Imidazolium ionic liquid-tagged palladium complex: An efficient catalyst for the Heck and Suzuki reactions in aqueous media

Article abstract of DOI:10.1039/c4gc00525b

An air stable, water soluble, and efficient ionic liquid-tagged Schiff base palladium complex was prepared. The synthesized complex was well characterized by NMR, mass spectrometry, FT-IR, UV-visible spectroscopy and powder X-ray diffraction. The complex was used as a catalyst for the Suzuki and Heck cross-coupling reactions in water. Good to excellent yields were achieved using a modest amount of the catalyst. In addition, the catalyst can be easily reused and recycled for six steps without much loss in activity, exhibiting an example of sustainable and green methodology. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00525b

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Imidazolium ionic liquid-tagged palladium
complex: an efficient catalyst for the Heck and
Suzuki reactions in aqueous media†
Cite this: DOI: 10.1039/c4gc00525b
Pankaj Nehra, Bharti Khungar,* Kasiviswanadharaju Pericherla, S. C. Sivasubramanian
and Anil Kumar*
An air stable, water soluble, and efficient ionic liquid-tagged Schiff base palladium complex was prepared.
The synthesized complex was well characterized by NMR, mass spectrometry, FT-IR, UV-visible spectro-
scopy and powder X-ray diffraction. The complex was used as a catalyst for the Suzuki and Heck cross-
coupling reactions in water. Good to excellent yields were achieved using a modest amount of the cata-
lyst. In addition, the catalyst can be easily reused and recycled for six steps without much loss in activity,
exhibiting an example of sustainable and green methodology.
Received 25th March 2014,
Accepted 9th June 2014
DOI: 10.1039/c4gc00525b
www.rsc.org/greenchem
general, it has been observed that neutral catalysts are not
retained in water and are lost during product isolation. There-
Introduction
Palladium-catalyzed cross-coupling reactions have made a fore, an effective approach is to use metal complexes with
noteworthy contribution to organic synthesis as they are used charged groups to avoid catalyst leaching into the organic
to build C–C bonds.1–4 An intriguing aspect of palladium cata- layer.19
lysis is its broad application in synthesis; it has played a
Ionic liquids are salts which have melting points below the
pivotal role in the synthesis of fine chemicals and advanced boiling point of water, and they are solely composed of ions in
functional materials.5 Palladium-catalyzed C–C coupling reac- their molten state. They have been termed ‘designer solvents’,
tions without the use of any ligands have been reported, but as their properties, such as the vapor pressure, thermal stabi-
the major drawbacks associated with them are the use of toxic lity, solubility, solvating ability etc., can be tuned as needed by
solvents and the requirement for high temperature and long appropriately varying the combination of ions. Chemists have
reaction time.6–8 There has always been a demand for the initially focused on using them as alternative solvents to vola-
development of a robust, efficient and cost effective catalyst tile organic solvents in various organic transformations.20–22
that can circumvent the limitations of the existing catalysts.9–11 Presently, ionic liquid chemistry is on the cutting edge of the
Moreover, environmental concerns and economic consider- development of sustainable processes, and has gained wide
ations also make it essential to develop catalytic systems that recognition due to the use of ionic liquids in a diverse range of
can be recovered and recycled, especially when noxious tran- applications.23,24 The concept of functionalized ionic liquids
sition metals are involved.12–14
(FILs), through the incorporation of additional functional
From the standpoint of green chemistry, water is known to groups as part of the cation and/or anion, has recently gained
be a potential replacement for organic solvents.15–17 The major remarkable interest.25 FILs that are obtained by incorporating
advantages associated with water are its low cost, non-flamm- a coordination center serve a dual purpose as an immobiliz-
ability and low toxicity, and there is no need to desiccate sub- ation solvent and a ligand to the catalyst.26 Many ligands teth-
strates prior to the reaction.18 Combining the use of water as a ered to imidazolium-based ionic liquids have been reported,
solvent with hydrophilic metal catalysts results in easy separ- however, these are not room temperature ionic liquids, and
ation and recycling of the catalyst from the product. In needed to be dissolved in another ionic liquid for catalytic
applications (Fig. 1).27–29 Wei et al. reported SiO2-supported
Department of Chemistry, Birla Institute of Technology and Science, Pilani, Pilani
Campus, Rajasthan, India. E-mail: bkhungar@pilani.bits-pilani.ac.in,
anilkumar@pilani.bits-pilani.ac.in; Fax: +91 5196 244183; Tel: +91 5196 5155652
†Electronic supplementary information (ESI) available: IR, UV-vis, powder XRD,
1H and 13C NMR, and mass spectrometry data for the ionic liquid-supported
Schiff base (4) and its Pd complex (5), and 1H and 13C NMR data for 8 & 10. See
DOI: 10.1039/c4gc00525b
Fig. 1 Ligands tethered to the imidazolium-based ionic liquid.
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imidazolium ionic liquid-immobilized PdEDTA as an efficient
and reusable catalyst for the Suzuki reaction in water.30 The
catalytic system is solid and provides a semi-homogeneous
environment for the reaction. The advantage of these types of
catalysts is that they are strongly retained during product
extraction.
Metal complexes of Schiff bases are the subject of detailed
investigation because of their wide applications in the fields of
synthesis and catalysis.31 As there has been extensive research
focused on the modification of Schiff base ligands, and as an
extension to our research on the development of ionic liquid-
tagged Schiff bases,32 we became interested in the synthesis of
water soluble ionic liquid-tagged metal complexes, and their
use as catalysts for organic transformations. Our efforts have
aimed to establish methodology for C–C coupling reactions
using an efficient and reusable catalyst in water and air,
making the process green and economically viable. Herein, we
report the synthesis and characterization of a novel imid-
azolium ionic liquid-tagged Schiff base complex of palladium,
and its catalytic application in the Heck and Suzuki reactions
in aqueous media. In comparison to other reported palladium
catalyzed reactions in aqueous media,33,34 the catalyst works
well under these conditions for water insoluble aryl halides in
both the Heck and Suzuki reactions, without the aid of a
phase transfer catalyst or organic solvents.
Scheme 2 Synthesis of the ionic liquid-tagged palladium complex.
After successful synthesis of the imidazolium ionic liquid-
tagged Schiff base 4, it was allowed to react with palladium
acetate in ethanol for 4 h to give the desired imidazolium
ionic liquid-tagged palladium complex
5 in 80% yield
(Scheme 2). The structure of palladium complex 5 was estab-
lished by spectroscopic and mass spectrometry analysis. In the
FTIR spectrum of 5, the azomethine peak was shifted towards
a lower frequency than in 4, to 1596 cm−1, and the peak in the
region of 3400 cm−1 (OH stretch) in the spectrum of 4 dis-
appeared, which indicated that the ligand had coordinated with
the metal ion through the nitrogen of the CvN group.35 The
1H and 13C NMR spectra of 5 were consistent with the struc-
ture of the complex. In the 1H NMR spectrum of 5, the shift of
the peak for the azomethine proton to δ 7.93 (Fig. 6, ESI†) and
the absence of a peak for the hydroxyl proton suggested that
complete complexation with the metal had occurred through
the nitrogen of the CvN group and oxygen of the OH group.
The MALDI mass spectrum of 5 showed a peak at m/z 777.32
that corresponds to the [M + H − 2Br]+ ion (Fig. 8, ESI†).
The electronic spectra of 4 and the complex 5 were recorded
using distilled DMSO as solvent. The free ligand 4 exhibited
two absorption bands at 280 nm and 321 nm, which can be
attributed to the π–π* and n–π* transitions of the ligand. The
shift of these bands towards shorter wavelengths (264 and
304 nm, respectively), and the appearance of a new band at
395 nm in the absorption spectrum of 5 suggested complexa-
tion of 4 with palladium. The powder XRD pattern of 4 showed
sharp peaks, revealing its crystalline nature (Fig. 4, ESI†). On
complexation with the metal, the intensity of the peaks in the
XRD patterns diminished and line broadening was observed,
indicating a change in nature from a crystalline to amorphous
state.36 The powder XRD pattern of the complex was also
recorded after heating it at 200 °C for 4 h, and it was found
that there was no significant change, indicating the high
thermal stability of 5.
Results and discussion
Synthesis of the imidazolium ionic liquid-tagged Schiff base
(4) was achieved, as depicted in Scheme 1. Selective O-alkyl-
ation of 2,4-dihydroxybenzaldehyde (1) with 1,3-dibromo-
propane gave 4-(3-bromopropoxy)-2-hydroxybenzaldehyde (2),
which on further reaction with 1-methylimidazole resulted in
the imidazolium ionic liquid-tagged aldehyde 3 in almost
quantitative yield. Condensation of 3 with aniline in ethanol
gave the imidazolium ionic liquid-tagged Schiff base 4 in 68%
yield.32 The structure of 4 was confirmed by IR, NMR and
mass spectrometry data. The FT-IR spectrum of 4 exhibited
absorption bands at 1620 cm−1 (CvN stretch) and 3441 cm−1
1
(OH stretch). In the H NMR spectrum of 4, a peak represent-
ing the azomethine proton (CHvN) appeared at δ 8.79, and
the hydroxyl proton resonated at δ 13.77. Peaks corresponding
to other protons were also shown. A peak at m/z 336.3 in the
ESI-MS spectrum of 4 corresponding to the [M − Br]+ ion
further confirmed the structure of 4.
We next focused our attention on exploring the catalytic
applications of 5 for carbon–carbon bond forming (Heck and
Suzuki) reactions in aqueous media. Initially, the reaction con-
ditions for the Heck reaction were optimized by performing a
model reaction between iodobenzene (6a) and benzyl acrylate
(7a). The results of the different reaction conditions are sum-
marized in Table 1. It was found that carrying out the reaction
at 80 °C with 1 mol% of 5 in the presence of K2CO3 gave
benzyl cinnamate (8aa′) in 96% yield (Table 1, entry 6). The
use of an organic base, such as triethylamine, resulted in a
Scheme 1 Synthesis of the ionic liquid-tagged Schiff base.
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Table 1 Optimization of the reaction conditions for the Heck reaction
catalyzed by 5a
Table 2 Heck reaction between aryl halides and alkenes catalyzed by 5
in watera
Time Yieldb
S. no.
X
I
Ar
R
Product (h)
(%)
1
2
3
4
5
6
7
8
C6H5
COOCH2C6H5 8aa′
COOCH2C6H5 8aa′
COOCH2C6H5 8aa′
COOCH3
C6H5
C6H5
C6H5
4-CH3C6H5
4-ClC6H5
4-BrC6H5
COOCH2C6H5 8ba′
COOCH2C6H5 8ca′
C6H5
4-CH3C6H5
4-ClC6H5
4.0
5.5
6.0
3.5
5.5
6.0
6.5
4.5
4.5
4.5
4.5
4.0
4.5
5.0
5.0
5.0
6.0
96
86
82
96
80
79
76
82
81
83
80
98
88
81
78
86
82
Br C6H5
Cl C6H5
I
I
Br C6H5
Cl C6H5
I
I
I
I
I
I
I
I
I
I
Entry
5 (mol%)
Base
Yieldb (%)
C6H5
C6H5
8ab′
8ac′
8ac′
8ac′
8ad′
8ae′
8af′
1
2
3
4
5
6
7
8
9
1.0
1.0
1.0
1.0
3.0
1.0
0.2
0.1
0.01
Et3N
26
74
84
88
98
96
88
87
86
NaOH
NaHCO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
C6H5
C6H5
C6H5
9
10
11
12
13
14
15
16
17
4-CH3C6H4
4-NO2C6H4
4-NO2C6H4
4-NO2C6H4
4-NO2C6H4
4-OCH3C6H4 COOCH2C6H5 8da′
2-C4H3S COOCH2C6H5 8ea′
8cc′
8cd′
8ce′
a Reaction conditions: 6a (1 mmol), 7a (2 mmol), 5 (x mol%), base
(2 mmol), water (2 mL), 4 h. b Isolated yield.
a Reaction conditions: aryl halide (1.0 mmol), alkene (2.0 mmol), 5
(1 mol%), K2CO3 (2.0 mmol), deionized water (2 mL). b Isolated yield.
Table 3 Optimization of the reaction conditions for the Suzuki reaction
catalyzed by 5a
Scheme 3 Heck reaction catalyzed by 5 in aqueous medium.
Entry
5 (mol%)
Base
Yieldb (%)
1
2
3
4
5
6
7
8
9
0.1
0.1
0.1
0.1
0.1
0.5
1
Et3N
69
68
71
76
80
80
81
82
80
poor yield of 8aa′, which may be attributed to the poor solubi-
lity of triethylamine in aqueous media.
NaOH
NaHCO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
With the optimized reaction conditions in hand, the scope
of the Heck reaction was investigated by employing substituted
aryl halides 6a–g to react with substituted olefins 7a′–f′
(Scheme 3). Aryl iodides with electron-donating methyl and
methoxy groups or an electron-withdrawing nitro group both
reacted well under these conditions to give the desired
coupled products in excellent yields (Table 2, entries 11–16).
Similarly, different substituted alkenes also reacted smoothly
to afford the corresponding coupled products in excellent
yields. It is worth mentioning that the presence of electron-
3
0.01
a Reaction conditions: 6a (1.0 mmol), 9a′ (1.2 mmol), 5 (x mol%), base
(2.0 mmol), deionized water (2 mL), 30 min. b Isolated yield.
Encouraged by the excellent results obtained by using 5 as
withdrawing groups on both substrates resulted in better a catalyst for the Heck reaction in aqueous media, we set out
yields compared to substrates bearing electron-donating to optimize the reaction conditions for the Suzuki reaction in
groups. The more demanding substrates, viz. aryl bromides aqueous media. As the Suzuki reaction is largely affected by
and aryl chlorides, were also allowed to react with styrene and the amount of catalyst and the type of base used, we optimized
benzyl acrylate under these conditions (Table 2, entries 2–3 the reaction conditions using the reaction between iodo-
and 6–7). To our delight, heteroaryl iodides, such as benzene (6a) and phenylboronic acid (9a′) to give biphenyl (10aa′)
2-iodothiophene (6e), also underwent Heck coupling with 7a′ as a model reaction. The results from various experiments are
to give the corresponding coupled product 8ea′ in 82% yield summarized in Table 3. It was found that using 0.1 mol% of 5
(Table 2, entry 17). The catalyst was found to be very effective in the presence of K2CO3 as the base were the most suitable
for various substrates, including heteroaromatic halides, and conditions for an efficient conversion at room temperature
gave the coupled products in good to excellent yields (76–98%) with an excellent yield of 10aa′ (Table 3, entry 5). The yield of
in a short reaction time. The identities of the products were 10aa′ was not affected much by increasing the concentration
1
confirmed by their melting points and H and 13C NMR data, of the catalyst up to 3 mol% (Table 3, entries 5–8). Although
which were found to be consistent with reported values (ESI†).
10aa′ was obtained in excellent yield (80%) using 0.01 mol% of
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Table 5 Reusability of 5 for the Heck reaction and the Suzuki reaction
Run
1
2
3
4
5
6
% Yielda of 8aa′
% Yielda of 10aa′
96
80
95
78
93
77
91
76
91
74
90
72
a Isolated yield.
Scheme 4 Suzuki reaction catalyzed by 5 in aqueous medium.
be one of the factors responsible for the gradual decrease in
the yield of the product during recycling.
Table 4 Suzuki reaction between aryl halides (6) and arylboronic acids
(9) catalyzed by 5 in watera
Conclusions
Time Yieldb
In summary, we have synthesized and systematically character-
ized a novel imidazolium ionic liquid-tagged palladium Schiff
base complex, and explored its catalytic activity for the Heck
and Suzuki reactions in aqueous media. The complex showed
high activity and excellent yields for these reactions. The cata-
lyst is moisture insensitive and highly stable under thermal
and aerobic conditions. The reaction conditions for the ionic
liquid-tagged palladium catalyzed coupling reactions are
simple and effective for more challenging substrates, such as
chlorides. The use of an aqueous medium and the recyclability
of the catalyst are the advantages that make this method econ-
omical and environmentally friendly.
S. no.
X
I
Ar
R
Product (min) (%)
1
2
3
4
5
6
7
8
C6H5
H
H
H
10aa′
10aa′
10aa′
30
50
55
50
55
55
30
40
50
35
55
60
40
45
30
80
76
71
79
73
70
75
89
65
89
74
70
76
74
82
Br C6H5
Cl C6H5
I
Br C6H5
Cl C6H5
C6H5
3,4,5-(OCH3)3 10ab′
3,4,5-(OCH3)3 10ab′
3,4,5-(OCH3)3 10ab′
I
I
I
I
4-CH3C6H4
H
H
H
H
H
H
H
H
H
10ba′
10ca′
10ha′
10da′
10da′
10ia′
10ja′
10ja′
10ea′
4-NO2C6H4
2-CH3C6H4
4-OCH3C6H4
9
10
11
12
13
14
15
Br 4-OCH3C6H4
I
Br
2-NH2C6H4
10H7
C
Cl C10H7
2-C4H3S
I
a Reaction conditions: arylhalide (1.0 mmol), arylboronic acid
(1.2 mmol), 5 (0.1 mol%), K2CO3 (2.0 mmol), water (2 mL). b Isolated
yield.
Experimental
Materials and methods
All of the reagents were purchased from Sigma-Aldrich, India,
and Spectrochem Pvt. Ltd, India, and were used without
additional purification. The solvents used were purchased
from Merck (India) and were distilled and dried before use.
Melting points were determined in open capillary tubes on a
MPA120 automated melting point apparatus and are un-
corrected. IR spectra were recorded on an ABB Bomen MB 3000
FTIR spectrophotometer using KBr pellets, and the electronic
spectra were recorded on a Shimadzu UV-1800 UV-Vis spectro-
photometer. 1H and 13C NMR spectra were recorded on Bruker
Heaven 11400 (400 MHz) and Varian (500 MHz) spectrometers
using CDCl3 and DMSO-d6 as solvents, and the chemical shifts
were expressed in ppm. Mass spectra were recorded on an AB
SCIEX TOF/TOF 5800 spectrometer. Powder X-ray diffraction
(XRD) data were obtained on a Rigaku Miniflex II diffract-
ometer with Cu Kα radiation, and diffraction patterns were
recorded over a range of 2θ angles from 10 to 50°. Synthesis of
the ionic liquid-supported Schiff base 4 was achieved from 2,4-
dihydroxybenzaldehyde by following our earlier reported
method.32
5 in the presence of K2CO3 (Table 3, entry 9), it was not poss-
ible to recycle the catalyst after two cycles under these con-
ditions, and thus we determined that the optimum loading for
the catalyst was 0.1 mol% for the Suzuki reaction.
The scope of the Suzuki reaction catalyzed by 5 in aqueous
medium was investigated by employing various aryl halides
and arylboronic acids (Scheme 4). The results are summarized
in Table 4. Reaction of substituted iodobenzenes (6a–e, 6h–j)
with arylboronic acids gave the corresponding coupled pro-
ducts in good to excellent yields (65–89%). Reaction of bromo-
benzene (6f) and chlorobenzene (6g) with arylboronic acids
also afforded good yields of the coupled products (70–76%)
(Table 4, entries 2–3, 5–6, 11 and 13–14). However, the times
required for the completion of the reactions for bromobenzene
(6f) and chlorobenzene (6g) were longer than that for iodo-
benzene (6a). The method is equally applicable for aryl iodides
substituted with either electron-withdrawing or electron-donat-
ing groups. For example, 4-nitro-, 4-methyl-, 4-methoxy- and
2-aminoiodobenzenes were smoothly converted to the corres-
ponding coupled products in high yields.
Synthesis of 4-(3-bromopropoxy)-2-hydroxybenzaldehyde (2).
The reusability of 5 was evaluated for both the model Heck
and Suzuki reactions. As shown in Table 5, the catalyst could
be effectively used for up to six cycles without much loss in
catalytic activity. Leaching of the catalyst during extraction may
A
round bottom flask containing 1,3-dibromopropane
(8.0 mmol), 2,4-dihydroxybenzaldehyde (6.0 mmol) and
sodium bicarbonate (6.0 mmol) in acetone (50 mL) was heated
at 60 °C for 60 h. After the reaction had gone to completion,
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the reaction mixture was purified over a silica gel column to Reusability and recovery of the catalyst
give pure 2 in 65% yield.
After the first run of the reaction was completed, the product
was directly extracted into the organic layer, and the catalyst
which remained in the aqueous layer was reused for the next
cycle of the reaction, following the same procedures as men-
tioned above for the Heck and Suzuki reactions.
Synthesis of the ionic liquid-tagged aldehyde (3). A mixture
of 1-methylimidazole (3.5 mmol) and 2 (3.5 mol) was stirred at
80 °C for 48 h to give a viscous liquid. The reaction mixture
was washed with a diethyl ether–ethyl acetate mixture to give
the ionic liquid-tagged aldehyde 3 in almost quantitative yield.
Synthesis of the ionic liquid-tagged Schiff base (4).
A mixture containing the ionic liquid-tagged aldehyde 3
(3.0 mmol) and aniline (4.0 mmol) in ethanol (15 mL) was
refluxed for 4 h. On completion of the reaction, ethanol
(30 mL) was added to the reaction mixture; the solid product
which formed was filtered off and washed with cold ethanol.
The crude product was purified by recrystallization from
ethanol–ethyl acetate (3 : 1 v/v).
Synthesis of the imidazolium ionic liquid-tagged palladium
complex (5). The ionic liquid-tagged Schiff base 4 (672 mg,
2.0 mmol) was stirred with ethanol in a 25 mL round bottom
flask for 15 min. Palladium acetate (224 mg, 1.0 mmol) was
added to the resulting solution, and the mixture was refluxed
for 4 h until the product had completely precipitated. After
cooling, the product 5 was separated by filtration and recrystal-
lized from a mixture of petroleum ether (2 mL) and methanol
(15 mL).
Acknowledgements
We gratefully acknowledge the DST, New Delhi (SR/FT/CS-040/
2008) for financial support. We also acknowledge financial
support from DST-FIST and UGC-SAP for instrumental facili-
ties. We thank the Advanced Instrumentation Research Facility
(AIRF), JNU, New Delhi for spectral analysis. PN thanks BITS
Pilani and UGC BSR for the research fellowship.
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General procedure for the Heck reaction in water
A 50 mL round bottom flask containing a mixture of aryl
halide (1.0 mmol), alkene (2.0 mmol), K2CO3 (2.0 mmol) and 5
(1.0 mol%) in water (2 mL) was heated to a temperature of
80 °C in an oil bath. The progress of the reaction was moni-
tored using TLC at regular time intervals. After completion of
the reaction, the reaction mixture was added to water (20 mL)
and extracted with ethyl acetate (3 × 10 mL). The organic layer
was dried with anhydrous Na2SO4, and concentrated to give
the crude product which was purified by column chromato-
graphy over silica gel (mesh 60–120), using n-hexane–ethyl
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