In situ prepared mesoporous silica nanosphere supported palladium(ii) 2-aminopyridine complex catalyst for Suzuki-Miyaura cross-coupling reaction in water

Article abstract of DOI:10.1039/c2jm33835a

We report the synthesis and catalytic activities of a mesoporous silica nanosphere supported palladium^II 2-aminopyridine complex (Pd-AMP-MSN). First, chloroalkyl functionalized mesoporous silica nanospheres (Cl-MSN) is prepared by an in situ co-condensation reaction of tetraethyl orthosilicate (TEOS) with ((chloromethyl)phenylethyl)trimethoxysilane (CMPE-TMS) using cetyltrimethylammonium bromide (CTAB) as the structure directing agent. The reaction of Cl-MSN with 2-aminopyridine followed by complexation with palladium acetate produces the catalyst Pd-AMP-MSN. FTIR spectroscopic analysis confirms the presence of 2-aminopyridine functionality inside the mesopores of the Pd-AMP-MSN. Nitrogen adsorption-desorption and X-ray diffraction analyses reveal mesoporous structures of the Pd-AMP-MSN catalyst with a specific surface area of 372 m² g^-1, a pore volume of 0.172 cm³ g^-1 and a narrow pore size distribution (D ～ 1.92 nm). FESEM and HRTEM results indeed show the formation of nanospheres with mesoporous structures. This catalytic system exhibits excellent activity in Suzuki-Miyaura cross-coupling reactions of aryl iodides, aryl bromides and also aryl chlorides with phenylboronic acids in water medium with high yields. This Pd-AMP-MSN catalyst could be quantitatively recovered by simple filtration and is found to be highly active without any significant loss of its catalytic activity in eight consecutive runs.

Full text of DOI:10.1039/c2jm33835a

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PAPER
In situ prepared mesoporous silica nanosphere supported palladium(II)
2-aminopyridine complex catalyst for Suzuki–Miyaura cross-coupling
reaction in water†
a
Paramita Mondal,‡ Sanjib Banerjee,‡ Anupam Singha Roy, Tarun K. Mandal and S. M. Islam
b
a
b
a
*
*
Received 14th June 2012, Accepted 10th August 2012
DOI: 10.1039/c2jm33835a
We report the synthesis and catalytic activities of a mesoporous silica nanosphere supported
palladiumII 2-aminopyridine complex (Pd-AMP-MSN). First, chloroalkyl functionalized mesoporous
silica nanospheres (Cl-MSN) is prepared by an in situ co-condensation reaction of tetraethyl
orthosilicate (TEOS) with ((chloromethyl)phenylethyl)trimethoxysilane (CMPE-TMS) using
cetyltrimethylammonium bromide (CTAB) as the structure directing agent. The reaction of Cl-MSN
with 2-aminopyridine followed by complexation with palladium acetate produces the catalyst Pd-
AMP-MSN. FTIR spectroscopic analysis confirms the presence of 2-aminopyridine functionality
inside the mesopores of the Pd-AMP-MSN. Nitrogen adsorption–desorption and X-ray diffraction
analyses reveal mesoporous structures of the Pd-AMP-MSN catalyst with a specific surface area of 372
m2 gꢀ1, a pore volume of 0.172 cm3 gꢀ1 and a narrow pore size distribution (D ꢁ 1.92 nm). FESEM and
HRTEM results indeed show the formation of nanospheres with mesoporous structures. This catalytic
system exhibits excellent activity in Suzuki–Miyaura cross-coupling reactions of aryl iodides, aryl
bromides and also aryl chlorides with phenylboronic acids in water medium with high yields. This Pd-
AMP-MSN catalyst could be quantitatively recovered by simple filtration and is found to be highly
active without any significant loss of its catalytic activity in eight consecutive runs.
a relatively broad particle size distribution. In a recent review,
Hu et al. have summarized the synthesis and application of
mesoporous hollow spheres of various inorganic materials.12
Again, it is very difficult to avoid agglomeration of the formed
particles in the above methods. Thus, some kind of organic
modification of the mesoporous particles is necessary in order to
prevent them from aggregation. Organically modified meso-
porous silica particles were prepared mainly via two approaches,
such as the post-synthesis grafting technique13 and organo-
siloxane/siloxane co-condensation methods.14 However, the co-
condensation strategy is usually preferred over the post-synthesis
grafting method, because the latter method usually produces
inhomogeneously grafted mesoporous materials due to the
blockage near the entries to the mesoporous channels by the
already grafted species.15 We are trying to utilize the co-
condensation strategy to prepare mesoporous silica nanospheres
for supporting palladium complex for heterogeneous catalysis of
different organic reactions.
1. Introduction
Mesoporous materials with controllable pore sizes (pore size: 2–
50 nm), narrow pore size distributions and high surface areas are
of interest for various applications such as in bioadsorption/
biocatalysis,1 adsorption,2 catalysis/catalyst support,3 drug
delivery4 as well as in packing materials for chromatography.5
Among these, one of the major applications of mesoporous silica
materials is their use as catalysts and catalysts support.6 Besides
silica, other mesoporous inorganic nanospheres were also used as
catalysts/photocatalysts.7 Consequently, several different
approaches have been reported for the synthesis of spherical
mesoporous particles such as oil–water interface templating,8 by
combination of the Stober synthesis with mesophase forma-
tion,9,10 spray-drying alcoholic sols,11 etc. But these methods
generate silica particles mostly in the micrometer size range with
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235,
W. B., India. E-mail: manir65@rediffmail.com; Fax: +91-33-2582-8282
bPolymer Science Unit, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata - 700032, India. E-mail: psutkm@iacs.res.in; Fax:
+91-33-2473-2805
In this context it is worthwhile mentioning that the palla-
dium-catalyzed Suzuki–Miyaura cross-coupling reaction of
aryl-boronic acids with aryl halides is one of the most powerful
and versatile methods for the selective formation of carbon–
carbon bonds and has been widely used in the synthesis of
pharmaceuticals, herbicides, natural products, and advanced
materials.16–20 Nowadays, chemists are increasingly looking for
† Electronic supplementary information (ESI) available: FESEM-EDX
spectra of Cl-MSN and Pd-AMP-MSN nanospheres and DLS data of
mesoporous Pd-AMP-MSN nanosphere suspension in water. See DOI:
10.1039/c2jm33835a
‡ These authors contributed equally to this manuscript.
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cleaner and more environmentally benign ways to make target
molecules.21–24 These efforts have primarily been directed
towards the development of homogeneous transition metal
catalyzed reactions in water.25–28 Homogeneous catalysts display
better efficiency and higher selectivity than their heterogeneous
counterparts. But tedious and time-consuming purification steps
are required to remove the expensive and toxic metal residue
contaminants from the final products, making recovery and
recycling of the catalyst difficult.29,30 Accordingly, use of
heterogeneous catalysts in aqueous organic transformations is
of considerable current interest since such catalysts can be easily
removed from aqueous medium for subsequent reuse and also
could be made readily amenable to continuous processing.31,32
Recently, many reports have appeared on Suzuki–Miyaura
coupling reactions catalyzed by supported Pd catalysts, which
was loaded on silica,33–39 NaY zeolite,40–43 diatomite,44 den-
drimer,45–48 magnetic nanoparticles,49,50 polymeric microspheres,
microcapsules and resin.51–55
modification of the method reported by Huh et al.62 Tetraethyl
orthosilicate (TEOS) was used as the silica precursor, ((chlor-
omethyl)phenylethyl)trimethoxysilane (CMPE-TMS) as the
silane
bearing
chloroalkyl
functional
moiety
and
cetyltrimethylammonium bromide (CTAB) as the template. The
ratio of the reaction components used according to
molar
ratio:
CTAB/TEOS/CMPE-TMS/NaOH/H2O
¼
1.0 : 8.16 : 1.05 : 2.55 : 4840. The synthesized Cl-MSN was
characterized by FTIR, small angle XRD, TGA, N2 sorption,
FESEM and TEM analysis.
Preparation of Cl-MSN supported Pd(II) complex (Pd-AMP-
MSN). The outline for the preparation of the mesoporous silica
supported palladiumII complex is given in Scheme 1. The silica
supported ligand was prepared according to a literature
method.56,59,60 First, the chloroalkyl functionalized silica,
Cl-MSN (0.5 g) was reacted with 2-aminopyridine (0.264 g) in
dioxane (10 mL) under reflux conditions for 48 h to afford a silica
supported 2-aminopyridine ligand, AMP-MSN. In the next step,
AMP-MSN (0.5 g) was subsequently reacted with palladium
acetate (0.1 g) in acetic acid (10 mL) at 80 ꢂC for 10 h to generate
the corresponding mesoporous silica supported palladiumII
complex catalyst (Pd-AMP-MSN).
It is pertinent to mention here that we have recently developed
a series of polymer anchored transition metal complex cata-
lysts.56–58 But only a few of them have proved to be effective
catalysts with good catalytic activity at ambient reaction condi-
tions. Also, some of these polymer supported complexes
efficiently catalyzed the oxidation and coupling reactions of
organic substrates in aqueous media.56,59,60 In this context,
functionalized mesoporous materials containing donor func-
tional groups at the pore surface can play an important role in
designing an efficient catalytic material.
2.3 General procedure for Suzuki–Miyaura cross-coupling
reaction in water
A mixture of aryl halide (1.0 mmol), phenylboronic acid
(1.2 mmol), K2CO3 (2.0 mmol), H2O (6.0 mL) and Pd-AMP-
MSN catalyst (0.03 g) was stirred at 80 ꢂC under air. To study the
progress of the reaction, the reaction mixture was collected at
different time intervals and quantified by GC analysis. At the end
of the reaction, the catalyst was separated by centrifugation. The
filtrate was extracted with CH2Cl2 and passed through a pad of
silica gel. The organic phase thus collected was dried over
Na2SO4, filtered, concentrated and the residue was purified by
flash chromatography on silica gel. The product was analyzed by
Here, we report a simple method to synthesize a mesoporous
silica nanosphere supported palladium(II) 2-aminopyridine
complex (Pd-AMP-MSN) catalyst. This method includes first the
synthesis of chloroalkyl functionalized mesoporous silica nano-
spheres (Cl-MSN) using an in situ co-condensation strategy. The
Pd-AMP-MSN catalyst is then prepared by reacting Cl-MSN
with 2-aminopyridine followed by complexation with palladiu-
m(II) acetate. The as-prepared Pd-AMP-MSN material exhibits
high catalytic activity and selectivity in the water-medium for the
Suzuki–Miyaura cross-coupling reaction. Meanwhile, this
nanocatalyst also rendered the excellent durability of the
heterogeneous catalyst, and this catalyst can be reused at least
eight times with negligible loss of catalytic activity.
1
GC-MS and H NMR. All products are known and compared
with authentic samples.
2.4 Characterization
2. Experimental
X-ray diffraction (XRD). X-ray diffraction measurement of the
dried mesoporous samples were performed on a Bruker AXS D8
diffractometer at an acceleration voltage of 40 kV with 40 mA
current intensity using a Cu tube (l ¼ 0.154 nm) as the radiation
source.
2.1 Materials
Tetraethyl orthosilicate (TEOS, Aldrich, purity 98%), cetyl-
trimethylammonium bromide (CTAB, Loba Chemie, India),
((chloromethyl)phenylethyl)trimethoxysilane
(CMPE-TMS,
Gelest Inc.), palladium acetate (Aldrich), 2-aminopyridine
(Aldrich) were used as received. Sodium hydroxide (NaOH) and
methanol (GR) were obtained from Merck, India and used as
received. Triple distilled water was used for making all the
aqueous solutions/dispersions.
Fourier transform infrared (FTIR) spectroscopy. FTIR spectra
of all the mesoporous materials were obtained on KBr pellets of
the samples prepared by mixing respective dry samples with KBr
in 1 : 100 (wt/wt) ratios in a Perkin Elmer FTIR Spectrum-400
spectrometer.
2.2 Preparation of the catalyst
Thermogravimetric analysis (TGA). The purified and dried
samples were subjected to thermogravimetric analysis using a TA
SDT Q 600 instrument at a heatꢂing rate of 10 ꢂC minꢀ1 under air
from room temperature to 800 C.
In situ synthesis of chloroalkyl modified mesoporous silica
nanospheres (Cl-MSN). The procedure used for the synthesis of
Cl-MSN is similar to our earlier report61 which is based on the
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Scheme 1 Schematic illustration of the synthesis of Pd-AMP-MSN catalyst.
Fig. 1 FTIR spectra of Cl-MSN, AMP-MSN and Pd-AMP-MSN
materials.
Fig. 2 Low angle XRD patterns of Cl-MSN, AMP-MSN and Pd-AMP-
MSN materials.
Nitrogen sorption. Nitrogen sorption isotherms were measured
using a Quantachrome Autosorb-1 at 77 K. Prior to the
measurements mesoporous silica samples were degassed at 100
ꢂC for four hours. The Brunauer–Emmett–Teller (BET) surface
area was deduced from the isotherm analysis. The pore size
distribution (PSD) was calculated by means of the Barrett–Joy-
ner–Halenda (BJH) method.
5 kV. The elemental compositions of the mesoporous materials
were measured from the energy-dispersive X-ray (EDX) associ-
ated with this FESEM.
Transmission electron microscopy (TEM). For TEM studies,
one drop of the ethanol dispersion of the powder samples was
placed on a carbon-coated copper grid and allowed to dry in air.
The grid was then observed on a JEOL JEM-2010 electron
microscope operated at an accelerating voltage of 200 kV.
Field emission scanning electron microscopy (FESEM). For
FESEM studies, mesoporous samples were dispersed in ethanol
and drop casted onto a cover slip, which was then placed on a
copper tape, supported on a metal stub and sputter coated with
platinum to minimize charging. The images of the samples were
taken by a JEOL JSM-6700F field emission scanning electron
microscope (FESEM) operated at an accelerating voltage of
Dynamic light scattering (DLS). DLS experiment was carried
ꢂ
out at 25 C on a Malvern Particle Size Analyzer (Model: ZEN
3690-Zetasizer NANO-ZS90). Mesoporous catalyst sample
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Fig. 3 (A) N2 adsorption–desorption isotherms and (B) BJH pore size distribution plots of Cl-MSN and Pd-AMP-MSN materials.
Fig. 4 Thermogravimetric analyses of Cl-MSN, AMP-MSN and Pd-
AMP-MSN materials.
Fig. 6 TEM image of mesoporous Cl-MSN and Pd-AMP-MSN nano-
spheres (left panel) and the corresponding high magnification TEM
image of the nanospheres (right panel).
Scheme 2 Suzuki–Miyaura cross-coupling reaction of phenyl bromide
with phenylboronic acid using Pd-AMP-MSN catalyst.
Fig. 5 FESEM image of mesoporous Cl-MSN and Pd-AMP-MSN
nanospheres.
co-condensation reaction of TEOS (main silica precursor) with
CMPE-TMS (silane with organic chloroalkyl functionality) at a
basic pH (ꢁ12) in the presence of the CTAB template and
subsequent removal of the surfactant template by an acid
extraction. 2-Aminopyridine functionalized mesoporous silica
nanospheres (AMP-MSN) and the mesoporous silica nano-
sphere supported palladiumII 2-aminopyridine complex (Pd-
AMP-MSN) catalyst were then prepared from the Cl-MSN
material by the reaction of Cl-MSN with 2-aminopyridine fol-
lowed by complexation with palladium acetate (Scheme 1).63
(Pd-AMP-MSN) (ꢁ0.01 mg) was dispersed in distilled and
filtered ethanol (1 mL).
3. Results and discussion
3.1 Preparation and characterization of the catalyst (Pd-ASM-
MSN)
(Chloromethyl)phenylethyl (CMPE) functionalized meso-
porous silica nanospheres (Cl-MSN) were prepared by a
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Table 1 Screening of reaction parameters for Suzuki–Miyaura cross-
coupling reaction catalyzed by Pd-AMP-MSNa
TGA thermograms of AMP-MSN and Pd-AMP-MSN materials
(Fig. 4) revealed a weight loss of 32.4% and 33.4% respectively in
the temperature range of 120 ꢂC to 800 ꢂC, whereas a weight loss
of 28.4% was registered for Cl-MSN in the same temperature
range (Fig. 4). Upon comparison of these TGA data, it is esti-
mated that 4% AMP moiety is successfully incorporated into the
Cl-MSN that actually complexed with 1% Pd in the Pd-AMP-
MSN material.
Temperature
(ꢂC)
Entry
Solvent
Base
Yield (%)b
The FESEM image of Cl-MSN shows spherical particles with
diameters ranging from 166–381 nm. The FESEM image of the
Pd-AMP-MSN catalyst (Fig. 5) also revealed spherical particles
with diameters in the range of 187–327 nm, indicating that there
is no shape change due to PdII incorporation. FESEM-EDX
analysis of the Cl-MSN (Fig. S1 in the ESI†) reveals the presence
of carbon along with Si and O in an atom% ratio of
2.4 : 53.1 : 44.5, which further suggests successful grafting of
CMPE functionality. Furthermore, the FESEM-EDX spectrum
of Pd-AMP-MSN material (Fig. S1 in the ESI†) reveals the
presence of Pd along with C, Si and O with the atom% ratio of
C : O : Si : Pd ¼ 46 : 36 : 14 : 2. The TEM image again confirms
the nanosphere shape of the Pd-AMP-MSN catalyst (Fig. 6).
High magnification TEM image (Fig. 6) clearly shows the pres-
ence of randomly oriented mesopores in the Pd-AMP-MSN
nanospheres. From TEM and FESEM images, it seems that the
nanospheres are somehow aggregated after functionalization.
But, this aggregation actually takes place during sample prepa-
ration. A Pd-AMP-MSN sample suspension was examined via
dynamic light scattering (DLS) to provide further support that
they are not aggregated or coalesced in suspension. The log
normal distributions of the sizes of the nanospheres indicate that
the hydrodynamic diameter (Dh) of the Pd-AMP-MSN is 256 nm
(Fig. S2 in the ESI†). The Dh value correlates well with the
diameters measured from FESEM and TEM.61 These results
indicate that there is no aggregation of the Pd-AMP-MSN
catalyst in aqueous suspension.
1
2
3
4
5
6
7
8
DMF
Water
EtOH
DMSO
CH3CN
THF
Water
Water
Water
Water
Water
Water
Water
Water
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
Na2CO3
NaHCO3
NaOAc
KF
80
80
80
80
80
80
80
80
80
80
80
80
25
50
99
99
72
65
39
28
59
80
66
25
30
11
Trace
54
9
10
11
12
13
14
NEt3
K2CO3
K2CO3
a
Reaction conditions: Iodobenzene (1.0 mmol), phenylboronic acid (1.2
mmol), base (2.0 mmol), and catalyst (0.03 g) and solvent (6 mL).
Yield was determined by GC analysis.
b
FTIR spectra of both AMP-MSN and Pd-AMP-MSN mate-
rials (Fig. 1) reveal bands at around 1087 cmꢀ1 (due to Si–O–Si
linkages of the silica network), 1633 cmꢀ1 and 1661 cmꢀ1 (due to
the aromatic moiety),64 3429 cmꢀ1 (due to N–H symmetric
stretching of 2-aminopyridine) and around 794 cmꢀ1 (due to
substituted aromatic ring).65,66 The appearance of all these bands
suggests the successful synthesis of AMP-MSN and Pd-AMP-
MSN materials.67 The XRD pattern of Cl-MSN material (Fig. 2)
featured only an intense (100) reflection peak at around 2q ¼
2.75ꢂ, indicating the presence of ordered channel like meso-
pores,68 even after incorporation of the organic moiety. XRD
patterns of AMP-MSN and Pd-AMP-MSN materials (Fig. 2)
also reveal an intense (100) reflection peak at around 2q ¼ 2.75ꢂ.
The appearance of this diffraction peak also suggests the pres-
ence of a mesoporous network in AMP-MSN and Pd-AMP-
MSN materials. The N2 adsorption–desorption isotherms at 77
K of the Cl-MSN materials reveal typical type IV isotherms
which suggests the presence of a mesoporous network.62 BET
specific surface area (SBET), (calculated by the BET analysis of
the isotherms) of Cl-MSN (SBET ¼ 1200 m2 gꢀ1) is quite high
with narrow pore size distributions of average pore diameter 1.96
nm and a total pore volume of 0.703 cm3 gꢀ1, respectively. The
N2 adsorption–desorption isotherm at 77 K of the Pd-AMP-
MSN material (Fig. 3A) also reveals a typical type IV isotherm,
characteristic of mesoporous materials.62 As expected, the SBET
3.2 Catalytic performances of Pd-AMP-MSN catalyst in
Suzuki–Miyaura cross-coupling reaction in water
Next, we evaluated this mesoporous silica nanosphere supported
palladium(II) complex, Pd-AMP-MSN, as a heterogeneous
catalyst in Suzuki–Miyaura cross-coupling reactions (Scheme 2).
Initial trials were performed to optimize the reaction conditions
for the coupling reaction of bromobenzene and phenylboronic
acid. We optimized several reaction parameters, including the
solvent, base and reaction temperature. We screened various
solvents in the model reaction. Among them, DMF and water
were found to be the best suitable media (yield ¼ 99%) for this
catalyst (Table 1, Entries 1 and 2). Other solvents, such as EtOH,
DMSO, CH3CN, and THF were not satisfactory (Table 1,
Entries 3–6). The Pd-AMP-MSN material was found to be very
stable in air and moisture and no change in its composition as
well as catalytic activity was observed when it was subjected to
the Suzuki–Miyaura cross-coupling reaction in air and water.
Water was used instead of organic solvent because it is non-toxic,
cheap, readily available, and environmentally benign.69,70 From a
greener point of view, water was used as the solvent for further
studies. Next, we examined the effects of bases on the Suzuki–
Miyaura cross-coupling reaction in water medium. The inorganic
of the Pd-AMP-MSN (372 m2
g
ꢀ1) (calculated by the BET
analysis of the isotherms) is much lower than that (SBET ¼ 1200
m2 gꢀ1) of Cl-MSN due to the incorporation of the palladium
complex. The Pd-AMP-MSN material also exhibits narrow pore
size distributions with an average diameter of around 1.92 nm
and a total pore volume of 0.172 cm3 gꢀ1 (Fig. 3B, estimated by
BJH analysis of the isotherms). The average pore size and the
total pore volume of the Pd-AMP-MSN catalyst also decreased
very slightly from 1.96 to 1.92 nm and 0.703 to 0.172 cm3 gꢀ1
.
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Table 2 The Suzuki–Miyaura cross-coupling reactions of aryl bromides with phenylboronic acid catalyzed by Pd-AMP-MSNa
Productc
Entry
Aryl halides
Time (h)
Yield (%)b
1
4
99
98
2
4
3
4
5
4
4
6
98
99
96
6
7
8
6
4
4
90
98
96
9
6
6
94
93
10
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Yield (%)b
88
Table 2 (Contd.)
Entry
Aryl halides
Productc
Time (h)
11
12
6
10
80
13
10
82
a
Reaction conditions: haloarenes (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2 mmol), catalyst (0.03 g), water (6.0 mL), and 80 ꢂC. b Yield of
product was determined by GC and GC-MS analysis using dodecane as the internal standard. c Products were identified by comparison of GC-MS, and
1H NMR spectral data with those reported in the literature.
Table 3 Comparison of the catalytic activity of aryl bromides or iodides
and phenylboronic acid with the Pd-AMP-MSN catalyst and other
catalysts reported in the literature
bases including K3PO4, K2CO3, Na2CO3, NaHCO3, NaOAc
and KF were investigated (Table 1, Entries 2, 7–11). As shown
in Table 1, K2CO3 was the best base for the reaction. Also, the
organic base NEt3 was studied but an unsatisfactory yield was
obtained (Table 1, Entry 12). This coupling reaction was found
to be highly sensitive to the reaction temperature. At lower
temperatures (25 and 50 ꢂC) only low to moderate yield (<54%)
was obtained (Table 1, Entries 13 and 14). At a reaction
temperature of 80 ꢂC, a maximum yield (99%) was obtained
and it was found to be optimal for the model reaction (Table 1,
Entry 2).
Temperature
(ꢂC)
Entry Catalyst
Time (h) Yield (%) Ref.
1
2
3
4
5
6
Pd-AMP-MSN
80
90
100
50
6
9
12
12
6
96
68
86
90
95
67
Present
72
38
73
74
pEVPBr-Pd
Pd-1/FSG
Fe3O4–Pd
To further verify the generality of the Pd-AMP-MSN catalyst
in the Suzuki–Miyaura cross-coupling reaction, we explored the
substrate scopes from aromatic to heterocyclic halides under the
optimized conditions. Treatment of phenylboronic acid with a
series of iodinated or brominated aromatics furnished the cor-
responding coupling products in good to excellent yields (90–
99%) (Table 2, Entries 1–9). The catalytic efficiency was not
significantly affected by the substituents on the aromatic ring of
the halides. It is clearly seen from Table 2 that the Suzuki–
Miyaura cross-coupling reaction of phenylboronic acid and aryl
iodides or aryl bromides bearing either electron donating or
electron withdrawing substituents afforded the corresponding
biphenyls in excellent yields (82–99%) in presence of Pd-AMP-
MSN catalyst in water. The Suzuki–Miyaura cross-coupling
reaction of sterically hindered aryl halides could also proceed
smoothly affording the desired coupled product in good yield
(94%) after 12 h (Table 2, Entry 9). A further extension of scopes
to heteroatom containing substrates was also investigated. At the
expense of times up to 10 h brominated heterocyclic derivatives
afforded good yields (>88%) when treated with phenylboronic
acid under the optimized conditions (Entries 10, 11). Notably,
the less reactive and less expensive phenyl chloride also showed
MCM-41-2N–PdII 90
Pd/NF 300
80
12
13
Fig. 7 Recycling efficiency for the Suzuki–Miyaura cross-coupling
reaction of bromobenzene and phenylboronic acid over Pd-AMP-MSN
catalyst.
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moderate reactivity (yield ¼ 82–83%) in the presence of the
catalyst (Table 2, Entries 12and 13). No reaction occurred in the
absence of the catalyst.
AMP-MSN) catalyst from Cl-MSN by a simple co-condensation
reaction followed by complexation. The catalytic activity of this
supported catalyst was investigated towards Suzuki–Miyaura
C–C cross-coupling reactions between aryl halides and boronic
acid compounds in water. The catalyst exhibited unprecedented
stability among Pd heterogeneous catalysts and can be reused
several times to achieve complete conversion without any addi-
tional activation treatment. The excellent catalytic performance
in water medium and the easy preparation and separation of the
catalyst makes it a good heterogeneous system and a useful
alternative to other heterogeneous palladium catalysts.
Table 3 shows the various palladium catalyst systems which
have been tested in the Suzuki–Miyaura cross-coupling of
various aryl bromides or iodides with phenylboronic acid to
produce the corresponding biaryl compounds solely in
water.38,71–74 Compared to the catalyst systems given in Table 3,
the in situ generated Pd-AMP-MSN material shows a higher
catalytic activity (yield ¼ 96%) in the coupling of aryl bromides
with phenylboronic acid under the optimized conditions.
3.3 Heterogeneity test
Acknowledgements
To check the leaching of metal into the solution during the
reaction, hot-filtration test was performed with Pd-AMP-MSN
catalyst for the Suzuki–Miyaura cross-coupling reaction of
bromobenzene with phenylboronic acid. After continuing the
reaction for 2 h, the catalyst was removed by filtration and the
conversion was determined to be 48%. The resulting filtrate was
subjected to heating for a further 3 h. GC results revealed that no
reaction occurred when the reaction was conducted with the
filtrate obtained after the removal of the solid catalyst. This
supported the observation that the active catalyst species did not
leach from the solid catalyst. The absence of palladium in the
filtrate was also confirmed by UV-vis spectroscopy and atomic
absorption spectroscopic (AAS) analysis. UV-vis spectra of the
filtrate did not show any absorption peaks characteristic of
palladium metal, indicating that the leaching of metal did not
take place during the course of the reaction and no palladium
was detected by AAS analysis of the filtrate after removal of the
catalyst. The FTIR spectrum of the used catalyst was quite
similar to that of the fresh sample. These studies clearly
demonstrated that the palladium was intact to a considerable
extent with the heterogeneous support, and no significant
amount of leaching occurred during reaction.
S. M. I. acknowledges Department of Science and Technology
(DST), Council of Scientific and Industrial Research (CSIR) and
University Grant Commission (UGC), New Delhi, India for
funding. This research was also partially supported by the grants
from the DST, New Delhi under the Nanoscience and Nano-
technology Initiative. P. M. thanks CSIR, New Delhi for her
respective senior research fellowships. S. B. thanks CSIR, Govt
of India, for providing Fellowships.
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