Palladium nanoparticles immobilized on Schiff base-functionalized mesoporous silica as a highly efficient and magnetically recoverable nanocatalyst for Heck coupling reaction

Article abstract of DOI:10.1002/aoc.4618

A new magnetically recoverable nanocatalyst was prepared by functionalization of mesoporous silica (SBA-15) with a Schiff base ligand, and then immobilization of palladium nanoparticles on it using a simple procedure. This heterogeneous catalyst was fully characterized using appropriate analyses and its catalytic efficiency was investigated in Heck reaction using iodo-, bromo- and chlorobenzene derivatives and styrene, with the aim of synthesizing stilbene derivatives, a class of compounds with a variety of pharmacological properties. Some of the characteristics of this nanocatalyst include good dispersion of palladium nanoparticles on the SBA-15 support, easy separation, catalyses the production of stilbene derivatives in a short time with excellent yields even for bromo- and chlorobenzene, and preservation of its catalytic activity after eight reaction cycles.

Full text of DOI:10.1002/aoc.4618

Received: 13 July 2018
DOI: 10.1002/aoc.4618
Revised: 3 September 2018
Accepted: 8 September 2018
F U L L P A P E R
Palladium nanoparticles immobilized on Schiff base‐
functionalized mesoporous silica as a highly efficient and
magnetically recoverable nanocatalyst for Heck coupling
reaction
1,2
2
Mohammad Mehdi Khodaei
| Mahsa Dehghan
1
Department of Organic Chemistry, Razi
A
new magnetically recoverable nanocatalyst was prepared by
University, Kermanshah 67149‐67346,
Iran
functionalization of mesoporous silica (SBA‐15) with a Schiff base ligand,
and then immobilization of palladium nanoparticles on it using a simple pro-
cedure. This heterogeneous catalyst was fully characterized using appropriate
analyses and its catalytic efficiency was investigated in Heck reaction using
iodo‐, bromo‐ and chlorobenzene derivatives and styrene, with the aim of syn-
thesizing stilbene derivatives, a class of compounds with a variety of pharma-
cological properties. Some of the characteristics of this nanocatalyst include
good dispersion of palladium nanoparticles on the SBA‐15 support, easy sepa-
ration, catalyses the production of stilbene derivatives in a short time with
excellent yields even for bromo‐ and chlorobenzene, and preservation of its cat-
alytic activity after eight reaction cycles.
2
Nanoscience and Nanotechnology
Research Center (NNRC), Razi University,
Kermanshah 67149‐67346, Iran
Correspondence
Mohammad Mehdi Khodaei, Department
of Organic Chemistry, Razi University,
Kermanshah 67149‐67346, Iran.
Email: mmkhoda@razi.ac.ir
Funding information
Razi University
KEYWORDS
Heck reaction, magnetically recoverable nanocatalyst, mesoporous silica, palladium nanoparticles
1
| INTRODUCTION
catalytic system with the least potential effects on the envi-
ronment, it is preferable to use palladium complexes sup-
The Heck reaction is one of the most important reactions
in the synthesis of agrochemicals, pharmaceuticals and
natural products. In addition, this reaction by
functionalization of various molecules with different sub-
stituents significantly reduces the number of synthetic
steps, and therefore has attracted much attention of
organic chemists. The award of the Nobel Prize to Rich-
ard Heck in 2010 clearly illustrates the importance of this
reaction. For the construction of carbon–carbon bonds in
coupling reactions (Heck, Suzuki, Stille and Sonogashira),
both homogeneous and heterogeneous catalysts can be
used. Palladium with phosphine ligands or other special
ligand systems has shown a high catalytic activity in the
Heck coupling reaction. Since researchers are always try-
ing to achieve a more efficient, simpler and greener
ported on solid supports as efficient and easily recyclable
heterogeneous catalytic systems, instead of using homoge-
[
2–4]
neous catalytic systems.
A number of materials have
been used to stabilize palladium nanoparticles (Pd NPs)
as active catalysts in the Heck reaction including char-
[
5]
[6]
[7]
[8]
[9]
coal, clay, silica, alumina, aerogels, micelles of
[1]
[10]
[11]
polymers
and dendrimers.
Pd NPs immobilized on
solid supports have important characteristics including:
[
12]
the ability to catalyse various chemical reactions, recov-
erability and thus elimination of economic and environ-
mental problems.
More recently, Pd NPs supported on mesoporous sil-
ica were successfully used for catalysing the Heck reac-
[
13–16]
tion of aryl halides and olefins.
Ordered
mesoporous silica materials are highly regarded by
Appl Organometal Chem. 2018;e4618.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
1 of 9
https://doi.org/10.1002/aoc.4618

2
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KHODAEI AND DEHGHAN
chemists for their widespread use in various fields includ-
ing as supports for the synthesis of a variety of catalysts,
such as field‐emission scanning electron microscopy
(FESEM), energy‐dispersive X‐ray (EDX) analysis, map-
ping, transmission electron microscopy (TEM), Fourier
transform infrared (FT‐IR) spectroscopy, Brunauer–
Emmett–Teller (BET)/Barrett–Joyner–Halenda (BJH)
analysis, and low‐angle and wide‐angle X‐ray diffraction
(XRD) (Scheme 1).
[10d,17–23]
adsorbents and sensors.
These hexagonally
packed mesoporous materials have many beneficial fea-
tures such as variability, stability, accessibility, low cost,
large and uniform pore sizes, high surface areas and facile
diffusion of sterically hindered molecules to their internal
[24,25]
active sites.
Moreover, the covalent bonds formed
between ligands and the surface of the support minimize
the leaching of grafted groups and metals. So, these fea-
tures make them an ideal support for the generation of
2
| MATERIALS AND METHODS
All materials and solvents were purchased from Merck
and Aldrich and used without further purification. A
Bruker ALPHA spectrophotometer was used to record
[26]
heterogeneous catalysts.
For the preparation of SBA‐
1
5‐supported palladium catalyst, functional modification
should be performed. After the modification of the sup-
port surface for creation of a suitable functional group,
the interaction between the modified support and metal
precursor is realized. Recently, a large number of catalytic
applications of Schiff base‐functionalized SBA‐15 have
−
1
FT‐IR spectra (range: 400–4000 cm ). XRD patterns
were acquired with an X’PertPro diffractometer using
Cu Kα radiation. TEM analysis was performed using a
Zeiss EM10C at an acceleration voltage of 80 kV. FESEM,
mapping and EDX were performed with a Zeiss SIGMA
VP‐500 instrument. Nitrogen adsorption–desorption iso-
therms were measured at −196°C using a BELSORP‐mini
II instrument.
[27]
been reported.
The Schiff base derivatives that have a
─
C═N─ functional group in their structures have various
properties, and hence they can be used in different fields
such as organic and bioinorganic chemistry, catalysis,
[
28]
synthesis of various compounds and liquid crystals.
The electron pair in nitrogen or carbon atoms can coordi-
nate to metal ions or various organic groups, and this
makes Schiff bases highly flexible in syntheses and
2.1 | Preparation of Schiff
base‐functionalized SBA‐15 (SF‐SBA‐15)
[29]
modifications.
The surfactant method of Zhao et al. was used to synthe-
[
30,31]
Herein, we focus on the synthesis and characteriza-
tion of a new magnetic nanocatalyst based on functional-
ized SBA‐15 support (Fe O ‐Pd@SF‐SBA‐15) as an
size SBA‐15.
Then, its modification was carried out
[
32]
using the method of Vunain et al.
Briefly, 50 ml of tol-
uene was introduced into a 100 ml round‐bottom flask,
and then 5.0 g of SBA‐15 and 5 ml (20.76 mmol) of 3‐
chloropropyltriethoxysilane (CPTES) were added. This
mixture was refluxed for 24 h, and the obtained solid
(SBA‐15‐Cl) was filtered, washed with toluene and meth-
3
4
effective and recyclable nanocatalyst for carbon–carbon
coupling reaction (Heck reaction). Palladium is used for
the olefination of aryl halides using styrene derivatives
to produce stilbene derivatives. The results indicate that
a good dispersion of Pd NPs on the surface of the support
[
33]
anol and dried at 110°C.
In another 100 ml round‐
(
SF‐SBA‐15) is achieved and this Fe O ‐Pd@SF‐SBA‐15
bottom flask, to a solution of 1.0 g of SBA‐15‐Cl in
50 ml of acetonitrile was added 0.83 g (5 mmol) of potas-
sium iodide and the mixture stirred for 1 h at 50°C. Then,
0.63 g (5 mmol) of melamine was added and this solution
3
4
nanocatalyst can successfully catalyse the formation of
carbon–carbon bonds in the Heck reaction. The final cat-
alyst was fully characterized using appropriate techniques
SCHEME 1 Preparation of Pd NPs
immobilized on Schiff base‐functionalized
SBA‐15 (Fe O ‐Pd@SF‐SBA‐15)
3 4

KHODAEI AND DEHGHAN
3 of 9
was refluxed for 9 h. After 9 h, the obtained solid was fil-
tered, rinsed with ethanol and dried at 80°C for 10 h.
Then, 1.0 g of the obtained solid was dissolved in 50 ml
of methanol and 1.67 g (11 mmol) of vanillin was added
to it and refluxed for 9 h. After completion of the reac-
tion, the obtained product (SF‐SBA‐15) was filtered,
washed with acetonitrile and air‐dried.
MgSO . Then the solvent was evaporated and the residue
4
was purified by column chromatography and character-
1
13
ized from its H NMR and C NMR spectra and melting
point.
2.5 | Spectral data for synthesized
products
2.2 | Preparation of Pd@SF‐SBA‐15
2.5.1 | (E)‐1,2‐Diphenylethene (1a)
1
Amounts of 0.5 g of SF‐SBA‐15 powder and 0.023 g
0.0001 mol) of palladium(II) acetate were dispersed in
0 ml of methanol. The mixture was stirred for 24 h at
White solid; m.p. 121–122°C. H NMR (300 MHz, CDCl ,
3
(
5
δ , ppm): 7.60 (d, J = 7.8 Hz, 4H), 7.47–7.42 (m, 4H), 7.34
(t, J = 7.8 Hz, 2H), 7.20 (s, 2H). C NMR (75 MHz,
H
13
ambient temperature. Then the orange solid was filtered
off and washed several times with methanol to afford
Pd @SF‐SBA‐15 complex. Then, 0.3 g of dry Pd @SF‐
SBA‐15 complex was dispersed in 30 ml of methanol
and 1 ml of hydrazine hydrate was added slowly over
CDCl , δ , ppm): 126.57, 127.68, 128.74, 137.38.
3
C
2+
2+
2.5.2 | (E)‐1‐Methoxy‐4‐styrylbenzene (1b)
1
White solid; m.p. 135–136°C. H NMR (300 MHz, CDCl ,
3
5
min under constant stirring at 60°C. The reduction of
2+
0
δ
H
, ppm):7.55–7.49 (m, 4H), 7.40 (t, J = 7.5 Hz, 2H), 7.30–
Pd to Pd started immediately, and the colour changed
from orange to black. The resulting black Pd @SF‐SBA‐
0
7.25 (m, 1H), 7.13 (d, J = 16.2, 1H), 7.02 (d, J = 16.2 Hz,
13
1
H), 6.96–6.97 (m, 2H), 3.88 (s, 3H). C NMR (75 MHz,
1
5 was collected and washed with distilled water several
CDCl , δ , ppm): 55.36, 114.16, 126.28, 126.64, 127.24,
3
C
times and then dried at 70°C. The loading of palladium
was measured to be 7.1 wt% using the inductively coupled
plasma optical emission spectrometry (ICP‐OES)
technique.
127.75, 128.23, 128.67, 130.17, 137.67, 159.33.
2.5.3 | (E)‐1‐Chloro‐4‐styrylbenzene (1c)
1
White solid; m.p. 127–128°C. H NMR (300 MHz, CDCl ,
3
2
.3 | Preparation of Fe O ‐Pd@SF‐SBA‐15
3
4
δH, ppm):7.55 (d, J = 6.9 Hz, 2H), 7.49 (d, J = 8.4 Hz, 2H),
7
.44–7.35 (m, 4H), 7.31 (d, J = 7.2 Hz, 1H), 7.11 (d,
In a 50 ml round‐bottom flask, 0.1 g of as‐prepared Fe O4
NPs was dispersed in 20 ml of absolute ethanol by
ultrasonication. In another 100 ml round‐bottom flask,
3
13
J = 2.4 Hz, 2H). C NMR (75 MHz, CDCl , δ , ppm):
3
C
1
1
26.58, 127.39, 127.69, 127.90, 128.77, 128.87, 129.33,
33.19, 135.87, 137.00.
0
.1 g of Pd@SF‐SBA‐15 was added to 40 ml of ethanol
and stirred for 30 min at ambient temperature. Then,
the dispersed Fe O NPs were added dropwise to this
solution and stirred for 24 h. After that, the solid product
was isolated using an external magnet and dried at 50°C.
3
4
2.5.4 | (E)‐1,2‐Bis(4‐methoxyphenyl)eth-
ene (1d)
1
White solid; m.p. 213–215°C. H NMR (300 MHz, CDCl ,
3
δ , ppm):7.33 (d, J = 8.7 Hz, 4H), 6.91 (d, J = 8.7 Hz, 4H),
H
2
.4 | General procedure for Heck coupling
13
5
.34 (s, 2H), 3.87 (s, 6H). C NMR (75 MHz, CDCl , δ ,
3 C
reaction
ppm): 55.32, 113.50, 126.20, 127.44, 129.45, 159.31.
In a round‐bottom flask, aryl halide (1 mmol), styrene
(
1.2 mmol), Et N (2 mmol) and 50 mg of Fe O ‐Pd@SF‐
2.5.5 | (E)‐1‐Chloro‐4‐(4‐methoxystyryl)
3
3 4
SBA‐15 (3.34 mmol% Pd) in 3 ml of dimethylformamide
DMF) were stirred at 120°C. The progress of the reaction
benzene (1e)
(
1
was monitored using TLC (n‐hexane–EtOAc, 8:2). After
completion of the reaction, the mixture was cooled to
room temperature and the catalyst was separated using
a magnet and washed several times with diethyl ether
and ethanol. The reaction mixture was diluted with
dichloromethane and the organic layer was dried over
White solid; m.p. 178–179°C. H NMR (300 MHz, CDCl ,
3
δ , ppm):7.50–7.44 (m, 4H), 7.36–7.30 (m, 2H), 7.08 (d,
H
1
3
J = 16.2 Hz, 1H), 6.98–6.93 (m, 3H), 3.88 (s, 3H).
C
NMR (75 MHz, CDCl , δ , ppm): 55.36, 114.21, 125.28,
3
C
127.42, 127.82, 128.81, 128.86, 129.80, 132.71, 136.21,
159.51.

4
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KHODAEI AND DEHGHAN
3
3
| RESULTS AND DISCUSSION
.1 | Catalyst characterization
In a comparative mode, the FT‐IR spectra for each step of
the synthesis of the Fe O ‐Pd@SF‐SBA‐15 nanocatalyst
3
4
−
1
are shown in Figure 1. The broad band at 3460 cm in
Figure 1a is assigned to O─H stretching of silanol groups
and adsorbed water molecules. The stretching and bend-
ing vibrations of Si─O─Si appeared at 1116 and 818 cm
−
1
,
respectively. The methylene group vibrations in the
propyl chain can be seen as two weak peaks at 2950 and
−
1
−1
2
880 cm in Figure 1b–e. The peak at around 1650 cm
is attributed to the characteristic vibration of imine group,
while this band shifts to lower frequency (around 1640 cm
−
1
)
in the spectrum of Pd@SF‐SBA‐15 (Figure 1d). The
shifting of this band to lower frequency confirmed the for-
mation of metal–ligand bond. The peak for the stretching
vibration of carbon–carbon double bond in aromatic ring
−
1
appeared at 1554 cm . A characteristic peak in Figure 1
−
1
e at 589 cm is attributed to the stretching vibration of
Fe─O. The presence of this peak indicates the presence
of Fe O NPs in the synthesized nanocomposite. Based
3
4
FIGURE 2 XRD patterns of (a) Fe O NPs, (b) Pd@SF‐SBA‐15
3
4
on the obtained results, it can be deduced that the organic
3 4
and (c) Fe O ‐Pd@SF‐SBA‐15
groups had been successfully grafted to the SBA‐15.
XRD patterns of Fe O4 NPs, Pd@SF‐SBA‐15 and
presence and stability of the crystalline phase of Fe O
3
3
4
Fe O ‐Pd@SF‐SBA‐15 are shown in Figure 2. In Figure 2
and Pd NPs on the amorphous silica walls (Figure 2c).
3
4
a, the strong peaks at 2θ = 30.2°, 36.4°, 43.7°, 53.5°,
6.3°, 62.3° and 73.8° are attributed to (220), (311), (400),
422), (511), (440) and (533) of highly crystalline cubic spi-
Mesoporous materials with hexagonal symmetry can
be effectively identified using low‐angle X‐ray scattering
measurements. Figure 3 shows the low‐angle XRD pat-
terns of SBA‐15 and Fe O ‐Pd@SF‐SBA‐15. It can be
5
(
[34,35]
nel structure of Fe O NPs
(JCPDS card file no. 19‐
3
4
3 4
0
629). A broad peak in Figure 2b at 2θ = 23° is attributed
observed that the three distinct reflections for Fe O ‐
3
4
[36]
to the amorphous silica walls
and the diffraction lines
Pd@SF‐SBA‐15 in the low‐angle region, indexed as (100),
[
30]
of (111), (200) and (220) at 2θ = 40.02°, 46.29° and 67.99°
of Pd(0) can be clearly observed in the pattern, which is
in good agreement with JCPDS card file no. 46‐1043. The
same sets of characteristic peaks in Figure 2a,b were also
observed for Fe O ‐Pd@SF‐SBA‐15, indicating the
(110) and (200),
are exactly the same as those for SBA‐
15. This observation indicates that the ordered hexagonal
mesoporous structure of SBA‐15 is preserved after
functionalization. A decrease in the intensity of reflections
for Fe O ‐Pd@SF‐SBA‐15 (Figure 3b) compared to SBA‐15
3
4
3 4
FIGURE 1 FT‐IR spectra of (a) SBA‐15, (b) SBA‐15‐Cl, (c) SF‐
SBA‐15, (d) Pd@SF‐SBA‐15 and (e) Fe ‐Pd@SF‐SBA‐15
FIGURE 3 Low‐angle powder XRD patterns of (a) SBA‐15 and
(b) Fe ‐Pd@SF‐SBA‐15
3
O
4
3 4
O

KHODAEI AND DEHGHAN
5 of 9
(
Figure 3a) indicates that the organic groups have been
and Si in the EDX analysis, it is concluded that the
functionalization of the SBA‐15 support and immobiliza-
tion of Pd NPs had been done successfully. Also, the map-
ping analysis suggests that Pd NPs and Schiff base are
successfully supported on SBA‐15.
grafted inside the channels of SBA‐15, such grafting reduc-
ing the scattering contrast of the mesoporous silica walls,
and thus reducing the intensity of reflections.
The presence of Pd NPs in the Fe O ‐Pd@SF‐SBA‐15
[37]
3
4
nanocomposite was confirmed using ICP‐OES and EDX
analyses. The morphology of the synthesized products
was characterized using FESEM and TEM. TEM images
clearly showed the presence of a well‐ordered two‐
dimensional hexagonal mesoporous structure for Fe O ‐
The nitrogen adsorption–desorption isotherms for
SBA‐15, SF‐SBA‐15 and Fe O ‐Pd@SF‐SBA‐15 are shown
3
4
in Figure 4. The textural properties are presented in
Table 1. As can be seen in Figure 4, all materials show
a type IV standard IUPAC isotherm hysteresis loop at a
3
4
[
39]
Pd@SF‐SBA‐15 (Figure S1a,b in supporting information).
It can be observed that Pd NPs have been inserted inside
the channels of SBA‐15 and suggests that SBA‐15, despite
the loading of metal NPs, has sufficient structural stabil-
ity to retain its morphology. The small black dots in
high relative pressure value (P/P = 0.65),
which is a
0
characteristic of mesoporous materials with a narrow
[
40]
pore size distribution.
However, their shapes are dif-
ferent, with a capillary condensation of nitrogen occur-
ring over a slightly lower P/P range when going from
0
[38]
[41]
Figure S1a,b are assigned to Pd(0) NPs
black dots attributed to Fe O NPs.
and larger
SBA‐15 to Fe O ‐Pd@SF‐SBA‐15.
It is observed that
3
4
with the same order for the adsorbed nitrogen volume,
the pore diameters decrease. The data of the isotherms
with the correlation coefficient from BET fitting were
used to calculate the physicochemical properties of the
materials. The reduction in the specific surface area and
pore size of functionalized SBA‐15 compared to SBA‐15
can be attributed to the presence of Schiff base, Pd and
Fe O , which block the channels (Table 1).
3
4
The FESEM images of SBA‐15 and Fe O ‐Pd@SF‐
3
4
SBA‐15 are shown in Figure S1c–e. As seen in Figure
S1c, the SBA‐15 sample consists of short rods with
wheat‐like morphologies and rod‐like structures that ran-
domly pack as bunches. It has been shown that these
types of morphologies have long‐range parallel channels
with two‐dimensional hexagonal mesostructures. The
morphology of SBA‐15 is preserved after Schiff base
functionalization and immobilization of Pd NPs (Figure
S1d,e). The EDX pattern and mapping analysis of
Fe O ‐Pd@SF‐SBA‐15 are shown in Figure S1f,g, respec-
3
4
3.2 | Evaluation of catalytic activity
3
4
tively. Based on the presence of peaks for Pd, N, O, C
The catalytic performance of the Fe O ‐Pd@SF‐SBA‐15
3
4
nanocatalyst was investigated in the synthesis of stilbene
derivatives. Thus, the Heck reaction between styrene and
iodobenzene was considered as a model reaction to opti-
mize the effects of base, solvent, temperature and Pd
loading. The results for optimization of conditions are
presented in Table 2. Entries 13–17 in Table 2 show the
effect of Pd loading on the catalytic performance. Based
on the obtained results, the best performance is when
3.34 mmol% of Pd is used; in other cases high yields of
the C─C coupled products were not achieved. As is evi-
dent from Table 2, the efficiency of the Heck reaction
was strongly affected by the nature of base. Thus, the
effect of base on the outcome of the reaction was explored
(Table 2, entries 6–9). Based on our experimental results,
FIGURE 4 Nitrogen adsorption–desorption isotherm curves for
(
3 4
a) SBA‐15, (b) SF‐SBA‐15 and (c) Fe O ‐Pd@SF‐SBA‐15
TABLE 1 Specific surface area, pore volume and mean pore diameter for SBA‐15, SF‐SBA‐15 and Fe
3
O
4
‐Pd@SF‐SBA‐15 nancatalyst
2
−1
3
−1
Material
Surface area (m g
)
Pore volume (cm g
)
Pore diameter (nm)
SBA‐15
575.33
401.36
307.39
0.92
0.69
0.48
7.69
6.99
5.90
SF‐SBA‐15
3 4
Fe O ‐Pd@SF‐SBA‐15
Total pore volume determined at P/P
0
= 0.99.
bMean pore diameter determined using the BJH method.

6
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KHODAEI AND DEHGHAN
TABLE 2 Optimization of reaction conditions
Entry
Catalyst
Solvent
Base
Pd loading (mmol%)
Temp. (°C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
Fe
3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
‐Pd@SF‐SBA‐15
NPs
Toluene
Et
Et
Et
Et
Et
3
3
3
3
3
N
N
N
N
N
3.34
3.34
3.34
3.34
—
80
80
90
90
31
CH
3
CN
26
EtOH
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
80
40
Trace
75
120
120
120
120
120
25
40
180
100
100
100
90
—
Na
2
CO
3
3.34
3.34
3.34
3.34
3.34
3.34
3.34
1.33
2.00
2.67
4.01
3.34
—
82
NaOAc
KOH
71
65
Et
3
Et
3
Et
3
Et
3
Et
3
Et
3
Et
3
Et
3
Et
3
Et
3
N
N
N
N
N
N
N
N
N
N
—
10
11
12
13
14
15
16
17
18
60
90
Trace
54
80
90
100
120
120
120
120
120
120
90
78
40
42
40
67
40
81
40
97
40
97
120
—
Reaction conditions: iodobenzene (1 mmol), styrene (1.2 mmol), base (2 mmol) and 3 ml of solvent.
bIsolated yield.
TABLE 3 Coupling of aryl halides with styrene derivatives in the presence of catalytic amounts of Fe
3
O
4
‐Pd@SF‐SBA‐15
Entry
X
R
R*
Product
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
I
H
H
1a
1a
1a
1b
1b
1b
1b
1b
1c
1c
1c
1d
40
97
96
94
97
97
92
95
92
91
90
94
93
Br
Cl
I
H
H
75
H
H
130
55
4‐OMe
H
I
H
4‐OMe
H
50
Br
Br
Cl
I
4‐OMe
90
H
4‐OMe
4‐OMe
4‐Cl
4‐Cl
4‐Cl
4‐OMe
90
H
170
60
H
1
1
1
0
1
2
Cl
Br
I
H
150
100
90
H
4‐OMe
(Continues)

KHODAEI AND DEHGHAN
7 of 9
TABLE 3 (Continued)
Entry
X
Br
I
R
R*
Product
Time (min)
Yield (%)
13
14
15
4‐OMe
4‐OMe
4‐OMe
4‐OMe
4‐Cl
4‐Cl
1d
1e
1e
180
80
89
95
92
Br
180
Isolated yield.
it was found that Et N afforded the best results for this
To investigate the role of magnetic NPs, a further
experiment was done in the presence of Fe O NPs as cat-
alyst (Table 2, entry 18). Based on the obtained results in
the presence of magnetic NPs, no stilbene product was
obtained. So, these NPs would only help in the easy sep-
aration of the catalyst from the reaction mixture.
3
reaction and allowed stilbene to be obtained in 97% yield
after 40 min. The other bases, Na CO , NaOAc and KOH,
3
4
2
3
were not as effective as Et N and only afforded moderate
3
to low yields of the corresponding coupling product.
Finally, the effects of solvent and temperature were
We applied the optimized reaction conditions for the
coupling of aryl halides with styrenes. All products were
obtained in high yields (Table 3). The mechanism of the
Heck reaction catalysed by the Fe O ‐Pd@SF‐SBA‐15
3
4
nanocatalyst is presented in Scheme 2.
Due to the high importance of the Heck reaction for
the synthesis of stilbene derivatives, a class of compounds
with diverse pharmacological properties, many
researchers have focused their attention on improving
the reaction conditions for the synthesis of stilbene deriv-
atives. A comparison based on the results of the present
research and those of previous reports is presented in
Table 4. The selected model reaction for these studies
was the coupling reaction between iodobenzene and sty-
rene using various Pd catalysts in DMF in the presence
SCHEME
2
3 4
Mechanism of Fe O ‐Pd@SF‐SBA‐15‐catalysed
Heck reaction
of 2 mmol of Et N at 120°C. The comparisons were per-
3
formed on the basis of reaction times and obtained yields
(
Table 4, entry 8 versus entries 1–7). Based on the
examined, the best result being obtained in DMF at
20°C for 40 min in the presence of 50 mg of Fe O ‐
obtained results, this catalytic system has characteristics
including easy separation, reusability for eight times with
negligible loss in catalytic activity, and giving excellent
1
3
4
Pd@SF‐SBA‐15 (3.34 mmol% Pd) using 2 mmol of Et N.
3
TABLE 4 Comparison of the results of this research with those of various previous reports
Entry
Catalyst
Time (h)
Yield (%)
Ref.
[
[
[
[
[
[
[
42]
43]
44]
45]
46]
47]
48]
1
2
3
4
5
6
7
8
Pd/MFC
2
2
66
75
68
71
96
96
92
97
Palladium acetate
Chloropalladated
48
23
2.5
1
PdCl
2
(SEt
2 2
)
NiFe
2 4
O
@TABMA‐Pd(0)
Pd/Met/GO
Diatomite‐supported Pd
2
Fe
3
O
4
‐Pd@SF‐SBA‐15
0.67
This work
Isolated yield.

8
of 9
KHODAEI AND DEHGHAN
base stabilizes the Pd NPs and prevents them leaching
from the surface of the support. Because of these advan-
tages and the importance of the Heck reaction, Heck
catalysis has great potential for further industrial
applications.
ACKNOWLEDGEMENT
The authors gratefully acknowledge Razi University for
financial support.
3 4
FIGURE 5 Reusability of Fe O ‐Pd@SF‐SBA‐15 nanocatalyst for
Heck reaction
ORCID
Mohammad Mehdi Khodaei
http://orcid.org/0000-0002-
yields of trans‐stilbene in a short reaction time, which
0385-9203
make it better than those of the previous works.
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How to cite this article: Khodaei MM, Dehghan
M. Palladium nanoparticles immobilized on Schiff
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efficient and magnetically recoverable nanocatalyst
for Heck coupling reaction. Appl Organometal
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