Synthesis and molecular structure of ruthenium(III) benzoylhydrazone complexes: Substituents effect on transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.jorganchem.2015.04.032

An easy and convenient synthesis of a new series of octahedral ruthenium(III) complexes bearing benzoylhydrazone of general formula [Ru(L)Cl(PPh₃)₂] (where L = 2-hydroxy-1-naphthaldehyde benzoylhydrazone) has been reported. The composition of all the complexes has been unequivocally characterized by microanalysis, IR, electronic, magnetic and EPR spectroscopic techniques. The substituted benzoylhydrazone ligands behave as a dianionic tridentate O, N and O donors and coordinate to ruthenium via the phenolic oxygen, the azomethine nitrogen and the deprotonated amide oxygen. The complexes exhibit moderately strong ligand-to-metal charge transfer transitions in the visible region and intraligand transition in the UV region. Magnetic moment of the complexes (298 K) lies in the range 1.72-1.97 μB reveals the presence of one unpaired electron in the metal centre. The low spin mononuclear Ru(III) benzoylhydrazone complexes display rhombic EPR spectral pattern in frozen solution. The molecular structure of two of the complexes has been established by single crystal X-ray crystallography and indicates the presence of a distorted octahedral geometry in these complexes. Further, the complexes 1-5 have been proven to catalyse the transfer hydrogenation of linear, cyclic and aromatic ketones to their corresponding secondary alcohols in the presence of i-PrOH/KOH at 82 °C and the maximum conversion is up to 99%. The effect of other variables on the transfer hydrogenation reaction such as solvent, base, temperature, time and catalyst loading is also reported.

Full text of DOI:10.1016/j.jorganchem.2015.04.032

Accepted Manuscript
Synthesis and Molecular Structure of Ruthenium(III) Benzoylhydrazone Complexes:
Substituents Effect on Transfer Hydrogenation of Ketones
Appukutti Kanchana Devi, Rengan Ramesh, Nattamai Bhuvanesh
PII:
S0022-328X(15)00244-2
10.1016/j.jorganchem.2015.04.032
JOM 19018
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 January 2015
Revised Date: 10 April 2015
Accepted Date: 15 April 2015
Please cite this article as: A. Kanchana Devi, R. Ramesh, N. Bhuvanesh, Synthesis and
Molecular Structure of Ruthenium(III) Benzoylhydrazone Complexes: Substituents Effect on
Transfer Hydrogenation of Ketones, Journal of Organometallic Chemistry (2015), doi: 10.1016/
j.jorganchem.2015.04.032.
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ACCEPTED MANUSCRIPT
Air stable paramagnetic Ru(III) complex bearing a benzoylhydrazone ligand
incorporating PPh and chloride as co-ligands. The molecular structures of two of the
3
complexes were confirmed by single crystal X-ray diffraction analysis. The catalytic
efficiency of the complex is investigated for different ketonessuch as
aromatic/heterocyclic/cyclic and aliphatic ketones in the presence of base with in 5 h. The
effect of other variables on the transfer hydrogenation reaction, such as solvent, base,
temperature, time, catalyst screening and catalyst loadingon the catalytic activity of the
complexes is also investigated

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Graphical abstract
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2°C
KOH
O
OH
OH
R2
O
R1
R2
R1
Conversion is up to 99%

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ABSTRACT
An easy and convenient synthesis of a new series of octahedral ruthenium(III)
complexes bearing benzoylhydrazone of general formula [Ru(L)Cl(PPh ) ] (where L = 2-
3
2
hydroxy-1-naphthaldehyde benzoylhydrazone) has been reported. The composition of all the
complexes has been unequivocally characterized by microanalysis, IR, electronic, magnetic
and EPR spectroscopic techniques. The substituted benzoylhydrazone ligands behave as a
dianionic tridentate O, N and O donors and coordinate to ruthenium via the phenolic oxygen,
the azomethine nitrogen and the deprotonated amide oxygen. The complexes exhibit
moderately strong ligand-to-metal charge transfer transitions in the visible region and
intraligand transition in the UV region. Magnetic moment of the complexes (298 K) lies in
the range 1.72-1.97 µB reveals the presence of one unpaired electron in the metal center. The
low spin mononuclear Ru(III) benzoylhydrazone complexes display rhombic EPR spectral
pattern in frozen solution. The molecular structure of two of the complexes has been
established by single crystal X-ray crystallography and indicates the presence of a distorted
octahedral geometry in these complexes. Further, the complexes 1-5 have been proven to
catalyse the transfer hydrogenation of linear, cyclic and aromatic ketones to their
corresponding secondary alcohols in the presence of i-PrOH/KOH at 82 °C and the maximum
conversion is up to 99%. The effect of other variables on the transfer hydrogenation reaction
such as solvent, base, temperature, time and catalyst loading is also reported.
Key words: Benzoyl hydrazone, Ru(III) complex, Synthesis, Characterization, X-ray
structure, Transfer hydrogenation.

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Synthesis and Molecular Structure of Ruthenium(III) Benzoylhydrazone
Complexes: Substituents Effect on Transfer Hydrogenation of Ketones
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a,
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Appukutti Kanchana Devi , Rengan Ramesh * and Nattamai Bhuvanesh
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School of Chemistry, Bharathidasan University, Tiruchirappalli-620024, India.
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Department of Chemistry, Texas A&M University, USA
*To whom correspondence should be made: +91-431-2407053, Fax: +91-431-2407045,
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INTRODUCTION
Generally, the coordination chemistry of hydrazone based ligands proved to be very
interesting, because of their excellent complexation ability towards transition metals and the
possibility of analytical applications. Hydrazones, produced from the reaction of hydrazides
with aldehydes or ketones, have very active sites for transition-metal atoms to coordinate
with C=N and C=O groups. The chemical interest arises from the ability of
benzoylhydrazone to adopt various coordination modes, leading to enormous structural
diversity of their complexes. They can easily be modified by variation of the parent
aldehydes or ketones used for their synthesis. These chelating ligands can exhibit amido-
imidol tautomerism (Scheme 1) and coordinate in either neutral [1], monoanionic [2],
dianionic [3-5] or tetraanionic [6] form bearing unusal coordination numbers, such as six and
seven [3, 7] in some mono or binuclear species. However, coordination modes are depended
on the reaction conditions, such as metal ion, its concentration, the pH of the medium and the
nature of the hydrazone ligand [2].The most predominant coordination mode is bidentate,
achieved through hydrazine nitrogen and carbonyl oxygen atoms [8]. When a third donor site
(D) is incorporated into the ligands, D, N, O- tricoordination also takes place. Though several
hydrazone metal complexes are known for their biological activities [9], catalytic applications
of these complexes are not much studied [10, 11].
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O
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Tautomerization
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R
R2
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Scheme 1. Amide and imidol forms of hydrazone
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The reduction of C=O and C=N bonds forming alcohols and amines, respectively, is
among the most fundamental molecular transformations. The development of new and
effective catalytic systems for transfer hydrogenation reaction by simple solvent as the proton
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source like 2-proponal, formaldehyde etc., continues to be the focus of many efforts designed
to develop new technologies for future industrial applications [12-14]. Organic synthesis
needs economically and technically more benign methods that are very general. From an
industrial point of view, catalytic transfer hydrogenation is an alternative source for high-
pressure catalytic hydrogenation with molecular hydrogen [15]. 2-proponal is the
conventional source having favourable properties; it is stable, operational simplicity (b.p. 82
°C), nontoxic, environmentally and economically favoured and then dissolves many organic
compounds [16]. The acetone by-product is readily removable [17]. The most active systems
are based on Ru, Rh and Ir containing nitrogen, oxygen-and/or phosphorous bearing ligands,
which lead to the easy formation of catalytically active intermediate species. Catalytic
hydrogenation using a phosphine in conjugation with an appropriate metal, usually ruthenium
is a versatile method which requires only very low levels of catalyst. Numerous ruthenium(II)
complexes have been reported as catalyst precursors for the transfer hydrogenation of ketones
and have shown high activity [18-23]. The most outstanding results have been obtained by
Noyori’s group who discovered and developed the catalyst [RuCl (diphosphine)(diamine)]
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with >99% yield [24]. W. Du group reported highly active Ru(II) complex catalysts bearing
bis(trifluoromethyl)pyrazolyl−pyridyl-based NNN ligands for the transfer hydrogenation of
ketones with high TOF [25]. As recently demonstrated by Pavel A. Duband co-workers,
exhibited a high activity [RuX (diphosphine)(1,2-diamine)] system showing excellent
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selectivity and reactivity for reduction of acetophenone derivatives [26]. Robert H. Morris
and co-workers described the complexes of the type trans- RuHCl(diphosphinite)(diamine)
and their application in the asymmetric transfer hydrogenation of ketones with 99%
conversion [27].
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Compared with ruthenium(II) analogues, there exist relatively few reports on
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paramagnetic ruthenium(III) complexes containing tertiaryphosphine/arsine and chlorides
and their catalytic applications to transfer hydrogenation [28-30]. Low-spin ruthenium(III)
complexes containing diphenylphosphanyl imidazole (1a) has been proved to be an efficient
pre-catalyst for transfer hydrogenation of ketones without any mechanism [31]. Further,
Zhengkun Yu et al reported Ru(III) complexes bearing a tridentate dimethylpyrazol and its
derivatives (1b) as efficient catalysts for transfer hydrogenation reaction [32]. In addition,
(pyridyl)benzoazole ruthenium(III) complexes (1c) were reported as catalysts for transfer
hydrogenation of ketones by Ojwach et al [33].
Me
nBu
N
N
PPh3
O
Cl
O
CF3
X
N
Ru
P
Ru
P
Cl
Cl
Cl
Cl
N
PPh3
PF6
N
N
N
N
Cl
N
H
N
N
Ru
Cl
Ru
Cl
Cl
Cl
CF3
Cl
N
N
Me
nBu
Cl
1
c
1
a
1b
1d
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In view of the growing interest on the catalytic activities of ruthenium complexes in
the transfer hydrogenation of ketones, in the present report, we have described the synthesis
and characterization of a series of mononuclear Ru(III) complexes with the tridentate 2-
hydroxy-1-napthaldehyde benzoylhydrazone along with triphenylphosphine and chloride as
ancillary ligands (1d). The composition of the complexes was accomplished by analytical and
spectral methods. The molecular structure of the two of the complexes 1 and 3 has been
determined by X-ray crystallography. Further, the catalytic efficiency of the complexes has
been explored for the transfer hydrogenation of a wide range of ketones.
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RESULTS AND DISCUSSION
Synthesis of the Complexes
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A new series of mononuclear ruthenium(III) benzoylhydrazone complexes of the
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type [Ru(L)(Cl)(PPh ) ] was achieved by reacting ruthenium(III) precursors [RuCl (PPh ) ]
3
2
3
3 3
with the tridentate benzoylhydrazone ligand (L -L ) in 1:1 molar ratio in the presence of
1
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catalytic quantity of triethylamine. The addition of triethylamine to the reaction mixture was
used to abstract a proton from the imidol oxygen and to facilitate the coordination of the
imidolate oxygen to the ruthenium(III) ion. The synthesized ruthenium(III) benzoylhydrazone
complexes are stable in air at room temperature, non-hygroscopic in nature and highly
soluble in common organic solvents such as dichloromethane, chloroform, acetonitrile, THF,
etc., producing intense colour in their solution.
O
N
PPh3 Cl
Ru
OH
N
O
O
PPh3
Benzene
H
HN
[RuCl3(PPh3)3]
H
N
Et3N / 3h reflux
R
1: R = -H
2
3
4
: R = -OH
: R = -OCH3
: R = -Cl
R
1
1
08
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5: R = -Br
Scheme 2. Synthesis of ruthenium(III) complexes
Characterization of the Complexes
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The free ligands showed a strong band in the regions 3370-3470 cm and 3107-3222
-1
cm which are characteristic of the -OH and –NH functional groups respectively. The free
-1
ligands also display ʋC=N and ʋC=O absorptions in the region 1592-1697 cm . The
disappearance of strong band due to ʋO-H in all the complexes indicates the deprotonation of
phenolic oxygen prior to coordination to the Ru(III) ion. The bands due to ʋN-H and ʋC=O
stretching vibrations are also not observed in the complexes indicating that the ligand
undergo tautomerization and subsequent coordination of the imidazole oxygen to the Ru(III)
-1
ion. This is also supported by the appearance of new bands in the region 1197-1252 cm and
-1
1564-1667 cm which may be attributed to the C-O and -C=N-N=C- group respectively of
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the coordinated ligand [34]. A sharp band near 330 cm has been observed for all the
complexes due to ʋRu-Cl stretching frequency. In addition, other characteristic bands due to
-1
ruthenium bound triphenylphosphine in the region 1472-1556 cm are also present in the
spectra of all the complexes [35]. The IR spectral data of all the complexes confirm the
coordination of the benzoylhydrazone ligand to ruthenium(III) ion via phenolate oxygen, the
azomethine nitrogen and the imidolate oxygen.
The UV-Vis spectra of all the Ru(III) complexes were obtained in CH Cl in the 200-
2
2
800 nm range at ambient temperature. All the new complexes display three absorption bands
in the ultraviolet and visible region. The moderately intense absorption band in the region
471–467 nm is probably due to charge transfer transitions taking place from the highest
*
occupied ruthenium t g orbital (HOMO) to the vacant π orbital of the ligand (LUMO).
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Another intense absorption in the higher energy region (273–226 nm) may be attributed to
*
*
usual n → π , π → π transitions occurring within the ligand orbitals. The pattern of the
electronic spectra of all the complexes indicate the presence of an octahedral environment
around ruthenium(III) ion similar to that of other reported ruthenium(III) octahedral
complexes [36].
The room temperature magnetic moment of the ruthenium(III) complexes (1-5) in the
powder state are in the range 1.72-1.97 μ . The value indicates the presence of one unpaired
B
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electron, confirming a low spin t2g configuration and a +3 oxidation state for ruthenium in all
the complexes [37, 38]. The electron paramagnetic resonance spectral studies are performed
both in pure powder samples and in solution at room temperature and liquid nitrogen
temperature respectively at X-band frequency. All the complexes exhibit well defined single
isotropic feature near g = 2.02 to 2.23 at room temperature. However, the EPR spectral
profiles of the complexes in dichloromethane solution at 77K show rhombic spectra with
three different ‘g’ values (g ≠ g ≠ g ) (Fig. S8, Supporting Information). Each complexes
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display three distinct signals indicating the distortion of the NO ClP coordination sphere
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around the metal centre from octahedral geometry. Overall the position of lines and nature of
the EPR spectra of the complexes are characteristic of low spin ruthenium(III) octahedral
complexes [39, 40].
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X-ray Crystallographic Studies.The molecular structure of the complexes 1 and 3 has
been determined by single crystal X-ray crystallography in order to determine the solid state
structural parameters. The ORTEP diagram of 1 and 3 are illustrated in Figs. 1 and 2
respectively. The summary of crystal data, data collection and refinement parameters are
given in Table 1. The bond parameters associated with the metal centre are listed in Table 2.
The benzoylhydrazone ligand coordinates meridionally to the ruthenium(III) ion in a
tridentate manner via the phenolate oxygen, the azomethine nitrogen and the deprotonated
amide oxygen in the benzoylhydrazone fragment forming one five membered and one six
membered chelate rings. The complex (1) crystallizes in the ‘P12 /n1' space group. The
1
coordinated benzoylhydrazone ligand and chloride ligand constitute one equatorial plane with
the metal at the centre. Further, the chloride ligand is trans to the azomethine nitrogen. The
two triphenylphosphine ligands are mutually trans to each other. Ruthenium(III) ion is
therefore sitting in a NO ClP coordination environment and adopts a distorted octahedral
2
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geometry as reflect in all the bond parameters around ruthenium(III) ion. The bond angles
around ruthenium(III) ion are O(1)-Ru(1)-P(1) = 91.43(5)°, O(2)-Ru(1)-P(1)=93.70°, O(1)-
Ru(1)-P(2) = 87.87(5)°, O(2)-Ru(1)-P(2) =87.56(4)°, N(1)-Ru(1)-Cl(1)= 171.40(5)° and bond
lengths of 1.9906(15) Å for Ru(1)-O(1), 2.0382(14)Å for Ru(1)-O(2), 2.0011(18)Å for
Ru(1)-N(1), 2.3686(7)Å for Ru(1)-Cl(1), 2.4021(7)Å for Ru(1)-P(1), 2.4071(7)Å for Ru(1)-
P(2) [41]. Further, the structure of the complex (3) adopts similar structure as in the complex
(1) with slight changes in bond angles and bond distances. As all the complexes display
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similar spectral properties, the other three complexes are assumed to have similar structure to
that of complexes (1) and (3). The X-ray determinations confirm the structure proposed on
the basis of spectroscopic data, consistent with metal trivalency and the diionized nature of
the ligands.
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Fig. 1. ORTEP diagram of the complex (1), hydrogen atoms have been omitted for clarity
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Fig. 2. ORTEP diagram of the complex (3), hydrogen atoms have been omitted for clarity.
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Table 1
Crystal data and structure refinement for complexes (1) and (3)
Complex 1
Complex 3
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C H ClN O P Ru
C H ClN O P Ru
54
42
2
2
2
55 44
979.38
150.15
0.71073
2
3 2
949.35
150.15
0.71073
Monoclinic
Monoclinic
P1211
11.668(2)
16.610(3)
12.540(2)
90
P12 /n1
1
12.493(3)
18.761(4)
19.725(4)
90
β (°)
γ (°)
Crystal size(mm )
Volume (Å )
103.211(2)
90
0.21x0.2x0.05
4500.9(16)
4
111.081(2)
90
0.32x0.18x0.1
2267.8(8)
2
3
3
Z
3
Dcalcd (Mg/m )
Absorption coefficient
F(000)
1.401
0.523 (mm )
1948
1.434
0.523 (mm )
-
1
-1
1006
Theta range for data collection (°) 1.517 to 27.461
1.740 to 27.558
-14<=h<=14,
-21<=k<=21,
-14<=l<=16
99.7%
Limiting indices
-16<=h<=16,
-
-
24<=k<=24,
25<=l<=25
Completeness to theta=25.242°
Absorption correction
99.9%
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Reflection collected
Refinement method
42033
20443
Full-matrix least-squares
Full-matrix least-squares on
2
2
on F
F
Data/restraints/parameters
Goodness-of-fit on F
10218 / 0 / 559
1.064
10110 / 1 / 579
1.065
2
Final R indices [I >2σ(I)]
R = 0.0345,
1
R = 0.0351,
1
wR = 0.0755
wR = 0.0708
2
2
R indices (all data)
R = 0.0485,
1
R = 0.0465,
1
wR = 0.0833
wR = 0.0774
2
2
-3
-3
Largest diff. peak & hole
0
.469 and -0.534 e. Å
0.670 and -0.413 e. Å
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Table 2
Selected bond lengths (Å) and angles (°) for the complexes (1) & (3)
1
1
97
98
Complex
Ru(1)-Cl(1)
1
3
2.3686(7)
2.4021(7)
2.4071(7)
1.9906(15)
2.0382(14)
2.0011(18)
1.328(3)
1.299(2)
1.403(2)
1.298(3)
1.306(3)
88.47(2)
89.08(2)
91.43(5)
87.87(5)
93.70(5)
87.56(4)
77.89(6)
171.40(5)
89.15(5)
93.43(5)
2.3567(13)
2.4088(11)
2.3990(11)
1.996(3)
2.026(3)
1.991(4)
1.324(5)
1.291(5)
1.398(5)
1.295(6)
1.311(6)
90.56(5)
91.34(5)
88.90(9)
90.71(9)
89.37(9)
90.61(9)
78.39(13)
170.91(12)
89.99(11)
88.15(11)
Ru(1)-P(1)
Ru(1)-P(2)
Ru(1)-O(1)
Ru(1)-O(2)
Ru(1)-N(1)
O(1)-C(3)
O(2)-C(12)
N(1)-N(2)
N(1)-C(1)
N(2)-C(12)
Cl(1)-Ru(1)-P(1)
Cl(1)-Ru(1)-P(2)
O(1)-Ru(1)-P(1)
O(1)-Ru(1)-P(2)
O(2)-Ru(1)-P(1)
O(2)-Ru(1)-P(2)
N(1)-Ru(1)-O(2)
N(1)-Ru(1)-Cl(1)
N(1)-Ru(1)-P(1)
N(1)-Ru(1)-P(2)
1
2
2
2
2
2
99
00
01
02
03
04
Catalytic transfer hydrogenation of ketones
The transfer hydrogenation reactions involving transfer of hydrogen from one
organic substrate to another are of great importance in organic synthesis, since one can avoid
the use of molecular hydrogen. We have performed the transfer hydrogenation of various
ketones using all Ru(III) hydrazone complexes as catalysts. The necessity of the ruthenium
complex to observe the ensuing transfer hydrogenation was ascertained by carrying out a
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series of blank or control experiments which suggest that none of RuCl .3H O or
3
2
benzoylhydrazone ligand or as a mixture causes the transformations under identical
conditions. In a typical experiment, acetophenone (0.2 mmol), ruthenium(III) complex (1 mol
%) and KOH (0.08 mmol) in 6 ml of isopropanol were taken into the round bottom flask and
are heated to reflux for 5 h at 82 °C. The catalyst was removed from the reaction mixture by
the addition of diethyl ether followed by filtration and subsequent neutralization with 1 M
HCl. The organic layer was filtered through the short path of silica gel by column
chromatography and is subjected to GC analysis. The catalyst performed efficiently in the
conversion of acetophenone to 1-phenyl ethanol. In order to optimize the reaction conditions
for better results, the effect of different solvents, bases, temperature, time and catalyst:
substrate (C:S) ratio was studied.
To study the influence of solvents in our catalytic system, we have chosen the
reaction between acetophenone (0.2 mmol), complex 4 (1 mol %) as the catalyst precursor in
the presence of various solvents and KOH as the base and the results are listed in Table 3.
Methanol, ethanol and isopropanol solvents were taken for investigation and isopropanol is
found to be a suitable system for the maximum conversion (99%) of acetophenone to 1-
phenylethanol. On the other hand ethanol solvent gave less conversion (47%) than
isopropanol. The lowest conversion (12%) was obtained with methanol solvent. Based on this
conclusion 2-propanol was taken as a proton source for our investigation.
2
2
24
25
Table 3
a
Effect of the solvent and base
Catalyst 4
OH
O
1
mol%
Solvent
Base, 82°C
5h, reflux
2
2
26
27
1
1

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b
Entry
Solvent
Base
KOH
KOH
KOH
---
Conversion (%)
1
2
3
4
5
6
7
8
9
Methanol
47
12
100
0
Ethanol
Iso-propanol
Iso-propanol
Iso-propanol
Iso-propanol
Iso-propanol
Iso-propanol
Iso-propanol
Iso-propanol
Iso-propanol
c
KOH
NaOH
NaHCO3
0
99
51
Na CO3
68
2
K CO3
2
72
1
1
0
1
Et N
Trace
Trace
3
Pyridine
a
2
2
2
2
28
29
30
31
Experimental conditions: reactions were carried out using acetophenone (0.2 mmol), catalyst (1 mol
%), base (0.08 mmol), iso-propanol (6 ml) at 82 °C for 5h.
b
Conversion was monitored by GC analysis and are average of two runs.
C
Reaction carried out in the absence of catalyst.
2
2
32
33
In order to improve the catalytic transfer hydrogenation, we examined the influence of
different bases. We initially carried out the reaction of acetophenone (0.2 mmol), using
complex 4 (1 mol %) as a test catalyst in the presence of isopropanol with different bases.
The strong inorganic bases like NaOH or KOH gave higher conversion of 99% and 100%
respectively. Whereas, weak inorganic bases such as Na CO and K CO shows moderate
2
2
2
2
2
2
2
2
34
35
36
37
38
39
40
41
2
3
2
3
conversion of 79% and 82% respectively. Organic bases such as triethylamine, pyridine we
observed only traces amount of alcohol was formed. The conversions are summarized in
Table 3. The results indicate that the conversion was strongly dependent upon the base
strength with the stronger bases giving the higher conversion.
2
2
2
42
43
44
Acetophenone has been chosen as a model substrate to explore the catalytic activity of
the complexes 1-5 under the optimized conditions. The reduction of acetophenone (0.2
mmol) to 1-phenyl ethanol by 2-propanol in the presence of KOH base has been to
1
2

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inspect the catalytic behaviour of the complexes 1-5 (1 mol %) in transfer hydrogenation.
All the complexes efficiently catalyze the transfer hydrogenation of acetophenone with
maximum conversion within 5 h. Among the tested complexes, complex 4 is highly
efficient in the transfer hydrogenation of ketones to alcohol with a high conversion of
100%. The result of transformations is given in Table 4. So, we decided to further explore
its catalytic activity in the reduction of ketones. Complexes containing chloro and bromo
substituents show effective conversions 100% and 98% respectively. The introduction of
electron withdrawing groups such as -Cl or -Br on the complexes shows high conversion
in comparison with electron releasing group. It was observed that the presence of electron
withdrawing group on the aromatic ring possibly decreased the electron density on the
metal center and hence the rate of transfer hydrogenation increases. The lowest
conversion observed is 79% for complex 2. The catalytic process is more efficient with
the complex 4 followed by 5, 1, 3 and complex 2 showing the least activity among all five
complexes.
2
59
Table 4
Transfer hydrogenation of acetophenone to 1-phenylethanol catalyzed by complexes (1-5)
a
2
2
60
61
Catalyst (1-5)
mol%
OH
O
1
Solvent
KOH, 82°C
h, reflux
5
2
2
62
63
b
Entry
Complex
Conversion (%)
1
2
3
4
5
1
2
3
4
5
85
79
82
100
98
a
2
2
2
64
65
66
Experimental conditions: reactions were carried out at 82 °C using acetophenone (0.2 mmol),
catalyst (1 mol %), base (0.08 mmol) and iso-propanol (6 mL).
b
Conversion was monitored by GC analysis and are average of two runs.
2
2
67
68
Low catalyst loading tests were competent to discover the effectiveness of the
catalyst. The designing of a new catalyst in a catalytic system is either to enhance the rate of
1
3

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2
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2
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2
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76
77
78
79
80
81
82
regeneration, to slow down the rate of decomposition of catalyst or to decreases the quantity
of catalyst used. Hence, the ability to use small amounts of catalyst and still achieve high
conversions is a great concern in transfer hydrogenation reactions due to the high cost of
metals and ligands used. In the course of our studies into developing efficient formation of
alcohol, we became interested to find the varying the catalyst/substrate ratio. In order to
optimize the effect of catalyst loading, different catalyst: substrate (C:S) ratios were tested in
the transfer hydrogenation reaction of acetophenone using complex 4 as catalyst in i-
PrOH/KOH. The reaction proceeds with high conversion (95%) when the catalyst loading is
0.2 mol %. When decreasing the catalyst loading to 0.15, 0.1 mol %, the reaction proceeds
with good conversions 76% and 70% respectively. Further, the catalyst works well with low
loading of catalyst 0.06 mol % and shows conversion of 60%. Thus it was concluded that the
catalyst loading of 0.2 mol % of Ru catalyst is the best compromise between optimum
reaction rate. The results are collected in Table 5 and the conversions reported are averages of
two runs in the case of all catalytic reaction.
2
2
83
84
Table 5
a
The effect of low loading catalyst on transfer hydrogenation reaction
2
2
85
86
OH
O
Catalyst 4
Solvent
2
2
87
88
KOH, 82°C
5h, reflux
b
c
Entry
Mol % Ru
1.0
Conversion (%)
TON
1
2
3
4
5
6
100
97
95
76
70
60
99
0.3
290
477
533
705
898
0.2
0.15
0.1
0.06
1
4

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a
2
2
2
2
89
90
91
92
Experimental conditions: reactions were carried out at 82 °C using acetophenone (0.2 mmol),
catalyst (0.06 –1 mol %), base (0.08 mmol), iso-propanol (6 ml).
b
Conversion was monitored by GC analysis and are average of two runs.
C
TON = Turnover number = ratio of moles of product formed to moles of catalyst used.
2
2
93
94
The progress of formation of 1-phenylethanol as a function of time using the above
optimized conditions is depicted in Fig. 3. The results indicate the formation of 1-
phenylethanol initially increased with the progress of the reaction, reached a maximum and
then remain unchanged. A high conversion (95%) for the formation of 1-phenylethanol was
observed at the optimum reaction time of 5 h. No noticeable improvement was observed even
after extending the reaction time to 7 h.
2
2
2
2
2
3
3
3
3
3
95
96
97
98
99
00
01
02
03
04
The results obtained from the optimization studies indicate clearly that excellent
conversion was achieved in the reduction of acetophenone to 1-phenylethanol when 4 was
used as the catalytic precursor with 0.2 mol % catalyst in 2-propanol, containing KOH as a
base, refluxed for 5 h at 82 °C.
3
05
3
3
3
06
07
08
Fig. 3. Influence of reaction time on the formation of 1-phenylethanol
To examine the generality of complex 4 as a catalyst for transfer hydrogenation, the
3
09
scope of ketones with different electronic effect was investigated under similar conditions
1
5

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11
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17
18
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29
30
31
32
33
34
and several representative results are summarized in Table 6. The complex exhibits
impressive efficiency towards the transfer hydrogenation of substituted ketones.
Acetophenone with substituents of varying electronic properties were efficiently reduced to
the corresponding secondary alcohols in excellent conversions. The conversion of
acetophenone to 1-phenyl ethanol is obtained in 95% conversion (entry 1). The presence of
electron-withdrawing or electron-donating substituents on the aromatic ring has significant
effect in the conversion. Aromatic halo substituents have an enhancing effect on the catalytic
activity, showing quantitative results on the formation of the corresponding alcohols (entries
2, 3 and 4) and catalyzed with excellent conversions of 97%, 98% and 99% respectively
when compared to that of acetophenone. Whereas, electron donating substituents (4’-methyl,
4’-methoxy and 4’- hydroxyl acetophenone) on the ring gave lower conversions to the
corresponding alcohols are 91%, 89% and 75% (entries 5, 6 and 7) respectively than that of
acetophenone. The introduction of electron withdrawing substituents to the para position of
the aryl ring of the acetophenone decreased the electron density on the C=O bond. So, that
the activity was improved giving rise to easier hydrogenation. Thus electron-withdrawing
groups on acetophenone benefit the catalytic activity while electron donating group decrease
it. Order of the reactivity of substituted acetophenone is NO > Br > Cl > H > Me > OMe >
2
OH. Further, sterically hindered ketone benzophenone (entry 8) underwent hydrogenation to
give the corresponding secondary alcohol in moderate conversion of 84%. The scope the
present catalyst is further explored to heterocyclic ketones and are less common.
Interestingly, heterocyclic aromatic ketone such as 2-acetylthiophene and 2-acetylpyridine
(entries 9 and 10) shows the conversion of 82% and 93% respectively. Moreover this catalyst
shows efficient activity for the transfer hydrogenation of five to seven membered cyclic
ketones i.e., cyclopentanol results 89%, cyclohexanol 78% and cyclohepatanol 66%
conversion (entries 11, 12 and 13). The complex efficiently catalyse the reduction of aliphatic
1
6

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3
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35
36
37
ketones such as ethyl methyl ketone, methyl propyl ketone and isobutyl methyl ketone to
their corresponding alcohols with 95%, 88% and 82% respectively (Figs. S28-S43,
Supporting Information).
3
3
38
39
Table 6
a
Transfer hydrogenation of various ketones with complex (4)
Catalyst 4
O
KOH, 82°C
OH
O
OH
CH3
R1
R2
R1
R2
H C
3
CH3
H C
3
5 h, reflux
3
40
b
c
Entry
1
Substrate
Product
Conversion (%)
TON
O
OH
9
5
477
O
O
OH
OH
OH
9
7
485
2
3
4
Cl
Br
Cl
Br
98
99
492
496
O
O N
2
O N
2
O
OH
OH
5
91
455
447
H C
3
H C
3
O
8
9
H CO
3
H CO
3
6
7
O
OH
75
378
HO
HO
1
7

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N
OH
8
9
84
93
82
89
78
66
422
463
412
447
390
330
474
O
OH
N
S
O
S
OH
1
0
1
2
O
OH
OH
1
1
O
O
HO
1
1
3
4
O
OH
9
8
5
8
O
OH
1
1
5
6
439
413
O
OH
82
a
3
3
3
3
41
42
43
44
Experimental conditions: reactions were carried out at 82 °C using ketone (0.2 mmol), catalyst (0.2
mol%), base (0.08 mmol), iso-propanol (6 ml).
b
Conversion was monitored by GC analysis and are average of two runs.
C
TON = Turnover number = ratio of moles of product formed to moles of catalyst used.
3
3
45
46
It is well known from the literature that the mechanism of transfer hydrogenation of
ketones by ruthenium complexes has been proposed to involve the formation of
3
47
1
8

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49
50
51
52
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54
ruthenium(II)-hydride as catalytic intermediate [42] and the mechanism involving Ru(III)
complexes is not known. However, the present Ru(III) complexes show excellent catalytic
performance with high conversions and the reasons are not clear. Further, it is believed that
the transfer hydrogenation may be associated with the reduction of Ru(III) to Ru(II) [33]. We
are currently investigating the scope and mechanism of these catalysts. The catalytic activity
of the present complexes is more efficient than the reported octahedral ruthenium complexes
in terms of conversion, reaction time and catalyst loading [43, 44].
3
3
3
3
3
3
3
3
3
3
3
3
3
55
56
57
58
59
60
61
62
63
64
65
66
67
Conclusions
The present work reports the synthesis of five ruthenium(III) complexes containing
benzoylhydrazone ligands. Synthesized complexes were characterized by various analytical
and spectroscopic techniques. All the complexes are one electron paramagnetic and display
5
rhombic EPR spectra typical of d low-spin metal centre. The molecular structure two of the
complexes has been confirmed by X-ray structure determination which confirms the O,N,O
coordination mode of hydrazones and reveals the presence of a distorted octahedral
geometry around ruthenium ion. Further, all the Ru(III) benzoylhydrazone complexes were
developed as effective and modular catalytic system for the transfer hydrogenation of a
range of ketones. In addition, the influence of the substituent of the ligands and the substrates
was studied and overall the catalytic rate increases with electron releasing substituents. The
reported complexes work well with the low catalyst loading of 0.06 mol % and further studies
on the mechanism of this interesting catalytic system are under progress.
3
3
3
68
69
70
EXPERIMENTAL SECTION
Materials
RuCl .3H O was purchased from Loba-Chemie Pvt. Ltd., and was used as supplied.
3
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76
Triphenylphosphine, 2-hydroxy-1-naphthaldehyde, benzhydrazide derivatives and the
ketones used for catalysis were purchased from Sigma-Aldrich and were used as received. All
the reagents used were chemically pure and analar grade. The solvents were freshly distilled
prior to use following the standard procedures [45] and degassed before use. The precursor
ruthenium(III) complex [RuCl (PPh ) ] [46] was prepared by reported literature method. The
3
3 3
substituted benzoyl hydrazone ligands were prepared by a reported literature method [47].
3
77
Physical measurements
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
Melting points were recorded in open capillaries with a Boetius micro-heating table
and are uncorrected. The analysis of carbon, hydrogen and nitrogen were performed by
analytical function testing Vario EL III CHNS elemental analyser at STIC, Cochin University
of Science and Technology, Cochin, India. FT-IR spectra were recorded in KBr pellets with a
-1
JASCO 400 plus spectrometer in the range 4000-400 cm . The electronic spectra were
recorded, in dichloromethane, on a Cary 300 Bio UV–Vis Varian spectrophotometer in the
6
range of 800-200 nm. The NMR spectra were recorded in DMSO-d for the ligands with a
Bruker 400 MHz instrument using TMS as the internal reference. EPR spectra of the
powdered samples were recorded on a JEOL JES-FA200 EPR spectrometer instrument in X-
band frequencies at RT and LNT, the field being calibrated with 2, 2-diphenyl-1-
picrylhydrazyl. Diamagnetic corrections were estimated from Pascal constants [48] were used
to obtain the molar paramagnetic susceptibilities. Organic compounds in catalysis were
identified by gas chromatography (GC) using Bruker 436-GC using GC-FID detector
equipped with a capillary column (15 m - 0.25 mm - 0.25 mm) and high purity nitrogen as
carrier gas.
3
3
93
94
Preparation of benzoylhydrazone ligands
To a stirred ethanolic solution (10 mL) of 4-substituted benzhydrazides (0.86g,
2
0

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95
96
97
98
99
00
5mmol), a solution of 2-hydroxy-1-naphtahldehyde (0.68-1 g, 5mmol) in ethanol (10 mL)
was added dropwise. The reaction mixture was refluxed for 6 h, the solution concentrated to
5mL and cooled to room temperature. The solid formed was filtered, washed with cold
methanol (5 mL) and dried in vaccum. The structures of the ligands were elucidated by
1
1
13
elemental analysis, IR and H NMR methods. The H and C NMR spectra for the ligands
are exactly as would be expected (Figs. S3-S7, Supporting Information).
4
01
Synthesis of ruthenium(III) benzoylhydrazone complexes
4
4
4
4
4
4
4
4
4
4
4
4
4
4
02
03
04
05
06
07
08
09
10
11
12
13
14
15
All the [Ru(L)(Cl)(PPh ) ] complexes have been synthesized by a general procedure.
3
2
Therefore the specific details are given below for a particular case. Solid [RuCl (PPh ) ] (50
3
3 3
mg, 0.05 mmol) was added to a benzene solution of L1-L5 (14.5-18.6 mg, 0.05 mmol) and
N(C H ) (0.5 mL) as the base. The reaction mixture was refluxed for 3 h, when a brown
2
5 3
colour solid was obtained. The reaction progress was monitored by using TLC. The brown
colour solid precipitated was collected by filtration, washed with hexane and finally dried in
air. Scheme 2 provides an overview of the complexes described in this work. The micro
analytical data for the complexes are satisfactory with the proposed molecular formula.
Analytical and spectral data for the complexes
[Ru(L1)(Cl)(PPh ) ] (1): Colour: Brown; Yield: 83%; M.p.: 208-210 °C (with
3
2
-1
decomposition); Anal. Calc. for C H ClN O P Ru (949.35 g mol ): C, 68.22%; H, 4.33%;
5
4
42
2
2 2
-1
N, 2.84%. Found: C, 68.31%; H, 4.46%; N, 2.95%. IR (KBr, cm ): 1197 ʋ(C-O), 1583 ʋ(-C=N-
3
-1
-1
). UV-Vis (CH Cl λ /nm; ɛ/dm mol cm ): 469(720), 270(5743), 227(8546).
N=C-
2
2,
max
Magnetic susceptibility (μ ): 1.72BM. ESR (g , g , g ) = 2.39, 2.22, 1.76.
eff
x
y
z
4
4
4
16
17
18
[Ru(L2)(Cl)(PPh ) ] (2): Colour: Brown; Yield: 81%; M.p.: 231-233 °C (with
3
2
-1
decomposition); Anal. Calc. for C H ClN O P Ru (965.14 g mol ): C, 67.26%; H, 4.46%;
5
4
42
2
3 2
-1
N, 2.81%. Found: C, 67.18%; H, 4.39%; N, 2.90%. IR (KBr, cm ): 1243 ʋ(C-O), 1564 ʋ(-C=N-
2
1

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3
-1
-1
4
4
19
20
). UV-Vis (CH Cl λ /nm; ɛ/dm mol cm ): 471(581), 273(4448), 226(6374).
N=C- 2 2, max
Magnetic susceptibility (μ ): 1.85BM. ESR (g , g , g ) = 2.59, 2.34, 1.74.
eff
x
y
z
4
4
4
4
4
21
22
23
24
25
[Ru(L3)(Cl)(PPh ) ] (3): Colour: Brown; Yield: 77%; M.p.: 245-247 °C (with
3
2
-1
decomposition); Anal. Calc. for C H ClN O P Ru (979.15 g mol ): C, 67.32%; H, 4.41%;
5
5
44
2
3 2
-1
N, 2.72%. Found: C, 67.45%; H, 4.53; N, 2.86%. IR (KBr, cm ): 1212 ʋ(C-O), 1597 ʋ(-C=N-N=C-
3
-1
-1
. UV-Vis (CH Cl λ /nm; ɛ/dm mol cm ): 467(558), 271(4492), 227(8953). Magnetic
)
2
2, max
susceptibility (μ ): 1.78BM. ESR (g , g , g ) = 2.40, 2.11, 1.80.
eff
x
y
z
4
4
4
4
4
26
27
28
29
30
Ru(L4)(Cl)(PPh ) ] (4): Colour: Brown; Yield: 86%; M.p.: 212-214 °C (with
3
2
-1
decomposition); Anal. Calc. for C H Cl N O P Ru (983.10 g mol ): C, 65.86%; H, 4.12%;
5
4
41
2
2
2 2
-1
N, 2.93%. Found: C, 65.92%; H, 4.20; N, 2.85%. IR (KBr, cm ): 1252 ʋ(C-O), 1634 ʋ(-C=N-N=C-
3
-1
-1
. UV-Vis (CH Cl λ /nm; ɛ/dm mol cm ): 471(722), 273(4325), 228(6189). Magnetic
)
2
2, max
susceptibility (μ ): 1.97BM. ESR (g , g , g ) = 2.36, 2.09, 1.75.
eff
x
y
z
4
4
4
4
4
31
32
33
34
35
Ru(L5)(Cl)(PPh ) ] (5): Colour: Brown; Yield: 89%; M.p.: 219-221 °C (with
3
2
-1
decomposition); Anal. Calc. for C H BrClN O P Ru (1027.05 g mol ): C, 63.16%; H,
5
4
41
2
2 2
-1
3.95%; N, 2.65%. Found: C, 63.07%; H, 4.02; N, 2.72%. IR (KBr, cm ): 1230 ʋ(C-O), 1667 ʋ(-
3
-1
-1
. UV-Vis (CH Cl λ /nm; ɛ/dm mol cm ): 471(603), 272(4460), 229(6292).
C=N-N=C-)
2
2, max
Magnetic susceptibility (μ ): 1.89 BM. ESR (g , g , g ) = 2.42, 2.22, 1.93.
eff
x
y
z
4
36
X-ray crystallography
4
4
4
4
4
37
38
39
40
41
Single crystals of complexes 1 and 3 were grown by slow evaporation of their
solution in dichloromethane-petroleum ether mixture. In each case, A BRUKER APEX 2 X-
ray (three-circle) diffractometer was employed for crystal screening, unit cell determination,
and data collection. The detector was set at 6.0 cm from the crystal sample (APEX2, 512x512
pixel). The X-ray radiation employed was generated from a Mo sealed X-ray tube (K =

2
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0.70173Å with a potential of 40 kV and a current of 40 mA) fitted with a graphite
monochromator in the parallel mode (175 mm collimator with 0.5 mm pinholes). Sixty data
frames were taken at widths of 0.5. These reflections were used in the auto-indexing
procedure to determine the unit cell. A suitable cell was found and refined by nonlinear least
squares and Bravais lattice procedures. Integrated intensity information for each reflection
was obtained by reduction of the data frames with the program [49]. The integration method
employed a three dimensional profiling algorithm and all data were corrected for Lorentz and
polarization factors, as well as for crystal decay effects. The absorption correction program
SADABS [50] was employed to correct the data for absorption. Hydrogen atoms were placed
in idealized positions and were set riding on the respective parent atoms. All non-hydrogen
atoms were refined with anisotropic thermal parameters. Absence of additional symmetry and
voids were confirmed using PLATON. The structure was refined (weighted least squares
2
refinement, XL [51] on F ) to convergence [52]. Olex2 was employed for the final data
presentation and structure plots.
4
56
Typical procedure for transfer hydrogenation of ketones
4
4
4
4
4
4
4
4
57
58
59
60
61
62
63
64
The mixture of a ketone (0.2 mmol) and base (0.08 mmol) containing the catalyst
(0.002 mmol) in i-PrOH (6ml) was stirred at 82 °C. After the reaction was complete, diethyl
ether could be added to the mixture and extract the ruthenium complexes followed by
filtration and neutralized with 1 N HCl, washed with water and dried over anhydrous Na SO .
2
4
Conversion obtained is related to the residual unreacted ketone. The alcohol products were
identified by comparison with the authentic samples. Acetone was identified as only by-
product in all the cases. As the catalyst is stable in all organic solvents and it can be
recovered and the work up process is also very simple for this catalytic system.
4
65
ACKNOWLEDGEMENTS
2
3

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One of the authors (A. K.) thank University Grants Commission (UGC), New Delhi,
for the award of UGC-BSR RFSMS. We are thankful to the SERB-DST New Delhi for GC
instrument facility through research project.
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Appendix A. Supplementary material
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H and C-NMR spectra of ligands, UV-Vis and EPR spectra of representative
complexes. CCDC 1023482, CCDC 1023481 contains the supplementary crystallographic
data for complex 1 and 3. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. GC analysis data of
reduced products.
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