Synthesis of Pd(II) organometal incorporated ordered Im3m mesostructural silica and its catalytic activity in a water-medium Sonogashira reaction

Article abstract of DOI:10.1039/c4nj00652f

A novel and reusable Pd(ii) organometal functionalized SBA-16 catalyst was synthesized by the co-condensation of Pd[PPh₂(CH₂) ₂Si(OCH₂CH₃)₃]₃Cl ₂ and tetraethyl orthos

Full text of DOI:10.1039/c4nj00652f

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Synthesis of Pd(II) organometal incorporated
ordered Im3m mesostructural silica and its
catalytic activity in a water-medium
Sonogashira reaction
Cite this: New J. Chem., 2014,
38, 4594
Fengxia Zhu, Lili Zhu, Xiaojun Sun, Litao An,* Pusu Zhao* and Hexing Li
A novel and reusable Pd(II) organometal functionalized SBA-16 catalyst was synthesized by the co-condensation
of Pd[PPh2(CH2)2Si(OCH2CH3)3]3Cl2 and tetraethyl orthosilicate (TEOS). This catalyst possessed bicontinuous
cubic Im3m mesostructure channels, which ensured the convenient diffusion of the reactant molecules into
the pore channels. The catalyst exhibited a very high activity for the Sonogashira coupling reaction of aryl
halides with phenylacetylene in water medium. Moreover, the synthesized catalyst could be separated from the
reaction mixture by simple filtration and reused for up to five runs without significant loss in activity.
Received (in Montpellier, France)
24th April 2014,
Accepted 26th June 2014
DOI: 10.1039/c4nj00652f
www.rsc.org/njc
reaction. Examples include PVC-supported Pd nanoparticles,12
a polystyrene resin-supported Pd(II) complex catalyst,13 a poly-
1. Introduction
The Sonogashira coupling reaction of acetylene with aryl halides mer supported macrocyclic Schiff base palladium complex,14
catalyzed by Pd(II) complexes is a powerful tool in organic synth- organopalladium(II) immobilized silicas,15 and so on.
esis and has been widely applied in various areas including
It is obvious that the functionalized supports are used to
biologically active molecules, natural product synthesis and mate- immobilize palladium complex catalysts, which show higher
rials science.1–4 This reaction is usually performed in a homo- activities than homogeneous catalysts in the Sonogashira
genous system with an organic solvent and a palladium– coupling reaction in water.16–19 Among the different supports
phosphorous complex.5–7 The use of toxic organic solvents such used for immobilized Pd(II) homogeneous catalysts, meso-
as DMSO, DMF, acetonitrile, etc. in the Sonogashira coupling porous silica materials remain the most popular choice
reaction not only increases the cost of the experiment, but because of their relatively low cost, large surface area, high
also pollutes the environment. Water is cheap, nontoxic, and thermal stability, mechanical stability, and good catalytic
nonflammable which could be considered as an alternative to performance.20 SBA-15 and MCM-41 are representative examples
expensive organic solvents and an attractive media for the devel- of these types of materials and have been used as supports for
opment of environmentally harmless chemical processes. Further- palladium catalysts.17,21–23
more, the separation of water-insoluble organic compounds from
the aqueous phase is very easy in many cases.8
Compared with two-dimensional hexagonal mesoporous
materials such as SBA-15, the mesoporous material SBA-16
Recently, homogenous Pd complex catalysts have also been (cubic, Im3m)24 is a new possible solid support for the immo-
employed in water-medium organic reactions, which could bilization of heterogeneous catalysts. The large surface area
diminish environmental pollution from organic solvents.9,10 and interesting 3D structure, consisting of ordered and inter-
Despite the high activity and selectivity of homogeneous palla- connected spherical mesopores, make it easily accessible for
dium organometallic catalysts there are disadvantages regarding guest molecules, which facilitate the transport of reactants and
separation and recycling. Moreover, the reaction suffers from products without pore blockage.25–28 Up to now, most SBA-16
severe problems related to the separation, recovery, and recycling supported Pd heterogeneous catalysts have been prepared by
of the expensive toxic metal catalysts, etc.11
either post-grafting or impregnation methods, while the
In this regard, different types of heterogeneous catalysts organic functionalities terminally bonded to the pore surface
have been prepared with the aim of achieving catalyst recovery might partially block the pore channels, which might negatively
and recycling for the water-medium Sonogashira coupling affect the catalytic efficiency.28,29 Moreover, they could easily
suffer from leaching during liquid phase reactions, leading to
poor durability.29
Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin
In this paper, we report the synthesis of functionalized SBA-16
with Pd(II) organometal incorporated into silica framework by
Normal University, Huaian, 223300, P. R. China. E-mail: anlitao@hotamil.com,
zhaopusu66@sohu.com; Fax: +86 517 83525083; Tel: +86 517 83525377
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surfactant-directed co-condensation between tetraethoxysilane N2 adsorption–desorption isotherms were measured at 77 K
(TEOS) and Pd(II)organometallicsilane. This functionalized meso- using a Quantachrome NOVA 4000e analyzer, from which the
porous material not only acts as an effective palladium catalyst for specific surface area (SBET) was calculated by applying the
the water-medium Sonogashira coupling reaction, but can also be Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–
reused five times without an obvious loss of catalytic activity. Its Halenda (BJH) model to the adsorption branches. Meanwhile,
activity is higher than the analogous catalysts which are prepared the average pore diameter (DP) and total pore volume (VP) were
from SBA-15.
calculated from the adsorption isotherms using the Barrett–
Joyner–Halenda (BJH) model. The surface morphologies and
porous structures were observed using transmission electron
microscopy (TEM, JEOL JEM2011). The surface electronic states
were analyzed using X-ray photoelectron spectroscopy (XPS,
Perkin-Elmer PHI 5000C). All the binding energy (BE) values
were calibrated by using the standard BE value of contaminant
carbon (C1S = 284.6 eV) as a reference.
2. Experimental
2.1. Catalyst preparation
Firstly, the Pd[PPh2(CH2)2Si(OCH2CH3)3]2Cl2 was synthesized by
mixing 10 mL of toluene, 3.5 mmol of Pd(COD)Cl2 and
0.70 mmol of PPh2CH2CH2Si(OCH2CH3)3 under argon atmo-
sphere, followed by stirring at 30 1C for 3 h. After being
concentrated to 6.0 mL pentane was added, leading to Pd[PPh2-
(CH2)2Si(OCH2CH3)3]2Cl2. Then, the organopalladium(II) functio-
nalized SBA-16, denoted as Pd(II)-PMO-SBA-16, was prepared
using the co-condensation method. In a typical synthesis, 1.0 g
of F127 and 2.6 g of K2SO4 was dissolved in 30 g of 0.50 M HCl
solution with stirring at 38 1C. After complete dissolution, 4.5 g
of tetraethyl orthosilicate (TEOS) was added to the homogeneous
solution. After being prehydrolyzed for 1 h, 0.42 g of Pd[PPh2-
(CH2)2Si(OCH2CH3)3]2Cl2 was added into the solution. This
mixture was left under vigorous stirring at 38 1C for 24 h.
Subsequently, hydrothermal treatment of the reactant mixture
was carried out at 100 1C for 24 h under static conditions in a
closed polypropylene bottle. Finally, the surfactants and other
organic substances were extracted by refluxing in ethanol
solution at 80 1C for 24 h. The Pd(II) loading was determined
as 3.2 wt% by ICP analysis.
2.3. Activity test
A mixture of aryl halide (1.0 mmol), phenylacetylene (1.5 mmol),
1,8-diazabicyclo [5.4.0] undec-7-ene (DBU, 0.40 mL), CuI
(0.16 mmol), n-decane (2.0 mmol) as an internal standard,
H2O (4.0 mL) and the catalyst with Pd(II) (0.0060 mmol), was
stirred in a flask at 90 1C for a given time (Scheme 1). After the
reaction was complete, the products were extracted with acetic
ether, followed by analysis using a gas chromatograph (SHIMADZU,
GC-17A) equipped with a JWDB-5, 95% dimethyl 1-(5%)-diphenyl-
polysiloxane column and a FID detector. The column temperature
was programmed from 80 to 250 1C at a ramp speed of
10 1C minÀ1. N2 was used as carrier gas. The reproducibility
was checked by repeating each result at least three times and
was found to be within Æ5%.
2.4. Catalyst recycling
A
mixture of iodobenzene (1.0 mmol), phenylacetylene
For comparison, the organopalladium(II) functionalized
SBA-15, denoted as Pd(II)-PMO-SBA-15 was also prepared by
surfactant-directed co-condensation of Pd[PPh2(CH2)2Si-
(OCH2CH3)3]2Cl2 and TEOS. Briefly, 0.5 g of P123 was dissolved
in 15 g of 2.0 M HCl solution and 3.9 g of distilled water, 1.0 g of
TEOS was added at 40 1C. After being prehydrolyzed for 1 h,
0.14 g of Pd[PPh2(CH2)2Si(OCH2CH3)3]2Cl2 was added into the
solution. The mixture was left under stirring for 24 h at 40 1C,
and subsequently heated for another 24 h at 100 1C under static
conditions in a closed polypropylene bottle. The yellow solid
product was collected and extracted in ethanol solution at 80 1C.
The Pd(II) loading was determined as 2.9 wt% by ICP analysis.
(1.5 mmol), DBU (0.40 mL), CuI (0.16 mmol), H2O (4.0 mL)
and Pd(II)-PMO-SBA-16 was stirred at 90 1C. At the end of the
reaction, the mixture was cooled to room temperature and
filtered to obtain the Pd(II)-PMO-SBA-16 catalyst. The separated
catalyst was washed with ethanol and dried under vacuum at
60 1C. The catalyst was separated from the liquid phase by high-
speed centrifugation. After this, the catalyst was reused with a
fresh charge of reactants for subsequent recycling under iden-
tical reaction conditions.
3. Results and discussion
3.1. Structural characteristics
2.2. Characterization
As mentioned in the experimental, the Pd[PPh2(CH2)2Si-
(OCH2CH3)3]2Cl2 was prepared from Pd(COD)Cl2 and
PPh2CH2CH2Si(OCH2CH3)3 under argon protection. Then, the
organopalladium(II) functionalized SBA-16, denoted as Pd(II)-
PMO-SBA-16, was prepared by co-condensation from the
The compositions and Pd(II) loadings were determined using
elemental analysis (Elementar Vario ELIII, Germany) and an
inductively coupled plasma optical emission spectrometer
(ICP-OES, Varian VISTA-MPX). Fourier transform infrared (FTIR)
spectra were collected using a Nicolet Magna 550 spectrometer
and the KBr method. Solid-state NMR spectra were recorded
using a Bruker AV-400 spectrometer. Thermogravimetric analysis
and differential thermal analysis (TG/DTA) were conducted using
a DT-60. Low-angle X-ray powder diffraction (XRD) patterns were
obtained using a Rigaku D/maxr B diffractometer with Cu Ka.
Scheme 1 Chemical equation of the Sonogashira coupling reaction
between aryl halide and phenylacetylene.
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obtained Pd[PPh2(CH2)2Si(OCH2ÀCH3)3]2Cl2 and tetraethyl ortho-
silicate (TEOS). Pd(II)-PMO-SBA-16 and Pd(II)-PMO-SBA-15 were
almost 2 mm in length and 500 nm in diameter. They were
determined using SEM analysis.
Elemental analysis showed that the Pd[PPh2(CH2)2Si(OCH2-
CH3)3]2Cl2 sample had a composition of C (55.24%) and H
(6.57%), which is in good accordance with that calculated for
C
60H87O9Cl2P3PdSi3 (C (55.18%) and H (6.67%)). ICP analysis
showed a P/Pd molar ratio of 2.03. The Pd(II) organometallic
complex incorporated into the silica matrix by the co-
condensation approach was investigated using FTIR, solid-
state NMR, and TG-DTA. Fig. 1 shows the FTIR spectra of
Pd(II)-PMO-SBA-16. No significant signals indicative of P123
surfactant molecules were observed, implying that they were
completely removed during EtOH extraction. There are two
peaks corresponding to Pd(II)-PMO-SBA-16, shown at 2985
and 2919 cmÀ1, which could be attributed to the asymmetric
and symmetric stretching modes of C–H bonds.30 The peak at
693 cmÀ1 is characteristic of the –H out-of-plane deformation
of the monosubstituted benzene ring. The peak around
1436 cmÀ1 was assigned to the vibrations of P–CH2.31 These
results confirm the successful incorporation of the Pd(II) orga-
nometallic complex into the silica supports. The absorption
peak at 1130–1090 cmÀ1 which is indicative of the P–Ph
vibration could not be clearly distinguished due to the overlap
with the intense Si–O vibration bands at 1100 cmÀ1 30
The
.
incorporation of the Pd(II) organometallic complex into
the SBA-16 network could be further characterized using the
solid-state NMR spectra.
Fig. 2 Solid NMR spectra of Pd(II)-PMO-SBA-16.
As shown in Fig. 2, the 13C CPMAS NMR spectrum of the
Pd(II)-PMO-SBA-16 sample displays two peaks at 14 and 57 ppm
which were assigned to the two C atoms in the –CH2–CH2–PPh2
group. A broad peak around 128 ppm was assigned to the C
atoms in the benzene ring.32,33 Meanwhile, the 29Si MAS NMR
spectrum of the Pd(II)-PMO-SBA-16 sample displayed three reso-
nance peaks up-field corresponding to Q4 (d = À112 ppm), Q3
(d = À102 ppm), and Q2 (d = À94 ppm) and two peaks down-field
corresponding to T3 (d = À66 ppm) and T2 (d = À59 ppm), where
Qn = Si(OSi)n–(OH)4Àn, n = 2–4, and Tm = RSi(OSi)m–(OH)3Àm, m =
1–3.33 The presence of Tm peaks suggest that the organic silica
moieties were successfully incorporated as a part of the silica
wall structure.
Fig. 3 TG and DTA curves of Pd(II)-PMO-SBA-16.
This was further confirmed by the TG/DTA. As shown in
Fig. 3, besides the weight loss due to the removal of physi-
sorbed water, and/or remaining ethanol from the solvent
extraction processes, the Pd(II)-PMO-SBA-16 displayed two
peaks around 383 and 452 1C. These peaks are possibly due
to the successive breakage of two PPh2–Pd–PPh2 bonds,24
corresponding to a total weight loss of around 16%.
As shown in Fig. 4, the XPS spectra show that all the Pd
species in the Pd(II)-PMO-SBA-16 were present in a +2 oxidation
state rather than in a metallic state, corresponding to the
binding energy (BE) of 343.1 and 337.8 eV in the Pd 3d5/2 and
Fig. 1 FTIR spectra of Pd(II)-PMO-SBA-16.
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Fig. 6 TEM images of Pd(II)-PMO-SBA-16.
Fig. 4 XPS spectra of Pd(PPh3)Cl2 and Pd(II)-PMO-SBA-16.
3d3/2 levels,34 respectively. In comparison with the BE of Pd in
Pd(PPh3)2Cl2, the BE of Pd in Pd(II)-PMO-SBA-16 shifted
negatively by 0.51 eV, indicating that the PPh2CH2CH2-ligand
exhibited stronger electron-donation than the PPh3-ligand,
making the Pd atom in the Pd(II)-PMO-SBA-16 more electron-
enriched. This can be easily understood by considering the
conjugated p–p system between P and Ph–. The PPh3-ligand
exhibits a conjugated p–p system comprising one P atom and
three Ph groups while the PPh2–CH2–CH2–ligand exhibits a
conjugated p–p system comprising one P atom and two Ph
groups.29 The bigger conjugated p–p system might dilute the
electron density on the P atom, which weakens the electron
donation ability of the P atom to the Pd atom in the coordina-
tion bonds.32
The small-angle XRD pattern of Pd(II)-PMO-SBA-16 in Fig. 5
exhibits a significant peak at 0.71 corresponding to the (110)
reflection and comparatively unresolved peaks at 2y values in
the range of 1.01–2.01. These peaks are related to the Im3m
body-centered cubic space group, thus providing an indication
of the Im3m structure of SBA-16.35 The TEM images (Fig. 6)
further confirmed the presence of the ordered cubic (space
group Im3m) mesostructure of Pd(II)-PMO-SBA-16.
Fig. 7 N2 adsorption–desorption isotherms of Pd(II)-PMO-SBA-16 and
pore size distribution curves (inset).
using the adsorption branch of the N2 isotherm with the Barrett–
Joyner–Halenda method (Fig. 7 inset), shows that the primary
mesopores of SBA-16 have a diameter of 6.4 nm, with the volume
of mesopores and the specific surface area estimated to be
0.51 cm3 gÀ1 and 676 m2 gÀ1, respectively.
A key consideration of the Pd(II)-PMO-SBA-16 catalyst is the
location of the Pd(II) species since they should come into
contact with reactant molecules. To determine the chemical
accessibility of the Pd(II) organometals incorporated into the
silica framework, the oxidation of Pd(II)-PMO-SBA-16 in KMnO4
aqueous solution was attempted. The Pd(II) content, deter-
mined by KMnO4 oxidation, could be considered as an acces-
sible Pd(II) species, while the total Pd(II) content was
determined using ICP analysis. Their molar ratio was deter-
mined as 92%, suggesting that most of the Pd(II) species in the
Pd(II)-PMO-SBA-16 catalyst were accessible for the chemical in
the liquid phase reaction.
Fig. 7 illustrates the nitrogen adsorption–desorption iso-
therms of the Pd(II)-PMO-SBA-16 sample. The prepared sample
exhibited a type IV N2 sorption isotherm with a H2-type hysteresis
loop, which is characteristic of mesoporous materials with a cage-
type porous structure, suggesting the existence of ink-bottle pores
in the above sample.36 The pore size distribution, calculated
3.2. Catalytic performances
In order to test the catalytic activity of Pd(II)-PMO-SBA-16 in the
Sonogashira coupling reaction, the reaction between iodoben-
zene and phenylacetylene was chosen as a model reaction,
using water as the solvent to investigate the influence of
different parameters on the catalytic activity. The parameters
included the catalyst, catalyst concentration, base, and reaction
temperature. The best result was obtained when the cross-
coupling was carried out with 0.6 mol% of Pd(II)-PMO-SBA-16
(Table 1, entry 2). The optimized reaction conditions were then
applied to the Sonogashira cross coupling reactions of various
aryl halides with phenylacetylene (Table 3).
The catalytic parameters of different Pd(II) organometallic
catalysts in the water-medium Sonogashira reaction are summar-
ized in Table 2. The Pd(II)-PMO-SBA-16 displayed a comparable
Fig. 5 Low-angle XRD pattern of Pd(II)-PMO-SBA-16.
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Table 1 The cross-coupling of iodobenzene and phenyl-acetylene under
different conditionsa
Table 3 Catalytic efficiencies of the Pd(II)-PMO-SBA-16 in water-medium
Sonogashira reactionsa
Catalyst content
(mol%)
Reaction
time (h)
Yield
(%)
Catalyst content
(mol%)
Temperature
(1C)
Yield
(%)
Entry
R
X
Entry
Base
1
2
3
4
5
6
7
0.60
0.60
0.60
0.60
0.90
0.90
0.90
H
I
I
I
I
Br
Br
Br
5
7
7
4
8
10
7
95
86
89
98
93
85
96
1
2
3
4
5
6
7
0.3
0.6
0.9
0.6
0.6
0.3
0.9
DBU
DBU
DBU
Et3N
K2CO3
DBU
DBU
50
70
90
90
90
90
90
62
86
95
87
75
51
97
CH3O–
CH3–
NO2–
H
CH3–
NO2–
a
a
Reactions conditions: aryl halide (1.0 mmol), phenylacetylene
(1.5 mmol), DBU (0.40 mL), CuI (0.16 mmol), n-decane (2.0 mmol),
H2O (4.0 mL), 90 1C and PMO-SBA-16.
Reactions conditions: iodobenzene (1.0 mmol), phenylacetylene
(1.5 mmol), base, CuI (0.16 mmol), n-decane (2.0 mmol), H2O (4.0 mL),
and Pd(II)-PMO-SBA-16.
strong electron-withdrawing group, such as –NO2, is known to
promote the direct coupling. Electron-donating groups, such as
–OCH3 on the iodoaryl partner gave excellent results. The results
above prompted us to investigate the reaction of aryl bromides.
With a catalyst loading of 0.9 mol%, Pd(II)-PMO-SBA-16 also
afforded a satisfactory yield for bromides containing –CH3 and
–OCH3 groups (Table 3, entries 5–7). Bromoarenes with either
electron-withdrawing substituents, such as –NO2, or electron-
donating substituents, such as –OCH3, coupled readily with
phenylacetylene in good yields. Aryl bromides containing
electron-withdrawing substituents reacted faster than those with
electron donating substituents (Table 3, entries 6 and 7).
Table 2 The catalytic efficiencies of different catalysts in the water-
medium Sonogashira reactiona
Entry
Catalyst
Yield (%)
1
2
3
Pd(PPh3)Cl2
Pd(II)-PMO-SBA-16
Pd(II)-PMO-SBA-15
97
95
88
a
Reactions conditions: iodobenzene (1.0 mmol), phenylacetylene
(1.5 mmol), DBU (0.40 mL), CuI (0.16 mmol), n-decane (2.0 mmol),
H2O (4.0 mL), 90 1C and an amount of Pd catalyst.
3.3. Recycling of the catalyst
efficiency to the Pd(PPh3)2Cl2 homogeneous catalyst. As expected,
the reaction catalyzed by Pd(II)-PMO-SBA-15 had a slower rate than
that of the Pd(II)-PMO-SBA-16 catalyst under the same reaction
conditions. The reason could be that Pd(II)-PMO-SBA-16 has an
easily accessible 3D structure consisting of ordered and intercon-
nected spherical mesopores for guest molecules, which facilitates
the transport of reactants and products without pore blockage.25–28
To confirm whether the heterogeneous or the dissolved
homogeneous Pd(II) species was the real catalyst responsible
for the coupling reaction between iodobenzene and phenyl-
acetylene, the following procedure was carried out, as proposed
by Sheldon et al.37 According to the proposed method, after
reacting for 4 h with the conversion exceeding 45%, the solid
catalyst was filtered out and the mother liquor was allowed to
react for another 5 h under identical reaction conditions. No
significant change in the conversion was observed, indicating
that the present catalysis was indeed heterogeneous in nature
rather than being due to the dissolved Pd(II) species leached
from Pd(II)-PMO-SBA-16.
One of the purposes for designing this heterogeneous catalyst is
to enable the recycling of the catalyst for use in subsequent
reactions. The reusability of the catalyst was tested by reacting
phenyliodide with phenylacetylene, as the representative reac-
tants, in the presence of 0.60 mol% of Pd(II), in order to study the
recyclability of this heterogeneous catalyst. Due to the unavoid-
able loss of solid catalyst during the course of recovery and
washing, we did three parallel reactions during each cycle, then
collected all the catalyst and portioned them for the next cycling.
Both Pd(II)-PMO-SBA-16 and Pd(II)-PMO-SBA-15 could be reused
for five cycles without a significant decrease in the catalytic
activity of the catalyst (Fig. 8). The excellent durability of the
In order to verify the higher activity of Pd(II)-PMO-SBA-16, its
activity towards aryl halide containing a variety of substituents
with phenylacetylene were examined. These results are summar-
ized in Table 3. The cross-coupling reaction of phenylacetylene
with a variety of aryl iodides proceeded smoothly, affording high
yields of 86–98% (Table 3, entries 1–4). Various electron-
donating and electron-withdrawing groups, such as –OCH3 and
–NO2 on the aryl iodide were well tolerated. The presence of a
Fig. 8 Recycling tests of Pd(II)-PMO-SBA-16.
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4. Conclusions
In summary, we have presented a Pd(II) organometal functionalized
SBA-16, which was efficiently used as a heterogeneous catalyst for
Sonogashira coupling reactions in water medium. The catalyst not
only shows high catalytic activity, but also offers many practical
advantages such as thermal stability and recyclability. The catalyst
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We are grateful to the National Natural Science Foundation of
China (20825724), University Natural Science Foundation of
Jiangsu Province (13KJB150008) and Administration of Science &
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