Anchoring of palladium(II) in chemically modified mesoporous silica: An efficient heterogeneous catalyst for Suzuki cross-coupling reaction

Article abstract of DOI:10.1016/j.ica.2010.07.076

The synthesis and characterization of a highly efficient and reusable catalyst, Pd(II) immobilized in mesoporous silica MCM-41, are described. Pd(II) Schiff-base moiety has been anchored onto mesoporous silica surface via silicon alkoxide chemistry. The catalyst has been characterized by small-angle X-ray diffraction (SAX), FTIR and electronic spectroscopy as well as elemental analysis. The catalyst is used in Suzuki cross-coupling reaction of various aryl halides, including less reactive chlorobenzene, and phenylboronic acid to give biaryls in excellent yields without any additive or ligand. High selectivity for the bi-aryl products containing both electron-donating and electron-withdrawing substituents, mild reaction conditions and possibility of easy recycle makes the catalyst highly desirable to address the industrial needs and environmental concerns.

Full text of DOI:10.1016/j.ica.2010.07.076

Inorganica Chimica Acta 363 (2010) 3993–3999
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Inorganica Chimica Acta
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Anchoring of palladium(II) in chemically modified mesoporous silica: An efficient
heterogeneous catalyst for Suzuki cross-coupling reaction
*
Susmita Bhunia, Rupam Sen, Subratanath Koner
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2010
Received in revised form 19 July 2010
Accepted 27 July 2010
Available online 1 August 2010
The synthesis and characterization of a highly efficient and reusable catalyst, Pd(II) immobilized in mes-
oporous silica MCM-41, are described. Pd(II) Schiff-base moiety has been anchored onto mesoporous sil-
ica surface via silicon alkoxide chemistry. The catalyst has been characterized by small-angle X-ray
diffraction (SAX), FTIR and electronic spectroscopy as well as elemental analysis. The catalyst is used
in Suzuki cross-coupling reaction of various aryl halides, including less reactive chlorobenzene, and phen-
ylboronic acid to give biaryls in excellent yields without any additive or ligand. High selectivity for the bi-
aryl products containing both electron-donating and electron-withdrawing substituents, mild reaction
conditions and possibility of easy recycle makes the catalyst highly desirable to address the industrial
needs and environmental concerns.
Keywords:
Suzuki reaction
Immobilization
Heterogeneous catalysis
Catalyst recycling
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
ores with a pore diameter in the range of 20–100 Å. As regards
the preparation and characterization of porous silica-based heter-
Suzuki coupling is an important reaction in organic chemistry
for the selective synthesis of biaryls [1]. Biaryls have widespread
applications in the synthesis of natural products, pharmaceuticals,
and advanced materials [2–5]. The utility of the Suzuki reaction
comes from its relative thermal stability, insensitivity to air or
moisture, broad functional group tolerance, as well as low toxicity
associated with boron compounds. Many palladium complexes
have been used as homogeneous catalysts for Suzuki reaction [6–
8]. Although homogeneous catalytic systems are known to exhibit
better activity than heterogeneous systems but for the large scale
applications in liquid phase reactions, it causes greater difficulties
such as purification of the final product, recycling of the catalyst,
and deactivation via aggregation into inactive Pd particles. Further
removal of Pd from organic products at the end of the reaction is
highly desirable because of its high cost and toxicity. So the devel-
opment of heterogeneous systems where the metal is grafted on
inorganic or organic supports has attracted much attention in
recent years [9–11]. The mesoporous silica-based materials desig-
nated as M41S have attracted a lot of attention in catalysis study
[12] since its invention by the scientists of Mobil Corporation
[13,14]. MCM-41 is one of the members of this family having
uniform hexagonal arrays of one-dimensional channels of mesop-
ogeneous catalyst for cross-coupling reactions few studies deserve
particular mention. It is reported that catalysts derived from
anchoring of Pd(II) complex into MCM-41 showed high catalytic
activity and recyclability for the Mizoroki–Heck reaction [15,16].
High activity and selectivity in the carbon–carbon coupling reac-
tion is also obtained using catalyst prepared by immobilization
of Pd on silica modified with methyl or phenyl groups [17]. Pd
nanoparticles dispersed on amine-functionalized zeolites and mes-
oporous silica are found to be highly active in Heck reactions
[18,19]. Recently, we have developed Pd(0) immobilized mesopor-
ous silica catalyst which efficiently catalyzes various C–C coupling
reactions like Heck, Sonogashira, Suzuki, Stille reactions [20,21].
Palladium complexes have been immobilized on mercaptopropyl-
functionalized SBA-16 and KIT-6 to catalyze Suzuki–Miyaura reac-
tion [22].
Wiedermann reported a series of square-planar trans-dichloro
palladium(II) complexes containing N-(2-thienylmethylene)-ani-
line and N-(2-thienylmethyl)-aniline ligands that exhibit high cat-
alytic activity in the Suzuki cross-coupling reaction [23]. Inspired
by this ligand system, we have synthesized heterogeneous catalyst
using similar type of ligand system.
Herein, we describe preparation, characterization and catalytic
efficacy of a new heterogeneous palladium based catalyst for Suzu-
ki cross-coupling reaction. The catalyst is active towards various
aryl halides including less active chlorobenzene as well as para-
chloroacetophenone under mild reaction conditions.
* Corresponding author. Tel.: +91 33 2414 6666x2778; fax: +91 33 2414 6223.
E-mail addresses: snkoner@chemistry.jdvu.ac.in, skoner55@hotmail.com
(S. Koner).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.076

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S. Bhunia et al. / Inorganica Chimica Acta 363 (2010) 3993–3999
nitrogen (CHN), was undertaken on a Perkin Elmer 240C elemental
2. Experimental
2.1. Materials
analyzer. Other instruments used in this study were the same as
reported earlier [25,26]. 1H NMR spectra were measured on a Bru-
ker Avance DPX 300 NMR (300 MHz) spectrometer using TMS as
the internal standard. To study the progress of the reaction the
products were collected at different time intervals and identified
and quantified by a Varian CP-3800 Gas chromatograph using a
CP-Sil 8 CB capillary column.
PdCl2. (>99.9%), tetraethyl orthosilicate (TEOS, 98%), the cationic
surfactant cetyltrimethylammonium bromide (CTAB, 98%),
3-mercaptopropyl-functionalized silica (SH–SiO2) [sulfur content
of SH–SiO2 = 3.95 wt%], and all other reagents were purchased
either from Sigma–Aldrich/Fluka or Alfa-Aesar and were used as
received without further purification. The solvents were purchased
from Merck (India) and were distilled and dried before use.
2.7. General experimental procedure for Suzuki cross-coupling reaction
The coupling reaction was carried out in a glass batch reactor.
At first aryl halide (3 mmol) and phenylboronic acid (0.4 g,
3.3 mmol) were dissolved in 4 ml ethanol. This reactant mixture
was then added to the solution of Na2CO3 (0.318 g, 3 mmol) in
1 ml of water under stirring condition. To this 0.01 g of Pd(II)–
MCM-41 catalyst was added and the reaction mixture was heated
to 60 °C in an oil bath. For isolation of products at the end of the
catalytic reaction, the catalyst was first separated out by filtration
and then the filtrate was extracted four times with diethyl ether
(4 Â 10 ml). The organic layers thus collected were combined and
washed with water to remove excess Na2CO3 and finally washed
with brine and dried over Na2SO4, filtered and concentrated in
vacuo. The residue was purified by column chromatography over
silica gel (mesh 60–120) using n-hexane/ethyl acetate mixture as
eluent to give the desired product. The product was analyzed by
GC and 1H NMR. All of the products were well-known and reported
in the literature. The spectroscopic data (1H NMR) of these prod-
ucts are consistent with those previously reported values.
2.2. Synthesis of siliceous MCM-41
MCM-41 was synthesized according to the literature method
[24] by the hydrolysis of the structure directing agent CTAB (cetyl-
trimethylammonium bromide) and tetraethyl orthosilicate (TEOS),
in basic solution with a molar composition of the reactants 1.0
CTAB:7.5 TEOS:1.8 NaOH:500 H2O. The gelatinous mixture was
then hydrothermally treated at 110 °C for 60 h in a Teflon lined
autoclave. After cooling to room temperature the resultant solid
was recovered by filtration, washed with deionized water and
dried in air. The collected product was calcined at 823 K for 8 h
to remove the occluded polymeric surfactants.
2.3. Organic modification of MCM-41 with (3-aminopropyl)-
triethoxysilane (3-APTES)
Post synthesis organic modification of the mesoporous material
was performed by stirring 0.3 g of MCM-41 with 0.004 mmol of
3-APTES in dry chloroform (20 ml) at room temperature for 12 h
under nitrogen atmosphere. The resulting white solid MCM-41-
(SiCH2CH2CH2NH2)x was filtered, and washed with chloroform
followed by dichloromethane and dried in air.
2.8. Heterogeneity tests
2.8.1. Hot filtration test
A
mixture of iodobenzene (3 mmol), phenylboronic acid
(0.4 g, 3.3 mmol), Na2CO3 (0.318 g, 3 mmol), Pd(II)–MCM-41
(2.82 Â 10À3 mol%) in 5 ml of 20% H2O/EtOH was stirred at 60 °C
and the progress of the reaction was monitored by GC analysis.
After 30% completion of the reaction (analyzed by GC), the catalyst
was separated at the reaction temperature (to avoid re-deposition
of leached Pd species on the catalyst surface during cooling of the
reaction mixture) by centrifugation and the reaction solution was
stirred at that temperature for another 2 h. There was no further
increase in the product concentration, as determined by GC
analysis.
2.4. Schiff base generation in APTES modified MCM-41 using
2-thiophenecarboxaldehyde
The white solid, MCM-41-(SiCH2CH2CH2NH2)x was refluxed
with 50 mg (0.45 mmol) of 2-thiophenecarboxaldehyde in metha-
nol at 60 °C for 8 h. The resulting cream colored solid was filtered,
washed with methanol followed by dichloromethane and dried in
air.
2.5. Anchoring of Pd(II) in modified MCM-41
2.8.2. Solid-phase poisoning test
A mixture of SH–SiO2 (0.10 g, 0.12 mmol) and 0.01 g of Pd(II)–
MCM-41 catalyst were taken in the solution containing arylhalides
(3 mmol), phenylboronic acid (0.4 g, 3.3 mmol) and Na2CO3
(0.318 g, 3 mmol) in 5 ml of 20% H2O/EtOH. The mixture was stir-
red continuously maintaining the temperature of the oil bath at
60 °C. The progress of the reactions was monitored by the method
described above.
Pd(II)–MCM-41 was prepared by refluxing Schiff base contain-
ing cream colored solid with 5 mg of PdCl2 in 20 ml acetone for
72 h. The light yellow solid thus formed was then filtered, dried
and washed with acetone using Soxhlet for 12 h to remove any
unanchored palladium species and dried under vacuum. The Pd
content of the catalyst found to be 0.3 (wt%) (2.82 Â 10À3 mol%).
Elemental analysis showed Pd:C and Pd:N were 15.62 and 1.84,
respectively.
3. Results and discussion
2.6. Catalyst characterization
3.1. XRD studies
The powder X-ray diffraction (XRD) patterns of the samples
were recorded on a Scintag XDS-2000 diffractometer using Cu K
a
The small-angle X-ray diffraction patterns of MCM-41 and the
catalyst Pd(II)–MCM-41 are shown in Fig. 1. The pristine MCM-
41 shows a typical three-peak pattern [14,27] with a very intense
d100 = 38.85 Å diffraction peak at 2h ꢀ 2.27° and two other weaker
peaks at 2h ꢀ 3.99° and 2h ꢀ 4.58° for d110 and d200, respectively.
MCM-41 shows an additional peak at 2h ꢀ 6.16° which can be
assigned to d210 reflection. All the peaks are well-resolved
radiation. Fourier transform infrared (FTIR) spectra of KBr pellets
were measured on a Perkin Elmer RX1 FTIR spectrometer. Elec-
tronic spectra were measured on a Shimadzu CP-3101 UV–Vis
spectrophotometer. The palladium content of the sample was
determined by Perkin Elmer A-Analyst 200 atomic absorption
spectrometer. Elemental analysis, for carbon, hydrogen, and

S. Bhunia et al. / Inorganica Chimica Acta 363 (2010) 3993–3999
3995
Fig. 2. IR spectra of: (a) MCM-41; (b) MCM-41-(SiCH2CH2CH2NH2)x; (c) Pd(II)–
MCM-41.
Fig. 1. Small-angle XRD pattern of: (a) MCM-41; (b) Pd(II)–MCM-41.
indicating MCM-41 has a well defined hexagonal symmetry and
have long-range ordering of structure. Comparison of X-ray pow-
der diffraction patterns of Pd(II)–MCM-41 and MCM-41 indicates
that the mesoporosity of the material remains intact even after for-
mation of the Pd complex in MCM-41 matrix although some de-
gree of amorphization takes place in the catalyst. Upon post
synthetic grafting, the diffraction lines are shifted to the higher an-
gles and broadening of the diffraction lines occurs. This is possibly
due to the lowering of the local order, such as variations in the wall
thickness, or might be the result of the reduction of scattering
contrast between the channel wall of silicate framework and
palladium complex present in the pores. A similar type of behavior
was observed by Burkett et al. in phenyl modified mesoporous
sieves [28,29] and by Lim and Stein for directly synthesized
thiol-MCM-41 [30]. Therefore, the changes of the diffraction lines
upon anchoring of Pd(II) into MCM-41 are not inconsistent.
Fig. 3. UV–Vis spectrum of Pd(II)–MCM-41.
3.2. FTIR spectra study
*
n ?
p
transitions of C@N and charge transfer transition arising
from
p
electron interactions between the metal and ligand which
The FTIR spectra of the MCM-41, MCM-41-(SiCH2CH2CH2NH2)x
and Pd(II)–MCM-41 are shown in Fig. 2. Presence of various N–H
and C–H vibration bands appeared in the 3100–2800 and
1550–1250 cmÀ1 region in the IR spectrum of MCM-41-
(SiCH2CH2CH2NH2)x that are absent in the case of MCM-41 clearly
indicates that 3-APTES has been anchored into the MCM-41 matrix.
The expected vibration band for azomethine group appears at ca.
1690 cmÀ1 in Pd(II)–MCM-41.
involves ligand to metal electron transfer typical for Pd–Schiff base
complex [31]. Relatively weak band appearing in the visible region
for d–d transitions could not be resolved owing to its very low
extinction coefficient and low palladium content of the catalyst.
Recently Pd(II) Schiff base complexes have been synthesized
and characterized by X-ray structure analysis and employed in
C–C cross-coupling reactions in homogeneous condition [23]. Sim-
ilar type of ligand arrangement is expected in this case also. Spec-
troscopic results and elemental analysis indicates that the
coordination geometry of the complex anchored to the silica wall
of MCM-41, where the nitrogen atom of azomethine group coordi-
nates to the metal centre in monodentate fashion leading to trans-
N–Pd–N and the central metal ion possesses the square-planar
geometry having trans-Cl–Pd–Cl arrangements (Scheme 1).
3.3. UV–Vis spectra
The solid-state electronic spectra of the catalyst Pd(II)–MCM-41
shown in Fig. 3, featured a strong band in the region of 290–
*
p ? p and
370 nm. This band can be ascribed to both the

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S. Bhunia et al. / Inorganica Chimica Acta 363 (2010) 3993–3999
O
O
O
EtO
OH
OH
OH
-3EtOH
a
Si
(CH2)
3NH2
+
Si(CH2)3NH2
EtO
+
S
EtO
3-APTES
CHO
MCM-41 -(SiCH2CH2CH2NH2)x
Silica wall
of MCM-41
b
O
O
O
Si
(CH2)
(CH2)
S
H
C
3N
C
H
O
O
O
c
Si
(CH2)
3 N
Cl
Pd
Cl
S
H
C
O
O
O
Si
3 N
S
Pd(II)-MCM-41
Scheme 1. (a) Modification of MCM-41 channel wall: APTES/chloroform; (b) condensation with 2-thiophenecarboxaldehyde in methanol; (c) metal complex formation:
PdCl2/acetone/reflux.
3.4. Suzuki cross-coupling reaction
condition. However, para-chlorophenol gives low yield in this
reaction. For Pd(II)–MCM-41, sodium carbonate is the best choice
as base, although triethylamine gave comparable yield in the cou-
pling reaction between iodobenzene and phenylboronic acid
(yield ꢁ 97%). In case of sodium acetate yield was very poor
(yield ꢁ 10%). In order to compare the efficacy of the catalyst
Pd(II)–MCM-41 with the analogous Pd(II)–Schiff base complex
used in homogeneous medium, we have synthesized the complex
containing similar [PdN2Cl2] chromophore (namely dichloro-
In Suzuki cross-coupling reaction, the catalyst is able to activate
the non-substituted as well as substituted aromatic halides con-
taining both electron-withdrawing and electron-donating groups
giving high yield of products under mild reaction conditions
(60 °C). The results of Suzuki cross-coupling reactions are shown
in Table 1. Among non-substituted aryl halides, iodobenzene and
bromobenzene (Table 1, entries 1 and 2) showed 100% conversion
with a 99% yield in 2–3 h. Aryl chlorides are of great interest due to
their low price and large availability. Generally it is difficult to acti-
vate C–Cl bonds because of their relative inertness. Suzuki coupling
of chlorobenzene with phenylboronic acid under both homoge-
neous and heterogeneous conditions over a variety of catalysts
has been studied in the recent past (Table 2). Zhou et al. have
shown Suzuki coupling of chlorobenzene by water-soluble
Pd(II)–diimine complex gives only 20% yield [32]. Further,
[PdBr2{et(impy)}2] complex of ligand yields no desired products
in case of chlorobenzene [33]. Palladium–guanidine complex
immobilized on SBA-16 has been used as catalyst in Suzuki cou-
pling of chlorobenzene that affords a yield of ca. 8% [34]. Recently
Dey et al. have used molecular sieves-supported palladium chlo-
ride in Suzuki coupling reaction of chloroarenes [35]. The yield of
chlorobenzene is satisfactory, but the reaction occurs at high tem-
perature (120 °C) in DMF medium. However, it is established that
leaching of Pd often occurs in C–C coupling reaction if DMF is used
as a solvent [36]. It is, therefore, desirable to avoid DMF being used
as solvent. It is noteworthy to mention that we have succeeded to
improve the yield of chlorobenzene in contrast to our recent work
using Pd(0)–MCM-41 as catalyst [21]. Chlorobenzene exhibited a
comparable yield in Suzuki reaction using (tert-butylarylphos-
phino)-polystyrene supported Pd catalyst under non-anhydrous
heterogeneous conditions [37]. Among the para-substituted
substrates, 4-bromoanisole, 4-iodoanisole, 4-nitro bromobenzene,
4-bromophenol, 4-iodophenol and para-bromoacetophenone
afford enhanced activity in Suzuki reactions. Pd(II)–MCM-41 is
able to activate para-chloroacetophenone also in the same reaction
bis-j-N-[N-(2-thienylmethylene)-1-propaneamine] palladium(II))
according to the literature method [23] and its catalytic activities
have been studied (Table 3). Although the conversion in Suzuki
coupling reactions for both the catalysts is comparable, Pd(II)–
MCM-41 showed markedly higher turnover.
Pd incorporated NaY zeolite [38] and Pd/CeO2 [39] have been
used as heterogeneous catalyst in cross-coupling reactions. Cai
et al. have used MCM-41 supported sulfur palladium(0) complex
in Suzuki coupling reaction [40]. However, the turn over numbers
(TON) in C–C coupling reaction for these catalysts are reported to
be low.
The catalyst Pd(II)–MCM-41 is not thermal, air or moisture
sensitive and so there was no need to carry out the reaction under
inert atmospheric condition. The catalyst is easily recoverable by
filtration and can be reused several times without any loss of cat-
alytic activity. For the recycling study, Suzuki coupling reaction
was performed with iodobenzene and phenylboronic acid main-
taining the same reaction condition. After first cycle of reaction
the catalyst was recovered by centrifugation and then washed
thoroughly with diethylether followed by copious amount of water
to remove the base present in the used catalyst and finally by
dichloromethane. The recovered catalyst was dried under vacuum
at 110 °C overnight. The performance of the recycled catalyst in
C–C coupling reactions up to four successive runs is summarized
in Table 4. The catalytic efficacy of the recovered catalyst remains
almost same in every run as shown in Fig. 4.
To ascertain the catalysis is indeed heterogeneous we have
performed hot filtration test and solid phase poisoning test. In

S. Bhunia et al. / Inorganica Chimica Acta 363 (2010) 3993–3999
3997
Table 1
Pd(II)–MCM-41 catalyzed Suzuki cross-coupling reaction of aryl halides and phenylboronic acid.a
B(OH)2
X
Pd(II)-MCM-41
+
R
20%H2O/EtOH
Na2CO3, 60 oC
R
R =
H, OCH3, COCH3 , NO2 ,
OH
X = I, Br, Cl
Entry
1
Aryl halide
Product
Time (h)
2
Conversionb (wt%)
100
Yieldc (wt%)
99
TOFd (hÀ1
)
5320
3546
414
Ph
Ph
Ph
I
2
3
4
3
24
12
100
93
99
60
96
Br
Cl
I
98
872
Ph
OMe
MeO
MeO
5
6
20
24
97
74
92
54
518
329
Ph
OMe
Br
Br
Ph
COCH3
H3COC
H3COC
O2N
7
24
21
17
93
Ph
COCH3
Cl
Br
Br
8
9
12
24
100
61
99
57
890
271
Ph
NO2
OH
Ph
HO
HO
HO
10
11
24
24
100
19
99
2
445
84
Ph
Ph
I
OH
Cl
OH
a
Reaction conditions: 3 mmol of aryl halide, 3.3 mmol of phenylboronic acid, 3 mmol of Na2CO3, 0.01 g of Pd(II)–MCM-41 (Pd content: 2.82 Â 10À3 mol%) in 20% H2O/EtOH
at 60 °C temperature.
b
Conversion of reactant is determined by GC.
Isolated yields were calculated from the mass of the product after separation by column chromatography. All the isolated products showed more than 99% of GC
c
purity.
d
TOF (turnover frequency) = moles converted/(mol of active site  time).

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S. Bhunia et al. / Inorganica Chimica Acta 363 (2010) 3993–3999
Table 2
Suzuki coupling of chlorobenzene with phenylboronic acid catalyzed by a variety of catalysts.
Entry
Catalyst
Time (h)
Base
Temp. (°C)
Yield (wt%)
Ref.
1
2
3
4
5
6
Pd(II)–MCM-41
Pd(0)–MCM-41
Pd(II)–diimine complex
[PdBr2{et(impy)}2]
Pd–L/SBA-16
24
24
24
16
3
Na2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4
K2CO3
60
80
120
85
50
120
60
24
20
0
8
84
This study
[21]
[32]
[33]
[34]
Molecular sieves-supported Pd catalyst
18
[35]
L = 1,1,3,3-tetramethyl-2-(3-trimethoxysilylpropyl)-guanidine, et(impy) = ethylene bridged imidazo[1,5-a]pyridine-3-ylidenes.
Table 3
Suzuki coupling of aryl halides and phenylboronic acid catalyzed by Pd-Schiff base complex and Pd(II)–MCM-41a.
Entry
1
Substrate
Time (h)
2
Catalyst
Conversionb (wt%)
Yieldc (wt%)
TOFd (hÀ1
)
Pd–Schiff base complex
Pd(II)–MCM-41
100
100
100
99
362
5320
I
2
3
24
24
Pd–Schiff base complex
Pd(II)–MCM-41
71
93
49
60
21
414
Cl
Br
Br
Br
Br
Pd–Schiff base complex
Pd(II)–MCM-41
93
61
64
57
28
271
OH
4
5
6
24
20
12
Pd–Schiff base complex
Pd(II)–MCM-41
83
74
62
54
25
329
COCH3
OMe
Pd–Schiff base complex
Pd(II)–MCM-41
98
97
84
92
36
518
Pd–Schiff base complex
Pd(II)–MCM-41
99
100
92
99
61
890
NO2
a
Reaction conditions: 3 mmol of aryl halide, 3.3 mmol of phenylboronic acid, 3 mmol of Na2CO3, 0.004 mmol of Pd–Schiff base complex (or 0.01 g of Pd(II)–MCM-41: Pd
content 2.82 Â 10À3 mol%) in 20% H2O/EtOH at 60 °C temperature.
b
Conversion of reactant is determined by GC.
Isolated yields were calculated from the mass of the product after separation by column chromatography. All the isolated products showed more than 99% of GC purity.
TOF (turnover frequency) = moles converted/(mol of active site  time).
c
d
hot filtration test, the solid catalyst was filtered off after 30% of the
coupling reaction was completed and the liquid phase of the reac-
tion mixture was kept in reaction condition for another 2 h. No in-
crease in the amount of product was noticed. This result suggests
that Pd is not being leached out from the solid catalyst during
the cross-coupling reactions.
According to Richardson and Jones [41] solid-phase poisoning
test is one of the important tests to ascertain the true heterogene-
ity of the Pd-based catalyst in C–C coupling reactions. We have
performed solid-phase poisoning tests using commercially avail-
able (purchased from Sigma–Aldrich) 3-mercaptopropyl-function-
alized silica (SH–SiO2) as an effective palladium scavenger which
selectively coordinates and deactivates the leached out palladium
[42]. Thereby, cessation of the reaction is expected if catalysis of
the coupling reaction is catalyzed by leached palladium species
from the solid support. A comparison of percentage of conversion
in absence and in presence of solid poison in C–C coupling reac-
tions (Table 5) clearly indicates that the catalytic activity of
Pd(II)–MCM-41 is not affected even in the presence of poisoning
agent SH–SiO2. The kinetic profiles of the typical Suzuki reactions
in the presence or absence of SH-SiO2 are shown in Fig. 5.
100
80
60
40
20
0
Table 4
Recycling of Pd(II)–MCM-41 catalysts in the Suzuki and cross-coupling reaction with
iodobenzene and phenylboronic acida.
Entry
Run
Conversionb (wt%)
Yieldc (wt%)
TOFd (hÀ1
)
1
2
3
4
1st
100
100
99
99
99
98
97
5320
5320
5267
5213
2nd
3rd
4th
98
a
Reactions were carried out in air using 3 mmol of aryl halide, 3.3 mmol of
phenylboronic acid, 3 mmol of Na2CO3, and 0.01 g of Pd(II)–MCM-41 (Pd content:
2.82 Â 10À3 mol%) in 20% H2O/EtOH at 60 °C temperature.
b
Conversion of reactant is determined by GC.
Isolated yields were calculated from the mass of the product after separation by
1
2
3
4
c
No. of cycle
column chromatography. All the isolated products showed more than 99% of GC
purity.
d
TOF (turnover frequency) = moles converted/(mol of active site  time).
Fig. 4. Chart showing recyclability of the catalyst.

S. Bhunia et al. / Inorganica Chimica Acta 363 (2010) 3993–3999
3999
Table 5
Performance of the Pd(II)–MCM-41 catalyst in solid-phase poisoning test.
Aryl halide
Phenylboronic acid
Coupling Reaction
T (°C)
SH–SiO2 added
Conversionb (yield) (wt%)
60
No
Yes
100 (99)
99 (98)
I
B(OH)2
Suzukia
60
No
Yes
97 (92)
96 (91)
OMe
Br
B(OH)2
a
Reactions were carried out in air using 3 mmol of aryl halide, 3.3 mmol of phenylboronic acid, 3 mmol of Na2CO3, 0.12 mmol of SH-SiO2, and 0.01 g of Pd(II)–MCM-41(Pd
content: 2.82 Â 10À3 mol%) in 20% H2O/EtOH at 60 °C temperature.
b
Conversion of reactant is determined by GC.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.07.076.
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4. Conclusion
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Acknowledgements
Financial support from the Ministry of Environment and Forest
(MoEF), Government of India, by a grant (F. No. 19/5/2005-RE) (to
SK) is gratefully acknowledged. Assistance from CAS programme,
Department of Chemistry, Jadavpur University and FIST pro-
gramme, Department of Science and Technology, Govt. of India is
also acknowledged.