Magnetic nanoparticles supported oxime palladacycle as a highly efficient and separable catalyst for room temperature Suzuki-Miyaura coupling reaction in aqueous media

Article abstract of DOI:10.1039/c5ra05077d

A novel magnetic nanoparticle-supported oxime palladacycle catalyst was successfully prepared and characterized. The magnetically recoverable catalyst was evaluated in the room temperature Suzuki-Miyaura cross-coupling reaction of aryl iodides and bromides in aqueous media. The catalyst was shown to be highly active under phosphine-free and low Pd loading (0.3 mol%) conditions. The catalyst could be easily separated from the reaction mixture using an external magnet and reused for six consecutive runs without significant loss of activity.

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Magnetic Nanoparticles Supported Oxime
Palladacycle as a Highly Efficient and Separable
Catalyst for Room Temperature Suzuki–Miyaura
Coupling Reaction in Aqueous Media
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Mohammad Gholinejad,*a Mehran Razeghi,a and Carmen Najerab
DOI: 10.1039/x0xx00000x
A novel magnetic nanoparticle-supported oxime palladacycle catalyst was successfully
prepared and characterized. The magnetically recoverable catalyst was evaluated in room
temperature Suzuki-Miyaura cross-coupling reaction of aryl iodides and bromides in aqueous
media. The catalyst was shown to be highly active under phosphine-free and low Pd loading
(0.3 mol%) conditions. The catalyst could be easily separated from the reaction mixture using
an external magnet and reused for six consecutive runs without a significant loss of activity.
www.rsc.org/
reactions as source of Pd clusters6 or NPs.7 Though, the
efficiency of oxime palladacycles, these catalysts suffers from
Introduction
Palladium catalyzed cross-coupling reactions are very
important and potent strategies for the formation of carbon-
carbon bonds.1 Particularly, Suzuki-Miyaura reaction, which is
the cross coupling reaction between aryl, vinyl, or alkyl halides
or pseudo-halides and organoboron reagents is one of the well-
known studied and advanced palladium catalyzed reactions for
the formation of different types of compounds, specially
biphenyls.2
Since the palladium is an expensive noble metal and also there
is a possibility of contamination of the products with palladium
which has high toxicity level, in the recent years, significant
progress in heterogenization of palladium catalysts have been
achieved. Even with substantial progress in this area, separating
of the catalyst from the reaction medium by conventional
methods such as filtration or centrifugation is not an easy task.3
One way to solve this problem, is using magnetic nanoparticles
(MNPs) with large ratio of surface area to volume,
superparamagnetic behavior and low toxicity as a support for
stabilization of metal catalysts.4 Along this line, in recent years,
extensive attention has been paid to use of magnetic
nanoparticles as a promising support for stabilization of
palladium catalysts and their application in different carbon-
carbon bond forming reactions.5
problems associate to the homogenous catalysts. In recent
years, few attempts have been made to solve the problem by
immobilization of oxime palladacycles on different supports.8
Polystyrene,9 [poly(ethyleneglycol) dimethacrylate],10 and other
resins,10,11 silica,12 and periodic mesoporous organosilica
(PMO),13 phosphonium tagged,14 and self-supported oxime
palladacycles15 are different strategies which have been used
for heterogenization of oxime palladacycles. However to the
best of our knowledge, there are no reports dealing with
immobilization of oxime palladacycles on magnetic
nanoparticles.
Recently, we have reported application of magnetite
nanoparticles as an efficient catalyst for Sonogashira–Hagihara
reaction under heterogeneous and ligand-free conditions in
ethylene glycol.16
Herein, for the first time, we report the use of magnetic
nanoparticles as a catalyst support for immobilization oxime
palladacycles and its application in room temperature Suzuki-
Miyaura cross-coupling reaction in aqueous media.
Results and discussion
Among the different types of palladium catalysts, oxime-based
palladacycles are one of the most promising, efficient and
easily prepared catalysts for carbon-carbon bond forming
Magnetic Fe3O4 nanoparticles were prepared without using any
capping agent or surfactant via conventional co-precipitation of
FeCl2 and FeCl3 according to the reported methods.16-17 TEM
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image of prepared Fe3O4 nanoparticles showed the formation of The obtained Fe3O4 nanoparticles were coated with a thin layer
mono-dispersed and uniformed Fe3O4 nanoparticles in average of silica using a sol-gel process to give core/shell Fe3O4
size of 10-15 nm (Figure 1).
nanoparticles (SiO2@Fe3O4 NPs). The obtained silica-coated
Fe3O4 nanoparticles were characterized using FT-IR analysis
which showed the Si-O-Fe bond in 1097 cm-1 (Figure 1,
supporting information). The oxime can be easily anchored
onto the surface of the SiO2@Fe3O4 by using synthesized 1-(4-
(1-(hydroxyimino)ethyl)phenyl)-3-(3-
(triethoxysilyl)propyl)urea (
aminophenyl)ethanone
1
) obtained from reaction of 1-(4-
oxime and triethoxy(3-
isocyanatopropyl) silane (Scheme 1)
Fig. 1 TEM image of prepared Fe3O4 nanoparticles
O
NOH
Si(OEt)4
dry Toluene
reflux, 24h
+
(EtO)3Si
N
N
H
H
Fe3O4 NPs
SiO2@Fe3O4 NPs
1
O
N OH
O
NOH
Li2PdCl4
THF, 72 h
O
O
Pd
Cl
O
O
Si
N
N
Si
N
N
H
H
H
H
OEt
OEt
3
2
Scheme 1 Synthesis of oxime supported SiO2@Fe3O4 NPs 3.
1
Formation of oxime
1
was confirmed using H NMR from the obtained simply by dissolving Li2PdCl4 in MeOH and treating
reaction mixture of 1-(4-aminophenyl)ethanone oxime and it with the oxime-functionalized SiO2@Fe3O4, with a loading of
triethoxy (3-isocyanatopropyl) silane (Scheme 2). 1H NMR 0.32 mmolg-1 determined by ICP analysis. FT-IR analysis of
showed complete reaction of 1-(4-aminophenyl)ethanone catalyst showed presence of (C=O) bond of catalyst as well as
oxime and formation of 1-(4-(1-(hydroxyimino)ethyl)phenyl)- (Fe-O-Si) bonds shift at 1634 and 1090 cm-1 respectively
3-(3-(triethoxysilyl)propyl)urea which was approved by (Figure 3, supporting information). Also, the presence of
downfield shift of protons of oxime product compared to palladium in the structure of catalyst was confirmed by energy-
starting oxime (Figure 2, supporting information).
dispersive X-ray spectroscopy (EDX) (Figure 3) obtained from
SEM analysis (Figure 4) of catalyst . It is worth mentioning
that SEM image confirm uniform and regular morphology of
the catalyst structure.
3
HON
O
NOH
CH3CN
+
(EtO)3Si
N
C O
(EtO)3Si
N
N
24h, r.t.
H
H
NH2
Scheme 2 Synthesis of 1-(4-(1-(hydroxyimino)ethyl)phenyl)-
3-(3-(triethoxysilyl)propyl)urea 1.
Loading of oxime on SiO2@Fe3O4 was determined using
elemental analysis, which showed 0.41 mmol oxime per gram
Fe3O4 NPs. In addition TG analysis of supported oxime
2
confirmed the presence of organic group with 0.38 mmolg-1
(Figure 2). The final oxime palladacycle catalyst
3 was
2 | J. Name., 2012, 00, 1-3
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coating and formation of core/shell nanoparticles. Furthermore,
a decreases in the magnetization value of the final catalyst
3
,
confirm immobilization of oxime palladacycle on core/shell
SiO2@Fe3O4 NPs.
Fig. 2 Thermogravimetric diagram of the the catalyst.
Fig. 5 Magnetization curves Fe3O4 NPs, SiO2@Fe3O4 and catalyst
The nitrogen adsorption–desorption isotherms for SiO2@Fe3O4,
supported oxime
2 and catalyst 3 are shown in Figure 6 and the
physical parameters measured with N2 sorption are summarized
in Table 1.
The results indicated that all samples possesses type-IV
isotherms with H3 hysteresis loop. The BET surface area and
pore volume decreased in SiO2@Fe3O4 compared with oxime
which may be attributed to the possibility that the pores of SiO2
were partially covered by the grafting oxime (Table 1). Also,
2
Fig. 3 EDX spectrum of the catalyst
1
the results indicated that surface area and pore volume
increased after introducing of palladium particles. This may be
related to complex formation of oxime palladacycle which
caused increasing in surface area and porous property of the
catalyst. Despite the low surface area of the catalyst, high
catalytic activity of catalyst (3) is may related to distribution of
the nanoparticles on the surface.
Moreover, pore size distributions calculated from the
adsorption branch using BJH method displayed uniform which
centered on around 6 nm (Table 1).
Fig. 4 SEM image of the catalyst
The superparamagnetic performance of the catalyst was
confirmed using magnetization curves of Fe3O4 NPs,
SiO2@Fe3O4 NPs, and final oxime palladacycle supported
catalyst at room temperature (Figure 5). Zero coercivity and
remanence on the magnetization loop was observed in all three
samples without presence of hysteresis loop. These results
indicated the superparamagnetic property and the ability of the
material to be separated from the reaction mixture by an
external magnet (Figure 5). In addition, the decrease in the
magnetization value of the SiO2@Fe3O4 (35 emug-1) in
comparison with Fe3O4 NPs (60 emug-1) confirms that the silica
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Fig. 6 N2 adsorption/desorption isotherms of SiO2@Fe3O4, supported oxime
palladacycle and catalyst
2
3
Table 1. The BET surface area, BJH pore volume and pore size for
SiO2@Fe3O4, supported oxime palladacycle 2 and catalyst 3
Sample
SiO2@Fe3O4
Supported oxime 2
Catalyst 3
SBETa (m2g-1)
Vtb (cm3g-1)
DBJHc (nm)
6.4
36
23
28
8.2
5.4
6.6
Table 2 Screening of different reaction conditions for the reaction of 4-
bromoanisole with phenylboronic acid in the presence of catalyst.a
6.02
6.4
Br
B(OH)2
Catalyst (0.3 mol%)
base, solvent
+
It is worth mentioning that we have prepared catalyst
3 using
MeO
MeO
Time
different ratio of Si/Pd and used them in the reaction of 4-
bromoanisol and phenylboronic acid. However, results
indicated no appreciate difference in catalytic activity of
catalysts (Table 1, supporting information).
Temp.
(˚C)
60
60
60
60
60
60
Entry
Base
Solvent
Yield(%)b
(h)
15
15
15
15
15
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
DABCO
NaOAc
K3PO4
t-ButOK
t-ButOK
t-ButOK
H2O
Toluene
DMF
CH3CN
THF
91
7
2
7
1
100
90
78c
84
5
49
12
86
97
70
88
In order to find information about state of palladium in the
catalyst
Li2PdCl4 as a source of Pd(II) species (Figure 4, supporting
information). Results showed a peak at 324 nm for catalyst
3, we studied solid UV spectrum of catalyst and
H2O/EtOH
EtOH
3
60
5
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O
50
25
25
25
25
25
25
25
2
which is very similar to Pd(II) peak in Li2PdClO4. This
observation confirmed formation of Pd(II) oxime palladacycle
complex in the catalyst.
24
24
24
24
24
24
24
24
The catalytic activity of prepared catalyst
Suzuki-Miyaura coupling reaction. In order to find optimal
reaction conditions, less reactive 4-bromoanisole and
3 was assessed in
a
phenylboronic acid were selected as substrates for a model
reaction (Table 2). Initially, the reaction was studied under
different solvents using K2CO3 as base at 25-60 °C (Table 2,
entries 1-9). Using aqueous ethanol (1:1) the reaction took
place quantitatively in only 1 h (Table 2, entry 6). However,
working at 25 ºC the reaction needed 1 d to give 4-
methoxybiphenyl in 84% yield (Table 2, entry 9). When the
reaction was performed in aqueous ethanol with different
organic and inorganic bases (Table 2, entries 10-16), t-BuOK
gave the highest 97% GC yield (Table 2, entry 14).
EtOH
25
a Reaction conditions: 4-bromoanisole (1 mmol), phenylboronic acid
(1.5 mmol), base (1.5 mmol), solvent (2 mL), catalyst (0.3 mol% Pd). b
GC yields. c 0.1 mol% Pd
With the optimized reaction conditions in hand, t-BuOK as
base, EtOH:H2O (1:1) as solvent and 0.3 mol% catalyst, the
substrate scope of the Suzuki-Miyaura reaction was
investigated for the reaction of structurally different aryl halides
with arylboronic acids (Table 3).
Table 3 Suzuki cross-coupling of structurally different aryl halides and arylboronic acids.a
Catalyst (0.3 mol%)
Ar1-Ar2
Ar1X + Ar2B(OH)2
t-BuOK, EtOH:H2O (1:1)
r.t.
Entry
1
Ar1
Ar2
Ph
Time (h)
0.3 (1)
Product
Yield (%)b
95(100)
I
2
3
Ph
0.7
0.7
99
92
S
Ph
Me
I
Me
Me
Me
4
5
Ph
1
92
I
MeO
Ph
0.3(1)
92(98)
MeO
I
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6
7
Ph
Ph
1
O2N
96
O2N
I
Br
18
85
8
Ph
1
100
Br
Br
OHC
OHC
OHC
OHC
MeO
O2N
9
4-MeOC6H4
4-MeOC6H4
Ph
2
100
OMe
10
11
Br
12 (24)
10
90 (98)
93
Br
O2N
Br
12
13
Ph
Ph
24
17
95
98
Br
14
15
16
17
18
19
Ph
17
1
Et
Br
Br
Br
Br
Et
98
99
Ph
NC
NC
4-MeC6H4
Ph
1
99
97
95
83
OHC
HO
Me
Me
OHC
HO
24
24
24
Ph
MeO
MeO
Br
Br
MeO
MeO
4-MeC6H4
CHO
Br
CHO
20
21
Ph
Ph
24
24
70
98
OHC
OHC
Br
22
23
MeO
O2N
Br
Br
4-MeOC6H4
4-MeOC6H4
24
3
91
98
OMe
NO2
MeO
MeO
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NO2
B(OH)2
24
5
96
Br
O2N
CN
B(OH)2
25
26
1
95
97
Br
NC
Br
MeOC
Ph
2
MeOC
27
28
Ph
Ph
24
24
67c
48c
Cl
Cl
NC
NC
O2N
O2N
aReaction conditions: 4-bromoanisole (1 mmol), phenylboronic acid (1.5 mmol), base (1.5 mmol), solvent (2 mL), catalyst (0.3 mol%). bYields are
isolated. cReactions were performed at 60 °C.
As shown in Table 3, the coupling reaction between aryl substrate. The result indicated that using 4a reaction proceeded
iodides and phenylboronic containing electron-donating groups under lower Pd loading and temperature with almost similar
(Table 2, entries 3-5) as well as nitro group (Table 3, entry 6) yields (Table 4, entries 8-10).
and thienyl iodide (Table 3, entry 2), proceeded effectively to
afford the corresponding products in excellent yields. Also,
reactions of wide range of aryl bromides containing both
electron- donating and electron-withdrawing groups with
different arylboronic acids proceeded well and desired coupling
products were obtained in high to excellent isolated yields. It is
Cl
Pd
MeO
O
OH
N
worth mentioning that reaction of
a less reactive 4-
bromophenol at room temperature afforded 4-hydroxybiphenyl
in 97% yield (Table 3, entry 17). Different substituted
arylboronic acids like 4-methoxyphenyl, 4-tolyl and 1-
naphthylboronic acids gave good yields (Table 3, entries 9, 10,
16, 19, and 22-25). All attempts to carry out the reaction of aryl
chlorides under optimized reaction conditions were
unsuccessful. However, increasing the reaction temperature to
60 °C, 4-chloro-benzonitrile and nitrobenzene reacted with
phenylboronic acid to give the corresponding 4-ciano and 4-
nitrobiphenyl in 67 and 48% yield, respectively (Table 3,
entries 27 and 28).
MeO
1a
Cl
(
OH
2
Cl
(
OH
2
Pd
N
Pd
N
NO2
PS
Cl
Cl
Comparison catalytic activity of presented catalyst with other
supported oxime palladacycles such as polymer supported
oxime palladacycles 1a11 and 2a10a and also unsupported oxime
palladacycle 3a7b (scheme 3) in Suzuki coupling reaction of 4-
bromoanisole with phenylboronic acid as a common substrate,
showed noticeably efficiency of presented magnetic supported
oxime palladacycle (Table 4, entries 1-7). We have also
compared catalytic activity of magnetic supported oxime
palladacycle with silica supported oxime palladacycle 5a12
reported by Corma and coworkers in the reaction of 4-
bromoacetophenone with phenylboronic acid as a common
2a
3a
O
N OH
O
O
Pd
Cl
Si
N
N
H
H
OEt
4a
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N OH
O
O
Si
S
Pd
Cl
( )10
O
OMe
5a
Scheme 3 Different oxime palladacycles reported in Suzuki
coupling reaction.
Table 4 Comparison catalytic activity of magnetic supported oxime
palladacycle with other reported oxime palladacycles.
Fig.
7
Recycling of catalyst
3
for the reaction of 4-bromoanisole with
Temp.
(°C)
50
Time
(h)
2
2
24
2
1
24
2
2
0.1
0.1
Pd
(mol%)
1
0.1
0.1
0.01
0.3
0.3
0.1
0.3
phenylboronic acid.
Entry
Catalyst
Yield%
1
2
3
4
5
6
7
8
9
1a
1a
2a
3a
4a
4a
4a
4a
4a
5a
96
62
89
94
100
95
78
97
95
99
Atomic absorption spectroscopy (AAS) of the solution obtained
after isolation of catalyst in the first run showed the amount of
leaching of Pd was 1.6%. Also, amount of palladium in the
recycled catalyst after 3rd run determined by ICP analysis to be
0.29 mmolg-1. In order to get information about stability of the
catalyst, TGA analysis of recycled catalyst after 3rd run was
studied. The results showed that the catalyst is thermally stable
up to 200 °C and organic functional group containing oxime
palladacyle was presented in the structure of the catalyst
(Figure 5, supporting information).
50
100
110
60
25
50
25
60
100
0.3
0.65
10
Furthermore, electron dispersive X-ray (EDX) of the recycled
catalyst after a 3rd run confirmed the presence of Fe and Pd
species in the structure of the recovered catalyst (Figure 8).
The recovery and reuse of catalyst becomes a main factor from
the point of view of economic and environmental sustainability.
Along this line, we have studied recyclability of magnetic
nanoparticles supported oxime palladacycle catalyst for the
reaction of less reactive 4-bromoanisole with phenylboronic
acid under the optimized reaction conditions. After the
completion of the reaction, the magnetic catalyst could be
simply recovered from the reaction mixture using an external
magnet, washed with water and diethyl ether, dried under
vacuum and reused in another batch. Recycling experiments
indicated that the catalyst
3 can be successfully used during six
consecutive runs without a significant loss of activity (Figure
7).
Br
B(OH)2
Fig. 8 EDX spectrum of the catalyst
3
after the 3rd run.
Catalyst (0.3 mol%)
+
MeO
t-BuOK, EtOH:H2O (1:1)
MeO
r.t.
Conclusions
In conclusion, new magnetic nanoparticle-supported oxime
palladacycle catalyst was successfully prepared and
characterized. Using this catalyst, structurally different aryl
iodides and bromides were reacted effectively with arylboronic
acids in aqueous media at room temperature. The magnetically
separable and phosphine free catalyst was successfully recycled
and used in six consecutive runs with similar catalytic activity.
Experimental Section
General
All chemicals were purchased from Sigma-Aldrich, Acros and
Merck and were used without further purification. Thin layer
chromatography was carried out on silica gel 254 analytical
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Journal Name
1
sheets obtained from Fluka. H NMR spectra were recorded at Then, reaction mixture was subjected to magnetic separation,
400 MHz and 13C NMR spectra were recorded at 100 MHz in and isolated material was washed sequentially with H2O (3×10
CDCl3 using TMS as internal standard. Thermogravimetric mL) and EtOH (3× 10 mL) and dried under vacuum for 24 h.
analysis (TGA) was conducted from room temperature to 700
°C in an oxygen flow using a NETZSCH STA 409 PC/PG
instrument. FT-IR spectra were recorded on a Bruker Vector
22. Energy dispersive X-ray analysis (EDX) was obtained using
Carl Zeiss ƩiGMA instrument. The structures of the prepared
materials were observed by transmission electron microscopy
(Philips CM-120). Adsorption-desorption analyses were
recorded using (BELsorp-max (Japan)).
General procedure for Suzuki-Miyaura reaction:
In a 10 mL flask, aryl halide (1 mmol), arylboronic acid (1.5
mmol), t-ButOK (1.5 mmol), catalyst (0.3 mol%) and 2 mL
water/ethanol (1:1) were added and stirred using mechanical
stirrer at r.t. to 60 °C (with respect to aryl halide). The progress
of the reactions were monitored by GC. After completion of the
reaction, the crude product was extracted using ethyl acetate.
The further purification was achieved by column
chromatography on silica using the hexane and ethyl acetate as
eluent.
Synthesis of Fe3O4 nanoparticles:
Magnetite (Fe3O4) nanoparticles were prepared following a
similar procedure reported in the literature,17 which will briefly
be explained in the following. FeCl3.6(H2O) (11.0 g) and
FeCl2.4(H2O) (4.0 g) were dissolved in 250 mL deionized water
under argon with vigorous stirring using mechanical stirrer. To
the mixture aqueous ammonia (25%, 40 mL) was added slowly
over 20 min under argon atmosphere and mixture was stirred at
80 °C for 4 h. A black precipitate (Fe3O4) were collected by
external magnet and washed three times with deionized water
and ethanol and dried under vacuum.
Acknowledgements
The authors are grateful to Institute for Advanced Studies in
Basic Sciences (IASBS) Research Council and Iran National
Science Foundation (INSF-Grant number of 93020713) for
support of this work.
Notes and references
Synthesis of silica-coated Fe3O4 nanoparticles
aDepartment of Chemistry, Institute for Advanced Studies in Basic
Sciences (IASBS), P. O. Box 45195-1159, Gavazang, Zanjan 45137-
6731, Iran.
1.0 g of synthesized Fe3O4 nanoparticles was sonicated in
ethanol (200 mL) for 30 min at room temperature. Then, to the
resulting suspension were added 2 mL of tetraethyl orthosilicate
(TEOS) and 6 mL aqueous ammonia and mixture was stirred
for 24 h at room temperature. Obtained silica-coated Fe3O4
nanoparticles were separated by an external magnet and washed
with H2O (3×10 mL) and EtOH (3×10 mL) and dried under
vacuum.
bDepartamento de Química Orgánica and Centro de Innovación en
Química Avanzada (ORFEO CINQA), Universidad de Alicante, Apdo.
99, E-03080 Alicante, Spain
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
Synthesis of 1-(4-aminophenyl)ethanone oxime
4-aminoacetophenone (1 mmol, 0.135 g) was dissolved in
mixture of 2 mL H2O and 0.5 mL EtOH at 70 °C. Then, NaOAc
(3 mmol, 0.246 g) and hydroxylamine hydrochloride (2 mmol,
0.140 g) were added and mixture was stirred for 1 h at 70 °C.
Then, reaction mixture was cooled to 0 °C in an ice bath to
precipitate the desired oxime product. The mixture was filtered
and obtained oxime was dried under a vacuum.
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Synthesis
of
1-(4-(1-(hydroxyimino)ethyl)phenyl)-3-(3-
(triethoxysilyl)propyl)urea (1)
To 10 mL flask containing 1-(4-aminophenyl)ethanone oxime
(1 mmol) was added 5 mL dry acetonitrile and mixture was
stirred
at
room
temperature.
Then
triethoxy(3-
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(triethoxysilyl)propyl)urea (
1
Synthesis of oxime-functionalized MNP (2)
Silica-coated Fe3O4 nanoparticles (500 mg) were sonicated in
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400 mg of compound
2 was sonicated at 5 mL MeOH for 30
min at room temperature. Then, Li2PdCl4 (0.075 mmol, 0.02 g)
and NaOAc (0.5 mmol, 0.041 g) were added. The mixture was
stirred under argon atmosphere for 72 h at room temperature.
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