Highly active and magnetically recoverable Pd-NHC catalyst immobilized on Fe3O4 nanoparticle-ionic liquid matrix for suzuki reaction in water

Article abstract of DOI:10.1055/s-0029-1217731

Pd-NHC-ionic liquid matrix was immobilized into ionic liquid layers coated on the surface of Fe3O4. The immobilized Pd-NHC was used as a catalyst for Suzuki coupling. The catalyst exhibited both high catalytic activity and stability for the coupling between aryl bromide and arylboronic acid in water. This catalyst was simply recovered by an external permanent magnet and recycled without a significant loss in the catalytic activity.

Full text of DOI:10.1055/s-0029-1217731

LETTER
2477
Highly Active and Magnetically Recoverable Pd-NHC Catalyst Immobilized
on Fe3O4 Nanoparticle–Ionic Liquid Matrix for Suzuki Reaction in Water
C
atalyst Immobiliz
b
ed on
N
anop
u
article–Ionic
L
iqui
T
d
Matrix aher, Jin-Beom Kim, Ji-Young Jung, Wha-Seung Ahn, Myung-Jong Jin*
School of Chemical Science and Engineering, Inha University, Incheon 402-751, Korea
Fax +82(32)8720959; E-mail: mjjin@inha.ac.kr
Received 21 April 2009
able research efforts have been devoted to the develop-
Abstract: Pd-NHC–ionic liquid matrix was immobilized into ionic
liquid layers coated on the surface of Fe3O4. The immobilized Pd-
NHC was used as a catalyst for Suzuki coupling. The catalyst exhib-
ited both high catalytic activity and stability for the coupling be-
ment of efficient heterogeneous catalysts. Ionic liquids
have received much attention as eco-friendly alternatives
to volatile organic solvents for catalytic reactions.3 Use of
tween aryl bromide and arylboronic acid in water. This catalyst was ionic liquids has been also studied to recycle homoge-
simply recovered by an external permanent magnet and recycled
without a significant loss in the catalytic activity.
neous catalysts. However, the reaction system requires a
large amount of expensive ionic liquid. It would be desir-
Key words: heterogeneous catalysis, Suzuki reaction, palladium,
magnetic nanoparticle, ionic liquids
able to reduce the amount of ionic liquid phase from an
economic and toxicological viewpoint.
Since the pioneering work of Mehnert, ionic liquids have
been used as support media combined into solid support
materials.4 This key feature was to coat thinly the surface
of silica gel with ionic liquid containing homogeneous
catalyst.5 As such, a major emphasis has been placed on a
facile method to construct heterogeneous catalysts with
one-step process. However, the leaching of catalysts is re-
sponsible for severe drawbacks in some cases. There is
still the need to develop more efficient and convenient
ionic liquid support catalysts. Our interest in this area has
prompted us to explore the immobilization of palladium–
N-heterocyclic carbene (Pd-NHC) complex with the sup-
ported ionic liquid. In this regard, we envisaged that sim-
ply recoverable Fe3O4–ionic liquid (IL) hybrid matrix
One practical limit to performing homogeneously cata-
lyzed reactions is the difficulty of removing the catalyst
from reaction mixture or separating the product continu-
ously. This limit is of significant environmental and eco-
nomic concern in large-scale organic syntheses. The use
of heterogeneous catalysts would be an attractive solution
to this problem because of their easy separation and facile
recycling.1 Therefore, successful development of homo-
geneous catalysts has been often followed by attempts to
immobilize the catalysts on a variety of insoluble supports
including cross-linked polymers and inorganic oxides.2
However, most of the heterogeneous catalysts show lower
activity than their homogeneous counterparts. Consider-
N
N
Cl–
Cl–
N
N
N
N
Cl–
Cl–
N
MeO
Si
N
MeO
N
N
Si
OH
N
N
OMe
O
O
OMe
OH
O O
Cl–
HO
Cl–
MeO Si
MeO
Si
O
OH
OH
OH
O
O
HO
O
O
O
O
O
HO
HO
Cl–
OMe
Si
Cl–
OMe
Si
i
O
O
O
O
O
ii
O
O
N
Fe3O4
N
Si
MeO
N
Fe3O4
N
Si
N
Cl–
Fe3O4
N
N
N
O
Cl–
MeO
OH
OH
HO
HO
O
O
O
O
O
O
Si
O
O
Si
OMe
Si
OMe
Si
OH
HO
Cl–
O
O
Si OMe
Cl–
OH
O
O
OMe
N
OMe
N
N
Si OMe
N
N
N
Cl–
Cl–
N
N
N
N
Cl–
Cl–
hydrophobic
environment
NH
N
2
3
Me
Bu
Bu
–
N
PF6
i) [tmim][Cl], toluene, 100 °C
ii) [bmim][PF6] + Pd-NHC 1, CH2Cl2, r.t.
–
N
N
N
PF6
=
=
Pd
N
– N
Me
PF6
Me
Bu
[bmim][PF6]
Pd-NHC
1
Scheme 1
SYNLETT 2009, No. 15, pp 2477–2482
x
x
.x
x
.2
0
0
9
Advanced online publication: 27.08.2009
DOI: 10.1055/s-0029-1217731; Art ID: U04009ST
© Georg Thieme Verlag Stuttgart · New York

2478
A. Taher et al.
LETTER
would be served as an attractive support material. Fe3O4
nanoparticles have been widely studied for biomedical ap-
plications such as targeted drug delivery, MRI contrast
enhancement, and biosensors.6 Recently, magnetite nano-
particles have emerged as promising supports for immo-
bilization.7 Most of immobilized catalysts are separated
by filtration or centrifugation. In contrast, magnetite-sup-
ported catalysts can be separated from the reaction system
by an external permanent magnet. This will circumvent
time-consuming and laborious separation steps. There-
fore, it makes possible the practical application for contin-
uous catalysis. In particular, the surface of nano-Fe3O4
having plenty of hydroxyl groups can be easily modified
with alkoxysilane reagents. Herein, we report the highly
active and magnetically recoverable Pd-NHC@Fe3O4–IL
catalyst for the Suzuki coupling reaction in water. Pd-
NHC compounds are well known to be a versatile class of
catalysts in homogeneous catalysis.8,9 Pd-NHC 1 was cho-
sen as a catalyst source for this study.
According to the reported method,10 Pd-NHC 1 was pre-
pared from 1-butyl-3-methylimidazolium hexafluoro-
phosphate {[bmim][PF6]} and Pd(OAc)2, which was
immobilized into thin layers of [bmim][PF6] coated on the
surface of Fe3O4 as follows (Scheme 1). Commercially
available nano-sized Fe3O4 was modified with a silanized
ionic liquid in order to enhance the interaction between
the surface and free ionic liquid. Modified Fe3O4 2 was
prepared by treatment of Fe3O4 with 1-(3-trimethoxysilyl-
propyl)-3-methylimidazolium chloride {[tmim][Cl]}11 in
refluxing toluene.12 The loading capacity of Fe3O4 was
confirmed to be 0.67 mequiv/g of [tmim][Cl] via both el-
emental analysis and weight gain. Pd-NHC@Fe3O4-IL 3
was obtained by immersing the modified Fe3O4 2 in
CH2Cl2 solution of 1 and [bmim][PF6], followed by evap-
orating CH2Cl2 from the mixture and washing with diethyl
ether.13 Pd-NHC complex 1 was encapsulated in the ionic
liquid layers during this process. The weight ratio of 3
is Fe3O4/[bmim][PF6]/Pd-NHC = 1.0/0.10/0.15, in which
the molar ratio of [bmim][PF6]/Pd-NHC is 1.6:1. Induc-
tively coupled plasma (ICP) analysis confirmed that the
Pd content is 0.17 mmol/g for 3. The absorbed ionic liquid
spreads as thin layers of free ionic liquid on the surface of
Fe3O4 support. The Pd-NHC in the ionic liquid layers
would act like a homogeneous species. This system thus
may possess the advantages of both homogeneous and
heterogeneous catalysts to some extent. Figure 1 shows
HR-TEM images of Pd-NHC@Fe3O4-IL 3 nanoparticles.
The majority of the particles is around 30–40 nm in size.
No formation of Pd particles could be found after the re-
action, which indicates that Pd-NHC 1 has been homoge-
neously embedded in ionic liquid layers on the surface of
Fe3O4 support.
Figure 1 HR-TEM images of Pd-NHC@Fe3O4-IL 3 (a) before use
and (b) after the first recovery
erally performed in organic solvents or water–organic sol-
vents. The use of water as an environmentally benign
solvent has recently attracted much attention from the
viewpoint of green chemistry. However, the coupling in
water is considerably limited by poor solubility of the sub-
strates. We reasoned that this problem might be solved by
a hydrophobic environment involving the ionic liquids
immobilized on the Fe3O4 support. Moreover, the surface
coated with hydrophobic ionic liquid can render the active
sites durable in water, resulting in less extent of leaching.
The Suzuki coupling of aryl bromide with phenylboronic
acid was tested in the presence of 0.5 mol% of 3 and
K3PO4 as base (Table 1).16 Aryl iodides have been em-
ployed as suitable coupling substrates in most of the het-
erogeneous coupling reactions. The use of inexpensive
aryl bromides would be highly desirable for the large-
scale applications. The initial investigation started with
the coupling between bromobenzene and phenylboronic
acid in pure water. Biphenyl was obtained in 90% conver-
sion at 85 °C after six hours (entry 1). A phase-transfer
agent, tetrabutylammonium bromide (TBAB), was added
to enhance the reactivity of the reaction. This reaction pro-
ceeded rapidly to afford the product in excellent yield (en-
try 2). It seems that the addition of TBAB increases the
substrate solubility in water. This catalytic system was
then applied to the coupling reactions of various aryl bro-
mides with phenylboronic acid.
Activated aryl bromides such as 1-bromo-4-nitrobenzene,
1-bromo-3-nitrobenzene, and 4-bromoacetophenone were
rapidly coupled in quantitative yields (entries 3–5). These
reactions proceeded well even at low temperature of 40 °C
(entries 6 and 7). High reactivity for deactivated 4-bro-
moanisole, 2-bromoanisole, 4-bromotoluene, and 2-bro-
motoluene was still observed (entries 8–11). As expected,
aryl bromides with electron-withdrawing group showed
somewhat higher reactivity than those with electron-do-
nating group. Furthermore, sterically hindered 2-bromo-
1,3-dimethyl benzene also reacted with phenylboronic
acid at 90 °C to give the coupled product in good yield
(entry 12).
We explored the catalytic ability of 3 for the Suzuki cou-
pling of aryl bromide with arylboronic acid in water
(Table 1). The Suzuki coupling is one of the most impor-
tant reactions for the construction of biaryl units in organ-
ic synthesis.14 Various kinds of palladium catalysts have
been used for the reaction.15 The Suzuki coupling is gen-
The high efficiency of catalyst 3 is probably attributed to
the fact that well-dispersed Pd-NHC in the ionic liquid
phase are more accessible to the reactants.
Synlett 2009, No. 15, 2477–2482 © Thieme Stuttgart · New York

LETTER
Catalyst Immobilized on Nanoparticle–Ionic Liquid Matrix
2479
Table 1 Suzuki Coupling of Aryl Bromide with Phenylboronic Acid in Watera
3
X
B(OH)2
+
K3PO4, TBAB
H2O
R
R
Entry
1c
Aryl bromide
Time (h)
6
Time (°C)
85
Yield (%)b
90 (92)
Br
Br
2
3
0.7
0.4
75
65
95 (99)
97 (100)
O2N
Br
Br
4
0.4
65
97 (100)
O2N
5
6
7
8
0.4
2.0
3.0
0.7
65
40
40
75
96 (100)
93 (97)
91 (97)
95 (99)
MeOC
Br
Br
MeOC
O2N
Br
MeO
Br
Br
9
10
11
1.0
1.0
1.0
75
75
85
94 (98)
96 (98)
94 (96)
OMe
Me
Br
Br
Br
Me
Me
12
1.5
90
87 (90)
Me
a Molar ratio: aryl bromide (1.0 equiv), phenylboronic acid (1.2 equiv), 3 (0.5 mol%), TBAB (0.5 equiv), and K3PO4 (2.0 equiv).
b Isolated yields (GC yield in parentheses).
c Reaction in the absence of TBAB.
Table 2 Suzuki Coupling of Aryl Bromide with Arylboronic Acid in Watera
3
B(OH)2
X
+
K3PO4, TBAB
H2O, 75 °C
R2
R2
R1
R1
Entry
Aryl bromide
Arylboronic acid
Time (h)
0.6
Yield (%)b
94 (95)
1
2
Br
(HO)2B
(HO)2B
Me
Me
0.6
92 (95)
MeO
Br
Synlett 2009, No. 15, 2477–2482 © Thieme Stuttgart · New York

2480
A. Taher et al.
LETTER
Table 2 Suzuki Coupling of Aryl Bromide with Arylboronic Acid in Watera (continued)
3
B(OH)2
X
+
K3PO4, TBAB
H2O, 75 °C
R2
R2
R1
R1
Entry
Aryl bromide
Arylboronic acid
Time (h)
1.0
Yield (%)b
3
4
95 (97)
91 (96)
MeO
Br
(HO)2B
(HO)2B
Me
Me
Br
1.0
Me
Br
5
6
0.6
1.0
94 (95)
90 (92)
(HO)2B
(HO)2B
OMe
OMe
MeO
Br
Br
7
1.0
8
95 (98)
91 (95)
(HO)2B
(HO)2B
OMe
Me
Me
Br
8c
Me
Br
9c
10
11
8
92 (97)
93 (92)
90 (93)
(HO)2B
(HO)2B
OMe
Cl
Me
0.6
1.0
MeO
Br
Br
(HO)2B
Cl
MeO
Br
(HO)2B
Cl
12
13
14
15
1.0
0.7
0.7
0.7
87 (92)
94 (96)
92 (95)
92 (96)
Me
(HO)2B
Br
NO2
(HO)2B
(HO)2B
MeO
Br
Br
NO2
MeO
NO2
Me
Br
16d
3.0
85 (90)
(HO)2B
OMe
Me
a Molar ratio: aryl bromide (1.0 equiv), arylboronic acid (1.2 equiv), 3 (0.5 mol%), TBAB (0.5 equiv), and K3PO4 (2.0 equiv).
b Isolated yields (GC yield in parentheses).
c Reaction in the absence of TBAB at 85 °C.
d Reaction temp was 90 °C.
Synlett 2009, No. 15, 2477–2482 © Thieme Stuttgart · New York

LETTER
Catalyst Immobilized on Nanoparticle–Ionic Liquid Matrix
2481
The catalytic behavior of 3 was also investigated in the ly be due to the weakly absorbed Pd-NHC outside of ionic
coupling of aryl bromides with different arylboronic acids liquid phase. The amounts of leached Pd were dropped
(Table 2). Deactivated aryl bromides were efficiently cou- upon reuse of 3. This excellent reusability and high stabil-
pled with electron-rich 4-methylphenylboronic acid and ity of 3 would be explained by strong binding of Pd-NHC
4-methoxyphenylboronic acid in high yields (entries 1–7). 1 to the modified Fe3O4. The presence of the supported
The coupling proceeded reasonably in the absence of ionic liquid phase contributes significantly to the direct
TBAB, although somewhat higher temperature and longer supporting of the Pd-NHC on the surface. In addition to
time were required (entries 8 and 9). Catalyst 3 also exhib- the high activity, the successful recycling of this catalytic
ited excellent activity in the coupling of the aryl bromides system allows for a more economic and environmentally
with electron-deficient arylboronic acids such as 4-chlo- friendly process.
rophenylboronic acid, 2-chlorophenylboronic acid, 3-ni-
trophenylboronic acid, and 3-nitrophenylboronic acid
In summary, Pd-NHC complex 1 has been immobilized
onto magnetite Fe3O4–ionic liquid hybrid matrix based on
(entries 10–15). Nearly complete conversions were
supported ionic liquid system. In this work, Pd-
achieved for the reactions within short reaction times. At
NHC@Fe3O4–IL 3 has shown excellent catalytic activity
and high stability for the Suzuki coupling reaction in wa-
somewhat higher temperature, reaction of sterically hin-
dered 2-bromo-1,3-dimethylbenzene with 4-methoxyphe-
ter. This heterogeneous catalyst can be recycled five times
nylboronic acid afforded the product in satisfactory yield
without a significant loss in the catalytic activity. Further-
(entry 16). We believe that this catalyst can be applicable
more, recovery of the catalyst by an external permanent
magnet is facile and efficient. Nano-Fe3O4 has been prov-
to a wide range of arylboronic acids in all cases.
en to be a promising support for the supported ionic liquid
catalysis. The applications of various magnetic nanoparti-
cle–organic hybrid matrix are now in progress.
Acknowledgment
This work was supported by the Carbon Dioxide Reduction &
Sequestration R&D Center of the 21st Century Frontier R&D
Programs.
References and Notes
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Separation and reuse of the catalyst are important aspects
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tive advantage of our catalyst 3 is its easy separation by
using external magnets. The loss of the catalyst can be ne-
glected in the process of separation. The recycling of 3
was investigated by the coupling of 4-bromoanisole with
phenylboronic acid in water (Figure 2).17 As shown
above, the catalyst after each run was recovered by simple
decantation, and then was reused in the next run. The cat-
alytic activity of 3 was significantly unchanged during
five repeated cycles. ICP analysis of the solution phase af-
ter the first reaction indicated that about 8.8% (9.7 ppm)
of the Pd was leached out from the surface. It might main-
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Synlett 2009, No. 15, 2477–2482 © Thieme Stuttgart · New York

2482
A. Taher et al.
LETTER
Pd content of 0.17 mmol/g was measured by inductively
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(16) General Procedure for the Suzuki Coupling Reaction
Aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K3PO4
(424 mg, 2.0 mmol), TBAB (161 mg, 0.5 mmol), dodecane
(40 mg, internal standard), and catalyst 3 (30 mg, 0.5 mol%)
were mixed in H2O (2.0 mL). The mixture was stirred at 75
°C in an air atmosphere. The reaction was periodically
monitored by GC. After magnetic separation of the catalyst,
the organic material was twice extracted with Et2O. The
organic phase was dried over MgSO4, and the solvent was
evaporated under reduced pressure. The crude was analyzed
by GC/GC-MS. The product was purified by short column
chromatography on silica gel.
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Tetrahedron 2005, 61, 259. (e) Lee, S.-M.; Yoon, H.-J.;
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M.-J. Green Chem. 2008, 10, 37.
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6714.
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2006, 12, 2880.
(12) Surface Modification of Nano-Fe3O4
To a solution of 1-(3-trimethoxysilylpropyl)-3-methyl-
imidazolium chloride (0.28 g, 1.0 mmol) in toluene was
added nano-sized Fe3O4 (Aldrich, 1.0 g). The mixture was
stirred at 100 °C for 10 h. After cooling, the nano-Fe3O4 was
magnetically separated from reaction mixture. Modified
Fe3O4 2 was washed with CH2Cl2 several times and dried at
60 °C under vacuum. Elemental analysis and weight gain
showed that 0.67 mmol of 1-(3-trimethoxysilylpropyl)-3-
methylimidazolium chloride was anchored on 1.0 g of 2.
(13) Immobilization of NHC-Pd onto Modified Nano-Fe3O4 2
To a solution of Pd-NHC 1 (195 mg, 0.29 mmol) and
1-butyl-3-methylimidazolium hexafluorophosphate (165
mg, 0.58 mmol) in CH2Cl2 (2 mL) modified Fe3O4 2 (1.0 g)
was added. The mixture was sonicated for 15 min at r.t., and
then CH2Cl2 was slowly removed under reduced pressure.
The resulting powder was washed with Et2O and dried under
vacuum at 60 °C to give Pd-NHC@Fe3O4–IL 3 (1.21 g). The
(17) Reuse of Pd-NHC@Fe3O4–IL 3
In the recycling experiment the reaction was performed by
using a mixture of 4-bromoanisole (187 mg, 1.0 mmol),
phenylboronic acid (134 mg, 1.2 mmol), K3PO4 (424 mg, 2.0
mmol), TBAB (161 mg, 0.5 mmol), and catalyst 3 (30 mg,
0.5 mol%) in H2O (2.0 mL) at 75 °C for 0.7 h. After
completion of the reaction, the catalyst was magnetically
separated from the solution. The solution was worked up as
described above. The separated catalyst was successively
reused for the next reaction without any pre-treatment.
Synlett 2009, No. 15, 2477–2482 © Thieme Stuttgart · New York

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