A suitable modified palladium immobilized on imidazolium supported ionic liquid catalysed transfer hydrogenation of nitroarenes

Article abstract of DOI:10.1016/j.jorganchem.2021.121935

The first well-defined modified palladium immobilized on imidazolium supported ionic liquid catalyst has been developed for the transfer hydrogenation of nitroarenes to anilines in good to excellent yields with formic acid as reducing agent. This methodology applies eco-friendly a reducing agent which is non-toxic, water soluble, more stable and simpler to handle. Particularly, the process constitutes a rare model of base-free transfer hydrogenations. The catalyst was reused up to nine consecutive cycles without any significance loss in its activity.

Full text of DOI:10.1016/j.jorganchem.2021.121935

Journal of Organometallic Chemistry 949 (2021) 121935
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A suitable modified palladium immobilized on imidazolium supported
ionic liquid catalysed transfer hydrogenation of nitroarenes
Ramasamy Shanmugapriyaa, Nallusamy Kanimozhib, Alagudurai Atheeswarib,
Parasuraman Karthikeyana,∗
a PG and Research Department of Chemistry Pachaiyappas College Campus, University of Madras, Chennai, 600 030, Tamilnadu, India
b Department of Chemistry, Dhanalakshmi Srinivasan College of Arts and Science for Women, Preambular 621 212, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The first well-defined modified palladium immobilized on imidazolium supported ionic liquid catalyst
has been developed for the transfer hydrogenation of nitroarenes to anilines in good to excellent yields
with formic acid as reducing agent. This methodology applies eco-friendly a reducing agent which is non-
toxic, water soluble, more stable and simpler to handle. Particularly, the process constitutes a rare model
of base-free transfer hydrogenations. The catalyst was reused up to nine consecutive cycles without any
significance loss in its activity.
Received 18 May 2021
Revised 1 June 2021
Accepted 8 June 2021
Available online 14 June 2021
Keywords:
Palladium
© 2021 Elsevier B.V. All rights reserved.
Metal complex
Green chemistry
Catalyst
Heterogeneous catalyst
1. Introduction
ing Rh [20-22], Ru [23-25], Cu [26-28], and Co [29-31] complexes.
However, some of these procedures suffer from drawbacks such as
Amines and their derivates are widely used as intermediates
in the synthesis of agrochemicals, pharmaceuticals, fine chemicals,
dyes, pigments, material science and biotechnology [1-4]. Due to
their importance, numerous methods have been developed for the
preparation of amines. Especially, the catalytic hydrogenation or
transfer hydrogenation of nitroarenes is one of the fundamental
reactions. However, primary aliphatic amines are produced by re-
ductive amination [5-7], reduction of nitriles [8-10], and direct am-
ination of alcohols [11-15], the most important method to obtain
primary anilines is based on the selective reduction of nitroarenes.
Besides, there is a strong need to develop suitable chemo-selective
catalysts for the reduction of nitro groups in the presence of sensi-
tive functional groups involving green chemistry protocols [16-18].
In the past few years, metal complexes are commonly used
as heterogeneous catalysts for the transfer hydrogenation of ni-
troarenes. However remarkable progress has been achieved in the
selective catalytic hydrogenation of nitroarenes by fine-tuning of
the corresponding heterogeneous catalysts [6-19], still the im-
provement of novel catalysts with broad functional group toler-
ance as well as high activity represents the substantial challenge.
Recently, many reports are on hydrogenation of nitroarenes us-
poor selectivity, long reaction times, high temperatures, and/or the
price of the metal are drawbacks of these protocols, low functional
group tolerance, and require tedious catalyst preparation and char-
acterization techniques.
To this point, glycine containing metal compound is a variety
of catalysts have been more applications into both laboratory and
industry [32-34]. In organometallic chemistry, amino acid contains
ligands concerned much interest with respect to their well-built
coordinating capacities to a variety of metal centers. The recovery
in calculation to that leaching can occur in the extractive work
up leading to a loss of the catalyst in the reaction mixture on
one hand and requests the extra effort toward purifies the ex-
tracted product. To overcome these complications, novel complex
was developed [32-34] by covalent linking of organo-catalytic unit
through an ionic liquid moiety (often chloroglycine). This suggest
that a low solubility of catalyst in the solvents used for extraction
of the product and high solubility in the reaction medium.
Currently, immobilization of metal has improved great value
in catalysis. To avoid the contamination of product with metal-
ligand, and to minimizes the organic waste, catalyst reusability,
cost-effective and less-toxic element is a model candidate to sub-
stitute precious metals of analogous activities as well as selectiv-
ity are achieved. In the recent years, our research team has been
involved in the development of green catalysts and environmen-
tally friendly procedures to organic transformations used in labo-
∗
Corresponding author.
E-mail address: pkarthikeyan99@gmail.com (P. Karthikeyan).
https://doi.org/10.1016/j.jorganchem.2021.121935
0022-328X/© 2021 Elsevier B.V. All rights reserved.

R. Shanmugapriya, N. Kanimozhi, A. Atheeswari et al.
Journal of Organometallic Chemistry 949 (2021) 121935
mixture = 5:1). All the obtained products are well-known in liter-
ature and were analyzed by 1H-NMR spectroscopy [2-5].
3. Results and discussion
At the initial of this project, the influence of various reaction
parameters has been investigated for the nitrobenzene reduction
as model reaction like best catalytic system, catalyst loading, tem-
perature, solvent and hydrogen source. Unfortunately, no reduc-
tion product was detected in the absence of catalyst (Table 1,
entry 1). To our delight, when using a palladium salts, the re-
duction reaction gave aniline in 2- 34 % (Table 1, entries 2-
6). Next, we studied the activity of synthesized 1-carboxy ethyl-
3-methyl imidazolium bromide [Cemim]Br, 1-(2-aminoethyl)-3-
methylimidazolium bromide [Aemim]Br, 1-glycyl-3-methyl imida-
zolium chloride [Gmim]Cl catalyst and it was found that [Gmim]Cl
catalysed furnished less yield of desired product (Table 1, en-
tries 7-9). An additionally we tested the combination of PdCl2
and the different ionic liquids led to significant activity to the ni-
trobenzene redaction reaction (Table 1, entries 10-12). However,
Among them homogeneous/heterogeneous [Gmim]Cl-Pd(II) cata-
Scheme 1. Catalytic reduction of nitrobenzene.
ratories and industries. The palladium immobilized on imidazolium
supported ionic liquid, [Gmim]Cl-Pd (II) with a nano sphere struc-
ture has been applied in catalysts owing to its advantages of envi-
ronmentally compatible, moisture insensitive, high dispersion, high
reactivity and easy separation. Recently, our group has developed
novel palladium complex and applied for carbon-carbon bond for-
mation reactions [40-42]. To the best of our knowledge, we have
first reported eco-friendly palladium immobilized glycine function-
alized ionic liquid and effectively applied transfer hydrogenation
of nitroarenes into anilines using HCOOH as a hydrogen source
(Scheme 1).
Table 1
Influence of the different catalyst.a
2. Experimental
Entry
Catalyst
Conversionb (%)
Yieldb (%)
TON/TOFh
1
–
00
05
10
18
26
40
21
25
28
54
59
65
99
99
95
99
94
99
00
02
06
13
22
34
19
23
26
52
54
63
94
94
92
94
90
94
00
2.1. Materials and methods
2
Pd2(dba)3
200/66
3
Pd(NO3)2
600/200
All solvents and chemicals were commercially available and
used without further purification unless otherwise stated. The 1H-
NMR spectra were recorded on a Bruker 500 MHz. FT-IR was
recorded on AVATRA 330 spectrometer with DTGS detector. Col-
umn chromatography was performed on silica gel (200-300 mesh).
Analytical thin-layer chromatography (TLC) was carried out on pre-
coated silica gel GF-254 plates.
4
Pd(OAc)2
1300/433
2200/733
3400/1133
1900/633
2300/766
2600/866
5200/1733
5400/1800
6300/2100
9400/3133
9400/3133
9200/3066
9400/3133
9000/3000
9400/3133
5
Pd(acac)2
6
PdCl2
7
[Cemim]Br
8
[Aemim]Br
9
[Gmim]Cl
10
11
12
13
14
15
16
17
18
PdCl2/[Cemim]Br
PdCl2/[Aemim]Br
PdCl2/[Gmim]Cl
[Gmim]Cl-Pd(II)
[Gmim]Cl-Pd(II)c
[Gmim]Cl-Pd(II)d
[Gmim]Cl-Pd(II)e
[Gmim]Cl-Pd(II)f
[Gmim]Cl-Pd(II)g
2.2. Synthesis of catalyst
The catalyst 1-carboxy ethyl-3-methyl imidazolium bro-
mide [Cemim]Br, 1-(2-aminoethyl)-3-methylimidazolium bromide
[Aemim]Br, 1-glycyl-3-methyl imidazolium chloride [Gmim]Cl and
[Gmim]Cl-Pd (II) was synthesized according to literature [38-40].
2.2.1. Spectral data of 1-carboxy ethyl-3-methyl imidazolium
bromide [Cemim]Br: FT-IR (KBr, νmax, cm−1): 3433, 1685; 1H-NMR
(500 MHz, DMSO-d6, TMS): δ 2.35 (t, 2H), 3.79 (s, 3H) 3.94 (t, 2H),
7.20 (d, 1H), 7.32 (d, 1H), 7.78 (s, 1H), 9.26 (s, 1H); 13C-NMR (125
MHz, DMSO-d6): δ 23.32, 36.70, 54.09, 123.25, 123.45, 177.55.
2.2.2. Spectral data of 1-(2-aminoethyl)-3-methylimidazolium
bromide [Aemim]Br: FT-IR (KBr, νmax, cm-1): 3295, 1637; 1H-NMR
(500 MHz, DMSO-d6, TMS): δ 2.49 (t, 2H), 3.94 (s, 3H), 5.11 (t, 2H),
5.74 (s, 2H), 7.1 (d, 1H), 7.5 (d, 1H), 7.7 (s, 1H); 13C-NMR (125 MHz,
DMSO-d6): δ 36.56, 38.34, 53.09, 129.19, 129.23, 139.19.
2.2.3. Spectral data of 1-glycyl-3-methyl imidazolium chloride
[Gmim]Cl: FT-IR (KBr, cm−1): 3429, 3372, 2933, 2855, 1628, 1526
and 1382. 1H-NMR (500MHz, DMSO-d6): δ 2.2 (s, 1H), 3.3 (s, 3H),
5.0 (s, 2H), 6.92 (d, 1H), 7.0 (d, 1H), 7.6 (s, 1H), 9.1 (s, 1H).
2.3. The general procedure for the reduction of nitrobenzene is
as follows.
a
Reaction conditions: nitrobenzene (1 mmol), different catalyst (0.01 mol%),
b
formic acid (3 mmol), temperature (25°C), time (3 h).
determined by GC.
C
d
e
catalyst concentration (0.02 mol%). catalyst concentration (0.009 mol%).
time (4 h). time (2 h). temperature (30°C). hTOF-turnover frequency (mol
product per mol catalyst h−1).
f
g
Table 2
Influence of the different solvent.a
Entry
Solvent
Conversionb (%)
Yieldb (%)
TON/TOFc
1
Et2O
6
4
400/133
2
DCM
18
18
10
42
3
13
14
08
38
1
1300/433
1400/466
800/266
3
n-heptane
p-xylene
Toluene
MeCN
DMF
4
5
3800/1266
100/33
6
7
27
73
59
56
48
75
78
15
99
24
69
52
50
43
72
75
13
94
2400/800
6900/2300
5200/1733
5000/1666
4300/1433
7200/2400
7500/2500
1300/433
9400/3133
8
THF
9
DMA
10
11
12
13
14
15
EtOH
MeOH
iPrOH
n-BuOH
Water
–
To a mixture of nitro compound (1 mmol), palladium catalyst
(0.01 mol %), formic acid (3 mmol) was added and then the re-
action mixture was stirred at 25°C for 3-4 h. The progress of re-
action was monitored by thin layered chromatography (TLC) and
gas chromatography (GC). All catalytic reactions were performed at
least twice to ensure reproducibility. After completion of the reac-
tion, the catalyst was separated by filtration method. The anilines
were purified by column chromatography (n-hexane/ethyl acetate
a
Reaction conditions: nitrobenzene (1 mmol), palladium cat. (0.01
mol%), formic acid (3 mmol), different solvent (5 mL), temperature
(25°C), time (3 h). determined by GC. cTOF-turnover frequency (mol
product per mol catalyst h−1) .
b
2

R. Shanmugapriya, N. Kanimozhi, A. Atheeswari et al.
Journal of Organometallic Chemistry 949 (2021) 121935
Table 3
full conversion and 99 % yield aniline (Table 1, entry 16). How-
ever, the yields were dropped significantly on decreasing the time
from 3 to 2 hour (Table 1, entry 17). In contrast to other proto-
cols, this reduction can be performed even at 30°C, while at 25°C
only 3 hours reaction time is necessary to complete the reduction
without any loss (Table 1, entry 18).
Influence of the different hydrogen donor.a
Entry
Hydrogen donor
Conversionb (%)
Yieldb (%)
TON/TOFc
1
–
00
04
20
16
10
36
46
03
99
05
99
00
01
17
14
07
34
45
02
94
03
94
-
2
CH3COOH
B2(OH)4
NH2NH2 H2O
H3PO2
100/33
3
1700/566
1400/466
700/233
3400/1133
4500/1500
200/66
4
Next influence of solvents was also examined and is shown in
Table 2. we screened various polar and non-polar solvents to the
model reaction. The reduction reaction couldn’t proceed smoothly
in nonpolar solvents (Table 2, entries 1-5) but they still show lit-
tle lower yield compared with H2O (Table 1, entry 14). Though,
the use of dipolar solvent such as DMF, MeCN, THF provided the
desired product with slower reaction rates (Table 1, entries 6-
8). Afterward, reduction of nitrobenzene to aniline often occurred
in polar solvents in poor to moderate yields (Table 1, entries 9-
13). However, under the same conditions, we have performed the
model reaction in absences of solvent and 94 % yield of reduction
was observed (Table 1, entry 15).
5
6
NaBH4
7
NH4HCO2
HCOONa
HCOOH
8
9
9400/3133
300/100
9400/3133
10
11
HCOOH/Et3N
HCOOHd
a
Reaction conditions: nitrobenzene (1 mmol), palladium cat. (0.01 mol%),
formic acid (3 mmol), different solvent (5 mL), temperature (25°C), time (3 h).
b
determined by GC. cTOF-turnover frequency (mol product per mol catalyst
d
h
−1). conc. of HCOOH (4 mmol).
The effect of hydrogen donor was also the subject of further in-
vestigations (see Table 3). Initially, no reduction reaction was found
when the absence of hydrogen donor (Table 3, entry 1). However,
reaction was carried using several hydrogen suppliers and conver-
sion 1- 45%yields (Table 3, entries 2-8). On the other hand, formic
acid is superior in comparison to various hydrogen donor (Table 3,
entry 9). To study the reduction of nitro group via a hydrogena-
tion method we performed the pallidum catalyzed reduction of ni-
lyst was found to be the best catalyst providing 99% conversion
in 3 hours (Table 1, entry 13). Subsequently, similar yield was
achieved when escalating the catalyst amount from 0.01 to 0.02
mol % (Table 1, entry 14) and it was found that conversion de-
creases with decrease in catalyst loading (Table 1, entry 15). Next,
the reaction conditions for the active catalyst system were further
optimized the reaction duration. The experiments showed that on
increasing the time from 3 to 4 hours gave the same results with
Table 4
Palladium-catalyzed reduction of substituted nitrobenzene.a-c
a
Reaction conditions: substituted nitrobenzene (1 mmol), palladium cat. (0.01 mol%), formic acid (3 mmol),
b
temperature (25°C), time (3-4 h). determined by GC. cTOF-turnover frequency (mol product per mol catalyst
−1). All the products were characterized by spectroscopic analysis 1H-NMR known compounds were compared
with authentic data [2-5].
h
3

R. Shanmugapriya, N. Kanimozhi, A. Atheeswari et al.
Journal of Organometallic Chemistry 949 (2021) 121935
Scheme 2. Proposed mechanistic pathways of nitrobenzene reduction.
Table 5
trobenzene at 3 bar of hydrogen pressure. Especially, when pres-
ence of HCOOH no reactivity was observed. Subsequently, reduc-
tion reaction was run the same condition in the presence of 3
equiv of HCOOH and an additional of nitrobenzene. In this test, we
observed if the catalytic active is designed by reduction reaction
with formic acid and then is able to use HCOOH as the “correct”
reducing agent.
Recycling of the catalyst for the reduction of nitrobenzene.a
Run
0
1
2
3
4
5
6
7
8
Yieldb (%)
94
94
93
93
91
90
88
88
86
a
Reaction condition: substituted nitrobenzene (1 mmol), palladium
b
cat. (0.01 mol%), formic acid (3 mmol), temperature (25°C), time (3 h).
determined by GC.
Later, the effect of base addition to the HCOOH was also tested,
triethylamine which is regularly required for transfer hydrogena-
tion, is not essential but causes even lower reactivity (Table 3, en-
try 10). However, to our best knowledge these this rare example
of a base-free catalytic transfer hydrogenation of nitro compounds.
Supplementary, increase of hydrogen donor concentration from 3
to 4 mmol has found no effect on selectivity (Table 3, entry 11).
Hence, the optimized reaction conditions for transfer hydrogena-
tion of nitroarene (1 mmol), palladium cat. (0.01 mol%) and formic
acid (3 mmol) at room temperature for 3-4 h.
ganic residue. The filtrated catalyst dried in oven and reused for
next run. The catalyst was found to be efficiently recycled up to
nine successive cycles with continuing high activity.
The direct reduction of the nitro group via condensation of
the initially formed nitrosoarene and the hydroxylamine is possi-
ble (Scheme 2). In this case, the subsequent condensation product
is supplementary reduced through azobenzene and 1,2-diphenyl-
hydrazine to give aniline. Nevertheless, in the reaction mixture of
the typical reaction, only negligible quantities of azobenzene are
observed as side products.Table 4
With optimized conditions to demonstrate the general appli-
cability of this catalyst system for wide range substituent aro-
matic nitro reduction reaction. Easy alkyl substituted nitroarenes
are reduced to corresponding amines in 85-90% yield (4a-b). Next,
o-, m-, and p-chloride substituted nitroarenes were studied and
it was found that very easily reduced into corresponding amines
with full conversion is achieved, giving yields of 85-88% within
3-4 h (4c-e). In addition, the position of the halide substituent
did not induce any substantial differences in reactivity. Hence, ni-
trobenzene containing o-, p- fluoro, bromo and m-, p- iodo was
successfully reduced to halo anilines in reasonable yields (4f-k).
However, the p-methoxy group in nitrobenzene could also be re-
duced into related amine successfully in 75% yield (4l). finally,
the reduction of o-, m-, and p- phenol substituted nitroarenes re-
duced into the desire products without affecting the -OH groups
(4m-o). In all the entries the established catalyst furnished high
conversion with excellent yield without forming negligible side
products such as hydroxylamine, hydrazine, azoarenes, or azoxy
arene.
In Scheme 2, the projected mechanism of our palladium-
catalyzed transfer hydrogenation is exposed. Coordination of for-
mate will lead to the neutral complex [HCOO Pd] (catalytic cycle
blue colour). β-hydride elimination from [HCOO Pd], which is well-
known in other formate decomposition reactions [35-37], liberates
CO2 and results in complex [HPd]. Consequent protonation from
HCOOH leads to the corresponding palladium dihydride complex
[H2Pd], which will reduce nitrobenzene to nitrosobenzene and wa-
ter. Following reduction of the latter intermediate in a similar cat-
alytic cycle leads to phenyl hydroxylamine and finally to aniline
(Reduction cycle in red color). As cited above, these intermediates
are not observed in the reaction and should be therefore faster re-
duced as compared to the initial nitrobenzene.
To compare the efficiency of our catalyst with some of the re-
ported catalysts for the nitrobenzene reduction reaction, we have
tabulated the results of these catalysts for the synthesis of aniline.
From the table, our catalyst is superior to some of the previously
reported catalysts in terms of reaction condition, reaction time, and
yield.Table 6
Next our attention on reusability of palladium catalyst for re-
duction reaction as shown in table 5. After completion of reaction,
catalyst was isolated by filtration method. The filtrate catalyst was
washed with ethyl acetate several times to remove the rest of or-
4

R. Shanmugapriya, N. Kanimozhi, A. Atheeswari et al.
Journal of Organometallic Chemistry 949 (2021) 121935
Table 6
Comparison of reported catalysts with [Gmim]Cl-Pd (II).
Entry
Catalyst and conditions
Time (h)
Yield (%)
Ref.
1
2
3
4
5
Pd(OAc)2 (0.01 mmol), ionic liquid (0.08 mmol), KOH (0.30 mmol), hydrazine hydrate (0.5 mL), water (2 mL), 50OC
8 h
95
99
[43]
PdCl4-me-Im@SBA-15 (2 mol%), ammonium formate (3 mmol), methanol (10 mL), 70O
Pd - BisILs[PF6] (5 μmol), hydrogen pressure (1 atm), distilled water (5 mL), 25°C.
P(DVB-DIIL)-Pd (2.0 mol%), ethanol (2 mL), H2 (0.1MPa)
C
1 h
[44]
8.5 h
1 h
100
99
[45]
[46]
Palladium cat. (0.01 mol%), formic acid (3 mmol), 25°C
3 h
94
Present Work
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
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Shanmugapriya:
Investigation,
Methodol-
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