Synthesis and Characterization of Pd Schiff Base Complex Immobilized onto Functionalized Nanoporous MCM-41 and its Catalytic Efficacy in the Suzuki, Heck and Stille Coupling Reactions

Article abstract of DOI:10.1007/s10562-016-1957-5

Abstract: The present work describes the use of palladium-vanillin-Schiff-base complex immobilized on MCM-41 nanostructure as efficient catalyst for the Suzuki–Miyaura, Stille and Mizoroki–Heck reactions of several aryl halides under aerobic conditions. All the reactions were carried out in green solvents (H₂O and PEG-400). The developed procedure results bring several benefits such as uses of inexpensive and non-toxic ligand (vanillin), easy catalyst/product separation and catalyst recycling. The catalyst can be reused at least for five consecutive cycles without a significant loss of its catalytic activity or metal leaching. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-016-1957-5

Catal Lett
DOI 10.1007/s10562-016-1957-5
Synthesis and Characterization of Pd Schiff Base Complex
Immobilized onto Functionalized Nanoporous MCM-41 and its
Catalytic Efficacy in the Suzuki, Heck and Stille Coupling
Reactions
Maryam Khanmoradi1 · Mohsen Nikoorazm1 · Arash Ghorbani-Choghamarani1
Received: 14 October 2016 / Accepted: 16 December 2016
© Springer Science+Business Media New York 2017
Abstract The present work describes the use of pal-
ladium-vanillin-Schiff-base complex immobilized on
MCM-41 nanostructure as efficient catalyst for the Suzuki–
Miyaura, Stille and Mizoroki–Heck reactions of several
aryl halides under aerobic conditions. All the reactions
were carried out in green solvents (H2O and PEG-400).
The developed procedure results bring several benefits
such as uses of inexpensive and non-toxic ligand (vanillin),
easy catalyst/product separation and catalyst recycling. The
catalyst can be reused at least for five consecutive cycles
without a significant loss of its catalytic activity or metal
leaching.
1 Introduction
The synthesis of biaryl compounds, using the Heck, Stille
and Suzuki–Miyaura reaction methods catalyzed by pal-
ladium complex have been recognized as a powerful syn-
thetic tools and a major area of interest in multiple organic
transformations for academic and industrial process includ-
ing the pharmaceuticals, polymers and fabrication of herbi-
cides [1–10]. The products of the latter reaction have been
used for the synthesis of natural products, agrochemicals,
several intermediates in pharmaceutical and fine chemicals.
For example, the synthesis of taxol, one of active antican-
cer drugs, involves an intramolecular Heck reaction [11].
The use of Schiff base complexes containing transition
metal ions is of significant importance due to their cata-
lytic and biological properties [12, 13]. As a catalyst, Pd
is very important in the pharmaceutical industry; however,
this metal is expensive and cannot be separated from the
reaction media efficiently. Therefore, to reduce the presence
of Pd in the product solution, immobilization of Pd-cata-
lysts onto a solid support allows formation of heterogene-
ous catalyst with decreased of Pd leaching into solution and
improves the stability of the catalyst [14]. There are several
reports of Pd-nanocatalyst immobilized on solid supports,
such as zeolites [15], carbon [16], metal oxides [17, 18],
polymers [19], graphite oxide [20] and mesoporous silica
[21] for C–C coupling reactions. Current research focuses
on the synthesis and application of modified mesoporous
MCM-41 materials, which have an active species attached
to the framework via host–guest interactions, creating dis-
crete and uniform catalyst sites on the inner walls of the
porous systems [22]. MCM-41 has a hexagonal arrange-
ment of one-dimensional mesoporous with diameters rang-
ing from 2 to 10 nm [23] and exhibits unique properties like
high surface area, homogeneity of the pores, tunable and
Graphical Abstract
Keywords Suzuki · Heck · Stille · Cross coupling
reactions · Mesoporous silica · Vanillin Palladium
* Mohsen Nikoorazm
e_nikoorazm@yahoo.com
1
Department of Chemistry, Faculty of Science, Ilam
University, P. O. Box 69315516, Ilam, Iran
Vol.:(0123456789)
1 3

M. Khanmoradi et al.
accessible pores. Likewise, When metal complexes immo-
bilized onto surface of modified MCM-41, due to the stabil-
ity of the metal complex, it is presumed that the metal will
remain in its immobilized state when anchored to a support
prior to reaction making it be less likely to leach into solu-
tion. Hence, MCM-41 has been focused in several research
areas such as nanoscience [24], environmental purification
[25], catalysis [26], drug delivery [27] and absorption [28].
The Pd immobilized on MCM-41 can be used as a catalyst
for Suzuki–Miyaura and Stille cross-coupling reaction of
aryl halides with phenylbronic acide or Ph3SnCl in water
and Mizoroki–Heck reaction with aryl halides and n-butyl
acrylate in PEG-400 under thermal conditions. The cata-
lytic system used here has several advantages like it is sta-
ble in air and moisture and the reaction can be done under
environmentally benign conditions. Vanillin (4-Hydroxy-
3-methoxybenzaldehyde), an aromatic flavor compound, is
one of the most commonly used food flavoring agents and
is extracted from pods of Vanilla planifolia. It holds tre-
mendous industrial applications in food, pharmaceuticals,
beverages, perfumes and as nutraceuticals. In this paper, we
report synthesis and characterization of a Pd-vanillin Schiff
base complex supported on functionalized nanostructured
MCM-41 (Pd-Vanillin-MCM-41), which has been applied
as a novel and efficient catalyst with excellent catalytic
activity and stereoselectivity in C–C cross coupling reac-
tions in green solvents.
2 Experimental
2.1 Materials
The cationic surfactant cetyltrimethylammonium bromide
(CTAB, 98%), tetraethylorthosilicate (TEOS, 98%), sol-
vents and other materials were purchased from Merck,
Aldrich or Fluka companies and used as received.
2.2 Techniques
IR (KBr) and Powder X-ray diffraction (PXRD) were
recorded using a NICOLET impact 410 spectrometer and
a Philips diffractometer with Co Kα radiation at 40 kV
and 30 mA, respectively. The particle size and morphol-
ogy were performed by a JEOLJEM-2010 scanning elec-
tron microscopy (SEM), on an accelerating voltage of
200 kV. The products component measurement was car-
ried out by the energy dispersive spectroscopy (EDS)
using a Tacnai TF20 high-resolution transmission elec-
tron microscope. Also, the content of Pd was measured by
OCH3
EtO
EtO
EtO
O
NH2
Si
O Si
O
NH2
OH
OH
OH
OHC
OH
n-hexane, 24 h, reflux
APTES-MCM-41
EtOH, 3h, reflux
N2 atmospher
MCM-41
N2 atmospher
OCH3
(1)
Pd(OAc)2
O
O Si
O
N
N
O
ethanol, 18 h reflux
NaBH4
OH
OH
O Si
O
N
OH
CH3
O
(2)
O Si
O
O
ethanol, 2 h reflux
OCH3
Pd-Vanillin-MCM-41
Vanillin-MCM-41
Scheme 1 Schematic representation of the synthesis heterogeneous catalyst of Pd-Vanillin-MCM-41
1 3

Synthesis and Characterization of Pd Schiff Base Complex Immobilized onto Functionalized…
inductively coupled plasma-optical emission spectrometry
(ICP-OES). The thermogravimetric analysis (TGA) curves
were collected using a PL-STA 1500 device manufactured
by Thermal Sciences. The textural properties of the synthe-
sized samples were evaluated by N2 sorption–desorption
stirred in water at 80°C for an appropriate time. The pro-
gress of reaction was monitored by TLC (eluent; n-hexane/
acetone, 8:2). After completion of the reaction, the solution
was allowed to cool to room temperature and was subse-
quently extracted with diethyl ether. Hereon, the pure prod-
uct was obtained.
1
technique (NOVA 1000). H NMR spectra was recorded
on a Bruker AVANCE300 spectrometer operating at
300.0 MHz. Melting point was measured with an Electro-
thermal 9100 apparatus.
2.6 General Procedure for the Synthesis of Butyl
Cinnamate Derivatives
2.3 Synthesis of MCM-41 Modified
with (3-Aminopropyl)-triethoxysilane
The general protocol for Heck coupling reaction is
described as follows: Aryl halide (1 mmol), butyl acrylate
(1.2 mmol), base (3 mmol), and PEG-400 (3 mL) were
added to a 25 mL round-bottom flask. After adding Pd-
Vanillin-MCM-41 (4 mg, 0.71 mol %) catalyst, the reaction
mixture was stirred at 120°C for an appropriate time. After
completion of the reaction [monitored by TLC (n-hexane/
acetone, 8:2)], the catalyst was filtered off and the reaction
mixture was separated with diethyl ether and water. After
evaporation of the diethyl ether, the product was obtained.
To a mixture of NaOH solution (2 M) and deionized
water (480 mL), 1 g of cetyltrimethyl ammonium bromide
(CTAB) was added. Then, 5 mL tetraethylorthosilicate
(TEOS) was continuously added and stirred at 80°C. The
resulting mixture was refluxed at the same temperature for
another 2 h. The obtained product was washed with deion-
ized water. MCM-41 was activated by heating at 70°C in
an oven for 20°C. The resulting white product calcined at
823 K for 5 h with rate of 2°C/min to remove the residual
surfactant. Functionalization of MCM-41 was performed
by refluxing 4.8 g of MCM-41 with 4.8 g of 3-aminopro-
pyltriethoxysilane (3-APTES) in n-hexane under nitro-
gen atmosphere for 24 h. The resulting white solid of the
functionalized MCM-41 (APTES-MCM-41) was filtered,
washed with n-hexane and dried under vacuum.
2500
2000
Pd-Vanillin-MCM-41
1500
MCM-41
1000
500
0
2.4 Preparation of Heterogeneous Pd- Schiff Base
Complex Grafted in Mesoporous Silica MCM-41
(Pd-Vanillin-MCM-41)
2
7
12
17
For the preparation of vanillin-MCM-41, 0.5 g APTES-
MCM-41 and vanillin (0.182 g, 1.2 mmol) in ethanol was
refluxed under N2 atmosphere for 3 h. The resulting solid
was washed with ethanol, collected by filtration and dried
under vacuum and designated as vanillin-MCM-41. Ulti-
mately, the Pd-Vanillin-MCM-41 was prepared by refluxing
of Pd(OAc)2 (0.168 g, 0.63 mmol) with 0.250 g vanillin-
MCM-41 in ethanol (20 mL) for 18 h. For the reduction
of Pd(II) present in the resulting catalyst, NaBH4 (0.022 g,
0.057 mmol) was added and then refluxed for 2 h under the
same conditions. The resulting black solid impregnated
with the metal complex was filtered and washed with etha-
nol to give the grey Pd(0) complex. The detailed prepara-
tion route for Pd-Vanillin-MCM-41 is shown in Scheme 1.
2 theta (degrees)
Fig. 1 Small angle XRD patterns of: MCM-41 and Pd-Vanillin-
MCM-41
250
200
150
100
50
0
Pd (111)
Pd (200)
Pd (220)
70
2.5 General Procedure for the Synthesis of Biphenyl
Derivatives
10
20
30
40
50
60
80
2 theta (degrees)
A mixture of aryl halide (1.0 mmol), PhB(OH)2 or Ph3SnCl
(0.5 mmol), base (3 mmol) and Pd-Vanillin-MCM-41 were
Fig. 2 XRD pattern of Pd-Vanillin-MCM-41 at high degree 2θ
1 3

M. Khanmoradi et al.
Fig. 3 FT-IR spectra of, a MCM-41, b APTES-MCM-41, c Vanillin-MCM-41 and d Pd-Vanillin-MCM-41
1 3

Synthesis and Characterization of Pd Schiff Base Complex Immobilized onto Functionalized…
Fig. 4 SEM images of, a
MCM-41 and b Pd-Vanillin-
MCM-41
Fig. 5 The EDS spectrum of
Pd-Vanillin-MCM-41
2.7 Selected Spectral Data
(m, 2 H), 1.75–1.69 (m, 2 H), 4.25–4.22 (t, J=6.4 Hz,
2 H), 6.50–6.46 (d, J=16 Hz, 1 H), 7.43–7.40 (m, 3 H),
7.56–7.54 (m, 2 H), 7.74–7.70 (d, J=16 Hz, 1 H) ppm.
Butyl 3-(4-methoxyphenyl)acrylate (Table 7, entry 4):
1H NMR (400 MHz, CDCl3): δH =1.01–0.97 (t, J=7.6 Hz,
3 H), 1.50–1.41 (sixt, J=8 Hz, 2 H), 1.72–1.66 (quint,
J=6.8 Hz, 2 H), 3.85 (s, 3 H), 4.23–4.20 (t, J=6.4 Hz,
2 H), 6.35–6.31 (d, J=16 Hz, 1 H), 6.92–6.91 (d,
J=7.2 Hz, 2 H), 7.50–7.48 (d, J=6.8 Hz, 2 H), 7.67–7.63
(d, J=15.6 Hz, 1 H) ppm.
1,1′-Biphenyl (Table 3, entry 1): 1H NMR (400 MHz,
CDCl3): δH =7.41–7.37 (tt, J=7.6, 1.2 Hz, 2 H), 7.50–7.40
(m, 4 H), δH= 7.64–7.62 (m, 4 H) ppm.
1
4-Methoxy-1,1′-biphenyl (Table 3, entry 4): H NMR
(400 MHz, CDCl3): δH =3.88 (s, 3 H), 7.03–7.0 (dt, J=8.8,
2.4 Hz, 2 H), 7.35–7.31 (tt, J=7.6, 1.2 Hz, 1 H), 7.46–7.41
(t, J=7.6 Hz, 2 H), 7.60–7.55 (m, 4 H) ppm.
Butyl cinnamate (Table 7, entry 1): 1H NMR (400 MHz,
CDCl3): δH =1.01–0.97 (t, J=7.6 Hz, 3 H), 1.50–1.44
1 3

M. Khanmoradi et al.
900
800
700
600
500
400
300
200
100
0
MCM-41 sample. The intensities of the XRD peaks for
Pd-Vanillin-MCM-41 are decreased insofar as for 110 and
200 reflections close to disappear in comparison with XRD
peaks of MCM-41. These intensity decreases indicating
that the palladium metal is highly dispersed on the inner
channel pores of MCM-41 [22]. In order to prove that the
Pd(0) is dispersed onto surface of MCM-41, the high angle
XRD test was conducted. As shown in Fig. 2, in higher
angle range three diffraction peaks can be observed at
2θ=39.51, 45.65 and 66.57 corresponding to d1 1 1, d2 0 0
and d2 2 0 planes respectively [29, 30]. These observations
indicated that the Pd(0) is immobilized on the inner chan-
nel pores.
a
b
0
0.2
0.4
0.6
0.8
1
1.2
Relative pressure, (p/p0 )
Fig. 6 Nitrogen adsorption–desorption isotherms for, a bulk MCM-
41 and b Pd-Vanillin-MCM-41
3.2 FT-IR Spectroscopy
Figure 3 shows the FT-IR spectra of the synthesized MCM-
41, APTES-MCM-41, Vanillin-MCM-41 and Pd-Vanillin-
MCM-41, respectively. For the MCM-41 sample (Fig. 3a)
the peaks at 1059, 962 and 454 cm−1 corresponded to
asymmetric stretching, symmetric stretching and bend-
ing vibrations of Si-O–Si respectively. The broad band
at 3441 cm−1, can be assigned to the hydrogen-bonding
silanol groups and adsorbed water and the band about
2900 cm−1 is associated with the CH2 vibrations corre-
sponding to the C–H stretching (Fig. 3b). The appearance
of the ν(C=N) (1649 cm−1) and ν (C=C) (1519 cm−1)
bands support the formation of the Schiff base ligand and
shows that amine groups of 4-hydroxy-3-methoxybenzal-
dehyde rings (vanillin) anchored onto MCM-41 (Fig. 3c).
The band at 1599 cm−1 assigned to (NH2) in the ligand is
shifted towards 1632 cm−1 in the Pd-Vanillin-MCM-41
identified the Pd-ligand bond formation (Fig. 3d).
Table 1 Textural parameters deduced from nitrogen sorption iso-
therms
Sample
SBET (m2/g) Pore diam. by
BJH method
Pore vol. (cm3/g)
(nm)
MCM-41
1372.27
464.18
2.45
1.21
1.521
0.6466
Pd-Vanillin-
MCM-41
3.3 SEM, EDS and ICP-OES
Figure 4 shows the scanning electron microscopy (SEM)
images of MCM-41 and Pd-Vanillin-MCM-41. This study
showed the morphologies of the MCM-41 and Pd-Vanillin-
MCM-41 are agglomerations of small spherical with uni-
form nanometre sized particles (Fig. 4). The energy disper-
sive spectrum (EDS) evidently shows the presence of Pd
in the catalyst (Fig. 5). The ICP-OES results showed that
the loading amount of Pd on Pd-Vanillin-MCM-41 was
1.77×10−3 mol/g.
Fig. 7 TGA thermograms of MCM-41 and Pd-Vanillin-MCM-41
3 Result and Discussion
3.1 X-ray Diffraction
3.4 N2 Adsorption–Desorption Studies
Nitrogen adsorption–desorption isotherms of the prepared
MCM-41 and Pd-Vanillin-MCM-41 show in Fig. 6. Pore
volume, surface areas, and average pore size were calcu-
lated by the Brunauer–Emmett–Teller (BET) and Bar-
rett–Joyner–Halenda (BJH) models. The BET data and
Figure 1 shows low-angle XRD patterns of the obtained
MCM-41 and Pd-Vanillin-MCM-41. Three characteris-
tic peaks 2θ=2.77, 4.67 and 5.13 assigned to (100), (110)
and (200) reflections were observed respectively for the
1 3

Synthesis and Characterization of Pd Schiff Base Complex Immobilized onto Functionalized…
Table 2 Optimization of the
solvent, base, temperature and
concentration of catalyst for the
synthesis of biphenyl in Suzuki
reaction
Entry
Temperature Solvent
(°C)
Base (eq.)
Catalyst (mol%)
Time(min)
Yielda
b
1
2
3
4
5
6
7
8
80
80
80
80
70
60
50
25
80
80
80
80
80
80
80
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
1,4-dioxane
PEG-400
H2O
H2O
H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
KOH
0
5 h
15
15
15
15
15
15
15
15
15
15
15
15
15
15
–
0.88
0.71
0.53
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.71
94
94
90
90
83
66
43
56
40
94
90
30
91
93
9
10
11
12
13
14
15
Et3N
NaHCO3
H2O
Reaction conditions: Aryl halide (1 mmol), PhB(OH)2 (1 mmol), base (3 mmol), solvent (3 mL). Reaction
time was 15 min
aIsolated yield
bNo reaction
BJH data of prepared materials are illustrated in Table 1.
The original pore volume of 1.521 cm3 g−1 and surface area
of 1372 m2 g−1 are reduced to 0.646 and 464 m2 g−1, after
increasing substance anchored on the surface of MCM-
41. Hence, this decrease of surface areas, pore volume
and average pore size of mesoporous materials were seen
with incorporation of Pd nanoparticles in the framework of
MCM-41 (Fig. 6b).
In continuation of our investigations on different organic
transformation [31–33], we were interested in finding a
simple and efficient method for C–C cross coupling reac-
tions via the reaction of phenylboronic acid or Ph3SnCl
with an aryl halide for synthesis of biphenyl derivatives and
aryl halides with n-butyl acrylate in model of Heck reaction
under the influence of Pd-Vanillin-MCM-41 as a non-toxic
reusable and heterogeneous catalyst with short reaction
times with good to excellent yields in green solvents under
aerobic conditions.
3.5 Thermogravimetric Analysis
At the onset of our work, the catalytic activity of com-
plex was evaluated in Suzuki–Miyaura cross coupling
reactions. For this propose the reaction between phenylb-
oronic acid (1 mmol) and phenyl iodide (1 mmol) using
K2CO3 as a base at 80 °C was chosen as model reaction.
When the experiments were carried out in the absence
of catalyst, the desired product was not achieved after
5 h (Table 2, entry 1). Thus, we tested different amount
of catalyst (0.53, 0.71 and 0.88 mol%) (Table 2, entries
2–4). As shown in (Table 2, entry 3) the best amount of
catalyst was found 4 mg (0.71 mol%) for this reaction.
In the next step, the influence of temperature on conver-
sion and product selectivity in the presence Pd-Vanillin-
MCM-41 was examined. When reaction temperature was
increased gradually from room temperature to 80°C, the
rate of the reaction increased (Table 2, entries 3, 5–8).
The thermogravimetric analysis (TGA) curves of MCM-41
and Pd-Vanillin-MCM-41 are shown in Fig. 7. The TGA
of the synthesized MCM-41 shows 3% weight loss in one
step at 25–100°C due to desorption of water. Pd-Vanil-
lin-MCM-41 sample showed several weight loss steps.
A mass loss below 200°C is due to the removal of physi-
cally adsorbed water and solvent inside the pores chan-
nels. Pd-Vanillin-MCM-41 showed the second step mass
loss (25%) within the temperature range of 200–500°C,
which is chiefly related to the decomposition of organically
modified framework. The third weight loss above 800°C
is likely associated with the decomposition of the silanol
groups. As a result, TGA diagram of Pd-Vanillin-MCM-41
showed several steps of weight loss that confirmed the for-
mation of Palladium Schiff-base complex.
1 3

M. Khanmoradi et al.
Table 3 C–C Coupling of aryl halides with phenylboronic acid in the presence of Pd-Vanillin-MCM-41 (4 mg, 0.71 mol%) in water at 80°C
X
Pd-Vanillin-MCM-41
+ PhB(OH)2
R
K2CO3, H2O, 80 °C
R
X= I, Br
Entry
Substrate
Product
Time (min)
Yield (%)a
Melting point (ºC)
[references]
TOF (h−1
)
1
2
3
15
20
35
94
92
90
65–67 [34]
45–47 [34]
107–108 [35]
529.3
388.7
217.3
4
15
25
55
60
55
40
45
55
60
95
94
85
87
92
80
89
93
90
83–85 [34]
64–66 [34]
82–84 [35]
55–57 [36]
161–164 [34]
113–114 [37]
51–54 [38]
125–127 [38]
60–63 [39]
535.2
317.7
130.6
122.5
141.3
136.3
167.1
142.89
126.7
5
6
7
8
9
10
11
12
13
45
91
Oil [40]
169.0
14
15
35
25
87
90
44–46 [34]
71–74 [34]
210.0
304.2
aIsolated yield
1 3

Synthesis and Characterization of Pd Schiff Base Complex Immobilized onto Functionalized…
Table 4 Catalytic results for
Entry
Temperature Solvent
(°C)
Base (eq.)
Catalyst (mol%)
Time (min)
Yielda
the Stille reaction
b
1
2
3
4
5
6
7
8
80
80
80
80
80
80
80
80
80
80
80
80
70
60
25
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
DMSO
EtOH
1,4-dioxane
PEG-400
H2O
H2O
H2O
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
0
10 h
40
40
40
40
40
40
40
40
40
40
40
40
40
40
–
0.71
0.88
1.06
1.06
1.06
1.06
1.06
1.06
1.06
1.06
1.06
1.06
1.06
1.06
76
87
92
91
Trace
90
90
KOH
Na2CO3
NaHCO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
9
30
10
11
12
13
14
15
Trace
b
–
90
82
40
b
–
The reaction mixture consisted of phenyl iodide (1.0 mmol), Ph3SnCl (0.5 mmol), base (K2CO3, 3 mmol)
and 3 mL of solvent. Reaction time was 40 min
aIsolated yield
bNo reaction
Later, various solvents were applied for this reaction.
Ethanol and 1,4-dioxane were found to be ineffective
under the present reaction conditions (Table 2, entries
9–11), whereas, PEG-400 and highly polar solvents like
water were found to give excellent yields (Table 2, entries
3, 11). Interestingly, the reaction proceeded smoothly in
water providing an excellent yield (94%). Also, a variety
of bases were employed to give desired product at 80 °C
in the presence of 0.71 mmol% catalyst (Table 2, entries
12–15). Using K2CO3 as a base, the reaction had pro-
ceeded in high yield (Table 2, entry 3). The results illus-
trated in Table 2.
catalyst (0.71, 0.88, 1.06 mol%) in water (Table 4, entries
2–4). Hereon, we chose 1.06% mol% to be the proper cata-
lyst loading. Also, the effect of a series of organic and inor-
ganic bases on the activity of Pd-Vanillin-MCM-41 catalyst
was studied and Na2CO3 was selected as the proper base
under optimization reaction conditions (Table 4, entries
5–8). Beside, in order to find appropriate solvent, we
designed this reaction in different solvents (Table 4, entries
9–11). However, H2O proved to be the best solvent. Fur-
ther optimization was performed at different temperatures
(Table 4, entries 12–15). A high conversion of biphenyl can
be achieved at 80°C. The results of the screening process
have been summarized in Table 3. Notably, the combina-
tion of H2O as solvent and K2CO3 as base at 80°C appeared
to be suitable for this coupling reaction in presence of our
developed Pd catalyst.
To examine the utility and generality of this approach,
we have performed the reaction under optimized reaction
conditions for the synthesis of a wide variety of biphe-
nyls. The biphenyl derivatives were obtained in high yields
(Table 3, entries 1–15).
After optimization, the Stille reaction of aryl halides
with Ph3SnCl in the presence of heterogeneous Pd-Vanillin-
MCM-41 catalyst (6 mg. 1.06 mol%) in water at 80°C were
carried out and the obtained results are listed in Table 5.
Also, the ability of Pd-Vanillin-MCM-41 to act as an
efficient catalyst in Heck reactions has been examined. The
reaction of aryl halides with n-butyl acrylate have been
employed as the model reaction. There are several param-
eters which have been tuned to get the optimized yield. The
reactions have been performed with 1.0 mmol of phenyl
The activity of the catalyst was also examined in the
Stille cross-coupling reactions. For this purpose, the reac-
tion between the phenyl iodide (1.0 mmol) and Ph3SnCl
(0.5 mmol) in the presence of base (3 mmol) was probed
as a model reaction. The catalytic reactions were first car-
ried out in the absence of catalyst, which did not give a
quantitative yield of biphenyl after 10 h (Table 4, entry 1).
To investigate the effect of the catalyst, systematic studies
were carried out in the presence of different amounts of the
1 3

M. Khanmoradi et al.
Table 5 The Stille reaction of
aryl halides with Ph3SnCl using
heterogeneous Pd-Vanillin-
MCM-41 catalyst (6 mg.
1.06 mol%) in water at 80°C
Entry Substrate
Product
Time (min) Yield (%)a Melting point TOF (h−1
)
(ºC) [refer-
ences]
1
2
3
40
45
55
92
93
94
64–66 [34]
45–47 [34]
Oil [34]
130.1
116.9
96.7
4
55
85
90
89
86
94
85
87
89
86–89 [34]
63–66 [34]
83–85 [35]
162–164 [34]
112–114 [37]
53–55 [38]
44–46 [34]
71–73 [34]
87.4
101.8
34.7
69.5
70.9
60.1
75.7
31.4
5
50
6
145
70
7
8
75
9
80
10
11
65
160
aIsolated yield
iodide and 1.2 mmol of n-butyl acrylate in the presence of
3 mmol of base under varying conditions, e.g. using differ-
ent solvents, temperature and various bases. As shown in
Table 6, when the reaction was carried out in the absence
of Pd-Vanillin-MCM-41, the low yield of product was
obtained (Table 6, entry 1). As shown in Table 6, among
the different amount of catalyst tested, 4 mg of Pd-Vanil-
lin-MCM-41 was found to be the most effective catalytic
amount since it gave the highest yield of the product. On
the other hand, when the amount of catalyst was reduced,
the yield of the product catalysed was decreased (Table 6,
entries 2–4). As entries 5–7 show in Table 6, the reaction
temperature plays an important role in the Pd-Vanillin-
MCM-41 catalysed Heck reaction, with the maximum
conversion reaching at 120°C. Solvents play an essential
role in the selectivity and reactivity performance, thus, the
choice of a proper solvent is necessary. To investigate the
effect of solvent, systematic studies are carried out in the
presence of different solvents (Table 6, entry 8–10). Thus,
the best yield is found in the presence of PEG-400 as sol-
vent. Then, a variety of bases were essayed to give desired
product in the presence of 0.71 mmol% complex. Using
K2CO3 as a base, the reaction had proceeded in high yield
(Table 6, entries 11–14). Finally, we found the following
optimized conditions: the reaction proceeded in 91% yield
with K2CO3 as a base in the presence of 4 mg (0.71 mol%)
of catalyst in PEG-400 at 120°C under stirring.
The reaction of various aryl halide derivatives, including
electron-donating and electron-withdrawing groups on the
aromatic ring, with n-butyl acrylate was then investigated
to confirm the generality of the present method. The results
of this study are summarized in Table 7.
1 3

Synthesis and Characterization of Pd Schiff Base Complex Immobilized onto Functionalized…
Table 6 Catalytic results for
Entry
Temperature Solvent
(°C)
Base (eq.)
Catalyst (mol%)
Time (min)
Yielda
the Heck reaction
b
1
2
3
4
5
6
7
8
120
120
120
120
130
110
100
120
120
120
120
120
120
120
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
EtOH
H2O
DMF
PEG-400
PEG-400
PEG-400
PEG-400
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
0
10 h
25
25
25
25
25
25
25
25
25
25
25
25
25
–
0.53
0.71
0.88
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.71
0.71
72
91
91
91
80
63
b
–
9
75
83
90
75
88
35
10
11
12
13
14
NaHCO3
Na2CO3
KOH
The reaction mixture consisted of aryl halide (1.0 mmol), n-butyl acrylate (1.2 mmol), base (K2CO3,
3 mmol), and 3 mL of solvent, reaction time was 25 min
aIsolated yield
bNo reaction
The catalyst was tested for their recyclability in these
reactions. For each cycle, the amounts of solvent and rea-
gents were adjusted depending on the amount of catalyst
that had been recovered from the previous run. The results
are presented in Fig. 8. In Suzuki coupling reaction, the
reusing possibility of the recovered catalyst was investi-
gated using phenyl iodide (5 mmol) and phenylboronic acid
(5 mmol) or Ph3SnCl (2.5 mmol) as a model reaction. After
each run, the supported catalyst was separated by filtra-
tion, then dried, and used directly for the next run (Fig. 8,
column a, b). In Mizoroki–Heck reaction, the reusing pos-
sibility of the recovered catalyst was investigated using
phenyl iodide and butyl acrylate as substrates. After each
run, the supported catalyst was extracted by filtration and
used directly for the next run (Fig. 8, column c). The result
showed that catalyst is recoverable and recycled for at least
five times without significant loss of its catalytic activity
(Fig. 8).
15 min and the obtained conversion was the same (53%).
And also, the hot filtration test was applied for syntheses
of biphenyl from reaction of phenyl iodide (1 mmol) and
PhB(OH)2 (1 mmol) in water at 80°C as model reaction in
the presence of catalyst (4 mg, 0.71 mol%). After comple-
tion of 50% of the coupling reaction the liquid phase is col-
lected by filtration at the reaction temperature. The mixture
was continued to be stirred at the same temperature, but the
coupling reaction dose not proceed further after the separa-
tion of the Pd-Vanillin-MCM-41 from the mixture.
4 Conclusions
In summary, palladium Schiff base complexes consisting
of mesoporous MCM-41 has been prepared by means of
Post-Grafting procedure and characterized using various
analytical techniques. The results indicated Pd-Vanillin-
MCM-41 catalyst was a highly efficient and recyclable
catalyst for C–C coupling reactions with variety aryl hal-
ides under mild conditions. Phenyl iodides and phenyl
bromides showed good reactivity to generate the corre-
sponding products in good to excellent yields for all reac-
tions. Short reaction times, easy purification, high yields,
recyclability and very low Pd leaching are main charac-
teristic of the process.
Finally, hot filtration test was investigated to check if
the leached metal species are responsible for the cata-
lytic activity in Mizoroki–Heck reaction of phenyl iodide
with butyl acrylate as a model substrate in the presence
of Pd-Vanillin-MCM-41 (4 mg, 0.71 mol%) at 120°C in
PEG-400. During the catalytic reaction, the solid cata-
lyst was removed from the reaction mixture by filtration
after 10 min and the determined conversion was 53%. The
residual solution was, then, allowed to react for the further
1 3

M. Khanmoradi et al.
Table 7 The Heck reaction of aryl halides with olefin using heterogeneous Pd-Vanillin-MCM-41 catalysts (0.71 mol%) at 120°C in PEG-400
Entry
1
Substrate
Product
Time (min)
25
Yield (%)a
91
References
TOF (h−1
304.2
)
Xu et al. [41]
2
3
60
45
90
89
Xu et al. [41]
Xu et al. [41]
126.7
167.3
4
5
30
25
91
86
Xu et al. [41]
Xu et al. [41]
256.3
290.7
6
7
40
45
90
90
Xu et al. [41]
Xu et al. [41]
190.14
169.0
8
55
85
Xu et al. [41]
130.6
9
70
60
75
89
Xu et al. [41]
Xu et al. [41]
90.54
10
125.35
aIsolated yield
1 3

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1 3