Palladium supported on poly (ionic liquid) entrapped magnetic nanoparticles as a highly efficient and reusable catalyst for the solvent-free Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2015.02.047

A novel palladium catalyst supported on poly (1-aminoethyl-3-vinylimidazolium bromide) entrapped magnetic nanoparticles (Pd/Fe₃O₄@PIL-NH₂) was prepared. The catalyst was characterized by TEM, FT-IR, VSM, XRD, and XPS. The Pd loading on the catalyst was 0.23 wt % as measured by AAS. The catalyst showed excellent activity for the solvent-free Heck reaction. In addition, this novel Pd catalyst could be simply recovered with an external magnet and run five times with excellent yields achieved.

Full text of DOI:10.1016/j.tetlet.2015.02.047

Accepted Manuscript
Palladium supported on poly (ionic liquid) entrapped magnetic nanoparticles as
a highly efficient and reusable catalyst for the solvent-free Heck reaction
Wendong Liu, Dongfang Wang, Yajing Duan, Yahui Zhang, Fengling Bian
PII:
S0040-4039(15)00318-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.02.047
TETL 45916
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
10 December 2014
3 February 2015
13 February 2015
Please cite this article as: Liu, W., Wang, D., Duan, Y., Zhang, Y., Bian, F., Palladium supported on poly (ionic
liquid) entrapped magnetic nanoparticles as a highly efficient and reusable catalyst for the solvent-free Heck reaction,
Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.02.047
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Graphical Abstract
Palladium supported on poly (ionic liquid) entrapped
magnetic nanoparticles as a highly efficient and
reusable catalyst for the solvent-free Heck reaction
Wendong Liu, Dongfang Wang, Yajing Duan, Yahui Zhang, Fengling Bian *

1
Tetrahedron Letters
journal homepage: www.els evier.com
Palladium supported on poly (ionic liquid) entrapped magnetic nanoparticles as
a highly efficient and reusable catalyst for the solvent-free Heck reaction
Wendong Liu a, Dongfang Wang a, Yajing Duan a, Yahui Zhang a, Fengling Bian a ,∗
a College of Chemistry and Chemical Engineering, Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou
University, 730000, PR China
ARTICLE INFO
ABSTRACT
Article history:
Received
A novel palladium catalyst supported on poly (1-Aminoethyl-3-vinylimidazolium bromide)
entrapped magnetic nanoparticles (Pd/Fe3O4@PIL-NH2) was prepared. The catalyst was
characterized by TEM, FT-IR, VSM, XRD and XPS. The Pd loading on the catalyst was 0.23
wt% as measured by AAS. The catalyst showed excellent activity for the solvent-free Heck
reaction. In addition, this novel Pd catalyst could be simply recovered with an external magnet
and run five times with excellent yields achieved.
Received in revised form
Accepted
Available online
Keywords:
Palladium
poly (ionic liquid)
magnetic nanoparticles
solvent-free
2014 Elsevier Ltd. All rights reserved.
Heck reaction
Heck reaction has proved to be a powerful method for C-C
bond formation in organic synthesis since it was reported by
Heck in 1968.1 In general, homogeneous palladium catalysts are
mostly used in the Heck reaction owning to their high efficiency.
However, the soluble palladium complexes suffer from
drawbacks associated with the catalyst separation and recovery
after reaction. A promising route to solve the problems is
heterogenize of homogeneous catalysts,2 and many support
materials have been explored to immobilize palladium such as
polymers, silica particles and metal oxides.3-6 Among the support
materials, magnetic nanoparticles can simplify the recovery
procedure with magnetic separation due to their magnetic
properties.7 Furthermore, magnetic nanocatalysts have the
advantage of nano size, which means the exposed surface area of
the active component of the nanocatalyst is high, and the activity
of catalyst may be enhanced consequently.8
catalysts exhibit good catalytic performance, their recovery is
still not easy due to the cumbersome and time-consuming
filtration or centrifugation.
In view of the advantages of magnetic nanoparticles and poly
(ILs), some magnetic poly (ILs) composites have been
developed.22-24 However, as far as we know, there is no literature
reported on applying the magnetic poly (ILs) supported Pd
catalyst to the solvent-free Heck reaction. It’s environmentally
benign and economically profitable to perform Heck reaction in
solvent-free condition. Herein, we wish to report a novel Pd
catalyst prepared by loading palladium on poly (amine-
functionalized ionic liquid) entrapped Fe3O4 nanoparticles. This
Pd catalyst (Pd/Fe3O4@PIL-NH2) exhibited higher efficiency for
the solvent-free Heck reaction compared with other supported Pd
catalysts reported in recent years,18,25-29 and could be used
consecutively for five times with excellent yields achieved. In
addition, the magnetic property of the catalyst made it very easy
to be recovered and recycled with a magnet after each run.
In the last few years, Ionic liquids (ILs) have attracted much
attention in catalytic systems because they could not only
stabilize the reaction catalytic species, but also improve the
catalytic activity.9 However, the properties of high viscosity and
homogeneity of ILs also limit their widespread use in organic
transformations. Recently, the concept of supported ionic liquid
phase (SILP) catalysts has emerged by combining the advantages
of ILs with various heterogeneous supports and functional groups
or active species (e.g. Brønsted acidic groups and metals).10
Based on this concept, many supported IL catalysts had been
developed for the C-C bond formation reactions.11-16 For example,
some poly (ILs) supported Pd catalysts had been prepared and
The procedure for synthesis of Pd/Fe3O4@PIL-NH2 was
depicted in Scheme 1. Firstly, Fe3O4 nanoparticles were
synthesized by chemical coprecipitation method,30 and then were
modified with 3-methacryloxypropyltrimethoxysilane (MPS) to
give Fe3O4~MPS.31 Secondly, the magnetic support Fe3O4@PIL-
NH2 was prepared by free radical polymerization of Fe3O4~MPS
and 1-aminoethyl-3-vinylimidazolium bromide hydrobromide
([VAIM]Br·HBr). Finally the catalyst Pd/Fe3O4@PIL-NH2 was
obtained by immobilizing PdCl2 on Fe3O4@PIL-NH2 and then
reducing Pd2+ to Pd0 with NaBH4. The amount of palladium in
applied to the Suzuki and Heck reaction.17-21 Although these
———
∗ Corresponding author. Tel.: +86-931-8912582; fax: +86-931-8912582; e-mail: bianfl@lzu.edu.cn

2
Tetrahedron
the catalyst was determined by atomic absorption spectroscopy
(AAS) to be 0.23 wt%.
Supplementary information Fig. S1). Both weight losses below
o
120 C are attributed to the loss of absorbed water on the
magnetic nanoparticles.22 In the range of 120-550oC, the weight
loss of 8.1 wt% for Fe3O4~MPS and the weight loss of 10.9 wt%
for Fe3O4@PIL-NH2 may prove that Fe3O4~MPS and
Fe3O4@PIL-NH2 have been fabricated successfully. According to
the weight losses, the ionic liquid content of Fe3O4@PIL-NH2 is
calculated to be 0.13 mmol g-1. VSM curves (see Supplementary
information Fig. S2) reveal that the resulted Fe3O4, Fe3O4@PIL-
NH2 and Pd/Fe3O4@PIL-NH2 exhibited superparamagnetic
behavior, and the saturated magnetization values are 79.07, 62.41
and 59.46 emu g-1, respectively.
The transmission electron microscope (TEM) images (Fig. 1)
show that magnetic nanoparticles Fe3O4, Fe3O4@PIL-NH2 and
Pd/Fe3O4@PIL-NH2 are all nearly spherical with average
diameters of 9.8 nm (Fig. 1a), 13.0 nm (Fig. 1b) and 13.2 nm
(Fig. 1c) respectively. Hence, this prepared catalyst may disperse
well in the catalytic system due to its nano size.8 In the FT-IR
spectra (Fig. 2), the samples all show Fe-O stretching vibration
bands at 584 cm-1.30 Besides, Fig. 2a shows the characteristic
stretching vibration of C-H (2931 cm-1), C=O (1705 cm-1) and
C=C (1635 cm-1) of the MPS groups, which demonstrate the
successful preparation of Fe3O4~MPS.32 In Fig. 2b, the band at
3419 cm-1 is attributed to the N-H stretching vibration of
[VAIM]Br. Absorbance bands at 1644 cm-1 and 1538 cm-1 are
attributed to the stretching vibration of C=N and C=C of
imidazolium rings, respectively.24,32 The deformation vibration of
N-H overlaps with the C=C adsorption at 1538 cm-1.33 All the
characteristic bands in Fig. 2b demonstrate that Fe3O4@PIL-NH2
is successfully prepared. The spectrum of Pd/Fe3O4@PIL-NH2
(Fig. 2c) shows no obvious change compared with Fig. 2b. The
stretching vibration (3419 cm-1) and deformation vibration (1538
cm-1) of N-H are slightly weaker and stronger respectively, which
may result from the coordination between NH2 and Pd
nanoparticles.33
X-ray diffraction (XRD) patterns (see Supplementary
information Fig. S3) of Fe3O4, Fe3O4@PIL-NH2 and
Pd/Fe3O4@PIL-NH2 all show diffraction peaks of standard Fe3O4
sample (JCPDS file No. 65-3107). In the XRD pattern of
Pd/Fe3O4@PIL-NH2, the absence of peaks of Pd nanoparticles
indicates that Pd nanoparticles may be highly dispersed on the
support.34-36 X-ray photoelectron spectroscopy (XPS) was applied
to determine the chemical state of palladium in fresh prepared
Pd/Fe3O4@PIL-NH2 (Fig. 3). The peak binding energies of 335.9
eV and 341.0 eV correspond to 3d5/2 and 3d3/2 of Pd0, while the
peak binding energies of 337.0 eV and 342.0 eV correspond to
3d5/2 and 3d3/2 of Pd2+.18,25 The area related to Pd2+ is much
smaller compared with that of Pd0, indicating that most palladium
Figure 1. TEM images of Fe3O4 (a), Fe3O4@PIL-NH2 (b),
Pd/Fe3O4@PIL-NH2 (c) and Pd/Fe3O4@PIL-NH2 after four
runs (d).
Thermal gravimetric analysis (TGA) was used to measure the
weight loss of Fe3O4~MPS and Fe3O4@PIL-NH2 (see

3
in the catalyst is in the form of Pd0.
the yield did not increase any more, which means that 40 min
was sufficient for the present reaction.
Table 1 Optimization of the Heck reaction conditionsa
CO2Bu-n
With the optimized conditions in hand, various substituted aryl
halides and olefins were investigated for the solvent-free Heck
reaction and the results were summarized in Table 2. In general,
the reaction afforded the desired products in good to excellent
isolated yields (85-97%) for aryl iodides and olefins (Table 2,
entries 1-17). Within the same reaction time (25 min),
iodobenzene bearing a methoxy group (Table 2, entries 7-9) in
ortho position gave lower yield (82%) than that with the methoxy
group at para or meta position due to steric hindrance.37 It is
noteworthy that when aryl iodides with electron-withdrawing
group (Table 2, entries 12-14) reacted with methyl acrylate, high
yields (91-94%) were achieved in 15 min, and the highest TOF
value (34182 h-1) emerged here. Furthermore, for 1-
iodonaphthalene (Table 2, entry 15), a yield of 96% was obtained
despite that 150 min was required; extension in reaction time is
probably caused by the hindrance of neighboring hydrogen atom
at 8 position of the naphthalene ring. In addition, the coupling of
methyl acrylate and styrene with 2-iodothiophene (Table 2,
entries 16, 17) gave the corresponding products in 80% (180 min)
and 34% (6 h) yields, respectively. Moreover, the isolated yields
could be further improved up to 95% and 71% respectively by
increasing the amount of [Pd] catalyst to 0.044 mol%.
Base,Catalyst
CO2Bu-n
I
+
Solvent-free, 120 o
C
Entry
Base
Catalyst (mol%)
0.011
0.011
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
NaOAc
Na2CO3
n-Bu3N
Et3N
Et3N
Et3N
40
40
40
40
40
40
30
60
trace
trace
90
0.011
0.011
0.022
93
93
0.0055
0.011
0.011
69
88
93
Et3N
Et3N
a Reaction conditions：iodobenzene, 1.0 mmol; n-butyl acrylate, 1.5 mmol;
Et3N, 1.5 mmol; [Pd] catalyst (Pd/Fe3O4@PIL-NH2); at 120 oC, under argon.
b Isolated yields.
Following the successful preparation of Pd/Fe3O4@PIL-NH2,
it was then applied to the solvent-free Heck reaction of various
aryl halides and olefins. In a model reaction of iodobenzene and
n-butyl acrylate, different reaction parameters such as base,
catalyst dosage and reaction time were optimized. As shown in
Table 1, trace products were given when using inorganic bases
(Table 1, entries 1, 2), which might be attributed to the poor
solubility of inorganic bases in the reaction system. When using
organic bases, excellent yields were obtained (Table 1, entries 3,
4), and Et3N was better for the present reaction than n-Bu3N.
Next, the effect of catalyst amounts was investigated (Table 1,
entries 4-6): when the amount of catalyst was raised from 0.0055
mol% to 0.011 mol%, the isolated yields increased from 69% to
93%; using more catalyst did not improve the yield further. In
addition, three reaction time were examined (Table 1, entries 4,
7, 8). Isolated yields rose from 88% to 93% as reaction time rose
from 30 min to 40 min. By prolonging reaction time to 60 min,
In view of the high efficiency of Pd/Fe3O4@PIL-NH2 for aryl
iodides, the catalyst was then applied to catalyze the solvent-free
Heck reaction of aryl bromides and aryl chloride with methyl
acrylate with 0.088 mol% [Pd] catalyst used at 130 oC (Table 2,
entries 18-20). It showed that 4-bromonitrobenzene and 3-
bromoquinoline achieved 80% (10 h) and 79% (24 h) isolated
yields, respectively. However, 4-chloronitrobenzene gave trace
product in 24 h under the same condition.
Table 2 Solvent-free Heck reaction of aryl halides and olefins catalyzed by Pd/Fe3O4@PIL-NH2a
R2
R1
R
R1
Et3N, Catalyst
X
+
R2 Solvent-free, 120 oC
R
Entry
1
Aryl halides
Alkenes
Products
T (min)
25
Yieldb (%)
94
TOFc (h-1)
20509
2
3
4
5
40
95
93
91d
92
12955
12682
19855
2952
40
25
170
6
7
100
92
5018
I
25
35
82
95
17891
14805
O
8
25
30
89
94
19418
17091
I
O

4
Tetrahedron
9
25
50
97
91
21164
9927
10
11
12
180
15
85
91
2576
33091
I
F
13
14
15
15
94
94
96
34182
34182
3491
I
Br
15
150
16
17
18
180
80
95e
2424
720
6 (h)
10 (h)
34
71e
515
269
80f
91
37
19
20
24 (h)
24 (h)
79f
Tracef
NO2
Cl
a Reaction conditions：aryl halides, 1.0 mmol; olefins, 1.5 mmol; Et3N, 1.5 mmol; [Pd] catalyst (Pd/Fe3O4@PIL-NH2), 0.011 mol%; at 120 oC, under argon.
b Isolated yields.
c TOF: [mol product][mol Pd]-1 h-1.
d Et3N, 3.0 mmol.
e [Pd] catalyst (Pd/Fe3O4@PIL-NH2), 0.044 mol%.
f [Pd] catalyst (Pd/Fe3O4@PIL-NH2), 0.088 mol%, at 130 oC.
In order to further evaluate the catalytic activity of
Pd/Fe3O4@PIL-NH2 in the present work, we compared this
catalyst with other supported catalysts reported recently for the
solvent-free Heck reaction of iodobenzene and n-butyl acrylate. It
clearly showed that Pd/Fe3O4@PIL-NH2 was the most efficient
catalyst with the highest TOF value of 12682 h-1 for the coupling
reaction (Table 3).
The reusability of supported catalysts is an important
parameter for practical application. Therefore, the reusability of
Pd/Fe3O4@PIL-NH2 for the solvent-free Heck reaction of
iodobenzene and methyl acrylate was also investigated. As seen
in Table 4, the catalyst could be used for five consecutive times
with excellent isolated yields despite that longer reaction time
was required after each run. Palladium leaching (0.08 ppm of Pd
was detected in the combined organic phase after first run

5
measured by AAS) and aggregation of the catalyst nanoparticles
(see TEM image of Fig. 1d) might be responsible for the
prolonging of reaction time. Nonetheless, 94% yield was still
obtained within 150 min at the fifth run, reflecting that
Pd/Fe3O4@PIL-NH2 kept high activity after being recovered. In
addition, the magnetic property of Pd/Fe3O4@PIL-NH2 made it
easily to be separated and recycled by an external magnet.
References and notes
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Entry Cat (mol %)
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(%)
(h-1)
Ref
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1
2
3
4
5
6
7
Pd/Fe3O4@PIL
-NH2 (0.011)
Et3N, 120 oC, 93
40 min
12682 This
Work
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PDVB-IL-Pd
(0.02)
Et3N, 120 oC, 95c
6 h
792
18
25
26
27
28
29
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SBA-TMG-Pd
(0.01)
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2 h
4650
184
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Fe2O3 (1)
Et3N, 100 ºC, 92
30 min
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n-Pr3N,
oC, 1 h
130 99
140 92
140 94
198
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6133
4029
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Pdnp/Pectin
(0.28)
n-Pr3N,
ºC, 5 min
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a Isolated yields.
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acrylatea
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1
2
3
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5
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Yieldb (%)
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94
30
96
35
93
65
93
150
94
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In conclusion, we have prepared a novel Pd catalyst supported
on poly (ionic liquid) entrapped magnetic nanoparticles. The
catalyst was highly effective for the solvent-free Heck reaction.
The catalyst could be used for five times with excellent yields
achieved. Along with the high activity of the catalyst, another
useful advantage of the catalyst is that it can be simply separated
from the reaction system and recovered with an external magnet,
a property which is attractive from the viewpoint of green
chemistry.
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Acknowledgments
37. Wang, Y. L.; Luo, J.; Liu, Z. L. J. Organomet. Chem. 2013, 739,
1-5.
The authors thank the National Science Foundation for
Fostering Talents in Basic Research of the National Natural
Science Foundation of China (J1103307).
Experiment details see Supplementary information.