Magnetically Recyclable Metal–Organic Framework@Fe3O4 Composite-Catalyzed Facile Reduction of Nitroarene Compounds in Aqueous Medium

Article abstract of DOI:10.1002/aoc.4132

A kind of Metal–organic framework (MOF) composite namely Cu-BTC@Fe₃O₄ (BTC?=?1,3,5-benzenetricarboxylate) was prepared and showed good catalytic activity toward the reduction of nitroarenes. This reaction proceeded smoothly under mild reaction conditions in aqueous medium using sodium borohydride as the reduction agent, affording the corresponding anilines in good to excellent yields. In addition, the catalyst could be easily recovered with an external permanent magnet and be reused for successive six runs with slight decrease in its activity.

Full text of DOI:10.1002/aoc.4132

Received: 4 August 2017
DOI: 10.1002/aoc.4132
Revised: 13 September 2017
Accepted: 15 September 2017
F U L L P A P E R
Magnetically Recyclable Metal–Organic Framework@Fe3O4
Composite‐Catalyzed Facile Reduction of Nitroarene
Compounds in Aqueous Medium
Sen Yang | Zhi‐Hui Zhang
| Qun Chen | Ming‐Yang He | Liang Wang
School of Petrochemical Engineering and
Jiangsu Key Laboratory of Advanced
Catalytic Materials & Technology,
Changzhou University, Changzhou
213164, P. R. China
A kind of Metal–organic framework (MOF) composite namely Cu‐BTC@Fe3O4
(BTC = 1,3,5‐benzenetricarboxylate) was prepared and showed good catalytic
activity toward the reduction of nitroarenes. This reaction proceeded smoothly
under mild reaction conditions in aqueous medium using sodium borohydride
as the reduction agent, affording the corresponding anilines in good to excellent
yields. In addition, the catalyst could be easily recovered with an external
permanent magnet and be reused for successive six runs with slight decrease
in its activity.
Correspondence
Zhi‐Hui Zhang and Liang Wang, School of
Petrochemical Engineering and Jiangsu
Key Laboratory of Advanced Catalytic
Materials & Technology, Changzhou
University, Changzhou, 213164, P. R.
China.
KEYWORDS
Email: zhangzh@cczu.edu;
aqueous medium, magnetic catalyst, metal–organic frameworks, nitro reduction
liangwang@cczu.edu.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 11775037
and 21302014
1 | INTRODUCTION
of catalysts, particularly the heterogeneous catalysts, have
been employed to activate the NaBH4.[11–18] Despite the
Anilines are very important compounds that found wide
applications in organic synthesis, material science, and
many other fields.[1] Generally, the reduction of nitroarene
compounds in the presence of a catalyst and a reductant is
one of the most fundamental and straightforward pro-
cesses to prepare anilines. Various reductants such as
H2,[2] NH2NH2,[3] NH4Cl,[4] alcohols,[5] acids,[6]
great advance in this field, the reported procedures still
suffered some drawbacks such as long reaction time, poor
recyclability of the catalysts as well as tedious work‐up.
Recently, the application of metal‐organic frameworks
(MOFs) in catalysis field has attracted much attention due
to their unique structural and catalytic properties.[19,20]
Although a large number of MOFs have been successfully
developed and applied, we believed that an ideal MOF
catalyst should be readily available or prepared, highly
stable, cost‐effective and easily recoverable. It is worth
noting that Cu‐BTC (H3BTC = 1,3,5‐benzenetricarboxylic
acid) has already been commercially available and also
has shown wide applications in diverse organic
reactions.[21–24] To further improve its stability and recy-
clability, Wang et al. prepared a well‐shaped magnetic
Cu‐BTC@Fe3O4 composite which exhibited good catalytic
activity and reusability toward the C‐H oxidation of
alkylbenzenes.[25] We also successfully applied such
NaHSO3,[7] NaBH4,[8] LiAlH4 and ammonia‐borane[10]
[9]
have already been successfully employed. Despite these
significant progress, there are still some issues to be
addressed. For example, (i) highly active and expensive
metal catalysts (Pd, Pt, or Au) are usually required for cat-
alytic hydrogenation processes; (ii) in some cases, low
selectivity and reaction rates are observed when using
alcohols as the hydrogen source; and (iii) some reductants
are not readily available or expensive. In this context,
NaBH4 turns out to be the most popular reductant since
it is commercially available and cheap. Meanwhile, a series
Appl Organometal Chem. 2017;e4132.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4132

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YANG ET AL.
NaBH4
completion of the reaction, the Cu‐BTC@Fe3O4 was
recovered with an external magnet, and washed with
EtOAc and water. The reaction mixture was extracted
with EtOAc (3 × 5 ml) and dried over Na2SO4. The
organic phase was then concentrated under reduced pres-
sure to give the crude product, which was further purified
by column chromatography with petroleum ether/ethyl
acetate to afford the pure product (if necessary).
NH2
NO2
Cu-BTC@Fe3O4
R
R
EtOH/H2O, 45 o
C
1
2
SCHEME
nitroarenes
1
Cu‐BTC@Fe3O4‐catalyzed the reduction of
magnetically recyclable MOF catalyst in one‐pot C(aryl)‐S‐
P bond formation reaction.[26] Given the importance of
aniline derivatives and in continuation of our interest in
MOF catalysis, herein, we report a facile reduction of
nitroarene compounds in aqueous medium utilizing a
magnetically recyclable Cu‐BTC@Fe3O4 composite
(Scheme 1).
3 | RESULTS AND DISCUSSION
The Cu‐BTC@Fe3O4 composite was prepared according
to our previous report through a secondary growth strat-
egy in non‐DMF solvent media.[25,26] The as‐prepared
composite was well characterized by PXRD, FT‐IR, TGA,
SEM‐EDS and N2 adsorption. The PXRD characterization
confirms the combination of the Cu‐BTC phase and the
Fe3O4 phase within the Cu‐BTC@Fe3O4 composite, as
2 | EXPERIMENTAL
2.1 | General remarks
All reagents were obtained from local commercial sup-
pliers and used without further purification. Thin‐layer
chromatography (TLC) was visualized using UV light.
1H and 13C NMR spectra were recorded on a Bruker
Advance 300 analyzer in Chloroform‐d (CDCl3) using
TMS as an internal standard. Fourier transform (FT)
Infrared data were recorded on an AVATAR‐370 (Nicolet)
spectrometer by transmission through the sample depos-
ited on a KBr pellet. Powder X‐ray diffraction (PXRD) pat-
terns of the as‐synthesized and recycled samples were
obtained using a Rigaku D/max 2500 PC X‐ray diffrac-
tometer with Cu Kα (1.5406 Å) radiation at 10 min−1
.
Thermogravimetric analysis (TGA) experiments were car-
ried out on a TG/DTA 6300 thermoanalyzer from room
temperature to 800 °C under nitrogen atmosphere at a
heating rate of 10 °C/min. Field‐emission Scanning Elec-
tron Microscopy (SEM) images of the samples were taken
at 30 kV with a ZEISS Supra 55 microscope. The composi-
tions of samples were analyzed by ICP (inductively
coupled plasma) analysis (Varian Vista‐AX). N2 adsorp-
tion isotherms were obtained using a Micromeritics ASAP
2460 instrument. All the products are known compounds
and were identified by comparing of their physical and
spectra data with those reported in the literature.
2.2 | General procedure for the reduction
of nitroarenes
In a round‐bottomed flask (10 ml) equipped with mag-
netic stirrer and condenser, nitroarene (1 mmol), EtOH/
H2O (3: 1, 4 ml) and Cu‐BTC@Fe3O4 (15 mg) were added
successively. Then NaBH4 (4 mmol) was added slowly at
room temperature. The reaction mixture was then stirred
at 45 °C for a given time (monitored by TLC). After
FIGURE 1 (a) the PXRD patterns of Cu‐BTC, Fe3O4 and Cu‐
BTC@Fe3O4 samples. (b) N2 adsorption/desorption isotherms and
particle size distribution (insert) of Cu‐BTC@Fe3O4

YANG ET AL.
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(Figure 1b). The composite shows a spherical morphology
with dimensions ranging from 30 to 200 nm and the
majority crystal size of 80–140 nm (Figure 1b insert).
Moreover, EDS and ICP analysis shows that the weight
percentage of Cu‐BTC in the composite is ca. 38.4% (For
details, see Supplementary Material).
With the composite in hand, nitrobenzene (1a) was
selected as the model substrate to optimize the reaction
conditions. Initially, the influence of temperature on the
reaction was examined under the following conditions:
1a (1 mmol), NaBH4 (4 equiv), Cu‐BTC@Fe3O4 (5 mg)
in EtOH (4 ml) under air atmosphere (Figure 2). The
results show that a higher reaction temperature could
benefit the conversion. However, when the reaction was
performed at 100 °C, only 41% yield was obtained (versus
35% yield at 45 °C). Thus 45 °C was chosen as the optimal
reaction temperature.
FIGURE 2 Initial survey on the reaction temperature
indicated in Figure 1a. The comparison of PXRD patterns
between the as‐synthesized composite and Cu‐BTC show
characteristic peaks of the Cu‐BTC moieties. The crystal
diffraction peaks of Fe3O4 can be observed in the PXRD
pattern of Cu‐BTC@Fe3O4 composite. Meanwhile N2
adsorption isotherms of Cu‐BTC@Fe3O4 composite
showed the BET surface area of 423 m2 g−1 and the coex-
istence of microporous and mesoporous adsorption
Further optimization of the reaction conditions are
listed in Table 1. No reaction occurred in the absence of
any catalyst. Catalyst loading survey clearly showed that
15 mg catalyst was the best choice, and further increase
of the catalyst loading to 20 mg did not affect the reaction
efficiency (entries 3~5). Methanol was also tried as the
solvent, however, an inferior result was obtained (47%,
entry 6). It was worth noting that addition of water to
TABLE 1 Optimized the reaction conditionsa
Entry
Catalyst (mg)
Solvent
NaBH4 (mmol)
Yield(%)[b]
1
‐
EtOH
4
n.r.
2
5
10
15
20
15
15
15
15
15
15
15
15
15
15
EtOH
4
4
4
4
4
4
4
4
4
4
4
3
5
4
35
42
3
EtOH
4
EtOH
66
5
EtOH
68
6
MeOH
47
7
EtOH/H2O (1:1)
MeOH/H2O (1:1)
EtOH/H2O (2:1)
EtOH/H2O (3:1)
EtOH/H2O (1:3)
H2O
85
8
78
9
91
10
11
12
13
14
15[c]
99
70
67
EtOH/H2O (3:1)
EtOH/H2O (3:1)
EtOH/H2O (3:1)
84
>99
n.r.
aReaction conditions: 1a (1 mmol), Cu‐BTC@Fe3O4 as the catalyst, NaBH4 as the reductant, solvent (4 ml), 45 °C, 3 h, air. [b] GC yields. [c] Fe3O4 nanoparticles as
the catalyst.

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YANG ET AL.
[a]
TABLE 2 Reduction of nitroarene compounds catalyzed by Cu‐BTC@Fe3O4
Entry
R
Product
2a
Time (h)
Yield (%)
1
C6H5
3
99
2
3
p‐CH3C6H4
p‐MeOC6H4
p‐ClC6H4
2b
2c
2d
2e
2f
3
3
3
3
3
2
2
2
3
3
3
3
3
3
2
3
3
97
94
4
93
5
p‐BrC6H4
93
6
p‐FC6H4
90
7
p‐OHC6H4
p‐NH2C6H4
p‐HOCH2C6H4
p‐NO2C6H4
p‐MeCOC6H4
m‐NO2C6H4
o‐OHC6H4
o‐BrC6H4
2g
2h
2i
98
8
99
9
96
10
11
12
13
14
15
16
17
18
2j
95[b]
99,[c] 95[d]
94[b]
96
2k
2l
2m
2n
2o
2p
2q
2r
93
o‐CH3C6H4
1‐naphthyl
Cyclohexyl
n‐hexyl
95
99
93[e]
91[e]
aReaction conditions: 1 (1 mmol), Cu‐BTC@Fe3O4 (15 mg), NaBH4 (4 equiv), EtOH/H2O (3:1, 4 ml), 45 °C, air, isolated yields. [b] NaBH4 (6 equiv), both the two
[c]
[d]
nitro groups were reduced.
NaBH4 (1 equiv), 20 min, 1‐(2‐nitrophenyl)ethan‐1‐ol was isolated as the product.
NaBH4 (6 equiv), 3 h, 1‐(2‐aminophenyl)
ethan‐1‐ol was isolated as the product. [e] GC yields.
the solvent greatly accelerated the reaction, and almost
quantitative yield was obtained in EtOH/H2O (3:1)
medium (entry 10). This result could be attributed that
water could promote the hydrolysis of amino‐borate
intermediate during the reaction.[8] However, reaction
using water as the solvent afforded a moderate yield
(entry 12). The amount of NaBH4 was then examined
and 4 equivalents of reductant were enough for this
transformation. Finally, to confirm the role of the catalyst,
a blank experiment was carried out by using Fe3O4
nanoparticles as the catalyst. The result clearly indicated
that the copper component was the actual catalyst center
(entry 15).
The scope and limitations of the present protocol was
then evaluated under the optimized reaction conditions.
As shown from Table 2, nitroarenes bearing with either
electron‐withdrawing or electron‐donating substituents
such as halide (Cl, Br and F), alkyl, methoxy, hydroxy,
amino groups underwent the reaction smoothly to afford
the corresponding products in excellent yields (90~99%).
Sterically hindered nitroarenes such as 1m~1o also
survived the reaction conditions to deliver to the corre-
sponding anilines in excellent yields (2m~2o, entries
13~15). It should be pointed out that chemoselective
reduction of keto group over nitro group was observed
for substrate 1k in the presence of only one equivalent
of NaBH4. Both keto group and nitro group were reduced
by employing six equivalents of NaBH4, affording 1‐(2‐
aminophenyl)ethan‐1‐ol as the major product in 95% yield
(entry 11). Moreover, this protocol was also suitable for
the reduction of dinitroarenes in the presence of six equiv-
alents of NaBH4 (entries 10 and 12). Finally, this protocol
was also extended to aliphatic nitro substrates such as
nitrocyclohexane and 1‐nitrohexane. The results showed
NaBH4 (4 equiv)
Cu-BTC@Fe3O4 (0.3 g)
NH2
NO2
EtOH/H2O (80 mL), 45 o
C
20 mmol scale
2a
1.77 g, 95% yield
1a
SCHEME 2 Gram‐scale synthesis of 2a

YANG ET AL.
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FIGURE 3 SEM images of Cu‐BTC@Fe3O4 before and after the catalysis
Finally, hot‐filtration experiment was performed using
the model reaction to investigate whether the reaction
proceeded via a heterogeneous or homogeneous fashion
(Figure 4). The catalyst was removed after 15 min while
the filtrate was kept stirring. The result showed that the
reaction was completely terminated after removal of the
catalyst. Meanwhile, the ICP analysis showed that copper
contents in the reaction mixture was less than 8 ppm,
indicating this catalytic system could be a heterogeneous
fashion.
4 | CONCLUSION
FIGURE 4 Hot‐filtration experiment
In summary, we have developed a facile and environmen-
tally benign protocol for reduction of nitroarenes using
Cu‐BTC@Fe3O4 composite in aqueous medium. The cata-
lyst is readily prepared and cost‐effective. A broad scope
of nitroarenes survived the reaction conditions to afford
the aniline derivatives in good to excellent yields under
mild reaction conditions. Excellent yields, good group
compatibility, as well as the reusability of the catalyst
are the major features of the present catalytic system.
that aliphatic nitro substrates were also well tolerated
under the reaction conditions to provide the correspond-
ing products in excellent yields (entries 17 and 18).
To demonstrate the practicality of the present
protocol, a scale‐up reaction (20 mmol) was carried out
under the optimized conditions (Scheme 2). To our
delight, this reaction provided the desired product 2a in
95% yield (1.77 g), showing a high potential application
for industry scale‐up.
ACKNOWLEDGEMENTS
This project was financially supported by the National
Natural Science Foundation of China (NO. 21302014
and 11775037).
The reusability of the catalyst is also an advantage of
the present protocol. Generally, after reaction, the catalyst
was recovered with an external magnet, and washed by
EtOAc and water. Using substrate 1a as example, the
results showed that the catalyst could be reused without
obvious loss in activity in six consecutive runs (99%,
99%, 99%, 95%, 93% and 92%, respectively). Moreover,
the recovered catalyst was further examined by scanning
electron microscopy (SEM). Results clearly showed that
no obvious aggregation of the catalyst was observed,
which might account for the good reusability of the
catalyst (Figure 3).
ORCID
Zhi‐Hui Zhang http://orcid.org/0000-0002-8744-7897
Liang Wang http://orcid.org/0000-0002-5367-1516
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How to cite this article: Yang S, Zhang Z‐H,
Chen Q, He M‐Y, Wang L. Magnetically Recyclable
Metal–Organic Framework@Fe3O4 Composite‐
Catalyzed Facile Reduction of Nitroarene
Compounds in Aqueous Medium. Appl
Organometal Chem. 2017;e4132. https://doi.org/
10.1002/aoc.4132
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