Base-free chemoselective transfer hydrogenation of nitroarenes to anilines with formic acid as hydrogen source by a reusable heterogeneous Pd/ZrP catalyst

Article abstract of DOI:10.1039/c4ra06174h

A highly efficient, chemoselective, environmentally-benign method is developed for the catalytic transfer hydrogenation (CTH) of nitroarenes using FA as a hydrogen source. Various supported Pd catalysts were examined for this transformation, and Pd supported ZrP (Pd/ZrP) proved to be the best catalyst for CTH of nitrobenzene. Applicability of the Pd/ZrP catalyst is also explored for hydrogenation of various substituted nitroarenes. The Pd/ZrP catalyst showed high specificity for hydrogenation of nitro groups even in the presence of other reducible functional groups such as -CC, -COOCH₃, and -CN. To investigate the reaction mechanism, a Hammett plot was obtained for CTH of p-substituted nitroarenes. The active site is thought to be in situ generated Pd(0) species as seen from XRD and TEM data. The Pd/ZrP catalyst is reusable at least up to 4 times while maintaining the same activity and selectivity. To the best of our knowledge, this is one of the best methodologies for CTH of nitroarenes under base-free conditions with high activity and chemoselectivity over heterogeneous Pd-based catalysts. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra06174h

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Base-free chemoselective transfer hydrogenation
of nitroarenes to anilines with formic acid as
hydrogen source by a reusable heterogeneous Pd/
ZrP catalyst†
Cite this: RSC Adv., 2014, 4, 38241
Jaya Tuteja, Shun Nishimura and Kohki Ebitani*
A highly efficient, chemoselective, environmentally-benign method is developed for the catalytic transfer
hydrogenation (CTH) of nitroarenes using FA as a hydrogen source. Various supported Pd catalysts were
examined for this transformation, and Pd supported ZrP (Pd/ZrP) proved to be the best catalyst for CTH
of nitrobenzene. Applicability of the Pd/ZrP catalyst is also explored for hydrogenation of various
substituted nitroarenes. The Pd/ZrP catalyst showed high specificity for hydrogenation of nitro groups
3
even in the presence of other reducible functional groups such as –C]C, –COOCH , and –C^N. To
investigate the reaction mechanism, a Hammett plot was obtained for CTH of p-substituted nitroarenes.
The active site is thought to be in situ generated Pd(0) species as seen from XRD and TEM data. The Pd/
ZrP catalyst is reusable at least up to 4 times while maintaining the same activity and selectivity. To the
best of our knowledge, this is one of the best methodologies for CTH of nitroarenes under base-free
conditions with high activity and chemoselectivity over heterogeneous Pd-based catalysts.
Received 24th June 2014
Accepted 8th August 2014
DOI: 10.1039/c4ra06174h
www.rsc.org/advances
Great attempts for serving high selectivity in the reduction of
nitro group even under the presence of other reducible func-
Introduction
Anilines are important intermediates and key precursors in the tional groups have been reported by using addition of stoi-
8
synthesis of dyes, pigments, agrochemicals, pharmaceuticals chiometric reducing agents such as sodium hydrosulte, iron,
1
9
10
11
pesticides, herbicides, and ne chemicals. The conventional tin or zinc in ammonium hydroxides, silanes, decaboranes,
12
synthesis methods for aniline are: transition metal-catalysed (a) and formates. However, these processes are not environmen-
cross-coupling reaction of ammonia with aryl halides, phenol tally sustainable, and the high selectivity is generally achieved
derivatives and aromatic boronic acid or (b) hydrogenation of on the expense of activity. Therefore, there is a strong incentive
corresponding nitroarenes. The latter approaches of catalytic to develop chemoselective methods for reduction of substituted
2
3
4
5
reduction of nitroarenes are particularly attractive owing to nitroarenes without using toxic additives and hydrogen gas.
their high atom efficiency and compatibility with large scale
Catalytic transfer hydrogenation (CTH), in which hydrogen
processes.
donors are used instead of pressurized-hydrogen gas, is more
Industrial preparation of aniline performed by Bechamp advantageous way from the view point of a cost and handling; it
6
process involving Fe/HCl as a reducing agent has now been eradicates the use of any special equipment for high pressured
replaced by more environmentally-benign catalytic protocols. gas. Moreover, it is said that CTH enhances the selectivity in the
Hydrogenation of nitroarenes using high pressure H
2
gas in reduction process via competitive adsorption of the liquid
conjunction with a range of heterogeneous catalysts has been hydrogen donor on catalyst surface. Therefore, CTH methods
the area of interest of a number of publications. Commercially induced the considerable technical improvement over the
13
7
available Ni or Pt based heterogeneous catalysts are commonly rather messy traditional reduction with homogeneous transi-
14
used for this transformation, however, the drawbacks of these tional metals and acid reported in previous literatures.
catalysts are their poor selectivity and inapplicability for
substituted nitroarenes.
A wide variety of homogeneous and heterogeneous catalytic
systems are developed for CTH of nitroarenes in combination
with iso-propanol–potassium hydroxide or formic acid-organic
15
base as hydrogen source. Other organic compounds known as
the hydrogen donor in CTH are sodium formate, methanol,
hydrazine, indoline, tetrahydroquinolines, 3-pyrroline, piperi-
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi, Ishikawa 923-1292, Japan. E-mail: ebitani@jaist.ac.jp; Fax: +81-761-
5
1-1149; Tel: +81-761-51-1610
16
†
Electronic supplementary information (ESI) available: Kinetics of nitroarenes, dine, pyrollidine, glucose, and glycerol.
1
13
H-NMR and C-NMR of isolated aniline, TEM images and particle size
distribution of Pd supported catalysts. See DOI: 10.1039/c4ra06174h
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Formic acid (FA), a non-toxic liquid at room temperature, Catalyst preparation
has been suggested as the promising hydrogen donor. It can be
traditionally obtained from biomass processing and produces
Synthesis of ZrP. Zirconium phosphate (ZrP) with a molar
ratio of P/Zr ¼ 2 has been prepared as mentioned in our
CO as sole product; i.e. it is a safe, cheap and high potential
21
2
2 2
previous report. An aqueous solution of 0.1 M ZrOCl $8H O
17
hydrogen donor. Though high efficiencies of FA as hydrogen
source have been obtained in combination with a high amount
of organic bases, these homogeneous bases result in increase of
complexity and cost in practice. Very recently, Beller and co-
(
100 mL) and 0.2 M sodium dihydrogen phosphate (100 mL)
was mixed at pH 1–2 dropwisely with continuous stirring at 343
K. The obtained gelatinous precipitates were digested for 1 h at
43 K, ltered, washed with water, and dried under vacuum at
3
18
workers reported base-free CTH reactions of nitroarenes with
FA using homogeneous iron complex as catalyst affording high
activity and selectivity under mild conditions. Despite accom-
plishment of many studies, search for an efficient, chemo-
selective, base-free, heterogeneously-catalysed, cost-effective
and environmentally-benign procedure for CTH of nitroarenes
is of continuing interest.
room temperature. The obtained material was converted to the
acid form by treating with 1 M HNO for 30 min with occasional
3
shaking. The sample was then separated from the acidic solu-
tion by decantation, and washed with distilled water for
removal of adhering acid. This acid treatment was repeated at
least ve times. Aer nal washing, the material was dried at
room temperature, and then characterized with XRD, N
2
The extensive research on hydrogenation reactions sug-
gested that the presence of Brønsted acid plays an crucial role
2
ꢀ1
adsorption (BET surface area 140 m
further studies.
g
) and applied for
19
for such reactions. We recently demonstrated zirconium
Synthesis of Pd/ZrP. Pd/ZrP was prepared by an adsorption
phosphate (ZrP) acted as an efficient solid acid support which
21
method as mentioned in our previous report with some
20
possessed a high ratio of Brønsted/Lewis acid sites for Pd-
catalysed hydrogenolysis of 5-hydroxymethyl furfural to 1,6-
modication. Pd(NO
solution (20 mL), and then ZrP (1 g) was mixed under stirring for
h at room temperature. The solid was ltered, washed with
3 2
) (46.1 mg) was resolved into an aqueous
21
hexanediol using FA as a hydrogen source. This result fasci-
nated us to pursue potential applications of Pd/ZrP as the
catalyst in CTH reactions of organic substrates such as
nitroarenes.
2
water and dried at room temperature for overnight under
vacuum. Then, 2.1 wt% Pd/ZrP catalyst as a brown coloured
powder was obtained.
In this study, a convenient and highly chemoselective
approach is developed for base-free CTH reactions of nitro-
arenes under environmentally-benign conditions using FA and
Pd/ZrP as a hydrogen source and a reusable heterogeneous
catalyst, respectively. The catalytic system was further applied
for the CTH of various substituted nitroarenes to the corre-
sponding anilines, and a plausible mechanistic pathway is
proposed based on the experimental results.
Characterization
Characterization of the catalyst was conducted using several
techniques. X-ray diffraction pattern (XRD) was recorded with a
Rigaku Smart Lab diffractometer using Cu Ka radiation within
q ¼ 2–80ꢁ range. Transmission Electron Microscopy (TEM)
2
study was carried out in a Hitachi H-7100 instrument operating
at an accelerating voltage at 100 kV. TEM samples were
prepared by dispersing the catalyst powder in ethanol under
ultrasonic radiation for 1–2 min, and then the resulted solution
was dropped on a copper grid followed by slow evaporation of
solvent under vacuum at room temperature. Inductive Coupled
Plasma-Atomic Emission Spectroscopy (ICP-AES) for determi-
nation of the actual amount of metal loading and metal leach-
ing test of reaction mixture was measured on Shimadzu ICPS-
Experimental
Reagents
All reagents and solvents were obtained from commercial
suppliers and used without further purication. Zirconium
chloride oxide octahydrate (ZrOCl
phosphate (NaH PO $2H O), nitric acid, hexane, toluene, N,N-
dimethylformamide (DMF), chloroform, dichloromethane
DCM), iso-propanol (i-PrOH), methanol, butanol, ethylacetate,
2 2
$8H O), sodium dihydrogen
2
4
2
7000 Ver. 2. Nuclear Magnetic Resonance (NMR) spectroscopy
was executed on Br u¨ cker advance 400 MHz instrument using
(
CDCl as a solvent.
3
ethanol, and naphthalene were purchased from Kanto Chem-
icals Co. Inc. Methyl-3-nitrobenzoate, methyl-4-aminobenzoate
and 4-chloronitrobenzene were provided by Aldrich. Formic
Catalytic reactions
acid, niobic acid, sulphated zirconia, palladium nitrate, nitro- Reaction of various nitroarenes was performed in a Schlenk
benzene, aniline, 3-nitrotoluene, 3-aminotoluene, 4-chloroani- glass tube attached to a reux condenser. In a typical CTH
line, 2-nitroanisole, 2-anisidine, methyl-3-aminobenzoate, 4- reaction, requisite amounts of substrate and catalyst were
nitrobenzonitrile, 4-aminobenzonitrile, and tetrahydrofuran weighed and mixed in ethanol solvent in the presence of FA.
were supplied by Wako Chemicals. 4-Nitrostyrene, 4-amino- The reactor was heated on an oil bath at 313 K for t h. Aer the
styrene, 2-nitrobiphenyl, 2-aminobiphenyl, 1-nitronaphthalene, completion of reaction for desired time, the reactant was cooled
and 1-aminonaphthalene were provided by Tokyo Chemical to room temperature, and naphthalene was added as an
Industry (TCI) Co. Ltd. HY zeolite (Si/Al ¼ 2.8, JRC-Z-HY-5.5), internal standard. The ltrate was analysed by a Shimadzu gas
ZSM-5 (Si/Al ¼ 45, JRC-Z5-90H(1)), and SiO
2
–Al
2
O
3
(Si/Al ¼ 2.1, chromatography (GC-17A) with an Agilent DB-1 column and a
JRC-SAH-1) were obtained from the Catalysis Society of Japan.
ame ionization detector. Conversion, yield and selectivity were
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calculated using the calibration curve method following these Table 1 Nitrobenzene reduction using various palladium catalysts
a
and FA
equations:
ꢀ
ꢁ
mmol of reactant detected
ꢂ 100
mmol of reactant taken
%
Conversion ¼ 100 ꢀ
mmol of product formed
ꢂ 100
mmol of reactant taken
%
Yield ¼
NB
AN
Particle
size /nm
b
Entry
1
Catalyst
Pd/ZrP
Conv./%
Yield/%
Sel./%
%
yield
ꢂ 100
conversion
%
Selectivity ¼ %
3.5
>99
>99
99
99
<1
100
61
98
90
87
87
8
>99
>99
99
99(97 )
0
98
59
92
84
79
75
4
>99
>99
100
100
0
98
97
94
93
91
86
—
c
2
d
3
e
g
4
f
5
Results and discussion
6
7
Pd/ZSM-5
2.8
c
21
22
Our own experience and literature survey of Brønsted acid-
mediated CTH reaction served as the starting point of our
studies. Pd was selected because it is known as an active metal
for several hydrogenation reactions. Initial experiments were
conducted to explore the role of support for Pd using FA for CTH
8
9
Pd/HY zeolite
3.8
Pd/Nb
Pd/g-Al
Pd/SiO
2
O
5
H.D.
H.D.
3.5
10
2
O
3
23
11
2 2 3
–Al O
2
ꢀ
1
1
2
3
Pd/SO
ZrP
4
/ZrO
2
H.D.
5
<1
<1
4
0
0
—
of nitrobenzene to aniline at 313 K. Numerous supports 14
Blank
Blank
0
0
20
f
including ZrP with high number of Brønsted acid sites,
15
zeolites, niobic acid, sulphated-zirconia, and g-alumina having
moderate Brønsted acid sites were chosen.
As shown in Table 1, all catalysts promoted the formation of
aniline to a considerable extent under the general condition
a
c
e
Reaction conditions: nitrobenzene (NB, 1 mmol, 5 mmol, 20 mmol),
c
e
c
e
ethanol (5 mL, 20 mL, 50 mL), FA (3 mmol, 15 mmol, 60 mmol),
temp. (313 K, 353 K), time (2 h, 3 h, 15 min, 12 h), 2.1 wt% Pd
catalyst (20 mg,
d
c
d
e
c,e
b
50 mg). Determined by TEM observation (See
f
g
Fig. S3, ESI). Without FA. Isolated yield. H.D.: Hardly distinguished.
(
1 mmol, 313 K, 2 h). Especially, Pd/ZrP and Pd/ZSM-5 stand out
with the high activities towards aniline as the sole product
entries 1 and 6), which could be attributed to high amount of
(
Brønsted acid sites of these catalysts. It has been understood reduction. Thus leaching of Brønsted acid sites and poisoning
that with the increase in Si/Al ratio, the Brønsted acid sites of Pd by sulfate ions could be one of the reasons for low activity
2
ꢀ
increases due to extra framework aluminium, thus the activity is of Pd/SO
associated with the tetrahedrally coordinated aluminium Though many supports including ZrP can afford small size
species in close proximity to Brønsted acid sites in ZSM-5. Pd particles between 2.8 and 3.8 nm, it was observed that there is
supported on HY and SiO –Al
having low Si/Al ratio were no correlation between Pd mean particle size and the activity of
4
2
/ZrO .
24
2
2 3
O
also found to be fairly active (entries 8 and 11). Though the catalyst. This fact suggested that the high number of Brønsted
number of Brønsted acid sites owing to Si–O–Al units would acid sites of support is one of the key factors for novel activity in
increase with decrease of Si/Al ratio, Pd/HY zeolite (Si/Al ¼ 2.8) CTH reaction of NB using FA. The high activities of the Pd/ZrP
showed little higher activity than Pd/SiO
is known that not only the amount of Brønsted acid sites but of nitrobenzene. Remarkably, Pd/ZrP catalyst showed higher
also the hydrophobicity derived from Si/Al ratio and topology activity than Pd/ZSM-5 (entries 2 and 7).
2
–Al
2
O
3
(Si/Al ¼ 2.1). It and Pd/ZSM-5 lead us to further examine the 5 mmol-scale CTH
strongly contributed on the catalysis over Si–O–Al type of
The metal-free ZrP showed very less activity with only 4%
materials. These factors also supposedly inuenced on their conversion of nitrobenzene (entry 13), resultantly, claries the
catalytic activities for the reaction. Notable yields (79–84%) of necessity of metal for the CTH process with FA. Furthermore,
aniline were attained with Pd supported on Nb O and g-Al O
the CTH reaction was also examined in the absence of Pd/ZrP
having moderate Brønsted acid sites (entries 9 and 10). catalyst and/or FA (entries 5, 14 and 15). In all three cases, the
2
5
2 3
2
ꢀ
Surprisingly, Pd supported on SO
4
2
/ZrO was found to be least reaction scarcely occurred. It follows that, the observed high
active for this CTH. It has been reported that there is high catalytic activity could be accounted for the presence of both Pd/
2
ꢀ
probability of leaching of Brønsted acid sites of SO
4
/ZrO
2
in ZrP and FA. Moreover, the present transfer hydrogenation
25
2ꢀ
polar protic solvents. The Pd loading onto SO
4
/ZrO was methodology is so effective that it can completely reduce
2
carried out in water in our case. This catalyst synthetic meth- nitrobenzene in just 15 minutes at 353 K without any loss in
odology is believed to result in loss of Brønsted acid sites of selectivity (entry 3).
2
ꢀ
SO4 /ZrO in accordance with previous literatures. Addition-
A 20 mmol-scale transfer hydrogenation was also performed
ally, the sulfate ions may also act as poison for the active Pd. It is to check the viability of the process. Remarkable activity (yield >
well known that BaSO poisons Pd's activity in the Rosenmund 99%) was observed at 313 K with a TON of 1977 (entry 4). Aer
2
4
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completion of the reaction, the catalyst was simply removed by a

ltration method. The ltrate was condensed under reduced
pressure to obtain aniline as crude product. Aniline was
1
13
obtained in 97% as isolated yield (entry 4g). H- and C-NMR
were conducted with the crude product only (see ESI, Fig. S1 and
S2†) and compared with NMR of authentic aniline. NMR anal-
ysis conrms that the NMR of crude product exactly matches
1
with authentic aniline and no extra peak was observed. H-NMR
3
(CDCl , TMS, 400 MHz): d 3.5 (s, 2H, amino), 6.6–7.1 (m, 5H,
phenyl ring) ppm.
Type of solvent was found to be crucial in transfer hydroge-
nation of nitrobenzene under our reaction conditions. Various
aprotic and protic solvents were attempted to use for the CTH
reaction of nitrobenzene over Pd/ZrP with FA as shown in Table
2. Alcoholic solvents showed good activity for the CTH, among
which ethanol led to the near full conversion of nitrobenzene
with almost 100% yield of aniline (entry 1). iso-Propanol which
would act both as a solvent and hydrogen donor showed good
activity with 76% conversion but with low selectivity (entry 2).
Butanol and methanol also gave mild or low activity (entries 3
and 4).
26
Fig. 1 Time course for transfer hydrogenation of nitrobenzene.
Reaction conditions: nitrobenzene (1 mmol), 2.1 wt% Pd/ZrP (20 mg),
ethanol (5 mL), FA (3 mmol), 313 K.
It is known that alcohols can serve as a hydrogen source in
that either there is no formation of intermediates or they react
immediately on catalytic surface before desorbing.
27
transfer hydrogenation, however, the reaction without FA in
ethanol solvent did not give any product in our case (Table 1,
entry 5). Accordingly, the contribution from alcohol solvents as
the hydrogen source in this reaction system is negligible. None
of the aprotic solvent catalysed the CTH of nitrobenzene to a
signicant extent (entries 5–10) under present reaction
conditions.
Reusability
Recyclability of the catalyst is an important criteria in hetero-
geneous catalytic system. On that account, the reusability of
catalyst for CTH of nitrobenzene was investigated. In each run,
aer the reaction, the catalyst was separated by centrifugation,
washed thoroughly with ethanol, dried in vacuum for overnight,
and then the dried catalyst was used for further reaction.
The results in Fig. 2 indicated that the Pd/ZrP catalyst can be
reused at least 4 times without losing the activity and selectivity.
The aliquots of the reaction mixture aer rst and fourth run
were analysed by ICP-AES, no Pd was detected in the solutions.
Time course of the reaction
To investigate the route of the reaction, the yield and selectivity
of aniline were monitored as a function of reaction time, as
shown in Fig. 1. Nitrobenzene conversion and aniline formation
exactly follows each other and selectivity was near 100%
throughout the reaction process under present reaction condi-
tions. Remarkably, no intermediate was detected; this suggests
Table 2 Screening of different solvents in the CTH reaction of nitro-
a
benzene towards aniline over Pd/ZrP
NB
AN
Entry
Solvent
Conv./%
Yield/%
Sel./%
1
2
3
4
5
6
7
8
9
Ethanol
iso-Propanol
n-Butanol
Methanol
Toluene
Tetrahydrofuran
N,N-Dimethylformamide
Ethylacetate
Chloroform
>99
76
54
40
40
10
7
7
6
4
>99
60
47
27
5
5
5
3
2
>99
79
87
67
—
—
—
—
—
—
1
0
Dichloromethane
2
a
Fig. 2 Reusability of Pd/ZrP for transfer hydrogenation of nitroben-
Reaction conditions: NB (1 mmol), 2.1 wt% Pd/ZrP (20 mg), solvent
(
5 mL), FA (3 mmol), 313 K, 2 h, 500 rpm. NB: nitrobenzne; AN: aniline. zene. Reaction conditions: nitrobenzene (1 mmol), 2.1 wt% Pd/ZrP
20 mg), ethanol (5 mL), FA (3 mmol), 313 K, 2 h.
(
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The hot ltration test was also conducted in order to re-affirm The results shown in Table 3 are under optimum rFeaction
the ideal heterogeneity of Pd/ZrP. For this, the catalyst was conditions for each substrate. It was seen that longer reaction
removed aer 1 h and reaction was continued with the ltrate. time is required for CTH reaction of substituted nitroarenes to
The results showed that the reaction didn't proceed. These achieve >90% yield of the corresponding anilines. Entries 2 and
conrmed that there was no signicant leaching of metal from 3 are for 3-nitrotoluene and 4-chloronitrobenzene, respectively.
the catalyst surface and the reaction is truly heterogeneous.
Notably, this catalyst system can efficiently transfer hydrogen to
produce substituted anilines at 313 K. Halo-nitroarenes have
tends to undergo hydrogenolysis (of C–X bond), but in our case
no hydrogenolysis was proceeded and 4-chloroaniline was
CTH of substituted nitroarenes
Aer optimizing the conditions for CTH reaction of nitroben- formed in 93% yield with >99% selectivity. Nitro groups
zene over Pd/ZrP, we further examined the scope of this catalytic attached to a naphthalene and biphenyl ring were also
system for CTH reactions of various substituted nitroarenes. successfully reduced with excellent yield and selectivity of the
a
Table 3 Transfer hydrogenation of various substituted nitro aromatic compounds over Pd/ZrP and FA
Entry
1
Reactant
Product
Time/h
2
Conv./%
>99
Yield/%
>99
Sel./%
100
2
6
18
16
92
94
96
92
93
94
100
99
b
3
4
5
99
16
24
12
94
94
93
91
99
98
96
6
7
>99
>99
8
9
20
18
98
93
95
91
97
97
c
1
0
6
97
90
92
a
b
c
Reaction conditions: substrate (1 mmol), 2.1 wt% Pd/ZrP (20 mg), FA (3 mmol), ethanol (5 mL), temp. (313 K), 500 rpm. Temp. (333 K). 2.1 wt%
Pd/ZrP (10 mg).
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corresponding anilines (entries 4 and 5). Reductions of nitro
groups in the presence of other functional groups such as
–
OCH , –COOCH (entries 6–8) proceeded with high tolerance
3 3
for these functional groups to afford the corresponding anilines
with >90% yield and selectivity.
The nitro group was selectively hydrogenated in the presence
of easily reducible nitrile group with 91% yield (97% selectivity)
of 4-aminobenzonitrile (entry 9). The most challenging task was
the chemoselective reduction of nitro group in the presence of
–C]C as the reactivity order for hydrogenation of olens are next
to nitro group. 4-Nitrostyrene was chemoselectively transformed
to 4-aminostyrene as major product with 92% selectivity (entry
10); in this case, the by-product of 4-ethylaniline was detected.
The cause for the high chemoselectivity might be explained in
terms of high electrostatic interactions of polar nitro group with
28
the catalyst surface as compared to non-polar –C]C bond.
Hammett plot
The reactivity of substrates in heterogeneous catalysis mainly
depends on the charge density at the reaction centre and steric
factor of substrates. Therefore, to study the reaction mechanism
of CTH, the initial rate for various substituted nitroarenes with
electron-withdrawing groups (EWG) like –COOCH , –Cl, –C^N
3
at para position were measured. Initial rates and rate constants
are shown in Table S1, in ESI.†
The graph is plotted for log(K/K
0
) against the Hammett
substitution constants (sigma) in Fig. 3, where K is an initial
rate constant for substituted nitrobenzene and K is an initial
29
0
rate constant for nitrobenzene. The plot shows a linear rela-
tionship with a slope of ꢀ1.0. The negative slope with such a
high value indicates that the present CTH proceeds via cationic
intermediate and EWG hinders the rate of reduction of
30
nitrobenzene.
Fig. 4 (a) XRD patterns for (1) ZrP support, (2) fresh Pd/ZrP, and Pd/ZrP
after (3) first run and (4) fourth run. (b) & (d) are TEM images and (c) & (e)
are particle size distribution for Pd/ZrP prior and after the reaction
XRD and TEM
Fig. 4 shows the XRD pattern for fresh as well as used catalysts. respectively.
The XRD pattern clearly demonstrates that there is no structural
deterioration of catalyst aer the reaction. However, an addi-
tional peak at 40 degree was observed for post reaction catalysts,
which corresponds to Pd(0). This indicates that Pd(II) is reduced
to Pd(0) during the reaction.
While performing the reaction, it was also seen that the
catalyst colour changes to black from its initial brown colour
within a few minutes of the reaction. The high reusability of
catalyst without any oxidation treatment of catalyst (shown in
Fig. 2) hints that the active site for transfer hydrogenation is in
situ formed Pd(0) instead of Pd(II). From decades, it's known
that the catalytic hydrogenation reaction requires metal (in
reduced form). FA has the ability and can convert Pd(II) from
31
Pd(0). Very recently, Wang et al., synthesised Pd nanoclusters
31
from Pd(II) using FA as a reducing agent at room temperature.
From Fig. 4(b–e), we observed that the average particle size
which was 3.5 nm for catalyst prior to reaction increases to 5.4
nm aer the catalytic run. An increase in particle size aer
Fig. 3 Hammett plot for reduction of p-substituted nitroarenes.
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reaction observed by TEM measurements may account the electron decient N of nitro group, respectively, to form 4
+
reduction of ionic Pd into Pd(0) nanoparticles in accordance (Fig. 5). One of the hydroxyl group in 4 attacks on the H (from
with the XRD measurements.
FA) to generate 5 eliminating a water molecule. Further protons
are trapped and the free aniline is desorbed from catalytic
surface as shown in 5 and 6 (Fig. 5).
Proposed mechanism
A high value negative slope was observed in the Hammett
plot signies a well-developed positive charge on the transition
state as discussed in Fig. 3. Thus, in strong agreement to the
Hammett plot a cationic intermediate was proposed for the
transformation.
A reaction mechanism is proposed for the CTH of nitroarenes
with FA as a hydrogen source as shown in Fig. 5 based on the
experimental results mentioned above. At the initial stage, FA
+
ꢀ
decomposes on the Pd/ZrP surface into CO
2
and H , H (1 in
The high activity of Pd/ZrP catalyst may be attributed to its
high capacity of holding nitroarenes by electrostatic interac-
tions between polar nitro group with H and H on Pd/ZrP. This
strong interaction leads to direct reduction of nitroarenes to
anilines without the formation of any intermediates.
Fig. 5), CO evolution is conrmed by GC-8A (TCD detector) of
2
32
collected gas from the reaction.
+
ꢀ
ꢀ
The H adsorbs on Pd surface to reduce Pd(II) into Pd(0) (2 in
Fig. 5). Since the nitro group attached to the benzene ring can
pull electrons more strongly from benzene ring, and can easily
be adsorbed on the surface of catalyst. Consequently, nitro-
arenes adsorbs on the catalyst surface by electrostatic interac-
tion as shown in 3 (Fig. 5).
Conclusions
+
ꢀ
Thereaer, H from Brønsted acid sites of ZrP and H (from In summary, a highly efficient, base-free, heterogeneously-cat-
FA) adsorbed on Pd are transferred to electron rich O and alysed, and chemoselective CTH methodology is developed for
Fig. 5 Proposed mechanism for transfer hydrogenation of nitroarenes over Pd/ZrP.
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the synthesis of industrially important anilines. The Pd/ZrP
catalyses the selective transfer hydrogenation of various func-
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The peak of CO was observed which clearly demonstrates
the decomposition of FA, which may illustrate that the FA
2
+
ꢀ
is dissociated into CO
2
, H and H underour reaction.
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