Palladium nanoparticles supported on functional ionic liquid modified magnetic nanoparticles as recyclable catalyst for room temperature Suzuki reaction

Article abstract of DOI:10.1016/j.tetlet.2012.11.009

Palladium nanoparticles were successfully immobilized on amine functionalized ionic liquid modified magnetic nanoparticles leading to a magnetically recoverable Pd catalyst, which exhibits high catalytic activity in the Suzuki coupling reaction at room temperature. The catalyst can be separated from the reaction mixture by applying a permanent magnet externally and can be reused for several times without significant loss of activity.

Full text of DOI:10.1016/j.tetlet.2012.11.009

Accepted Manuscript
Palladium nanoparticles supported on functional ionic liquid modified magnetic
nanoparticles as recyclable catalyst for room temperature Suzuki reaction
Jiayi Wang, Beiling Xu, Haiyang Sun, Gonghua Song
PII:
S0040-4039(12)01930-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2012.11.009
TETL 42101
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
15 August 2012
22 October 2012
2 November 2012
Please cite this article as: Wang, J., Xu, B., Sun, H., Song, G., Palladium nanoparticles supported on functional ionic
liquid modified magnetic nanoparticles as recyclable catalyst for room temperature Suzuki reaction, Tetrahedron
Letters (2012), doi: http://dx.doi.org/10.1016/j.tetlet.2012.11.009
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Palladium nanoparticles supported on
functional ionic liquid modified magnetic
nanoparticles as recyclable catalyst for room
temperature Suzuki reaction
Leave this area blank for abstract info.
Jiayi Wang, Beiling Xu, Haiyang Sun and Gonghua Song*
Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and
Technology, P. R. China. 200237
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H N
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Catal:
NH
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1
Tetrahedron Letters
journal homepage: www.elsevier.com
Palladium nanoparticles supported on functional ionic liquid modified magnetic
nanoparticles as recyclable catalyst for room temperature Suzuki reaction
Jiayi Wang, Beiling Xu, Haiyang Sun and Gonghua Song ∗
Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology, P. R. China. 200237
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Palladium nanoparticles were successfully immobilized on amine functionalized ionic liquid
modified magnetic nanoparticles leading to a magnetically recoverable Pd catalyst, which
exhibits high catalytic activity in Suzuki coupling reaction at room temperature. The catalyst can
be separated from the reaction mixture by applying a permanent magnet externally and can be
reused for several times without significant loss of activity.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
magnetic nanoparticles
Suzuki reaction
ionic liquid
palladium
It is well-known that palladium catalyzed Suzuki reactions
between organoboronic acids and halides are very important and
powerful methodologies for C-C bond formation in organic
synthesis.1 In general, soluble palladium complexes with
phosphine ligands are applied as the catalysts. One main
disadvantage of that approach is the difficulty in recovering and
reusing the expensive palladium catalysts. In view of the
increasing demand for economical, safe and environmental
benign processes in organic chemistry, much work has been
devoted to the development of Pd catalysts which can be
recovered by filtration, including immobilizing Pd on organic
polymers2 or inorganic solids3, supported Pd-NHC catalysts4 and
so on.5 Magnetic nanoparticles as promising supports for
immobilization of Pd catalysts, which can be easily separated
from the reaction medium by an external permanent magnet,
have gained more and more attention in the past years.6 However,
either prolonged reaction time or increased amount of Pd catalyst
was generally necessary. Thus, the development of effective and
stable magnetically recoverable Pd catalysts is highly desirable.
Pd-NHC complexes supported on iron oxide nanoparticle could
effectively catalyze Suzuki reactions with a little big amount of
o
Pd (7.3 mol%) at 50 C for 12 hours.9 Alper also reported Pt
nanoparticles supported on ionic liquid-modified magnetic
nanoparticles, which showed selectivity in the hydrogenation of
alkynes to cis-alkenes.10 He’s group also reported that the
Pd/(SiO2/Fe3O4) catalyst can be applied in Heck reactions,
although the reusing activity was poor.11 Those examples implied
that the linkers between magnetic nanoparticles and stabilizing
groups or ligands had important effects on catalytic activity.
Herein, we report a novel catalyst composed of a Fe3O4 core,
which making the catalyst magnetically recoverable, and a shell
consisting of Pd nanoparticles stabilized by an amine-
functionalized ionic liquid. The amine-functionalized ionic liquid
acts not only as an effective stabilizer to Pd nanoparticles, but
also making the catalyst more compatible with organic substrates
and solvent. The catalyst can efficiently catalyze the Suzuki
reactions with 0.5 mol% of Pd at room temperature, and can be
easily recovered and reused for several times without significant
loss of activity.
Ionic liquid has been widely used as a significant alternative
medium for the traditional organic synthesis and catalytic
reactions in recent years.7 It has been found that the metal
catalysts as well as their nanoparticles can be stabilized and have
better catalytic activities in ionic liquids.8 Gao reported that the
———
∗ Corresponding author. Tel.: +86 21 64253140; fax: +86 21 6425 2603; e-mail: ghsong@ecust.edu.cn

2
respectively, demonstrating that the ionic moiety was
successfully immobilized on silica substrates.
a
b
Figure 1. TEM images of freshly prepared Pd/IL-NH2/SiO2/Fe3O4 (a) and
after being reused three times (b).
(a)
(b)
Scheme 1. Preparation of Pd/IL-NH2/SiO2/Fe3O4.
10
20
30
40
50
60
70
80
The preparation of the catalyst Pd/IL-NH2/SiO2/Fe3O4 is
schematically described in Scheme 1. Firstly, commercially
available Fe3O4 nanoparticles with average diameter of 20 nm
were coated with silica to obtain the silica-coated Fe3O4
nanoparticles (SiO2/Fe3O4).6h Then, the SiO2/Fe3O4 nanoparticles
were functionalized with (3-chloropropyl) triethoxysilane by
refluxing in anhydrous toluene. Next, the ionic liquid moiety can
be anchored easily onto the surface of SiO2/Fe3O4 to obtain
amine functionalized ionic liquid modified magnetic
nanoparticles (IL-NH2/SiO2/Fe3O4).12 The loading amount of NH2
was 0.19 mmol per gram solid based on the nitrogen content
determination via CHN microanalyses. Finally, the IL-
NH2/SiO2/Fe3O4 was added to aqueous Na2PdCl4 solution with
mechanically stirring, followed by reducing Pd2+ to Pd0 with
hydrazine and dry under vacuum to afford the supported Pd
catalyst (Pd/IL-NH2/SiO2/Fe3O4).13 The loading amount of
palladium was 1.4 wt% as determined by inductively coupled
plasma atomic emission spectroscopy (ICP-AES). The as-
synthesized catalyst can be well dispersed in various polar or
nonpolar solvents, such as water, ethanol, acetone, DMF, ethyl
acetate, cyclohexane etc.
2 Theta (degree)
Figure 2. XRD patterns of (a) IL-NH2/SiO2/Fe3O4 and (b) Pd/IL-
NH2/SiO2/Fe3O4.
The synthesized catalyst was then applied in the Suzuki
coupling reactions. The catalytic activities of the Pd/IL-
NH2/SiO2/Fe3O4 catalyst were evaluated in the model reaction of
iodobenzene with phenylboronic acid in different solvents
o
without other ligands at room temperature (25 C) in air for 3
hours.14 The results are listed in Table 1. It was found that Pd/IL-
NH2/SiO2/Fe3O4 catalyst could effectively catalyze the mode
reaction with highest yield in ethanol-water (1:1, v/v) among the
solvents investigated (Table 1, entries 1-7). Relatively strong
inorganic base NaOH overmatched both weak inorganic base
K2CO3 and organic base NEt3 (Table 1, entries 7-9). Longer
reaction times were needed to complete the reactions when the
amount of catalyst decreased (Table 1, entries 10-12). It was also
noted that no homo-coupling product of phenylboronic acid was
observed after stirring for hours in the absence of iodobenzene,
which demonstrated good selectivity of the catalyst to Suzuki
reaction over the homo-coupling of phenylboronic acid. We then
chose the optimized reaction conditions for next investigation as
follows: 0.5 mol% Pd as catalyst, ethanol-water (1:1, v/v) as
solvent, NaOH as base, and reaction at room temperature (25 oC).
The catalyst was also characterized by TEM, XRD and FTIR.
The Pd nanoparticles were well dispersed on catalyst surface as
shown in Fig. 1a. XRD measurements of the catalyst exhibited
diffraction peaks of the spinel maghemite structure and layered
amorphous silica. The absence of peaks of palladium
nanoparticles (Fig. 2) implied that the Pd species were highly
dispersed on the catalyst surface.6d,6k,6l The FTIR spectrum of the
Pd/IL-NH2/SiO2/Fe3O4 catalyst demonstrated that no obvious
changes occur after immobilization of palladium on the
Using the optimized condition, the catalyst was applied to a
wide range of aryl iodides and bromides (Table 2). Both electron
rich and electron deficient aryl iodides were superior and
delivered products with good to excellent yields within 4 h.
(Table 2, entries 1-10). Prolonged reaction time was required for
the less active aryl bromides to fulfill the reaction (Table 2, entry
12-17). The sterically demanding aryl iodide also gave moderate
yield with a longer reaction time (Table 2, entry 11). The
coupling of 4-bromochlorobenzene and 4-iodochlorobenzene
with arylboronic acids delivered the products with high yields,
showing good selectivity (Table 2, entries 4, 8, 15 and 16). The
yield for the coupling of phenyl chloride with phenylboronic acid
was almost inactive even with strengthened reaction condition
(Table 2, entry 18). On the whole, the Pd/IL-NH2/SiO2/Fe3O4
magnetite
nanoparticles
surface
(see
Supplementary
Information), which was coincident with the results reported by
Ma 6o. The absorption band at 3420 cm-1 was attributed to both
symmetrical and asymmetrical modes of the O-H bonds attached
to the surface of silica. The absorption bands at 1090 and 588 cm-
1 were attributed to the Si-O bonds and Fe-O bonds respectively.
Alkyl C-H stretches were found at 2930 and 2850 cm-1, while
amine C-N and C=C stretch were found at 1380 and 1560 cm-1

3
catalyst showed better catalytic activity than the reported BIO–
times showed that the Pd nanoparticles were still well dispersed
IM–Pd,5h PS-Pd-SILC,6f Pd@Fe3O4,
Pd@SMP6p and Iron
and no obvious aggregation of Pd was observed (Fig. 1b). To
investigate the heterogeneity of the catalyst, we conducted a
filtration test for the Suzuki reaction between iodobenzene and
phenylboronic acid using Pd/IL-NH2/SiO2/Fe3O4 as catalyst. The
reaction was allowed to proceed for 1 h (GC yield: 87%), and
then the catalyst was separated. However, there’s no increasing
in yield of the desired product when the reaction was continued
for further 2 h after the catalyst was magnetically separated,
confirming the heterogeneous character of the catalyst.
6m
Oxide–Pd catalysts,9 with respect to either shorter reaction time,
room temperature reaction or less amount of catalyst.
Table 1. Optimization of the reaction conditionsa
Entry
1
2
3
4
5
6
7
8
Solvent
EtOH
EtOH
Acetone
DMF
H2O
Acetone: H2O (1:1)
EtOH: H2O (1:1)
EtOH: H2O (1:1)
EtOH: H2O (1:1)
EtOH: H2O (1:1)
EtOH: H2O (1:1)
EtOH: H2O (1:1)
Base
Time (h)
3
3
3
3
3
3
3
3
3
6
10
14
Yieldb (%)
56
71
4
trace
21
34
99
94
K2CO3
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
K2CO3
NEt3
9
66
99c
99d
99e
Figure 3. Photo of (a) the Pd/IL-NH2/SiO2/Fe3O4 catalyst in the reaction
mixture, (b) the catalyst dispersion in the reaction and (c) a magnet attracted
the magnetic stirring bar and catalyst.
10
11
12
NaOH
NaOH
NaOH
a For each reaction, 0.5 mmol iodobenzene, 0.75 mmol phenylboronic acid, 1
mmol base, 19 mg Pd/IL-NH2/SiO2/Fe3O4 (0.5 mol% Pd) and 3 ml solvent
a
Table 3. Recycle and reuse of Pd/IL-NH2/SiO2/Fe3O4
b
were used. The amount of Pd was based on iodobenzene. GC yields
Cycle
0
1
2
3
4
5
6
7
Yieldb (%)
99
99
99
91
95
93
96
98
(internal standard: dodecane). c 0.2 mol% Pd used. d 0.1 mol% Pd used. e 0.05
mol% Pd used.
a Reaction conditions were the same as that of entry 7 in Table 1.
b GC yields (internal standard: dodecane).
Table 2. Suzuki coupling reactions of aryl halides and
arylboronic acidsa
In conclusion, we have developed a convenient protocol to
prepare magnetically recoverable Pd catalyst by immobilizing
palladium nanoparticles on functional ionic liquid modified
magnetic nanoparticles. The amine functionalized ionic liquid
moieties immobilizing Pd nanoparticles in the Pd/IL-
NH2/SiO2/Fe3O4 catalyst result in excellent catalytic activity for a
wide range of aryl iodides and bromides in Suzuki coupling
reactions at room temperature. Moreover, the catalyst can be well
dispersed in the reaction medium, facile magnetically recovered
from reaction mixture and reused for several times without
significant loss of activity. All these advantages make the
protocol be a green and convenient process for other metal
catalyzed important reactions.
Entry
1
R1
H
X
I
R2
H
Time (h)
3
Yieldb (%)
96
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
p-Me
p-OMe
p-Cl
H
p-Me
p-OMe
p-Cl
H
p-Cl
o-CF3
H
p-Me
p-Me
p-Cl
p-Cl
m-NO2
H
I
I
I
I
I
I
I
I
H
H
H
p-Me
p-Me
p-Me
p-Me
m-NO2
m-NO2
H
H
H
p-Me
H
p-Me
H
3.5
3.5
3
3
3
4
3
3
3
18
5
6
4.5
6
4.5
10
24
90
88
96
98
98
91
94
90
92
60
87
82
92
86
98
Acknowledgments
I
I
Br
Br
Br
Br
Br
Br
Cl
Financial support for this work from National Basic Research
Program of China (973 Program) (Grant 2010CB126101),
National Key Technology R&D Program (Grant No.
2011BAE06B05-4) and DC Foundation are gratefully
acknowledged.
88
18
a
H
3c,d
For each reaction, 0.5 mmol aryl halide, 0.75 mmol phenylboronic acid, 1
mmol NaOH, 19 mg Pd/IL-NH2/SiO2/Fe3O4 (0.5 mol% Pd) and 3 ml ethanol-
References and notes
b
water (1:1, v/v) were used. The amount of Pd was based on aryl halides.
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c
d
Isolated yields. The reaction temperature is 80 oC. GC yield (internal
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The Suzuki coupling reaction of iodobenzene and
phenylboronic acid was used to test the reusability of the Pd/IL-
NH2/SiO2/Fe3O4 catalyst. After each cycle, the catalyst was
magnetically recovered (Fig. 3), successively washed with ethyl
acetate and water. After drying at room temperature, the
magnetically recovered catalyst was used for the next run. As
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catalyst was observed after seven cycles. The content of Pd in the
catalyst reused for five times remained 1.4 wt% as determined by
ICP-AES, which implied that the catalyst was highly active and
stable. The TEM image of the catalyst after being reused three

4
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nanoparticles (IL-NH2/SiO2/Fe3O4): SiO2/Fe3O4 (5.0 g) was
functionalized with (3-chloropropyl)triethoxysilane (15 mL, 62.4
mmol) by refluxing in toluene (200 mL) for 48 h, then imidazole
(8.4 g, 124.8 mmol) was added and the reaction was continued for
8 h. The nanoparticles were magnetically separated and washed
three times with ethanol (50 mL), and re-dispersed in acetonitrile
(200 mL) under mechanically stirring. Then, 2-bromoethanamine
hydrobromide (10.1 g, 50 mmol) was added and the reaction was
o
continued for 24 h at 80 C. The nanoparticles were magnetically
separated and washed three times with ethanol (50 mL), and re-
dispersed in water (200 mL) under mechanically stirring. Then, 2
g of NaOH (50 mmol) in 50 mL of water was added. The stirring
was continued for 4 h. The nanoparticles were magnetically
separated and washed three times with ethanol (50 mL), dried
under vacuum for 12 h. The brownish-black powder of IL-
NH2/SiO2/Fe3O4 (4.5 g) could be obtained. The loading amount of
NH2 was 0.19 mmol per gram, which was quantified via CHN
microanalyses based on the nitrogen content determination. IR
spectrum (KBr, cm-1): 3420, 2930, 2840, 1630, 1540, 1440, 1400,
1090, 964, 810, 638, 588 cm-1.
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13. Preparation of Pd/IL-NH2/SiO2/Fe3O4: A mixture of IL-
NH2/SiO2/Fe3O4 (1.0 g) and Na2PdCl4 (2.5 mM, 100 mL) was
mechanically stirred for 72 h. The nanoparticles were
magnetically separated and washed three times with water (50
mL), and re-dispersed in water (100 mL) under mechanically
stirring. Then, hydrazine hydrate (80%) (1 mL) was added. The
stirring was continued for 18 h. The nanoparticles were
magnetically separated and washed three times with ethanol (50
mL), dried under vacuum to afford the desired Pd/IL-
NH2/SiO2/Fe3O4 (0.95 g) as brownish-black powder. IR spectrum
(KBr, cm-1): 3420, 2930, 2850, 1630, 1560, 1420, 1380, 1090,
960, 804, 633, 588 cm-1. The content of Pd in the catalyst was 1.4
wt% as determined by ICP-AES.
14. General procedure for Suzuki reaction: The Pd/IL-
NH2/SiO2/Fe3O4 catalyst (19 mg, Pd: 0.5 mmol %), aryl halide (0.5
mmol), arylboronic acid (0.75 mmol), base (1 mmol), and solvent
(3 mL) were added into a 10 mL glass test tube. Then the reaction
was allowed to proceed at room temperature (25 oC). After the
reaction was finished as monitored by thin layer chromatography
(TLC), the catalyst was magnetically separated and washed two
times with ethyl acetate (1 mL). To obtain the GC yield, the
combined organic phase was analyzed by a gas chromatograph
(GC) equipped with a flame ionization detector (FID) detector
using dodecane as an internal standard (Angilent 7890A, HP-5, 30
m × 0.32 mm). To obtain the isolated yield, the combined organic
phase was concentrated and the products were purified by column
chromatography over silica gel. For reuse of the catalyst, after
magnetically recovered, the catalyst was successively rinsed with
ethyl acetate (1 mL × 2) and water (0.5 mL), and dried at room
temperature ready for the next cycle.