Sulfonated palladium(II) N-heterocyclic carbene complex immobilized on nano-micro size poly(4-vinylpyridinium chloride) for Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1002/aoc.3350

The sulfonated palladium(II) N-heterocyclic carbene complex Pd^II(NHC)SO₃^-, supported on poly(4-vinylpyridinium chloride), was used as a heterogeneous, recyclable and active catalyst for the Suzuki-Miyaura reaction. This catalyst was applied for coupling of various aryl halides with phenylboronic acid and the corresponding products were obtained in excellent yields and short reaction times. The catalyst was characterized using Fourier transform infrared and diffuse reflectance UV-visible spectroscopies, scanning electron microscopy and elemental analysis. After each reaction, the catalyst was recovered easily by simple filtration and reused several times without significant loss of its catalytic activity.

Full text of DOI:10.1002/aoc.3350

Full paper
Received: 21 April 2015
Revised: 25 May 2015
Accepted: 13 June 2015
Published online in Wiley Online Library: 6 August 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3350
Sulfonated palladium(II) N-heterocyclic carbene
complex immobilized on nano–micro size
poly(4-vinylpyridinium chloride) for Suzuki-
Miyaura cross-coupling reaction
Zari Pahlevanneshan, Majid Moghadam*, Valiollah Mirkhani*,
Shahram Tangestaninejad, Iraj Mohammadpoor-Baltork and Saghar Rezaei
The sulfonated palladium(II) N-heterocyclic carbene complex PdII(NHC)SO3À, supported on poly(4-vinylpyridinium chloride), was
used as a heterogeneous, recyclable and active catalyst for the Suzuki–Miyaura reaction. This catalyst was applied for coupling
of various aryl halides with phenylboronic acid and the corresponding products were obtained in excellent yields and short reac-
tion times. The catalyst was characterized using Fourier transform infrared and diffuse reflectance UV–visible spectroscopies,
scanning electron microscopy and elemental analysis. After each reaction, the catalyst was recovered easily by simple filtration
and reused several times without significant loss of its catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: palladium(II) complex; N-heterocyclic carbene; heterogeneous catalyst; polyvinylpyridine; C–C coupling reactions
novel chemical properties as acidity, basicity and hydrophilic–
Introduction
hydrophilic balance, 4-PVP is of great importance.[33] The polymers
Recently, N-heterocyclic carbenes (NHCs) have attracted much atten-
tion because of their application as versatile ligands in homogeneous
transition metal catalysis. This is due to the strong σ-donor properties,
ease of preparation and effective binding ability to any transition
metal.[1–7] NHC complexes as homogeneous catalysts have been used
in a number of organic transformations such as olefin metathesis and
C–C and C–N bond formation reactions.[8–19] Therefore, the discovery
of catalytic properties of Pd–NHC complexes in the Mizoroki–Heck
and Suzuki–Miyaura cross-coupling reactions is of great importance
since these reactions have very many applications in the synthesis
of natural products, numerous drugs and high-performance modern
organic materials.[20–25] Despite the wide application of these homo-
geneous catalysts in organic transformations, their recycling is compli-
cated and their separation is very difficult. Moreover, in consequence
of the toxicity of palladium residuals, acceptable limits of palladium
traces in pharmaceuticals are set usually at the parts per million level.
Application of supported catalysts can solve this problem. In this re-
gard, polymer-supported catalysts have received much attention
due to the stability and uniformity of the obtained materials, and
the possibility of easier separation of product and catalyst, combining
the advantages of homogeneous and heterogeneous catalysis. For in-
stance, the immobilization of Pd–NHCs on polystyrene[26] or silica[27,28]
has been reported. Such systems have several advantages such as
simple recycling of the catalysts by filtration, preventing the loss of
both ligands and (heavy) metal, thus considerably decreasing the en-
vironmental problems of waste materials compared to homogeneous
catalysis.[29,30]
and copolymers of 4-vinylpyridine are intriguing materials with
many applications such as in sensors and actuators, host–ligand
of metal-containing chromophores, antimicrobial materials, and
so on.[34–36] PVP is an attractive polymer for immobilization of
nanoparticles because of the strong affinity of the pyridyl group
to metals and because of its ability to undergo hydrogen bonding
with polar species. Although cross-linked pyridinic resins are hy-
drophilic, their water affinity can be increased by the
quaternization of pyridine.[34,37] In addition, PVP can interact elec-
trostatically in quaternized or protonated forms with charged
surfaces.[38–40]
Any ion-exchange resin should have two features: high ion-
exchange capacity and fast ion-exchange kinetics. These character-
istics could be obtained in network materials as a result of the func-
tional groups present in the chemical structure and of their
porosity.
In continuation of our previous work using supported catalysts
in organic synthesis,[41–45] here we report the preparation and
characterization of the sulfonated palladium(II) NHC complex
PdII(NHC)SOÀ3 , supported on poly(4-vinylpyridinium chloride), as
a
heterogeneous catalyst in Suzuki-Miyaura C–C coupling
reactions (Scheme 1).
*
Correspondence to: Majid Moghadam or Valiollah Mirkhani, Department of
Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran.
E-mail: moghadamm@sci.ui.ac.ir; mirkhani@sci.ui.ac.ir
Poly(4-vinylpyridine) (4-PVP) and poly(2-vinylpyridine) (2-PVP)
have been known for many years and have found several indus-
trial as well as laboratory applications.[31,32] Considering such
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-
73441, Iran
Appl. Organometal. Chem. 2015, 29, 678–682
Copyright © 2015 John Wiley & Sons, Ltd.

Suzuki–Miyaura reaction catalysed by a supported Pd catalyst
Results and discussion
Characterization of PdII(NHC)SO3À@PVP
The preparation route for the catalyst is shown in Scheme 2. As can
be seen, the sulfonated PdII(NHC), PdII(NHC)SO3À, can be immobilized
on poly(4-vinylpyridinium chloride) via electrostatic interactions,
producing the heterogeneous PdII(NHC)SO3À@PVP catalyst.
The prepared catalyst was characterized using elemental and ICP
analyses, SEM, and DR UV–vis and FT-IR spectroscopies.
The DR UV–vis spectrum of the homogeneous catalyst shows an
absorption peak at 322 nm (Fig. 1(A)), while for PVP two absorption
peaks appear at 280 and 248 nm (Fig. 1(B)). The spectrum of the
catalyst shows the characteristic peaks of the homogeneous catalyst
and also the support. In the DR UV–vis spectrum of PdII(NHC)
SO3À@PVP, the absorption peaks appear at 323, 282 and 249 nm
Scheme 1. Suzuki-Miyaura C–C coupling reaction catalysed by
PdII(NHC)SO3À@PVP.
Experimental
General remarks
À
(Fig. 1(C)). These observations clearly confirm that the PdII(NHC)SO3
All the reagents were of analytical grade and used without further
purification. The chemicals used in this work were purchased from
Merck, Fluka and Sigma-Aldrich. Fourier transform infrared (FT-IR)
complex is supported on poly(4-vinylpyridinum chloride).
The FT-IR spectrum of the parent PdII(NHC)SO3À shows characteristic
SO vibrational bands at 1051 and 1187 cmÀ1. These bands also
appear in the FT-IR spectrum of PdII(NHC)SO3À@PVP, which confirms
the supporting of PdII(NHC)SO3À on PVP (Fig. 2). Also, the presence of
sulfur in the elemental analysis of the catalyst is a good indication for
the immobilization of PdII(NHC)SO3À on PVP.
1
spectra were recorded with a JASCO 6300 spectrophotometer. H
NMR spectra were recorded with a Bruker Avance 400 MHz spec-
trometer using DMSO-d6 as solvent. The morphological features
were investigated using a Hitachi S-4700 field emission scanning
electron microscopy (SEM) instrument. The Pd content of the
catalyst was determined using inductively coupled plasma (ICP)
analysis (PerkinElmer). Substances were identified and quantified
using GC with an Agilent GC 6890 equipped with a 19096C006
80/100 WHP packed column and a flame ionization detector. Dif-
fuse reflectance UV–visible (DR UV–vis) spectra were recorded
with a JASCO V-670 spectrophotometer. The PdII(NHC)SO3À com-
plex was prepared according to a literature method.[46] The pro-
tonation of 4-PVP was carried out according to a literature
method.[47]
The SEM images of PdII(NHC)SO3À@PVP show that the PVP particles
are aggregated as spherical particles ( Fig. 3). The histogram of particle
size distribution shows that about 78% of PVP particles have sizes
less than 100 nm (Fig. 3(C)). The Pd content of PdII(NHC)SO3À@PVP,
determined using ICP analysis, is 0.046 mmol gÀ1
.
Suzuki–Miyaura cross-coupling of aryl halides with
phenylboronic acid in presence of PdII(NHC)SO3À@PVP
After preparation and characterization of the catalyst, its catalytic
activity was investigated in the Suzuki–Miaura C–C coupling reac-
tion. First, the reaction parameters such as kind of base and solvent,
temperature and catalyst amount were optimized in the reaction of
4-iodoanisole (1 mmol) with phenylboronic acid (1.2 mmol). The
progress of the reaction was monitored using GC. The results are
summarized in Table 1. The results show that the optimized amount
of catalyst is 0.00096 mmol (0.01 mol%) of PdII(NHC)SO3À@PVP
(Table 1, entries 1–4). This coupling reaction is found to be highly
sensitive to the reaction temperature. The highest yield is obtained
at 90°C (Table 1, entries 3 and 5–7). When the model reaction is car-
ried out in various aqueous–organic media, the reaction is more
Immobilization of PdII(NHC) on poly(4-vinylpyridinum chlo-
ride): PdII(NHC)SO3À@PVP
First, commercially obtained 4-PVP was protonated by reaction with
excess HCl (1 N). The resulting protonated polymer was repeatedly
washed with deionized water to ensure complete removal of thÀe
unreacted free acid from the gel. To a solution of PdII(NHC)SO3
(500 mg) in water (50 ml), poly(4-vinylpyridinum chloride) (5 g)
was added and the mixture stirred for 24 h at room temperature.
At the end of the reaction, the catalyst, PdII(NHC)SO3À@PVP, was fil-
tered, washed successively with dimethylformamide (DMF), water
and acetone, and dried under vacuum. CHNS analysis (%): C,
73.02; H, 6.42; N, 11.46; S, 0.31.
General procedure for Suzuki-Miyaura cross-coupling reaction
of aryl halides with phenylboronic acid in presence of PdII(NHC)
SO3À@PVP
A mixture of aryl halide (1 mmol), arylboronic acid (1.2 mmol),
K2CO3 (207 mg, 1.5 mmol) and catalyst (0.096 mol%) in 4 ml of
DMF–H2O (2:1) was stirred at 90°C under air atmosphere. The prog-
ress of the reaction was monitored using GC. After completion of
the reaction, ethyl acetate (15 ml) was added and the catalyst was
filtered. The organic phase was washed with H2O (2 × 10 ml), dried
over anhydrous MgSO4 and evaporated. The residue was recrys-
tallized from ethyl acetate and ether (1:3) to afford the pure
product.
Scheme 2. Preparation of PdII(NHC)SO3À@PVP catalyst.
Appl. Organometal. Chem. 2015, 29, 678–682
Copyright © 2015 John Wiley & Sons, Ltd.
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Z. Pahlevanneshan et al.
322
200
300
400
500
600
600
600
700
700
700
800
800
800
Wavelength(nm)
Figure 2. FT-IR spectra: (a) PVP; (b) PdII(NHC)SO3À; (c) PdII(NHC)SO3À@PVP.
(A)
280
248
entries 3, 11 and 12). When the model reaction is carried out in the
absence of base, no appreciable amount of product is produced.
Therefore, the presence of a base is crucial in these reactions. Vari-
ous organic and inorganic bases such as NEt3, Na3PO4 and K2CO3
were applied in the model reaction. Among them, K2CO3 is found
to be the most efficient base. Comparison of inorganic bases uti-
lized shows that carbonate base is more stable than the other inor-
ganic base, and an organic base like Et3N is not as efficient as K2CO3
(Table 1, entries 3, 8–10). Therefore, it is concluded that the optimum
reaction conditions involve 4-iodoanisole (1 mmol), phenylboronic
acid (1.2 mmol), K2CO3 (1.5 mmol) and PdII(NHC)SO3À@PVP (0.096
mol% Pd) in DMF–H2O (2:1, 4 ml) at 90°C under air (Table 1, entry 3).
The scope and generality of this catalytic system was examined in
the Suzuki–Miyaura cross-coupling of aryl halides with phenylboronic
acid. Under the optimized reaction conditions, vatious aryl halides
were reacted with phenylboronic acid to produce substituted biphe-
nyls (Table 2). Various aryl iodides, bromides and chlorides bearing
electron-donating and electron-withdrawing groups react efficiently
with phenylboronic acid and the desired cross-coupling products
are produced in high yields. The results show that the electronic
properties of the substituents on the aromatic rings of aryl halide
have no significant influence on the reaction. As expected, aryl io-
dides are found to be more reactive than aryl bromides and
chlorides.
200
300
400
500
Wavelength(nm)
(B)
323
282
249
À
The catalytic activity of homogeneous PdII(NHC)SO3 was also
investigated in the model reaction. The results show that in the
reaction involving 4-iodoanisole (1 mmol), phenylboronic acid
(1.2 mmol), K2CO3 (1.5 mmol) and PdII(NHC)SO3À (0.096 mol% Pd)
in DMF–H2O (2:1, 4 ml) at 90°C under air, the amount of desired
product is 100% after 12 min. The main disadvantage of homoge-
neous PdII(NHC)SO3À is its solubility in the reaction mixture, and it
cannot be recovered and reused.
200
300
400
500
Wavelength(nm)
Catalyst recovery and reuse
(C)
Since the homogeneous catalyst cannot be recovered and reused
even once, the reusability of the heterogeneous catalyst was inves-
tigated in the reaction of 4-iodoanisole with phenylboronic acid in
the presence of K2CO3. At the end of each reaction, the catalyst
was filtered and washed thoroughly successively with acetone,
diethyl ether and water, and reused with fresh 4-iodoanisole and
phenylboronic acid. The results are summarized in Table 3. As can
be seen, the catalyst can be recovered and recycled five consecutive
Figure 1. DR UV–vis spectra: (A) PdII(NHC)SO3À; (B) PVP; (C) PdII(NHC)SO3À@PVP.
effective in DMF–H2O mixture (2:1). When EtOH–H2O mixture is
used as solvent, lower yield of the desired cross-coupling product
is obtained, while in toluene–H2O the product yield is very low.
Therefore, all reactions are carried out in DMF–H2O mixture (Table 1,
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 678–682

Suzuki–Miyaura reaction catalysed by a supported Pd catalyst
Table 1. Optimization of reaction conditions in the Suzuki-Miyaura
reaction of À4-iodoanisole with phenylboronic acid catalysed by
PdII(NHC)SO3 @PVPa
Entry
Solvent
Base
Pd
(mol%)
Temperature
(°C)
Yield
(%)b
1
2
3
4
5
6
7
8
9
DMF–H2O (2:1)
DMF–H2O (2:1)
DMF–H2O (2:1)
DMF–H2O (2:1)
DMF–H2O (2:1)
DMF–H2O (2:1)
DMF–H2O (2:1)
DMF–H2O (2:1)
DMF–H2O (2:1)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na3PO4
NEt3
0
90
0
70
97
98
16
43
98
73
25
6
0.048
0.096
0.144
0.096
0.096
0.096
0.096
0.096
0.096
0.096
0.096
90
90
90
Room temp.
50
110
90
(A)
90
10 DMF–H2O (2:1)
11 EtOH–H2O (2:1)
No base
K2CO3
90
90
81
45
12 Toluene–H2O (2:1) K2CO3
90
aReaction conditions: 4-iodoanisole (1 mmol), phenylboronic acid (1.2
mmol), base (1.5 mmol), solvent (4 ml) under air atmosphere.
bGC yield after 100 min.
Table 2. Suzuki-Miyaura cross-coupling reaction of aryl halides and
phenylboronic acid catalysed by PdII(NHC)SO3À@PVPa
(B)
Entry
R
X
PdII(NHC)SOÀ3 @PVPb
Time (h)
Yield (%)c
30
25
20
15
10
5
1
H
I
1.55
98
2
4-Me
4-MeO
4-NO2
H
I
1.6
1.67
1.55
5
97
97
99
93
87
93
99
68
99
31
97
3
I
4
I
5
Br
Br
Br
Br
Cl
Cl
Cl
Cl
6
4-MeO
4-NO2
4-Ac
4
7
3.4
4
8
9
H
12
4
0
20
40
60
80 100 120 140 160 180
Diameter(nm)
10
11
12
4-Ac
4-MeO
4-NO2
12
11
(C)
aReaction conditions: aryl halide (1 mmol), phenylboronic acid (1.2
mmol), K2CO3 (1.5 mmol), DMF–H2O (2:1, 4 ml) under air.
b90°C, catalyst (0.096 mol% Pd).
cDetermined using GC.
Figure 3. (A, B) SEM images of PdII(NHC)SO3À@PVP. (C) Histogram of
particle size distribution of catalyst.
times. The yield for the fifth run is 88% for the heterogeneous cata-
lyst. The amount of Pd leached was determined using ICP analysis.
As evident from Table 3, no Pd is detected in the filtrates after the
first two runs. The nature of the recovered catalyst was studied
using FT-IR spectroscopy. The FT-IR spectrum of the recovered cata-
lyst is the same as that of the fresh catalyst (Fig. 4). A probable
reaction is that the K2CO3 deprotonates the pyridinium units in
the PVP polymer, and then the palladium catalyst attaches to the
pyridines. The absence of a band corresponding to Pd–NPy vibration
(418 cmÀ1) in the FT-IR spectrum of the recovered catalyst confirms
that the catalyst retains its initial structure during the catalytic
reactions.[48]
Appl. Organometal. Chem. 2015, 29, 678–682
Copyright © 2015 John Wiley & Sons, Ltd.
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Z. Pahlevanneshan et al.
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Conclusions
The NHC complex PdII(NHC)SO3À, supported on nano–micro size
poly(4-vinylpyridinium chloride), was used as a heterogeneous,
recyclable and active catalyst for the Suzuki–Miyaura reaction.
Simple preparation of catalyst, good catalytic activity and high
reusability are noteworthy advantages of this catalytic system in
the C–C coupling reactions.
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Acknowledgements
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The authors are grateful to the Research Council of the University of
Isfahan for financial support of this work.
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Appl. Organometal. Chem. 2015, 29, 678–682