Hydrogenation of nitroaromatics to anilines catalyzed by air-stable arene ruthenium (II)–NNN pincer complexes

Article abstract of DOI:10.1002/aoc.4689

A series of air-stable, phosphine-free arene ruthenium (II)–NNN pincer complexes (RuL, RuL¹, RuL² and RuL³) have been synthesized and characterized by spectroscopic and single-crystal X-ray analysis. Further, arene ruthenium (II)–NNN pincer complexes have been used as catalyst for hydrogenation of nitroaromatics into aniline in the presence of NaBH₄ at room temperature. The catalytic process suggested highly chemo-selective nitroreduction with wide functional group tolerance.

Full text of DOI:10.1002/aoc.4689

Received: 11 July 2018
DOI: 10.1002/aoc.4689
Revised: 12 October 2018
Accepted: 15 October 2018
F U L L P A P E R
Hydrogenation of nitroaromatics to anilines catalyzed by
air‐stable arene ruthenium (II)–NNN pincer complexes
Jayaraman Pitchaimani1 | Nanjappan Gunasekaran2 | Savarimuthu Philip Anthony3 |
Dohyun Moon4 | Vedichi Madhu1
1 Department of Chemistry, Karunya
A series of air‐stable, phosphine‐free arene ruthenium (II)–NNN pincer com-
Institute of Technology and Sciences
plexes (RuL, RuL1, RuL2 and RuL3) have been synthesized and characterized
(Deemed to be University), Coimbatore
641 114Tamil Nadu, India
2 Centre for Nanotechnology & Advanced
by spectroscopic and single‐crystal X‐ray analysis. Further, arene ruthenium
(II)–NNN pincer complexes have been used as catalyst for hydrogenation of
Biomaterials (CenTAB), SASTRA Deemed
University, Thanjavur 613401Tamil Nadu,
India
3 School of Chemical & Biotechnology,
SASTRA Deemed University, Thanjavur
613401Tamil Nadu, India
4 Beamline Department, Pohang
Accelerator Laboratory, 80 Jigokro‐127
beongil, Nam‐gu, Pohang, Gyeongbuk,
Korea
nitroaromatics into aniline in the presence of NaBH4 at room temperature.
The catalytic process suggested highly chemo‐selective nitroreduction with
wide functional group tolerance.
KEYWORDS
arene ruthenium (II) complex, hydrogenation of nitroaromatics, NNN pincer ligand, phosphine‐free
ligand
Correspondence
Vedichi Madhu, Department of
Chemistry, Karunya Institute of
Technology and Sciences (Deemed to be
University), Coimbatore‐641 114, Tamil
Nadu, India.
Email: madhu@karunya.edu;
vmadhuu1@gmail.com Dohyun Moon,
Beamline Department, Pohang
Accelerator Laboratory, 80 Jigokro‐127
beongil, Nam‐gu, Pohang, Gyeongbuk
Korea.
Email: dmoon@postech.ac.kr
Funding information
Science and Engineering Research Board,
Grant/Award Numbers: EMR/2016/
006106, SB/FT/CS‐182/2011 and SB/FT/
CS‐189/2013; University Grants Commis-
sion, Grant/Award Number: 30‐11/2015
(SA‐II
1 | INTRODUCTION
agrochemicals, antioxidants, pharmaceuticals, lubricants,
fungicides, diazonium salts, dyes and pigments indus-
Aromatic amines are very important functional deriva-
tives in synthetic chemistry. They are widely used in
tries.[1] In contrast, nitroaromatic compounds (NACs)
are toxic and have widely been used as explosives.
Appl Organometal Chem. 2019;e4689.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4689

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PITCHAIMANI ET AL.
Contamination of NACs in water and soils causes serious
environmental pollution and harms the ecosystem.[2]
Thus, finding a suitable catalyst for the reduction of
nitroaromatics into their anilines has been of great inter-
est both for academic research as well as industrial appli-
cations. In this context, different heterogeneous/
homogeneous catalysts have often been employed for
the conversion of nitroarenes into aromatic amines.
Selective reduction of nitroaromatics at ambient tempera-
ture using nitrogen‐doped TiO2@carbon nanosheets‐
based catalysts has been demonstrated.[3] Huang and
co‐workers described selective platinum nanocatalyst for
hydrogenation of nitroaromatics to anilines at room
temperature.[4] Nickel‐oxide‐supported nano‐Pd catalyst
has also been reported for selective reduction of
nitroaromatics into anilines.[5] Furthermore, CeO2
nanorod‐based catalysts have also been reported for
hydrogenation of nitroaromatics with N2H4 as a reducing
agent.[6] Also, iron pyrite (FeS2) catalyst has been
reported for selective hydrogenation of various
substituted nitroarenes into their aniline derivatives using
molecular hydrogen.[7] Au−Cu alloy showed visible‐light‐
induced photocatalytic activity for the reduction of
nitroaromatics to anilines.[8] Mesoporous Au‐loaded
TiO2 nanoparticle assemblies showed efficient catalytic
activity for the chemoselective reduction of nitroarenes
into amines with NaBH4.[9] Wu and co‐workers described
a metal‐free reduction of NACs into corresponding
amines using B2pin2 and KOtBu in isopropanol.[10] Yu
and co‐workers reported the Ru‐NNN catalytic system
consisting of pyridyl‐benzimidazolyl‐based NNN ligand
for transfer hydrogenation of ketones.[11] Also, Yang,
Chen and co‐workers described the catalytic activity of
ruthenium‐NNN complex for transfer hydrogenation of
ketones.[12] Furthermore, Jia's group reported arene
ruthenium (II) complexes based Schiff‐base ligands as
homogeneous catalysts for reducing nitroarenes.[13]
Molecularly defined transition metal–pincer (pincer =
PNP, PCP, PNN, SNS, NCN and CNN) complexes have
drawn increased attention due to their wide range of cat-
alytic applications, such as C−H bond activation,
amination of alcohols, hydrogenation, production of bio-
fuel, water oxidation, conversion of nitriles to secondary
imines, hydrogenation of CO2 and alcohols to acetals.
However, most of the reported metal–pincer complexes
are not stable as they encompass phosphines or phos-
phites as electron‐donor groups.[14] Hence, synthesizing
air‐stable pincer complexes via replacing phosphine
donor by stable and strong coordinating amine donor
groups received significant interest in recent years.[15] In
this context, NNN pincer molecules could be potential
chelating ligands for replacing phosphines and producing
air‐stable arene ruthenium (II)–pincer complexes, and
synthetic accessibility of ligands also provided the oppor-
tunity to develop versatile pincer complexes that might
exhibit selective and enhanced catalytic activity. In this
manuscript, we report the synthesis and structural studies
of a series of phosphine‐free and air‐stable arene ruthe-
nium (II)–NNN pincer complexes, and explored for selec-
tive hydrogenation of nitroaromatics to anilines in the
presence of NaBH4. The structure of the complex was
unambiguously characterized using single‐crystal X‐ray
diffraction. The strong catalytic activities of arene ruthe-
nium (II)–NNN pincer complexes with various functional
groups (electron donor and acceptor)‐attached substrate
demonstrate wide functional group tolerance.
2 | EXPERIMENTAL
2.1 | Materials and methods
All solvents and chemicals were purchased from com-
mercial suppliers (Aldrich, Alfa Aesar and TCI
chemicals), and were used as received. Solvents were
purified with appropriate drying agents when required.
All moisture‐sensitive reactions were carried out under
nitrogen atmosphere. UV–Vis absorption spectra were
recorded with a Perking Elmer Lambda 1050 spectropho-
tometer. Fourier transform‐infrared (FT‐IR) measure-
ments were carried out with Shimadzu IRAffinity‐1S
spectrophotometer with KBr pellets. The course of the
reactions was analyzed by a Shimadzu GC‐2014 gas chro-
matograph equipped with a RTX‐5 column (60 m length,
0.32 mm inner diameter) and flame ionization detector. A
single crystal of RuL was coated with paratone‐N oil, and
the diffraction data were measured at 173 K with syn-
chrotron radiation (l = 0.60999 Å) on an ADSC
Quantum‐210 detector at 2D SMC with a silicon (111)
double‐crystal monochromator (DCM) at the Pohang
Accelerator Laboratory, Korea. The ADSC Q210 ADX
program[16] was used for data collection (the detector dis-
tance is 63 mm, omega scan; Do = 31, the exposure time
is 2 s per frame) and HKL3000sm (Ver. 703r)[17] was used
for cell refinement, reduction and absorption correction.
The crystal structure of RuL was solved by the direct
method using the SHELX‐XT (2014/4) program and
refined by full‐matrix least‐square calculations using the
SHELX‐XL (2014/7) program package.[18] All non‐
hydrogen atoms were refined anisotropically. The hydro-
gen atoms were assigned an isotropic displacement coeffi-
cient of Uiso(H) = 1.2 or 1.5 Ueq, and their coordinates
were allowed to ride on their respective atoms. Crystallo-
graphic data of RuL were deposited with the Cambridge
Crystallographic Data Center as supplementary publica-
tion no. CCDC‐1576819. These data can be obtained free

PITCHAIMANI ET AL.
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of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
1461 (m), 1267 (m), 742 (s). 1H‐NMR (300 MHz,
CDCl3): δ 7.68–759 (t. 1H), 7.45–7.29 (d, 2H), 3.92–
3.64 (s, 4H), 3.15–2.72 (m, 8H), 2.71–2.42 (s, 8H),
2.43–2.21 (s, 2H). HRMS (ESI): calcd for C15H25N5
[M + H]+ 275.39; found 276.21.
2.2 | Synthesis and characterization data
2.2.1 | Synthesis of Py‐NNNmor (L)
2.2.3 | Synthesis of Py‐NNNMepip (L2)
A solution of 2,6‐bis (bromomethyl) pyridine (0.3 g, 1.13
mmol) in CH3CN (30 ml) was added dropwise into the
stirred solution of morpholine (0.197 g, 2.26 mmol) and
K2CO3 (0.468 g, 3.39 mmol) in CH3CN (10 ml). The solu-
tion was allowed to stir at 85°C for 14 hr. After the com-
pletion of the reaction, the reaction mixture was cooled to
room temperature and filtered. The filtrate was evapo-
rated, extracted in chloroform and dried over anhydrous
Na2SO4 that afforded white crystalline powder as a prod-
uct (yield 0.280 g, 89%). IR (KBr): ν in cm−1 = 2800 (m),
1575 (m), 1454 (m), 1298 (m), 1111 (s), 906 (m), 864
2,6‐Bis (bromomethyl) pyridine (0.8 g, 3.0 mmol) in
CH3CN (45 ml) was added dropwise to the stirred solu-
tion of 1‐methylpiperazine (0.6 g, 6.0 mmol). After addi-
tion, K2CO3 (1.249 g, 9.0 mmol) in CH3CN (20 ml) was
added. The resulting reaction mixture was stirred for 14
hr at 85°C. After completion of the reaction, the com-
pound was extracted using chloroform and water. The
organic layer was collected, and evaporated in vacuum
under the reduced pressure (yield 0.815 g, 90%). IR
(KBr): ν in cm−1 = 3396 (m), 2934 (m), 1610 (m), 1465
(s), 1296 (m), 1120 (m), 979 (m). 1H‐NMR (500 MHz,
CDCl3): δ 7.59 (s, 1H), 7.28 (s, 2H), 3.66 (s, 4H), 2.54 (s,
8H), 2.46 (s, 8H), 2.28 (s, 6 H). 13C‐NMR (126 MHz,
CDCl3): δ 158.0, 136.5, 121.2, 77.2, 76.7, 64.4, 55.1, 53.2,
46.1. ESI‐MS: calcd for C17H29N5: 303.44; found
[M + H]+: 304.
1
(m). H‐NMR (500 MHz, CDCl3): δ 7.65–7.52 (m, 1H),
7.31 (d, J = 7.6 Hz, 2H), 3.84–3.69 (m, 8H), 3.66 (s, 4H),
2.51 (s, 8H). 13C‐NMR (126 MHz, CDCl3): δ 157.7, 136.7,
121.4, 77.3, 76.7, 66.9, 64.8, 53.7. ESI‐MS: calcd for
C15H23N3O2: 277.36; found [M + H]+: 278.
2.2.2 | Synthesis of Py‐NNNpip (L1)
Step 1. Synthesis of PMP1: a solution of 2,6‐bis
(bromomethyl) pyridine (1 g, 3.77 mmol) in CH3CN
(40 ml) was added dropwise into the stirred solution
of 1‐boc‐peprazine (1.4037 g, 7.54 mmol) and K2CO3
(1.56 g, 1.13 mmol) in CH3CN (20 ml). The solution
was allowed to stir at 85°C for 14 hr. After completion
of the reaction, the reaction mixture was cooled and
filtered. The filtrate was evaporated, extracted in chlo-
roform and dried over anhydrous Na2SO4 that
afforded pale yellow oil as a product (PMP1; yield
2.2.4 | Synthesis of Py‐NNNpidine (L3)
2,6‐Bis (bromomethyl) pyridine (0.3 g, 1.13 mmol) in
CH3CN (30 ml) was added dropwise to the mixture of
piperidine (0.192 g, 2.26 mmol) and K2CO3 (0.468 g,
3.39 mmol) in CH3CN (15 ml), and the resulting reaction
mixture was stirred for 14 hr at 85°C. The compound was
extracted with chloroform and evaporated in vacuum
under reduced pressure (yield 0.282 g, 91%). IR (KBr): ν
in cm−1 = 2939 (m), 2372 (m), 1627 (m), 1581 (m), 1456
1
0.8 g, 80%). H‐NMR (400 MHz, CDCl3): δ 7.78–7.48
1
9 m), 1276 (m). H‐NMR (500 MHz, CDCl3): δ 7.58–7.48
(m, 1H), 7.31 (dd, J = 13.3, 7.0 Hz, 2H), 4.12–3.52
(m, 4H), 3.58–3.30 (m, 8H), 2.72–2.28 (m, 7H), 1.94
(s, 1H), 1.73–1.18 (m, 18H). 13C‐NMR (400 MHz,
CDCl3): δ 157.77, 154.81, 136.74, 121.43, 79.60, 64.45,
53.08, 28.43. HRMS (ESI): calcd for C25H41N5O4
[M + H]+ 475.62; found 476.3244.
Step 2. Synthesis of L1: to a solution of PMP1 (0.8 g,
1.68 mmol) in methanol (15 ml), 1 N HCl (8 ml) was
added and allowed to stir for 6 hr at 60°C. After
cooling to room temperature, the reaction mixture
was neutralized with 5% NaHCO3. Then the solvent
was evaporated. The resulting residue was washed
with ethanol and concentrated in vacuum to get the
yellow oily product of L1 (yield 0.750 g, 75%). IR
(KBr): ν in cm−1 = 3429 (w), 2923 (m), 1639 (s),
(m, 1H), 7.20 (d, J = 5.0 Hz, 2H), 3.53 (s, 4H), 2.36 (s,
8H), 1.57–1.49 (m, 8H), 1.37 (d, J = 4.7 Hz, 4H). 13C‐
NMR (400 MHz, CDCl3): δ 159.83, 146.45, 137.89,
136.53, 127.67, 124.37, 121.37, 65.23, 58.31, 54.80, 25.97,
24.22, 20.20. ESI‐MS: calcd for C17H27N3: 273.41; found
[M + H]+: 274.
2.2.5 | Synthesis of RuL
[Ru(p‐cymene)Cl2]2 (0.056 g, 0.09 mmol) was added into
the ligand L (0.051 g, 0.18 mmol) in methanol (10 ml)
under stirring at room temperature. The reaction mixture
was allowed to stir for 3 hr at room temperature. After
completion of the reaction the solvent was evaporated

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PITCHAIMANI ET AL.
under reduced pressure. The resulting yellow solid was
dissolved in methanol, then AgBF4 (0.017 g, 0.09 mmol)
was added and stirred for 0.5 hr. Diffusion of diethylether
into the filtrate solution for 3 days afforded the red crys-
tals, which are suitable for single‐crystal X‐ray diffraction
(yield 0.093 g, 84%). IR (KBr): ν in cm−1 = 2964 (m), 2854
(m), 1610 (m), 1574 (m), 1456 (s), 1389 (m), 1293 (m),
1058(s), 865(s), 794(m), 668(m). 1H‐NMR (500 MHz,
DMSO): δ 7.96 (dd, J = 52.3, 7.8 Hz, 2H), 7.61 (d, J =
7.4 Hz, 1H), 6.22–5.68 (m, 4H), 4.68 (dd, J = 38.4, 15.2
Hz, 2H), 4.32–4.12 (m, 2H), 4.13–3.93 (m, 2H), 3.78 (s,
8H), 3.60 (d, J = 12.2 Hz, 4H), 2.82 (s, 1H), 2.46–2.32
(m, 3H), 1.88 (s, 2H), 1.38–0.89 (m, 6H). 13C‐NMR (101
MHz, D2O): δ 159.93 (s), 158.97 (s), 141.46 (s), 140.26
(s), 130.14 (s), 126.62 (s), 124.42 (s), 123.20 (s), 107.12
(s), 97.27 (s), 85.74 (s), 85.15 (s), 84.75 (s), 83.86 (s),
83.50 (s), 81.93 (s), 81.26 (s), 80.58 (s), 76.38 (s), 75.81
(s), 65.98 (s), 65.37 (s), 64.84 (s), 64.44 (s), 63.80 (d, J =
22.6 Hz), 63.50–62.45 (m), 56.05 (s), 54.49 (s), 52.68 (s),
52.07 (s), 51.64 (s), 48.84 (s), 30.98 (s), 30.77 (s), 23.14
(d, J = 16.0 Hz), 21.81 (s), 19.64 (s), 19.01 (s), 17.59 (s),
17.00 (s), 16.80 (s).
1606 (m), 1465 (s), 1261 (m), 1099 (m), 803 (m). 1H‐
NMR (400 MHz, D2O): δ 7.95 (d, J = 5.8 Hz, 1H), 7.51
(d, J = 6.7 Hz, 2H), 5.83 (dd, J = 45.6, 11.6 Hz, 4H), 5.25
(t, J = 12.7 Hz, 4H), 5.06 (d, J = 6.0 Hz, 4H), 3.72 (d, J
= 22.4 Hz, 4H), 3.21 (d, J = 10.9 Hz, 8H), 3.04 (d, J =
10.9 Hz, 4H), 2.80 (s, 3H), 1.98 (d, J = 12.0 Hz, 6H), 1.11
(s, 6H). 13C‐NMR (400 MHz, D2O): δ 163.59 (s), 140.40
(s), 139.05 (s), 126.46 (s), 123.15 (s), 98.42 (s), 92.69 (s),
84.36 (s), 76.38 (s), 75.80 (s), 62.46 (s), 60.14 (s), 53.56
(s), 52.97 (s), 49.56 (s), 48.84 (s), 43.33 (s), 42.72 (s),
38.48 (s), 30.98 (s), 30.45 (s), 23.29 (s), 21.80 (s), 18.85
(s), 17.58 (s), 16.82 (s).
2.2.8 | Synthesis of RuL3
[Ru(p‐cymene)Cl2]2 (0.06 g, 0.098 mmol) was added into
ligand L3 (0.07 g, 0.2 mmol) in methanol (10 ml) and
stirred for 3 hr at room temperature. After completion
of the reaction, the solvent was evaporated under reduced
pressure to afford orange solid (yield 0.146 g, 89%). IR
(KBr): ν in cm−1 = 2926 (s), 1649 (m), v(C=N) 1600
(m), 1460 (s), 1159 (m), 1031 (m), 864 (m), 805 (s), 559
1
(m). H‐NMR (500 MHz, CDCl3): δ 7.80 (s, 2H), 7.53 (s,
2.2.6 | Synthesis of RuL1
1H), 5.82 (d, J = 5.2 Hz, 1H), 5.49 (d, J = 5.5 Hz, 2H),
5.36 (t, J = 7.9 Hz, 1H), 4.82–4.55 (m, 2H), 3.80 (t, J =
11.1 Hz, 2H), 3.55–3.46 (m, 1H), 3.07 (dddd, J = 82.6,
69.0, 13.7, 6.8 Hz, 3H), 2.70–2.49 (m, 4H), 2.32–2.17 (m,
4H), 1.94 (s, 4H), 1.69 (d, J = 7.1 Hz, 4H), 1.46–1.35 (m,
6H), 1.23 (qd, J = 13.5, 6.4 Hz, 4H). 13C‐NMR (101 MHz,
D2O): δ 161.36 (s), 153.76 (s), 140.94 (s), 139.49 (s),
128.79 (d, J = 17.0 Hz), 124.82 (s), 98.42 (s), 92.70 (s),
85.73 (s), 85.11 (s), 82.07 (s), 76.38 (s), 75.80 (s), 65.99
(s), 65.47 (s), 62.42 (s), 56.51 (s), 53.69 (s), 30.78 (s),
30.45 (s), 23.03 (d, J = 16.6 Hz), 22.83–22.27 (m), 22.27–
22.15 (m), 21.72 (d, J = 14.8 Hz), 20.93 (s), 19.94 (s),
19.60 (s), 18.66 (s), 17.58 (s), 16.19 (s), 14.07 (s).
[Ru(p‐cymene)Cl2]2 (0.06 g, 0.098 mmol) was added into
ligand L1 (0.054 g, 0.19 mmol) in methanol (7 ml) and
stirred for 3 hr at room temperature. After completion
of the reaction, the solvent was evaporated under reduced
pressure to afford yellow solid (yield 0.116 g, 93%). IR
(KBr): ν in cm−1 = 3376 (w), 3057 (m), 2960 (m), 1658
(m), v(C=N) 1601 (m), 1460 (s), 1382 (m), 1179 (m),
1
1087(m), 1001(m), 870 (s), 802 (m), 681 (m). H‐NMR
(500 MHz, CDCl3): δ 7.58 (dd, J = 14.1, 6.5 Hz, 2H),
7.18–7.00 (m, 1H), 5.31 (d, J = 5.7 Hz, 1H), 5.27 (dd, J =
17.0, 5.8 Hz, 2H), 5.20–5.02 (m, 1H), 4.26 (d, J = 23.6
Hz, 1H), 3.75–3.45 (m, 4H), 3.41 (s, 2H), 3.19 (dt, J =
23.1, 11.5 Hz, 2H), 2.99–2.70 (m, 4H), 2.56 (s, 3H), 2.49–
2.40 (m, 2H), 2.35–1.89 (m, 8H), 1.21–1.11 (m, 6H). 13C‐
NMR (101 MHz, D2O): δ 156.66 (s), 123.26 (s), 98.42 (s),
92.70 (s), 84.59 (s), 80.38 (s), 76.38 (s), 75.79 (s), 62.79
(s), 49.00 (d, J = 33.3 Hz), 43.49 (s), 31.01 (s), 30.45 (s),
27.51 (s), 23.26 (s), 21.78 (s), 18.83 (s), 17.57 (s), 16.82 (s).
2.2.9 | General procedure for catalytic run
To a solution of RuL catalyst (0.0018 g, 0.003 mmol) in
ethanol (2.0 ml), appropriate nitroaromatics (0.3 mmol)
and NaBH4 (0.045 g, 1.2 mmol) were added. The reaction
mixture was stirred for 0.5 hr at room temperature. Reac-
tion completion was monitored by thin‐layered chroma-
tography, and then the crude reaction mixture was
extracted with ethyl acetate twice. The organic fractions
were removed in vacuum and again dissolved in metha-
nol. The product was analysed by using gas chromatogra-
phy. The percentage of yield was calculated by the
comparison of retention times and corresponding peak
area under the experimental conditions. The area
2.2.7 | Synthesis of RuL2
[Ru(p‐cymene)Cl2]2 (0.06 g, 0.098 mmol) was added into
ligand L2 (0.06 g, 0.19 mmol) in methanol (7 ml) and
stirred for 3 hr at room temperature. After completion
of the reaction, the solvent was evaporated under reduced
pressure to afford orange solid (yield 0.118 g, 89%). IR
(KBr): ν in cm−1 = 3351 (w), 2963 (m), 1658 (s), v(C=N)

PITCHAIMANI ET AL.
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normalization method was used to calculate percentage
of product formed.
3 | RESULTS AND DISCUSSION
Arene ruthenium (II)–pincer complexes have been syn-
thesized by reacting tridentate NNN pincer ligands (L,
L1, L2 and L3; Scheme 1) with dichloro(p‐cymene) ruthe-
nium (II) dimer in MeOH at room temperature
(Scheme 2). The complexes were characterized by using
NMR, mass, IR and UV–Vis spectroscopic studies. Fur-
ther, to gain insight on the structure of arene Ru (II)–pin-
cer complexes, we have attempted to grow single crystals
of all arene Ru (II)–pincer complexes. However, only
RuL complex produced quality single‐crystals by slow
diffusion of diethyl ether into CH3OH. The complex
RuL crystallized in the monoclinic space group P21/c. A
thermal ellipsoidal plot of RuL complex is shown in
Figure 1. The crystal refinement data of RuL are given in
Table 1. The crystal structure of RuL describes that Ru
(II) center adopted a typical ‘three leg piano‐stool’ geom-
etry via coordination of two nitrogen (N1 and N2) atoms
of the NNN pincer ligand, one chloride ion and π‐bonded
FIGURE 1 Ellipsoid plots of complex RuL at the 30% probability
level. All hydrogen atoms and BF4− anions are omitted for clarity
TABLE 1 Crystal data and structure refinement for RuL
RuL
Empirical formula
Formula weight
Temperature
C25H37BClF4N3O2Ru
634.90
6
173(2) K
to the p‐cymene ring in ɳ‐coordination mode. For com-
plex RuL, the distance between Ru metal center and cen-
troid of p‐cymene is 1.6554(4) Å. The Ru–C bond
distances fall in the range of 2.181(1)–2.229(2) Å. The
Wavelength
0.610 Å
Crystal system
Space group
Monoclinic
P21/c
Unit cell dimensions
a = 15.984(2) Å
b = 9.689(2) Å
c = 18.117(2) Å
Volume
2753.3(7) Å3
Z
4
Density (calculated)
Absorption coefficient
F (000)
1.532 mg m−3
0.475 mm−1
1304
Crystal size
0.275 × 0.245 × 0.195 mm3
Theta range for data collection
Index ranges
2.055–24.998°
−22 ≤ h ≤ 22, −12 ≤ k ≤ 12,
−25 ≤ l ≤ 25
SCHEME 1 Synthesis of tridentate NNN pincer ligands (L–L3)
Reflections collected
14 373
Independent reflections
Completeness to theta = 21.469°
Max. and min. transmission
Data/restraints/parameters
Goodness‐of‐fit on F 2
7428 [R (int) = 0.0223]
97.7%
1.000 and 0.846
7428/0/338
1.028
Final R indices [I > 2sigma(I)]
R indices (all data)
R1 = 0.0271, wR2 = 0.0704
R1 = 0.0336, wR2 = 0.0722
0.564 and −0.846 e Å−3
SCHEME 2 Synthetic pathway for the preparation of arene
ruthenium (II)–NNN pincer complexes
Largest diff. peak and hole

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PITCHAIMANI ET AL.
Ru(1)‐Cl(1), Ru(1)‐N(1) and Ru(1)‐N(3) bond lengths
were 2.4058(5), 2.1481(13) and 2.2541(14) Å, respectively.
The coordination sphere and bond lengths were consis-
tent with analogous p‐cymene ruthenium (II) complexes
reported in the literature.[19] Single‐crystal structural
analysis also revealed that monomeric RuL complex
shows C–H···O intermolecular hydrogen bonding interac-
tions and leads to a supramolecular dimeric structure
(Figure 2). The C(19)–H(19)···O(2) hydrogen bonding dis-
tance is 2.303(1) Å and the corresponding bond angle is
163.09(1)°. Interestingly, NNN pincer ligand unsymmetri-
cally coordinated to the ruthenium metal center in the
NNN pincer complex. In other words, one side‐arm of
the pincer ligand of RuL complex was free and flexible
that may cooperatively coordinate to the ruthenium
metal center, which is important for better catalysis. It
is noted that Ru–pincer complexes/catalysts with such
coordination patterns have been reported scarcely. The
UV–Vis spectra of the ligands and their respective com-
plexes were obtained in CH3OH solvent (Figure 3). The
absorption spectra of arene‐Ru complexes display
absorption bands in the range of 200–270 nm due to
π–π* transition. The absorption band between 350 and
470 nm is attributed to the metal‐to‐ligand charge trans-
fer [MLCT = dπ (Ru) → π*(L)].[20]
Fourier transform‐infrared spectra of free ligands
exhibited a characteristic ν(C=N) stretching vibration
band in the range of 1583–1653 cm−1 (Figures S1–S4).
Upon coordination of the pincer ligand with metal center,
the ν(C=N) stretching vibration bands appeared in the
range between 1600 and 1658 cm−1. The IR stretching
vibration changes support NNN pincer ligands coordi-
1
nated to Ru metal center (Figures S5–S8). Further, H‐
and 13C‐NMR spectra of all ligands and their arene Ru
(II) complexes were recorded in CDCl3 and D2O (Figures
S9–S25). 1H‐NMR spectra of all ligands exhibited two sets
of pyridine proton resonance in the range of 7.31–8.22
ppm, and aliphatic protons are resonated between 1.92
and 4.50 ppm. The ligand L1 shows terminal NH signals
at 2.21–2.45 ppm. The N‐CH3 protons of ligand L2
appeared at 2.25–2.29 ppm, and its relevant 13C peak res-
onated at 46.54 ppm. 13C signals of pyridine and aliphatic
carbons are resonated in the range of 120–165 ppm and
1
38–65 ppm, respectively. The H‐NMR spectra of arene
ruthenium (II)–NNN pincer complexes exhibited proton
signals between 5.18 and 6.12 ppm, which were assigned
as p‐cymene protons. The isopropyl and methyl protons
of all complexes were resonated between 1.10–1.21 ppm
and 1.78–2.56 ppm, respectively. The 13C‐NMR peaks
were in the range of 75.81–92.69 ppm and 16.80–23.22
ppm due to p‐cymene and isopropyl/methyl carbons.
The arene ruthenium (II)–NNN pincer complexes
have been studied for the hydrogenation/reduction of
nitrobenzene with NaBH4 at room temperature
(Scheme 3). Significantly, the complex RuL exhibited bet-
ter catalytic activity among all arene ruthenium (II)–
NNN pincer complexes, and the results are presented in
Table 2. It is noted that no hydrogenation of nitrobenzene
was observed in the absence of catalyst (Table 2, entry 6).
Also, in the presence of complex [Ru(p‐cymene)Cl2]2 and
without reducing agents also showed no hydrogenation of
nitrobenzene (Table 2, entries 5 and 10). Further, the cat-
alytic activity of RuL for the reduction of nitroaromatics
has also been examined in the presence of various reduc-
ing agents (Table 2, entries 1, 8 and 9). Interestingly, con-
version of nitroaromatics to aniline was obtained at high
yield (95%) when NaBH4 reducing agent was used in
FIGURE 2 Supramolecular interactions in the crystal structure
of RuL
FIGURE 3 Absorption spectra of arene Ru (II)–NNN pincer
SCHEME
3
Arene ruthenium (II)–NNN pincer‐catalyzed
complexes in methanol (1 × 10–4 M)
hydrogenation of nitroaromatics to anilines

PITCHAIMANI ET AL.
7 of 9
TABLE 2 Optimization of catalyst in the reduction of nitrobenzenea
Entry
Catalyst
Solvent
Time (hr)
Reducing agent
Yield (%)b
1
RuL
RuL1
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Methanol
Acetonitrile
DCM
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
NaBH4
95
2
NaBH4
85
3
RuL2
NaBH4
83
4
RuL3
NaBH4
82
5
[Ru(p‐cymene)Cl2]2
NaBH4
No reaction
6
No catalyst
Free ligand
RuL
NaBH4
No reaction
7
NaBH4
No reaction
8
N2H4
2
9
RuL
Ascorbic acid
No reducing agent
NaBH4
No reaction
10
11
12
13
14
15
16
RuL
No reaction
RuL
68
2
RuL
NaBH4
RuL
NaBH4
7
RuL
THF
NaBH4
33
0
RuL
Toluene
iPrOH
NaBH4
RuL
NaBH4
0
aReaction conditions: nitrobenzene (0.0369 g, 0.3 mmol), NaBH4 (0.045 g, 1.2 mmol), RuL (0.0018 g, 0.003 mmol) in ethanol (2 ml) at room temperature for
0.5 hr.
bYield is determined by gas chromatography with area normalization and gas chromatography conditions for analysis: RTX‐5 column, 60 m × 0.32 mm, initial
column temperature was increased from 130 to 210°C at a rate of 7°C per min, and from 210 to 250°C at a rate of 20°C per min; FID detector, 280°C; injector,
260°C; carrier gas, N2; rate, 2.00 ml min−1
.
ethanol at room temperature (Table 2, entry 1). All other
reducing agents such as N2H4 and ascorbic acid did not
produce significant reduction. The catalytic reaction has
also been performed with other solvents like methanol,
acetonitrile, dichloromethane, tetrahydrofuran, toluene,
i‐PrOH (Table 2, entries 11–16). The conversion of nitro-
benzene to aniline with 68% and 33% was obtained when
methanol and THF were used as solvents, respectively.
Notably, reaction of NaBH4 in protic solvents, such as
methanol and ethanol, afforded unstable acid–base
adduct ROH•BH3, which leads to further reaction in the
presence of catalyst. However, when we use the solvent
iPrOH no nitrobenzene reduction was observed under
standard conditions due to the formation of stable acid–
TABLE 3 Screening of substrates for nitroaromatic reduction
catalyzed by RuLa
Time Yield
Entry
Substrate
Product
(hr)
(%)b
1
0.5
95
2
3
0.5
0.5
83
96
4
0.5
98
i
base adduct PrOH•BH3, which could halt the reac-
tion.[21] These optimized reaction conditions have been
utilized for the reduction of a series of NACs having dif-
ferent electron‐donating and electron‐withdrawing
groups (Table 3). The chemo‐selective hydrogenation of
NACs with reducible functional groups such as acetyl,
acyl, aldehyde and halogen have also been investigated
and showed good tolerance. To gain insight into the reac-
tion mechanism of the reduction of nitroaromatics, the
reaction of RuL with NaBH4 in ethanol was performed.
The intermediate was isolated and investigated using
1H‐NMR, the proton signal at −12.96 ppm suggested
5
6
0.5
0.5
73
88
7
0.5
50
(Continues)

8 of 9
PITCHAIMANI ET AL.
TABLE 3 (Continued)
suggests that RuL exhibited better catalytic activity, selec-
tivity and wide functional group tolerance. The mecha-
nistic studies showed that formation of Ru‐H
intermediate was a key step for activation of catalyst.
Time Yield
(hr)
Entry
Substrate
Product
(%)b
8
0.5
63
ACKNOWLEDGEMENTS
This research is supported by Science and Engineering
Research Board (SERB; Project No. SB/FT/CS‐182/2011
and EMR/2016/006106) and University Grants Commis-
sion (UGC) (No: 30‐11/2015; SA‐II). NG acknowledges
SERB (SB/FT/CS‐189/2013).
9
0.5
0.5
0.5
76
97
92
10
11
ORCID
Vedichi Madhu https://orcid.org/0000-0003-0253-6864
12
13
0.5
0.5
55
79
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Gunasekaran N, Anthony SP, Moon D, Madhu V.
Hydrogenation of nitroaromatics to anilines
catalyzed by air‐stable arene ruthenium (II)–NNN
pincer complexes. Appl Organometal Chem. 2019;
e4689. https://doi.org/10.1002/aoc.4689
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