Hydrogenation of nitroarenes catalyzed by a dipalladium complex

Article abstract of DOI:10.1002/aoc.3976

A dipalladium complex [Pd₂(L)Cl₂](PF₆)₂ (2), via the substitution of (PhCN)₂PdCl₂ with 5-phenyl-2,8-bis(6′-bipyridinyl)-1,9,10-anthyridine (L) followed by the anion exchange, was found to be a good pre-catalyst for the reduction of nitroarenes to yield the corresponding anilines under atmospheric pressure of hydrogen in methanol. This method provides a straightforward access to a diverse array of functionalized anilines, exhibiting a possible application in synthetic chemistry. The catalytic activity of this complex is enhanced by the di-metallic system via the synergistic effect.

Full text of DOI:10.1002/aoc.3976

Received: 1 May 2017
DOI: 10.1002/aoc.3976
Revised: 15 June 2017
Accepted: 19 June 2017
F U L L P A P E R
Hydrogenation of nitroarenes catalyzed by a dipalladium
complex
Ming‐Uei Hung | Shu‐Ting Yang | Mani Ramanathan | Shiuh‐Tzung Liu
Department of Chemistry, National
A dipalladium complex [Pd (L)Cl ](PF ) (2), via the substitution of
Taiwan University, Taipei 106, Taiwan
2
2
6 2
(
PhCN) PdCl2 with 5‐phenyl‐2,8‐bis(6′‐bipyridinyl)‐1,9,10‐anthyridine (L)
2
Correspondence
followed by the anion exchange, was found to be a good pre‐catalyst for the
reduction of nitroarenes to yield the corresponding anilines under atmospheric
pressure of hydrogen in methanol. This method provides a straightforward
access to a diverse array of functionalized anilines, exhibiting a possible appli-
cation in synthetic chemistry. The catalytic activity of this complex is enhanced
by the di‐metallic system via the synergistic effect.
Shiuh‐Tzung Liu, 1, Sec.4, Roosevelt Road,
Department of Chemistry, National
Taiwan University, Taipei 106, Taiwan.
Email: stliu@ntu.edu.tw
Funding information
Ministry of Science and Technology,
Taiwan, Grant/Award Number:
MOST103‐2113‐M‐002‐002‐MY3
KEYWORDS
dinuclear, nitroarene, palladium, reduction, synergistic effect
1
| INTRODUCTION
covalently linked via the anthyridine moiety
[
10]
(
Scheme 1). The two metal centers, fixed within a short
Dinuclear transition metal complexes are an important
class of coordination compounds for the investigation of
distance, cooperate with each other and enhance the
catalytic activity. Thus, the dinuclear copper complex
with L shows to be a good catalyst for the oxidative
coupling of primary alcohols into the corresponding
[1]
metal–metal interactions, synergistic effect in cataly-
[2]
[3]
sis, and mimicking enzymes. In particular, chemists
have great interest in the use of bimetallic compounds
as catalysts to impart new reactivity of chemical reac-
tions. Among various transition metal ions, dipalladium
complexes have received much attention due to the
[
10a]
esters.
To continue our interest in the study of
dimetallic complexes, we report the preparation of
dipalladium complexes containing the anthyridine‐based
ligand L and the catalytic activity of the resulting com-
plexes toward the reduction of nitroarenes.
[4]
remarkable activity of this metal ions. Few di‐nuclear
palladium complexes exhibit the catalytic activities in
[5–7]
[8]
cross‐coupling,
olefin polymerization,
and C‐H
[9]
activation. It is noted that the dimetallic centers in
close proximity might play a key role leading to a better
activity in comparison to those for the related monome-
tallic analogues or species. Thus, the challenging task
in designing bimetallic catalysts is the choosing of
ligands that are capable of accommodating metal ions
and allowing them to cooperate with each other during
the catalytic reaction.
2
| EXPERIMENTAL
.1 | General
2
Chemicals and solvents were of analytical grade and used
without further purification. Nuclear magnetic resonance
spectra were recorded on a 400 MHz spectrometer. Chem-
ical shifts are given in parts per million relative to Me Si
for H and C NMR. Ligand L,
4
1
13
[10]
Recently, we reported that the metal centers are sepa-
rated by ca. 5 Å in dinuclear metal complexes hosted by
the rigid ligand L, a two terpyridinyl compartments
[(terpy)PdCl](PF6),
[(bpy)PdCl ] and [(dien)PdCl]Cl were prepared accord-
ing to the reported procedure.
2
[
11]
Appl Organometal Chem. 2017;e3976.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3976

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HUNG ET AL.
4
4
4
‐fluoroaniline, 4‐chloroaniline, 4‐aminobenzyl alcohol,
‐aminophenylacetonitrile,
4‐aminophenyl
acetate,
‐(4‐morpholinyl)aniline, 4‐aminoacetophenone, m‐phe-
nylenediamine, 3‐aminobenzyl alcohol, 3‐aminophenol,
o‐phenylenediamine, 2‐chloroaniline, 2‐aminobiphenyl,
2,6‐xylidine, 1‐naphthylamine, and 2‐aminofluorene.
SCHEME 1 Dimetallic complexes with an anthyridine‐based
multiple dentate L
3
3
| RESULTS AND DISCUSSION
[10]
.1 | Preparation and characterization of
2.2 | Preparation of dipalladium
complexes
complex (2)
Ligand L was prepared according to our previously
reported method.
A mixture of L (60 mg, 0.11 mmol) and Pd(MeCN) Cl2
[10a]
2
Complexation of Pd(MeCN) Cl
2 2
(
60 mg, 0.23 mmol) in CH Cl (15 ml) was stirred at room
2 2
with L provided the desired di‐palladium(II) complex
(L)Pd Cl ]Cl (1) (Scheme 2). This complex is insoluble
temperature under N for 2 h. KPF (210 mg, 1.1 mmol)
2
6
[
2
2
2
was then added and the mixture was stirred overnight.
The solvent was removed under reduced pressure and
in most of organic solvents except dimethylsulfoxide
dmso). In order to improve its solubility, anion metathe-
sis of 1 with KPF gave [(L)Pd Cl ](PF ) (2), which is
(
CH CN (around 20 ml) was added to the residue. The sus-
3
6
2
2
6 2
pension was passed through a pad of Celite and concen-
slightly soluble in CH CN and CH NO .
3
3
2
trated to around 5 ml. Et O was added to the solution to
1
2
The H–NMR spectrum of 2 in CD CN appears nine
3
yield yellow solids, which was collected and washed with
sets of signals for the terpyridinyl part of the ligand and
the C NMR spectrum of 2 shows 20 shifts for the com-
plex. These observations clearly indicate the symmetrical
environment of the complex, i.e. both terpyridinyl pockets
are coordinated toward the palladium ions. The dimetallic
nature of the complex is further confirmed by the mass
determination. ESI‐HRMS of 2 in CH CN matrix shows
a peak at m/z = 424.4729, which is matched with the
theoretical value of 424.4712 of [2–2 (PF )] . Besides this
signal, there is another peak at m/z = 415.4925, which is
MeOH and Et O. After drying under vacuum, the desired
13
2
1
product was obtained as brown solids (104 mg, 86%). H
NMR (CD CN, 400 MHz) δ 9.02 (d, J = 4.8 Hz, 2 H),
3
8
.42 (t, J = 8.0 Hz, 2 H), 8.37 (td, J = 7.7, 1.3 Hz, 2 H),
8
.33–8.28 (m, 2 H), 8.26 (d, J = 8.0 Hz, 2 H), 8.22
(d, J = 8.0 Hz, 2 H), 8.09–8.02 (m, 2 H), 7.89 (d,
3
J = 8.0 Hz, 2 H), 7.86–7.80 (m, 2 H), 7.48–7.42 (m, 5 H);
13
C NMR (CD CN, 125 MHz) δ 159.52, 156.78, 156.43,
.
2+
3
6
1
1
1
56.17, 153.14, 146.66, 143.92, 143.88, 143.72, 129.99,
29.89, 129.88, 129.84, 129.61, 129.30, 127.31, 162.22,
25.83, 125.45, 120.15; HRMS (ESI) m/z [M – 2PF6]
2+
consistent with the formula of [Pd (L )Cl(OH)] , pre-
2
1
2+
sumably due to the partial hydrolysis of 2 during the ion-
ization. Conductivity measurement for 2 using dmso and
CH NO as the solvents provided the data Λ = 160 and
Calcd. for C H Cl N Pd : 424.4735, Found: 424.4712.
3
7
23
2
7
2
.
Anal. Calcd. for C H Cl F N P Pd 6H O: C 35.63; H,
37
23
2
12
7
2
2
2
3
2
2
.83; N, 7.86. Found: C, 35.42; H, 2.78; N, 7.37.
−1
2
−1
180 (ohm cm mol ), respectively, suggesting that the
complex is a 2:1 electrolyte. All these characterization
conclude the structure of the dipalladium complex 2.
2.3 | Catalytic reduction of
nitroarenes‐typical procedure
3
.2 | Catalysis‐reduction of aromatic nitro
A mixture of nitroarene (0.5 mmol), 2 (2.8 mg,
compounds.
0
.0025 mmol), NaBH CN (0.9 mg, 0.012 mmol) in a reac-
3
tion tube was degassed and refilled with H (1 atm). Meth-
Metal‐catalyzed reduction of nitro group under hydrogen
2
[
12]
anol (0.5 ml) was then added and the resulting solution
was stirred at 50 °C for 6 h. After completion, the reaction
mixture was diluted with Et O or CH Cl and passed
atmosphere is well documented,
but not with a
2
2
2
through Celite. The filtrate was concentrated and purified
by column chromatography on silica gel to yield the pure
product. The following obtained compounds have been
1
13
characterized by comparing their H and C spectral data
see supporting information) with those reported in the lit-
erature: p‐toluidine, 4‐aminophenol, p‐phenylenediamine,
‐fluoroaniline, 4‐chloroaniline, 4‐aminobenzyl alcohol,
(
4
SCHEME 2 Preparation of dipalladium complexes

HUNG ET AL.
3 of 6
dipalladium complex as a catalyst. Thus, we explored the
activity of 2 in this regard. First, the reduction of p‐
nitrotoluene into p‐toluidine as the model reaction was
chosen to establish the optimized catalytic conditions.
After several trials, we found that complex 2 appeared
no catalytic activity under atmospheric pressure of hydro-
gen, presumably that the Pd(II) centers were not able to
be reduced to Pd(0) under this condition. Thus, a number
of reducing agents were tested for the initiation of the
reduction and the results are summarized in Table 1. With
the use of various hydrides, complex 2 showed catalytic
activity on this reduction (Table 1, entries 2–7). Among
them, sodium cyanoborohydride gave the best result.
Obviously, this catalytic hydrogen reduction of nitro com-
pounds does require the reduction of palladium(II) ions
initially. There is no reduction of nitro compound under
N atmosphere (entry 8), indicating that the hydrogen
2
molecules are responsible for the reduction.
Further study on the optimization conditions was
investigated (Table 2). By varying the reaction tempera-
ture, we observed that the reaction temperature had a sig-
nificant impact on the yield of the product. At 50 °C, the
catalysis led to the desired product quantitatively. At the
beginning, a mixed solvent of MeOH/MeCN was used
for the study due to the solubility of the complex. How-
ever, we still surveyed the solvent effect of the reduction
(
Table 2, entries 5–10). The reaction was indeed acceler-
a
ated by using methanol as the solvents, even in pure
methanol (entries 10 and 14). A high conversion was
found with the use of a mixed solvent of methanol and
acetonitrile in a ratio of 1:1, but a decreasing yield in
1:5. A significant decrease of product was observed when
acetonitrile was used as the single solvent. Other less
polar solvents such as THF and toluene were not good
for the catalysis as a large amount of starting material
was recovered from reactions attempted in these solvents.
After the solvent and reaction temperature screening, we
next looked at the catalyst loading. At 50 °C, a catalyst
loading of 0.5 mol % was sufficient to have a quantitative
conversion of p‐nitrotoluene into p‐toluidine (entry 12),
but a dramatic decrease with 0.1 mol % loading. It is
noticed that the reaction proceeds well under a
TABLE 1 Initiation for the catalytic reduction
b
b
Entry
Reducing agent
Conv.(%)
Yield (%)
1
None
0
0
2
3
4
5
6
7
8
NaBH
3
CN
100
100
100
100
100
100
< 5
100
87
91
84
82
51
‐
4
Et NBH
4
NaBH
NaH
4
HCO
HCO
2
2
Na
NH
4
3
NaBH CN
a
Reaction conditions: p‐nitrotoluene (0.20 mmol), 2 (0.004 mmol, 2 mol%),
reducing agent (0.02 mmol) in MeOH/MeCN (1/1, 2 ml) at 50 °C under H
2
b
1
(
1 atm) for 6 h. Conv. and yields were based on H NMR integration.
a
TABLE 2 Optimization for the reduction
b
Entry
2/NaH
3
BCN (in mmol)
Temp (°C)
Solvent (2 ml)
Yield
−
3
−2
−2
−2
−2
−2
−2
−2
−2
−2
−2
−2
−3
1
4 x 10 /2 x 10
20
MeOH/MeCN(1:1)
26%
−3
2
3
4
5
6
7
8
9
4 x 10 /2 x 10
30
40
50
50
50
50
50
50
50
50
50
50
50
50
MeOH/MeCN(1:1)
MeOH/MeCN(1:1)
MeOH/MeCN(1:1)
MeOH
32%
−3
4 x 10 /2 x 10
74%
−3
4 x 10 /2 x 10
100%
100%
100%
71%
−3
4 x 10 /2 x 10
−3
4 x 10 /2 x 10
MeOH/MeCN(5:1)
MeOH/MeCN(1:5)
MeCN
−3
4 x 10 /2 x 10
−3
4 x 10 /2 x 10
49%
−3
4 x 10 /2 x 10
THF
15%
−3
1
1
1
1
1
1
0
1
2
3
4
5
4 x 10 /2 x 10
Toluene
0
−3
2 x 10 /1 x 10
MeOH
100%
100%
71%
−3
1 x 10 /5 x 10
MeOH
−4
−3
5 x10 /5 x 10
MeOH
−3
−3
−2
1 x 10 /5 x 10
−3
MeOH (0.5 mL)
MeOH
100%
Trace
c
4 x 10 /2 x 10
a
2
Reaction conditions: p‐nitrotoluene (0.20 mmol), 2 and reducing agent in solvent under H (1 atm) for 6 h.
b
NMR yield.
atmosphere.
c
under N
2

4
of 6
HUNG ET AL.
concentration of 1 M (entry 14), which may reduce the
amount of solvent used.
substrates such as 1‐nitronaphthylene, 2‐nitro‐9H‐
fluorene and 4‐(4‐nitrophenyl)morpholine were reduced
to the corresponding amine compounds (entries
1
8 ~ 20). In this invesitgation, we found that functional
3
.3 | Reduction of nitro‐compounds‐
groups such as chloro, cyano, keto, and carboxylate were
tolerated under the catalytic conditions. It is also noticed
that dinitrobenzenes could be reduced to the correspond-
ing phenylenediamines, but requiring a higher loading of
the catalyst (2.5 mol%) and a longer reaction time (12 h).
For examples, both o‐ and m‐substituted dinitrobenzenes
gave full conversions of phenylenediamines.
reaction scope
With the above optimized conditions, exploration of
this reaction toward various aromatic nitro compounds
was investigated (Table 3). Various nitroarenes
including p‐methylnitrobenzene, p‐hydroxynitrobenzene,
p‐(hydroxymethyl)nitrobenzene, p‐aminonitrobenzene,
p‐cyanonitrobenzene,
p‐chloronitrobenzene,
p‐
fluoronitrobenzene, p‐acetoxynitrobenzene and p‐
acetylnitrobenzene, were reduced cleanly in excellent
yields (Table 3, entries 1 ~ 10). The presence of electron
withdrawing (−F, −COCH3) or electron donating
3
.4 | Synergistic effect of 2 in the
reduction
In order to demonstrate the uniqueness of the dimetallic
complex in the catalysis, comparison of various palladium
complexes as catalysts for the reduction of 4‐
nitroacetophenone was evaluated and the observation is
summarized in Table 4. Quite obviously, complex 2 proves
to be the best catalyst in this reduction. Other mono‐pal-
ladium complexes as catalyst precursors exhibited some
activity, but not as good as complex 2. In all mono‐palla-
dium catalysis, the hydroxylamine intermediate were iso-
lated, in particular with the use of [(dien)PdCl]Cl as the
catalyst (Table 4, entry 8). Some rhodium and iridium
complexes were also subjected to this investigation, but
not quite active. In monitoring the catalytic reaction
(
−OCH , −NH ) substituents on benzene rings has no sig-
3 2
nificant effect on the reduction. This catalytic system is
also applicable to the ortho‐ and meta‐substituted com-
pounds (entries 11 ~ 16). The dipalladium complex also
catalyzed the reduction of a bulky substrate, 2,6‐dimethyl
nitrobenzene, effectively (entry 17). Other arene
a
2 2
TABLE 3 Reaction scope of Ar‐NO ➔ Ar‐NH
Entry
Ar =
Isolated yield
1
C
6
H
5
‐
94%
2
3
4
5
6
7
8
9
p‐MeC
p‐HOC
p‐H NC
p‐FC
p‐ClC
p‐MeCOC
p‐(HOCH
p‐(NCCH
p‐MeCO
m‐H NC
m‐(HOCH
m‐HOC
o‐H NC
o‐ClC
o‐PhC
2,6‐Me
1‐naphthyl
6
H
4
‐
96%
6
H
4
‐
100%
100%
100%
79%
TABLE 4 Comparison of various complexes on the reduction
2
6
H
4
‐
b
c
c
6
H
4
‐
Entry Pd complex
Yield of 3 Yield of 4
1
2
3
4
5
6
7
8
2 (0.5 mol%)
[(terpy)PdCl](PF
[(bpy)PdCl ] (1 mol%)
Pd(CH CN) Cl
[Rh(COD)Cl]
[IrCOD)Cl] (1 mol%)
[Rh(COD)Cl] /PPh
[(dien)PdCl]cl (1 mol%)
99%
74%
68%
0
6
4
H ‐
H
4
‐
98%
6
) (1 mol%)
7%
6
)C
6
H
4
‐
100%
100%
88%
2
9%
2
)C
6 4
H ‐
3
2
2
/dppe (1 mol%) 62%
Trace
18%
‐
2
2
(1 mol%)
25%
82%
24%
8%
10
11
12
13
14
15
16
17
18
19
2 6 4
C H ‐
H
4
‐
100%
100%
99%
2
2
6
)C
6
H
4
‐
2
3
Trace
92%
2
6
4
H ‐
a
H
4
‐
100%
62%
Reaction conditions: p‐nitroacetophenone (0.5 mmol), Pd complex,
2
6
3 2
NaBH CN (2.5 mol%) in MeOH (0.5 ml) at 50 °C under H (1 atm) for 6 h.
6
4
H ‐
b
structures of some Pd complexes given in scheme 3.
NMR yields.
S3
6
H
4
‐
72%
c
2
C
6
H
4
‐
100%
99%
99%
2
0
100%
a
Reaction conditions: nitroarene (0.5 mmol), 2 (0.0025 mmol), NaBH
0.012 mmol) in MeOH (0.5 ml) at 50 °C under H (1 atm) for 6 h.
3
CN
SCHEME 3 Structures of mono‐nuclear Pd(II) complexes used
for comparison
(
2

HUNG ET AL.
5 of 6
catalyzed by 2, we found that a trace amount of hydroxyl-
amine species was also produced during the reaction.
Hydroxylamine compound II is known to be one of the
key intermediates for the reduction of nitroarenes
(Scheme 4). Thus, the observation of the formation of
hydroxylamine is expected, but relative amounts pro-
duced illustrate the reactivity difference among various
catalysts in the reduction.
It is noticed that quite a number of palladium nano‐
particle catalysts have been reported on the reduction
nitroarenes, but few palladium(II) complexes in homoge-
neous phase. For example, Maleczka's group investigated
SCHEME 5 Possible intermediates catalyzed by 2
dipalladium catalyzed reduction, not other mono‐nuclear
catalysts, is presumably due to the synergistic assistance
of metal ions as shown in Scheme 5. Since the two palla-
dium ions are in a close proximity, the coordinating inter-
mediates of nitrosoarene and arylhydroxylamine on each
metal ion are in a distance readily for N‐N coupling
leading to the azoxy species, which is absent in a mono‐
nuclear complex. This suggests that the reduction path-
way catalyzed by 2 prefers to proceed the pathway B.
the reduction of nitrobenzene using Pd(OAc) in THF
2
[
13a]
with polymethylhydrosiloxane as the hydride source.
On the other hand, palladium nanoparticles stabilized
by alkyne derivatives are able to catalyze the reduction
of nitrobenzene to aniline under atmospheric pressure of
3
[13b]
H with a turnover number of 10 .
In comparison
with other procedures, the catalytic activity operated by
in methanol for reduction of nitro aromatic compounds
2
2
in high yields is comparable to those of palladium
nanoparticles.
Another interesting observation in monitoring the
reduction of p‐nitroacetophenone was the detection of
the azoxy species 5 when using 2 as the pre‐catalyst
4
| CONCLUSIONS
We have prepared a new dipalladium complex 2 with an
anthyridine‐based ligand, in which the two metal ions
are in close proximity. Upon in situ reduction with
(Figure 1), but not other mono‐nuclear palladium cata-
lysts. As shown in scheme 4 pathway B, azoxybenzene
III is one of the key intermediates in the reduction of
nitroarenes due to the coupling of nitrosoarene and
hydroxylamine. The formation of such a species in the
NaBH CN, complex 2 shows to be a good catalyst on the
3
reduction of nitroarenes into the corresponding anilines
under atmospheric pressure hydrogen. This method pro-
vides a straightforward access to a diverse array of func-
tionalized anilines. Detection of the azoxybenzene
intermediate in the di‐palladium catalyzed system gives
the clue of the possible synergistic effect on the dimetallic
species in the catalysis.
ACKNOWLEDGEMENTS
We thank the Ministry of Science and Technology,
Taiwan for the financial support (MOST103‐2113‐M‐002‐
002‐MY3).
SCHEME 4 Reaction pathways of the reduction of nitroarenes
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