HNT (95:5, w/w) (a, b) before and (c, d) after
SM reactions.
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Appl. Organometal. Chem. 2014, 28, 156–161

Pd nanoparticles immobilized on PNIPAM–halloysite
Table 1. Suzuki coupling reactions of 4-bromoacetophenone 1 h
and phenylboronic acid 2a with C1–C3 as a function of temperature
and the amount of C1a
optimized as C1, 70 °C and 1 mmol% Pd, which was applied to
the conversion reactions of a variety of substrates as below.
The reactions of a variety of aryl bromides 1 with arylboronic acids
2 are listed in Table 2, under the optimized reaction conditions
(i.e. 1 mmol% C1, at 70 °C, in water). The coupling reactions of
phenylboronic acid 2a with a wide range of aryl bromides with elec-
tron-donating groups (entries 1–3) provided excellent conversions
above 95%. However, 4-methylaryl bromide 1d afforded only a mod-
erate yield of the corresponding product (82%, entry 4). In addition,
aryl bromides with electron-withdrawing groups also showed excel-
lent conversion yields (>94%) (entries 5–10). Bromobenzene 1a and
4-methylphenylboronic acids 2b were converted to the correspond-
ing biaryl product with a high yield of 95% (entry 11). On the other
hand, 4-methoxy and 4-chlorophenylboronic acids gave moderate
yields of 80% and 74%, respectively (entries 12 and 13). Furthermore,
C1 shows an excellent reactivity even in the conversion reaction
between 4-bromoacetophenone 1h and various arylboronic acids
with electron-donating and withdrawing groups (yields > 94%,
entries 14–17).
Entry
Catalyst (NIPAM:HNT)
T (°C) Time (h) Yield (%)b
1
2
3
4
5
6
7
HNT (5%), 5 mmol% Pd
HNT (15%), 5 mmol% Pd
HNT (30%), 5 mmol% Pd
HNT (5%), 5 mmol% Pd
HNT (5%), 5 mmol% Pd
HNT (5%), 5 mmol% Pd
HNT (5%), 1 mmol% Pd
60
60
60
50
70
70
70
1.5
2
98
99
2
55
2
99
0.5
0.8
1
99
Lastly, we estimated recyclability of C1 by performing two
model reactions at 70 °C in water: (i) 4-bromoacetophenone 1 h
and phenylboronic acid 2a for the first five runs; and (ii)
bromobenzene 1a and 4-methylphenylboronic acid 2b for the
next five runs, respectively (Table 3). It was found that C1 showed
Quant.
Quant.
aReaction conditions: 4-bromoacetophenone (1 h, 1 mmol),
phenylboronic acid (2a, 1.5 mmol, 1.5 equiv.), K2CO3 (3 mmol, 3
equiv.), H2O (2 ml) and Pd catalyst.
bIsolated yield (the purity of the isolated product was confirmed
by 1H NMR).
Table 2. Suzuki coupling reactions of various aryl bromides 1 and
arylboronic acids 2 with C1 at 70 °C in watera
of palladium, aluminum and silicon in C1 (Fig. S1). The powder XRD
pattern of C1 shows the diffraction peaks at about 40°, 46.6° and
68°, which were unambiguously indexed as (111), (200) and (220)
reflections, respectively, on the basis of ICDD file 46–1043 for crystal-
line metallic Pd(0) (Fig. S2). The broad peaks at about 20° and 11°
were attributed to amorphous PNIPAM and HNT, respectively.[77,78]
The catalytic activities of as-prepared C1–C3 were estimated
via a coupling reaction between 4-bromoacetophenone 1 h and
phenylboronic acid 2a (Table 1). First, C1–C3 were applied for the
reaction under the conditions of 5 mmol% Pd catalysts and K2CO3
(3 equiv.) as a base at 60 °C in water, respectively (entries 1–3,
Table 1). C1 and C2 afforded 98–99% yields of 4-acetylbiphenyl
3 h. However, reaction time with C1 was shorter than that of C2
(1.5h vs. 2 h). The coupling reaction with C3 showed a moderate
conversion yield of 55% even in 2 h. Based on these results,
we chose C1 as the optimum catalyst for the SM reaction. To opti-
mize reaction conditions such as reaction temperature and amount
of palladium catalysts, we performed several test reactions with C1
(Table 1, entries 1 and 4–7). First, in order to optimize reaction
temperatures using C1, we investigated conversion yields with
5 mmol% Pd catalyst as a function of temperature from 50 to 70 °
C (Table 1, entries 1 and 4–6). As the reaction temperature in-
creased, the reaction time was significantly decreased from 1.5 to
0.5h, affording similar conversion yields of about 98–99%. Further-
more, the reaction was quantitatively completed within 0.8h at 70 °
C (entry 6, Table 1). To estimate the effects of the amount of Pd cat-
alysts on the conversion reaction, we performed an additional reac-
tion with 1 mmol% Pd catalysts (Table 1, entry 7), resulting in
quantitative conversion within a slightly longer reaction time of
1 h compared to that of 5 mmol% Pd catalysts. Considering the
amount of catalysts used in the reaction, 1 mmol% Pd catalyst is
more effective than 5 mmol% Pd. Thus the reaction condition was
Entry
1 (R1)
2 (R2)
Time (h) Yield (%)b
1
1a (―H)
2a (―H)
2a
1.5
3
97
97
2
1b (―NH2)
1c (―OH)
1d (―CH3)
1e (―NO2)
1f (―CN)
3
2a
1
Quant.
82
4
2a
4
5
2a
1
95
6
2a
0.5
4
Quant.
94
7
1 g (―CO2CH3) 2a
8
1 h (―COCH3)
2a
1
Quant.
99
9
1i (―CHO)
2a
3
10
11
12
13
14
15
16
17
1j (―CF3)
1a
2a
2
95
2b (―CH3)
2c (―OCH3)
2d (―Cl)
2b
1
95
1a
1
80
1a
4
74
1 h
0.5
1
94
1 h
2c
97
1 h
2d
2
98
1 h
2e (―F)
3
98
aReaction conditions: aryl bromide (1, 1 mmol), arylboronic acid (2,
1.5 mmol, 1.5 equiv.), K2CO3 (3 mmol, 3 equiv.), H2O (2 ml) and
1 mmol% Pd catalyst (C1).
bIsolated yield (the purity of the isolated product was confirmed
1
by H NMR).
Appl. Organometal. Chem. 2014, 28, 156–161
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M. C. Hong et al.
Table 3. Recycling experiments of C1 in Suzuki coupling reactions at 70 °C in water.
Trial
1st
2nd
3rd
4th
5th
Yield (%)a
TOFb
99
94
99
99
97
198
188
198
198
194
Reaction conditions: 4-bromoacetophenone (1 h, 1 mmol), phenylboronic acid (2a, 1.5 mmol, 1.5 equiv.), K2CO3 (3 mmol, 3 equiv.), H2O (2 ml),
1 mmol% Pd catalyst (C1) and a reaction time of 40 min.
Trial
6th
7th
8th
9th
10th
Yield (%)a
TOFb
98
97
91
98
90
196
194
182
196
180
Reaction conditions: bromobenzene (1a, 1 mmol), 4-methylphenylboronic acid (2b, 1.5 mmol, 1.5 equiv.), K2CO3 (3 mmol, 3 equiv.), H2O (2 ml),
1 mmol % Pd catalyst (C1) and a reaction time of 30 min.
aIsolated yield (the purity of the isolated product was confirmed by 1H NMR).
bTurnover frequency defined as moles of 4-bromoacetophenone or bromobenzene consumed per mole of total metal per hour.
excellent recyclability for the first five runs, resulting in high
yields above 94–99% in 40 min (turnover frequency (TOF) of
above 188 hÀ1). The high catalytic activity was preserved for the
following five runs, affording 90–98% conversion (TOF of above
180 hÀ1). After each run, the amount of Pd leaching from C1
was estimated by performing ICP measurements on the superna-
tant solutions. The solutions were collected by filtering the
catalyst from the reaction mixture after the reactions were
complete. No significant Pd leaching (<1 ppm) was observed. In
addition, there was no significant agglomeration of Pd
nanoparticles after SM reactions on the basis of the TEM images
(Fig. 1c, d). This suggests that the excellent recyclability is
attributed to the existence of HNT as a solid support of Pd
nanoparticles in the network of PNIPAM.
the unique advantages of HNT and PNIPAM were successfully uti-
lized for the SM reactions in this work. Applications of similar
approaches to other Pd-catalyzed reactions such as Heck, Stille,
Sonogashira, Hiyama, Negishi and oxidation/reduction reac-
tions can be readily envisaged.
Acknowledgements
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF), funded by the Ministry of Education (2012R1A1A2001005
and 2011–0023512) and by a grant from the Eco-Innovation
project funded by the Ministry of Environment of Korea
(No. 405-111-004).
In conclusion, Pd nanoparticles were successfully anchored on
PNIPAM/HNT hydrogel nanocomposites. The Pd-anchored
PNIPAM/HNT catalysts demonstrated excellent catalytic activities
for various SM reactions even under mild reaction conditions,
such as with water as sole solvent at 70 °C. In particular, catalyst
C1 showed superior recyclability to that of the Pd-grafted
PNIPAM system. Although various supports such as carbon,
polymers and modified SiO2 have been utilized to develop
heterogeneous Pd catalytic systems, most of them needed either
a higher reaction temperature, above 80 °C,[21] or an aqueous
solvent mixed with ethanol[23,30,79,80] or DMF[26,81] to achieve com-
parable catalytic activities to our system. We therefore believe that
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Appl. Organometal. Chem. 2014, 28, 156–161
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