Palladium nanoparticles supported on 1,3-dicyclohexylguanidine functionalized mesoporous silica SBA-15 as highly active and reusable catalyst for the Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1039/c4ra14668a

This paper describes a simple procedure for the synthesis and application of SBA-15/1,3-DCG/Pd as a novel nano catalyst in Suzuki-Miyaura cross-coupling reactions under atmospheric conditions in aqueous media. The SBA-15/1,3-DCG/Pd is air and moisture stable and readily prepared and the results indicated that palladium nanoparticles (Pd NPs) with an average size of 10 nm were uniformly dispersed onto the inorganic support. Aryl halides, coupled with phenyl boronic acid, smoothly afford the corresponding cross-coupling products with good to excellent yields (83-99%). It was found that the catalytic system showed excellent activity for the Suzuki-Miyaura cross-coupling reaction of less reactive aryl chlorides with phenyl boronic acid. The reaction conditions have been evaluated considering the effect of various parameters such as reaction time, temperature, amount of catalyst, amount of base and its nature and solvent. The catalyst is completely recoverable in a facile manner from the reaction mixture and the efficiency of the nano catalyst remains unaltered even after five cycles. This journal is

Full text of DOI:10.1039/c4ra14668a

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PAPER
Palladium nanoparticles supported on 1,3-
dicyclohexylguanidine functionalized mesoporous
silica SBA-15 as highly active and reusable catalyst
for the Suzuki–Miyaura cross-coupling reaction
Cite this: RSC Adv., 2015, 5, 20098
a
a
a
b
Hojat Veisi,* Abbas Amini Manesh, Neda Eivazi and Ali Reza Faraji
This paper describes a simple procedure for the synthesis and application of SBA-15/1,3-DCG/Pd as a novel
nano catalyst in Suzuki–Miyaura cross-coupling reactions under atmospheric conditions in aqueous media.
The SBA-15/1,3-DCG/Pd is air and moisture stable and readily prepared and the results indicated that
palladium nanoparticles (Pd NPs) with an average size of 10 nm were uniformly dispersed onto the
inorganic support. Aryl halides, coupled with phenyl boronic acid, smoothly afford the corresponding
cross-coupling products with good to excellent yields (83–99%). It was found that the catalytic system
showed excellent activity for the Suzuki–Miyaura cross-coupling reaction of less reactive aryl chlorides
with phenyl boronic acid. The reaction conditions have been evaluated considering the effect of various
parameters such as reaction time, temperature, amount of catalyst, amount of base and its nature and
solvent. The catalyst is completely recoverable in a facile manner from the reaction mixture and the
efficiency of the nano catalyst remains unaltered even after five cycles.
Received 16th November 2014
Accepted 11th February 2015
DOI: 10.1039/c4ra14668a
www.rsc.org/advances
hydrothermal stability, and possesses hexagonal arrays of
uniform pores with high special surface area and large pore
Today, the heterogenisation of metal complexes with catalytic volume.
Introduction
6
–10
1
–3
4,5
activity within inorganic matrixes such as SiO , MCM-41,
In past two decade, the use of palladium nanoparticles (Pd
clay, zeolite and mesoporous materials is a eld NPs) in catalysis has expanded considerably and has led to
that has progressed due to the combination of high reactivity interesting applications both for fundamental research and
2
6
–11
SBA-15,
2
1
and chemical selectivity (like homogenous catalysts) with industrial manufacturing.
The catalytic formation of
facile separation of the catalyst from the reaction mixture. carbon–carbon bonds (such as the Suzuki, Heck, Sonoga-
Due to its properties, particular attention has been paid to shira, Stille, Hiyama and Negishi reactions) is one of the most
reusability, thermal stability and low cost of the catalysts. In important reaction in organic synthesis. Among of these
order to scientists and technologists have devoted much effort reactions, the Suzuki–Miyaura cross-coupling is one of the
to the development of functionalized ordered mesoporous most important and useful transformations for the selective
1
2,13
silicas
tailored composition has received great attention. These synthesis of natural products, pharmaceuticals intermedi-
with high surface areas, tunable pore size and construction of biaryls. This compound is applied in the
1
4
materials have broad application ranging gas separation,
from adsorbent, and catalysis
ates, optoelectronic devices, liquid crystal chemistry and
1
5
16–19
20
22–25
to biological uses.
advanced material.
Due to the broad availability and low
Because of these materials have excellent properties such as toxicity of its substrates (aryl boronic acids or borates and aryl
mechanically stable structure, high surface area, and large, halides or triates) and its tolerance towards broad func-
ordered pores with narrow size distribution of an inorganic tional group and solvents, the SM cross-coupling reaction has
backbone. This organic mesoporous functionalization has found widespread applications in synthetic organic chem-
been widely achieved by the graing of functional groups. In istry. Encouraged to our previous works on C–C-coupling
2
6–28
particularly SBA-15 between other silica materials, ordered reactions,
in the present study, we designed and
mesoporous silicas (OMSs) have relatively high to good prepared SBA-15/1,3-DCG/Pd nanocatalyst (Scheme 1) by
graing of 1,3-dicyclohexylguanidine (1,3-DCG) groups on
SBA-15 and subsequent deposition of Pd nanoparticles as a
Department of Chemistry, Payame Noor University (PNU), 19395-4697 Tehran, Iran. new and stable catalyst and investigated their performance in
a
E-mail: hojatveisi@yahoo.com; Fax: +98 838 4227463
the Suzuki–Miyaura reactions for synthesis of biaryls
Scheme 2).
b
Young Researchers & Elite Club, Pharmaceutical Sciences Branch, Islamic Azad
(
University, Tehran, Iran
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Scheme 1 Preparation of SBA-15/1,3-DCG/Pd.
Pd are listed in Table 1. The SBET, VBJH and MPD of the SBA-
Result and discussion
15/1,3-DCG were signicantly reduced compared to the parent
The formation of palladium catalyst onto the SBA-15 was veri- SBA-15 (this decrease indicates the decrease in interaction
ed using CHN analysis, UV-Vis, FT-IR, size distribution (PSD), between adsorbate, N molecules, and the nano-sized SBA-15

2
scanning electron microscope (SEM), transmission electron surface aer modication with organic groups). Therefore this
microscope (TEM), wavelength-dispersive X-ray spectroscopy observation conrmed that the organic chain prefer to be
(
WDX), inductively coupled plasma (ICP-AES), energy-dispersive graed in the ordered nano pores.
X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray
adsorption isotherms of SBA-15 and SBA-15/1,3-DCG/Pd
N
2
photoelectron spectroscopy (XPS). The elemental analysis are also shown in Fig. 1. The isotherms were all type IV with a
report along with summary of measured and calculated values H1 hysteresis loop and a steep increase in adsorption at relative
for C, H and N is presented in Table 1. These data indicated the pressures of 0.58–0.80 for SBA-15 samples approximately
fact that the CHN content increased with the organic chain size attributed to capillary nitrogen condensation according to
because of immobilization of organic groups in the inorganic IUPAC classication. This is typical for mesoporous materials
9,10
matrix.
with ordered pore structures.
Additionally, upon modica-
By standard Brunauer–Emmett–Teller technique (BET) the tion the surface area and pore volume decreased obviously.
specic surface area was analyzed. Nitrogen adsorption exper- These results are in good agreement with the fact that the
iments were performed using 1 g of the nano catalyst. The surface modication indeed occurred inside the primary mes-
surface area (SBET), pore size (MPD) and volume (VBJH) of the opores of the SBA-15.
SBA-15, SBA-15-APTMS, SBA-15/1,3-DCG and SBA-15/1,3-DCG/
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Fig. 1
2
N -adsorption desorption isotherms of SBA-15 and SBA-15/
1
,3-DCG/Pd.
ꢀ
1
cm indicates that the successful functionalization of the SBA-
5 with trimethoxysilylpropylamine. Therefore, these results
conrmed that the SBA-15 was successfully modied by amine
Scheme 2 Suzuki–Miyaura reaction catalyzed by SBA-15/1,3-DCG/ spacer groups. The C]N imine vibrations signal was observed
1
ꢀ
1
Pd.
at 1650 cm , which shows the condensation reaction between
DCC with amino-functionalized SBA-15/pr-NH (see Fig. 3).
2
Typical SEM images with different magnications for SBA-
The loading of palladium in the catalyst was determinated by 15/1,3-DCG/Pd are shown in Fig. 4. It reveals that the
EDX and ICP analyses. The amount of Pd in nano catalyst was morphology of SBA-15/1,3-DCG/Pd consists of rod-shaped
determined with an ICP-EAS instrument and nal Pd content particles aggregated into bundles.
ꢀ
1
was measured around 0.30 mmol g . This amount indicating
that 75.6% of the immobilized ligands were complexed with prepared catalyst are shown in Fig. 5, from which it can be seen
palladium ions (see Table 1).
that the catalyst shows that the attachment of organic compo-
The Pd loading of the prepared catalyst, measured by ICP, nents to the SBA-15 materials has no distinct inuence on the
The selected TEM images and its size distribution of the
ꢀ1
showed a value of about 0.30 mmol g
of catalyst. The morphology of composition. It is seen that the average size Pd is
elemental composition of the SBA-15/1,3-DCG/Pd samples was in the range of 10 nm and palladium nanoparticles are
determined by EDX analysis. The result shown in Fig. 2 reveals dispersed on SBA-15. Particle Size Distribution Analysis (PSD)
that the nano catalyst contain Si, O, C and Pd.
on catalysts is a measurement designed to determine and report
FT-IR spectrum of SBA-15/1,3-DCG is shown in Fig. 2. In this information about the size and range of a set of particles
section the (n), (s), ( ) and ( ) represents stretching, bending, representative of a material. Therefore, PSD analysis of catalyst
s
as
symmetric and asymmetric vibration, respectively. As it is seen carried out. This analysis indicated that the average particle size
9,10
in the spectrum of SBA-15, the structure is veried by the IR in reverse microemulsion solution was around 10 nm (see
ꢀ1
bands located at 3400 cm (nas O–H of silanols groups and Fig. 5b).
ꢀ
1
ꢀ1
adsorbed water), 1085 cm (nas Si–O), 807 cm (n
4
s
Si–O),
The low angle XRD patterns of SBA-15 and its functionalized
ꢀ1
67 cm (s Si–O–Si). C–H bending, vibration modes of the derivatives and are shown in Fig. 6. Small angle X-ray powder
propyl in APTMS groups are observed between 1470 cm and diffraction of the parent SBA-15 gave the peaks in the 2u range
ꢀ
1
ꢀ
1
7,8
9,29
2850–2935 cm
.
The N–H deformation peak at 1540–1561 of 1–2.38 corresponding to 2D hexagonal p6mm symmetry.
Table 1 Chemical composition and structural properties of the immobilized Pd-nano-catalyst on the SBA-15
Elemental
analysis (wt%)
a
d
Organic
functional group Schiff base-
Pd (mmol g
Immobilized-Pd
% Coordinated
Schiff base groups
to Pd ions
Structural parameters
b
ꢀ1
c
ꢀ1
2
ꢀ1
3
ꢀ1
ꢀ
Sample
C
N
)
complex (mmol g
)
S
BET (m g
)
V
BJH (cm g
)
MPD (A)
SBA-15
0.46
7.35 4.13
13.2
—
—
—
—
—
2.95
4.35
—
—
—
0.30
—
—
—
75.6
782
432
328
285
0.99
0.45
0.23
0.18
59
43
35
27
SBA-15-APTMS
SBA-15/1,3-DCG
SBA-15/1,3-DCG/Pd 11.3
6.10
2.10 1.3 1.50
a
b
c
Nitrogen was estimated from the elemental analyses. Pd content determined from EDX analysis. Determined from the N-contents. Determined
d
from the Pd-content, assume that palladium ions coordinated with 1.3-DCG ligands. The pore size determined using the BJH method.
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Fig. 2 EDS spectrum of SBA-15/1,3-DCG/Pd.
Fig. 3 FTIR spectra of SBA-15/1,3-DCG/Pd.
The (1 1 0) and (2 0 0) reections reveal long-range, mesopore
The X-ray diffraction (XRD) pattern in Fig. 7 shows the
ordering typical of SBA-15. Aer functionalization of SBA-15, the crystalline structure of the SBA-15/1,3-DCG/Pd catalyst. The
1 0 0) and (2 0 0) reections of the SBA-15 were remained but XRD pattern of the SBA-15/1,3-DCG/Pd catalyst also shows Pd-
(
the intensity of peak was decreased. In addition, the (2 0 0) NPs on the silica surface. The f-SBA-15-Pd nanocatalyst clearly
reection was broadened and shied marginally to higher 2q exhibited (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystallographic
ꢁ
ꢁ
ꢁ
values. The (1 1 0) reection became weak and broad, which planes of face-centered cubic (fcc) palladium at 39 , 46 , 67 and
could be due to contrast matching between the silicate frame- 82 respectively (JCPDS no. 89-4897).
ꢁ
work and organic moieties which are located inside the chan-
The XPS spectroscopic analysis of the heterogeneous catalyst
nels of SBA-15. These results indicated that the basic structure is a quantitative technique to indicate the electron properties of
of the parent SBA-15 was not damaged in the whole process of the species immobilized on the surface, such as oxidation state,
catalyst preparation. These results agree well with those repor- the electron environment and the binding of the core electron (E
9,10
ted by others.
binding) of the metal. Fig. 8 shows the XPS spectrum of SBA-15/1,3-
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Fig. 4 SEM images of SBA-15/1,3-DCG/Pd.
DCG/Pd. The XPS spectrum of Si 2p contains a peak. This strong
peak located at 104.3 eV, which is the typical Si 2p1/2. Moreover, the
study of the SBA-15/1,3-DCG/Pd at the Pd 3p level shows peaks at
531.7 and 554.2 eV for Pd 3p3/2, which clearly indicates that the Pd
nanoparticles are stable as metallic state in the nanocomposite
0
structure. In comparison to the standard binding energy of Pd ,
with Pd 3p3/2 of about 532.4 eV and Pd 3p1/2 of about 560.2 eV, it
can be concluded that the Pd peaks in the SBA-15/1,3-DCG/Pd
Fig. 6 XRD small angle patterns of the parent SBA-15 and SBA-15/1,3-
DCG/Pd.
0
shied to lower binding energy than Pd standard binding
30,31
energy. The previous studies
indicated that the position of Pd
3
p peak is usually inuenced by the local chemical/physical envi-
representative SEM and corresponding elemental maps (WDX)
images for the synthesized catalyst. It can be seen that the
particles are not fully spherical. In addition, some particle
aggregations are observed, which is likely to be caused by the
hydrogen interactions between particles. The WDX analysis
reveals that Pd metal particles are well dispersed in the catalyst,
which agrees well with the XRD and TEM analysis.
Aer structure characterization of the prepared nano-
catalysts (SBA-15/1,3-DCG/Pd), its catalytic activity was evalu-
ated in the Suzuki reaction (Scheme 2).
ronment around Pd species besides the formal oxidation state, and
shis to lower binding energy when the charge density around it
increases. Therefore, the peaks at 554.2 and 531.7 could be due to
0
Pd species bound directly to 1,3-DCG in the SBA-15/1,3-DCG/Pd.
The characteristic peaks corresponding to carbon (C 1s),
nitrogen (N 1s) and oxygen (O 2s) are also clearly observed in XPS
elemental survey of the SBA-15/1,3-DCG/Pd(0).
In combination with scanning electron microscopy (SEM),
wavelength-dispersive X-ray spectroscopy (WDX) can provide
qualitative information about the distribution of different
chemical elements in the catalyst matrix. Fig. 9 collects
The next investigation of this study was to optimize the
conditions of the Suzuki coupling reaction by SBA-15/1,3-DCG/
Fig. 5 (a) TEM images and (b) PSD particle size distribution on the corresponding to SBA-15/1,3-DCG/Pd sample.
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temperature biaryl could be synthesized in lower yield than
ꢁ
4
0 C, although the mixture was stirred for prolonged reaction
ꢁ
time (Table 2, entry 1). While the temperature rises to 40 C the
yield of biaryl increased rapidly (Table 2, entry 5).
In order to show the effect of solvents upon the coupling
reaction, the reaction of phenyl iodide reacted with phenyl-
boronic acid in various solvent (nonpolar, protic and aprotic) in
ꢁ
the presence of SBA-15/1,3-DCG/Pd at 40 C. For this study
2
many solvents such as DMF, 1,4-dioxane, EtOH, H O, THF,
toluene and mixtures of two solvents (DMF/H O, DMF/EtOH
2
and H O/EtOH) are considered. Using H O, EtOH, THF, or
2
2
toluene did not favor the reaction, even on prolonging the
reaction time to 5 h (Table 3, entries 1–6). But, when we adopted
8,11
2
the H O/EtOH co-solvent, high yield of biaryl was obtained.
The explanation may be attributed to the good solubility of the
organic reactants and the inorganic base (Table 3, entries
Fig. 7 XRD pattern of SBA-15/1,3-DCG/Pd.
10 and 11). The results indicated that the cross-coupling reac-
Pd. In our initial screens, more reaction parameters such as tion proceeded in a higher yield (99%) aer 0.5 h. Therefore,
reaction temperature, solvent, type and amount base were H O/EtOH was recognized to be a more suitable reaction media
2
investigated. The model substrate for initial screening was for this coupling reaction.
phenyl iodide and phenylboronic acid leading to biaryl. Control
The inuence of the type and amount of different bases were
experiments performed in the absence of the Pd NPs conrmed also studied and the results are listed in Table 4. Various
the crucial role of catalyst. The inuence of the reaction amounts of bases such as KOH, EtONa, t-BuONa, Na CO ,
2
3
temperature on the cross-coupling of phenyl iodide and phe- K CO , Cs CO , NaOAc, NaOH and Et N were explored. It can be
2
3
2
3
3
nylboronic acid catalyzed by SBA-15/1,3-DCG/Pd are summarized found that base and amount of it played a critical role in the
in Table 1. As shown in Table 2, as the reaction temperature was cross-coupling system. The SM cross-coupling reaction stopped
ꢁ
increased to 60 C, the yield of biaryl slightly increased. At room completely in the absence of K CO (entry 1), indicating that the
2
3
Fig. 8 XPS image of the SBA-15/1,3-DCG/Pd.
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Fig. 9 SEM image of SBA-15/1,3-DCG/Pd and elemental maps of Si, O, C, N and Pd atoms of SBA-15/1,3-DCG/Pd.
a
a
Table 2 Effect of temperature
Table 4 Effect of base
ꢁ
Entry
Nano catalyst (mg)
T ( C)
Time (h)
Yield (%) Entry
Base (base : Ph(OH)
2
/mol : mol)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
15
15
—
5
10
5
10
10
10
25
30
40
40
40
50
50
60
70
10
3
10
3
0.5
3
0.5
0.5
0.5
47
54
Trace
78
99
81
99
99
99
1
2
3
4
5
6
7
8
9
1
1
—
10
3
3
3
3
—
9
EtONa (2 : 1)
t-BuONa (2 : 1)
Na
NaOAc (2 : 1)
NaOH (3 : 1)
10
74
11
10
61
99
99
64
74
76
83
2
CO
3
(2 : 1)
3
0.5
K
2
K
2
K
2
K
2
CO
CO
CO
CO
3
3
3
3
(1 : 1)
(2 : 1)
(3 : 1)
(1 : 1)
0.5
0.5
3
0.5
0.5
0.5
0
1
a
Reaction conditions: phenyl iodide (0.116 g, 1 mmol); phenyl boronic
Et
3
Et
3
Et
3
N (1 : 1)
N (2 : 1)
N (3 : 1)
acid (0.134 g, 1.1 mmol); K
2
CO
3
(0.276 mg, 2 mmol); water–ethanol
3
12
(1 : 1), 4 cm .
13
a
Reaction conditions: nano catalyst (10 mg); phenyl iodide (0.116 g,
1
mmol); phenyl boronic acid (0.134 g, 1.1 mmol); water–ethanol
a
Table 3 Effect of solvent and co-solvent
3
ꢁ
(1 : 1), 4 cm ; T ¼ 40 C.
Entry
Solvent
Time (h)
Yield (%)
ratio of K
yield increased with the amount of K
entries 7 and 8), however, no signicant difference was
observed when the amount of K CO was higher than 2 mmol
entry 9). In our optimization studies, results showed that the
amount of K CO was 2 eq. to aryl halides (Table 4, entry 7).
2
CO
3
could accelerate the cross-coupling reaction. The
1
2
3
4
5
6
7
8
9
DMF
Toluene
THF
1,4-Dioxane
EtOH
5
5
5
5
5
5
5
5
0.5
0.5
0.5
0.5
1
13
11
14
8
43
46
62
39
76
78
80
99
82
2
CO rose from 1 to 2 mmol
3
(
2
3
(
H
2
O
DMF/H
DMF/EtOH
2
O
2
3
To widen the range of substrate, we examined the aryl
iodides, bromides and chlorides with electron-withdrawing and
electron-releasing groups and phenylboronic acid in the
Suzuki–Miyaura reaction (Table 5). For characterize the prod-
H
2
H
2
H
2
H
2
H
2
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 2)
O/EtOH (1 : 2)
1
1
1
1
0
1
2
3
1
13
ucts used the H NMR, C NMR, FT-IR spectroscopy and
elemental analysis. As shown in Table 5, high yields were
obtained with both activated and non-activated aryl iodides
(Table 5, entries 1, 4 and 10). Aryl iodides without any substit-
uent and electron-decient aryl halides are reactive substrates
in Suzuki–Miyaura reactions and the related reactions went to
a
Reaction conditions: nano catalyst (10 mg); phenyl iodide (0.116 g,
1
2
mmol); phenyl boronic acid (0.134 g, 1.1 mmol); K
2
CO
3
(0.276 mg,
ꢁ
mmol); T ¼ 40 C.
II
Pd NPs has excellent performance for activating Ar–Pd –X in completion in shorter reaction times. Next, the ability of Pd NPs
2 3
the presence of K CO . It also seemed that the reaction rate was to promote the SM cross-coupling for aryl chlorides and aryl
related closely with the amount of K
2
CO
3
. The higher molar bromides were studied. Indeed, the activation of aryl chlorides
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Table 5 Suzuki–Miyaura cross-coupling reaction of aryl halides with Table 5 (Contd.)
phenylboronic acid in aqueous media
a
Entry
Aryl halide
Time (h)
0.5
Yield (%)
a
Entry
1
Aryl halide
Time (h)
0.5
Yield (%)
16
17
18
96
94
85
99
98
2.0
10
2
3
4
5
6
7
8
9
1.0
8
82
92
95
84
97
90
86
97
93
80
97
91
83
0.5
1.5
10
19
2.0
94
2
2
0
1
0.5
1.2
99
94
0.5
1.2
10
2
2
3
4
12
82
94
90
2
1.2
2.2
2
a
Reaction conditions: nano catalyst (10 mg), aryl halide (1 mmol),
CO (0.276 mg, 2 mmol),
10
11
12
13
14
15
0.75
2.5
phenylboronic acid (0.134 g, 1.1 mmol), K
water–ethanol (1 : 1), 4 cm , T ¼ 40 C.
2
3
3
ꢁ
and aryl bromides by Pd NPs is a challenge for catalysis, in
2
2–25
Suzuki–Miyaura reactions. Base on previous reports
for the
aryl chlorides, harsher conditions are needed than those used
for the activation of aryl bromides.
12
Further investigation showed that the relative reactivity of
aryl halides in SBA-15/1,3-DCG/Pd catalyzed reactions is in the
following order: R–I > R–Br > R–Cl. These results reect the
reactivity toward oxidative addition. Various substituents on
aryl halides can inuence on the Suzuki–Miyaura cross-
coupling reaction. Therefore, electron-withdrawing substitu-
ents on aryl halides increase the reaction rate and electron-
donating substituents on aryl halides decrease the reaction
rate. Suzuki–Miyaura cross-coupling reactions of aryl halide
with electron-withdrawing groups (Table 5, entries 4–6) pro-
ceeded in shorter reaction times and gave coupled products in
very good yields. By comparison of 4-bromobenzene and 2-
bromobenzene indicated that an aryl bromide with an ortho
1.2
3.5
6.0
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atoms in the Suzuki cross coupling reaction. This new hetero-
geneous catalyst showed the main advantages such as simple
preparation, easy separation aer reaction, mild condition
reaction, high yield and selectively, widely used for different aryl
halides and reusability for several times without any signicant
loss in its catalytic activity. Other extension of the method such
as applying different catalysts is currently under investigation.
Experimental
Preparation of 1,3-dicyclohexylguanidine groups in SBA-15
(SBA-15/1,3-DCG)
To a 100 mL of round-bottom ask were introduced 30 mL of
anhydrous toluene and 3.0 g of SBA-15 and 0.18 g (1.5 mmol) of
3
-aminopropyl trimethoxysilane (APTMS) were added. The
solution was reuxed for 24 h under an inert atmosphere,
ltered and washed subsequently with toluene, dichloro-
Fig. 10 Reusability of the nano catalyst in coupling reaction.

methane, and methanol, and dried under reduced pressure at
ꢁ
substituent gave a lower yield than those with para or meta 80 C for 24 h. In another 100 mL round-bottom ask, to a
substituents which is presumably due to the absence of a steric solution of 3 g of aminopropyl functionalized SBA-15 in 35 mL
effect from the 2-substituent (Table 5, entries 6 and 9). In the case of toluene, 1 g of dicyclohexylcarbodiimide (DCC) was added
of aryl chlorides with phenyl boronic acid, many of catalytic and reuxed for 24 h. Aer completion of the reaction, the solid
system did not efficiency precede the coupling even on harsh products were ltered and dried, and again washed with
ꢁ
condition such as high temperature and extending the reaction deionized water and then acetone and dried at 50 C for 12 h
time up to 24 h. In this investigation, we proved that the aryl (Scheme 1).
chlorides with phenyl boronic acid were coupled without any
limitation. The coupling reactions of aryl chlorides with phenyl Immobilization of Pd(II) ions on the surface of SBA-15/1,3-
boronic acid required longer times and gave moderate yields DCG
against with other aryl halide (Table 5, entries 3, 12, 15, and 18). In
2
.0 g of SBA-15/1,3-DCG and 0.177 g of palladium chloride
addition, 2-bromothiophene and 2-iodothiophene were coupled
with phenyl boronic acid and gave the desired products with no
poisoning of the palladium catalyst (Table 5, entry 23 and 24).
The reusability of the SBA-15/1,3-DCG/Pd was also checked
(1 mmol) in 50 mL of acetonitrile was stirred at room temper-
ature for 4 h (Scheme 1). The brown resulting solid was ltered,
washed with acetone and THF and dried in vacuum at 80 C for
3
ꢁ
h to give SBA-15/1,3-DCG/Pd(II).
(see Fig. 10). The cross-coupling reaction was carried out with
phenyl iodide and phenyl boronic acid under the optimized
reaction conditions. The nano-catalyst was separated from the
reaction mixture by centrifugation and washed with water and
chloroform. Then the solid catalyst was dried and reused for ve
times without signicant loss in its catalytic activity.
In order to examine the leaching of Pd from the SBA-15/1,3-
DCG/Pd as catalyst, we have used the hot ltration test. So, the
reaction of phenyl iodide and phenyl boronic acid in the pres-
ence of SBA-15/1,3-DCG/Pd under optimized reaction condition
was studied. In hot ltration test, the solid catalyst was ltered
off aer 45% of the coupling reaction was completed and the
Reduction of Pd(II) ions on the surface of SBA-15/1,3-DCG
The reduction of SBA-15/1,3-DCG/Pd(II) by hydrazine hydrate
was performed as follows: 50 mg of SBA-15/1,3-DCG/Pd(II) was
dispersed in 60 mL of water, and then 100 mL of hydrazine
hydrate (80%) was added. The pH of the mixture was adjusted to
1
0 with 25% ammonium hydroxide and the reaction was carried
ꢁ
out at 95 C for 2 h. The nal product SBA-15/1,3-DCG/Pd(0) was
ꢁ
washed with water and acetone, and dried in vacuum at 50 C.
Suzuki–Miyaura coupling reaction
liquid phase of the reaction mixture was kept in reaction In a typical reaction, 10 mg of the nano catalyst (10 mg ¼
condition for another 24 h. No increase in the amount of 0.003 mmol Pd) was placed in a 25 mL balloon, 1 mmol of the
product was noticed. This result suggests that Pd is not being aryl halide in 4 mL of water–ethanol (1 : 2) was added 0.134 g
leached out from the solid catalyst during the cross-coupling (1.1 mmol) of phenyl boronic acid, 0.276 mg of K CO (2 mmol).
2
3
ꢁ
reactions and the SBA-15/1,3-DCG/Pd behaved like a heteroge- The mixture was then stirred for the desired time at 40 C. The
neous catalyst, which could be recovered and reused.
reaction was monitored by thin layer chromatography (TLC).
Aer completion of reaction, the reaction mixture was cooled to
room temperature and the catalyst was recovered by centrifuge
and washed with ethyl acetate and ethanol. The combined
Conclusion
In this research, we have designed and synthesized a simple and organic layer was dried over anhydrous sodium sulfate and
efficient catalysis system of SBA-15 nano sized anchored palla- evaporated in a rotary evaporator under reduced pressure. The
dium complexes with a number of bidentate ligands of N,N crude product was puried by column chromatography.
20106 | RSC Adv., 2015, 5, 20098–20107
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RSC Advances
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Acknowledgements
We are thankful to Payame Noor University (PNU) for partial
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