Phosphine free SBA-15-EDTA-Pd highly active recyclable catalyst: Synthesis characterization and application for Suzuki and Sonogashira reaction

Article abstract of DOI:10.1039/c4cy00144c

Phosphine obstructed highly efficient and reusable SBA-15-EDTA-Pd(11) has been synthesized by anchoring a Pd-EDTA complex over the surface of organo-functionalized SBA-15. The physiochemical properties of the organo-functionalized catalyst were analyzed b

Full text of DOI:10.1039/c4cy00144c

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Phosphine free SBA-15–EDTA–Pd highly active
recyclable catalyst: synthesis characterization and
application for Suzuki and Sonogashira reaction†
Cite this: Catal. Sci. Technol., 2014,
4, 2978
*
Priti Sharma and A. P. Singh
Phosphine obstructed highly efficient and reusable SBA-15–EDTA–Pd(11) has been synthesized by
anchoring a Pd–EDTA complex over the surface of organo-functionalized SBA-15. The physiochemical
properties of the organo-functionalized catalyst were analyzed by elemental analysis, ICP-OES, XRD,
N2 sorption measurement isotherms, TGA and DTA, solid state 13C, 29Si NMR spectroscopy FT-IR, XPS
DRS UV-visible, SEM and TEM. The XRD and N2 sorption analyses of the synthesized catalyst confirm that
the ordered mesoporous channel structure was retained even after the multistep synthetic procedures.
The (100), (110) and (200) reflections in SBA-15 provide a good structural stability, an the existence of
a long range order and high pore wall thickness. The TGA-DTA results reveal that the thermal stability
of the synthesized catalyst SBA-15–EDTA–Pd(11) was maintained at higher temperature. The organic
moieties anchored over the surface of the SBA-15 and inside the pore wall were demonstrated by solid
state 13C NMR and FT-IR spectroscopy. Further, solid state 29Si NMR spectroscopy provides information
about the degree of functionalization of the surface silanol groups, of the SBA-15 with organic moieties.
The electronic environment and oxidation state of the Pd metal in the SBA-15–EDTA–Pd(11) were moni-
tored by XPS, and DRS UV-visible techniques. Moreover, the morphologies and topographic information
of the synthesized catalyst were confirmed by SEM and TEM spectroscopy. The synthesized catalyst
SBA-15–EDTA–Pd(11) was screened for the Suzuki and Sonogashira coupling reactions and shows a
higher catalytic activity with higher TON (turn over number). The anchored solid catalyst can be recycled
efficiently and reused five times, without a major loss in the reactivity.
Received 5th February 2014,
Accepted 22nd March 2014
DOI: 10.1039/c4cy00144c
www.rsc.org/catalysis
the most popular member of the group, possessing extremely
high surface areas, ease of accessibility, uniform pore sizes
Introduction
Various homogenous complexes are widely used for organic
transformations; however, the separation and recycling of the
rather expensive catalysts imparts difficulties. The hetero-
genization of such homogenous catalysts on solid supports
can mitigate these problems. Furthermore, heterogeneous
catalysts have clear advantages over their homogeneous
counterparts; they can be easily separated from the reaction
medium. “Heterogeneous catalytic system includes polymer/
dendrimer supported palladium catalysts palladium on car-
bon palladium supported metal oxides clays and molecular
sieves”.1 Since the synthesis of ordered mesoporous materials
in 1992 sparked worldwide interest in the field of heteroge-
neous catalysis and separation science; SBA-15 has become
and stability.2
To extend the applicability of SBA-15 materials, it is neces-
sary to modify the surface by organic functional groups, to
anchor metals and metal complexes. The grafting of func-
tional organosilanes by using the surface hydroxyl groups as
the anchor points has been widely used. Furthermore, the
leaching of the active site can also be avoided as the organic
moieties are covalently attached to the inorganic support.
Palladium complexes, with or without phosphine ligands,
can catalyze C–C coupling reactions. The phosphine-assisted
approach is the classical and well-established method,3
which gives excellent results in the majority of cases; whereas
phosphine ligands are expensive, toxic, and unrecoverable.
Additionally, a major drawback of phosphine ligands in a cat-
alytic reaction is the oxidation of phosphine to a phosphine
oxide, as well as cleavage of the P–C bond, causing degrada-
tion of the catalytic cycle. In large-scale industrial applica-
tions, phosphines might be a more serious economical
burden than even palladium itself, which can be recovered at
any stage of the production, or from the waste. Therefore, the
Catalysis Division, CSIR-National Chemical Laboratory, Pune 411008, India.
E-mail: ap.singh@ncl.res.in; Fax: (+)91 20 2590 2633; Tel: (+)91 20 2590 2497
† Electronic supplementary information (ESI) available: Suzuki, Sonogashira
product 1H, 13C NMR, details of experimental section, pore size distribution
plot, TGA, DTA plot, X-ray photoelectron spectroscopy (XPS), solvent, tempera-
ture and base optimization for the Suzuki and Sonogashira coupling reaction.
See DOI: 10.1039/c4cy00144c
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development of phosphine-free catalysts for C–C bond-
forming reactions would be an important topic, of interest to
current industrial research.4
achieved by a post synthesis grafting method (Scheme 1A)
furthermore, free –OH groups present in NH2–SBA-15
were protected (Scheme 1B) by published procedure.21 Finally
Pd–EDTA complex was covalently grafted over the organo-
modified surface of the SBA-15.8 The products were dried
and characterized systematically by nitrogen sorption iso-
therms; cross polarization magic angle spinning (CPMAS)
NMR and infrared spectroscopies; elemental analysis;
and thermogravimetric and differential thermogravimetric
(TGA-DTA) analysis to gain complete structural and composi-
tional information.
The accurate selection of the ligands is the key factor for
the synthesis of the complex. Recently, a number of nitrogen
based compounds in phosphine free conditions have com-
monly been used as the ligand, such as C based heterocyclic
carbenes, C–N based 2-aryl-2-oxazolines, aryl (heteroaryl)
oximes, arylimines, N,N-based diazobutadienes, DMG and
salen complexes.5 However, these ligands are not easily avail-
able and contain tedious and expensive synthesis processes
and are hardly stable in the catalytic systems. Therefore, a
simple, easily accessible, stable catalyst is desired for these
high temperature reactions.
Cross coupling reactions are one of the broadest areas
for the synthesis of symmetrical and unsymmetrical binary
compounds, which are the key components of the several
natural products, as well as in the field of engineering mate-
rials, such as conducting polymers, molecular wires and liq-
uid crystals.6 Among the basic types of palladium catalyzed
transformations, the Suzuki and Sonogashira reactions
related chemistry occupy a special place.7 The Suzuki cross-
coupling reaction of organoboron reagents with organic
halides represents one of the most versatile and straightfor-
ward methods for carbon–carbon bond formation. The reac-
tion is largely unaffected by water, tolerating a large range of
functionality and yielding non-toxic byproducts. Furthermore,
the Sonogashira coupling of phenyl acetylenes and aryl
halides is one example of a palladium catalyzed reaction that
allows the connection of a CC triple bond substituent to an
aromatic ring.
In 2005, Korolev et al. described the synthesis of a
PdCl2–EDTA complex as a homogeneous catalyst for the
Suzuki–Miyaura reaction in water.8 However, this catalytic
system was not stable enough to store for a long time period
and unfortunately, no catalyst recovery was possible. In this
context, we decided to immobilize the Pd-complex on the sur-
face of an organo-functionalized SBA-15, in order to recycle
the catalyst. Herein, we report grafting of Pd–EDTA complex,
and its derivatives into SBA-15 phases and their catalytic
properties in the Suzuki and Sonogashira coupling reactions,
as a heterogeneous catalyst. The immediate goals of our
study were (i) to evaluate the heterogenization method of the
Pd–ethylenediaminetetraacetic acid complex over an organo-
modified mesoporous SBA-15 support, (ii) to measure the cat-
alytic properties in the Suzuki and Sonogashaira C–C cou-
pling reactions (iii) to optimize the reaction parameters, such
as the temperature, solvent and base in the both coupling
reactions, (iv) to determine the extent of the stability of the
catalysts, as well as their recycling properties.
The XRD patterns of (a) calcined SBA-15, (b) –OH
protected NH2–SBA-15 and (c) Pd–EDTA–SBA-15(11) com-
plexes are visualized in Fig. 1 and (a) SBA-15–EDTA–Pd(7),
(b) SBA-15–EDTA–Pd(11), (c) SBA-15–EDTA–Pd(15) in Fig. S1
(ESI†). The typical hexagonal phase of the SBA-15 [main
(100), (200), and (210)] reflections are clearly visible in the
calcined SBA-15. In all the samples, the (110) reflection is
more intense than the (200) reflection. It favors a more com-
plete condensation of the wall structure, due to the higher
temperature preferred in the hydrothermal synthesis and fur-
ther calcinations. As evident from Fig. 1 the XRD patterns of
the samples synthesized after treatment with 3-APTMS and
[(MeO)2SiMe2] are almost similar to the parent SBA-15 sam-
ple, with a small decrease in the overall intensity to the (100),
(110) and (200) reflections. From the XRD pattern, it is clear
that after the anchoring of the Pd–EDTA complexes with dif-
ferent weight% ratios (7%, 11%, 15%) an inconsequential
decrease in the peak intensities to the (100), (110) and (200)
reflections was observed without changing the peak positions
(Fig. S1†). This perseverance of the peak positions indicates
that even the presence of a large amount of Pd–EDTA com-
plex moieties, by the partial filling inside the mesopores, is
less detrimental to the quality of the SBA-15 material.9 The
persistence of the (100), (110) and (200) reflections (Fig. 1)
not only proved the structural stability and existence of a
long range ordering to the mesophase, but also the survival
of the undisturbed pore wall thickness, even after a number
of treatments with organic molecules in solvents.
The nitrogen adsorption–desorption results of the cal-
cined SBA-15 and SBA-15–EDTA–Pd(7), SBA-15–EDTA–Pd(11),
SBA-15–EDTA–Pd(15) samples and their corresponding pore
size distribution curves are plotted in Fig. 2 and S2,† respec-
tively. The N2 adsorption–desorption isotherms of all the sam-
ples show a type IV isotherm. The surface area, average pore
diameter, pore volume and wall thickness were observed for the
calcined SBA-15 and SBA-15–EDTA–Pd(7), −11 and −15 deriva-
tives samples and are summarized in Table 1. All the samples
show type IV adsorption isotherms, according to the IUPAC
classification, indicating the uniformity of the mesopores due
to capillary condensation of N2 within the mesopores with a
completely reversible nature, uniformly sized mesopores, with a
capillary condensation step at P/Po = 0.3–0.4.
Results and discussion
The heterogenized palladium catalyst, SBA-15–EDTA–Pd, was
obtained by the procedure outlined in Scheme 1. Starting
from the synthesis of SBA-15 and surface modification was
The total surface area, average pore diameter and pore vol-
ume observed for the calcined SBA-15, SBA-15–EDTA–Pd(7),
SBA-15–EDTA–Pd(11) and SBA-15–EDTA–Pd(15) were found
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Scheme 1 Schematic diagram of SBA-15 functionalization and heterogenization of Pd–EDTA–SBA-15. 1(A) Amino (–NH2) functionization,
1(B) capping of SBA-15, 1(C) Pd–EDTA complex formation, 1(D) anchoring of Pd–EDTA complex over modified surface of SBA-15.
Fig. 2 Nitrogen adsorption–desorption isotherm of (a) calcined SBA-15,
Fig. 1 XRD pattern of (a) calcined SBA-15 (b) –OH protected –NH2–SBA-15 (c)
(b) SBA-15–EDTA–Pd (7), (c) SBA-15–EDTA–Pd(11), (d) SBA-15–EDTA–Pd(15).
SBA-15–EDTA–Pd(11).
to be 739 m2 g−1, 65 Å, 1.173 cm3 g−1, 665 cm3 g−1, 64.3 Å,
metal Pd–EDTA complex immobilization over organo-
modified SBA-15 is indicative of the grafting of the complex
Pd–EDTA inside the channels of the mesoporous SBA-15. It
is clear from Table 1 that even though silylation procedures
changed the textural properties of the mesoporous material,
0.439 cm3 g−1, 363 m2 g−1, 56 Å, 0.478 cm3 g−1 and 187 m2 g−1
,
48.5 Å, 0.36 cm3 g−1, respectively. The decrease in the total
mesoporous surface area (10%, 50%, 74%), pore diameter
(2%, 13%, 26%) and pore volume (62%, 59%, 69%) after
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Table 1 Textural properties of mesoporous calcined SBA-15 & SBA-15–EDTA–Pd
Loading of Pdb (wt.%)
BET surface
area (m2 g−1
Average pore
diameter (DP) (Å)
Pore volume
Sample
Na (wt.%)
Input
Output
)
(Vp, cm3 g−1
)
Calcined SBA-15
739
665
363
187
65
64
56
48
1.17
0.43
0.47
0.36
SBA-15–EDTA–Pd(7)
SBA-15–EDTA–Pd(11)
SBA-15–EDTA–Pd(15)
2.2
2.5
4.3
7
11
15
5.7
6.2
10.8
a
b
Calculated based on elemental (nitrogen) analysis value. Input is based on the amount of Pd during synthesis reaction; output is based on
the ICP-OES analysis.
the decrease is more prominent after Pd–EDTA complex
immobilization, since the bulkier organic moieties inside
the pore channels occupy a large area of the void space.
The capillary condensation steps of SBA-15–EDTA–Pd(7),
SBA-15–EDTA–Pd(11), and SBA-15–EDTA–Pd(15) due to
anchoring of Pd–EDTA complexes are reduced to lower P/Po
values. The shift to a slightly lower partial pressure shows a
possible reduction in the pore size and a partial distortion in
the pore arrangement, consistent with the XRD results. It is
known that the inflection position in the N2 sorption iso-
therm depends on the diameter of the mesopores and the
sharpness usually indicates the uniformity of the mesopores,
due to capillary condensation of N2 within the mesopores.
The presence of isolated surface silanols, hydrogen
bonded hydroxyl groups, anchored on the complex Pd–EDTA
are evidenced from the IR spectrum of the calcined SBA-15
and its modified samples. Fig. 3 shows the FT-IR spectra of
(a) EDTA, (b) calcined SBA-15, (c) –OH protected NH2–SBA-15,
(d) SBA-15–EDTA–Pd(7), (e) SBA-15–EDTA–Pd(11), (f) SBA-15–
EDTA–Pd(15). In the visualized IR spectrum of calcined SBA-15
and –OH protected NH2–SBA-15 the ν–OH stretching vibra-
tions observed in the range 3600–3400 cm−1 region are attrib-
uted to the hydrogen-bonded silanol groups and the sharp
band at 3757 cm−1 corresponds to the isolated surface silanol
groups (Fig. 3b) In the present analysis, after 3-APTMS
functionalization, a sharp decrease in the intensity of peak at
3757 cm−1 with a peak shift to a lower value is seen, demon-
strating the role of the surface silanols in modifications
(Fig. 3c). In Fig. 3 the bands observed near 797 cm−1 and
1076 cm−1 are due to the symmetric and asymmetric vibra-
tions of the Si–O–Si group, respectively.10 Two strong bands
were observed in the case of SBA-15–EDTA–Pd(11) complexes
at 1620 and 1587 cm−1; these correspond to the C–O
stretching vibrations and N–H bending vibrations, respec-
tively.11 Further, the intensity of the C–O stretching vibra-
tions and N–H bending vibrations increases as the loading of
Pd–EDTA complex increases. Since the Pd–EDTA was
anchored over the modified surface of the SBA-15, the band
at 3294 cm−1 ascribed to the NH2 stretching vibrations
disappeared, while a broad weak band at 3257 cm−1 was
observed, which is attributed to the stretching vibration of
–[NH3] + resulting from the EDTA modification.12 This broad
band is much broader in the case of the SBA-15–EDTA–Pd(15)
catalyst, which might be due to the higher loading of the
Pd–EDTA complex. Further, new bands at 1383 and 1627 cm−1
were also observed for the Pd–EDTA modified SBA-15, which
are attributed to the –COO– group symmetrical and asymmetri-
cal stretching vibrations, respectively. These observed results
indicate that all the SBA-15–EDTA–Pd complexes, along with
different loading, are systematically synthesized.
Fig. 4 show the solid state 13C CP/MAS NMR spectra of
(a) NH2–SBA-15, (b) –OH protected – NH2–SBA-15, (c) SBA-15–
EDTA–Pd(11). In the solid state 13C CP/MAS NMR spectra of
NH2–SBA-15, –OH protected NH2–SBA-15 and SBA-15–EDTA–
Pd(11), the peak observed at 9.3 ppm can be accounted for by
the carbon (C1) atom bonded to the silicon. The signal at
21.3 ppm corresponding to a methylene carbon (C2) and the
signal at 42.5 ppm can be attributed to the carbon atom
attached to the NH2 group (Fig. 4a). After the –OH group pro-
tection of NH2–SBA-15 by dimethoxydimethylsilane, along
with all three peaks of NH2–SBA-15, one extra peak is clearly
visible at −2.2 ppm, which corresponds to the methyl group
attached the to capping agent dimethoxydimethylsilane
(Fig. 4b). Additionally, no peak was observed for the methoxy
group, which confirms the successful grafting of the
3-APTMS over the support (SBA-15). Two consecutive peaks,
observed at 171 ppm and 180.6 ppm, correspond to the car-
bon atoms of the amide group formed due to the carboxylic
group being anchored to the linker 3-APTMS of the SBA-15.
Fig. 3 FT-IR spectrum of (a) EDTA, (b) calcined SBA-15, (c) –OH
protected NH2–SBA-15, (d) SBA-15–EDTA–Pd(7), (e) SBA-15–EDTA–Pd(11),
(f) SBA-15–EDTA–Pd(15).
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Fig. 5 Solid state 29Si CP/MAS NMR spectrum of (a) calcined SBA-15,
(b) NH2–SBA-15, (c) –OH protected –NH2–SBA-15 (d) SBA-15–EDTA–Pd(11).
Fig. 4 Solid state 13C CP/MAS NMR spectrum of (a) NH2–SBA-15,
(b) –OH protected –NH2–SBA-15 (c) SBA-15–EDTA–Pd(11).
methoxy group of the dimethoxydimethylsilane (Fig. 5c). The
absence of the T0(SiC(OH)3 sites confirms that the EDTA–Pd
complex is covalently anchored to the modified surface of the
SBA-15 (Fig. 5d).
Furthermore, another peak observed in the case of SBA-15–
EDTA–Pd(11), at 63.85 ppm, corresponds to the carbon of the
ethylene group in EDTA, which is directly attached to the
amide group and the peak at 55.4 ppm is due to the ethylene
group, which is directly attached to the carboxylic acid group
(Fig. 4c). The reason for the appearance of two types of peak
for the carboxylic acid might be that all the carboxylic groups
did not get anchored over the 3-APTMS modified surface of
the SBA-15.
The thermal stability of all the synthesized materials was
studied by thermogravimetric analysis (TGA), under an air
atmosphere, from ambient temperature to 1000 °C with a
temperature increment of 10 °C min−1. The TGA plots of all
the synthesized and modified SBA-15 samples show an
approximately 5% weight loss below 120 °C, caused by the
desorption of physisorbed water molecules (Fig. S3†). In
the TGA plot, a loss of ~42 weight% from the as-synthesized
SBA-15 was observed between 132 °C and 195 °C, which cor-
responds to the removal of the trapped surfactant within the
closed pores (Fig. S3,† A, B, a). In comparison, nearly no
weight loss in the TGA and DTA was observed in the calcined
SBA-15 between 132 °C and 195 °C, which indicates the com-
plete removal of surfactant from SBA-15 (ref. 14) (Fig. S3,†
A, B, b). These data evidently support the complete removal
of the organic surfactant from the calcined SBA-15. The
TGA results of the –OH protected NH2–SBA-15 sample shows
a weight loss in three steps. In the first step, a weight
loss between 70 °C and 150 °C corresponds to the loss of
loosely bounded water, adsorbed moisture, which is clearly
evidenced by the small exothermic peak of the DTA. In
the second step, a weight loss was observed in the region of
245–385 °C. The TGA analysis and a sharp visible exothermic
peak in the DTA analysis in the same temperature region
(245–385 °C) are attributed to 3-APTMS. The TGA plot of –OH
protected NH2–SBA-15 quantitatively shows an ~21.27%
weight loss, which is greater than the calcined SBA-15; this
strongly supports the successful anchoring of the 3-APTMS
over SBA-15. The third weight loss visible in the TGA analysis
in the 380–480 °C range corresponds to the removal of
dimethoxydimethylsilane [(MeO)2SiMe2], which is evidently
supported by the DTA analysis showing one strong exother-
mic peak in the same temperature range (Fig. S3,† B, c). In
The 29Si MAS NMR spectra of (a) calcined SBA-15,
(b) NH2–SBA-15, (c) –OH protected –NH2–SBA-15, (d) SBA-15–
EDTA–Pd(11) are exhibited in Fig. 5. The peaks seen in the
spectra, at −112, −102, −68 and −61 ppm, are usually assigned
to the Q4 [Si(OSi)4, siloxane], Q3 [Si(OH)(OSi)3, single silanol]
and T3 [SiR(OSi)3] sites, respectively (Fig. 5a–d). The calcined
SBA-15 sample shows the presence of broad resonance peaks
from −126 to −98 ppm, indicative of a range of Si–O–Si bond
angles, and it is noteworthy that the sample contains large
amounts of Q4 sites, showing a high framework cross-
linking.13 The low intensity in the Q3 value shows that these
silanol groups are highly accessible to the silylating agents
(Fig. 5a). In the –OH protected-NH2–SBA-15, the Si spectrum
shows one extra peak at −68.48 ppm, due to the capping
agent dimethoxydimethylsilane, which is assigned to a mix-
ture of T3 [SiR(OSi)3] and T2 [Si(OH)R(OSi)2] organosilicon,
respectively. The peak at −16.87 ppm corresponds to the Si
of the (dimethoxydimethylsilane) capping agent (Fig. 5c).
After protection of the –OH group in NH2–SBA-15 by
dimethoxydimethylsilane, it is clearly visible that the T2 site
[Si(OH)R(OSi)2] disappeared, along with the appearance of a
new peak at 16 ppm. These two significant changes evidently
proved the free silanol group of NH2–SBA-15 the T2 site
[Si(OH)R(OSi)2] becomes blocked by a condensation with the
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the case of the heterogenized metal complex SBA-15–EDTA–
Pd(11) one extra peak was observed along with two peaks
shown in the –OH protected NH2–SBA-15, in the region of
437–556 °C assigned to the Pd–EDTA complex. Note that
decomposition of the Pd–EDTA complex occurred at an ele-
vated temperature revealing the high thermal stability of the
complex. A direct comparison of the weight loss in the case
of heterogenized SBA-15–EDTA–Pd(11), and in the capped
amino functionalized SBA-15 shows an ~7 weight% loading
of the complex material (Fig. S3,† A, B, d).
A TEM image of the calcined SBA-15 and SBA-15–EDTA–Pd(11)
provide structural evidence that the material is organized into
ordered arrays of two-dimensional hexagonal mesopores (Fig. 6).
A significant difference in the TEM patterns was not observed
between the two Fig. 6A, B. However, after the anchoring of
the EDTA–Pd complex inside the mesoporous channels of
SBA-15 the image show distinct, deep contrasting meso paral-
lel channels, with respect to the light shaded surface. This
might be interpreted as being due to the presence of the
EDTA–Pd complex inside the SBA-15, but not on the surface.
If the EDTA–Pd complex was anchored on the surface of the
functionalized SBA-15, then the high-contrast dark meso par-
allel channels would have appeared along the boundary of the
visualized SBA-15 and not inside the porous body, as observed
previously by Shephard et al.15 Thus, the immobilization of
the Pd-complex inside the pore-channels, by anchoring to the
interior walls of these porous channels may be supported by
the TEM analysis.
clearly, a molecular-scale periodicity in the SEM images.
Further, the SBA-15–EDTA–Pd(11) is demonstrated to be
a molecular-based material; the large molecular system
becomes denser in comparison to the calcined SBA-15 after
the Pd–EDTA complex is anchored over the mesoporous surface.
X-ray photoelectron spectroscopy (XPS) is a powerful tool
to investigate the electronic properties of the species formed
on the surface, such as the electronic environment, e.g. oxida-
tion state and or multiplicity influences in the binding energy
of the core electron of the metal. The synthesized material
SBA-15–EDTA–Pd(11) was characterized by X-ray photoelec-
tron spectroscopy (XPS) to ascertain the oxidation state of the
Pd species. In Fig. S4 (ESI†) the Pd binding energy of the
SBA-15–EDTA–Pd(11) exhibits two strong peaks, centered at
336.7 eV and 341.5 eV, respectively, which are assigned to the
Pd 3d5/2 and Pd 3d3/2 signal, respectively. The observed peaks
correspond to the Pd2+ oxidation state in the synthesized
SBA-15–EDTA–Pd(11). According to the literature, the pure
PdCl2 metal salt binding energy for the Pd 3d3/2 and Pd 3d5/2
orbitals appear at 342.8 and 337.6 eV, respectively.16 In com-
parison to the literature values, the synthesized SBA-15–
EDTA–Pd(11) shows a shift in binding energy of Pd towards a
lower value viz. 341.5 Pd 3d3/2 and 336.7 eV Pd 3d5/2, respec-
tively.17 The shift in the binding energy towards lower values
indicates that the Pd state in SBA-15–EDTA–Pd is a more
electron rich state than the PdCl2. The reason for this might
be the possibility of an electron donation from the EDTA to
the palladium and it suggests a strong interaction between
the Pd metal species and the EDTA ligand was present in the
SBA-15–EDTA–Pd catalyst. These results are in agreement
with the UV-vis observations.
The morphologies of the calcined SBA-15 and SBA-15–
EDTA–Pd(11) are shown in Fig. 7A, B, respectively. The cal-
cined SBA-15 shows uniform arrays of mesochannels and
Diffuse reflectance UV-vis measurement is a useful tech-
nique to obtain information about the oxidation state of an
incorporated metal species. The UV-vis spectra of (a) calcined
SBA-15, (b) SBA-15–EDTA–Pd(7), (c) SBA-15–EDTA–Pd(11), (d)
SBA-15–EDTA–Pd(15) are shown in Fig. 8. Calcined SBA-15
shows the characteristic absorption at 254 nm, which corre-
sponds to the siliceous material (Fig. 8a). The diffuse
Fig. 6 TEM images of calcined (A) SBA-15 and (B) SBA-15–EDTA–Pd(11).
Fig. 8 UV absorbance spectra of (a) calcined SBA-15, (b) SBA-15–
EDTA–Pd(7), (c) SBA-15–EDTA–Pd(11), (d) SBA-15–EDTA–Pd(15).
Fig. 7 SEM images of calcined (A) SBA-15 and (B) SBA-15–EDTA–Pd(11).
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reflectance spectra (200–800 nm) of the SBA-15–EDTA–Pd
(Pd–EDTA loading 7, 11, 15%) catalysts display nearly iden-
tical features in the absorption bands in the UV region in
the range 205–680 nm, with reference to a BaSO4 standard.
The UV visible spectra of the SBA-15–EDTA–Pd (Fig. 8a–c) shows
characteristics bands at 205–290 nm and 325–362 nm and a
broad peak at 682 nm. The weak bands in the 205–290 nm
region are assigned to the weak p–p* transitions (Fig. 8b–d).
The bands in the 325–362 nm region are assigned to the d–d
transition of the metal;18 this strongly supports the Pd(II)
oxidation state. Another band appears at 682 nm, which
might be due to an n–p* transition after incorporation of the
Pd–EDTA over the modified surface of the SBA-15.
presence of the base potassium carbonate (K2CO3) at 120 °C,
in a model reaction of the Suzuki coupling between iodo-
benzene and boronic acid. Among all the used solvents,
DMF, DMSO and NMP were able to give a significant yield
(85–100%) in a 5 h reaction time period (Fig. S5†). However,
the non-polar or less polar solvents such as THF and toluene
progress with a strong rationale for the intermediate stabili-
zation via the coordinating ability and the polarity. From the
obtained results of the solvent optimization for the conver-
sion of iodobenzene, the reactivity order emerged as follows:
DMF (100%) > DMSO (89%) > NMP (83%) > THF (39%) >
toluene (17%) > 1,4-dioxane (15%) > xylene (10%), respec-
tively (Fig. S5†). In addition, the conversion of iodobenzene
was also carried out using SBA-15–EDTA–Pd(11) in the pres-
ence of water as the solvent under similar reaction condi-
tions. The yield of the biphenyl was found to be 33 wt.% in a
12 h reaction time. Since after the oxidative addition of the
aryl halide, charge separation would take place at the Pd
metal centre to a much greater extent. To stabilize the gener-
ated high charge during the oxidative step at the Pd centre
more coordination is required which is possible from the
high polar solvents. The Suzuki coupling reaction of iodo-
benzene (1 mmol) and a boronic acid (1.5 mmol) in the
presence of K2CO3 using the solvent DMF (3.5 ml) over the
SBA-15–EDTA–Pd(11) (15 mg) catalyst was examined to see
the influence of the temperature in the range 60 °C to 120 °C
on the product yield (Fig. S6†). A lower temperature does not
favour the formation of the product (biphenyl); however the
yield of the biphenyl increased sharply with an increase in
the reaction temperature and reached the value of 99% in 6 h
at 120 °C. Generally, the coupling reaction was favoured at
high temperature, since a high activation energy is required
to dissociate the aryl halide bond at the oxidative addition
step. It is noteworthy to mention here that no biphenyl prod-
uct was obtained below 55 °C. Hence, the optimum reaction
temperature with respect to conversion towards the biphenyl
product, under present reaction conditions, was found to be
120 °C (Fig. S6†).
Various bases such as NaOH, NaHCO3, K2CO3 and NEt3
were screened for the reaction. Among all the used bases,
organic bases like triethylamine (NEt3) were found to be
unreactive in comparison to the inorganic bases. The order
of reactivity of the iodobenzene in the presence of various
bases could be arranged in the decreasing order as: K2CO3
(100) > NaOH (60) > NaHCO3 (45) ≫ NEt3 (32). It is clear
that the reactivity of K2CO3 was found to be quite high
among the used inorganic bases (Fig. S7†). Potassium car-
bonate was able to give a 95% yield of the coupled product
(biphenyl) in 5 h at 120 °C. The Suzuki coupling reaction is
known to be affected by salt particles, yet their mode of
action has not been completely clarified. According to one
hypothesis,20 in the reaction, the anions at the surface of the
solid salt particles act as electron donors for the Pd metal
centre, which increase the electron density of the Pd metal
centre. This effect could promote the oxidative step, to form
the intermediate Ar–Pd–X (3). Therefore, the reaction is
Suzuki coupling
The catalysts SBA-15–EDTA–Pd with different wt.% loadings
of Pd–EDTA (7%, 11%, 15%) were screened in the Suzuki
coupling reaction using the following reaction conditions:
arylboronic acid (1.5 mmol), aryl halide (1 mmol), potassium
carbonate (3 mmol), DMF (3.5 ml), SBA-15–EDTA–Pd (15 mg)
at 120 °C. The conversion of iodobenzene and TON with
respect to Pd loading were found to be 70%, 99%, 99%
and 100, 130.2 and 74.2, respectively. The reaction proceeds
at the active Pd metal centre. Further, the TON increases
from SBA-15–EDTA–Pd(7) to SBA-15–EDTA–Pd(11) and there-
after decreases.
The heterogeneous Suzuki cross-coupling of a boronic acid
with an aryl iodide may proceed through a catalytic cycle, analo-
gous to that proposed for homogeneous palladium catalysts.19 In
the first step of the reaction insight, an oxidative addition of the
aryl halide ArX (2) to the SBA-15–EDTA–Pd(11) (1) complex pro-
vides an SBA-15 bound aryl palladium(II) complex (3). The leav-
ing anion adds to the metal center, to give the intermediate (3).
The displacement of a halide ion (X) from SBA-15-L-(Ar–Pd–X)
(3), to give the more reactive organopalladium alkoxide SBA-15-
L-(Ar–Pd–CO3K+) or organopalladium hydroxide (R–Pd–OH),
depends on the base used. Further, in the second step of the
reaction mechanism, transmetalation between the SBA-15-L-
(Ar–Pd–CO3K+) aryl palladium(II) complex (6) and the boronic
acid (7) provides the SBA-15-L-(Arl–Pd–Ar) reaction interme-
diate (10), with the subsequent removal of the byproduct
B(OH)2(CO3 K+)2 (9). In the last step of the reaction mechanism,
−
the reductive elimination of biphenyl Arl–Ar (11) from the
unsymmetrical intermediate (10) regenerates the SBA-15-L-Pd (1)
complex.
Since the mechanism of the Suzuki coupling reaction is a
multistep procedure, a small variation in the physical and
reaction parameters can change the product yield and rate of
the reaction drastically. Hence, the influences of the solvent,
reaction temperature and various bases were evaluated on
the product yield using SBA-15–EDTA–Pd(11) as the catalyst
with iodobenzene and boronic acid.
In order to probe the role of the solvents in the Suzuki
coupling reaction, a series of solvents, such as DMSO, DMF,
HMPA, THF, 1,4-dioxane, and toluene were used, in the
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accelerated because this is the rate determining step. In
other words, the electron-donation effect could enhance the
activity of the catalyst with an increase in the electron density
of the Pd.
TON were found to be 81.2, 94.9 and 128.9, respectively
(Table 2, entries 1, 2, 3). In the step of the oxidative addition of
the catalytic cycle of the halogenated (X = Cl, Br, I) substrates,
coupling reactions involving these substrates typically decrease
in the order R–Cl > R–Br > R–I. This can be explained in terms
of the R–X bond dissociation enthalpies (BDE). For example,
the X–Ph BDE ranges from X = Cl 95.5 1.5 kcal mol−1; X = Br
80.4 1.5 kcal mol−1 to X = I 65.0 1 kcal mol−1. It is evident
from Table 2 that the reactivity of the aryl chloride and bro-
mides with phenyl boronic acid were found to be lower than
aryl iodides and require comparatively longer reaction times
for the completion of the reaction.
Furthermore, electron rich and electron poor aryl halides
react smoothly with phenylboronic acid in similar reaction con-
ditions. Electron poor aryl halides like 4-chloronitrobenzene,
4-bromonitrobenzene and 4-iodonitrobenzene with phenyl
boronic acid gave coupled product (4-nitro biphenyl) in 98%,
99%, 95% with TON 128.9, 130.2, 125, respectively (Table 2,
entries 4, 5, 6). The relative reactivity of aryl halide towards the
metal centre decreases in the order: I > Br ≫ Cl. Aryl halides
activated by the proximity of electron-withdrawing groups are
The characteristics of the organoboron reagent (i.e., high
selectivity in cross-coupling reactions, stability, non-toxic
nature, and tolerance towards functional groups) often give
the Suzuki coupling a practical advantage over other cross-
coupling processes. After optimizing the reaction parameters
of the Suzuki coupling reaction between iodobenzene and
phenyl boronic acid, several substituted and non-substituted
aryl halides were employed in the reaction and the results are
summarized in Table 2. The desired corresponding products
were obtained in good yields, with high TONs. As shown in
Table 2, the Suzuki coupling reaction of phenyl boronic acid
with a variety of aryl halided proceeds smoothly under mild
reaction conditions, giving the corresponding coupled prod-
ucts in high yields (75%–99%) (Table 2, entries 1–10).
Monosubstituted aryl halides, such as chlorobenzene,
bromobenzene and iodobenzene with phenylboronic acid gave
71%, 83%, 98% yields of the biphenyl and the corresponding
Table 2 Reactivity of SBA-15–EDTA–Pd(11) catalyst for Suzuki reactiona
S. no
1
Aryl halide
Product
Time (h)
30
Yield (%)
71
TON
81
2a
2
3
4
5
6
8
5
6
6
3
83
98
98
99
95
94
128
128
130
125
2a
2a
2b
2b
2b
7
9
96
109
2c
8
7
9
1
94
107
101
130
2c
9
89
2d
10
99b
2a
a
Reaction conditions: (1.5 mmol) arylboronic acid, (1 mmol) aryl halide, (3 mmol) potassium carbonate (base), 3.5 ml DMF with (15 mg)
b
heterogeneous SBA-15–EDTA–Pd(11) catalyst, temperature 120 °C. Carried out in homogeneous catalyst EDTA–Pd. (3 mol%).
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more reactive to the oxidative addition than those with donat-
ing groups, thus allowing the use of electron deficient halides,
such as 4-chloronitrobenzene for the cross-coupling reaction.
Further, aryl iodides are more reactive than the bromides and
chlorides. The substituent effect in the aryl iodides appeared to
be less significant than in the aryl chlorides and bromides.
Subsequently, the electron rich aryl halides also show moder-
ate to excellent reactivity (89–96% yield) in the formation of the
corresponding products (4-methylbiphenyl, 4-methoxybiphenyl)
in the Suzuki coupling reaction under similar reaction condi-
tions. The electron rich para substituted aryl halides, such as
4-bromotoluene, 4-iodotoluene 4-bromoanisole, with boronic
acid afforded 96%, 94%, and 89% yields after 9 h, 7 h, 9 h,
respectively (Table 2, entries 7, 8, 9). The coupling reaction of
chloronapthalene with phenyl boronic acid was investigated
under similar reaction conditions, but no desired coupled
product was obtained, even after 24 h of reaction time. In addi-
tion, the catalytic activity of the SBA-15–EDTA–Pd(11) was com-
pared with a homogeneous counterpart. The product yield over
SBA-15–EDTA–Pd(11) and EDTA–Pd were found to be 98 and
99 wt.% conversion in 5 h and 1 h, respectively (Table 2, entries 3
and 10).
strong enough to abstract a proton from the alkyne, a trans-
metalation step could be excluded.21 The terminal alkyne C–H
bond activation is accomplished by the coordination of the
alkyne to the SBA-15-L-(ArI–Pd–X) (3) complex. Upon coordina-
tion, the C–H bond is weakened, and H–X is removed from the
SBA-15-L-(ArI–Pd–X) (3) in the presence of a base (triethylamine)
(6) to form the reaction intermediate (5) with the subsequent
byproduct of triethylamine halide (7). In the last step of the reac-
tion mechanism a reductive elimination of diphenylacetylene
Arl–Ar (7) from the unsymmetrical intermediate (6) regenerates
the SBA-15-L-(Pd) complex.21
In order to probe the role of the solvent in the Sonogashira
coupling reactions, a series of bases such as NaOH, Na2CO3,
K2CO3, NEt3 and NaHCO3 were screened in the reaction.21
Among all the organic bases used, triethylamine (NEt3) was
found to be most reactive in comparison to the inorganic
bases. The order of reactivity of iodobenzene in the presence of
various bases could be arranged in the decreasing order as:
NEt3 (100%) > K2CO3 (58%) > Na2CO3 (45%) > NaOH (35%) >
NaHCO3 (10%). It is clear from the comparison that the triethyl-
amine base plays a highly active role in the Sonogashira cou-
pling reaction (Fig. S8†). These organic bases were superior to
inorganic bases such as K2CO3, Na2CO3, NaOH, NaHCO3, and
NEt3. This may be due to the partial inhomogeneity of the inor-
ganic bases with the organic substrate, reagent and solvent,
which lowered the conversion and increased the reaction times
compared to the organic bases (10–100% in 2 h).
Sonogashira reactivity
The original Sonogashira reaction often required a very dry
organic solvent, inert atmosphere, strong base, prolonged reac-
tion time and a phosphine containing catalyst. Generally a
copper co-catalyst was needed in the Sonogashira coupling
reaction. However, the addition of copper, although beneficial
in terms of increasing the reactivity of the system added some
shortcomings, the principal being the necessity of avoiding the
presence of oxygen in order to block the undesirable alkyne
homocoupling through a copper mediated Hay/Glaser reaction.
The copper–acetylides formed in situ could undergo an oxida-
tive dimerization to give diaryldiacetylenes when they are
exposed to air or an oxidant (a reaction known as the Glaser
coupling). These byproducts are generally difficult to separate
from the desired products. Furthermore, the copper acetylide
is a potential explosive reagent. To address such a problem, a
solution was to eliminate the copper in the so-called “copper-
free” Sonogashira reaction.21
Here, we have shown that the synthesized catalyst SBA-15–
EDTA–Pd(11) catalyst can be effectively handled in phosphine
and copper free conditions for the Sonogashira coupling
reaction. Without the involvement of copper, there are even
fewer mechanistic suggestions to be found in a homogeneous
catalytic system.22 The heterogeneous Sonogashira coupling
of phenyl acetylene with an aryl halide may proceed through
a catalytic cycle analogous to that proposed for the homoge-
neous palladium catalysts.
After optimizing the reaction parameters, various substituted
and non-substituted aryl halides were also investigated in the
Sonogashira coupling reaction between iodobenzene and phe-
nyl acetylene, in Cu free reaction conditions and the results are
summarized in Table 3.
Various aryl halides were coupled with phenylacetylene in
the presence of SBA-15–EDTA–Pd(11), triethylamine (3 mmol)
and with DMF as a solvent at 120 °C (Table 3). As evident
from the Table 3, all the substituted and non-substituted aryl
halides reacted with phenyl acetylene under mild reaction
conditions to give their corresponding products (Table 3,
entries 1–8). Monosubstituted aryl halides, such as chloro-
benzene, bromobenzene and iodobenzene, with phenyl acety-
lene gave the coupled product (biphenyl) in 75%, 85%, 100%
yields with a TON of 85, 97, 114, respectively. As discussed
earlier, the chlorides show a poor reactivity compared to the
bromides and iodides under similar reaction conditions. As
is clear from the Table 3, the Sonogashira coupling reactions
proceeded quickly with the aryl iodides and bromides, in
comparison to the chlorides (Table 3, entries 1, 2, 3).
The electron withdrawing (deficient) nitro (–NO2) bearing aryl
halides, such as 4-chloronitrobenzene, 4-bromonitrobenzene,
4-iodonitrobenene, with phenyl acetylene, gave the coupled
product 4-nitrobiphenyl in 99%, 98%, 100% yields and 113,
112, 114 TON, respectively. From the catalytic cycle point of
view in the Sonogashira coupling reaction, the oxidative addi-
tion of the aryl halide to the Pd metal centre result in the tran-
sition state (3) in the first step. The relative reactivity decreases
in the order of I > Br ≫ Cl. Aryl halides activated by the
In the first step the catalytic cycle is initiated by an oxida-
tive addition of the aryl halide (Ar–X) to species (1), forming
the oxidative addition adduct reaction intermediate (2) spe-
cies. The second step is the activation of the terminal alkyne.
Because no copper salt was employed, and the base must be
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Table 3 Reactivity of SBA-15–EDTA–Pd(11) catalyst for Sonogashira reactiona
S. no
1
Aryl halide
Product
Time (h)
24
Yield (%)
75
TON
85
3a
3a
3a
2
3
8
6
85
97
100
114
4
5
6
7
1
7
1
9
99
98
113
112
114
101
3b
3b
100
89
3b
3c
8
7
99
113
3c
a
Reaction conditions: (1.15 mmol) phenyl acetylene, (1 mmol) aryl halide, (3 mmol) triethylamine (base), 3.5 ml DMF with (15 mg)
heterogeneous SBA-15–EDTA–Pd(11) catalyst, temperature 120 °C.
proximity of electron-withdrawing groups are more reactive to
the oxidative addition than those with donating groups.
Electron withdrawing groups increase the reactivity of aryl
halides in the coupling reaction; which is clear from Table 3
(entries 4, 5, 6). However, the electron donating groups such as
methyl bearing aryl halides like 4-bromotoluene, 4-iodotoluene
with phenylacetylene, gave the corresponding products in 89%
and 99% yields with 101 and 113 TON, respectively (Table 3
entries 7, 8).
Generally, electron donating bearing aryl halides show a
lower reactivity in coupling reactions, since the electron density
increases over the Pd intermediate (3) results in a lower feasi-
bility of further transmetalation and the reductive elimination
step, respectively. It is clear from Table 3 (entries 5–8) that aryl
halides bearing methyl and nitro groups react with phenyl acet-
ylene to give excellent yields of the corresponding biaryl,
whereas the aryl chlorides possessing para-substituents, give
good yields. Electron-rich substrates, such as 4-bromotoluene
and 4-iodotoluene show lower reactivity in comparison to the
electron withdrawing bearing aryl halides and take a longer
reaction period for the completion of the reaction (Table 3,
entries 7, 8). The coupling reaction of sterically hindered
2-chloronapthalene with phenyl acetylene did not proceed,
even after a longer reaction time (24 h).
Heterogeneity and recycling studies of catalyst SBA-15–EDTA–Pd(11)
To test if the metal was leached out from the solid catalyst
during the reaction, a hot filtration test was performed. In
this process the Sonogashira coupling reaction mixture was
collected by filtration at the reaction temperature (120 °C)
after a reaction time of 1 h, which gave a 58% conversions of
iodobenzene. The residual activity of the supernatant solu-
tion was studied. It was noticed that after filtration of the
SBA-15–EDTA–Pd(11) catalyst from the reaction mixture at
the elevated reaction temperature (120 °C), (in order to avoid
a possible recoordination or precipitation. of soluble palla-
dium upon cooling) the coupling reactions did not proceed
further. Thus, the results of the hot filtration test suggest that
Pd was not being leached out from the solid catalyst during
the coupling reactions. These results confirm that the palla-
dium catalyst remains on the support even at elevated tem-
peratures during the reaction.
Further, to evaluate the reusability, after carrying out the
reaction, the mixture was filtered using a sintered glass funnel,
and the residue washed with DMF (3–5 ml), dichloromethane
(2–5 ml) and acetone (2–5 ml). After being dried in an oven
(overnight), the catalyst could be reused directly without fur-
ther purification (Fig. 9). The amount of Pd leaching into
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efficiently and reused five times, without a major loss in
activity, due to the well modified surface properties of the
SBA-15 as a support.
Experimental
SBA-15 was synthesized according to the reported procedure
using tri-block P123 as a template under acidic conditions.23
Pluronic 123 (2 g) was dissolved in a solution of HCl (60 mL
of 2.0 M) and H2O (60 mL). The solution was stirred at 38 °C
for 1 h. After that time, tetraethylorthosilicate (TEOS) was
added, and the resultant solution was left stirring at 38 °C
for a further 4 h and then aged at 100 °C for 24 h. For the
surface medication, see the ESI.†
Fig. 9 Recycling study of SBA-15–EDTA–Pd(11) catalyst.
SBA-15–Pd–EDTA synthesis
PdCl2 (0.5 mmol) in 5 ml distilled water was treated with
0.186 g (0.5 mmol) of EDTA and 1 mmol of sodium carbonate
(Na2CO3) and stirred to give yellow red solution. To the resul-
tant solution, the calculated amount (1.0 g) of organo-
modified SBA-15 was added, along with the slow addition
of 25 ml of Millipore water.8 The final mixture was stirred at
75 °C for 24 h and washed with distilled water and soxhlet
extracted to remove the unanchored materials from the
SBA-15 surface (Scheme 1D). The resultant material was named
SBA-15–EDTA–Pd(11). Similarly, 7 and 15 wt.% of Pd loading
were also synthesized by using corresponding amount of
modified SBA-15 and PdCl2 and the obtained materials were
designated as SBA-15–EDTA–Pd(7), SBA-15–EDTA–Pd(11),
SBA-15–EDTA–Pd(15).
solution for the Suzuki reactions were detected through ICP.
The amount of lost Pd for the reaction was less than 1.0 weight
% of the total Pd content. Even though a small amount of Pd
loss could be detected, the catalyst still showed a relatively high
reusability and stabilities for Sonogashira coupling reactions.
The present study indicates that the catalyst can be recycled a
number of times without losing its activity to a greater extent.
Conclusion
In summary, a highly stable and recyclable SBA-15–EDTA–Pd
catalyst has been synthesized by anchoring a Pd–EDTA com-
plex over the inner surface of organo-functionalized SBA-15
with different wt.% loading (7%, 11%, and 15%). XRD and
N2 sorption analyses, which reveal the morphological and tex-
tural properties of the synthesized catalyst, confirm that the
metal complex Pd–EDTA is firmly attached to the organo-
modified SBA-15 support and the ordered mesoporous chan-
nel structure was retained even after multistep synthetic
procedures. The TGA-DTA results reveal the thermal stability
of the synthesized catalyst SBA-15–EDTA–Pd(11) at elevated
temperatures. The organic moieties anchored over the sur-
face of the SBA-15 and inside the pore wall were evident by
solid state 13C NMR spectra and FT-IR spectroscopy. Further,
solid state 29Si NMR spectroscopy provides information about
the degree of silylation and functionalization with the
organic moieties. The electronic environment and oxidation
state of the Pd metal in the SBA-15–EDTA–Pd(11) were con-
firmed by XPS and DRS UV-visible techniques. Subsequently,
the morphology information of the synthesized catalyst was
monitored by SEM and TEM spectroscopy. The synthesized
heterogeneous catalyst SBA-15–EDTA–Pd(11) was screened in
the Suzuki and Sonogashira coupling reactions and shows a
higher catalytic activity with a higher TON in phosphine free
conditions. The present SBA-15–EDTA–Pd(11) catalytic system
tolerates a broad range of functional groups of the aryl halide
in both the Suzuki and Sonogashira coupling reactions, with-
out using a phosphine ligand or Cu co-catalyst. The hetero-
genized solid catalyst SBA-15–EDTA–Pd(11) can be recycled
General procedure for Suzuki coupling reactions
The Suzuki reaction was carried out in a 25 ml oven dried,
two necked round bottom flask, heated over a reactor with
high stirring 700+ rpm. In a typical run, 1 mmol of aryl
halide, 1.15 mmol of aryl boronic acid, 3 mmol potassium
carbonate (K2CO3) and 15 mg of SBA-15–EDTA–Pd(11)
(heterogeneous catalyst) were allowed to stir at 120 °C with
the solvent DMF (3.5 ml). The reaction mixture was sampled
at measured time intervals and analyzed by gas chromatogra-
phy. The samples were centrifuged with high rpm before
injection into a gas chromatograph.
General procedure for Sonogashira coupling reactions
The catalyst screening for the Sonogashira coupling reaction
was carried out in 25 ml oven dried, two necked round bottom
flask, heated over a reactor with high stirring 700+ rpm. In a
typical, run 1 mmol of aryl halide, 1.5 mmol of phenylacetylene,
3 mmol triethylamine (base) and 15 mg of SBA-15–EDTA–Pd
(11) (heterogeneous catalyst) were allowed to run at 120 °C with
DMF as the solvent. The reaction mixture was sampled at mea-
sured time intervals and analyzed by gas chromatography.
The samples were centrifuged with high rpm before injecting
into the gas chromatograph. The products were analyzed by
GCMS and 1H, 13C CP MAS NMR.
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Paper
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Acknowledgements
The authors gratefully acknowledge the CSIR networking pro-
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Delhi, India for a Senior Research Fellowship. The authors are
thankful to Dr. Satynarayana Chlikuri for the adsorption mea-
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