Synthesis, spectral characterization and catalytic studies of new ruthenium(II) chalcone thiosemicarbazone complexes

Article abstract of DOI:10.2478/s11532-009-0116-x

A series of new hexa-coordinated ruthenium(II) complexes of the type [Ru(CO)(EPh₃)(B)(L)] (E = P or As; B = PPh₃, AsPh₃ or Py; L = chalcone thiosemicarbazone) have been prepared by reacting [RuHCl(CO)(EPh₃)₂(B)] (E = P or As; B = PPh₃, AsPh₃ or Py) with chalcone thiosemicarbazones in benzene under reflux. The new complexes have been characterized by analytical and spectroscopic (IR, UV-vis, ¹H, ³¹P and ¹³C NMR) methods. On the basis of data obtained, an octahedral structure was assigned for all of the complexes. The chalcone thiosemicarbazones behave as dianionic tridentate O, N, S donors and coordinate to ruthenium via the phenolic oxygen of chalcone, the imine nitrogen of thiosemicarbazone and thienol sulfur. The new complexes exhibit catalytic activity for the oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones and they were also found to be efficient catalysts for the transfer hydrogenation of carbonyl compounds. Versita Warsaw and Springer-Verlag Berlin Heidelberg.

Full text of DOI:10.2478/s11532-009-0116-x

Cent. Eur. J. Chem. • 8(1) • 2010 • 229–240
DOI: 10.2478/s11532-009-0116-x
Central European Journal of Chemistry
Synthesis, spectral characterization and
catalytic studies of new ruthenium(II) chalcone
thiosemicarbazone complexes
Research Article
Manisekar Muthukumar, Periasamy Viswanathamurthi*
Department of Chemistry, Periyar University,
Salem 636011, India
Received 4 August 2009; Accepted 26 September 2009
Abstract: A series of new hexa-coordinated ruthenium(II) complexes of the type [Ru(CO)(EPh3)(B)(L)] (E = P or As; B = PPh3, AsPh3 or Py; L
= chalcone thiosemicarbazone) have been prepared by reacting [RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3, AsPh3 or Py) with
chalcone thiosemicarbazones in benzene under reflux. The new complexes have been characterized by analytical and spectroscopic
(IR, UV-vis, 1H, 31P and 13C NMR) methods. On the basis of data obtained, an octahedral structure was assigned for all of the complexes.
The chalcone thiosemicarbazones behave as dianionic tridentate O, N, S donors and coordinate to ruthenium via the phenolic oxygen
of chalcone, the imine nitrogen of thiosemicarbazone and thienol sulfur. The new complexes exhibit catalytic activity for the oxidation
of primary and secondary alcohols to their corresponding aldehydes and ketones and they were also found to be efficient catalysts for
the transfer hydrogenation of carbonyl compounds.
Keywords: Spectroscopic characterization • Catalytic oxidation • Catalytic transfer hydrogenation • Chalcone thiosemicarbazone complexes
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.
Transition
metal
complexes
containing
1. Introduction
triphenylphosphine have been extensively synthesized
and investigated because phosphine ligands are
reactive and versatile homogeneous catalysts in alkene
polymerization [13], Heck [14] and Suzuki [15] reactions
and oxidation of alcohols [16,17]. Oxidation of alcohols
are of paramount importance in organic syntheses
because of the importance of the product carbonyl
compounds as intermediates for medicines, agricultural
chemicals, and fragrances [18].
The reduction of ketones and aldehydes to the
corresponding alcohols, which can be performed by a
variety of methods, is another fundamental reaction in
organic synthesis. Among the methods used, transfer
hydrogenation [19] is an elegant strategy that can be
accomplished with homogeneous [20], heterogeneous
[21], perfluorinated [22], and water-soluble catalysts
[23], using different hydrogen sources, such as
Thiosemicarbazones have received considerable
attention due to their antibacterial [1,2] and cytotoxic
activity [3] against cancer cells. In recent years,
various metal complexes of thiosemicarbazide
derivatives have been the subject of interest due to
their wide range of pharmacological properties, such
as antimicrobial [4,5], antiameobic [6], antitumor [7]
and antiviral activity [8]. Transition metal complexes of
thiosemicarbazones possess some degree of cytotoxic
activity [9]. The chemistry of complexes of ruthenium
with thiosemicarbazones, which can coordinate to the
metal either in neutral thione form or in the anionic
thiolate form, has received attention in recent years
primarily due to their varied coordination mode, novel
electrochemical and electronic properties, as well as
their biological importance [10-12].
* E-mail: viswanathamurthi@rediffmail.com
229

Synthesis, spectral characterization and catalytic studies
of new ruthenium(II) chalcone thiosemicarbazone complexes
isopropyl alcohol and formic acid derivatives. Transfer
hydrogenation of carbonyl compounds using isopropyl
alcohol as reducing agent as well as solvent has
attracted a lot of attention over the last few years due
to the comparatively benign nature of the reagents [24].
Althoughanumberoftransitionmetalcatalystshavebeen
used for this transformation, ruthenium(II) complexes
[24,25] have attracted the most interest. In contrast to
conventional hydrogenation using dihydrogen and metal
catalysts, which frequently require a high hydrogen
pressure [26], the transfer hydrogenation has some
unique advantages in its simplicity and avoidance of
cumbersome reducing agents. However, in most cases,
the reactions are effective only with ketones. It has been
shown, however, that ruthenium complexes containing
phosphine or acetonitrile ligands are effective catalysts
for reduction of aldehydes [27]. Encouraged by these
reports, we wanted to explore the efficacy of the newly
synthesized [ONS]-type chalcone thiosemicarbazone
ruthenium(II) complexes containing triphenylphosphine/
arsine.
H3CO
OH
S
C
N
NH C NH2
CH
CHR
Ligand
R
L1
L2
L3
L4
4-(CH3)C6H4
4-(OCH3)C6H4
4-(Cl)C6H4
3,4-(OCH3)2C6H3
Figure 1. Structure of chalcone thiosemicarbazones
13C) were recorded on a Jeol GSX-500 spectrometer in
CDCl3. 1H NMR and 13C NMR spectra were obtained at
room temperature using TMS as the internal standard.
31P NMR spectra of the complexes were obtained using
orthophosphoric acid as a reference. The catalytic
yields were determined using ACME 6000 series gas
chromatography instrument equipped with a flame
ionization detector (FID) using a DP-5 column of 30 m
length, 0.53 mm diameter and 5.00 µm film thickness.
Melting points were recorded on a Technico micro
heating table and are uncorrected.
Hence, in this paper we report the synthesis,
characterization and catalytic activities of
a
series of hexacoordinated ruthenium(II) chalcone
thiosemicarbazone complexes containing PPh3/
AsPh3. The characterizations of the complexes were
accomplished by analytical and spectroscopic (IR, UV-
vis, 1H, 31P and 13C NMR) methods.
2. Experimental procedure
2.1. Reagents and materials
2.3. Preparation of ONS tridentate chalcone
All the reagents used were chemically pure and AR
grade. The solvents were purified and dried according to
standard procedures [28]. RuCl3•3H2O was purchased
from Loba Chemie Pvt. Ltd., and was used without
further purification. The starting complexes [RuHCl(CO)
(PPh3)3] [29], [RuHCl(CO)(AsPh3)3] [30] and [RuHCl(CO)
(Py)(PPh3)2] [31] were prepared according to literature
methods. The general structure of the chalcone
thiosemicarbazone ligands used in this study is given
below (Fig. 1).
thiosemicarbazone ligands
Ligands were prepared by a two-step processes. In
the first step, the 2-hydroxychalcone ligands were
prepared by stirring 4-substituted benzaldehyde
(0.6008-0.8309 g,
5
mmol) with 2-hydroxy-4-
methoxyacetophenone (0.8309 g, 5 mmol) in the
presence of 50 cm3 alcoholic sodium hydroxide solutions
(20%).After 24 h stirring, the product was precipitated by
addition of concentrated hydrochloric acid, then filtered
and recrystallized from ethanol. In the second step,
2-hydroxychalcones (0.2683-0.3143 g, 1 mmol) were
reacted with thiosemicarbazide (0.4253 g, 4.7 mmol) in
ethanol (100 cm3) containing concentrated hydrochloric
acid (0.5 cm3). The reaction mixture was refluxed for 8
h and the solvent was evaporated under vacuum. The
resulting residue (yield 60-65%) was recrystallized from
ethanol. The ligands were characterized by analytical
and spectroscopic methods (Tables 1-4).
2.2. Instruments
CHN analyses were performed by using Carlo-Erba
1108 model analyzer at Central Drug Research Institute
(CDRI), Lucknow, India. FT-IR spectra were recorded
in KBr pellets with a Nicolet FT-IR spectrophotometer
over the 400-4000 cm-1 range. UV-vis spectra of the
complexes were recorded in dichloromethane solution
on a Shimadzu UV-visible 1650 PC spectrophotometer
over the 200-800 nm range. NMR spectra (1H, 31P and
230

M. Muthukumar, P. Viswanathamurthi
2.4. Synthesis of new ruthenium(II) chalcone 2.5. Procedure for catalytic oxidation of
thiosemicarbazone complexes
alcohols
All complexes were prepared by the following common Catalytic oxidation of primary alcohols to the
procedure: To a solution of [RuHCl(CO)(EPh3)2(B)] corresponding aldehydes and secondary alcohols to
(E = P or As; B = PPh3, AsPh3 or Py) (0.1 g) in benzene ketones by ruthenium(II) chalcone thiosemicarbazone
(20 cm3), the appropriate chalcone thiosemicarbazone complexes were studied in the presence of NMO
(0.0315-0.0504 g) was added in 1:1 molar ratio. The as co-oxidant. A typical reaction using the complex
mixture was heated under reflux for 6 h on a water as a catalyst and primary or secondary alcohols as
bath. The reaction mixture gradually became intensely substrates at 1:100 molar ratio was as follows:Asolution
colored during heating. The contents were then of ruthenium complex (0.01 mmol) in dichloromethane
concentrated to approximately 3 cm3 by removing the (20 cm3) was added to the mixture containing substrate
solvent under reduced pressure. The contents were (1 mmol), NMO (3 mmol) and molecular sieves. The
cooled and the product was separated by the addition of solution mixture was reacted under reflux for 10 min,
10 cm3 of petroleum ether (60-80°C). The product was and the solvent was then evaporated from the mother
recrystallized from a dichloromethane/petroleum ether liquor under reduced pressure. The solid residue was
mixture. The compounds were dried under vacuum over then extracted with diethyl ether (60-80°C) (20 cm3)
fused calcium chloride and the purity of the complexes and concentrated to approximately 1 cm3. The extract
was checked by TLC (Yield 76-89%).
was dried over anhydrous sodium sulfate and analyzed
Table 1. Analytical data of free ligands and their ruthenium(II) chalcone thiosemicarbazone complexes.
Yield
(%)
M.Pt
Compound
Formula
Calculated (found) (%)
H
(˚C)
C
N
L1
C18H19O2N3S
C18H19O3N3S
60
63
61
65
86
85
82
78
87
79
81
89
76
86
78
89
094
163
185
180
156
227
224
232
138
165
154
134
224
222
173
170
63.32(63.38)
60.48(60.54)
56.43(56.39)
58.89(58.93)
66.52(66.61)
65.45(65.48)
63.99(64.10)
64.73(64.72)
61.11(61.16)
60.22(60.18)
58.88(58.79)
59.68(59.60)
62.28(62.24)
61.08(61.13)
59.30(59.26)
60.34(60.36)
5.61(5.58)
5.35(5.31)
4.45(4.43)
5.46(5.45)
4.77(4.73)
4.69(4.70)
4.37(4.35)
4.75(4.72)
4.38(4.43)
4.32(4.28)
4.02(3.98)
4.38(4.39)
4.60(4.58)
4.51(4.56)
4.12(4.11)
4.59(4.61)
12.31(12.37)
11.75(11.70)
11.61(11.58)
10.84(10.79)
4.23(4.21)
4.16(4.20)
4.14(4.10)
4.04(4.00)
3.88(3.90)
3.83(3.85)
3.81(3.79)
3.73(3.71)
6.91(6.89)
6.78(6.72)
6.74(6.69)
6.54(6.50)
L2
L3
C17H16O2ClN3S
L4
C19H21O4N3S
[Ru(CO)(PPh3)2(L1)]
[Ru(CO)(PPh3)2(L2)]
[Ru(CO)(PPh3)2(L3)]
[Ru(CO)(PPh3)2(L4)]
[Ru(CO)(AsPh3)2(L1)]
[Ru(CO)(AsPh3)2(L2)]
[Ru(CO)(AsPh3)2(L3)]
[Ru(CO)(AsPh3)2(L4)]
[Ru(CO)(Py)(PPh3)(L1)]
[Ru(CO)(Py)(PPh3)(L2)]
[Ru(CO)(Py)(PPh3)(L3)]
[Ru(CO)(Py)(PPh3)(L4)]
C55H47O3N3SP2Ru
C55H47O4N3SP2Ru
C54H44O3N3SClP2Ru
C56H49O5N3SP2Ru
C55H47O3N3SAs2Ru
C55H47O4N3SAs2Ru
C54H44O3N3SClAs2Ru
C56H49O5N3SAs2Ru
C42H37O3N4SPRu
C42H37O4N4SPRu
C41H34O3N4SClPRu
C43H39O5N4SPRu
231
231

Synthesis, spectral characterization and catalytic studies
of new ruthenium(II) chalcone thiosemicarbazone complexes
Table 2. IR absorption frequencies (cm-1) of free ligands and their ruthenium(II) chalcone thiosemicarbazone complexes
Compound
PPh3/AsPh3
νC≡O
νC=N
ν
C=N + νC=C
νC-O
νC-S
L1
-
1610
1609
1605
1606
1590
1582
-
1330
1343
1332
1328
1368
1365
816
835
830
822
745
746
-
L2
-
-
-
L3
-
-
-
L4
-
-
-
[Ru(CO)(PPh3)2(L1)]
[Ru(CO)(PPh3)2(L2)]
1930
1929
1556
1559
1433, 1092, 696
1433, 1092, 697
[Ru(CO)(PPh3)2(L3)]
1929
1585
1563
1344
746
1432, 1091, 697
[Ru(CO)(PPh3)2(L4)]
[Ru(CO)(AsPh3)2(L1)]
[Ru(CO)(AsPh3)2(L2)]
[Ru(CO)(AsPh3)2(L3)]
[Ru(CO)(AsPh3)2(L4)]
[Ru(CO)(Py)(PPh3)(L1)]
[Ru(CO)(Py)(PPh3)(L2)]
[Ru(CO)(Py)(PPh3)(L3)]
[Ru(CO)(Py)(PPh3)(L4)]
1917
1929
1943
1935
1935
1930
1926
1930
1930
1585
1590
1596
1585
1584
1582
1583
1581
1583
1561
1554
1559
1560
1562
1559
1554
1553
1559
1371
1343
1362
1342
1361
1343
1358
1368
1367
746
739
739
740
740
746
746
746
746
1432, 1092, 697
1434, 1082, 694
1435, 1086, 694
1435, 1086, 694
1436, 1079, 694
1432, 1092, 697
1432, 1092, 697
1432, 1091, 696
1432, 1092, 697
Table 3. UV-vis spectroscopic data of free ligands and their ruthenium(II) chalcone thiosemicarbazone complexes.
Compound
λλmax, λ nm (ε) (dm3 mol-1 cm-1
)
L1
337(10562), 302(11630), 237(21956)
L2
341(10358), 302(11630), 236(22182)
L3
337(10562), 303(10849), 233(23956)
L4
337(10562), 302(11630), 290(13425), 238(21242)
432(6574), 361(7986), 272(17130), 235(22890)
432(6574), 362(7543), 272(17130), 234(23215)
[Ru(CO)(PPh3)2(L1)]
[Ru(CO)(PPh3)2(L2)]
[Ru(CO)(PPh3)2(L3)]
436(6285), 362(7543), 270(18213), 238(21242)
[Ru(CO)(PPh3)2(L4)]
[Ru(CO)(AsPh3)2(L1)]
[Ru(CO)(AsPh3)2(L2)]
[Ru(CO)(AsPh3)2(L3)]
[Ru(CO)(AsPh3)2(L4)]
[Ru(CO)(Py)(PPh3)(L1)]
[Ru(CO)(Py)(PPh3)(L2)]
[Ru(CO)(Py)(PPh3)(L3)]
[Ru(CO)(Py)(PPh3)(L4)]
430(6763), 362(7543), 275(16025), 237(21956)
351(9596), 267(19128), 235(22890)
353(8318), 265(19956), 234(23215)
351(9596), 267(19128), 234(23215)
352(8952), 267(19128), 236(22182)
431(6692), 363(7121), 272(17130), 235(22890)
431(6695), 362(7543), 272(17130), 235(22890)
431(6698), 362(7543), 270(18213), 234(23215)
428(6993), 362(7543), 271(17986), 235(22890)
232

M. Muthukumar, P. Viswanathamurthi
1
Table 4. H NMR data of free ligands and their ruthenium(II) chalcone thiosemicarbazone complexes.
Compound
L1
δ (ppm)
8.67 (NH), 6.96-7.88 (m, aromatic and -CH=CH-), 6.32 (OH), 5.10 (NH2), 3.81 (OCH3), 2.39 (CH3)
8.59 (NH), 6.62-7.88 (m, aromatic and -CH=CH-), 6.30 (OH), 5.12 (NH2), 3.85, 3.81 (OCH3)
8.63 (NH), 6.85-7.76 (m, aromatic and -CH=CH-), 6.31 (OH), 5.10 (NH2), 3.81 (OCH3)
7.27-7.36 (m, aromatic and -CH=CH-), 4.38 (NH2), 3.50 (OCH3), 2.38 (CH3)
6.95-7.36 (m, aromatic and -CH=CH-), 4.42 (NH2), 3.84, 3.50 (OCH3)
7.27-7.38 (m, aromatic and -CH=CH-), 4.39 (NH2), 3.51 (OCH3)
L2
L3
[Ru(CO)(PPh3)2(L1)]
[Ru(CO)(PPh3)2(L2)]
[Ru(CO)(PPh3)2(L3)]
[Ru(CO)(AsPh3)2(L1)]
[Ru(CO)(AsPh3)2(L2)]
[Ru(CO)(Py)(PPh3)(L2)]
[Ru(CO)(Py)(PPh3)(L3)]
7.15-7.35 (m, aromatic and -CH=CH-), 4.38 (NH2), 3.50 (OCH3), 2.39 (CH3)
6.85-7.32 (m, aromatic and -CH=CH-), 4.42 (NH2), 3.84, 3.51 (OCH3)
6.91-7.30 (m, aromatic and -CH=CH-), 4.40 (NH2), 3.85, 3.50 (OCH3)
7.18-7.36 (m, aromatic and -CH=CH-), 4.39 (NH2), 3.51 (OCH3)
by GC. The oxidation products were identified by GC thiosemicarbazone ligands in dry benzene in an equal
co-injection with authentic samples of the appropriate molar ratio (Scheme 1).
aldehyde or ketone.
In all of these reactions, it was observed
that the chalcone thiosemicarbazones behave as
dibasic tridentate chelating ligands by replacing a
triphenylphosphine/arsine, chloride and hydride ion
from the starting complexes.
2.6. Procedure for catalytic transfer
hydrogenation of carbonyl compounds
The catalytic transfer hydrogenation reactions
were also studied using ruthenium(II) chalcone
thiosemicarbazone complex as a catalyst, carbonyl
compounds as substrate and KOH as promoter at
1:300:2.5 molar ratios. The procedure was as follows:
A mixture containing carbonyl compounds (3.75 mmol),
the ruthenium complex (0.0125 mmol) and KOH (0.03
mmol) in isopropyl alcohol (10 cm3) was reacted under
reflux in a water bath for different time periods. After
completion of the reaction the catalyst was removed
from the reaction mixture by the addition of diethyl ether
followed by filtration and subsequent neutralization with
1M HCl. The ether layer was filtered through a short path
of silica gel by column chromatography. The filtrate was
concentrated to approximately 1 cm3 and subjected to
GC analysis. The hydrogenated product was identified
and determined by comparison with authentic samples
of the appropriate alcohol.
All of the complexes are stable in air at room
temperature, yellowish-green in color, non-
hygroscopic and highly soluble in common organic
solvents such as dichloromethane, acetonitrile,
chloroform, benzene and DMSO. The analytical
data are listed in Table 1 and are in good agreement
H3CO
OH
S
C
[RuHCl(CO)(EPh3)2(B)]
+
C
N
NH
NH2
CH
CHR
Benzene/
Reflux 6h
EPh3
Ru
O
CO
S
H3CO
3. Results and discussion
C
N
B
C
CH
Diamagnetic, hexa-coordinated low spin ruthenium
(II) complexes of general formula [Ru(CO)(EPh3)(B)
(L)] (E = P or As; B = PPh3, AsPh3 or Py; L = chalcone
thiosemicarbazone) were synthesized in good
yields from the reaction of [RuHCl(CO)(EPh3)2(B)]
(E = P or As; B = PPh3, AsPh3 or Py) with chalcone
N
NH2
CHR
(E = P or As; B = PPh3, AsPh3 or Py; R = 4-(CH3)C6H4, 4-(OCH3)C6H4, 4-(Cl)
C6H4 or 3,4-(OCH3)2C6H3)
Scheme1.Formationofruthenium(II)chalconethiosemicarbazone
complexes
233
233

Synthesis, spectral characterization and catalytic studies
of new ruthenium(II) chalcone thiosemicarbazone complexes
triphenylarsine (around 1440, 1090 and 700 cm-1) were
also present in the spectra of all complexes [17].
with the general molecular formula proposed for the
complexes.
3.2. Electronic spectroscopic analysis
3.1. Infrared Spectroscopic analysis
The important IR absorption frequencies of the ligands
and their metal complexes, along with their assignments,
are listed in Table 2. The ligands used in the present study
can exhibit thione-thienol tautomerism (Scheme 2).
All the chalcone thiosemicarbazone ruthenium
complexes are diamagnetic, indicating the presence of
ruthenium in the +2 oxidation state. The ground state
1
of ruthenium(II) in an octahedral environment is A1g,
arising from the t62g configuration, and the excited states
H3CO
OH
H3CO
OH
S
C
SH
C
3
3
corresponding to the t52ge1g configuration are T1g, T2g,
C
N
NH
NH2
C
N
N
NH2
1
1T1g and T2g. Hence, four bands corresponding to the
CH
CH
transitions 1A1g→3T1g, 1A1g→3T2g, 1A1g→1T1g and 1A1g→1T2g
are possible in order of increasing energy.
CHR
CHR
The electronic spectral data of the free ligands
and their complexes in dichloromethane are listed in
Table 3. The spectra of all the free ligands showed two
types of transitions. The absorbance that appears in the
233–238 nm range was assigned to π- π* transitions
involving molecular orbitals located on the phenolic
chromophore. These peaks have been shifted in
the spectra of the complexes; this may be due to the
donation of a lone pair of electrons by the oxygen of
the phenoxy group to the central metal atom [36]. The
second type of transitions appeared at the 290–341 nm
range and were assigned to n-π* transitions involving
molecular orbitals of the -C=N chromophore and the
benzene ring. These bands were also shifted upon
complexation, indicating that the imine group nitrogen
atom appears to be coordinated to the metal ion [37].
The spectra of all the complexes show another type
of transition (Fig. 2) different from the free ligands. The
broad bands appeared at the 351–436 nm range, which
can be assigned to charge transfer (C.T) transitions [38].
The nature of the observed electronic spectra and the
position of absorption bands are consistent with those of
other similar ruthenium(II) octahedral complexes [38].
Scheme 2. Tautomerism in chalcone thiosemicarbazones
The free chalcone thiosemicarbazone ligands
showed a strong band in the region 1605-1610 cm-1 due
to νC═N. This band was shifted to a lower wave number
(1581-1596 cm-1) in the ruthenium complexes indicating
the coordination of the ligands to ruthenium through
the azomethine nitrogen atom [32] and this lowering of
the wave number may be attributed to the decrease in
electron density on the nitrogen atom of the azomethine
group. In all of the complexes, the bands that appeared
in the region 1536-1551 cm-1 were assigned to the mixed
mode of vibration arising from νC=N and νC=C vibrations
[33]. This also indicates that the coordination is through
the nitrogen atom.Astrong band observed at 1328-1343 cm-1
in the free chalcone thiosemicarbazone ligands was
assignedtophenolicνC-O stretching.Thisbandwasshifted
to higher wave number (1342-1371 cm-1) in the spectra
of the complexes due to its coordination to ruthenium
ion through the oxygen atom of the phenolic group [34].
This was further supported by the disappearance of the
broad νOH band around 3400-3600 cm-1 in the complexes,
indicating deprotonation of the phenolic proton prior to
coordination to the ruthenium metal. The band due to
νC-S appeared around 816-835 cm-1 in the free ligands
but disappeared upon complexation and a new band
appeared around 739-746 cm-1. These observations may
be attributed to thienolisation of the –NH-C=S group and
subsequent coordination through the deprotonated sulfur
[35]. Hence, from the infrared spectroscopic data, it is
inferred that the azomethine nitrogen, phenolic oxygen
and thienol sulfur atoms are involved in the coordination
of the chalcone thiosemicarbazone to ruthenium ion in
all complexes. In addition, the appearance of a strong
band at 1917-1943 cm-1 and a medium intensity band
at 1000-1030 cm-1 indicate the presence of carbon
monoxide and nitrogen base [34], respectively. All the
other characteristic bands due to triphenylphosphine or
3.3. 1H NMR Spectroscopic analysis
The 1H NMR spectra of the ruthenium(II) chalcone
thiosemicarbazone complexes and ligands were
recorded in CDCl3 to confirm the binding of chalcone
thiosemicarbazone to the ruthenium ion. All of the
complexes exhibit a series of overlapping multiplets
in the region 7.38-6.85 ppm in their spectra (Table 4),
which were assigned to the protons of the phenyl groups
present in the PPh3 or AsPh3 or Py ligands and chalcone
thiosemicarbazone [39]. The signal due to two alkene
protons also appeared in the region 7.3-6.9 ppm and
hence merged with the multiplets of aromatic protons.
This clearly revealed the absence of alkene coordination
234

M. Muthukumar, P. Viswanathamurthi
to the metal, as alkene carbon coordination would result
in shifts of the olefinic protons to lower δ value at least
by 2 ppm [39]. The signal for OH protons appeared
as a broad singlet in the region 6.32-6.30 ppm in free
chalcone thiosemicarbazone ligands, but in complexes
no peak was observed for the OH protons, supporting
the assumption that the phenolic group is deprotonated.
This suggests coordination occurs through the phenolic
oxygenofchalconethiosemicarbazonestotheruthenium
ion. The signal for the terminal NH2 protons appeared
as a singlet at 4.40-4.37 ppm in complexes, which
corresponds to an up-field shift in comparison with that
of the NH2 resonance free ligands. Where present on the
chalcone thiosemicarbazones, the methoxy protons on
the benzaldehyde-derived aromatic ring were observed
in 3.85-3.84 ppm region in both complexes and the free
ligands. The acetophenone-derived methoxy protons
appeared as a singlet in 3.51-3.50 ppm region in the
metal complexes, corresponding to an up-field shift in
comparison with that of the corresponding free ligands,
which indicates a shielding effect of an aromatic ring.
Further, the signal for methyl proton appeared as a
singlet at 2.39 ppm in both complexes and the free
ligand.
Figure 3. 31P NMR spectrum of [Ru(CO)(PPh3)2(L2)]
ligands, a sharp singlet was observed around 36.15-
36.21 ppm for magnetically equivalent phosphorus
atoms trans to each other [35]. The spectra of all other
complexes exhibited a singlet around 34.82-34.86 ppm,
corresponding to the presence of a triphenylphosphine
group trans to a heterocyclic nitrogen base [39].
3.4. 31P NMR Spectroscopic analysis
31P NMR spectra of some of the complexes were
recorded to confirm the presence of triphenylphosphine
groups in the new complexes (Table 5, Fig. 3). In the
case of complexes containing two triphenylphosphine
3.5. 13C NMR Spectroscopic analysis
The 13C spectra of ruthenium(II) chalcone
thiosemicarbazone complexes (Table 5) exhibit
a strong signal at 205.36-204.64 ppm and were
assigned to CO carbon. A sharp singlet appeared at
177.98-176.35 ppm that was assigned to phenolic
–C-O carbon. The appearance of a peak at 159.68-
159.46 and 157.69-157.58 ppm were assigned to
-C-S and –C=N carbons, respectively. Multiplets
appeared around 138.22-114.27 ppm region have
been assigned to aromatic carbons. The alkene
carbons appeared in the region 130.58-115.45 ppm,
and hence merged with the aromatic carbons. The
presence of peak at 96.03-96.02 ppm was assigned
to α =CH- carbon. Sharp singlets at 55.38 and 54.98-
54.86 ppm were assigned to the methoxy carbon of
benzaldehyde-derived and acetophenone-derived
groups, respectively. In addition, a sharp singlet at
21.19-21.15 ppm was assigned to methyl carbons.
Based on the analytical and spectroscopic (IR, UV-
vis, 1H, 31P and 13C NMR) data, an octahedral structure
(Fig. 4) has been tentatively proposed for all the
ruthenium(II) chalcone thiosemicarbazone complexes.
Figure 2. Electronic spectrum of [Ru(CO)(PPh3)2(L4)]
235
235

Synthesis, spectral characterization and catalytic studies
of new ruthenium(II) chalcone thiosemicarbazone complexes
Table 5. 13C NMR and 31P NMR data (δδ in ppm) of ruthenium(II) chalcone thiosemicarbazone complexes.
Complex
13C NMR
31P NMR
204.89 (s), 176.54 (s), 159.53 (s), 157.63 (s),
138.22-120.50 (m), 96.03 (s), 54.93 (s), 21.19 (s)
[Ru(CO)(PPh3)2(L1)]
36.21
205.36 (s), 176.24 (s), 159.68 (s), 157.69 (s),
133.91-114.27 (m), 96.02 (s), 55.38 (s), 54.92 (s)
[Ru(CO)(PPh3)2(L2)]
[Ru(CO)(Py)(PPh3)(L1)]
[Ru(CO)(Py)(PPh3)(L3)]
36.15
34.86
34.82
204.64 (s), 176.35 (s), 159.46 (s), 157.58 (s),
134.54-116.36 (m), 96.03 (s), 54.98 (s), 21.15 (s)
205.12 (s), 177.98 (s), 159.62 (s), 157.67 (s),
136.52-117.13 (m), 96.02 (s), 54.86 (s)
3.6. Catalytic oxidation
EPh3
Ru
Catalytic oxidation of primary and secondary
alcohols by the synthesized ruthenium (II) chalcone
thiosemicarbazone complexes were carried out in
dichloromethane in the presence of NMO, and the
by-product of water was removed by using molecular
sieves. All of the complexes oxidize primary alcohols
to the corresponding aldehydes, and secondary
alcohols are oxidized to ketones in high yields and
O
H3CO
CO
S
C
N
B
C
CH
N
NH2
CHR
the results are listed in Table 6. The oxidation of Figure 4. Proposed structure of the new ruthenium(II) chalcone
thiosemicarbazone complexes (E = P or As; B = PPh3,
AsPh3 or Py; R = 4-(CH3)C6H4, 4-(OCH3)C6H4, 4-(Cl)C6H4 or
3,4-(OCH3)2C6H3)
benzyl alcohol to benzaldehyde and cinnamyl alcohol
to cinnamaldehyde resulted in 94-99% and 96-99%
yields, respectively.
O
N
O
The present investigations suggest that the
complexes react efficiently with NMO to yield a high
valent ruthenium-oxo species [40] capable of oxygen
atom transfer to alcohols. This was further supported
by spectroscopic changes that occur on addition of
NMO to a dichloromethane solution of the ruthenium(II)
complexes. The appearance of a band at 390 nm in
UV-vis spectra is attributed to the formation of high-
valent RuIV=O species, which was also found for other
oxo ruthenium(IV) complexes [41,42]. Further support
comes from FT-IR of the solid (obtained by evaporation
of the resultant solution to dryness), which shows a band
at 860 cm-1, characteristic of RuIV=O species (Scheme 3)
[38], which is absent in the ruthenium catalyst. Except
for the above difference noted, the IR spectra of the
catalyst and solid appear quite similar, which suggest
that the coordinated chalcone thiosemicarbazone
ligand remains intact in the oxidation process. It follows
that the catalytic oxidation proceeds through metal-oxo
intermediate.
(ligand)RuII(PPh3)2
H2O
+
2PPh3
+
H3C
O
OH
O
N
(ligand)RuIV
O
2PPh3
+
CH3
Scheme 3. Proposed catalytic cycle for the oxidation of alcohols
by the ruthenium(II) chalcone thiosemicarbazone
complexes
3.7. Catalytic transfer hydrogenation of
carbonyl compounds
One of the newly synthesized complexes, [Ru(CO)
(AsPh3)2(L1)], was employed as a “model” catalyst
and the catalytic activity in transfer hydrogenation of
various types of aldehydes and ketones in the presence
of isopropyl alcohol and KOH was explored. In all of
the reactions, this complex catalyzes the reduction of
aldehydes and ketones to the corresponding alcohols
236

M. Muthukumar, P. Viswanathamurthi
Table 6. Catalytic oxidation data of ruthenium(II) chalcone thiosemicarbazone complexes.
Complex
Substrate
Product
Yield (%)a
Benzyl alcohol
A
B
98
99
[Ru(CO)(PPh3)2(L1)]
Cinnamyl alcohol
Benzyl alcohol
A
B
97
99
[Ru(CO)(PPh3)2(L2)]
[Ru(CO)(PPh3)2(L3)]
[Ru(CO)(PPh3)2(L4)]
[Ru(CO)(AsPh3)2(L1)]
[Ru(CO)(AsPh3)2(L2)]
[Ru(CO)(AsPh3)2(L3)]
[Ru(CO)(AsPh3)2(L4)]
[Ru(CO)(Py)(PPh3)(L1)]
[Ru(CO)(Py)(PPh3)(L2)]
Cinnamyl alcohol
Benzyl alcohol
A
B
99
99
Cinnamyl alcohol
Benzyl alcohol
A
B
95
97
Cinnamyl alcohol
Benzyl alcohol
A
B
99
99
Cinnamyl alcohol
Benzyl alcohol
A
B
96
98
Cinnamyl alcohol
Benzyl alcohol
A
B
98
99
Cinnamyl alcohol
Benzyl alcohol
A
B
99
99
Cinnamyl alcohol
Benzyl alcohol
A
B
95
97
Cinnamyl alcohol
Benzyl alcohol
A
B
94
96
Cinnamyl alcohol
Benzyl alcohol
A
B
98
99
[Ru(CO)(Py)(PPh3)(L3)]
Cinnamyl alcohol
Benzyl alcohol
A
B
99
99
[Ru(CO)(Py)(PPh3)(L4)]
Cinnamyl alcohol
A: benzaldehyde; B: Cinnamaldehyde
a Yield determined by GC and comparing with the analyses of authentic samples.
O
OH
via hydrogen transfer from isopropyl alcohol with KOH
as a promoter (Scheme 4, Table 7).
Catalyst : 0.0125 mmol
isoPrOH/KOH
Mechanstically, the base facilitates the formation
of ruthenium alkoxide by abstracting proton from
isopropyl alcohol and subsequently the alkoxide
undergoes β-elimination to give ruthenium hydride,
which is an active species. No transfer hydrogenation
was observed in the absence of base. In addition,
the work-up process is very simple for this catalytic
system and as the catalyst is stable in all organic
solvents, it can be recovered.
R
R'
R
R'
R, R' = alkyl or aryl
Scheme 4. Reaction of catalytic transfer hydrogenation
4. Conclusions
Several new ruthenium(II) chalcone thiosemicarbazone
complexes were synthesized using chalcone
237
237

Synthesis, spectral characterization and catalytic studies
of new ruthenium(II) chalcone thiosemicarbazone complexes
Table 7. Catalytic transfer hydrogenation of carbonyl compounds by [Ru(CO)(AsPh3)2(L1)] with isopropyl alcohola
Entry
Substrate
Product
Time (min)
Conversion (%)b
CHO
OH
1
2
3
30
30
30
91
86
89
CHO
OH
H3C
H3C
CHO
OH
CHO
OH
4
30
86
H3CO
O2N
Cl
H3CO
O2N
Cl
CHO
CHO
OH
OH
5
6
7
60
60
60
88
84
95
O
O
OH
O
OH
8
240
85
OH
9
240
240
82
43
O
OH
10
aConditions: reactions were carried out by heating at reflux, using 3.75 mmol of ketone (in 10 cm3 isopropyl alcohol); catalyst/ketone/KOH ratio
1:300:2.5.
bYield of product was determined using a ACME 6000 series GC-FID with a DP-5 column of 30 m length, 0.53 mm diameter and 5.00 µm film thickness
and by comparison with authentic samples.
238

M. Muthukumar, P. Viswanathamurthi
conversions (>80%) in the presence of isopropyl alcohol
and KOH as promoter.
thiosemicarbazones formed from derivatives of
2-hydroxychalcone and thiosemicarbazide. The new
complexes were characterized by analytical and
spectroscopic methods. An octahedral structure was
tentatively proposed for all of the complexes. The
complexes showed efficient catalytic activity for the
oxidation of both primary and secondary alcohols with
excellent yields in short reaction time in the presence
of N-methylmorpholine-N-oxide, and also for transfer
hydrogenation of aldehydes and ketones with high
Acknowledgement
The authors express their sincere thanks to Council of
Scientific and Industrial Research (CSIR), New Delhi
[Grant No. 01(2065)/06/EMR-II] for financial support.
One of the authors (M.M) thanks CSIR for the award of
Senior Research Fellowship.
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