Palladium stabilized by 3,4-dihydroxypyridine-functionalized magnetic Fe3O4 nanoparticles as a reusable and efficient heterogeneous catalyst for Suzuki reactions

Article abstract of DOI:10.1039/c6ra01734g

A Pd supported on 3,4-dihydroxypyridine (Py)-functionalized Fe₃O₄ (Fe₃O₄/Py/Pd) hybrid material has been synthesized for the first time. Inductively coupled plasma (ICP), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and field emission scanning electron microscopy (FESEM) and Fourier transform infrared (FTIR) studies have been used to characterize the catalyst. The catalyst exhibited very high activity for Suzuki coupling reaction of several aryl halides with phenylboronic acids in aqueous phase at room temperature. The most interesting result of this work is the possibility to perform Suzuki reactions of aryl chlorides in the presence of the prepared catalyst. The yields of the products were in the range from 70% to 98%. In this process, the novel catalyst could be recovered in a facile manner from the reaction mixture by applying an external magnet device, and reused more than eight times without any significant loss in activity.

Full text of DOI:10.1039/c6ra01734g

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Palladium stabilized by 3,4-dihydroxypyridine-
functionalized magnetic Fe3O4 nanoparticles as
a reusable and efficient heterogeneous catalyst for
Suzuki reactions
Cite this: RSC Adv., 2016, 6, 27252
a
b
Mozhgan pirhayati, Hojat Veisi and Ali Kakanejadifarda
*
A Pd supported on 3,4-dihydroxypyridine (Py)-functionalized Fe3O4 (Fe3O4/Py/Pd) hybrid material has been
synthesized for the first time. Inductively coupled plasma (ICP), energy dispersive X-ray spectroscopy (EDX),
transmission electron microscopy (TEM), scanning electron microscopy (SEM), and field emission scanning
electron microscopy (FESEM) and Fourier transform infrared (FTIR) studies have been used to characterize
the catalyst. The catalyst exhibited very high activity for Suzuki coupling reaction of several aryl halides with
phenylboronic acids in aqueous phase at room temperature. The most interesting result of this work is the
possibility to perform Suzuki reactions of aryl chlorides in the presence of the prepared catalyst. The yields
of the products were in the range from 70% to 98%. In this process, the novel catalyst could be recovered in
a facile manner from the reaction mixture by applying an external magnet device, and reused more than
eight times without any significant loss in activity.
Received 20th January 2016
Accepted 1st March 2016
DOI: 10.1039/c6ra01734g
www.rsc.org/advances
nanostructured catalysts have been extensively investigated due
to their much higher activities than that of the corresponding
1. Introduction
bulk materials, however isolation and recovery of these tiny
nanocatalysts from the reaction mixture is not easy.2–6
Conventional techniques (such as ltration) are not efficient
because of the nano size of the catalyst particles so to overcome
this issue, the use of magnetic nanoparticles has emerged as
a viable solution. One of the attractive features of MNPs is the
possibility of fast and cost-efficient separation by applying an
external magnetic eld, which makes them as the ideal candi-
dates for practical use in catalysis process. This new direction in
catalysis galvanized the academic research and led to the
development of a great number of magnetic-based catalysts,
especially magnetite (Fe3O4) that found application in the
various reactions.7–16 Transition metal-catalyzed protocols,
especially palladium reactions, have become powerful tools in
chemical synthesis of many materials and compared with
frequently used palladium(II) complexes catalysts, palladium
nanoparticles were found to exhibit remarkable catalytic activ-
ities for Suzuki, Heck and Sonogashira cross-coupling reac-
tions,17–25 hydrogenation of many organic compounds,26–28
oxidation of hydrocarbon,29 isomerization,30 and decomposi-
tion of nitrogen monoxide,31 which have been extensively used
in the synthesis of drugs and ne chemicals.32–37 There have
been a variety of reports describing the use of magnetic nano-
composites for the immobilization of Pd nanoparticles, the
catalytic activity of Pd nanoparticles can be retained, and the
stability also can be improved to some extent.38–40 In continuing
our attempts toward the extension of efficient and
Green catalysis is a subchapter of green chemistry but probably
the most important one generally. Catalysts are divided into two
groups: most examples are homogeneous systems and the
others are heterogeneous systems. Homogeneous catalysts have
a number of other advantages, such as high selectivity, better
yield, and easy optimization of catalytic systems by rectication
of ligands and metals, but the difficulty of catalyst separation
from the nal product limits their use in industrial and
synthetic applications. To address the separation problems in
homogeneous catalysis, chemists and engineers have investi-
gated a wide range of strategies resulting in the use of hetero-
geneous catalyst systems which appeared to be the best logical
solution.1 The benet of heterogeneous catalysts is very simple
separation from the reaction but, a substantial decrease in the
activity of the immobilized catalyst is frequently observed due to
the loss of the catalyst in the separation processes and/or
diffusion factors. Consequently, we need a catalyst system
that not only shows high activity and selectivity (like a homo-
geneous system) but also possesses the ease of catalyst separa-
tion and recovery (like a heterogeneous system). These goals can
be achieved by nanocatalysts. Nanocatalysts bridge the gap
between homogeneous and heterogeneous catalysis, preserving
the desirable attributes of both systems. Recently,
aDepartment of Chemistry, Faculty of Science, Lorestan University, Khoramabad, Iran
bDepartment of Chemistry, Payame Noor University (PNU), 19395-4697 Tehran, Iran.
E-mail: hojatveisi@yahoo.com
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Scheme 1 Preparation of Fe3O4/Py/Pd(0) nanocatalyst.
environmentally benign heterogeneous catalysts,41–44 herein, we a magnet. By utilizing this property, it could be reused for eight
will report a simple preparation of palladium nanoparticles cycles without losing its catalytic activity, indicative of a poten-
incorporated into Fe3O4/Py/Pd(0) nanocomposite as a new tial application in industry.
magnetically recoverable heterogeneous catalyst (Scheme 1).
This recyclable heterogeneous catalyst was used to catalyze C–C
coupling reaction in an aqueous solvent by Suzuki reaction
(Scheme 2). Most important, the synthesized Fe3O4/Py/Pd(0)
nanocomposite presented good magnetic property, and could
2. Experimental
2.1. Preparation of the magnetic Fe3O4 nanoparticles
(MNPs)
be easily separated from the reaction mixture by using Magnetic nanoparticles were prepared via co-precipitation of
Fe(III) and Fe(II) ions with a molar ratio of 2 : 1 in the presence of
ammonium hydroxide. Generally, a mixture of FeCl3$6H2O
(5.838 g, 0.0216 mol) and FeCl2$4H2O (2.147ꢀg, 0.0108 mol) was
dissolved in 100 mL deionized water at 85 C under N2 atmo-
sphere and intense mechanical stirring (500 rpm). Aerwards,
10 mL of 25% NH4OH was quickly injected into the reaction
mixture in one section. The addition of the base to the Fe2+/Fe3+
Scheme 2 Fe3O4/Py/Pd for Suzuki cross-coupling reaction.
salt solution resulted in the formation of the black precipitate of
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MNPs instantly. The reaction continued for another 25 min and product was re-dispersed in ethanol by sonication and then was
the mixture was cooled to room temperature. The magnetic isolated with magnetic decantation for 5 times. The precipitated
nanoparticles as a dark solid were isolated from the solution by product (Fe3O4/Py) was dried at room temperature under
magnetic separation and washed several times by DI water.
vacuum.
2.2. Preparation of the Fe3O4/Py
2.3. Preparation of the Fe3O4/Py/Pd(0)
The obtained MNPs powder (500 mg) was dispersed in 50 mL The Fe3O4/Py (500 mg) were dispersed in CH3CN (30 mL) by
ethanol/water (volume ratio, 1 : 1) solution by sonication for 20 ultrasonic bath for 30 min. Subsequently, a yellow solution of
min, and 3,4-dihydroxypyridine (Py) (250 mg) was added to the PdCl2 (30 mg) in 30 mL acetonitrile was added to dispersion of
mixture. The mixture was stirred for 5 h at 40 ꢀC. The suspended Fe3O4/Py and the mixture was stirred for 10 h at room temper-
substance was separated with centrifugation. The settled ature. Then, the Fe3O4/Py/Pd(II) was separated by magnetic
Fig. 1 FT-IR spectra of (a) Fe3O4, (b) Fe3O4/Py, and (c) Fe3O4/Py/Pd.
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decantation and washed by CH3CN, H2O and acetone respec- ordination48 presumably leads to a shi of this peak to lower
tively to remove the unattached substrates. Scheme 1 depicted frequency. This shi can be observed comparing curve c. This
the synthetic procedure of Fe3O4/Py/Pd(II). The reduction of peak at curves b and c displayed the successful attachment of
Fe3O4/Py/Pd(II) by sodium borohydride was performed. The nal pyridine organic groups and subsequent coordination of Pd
product Fe3O4/Py/Pd(0) was washed by water and ethanol and nanoparticles within the hybrid material.
dried in vacuum at 40 ꢀC. The concentration of palladium in
Fe3O4/Py/Pd(0) was 3.75 wt% (0.353 mmol gꢁ1), which was Fe3O4/Py/Pd catalyst announced that the Pd nanoparticles with
The transmission electron microscopy (TEM) image of the
determined by ICP-AES.
nearly spherical morphology were formed on the surface of the
modied Fe3O4 nanoparticles (Fig. 2).
Field Emission Scanning Electron Microscope (FESEM)
shows the image of the synthesized Fe3O4/Py/Pd loaded
magnetite nanoparticles. It was conrmed that the catalyst was
made up of monotonic nanometer-sized particles (Fig. 3).
The existence of metallic Pd in the catalyst was also
conrmed by the EDX detector coupled to the SEM which
showed the presence of Fe, C, O and N in Fig. 4.
Fig. 5 shows a representative SEM image and corresponding
elemental maps for the synthesized catalyst. It can be seen that
Pd metal particles are well dispersed in the composite. The
selected-area elemental analysis gure reveals the presence of C,
O, Fe and Pd throughout the sample in a homogeneous manner,
which conrms the regular uniformity of the prepared sample.
The XRD pattern of the Fe3O4/Py/Pd is presented in Fig. 6.
The characteristic diffraction peaks in the sample at 2q of 30.3ꢀ,
35.7ꢀ, 43.4ꢀ, 53.8ꢀ, 57.2ꢀ and 62.8ꢀ are corresponded to the
diffraction of (220), (311), (400), (422), (511), and (440) of the
2.4. Suzuki coupling reactions
Catalytic activity of the synthesized Fe3O4/Py/Pd nanocomposite
was tested through the Suzuki reaction of aryl halides with
phenylboronic acid. In the typical experiment, a mixture of aryl
halide (1.0 mmol), phenylboronic acid (0.134 g, 1.1 mmol),
K2CO3 (0.276 mg, 2 mmol), and 7.0 mg Fe3O4/Py/Pd nano-
composite (7 mg z 0.021 mol% Pd) were added into 5 mL
water/ethanol (1 : 1) solution in a 25 mL Schlenk tube. The
mixture was then stirred for the desired time at room temper-
ature. The reaction was monitored by thin layer chromatog-
raphy (TLC, n-hexane/acetone; 4 : 1). Aer completion of the
reaction, 5 mL ethanol was added, and the catalyst was removed
by external magnet. Further purication was achieved by
column chromatography.
3. Results and discussion
3.1. Catalyst characterization
The Fe3O4 particles were synthesized according to the literature
using ferric chloride hexahydrate and ferrous chloride tetrahy-
drate upon addition of NH4OH. The Fe3O4/Py particles were
prepared by coating 3,4-dihydroxypyridine (Py) on the surface of
Fe3O4 particles. For the preparation of palladium loaded
materials, Fe3O4/Py particles were treated with a PdCl2 (Scheme
1), and then a sodium borohydride was added. Finally, the
mixture was collected by an external magnet, followed by drying
in vacuum. The Pd content in Fe3O4/Py/Pd catalyst was deter-
mined 3.75 wt% (0.353 mmol gꢁ1) by ICP-AES. The prepared
catalyst was characterized by, FTIR, SEM, FESEM, TEM, ICP and
EDX.
Fig. 1 shows Fourier transform infrared (FTIR) spectra for
Fe3O4, Fe3O4/Py, and Fe3O4/Py/Pd. The FTIR spectrum for the
MNPs alone shows a stretching vibration at 3408 cmꢁ1 which
incorporates the contributions from both symmetrical and
asymmetrical modes of the O–H bonds which are attached to
the surface iron atoms.46 The bands at low wave numbers (#700
cmꢁ1) come from vibrations of Fe–O bonds of iron oxide, in
which for the bulk Fe3O4 samples appear at 570 and 375 cmꢁ1
but for Fe3O4 nanoparticles at 624 and 572 cmꢁ1 as a blue shi,
due to the size reduction.47–49 The presence of an adsorbed water
layer is conrmed by a stretch for the vibrational mode of water
found at 1625 cmꢁ1. The FTIR spectra of Fe3O4/Py and Fe3O4/Py/
Pd show Fe–O vibrations in the same vicinity. The introduction
of pyridine groups to the surface of MNPs is conrmed by the
bands at 1627, 1432 and 3000 cmꢁ1 assigned to the C]N, C]C
and C–H stretching vibrations respectively. The C]N signal
Fig. 2 TEM image of Fe3O4/Py/Pd.
appeared at 1627 cmꢁ1 in curve b for the metal–ligand co- Fig. 3 FESEM images of Fe3O4/Py/Pd.
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Fig. 4 EDX data for the catalyst.
Fe3O4. The entire diffraction peaks match with the magnetic separate and recycle the prepared catalyst from the solution by
cubic structure of Fe3O4 (JCPDS 65-3107).50 Typical diffraction an external magnetic force.
peaks of Pd(0) correspond to (111), (200) and (220) are observed
at 40.1ꢀ, 46.7ꢀ and 68.2ꢀ (JCPDS 87-0638) in the pattern. The
results from XRD imply that the Pd nanoparticles have been
successfully immobilized onto the surface of the magnetic
3.2. Catalytic activity
In continuation of our interest in environmentally benign
chemical processes,41–44 aer characterization of the
particles.
prepared nanocatalyst to evaluate the catalytic ability of the
The magnetic measurements were carried out by VSM at
composites, Suzuki cross-coupling reaction was carried out
room temperature. The magnetization curves measured for
as model reactions (Scheme 2). It is well-known that the
Fe3O4/Py/Pd is presented in Fig. 7. The magnetic saturation
value is 55.6 emu gꢁ1. The decrease in the saturation magneti-
Suzuki cross-coupling reaction of arylhalides and phenyl-
boronic acid provides an efficient route to form C–C bond
under relatively mild conditions.45 The Fe3O4/Py/Pd catalyzed
Suzuki reaction between phenylboronic acid and bromo-
benzene was chosen as a model reaction to evaluate the
zation is due to the presence of the organic shell and Pd
nanoparticles on the Fe3O4 surface. Therefore, the above
mentioned results indicated an easy and efficient way to
effects of solvent (nonpolar, protic and aprotic), base (Et3N,
Fig. 5 SEM image of Fe3O4/Py/Pd shows the presence of C, O, Fe, Pd and Fe/Pd atoms in the catalyst.
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Table 2 Heterogeneous Suzuki–Miyaura reaction of aryl halides with
phenylboronic acids catalyzed by Fe3O4/Py/Pd at room temperaturea
Entry
R1C6H4X
R2C6H4B(OH)2
X
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
I
0.16
12
0.66
12
0.20
1
0.8
0.3
12
0.3
1.5
1
0.5
3
5
1
5
98
70
98
75
98
96
96
98
65
98
96
96
96
88
70
96
90
96
85
80
92
92
Cl
Br
Cl
I
Br
Br
I
Cl
I
Br
Br
I
Br
Br
I
Br
I
Br
Br
Br
Br
4-CH3
4-CH3
4-CH3
4-COCH3
4-COCH3
4-COCH3
4-CH3O
4-CH3O
4-Cl
3-NO2
3-NO2
2-CHO
2-Thienyl
2-Thienyl
1-Naphthyl
H
9
Fig. 6 XRD patterns of Fe3O4/Py/Pd.
10
11
12
13
14
15
16
17
18
19
20
21
22
H
H
4-NO2
4-NO2
4-CH3
4-CH3
2
3
3
1
4-COCH3
H
4-COCH3
1.5
a
Reactions were carried out under aerobic conditions in 3 mL of H2O/
EtOH (1 : 1), 1.0 mmol arylhalide, 1.1 mmol phenylboronic acids and 2
mmol K2CO3 in the presence of catalyst (0.007 g, 0.2 mol% Pd).
b
Isolated yield.
Fig. 7 VSM spectra of Fe3O4/Py/Pd.
scope of the reaction was explored with various aryl halides (I,
Br, Cl) and phenylboronic acids under the optimized conditions
(Table 2, entries 1–22). Under the optimized conditions, phenyl
iodides, bromides and chlorides all reacted efficiently with
phenylboronic acid (Table 2, entries 1–15). Aryl halides with
electron-withdrawing or releasing groups reacted with phenyl-
boronic acid to afford the corresponding products in high
yields. It was found that the yield when using an ortho
substituted aryl bromide was lower (Table 2, entry 15) than
those obtained with para- or meta-substituted aryl bromides
(Table 2, entries 4–12, 13 and 14). Notably, heteroaryl halides
such as 2-bromothiophene and 2-iodothiophene with phenyl-
boronic acid gave the corresponding coupled products in 96%
and 90% yields, respectively (Table 2, entries 16 and 17). The
most interesting result of this work is the possibility to perform
Suzuki reactions of aryl chlorides in the presence of prepared
NaOAc and K2CO3) and amount of catalyst at room temper-
ature. Optimization conditions studies are summarized in
Table 1.
The reactions were conducted using H2O/EtOH (1 : 1) as the
best solvent. Among the bases evaluated, K2CO3 was found to be
the most effective. The effect of catalyst loading was investi-
gated employing several quantities of the catalyst ranging from
0.1 mol% to 0.3 mol% (Table 1, entries 6, 8 and 9). The best
yield was obtained with 0.007 g (0.2 mol%) of the catalyst (Table
1, entry 6). To study the generality of this cross-coupling, the
Table
1 Optimization of the conditions for the Suzuki–Miyaura
reaction of bromobenzene with phenylboronic acida
Entry Solvent
Pd (mol%) Base
Time (min) Yieldb (%)
1
2
3
4
5
6
7
8
9
10
DMF
0.2
0.2
0.2
0.2
K2CO3
80
60
60
90
60
40
90
60
60
80
65
77
55
65
98
80
75
Toluene
EtOH
H2O
K2CO3
K2CO3
K2CO3
NaOAc
K2CO3
Et3N
K2CO3
K2CO3
No base 90
EtOH/H2Oc 0.2
EtOH/H2Oc 0.2
EtOH/H2Oc 0.2
EtOH/H2Oc 0.1
EtOH/H2Oc 0.3
EtOH/H2Oc 0.2
96
Trace
Fig. 8 (left) The recycling of the Fe3O4/Py/Pd for the Suzuki coupling
reaction under similar conditions; (right) recycled Fe3O4/Py/Pd cata-
lyst from reaction mixture.
a
Reaction conditions: bromobenzene (1 mmol), PhB(OH)2 (1.1 mmol),
Fe3O4/Py/Pd, solvent (3 mL). Isolated yield. c EtOH/H2O ¼ 1 : 1.
b
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Table 3 Catalytic performance of different catalysts in the coupling reaction of iodobenzene and bromobenzene with phenylbronic acid
Entry
Catalyst (mol%)
Conditions
X
Time (h)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
Bis(oxamato)palladate(II) complex (5)
NHC–Pd(II) complex (0.2)
SiO2-pA-Cyan-Cys-Pd (0.5)
Pd3(dba) (1)
Et3N, n-Bu4NBr, 120 ꢀ
C
I, Br
I, Br
I, Br
Br
2
5, 6
5, 5.5
24
6
0.5, 0.5
0.5, 0.75
0.16, 0.66
78, 65
98, 90
95, 88
77.7
51
52
53
54
55
56
57
K3PO4$3H2O, H2O, TBAB, 40 ꢀ
C
K2CO3, H2O, 100 ꢀ
C
K3PO4, THF, 80 ꢀ
C
Pd–BOX (2)
K2CO3, DMF, 70 ꢀC
Et3N, DMF, 100 ꢀC
I
100
g-Fe2O3–acetamidine–Pd (0.12)
Pd-isatin Schiff base-g-Fe2O3 (0.5, 1.5)
Fe3O4/Py/Pd (0.02)
I, Br
I, Br
I, Br
96, 96
95, 90
98, 98
Et3N, solvent-free, 100 ꢀ
K2CO3, H2O/EtOH, rt
C
This work
catalyst with good yields (Table 2, entries 2, 4 and 9). The
A comparison of the activity of various Pd catalysts with
heterogeneity of the catalyst was evaluated to study whether the Fe3O4/Py/Pd in the Suzuki coupling reaction published in the
reaction using solid Pd catalysts occurred on the Fe3O4/Py literature is listed in Table 3. From table, it can be seen that
surface or was catalyzed by Pd species leached in the liquid present catalyst exhibited higher conversions and yields
phase. To address this issue, two separate experiments were compared to the other reported system.
conducted with bromobenzene and phenyl boronic acid. In the
rst experiment, the reaction was terminated aer 20 min; in
4. Conclusion
this connection, the catalyst was separated from the reaction
mixture by an external magnet and the reaction was continued
with the ltrate for an additional 60 min. In the second exper-
The immobilized palladium on 3,4-dihydroxypyridine (Py)-
coated Fe3O4 was found to be effective in the Suzuki coupling
iment, the reaction was terminated aer 20 min. In both cases,
of various aryl halides and especially less reactive aryl chlorides
the desired product was obtained in the same yield (45%). Pd
with phenyl boronic acid. The benets of this catalyst are the
was not detected in the ltrate in either experiment by ICP-AES.
cheap and uncomplicated synthetic pathway, and mild and
These studies demonstrate that only the Pd bound to Fe3O4/Py
efficient C–C coupling reactions in green media. Notably, the
during the reaction is active, and the reaction proceeds on the
recyclability of the catalyst by a external magnet, with no
heterogeneous surface.
palladium leaching from Fe3O4/Py/Pd is good characteristics. It
The recyclability of Fe3O4/Py/Pd was further studied because
seems that 3,4-dihydroxypyridine have played an important role
the recycling of the heterogeneous catalyst was an important
in stabilization of catalyst particles owing to its structural
issue for practical applications. The recovered catalyst was
diversity.
added to reaction mixture under the same conditions (Table 1)
for eight cycles without a signicant loss of yield and catalytic
activity (Fig. 8, le side). This result also demonstrated that the
palladium leaching of the catalyst was low. In order to regen-
Acknowledgements
erate the catalyst, aer each cycle, it was separated by a magnet
We are thankful to Payame Noor University (PNU) and Lorestan
University for partial support of this work.
(Fig. 8, right side), and washed several times with deionized
water and ethanol. Then, it was dried in oven at 50 ꢀC. In
continuation of our works, a blank test by Fe3O4@Pd was per-
formed under same condition for this reaction coupling.
Interestingly, we have found that the catalyst was not recover-
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