A hydroquinone based palladium catalyst for room temperature nitro reduction in water

Article abstract of DOI:10.1039/c4ra06547f

A hydroquinone based palladium complex [Pd(H₂L)(Cl)₂] (1), acts as an efficient room temperature catalyst for reduction of nitroarenes in water as solvent. 1 also acts as a tandem catalyst for Suzuki-Miyaura cross coupling in ethanol followed by reduction of nitroarenes in one pot with a loading of 0.25 mol% catalyst.

Full text of DOI:10.1039/c4ra06547f

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
A hydroquinone based palladium catalyst for
room temperature nitro reduction in water†
Cite this: RSC Adv., 2014, 4, 35233
*
Alok Kumar,‡ Kallol Purkait,‡ Suman Kr. Dey, Amrita Sarkar and Arindam Mukherjee
Received 2nd July 2014
Accepted 30th July 2014
DOI: 10.1039/c4ra06547f
www.rsc.org/advances
A hydroquinone based palladium complex [Pd(H2L)(Cl)2] (1), acts as an solvent like water at room temperature with a discrete metal
efficient room temperature catalyst for reduction of nitroarenes in complex is challenging. Very few catalysts perform the reduc-
water as solvent. 1 also acts as a tandem catalyst for Suzuki–Miyaura tion of nitroarenes at room temperature with water as the
cross coupling in ethanol followed by reduction of nitroarenes in one solvent (Table 1) viz. Zn-dust (10 eq.) in surfactant (TPGS-750 M)
pot with a loading of 0.25 mol% catalyst.
water mixture8 and the ionic liquid based PVDF–[C6(mpy)2]
[NiCl4]22 (5 mol%) catalyst9 are needed in less amounts, are less
toxic to environment are of high interest for their potential
applications.10 Such catalysts are air stable, efficient, economi-
cally viable and hence greener. The hydroquinone ligand used
in this work is synthesized in a single step and is stable. The
Palladium catalyst is active in lesser mol% without a phosphine
or carbene ligand. To the best of our knowledge the use of
hydroquinone in nitroarene reduction is not known.
The reaction between a pyrazole derivatized hydroquinone
ligand H2L and PdII(MeCN)2Cl2 in acetonitrile under reux
condition led to formation of complex 1 which was further
puried by crystallization from acetonitrile solution (Scheme 1).
The single crystal X-ray diffraction showed the complex crys-
tallizes in orthorhombic, space group Cmc2(1). The N atoms
from the two pyrazoles chelate to the PdII forming a seven
membered ring and the remaining two coordination sites of the
PdII are satised by two chloride ions. The two Pd–N bond
Palladium based catalysts are well known in the literature for
their efficiency in a wide spectrum of coupling reactions and
reduction of various functionalities.1 Such catalytic reactions
are of importance for their use in industrial applications
spanning from pharmaceuticals to polymers or syntheses of
natural products, dyes.2 Aryl amine derivatives are very impor-
tant intermediates in syntheses of many of the above using
nitroarene precursors.3 Room temperature reduction of nitro-
arenes is advantageous since there are compounds which are
susceptible to degradation when heated during reduction.
Several transition metal based catalysts are known to perform
reduction of nitroarenes.4 Most metal catalyzed nitro reduction
reactions require higher temperatures,4f,5 hydrogen gas,3b,6 or
use of expensive hydride source like silanes.4b,5e,g,7 Among the
most recent ones the work of Beller and co-workers used a Fe–
ꢀ
1
ꢀ
phenanthroline catalyst (1.0 mol%) at 100 C in THF with four
˚
distances are ca. 2.024 A with an angle of ca. 85.5 . H NMR of
the complex indicate that the complex is pure and supports that
the solution structure is same as in the solid state where the Pd
is coordinated via the two pyrazole nitrogen and the hydrogens
molar equivalent hydrazine as the hydrogen source for efficient
and very selective reduction of nitroarenes.5b However, metal
complexes that reduces nitroarenes at room temperature are
scarce (Table 1). Most of such catalysts are nano particles (Table
1). Hence performing the nitroarene reduction in a green
1
of the two –OH groups of hydroquinone are visible in the H
NMR spectra.
The catalytic activity studies of 1 towards the reduction of
aromatic nitro compounds showed the catalyst to be a prom-
ising candidate. Aer optimization (Tables S3–S5, ESI†) it was
found that with 0.25 mol% of catalyst at room temperature in
water would provide with excellent isolated yield. Hence we
probed the aromatic nitro reduction reactions with various
substrates (Table 2) containing different functional group and
found good efficiency and selectivity towards both aromatic and
hetero aromatic nitro compounds. Most of the nitro substrates
used are barely soluble in water but as the reaction proceeds
Department of Chemical Sciences, Indian Institute of Science Education and Research
Kolkata, Mohanpur Campus, Mohanpur-741246, India. E-mail: a.mukherjee@iiserkol.
ac.in; Fax: +91 33 25873020
† Electronic supplementary information (ESI) available: General synthesis
procedure of ligand and metal complex, characterization data, single crystal
X-ray data of 1, NMR spectra, complete optimization data of catalytic reaction,
standard procedure of catalysis, mechanism of catalytic cycle, table of
Suzuki–Miyaura cross coupling reaction. CCDC 996012. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra06547f
‡ The authors pay equal contributions to this work.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 35233–35237 | 35233

View Article Online
RSC Advances
Communication
Table 1 Selected data on literature reported transition metal catalysed nitro reduction at room temperature
Sl. no.
Catalyst
t
mol%
300
H-source and other additive
Solvent
Ref.
1
2
3
4
5
6
7
8
Fe-NPs
2–3 h
Water
Water
Ethanol
Water
2-MeTHF and water
Ethanol
Methanol
Methanol–water
Water
THF
Water
Ethanol
Water
4a
4b
4d
4e
6c
6d
11
12
7b
9
Au-NPs
10 min to 24 h
1 min to 4 h
7 h
0.5
TMDSa (4.5 eq.)b
NAP-Mg–Au(0)c
Pd/C
NaBH4 (47 eq.)e
d
2.5
0.2
H3PO2 (1 eq.), NaH2PO2 (3 eq.)
Pd-polymer
Si-supported Pt
Co–Co2Bf
2 h
H2-1 atm, KOH (0.1 mol Lꢁ1
H2-baloon
)
0.5–2 h
10–50 min
2 h
30 min
<15 min
30 min
1–4 h
0.5–1.0
g
NaBH4 (4 eq.)/N2H4 (0.25 mL mmolꢁ1
NaBH4 (3 equiv.)
)
Ni–PAMAM–PVAm/SBA-15h
Pd(OAc)2
—
5
5
9
PMHSi (3–5 eq.), KF (2 eq.)
NaBH4 (8 eq.)
j
10
11
12
PVDF–[C6(mpy)2][NiCl4]22
Au/TiO2 NPs
Zn-dust
0.1
NH3BH3 (1.5 eq.)
13
8
k
NH4Cl (2 eq.), TPGS-750 Ml
a
b
c
d
TMDS ¼ 1,1,3,3-tetramethyldisiloxane. Argon atmosphere. NAP ¼ Nano Active™ Magnesium Oxide Plus. 75 mg per mol of substrate.
e
f
g
h
Nitrogen atmosphere. Nanocomposite. Used 20 mg molꢁ1 of substrate. PAMAM–PVAm/SBA-15 ¼ polyamidoamine–polyvinylamine/
mesoporous silica catalyst-0.1 g molꢁ1 of substrate. PHMS ¼ polymethylhydro-siloxane. PVDF ¼ polyvinylidene uoride, mpy ¼ 3-
i
j
methylpyridinium. k 10 equivalent. l TPGS-750 M ¼ DL-a-tocopherol methoxypolyethylene glycol succinate solution.
A careful look at Table 2 shows that with bromo or iodo
substituted nitroarenes, the reduction reaction is coupled with
dehalogenation. This indicated the probability of Suzuki–
Miyaura cross-coupling by 1 where the dehalogenation is a part
1
of the mechanistic step (Scheme S1, ESI†). The H NMR of the
Table 2 Reduction of aryl nitro compound catalyzed by 1a
Scheme
1 Synthetic methodology of 1 along with X-ray crystal
structure displaying 30% probability thermal ellipsoids. All hydrogen
atoms and solvent molecules are omitted for the sake of clarity.
they become soluble once converted to the respective amine.
The progress of the reduction of 4-nitrobenzonitrile reaction
was monitored by 1H NMR study in D2O at 25 ꢀC (Fig. S3, ESI†)
with catalyst 1, where we found the increase in product
concentration, with respect to 1,4-dioxane used as standard,
with increase in time, using 0.25 mol% catalyst. The NMR
spectra also show the appearance of two peaks at 7.15 and 7.67
ppm due to the formation of hydroxylamine as an intermediate,
which vanishes with time (Fig. S3†). It should be noted that the
nitroarene reduction conditions used may also generate nano-
particles of Pd in solution although quantitative conversions
were obtained using stoichiometric NaBH4. Hence to under-
stand the importance of the ligand we studied the catalysis with
other most common palladium substrates viz. Pd(OAc)2, PdCl2
and Pd(MeCN)2Cl2 only without the ligand H2L keeping other
reaction conditions same (Table S6, ESI†). If only Pd is
responsible for catalysis we would obtain similar yields in the
nitroarene reduction. Ethanol was used as solvent since the
palladium complexes used were better soluble in ethanol.
Hence, the comparison would be more accurate in ethanol and
the nitroarene reduction is equally efficient in ethanol. We
found that using upto 1 mol% of the above three compounds
the nitroarene reduction yield was less than 40% in 6 h whereas
for the same substrate the [Pd(H2L)(Cl)2] (1) gave complete
conversion to aryl amine in ca. 10 min emphasizing the effi-
ciency of 1.
Entry Substrate
Product
Isolated yieldb (%) Time (h)
1
2
98
98
0.42
0.67
3
4
98
98
0.17
0.5
5
6
98
99
0.33
0.25
7
98
0.42
a
Reaction conditions: 1.0 mmol nitro benzene, 4.0 mmol sodium
b
borohydride, 0.25 mol% catalyst 1, water, 27 ꢀC. Isolated yields were
reported aer column chromatography.
35234 | RSC Adv., 2014, 4, 35233–35237
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
Table
3
One pot tandem catalytic reaction of aryl nitro halide
reaction between iodobenzene and NaBH4 in presence of 1
showed that the formation of a single peak at 7.32 ppm corre-
sponding to the formation of benzene by dehalogenation of
iodobenzene. There is also a weak signal at ꢁ17 ppm which
might be due to the presence of a transient Pd–H intermediate14
(Fig. S1–S2, ESI†) indicating the role of Pd in dehalogenation
during the reduction reaction. However, a direct hydride attack
at the Pd bound benzene ring also cannot be excluded. The
NMR shows that most of the substrate is converted to dehalo-
genated product by the time we recorded the NMR spectra (5–6
min) (Fig. S1, ESI†). Hence this catalyst has potential for
dehalogenation and reduction in a single pot when the halogen
atoms attached to the nitroarene is a bromine or iodine. A
nitroarene reduction mechanism involving Pd–H intermediate
may be as proposed in Scheme 2 (more details in Fig. S2, ESI†)
which also takes into account the simultaneous dehalogena-
tion. The mechanism is also supported by the literature
knowledge of the transformation of quinone to hydroquinone
in presence of hydride.15 In presence of sodium borohydride the
quinone PdII complex 1 is reduced to a hydroquinone PdII
complex which further changes to a quinone Pd0 complex and
donates two electrons to the substrate/intermediate to perform
two electron reduction in each step and the hydrogens are
coming from solvent. Aer each step the quinone PdII complex
1 is reformed and the cycle continues. Hence, NaBH4 helps
recycle the catalyst whereas the Pd centre donates electron to
the substrate and consecutive intermediates helping the 6eꢁ
catalytic reduction (Schemes 2 and S2, ESI†). The chloro
substituted substrates were not dehalogenated under similar
conditions which reect that it may be possible to impart
selectivity of dehalogenation due to lower activity of the catalyst
towards dehalogenation of chloro group. Since we found that
the catalyst has possibility in Suzuki–Miyaura cross-coupling we
probed it for the same as that would open a possibility of
tandem catalysis using the same catalyst. The literature data
suggested that a catalyst that would perform both Suzuki–
Miyaura (with aryl halides and phenylboronic acids) and
reduction of nitroarenes in a single pot in two consecutive steps
in room temperature is not known.
compound catalysed by 1a
R
X
I
Product
Isolated yieldb (%)
p-NO2
98
m-NO2
I
98
97
p-NO2
Br
a
Reaction condition: 1.0 mmol aryl nitro halide, 1.5 mmol
phenylboronic acid, 3 mmol K2CO3, 0.25 mol% catalyst 1, ethanol at
27 ꢀC; Followed by addition of 4 mmol NaBH4 in the same pot, 27 ꢀC.
b
Isolated yields were reported aer column chromatography.
The study of catalytic activity of 1 towards Suzuki–Miyaura
cross coupling reaction was initially done with 4-bromoanisole
which showed complete conversion with 0.5 mol% of catalyst.
Following which the conditions were standardised varying the
solvent (Table S7, ESI†), base (Table S8, ESI†) and catalyst
loading (Table S9, ESI†) using the same halide substrate and
phenylboronic acid. We found that ethanol and K2CO3 are the
best solvent and base and only 0.25 mol% catalyst loading can
successfully convert reactants to product within 1–4.5 h at room
temperature using a green solvent like ethanol (with other
substrates, Table S11, ESI†). It is well known that the palladium
has to reduce to the Pd0 state to perform the catalytic cycle of
Suzuki–Miyaura cross-coupling, here the hydroquinone ligand
would be useful since the it is capable of undergoing two elec-
tron oxidation. Hence, the mechanistic steps proposed take this
into account (Scheme S1, ESI†).
We attempted one pot tandem type catalysis to synthesize
biaryl amines from nitro substituted aryl halide using
1. 0.25 mol% of 1 was loaded at the beginning of the C–C bond
coupling reaction along with an aryl halide and phenylboronic
acid in presence of K2CO3. Following the completion of C–C
bond coupling monitored by silica gel TLC, NaBH4 was added as
hydrogen source without adding any more catalyst and the
biaryl amines were isolated in more than 97% yield of the
desired products (Table 3).
Conclusions
In conclusions, we have presented well characterized, easy to
synthesize air stable hydroquinone based palladium complex 1
which shows promising activity towards reduction of
Scheme 2 Representative dehalogenation and nitroarene reduction
mechanism of 1-bromo-4-nitrobenzene as
a model substrate
(detailed mechanism in ESI†).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 35233–35237 | 35235

View Article Online
RSC Advances
Communication
nitroarenes in presence of other functionalities like nitriles. The
amount of catalyst required is low (0.25 mol%) and moreover
the same catalyst is also capable of C–C cross coupling followed
by reduction of nitroarenes in one pot without any further
catalyst addition which renders it suitable as a Tandem catalyst
for the above purpose. Finally, for halide substrates the catalyst
may have potential for simultaneous dehalogenation and nitro
reduction if the halides are bromine or iodine. The potential of
the catalyst warrants further work with a wider spectrum of
substrate to check upon its functionality tolerance and use
of the dehalogenation reaction for other possible substitutions
along with detailed mechanistic studies.
3 (a) R. S. Downing, P. J. Kunkeler and H. van Bekkum, Catal.
Today, 1997, 37, 121; (b) A. M. Tafesh and J. Weiguny, Chem.
Rev., 1996, 96, 2035; (c) R. J. Rahaim, Jr and R. E. Maleczka,
Jr, Org. Lett., 2005, 7, 5087; (d) S. Chandrasekhar,
S. J. Prakash and C. L. Rao, J. Org. Chem., 2006, 71, 2196;
(e) B. Chen, U. Dingerdissen, J. G. E. Krauter,
H. G. J. Lansink Rotgerink, K. Moebus, D. J. Ostgard,
P. Panster, T. H. Riermeier, S. Seebald, T. Tacke and
H. Trauthwein, Appl. Catal., A, 2005, 280, 17; (f)
P. K. Mandal and J. S. McMurray, J. Org. Chem., 2007, 72,
6599.
4 (a) R. Dey, N. Mukherjee, S. Ahammed and B. C. Ranu, Chem.
Commun., 2012, 48, 7982; (b) S. Park, I. S. Lee and J. Park, Org.
Biomol. Chem., 2013, 11, 395; (c) X. Liu, S. Ye, H.-Q. Li,
Y.-M. Liu, Y. Cao and K.-N. Fan, Catal. Sci. Technol., 2013,
3, 3200; (d) K. Layek, M. L. Kantam, M. Shirai, D. Nishio-
Hamane, T. Sasaki and H. Maheswaran, Green Chem.,
2012, 14, 3164; (e) M. Baron, E. Metay, M. Lemaire and
F. Popowycz, Green Chem., 2013, 15, 1006; (f) A. K. Shil and
P. Das, Green Chem., 2013, 15, 3421.
5 (a) I. H. Abd El Maksod, E. Z. Hegazy, S. H. Kenawy and
T. S. Saleh, Adv. Synth. Catal., 2010, 352, 1169; (b)
R. V. Jagadeesh, G. Wienhoefer, F. A. Westerhaus,
A.-E. Surkus, M.-M. Pohl, H. Junge, K. Junge and M. Beller,
Chem. Commun., 2011, 47, 10972; (c) P. P. Sarmah and
D. K. Dutta, Green Chem., 2012, 14, 1086; (d) T. Schabel,
C. Belger and B. Plietker, Org. Lett., 2013, 15, 2858; (e)
M. Sutter, L. Pehlivan, R. Lafon, W. Dayoub, Y. Raoul,
E. Metay and M. Lemaire, Green Chem., 2013, 15, 3020; (f)
F. Yuste, M. Saldana and F. Walls, Tetrahedron Lett., 1982,
23, 147; (g) K. Junge, B. Wendt, N. Shaikh and M. Beller,
Chem. Commun., 2010, 46, 1769; (h) U. Sharma,
P. K. Verma, N. Kumar, V. Kumar, M. Bala and B. Singh,
Chem.–Eur. J., 2011, 17, 5903.
6 (a) G. Brieger and T. J. Nestrick, Chem. Rev., 1974, 74, 567; (b)
P. Lara, A. Suarez, V. Colliere, K. Philippot and B. Chaudret,
ChemCatChem, 2014, 6, 87; (c) X. Xi, Y. Liu, J. Shi and S. Cao,
J. Mol. Catal. A: Chem., 2003, 192, 1; (d) V. Pandarus,
R. Ciriminna, F. Beland and M. Pagliaro, Adv. Synth. Catal.,
2011, 353, 1306; (e) R. F. D'Vries, M. Iglesias, N. Snejko,
S. Alvarez-Garcia, E. Gutierrez-Puebla and M. A. Monge, J.
Mater. Chem., 2012, 22, 1191.
Acknowledgements
We sincerely acknowledge DST for nancial support vide
project no SR/S1/IC-36/2010. We also thank IISER Kolkata for
the nancial and infra-structural support, including NMR,
single crystal X-ray facilities and other analytical instruments.
A.K. is thankful to DST for Inspire fellowship. K.P. thanks UGC,
A.S. thanks IISER Kolkata and S.K.D. thanks CSIR-India for
providing research fellowship.
Notes and references
1 (a) M. K. Lakshman, J. Organomet. Chem., 2002, 653, 234; (b)
A. Suzuki, J. Organomet. Chem., 1999, 576, 147; (c)
U. Christmann and R. Vilar, Angew. Chem., Int. Ed., 2005,
44, 366; (d) S.-D. Yang, C.-L. Sun, Z. Fang, B.-J. Li, Y.-Z. Li
and Z.-J. Shi, Angew. Chem., Int. Ed., 2008, 47, 1473; (e)
A. Suzuki, Angew. Chem., Int. Ed., 2011, 50, 6722; (f)
C.-L. Sun, B.-J. Li and Z.-J. Shi, Chem. Commun., 2010, 46,
677; (g) F. K. Sheffy, J. P. Godschalx and J. K. Stille, J. Am.
Chem. Soc., 1984, 106, 4833; (h) R. Giri, N. Maugel, J.-J. Li,
D.-H. Wang, S. P. Breazzano, L. B. Saunders and J.-Q. Yu,
J. Am. Chem. Soc., 2007, 129, 3510; (i) K. L. Hull and
M. S. Sanford, J. Am. Chem. Soc., 2007, 129, 11904; (j)
N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457; (k)
A. F. Littke and G. C. Fu, Angew. Chem., Int. Ed., 2002, 41,
4176; (l) X. Chen, K. M. Engle, D.-H. Wang and J.-Q. Yu,
Angew. Chem., Int. Ed., 2009, 48, 5094; (m) K. C. Nicolaou,
P. G. Bulger and D. Sarlah, Angew. Chem., Int. Ed., 2005, 44,
4442; (n) A. F. Littke, C. Dai and G. C. Fu, J. Am. Chem.
Soc., 2000, 122, 4020.
7 (a) L. Pehlivan, E. Metay, S. Laval, W. Dayoub,
P. Demonchaux, G. Mignani and M. Lemaire, Tetrahedron
Lett., 2010, 51, 1939; (b) R. J. Rahaim, Jr and
R. E. Maleczka, Jr, Synthesis, 2006, 3316.
8 S. M. Kelly and B. H. Lipshutz, Org. Lett., 2014, 16, 98.
9 A. Chinnappan and H. Kim, RSC Adv., 2013, 3, 3399.
2 (a) D. Hollmann, S. Baehn, A. Tillack and M. Beller, Angew.
Chem., Int. Ed., 2007, 46, 8291; (b) S. U. Sonavane,
M. B. Gawande, S. S. Deshpande, A. Venkataraman and
R. V. Jayaram, Catal. Commun., 2007, 8, 1803; (c) X. Yuan,
N. Yan, C. Xiao, C. Li, Z. Fei, Z. Cai, Y. Kou and P. J. Dyson, 10 (a) P. Anastas and J. Warner, Green Chemistry: Theory and
Green Chem., 2010, 12, 228; (d) F. Wang, J. Liu and X. Xu,
Chem. Commun., 2008, 2040; (e) A. Yokoyama, H. Suzuki,
Y. Kubota, K. Ohuchi, H. Higashimura and T. Yokozawa, J.
Practice, 1998; (b) J. Clark and D. MacQuarrie, Handbook of
Green Chemistry & Technology, 2002; (c) M. Lancaster, Green
Chemistry: An Introductory Text, 2002.
Am. Chem. Soc., 2007, 129, 7236; (f) K. C. Nicolaou, 11 A. A. Vernekar, S. Patil, C. Bhat and S. G. Tilve, RSC Adv.,
J. M. Ramanjulu, S. Natarajan, S. Brase, H. Li, 2013, 3, 13243.
C. N. C. Boddy and F. Rubsam, Chem. Commun., 1997, 12 R. J. Kalbasi and F. Zamani, RSC Adv., 2014, 4, 7444.
1899; (g) O. Baudoin, M. Cesario, D. Guenard and
F. Gueritte, J. Org. Chem., 2002, 67, 1199.
35236 | RSC Adv., 2014, 4, 35233–35237
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
13 E. Vasilikogiannaki, C. Gryparis, V. Kotzabasaki,
I. N. Lykakis and M. Stratakis, Adv. Synth. Catal., 2013, 355,
907.
14 (a) P. Leoni, M. Sommovigo, M. Pasquali, S. Midollini,
D. Braga and P. Sabatino, Organometallics, 1991, 10, 1038;
(b) Y. Zhu, C.-H. Chen, C. M. Fafard, B. M. Foxman and
O. V. Ozerov, Inorg. Chem., 2011, 50, 7980.
15 (a) J. Cheng, K. L. Handoo, J. Xue and V. D. Parker, J. Org.
Chem., 1993, 58, 5050; (b) X.-Q. Zhu, C.-H. Wang, H. Liang
and J.-P. Cheng, J. Org. Chem., 2007, 72, 945.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 35233–35237 | 35237