Polymerized functional ionic liquid supported Pd nanoparticle catalyst for reductive homocoupling of aryl halides

Article abstract of DOI:10.1007/s00706-013-0925-7

Homopolymer of 3-(cyanomethyl)-1-vinylimidazolium hexafluorophosphate was synthesized and used as a support for Pd nanoparticles. The Pd@poly-CN-PF ₆ catalyst (2 mol % Pd) selectively catalyzes the reductive homocoupling reaction of aryl iodides and bromides at 100 C in water, and the catalyst can be recycled and reused several times with slight loss of activity.

Full text of DOI:10.1007/s00706-013-0925-7

Monatsh Chem (2013) 144:1159–1163
DOI 10.1007/s00706-013-0925-7
ORIGINAL PAPER
Polymerized functional ionic liquid supported Pd nanoparticle
catalyst for reductive homocoupling of aryl halides
•
•
•
Jiayi Wang Yuan Li Pinzhen Li
Gonghua Song
Received: 8 November 2012 / Accepted: 9 January 2013 / Published online: 28 February 2013
Ó Springer-Verlag Wien 2013
Abstract Homopolymer of 3-(cyanomethyl)-1-vinylimi-
dazolium hexafluorophosphate was synthesized and used as
a support for Pd nanoparticles. The Pd@poly-CN-PF6
catalyst (2 mol % Pd) selectively catalyzes the reductive
homocoupling reaction of aryl iodides and bromides at
100 °C in water, and the catalyst can be recycled and
reused several times with slight loss of activity.
reducing agents, including hydroquinone [5, 6], alcohols
[7–10], benzoin [11], poly(ethylene glycol) [12], glucose
[13], ascorbic acid [14], hydrogen gas [15], amines [16],
formate salt [17, 18], zinc [19–22], and indium [23] have
been utilized as hydrogen donor and/or electron source to
regenerate the reductive Pd0 active species from the
oxidative Pd2? species to complete the catalytic redox
cycle. But most studies focused on homogeneous catalysts,
and the recycling and reuse of the palladium catalysts were
rarely discussed [12, 13, 18, 24, 25]. Thus, the develop-
ment of recyclable heterogeneous palladium catalysts is
highly desirable.
Keywords Heterogeneous catalysis Á Polymerization Á
Reductive homocoupling Á Aryl halides Á Palladium
Introduction
Polymerized ionic liquids (PILs) [26, 27], which combine
the unique properties of ionic liquids and macromolecular
architectures, have recently attracted more and more atten-
tion. The application of PILs as supports in noble-metal-
catalyzed reactions is an important aspect of this chemistry
[28–32]. Exploiting the fact that the nitrile group is a good
stabilizer of Pd nanoparticles [33, 34], we report herein the
reductive homocoupling of aryl halides catalyzed by Pd
nanoparticles supported and stabilized by a nitrile func-
tionalized ionic liquid homopolymer. The nitrile
functionalized ionic liquid moieties in the polymer act not
only as stabilizer to Pd nanoparticles, but also make the
catalyst more compatible with the organic substrates.
The immobilized Pd nanoparticles can efficiently catalyze
the homocoupling reactions of aryl halides in water at
100 °C with good yields, and the catalyst can be recycled
and reused several times.
The reductive dimerization of aryl halides is one of the
most important C–C bond-forming methods in organic
synthesis because biaryls are useful building blocks for
many biologically significant pharmaceuticals and agro-
chemicals. In general, the biaryls are synthesized by
copper-mediated Ullmann homocoupling of aryl halides
[1]. However, the need for a stoichiometric amount of
copper catalysts and high temperature limited its wide
application. In recent years, researchers have made
improvements to this methodology by utilizing other
transition metals, including palladium, zinc [2], indium [3],
and nickel [4] to promote the Ullmann homocoupling
reactions under milder conditions. For the palladium-cat-
alyzed reductive homocoupling of aryl halides, various
J. Wang Á Y. Li Á P. Li Á G. Song (&)
Shanghai Key Laboratory of Chemical Biology,
School of Pharmacy, East China University
of Science and Technology, Shanghai 200237,
People’s Republic of China
Results and discussion
As shown in Scheme 1, the functionalized ionic liquid mono-
mer 3 was synthesized by the reaction of 1-vinylimidazole (1)
e-mail: ghsong@ecust.edu.cn
123

1160
J. Wang et al.
and 2-chloroacetonitrile (2), and then treated with potassium
hexafluorophosphate to obtain the monomer. The azodiisobu-
tyronitrile (AIBN)-catalyzed free radical homo-polymerization
of the monomer afforded poly-CN-PF6. Finally, the polymer
was added to the aqueous H2PdCl4 solution with vigorous
stirring, followed by reduction of Pd2? to Pd0 with NaBH4 and
drying under vacuum to afford the catalyst Pd@poly-CN-PF6.
A high loading amount of Pd on the catalyst (10 wt %) was
achieved as determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES). The as-synthesized catalyst
was found to be insoluble in water and hardly soluble in com-
monly used organic solvents, such as methanol, ethanol,
acetonitrile, acetone, dichloromethane, toluene, and ethyl
acetate. Therefore, the catalyst can be easily recycled by simple
filtration after the reaction. The catalyst was also characterized
by transmission electron microscopy (TEM). The Pd nano-
particles were well dispersed in the polymer with an average
diameter of 3 nm as shown in Fig. 1a.
additive (Table 1, entry 1). When one equivalent of
glucose, ascorbic acid, or hydroquinone was used as an
additive, the yields of product were dramatically improved
to 51, 58, and 45 %, respectively (Table 1, entries 2–4).
Other additives, such as ethylene glycol, glycerol, and
b-CD, gave no product and the starting material remained
unchanged (Table 1, entries 5–7). We therefore chose
ascorbic acid as the most effective additive for further
investigation. The yield of reaction product went up
smoothly as the reaction temperature increased and a yield
of 85 % after reaction for 9 h was achieved at 100 °C
(Table 1, entries 8–10). Further investigation demonstrated
that 2 mol % Pd based upon the amount of 4c could afford
the homocoupling product 5c in a high yield of 97 %
(Table 1, entry 11). It was also noted that little reductive
deiodination product was observed according to GC anal-
ysis, implying good selectivity of the catalyst for reductive
homocoupling over reductive deiodination. Finally, we
chose the following optimized reaction conditions:
2 mol % Pd in Pd@poly-CN-PF6 as catalyst, water as
solvent, NaOH as base, ascorbic acid as additive, and
reaction at 100 °C.
The synthesized catalyst was then applied in the
reductive homocoupling of aryl halides. The catalytic
activities of Pd@poly-CN-PF6 under different conditions
were evaluated in the model reductive homocoupling
reaction of 1-iodo-4-methoxybenzene (4c, Scheme 2). The
results are listed in Table 1. A low yield of 10 % was
obtained with 1 mol % Pd as catalyst and without any
Under the optimized conditions, the palladium-cata-
lyzed reductive homocoupling reaction was extended to
various aryl halides, and the results are summarized in
Table 2 (Scheme 2). The reactions proceeded smoothly
with both electron-deficient and electron-rich aryl iodides
in good yields (Table 2, entries 1–4). For the less active
aryl bromides, longer reaction times were required to get
good yields (Table 2, entries 6–10). The highly sterically
demanding 1-iodo-2-(trifluoromethyl)benzene gave the
oC
3h
.
1 75
,
,
N
N
C
C
l
C
N
N
N
N
.
2 KPF6
H
2O
3
1
2
PF6
AIBN
H
N
3C
C
N
r
e
N
N
.
1 H Pd
l
C 4
2
m
N
N
N
C
y
Pd@poly-CN-PF6
l
N
N
o
n
.
a
2 N BH4
Base, additive
p
L
X
I
PF6
N
H2O, T, Time
R
R
R
o
-
-
o
l
p
y
-
N PF
6
-
Pd
@
l
y
N PF
C
C
p
6
4a-4k
5a-5f
Scheme 2
Scheme 1
Fig. 1 TEM images of freshly
prepared Pd@poly-CN-PF6
(a) and reused four times (b)
123

Polymerized functional ionic liquid supported Pd nanoparticle catalyst
1161
product in a yield of 10 % after 9 h. The homocoupling
of 1-iodo-4-chlorobenzene and 1-bromo-4-chlorobenzene
delivered the dichloro products with good yields, which
exhibited good selectivity of iodo and bromo groups over
the chloro group (Table 2, entries 4 and 9). Poor catalytic
activity for chlorobenzene was observed as only a yield of
8 % was obtained even with 6 mol % Pd and reacting for
24 h (Table 2, entry 11). On the whole, the Pd@poly-CN-
PF6 catalyst system showed better catalytic activity than the
reported PdCl2/EDTA/ascorbic acid catalyst system with
respect to higher yields and lower amount of catalyst [14].
The homocoupling reaction of 1-iodo-4-methoxyben-
zene (4c) was used to test the reusability of the Pd@poly-
CN-PF6 catalyst. After each cycle, the catalyst was
recovered by filtration, washed with ethyl acetate and
water, and dried ready for the next run. As shown in
Table 3, the catalyst activity decreased slightly as the cycle
times increased. To identify the reason for this catalytic
activity decline, palladium leaching was determined by
ICP-AES as 20 ppm in 3 cm3 water, which was consistent
with the loading amount of palladium of the first recycled
catalyst (9.7 wt %, ICP-AES). The TEM image of the
catalyst after being reused four times (Fig. 1b) showed that
bigger Pd nanoparticles than the original ones were formed
with an average diameter of about 5 nm. In a comparative
experiment, the reaction was carried out at room temper-
ature, but no product was detected and the palladium
leaching in the reaction was also negligible (below detec-
tion limit of ICP-AES). Thus, owing to the combined
action of palladium leaching at high temperature and
aggregation of palladium nanoparticles [35, 36], a moder-
ate yield of 74 % was obtained in the fourth cycle even
after prolonging the reaction time to 11 h.
Table 1 Effect of the reaction conditions on the formation of 5c
Entry
Additive
Pd/mol %
T/°C
Yielda/%
1
–
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
70
70
10
51
58
45
0
2
Glucose
3
Ascorbic acid
Hydroquinone
Ethylene glycol
Glycerol
70
4
70
5
70
6
70
0
In conclusion, a nitrile-functionalized ionic liquid
polymer was synthesized and applied to the immobilization
of Pd nanoparticles. The Pd@poly-CN-PF6 catalyst
(2 mol % Pd) selectively catalyzes the reductive homo-
coupling of aryl iodides and bromides at 100 °C in water.
The catalyst can be easily recycled and its activity
decreases slightly as the recycling times increased owing to
the combined action of palladium leaching at high tem-
perature and aggregation of palladium nanoparticles.
Further application of the catalyst to other palladium-cat-
alyzed reactions at lower temperature is ongoing in our lab.
7
b-CD
70
0
8
Ascorbic acid
Ascorbic acid
Ascorbic acid
Ascorbic acid
80
63
70
85
97
9
90
10
11
100
100
Reaction conditions: 1 mmol 1-iodo-4-methoxybenzene (4c), 1 mmol
additive, 2 mmol NaOH, 3 cm3 H2O, 9 h
a
GC yields
Table 2 Reductive homocoupling reactions of aryl halides
Entry Aryl halide
Time/h Product Yielda/% M.p./°C
Experimental
1
4a, R = H, X = I
9
9
5a
5b
5c
5d
5e
5a
5b
5c
5d
5f
93
95
92
90
10
90
90
87
84
83
8b,c
68–69
All chemicals and reagents were purchased from standard
commercial suppliers. All the homocoupling products are
known compounds; their physical and spectroscopic data
were compared with those reported in the literature [9, 25,
37–39] and found to be identical. Melting points were
recorded on an EZ-melt MPA120 (Stanford Research
Systems, Inc., USA). 1H NMR and 13C NMR spectra were
2
4b, R = p-Me, X = I
4c, R = p-OMe, X = I
4d, R = p-Cl, X = I
4e, R = o-CF3, X = I
4f, R = H, X = Br
120–121
177–178
146–147
67–68
3
9
4
9
5
9
6
12
12
68–69
7
4g, R = p-Me, X = Br
120–121
177–178
146–147
40–41
8
4h, R = p-OMe, X = Br 12
4i, R = p-Cl, X = Br 12
4j, R = m-OMe, X = Br 12
4k, R = H, X = Cl 24
9
10
11
Table 3 Recycling and reuse of Pd@poly-CN-PF6
5a
68–69
Cycle
0
1
2
3
4b
Reaction conditions: aryl halide (1 mmol), Pd@poly-CN-PF6 (2 mol %),
ascorbic acid (1 mmol), NaOH (2 mmol) in 3 cm3 water under air
atmosphere
Yielda/%
92
90
88
85
74
a
Reaction conditions were the same as those of entry 3 in Table 2
a
Isolated yields
Isolated yields
b
GC yield
b
c
Prolonged to 11 h
6 mol % Pd was used
123

1162
J. Wang et al.
1
light yellow powder (8.0 g, 95 %). H NMR (400 MHz,
DMSO-d6): d = 9.57 (s, 1H), 8.26 (t, J = 1.6 Hz, 1H),
8.02 (s, 1H), 7.35 (dd, J1 = 8.0 Hz, J2 = 4.4 Hz, 1H), 6.00
(dd, J1 = 9.6 Hz, J2 = 1.2 Hz, 1H), 5.59 (s, 2H), 5.49 (dd,
J1 = 4.4 Hz, J2 = 1.2 Hz, 1H) ppm; 13C NMR (100 MHz,
DMSO-d6): d = 137.21, 129.18, 123.92, 120.02, 114.85,
110.14, 37.64 ppm; HRMS (ESI): calcd for C7H8N3
134.0718 [M - PF6]?, found 134.0715, calcd for
C14H16F6N6P 413.1078 [2M - PF6]?, found 413.1075.
recorded on a Bruker AM-400 (400 MHz) spectrometer
with CDCl3 or DMSO-d6 as the solvent and TMS as the
internal standard. Chemical shifts are reported in d (parts
per million) values. Coupling constants (J) are reported in
Hz. Electrospray ionization (ESI) analyses were recorded
by a Voyager RP MALDI-TOF spectrometer (Micromass
LCTTM, Mass Spectrometry Instruments Ltd.). Elemental
analyses were conducted on an Elementar Vario EL III
(Elementar Analysensysteme GmbH, Germany). ICP-AES
was measured by a Varian 710-ES. Gel permeation chro-
matography was conducted on a Waters Breeze HPLC
system. Analytical thin-layer chromatography (TLC) was
carried out on precoated plates (silica gel 60 F254), and
spots were visualized under ultraviolet light.
Synthesis of poly-CN-PF6
In a 100-cm3 round-bottom flask, 5.58 g 3-(cyanomethyl)-
1-vinylimidazolium hexafluorophosphate (20 mmol) was
dissolved in 50 cm3 acetonitrile and the reaction was
heated to 80 °C. Then, 32.8 mg AIBN (1 mol %) was
added and the reaction was continued for 72 h under
nitrogen. On completion, the polymer was collected by
filtration, washed with acetonitrile (50 cm3 9 2), and dried
under vacuum to afford the product as a light yellow solid
powder (4.9 g, 80 %). The Mn and Mw of the polymer were
23,766 and 39,848, respectively, as determined by gel
permeation chromatography (GPC).
Gas chromatography was performed on an Agilent
7890A equipped with an HP-5 capillary column
(30.0 m 9 0.32 mm 9 0.25 lm). The injector and flame
ionization detector (FID) temperatures were set to 280 and
300 °C, respectively. Gas flow through the column was set
at 8 cm3/min. The oven initial temperature was held at
70 °C for 2 min, increased to 300 °C at a rate of 20 °C/
min, then cooled to 60 °C and held at that temperature until
the next injection. The response peak area ratios of product
5c and internal standard dodecane (A5c/Ad = x) were
obtained from Agilent 7890A. The GC yields of 5c
(y) were calculated by the formula of curve fitting
(y = 0.59223x - 0.00307).
Preparation of Pd@poly-CN-PF6
A mixture of 1 g poly-CN-PF6 and 100 cm3 H2PdCl4
(0.01 M) was stirred at room temperature for 72 h, then
filtered and washed with water (20 cm3 9 3). After drying,
the obtained gray solid was re-dispersed in water (50 cm3)
and reduced by a freshly prepared solution of 185 mg
(5 mmol) NaBH4 in 10 cm3 water. Then the reaction
mixture was filtered, washed with water (20 cm3 9 3) and
ethanol (20 cm3 9 3), and dried under vacuum to afford
the desired Pd@poly-CN-PF6 as a black powder. The
content of Pd in the catalyst was 10 wt % as determined by
ICP-AES.
3-(Cyanomethyl)-1-vinylimidazolium hexafluorophosphate
(3, C7H8F6N3P)
A mixture of 4.7 g 1-vinylimidazole (50 mmol) and 3.75 g
2-chloroacetonitrile (50 mmol) was heated with stirring at
75 °C for 3 h. Then, the product was washed with diethyl
ether (50 cm3 9 2) and EtOAc (50 cm3 9 2). After filtra-
tion and drying, 3-(cyanomethyl)-1-vinylimidazolium
chloride was obtained as a light yellow solid powder
(8.1 g, 96 %). 1H NMR (400 MHz, D2O): d = 9.32
(s, 1H), 7.91 (s, 1H), 7.79 (s, 1 H), 7.29 (dd,
J1 = 7.2 Hz, J2 = 4.4 Hz, 1H), 5.88 (dd, J1 = 7.6 Hz,
J2 = 4.0 Hz, 1H), 5.55 (s, 2H), 5.51 (dd, J1 = 4.8 Hz,
J2 = 4.0 Hz, 1H) ppm; 13C NMR (100 MHz, D2O):
d = 135.81, 128.08, 123.22, 120.46, 113.63, 110.98,
37.45 ppm; HRMS (ESI): calcd for C7H8N3 134.0718
[M - Cl]?, found 134.0711, calcd. for C14H16ClN6
303.1125 [2M - Cl]?, found 303.1121.
General procedure for reductive homocoupling
of aryl halides
Pd@poly-CN-PF6 catalyst (21.2 mg, 2 mmol % Pd based
on aryl halide), aryl halide (1 mmol), 80 mg NaOH
(2 mmol), 176 mg ascorbic acid (1 mmol), and 3 cm3
water were added into a 25-cm3 round-bottom flask, which
was then heated to 100 °C for several hours. The reaction
was monitored by TLC. After the reaction was finished, the
catalyst was filtered and washed with ethyl acetate
(4 cm3 9 3) and water (4 cm3 9 3), which was then dried
ready for the next use. The filtrate was extracted with
EtOAc (4 cm3 9 3). To obtain the GC yield of 5c for
optimization of reaction conditions, the combined organic
phase was analyzed by GC on an Agilent 7890A using
100 mg dodecane as an internal standard. To obtain the
isolated yield, the combined EtOAc extracts were evapo-
rated to leave the crude products which were purified by
Then, 5.07 g 3-(cyanomethyl)-1-vinylimidazolium chlo-
ride (30 mmol) was dissolved in 50 cm3 water, and 5.53 g
potassium hexafluorophosphate (30 mmol) was added with
vigorous stirring. After reaction for 48 h at room temper-
ature, the reaction mixture was filtered. Then, the filter
cake was washed with water (50 cm3 9 2) and EtOAc
(50 cm3 9 2), and dried under vacuum to obtain 3-(cy-
anomethyl)-1-vinylimidazolium hexafluorophosphate as a
123

Polymerized functional ionic liquid supported Pd nanoparticle catalyst
1163
`
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this sublimation could be avoided by adding three drops of
toluene (about 0.1–0.15 cm3) into the reaction mixture.
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Acknowledgments Financial support for this work from the
National Natural Science Foundation of China (NSFC, Grant
20676033), National Basic Research Program of China (973 Program)
(Grant 2010CB126101), National Key Technology R and D Program
(Grant No. 2011BAE06B05-4), and DC Foundation ?is gratefully
acknowledged.
19. Venkatraman S, Li CJ (1999) Org Lett 1:1133
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