New strategy by a two-component heterogeneous catalytic system composed of Pd–PVP–Fe and heteropoly acid as co-catalyst for Suzuki coupling reaction

Article abstract of DOI:10.1007/s11164-015-2387-5

We have developed a simple and efficient catalytic protocol composed of hollow palladium-poly(N-vinylpyrrolidone)-nano zero valent iron and H₅PMo₁₀V₂O₄₀ (Mo₁₀V₂) supported on Fe₂O₃@SiO₂ core–shell nano particles, as reusable catalytic system for Suzuki coupling reaction under ligand- and base-free conditions. These reusable solid catalysts exhibited excellent activity and the methodology is applicable to diverse substrates providing good-to-excellent yields of desired products. This method has advantages of high yields, low reaction times, elimination of ligand and base, heterogeneous catalysts, and simple methodology. In order to study the role of Fe@Si–Mo₁₀V₂ in the Suzuki coupling reaction, electron transfer property of Fe@Si–Mo₁₀V₂ and Pd–PVP–Fe by means of cyclic voltammetry measurements were investigated. Moreover, this catalytic system could be recovered in a facile manner from the reaction mixture and recycled several times without any significant loss in activity. In this study, the heterogeneity of both component of our catalytic system was investigated and the content of palladium (Pd) and Mo₁₀V₂ into filtrates was evaluated quantitatively by inductively coupled plasma atomic emission spectroscopy (ICP-AES). According to the obtained results from the ICP-AES measurements, the small amount of Pd and Mo₁₀V₂ leach have been obtained.

Full text of DOI:10.1007/s11164-015-2387-5

Res Chem Intermed
DOI 10.1007/s11164-015-2387-5
New strategy by a two-component heterogeneous
catalytic system composed of Pd–PVP–Fe
and heteropoly acid as co-catalyst for Suzuki coupling
reaction
1
,2
1
1,2
•
Ezzat Rafiee
•
Masoud Kahrizi
Parvaneh Ghaderi-Sheikhi Abadi
•
Mohammad Joshaghani
2
Received: 18 September 2015 / Accepted: 7 December 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract We have developed a simple and efficient catalytic protocol composed
of hollow palladium-poly(N-vinylpyrrolidone)-nano zero valent iron and
H PMo V O (Mo V ) supported on Fe O @SiO core–shell nano particles, as
5
10
2
40
10
2
2
3
2
reusable catalytic system for Suzuki coupling reaction under ligand- and base-free
conditions. These reusable solid catalysts exhibited excellent activity and the
methodology is applicable to diverse substrates providing good-to-excellent yields
of desired products. This method has advantages of high yields, low reaction times,
elimination of ligand and base, heterogeneous catalysts, and simple methodology. In
order to study the role of Fe@Si–Mo V in the Suzuki coupling reaction, electron
1
0 2
transfer property of Fe@Si–Mo V and Pd–PVP–Fe by means of cyclic voltam-
1
0 2
metry measurements were investigated. Moreover, this catalytic system could be
recovered in a facile manner from the reaction mixture and recycled several times
without any significant loss in activity. In this study, the heterogeneity of both
component of our catalytic system was investigated and the content of palladium
(
Pd) and Mo V into filtrates was evaluated quantitatively by inductively coupled
10 2
plasma atomic emission spectroscopy (ICP-AES). According to the obtained results
from the ICP-AES measurements, the small amount of Pd and Mo V leach have
been obtained.
1
0 2
Keywords Suzuki coupling reaction Á Palladium Á Heterogeneous catalyst Á
Heteropoly acid Á Bifunctional catalytic system
&
Ezzat Rafiee
ezzat_rafiee@yahoo.com; e.rafiei@razi.ac.ir
1
2
Faculty of Chemistry, Razi University, Kermanshah 67149, Iran
Institute of Nano Science and Nano Technology, Razi University, Kermanshah 67149, Iran
123

E. Rafiee et al.
Introduction
Palladium (Pd)-catalyzed C–C bond formation reactions have drawn much attention
in organic synthesis due to its industrial potential for the synthesis of chemicals and
their intermediates [1–4]. Among these, the coupling of aryl halides with aryl
boronic acids (Suzuki coupling) [5–8] represents the most successful method for the
synthesis of biaryls.
This reaction is generally considered to be a homogeneous reaction most
commonly catalyzed by soluble Pd(II) species with a variety of ligands, such as
palladium-phosphine complexes [9], oxime palladacycles [10], palladium–salen
complexes [11] and palladium-N-heterocyclic carbine complexes [12]. Homoge-
neous catalysts have the advantages of being dissolved in reaction medium, thus
providing mild reaction conditions and higher activities compared to their
heterogeneous counterparts [13]. Nevertheless, separation of the expensive catalyst
from the product for reusing is often troublesome in these homogeneous systems.
Moreover, aggregation and precipitation of Pd metal in the homogeneous systems
always leads to lost activity of the catalysts [14].
In order to address these problems, heterogeneous Pd supported on some
materials has been employed in recent years [15–22]. Among these supports, iron(0)
nano particles (FeNP) have been the subject of increasing interest, mostly due to
economic considerations. Because of the nano-scale size of FeNP, these particles
have higher catalyst loading capacities and higher dispersion than many conven-
tional supports, leading to the improved catalytic activity of these catalysts. Another
attractive feature of FeNPs is the possibility of fast and cost-efficient separation by
applying an external magnetic field, which makes them ideal candidates for
practical use in catalytic process. Moreover, FeNP can reduce Pd(II) and regenerate
active Pd(0) species. It should be noted that, magnetically, interactions between
particles makes FeNPs susceptible to agglomeration. Thus, in order to decrease the
agglomeration degree, effective coating of magnetite particles is essential. Several
molecules have been used to coat the surface of metal nanoparticles. Poly(N-
vinylpyrrolidone), PVP, is particularly useful in this regard [23, 24], since it is
readily available and non-toxic. Furthermore, the twisty hydrocarbon tails of PVP
may help to stabilize the Pd nanoparticles and prevent them from aggregating with
each other. Also, the electron transfer between Fe(0) and Pd(II) generates Fe(II)
species, which receive lone-pair electrons from PVP shell and reduce into Fe(0)
[
25].
In recent years, heteropoly acids (HPAs) have received a lot of attention in
coupling reactions and growing studies have been done on the use of them in this
field of organic transformations [26, 27].
One of the main advantages of HPAs is their electron transfer ability. The
reduction potentials of HPAs containing Mo and V are high, as these ions are easily
reduced [28]. These structural features make these compounds an appropriate choice
for oxidation/reduction reactions. Especially in coupling reactions, HPAs can
facilitate the oxidative/addition and reductive/elimination steps. Bases are one of the
substantial, required components of the Suzuki coupling reaction. The use of a high
1
23

New strategy by two-component heterogeneous catalytic…
amount of bases poses several problems, such as an inability to recover,
contamination of environment, corrosion of equipment, and high toxicity. Hence,
there is a need for an efficient, eco-friendly, and recoverable alternative for
replacement of bases in the Suzuki reaction. HPAs with electron transfer ability can
act as an excellent alternative for bases in the reductive elimination step in coupling
reactions. Along this line, using heterogeneous HPAs, which are low in toxicity,
highly stable towards humidity, recyclable, and air stable, can be useful. Supported
HPAs are advantageous over conventional bases, as they can be easily recovered
from the reaction mixture by simple filtration and can be reused without activation,
thereby making the process economically and environmentally viable.
As a part of our project about the use of magnetic catalysts in organic synthesis
29, 30] and emanating from our interest in Pd-catalyzed cross-coupling reactions
31–34], the present study evaluates the usefulness of a combination of palladium–
[
[
PVP-nano zero valent iron (Pd–PVP–Fe) [35] and H PMo V O (Mo V ) [36]
10 2
5
10
2
40
immobilized on Fe O @SiO core–shell nano particles (Fe@Si–Mo V ) in the
2
3
2
10 2
Suzuki coupling reaction. In this system, low amounts of Fe@Si–Mo V are used
0 2
1
as an efficient base alternative, which reduces costs because of recyclability and
increased efficiency in the Suzuki reaction.
Experimental
Materials and methods
All reactions were performed under an atmosphere of dry nitrogen. All chemicals
used in this work were obtained from Fluka, Aldrich, or Merck companies and used
without further purification. The FT-IR spectra of the samples were recorded at
room temperature using a Bruker, ALPHA spectrophotometer at a spectral
-
1
resolution of 4 cm using KBr pellets. The X-ray diffraction (XRD) patterns
were obtained on Inel French, EQUINOX 3000 model X-ray diffractometer using
Cu-Ka radiation. The Pd and molybdenum (Mo) content was measured using
inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Spectro
Ciros CCD spectrometer. Thin-layer chromatography (TLC) on precoated silica gel
Fluorescent 254 nm (0.2 mm) on aluminum plates were used for monitoring the
1
reactions. The cross-coupling products were characterized by their H NMR spectra.
The electrochemical reduction of catalyst was studied in phosphate buffer (0.1 M),
pH = 7. A glassy carbon electrode was used as the working electrode and Ag/AgCl
1
-
as the reference electrode. Cyclic voltammograms (m, 0.1 Vs ) were obtained at
ambient temperature (20 ± 5 °C).
Preparation of Pd–PVP–Fe
In a typical synthesis, 0.00113 g (0.0063 mmol) PdCl and 0.058 g (0.0014 mmol)
2
PVP were dissolved in distilled water: N,N-dimethylformamide (DMF) (1:5 V/V)
under stirring for 5 min until the solution became homogeneous. In a separate
beaker, 0.015 g (0.075 mmol) of FeCl Á4H O and 0.058 g (0.0014 mmol) of PVP
2
2
123

E. Rafiee et al.
were dissolved in similar solvent under stirring and added dropwise to PdCl2
solution under an ultrasonic bath. The mixture was sonicated for 1 min under mild
power to form a homogeneous solution, and then the resulting homogeneous
solution was refluxed at 110 °C for 16 h. After thermal treatment, the resulting
precipitate was collected, filtered, and washed with distilled water and absolute
ethanol several times to remove any possible impurities, and finally dried in vacuo
before reaction [35].
Preparation of Mo V
0 2
1
Mo V was prepared according to the following procedure: sodium metavanadate
1
0 2
(
Na HPO in 100 ml of water. Then this solution was cooled, 5 ml of concentrated
24.4 g) was dissolved by boiling in 100 ml of water and then mixed with 7.1 g of
2
4
sulfuric acid was added, and the solution developed a red color. An addition of
1
21.0 g of Na MoO Á2H O dissolved in 200 ml of water was then made. While the
2
4
2
solution was vigorously stirred, 85 ml of concentrated sulfuric acid was added
slowly, and the hot solution was allowed to cool to room temperature. Mo V was
1
0 2
then extracted with 500 ml of diethyl ether. Evaporation of the solvent afforded a
crude product which dissolved in water, concentrated to first crystal formation, and
then allowed to crystallize further. The large red crystals that formed were filtered,
washed with water, and air dried [36].
Preparation of Fe@Si–Mo V
0 2
1
FeCl Á4H O (2.0 g) and FeCl Á6H O (5.4 g) were dissolved in water (20 ml)
2
2
3
2
separately, followed by the two iron salt solutions being mixed under vigorous
stirring. An NH OH solution (0.6 M, 200 ml) was added to the stirring mixture at
4
room temperature, immediately followed by the addition of a concentrated NH OH
4
solution (25 % w/w, 30 ml). The resulting black dispersion was continuously stirred
for 1 h at room temperature and then heated to reflux for 1 h to yield a brown
dispersion. It was then purified by a four-times-repeated centrifugation, decantation,
and redispersion cycle until a stable brown magnetic dispersion was obtained.
Coating of a layer of silica on the surface of the nanoparticles was achieved by
premixing a dispersion of the purified nanoparticles (8.5 % w/w, 20 ml) obtained
previously with EtOH (80 ml) for 1 h at 40 °C. A concentrated ammonia solution
was added and the resulting mixture stirred at 40 °C for 30 min. Subsequently,
TEOS (1.0 ml) was charged to the reaction vessel and the mixture continuously
stirred at 40 °C for 24 h. It was collected using a permanent magnet, followed by
washing three times with EtOH, diethyl ether, and drying under vacuum for 24 h;
0
dropwise to a suspension of 1.0 g Fe O @SiO in MeOH (50 ml) while being
.7 g of Mo V was dissolved in 5 ml of dry MeOH. This solution was added
10 2
2
3
2
dispersed by sonication. The mixture was heated at 70 °C for 72 h under vacuum
while being mechanically stirred to obtain Fe@Si–Mo V . The catalyst was
collected by a permanent magnet and dried under vacuum overnight [37].
1
0 2
1
23

New strategy by two-component heterogeneous catalytic…
General procedure for the Suzuki coupling reaction
For Suzuki coupling reaction, a reaction tube was charged with aryl halide
(4 mmol), phenylboronic acid (4 mmol), TBAB (4 mmol), Pd–PVP–Fe (0.03 g),
Fe@Si–Mo V (0.02 g), and 9 ml of EtOH/H O (6:3) and the resulting mixture
1
0
2
2
was refluxed at 90 °C. Progress of the reaction was monitored by TLC. Upon
completion of the reaction, the reaction mixture was then cooled to room
temperature and both components of our catalytic system were separated by magnet,
washed with diethyl ether (2 9 10 ml), water (2 9 10 ml), and then dried under
vacuum for reusability. The residual mixture was extracted by CH Cl (3 9 20 ml),
2
2
the combined organic layer was dried over MgSO . The solvent was evaporated and
4
1
the crude products were characterized by comparing their H NMR spectra with
standard data [38–50].
a
Table 1 Optimization of the reaction conditions
b
Entry
Catalyst (g)
Solvent
Time (min)
Yield (%)
Pd–PVP–Fe (mol%)
HPA
Mo10
-
4
4
1
2
3
4
5
6
7
8
9
0.02 (3.76 9 10
0.02 (3.76 9 10
)
)
V
2
(0.01)
H
H
H
H
H
H
H
2
2
2
2
2
2
2
O/EtOH
O/EtOH
O/EtOH
O/EtOH
O/EtOH
O/DMF
O
30
60
89
89
78
22
4
-
Fe@Si-Mo10
V
2
(0.01)
(0.01)
-
4
002 (3.76 9 10
–
)
(NH Mo10
4
)
5
V
2
120
120
120
60
Pd2.5Mo10
2
V (0.01)
-
-
-
-
-
-
-
4
4
4
4
4
4
4
0.02 (3.76 9 10
0.02 (3.76 9 10
0.02 (3.76 9 10
0.02 (3.76 9 10
0.02 (3.76 9 10
0.02 (3.76 9 10
0.02 (3.76 9 10
0.02 (3.76 9 10
0.01 (1.88 9 10
0.03 (5.64 9 10
0.04 (7.52 9 10
)
)
)
)
)
)
)
)
)
)
)
–
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
Fe@Si–Mo10
V
V
V
V
V
V
V
V
V
V
2
2
2
2
2
2
2
2
2
2
(0.02)
(0.02)
(0.02)
(0.02)
(0.03)
(0.02)
(0.02)
(0.02)
(0.02)
(0.02)
79
65
44
23
94
91
12
73
96
91
60
DMF
60
MeCN
120
40
10
11
12
13
14
15
a
H
H
H
H
H
H
2
O/EtOH
2
O/EtOH
2
O/EtOH
2
O/EtOH
2
O/EtOH
2
O/EtOH
40
c
-4
120
60
-
-
-
4
4
4
30
30
Reaction conditions: Phenylboronic acid (4 mmol), bromobenzene (4 mmol), tetrabutylammonium
bromide, TBAB (4 mmol), solvent (9 ml) and reflux condition
b
Isolated yield
c
Reaction proceeded in room temperature
123

E. Rafiee et al.
Results and discussion
Initial investigation started with the cross-coupling reaction of phenylboronic acid and
bromobenzene at reflux and under ligand- and base-free conditions as a model system.
Activity of various HPAs such as H PMo V O (Mo V ), Pd PMo V O ,
5
10
2
40
10
2
2.5
10 2 40
(
reaction was investigated (Table 1, entries 1–4). Mo V and Fe@Si–Mo V were
NH ) PMo V O and Fe@Si–Mo V accompanied by Pd–PVP–Fe in the model
4 5 10 2 40 10 2
1
0
2
10 2
found to be the most effective (entries 1, 2), but there is a problem here; Mo V in
0 2
1
acidic form is a homogeneous catalyst in this reaction media. The major problem,
limiting the utility of homogeneously catalyzed processes, is the well-known
difficulty in catalyst recovery and recycling. There are two ways for heterogenization
of HPAs. One is using salt of HPAs and the other way is making a supported form. Pd
salt of Mo V (entry 4) shows very low catalytic activity. Therefore, we chose
1
0 2
Fe@Si–Mo V as a heterogeneous recoverable catalyst. It is noteworthy that in the
1
0 2
absence of HPA, the reaction did not proceed well (entry 5) and the starting materials
remained intact after 120 min (Table 1).
To establish the best reaction conditions, the effect of various solvents, such as
H O, MeCN, DMF, and mixed solvent systems such as H O/EtOH and H O/DMF
have been studied (entries 6–9, 11). H O/EtOH were found to be the choice in terms
2
2
2
2
a
Table 2 Suzuki reaction of various aryl halides with phenylboronic acid
b
c
Entry
Aryl halide
Time (min)
Yield (%)
TON
1
2
3
4
5
6
7
8
9
Bromobenzene
30
96
94
92
91
91
97
89
88
91
86
78
93
84
42
175
171
167
165
165
177
162
160
165
156
142
169
153
76
4-Methoxy bromobenzene
4-Methyl bromobenzene
4-Bromo aniline
40
40
50
4-Bromo benzaldehyde
4-Nitro bromobenzene
1-Bromo naphthalene
Chlorobenzene
30
30
40
50
4-Methyl chlorobenzene
4-Methoxy chlorobenzene
4-Chloro aniline
60
1
1
1
1
1
a
0
1
2
3
4
70
80
4-Chloro benzaldehyde
4-Chloro bromobenzene
Fluorobenzene
40
40
24 (h)
Reaction conditions: Phenylboronic acid (4 mmol), aryl halide (4 mmol), TBAB (4 mmol), H
9 ml: 2/1) and reflux conditions
2
O/EtOH
(
b
1
Isolated yield. All products were characterized by comparing their H-NMR data with authentic
samples [37–44]
c
TON = mmol of product/mmol of Pd catalyst
1
23

New strategy by two-component heterogeneous catalytic…
Table 3 Comparison of the reaction data with other reported methods
Entry Catalyst
Solvent
EtOH
Temp. (°C) Time (h) Yield (%) Ref
1
2
3
4
5
6
7
9
1
1
1
1
Graphene oxide/Pd
Reflux
90
2.5
2
86
94
97
94
94
90
93
90.8
90
82
98
98
39
40
41
42
43
44
45
46
47
48
49
50
Cyclopalladated ferrocenylimine
Fe@Pd/C
H
H
H
H
H
H
2
2
2
2
2
2
O
O
80
0.5
8
Supported Pd(II) complex
Pd(salen)-HPA
3
O/CH OH R.T
O/EtOH
O
50
5
Pd/NH
Carbon-Pd catalyst
Pd/Fe @SiO @KCC-1
2
–SiO
2
60–70
100
80
1
O/DMF
3
3
O
4
2
EtOH
6
0
1
2
3
Pd/MSN-IPr
Mag-IL-Pd
IsoPrOH
80
2
H
H
2
2
O
O
60
7.5
1
Pd@SBA-15C
80
Pd(II) bounded 2D-hexagonally
ordered functionalized MCM-41
DMF
100
4
14
This work
H
2
O/EtOH
Reflux
0.5
96
–
a
The reaction is performed in the presence of Me-PhB(OH)
2
of yield and reaction time (entry 11). Next, the model reaction was carried out in the
presence of different amounts of Fe@Si–Mo V and Pd–PVP–Fe as catalytic
system. According to the obtained results, 0.02 g of the Fe@Si–Mo V and 0.03 g
1
0 2
1
0 2
of Pd–PVP–Fe were chosen as the optimum amount of catalytic system for further
reactions (entry 14).
Considering these optimized reaction conditions, various aryl halides were
coupled with phenylboronic acid (Table 2). Aryl bromides gave excellent yields
with shorter reaction times (entries 1–7). Aryl chlorides react efficiently with
phenylboronic acid giving affordable yields of Suzuki product with longer reaction
times (entries 8–13). Aryl halides containing electron-withdrawing functional
groups give excellent yields with lower reaction times compared to those containing
electron-donating functional groups.
To our surprise, the reaction of phenylboronic acids with fluorobenzene afforded
the respective product in a moderate yield (entry 14), while previously due to high
strength of C–F bond, the fluoroarenes were rarely used [38].
As shown in Table 2, the remarkable reactivity of Pd–PVP–Fe and Fe@Si–
Mo V as catalytic system in Suzuki cross-coupling reactions with various
1
0 2
functionalized substrates also proved to exhibit an acceptable impact on the turn
over number (TON) of these reactions (the Pd loading in Pd–PVP–Fe as a main
catalyst was 7.8 %, which was determined using ICP-AES measurement).
The reaction data, along with some literature data for comparison, are given in
Table 3. These catalysts showed good reactivity. However, the use of base, ligand,
123

E. Rafiee et al.
Table 4 Mo10
V
2
leaching measured by ICP-AES
1
Run number
2
3
4
Leach (wt%)
9.75
6.1
4.87
3.67
(a)
(b)
E/V
E/V
2
V and b Pd–PVP–Fe-modified glassy carbon electrode
Fig. 1 Cyclic voltammogram of a Fe@Si–Mo10
in a phosphate buffer solution of pH = 7, 100 mV/s
and non-recyclable catalysts in these reported systems is not beneficial to industrial
and synthetic applications.
In order to study on the role of Fe@Si–Mo V on the Suzuki coupling reaction,
1
0 2
we decided to investigate the electron transfer property of Fe@Si–Mo V and Pd–
10 2
PVP–Fe by means of CV measurements. The CV responses of Fe@Si–Mo V and
1
0 2
Pd–PVP–Fe modified glassy carbon electrode in a phosphate buffer solution of
pH = 7 is shown in Fig. 1. In the potential range between E = -0.1 V and
E = 1.00 V, three pairs of redox peaks were observed for Fe@Si–Mo V modified
1
0 2
electrode. It can be attributed to electron-transfer reaction of Mo(V)/Mo(IV),
Mo(IV)/Mo(III), and V(V)/V(IV) couples. Also, the characteristic symmetrical
reduction wave of palladium is identified with its reduction peak located at
E = ?0.17 V. Comparing with the CV of Fe@Si–Mo V , we can deduce that the
1
0 2
presence of HPA species can facilitate the oxidation and reduction of palladium in
catalytic cycles of Suzuki coupling reaction.
The reusability of the catalysts is a very important theme, especially for
commercial applications. The advantage of our catalytic system lied in not only the
high catalytic activity attributed to the formation of C–C bond but also in the easy
separation and recyclability. Recycling experiments were carried out for the cross-
coupling of bromobenzene and phenylboronic acid under the optimum conditions in
Table 1. At the fourth reaction runs, 74 wt% of the catalyst (compared with the
amount of fresh one used in the first run) was recovered and yield of the product
decreased from 91 to 80 % after fourth runs (Fig. 2). This decrease in the product
yield may be due to the leaching of the active site.
To check whether this catalyst is completely heterogeneous, the content of Pd
and Mo V into filtrates was evaluated quantitatively by inductively coupled
1
0 2
1
23

New strategy by two-component heterogeneous catalytic…
plasma atomic emission spectroscopy (ICP-AES). According to the obtained results
from the ICP-AES measurements, the Pd leach is negligible and a small amount of
Mo V leach was obtained (Table 4).
1
0 2
Another test was performed to check if this active catalytic system is indeed
heterogeneous (Fig. 3). First, hot filtration of the catalysts after obtaining a
conversion of about 50 % was done. Thereafter, the reaction was continued with the
residual solution. No further conversion was observed in the reaction as a proof that
no homogeneous catalyst was present in the reaction mixture.
In order to show the effect of reuse on the structural features of the catalysts, the
FT-IR spectra and XRD pattern of the fresh Pd–PVP–Fe and Fe@Si–Mo V and
1
0 2
reused ones are presented in Fig. 4. It is notable that, in this catalytic system, Pd–
PVP–Fe and Fe@Si–Mo V , both components separated at the same time.
1
0 2
-
1
For Pd–PVP–Fe, the band at 750 cm
-
corresponds to the C–C stretching
vibration, and 1100 cm is related to the C–N stretching vibration (pyridine ring).
1
-
1
The peaks at about 1300 and 1550 cm are related to C–H stretching vibrations
and stretching vibration of C=O, but in the case of Fe@Si–Mo V , the bands for
the Keggin structure of neat Mo V occurred at about 1086, 1024, 927, and
1
0 2
1
0 2
-
1
-1
8
35 cm . The presence of two bands at about 400–1000 cm indicates the
presence of Fe O in the structure of Fe@Si–Mo V . The FT-IR spectrum of the
3
4
10 2
recovered catalysts shows all of the characteristic peaks of the fresh catalysts, which
confirms that the structure of both components of the catalytic system during the
reuse and recovery processes remains unchanged.
2
Fig. 2 Reusability of FeSi–Mo10V and Pd–PVP–Fe in the model reaction and catalysts recovery
123

E. Rafiee et al.
1
00
8
6
4
2
0
0
0
0
0
Hot filtration of the catalysts
In the presence of the catalysts
0
10
20
30
40
50
60
Time (min)
Fig. 3 Reaction progress before and after filtration of the catalysts
Also, the XRD patterns of the fresh and recovered catalysts are shown in Fig. 4.
As presented, in XRD patterns of recovered Pd–PVP–Fe and Fe@Si–Mo V , the
1
0 2
0
Pd have a crystalline structure with peaks appearing at around 2h = 40.12°, 46.36°,
8.12°, 82.10°, and 86.62°, while the diffraction peaks observed at 45.90°, 53.30°,
6
and 77.75° correspond to the presence of nano zero valent irons in the Pd–PVP–Fe
structure. As can be seen, in the overall XRD pattern of recovered catalyst and co-
catalyst, the characteristic peaks of both Pd–PVP–Fe and Fe@Si–Mo V were
1
0 2
observed clearly. From the standpoint of this, we can conclude that during the
recovery and reusing processes, all of the fresh and reused catalysts have similar
catalytic behaviors and structural properties.
We believe that HPA play a significant role in the transmetalation step in Suzuki
coupling reactions. As is shown in Scheme 1, the mechanism begins with oxidative
addition of the arylhalides to the Pd(0) to form a Pd(II) complex. At this point, the
Pd–X bond activation is done by Fe@Si–Mo V acidic hydrogens. H–X is a very
1
0 2
good leaving group, and, produced intermediately, is a very active component for
alkylation. A produced polyoxoanion then adds to the organoborane to form a
0
borate reagent making its Ar group more nucleophilic. Transmetalation with the
0
borate then follows, where its Ar group replaces the H–X on the palladium
complex. Reductive elimination then gives the final coupled product, regenerates
the palladium catalyst, and then the catalytic cycle can begin again. Also, further
research is under investigation in our laboratory.
Conclusions
In conclusion, we have applied a green protocol based on the simultaneous use of
Pd–PVP–Fe and Fe@Si–Mo V . This catalytic system demonstrated excellent
1
0 2
catalytic activity for the carbon–carbon cross-coupling Suzuki reaction. The
presence of Fe@Si–Mo V is essential for obtaining high performances of the
1
0 2
1
23

New strategy by two-component heterogeneous catalytic…
2
Fig. 4 FT-IR spectra and XRD pattern of fresh Pd–PVP–Fe, Fe@Si–Mo10V , and recovered ones in the
presence of Fe@Si–Mo10V
2
catalytic system; most probably it can activate precursors and regenerate the Fe(II)
and Pd(II) to active catalytic species. High stability, easy removal from the reaction
mixture, and reusability of the catalyst up to four consecutive cycles with minimal
123

E. Rafiee et al.
Ar-X
Ar'-Ar
Pd (0)
Oxidative Addition
Reductive Elimination
Ar-Pd-X
Ar-Pd-Ar'
Transmetalation
HPA
[
XB(OH) ]
2
+
HPA
[
Ar-Pd-XH]
PA
[
Ar'-B(OH) PA]
Ar'-B(OH)2
2
Scheme 1 Proposed mechanism
loss of activity are the major advantages of this catalytic system. According to the
obtained results from the ICP-AES measurements, a small amount of Pd and
Mo V leach have been obtained. Further work is in progress to broaden the scope
1
0 2
of these types of catalytic systems for various organic transformations.
Acknowledgments The authors thank the Razi University Research Council for support of this work.
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