A new periodic mesoporous organosilica containing diimine-phloroglucinol, Pd(ii)-grafting and its excellent catalytic activity and trans-selectivity in C-C coupling reactions

Article abstract of DOI:10.1039/c0jm01180k

A new organosilane precursor has been designed via Vilsmeier-Haack formylation of phloroglucinol followed by its Schiff base condensation with 3-aminopropyl-triethoxysilane (APTES). A novel organic-inorganic hybrid periodic mesoporous organosilica (PMO) LHMS-3 containing the highly coordinating bis(propyliminomethyl)-phloroglucinol moiety inside the pore wall has been synthesized by using this precursor organosilane molecule. Phenolic-OH and imine-N donor sites present in this PMO material have been utilized to anchor Pd(ii) species at the surface of the mesopores. Small angle neutron scattering, XRD, HR TEM, SEM, ¹³C and ²⁹Si solid state MAS NMR, UV-vis and FT IR spectroscopic tools are utilized to characterize the 2D-hexagonal mesophase and the presence of the bis(propyliminomethyl)-phloroglucinol moiety inside the pore wall. This Pd-anchored material Pd-LHMS-3 showed excellent catalytic activity and trans-selectivity in Heck C-C bond formation reactions for the synthesis of a series of value-added aromatic and aliphatic olefins.

Full text of DOI:10.1039/c0jm01180k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A new periodic mesoporous organosilica containing diimine-phloroglucinol,
Pd(II)-grafting and its excellent catalytic activity and trans-selectivity in C–C
coupling reactions†
a
Arindam Modak, John Mondal, Vinod K. Aswal and Asim Bhaumik
a
b
a
*
Received 23rd April 2010, Accepted 23rd June 2010
DOI: 10.1039/c0jm01180k
A new organosilane precursor has been designed via Vilsmeier–Haack formylation of phloroglucinol
followed by its Schiff base condensation with 3-aminopropyl-triethoxysilane (APTES). A novel
organic–inorganic hybrid periodic mesoporous organosilica (PMO) LHMS-3 containing the highly
coordinating bis(propyliminomethyl)-phloroglucinol moiety inside the pore wall has been synthesized
by using this precursor organosilane molecule. Phenolic-OH and imine-N donor sites present in this
PMO material have been utilized to anchor Pd(II) species at the surface of the mesopores. Small angle
neutron scattering, XRD, HR TEM, SEM, 13C and 29Si solid state MAS NMR, UV-vis and FT IR
spectroscopic tools are utilized to characterize the 2D-hexagonal mesophase and the presence of the
bis(propyliminomethyl)-phloroglucinol moiety inside the pore wall. This Pd-anchored material Pd-
LHMS-3 showed excellent catalytic activity and trans-selectivity in Heck C–C bond formation
reactions for the synthesis of a series of value-added aromatic and aliphatic olefins.
PMO materials and the resulting materials have found exciting
Introduction
advanced applications in catalysis,18 biosensors,19 enantiomeric
separation,20 light harvesting,21 drug release22 and so on. Recently
we have reported strong photoluminescence from a fluorophore
grafted organic–inorganic hybrid mesoporous silica, synthesized
through Schiff base condensation between an aromatic
dialdehyde and aminopropyl-trimethoxysilane (APTES).23 Due
to the presence of highly coordinating phenolic-OH and imine-N
donor groups in the framework these fluorophore grafted mate-
rials23,24 showed excellent metal ion sensing properties.
Since the first report of highly ordered mesoporous silica using
the supramolecular templating approach,1 a large variety of
precursor molecules have been utilized for the synthesis of novel
functionalized mesoporous materials to explore their appli-
cations in different frontier areas like sensing,2 adsorption,3
catalysis,4,5 ion-exchange,6 optoelectronics,7 and so on. In order
to functionalize the surface of mesoporous silica and induce
hydrophobicity a large number of organosilanes have been used
as precursors along with tetraethyl orthosilicate for the synthesis
of organically modified mesoporous materials.8,9 Further, on
loading of an active metal in the framework of these organically
modified mesoporous materials showed much enhanced catalytic
activity vis-aꢀ-vis pure mesoporous metallosilicates.10–12 Thus
designing active metal anchored mesoporous material having
high surface area has huge potential to be exploited in catalytic
reactions. In this context another versatile strategy has been
developed for the synthesis of periodic mesoporous silicas
(PMOs),13 where an organosilane compound [(R0O)3Si–R–
Si(OR0)3, R ¼ CH2CH2, C6H4, etc., R0 ¼ CH3, C2H5 etc.] having
two trialkoxysilyl groups connected by an organic bridge alone
can be used as the precursor molecule for the synthesis of
mesoporous materials.13–17 These PMOs have attracted wide-
spread attention in recent times because of huge possibilities to
tune the nature of organic functional group in the precursor
organosilane compound and thereby controlling their physico-
chemical and related surface properties. Different functional
organic moieties have been grafted inside the pore wall of these
On the other hand, the Pd-containing catalysts have gained
increasing attention over the years in a series of C–C coupling
reactions especially in Heck25 and Suzuki26 reactions for the
synthesis of value-added organic compounds.27 For homo-
geneous systems these Pd-containing catalysts suffer the draw-
back of complicated catalyst recovery and difficult regeneration
processes. Very recently Pd-containing heterogeneous catalytic
systems have been intensively studied in Heck reactions, where
the Pd species is anchored inside the pore walls of mesoporous
material.28–30 However, the nature of binding of Pd species to the
pore-wall, which essentially determines the stability of metal-
anchoring and thus reducing the possibility to leach the Pd
species during the catalytic reaction at elevated temperature still
remains a challenge today over Pd-functionalized materials
under liquid phase catalytic reaction conditions. The presence of
multiple imine-N and phenolic-OH groups in the bis(propyli-
minomethyl)-phloroglucinol moiety in the mesopore wall could
anchor Pd(II) species more strongly in the mesopore surface and
thus minimizing the possibility to leach-out the Pd species during
the reaction. Herein, we first report a new organic–inorganic
hybrid periodic mesoporous organosilica (LHMS-3) containing
a highly coordinating diimine-phloroglucinol moiety (designed
through Vilsmeier–Haack formylation reaction of phlor-
oglucinol followed by its Schiff base condensation with 3-ami-
nopropyl-trimethoxysilane) inside the pore wall, which can
aDepartment of Materials Science, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata, 700 032, India. E-mail: msab@iacs.res.in
bSolid State Physics Division, Bhabha Atomic Research Centre, Trombay,
Mumbai, 400 085, India
† Electronic supplementary information (ESI) available: Further
spectroscopic data. See DOI: 10.1039/c0jm01180k
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effectively anchor Pd(II) species strongly on its mesopore surface,
and thereby show excellent catalytic activity and trans-selectivity
in the Heck C–C bond formation reaction between aryl halides
and alkenes (both aromatic and aliphatic) for the synthesis of
value-added organics.
a two-neck round-bottom flask equipped with a condenser and
an addition funnel. A solution of APTES (2.38 g, 0.0108 mol) in
dry ethanol (5 ml) was added drop-wise over 30 min. Within
5 min after initiating the addition, the reaction became mildly
exothermic. The reaction mixture was kept at reflux for 2 h after
the complete addition of APTES. The mixture was allowed to
cool to room temperature and evaporated using a rotary evap-
orator, leaving behind a very viscous dark red gel, which dried in
air to yield a dark red solid. This red solid was characterized by
solid state 13C (Fig. 1, d ¼ 9.6 (2C), 18.0 (2C), 23.8 (6C), 52.2
(2C), 58.2 (6C), 99.6 (1C), 105.5 (2C), 157.8 (2C), 167.3 (3C)
ppm) and 29Si (Fig. 2: d ¼ ꢀ51.2 ppm) MAS NMR spectroscopy.
Experimental
Synthesis of diformylphloroglucinol
Diformylphloroglucinol was synthesized through Vilsmeier–
Haack formylation31 of phloroglucinol (Scheme 1). In a typical
synthesis 1.0 g (0.008 mol) phloroglucinol (Merck) was dissolved
in 10 ml distilled dioxane (Merck) under rapid stirring and mild
heating. In another beaker 1.75 gm DMF (0.024 mol, Merck)
was taken and then 3.67 gm phosphorous oxychloride
(0.024 mol, Lobie Chemie) was added drop-wise maintaining the
temperature of the mixture at about 303 K through mild heating.
During this addition a highly viscous and reactive yellow liquid
was formed. Then this yellow iminium salt was added very slowly
to the phloroglucinol solution under rapid stirring maintaining
the temperature at ca. 313–318 K. After complete addition, the
reactant mixture was stirred for several hours until a yellow
crystalline solid is formed. Then the solid was dissolved in the
minimum amount of distilled water and then twice extracted with
ethyl acetate. The organic layer was washed with Brine solution
and then dried in sodium sulfate for about 30 min. After
removing ethyl acetate using a rotary evaporator, an orange
yellow solid was recovered. The solid was further heated at 363 K
until it was completely solvent free. Then it was re-crystallized
from ethylacetate–petroleum ether (1 : 5) and characterized by
solid state 1H NMR (300 MHz, DMSO-d6: d 12.49 (br, 3H, OH),
9.992 (s, 2H, CHO), 5.88 (s, 1H, Ar-H) ppm), 13C NMR
(75 MHz, DMSO-d6: d ¼ 94.5 (CH), 104.2 (C), 169.9 (C), 191.85
Synthesis of LHMS-3
I was used as organosilane source along with tetraethyl-ortho-
silicate (TEOS, Aldrich) in 1 : 3 molar ratio for the synthesis of
LHMS-3. Cetyl pyridinium chloride (CPC, Loba Chemie) was
used as a structure-directing agent and tartaric acid (TA, Loba
Chemie) was used to adjust the initial pH of the synthesis
mixture. In a typical synthesis 3.58 g (0.01 mol) of CPC was
dissolved in an aqueous solution of TA (0.6 g in 70 g H2O) under
vigorous stirring. To this 1.2 g Brij-35 (C12H25–(OC2H4)23–OH,
co-surfactant, Loba Chemie) was added. The resulting mixture
was stirred for about 30 min to obtain a clear solution. Then the
(CH) ppm) and FT IR (KBr: ꢀ ¼ 2890, 1641.2, 2599, 2783.4,
´
1402.9, 1260, 1196.8, 1097 cmꢀ1).
Synthesis of Schiff base bridged organosilane precursor I
3-Aminopropyl triethoxysilane (APTES, Adrich) was used as Si
source for its condensation with the aromatic aldehyde moiety of
diformylphloroglucinol (Scheme 1). For the synthesis of this
Fig. 1 13C CP MAS NMR spectrum of the precursor I.
Schiff base,
a solution of diformylphloroglucinol (1.0 g,
0.0054 mol) in dry ethanol (20 ml) was placed under argon in
Scheme 1 Synthesis of I: Vilsmeier–Haack hydroformylation followed
by Schiff base condensation.
Fig. 2 29Si MAS NMR spectrum of the precursor I.
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viscous gel was poured into the premixed surfactant solution and
stirred vigorously for about 2 h. After 2 h stirring, 2M NaOH
solution was added drop-wise into the mixture until the pH
reached ca. 12.0. The resulting solution was kept under stirring
for another 14 h at room temperature and then hydrothermally
treated at 353 K for 72 h under static conditions. The red powder
was recovered under filtration, washed several times with water
and dried under vacuum. Surfactant CPC was removed from the
as-synthesized solid by two repeating ethanol–HCl extraction at
room temperature and this sample is designated as LHMS-3. For
Pd-loading 1 g of LHMS-3 was suspended in acetic acid solution
(15 ml) of palladium(II) acetate (0.75 g) and the suspension was
refluxed for 24 h at 393 K, the dark red mesoporous material
slowly changed color to deep brown and no further change in
color was observed on further reflux. The resulting mesoporous
material was filtered and washed thoroughly with tetrahydro-
furan and methanol and dried under vacuum to obtain
Pd-LHMS-3.
adsorption/desorption isotherms are obtained using a Bel Japan
Inc. Belsorp-HP at 77 K. Prior to gas adsorption, samples were
degassed for 4 h at 393 K under high vacuum conditions. 13C and
29Si solid state MAS NMR measurements were performed on
a Bruker Avance 500 MHz NMR spectrometer using a 4 mm
MAS probe under static conditions. The 90ꢂ pulse lengths used
for 13C in the experiments were typically 3.0 ms and a recycle time
of 5 s. The chemical shifts for 29Si spectra are referenced to TMS
at 0 ppm. Thermogravimetry (TG) and differential thermal
analyses (DTA) of the samples were carried out using a TGA
Instruments thermal analyzer TA-SDT Q-600. Atomic absorp-
tion spectroscopic (AAS) measurements were carried-out by
using a Shimadzu AA-6300 AAS spectrometer fitted with
a double beam monochromator. UV-visible diffuse reflectance
spectra were recorded on a Shimadzu UV 2401PC with an inte-
grating sphere attachment. BaSO4 was used as background
standard. FT IR spectra of these samples were recorded using
a Nicolet MAGNA-FT IR 750 Spectrometer Series II.
For a typical catalytic reaction under nitrogen, an oven-dried
two-necked round-bottom flask containing a stirring bar was
charged with an organic halide (1.00 mmol), olefin (1.00 mmol),
Pd-LHMS-3 catalyst (0.05 g) and triethanolamine (5.0 ml). The
mixture was refluxed at 373 K for ca. 12–14 h under vigorous
stirring. Then the mixture was allowed to cool to room temper-
ature and filtered immediately to separate the catalyst from the
residual mixture. The filtrate was diluted by addition of diethyl
ether (15 ml) and then it was extracted using ethyl acetate. The
combined organic layers were washed with water and brine and
then dried with MgSO4 and concentrated under reduced pressure
to obtain either a solid residue or a viscous liquid. The product
was finally purified through re-crystalization in an ethylacetate–
hexane mixture to obtain the yield.
Results and discussion
Nanostructure
Fig. 3 shows the SANS data of the template-free LHMS-3
sample. The scattering intensity clearly depicts a Bragg peak at
ꢀ1
ꢁ
Q ¼ 0.151 A corresponding to the ordered mesoporous struc-
ture in the sample. The Bragg peak occurs at Q ¼ 2p/d, where d is
the repeating distance between the pores. From this figure we
obtain value of d ¼ 4.16 nm, which agrees very well with small
angle powder XRD data (d100 ¼ 4.1 nm, Fig. 4b). The powder
X-ray diffraction patterns for as-synthesized and template-
extracted LHMS-3 samples are shown in Fig. 4. The materials
are highly ordered, showing three strong diffractions for the 100,
110, 200 planes corresponding to 2D-hexagonal mesophase.1 A
representative TEM image of LHMS-3 is shown in Fig. 5. In this
image low electron density spots (pores) can be seen throughout
the specimen and these are arranged in a honey-comb like
hexagonal array.1,13,23 A closer look into this image suggested the
dimension of the pores to be ca. 2.8 nm. This was further
confirmed from the FFT pattern shown in the inset of Fig. 5.
Instrumentation
1H and 13C NMR experiments (liquid state) were carried out on
a Bruker DPX-300 NMR spectrometer. Mass spectrometric data
were acquired using the electron spray ionization (ESI) technique
at 25–70 eV in a Micromass Q-tof-Micro Quadruple mass
spectrophotometer. Carbon, hydrogen and nitrogen contents
were analyzed using a Perkin Elmer 2400 Series II CHN analyzer.
X-Ray diffraction patterns of the powder samples were obtained
with a Bruker AXS D8 Advanced SWAX diffractometer using
Cu Ka (l ¼ 0.15406 nm) radiation. Using small-angle neutron
scattering (SANS), we measured the scattering intensity I(Q) as
a function of wave vector transfer Q (¼ 4p ꢁ sinq/l, where l is
the wavelength of the incident neutrons and 2q is the scattering
angle). The measurements were performed on the SANS
instrument at the Guide Tube Laboratory, Dhruva reactor,
BARC, India. This diffractometer uses a polycrystalline BeO
filter as a monochromator and the mean wavelength of the
ꢁ
incident neutron beam is 5.2 A with a resolution (Dl/l) of about
15%. The angular distribution of the scattered neutron was
recorded using a linear 1 m long He3 position sensitive detector.
The data were recorded in the accessible Q range of 0.017–
0.35 Aꢀ1. The measured SANS data were corrected for the
ꢁ
background and the empty cell contributions.
High resolution transmission electron microscopy images are
recorded on a JEOL 2010 TEM operated at 200 kV. Nitrogen
Fig. 3 Small angle neutron scattering pattern of LHMS-3.
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Fig. 4 Small angle XRD patterns of as-synthesized (a) and template-
extracted (b) LHMS-3 samples.
Fig. 6 N2 adsorption/desorption isotherms of LHMS-3 77 K. Adsorp-
tion points are marked by filled circles and desorption points by empty
circles. Pore size distribution is shown in the inset.
Solid state NMR and chemical environment
13C and 29Si MAS NMR experiments often provide useful
information regarding the chemical environment around the C
and Si nuclei and the presence of organic functional groups in the
hybrid frameworks. In Fig. 7 a 13C CP MAS NMR spectrum for
template-extracted LHMS-3 is shown. The spectrum exhibits
strong signals at 9.6, 13.9, 22.8, 52.2, 99.6, 104.8, 158.2 and
160 ppm, which resembles different carbon atoms of I as shown
in Fig. 1. Respective peaks of I species are marked in the inset.
This result indicates that the diimine fragment (I) is covalently
grafted inside the pore walls of LHMS-3. Further, very small
signals at 173.5 and 189 ppm suggested the presence of some
unreacted aldehyde species in LHMS-3 material also. 29Si MAS
NMR spectra of LHMS-3 sample (Fig. 8) shows downfield
chemical shifts at ꢀ57.3 and ꢀ51.9 ppm, which could be
Fig. 5 Transmission electron micrograph of LHMS-3. Fast Fourier
Transform (FFT) pattern is shown in the inset.
Bright diffraction spots and their hexagonal arrangements are
quite clear.
Mesoporosity and pore dimension
The N2 sorption isotherms of template-free LHMS-3 are shown
in Fig. 6. These isotherms are classified as type IV, characteristic
of the mesoporous materials.1 The sharp increase in N2 uptake
for adsorption is observed at P/P0 ¼ 0.28–0.36, clearly indicating
that the mesopores are uniform in size. Pore size distribution of
these samples estimated by employing the BJH method is shown
in the inset of Fig. 6, suggesting uniform pores with dimensions of
ca. 2.78 nm in this material. Unit cell for the 2D-hexagonal
structure of LHMS-3: a ¼ 4.73 nm and wall thickness t ¼ (2d100
/
O3 ꢀ pore diameter) 1.95 nm, agrees very well with HR TEM
results. The BET surface area and pore volume of LHMS-3 were
478 m2 gꢀ1 and 0.37 cc gꢀ1, respectively. Pores of dimension of ca.
2.8 nm are good for loading Pd(II) on the surface and carrying out
the Heck C–C coupling reaction of the bulky organic reactants.
Fig. 7 13C CP MAS NMR spectrum of LHMS-3.
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Fig. 8 29Si MAS NMR spectrum of LHMS-3.
attributed to the T2 ((OH)2(OSi)Si–R–Si(OSi)2(OH)) and T1
((OH)2(OSi)Si–R–Si(OSi)(OH)2) species, respectively13,23 in
addition to peaks at ꢀ103.8 (Q3) and ꢀ95.0 ppm (Q2) further
suggesting the presence of species I (Scheme 1) in the framework.
Scheme 2 Binding sites of Pd(II) in Pd-LHMS-3.
addition to the broad absorption maximums due to the
bis(propyliminomethyl)-phloroglucinol moiety. This could be
attributed to the ligand chelation, which leads to ligand-to-metal
charge transfer (LMCT) bands in addition to the p / p*
transitions due to the phenolic and diimine moieties.32 In the
presence of Pd(II) the imine-N and phenolic-OH donors form
strong coordination and covalent bonds (Scheme 2). This leads
to low energy absorption. On grafting of Pd(II) through bonding
with the imine-N and phenolic–OH donor sites of the pore walls
of LHMS-3 could stabilize via six-membered rings as shown in
Scheme 2.33 Hence this spectroscopic result further suggested
that the bridged organosilane moiety I is present inside the pore
walls of the LHMS-3 framework.
UV-vis absorption
UV-visible spectroscopy was used to characterize the optical
absorbance of the bis(propyliminomethyl)-phloroglucinol
moiety present in the pore walls of the LHMS-3 and Pd-LHMS-3
samples. LHMS-3 (as-synthesized and template-free forms) and
Pd(II)-exchanged samples exhibited major UV-visible absorption
bands in the range of 200 to 600 nm (Fig. 9). Three strong
absorption bands appeared at 255, 350 and 446 nm for the
template-free LHMS-3 sample. The Pd-LHMS-3 material
exhibits large absorption in the higher wavelength region in
Framework and bonding
Both the functionalized PMO material LHMS-3 and Pd-LHMS-
3 exhibit FT IR peaks (Fig. 10) in the region 3650–3300 (n O–H),
3100–2800 cmꢀ1 (n C–H), ꢃ1620, ꢃ1512 and 1452 cmꢀ1 (skeletal
vibrations). Apart from the C–H vibrations at 2930 and
2850 cmꢀ1 N–C vibrations at ca. 800 cmꢀ1 are observed for the
template-free LHMS-3 samples. The formation of Schiff base in
LHMS-3 has been confirmed by the appearance of a sharp peak
at 1621 cmꢀ1 (n > C]N–). In the palladium(II) anchored meso-
porous material Pd-LHMS-3, the n >C]N– peak is quite broad,
the n > C]N– (free) band at 1621 cmꢀ1 has been shifted at
1606 cmꢀ1 for n > C]N– (coordinated). Mild shifts in three
medium intensity peaks to 1606, 1512 and 1452 cmꢀ1 for the Pd-
LHMS-3 sample suggested metalation.26 This spectral evidence
suggests the presence of the bis(propyliminomethyl)-phloro-
glucinol moiety in LHMS-3 and palladium(II) species are
strongly anchored to the mesopore surface via covalent bridges.
Fig. 9 UV-vis diffuse reflectance spectra of as-synthesized (a) and
surfactant-free LHMS-3 (b), and Pd-LHMS-3 (c).
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Pd-LHMS-3 showed excellent catalytic activity (yield 75–85%)
for the C–C coupling of aryl halides with a series of styrene and
acrylic acid functionalities. After the fourth catalytic cycle, the
yield for trans-stilbene is ca. 81%, suggesting high catalytic effi-
ciency and trans-selectivity for Pd-LHMS-3.
Proof of heterogeneity of the catalyst (hot filtration test)
We have chosen the Heck reaction between styrene and iodo-
benzene to examine whether Pd is being leached out from the
catalyst to the solution. The hot filtration test was conducted in
a reaction mixture consisting of 1.0 mmol of iodobenzene,
1.0 mmol of styrene, 0.05 g Pd-LHMS-3 catalyst and 5 ml of
triethanolamine. The solution was refluxed in a round-bottom
flask at 110 ꢂC under inert an atmosphere placed in a temperature
controlled oil bath. After 5 h, the reaction mixture was allowed to
cool to room temperature and the viscous solution was diluted
with water and then filtered. Atomic absorption spectrometric
analysis was carried out with the filtrate and no Pd was detected.
This result clearly suggests that no Pd is being leached out from
the catalyst during the course of this reaction. For another part of
the hot reaction mixture for this same experiment (after 5 h
reaction) the catalyst was filtered out immediately and the rest of
the coupling reaction was continued in the absence of the catalyst
for another 5 h. Conversion remained the same (46.2 and
46.7 mol% after and before the hot filtration test, respectively). It
is noteworthy that the solution remains completely colorless after
the Heck coupling reaction, in the presence of the Pd-catalyst. All
of these experimental observations suggest that our novel Pd-
loaded PMO catalyst is truly heterogeneous in nature for this
Heck C–C coupling reaction.
Fig. 10 FT IR spectra of surfactant-free LHMS-3 (a) and Pd-LHMS-3
(b).
The AAS spectroscopic tool was employed to determine the
amount of palladium(II) loading in this mesoporous surface and
it was found that Pd-LHMS-3 contains 7.4 wt% Pd, which is
sufficient to carry out liquid phase catalytic reactions.
Catalysis
In Table 1 we have summarized our results for Pd-LHMS-3
catalyzed Heck reactions of aryl halides with aromatic (styrene)
as well as aliphatic olefins (acrylic acid). The reaction pathway is
schematically presented in Scheme 3. As seen from Table 1,
Recycling of the catalyst
The recycling of the catalyst is an important issue in hetero-
geneous catalysis. Thus we have turned our attention to prove
the reusability of our Pd-PMO catalyst. We have recycled the
catalyst four times for the reaction between iodobenzene and
styrene. The catalyst was recovered and reused by the following
steps. After the reaction was completed, the mixture was allowed
to cool to room temperature and diluted with water. The catalyst
was filtered, washed extensively with water, ethyl acetate, hexane,
acetone until GC analysis of the filtrate indicated no detectable
amount of reagents or products. Then the catalyst was dried
under high vacuum overnight at 120 ꢂC. It was then reused in the
next run without changing the reaction conditions. After
carrying out the reactions for the fourth catalytic cycles the
catalyst showed consistent catalytic activity as shown in Table 2.
Scheme 3 Heck C–C bond formation reaction catalyzed by Pd-LHMS-3.
Table 1 Heck coupling reactions catalyzed by Pd-LHMS-3
Reference
Time (h) Yield (%)a (see ESI)
Entry
R
X
Olefin
1
2
3
4
5
6
7
8
9
10
H
I
I
I
Br
Styrene
Styrene
Styrene
Styrene
10
12
10
14
12
85
80
80
75
75
80
75
85
75
75
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
Table 2 Catalytic activity in different cycles over Pd-LHMS-3 in the
reaction between iodobenzene and styrenea
4-OCH3
4-NO2
H
-CHO
H
4-OCH3
4-NO2
H
Number of reaction cycles
Product yield (%)
4-Br Styrene
I
I
I
Br
I
Acrylic acid 10
Acrylic acid 12
Acrylic acid 10
Acrylic acid 12
Acrylic acid 12
1
2
3
4
85.6
85.0
83.4
82.1
4-CH3
a
a
1
Yields refer to those of purified products characterized by H and 13C
NMR data.
Reactions were carried out at 110 ꢂC and under a N2 atmosphere in the
presence of triethanolamine solvent under rapid stirring.
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From this table it is clear that our novel mesoporous Pd-
containing PMO material Pd-LHMS-3 has been successfully
utilized in Heck coupling reactions and the catalyst can be easily
filtered and reused for four catalytic cycles without any signifi-
cant decrease in its catalytic activity.
step is the oxidative addition of an aryl halide to this 14-electron
Pd0 species to yield Ar–Pd–X species.39 The third step is the syn
addition of an alkyl or aryl vinyl group to the Ar–Pd–X species to
generate the intermediate adduct. The fourth step has been
accomplished by b–H elimination to obtain the desired alkene
with high selectivity (E/Z).35 In the final step TEA abstracts
a proton from H–Pd–X to regenerate the catalyst for further
reaction. Thus the Pd-LHMS-3 heterogeneous catalyst contain-
ing the bis(propyliminomethyl)-phloroglucinol moiety inside the
pore wall has a strong ability to trap Pd2+ ions on the surface of
the mesopores through metal legation to generate a highly active
heterogeneous catalyst for C–C coupling reaction. The active
species (Pd0) of the Heck coupling reaction has been stabilized by
strongly coordinating with neutral imine-N donor sites at the
surface of the mesopores, which circumvents the leaching of Pd0
from the catalyst surface. Thus, due to the presence of a large
number of accessible donor sites, this material can serve as an
excellent scaffold for metal-mediated catalytic reactions.
Salient features of our Pd-LHMS-3 catalytic system in Heck
C–C coupling reaction are (i) recycling ability of the solvent
triethanol amine (TEA), (ii) dual role of TEA: both as base as
well as solvent, in addition to this it can stabilize the active metal
species due to its significant ligand character, (iii) this C–C
coupling can be performed very effectively without the addition
of triphenylphosphine,34 which is conventionally used in the
Heck coupling reaction and is environmentally toxic. Thus final
work-up can be done easily here in an environmentally friendly
way. Although Heck reaction between aryl halides and butyl
acrylate catalyzed by Pd(II) complexes35–37 gives good catalytic
activity for activated aryl halides, recycling of the catalyst is
a major drawback in such cases. Pd-functionalized silica
surfaces,38–40 where the Pd(II) moiety is grafted at the surface of
the material have the additional advantage of high recycling
efficiency in the Heck C–C coupling reaction.
Conclusions
High concentration of phenolic-OH groups due to the
presence of the phloroglucinol in the pore wall of Pd-LHMS-3
material may support the anion ligated palladium(0) mechanism
for Heck coupling as proposed by Amatore and Jutand.41 In
Scheme 4 we have proposed a catalytic cycle for this C–C bond
formation reaction over Pd-LHMS-3. In the first step of this
cycle, Pd+2 is reduced to Pd0 by the solvent TEA42 at the surface
of the mesopores. Pd0 is the active species of this reaction, which
could be stabilized by the neighboring imine-N donor sites of the
PMO and also by the solvent triethanolamine (TEA). The next
From our above experimental results we can conclude that a new
organic–inorganic hybrid PMO material can be synthesized
by designing an organosilane precursor prepared through
Vilsmeier–Haack hydroformylation of phloroglucinol followed
by its Schiff base condensation with 3-aminopropyl-trimethoxy-
silane. The highly donating bis(propyliminomethyl)-phloro-
glucinol moiety in the pore wall of this PMO material is
responsible for anchoring Pd(II) ions, and the resulting Pd-
LHMS-3 showed excellent catalytic activity and trans-selectivity
in the Heck C–C bond formation reaction between aryl halides
and alkenes in the synthesis of value-added organics. Thus the
ability to functionalize the pore wall of a mesoporous material
with a highly coordinating species for a metal ion scavenger
together with uniform systems of mesopores and visible absorp-
tion could open-up new avenues in designing functionalized PMO
materials for fine chemical synthesis via green chemical routes.
Acknowledgements
AB wishes to thank DST, New Delhi for research grants. This
work was partly funded by the NanoScience and Technology
Initiative of DST. AM and JM are grateful to CSIR, New Delhi
for their Junior Research Fellowships.
References
1 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and
J. S. Beck, Nature, 1992, 359, 710–712.
2 (a) S. A. El-Safty, D. Prabhakaran, A. A. Ismail, H. Matsunaga and
F. Mizukami, Adv. Funct. Mater., 2007, 17, 3731; (b) K. Sarkar,
K. Dhara, M. Nandi, P. Roy, A. Bhaumik and P. Banerjee, Adv.
Funct. Mater., 2009, 19, 223–234.
3 (a) M. Kruk, M. Jaroniec and A. Sayari, Microporous Mesoporous
Mater., 2000, 35–36, 545–553; (b) W. G. Borghard, D. C. Calabro,
F. P. DiSanzo, M. M. Disko, J. W. Diehl, J. C. Fried,
M. A. Markowitz, M. Zeinali, B. J. Melde and A. E. Riley,
Langmuir, 2009, 25, 12661–12669.
4 R. Garro, M. T. Navarro, J. Primo and A. Corma, J. Catal., 2005,
233, 342–350.
5 (a) T. Sakai, Y. Tsutsumi and T. Ema, Green Chem., 2008, 10,
337–341; (b) M. Shakeri and K. Kawakami, Catal. Commun., 2008,
Scheme 4 Proposed reaction pathway for the Heck reaction over Pd-
LHMS-3.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 8099–8106 | 8105

View Article Online
10, 165–168; (c) P. Y. Wang, X. Liu, J. Yang, Y. Yang, L. Zhang,
Q. H. Yang and C. Li, J. Mater. Chem., 2009, 19, 8009–8014.
6 D. Chandra and A. Bhaumik, J. Mater. Chem., 2009, 19, 1901–1907.
7 C. Bouvy, F. Piret, W. Marine and B. L. Su, Chem. Phys. Lett., 2007,
433, 350–354.
24 D. Chandra, A. Dutta and A. Bhaumik, Eur. J. Inorg. Chem., 2009,
4062–4068.
25 C. Gonzalez-Arellano, A. Corma, M. Iglesias and F. Sanchez, Adv.
Synth. Catal., 2004, 346, 1758–1764.
26 (a) L. X. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133–173; (b)
P. T. Patil, A. Dimitrov, J. Radnik and E. Kemnitz, J. Mater.
Chem., 2008, 18, 1632–1635; (c) K. Sarkar, M. Nandi, M. Islam,
T. Mubarak and A. Bhaumik, Appl. Catal., A, 2009, 352, 81–86.
27 V. Polshettiwar, P. Hesemann and J. J. E. Moreau, Tetrahedron, 2007,
63, 6784–6790.
28 X. Ma, Y. X. Zhou, J. C. Zhang, A. L. Zhu, T. Jiang and B. X. Han,
Green Chem., 2008, 10, 59–66.
29 Y. Wan, H. Y. Wang, Q. F. Zhao, M. Klingstedt, O. Terasaki and
D. Y. Zhao, J. Am. Chem. Soc., 2009, 131, 4541–4550.
30 M. Choi, D. H. Lee, K. Na, B. Yu and R. Ryoo, Angew. Chem., Int.
Ed., 2009, 48, 3673–3676.
31 R. Paolesse, S. Nardis, M. Venanzi, M. Mastroianni, M. Rosso,
F. R. Fronczek and M. G. H. Vicente, Chem.–Eur. J., 2003, 9,
1192–1197.
8 S. L. Burkett, S. D. Sims and S. Mann, Chem. Commun., 1996,
1367–1368.
9 F. Pereira, K. Valle, P. Belleville, A. Morin, S. Lambert and
C. Sanchez, Chem. Mater., 2008, 20, 1710–1718.
10 A. Bhaumik and T. Tatsumi, J. Catal., 2000, 189, 31–39.
11 A. Corma and H. Garcia, Adv. Synth. Catal., 2006, 348, 1391–1412.
12 E. L. Margelefsky, R. K. Zeidan and M. E. Davis, Chem. Soc. Rev.,
2008, 37, 1118–1126.
13 S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki,
J. Am. Chem. Soc., 1999, 121, 9611–9614.
14 T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature,
1999, 402, 867–871.
15 A. Stein, B. J. Melde and R. C. Schroden, Adv. Mater., 2000, 12,
1403.
16 O. Olkhovyk and M. Jaroniec, J. Am. Chem. Soc., 2005, 127, 60.
17 Y. C. Liang, M. Hanzlik and R. Anwander, J. Mater. Chem., 2006,
16, 1238–1253.
18 (a) K. Yamamoto, Y. Nohara and T. Tatsumi, Chem. Lett., 2001,
648–649; (b) P. Nguyen, P. Hesemann, P. Gaveau and
J. J. E. Moreau, J. Mater. Chem., 2009, 19, 4164–4171.
19 B. Johnson-White, M. Zeinali, K. M. Shaffer, C. H. Patterson,
P. T. Charles and M. A. Markowitz, Biosens. Bioelectron., 2007, 22,
1154–1162.
20 S. Inagaki, S. Guan, Q. Yang, M. P. Kapoor and T. Shimada, Chem.
Commun., 2008, 202–204.
21 H. Takeda, Y. Goto, Y. Maegawa, T. Ohsuna, T. Tani,
K. Matsumoto, T. Shimada and S. Inagaki, Chem. Commun., 2009,
6032–6034.
32 W. K. Dong, Y. X. Sun, Y. P. Zhang, L. Li, X. N. He and X. L. Tang,
Inorg. Chim. Acta, 2009, 362, 117–124.
33 S. Wang, W. J. Oldham Jr., R. A. Hudack, Jr. and G. C. Bazan,
J. Am. Chem. Soc., 2000, 122, 5695–5709.
34 R. F. Heck and J. P. Nolley, J. Org. Chem., 1972, 37, 2320–2322.
35 A. F. Littke and G. C. Fu, J. Am. Chem. Soc., 2001, 123, 6989–7000.
36 J. M. Chitanda, D. E. Prokopchuk, J. W. Quail and S. R. Foley,
Dalton Trans., 2008, 6023–6029.
37 K. H. Shaughnessy and R. B. DeVasher, Curr. Org. Chem., 2005, 9,
585–604.
38 L. Firmansjah and G. C. Fu, J. Am. Chem. Soc., 2007, 129,
11340–11341.
39 D. J. Macquarrie and S. E. Fairfield, J. Mater. Chem., 1997, 7,
2201–2204.
€
40 M. Cornelius, F. Hoffmann and M. Froba, Chem. Mater., 2005, 17,
22 C. X. Lin, S. Z. Qiao, C. Z. Yu, S. Ismadji and G. Q. Lu, Microporous
Mesoporous Mater., 2009, 117, 213–219.
23 D. Chandra, T. Yokoi, T. Tatsumi and A. Bhaumik, Chem. Mater.,
2007, 19, 5347–5354.
6674–6678.
41 C. Amatore and A. Jutand, Acc. Chem. Res., 2000, 33, 314–321.
42 H. J. Li and L. Wang, Eur. J. Org. Chem., 2006, 5099–5102.
8106 | J. Mater. Chem., 2010, 20, 8099–8106
This journal is ª The Royal Society of Chemistry 2010