DNA-binding, catalytic oxidation, CAC coupling reactions and antibacterial activities of binuclear Ru(II) thiosemicarbazone complexes: Synthesis and spectral characterization

Journal of Advanced Research (2012) 3, 233–243

Cairo University

Journal of Advanced Research

ORIGINAL ARTICLE

DNA-binding, catalytic oxidation, CAC coupling
reactions and antibacterial activities of binuclear
Ru(II) thiosemicarbazone complexes: Synthesis and
spectral characterization
Arumugam Manimaran a, Chinnasamy Jayabalakrishnan

b,*

a
Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamanagalam-638 401, Erode District,
Tamil Nadu, India
b
Post Graduate and Research Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science,
Coimbatore-20, Tamil Nadu, India

Received 8 January 2011; revised 2 July 2011; accepted 18 July 2011
Available online 31 August 2011

KEYWORDS
Binuclear ruthenium(II);
Thiosemicarbazone;
CT-DNA;
Catalytic reactions;

Antibacterial activity

Abstract New hexa-coordinated binuclear Ru(II) thiosemicarbazone complexes of the type
{[(B)(EPh3)(CO)ClRu]2L} (where, E = P or As; B = PPh3 or AsPh3 or pyridine; L = mononucleating NS donor of N-substituted thiosemicarbazones) have been synthesized and characterized by
elemental analysis, FT-IR, UV–vis and 31P{1H} NMR cyclic voltammetric studies. The DNA-binding studies of Ru(II) complexes with calf thymus DNA (CT-DNA) were investigated by UV–vis,
viscosity measurements, gel-electrophoresis and fluorescence spectroscopy. The new complexes have
been used as catalysts in CAC coupling reaction and in the oxidation of alcohols to their corresponding carbonyl compounds by using NMO as co-oxidant and molecular oxygen (O2) atmosphere at ambient temperature. Further, the new binucleating thiosemicarbazone ligands and
their Ru(II) complexes were also screened for their antibacterial activity against Klebsiella pneumoniae, Shigella sp., Micrococcus luteus, Escherichia coli and Salmonella typhi. From this study, it was

* Corresponding author. Tel.: +91 4222692461/+91 9442001793; fax:
+91 4222693812.
E-mail address: (C. Jayabalakrishnan).
2090-1232 ª 2011 Cairo University. Production and hosting by
Elsevier B.V. All rights reserved.
Peer review under responsibility of Cairo University.
doi:10.1016/j.jare.2011.07.005

Production and hosting by Elsevier


234

A. Manimaran and C. Jayabalakrishnan
found out that the activity of the complexes almost reaches the effectiveness of the conventional
bacteriocide.
ª 2011 Cairo University. Production and hosting by Elsevier B.V. All rights reserved.

Introduction
Thiosemicarbazones are an important class of N, S donor ligands which have considerable interest because of their chemistry and biological activities, such as antitumor, antibacterial,
antiviral, antiamoebic and antimalarial activities [1,2]. They

have also been used for device applications in telecommunications, optical computing, optical storage and optical information processing [3]. The chemistry of complexes of ruthenium
with their thiosemicarbazones, which can coordinate with the
metal either in neutral thione form or in the anionic thiolate
form, has received attention in recent years primarily due to
its varied coordination mode, novel electrochemical and electronic properties [4–6]. In particular, the use of ruthenium
complexes as chemo-therapeutic agents for the treatment of
cancer is also well established [7]. Binding to DNA is usually
accompanied by marked absorbance changes in the UV–vis
frequency range, fluorescence and gel-electrophoresis. Among
the various mechanisms through which they carry on their action, of utmost importance is their direct interaction with
DNA. The capability of DNA metallointercalators is determined by several factors, such as the planarity of ligand, atom
type of ligand donor and the coordination geometry [8]. Herein, in connection with our ongoing interest [9,10], we describe
the synthesis and characterization of a series of new class of
binuclear ruthenium(II) thiosemicarbazone complexes along
with their CT-DNA binding studies, CAC coupling reaction
and oxidation of alcohols. Further, the new ligands and
their ruthenium(II) thiosemicarbazone complexes were also
screened for their antibacterial activity against Klebsiella pneumoniae, Shigella species, Micrococcus luteus, Escherichia coli,
and Salmonella typhi.

Experimental
Material and methods
All the chemicals and solvents used were purified and dried by
standard methods. RuCl3Æ3H2O was purchased from Lobachemie Pvt. Ltd. Bombay, India and was used without further
purification. FT-IR spectra were recorded as KBr pellets with
a Nicolet FT-IR spectrometer in 4000–400 cmÀ1 range.
Electronic spectra of the complexes were recorded in DMSO
solutions using Systronics double beam UV–vis spectrophotometer-2202 in the range 800–200 nm. Microanalyses were
carried out with a Vario El AMX-400 elemental analyzer.
31

P{1H} NMR spectra were monitored on a Bruker AMX400 NMR spectrophotometer using DMSOÆd6 as solvent and
tetramethylsilane (1H) and orthophosphoric acid (31P) as internal standards at Indian Institute of Science (IISc), Bangalore,
India. Cyclic voltammetric studies were carried out with a
BAS CV-27 model electrochemical analyzer in acetonitrile
using glassy carbon working electrode and the potentials were
referenced to Ag–AgCl electrode. Melting points were recorded
with a Veego VMP-DS model heating table and were

uncorrected. The starting complexes [RuHCl(CO)(PPh3)3]
[11], [RuHCl(CO)(AsPh3)3] [12] and [RuHCl(CO)(PPh3)2(py)]
[13] were prepared by the reported literature methods.
Preparation of binucleating thiosemicarbazone ligands
The binucleating thiosemicarbazone ligands have been
prepared by adding ethanolic solution of thiosemicarbazide,
(H2L1) (0.1820 g, 2 mmol)/N(4)-methylthiosemicarbazide,
(H2L2) (0.2101 g, 2 mmol)/N(4)-phenylthiosemicarbazide,
(H2L3) (0.3341 g, 2 mmol)/N(4)-(2-chlorophenyl)thiosemicarbazide, (H2L4) (0.4020 g, 2 mmol) with ethanolic solution of
terephthalaldehyde (0.134 g, 1 mmol) in 2:1 molar ratio.
The reaction mixture was then refluxed on a water bath for
about 6 h. The condensation product was filtered, thoroughly
washed with ethanol and ether, recrystallized with ethanol and
dried under reduced pressure over anhydrous CaCl2. Purity of
the synthesized thiosemicarbazone ligands was monitored by
TLC using silica gel. The analytical data, FT-IR, 1H NMR
spectral data confirm the proposed molecular formula and
the structure of the thiosemicarbazone ligands (Scheme 1)
(Yield: 76–85%).
Synthesis of binuclear ruthenium(II) thiosemicarbazone
complexes
To a solution of [RuHCl(CO)(PPh3)3] (0.1904 g, 0.2 mmol)/

[RuHCl(CO)(AsPh3)3] (0.2170 g, 0.2 mmol)/[RuHCl(CO)(PPh3)2(py)] (0.1540 g, 0.2 mmol) in equimolar ratio of
DMSO/benzene (20 cm3), an appropriate thiosemicarbazone
ligand, H2L1AH2L4 (0.0280–0.0500 g, 0.1 mmol) (Scheme 2)
was added (2:1 ratio) and heated under reflux for 4–6 h and
then it was concentrated to 5 cm3. The new complex was separated by the addition of 10 cm3 of petroleum ether (60–
80 °C). The product was filtered, washed with petroleum ether
and recrystallized from methanol/CH2Cl2 mixture and dried in
vacuo. The purity of the complexes was checked by TLC (Yield
%80%).
Catalytic oxidation reactions by binuclear Ru(II)
thiosemicarbazone complexes
Catalytic oxidation of primary and secondary alcohols to corresponding aldehydes and ketones, respectively, by ruthenium(II) thiosemicarbazone complexes was studied in the
presence of NMO as co-oxidant and O2 atmosphere at

N
HN
RHN
S

NHR
NH
N

N
N

S
RHN
HS

'Keto' form

NHR
N
N
'Enol' form

Where, R=H (H2L1) or CH3 (H2L2) or C6H5 (H2L3) or o-Cl-C6H4

Scheme 1

SH

Structure of thiosemicarbazone ligands.


Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes
N
HN
RHN
S

S

2 [RuHCl(CO)(EPh3)2(B)]

+

NHR
NH

N

DMSO/Benzene
Reflux 4-6 h

OC
Cl
Ph3E Ru B
N
N

S

RHN

Antibacterial studies

Where R= H / CH3 / phenyl / o-Cl-phenyl; E= P / As; B=PPh 3 / AsPh 3 / pyridine

Scheme 2 Formation of binuclear Ru(II) thiosemicarbazone
complexes.

MN

Ru 2+ + NMO
R1

R1

+

Ru 4+
O

CH OH
R2

R1

H
CH O

Ru 4+
O

R2

Ru 4+
O

R2

H

+

C OH
R2

+ NM

Ru3+
O

R1

R1

C O

C OH

Ru 3+
O

+

H+

R2

R2
H

Ru 4+
O

R1

H

CH O

+

added with stirring and the mixture was heated under reflux.
The remaining PhBr in Et2O (5 cm3) was added dropwise
and the mixture was refluxed for 40 min. To this mixture,
1.03 cm3 (0.01 mol) of PhBr in anhydrous Et2O (5 cm3) and
the Ru(II) thiosemicarbazone complex (0.05 mmol) chosen
for investigation were added and heated under reflux for 6 h.
The reaction mixture was cooled and hydrolyzed with a saturated solution of aqueous NH4Cl and the ether extract on
evaporation gave a crude product which was chromatographed to get pure biphenyl and compared well with an
authentic sample [14,15].

NHR

N
S
N
Ph3E Ru B
Cl
CO

MN

235

H+

Ru2+

+ H 2O

(where, R1 = aryl / alkyl; R2 = alkyl / H)

Scheme 3 Proposed mechanism for the oxidation of alcohols
using ruthenium/NMO.

ambient temperature separately. A typical reaction using the
ruthenium complexes as catalyst and benzyl alcohol, cinnamyl
alcohol and cyclohexanol as substrates at 1:100 molar ratio is
described as follows. A solution of ruthenium complex
(0.01 mmol) in 20 cm3 CH2Cl2 was added to the solution of
substrate (1 mmol) and NMO (3 mmol) and O2 atmosphere
at ambient temperature. The solution mixture was stirred for
6 h and the solvent was then evaporated from the mother liquor under reduced pressure. The solid residue was then extracted with petroleum ether (60–80 °C) (20 cm3) and the
ether extracts were evaporated to give corresponding aldehydes/ketone, which were then isolated and quantified as their
2,4-dinitrophenylhydrazone derivative [14].

The binucleating thiosemicarbazone ligands and their ruthenium(II) complexes have been tested in vitro to asses their
growth inhibitory activity against K. pneumoniae, Shigella species, M luteus, E. coli, and S. typhi by Kirby Bauer method [16].
The test organisms were grown on nutrient agar medium in petri plates. The compounds to be tested were dissolved in DMSO
and soaked in a filter paper disc of 5 mm diameter and 1 mm
thickness. The concentrations used in this study were 0.5%,
1.0%, 1.5%, 2.0% and 2.5%. The discs were placed on the previously seeded plates and incubated at 37 °C for 24 h.Amoxycilin, ampicillin, erythromycin and streptomycin were used as
standards with different concentrations. A control test with
the solvent was also carried out under identical conditions.
DNA-binding experiments
Stock solutions of disodium salt of CT-DNA solutions were
prepared by DNA in buffer, 50 mmol NaCl/5 mmol Tris–

HCl in water. DNA concentration was determined by UV
absorbance at 260 nm after 1:100 dilutions. Stock solutions
were kept at 4 °C and used within 4 days. Doubly distilled
water was used to prepare the buffer. All the titrations were
done using DNA stock solutions pretreated with metal complex to take care of the dilution effects. Solution of DNA
gave the ratio of UV absorbance at 260 and 280 nm, A260/
A280 $ 1.9 indicating that the DNA was sufficiently free of
protein [17].
Absorption spectra were recorded on a Systronics double
beam UV–vis spectrophotometer-2202 using cuvettes of 1 cm
path length. For UV–vis spectral titrations, 4 · 10À5 mol concentration of ruthenium solutions were used and calf thymus
DNA (10.5 mmol) was added in steps till R = 40 (Retention
Time). Intense MLCT bands were monitored to follow the
interaction of the complex with CT-DNA. The intrinsic binding constant Kb of the complex to CT-DNA was determined
from Eq. (1), through a plot of [DNA]/ea–ef vs [DNA], where
[DNA] represents the concentration of DNA and ea, ef and eb,
the apparent extinction coefficient (Aobs/[M]), the extinction
coefficient for free metal [M] complex and the extinction coefficient for the free metal [M] complex in the fully bound form,
respectively. In plots of [DNA]/ea–ef vs [DNA], Kb is given by
the ratio of slope to intercept [18].

CAC coupling reaction by binuclear ruthenium(II)
thiosemicarbazone complexes

½DNAŠ ½DNAŠ
1
¼
þ
ea À ef
eb À ef Kb ðeb À ef Þ

Magnesium turnings (0.320 g) were placed in a flask equipped
with a CaCl2 guard tube. A crystal of iodine was added. PhBr
(0.75 cm3 of total 1.88 cm3) in anhydrous Et2O (5 cm3) was

For viscosity measurements, the Ubbelohde viscometer was
thermostated at 25 °C in a constant temperature bath. The
concentration of DNA was 160 lM and the flow-times were

ð1Þ


486
486
484
490

402,
368,
403,
369,
369,
368,
372,
368,
370,
402,
368,
550
424,

424,
420,
425,
1558
1559
1575
1560
1558
1564
1560
1578
1582
1580
1559
1575
1602
1608
1612
1609
3.87 (3.83)
3.81 (3.82)
3.55 (3.50)
3.42 (3.39)
3.50 (3.53)
3.45 (3.49)
3.23 (3.20)
3.12 (3.06)
4.65 (4.66)
4.56 (4.51)
4.19 (4.17)

4.01 (3.97)
22.87 (22.83)
20.79 (20.80)
14.83 (14.81)
12.79 (12.76)

determined with a digital timer (1/R = [Ru]/[DNA] = 0.5).
Emission spectra were determined with Hitachi F-2500
fluorescence spectrophotometer at room temperature.
Fluorescence experiments were conducted by adding aliquots
of 0–10 · 10À5 mol solutions of the complexes C9AC12 to
samples containing 2 · 10À5 mol ethidiumbromide and
1 · 10À4 mol CT-DNA in Tris–HCl buffer.
The DNA cleavage activity of ruthenium(II) thiosemicarbazone complexes were monitored by agarose gel electrophoresis
on CT-DNA. The tests were performed under aerobic conditions with H2O2 as an oxidant. Each reaction mixture (20 lL
total volume) containing 240 ng of CT-DNA, 50 mmol phosphate buffer (pH 7.8), in the presence or absence of 120 lM
of H2O2, and varying concentrations of ruthenium(II) complexes C9AC12 in the range 1–50 lM, were incubated at
37 °C for different periods of time. After incubation, the
solution was subjected to electrophoresis on a 1% agarose
gel in 1· TAE buffer (40 mmol Tris–acetate, 1 mmol EDTA)
at 100 V, for 2 h. After electrophoresis the gel was stained with
1 lg/mL ethidiumbromide (EB) for 30 min prior to being
photographed under UV light.

The elemental analysis (C, H, N, S) of all thiosemicarbazone
ligands and their Ru(II) complexes are in good agreement with
the calculated values thus confirming the proposed binuclear
composition for all the complexes (Table 1). The complexes
were obtained in powder form. Various attempts have been
made to obtain the single crystals of the complexes but it has

been unsuccessful. So for all the ligands and their complexes
are stable at room temperature, non-hygroscopic and insoluble
in water, methanol, ethanol and soluble in CH2Cl2, CHCl3,
DMF, DMSO and CH3CN.
IR spectra

C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
L1
L2
L3
L4

{[(PPh3)2(CO)ClRu]2L1} C84H70Cl2N6O2P4Ru2S2
{[(PPh3)2(CO)ClRu]2L2} C86H74Cl2N6O2P4Ru2S2
{[(PPh3)2(CO)ClRu]2L3} C96H78Cl2N6O2P4Ru2S2
{[(PPh3)2(CO)ClRu]2L4} C96H76Cl4N6O2P4Ru2S2
{[(AsPh3)2(CO)ClRu]2L1} C84H70As4Cl2N6O2Ru2S2
{[(AsPh3)2(CO)ClRu]2L2} C86H74As4Cl2N6O2Ru2S2
{[(AsPh3)2(CO)ClRu]2L3} C96H78As4Cl2N6O2Ru2S2

{[(AsPh3)2(CO)ClRu]2L4} C96H76As4Cl4N6O2Ru2S2
{[(py)(PPh3)(CO)ClRu]2L1} C58H50P2Cl2N8O2Ru2S2
{[(py)(PPh3)(CO)ClRu]2L2} C60H54P2Cl2N8O2Ru2S2
{[(py)(PPh3)(CO)ClRu]2L3} C70H58P2Cl2N8O2Ru2S2
{[(py)(PPh3)(CO)ClRu]2L4} C70H56P2Cl4N8O2Ru2S2
H2L1 C10H12N6S2
H2L2 C12H16N6S2
H2L3 C22H20N6S2
H2L4 C22H18Cl2N6S2

128
170
120
118
56
61
64
110
58
>250
70
>250
183
130
220
195

60.90
61.31
63.75

61.41
55.06
55.52
58.10
56.15
50.55
51.25
54.94
52.57
42.84
46.73
61.09
52.69

(4.25)
(4.41)
(4.32)
(4.05)
(3.90)
(3.97)
(4.00)
(3.70)
(3.70)
(3.85)
(3.86)
(3.53)
(4.30)
(5.20)
(4.70)
(3.60)

5.07 (5.01)
4.99 (5.00)
4.65 (4.68)
4.48 (4.50)
4.59 (4.60)
4.52 (4.48)
4.23 (4.23)
4.09 (4.05)
8.13 (8.10)
7.97 (8.01)
7.32 (7.30)
7.01 (7.05)
29.97 (30.01)
27.25 (27.21)
19.43 (19.34)
16.76 (16.80)
4.26
4.43
4.35
4.08
3.85
4.01
3.96
3.73
3.66
3.87
3.82
3.53
4.31

5.23
4.66
3.62

Results and discussion

(61.00)
(61.30)
(63.72)
(61.39)
(55.01)
(55.48)
(58.06)
(56.12)
(50.51)
(51.23)
(55.00)
(52.53)
(42.86)
(46.69)
(61.06)
(52.65)

1535
1532
1540
1538
1532
1539
1535

1536
1540
1538
1539
1533
–
–
–
–

750
745
746
740
752
750
753
758
754
748
744
747
848
856
810
832

1926
1927
1933

1932
1927
1928
1930
1942
1924
1926
1928
1927
–
–
–
–

254,
254,
255,
254,
254,
255,
254,
254,
254,
254,
254,
250,
232,
235,
230,
235,

369,
302,
369,
303,
304,
300,
294,
297,
297,
297,
298,
360,
280,
287,
282,
288,

mC‚N mN‚N‚C mC„S mC„O kmax (nm)
(cmÀ1) (cmÀ1)
(cmÀ1) (cmÀ1)
S

Melting Elemental analysis found (Calcd.)%
point
C
H
N
(°C)
S. No. Ligand/complexes

Table 1

Analytical, FT-IR and UV–vis spectral data of binuclear ruthenium(II) thiosemicarbazone complexes.

FT-IR spectral data

UV–vis

429,
403,
434,
403,
403,
403,
403,
402,
391,
430,
403,

537
433,
540,
434,
434,
433,
609
530
528

461,
433,

606
464, 618

A. Manimaran and C. Jayabalakrishnan
576
628
464, 639
468, 609
465, 615

236

The FT-IR spectra of the free thiosemicarbazone ligands were
compared with those of the new complexes in order to confirm
the coordination of ligand to the ruthenium metal (Table 1).
The bands present around 3100 cmÀ1 in the spectrum of the ligand (H2L1) are assigned to masym and msym vibration of the terminal NH2 group. This band is also present in the spectra of the
complexes C1, C5 and C9 indicating the non-involvement of
this group in coordination. The absorption due to C‚N group
of the free ligand present around 1602–1612 cmÀ1 region
undergoes a negative shift by 20–53 cmÀ1 (1558–1582 cmÀ1)
in the spectra of the complexes indicating the coordination of
azomethine nitrogen to the metal. The m(NAN) bands of the ligands are present around 1058 cmÀ1. The increase in frequency
of this band 1060–1080 cmÀ1 in the spectra of the complexes
provides evidence for the co-ordination via the azomethine
nitrogen [19]. A strong band in the 2988–3020 cmÀ1 region
attributed to m(NAH) group of ANHAN‚CA found in the spectra of free ligands (H2L2, H2L3 and H2L4) is not present in the
spectra of the complexes. The band present around 810–856 cmÀ1

in the spectra of thiosemicarbazone ligands is assigned to
AHNAC‚S group. This band is also not present in the spectra
of the complexes. But new bands are present at 1532–1540 cmÀ1
and 740–758 cmÀ1 which are assigned to the new azomethine
group AC‚NANA and ACAS, respectively. These


Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes
Table 2
S. No.

L1
L2
L3
L4
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12

237

31

P{1H} NMR spectra of binucleating thiosemicarbazone ligands and their ruthenium(II) complexes.
Ligand/complex
1

H2L
H2L2
H2L3
H2L4
{[(PPh3)2(CO)ClRu]2L1}
{[(PPh3)2(CO)ClRu]2L2}
{[(PPh3)2(CO)ClRu]2L3}
{[(PPh3)2(CO)ClRu]2L4}
{[(AsPh3)2(CO)ClRu]2L1}
{[(AsPh3)2(CO)ClRu]2L2}
{[(AsPh3)2(CO)ClRu]2L3}
{[(AsPh3)2(CO)ClRu]2L4}
{[(py)(PPh3)(CO)ClRu]2L1}
{[(py)(PPh3)(CO)ClRu]2L2}
{[(py)(PPh3)(CO)ClRu]2L3}
{[(py)(PPh3)(CO)ClRu]2L4}

1

H NMR (d ppm)

dACH‚NA

daromatic

dASH

dANH/NHA

dACH3

9.1
8.2
9.8
10.4
10.5
9.6
10.0
9.5
10.4
9.2
8.8
9.8
10.0
10.0
9.9
9.4

6.2–6.8
7.8–8.1
6.4–7.0
6.6–7.3
5.6–7.0
6.6–7.4

6.8–8.2
6.8–7.4
5.8–7.2
6.7–7.6
6.2–7.2
6.2–7.0
6.4–8.2
6.4–7.2
6.2–7.0
6.4–7.2

11.9
11.5
10.4
12.4
–
–
–
–
–
–
–
–
–
–
–
–

4.6
3.3

4.4
3.8
3.8
5.0
3.4
3.4
3.8
4.0
4.4
4.4
2.5
3.4
4.4
3.4

–
2.4
–
–
–
3.2
–
–
–
2.8
–
–
–
1.8
–

–

31
P NMR
(dppm)

–
–
–
–
27.4,
27.0,
27.5,
26.9,
–
–
–
–
57.3
57.3
57.7
58.2

44.3
38.3
38.3
38.7

observations indicate the enolization of the ANHAC‚S group
and subsequent deprotonation before coordination to the metal. In all the complexes, the band due to free m(C„O) group is

present at 1924–1942 cmÀ1. The absorption due to coordinated
pyridine is seen in the complexes, C9AC12 around 1000 cmÀ1.
The characteristic bands due to PPh3/AsPh3 are also present
around 1417–1456 cmÀ1, 1091–1093 cmÀ1, 736–796 cmÀ1 and
516–518 cmÀ1 in all the complexes. The replacement of the hydride ion in the starting complexes by the ligands has been confirmed by the absence of a band around 2020 cmÀ1 in all the
complexes [20].
UV–vis spectra
Electronic spectra of ruthenium(II) thiosemicarbazone complexes in DMSO showed absorption in the range of 250–
639 nm (Table 1) which can be assigned, metal to ligand charge
transfer followed by intra-ligand transitions, respectively. The
ground state of Ru(II) in an octahedral environment is 1A1g,
arising from the t2g6 configuration. The excited state terms
are 3T1g, 3T2g, 1T1g and 1T2g. Hence, four bands corresponding to the transitions 1A1g fi 3T1g, 1A1g fi 3T2g, 1A1g fi 1T1g
and 1A1g fi 1T2g are possible in the order of increasing energy.
The electronic spectral bands around 528–639 nm are assigned
to 1A1g fi 1T2g. The other high intensity band in the visible region around 402–468 nm has been assigned to charge transfer
transitions arising from the metal t2g level to the unfilled P*
molecular orbital of the ligand. The high intensity bands
around 300–372 and 250–298 nm has been designated as
n fi P* and fi P fi P* transitions, respectively. The pattern
of the electronic spectra for all the complexes indicates the
presence of an octahedral environment around ruthenium(II)
similar to that of other ruthenium octahedral complexes [21].
1
H NMR spectra of thiosemicarbazone ligands and their Ru(II)
complexes

The ligand to metal bonding is further supported by 1H NMR
spectra. The nuclear magnetic resonance spectrum of the thiosemicarbazone ligands (Table 2 and Fig. 1) and ruthenium(II)
diamagnetic compounds (Table 2 and Fig. 1) taken in

Fig. 1 1H NMR spectra of binucleating H2L1 ligand and
{[(PPh3)2(CO)ClRu]2L1} complex.

DMSOÆd6 solution confirm the complex formation. In the
spectra of all the complexes a sharp singlet appeared at 8.8–
10.5 ppm has been assigned to azomethine proton (AHC‚N).
The positions of azomethine signal in the complexes are up
field/down field compared to that of the free ligands observed
at 8.2–10.4 ppm, indicating coordination through the azomethine nitrogen atom. Enolization of thiocarbonyl group is indicated by the singlet present at 10.4–12.0 ppm in the spectra of
the ligands, which are attributed to ACASH protons of thioamide group. The absence of thionyl group in the complexes
indicates deprotonation of this group of the thiosemicarbazone
ligands on complexation and coordination to ruthenium
through thionyl sulfur.
The terminal ANH2 protons in the complexes C1, C5 and
C9 and ANH protons in the complexes C2AC4, C6AC8 and


238

Fig. 2

A. Manimaran and C. Jayabalakrishnan

31

P NMR spectra of binuclear C1 and C9 complexes.

C10AC12 are seen in the positions with slight deviation as in
the ligands spectrum around 2.5–3.8 and 3.4–5.0 ppm confirming the non-involvement of this group in coordination with the

metal. Multiplets are observed around 5.6–8.2 ppm in all the
complexes and have been assigned to the aromatic protons
of triphenylphosphine, triphenylarsine, pyridine and phenyl
of the thiosemicarbazone ligands. Methyl protons appear as
singlets in the region of 1.8–3.2 ppm in the complexes C2, C6
and C10. Furthermore, in all the complexes, largest Dd are observed for the protons that are located close to the coordinating atoms. So, the deshielding effect of the metal is apparent to
such protons [22].
31

P NMR spectra of Ru(II) complexes

The 31P NMR spectra were recorded for C1AC4 and C9AC12
to confirm the presence of triphenylphophine group and heterocyclic nitrogen base in the binuclear ruthenium(II) thiosemicarbazone complexes (Table 2 and Fig. 2). The signal
appeared at 26.9–27.5 and 38.3–44.3 ppm in the spectrum of
C1AC4 attributed to the two phophine ligands are cis to each
other in these complexes. However, complexes C9AC12 exhibited only one signal at 57.3–58.2 ppm consistent with the presence of one triphenylphosphine group. This observation
indicates the retention of coordinated pyridine in the complexes even after the coordination of bidentate thiosemicarbazones. The 31P NMR spectral studies clearly indicate a more
labile RuAP bond compared to RuAN bond in these complexes, which is a reflection of better r-donating ability of
the nitrogen bases compared to that of phosphorus in PPh3
[23].
Catalytic oxidation reactions
The catalytic activity of the newly synthesized binuclear ruthenium(II) thiosemicarbazone complexes was examined in the
presence of N-methylmorpholine-N-oxide (NMO) and molecular oxygen (Table 3) as co-oxidants for the oxidation of primary and secondary alcohols in CH2Cl2. Results of the
present investigation suggest that the complexes are able to react more efficiently with NMO than with molecular oxygen as
indicated by the low product yield when molecular oxygen is
employed as the co-oxidant. This is in accordance with a
previous observation [24]. Formation of a high valent

ruthenium-oxo-species, RuIV‚O, is proposed as the intermediate in ruthenium(II) complexes catalyzed oxidation with
NMO. Ruthenium compounds are known to be good hydride

ion abstracting agents. N-methylmorpholine and water are the
by products during the course of the reaction (Scheme 3). This
was further supported by spectral changes that occur by addition of NMO to a dichloromethane solution of the ruthenium(II) complexes. The appearance of a peak at 390 nm is
attributed to the formation of RuIV‚O species which is in
conformity with other oxo ruthenium complexes [25–27].
The proposed mechanism in the presence of molecular oxygen includes the oxidation of alcohol by a Run+ complex to
form aldehyde and Ru(nÀ2)+, followed by the oxidation of
Ru(nÀ2)+ to Run+ with O2 [28]. However, one cannot rule
out a concerted mechanism, for example one including the oxidation of alcohol by O2 in the coordination sphere of Run+.
Metal complexes can act as an oxygen atom transfer reagent
resulting in the formation of metal-oxo species [29]. An important characteristic of ruthenium NMO or O2 system results in
the selective oxidation at the alcoholic group of ACH2 benzyl
alcohol to benzaldehyde while the ACH2 group remains unaffected. Saturated aliphatic alcohols such as cyclohexanol, Propan-1-ol and 2-methylpropyl alcohol are converted into the
corresponding aldehydes/ketones with high conversion.
All the synthesized ruthenium complexes were found to catalyze the oxidation of alcohols to aldehydes/ketones, but the
yields and the turnover were found to vary with different catalysts. The relatively higher product yield obtained for the oxidation of benzyl alcohol than for cyclohexanol, Propan-1-ol
and 2-methylpropyl alcohol was due to the fact that the aCH moiety of benzyl alcohol is more acidic than that of cyclohexanol [28], Propan-1-ol and 2-methyl propyl alcohol. It has
also been found that triphenylphosphine complexes possess
higher catalytic activity than the triphenylarsine complexes
[29] and pyridine substituted complexes. This may be due to
the higher donor ability of the arsine and pyridine ligand compared with that of phosphine. It has also been found that
ruthenium(II) complexes, C2, C6 and C10 derived from
N(4)-methyl thiosemicarbazone ligands showed less catalytic
activity when compared with other complexes, which could
be attributed to the presence of the electron-releasing methyl
group in these complexes, which decreases the catalytic activity
by increasing the electron density on the metal ion [30].
The catalytic activity study reveals that the ligand system
influences the catalytic behavior of the complexes. As expected, the ligands containing electron donating group increases the catalytic activity of the complexes.
CAC coupling reactions

The CAC coupling reactions by the synthesized binuclear
ruthenium(II) thiosemicarbzone complexes {[(B)(EPh3)Cl(CO)Ru]2L} (where, L = binucleating thiosemicarbazone ligands; E = P/As; B = PPh3/pyridine) were carried out and
the results of this study are listed in Table 3. The system chosen
for our study is the coupling of phenylmagnesium bromide
with bromobenzene that was first converted into the corresponding Grignard reagent [31]. Then bromobenzene, followed by the complex chosen for the investigation, was
added to the above reagent and the mixture was heated under
reflux for 6 h. After work up, the mixture yielded biphenyl.
Only a very little amount of biphenyl is formed when the


Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes
Table 3

239

Catalytic activity data of new binuclear Ru(II) thiosemicarbazones complexes

Complexes Catalytic oxidation of alcohols
Substrate

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

C11

C12

a

Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol

Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol
Propan-1-ol
Benzyl alcohol
2-methyl propyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol
Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methylpropyl alcohol
Benzylalcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol

Benzyl alcohol
Cyclohexanol
Propan-1-ol
2-methyl propyl alcohol

Product

Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde

Benzaldehyde
2-Methyl-propionaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde
Benzaldehyde
Cyclohexanone
Propionaldehyde
2-Methyl-propionaldehyde

CAC coupling reaction
In presence of NMO

In presence of O2 atm

Yield
(%)

Turnover
numbera

Yield
(%)

Turnover
numbera

80
93
60
65
75
85
59
63
86
97
68
72
89
98
73
77
73

79
58
58
63
74
55
73
59
79
61
64
78
80
67
69
71
74
55
56
64
74
67
49
70
74
57
58
72
77
63

67

88
96
79
84
77
89
78
82
90
101
87
93
90
103
92
96
77
82
78
77
66
78
75
75
79
82
80
83

80
83
84
88
74
75
74
76
67
72
65
78
72
76
77
78
76
81
80
88

75
85
56
61
66
74
50
56
71

81
59
65
78
83
64
68
5
668
53
55
57
60
45
62
51
70
53
55
64
70
54
64
62
66
47
50
55
59
40

45
59
60
41
45
61
66
50
54

78
89
72
80
70
76
68
72
73
84
77
83
80
86
82
88
67
70
71
75

58
64
62
65
70
72
70
73
67
73
70
79
64
98
64
69
57
72
55
62
62
63
57
62
64
69
68
72

Yield (mg)

Yield (%)

0.420
44

0.429
45

0.285
45

0.269
28

0.368
38

0.312
32

0.225
23

0.220
23

0.305
32

0.226
24

0.250
26

0.268
28

Moles of product per mole of catalyst.

reaction is carried out without the catalyst. This is an insignificant amount compared to the yields of biphenyl that have
been obtained from the reactions catalyzed by binuclear Ru(II)
complexes. The catalytic properties of the new binuclear complexes have also been compared with those already reported
binuclear complexes [32]. It has been observed that the binuclear ruthenium(II) thiosemicarbazone complexes are moderate active catalysts.

3.7. Antibacterial activities
The variation in the effectiveness of the different compounds
against different organisms depends on their impermeability
of the microbial cells or on the difference in the ribosome of
the microbial cells. In general, the complexes (Table 4) are
more active than that of parent ligands and ruthenium(II)
starting complexes. The increase in the antibacterial activity


240

Table 4
Ligand/
complex

Antimicrobial activities of new thiosemicarbazone ligands and their binuclear Ru(II) complexes.
Inhibition zone concentration in mm
Shigella sp.

K. Pneumoniae

E. coli

S. typhi

0.5%

1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5%

9
8
9
10
11
10
10
12
11
12
11
11
10
12
11

12
D
–

9
9
10
10
11
12
13
12
12
12
12
11
10
13
12
12
E
12

10
9
10
10
12
12
13

14
13
11
11
10
11
13
12
12
Ax
8

11
10
11
9
12
10
12
12
12
10
12
11
12
12
12
13
Ak
23

10
10
11
8
12
11
12
12
11
11
11
11
12
13
13
13
S
17

7
8
8
8
10
11
10
12
10
10

10
11
12
14
14
14
D
–

8
8
8
8
10
12
12
12
10
11
10
14
12
14
14
15
E
16

8
9

9
8
10
12
12
10
10
11
14
14
13
13
15
15
Ax
10

9
8
9
9
11
12
14
11
12
11
14
14
14

15
15
16
Ak
21

9
8
10
9
12
13
14
11
14
11
14
15
14
15
15
16
S
10

7
6
7
8
9

11
10
11
12
12
10
9
10
11
11
11
D
–

Where, D, DMSO, E, erythromycin, Ax, amoxycilin, Ak, ampicillin; and S, streptomycin.

7
8
7
8
9
11
10
11
12
12
9
9
10
12

12
11
Ak
15

8
8
7
9
10
12
10
11
12
13
11
10
11
12
13
12
Ax
–

8
9
8
8
10
12

13
12
11
13
11
10
12
12
14
14
S
9

9
9
8
7
11
12
13
12
11
11
12
12
12
10
14
14
E

16

7
7
8
7
10
13
12
10
10
11
12
12
11
12
11
12
D
–

7
7
8
8
10
13
12
10
12

11
12
12
11
12
12
12
E
–

8
8
7
9
10
12
14
10
12
11
12
14
13
14
14
14
Ax
–

8

8
7
9
11
14
14
12
14
12
13
14
13
14
14
14
Ak
–

8
9
9
9
11
14
14
12
14
10
13
14

12
12
15
10
S
–

9
8
9
9
11
12
12
10
11
11
11
12
12
12
12
12
Ak
16

10
10
8
10

11
12
12
13
12
12
12
12
14
12
12
12
Ax
–

10
10
8
10
10
13
14
13
12
12
10
12
14
11
14

13
E
10

11
11
9
10
12
14
14
12
14
14
10
10
15
13
14
13
S
–

11
11
10
8
12
14
12

12
14
14
10
11
12
13
14
14
D
8

A. Manimaran and C. Jayabalakrishnan

H2L1
H2L2
H2L3
H2L4
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12

Standards

M. Luteus


Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes

241

1.20
a
b
c
d
e

η/η0,

Relative specific viscosity

1.18
1.16
1.14
1.12
1.10
1.08
1.06
1.04
1.02
1.00

Fig. 3

UV–vis spectra of C11-DNA complexes.

0

10

20

30

40

50

-3

x 10

1/R = {[Ru-complex]/[DNA]}
1.8

Fig. 5 On the effect of C9 (a), C10 (b), C11 (c), C12 (d) and CTDNA (e) on the viscosity of CT-DNA at 1/R = 0.5 relative
viscosity vs 1/R.

(Eo-Ef)/(Eb-Ef)

1.6

1.4
1.2
1
0.8
0.6
0.4
0.2
0
0

0.001

0.002

0.003

0.004

0.005

0.006

[DNA], M

Fig. 4

Plot of (eo–ef)/(eb–ef) vs [DNA] for C11 complex.

of the metal chelates with increase in concentration is due to
the effect of metal ion on normal cell process. Such increase

in the activity of the metal chelates can be explained on the basis of Overtone’s concept [33] and Chelation theory [34,35].
3.8. DNA binding studies
The binuclear ruthenium(II) thiosemicarbazone complexes,
C1AC4 exhibit intense MLCT as well as ligand based P–P*
transitions with very high molar absorptivities (Fig. 3). When
titrated with a solution of calf thymus DNA (R = [DNA]/[Ru
complex] = 1–40), all of them show hypochromism and redshifts in the MLCT band [36]. The complexes C1AC8 show
small or no redshift. This is supported by lower values of R
at which maximum hypochromism was observed for
C9AC12 (R = 8–15 for C1AC4; R = 25 for C5AC8;
R = 30 for C9AC12. The complexes C9 and C10 with only
thiosemicarbazide and N(4)-methylthiosemicarbazide ligands
show initially hypochromism (R = 5), then hyperchromism
(R = 15) and again hypochromism (R = 40) with a red-shift
during titration with CT-DNA revealing the presence of two
or more DNA binding regimes.
As the titration progressed, a response curve was plotted
from the experimental data (Fig. 4) as the measured emission
as a function of DNA concentration, allowing the progress
of the experiment to be monitored. It is assumed that when
no further significant increase in fluorescence is detected, i.e.

Fig. 6

Fluorescence spectrum C11-DNA complex.

the response curve plateaus, that the metal complex has
reached saturation. Saturation is defined as the point where
all binding sites on the DNA are occupied. Corrected emission
is defined as the observed emission minus emission of the metal

complex alone. The DNA binding constants obtained reveal
several interesting trends in DNA binding. The complexes
C9 and C10 have been shown to exhibit a DNA binding affinity higher than its N(4)-phenylthiosemicarbazone complex,
C11 and N(4)-o-chlorophenylthiosemicarbazone complex,
C12, which is well known to exhibit partial intercalation with
DNA. This is interesting as phenyl substitution on N(4) is expected to hinder the partial insertion of phenyl ring in between
the DNA base pairs. So it is obvious that hydrophobic
interaction of the phenyl groups with DNA contributes to
the stabilization of the bound complex. This is supported by
the observed decrease in values of K in the order, H2L4 >
H2L3 > H2L2 > H2L1 of Ru(II) complexes. It is clear that


242

A. Manimaran and C. Jayabalakrishnan
References

Fig. 7 Agarose gel-electrophoresis photograph of C11-DNA
complex.

the C11 and C12 complexes fit most closely against the DNA
helical structure with van der Waals interactions between complex and DNA being maximum. These ligands favor hydrophobic interaction with DNA rather than with water
molecules leading to the release of water molecules from
DNA on binding, enhancement in entropy and stabilization
of the DNA-bound complex. All the complexes possess a binding site size of 4–6 base pairs, which is consistent with earlier
reports [37].
When CT DNA (50 lM) was titrated with ruthenium(II)
complexes (1/R = 0–0.5 = [Ru]/[DNA]), the relative viscosity
of the CT DNA increases for most of the complexes (Fig. 5).

The increase in relative viscosity of C11 and C12 is higher than
that of C9 and C10, which is due to the aggregation of C9 and
C10 on the helical DNA as nano template via hydrophobic
interaction [38]. The mixed ligand complexes C11 and C12 also
show an increase in relative viscosity of CT DNA but less than
C9 and C10, obviously due to partially intercalating phenyl
ring in them.
The fluorescence titration spectra have been confirmed to
be effective for characterizing the binding mode of the metal
complexes to DNA [39]. The enhancements in the emission
intensity of the ligand and its ruthenium(II) complexes with
increasing CT-DNA concentrations are shown in Fig. 6. The
ligand and its ruthenium(II) complexes could emit luminescence in Tris–HCl buffer with a maximum appearing at
510 nm. This phenomenon was related to the extent to which
the compounds penetrate into the hydrophobic environment
inside the DNA, thereby avoiding the quenching effect of solvent water molecules. The marked increase in emission intensity of these three compounds were also in accordance with
those observed in the fluorescence titration spectra studies of
other intercalators [40]. Consequently, it might be concluded
that the Ru(II) complexes bound DNA via intercalation mode.
Cleavage of CT-DNA upon ruthenium(II) thiosemicarbazone
complexes
The potential of the present complex, C11 to cleave DNA was
studied by gel electrophoresis Fig. 7 using CT-DNA in Tris–
HCl buffer (pH 7.8). Cleavage studies of CT-DNA were observed for the present binuclear ruthenium(II) complex, C11.
The cleavage efficiency of this ruthenium(II) complex is 60%.
Acknowledgment
One of the authors (C. Jayabalakrishnan) thanks University
Grants Commission (UGC), Hyderabad, India [Ref. No.
MRP-2240/06] for the financial support. We are thankful for
providing 31P{1H} NMR spectra to NMR Research Centre,

Indian Institute of Science (IISc), Bangalore, India.

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