Multi walled carbon nanotubes supported N-heterocyclic carbene-cobalt (II) as a novel, efficient and inexpensive catalyst for the Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.catcom.2015.12.027

In this paper, an N-heterocyclic carbene-cobalt complex (NHC-Co²⁺) was immobilized onto the surface of multi-walled carbon nanotubes (MWCNTs) via direct grafting amination approach for the first time. The resultant composite (Co - NHC@MWCNTs) was characterized by FT-IR, TGA, XRD, ICP-OES, FE-SEM, TEM and CHN analyses. It was demonstrated that Co-NHC@MWCNTs can act as an efficient and inexpensive catalyst for Heck reactions in normal conditions which provided the corresponding products in moderate to good yields. More importantly, this phosphine and palladium-free catalyst can be reused for at least six successive runs without any discernible decrease in its catalytic activity and no remarkable changes were observed in catalyst structure.

Full text of DOI:10.1016/j.catcom.2015.12.027

Multi walled carbon nanotubes supported N-heterocyclic carbene–cobalt () as
a novel, efficient and inexpensive catalyst for the Mizoroki–Heck reaction
Abdol R. Hajipour, Zahra Khorsandi
PII:
DOI:
Reference:
S1566-7367(15)30180-1
doi: 10.1016/j.catcom.2015.12.027
CATCOM 4544
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 November 2015
23 December 2015
24 December 2015
Please cite this article as: Abdol R. Hajipour, Zahra Khorsandi, Multi walled carbon
nanotubes supported N-heterocyclic carbene–cobalt () as a novel, efficient and inex-
pensive catalyst for the Mizoroki–Heck reaction, Catalysis Communications (2015), doi:
10.1016/j.catcom.2015.12.027
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ACCEPTED MANUSCRIPT
Multi walled carbon nanotubes supported N-heterocyclic carbene cobalt
(ΙΙ) as a novel, efficient and inexpensive catalyst for the Mizoroki–Heck
reaction
Abdol R. Hajipoura,b, Zahra Khorsandia
a Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan University of
Technology, Isfahan 84156, Islamic Republic of Iran
b Department of Neuroscience, University of Wisconsin, Medical School, Madison, WI53706-
1532, USA
Abstract
In this paper, an N-heterocyclic carbene–cobalt complex (NHC-Co2+) was immobilized onto the
surface of multi-walled carbon nanotubes (MWCNTs) via direct grafting amination approach for
the first time. The resultant composite (Co-NHC@MWCNTs) was characterized by FT-IR,
TGA, XRD, ICP-OES, FE-SEM, TEM and CHN analysis. It was demonstrated that Co-
NHC@MWCNTs can act as an efficient and inexpensive catalyst for Heck reactions in normal
conditions which provided the corresponding products in moderate to good yields. More
importantly, this phosphine and palladium-free catalyst can be reused for at least six successive
runs without any discernible decrease in its catalytic activity and no remarkable changes were
observed in catalyst structure.
Corresponding author: Tel.: + 98 31 3391 3262; fax: + 98 31 3391 2350; e-mail:
haji@cc.iut.ac.ir.
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Key words: MWCNTs, NHC ligand, Mizoroki–Heck, heterogonous catalyst
Introduction
The formation of C-C bonds is a fundamental reaction in organic synthesis for its many vast
applications in the synthesis of natural products, numerous drugs and high-performance modern
materials. The Mizoroki-Heck coupling have received considerable attention as one of the most
important C-C bonds forming reaction [1]; therefore, a number of efficient catalytic systems
have been developed for this transformations [2]. The standard Heck reaction contains palladium
salt and phosphine ligands suffers from limitations that have so far precluded many industrial
applications [3]. In addition to, due to the high cost and toxicity of palladium; researchers are
interested in Heck reaction catalyzed by inexpensive transition metals such as Ni [4], Cu [5] Fe
[6] and Co [7-8]. To overcome other disadvantages of traditional methods such as using
expensive, toxic and unrecoverable phosphine ligands and complicated recovery, several types of
nitrogen-based ligands such as N-heterocyclic carbenes (NHCs) metal complexes have been
developed by immobilizing them on solid supports [9-10].
Multi walled carbon nanotubes (MWCNTs) are one of the hottest nano materials in the
synthesis, characterization, and applications because of their unique thermal, mechanical and
chemical properties [11-15]. Furthermore, MWCNTs can be used as wonderful catalyst support
because of high surface area and their insolubility in the most solvents. In this regard, diamines
are more favorite molecules, which could behave as a medium to attach miscellaneous
compounds to MWCNTs. Actually, the diamines consist of two amino groups (–NH2): one could
attach on MWCNTs, and one is available for bonding to other functional groups [16-18].
Actually, the described method is one of the most remarkable pre-functionalized strategies,
which allows the covalent binding of different groups on the surface of MWCNTs. Inspired by
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the widespread successful applications of NHCs catalytic systems [19] and disadvantages of
palladium complexes, we decided to improve more cost-effective and environmentally
heterogeneous palladium-free catalyst for C–C coupling reaction by immobilizing first-row
transition metal-NHCs complex on MWCNTs.
Recently, cobalt-catalyzed organic synthesis have attracted much attention; because cobalt is
readily available, non-toxic, low-cost, stable in air and exhibits powerful catalytic activities [20-
22]. Studies on the structure of Co-NHC complex have been reported [23]; but, to the best of our
knowledge, olefin arylation reactions employing such complexes did not report previously. In the
field of C–C bond formation using cobalt catalysts, the reductive coupling reaction of alkyl
halides with aryl Grignard are active topics in organic synthesis [24-25] However, the reactions
between aryl halides and olefins in the presence of a base (Heck reaction) using a cobalt catalyst
are rare and as we know, only a few reports refer to the application of cobalt catalysts in this
coupling reaction [26-29]. For example, in 2009, Qi et al. reported Heck reaction using cobalt
nanoparticles during long reaction time at extremely high temperature (150 °C, Scheme 1a) [26].
Application of uniform Co−B amorphous alloy nanoparticles as catalyst in C–C coupling
reaction was also described by Zhu and his co-workers in 2012 (at 120 °C using DMF/water as
solvent and during 12–24 h, Scheme 1b) [27]. These reported cobalt catalysts have various
advantages such as good mechanical and excellent chemical stabilities and better economic and
ecological properties in compared to palladium catalysts. In contrast, they exhibited lower
activity in Mizoroki–Heck reaction in expanded range of aryl halides, in spite of spending high
temperature and long reaction time. Therefore, to overcome these problems, more convenient
approaches using a cobalt catalyst are required. In continuation of our recent investigations on
the application of heterogeneous catalytic systems in cross-coupling reaction [30-32]; in this
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report, NHC-Co(ΙΙ) complex was supported on MWCNTs via direct grafting amination approach
for the first time and its catalytic activity was demonstrated by employing the Mizoroki–Heck
coupling reaction. Using our catalytic system, the reaction conditions can be improved and
accelerated the Heck reaction without using palladium and phosphine under mild conditions in
present poly ethylene glycol as green solvent (Scheme 1c).
Scheme 1
2. Experimental
2.1. Catalyst preparation
The Co-NHC@MWCNTs catalyst was prepared using multistep synthesis process which
demonstrated in Scheme 2. Detailed procedures are included in the supporting information.
Scheme 2
2.2. Characterization
The products of each step was characterized by FT-IR (Figure 1). The FT-IR spectrum of 4
is shown absorption bands at 2927 cm−1 (C–H), 3433 cm−1 (N–H stretching vibration) and 1579
cm−1 (C=C), shifting peak (in a comparison of 2 with 4 spectrum) indicates a successful
modification of the NHC precursor [33]. Additionally, several signals (1130 and 608 cm−1) is
exposed the chloride group bonds of 4 clearly.
Fig. 1
The nitrogen content of the support was 6.1 %. According to this value, the amount of NHCs
ligand which is available in catalyst is 1.55 mmol/g. The Co loading of catalyst was measured by
ICP and it was about 2.2 % (0.35 mmol/g). The structural properties of catalyst was analyzed by
X-ray diffraction (XRD). The resulted curve was compared with the pure MWCNTs. All peaks
demonstrate the structure of this Co (ΙΙ) complex (Fig. S1, supplemental data), the similar XRD
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pattern observation were reported before for Co (II) complex [34]. The thermal stability of
compound 4 was also evaluated by the TGA, and the thermograms are given in Fig. S2.
According to these curves, the weight loss below 300 ºC is 2 %. This observation can be
attributed to the formation of a stable cobalt complex. The degree of functionalization on the
MWCNTs could be calculated from the weight loss between 200 °C and 800 °C on TGA and
elemental analysis. The observed total weight losses for catalyst is 67 %, on the basis of this
value, the amount of organic moieties was about 1.73 mmol/g of catalyst. A good agreement was
observed between elemental analysis and TGA data for these conversions. The morphology of
the surfaces of MWCNTs and catalyst were studied by field emission scanning electron
microscopy (FE-SEM). As can be seen the nanotubes are aggregated and has retained their
nanotube nature upon functionalization (Fig. S3). In Figure 2, the transmission electron
microscopy (TEM) images of catalyst 4 clearly show the immobilization of cobalt complex on
MWCNTs. Moreover, the large metal nanoparticles which appear in TEM images could be
related to some aggregating [35].
Figure 2
3. Results and Discussion
To explore the activity of the nanocatalyst, we applied it in the Mizoroki–Heck cross-
coupling reaction. A model reaction of iodobenzene with methyl acrylate was used to optimize
the reaction conditions. The activity of the catalyst in different amounts, using various base,
solvent and reaction temperature was studied (Table 1). According to the optimization results
and the goals of green chemistry, all the reactions were carried out in poly ethylene glycol (PEG)
at 80 ºC. Table 1
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Encouraged by our initial studies; the generality and versatility of this catalyst have been
examined. The results are shown in Table 2. Aryl halides with electron-donating groups, in
comparison with aryl halides with electron-withdrawing groups, showed better conversions in
shorter reaction times. Table 2
Interestingly; as shown in Table 2, the supported NHC-cobalt (ΙΙ) nanocatalyst was
compatible with a wide range of functional groups and exhibited good activity in the Mizoroki–
Heck reaction using PEG as a solvent in mild conditions. Our catalyst was successfully reused at
least six times without any significant losing of its activity (Table 3). The scale of the reaction
was increased to 10.0 mmol, keeping the reaction stoichiometry intact. The reaction was found to
proceed successfully, and the corresponding product was obtained in 78% yield via described
method respectively. Table 3
The recovered catalyst was analyzed using TEM and FT-IR (Fig. S4, S5). According to
these results no remarkable changes were observed in catalyst structure. To explore the leaching
of the catalyst, the Heck coupling in model reaction was studied. After 30% of the coupling
reaction had proceeded, the catalyst was separated with a centrifuge and ICP analysis of the
remaining solution revealed that there was no cobalt in the reaction mixture. These results
suggested that the cobalt was not being leached out from the solid surface of the catalyst during
the coupling reaction and clearly demonstrated that this novel catalyst was heterogeneous in
nature.
4. Conclusions
In summary, cobalt supported on direct functionalization MWCNTs as NHCs precursor was
fabricated for the first time. The prepared Co-NHC@MWCNTs catalyst exhibited high catalytic
activity for the Mizoroki–Heck reaction. The current strategy is a novel, environmentally
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friendly and economical way for the Heck reaction using significantly low cobalt loading. It
could be recycled and reused six times without obvious cobalt leaching. Further applications of
the catalytic system in other reactions are under investigation in our laboratory.
Acknowledgments
The authors thank Isfahan University of Technology (IUT), IR Iran.
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Legend for Scheme
Scheme 1 Heck reaction using various types of cobalt catalytic systems under different
conditions.
a) Qi et.al
I(Br)
W
Co-NPs
W
K2CO3, NMP
150 oC, 12-30 h
R
b) Zhu et. al
I
Co-B
K2CO3, water/DMF
120 oC, 12-24 h
R
c) This work
X
W
Co-NHC@MWCNTs
W
Li2CO3, PEG (200)
80 oC, 5-10 h
R
X = I, Br
W = MA, Ph
Scheme 2. The preparation of Co-NHC@ MWCNTs catalyst
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Legend for Figure
Fig. 1. FT-IR spectra of products of catalyst preparation steps
Fig. 2.TEM image of Co-NHC@MWCNTs
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Table 1. Optimization of reaction conditionsa
Entry
1
Solvent
PEG (200)
Base (equiv.)
K2CO3 (2)
Cat (mg)
10
T (ºC)
100
Yieldb (%)
71
2
3
PEG (200)
PEG (200)
PEG (200)
PEG (200)
PEG (200)
PEG (200)
DMSO
KOH (2)
LiOH (2)
10
10
10
10
10
10
10
10
10
10
5
100
100
100
100
100
100
100
100
100
100
100
100
80
68
73
78
82
86
76
74
69
54
58
79
89
85
61
4
Li2CO3 (2)
Li2CO3 (3)
Li2CO3 (4)
Li2CO3 (1.5)
Li2CO3 (4)
Li2CO3 (4)
Li2CO3 (4)
Li2CO3 (4)
Li2CO3 (4)
Li2CO3 (4)
Li2CO3 (4)
Li2CO3(4)
5
6
7
8
9
NMP
10
11
12
13
14
15
Toluene
EtOH
PEG (200)
PEG (200)
PEG (200)
PEG (200)
20
10
10
60
aReaction conditions: iodobenzene (1 mmol, 120 μL), MA (1.5 equiv., 136 μL), base, solvent (3
mL); bGC conversion.
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Table 2. Heck reactions of aryl halides with olefin catalysed by 4
Entry
X
R
Olefin
Time (h)
Yieldb(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
I
I
I
H
4-OMe
4-NO2
3-NO2
H
4-NO2
4-OMe
4-Cl
4-CN
4-COCH3
H
4-NO2
3-NO2
4-OMe
H
4-NO2
4-OMe
4-Cl
4-CN
H
4-OMe
4-NO2
H
MA
MA
MA
MA
MA
MA
MA
MA
MA
MA
EA
EA
EA
EA
EA
EA
EA
5
5
5
5
6
6
6
6
6
6
5
5
5
5
6
6
6
6
6
8
8
8
10
10
85
92
87
81
77
74
80
76
73
64
82
77
79
81
72
65
63
58
60
78
82
74
67
61
I
Br
Br
Br
Br
Br
Br
I
I
I
I
Br
Br
Br
Br
Br
I
I
I
Br
Br
EA
EA
Styrene
Styrene
Styrene
Styrene
Styrene
4-NO2
a GC conversion.
Table 3. Reusability of the catalyst
Run
Yield a (%)
Run
4
5
Yielda (%)
1
2
3
85
85
83
81
80
80
6
aIsolated yield
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Graphical abstract
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Highlights

The first time report of N-heterocyclic carbene–cobalt complex (NHC-Co2+) immobilized onto
the surface of multi-walled carbon nanotubes (MWCNTs) via direct grafting amination approach.
The introduced catalyst is air stable, highly active and reusable without toxic ligands.
This paper presents a facility and high efficiency strategy for the Heck reaction using green
solvents and mild conditions.


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