CO-free, aqueous mediated, instant and selective reduction of nitrobenzeneviarobustly stable chalcogen stabilised iron carbonyl clusters (Fe3E2(CO)9, E = S, Se, Te)

Article abstract of DOI:10.1039/d0ra04491a

Highly stable and thermally robust iron chalcogenide carbonyl clusters Fe₃E₂(CO)₉(E = S, Se or Te) have been explored for the reduction of nitrobenzene. A 15 min thermal heating of an aqueous solution of nitrobenzene and hydrazine hydrate in the catalytic presence of Fe₃E₂(CO)₉(E = S, Se or Te) clusters yield average to excellent aniline transformations. Among the S, Se and Te based iron chalcogenised carbonyl clusters, the diselenide cluster was found to be most efficient and produce almost 90% yield of the desired amino product, the disulfide cluster was also found to be significantly active, produce the 85% yield of amino product, while the ditelluride cluster was not found to be active and produced only 49% yield of the desired product. The catalyst can be reused up to three catalytic cycles and it needs to be dried in an oven for one hour prior to reuse for the best results. The developed method is inexpensive, environmentally benign, does not require any precious metal or a high pressure of toxic CO gas and exclusively brings the selective reduction of the nitro group under feasible and inert free conditions.

Full text of DOI:10.1039/d0ra04491a

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CO-free, aqueous mediated, instant and selective
reduction of nitrobenzene via robustly stable
chalcogen stabilised iron carbonyl clusters
Fe E (CO) , E ¼ S, Se, Te)†
Cite this: RSC Adv., 2020, 10, 32516
(
3
2
9
Charu Sharma, Avinash Kumar Srivastava,
and Raj Kumar Joshi
Aditi Soni, Sangeeta Kumari
*
Highly stable and thermally robust iron chalcogenide carbonyl clusters Fe
been explored for the reduction of nitrobenzene. A 15 min thermal heating of an aqueous solution of
nitrobenzene and hydrazine hydrate in the catalytic presence of Fe (CO)
(E ¼ S, Se or Te) clusters
3 2
E
(CO)
9
(E ¼ S, Se or Te) have
E
3 2
9
yield average to excellent aniline transformations. Among the S, Se and Te based iron chalcogenised
carbonyl clusters, the diselenide cluster was found to be most efficient and produce almost 90% yield of
the desired amino product, the disulfide cluster was also found to be significantly active, produce the
5% yield of amino product, while the ditelluride cluster was not found to be active and produced only
9% yield of the desired product. The catalyst can be reused up to three catalytic cycles and it needs to
be dried in an oven for one hour prior to reuse for the best results. The developed method is
inexpensive, environmentally benign, does not require any precious metal or a high pressure of toxic CO
gas and exclusively brings the selective reduction of the nitro group under feasible and inert free conditions.
8
Received 20th May 2020
Accepted 8th August 2020
4
DOI: 10.1039/d0ra04491a
rsc.li/rsc-advances
5
reaction. Nevertheless, due to the high catalytic potential of
Fe(CO) , it has been extensively used for various carbonylation
5
Introduction
Since the initial application of an iron catalyst by Reppe and reactions as well as as for high-nuclearity clustere forma-
14
1
6–11
12
13
Vetter in 1953, iron-based catalysis has inuenced the estab- tions. Electron-transfer, hydrogenation, hydride transfer
15
lishment of various catalysts based on 4d and 5d transition and electrochemical methods are some of the important
elements. However, before Reppe and Vetter's pioneering work, available methodologies to furnish distinct amines by utilizing
2,3
Fe in the form of iron carbonyl had been known since 1891.
a nitro precursor. The reduction of nitrobenzene has gained
Being the second most abundant metal in the earth's crust, Fe is tremendous interest due to its hazardous effect on the envi-
always under the limelight to explore as a catalyst for a variety of ronment. The process of reduction is quite complicated as the
organic reactions. Since, an efficient, cost-effective, fast, clean nitro group reduction advances in stages and stops due to the
and selective catalyst is needed in modern research, iron is the formation of hydroxylamine and azoarene as side products at an
4
16
most attractive option available in the surroundings. However, intermediate stage. Traditionally, catalytic transfer hydroge-
17
18
19
catalysts derived from 4d and 5d transition metals and other nation has been employed in the presence of Ru, Rh, Pd, or
rare earth metals are more efficient than 3d transition metals, Ni (ref. 20) metal catalysts. Very few studies have utilized inex-
21
22
but also expensive in meeting present and upcoming societal pensive Co, Fe, or Zn (ref. 23) metals for transfer hydroge-
4
demands. Zero-valent iron in the form of Fe(CO)
5
is one of the nation; however, economic consequences strongly favour the
least expensive forms and of ubiquitous availability. In situ iron-based system due to its greater environmental
activation and catalysis by Fe(CO)5 require a strong base, sustainability.
moreover, the di-sodium tetracarbonyl ferrate Na Fe(CO) and
Traditionally, a plentiful inexpensive iron powder was used
2
4
iron tetra carbonyl hydride H Fe(CO)
2 4
species were formed for nitro reduction, but it has low efficiency and produces
during the reaction, these species are highly temperature and a large amount of wastage. The nano-form of zero-valent iron
light sensitive and drastically affect the productivity of the (nZVI) was used for the reduction of nitro derivatives, but it was
unsuccessful, as the catalytic activity of NPs is signicantly
related to their specic morphology, which is difficult to
Department of Chemistry, Malaviya National Institute of Technology, Jaipur 302017, produce in bulk. Moreover, the formation of nZVI is a dynamic,
Rajasthan, India. E-mail: rkjoshi.chy@mnit.ac.in
time-consuming and laborious process. Apart from that, the
accumulation of nZVI reduces its mobility, dispersibility and
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra04491a
1
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simultaneously reduces its catalytic reactivity. Zero-valent a large scale. The present method produces signicant trans-
metal carbonyl clusters have some unique inherent properties formations and does not generate any waste (Scheme 2). The
of catalysis and thermal stability. Moreover, the absence of large catalyst is found to be efficient and brings the desired trans-
bulk phases results in a high surface to volume ratio, which is formation in just 15 min. Moreover, it works smoothly in
ꢀ
benecial for catalysis, since it minimizes the reaction rate per aqueous media at 110 C and is applicable for a large variety of
25
unit amount of catalyst, which likewise limits the cost. Metal functionalised nitrobenzenes.
carbonyls and clusters have been used for a wide range of
2
6
27
catalysis: in particular, for carbonylation, C–C coupling,
28
29
hydroformylation, hydroesterication, reduction of nitro- Results and discussion
30–32
aranes
formations.
1
and many more value-added organic trans-
Iron complexes have been established as excellent catalysts for
various organic transformations, our group has also exploited
6–11
Initially (Scheme 1), Pittman and co-workers in
979 utilised Rh (CO) for nitro reduction. Later, in 2014,
Kaneda and co-workers used a composite dendrimer of Rh
carbonyl cluster and polyamine composite for nitro reduction.
Recently in 2017, Lipshutz and co-workers used carbonyl iron
powder under aqueous micellar catalysis condition. Fe(CO)
and Fe (CO)12 have also been used for nitro reduction, but all
the methods require rather drastic reaction condition,
including long duration, high pressure of CO gas, handling of
3
3
6 12
6,47
quite a few iron based organic transformations.
The litera-
5
ture encouraged us to investigate the catalytic activity of
chalcogen-stabilized iron carbonyl complexes. This is the rst
report which utilises Fe E (CO) (E ¼ S, Se or Te) clusters for the
34
35
3
2
9
5
reduction of nitrobenzene and its derivatives to their corre-
sponding arylamines. In the very rst reaction, a dioxane solu-
tion of nitrobenzene and hydrazine (a source of hydrogen) with
3
3 2 9
an Fe Se (CO) (3 mol%) cluster was reuxed with continuous
highly toxic Fe(CO) , reducing agent and various supporting
5
monitoring of the reaction on a TLC. Aer 6 h, complete
consumption of reactant along with the signicant formation of
the desired reduction product was experienced. The reaction
was reinvestigated in the absence of Fe-catalyst, but no chem-
ical transformation was recorded. Then, the reaction was opti-
mized in search of the best suitable parameters for maximum
transformation (Table 1). Initially, the amount of iron catalyst
was optimised: 1 mol% of the catalyst failed to initiate the
reaction, while 1.5 mol% of the catalyst started to produce the
desire product in below to average amount (29%), however, the
yield was improved from 29 to 47, 68 and 89% with 2, 2.5 and
36
reagents for to promote the reaction.
Recently, chalcogen and metal chalcogenide complexes have
come under the spotlight, these complexes are continuously
breaking the stereotype of air and moisture sensitive metal
complexes. Due to good electron donor properties of chalcogen
atoms, chalcogenide complexes have emerged as a potential
alternative to phosphine complexes. Their ability to withstand
air/moisture and their permanence during catalysis makes
them suitable catalysts for long-duration reactions, which
37,38
39
include C–C couplings
transformations.
C–N couplings and other organic
40
3
mol% of the Fe-catalyst, respectively. No substantial
improvement in the yield of aniline was experienced with 3.5 or
mol% of catalyst loading. Hence, 3 mol% of catalyst was the
ideal amount for a creditable transformation (Table 1, entries 2–
). The other chalcogen-stabilised iron carbonyl clusters
Fe (CO) and Fe Te (CO) were also explored for the present
reaction (Table 1, entries 9 and 10). It was observed that
Fe (CO) shows almost comparable catalytic efficiency to the
selenium analogue, (82% yield of aniline), while the Fe Te (-
In addition to gaseous hydrogen, several other reducing
agents have been used that allow the efficient reduction of
nitrobenzene when used in combination with metals, including
boranes, NaBH4, silanes and hydrazine.
hydrate is used as a very suitable reducing reagent as it gener-
ates only N as a by-product. It is fairly safe and easy to handle
compared with its unstable anhydrous form. Many nitro
reduction protocols using a combination of iron complexes and
hydrazine hydrate have been published in the last few years.
This paper is designed to report a selective reduction of
nitroarenes with hydrazine, catalysed by zero-valent Fe Se (CO)
9
and Fe S (CO) carbonyl clusters. These clusters are robustly
stable, insensitive towards air, moisture, water and sunlight,
can be stored for a long time and can be easily synthesized on
4
41
42
43
44–46
Hydrazine
8
3
S
2
9
3
2
9
2
3
S
2
9
4
7–49
3
2
CO)9 was not efficient as it yielded average (49%) trans-
formation of the desire product. Hence, the order of catalytic
3
2
efficiency for these catalysts is Fe
Fe Te (CO) . The difference in catalytic efficiency may be due to
the electron-donor properties of S/Se/Te.
3 2 9 3 2 9
Se (CO) z Fe S (CO) >
3
2
9
3
2
9
37
In a further attempt to optimize the amount of hydrazine
hydrate which was used as a hydrogen source in the reaction,
a 0.5 mmol amount of hydrazine produced 19% yield of aniline,
while increasing the amount of hydrazine signicantly
increased the yield of aniline; however, no considerable
Scheme
reduction reactions. Most metal carbonyl catalysed nitro reduction
reactions require a high pressure of CO and H2(g)
1 Previous reports on metal carbonyl catalysed nitro
.
3 2 9
Scheme 2 Nitro reductions catalysed by an Fe Se (CO) cluster.
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Table 1 Optimization of various parameters with nitrobenzene
ꢀ
a
S. no.
Solvent
Fe cat
Cat, mol%
N
2
H
4
, mmol
Temp., C
Time (min)
Yield , %
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
—
b
b
b
b
b
b
b
a
—
1
1.5
2
2.5
3
3.5
4
3
3
3
3
3
3
3
3
3
3
3
3
2
2
2
2
2
2
2
2
2
2
0.5
1
1.5
2
2.5
3.0
2
2
2
2
2
2
2
2
2
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
80
120
130
150
110
110
110
110
110
110
110
110
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
360
10
7
nd
nd
29
47
68
89
90
90
85
49
19
37
69
87
88
90
85
88
87
89
27
75
69
79
82
45
5
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
c
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
2
2-Propanol
Butanol
sec-Butanol
EtOH
MeOH
Toluene
3
3
3
3
3
3
3
3
15
15
15
15
15
15
15
15
2
2
2
5
6
7
2
2
2
Dimethyl formamide
Dioxane
19
a
Isolated yields, (a) Fe
3 2 9 3 2 9 3 2 9 2 4 2
S (CO) , (b) Fe Se (CO) , (c) Fe Te (CO) , optimised conditions: nitrobenzene (1 mmol), N H $H O (2 mmol), catalyst
ꢀ
Fe
3
Se (CO) (3 mol%), temperature 110 C, time 15 min, solvent water.
2
9
improvement was observed beyond the 2 mmol amount of 1, entries 26–27). Besides hydrazine, other sources of hydrogen
hydrazine (Table 1, entries 11–16). Here, 2 mmol of hydrazine (Table 2), including isopropyl alcohol with KOH, and acetic acid
hydrate serves as the optimal amount to achieve the desired or formic acid with triethylamine, were considered, but none of
quantity of hydrogen for the reaction. During the temperature them mimicked the reaction. Moreover, not even a trace of
optimization, the reaction was found a bit sluggish at below product was observed when the reaction mixture was reuxed
ꢀ
ꢀ
6
8
0 C; moreover, it took 6 h to produce 85% transformation at for up to 12 h. An insignicant transformation was obtained in
0 C (Table 1, entry 17). But, increasing the temperature, the an isopropyl alcohol and KOH solution, and it was not worth
ꢀ
duration of the reaction is drastically reduced, and at 110 C, considering.
the reaction took just 15 min to brings the 90% transformation,
ꢀ
while at 120, 130 and 150 C, the reaction was completed in just
1
0, 7 and 2 min, respectively (Table 1, entries 18–20). In the
Table 2 Optimisation of reagents used as in situ hydrogen sources for
the reaction
present method, water was used as the solvent, and the reaction
was reuxed between 100 and 110 C. The solvent was also
a
ꢀ
optimized, and the highest yield was achieved in water, which is S. no.
a green and economical solvent.
Reagents
% yield
1
Isopropyl alcohol and KOH
Acetic acid with triethylamine
Formic acid with triethylamine
Hydrazine
<20
—
—
Reaction also shows the signicant transformations in
methanol, ethanol, butanol, and sec-butyl alcohols however,
isopropyl alcohol was not found suitable as it produces only
2
3
4
89
2
7% product (Table 1, entries 21–25). Other organic solvents,
a
Nitrobenzene (1 mmol), catalyst Fe
10 C, time 15 min, solvent water.
3
Se
2
(CO)
9
(3 mol%), temperature
including toluene, 1,4-dioxane and dimethylformamide
produce 45%, 19% and 5% yields of aniline, respectively (Table
ꢀ
1
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Substrate scope
was repeated with an increased quantity of hydrazine. A slightly
reduced reduction of 2-aminonitrobenzene to 1,2-dia-
minobenzene (11, 76%) was observed due to the ortho positions.
However, a signicantly enhanced yield (12, 86%) of the meta-
In order to check the scope of the reaction, various functionally
different nitroarenes were screened at the optimized reaction
parameters (Table 3). In general, most of the reactions produced
excellent yield of the desired amine products. However, some
variation was also observed with the change in the position of
the functional group at the benzene ring. In comparison to
para- and meta-substitution, the ortho-functionalised nitroben-
zene derivatives show a bit less transformation. The electronic
nature of the functional groups also inuences the desired
transformation, and a slightly better yield was obtained with
electron-withdrawing group attached to nitrobenzene. The
initial reaction of nitrobenzene produces an excellent 89% yield
of reductive product, i.e., aniline (1). Furthermore, the substrate
scope was extended to halide derivatives of nitrobenzene: 4-
chloro-, 4-bromo- and 4-iodonitrobenzene react smoothly and
produce 86%, 84% and 83% yield of the respective aniline
products. The position of the substituent also inuences the
substituted aminonitrobenzene was recorded.
A similar
reduced trend was also observed with other ortho- and para-
functional derivatives. The ortho- and para-derivatives of methyl
nitrobenzene produced 71% (13) and 73% (14) transformation
of their respective products. A marginally increased yield was
recorded with para-methoxy nitrobenzene (15, 80%). Functional
group interference was expected, but para-nitro acetophenone
produced an excellent 83% (16) yield of desired product.
Furthermore, the reaction of para-nitro benzyl alcohol conve-
niently react to produce 79% (17) of the corresponding product.
Similarly, 1-nitronaphthalene also participated well in the
reaction to produce 87% (18).
Catalyst recycling
productivity: a slightly reduced yield was obtained with 2-bro- The catalyst was isolated from the organic layer and used again
monitro benzene (5, 79%). 2,5-di-bromonitrobenzene also with an aqueous layer for another catalytic cycle. In the second
reacted efficiently and produced an excellent yield (6, 82%) of and third catalytic cycles, 63 and 46% transformations of the
the respective product. Various electron-withdrawing groups desired product were obtained, respectively. This indicates the
were also tested for the present reaction, para-hydroxy and para- decreasing catalytic efficiency of the catalyst. Besides this, it was
cyanonitrobenzene both yield highly signicant trans- also noted that there was 20% loss of catalyst in each catalytic
formations (7, 87%) and (8, 85%) of the respective aniline run. This may be due to the presence of moisture on the surface
ꢁ
1
products. The 1,4-dinitrobenzene reacted under the optimised of the catalyst, as a broad signal for moisture (at 3409 cm ) was
conditions to give an outstanding yield with a selective reduc- recorded in the FTIR spectrum of the used catalyst (Fig. S1,
tion of one nitro group, and para-nitroaniline (9) was isolated ESI†). Therefore, the catalyst has to be dried in an oven for 1 h
with 88% transformation. Moreover, a 75% transformation of before using it for the next catalytic cycle.
1,4-diaminobenzene (10) was observed when the same reaction
Plausible mechanism
While performing control experiments it was observed that the
reaction has good selectivity towards the electron-withdrawing
functionalities. A reaction mixture of 4-methylnitrobenzene
and 4-cyanonitrobenzene (1 equivalent of each) under opti-
mised reaction condition produces para-cyano aniline (73%) as
a major product and para-methyl aniline (27%) as minor
product (Scheme 3).
Table 3 General scope of the reaction with various derivatives of
nitrobenzene
a
In another control experiment maintaining similar reaction
conditions and the same reagents, aer 15 minutes of the
reaction, furthermore 2 equiv. of hydrazine was added and the
reaction was again run for next 15 minutes. Aer two consec-
utive cycles, 91 and 56% yields of p-cyano aniline and p-methyl
aniline were obtained respectively. In the second run, the
required hydrazine was added, but the activity of the catalyst
was considerably compromised. Therefore only a marginal
increase in the yield of p-cyano aniline was observed, while due
to the signicant quantity of unreacted 4-methyl nitrobenzene it
showed a signicant increase in transformations.
a
Reaction conditions: Fe
$H
3
Se
2
(CO)
9
(3 mol%), nitrobenzene derivatives
ꢀ
(1 mmol), N
2
H
4
2
O (2 mmol), temperature 110 C, time 15 min,
solvent water, isolated yields.
Scheme 3 Control experiment showing affinity towards the with-
drawing group.
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ꢀ
However, neither a detailed study conducted nor any inter- stirring at 110 C for 15 min. Aer completion of the reaction,
mediate species were isolated during the present trans- the reaction mixture was cooled to room temperature, and the
formation, but a tentative mechanism has been proposed on desired organic product was extracted in an organic layer of
the basis of evidence available in the literature (Scheme 4). ethyl acetate. The organic layer was then dried over anhydrous
There are strong evidences in the literature suggests that Fe(0) Na
2 4
SO , and ethyl acetate was evaporated on a rotavapor at
and Se both are individually excellent catalysts for nitro reduced pressure to obtain the crude product. Finally, the crude
48–55
reduction.
product was subjected to a chromatographic work-up and
Initially, the Fe-cluster (1) interacts with nitrobenzene in the further purication and spectral characterization.
presence of water and is oxidised and releases the required
electrons with the simultaneous loss of CO to form intermediate
Conclusion
53
(2), which facilitates the formation of nitrosobenzene (a). The
hydrazine coordinates with intermediate (2) to produce (3). The
interaction of the proton present in the close vicinity of the
bridged Se in intermediate (3) leads to the loss of the Fe–Se
bond. The deprotonation of intermediate (3) results in inter-
mediate (4). Moreover, a simultaneous protonation of nitro-
sobenzene forms intermediate arylhydroxylamine (b). Further
consecutive deprotonation of intermediate 4 and protonation of
arylhydroxylamine form intermediate 5 and arylamine, respec-
tively. Loss of N2 and reunion of CO with intermediate 5
regenerate the catalyst (1). The nitro reduction is possible via
either four or six electron transfer. The six electron transfer
In conclusion, a robustly stable, reusable, highly economical,
easy to synthesise and zero-valent iron chalcogenide carbonyl
cluster Fe Se (CO) has been explored for the reduction of
3 2 9
nitroarenes. The chalcogen-stabilised Fe-cluster brings about
an excellent transformation to a range of functionalised nitro-
arenes under the inert free conditions and in an aqueous
medium. The methodology was found to be strongly feasible
and worked in the green solvent water. Moreover, it also avoids
the use of a precious metal catalyst or supporting reagents. The
catalyst can be reused for three catalytic cycles. The present
methodology also provides a selective but temperature-
dependent timeframe (2–15 min) to accomplish the reaction.
This is the rst report where zero-valent chalcogen-stabilised
iron carbonyl clusters were explored for nitro reduction in the
absence of high CO pressure.
47
strongly needs an alkaline medium and during the six electron
transfer, nitrosobenzenes may undergo condensation. This is
quite rare, but the formation of azoxybenzene followed by azo-
benzene and hydrazobenzene to aniline is frequently
50
observed.
Conflicts of interest
Experimental section
General procedure for the catalytic synthesis
There are no conicts to declare.
In a clean reaction tube were placed the Fe Se (CO) catalyst
3
2
9
(
13.5 mg, 3 mol%) and the derivative of nitroarenes (1 mmol). Acknowledgements
To this were added hydrazine hydrate (64 mL, 2 mmol) and water
Raj K. Joshi thanks the CSIR (01(2996)/19/EMR-II) for nancial
assistance. Charu Sharma, Sangeeta Kumari and Avinash K.
Srivastava thank MNIT for the research fellowships. Aditi Soni
thanks UGC for a research fellowship. The authors also
acknowledge the MRC, MNIT for providing characterisation
facilities.
as a solvent, the reaction tube was heated with continuous
Notes and references
1
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1
2
3
4
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5
6
7
8
1
2
Scheme 4 A plausible mechanism for the nitro reduction.
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