Catalytic transfer hydrogenation of ketones by ruthenium(II) cyclometallated complex containing para-chloroacetophenone thiosemicarbazone

Article abstract of DOI:10.1016/j.inoche.2011.02.006

Ruthenium(II) cyclometallated complex containing para-chloroacetophenone thiosemicarbazone (L) of formula [Ru(L)(CO)(PPh₃)₂] has been synthesized and characterized by elemental and spectral analyses. The thiosemicarbazone ligand coor

Full text of DOI:10.1016/j.inoche.2011.02.006

Inorganic Chemistry Communications 14 (2011) 686–689
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Catalytic transfer hydrogenation of ketones by ruthenium(II) cyclometallated
complex containing para-chloroacetophenone thiosemicarbazone
⁎
Devaraj Pandiarajan, 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
Article history:
Ruthenium(II) cyclometallated complex containing para-chloroacetophenone thiosemicarbazone (L) of
formula [Ru(L)(CO)(PPh3)2] has been synthesized and characterized by elemental and spectral analyses. The
thiosemicarbazone ligand coordinates to ruthenium as a terdentate C, N, and S donor generating two five
membered metallacycles. The crystal structure analysis of the complex [Ru(L)(CO)(PPh3)2] indicates the
presence of a distorted octahedral geometry. The complex shows a quasi-reversible one electron oxidation
(RuIII/RuII) at 0.78 V vs SCE. Further, the catalytic transfer hydrogenation of substituted acetophenones by the
titled complex was carried out with conversions up to 99.3% in the presence of i-prOH/KOH.
© 2011 Elsevier B.V. All rights reserved.
Received 11 December 2010
Accepted 12 February 2011
Available online 19 February 2011
Keywords:
Cyclometallation
X-ray structure
Redox behavior
Catalytic transfer hydrogenation
The chemistry of transition metal complexes of thiosemicarbazone
has been receiving considerable attention primarily because of their
bioinorganic relevance [1] in general and for their biological activities
[2]. Thiosemicarbazones usually react as chelating ligands with
transition metal ions by bonding through the sulfur and the
azomethine nitrogen atoms and in some cases they behave as
tridentate ligands and bond through the sulfur and two nitrogen
atoms [3]. Complexation of the thiosemicarbazone usually occurs via
dissociation of the acidic thiol proton, resulting in the formation of a
five-membered chelate ring. Particularly, thiosemicarbazones have
emerged as an important class of sulfur donor ligands for transition
metal ions because of their mixed hard–soft donor character and
versatile coordination behavior [4]. Thiosemicarbazones and semi-
carbazones of aromatic aldehydes or ketones are known to act as
tridentate ligands can yield cyclometallated complexes having two
fused five-membered chelate rings at the metal center [5].
Among the different metal catalyzed hydrogenation reactions,
ruthenium-based catalytic systems are found to be effective in the
transfer hydrogenation of ketones [6] and imines [7]. The ability of
ruthenium complexes to dehydrogenate alcohols and deliver the
hydrides to a ketone [8] or an α,β-unsaturated ketone has made
them useful as transfer hydrogenation catalysts [9]. Ruthenium
complexes containing pincer ligands have been found to be the
efficient catalysts in transfer hydrogenation of aliphatic and aromatic
ketones [10]. Recently rhodium NCN pincer complexes have been
reported from our laboratory as excellent catalyst for transfer
hydrogenation reaction [11]. However, the catalytic activity of
cyclometallated ruthenium thiosemicarbazone has been neglected
from the literature. Hence, we describe here the synthesis and
characterization of cyclometallated ruthenium(II) complex bearing
para-chloroacetophenone thiosemicarbazone. Further, the redox
behavior and catalytic property of the synthesized complex have
been investigated.
para-chloroacetophenone thiosemicarbazone was prepared by
reported procedure [12]. The general procedure for the preparation
of the complex is as follows. To a methanolic solution of 0.1 mmol
[RuHCl(CO)(PPh3)3] was added the corresponding 0.2 mmol para-
chloroacetophenone thiosemicarbazone ligand, 0.1 mmol, lithium
bromide and 0.2 mmol sodium acetate. The mixture was refluxed
for 5 h under nitrogen atmosphere. The precipitate which formed was
filtered off, washed with methanol and dried in vacuo (Scheme 1).
Yield: 64%.
The complex is orange yellow in color and found to be air stable
and non-hygroscopic in nature. The synthesized ruthenium(II)
cyclometallated complex is highly soluble in common solvents such
as chloroform and dichloromethane. The analytical data (C, H, N and
S; Calcd: C, 65.39; H, 4.40; N, 4.40; and S, 3.36; Found: C, 65.26; H,
4.48; N, 4.34; and S, 3.39) are in good agreement with the
compositions proposed for the complex. The free ligand displays
υC_S absorption at 788–830 cm−1 and the N―H absorption at
3181 cm−1. The disappearance of υC_S and υN―H in the complex
confirms that the ligand coordinates to the metal in the thiol form and
a new band that appeared around 1295–1320 cm−1 region which
corresponds to υC―S. A strong band that is observed at 1601 cm−1
[13] is characteristic of the azomethine group (C N). A new band that
appeared at 1924 cm−1 is due to the terminally coordinated carbonyl
group and is observed at slightly higher frequency than in the
precursor complex. The IR spectrum that shows a band at 3427 cm−1 is
⁎
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045/2407020.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.02.006

D. Pandiarajan, R. Ramesh / Inorganic Chemistry Communications 14 (2011) 686–689
687
Table 2
Selected bond lengths (A°) and bond angles (°) of complex [Ru(L)(CO)(PPh3)2]a.
Bond lengths (A°)
Bond angles (°)
Ru1―C1
Ru1―S1
Ru1―P1
Ru1―P2
Ru1―N1
Ru1―C52
N1―N2
S1―C9
2.068(3)
2.4740(9)
2.3813(11)
2.3672(11)
2.093(2)
1.835(3)
1.384(4)
1.744(4)
1.307(4)
N1―Ru1―C1
S1―Ru1―N1
C1―Ru1―C52
S1―Ru1―C52
N1―Ru1―C1
S1―Ru1―P1
S1―Ru1―P2
P1―Ru1―P2
P1―Ru1―N1
P1―Ru1―C1
P1―Ru1―C52
P2―Ru1―N1
P2―Ru1―C1
P2―Ru1―C52
Ru1―S1―C9
77.09(12)
79.11(7)
95.44(14)
108.36(11)
77.09(12)
89.23(3)
88.00(3)
177.18(3)
89.54(8)
90.78(11)
89.75(13)
90.41(8)
91.96(11)
90.67(13)
94.99(11)
N1―C7
Scheme 1. Cyclometallation of thiosemicarbazone.
due to the thiosemicarbazone phenyl attached ―NH group [14]. This
absorption remains the same even after the coordination indicates that
the group is not involved in the coordination. In addition, the bands near
535, 695, 740 and 1556 cm−1 in the spectrum of the complex are
attributed to the presence of PPh3 ligands. The electronic spectrum of
the complex in CHCl3 shows intense band at 479 nm is assigned for
metal to ligand charge transfer transition (MLCT). The absorptions at
319 and 279 nm corresponds to usual n–π* and π–π* transitions
respectively. The pattern of the electronic spectrum of the complex
indicates the presence of an octahedral geometry around the
ruthenium(II) ion [15]. The 1H NMR spectrum shows a multiplet
around 6.2–7.8 ppm which is assigned to aromatic protons of the ligand
and the phenyl group of triphenylphosphine. The ―CH3 proton appears
as a singlet at 1.8 ppm and ―NH proton is observed as a broad peak in
the 4.6 ppm. Cyclic voltammogram displays a quasi-reversible one
electron oxidation (RuIII/RuII) in dichloromethane solution at the scan
rate of 100 mV s−1 and the E1/2 value of the complex is +0.78 V
(ΔEp=150 mV).
a
ESD in parenthesis.
(2)Å and 2.474(9)Å respectively. The two PPh3 groups occupy mutually
trans to each other and the Ru1―P1 2.381(11) and Ru1―P2 2.367(11)
bond distances are similar from the metal center [16]. The Ru―C bond
length is 1.835(3)Å in Ru1―C52 fragment which is quite normal
compared to the structurally characterized carbonyl complexes of
ruthenium [17]. The C(9)―S(1) distance 1.744(4) is consistent with
increased single bond character and C(9)―N(2) distance 1.300(4) is
increased to double bond character in deprotonated form.
Transfer hydrogenation of acetophenone in the presence of cyclo-
metallated Ru(II) thiosemicarbazone has been studied in isopropanol/
KOH medium (Scheme 2). The reactions were conducted at a catalyst,
substrate and base (C/S/base) in molar ratio 1:500:2.5 respectively.
In order to optimize the reaction conditions, different catalyst:substrate
(C/S) ratios were tested and the results are summarized in Table 3.
For this initial experiments, acetophenone was selected as a test-
substrate and was allowed to react in 2-propanol with catalytic quantity
of [Ru(L)(CO)(PPh3)2] in the presence of KOH. When increasing the C/S
ratio to 1:1000, 1:1500 in 2-propanol, the reaction still proceeds with a
reasonable conversions. Thus, it was concluded that catalyst:substrate
ratio of 1:500 is the best compromise between optimum reaction rate
and S/C ratio in 2-propanol.
The summary of single crystal X-ray structure refinement is shown
in Table 1 and selected bond lengths and bond angles are given in
Table 2. The thiosemicarbazone ligand coordinates to the metal center
with C, N and S forming two five membered metallacycles (Fig. 1) with
bite angles of N1―Ru1―C1=77.09(12)°; S1―Ru1―N1=79.11(7)°;
C1―Ru1―C52=95.44(14)°; and S1―Ru1―C52=108.36(11)°. The
bond lengths of Ru1―C1, Ru1―N1 and Ru1―S1 are 2.068(3)Å, 2.093
Table 1
The complex reduces the acetophenone and its derivatives with
good conversions over a period of 2 h (Table 4). The conversion of
acetophenone to 1-Phenyl-ethanol was completed with 99.3% isolated
yield. In this reaction, the base facilitates the formation of ruthenium
alkoxide by abstracting the proton of the alcohol and subsequent
β-elimination of alkoxide generates ruthenium-hydride, which is an
active species in this reactions. Although no mechanistic studies have
been performed, the catalytic transformation of acetophenone most
probably follows the classical pathway in which ketones coordinate to
hydride ruthenium metal intermediate [18]. It has been observed
that the catalytic activity varies with respect to substituent in the
acetophenone fragment. The electron donating substituent present in
the substrates found to be moderately active in the catalytic reactions. In
the case of 4′-methylacetophenone and 4′-methoxyacetophenone the
conversions to their corresponding alcohols are 93.3% and 89.4%
respectively (entry 2 and 3). Substrate with electron withdrawing
substituents such as Cl, Br, and NO2 plays a significant role in the
conversion of ketones to alcohols, (entries 4, 5, and 6) and catalyze with
good conversions of 96.5%, 97.5% and 98.5% respectively. The introduc-
tion of electron-withdrawing substituents to the para position of the
aryl ring of the ketones decreased the electron density on the C O bond
so that the activity (entries 2, 3, and 6) was improved giving rise to
easier hydrogenation. Although several catalytic systems have been
reported to support transfer hydrogenation reactions of ketones, a
catalyst of this type is novel for its CNS donor, PPh3 and CO ligand
Crystal data and structure refinement for complex [Ru(L)(CO)(PPh3)2].
Formula
C52H42ClN3OP2RuS
955.42
Triclinic
P-1 No. (2)
12.5823(2)
12.8009(2)
16.0076(2)
88.045(1)
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
74.654(1)
γ (°)
65.717(1)
2258.15(6)
2
1.405
0.566
Volume [Å3]
Z
Dcalc [g/cm3]
Mu(MoKa) [/mm]
F(000)
980
Crystal size [mm]
Temperature (K)
Radiation [angstrom]
Theta min.–max. [deg]
Dataset
Tot.,uniq. data, R (int)
Observed data [IN2.0 sigma(I)]
Nref, Npar
0.19×0.30×0.43
293
MoKa 0.71073
1.8, 24.3
−14:14; −14:14; −18:18
38420, 7289, 0.035
5897
7289, 551
R, wR2, S
R indices (all data)
0.0376, 0.1010, 1.044
0.0520, 0.0935, 1.044
w=1/[\s^2^(Fo^2^)+0.0484P)^2^+2.1375P] where P=(Fo^2^+2Fc^2^)/3
Max. and av. shift/error
0.00, 0.00
Min. and max. resd. dens. [e/Ang^3]
−0.62, 1.52

688
D. Pandiarajan, R. Ramesh / Inorganic Chemistry Communications 14 (2011) 686–689
Fig. 1. The ORTEP diagram of the complex [Ru(L)(CO)(PPh3)2].
Scheme 2. Transfer hydrogenation of aromatic ketones.
environment. The catalytic activity of the present complex is low when
compared to ruthenium complexes containing Schiff base or pincer
ligands [11,19]. However, the catalytic efficiencyof the complex is better
than the other ruthenium complexes in the transfer hydrogenation of
acetophenones with respect to catalyst/substrate ratio (1:500) and turn
over numbers [20,21].
1H NMR) methods which confirm the coordination mode of the ligand
to the metal through C, N, and S donors. X-ray diffraction study indicates
the presence of a distorted octahedral geometry around Ru(II). A one
electron quasi-reversible oxidation is observed. The catalytic activity
of the complex has been determined with a series of acetophenones
and the maximum conversion was found to be 99.3%.
The present work describes the synthesis of cyclometallated
ruthenium(II) thiosemicarbazone complex via C―H activation of
para-chloroacetophenone thiosemicarbazone. The characterization of
the complex was accomplished by analytical and spectral (IR, UV–Vis,
Acknowledgement
The authors express their sincere thanks to Council of Scientific
and Industrial Research (CSIR), New Delhi [Grant 01(2156)/07/EMR-
II] for financial support. One of the authors S.D.P thank CSIR for the
award of Senior Research Fellowship (SRF). We thank DST-FIST, India
for X-ray data collection at School of chemistry, Bharathidasan
University, Tiruchirappalli.
Table 3
Catalytic transfer hydrogenation of by [Ru(L)(CO)(PPh3)2]/i-PrOH/KOHa.
Entry
C/S
Time (h)
Conversionb (%)
Appendix A. Supplementary data
1
2
3
1:1500
1:1000
1:500
2
2
2
78.6
87.5
99.3
CCDC 759727 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Centre via www.ccdc.cam.ac.uk/data.
request/cif.
a
Conditions: reactions were carried out at 80 °C using substrate (5–15 mmol),
catalyst (0.01 mmol) in 10 ml of isopropanol, KOH (0.025 mmol).
b
Conversion of product was determined using a 1H NMR.

D. Pandiarajan, R. Ramesh / Inorganic Chemistry Communications 14 (2011) 686–689
689
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Table 4
Catalytic transfer hydrogenation of substituted acetophenone by [Ru(L)(CO)(PPh3)2]/i-
PrOH/KOHa.
Entry Substrate
Product
Conversion (%)b TONc
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OH
CH3
O
1
99.3
497
CH3
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H
H
93.3
467
OH
CH3
2
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H3C
H3C
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447
483
488
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4
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Cl
Cl
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6
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CH3
Br
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O
OH
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CH3
O2N
O2N
Conditions: reactions were carried out at 80°C using 5 mmol of ketone (10 ml
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b
Conversion of product was determined using a 1H NMR.
TON = moles of product per mole of catalyst.
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Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2011.02.006.
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