Chemoselective transfer hydrogenation of nitroarenes by highly dispersed Ni-Co BMNPs

Article abstract of DOI:10.1016/j.catcom.2016.05.023

Highly dispread Ni-Co bimetallic nanoparticles (Ni-Co BMNPs) are synthesized and applied as an efficient catalyst in the chemoselective transfer hydrogenation of nitroarenes (CTH) using hydrazine hydrate as the hydrogen donor. The BMNPs can efficiently catalyze the reduction reaction without any additives under mild conditions with high TOF. Significantly higher activity is achieved when compared with corresponding single-component catalysts, optimal composition of the Ni-Co BMNPs was screened which was proved to be crucial in both the selectivity and yields. Excellent performance of Ni-Co BMNPs can be ascribed to the improved dispersion of active sites on the BMNPs surface (compared with Ni NPs) and the electron transfer from cobalt to nickel.

Full text of DOI:10.1016/j.catcom.2016.05.023

Chemoselective transfer hydrogenation of nitroarenes by highly dispersed
Ni-Co BMNPs
Jia-wei Zhang, Guo-ping Lu, Chun Cai
PII:
DOI:
Reference:
S1566-7367(16)30181-9
doi: 10.1016/j.catcom.2016.05.023
CATCOM 4670
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12 March 2016
25 May 2016
26 May 2016
Please cite this article as: Jia-wei Zhang, Guo-ping Lu, Chun Cai, Chemoselective trans-
fer hydrogenation of nitroarenes by highly dispersed Ni-Co BMNPs, Catalysis Communi-
cations (2016), doi: 10.1016/j.catcom.2016.05.023
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ACCEPTED MANUSCRIPT
Journal Name
journal homepage: www.elsevier.com
Chemoselective transfer hydrogenation of nitroarenes by highly dispersed Ni-Co BMNPs
Jia-wei Zhang, Guo-ping Lu and Chun Cai 
Chemical Engineering College, Nanjing University of Science & Technology, 200 Xiaolingwei, Nanjing 210094, P. R. China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Available online
Highly dispread Ni-Co bimetallic nanoparticles (Ni-Co BMNPs) are synthesized
and applied as an efficient catalyst in the chemoselective transfer hydrogenation
of nitroarenes (CTH) using hydrazine hydrate as the hydrogen donor. The BMNPs
can efficiently catalyze the reduction reaction without any additives under mild
conditions with high TOF. Significantly higher activity is achieved when
compared with corresponding single-component catalysts, optimal composition of
the Ni-Co BMNPs was screened which was proved to be crucial in both the
selectivity and yields. Excellent performance of Ni-Co BMNPs can be ascribed to
the improved dispersion of active sites on the BMNPs surface (compared with Ni
NPs) and the electron transfer from cobalt to nickel.
Keywords:
Bimetallic nanoparticles
Nitroarenes
Reduction
Transfer hydrogenation
2016 Elsevier B.V. All rights reserved.
1
. Introduction
Aromatic amines are ubiquitous building blocks in organic
associated with monometallic counterparts and a significant
enhancement due to synergistic effects [25-27]. Considering
those excellent properties of BMNPs, it can be a potential way
to solve the issues that most non-noble metals catalysts
suffered [28]. More recently, initial attempts have been made
to perform Pt-Ni [29] and Fe-Ni BMNPs [30] for the CTH of
nitroarenes. High catalytic activities are achieved, but use of
synthesis, widely used in the manufacture of fine
agrochemicals, dyes, pigments, and pharmaceuticals) and
(
bulk chemicals (polymers) [1-3]. Aromatic amines can be
obtained by various methods including reduction of aryl
azides [4], amination of aryl halides [5], catalytic reduction of
nitroarenes [6] and so on. Among these methods, the
chemoselective transfer hydrogenation (CTH) of nitroarenes
represents a more cost-efficient and greener route for the
synthesis of aryl amines [7-9]. However, CTH of nitroarenes
reported up to date still rely on use of noble metal catalysts,
such as Pt [10], Pd [11], Au [12], and Ir [13], in which
widespread applications are limited due to their low
earthabundance, high price and toxicity. Therefore, much
attention has been transferred to the research of effective
catalysts based on earth-abundant metal elements (such as Fe,
Co, Ni) [14, 15]. Nevertheless, high catalytic loading and
drastic conditions are often required, and some catalytic
systems are found to be less chemoselective towards nitro
group [16-19].
robust reducing reagent (NaBH ) and high catalyst loading
4
also lead to manipulate inconvenience and costly reagent.
Along this line, we present here a Ni-Co BMNPs catalyst for
the CTH of nitroarenes using easy-to-handle hydrazine
hydrate (N H ·H O) [31, 32] as the hydrogen donor, this
2
4
2
protocol allow an efficient transformation of nitroarenes under
mild conditions.
2. Experimental Section
2
.1. Catalyst preparation
Ni, Co, and Ni M
(M = Fe, Co, Zn and Cu) BMNPs (x
100-x
x
represents the molar percentage of Ni in the BMNPs) catalysts
were prepared by chemical reduction, PVP was applied as
stabilizer and the reductant was NaBH . In a typical
4
preparation process for Ni Co BMNPs, NiCl ·6H O (0.5
In the past decades, nanoparticles have attracted much
attention due to their outstanding properties, and they are
uniquely suitable for applications in catalysis owing to the
vast surface to volume ratio and easily available large
numbers of active sites per unit area [20-24]. Among these,
bimetallic nanoparticles (BMNPs), composed of two different
metal elements, usually show a combination of the properties
50
50
2
2
mg), CoCl (0.26 mg) and PVP (160 mg, average molecule
2
weight = 40,000) were dissolved in ethanol (1.5 mL) and
charged into a 10 mL reactor with a magnetic stirrer. Then a
fresh prepared ethanol solution of NaBH (0.8 mg, in 0.5 mL
4
ethanol) was added into the reactor quickly under vigorous
o
stirring (1000 rpm) at room temperature (25 C) in argon

Corresponding author. Tel.: +86-25-84315514; fax: +86-25-84315030; e-mail: c.cai@njust.edu.cn

ACCEPTED MANUSCRIPT
2
atmosphere. The color of the colloid mixture would turn to
remains active for one week and should be kept under an inert
atmosphere
black immediately which indicates that metal salts have been
reduced to metal particles. The catalysts prepared were
directly used for catalytic reactions or freeze-dried for further
analysis. Catalysts used in reactions should be freshly
prepared because over-exposure of catalysts containing Ni to
air will result in significantly decrease in catalytic activity due
to oxidation [33].
The X-ray powder diffraction (XRD) pattern of Ni Co is
50
50
shown in Fig. 2, suggesting a good crystallinity of Ni and Co
in the catalyst. Peaks at 2θ= 44.4°, 51.7° and 76.1° can be
assigned to (111), (200), (220) crystal planes of Ni (JCPDS
file:65-0380), besides, the diffraction peaks at 44.2° and 51.5°
are the characteristic (111) and (200) reflections of cubic Co,
respectively (JCPDS file:15-0806), the position of peaks have
slightly shifts due to formation of alloy [33].
2
.2. Catalytic reduction of nitroarenes.
Hydrazine hydrate was chosen as the hydrogen donor for
the low emission of pollutants. In a typical procedure,
hydrazine hydrate (4 equiv) was added into the reactor which
containing fresh prepared catalyst as described above. Then
the reactor was put into a preheated oil bath with a stirring
speed of 500 rpm, and the substrate (1 mmol) dissolved in 1
mL ethanol was added drop-wisely under argon. The reactions
were monitored by TLC. After the reaction, the reaction
mixture was vacuum filtered through a pad of silica on a
glass-fritted funnel and an additional 15 mL of ethyl acetate
(
5 mL portions) was used to rinse the product from the silica,
the filtrate was concentrated in vacuum and analyzed by GC.
Products were purified by column chromatography and
1
13
identified by H NMR and C NMR.
3
. Results and discussion
3
.1. Characterization of Ni-Co BMNPs
Fig. 1. TEM image of Ni Co BMNPs. The inlet
50
50
The morphologies of Ni Co
BMNPs catalyst is
image shows the size distribution of Ni50Co50BMNPs
5
0
50
examined by TEM, which indicates that the as prepared
Ni Co BMNPs are well-proportioned anomalous spherical
5
0
50
with an average diameter of 2.5 nm (Fig.1), a little smaller
than Ni NPs and Co NPs (Fig. S1). The catalyst suspension
Fig. 2. (a) Detail Views of XRD pattern of the Ni Co BMNPs from 2θ=40° to 50°; (b) XRD pattern of the Ni Co BMNPs
5
0
50
50
50
from 2θ=45° to 55°

ACCEPTED MANUSCRIPT
Fig. 3. XPS spectra of the Ni Co catalyst: (a) Ni 2p region and (b) Co 2p region.
5
0
50
Table 1
a
Evaluation of the reaction parameters on the reduction of 4-nitrotoluene to p-toluidine .
o
b
b
Entry
Catalyst
Ni Fe
Temp( C)
Solvent
Selectivity
%)
Yield (%)
(
c
1
2
3
4
80
80
80
80
80
80
80
80
80
80
80
80
80
80
60
70
90
100
80
80
80
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
37
<10
26
63
0
17
<10
18
53
0
5
0
50
c
Ni Zn
50
50
50
50
c
Ni Cu
5
0
c
Ni Co
5
0
5
6
7
8
9
/
Ni Co
>99
70
92
88
69
56
45
<10
<1
64
74
99
94
80
>99
79
98
70
92
85
47
42
38
<10
<1
51
58
97
87
37
25
68
5
0
50
25
Ni Co
7
5
Ni62.5Co37.5
Ni37.5Co62.5
Ni Co
1
0
2
5
75
d
1
1
1
1
2
3
Ni NPs
Co NPs
e
f
NiCl + CoCl
2
2
g
1
4
Ni Co
5
0
50
50
50
50
50
50
50
50
1
5
6
7
8
Ni Co
5
0
1
1
1
Ni Co
5
0
Ni Co
5
0
Ni Co
EtOH
5
0
h
1
9
Ni Co
PEG400
t-BuOH
Ethylene
glycol
THF
5
0
2
2
0
1
Ni Co
50
Ni Co
5
0
2
2
2
2
3
4
Ni Co
80
80
80
80
80
80
53
86
73
>99
57
>99
38
73
69
>99
31
>99
5
0
50
50
50
50
50
50
Ni Co
1, 4-dioxane
5
0
Ni Co
H O
5
0
2
i
2
2
5
6
Ni Co
H O
5
0
2
j
Ni Co
EtOH
EtOH
5
0
k
2
7
Ni Co
5
0
a
Reaction conditions: 1 mmol of nitroarenes, 0.4 mmol % catalyst (based on metal), 4 equiv of
-
3
N H ·H O, 2mL EtOH as solvent, 4×10 mmol Ni-Co BMNPs and Ni-Co ratio was 1:1, The
2
4
2
reactions were carried out at corresponding temperature in a sealed tube under argon atmosphere
for 1 h.
b
Yield and selectivity were determined by GC.
c
0
.3 mmol % catalyst.
d
e
f
-3
4
×10 mmol Ni NPs was added instead of Ni-Co BMNPs.
-3
4
×10 mmol Co NPs was added instead of Ni-Co BMNPs.
-3 -3
2
×10 mmol NiCl and 2×10 mmol CoCl were added instead of Ni-Co BMNPs.
2 2
g
h
i
no N H ·H O was added.
2
4
2
PEG 400 was added as stabilizer and solvent.
-nitrophenol was used as the substrate.
4

ACCEPTED MANUSCRIPT
Et SiH was used as hydrogen donor
NaBH was used as reducing reagent
4
Tetrahedron
j
3
k
4
The XPS spectra points out that the ratio of the surface
concentration between Ni and Co is 1.94, which indicates that
(Ni/M=1, M = Fe, Co, Zn and Cu) BMNPs was screened for
the CTH of 4-nitrotoluene, uncomplete reduction was found
and about 70% of the products were byproducts (Table 1,
entries 1-3), 4,4-dimethylazobenze and 4,4-dimethylazoxy-
benzene were found to be the major by products as confirmed
by GC-MS. Ni-Co BMNPs was found to be the optimal
catalyst (entry 4). Catalyst loading was an important
parameter in the CTH of 4-nitrotoluene, no reaction took
place without catalyst (entry 5). The yield was increased to
98% using 0.4 mmol% of Ni Co (entry 6). Then, we
evaluated the effects of Ni/Co ratio on the activity (entries 7-
10), and the initial Ni/Co ratio of 1:1 was found to be the
optimal, Ni NPs and Co NPs shown low activity and provide
poor yields (entries 11-12).
more Ni presents in the surface of the Ni Co BMNPs. The
5
0
50
highest doublet (853.1 eV and 870.3 eV) in Fig. 3a is due to
2
+
Ni in NiO and the second intensive doublet (857.5 eV and
2
+
8
75.0 eV) can be attributed to Ni of Ni(OH) , the reason for
2
the presence of NiO and Ni(OH) is that the surface atoms of
2
the catalysts can be easily oxidized and hydroxide when
exposed to oxygen and moisture in air [34]. The doublets in
Fig. 3b (778.78 eV and 793.68 eV) were in accordance with
50
50
0
the formation of metallic Co , while peaks around 782.68eV
and 798.98 eV also indicates that cobalt in the surface of the
catalyst was oxidized [16].
In situ UV-vis spectra were recorded to the preparation
process of the Ni, Co and Ni Co catalyst under diluted
conditions (Fig. S3 for UV-vis), a large increase in absorption
The high catalytic performance of Ni-Co BMNPs can be
attributed to the synergistic effects of Ni and Co. TEM results
indicated that the Ni-Co BMNPs have smaller particle size
than Ni and Co NPs. Ni as the major active sites presents in
the surface of Ni-Co BMNPs, which has been confirmed by
XPS. This kind nanostructure provides more chance for
contact between active sites and reactant molecules.
Moreover, in the alloy form, electrons transfer from Co to Ni
increases the local electron density, facilitating the efficient
electron flow from the catalyst surface to the adsorbed nitro
compounds [33, 34], which may be the main reason for the
activity and selectivity of BMNPs.
5
0
50
was determined when the NaBH solution was added, and the
4
metal salts were completely reduced into metal NPs in two
minutes. The inductively coupled plasma mass spectrometry
(
ICP-MS) confirmed that the Ni/Co ratio were in accordance
with the designed compositions of the Ni Co BMNPs
5
0
50
(
Table S1).
.2. Catalytic Reduction of 4-Nitrotoluene
-Nitrotoluene was chosen as the model substrate for the
3
4
optimization of the reaction conditions. Initially, ratio of the
metal was set as 1:1, and the activity of different Ni M
50
50
Table 2
a
Chemoselective Reduction of Nitroarenes catalyzed by Ni Co BMNPs .
50
50
Entr
y
Substrate
Product
Yield
(%)
Entr
y
Substrate
Product
Yield
(%)
b
b
e
1
2
99(96 )
20
92
91
94
99
2
1
3
4
5
6
7
22
23
24
25
26
90
99
98
96
95
96
e
95(89 )
89
94

ACCEPTED MANUSCRIPT
e
8
9
98
27
83(79 )
e
e
96(94 )
28
29
30
91(85 )
e
1
0
1
99
89(87 )
e
1
1
1
1
1
92
83(81 )
c
e
2
3
4
5
99(98 )
31
32
81(76 )
c
99(99 )
71
91
94
d
98
93
33
34
e
1
1
6
7
92
98
35
36
81(77 )
e
87(81 )
e
1
1
8
9
90
97
37
92(89 )
a
.
o
Reaction conditions: nitroarenes (1 mmol), Ni-Co BMNPs (0.4 mmol%), N H H O(4 equiv), 80 C, 1h, under argon.
2
4
2
b
c
d
.
4
e
GC yields. Water was used as solvent. N H H O (5 equiv). Isolated yield.
2
2
Subsequently, controlled experiment between the developed
BMNPs versus commercial available nickel or cobalt salt
gave apparent difference, the sole metal salt almost afford no
product (entry 13). As the benchmark, no reaction took place
without addition of N H ·H O (entry 14). Temperature is also
Due to the high activity of our catalyst, we attempted
reduction of 4-nitrotoluene at 1g scale, and the starting
material was equivalently converted into p-toluidine with an
isolated yield about 91%, almost the same as the yield at
small scale. The catalyst can be separated from reaction
mixture, recycle study can be found in supplementary content.
To further show the excellent catalytic activity of this
BMNPs, Turn over frequency (TOF) was calculated on the
basis of the conversion at a reaction time of 15 min (TOF =
2
4
2
crucial for this reaction. The selectivity to p-toluidine was
o
pretty low (entry 15) at 60 C, while both the yield and
selectivity decreased when the reaction temperature was
o
higher than 90 C due to the aggregation of NPs (entries 17-
-
1
1
8) [35]. Screening of organic solvent and water revealed that
0.24 s ).
ethanol was the best candidate (entries 19-24). Water is not an
ideal solvent (entry 24) for the reduction of 4-nitrotoluene, but
it is an excellent medium for the water-soluble substrate (4-
nitrophenol) (entry 25). Reductive potential of the hydrogen
donors also significantly impact the hydrogenation of the nitro
group. Sodium borohydride could yield complete conversion
from nitroarenes to amines (entry 27), while silanes, typically
3
.3. CTH of Nitroarenes into Amines with Hydrazine Hydrate
Catalyzed by Ni-Co BMNPs
The functional group tolerance of the transfer
hydrogenation process catalyzed by Ni Co BMNPs had
been
functionalized nitro compounds under optimized conditions.
The substituted nitrobenzene having electron-donating or
withdraw groups were furnished with excellent yields (Table
50
50
demonstrated
applying
structurally
diverse
Et SiH, proved to be incompatible with poor conversion
3
(
entry 26). Hydrazine hydrate was chosen as the optimal
hydrogen donor in our system because it generates only
nitrogen gas as a byproduct and it is relatively safe and easy
to handle [32].
2
, entries 1-11). When it comes to some water soluble
nitroarenes, reaction can also be carried out at water afford
corresponding amines with excellent yields (entries 12-13).
Halogenated anilines, which are commonly used as precursors
for agrochemicals, were obtained by reduction of halogenated

ACCEPTED MANUSCRIPT
6
nitroarenes without the formation of dehalogenation
Acknowledgments
products (entries 14-20).
This work was a project funded by the Priority Academic
Development of Jiangsu Higher Education Institutions. We
gratefully acknowledge Nature Science Foundation of Jiangsu
Province (BK 20131346, BK 20140776) the Fundamental
Research Funds for the Central Universities (30915011315)
for financial support. This work was also supported by
National Natural Science Foundation of China (21476116,
Furthermore, some multi-substituted nitroarenes can be
completely reduced into amines (entries 21-26). Amino-
substituted N-heterocycles, which are important intermediates
in pharmaceutical and agrochemical industries, were obtained
in good to excellent yields (entries 27-31). Regioselctive
reduction of dinitro group has been investigated, it was found
that only one nitro group was reduced into the respective
amine without affection another nitro group (entry 32), what’s
more, the dinitro compounds can be completely reduced to
provide the diamine products (entries 33-34) by increasing the
amount of hydrazine hydrate to 5 equiv. Importantly, the nitro
groups were reduced into amino groups with good selectivity
in the presence of C=C, C≡C and C≡N bonds (entries 35-37),
which are considered as the main challenge in the CTH of
nitroarenes [16]. However, nitro can’t be selectively reduced
in the presence of formyl substituent, when 4-
nitrobenzaldehyde was selected as substrate, both nitro group
and carbonyl were reduced, and aldimine condensation
product has also been detected.
2
1402093) and Chinese Postdoctoral Science Foundation
(
2015M571761) for financial support.
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[
[
[
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2
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[
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7
prepared bimetallic alloys toward new applications in organic
synthesis, ongoing efforts are dedicated to explore the
mechanism of the enhanced activity and selectivity so as to
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Graphical abstract

ACCEPTED MANUSCRIPT
Highlights:
I. Highly dispread Ni-Co bimetallic nanoparticles
catalysts were prepared
II. The catalysts were used in the CTH of
nitroarenes
III.Ni Co was found to be the best catalyst.
5
0
50
IV.Synergistic effects between Ni and Co are
keys for high activity