Fibrous nano-silica (KCC-1)-supported palladium catalyst: Suzuki coupling reactions under sustainable conditions

Article abstract of DOI:10.1002/cssc.201100379

Noble amines recycled: Fibrous high-surface-area nano-silica functionalized with aminopropyl groups and loaded with well-dispersed Pd nanoparticles is evaluated for the Suzuki coupling of aromatic halides. It is active for the reaction of a range of aryl bromides and iodides as well as chlorides with aryl boronic acids in good to excellent yields. The catalyst can be recovered and reused for a number of cycles with negligible loss in activity. Copyright

Full text of DOI:10.1002/cssc.201100379

DOI: 10.1002/cssc.201100379
Fibrous Nano-Silica (KCC-1)-Supported Palladium Catalyst: Suzuki Coupling
Reactions Under Sustainable Conditions
[
a]
Aziz Fihri, Dongkyu Cha, Mohamed Bouhrara, Noor Almana, and Vivek Polshettiwar*
Since the discovery of the Ullmann reaction over a century
In a continuation of our search for green and sustainable
[
1]
[27–34]
ago, in 1901, the transition-metal-catalyzed cross-coupling re-
nano-catalytic protocols,
we herein report novel Pd-nano-
action has played an important role in the synthesis of CÀC
catalysts supported on our recently discovered high-surface-
[
2]
[27,28]
bonds. In 1981, Suzuki discovered a novel Pd-catalyzed cross-
area silica exhibiting a unique fibrous morphology (KCC-1).
[
3]
coupling reaction of aryl boronic acids and aryl halides,
We discovered that the high surface area of KCC-1 is attribut-
able to fibers and not pores, which dramatically increases its
[
4–11]
which has been applied widely
and for which he received
[27]
the 2010 Nobel prize in chemistry. This reaction has become
an extremely powerful process for the synthesis of biaryls,
which have a diverse spectrum of applications, ranging from
accessibility. We believe that this unique property will be
very useful for the design of silica-supported catalysts, for
which the accessibility of active sites can be increased signifi-
cantly. After demonstrating the validity of this concept for the
hydro-metathesis of olefins by using a KCC-1/TaH catalyst
[
12–15]
pharmaceuticals to materials science.
Recently, the use of nanocatalysts has increased rapidly and
has resulted in the development of several active and efficient
[28]
system, we designed highly disperse Pd-nanoparticles sup-
ported on fibers of KCC-1 to examine the advantages of fi-
brous KCC-1 as a catalyst support in Suzuki coupling reactions.
The first step in accomplishing this catalyst design was the
functionalization of KCC-1 with amino groups, which could
then act as pseudo chelators or ligands to control the metal
leaching during the reaction. Functionalization was achieved
by post-synthetic modification of the fibers of KCC-1 by reac-
tion with 3-aminopropyltriethoxysilane (Scheme 1) to produce
KCC-1-NH2.
[
16–20]
nano-catalysts for various protocols.
These systems have
several advantages over conventional catalysts, such as superi-
or activity and improved stability. Combining metal nanoparti-
cles with a support of choice provides a large field for the
discovery of new, highly active nanocatalysts for important
and challenging reactions, which also offer the additional ad-
vantage of recyclability. The preparation of Pd nanoparticles is
usually based on the reduction of a metal salt in the presence
of a reducing agent and a stabilizer. Many substrates, such as
[
21]
[22]
polymers,
dendrimers, ionic
[
23]
liquids,
ordered mesoporous
and carbon nano-
[
24]
silica,
[
25]
tubes, have been used as sta-
bilizers and supports for Pd
nanoparticles. We recently re-
viewed these nano-catalysts for
[
26]
Suzuki coupling reactions and
observed that an extensive
range of nano-catalyst systems
was developed for this process
in
a short period of time.
Although most of these nano-
catalyst systems are active and
usable, two main challenges still
remain unresolved: 1) stable
nano-catalysts that avoid activity
loss from particle-size growth
during the reaction caused by
Ostwald ripening and 2) active
nano-catalysts that use challeng-
ing, but economical chloroar-
enes as substrates.
Scheme 1. Synthesis of KCC-1-NH
2
/Pd nano-catalysts (only two fibers are shown for clarity).
Nano-composite KCC-1-NH was then characterized by using
2
[
a] Dr. A. Fihri, Dr. D. Cha, Dr. M. Bouhrara, N. Almana, Prof. Dr. V. Polshettiwar
29
13
solid-state Si and C cross-polarization magic-angle-spinning
Nano-Catalysis Laboratory (NanoCat), KAUST Catalysis Centre (KCC)
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955 (Saudi Arabia)
29
(
CP-MAS) NMR spectroscopy. The Si CP-MAS NMR spectrum
(
Figure 1a) shows two strong characteristic signals at d=
E-mail: vivek.pol@kaust.edu.sa
À99.7 and À109.2 ppm that can be assigned to the Q3 and Q4
ChemSusChem 2012, 5, 85 – 89
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
85

(
Scheme 2). Initial trials were per-
formed to optimize the reaction
conditions for the coupling reac-
tion of bromobenzene and phe-
nylboronic acid. After screening
a range of usual inorganic and
organic bases and exploring the
scope of various solvents, this
nano-catalyst was found to be
most active in a water–ethanol
mixture used as a solvent with
potassium phosphate as a base.
Using these optimized reaction
conditions, the efficiency of the
catalyst system was studied for
the Suzuki coupling reaction of
various aryl halides and aryl bor-
onic acids (Table 1).
Clearly, KCC-1-NH /Pd is an
2
active and efficient catalyst for
coupling a range of aryl iodides
and bromides with several aryl
boronic acids, producing the
corresponding biaryl compounds
in good to excellent yields
(Table 1). Although aryl halides
with various functional groups
2
9
13
2
Figure 1. a) Si CP-MAS NMR spectrum, b) C CP-MAS NMR spectrum, and c) TGA spectrum of KCC-1-NH .
sites corresponding to silica substructures with different de-
grees of condensation. A weak Q2 signal at d=À90.2 ppm is
also observed. The signals at d=À57.3 and À66.8 ppm are
efficiently couple with a range of boronic acids (Entries 1–13),
our attempts to use 2-furanboronic acid (entry 14) resulted in
no product formation, which may be attributable to the sub-
strate’s instability in water. However, a phenylboronic acid with
a sensitive 1,3-dioxolane ring underwent the Suzuki coupling
characteristic for the T2 and T3 sites of RSiO units of as-syn-
3
thesized nano-composite KCC-1-NH , indicating the covalent
2
attachment of organic molecules to silica. To confirm that the
13
aminopropyl group is attached to KCC-1, a C CP-MAS experi-
ment was performed. The resultant spectrum (Figure 1b) has
signals at d=8.4, 21, and 41.5 ppm corresponding to the three
carbon atoms of the aminopropyl group, confirming the pres-
ence of this organic functionality. To calculate the amount of
amino group loading, thermogravimetric analysis (TGA; Fig-
ure 1c) of the KCC-1-NH silica spheres was performed under
2
an inert atmosphere. After an initial weight loss attributable to
water and moisture up to 1108C, the nano-composite was rela-
tively stable up to 3508C; from 350–6508C, a 10.5% weight
loss was observed, which could be attributed to the loss of co-
valently bound aminopropyl groups.
The as-synthesized KCC-1-NH composite was then treated
2
II
0
with PdCl , followed by the reduction of Pd to Pd by using a
2
hydrogen reduction method, to produce the KCC-1-NH /Pd
2
Figure 2. TEM images of KCC-1-NH /Pd nano-catalysts.
2
nanocatalyst (Scheme 1). TEM images of this catalyst revealed
that the fibers of KCC-1 were fully loaded with well-dispersed
Pd nanoparticles with an average size range of 1–5 nm
(Figure 2). Inductively coupled plasma coupled with atomic ab-
sorption spectroscopy (ICP-AAS) was used for elemental analy-
sis, and the results revealed a 6.3% concentration of Pd in the
as-synthesized catalyst system.
Next, we evaluated this nano-composite (KCC-1-NH /Pd) as a
2
heterogeneous catalyst in Suzuki coupling reactions
2
Scheme 2. Suzuki coupling reactions using the KCC-1-NH /Pd nano-catalyst.
8
6
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2012, 5, 85 – 89

mized conditions, we tested several aryl chlorides for Suzuki
coupling reactions, which are summarized in Table 2.
Table 1. Suzuki coupling reactions of aryl halides with aryl boronic acid
[
a]
using KCC-1-NH
2
/Pd.
Interestingly, KCC-1-NH /Pd was also active for a range of
2
1
2
[b]
Entry X
R
R
Product
Yield
%]
chloro aromatics with good to excellent yields. This is an
additional and important attribute of this nano-catalyst pro-
tocol, as few heterogeneous catalyst systems show good ac-
tivity toward aryl chlorides, which are sustainable substrates
for Suzuki coupling reactions, under ligand-free condi-
[
1
2
3
4
5
6
7
8
9
1
I
H
H
H
H
H
H
95
86
94
81
94
93
97
81
97
77
95
88
92
Br
I
[26]
tions.
Catalyst stability, heterogeneity, and leaching are impor-
tant factors to evaluate the sustainability of the catalyst
system. We conducted a recycling and leaching study for
MeO
MeO
H
Br
the KCC-1-NH /Pd catalyst system by using the Suzuki cou-
2
pling of 4’-bromoacetophenone and phenylboronic acid as
a test reaction. After completion of the first reaction to
yield the corresponding biaryl, the catalyst was recovered,
washed sequentially with ethanol and acetone, and finally
dried under vacuum. Utilizing the used catalyst, the above
test reaction was performed again under the same condi-
tions with similar catalyst activity. We were able to reuse
the catalyst for at least seven times with a negligible
change in its activity (Table 3). Thus, the as-synthesized
nanocatalyst showed excellent recyclability, and catalyst de-
terioration was not observed, confirming its high stability.
Also, the Pd-metal concentration of the catalyst before and
after the reaction was very similar, and no Pd was detected
(by using ICP-MS analysis) in the isolated biaryl product.
The reason for negligible leaching is the presence of the
amino group on the surface of KCC-1 that acts as a pseudo
ligand or chelating agent for the metal and thus prevents
Br NO
2
Br CN
H
Br MeCO
Br Cl
Br Cl
Br OH
H
H
[c]
H
0
H
11
Br NO
2
MeO
MeO
MeO
1
1
2
3
Br CN
[31]
leaching to the reaction solvent.
Br MeCO
1
4
5
Br
Br
H
H
0
[
a]
Table 2. Suzuki coupling reactions of chloroarenes using KCC-1-NH
2
-Pd.
[
d]
1
95
[
K
1
a] Reaction conditions: aryl halide (1 equiv), arylboronic acid (1.5 equiv), and
PO (3 equiv) were heated in 5 mL of 3:2 mixture of H O and Ethanol at
008C for 4 h in the presence of KCC-1-NH /Pd (0.5 mol% of Pd). [b] Yields
determined by using GC. [c] N-Methyl-2-pyrrolidone was used as solvent at
408C. [d] The reaction was run for 6 h.
3
4
2
1
2
[b]
Entry R
R
Product
Yield
2
[%]
1
1
2
3
4
MeCO
MeCO
CN
H
94
MeO
MeO
MeO
95
93
91
smoothly (entry 15), notably with the 1,3-dioxolane ring re-
maining intact.
Although the catalyst was active for bromo- and iodoaryl
compounds, our main objective was to design a nano-catalyst
for the Suzuki coupling of challenging, but economical sub-
strates, that is, aryl chlorides. In our initial attempts, we used
the same optimized conditions as for the bromo and iodo aro-
matics, and the reactions proceeded under these conditions,
but with lower conversions. Therefore, we optimized several
reaction parameters, including the solvent, base, and reaction
temperature, and finally found that the use of tetra-n-butylam-
monium bromide (TBAB) and changing the base from K PO to
NO
H
2
5
95
[
a] Reaction conditions: aryl chloride (1 equiv), arylboronic acid
(1.5 equiv), and NaOH (3 equiv) were heated in 5 mL N-Methyl-2-pyrroli-
done (NMP) at 1408C for 10 h in the presence of tetra-n-butylammonium
bromide (0.1 equiv) and KCC-1-NH
mined by using GC.
2
/Pd (2.5 mol% of Pd). [b] Yield deter-
3
4
NaOH was necessary to achieve good yields. Using these opti-
ChemSusChem 2012, 5, 85 – 89
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
87

and distribution of Pd-nano-particles remains nearly the same
Figure 3), and thus, the system maintains nearly the same cat-
Table 3. Recycling study of KCC-1-NH
tion of 4’-bromoacetophenone and phenylboronic acid.
2
-Pd using the Suzuki coupling reac-
(
alytic activity. These results clearly indicate the advantage of
the fibrous nature of KCC-1.
Run
Yield [%]
Conclusions
1
2
3
4
5
6
7
97
95
96
94
93
95
90
Fibrous high-surface-area nano-silica KCC-1 was functionalized
with aminopropyl groups and was loaded with Pd nanoparti-
cles by treating the functionalized surface with PdCl , followed
2
by hydrogen reduction. TEM images revealed that fibers of
KCC-1 were fully loaded with well-dispersed Pd nanoparticles
(
1–5 nm). The as-synthesized KCC-1-NH /Pd nanocatalyst was
2
then evaluated for the Suzuki coupling of aromatic halides and
was found to be active for the reaction of a range of aryl bro-
mides and iodides with aryl boronic acids in good to excellent
yields. Notably, the catalyst was also active for aryl chlorides,
which are difficult, but more “sustainable substrates”. The cata-
lyst was easily recovered from the reaction mixture and was
reused for a number of cycles with negligible change in the
catalyst activity, confirming the heterogeneity and good sta-
bility of this catalyst system. TEM images of the fresh and used
catalyst showed that the Pd nanoparticles supported on
fibrous nano-silica remained unchanged, indicating no signifi-
Why does the KCC-1-NH /Pd nano-catalyst not deactivate
2
even after several reactions? Restricted Ostwald ripening pro-
vides the explanation. The catalytic activity of nano-Pd de-
pends on the size of the particles, and we observed that
Pd-nanoparticles with sizes ranging from 1–5 nm are effective
in catalyzing Suzuki coupling reactions. However, these nano-
particles generally lose their activity slowly during every reac-
tion cycle. This loss in activity is a result of the nanoparticles
growing larger in size through the Ostwald ripening process,
in which small nanoparticles merge together to form larger
[
35]
nanoparticles.
In the case of the KCC-1-NH /Pd catalyst
2
cant Ostwald ripening. Thus, the KCC-1-NH /Pd nanocatalyst
2
system, we observed no appreciable Ostwald ripening, and the
particle size and distribution remained the same even after the
reaction (Figure 3), which in turn maintained the catalytic activ-
ity. Ostwald ripening may be absent because most of the Pd-
nanoparticles are present between two fibers of KCC-1 (where
the maximum distance between two fibers is 5 nm), which im-
plies that the nano-particles cannot grow larger than 5 nm in
synthesized by using a simple protocol displays superior
activity for Suzuki coupling reactions even with challenging
substrates like aryl chlorides under sustainable ligand-free
conditions.
size because such a growth would be restricted by the fibers Experimental
of KCC-1. Therefore, even after several reaction cycles, the size
Preparation of KCC-1-NH2
In a 250 mL round-bottom flask, anhydrous toluene (150 mL),
KCC-1 (12.00 g), and 3-aminopropyltriethoxysilane (APTS, 40 mL)
were introduced successively. The mixture was refluxed for 48 h.
The solution was filtered, the solid was washed with acetone and
chloroform, and then the solid was dried overnight at 658C under
vacuum to yield the KCC-1-NH nano-composite.
2
Preparation of KCC-1-NH /Pd
2
A Schlenk flask was charged with of KCC-1-NH (1 g), PdCl (0.25 g),
2
2
and deionized water (40 mL), and the mixture was then sonicated
for 2 h. The obtained suspension was stirred at room temperature
for 24 h. The solid collected by centrifugation was washed several
times with water, ethanol, and acetone and dried overnight under
vacuum at 658C. The reduction was performed in a fixed-bed con-
tinuous-flow reactor. The unreduced catalyst (200 mg) was placed
in a stainless steel tabular reactor with a 9 mm internal diameter
À1
and then reduced in a stream of hydrogen (20 mLmin ) at 4008C
for 15 h. The temperature in the reactor was controlled by using a
PID temperature controller connected to a thermocouple placed
inside the catalyst bed. The Pd content in the final material was de-
termined by using ICP-elemental analysis to be 6.3%.
2
Figure 3. TEM images of KCC-1-NH /Pd nano-catalysts a) before reaction and
b) after reaction. Particle size distribution c) before reaction and d) after reac-
tion.
8
8
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2012, 5, 85 – 89

Representative procedure for the KCC-1-NH /Pd-catalyzed
Suzuki coupling reaction
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Keywords: amines · nanoparticles · palladium · supported
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Received: July 19, 2011
Published online on November 15, 2011
ChemSusChem 2012, 5, 85 – 89
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
89