A new functionalized mesoporous matrix supported Pd(ii)-Schiff base complex: An efficient catalyst for the Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1039/c003142a

A new Pd(ii) bounded 2D-hexagonally ordered functionalized MCM-41 type material (IV) has been synthesized. Functionalization was carried out by the anchoring of 3-aminopropyltriethoxysilane in the MCM-41 type mesoporous material, followed by grafting with 2,6-diacetylpyridine (DAP) to give a N ₃-type Schiff base chelating attachment for the Pd(ii) species. Fourier transform infrared (FTIR), powder X-ray diffraction (PXRD) and high resolution transmission electron microscopy (HRTEM) studies have been used to characterize the material. Material IV behaves as a highly active catalyst towards Suzuki-Miyaura cross-coupling reaction for the synthesis of biaryl organics. In addition, IV acts as a true heterogeneous catalyst in coupling reaction. It was found that this catalyst is highly efficient and recyclable towards Suzuki-Miyaura reaction with high turn over frequencies. X-Ray photoelectron spectroscopic (XPS) analysis was employed to understand the oxidation state of the palladium atom in the catalyst (IV) and its loading in the material. The Royal Society of Chemistry.

Full text of DOI:10.1039/c003142a

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A new functionalized mesoporous matrix supported Pd(II)-Schiff base
complex: an efficient catalyst for the Suzuki–Miyaura coupling reaction†
Koushik Dhara,*a Krishanu Sarkar,b Dipankar Srimani,c Subrata Kumar Saha,d Pabitra Chattopadhyaye and
Asim Bhaumik* f
Received 15th February 2010, Accepted 7th May 2010
First published as an Advance Article on the web 7th June 2010
DOI: 10.1039/c003142a
A new Pd(II) bounded 2D-hexagonally ordered functionalized MCM-41 type material (IV) has been
synthesized. Functionalization was carried out by the anchoring of 3-aminopropyltriethoxysilane in the
MCM-41 type mesoporous material, followed by grafting with 2,6-diacetylpyridine (DAP) to give a
N3-type Schiff base chelating attachment for the Pd(II) species. Fourier transform infrared (FTIR),
powder X-ray diffraction (PXRD) and high resolution transmission electron microscopy (HRTEM)
studies have been used to characterize the material. Material IV behaves as a highly active catalyst
towards Suzuki–Miyaura cross-coupling reaction for the synthesis of biaryl organics. In addition, IV
acts as a true heterogeneous catalyst in coupling reaction. It was found that this catalyst is highly
efficient and recyclable towards Suzuki–Miyaura reaction with high turn over frequencies. X-Ray
photoelectron spectroscopic (XPS) analysis was employed to understand the oxidation state of the
palladium atom in the catalyst (IV) and its loading in the material.
offers considerable complexity to the reaction that may render
Introduction
it industrially unacceptable.4 Thus the heterogenization of a Pd-
Palladium is one of the most crucial metals in catalysis and it
catalyst is a logical step to avoid these problems, as it allows
is commonly used to catalyze a large variety of carbon–carbon
for easy removal from the reaction mixture. However, inorganic
bond forming reactions.1 Of these reactions, the Suzuki–Miyaura
heterogeneous Pd-catalysts show low stability due to leaching of
palladium from the surfaces.5 In fact the leached homogeneous
Pd species is responsible for the catalytic activity in most of
the cases.6 Therefore the search for new efficient and recyclable
heterogeneous catalysts has received much attention.7 Different
approaches like encapsulation or immobilization of catalytically
active metal complexes in solid supports such as zeolites,8 covalent
grafting of such active complexes onto reactive polymer surfaces9
or inorganic porous matrices10 etc. have been utilized in developing
an efficient heterogeneous catalyst. Thus, functionalized ordered
mesoporous silica with high surface area, e.g. MCM-41, MCM-
48 and SBA-15 having attractive nanoscale pore structure are the
natural choice for heterogenization.11
Modified Pd-silica catalysts have been investigated by Molnar
et al.12 and found that Pd immobilized on silica modified with
methyl or phenyl groups exhibited high activity and selectivity in
the Mizoroki–Heck coupling reaction. So the choice of ligand
for functionalization of silica matrices has an important role
on catalytic activity and stability. Mandal et al.13 and Bedford
et al.14 reported the immobilization of Pd nanoparticles on amine-
functionalized zeolites and mesoporous silica, respectively, and
observed high activity and stability of both catalysts in the
Mizoroki–Heck reaction. Tsai et al.15 and Gonzalez-Arellano
et al.16 investigated the nanosized Pd-bipyridyl complex and a
Pd(II)-Schiff base complex anchored onto MCM-41 which showed
high reactivity and recyclability for the Mizoroki–Heck reaction.
An active catalyst for the Ullmann reaction of iodobenzene was
phenyl-PMO supported Pd(II) complex that was reported by
Wan et al.17 Recent developments on the mesoporous material
reaction is very important for constructing unsymmetrical biaryl
compounds compared to other coupling reactions, which employ
more reactive organometallics such as organozinc or Grignard
reagents. Complexes of palladium(II) with nitrogen containing
ligands are well-known catalysts for Suzuki–Miyaura coupling
reactions and thus there is a large interest to employ this reaction
in the formation of C–C bonds.2 The organoboronic acids (and
esters) commonly employed in the Suzuki–Miyaura reaction,
which are air- and moisture-stable and have relatively low toxicity.3
However, common catalysts for the Suzuki–Miyaura reaction are
generally based on homogeneous palladium complexes of Pd(0)
or Pd(II), which causes difficulties in purification of the final
product, recycling of the catalyst, and deactivation via aggregation
into Pd nanoparticles. Further removal of residual palladium
aDepartment of Chemistry, Sambhu Nath College, Labpur, Birbhum,
731303, West Bengal, India. E-mail: koushikdhara@rediffmail.com;
Fax: +913324732805; Tel: +919433125327
bDepartment of Chemistry, Netaji Mahavidyalaya, Arambagh, Hooghly,
712601
cDepartment of Organic Chemistry, Indian Association for the Cultivation
of Science, Kolkata, 700 032, India
dDepartment of Physics, Sambhu Nath College, Labpur, Birbhum, 731303,
West Bengal, India
eDepartment of Chemistry, Burdwan University, Golapbug, Burdwan, 713
104, West Bengal, India
f
Department of Materials Science, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata, 700 032, India. E-mail: msab@iacs.res.in
† Electronic supplementary information (ESI) available: Detailed 1H and
13C data of the coupling products. See DOI: 10.1039/c003142a
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SBA-15 provided a new possible candidate for a solid support for
immobilization of homogeneous catalysts.11f,18 Crudden et al.,19
Shimizu et al.,20 and Ji et al.21 reported that Pd complexes
immobilized on mercapto-functionalized SBA-15 and FSM-16
also displayed high activity and excellent recyclability in the
Suzuki–Miyaura reaction. Recently Sarkar et al.22a reported facile
Suzuki coupling over ortho-metalated Pd(II) complex anchored on
2D-hexagonal mesoporous organosilica. It is generally believed
that high surface area of a heterogeneous catalyst results in high
catalytic activity. Considering the fact that MCM-41 support
has an extremely high surface area and the catalytic palladium
species is anchored on the inner surface of the mesopores of the
MCM-41 support, we expect that a MCM-41 supported palladium
catalyst will exhibit high activity, stability and good reusability.
To the best of our knowledge for the reaction of organoboron
compounds with organic halides, only very few palladium(II)
complexes anchored on functionalized MCM-41 support have
been prepared and successfully used in organic reactions.22
In this paper, we report the synthesis (Scheme 1) of a new
Pd(II) bound heterogeneous catalyst (IV) via functionalization
of 2D-hexagonally ordered MCM-41 type material with 3-
aminopropyltriethoxysilane followed by the grafting with 2,6-
diacetylpyridine (DAP). Fourier transform infrared (FTIR),
powder X-ray diffraction (PXRD), high resolution transmission
electron microscopy (HRTEM) and X-ray photoelectron spectra
(XPS) studies have been used to characterize the material. We
wish to focus the application of this Pd(II)-complex supported
on functionalized MCM-41, which involves the use of DAP
based N3-Schiff base as a chelating attachment, for the Suzuki–
Miyaura coupling. This functionalized mesoporous material (IV)
acts as a true heterogeneous catalyst together with high reactivity
and recycling efficiency towards Suzuki–Miyaura C–C coupling
reaction.
Results and discussion
Fourier transform IR spectroscopy analysis
FTIR spectra of I, II, III and IV are shown in Fig. 1. C–H and
N–H vibration bands in the 3500–2800 cm-1 and 1650–1500 cm-1
regions were found in the FTIR spectra of II and these bands are
surely absent in the case of I. This result shows that 3-APTES
has been attached into the mesoporous silica (I) matrix. Now,
II is allowed to react with the 2,6-diacetylpyridine (DAP) and
it produces a functionalized mesoporous Schiff-base compound
(III) (Scheme 1).23 The bands observed at 1670 cm-1 and 1527 cm-1
for III could be attributed to the C N and C C (pyridine ring)
stretching frequency respectively. The free C O (acetyl group)
of DAP shows a band at 1695 cm-1 and is absent in the case of
III which indicates the absence of free –CH3C O in III. Thus it
is evident from the FTIR spectrum that both the acetyl groups
are converted into Schiff base. The spectrum of isolated Pd(II)-
=
=
=
=
=
bound complex (IV) is shown in Fig. 1 where the C N stretching
frequency is red shifted by 64 cm-1 and appears at 1606 cm-1. The
stretching frequency at 564 cm-1 in IV indicates uPd–N 11e In order to
.
establish the homogeneous distribution of functional groups inside
material II, FTIR spectra have been carried out in a quantitative
manner. Percentage transmittances (–CH2– groups taken as probe)
have been plotted against the weight taken of the sample and a
linear curve fit (Fig. 2) is obtained, signifying nearly homogeneous
distribution of functional groups.23
Fig. 1 FTIR spectra of samples I–IV.
Powder XRD
The small angle powder X-ray diffraction patterns for I, II, III and
IV are shown in Fig. 3. Sample I is highly ordered, showing three
strong diffractions for the 100, 110, 200 planes and a weak one
for the 210 plane of 2D-hexagonal mesophase.11a,24 Comparison
of the diffraction patterns for samples I–III indicates that the 2D-
hexagonal ordering have been almost retained after the binding
of 3-APTES and also after the grafting of DAP molecule. An
overall decrease in the intensity of the peaks is observed after the
binding with DAP. This is attributed to the lowering of local order,
such as variations in the wall thickness, or it may be due to the
reduction of scattering contrast between the channel wall of the
silicate framework and Schiff base ligand present in III mentioned
Scheme 1 (a) Functionalization of the pore wall of the MCM-41 type
material with 3-aminopropyltriethoxysilane (3-APTES), (b) grafting of
the 2,6-diacetylpyridine (DAP) to the surface of II and (c) Pd(II) (PdCl2)
binding with III. All three Si–OH groups, attached to the mesopore surface,
are shown in the scheme for clarity.
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Fig. 4 HRTEM images of material IV in the direction of the pore axis.
FFT pattern is shown in the inset.
Fig. 2 % Transmittance (T) vs. weight of the material II obtained from
FTIR.
Fig. 5 EDS pattern of material IV. [Signals of Cu are due to the sample
grid.]
N2 sorption
Fig. 6(a) indicates the N2 adsorption/desorption isotherms of
materials I and IV. The isotherm of material I could be classified
as type IV characteristic of the mesoporous materials.24–26 Sharp
capillary uptake is observed for the adsorption of N2 at P/P0 =
0.3–0.5, indicating the mesopores were uniform in size. Pore size
distribution (PSD) of these samples I and IV estimated by the non-
local density functional theory (NLDFT) is shown in Fig. 6(b).
This PSD pattern suggested the peak pore diameter of 4.0 nm
and 2.4 nm for samples I and IV, respectively. Pore diameter
estimated from the respective TEM image analysis resembles nicely
Fig. 3 Small angle XRD patterns for samples I–IV.
previously by Lim and Stein.25 XRD pattern is retained in the case
of Pd(II)-bounded material IV, where the d-spacing is found to
increase a little, and this is caused probably due to the coordination
of the Pd(II) with the chelating N3-type ligand unit inside the pore.
No wide-angle XRD peak (not shown here) was seen in material
IV suggesting the absence of metallic Pd in the material.
HRTEM
High resolution transmission electron micrographs (HRTEM) of
Pd(II)-catalyst (IV) are shown in Fig. 4 in the perpendicular direc-
tion of the pore axis. The 2D-hexagonal arrangement of the pores
with a different contrast than that of the pore walls throughout the
specimen is quite clear. Hexagonal pore ordering after the DAP
grafting and also for Pd(II)-binding with the composite has been
observed. It is further confirmed by the respective SAED (selected-
area electron diffraction) pattern (Fig. 4, inset) corroborating with
the hexagonal ordering of the pores. From the TEM image and
XRD data, it can be concluded that the ordered mesoporous silica
based matrix remains almost intact throughout all the reaction
steps starting from the surface functionalization to the binding of
Pd(II). The metal content of the material IV was investigated using
EDS. EDS spectra at different points of the image (Fig. 5) confirm
the presence of Pd in the mesoporous silica matrix.
Fig. 6 (a) N2 adsorption/desorption isotherms of I and IV, (b) pore
size distributions, determined by the non-local density functional theory
(NLDFT) method.
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Table 1 XPS results for different elements present in catalyst (IV)
compared to more easily coupled electron-deficient species, such
as aryl iodides, and the data are depicted graphically in Fig. 8. The
optimum activity (measuring percentage conversion in 8 h) of the
catalyst (IV) was found in DMF solvent together with KF acting
as base. These conditions were thus used for all further studies.
Elements
Pd
Binding energy
Si
C
3d5/2
3d3/2
N
Observed/eV
Corrected/eVa
Literature/eVb
107
103
103.5
289
285
285
342
338
337.5
347
343
343
404
400
402
a Corrected to C 1s with binding energy of 285 eV using adventitious
carbon. b Handbook of X-Ray Photoelectron Spectroscopy, Physical Elec-
tronics, Perkin Elmer.
with these values. The BET surface area and pore volume of
sample I was 952 m2g-1 and 0.79 ccg-1, whereas for sample IV this
value decreased to 154 m2g-1 and 0.14 ccg-1, respectively. During
functionalization via 3-APTES and later grafting by the DAP, the
pore volumes are drastically reduced. This could be attributed to
the more loading of organics, which occupy the void space inside
the mesopores. It is clearly suggested that the chromophores are
grafted inside the mesopores.
X-Ray photoelectron spectra
Fig. 8 Bar diagram of catalyst activity in the solvent/base optimization
for catalyst IV (coupling of phenylboronic acid with 4-bromoanisole in
DMF/KF at 100 ◦C).
X-Ray photoelectron spectra (XPS) were carried out for material
IV in order to define the oxidation state of the palladium atom in
the catalyst (IV) and also to detect the presence of the desired
element in the catalyst (IV). Fig. 7 showed the typical Pd(II)
oxidation state (Pd 3d5/2 = 338 eV, Pd 3d3/2 = 343 eV) and no
cluster formation of Pd metal in the catalyst.11e Table 1 shows
the XPS binding energy values for different elements present in
the catalyst (IV). These results suggest a nice correlation of the
observed and literature binding energies and thus the presence of
Pd(II), Si, C and N for material IV as shown in Scheme 1.
Suzuki–Miyaura reaction
To evaluate the activity of the catalysts, the Suzuki–Miyaura cou-
pling reaction of organoboron compounds with organic halides
was studied. The choice of the conditions was based on those
frequently employed for homogeneous and heterogeneous cat-
alyzed Suzuki–Miyaura coupling reactions. A range of substituted
aryl bromides were used as substrates. The entries in Table 2
reveal that both electron withdrawing (entries 1–6) and electron-
donating (entries 8–12) substituents yielded excellent results. Even
2-substituted aryl bromides (5, 10) led to excellent yield of the
desired biaryl.
Heterogeneity test
Leaching of palladium in solution and also recovery and reuse of
the catalyst are the vital topics in coupling reaction especially in
heterogeneous reaction condition.27 To elucidate this matter, we
have performed a series of tests as described below.
Hot filtration test28
Fig. 7 X-Ray photoelectron spectra (XPS) of catalyst (material IV) (A:
Si 2p3/2; B: C 1s1/2; C: Pd 3d5/2, 3d3/2; D: 2p3/2).
After completion of 30% of the coupling reaction the liquid phase
is collected by filtration at the reaction temperature. If palladium
is leaching from the solid surface during the reaction, the liquid
phase should contain palladium. Atomic absorption spectrometric
analysis of the liquid phase reveals that any detectable palladium
is absent in the reaction mixture. Simultaneously the coupling
reaction does not proceed further after the filtration of the
Pd(II)-catalyst (IV) from the batch reactor at the same reaction
temperature. It is further noted that palladium is also not detected
in the liquid phase after the completion of the reaction and
remarkably the filtrate remains completely colorless. These results
Catalytic testing
Initially, a solvent/base optimization study was carried out for the
coupling of phenylboronic acid with 4-bromoanisole catalyzed
by catalyst IV, using 1,4-dioxane, toluene and DMF as solvents
and KF : K3PO4 (1 : 1), KF, K2CO3, K3PO4 and Cs2CO3 as bases.
4-Bromoanisole was chosen as the test substrate because it is
electronically deactivated with respect to oxidative addition and
therefore gives a more accurate assessment of catalyst activity as
6398 | Dalton Trans., 2010, 39, 6395–6402
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