Half-sandwich ruthenium(II)complexes containing O, N bidentate azo ligands: Synthesis, structure and their catalytic activity towards one-pot conversion of aldehydes to primary amides and transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.jorganchem.2019.05.017

The ruthenium(II)complexes of the general formula [Ru(η⁶?p?cymene)(Cl)(L_1?5)](L = O, N-donors of biphenylazo derivatives), formed by reacting 2?(biphenylazo)phenol (HL₁ ? HL₄)and 1?(biphenylazo)naphthol ligands (HL₅)with [{η⁶?p?cymene)RuCl}₂(μ?Cl)₂]have been synthesized. The compositions of the complexes have been established by IR, UV–Vis, ¹H NMR spectral methods and X-ray crystallography. The synthesized complex could act as an efficient, reusable homogeneous catalyst for transformation of aldehydes to the corresponding primary amides in the presence of NH₂OH·HCl, thus resulting an expansion of Beckmann rearrangement. The effect of solvent, base, temperature, time, catalyst loading and recyclability was also investigated. They also effectively catalyze the transfer hydrogenation reaction of various ketones with 2-propanol.

Full text of DOI:10.1016/j.jorganchem.2019.05.017

Accepted Manuscript
Half-sandwich ruthenium(II) complexes containing O, N bidentate azo ligands:
Synthesis, structure and their catalytic activity towards one-pot conversion of
aldehydes to primary amides and transfer hydrogenation of ketones
Raja Nandhini, Venkatachalam Galmari
PII:
S0022-328X(19)30206-2
DOI:
https://doi.org/10.1016/j.jorganchem.2019.05.017
JOM 20799
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 March 2019
Revised Date: 15 May 2019
Accepted Date: 17 May 2019
Please cite this article as: R. Nandhini, V. Galmari, Half-sandwich ruthenium(II) complexes containing
O, N bidentate azo ligands: Synthesis, structure and their catalytic activity towards one-pot conversion
of aldehydes to primary amides and transfer hydrogenation of ketones, Journal of Organometallic
Chemistry (2019), doi: https://doi.org/10.1016/j.jorganchem.2019.05.017.
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ACCEPTED MANUSCRIPT
Half-Sandwich ruthenium(II) complexes containing O, N bidentate
azo ligands: Synthesis, Structure and their catalytic activity towards
one-pot conversion of aldehydes to primary amides and Transfer
hydrogenation of ketones
Raja Nandhini and Galmari Venkatachalam*
PG & Research Department of Chemistry, Government Arts College, Dharmapuri- 636 705,
Tamilnadu, India.
Corresponding Author: *E-mail address: gvchem@gmail.com [G. Venkatachalam]
Abstract
6
The ruthenium(II) complexes of the general formula [Ru(η ‒p‒cymene)(Cl)(L1‒5)] (L =
O, N-donors of biphenylazo derivatives), formed by reacting 2‒(biphenylazo)phenol (HL ‒
1
6
HL ) and 1‒(biphenylazo)naphthol ligands (HL ) with [{η ‒p‒cymene)RuCl} (µ‒Cl) ] have
4
5
2
2
1
been synthesized. The compositions of the complexes have been established by IR, UV‒Vis, H‒
NMR spectral methods and X-ray crystallography. The synthesized complex could act as an
efficient, reusable homogeneous catalyst for transformation of aldehydes to the corresponding
2
primary amides in the presence of NH OH·HCl, thus resulting an expansion of Beckmann
rearrangement. The effect of solvent, base, temperature, time, catalyst loading and recyclability
was also investigated. They also effectively catalyze the transfer hydrogenation reaction of
various ketones with 2-propanol.
Keywords: Biphenylazo ligands, Ruthenium‒p‒cymene complexes, Structure, Amidation
reaction, Beckmann rearrangement,Catalytic transfer hydrogenation.

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1
. Introduction
Half-sandwich η ‒p‒cymene ruthenium complexes containg various ligands have played
6
an important role in the development of modern organometallic chemistry. They have attracted
an enormous interest due to their potential applications in catalytic activity in wide range of
organic reactions [1, 2] and also display promising anti-cancer activity [3, 4]. Current interest in
the organometallic chemistry of ruthenium(II) complexes lies in the development of the new
catalytic system for different organic transformations such as oxidation [5‒7], transfer
hydrogenation reactions [8‒11], amide conversion[12, 13], reduction of nitro compounds [14‒
1
6] etc.
The development of efficient methods for the synthesis of amides is very important
because of their usefulness in a wide variety of applications in academia as well as in industry,
especially as intermediates in organic synthesis, raw materials for engineering plastics,
detergents, lubricants and pharmaceuticals. The conversion of carbonyl compounds, such as
aldehydes, ketones, and oximes, is a good candidate for the synthesis of amides [17–20].
Beckmann rearrangement is commonly used to transform oximes into the corresponding amides
[21]. This rearrangement is commonly used to transform ketoximes into the corresponding N-
substituted amides requiring the use of strong acids [22]. Further, the synthesis of primary
amides from aldoximes is very difficult and reactive reagents have to be used in stoichiometric
amounts for the transformation to occur. The one-pot synthesis of amide from aldehyde with
amines can be a potentially elegant alternative pathway. It has attracted much attention because it
(i) eliminates the isolation of unstable intermediates, (ii) reduces hazardous wastage, (iii) more
efficient and selective and (iv) no by-product formation is observed [23]. Significant efforts have
been developed in recent years to the development of one-pot process enabling the direct

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formation of primary amides from aldehydes and hydroxylamine derivatives via rearrangement
of the in situ formed aldoximes.
The powerful of ruthenium metal compounds to dehydrogenate alcohol and delivered the
hydride to a ketone [24, 25] or an α, β-unsaturated ketone has make them useful as transfer
hydrogenation catalysts [26‒30]. The reaction conditions for transfer hydrogenations are
economic, relatively mild and environmentally friendly. Experimental and theoretical studies to
improve the new catalysts for transfer hydrogenation is still of substantiate importance, in order
to find most efficient catalysts.
Ruthenium(II) p-cymene complexes showed a piano-stool three legged structure in the
ruthenium metal center have a similar octahedral geometry and this structural feature opens the
possibility to introducing in the molecules of two type of stereogenic center: (i) the ligand and
(ii) the metal. It has been found in the reported literature that the azo phenol ligand is well known
to coordinate center metal ions usually with O, N bidentate ligands forming a five and six
membered rings. The arylazo groups due to its more acidic nature stabilizes ruthenium metal in
lowest oxidation state while phenolate oxygen atom being a hard base stabilized the higher
oxidation states of the ruthenium metal ion [31].
We describe here, the synthesis and characterization of new mononuclear ruthenium(II)
complexes containing 2‒(biphenylazo)phenol and 1‒(biphenylazo)naphthol ligands. The
molecular structure of the complexes is investigated with the help of the single crystal XRD
6
structure in combination with spectral studies. Further, the catalytic study of the (η ‒p‒cyemene)
ruthenium(II) complexes for both aldehyde to amide conversion and transfer hydrogenation of
ketones have been carried out. Density Functional Theory calculations (DFT) also performed to
support the experimental findings.

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2
. Results and Discussion
The reaction of 2‒(biphenylazo)phenol and 1‒(biphenylazo)naphthol ligands with
6
6
chloro‒bridged (η ‒p‒cymene) ruthenium precursor complex [{η ‒p‒cymene)RuCl }(µ‒Cl) ] in
2
2
methanol at room temperature in 1:2 molar ratio resulted in the formation of conformationally
rigid new monomeric ruthenium(II) complexes (Scheme 1). The complexes were found to be air
stable and are soluble in polar solvents such as dichloromethane and acetone, but insoluble in
non-polar solvents such as pentane and hexane. The data obtained from elemental analysis are in
good agreement with the compositions proposed for the structure of complexes. It is noted that
the present ligand system binds the metal center as five membered chelate rings in complexes 1-
4
and six membered chelate rings in complex 5.
Cl
Cl
R
OH
Ru
Ru
Cl
MeOH/ Stirring/ 4h
R
O
Cl
Ru
Cl
N
N
N N
R = CH (L ; 1), OCH (L ; 2), C(CH )
3 3
3
1
3
2
R = CH (HL ); OCH (HL );
3
1
3
2
(
L ; 3), Cl (L ; 4)
3 4
C(CH ) (HL ); Cl (HL )
3
3
3
4

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Cl
Cl
OH
N
O
Ru
Ru
Cl
MeOH/ Stirring/ 4h
Ru
Cl
N
N
N
Cl
HL5
L5 ; 5
Scheme 1. Synthesis of p-cymene Ru(II) 2‒(biphenylazo)phenolate (1‒4) and 1‒
biphenylazo)naphtholate complexes (5)
(
2
.1. Characterization of the complexes
Infrared spectra of all the ligands displayed strong bands around 1403–1445 and 1257–
‒
1
1
270 cm corresponding to υ–N=N– and phenolic υC–O stretching, respectively. After
‒
1
complexation υ–N=N– appears at 1384–1389 cm and the red shifting is corroborated with N
azo) coordination. FT‒IR spectra of complexes 1‒5 are given in Figs. S1‒S5 (see supporting
information). The coordination through phenolic oxygen is confirmed by the increase of C–O at
(
‒
1
higher frequencies in the region 1305–1323 cm in all the complexes. This is further supported
‒
1
by the disappearance of υ–OH band in the range 3440–3457 cm in all the complexes [32–34].
The UV‒Vis spectra of all complexes in chloroform solutions exhibit characteristic
absorptions in the region 200‒800 nm. The representative UV-Vis spectra of complexes 1‒5 are
given in Figs. S6‒S10 (see supporting information). The absorption at 445–460 nm is probably
due to metal-to-ligand charge transfer transitions. The high intensity bands around 340–325 nm
and 260–250 nm has been designated as n–π* and π–π* transitions respectively for the
biphenylazo ligands in these complexes. The obtained electronic spectral pattern of all

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complexes clearly shows the presence of an octahedral environment around the ruthenium(II) ion
which is consistent with other reported ruthenium octahedral complexes [35, 36, 50].
1
The H‒NMR spectra of the complexes 1–5 have been recorded in CDCl and were in
3
agreement with their molecular structures. Complexes 1–5 are given in the supporting
information (S11‒S15). All the complexes show multiplets at δ 6.4–7.9 ppm for the presence of
biphenylazo phenol ligands. The methyl protons appear as singlet and isopropyl protons appear
as two doublets of the p-cymene ligand in the range of δ 2.1–2.2 ppm and δ 0.8–1.4 ppm,
respectively. The isopropyl CH protons appear as a septet in the range of δ 2.3–2.8 ppm and the
p-cymene ring protons are observed in the ranges of δ 4.2–5.8 ppm as either four doublets (4H)
or two doublets (2H) and one singlet (2H) for all the complexes. In additional, methyl signals are
observed as singlet for complex 1 at δ 2.3 and methoxy signals are observed as singlet for
complex 2 at δ 3.7 ppm. A sharp singlet appeared for OH protons of all the ligands (HL –HL ) in
1
5
the region δ 11.7 ppm was disappeared in all the complexes.
.2. X-ray crystallographic studies
The structure of the compound (1), (2) and (5) consist of neutral arene ring bonded to the
2
ruthenium along with chloride and O, N-donors of 2‒(biphenylazo) phenol ligands. The
molecular structure of complexes (1), (2) and (5) with atom labeling scheme is shown in Fig 1 &
2
. The summary of single crystal X-ray structure refinement is given in the supporting
information S16. The selected bond parameters are presented in Table 1. In the crystal structure
of 1, the complex crystallized in the triclinic with the P -1 space group whereas the complexes 2
and 5 are recrystallized in monoclinic with the P 21/c space group. The complexes 1, 2, 5 adopt a
typical three-legged piano stool conformation with N, O and Cl atoms as the legs. More
importantly the isopropyl group of the complex 2 is the successive refinements lead to the

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permissible R factor values. The position and the bond parameters of the isopropyl group are
supported and confirmed from the DFT calculations. Computational methods and experimental
findings are being used together nowadays to complete the structural characterization [37].
1
5
Fig 1. ORTEP drawing of complexes 1 and 5 with hydrogen atoms being omitted for clarity.
2
(a)
(b)
Fig 2. (a) ORTEP drawing of complex 2 (with hydrogen atoms being omitted for clarity) and (b)
DFT (BP86/TZVP) optimized geometry with isopropyl group.

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The 2‒(biphenylazo) phenolate ligands bind the metal center at O and N forming the five
membered chelate ring with bite angle O(1)–Ru(1)–N(1) 79.10(15), 79.34(9) and bond lengths of
Ru(1)–O(1) and Ru(1)–N(1) are 2.059(4), 2.0520(1) and 2.052(2), 2.100(2) in 1 and 2. The 1‒
(biphenylazo) naphtholate ligand bind the metal center at O and N forming the six membered
chelate ring with bite angle of O(1)–Ru(1)–N(2) 87.49(9) and bond lengths of Ru(1)–O(1) and
Ru(1)–N(2) are 2.005(2) and 2.058(2) in 5 respectively. The Ru–Cl bond length is found to be
2
.4215(15), 2.4018(9) and 2.4202(8). As all the complexes display similar spectral properties,
the other complexes are assumed to have similar structure to that of complexes 1, 2 and 5. In
6
half-sandwich complexes of (η ‒p‒cymene) ruthenium with several nitrogen ligands, Ru–N bond
lengths have been reported [38, 39] generally between 2.060, 2.100(2) and 2.156 Å, which are
consistent with the present values.
Table 1 Selected bond lengths (Å) and angles (°) in complexes 1, 2 (DFT) and 5
Distances / angles
1
2
(DFT)
5
Ru(1)-O(1)
Ru(1)-N(1)
Ru(1)-C(28)
Ru(1)-Cl(1)
N(2)-N(1)
N(2)-C(8)
C(1)-O(1)
2.059(4)
2.103(4)
2.173(5)
2.4215(15)
1.268(6)
1.420(6)
1.302(7)
2.052(2) (2.047)
2.100(2) (2.101)
2.2010(1) (2.180)
2.4018(9) (2.394)
1.271(3) (1.284)
1.4117(1) (1.401)
1.303(4) (1.306)
2.055(2)
2.058(2)
2.182(3)
2.4202(8)
1.251(3)
1.388(4)
1.272(4)
O(1)-Ru(1)-N(1)
O(1)-Ru(1)-C(28)
O(1)-Ru(1)-Cl(1)
N(1)-Ru(1)-Cl(1)
C(28)-Ru(1)-Cl(1)
N(2)-N(1)-Ru(1)
79.10(15)
129.08(19)
85.52(13)
85.50(11)
144.82(15)
133.1(3)
79.34(9) (79.47)
87.49(9)
89.27(10)
84.88(7)
83.94(7)
153.63(9)
130.5(2)
108.56
(107.8)
85.14(7) (84.94)
84.44(7) (85.23)
146.09
(145.98)
132.2(2) (132.1)

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2
.3. Catalytic one-pot conversion of aldehydes to amides
-nitrobenzaldehyde has been chosen as a model substrate to explore the catalytic activity
4
of the complexes 1-5 under the optimized conditions. Among the tested complexes, complex 1 is
highly efficient in the aldehyde to amide with a high conversion of 96%. The result of
transformations is given in Table 2. We found that complex 1 showed in high yield than 2, 3, 4
and 5 at the same reaction time and at the same temperature. In the results obtained, complex 1
shows good catalytic activity among the other four complexes. Hence, complex 1 was selected as
the model catalyst for conversion of 4-nitrobenzaldehyde to 4-nitrobenzamide using
NH
2
OH.HCl by refluxing in acetonitrile with NaHCO
3
as the base.
Table 2
Optimized conditions for the 4-nitrobenzaldehyde to 4-nitrobenzamide conversion using
a
complexes 1-5 .
O
O
Complex
Catalysts (1mol%)
H
NH2
O N
2
CH CN/ NaHCO / O N
3
NH OH.HCl
3
2
2
______________________________________________________________________
b
Entry
Complexes
Yield (%)
______________________________________________________________________
1
1
2
3
4
5
96
82
79
70
62
2
3
4
5
_
_____________________________________________________________________
a
2
Reaction coditions: aldehyde (1 mmol), NH OH.HCl (1 mmol), catalyst (1 mol%)
and 2 mL of CH CN were refluxed for 5 h.
3
b
Isolated yield after column chromatography.

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In order to optimize the effect of catalyst loading, different catalyst: substrate (C:S) ratios
were tested in the one-pot conversion of 4-nitrobenzaldehyde to 4-nitrobenzamide using complex
1
as a catalyst and the results are summarized in Table 3. The reaction proceeds with good
isolated yield when the C:S ratio is 1:100. When changing the C:S ratio to 1:200, 1:300, 1:500
and 1:1000, the reaction still proceeds smoothly accompanied by a drop in the isolated yield.
Thus, it was concluded that catalyst: substrate ratio of 1:100 is the best compromise between
optimal reaction rates in acetonitrile and we obtained 96% yield of amide (entry 1).
Table 3
Effect of catalyst: substrate (C:S) ratio in the one-pot conversion of 4-nitrobenzaldehyde to 4-
a
nitrobenzamide using complex [Ru(p‒cymene)(Cl)(L )] 1
1
O
O
Complex 1
H
NH2
O N
2
CH CN/ NaHCO3
O N
2
3
______________________________________________________________________
b
Entry
Ratio
Yield (%)
______________________________________________________________________
1
2
3
4
5
1:100
1:200
1:300
1:500
1:1000
96
68
52
41
35
_
_____________________________________________________________________
a
Reaction conditions: Substrate (1 mmol), NH OH.HCl (1 mmol) and 2 mL of CH CN
2
3
were refluxed for 5 h.
b
Isolated yield after column chromatography.

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The catalytic activity of the ruthenium complex 1 was explored for the one-pot synthesis
of amides from various aldehydes with hydroxylamine hydrochloride. For the entire
optimization, 4-nitrobenzaldehyde was taken as a test substrate for different conditions. To study
the influence of solvents in our catalytic system, we have chosen the reaction between 4-
nitrobenzaldehyde (1 mmol), NH OH·HCl (1 mmol), complex 1 (1 mol%) as the catalyst
2
precursor in the presence of various solvents and NaHCO (1 mmol) as the base. Xylene,
3
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 (96%) at a lower temperature.
The choice of the base was chosen, as a next step for the optimization in Table 4. It has been
observed that in acetonitrile solvent, NaHCO and KHCO gave excellent isolated yields of 96%
3
3
(entry 9) and 88% (entry 6) respectively, when compared to a much weaker base like
CH COONa or Et N. Thus, it was concluded that NaHCO as a base in acetonitrile solvent at 78
3
3
3
°
C is the optimized condition for this conversion.

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Table 4
Optimization of solvent and base in the one-pot conversion of 4-nitrobenzaldehyde to 4-
a
nitrobenzamide using complex [Ru(p‒cymene)(Cl)(L )] 1
1
O
O
Complex
Catalyst 1 (1 mol%)
H
NH2
O N
Solvent/ Base
O N
2
2
_
_
_____________________________________________________________________________
o
b
Entry
Solvent
Base
Temp ( C)
Time (h)
Yield (%)
_____________________________________________________________________________
1
2
3
4
5
6
7
8
9
Xylene
Toluene
Benzene
CHCl3
NaHCO3
NaHCO3
140
110
80
24
24
24
24
24
5
30
68
NaHCO
3
28
NaHCO3
NaHCO3
61
32
CH Cl2
40
<25
88
2
CH
CH
3
3
CN
CN
KHCO
3
78
Et
3
N
78
5
<20
<20
96
CH CN
CH COONa
78
5
3
3
CH CN
NaHCO3
78
5
3
_
_____________________________________________________________________________
a
Reaction conditions: 4-nitrobenzaldehyde (1 mmol), NH OH.HCl (1 mmol), base (1 mmol)
2
and 2 mL of solvent.
b
Isolated yield after column chromatography.

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a
Table 5 One-pot conversion of aldehydes to amides using complex [Ru(p‒cymene)(Cl)(L
1
)] 1
Complex
Catalyst 1 (1 mol%)
O
O
NH OH.HCl/ NaHCO
2
3
Ar
Ar
NH2
H
CH CN/ 5h
3
b
Entry
1
Substrate
Product
Yield (%)
O
CHO
NH2
93
O
CHO
NH2
H C
H C
3
3
2
3
82
76
O
CHO
NH2
H CO
H CO
3
3
O
CHO
OCH3
CHO
NH2
H CO
H CO
OCH3
3
3
4
5
71
90
O
NH2
Cl
Cl
O
CHO
CHO
NH2
NH2
O N
O N
2
2
6
96
N
N
O
7
8
72
67
NH2
NH2
O
CHO
O
O
O
CHO
HO
HO
9
75

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O
O
H
NH2
1
0
70
a
Conditions: Catalyst (1 mol%), aldehyde (1 mmol), NH OH.HCl (1 mmol), NaHCO (1 mmol)
and 2 mL CH CN. Isolated yield after column chromatography.
2
3
b
3
To explore the scope of the new catalyst, a range of other substituted aromatic and
heterocyclic aldehydes were converted to primary amides using catalyst 1 under the optimized
condition. All the reactions were carried out under identical reaction conditions to allow
comparison of results. A broad range of amides were successfully synthesized with good to high
isolated yields using the above optimized protocol. The results collected from the catalytic
reactions are listed in Table 5. The electron donating groups like –CH , –OCH and –OH (entries
3
3
2
, 3 and 9) on benzaldehyde alters the reactions and the corresponding amides were obtained in
good yields of 82%, 76% and 75% respectively and gave slightly lower yields compared with
benzaldehyde. On the other hand, electron withdrawing substituents, such as the –Cl, and –NO2
substituents (entries 5 and 6) offering excellent yields (90%, and 96%) when compared to
substrate containing electron donating group. The introduction of electron withdrawing
substituents to the para position of the aryl ring of the aldehyde decreased the electron density on
the C=O bond so that the activity was improved giving rise to easier amidation reaction. The
conversion proceeded smoothly even in the presence of heteroatoms such as N and O in the
substrates (entries 7 and 8) and a range of heterocyclic aromatic amides were obtained in good
isolated yields. 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 [40]. The present Ru(II)

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catalyst is more efficient in amidation reaction than the reported ruthenium(II) complexes in
terms of reaction time, catalyst loading and isolated yields [41‒43].
2
.4. Catalytic transfer hydrogenation of ketones
Catalytic transfer hydrogenation reaction in which hydrogen is transferred from one
organic molecule to another by ruthenium complexes is well known since one can avoid the use
of molecular hydrogen and this prompted us to carry out this type of reactions. Representative
types of aliphatic alkyl and aryl ketones were chosen to evaluate the performance of catalyst
1
complex [Ru(p‒cymene)(Cl)(L )] 1 in transfer hydrogenation reaction in the presence of iso-
propanol and KOH as promoter.
Table 6
Catalytic transfer hydrogenation of acetophenone using complex 1.
O
OH
complex 1
i-prOH /KOH /2h
_
_
____________________________________________________________
Entry
C/S
Conversion (%)
TON
____________________________________________________________
1
2
3
4
1:900
1:600
60
225
180
285
-
75
1:300
95
absence of catalyst
product not detected
_____________________________________________________________
In order to optimize the reaction conditions, different catalyst: substrate ratios were tested
and the results are summarized in Table 6. For this initial experiments, acetophenone was
selected as a test-substrate and allowed it to react in 2-propanol with catalytic quantities of

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[
Ru(p‒cymene)(Cl)(L
1
)] 1 complex in the presence of KOH. When increasing the C:S ratio to
1
:300, 1:600 and 1:900 in 2-propanol, the reaction still proceeds smoothly accompanied by a
moderate drop in conversion. Thus, it was concluded that catalyst: substrate ratio of 1:300 is the
best compromise between optimal reaction rate and C/S ratio.
Table 7
6
Catalytic Transfer Hydrogenation of Ketones by [Ru(η ‒p‒cymene)(Cl)(L(1‒5)] 1‒5.
OH
O
O
Catalyst (1-5) 0.3mol%
i-PrOH/ KOH/ 2h
+
R
R'
R
R'
a
Conversion (%)
Complexes
b
Entry
Substrate
Product
TON
1
2
3
4
5
O
OH
1
75 70 87 60 70 225 210 261 180 210
95 80 90 85 75 285 240 270 255 225
O
OH
CH3
CH3
2
c
O
OH
OH
3
90 82 85 80 60
270 246 255 240 180
O
4
5
90 96 92 85 80 270 288 276 255 240
82 72 85 80 70 246 270 255 240 210
O
OH
CH3
CH3
H CO
3
H CO
3

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O
OH
6
7
70 73 64 68 75 210 219 192 204 225
O
OH
c
90 85 80 75 80 270 255 240 225 240
a
b
c
GC analysis; TON = ratio of moles of product obtained the moles of catalyst used; Time 6h
The catalyst performed efficiently in the conversion of ketones to alcohols and the results
of this organic transformation are presented in Table 7. Both aliphatic and aromatic ketones are
converted into their corresponding secondary alcohols with good conversions and turnover. The
efficient conversion (upto 95%) was found in the case of acetophenone (entry 2) among the
aromatic ketones with all the complex catalysts 1‒5. The complexes 1‒5 efficiently catalyzed the
reduction of acyclic ketones such as ethyl methyl ketone and isobutyl methyl ketone to their
corresponding alcohols with 90% conversion respectively. These ketones took longer time (entry
3
& 7) to react compared to their cyclic counter parts. Moreover, these catalysts show good
activity for the transfer hydrogenation of five and six membered cyclic ketones with excellent
conversions upto 96%. This trend of higher conversion of cyclic ketones can be attributable to
steric hindrance with respect to access to the C=O bond [44]. The complexes of biphenylazo
ligands (L ) do not differ significantly in catalytic efficiency. The catalytic results obtained are
1
‒5
comparable with other ruthenium complexes containg different types of O, N donor ligands [45].
No transfer hydrogenation was observed in the absence of base. In the transfer hydrogenation
reaction, the base facilitates the formation of ruthenium alkoxide by abstracting proton from the
alcohol and subsequently alkoxide undergoes
β-elimination to give ruthenium hydride, which is
an active species in this reaction. This is the mechanism proposed by several workers on the

ACCEPTED MANUSCRIPT
studies of ruthenium catalyzed transfer hydrogenation reaction by metal hydride intermediates
[
46
‒
50].
. Conclusion
A family of five half-sandwich ruthenium(II) complexes of the general formula [Ru(η ‒
cymene)(Cl)(L )] have been synthesized and characterized. The structure of complexes 1, 2
3
6
p
‒
1
‒5
and 5 were confirmed by single crystal X-ray differaction, to have the pseudo-octahedral three
legged piano stool geometry. These ruthenium complexes exhibit a high activity in an
exceptionally short reaction time, effective of solvent, base assisted aldehydes to primary
amines. Among the five complexes, the complex 1 is the most active one. The ruthenium
complexes also effectively catalyze the transfer hydrogenation of various ketones in presence of
i-prOH.
4
. Experimental Procedures
4
.1. Materials and physical measurements
Commercial RuCl .3H O was purchased from Himedia. All the used reagents were
3
2
chemically pure or analytical reagent grade. Solvents were purified and dried according to
standard procedures. The 2 (biphenylazo)phenol and 1 (biphenylazo)naphthol ligands [51] were
‒
‒
prepared by diazotization reaction of 2-aminobiphenyl with corresponding para-substituted
6
phenols. The metal precursor [{η ‒
p
‒
cymene)RuCl}
2
(
µ‒Cl)
2
] was prepared by standard
procedure [52]. The substrates used in the catalytic studies, were purchased from Merck and
Aldrich. The IR spectra of the complexes were recorded on an agilent resolution pro model in
‒
1
4
000–400 cm range. Electronic spectra of the complexes were recorded in CHCl solution in a
3
1
Cary 300 Bio UV–Vis Varian spectrophotometer in the range 800
‒
200 nm. The H NMR spectra
were recorded in CDCl with Bruker 300MHz instrument using TMS as internal reference.
3
4
.2. General method of synthesis of Half-Sandwich Ruthenium complexes (1‒5)

ACCEPTED MANUSCRIPT
The RB flask containing ruthenium precursor [RuCl
-(biphenylazo)phenol and 1 (biphenylazo)naphthol ligands (HL1
.2mmol) were taken in methanol (20 mL) and the mixture was stirred for 4h. Then, the solvent
2
(p
‒
cymene)]
2
(0.06g; 0.1mmol) and
2
0
‒
‒
HL ) (0.0568
‒0.0640g;
5
was removed through vaccum, the dark red mass obtained was dissolved in chloroform (15 mL)
and filtered through short path of silica gel column to remove insoluble materials. The red
solution was then concentrated into 2 mL, dark-red product separated out an addition of excess
hexane and washed with diethyl ether and dried in vaccum.
[Ru(p
1
‒cymene)(Cl)(L )] (1)
o
Yield: 70%; brown solid; M.p.196 C, Anal.Cal. for C29
H
24
N
2
OClRu: C, 62.92; H, 4.33;
IR (cm ): 1384 (N=N), 1313 (C O).
H), 5.8
max, (nm):
‒
1
N, 5.06; Found: C, 62.94; H, 4.32; N, 5.10. FT
‒
ν
ν ‒
1
H‒
NMR (300 MHz, CDCl3):
d 1H, cymene Ar H), 0.9–1.3 (dd, 6H, 2CH of p
50, 340, 250.
Ru(p cymene)(Cl)(L )] (2)
δ
(ppm) = 6.9–7.9 (m, Ar-H), 4.3 (d 1H, cymene Ar
‒
(
‒
‒cymene), 2.3 (s, CH ). UV–Vis (
λ
3
3
4
[
‒
2
o
Yield: 85%; brownish red solid; M.p. 212 C, Anal. Cal. For C29
H
26
N
2
O
2
ClRu: C, 60.94;
(N=N), 1305 (C O).
H), 5.8 (d
cymene), 3.7 (s,
−
1
H, 4.55; N, 4.90; Found: C, 60.92; H, 4.57; N, 4.88. FT
‒
IR (cm ): 1388
ν
ν ‒
1
H
‒
NMR (300 MHz, CDCl3):
H, cymene Ar H), 2.4 (s, 3H, CH of p
OCH ). UV–Vis (
δ (ppm) = 6.7–7.9 (m, Ar-H), 4.15 (d 1H, cymene Ar‒
1
‒
‒
cymene), 0.8–1.4 (dd, 6H, 2CH of p
‒
3
3
λ
max, (nm): 446, 330, 250.
3
[
Ru(p‒cymene)(Cl)(L )] (3)
3
o
Yield: 60%; brown solid; M.p. 206 C, Anal.Cal. For C H N OClRu: C, 64.10; H, 5.84;
3
2
35
2
−
1
N, 4.67; Found: C, 64.12; H, 5.85; N, 4.61. FT
‒
IR (cm ): 1389
ν
(N=N), 1317
ν(C‒O).
1
H‒
NMR (300 MHz, CDCl
3
):
δ
(ppm) = 6.4–7.9 (m, Ar-H), 4.1 (d 1H, cymene Ar
‒
H), 5.3 (d

ACCEPTED MANUSCRIPT
1
H, cymene Ar‒H), 2.3 (s, 3H, CH
3 3
of p‒cymene), 1.1–1.3 (dd, 6H, 2CH of p‒cymene), 2.9 (m,
(
CH ) C). UV–Vis (
λmax, (nm): 444, 325, 255.
3
3
[Ru(p
‒
cymene)(Cl)(L )] (4)
4
o
Yield: 75%; brown solid; M.p. 142 C, Anal. Cal. For C H N OCl Ru: C, 58.13; H,
2
8
27
2
2
−
1
4
.67; N, 4.84; Found: C, 58.10; H, 4.70; N, 4.82. FT
‒
IR (cm ): 1388
ν
(N=N), 1310 ν ‒O).
(C
H), 2.2
λmax, (nm): 455,
1
H
‒
NMR (300 MHz, CDCl
3
):
δ
(ppm) = 6.6–7.7 (m, Ar-H), 4.15–5.6 (d 1H, cymene Ar
‒
(s, 3H, CH of p cymene), 0.9–1.2 (dd, 6H, 2CH
3
‒
3
of p cymene). UV–Vis (
‒
3
30, 260.
[Ru(p
‒
cymene)(Cl)(L )] (5)
5
o
Yield: 80%; brown solid; M.p. 240 C, Anal. Cal. For C H N OClRu: C, 65.18; H,
3
2
24
2
−
1
4
.07; N, 4.75; Found: C, 65.19; H, 4.09; N, 4.76. FT
‒
IR (cm ): 1384
ν
(N=N), 1323
H), 5.8 (d
cymene).
ν(C‒O).
1
H
‒
NMR (300 MHz, CDCl3):
H, cymene Ar H), 2.5–2.8 (s, 3H, CH of p
UV–Vis ( max, (nm): 448, 330, 250.
.3. Typical procedure for the one-pot conversion of aldehydes to amides
To an oven-dried round-bottom flask equipped with magnetic stirring bar was added
complex (1) (1 mol%), the aldehyde (1 mmol), NH OH.HCl (1 mmol) and NaHCO (1 mmol)
δ
(ppm) = 6.6–7.8 (m, Ar-H), 4.2 (d 1H, cymene Ar
‒
1
‒
‒
cymene), 0.85–1.3 (dd, 6H, 2CH of p
‒
3
3
λ
4
2
3
and the mixture was placed under an atmosphere of N . Dry and degassed MeCN (2 mL) was
2
added and the reaction mixture was refluxed for the time specified under an N atmosphere. The
2
reaction was cooled to room temperature and the solvent evaporated. The residue was dissolved
2 2
in CH Cl , filtered and the solvent removed. The crude product was then purified using silica gel

ACCEPTED MANUSCRIPT
chromatography (CHCl
3
/MeOH) giving the amides in high isolated yields. Characterization
details for each amide are given in the supporting information S16-S20.
4
.4. General procedure for catalytic transfer hydrogenation reaction
A solution of the ketone (3.75mmol), KOH (0.03mmol) and the catalyst complexes 1‒5
o
(
0.0125mmol) was heated under reflux (80 C) in 5 mL of 2-propanol for 2h. The solvent was
removed under vaccum, and an aliquot of the remaining product was extracted with diethylether,
filtered through a short column of silica gel. The column was washed with diethylether. The
filtrate and washings of the column were mixed and evaporated on a rotary evaporator and
analyzed by GC.
4
.5. X-ray crystallography
Single crystals of [Ru(η ‒
6
6
p‒
cymene)(Cl)(L )] (1), [Ru(η ‒
p‒
cymene)(Cl)(L )] (2) and
1
2
6
[
Ru(η ‒
solution at room temperature. Crystals were mounted on a Stoe Mark II-Image Plate
diffractometer using monochromated Mo K radiation ( = 0.71073). Data were collected at
96 K. Structures were solved with direct method using SHELXS [53] and were refined by full-
p‒cymene)(Cl)(L )] (5) were grown by slow evaporation of a chloroform–pentane
5
α
λ
2
2
matrix least-squares method [54] on F with SHELXL. Non hydrogen atoms were refined with
anisotropy thermal parameters.
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

ACCEPTED MANUSCRIPT
1
FT-IR, UV-Vis and H-NMR spectra of representative complexes. CCDC 1868196,
CCDC 1915333 and CCDC 1863103 contain the supplementary crystallographic data for
complex 1, 2 and 5. 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).
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