Palladium Supported on Metformin-Functionalized Magnetic Polymer Nanocomposites: A Highly Efficient and Reusable Catalyst for the Suzuki–Miyaura Coupling Reaction

Article abstract of DOI:10.1002/cctc.201600994

In this paper, a novel magnetically responsive nanocatalyst, Pd/Fe₃O₄@PMAA-Met (PMAA=poly(methacrylic acid), Met=metformin), has been successfully prepared by immobilizing palladium on the surface of metformin-functionalized magnetic polymer nanocomposites. This novel nanocatalyst was characterized by FTIR, XRD, TEM, X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry (VSM). The prepared nanocatalyst showed excellent catalytic activity for Suzuki–Miyaura coupling reactions of aryl halides (I, Br, Cl) with arylboronic acids even at palladium loadings of 0.01–0.0007 mol % and the turnover frequency (TOF) reached up to 125 714 h^?1. The excellent activity of this Pd/Fe₃O₄@PMAA-Met catalyst could be attributed to a “release and catch” catalytic mechanism as well as its nanometer size. This nanocatalyst could be simply separated with an external magnet and reused at least twelve times with excellent yields achieved.

Full text of DOI:10.1002/cctc.201600994

Accepted Article
Title: Palladium supported on metformin-functionalized magnetic
polymer nanocomposites：A highly efficient and reusable catalyst
for the Suzuki-Miyaura coupling reactions
Authors: Pengbo Yang, Rong Ma, and Fengling Bian
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201600994
Link to VoR: http://dx.doi.org/10.1002/cctc.201600994
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ChemCatChem
10.1002/cctc.201600994
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Palladium supported on metformin-functionalized magnetic
polymer nanocomposites：A highly efficient and reusable
catalyst for the Suzuki-Miyaura coupling reactions
[a]
[a]
[a]
Pengbo Yang, Rong Ma, and Fengling Bian*
Abstract: In this paper,
a
novel magnetically responsive
expected to bridge the gap between homogeneous and
heterogeneous catalysts. Up to now, many well defined
nanocatalyst Pd/Fe @PMAA-Met has been successfully prepared
3 4
O
by immobilizing palladium on the surface of metformin-functionalized
magnetic polymer nanocomposites. This novel nanocatalyst was
characterized by FT-IR, XRD, TEM, XPS and VSM. The prepared
nanocatalyst showed excellent catalytic activity for the Suzuki-
Miyaura coupling reactions of aryl halides (I, Br, Cl) with arylboronic
acids even at palladium loading of 0.01-0.0007 mol% and the TOF
magnetic
polymer
nanocomposites-supported
palladium
[
5a, b, 9]
catalysts have been developed for the Suzuki reactions.
Wang et al.
polymer nanocomposites for immobilizing palladium species.
[
5a]
recently prepared NHC-functionalized magnetic
[
5c]
Wang and colleagues
anchoring palladium on magnetic polymer nanocomposites
Fe @PUVS. All of these catalysts are highly active and
developed a novel catalyst by
-1
value could reach up to 125714 h . The excellent activity of this
Pd/Fe @PMAA-Met could be attributed to a “release and catch”
3 4
O
O
3 4
magnetic separation for Suzuki reactions.
catalytic mechanism as well as its nanometer size. This nanocatalyst
could be simply separated with an external magnet and reused at
least twelve times with excellent yields achieved.
Recently, Palladium-guanidine complexes are found to be
[
10]
highly active in the Suzuki reactions.
Besides their excellent
catalytic performence, the palladium-guanidine complexes are
air-stable, low toxic, and easily accessibile via chemical
synthesis, and thus are very attracive in practical applications.
However, few papers were reported about using guanidine as
ligand in heterogeneous system for the Suzuki coupling
Introduction
[
10c]
reactions. Yang and colleagues
prepared a novel catalyst by
Palladium-catalyzed Suzuki-Miyaura coupling is among the most
important reactions for the formation of carbon-carbon bonds in
immobilizing palladium-guanidine complex on SBA-16.
[11]
Beygzadeh et al.
Fe /SiO -Met-Pd(OAc)
groups) on the surface of Fe
developed
) by modifying metformin (biguanide
/SiO nanoparticles. These
a
nanosized catalyst
[
1]
organic transformations. Although significant progress has
been made by homogeneous catalysts in this reaction, it suffers
from drawbacks associated with tedious separation and difficult
(
3
O
4
2
2
3
O
4
2
catalysts all exhibited excellent catalytic activity and recyclability
in Suzuki coupling reactions.
[
2]
recovery.
could be an attractive solution to these problems. However,
compared with similar homogeneous catalysts, the
heterogeneous catalysts usually show a decrease in catalytic
Heterogenization of the homogeneous catalysts
[
3]
In continuation of our efforts to develop efficient and eco-
[5c, 12]
friendly heterogeneous catalysts,
herein we report the full
details of the development of a novel metformin-functionalized
magnetic polymer nanocomposites-supported palladium catalyst
[
4]
activity and selectivity, thus the rush towards the development
of more active and recyclable catalysts has been more and more
important.
(
Pd/Fe
3
O
4
@PMAA-Met). This catalyst showed excellent catalytic
O at low
activity in the Suzuki-Miyaura reactions in EtOH/H
2
Magnetic polymer nanocomposites-supported palladium
catalysts have caught great attentions duing to following unique
palladium loading, and could be used consecutively for twelve
times with excellent yields achieved.
[
5]
properties. Firstly, the nanometer size of the catalysts could
make them show excellent catalytic performance because of
their large specific surface area and high densities of active
Results and Discussion
[
6]
sites. Secondly, the magnetic nanocatalysts could be easily
separated by using an external magnet because of their
magnetic nature, thus obviating tedious procedures like filtration
The preparation procedures of Pd/Fe
3
O
4
@PMAA-Met were
was modified with 3-
Trimethoxysilyl)propylmethacrylate (MPS) by stirring a mixture
illustrated in Scheme 1. Firstly, Fe
3
O
4
[
7]
or centrifugation. Lastly, a polymer shell could stabilize the
magnetic nanoparticles by protecting them from agglomeration
and oxidation. Moreover, the polymer shell is easily chemically
modified with functional groups for anchoring palladium, and
(
o
of Fe
3
O
4
, ammonia and MPS at 60 C in EtOH/H
2
O for 24 h.
@PMAA
were prepared by reflux precipitation polymerization of
methacrylic acid and Fe @MPS. Next, Fe @PMAA-Met
nanoparticles were synthesized by the reaction between free
metformin and acyl chloride Fe @PMAA. Finally, the catalyst
Pd/Fe @PMAA-Met was obtained by immobilizing PdCl on
Fe @PMAA-Met nanoparticles in water. The content of
palladium loaded on Pd/Fe @PMAA-Met was determined to
be 0.71 wt% based on Atomic absorption spectroscopy (AAS).
3 4
Then, the magnetic polymer nanocomposites Fe O
[
8]
thus the stability of the catalysts could be improved. Therefore,
the magnetic polymer nanocomposites-supported catalysts are
3
O
4
3 4
O
3
O
4
3
O
4
2
[
a]
P. Yang, R. Ma, Dr. F. Bian
College of Chemistry and Chemical Engineering,
Lanzhou University
3
O
4
3
O
4
Lanzhou 730000 (PR China)
E-mail: bianfl@lzu.edu.cn
Supporting information for this article can be found under
1
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ChemCatChem
10.1002/cctc.201600994
FULL PAPER
3 4
Scheme 1. Synthesis of Pd/Fe O @PMAA-Met catalyst.
Figure 2. Room temperature magnetization curves of a)
Fe @MPS, b) Fe @PMAA-Met and c) Pd/Fe @PMAA-
Met catalyst.
O
3 4
O
3 4
3 4
O
Figure 1. FT-IR spectra of a) Fe
and c) Fe @PMAA-Met.
3 4 3 4
O @MPS, b) Fe O @PMAA
3
O
4
Figure 1 shows the FT-IR spectra of a) Fe
@PMAA and c) Fe @PMAA-Met. In Figure 1a, the
characteristic absorption band of Fe-O is clearly observed at 592
3 4
O @MPS, b)
Fe
3
O
4
3 4
O
-
1
-1
cm . The presence of absorption bands at 1633 cm (C=C),
-1
-1
1
705 cm (C=O), and 2928 cm (C-H) confirms the successful
attachment of MPS on the surface of Fe
[
12a, 13]
3
O
4
.
Compared
with Figure 1a, there are some changes in the spectrum of
-
Fe
3
O
4
@PMAA (Figure 1b). The intensity of the band at 1703 cm
increases due to the modification of MAA on the Fe @MPS
surface. There are also two new bands appearing at 1637 cm
1
3
O
4
-
1
-1
and 1524 cm , which are attributed to C=O stretching vibration
[
14]
Figure 3. The magnetic separation and redispersion of
Pd/Fe @PMAA-Met catalyst.
and N-H bending vibration of MBA.
In Figure 1c, the
absorption band at 1679 cm corresponds to the C=N stretching
vibration of metformin. The intensity of the band at 1655-1630
-1
3 4
O
3 4
The magnetic properties of a) Fe O @MPS, b)
-1
cm (-CONH-) increases significantly than the spectrum of
Fe
3
O
4
@PMAA-Met and c) Pd/Fe @PMAA-Met catalyst were
3 4
O
Fe
Fe
3
O
O
4
@PMAA, suggesting metformin has reacted with
measured by a vibrating sample magnetometer (VSM) at room
temperature. As shown in Figure 2, the saturation magnetization
[15]
3
4
@PMAA nanoparticles successfully.
All these
characteristic absorption bands confirm the successful
preparation of Fe @PMAA-Met. Elemental analysis gave a
metformin loading of 0.16 mmol/g in Fe @PMAA-Met.
values of Fe
and 38.38 emu/g, respectively, indicating that Fe
nanoparticles has been successfully modified with polymer and
3
O
4
@MPS and Fe
3
O
4
@PMAA-Met are 64.65 emu/g
3
O
4
3 4
O
@MPS
3
O
4
2
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ChemCatChem
10.1002/cctc.201600994
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metformin. After immobilizing palladium, the saturation
magnetization value of Pd/Fe
6.90 emu/g, but it was still enough for complete separation by
an external magnet. In addition, Pd/Fe @PMAA-Met could be
3 4
O @PMAA-Met is reduced to
3
3
O
4
easily dispersed in solution again after the magnetic separation
due to its superparamagnetism. The pictures of the magnetic
3 4
separation and redispersion of Pd/Fe O @PMAA-Met catalyst
were shown in Figure 3.
The transmission electron microscope (TEM) images of a)
@PMAA-Met and b) Pd/Fe @PMAA-Met catalyst were
Fe
3
O
4
3 4
O
shown in Figure 4. It can be seen that both magnetic
nanoparticles are nearly spherical with average diameters of
Figure 6. XPS spectra of a) fresh Pd/Fe
3 4
O @PMAA-Met
catalyst and b) Pd/Fe @PMAA-Met catalyst after one run.
3 4
O
1
3.5 nm (Fig. 4a) and 14.3 nm (Fig. 4b), respectively. The
conducted to investigate the chemical oxidation state of
palladium in the fresh and the used Pd/Fe @PMAA-Met
catalyst. As shown in Figure 6a, there are two peaks at the
binding energy of 342.4 eV and 337.5 eV in the fresh
nanometer size of the catalyst could make its active sites easily
contact with the reactants, which greatly benefits the catalytic
3
O
4
[
6a]
activity of Pd/Fe
Figure 5 shows the X-ray diffraction (XRD) patterns of a)
@MPS, b) Fe @PMAA-Met and c) fresh
@PMAA-Met catalyst. All the samples have six
3 4
O @PMAA-Met.
3 4
Pd/Fe O @PMAA-Met catalyst, which correspond to Pd(II) 3d3/2
Fe
Pd/Fe
diffraction peaks at 30.2 ，35.6 ，43.3 ，53.6 ，57.2 , 62.8
which correspond to the crystal face (220), (311), (400), (422),
511), (440) of Fe (JCPDS file No. 19-0629). In addition, the
3
O
4
3 4
O
and Pd(II) 3d5/2. This result indicates that the chemical state of
palladium is divalent in fresh Pd/Fe O @PMAA-Met catalyst.
3
O
4
o
o
o
o
o
o
3
4
Compared to Figure 6a, some differences are found in the used
Pd/Fe @PMAA-Met catalyst (Figure 6b). Except for the peaks
3
O
4
(
3 4
O
of Pd(II), two new peaks appear at the binding energy of 340.2
eV and 335.0 eV, which are assigned to 3d3/2 and 3d5/2 of Pd(0),
respectively, indicating that most of Pd(II) were reduced to
characteristic diffraction peaks of Pd nanoparticles do not
appear in Figure 5c, probably because the palladium exists as
3 4
Pd(II) in fresh Pd/Fe O @PMAA-Met catalyst.
[
16]
metallic Pd(0) in the Suzuki reactions.
After the successful preparation of Pd/Fe
catalyst, the catalytic performance was evaluated in the Suzuki-
Miyaura coupling reactions. In model reaction of
X-ray photoelectron spectroscopy (XPS) was also
3
4
O @PMAA-Met
a
bromobenzene and phenylboronic acid, different reaction
conditions including the base, the solvent, and the catalyst
dosage were tested to find the optimized reaction conditions
(
Table 1). Initially, the effect of different bases on the model
reaction was explored in EtOH/H O (v/v=1:1). Results indicated
that K CO was more appropriate for the Suzuki reactions than
other bases(NaOH, Et N and n-Bu N) and gave the highest yield
of 96% (entries 1-4). Next, the different solvents were studied in
the Suzuki reactions using K CO as a base (entries 4-8).
2
2
3
3
3
2
3
Figure 4. TEM images of a) Fe
Pd/Fe @PMAA-Met catalyst.
3 4
O @PMAA-Met and b)
EtOH/H O (v/v=1:1) was proved to be the most favorable
2
3
O
4
Table 1. Optimization of the Suzuki-Miyaura coupling reaction of
bromobenzene
with
phenylboronic
acid
catalyzed
by
[
a]
Pd/Fe @PMAA-Met.
3 4
O
[
b]
[c]
Catalyst
mol%)
Yield
(%)
Entry
Base
solvent
(
1
2
3
4
5
6
7
8
9
Et
n-Bu
NaOH
3
N
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.005
EtOH:H
2
2
2
2
O=1:1
54%
21%
87%
96%
54%
44%
67%
91%
96%
89%
3
N
EtOH:H
EtOH:H
EtOH:H
O=1:1
O=1:1
O=1:1
K
K
K
K
K
K
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
2
H O
EtOH
EtOH:H
2
2
2
2
O=2:1
EtOH:H
EtOH:H
EtOH:H
O=1:2
O=1:1
O=1:1
10
[
a] Reaction conditions: bromobenzene (2 mmol), phenylboronic acid
Figure 5. XRD patterns of a) Fe
3 4
O
@MPS, b) Fe
3 4
O @PMAA-
o
(
2.4 mmol), base (5 mmol), solvent (6 mL), at 70 C for 30 min. [b]
Met and c) fresh Pd/Fe @PMAA-Met catalyst.
3 4
O
Catalyst (based on bromobenzene). [c] Isolated yield.
3
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ChemCatChem
10.1002/cctc.201600994
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proceeded extremely well and quantitatively gave the desired
products under optimized reaction conditions (entries 1-2). To
explore the potential of the catalyst, we decreased the catalyst
dosage to 0.001 mol% in the reactions of iodobenzene and 4-
iodotoluene with phenylboronic acid. The desired products were
-1
quantitatively obtained with the TOF values of 99000 h and
-1
9
8000 h , respectively. Further decreasing the catalyst dosage
to 0.0007 mol% did not have a signifiacnt effect on reactions, for
-1
-1
which high TOF values of 125714 h and 124285 h were still
obtained. These TOF values, to the best of our knowledge, were
the highest so far have been achieved by heterogeneous
[
17]
catalysts for the Suzuki-Miyaura reactions of aryl iodides.
The reactions of various aryl bromides were also performed
well to afford the corresponding biaryl products in high to
excellent isolated yield (entries 3-14). For the methyl-substituted
aryl bromides (entries 10-13), the position of the methyl group
had a significant impact on the yield of the corresponding biaryl
products. The highest yield was obtained when the methyl group
was at para position, followed by meta and ortho position. Under
the same conditions, 3, 5-dimethylbromobenzene gave the
corresponding biaryl product in 84% yield, but a satisfactory
O, EtOH, yield of 94% was achieved by prolonging reaction time to 45 min.
3 4
Figure 7. Kinetic profiles of the Pd/Fe O @PMAA-Met in the
Suzuki reaction. Reaction conditions: bromobenzene (2
mmol), phenylboronic acid (2.4 mmol), K CO (5 mmol),
Pd/Fe @PMAA-Met (0.01 mol% Pd), solvent (6 mL, EtOH:
O=1:1), at 70 C.
2
3
3 4
O
o
H
2
medium among a series of different solvents such as H
2
EtOH/H
EtOH/H
2
O (v/v=2:1), EtOH/H
O (v/v=1:1) as solvent, the effect of the catalyst dosage
2
O (v/v=1:2). With K
2
CO
3
as base,
Furthermore, various arylboronic acids such as 4-
methylphenylboronic acid and 4-chlorophenylboronic acid were
also coupled with aryl bromides efficiently under the optimized
reaction conditions and the excellent yields in the range of 95%-
99% were achieved (entries 15-18).
2
was also examined in the model reaction (entries 4, 9, 10), and
the best result was obtained using 0.01 mol% loading of
palladium.
Furthermore, reaction kinetics of the Suzuki reaction of
bromobenzene with phenylboronic acid was investigated. As
shown in Figure 7, when the reaction time was raised from 5 min
to 30 min, the isolated yield of biphenyl increased from 28% to
From a practical point of view, the use of aryl chlorides is
becoming increasingly attractive in the Suzuki-Miyaura reactions
because they are inexpensive and readily available. The
applicability of this catalyst to aryl chlorides was then tested in
the presence of phase transfer agent TBAB which is considered
to stabilize the dissolved palladium species in solution (Table
96%. But the yield did not increase any more by prolonging
reaction time to 1 h, which meant that 30 min is the appropriate
time for the Suzuki reactions.
[
18]
2).
At a low Pd loading level of 0.01 mol%, the reaction of
With the optimized reaction conditions in hand (K
EtOH/H O (v/v=1:1), 0.01 mol% palladium loading, 30 min)，the
adaptability of the Pd/Fe @PMAA-Met catalyst in the Suzuki-
Miyaura coupling reactions of different aryl halides with
arylboronic acids was investigated. As shown in Table 2, the
coupling reactions of aryl iodides with phenylboronic acid
2
CO
3
,
electron-poor aryl chlorides with arylboronic acids proceeded
well albeit at a slower rate, giving the desired products in 74%-
91% yields (entries 19-24). By contrast, the electron-rich 4-
chlorotoluene is hardly reactive under the same conditions (entry
25).
2
3
O
4
[
a]
3 4
Table 2. Suzuki-Miyaura reactions between aryl halides and arylboronic acids in the presence of Pd/Fe O @PMAA-Met catalyst.
[
b]
[c] -1
Entry
1
Aryl halide
Arylboronic acid
Pd/mol%
T
Yield
TOF /h
0
.01
0.001
.0007
30 min
1 h
99%
99%
88%
19800
99000
125714
0
1 h
0
.01
0.001
.0007
30 min
1 h
99%
98%
87%
19800
98000
124285
2
0
1 h
4
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ChemCatChem
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FULL PAPER
3
4
5
6
7
8
9
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
30 min
30 min
30 min
30 min
30 min
30 min
30 min
30 min
96%
98%
98%
92%
92%
95%
97%
97%
19200
19600
19600
18400
18400
19000
19400
19400
1
0
1
1
0.01
0.01
30 min
30 min
96%
93%
19200
18600
12
13
14
3
0 min
5 min
84%
94%
16800
12533
0.01
0.01
4
30 min
98%
19600
15
16
17
18
19
20
21
22
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
30 min
30 min
30 min
30 min
6 h
97%
98%
95%
96%
19400
19600
19000
19200
1517
[
d]
91%
88%
84%
85%
[
d]
d]
8 h
1100
[
8h
1050
[
e]
8 h
1063
5
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ChemCatChem
10.1002/cctc.201600994
FULL PAPER
[
[
e]
e]
23
24
25
0.01
0.01
0.01
8 h
8 h
83%
74%
1038
925
[
d]
24 h
Trace
--
o
[
a] Reaction conditions: aryl halides (2 mmol), arylboronic acids (2.4 mmol), K
2
CO
3
(5 mmol), solvent (6 mL, EtOH：H
2
O=1:1), at 70 C. [b]
-1
-1
Isolated yield. [c] TOF: [mol product] [mol Pd] h . [d] In the presence of 2 mmol TBAB, at reflux. [e] At reflux.
To
Pd/Fe
further
evaluate
the
catalytic
activity
of
3 4
acid. As shown in Table 3, the current Pd/Fe O @PMAA-Met
3
O
4
@PMAA-Met, a comparison was made with various
catalyst is superior to other magnetic nanoparticles-supported
catalysts in terms of catalytic activity for the Suzuki reactions, as
magnetic nanoparticles-supported catalysts reported for the
Suzuki-Miyaura reaction of bromobenzene with phenylboronic
-1
reflected from its extremely high TOF values (19200 h ).
Table 3 Performance of various magnetic nanoparticles-supported catalysts for the Suzuki-Miyaura reaction of bromobenzene with
phenylboronic acid.
[
b]
Solvent/
TOF
[
a]
Entry
Pd catalyst (mol%)
Yield
Ref
-1
temperature/time
(h )
o
1
2
3
4
5
6
7
8
9
Pd/Fe
3
O
4
@PMAA-Met (0.01)
EtOH/H
2
O/70 C/30 min
96%
95%
84%
98%
19200
814
28
This work
o
[11]
3 4
Fe O /SiO
2
-Met-Pd(OAc)
2
(0.14)
EtOH/H
2
O/80 C/50 min
o
[19]
3 4
Fe O @SiO
2
@C22–Pd(II) (0.5)
DMF/H
2
O/75 C/6 h
[
20]
Fe
3
O
4
/DAG/Pd (0.2)
EtOH/H
2
O/r.t./0.66 h
742
17
o
[c]
[16b]
[9a]
Phosphinite functionalized Fe
O
3 4
supported palladium (0.3)
EtOH/H
DMF/H
EtOH/H
2
O/60 C/18 h
90%
98%
96%
95%
93%
87%
o
Fe
3
O
4
/P(GMA-AA-MMA)-Schiffbase–Pd (0.1)
2
O/80 C/1 h
980
160
950
1033
687
o
[9b]
3 4
Fe O /P(GMA-AA-MMA)-Pd (0.2)
2
O/80 C/3 h
o
[12b]
[5c]
Fe
3
O
4
@PUNP-Pd (0.1)
H
2
O/90 C/1 h
o
3
Fe O
4
@PUVS-Pd (0.09)
H
2
O/90 C/1 h
[
21]
10
Fe
3
4
O /SiO
2
/HPG-OPPh
2
-PNP (0.76)
2
DMF/H O/r.t./10 min
-1
-1
[
a] Isolated yield. [b] TOF: [mol product] [mol Pd] h . [c] GC yield.
[
22]
o
To explore if any “release and catch” mechanism
existing in the Suzuki reactions in the presence of
Pd/Fe @PMAA-Met, several experiments were performed
Table 4). In the first two experiments (entries 1-2),
mixture of Pd/Fe @PMAA-Met and EtOH/H O was stirred at
0 C for 30 min, then the magnetic catalyst was separated with
was
an external magnet at 70 C (entry 1) and room temperature
(entry 2), respectively. Afterwards, the supernate was mixed with
3
O
4
2 3
bromobenzene, phenylboronic acid, and K CO to carry out the
[23]
o
(
the
Suzuki reaction for 30 min at 70 C. When the magnetic
o
3
O
4
2
separation was performed at 70 C, the corresponding biphenyl
o
7
product was obtained in 36% yield (the fresh catalyst) and 13%
6
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ChemCatChem
10.1002/cctc.201600994
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nd
yield (the catalyst used in 2 run), respectively. However, both
polymer nanocomposites at room temperature. In addition, when
nd
o
fresh catalyst and the catalyst used in 2 run gave 0% yield of
the magnetic separation was performed at 70 C, the fresh
nd
diphenyl when the magnetic separation was performed at room
temperature. The results showed that the Pd(II) could be
catalyst gave a higher yield of diphenyl than catalyst used in 2
run, which may because that most of the Pd(II) has been
reduced to Pd(0) after the first run.
o
released in solution at 70 C and recaptured on magnetic
[
a]
Table 4 Suzuki-Miyaura coupling reactions of bromobenzene with phenylboronic acid catalyzed by Pd/Fe
3 4
O @PMAA-Met.
[
b]
Yield
The temperature of magnetic
separation
Entry
1
Precedure
Fresh
catalyst
Catalyst used
in 2 run
nd
o
o
Catalyst and solvent were stirred for 30 min at 70 C,
70 C
36%
13%
magnetic separated. The supernate mixed with reagents
2 3
(bromobenzene, phenylboronic acid and K CO ) to continue
o
the reaction for 30 min at 70 C.
2
3
4
5
r.t.
0%
0%
o
The normal reaction was stopped and analyzed after 5 min.
70 C
28%
70%
31%
25%
62%
28%
o
70 C
The normal reaction was stopped after 5 min, magnetic
separated. The supernate continued to react for another 25
o
min at 70 C.
r.t.
[
a] Reaction conditions: bromobenzene (2 mmol), phenylboronic acid (2.4 mmol), K
2 3 3 4
CO (5 mmol), Pd/Fe O @PMAA-Met (0.01 mol%),
solvent (6 mL, EtOH: H O=1:1). [b] Isolated yield.
2
In order to further investigate the “release and catch”
mechanism in the normal reactions in the presence of the
[
24]
reagents (bromobenzene, phenylboronic acid and K
Suzuki reactions catalyzed by Pd/Fe @PMAA-Met in
O at 70 C was analyzed in the subsequent three
2 3
CO ), the
3
O
4
o
EtOH/H
2
experiments (Table 4 entries 3-5). The third experiment (entry 3)
was stopped after 5 min and gave the corresponding biphenyl
product in 28% yield with fresh catalyst. The fourth and fifth
experiments were stopped after 5 min, then the catalyst was
o
magnetically separated at 70 C (entry 4) and room temperature
(
entry 5), respectively. After the supernate continuing to react for
another 25 min, 70% and 31% yields were respectively obtained
using fresh catalyst. These results showed that “release and
catch” mechanism did occurred in the normal Suzuki reactions.
In this system, the catalytic moiety was released from solid
o
catalyst at 70 C and recaptured on the support at room
3 4
temperature, which allowed Pd/Fe O @PMAA-Met to combine
the advantages of homogeneous catalyst in high catalytic
activity and magnetic heterogeneous catalyst in easy
[
22]
nd
regeneration.
In addition, the catalyst used in 2 run got
similar results of the fresh catalyst in the presence of reagents,
Figure 8. Recycling and reuse of Pd/Fe
3 4
O
@PMAA-Met in the
o
indicating that Pd(0) could also be released at 70 C in this case
Suzuki-Miyaura
reaction
of
4-bromoacetophenone with
[18b, 25]
because of the oxidative addition of aryl bromides.
phenylboronic acid.
7
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The recyclability is a crucial factor to evaluate the supported
catalysts in view of industrial application and green chemistry.
Characterization
Fourier-transform infrared (FT-IR) spectra were measured on a
NEXUS670 (Nicolet, USA) spectrophotometer using KBr pellets.
Transmission electron microscopy (TEM) images were obtained
from a Tecnai-G2-F30 (FEI, USA) under an operating voltage of
3 4
The recycling experiments of Pd/Fe O @PMAA-Met catalyst
were carried out in the coupling reaction of 4-
bromoacetophenone with phenylboronic acid. After each run, the
3 4
Pd/Fe O @PMAA-Met catalyst was separated by a permanent
300 kV. The Pd content in the supported catalyst was
magnet, washed with deionized water and ethanol and dried
under vacuum, and then the catalyst was directly reused in the
next run. As shown in Figure 8, this magnetic nanocatalyst could
be used for at least twelve times with no obvious decrease in
isolated yield, despite that the reaction time needs to be
gradually prolonged. The prolonging of the reaction time is
probably caused by palladium leaching. The palladium leaching
was determined using AAS to be 4% (0.14 ppm) in the reaction
mixture after the third run. In addition, AAS analysis showed that
determined by an AA240 (Varian Corporation, USA) atomic
absorption spectrometer (AAS). X-ray diffraction (XRD)
measurements were carried out at room temperature using a
Shimadzu XRD-6000 spectrometer (Japan) with Nicker-filter Cu
Kα radiation (λ=0.15418 nm). The magnetic properties of the
nanoparticles were determined by
a
vibrating sample
magnetometer (VSM) (Lake Shore 7304, USA). X-ray
photoelectron spectrograph (XPS) was collected on an
Axis Ultra DLD electron spectrometer (Kratos, UK) with the
1
13
C1s=284.8 eV as internal standard. H and C NMR (400 and
00 MHz) spectra were measured on an AVANCE III 400 NMR
72% of palladium was retained on the magnetic polymer
1
nanocomposites after twelve cycles. Nonetheless, 88% yield
was still obtained in the twelfth run, indicating that this catalyst
has excellent recyclability in Suzuki reactions.
spectrometer (Bruker, Germany) using CDCl3 or DMSO-d6 as
solvents and TMS as the internal standard.
3 4
Preparation of Fe O nanoparticles
Fe
precipitation method.
.815 g of FeCl ·4H
water under vigorous mechanical stirring. Afterward, 10 mL of
3
O
4
nanoparticles12aw] ere synthesized by chemical co-
Conclusions
[
Typically, 2.070 g of FeCl
3 2
·6H O and
0
2
2
O were dissolved in 100 mL of deionized
In conclusion, we have developed
functionalized magnetic polymer nanocomposites-supported
palladium catalyst Pd/Fe @PMAA-Met, which showed
a
novel metformin-
NH
3
o
·H
2
O (25 %) was quickly injected into the reaction mixture at
3
O
4
50 C under Ar atmosphere. Then the solution was heated up to
o
excellent catalytic activity in the Suzuki-Miyaura reactions of aryl
halides (I, Br, Cl) with arylboronic acids. This nanocatalyst could
be easily recovered for further use, and have been used
consecutively for twelve times with excellent yields achieved.
Moreover, this catalyst possesses a high catalytic activity that
surpasses all the previously reported magnetic nanoparticles-
supported catalysts in terms of TOF values. This high activity
can be attributed not only to its nanometer size, but also to a
85 C and kept for 1 h. After the reaction completed, the black
Fe nanoparticles were separated using a permanent magnet
3
O
4
and washed with deionized water to neutral. Finally, the resulting
o
product was dried under vacuum at 50 C for 24 h.
3 4
Preparation of Fe O @MPS nanoparticles
The Fe
literature procedure. 500 mg of Fe
of NH ·H O (25 %) were homogenized in 50 mL of ethanol/water
volume ratio of 4:1) with an ultrasound mixer for 30 min.
3 4
O @MPS nanoparticles were prepared according to the
[26]
3
4
O nanoparticles and 2 mL
“
release and catch” catalytic mechanism that allows the benefits
3
2
of homogeneous catalysis to be combined with heterogeneous
catalysis. These beneficial features make the metformin-
functionalized nanocomposites-supported palladium catalyst
attractive for practical application.
(
Subsequently, 2 mL of MPS was added and the mixture was
o
maintained at 60 C for 24 h. After the reaction completed, the
resulting nanoparticles were collected by a permanent magnet
and washed with ethanol, water for three times, respectively.
Finally, the obtained product was dried under vacuum for 12 h to
give Fe
3
O
4
@MPS nanoparticles.
Experimental Section
Preparation of Fe
3
O
4
@PMAA
Materials
The distillation-precipitation polymerization was used to
synthesize magnetic polymer nanocomposites
Fe @poly(methacrylic acid) (Fe @PMAA). First, 100 mg
of Fe @MPS nanoparticles were dispersed in 80 mL of
acetonitrile with the help of an ultrasound mixer for 30 min. Then,
a mixture of 400 mg of methacrylic acid (MAA), 100 mg of BIS,
Dimethylamine hydrochloride (≥ 98%), Dicyandiamide (≥ 99%)
and Thionyl chloride (1.0 mol/L in methylene chloride,
EnergySeal) were purchased from Shanghai Energy Chemical
Company and used as received. Ferric chloride hexahydrate
[14]
3
O
4
3 4
O
3
O
4
(
FeCl
provided
Trimethoxysilyl)propylmethacrylate (MPS, ≥ 98%) was obtained
from Beijing J & K Scientific Company. Methacrylic acid (MAA, ≥
8%) was provided by Tianjin Guangfu Fine Chemicals and
purified by vacuum distillation. N, N’-Methylenebisacrylamide
BIS, ≥ 98%) was supplied by Sinopharm Chemical Reagent Co.
Ltd. and used without further purification. Azodiisobutyronitrile
AIBN) was obtained from Xi’an Chemical Reagent Factory
Company and recrystallized in ethanol. Palladium (II) chloride
PdCl , ≥ 59.5%) was provided by Shenyang Keda Reagents
3
·6H
2
O) and iron chloride tetrahydrate (FeCl
2
·4H
2
O) were
by Tianjin Guangfu Fine Chemicals.
3-
10 mg of AIBN was added to the aforementioned suspension.
(
The reaction mixture was bubbled with Ar gas for 30 min, and
heated up to boiling state. After that, 40 mL of acetonitrile was
distilled from the reaction mixture within 2 h. Finally, the
Fe O @PMAA nanocomposites were magnetically separated
3 4
and washed with absolute ethanol for several times and dried
under vacuum overnight.
9
(
(
3 4
Preparation of Fe O @PMAA-Met
(
2
Company. Other materials were of analytical grade and used as
received.
[
15, 27]
Synthesis of free Metformin
: A mixture of 4.160 g of
dimethylamine hydrochloride and 4.671 g of dicyandiamide was
8
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ChemCatChem
10.1002/cctc.201600994
FULL PAPER
put in a round-bottomed flask. Then the flask was heated in an
oil-bath at 145 C for 2 h. The obtained product was purified by
separated by an external magnet and washed with water (2 mL
×3) and ethyl acetate (2 mL×3), respectively, then dried and
directly used in the next run.
o
recrystallization in absolute ethanol for three times. After dried
o
under vacuum at 60 C for 8 h, the metformin hydrochloride was
obtained in 58% yield. Afterward, in another round-bottomed
flask, a mixture of 0.828 g (5 mmol) of metformin hydrochloride,
Acknowledgements
0.200 g (5 mmol) of NaOH and 50 mL of ethanol was stirred for
h at room temperature. Then, the suspension was filtered and
5
ethanol was removed with rotary evaporation leading to free
We gratefully acknowledge financial support from the National
: FT-IR (KBr cm ): 3372, Science Foundation for Fostering Talents in Basic Research of
[
27-28]
-1
metformin. Metformin hydrochloride
299, 3160, 1448, 1418, 1167, 1063. H NMR (400 MHz,
DMSO-d ): δ (ppm) = 7.22 (s, 2H, imine), 6.79 (s, 4H, amide),
1
3
the National Natural Science Foundation of China (J1103307).
6
13
2
.92 (s, 6H, CH
3 2
); C NMR (100 MHz, D O): δ (ppm) = 160.6,
Keywords: metformin • magnetic polymer nanocomposites •
158.9, 38.0.
palladium • Suzuki coupling reactions • heterogeneous catalyst
3 4 3 4
Chloroformylation of Fe O @PMAA: 150 mg of Fe O @PMAA
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[
[
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prepared as follows: A mixture of 0.646 g (5 mmol) of freshly
prepared free metformin, 1 mL of Et N and 1 mL of DMF was
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chloride Fe @PMAA nanoparticles and 30 mL of DMF was
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O
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@PMAA-Met: The metformin-functionalized
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kept at 120 C for 3 d. When the reaction completed, the
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P. Yang, R. Ma, F. Bian*
■ –■■
■
Palladium supported on metformin-
functionalized magnetic polymer
nanocomposites：A highly efficient
and reusable catalyst for the Suzuki-
Miyaura coupling reactions
Photo finish: A novel metformin-functionalized magnetic polymer nanocomposite
Fe O @PMAA-Met was used as a support for Palladium. The obtained catalyst was
3 4
magnetically separable and highly active for the Suzuki-Miyaura reactions. The
catalyst could be reused for 12 times with excellent yields achieved. A “release and
catch” mechanism proved to exist in the Suzuki reactions.
1
1
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