Silica-acac-supported palladium nanoparticles as an efficient and reusable heterogeneous catalyst in the Suzuki-Miyaura cross-coupling reaction in water

Article abstract of DOI:10.1007/s12039-013-0561-0

Supported Pd nanoparticles on acetyl acetone modified silica gel was prepared and its catalytic application in the Suzuki-Miyaura reaction of various aryl halides with phenylboronic acid was investigated. The reaction was carried out in water as solvent under green conditions in the presence of NaHCO ₃ as the base. The catalyst was characterized by a combination of powder XRD, XPS, TEM and SEM. The catalyst is not sensitive to air and moisture, so reactions were carried out without using inert atmosphere. The catalyst can be reused up to six times without significant loss of catalyst activity. The cumulative turnover number (TON) over six runs is 13525. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-013-0561-0

c
J. Chem. Sci. Vol. 126, No. 1, January 2014, pp. 85–93. ꢀ Indian Academy of Sciences.
Silica-acac-supported palladium nanoparticles as an efficient
and reusable heterogeneous catalyst in the Suzuki–Miyaura
cross-coupling reaction in water
ABDOL REZA HAJIPOURa,b,∗, ZOHRE SHIRDASHTZADEa and GHOBAD AZIZIa
aPharmaceutical Research Laboratory, Department of Chemistry, Isfahan University of Technology,
Isfahan 84156, IR Iran
bDepartment of Neuroscience, University of Wisconsin, Medical School, 1300 University Avenue,
Madison, WI 53706-1532, USA
e-mail: haji@cc.iut.ac.ir
MS received 9 August 2013; revised 21 October 2013; accepted 25 October 2013
Abstract. Supported Pd nanoparticles on acetyl acetone modified silica gel was prepared and its catalytic
application in the Suzuki–Miyaura reaction of various aryl halides with phenylboronic acid was investigated.
The reaction was carried out in water as solvent under green conditions in the presence of NaHCO3 as the base.
The catalyst was characterized by a combination of powder XRD, XPS, TEM and SEM. The catalyst is not
sensitive to air and moisture, so reactions were carried out without using inert atmosphere. The catalyst can be
reused up to six times without significant loss of catalyst activity. The cumulative turnover number (TON) over
six runs is 13525.
Keywords. Supported palladium nanoparticles; Suzuki–Miyaura reaction; aryl halide; phenylboronic acid;
heterogeneous catalyst.
1. Introduction
could be an attractive method since the heterogeneous
catalysts can be easily separated from the reaction
mixture.21–23 Thus, many studies have been concerned
with the development of palladium catalysts, includ-
ing immobilizing palladium complexes or nanoparticles
on several supports.24–35 In the case of silica-supported
palladium nanoparticles, several amorphous and porous
supports have been used for Suzuki reaction.36–47
Recently, many studies have been devoted to the
development of milder reaction conditions, and water
has been currently used as a benign solvent in the
Suzuki reaction.48–53 Water has obvious advantages as
a solvent in organic synthesis, since it is the most
abundant and inexpensive liquid on earth. Also, it is
eco-friendly, nontoxic, and non-flammable solvent.54,55
Herein, we report a new heterogeneous and reusable
catalyst for Suzuki–Miyaura cross-coupling reaction in
water. The catalyst is not sensitive to air and displays
high efficiency for biaryl synthesis. Moreover, this he-
terogeneous catalyst performed up to six runs without
significant loss of activity.
The Suzuki–Miyaura cross-coupling reaction is an
important method for aryl–aryl bond formation, using
aryl halides and phenylboronic acids.1–3 Significant
studies that had been done in this area in the last decade
are derived from numerous advantages of this reac-
tion such as high stability of organoboranes towards
air and moisture, low toxicity associated with boron
compounds, mild and easy reaction conditions, use of
aqueous inorganic bases and coupling of stearic hin-
dered substrates.4–6 The Pd-catalysed Suzuki reactions
are well-known as influential methods to construct C–C
bond in modern chemical transformations,7–9 and have
found extensive use as much as of the utility of its pro-
ducts in the synthesis of natural products, pharmaceuti-
cal intermediates, agrochemical drug, advanced materi-
als and liquid crystals.10–15
In spite of the fact that homogeneous palladium cata-
lysts based on various Pd complexes along with some
ligands16–18 or free of ligands,19,20 have been exten-
sively used for Suzuki reaction, an outstanding disad-
vantage of these procedures is the difficulty in recov-
ering and reusing the expensive catalyst. Overcoming
this drawback using heterogeneous palladium catalysts
2. Experimental
All starting materials were purchased from Merck and
1
∗For correspondence
used without further purification. H and 13C NMR
85

86
Abdol Reza et al.
spectra were recorded on a Bruker 300 spectrome- 24 h. The solids were washed with water and soxhlated
ter using TMS as an internal standard in CDCl3 and with ethanol for 5 h and dried under reduced pressure.
FT-IR spectra were obtained as KBr pellets on a Finally, the resulting solid was stirred with acetonitrile
JASCO 680-PLUS spectrophotometer. The XPS spec- in the presence of 0.05 g of Pd(OAc)2 and after wash-
tra were obtained by X-ray photoelectron spectroscopy, ing the solids by acetonitrile, the Pd(II)-acac-supported
Twin Anode X-Ray Soursce Systems (X-Ray 8025- silica refluxed in acetonitrile to form a black solid. The
Bestes). Also we used GC (BEIFIN 3420 Gas Chro- black solid was washed with acetone and dried under
matograph equipped with a Varian CP SIL 5CB col- vacuum at room temperature to obtain the desired cata-
umn – 30 m, 0.32 mm, 0.25 μm) for examination of lyst. The Pd-amount was 0.04 mmol/g (4.26 mg/g)
reactions conversions. X-ray powder diffraction (XRD) determined using AAS.
were obtained by Philips X’Pert PRO over the 2θ
2.2 General procedure for the Suzuki reaction of aryl
halides with phenylboronic acid
range from 10–100◦ at the rate of 2.58◦/min, using
Cu-Ka radiation (λ = 1.54060 Å). Sizes of the syn-
thesized particles were observed using transmission
A mixture of the appropriate aryl halide (1 mmol),
electron microscope (TEM, HITACHI H-700H) at an
phenylboronic acid (1.2 mmol), SiO2-acac-PdNPs cata-
accelerating voltage of 150 kV. Samples were pre-
lyst (10 mg, 0.0004 mmol, 0.04 mol%), NaHCO3
pared by placing drops of diluted ethanol dispersed of
(2 mmol) was added to water (3 mL) in a round-
catalyst particles on the surface of cooper grids and
bottom flask equipped with condenser and refluxed
under heating conditions in reflux temperature. The
evaporation.
mixture was stirred continuously during the reaction.
After the reaction was completed (monitored by TLC,
90/10 hexane/EtOAc), the mixture was cooled to room
temperature and the catalyst was separated by fil-
2.1 General procedure for the Synthesis
of the silica-acac-supported palladium nanoparticles
Initially, to prepare activated silica, silica gel 60 (5 g) tration, washed thrice with water and acetone and
was soaked in concentrated HCl overnight and then re- allowed to dry at room temperature for 30 min under
suspended in deionized water several times until the pH vacuum for the next run. The reaction mixture was
of the filtrate was higher than 6. The acid-treated sili- poured into a separating funnel and water (20 mL)
ca gel was dried at 150◦C overnight and cooled in a and n-hexane (10 mL) were added and extracted
desiccator. To prepare 3-iodopropyl trimethoxysilane- for thrice. The combined organic phases were dried
supported silica gel, activated silica gel (5 g) was sus- over CaCl2, filtered, and the solvent was evaporated.
pended in solution of 3-iodopropyl trimethoxysilane The residue was the products which were various
(5 mmol) in toluene. The suspension was refluxed for biaryls. In some cases, the residue was purified by col-
24 h. After cooling, the particles were collected by umn chromatography (silica gel 60, EtOAc: n-hexane
filtration and exhaustively washed with ethanol, and 10:90). All the products were known and character-
1
dried under reduced pressure to give iodoropropyl-silica ized by melting point, IR and H NMR spectroscopy
gel. Iodoropropyl-silica gel re-suspended in 30 mL and were compared with authentic samples. The data
acetonitrile and 5 mmol (0.50 g) acetyl acetone and were found to be identical to those reported in the
5 mmol (0.69 g) K2CO3 were added and refluxed for literature.
Scheme 1. Synthetic route for preparation of the catalyst.

Suzuki-Miyaura cross-coupling reaction in water
87
Figure 1. Powder X-ray diffraction (XRD) pattern of pal-
ladium nanoparticles supported on silica.
3. Results and discussion
Synthetic route for preparation of the catalyst is shown
in scheme 1. The catalyst is represented as SiO2-acac-
PdNPs. In this study, we utilize acetonitrile as reduc-
ing agent for reduction of palladium(II) to palladium(0)
nanoparticles. As Susharina and co-workers mentioned
earlier on the oxidation of acetonitrile by using plat-
inum catalysts, oxidation of acetonitrile in the presence
of oxygen leads to formation of CO2, H2O, N2 and
NOx at high temperatures.56 As we did not observe any
changes in GC chromatograms (FID detector) of ace-
tonitrile (HPLC grade) after and before reduction of
palladium, we believe that the formation of these prod-
ucts may be probable during reduction of palladium
with acetonitrile.
Characterization of heterogeneous catalyst was stud-
ied using XRD, XPS, SEM and TEM analysis. Results
indicate that palladium content of the catalyst is
0.04 mmol/g (4.26 mg/g). As shown in figure 1, XRD
pattern of catalyst that matched well with that of it
(Reference code: 00-005-0681) shows the presence of
metallic palladium phases (2 theta = 40.1, 46.5, 68.0
and 86.0) and amorphous silica (2 theta = 22.0) and
is consistent with the metallic palladium(0) data in
the literature.57 Also, we can see that no characteris-
tic peaks of impurities were observed. Results from
XRD indicate that palladium nanoparticles have been
successfully immobilized on the surface of silica-acac.
Figure 3. XPS spectra of the catalyst: (a) survey spectrum,
(b) C1s, and (c) Pd3d.
The SEM image (figure 2a) in accordance with broad
peak of silica in XRD spectra, shows that the catalyst
was almost amorphous. Dark spots in the TEM image
(figure 2b) indicate the presence of palladium nanopar-
ticles in the range of 6–12 nm.
Figure 2. (a) SEM and (b) TEM images of the palladium
nanoparticles supported on the surface of silica gel.

88
Abdol Reza et al.
Table 1. Optimization of base and solvent for the Suzuki reaction of 4-
bromoacetophenone with phenylboronic acid.a
Entry
Base
Solvent
Conversion(%)b
1
2
3
4
5
6
7
8
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
Na2CO3
NaOH
EtOH
MeOH
H2O
DMF
NMP
CH3CN
Dioxane
H2O
85
87
90
60
25
50
10
40
0
9
H2O
10
11
12
Et3N
Na(OAC)
K2CO3
H2O
H2O
H2O
0
20
30
aReaction conditions: 4-bromoacetophenone (1 mmol), phenylboronic acid
(1.2 mmol), 20 mg catalyst, various bases and solvents
bGC conversion
The catalyst was also characterized by XPS analysis, resolved into two spins – orbit pairs with 3d5/2 bind-
and the results are shown in figure 3. Figure 3a shows ing energies of 336 and 337 eV. Peak binding ener-
the survey spectrum of catalyst and it is obvious that gies of 336 eV (Pd3d5/2) and 341.0 eV (Pd3d3/2) cor-
C and Pd were present in the catalyst. Figure 3b illus- respond to fully reduced palladium, while the peaks
trates the C1s spectrum, in which the peak centred at at 337 and 342.1 eV are assigned to the palladium(II)
293 eV is attributed to C species, which originated from species, which may result from incomplete reduction of
the acac and propyl groups, suggesting that organic palladium. It can be seen from the XPS spectrum that
moieties existed in the surface of the catalyst. From most of Pd2+ was reduced to Pd0 because the area of the
figure 2c, it can be seen that the Pd3d spectrum could be peak of Pd2+ ion was relatively small.
Table 2. Optimization of base and catalyst concentration for the Suzuki reaction of 4-bromoacetophenone with
phenylboronic.a
Entry
Cat.(mg)
Base (mmol)
Conversion (%)b
1
2
3
4
5
6
7
5
2
2
2
2
1
2
4
80
> 99
> 99
100
65
99
99
10
20
40
10
10
10
aReaction conditions: 4-bromoacetophenone (1 mmol), phenylboronic acid (1.2 mmol), water (1.5 ml), reflux
bGC conversion

Suzuki-Miyaura cross-coupling reaction in water
89
In order to optimize the reaction conditions, the table 3, the reaction did not show any significant sensi-
reaction in various solvents in the presence of dif- tivity to electronic effects. Also, selectivity of the cata-
ferent bases was tried. Initially, the Suzuki reac- lyst was examined by using ortho-, meta- and para- bro-
tion conditions were optimized for the reaction of mochlorobenzene (table 3, entries 4–6); in each case,
4-bromoacetophenone and phenylboronic acid in the only Br as the better leaving group was substituted and
presence of various solvents and bases using 20 mg of “Cl did not enter the reactions;” this means that the
catalyst. According to data present in table 1, water was reaction acts chemo-selectively and in comparison to
the most efficient solvent and NaHCO3 was chosen as Cl, Br is a better leaving group.
the best base for this reaction (table 1, entry 3).
In addition, we found that most hindered aryl halides
The concentration of base and catalyst was also usually produce corresponding biaryls in longer reac-
optimized by employing various amounts of base tion times and lower yields. To show the effect of
and catalyst for the reaction of 4-bromoacetophenone stearic hindrance in the Suzuki coupling reaction and
and phenylbronic acid, using 10 mg (0.0004 mmol, applicability of the catalyst for stearically hindered
0.04 mol% Pd) of catalyst and 2 equivalents of base aryl halides, we tried the coupling of 1-bromo-2-
gave the best results (table 2, entry 2). The catalyst was nitrobenzene, 2-bromo toluene and 2, 6-dimethyl bro-
completely stable and thus the reactions could be car- mobenzene and found that the reaction of these com-
ried out without using inert atmosphere. These reaction pounds took place in longer times (entries 17, 22 and
conditions were applied in the Suzuki reaction of differ- 23).
ent types of aryl halides with phenylboronic acids under
reflux conditions (table 3).
Recycling of the catalyst was examined for the
coupling reaction of iodobenzene with phenylboronic
As demonstrated in table 3, the catalyst can be used acid under optimized reaction conditions (iodobenzene
for cross-coupling reaction of aryl iodides, bromides (0.25 mmol), phenylboronic acid (0.3 mmol), NaHCO3
and even less reactive aryl chlorides under heating (0.5 mmol), catalyst (0.04 mol%) and water 3 ml) in
conditions in reflux temperature, and they were trans- reflux temperature. After the first run of the catalyst,
formed to the corresponding coupled products in good the reaction mixture was allowed to cool to room tem-
to excellent yields in short reaction times. However, aryl perature and the catalyst was separated by filtration,
chlorides were reacted more slowly in comparison to washed with H2O and acetone (thrice) and allowed to
the iodides and bromides derivatives. Reactions were dry at room temperature for 30 min under vacuum. The
performed with both electron-donating and electron- recycled catalyst was reused under the same reaction
withdrawing groups and the results are demonstrated in conditions. The catalyst could be recycled four times
Table 3. Suzuki reaction of aryl halides with phenylboronic acid in water at reflux temperaturea.
Entry
ArX
Product
Time (min)
Yield (%)
1
TONb
TOF/hc
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Total TON
2350
2375
2300
2250
2200
2050
13525
392
396
383
375
314
292
6
6
6
6
7
7
94
95
92
90
88
82

90
Abdol Reza et al.
Table 3.
(continued)
Entry
ArX
Product
Time (min)
Yield (%)
94
2
3
8
20
84
4
5
6
6
7
8
90
91
85
CI
7
50
92
8
9
95
87
58
125
10
11
20
17
91
89

Suzuki-Miyaura cross-coupling reaction in water
91
Table 3.
(continued)
Entry
ArX
Product
Time (min)
19
Yield (%)
90
12
13
14
15
13
10
45
91
92
94
16
17
45
91
80
6 h
18
19
4 h
17
93
88
20
21
12
15
86
87

92
Abdol Reza et al.
Table 3.
(continued)
Entry
ArX
Product
Time (min)
8 h
Yield (%)
71
22
23
9.5 h
66
aReaction conditions: aryl halide (1 mmol), phenylboronic acid (1.2 mmol), base (2 mmol), 10 mg catalyst and
3 ml water as solvent
bTON (turnover number) = product (mmol)/catalyst (mmol)
cTOF (turnover frequency) = TON/reaction time (h)
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six times, yield of product could still be obtained at
82% (table 3, entry 1). The cumulative turnover num-
ber (TON) over six runs was 13525. We performed the
FT-IR analysis for studying the fate of acac in the cata-
lyst after six catalytic runs. The FT-IR spectra clearly
showed the absorption bands of acac and confirmed that
acac is still present on the catalyst surface.
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12. Phakhodee W, Toyoda M, Chou
C
M,
In summary, we found that palladium nanoparticles
could be stably immobilized onto the surface of ligand-
modified silica gel, and thus could act as highly
active and reusable catalyst for promoting the Suzuki–
Miyaura cross-coupling reaction of various aryl halides
with phenylboronic acids to produce the corresponding
products in excellent yields with high chemo-selectivity
and acceptable reaction times.
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Acknowledgements
We thank the Isfahan University of Technology (IUT),
IR Iran for funding support. Further, financial support
from the Center of Excellence in Sensor and Green
Chemistry Research (IUT) is also thanked.
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