Pd-chelated 1,3,5-triazine organosilica as an active catalyst for Suzuki and Heck reactions

Article abstract of DOI:10.1016/j.mcat.2019.110521

Design of novel functional periodic mesoporous organosilica with molecular level control finds applications in several fields of energy and environmental research including adsorption, catalysis, nanotechnology, and energy harvesting. Herein we present a melamine (1,3,5-triazine) functionalized periodic mesoporous silica (MPMO) by self-assembly of N²,N⁴,N⁶-tris(3-(triethoxysilyl)propyl)-1,3,5-triazine-2,4,6-triamine (TTET) with tetraethylorthosilicate (TEOS) via cocondensation strategy. The TTET silsesquioxane precursor was synthesized by the condensation reaction between cyanuric chloride and 3-aminopropyl triethoxysilane. The resultant MPMO material serves as an effective solid chelating agent through amine and triazine functionalities for Pd(II) to provide Pd-MPMO. The Pd-MPMO material was thoroughly characterized by a small-angle XRD, HRTEM, N₂ sorption, ¹³C CP-MAS NMR, ²⁹Si CP-MAS NMR, and ICP analyses. The Pd-MPMO serves as an active catalyst for C–C bond formation reactions by Suzuki- and Heck cross-coupling methodologies under ligand- and cocatalyst-free conditions. Notably, the present catalytic protocol demonstrates a wide spectrum of substrate scope towards Suzuki coupling between aryl halides (I^?, Br^?, Cl^?) and aryl boronic acids with high turn-over-number (TON) in aqueous phase under air ambience. Whereas for Heck-coupling reaction, the phenyl iodides furnished high TON than the other aryl halides. Investigation of Pd-leaching by a hot filtration test as well as reusability experiments confirms the true heterogeneous nature of present Pd-MPMO and its robustness in terms of substrate scope, catalyst stability, and durability.

Full text of DOI:10.1016/j.mcat.2019.110521

MolecularCatalysis476(2019)110521
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Molecular Catalysis
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Pd-chelated 1,3,5-triazine organosilica as an active catalyst for Suzuki and
Heck reactions
⁎
S. Elavarasana, K. Kalaa, Ibrahim Muhammada, A. Bhaumikb, M. Sasidharana,
a SRM Research Institute and Department of Chemistry, SRM Institute of Science & Technology, Kattankulathur, Chennai, 603203, India
b School of Materials Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Design of novel functional periodic mesoporous organosilica with molecular level control finds applications in
several fields of energy and environmental research including adsorption, catalysis, nanotechnology, and energy
harvesting. Herein we present a melamine (1,3,5-triazine) functionalized periodic mesoporous silica (MPMO) by
self-assembly of N2,N4,N6-tris(3-(triethoxysilyl)propyl)-1,3,5-triazine-2,4,6-triamine (TTET) with tetra-
ethylorthosilicate (TEOS) via cocondensation strategy. The TTET silsesquioxane precursor was synthesized by
the condensation reaction between cyanuric chloride and 3-aminopropyl triethoxysilane. The resultant MPMO
material serves as an effective solid chelating agent through amine and triazine functionalities for Pd(II) to
provide Pd-MPMO. The Pd-MPMO material was thoroughly characterized by a small-angle XRD, HRTEM, N2
sorption, 13C CP-MAS NMR, 29Si CP-MAS NMR, and ICP analyses. The Pd-MPMO serves as an active catalyst for
CeC bond formation reactions by Suzuki- and Heck cross-coupling methodologies under ligand- and cocatalyst-
free conditions. Notably, the present catalytic protocol demonstrates a wide spectrum of substrate scope towards
Suzuki coupling between aryl halides (I−, Br−, Cl−) and aryl boronic acids with high turn-over-number (TON) in
aqueous phase under air ambience. Whereas for Heck-coupling reaction, the phenyl iodides furnished high TON
than the other aryl halides. Investigation of Pd-leaching by a hot filtration test as well as reusability experiments
confirms the true heterogeneous nature of present Pd-MPMO and its robustness in terms of substrate scope,
catalyst stability, and durability.
Melamine silica
Silsesquioxane
Pd-chelation
Suzuki coupling
Heck coupling
1. Introduction
devoted to the self-organization of silsesquioxane via supramolecular
self-assembly. Later on, immobilization of organometallics onto PMOs
The finding of periodic mesoporous organosilicas (PMOs) by con-
densation of silsesquioxane of the form (R′O)3SieReSi(OR′)3 (where R
is linking organic functionality) by templating sol-gel route garnered
much interest in crafting functional materials with controlled properties
at the atomic level [1–3]. The noteworthy feature of PMOs is that the
bridging organic groups (R) do not obstruct the mesoporous channels or
mouth and endows PMOs with uniformly distributed organic func-
tionalities within the silica framework as well as tunable hydro-
phobicity/hydrophilicity compared to conventional mesoporous silicas
(MCM-41, SBA-15 etc.). As this technique permits stoichiometric in-
tegration of R groups in the silicate network, catalytic properties can be
envisioned by incorporating appropriate R groups into the porous wall
structures. A variety of PMOs with linking spacers (R) originating from
eCH2e, eCH2eCH2e, eCH]CHe, eC6H5e, biphenylene, vinylene-
phenylene, etc. have been covalently embedded within the framework
of mesoporous materials [4–11]. Thus, enormous efforts have been
with appropriate organic functionality shown to exhibit high catalytic
activity due to fine distribution of active sites as well as superior hy-
drophobicity which facilitates the diffusion of organic molecules [12].
For practical applications, embedding metal complexes on porous ma-
trix is an ideal strategy and indeed metal complex supported hetero-
geneous-systems exhibit comparable or even better performance than
the homogeneous counterparts [13,14]. In this context, the develop-
ment of new mesoporous supports with imprinted ligands for facile
metal coordination without additional tethering linker would be highly
desirable, but challenging. There are few reports on hybrid PMOs with
organic groups positioned within the frameworks having metal-ligating
capability [15,16]. Recently, Inagaki et al. reported crystal-like PMOs
containing divinyl pyridine and phenyl-pyridine groups which effec-
tively served as a solid ligand for the creation of Ir- and Ru-metal
complexes on the pore surfaces by post-synthesis approach [17–19].
Thus immobilized metal complexes expected to have less diffusion
⁎ Corresponding author.
E-mail address: sasidharan.m@res.srmuniv.ac.in (M. Sasidharan).
https://doi.org/10.1016/j.mcat.2019.110521
Received 11 April 2019; Received in revised form 15 July 2019; Accepted 18 July 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
limitation towards reactants and products due to wide uniform chan-
nels which would be expected to enhance the catalytic performances.
Later on, PMOs containing 2,2′-bipyridine ligands with crystalline pore
wall structures served as versatile support for complex formation with
Ru, Ir, Re, and Pd metals offering good catalytic activity and recycl-
ability for catalytic reactions [20,21].
organosilica, the following molar composition 0.9SiO2:0.1TTTS:0.012
EO-PO-EO: 6.5 HCl: 169 H2O was used. For synthesis, P123 triblock
copolymer (2 g) was dissolved in Millipore water with subsequent ad-
dition of HCl (36 wt%). Then, TEOS silica precursor as per the com-
position was added and stirred for 2 h, followed by drop wise addition
of TTTS to the above partially hydrolyzed oligomeric suspension. The
mixture was heated at 313 K with continuous stirring for 20 h and
further aged at 373 K for two days. The white product obtained by
hydrothermal treatment was dried and finally, the surfactant was ex-
tracted with ethanol containing 3% HCl at 353 K and this procedure
was repeated thrice to obtain melamine periodic mesoporous silica
(MPMO). For the preparation of Pd- MPMO, the MPMO (1 g) was sus-
pended in acetone (10 mL) containing an appropriate amount of Pd
(OAc)2, followed by sonication for 15 min and prolonged stirring for
another 3 h. Finally, the material was filtered off and washed thor-
oughly with acetone, ethanol, and dried at 353 K to obtain the final Pd-
MPMO.
In the field of catalysis, Suzuki cross coupling and Heck coupling
reactions (for CeC bond formation) are traditionally performed using a
homogeneous Pd-catalyst involving phosphine ligands under alkaline
medium in presence of a Cu(I)-halide as co-catalyst in the inert atmo-
sphere [22–25]. However, these homogeneous protocols often suffer
from high cost of ligands, issues with reactant/product/heavy metal
separation and high purification cost [26–29]. In an effort to achieve
heterogeneous catalysts devoid of these issues, phosphorous ligands
and cocatalysts anchored heterogeneous supports were explored to
chelate noble metals albeit with moderate success [30,31]. In this
context, metal-supported catalysts, especially Pd-nanoparticles were
widely explored by utilizing the beneficial attributes of both metal as
well as supports such as high surface area vis-à-vis catalytic sites, large
pore volume, and highly dispersed active sites. Towards this end, sev-
eral porous metal oxides, silica materials, MWCNT, graphene, and or-
ganic polymers have been investigated as supports for anchoring metals
due to their beneficial textural properties, thermal/chemical stabilities,
and amenability for surface functionalization [32–37]. In this context,
organic polymers integrated with suitable chelating functionalities have
also been explored as supports for chelating metal ions that could
subsequently serve as a potential solid catalyst. For instance, Zhang
et al. synthesized polydivinylbenzene-based (PDVB) porous polymers
with good long-term stabilities in a wide pH range by simple functio-
nalization strategy [38,39].
2.3. Materials characterization
The porous Pd-MPMO material was characterized by XRD with a
Bruker diffractometer (D8 Advance, Davinci) with CuKα radiation (λ
=1.5418 Å). The morphology was observed with high-resolution TEM
with a JEOL (JEM-2100 Plus) operating at 200 kV by coating samples
on a copper grid layered with carbon. The 29Si MAS NMR and 13C CP-
MAS NMR spectra were acquired on a Bruker MSL-300WB spectrometer
at 59.62 and 75.47 MHz for 29Si and 13C, respectively. Their chemical
shifts were provided with respect to tetramethylsilane and glycine, re-
spectively. BET surface area, pore size distribution, and pore volume
details were obtained from N2 sorption isotherms using Autosorb IQ
series (Quantachrome Instruments). Elemental analysis of Pd was per-
formed with a Perkin Elmer Plasma 400 ICP-OES. The progress of the
reaction was monitored using gas chromatograph fitted with a capillary
column with an FID detector (Agilent 7980 B) and products were
confirmed with the authentic samples.
Recently, we have studied the post-synthetic modification of ethy-
lene-silica to derive materials with antagonistic dual functionalization
of acid- and bases within a single matrix [40,41]. In the present report,
we demonstrate new PMOs with 1,3,5-triazine (melamine) as a bridging
group using N2,N4,N6-tris(3-(triethoxysilyl)propyl)-1,3,5-triazine-2,4,6-
triamine (TTET) with tetraethylorthosilicate (TEOS) by cocondensation
strategy. The abundant imines and triazine functionalities present in the
organic part was effectively utilized to chelate Pd(II) ions to provide Pd-
MPMO material for catalytic applications. Herein, we demonstrate the
scope of Pd-MPMO for Suzuki and Heck cross-coupling reactions under
ligand as well as cocatalyst-free conditions under water/air ambience.
2.4. Suzuki cross-coupling reactions
Aryl halides (2 mmol), phenylboronic acids (2.2 mmol), K2CO3
(4 mmol), Pd-MPMO (10 wt%) and water (5 mL) were charged to a
25 mL round bottom flask. The flask was fitted with a reflux condenser
and ice-cold water was circulated during the reaction. The reaction
mixture was stirred at 353 K for 3 h and after completion of the reac-
tion, the catalyst was removed by filtration. The organics were twice
extracted using ethyl acetate and the combined extracts were con-
centrated to obtain the crude product. The reaction progress was
monitored by TLC as well as gas chromatography (GC). The yields were
determined by gas chromatography on the basis of aryl halides.
2. Experimental
2.1. Materials
Cyanuric chloride, 3-aminopropyl triethoxysilanes, dimethyl sulf-
oxide (DMF), Pd(OAc)2 (99.8%), triethylamine, and other organic
chemicals for catalytic reactions with AR grade were obtained from
Sigma Aldrich, which were used as received. Other chemicals such as
KOH, K2CO3, and KOtBu+ and solvents employed for this study were
purchased from AVRA and TCI-chemicals with analytical grade unless
otherwise specified. All these chemicals also used without any further
purification.
2.5. Heck coupling reactions
Iodobenzene (1 mmol), ethyl acrylate (1.2 mmol), triethylamine
(1 mmol), DMF (4 mL) and catalyst (15 wt%) were taken in a RB flask
fitted with reflux condenser. Then the reaction mixture was stirred at
373 K for a period of 4 h and after completion of the reaction, the
catalyst was removed by filtration with subsequent water work-up and
then, the organic layer was extracted twice with ethyl acetate. The
organic extract was dried using anhydrous Na2SO4, and the solvent was
removed under low-pressure to yield crude compound. The reaction
progress was monitored by TLC and products were quantified with the
help of the GC.
2.2. Material synthesis
The synthesis of silsesquioxane (TTET) was performed in the la-
boratory using cyanuric chloride and 3-aminopropyl triethoxysilane. In
a typical synthesis, cyanuric chloride (5 mmol) solubilised in dioxane
(50 mL) was stirred in a 200 mL RB flask. Subsequently, 3-aminopropyl
triethyoxysilane (16 mmol) dissolved in dioxane (50 mL) was mixed to
the above content and then Na2CO3 base (10 mmol) was added. The
content was refluxed for a period of 12 h and the precipitate was se-
parated and washed with ethanol followed by drying under vacuum to
get the target silsesquioxane. In a typical synthesis of melamine-based
2.6. Leaching and recyclability tests
To investigate any metal leaching, a hot filtration test was per-
formed with Suzuki reaction involving phenylboronic acid and aryl
2

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
Scheme 1. Synthesis of N2,N4,N6-tris(3-(triethoxysilyl)propyl)-1,3,5-triazine-2,4,6-triamine.
Scheme 2. Synthesis of periodic mesoporous organosilica via the co-condensation procedure.
Fig. 1. TEM micrographs of (A) & (B) pristine MPMO material; (C) Pd-MPMO and (D) electron diffraction pattern of Pd-MPMO.
iodide as a model test reaction. Initially, the reaction was performed for
a period of 1.5 h at 48% conversion level and then, the catalyst was
recovered from the reaction flask by filtration and continued the re-
action with the filtrate for remaining 1.5 h under the same reaction
conditions and the filtrate was analyzed by GC. For catalyst recycl-
ability test, the recovered catalyst from the first reaction run was
separated by centrifugation, washed thoroughly with ethanol and
acetone, dried in a vacuum oven at 373 K for overnight to remove any
occluded organics on the catalyst surface. The retrieved catalyst was
recycled for another 4 cycles under similar reaction conditions by fol-
lowing the above-described procedures.
3

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
3. Results and discussion
stripes displays an array of mesoporous channels when viewed along
the pore direction confirming the existence of regular hexagonal me-
sopores (Fig. 1B). It is further seen from Fig. 1C that the mesoporosity is
clearly retained after Pd-chelation. The electron diffraction pattern of
Pd-MPMO clearly confirms the presence of Pd-species from the ob-
served (111), (200), and (220) reflections (Fig. 1D). The amount of Pd
loading in the material was found to be 1.65 wt% as estimated from
inductively coupled plasma analysis (ICP).
3.1. Characterization of materials
The preparation of N2,N4,N6-tris(3-(triethoxysilyl)propyl)-1,3,5-
triazine-2,4,6-triamine (TTET) by condensation of cyanuric chloride
with 3-aminopropyl triethoxysilane is shown in Scheme 1. The 1H NMR,
13C NMR, and mass spectral data (Figures S1-S3, supplementary in-
formation) of TTET clearly confirmed the formation of tri-silses-
quioxane without any impurity compound due to incomplete con-
The textural properties of MPMO and Pd-MPMO were further stu-
died by nitrogen sorption studies as shown in Fig. 2. The sorption iso-
therm data shown in Fig. 2 features Type IV isotherms according to
International Union of Pure and Applied Chemistry (IUPAC) classifi-
cation suggesting the presence of regular uniform mesopores in the
melamine-constructed PMOs [2,3]. The BET surface area and pore vo-
lume were found to be 535 m2 g−1 and 0.45 mL/g (Fig. 2A) for MPMO
material. In addition, the BJH adsorption pore-size distribution curve
shows mesopores of average diameter of 8.0 nm (Fig. 2B) in accordance
with TEM observation. Fig. 2C shows the sorption isotherms of Pd-
MPMO materials with rather a narrow hysteresis loop featured between
P/P0 = 4.4-0.8 compared to pristine MPMO. The BET surface area and
pore volume of Pd-MPMO were found to be 521 m2 g−1 and 0.43 mL/g
with the pore-size distribution of ∼6.2 nm. It is worthy to note that the
textural properties remain nearly intact after Pd-loading indicating the
advantageous feature of periodic mesoporous organosilicas. However,
the average mesopore size reduced to 6.2 nm due to implantation of Pd-
species along the channels.
densation. Scheme
2 illustrates the self-assembly of TTET tri-
silsesquioxane with tetraethylorthosilicate (TEOS) under acidic
medium following our earlier reports [40,41] using Pluronic triblock
copolymer (EO-PO-EO) surfactant to provide melamine-PMO (MPMO).
Then the MPMO was converted to Pd-MPMO by simply treating with Pd
(OAc)2 in acetone solution to give Pd-MPMO. To evaluate the crystal-
linity of the Pd-MPMO, low- and wide-angle X-ray diffractograms were
recorded (Figure S4A, supplementary), where the presence of a broad
reflection centred at 0.9° corresponding to (100) reflection along with
very weak (110) and (200) reflections were noted. These results clearly
indicate the mesoporous nature of the melamine constructed periodic
mesoporous organosilica. Furthermore, XRD characterization at high-
angle region showed a very broad reflection at ˜24.8° confirming the
amorphous nature of mesoporous wall structures (Figure S4B, supple-
mentary). The absence of any reflections corresponding to Pd-species
indicates a very fine distribution of Pd-species throughout the silica
framework. The observation of mesopores by XRD also corroborated
with HRTEM observation, where the honeycomb-like pore architecture
is clearly noticed (Fig. 1A). Furthermore, the appearance of bright spots
when viewed perpendicular to the pore axis (Fig. 1A) indicates the
mesopores of size ∼6.5 nm. Similarly, the appearance of bright contrast
To get more insight into the silicon local environments and presence
of organic functionalities, 29Si MAS NMR and by 13C CP-MAS NMR with
Pd-MPMO material (Fig. 3A) were performed. The 29Si MAS NMR
spectrum exhibits signals indicating the presence of both “T” and “Q”
sites in accordance with previous results [7–9,42,43]. The two major
Fig. 2. N2 sorption isotherms (A) and pore size distribution curve of (B) of pristine MPMO; and (C) & (D) represents data of Pd-MPMO.
4

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
Fig. 3. (A) & (B) refers to 29Si and 13C NMR spectra of Pd-MPMO material.
peaks at 68.9 and 76.5 ppm corresponding to covalently bonded silicon
with carbon atom of 1,3,5-triazine unit (melamine), ca.
(N3C3)‒Si‒(OSi)2(OH) (T2 site) and (N3C3)‒Si‒(OSi)3 (T3 site), re-
spectively [40,41]. Since the Pd-MPMO was made by co-condensation
strategy, the presence of Q-type species is also evident. The prominent
signal at 99.8 ppm indicates the presence of (3Si)-Si-(OH) (Q3) along
with (4Si)-Si (Q4) and a minor amount of (2Si)-Si-(OH)2 (Q2) at 108.7
and 90.8 ppm, respectively. The broad signal feature of Q3-species
suggests the existence of connectivity defects to a certain degree [44].
In addition, the 13C CP-MAS NMR exhibits an intense resonance signal
at ∼163 ppm corresponding to the melamine unit linked to the carbon.
The resonance signals at 24.1, 24.7, and 41.7 are ascribed to the side
chain eCH2 groups linked to the silicon in the hybrid material. We have
also noted a feeble signal at 8.6 ppm due to hydrolysis of Si-(CH2)3-
groups. The other major signal at 71.4 ppm is assigned to the ether
groups of the EO-PO-EO from surfactant residue [45,46]. Thus, both the
29Si MAS NMR and 13C CP-MAS NMR spectra clearly prove the presence
of both inorganic silica units and organic melamine units, thereby
confirming the formation of melamine-periodic mesoporous organosi-
lica (MPMO).
3.2. Suzuki-Miyaura coupling reaction
A typical Suzuki cross-coupling reaction between bromobenzene
and phenylboronic acids carried out in the presence of Pd-MPMO cat-
alyst is shown in Table 1. Although several supported Pd-nanoparticles
[47] and Pd-immobilized mesoporous polymer [48] were found to be
efficient in organic solvents, their performance in water solvent was
poor. Table 1 illustrates the optimization of various reaction parameters
between bromobenzene and phenylboronic acid as a model reaction.
Preliminary optimization reactions by treatment of 1i (2.0 mmol) with
slight excess of 2a (2.2 mmol) using Pd-MPMO (10 wt%) in presence of
K2CO3 (4 mmol) in aqueous medium resulted in the desired biphenyl
compound (3aa) with 99.0% yield at 80 °C after 3 h (Table 1, entry 1).
Among the various bases scrutinized, K2CO3 was found to be a suitable
base to provide high conversion and selectivity for biphenyl compound
Table 1
Optimization of Suzuki-coupling between bromobenzene and aryl boronic acids using Pd-MPMO.a
Entries
Catalyst, Wt %
Base
Temp. (K)
Solvent
Yieldb (%)
1
10
K2CO3
Cs2CO3
KOH
353
353
353
353
298
333
373
353
353
353
353
353
353
353
353
353
353
Water
Water
Water
Water
Water
Water
Water
Ethanol
Glycerol
Acetonitrile
Water
Water
Water
Water
Water
Water
Water
99.0
83.0
87.0
71.0
43.0
86.0
99.0
68.0
58.0
51.0
77.0
99.0
38.0
69.0
Nil
2
10
3
10
4
10
Et3N
5
10
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO
6
10
7
10
8
10
9
10
10
11
12
13c
14d
15
16
17
10
5
15
10
10
No Catalysts
Pristine MPMO
10
K2CO3
No base
Nil
11.0
a
b
c
Reaction conditions: Bromobenzene (2.0 mmol), phenylboronic acid (2.20 mmol), base (4.0 mmol) and H2O (5 mL) were stirred for 3 h unless otherwise noted.
GC analysis and yields based on aryl halides.
reaction time 1 h.
d
reaction time 2 h.
5

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
[a]
Scheme 3. Biphenyl synthesis using various halo benzenes and different phenylboronic acid over Pd-MPMO
.
[a]Reaction conditions: Bromobenzene (2.0 mmol), phenylboronic acids (2.20 mmol),10 Wt% Pd-MPMO, K2CO3 (4.0 mmol) and H2O (5 mL) were stirred for 3 h at
353 K. unless otherwise noted. [b]GC analysis and yields based on aryl halides. [c]The value in the parenthesis refers to turn-over-number. [d]reaction stirred for 12 h
at 373 K and 20 Wt% catalyst loading.
(Table 1, entries 1–4). Generally, heterogeneous catalysts require rather
high energy for activation, the temperature influence was scrutinized in
the presence K2CO3 (Table 1, entries 1, 5–7). As the temperature in-
creases from 25 to 100 °C, the conversion increases monotonically but
little difference in the activity between 80 °C (98. 8%) and 100 °C
(99.1%) was observed. Therefore, 353 K was chosen as the optimal
temperature for further investigation. Water was found to be the best
solvent among various solvents (Table 1, entries 1, 8–10) investigated
and glycerol leads to low conversion despite being hydrophilic in
nature possibly due to a high viscosity which suppresses diffusion of
reactants. Furthermore, lowering the catalyst loading and reaction time
led to lower conversion vis-ȧ-vis yield and optimum reaction time of 3 h
6

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
was fixed (Table 1, entries 1, 11–14). It is important to note that ab-
sence of a catalyst or with pristine MPMO, produced no products (en-
tries 15 and 16); however, very little conversion of about 11.0% was
observed in the absence of K2CO3 (entry 17).
3.3. Heck coupling reactions
Next, we investigated the activity of Pd-MPMO for Heck coupling
between aryl iodide and ethyl acrylate (Table 2) and several controlled
experiments were carried out by considering catalyst loading, solvents,
bases, and reaction temperatures. As the catalyst loading increases, the
yield for the desired product raises monotonically and the critical cat-
alyst loading was found to be 20 wt% (Table 2, entries 1–3). Experi-
ments with different solvents such as dimethylformamide (DMF), water,
acetonitrile, and toluene (Table 2, entries 3–6) indicated DMF as a
suitable solvent with 93.0% yield at 20 wt% catalyst loading. Among
different organic and inorganic bases tested, triethylamine exhibits the
best activity (Table 2, entry 3) than the inorganic bases such as Cs2CO3,
K2CO3, and KOH (entries 8–10). Thus, the base screening results sug-
gest that triethylamines effectively abstracts hydrogen from iodo-
benzene during the course of CeC bond formation than the inorganic
bases. Since activation energy plays a critical role in any catalytic re-
actions, the influence of reaction temperature was also studied but the
lower temperatures at 333 K and 353 K show lower yields (entries 3, 11,
& 12). Furthermore, reactions performed in the absence of catalysts as
well as the base did not show any conversion (entries 13 & 14).
Having identified the standard reaction protocol for Heck coupling
reaction, the scope of Pd-MPMO with a library of aryl halides and ethyl
acrylate was screened. For ethyl acrylate as substrate (Scheme 4), the
substituted aryl iodides such as p‒NO2, p‒CF3 and p‒CN furnished ex-
cellent yields (entries 6ba-6 da); whereas the electron donating p‒CH3
and p‒OH (entries 6ea-6fa) showed lower reactivities with 78.0 &
62.0% yield, respectively. The decrease in reactivity is attributed to the
inductive effect arising from electron-donating groups which deactivate
the aryl iodides in contrary to electron-withdrawing functionalities.
Among the substituted aryl iodides, the p‒CF3 compound exhibits the
highest TON of 259. Screening of reactivity pattern of different aryl
halides under identical conditions showed high reactivity for iodo-
benzene than bromo-compound (6 ha); whereas chlorobenzene (6ia) as
substrates exhibited no reactivity. The difference in reactivity among
halide substrates is attributed to the facile leaving ability of I− (iodide)
compared to Br− or Cl− groups. With styrene as a substrate unlike ethyl
acrylate, both activated and deactivated aryl iodides (6ab-6bb, 6eb &
6gb) furnished lower yields possibly due to the combined steric- and
electronic-effects of substituents. The plausible catalytic cycle was re-
presented in Scheme 6 for the Heck reaction.
Having established the optimized reaction conditions for Suzuki-
coupling, next the scope of Pd-MPMO was investigated with diverse
substrates. Scheme 3 presents the reaction between various substituted
iodobenzenes and phenylboronic acids over Pd-MPMO catalyst. The
reaction progress as well as products were confirmed using authentic
samples with the GC. The un-substituted iodobenzene affords the cor-
responding product in quantitative yield (3aa, 99%) compared to sub-
stituted analogues. The presence of substituents at both p‒ or m‒ po-
sitions of phenyl ring of aryl halides led to facile reaction (3ba-3 ha) but
the electron-withdrawing groups (eCN, eNO2, eCF3, eCOOH) at
p‒position showed marginal higher activity than the electron-donating
functionality at the m‒position (3ea). The noteworthy feature is that
Pd-MPMO catalytic system shows high turn-over-number (TON,
number of moles of product/number of moles of Pd) which varies in the
range of 205–265 (iodobenzene as substrate) depending on the sub-
stituents nature. Furthermore, phenylboronic acids with electron-
withdrawing and electron-donating substituents at p‒position de-
creased the yield to 77 & 71% (3ac & 3ab). However, heterocyclic
furan-based boronic acid analogue offered merely 34% (3ad) yield.
Entries 3ia-3na and 3ib-3ic exhibit the reactivity profiles of diverse
phenyl bromides as substrates. To our delight, both p‒ or m‒bromo
compounds gave excellent yields (entries 3ia-3na) along with high TON
regardless of electronic effects. In addition, substituted phenylboronic
acids also led to very good yields (entries 3ib & 3ic) under identical
reaction conditions. Encouraged with the above results, we next in-
vestigated the coupling reaction with highly challenging phenyl
chloride as a substrate which furnishes moderate yield albeit with
longer reaction duration (12 h), high catalyst loading (20 wt%) and
high temperature (373 K, entries 3oa & 3ob). A similar observation was
also made in the previous study [49]. Based on the literature reports
[50,51], the plausible mechanism (Scheme 5) involves the initial oxi-
dative addition of aryl halides with Pd species followed by nucleophilic
addition of phenylboronic acid resulting in an insertion of aryl group in
Pd-intermediate. Finally, reductive elimination of the intermediate
compound leads to a final product with simultaneous regeneration of
the catalyst.
Table 2
Optimization of various reaction parameters of Heck reaction between Iodobenzene and Ethyl acrylate using Pd-MPMO.a
Entry
Catalyst,Wt %
Base
Temp. (K)
Solvent
Yieldb (%)
1
10.0
Triethylamine
Triethylamine
Triethylamine
Triethylamine
Triethylamine
Triethylamine
Pyrrolidine
Cs2CO3
373
373
373
373
373
373
373
373
373
373
353
333
373
373
DMF
70.0
80.0
93.0
52.0
8.0
2
15.0
DMF
3
20.0
DMF
4
20.0
Acetonitrile
Water
Toluene
DMF
5
20.0
6
20.0
49.0
63.0
Nil
7
20.0
8
20.0
DMF
9
20.0
K2CO3
DMF
Nil
10
11
12
13
14
20.0
KOH
DMF
Nil
20.0
Triethylamine
Triethylamine
Triethylamine
No base
DMF
56.0
17.4
Nil
20.0
DMF
No catalysts
20.0
DMF
DMF
6.0
a
b
Reaction conditions: Iodobenzene (1.0 mmol), Ethyl acrylate (1.0 mmol), base (2.0 mmol), H2O (4 mL), were stirred at 4 h unless otherwise noted.
GC analysis and yields based on aryl halides.
7

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
[a]
Scheme 4. Heck coupling between aryl halides and different alkenes over Pd-MPMO
.
[a]Reaction conditions: Iodobenzene (1.0 mmol), Ethyl acrylates/Styrenes (1.0 mmol), 20 Wt% catalyst, Et3N (2.0 mmol), DMF (4 mL) were stirred for 4h at 373 K.
[b]GC analysis and yields based on aryl halides. [c]The value in the parenthesis refers to turn-over-number.
3.4. Metal-leaching and recyclability test
To investigate whether any Pd is leached out of mesoporous silica
matrix into the solution, a hot filtration experiment was carried out. For
this purpose, Suzuki reaction involving iodobenzene and phenylboronic
acid was considered as a test reaction for the hot-filtration test. Initially,
the reaction was performed for a period of 1.5 h at 48.5% conversion
level and then, the catalyst was filtered off. Using the above filtrate, the
reaction was further continued for 1.5 h under identical conditions.
After completion of 3 h, GC analysis of the filtrate showed no progress
in the reaction which directly confirms true heterogeneous nature of the
Pd-MPMO catalyst (Supporting Information, Figure S5). Analysis by
atomic absorption spectroscopy for trace Pd metal also confirmed the
absence of Pd and corroborate the hot-filtration test.
To assess catalyst reusability of present catalytic protocol, again
Suzuki cross-coupling reaction was performed as a model reaction using
Pd-MPMO. After completion of the first run, the catalyst was recovered
by filtration, successively washed with water, ethanol, and acetone till
GC analysis of the filtrate shows no traces of reactants or products. Then
the recovered catalyst was dried at 373 K under vacuum for 2 h to re-
move any adsorbed solvents on the surface. The catalyst obtained from
the first run was reused for 4 successive runs and showed ˜9% loss in the
Scheme 5. A plausible mechanism of Suzuki coupling reaction over Pd-MPMO.
8

S. Elavarasan, et al.
MolecularCatalysis476(2019)110521
Scheme 6. A plausible mechanism of Heck coupling reaction over Pd-MPMO.
activity (Supporting Information, Figure S6). Thus the novel Pd-MPMO
with high activity coupled with wide substrate scope for Suzuki reac-
tion in the aqueous medium could serve as an alternative robust and
environmental benign catalyst.
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