Arene ruthenium(II) p-chloroacetophenone phenylthiosemicarbazone complex mediated transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.inoche.2010.07.026

A series of cationic half-sandwich arene ruthenium(II) complexes of general formula [Ru(η⁶-p-cymene)Cl(L)]Cl have been synthesized from the reaction of [Ru(η⁶-p-cymene)Cl₂]₂ with thiosemicarbazone derivatives (L). Characterization of the complexes were accomplished by analytical and spectral (FT-IR, UV-Vis, ¹H NMR) methods. Single crystal structure determination reveals the presence of a pseudooctahedral three-legged piano stool conformation. All the complexes exhibit a quasi-reversible one electron reduction in the range from -0.75 to -0.85 V. Further, the catalytic activity of the titled complex has been investigated in the transfer hydrogenation of ketones in the presence of isopropanol/NaOH.

Full text of DOI:10.1016/j.inoche.2010.07.026

Inorganic Chemistry Communications 13 (2010) 1321–1324
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Arene ruthenium(II) p-chloroacetophenone phenylthiosemicarbazone complex
mediated transfer hydrogenation of ketones
⁎
Mathiyazhagan Ulaganatha Raja, Elangovan Sindhuja, Rengan Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
a r t i c l e i n f o
a b s t r a c t
A series of cationic half-sandwich arene ruthenium(II) complexes of general formula [Ru(η6-p-cymene)Cl
(L)]Cl have been synthesized from the reaction of [Ru(η6-p-cymene)Cl2]2 with thiosemicarbazone
derivatives (L). Characterization of the complexes were accomplished by analytical and spectral (FT-IR,
UV–Vis, 1H NMR) methods. Single crystal structure determination reveals the presence of a pseudooctahe-
dral three-legged piano stool conformation. All the complexes exhibit a quasi-reversible one electron
reduction in the range from −0.75 to −0.85 V. Further, the catalytic activity of the titled complex has been
investigated in the transfer hydrogenation of ketones in the presence of isopropanol/NaOH.
© 2010 Elsevier B.V. All rights reserved.
Article history:
Received 6 June 2010
Accepted 21 July 2010
Available online 29 July 2010
Keywords:
Synthesis
Ruthenium(II)-p-cymene
Phenylthiosemicarbazone ligands
Structural characterization
Transfer hydrogenation
There has been much interest in the chemistry of half-sandwich
arene ruthenium(II) complexes [1,2] since the development of useful
synthetic precursors with the general formula [RuCl2(η6-arene)]n.
Syntheses of new and highly active transition-metal based catalysts
derived from arene ligands are used in different catalytic reactions
including addition of carboxylic acids to alkynes [3], transfer
hydrogenation of ketones [4], polymerization of olefins [5] and
isomerization of alkenes [6]. Further, several arene ruthenium
complexes are found in applications as anticancer drugs [7,8].
The reduction of carbonyl compounds is an important transfor-
mation in organic synthesis from both academic and industrial points
of view [9]. Transfer hydrogenation of ketones by using an alcohol,
preferably isopropyl alcohol, is an interesting alternative to the
classical hydrogenation process requiring the use of dihydrogen gas
[10]. Homogeneous ruthenium complexes are considered to be the
most attractive catalysts for transfer hydrogenation reactions, though
other metal complexes have also been used successfully [11]. Transfer
hydrogenation of unsaturated compounds like ketones, imines etc. by
using isopropanol/NaOH as hydrogen source is a widely investigated
reaction that is promoted by many transition metal complexes [12].
Cationic arene ruthenium(II) complexes containing 2,2'-bipyrimi-
dine ligands are used as active catalysts for transfer hydrogenation of
ketones in water in the presence of HCOOH/HCOONa [13]. Similarly,
asymmetric transfer hydrogenation of ketones carried out by [RuCl2
(p-cymene)]2 in the presence of NaOH and t-BuOK has been described
[14]. Bimetallic ruthenium(II) complexes bearing bis(aminopho-
sphine) and bis(phosphinite) monodentate ligands were used as
catalyst in the transfer hydrogenation of substituted acetophenone
[15]. Transfer hydrogenation of benzophenone and acetophenone by
arene ruthenium(II) complexes containing bis(pyrazolylazines)
ligands has also been described [16]. Further, the catalytic activity of
conformationally rigid diphosphine arene ruthenium(II) complexes in
the transfer hydrogenation of various ketones has been reported [17].
Furthermore, transfer hydrogenation of aromatic ketones were
successfully achieved by water-soluble arene ruthenium complexes
containing an N,N-chelating 2,2'-dipyridylamine ligands [18].
As a part of ongoing efforts to synthesize novel ruthenium
complexes and to study their catalytic efficiency in transfer hydroge-
nation reactions [19], herein we describe the synthesis and char-
acterisation of a series of cationic arene ruthenium(II) complexes
containing thiosemicarbazones along with their catalytic activity in
transfer hydrogenation of ketones.
The substituted acetophenone thiosemicarbazone ligands (L1–L5)
were prepared by modification of the reported procedure [20]. The
general procedure for the preparation of the complexes is as follows.
To a solution of 0.1 mmol of [{(η6-p-cymene)RuCl}2(μ-Cl)2)] in dry
benzene (25 ml) was added 0.2 mmol thiosemicarbazone ligand
(mole ratio of ruthenium complex and ligand is 1:2 respectively)
and the mixture was refluxed for 2–3 h (Scheme 1). The solution was
concentrated to about 3 ml, pet. ether (15 ml) was added to
precipitate the cationic η6-p-cymene ruthenium(II)phenylthiosemi-
carbazone complexes. The resulting complexes were recrystallized
from CH2Cl2/pet.ether and dried under vacuum. The overall yield
obtained for all the complexes were 66–80%.
All the newly synthesized η6-p-cymene ruthenium(II)phenylthio-
semicarbazones complexes are colored, stable in air, non-hygroscopic in
⁎
Corresponding author. Tel.: +91 431 2407020; fax: +91 431 2407045.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.07.026

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M.U. Raja et al. / Inorganic Chemistry Communications 13 (2010) 1321–1324
Scheme 1. Synthesis of η6-p-cymene Ru(II)phenylthiosemicarbazone complexes.
nature and highly soluble in common solvents such as dichloromethane,
acetonitrile and chloroform. The elemental analyses are in good
agreement with the molecular formula proposed for all the complexes
(Table 1). In the IR spectra of all the complexes, the thiosemicarbazone
ligands behave as neutral bidentate ligand and coordinated to the metal
around ruthenium consists of the η6-p-cymene ring, sulphur, nitrogen
of thiosemicarbazone and a chloride and all are disposed in a classical
pseudooctahedral three-legged piano-stool geometry, similar to that
of other ruthenium(II)-p-cymene complexes (Fig. 1) [23]. The
phenylthiosemicarbazone ligands bind the metal center at S and N
forming the five membered chelate ring with bite angle around Ru(II)
ion being 81.13(9) for S(1)–Ru(1)–N(1) and bond lengths for Ru(1)–S
(1) and Ru(1)–N(1) are 2.3376(12) Å and 2.130(3) Å respectively. The
Ru–Cl bond length is found to be 2.4218(12) Å. Ruthenium(II) ion is
therefore sitting in a SNCl(η6-p-cymene) coordination environment,
which is pseudooctahedral in nature as reflected in all the bond
parameters around Ru(II) ion. As all the complexes display similar
spectral and electrochemical properties, the other four complexes
are assumed to have similar structure to that of complex [Ru(η6-p-
cymene)Cl(L2)]Cl.
ions through azomethine (NC=N) nitrogen (νC=N: 1606–1555 cm−1
)
and the thione sulphur (νC=S: 828–809 cm−1) (Table 1). Bands in the
region 464–440 cm−1 and 428–417 cm−1 are probably due to Ru–N and
Ru–S bond, respectively [21]. The electronic spectra of all the complexes
in chloroform atroom temperature showedthe mediumintensitybands
around 435–449 nm region due to metal to ligand charge transfer
⁎
⁎
transition (MLCT) and intense n-π , π–π ligand-centered transitions
bands with maxima in the 243–284 nm region (Table 1). The pattern of
the electronic spectra of all the complexes indicates the presence of an
octahedral environment around the ruthenium(II) ion [22]. The 1H NMR
spectra of the complexes were recorded in CDCl3. The multiplet
observed in the region around δ 6.8–8.4 ppm in all the listed complexes
have been assigned to the aromatic protons of the phenyl group of the
phenylthiosemicarbazone ligand. The singlet in the region δ 2.6–
3.0 ppm is due to methyl protons nearer to coordinated azomethine
group. The singlet around δ 3.8 ppm is due to methoxy protons in the
complex 5. In addition, the two isopropyl methyl protons of the p-
cymene appeared as doublet in the region of δ 0.9–1.7 ppm and the
methine protons comes in the region of δ 2.5–2.8 ppm as septet. Further,
the methyl group of the p-cymene comes as singlet around the region of
δ 1.8–1.9 ppm. Also, the hydrazinic proton and the imine (CH=N)
proton of thiosemicarbazone ligands shift downfield by 0.15–0.25 ppm
upon coordination to the ruthenium(II) ion. Cyclic voltammogram of
these complexes exhibit one quasi reversible reduction (ΔEp=150–
210 mV) at a scan rate of 100 mV s−1 and the E1/2 lies in the range from
−0.75 to −0.85 V (RuIII/RuII).
One of the complexes [Ru(η6-p-cymene)Cl(L2)]Cl is taken as
model catalyst and the catalytic activity in transfer hydrogenation of
aliphatic and aromatic ketones in the presence of isopropanol as
hydrogen source and base as promoter has been explored (Scheme 2).
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 studies of
ruthenium catalyzed transfer hydrogenation reaction by metal
hydride intermediates [24].
Since the base facilitates the formation of a ruthenium alkoxide by
abstracting the proton from isopropanol, different bases were used as
promoters in the transfer hydrogenation of ketones (Table 2).
Acetophenone was kept as a test substrate and allowed it to react in
isopropanol with catalytic quantities of complex 2 in the presence of
different bases like NaOH, KOH, t-BuOK and NaOAc. It has been
observed that NaOH and KOH are shown to have good conversions
The complex [Ru(η6-p-cymene)Cl(L2)]Cl (2) has been structurally
characterized by X-ray crystallography. The coordination sphere
Table 1
Analytical, IR and UV–Vis spectral data of η6-p-cymene ruthenium(II)phenylthiosemicarbazone complexes.
Complexes
Found (calculated) %
νC=N (cm−1
)
νC=S (cm−1
)
λmax, nm (ε, M−1 cm−1
)
C
H
N
S
(1) [Ru(η6-p-cymene)(Cl)(L1)]Cl
(2) [Ru(η6-p-cymene)(Cl)(L2)]Cl
(3) [Ru(η6-p-cymene)(Cl)(L3)]Cl
(4) [Ru(η6-p-cymene)(Cl)(L4)]Cl
(5) [Ru(η6-p-cymene)(Cl)(L5)]Cl
55.69 (55.62)
52.35 (52.32)
48.59 (48.56)
51.40 (51.41)
54.87 (54.79)
5.23 (5.27)
4.74 (4.78)
4.40 (4.46)
4.66 (4.68)
5.31 (5.37)
7.79 (7.83)
7.33 (7.29)
6.80 (6.75)
9.59 (9.63)
7.38 (7.25)
5.95 (5.92)
5.59 (5.60)
5.19 (5.18)
5.49 (5.53)
5.63 (5.59)
1557
1606
1555
1595
1604
810
828
821
809
825
449a (492), 282b (13180), 244c (13666)
440a (538), 282b (13408), 243c (14577)
435a (499), 283b (13185), 245c (13743)
442a (521), 278b (13664), 251c (14504)
445a (495), 284b (13560), 248c (13050)
a
MLCT.
n−π*.
π−π*.
b
c

M.U. Raja et al. / Inorganic Chemistry Communications 13 (2010) 1321–1324
1323
Fig. 1. ORTEP view of the complex [Ru(η6-p-cymene)Cl(L2)]Cl. Selected bond lengths (Å) and bond angles (°): Ru(1)–N(1) 2.130(3), Ru(1)–S(1) 2.3376(12), Ru(1)–Cl(1) 2.4218
(12) and N(1)–Ru(1)–S(1) 81.13(9), N(1)–Ru(1)–Cl(1) 86.00(9), S(1)–Ru(1)–Cl(1) 85.57(5).
when compared to the t-BuOK and NaOAc in the hydrogenation
reactions and the influence of bases in hydrogen promotion is in the
order of NaOHNKOHNt-BuOKNNaOAc. Hence, it is decided that base
NaOH is the best compromise between optimum reaction rate in
isopropanol and reaching 99% conversion for acetophenone within
20 h. Under the same experimental condition, transfer hydrogenation
of a series of aliphatic and aromatic ketones was explored by means of
complex 2 as catalyst.
In all the reactions, this complex catalyzes the reduction of ketones
to the corresponding alcohols via hydrogen transfer from isopropanol.
In a typical experiment, the ketone (3.75 mmol), arene ruthenium(II)
complex (0.0125 mmol) and NaOH (0.03 mmol) in 5 ml of isopropanol
(catalyst/substrate/base molar ratio 1:300:2.5) were taken into the
round bottom flask and are heated to reflux for 20 h at 82 °C. The
catalyst was removed from the reaction mixture by the addition of 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 2
performed efficiently in the conversion of ketones to alcohols in 71 to
99% and the results of this organic transformation are presented in
(Table 3). The conversion of ketone to secondary alcohol in the case of
acetophenone was 99% (20 h) (entry 1), which takes place at a faster
rate than that for 4-methoxy acetophenone (88%). The presence of
electron withdrawing (Cl) or electron donating (OCH3) substituent on
acetophenone (entry 2 and 3) has significant effect on the reduction of
ketones to their corresponding alcohols. The maximum conversion of 4-
chloro acetophenone to corresponding alcohol was achieved over a
period of 20 h. 4-methoxy acetophenone was converted to its
corresponding alcohol in 88% conversion (entry 3). Interestingly, this
catalyst shows excellent activity for the conversions of six, seven and
eight membered cyclic ketones (entries 4–7) to their corresponding
alcohols with 71 to 80% conversions. No transfer hydrogenation was
observed in the absence of base. The work up process is very simple for
this catalytic system and as the catalyst is stable in all organic solvents
and it can be recovered. The catalytic efficiency of the present complex is
less when compared to the ruthenium complexes containingSchiff base,
aryl azo and pincer ligands [25]. However, it has been found that the
Table 2
Catalytic transfer hydrogenation of acetopheneone by [Ru(η6-p-cymene)Cl(L2)]Cl with
i-PrOH.a
Entry
Bases
Time (h)
Conversion (%)
TON
1.
2.
3.
4.
NaOH
KOH
t-BuOK
AcONa
5
5
5
5
83
78
48
9
249
234
144
27
a
Conditions: reactions were carried out at 82 °C using 3.75 mmol of acetophenone
(5 ml isopropanol); catalyst/ketone/base ratio 1: 300: 2.5. TON=ratio of moles of
product obtained to the moles of catalyst used.
Scheme 2. Catalytic transfer hydrogenation of ketones.

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M.U. Raja et al. / Inorganic Chemistry Communications 13 (2010) 1321–1324
Table 3
www.ccdc.cam.ac.uk/data_request/cif, or from the Cambridge Crys-
tallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK.
Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.inoche.2010.07.026.
Catalytic transfer hydrogenation of ketones by [Ru(η6-p-cymene)Cl(L2)]Cl with
i-PrOH/NaOH.a
Entry
1
Ketone
Time (h)
5/20
Conversion (%)
83/99
TON
249/297
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We gratefully acknowledge financial support from the Department
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Appendix A. Supplementary data
CCDC 760991 contains the supplementary crystallographic data
for this paper. This data can be obtained free of charge via http://
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