Ruthenium(II) half-sandwich complexes containing thioamides: Synthesis, structures and catalytic transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.jorganchem.2012.10.003

A new family of cationic half-sandwich complexes of the type [(η⁶-cymene)Ru(PPh₃)(L)]⁺ (L = bidentate monoanionic thioamide) have been synthesized and isolated as their tetraphenylborate salts. All the synthesized ruthenium(II) arene complexes are air stable and are fully characterized by elemental analysis, spectral and X-ray diffraction methods. In chloroform solution all the complexes exhibit characteristic metal to ligand charge transfer (MLCT) absorptions and ligand based transitions. Molecular structure of the complexes 2, 3 and 4 has been determined by single crystal X-ray crystallography indicates that the thioamide ligands are coordinated to ruthenium as a bidentate O, S donor and a typical piano stool geometry was observed around ruthenium(II) metal center. Complexes 1-5 were tested as catalysts in the transfer hydrogenation of aliphatic and aromatic ketones to secondary alcohols in the presence of 2-propanol/KOH. Further, the influence of base, reaction temperature and catalyst loading in this reaction was also evaluated to find out the most active catalyst.

Full text of DOI:10.1016/j.jorganchem.2012.10.003

Journal of Organometallic Chemistry 723 (2013) 26e35
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ruthenium(II) half-sandwich complexes containing thioamides: Synthesis,
structures and catalytic transfer hydrogenation of ketones
*
Devaraj Pandiarajan, Rengan Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli, Tamil Nadu 620 024, India
a r t i c l e i n f o
a b s t r a c t
6-cymene)Ru(PPh3)(L)]þ (L ¼ bidentate
Article history:
A new family of cationic half-sandwich complexes of the type [(
h
Received 10 August 2012
Received in revised form
28 September 2012
monoanionic thioamide) have been synthesized and isolated as their tetraphenylborate salts. All the
synthesized ruthenium(II) arene complexes are air stable and are fully characterized by elemental
analysis, spectral and X-ray diffraction methods. In chloroform solution all the complexes exhibit char-
acteristic metal to ligand charge transfer (MLCT) absorptions and ligand based transitions. Molecular
structure of the complexes 2, 3 and 4 has been determined by single crystal X-ray crystallography
indicates that the thioamide ligands are coordinated to ruthenium as a bidentate O, S donor and a typical
piano stool geometry was observed around ruthenium(II) metal center. Complexes 1e5 were tested as
catalysts in the transfer hydrogenation of aliphatic and aromatic ketones to secondary alcohols in the
presence of 2-propanol/KOH. Further, the influence of base, reaction temperature and catalyst loading in
this reaction was also evaluated to find out the most active catalyst.
Accepted 2 October 2012
Keywords:
Thioamide ligand
Ruthenium(II) arene complex
Crystal structure
Catalytic transfer hydrogenation
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
established in the literature. Moreover complexes containing
phosphine ligands continue to enjoy immense popularity in
A great deal of work has been devoted to the study of thio-
amides because of their flexible coordination capability. Apart from
the coordination, the thioamides are useful synthetic intermediates
[1e3] and often appear in biologically active molecules [4e7]. The
chemical versatility of thioamide ligands is a good reason for their
study: they are starting materials for the preparation of thiazoles,
amidenes, amidrazones formation; they coordinate strongly with
metal ions. The first thioamide was synthesized in 1815 by Gay-
Lussac [8]. Because of the numerous applications of metal
complexes containing thioamides, considerable effort has been
directed toward the synthesis and structural characterization of
metal complexes of aromatic thioamides in recent years.
connection with homogenous catalysis and organometallic chem-
istry [18e23]. A large number of ruthenium complexes with arene
ligands have been employed in different catalytic reactions
including allylic alkylation [24], amination [25], cyclization [26],
cycloisomerization of dienes [27] and hydroformylation [28] and
transfer hydrogenation.
The reduction of organic compounds is a subject of remarkable
interest from both academic and industrial perspectives. Several
viable methodologies have been established for this purpose, and
most of them make use of a metal, in either stoichiometric or
catalytic amounts, to promote the reaction between the reducing
agent and the substrate. Transfer hydrogenation is arguably one of
the most researched areas in homogeneous catalysis [29,30].
Catalytic transfer hydrogenation of ketones is a fundamental and
key process for the production of a wide range of alcohols including
chiral compounds, which are valuable final products and precur-
sors for the pharmaceutical, agrochemical, flavor, fragrance, mate-
rials, and fine chemical industries [31e37]. Among the transition
metal complexes particularly ruthenium based complexes are
important and a large number of ruthenium complexes have been
reported as catalyst precursors for the transfer hydrogenation of
ketones and have shown high activity [38e45].
Half-sandwich ruthenium complexes are excellent catalysts for
a wide variety of organic reactions [9,10] and have shown promise
as anticancer agents [11e16]. Interest in arene ruthenium chem-
istry began in 1957 with the discovery of complexes of the type
[Ru(
complexes adopt ‘‘three legged piano stool’’ ruthenium complexes,
where the
6-coordinated arene is the ‘‘bench’’, and the other
h
6-arene)2]2þ [17]. Generally ruthenium half sandwich
h
ligands ‘‘the legs’’ of the stool. The ruthenium arene complexes
having potential application in organic synthesis are well
Transfer hydrogenation reaction in which hydrogen is trans-
ferred from one organic molecule to another is of great importance
in organic synthesis since one can avoid the use of molecular
* Corresponding author. Fax: þ91 431 2407045/2407020.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.10.003

D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
27
hydrogen [46,47]. Even though a variety of proton donor have been
published in the literature, 2-propanol is considered to be a popular
reactive solvent for transfer hydrogenation, as it is easy to handle
and is relatively nontoxic, environmentally benign, and inexpen-
sive. Based on the earlier reports on catalytic transfer hydrogena-
tion, we have also fascinated to study the catalytic performance of
the cationic ruthenium(II) arene complexes 1e5 in the transfer
hydrogenation of ketones.
synthesized thioamide ligands are confirmed by analytical and 1H
NMR spectroscopy. All the ligands are obtained as single crystals,
among the five ligands (HL1eHL5) one of the ligand structures was
solved by single crystal X-ray crystallography. The ball and stick
representation of the ligand (HL3) is given in Fig. 1.
2.2. Synthesis of arene ruthenium(II) thioamide complexes
In the early 1990s Noyori et al. synthesized the Ru(II)
All the ligands used in the present study are abbreviated in
general as HL. Reaction of 1:2 ratio of [(h m-Cl)Cl]2
6-p-cymene)Ru(
half-sandwich
complex
ligated
by
chiral
N-tosyl-1,2-
diphenylethylenediamine and was reported as very powerful
catalyst precursor for the transfer hydrogenation reaction [48].
More recently, Baratta et al. have reported 1-(pyridin-2-yl)meth-
anamine based ruthenium catalysts for fast transfer hydrogenation
of carbonyl compounds in 2-propanol [49]. Numerous different
Ru(II)-arene systems bearing PP, NN, NO, and NPN ligands have
been described for this process [50e54]. Other ruthenium and
iridium catalysts containing tetradentate diaminodiphosphine and
aminophosphine ligands have also been shown to be very effective
for the transfer hydrogenation of ketones [54e57]. Further, ruthe-
nium pincer complexes have been found to be the efficient catalysts
in transfer hydrogenation of aliphatic and aromatic ketones [58,59].
Organometallic compounds containing metalacycle are also proved
to be the excellent precatalyst for the transfer hydrogenation
reactions [60]. In order to design new ruthenium complexes with
potential catalytic activity in the reduction of ketones, we decided
to synthesize the arene ruthenium(II) thioamide complexes (1e5).
In view of the promising results obtained on the synthesize and
catalytic applications of Ru, Rh and Pd complexes in various organic
transformations [61e63], the present work describes the synthesis
and characterization of Ru(II) arene complexes containing biden-
tate thioamide ligands along with transfer hydrogenation of a series
of ketones. All the synthesized complexes have been characterized
by elemental analysis, conductance measurements, UVevis and 1H,
31P NMR in solution. The molecular structure of the complexes 2, 3
and 4 is confirmed through single crystal X-ray diffraction. In
addition, transfer hydrogenation reaction was optimized by varying
base, time and catalyst loading and temperature in order to
determine the most active catalyst.
with bidentate thioamide ligands and PPh3 was taken in methanol
respectively. In presence of triethylamine the reaction proceeds
smoothly at room temperature under nitrogen atmosphere for 3 h
(Scheme 2). Coordination was immediate, as a change of color from
red to orange. The solid residue thus obtained by the addition of
tetraphenylborate salt to the reaction mixture was filtered and
washed with cold methanol and diethyl ether. All new ruth-
enium(II) thioamide complexes have been isolated as yellow or
orange red colored air stable solids and are non-hygroscopic in
nature. The synthesized ruthenium complexes are highly soluble in
polar organic solvents such as chloroform, dichloromethane,
DMSO, DMF and it is almost insoluble in diethyl ether and petro-
leum ether. The analytical data (C, H, N and S) are in good agree-
ment with the compositions proposed for all the complexes and are
presented in Table 1. The molar conductance value of all the ruth-
enium(II) arene thioamide complexes is in the range 103.8e
104.9 Uꢁ1 cm2 molꢁ1 in acetonitrile solution at 25 ꢀC and shows
1:1 electrolytic nature [64e66].
2.3. Spectral characterization
IR spectra of all the ruthenium(II) arene thioamide complexes
are recorded and the important IR spectroscopic vibrational bands
for the complexes are given in Supporting information. The ligand
showed a band in the region 698e740 cmꢁ1 which is characteristic
of C]S stretching frequency [67]. The appearance of new bands
near 1318e1336 cmꢁ1 indicates the coordination of phenolic
oxygen (nCeO) to the ruthenium metal. In addition, these complexes
show bands near 540, 690, 740 and 1434 cmꢁ1 which are due to
PPh3 ligand. Electronic spectra of all the complexes have been
recorded in dry chloroform solution in the range 200e800 nm
(Supporting information). All the complexes display three intense
absorptions in the region 500e230 nm. The absorption spectra of
the ruthenium(II) arene thioamide complexes exhibited very
intense bands around 292e299 nm and 246e268 nm are assigned
2. Results and discussion
2.1. Synthesis of 5-substituted thioamide ligands
Ligands of this class have been synthesized by taking the reac-
tion mixture of sulfur, pyrrolidine, 5-substituted salicylaldehyde in
acetic acid in the presence of 20 cm3 of N,N0-dimethylformamide
(DMF) and was heated to 100 ꢀC for 4 h (Scheme 1). The reaction
progress was monitored by thin layer chromatography (TLC). After
completion of the reaction, the reaction mixture was cooled to
room temperature. The desired product was solidified by pouring
the reaction mixture to the ice cold water. The crude solid product
was filtered and purified by column chromatography. The
to ligand-centered (LC)
p /
p* and n / p* transitions respectively.
β
α
'
β
N
S
H
O
S8
α
'
OH
OH
DMF/ACOH
100o C
+
N
R
R
H
R = H, Cl, Br, OCH3, NO2
Fig. 1. Ball and stick representation of the ligand (HL3) shown at the 50% probability
Scheme 1. Preparation of thioamides.
level. Hydrogen atoms have been omitted for clarity.

28
D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
+
CH3
H3C
BPh4
Cl
Cl
Cl
N
PPh3
N
Ru
S
Ru
+
S
Ru
Cl
TEA
CH3
R
OH
R
O
PPh3
NaBPh4
MeOH
R = H (1), Cl (2), Br (3), OCH3 (4), NO2 (5)
Scheme 2. Synthesis of ruthenium(II) arene thioamide complexes.
Table 1
Analytical data for ruthenium(II) arene thioamide complexes.
Sl. no
Complexes
Color
M. pt
Calculated (found) %
LM (acetonitrile,
10ꢁ3 M) Uꢁ1
C
H
N
S
1
2
3
3
5
[(
[(
[(
[(
[(
h
h
h
h
h
6-cymene)Ru(PPh3)(L1)]þ BPh4
6-cymene)Ru(PPh3)(L2)]þ BPh4
6-cymene)Ru(PPh3)(L3)]þ BPh4
6-cymene)Ru(PPh3)(L4)]þ BPh4
6-cymene)Ru(PPh3)(L5)]þ BPh4
Orange
Orange
Orange
Yellow
Yellow
183
176
168
172
191
73.96(73.88)
71.55(71.43)
68.66(68.52)
72.99(72.92)
70.84(70.73)
6.01(6.07)
5.72(5.68)
5.49(5.53)
6.03(6.08)
5.66(5.58)
1.37 (1.27)
1.32 (1.29)
1.27 (1.23)
1.33 (1.22)
2.62 (2.53)
3.13 (3.29)
3.03 (3.09)
2.91 (2.86)
3.04 (2.92)
3.00 (3.09)
104.1
104.5
104.9
104.8
104.3
The lowest energy absorption bands in the electronic spectra of
these complexes in the visible region 331e354 nm are ascribed to
metal to ligand charge transfer (MLCT) transitions.
anisotropic aromatic ring current and the shielded 1H NMR signals
(3.4e3.6 ppm) are assigned to the CH2 group. In addition, the
remaining two b-, b
0-CH2 protons of pyrrolidine ring are observed
as two multiplets in the range of 1.9e2.5 ppm. The 31Pe{1H}NMR
spectra of all the complexes 1e5 showed a singlet around
2.4. 1H and 31P NMR spectra
d
29.19e30.52 ppm which confirm the coordination of one tri-
phenylphosphine group to the ruthenium metal in the complexes.
In all these complexes the 31P nuclei exhibited a down field shift as
compared to that in the free phosphines and the respective
precursor complexes. The deshielding of the phosphorus may be
caused by relatively less donation of electron density from the
Ru(II) center through back bonding. This suggests that the degree of
The bonding arrangement is further confirmed by NMR spec-
troscopy. The NMR spectroscopic data for complexes 1e5 are
recorded in CDCl3 solution and the results are consistent with the
molecular structures revealed by single-crystal X-ray diffraction
(Table 2). The 1H and 31P{1H} NMR spectra of all the complexes are
given in the Supporting information. All the complexes show
dpepp back bonding influences the chemical shift of the phos-
phorus nuclei.
multiplets at
d
¼ 6.8e7.8 ppm for the presence of thioamide
ligands, triphenylphosphine and the tetraphenylborate aromatic
protons. In addition the eOH protons are disappeared in all the
complexes indicating that the ruthenium is coordinated through
2.5. X-ray structure determination
phenolic oxygen. The protons of isopropyl group of the
ene ligand appear as two sets of doublets around
¼ 0.8e1.0 ppm
[68]. The methyl protons appear as singlet at
¼ 1.5e1.8 ppm. The
isopropyl CH proton appears as a septet in the range of
¼ 2.4e
2.5 ppm. The four aromatic hydrogens of the p-cymene ligand
appear as four sets of doublets in the range of
¼ 3.8e5.4 ppm.
Additionally methoxy protons are observed as singlet for complex
4 at
h
6-p-cym-
d
As part of the structural characterization, the molecular struc-
ture of the ligand (HL3) and the complexes 2, 3 and 4 have been
determined by single crystal X-ray analysis. The single crystals of
the complex 2, 3 and 4 along with tetraphenylborate were obtained
from the slow evaporation method. The ball and stick representa-
tion at 50% thermal ellipsoid probability with atom numbering
scheme is shown in Figs. 1e4. The summary of single crystal X-ray
structure refinement is shown in Table 3 and the selected bond
angle and the bond length are given in Table 4.
d
d
d
d
¼ 3.6 ppm. In the NMR spectra four methylene groups
(pyrrolidine ring) on the thioamide-N atoms appeared separately,
suggesting their non-equivalence and slow rotation of the thio-
amide CeN bond on the NMR time scale [69]. The
a
-,
a
0-CH2
[(
monoclinic space group P21/n. The structure of the complex is
h
6-Cymene)Ru(PPh3)(L2)]þ (BPh4) (2) is crystallized in the
protons from the pyrrolidine are in the shielding region of an
Table 2
1H and 31P NMR spectral data of ruthenium(II) arene thioamide complexes.
Complexes
Cymene
Ligand
31P
AreH
eCH3
eCH
eC(CH3)2
AreH
a
-CH2
b-CH2
eOCH3
1
2
3
4
5
4.47e5.6
4.47e5.6
4.47e5.6
4.47e5.6
4.47e5.6
1.79
1.81
1.79
1.79
1.79
1.9
1.9
1.9
1.9
1.9
0.85e0.98
0.85e0.98
0.85e0.98
0.85e0.98
0.85e0.98
6.8e7.8
6.8e7.8
6.8e7.8
6.8e7.8
6.8e7.8
3.49e3.60
3.49e3.60
3.49e3.60
3.49e3.60
3.49e3.60
1.9e2.5
1.9e2.5
1.9e2.5
1.9e2.5
1.9e2.5
e
e
29.72
29.69
29.71
29.19
30.52
3.74
e

D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
29
Fig. 4. Molecular structure of [(Ru(
h
6-p-cymene)PPh3)L3] (4), showing the 50% prob-
ability level and the counter ion (BPh4) is not shown for clarity. All hydrogen atoms are
omitted for clarity.
ꢀ
ꢀ
2.102(3) A bond length is shorter than Ru1eS1 2.3695(11) A. The
ꢀ
Ru(1)eP(1) bond length is 2.3553(12) A is similar to those reported
Fig. 2. Molecular structure of [(Ru(h
6-p-cymene)PPh3)L2] (2), showing two indepen-
dent molecule 1 and 2. Thermal ellipsoids are drawn at 50% probability level and the
counter ion (BPh4) is not shown for clarity. All hydrogen atoms are omitted for clarity.
for other ruthenium phosphine complexes [72]. It was observed
that the complexes 3 and 4 also adopt similar geometrical envi-
ronment as in the complex 2 with slight variation in the bond angles
and bond distances. In all the complexes the C]S bond distances
are longer than that of the free thioamide ligand but the phenolic
CeO bond length is slightly reduced upon coordination of the
ruthenium to the thioamide ligands. The CeN bond distances of the
complexes are slightly elongated while on coordination with
ruthenium metal.
shown in Fig. 2. The unit cell contains two independent molecules
which are essentially identical (1 and 2). The molecular structure of
complex 2 shows typical piano-stool geometry with the metal
center being coordinated by the chelating thioamide O, S donors,
triphenylphosphine and the fourth position can be envisaged by
the centroid of the aromatic ring of the arene ligand. The thioamide
ligand binds to the ruthenium metal center via O, S atom forming
one six-membered chelate ring with bite angles of 87.79(9)ꢀ O1e
Ru1eP1, 87.19(8)ꢀ O1eRu1eS1 and 86.36(4)ꢀ P1eRu1eS1, 114.6(2)ꢀ
C13eO1eRu1. The p-cymene ring is planar and the RueC average
2.6. Catalytic transfer hydrogenation of ketones
Transition-metal-catalyzed transfer hydrogenation of a wide
variety of functional groups by different hydrogen donors is an
interesting alternative to conventional hydrogenation [46,47]. Due
to the encouraged applications of half-sandwich arene ruthenium
compounds in transfer hydrogenation catalysis, we have also
examined the catalytic activity of the complexes 1e5 toward
reduction of carbonyl group of ketones. Acetophenone has been
chosen as a model substrate to explore the catalytic performance of
the complexes.
ꢀ
ꢀ
bond length of 2.6762 (5) A [range 2.196(5)e2.260(5) A]. The CeC
bond distances in the p-cymene ligands of the complexes alter-
nate in a long-short-long pattern, which is typically found for are-
nes coordinated to ruthenium [70,71]. The bond lengths for this
cationic complex are very similar to those reported for related,
three-legged piano-stool ruthenium complexes [18]. The Ru1eO1
Initially we tried the transfer hydrogenation reaction of aceto-
phenone using complex 3 as a test catalyst and green solvents such
as water, glycerol and KOH as base. We found that there is no
detectable conversion even after 24 h at reflux. Based on this
conclusion 2-propanol was taken as a proton source for our
investigation. At room temperature transfer hydrogenation using
complex 3 in the presence of 2-propanol/KOH shows no noticeable
conversion. When the temperature is increased to refluxing
condition the reaction proceeds well. The volatile acetone product
can also be easily removed to shift an unfavorable equilibrium.
2.7. Base variation study
In order to improve the efficiency of our catalytic transfer
hydrogenation, we examined the influence of bases. The base
variation experiments were performed using 1:500 ratio of the
catalyst 3 and substrate in 10 ml isopropanol with different bases.
First the reaction was conducted without any base and no reaction
was observed even after 24 h. Addition of inorganic bases like
Na2CO3 and K2CO3 shows moderate conversions of 86% and 82%
respectively (Table 5; entries 1 and 2). In the case of NaOH and KOH
the conversions are 96% and 98% respectively (entries 3 and 4).
Fig. 3. Molecular structure of [(Ru(h
6-p-cymene)PPh3)L3] (3), showing two indepen-
dent molecule 1 and 1A. Thermal ellipsoids are drawn at 50% probability level and the
counter ion (BPh4) is not shown for clarity. All hydrogen atoms are omitted for clarity.

30
D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
Table 3
Crystal data and structure refinement parameters for HL3, complexes 2, 3 and 4.
Compound
HL3
2
3
4
Formula
C
11H12BrNOS
C126H120B2Cl2N2O2P2Ru2S2
2114.78
Monoclinic
C126H120B2Br2N2O2P2Ru2S2
2203.88
Monoclinic
C64H62BNO2PRuS
1052.06
Formula weight
Crystal system
Space group
284.98
Monoclinic
P2(1)/c
Orthorhombic
Pbca
a ¼ 14.5730(7) A,
P21/n
a ¼ 18.470(3) A ,
P2(1)/n
a ¼ 18.5279(5) A,
a ¼ 10.7661(3) A,
a
¼ 90 (ꢀ)
a
b
¼ 90.00 (ꢀ)
a
¼ 90 (ꢀ)
ꢀ
ꢀ
ꢀ
ꢀ
a
¼ 90.00 (ꢀ)
¼ 102.782(2) (ꢀ)
ꢀ
ꢀ
b ¼ 10.5257(4) A,
b ¼ 16.593(2) A,
b
b ¼ 16.5665(5) A,
¼ 102.875(2) (ꢀ)
b ¼ 21.1800(10) A,
¼ 90 (ꢀ)
¼ 90 (ꢀ)
ꢀ
ꢀ
b
b
¼ 102.207(2) (ꢀ)
c ¼ 10.6621(4) A,
c ¼ 36.241(5) A,
g
¼ 90 (ꢀ)
¼ 90.00 (ꢀ)
c ¼ 34.9110(17) A,
g
ꢀ
ꢀ
ꢀ
ꢀ
c ¼ 36.4225(10) A,
g
g
¼ 90.00 (ꢀ)
3
ꢀ
Volume [A ]
Z
1180.92(7)
4
10,831(3)
60
10,898.5(5)
4
10,775.5(9)
8
Reflections collected
Dcalc [g/cm3]
F(000)
3227
1.610
576
102,540
1.2969
4394.5
7280
1.343
4544
9010
1.297
4392
Crystal size [mm]
Temperature (K)
Radiation [A]
0.08 ꢂ 0.08 ꢂ 0.07
0.30 ꢂ 0.26 ꢂ 0.22
0.25 ꢂ 0.20 ꢂ 0.15
0.20 ꢂ 0.20 ꢂ 0.15
293(2)
298.15
293(2)
293(2)
ꢀ
MoKa 0.71073
Full-matrix least-squares
on F2
1.94, 29.26
0.813
0.0411, 0.2115
0.0697, 0.1645
0.693 and ꢁ0.791 e$Aꢁ3
MoKa 0.71073
MoKa 0.71073
MoKa 0.71073
Refinement method
Full-matrix least-squares on F2
Full-matrix least-squares on F2
Full-matrix least-squares on F2
Theta minemax [ꢀ]
Goodness-of-fit on F2
R, wR2
R indices (all data)
Largest diff. peak and hole
1.15, 25.06
1.060
0.0587
1.15, 17.88
1.040
0.0434, 0.1540
0.0722, 0.1229
1.351 and ꢁ0.292 e$Aꢁ3
1.17, 25.00
1.035
0.0454, 0.1101
0.0884, 0.1462
0.441 and ꢁ0.437 e$Aꢁ3
0.0870, 0.1670
0.84 and ꢁ0.73 e$Aꢁ3
Table 4
Selected bond length and bond angles.
L3
2
3
4
ꢀ
Bond lengths (A)
O1eRu1
S1eRu1
Ru1eP1
N1eC11
S1eC11
C13eO1
Ru1eC (arene)
2.102(3)
O1eRu1
S1eRu1
Ru1eP1
C7eN1
C7eS1
C1eO1
2.102(6)
2.374(3)
2.360(3)
1.337(11)
1.697(10)
1.317(12)
2.203(10)e2.242(10)
O(1)eRu(1)
2.082(2)
2.3572(11)
2.3537(12)
1.311(5)
1.710(4)
1.332(4)
2.3695(11)
2.3553(12)
1.313(5)
1.703(4)
1.322(5)
S(1)eRu(1)
P(1)eRu(1)
C(36)eN(1)
C(36)eS(1)
C(29)eO(1)
N1eC7
S1eC7
O1eC1
1.318(4)
1.670(3)
1.336(4)
2.196(5)e2.260(5)
Ru1eC (arene)
Ru1eC (arene)
2.203(4)e2.269(4)
Bond angles (ꢀ)
N1eC7eS1
O1eC1eC2
121.8(2)
123.7(3)
O1eRu1eP1
O1eRu1eS1
P1eRu1eS1
C13eO1eRu1
87.79(9)
87.19(8)
86.36(4)
114.6(2)
O1eRu1eP1
O1eRu1eS1
P1eRu1eS1
C1eO1eRu1
87.32(19)
87.20(18)
86.60(9)
114.2(6)
O(1)eRu(1)eP(1)
O(1)eRu(1)eS(1)
P(1)eRu(1)eS(1)
C(29)eO(1)eRu(1)
86.21(8)
87.75(7)
85.23(4)
113.9(2)
Table 5
Base variation study.a
[Ru] (3)
O
CH3
O
OH
CH3
OH
+
+
H3C
CH3
H3C
CH3
IPA / Base
Reflux
Entry
Base
Conversionb (%)
1
2
3
4
5
6
Na2CO3
K2CO3
NaOH
KOH
CH3COONa
TEA
86
82
96
98
36
Traces
a
Experimental conditions: reactions were carried out at 80 ꢀC using acetophenone (5 mmol), catalyst (0.01 mmol), base (0.025 mmol) in 10 ml of isopropanol for 8 h.
Conversion of the product was determined by 1H NMR spectroscopy based on acetophenone.; average of two runs.
b

D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
31
Table 6
Conversion vs reaction time (h).a
[Ru]
O
CH3
O
OH
CH3
CH3
OH
+
+
H3C
CH3
H3C
IPA / KOH
Reflux
Complexes
Conversionb (%)
1 h
3 h
5 h
6 h
8 h
1
2
3
4
5
48
54
44
58
42
62
68
56
72
66
69
76
64
91
78
82
92
89
96
81
96
98
94
99
85
a
Experimental conditions: reactions were carried out at 80 ꢀC using acetophenone (5 mmol), catalyst (0.01 mmol), base (0.025 mmol) in 10 ml of isopropanol.
Conversion of the product was determined by 1H NMR spectroscopy based on acetophenone; average of two runs.
b
Among the different bases used for our studies, the KOH was the
best choice of base and the conversion of product could be
increased to 99% (entry 4). Very low conversion was noted
in CH3COONa condition. With the organic base triethylamine,
we observed the traces of alcohol formation. From the above
results the conversion was strongly dependent upon the base
strength with the stronger bases giving higher conversions
(TEA < CH3COONa < Na2CO3 < K2CO3 < NaOH < KOH).
2.8. Catalyst screening and time variation study
As a benchmark reaction for the catalyst screening and reaction
time, the catalytic activity of the complexes 1e5 in the transfer
hydrogenation of acetophenone using 1:2.5:500 for the catalyst,
KOH and substrate in 10 ml isopropanol. The reaction progress was
periodically monitored by 1H NMR spectroscopy. All the complexes
efficiently catalyze the transfer hydrogenation of acetophenone
with maximum conversions within 8 h. The results of catalyst
screening and time variation study are displayed in Table 6. From
the results it has been found that all the five complexes (1e5)
have proven to be active and efficient catalysts leading to nearly
quantitative conversions (85e98%) of acetophenone into 1-
phenylethanol. Among all, complex 4 shows maximum conver-
sion in the transfer hydrogenation of acetophenone (Table 6; entry
Fig. 5. Time dependence of the catalytic transfer hydrogenation of acetophenone;
catalyst (1e5, 0.01 mmol), KOH (0.025 mmol), acetophenone (10 mmol), and 2-
propanol (10 ml), T ¼ 355 K.
Table 7
Effect of low catalyst loading.a
[Ru] (4)
O
CH3
O
OH
CH3
CH3
OH
+
+
H3C
CH3
H3C
IPA / KOH
Reflux
Entry
C:S ratio
Time (h)
Conversionb (%)
TON
1
2
3
4
5
6
7
1:100 (RT)
1:100
1:300
24
8
8
8
8
NRc
>99
>99
98
96
58
e
99
297
480
960
870
1080
1:500
1:1000
1:1500
1:3000
8
8
36
a
Experimental conditions: reactions were carried out at 80 ꢀC using acetophenone, catalyst (0.01 mmol), base (0.025 mmol) in 10 ml of isopropanol.
Conversion of the product was determined by 1H NMR spectroscopy based on acetophenone; average of two runs.
No reaction.
b
c

32
D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
4). The catalytic activity of complex 4 sharply increased after the
O
[Ru]
OH
O
OH
R'
+
+
reaction progressed for 1 h, although its initial activity was the
highest (Fig. 5). This activity is significantly higher than those of
related ruthenium arene complexes [73]. On the other hand, only
H3C CH3
H3C CH3
R
R'
R
Scheme 3. Catalytic transfer hydrogenation of ketones.
Table 8
Catalytic transfer hydrogenation of ketones with complex (4).a
Entry
Substrate
Product
Conversionb (%)
97
TONc
970
TOFd (hꢁ1
)
O
OH
1
121.2
CH3
O
CH3
OH
2
3
4
96
98
92
960
980
920
120
CH3
CH3
CH3
Cl
Cl
O
OH
CH3
122.5
115.0
Br
Br
O
OH
CH3
CH3
H3C
H3C
O
OH
CH3
5
6
76
99
760
990
95.0
CH3
CH3
O
O
O
OH
CH3
123.7
O2N
O2N
O
OH
CH3
7
8
9
68/85
680/850
85/106.25
116.3
CH3
HO
HO
O
F
OH
CH3
93
930
CH3
F
O
OH
CH3
CH3
73
730
91.3
H3C
H3C
CH
CH
O
OH
CH3
MeO
MeO
MeO
MeO
10
11
76
78
760
780
95
CH3
CH3
OMe O
OMe OH
CH3
MeO
MeO
MeO
MeO
97.5

D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
33
Table 8 (continued )
Entry
Substrate
Product
Conversionb (%)
52
TONc
520
TOFd (hꢁ1
)
O
OH
CH3
12
13
65
CH3
O
OH
89
890
111.3
O
OH
14
15
92
96
920
960
115
120
O
OH
O
OH
16
17
90
900
112.5
O
OH
CH3
86/99e
48/62e
860/990
480/620
107.5/82.5
60.0/51.7
CH3
N
N
O
OH
CH3
S
S
18
CH3
a
Experimental conditions: reactions were carried out at 80 ꢀC using acetophenone (10 mmol), catalyst (0.01 mmol), base (0.025 mmol) in 10 ml of isopropanol for 8 h.
Conversion of product was determined using a 1H NMR.
b
c
TON ¼ moles of product per mole of catalyst.
d
e
Turnover frequency: moles of product per mole of catalyst per hour, in hꢁ1
The reaction after 12 h.
.
moderate yields were obtained by complexes 2 and 3, although the
reaction time was continued to 8 h. Using complex 1, we obtained
a reasonable conversion of 96% in 8 h (entry 1). The lowest
conversion obtained from our results is 85% in the presence of
complex 5 (entry 5).
still proceeds with moderate conversions. The high turnover
number (TON) of 1080 was observed in the case of 1:3000 C/S ratio.
When we change the C/S ratios of 1:100, 1:300, 1:500, and 1:1000
the reaction proceeds with excellent conversions. Thus, it was
concluded that catalyst:substrate ratio of 1:1000 is the best
compromise between optimum reaction rate and C/S ratio.
Complexes containing chloro and bromo substituents show
effective conversions 93% and 91% respectively (entries 2 and 3).
The lowest conversion observed is 85% (entry 5). From the observed
results it was found that the catalytic efficiency of the complexes
follows the order (5 < 3 < 2 < 1 < 4). The catalytic activity of these
complexes 1e5 has been explained on the basis of the substituents
present in the complexes. The introduction of electron withdrawing
groups such as eCl, eBr and eNO2 on the complexes shows less
conversion in comparison with electron releasing group. It was
observed that the presence of electron releasing eOCH3 group on
the 5th position of aromatic ring possibly increased the electron
density on the metal center and hence the rate of transfer hydro-
genation increases.
2.10. Catalytic transfer hydrogenation of substrates 1e18
With these optimized conditions, the catalytic transfer hydro-
genation of ketones in the presence of catalyst 4 has been studied in
an isopropanol/KOH medium using a molar ratio of 1:2.5:1000 for
the catalyst, KOH and substrate in 10 ml isopropanol (Scheme 3).
The complex exhibited excellent catalytic activities for the transfer
hydrogenation of substituted acetophenones (Table 8, entries 1e
11). The details of percent conversions, TONs and TOFs are given
in Table 8. It has also been observed that the catalytic activity varies
with respect to substituent in the ketones. The electron donating
substituent present in the substrates showed a moderate effect on
the catalytic activity. In the case of 40-methylacetophenone and 40-
methoxyacetophenone the conversions to their corresponding
alcohols are 82% and 76% respectively (entries 4 and 5). Substrate
with electron-withdrawing substituents such as Cl, Br, NO2 plays
a significant role in the conversion of ketones to alcohols (entries
2,3, and 6) and catalyze with good conversions of 96%, 98% and 99%
respectively. The introduction of electron-withdrawing substitu-
ents to the para position of the aryl ring of the ketone decreased the
2.9. Effect of low catalyst loading
In order to optimize the reaction conditions for the transfer
hydrogenation of acetophenone different catalyst:substrate (C/S)
ratios were taken and the results are summarized in Table 7. Low
catalyst loading tests were performed to determine the catalytic
efficiency of the catalyst (4) in the presence of 2-propanol and KOH.
When increasing the C/S ratio to 1:1500 and 1:3000 the reaction

34
D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
electron density on the C]O bond so that the activity was
improved giving rise to easier hydrogenation. In the case of methyl
and methoxy substituted acetophenone only moderate conversions
(entries 9e11) are obtained. The catalytic reduction of 2-
acetylnaphthalene and benzophenone shows less conversion of
52% and 89% respectively. In the case of acyclic ketones (entries 14e
16) good conversions 92%, 96% and 90% are obtained respectively.
Interestingly, complex 4 is found to be an efficient catalyst for the
transfer hydrogenation of 2-acetylpyridine and 2-acetylthiophene
and shows the conversions of 99% and 62% respectively in 24 h
(entries 15 and 16). Probably the competing coordination between
the N-donor atom and the carbonyl group in substrate with Ru(II)
metal center caused the decrease in the catalyst activity. It is worth
to note that the observed results are significantly better than those
previously reported [74e76]. Although no mechanistic studies have
been performed, the catalytic transformation of ketones most
probably follows the classical pathway in which ketones coordinate
to hydride ruthenium metal intermediate [26,43,77,78].
4.3. Preparation of thioamide ligands (HLs)
A mixture of sulfur (0.5 g, 1 mmol), pyrrolidine (1.109 g,1 mmol),
5-substituted salicylaldehyde (1.25 g, 0.66 mmol), and acetic acid
(0.3 ml) in 20 cm3 of N,N-dimethylformamide (DMF) was heated to
100 ꢀC over 1 h with stirring. The mixture was stirred for an
additional 3 h at this temperature and cooled to room temperature.
The resulting mixture was poured with stirring into 300 cm3 of
water cooled in an ice bath and the stirring was continued until the
oily product had solidified. The solids were filtered, washed with
water, dried in air, and dissolved in 600 cm3 of hot methanol. The
remaining solids (mainly unreacted sulfur) were removed by
filtration and the red solution was treated with charcoal. The
filtered solution was evaporated to 30 cm3 under reduced pressure
to give a faint yellow precipitate, which was recrystallized from
warm diethyl ether. Yield ¼ 57e75%.
4.4. Synthesis of ruthenium(II) arene complexes
A
mixture containing starting [(h m-Cl)Cl]2
6-p-cymene)Ru(
3. Conclusion
(100 mg, 1 mmol), thioamide ligand (468 mg, 2 mmol), triphenyl-
phosphine (1 mmol) and triethylamine (0.3 mL) in methanol
(30 ml) was taken in a clean 50 ml round bottom flask. The resulting
mixture was allowed to react with stirring at room temperature for
3 h. The reaction progress was monitored through thin layer
chromatography. After the reaction completions add NaBPh4
(1 mmol) results an orange solid formation, which was collected by
suction filtration, washed once with cold methanol, and dried in
vacuo. Yield ¼ 65e86%.
In summary, ruthenium(II) arene complexes containing thio-
amide and triphenylphosphine ligands are demonstrated as useful
catalysts for the transfer hydrogenation of ketones. All the
synthesized complexes are completely characterized by analytical
and spectral (UVevis, 1H and 31P NMR) methods. Three of the
complexes were structurally characterized by single crystal X-ray
diffraction method which reveals thioamide ligands coordinate to
ruthenium via O and S and witnessed the presence of a typical three
legged piano stool geometry around ruthenium. Further, complex 4
is found to be the most efficient catalyst among the series in the
conversion of ketones to alcohols with maximum conversion of
99%.
4.5. Typical procedure for the catalytic transfer hydrogenation of
ketones
In a dry two-necked round bottom flask under an atmosphere of
nitrogen were placed an appropriate amount of catalyst 1e5
(0.01 mmol), (0.025 mmol) of KOH and (10 mmol) of aryl ketone
in 2-propanol (10 ml) was added and the resulting mixture was
refluxed under an atmosphere of nitrogen and the course of the
reaction was monitored by 1H NMR analysis. After completion of
the reaction, the solvent was removed under reduced pressure. The
catalyst was removed by the addition of 15 ml of ether (b.p., 40e
60 ꢀC) followed by filtration and subsequent neutralization with
dilute HCl. The combined organic fractions were dried over anhy-
drous Na2SO4. The solvent was distilled off to obtain a crude
mixture containing ketone and its hydrogenated product.
Percentage conversion was calculated by 1H NMR spectra of the
crude mixture. The only side product formed is acetone, which is
easily removed by distillation during workup.
4. Experimental section
4.1. Reagents and materials
All reagents and solvents were of high purity grade and were
purchased from commercial sources and used as received unless
noted otherwise. Commercial RuCl3$3H2O was purchased from
Loba-Chemie Pvt. Ltd. The solvents were purified and dried
according to standard procedures [79]. 5-Substituted salicylalde-
hyde, pyrrolidine and sulfur were obtained from Aldrich. The
starting precursor complex [(h m-Cl)Cl]2 [80] was
6-p-cymene)Ru(
prepared according to the reported procedure.
4.2. Physical measurements
4.6. X-ray crystallography
Microanalytical (C, H, N) data were obtained with a Perkine
Elmer model 240C elemental analyzer. Melting Points were recor-
ded in the Boetius micro heating table and are uncorrected. Infrared
spectra of complexes were recorded in KBr pellets with a Perkine
Elmer 597 spectrophotometer in the range 4000e400 cmꢁ1. Elec-
tronic spectra were recorded on a Varian Cary 300 Bio UVeVis
spectrophotometer using cuvettes of 1 cm path length. The solu-
Single crystals of ligand (HL3), [(
h
6-cymene)Ru(PPh3)(L2)]þ BPh4
6-cymene)
(2), [(
h
6-cymene)Ru(PPh3) (L3)]þ BPh4 (3) and [(
h
Ru(PPh3) (L4)]þ BPh4 (4) are grown by slow evaporation at room
temperature. A single crystal of suitable size was mounted on the
top of a glass fiber, and transferred to a Stoe IPDS diffractometer
using monochromated MoKa radiation. Corrections were made for
Lorentz and polarization effects as well as for absorption (numer-
ical). The structures were solved and refined by full-matrix least-
squares techniques on F2 using the SHELX-97 program [81,82]. The
absorption corrections were done by the multi scan technique. All
data were corrected for Lorentz and polarization effects, and the
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were included in the refinement process as per the riding model.
tion electrical conductivity measurements (L , reported as
M
Uꢁ1 cm2 molꢁ1) of acetonitrile solutions of the complexes were
taken using Systronic 305 conductivity bridge at room tempera-
ture. The 1H and 31P{1H} NMR (
d in ppm) spectra were recorded
using a Bruker 400 MHz spectrometer operating at the appropriate
frequencies using TMS and 85% H3PO4 as internal and external
references, respectively.

D. Pandiarajan, R. Ramesh / Journal of Organometallic Chemistry 723 (2013) 26e35
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