Magnetic iron oxide nanoparticles–N-heterocyclic carbene–palladium(II): a new, efficient and robust recyclable catalyst for Mizoroki–Heck and Suzuki–Miyaura coupling reactions

Article abstract of DOI:10.1002/aoc.3475

A new and efficient nanoparticle–N-heterocyclic carbene–palladium complex was synthesized and characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction, transmission electron microscopy, elemental analysis, inductively coupled plasma analysis and vibrating sample magnetometry. This catalytic system was found to be a highly active catalyst in the Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions. These reactions were best performed in dimethylformamide and water, respectively, in the presence of only 0.054 mol% of palladium under mild conditions. Moreover, the catalyst could be recovered easily and reused at least ten times without any considerable loss of its catalytic activity. Copyright

Full text of DOI:10.1002/aoc.3475

Full paper
Received: 8 December 2015
Revised: 9 February 2016
Accepted: 11 February 2016
Published online in Wiley Online Library: 6 April 2016
(wileyonlinelibrary.com) DOI 10.1002/aoc.3475
Magnetic iron oxide nanoparticles–N-heterocyclic
carbene–palladium(II): a new, efficient and robust
recyclable catalyst for Mizoroki–Heck and
Suzuki–Miyaura coupling reactions
Abdol R. Hajipoura,b*, Nayereh S. Tadayonia and Zahra Khorsandia
A new and efficient nanoparticle–N-heterocyclic carbene–palladium complex was synthesized and characterized using Fourier
transform infrared spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy, energy-dispersive
X-ray analysis, X-ray diffraction, transmission electron microscopy, elemental analysis, inductively coupled plasma analysis and
vibrating sample magnetometry. This catalytic system was found to be a highly active catalyst in the Mizoroki–Heck and
Suzuki–Miyaura cross-coupling reactions. These reactions were best performed in dimethylformamide and water, respectively,
in the presence of only 0.054mol% of palladium under mild conditions. Moreover, the catalyst could be recovered easily and
reused at least ten times without any considerable loss of its catalytic activity. Copyright © 2016 John Wiley & Sons, Ltd.
Keywords: Mizoroki–Heck reaction; Suzuki–Miyaura reaction; magnetic nanoparticles; N-heterocyclic carbenes
develop more stable, efficient and user-friendly catalysts.[40–43]
The direct syntheses of imidazolium salt on MNPs and preparation
Introduction
The first use of N-heterocyclic carbene (NHC) ligands in catalytic
applications was first reported by Herrmann and co-workers[1]
of palladium–NHC complexes with catalytic activity in Mizoroki–
Heck and Suzuki–Miyaura reactions are rare and, to the best of
our knowledge, only a few examples have so far been repor-
ted.[21–25] Furthermore, superior catalytic activities of palladium–
NHC complexes immobilized on MNPs have prompted us to
develop novel and facile ways for the preparation of robust and
recyclable palladium–NHC catalysts.
In a continuation of our recent investigations of the application
of heterogeneous catalytic systems in cross-coupling
reactions,[44–50] herein we report a study of the efficiency of our
new catalyst in the Mizoroki–Heck and Suzuki–Miyaura cross-
coupling reactions.
;
subsequently, they have attracted many chemists’ attention as
suitable transition metal ligands. Compared with phosphine
ligands, NHCs have the same σ-donor and low σ-acceptor ability
and can form stronger bonds with the majority of metals; in addi-
tion, they have lower toxicity, higher dissociation energies and
more stability in air and moisture.[2–12] Therefore, NHC complexes
have been used widely as efficient homogeneous catalysts in a
number of organic transformations such as Mizoroki–Heck and
Suzuki–Miyaura reactions.[13–30] Palladium–NHC complexes show
excellent catalytic activities in these cross-coupling transformation
as fundamental reactions in organic synthesis which have many
applications in the synthesis of pharmaceuticals, fine chemicals
and natural products.[31,32]
Results and discussion
Despite the higher catalytic activities of homogeneous catalysts
than their heterogeneous counterparts, the application of the for-
mer has some drawbacks such as their cost and non-reusability,
but these disadvantages can be mostly overcome by using suitable
supports to generate heterogeneous catalysts. Several types of het-
erogeneous NHC–palladium catalysts have been developed by
immobilizing palladium on various solid supports.[33–37] However,
these supported catalysts suffer from several problems such as easy
agglomeration and leaching of palladium from the catalyst to the
reaction mixture. Therefore, the development of new catalytic sys-
tems that are not only stable and active, but also easy to separate
and re-use is highly desirable to facilitate actual practical applica-
tions in carbon–carbon coupling reactions. In this field, magnetic
separation provides a very useful and efficient approach.[38,39] An-
choring of palladium compounds on the surfaces of magnetic
nanoparticles (MNPs), by grafting complex agents, can be used to
The iron oxide MNPs–NHC–palladium(II) (MNPs-NHC-Pd(ΙΙ)) catalyst
was prepared using a procedure shown in Scheme 1. The proposed
structure of the catalyst contains pyridine according to a previous
report.[51]
The products of each step were characterized using Fourier
transform infrared (FT-IR) spectra (Figs 1 and 2). FT-IR spectrum (a)
*
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156, IR
Iran. E-mail: haji@cc.iut.ac.ir
a Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, IR, Iran
b Department of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison, WI, 53706-1532, USA
Appl. Organometal. Chem. (2016), 30, 590–595
Copyright © 2016 John Wiley & Sons, Ltd.

Mizoroki–Heck and Suzuki–Miyaura coupling reactions
structure of the catalyst. The FT-IR spectrum is very similar to that
of a previously reported NHC–Pd catalyst.[52]
The morphology of the surfaces of MNPs-NHC-Pd(ΙΙ) was studied
using field emission scanning electron microscopy (FE-SEM; Fig. 3
(a)). The energy-dispersive X-ray analysis (EDX) results, obtained
from SEM analysis of MNPs-NHC-Pd(ΙΙ), clearly show the presence
of Pd nanoparticles in the catalyst (Fig. 3(b)).
Further characterization of MNPs-NHC-Pd(ΙΙ) was performed
using transmission electron microscopy (TEM). The TEM images of
the catalyst show well-defined dispersed spherical palladium parti-
cles in a silica bed (Fig. 4(a)). From the particle size histogram in
Fig. 4(b), the nanoparticle size is about 15–35 nm.
Scheme 1. Synthesis of catalyst.
The catalyst was subjected to CHN analysis. The nitrogen content
is found to be 2.01%. According to this value, the amount of at-
tached NHC ligand is 0.45 mmol gꢀ1. The Pd loading of the catalyst
was measured using inductively coupled plasma (ICP) analysis, and
is 0.017 mmol gꢀ1
.
The X-ray diffraction (XRD) pattern indicates a crystalline struc-
ture with peaks attributed to Fe3O4 at 2θ of 30.1°, 35.4°, 43.1°,
53.4°, 56.9° and 62.5° and a peak at 2θ of 15–25° ascribed to
Figure 1. FT-IR spectra: (a) MNPs; (b) silica-coated magnetite nanoparticles
(SiO2@Fe3O4); (c) chloropropyltrimethoxysilane on the surface of MNPs (1).
Figure 3. (a) FE-SEM images and (b) SEM–EDX spectrum of catalyst 3.
Figure 2. FT-IR spectra: (d) phenylimidazolium chloride–MNPs (2); (e)
MNPs-NHC-Pd(ΙΙ) (3).
in Fig. 1 shows absorption bands at 560–570 cmꢀ1 (Fe–O stretching
vibration). In spectra (b) and (c), the characteristic peaks at
1090 cmꢀ1 (Si–O) and 797 cmꢀ1 (Si–O–Si) confirm the successful
coating of silica and chloropropyltrimethoxysilane on the surface
of the MNPs. Furthermore, the absorption bands at 1740 cmꢀ1
(C¼N) in spectrum (d) in Fig. 2 indicates modification of the NHC
precursor. Finally several signals characteristic of Pd–C vibrations
detected in spectrum (e) (1645, 1458, 685 cmꢀ1) confirm the
Figure 4. (a) TEM images and (b) particle size distribution for catalyst 3.
Appl. Organometal. Chem. (2016), 30, 590–595
Copyright © 2016 John Wiley & Sons, Ltd.
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A. R. Hajipour, N. S. Tadayoni and Z. Khorsandi
SiO2.[42,43] The XRD pattern confirms that Pd(ΙΙ) is successfully
loaded on the catalyst (Fig. 5).
The thermal stability of the catalyst was evaluated using thermo-
gravimetric analysis (TGA), carried out in nitrogen atmosphere be-
tween 80 and 800°C at a rate of 10°C minꢀ1. The TGA plots of the
catalyst depict a two-step thermal decomposition. The first step of
weight loss (below 200°C) corresponds to the removal of physically
adsorbed water. The main weight loss of the second step is due to
the removal of organic moieties on the surface. This observation can
be attributed to the development of a stable palladium complex
(Fig. 6). The degree of functionalization on the MNPs can be calcu-
lated from the weight loss between 200 and 800°C of TGA and ele-
mental analysis. The observed total weight loss for the catalyst is
15.1%. On the basis of this value, the amount of organic moieties
is about 0.41 mmol gꢀ1 of catalyst. A good agreement is observed
between elemental analysis and TGA data for these conversions.
The magnetization curve of the catalyst is shown in Fig. 7. The mag-
netization of Fe3O4 is 69.4 emu gꢀ1 and that of the catalyst is
13.5 emu gꢀ1. These results indicate the paramagnetic nature of the
catalyst. Because of the coating of MNPs with silica, the magnetization
value of the catalyst is significantly lower than that of bulk magnetite.
Compound 3 (Scheme 1) was obtained as an air- and moisture-
stable catalyst and its catalytic activity was investigated in
Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions.
Firstly, base, solvent and reaction temperature were examined in
the coupling reaction of bromobenzene (1.0mmol) and styrene
(1.2 mmol) as a model reaction of Mizoroki–Heck cross-coupling (
Table 1). Various solvents such as H2O, EtOH–H2O and
dimethylformamide (DMF) were examined. It is also found that
the reaction temperature has a great influence on this reaction. A
Figure 7. Room temperature magnetization curve of catalyst 3.
Table 1. Optimization of reaction conditions for Mizoroki–Heck reac-
tion of bromobenzene with styrene in the presence of 3a
Entry Catalyst 3 (mg)
Base
Solvent
DMF
Temp. (°C) Yield (%)b
1
10
10
10
10
10
10
10
10
10
10
10
20
5
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
100
70
80
r.t.
70
70
70
70
70
70
70
70
70
87
87
85
43
50
87
69
66
57
61
48
89
74
2
DMF
3
DMF
4
DMF
5
EtOH–H2O
DMSO
NMP
6
7
8
DMF
9
K3PO4
Na2CO3
DMF
10
11
12
13
DMF
NaHSO4 DMF
K2CO3
K2CO3
DMF
DMF
aReaction carried out with bromobenzene (1.0 mmol), styrene
(1.2 mmol), base (2 equiv.), solvent (2.0 ml) for 20 min.
bGC yield.
relative improvement in the conversion (87%) is achieved at 70°C.
At lower reaction temperatures (50°C and room temperature), the
yield decreases even after long reaction times; higher reaction tem-
perature (100°C) gives no better yield. Therefore, all reactions were
carried out at 70°C. The influence of base in this reaction was also
studied; using bases such as K3PO4 or KOH instead of K2CO3 clearly
gives a lower conversion. The influence of amount of catalyst was
also investigated, with the result that 10 mg of catalyst (0.018 mol
% of Pd; calculated using ICP analysis) was applied as the optimal
amount (Table 1, entry 2).
Figure 5. XRD pattern of catalyst 3.
These optimized reaction conditions were used for Heck
arylation of various aryl halides with a variety of olefins. As is evi-
dent from Table 2, variously substituted aryl iodides, aryl bromides
and aryl chlorides are transformed into the corresponding products
in good to excellent yields.
The results show that aryl halides with electron-withdrawing sub-
stituents give lower yields than derivatives containing electron-
donating groups. However, aryl chlorides react more slowly than
aryl bromide and aryl iodide derivatives. Among aryl halides, aryl
Figure 6. TGA curve of pure catalyst 3.
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Appl. Organometal. Chem. (2016), 30, 590–595

Mizoroki–Heck and Suzuki–Miyaura coupling reactions
Table 2. Scope of Mizoroki–Heck cross-coupling reactiona
the reaction of bromobenzene with phenylboronic acid was chosen
as a model reaction and the effect of various bases and solvents
was studied (Table 3). It seems that K2CO3 acts as a better base than
the others and EtOH–H2O (1:1) gives the best results among various
solvents. The reactions at various temperatures were also examined
and room temperature is found to be optimal. Increasing the tem-
perature does not have any significant effect on the yields and the
time of reactions.
Entry
X
R
Olefinb
MA
Yield (%)c
1
I
I
I
I
H
98
89
90
96
95
98
90
97
55
90
95
60
90
88
97
96
65
73
85
42
63
59
87
76
45
87
91
73
79
61
2
3-Br
MA
Under the optimized reaction conditions (Table 3, entry 1), aryl io-
dides and aryl bromides with various substituents are converted to
their corresponding products in good to excellent yields (Table 4).
For understanding the effect of iron fragment on catalyst perfor-
mance, a new catalyst without Fe3O4 fragment with silica was synthe-
sized and the Heck model reaction (bromobenzene (1.0 mmol) and
styrene (1.2 mmol)) was performed again. The activity was compared
to that for our catalyst. No significant change is observed, so it can be
concluded that Fe3O4 does not have an effect on catalytic activity.
Leaching of metal ions is a serious problem for supported metal
catalysts, and prevents catalyst separation and recycling. To explore
the leaching of the catalyst, two similar control experiments were per-
formed. To this end, the reaction between bromobenzene and sty-
rene was carried out simultaneously in two different vessels under
exactly the same conditions. In each case, the reaction progress was
monitored using GC. After 10 min the catalyst was separated from
one of the vessels using a magnetic bar, while the other remained un-
changed. Thereafter, both of the reactions were continued under the
same conditions. After another 10 min, both reactions were stopped
and their completion was again examined using GC. The results show
that the reaction in the absence of the catalyst no longer proceeds,
while completion of the reaction in the other vessel is improved. This
observation confirms that no leaching occurs in the reaction medium
during the process. In fact, the leaching phenomenon that is often is
seen for heterogeneous catalytic systems does not occur here, per-
haps because of the specific nature of the designed catalyst.
Catalyst recycling is an essential property from economic,
environmental and industrial points of view. Therefore, the catalyst
reusability was checked in the Mizoroki–Heck reaction of
bromobenzene (1.0 mmol) and styrene (1.2 mmol). At the end of
each reaction, the catalyst was separated, using an external
magnetic field, washed with ethanol and acetone and reused. As
3
4-NO2
4-OMe
H
MA
4
MA
5
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
I
MA
6
4-OMe
4-NH2
3-OMe
2-NO2
4-Br
MA
7
MA
8
MA
9
MA
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
MA
4-OCH3
2-OMe
4-Cl
MA
MA
MA
4-CN
4-Cl
MA
MA
4-CH3
4-NO2
H
MA
MA
MA
4-OMe
4-NH2
4-OMe
H
MA
MA
Styrene
Styrene
Styrene
Styrene
Styrene
MMA
MMA
MMA
MMA
MMA
I
Br
Br
Cl
I
H
4-OMe
4-OMe
4-OMe
H
I
Br
Br
Cl
4-OMe
H
4-OMe
aReaction carried out with aryl halide (1.0 mmol), alkene (1.2 mmol),
K2CO3 (2.0 equiv.) in 2.0 ml of DMF, 30 mg of catalyst (0.054 mol%
of Pd) at 70°C for 20 min.
bMA, methyl acrylate; MMA, methyl methacrylate.
cIsolated yield.
Table 3. Optimization of reaction of bromobenzene with
phenylboronic acida
chlorides are ideal substrates for coupling reactions due to their be-
ing inexpensive and widely available in comparison to their bro-
mide or iodide counterparts. The first step in catalytic coupling
reactions (oxidative addition) is not favored with aryl chloride sub-
strates because of the high strength of the C–Cl bond. The most re-
ported methods require high loadings of palladium catalyst and
harsh conditions; anyway those catalysts show little or no activity
with aryl chloride substrates. In contrast, Heck coupling of even less
reactive aryl chloride derivatives with olefins gives good yields for
the present method as evident from Table 2.
The MNPs-NHC-Pd(ΙΙ) catalyst displays high activities which may
be related to good dispersion and small size of the palladium nano-
particles. It is noteworthy that the reactions are completed within
20 min at room temperature under mild conditions, and the prod-
ucts are obtained in excellent yields.
Entry
Base
Solvent
Temp. (°C)
Yield (%)b
1
2
3
4
5
6
7
8
K2CO3
KOH
EtOH–H2O
EtOH–H2O
EtOH–H2O
H2O
r.t.
r.t.
r.t.
r.t.
r.t
95
73
84
36
59
68
95
96
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
EtOH
PEG (200)
EtOH–H2O
EtOH–H2O
r.t.
80
100
aReaction carried out with bromobenzene (1.0 mmol), phenylboronic
acid (1.1 mmol) and solvent (2.0 ml) for 30 min.
bIsolated products.
Encouraged by the excellent results obtained for the Heck reac-
tion, we then examined the activity of this catalytic system for the
Suzuki–Miyaura reaction. In order to optimize reaction conditions,
Appl. Organometal. Chem. (2016), 30, 590–595
Copyright © 2016 John Wiley & Sons, Ltd.
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A. R. Hajipour, N. S. Tadayoni and Z. Khorsandi
Table 4. Suzuki–Miyaura cross-coupling of aryl halides and
phenylboronic acid in the presence of 3a
Entry
X
RC6H4X
Time (h)
Yield (%)b
1
I
I
I
H
0.5
0.5
0.5
1
97
96
98
93
78
75
98
91
89
96
94
93
95
96
94
65
71
74
2
4-OMe
4-NO2
H
3
4
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
5
4-NO2
4-Cl
1
Figure 8. FT-IR spectra of fresh MNPs-NHC-Pd(ΙΙ) (red) and of reused MNPs-
NHC-Pd(ΙΙ) after ten runs (green).
6
1
7
2-Cl
2
8
4-OMe
4-Me
4-OCH3
3-OCH3
2-OCH3
3-Cl
1
faster reaction times (20 and 30 min, respectively), lower tempera-
tures and greener media (room temperature and aqueous media
for Suzuki reaction) in comparison with current methods.
9
1
10
11
12
13
14
15
16
17
18
1
1
2
1
Conclusions
2-Cl
2
4-CHO
4-NO2
4-OMe
H
1
In summary, the synthesis of a new, efficient and facile recyclable
nanocatalyst (MNPs-NHC-Pd(ΙΙ)) is reported and its application to
the Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions
was investigated. This approach is a highly efficient one, which pro-
vides high yields of products in short reaction times and under mild
conditions with the use of a robust and recyclable catalyst.
2
2
2
aReaction carried out with aryl halide (1.0 mmol), phenylboronic acid
(1.1 mmol) using K2CO3 (1.5 mmol) and 2 ml of EtOH–H2O (1:1 v/v)
in the presence of 10 mg of catalyst (0.036 mol% of Pd) at room
temperature.
bGC yield.
Experimental
Materials
evident from Table 5, the catalyst can be reused ten times without
significant loss of activity. The yield for the tenth run is 90%.
The recovered catalyst was analyzed using FT-IR spectroscopy, and
the result is shown in Fig. 8. Surprisingly, no marked changes are
observed in catalyst structure after being used for 10 reaction rums.
Furthermore, ICP and CHN analyses (N = 1.91%) of 3 were carried out
after ten reuses and the results show that only a very small amount
(less than 1%) of palladium metal is removed from the catalyst.
It should be noted that, to the best of our knowledge, this
phosphine-free palladium catalyst is a rare demonstration of the
combination of NHC and Pd supported on MNPs to generate a
highly efficient and robust recyclable catalyst that can be reused
more than 10 times without any considerable loss of its catalytic ac-
tivity. In addition, this efficient catalytic system results in Mizoroki–
Heck and Suzuki–Miyaura cross-coupling reactions in significantly
All chemicals were purchased from Merck Chemical Co. (Germany)
and Sigma-Aldrich. The reagents were used without any further
purification. All the organic products were known and identified
by comparison of their physical and spectral data with those of
authentic samples.
Instrumentation and analyses
1H NMR and 13C NMR spectra were obtained using a Bruker Avance
500 MHz spectrometer with samples in CDCl3. FT-IR spectra were
recorded with a Jasco-680 (Japan) spectrophotometer in KBr pel-
lets. Powder XRD patterns were obtained using an X’Pert MPD, with
Cu Kα radiation (40 kV, 30 mA). SEM images were obtained using a
Philips XL20 microscope. TEM images were obtained using a Philips
CM10 microscope. The palladium content in the catalyst was mea-
sured using ICP analysis. Magnetic measurements were done with a
vibrating sample magnetometer in the range 0.0001–50 emu. TGA
was conducted with a heating rate of 10°C minꢀ1 in the range
30–600°C under nitrogen atmosphere.
Table 5. Recyclability of catalyst in Mizoroki–Heck reactiona
Run
Yield (%)b
Run
Yield (%)b
1
2
3
4
5
87
85
86
86
86
6
85
86
84
82
80
Synthesis of catalyst
7
MNPs were prepared according to the literature[40] based on pre-
cipitation of magnetite nanoparticles from a mixture of iron(III)
chloride and iron(II) sulfate with ammonia (25% solution in water).
Next, in a round-bottom flask equipped with mechanical stirrer
and condenser, a mixture of SiO2@Fe3O4 (4.03 g) and chloro-
propyltrimethoxysilane (0.4 mmol, 0.74 ml) was refluxed in dry tolu-
ene (100 ml) at 110°C for 3 days. After this time, the MNPs were
8
9
10
aReaction carried out with bromobenzene (1.0 mmol), styrene
(1.2 mmol), base (2 equiv.), solvent (2.0 ml) for 20 min.
bIsolated yield.
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Mizoroki–Heck and Suzuki–Miyaura coupling reactions
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Acknowledgments
Financial support from Isfahan University of Technology (IUT), Iran is
appreciated. Additional financial support from the Center of Excel-
lence in Sensor and Green Chemistry Research (IUT) and the Iran
Nanotechnology Initiative Council is gratefully acknowledged.
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