Palladium nanoparticles embedded in improved mesoporous silica: A pH-triggered phase transfer catalyst for Sonogashira reaction

Article abstract of DOI:10.1002/aoc.3349

An efficient and reusable pH-responsive mesoporous silica nanocomposite shuttle-supported palladium catalyst was synthesized, which efficiently promotes the Sonogashira reaction in water-based biphasic systems. This catalyst of shell-embedded palladium nanoparticles is highly dispersed in organic phase in a pH range from 9 to 10 just like a homogeneous catalyst, and can be separated and reused like a heterogeneous one by adjusting the pH value of the aqueous medium. In addition, Sonogashira reactions can be performed without a copper co-catalyst.

Full text of DOI:10.1002/aoc.3349

Full paper
Received: 26 April 2015
Revised: 24 May 2015
Accepted: 7 June 2015
Published online in Wiley Online Library: 20 July 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3349
Palladium nanoparticles embedded in
improved mesoporous silica: a pH-triggered
phase transfer catalyst for Sonogashira reaction
a
b
b
b
Xiaohua Zhao *, Xiang Liu , Yaoqin Zhu and Ming Lu
An efficient and reusable pH-responsive mesoporous silica nanocomposite shuttle-supported palladium catalyst was synthesized,
which efficiently promotes the Sonogashira reaction in water-based biphasic systems. This catalyst of shell-embedded palladium
nanoparticles is highly dispersed in organic phase in a pH range from 9 to 10 just like a homogeneous catalyst, and can be sepa-
rated and reused like a heterogeneous one by adjusting the pH value of the aqueous medium. In addition, Sonogashira reactions
can be performed without a copper co-catalyst. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: mesoporous silica; pH-responsive; Sonogashira reaction
reported in this contribution, we further explored its innovative
application for the Sonogashira reaction using palladium NPs
Introduction
Palladium nanoparticles (NPs) are a promising catalyst in chemical
synthesis due to their high surface-to-volume ratio and large
embedded in the mesoporous silica shuttle as catalyst. The result
demonstrates that the palladium catalyst immobilized on the
pH-responsive nanocomposite shuttle is very efficient. A possible
reason is that the pH-responsive shuttle-supported palladium
catalyst can be highly dispersed in organic phase just by adjusting
the pH (9–10) of the reaction medium, preserving the benefits of
homogeneous catalysis. In addition, it can be easily separated from
the reaction mixture simply by adjusting the pH (3–4) and reused
without obvious loss of catalytic activity.
[
1–3]
fraction of atoms exposed to reactant molecules.
However,
the direct application of noble palladium NPs in catalysis is often
difficult because of their strong tendency towards agglomeration.
To solve this problem, palladium NPs can be synthesized in the
[
4,5]
presence of various stabilizers such as surfactants,
functioned
[6–8]
[9,10]
polymers
and inorganic carriers.
Among various materials,
mesoporous materials may be excellent candidates due to their
unique architecture. To date, mesoporous materials containing
noble metal NPs have attracted much attention owing to their
[
11–14]
potential application in catalysis.
More specifically, recent
Experimental
research indicates that template-containing mesoporous silica is
[15–20]
an interesting composite for various applications.
Materials and instrumentation
Liquid–liquid biphasic catalysis has been widely studied,
All reagents of analytical reagent grade were purchased and used
as received without further purification. Instruments used were a
GC instrument (Bruker GC-450), X-ray diffractometer (Bruker D8
Advance), elemental analyzer (Vano MICRO), inductively coupled
plasma mass spectrometer (OPTIMA 5300DV) and surface area
and particle size distribution analyzer (ASAP 2020).
provided that the catalyst and the reaction product are in two
[
21–23]
different liquid phases at the end of the transformation.
It is
even better that an extraction solvent can be avoided by develop-
ing biphasic catalytic protocols. Although the use of fluorous
biphase systems and ionic liquid biphase systems has attracted a
great deal of attention, a water-based biphasic catalytic system
remains the most interesting choice in view of its lower cost and
[
24–28]
it being less hazardous.
However, while biphasic conditions
General procedure for synthesis of nanocomposite
[30]
facilitate separation of catalyst and product and catalyst reuse,
multiple phases introduce kinetic barriers and mass transfer limita-
tions. An interesting approach is that of anchoring homogeneous
catalysts to thermomorphic polymers that are completely water
soluble at room temperature but become more lipophilic and
migrate towards the organic phase at the higher temperatures
shuttle-supported palladium NPs
A mixture of 505 ml of methanol, 400 ml of water, 3.0g of
cetyltriethylammonium chloride) and 2.2ml of aqueous NaOH
*
Correspondence to: Xiaohua Zhao, School of Materials Science and Engineering,
Jiangsu University, zhenjiang 212013, China. E-mail: zhao12_19@163.com
[
29]
used for the catalytic reaction. In this case, the key problem is
how to avoid significant catalyst losses at the separation stage.
Yang’s group and our group reported the synthesis of a
pH-responsive mesoporous silica nanocomposite shuttle, which
exhibited excellent potential as a suitable scaffold to immobilize
a School of Materials Science and Engineering, Jiangsu University, Zhenjiang
12013, China
2
b School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China
[
30,31]
noble metal NPs without destroying their activity.
In the work
Appl. Organometal. Chem. 2015, 29, 674–677
Copyright © 2015 John Wiley & Sons, Ltd.

A pH-triggered phase transfer catalyst for Sonogashira reaction
À1
(
1mol l ) solution was stirred for 30min at 25 °C. An amount of
8.61 mmol of tetramethyl orthosilicate (TMOS) in combination with
x/(x + y) × 1.52 mmol of octyltrimethoxysilane was added into this
mixture under stirring, where x (=5, 4, 3, 2) and y (=1) represent
the molar fractions of the used siliceous precursors for core forma-
tion and shell formation, respectively. Taking 2C@1 N as an exam-
ple, the molar fraction of siliceous precursors added in the first
step for the core formation is two-thirds of the total siliceous pre-
cursors, and the molar fraction of the siliceous precursors added
in the second step for the shell formation is one-third of the total
siliceous precursors. Then, after stirring for 1.5h, 8.61 mmol of
TMOS together with y/(x + y) × 1.52 mmol of (MeO) SiCH CH
3
2
2
CH NHCH CH NHCH CH NH was added into the above mixture.
2
2
2
2
2
2
After further stirring for 10 h and aging overnight, the precipitate
was isolated and washed with water. After being dried, the pow-
ders were extracted with hot ethanol (12 h, repeated three times)
to afford xC-yN. Then 1 g of xC-yN was added into 12 ml of toluene
Figure 1. Small-angle XRD pattern of nanocomposite 5C-1 N.
containing 0.017 g of Pd(OAc) . After stirring for 4 h at room
2
temperature, the palladium-adsorbed solid was reduced with
NaBH in a mixture of toluene and ethanol (20/1v/v), affording
4
the xC-yN-supported palladium NPs (Pd-xC-yN).
General conditions for Sonogashira reactions
Aryl halide (1.0mmol), terminal alkynes (1.2 mmol), base (4.0mmol)
and palladium (1.0 mol%) were added to a vessel with 4 ml of water
and 4 ml of ether. The mixture was continuously stirred at room
temperature for the desired time until complete consumption of
the starting aryl halide. The reaction progress was monitored using
GC. At the end of reaction, a few drops of diluted HCl solution
À1
(
1mol l ) were added into the reaction system, and the pH of wa-
ter was adjusted to 3–4. After stirring, the catalyst transferred into
the water, and the upper organic layer was isolated for GC analysis
and further purification.
Results and discussion
Figure 2. XRD patterns of 5C-1 N and Pd-5C-1 N.
Synthesis and characterization
accordance with the increasing ratio of C (Fig. S2, supporting infor-
mation). The content of Pd agrees with the analysis of XRD, and the
Pd loading of 5C-1 N is estimated as 1.5 wt% based on inductively
coupled plasma atomic emission spectrometry (ICP-AES).
Brunner–Emmet–Teller (BET) measurements were carried out to
investigate the nanocomposites. The specific surface area of the
2
À1
representative sample 5C-1 N is 20 m g , caused by the low tem-
perature of about À196 °C of the nitrogen sorption measurements.
Under these conditions, the organic ligands in nanopores are so
rigid that nitrogen cannot sufficiently enter the interior before
reaching the measuring equilibrium. In addition, the pore size of
Transmission electron micrographs of 5C-1 N and Pd-5C-1N are
shown in Fig. 3. It is clear that both 5C-1 N and Pd-5C-1 N have
uniform discrete microspheres with a diameter of 200–300 nm and
a shell–core structure. There is also a clear boundary between the
core and shell (Fig. 3(A)). Pores in these images can be easily seen.
And pores between shell and core are more than that for other parts.
After loading, the color of the discrete microspheres becomes
deeper, suggesting that pores are filled with Pd NPs (Fig. 3(B)).
Through calculation, the size of the Pd NPs is about 0.4 nm.
To ascertain the oxidation state of the palladium, X-ray photo-
electron spectroscopy studies were carried out. In Fig. 4, the
palladium binding energy of Pd-5C-1 N exhibits two strong peaks
centered at 340.5 and 335.2 eV, which are assigned to Pd 3d3/2
and Pd 3d5/2, respectively. These values agree with the Pd(0)
binding energy of Pd NPs.
3
À1
5C-1 N is 8.21 nm and the pore volume is 0.03 cm g . This also
indicates that the nanocomposites are mesoporous materials with
medium pore volumes, thus benefitting the loading of palladium
NPs. Meanwhile, both small-angle and wide-angle X-ray diffraction
(
XRD) experiments were performed. As shown in Fig. 1, the small-
angle XRD pattern of sample 5C-1N shows a diffraction peak at
θ ≈ 2.5°, indicating a mesoporous structure. All other samples
2
show the same diffraction peak (Fig. S1, supporting information).
This also supports the BET results.
Comparing the wide-angle XRD pattern of Pd-5C-1 N with that of
5C-1 N, a strong peak at 40° in the pattern of the former, shown in
Fig. 2, is clearly observed. This is well known as characteristic of
palladium according to the standard XRD spectra database. There-
fore, we suppose the successful loading of Pd NPs. Also, the
strength of the Pd peak increases with increasing ratio of C. This
also indicates that the loading amount of Pd NPs also increases in
Sonogashira reaction catalyzed by Pd-5C-1 N
The Sonogashira reaction of terminal alkynes with aryl halides
provides a powerful tool for C–C bond formation, which has been
Appl. Organometal. Chem. 2015, 29, 674–677
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

X. Zhao et al.
a
Table 1. Sonogashira reaction of iodobenzene with phenylacetylene
A:
B:
b
Entry
Base
Solvent
Toluene
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
Et
3
3
3
3
3
N
18
18
24
15
15
24
24
24
24
24
24
78
75
50
93
96
78
82
85
35
28
10
N
N
N
N
Benzene
Dioxane
Tetrahydrofuran
Ether
Figure 3. Transmission electron microscopy images of (A) 5C-1 N and (B)
Pd-5C-1 N at different resolutions.
Pyridine
Ether
Morpholine
Piperidine
Ether
Ether
K
2
CO
3
Ether
1
0
1
NaOAc
KF
Ether
1
Ether
a
Reaction conditions: iodobenzene (1.0 mmol), phenylacetylene
(
2
1.5 mmol), base (4.0 mmol), H O solvent (4 ml/4 ml), Pd-5C-1 N catalyst
(
b
1.0 mol% Pd).
GC yield.
3
temperature (Table 1, entry 5). A possible reason is that Et N has
greater solubility in ether, where the Sonogashria reaction occurs
in a homogeneous environment.
The scope and limitation of the current protocol were investi-
gated under the optimized conditions. The results are summarized
in Table 2. Electron-donating groups such as –Me and –OMe at the
Figure 4. X-ray photoelectron spectrum of Pd-5C-1 N showing Pd 3d5/2
and Pd 3d3/2 binding energies.
[
32–34]
reported in water media.
We next focused our attention on
exploring catalytic applications of the pH-responsive nanocompos-
ite shuttle-supported palladium catalyst for the Sonogashira
reaction in water-based biphasic systems (Fig. 5).
Table 2. Sonogashira reaction between aryl halides and terminal al-
kynes in water
a
Initially, the cross-coupling between iodobenzene and
phenylacetylene was chosen as a model reaction for screening
the best reaction conditions. As evident from Table 1, the solvent
effect is found to play a crucial role in the Sonogashira reactions.
Among all the examined solvents, ether is the best reaction
medium. The difference is largely due to the transferability in
different solvents. It is found that the phase transfer does not occur
in other chosen solvents, and the catalyst remains in the organic
phase despite changing the pH. Hence, the catalyst is difficult to
recycle. According to the literature, the base is a significant factor
that determines the efficiency of the Sonogashira reaction. Various
bases were investigated and the results show that the use of
inorganic base, such as K CO and NaOAc (Table 1, entries 9–11),
b
c
Entry
R
1
X
R
2
Time (h)
Yield (%)
1
4-OMe
4-Me
I
H
H
H
H
H
H
H
H
H
H
H
H
H
15
15
10
10
10
10
10
15
15
24
24
24
24
10
10
10
77 (70)
80 (75)
95 (90)
93 (82)
99 (95)
90 (86)
92 (86)
73 (70)
80 (75)
81 (70)
86 (80)
46
2
I
3
4-COMe
4-CN
4-NO2
4-Br
I
4
I
5
I
6
I
7
4-Cl
I
2
3
8
2-Cl
I
affords little product. In contrast, organic bases are able to afford
a good to excellent coupling yield (Table 1, entries 5–8). Among
9
3-Cl
I
10
11
3-Iodopyridine
I
them, Et N provides a much better result than the others at room
2-Iodothiophene
I
3
12
13
14
15
16
4-NO
4-CN
2
Br
Br
I
33
4-COMe
4-COMe
4-COMe
4-NH
2
91 (86)
92 (85)
88 (80)
I
4-CF
4-Br
3
I
a
Reaction conditions: ArX (1.0 mmol), terminal alkynes (1.5 eq.), Et
O–ether (4 ml/4 ml), Pd-5C-1 N catalyst (1.0 mol% Pd).
Reaction was monitored by GC.
3
N
(4.0 eq.), H
2
b
Figure 5. Sonogashira reaction catalyzed by pH-responsive palladium
nanocomposite (Pd-5C-1 N).
c
GC yield; isolated yields given in parentheses.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 674–677

A pH-triggered phase transfer catalyst for Sonogashira reaction
Table 3. Recycling experiments in Sonogashira coupling between
Iodobenzene and phenylacetylene
Conclusions
a
We developed a catalyst composed of nanocomposite shuttle-
supported palladium NPs. Its employment in the Sonogashira reac-
tion of aryl iodides and terminal acetylenes resulted in high
efficiency. In addition, the catalyst can be pH-triggered and recycled
like a heterogeneous catalyst by adjusting the pH value of the aque-
ous medium. Moreover, the Sonogashira reactions were performed
without copper co-catalyst.
Run
1
2
3
4
5
b
Yield (%)
96
94
94
92
90
a
Reaction condition: ArX (1.0 mmol), terminal alkynes (1.5 eq.), Et
3
N
Acknowledgments
(
4.0 eq.), H
2
O–ether (4 ml/4 ml), Pd-5C-1 N catalyst (1.0 mol% Pd).
b
GC yield.
This work was supported by Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions (1281220030).
para position of aryl iodide couple smoothly with phenylacetylene
to give good to excellent yields. It is observed that electron-
deficient aryl iodides undergo cross-coupling within a short
reaction time with excellent yield. It should also be noted that aryl
iodides can be selectively coupled in the presence of aryl bromide
or aryl chloride. When the substrate contains iodo and
bromo/chloro substituents, phenylacetylene couples readily only
with the iodo side (Table 2, entries 8 and 9). The steric
properties of the substituents also show obvious influences
on the reaction yield and rate. Substituent located at meta
or para position gives rise to the reaction in comparison to
that at the ortho position due to the steric effect. Interest-
ingly, the coupling reactions of heteroaryl iodides such as
References
[
1] R. Ricciardi, J. Huskens, M. Holtkamp, K. Michael, U. Karst, W. Verboom,
ChemCatChem 2015, 7, 936.
[
[
2] T. Borkowski, J. Dobosz, W. Tylus, A. M. Trzeciak, J. Catal. 2014, 319, 87.
3] Q. Xiao, S. Sarina, D. P. Arnold, H. Liu, H. Zhu, Green Chem. 2014,
16, 4272.
[
4] E. E. Drinkel, R. R. Campedelli, A. M. Manfredi, H. D. Fiedler, F. Nome,
J. Org. Chem. 2014, 79, 2574.
[5] B. S. Souza, E. C. Leopoldino, D. W. Tondo, J. Dupont, F. Nome, Langmuir
012, 28, 833.
2
[6] H. X. Zhao, Y. X. Wang, R. H. Wang, Chem. Commun. 2014, 50, 10871.
[7] L. Y. Li, H. X. Zhao, J. Y. Wang, R. H. Wang, ACS Nano 2014, 8, 5352.
[8] C. H. Lu, F. C. Chang, ACS Catal. 2011, 1, 481.
[9] L. Xu, C. G. Li, K. Zhang, P. Wu, ACS Catal. 2014, 4, 2959.
[
[
10] Y. Ko, H. Baek, Y. Kim, M. Yoon, J. Cho, ACS Nano 2013, 7, 143.
11] Z. P. Dong, K. Liang, C. X. Dong, X. L. Li, X. D. Le, J. Ma, RSC Adv. 2015, 5,
3
-iodopyridine and 2-iodothiophene with terminal alkynes
can also proceed smoothly under the same conditions
affording the corresponding coupling products in good yields
after 24 h of reaction time. As for aryl bromides, the substrates
are converted to the corresponding coupling products in rela-
tively low yields. When other terminal acetylenes are
examined to couple with 4-acetylphenyl iodide, excellent
yields of products are obtained without obvious electronic
discrimination.
20987.
[
12] B. Karimi, A. Zamani, F. Mansouri, RSC Adv. 2014, 4, 57639.
[13] J. Saha, K. Bhowmik, I. Das, G. De, Dalton Trans. 2014, 43, 13325.
[14] X. Xu, Y. Li, Y. Gong, P. Zhang, H. Li, Y. Wang, J. Am. Chem. Soc. 2012,
1
34, 16987.
15] T. Yuan, H. Gong, K. Kailasam, Y. Zhao, A. Thomas, J. J. Zhu, J. Catal.
015, 326, 38.
[
[
2
16] A. Satapathy, S. T. Gadge, E. N. Kusumawati, B. M. Bhanage, Catal. Lett.
2015, 145, 824.
[
[
[
[
17] X. Zhang, P. P. Zhang, H. Yu, Z. Ma, S. H. Zhou, Catal. Lett. 2015, 145, 784.
18] F. W. Zhang, H. Q. Yang, Catal. Sci. Technol. 2015, 5, 572.
19] S. Das, S. Bhunia, T. Maity, S. Koner, J. Mol. Catal. A 2014, 394, 188.
20] B. Basu, S. Paul, Appl. Organometal. Chem. 2013, 27, 588.
The recycling and recovery of a catalytic system are very
important issues from both practical and environmental
points of view. After completion of reactions, a few drops of
diluted HCl were added into the reaction mixture and the
pH was adjusted to 3–4. Then the catalyst returned to the
water phase. The upper organic layer was separated and
[21] H. P. Steinrueck, P. Wasserscheid, Catal. Lett. 2015, 145, 380.
[22] J. F. B. Hall, X. Han, M. Poliakoff, R. A. Bourne, M. W. George, Chem.
Commun. 2012, 48, 3073.
[
23] X. Zhang, A. F. Cardozo, E. Manoury, F. D. Agosto, R. Poli, Chem. Eur. J.
014, 20, 15505.
24] M. Hong, C. Cai, J. Chem. Res. 2010, 34, 568.
worked up. Then the pH was readjusted to 9–10 with Et N
3
2
when fresh ether and substrates were added into the above
mixture. The catalyst came back to the organic phase again,
making the reaction homogeneous in the organic phase.
Afterwards, the next reaction could be carried out. However,
this method does not work when the substrates are acid
sensitive. As evident from Table 3, the activity shows no obvi-
ous decrease from the second cycle to the fifth cycle. To
further evaluate the stability of the catalyst, we determined
the residual Pd contents in the ether layer and water layer
with ICP-AES. In the first run, the residual Pd concentration
in ether is determined as 0.25 ppm after the catalyst is trans-
ferred into water. The Pd concentration in water is as low as
[
[25] P. F. Gao, A. Li, H. H. Lee, D. I. C. Wang, Z. Li, ACS Catal. 2014, 4, 3763.
[
[
26] L. Y. Kong, Q. Yang, H. Xing, B. Su, Q. L. Ren, Green Chem. 2014, 16, 102.
27] G. Cravotto, E. Borretto, M. Oliverio, A. Procopio, A. Penoni, Catal.
Commun. 2015, 63, 2.
[
28] V. Mazan, I. Billard, N. Papaiconomou, RSC Adv. 2014, 4, 13371.
[29] C. Liu, X. M. Li, Z. L. Jin, Catal. Today 2015, 247, 82.
[30] H. X. Wang, H. Yang, H. Liu, Y. Yu, H. C. Xin, Langmuir 2013, 29, 6687.
[
31] Y. Q. Zhu, X. Liu, M. N. Shen, Y. G. Xia, M. Lu, Catal. Commun. 2015,
3, 21.
6
[
[
32] B. Liang, M. J. Dai, J. H. Chen, Z. Yang, J. Org. Chem. 2005, 70, 391.
33] B. H. Lipshutz, D. W. Chung, B. Rich, Org. Lett. 2008, 10, 3793.
[34] A. Khalafi-Nezhad, F. Panahi, Green Chem. 2011, 13, 2408.
0
.018 ppm after the catalyst moves back to the ether phase.
In the fifth cycle, the residual Pd in ether is 0.3 ppm, and
the accumulative Pd residue in the water is 0.057 ppm. The
low concentrations of the residual Pd in both water and
organic phase are due to the complete phase transfer, indicat-
ing that the catalyst has sufficient stability.
Supporting information
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web site.
Appl. Organometal. Chem. 2015, 29, 674–677
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc