In site preparation of Pd(II)-MoS2 complex: A new high-efficiency catalyst for alkenylation of heteroaromatics by direct C-H bond activation

Article abstract of DOI:10.1016/j.apcata.2015.10.014

Pd(II)-MoS₂ catalyst was designed by affording Pd²⁺ salt on the surface of single-layered molybdenum disulfide nanosheets._. Based on the dispersible template, this metal salt and molybdenum disulfide complex offered several advantages, including easy preparation, low cost, and capability to prepare excellent dispersible metal ion cluster nanostructures. The application of this complex in alkenylation of heteroaromatics was reported. Direct C-H activation of indoles, furans and thiophenes was carried out to combine with several kinds of terminal olefins. This catalyst also performed high efficiency in several other kinds of coupling reactions. Thus, a universal suitable catalyst for C-H activation and CC bond formation was invented by a convenient in situ preparation.

Full text of DOI:10.1016/j.apcata.2015.10.014

Accepted Manuscript
Title: In site preparation of Pd(II)-MoS2 Complex: a new
high-efficiency catalyst for alkenylation of heteroaromatics by
direct C-H bond activation
Author: Sheng Wang Guoqing Deng Jiangjiang Gu Wenwen
Hua Xudong Jia Kai Xi
PII:
S0926-860X(15)30191-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.10.014
APCATA 15590
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Applied Catalysis A: General
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Revised date:
Accepted date:
31-8-2015
5-10-2015
9-10-2015
Please cite this article as: Sheng Wang, Guoqing Deng, Jiangjiang Gu, Wenwen Hua,
Xudong Jia, Kai Xi, In site preparation of Pd(II)-MoS2 Complex: a new high-efficiency
catalyst for alkenylation of heteroaromatics by direct C-H bond activation, Applied
Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.10.014
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In site preparation of Pd(II)-MoS2 Complex: a new high-efficiency catalyst for
alkenylation of heteroaromatics by direct C-H bond activation
Sheng Wanga boena@139.com, Guoqing Dengb denggqq@sina.com, Jiangjiang Gua
gjjdgqf@gmail.com, Wenwen Huab hww19900523@126.com, Xudong Jiaa*
jiaxd@nju.edu.cn, Kai Xib* xikai@nju.edu.cn
aState Key Laboratory of Coordination Chemistry, Department of Polymer Science &
Engineering, Nanjing National Laboratory of Microstructures, Nanjing University,
Nanjing 210093, P.R. China.
bDepartment of Polymer Science & Engineering, Nanjing University, Nanjing 210093,
P.R. China.
*Corresponding author: Fax: +86-25-83621337 (X. D. Jia); Fax: +86-25-83686197 (K.
Xi)

Graphical Abstract

Highlights
1. A Pd catalyst was designed by affording Pd2+ salt on the surface of single-layered
MoS2.
2. This metal salt and molybdenum disulfide complex offered several advantages,
including easy preparation, low cost, and capability to prepare excellent
dispersible metal ion cluster nanostructures with controllable size.
3. Direct C-H activation of indoles, furans and thiophenes was carried out to
combine with several kinds of terminal olefins with 0.2% amount of the catalyst.
4. This easy-prepared catalyst also performed high efficiency in several other kinds
of C-C coupling reactions.
Abstract
Pd(II)-MoS2 catalyst was designed by affording Pd2+ salt on the surface of
single-layered molybdenum disulfide nanosheets. Based on the dispersible template,
this metal salt and molybdenum disulfide complex offered several advantages,
including easy preparation, low cost, and capability to prepare excellent dispersible
metal ion cluster nanostructures. The application of this complex in alkenylation of
heteroaromatics was reported. Direct C-H activation of indoles, furans and thiophenes
was carried out to combine with several kinds of terminal olefins. This catalyst also
performed high efficiency in several other kinds of coupling reactions. Thus, a
universal suitable catalyst for C-H activation and C-C bond formation was invented
by a convenient in situ preparation.
Keywords: Transition metal dichalcogenides; C-H activation; Heterogeneous
catalysis; MoS2

1. Introduction
In the last decade, tremendous attention has been paid to two-dimensional crystal
with exceptional electronic, optical and mechanical properties [1-4]. Although
graphene is the most well-known layered material, transition metal dichalcogenides
(TMDs) have acquired increasing attention, because of their unique properties and
various applications in energy storage, sensing, electronic devices and photo-catalysis
[5-11]. Among these TMDs, MoS2 is a representative example. The monolayer MoS2
can be applied in the field effect transistor (FET) [12] and NO detection [13].
Moreover, MoS2 nanosheets are also promising materials for catalytic hydrogenation
and potential building blocks for thermoelectric and energy storage devices [11,14,15].
However, application of MoS2 nanosheets as a catalyst support in chemical catalysis
has been rarely studied so far.
With regards to metal catalytic applications in the last decades, palladium
catalyzed carbon-carbon bond formations are a powerful set of synthetic tools [16-22].
Normally, aryl halide is catalyzed by palladium(0) in the procedure in which aryl or
alkyl palladium(II) intermediates are formed [23-25]. These intermediates can be
subsequently functionalized to form carbon-carbon bonds through the reaction with
organometallic reagents [26-29]. Due to the need of a more straightforward and
economical process to prepare such intermediates, the C-H activation reactions have
thus been greatly improved [30-32]. Palladium(II) catalyzed direct C-H activation
reactions are now attracting more and more attention [33-38]. Traditional palladium(II)
catalysts used in C-H activation is usually palladium salts in a large amount, up to
10%. Therefore, it is significant to find a suitable support for palladium which will
bring a remarkable reduce of catalyst amount [39-43].
Herein, a generally applicable catalyst was acquired by in situ preparation of two
stable materials, providing a cheap and efficient palladium catalyst for C-H activation.
The preparation of single-layer MoS2 nanosheets could be realized by using n-butyl
lithium as the intercalation agent to insert lithium ions into the layered structures,
followed by exfoliation in water with ultrasonication [44-46]. After dried through
cryodesiccation, the two-dimensional MoS2 was dispersed in DMF under

ultrasonication, which could be used as catalyst carrier by a straightforward mixing
with PdCl2 solution [47]. This catalyst could be widely used in alkenylation of
heteroaromatics by direct C-H bond activation. In all the examples we had tried, the
reactions were carried out with a lower catalyst amount compared to traditional
catalysts, and in high yield [48-50]. Meanwhile, Mizoroki-Heck reactions,
Suzuki-Miyaura reactions and functionalization reactions of thiophene could also be
catalyzed by this complex. By this means, a new generally suitable palladium catalyst
had been developed.
2. Experimental details
2.1 Catalyst preparation
To obtain large quantities of MoS2 nanosheets, solution based exfoliation
methods had been used [46,51]. 0.1 ml of PdCl2 solution in DMF (0.01 M) and 0.4
mg of MoS2 nanosheets dispersed in 2 ml of DMF were directly mixed for just ten
minutes at room temperature. Pd(II)-MoS2 complex could be in situ prepared by this
way.
2.2 Catalyst characterization
Fig. 1a showed the chemical exfoliated MoS2 nanosheets, which were single
layered. The ion cluster of palladium was supposed to be formed after the Pd2+
depositing onto the surface of the MoS2 nanosheets (Fig. 1b). Interestingly, the ion
cluster was inclined to form on the edge sites of MoS2 (Fig. 1b insert), which
indicated that the edge was more active than the basal plane [52]. As reported,
because of the violent nature of chemical exfoliation reaction, the crystal structure of
MoS2 became deformed [53], and the defects could be visible [54]. The MoS2
nanosheets had been proved to contain negative charge after chemical exfoliation [55].
It was supposed that the negative charge might gather around the defects, where
molybdenum atoms were unsaturated. Thus, palladium ion with positive charge would
deposit around the defects, especially onto the defect-rich edge sites with high surface
energy [44,56]. Meanwhile, as studied before, using DMF as the solvent and capping
agent, sulphur atoms had strong interaction with palladium, leading palladium clusters
to decorate on the surface of two-dimension nanosheets with well-defined size and

shape [57]. The sulphur atoms on the surface of our single-layered MoS2 might also
have strong interaction with palladium ion in DMF, therefore perfect dispersion of
palladium ion cluster could also be observed from this image, which might lead to
good catalytic performance.
The XPS spectra of Pd doped MoS2 nanosheets were showed in Fig. 2. The Mo
3d showed two peaks at 228.2 eV and 231.4 eV, respectively, indicating the MoS2
nanosheets were in 1T phase [51]. The binding energy of Pd 3d peaks at 336.4 eV and
341.8 eV, respectively suggested the successful deposition of palladium(II) onto the
surface of the MoS2. Metallic Pd doublet peaks (at about 335.0eV and 341.0eV) were
not observed in the spectra.
The particle size distribution of the Pd(II) ion clusters on the MoS2 supports was
also determined by TEM analysis, and the average particle diameter was 6.28 nm (Fig.
3a). The small size of the Pd ion clusters contributed greatly to the high catalytic
activity [58].
After centrifugation of the catalyst suspension in DMF, the Pd(II)-MoS2 complex
was separated. The EDX result of the catalyst showed the palladium content was
amounted to 6.07% by weight, which could be converted into 3.19% by number of
atom (Fig. 3b, calculation see in ESI). Compared to the amount of PdCl2 added to the
MoS2 suspension, 25% of the Pd(II) ion were deposited onto the surface of the MoS2
nanosheets.
3. Results and discussion
3.1 Application of catalyst in C-H activation and oxidative Heck reaction
The indole motif is a ubiquitous feature of alkaloid and peptide natural product,
and the functionalization of indoles is an important transformation in organic
chemistry [59,60]. Meanwhile, other five-membered cycles, especially thiophenes and
furans, are common structural motifs found in abroad range of natural products, drug
molecules, and industrial materials [61]. They are considered to be important targets
[59,62]. Many useful and practical processes exist for the modification of them, and
metal-catalyzed coupling reactions are of particular utility [63-66].
Traditional Heck reactions contain an unconquered disadvantage, that is, the

overall coupling of the two fragments requires two discrete activation steps: the
formation of an aryl or vinyl halide and the palladium(0)-catalyzed union of the
reaction partners [27-29,67,68]. A direct oxidative Heck through palladium-catalyzed
C-H activation/C-C bond-forming processes would bypass this disadvantage and lead
to a more efficient process [69]. Here we applied our in site prepared Pd(II)-MoS2
complex in the direct C-H activation, to our delight, a high efficiency Heck product
was acquired [48,70].
We began to explore the reaction with N-methylindole and n-butyl acrylate by
use of DMF as the solvent and Cu(OAc)2 as the oxidant. According to the report,
ligand conjugation of chemically exfoliated MoS2 brought out the thiol
functionalization route to modify MoS2 sheets. Here we prepared MoS2 decorated by
mercaptoethylamine, mercaptoethanol and mercaptoacetic acid using this method.
Undecorated MoS2 and MoS2 decorated by mercaptoethylamine, mercaptoethanol or
mercaptoacetic acid, were dispersed in DMF (2 mL, 0.2 mg/mL) and mixed with
PdCl2 (0.1 ml, 0.01 M) solution. Then it was used as the catalyst in the reaction.
Beyond our expectation, undecorated MoS2 performed much better than decorated
MoS2 (Table 1). As MoS2 could be decorated along with the defects on the surface
trapping sulfydryl group, once the MoS2 sheets completely modified, Pd ion would
not be concentrated on the surface of MoS2 thereof. Once the reaction was carried out
using only PdCl2 without support, the reaction scarely happened.
Table 2 showed a range of five numbered heterocyclic compounds reacted with
alkenes. It could be found that the C-H activation and oxidative coupling reactions
were catalyzed by Pd(II)-MoS2. These reactions were carried out with Pd(II)-MoS2
(0.1 ml of 0.01M PdCl2 with 2 ml of MoS2 suspension), hetero cycles (1a~1c) (0.5
mmol, 1 equiv), olefins (2a~2e) (1 mmol, 2 equiv), and Cu(OAc)2 (0.9 mmol, 1.8
equiv). The C-H activation and coupling proceeded smoothly to give the
corresponding products at 70 oC for 18 h.

Compared to traditional palladium catalysts, we reduced the amount of
palladium catalyst from 10% to 0.2% of the reactant. To confirm the role of the
Pd(II)-complex in catalysis, the in site prepared catalyst was also separated from the
solvent with the dissociative PdCl2 removed. After redispersed in DMF, the catalyst
was used to the reactions of compound 1a with 2a. The result had none different
manifestations with the previous reactions which certified that the Pd(II)-MoS2, not
the dissociated Pd ion played a main role in catalysis.
We proposed a Pd(II) involved C-H activation mechanism (Fig. 4), in relation to
a Pd(II)-Pd(0)-Pd(II) catalytic cycle [71]. But since everything has two sides, our
convenient and high-efficiency catalyst were not recyclable, because the
palladium(II)-MoS2 could not be easily separated. In the reaction, 165 mg copper salt
was used. Although we tried many ways, the only 0.4 mg MoS2 support in the
reaction system could not be separated from the large amount of copper salt.
3.2 Application of catalyst in other C-C bond formation reactions
This catalyst showed not only high efficiency in C-H activation reaction, but also
good performance in traditional coupling reaction, such as Suzuki reaction, Heck
reaction and functionalization of hetero cycles by aryl halides. The Suzuki reaction
between aryl halide (0.50 mmol) and boronic acid (0.55 mmol) proceeded in the
presence of Pd(II)-MoS2 (0.01 ml of 0.01 M PdCl2 with 2 ml of MoS2 suspension) in
EtOH and water, using Na2CO3 as the base under atmosphere, to acquire the coupling
product with up to 95% yield. The Heck reaction of aryl bromide (0.50 mmol) and
olefins (1.0 mmol) was performed with Pd(II)-MoS2 (0.05 ml of 0.01M PdCl2 with 2
ml MoS2 suspension), triethylamine and tetrabutylammoniumacetate (TBAA) in DMF
o
at 120 C, to acquire the corresponding cinnamates with up to 89% yield. Beside the
direct C-H activation and oxidative coupling, hetero cycles could also be
functionalized by aryl halide in the reaction catalyzed by palladium catalyst [72,73].
The reactions were carried out with thiophene (1.0 mmol), aryl halide (0.50 mmol),
Pd(II)-MoS2 (0.25 ml of 0.01M PdCl2 with 2 ml of MoS2 suspension) and CsOAc (2
equiv) in DMF. This reaction proceeded successfully.

4. Conclusions
In summary, Pd2+ combined to single layered MoS2 nanosheets showed favorable
catalytic property in several kinds of coupling reactions, including the C-H activation
and oxidative functionalization of hetero cycles. Existence form of palladium as a
Pd(II) salt brought many benefits, especially, a more straightforward and economical
process to prepare intermediates in C-C bond formation. Low cost, simple material,
and high stability were also significant advantages. This catalyst could also be widely
used in Suzuki-Miyaura reaction, Heck reaction, and C-H bond functionalization
reactions of thiophenes. By this means, by the aid of a reliable support, commercial
Pd(II) salt could turn to be an efficiency and normal applicable C-C bond catalyst.
This new methodology in designing catalyst may promise a broad prospect in virtue
of its productiveness, as atom economy is encouraged in current chemistry.
Acknowledgements
We thank the Program for Changjiang Scholars and Innovative Research Team in
University and the Fundamental Research Funds for the Central Universities
(1104020505) for financially supporting in this research.

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Figure Captions
Fig. 1. Characterization of the Pd(II)-MoS2 catalyst. (a) TEM image of the MoS2
nanosheets scale. (b) TEM image of the Pd(II)-MoS2 complex

Fig. 2. Characterization of the Pd(II)-MoS2 catalyst (a) Binding energy of Mo 3d (b)
Binding energy of Pd 3d

Fig. 3. (a) Particle size distributions of Pd(II) ion cluster (b) EDX analysis for
elements

Fig. 4. Supposed mechanism of Pd(II)-MoS2 catalyzed C-H activation

Scheme 1. Application of Pd(II)-MoS2 in coupling reaction

Tables
Table 1. Catalytic activity of Pd(II)-support in C-H activation
Catalyst Support
MoS2
Conversion（%）
100
30
0
MoS2 decorated by mercaptoethylamine
MoS2 decorated by mercaptoethanol
MoS2 decorated by mercaptoacetic acid
None
0
5

Table 2. System Expansion of C-H activation and oxidative coupling reaction
Entry
1
Compound 1
Compound 2
Product
3aa
Yield（%）
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1c
1c
1c
1c
1c
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
2a
2b
2c
2d
2e
75
86
68
55
84
82
83
71
61
89
74
73
67
64
82
2
3ab
3ac
3
4
3ad
3ae
5
6
3ba
3bb
3bc
3bd
3be
3ca
7
8
9
10
11
12
13
14
15
3cb
3cc
3cd
3ce