Half-sandwich ruthenium(II) complexes containing biphenylamine based Schiff base ligands: Synthesis, structure and catalytic activity in amidation of various aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2020.121175

New half-sandwich ruthenium (II) complexes [η⁶?p-cymene)Ru (L_1-3)Cl] (1–3) containing biphenylamine based Schiff base ligands (HL_1-3) have been synthesized and characterized by analytical and spectroscopic methods. Additionally, the solid state structure of 2 has been determined by single crystal X-ray diffraction study. The complex 2 serves as a catalyst for the amidation of various aldehydes to amides in good yield.

Full text of DOI:10.1016/j.jorganchem.2020.121175

Journal Pre-proof
Half-sandwich ruthenium(II) complexes containing biphenylamine based Schiff base
ligands: Synthesis, structure and catalytic activity in amidation of various aldehydes
Veerasamy Nagalakshmi, Raja Nandhini, Veerappan Brindha, Bellie Sundaram
Krishnamoorthy, Kasthuri Balasubramani
PII:
S0022-328X(20)30076-0
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121175
JOM 121175
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 November 2019
Revised Date: 31 January 2020
Accepted Date: 7 February 2020
Please cite this article as: V. Nagalakshmi, R. Nandhini, V. Brindha, B.S. Krishnamoorthy, K.
Balasubramani, Half-sandwich ruthenium(II) complexes containing biphenylamine based Schiff
base ligands: Synthesis, structure and catalytic activity in amidation of various aldehydes, Journal of
Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121175.
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Half-Sandwich Ruthenium(II) Complexes Containing Biphenylamine based
Schiff base ligands: Synthesis, Structure and Catalytic activity in amidation of
various aldehydes
Veerasamy Nagalakshmia, Raja Nandhinib, Veerappan Brindhac, Bellie Sundaram
Krishnamoorthyc* and Kasthuri Balasubramani*a
aDepartment of Chemistry, Government Arts College (Autonomous), Karur, 639 005, Tamil
Nadu, India.
bPG & Research Department of Chemistry, Government Arts College, Dharmapuri- 636 705,
Tamilnadu, India.
cPG & Research Department of Chemistry, Vivekanandha college of Arts and Science for
Women, Tiruchengode, Namakkal, Tamilnadu, India.
Corresponding Author: *E-mail address: manavaibala@gmail.com [Dr. K. Balasubramani].
Abstract
New
half-sandwich
ruthenium(II)
complexes
[η6‒p-cymene)Ru(L1-3)Cl]
(1-3) containing biphenylamine based Schiff base ligands (HL1-3) have been synthesized and
characterized by analytical and spectroscopic methods. Additionally, the solid state structure of 2
has been determined by single crystal X-ray diffraction study. The complex 2 serves as a catalyst
for the amidation of various aldehydes to amides in good yield.
Keywords: Half-Sandwich; Ruthenium; Schiff base; Synthesis; Crystal structure; Amidation.

Introduction
The chemistry of Schiff bases is very generous in providing ligands. The number of
aldehyde and keto precursors with amine groups is available for condensation reactions leading
to azomethine compounds is practically unlimited. In the same aspect, a large number of Schiff
base and their complexes have been studied for their interesting and important properties of their
wide range of applications in numerous scientific areas [1]. Ruthenium complexes have been
used as catalysts or catalyst precursors for a variety of purposes, including hydrogenation,
oxidation, amidation, isomerisation, polymerization, nucleophilic addition to multiple bonds and
carbon–carbon bond formation [2]. Particularly, half-sandwich ruthenium(II) complexes
containing suitable combinations of O and N mixed donor ligands have proven to be highly
efficient homogeneous catalysts for the synthesis of valuable organic compounds[3‒5].
Generally, ruthenium half-sandwich complexes adopt ‘‘three legged piano stool’’ ruthenium
complexes, where the η6-coordinated arene is the ‘‘bench’’, and the other ligands ‘‘the legs’’ of
the stool. The ruthenium arene complexes having potential application in organic synthesis are
well established and several arene ruthenium(II) complexes containing different ligands such as
Schiff bases [6], aryl azo [7], pincer [8], carbene [9] and tripodal [10] are known in the literature.
The rearrangement of aldoximes into amides has been reported using transition metal catalysts
containing nickel [11], copper [12], palladium [13], iridium [14, 15], rhodium [16] and zinc [17].
Transition metal catalyzed one-pot conversion of aldehydes to amides in the presence of
NH2OH has been recently investigated. Ru(II) complexes, viz., [Ru(terpy)(PPh3)2Cl2] [18] and
[Ru(DMSO)4Cl2] [19] were also used as homogeneous catalysts for this transformation using
NH2OH.HCl and NaHCO3. However, ruthenium(II/III) with potential arylazo ligands were
reported as efficient catalyst for the conversion of aldehydes to amides [20]. Recently, new five

coordinated ruthenium(II) Schiff base complexes were found to be good homogeneous catalysts
for the conversion of aldehydes into primary amides using NH2OH.HCl and NaHCO3 in
moderate conversions [21]. Singh and co-workers have reported that the phosphine free
ruthenium arene complexes act as catalyst for one-pot conversion of aldehydes to primary
amides in a water medium [22].
In this context, we report on the study of synthesis, structure, and catalytic activity of
half-sandwich ruthenium(II) Schiff base complexes that formed from Schiff base ligands bearing
2-aminobiphenyl with the various aldehyde group on the ligand system. All the synthesized
complexes have been characterized by elemental analysis, FT-IR, UV-Vis and H-NMR
spectroscopy techniques. The molecular structure of complex 2 was confirmed by single crystal
X-ray diffraction study and tested as catalyst for the conversion of aldehyde into their
corresponding amides. Density functional theory (DFT) calculations are performed to get more
insight into the geometrical and electronic structural features of the complex 2. Further, the
origin of the catalytic activity of the synthesized complex 2 was explored with the help of
reactivity descriptors from the electronic structure calculations. On the other hand, the synergetic
effect of the presence of Ru-p-cymene moiety and the bioactive Schiff’s base presence in these
complexes can lead to effective anti-cancer activity and modeling of these complexes with
biomolecules like proteins is also a thirst-ful area of research.

Results and discussion
The Schiff base ligand derivatives were conveniently prepared in excellent yield by the
condensation of substituted aldehydes with 2-aminobiphenyl in an equimolar ratio. These ligands
were allowed to react with the ruthenium(II) precursor [(η6-p-cymene)RuCl2]2 in a 1 : 2 molar
ratio in the presence of triethylamine as a base and the new ruthenium(II) complexes of the
general formula [(η6-p-cymene)Ru(L1-3)Cl] (L-Schiff base ligands) (Scheme 1) were obtained in
high yields. All the complexes are air-stable and highly soluble in most organic solvents. The
analytical data of all the ruthenium(II) Schiff base complexes are in good agreement with the
molecular formula proposed.
Cl
Cl
Ru
R
R
Ru
Cl
Cl
OH
N
O
N
Ru
H
H
Cl
CH2Cl2/ Stirred/ 4h
R = H; (HL1), OCH3; (HL2)
R = H (L1; 1), OCH3(L2; 2)
Cl
Cl
Ru
Ru
Cl
Cl
O
OH
Ru
N
N
H
H
Cl
CH2Cl2/ Stirred/ 4h
HL3
L3; (3)
Scheme 1. Synthesis of half-sandwich (η6-p-cymene) ruthenium(II) Schiff base complexes.

Characterization of the complexes
The FT-IR spectra of the free ligands also displayed υC=N and υC-O absorptions in the
region of 1548–1576 cm−1 and 1610–1653 cm−1, respectively, which indicate that the ligands
exist in the amide form in the solid state. Coordination of the ligand to the ruthenium(II) ion
through an azomethine nitrogen is expected to reduce the electron density in the azomethine link
and thus lower the absorption frequency upon complexation at 1601–1615 cm−1. The phenolic –
OH group expected to observe in the region 3400 cm-1 was absent in these complexes. The IR
spectra of all the complexes therefore confirm the mode of coordination of the Schiff base ligand
to the ruthenium(II) ion via the azomethine nitrogen and phenolic oxygen [23]. FT-IR spectra of
complexes are given in Figs S1-S3 (see supporting information).
The absorption spectra of the ruthenium(II) complexes in chloroform exhibited bands in
the ultraviolet region below 270–290 and 360–370 nm that are very similar and are attributable
to the transitions within the ligand orbitals (n–π*, π–π*) taking place in the Schiff base ligands.
The lowest energy absorption bands in the electronic spectra of the complexes in the visible
region 520–540 nm are ascribed to MLCT transitions. Based on the pattern of the electronic
spectra of all the complexes an octahedral environment around the ruthenium(II) ion has been
proposed similar to that of the other octahedral ruthenium(II) complexes [24]. UV-Vis spectra of
the complexes are given in Figs S4 (see supporting information).
1
13
The H–NMR spectra of ligands (HL1 – HL3), complexes 1-3 and C–NMR spectra of
the complexes 1-3 were recorded in CDCl3 and are given in Figs S5-S13 (see supporting
information). The multiplet observed in the region around δ 6.8–7.9 ppm in all the listed
complexes has been assigned to the aromatic protons of the biphenyl group of the Schiff base
ligands. The signal due to the azomethine proton (CH=N) appears at δ 8.7‒8.8 ppm. The position

of the azomethine signal in the complexes is slightly downfield in comparison with that of the
free ligands, suggesting de-shielding of azomethine proton due to its coordination to
ruthenium(II) ion. A sharp singlet due to the –OH proton of the free Schiff base ligands in the
region δ 11.8‒13.15 ppm was disappeared in all the complexes, further supporting coordination
of the phenolic oxygen [25-28]. Therefore, the 1H–NMR spectra of the all complexes confirm the
bidentate coordination mode of the Schiff base ligands to ruthenium(II) ions. In all the
complexes, the cymene protons appear in the region of δ 4.1–5.2 ppm [29‒31]. The isopropyl
methyl protons of the p-cymene appeared as two doublets in the region of δ 1.0‒1.8 ppm. The
singlet around δ 3.2 ppm is due to methoxy protons in the complex 2.
X-ray crystallographic studies
Attempts were made to grow single crystals for all the complexes to confirm the
coordination mode of the ligand to metal and the geometry of the complexes. Molecular structure
of [Ru(η6-p-cymene)(Cl)(L2)] (2) have been determined by single-crystal X-ray diffraction
analyses. Crystals of 2 grew from slow diffusion of dichloromethane into hexane solutions and
crystallized in the triclinic system with the Pi space group. The structural refinement parameter is
given in Table 1 and the selected bond lengths and bond angles are given in Table 2. The
ORTEP views of the molecules with the atom numbering are shown in Figure 1. The molecular
structure of complex 2 clearly indicates that the Schiff base ligand coordinates in a bidentate
manner to ruthenium ions via the azomethine nitrogen and phenolic oxygen in addition to one
chlorine and one arene group. The complex adopts the commonly observed piano-stool geometry
as reported in many half-sandwich arene ruthenium(II) complexes [32-34].

Figure 1. ORTEP diagram showing the molecular structure of the complex 2 with piano-
stool geometry (50% thermal ellipsoids).
In this case, the arene ring forms the seat of the piano-stool, while the bidentate N, O and
Cl ligands form the three legs of the stool. Therefore, the ruthenium(II) ion is sitting in a NOCl
(η6-arene) coordination environment. The Schiff base ligand binds to the metal centre at N and O
forming the five membered chelate ring with bite angles 88.37(7)° O(1)–Ru(1)–N(2) and
82.73(6)°, N(2)–Ru(1)–Cl(1). The bond lengths of Ru(1)–N(2) and Ru(1)–O(1) are 2.1040(19) Å
and 2.0516(16) Å respectively. The Ru–Cl bond length is found to be 2.4358(6) Å and the bond
length is in agreement with other structurally characterized p-cymene ruthenium complexes. The
ruthenium atom is π bonded to the arene ring with an average Ru–C distance of 2.173(3) Å,
whereas the average C–C bond length in the arene ring is 1.482(4) Å with alternating short and
long bonds. As all the complexes display similar spectral properties, the other two complexes are
assumed to have a similar structure to that of [Ru(η6-p-cymene)(Cl)(L2)] (2).

Table 1. Selected crystal data and structure refinement summary of complex 2.
2
Empirical formula
Formula weight
Temperature
Wavelength/ Å
Crystal system
Space group
a/ Å
C30 H27Cl N O2 Ru
573.06
298(2)
0.71073 Å
Triclinic
P-1
10.2040(4) Å
10.2074(4) Å
12.4855(5) Å
88.231(2)
88.647(2)
86.579(2)
b/ Å
c/ Å
α/°
β/°
γ/°
3
Volume/ Å
Z
1297.20(9)
2
3
Density (calculated)/ Mg/m
1.441
-1
Absorption coefficient/ mm
F(000)
0.736
577
Theta range for data collection/°
Reflections collected
2.000 to 29.054
6416
Independent reflections
[R(int) = 0.051]
2
Goodness-of-fit on F
1.286
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0323, wR2 = 0.1104
R1 = 0.0388, wR2 = 0.0984
Largest difference in peak and hole 0.821 and -1.118

Table 2. Selected bond parameters from DFT (BP86/Def2-TZVP) method and XRD method for
the complex 2.DFT [Exp.]
Bond Angle ( ◌)
87.87 [88.37(7)]
85.72 [85.64(6)]
83.46 [82.73(6)]
86.93 [89.42(7)]
95.31 [94.34(9)]
89.98 [90.05(9)]
150.18 [149.82(7)]
156.92 [156.36(7)]
150.90 [151.41(7)]
166.61 [165.10(8)]
161.53 [162.44(6)]
Atoms
Ru1-N28
Ru1-O3
Ru1-C4
Ru1-C6
Ru1-C8
Ru1-C9
Ru1-C11
Ru1-C13
N28-C29
N28-C26
Ru1-Cl2
Bond Length (Ǻ)
2.1021 [2.1040(19)]
2.0654 [2.0516(16)]
2.2156 [2.192(2)]
2.1834 [2.170(2)]
2.2216 [2.202(3)]
2.1747 [2.176(2)]
2.1933 [2.190(2)]
2.2236 [2.192(2)]
1.4394 [1.437(3)]
1.3090 [1.288(3)]
2.41786 [2.4358(6)]
Bonds
N28-Ru1-O3
O3-Ru1-Cl2
N28-Ru1-Cl2
Cl2-Ru1-C9
N28-Ru1-C6
O3-Ru1-C11
O3-Ru1-C22
O3-Ru1-C23
N28-Ru1-C24
N28-Ru1-C25
Cl2-Ru1-C23
Computational Studies
The molecular structure of the synthesized complex 2 has been confirmed further by DFT
calculations. Computational methods are being used routinely to overcome the limitations
experienced by the experimental chemists. Arene-Ruthenium complexes are of much interest due
to their applications in the biological field. But these type complexes are often lacking in
structural characterization due to the poor quality of the crystal which leads to disorder in the
crystal structure. Localization of higher electron density on specific atoms also leads to a poor
structure solution. These can be easily overcome by suitable computational approach. Here we
have optimized the molecular structure of 2 to get more insight about its electronic and
geometrical preferences. The DFT optimized geometry of the complex 2 is provided in Figure 2.
DFT computed bond lengths ad bond angle values are in good agreement with the experimental
values obtained from X-ray crystallographic method (Table 2). The distorted octahedral
geometry around the ruthenium atom is confirmed from DFT and XRD method which is similar
to the already reported ruthenium arene complexes. The DFT (BP86/Def2-TZVP) computed Ru–

Cl bond length of 2.4179 A0 is close to the experimental value of 2.4358(6) A0 The DFT
.
(BP86/Def2-TZVP) computed Ru-O bond length of 2.0654 A0 is also close to the experimental
value of 2.0516(16) A0. The Frontier Molecular Orbitals computed from the DFT (BP86/Def2-
TZVP) method for the complex 2 are provided in Figure 3.
Figure 2. DFT (BP86/Def2-TZVP) optimized geometry of the complex 2 showing zig-
zag arrangement of the molecules generating a chains linked through Cl and N atoms.

HOMO (-0.1588)
HOMO-1 (-0.1792)
HOMO-2 (-0.1842)
HOMO-3 (-0.1949)
LUMO (-0.0897)
LUMO+1 (-0.0826)
LUMO+2 (-0.0754)
LUMO+3 (-0.0645)

Figure 3. Pictures of the Frontier Molecular Orbitals computed from DFT (BP86/Def2-TZVP)
method
Molecular orbital pictures reveal that there is considerable delocalization of electrons on
Ru, O, N and Cl atoms except the biphenyl ring in the HOMO and LUMO. The electron of
biphenyl ring is mainly involved in LUMO+1, LUMO+2 and LUMO+3 orbitals.
DFT (BP86/Def2-TZVP) computed reactivity descriptors are provided in the Table 3.
Though the DFT computed ELUMO-HOMO energy gap of 2.0 eV confirm the stability of the
complex 2, the electrophilicity value of 6.0 eV suggest the electrophlic nature of this molecule.
The DFT (BP86/Def2-TZVP) computed dipole moment value of 4.8 Debye also suggest the
possible NLO active nature of this molecule which is confirmed by the second order
hyperpolarizability values. In fact, the Non Linear Optical activity is caused by the interaction of
intense light with matter. Usually the presence of considerable charge separation resulting in
polarizability and high dipole moment values lead to the NLO activity of the molecule. The
energy gap between the HOMO and LUMO is also crucial in determining the electronic, redox,
transport and optical properties of the materials. Thus the obtained polaizability values for
compounds 2 is due to the charge separation raised from the donor-acceptor groups present in the
molecule.

Table 3. DFT (BP86/Def2-TZVP) computed reactivity descriptors for the complex 2
Cluster
2
HOMO
-4.3104
-2.4278
1.8826
-3.3691
0.9413
1.0624
6.0293
4.3104
2.4278
LUMO
E LUMO – HOMO
Chemical potential (µ) = ELUMO+HOMO/2
Hardness (Ƞ) = E LUMO –HOMO/2
Softness (S) = 1/Ƞ
Electrophilicity (Ѡ) = µ2/2Ƞ
Ionisation potential IP (eV)
Electron Affinity EA (eV)
Dipole moment (Debye)
4.8148
Catalytic one-pot conversion of aldehyde to amides
Under the optimized reaction condition, the complex [Ru(η6-p-cymene)(Cl)(L2)] 2 was
tested as a catalyst for various aldehydes to amide transformations. The reaction conditions were
loaded with catalyst and 4- nitrobenzaldehyde as substrate with different catalyst and substrate
ratio (C/S=1:600, 1:300 and 1:100) in the presence of NH2OH.HCl and base NaHCO3 in
acetonitrile for 7 h. The results of conversion of 4- nitrobenzaldehyde to 4-nitrobenzamide are
summarized in Table 4. It has been observed that the C/S ratio 1:100 is the best choice of
isolating the yield of amide product.

Table 4. Reaction optimization for the catalytic one-pot conversion of 4-nitrobenzaldehyde to 4-
nitrobenzamide using complex (2)a.
O
CHO
Catalyst 2
NH2
NH2OH.HCl/ NaHCO3/
CH3CN/ 7h
O2N
O2N
Entry
Catalyst/Substrate
Yield (%)
68
1:600
1:300
1:100
1
2
70
98
3
aConditions: Catalyst (1 mol%) aldehyde (1 mmol), NH2OH·HCl (1 mmol),
NaHCO3 (1 mmol) and 3 mL CH3CN. bIsolated yield after column
chromatography (average of two runs).
To study the influence of solvents in our catalytic system, we have chosen the reaction
between 4-nitrobenzaldehyde (1 mmol), NH2OH·HCl (1 mmol), complex 2 (1 mol%) as the
catalyst precursor in the presence of various solvents and different bases (1 mmol). Xylene,
toluene, benzene, chloroform, dichloromethane and acetonitrile are taken for our solvent
variation study. The extent of conversion is solvent-dependent and low conversions were
observed in benzene and xylene as solvent even at a higher temperature. Acetonitrile was found
to be the solvent of choice with excellent isolated yield of amide (98%) at a lower temperature.
The choice of the base was chosen as a next step for the optimization and are given in Table 5. It
has been observed that in acetonitrile solvent, NaHCO3 and KHCO3 gave excellent isolated
yields when compared to a much weaker base like CH3COONa or Et3N. Thus, it was concluded

that NaHCO3 as a base in acetonitrile solvent at 80 °C is the optimized condition for this
conversion.
Table 5
Optimization of solvent and base in the conversion of 4-nitrobenzaldehyde to 4-nitrobenzamide
using complex [Ru(p‒cymene)(Cl)(L2)] (2)a
O
O
H
Catalyst 2
(1 mol%)
NH2
O2N
O2N
Solvent/ Base
Temp (oC)
Entry
Time (h) Yield (%)b
Base
Solvent
Xylene
1
2
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
KHCO3
Et3N
32
58
140
110
80
24
24
24
24
24
5
Toluene
Benzene
25
3
4
5
6
30
CHCl3
60
<20
CH2Cl2
CH3CN
CH3CN
40
80
80
82
7
5
5
<15
80
CH3CN
CH3CN
CH3COONa
<15
98
8
9
5
NaHCO3
80
aReaction conditions:4-nitrobenzaldehyde (1 mmol), NH2OH·HCl(1 mmol), catalyst
(1 mol%), base (1 mmol) and 2 mL solvent.
bIsolated yield after column chromatography.
Present investigation suggested that a range of various substituted aromatic and
heterocyclic aldehydes was converted to a primary amides using [Ru(η6
-p-cymene)(Cl)(L2)] (2)

as an active catalyst. The results are listed in Table 6. Benzaldehyde was converted into
Benzamide with 90% yield.
Table 6 Conversion of aldehydes to amides using complex [Ru(p‒cymene)(Cl)(L2)] (2)a
O
O
(1 mol%)
Catalyst 2
NH2OH.HCl/ NaHCO3
CH3CN/ 7h
Ar
Ar
NH2
H
Entry
1
Product
Yield (%)b
90
Substrate
O
CHO
NH2
O
O
O
CHO
CHO
NH2
NH2
NH2
82
74
2
H3C
H3CO
H3CO
H3C
3
4
H3CO
H3CO
CHO
70
98
OCH3
OCH3
O
CHO
NH2
5
6
O2N
O2N
Cl
O
CHO
CHO
NH2
92
Cl
O
NH2
7
70
73
HO
HO
8
9
NH2
O
N
CHO
CHO
N
O
65
NH2
O
O
aConditions: Catalyst (1 mol%) aldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol)
and 2 mL CH3CN. bIsolated yield after column chromatography

Interestingly, the presence of electron donating group in the para position of the
benzaldehyde, such as CH3, OCH3 substituents (entries 2, 3) on benzaldehyde converted to
corresponding amides was obtained a good yield of 82% and 74%. The electron withdrawing
groups like –NO2, –Cl substituents (entries 5 and 6) offers excellent yields (98%, 92%)
compared to substrates containing electron donating groups. Further, the complex 2 efficiently
catalyzes heterocyclic aldehydes to respective amides in good yield up to 73%. Further, we
believe that the catalytic transformation proceeds via the oxidative addition of the aldoxime N–
OH bond to Ru(II), followed by nucleophilic attack on the coordinated imine, then β-elimination
of cyclometalated, and finally reductive elimination to give the amide according to the
mechanism proposed by Crabtree et al [35].
Conclusion
We have synthesized new air stable (η6‒p‒cymene) ruthenium(II) complexes containing
bidentate Schiff base ligands derived from biphenylamine with corresponding aldehydes. The
steric hindrance of the bidentate Schiff base ligands (HL1‒HL3) seems to govern the
stoichiometry of the resulting ruthenium(II) complexes. These complexes were fully
1
characterized by elemental analysis and spectroscopic (IR, UV–Vis & H
‒
NMR) methods. The
structure of the complex 2 was confirmed by single crystal X-ray diffraction and by DFT
(BP86/Def2-TZVP) calculations, to have the pseudo-octahedral three legged piano stool
geometry. The new six coordinated [Ru(η6‒p‒cymene)(Cl)(L2)] (2) complex was tested as a
catalyst for the conversion of aldehyde to primary amides and showed excellent activity.

Experimental section
Materials and Physical measurements
All chemicals and solvents were obtained from commercial sources and used as such
without further purifications. RuCl3.3H2O, was purchased from Himedia and 2–aminobiphenyl,
2–hydroxy–1–naphthlaldehyde, 3–methoxy salicylaldehyde, salicylaldehyde and different
aromatic aldehydes for catalytic studies were purchased from Sigma–Aldrich. The microanalysis
of carbon, hydrogen and nitrogen was carried out using a Vario EL III Elemental analyzer at
SAIF – Cochin, India. The FT–IR spectra of the complexes were recorded in Agilent Resolution
pro system spectrometer over the range 4000 to 400 cm‒1. Electronic spectra of the complexes
were recorded in CHCl3 solution in a Cary 300 Bio UV–Vis Varian spectrophotometer in the
1
range 800
‒
200 nm. The H–NMR spectra were recorded in CDCl3 with Bruker 300MHz
instrument using TMS as internal reference.
Preparation of Schiff base ligand (HL1 HL3)
The reaction mixture containing 2 aminobiphenyl (0.181g; 1mmol) and substituted
‒
‒
aldehydes (0.172g; 1mmol) in ethanol was stirred for 1 hr. After a few minutes an orange yellow
precipitate was obtained which was filtered, washed with ethanol and dried. Yield: 80-90 %.
HL1: IR (cm‒1): 1611(HC=N), 3037 (OH), 1H
‒NMR (CDCl3) δ (ppm): 6.9-8.2 (m, Ar), 8.7 (1H,
s,
‒ HC=N), 13.3 (1H, s, OH).
HL2: IR (cm‒1): 1616 (HC=N), 3220 (OH), 1H
‒NMR (CDCl3)
δ
(ppm): 6.8-7.4 (m, Ar), 8.5
(1H, s,
‒HC=N), 3.8 (3H, s, OCH3), 12.9 (1H, s, OH).
HL3: IR (cm‒1): 1615 (HC=N), 2921(OH), 1H
‒
NMR (CDCl3) δ (ppm): 6.9-8.0 (m, Ar), 9.3
(1H, s, ‒HC=N), 14.8 (1H, s, OH).

General method of synthesis of Half-Sandwich Ruthenium(II) Schiff base complexes (1
The RB flask containing ruthenium precursor [RuCl2(p‒cymene)]2 (0.06g; 0.1mmol) and
Schiff base ligands (HL1 HL3) (0.0568 0.0640g; 0.2mmol) were taken in methanol (20 mL) and
‒3)
‒
‒
the mixture was stirred for 4 h. Then, the solvent was removed through vacuum, the dark red
mass obtained was dissolved in chloroform (15 mL) and filtered through a short path of silica gel
column to remove insoluble materials. The red solution was then concentrated into 2 mL, dark-
red product separated out with an addition of excess hexane and washed with diethyl ether and
dried in a vacuum.
[Ru(p‒cymene)(Cl)(L1)] (1)
o
Yield: 85%; orange solid; M.p.250 C, Anal.Cal. for C29H27NOClRu: C, 64.32; H, 4.99;
N, 2.58; Found: C, 64.30; H, 4.97; N, 2.59. FT (C
1H
δ (ppm) = 6.9‒7.9 (m, Ar-H), 4.4 (d 1H, cymene Ar‒H), 5.1 (d
‒ ν(C=N), 1324 ν ‒O).
IR (cm‒1): 1607
‒
NMR (300 MHz, CDCl3):
1.7 (dd, 6H, 2CH3 of p‒cymene). 13C-NMR (300, MHz, CDCl3):
1H, cymene Ar
‒
H), 1.0
‒
δ
(ppm) 170 (H-C=N), 125 -130 (C=C, Ar), 10 – 55 (cymene-CH3, -CH(CH3)2. UV–Vis (
λmax,
(nm)): 360, 520.
[Ru(p‒cymene)(Cl)(L1)] (2)
o
Yield: 80%; Brown solid; M.p.230 C, Anal.Cal. for C30H28NOClRu: C, 63.15; H, 4.91;
N, 2.45; Found: C, 63.17; H, 4.93; N, 2.43. FT (C
1H
δ (ppm) = 6.8-7.7 (m, Ar-H), 4.4 (d 1H, cymene Ar‒H), 5.2 (d 1H,
‒ ν(C=N), 1330 ν ‒O).
IR (cm‒1): 1601
‒
NMR (300 MHz, CDCl3):
1.7 (dd, 6H, 2CH3 of p‒cymene), 3.2 (s, OCH3). 13C-NMR (300, MHz,
cymene Ar
CDCl3):
CH(CH3)2. UV–Vis (
‒H), 1.0‒
δ
(ppm) 45 (OCH3), 170 (H-C=N), 120-130 (C=C, Ar), 10 – 55 (cymene-CH3, -
max, (nm): 370, 540.
λ

[Ru(p‒cymene)(Cl)(L1)] (3)
o
Yield: 78%; red solid; M.p.210 C, Anal.Cal. for C33H29NOClRu: C, 64.00; H, 4.90; N,
1
2.36; Found: C, 64.03; H, 4.92; N, 2.34. FT
‒
IR (cm‒1): 1615
ν
(C=N), 1310
ν
(C
‒
O). H
‒
NMR
(300 MHz, CDCl3):
δ (ppm) = 7.0‒7.6 (m, Ar-H), 5.1 (d 1H, cymene Ar‒H), 4.1 (d 1H, cymene
Ar‒
H), 1.0
‒
1.8 (dd, 6H, 2CH3 of p‒cymene). 13C-NMR (300, MHz, CDCl3):
δ
(ppm) 170 (H-
C=N), 122-130 (C=C, Ar), 10 – 55 (cymene-CH3, -CH(CH3)2.UV–Vis (
Typical procedure for the one-pot conversion of aldehydes to amides
λmax, (nm)): 360, 530.
The reaction mixture containing complex catalyst 2 (1 mol%), the aldehyde (1 mmol),
NH2OH.HCl (1 mmol) and NaHCO3 (1 mmol) in 5 mL of acetonitrile was refluxed for the time
specified under an N2 atmosphere. Then, the resulting was cooled to room temperature and the
solvent was evaporated. The residue obtained was dissolved in CH2Cl2 and the solvent was
removed. The crude product was then purified using silica gel chromatography using
CHCl3/MeOH as an eluent.
X-ray Structure Determination
Diffraction data of 2 were collected on a Bruker AXS SMART APEX diffractometer,
equipped with a CCD area detector using MoKα radiation (λ = 0.71073 Å). All the data were
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) [36], SADABS [37] 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
DFT calculations were carried out using the software ORCA developed by Frank Neese
and co-workers [38a]. All calculations were performed using the BP86 density functional [38b-
38e] and TZVP basis set 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 frequency calculations, which show no imaginary
frequencies. Pictures of the optimized geometries and the frontier molecular orbitals are taken
using the graphics programme ChemCraft [38f].
Acknowledgments
The authors express sincere thanks to Single Crystal X‒ray Diffraction Facility, SAIF
Guwahati University, for providing X‒ray data collection.
Supporting Information
FT-IR, UV-Vis and NMR spectra of ligands and complexes. Crystallographic data for
complex 2 (CCDC1963806) can also be obtained free of charge from the Cambridge
Crystallographic Data Centre (CCDC) via www.ccdc.cam.ac.uk/data_request/cif.

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Table 4. Reaction optimization for the catalytic one-pot conversion of 4-nitrobenzaldehyde to 4-
nitrobenzamide using complex (2)a.
O
CHO
Catalyst 2
NH2
NH2OH.HCl/ NaHCO3/
CH3CN/ 7h
O2N
O2N
____________________________________________________
yield (%)b
Entry
C/S
_______________________________________________
1
2
3
1:600
1:300
1:100
68
70
90
_______________________________________________
a Conditions: Catalyst (1 mol%) 4-nitrobenzaldehyde (1mmol), NH2OH·HCl
(1 mmol), NaHCO3 (1 mmol) and 3 mL CH3CN; b Isolated yield after column
chromatography (average of two runs).

Table 5
Optimization of solvent and base in the conversion of 4-nitrobenzaldehyde to 4-nitrobenzamide
using complex [Ru(p‒cymene)(Cl)(L2)] (2)a
O
O
H
Catalyst 2
(1 mol%)
Solvent/ Base
_____________________________________________________________________________________
Entry Solvent Base Temp ( C) Time (h)
Yield (%)b
______________________________________________________________________________
NH2
O2N
O2N
ᵒ
1
2
3
4
5
6
7
8
9
Xylene
Toluene
Benzene
CHCl3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
KHCO3
140
110
80
24
24
24
24
24
5
32
58
25
60
30
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
40
<20
82
80
Et3N
80
5
<15
<15
98
CH3COONa
NaHCO3
80
5
80
5
_____________________________________________________________________________
a Conditions: Catalyst (1 mol%) 4-nitrobenzaldehyde (1 mmol), base (1 mmol), solvent 3 mL;
b Isolated yield after column chromatography.

Table 6 Conversion of aldehydes to amides using complex [Ru(p‒cymene)(Cl)(L2)] (2)a
O
O
(1 mol%)
Catalyst 2
NH2OH.HCl/ NaHCO3
CH3CN/ 7h
Ar
Ar
NH2
H
Entry
Substrates
Products
O
Yields (%)b
CHO
CHO
NH2
1
2
3
90
82
74
O
NH2
H3C
H3C
O
CHO
CHO
NH2
H3CO
H3CO
H3CO
H3CO
O
NH2
OCH3
CHO
OCH3
4
5
70
92
O
H3CO
H3CO
H3CO
H3CO
NH2
NH2
OCH3
OCH3
O
CHO
CHO
CHO
O2N
Cl
O2N
6
7
98
70
O
NH2
Cl
O
O
N
O
N
NH2
NH2
8
9
73
65
O
CHO
______________________________________________________________________________
a Conditions: Catalyst (1 mol%) aldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol)
and 3 mL CH3CN. b Isolated yield after column chromatography (average of two runs).

Highlights
‹ A new family of 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 varies aldehydes to amines.