N-heterocyclic carbene–palladium(II) complex supported on magnetic mesoporous silica for Heck cross-coupling reaction

Article abstract of DOI:10.1002/aoc.4904

Magnetic mesoporous silica was prepared via embedding magnetite nanoparticles between channels of mesoporous silica (SBA-15). The prepared composite (Fe₃O₄@SiO₂-SBA) was then reacted with 3-chloropropyltriethoxysilane, sodium imidazolide and 2-bromopyridine to give 3-(pyridin-2-yl)-1H-imidazol-3-iumpropyl-functionalized Fe₃O₄@SiO₂-SBA as a supported pincer ligand for Pd(II). The functionalized magnetic mesoporous silica was further reacted with [PdCl₂(SMe₂)₂] to produce a supported N-heterocyclic carbene–Pd(II) complex. The obtained catalyst was characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, vibrating sample magnetometry, Brunauer–Emmett–Teller surface area measurement and X-ray diffraction. The amount of the loaded complex was 80.3?mg?g^?1, as calculated through thermogravimetric analysis. The formation of the ordered mesoporous structure of SBA-15 was confirmed using low-angle X-ray diffraction and transmission electron microscopy. Also, X-ray photoelectron spectroscopy confirmed the presence of the Pd(II) complex on the magnetic support. The prepared magnetic catalyst was then effectively used in the coupling reaction of olefins with aryl halides, i.e. the Heck reaction, in the presence of a base. The reaction parameters, such as solvent, base, temperature, amount of catalyst and reactant ratio, were optimized by choosing the coupling reaction of 1-bromonaphthalene and styrene as a model Heck reaction. N-Methylpyrrolidone as solvent, 0.25?mol% catalyst, K₂CO₃ as base, reaction temperature of 120°C and ultrasonication of the catalyst for 10?min before use provided the best conditions for the Heck cross-coupling reaction. The best results were observed for aryl bromides and iodides while aryl chlorides were found to be less reactive. The catalyst exhibited noticeable stability and reusability.

Full text of DOI:10.1002/aoc.4904

Received: 8 August 2018
DOI: 10.1002/aoc.4904
Revised: 19 February 2019
Accepted: 21 February 2019
F U L L P A P E R
N‐heterocyclic carbene–palladium(II) complex supported
on magnetic mesoporous silica for Heck cross‐coupling
reaction
Ayat Nuri | Yagoub Mansoori
| Abolfazl Bezaatpour
Department of Applied Chemistry,
Faculty of Science, University of
Mohaghegh Ardabili, Ardabil 56199‐
Magnetic mesoporous silica was prepared via embedding magnetite nanoparti-
cles between channels of mesoporous silica (SBA‐15). The prepared composite
11367, Iran
(
Fe O @SiO ‐SBA) was then reacted with 3‐chloropropyltriethoxysilane,
3
4
2
Correspondence
sodium imidazolide and 2‐bromopyridine to give 3‐(pyridin‐2‐yl)‐1H‐
imidazol‐3‐iumpropyl‐functionalized Fe O @SiO ‐SBA as a supported pincer
Yagoub Mansoori, Department of Applied
Chemistry, Faculty of Science, University
of Mohaghegh Ardabili, Ardabil, 56199‐
3
4
2
ligand for Pd(II). The functionalized magnetic mesoporous silica was further
1
1367, Iran.
reacted with [PdCl (SMe ) ] to produce a supported N‐heterocyclic carbene–
2
2 2
Email: ya_mansoori@yahoo.com;
ya_mansoori@uma.ac.ir
Pd(II) complex. The obtained catalyst was characterized using Fourier trans-
form infrared spectroscopy, scanning electron microscopy, energy‐dispersive
X‐ray analysis, vibrating sample magnetometry, Brunauer–Emmett–Teller sur-
face area measurement and X‐ray diffraction. The amount of the loaded com-
−
1
plex was 80.3 mg g , as calculated through thermogravimetric analysis. The
formation of the ordered mesoporous structure of SBA‐15 was confirmed using
low‐angle X‐ray diffraction and transmission electron microscopy. Also, X‐ray
photoelectron spectroscopy confirmed the presence of the Pd(II) complex on
the magnetic support. The prepared magnetic catalyst was then effectively used
in the coupling reaction of olefins with aryl halides, i.e. the Heck reaction, in
the presence of a base. The reaction parameters, such as solvent, base, temper-
ature, amount of catalyst and reactant ratio, were optimized by choosing the
coupling reaction of 1‐bromonaphthalene and styrene as a model Heck reac-
tion. N‐Methylpyrrolidone as solvent, 0.25 mol% catalyst, K CO as base, reac-
2
3
tion temperature of 120°C and ultrasonication of the catalyst for 10 min before
use provided the best conditions for the Heck cross‐coupling reaction. The best
results were observed for aryl bromides and iodides while aryl chlorides were
found to be less reactive. The catalyst exhibited noticeable stability and
reusability.
KEYWORDS
Heck reaction, heterogeneous catalyst, magnetic mesoporous silica, NHC–Pd(II) complex
Appl Organometal Chem. 2019;e4904.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 15
https://doi.org/10.1002/aoc.4904

2
of 15
NURI ET AL.
1
| INTRODUCTION
Anchoring of catalysts on magnetic nanoparticles
MNPs) has been the focus of many research
(
[
11–14]
In 2010, the Nobel Prize for Chemistry was awarded
to Heck, Negishi and Suzuki for their research on
palladium‐catalysed cross‐coupling reactions in organic
synthesis since these reactions are strategically very impor-
tant and powerful for the formation of C―C bonds. In
recent years, intensive studies have focused on the prepa-
ration and supporting of palladium complexes with vari-
ous types of ligands as effective catalysts for coupling
reactions. Such efforts have resulted in improvements in
the efficiency of organic syntheses and a reduction in pro-
activities in recent years.
MNPs have advantages
of non‐toxicity, high dispersibility, biocompatibility, a
noticeably high surface‐to‐volume ratio, high coercivity,
superparamagnetic properties, great extraction efficiency
[
1]
[15,16]
and convenient separation.
In addition, MNP‐
supported catalysts can be easily and rapidly collected
from various reaction mixtures by means of an external
magnet.
Although bare MNPs have been reported as catalyst
[
17,18]
supports,
they have an intrinsic tendency towards
[
2]
cess time, reaction costs and generation of byproducts.
agglomeration. In order to overcome this limitation, it is
recommended to use MNPs in the form of composites.
Various materials, such as silica, oleic acid and polysty-
rene, have been used for this purpose. Among all reported
matrices, mesoporous silica has been more emphasized
These reactions have a great potential for industrial
applications; however, due to the properties and activities
of the existing catalysts, limited industrial applications
have been observed for the Heck reaction. On the other
hand, many researchers have noted that the Heck coupling
reaction is regioselective. For example, Hallberg et al.
showed that the regiochemistry of enol ether arylation
can be controlled by various parameters, such as the
[3]
[
19]
due to its biocompatibility,
high surface area, large
pore size and the other aforementioned advantages.
Combination of MNPs and mesoporous silica pro-
vides the advantages of both high surface area and mag-
netic properties. That is why many researchers have
investigated the modification of MNPs by mesoporous
silica materials, e.g. SBA‐15 and various kinds of MCMs,
[
4]
electronic nature and halogen type of its aromatic ring.
In addition to regioselectivity and industrial applicability,
the high atomic economy and low‐cost reagents of
palladium‐catalysed C―H functionalization in C―C cou-
pling reactions have led to the exploration of many to Pd
complexes in modern organic chemistry. Developing
highly regioselective reactions and enhancing the catalytic
activity by using particular ligands are the focal points of
[
18,20,21]
in recent years.
Incorporation of MNPs within
the channels of SBA‐15 leads to the formation of
Fe O @SBA‐15 particles that can be employed as mag-
x
y
[
22,23]
netically retrievable catalyst supports.
Application
of such support material was reported by Tai et al.,
who studied the synthesis of magnetic core–shell struc-
tures of mesoporous silica functionalized with sulfonic
acid and used the obtained nanoparticles for catalysis
of esterification. Their porous magnetic nanocomposites
[5]
most recent studies in this field.
Highly ordered mesoporous silica materials have
attracted a considerable deal of attention in the various
applications, such as diagnosis, drug delivery, extraction
and hyperthermia. Their non‐toxic nature, high surface
area (700 to 1200 m g ), abundant Si―OH bonds, tun-
able volume and pore diameter (2 to 50 nm) and potency
of surface modification mean that they are attractive candi-
dates for coating and catalyst support purposes. An exam-
ple of such materials is SBA‐15, which can present high
hydrothermal stability, large pore size (4.6 to 10.0 nm),
two‐dimensional hexagonal (p6mm) mesostructure and
thick uniform silica walls (31 to 64 Å). For these reasons,
SBA‐15 has been considered for simple internal surface
showed high activity towards esterification of propionic
2
−1
[24]
acid, in addition to facile recovery and re‐use.
Also,
Snoussi et al. synthesized a core–double shell nanocom-
posite, i.e. Fe O @NH ‐mesoporous silica@polypyrrole/
3
4
2
Pd, via a sonochemical procedure. The catalytic activity
of their obtained magnetic material was assessed in the
reduction of 4‐nitrophenol to 4‐aminophenol, which
gave a yield of above 94%, and in the degradation of
[25]
methyl orange in aqueous media (with 99% yield).
In a recent study, Keypour et al. synthesized a Pd(II)–
Schiff base complex supported on MNPs as
[
6–9]
functionalization and release of large molecules.
For
a
example, Lazar et al. designed a highly efficient reusable
heterogeneous catalyst using SBA‐15, i.e. Pd(II)‐
DHBP@SBA‐15, for arylation through the Heck reaction
and hydrogenation of alkenes. Their supported catalyst
retrievable heterogeneous catalyst. Their supported cata-
lyst was used in the Suzuki cross‐coupling reaction of
various aryl halides (I, Br, and Cl) with phenylboronic
acid and in the Heck coupling reaction of aryl halides
[
26]
was obtained via anchoring
a
2,2‐dihydroxyben-
(I and Br) with styrene.
zopheneone ligand onto amino‐functionalized SBA‐15
and subsequent metalation by Pd(II). Their obtained solid
catalyst was recycled effectively and reused several times
Though mesoporous silica composites have been
widely applied to coupling reactions, limited studies have
been reported about anchoring homogeneous catalysts on
mesoporous magnetic silica for C―C coupling reactions.
[10]
without any significant loss of activity.

NURI ET AL.
3 of 15
One of the rare examples of such research is the study of
Nikoorazam et al., who synthesized and characterized a
Pd(0)–Schiff base complex supported on magnetic
nanoporous MCM‐41 as a catalyst for the Suzuki and
Heck reactions. Their catalyst could be easily recovered
from the reaction mixture using an external magnet and
QUANTAX QX2 (RONTEC, Germany). X‐ray photoelec-
tron spectroscopy (XPS) was carried out with a Kratos
Axis Ultra DLD electron spectrometer using
a
monochromated Al Kα source operated at 150 W.
Analyser pass energies of 160 and 20 eV were applied
for acquiring wide spectra and individual photoelectron
lines, respectively. The spectrometer charge neutraliza-
tion system was used for stabilization of surface poten-
tial. The Si 2p line of silica, set at 103.3 eV, was used
as reference of the binding energy scale. Kratos software
was used to process the spectra. Powder samples were
hand‐pressed into pellets directly on a sample holder
using a clean Ni spatula. Quantitative GC analyses were
performed with an Agilent 7890 instrument using a cap-
illary column in conjunction with a flame ionization
detector. Column temperature was programmed
[27]
reused several times.
With respect to the advantages and drawbacks of the
mentioned materials, herein, we report the synthesis and
characterization of a novel N‐heterocyclic carbene
(
NHC)–Pd(II) complex anchored on Fe O @SiO ‐SBA.
3
4
2
The obtained retrievable magnetic catalyst showed excel-
lent efficiency in the Heck reaction. To the best of our
knowledge, the present study is the first report of the use
of Fe O @SiO ‐SBA as a magnetic support for the Heck
3
4
2
reaction.
−
1
between 180 and 200°C (2°C min ). Nitrogen was used
−
1
as a carrier gas at a flow rate of 20 ml min .
2
2
| EXPERIMENTAL
.1 | Instrumentation
2.2 | Materials
[
28]
[29]
MNPs,
(
Fe O @SiO ‐SBA
and trans‐[Pd(Cl)2
3
4
2
NMR spectra were obtained with a Bruker Avance
50 MHz spectrometer using tetramethylsilane. Fourier
[
30]
SMe ) ] complex
2 2
were prepared according to the liter-
2
ature. 3‐Chloropropyltrimethoxysilane (CPTS) and
pluronic P‐123 were obtained from Exir (Austria) and used
as received. All solvents were of laboratory grade and dried
according to procedures described in the literature. The
other laboratory‐grade chemicals were obtained from
Merck and used without further purification.
transform infrared (FT‐IR) spectra were recorded with a
PerkinElmer RXI spectrophotometer using KBr. Thermo-
gravimetric analysis (TGA) was conducted with a
Linseis STA PT 1000 instrument using a scanning rate
[31]
−
1
of 10°C min . The inductively coupled plasma optical
emission spectrometry (ICP‐OES) technique (Perkin-
Elmer, Optima 5300 DV) was employed for the determi-
nation of the amount of palladium on the supported
NHC–Pd(II) complex. Wide angle X‐ray diffraction
2
.3 | Functionalization of Fe O @SiO ‐SBA
3
4
2
with CPTS
(XRD) and low‐angle XRD in the 2θ ranges 10–80°
and 0.7–10° were conducted with Philips PW1730 (scan-
The MNPs of Fe O @SiO ‐SBA (1.000 g) were dispersed
3
4
2
−
1
ning rate of 0.01° min ) and Philips X‐Pro (scanning
in 40 ml toluene. Then, CPTS (4.1 ml, 17.1 mmol) was
added to the suspension dropwise and the reaction mix-
ture was refluxed for 24 h. The functionalized magnetic
mesoporous silica was decanted magnetically, washed
with methanol and acetone several times and then dried
in a vacuum oven at 45°C.
−
1
rate of 1° min ) instruments at room temperature,
respectively. Both instruments were equipped with an
X‐ray radiation source using Cu K (λ = 0.15406 nm).
α
Scanning electron microscopy (SEM) images were
obtained with a LEO 1430VP instrument. A vibrating
sample magnetometry (VSM) instrument (Maghnatis
Danesh‐pajooh Kashan Co. Iran) with a maximum mag-
netic field of 10 kOe was used for magnetization mea-
surements at room temperature. Transmission electron
microscopy (TEM) images of the supported NHC–Pd(II)
complex were obtained with a LEO 912 AB microscope
with an acceleration voltage of 120 kV. Multipoint nitro-
gen adsorption–desorption analysis according to the
Brunauer–Emmett–Teller (BET) method was performed
at −197.018°C using a TriStar II plus Micromeritics
automated gas adsorption analyser. Energy‐dispersive
X‐ray (EDX) microanalysis was performed with a
2.4 | Synthesis of Fe O @SiO ‐SBA‐(CH ) ‐
3
4
2
2 3
imidazole
Sodium hydride (0.410 g, 17.1 mmol) was added to a mag-
netically stirred solution of imidazole (1.160 g, 17.1 mmol
in 25 ml of sodium‐dried toluene), and stirred for 30 min
under an argon atmosphere at room temperature to give
sodium imidazolide. Then, Fe O @SiO ‐SBA‐(CH ) ‐Cl
3
4
2
2 3
(1.000 g) was added and the mixture was refluxed under an
argon atmosphere for 24 h. The imidazole‐functionalized

4
of 15
NURI ET AL.
magnetic mesoporous silica was decanted magnetically,
washed with ethanol several times and then dried in a vac-
uum oven at 45°C.
3 | RESULTS AND DISCUSSION
3.1 | Catalyst synthesis and
characterization
2
.5 | Synthesis of 3‐(pyridin‐2‐yl)‐1H‐
imidazol‐3‐iumpropyl (PIP)‐functionalized
Fe O @SiO ‐SBA
The MNPs were prepared by a co‐precipitation method
and then incorporated between the channels of SBA‐15.
The as‐prepared magnetic mesoporous particles were
3
4
2
[
32]
then functionalized with PIP–Pd(II) complex
the supported NHC–Pd(II) complex (Scheme 1).
to afford
The synthesized Fe O @SiO ‐SBA‐(CH ) ‐imidazole and
3
4
2
2 3
2‐bromopyridine (0.770 ml, 8.07 mmol) were stirred for
The FT‐IR spectra of MNPs, MNP@SiO ‐SBA and
2
2
0 h at 160°C. The mixture was then cooled to room tem-
supported NHC–Pd(II) complex are shown in Figure 1.
perature and the resultant functionalized magnetic meso-
porous silica was washed several times with the Et O–
CH Cl mixture (80/20, 20 ml) and decanted magneti-
cally. The obtained MNPs were then dried in a vacuum
oven at 45°C.
In all spectra, the presence of ν (Si―O―Si) band
as
−1
2
at 1110 cm and the characteristic strong peak of mag-
−1
2
2
netite at 589 cm
between SBA‐15 channels.
reveal the embedding of MNPs
[33]
In the spectrum of the
NHC–Pd(II) complex (Figure 1c), the bands related to
−
1
imidazole ring appear at 1854 and 1448 cm . Also,
the C&dbond;N stretching frequency, probably some-
what coupled with the pyridine ring vibration, is
2
.6 | Synthesis of supported NHC–Pd(II)
complex
−
1 [34]
observed at 1624 cm .
13
The assigned C CP‐MAS NMR spectrum of the sup-
ported catalyst is shown in Figure 2. The C‐2 and C‐6 car-
bons of pyridine ring (C and C ) appear at 146.2 and
PIP‐functionalized Fe O @SiO ‐SBA and trans‐[Pd(Cl)
3
4
2
2
(
(
(
SMe ) ] were dispersed in 20 ml of dry tetrahydrofuran
THF). Then, a THF solution of potassium t‐butoxide
0.030 g, 0.27 mmol) was added to the mixture at −80°C
2 2
a
b
148.1 ppm, respectively. The C‐2 atom of five‐membered
ring (C ) and C‐4 atom of pyridine ring (C ) are seen at
c
d
under continuous stirring. The reaction mixture was
warmed to room temperature over 1.5 h and stirred for
another 12 h. The supported NHC–Pd(II) complex was
decanted magnetically and washed with Et O–CH Cl
mixture (80/20) several times. In order to replace the
chloride ion of the supported complex with bromide
ion, the obtained MNPs were stirred for 4 h in 20 ml of
a dry acetone solution containing NaBr (0.125 g,
137.5 and 139.2 ppm, respectively. The remaining aro-
matic carbon atoms, C‐4 and C‐5 carbons of five‐
membered ring (Ch and C ), are observed at 111.7–
g
2
2
2
130.0 ppm. In the aliphatic region, the propyl carbons
appear at 50.3 ppm (C ), 23.2 ppm (C ) and 7.5 ppm
i
j
[
35,36]
(
Ck).
−
1
The amount of loaded Pd(II) (0.25 mmol g ;
2.64 wt%) was measured by ICP‐OES. TGA was used to
1
.22 mmol). The final product was separated using an
estimate the quantity of the loaded Pd(II) complex.
external magnet, washed with acetone and dried in a vac-
uum oven at 45°C.
[
32]
Removal of physically and chemically adsorbed H O mol-
2
ecules is shown by a mass loss of about 3.17% for
MNP@SiO ‐SBA from ambient temperature to 690°C
2
2
.7 | Heck coupling reaction
(Figure 3a). Under the same conditions, a very distinct
mass loss of about 11.2% is observed for the supported
NHC–Pd(II) complex due to the thermal breakdown of
the complex (Figure 3b). The difference between these
two values (Δm = 8.03%) can be attributed to the amount
In a 25 ml round‐bottom flask, the supported NHC–Pd(II)
complex was dispersed in 1 ml of dry N‐methylpyrrolidone
(NMP) with 8 min of ultrasonication. Aryl halide
(
1.0 mmol), olefin (1.2 mmol) and K CO (1.2 mmol) were
of complex loaded onto MNP@SiO ‐SBA (8.03% or
80.3 mg g of magnetic mesoporous silica).
2
2
3
−
1
then added to the catalyst suspension and the reaction
mixture was stirred at 120°C. The optimal amount of
Pd(II) was 0.25 mol% of aryl halide. The progress of the
reaction was monitored by TLC. After completion of the
reaction, the catalyst was separated magnetically and the
residue was analysed using GC in order to determine its
final composition. Further purification for NMR analysis
was achieved by conducting chromatography on glass
plates using silica gel as the stationary phase.
The SEM images of the MNPs and supported catalyst
are shown in Figure 4. The MNPs have a uniform spher-
ical morphology with diameters below 50 nm (Figure 4a).
Also, the particles of the supported catalyst have a uni-
form size distribution and possess diameters greater than
100 nm (Figure 4b). These particles seem to be aggregated
possibly due to the very high hydrophilicity of the meso-
[
37]
porous silica and moisture adsorption.

NURI ET AL.
5 of 15
SCHEME 1 Synthesis of the supported NHC–Pd(II) complex
FIGURE 1 FT‐IR spectra (KBr): (a)
MNPs; (b) MNP@SiO ‐SBA; (c) supported
2
NHC–Pd(II) complex

6
of 15
NURI ET AL.
magnetite remains unchanged in the final product.
The attenuation of the diffraction peaks can be attrib-
uted to the embedding of MNPs between mesoporous
silica channels (Figure 5b). The observed peaks at
2θ = 39° and 47° may be related to the formation of
palladium nanoparticles (111 and 200 hkl, respectively),
which has been reported for the unsupported
[
32]
catalyst.
The low‐angle XRD pattern of MNP@SiO @SBA is
2
depicted in Figure 6a. This pattern displays a well‐
recognized diffraction peak at 2θ = 1.06° corresponding
to the (100) diffraction, characteristic of a well‐ordered
two‐dimensional hexagonal (p6mm) mesoporous struc-
[
39]
ture.
Pure SBA‐15 sample exhibits two other peaks
corresponding to (110) and (200) diffractions at
[
40]
2θ = 1.58° and 1.82°.
Since the introduction of
magnetite between the channels of mesoporous silica
decreases the ordering of SBA‐15, the peaks correspond-
ing these diffractions are absent in the pattern of
[41,42]
MNP@SiO ‐SBA, as can be seen in the literature.
2
The retention of the peak corresponding to the (100)
diffraction in the low‐angle XRD pattern of the sup-
ported NHC–Pd(II) complex (Figure 6b) indicates that
the ordered mesoporous structure was maintained in
the magnetic solid catalyst. Attenuation of the peaks is
probably due to the difference in the scattering contrasts
of the pores and walls and formation of the Pd(II)
1
3
FIGURE 2 Solid‐state C NMR spectrum of supported NHC–
Pd(II) complex
complex inside the channels of MNP@SiO @SBA. The
2
shift of the diffraction peak can be attributed to the
functionalization of the interior walls of the pores by
the complex.
The ordered mesoporous structure of SBA‐15 that
is observed using low‐angle XRD was further
confirmed by TEM (Figure 7). The TEM images of
The XRD patterns of the bare MNPs and supported
NHC–Pd(II) complex are presented in Figure 5 and out-
line the characteristic peaks of magnetite. Therefore,
Figure 5a confirms the formation of cubic magnetite
nanoparticles (JCPDS file no. 01‐1111) with the Fd‐3m
[
43]
[38]
space group.
Furthermore, the crystal structure of
MNP@SiO @SBA show highly ordered arrays of one‐
2
FIGURE 3 TGA thermograms
(
nitrogen atmosphere, scan rate of
−
1
1
2
0°C min ): (a) MNP@SiO ‐SBA; (b)
supported NHC–Pd(II) complex

NURI ET AL.
7 of 15
FIGURE 6 Low‐angle powder XRD patterns: (a) MNP@SiO
2
‐
SBA; (b) supported NHC–Pd(II) complex
dimensional mesoporous channels. As illustrated in
Figure 7, the Fe O nanoparticles of 20 to 50 nm in diam-
3
4
eter are dispersed between the channels.
The qualitative elemental composition of the sup-
ported NHC–Pd(II) complex was determined using
EDX analysis at random points on the surface. The
weight and atomic percentages of the elements
present in the samples are listed in Table 1. This mea-
surement confirmed the presence of palladium atoms
together with iron, carbon, nitrogen, oxygen, bromine
and silicon atoms.
The magnetization curves of MNPs, MNP@SiO and
2
the supported NHC–Pd(II) complex are shown in
Figure 8 and Table 2 summarizes the results. As can be
seen, the saturation magnetization values (M ) of the pre-
s
−
1
pared materials are in the range 6.83 to 78.14 emu g .
These values are less than those reported for bulk magne-
tite particles (92 to 100 emu g ) due to the superpara-
FIGURE 4 SEM images: (a) MNPs; (b) supported NHC–Pd(II)
complex
−
1
magnetic behaviour that can be observed for single‐
[
44]
domain MNPs below a critical size.
Embedding of the
MNPs between the non‐magnetic mesoporous silica chan-
nels of SBA is responsible for decreasing the M values.
s
Moreover, small‐field coercivity of the supported NHC–
−
1
Pd(II) complex (H = 36.81 Oe, M = 0.44 emu g and
c
r
M /M = 0.06) shows that the supported catalyst particles
r
s
[
45]
are superparamagnetic.
The BET surface areas, pore volumes and average pore
sizes of the magnetic mesoporous silica and the supported
catalyst are summarized in Table 3. The nitrogen
adsorption–desorption isotherms of MNP@SiO ‐SBA and
2
the supported NHC–Pd(II) complex are shown in
Figure 9. As can be seen, MNP@SiO ‐SBA exhibits a typi-
2
[
46]
cal type‐IV curve with an H2 hysteresis loop.
The aver-
age pore diameter of MNP@SiO ‐SBA is 7.63 nm, the
average pore volume is 0.77 cm g and the BET surface
FIGURE 5 Wide‐angle powder XRD patterns: (a) MNPs; (b)
supported NHC–Pd(II) complex
2
−1
3

8
of 15
NURI ET AL.
2
FIGURE 7 TEM images of MNP@SiO ‐SBA
2
−1
TABLE 1 Amounts of various elements present in supported
NHC–Pd(II) complex
area is 483 m g . The corresponding values for the sup-
3
−1
2
−1
ported catalyst are 6.56 nm, 0.20 cm g and 105 m g .
This can be an indication that the PIP–Pd(II) complex
has been successfully supported into the pores. The
obtained results indicate that MNPs do not block the
intrachannel space, allowing molecules to enter the chan-
nels for subsequent modifications. Moreover, the value
found for the pore diameter of the final catalyst is in the
range of accepted values for mesoporous materials (2–
Element
Amount (wt%)
Amount (at%)
Carbon
Nitrogen
Oxygen
Sodium
Silicon
0.75
4.31
1.35
6.67
43.81
6.50
59.40
6.14
23.02
13.16
1.30
17.78
5.11
[
46]
0
50 nm).
The sharp increase of P/P from 0.5 to 1.0 and
the narrow size distribution (7.63 nm) highlight the forma-
tion of a highly ordered mesoporous material with hexag-
Iron
Bromine
Palladium
0.35
[
22]
2.58
0.53
onal cylindrical channels.
FIGURE 8 Magnetization curves for MNPs, MNP@SiO
2
and supported NHC–Pd(II) complex

NURI ET AL.
9 of 15
TABLE 2 Magnetic properties of prepared materials
M
s
−
1 a
(emu g
)
M
(emu g
r
H
(Oe)
c
M
M
r
/
−
1 b
c
d
s
Sample
)
MNPs
78.14
34.52
6.83
4.03
1.91
0.44
18.62 0.05
22.24 0.06
36.81 0.06
MNP@SiO
2
Supported NHC complex
a
Saturation magnetization.
b
c
Remanent magnetization.
Coercive force.
d
Remanence ratio.
TABLE 3 Physical properties of prepared materials
pore Pore volume SBET
D
3
−1
2
−1
Sample
(nm) (cm g
)
(m g )
MNP@SiO
2
‐SBA
7.63
6.56
0.77
0.20
483
105
Supported NHC–Pd(II)
complex
FIGURE 10 High‐resolution XPS spectra of supported NHC–
Pd(II) complex: (a) C 1s region; (b) Pd 3d5/2 and Pd 3d3/2 core
levels; (c) N 1s region
In the C 1s XPS spectrum of the supported NHC–Pd(II)
complex (Figure 10a), five main components are observed.
These components have binding energies at 284.8, 285.8,
286.8, 288.3 and 289.6 eV and can be assigned to carbon
in C―C/C―H, C―N(C¼N), C―O, O―C¼O and carbon-
ate bonds, respectively. The incomplete chemical calcina-
tion of pluronic P‐123 might be responsible for the
FIGURE 9 Nitrogen adsorption–desorption isotherms (a) and
pore size distribution (b) of MNP@SiO
Pd(II) complex
2
‐SBA and supported NHC–

10 of 15
NURI ET AL.
[
25]
appearance of the C―O bonds.
Figure 10b shows the
3.2 | Heck coupling reaction
XPS spectrum of the Pd 3d core level of the supported cat-
alyst. Two Pd 3d doublets located at 343.2 and 338.0 eV (Pd
3.2.1 | Optimization of reaction conditions
3
d5/2), and 341.0 and 335.8 eV (Pd 3d3/2) confirm that
both Pd(II) and Pd(0) species are present in the sam-
The reaction conditions were optimized by focusing on
the coupling reaction of 1‐bromonaphthalene and sty-
rene, as model substrates for the Heck reaction, and vary-
ing the reaction parameters including solvent, catalyst
dosage, base, reactant molar ratio, reaction temperature
and sonication of the magnetic catalyst prior to use. The
results are presented in Table 4. After a careful screening
of various solvents (entries 1–7), NMP turned out to be
the best by giving the highest yield of the coupling prod-
uct (entry 7, 54% yield). Then, the effect of catalyst load-
ing was optimized. The best conversion was observed
using 0.25 mol% catalyst (entry 11, 90% yield). Evaluation
[47,48]
ple.
Appearance and increase in intensity of Pd
3
d doublet corresponding to Pd(0) were observed from
XPS measurements followed by a decrease in Pd(II)
doublet intensity. This effect seems to be induced by
X‐ray photon bombardment together with low‐energy
[
49]
electrons used for the surface charge compensation.
Lastly, the N 1s region, Figure 10c, contains two compo-
nents at binding energies of 399.4 and 400.9 eV, which
may be assigned to the nitrogen atom in the pyridine
ring and to the partially positively charged nitrogen
atoms in five‐membered ring, respectively.
a
TABLE 4 Optimization of conditions for Heck reaction
b
c
−1 d
Entry
Solvent
Pd (mol%)
Base
ArX:styrene
Temperature (°C)
Conversion (%)
TON /TOF (h
)
e
1
2
3
4
5
6
7
8
9
Ethyl acetate
Acetone
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.22
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
K
K
K
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1.2
1:1.2
—
Trace
43
—
331/110
—
e
—
e
CHCl
CH CN
3
—
Trace
Trace
Trace
15
e
3
—
—
DMF
DMAc
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
120
120
120
120
80
—
116/39
416/139
386/129
328/109
352/117
360/120
360/120
278/93
184/61
343/115
284/95
252/84
368/123
384/128
54
85
82
10
11
12
13
14
15
16
17
18
19
100
120
150
120
120
120
120
120
120
120
88
90
90
Free
Et
CsF
70
3
N
46
86
Pyridine
NaOAc
71
63
K
K
2
2
CO
3
3
92
f
CO
96
a
Reaction conditions: 1‐bromonaphthalene (1.0 mmol), styrene (1.0 mmol), solvent (1 ml), base (1.2 mmol) and reaction time 3 h.
b
c
Determined by GC analysis.
TON = number of moles of desired product/number of moles of metal active sites.
d
TOF = number of moles of reactant converted/(number of moles of metal active sites × time in hours).
Reflux temperature.
e
f
Supported NHC–Pd(II) complex was sonicated in an ultrasonic bath for 10 min before reaction.

NURI ET AL.
11 of 15
a
TABLE 5 Heck reaction of aryl halide swith olefins catalysed by supported NHC–Pd(II) complex
M.p. (°C)
−1
Entry Aryl halide Olefin
Product
Found Reported
Time (h) TON/TOF (h
)
Conversion (%)
[50]
1
74–76
76–78
2.5
360/144
90
[
51]
2
Oil
Oil
Oil
2.0
384/192
96
[52]
3
4
Oil
1.5
3.0
396/264
393/131
99
98
[53]
108–110 105–108
[
52]
5
89–91
37–39
88–89
36–38
1.5
401/267
100
[52]
6
7
2.0
2.5
396/198
395/158
99
99
[47]
145–147 148–150
135–137 135–137
[47]
8
9
2.0
2.0
396/198
384/192
99
96
[54]
118–120 118.5–121.5
[
52]
1
1
0
1
Oil
Oil
2.0
3.0
396/198
384/128
99
96
[
47]
52]
71–73
70–71
36–38
[
1
1
2
3
37–39
2.5
2.5
395/158
385/154
99
96
[54]
118–120 118.5–121.5
(Continues)

12 of 15
NURI ET AL.
TABLE 5 (Continued)
M.p. (°C)
−1
Entry Aryl halide Olefin
Product
Found Reported
Time (h) TON/TOF (h
)
Conversion (%)
[52]
1
4
37–39
36–38
2.5
—
Trace
[54]
1
5
118–120 118.5–121.5
2.5
—
Trace
a
Reaction conditions: aryl halide (1.0 mmol), olefin (1.2 mmol), solvent (1 ml) and base (1.2 mmol).
of the effect of various reaction temperatures and bases
subsequent migration, β‐hydride elimination and base‐
induced removal of hydrogen halide give the desired
product and regenerate the catalyst.
(entries 9–17) showed that the highest advancement of
the coupling reaction was for 120°C and K CO as base
2
3
(
entry 11). It should be added that the product yield was
not satisfactory in the absence of any base (entry 13,
0% yield). Also, the molar ratio of ArX/olefin was opti-
In Table 6, the catalytic activity of the supported
NHC–Pd(II) complex in the Heck cross‐coupling
reaction of iodobenzene with styrene is compared with
that of a few catalysts reported in the literature. As
seen, the as‐prepared supported catalyst is more effective
than the other catalysts for the studied reaction
7
mized and the best result was observed using a 1:1.2
molar ratio (entry 18, 92% yield). Finally, it was observed
that sonicating the magnetic supported catalyst for
1
9
−
1
0 min improves the progress of the reaction (entry 19,
6% yield).
(Qm = 6.02 mg g ). Magnetic separation is another
advantage of the as‐prepared supported catalyst.
After optimizing the reaction conditions, the scope of
the reaction was investigated by applying the supported
NHC–Pd(II) catalyst to the Heck coupling reaction of a
wide range of aryl halides and olefins. The results are
listed in Table 5. As seen, although the palladium loading
3.2.2 | Recyclability of catalyst
−
1
was as low as 0.25 mmol g , excellent yields and turn-
over frequencies (TOFs) were obtained for the examined
aryl iodides and bromides (entries 1 to 13). Meantime,
the utilized aryl chlorides demonstrated a quite low reac-
tivity under the optimal reaction conditions (entries 14
and 15). Therefore, the catalyst can efficiently promote
the coupling reaction of aryl iodides and bromides with
various olefins.
Complicated and expensive routes are usually applied
for preparing supported catalysts. Therefore, sufficient
attention must be paid to the retrieval of the used
catalyst from a economic point of view. In the present
work, the stability and reusability of the supported
NHC–Pd(II) complex were also investigated in the Heck
cross‐coupling reaction of iodobenzene and styrene, as
model substrates, under the optimal conditions. It was
observed that the reaction conversion reaches 90%
during 1.0 h and gradually increases to 96% within
1.5 h in the first run. In order to regenerate the catalyst,
after each cycle, the catalyst was recovered easily
by simple magnetic decantation, washed with diethyl
ether and dried in vacuum. As can be seen in
Figure 11, the supported catalyst can be reused at least
five times with no significant loss of catalytic activity.
This experiment revealed that the synthesized composite
is not only a highly active catalyst but it is also a
stable and recyclable material under the described reac-
tion conditions.
A possible mechanism for the Heck cross‐coupling
reaction catalysed by the NHC–Pd(II) complex can be
proposed, as shown in Scheme 2. The mechanism is
suggested based on the mechanism described by
[55,56]
Shaw,
in which a Pd(II)/Pd(IV) cycle is involved
for pincer‐type Pd(II) complexes. According to the pro-
posed mechanism, following the coordination of olefin
−
3
to the supported Pd(II), the nucleophile (KCO )
attacks the alkene‐coordinated complex to give an alkyl
σ‐complex. The aryl halide oxidizes the Pd(II) centre,
the attached nucleophile is released and the alkene
coordinates to the Pd(IV) complex again. Then, the

NURI ET AL.
13 of 15
SCHEME 2 Proposed Pd(II)/Pd(IV)
catalytic cycle for the Heck cross‐coupling
reaction in the presence of the supported
NHC–Pd(II) complex
TABLE 6 Comparison of prepared catalyst with other catalysts for Heck cross‐coupling reactions of iodobenzene with styrene
−1
Entry
Catalyst
Conditions
Time (h)
Yield (%)
TON/TOF (h
)
Ref.
[
[
[
[
[
[
[
57]
58]
59]
60]
61]
62]
63]
1
2
3
4
5
6
7
8
Agarose‐Pd(0)
PdNP‐SβCD
Palladacycle
Pd/C particles
Pd–MPTA‐1
Solvent‐free, Et
3
N, 100–120°C
2
2
90
99
174/87
182/91
H
2
O, K
2
CO
N, 140°C
N, 100°C
CO , 100°C
COONa, 80°C
N, DMA, 80°C
NMP, K CO , 120°C
3
, 100°C
DMA, Et
3
18
24
6
100
23
504/28
H
2
2
O, Et
3
5.76/0.24
1302/217
93/62
H
O, Cs
2
3
92
Pd (OAc)
2
(ligand‐free)
PEG‐400, CH
3
1.5
3
93
1′‐Carbopalladated complexes
Supported NHC–Pd(II) complex
Et
3
80
159/53
2
3
2
96
384/192
This work
FIGURE 11 Recycling of supported
NHC–Pd(II) complex for Heck cross‐
coupling reaction of iodobenzene and
styrene. Reaction conditions: iodobenzene
(
(
1.0 mmol), styrene (1.0 mmol), solvent
1 ml), base (1.2 mmol) and reaction time
1
.5 h

14 of 15
NURI ET AL.
[
[
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4
| CONCLUSIONS
17] S. Rostamnia, A. Nuri, H. Xin, A. Pourjavadi, S. H. Hosseini,
Tetrahedron Lett. 2013, 54, 3344.
In summary, a highly efficient and retrievable NHC–
Pd(II) catalyst supported on Fe O @SiO ‐SBA was syn-
[18] R. Hudson, Y. Feng, R. S. Varma, A. Moores, Green Chem.
014, 16, 4493.
3
4
2
2
thesized. The obtained magnetic supported catalyst was
fully characterized using FT‐IR, TGA, SEM, TEM, XRD,
EDX, BET, VSM and XPS analyses and it was successfully
used for the Heck cross‐coupling reaction of aryl halides
and olefins. In addition, the catalyst was found to be
effective in the Heck cross‐coupling reactions of a series
of substituted aryl halides and olefins, as it provided
excellent yields. The applied catalyst could be easily
retrieved from the reaction mixture through magnetic
separation and re‐used for at least five consecutive cycles.
[
[
[
[
[
[
[
[
[
[
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ACKNOWLEDGEMENTS
Catal. Commun. 2017, 92, 56.
25] Y. Snoussi, S. Bastide, M. Abderrabba, M. M. Chehimi,
Financial support provided by the Graduate Council of
the University of Mohaghegh Ardabili is gratefully
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ORCID
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Yagoub Mansoori https://orcid.org/0000-0002-0562-3704
Azizian‐Kalandaragh, M. Pooresmaeil, RSC Adv. 2016, 6,
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How to cite this article: Nuri A, Mansoori Y,
Bezaatpour A. N‐heterocyclic carbene–
palladium(II) complex supported on magnetic
mesoporous silica for Heck cross‐coupling reaction.
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