Pd immobilized on modified magnetic Fe3O4 nanoparticles: Magnetically recoverable and reusable Pd nanocatalyst for Suzuki-Miyaura coupling reactions and Ullmann-type N-arylation of indoles

Article abstract of DOI:10.1007/s12039-016-1098-9

The Pd supported on amidoxime (AO)-functionalized Fe₃O₄ (Fe₃O₄/AO/Pd) hybrid material was used as an effective and recyclable nanocatalyst in Suzuki-Miyaura coupling reactions. The catalyst was very effective for the Suzuki-Miyaura reaction of aryl halides (Ar–I, Ar–Br, Ar–Cl) with phenylboronic acid and conversion was excellent in most cases. The yields of the products were in the range from 7–98%. The catalyst showed good stability and could be recovered and reused for six reaction cycles without a significant loss in its catalytic activity. Also, a wide range of N-arylated indoles are selectively synthesized through intermolecular C(aryl)–N bond formation from the corresponding aryl iodides and indoles through Ullmann-type coupling reactions in the presence of the prepared catalyst. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1098-9

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1098-9
Pd immobilized on modified magnetic Fe3O4 nanoparticles: Magnetically
recoverable and reusable Pd nanocatalyst for Suzuki-Miyaura coupling
reactions and Ullmann-type N-arylation of indoles
RAMIN GHORBANI-VAGHEIa,∗, SABA HEMMATIa and MALAK HEKMATIb
aDepartment of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, 6517838683 Hamedan, Iran
bDepartment of Pharmaceutical Chemistry, Faculty of Pharmaceutical Chemistry, Pharmaceutical Sciences
Branch, (IAUPS), Tehran, Iran
e-mail: rgvaghei@yahoo.com
MS received 21 January 2016; revised 3 April 2016; accepted 30 April 2016
Abstract. The Pd supported on amidoxime (AO)-functionalized Fe3O4 (Fe3O4/AO/Pd) hybrid material was
used as an effective and recyclable nanocatalyst in Suzuki-Miyaura coupling reactions. The catalyst was very
effective for the Suzuki-Miyaura reaction of aryl halides (Ar–I, Ar–Br, Ar–Cl) with phenylboronic acid and
conversion was excellent in most cases. The yields of the products were in the range from 7–98%. The catalyst
showed good stability and could be recovered and reused for six reaction cycles without a significant loss in its
catalytic activity. Also, a wide range of N-arylated indoles are selectively synthesized through intermolecular
C(aryl)–N bond formation from the corresponding aryl iodides and indoles through Ullmann-type coupling
reactions in the presence of the prepared catalyst.
Keywords. Fe3O4/AO/Pd; magnetic nanoparticles; heterogeneous catalyst; Pd nanoparticles; suzuki;
ullmann.
1. Introduction
of the support followed by an adequate thermal treat-
ment.12 Mesoporous supports (MCM-41, SBA) are
frequently used supports of PdNPs-MNPs.13,14a−c Other
supports, such as polymer-coated MNPs14d−g ionic-
liquid-modified MNPs,14h sulfonated graphene-deco-
rated MNPs,14i are also employed for the stabilization
of PdNPs.
The immobilization methods used to deposit palla-
dium into heterogeneous solid beds have been studied
extensively, and diverse supports on clay,15 carbon nano-
fiber,16 montmorillonite,17 magnetic mesoporous silica,18
zeolite,19 and metal oxides20 have been investigated. A
current challenge in this area is the development of effi-
cient immobilized systems that could simultaneously
fulfil the usual targets of achieving high TON values
and facilitate recovering and reuse Further, there is the
need for obtaining Pd-free final products,21,22 comply-
ing with the strict purity specifications of the pharma-
ceutical industry.23,24 In particular, iron oxide magnetic
nanoparticles (MNPs) have received considerable atten-
tion as they are biocompatible and can be recovered
easily from reaction mixtures by using a simple external
magnetic field.
Catalyst plays a significant role in the production of
chemicals today and nanomaterials have the potential
for improving efficiency, selectivity, and yield of cataly-
tic process. The higher surface to volume ratio means
that many more catalysts are actively participating
in the reaction. The potential for cost saving is tremen-
dous from the perspective of material, equipment,
labour and time. Higher selectivity means less waste
and fewer impurities, which could lead to safer and
reduced environmental impact. The study of applica-
tion of metal nanoparticles in catalysis, particularly, on
organic transformations, has become a frontier area of
research in nanocatalysis.1 Among the different metal
nanocatalysts, palladium nanoparticles (PdNPs) have
gained much reputation, because palladium is a versa-
tile catalyst in modern organic synthesis and is widely
used for a significant number of synthetic transforma-
tions2–4 suchas, Heck, Suzuki, Stille, Sonogashiracross-
coupling reactions and N-arylation of heterocycles.5–11
Classically, the fixation of NPs on supports uses
mostly the reduction of metal salts in the presence
Recently, we have reported modified single-walled
carbon nanotubes and modified- mesoporous SBA-15
∗For correspondence

Ramin Ghorbani-Vaghei et al.
as a support to stabilize palladium nanoparticles 2. Experimental
and its applications in Suzuki-Miyaura and Ullmann
coupling reactions.25 Also, very recently, we prepared
a novel amidoxime-functionalized Fe3O4 nanoparti-
cles for deposition of palladium nanoparticles (Fe3O4/
AO/Pd) as an efficient and separable catalyst for Sono-
gashira coupling reaction.13f In continuation of our
interest on the synthesis of novel supported catalysts,26
we decided to use amidoxime-functionalized Fe3O4
nanoparticles for stabilization of palladium nanoparti-
2.1 Preparation of the magnetic Fe3O4 nanoparticles
(MNPs)
Naked Fe3O4 nanoparticles were prepared according to
the literature method.13c
2.2 Preparation of the Fe3O4/AO
cles (figure 1) and its application as a catalyst in Suzuki- The obtained MNPs powder (500 mg) was dispersed
Miyaura and Ullmann coupling reaction under mild and in 50 mL toluene solution by sonication for 20 min,
low palladium loading conditions.
and then triethoxyethylcyanide (3 mmol) was added
OH
HO
OH
Fe3O4
HO
HO
OH
OH
OH
Fe3O4 MNPs
1. triethoxyethylcyanide
2. NH2OH
3. PdCl2
4. NaBH4
OH
AO
N
C
OA
Pd
O
O
O
Si
NH2
Fe3O4
OA
OA
AO
Fe3O4/AO/Pd
AO
Figure 1. Preparation of Fe3O4/AO/Pd and its SEM and TEM images.

Pd supported on amidoxime (AO)-functionalized Fe3O4 NPs
to the mixture. The mixture was refluxed under argon 25 mL balloon, 1.1 mmol of the aryl iodides in 3 mL
atmosphere at 100◦C for 48 h. The product was sepa- of DMF was added 1 mmol of indoles and 2 mmol
rated by filtration, washed with ethanol, and dried of Et3N. The mixture was then refluxed for 2 h at
under vacuum for 24 h at 50◦C The precipitated prod- 110◦C. After completion of reaction monitored by TLC
uct (Fe3O4/Ethyl-CN) was dried at room temperature (n-Hexane/acetone, 4:1), the reaction mixture was
under vacuum. Then, the [2-cyanoethyl]-functionalized cooled to room temperature (the catalyst was recovered
MNPs (1 g) were immersed in NH2OH aqueous solu- by magnet) and was extracted with diethyl ether three
tion (50 mL, 50%) at 65◦C for 5 h (figure 1). The ami- times (3 × 10 mL). The combined organic layers were
doxime groups in MNPs (Fe3O4/AO) were filtrated off, washed with brine solution and dried over anhydrous
washed with distilled water for several times and dried. Na2SO4 and concentrated in vacuum. The crude prod-
uct was purified by column chromatography on silica
2.3 Preparation of the Fe3O4AO/Pd(0)13c
gel (60–120 mesh) to provide the N-aryl indoles.
The Fe3O4/AO (500 mg) were dispersed in CH3CN
(30 mL) by ultrasonic bath for 30 min. Subsequently,
a yellow solution of PdCl2 (30 mg) in 30 mL ace-
tonitrile was added to dispersion of Fe3O4/AO and the
mixture was stirred for 10 h at 20–25◦C. Then, the
Fe3O4/AO/Pd(II) was separated by magnetic decanta-
tion and washed by CH3CN, H2O and acetone, respec-
tively, to remove the unattached substrates.
The reduction of Fe3O4/AO/Pd(II) by hydrazine
hydrate was performed as follows: 50 mg of Fe3O4/
AO/Pd(II) was dispersed in 60 mL of water, and then
100 μL of hydrazine hydrate (80%) was added. The pH
of the mixture was adjusted to 10 with 25% ammonium
hydroxide and the reaction was carried out at 95◦C for
2 h. The final product Fe3O4/AO/Pd(0) was washed with
water and dried in vacuum at 40◦C. Figure 1 depicted
the synthetic procedure of Fe3O4/AO/Pd. The concen-
tration of palladium in Fe3O4/AO/Pd was 1.83 wt%
(0.17 mmol/g), which was determined by ICP-AES and
EDS.
3. Results and discussion
As a part of our ongoing research program to develop
new synthetic methodologies, after successful fabri-
cation of the catalyst,13f we have again investigated
the morphology surface by FESEM and TEM images
(figure 1). The FESEM image (right) of the synthesized
Fe3O4/AO/Pd confirmed that the catalyst was made
up of uniform nanometer-sized particles. The TEM
image of the Fe3O4/AO/Pd catalyst (left) revealed that
the Pd nanoparticles with nearly spherical morphol-
ogy were formed on the surface of the modified Fe3O4
nanoparticles. In the transmission electron microscopy
images, iron oxide nanoparticles of 10–15 nm in diam-
eter and palladium nanoparticles of ∼3 nm, entrapped
in iron oxide are observed. Also, the TEM image of
the Fe3O4/AO/Pd catalyst shows that the functional-
ized magnetic nanoparticles possess almost spherical
morphology with nearly monodispersity.
The catalytic activity of prepared catalyst (Fe3O4/
AO/Pd) was evaluated in Suzuki coupling reaction. In
order to find the optimized reaction conditions, the
reaction between 4-methyl-iododbenzene and phenyl-
boronic acid was chosen as model reaction (table 1).
The effect of different factors such as solvents, bases
and amount of the catalyst were studied at room tem-
perature (table 1). Results indicated that using K2CO3
as a base, H2O/EtOH (1:1) as a solvent, and 0.1 mol%
catalyst at 25◦C afforded highest yield of the product
(table 1, Entry 5).
Using these optimized reaction conditions, the effi-
ciency of this catalyst was studied for Suzuki reaction
of various aryl halides and the results are summarized
in table 2. Aryl iodide (entries 1–6) and aryl bromide
(entries 7–13) with different functional groups effi-
ciently reacted with phenylboronic acid to produce
biaryl products in good to excellent yields (table 2).
2.4 Suzuki-Miyaura coupling reaction13c
In a typical reaction, 7 mg of the Fe3O4/AO/Pd (10 mg
= 0.0017 mmol Pd) was placed in a 25 mL Schlenk
tube, 1 mmol of the aryl halide in 5 mL of water/ethanol
(1:1) was added 0.134 g (1.1 mmol) of phenyl boronic
acid, 0.276 mg of K2CO3 (2 mmol). The mixture was
then stirred for the desired time at room temperature.
The reaction was monitored by thin layer chromatog-
raphy (TLC). After completion of the reaction, 5 mL
ethanol was added, and the catalyst was removed by
external magnet. Further purification was achieved by
column chromatography. All the products were identi-
fied by melting point and NMR spectroscopy.
2.5 Ullmann-type N-Arylation of indoles
In a typical reaction, 10 mg of the catalyst (10 mg cata- This catalytic system was also applied for the aryl chlo-
lyst = 0.0017 mmol Pd, 0.1 mol%) was placed in a rides (table 2, entries 13–16). It is clear from table 2

Ramin Ghorbani-Vaghei et al.
Table 1. Optimization of the conditions for the Suzuki-Miyaura reaction of
4-methyl-iododbenzene with phenylboronic acid.a
Fe3O4/AO/Pd
CH3
B(OH)2
H3C
I
+
Entry
Solvent
Pd (mol%)
Base
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
DMF
toluene
EtOH
H2O
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.2
0.1
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOAc
Et3N
K2CO3
K2CO3
No base
90
90
80
180
60
60
60
60
60
90
82
60
75
50
96
60
70
75
96
EtOH/H2Oc
EtOH/H2Oc
EtOH/H2Oc
EtOH/H2Oc
EtOH/H2Oc
EtOH/H2Oc
Trace
aReaction conditions: 4-methyl-idodbenzene (1 mmol), PhB(OH)2 (1.1 mmol),
Fe3O4/AO/Pd, solvent (3 mL).
bIsolated yield.
cEtOH/ H2O = 1:1.
Table 2. Heterogeneous Suzuki-Miyaura reaction of aryl halides with phenyl-
boronic acid catalyzed by Fe3O4/AO/Pd at room temperature.a
R
R
Fe3O4/AO/Pd
K2CO3, EtOH-H2O, r.t.
B(OH)2
+
X
X = I, Br, Cl
X
Entry
R
H
Time (h)
Yield (%)b
M.p. (◦C)
M.p. (◦C)ref
1
2
3
4
I
I
I
I
I
I
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
1
1
1
2
2
2
2
4
4
6
4
6
5
15
24
24
96
96
95
85
95
90
96
90
90
85
90
88
96
70
65
60
68-70
88-90
47-49
Oil
69-7027
89-9027
45-4927
Oil28
4-CH3O
4-CH3
2-CH3O
4-F
4-NO2
H
4-CH3O
4-CH3
2-CH3O
4-F
4-NO2
4-COCH3
H
5
6
7
8
73-75
114-115
68-70
88-90
47-49
Oil
73-75
114-115
120-122
68-70
47-49
88-90
73-7428
114-11528
69-7027
89-9027
45-4927
Oil28
9
10
11
12
13
14
15
16
73-7428
114-11528
120-12128
69-7027
45-4927
89-9027
4-CH3
4-CH3O
aReaction conditions: Arylhalide (1 mmol), phenylboronic acid (1.2 mmol), K2CO3
(2 mmol), H2O/EtOH (1:1) (3 mL), catalyst (0.1 mol%) and 25◦C.
bIsolated yield.
that aryl chlorides coupled with phenylboronic acid in experiment, the reaction was terminated after 30 min;
moderate yields within 15–24 h reaction times.
at this temperature, the catalyst was separated from the
In order to prove the heterogeneous nature of the cat- reaction mixture by an external magnet and the reaction
alyst, heterogeneity test was performed, in which two was continued with the filtrate for an additional 60 min.
separate experiments were performed with 4-methyl- In the second experiment, the reaction was terminated
iododbenzene and phenylboronic acid. In the first after 30 min. In both cases, the desired product was

Pd supported on amidoxime (AO)-functionalized Fe3O4 NPs
obtained in the same yield (55%). The results indicate of indoles. To find the best conditions, the reaction
that the nature of reaction process was heterogeneous between iodobenzene and indole was chosen as model
and there was no reaction in the homogeneous phase.
reaction. Influences of different parameters were exam-
One of the main advantages of magnetite-NPs-sup- ined to obtain the best possible combination. The para-
ported catalysts is the easy separation and recycling of meters included solvent, reaction temperature, base,
the catalyst. To study the recycling properties of this catalyst concentration without the protection by an inert
catalyst, the reaction of 4-methyl-iododbenzene with gas. Observation indicated that the best conditions for
phenylboronic acid under same conditions was selected. the N-arylation of indole with iodobenzene involve the
The results showed that the catalyst is recyclable in six use of 0.1 mol% of Pd catalyst, 2 equiv. of Et3N in DMF
consecutive runs just by decantation with a magnetic at 110◦C for 2 h. So, based on the optimized conditions,
bar without any significant loss of catalytic activity the scope and generality of the developed protocol with
(figure 2). Palladium leaching of the catalyst was stud- respect to various aryl iodides were investigated using
ied before and after the reaction by ICP-AES analysis. our catalyst. The results are summarized in table 3.
The Pd content was found to be 1.80 wt% and 1.70 wt% When indole was coupled with aryl iodides contain-
before and after the six reaction series, respectively, ing both electron-donating and electron-withdrawing
which implied insignificant Pd leaching.
groups, the corresponding products were obtained in
After its successful application of the prepared cat- excellent yields (table 3, entries 1–6). It is very impor-
alyst in the Suzuki coupling reaction, its catalytic tant to mention that the reaction is very selective to give
activity was investigated for Ullmann type N-arylation only N-arylated product and in none of these cases C-
arylation of indole was observed. To ensure that there
was no C-arylation in the reaction, the reaction was
carried out with 1-methyl indole and it was found that
no product was formed (table 3, Entry 7). This shows
the high selectivity of Fe3O4/AO/Pd as catalyst in the
N-arylation of indole by aryl iodides.
4. Conclusions
In conclusion, we have prepared and characterized
novel Fe3O4 nanoparticles supported palladium nano-
particles. The catalyst showed high activity in Suzuki
coupling reaction of aryl halides (X = I, Br, Cl). The
Figure 2. The recycling of the Fe3O4/AO/Pd for the
Suzuki coupling reaction under similar conditions.
catalyst showed good stability and was recycled for
Table 3. Synthesis of N-aryl-indoles catalyzed by Fe3O4/AO/Pd.a
R2
Fe3O4/AO/Pd
Et3N, DMF, 110 C
N
+
I
R2
N
R1
Entry
R1
R2
Time (h)
Yield (%)b
1
2
3
4
5
6
7
H
H
H
H
H
H
CH3
H
2
2
4
2
2
2
10
96
96
90
85
95
90
0.0
4-OCH3
2-OCH3
4-CH3
4-Cl
4-NO2
H
aReactions were carried out under aerobic conditions in 3 mL of DMF,
1.1 mmol aryliodide, 1.0 mmol indole and 2 mmol Et3N in the presence
of Pd catalyst (0.010 g, 0.1 mol% Pd) and 110◦C for 2–4 h.
bIsolated yield.

Ramin Ghorbani-Vaghei et al.
Mohammadi P and Noroozi M 2015 J. Mol. Catal. A:
several times with small decrease in its catalytic activ-
ity. These advantages make the process highly valuable
from the synthetic and environmental points of view.
Also, we have developed a novel and practical method
for the synthesis of N-arylindoles through the reac-
tion of indoles with aryl iodides by using Fe3O4/AO/Pd
catalyst under mild reaction conditions.
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Acknowledgement
We are thankful to Bu-Ali Sina University, Center of
Excellence and Development of Chemical Methods
(CEDCM) for partial support of this work.
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