Binuclear half-sandwich ruthenium(II) Schiff base complexes: Synthesis, characterization, DFT study and catalytic activity for the reduction of nitroarenes

Article abstract of DOI:10.1016/j.jorganchem.2019.120984

The binuclear ruthenium(II) p-cymene complexes containing Schiff base ligands of general composition [(Ru(p-cymene)Cl)₂L_1-6] (1-6) have been synthesized. The complexes were characterized by analytical and spectral (FT?IR, UV–Vis & ¹H NMR) methods. The molecular structure of the representative complex [(Ru(p-cymene)Cl)₂(L₆)] (6) was determined by single-crystal X-ray diffraction and density functional theory (DFT) calculations. Further, these half-sandwich ruthenium complexes are active catalysts for the mild hydrogenation of nitroarenes to aromatic anilines in the presence of NaBH₄ in ethanol. The most efficient catalyst 6, was found be compatible with nitroarenes of various functional groups.

Full text of DOI:10.1016/j.jorganchem.2019.120984

Journal Pre-proof
Binuclear half-sandwich ruthenium(II) Schiff base complexes: Synthesis,
characterization, DFT study and catalytic activity for the reduction of nitroarenes
Raja Nandhini, Bellie Sundaram Krishnamoorthy, Galmari Venkatachalam
PII:
S0022-328X(19)30427-9
DOI:
https://doi.org/10.1016/j.jorganchem.2019.120984
JOM 120984
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 5 September 2019
Revised Date: 11 October 2019
Accepted Date: 14 October 2019
Please cite this article as: R. Nandhini, B.S. Krishnamoorthy, G. Venkatachalam, Binuclear half-
sandwich ruthenium(II) Schiff base complexes: Synthesis, characterization, DFT study and catalytic
activity for the reduction of nitroarenes, Journal of Organometallic Chemistry (2019), doi: https://
doi.org/10.1016/j.jorganchem.2019.120984.
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Binuclear Half-Sandwich Ruthenium(II) Schiff Base Complexes: Synthesis,
Characterization, DFT Study and Catalytic Activity for the Reduction of
Nitroarenes
Raja Nandhinia, Bellie Sundaram Krishnamoorthyb and Galmari Venkatachalam*a
aPG & Research Department of Chemistry, Government Arts College, Dharmapuri- 636 705,
Tamilnadu, India.
bPG & Research Department of Chemistry, Vivekanandha college of Arts and Science for
Women, Tiruchengode, Salem, Tamilnadu, India.
Corresponding Author: *E-mail address: gvchem@gmail.com [G. Venkatachalam].
ABSTRACT
The binuclear ruthenium(II) p-cymene complexes containing Schiff base ligands of
general composition [(Ru(p-cymene)Cl)2L1-6] (1-6) have been synthesized. The complexes were
1
characterized by analytical and spectral (FT−IR, UV−Vis & H−NMR) methods. The molecular
structure of the representative complex [(Ru(p-cymene)Cl)2(L6)] (6) was determined by single-
crystal X-ray diffraction and density functional theory (DFT) calculations. Further, these half-
sandwich ruthenium complexes are active catalysts for the mild hydrogenation of nitroarenes to
aromatic anilines in the presence of NaBH4 in ethanol. The most efficient catalyst 6, was found
be compatible with nitroarenes of various functional groups.
Keywords: Binuclear Ruthenium; Schiff base; Synthesis; Crystal Structure; DFT; Reduction of
Nitroarene.
1

INTRODUCTION
The arene ruthenium complexes play an important role in organometallic chemistry.
Synthesis of half-sandwich ruthenium(II) complexes received a considerable attention owing to
their catalytic properties [1] and interesting anti-tumor activity [2]. The half-sandwich ruthenium
units with three-legged, piano-stool fragments are easy to prepare and are air-stable, where the
piano-stool legs can be variably connected with O, N donor ligands [3‒12]. The majority of half-
sandwich ruthenium complexes have been prepared directly via reaction between various organic
donors and half-sandwich ruthenium dimer precursors. Schiff base compounds are important O,
N-based ligands which are readily modifiable to allow fine tuning of steric and electronic
properties of the metal center [13].
The η6-arene-ruthenium(II) complexes represent an important class of compounds, [14]
well-known for promoting a great variety of catalytic reactions, such as C-H activation, [15]
nitrile hydration [16] or transfer hydrogenation processes [17]. Aromatic anilines are important
intermediates and precursors in the preparation of dyes, agrochemicals, pharmaceuticals, and
pigments [18,19]. The most commonly used method for the synthesis of aromatic anilines is the
reduction of nitroarenes. Catalytic hydrogenation using heterogeneous or homogeneous
transition-metal catalysts is a well-established technique and is often employed for the reduction
of nitroarenes to aromatic anilines [20−26]. There are many reports on the hydrogenation
reaction of nitroarenes using transition metal catalysts in the presence of a reducing agent such as
hydrogen gas, sodium tetrahydroborate, hydrazine, and alcohol as the hydrogen source.
However, the use of well-defined half-sandwich ruthenium complexes containing Schiff-base
ligands as homogeneous catalysts for hydrogenation of nitroarenes has rarely been reported. In
comparison with previously reported half-sandwich ruthenium systems, ruthenium catalysts
2

containing bulky naphthalene groups may exhibit higher catalytic activity for nitroarene
reduction. The promising efficiency of these catalysts may be partially attributed to the strong
electron-donating ability of the nitrogen and oxygen donors from Schiff base ligands.
We, prepare new half-sandwich binuclear ruthenium complexes with Schiff base ligands
and hope to exploit their chemistry in catalytic reduction of nitroarenes. Preliminary results
indicate that these complexes can catalyze nitroarene reduction in the presence of NaBH4 in
ethanol. The electronic effects on their catalytic behavior of nitroarene reduction are also
discussed. Computational chemistry tools are being used routinely to get more insight about the
structural preferences and bonding nature of the metal complexes. Density Functional Theory
(DFT) calculations at BP86/TZVP level are performed on the complex 6, as a model to elucidate
the electronic as well as geometrical structural features and bonding nature of these complexes.
RESULT AND DISCUSSION
The dark red half-sandwich binuclear ruthenium(II) complexes (1-6) were obtained by
the reaction of [Ru(p-cymene)(µ-Cl)Cl]2 with an equivalent of the Schiff-base ligands in the
presence of Et3N in CH2Cl2 under stirring at room temperature for 4h (Scheme 1). Ruthenium
complexes were isolated as pure complexes by chromatography on silica gel using EtOAC as an
1
eluent in yields of 60−80%. All complexes have been characterized by IR, UV-Vis, H-NMR
and mass spectrometry. The half-sandwich ruthenium complexes are air and moisture stable,
soluble in CH2Cl2, CHCl3 and CH3CN.
3

R'
HO
N
Cl
Ru
Cl
Ru
Cl
Ru
R'
O
N
O
N
R
R'
Ru
R
CH2Cl2/Stirr.4h/Et3N
Cl
N
HO
R'
R = CH2; R' = H (L1; 1), -O- ; R' = H (L2; 2)
R = CH2; R' = OCH3 (L3; 3), -O- ; R' = OCH3 (L4; 4);
R = CH2 (L5; 5), -O- (L6; 6)
R = CH2; R' = H (H2L1), -O- ; R' = H (H2L2)
R = CH2; R' = OCH3 (H2L3), -O- ; R' = OCH3 (H2L4)
R = CH2 (H2L5); -O- (H2L6)
Scheme 1. Synthesis of Half-Sandwich Binuclear Ruthenium complexes (1-6).
The IR spectra of the Schiff bases (H2L1‒H2L6) showed strong bands around 1619‒1612
cm‒1 region are assigned for −CH=N stretching frequency. The C=N stretching frequencies
displayed by the ruthenium(II) Schiff base complexes are lower than that for the uncomplexed
ligands. This lowering of the C=N stretching frequency observed at 1595‒1604 cm‒1 for these
ruthenium complexes is attributed to coordination of C=N bonding in the chelate ring to the
metal center. The FT‒IR spectra of complexes 1‒6 are given in Figs. S1‒S6 (see supporting
information).
Electronic spectra of all the complexes were recorded in chloroform solution at room
temperature showed three bands in the region 200–800 nm. The UV-vis spectra of complexes 1‒
6 are given in Figs. S7‒S12 (see supporting information). The high intensity bands in the 240–
270 nm region were assigned to π– π* transitions of the ligands. The bands in the region 340–
350 nm have been designated as n–π* transition of the non-bonding electron present on the
nitrogen of the azomethine group [27]. In all the complexes, the lowest energy band observed in
4

the region 430–450 nm were attributed to the metal to ligand charge transfer (MLCT) transitions
[28–30].
1
The H‒NMR spectra of all the ligands and complexes were recorded in CDCl3 to
confirm the coordination mode of the Schiff base ligand to the ruthenium(II) ion. The presence
of the azomethine proton (−CH=N) attached to the phenyl ring was confirmed by the singlet
1
around 8.3‒8.9 ppm. The H-NMR spectra of complexes 1‒6 are given in Figs. S13‒S18 (see
supporting information)The aromatic protons of all these complexes are observed as multiplets
around 6.8‒7.9 ppm. In all the complexes, the aromatic hydrogen’s of p-cymene protons appear
in the region of δ 4.4–5.5 ppm. Additionally, the methyl group of the p-cymene comes as a
singlet around the region of δ 2.22–2.35 ppm. The isopropyl methyl protons of the p‒cymene
appeared as two sets of doublets in the region of δ 1.12–1.29 ppm. The methylene (CH2) proton
bridging the two phenyl rings in the complexes 1 and 5 was observed around 3.81 and 3.5 as a
singlet. Additionally methoxy protons are observed as singlet for complexes 3 and 4 at δ 3.5 and
3.84 ppm respectively. The electrospray ionization (ESI) mass spectra of 4 and 5, obtained in
positive mode, give peaks at m/z = 1008 and 1046 respectively. The loss of one chloride atom
for half-sandwich complexes is quite common under ESI-MS conditions [31‒33].
The molecular structure of complex 6 was confirmed by single crystal X-ray diffraction
and DFT methods. The crystals were grown by slow diffusion of chloroform into a concentrated
solution of the complexes in pentane solution. The crystallographic data for binuclear half-
sandwich ruthenium complexes 6 is summarized in Table 1. The molecular structure of 6 is
shown in Figure 1. The crystal packing was solved with monoclinic C2/c space groups. Each
ruthenium is coordinated by one chlorine, one nitrogen and one oxygen atom from the Schiff
base ligand and one of the p‒cymene rings. The complex adopt the three legged piano stool
5

geometry with σ-bonded Cl, N and O atoms as three legs and the arene ring of the p-cymene
group forming the seat.
Figure 1. ORTEP diagram of the complex [ Ru2Cl2(η6‒p‒cymene)2Cl(L6)] (6).
The Ru‒O distances (2.061(3) and 2.057(3) Å and Ru−N distances (2.082(3) and
2.087(3) Å are comparable to those of a half‒sandwich ruthenium complex containing [N, O]
anionic bidentate ligands [34‒36]. The DFT (BP86/TZVP) computed Ru-O distance of 2.039 Å
and Ru−N distance of 2.088 Å are very close to the values obtained from experimental XRD
method. The DFT (BP86/TZVP) computed Ru-Cl distance [2.427 Å] also very close to the
experimentally obtained value 2.4339(13) Å. The metrical parameters obtained from XRD and
DFT (BP86/TZVP) methods are provide in Table 2.
6

Table 1. Selected crystal data and structure refinement summary of complex 6.
6
Empirical formula
Formula weight
Temperature
Wavelength/ Å
Crystal system
Space group
a/ Å
C56 H51 N2 O3 Cl5Ru2
1166
298
0.71073 Å
monoclinic
C2/c
22.4292 (11) Å
13.5674 (11) Å
19.3384 (13) Å
90
b/ Å
c/ Å
α/°
β/°
107.202(5)
90
γ/°
3
Volume/ Å
Z
5621.6 (7)
4
3
Density (calculated)/ Mg/m
1.513
-1
Absorption coefficient/ mm
F(000)
0.962
2576
Theta range for data collection/°
Reflections collected
1.777 to 29.110
50476
Independent reflections
Data / restraints / parameters
7556 [R(int) = 0.066]
7521 / 0 / 333
2
Goodness-of-fit on F
1.314
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.062, wR2 = 0.2141
R1 = 0.1284, wR2 = 0.1831
7

Table 2. Selected bond lengths (Å) and Bond Angles (o) for Ruthenium Complex
Bond Length (Å)
Bond Angle (o)
87.21(14) [86.80]
Ru1-Cl2 2.4339(13) [2.427] N5-Ru1-O4
Ru1-N5
Ru1-O4
Ru1-C36
Ru1-C39
C36-C48
C41-C44
C11-O3
2.073(4) [2.088]
2.038(3) [2.039]
2.189(5) [2.197]
2.176(5) [2.191]
1.349(8) [1.438]
1.432(8) [1.486]
1.382(5) [1.398]
N5-Ru1-Cl2
O4-Ru1-Cl2
N5-Ru1-C42
O4-Ru1-C36
O4-Ru1-C42
N5-Ru1-C36
O4-Ru1-C39
N5-Ru1-C42
C36-Ru1-C42
C39-Ru1-Cl2
C42-Ru1-Cl2
C11-O3-C65
85.90(10) [85.08]
83.37(11) [83.20]
94.38(17) [95.23]
90.58(19) [89.78]
155.5(2) [155.72]
119.09(19) [120.46]
115.95(19) [115.97]
156.2(2) [156.40]
67.7(2) [68.32]
91.22(16) [91.29]
121.15(17) [121.08]
118;42 [117.87]
In the solid state, the complex exists as a solvate with chloroform. There are four
molecules present in the asymmetric unit. The Ru‒O distances 2.038(3) Å and Ru−N distances
2.073(4) Å are comparable to those of a half‒sandwich ruthenium complex containing [N, O]
anionic bidentate ligands [37‒39]. The DFT (BP86/def2-TZVP) computed Ru-O distance of
2.039 Å and Ru−N distance of 2.088 Å are very close to the values obtained from experimental
XRD method. The DFT (BP86/def2-TZVP) computed Ru-Cl distance 2.427 Å also very close to
the experimentally obtained value 2.4339(13) Å. The metrical parameters obtained from XRD
and DFT (BP86/def2-TZVP) methods are provided in Table 2 and the DFT optimized geometry
is shown in Figure 2.
8

Figure 2. The DFT (BFP86/def2-TZVP) optimized geometry of the complex 6
The Ru-N, Ru-O and Ru-Cl bond lengths are close to the values of the similar type of
geometry in the complexes and the geometrical features are important in deciding their structure-
activity towards cancer cells [40]. In the solid state, the packing diagram of the compound 6
shows chain like array interconnected through the chlorine atoms and Schiff bases (Figure S1)
(See supporting information). Thus the presence of chlorine atom or halogen atom as leaving
group in this type of molecules, ie., piano-stool ruthenium-arene complexes is already known,
and which stabilizes the binuclear complex significantly in both solution and solid state.
In order to overcome the discrepancies experienced while solving the crystal structure,
we have performed DFT calculations at BP86/def2-TZVP level, which helps to arrive the
definite conclusion about the molecular structures of the complexes obtained. The charge on the
metal atom is a crucial factor, which govern the catalytic activity of the complex. The DFT
(BP86/def2-TZVP) computed charges on the two Ru atoms are +2 oxidation states. The
information about the stability of the metal complexes are usually obtained by means
computational methods. One of the best indicator is the energy gap ELUMO-HOMO, as per
9

Koopmanns’ theorm. The significant ELUMO-HOMO gap value of 2.03 eV is in support with the
stability of binuclear ruthenium complex 6 (Table 3). The DFT (BP86/def2-TZVP) computed
frontier molecular orbitals (FMO) are highly useful in studying the stability of the bimetallic
complex through the orbital energies and delocalization of electrons. The molecular orbital
pictures of the FMOs (HOMO, LUMO, HOMO-1 and LUMO+1) obtained from DFT
computation is presented in Figure 3. From the HOMO picture of the complex, significant
amount electron localization on the Ru atoms and Cl atoms whereas in the LUMO the no orbitals
of the halogen atoms are involved. The bonding interactions between one of the Ru atom with Cl
and arene ring is higher in HOMO, where as in HOMO-1 the interaction of second Ru atom with
Cl and arene ring is higher. The involvement of the orbitals of O and N and the delocalization of
electrons on the ring stabilizes the LUMO and
LUMO+1orbitals. The DFT (BP86/def2-
TZVP) computed reactivity descriptors like, Chemical potential(µ), Hardness (Ƞ), Softness(S),
Electrophilicity (Ѡ), Ionization potential (IP) and Electron Affinity (EA) values are provided in
Table 3. The DFT computed hardness value is in line with the stability of complex 6, and the
electron affinity and electrophilicity values suggest the electrophilic nature of diruthenium-Schiff
base complex.
10

Figure 3. The DFT (BFP86/def2-TZVP) computed frontier molecular orbitals (FMOs) of the
complex 6 with iso surface value of 0.03.
11

Table 3. DFT (BFP86/def2-TZVP) computed reactivity descriptors like, Chemical potential
(µ), Hardness (
Ƞ), Softness(S), Electrophilicity (Ѡ), Ionization potential (IP) and Electron
Affinity (EA) values (eV).
Complex
6
HOMO
-4.4496 eV
LUMO
-2.4226 eV
2.0270 eV
E LUMO – HOMO
Chemical potential
(µ) = ELUMO+HOMO/2
Hardness
-3.4361 eV
1.0135 eV
0.9867 eV
5.8248 eV
4.4496 eV
2.4226 eV
(Ƞ) = E LUMO –HOMO/2
Softness
(S) = 1/
Electrophilicity
) = µ2/2
Ƞ
(Ѡ
Ƞ
Ionization potential
(IP) (eV)
Electron Affinity
(EA) (eV)
The catalytic performance of half-sandwich binuclear ruthenium complexes 1‒6 has been
evaluated for reduction of nitroarenes with 1-chloro-4-nitrobenzene as a model substrate. As
shown in table 4, the binuclear half-sandwich ruthenium complex 6 was very active towards
reduction of nitro compounds. The complex 6 was chosen as the optimal catalyst to screen
various solvents (Table 4, entries 1‒16). A high yield of 95% of the desired products was
achieved in ethanol solvent. No desired products were detected in water due to the poor
solubility of the catalyst (Table 4, entry 11). As expected, no reaction was observed in the
12

control experiment performed without both base and the catalyst (Table 4, entry 14). From these
studies, the optimal conditions for the reduction of nitroarenes were using 6 as catalyst (1mol %)
in the presence of four equivalents of NaBH4 at room temperature.
Table 4 Screening of Half-Sandwich Ruthenium Complexes Catalysts for Nitroarene
Reductiona
NO2
NH2
Ru catalyst
NaBH4/ EtOH/ RT
Cl
Cl
Entry
1
Catalyst
Solvent
EtOH
Time (h)
3
Yield (%)b
60
1
2
2
3
4
5
6
6
6
6
6
6
6
6
-
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
DCM
Toluene
DMF
THF
3
73
3
3
70
4
3
75
5
3
83
6
3
95
7
6
30
8
5
55
9
8
28
10
11
12
13
14
15
16
10
12
10
24
3
48
10
MeCN
H2O
42
No Product
No Product
92c
EtOH
EtOH
EtOH
6
6
3
5
88d
a Reaction conditions: 0.3mmol 1-chloro-4-nitrobenzene, 1.2mmol NaBH4, Ru catalyst (3mol%),
solvent (2mL), room temperature, bIsolated yield, c6 (1mol%),d6 (0.5mol%).
13

Table 5 Screening of Substrate for Nitroarene Reduction Catalyzed by Complex 6a
Entry
Substrate
Product
Yield (%)b
NH2
NO2
1
2
3
88
95
94
NH2
NO2
Cl
Cl
NH2
NO2
HOH2C
HO
OHC
NH2
NO2
O
Me
Me
4
90
HOOC
COOH
OH
NH2
NH2
OH
H2N
HO
O2N
HO
NH2
NO2
5
6
80
92
NO2
NH2
H2N
H2N
7
8
9
96
90
95
NH2
CH3
NO2
CH3
NO2
NH2
H3C
H3C
14

NH2
NO2
H3CO
H3CO
OHC
10
11
92
82
NO2
NH2
OH
OH
HOH2C
aReaction conditions: 0.3 mmol nitroarene, 1.2 mmol NaBH4, catalyst (1mol%), solvent (2 mL),
room temperature (3h), bIsolated yield.
With the optimal reaction condition in hand, we started to expand the scope and
efficiency of this methodology. Many functionalized anilines were obtained in excellent yields
(Table 5). As shown in Table 4, the catalyst 6 was found to be very active toward the reduction
of various nitro compounds. Electron-withdrawing and electron-donating substituents on the
aromatic ring were converted to the desired products in high yields. However, reduction of
nitrotoluene generally takes a long time for an electron-donating group such as –CH3 or –OCH3
(Table 5, entries 8
‒
10). Chemoselective reduction of the nitro group in the presence of other
5). Overall,
substituents such as –CHO, –COMe and –COOH was performed (Table 5, entries 3
‒
the present nitroarene reduction protocol is simple to operate and gives the corresponding aniline
in good to excellent yields.
A possible reaction mechanism for the reduction of nitroarenes is shown in Scheme 2.
The proton signal at -10.18 ppm corresponds to a hydride, which indicates the presence of a Ru-
H intermediate. Then, the reduction of the nitroarene may be proposed to occur via the outer-
sphere mechanism (intermediate B), which gives the hydroxyl amine first and then the final aryl
amine [41].
15

Cl
Ru
H
Ru
O
O
N
N
NaBH4
O
O
N
N
Ru
O
Cl
Ru
H
O
Complex (6)
RuH
NaBH4
NO
Cl
-
NaBH4
Cl
O
H
N
O
Ru
H
Ru
O
N
O
O
NH
NH
H
O
N
Ru
H
NO2
Cl
H
O
O
Ru
H
O
N
O
Intermediate A
Cl
Intermediate B
NO2
NO
NHOH
NH2
A
A
A
-H2O
-H2O
Cl
I
Cl
Cl
Cl
Product
II
III
Scheme 2. Probable Mechanism for Reduction of Nitroarenes Catalyzed by Half-Sandwich
Binuclear Ruthenium Complexes
CONCLUSION
In summary, we have synthesized and characterized a series of novel half-sandwich
binuclear ruthenium(II) complexes with binucleating Schiff-base ligands and examined their
ability as catalysts in hydrogenation of nitroarenes using sodium tetrahydroborate as hydride
source in ethanol solvent. The hydrogenation reaction offers varied functional group
16

compatibility and broad substrate scope and provides aromatic anilines in excellent yields The
advantages of these homogeneous half-sandwich ruthenium catalyst systems include the case of
catalyst synthesis and high catalytic activities toward nitroarene reduction in the presence of
NaBH4 in ethanol at room temperature. Moreover, a combination of spectroscopic studies and X-
ray crystallographic and DFT (BP86/def2-TZVP) studies also confirmed the piano-stool
geometry for the molecular structure of the half-sandwich binuclear ruthenium complexes.
Molecular Orbital calculations and computation of reactivity descriptors also confirm the
stability and electrophilic nature of these complexes.
EXPERIMENTAL METHODS
Materials and Physical measurements
All the reagents used were chemically pure and of analytical grade. The solvents were
freshly distilled before use. RuCl3·3H2O was purchased from Himedia Pvt. Ltd., and was used
without further purification. α-Phellandrene, 2-hydroxy -1- naphthaldehyde, O-vanillin and 4,4’-
diaminophenyl methane, 4,4’-diaminiphenyl ether were purchased from Sigma-Aldrich. The
Schiff base ligands were prepared according to literature reports [42, 43]. The starting materials
[(η6
-p-cymene)2Ru2(Cl2)2], were prepared according to published methods [44]. The IR spectra
of the complexes were recorded on an agilent resolution pro model in 4000–400 cm−1 range.
Electronic spectra of the complexes were recorded in CHCl3 solution in a Cary 300 Bio UV–Vis
1
Varian spectrophotometer in the range 800
‒
200 nm. The H
‒
NMR spectra were recorded in
CDCl3 with Bruker 300MHz instrument using TMS as internal reference.
Synthesis of Half-Sandwich Binuclear Ruthenium complexes (1 6)
A solution of the chloro bridged dimeric complex (0.06g; 0.1mmol), Schiff base ligands
‒
(0.040
‒
0.0501g; 0.1mmol) and triethylamine in dichloromethane (30 mL) was added and
17

mixture was stirred for 4 h at room temperature. A color change of the solution from dark red
was observed. The solution was concentrated to 2 mL, and hexane was added to initiate
precipitation of the complex. The red-brown powder was filtered and washed with hexane, and
purified using column chromatography.
[(Ru(p-cymene)Cl)2L1](1)
o
Red-brown solid. Yield: 70%. M.p. 180 C. Anal. Calcd for C47H40N2Cl2O2Ru2: C,
1
60.25;H, 4.27; N, 2.99. Found: C, 59.02; H, 4.12; N, 2.59. FT
1325 (C O). UV–Vis
(m, Ar-H), 8.9 (s, HC=N), 3.8 (s, CH2), 4.4 (d 1H, cymene Ar
(d 1H, cymene Ar H), 3.2 (m, 1H, CH of p‒cymene), 2.3 (s,3H, CH3 of p‒cymene), 1.2 (dd, 6H,
‒
IR cm‒ ): 1610
ν
(CH=N),
(ppm) = 6.9–7.8
H), 5.1 (d 1H, cymene Ar H), 5.4
ν
‒
λ δ
max (nm): 450,340, 270. 1H NMR (300 MHz, CDCl3):
‒
‒
‒
2CH3 of p‒cymene).
[(Ru(p-cymene)Cl)2L2](2)
Red-brown solid. Yield: 78%. M.p.200oC. Anal. Calcd for C46H38N2Cl2O3Ru2: C, 58.8;
1
H, 4.05; N, 2.98. Found: C, 58.02; H, 3.95; N, 2.86. FT
‒
IR cm‒ ): 1607
ν
(CH=N), 1328
(ppm) = 7.1–7.9 (m, Ar-H),
H), 5.3 5.5 (d 1H, cymene Ar
ν(C‒O).
UV–Vis
λ
max (nm): 440, 330, 250.1H NMR (300 MHz, CDCl3):
δ
8.5 (s, HC=N), 4.4 (d 1H, cymene Ar
‒
H), 5.1 (d 1H, cymene Ar
‒
‒
‒
H), 3.2 (m, 1H, CH of p‒cymene), 2.2 (s,3H, CH3 of p‒cymene), 1.12 (dd, 6H, 2CH3 of p‒
cymene).
[(Ru(p-cymene)Cl)2L3](3)
Red solid. Yield: 75%. M.p.230oC. Anal. Calcd for C49H46N2Cl2O4Ru2: C, 58.44; H,
1
4.37; N, 2.80. Found: C, 58.39; H, 4.15; N, 2.76. FT
‒
IR cm‒ ): 1595
ν
(CH=N), 1330
ν(C‒O).
1
UV–Vis
λ
max (nm): 450, 340, 270. H NMR (300 MHz, CDCl3):
δ
(ppm) = 6.9–7.9 (m, Ar-H),
8.3 (s, HC=N), 3.8 (s, CH2), 3.5 (s, 3H, OCH3), 4.4 (d 1H, cymene Ar‒H), 5.1 (d 1H, cymene
18

Ar‒H), 5.5 (d 1H, cymene Ar‒H), 2.3 (s,3H, CH3 of p‒cymene), 1.29 (dd, 6H, 2CH3 of p‒
cymene).
[(Ru(p-cymene)Cl)2L4](4)
Brown solid. Yield: 78%. M.p.190oC. Anal. Calcd for C48H44N2Cl2O5Ru2: C, 57.14; H,
1
4.16; N, 2.8. Found: C, 57.06; H, 4.02; N, 2.72. FT
‒
IR cm‒ ): 1604
ν
(CH=N), 1330
(ppm) = 6.8–7.8 (m, Ar-H),
H), 5.4 (d 1H, cymene Ar H),
ν(C‒O).
1
UV–Vis
λ
max (nm): 450, 320, 270. H NMR (300 MHz, CDCl3):
δ
8.4 (s, HC=N), 4.4 (d 1H, cymene Ar
‒
H), 5.2 (d 1H, cymene Ar
‒
‒
3.84 (s, 3H, OCH3), 3.2 (m, 1H, CH of p‒cymene), 2.3 (s,3H, CH3 of p‒cymene), 1.2 (dd, 6H,
2CH3 of p‒cymene).
[(Ru(p-cymene)Cl)2L5](5)
Red
‒
brown solid. Yield: 80%. M.p.210oC. Anal. Calcd for C55H52N2Cl2O2Ru2: C, 63.09;
1
H, 4.97; N, 2.67. Found: C, 62.80; H, 4.90; N, 2.50. FT
‒
IR cm‒ ): 1600
ν
(CH=N), 1340
(ppm) = 6.8–7.9 (m, Ar-H),
H), 3.1 (m, 1H, CH of p‒cymene), 2.2 (s,3H,
ν(C‒O).
1
UV–Vis
λ
max (nm): 460, 330, 270. H NMR (300 MHz, CDCl3):
δ
8.6 (s, HC=N), 3.8 (s, CH2), 5.2 (d 1H, cymene Ar
‒
CH3 of p‒cymene), 1.21 (dd, 6H, 2CH3 of p‒cymene).
[(Ru(p-cymene)Cl)2L6](6)
Red
‒
brown solid. Yield: 78%. M.p.185oC. Anal. Calcd for C54H50N2Cl2O3Ru2: C, 61.83;
1
H, 4.77; N, 2.67. Found: C, 61.73; H, 4.65; N, 2.58. FT
‒
IR cm‒ ): 1601
ν
(CH=N), 1340
(ppm) = 6.8–7.9 (m, Ar-H),
H), 5.2 (d 1H, cymene Ar H),
ν(C‒O).
1
UV–Vis
λ
max (nm): 450, 320, 270. H NMR (300 MHz, CDCl3):
δ
8.7 (s, HC=N),4.4 (d 1H, cymene Ar
‒
H), 5.0 (d 1H, cymene Ar
‒
‒
3.2 (m, 1H, CH of p‒cymene), 1.29 (dd, 6H, 2CH3 of p‒cymene).
19

General Procedure for the Reduction of Nitroarenes to Anilines with Half-Sandwich
Binuclear Ruthenium Catalysts
A half-sandwich binuclear ruthenium complex (1 mol%) was dissolved in EtOH (2.0
mL); then appropriate nitroarenes (0.3 mmol) and NaBH4 (1.2 mmol) were added. The resulting
mixture was stirred at room temperature in a closed vessel. After completion of the reaction
(monitored by TLC), the crude reaction mixture was extracted with ether (3×2 mL). After the
solvents were removed in vacuo from the combined organic extracts, the crude products were
loaded directly onto a column of silica gel and purified by column chromatography using
petroleum ether and ethyl acetate (1:3) to get the corresponding products.
Aniline. Yellow oil, yield: 88%. 1H-NMR (500 MHz, CDCl3):
δ 7.32 (m, 2H), 6.94 (m, 1H), 6.48
(m, 2H), 3.6 (br, 2H).
1
4-Chloroaniline. Colorless solid, Yield: 95%. H-NMR (300 MHz, CDCl3):
δ 7.1 (d, 2H), 6.62
(d, 2H), 3.62 (br, 2H).
1
(4-Aminophenyl)methanol. Pale yellow solid; yield: 94%. H-NMR (300 MHz, CDCl3):
δ 7.12
(d, 2H), 6.69 (d, 2H), 4.4 (s, 2H), 3.4 (br, 2H).
1-(4-Aminophenyl)ethanol. Pale yellow solid; yield: 90%. 1H-NMR (300 MHz, CDCl3):
δ 7.2 (d,
2H), 6.65 (d, 2H), 4.73 (m, 1H), 3.2 (br, 2H), 1.45 (d,3H).
1
4-Aminophenol. Brown solid; yield: 92%. H-NMR (300 MHz, CDCl3):
δ 6.51 (t, 4H), 4.35 (s,
NH2).
1,4-Phenylenediamine. Red solid; yield: 96%. 1H-NMR (300 MHz, CDCl3):
δ6.6 (s, 4H), 3.3 (br,
4H),
2-Toluidine. Yellow oil; yield: 90%.1H-NMR (300 MHz, CDCl3):
δ 7.2 (t, 2H), 6.8 (t, 1H), 6.6
(d, 1H), 4.3 (br, 2H), 2.28 (s, 3H).
20

1
4-Toluidine. Colorless solid; yield: 95%. H-NMR (300 MHz, CDCl3):
δ 7.1 (d, 2H), 6.55 (d,
2H), 3.48 (br, 2H), 2.35 (s, 3H).
p-Anisidine. Orange solid; yield: 92%. 1H-NMR (300 MHz, CDCl3):
3.9 (s, 3H), 3.2 (br, 2H).
δ 6.6 (d, 2H), 6.51 (d, 2H),
X-ray Structure Determination
Diffraction data of (6) were collected on a Bruker AXS SMART APEX diffractometer,
equipped with a CCD area detector using Mo K radiation ( = 0.710 73 Å). All the data were
α
λ
collected at 298 K, and the structures were solved by direct methods and subsequently refined on
F2 by using full-matrix least-squares techniques (SHELXL) [45], SADABS [46] absorption
corrections were applied to the data, all non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were located at calculated positions. All calculations were performed using the
Bruker Smart program.
Computational Details
Computational chemistry tools like DFT methods are already used successfully for the
complete characterization of challenging molecules [47a-47e]. In order to overcome the effect of
solvation and disorder resulted in the crystal structure of the molecule 6, DFT calculations were
used as a support for the predicted molecular structure. DFT calculations were carried out using
the software ORCA developed by Frank Neese and co-workers [48a]. All calculations were
performed using the BP86 density functional [48b-48e] and def2-TZVP basis set [48f] included
in the ORCA programme, which is free for academic use. Molecular structure from X-Ray
diffraction method was taken as initial geometry for the DFT calculations. Self-Consistent Field
(SCF) calculations were performed using the TIGHTSCF convergence criteria. The geometry
optimization was carried out and the resulting geometries are confirmed as minima through the
21

frequency calculations, which show no imaginary frequencies. The reactivity descriptors like,
Chemical potential (µ), Hardness ( ), Softness(S), Electrophilicity ( ), Ionization potential (IP)
Ƞ
Ѡ
and Electron Affinity (EA) are also calculated to predict the stability of the complexes. Pictures
of the optimized geometries and the frontier molecular orbitals are taken using the graphics
programme ChemCraft [48g].
Acknowledgments
The authors express sincere thanks to Single Crystal X‒ray Diffraction Facility, SAIF
Guwahati University, for providing X‒ray data collection.
Appendix A. Supplementary data
1
FT-IR, UV-Vis and H-NMR spectra of representative complexes. CCDC 1942404
contains the supplementary crystallographic data for complex 6. These data can be obtained free
of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ UK (fax: +44 1223
336033; email: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
References
[1] C. S. Hauser, C. Slugove, K. Mereiter, R. Schmid, K. Kirchner, L. Xiao, Weissensteiner, J.
Chem. Soc., Dalton Trans. (2001) 2989.
[2] (a) C. S. Allardyce, P. J. Dyson, D. J. Ellis, S. L. Heath, Chem. Commun. (2001) 1396
(b) H. Chen, J. A. Parkinson, S. Parsons, R. A. Coxall, R. O. Gould, P. J. Sadler, J. Am.
Chem. Soc. 124 (2002) 3064 3082; (c) R. E. Aird, J. Cummings, A. A. Ritchie, M. Muir, R.
E. Morris, H. Chen, P. J. Sadler, D. I. Jodrell, Br. J. Cancer 86 (2002) 1652 1657; (d) R. E.
Morris, R. E. Aird, S. Murdoch, H. Chen, J. Cummings, N. D. Hughes, S. Parsons, A. Parkin,
‒1397;
‒
‒
G. Boyd, D. I. Jodrell, P. J. Sadler, J. Med. Chem. 44 (2001) 3616‒3621.
22

[3] H. Li, Z.-J. Yao, D. Liu, G.-X. Jin, Coord. Chem. Rev., 139 (2015) 293–294.
[4] Y.-F. Han, G.-X. Jin. Chem. Soc. Rev., 43 (2014) 2799 2823.
‒
[5] Y.-F. Han, H. Li, Y. Fei, Y.-J. Lin, W.-Z. Zhang, G.-X. Jin. Dalton Trans
7124.
., 39 (2010) 7119‒
[6] G. L. Wang, Y.-J. Lin, H. Berke, G.-X. Jin, Inorg. Chem., 49 (2010) 2193
[7] W.-Z. Zhang, Y.-F. Han, Y.-J. Lin, G.-X. Jin, Dalton Trans., 39 (2009) 8426
[8] Y.-F. Han, H. Li, G.-X. Jin, Chem. Commun., 46, (2010) 6879 6890.
[9] P. Singh, A. K. Singh, Organometallics, 29 (2010) 6433 6442.
[10] R. M. Lord, A. J. Hebden, C. M. Pask, I. R. Henderson, S. J. Allison, S. L. Shepherd, R. M.
Phillips, P. C. McGowan, J. Med. Chem., 58 (2015) 4940 4953.
[11] M. Ito, S. Kitahara, Ikariya. J. Am. Chem. Soc., 127 (2005) 6172
‒
2201.
‒
8431.
‒
‒
‒
‒6173.
[12] E. Rodríguez Arce, C. Sarniguet, T. S. Moraes, M. Vieites, A. I. Tomaz, A. Medeiros, M. A.
Comini, J. Varela, H. Cerecetto, M. González, F. Marques, M. H. García, L. Otero, D.
Gambino, J. Coord. Chem., 68 (2015) 2923
[13] M. J. Chow, C. Licona, D. Y. Q. Wong, G. Pastorin, C. Gaiddon, W. H. Ang, J. Med.
Chem., 57 (2014) 6043 6059.
[14] (a) F. C. Pigge, J. J. Coniglio, Curr. Org. Chem. 5 (2001) 757
Kondratenko, Top. Organomet. Chem. 7 (2004) 181 204; (c) J. Gimeno, V. Cadierno, P.
Crochet, R. H. Crabtree, D. M. P. Mingos (Eds.), Comprehensive Organomet. Chem., III,
vol. 6, Elsevier, Oxford, (2007) 465 550; (d) B. Therrien, Coord. Chem. Rev. 253 (2009)
493 519; (e) L. Delaude, A. Demonceau, Dalton Trans. 41 (2012) 9257 9268; (f) P.
Kumar, R. K. Gupta, D. S. Pandey, Chem. Soc. Rev. 43 (2014) 707 733; (g) B. Therrien,
CrysEngComm. 17 (2015) 484 491.
‒2937.
‒
‒
784; (b) J. H. M. A. Rigby,
‒
‒
‒
‒
‒
‒
23

[15] (a) S. I. Kozhushkov, L. Ackermann, Chem. Sci. 4 (2013) 886
Liu, S. I. Kozhushkov, L. Ackermann, Adv. Synth. Catal. 356 (2014) 1461
[16] (a) R. García-Álvarez, J. Francos, E. Tomás-Mendivil, P. Crochet, V. Cadierno, J.
Organomet. Chem. 771 (2014) 93 104; (b) P. Crochet, V. Cadierno, Top. Organomet.
Chem. 48 (2014) 81 118.
[17] (a) A. L. Noffke, A. Habtemariam, A. M. Pizarro, P. J. Sadler, Chem. Commun. 48 (2012)
5219 5246; (b) F. Foubelo, C. Nájera, M. Yus, Tetrahedron: Asymmetry 26 (2015) 769
790; (c) D. Wang, D. Astruc, Chem. Rev.115 (2015) 6621 6686.
‒
896; (b) S. De Sarkar, W.
‒1479.
‒
‒
‒
‒
‒
[18] M. J. Vaidya, S. M. Kulkarni, R. V. Chaudhari, Org. Process Res. Dev. 7 (2003) 202−208.
[19] A. M. Tafesh, Weiguny, J. Chem. Rev. 96 (1996) 2035−2052.
[20] R. K. Rai, A. Mahata, S. Mukhopadhyay, S. Gupta, P. -Z. Li, K. T. Nguyen, Y. Zhao, B.
Pathak, S. K. Singh, Inorg. Chem. 53 (2014) 2904−2909.
[21] J. Liu, J. Cui, F. Vilela, J. He, M. Zeller, A. D. Hunter, Z. Xu, Chem. Commun. 51 (2015)
12197−12200.
[22] W.-G. Jia, Y.-C. Dai, H.-N. Zhang, X. Lu, E.-H. Sheng, RSC Adv. 5 (2015) 29491−29496.
[23] Z. Zhao, H. Yang, Y. Li, X. Guo, Green Chem. 16 (2014) 1274−1281.
[24] X.-J.Yang, B. Chen, L.-Q. Zheng, L.-Z. Wu, C.-H. Tung, Green Chem. 16 (2014)
1082−1086.
[25] X. Liu, S. Ye, H. –Q. Li, Y.-M. Liu, Y. Cao, K.-N. Fan, Catal. Sci. Technol. 3 (2013)
3200−3206.
[26] R. G. De Noronha, C. C. Romão, A. C. Fernandes, J. Org. Chem. 74 (2009) 6960−6964.
[27] A. B. P. Lever, Inorganic Electronic Spectroscopy, 2nd ed., Elsevier, New York, (1984).
[28] K. Ghosh, R. Kumar, K. Kumar, A. Ratnam, U. P. Singh, RSC Adv. 4 (2014) 43599–
24

43605
[29] T. D. Thangadurai, K. Natarajan, Trans. Met. Chem. 25 (2000) 347–351.
[30] J. Dutta, M. G. Richmond, S. Bhattacharya, Eur. J. Inorg. Chem. (2014) 4600–4610.
[31] T.-T. Thai, B. Therrien, G. Süss-Fink, Inorg. Chem. Commun. 12 (2009) 806
[32] R. Payne, P. Govender, B. Therrien, C. M. Clavel, P. J. Dyson, G. S. Smith, J. Organomet.
Chem. 729 (2013) 20 27.
[33] P. Govindaswamy, B. Therrien, G. Süss-Fink, P. St
692 (2007) 1661 1671.
‒807.
‒
ĕpnicka, J. Ludvík, J. Organomet. Chem.
‒
[34] R. Pettinari, F. Marchetti, C. Pettinari, A. Petrini, R. Scopelliti, C. M. Clavel, P. J. Dyson,
Inorg. Chem. 53 (2014) 13105−13111.
.
[35] R. M. Lord, A. J. Hebden, C. M. Pask, I. R. Henderson, S. J. Allison, S. L. Shepherd, R. M.
Phillips, P. C. McGowan, J. Med. Chem. 58 (2015) 4940−4953.
[36] J. G. Malecki, R. Kruszynski, M. Jaworska, P. Lodowski, Z. Mazurak, Z. J. Organomet.
Chem. 693 (2008) 1096−1108.
[37] R. Pettinari, C. Pettinari, F. Marchetti, C. M. Clavel, R. Scopelliti, P. J. Dyson,
Organometallics 32 (2013) 309−316.
[38] J. G. Malecki, Struct. Chem. 23 (2012) 461−472.
[39] D. Zuccaccia, G. Bellachioma, G. Cardaci, C. Zuccaccia, A. Macchioni, Dalton Trans.
(2006) 1963−1971.
[40] (a) A. Habtemariam, M. Melchart, R. Fernandez, S. Parsons, I. D. Oswald, A. Parkin, F.
P. Fabbiani, J. E. Davidson, A. Dawson, R. E. Aird, D. I. Jodrell, P. J. Sadler, J. Med.
Chem., 49 (2006) 6858-6868.; (b) Leli Zeng, Pranav Gupta, Yanglu Chen, Enju Wang,
Liangnian Ji, Hui Chao, Zhe-Sheng Chen, Chem. Soc. Rev., 46 (2017) 5771-5804.
25

[41] A. K. Shil, P. Das, Green Chem. 15 (2013) 3421−3428.
[42] R. Nandhini, G. Venkatachalam, M. Deepan Kumar, M. Jaccob, Polyhedron 158 (2019)
183–192
[43] M. A. Bennett, A. K. Smith, J. Chem. Soc. Dalton Trans. (1974) 233‒240.
[44] W.-G. Jia, H. Zhang, T. Zhang, S. Ling. Inorg. Chem. Commun., 66 (2016) 15.
[45] G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., Sect. A, 64 (2008) 112
‒
122.
[46] G. Sheldrick, G. SADABS v. 2.01, Bruker/Siemens area detector absorption correction
program; Bruker AXS: Madison, WI, USA, (1998).
[47] (a) K. Bharathi, L. Beerma, C. Santhi, B. S. Krishnamoorthy, J.-F. Halet, J. Organomet.
Chem.,792 (2015) 220-228. doi:10.1016/j.jorganchem.2015.05.057; (b). B. S.
Krishnamoorthy, S. Kahlal, S. Ghosh, J.-F. Halet, Theor. Chem. Acc. 132 (2013) 1356.
(doi:10.1007/s00214-013-1356-6).; (c) B. S. Krishnamoorthy, A. Thakur, K. K. V.
Chakrahari, S. K. Bose, P. Hamon, T. Roisnel, S. Kahlal, S. Ghosh, J. -F. Halet, Inorg.
Chem., 51 (2012) 10375. (DOI:10.1021/ic301571e).; (d) B. S. Krishnamoorthy, S.
Kahlal, B. Le Guennic, J.-Y. Saillard, S. Ghosh, J.-F. Halet, Solid State Sciences 14
(2012) 1617. (doi:10.1016/j.solidstatesciences.2012.03.026).; (e) K. Geetharani, B. S.
Krishnamoorthy, S. Kahlal, S. M. Mobin, J.-F. Halet, S. Ghosh, Inorg. Chem., 51 (2012)
10176 (DOI:10.1021/ic300848f).
[48] (a) F. Neese, The ORCA program system. Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2
(2012) 73–78. (b) S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys., 58 (1980) 1200-1211. c)
A. D. Becke, J. Chem. Phys., 84 (1986) 4524-4529. d) A. D. Becke, Phys. Rev. A, 38
(1986) 3098-3100. (e) J. P. Perdew, Phys. Rev. B, 33 (1986) 8822-8824. (f) F. Weigend,
26

R. Ahlrichs, Phys. Chem. Chem. Phys., 7 3297. g) Chem craft - graphical software for
visualization of quantum chemistry computations. https://www.chemcraftprog.com
27

Highlights
‹ A new family of binuclear Ruthenium(II) p-cymene complexes have been synthesized.
‹ Characterization of the complexes were performed by spectral and SC-XRD.
‹ DFT calculations were performed.
‹ These complexes were used as an efficient catalyst for reduction of nitroarenes to amines.

Declaration of interests
☐ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
No conflict of interest.