Synthesis and characterization of mesoporous Pd(II) organometal nanoplatelet catalyst for copper-free Sonogashira reaction in water

Article abstract of DOI:10.1016/j.jorganchem.2018.02.004

Periodic mesoporous organopalladium(II)-bridged silica with platelet morphology (Pd(II)-PMO-P) was synthesized by the co-condensation of TEOS and Pd[PPh₂(CH₂)₂Si(OC₂H₅)₃]₂Cl

Full text of DOI:10.1016/j.jorganchem.2018.02.004

Accepted Manuscript
Synthesis and characterization of mesoporous Pd(II) organometal nanoplatelet
catalyst for copper-free Sonogashira reaction in water
Fengxia Zhu, Pusu Zhao, Qiaoqi Li, Didi Yang
PII:
S0022-328X(18)30092-5
10.1016/j.jorganchem.2018.02.004
JOM 20303
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 October 2017
Revised Date: 29 January 2018
Accepted Date: 2 February 2018
Please cite this article as: F. Zhu, P. Zhao, Q. Li, D. Yang, Synthesis and characterization of
mesoporous Pd(II) organometal nanoplatelet catalyst for copper-free Sonogashira reaction in water,
Journal of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.02.004.
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ACCEPTED MANUSCRIPT
Synthesis and characterization of mesoporous Pd(II) organometal
nanoplatelet catalyst for copper-free Sonogashira reaction in water
a,b
a*
a
b
Fengxia Zhu, Pusu Zhao, Qiaoqi Li and Didi Yang
a.
Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and
Chemical Engineering, Huaiyin Normal University, Huaian, 223300, P. R. China.
b.
The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth
Functional Materials, Shanghai Normal University, Shanghai 200234, P. R. China.
Abstract Periodic mesoporous organopalladium(II)-bridged silica with platelet morphology
(Pd(II)-PMO-P)
was
synthesized
by
the
co-condensation
of
TEOS
and
Pd[PPh (CH ) Si(OC H ) ] Cl using P123 as structure-directing agent. The as-made
2
2 2
2
5 3 2
2
Pd(II)-PMO-P samples were characterized with XRD, BET, ICP, SEM, TEM, solid NMR, XPS,
as well as FT-IR. It was found that these products were composed of platelets (300-400 nm) with
short and straight mesoporous channels (100-200 nm). Organopalladium (II) was integrally
incorporated into the walls of silica. Compared to the organopalladium(II)-bridged PMO with
irregular pore structure, Pd(II)-PMO-P catalyst exhibited high catalytic efficiency in the
water-medium Sonogashira reaction because its short pore channels and nano-sized structures
could accelerate the diffusion and adsorption of reactant molecules. Furthermore, the
heterogeneous catalyst could be reused several times without significant loss of activity, which
efficiently decreases the cost as well as the pollution in order to meet the criteria of green
chemistry.
Keywords: mesoporous Pd(II) organometal; nanoplatelet; Sonogashira reaction; water-medium
1

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1
. Introduction
Since the discovery of M41S, nanostructured mesoporous silica comprises a
promising class of catalyst support with singular properties, such as high surface area, pore
size uniformity, thermal stability, and synthetic reproducibility [1, 2]. In addition, the
material hydrophobicity can be improved by functionalization with alkoxysilane groups
bearing ethyl, benzyl, thiol moieties [3, 4]. Previously, we established a new approach to
integrally incorporate organometals into the silica walls, leading to the
organometal-bridged periodic mesoporous silica[5-7], which further improved the catalytic
performance owing to the diminished diffusion limit and also improved the durability of
catalyst by inhibiting the leaching of metallic active sites [8-11]. Recently, hierarchically
meso-/macroporous nanocatalysts represent an interesting class of solid catalysts since the
incorporation of interconnected macropores in mesoporous materials. This kind of
mesoporous materials provide an novel platform to fabricate highly active catalysts for the
water-medium clean organic synthesis because macropores can favor mass transfer and
reduce transport limitations[12]. However, classic mesoporous solid supports, such as
SBA-15 possessing the mesostructure of micrometer-long to afford inefficient in-pore
diffusion of substrates, resulting in poor catalytic turnover[13, 14]. Obviously, the silica
supports with the short pore channels and the regular morphology should be more
favorable for the diffusion and adsorption of reactant molecules. Mesoporous
organopalladium heterogeneous catalysts with short channels have been reported [11, 15,
1
6], but most of them were prepared by grafting method or using terminal functionalized
2

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silica precursor, leading to the pore blockage for their organic groups terminally bonded to
the mesopore wall.
Transition metal mediated cross-coupling reactions have proven to be powerful tools
for mild, highly efficient carbon-carbon bond formation [17]. Among these processes,
palladium catalyze carbon-carbon coupling reactions, the Sonogashira cross-coupling
2
stands out as a unique method enabling the coupling between (sp) carbon and (sp ) carbon
atoms[18-22]. The reaction products containing such bonds are important intermediates in
organic synthesis, and for the preparation of natural products [23, 24], biologically active
molecules [25-28], molecular electronics [28-30], and polymers[31-33].The original
Sonogashira reaction was generally performed in the presence of palladium and copper (I)
iodide as a co-catalyst in organic solvents [34]. The presence of copper increases the
reactivity of the acetylene by the formation of a copper acetylide. However, Glaser-type
oxidative dimerization of the terminalalkynes cannot be avoided in copper-mediated
reactions [34], in which side products (diynes) are generally difficult to separate from the
desired products, meanwhile copper acetylide is a potentially explosive reagent.
In view of the environmental pollution caused by the use of the hazardous, volatile
organic solvents and by-product, the use of an aqueous medium in metal-catalyzed
reactions has received considerable attention because water is harmless, non-combustible
and readily available. The main advantage of water as a solvent offers simplification of
work-up procedures and acceleration of reaction rates[35]. Most studies focused on
homogeneous and more expensive palladium complex catalysts for the water-medium
Sonogashira reaction due to the relatively low solubility of organic compounds[36-38].
3

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Despite the high activity and selectivity, they were limited in practical application because
of the complicated separation procedures from the reaction mixture as well as the
difficulties in recycling, which will inevitably lead to high cost and even environmental
pollutions from heavy metallic ion[39, 40]. Thus, it is desirable to develop highly efficient
heterogeneous palladium catalyst for the copper-free Sonogashira coupling reaction.
Herein, we reported for the first time the uniformly organopalladium(II)-bridged PMO
catalysts with platelet morphology (Pd(II)-PMO-P) synthesized by the co-condensation of
organopalladium(II)-bridged silane and TEOS using P123 as structure-directing agent.
This catalyst showed higher efficiency than the common organopalladium(II)-bridged
PMO catalysts (Pd(II)-PMO-T) with irregular shape prepared by traditional method in the
water-medium Sonogashira reaction.
2
. Experimental sections
2
.1. Materials
Pluronic
P123
(EO PO EO ,
M=
5800),
2-(diphenylphos-phino)-
2
0
70
20
ethyltriethoxysilane (PPh CH CH Si(OC H ) )was obtained from Gelest Inc. and
2
2
2
2
5 3
dichlorobis(triphenyl phosphine) palladium(II) (Pd(PPh ) Cl ) was purchased from
3
2
2
Sigma-Aldrich Company Ltd. Other chemicals used, including tetraethyl orthosilicate
TEOS), ZrOCl ·H O, inorganic acids, and organic reagents, were analytically pure and
(
2
2
purchased from Sinopharm Chemical Reagent Co., Ltd. The solvents are of analytical
quality and dried by standard methods.
2
.2. Preparation of mesoporous silica SBA-15-P
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Platelet SBA-15 (SBA-15-P) was prepared according to the following procedures. In
a typical process, 1.0 g of P123 was dissolved in 45 mL of 2.0 mol/L HCl solution
o
containing 0.32 g of ZrOCl ·H O at 35 C. After being completely dissolved, 10 mmol of
2
2
TEOS was added dropwise slowly. The mixture solution was stirred for 40 min. The
o
o
mixture was stirred at 35 C for 24 h and then aged in an autoclave at 100 C for another 24
h in an oven. Finally, the solid powder filtered and dried was subjected to refluxing in
o
ethanol solution for 24 h at 80 C to remove P123, leading to the white powder SBA-15-P.
2
.3. Catalyst Preparation
In a typical process, Pd(II) organometallicsilane (Pd[PPh (CH ) Si(OC H ) ] Cl ) was
2
2 2
2
5 3 2
2
synthesized in a dry and oxygen-free argon atmosphere using Schlenk techniques[41].
Organopalladium(II)-bridged PMO catalysts with platelet morphology denoted as
Pd(II)-PMO-P,
was
prepared
by
adding
a
certern
amount
of
(
Pd[PPh (CH ) Si(OC H ) ] Cl ) in the platelet SBA-15 synthesis solution. 1.0 g of P123
2
2 2
2
5 3 2
2
was dissolved in 45 mL of 2.0 mol/L HCl solution containing 0.32 g of ZrOCl ·H O at 35
2
2
o
C. After being completely dissolved, a certain amount of TEOS was added dropwise
slowly. The mixture solution was stirred for 40 min, and then the as-prepared
Pd[PPh (CH ) Si(OC H ) ] Cl was added. The final reactant molar composition is 0.017
2
2 2
2
5 3 2
2
P123: 9.0 HCl: 0.10 Zr: 208 H O: 1.0 Si, where Si referred to the total silica source. The
2
o
mixture was stirred at 35 C for 24 h. The mixture was aged, filtrated and treated according
to the procedure used in preparing SBA-15-P leading to the primrose powder Pd
(
II)-PMO-P catalyst. The Pd (II) loading was adjusted by changing the amount of
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Pd[PPh (CH ) Si(OC H ) ] Cl in the initial mixture, corresponding to Pd (II)-PMO-P-1,
2
2 2
2
5 3 2
2
Pd (II)-PMO-P-2, and Pd (II)-PMO-P-3.
Pd(II)-PMO-T catalyst was also synthesized by traditional co-condensation method. A
certain amount of TEOS was added to the solution contain 1.0 g of P123, 7.5 mL of
o
deionized water and 30 mL of 2.0 mol/L HCl at 35 C. After being stirred for 40 min,
Pd[PPh (CH ) Si(OC H ) ] Cl was added. Then the mixture was stirred for 24 h at the
2
2 2
2
5 3 2
2
o
same temperature and aged at 100 C for another 24 h to get the primrose solid precipitate.
The precipitate was filtrated and treated according to the procedure used in preparing Pd
(
II)-PMO-P.
2
.4. Catalyst Characterization
Structural characteristics were characterized by X-ray diffraction (XRD, Rigaku
D/Max-RB. CuK ), scanning electron microscope (SEM, Hitachi S-4800), transmission
α
electron microscopy (TEM, JEOL JEM2100), N2 adsorption-desorption isotherms
(
Quantachrome NOVA 4000e) and solid state nuclear magnetic resonance (NMR, Bruker
AV-400). Thermo gravimetric analysis and differential thermal analysis (TG/DTA) were
conducted on a DT-60. Specific surface areas (SBET) and average pore diameter (D ) were
P
calculated by using BET and BJH models, respectively. The surface electronic states were
analyzed by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA). All
the binding energy values were calibrated by using C1S = 284.6 eV as a reference. The
metal loadings were determined by inductively coupled plasma optical emission
spectrometer (ICP-OES, Varian VISTA-MPX).
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2
.5. Activity Test
The water-medium Sonogashira reaction (Scheme1) of aryl iodide with
phenylacetylene was chosen as a probe to evaluate the catalytic performance. It was carried
o
out at 60 C in a 25 mL Schlenk flask containing a catalyst with 0.0030 mmol of Pd
catalyst, 1.0 mmol of iodobenzene, 1.5 mmol of phenylacetylene, 2.0 mmol of
triethylamine and 5.0 mL of distilled water. After stirring for 5 h, the products were
extracted by ethyl acetate, followed by analysis on a GC-17A gas chromatograph
(
SHIMADZU) equipped with a JWDB-5, 95% dimethyl 1-(5%)-diphenylpolysiloxane
o
column and a FID detector. The column temperature was programmed from 80 to 250 C
o
at a speed of 10 C/min and N was used as carrier gas. The conversions and yields
2
determined on the GC were calibrated by using commercially available samples. The
reproducibility was checked by repeating each result at least three times and is found to be
within ±5%.
Scheme 1. Sonogashira reaction of phenylacetylene with aryl halide.
2
.6. Catalyst recyclability study
In order to determine the durability, the catalyst was allowed to centrifuge after each
run of the reactions, following by washed thoroughly with distilled water and ethanol,
o
drying at 80 C overnight under vacuum condition. Then, the catalyst was reused with
7

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fresh charge of water and reactants for subsequent recycling runs under the same reaction
conditions.
2
.7. Adsorption test
The adsorption for rhodamine B (Rh B) in aqueous solution was taken as a probe to
evaluate the adsorption performance of different catalysts. In a typical run of tests, catalyst
o
(
100.0 mg) was soaked in water (200 mL) and oscillated at 25 C for 12 h, followed by
adding aqueous solution (50.0 mL) containing 2.50 mg Rh B. The solution was sampled at
given time intervals and the concentration of the left Rh B in the solution was determined
on an UV-visible spectrophotometer (UV 7504/PC) at the characteristic adsorption
wavelength of 523 nm.
3
. Results and discussion
3
.1. Structural characteristics
8

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SBA-15-P
Pd(II)-PMO-P-1
Pd(II)-PMO-T
Pd(II)-PMO-P-2
Pd(II)-PMO-P-3
1.0
1.5
2.0
2.5
3.0
3.5
2θ
(degree)
Fig. 1. Low-angle XRD patterns of different samples.
The small-angle X-ray diffraction (XRD) patterns of the samples were shown in Fig.
1
. All of SBA-15-P and organometal Pd(II) catalyst exhibited three well resolved peaks in
o
the XRD patterns. An intense diffraction around 2
θ
= 0.8-0.9 indicative of (100) plane
suggested the 2D p6mm hexagonal mesoporous structures [42]. The weak diffraction peaks
in higher angle corresponded to (110) and (200) planes, suggesting the ordered mesoporous
structure[42], which could be further confirmed by TEM images in Fig. 3. However, the
weak diffraction peaks disappeared and the intense diffraction became wider and weaker in
Pd (II)-PMO-P-3, suggesting that when Pd(II) loading increased, the mesoporous structure
tended to be disordered. It could be due to that the Pd(II) organometallic complexes were
mainly embedded in the pore walls and thus reduced the ordering degree of mesoporous
structure. The Pd(II)-PMO-P-2 contained well-ordered mesoporous channels similar to the
SBA-15-P and Pd(II)-PMO-T.
9

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The ordered mesoporous structures of different samples were further confirmed by
nitrogen adsorption-desorption isotherms. As shown in Fig.4, a series of Pd(II)-PMO-P and
Pd(II)-PMO-T catalysts exhibited type IV isotherms with H -type hysteresis loops, which
1
represented the mesoporous structures [43-45]. The BET surface area, BJH pore diameter,
and pore volume of samples were listed in Table 1.
Fig. 2. FESEM images of Pd (II)-PMO-P-2(a), Pd (II)-PMO-T(b).
(
a)
(b)
Fig. 3. TEM images of Pd (II)-PMO-P-2 (a) and Pd (II)-PMO-T (b).
1
0

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Pd(II)-PMO-P-1
Pd(II)-PMO-P-2
Pd(II)-PMO-P-3
Pd(II)-PMO-T
700
600
500
400
300
200
100
0.2
0.4
0.6
0.8
p/p0
Fig. 4. N adsorption-desorption isotherms of different samples
2
Table 1
The structure parameters of and catalytic performances of different catalysts in
a
Sonogashira reaction .
Pd Loading
SBET
DP
VP
Conv. Yield
Samples
2
3
(
mmol/g)
/
(m /g) (nm) (cm /g)
(%)
0
(%)
0
SBA-15-P
819
803
749
601
655
5.3
5.2
5.2
5.1
5.3
0.93
0.94
0.92
0.67
0.73
Pd (II)-PMO-P-1
Pd (II)-PMO-P-2
Pd (II)-PMO-P-3
Pd (II)-PMO-P-T
0.24
0.43
0.58
0.42
92
96
89
83
92
96
89
83
a
Reaction conditions: a catalyst containing 0.0030 mmol Pd, 1.0 mmol iodobenzene, 1.5 mmol
o
phenylacetylene, 6.0 mmol triethylamine, 5.0 mL H O, 50 C, 5 h.
2
The FESEM images in Fig. 2 demonstrated that Pd-PMO-P-2 was present in uniform
platelet. The average width and thickness of the platelets were 300-400 nm and 100-200
nm, respectively. Well ordered pores arranged and aligned along the thickness of the thin platelets
1
1

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are observed on the TEM images (Fig. 3). The platelet morphology and short mesochannels of these
materials are quite different from those of Pd-PMO-T, which possessed irregular morphology and
long mesochannels in micrometers. It was due to that the Zr(IV) ions increased the average
micellar radius of P123 micelles, affected the self-assembly of P123 micelles and further
accelerated formation of ordered mesoporus silica[46].
337.7 eV
338.0 eV
Pd(II)-PMO-P-2
Pd(II)-PMO-T
Pd(PPh ) Cl
3 2 2
333
336
339
342
345
348
Binding Energy (eV)
Fig. 5. The XPS spectra of different samples.
The XPS spectra revealed that all the Pd species in the Pd(II)-PMO-P-2 sample were
present in +2 oxidation state (Fig. 5), corresponding to the binding energy (BE) of 342.9
and 337.7 eV in the Pd 3d5/2 and 3d3/2 levels, respectively[47]. In comparison with the BE
of the Pd in Pd(PPh ) Cl , the BE of the Pd in the Pd(II)-PMO-P-2shifted negatively by 0.3
3
2
2
eV, indicating that the PPh -CH -CH -ligand exhibited stronger electron-donation than the
2
2
2
PPh -ligand, making Pd atom in the Pd (II)-PMO-P-2 more electron-enriched[47]. It could
3
be also seen from Fig. 5 that all the Pd species in the Pd(II)-PMO-P-2 and Pd(II)-PMO-T
1
2

ACCEPTED MANUSCRIPT
sample had similar chemical environment. The two samples had similar solid NMR spectra
2
9
as shown in Fig. 6. The Si CP MAS NMR spectrum displayed three resonance up-field
4
3
2
peaks corresponding to Q (
δ
= -110 ppm), Q (δ = -102 ppm), and Q (
δ
= -92 ppm), and
3
2
two downfield peaks corresponding to T (δ = -65 ppm) and T (δ = -57 ppm). The
m
presence of T peaks indicated the incorporation of the organometal silica moieties as a
1
3
part of the silica wall structure[48]. Meanwhile, the C CP MAS NMR spectrum showed
two peaks at 15 and 58 ppm indicative of two C atoms in the -CH -CH -PPh group. A
2
2
2
broad peak around 130 ppm was assigned to the C atoms in the benzene ring[49]. Other
peaks marked with asterisks were attributed to rotational sidebands and were confirmed by
changing the rotation speed [50]. Besides rotational sidebands marked with asterisks, only
3
1
one intense peak was observed around 19 ppm in the P CPMAS NMR spectrum of the Pd
(
II)-PMO-P-2, indicating a unique coordination model between P and Pd(II)[16].
Meanwhile, the TG/DTA curves in Fig. 7 reveal that there is a weak endothermic peak
o
around 100 C with weight loss of about 14% due to the desorption of water and/or ethanol
o
remaining from solvent extraction processes. The exothermic peak around 300 C with
weight loss of 8 % could be assigned to the oxidation of Pd (II) organometallic complex in
o
the pore channel. The other exothermic peak at 500 C with the weight loss of 22% was
resulted from the oxidation of the phenyl fragments embedded in the pore wall[49]. These
results also confirmed the successful incorporation of both the Pd (II) organometallic
complexes and the phenyl groups onto the silica support.
1
3

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2
9
Si CP MAS
3
Q
2
4
Q
Q
2
T
3
T
0
-50
-100
-150
-200
Chemical shift (ppm)
1
3
C CP MAS
CPh-
C1
C2
*
*
2
50
200
150
100
50
0
Chemical shift (ppm)
3
1
P CP MAS
1
9 ppm
*
*
*
*
-
100
0
100
200
Chemical shift (ppm)
Fig. 6. The solid NMR spectra of Pd (II)-PMO-P-2 catalyst.
1
00
4
2
0
0
0
9
8
7
0
0
0
-
-
-
20
40
60
80
-
2
00
400
600
800
1000
o
Temperature (
C)
Fig. 7. TG/DTA curves of Pd (II)-PMO-P-2
1
4

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3
.2. Catalytic Performances
It was summarized that the catalytic activity of different kinds of Pd(II) catalysts
performed in the water-medium Sonogashira reactions between iodobenzene and
phenylacetylene (Table 1). SBA-15-P was inactive, implying that the Pd(II) active sites
were essential for the present reactions. Since no copper salt was used, undesired
Glaser-type oxidative homocoupling was avoided, indicating that all these catalysts were
almost absolutely selective. From Table 1, it could be seen that the activity of the
Pd(II)-PMO-P catalysts with platelet morphology first increased and then decreased with
the increase of Pd loading. The Pd(II)-PMO-P-1 catalyst with very low Pd(II) loading (0.24
mmol/g) exhibited poor activity due to the relatively long distance between the
neighboring Pd(II) active sites. Taking into account that the Sonogashira reaction needed
two kinds of molecules adsorbed on the neighboring Pd(II) active sites, the big space
between the neighboring Pd(II) active sites would diminish their synergetic effect.
Meanwhile, the Pd(II)-PMO-P-3 with high Pd (II) loading (0.58 mmol/g) also showed poor
activity, possibly due to the steric hindrance that retarded the diffusion and even the
adsorption of organic molecules on the Pd(II) active sites. So the optimum catalyst was
determined as Pd(II)-PMO-P-2 with Pd content of 0.43 mmol/g. Comparing the catalytic
performance between Pd(II)-PMO-P-2 and Pd(II)-PMO-P-T with similar Pd loadings, it
was obvious that the former showed higher catalytic activity in water-medium Sonogashira
reaction, which was further confirmed (Table 2). This could be mainly attributed to the
uniform platelet morphology with short mesoporous channels, which facilitate the
diffusion and mass transfer of the reactant and product molecules. According to the
1
5

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adsorption test in Fig. 8, the Pd(II)-PMO-P-2 exhibited not only higher saturated
adsorption capacity than the Pd(II)-PMO-T, but also showed faster adsorption rate than
Pd(II)-PMO-T, implying that the short pore channels might promoted the diffusion of RhB
molecules. Thus, the higher activity of the Pd(II)-PMO-P-2 than that of the Pd(II)-PMO-T
could be attributed to both the higher adsorption for reactant molecules and the less
diffusion limit.
70
60
50
40
30
20
10
0
Pd(II)-PMO-P-2
Pd(II)-PMO-T
0
2
4
6
8
Adsorption Time (h)
Fig. 8. Adsorption test of the different catalysts.
In order to verify the higher activity of Pd (II)-PMO-P-2, the Sonogashira reaction of
different aryl halide with phenylacetylene was set up (Table 2). As expected, a series of
functional groups on the phenyl ring of aryl halides, such as methoxy and nitro, were
compatible under this procedure. 4-iodonitrobenzene and 4-bromonitrobenzene containing
electron-withdrawing group exhibited higher activity and yield to target products than
4
-iodomethoxybenzene, 4-iodomethylbenzene and 4-bromomethoxybenzene containing
electron-donating group because the electron-withdrawing groups make the carbon of the
1
6

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C-I and C-Br bonds more positive which favors the Sonogashira reaction (Table 2, entries
3
-8, 11-14).
Table 2
a
Performances of different catalysts in water-medium Sonogashira reaction
Entry
Reaction Time
Conv.
(%)
96
Catalyst
R
X
Yield (%)
(
h)
5
1
2
3
4
5
6
7
8
9
Pd (II)-PMO-P-2
Pd (II)-PMO-P-T
H
H
I
I
96
83
82
67
84
71
97
94
95
86
87
74
98
83
5
5
5
5
5
3
3
8
8
83
82
67
84
71
97
94
95
86
87
74
98
83
Pd (II)-PMO-P-2 OCH3
Pd (II)-PMO-P-T OCH3
I
I
Pd (II)-PMO-P-2
Pd (II)-PMO-P-T
Pd (II)-PMO-P-2
Pd (II)-PMO-P-T
Pd (II)-PMO-P-2
Pd (II)-PMO-P-T
Pd (II)-PMO-P-2
Pd (II)-PMO-P-T
Pd (II)-PMO-P-2
Pd (II)-PMO-P-T
CH3
CH3
NO2
NO2
H
I
I
I
I
Br
Br
Br
Br
1
1
1
1
1
0
1
2
3
4
H
CH3
CH3
11
11
6
NO2 Br
NO2 Br
6
a
Reaction conditions: a catalyst containing 0.0030 mmol Pd, 1.0 mmol aryl halid, 1.5 mmol
o
phenylacetylene, 2.0 mmol triethylamine, 5.0 mL H O, 60 C.
2
To make sure whether the heterogeneous or the dissolved homogeneous Pd (II)
species was the real catalyst for the coupling reactions, the Sonogashira reaction of
4
-nitroiodobenzene and phenylacetylene was chosen as a probe and the following
experiments were carried out according to the procedure proposed by Sheldon et al[51].
1
7

ACCEPTED MANUSCRIPT
Firstly, the reaction was allowed to proceed for 2 h until the 4-nitroiodobenzene conversion
exceeded 45%. Then, the solid catalyst was filtered out and the mother liquid was allowed
to react for another 3 h under the identical reaction conditions. No significant change in the
conversion was observed, indicating that the present catalysis was indeed heterogeneous in
nature rather than the dissolved Pd (II) species leached from Pd (II)-PMO-P-2.
An important merit of heterogeneous catalysts was their convenient recycling and
reuse. As shown in Fig.9, the Pd (II)-PMO-P-2 could be reused in water-medium
Sonogashira reaction of 4-iodomethylbenzene with phenylacetylene for at least 7 times
without significant decrease in catalytic efficiency, showing good durability of the catalyst.
On the one hand, the Pd (II) organometals embedded in silica walls could effectively
inhibit the leaching of Pd (II) active sites. According to the ICP analysis, Pd (II) species in
the solution was less than 0.5 ppm after being used 7 times, suggesting that the Pd
(
II)-leaching could be essentially neglected. On the other hand, the Pd (II) organometals
embedded in silica walls might also enhance the hydrothermal stability of Pd
II)-PMO-P-2[52]. As shown in Fig. 10, both the low-angle XRD pattern and TEM image
(
demonstrated the preservation of ordered mesoporous structure in the Pd(II)-PMO-P-2
catalyst after being used repetitively for 7 times.
1
00
8
6
4
2
0
0
0
0
0
1
2
3
4
5
6
7
Recycling Number
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Fig. 9. The recycling tests of Pd(II)-PMO-P-2 catalyst in water-medium Sonogashira
reaction.
1
2
3
4
2
Theta Degree
Fig. 10. Low-angle XRD patterns (left) and TEM image (right) of Pd(II)-PMO-P-2 catalyst
after being used repetitively for 7 times.
Conclusions
In summary, a facile approach of surfactant-directed co-condensation for preparing a
novel organopalladium(II)-bridged periodic mesoporous organosilica, into which
palladium(II) integrally incorporated, was developed. The as-prepared Pd(II)-PMO-P
catalyst possessed platelet morphology and ordered mesoporous channels in the range of
3
00-400 nm, which facilitated the diffusion and mass transfer. The catalyst showed higher
activity and stronger durability than traditional Pd(II)-PMO-P-T in water-medium
Sonogashira reactions. The Pd(II)-PMO-P could be reused for 7 times without significant
deactivation. Other periodic mesoporous organometal silica catalysts including Rh(I),
Ru(II), Au(I), Ni(II), etc., could also be designed based on the present method, which may
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offer more opportunities for designing powerful immobilized homogeneous catalysts for
water-medium clean organic reactions.
Acknowledgements
This work was supported by the Project of College Natural Science Research Plan of
Jiangsu province (13KJB150008) and the Natural Science Foundation of Jiangsu
Provincial Department of Education (17KJA150002, 15KJA15003, 17KJB150008).
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Highlights
1
. A mesoporous organometal Pd(II) catalyst with short channel was synthesized by
surfactant-directed co-condensation.
. The catalyst with active sites inside silica walls was in uniform platelet (300~400
nm).
2
3
. The Pd(II)-PMO-P catalyst exhibited higher efficiency than traditional
Pd(II)-PMO-T with irregular shape in water-medium Suzuki reaction.
4
. This Pd(II)-PMO-P catalyst could be used repetitively showing the excellent
durability.