Pronounced effect of pore dimension of silica support on Pd-catalyzed Suzuki coupling reaction under ambient conditions

Article abstract of DOI:10.1039/c8nj00254a

The pore size of the mesoporous silica support is found to have an enormous effect on the reactivity of the Pd-catalysts in the Suzuki coupling reaction. Heterogeneous palladium catalysts, MCM-Pd and SBA-Pd, have been synthesized using two different types of silica supports that differ from each other in their pore dimension. The silica surfaces are grafted with aminopropyl silane groups and functionalized with tris(4-formylphenyl)amine engaging one of the formyl groups while the other two formyl groups are reacted with two 2-aminothiophenol moieties. This results in two Schiff base sites with adjacent -SH groups leading to active metal binding sites in the samples and increases the hydrophobicity of the framework. The materials have been characterized by powder X-ray diffraction, nitrogen sorption studies, transmission electron microscopy, thermal analysis, and different spectroscopic techniques. The samples are used as catalysts for Suzuki cross-coupling reactions of aryl halides with phenylboronic acid under ambient conditions. The products of the reactions are identified and estimated by ¹H NMR and gas chromatography. SBA-Pd performs much better compared to MCM-Pd and a yield of up to 98% is achieved in CH₃OH within 24 h of the reactions. This is attributed to the higher pore dimension of SBA-Pd that allows better interaction of the substrate molecules with the active centers on the surface of the pores.

Full text of DOI:10.1039/c8nj00254a

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Page 1 of 26
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DOI: 10.1039/C8NJ00254A
Pronounced effect of pore dimension of silica support on Pd-catalyzed Suzuki
coupling reaction under ambient conditions
Trisha Das,a Hiroshi Uyamab, Mahasweta Nandi* a
aIntegrated Science Education and Research Centre, Siksha Bhavana, Visva-Bharati University, Santiniketan 731
235, India
bDepartment of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita,
Osaka, 565 0871, Japan
Email: mahasweta.nandi@visvaꢀbharati.ac.in
Abstract
Pore size of the mesoporous silica support is found to have enormous effect on the reactivity of the Pdꢀcatalysts in
Suzuki coupling reaction. Heterogeneous palladium catalysts, MCM-Pd and SBA-Pd, have been synthesized
using two different types of silica supports which differ from each other in their pore dimension. The silica
surfaces are grafted with aminopropyl silane groups and functionalized with tris(4ꢀformylphenyl)amine engaging
one of the formyl groups while the other two formyl groups are reacted with two 2ꢀaminothiophenol moieties.
This gives rise to two Schiff base sites with adjacent –SH groups leading to active metal binding sites in the
samples and increases hydrophobicity of the framework. The materials have been characterized by powder Xꢀray
diffraction, nitrogen sorption studies, transmission electron microscopy, thermal analysis, and different
spectroscopic techniques. The samples are used as catalysts for Suzuki crossꢀcoupling reactions of aryl halides
1
with phenylboronic acid at ambient condition. The products of the reactions are identified and estimated by H
NMR and gas chromatography. SBA-Pd performs much better compared to MCM-Pd and yield up to 98% is
achieved in CH3OH within 24 h of the reactions. This is attributed to the higher pore dimension of SBA-Pd that
allows better interaction of the substrate molecules with the active centers on the surface of the pores.

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Introduction
Organic transformations catalyzed by transition metal have gained widespread attention over the years as they
yield valuable products and new functional groups can be introduced into relatively inert substrates. Palladium
catalyzed Suzuki crossꢀcoupling reaction for the formation of the C–C bond is one such commercially important
reaction which has been extensively studied since its first report in 1979.1,2 It has enormous importance in
synthetic organic chemistry3 and different catalytic systems for this reaction have been reported in the literature.4ꢀ6
However, most of the catalytic studies have been carried out in homogeneous systems using palladium complexes
that are soluble in reaction media.7,8 Though homogeneous catalysts generally require lower temperature than
heterogeneous catalysts they have serious disadvantages, like complicated recovery and regeneration process of
the expensive catalyst.9ꢀ12 But for heterogeneous catalysis, the catalyst being in a different phase in the reaction
media, separation of the product and the catalyst can be done easily, mostly simply by filtration or centrifugation.
Thus, heterogeneous catalysts can be recycled and reused for several reaction cycles without significant loss in
their structure or reactivity. Thus, efforts have been made to design safe, effective and recyclable heterogeneous
catalysts for a number of reactions, including CꢀC bond formation reactions.13,14 However, leaching of metal ion
into the reaction media remains a drawback of heterogeneous catalysts. In such cases it becomes difficult to
conclude on the heterogeneity of the catalyst as then it can be both the metal ions leached in to solution or bound
to the solid support that catalyzes the reaction. Thus, the metal ion should be strongly bound to the solid support
such that it does not dissociate from the framework during the liquid phase catalytic reactions.
To prepare a heterogeneous catalyst, a solid support in which the functional groups can be introduced and
the metal atoms can be strongly immobilized is necessary. Different types of catalyst supports are known in the
literature. Some of them include zeolite,15 alumina,16 silica,17ꢀ19 polymer,20ꢀ22 mesoporous carbon,23,24 carbon
nanotubes,25,26 graphene,27,28 metalꢀorganic framework,29,30 metal oxide nanoparticles,31,32 immobilized
nanoparticles,33,34 etc.35,36 Among these, mesoporous silica is often chosen as a catalyst support for its numerous
advantages like high surface area, large pore volume and precise tuning of pore size according to requirement.37
These properties allow suitable interaction of the substrate with the active centers on the surface of the catalyst
and increases the efficiency.13 Moreover, silica framework is rigid, thermally and chemically stable, and
modification can be easily done to introduce different functional groups that enable strong binding of metal ion. A
large number of organic precursors have been used for the synthesis of organically modified hybrid mesoporous
silica, to functionalize their surface and induce hydrophobicity.38,39 They bear suitable donor sites that efficiently
bind metal centers through covalent bonding. The nature of interaction between the metal center and the poreꢀwall
make the materials extraordinarily stable and minimize the possibility of metal leaching during catalytic reactions.
For Suzuki crossꢀcoupling reactions, most of the research has been carried out on development in reaction
scope, like use of aryl chlorides,40,41 nonꢀactivated halides42 and esters43 as substrates, selection of boron reagents
for the reactions,44 effect of ligand45, 46 and ligand free conditions,47 ability to carry out reaction at ambient

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temperature,48, 49 in aqueous medium,21,47,49,50 under solventꢀfree conditions,51 in green solvents,52 with very low
catalyst loadings,53, 54 etc. But, to the best of our knowledge, there are so far no systematic studies reported that
deal with the effect of the pore size of the mesoporous silica support on the activity of Pdꢀcatalysts in Suzuki
coupling reaction. Since the functionalization of the organics and hence anchoring of the metal centers take place
inside the pores of the mesoporous framework, the residual pore space plays an important role in determining the
catalytic performance. It is known that catalytic activity improves with greater surface exposure of the substrates
to the active centers and their accessibility into the porous structure. Temperature also plays an important role
here, as in any reaction, by providing the activation energy; higher the temperature faster is the reaction. Hence at
higher temperatures it is difficult to carry out comparative study of catalysis using systems that differ only in their
pore structure. Thus, catalytic systems studied at room temperature, with judicial choice of donor atoms that binds
strongly with specific metal ions and differing only in pore dimension can add substantially to the understanding
of such CꢀC bond formation reactions.
In this respect, we have synthesized, characterized and studied heterogeneous catalytic properties of two
palladium immobilized mesoporous silica materials, MCM-Pd and SBA-Pd (Scheme 1). The two silica supports
used for this study belong to two different families of silicates. While the MCMꢀ41 type silica55 with pore
diameter of ca. 3.8 nm is synthesized using ionic surfactant template, the SBAꢀ15 silica56 with 7.6 nm size is
prepared using a nonꢀionic blockꢀcopolymer template. The silica surfaces are grafted with aminopropyl silane
moieties and functionalized with trialdehyde, tris(4ꢀformylphenyl)amine employing one of the aldehyde groups.
Incorporation of the bulky organic moiety enhances hydrophobicity of the silica framework and checks its
hydrolysis in polar solvent. The other two aldehyde groups are allowed to undergo Schiffꢀbase condensation with
2ꢀaminothiophenol which results in introduction of multiple imineꢀN and thiol –SH groups in the mesopore wall.
These donor atoms can bind strongly to the palladium species and the soft sulfur center preferentially stabilizes
the soft metal.57,58 Introduction of two binding sites in one functional system in contrast to other systems reported
earlier19,21 with one binding site per functionality increases the number of active sites available for metal binding.
The sequential steps of functionalization and metal loading lead to reduction in the mesopore volume in the
materials. The objective of this work is to study the reactivity of the two heterogeneous catalysts which differ in
their pore structure in Suzuki coupling reactions at roomꢀtemperature. It is indeed found that the pore size and
volume has a huge effect on the catalytic properties.

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Experimental
Materials and physical measurements
All chemicals used in the syntheses are obtained from commercial sources and used without further
purification. The Powder Xꢀray diffraction patterns of the samples have been collected on a Bruker Dꢀ8 Advance
instrument using Niꢀfiltered CuꢀKα radiation (λ = 1.5406 Å) operated at 40 kV and 40 mA. Nitrogen sorption
isotherms of the samples are measured at 77 K using a NOVA 2200e Surface Area and Pore Size Analyzer,
Quantachrome Instruments, USA. Prior to the measurements, the samples are degassed for 8ꢀ12 h at 393 or 423 K
depending on the nature of the framework and the specific surface areas are obtained from the isotherms using
BrunauerꢀEmmettꢀTeller (BET) and Langmuir method. For the pore size distribution profiles, nonꢀlocal density
functional theory (NLDFT) model has been used. The transmission electron microscopic (TEM) images have
been recorded in a JEOL JEMꢀ1400 transmission electron microscope. Before the analysis sample grids have
been prepared by putting one drop of the samples dispersed in C2H5OH on a thin layer of amorphous carbon
coated copper grid of 400 mesh. FTꢀIR spectra of the samples at different stages have been recorded on a Perkin
Elmer spectrometer (Spectrum Two) using attenuated total reflectance (ATR) technique. Solid state 13C CP and
29Si MAS NMR analyses are carried out in a CHEMAGNETICS 300 MHz CMX 300 spectrometer.
Thermogravimetric studies have been done under nitrogen atmosphere (flow rate: 50 cc/min) from room
temperature to 1200 ºC (heating rate of 2 ºC/min) using a Perkin Elmer STA ꢀ6000 thermal analyzer. Palladium
contents of the samples are measured by inductively coupled plasma atomic emission spectroscopy (ICPꢀAES)
using a Shimadzu ICPSꢀ7510 Sequential Plasma Spectrometer. The solutions for analyses are prepared by
digesting 30 mg of the samples in 1 mL of hydrofluoric acid to dissolve the silica part followed by the addition of
1 mL of nitric acid, which solubilizes the materials completely. The volumes are made up to 25 mL with MilliꢀQ
water, the solutions are membrane filtered and fed into the autoꢀsampler of the spectrometer. Gas chromatography
studies are carried out using a Shimadzu GCꢀ2014 gas chromatograph equipped with a fused silica capillary
column and a FID detector. 1H and 13C NMR spectra are recorded in CDCl3 solvent using a Bruker 400 MHz
spectrometer with tetramethylsilane (δ = 0) as the internal standard. The chemical shifts are expressed in parts per
million (δ).
Syntheses of the catalysts
Syntheses of mesoporous MCM-41 and SBA-15 mesoporous silica and their functionalization with 3-aminopropyl
triethoxy silane
For the synthesis of MCMꢀ41,55 an acidic aqueous solution was made by dissolving 0.78 g of tartaric acid in 60 g
of water. Then 2.96 g of cetyltrimethylammonium bromide (CTAB) and 1.5 g of Brijꢀ35 (C12H12 (OC2H4)23OH), a
polyether and aliphatic hydrocarbon chain surfactant) were dissolved in that acidic solution under stirring at room
temperature for 30 minutes. Then 3.5 g of tetraethyl orthosilane was added drop wise to the mixture under

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continuous stirring and the resulting gel was stirred for 2 h. Then 1(M) NaOH solution was added dropwise until
the pH became approximately 11. The resulting white mixture was aged overnight under stirring at room
temperature and then heated at 348 K for 72 h without stirring in a polypropylene bottle. The solid product was
then recovered by filtration, washed several times with water, and dried under vacuum. The resulting powder was
calcined in a flow of air at 723 K for 8 h for complete removal of the organic structure directing agent.
For synthesis of SBAꢀ15,56 1.7 g of Pluronic P123 (a block copolymer, Poly(ethylene glycol)ꢀblockꢀ
poly(propylene glycol)ꢀblockꢀpoly(ethylene glycol)) was dissolved in 62 ml of water (taken in a polypropylene
bottle) under stirring. 6 g of 35% HCl was added to the resulting clear solution and stirred again. Then 3.5 g of
tetraethyl orthosilane was added drop wise to the mixture and stirred at 313 K for 20 h. The resulting gel was then
aged at 373 K for 20 h without stirring. After cooling to room temperature, the product was isolated by filtration
and washed repeatedly with water and C2H5OH and dried at 373 K. The mesoporous silica was then obtained as a
white powder after calcination at 773 K in a flow of air.
Both the mesoporous silica materials, MCMꢀ41 and SBAꢀ15 were functionalized with 3ꢀaminopropyl
triethoxy silane (3ꢀAPTES) by stirring 0.5 g of the respective silica with 0.9 g of 3ꢀAPTES for 12 h in chloroform
at room temperature under N2 atmosphere. The ethoxy groups of 3ꢀAPTES condense with the residual –OH
groups on the surface of the mesoporous silica. The white solid 3ꢀaminopropyl functionalized MCMꢀ41 and SBAꢀ
15 were obtained after filtering the suspension, washing the product repeatedly with chloroform followed by
dichloromethane and drying in air.59
Grafting of tris(4-formyl phenyl) amine on 3-aminopropyl triethoxy silane functionalized MCM-41 and SBA-15
silica
The trialdehyde, tris(4ꢀformyl phenyl) amine, has been synthesized through VilsmeierꢀHaack formylation
reaction of triphenylamine following a reported procedure.60 The 3ꢀAPTES functionalized MCMꢀ41 silica was
refluxed with the trialdehyde dissolved in CH3OH for 4 h (Scheme 1). The mole ratio of amine to trialdehyde was
kept 1:1 so that only one formyl group undergoes Schiff base condensation with the 3ꢀAPTES amine and the
remaining formyl groups in each molecule stay unreacted. The resulting light yellow solid was collected through
filtration, washed repeatedly with CH3OH until the filtrate become colorless and finally dried in a vacuum
desiccator. Similar process was carried out with 3ꢀAPTES functionalized SBAꢀ15 silica to get the corresponding
tris(4ꢀformyl phenyl) amine Schiffꢀbase product.

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+
MCM-Pd/SBA-Pd
Scheme 1. Grafting of tris(4ꢀformyl phenyl) amine on 3ꢀaminopropyl triethoxy silane functionalized silica and
Syntheses of MCM-Pd and SBA-Pd
Incorporation of –N and –S donor sites into tris(4-formyl phenyl) amine grafted mesoporous silica and synthesis
of the catalysts, MCM-Pd and SBA-Pd
The tris(4ꢀformyl phenyl) amine grafted mesoporous MCMꢀ41 silica was treated with 2ꢀaminothiophenol which
undergo Schiff base condensation with the two unreacted –CHO groups (Scheme 1). 2ꢀaminothiophenol was
taken two times with respect to tris(4ꢀformyl phenyl) amine and the suspension in CH3OH was stirred for 6 h at
room temperature. The dark yellow material was recovered by filtration, washed with CH3OH and dried in a
vacuum desiccator. In a similar way, tris(4ꢀformyl phenyl) amine grafted mesoporous SBAꢀ15 silica was also
functionalized with 2ꢀaminothiophenol.
1.0 g of each of the 2ꢀaminothiophenol functionalized silica materials are stirred for 2 h with 0.1 g of
palladium acetate dissolved in 20 mL of CH3OH at room temperature. The dark brown solid catalysts formed
were filtered and washed with CH3OH to remove any unreacted salt and dried under vacuum. The Pdꢀ
immobilized materials thus obtained are named as MCM-Pd and SBA-Pd when the silica support is
functionalized MCMꢀ41 and SBAꢀ15, respectively.

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Catalysis: To determine the efficiency of MCM-Pd and SBA-Pd catalysts in Suzuki coupling reaction several
parameters like solvent, base and reaction time have been explored. In a typical catalytic cycle, 1.0 mmol of the
aryl halide, 1.2 mmol of aryl boronic acid and 2.5 mmol of base were taken in 10 mL of solvent and the mixture
was stirred at room temperature. The reaction was started with the addition of 0.01 g of the catalyst. The reactions
were continued for the desired time and the solid catalyst was separated from reaction mixture by centrifugation.
Then products were extracted in diethyl ether and organic layer was repeatedly washed with brine solution and
water. The solvent was evaporated, the product was purified using column chromatography and from here isolated
1
yield was recorded. Finally the compounds were characterized by H NMR spectroscopy in CDCl3 solvent.
1
Beside H NMR analyses, gas chromatography studies are also carried out to identify the products formed,
determine the yield and turnover frequencies. For reusing the catalysts, the solid separated by centrifugation was
washed repeatedly and then stirred with C2H5OH at room temperature for 2 h to remove any adhered organic
compound from the previous catalytic cycle. After drying, the catalysts were used in the next run following the
same procedure described above. The heterogeneity of the catalysts was checked by performing filtration tests.
For that the catalysts were separated from the reaction mixture after 2 h and then the reactions were continued for
the typical duration.
Results and Discussion
The microstructure of the samples prepared over two different types of supports has been carried out using
powder Xꢀray diffraction study. There are two set of data; one for the MCMꢀ41 series of samples and the other for
the SBAꢀ15 series. The patterns for calcined MCMꢀ41, 3ꢀAPTES functionalized MCMꢀ41, tris(4ꢀformyl phenyl)
amine loaded MCMꢀ41and MCM-Pd are given in Fig. 1. The corresponding diffraction patterns for the SBAꢀ15
family of samples along with SBA-Pd are given in Fig. 2. Both set of samples show 2Dꢀordered hexagonal
mesostructure that is indicated by the presence of three distinct diffraction peaks and can be assigned to 100, 110
and 200 planes along with a weak signal for 210 plane.61, 62 However, with gradual functionalization the ordering
of pores decreased somewhat along with reduction in pore size; these are reflected from the lowering of peak
intensities and shifting of the peaks to higher 2θ values, respectively. The diffraction patterns of MCM-Pd and
SBA-Pd, obtained by attaching 2ꢀaminothiophenol moiety to the functionalized silica supports and then loading
palladium centers over them, remain almost unchanged as compared to the functionalized support which indicates
that there is no significant shrinkage in size of the pores. However, the intensity of the peaks reduces to some
extent, which is again attributed to the lowering of ordering of the pores in the samples in comparison to the silica
support used for further functionalization.

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a
b
c
d
2
4
6
8
10
2θ in Degrees
Fig. 1. Powder Xꢀray diffraction patterns of (a) calcined MCMꢀ41, (b) 3ꢀAPTES functionalized MCMꢀ41, (c)
tris(4ꢀformyl phenyl) amine loaded MCMꢀ41 and (d) MCM-Pd.
a
b
c
d
1
2
3
4
5
2θ
in Degrees
Fig. 2. Powder Xꢀray diffraction patterns of (a) calcined SBAꢀ15, (b) 3ꢀAPTES functionalized SBAꢀ15, (c) tris(4ꢀ
formyl phenyl) amine loaded SBAꢀ15 and (d) SBA-Pd.

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Table 1: Surface area, pore volume and pore size of the samples at various stages
Sl.
No.
Sample
BET
Surface Area
(m2/g)
Langmuir
Surface Area
(m2/g)
Pore
Volume
(cc/g)
Pore
Size
(nm)
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
MCMꢀ41
963
505
241
160
140
704
408
242
177
129
1422
838
375
248
222
1091
661
390
291
207
1.548
0.794
0.576
0.532
0.408
0.958
0.567
0.254
0.27
3.78
2.95
2.59
ꢀꢀꢀ
3ꢀAPTES functionalized MCMꢀ41
Tris(4ꢀformyl phenyl) amine loaded on (b)
2ꢀaminothiophenol grafted on (c)
MCM-Pd
ꢀꢀꢀ
SBAꢀ15
7.588
6.794
5.880
ꢀꢀꢀ
3ꢀAPTES functionalized SBAꢀ15
Tris(4ꢀformyl phenyl) amine loaded on (g)
2ꢀaminothiophenol grafted on (h)
SBA-Pd
(j)
0.192
ꢀꢀꢀ
The nitrogen adsorption/desorption isotherms have been studied at various stages of functionalization of
both kind of silica support which vary in their pore sizes. The isotherms for calcined MCMꢀ41, 3ꢀAPTES
functionalized MCMꢀ41, tris(4ꢀformyl phenyl) amine loaded MCMꢀ41, 2ꢀaminothiophenol grafted tris(4ꢀformyl
phenyl) amine MCMꢀ41 and MCM-Pd are given in Fig. 3. The corresponding isotherms for calcined SBAꢀ15, 3ꢀ
APTES functionalized SBAꢀ15, tris(4ꢀformyl phenyl) amine loaded SBAꢀ15, 2ꢀaminothiophenol grafted tris(4ꢀ
formyl phenyl) amine SBAꢀ15 and SBA-Pd are shown in Fig. 4. The BET (BrunauerꢀEmmettꢀTeller) and
Langmuir surface areas, pore size and pore volume of the samples are given in Table 1. It can be seen in both the
cases that gradual decrease in the surface area takes place after each stage of functionalization. The Langmuir
surface areas are higher compared to BET surface areas which are attributed to the assumptions taken in the
model chosen and the process involved in surface area determination. Both series of samples showed typical type
IV isotherms with steep rise at higher pressure due to capillary condensation indicating the mesoporous nature of
the materials.63, 64 Desorption hysteresis is observed for MCMꢀ41 based samples which arises from intercrystallite
adsorption.65 In case of SBAꢀ15 set of samples, quite broad H1ꢀtype hysteresis can be seen typical of mesoporous
materials with 1D cylindrical channels.66 The average pore diameter of the starting mesoporous silica, MCMꢀ41
and SBAꢀ15 obtained by NLDFT model are ca. 3.8 and 7.6 nm, respectively (Fig. 5). As expected the size of the
pore decreases gradually when functionalization takes place on the walls of the pores. This trend is also followed
by the pore volume and it is noteworthy that though the pore sizes of SBAꢀ15 based samples are higher the
corresponding pore volumes are lower compared to the MCMꢀ41 series of samples.

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1200
800
400
0
a
b
c
d
e
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. 3. Nitrogen sorption isotherms of (a) calcined MCMꢀ41, (b) 3ꢀAPTES functionalized MCMꢀ41, (c) tris(4ꢀ
formyl phenyl) amine loaded MCMꢀ41, (d) 2ꢀaminothiophenol grafted tris(4ꢀformyl phenyl) amine MCMꢀ41 and
(e) MCM-Pd. For clarity, Yꢀaxis values have been made offset by 100 cc/g, 70 cc/g, 130 cc/g and 70 cc/g, for
plot a, b, c and d, respectively. (Adsorption points: filled symbols; desorption points: empty symbols)
800
a
600
b
c
400
d
200
e
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. 4. Nitrogen sorption isotherms of (a) calcined SBAꢀ15, (b) 3ꢀAPTES functionalized SBAꢀ15, (c) tris(4ꢀ
formyl phenyl) amine loaded SBAꢀ15, (d) 2ꢀaminothiophenol grafted tris(4ꢀformyl phenyl) amine SBAꢀ15 and
(e) SBA-Pd. For clarity, Yꢀaxis values have been made offset by 100 cc/g, 70 cc/g, 50 cc/g and 50 cc/g, for plot a,
b, c and d, respectively. (Adsorption points: filled symbols; desorption points: empty symbols)

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1.0
0.8
0.6
0.4
0.2
0.0
a
b
0
4
8
12
16
20
Pore Width (nm)
Fig. 5. Pore size distribution of (a) MCMꢀ41 and (b) SBAꢀ15 using NLDFT model
The BET surface areas of the 2ꢀaminothiophenol grafted tris(4ꢀformyl phenyl) amine MCMꢀ41 and
MCM-Pd are 160 and 140 m2g−1, and the pore volumes are 0.532 and 0.408 cc/g, respectively (Table 1). For the
2ꢀaminothiophenol grafted tris(4ꢀformyl phenyl) amine SBAꢀ15 and SBA-Pd, the BET surface areas are 177 and
129 m2g−1, and the pore volumes are 0.270 and 0.192 cc/g, respectively. The basic character of the samples
remains the same; with typeꢀIV isotherms for all and broader hysteresis for SBAꢀ15 based materials than the
corresponding MCMꢀ41 types. However, the pore sizes of these samples obtained using NLDFT model is very
conclusive. This may happen due to anchoring of excessive functional groups on the pore surface of the silica
support which hinders the access of nitrogen molecules for adsorption on the surface of mesoporous silica.
However, the TEM images of the samples (shown below) confirm that the basic hexagonal arrangement of pores
is retained in the silica supports after all the functionalization and metal loading process.

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Fig. 6. TEM images of (a) MCM-Pd, (b) SBA-Pd, (c) tris(4ꢀformyl phenyl) amine loaded MCMꢀ41 and (d)
tris(4ꢀformyl phenyl) amine loaded SBAꢀ15
The TEM images of MCM-Pd and SBA-Pd, obtained after successive functionalization of MCMꢀ41 and
SBAꢀ15, respectively, have been illustrated in Fig. 6 (a and b, respectively). Both the samples show hexagonal
arrangement of pores having a different contrast with respect to the pore walls. The TEM images of samples at an
intermediate step after tris(4ꢀformyl phenyl) amine is loaded on MCMꢀ41 and SBAꢀ15 has been illustrated in Fig.
6 (c and d, respectively). It is observed that with increase in functionalization the ordering of the pores are
affected to some extent, which is quite natural. However, the basic hexagonal ordering of pore is retained more or
less in both the samples. Thus, from powder Xꢀray diffraction patterns, gas adsorption studies and TEM image
analyses the existence of mesopores in the samples could be established.
The FTꢀIR spectra of the samples at various stages of synthesis using both kind of silica support, MCMꢀ
41 and SBAꢀ15, have been recorded by ATR technique and the results are given in Fig. s1 and s2, respectively.
Both set of samples showed similar kind of characteristic peaks, as expected, since the functional groups
introduced are essentially the same on two different SiO2 matrices which differ only in their pore size (plot a in
both the figures). After functionalization with 3ꢀAPTES the mesoporous silicates (plot b) show a broad band in

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the range of 3600ꢀ2850 cmꢀ1 which can be attributed to the presence of –NH2 and –CH2– groups. For the tris(4ꢀ
formyl phenyl) amine grafted Schiffꢀbase product, new bands appear around 1695 and 1645 cmꢀ1 (plot c). The
band at 1695 cmꢀ1 may be assigned to the unreacted carbonyl groups of tris(4ꢀformyl phenyl) amine and that at
1645 cmꢀ1 originates due to the formation of azomethine moiety. After another Schiff base condensation with 2ꢀ
aminothiophenol (plot d), the band for the carbonyl groups disappear completely indicating conversion of all –
CHO to –C=N. Finally, after palladium acetate binds with the solid supports, the bands for azomethine groups
shift to somewhat lower wavenumber region (ca. 1620 cmꢀ1) indicating the participation of the azomethine bond
in complexation.
13C
d
c
b
a
250
200
150
100
50
0
-50
Chemical Shift (ppm)
Fig. 7. Solid state 13C CPꢀMAS NMR spectra of (a) 3ꢀAPTES functionalized MCMꢀ41, (b) tris(4ꢀformyl phenyl)
amine loaded MCMꢀ41, (c) 2ꢀaminothiophenol grafted tris(4ꢀformyl phenyl) amine MCMꢀ41 and (d) MCM-Pd
Solid state MAS NMR studies often provide valuable information about the structure of new materials
formed, incorporation of organic moieties and metal binding. As established from FTꢀIR spectra that for both type
of SiO2 frameworks the functional groups introduced in the silica matrices at each step of functionalization are
same, the MAS NMR spectra of only MCMꢀ41 based samples have been carried out. The 13C CP MAS NMR
spectra of all the samples except MCMꢀ41 (which does not have any carbon containing counterpart) are shown in
Fig. 7. The spectrum of 3ꢀaminopropyl triethoxy silane grafted silica shows peaks at 8.0, 19.7 and 40.4 ppm (Fig.
7a) that can be attributed to the aliphatic carbons of the aminopropyl units. In the tris(4ꢀformyl phenyl) amine
grafted sample (Fig. 7b) peaks are observed at 10.3, 21.9, 42.3, 61.5, 73.7, 95.1, 96.4, 121.7, 125.8, 127.4, 130.7,
164.8 and 180.4 ppm which indicate the presence of aliphatic and aromatic carbons. This confirms that Schiffꢀ
base reaction has occurred between the amine group of the amino propyl moiety and the aldehyde of tris(4ꢀformyl
phenyl) amine. In Fig. 7c and 7d, the spectra after condensation of residual –CHO groups with 2ꢀaminothiophenol

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and MCM-Pd are shown. These are more or less similar to the previous spectrum with identical number of peaks
for carbon atoms and small chemical shifts confirming retention of the heterogeneous support after condensation
and complex formation.
29Si
e
d
c
b
a
200
100
0
-100
-200
Chemical Shift (ppm)
Fig. 8. Solid state 29Si MAS NMR spectra of (a) calcined MCMꢀ41, (b) 3ꢀAPTES functionalized MCMꢀ41, (c)
tris(4ꢀformyl phenyl) amine loaded MCMꢀ41, (d) 2ꢀaminothiophenol grafted tris(4ꢀformyl phenyl) amine MCMꢀ
41 and (e) MCM-Pd
To establish the chemical environment around the silicon atom in the samples 29Si MAS NMR studies of
the materials are performed and the results are given in Fig. 8. For the calcined MCMꢀ41, peaks are observed at
ca. ‒112.0 and ‒103.0 ppm (Fig. 8a) which can be ascribed to the Q4 and Q3 silica centers of the Si(OSi)n(OH)4‒n
units. For the APTES functionalized silica (Fig. 8b), apart from the Q4 and Q3 silica species additional peaks
appear at ‒69.0 and ‒58.3 ppm due to incorporation of the aminopropyl groups. There are literature reports which
show that similar kind of hybrid mesoporous silica shows chemical shifts at ca. 67 and 58 ppm. These peaks have
been attributed to the T3 ((SiO)3SiꢀRꢀSi(OSi)3) and T2 ((HO)2(OSi)SiꢀRꢀSi(OSi)2(OH)) species,67,68 respectively.
The spectra for the other two samples, both containing Schiffꢀbase grafted complexes, and the Pdꢀloaded catalyst,
MCM-Pd (Fig. 8cꢀe) are similar to the APTES grafted sample, indicating the formation of imine bond and its
retention after metal coordination in the catalyst.
Thermogravimetric analysis has been carried out for calcined mesoporous silica, 3ꢀAPTES functionalized
silica and tris(4ꢀformyl phenyl) amine loaded silica to determine stability of the frameworks and the organic
content incorporated into silica support due to functionalization. The study has been carried out from room

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temperature to 1200 K and the results for the MCM and SBA series of samples are given in supporting
information, Fig. s3 and s4, respectively. From the plots, the amount of APTES and tris(4ꢀformyl phenyl) amine
loaded on the frameworks are calculated. They are found to be ca. 6.12 and 1.11 wt.% (0.66 and 0.11 mmol/g) for
the MCM supported samples and 6.56 and 1.66 wt.% (0.63 and 0.17 mmol/g) for the SBA supported samples.
The contents of palladium in MCM-Pd and SBA-Pd has been estimated by ICPꢀAES analysis and the amounts
have been found to be 3.8 mg/g (0.0356 mmol/g) and 4.2 mg/g (0.0393 mmol/g), respectively.
Catalysis
The catalytic property of MCM-Pd and SBA-Pd in Suzuki coupling reactions has been studied using
various substrates under ambient conditions. At first, the reaction conditions are optimized by tuning various
parameters. The coupling reactions are carried out for different reaction times in presence of 10 mg of the catalyst
with 1.0 mmol of iodobenzene, 1.2 mmol of phenylboronic acid and 2.5 mmol of base using different solvents
and bases which give biaryl derivatives as the products.69ꢀ71 The molar ratio of catalyst/iodobenzene for MCM-Pd
and SBA-Pd is 1.09/1000 and 1.659/1000, respectively, calculated with respect to active Pdꢀcenter. To select the
most suitable solvent, the first optimization is carried out using different solvents, namely, DMF, CH3CN,
C2H5OH and CH3OH. The reactions are carried out at room temperature using K2CO3 as the base for 24 h and the
results are given in Table 2.
Table 2. Suzuki coupling reactions in different solvents
Entry
Solvent
Time (h)
Yield* (Isolated Yield**) in %
MCM-Pd SBA-Pd
1.
2.
3.
DMF
24
24
24
34
52
CH3CN
C2H5OH
45
67
58
91
74 (69)
94 (90)
4.
CH3OH
24
*Obtained from gas chromatography, **Purified products characterized by 1H NMR spectroscopy

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It is evident that relatively poor yield is obtained when DMF is the solvent (Table 2, entry 1) and the yield
increases only marginally in case of acetonitrile (Table 2, entry 2). The yields are best in alcoholic solvents viz.
C2H5OH and CH3OH (Table 2, entries 3 and 4, respectively). For the catalytic reactions aqueous medium is not
used as silica framework is susceptible to hydrolysis on prolonged exposure to water under the coupling
conditions. However, modification of the silica framework with several organic groups somewhat increases its
hydrophobicity and enhances its ability to act as a heterogeneous catalyst in polar alcoholic solvents. Thus, from
the above studies, CH3OH has been selected as the solvent for all other reactions.
Suzuki coupling reactions are generally carried out in basic medium where the base acts as a coꢀcatalyst
for the reaction. It not only neutralizes the acidic byꢀproducts of the reaction, it also contributes to increasing the
reactivity of the catalyst. For optimizing the conditions, different bases have been used and the reactions are
carried out at room temperature in CH3OH medium for 24 h. The results have been shown in Table 3. The
reactivity is found to decrease in the following order: Cs2CO3> K2CO3> NaOH > Na2CO3> NaHCO3> Li2CO3.
Since K2CO3is more economic, it is preferred as the base to carry out further studies though it gives slightly lower
yield in comparison to Cs2CO3.
Table 3. Suzuki coupling reactions in different bases
OH
Catalyst, MeOH
B
I
+
24 h, R.T., Base
OH
Entry
Base
Time (h)
Yield* (Isolated Yield**) in %
MCM-Pd SBA-Pd
1.
2.
3.
4.
5.
6.
Li2CO3
Na2CO3
K2CO3
24
24
24
24
24
24
67
71
74
76
67
72
84
88
94
Cs2CO3
NaHCO3
NaOH
94.5
87
90
*Obtained from gas chromatography, **Purified products characterized by 1H NMR spectroscopy

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Time and temperature are other two very important parameters for these reactions. At first, the coupling reactions
are carried out at room temperature for different time periods (Table 4, entries 1ꢀ5). As expected, with increase in
time the yield of biphenyl product gradually increases for both the catalysts. It has been observed that the yield for
MCM-Pd always remains less than SBA-Pd and even after 72 h of reaction the yield reaches only up to 81 %.
However, for SBA-Pd the yield is found to be 94% within 24 h of the reaction. Hence all the reactions have been
carried out for 24 h. Regarding the temperature, it is expected that the rate of a reaction will increase with the
increase in temperature. It is nothing but providing energy externally in the form of heat to enhance the rate of the
reaction. Thus, for both the catalyst it has been found that at elevated temperature (333 K) the conversions are
reasonably high within 6ꢀ12 h (Table 4, entries 6 and 7). The yields for MCM-Pd and SBA-Pd reach to 96 and
99 %, respectively, in 12 h. However, the objective of this study is to see the effect of pore dimension on the
reactivity of the catalysts and hence the results obtained at ambient temperature have been considered for the
purpose of comparison.
Table 4. Suzuki coupling reactions at different time and temperature
Entry
Time (h)
Temperature (K)
Yield* (Isolated Yield**) in %
MCM-Pd SBA-Pd
1.
2.
3.
4.
5.
6.
7.
12
24
36
48
72
6
300 K
300 K
300 K
300 K
300 K
333 K
333 K
56
74 (69)
75
72
94 (90)
95
78
97
81
98
72
90
12
96
> 99
*Obtained from gas chromatography, **Purified products characterized by 1H NMR spectroscopy
After the optimization studies, Suzuki coupling reactions are carried out between various bromoꢀ/iodoꢀ
derivatives and phenylboronic acid in CH3OH medium using K2CO3 as the base at room temperature for 24 h. For
the comparative study, apart from bromoꢀ and iodoꢀbenzene, other electronꢀwithdrawing and electronꢀdonating

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substrates have also been used and the results are shown in Table 5. It is found that the yield of the biaryl
derivatives vary depending on the catalyst used. The corresponding TOF values for each entry are also calculated
and given in the table.
Table 5. Suzuki coupling reactions over different substrates under optimized reaction condition
OH
OH
Catalyst, MeOH
X
B
+
24 h, R.T., K2CO3
R
R
Entry
R
X
Time
(h)
Yield* (Isolated Yield**) in %
TOF***
MCM-Pd
SBA-Pd
78 (74)
94 (90)
90 (85)
93 (89)
93 (88)
90 (84)
93 (88)
88 (81)
94 (89)
98 (94)
92 (87)
94 (90)
92 (86)
96 (91)
MCM-Pd
60
SBA-Pd
82
1.
2.
H
H
Br
I
24
24
24
24
24
24
24
24
24
24
24
24
24
24
50
74 (69)
72
88
99
3.
4−CH3
4−CH3
Br
I
85
93
4.
81
96
98
5.
2−CH3
Br
Br
I
84
100
86
98
6.
4−OCH3
4−OCH3
4−Cl
73
93
7.
75
88
98
8.
Br
Br
I
68
80
93
9.
4−COCH3
4−COCH3
4−NO2
78
92
99
10.
11.
12.
13.
14.
85
100
78
104
97
Br
I
65
4−NO2
66
77
99
4−COOH
4−COOH
Br
I
69
81
97
70
82
101
*Obtained from gas chromatography, **Purified products characterized by 1H NMR spectroscopy, ***TOF: turnover
frequency (moles of substrate converted per mole of metal center per hour)
The 1H NMR spectra and data72 of the purified samples have been given in the supporting information.

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The conversion of bromobenzene is considerably less in comparison to iodobenzene and in all the cases it
is seen that the iodoꢀderivative is more reactive. For all the substrates, the yield of product with MCM-Pd is
found to be significantly lower than SBA-Pd. For both the catalysts the conversion of electronꢀdonating
substituents (Table 5, entries 3ꢀ8) is lower compared to those with electronꢀwithdrawing substituents (Table 5,
entries 9ꢀ14). The possible explanation for the observations in Table 5 has been given later.
Recyclability and test for heterogeneity
Heterogeneous catalysts are advantageous because they are recyclable. In order to study the recyclability of
MCM-Pd and SBA-Pd, the catalysts have been recovered after each reaction cycle, regenerated and used in
subsequent cycles under identical conditions. Fig. 9 shows the plot of conversion (%) against the number of
reaction cycles for both the catalysts for the coupling reaction of iodobenzene with phenylboronic acid in the
presence of K2CO3 for 24 h at ambient temperature. Three successive reaction cycles have been performed and it
can be seen that there isn’t any substantial loss of activity for any of the catalysts after the repeated reactions. The
TEM image of SBA-Pd after three reaction cycles has been given in supporting information, Figure s5. It can be
seen that the porous structure is retained and also there is no formation of Pdꢀnanoparticles. The color of the
catalysts after the reactions does not blacken which also indicate that metal nanoparticles are not involved in the
catalytic cycle. These results suggest that both MCM-Pd and SBA-Pd preserve their characteristic properties
after the reaction cycles and hence function as recyclable catalysts. Since the aim of this work is to study the
effect of pore dimension of the solid catalyst support on the reactivity, reactions are not conducted further after
the third cycle.
MCM-Pd
SBA-Pd
100
80
60
40
20
0
1
2
3
Number of cycle
Fig. 9. Recycling efficiency of MCM-Pd and SBA-Pd for Suzuki coupling of iodobenzene and phenylboronic
acid under ambient temperature

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To confirm the heterogeneity of MCM-Pd and SBA-Pd, coupling reactions of iodobenzene have been
carried out with phenylboronic acid under optimized condition and after 4 h of the reactions the catalysts are
removed from the reaction mixtures. After that the reactions are continued for the usual reaction time of 24 h. For
both the catalysts, it is found that after separation from the reaction medium no increase in conversion takes place.
This suggests that the reactions proceed only in the presence of the heterogeneous solid catalysts. In addition to
this, the reaction mixtures collected by filtration are analyzed by ICPꢀAES and it is found that the palladium
contents are below detection limit. The results confirm that palladium is not leached out from the solid support
into the reaction mixture and the catalyst is truly heterogeneous in nature where the active palladium species is
bound strongly by covalent bond to the functionalized silica support.
Mechanism and effect of pore size
In the present work we are studying two catalytic systems, MCM-Pd and SBA-Pd, which are different
from each other with respect to the solid support used. For MCM-Pd, the silica support MCMꢀ41 is synthesized
with CTAB template and the average size of the pores is 3.8 nm (Fig. 5). On the other hand, for SBA-Pd Pluronic
P123 is used as template which gives rise to bigger pore sizes with an average of 7.6 nm (Fig. 5). The pore sizes
are dependent on the dimension of the selfꢀassemblies formed by the template molecules. In both the catalysts,
palladium is coordinated to the N and S atoms of the Schiffꢀbase and thiol group of 2ꢀaminothiol, respectively,
which are immobilized over the mesoporous silica supports. It is interesting to note that the first Schiff base
condensation of tris(4ꢀformyl phenyl) amine with the aminoꢀgroup of APTES introduces two more aldehyde
groups into the framework. These two aldehydes in the next step of Schiff base condensation with 2ꢀaminothiol
give two N,Sꢀdonor sites which can bind to two palladium centers of palladium acetate. Thus, two successive
steps of Schiff base condensation increase the number of active sites capable of binding the metal ion in the
catalysts. The most preferred geometry for palladium is square planar and the other two coordination sites are
fulfilled by two acetate ions (OAcꢁ) from the metal precursor Pd(OAc)2. The oxidation state of Pd is +II in the
precatalyst,73, 74 from which Pd(0) is formed in situ in the reaction mixture in presence of phenyl boronic acid and
the base. A brief mechanism of Suzuki coupling reaction is given in supporting information, Fig. s6.
For both MCM-Pd and SBA-Pd the steps involved in the coupling reaction are essentially the same. So
the difference in reactivity must be arising from some other aspect. It is known that all reactions take place at the
surface. Greater the surface available more will be the reactivity of a catalyst. Now for MCMꢀ41 based catalyst,
MCM-Pd the size of the pore is smaller compared to the SBAꢀ15 based catalyst, SBA-Pd. Thus, the surface
exposure available for the former is lesser compared to the latter. Apart from that, greater pore space is available
for SBA-Pd which allows substrates to comfortably diffuse inside the pores and come in contact with the active
sites on the surface of the catalyst. Thus both the availability of more active sites on catalyst surface as well as
bigger size of the pores contributes to the higher reactivity of SBA-Pd than MCM-Pd. However, this result is

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only true when the catalysis is carried out at room temperature when no activation energy is supplied externally.
However, when the reactions are carried out at 333 K, it is found that MCM-Pd behaves as a very good catalyst.
Also there are ample examples of MCMꢀ41 based Pdꢀmaterials as very effective catalysts in Suzuki Coupling
reactions at higher temperature.17,67 This result indicates that the mesopores in MCMꢀtype of materials are also
superior compared to bulk or microporous catalysts68 but the reactivity can be better with bigger mesopores. That
is the reason why SBA-Pd derived from a solid support with bigger pore size shows very high reactivity even
under ambient conditions. On the other hand, for MCM-Pd elevated temperature i.e. supply of external energy is
required to attain the same extent of reactivity.
Now coming to the reactivity of various substrates in the Suzuki coupling reactions, the oxidative
addition step must be considered. In most cases this is the rate determining step in the Tolman’s cycle where an
aryl halide gets inserted into the palladium center to give an organoꢀpalladium species. As a result of this Pd(0) is
oxidized to Pd(II). This step involves the carbonꢀhalogen bond breaking and palladium binds to the halogen as
well as the Ar group. Now, if there is any electronꢀwithdrawing group present in the para position of the aryl
halide (with respect to halide), the carbonꢀhalogen bond cleavage becomes more facile. Thus, the reactivity of
these substrates increase compared to those with an electronꢀdonating group at the para position. This explains
the results that are obtained and given in Table 5 where the electronꢀwithdrawing substituents, ꢀCOCH3, ꢀNO2 and
ꢀCOOH give higher yield than electronꢀdonating substitutions, ꢀCH3, ꢀOCH3 and ꢀCl.
Conclusions
In the present article, we report two different Pdꢀcatalysts, MCM-Pd and SBA-Pd based on two different
types of silica supports with different pore sizes. The silica supports are functionalized taking advantage of the
aminopropyl groups grafted into the framework and subjected to two steps of Schiff base condensation to
introduce two N,Sꢀdonor sites which can bind to Pdꢀcenters. The materials at different stage are characterized for
their microstructure, porosity, functionalization and metal binding. Both the catalysts can heterogeneously
catalyze Suzuki coupling reactions of a range of substrates, which includes both electronꢀdonating and electronꢀ
withdrawing ones, but to different extents. The catalyst based on a smaller mesopore is less reactive compared to
the higher poreꢀsize counterpart when the reactions are carried out under ambient conditions. However, the
reactivity of both the catalysts is leveled at high temperature where energy is supplied externally. This indicates
that bigger pore dimension of the heterogeneous support facilitates the catalytic reactions. This can be attributed
to both the availability of higher pore wall surface exposed for the reactions as well as the ease of accessibility of
the substrates inside the pocket of the pores. This is a rare example of systematic study of heterogeneous Suzuki
coupling reaction where the effect of pore size of the solid catalyst support has been studied.
Acknowledgements

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MN acknowledges financial support from DSTꢀSERB (SB/FT/CSꢀ004/2014 dated 27/06/2014), New
Delhi, India and VisvaꢀBharati. Authors are thankful to Narayan Pradhan and Partha Roy of Indian Association
for the Cultivation of Science and Jadavpur University, respectively, for providing TEM and GC facilities.

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New Journal of Chemistry
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DOI: 10.1039/C8NJ00254A
Table of Content
Pronounced effect of pore dimension of silica support on
Pd-catalyzed Suzuki coupling reaction under ambient
conditions
Trisha Das, Hiroshi Uyama and Mahasweta Nandi
The pore size of mesoporous silica support affects the
reactivity of heterogeneous Palladium catalysts in Suzuki
reaction under ambient conditions which is leveled at higher
temperature.