Suzuki coupling reaction catalyzed heterogeneously by Pd(salen)/ polyoxometalate compound: Another example for synergistic effect of organic/inorganic hybrid

Article abstract of DOI:10.1002/aoc.3086

A hybrid compound consisting of palladium(salen) [salen = N,N′-bis(salicylidene)ethylenediamine] complex covalently linked to a lacunary Keggin-type polyoxometalate, K₈[SiW₁₁O ₃₉](POM), was synthesized and characterized by FT-IR, elemental analysis, inductively coupled plasma and diffuse reflectance UV-visible spectroscopic methods. The hybrid, [Pd(salen)-POM], was investigated in the Suzuki cross-coupling in EtOH/H₂O under mild reaction conditions. In comparison to the corresponding organic and inorganic moiety, the hybrid has shown greatly improved catalytic activity, and much higher yields toward coupling products were obtained with a low catalyst loading for various aryl halides, including unreactive and sterically hindered ones. The catalyst also exhibited prominent recyclable performance and no obvious loss of activity was observed after six consecutive runs.

Full text of DOI:10.1002/aoc.3086

Full Paper
Received: 27 June 2013
Revised: 8 October 2013
Accepted: 8 October 2013
Published online in Wiley Online Library: 6 December 2013
(wileyonlinelibrary.com) DOI 10.1002/aoc.3086
Suzuki coupling reaction catalyzed
heterogeneously by Pd(salen)/polyoxometalate
compound: another example for synergistic
effect of organic/inorganic hybrid
Jinhui Tonga,b, Haiyan Wanga,b, Xiaodong Caia,b, Qianping Zhanga,b,
Hengchang Maa,b and Ziqiang Leia,b*
A hybrid compound consisting of palladium(salen) [salen = N,N′-bis(salicylidene)ethylenediamine] complex covalently linked
to a lacunary Keggin-type polyoxometalate, K8[SiW11O39](POM), was synthesized and characterized by FT-IR, elemental
analysis, inductively coupled plasma and diffuse reflectance UV–visible spectroscopic methods. The hybrid, [Pd(salen)–POM],
was investigated in the Suzuki cross-coupling in EtOH/H2O under mild reaction conditions. In comparison to the corresponding
organic and inorganic moiety, the hybrid has shown greatly improved catalytic activity, and much higher yields toward coupling
products were obtained with a low catalyst loading for various aryl halides, including unreactive and sterically hindered ones.
The catalyst also exhibited prominent recyclable performance and no obvious loss of activity was observed after six consecutive
runs. Copyright © 2013 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: cross-coupling; heterogeneous catalysis; organic–inorganic hybrid composites; polyoxometalates; Pd(salen)
particular, polyoxometalates (POMs) have been recently associ-
ated with a variety of organic molecules in order to prepare
Introduction
Palladium-catalyzed cross-coupling of aryl halides with arylboronic
acids, known as Suzuki–Miyaura coupling, has contributed greatly
to the straightforward and facile construction of carbon–carbon
bonds.[1] This reaction is generally considered to be a homogeneous
reaction most commonly catalyzed by soluble Pd(II) species with a
variety of ligands, such as palladium–phosphine complexes,[2] oxime
palladacycles,[3] palladium-salen complexes[4] and palladium–N-het-
erocyclic carbine complexes.[5] However, separation of the expensive
catalyst from the product for reuse is often problematic in these
homogeneous systems. Moreover, aggregation and precipitation of
palladium metal in the homogeneous systems always leads to lost
activity of the catalysts. Thus heterogeneous catalysts are highly
desirable, especially in large-scale synthesis, from both environmental
and economic aspects. Much effort has been paid to developing
heterogeneous catalytic systems that can be efficiently reused.[6]
Schiff bases are an important class of ligands and have been
studied extensively. Much effort has been paid to investigating
the viability of Schiff-base type ligands for coupling reactions,
and the salen and half-salen Pd(II) complexes were found to act
as efficient catalysts in coupling reactions.[4a–4c,7] For reuse of
the catalysts, many heterogenized systems have been founded
by supporting the salen Pd(II) complexes on some supports, such
as Merrifield resin and functionalized silica,[4d,8] and magnetite
nanoparticles.[9] However, most of the catalytic systems suffer
from harsh reaction conditions and low catalytic efficiency.
Interest in the preparation of organic/inorganic molecular
hybrids with diverse properties that may lead to new applications
has been continuously increasing in the past decades.[10] In
new compounds with interesting optical, magnetic or electric
properties.[10] In the field of catalysis, these compounds may be
expected to lead to more efficient recyclable multifunctional
catalysts and originate systems in which the catalytic abilities of
the two species reinforce each other.[10]
Recently, numerous POM-based hybrids have been
reported. POM-based N-heterocyclic carbene palladium com-
plexes have been used in catalyzing C–C cross-coupling
and aromatic dehalogenation reactions. In such systems,
POM-based hybrids display an extended polyanionic surface.
This setup is expected to contribute sterically and by virtue
of electrostatic repulsions to prevent agglomeration of
incipient Pd0, which generally deteriorates the turnover
efficiency of the systems.[11] Several metallosalen–POM
(M(salen); M = Mn, Co, Ni, Pd) hybrid compounds in which
metallosalen complex was covalently linked to a Keggin-type
POM have been synthesized by Neumann’s group.[12] Followed
*
Correspondence to: Ziqiang Lei, Key Laboratory of Eco-Environment-Related
Polymer Materials, Ministry of Education, Lanzhou 730070, People’s Republic
of China. Email: leizq@nwnu.edu.cn
a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of
Education, Lanzhou 730070, People’s Republic of China
b Key Laboratory of Gansu Polymer Materials, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s
Republic of China
Appl. Organometal. Chem. 2014, 28, 95–100
Copyright © 2013 John Wiley & Sons, Ltd.

Jinhui Tong et al.
by Mirkhani et al., the hybrid compounds were applied as catalysts
for oxidation of alkanes and alkenes,[13] benzyl halide[14] and alkene
epoxidation.[15] In these systems, obvious synergistic effects
between the metallosalens and polyoxometalates were observed
and this endowed the hybrids with much higher catalytic activities
than the metallosalen moieties.
Inspired by above works, we demonstrate herein the catalytic
performance of a hybrid compound Pd(salen)–POM in the Suzuki–
Miyaura coupling reaction in a solvent mixture of ethanol–water
(Scheme 1) under mild reaction conditions. In comparison with
homogeneous Pd(salen) catalyst, much higher yield and turnover
number were obtained on heterogeneous Pd(salen)–POM catalyst.
The effects of various bases, solvents, reaction temperature and reac-
tion time on the activity of the catalyst were investigated. The sub-
strates’ scope and reusability of the catalyst were also investigated.
Scheme 2. Preparation procedure of Pd(salen)–POM.
bromobenzene to base is was 1:2. The resulting mixture was
stirred in an oil bath at the specified temperature for the desired
time. After the reaction, ethanol was removed by rotary evapora-
tion, then the reaction mixture was diluted with water (3.0 ml)
and extracted with ethyl acetate (10.0 ml) three times. The
organic layers were combined, dried over MgSO4, filtered and
evaporated under reduced pressure. The residue was purified
by column chromatography on silica gel to give the pure prod-
Experimental
1
ucts. The identities of the products were confirmed by H NMR
General
and 13C NMR spectral data.
All reagents were of analytical grade and used as received. FT-IR
spectra were recorded as KBr pellets in the range 400–4000 cmꢀ1
on a Nicolet Nexus 670 FT-IR spectrophotometer. Diffuse reflectance
UV–visible spectra were measured on a Shimadzu UV-3100 spectro-
photometer. Elemental analysis was made using an Elementar Vario
EL system. Pd content was determined by inductively coupled
plasma (ICP) on a PerkinElmer ICP/6500 atomic emission spectrom-
eter. 1H NMR and 13C NMR spectra were recorded on a Mercury 400
MHz nuclear magnetic resonance instrument and the data were
supplied in the supporting information.
Results and Discussion
Characterization of Pd(Salen)–POM
The prepared samples were characterized by elemental analysis,
FT-IR, ICP and diffuse reflectance UV–visible spectroscopy. Initial
evidence of the electronic effect of the POM on the metallosalen
was observed in the UV–visible spectra. As can be seen in Fig. 1, Pd
(salen) has a peak at λmax = 399 nm (Fig. 1A), which is attributed to
the O(pπ)→Pd(dπ) charge transfer transition.[12,15] This peak shifted
to 379 nm in the Pd(salen)–POM compound (Fig. 1B). This suggests
an intramolecular effect of the POM on the metallosalen.[12]
The FT-IR spectrum of K8[SiW11O39] shows characteristic bands of
Keggin POMs at 795, 887 and 954 cmꢀ1 (W–O–W bonds) (Fig. 2A).
These bands shifted to 800, 919 and 963 cmꢀ1, respectively, in
the spectrum of Pd(salen)–POM (Fig. 2C). The spectrum of Pd
(salen) shows a band at 1630 cmꢀ1 due to C¼N stretching vibra-
tions (Fig. 2B). However, this band shifted to 1618 cmꢀ1 in the
spectrum of Pd(salen)–POM (Fig. 2C). These observations clearly
indicate the attachment of Pd to Salen–POM. In the spectrum of
Preparation of the Catalysts
The lacunary Keggin-type polyoxometalate, K8[SiW11O39] and the
hybrid Pd(salen)–POM complex were synthesized according to
published procedures as described in Scheme 2.[12] The salen
ligand and corresponding Pd(salen) complex were synthesized
according to the procedures described in the literature.[16]
Gram-Scale Procedure of Suzuki Coupling
All Suzuki–Miyaura coupling reactions were carried out in a 10 ml
round-bottom flask equipped with a reflux condenser. In a typical
experiment, bromobenzene (78 mg, 0.5 mmol), phenylboronic
acid (73 mg, 0.6 mmol), desired amounts of base and catalyst,
and 1.0 ml solvents were added to the flask. The molar ratio of
Scheme 1. Suzuki cross-coupling of aromatic halides and phenyboronic
acid catalyzed by Pd(salen)–POM.
Figure 1. Diffuse reflectance UV–visible spectrum of Pd–Salen (A) and
Pd(salen)–POM (B).
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Appl. Organometal. Chem. 2014, 28, 95–100

Suzuki coupling reaction, heterogeneous catalysis
Table 1. Suzuki cross-coupling of bromobenzene and
phenylboronic acid catalyzed by different catalystsa
HO
OH
Br
B
Catalyst, K2CO3
50 oC, 5 h
+
Entry
Catalyst
Solvent
Yield (%)b
TONc
1
Pd(salen)–POM
Pd(salen)
EtOH/H2O
EtOH/H2O
EtOH/H2O
DMF/H2O
94
69
—
88
607
536
—
176e
2
3
4d
K8SiW11O39
Pd(salen)
aReaction conditions: bromobenzene (78 mg, 0.5 mmol);
arylboronic acids (73 mg, 0.6 mmol); K2CO3 (138 mg, 1.0
mmol); catalyst (0.2 mol%); EtOH/H2O (v/v, 1:1; 1.0 ml); T = 50°C;
t = 5 h.
Figure 2. FT-IR spectrum of K8SiW11O39 (A), Pd(salen) (B) and Pd(salen)–
POM (C).
bIsolated yield.
cTON, moles of cross-coupling product produced per mole of
Pd(salen), the bands at 1400–1500 cmꢀ1 are assigned to
stretching vibrations of benzene rings of the salen ligand; the
bands at 1196 and 1150 cmꢀ1 are attributed to stretching vibra-
tions of C–O bonds; the bands at 2920 and 1312 cmꢀ1 are
assigned to stretching and bending vibration of C–H bonds in
methylene respectively; and the band at 740 cmꢀ1 is due to
bending vibration of C–H bonds in benzene rings.
palladium.
dResults from Borhade and Waghmode.[7b] Reaction was carried
out at 110°C, in DMF/H2O (v/v, 1:1; 1.0 ml).
eTON is calculated by ourselves based on the published reaction data.
Elemental analysis for Pd(salen)–POM: Calc. C, 30.33; H, 5.05; N,
1.83. Found: C, 30.19; H, 5.12; N, 1.79. The formula is
catalyst at 50°C for 5 h. The results are shown in Table 2. Among
the investigated solvents, a mixture of ethanol and water with 1:1
volume ratio was found to be the best one and the highest yield
of 94% was obtained (Table 2, entry 6). Base is considered to be
crucial in the Suzuki coupling reaction. Among the investigated
bases, NaOH and K2CO3 were found to be the top two best ones
under our reaction conditions and the highest yields of 96% and
94% were obtained, respectively (Table 3). In consideration of
strong corrosion by NaOH, K2CO3 was employed as base in the
following investigations. The results of investigations on effects
of the catalyst amount are summarized in Table 4. As can be
seen, the yield of cross-coupling product increased with an
increase in catalyst loading at first, but then decreased slightly
due to formation of more dehalogenation by-products. The
C
116H230N6O42Si3W11Pd. Elemental analysis for Pd(salen): Calc.
C, 51.56; H, 3.79; N, 7.52. Found: C, 51.38; H, 3.84; N, 7.44. The for-
mula is C16H14N2O2Pd.
Pd contents in Pd(salen) and Pd(salen)–POM determined by ICP
are 17.04% (1.6 × 10ꢀ3 mol gꢀ1) and 1.64% (1.5 × 10ꢀ4 mol gꢀ1
respectively.
)
Comparison of Catalytic Activities of Pd(Salen) and Pd
(Salen)–POM
The catalytic activity of the hybrid Pd(salen)–POM was investigated
by choosing the coupling of bromobenzene and phenylboronic
acid as a model reaction. The reaction was performed at 50°C in
an EtOH–H2O mixture using K2CO3 as base, with a molar ratio of
1:1.2:2 for bromobenzene, phenylboronic acid and K2CO3. For
comparison, the reactions catalyzed by Pd(salen) and POM were
also investigated under the same conditions and the results are
listed in Table 1. It is clear that the hybrid catalyst showed much
higher catalytic activity than both Pd(salen) and POM moieties. A
high yield of 94% and turnover number (TON, here defined as
moles of cross coupling product produced per mole of palladium)
of 607 was obtained on Pd(salen)–POM, while no reactions
occurred in the case of the POM and only 69% yield and 536 TON
were obtained on Pd(salen). The hybrid is also more efficient than
the reported analogous Pd–salen (entries 4), and a much higher
TON was obtained under milder reaction conditions. Thus the
hybrid compound was chosen as a catalyst to optimize the reaction
conditions in the model reaction.
Table 2. Effect of solvent on Suzuki coupling catalyzed by Pd
(salen)–POMa
Entry
Solvent
Yield (%)b
TONc
1
2
3
4
5
6
7
CH3CN
—
—
71
53
—
94
—
—
—
CH3OCH3
H2O
458
342
—
EtOH
DMF
EtOH–H2O (v/v, 1:1)
Toluene
607
—
aReaction conditions: bromobenzene (78 mg, 0.5 mmol);
phenyloboric acid (73 mg, 0.6 mmol); K2CO3 (138 mg, 1.0 mmol);
catalyst (0.2 mol%); solvent (1.0 ml); T = 50°C; t = 5 h.
bIsolated yield.
Optimization of the Reaction Conditions
cTON = moles of cross-coupling product produced per mole of
First, we investigated the effects of different solvents on the
selected model reaction in the presence of 0.2 mol% of the
palladium.
Appl. Organometal. Chem. 2014, 28, 95–100
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Jinhui Tong et al.
Table 3. Effect of base on Suzuki coupling catalyzed by Pd(salen)–
Table 5. Effect of temperature on Suzuki coupling catalyzed by Pd
(salen)–POMa
POMa
Entry
Base
Yield (%)b
TONc
Entry
Temperature (°C)
Yield (%)b
TONc
1
2
3
4
5
6
Cs2CO3
KF
57
62
18
—
96
94
368
400
116
—
1
2
3
4
5
25
35
50
60
65
13
16
280
344
Et3N
95
2047
2090
2155
NaH2PO4
NaOH
K2CO3
97
620
607
>99
aReaction conditions: bromobenzene (78 mg, 0.5 mmol); arylboronic
acids (73 mg, 0.6 mmol); K2CO3 (138 mg, 1.0 mmol); catalyst
(0.06 mol%); EtOH–H2O (v/v, 1:1; 1.0 ml); t = 5 h.
aReaction conditions: bromobenzene (73 mg, 0.5 mmol);
arylboronic acids (78 mg, 0.6 mmol); base (1.0 mmol); catalyst
(0.2 mol%); EtOH–H2O (v/v, 1:1; 1.0 ml); T = 50°C; t = 5 h.
bIsolated yield.
bIsolated yield.
cTON, moles of cross-coupling product produced per mole of
cTON, moles of cross-coupling product produced per mole of
palladium.
palladium.
Table 6. Effect of time on Suzuki coupling catalyzed by Pd(salen)–
POMa
Table 4. Effect of catalyst amount on Suzuki coupling catalyzed by
Pd(salen)–POMa
Entry
Time (h)
Yield (%)b
TONc
Entry
Catalyst (mol%)
Yield (%)b
TONc
1
0.5
1
83
86
1788
1853
1940
2047
2155
21
2
1
2
3
4
5
0.02
0.04
0.06
0.08
0.2
82
90
95
94
94
5300
2908
2047
1519
607
3
2
90
4
3
95
5
4
>99
1
6d
7e
4
4
—
—
aReaction conditions: bromobenzene (78 mg, 0.5 mmol);
arylboronic acids (73 mg, 0.6 mmol); K2CO3 (138 mg, 1.0 mmol);
EtOH–H2O (v/v, 1:1; 1.0 ml); T = 50°C; t = 5 h.
aReaction conditions: bromobenzene (78 mg, 0.5 mmol);
arylboronic acids (73 mg, 0.6 mmol); K2CO3 (138 mg, 1.0 mmol);
catalyst (0.06 mol%); EtOH–H2O (v/v, 1:1; 1.0 ml); T= 65°C.
bIsolated yield.
bIsolated yield.
cTON, moles of cross-coupling product produced per mole of
cTON = moles of cross-coupling product produced per mole of
palladium.
palladium.
dSubstrate: arylboronic acids 73 mg (0.6 mmol).
eSubstrate: bromobenzene 78 mg (0.5 mmol).
highest yield of 95% was obtained in the presence of 0.06 mol%
of the catalyst. The effects of reaction temperature and reaction
time on the model reaction were also investigated and the re-
sults are listed in Tables 5 and 6. It is clear that increase of tem-
perature or reaction time favors improvement of the yield. As a
result, the reaction can complete at 65°C in 4 h. Considering
the influences of homo-coupling of phenylboronic acid and
bromobenzene, control experiments were carried out under
the optimized conditions and the results have shown that only
1% yield was obtained in homo-coupling reaction of
phenylboronic acid, while no product was detected in that of
bromobenzene (Table 6, entries 6 and 7).
contains a nitrogen donor are typically difficult to accomplish.[1]
It is therefore noteworthy that the hybrid catalyst successfully
promoted the coupling reactions of 4-bromoaniline with
phenylboronic acid, 4-methyl- and 4-chloro-phenylboronic
acids, and up to 90% yield was obtained (entries 5–7).
Polyarylbenzenes have attracted much attention in the field of
organic optical and electronic devices.[17] A one-pot multiple
Suzuki coupling leading to such polyarylbenzenes can be more
challenging. The hybrid catalyst can effectively catalyze one-pot
two and triple coupling reactions with low loading of 0.06 mol%.
1,3,5-Triarylbenzenes were obtained with a yield of 72% in the
coupling of 1,3,5-tribromobenzene with phenylboronic acid and
4-methylphenylboronic acid at 65°C, respectively (Table 8, entries
1 and 2). This method would offer the attraction of reducing the
number of steps required to access highly aryl-substituted arenes.
Unfortunately, for coupling reactions of aryl chlorides, which are
much more difficult to activate than aryl iodides and bromides, only
low yields were obtained (Table 7, entries 24–26; Table 8, entries
3–5). Although the hybrid has shown superior catalytic activities
in coupling reaction of aryl bromides and iodides compared to
homogeneous[4a,4c,7] and heterogeneous[4b,8,9] analogues reported
under milder conditions, much more effort is needed to develop
Extension of Substrate Scope
Based on the above investigations, general applicability of the
hybrid catalyst in the coupling of various aryl halides and
arylboronic acids were investigated under the optimized condi-
tions. As summarized in Table 7, outstanding catalytic activities
were observed for the coupling reactions of aryl bromides and
iodides, including not only activated ones with electron-
withdrawing substituents (entries 1–4) but also deactivated ones
with electron-donating substituents (entries 5–23); moderate to
excellent yields of 77–99% and high TON of up to 2155 were
obtained. Coupling reactions in which one coupling partner
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Suzuki coupling reaction, heterogeneous catalysis
Table 7. Suzuki cross-couplings of various aryl halides and arylboronic acidsa
R1
HO
OH
X
B
Pd(salen)-POM
K CO ,
EtOH/H2O
2
3
+
R1
R2
R2
Entry
X
R1
R2
Time (h)
Yield (%)b
TONc
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
4-O2N
4-O2N
4-O2N
4-HOOC
4-H2N
4-H2N
4-H2N
4-MeO
4-MeO
4-MeO
4-HO
H
4
6
>99
>99
77
2155
2155
1659
2155
1940
1897
1897
2155
2004
1897
2155
2155
1767
2155
2155
2068
2155
2112
1940
2155
2155
1982
1875
431
2
4-Cl
4-Me
H
3
6
4
8
>99
90
5
H
4
6
4-Me
4-Cl
H
4
88
7
4
88
8
8
>99
93
9
4-Me
4-Cl
H
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24d
25d
26d
8
88
3
>99
>99
82
4-HO
4-Cl
4-Me
4-Cl
4-Me
H
6
4-HO
6
4-Me
4
>99
>99
96
4-Me
4
4-Me
3
H
4-Me
4-Me
H
3
>99
98
2-Me
6
2-Me
4
90
4-MeO
4-MeO
4-MeO
4-MeO
4-O2N
4-Me
4-Me
H
6
>99
>99
92
I
5
I
4-Cl
3
I
3-NO2
4-Me
4-Me
4-Me
6
87
Cl
Cl
Cl
12
12
12
20
43
926
H
36
776
aReaction conditions: aryl halide (0.5 mmol); arylboronic acids (0.6 mmol); K2CO3 (1.0 mmol); catalyst (0.06 mol%); EtOH–H2O (v/v, 1:1; 1.0 ml);
T = 65°C.
bIsolated yield.
cTON, moles of cross-coupling product produced per mole of palladium.
dT = 90°C.
Table 8. One-pot two and triple Suzuki cross-couplinga
b
Entry
Aryl halide
Arylboronic acid
Product
Time(h)
Yield (%)
TONc
1
1,3,5-Tribromobenzene
1,3,5-Tribromobenzene
1,3,5-Trichlorobenzene
1,3,5-Trichlorobenzene
1,2-Dichlorobenzene
Phenylboronic acid
p-Tolylboronic acid
Phenylboronic acid
p-Tolylboronic acid
Phenylboronic acid
1,3,5-Triphenylbenzene
24
24
36
24
24
72
72
21
12
20
1551
1551
452
2
1,3,5-Tris(4-methylphenyl)benzene
1,3,5-Triphenylbenzene
3d
4d
5e
1,3,5-Tris(4-methylphenyl)benzene
1,2-Diphenylbenzene
258
431
aReaction conditions: aryl halide (0.5 mmol); arylboronic acids (1.8 mmol); K2CO3 (3.0 mmol); catalyst (0.06 mol%); EtOH/H2O (v/v, 1:1; 2.0 ml);
T = 65°C.
bIsolated yield.
cTON, moles of cross-coupling product produced per mole of palladium.
dT = 90°C.
eArylboronic acids (1.2 mmol), K2CO3 (2.0 mmol); T = 90°C.
Appl. Organometal. Chem. 2014, 28, 95–100
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Jinhui Tong et al.
4-methyl- and 4-chloro-phenylboronic acids. In particular, the
hybrid exhibited excellent reusability and the performance of
the catalyst was almost fully retained during the reuse process.
This work is another vivid example of the synergistic effect of
metal complex–POM hybrids.
Acknowledgments
The authors are grateful to the National Natural Science Founda-
tion of China (21363021, 21174114), Scientific Research Fund of
Northwest Normal University (NWNU-LKQN-10-28) and Program
for Changjiang Scholars and Innovative Research Team in
University (IRT1177) for financial support.
References
Figure 3. Reuse of catalyst. Reaction conditions: bromobenzene (78 mg,
0.5 mmol); phenylboronic acid (73 mg, 0.6 mmol); K2CO3 (138 mg,
1.0 mmol); catalyst (1.5 mg, 0.06 mol%); EtOH/H2O (v/v, 1:1; 1.0 ml);
T = 65°C; t = 4 h.
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Reusability of the Catalyst
The reusability of the hybrid catalyst was also evaluated in the
coupling of bromobenzene and phenylboronic acid. After the
reaction, the catalyst was separated by centrifugation, washed
with ethyl acetate three times, and then subjected to the second
run under the same conditions. Palladium content in the solution
was determined using ICP–atomic emission spectroscopy. The
results indicated that on average 0.04% of palladium had leached
out from the catalyst. Furthermore, the filtered solution did not
exhibit any further reactivity. As can be seen from Fig. 3, no
decrease in yield was observed in the first four runs; however,
the yield dropped from 99% to 92% in the following two runs.
This decrease could be mainly attributed to unavoidable loss of
the catalyst during collection. The results confirm that the Pd
(salen)–POM hybrid has good stability and recyclable applicability
for the Suzuki cross-coupling reaction.
Conclusions
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a
heterogeneous catalyst for Suzuki cross-coupling reactions. Pd
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the corresponding Pd(salen) and POM moieties. High levels of
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iodides with arylboric acids, including deactivated and sterically
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hybrid also showed high catalytic activities for one-pot triple
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Supporting Information
Additional supporting information may be found in the online
version of this article at the publisher’ web-site.
wileyonlinelibrary.com/journal/aoc
Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 95–100