A reusable heterogeneous catalyst without leaking palladium for highly-efficient Suzuki-Miyaura reaction in pure water under air

Article abstract of DOI:10.1039/c6ra11736h

Herein, we report a heterogenous catalyst (Pd@FSM) by immobilization of a novel Pd²⁺ sensor as promoter over mesoporous silica. Pd@FSM with a high palladium loading of ca. 11 mg g^-1 exhibited superior catalytic activity for Suzuki-Miyaura cross-couplings and a catalyst loading of 0.05 mol% is typically sufficient to achieve excellent reaction yields. Notably, the reaction is typically carried out in water without removing atmospheric oxygen. The catalyst is conveniently recycled and remains highly active even after being recycled 5 times. During this process, loss of palladium from the solid support of the catalyst is negligible. Furthermore, the catalyst can be stored in air for at least three months without loss of its catalytic activity. This work provides a new approach to developing heterogeneous palladium catalysts by combing materials and fluorescent sensors.

Full text of DOI:10.1039/c6ra11736h

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A reusable heterogeneous catalyst without leaking
palladium for highly-efficient Suzuki–Miyaura
reaction in pure water under air†
Cite this: RSC Adv., 2016, 6, 60996
*
*
Qi Cai, Gaosheng Liang, Yufang Xu, Xuhong Qian and Weiping Zhu
Herein, we report a heterogenous catalyst (Pd@FSM) by immobilization of a novel Pd2+ sensor as promoter
over mesoporous silica. Pd@FSM with a high palladium loading of ca. 11 mg gꢀ1 exhibited superior catalytic
activity for Suzuki–Miyaura cross-couplings and a catalyst loading of 0.05 mol% is typically sufficient to
achieve excellent reaction yields. Notably, the reaction is typically carried out in water without removing
atmospheric oxygen. The catalyst is conveniently recycled and remains highly active even after being
recycled 5 times. During this process, loss of palladium from the solid support of the catalyst is
negligible. Furthermore, the catalyst can be stored in air for at least three months without loss of its
catalytic activity. This work provides a new approach to developing heterogeneous palladium catalysts by
combing materials and fluorescent sensors.
Received 5th May 2016
Accepted 17th June 2016
DOI: 10.1039/c6ra11736h
www.rsc.org/advances
separation of heavy metal ions, such as palladium ions, has
gained extensive interest. These studies have inspired con-
1. Introduction
structing multifunctional platforms for ion detection, separa-
tion and reusability onto host materials.24–26 Recently, Bhalla
Palladium catalysts are widely employed in carbon–carbon
bond formation and play an indispensable role in the synthesis
of complex structures, such as natural products, pharmaceuti-
cals, functional materials, polymers and supermolecules.1–4
Homogeneous palladium catalysts are routinely employed in
the Suzuki–Miyaura reaction. Though they have found great
success, limitations such as high expenses and sensitivity to
oxygen have been encountered.5 Recycling of a catalyst may
greatly lower its cost and is particularly desired in industrial
applications.6 Besides, contamination of the reaction products
by palladium is not efficiently removed and represents an
important concern of such processes.7 Recently, development of
heterogeneous palladium catalysts have attracted considerable
attention since they are conveniently recycled for reuse. They
are typically constructed by covalent or supra-molecular
immobilization8,9 of metal catalysts on the surface of various
materials through interactions, including silica,10,11 gra-
phene,12,13 carbon nano tube,14,15 and metallic oxide,16 and the
organic matrix, such as polymers and dendrimers.17,18
group reported
a multi-functional hetero-oligophenylene
derivative, which could detect palladium ions and induce to
form palladium nanoparticles as novel catalysts for carbon–
carbon coupling.27 In another aspect, various nanoparticles or
micro-particles have been reported and widely used as the
carrier of solid catalysts.10,11,16,19,20,28,29 Among those materials,
the mesoporous silica nanoparticles is highly attractive by
virtue of its large specic surface area, facile modication for
diverse functions and heat- and hydro-stability.30–32 Amini et al.
reported a heterogeneous palladium catalyst by anchoring tri-
dentate pyridine ligand to mesoporous silica, which showed
good capacity in catalyzing Mizoroki–Heck and Suzuki–Miyaura
reaction with 1 mol%.It could be recycled for four times but loss
of catalytic activity aer each recycle was noted.33 Furthermore,
Lin et al. reported a magnetic palladium catalyst, exhibiting
much higher activity with only 0.02 mol% and excellent recy-
clability for more than 20 times without notable loss of activity.
However, the leaking of palladium and requiring the use of
organic co-solvent are still concerns.34 Combination of selective
and specic uorescence probes with various matrixes is highly
desirable endowing them excellent performance, such as
broader linear detection range, convenience and reusability in
separation or even lower detection limit.24–26,35
Various ligands that have been immobilized for chelating of
palladium ions include thiol groups, heterocyclic ring structure,
multi-oxygen or nitrogen groups and other groups.11,19–23 In
recent years, the uorescent probes for the detection and
State Key Laboratory of Bioreactor Engineering and Shanghai Key Laboratory of
Chemical Biology, School of Pharmacy, East China University of Science and
Technology, 130 Meilong Road, Shanghai, 200237, P. R. China. E-mail: wpzhu@
ecust.edu.cn; xhqian@ecsut.edu.cn; Fax: +86-21-64252603
We previously reported a uorescent material FSM by
anchoring a palladium uorescent probe on mesoporous silica
for detection and separation of palladium ions in various
solutions.35 Further investigation showed that the palladium
ions were tightly bound with FSM, and could not be easily
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra11736h
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Table 1 Catalytic activity evaluation of Pd@FSMa
pulled down by the H+, EDTA or other chelating agents. The
high stability of the complex has promoted us to test its
potentials as a heterogeneous palladium catalyst. Herein, based
on the palladium uorescent probe FSM, we report a reusable
heterogeneous catalyst Pd@FSM for Suzuki–Miyaura reaction.
The catalyst exhibited excellent ability in catalyzing the Suzuki–
Miyaura cross-coupling reactions of aryl halides with phenyl-
boronic acids in water with only 0.05 mol% amount and kept its
activity stable even aer exposed to the air for three months or
recycled for ve times without palladium leaking.
Catalyst (mol%)
Yield (%)b
0.005
27
0.01
90
0.05
97
10
0.2
98
10
Time (h)
24
24
a
Condition: 1.0 mmol B1, 1.5 mmol boronic acid B2, 2.0 mmol K2CO3
and Pd@FSM in 5 mL water solution under 80 ꢁC. b Isolated yield, mean
of three experiments.
2. Experimental
The synthesis of FSM was reported by our previous work.35
Mainly by integrating the highly specic Pd2+ uorescent probe
with mesoporous silica through silicon oxygen bond.
melting point. In the recycle test, 4-bromoacetophenone (199
mg, 1.0 mmol), phenylboronic acid (183 mg, 1.5 mmol), K2CO3
(276 mg, 2.0 mmol), Pd@FSM (5.0 mg, 0.05 mol%) and water
(25 mL) were used. Aer each cycle, the solution was extracted
with CH2Cl2, and the combined organic layers were dried over
MgSO4 and concentrated in vacuo. The products were sent for
2.1 Preparation of Pd@FSM
FSM (500 mg) and PdCl2 solution in water (10 mM, 20 mL) were
stirred for 3 h at room temperature. Then the mixture was
ltered and washed with water for three times. Aer that, the
residue was further reuxed in methanol for another 24 h. The
solid catalysts were then ltered and washed with methanol and
water for three times respectively. The nal product Pd@FSM
was dried under vacuum. The amount of palladium anchored to
Pd@FSM was 11 mg gꢀ1 by the inductively coupled plasma
atomic emission spectrometry (ICP-AES). The palladium ion
was captured by alkynyl group and aminothiophene owing to
their rich electron density as reported (Fig. 1),36 which caused
the decrease of uorescence of FSM.35
Table 2 Different aryl halides for Suzuki–Miyaura reactiona
Entry
R1
R2–B(OH)2
Yieldb (%)
1
2
41
91
2.2 General procedure for Suzuki–Miyaura reactions
Aryl halide (1 mmol), phenylboronic acid (1.5 mmol), K2CO3
(276 mg, 2.0 mmol), Pd@FSM (5.0 mg, 0.05 mol%) and water (5
mL) were mixed in 25 mL round-bottom ask and stirred at 80
ꢁC for 10 h. Aer the reaction completed, the reaction mixture
was extracted with CH2Cl2 (3 ꢂ 50 mL). The combined organic
layers were dried over MgSO4 and concentrated in vacuo. The
remaining residue was puried by column chromatography on
3
4
87
97
5
6
7
8
91
81
81
91
1
silica gel and conrmed with H NMR, mass spectral data and
9
94
10
84
86
11
a
Condition: 1.0 mmol R1–X, 1.5 mmol boronic acid, 2.0 mmol K2CO3
and Pd@FSM (0.05 mol% Pd) in 5 mL water solution under 80 ꢁC for
10 h. Isolated yield, mean of three experiments.
Fig. 1 Preparation of Pd@FSM and its application in suzuki coupling
reaction.
b
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ICP-MS analysis to detect the leached palladium. The water important feature in practical industrial application. Therefore,
phase was centrifuged aer extraction, washed with extensive the following catalysis experiments were all carried out with
methanol and Milli Q-water, then dried under vacuum and then 0.05 mol% of palladium under 80 ꢁC in water for 10 h in
used directly without any further treatment.
ambient air.
A broad substrate scope plays an essential role in evaluating
the efficiency of a catalyst. Therefore, we screened different aryl
halides with phenylboronic acid to further explore the cata-
lyzing capability of Pd@FSM. As shown in Table 2, Pd@FSM
3. Results and discussion
As a safe and cost-effective solvent, water has been widely
applied in catalyzing the Suzuki–Miyaura coupling reaction, as
the solvent or a co-solvent.34,37–39 In this research, the Pd@FSM
catalyst performed excellent catalytic property in pure water
under aerobic conditions.
Table 4 Widespread application of Pd@FSM in catalyzing Suzuki–
Miyaura reaction with different phenylboronic acidsa
Firstly, we examined the catalytic activity of Pd@FSM with
a different amount of catalyst loading in water under air
atmosphere. The results showed that Pd@FSM of 0.01 mol%
can afford the product in 90% yield in 24 h. Raising the catalyst
loading to 0.05 mol% led to improving the reaction yield to 97%
but only in 10 h. These outcomes illustrate the much higher
activity of Pd@FSM than those homogeneous or heterogeneous
palladium catalysts under the condition of water solution and
air atmosphere.11,36,40 Aer that, the conversion rate had no
signicant increase even with much more catalyst (Table 1).
Besides, the results also proved that the presence of oxygen does
not inhibit the catalytic activity of Pd@FSM. This is an
Entry
19
R1–X
R2–B(OH)2
Yieldb (%)
>99
20
21
>99
>99
Table 3 Steric hindrance effect of Suzuki–Miyaura reactiona
22
23
24
>99
94
Entry
12
R1–X
R2–B(OH)2
Yieldb (%)
94
>99
13
14
15
16
17
91
95
46
85
83
25
99
26
27
28
96
91
32
29
69
48
30
18
26
a
Condition: 1.0 mmol 4-bromoacetophenone, 1.5 mmol boronic acid
a
Condition: 1.0 mmol R1–X, 1.5 mmol boronic acid R2–B(OH)2, 2.0 R2–B(OH)2, 2.0 mmol K2CO3 and Pd@FSM (0.05 mol% Pd) in 5 mL
b
mmol K2CO3 and Pd@FSM (0.05 mol% Pd) in 5 mL water solution water solution under 80 ꢁC for 10 h. Isolated yield, mean of three
b
under 80 ꢁC for 10 h. Isolated yield, mean of three experiments.
experiments.
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Table 5 Recyclability and palladium leaking of Pd@FSM
Heavy metal pollution to the products, like drugs and food
additives, have been an important problem for palladium catalysts
due to the difficulty in extraction of homogeneous catalysts from
the solution or palladium leaking from heterogeneous catalysts
which although could relieve that problem to a certain extent, not
to mention the increasing costs.42–45 Among those heterogeneous
catalysts' advantages, easy separation and recyclability are the key
solution to this issue, and naturally become the standard in
assessing the performance of catalysts. Even though, Soomro once
reported the dissolution and redeposition mechanism in the
catalyzing process of heterogeneous palladium catalysts, which
caused the palladium leaching and decrease of catalysis eff-
cience.46,47 In this research, the activity of Pd@FSM kept remained
even aer recycled for ve times without the leaking of palladium
observed in the product (Table 5). We supposed that the rich
electron ligands (alkyne and thiophenemethylamine) and proper
cavity size in FSM provided high combing ability for Pd2+. The post
processing of prepared Pd@FSM by washing with methanol under
reux and difficulties in pulling Pd2+ down by other palladium
chelating agents could also be good evidences.
Stability of one catalyst in the air is another concern.
Pd(PPh3)4, the most commonly applied homogeneous palla-
dium catalyst, was limited in application due to its sensitivity to
oxygen. Therefore, the protection by inert gas during the
process is necessary and leads to a raise in cost. In this research,
Pd@FSM could achieve high yield using even lower catalyst
loading in water at 80 ꢁC without argon protection. In
comparison, the catalytic activity of Pd(PPh3)4 dropped mark-
edly if the reaction is carried out without deoxygenation, and
the PdCl2 could only achieve 81% yield even with 5 mol%
amount. Nevertheless, when the palladium ions were captured
by the FSM from the water phase of PdCl2 catalyzed solution
aer extracted with chloroform, they showed remarkable
activity with only 0.05 mol%. These studies clearly conrmed
the advantages of the specic and selective palladium uores-
cence probe which could not only anchor the Pd2+ to the solid
part, but also perform as the ligand assisting the catalytic
process with signicantly increased activity. Moreover,
Pd@FSM could also remain its catalytic performance even aer
exposed to the air for three months (Table 6), which was
unprecedented.48
Run
1
2
3
4
97
nd
5
96
nd
Yield (%)
97
97
nd
97
nd
Leaking of Pd2+
nda
a
nd ¼ not detected.
had good performance for different substrates with high yields
in water, especially for bromo- and iodo-benzenes. It could
achieve uniformly high yields for a wide selection of substrates
in pure water. And a lower catalyst loading is used compared
with other palladium catalysts.40,41
Besides, the results in Table 2 showed that substitution on
the ortho-/para of aryl halides had no obvious regular effect on
the yields, no matter it was electron-drawing (acetyl-, nitro- and
cyan-) or donating group (methoxy-, methyl-). The above
phenomenon was further conrmed by the results in Table 3
(entries 12 and 13). However, we also found that the sterically
demanding ortho-position on the phenylboronic acids would
signicantly lower the yields (Table 3, entries 14 and 15),
considering the equal contribution of methoxyl group to elec-
tron density increase in ortho- and para-position. The steric
hindrance from both the ortho-substituted substrates caused
sharp reduction of the product (Table 3, entry 18).
Furthermore, we probed Pd@FSM catalyst in phenylboronic
acids with various substitutions by reacting with 4-bromo-
acetophenone which had relatively higher activity compared with
other aryl halides (Table 2, entry 4). Phenylboronic acids
substituted with methyl-, uoro-and chloro-groups are all well
tolerated even much higher yields (Table 4, entry 19–26), while the
cyan-group in para-position came up with slight decrease of the
yield (Table 4, entry 27). The formyl- and nitro-groups in the meta-
position caused signicant decrease of the yields of products (Table
4, entries 28 and 29). Besides, we also achieved the hydroxythio-
group substituted product in the para-position for the rst time.
Table 6 Stability and high activity of Pd@FSM compared to the reported catalysts
Catalyst
Pd@FSM
97
0.05
Pure water
Air
Yes
Pd@FSMa
97
0.05
Pure water
Air
Yes
(Ph3P)4Pd
58
0.1
DMF
Air
No
(Ph3P)4Pd
99
0.1
DMF
Argon
No
PdCl2
81
5
Water
Air
No
Pd@silica (ref. 11)
96
0.2
Organic/water
Not mentioned
Yes
Yield (%)
Cat. (mol%)
Solvent
Atmosphere
Recyclability
a
Pd@FSM was exposed to air for 3 months before used.
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In summary, we have successfully designed and prepared
a novel palladium catalyst by graing highly selective and
sensitive uorescent sensor onto the surface of MSNs for
anchoring Pd2+.The as-synthesized catalyst Pd@FSM displayed
remarkable catalytic activity towards Suzuki–Miyaura coupling
reaction in water with only 0.05 mol% of palladium and good
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Acknowledgements
We thank the nancial support from National High Technology
Research and Development Program of China (863 Program,
2012AA061601), National Natural Science Foundation of China
(Grants 21236002, 21476077), and Shanghai Pujiang Program. W.
Zhu is grateful for the support from Chinese Scholarship Council.
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