SBA-15-functionalized palladium complex partially confined with ionic liquid: An efficient and reusable catalyst system for aqueous-phase Suzuki reaction

Article abstract of DOI:10.1039/c2ob07176b

A novel SBA-15 functionalized palladium complex partially confined with 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid (Material 4) was found to be a very efficient and reusable catalyst in the Suzuki-Miyaura coupling reaction of aryl halides including aryl chlorides and heteroaryl halides with different aryl boronic acids under aqueous conditions without any organic co-solvents. Our studies showed that 4 is a more efficient catalyst in comparison with the catalyst not containing IL or catalyst with a higher ratio of IL. The materials were characterized by N₂-sorption analysis, TGA and transmission electron microscopy before and after catalysis. While our studies showed that the catalyst can be successfully recycled and reused in at least 4 reaction runs, in contrast, several poisoning experiments and kinetic studies provide the notion that homogeneous (dissolved) species are responsible for the observed catalysis.

Full text of DOI:10.1039/c2ob07176b

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PAPER
SBA-15-functionalized palladium complex partially confined with ionic
liquid: an efficient and reusable catalyst system for aqueous-phase Suzuki
reaction†
Babak Karimi* and Asghar Zamani
Received 24th December 2011, Accepted 10th April 2012
DOI: 10.1039/c2ob07176b
A novel SBA-15 functionalized palladium complex partially confined with 1-butyl-3-methylimidazolium
hexafluorophosphate ionic liquid (Material 4) was found to be a very efficient and reusable catalyst in the
Suzuki–Miyaura coupling reaction of aryl halides including aryl chlorides and heteroaryl halides with
different aryl boronic acids under aqueous conditions without any organic co-solvents. Our studies
showed that 4 is a more efficient catalyst in comparison with the catalyst not containing IL or catalyst
with a higher ratio of IL. The materials were characterized by N2-sorption analysis, TGA and
transmission electron microscopy before and after catalysis. While our studies showed that the catalyst can
be successfully recycled and reused in at least 4 reaction runs, in contrast, several poisoning experiments
and kinetic studies provide the notion that homogeneous (dissolved) species are responsible for the
observed catalysis.
the practical applications, are still limited. However, the major
problem when using metal nanoparticles as catalysts in coupling
Introduction
The Suzuki–Miyaura cross-coupling of boronic acids or esters
with organic halides is one of the most important transformations
in organic chemistry for the construction of biaryl units from
both an academic and industrial point of view.1–8 Biaryls are
valuable compounds used in production of fine and bulk chemi-
cals such as pharmaceuticals, agrochemicals and natural
products.9–17 While the Suzuki coupling has been traditionally
performed using palladium catalyst in the presence of various
phosphine ligands under homogeneous conditions, they suffer
from the problems of separation of catalyst from reaction
mixture, recycling of catalyst, deactivation through the aggrega-
tion of metallic Pd particles, and product contamination, which
is particularly drawback for pharmaceutical industries.1,2 Hence
efforts have been made to immobilize the homogeneous palla-
dium catalysts onto a diverse solid supports in the form of either
metal complex and metal nanoparticles in order to facilitate their
separation from the products.13–16 Furthermore, heterogeneous
catalysts with reasonable recyclability, which is the key factor for
reactions is their aggregation into less active large particles due
to the high surface energy of the nanoparticles. Therefore, one of
the most challenging topics in this area is the development of
more appropriate heterogeneous systems to effectively to prevent
the aggregation and agglomeration of Pd nanoclusters while
keeping their durable activity in coupling reactions.
In recent years, it has been revealed that the use of ionic liquid
functional groups either grafted17–19 or impregnated on the solid
supports played an important role in preventing the aggregation
of Pd nanoparticles.20–26 Among different supports, ordered
mesoporous materials are very attractive for immobilizing palla-
dium catalysts, because of their large pore size (1.5–40 nm),
high surface area (up to 2000 m2 g−1), and tunable
structure.27–31
Quite recently, we showed that the confinement of the ionic
liquid 1-butyl-3-methylimidazolium bromide ([BMIm] Br)
inside the nanospace of SBA-15-functionalized TEMPO resulted
in an efficient catalyst system for aerobic oxidation of alcohols.32
Our investigation revealed that using this combined catalyst
system much higher selectivities can be attained in the oxidation
of allylic alcohols in comparison with SBA-15-TEMPO not
charged with IL.32 In continuation of this study, herein, we show
that the partial confinement of nanospaces of a novel SBA-15-
functionalized palladium complex with 1-butyl-3-methylimida-
zolium hexafluorophosphate significantly improves the catalyst
performance and the catalyst selectivity in the Suzuki–Miyaura
coupling of aryl halides with aryl boronic acids in water.
Department of Chemistry, Institute for Advanced Studies in Basic
Sciences (IASBS), P.O. Box 45195-1159, Gava Zang, Zanjan, Iran.
E-mail: karimi@iasbs.ac.ir; Fax: +98-241-4214949;
Tel: +98-241-4153225
†Electronic supplementary information (ESI) available: The experimen-
tal details for the preparation of catalyst 4, N2 adsorption-desorption iso-
therms and TGA analysis of catalyst 4 and all intermediates, 1H- and
13C-NMR for all coupling products. See DOI: 10.1039/c2ob07176b
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 4531–4536 | 4531

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area, Barrett–Joyner–Halenda (BJH) average pore size distri-
butions and primary mesoporous volume were shown to be
599 m2 g−1, 8.2 nm and 0.99 cm3 g−1, respectively.34 On the
other hand, IL@SBA-15-Pd 4 display considerably lower BET
surface area, and primary mesoporous volume in comparison to
SBA-15-Pd. It is also worth nothing that the BJH average pore
size of 4 was also slightly decreased by 6.7 nm. These results
suggest that ionic liquid [BMIm][PF6] were successfully intro-
duced into the interior of nanospaces of SBA-15, where co-
valently immobilized palladium complexes are located.
The results also point to the fact that the mesopores of the
material 4 are not fully filled by the ionic liquid (Fig. 5S vs. 6S,
ESI†). The thermal stability of both 3 and 4 was determined by
thermogravimetric analysis (TGA) with heating from room temp-
erature to 800 °C under an air flow. The organic materials
present in 3 were also determined by elemental analysis. Finally
the Pd content of all prepared catalysts was calculated using
atomic absorption spectroscopy of the acid washed solution
using standard addition method. The catalytic performance of 4
was then evaluated in the Suzuki–Miyaura coupling reaction of
arylboronic acids with various types of aryl halides in water. The
reaction was initially tested using 4-bromoanisole and phenyl-
boronic acid coupling partners and K2CO3 as base in neat water
at 60 °C (Table 1).35 As shown in Table 1, 4 is a more efficient
catalyst in comparison with the catalyst not containing IL
(Table 1, entry 4) or catalyst having higher ratio of IL to
SBA-15-Pd (SBA-15-Pd/IL = 1 g/1 g, Table 1, entry 5).
Results and discussion
The preparation procedure to obtain the catalyst is outlined in
Scheme 1. We chose to employ the ordered mesoporous silica
SBA-1529,30 as a support for a bidentate ligand because it has
regular porosity, high surface area, and high thermal and hydro-
thermal stabilities. SBA-15 was obtained in the presence of
pluronic P123 (EO20PO70EO20, EO = ethylene oxide, PO = pro-
pylene oxide, MAV = 5800, Aldrich) as a lyotropic structure
directing agent and (EtO)4Si as silica precursor under acidic con-
ditions following the reported literature procedure.30 The result-
ing
SBA-15
was
then
functionalized
with
3-
aminopropyltrimethoxysilane followed by the condensation with
2-acetylpyridine using known procedures33 to give the corre-
sponding iminopyridine functionalized SBA-15 2. The obtained
iminopyridine functionalized SBA-15 was then reacted with an
acetone solution of Pd(OAc)2 at room temperature, as described
in the Experimental, affording the immobilized palladium
complex (SBA-15-Pd, 3). To this end, 3 was allowed to mix
with an acetone solution of 1-butyl-3-methylimidazolium hexa-
fluorophosphate [BMIm][PF6] with appropriate concentration
(1 g IL in 0.5 ml acetone). After stirring for 24 h and evapo-
ration of the volatile solvent under reduced pressure, eventually
yield a free-flowing brown solid, which is referred to as
IL@SBA-15-Pd 4 (Scheme 1).
For the purpose of comparison, a diverse array of SBA-15
supported palladium catalyst was prepared by the confinement of
other ionic liquids bearing alkyl chain functionality in order to
study the effect of alkyl chains in the confined ionic liquid on
catalyst activity.
The textural properties of the original palladium supported
SBA-15 3 were measured by nitrogen adsorption–desorption iso-
therm at 77 K (Fig. 5S, ESI†). The material exhibited a type IV
isotherm with sharp hysteresis loop in relative pressures P/P0 ≈
0.6–0.8, which is the characteristic of ordered mesoporous
material with open pore structure and a narrow pore size distri-
bution. The calculated Brunauer–Emmett–Teller (BET) surface
Also the results showed that the use of catalysts containing
n-octyl and n-dodecyl-3-methylimidazolium hexafluoropho-
sphate and 1-butyl-3-methylimidazolium bromide gave a much
lower yields of the corresponding products (Table 1, entries 2, 3
and 6).
The resulting IL@SBA-15-Pd (IL = 1-butyl-3-methylimida-
zolium hexafluorophosphate) catalyst was then tested in the
Suzuki cross-coupling reaction of arylboronic acids with various
types of aryl halide under similar reaction conditions. The reac-
tions were conveniently carried out in a batch reactor in the
Scheme 1 A schematic pathway for preparation of SBA-15-functionalized palladium complex partially confined with [BMIm][PF6] as ionic liquid.
4532 | Org. Biomol. Chem., 2012, 10, 4531–4536 This journal is © The Royal Society of Chemistry 2012

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temperature range 60–90 °C. As expected, we observed that the
coupling of 4-iodoanisole (1 mmol) and phenylboronic acid
(1.1 mmol) in the presence of IL@SBA-15-Pd (0.05 mol%) and
K2CO3 (1.5 mmol) in distilled water (3 mL) at 60 °C proceeded
rapidly to give 4-phenylanisole in 92% isolated yield after 2 h
(Table 2, entry 1). We then investigated Suzuki coupling reaction
of aryl bromides using 4 as the catalyst. As shown in Table 2,
the reactions of various aryl bromides including electron-rich
(deactivated) and electron-deficient (activated) substrates pro-
ceeded readily at very low catalyst loading (0.05–0.1 mol%) in
water to afford excellent yields of the corresponding biphenyls
(Table 2, entries 3–10).
We found also that catalyst 4 (0.2 mol%) shows high reactivity
for Suzuki–Miyaura cross-coupling of activated aryl chlorides
with phenyl and p-tolyl boronic acid in water (Table 2, entries 11
and 12). Since many drugs contain heteroaromatic fragments
chemists are very interested in cross-coupling of heteroaryl
halides, but the presence of heteroatoms such as sulfur and nitro-
gen often leads to low reactivity in transition-metal catalyzed
coupling reactions.36 Along this line, commercially available,
substrates, 3-bromo pyridine, 2-bromo pyridine and 2-bromo
thiophene were tested (Table 2, entries 13–17). These substrates
were found to give an excellent yield of the Suzuki product
without loss of activity of catalyst. Moreover, various functional
groups, such as cyano, acyl, CHO, ethyl, and methoxy were tol-
erated and the corresponding coupled products were obtained in
excellent isolated yields (Table 2, entries 1–12).
It should be also noted that performing the Suzuki–Miyaura
cross-coupling of 4-bromoanisol with phenyl boronic acid in
open air resulted in almost the same yield of cross-coupling
product (Table 2, entry 10) in comparison with the same trans-
formation performed under an argon atmosphere (ca. 97%),
confirming that the atmosphere does not significantly impact on
the efficiency of the present catalyst system.
Table 1 Suzuki–Miyaura coupling of 4-bromoanisole with phenyl-
boronic acid using various type of IL confined SBA-15-Pd catalyst
systems
The reusability and recovery of the catalysts are important
issues, especially when the reactions use solid catalysts. By
using the coupling reaction of phenylboronic acid and 4-bromo
benzaldehyde as a test model, it was found that this supported
catalyst has been recovered and reused up to 4 times without
considerable loss of its reactivity (1st reuse: 95%, 2nd reuse:
93%, 3rd reuse: 96%, 4th reuse: 90%, Fig. 1).
To assess whether the catalyst is actually operating in a hetero-
geneous pathway, or whether it is merely a reservoir for more
active or naked Pd nanoparticles, a set of three independent
experiments including a Hg0 test and two three-phase poisoning
tests were designed for the coupling reaction of 4-bromoanisole
Entry
R
X
Loading of IL (g IL/g 3)
Yield (%)
1a
2
nBu
nOct
nDodec
—
PF6
PF6
PF6
—
PF6
Br
0.5
0.5
0.5
—
1
95
41
23
58
33
69
3
4b
5
nBu
6
nBu
0.5
a IL@SBA-15-Pd 4. b SBA-15-Pd 3.
Table 2 Suzuki–Miyaura coupling of aryl halides with aryl boronic acids using IL@SBA-15-Pd, 4
Entry
X
R
R′
IL@SBA-15-Pd (mol%)
TBABa (equiv.)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
I
I
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
MeO
Me
CHO
CN
H
Me
H
H
Me
H
Me
H
Me
H
H
Me
H
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.2
0.2
0.1
0.1
0.1
0.1
0.1
—
—
—
—
—
—
—
0.5
0.5
1
2
2
1
1
2
2
2
2
2
2
2
4
92
96
95
93
95
100
96
100
92
CN
NO2
NO2
H
9
Me
4
10
11
12
13
14
15
16
17
MeO
NO2
NO2
4.5
10
10
8
95
84c
90c
94c
99c
99c
97c
91c
3-Bromopyridine
3-Bromopyridine
2-Bromothiophene
2-Bromothiophene
2-Bromopyridine
Me
H
Me
H
8
1
1
1
10
10
8
a TBAB: tetra butyl ammonium bromide. b Isolated yields. c Temperature: 80 °C.
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 4531–4536 | 4533

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Fig. 2 Reaction progress as a function of time for the Suzuki coupling
of 4-bromoanisole with phenylboronic acid by using IL@SBA-15-Pd
catalyst in water under: (a) normal conditions (blue diamond), (b) in the
presence of 400 equiv. Hg0 (red square), (c) in the presence of 400
equiv. PVP (violet cross), and (d) in the presence of 400 equiv. mercap-
topropyl silica (green triangle).
Fig. 1 Representation of 4-phenyl benzaldehyde yield at different
reuses of the catalyst.
with phenyl boronic acid under the standard reaction con-
ditions.37,38 To conduct the test, three different flasks were
charged with 4-bromoanisole (1 mmol), phenyl boronic acid
(1.1 mmol), IL@SBA-15-Pd (0.05 mol%), and K2CO3
(1.5 mmol) in water (3 mL) as the solvent. The mixture of three
flasks were then heated to 60 °C, while mercury (Hg0) was
added to the first flask, amorphous silica-supported mercaptopro-
pyl (mercaptopropyl silica) to the second one, and poly-(4-vinyl-
pyridine) (PVP) to the third flask in a ratio of 400 equiv. to the
initial palladium content of the catalyst. These control exper-
iments showed that the reactions were quenched completely over
all three flasks upon addition of the poison (Fig. 2). These
results suggest that for the present reaction, supported palladium
catalyst serves as a reservoir for active Pd species that are dis-
solved from the solid catalyst under the described reaction
conditions.
To further study the contribution of homogeneous catalysis,
we also conducted a hot-filtration test after the reaction of 4-bro-
moanisole with phenyl boronic acid was initiated and before the
substrates were consumed. We found that a further 21% conver-
sion of the coupling reaction was observed upon the heating of
the catalyst-free solution for 24 h at 60 °C. The positive hot-
filteration test is certainly in good agreement with the results
obtained from the poisoning tests and highlight the notion that
IL@SBA-15-Pd catalyst system is indeed a pre-catalyst for the
soluble (homogeneous) palladium species in the described
Suzuki–Miyaura coupling reaction.
To understand the changes in our catalyst system on a molecu-
lar level during the reaction, the recovered catalyst was further
subjected to N2-adsorption–desorption analysis, transmission
electron microscopy (TEM) and elemental microanalysis. N2-
sorption studies and TEM analysis provide valuable information
about the surface properties and ultrastructure of the recovered
catalyst. It is noteworthy that even after the reaction and recy-
cling, the recovered catalysts exhibit a type IV isotherm with a
sharp condensation–evaporation stage, which indicates that the
ordered mesostructures have mostly survived. Notably, the
materials also showed a total pore volume slightly lower than
that of the fresh catalyst charged with ionic liquid. While this
Fig. 3 Transmission electron microscopy (TEM) image of recovered
IL@SBA-15-Pd material showing two dimensional hexagonal sym-
metry with superior uniformity of the mesoporous channels. Scale bar:
20 nm.
result indicates a partial desorption of ionic liquid during the cat-
alysis and work-up stages, it strongly points to the fact that the
total loss of ionic liquid [BMIm][PF6] was still not significant in
the recovered catalyst.34
Both TGA and elemental microanalysis demonstrate that the
loss of ionic liquid is not significant (less than 5% after the 4th
reaction cycle), which could account to some extent the preser-
vation of catalytic activity during 4 reaction runs.
TEM image of the catalyst after the 4th reaction cycle is dis-
played in Fig. 3. This micrograph demonstrates that not only the
2D-hexagonal structure of the catalyst is largely survived but
also a well-distribution of Pd nanoparticles with sizes mostly
less than 5 nm throughout the regular channels of the parent
SBA-15. This observation reveals, interestingly, that while
4534 | Org. Biomol. Chem., 2012, 10, 4531–4536
This journal is © The Royal Society of Chemistry 2012

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Experimental
Preparation of SBA-15
The synthesis of SBA-15 has been achieved using a known pro-
cedure described by Stucky and co-workers (see ref. 30). In a
typical preparation procedure, 4.0 g of Pluronic P123 (Aldrich,
average Mw = 5800) was dissolved in 30 g of water and 120 g of
2 M HCl solution with stirring at 35 °C. Then 8.50 g of tetra-
ethoxysilane (TEOS) was added into that solution while stirring
at 35 °C for 20 h. The mixture was aged at 80 °C overnight
without stirring. The solid was filtered off and washed
thoroughly with hot ethanol/water using a Soxhelet apparatus for
18 h to remove the surfactant molecules. It was dried in air at
110 °C overnight.
Preparation of SBA-15-amine
Refluxing the SBA-15 (10 g) with 3-aminopropyltrimethoxy-
silane (10 mmol) in dry toluene for 18 h. The solid materials
were filtered off and washed with hot toluene for 12 h in a con-
tinuous extraction apparatus (Soxhelet) and then dried in oven at
90 °C overnight to give the SBA-15 supported amine (SBA-15-
amine).
Scheme 2 The proposed pathway for the generation, stabilization, and
re-capture of Pd-nanoparticles into the interior of the mesochannels of 4
where the physically adsorbed ionic liquid [BMIm][PF6] layer is
located.
Preparation of SBA-15-lig
2-Acetylpyridine (0.97 g, 8 mmol) was added to a mixture of
the oven dried SBA-15-amine (5 g) in super dry ethanol
(150 mL) in a 250 mL round bottomed flask. The reaction
mixture was stirred at 60 °C for 24 h. The ligand-grafted silica
was filtered at the reaction temperature and the resulting solid
was washed thoroughly with hot toluene and ethanol to remove
unreacted ketone. It was dried in air at 90 °C overnight to
furnish the corresponding SBA-15 supported bidentate ligand
(SBA-15-lig).
IL@SBA-15-Pd acts as a reservoir for highly active and soluble
Pd species, it may also operate as a nanoscaffold to recapture Pd
nanoparticles inside the mesopores where ionic liquid are located
as stabilizer, thus preventing Pd aggregation (Fig. 3).
The result of TEM analysis accompanied with our complimen-
tary kinetic and poisoning experiments revealed that while cata-
lyst system 4 could in fact act as a reservoir for the soluble
palladium species in the described Suzuki–Miyaura coupling
protocol but it is also a powerful support for the stabilization of
re-captured active Pd-nanoparticles (PdNPs) in the interior of
SBA-15 mesochannels where physically adsorbed ionic liquid
layer is located (Scheme 2).
Preparation of SBA-15-Pd
SBA-15-lig (3 g) was added to a solution of palladium acetate
(0.035 g Merck) in dry acetone (100 mL). The reaction mixture
was stirred at room temperature for 24 h. After stirring, the solid
was filtered, washed with acetone, ethanol and ether in order to
remove any adsorbed palladium on the surface. It was then dried
in an oven at 90 °C overnight to furnish the corresponding
SBA-15 supported complex (SBA-15-Pd).
Conclusions
We have demonstrated that a novel SBA-15 functionalized palla-
dium complex partially confined with imidazolium type ionic
liquids efficiently catalyzes Suzuki–Miyaura coupling of aryl
halides. This reusable catalyst is also quite effective for the coup-
ling of heteroaryl halides. Interestingly, it was found that while
hot filtration tests and kinetic experiments showed the presence
of soluble Pd species during the reaction process, recovery
studies illustrated that no significant decrease has occurred in the
activity and metal content of recovered IL@SBA-15-Pd. This
observation implies that while catalyst system 4 could in fact act
as a reservoir for the soluble palladium species in the described
Suzuki–Miyaura coupling protocol, it is also a powerful support
for the generation and stabilization of active Pd-nanoparticles
(PdNPs) in the interior of SBA-15 mesochannels where phys-
ically adsorbed ionic layers are located.
Preparation of IL@SBA-15-Pd
SBA-15-Pd (3 g) was added to a solution of 1-butyl-3-methyl-
imidazolium hexafluorophosphate (1.5 g) in dry acetone
(100 mL). The reaction mixture was stirred at room temperature
for 24 h. After stirring, acetone was slowly removed under
reduced pressure. The resulting powder was then dried in an
oven at 90 °C overnight to give final catalyst IL@SBA-15-Pd at
a loading ca. 0.032 0.001 mmol g−1 (atomic absorption spec-
troscopy (AA)).
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 4531–4536 | 4535

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15 H. Yoon, S. Ko and J. Jang, Chem. Commun., 2007, 1468.
General procedure for heterogeneous Suzuki reaction
16 For excellent reviews on supported Pd catalyst for coupling reactions see:
(a) Á. Molnár, Chem. Rev., 2011, 111, 2251; (b) B. Karimi,
H. Behzadnia, E. Farhangi, E. Jafari and A. Zamani, Curr. Org. Synth.,
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2002, 102, 3385.
17 B. Karimi and D. Enders, Org. Lett., 2006, 8, 1237.
18 B. Karimi, D. Elhamifar, J. H. Clark and A. J. Hunt, Chem.–Eur. J.,
2010, 16, 8047.
The Suzuki reaction was performed by using arylboronic acid
(1.1 mmol), aryl halide (1.0 mmol), K2CO3 (1.5 mmol), and cat-
alyst (0.05–0.2 mol%) in distilled water at 60–80 °C. The reac-
tion progress was monitored by GC analysis after completion of
the reaction; the mixture was allowed to cool to room tempera-
ture and was then filtered and washed with H2O and ethyl
acetate. The organic phase was separated and dried over MgSO4
and the solvent was then removed under reduced pressure. Pure
products were obtained after recrystallization or by isolation of
the residue by column chromatography on silica.
19 X.-D. Mu, J.-Q. Meng, Z.-C. Li and Y. Kou, J. Am. Chem. Soc., 2005,
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20 J. Huang, T. Jiang, H. Gao, B. Han, Z. Liu, W. Wu, Y. Chang and
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We also acknowledge Dr Hojatollah Vali from McGill University
for TEM images.
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