Palladium supported on a magnetic microgel: An efficient and recyclable catalyst for Suzuki and Heck reactions in water

Article abstract of DOI:10.1039/c3gc40941d

A novel heterogeneous Pd catalyst was synthesized by anchoring palladium(ii) onto poly(undecylenic acid-co-N-isopropylacrylamide-co-potassium 4-acryloxyoylpyridine-2,6-dicarboxylate)-coated Fe₃O₄ (Fe₃O₄@PUNP) magnetic microgel. The catalyst (Fe ₃O₄@PUNP-Pd) was characterized by FT-IR, TEM, VSM, XRD and XPS, and the loading level of Pd in Fe₃O₄@PUNP-Pd catalyst was measured to be 0.330 mmol g^-1 by AAS. This catalyst exhibits excellent catalytic activity for the Suzuki and Heck reactions in water. In addition, the Fe₃O₄@PUNP-Pd catalyst can be easily separated and recovered with an external permanent magnet, and the reuse experiment shows that it can be used consecutively six times without significant loss in catalytic activity.

Full text of DOI:10.1039/c3gc40941d

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ARTICLE TYPE
Palladium supported on magnetic microgel: an efficient and recyclable
catalyst for Suzuki and Heck reactions in water
a
a
a
a
a
b
Jianhua Yang, Dongfang Wang, Wendong Liu, Xi Zhang, Fengling Bian, * Wei Yu *
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
A novel heterogeneous Pd catalyst was synthesized by anchoring palladium (II) onto poly (undecylenic
acid-co-N-isopropylacrylamide-co-potassium 4-acryloxyoylpyridine-2,6-dicarboxylate)-coated Fe
Fe @PUNP) magnetic microgel. The catalyst (Fe @PUNP-Pd) was characterized by FT-IR, TEM,
VSM, XRD and XPS, and the loading level of Pd in Fe @PUNP-Pd catalyst was measured to be 0.330
mmol g by AAS. This catalyst exhibits excellent catalytic activity for the Suzuki and Heck reactions in
water. In addition, the Fe @PUNP-Pd catalyst can be easily separated and recovered with an external
3 4
O
(
O
3 4
3 4
O
3 4
O
-1
1
0
3 4
O
permanent magnet, and the reuse experiment shows that it can be used consecutively six times without
significant loss in catalytic activity.
time-consuming centrifugation and filtration.
1
Introduction
Recently, the magnetic nanoparticles have been widely applied as
10-12
solid supports for catalyst.
The magnetic nanoparticle-
1
2
2
3
3
4
4
5
0
5
0
5
0
5
Palladium-catalyzed Suzuki and Heck cross-coupling reactions
constitute important methods for C-C bond formation in organic
5
5
6
6
7
0
5
0
5
0
supported catalysts can be simply and efficiently separated from
the reaction system by an external permanent magnet and thus
1
,2
synthesis and industrial processes. Currently the Heck and
Suzuki reactions are performed mostly in organic solvents with
homogeneous palladium salts and complexes, which bring about
concerns of high cost and environmental issues at large scale.
Therefore, it is highly desirable to develop reusable
heterogeneous palladium catalysts which can be easily separated
from the reaction systems. In addition, from the viewpoint of
green chemistry, it is preferable to use water to replace toxic
organic solvents as the reaction medium. However, many
developed heterogeneous catalytic systems suffer from the
drawbacks of low catalytic activity and selectivity, and are not
suitable to be used to catalyze the reactions in aqueous media
because most organic substrates dissolve poorly in water. How to
solve these problems is now a major issue to chemists working in
13,14
avoid cumbersome operating process.
The organic polymer
coated-magnetic nanoparticles can be stabilized in reaction media
by the prevention of nanoparticle aggregation. In addition, as it is
easier to functionalize the polymers by introducing various kinds
of ligands, the immobilization of metal catalysts can be greatly
15
16
facilitated. Recently, Stevens and his co-workers prepared the
polystyrene coated-γ-Fe nanoparticle-supported-Pd catalyst,
and used it for the Suzuki reaction of iodo- and bromoaromatics
in DMF/H O. The catalyst can be easily recovered and reused by
2 3
O
2
17
an external permanent magnet. Amorin and his coworkers
synthesized a very efficient dendritic Pd (II) catalyst grafted on γ-
2 3
Fe O /polymer nanoparticles, which showed high reactivity and
recoverability for the Sonogashira coupling in water.
On continuation of our research interest in developing
3
this area.
heterogeneous transitional metal catalysts loaded on hydrogels
Polymers are commonly used as supports for catalyst loading.
Among these systems, microgels, with the advantages of
nanometer scale, large surface area, and easy functionalization,
has attracted widespread attention. The microgels modified by
introducing a specific ligand can be uniformly dispersed in
reaction media, and as a catalyst support, they can increase the
probability of contact of the reactants with the active sites of
catalyst, and thus improve the latter’s catalytic activity.
Biffis and his co-workers synthesized 4-vinyl pyridine-
functionalized microgel-supported-Pd catalyst and applied it to
18,19
and magnetic nanoparticles,
we endeavored to combine the
high catalytic activity of microgel supported-catalyst with the
advantages of easy separation and recycling of magnetic
nanoparticles supported-catalyst by preparing a novel magnetic
4
,5
3 4
microgel supported-Pd catalyst (Fe O @PUNP-Pd). This catalyst
shows high catalytic activity for the Suzuki and Heck reactions in
water, and could be used six times without significant loss in
catalytic activity. Herein we wish to report this work in detail.
6,7
7
5
2 Experimental
8
9
2
the Suzuki reaction in DMA/H O. Zhang et al. prepared pH-
responsive polymeric microsphere-immobilized palladium-
iminodiacetic acid catalyst for the Suzuki and Heck reactions in
aqueous solution. While satisfactory results were obtained in
these works, but the recovery of catalyst needs cumbersome and
2.1 Materials
N-isopropylacrylamide (NIPAM, ≥ 99%) was purchased from
J&K Chemicals and purified by recrystallization from hexane
before use. N, N’-methylenebisacrylamide (BIS, ≥ 98%) was
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purchased from Sinopharm Chemical Reagent Co., Ltd. and used
as received. Potassium persulfate (KPS, ≥ 99.5%) was obtained 60 Fe
from Xi'an Chemical Reagent Factory and was purified by
recrystallization from water. The chelidamic acid-functionalized
monomer of potassium 4-acryloxyoylpyridine-2,6-dicarboxylate
Next, In a 100 mL four-neck round-bottomed flask, 20 mg
3 4
O @UA was mixed with 30 mL of aqueous solution
containing NIPAM (2 mmol), PAP (0.32 mmol) and BIS (0.147
mmol). After being degassed under argon gas for 30 min., the
-
1
5
0
5
0
5
0
5
0
5
0
5
solution was heated up to 80 ºC, and 0.18 mL of 0.05 g mL KPS
solution was injected in to initiate the polymerization. After the
19
(
PAP) was synthesized as described elsewhere. Palladium (II)
2
chloride (PdCl , ≥ 59.5%) was provided by Shenyang Keda 65 polymerization process maintained for 4 h under argon gas with
Reagents Company. Tetrabutyl ammonium bromide (TBAB, ≥
9%) was purchased from Tianjin Guangfu Fine Chemicals and
used as received. Other reagents were all in analytical grade and
used without further purification. Deionized water was used in
the present experiments.
vigorous stirring, the final product was collected by magnetic
separation, washed with deionized water for 5 times, and finally
dried under vacuum.
9
1
1
2
2
3
3
4
4
5
5
2.4 Synthesis of Fe
@PUNP-Pd catalyst was prepared in a typical procedure as
follows: Fe @PUNP (3 mg) and PdCl (0.001 mmol) were
3 4
O @PUNP-Pd catalyst
3 4
70 Fe O
2
.2 Characterizations
Transmission electron microscopy (TEM) images were obtained
with Tecnai-G2-F30 (FEI, USA) transmission electron
microscopy operating at 300 kV. At least 50 particles were
O
3 4
2
added into a screw-capped vial, where the molar ratio of the
ligand of PAP to Pd was 3:1. The mixture was stirred at room
temperature for 12 h. During the period the color of the solution
a
measured to obtain the average diameter of the nanoparticles. 75 turned into transparent from light yellow. The obtained catalyst
2+
Fourier transform infrared spectra (FT-IR) were measured on a
NEXUS670 (Nicolet, USA) spectrophotometer using KBr pellets
of samples. X-ray diffraction (XRD) of the samples were
recorded on a Shimadzu XRD-6000 spectrometer using Nicker-
was washed with deionized water to remove the excessive Pd ,
and then was used directly in the coupling reaction.
2.5 General procedure for Suzuki reaction in water
To a 10 mL overpressure screw-capped vial were added
α
filter Cu K radiation (λ=0.15418 nm). The magnetic properties 80 sequentially aryl halide (1 mmol), arylboronic acid (1.5 mmol),
of nanoparticles were measured on a Model 7304 (Lake Shore,
USA) vibrating sample magnetometer (VSM) at room
2 3 3 4
K CO (3 mmol), the Fe O @PUNP-Pd catalyst (0.1 mol% Pd
based on Aryl halide) and 2 mL deionized water. The mixture
was degassed under argon gas for 10 min, and then vigorously
stirred at 90 ºC for 1 h. After being cooled to room temperature,
2
temperature. The N adsorption–desorption isotherms were
measured at 76 K using a Micromeritics ASAP 2020M
instrument. The specific surface area was calculated by the 85 the solution was diluted with deionized water, and the catalyst
Brunauer–Emmett–Teller (BET) method. The quantity of
leaching Pd was detected using an AA240 (Varian Corporation,
USA) atomic absorption spectroscopy (AAS). X-ray photo-
electron spectroscopy (XPS) measurements were performed on a
was separated by using an external permanent magnet. The crude
product was extracted with diethyl ether (3×10 mL). (for
substrates containing –OH or -COOH, the solution was diluted
-1
with deionized water, acidified with 1 mol L HCl and then
Thermon Scientific ESCALAB250xi electron spectrometer with 90 extracted with diethyl ether.) The organic phase was washed with
1
contaminated C as internal standard (C1s=284.8 eV). The H
4
brine, dried with MgSO , and then the solvent was evaporated
1
3
NMR (400 MHz) and C NMR spectra were recorded in CDCl
or DMSO-d on a Bruker AVANCE III 400 NMR spectrometer
with TMS as internal reference.
.3 Synthesis of the Fe @PUNP magnetic microgel
Fe @Poly (undecylenic acid-co-N-isopropylacrylamide-co-
potassium 4-acryloxyoylpyridine-2,6-dicarboxylate) (Fe
3
under reduced pressure. The resulting residual was
chromatographed on silica gel (hexane/ ethyl acetate) to give pure
product which was analyzed by H and C NMR spectra.
95 For kinetics study, to a 10 mL overpressure screw-capped vial
were added sequentially 4-iodoanisole (1 mmol), arylboronic acid
6
1
13
2
3 4
O
3 4
O
O
3 4
@
2 3 3 4
(1.5 mmol), K CO (3 mmol), the Fe O @PUNP-Pd catalyst (0.1
PUNP) magnetic microgel was synthesized in a two-step
procedure:
mol % Pd based on Aryl halide) and 2 mL deionized water. The
mixture was degassed under argon gas for 10 min, and then
Firstly, undecylenic acid-coated magnetic nanoparticles 100 vigorously stirred at 90 ºC for the indicated time. The reaction
(
Fe
chemical co-precipitation method. As a typical procedure, 0.86
g of FeCl and 2.35 g of FeCl were dissolved in 40 mL of
deionized water, and then the solution was degassed for 30 min.
3
O
4
@UA) were prepared according to a previous report by
mixture was then treated following the aforementioned
procedure, and the yield was determined according to the isolated
product.
20
2
3
2.6 General procedure for Heck reaction in water
After the solution was heated to 80 °C, a solution of 0.1 g of 105 To a 10 mL overpressure screw-capped vial were added aryl
undecylenic acid in 5 mL of acetone was added, followed by 5 halide (1 mmol), acrylic acid (1.5 mmol), K CO (3 mmol),
mL of 28% (w/w) NH ·H O. Then 1.0 g (5 × 0.2 g) of Fe @PUNP-Pd catalyst (0.1 mol% Pd) and 2 mL deionized
2
3
3
2
3 4
O
undecylenic acid was added into the mixture. The mixture was
stirred vigorously for 30 min at 80 °C, and was then cooled
water. The mixture was degassed under argon gas for 10 min.,
and then vigorously stirred at reflux temperature (94 ºC) for 12 h.
slowly to room temperature. The mixture was precipitated with 110 After being cooled to room temperature, the solution was diluted
mixed solvent of acetone and MeOH (1:1 by volume), and the
precipitates were isolated from the solvent by magnetic
separation method. Finally, the precipitates were washed with the
same mixed solvent for five times to remove the excessive
with deionized water, and the catalyst was separated by using an
external permanent magnet. The reaction solution was acidified
with 1 mol L HCl. The solid product was filtered, washed with
cold water, and then recrystallized from water and ethanol. Their
-1
1
13
undecylenic acid. The prepared Fe
vacuum.
3 4
O @UA was dried under 115 structures were analyzed by H and C NMR spectra.
2.7 Reusability of the Fe @PUNP-Pd catalyst
3 4
O
2
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3 4
The reusability of the Fe O @PUNP-Pd catalyst was examined
by applying it to the Suzuki coupling of 4-bromoacetophenone
with arylboronic acid. After the first run completed, the
3 4
Fe O @PUNP-Pd catalyst was separated with an external
5
permanent magnet and washed with deionized water and diethyl
ether alternately. Then the recovered catalyst was reused directly
in the next run of the reaction. This process was repeated multiple
times.
3
Result and Discussions
1
1
2
0
5
0
3.1 Synthesis and characterization of Fe
microgel and Fe @PUNP-Pd catalyst
3 4
O @PUNP magnetic
3 4
O
Using well designed microgel as catalyst scaffold is advantageous
in that they can be uniformly dispersed in the reaction medium,
and thus the contact of reactants with active sites of the catalyst
Fig. 1 FT-IR spectra of (a) Fe
3
O
4
@UA, (b) Fe
3 4
O @PUNP, (c) fresh
6
can be greatly improved. On the other hand, when the magnetic
4
5
Fe @PUNP-Pd and (d) Fe O
3
O
4
3 4
@PUNP-Pd after six runs
nanoparticles act as supports for catalyst, they can be easily
separated and recycled by an external permanent magnet,
obviating the cumbersome operating process of precipitation or
The FT-IR spectra of (a) Fe
fresh Fe @PUNP-Pd are shown in Fig. 1. In Fig. 1a, two bands
located at 2925 cm and 2853 cm can be attributed to the
3 4 3 4
O @UA, (b) Fe O @PUNP and (c)
3 4
O
-1
-1
21
centrifugation. We envisioned that if magnetic nanoparticles
coated with functionalized microgels are used to load catalyst, the
advantages of both microgels and magnetic nanoparticles would
-
1
aliphatic C-H stretching vibrations, and the bands at 1638 cm
-1
5
5
6
6
0
5
0
5
and 991 cm are assigned to the C=C stretching vibration and the
C=C out-of-plane bend vibration respectively. The band at 910
be combined. In this view, we prepared a novel Fe
magnetic microgel via radical polymerization in water.
3 4
O @PUNP
-
1
cm is assigned to the C-H out-of-plane bend vibration for the
-1
=
CH
2
group, and the band at 588 cm can be attributed to Fe-O
@UA are
consistent with those reported by Shen et al. In Fig. 1b, the band
stretching vibration. These characteristic bands of Fe
3 4
O
22
-1
at 1706 cm corresponds to the C=O stretching vibration for the
-1
-1
-1
PAP groups. The bands at 1625 cm , 1570cm ，1436 cm can
be attributed to the C=O stretching vibration (amide I band), N-H
deformation vibration (amide II band), and the methyl group
asymmetric bending vibrations in the PNIPAM segments
respectively. The band at 582 cm is consistent with the Fe-O
stretching vibration. All the characteristic bands in the FT-IR
spectrum (Fig. 1b) demonstrate that the Fe
microgel has been successfully prepared. On complexation with
Pd(II) (Fig. 1c), the intensity of the band at about 1710 cm
-1
3 4
O @PUNP magnetic
-1
2
5
3 4
Scheme 1 Preparation of Fe O @PUNP magnetic microgel and
Fe @PUNP-Pd catalyst
3
O
4
become obviously weaker than that of Fe
3 4
O @PUNP, consistent
with the formation of a metal-ligand bond.
3 4
As shown in Scheme 1, the Fe O @PUNP magnetic microgel
was synthesized in two steps from commercially available stuffs.
In the first step, the undecylenic acid-coated magnetic
3
3
4
0
5
0
nanoparticles (Fe
precipitation. The purpose of this operation was to introduce onto
the surface of Fe nanoparticles the C=C bond which would
find use in the subsequent copolymerization. Next, the
copolymerization of Fe @UA with NIPAM and PAP was
realized in water by using K
expected that thus formed Fe
favorably as PAP possesses excellent capacity for anchoring Pd
3 4
O @UA) were prepared by chemical co-
3 4
O
3 4
O
2 2
S
O
8
as the radical initiator. It was
2+
3
4
O @PUNP would bind Pd
2
+
19
.
As such, Fe
solution of PdCl
catalyst. The content of Pd
3 4
O
@PUNP was then treated with an aqueous
to afford the designed Fe @PUNP-Pd
@PUNP-Pd was
2
3 4
O
2+
in Fe
determined to be 0.330 mmol g by AAS.
3 4
O
-1
3 4 3 4
Fig. 2 Magnetic hysteresis curves of (a) Fe O @UA, (b) Fe O @PUNP
7
0
and (c) fresh Fe @PUNP-Pd.
3 4
O
The magnetic property of magnetic materials was investigated
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3

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Page 4 of 9
with a vibrating sample magnetometer (VSM) at 300 K. As
shown in Fig. 2, the magnetic hysteresis curves of (a) Fe @UA,
b) Fe @PUNP and (c) fresh Fe @PUNP-Pd reveal that
they possess superparamagnetic behavior, and their saturation
3 4
O
(
O
3 4
3 4
O
-1
5
magnetization values are 62.5, 55.3 and 36.0 emu g respectively,
indicating that these magnetic materials can be efficiently
separated by an external permanent magnet.
3
5
Fig. 5 TEM images of (a) Fe
Fe @PUNP -Pd, (d) Fe
Fe @PUNP-Pd after six runs.
3
O
4
3 4
@ UA, (b) Fe O @PUNP, (c) fresh
3
O
4
3 4
O @PUNP-Pd after one run and (e)
3
O
4
3
.2 Fe
3
O
4
@PUNP-Pd catalyzed Suzuki reaction in water
@PUNP-Pd, its
4
0
Following the successful preparation of Fe O
3 4
effectiveness for the Suzuki coupling reaction was examined next.
It is known that the Suzuki reaction is largely effected by the type
25-29
of alkaline and the amount of catalyst used.
Therefore, firstly
the reaction of 4-iodoanisole with phenylboronic acid was used as
Fig. 3 XRD patterns of (a) Fe
Fe
3
O
4
@UA, (b) Fe
3
O
4
@PUNP, (c) fresh
1
0
3
O
4
@PUNP-Pd and (d) Fe
3
O
4
@PUNP-Pd after six runs.
45 a model reaction for the screening of bases, and the results were
summarized in Table 1 (entries 1-5). With the amount of catalyst
fixed as 0.1 mol %, the reaction proceeded well when organic
bases such as triethylamine and piperidine were used, affording
the desired coupling product in excellent yields (entries 1 and 2),.
However, the yield was considerable lower with tributylamine as
the base (entry 3). The decrease in yield with tributylamine is
probably because the solubility of tributylamine is low in water.
The XRD patterns of the (a) Fe
c) fresh Fe
characteristic diffraction peaks in the samples at 2θ of 30.2º, 35.5º,
3.3º, 53.8º, 57.2º, and 62.8º correspond to the diffraction of
(220), (311), (400), (422), (511), and (440) of the Fe . All the
diffraction peaks match with the magnetic cubic structure of
3 4 3 4
O @UA, (b) Fe O @PUNP and
(
3 4
O @PUNP-Pd are presented in Fig. 3. The
4
5
5
6
0
5
0
1
5
3 4
O
23
3 4
Fe O (JCPDS 65-3107).
Excellent result was also obtained when inorganic base K
entry 5) was used. By comparison, NaHCO , a weaker inorganic
base, was less effective for the reaction (entry 4). As K CO was
2 3
CO
(
3
2
3
the most efficient base for the present catalytic system, it was
chosen for subsequent investigations. Further experiments
showed that 0.1 mol% amount of catalyst were sufficient to
guarantee a clean and complete conversion. When the amount of
catalyst was reduced to 0.05 mol% under the otherwise same
conditions as shown in entry 5, Table 1, the yield decreased to
7
2% (entry 6). On the other hand, using 0.2 mol% catalyst didn’t
Fig. 4 XPS Pd 3d spectra of (a) fresh Fe
3 4
O @PUNP-Pd and (b)
increase the yield significantly (entry 7).
2
0
3 4
Fe O @PUNP-Pd after one run.
Table 1 Optimization of the reaction conditions for Suzuki reaction of 4-
a
To establish the oxidation state of Pd in Fe
analysis was conducted, and the result is shown in Fig. 4. The
binding energy of Pd 3d5/2 and 3d3/2 for the fresh Fe @PUNP-
Pd is found to be 338.2 and 343.2 eV (Fig. 4a), indicating that the
3
O
4
@PUNP-Pd, XPS 65 iodoanisole with phenylboronic acid catalyzed by Fe
3
O
4
@PUNP-Pd
3 4
O
b
Entry
Base
Et
Piperidine
t-Bu
NaHCO
Catalyst (mol %) Yield (%)
24
2
5
loaded palladium is in its II state.
The TEM images of (a) Fe @UA, (b) Fe
fresh Fe @PUNP-Pd catalyst are presented in Fig. 5. The
obtained Fe @UA nanoparticles have an average diameter of
.05 ±1.91 nm (Fig. 5a). Fe @PUNP microgels have the
average diameter of 10.85±2.52 nm (Fig. 5b). After the loading
1
2
3
4
5
6
7
3
N
0.1
0.1
0.1
0.1
0.1
0.05
0.2
95
93
80
85
96
72
97
O
3 4
3 4
O @PUNP and (c)
3
N
3 4
O
3
3 4
O
K
2
CO
CO
CO
3
3
3
9
O
3 4
K
2
K
2
3
0
2+
a
of Pd (Fig. 5c), the fresh Fe
3 4
O @PUNP-Pd catalyst had an
Reaction conditions: 1 mmol 4-iodoanisole, 1.5 mmol phenylboronic
@PUNP-Pd catalyst, 2 mL deionized water and
average diameter of 11.55±2.14 nm. The nitrogen adsorption–
acid, 3 mmol base, Fe
argon atmosphere. Isolated yield.
3
O
4
b
desorption measurement indicates that the catalyst had a specific
2
-1
surface area of 31.9 m g (see ESI, Fig. S1).
With the optimized reaction conditions in hand, the scope of the
7
0
Fe
3 4
O @PUNP-Pd-catalyzed Suzuki reactions was investigated by
4
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Green Chemistry
a
3 4
Table 2 Fe O @PUNP-Pd-catalyzed Suzuki reactions in water
employing various substituted aryl halides to react with
arylboronic acid. The results are summarized in Table 2. With
iodobenzene and 4-iodoanisole as the reactants, the reaction
finished in 1 h, forming the corresponding coupling products in
yields of 98% and 96% (entries 1 and 2) respectively. The
Entry
Aryl halide
Product
Temp. Time Yield
b
(
ºC)
(h)
(%)
98
96
95
96
97
96
96
80
5
0
5
0
5
0
5
0
5
0
reaction of substituted bromobenzenes, such as p-OH, p-NO
2
, p-
1
2
3
4
5
6
7
8
90
90
90
90
90
90
90
90
1
COOH, p-COCH as well as bromobenzene, also afforded
3
satisfactory yields (95-97%) in 1 h (entries 3-7). The yield was a
little lower when 4-bromofluorobenzene was used as the substrate
(entry 8). The reaction of 4-bromotoluene with phenylboronic
acid was not complete in 1 h, but when the reaction time was
extended to 3 h, the coupling product was obtained in 91% yield
1
1
1
1
2
2
3
3
4
4
5
1
1
(
entry 9). On the other hand, the Suzuki reaction of 4-
bromoanisole with phenylboronic acid proceeded poorly under
the same conditions, and only 10 % yield was obtained. However,
when TBAB was added to the reaction mixture, a satisfactory
yield of 90% was achieved in 1 h (entry 10). Similar phenomena
were also observed when the protocol was applied to the ortho-
and meta-substituted aryl halides (entries 11 and 12). The
beneficial effect of TBAB was exhibited the most in the case of
COOH
1
1
1
1
3
1
1
1
1
40
91
10
9
90
90
90
3,5-ditrifluoromethyl bromobenzene, where the reaction hardly
c
1
0
90
took place without TBAB, but proceeded very well in its
presence (entries 13). It is noteworthy that hydrophilic 4-
chlorobenzoic acid could also react with phenylboronic acid by
40
c
11
92
1
5
5
28
53
83
the action of 0.1 mol% Fe
product in 68% yield (entry 14), albeit that elevated temperature
reflux) and longer time (5 h) were required.
3 4
O @PUNP-Pd to afford the coupling
1
2
90
c
(
Mechanistically, the Suzuki coupling reactions start with the
oxidative addition of C-X bond to Pd (0), and under all the
circumstances, the active catalyst is Pd (0). In cases where Pd (II)
is used as the catalyst, it has to be firstly reduced in situ to Pd (0).
13
1
3
Trace
90
c
9
0
1
5
Trace
68
1
4
reflux
During the preparation of Fe
reductant to reduce Pd (II) to Pd (0). In deed, the XPS analysis of
the freshly prepared Fe @PUNP-Pd indicates that there is no
3 4
O @PUNP-Pd, we did not use any
a
Reaction conditions: 1 mmol aryl halide, 1.5 mmol phenylboronic acid,
.1 mol% Fe @PUNP-Pd catalyst, 3 mmol K CO , 2 mL deionized
0
3
O
4
2
3
b
c
3 4
O
5
5
water and argon atmosphere. Isolated yield. 2 mmol TBAB was used.
Pd (0) species on its surface. Therefore, there must be some
species in the reaction system responsible for the generation of Pd
3 4
In order to evaluate the efficiency of the Fe O @PUNP-Pd
catalyst, the present catalytic system was compared with some
other magnetic nanoparticles-supported catalysts. As shown in
Table 3, for the reaction of bromobenzene with phenylboronic
acid, Fe
highest apparent turnover frequencies (TOF) of 950 h (entry 1).
In addition, with Fe @PUNP-Pd as the catalyst, the reaction
(
0). One possibility is that phenylboronic acid could act as the
30
reductant, as proposed by Köhler et al. On the other hand,
31
Veinot et al. recently reported a solid palladium catalyst (II)
supported on iron/iron oxide nanoparticles which are also
effective for the Suzuki coupling reactions. They believed that the
iron/iron oxide nanoparticles themselves could reduce palladium
6
0
3 4
O @PUNP-Pd was the most efficient catalyst, with the
-1
3 4
O
can be realized in water which is not only cheap and readily
available, but also non-toxic and non-flammable.
(
II). Considering the relevance of these studies to ours, we think
that both of these mechanisms might explain the high catalytic
capacity of Fe @PUNP-Pd. Evidence to support the hypothesis
3 4
O
was provided by the XPS study of the catalyst after reaction. As
shown in Fig. 4b, the XPS Pd 3d spectrum of the recovered
Fe
3 4
O @PUNP-Pd shows the binding energies of Pd 3d5/2 and Pd
3
d3/2 at about 335.2 and 340.6 eV respectively, which can be
31,32
attributed to the Pd (0) species.
This result confims tha
formation of Pd (0) during the reaction process.
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C3GC40941D
Green Chemistry
Page 6 of 9
Table 3 Performance of different magnetic nanoparticle-supported Pd
catalysts for the reaction of bromobenzene with phenylboronic acid
to effect in the reaction of aryl halides with vinyl substrates in
water. As shown in Table 4, the Heck reactions of various aryl
iodides with hydrophilic acrylic acid proceeded well in H O at
2
a
b
Entry
Pd catalyst Time (h)/ Medium Yield
TOF
Ref
-
1
(
mol% )
Temp
(
(%)
(h )
950
46.2
16.3
17.8
4.2
reflux temperature, resulting in the formation of cinnamic acids in
yields of 91-96 % (entries 1-4). The substituents at the phenyl
ring of aryl iodides had no significant effect on the reaction
consequence. Iodobenzene did not react with hydrophobic styrene
in H O, but when a mixed solvent of H O/DMF (1:1 by volume)
o
C)
@PUNP- 1/90
Pd (0.1)
3
3
4
0
5
0
1
2
3
4
5
Fe
3
O
4
H
2
O
95
This
work
32
Pd-Fe
3
O
1)
O
6)
4
@C
2/60
EtOH
EtOH
99
(
2
2
Pd-Fe
(
3
4
@C 1/reflux
98
33
34
35
was used, the reaction took place, and a satisfactory yield of 95%
was achieved in 8 h (entry 5). This protocol is also applicable to
aryl bromides. Both electron-withdrawing and electron-donating
group substituted aryl bromides, such as 4-bromoacetophenone,
Xerogel g1-
MNPs (1)
5 /60
MeOH
Toluene
89
Fe
Pd(OAc)
2）
3
O
4
-Bpy-
12/80
> 99
2
4
-nitroacetophenone and 4-bromotoluene, can be smoothly
（
converted to the corresponding coupling products in high yields
(entries 6-8).
6
7
8
Pd@Mag-
MSN (1)
6/80
Dioxane
77
12.8
47.5
101
36
31
37
Fe@Fe
0.5)
Pd/NiFe
00 (0.08)
x
O
y
/Pd 4/ R.T
H
2
O/EtOH 95
(1:1)
a
3 4
Table 4 Fe O @PUNP-Pd-catalyzed Heck reactions in water
(
2
O
4
-
12/80 NMP/H
（5:2）
(1) 24/reflux DME/H
3:1）
(1) 0.5/ 90 DMF/H
1:1)
2
O
97
70
98
3
9
Pd/Fe
3
O
4
2
O
2.9
38
39
Entry
Aryl halide
Product
Time Yield
b
（
(
h) (%)
1
0
Pd/NiFe
2
O
4
2
O
196
1
2
96
(
1
2
3
4
a
b
Isolated yield. The apparent TOF value was measured as moles of
12 91
94
product with per mole of Pd catalyst per hour.
1
2
5
Furthermore, the reaction kinetics of Suzuki reaction of 4-
iodoanisole with phenylboronic acid with 0.1 mol%
Fe O @PUNP-Pd as catalyst was investigated, and the result is
3 4
12 96
shown in Fig. 6a. The yield of 4-methoxybiphenyl increased
quickly with reaction time until it reached 96% at 60 min. The
1
2
2
trace
I
1
0
high efficiency of Fe
the uniformly dispersion Fe
3
O
4
@PUNP-Pd catalyst can be attributed to
@PUNP-Pd in the reaction
5
c
8
1
95
3 4
O
89
medium and its large surface area. Both factors would increase
the probability of contact between reactants and the active sites of
6
7
8
40
12 90
91
3 4
Fe O @PUNP-Pd, and thus improve the catalyst’s performance.
1
2
a
Reaction conditions: 1 mmol aryl halide, 1.5 mmol vinyl substrates, 0.1
mol% Fe @PUNP-Pd, 3 mmol K CO , 2 mL deionized water and
3
O
4
2
c
3
b
argon atmosphere. Isolated yield. A mixed solvent of H
by volume) was used.
2
O/DMF (1:1
4
5
3
3 4
.4 The reusability of the Fe O @PUNP-Pd catalyst
The reusability of the Fe @PUNP-Pd catalyst was evaluated
3 4
O
by using the Suzuki coupling of 4-bromoacetophenone with
phenylboronic acid as the model reaction. The procedure is
5
5
6
0
5
0
3 4
simple: the Fe O @PUNP-Pd catalyst was firstly applied to the
reaction, and then recovered after reaction complete by separating
it from the reaction mixture with an external permanent magnet,
2
washing it with H O and diethyl ether alternatively. It was reused
1
2
5
0
Fig. 6 The kinetic plots of Suzuki reaction of 4-iodoanisole with
phenylboronic acid. (a) Normal reaction kinetics; (b) Reaction kinetics
after the catalyst being removed from the reaction at 10 minutes. Reaction
conditions: 1 mmol 4-iodoanisole, 1.5 mmol phenylboronic acid, 0.1
directly for the next cycle without further treatment. Our
recycling experiment showed that the yield maintained in the
range of 94-96% before the fifth run, and still reached 88% at the
sixth run (Table 5).
Subsequently, the leaching of Pd after the sixth run was analyzed
by AAS, which gave the values of 0.24 ppm and 0.47 ppm in the
diethyl ether and aqueous phases respectively. The result
3 4 2 3
mol% Fe O @PUNP-Pd catalyst, 3 mmol K CO , 2 mL deionized water
and argon atmosphere. The yields are isolated yields.
3
.3 Fe
Encouraged by the satisfactory results with the Suzuki reaction,
the Fe @PUNP-Pd catalyst was applied to the Heck reaction
next. Toward this end, 0.1 mol% of Fe @PUNP-Pd was used
3 4
O @PUNP-Pd catalyzed Heck reaction in water
3 4
indicates that the Fe O @PUNP-Pd catalyst are stable under the
experiment conditions. A few catalytic tests were further
3 4
O
2
5
3 4
O
6
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C3GC40941D
Page 7 of 9
Green Chemistry
conducted to assess the impact of palladium leaching: (1) the
magnetic catalyst was removed from the reaction system after 10
This work was supported by the Fundamental Research Funds for
the Central Universities (lzujbky-2010-32) and the National
minutes and the reaction kinetics was monitored from then on. As 55 Science Foundation for Fostering Talents in Basic Research of
shown in Fig. 6b, after the catalyst was removed, the yield
stopped to increase, remaining almost constant (44-47%); (2) as
mentioned above, there was 0.47 ppm of Pd detected leaching
into the supernatant during the course of the reaction.
Correspondingly, a control experiment was conducted with the
the National Natural Science Foundation of China (J1103307).
5
Notes and references
a
Key Laboratory of Nonferrous Metal Chemistry and Resources
2
same equivalent of PdCl (0.47 ppm) as the catalyst. We found
6
6
0
5
Utilization of Gansu Province, College of Chemistry and Chemical
Engineering, Lanzhou University, 730000, China. Fax: +86 931 8912582;
Tel: +86 931 8912582; E-mail: bianfl@lzu.edu.cn (F.L.Bian)
1
0
that only a tiny amount of product (2%, GC yield) formed after
one hour, with most of the reactant remained. From these
analyses we concluded that effect of palladium leaching was
negligible for the present catalyst system.
b
b State Key Laboratory of Applied Organic Chemistry, Lanzhou
University, Lanzhou, 730000, China. Fax: +86 931 8912582; Tel: +86
931 8912582; E-mail: yuwei@lzu.edu.cn (W. Yu)
†
Electronic Supplementary Information (ESI) available: [BET surface
Table 5 Reusability of Fe
4-bromoacetophenone with phenylboronic acid
3 4
O @PUNP-Pd catalyst in the Suzuki reaction
a
1 13
1
5
analysis,
DOI: 10.1039/b000000x/
H
and
C NMR spectra data of the products]. See
Runs
Yield (%)
1
96
2
95
3
96
4
94
5
90
6
88
b
70
75
80
1
2
A. Molnár, Chem. Rev., 2011, 111, 2251-2320.
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a
Reaction conditions: 1 mmol 4-bromoacetophenone, 1.5 mmol
phenylboronic acid, 0.1 mol% Fe @PUNP-Pd, 3 mmol K CO
deionized water and argon atmosphere. Isolated yield.
4
3
O
4
2
3
, 2 mL
b
3
4
5
3 4
To gain a deep insight on the stability of Fe O @PUNP-Pd, its
structure was analyzed by FT-IR, XRD and TEM after being
recovered from reaction mixture. Fig. 1d shows the FT-IR
7
708.
2
2
3
3
0
5
0
5
6
7
8
9
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3
O
4
@PUNP-Pd after three runs. The weak band at
about 1710 cm indicates that the metal-ligand bond still exists in
the reused Fe @PUNP catalyst. Fig. 3d shows that the XRD
image of Fe @PUNP-Pd after the six run, from which we can
see that the crystalline phase of Fe did not change during the
reaction. Compared with Fig. 3c, new peaks appear at 2θ of 40.1 º,
6.0 º and 67.3º. These peaks are due to the (111), (200), and
-
1
3 4
O
1
1
0
P.-H. Li, L. Wang, L. Zhang and G.-W. Wang, Adv. Synth. Catal.,
3 4
O
2
012, 354, 1307-1318.
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O
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9
9
5
0
5
4
(
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that Pd (0) nanoparticles were formed during the reaction.
The TEM images of the recycled Fe O @PUNP-Pd shows that
3 4
after the first run, there were a part of palladium (0) nanoparticles
dispersed onto the surface of the magnetic nanoparticles support
(
aggregated into bigger particles on the surfaces of the matrices.
The aggregation of Pd probably led to the reduction in catalytic
activity after multiple runs.
1
4
15 D. Rosario-Amorin, M. Gaboyard, R. Clérac, L. Vellutini, S. Nlate
and K. Heuzé, Chem. Eur. J. 2012, 18, 3305-3315.
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Conclusions
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00
0 L.-F Shen, P. E. Laibinis and T. A. Hatton, Langmuir, 1999, 15, 447-
In summary, a novel heterogeneous Pd catalyst has been
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4
53.
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4
4
5
0
5
0
3 4
O
3 4
O
2
reactions in water, but also possesses high stability. It can be used
for at least six consecutive runs without significant loss of its
catalytic activity. In addition, as magnetic nanoparticles were
used as the solid support, the present catalyst can be simply
recovered from the reaction mixture by an external permanent
magnet, and then put to use again after washing with water and
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2
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3 4
ethyl ether. The merits associated with Fe O @PUNP-Pd
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Acknowledgements
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DOI: 10.1039/C3GC40941D
Green Chemistry
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| Journal Name, [year], [vol], 00–00
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Page 9 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C3GC40941D
Palladium supported on magnetic
microgel serves as an efficient and
recyclable
catalyst
for
the
water-phase Suzuki and Heck
reactions.