Air stable ligandless heterogeneous catalyst systems based on Pd and Au supported in SiO2 and MCM-41 for Suzuki-Miyaura cross-coupling in aqueous medium

Article abstract of DOI:10.1016/j.apcata.2013.04.034

Palladium and palladium-gold containing siliceous-MCM-41 and sol-gel palladium and palladium-gold silica composites have been prepared. The catalytic performance of incorporated Pd/MCM-41, Au-Pd/MCM-41 and Pd and Au-Pd/SiO ₂ sol-gel catalysts for the Suzuki-Miyaura cross-coupling reaction was determined and the influence of the matrix and the catalyst composition on the catalytic activity were also studied. The catalysts were characterized by N₂ physisorption (BET/BJH methods), X-ray diffraction, temperature programmed reduction analysis, H₂ chemisorption, atomic absorption spectrophotometry and Raman spectroscopy. The silica-containing palladium and palladium-gold catalysts prepared using the MCM-41 matrix showed greater catalytic activity than using the conventional sol-gel method; however, gold had a significant influence on this reaction. The catalyst did not undergo metal leaching and could be easily recovered and re-used (reused).

Full text of DOI:10.1016/j.apcata.2013.04.034

Applied Catalysis A: General 462–463 (2013) 39–45
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Air stable ligandless heterogeneous catalyst systems based on Pd and
Au supported in SiO and MCM-41 for Suzuki–Miyaura cross-coupling
2
ଝ
in aqueous medium
a
b
Marcelo Gomes Speziali , Anderson Gabriel Marques da Silva ,
b
a
Débora Michelini Vaz de Miranda , Adriano Lisboa Monteiro ,
Patricia Alejandra Robles-Dutenhefner
Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil
Departamento de Química, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil
b,∗
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium and palladium-gold containing siliceous-MCM-41 and sol–gel palladium and palladium-gold
silica composites have been prepared. The catalytic performance of incorporated Pd/MCM-41, Au-
Pd/MCM-41 and Pd and Au-Pd/SiO2 sol–gel catalysts for the Suzuki–Miyaura cross-coupling reaction was
determined and the influence of the matrix and the catalyst composition on the catalytic activity were
also studied. The catalysts were characterized by N2 physisorption (BET/BJH methods), X-ray diffraction,
temperature programmed reduction analysis, H2 chemisorption, atomic absorption spectrophotometry
and Raman spectroscopy. The silica-containing palladium and palladium-gold catalysts prepared using
the MCM-41 matrix showed greater catalytic activity than using the conventional sol–gel method; how-
ever, gold had a significant influence on this reaction. The catalyst did not undergo metal leaching and
could be easily recovered and re-used (reused).
Received 27 January 2013
Received in revised form 15 April 2013
Accepted 21 April 2013
Available online 3 May 2013
Keywords:
Suzuki cross-coupling
Palladium and Palladium-gold catalysts
Mesoporous molecular sieves
©
2013 The Authors. Published by Elsevier B.V. All rights reserved.
1
. Introduction
solvents in many reactions, including C C coupling reactions, it is
aligned with the development of green chemistry. The use of water
as a solvent has several advantages such as its low cost, abundance
and easy separation of lipophilic products from the aqueous phase
[18–20].
Aimingto promotethe developmentof green chemistry, interest
has increased in studying methods for the preparation of hetero-
geneous catalysts to achieve a catalyst that can be efficient, stable
and recyclable for several applications. Silica [21], MCM-41 [22],
SBA-15 [23], activated carbon [24] and metallic nanoparticles [25]
have been successfully used as supports in Suzuki coupling with
several advantages compared to homogeneous systems. However,
in several cases, Pd leaching has been observed and the distinction
between a homogeneous or heterogeneous catalyst is not an easy
task [26]. Palladium is the most commonly used metal catalyst in
Suzuki coupling, but gold has recently been studied as a catalyst for
these reactions as well [20,27–29].
The sol–gel method has great potential for the preparation of
materials, by allowing the structural control of ceramics [30]. The
materials obtained with this method have a high surface area that
allows the incorporation of different components dispersed in the
matrix [31–35]. Another widely known matrix is the ordered (sug-
gested: commercial) mesoporous molecular sieve produced by the
company Mobil, denoted with the number 41 (MCM-41 [36]. This
The Suzuki reaction [1–3], involving the Pd-catalyzed C-C cross-
coupling reaction of aryl boronic acids with aryl halides, has become
an attractive process for the synthesis of biaryls, which have a
broad and diverse range of applications from pharmaceuticals to
science materials [4–6]. Thus, efforts have been directed to achieve
improvements in the Suzuki reaction by designing new ligands
and/or precursors of palladium, such as phosphine ligands [7–10],
N-heterocyclic carbenes [11–16] and palladacycles [13,17], in order
to achieve an efficient cross-coupling reaction under mild and
environmentally benign conditions, which includes the choice of
solvent, low catalyst loading and/or easy separation and recycla-
bility of the catalyst. In this regard, reducing the use of organic
ଝ
This is an open-access article distributed under the terms of the Creative Com-
mons Attribution-NonCommercial-No Derivative Works License, which permits
non-commercial use, distribution, and reproduction in any medium, provided the
original author and source are credited.
∗ Corresponding author at: Patrícia Alejandra Robles-Dutenhefner Universidade
Federal de Ouro Preto, Instituto de Ciências Exatas e Biológicas, Departamento de
Química, Campus Universitário s/n Bauxita, 35400-000, Ouro Preto MG, Brazil.
Tel.: +55 31 3559 1230.
E-mail address: pard@iceb.ufop.br (P.A. Robles-Dutenhefner).
0
926-860X/$ – see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.04.034

4
0
M.G. Speziali et al. / Applied Catalysis A: General 462–463 (2013) 39–45
material possesses regular arrays of large uniform channels and a
high surface area (above 700 m ). These mesoporous materials are
residual organics. The TEOS/C16-TAB/TMAOH/water molar ratio
was 1.0/0.12/0.3/22.0.
2
very attractive for the development of new heterogeneous catalysts
for many reactions, e.g., the oxidation of hydrocarbons, hydrogena-
tion, etc. [37]. The presence of silanol groups, which are capable
of being functionalized, make them useful as supports for organic
compounds and metal oxides on which very high dispersion of
the active phase may be achieve [38,39]. Mesoporous molecular
sieves containing different transition metals have great potential
as heterogeneous catalysts in liquid-phase reactions.
2
.4. Catalyst characterization
The powder X-ray diffractometry (XRD) measurements were
performed on a Rigaku model Geigerflex-3034 equipment using
a CuK˛ radiation (40 kV, 40 mA, ꢀ = 0.15418 nm). The diffraction
pattern was measured between 12 and 50 2ꢁ with a step size of
0
◦
◦
◦
.05 2ꢁ and a step time of 4.0 s. Samples were previously dried at
In this study, we evaluated the catalytic activity in Suzuki cou-
pling under a normal air atmosphere using water as the solvent and
1
10 C overnight and pulverized.
The reducibility of the catalyst surfaces was determined by
palladium- and/or gold-based catalysts, i.e., Pd and Au-Pd/SiO and
2
Temperature-Programmed Reduction (TPR) in a Quantachrome
equipment ChemBET-300 equipped with a thermal conductivity
detector. Prior to analysis ca. 150 mg was packed into a quartz cell,
heated for 2 h at 200 C under a He stream and then it was cooled
to room temperature. The experiments were performed between
Pd and Au-Pd/MCM-41, without any ancillary ligands. The effects
of gold on the catalytic properties were also studied. Recycling of
the catalysts was also performed and demonstrated the possibility
of reusing the catalytic system.
◦
◦
3
0 and 900 C in flow 5%H /N , increasing the temperature linearly
2
2
◦
−1
at a rate of 10 C min
.
2
. Experimental
Textural characteristics of the matrices were determined from
◦
nitrogen adsorption isotherms at -196 C using an Autosorb
IQ-Quantachrome Instrument. The samples (ca. 200 mg) were
degassed for 2 h at 300 C before analysis. Specific surface areas
were determined by the Brunauer–Emmett–Teller equation (BET
method) from adsorption isotherm in a relative pressure range
2.1. Chemicals
◦
All reagents were purchased from commercial sources and used
as received.
0
.07 < P/Po < 0.3. The total pore volume was obtained of the amount
2
.2. Catalyst preparation Pd/SiO2 and Au-Pd/SiO2
of N2 adsorbed at a relative pressure close to unity. The average
pore diameter was determined by Barrett–Joyner–Halenda (BJH)
from the N2 desorption isotherms.
The palladium dispersion of catalysts was determined from
combined isotherm measured for hydrogen using the extrapola-
tion method in an Autosorb-Quantachrome-iQ-C. Before analysis,
The 5.0 wt.% Pd/SiO2 and 3.0–3.0 wt.% Au-Pd/SiO2 (3.0 wt.%
for each metal) catalysts were prepared by the sol–gel method
using tetraethoxysilane (15.1 g, TEOS, 98%, Sigma–Aldrich), PdCl2
(
Sigma–Aldrich) and HAuCl (Sigma–Aldrich) as precursors. The sol
4
was obtained from a TEOS/ethanol/water mixture in a 1/3/10 molar
ratio with the addition of HCl and HF (up to pH 2.0) as the catalysts.
The sample was dried at 110 C for 24 h and thermally treated for
◦
the catalysts were dried at 200 C for 2 h under vacuum. Then,
the catalysts were reduced at 500 C (10 C min ) in flowing H2
◦
◦
−1
◦
3
(40 cm /min). Following reduction, the samples were evacuated
◦
2
h at 500 C in air.
for 1 h at reduction temperature adsorption temperature under
vacuum.
The small angle X-ray scattering (SAXS) measurements were
obtained using synchrotron radiation with ꢀ = 1.488 nm. Syn-
chrotron radiation measurements were carried out at the
D11A-SAXS beamline of the LNLS (Campinas, Brazil) using a Huber-
2
.3. Catalyst preparation Pd/MCM-41 and Au-Pd/MCM-41
The 5.0 wt.% Pd/MCM-41 and 3.0–3.0 wt.% Au-Pd/MCM-41
(
3.0 wt.% for each metal) catalysts were prepared by the direct
4
23 3-circle diffractometer. The SAXS setup was equipped with a Si
incorporation of Pd and Au into the MCM-41 [40] framework,
aiming for the isomorphous substitution of Si by Pd and Au-Pd
ions. TEOS, PdCl2 (Sigma–Aldrich) and HAuCl4 (Sigma–Aldrich)
were used as the precursors and hexadecyltrimethylammonium
bromide (C16-TAB, Sigma–Aldrich) as the structural template. A
C16-TAB solution in water was added to the solution of TEOS (2.5 g)
in aqueous tetramethylammonium hydroxide (TMAOH, 25 wt.%,
Sigma–Aldrich), and the mixture was stirred for 30 min. Then,
PdCl , HAuCl and the remaining TEOS (21.0 g) were added. After
additional mixing (stirring) at 40 C for 24 h, the mixture was placed
in the autoclave at 100 C for 24 h and then cooled to room temper-
ature. The obtained solid was separated by filtration, washed with
deionized water and ethanol and dried at 40 C. Then, the solid was
(1 1 1) monochromator, giving a horizontally focused X-ray beam.
The incident X-ray wavelength ꢀ was 1.488 nm and the scattering
◦
angle 2ꢁ was approximately 0–10 Table 1.
.
2.5. General procedure for Suzuki cross-coupling
In a typical experiment of the Suzuki cross-coupling reaction,
an aqueous suspension (20 mL of deionized water) containing
the corresponding aryl halide (1.0 mmol), phenylboronic acid
(1.2 mmol), KOH (4 mmol), catalyst (130 mg) and tetradecane
(internal standard; ∼120 mg) was stirred for an appropriate period
2
4
◦
◦
◦
◦
◦
heated from room temperature to 550 C under a nitrogen flow
and calcined for 3 h at 550 C under an air flow to remove the
of time in a glass reactor with magnetic stirring at 80 C. After the
◦
desired time (2 or 4 h, c.f. Tables 2 and 3), the reaction mixture was
Table 1
Elemental analysis data and surface properties of Pd and Au-Pd catalysts.
Surface area (m2
g
−1
)
Total pore volume (cm /g)
3
Average pore diameter (A)
˚
Catalyst
Metal content
Dispersion D (%)
Au
Pd
Au
Pd
Pd/SiO2
Au-Pd/SiO2
Pd/MCM
–
2.9
–
4.9
2.8
4.8
2.7
–
none
–
73
57
86
78
259
268
269
580
0.90
0.92
1.35
1.58
33
47
19
20
Au-Pd/MCM
2.9
none

M.G. Speziali et al. / Applied Catalysis A: General 462–463 (2013) 39–45
41
Table 2
Suzuki–Miyaura reactions catalyzed by Pd/SiO2 and Au-Pd/SiO2 (sol–gel).
a
Entry
Aryl halide
Product
Catalyst (M/SiO2)
Pd
Yield (%)b,c
2
h
4 h
1
57
69(65d)
Br
Ph
MeO
81(78d)
96
MeO
2
3
Au-Pd
Pd
68
90
Br
Ph
H C
3
H C
3
4
5
Au-Pd
Pd
92
41
99
53
Br
Ph
OCH
OCH
6
7
Au-Pd
Pd-
44
26
60
48
Br
Ph
O 2
N
O N
2
8
9
Au-Pd
Pd
40
34
61
50
CH Cl
CH Ph
2
2
1
0
Au-Pd
39
59
a
b
c
◦
Reaction conditions: [aryl boronic acid] = 1.2 mmol, [aryl halide] = 1.0 mmol, catalyst = 0.130 g (%mol), H2O (20 mL), T = 80 C.
Yields were determined by GC using tetradecane as the internal standard and based on the amount of aryl halide consumed.
By-products were detected as the minimum amount of homocoupling products and reduced aryl halide.
d
3
rd recycling of the catalyst.
cooled to room temperature and extracted with dichloromethane
(
3 × 20 mL). The organic phases were uncolored and clear and
*
*
no visible particles of catalyst could be observed. The organic
phases were mixed and the products were identified and quanti-
fied by gas chromatography using a Shimadzu GC-2014 apparatus
Au-Pd/MCM-41
Au-Pd/SiO2
Δ
*
*
(
using tetradecano as internal standard with a respective calibra-
tion curve). The products were also analyzed by GC-MS on a 17B
Shimadzu GCMS-QP5000. The fragmentation patterns were com-
pared with standards and library data.
For the recycling experiments, after extraction of the organic
products, the reagents were added to the liquid phase con-
taining the catalyst [aryl halide (1.0 mmol), phenylboronic acid
Δ
Δ
Pd/MCM-41
(
1.2 mmol), KOH (2 mmol), tetradecane (∼120 mg)]. Thus, the mix-
ture was allowed to react further and the analysis was performed
as described above.
Δ
Pd/SiO2
3
. Results and discussion
Δ
3.1. Characterization of the catalysts
The XRD patterns of sol–gel silica and MCM-41 doped with Pd
and Au-Pd are shown in Fig. 1. The diffractogram of the sample
◦
Pd/SiO2 indicated the presence of a low intensity peak at 2ꢁ = 33.7
15
20
25
30
35
40
45
50
corresponding to crystalline phase of palladinite PdO(1 0 1) [41],
formed by the thermal decomposition of the precursor PdCl2
under air atmosphere. The XRD pattern of Pd/MCM-41 indicated
2θ
Fig. 1. XRD patterns for the silica-containing palladium and palladium-gold cata-
◦
◦
◦
lysts treated at 550 C (* Au(0)/ꢂ PdO).
the presence of two peaks at 2ꢁ = 33.7 and 42.0 corresponding

4
2
M.G. Speziali et al. / Applied Catalysis A: General 462–463 (2013) 39–45
Table 3
Suzuki–Miyaura reactions catalyzed by Pd/MCM-41 and Au-Pd/MCM-41.
a
Entry
Aryl halide
Product
Catalyst (M/MCM-41)
Pd
Yield (%)b,c
2
h
4 h
1
70
85(83d)
Br
Ph
MeO
MeO
2
3
Au-Pd
Pd
76
95
88(87d)
99
Br
Ph
H C
3
H C
3
4
5
Au-Pd
Pd
94
55
99
70
Br
Ph
OCH
OCH
6
7
Au-Pd
Pd
58
36
84
72
Br
Ph
O 2
N
O N
2
8
9
Au-Pd
Pd
50
55
79
72
CH Cl
CH 2Ph
2
1
0
Au-Pd
62
88
a
b
c
◦
Reaction conditions: [aryl boronic acid] = 1.2 mmol, [aryl halide] = 1.0 mmol, catalyst = 0.130 g, H2O (20 mL), T = 80 C.
Yields were determined by GC using tetradecane as the internal standard and based on the amount of aryl halide consumed.
By-products were detected as the minimum amount of homocoupling products and reduced aryl halide.
d
3
rd recycling of the catalyst.
to Palladinite phases PdO(1 0 1) and PdO(1 1 0), respectively. The
lower intensity of the palladinite in Pd/SiO catalyst than Pd/MCM-
2
4
1 indicates a greater reduction of PdCl2 precursor during the
Pd-Au/MCM-41
Pd-Au/Sol-gel
preparation of the composite and it was not reoxidized after the
calcination step. The XRD patterns of the Au-Pd/MCM-41 and Au-
Pd/SiO2 catalysts indicated two diffraction peaks at 2ꢁ = 37.9 and
◦
◦
4
4.6 attributed to metallic gold Au(1 1 1) and Au(2 0 0) [42] and a
peak corresponding to Palladinite (PdO) coinciding with the angles
described in the XRD patterns of palladium catalysts. The band
observed at 2ꢁ = 18 is related to amorphous SiO2 that was formed
◦
during calcination of catalysts [43].
The profiles of temperature programmed reduction analysis
(
TPR) of catalysts are showed in Fig. 2 The TPR analysis results
revealed that Pd/SiO2 and Au-Pd/SiO2 showed no reducible phases
of palladium and gold oxides. The TPR profiles of Pd/MCM-41
and Au-Pd/MCM-41 indicated reduction of palladium oxide phases
to metallic palladium (100–240 C and 120–225 C respectively).
These results are consistent with the observations obtained by XRD
which indicated predominance of Au(0) for all samples. For the Pd
phases, it was observed that there are the predominance of Pd(0)
for the samples prepared by sol–gel method and Pd (II) for MCM-
Pd/MCM-41
Pd/Sol-gel
◦
◦
4
1 catalysts. For the Pd-containing catalysts, it was observed in
◦
the TPR profiles a strong and negative peak near 100 C character-
istic of the ␤-hydride palladium decomposition (␤-H-Pd) formed
during TPR analysis [44,45]. This indicates that the PdO species
previously observed by XRD were reduced at low temperature,
followed by hydrogen sorption, forming hydrides over metallic
Pd particle surfaces [46,47]. The temperature range to palladium
200
400
600
800
Temperature(ºC)
Fig. 2. TPR profiles of the Pd and Au-Pd catalysts.

M.G. Speziali et al. / Applied Catalysis A: General 462–463 (2013) 39–45
43
1
1
100
000
(
a)
Pd/Sol-gel
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
Pd-Au/sol-gel
Pd/MCM-41
Pd-Au/MCM-41
(100)
Pd-Au/MCM-41
(110)(200)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure/ P/Po
2
3
4
5
6
7
8
2
θ
Fig. 3. N2-Adsorption–desorption isotherms of Pd/SiO2, Au-Pd/SiO2 Pd/MCM-41
and Au-Pd/MCM-41 and their pore size distribution curves using the BJH method.
(
b)
hydride decomposition has been reported between 323 and 373 K
[
48,49] and is in agreement with those observed for the composites
prepared.
Table 1 shows the values of the textural characteristics of the
catalysts measured by N2 physisorption and H2 chemisorption
(100)
techniques. Fig. 4 shows the N -adsorption–desorption isotherms
2
and their respective BJH distribution of pore sizes graphs. The cat-
alysts showed values of specific surface area between 259 and
2
−1
5
80 m g that are within the expected range for the composite
preparation of silica sol–gel and MCM-41.
The specific surface area of the catalysts decreases in the follow-
ing order: Au-Pd/MCM-41> Pd/MCM-41 > Au-Pd/SiO > Pd/SiO It
2
2.
was observed that the bimetallic catalysts show a small increase
in area as compared to the monometallic catalysts for both meth-
ods (correct, Table 1). The presence of gold can have influenced the
increase in surface area of bimetallic composites. The high surface
area of the oxide support normally can lead to increased disper-
sion of the metal and this tendency was observed between the
(
110)
Pd/MCM-41
2
3
4
5
6
7
8
2
θ
sol–gel and MCM-41 samples (Table 1, D (%) = 73 vs. 86, Pd/SiO2
Fig. 4. SAXS patterns for: (a) Au-Pd/MCM-41 (b) Pd/MCM-41 samples.
and Pd/MCM-41 respectively). However, Au-Pd catalysts showed
lower dispersion of palladium than Pd catalysts (Table 1, (D (%) = 73
for Pd/SiO2 and D (%) = 57 for Au-Pd/SiO2 and D (%) 86 vs. 78 for
MCM-41 catalysts).
The isotherms of Pd/SiO2 and Au-Pd/SiO2 catalysts are corre-
sponding to type IV (in the IUPAC classification), which is typical
of mesoporous materials (Fig. 3) [50,51]. The appearance of the
hysteresis loop type H1 is presented at high relative pressures
attributed to 2-D hexagonal symmetry, indicating a highly ordered
hexagonal structure [40]. This confirms that the silica-containing
Pd and Au-Pd catalysts have the typical structure of the MCM-41
silica, which is in agreement with the pore size distribution curves.
3.2. Catalytic tests: Suzuki cross-coupling
(
P/Po ≈ 0.7 to 0.9) and can be related to the formation of textural
mesoporosity. Pore size distribution curves (Fig. 4) for the catalysts
exhibited unimodal profile with a narrow range of variation of pore
size, ranging from 2 to 6 nm for Au/SiO2 and Au-Pd/SiO2.
Pd/MCM-41 and Au-Pd/MCM-41 catalysts exhibits a type IV
isotherm (in the IUPAC classification) with a well defined H3 loops
that do not level off at relative pressures close to the saturation
vapor pressure were reported for aggregates of plate-like parti-
cles giving rise to slit-shaped pores (Fig. 3) [50,51]. However, the
average pore diameter in sol–gel materials nm are larger than that
in MCM-41 materials, as a peak width at half-maximum is 1.9 nm
for both MCM-41 materials vs 2.6 and 4.6 nm for Pd/SiO2 and Au-
Pd/SiO2 respectively.
Having obtained and characterized the Pd and Au-Pd/SiO2 and
Pd and Au-Pd/MCM-41, we evaluated them for the Suzuki cross-
coupling reaction. The performance of metal-MCM-41 (metal = Pd
or Au-Pd) was compared with that of a silica-included metal
material prepared by a conventional sol–gel method without the
surfactant. We also evaluated the influence of gold in the per-
formance of catalysts. However, this study aims to apply the
heterogeneous catalysts of Pd and Au-Pd under an air atmosphere,
without any ancillary ligands and using water as the solvent in the
Suzuki cross-coupling reaction. We chose as a reaction model the
reaction of electron-rich p-bromoanisole with phenylboronic acid.
After the initial screening, we chose to run the coupling reac-
◦
The SAXS pattern of Au-Pd/MCM-41 (Fig. 4(a)) showed (1 0 0),
tions using KOH at 80 C. The Pd and Au-Pd/SiO and Pd and
2
◦
(
1 1 0) and (2 0 0) reflections at 2ꢁ = 1.67, 2.91 and 3.33 , respec-
Au-Pd/MCM-41 catalysts were evaluated for the coupling reac-
tion of different aryl halides with phenylboronic acid (Scheme 1),
affording the corresponding coupling products in good to excel-
lent yields (Tables 2 and 3). As shown in Table 2, for both the
tively. The synchrotron radiation small-angle pattern for the
Pd/MCM-41 catalyst showed (1 0 0) and (1 1 0) reflections at
◦
2
ꢁ = 1.62 and 2.85 , respectively (Fig. 4(b)). The peaks can be

4
4
M.G. Speziali et al. / Applied Catalysis A: General 462–463 (2013) 39–45
Br
(HO) B
catalyst
KOH (4 equiv.)
2
R´
+
tetradecane, H O
2
o
R´
80 C, 2-4 h
Scheme 1. Suzuki–Miyaura cross coupling catalyzed by Pd and Au-Pd.
Pd and Au- Pd/SiO2 catalysts, the Suzuki–Miyaura coupling reac-
tion of p-bromotoluene gave the corresponding coupled products
in almost quantitative yield (Table 2, entries 3 and 4). Up to 80%
yield of the coupling product was obtained for the p-bromoanisole
and the catalyst was recycled three times without a noticeable
change in the yield (Table 2, entries 1 and 2). The presence of a
strong electron-donating group such as methoxy would decrease
the oxidative addition rate and the overall activity if the oxida-
tive addition is the rate-determining step [52]. In this context, the
lower yields obtained for the coupling with p-bromonitrobenzene
or p-bromobenzaldehyde were unexpected (Table 2, entries 5–8).
Further studies are required to explain these results, which could be
associated with surface effects on the rate-determining step. Ben-
zyl chloride was coupled only in moderate yields (Table 2, entries 9
and 10). Since benzyl chlorides are active substrates for the Suzuki
4. Conclusions
We have developed
a ligand-free, air-resistant practical
and efficient catalyst system that allowed us to perform the
Suzuki–Miyaura cross-coupling reaction of aryl bromides with
arylboronic acids in water using Pd or Au-Pd immobilized in sil-
ica or MCM-41 as the catalyst. MCM-41 was a better support than
silica sol–gel for the catalytic activity of the Pd and Au-Pd cata-
lysts, and the bimetallic catalyst gave higher yields, demonstrating
the beneficial effect of gold. Furthermore, the reactions proceeded
under mild conditions using an aqueous and recoverable medium.
The ligand-less system, mild conditions, air stability and recyclabil-
ity as well as the use of water as the solvent are consistent with the
principles of green chemistry for the production of cleaner chemi-
cals.
coupling reaction in homogeneous systems such as Pd(OAc) /PPh3
2
[
53], the results also indicate the presence of surface effects and,
Acknowledgments
as a consequence, that our catalytic system actually operates as a
heterogeneous catalyst.
We thank CNPq, PRONEX, FAPEMIG, FAPERGS and INCT-Catálise
for partial financial support. We also thank CNPq (M.G.S., A.G.M.S.
and D. M.) for the scholarships.
In terms of the different aryl halides evaluated, a similar trend
was observed for the Pd and Au-Pd/MCM-41 catalysts (Table 3).
However, under the same conditions, the catalysts supported in
MCM-41 gave higher yields than the silica sol–gel catalysts. The
XRD and TPR analysis indicated a predominance of Pd(0) and Au(0)
in the silica sol–gel materials over MCM-41. Since the zero valent
species are the active site for the catalytic cycle, higher activity
would be expected for the silica sol–gel materials. MCM-41 was a
better support than silica for the catalytic activity of Pd and Au-Pd
catalysts, probably due to the larger surface area and pore volume,
which may improve the dispersion of the metals and the accessi-
bility of organic substrates to the metal centers.
The beneficial effect of gold can be verified in every entry. In both
supports, the relative increase in the yields was verified through
experiments; it did not matter if the substituent group in the aryl
halide was electron-donating or withdrawing (Tables 2 and 3).
Some reports in the literature have shown increased efficiency of a
catalyst by adding gold to palladium-doped materials. This increase
has been attributed to changes in the electronic structure of the
catalytically active metals, formation of gold nanoparticles below
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