Synthesis, Characterization, and Catalytic Activity of Pd(II) Salen-Functionalized Mesoporous Silica

Article abstract of DOI:10.1155/2017/6208132

Salen ligand synthesized from 2-hydroxybenzaldehyde and 2-hydroxy-1-naphthaldehyde was used as a palladium chelating ligand for the immobilization of the catalytic site. Mesoporous silica supported palladium catalysts were prepared by immobilizing Pd(OAc)₂ onto a mesoporous silica gel through the coordination of the imine-functionalized mesoporous silica gel. The prepared catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), inductivity couple plasma (ICP), nitrogen adsorption-desorption, and Fourier transform infrared (FT-IR) spectroscopy. The solid catalysts showed higher activity for the hydroamination of C-(tetra-O-acetyl-β-D-galactopyranosyl)allene with aromatic amines compared with the corresponding homogenous catalyst. The heterogeneous catalytic system can be easily recovered by simple filtration and reused for up to five cycles with no significant loss of catalytic activity.

Full text of DOI:10.1155/2017/6208132

Hindawi
Journal of Chemistry
Volume 2017, Article ID 6208132, 12 pages
https://doi.org/10.1155/2017/6208132
Research Article
Synthesis, Characterization, and Catalytic Activity of
Pd(II) Salen-Functionalized Mesoporous Silica
Rotcharin Sawisai, Ratchaneekorn Wanchanthuek,
Widchaya Radchatawedchakoon, and Uthai Sakee
Creative Chemistry and Innovation Research Unit, Center of Excellence for Innovation in Chemistry (PERCH-CIC),
Department of Chemistry, Faculty of Science, Mahasarakham University, Mahasarakham 44150, ailand
Correspondence should be addressed to Uthai Sakee; uthai.s@msu.ac.th
Received 15 March 2017; Revised 24 April 2017; Accepted 14 May 2017; Published 3 July 2017
Academic Editor: Bartolo Gabriele
Copyright © 2017 Rotcharin Sawisai et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
Salen ligand synthesized from 2-hydroxybenzaldehyde and 2-hydroxy-1-naphthaldehyde was used as a palladium chelating ligand
for the immobilization of the catalytic site. Mesoporous silica supported palladium catalysts were prepared by immobilizing
Pd(OAc) onto a mesoporous silica gel through the coordination of the imine-functionalized mesoporous silica gel. e prepared
2
catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX),
inductivity couple plasma (ICP), nitrogen adsorption-desorption, and Fourier transform infrared (FT-IR) spectroscopy. e solid
catalysts showed higher activity for the hydroamination of C-(tetra-O-acetyl-ꢀ-D-galactopyranosyl)allene with aromatic amines
compared with the corresponding homogenous catalyst. e heterogeneous catalytic system can be easily recovered by simple
filtration and reused for up to five cycles with no significant loss of catalytic activity.
1. Introduction
amine with a moderate to good yield [16]. However, this
reported method suffers from disadvantages, such as long
reaction time, expensive catalyst, and unstable operating
state. e practical application of the catalyst in liquid phase
reactions is hindered by both high cost and difficulties in
catalyst separation and recycling. e common catalysts for
these reactions are generally homogeneous systems, which
cause difficulties in recycling the catalyst and aggregation
of metal nanoparticles [17]. us, a heterogeneous system
for the metal catalyst is a reasonable step to avoid these
problems, and it is feasible to overcome such limitations by
employing Pd on a solid support [18]. When considering this,
silica fulfils many of the required aspects as an appropriate
solid support for the deposition of palladium catalysts [19,
20]. ere have been several methods established towards
the preparation of supported Pd-catalysts [21]. Silica, due
to its inexpensive, comfortable, accessibility, high thermal
resistant, high pore volume, and narrow pore size distribution
characteristics, has been the focus of significant consideration
e synthesis of nitrogen containing compounds is of great
importance in basic research and industrial processes due to
these important structures playing vital roles in drug research
and development [1, 2]. Among the several methods to form
C–N bond from N–H bond, hydroamination of unsaturated
C=C bonds has the benefit of being cost effective and feasible,
and it is widely used by many researchers worldwide [3–7].
Allenes are a unique class of organic compounds with two
cumulated double bonds [8]. ere has been much interest
in using them as starting material for the synthesis of ally-
lamines. Only a small number of transition-metal catalyzed
intermolecular hydroamination reactions of allenes have
been described [9–14]. Previously, we reported the addition
of amines to allene using Pd(OAc) , from which a mixture of
2
the desired allylic amine and a minor amount of dienic amine
was obtained [15]. Following this, the gold catalytic hydroam-
ination of allene was investigated to give the desired allylic

2
Journal of Chemistry
compared to other operative supports like polymeric ionic
liquid, alumina, MCM-41, SBA-15, and so on [22, 23].
Among the various N-based ligands, Schiff bases are well
known as ligands for the complexation of metal ions and as
catalysis because of their easy preparation and being simple
and cost effective starting materials with high resistance
under a variety of conditions. erefore, many researchers
have developed several approaches to immobilize palladium
Schiff-base complexes on solid supports [24]. To date, only
a few palladium(II) complexes attached to functionalized
mesoporous silica supports have been synthesized and suc-
cessfully applied in organic reactions. However, no catalytic
study has focused on the hydroamination of allene using
palladium supported on mesoporous materials as catalysts. In
our ongoing efforts to develop greener organic reactions, we
now report a simple strategy for the preparation and charac-
terization of a novel Salen-functionalized mesoporous silica
supported Pd Schiff-base complex as a reusable catalyst and
its application for the hydroamination of C-(tetra-O-acetyl-
ꢀ-D-galactopyranosyl)allene (1) with aromatic amines.
at a refluxing temperature under a nitrogen atmosphere for
7 h, and then the solids were filtered and washed thoroughly
with toluene until the washings were colorless. e solid
product was dried in air at 120∘C overnight before being used
in the next step, and the resulting materials were denoted as
SiO @ imineSA and SiO @ imineNA for salicylaldehyde and
2
2
2-hydroxy-1-naphthaldehyde, respectively.
Method B. Silica gel (20 g) was added to 60 mL of toluene. To
this slurry, 42.86 mole of APTES was added and the resultant
slurry was heated at 110∘C under a nitrogen atmosphere for
7 h, and then salicylaldehyde or 2-hydroxy-1-naphthaldehyde
(42.86 mol) was added to the slurry and stirred at a refluxing
temperature under a nitrogen atmosphere for 7 h. Following
this, the solids were filtered and washed thoroughly with
toluene until the washings were colorless. e solid product
was dried in air at 120∘C overnight and denoted as SiO @
2
imineSB and SiO @ imineNB for salicylaldehyde and 2-
2
hydroxy-1-naphthaldehyde, respectively.
2.3.2. General Procedure for Immobilization of
Pd(OAc)2 onto Salen-Silica
2. Experimental
2.1. General. All the starting materials, reagents, solvents,
and eluents were commercial and used as purchased without
purification. Flash column chromatography was performed
using a silica gel 60 (230–400 mesh). Allene (1) was prepared
according to a previously described method [25, 26].
Method I. To a round-bottomed flask, palladium acetate
(415.68 mg, 1.73 mmol) and acetone (100 mL) were added.
e solution was stirred at room temperature for 30 min
under a nitrogen atmosphere and then 5 g of SiO @ imineSA
2
was added. e mixture was stirred at room temperature
for 24 h. e deposited solids were filtered from the solvent
and washed repeatedly through the Soxhlet extraction with
ethanol and acetone until the washings were colorless, and
2.2. Material Characterization. e prepared catalyst was
characterized by various techniques, such as SEM, SEM-EDX,
XRD, FT-IR, and BET. XRD measurements were performed
with a D8 Advance Bruker diffractometer with the Cu-Kꢁ1
and Cu-Kꢁ2 radiations over a wide 2ꢂ range of 0∘–80∘.
Scanning electron micrographs (SEM) and energy dispersive
X-ray (EDX) analyses were operated on a JEOL JSM-6460LV
microscope. e IR spectrum was obtained using a FT-IR
spectrophotometer (Bruker, Tensor 27). e surface area and
porosity were measured with a Quantachrome Autosorb 1-
MP. e palladium content was analyzed by an inductively
coupled plasma optical emission spectrometer (ICP-OES,
Optima 3000 DV, Perkin Elmer, EUA).
they were dried overnight at 120∘C and denoted as SiO @
2
imineSA-Pd-I.
Method II. To a round-bottomed flask, palladium acetate
(415.68 mg, 1.73 mmol) and acetone (100 mL) were added.
e solution was stirred at room temperature for 30 min
under a nitrogen atmosphere and then 5 g of Salen-silica
was added. e mixture was refluxed while being stirred for
4 h. e deposited solids were separated from the solvent by
filtration and washed repeatedly for Soxhlet extraction with
ethanol and acetone, until the washings were colorless, and
they were dried overnight at 120∘C and denoted as SiO @
2
2.3. Catalyst Preparation. e catalyst was easily prepared
from commercially available starting materials as shown in
Scheme 1.
imineSA-Pd-II, SiO @ imineSB-Pd-II, SiO @ imineNA-Pd-
2
2
II, and SiO @ imineNB-Pd-II.
2
2.4. Procedure for Hydroamination of Allene (1). All reactions
were run under an air atmosphere. A reaction tube was
charged with allene (1) (0.27 mmol), amine (0.81 mmol), Pd
catalyst (90 mg, 5 mol%), trifluoroacetic acid (20 mol%), and
THF (0.5 mL). e reaction mixture was allowed to proceed
while being stirred at room temperature. e progress of
the reaction was monitored by TLC. Afer completion of
the reaction, the catalyst was filtered off and the solvent was
evaporated under reduced pressure, afer which the residue
was purified by flash column chromatography on silica gel to
yield the product. e solid compounds were characterized
2.3.1. General Procedure for Preparation of Imine-
Functionalized Silica Gel (Salen-Silica): SiO2@ Imine. A
commercially available silica gel (230–400 mesh) was dried
by heating at 120∘C for 12 h prior to use.
Method A. Salicylaldehyde or 3-hydroxy-2-naphthaldehyde
(42.86 mol) and then 3-aminopropyltriethoxysilane (APTES)
(10 mL, 42.86 mol) were added to toluene (60 mL). A yellow-
ish color showed immediately due to the formation of the
imine. e resulting solution was stirred at a refluxing tem-
perature under a nitrogen atmosphere for 7 h. Afer cooling,
the solid silica gel (20 g) was added to the mixture and stirred
by 1H and 13C NMR spectroscopy.

Journal of Chemistry
3
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OEt
EtO Si
OEt
NH2
N
Si(OEt)3
(a)
+
Reflux, 7 h
O
OH
H
OH
Reflux, 7 h
Silica
HO
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
O
O
O
OH
OH
O
Si
NH2
NH2
OEt
EtO Si
OEt
OH
NH2
H
N
N
(b)
OH
Reflux, 7 h
OH
O
O
O
Toluene
Reflux, 7 h
Si
O
Si
O
Si
O
O
OHOHOHO
O OH
OH
Silica
Acetone
Pd(OAc)2
O
O
Pd
N
N
Si
O
Si
O
O
O
OHOHOH O
O OH
Scheme 1: Synthetic strategy for the preparation of the silica gel supported Pd catalyst. (a) Homogeneous route for mesoporous silica
functionalization (method A) and (b) heterogeneous route for mesoporous silica functionalization (method B).
3a: 1H-NMR (400 MHz, CDCl ): ꢃ 7. 17 (2H, apt, J 8.4 and
J 15.6 and 5.2), 5.37 (1H, m), 5.31 (1H, dd, J 10.4 and 6.0), 5.13
(1H, dd, J 10.4 and 3.2), 4.81 (1H, apt, J 6.0 and 5.2), 4.00–4.15
3
7. 2), 6. 7 1 (1H, apt, J 8.4 and 7.2), 6.60 (2H, d, J 7. 6), 5. 98 (1H,
dt, J 15.6 and 5.2), 5.82 (1H, q, J 15.6 and 5.2), 5.37 (1H, m),
5.28 (1H, dd, J 10.4 and 6.0), 5.10 (1H, dd, J 10.4 and 3.6), 4.81
(1H, apt, J 6.0 and 5.2), 4.00–4.16 (3H, m), 3.75–3.95 (3H, m),
(3H, m), 3.75–3.87 (3H, m), 3.74 (3H, s), 1.99–2.15 (12H, 4 × s);
13C-NMR (100 MHz, CDCl ): ꢃ 170.53, 170.23, 170.13, 170.08,
3
152.44, 141.82, 134.79, 123.41, 114.92, 114.57, 72.50, 68.40, 68.33,
68.04, 68.01, 61.87, 45.94, 20.72, 20.70, 20.66.
1.97–2.15 (12H, 4 × s); 13C-NMR (100 MHz, CDCl ): ꢃ 170.52,
3
3c: 1H-NMR (400 MHz, CDCl ): ꢃ 7. 09 (1H, apt, J 8.4 and
170.19, 170.06, 169.87, 147.64, 134.47, 129.27, 123.49, 117.87, 113.14,
3
72.53, 68.31, 68.29, 68.01, 61.83, 45.62, 20.73, 20.69, 20.66.
8.0), 6.15–6.30 (3H, m), 5.99 (1H, dt, J 15.6 and 5.2), 5.87 (1H,
q, J 15.6 and 5.6), 5.38–5.40 (1H, m), 5.29 (1H, dd, J 10.4 and
6.0), 5.11 (1H, dd, J 10.4 and 3.2), 4.80 (1H, apt, J 6.0 and 5.2),
1
3b: H-NMR (400 MHz, CDCl ): ꢃ 6.79 (2H, d, J 8.8),
3
6.60 (2H, d, J 8.8), 5.97 (1H, dt, J 15.6 and 5.2), 5.85 (1H, q,

4
Journal of Chemistry
4.00–4.15 (4H, m), 3.82 (2H, d 5.2), 3.75 (3H, s), 1.96–2.16
2.5. Recycling Studies. Recycling studies were carried out
using the procedure for the hydroamination of allene
(1) (0.27 mmol) with aniline (0.81 mmol), Pd catalyst
(12H, 4 × s); 13C-NMR (100 MHz, CDCl ): ꢃ 170.53, 170.20,
3
170.04, 169.89, 160.84, 149.10, 134.52, 130.04, 123.49, 106.18,
102.82, 99.28, 72.52, 68.29, 68.07, 68.00, 61.86, 55.01, 45.61,
(SiO @ imineNB-Pd-II, 90 mg, 5 mol%), trifluoroacetic
2
20.73, 20.68, 20.63.
acid (20 mol%), and THF (0.5 mL). Afer the reaction, the
catalyst was separated from the reaction mixture by filtration
through a sintered glass funnel. e separated catalyst was
washed with dichloromethane (3 × 1 mL), dried in a vacuum,
and reused again to check its recycling efficiency.
1
3d: H-NMR (400 MHz, CDCl ): ꢃ 6.87 (1H, dt, J 7. 6
3
and 1.6), 6.78 (1H, dd, J 8.0 and 1.2), 6.69 (1H, dt, J 7. 6 and
1.2), 6.58 (1H, dd, J 8.0 and 1.2), 6.02 (1H, dt, J 15.6 and 5.2),
5.87 (1H, q, J 15.6 and 5.6), 5.37 (1H, d, J 3.2), 5.29 (1H, dd,
J 10.4 and 6.0), 5.12 (1H, dd, J 10.4 and 3.2), 4.82 (1H, apt,
J 6.0 and 5.2), 4.00–4.16 (3H, m), 3.75–3.86 (3H, m), 3.85
3. Results and Discussion
(3H, s), 1.98– 2.13 (12H, 4 × s); 13C-NMR (100 MHz, CDCl ):
3
3.1. Synthesis and Characterization of Catalyst. e prepa-
ration of the palladium immobilized on the Salen-silica
is shown in Scheme 1. Salen was covalently bonded to
the commercially available silica gel (230–400 mesh) giving
the Salen-silica. It was readily prepared by two different
approaches. e first route for the functionalized meso-
porous silica used the homogenous system (method A) for
the preparation of the silylating agent by the reaction of
3-aminopropyltriethoxysilane (APTES) with salicylaldehyde
or 3-hydroxy-2-naphthaldehyde. e product was condensed
with the silanol group of the solid silica gel to give the
imine-functionalized silica gel (Scheme 1(a)). e solid pow-
ders from salicylaldehyde and 3-hydroxy-2-naphthaldehyde
were designated as SiO @ imineSA and SiO @ imineNA,
ꢃ 170.47, 170.20, 170.04, 169.88, 146.95, 137.59, 134.84, 123.30,
121.23, 117.01, 109.57, 72.62, 68.29, 68.09, 68.05, 61.88, 55.41,
45.47, 20.75, 20.72, 20.68, 20.65.
1
3e: H-NMR (400 MHz, CDCl ): ꢃ 6.99 (2H, d, J 8.4),
3
6.55 (2H, d, J 8.4), 6.00 (1H, dt, J 15.6 and 5.2), 5.86 (1H, q,
J 15.6 and 5.6), 5.37 (1H, d, J 3.2), 5.31 (1H, dd, J 10.4 and
6.0), 5.11 (1H, dd, J 10.4 and 3.6), 4.79 (1H, apt, J 6.0 and 5.6),
4.00–4.16 (4H, m), 3.82 (2H, d J 5.2), 2.22 (3H, s), 1.99–2.13
(12H, 4 × s), 13C-NMR (100 MHz, CDCl ): ꢃ 170.50, 170.19,
3
170.04, 169.86, 145.37, 134.73, 129.76, 127.09, 123.34, 113.35,
72.59, 68.24, 68.05, 68.00, 61.83, 46.03, 20.70, 20.67, 20.34.
3f: 1H-NMR (400 MHz, CDCl ): ꢃ 7. 06 (1H, t, J 7. 6), 6. 55
3
(1H, d, J 7.6), 6.38–6.48 (2H, d, m), 6.01 (1H, dt, J 15.6 and 5.2),
5.87 (1H, q, J 15.6 and 5.2), 5.39 (1H, dd, J 3.6 and 1.6), 5.31
(1H, dd, J 10.4 and 6.0), 5.12 (1H, dd, J 10.4 and 3.6), 4.81 (1H,
apt, J 6.0 and 5.2), 4.00–4.15 (4H, m), 3.84 (2H, d, J 5.2), 2.27
2
2
respectively. In the second route, the heterogeneous system
(method B) involved the reaction of the silica gel with the
APTES to give aminosilica followed by a condensation with
salicylaldehyde or 3-hydroxy-2-naphthaldehyde to provide
the Salen-silica (Scheme 1(b)). e solid powders from the
reaction of salicylaldehyde and 3-hydroxy-2-naphthaldehyde
were designated as SiO @ imineSB and SiO @ imineNB,
(3H, s), 1.97–2.14 (12H, 4 × s); 13C-NMR (100 MHz, CDCl ):
3
ꢃ 170.48, 170.16, 170.01, 169.83, 147.67, 138.98, 134.61, 129.13,
123.31, 118.75, 113.88, 110.19, 72.57, 68.26, 68.02, 67.96, 61.80,
45.65, 21.54, 20.70, 20.65.
1
3g: H-NMR (400 MHz, CDCl ): ꢃ 8.09 (2H, d, J 8.8),
2
2
3
respectively. Finally, the precursor SiO @ imineSA was
6.58 (2H, d, J 8.8), 6.01 (1H, dt, J 15.6 and 5.2), 5.88 (1H, q,
J 16.0 and 5.2), 5.38–5.40 (1H, m), 5.30 (1H, dd, J 10.4 and
5.6), 5.11 (1H, dd, J 10.4 and 2.8), 4.82 (1H, apt, J 6.0 and 5.2),
3.75–4.18 (6H, m), 1.93–2.14 (12H, 4 × s); 13C NMR (100 MHz,
2
applied for the synthesis of the Pd complex by reacting with
Pd(OAc) in acetone at room temperature for 24 h (method
2
I) to give the product, designated as SiO @ imineSA-Pd-
2
I. e Pd complexes of SiO @ imineSA, SiO @ imineSB,
CDCl ): ꢃ 170.64, 170.17, 170.08, 169.77, 152.98, 138.32, 132.15,
2
2
3
SiO @ imineNA, and SiO @ imineNB were also obtained by
126.36, 124.85, 111.39, 72.24, 68.45, 68.23, 67.95, 67.82, 61.73,
44.89, 20.73, 20.69, 20.64.
2
2
reacting with Pd(OAc) in acetone under reflux conditions
2
3h: 1H-NMR (400 MHz, CDCl ): ꢃ 7. 55 (1H, d, J 8.0), 7.40
for 4 h (method II) to provide the resulting products, and they
3
were designated as SiO @ imineSA-Pd-II, SiO @ imineSB-
(1H, s), 7.32 (1H, apt, J 8.8 and 8.0), 6.90 (1H, d, J 8.0), 5.99 (1H,
dt, J 15.6 and 5.2), 5.92 (1H, q, J 15.6 and 5.2), 5.37–5.40 (1H,
m), 5.31 (1H, dd, J 10.4 and 5.6), 5.10 (1H, dd, J 10.4 and 2.8),
4.82 (1H, apt, J 6.0 and 5.2), 4.00–4.19 (4H, m), 3.93 (2H, d,
2
2
Pd-II, SiO @ imineNA-Pd-II, and SiO @ imineNB-Pd-II,
2
2
respectively. e characterization of the SiO @ imine and
2
Salen-silica supported Pd complex were done on the basis of
their properties, such as via FT-IR, XRD, SEM, SEM-EDX,
J 5.2), 1.96–2.17 (12H, 4 × s); 13C-NMR (100 MHz, CDCl ):
3
ICP-OES, and N adsorption-desorption spectra data.
ꢃ 170.61, 170.18, 170.07, 169.85, 149.44, 148.44, 132.92, 129.81,
2
e silanol (≡Si–OH) groups on the surface of the silica
showed a significant qualification through silylation. Figure 1
shows the FT-IR spectra of (I) SiO , (II) SiO @ imineSA,
124.40, 119.04, 112.38, 106.52, 72.39, 68.35, 68.25, 67.92, 61.73,
45.31, 20.72, 20.69, 20.66.
1
3i: H-NMR (400 MHz, CDCl ): ꢃ 7. 84 (2H, d, J 8.8),
2
2
3
(III) SiO @ imineSB, (IV) SiO @ imineSA-Pd-II, (V) SiO @
6.59 (2H, d J 8.8), 5.97 (1H, dt, J 15.6 and 5.2), 5.85 (1H,
q, J 15.6 and 5.2), 5.35–5.40 (1H, m), 5.29 (1H, dd, J 10.4
and 5.6), 5.10 (1H, dd, J 10.4 and 3.2), 4.81 (1H, apt, J 6.0
and 5.2), 4.0–4.17 (4H, m), 3.93 (2H, d, J 4.8), 2.50 (3H, s),
2
2
2
imineSB-Pd-II, and (VI) SiO @ imineSA-Pd-II. According
2
to Figure 1, the characteristic silica bands combined with
a silica backbone can be clearly detected in all spectra.
e characteristic Si–O–Si bands at 1057 and 799 cm−1,
present in all samples, are assigned to the silica network. e
spectrum of the free silica displays a typical broad band at
3361 cm−1 due to the vibration of the H bond of the silanol
1.96–2.16 (12H, 4 × s); 13C-NMR (100 MHz, CDCl ): ꢃ 196.43,
3
170.53, 170.15, 170.03, 169.78, 151.74, 133.05, 130.78, 130.73,
127.01, 124.12, 111.73, 111.66, 72.32, 68.35, 67.92, 67.85, 61.74,
44.83, 25.98, 20.70, 20.65, 20.60.

Journal of Chemistry
5
(VI)
(V)
(VI)
(V)
752
753
1446
1625 1539
797
2935
2931
1053
798
1446
1541
(IV)
(III)
(IV)
(III)
(II)
1625
1053
1054
752
754
796
795
1446
1539
2925
1625
1635
(II)
(I)
1496
1498
3062
2937
2939
1051
756
796
3066
(I)
1635
1050
1057
799
1631
3361
3900 3400 2900 2400 1900 1400
Wavelength (cm−1
1400 1200 1000
800
)
600
)
Wavelength (cm−1
(a)
(X)
(X)
1431
1542
3060
2933
744
744
745
744
795
(IX)
(IX)
1620
1052
1433
1544
796
795
3057
(VIII)
2931
(VIII)
(VII)
1620
1635
1053
(VII)
3058
2933
1544
1544
1051
796
799
(I)
3058
2933
1050
(I)
1635
975
1631
3361
1057
1000
Wavelength (cm
3900 3400 2900 2400 1900 1400
1400
600
800
1
1200
1
Wavelength (cm
(b)
Figure 1: (a) FT-IR of (I) SiO , (II) SiO @ imineSA, (III) SiO @ imineSB, (IV) SiO @ imineSA-Pd-II, (V) SiO @ imineSB-Pd-II, and (VI)
2
2
2
2
2
SiO @ imineSA-Pd-II. (b) FT-IR of (I) SiO , (VII) SiO @ imineNA, (VIII) SiO @ imineNB, (IX) SiO @ imineNA-Pd-II, and (X) SiO @
2
2
2
2
2
2
imineNB-Pd-II.

6
Journal of Chemistry
(VI)
(V)
(IV)
(III)
(II)
(I)
10
20
30
40
50
60
2-theta (degrees)
Figure 2: XRD patterns of (I) SiO , (II) SiO @ imineSA, (III) SiO @ imineSB, (IV) SiO @ imineSA-Pd-II, (V) SiO @ imineSB-Pd-II, and
2
2
2
2
2
(VI) SiO @ imineSA-Pd-II.
2
group and water of the siloxane backbone. e spectra
of all the samples (Figures 1(a) and 1(b)) of the modified
silica were identified by the presence of two weak bands at
3058–3066 and 2931–2939 cm−1 due to the vibration of the
aromatic and aliphatic C-H groups, respectively. Meanwhile,
the bending vibration of the Si–OH on the free silica at
975 cm−1 significantly decreased afer grafing with APTES.
in the silica particle size from the preparation with method
I. Although the palladium particles and their dispersions
are not clearly visible from the SEM spectra, considering
the results of the SEM-EDX (Figure 4) and the ICP-OES,
it was possible to define the palladium attachment on the
surface of the silica supports. e presence of palladium in the
Pd(II)-Schiff-base@SiO samples could be clearly seen from
2
e Schiff bases display strong bands at 1635 and 1498 cm−1
due to the azomethine (C=N) and phenolic (C–O) stretching
Figure 4. e amount of adhered palladium was evaluated by
ICP-OES, as shown in Table 1.
modes, respectively. e band at 1635 cm−1, upon the reaction
of the Salen complex with Pd, shifed to a lower frequency
(1625 cm−1 for SiO @ imineSA-Pd and SiO @ imineSB-Pd;
e results from the N adsorption-desorption isotherms
2
of the silica-based samples and the respective textural param-
eters containing the specific surface area (SBET), the pore
diameter, and the total pore volume (ꢄtotal) are summarized
in Table 1. e isotherms of all samples showed typical type IV
patterns corresponding to the IUPAC definitions of porosity
(Figure 5). e specific surface area provided by the BET
2
2
1625 cm−1 for SiO @ imineNA-Pd and SiO @ imineNB-Pd),
2
2
which indicated the formation of a Pd complex. e powder
X-ray diffraction patterns of the parent silica gel (SiO ) and
2
palladium immobilized Salen-silica are shown in Figure 2.
method for the free SiO was 411.16 m2g−1, which upon func-
∘
e patterns have a broad peak of 2ꢂ = 22.4 , which
2
tionalization by methods A and B with salicylaldehyde and
3-hydroxy-2-naphthaldehyde gave SiO @ imineSA, SiO @
indicated that the material had low crystallinity or the amor-
phous nature of the silica. A few decreases in the intensity
were identified for the Pd(II)-Salen-silica that confirmed the
immobilization of the Salen-silica. is decrease in intensity
could be due to the covering of the pores in the silica surface
during the metalation.
2
2
imineSB, SiO @ imineNA, and SiO @ imineNB, which
2
2
were reduced to 340.58, 366.92, 273.49, and 268.89 m2g−1,
respectively. In addition, the nitrogen adsorption-desorption
studies indicated that a significant decrease in pore size by the
imine-functionalization of the silica channels was monitored.
ere was not much difference in the surface areas of the
various catalysts (entries 6–10, Table 1), with values between
ca. 302 and 358 m2g−1. Different surface areas and pore
volumes can support different activities of the porous solid
catalysts, but there was no support due to different pore
volumes and BET surface areas in this study.
In good agreement with the XRD, the SEM images of the
free silica and supported Pd(II)-Schiff-base@SiO samples
(Figure 3) showed the ordered channel structure of the
mesoporous materials, which is stored during the complex
grafing. e silica gel did not change significantly in size or
gathered state under the supported processes in method II,
but the presence of palladium caused a significant decrease
2

Journal of Chemistry
7
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
Figure 3: SEM image of (a) SiO , (b) SiO @ imineSA, (c) SiO @ imineSA-Pd-II, (d) SiO @ imineSB, (e) SiO @ imineSB-Pd-II, and (f) SiO @
2
2
2
2
2
2
imineSA-Pd-I, (g) SiO @ imineNA, (h) SiO @ imineNA-Pd-II, (i) SiO @ imineNB, and (j) SiO @ imineNB-Pd-II.
2
2
2
2
3.2. Catalytic Hydroamination. Afer structural character-
ization of the prepared Pd(II) immobilized on Salen-
functionalized silica, SiO @ imineSA-Pd-II, SiO @ imineSA-
SiO @ imineNB-Pd-II, its catalytic activity was investigated
2
via the hydroamination of allene (1). e starting material
was prepared as previously described. e reaction between
the allene (1) and the aniline (2a) was selected as the model
2
2
Pd-I, SiO @ imineSB-Pd-II, SiO @ imineNA-Pd-II, and
2
2

8
Journal of Chemistry
Si
2
Si
2
Si
2
O
O
C
O
C
0
4
6
8
10 12 14 16 18 20
(keV)
0
4
6
8
10 12 14 16 18 20
(keV)
0
4
6
8
10 12 14 16 18 20
(keV)
Full scale 9092 cts Cursor: 0.000 keV
Full scale 9092 cts Cursor: 0.000 keV
Full scale 9092 cts Cursor: 0.000 keV
(a)
(b)
(c)
Si
Si
Si
O
O
O
C
Pd
C
Pd
C
Pd
0
2
4
6
8
10 12 14 16 18 20
(keV)
0
2
4
6
8
10 12 14 16 18 20
(keV)
0
2
4
6
8
10 12 14 16 18 20
(keV)
Full scale 2789cts Cursor: 14.357 keV (1 cts)
Full scale 2789 cts Cursor: 14.357 keV (1 cts)
Full scale 2789cts Cursor: 14.357 keV (0 cts)
(d)
(e)
(f)
Si
Si
Si
O
C
O
O
C
C
Pd
0
2
4
6
8
10 12 14 16 18 20
(keV)
0
2
4
6
8
10 12 14 16 18 20
(keV)
0
2
4
6
8
10 12 14 16 18 20
(keV)
Full scale 2789cts Cursor: 14.357 keV (0 cts)
Full scale 2789 cts Cursor: 14.357 keV (1 cts)
Full scale 2789 cts Cursor: 14.357 keV (1 cts)
(g)
(h)
(i)
Si
O
C
Pd
0
2
4
6
8
10 12 14 16 18 20
(keV)
Full scale 2789 cts Cursor: 14.357 keV (1 cts)
(j)
Figure 4: EDS spectrum of (a) SiO , (b) SiO @ imineSA, (c) SiO @ imineSB, (d) SiO @ imineSA-Pd-II, (e) SiO @ imineSB-Pd-II, (f) SiO @
2
2
2
2
2
2
imineSA-Pd-I, (g) SiO @ imineNA, (h) SiO @ imineNB, (i) SiO @ imineNA-Pd-II, and (j) SiO @ imineNB-Pd-II.
2
2
2
2
reaction using THF as the solvent and TFA as the additive
with 5 mol% palladium catalyst, as shown in Table 2.
e reaction was successfully operated at room tempera-
ture under air without any special precautions. To verify the
SiO @ imineNA-Pd-II, SiO @ imineNB-Pd-II, and homo-
2
2
geneous [Pd(OAc )] were compared as catalysts (with the
2
same palladium content) under identical conditions. It is
necessary to note that SiO @ imineSA-Pd-I exhibited the
2
effect of the imine-functionalization on the catalysis, SiO @
same yield as indicated by the homogeneous [Pd(OAc )]
2
2
imineSA-Pd-II, SiO @ imineSA-Pd-I, SiO @ imineSB-Pd-II,
(entries 1 and 3, Table 2). en, when comparing with
2
2

Journal of Chemistry
9
)
Table 1: Surface area, total pore volume, pore size distribution, and amount of Pd loaded on the surface of the silica-based materials.
Entry
Materials
ꢅ
a (m2 g−1
411.16
)
Total pore volumeb (cm3g−1
)
Mean pore diameterc (nm) Pd contentd SDe (mg g−1
BET
1
SiO
0.860
0.503
0.589
0.430
0.416
0.505
0.429
0.539
0.416
0.407
4.18
2.95
3.21
3.14
3.10
2.82
2.83
2.95
2.58
2.65
NAf
NAf
2
2
3
4
5
6
7
8
9
10
SiO @ imineSA
SiO @ imineSB
SiO @ imineNA
SiO @ imineNB
340.58
366.92
273.49
268.89
358.63
302.64
365.38
322.64
305.41
2
NAf
2
NAf
2
NAf
2
SiO @ imineSA-Pd-II
SiO @ imineSA-Pd-I
SiO @ imineSB-Pd-II
SiO @ imineNA-Pd-II
30.74 0.42
33.6 1.52
29.58 1.63
25.85 0.72
25.12 0.77
2
2
2
2
SiO @ imineNB-Pd-II
2
aBET method used in N sorption. bSingle-point pore volume at ꢀꢁꢀ = 0.975. cAdsorption average pore diameter by BET method. dDetermined by ICP-OES
2
ꢂ
analysis. eAverage of triplicates SD. f Not applicable.
Table 2: Optimization of reaction conditions of C-(tetra-O-acetyl-ꢀ-D-galactopyranosyl)allene (1) with aniline (2a).a
Yieldb (%)
Entry
Catalyst
3a
30
17
34
16
50
60
1
Pd(OAc)
2
2
3
4
5
6
SiO @ imineSA-Pd-II
2
SiO @ imineSA-Pd-I
2
SiO @ imineSB-Pd-II
2
SiO @ imineNA-Pd-II
2
SiO @ imineNB-Pd-II
2
OAc
NH2
OAc
OAc
O
NHPh
OAc
O
5 mol% catalyst
+
C
AcO
20 mol% TFA, THF, rt, 48 h
AcO
OAc
OAc
1
2a
3a
aAll reactions were carried out with conditions: C-(tetra-O-acetyl-ꢃ-D-galactopyranosyl)allene (1) (0.27 mmol) and aniline (0.81 mmol) in 0.5 mL of solvent
for 48 h. bIsolated yields.
the imine-functionalization of silica from salicylaldehyde,
the activity was significantly decreased with imineSA-Pd-
observed afer the fifh cycle. Additionally, palladium leach-
ing in SiO @ imineNB-Pd-II was determined. e palladium
2
II and SiO @ imineSB-Pd-II (entries 2 and 4, Table 2).
content of the clear filtrates obtained upon filtration afer the
reaction was < 0.2 ppm, from ICP-OES analyses.
2
However, in the presence of the imine-functionalization of
silica from 3-hydroxy-2-naphthaldehyde, SiO @ imineNA-
2
To test the extent and restrictions of our catalyst, SiO @
2
Pd-II, and SiO @ imineNB-Pd-II, the reaction proceeded
2
imineNB-Pd-II, we also studied the hydroamination of 1
with substituted anilines (2b–i) bearing an electron-poor or
an electron-rich group on the benzene ring provided to the
corresponding allylamines (3b–i) (Table 3).
with an improved yield of the desired product (3a) (entries
5 and 6, Table 2). In addition, the formation of diallylated
acetate and diallylated amine was not observed as minor
products. e recycling ability of SiO @ imineNB-Pd-II for
2
According to previous reports, the results also showed
that the onset potential for the heterogeneous catalyst was
more positive than under the homogenous condition. is
could be due to the reduced formation of diallylated amine
and diallylated acetate as byproducts.
the hydroamination of 1 with aniline was evaluated. Afer
the recovery of the catalyst, it was reused directly without
further purification. e catalyst could be successfully recy-
cled for five consecutive cycles for the hydroamination of 1
with aniline and no significant loss of catalytic activity was

10
Journal of Chemistry
Table 3: Hydroamination of C-(tetra-O-acetyl-ꢀ-D-galactopyranosyl)allene (1) with aromatic aminesa.
Product
Entry
1
Amine
b
(%)
NH2
MeO
3b (26)
3c (32)
2b
2
3
NH2
MeO
2c
3d (41)
NH2
MeO
2d
2e
H3C
NH2
4
5
6
7
3e (76)
3f (78)
3g (18)
3h (18)
H3C
O2N
NH2
2f
NH2
2g
O2N
O
NH2
NH2
2h
2i
8
3i (23)
R1
OAc
OAc
NH2
OAc
O
OAc
O
NH
SiO2@ imineNB-Pd-II
C
R1
AcO
AcO
+
20 mol% TFA
THF, rt, 48 h
OAc
OAc
1
2b–i
3b–i
aReaction condition: C -(tetra-O-acetyl-ꢃ-D-galactopyranosyl)allene (1) (0.27 mmole) and aniline (0.81 mmole) in 0.5 mL of solvent for 48 h; SiO @ imineNB-
2
Pd-II (5 mol%); TFA (20 mol%). bIsolated yields.
4. Conclusion
of aromatic amines. It could be easily recovered by simple
separation even afer the catalyst was reused for a fifh cycle
with no significant loss of catalytic activity.
A novel mesoporous silica supported palladium(II)-based
heterogeneous catalyst has been successfully synthesized by
immobilizing Pd(OAc) onto a Salen-functionalized silica
2
Conflicts of Interest
gel via coordination. e supported catalyst exhibited good
activity for the hydroamination of allene (1) with a variety
e authors declare that they have no conflicts of interest.

Journal of Chemistry
11
(f)
(k)
(j)
(e)
(d)
(i)
(c)
(b)
(h)
(g)
(a)
Relative pressure (P/P )
Relative pressure (P/P )
0
0
Figure 5: N adsorption-desorption isotherms of the silica-based materials: (a) SiO , (b) SiO @ imineSA, (c) SiO @ imineSB, (d) SiO @
2
2
2
2
2
imineSA-Pd-II, (e) SiO @ imineSB-Pd-II, (f) SiO @ imineSA-Pd-I, (g) SiO , (h) SiO @ imineNA, (i) SiO @ imineNB, (j) SiO @ imineNA-
2
2
2
2
2
2
Pd-II, and (k) SiO @ imineNB-Pd-II.
2
Acknowledgments
hydroamidation,” Chemical Reviews, vol. 115, no. 7, pp. 2596–
2697, 2015.
is work was supported by Mahasarakham University,
Human Resource Development in Science Project (Science
Achievement Scholarship of ailand (SAST)), the Center of
Excellence for Innovation in Chemistry (PERCH-CIC).
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