Stabilized well-dispersed Pd(0) nanoparticles for aminocarbonylation of aryl halides

Article abstract of DOI:10.1039/c1dt10927h

Well-dispersed palladium (0) nanoparticles stabilized with phosphonium based ionic liquid were synthesized conveniently and fully characterized. A catalyst system comprising of the Pd(0) nanoparticles and a base was found to be recyclable and efficient for the aminocarbonylation reaction of aryl iodide in ionic liquid media. In the presence of potassium tert-butyloxide, for the relatively stable aryl chloride and bromide substrates, medium activities were achieved for the catalyst. The catalyst composites can be recycled at least five times with sustained activity.

Full text of DOI:10.1039/c1dt10927h

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PAPER
Stabilized well-dispersed Pd(0) nanoparticles for aminocarbonylation of aryl
halides†
Yinghuai Zhu,* Li Chuanzhao, Algin Oh Biying, Meriska Sudarmadji, Anqi Chen, Dang Thanh Tuan and
Abdul M Seayad
Received 17th May 2011, Accepted 2nd July 2011
DOI: 10.1039/c1dt10927h
Well-dispersed palladium (0) nanoparticles stabilized with phosphonium based ionic liquid were
synthesized conveniently and fully characterized. A catalyst system comprising of the Pd(0)
nanoparticles and a base was found to be recyclable and efficient for the aminocarbonylation reaction
of aryl iodide in ionic liquid media. In the presence of potassium tert-butyloxide, for the relatively stable
aryl chloride and bromide substrates, medium activities were achieved for the catalyst. The catalyst
composites can be recycled at least five times with sustained activity.
On the other hand, the use of metal nanoparticle based catalysts,
which are either immobilized on solid supports or suspended
Direct coupling reactions between an amine and a carboxylic acid
in colloid systems, has attracted increasing attention both in
Introduction
academia and industry.39–47 It has been found that catalyst systems
comprising of an ionic liquid and metal nanoparticles can be used
conveniently in organic synthesis and easily recovered.43,44 Since
ionic liquids combine the advantages of a solid for supporting
the catalyst, and the advantages of liquid reaction media for
allowing the catalyst to move freely, various catalytic organic
reactions have been studied in ionic liquids.48 Inspired by all
the attractive preliminary results and in an attempt to achieve
an efficient, economical and green aminocarbonylation reaction,
we have studied the potential of using ionic liquid stabilized
palladium(0) nanoparticles as a catalyst for aminocarbonhylation
reactions. The results obtained show that the new nano-Pd(0)
catalyst is both efficient and recyclable.
or its derivative is one of the most commonly used methods
to prepare amides.1–4 However, in general, a large amount of
coupling reagent is used to conduct the reactions in the process.3,4
Further, either highly hazardous halogenating or dehydration
reagents are needed to synthesize the substrates such as carboxylic
acid derivatives.3,4 Therefore, the processes are not considered
as economical and green. It is necessary to develop an atom-
efficient and environmental benign method to meet the sustain-
able requirements. During the investigation of novel synthetic
techniques, palladium complexes catalyzed aminocarbonylation
of aryl halides to form an amide was discovered, as shown in eqn
(1).5–31 Besides the extensively used gaseous CO, alternatives such
as W(CO)6 and BH3·CO have also been examined as potential
CO sources, giving reasonable yields.32–38 However, as with other
homogeneous catalytic processes, the recovery and recycling of the
palladium complexes based catalyst composites from the product
mixture remains a big challenge that will hinder their further
applications on an industrial scale. To avoid the contamination
of heavy metals in the products and reuse of the high-cost
ligands, it is necessary to address the recycling and sustainability
problems. Therefore, palladium complex based homogeneous
catalysts have been heterogenized by immobilization on various
solid supports.5–7,31 An impressive example is the direct use of
commercially available Pd/C as a catalyst for aminocarbonylation
reactions with assistance of microwave irradiation, in which a yield
of 95% was reached.31
(1)
Results and discussion
Syntheses and characterization of catalyst and ligand
Palladium nanoparticles were prepared by slow reduction of
bis(benzonitrile)dichloropalladium(II) with ethylene glycol at
135 ◦C under an argon atmosphere. The nanoparticles were
then extracted with ionic liquid tri-n-butyltetradecylphosphonium
dodecylbenzenesulfonate ([TBTdP][DBS]), followed by washing
with diethyl ether and oxygen-free deionized water in sequence to
remove all by-products and impurities. After drying in the vacuum,
the composite could be used for aminocarbonylation reactions.
The synthesized Pd(0) nanoparticles were analyzed by TEM
Institute of Chemical and Engineering Sciences, No 1 Pesek Road, Ju-
rong Island, Singapore, 627833. E-mail: zhu_yinghuai@ices.a-star.edu.sg;
Tel: 6567963801
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1dt10927h
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Table 1 Effects of bases on the aminocarbonylation between iodobenzene and aniline a,b
Base
K2CO3
Na3PO4
64
Pyridine
57
Et3N
51
NaOH
33
Yield (%)
77 (75b,c
)
a Reaction conditions: 1.0 mmol of aryl iodide, 1.05 mmol of aniline, 3.0 equiv. of base, 1 mol% Pd, ionic liquid [TBTdP][DBS] 10 mL, PCO = 180 psi,
reacted at 130 ◦C for 5 h. b Isolated yield (%), average of 2 runs. c In 10-fold scale.
and XPS. The TEM image in Fig. 1 shows uniform crystalline
nanoparticles with an average particle size of ~4.2 nm (Fig.
1B, determined from the measurement of ~130 particles). These
results are comparable with reported results obtained from other
methods.49 Samples for XPS analysis were prepared according
to the literature.43–45 The XPS analysis in Fig. 2 shows typical
base. Since phosphonium based ionic liquids were considered to
have the advantages of enhanced chemical stability, thermal sta-
bility, and compatibility to strong alkaline solutes, [TBTdP][DBS]
was used as the reaction media.51 For comparison, imidazolium
based ionic liquid, [HMIM][CF3SO3], was also used as the reaction
media. The aminocarbonylation between aniline and iodobenzene
was selected as a standard reaction to investigate the base and
temperature effects. The reactions were conducted at 130 ◦C
with a CO pressure of 180 psi. The conditions are similar to
literature procedures, without using microwave irradiation.31 The
commercially available bases, including K2CO3, Na3PO4, pyridine,
Et3N and NaOH, were examined. The yields ranged from 33% for
NaOH to 77% obtained from K2CO3 (see Table 1). We attribute
the result to the fact that a strong base may cause decomposition
of the reactants and/or products. Middle yields of 57% and
51% have been achieved from using an organic base, pyridine
and Et3N respectively. The most advantageous combination of
[TBTdP][DBS] stabilized nano-Pd(0) and K2CO3 were then used
to perform aminocarbonylation reactions of other aryl halides.
In addition, to check the temperature effects, the same reaction
was also performed at 45 ◦C, and a yield of 13% was achieved
based on 1HNMR analysis. The result is consistent with literature
and further confirms that high temperatures favor this type of
reaction.13 After the reaction, the product mixture was extracted
with ethyl acetate–hexane and subjected to purifying with col-
umn chromatography to produce the corresponding amides. The
Pd(0) absorptions at 340.8 eV and 335.5 eV for 3d3/2 and 3d5/2
,
respectively. The results are consistent with literature for Pd(0).50
The [TBTdP][DBS] stabilized Pd(0) nanoparticles can be kept for
more than four weeks under an argon atmosphere without causing
observable changes in particle size (see ESI†).
Fig. 1 TEM picture (A) and size histogram of Pd(0) nanoparticles (B).
1
products were analyzed by elemental analysis, H and 13C NMR
spectroscopy (see ESI†). The catalytic results are summarized in
Table 2. As can be seen from Table 2, both electron-rich and
electron-poor aryl iodides were successfully transformed into the
corresponding amides in moderate to high yields. Yields of the
aminocarbonylation reaction varied depending on both amine and
aryl iodide substrates. On the other hand, globally, the catalytic
activity in phosphonium based ionic liquid, [TBTdP][DBS], was
relatively high compared to in the imidazolium based ionic liquid,
[HMIM][CF3SO3]. In our conditions, both primary and secondary
amines performed well with aryl iodides. It should be pointed
out that the nano-Pd(0) catalyst could be easily recovered by
extracting products from the reaction mixture with a mixed solvent
of ethyl acetate–hexane (v/v = 1 : 1) (Fig. 2 B). The recovered
catalyst could be reused at least five times, giving consistent
isolated yields of 76%, 74%, 76%, 75% and 77%, respectively
for the aminocarbonylation of aryl iodide with aniline and CO
gas. The recycled catalyst demonstrated sustained activity. After
5 runs, the nano-Pd(0) catalyst was re-analyzed with TEM and
Fig. 2 XPS spectrum (A) and reaction mixture (B, product extraction
(left), reaction phase (right)).
Evaluation of catalyst activity
For the aminocarbonylation of aryl halides, the ionic liquid
[TBTdP][DBS] stabilized Pd(0) nanoparticles were used as the
catalyst in 1 mol% ratio of [Pd] to aryl halide in the presence of a
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Table 2 IL-stabilized nano-Pd(0) catalyzed aminocarbonylation of ArIa,b
R1
H
p-MeO
p-NO2
Amine
74A, 65Bc,
60A, 47B
80A,85B
67A,55B
86A,79B
74A,66B
83A,71B
71A,79B
89A,65B
68A,63B
77A(13d), 72B
65A,74B
a Reaction conditions: 1.0 mmol of aryl iodide, 1.05 mmol of amine, 3.0 equiv. of K2CO3 (anhydrous powder), 1 mol% Pd, ionic liquid 10 mL, PCO = 180 psi,
reacted at 130 ◦C for 5 h. b Isolated yield (%), average of 2 runs. c A, yield from media [TBTdP][DBS], B yield from media [HMIM][CF3SO3]. d Reaction
was run at 45 ◦C.
Table 3 IL-stabilized nano-Pd(0) catalyzed diaminocarbonylation of iodobenzenea,b
Diamine
ArX
72
68
59
76
a Reaction conditions: 2.0 mmol of aryl iodide, 1.05 mmol of diamine, 6.0 equiv. of K2CO3 (anhydrous powder), 1 mol% Pd, ionic liquid [TBTdP][DBS]
10 mL, PCO = 180 psi, reacted at 130 ◦C for 5 h. b Isolated yield (%), average of 2 runs.
XPS. The analysis has demonstrated that both particle size and
oxidation state of nano-Pd(0) has essentially remained unchanged
(see ESI†). After the reaction procedure, the concentrations of
nano-Pd(0) leached to the aqueous solution obtained from the
washing of catalyst composite with oxygen-free deionized water
to remove excess K2CO3 were analyzed by ICP and found to
be less than 2.0 ppm. This reaction was also conducted in 10-
fold scale and an isolated yield of 75% was achieved (Table 1).
Under similar conditions, reactions of two equivalent aryl
iodides with CO and diamines such as o-phenylenediamine
and racemic diaminocyclohexane, etc., produced the cor-
responding diamides, as summarized in Table 3. Medium
to good yields were achieved for the diaminocarbonylation
reactions.
The ionic liquid stabilized nano-Pd(0) catalyst was also found
to be active for the aminocarbonylation of both aryl chloride and
bromide to generate the corresponding amides, as summarized
in Table 4. For the inert halides, a strong base, potassi◦um tert-
butoxide (KOBut), and a relatively high temperature (150 C) were
employed to enhance the catalyst activity. The base-compatible
ionic liquid, [TBTdP][DBS], was once again used as reaction
media. It can be seen from Table 4 that in the presence of KOBut,
a medium to good yield was achieved for the aryl halide and
bromide, respectively. In addition, aryl halides could undergo
intramolecular aminocarbonylation to form a cyclic amide, as
shown in eqn (2). With the ionic liquid stabilized nano-Pd(0)
catalyst, yields of 37% and 9% were achieved for bromide and
chloride substrates, respectively.
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Table 4 IL-stabilized nano-Pd(0) catalyzed aminocarbonylation of ArCl(Br)a,b
Amine
ArX
47
77
31
59
39
68
25
50
a Reaction conditions: 1.0 mmol of aryl halide, 1.05 mmol of amine, 3.0 equiv. of KOBut, 1 mol% Pd, ionic liquid [TBTdP][DBS] 10 mL, PCO = 180 psi,
reacted at 150 ◦C for 5 h. b Isolated yield (%).
the ionic liquid stabilized nano-Pd(0) is the true catalyst.44 In
the catalyst cycle, oxidative addition of an aryl halide to nano-
Pd(0) to form an Ar[nano-Pd(II)]X intermediate, CO insertion
and reductive elimination to form products have been well recog-
nized for palladium-catalyzed carbonylation reactions.10,18,24,28,36 In
(2)
addition, in IR spectroscopy, a new peak of nCO at 1904 cm-1 ap-
peared for the [TBTdP][DBS] stabilized palladium nanoparticles
compared with the CO absorption (n = 2135 cm-1, broad) in ionic
liquid [TBTdP][DBS] (see Fig. 3). The result is consistent with
the CO absorption on a nano-Pd(0) film and further confirms the
formation of CO–nano-Pd bond.52 Combining all the information,
it is reasonable to propose the mechanism in Scheme 1. A number
of factors, including substrate, temperature and CO pressure
may influence the reaction rates. Nevertheless, in our system the
formation of an acylpalladium intermediate (ArCO-[nano-Pd(II)])
could be the key step due to the poor transport of CO into the
reaction phase.
Mechanism study
Although the real mechanism is not clear at this stage, in agreement
with previous studies on the catalytic aminocarbonylation reac-
tions and our current observations,11,21,27,36 the catalytic mechanism
as shown in Scheme 1 can be proposed. In order to exclude the
possibility of residual homogeneous palladium catalyst initiating
the aminocarbonylation reaction, mercury poisoning studies were
conducted. In the presence of mercury, no products could be
obtained under the same conditions. This result confirmed that
Fig. 3 IR spectrum of CO adsorbed on [TBTdP][DBS] stabilized nano-Pd
(below, ionic liquid and CO absorption were subtracted) and the dissolved
CO in the ionic liquid only (above, ionic liquid was subtracted).
Scheme 1 Proposed mechanism for nano-Pd catalyzed aminocarbo-
nylation.
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amine, 3.0 equiv. of base, 0.01 mmol Pd nanoparticles and 10 mL
of [TBTdP][DBS]. For diaminocarbonylation reactions, 2.0 mmol
aryl iodide and 6.0 equiv. of base were used. The reaction was
Conclusion
In summary, ionic liquid stabilized, well-dispersed palladium
nanoparticles have been synthesized and used as the catalyst to
perform aminocarbonylation reactions for the first time. Under
our conditions, the catalyst was found to be active for aryl
chloride, bromide and iodide substrates. The catalyst could be
recovered conveniently and reused. Studies on the effects of
catalyst components and other conditions are ongoing in our
laboratory. The catalyst may provide an alternative to the existing
palladium complex based homogeneous catalytic process. We
are expecting their wide applications in organic synthesis and
pharmaceutical industry.
◦
◦
performed at a temperature of 130 C (150 C for aryl chloride
and bromide) for 5 h under a CO pressure of 180 psi. The reaction
was also run at 45 ◦C. After aminocarbonylation, the reaction
residue was extracted with a solvent mixture of hexane–ethyl
acetate (2 ¥ 30 mL). For catalyst recycle runs, after extracting
the crude product, the catalyst mixture was washed with oxygen-
free deionized water (2 ¥ 10 mL) to removed excess K2CO3 and
◦
other inorganic salts followed by drying in vacuum at 40 C for
15 h before being used for the next runs. The solvent was removed
from the extract solution under reduced pressure with a rotary
evaporator. The resulting crude products were purified with flash
column chromatography (SiO2, eluted with solvent mixture of
hexane and ethyl acetate in a ratio of 1 ~ 15 : 1) to obtain the
amide products. All products are known and were analyzed by 1H
and 13CNMR spectroscopy (see ESI†). Results are summarized in
Table 2.
Experimental section
General
All operations and experiments were performed under an argon
atmosphere using a glove box or standard Schlenk techniques.
Solvents were dried with standard methods and distilled prior to
use.53 All aryl iodides and amines were predried, recrystallized
or distilled before use. Transmission electron microscopy (TEM)
measurements were carried out on a JEOL Tecnai-G2, FEI
analyzer at 200Kv. ICP analysis was determined using a VISTA-
MPX, CCD Simultaneous ICP-OES analyzer. X-Ray photoelec-
tron spectrometer (XPS) was performed with an ESCALAB 250
Mercury poisoning tests. We designed and conducted two
experiments to investigate the effects of Hg on the precatalyst and
the preformed nano-Pd(0) catalysts for the aminocarbonylation
between aniline and iodobenzene. First, after the formation
of nano-Pd(0), a large excess (350 equiv.) of Hg was added
to the ionic liquid solution.54 After stirring for 2 h at room
temperature under argon, the mixture containing Hg was used
in the aminocarbonylation reaction. After carrying out the same
operation to collect the product as described above, no isolable
products were obtained. In the second experiment, excess Hg was
added to an active catalyst solution. In the experiment, after a
2 h reaction process, the reaction was interrupted by cooling the
mixture to room temperature. The CO pressure was then released
and 350 equivalents of Hg under a CO atmosphere was carefully
adding. After stirring for about 2 h at room temperature, the
mixture was reheated to 130 ◦C and further reacted for 3 h. After
similar purification steps, 85 mg product of N-phenylbenzamide
was obtained in 43% yield.
1
analyzer. H, 13C NMR spectra were recorded on a Bruker 400
spectrometer operating at 400.13, 100.62 MHz, respectively. The
in situ FTIR measurements were carried out using a transmission
liquid cell which was thermostated at 60 ◦C and pressurized with
2 bar CO. The FT-IR cell was seated in a Bruker Vertex 70
spectrometer and a resolution of 2 cm-1 was used.
Synthesis of palladium (0) nanoparticles in ionic liquid. 0.98 g
(2.53 mol) of PdCl2(PhCN)2 was added to 40 mL of ethylene glycol
and dispersed with an ultrasonic bath. The resulting solution was
heated to 135 ◦C slowly with violent stirring. After 20 min, a 5 mL
ionic liquid [TBTdP][DBS] was added and the mixture was further
reacted for 2 h before being cooled to room temperature. The
ethylene glycol was then removed. The residue was washed with
diethyl ether (2 ¥ 5 mL) and deionized water (2 ¥ 5 mL) in sequence,
followed by drying in vacuum for 2 days at 40 ◦C. To isolate
the palladium nanoparticles, the ionic liquid stabilized composite
was diluted with dichloromethane and separated by centrifugation
(5000 rpm, 25 min), followed by washing with dichloromethane
(2 ¥ 10 mL) and drying in vacuum. The nanoparticles were
characterized by XPS and TEM.
Acknowledgements
This work was supported by the Institute of Chemical and Engi-
neering Sciences (ICES), GlaxoSmithKline (GSK) and the Singa-
pore Economic Development Board (EDB). We thank Dr Rex Ren
at IL-TECH Inc. for providing the ionic liquid [TBTdP][DBS]. We
also thank Ms Wang Zhan in ICES for XPS analysis.
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