Synthesis and applications of polymeric N-heterocyclic carbene palladium complex-grafted silica as a novel recyclable nano-catalyst for Heck and Sonogashira coupling reactions

Article abstract of DOI:10.1039/c3nj41137k

A new catalytic system based on palladium nanoparticles supported on polymeric N-heterocyclic carbene-grafted silica (Si-PNHC-Pd) is introduced. Aminopropylsilica was reacted with benzoylchloride to form acrylamidopropylsilica. Onto this functionalized silica, vinylimidazole monomer was copolymerized by free radical polymerization. Poly(vinylimidazole)-grafted silica was treated with methyl iodide to form a quaternary salt of poly(vinylimidazole). Si-PNHC-Pd was obtained by subsequent treatment of imidazolium salt with PdCl₂. Determination of Pd content was performed using an inductively coupled plasma (ICP) analyzer and the crystalline character was determined by an XRD technique. The topography and particle size of the catalyst was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM images showed that palladium was dispersed through the polymer surface in nano particle size. This catalytic system exhibited excellent activity in Heck and Sonogashira coupling reactions with various aryl halides. The catalyst was successfully re-used up to 12 runs without appreciable change in its activity. High efficiency of the catalyst along with high yields of products, easy purification, large scale synthesis and high TON and TOF are among the advantages of this heterogeneous catalyst.

Full text of DOI:10.1039/c3nj41137k

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Synthesis and applications of polymeric N-heterocyclic
carbene palladium complex-grafted silica as a novel
recyclable nano-catalyst for Heck and Sonogashira
coupling reactions†
Cite this: NewJ.Chem., 2013,
37, 2011
Bahman Tamami,*a Fatemeh Farjadian,ab Soheila Ghasemia and Hamed Allahyaria
A new catalytic system based on palladium nanoparticles supported on polymeric N-heterocyclic carbene-
grafted silica (Si–PNHC–Pd) is introduced. Aminopropylsilica was reacted with benzoylchloride to form
acrylamidopropylsilica. Onto this functionalized silica, vinylimidazole monomer was copolymerized by free
radical polymerization. Poly(vinylimidazole)-grafted silica was treated with methyl iodide to form a quaternary
salt of poly(vinylimidazole). Si–PNHC–Pd was obtained by subsequent treatment of imidazolium salt with PdCl2.
Determination of Pd content was performed using an inductively coupled plasma (ICP) analyzer and the
crystalline character was determined by an XRD technique. The topography and particle size of the catalyst
was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force
microscopy (AFM). TEM images showed that palladium was dispersed through the polymer surface in nano
particle size. This catalytic system exhibited excellent activity in Heck and Sonogashira coupling reactions with
various aryl halides. The catalyst was successfully re-used up to 12 runs without appreciable change in its
activity. High efficiency of the catalyst along with high yields of products, easy purification, large scale synthesis
and high TON and TOF are among the advantages of this heterogeneous catalyst.
Received (in Montpellier, France)
14th December 2012,
Accepted 22nd March 2013
DOI: 10.1039/c3nj41137k
www.rsc.org/njc
Palladium-catalyzed cross-coupling reactions are one of the
most employed organic transformations.6 Among these trans-
Introduction
The use of N-heterocyclic carbenes (NHCs) as ligands and their
metal complexes have been receiving wide attention recently.1
Some of the beneficial features of these compounds that make
them unique in organometallic chemistry and are responsible
for their rapid growing are; s-electron-donation and to a lesser
degree p-back-donation properties that lead to stronger bonds
with metal centers than most classical ligands, resistance of
metal complexes to decomposition, thermal and oxidative
stability and better steric protection of the active site within
the inner metal coordination sphere.2 These properties make
them the only class of ligands that has been able to challenge the
widely employed tertiary phosphines.3 The first metal complex
with a NHC ligand was reported in 1968 by Ofele and Wanzlick.4
Since then, many complexes of transition metals with enhanced
catalytic performance and higher stability have been introduced.5
formations, the coupling of aryl, vinyl, benzyl or allyl halides,
acetates or triflates with alkenes in the Heck–Mizoraki reaction,7
and the reaction of terminal alkynes with organic halides in the
Sonogashira reaction8 for the synthesis of substituted arylated
alkyne compounds, are interesting examples of carbon–carbon
[C(sp2)–C(sp)] bond formations. These reactions are carried out
in the presence of Pd catalysts involving ligands such as
phosphines, amines, NHCs, dibenzylidineacetone (dba), etc.6a
Among various ligands, NHCs have gained attention due to
their capability to form moisture- and air-stable catalysts with
remarkable activity in carbon–carbon bond formation reactions.5a
The NHC ligand Pd complex was first introduced by Herrmann and
Kocher in 1997 to the Suzuki and Heck reactions9 and is now widely
applied in C–C bond formation reactions.10 Many homogeneous
palladium complexes of NHC ligands are reported as potential
alternatives to phosphine ligands for cross-coupling reactions.10b
However, homogenous systems are accompanied with limita-
tions such as inefficient separation of products from the reaction
mixture and the loss of expensive metal complexes. On the other
hand, the immobilization of homogeneous catalysts onto insoluble
a Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454,
Iran. E-mail: tamami@chem.susc.ac.ir
b Center for Pharmaceutical Nanotechnology and Biomaterials,
Shiraz University of Medical Sciences, Shiraz 71345, Iran
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nj41137k supports, polymeric organics11 or inorganics,12 is an expanding
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research area offering the advantages of easy product separation,
Aminopropylsilica (AMPS) (I) was reacted with acryloylchloride
catalyst recovery, recycling and the inhibition of metal loss. Silica or to form acrylamidopropylsilica (II). This modified silica, with a
modified silica-supported catalysts are good alternatives to tradi- polymerizable double bond, was copolymerized with vinylimidazole
tional homogeneous catalysts; they are environmentally friendly, monomer through a free radical polymerization reaction. FT-IR
have excellent (chemical and thermal) stability, good accessibility, spectrum of the polyvinylimidazole-grafted silica (Si–PVI) (III)
and good dispersion of catalytic sites.12 Several advantages of the showed the characteristic band of the N–CQN group at
immobilization of homogeneous catalysts, and the preparation and 1647 cmÀ1 and CQC group of imodazole at 1504 cmÀ1 respectively
use of NHC Pd complexes supported either on polymeric13 or (Fig. 1).
silica14 supports have also been investigated. Other inspiring work
The amount of PVI-grafted silica was determined by TGA
in this field, was preparation of a main-chain NHC–palladium and was found to be 1.3 mmol gÀ1 (Fig. 2). Polymer-grafted
polymer as a recyclable self-supported catalyst for the Suzuki silica (III) was treated with methyl iodide followed by stirring
reaction, which was reported by Karimi et al.15
with sodium chloride solution to form a quaternary salt of
Grafting polymer chains onto the silica surface provide a system polyvinylimidazole (Si–QPVI) (IV). The FT-IR spectrum of (IV)
that can be completely compatible with solvents and substrates, with exhibit two bands at 1677 and 1522 cmÀ1, which are assigned to
good dispersion. Polymer-grafted silica has found applications in the C–H bending of the imidazolium group, and at 1141 cmÀ1
practical and fundamental studies of interfacial phenomena.16 There was the C–H band of the C2 position of the imidazoilim ring
are some reports in the literature on the applications of polymeric (Fig. 1). The permanent charge density of the imidazole groups
metal complex-grafted silica as catalysts in organic transformations. was determined using argentimetric titration and was calculated
The first concept of this type appeared in 1991 by Challa et.al, who to be 1.1 mmol gÀ1. The imidazolium chloride salt was reacted
used Cu(II) complexes of poly(styrene-co-N-vinylimidazole) grafted on with PdCl2 in the presence of cesium carbonate as a base and
silica in an oxidative coupling reaction.17 Other reports are on the DMF as a solvent to obtain the polymeric N-heterocyclic carbene–
use of Co(II) tetraphenylporphyrins immobilized on poly(4-vinylpyr- palladium-grafted silica (Si–PNHC–Pd) (V) (Scheme 1). The FT-IR
idine-co-styrene)-grafted silica for the aerobic oxidation of ethylben- Spectrum of (V) showed the absorption frequencies of a double
zenes,18 a poly(4-vinylpyridine)–Cu(II) complex for use in oxidation bond at 1642 and the C–Pd absorption frequency at 792 cmÀ1
,
reactions,19 and poly(N-vinyl-2-pyrrolidone)–Ru(0) complex-grafted along with the disappearance of the C–H band of the C2 position
silica for the hydrogenation of aromatic compounds.20 However, of the imidazolium ring due to NHC–Pd formation (Fig. 1).13b
as far as we know there is no report in the literature on hetero- Determination of Pd content of (V) was performed by digestion of
geneous polymeric N-heterocyclic carbene–Pd complex-grafted silica the catalyst followed by ICP analysis, which revealed the presence
for use in cross-coupling reactions. In continuation of our studies on of 0.26 mmol Pd gÀ1 for this heterogeneous catalyst.
heterogeneous Pd catalysts based on polymer-grafted silica,21 herein,
The powder X-ray diffraction (XRD) patterns for (V) showed
we report the synthesis and applications of a novel polymeric the expected crystallinity of Pd(0) nanoparticles and amorphous
N-heterocyclic carbene–palladium complex based on poly(N-methyl silica (Fig. 3). The diffraction rings can be ascribed to the (111),
vinylimidazolium)-type polymer-grafted silica as a recoverable (200), (220) and (311) crystallographic planes of the Pd(0)
catalyst in Heck and Sonogashira coupling reactions.
nanoparticles.
Scanning Electron Microscopy (SEM) images of the catalyst
are shown in Fig. 4. The SEM images show the smooth surface
of the cubic like Si–PNHC–Pd. According to the Transmission
Electron Microscopy (TEM) image (Fig. 5a), the palladium
particles are well dispersed through the catalyst surface, with
a certain degree of homogeneity. The average size of the
palladium particles were measured from the TEM image and
Results and discussion
Synthesis and characterization of polymeric N-heterocyclic
carbene–Pd complex-grafted silica (Si–PNHC–Pd)
In this study we disclose a new type of polymeric NHC–Pd
complex grafted onto silica (Scheme 1).
Scheme 1 Synthesis of the polymeric N-heterocyclic carbene–Pd complex-grafted silica (Si–PNHC–Pd).
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Fig. 4 (A) SEM image of polymeric N-heterocyclic carbene–Pd complex-grafted
silica (Si–PNHC–Pd) (V) (B) SEM close-up of image (A).
Fig. 1 FT-IR Spectrum of poly(N-vinylimidazole)-grafted silica (Si–PVI) (III),
quaternary chloride salt of polyvinylimidazole-grafted silica (Si–QPVI) (IV) and
polymeric N-heterocyclic carbene–Pd complex-grafted silica (Si–PNHC–Pd) (V).
Fig. 5 (a) TEM image of polymeric N-heterocyclic carbene–Pd complex-grafted
silica (Si–PNHC–Pd) (V).
Fig. 2 Thermogravimetric analysis of poly(N-vinylimidazole)-grafted silica
(Si–PVI) (III).
were performed from (V) to obtain the topography of the Pd
found to be in the range of 7–10 nm. AFM was used to study the particle dispersion in 3 dimensions of x, y, z. In (Fig. 6a) there is
topography of the elements in micro and nano scales.23 a two dimensional (2-D) AFM image, which shows that the
By means of this digital instrument nanoscope, several scans average diameter of the Pd nanoparticles is between 6–7 nm.
Fig. 3 XRD spectrum of polymeric N-heterocyclic carbene–Pd complex-grafted silica (Si–PNHC–Pd) (V).
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Table 1 Optimization of base, solvent and Pd catalyst (V) for the Heck reaction
of iodobenzene with n-butyl acrylatea
Entry Base
Solvent Pd (mol%) Time (h) Conversionb (%)
1
2
3
4
5
6
7
8
9
K2CO3 DMF
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.3
0.2
3.5
8
4
2
10
4
7
2.5
3
100
90
60
100
70
90
80
90
87
K2CO3 EtOH
K2CO3 THF
K2CO3 NMP
LiF
NaOAc NMP
NaOH NMP
NMP
K2CO3 NMP
K2CO3 NMP
a
Reaction conditions: iodobenzene (1.0 mmol), n-butyl acrylate
(1.2 mmol), base (2.0 mmol), Pd catalyst (0.5–0.2 mol%) and 3 mL of
b
solvent at 120 1C. Conversions based on iodobenzene.
Fig. 6 AFM images of the polymeric N-heterocyclic carbene–Pd complex-
grafted silica (Si–PNHC–Pd) (V) (a) 2-D image, (b) 3-D image and (c) voltage
profile.
styrene as the olefinic substrate with different aryl halides
(Table 2). The electron-neutral, electron-rich and electron-poor
aryl iodides and bromides reacted with n-butyl acrylate very well
to generate the corresponding cross-coupling products in good
to excellent yields; however, aryl chlorides were inert under the
same conditions. We used tetrabutylammonium bromide
(TBAB) as the Jeffery catalyst24 in the reaction of aryl chlorides
with n-butyl acrylate, but they were unreactive and remained
intact even after long reaction times. Only 4-chloro nitro
benzene, as an activated substrate, transformed to the coupled
product in low yield after 24 h (Table 2, entry 13).
This concept was also confirmed by the TEM image. In the 3-D
image (Fig. 6b) more details were obtained about the height of
the Pd particles. The maximum height scanned was 10 nm. The
corresponding histogram (Fig. 6c) shows a symmetric distribution
of scanned depth. The maximum height of the particles was
9.3 nm. By the symmetric shape of the histogram, we can
conclude that the Pd nanoparticles dispersed smoothly, without
or with a negligible amount of agglomeration.
We also examined the Sonogashira cross-coupling reaction
for the generation of arylated–alkyne compounds. The copper-,
amine- and phosphine-free Sonogashira reaction of phenylacetylene
with aryl iodides, bromides and some chlorides were performed with
this catalyst. In a model reaction, the coupling of iodobenzene with
phenylacetylene was studied in the presence of different bases and
solvents in the presence of the catalyst. The reaction parameters
were optimized. DMF as solvent and K2CO3 as base using 0.5 mol%
of the catalyst at 120 1C gave the best result. The amount of the
catalyst could be reduced to 0.1 mol% by prolonging the reaction
time to 2.5 h. The generality of this reaction was shown when other
coupling reactions were carried out using phenylacetylene with
different aryl halides (Table 3). The electron-neutral, electron-rich
and electron-poor aryl halides reacted with phenylacetylene very well
to generate the corresponding cross-coupling products in good to
excellent yields. Although aryl chlorides are not as reactive and are
less commonly employed in palladium-catalyzed coupling reactions,
Sonogashira reactions could be performed with this catalyst using
phenylacetylene and aryl chlorides in the presence of a catalytic
amount of TBAB (Table 3, entries 11–13).
Pd nanoparticle formation during NHC–Pd complexation is
inevitable, since IV was reacted with PdCl2 in the presence of
CsCO3 as base and DMF as solvent. Although these conditions
are favored for NHC–Pd complexation, it can also prompt Pd
nanoparticle formation by implementing the transformation of
Pd(II) to Pd(0) in the presence of the base6c and DMF,22 which
are also working as reducing agents. We also observed stabilized
Pd nanoparticles by TEM and AFM without aggregation, as a
result, we concluded that the ionic structure of IV protected the
Pd nanoparticles from aggregation, as suggested and confirmed
by Burguete et.al., who had the same observation in their NHC–
Pd complex formation.13g
Heck and Sonogashira coupling reactions
The catalytic activity of the Pd catalyst (V) was tested for a Heck
reaction of aryl halides with styrene and n-butyl acrylate. In a
model reaction, the coupling of iodobenzene with n-butyl
acrylate was studied in the presence of a base and the catalyst
in a solvent at 120 1C. The reaction conditions were optimized,
and the results are presented in Table 1. It was found that the
best system was NMP as the solvent and K2CO3 as the base
Heterogeneity test and recycling of the catalyst
using 0.5 mol% of catalyst at 120 1C. The amount of catalyst can For practical applications of a heterogeneous catalytic system,
be reduced down to 0.2 mol% of Pd by prolonging the reaction the reusability of the supported catalyst is very important. Thus,
time to 3 h.
the recovery and reusability of the supported catalyst (V) was
The generality of this reaction system was shown when other investigated using iodobenzene and n-butyl acrylate as model
coupling reactions were carried out using n-butyl acrylate or substrates. We could successfully use this catalyst in 12 subsequent
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Table 2 Heck reaction of n-butyl acrylate or styrene with aryl halidesa
Table 3 Sonogashira reaction of phenylacetylene with aryl halidesa
Entry Aryl halide
1
R2
Time (h) Isolated yieldb (%)
Entry
1
Aryl halide
Time (h)
1.25
Isolated yieldb (%)
93
CO2Bun
2
6
95
95
2
3
CO2Bun
CO2Bun
2
3
1.5
1.5
85
77
1.5
87
4
5
6
CO2Bun
CO2Bun
CO2Bun
1.45
8
78
80
90
4
5
6
7
8
9
1.45
1.5
1
70
77
92
95
85
78
5
7
CO2Bun
4
93
1
8
CO2Bun
CO2Bun
5
3
80
75
50
2
9
2
10
CO2Bun 24
CO2Bun 24
10
24
24
70
11
12
30
40
11
12
85c
CO2Bun 24
24
24
65c
80c
13
13
14
15
16
CO2Bun 24
CO2Bun 24
30c
Tracec
90
a
Molar ratios of ArX: phenylacetylene: K2CO3: palladium catalyst =
b
1.0 : 1.2 : 2.0 : 0.005. Reaction conditions: DMF at 120 1C. The charac-
terization of products was performed by comparison of their FT-IR,
1H-NMR, 13C-NMR and physical data with those of authentic samples.26
c
With additional tetrabutylammonium bromide (0.01 mmol).
Ph
Ph
Ph
7
12
10
reactions and the catalyst retained its catalytic activity in these
repeating cycles. The Pd catalyst did not show an appreciable
change in its catalytic activity but prolonging the reaction time
was necessary to achieve full conversion. The results are presented
in Table 4. To check the heterogeneity of this catalyst, which is
an important factor, the filtrate of each cycle was analyzed by an
ICP technique. Very low palladium contamination was observed
during these experiments. The turn over number (TON = mmole
of product/mmole of Pd catalyst) of the catalyst was calculated
for 12 runs of recycling to be 2130 and an average turnover
80
17
88
a
Molar ratios of ArX: n-butyl acrylate or styrene: K2CO3: palladium
catalyst = 1.0 : 1.2 : 2.0 : 0.005. Reaction conditions: NMP at 120 1C.
b
The characterization of products was performed by comparison
of their FT-IR, 1H-NMR, 13C-NMR and physical data with those of
authentic samples.25 c With additional tetrabutylammonium bromide
(0.01 mmol).
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Table 4 Recycling of Pd catalyst (V) for the Heck reaction of iodobenzene with
n-butyl acrylatea
by SEM, XL-30 FEG SEM, Philips, at 20 kV. Pd loading and leaching
tests were carried out with an Inductively Coupled Plasma (ICP)
analyzer (Varian, vista-pro). Particle size distribution (PSD) was
determined by a dynamic light scattering particle size analyzer
Horiba-LB-550 instrument. Atomic force microscopy (AFM) analysis
was performed with Nanoscope IIIa Multimode AFM (from Veeco)
and images are recorded in Tapping Mode using a 10 micron
scanner and standard silicon cantilevers.
Entry
Cycle
Time (h) Isolated yield (%) TONb
TOFc (hÀ1
)
1
2
3
4
5
6
7
8
9
1st
2
95
95
95
93
93
90
87
87
85
85
80
80
190
190
190
186
186
180
174
174
170
170
160
160
95
76
76
74.4
67.6
60
2nd
3rd
4th
5th
6th
7th
8th
9th
10th
11th
12th
2.5
2.5
2.5
2.75
3
3.5
3.5
3.5
4
49.7
49.7
48.5
42.5
37.6
37.6
Preparation of poly(N-vinylimidazole)-grafted silica (Si–PVI) (III)
Acrylamidopropylsilica was prepared according to our reported
procedure.21 To a suspension of acrylamidopropylsilica (2.0 g),
N-vinylimidazole (4 mL) and recrystallized benzoyl peroxide
(0.05 g) were added in a 10 mL sealed tube. The mixture was
heated at 90 1C in an oven for 15 h. The polymer-grafted silica
was Soxhlet-extracted with CHCl3 (200 mL) for 24 h, followed by
washing and drying. The amount of poly(N-vinylimidazole)-
10
11
12
4.25
4.25
TON for twelve 2130
Av. TOF (hÀ1) 59.5
runs
a
Reaction conditions: molar ratio of iodobenzene: n-butyl acrylate:
b
K2CO3: Pd catalyst = 1.0 : 1.2 : 2.0 : 0.005 in NMP at 120 1C. TON =
mmol of products/mmol of Pd catalyst. TOF = TON/time.
c
grafted silica was determined by TGA to be 1.3 mmol gÀ1
.
frequency (TOF (hÀ1) = TON/time) for 12 runs was calculated to
be 59.5.
Preparation of polymeric N-heterocyclic carbene–Pd complex-
grafted silica (Si–PNHC–Pd) (V)
A hot filtration test was also used to ascertain whether the
catalyst was behaving in a truly heterogeneous manner, or
whether it was merely a reservoir for a more active soluble
form of Pd. In a typical experiment, Pd complex (0.5 mol%),
iodobenzene (1.0 mmol), n-butyl acrylate (1.2 mmol), K2CO3
(2.0 mmol) and NMP (3 mL) were taken in a round-bottomed
flask and stirred at 120 1C for 15 min. At this stage, the catalyst
was filtered off and the experiment was continued with the
filtrate for another 24 h. There was no detectable increase in the
product concentration, as is evident from the GC analysis.
Furthermore, the ICP analysis does not show any Pd leaching
in this stage. This experiment confirmed the heterogeneous
character of the catalytically active species.
To a suspension of (Si–PVI) (2.0 g) in DMF (30 mL) was added
methyl iodide (5 mmol, 0.311 mL). The slurry was stirred at
80 1C for 20 h and then filtered off. The corresponding solid
was washed with DMF and ethanol and then dried under
reduced pressure. The resulting powder stirred in NaCl solution
(5%) (35 mL) at room temperature for one day. The mixture was
filtered off, washed thoroughly with H2O and acetone and then
dried in an oven under vacuum at 60 1C for 10 h. The chloride
ion capacity of the imidazolium type polymer was found using
an argentimetric titration method.27 It was calculated to be
1.1 mmol g .
À1 28 The resulting imidazolium chloride salt (2.0 g)
was reacted with PdCl2 (5 mmol) in the presence of Cs2CO3
(10 mmol) as base and DMF (30 mL) as solvent at 80 1C for 20 h.
The mixture was filtered and washed thoroughly with DMF,
H2O and acetone. The black powder was then treated for 24 h in
DMF solvent in order to remove any physisorbed palladium
particles. According to the ICP analysis, the Pd content in the
Experimental section
General remarks
Materials were purchased from Fluka AG and Merck Chemical heterogeneous catalyst was determined to be 0.26 mmol gÀ1
.
Companies. Aminopropylsilica, with an average particle size of
0.015–0.035 mm (>400 mesh ASTA) and a loading of 0.95 mmol gÀ1
,
General procedure for the Heck reaction
was supplied by Fluka AG. All products were characterized by A suspension of aryl halide (1.0 mmol), K2CO3 (2.0 mmol), Pd
comparison of their IR and NMR spectra and physical data complex (0.005 mmol) in NMP (5 mL) was mixed in a reaction
with those reported in the literature. Thin-layer chromatography flask and n-butyl acrylate or styrene (1.2 mmol) was added. The
was performed on silica-gel Polygram SIL/UV 254 plates. Petroleum reaction mixture was stirred at 120 1C for an appropriate length
ether boiling point used for TLC monitoring is in the range of of time. The progress of the reaction was monitored by TLC
40–60 1C. Gas chromatographic analyses were performed on a using a petroleum ether–ethyl acetate (95/5) and/or GC until no
Shimadzu GC 10-A instrument with hydrogen flame ionization traces of starting aryl halide was observed. On completion of
detector. Infrared spectra were recorded on a Shimadzu FT-IR- the reaction, the mixture was filtered and the filtrate poured
8300 spectrophotometer. NMR spectra for 1H and 13C were into water (50 mL) and extracted with CH2Cl2 (3 Â 15 mL). The
recorded on a Bruker Avance DPX instrument (250 MHz). combined organic phases were dried over Mg2SO4, filtered and
Thermogravimetric analysis (TGA) was performed using Perkin the solvent evaporated under vacuum. The product was isolated
Elmer instrument under N2 gas and a heating rate of 20 1C minÀ1
.
by flash chromatography using a mixture of petroleum ether
X-ray diffraction data obtained with XRD, D8, Advance, Bruker, and ethyl acetate as eluent. Characterization of the products was
axs. TEM analysis was performed on a Philips model CM performed by comparison of their FT-IR, 1H-NMR, 13C-NMR, and
10 instrument. Scanning electron micrographs were obtained physical data with those of the authentic samples. The used
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under vacuum and reused without any pretreatment for repeating
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General procedure for the Sonogashira reaction
To a suspension of aryl halide (1.0 mmol), K2CO3 (2.0 mmol),
Pd complex (0.005 mmol) in DMF (5 mL), phenylacetylene
(1.2 mmol) was added. The reaction mixture was stirred at
120 1C for an appropriate length of time. The progress of the
reaction was monitored by TLC using a petroleum ether–chloroform
mixture (90 : 10) and/or GC until no traces of starting aryl halide was
observed. Upon completion of the reaction, the procedure outlined
above was followed.
Conclusion
In conclusion, we introduced a novel heterogeneous catalyst
based on polymeric N-heterocyclic carbene–Pd complex-grafted
silica. The XRD technique was used to ascertain the presence of
Pd(0). TEM image showed the presence of Pd nanoparticles
dispersed through the surface of this polymer. In addition, the
AFM histogram confirmed the presence of palladium in the
nanoparticle size range. The high efficiency and stability of this
catalytic system was shown for Heck and Sonogashira coupling
reactions with a number of aryl halides. The catalyst showed
high thermal stability and could be successfully reused for 12
reaction cycles giving a total TON = 2130. Simple filtration of
the catalyst, excellent dispersity of Pd particles, short reaction
times and high yields are among the other advantages of this
catalytic system.
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Acknowledgements
The authors gratefully acknowledge the partial support of this
study by the Shiraz University Research Council. The author
would like to thank Professor Dr Mathias Ulbricht (Lehrstuhl
Technische Chemie II–Universitat Duisburg-Essen) for his
valuable comments and Dr Steffan Franzka (Fakultat fu¨r Chemie–
¨
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.
c
2018 New J. Chem., 2013, 37, 2011--2018
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013