Dimethyl sulfoxide ruthenium(II) complexes of thiosemicarbazones and semicarbazone: Synthesis, characterization and biological studies

Article abstract of DOI:10.1016/j.poly.2008.05.034

In an attempt to synthesize Ru(II)-dmso-thiosemicarbazone complexes, a series of carbonyl compounds were selected to condense with thiosemicarbazide in ethanol in order to get the respective thiosemicarbazone ligand as the first step. All the selected carbonyl compounds yielded the expected thiosemicarbazone ligand, but the product obtained by the reaction of benzaldehyde with thiosemicarbazide in ethanol was found to show sharp IR peak characteristic of C{double bond, long}O group and thought to be benzaldehyde semicarbazone. When the ligands were treated with cis-[RuCl₂(dmso)₄)] in ethanol, all the ligands yielded thiosemicarbazone complexes, while the suspected semicarbazone ligand resulted in orange-yellow crystalline product which has been found to be a semicarbazone complex by XRD studies. A mechanism has been proposed for the conversion of C{double bond, long}S to C{double bond, long}O during ligand preparation, which involves the role of adventitious water in ethanol. All the complexes were characterized by analytical and spectroscopic (IR, UV-Vis and ¹H NMR) methods. The redox behaviors of the complexes were studied by cyclic voltammetry. The preliminary DNA-binding ability of the complexes was studied by recording electronic absorption spectra of the complexes in presence of herring sperm DNA. Antibacterial activities of the complexes were also been evaluated against five pathogenic bacteria.

Full text of DOI:10.1016/j.poly.2008.05.034

Polyhedron 27 (2008) 2743–2750
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Dimethyl sulfoxide ruthenium(II) complexes of thiosemicarbazones
and semicarbazone: Synthesis, characterization and biological studies
Viswanathan Mahalingam a, Nataraj Chitrapriya a, Frank R. Fronczek b, Karuppannan Natarajan a,
*
a Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
In an attempt to synthesize Ru(II)-dmso-thiosemicarbazone complexes, a series of carbonyl compounds
were selected to condense with thiosemicarbazide in ethanol in order to get the respective thiosemicar-
bazone ligand as the first step. All the selected carbonyl compounds yielded the expected thiosemicarba-
zone ligand, but the product obtained by the reaction of benzaldehyde with thiosemicarbazide in ethanol
was found to show sharp IR peak characteristic of C@O group and thought to be benzaldehyde semicar-
bazone. When the ligands were treated with cis-[RuCl2(dmso)4)] in ethanol, all the ligands yielded thio-
semicarbazone complexes, while the suspected semicarbazone ligand resulted in orange-yellow
crystalline product which has been found to be a semicarbazone complex by XRD studies. A mechanism
has been proposed for the conversion of C@S to C@O during ligand preparation, which involves the role of
adventitious water in ethanol. All the complexes were characterized by analytical and spectroscopic (IR,
UV–Vis and 1H NMR) methods. The redox behaviors of the complexes were studied by cyclic voltamme-
try. The preliminary DNA-binding ability of the complexes was studied by recording electronic absorp-
tion spectra of the complexes in presence of herring sperm DNA. Antibacterial activities of the
complexes were also been evaluated against five pathogenic bacteria.
Received 12 February 2008
Accepted 23 May 2008
Available online 9 July 2008
Keywords:
Ruthenium(II)
Thiosemicarbazone
Cyclic voltammetry
DNA binding
Antibacterial activity
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
of similar compounds containing ligands with hard donor sets such
as O,O [7] and O,N [8].
In the development of novel metal-based therapeutics, detailed
studies on the interactions between biomolecular targets such as
DNA and structurally defined transition metal complexes can pro-
vide invaluable information [1]. Among the non-platinum com-
pounds exhibiting anticancer properties, those of ruthenium are
very promising, showing activity on even such tumors which
developed resistance to cisplatin or in which cisplatin is totally
inactive. Moreover, in contrast to cisplatin, the toxic side effects
of these ruthenium complexes are relatively low [2]. Ruthenium
complexes having sulfoxide, chloro and N-donor ligands are of re-
cent interest for their antimetastatic properties [3]. The successful
completion of preclinical tests [4–6] and phase I clinical trials of
NAMI-A (imidazolium trans-imidazole dimethylsulfoxide tetra-
chloro ruthenate) has promoted the investigation on the synthesis
Though several Cl–Ru(II)-dmso complexes [9–11] are available
as precursors and few other such Ru(III) complexes namely mer-
[RuCl3(dmso)2Im] and Na[trans-RuCl4(dmso)Im] have been studied
for their anticancer activities [12,13], in our present exploration a
well-known Ru(II)-dmso complex namely cis-[RuCl2(dmso)4] is
chosen due to the obvious reason that Ru(III) is paramagnetic and
limits the use of NMR spectroscopy for characterization purposes.
Furthermore, cis-[RuCl2(dmso)4] possesses mutagenic properties
[14], exhibits good antineoplastic activity against several murine
metastasizing tumors [15] and interacts in vitro with DNA to form
covalent bonds with the nucleobases, especially guanine (N7)
[16]. To go par with the remarkable antitumor activities of cis-
[RuCl2(dmso)4], we chose thiosemicarbazones as ligand system, be-
cause of the large number of research reports on the coordination
chemistry [17] and biological activities [18] of thiosemicarbazone
complexes. Another impelling factor is that the biological proper-
ties are predominantly reliant on the nature of the aldehyde or ke-
tone [19] and especially if these are heteroaromatic systems, their
nature seems to perk up their activity [20–24]. With this general
framework, the present paper deals with the reaction of a series
of thiosemicarbazones of aromatic and heteroaromatic
carbonyl compounds and a semicarbazone with cis-[RuCl2(dmso)4],
Abbreviations: TBAP, tetra butyl ammonium perchlorate; Tris, tris-(hydroxy-
methyl) aminomethane; UV–Vis, ultraviolet–visible; Cfu, colony forming units;
NCIM, National Centre for Industrial Microorganisms; MTCC,microbial type culture
collection; NCCLS, National Committee for Clinical Laboratory Standards.
* Corresponding author. Tel.: +91 4222424655; fax: +91 4222422387.
E-mail address: k_natraj6@yahoo.com (K. Natarajan).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.05.034

2744
V. Mahalingam et al. / Polyhedron 27 (2008) 2743–2750
characterization, electrochemistry, DNA-binding potential and
antibacterial activity of the resulting complexes. The general struc-
ture of the ligands used in this study is given in Chart 1.
on Varian-Australia AMX-400 instrument. Micro analyses (C, H, N
and S) were performed on a Vario EL III Elementar analyzer. The elec-
trochemical analyzer (CHI 1120A) equipped with a three electrode
compartment consisting of Platinum disc working electrode, Plati-
num wire counter electrode and Ag/AgCl reference electrode was
used to record the cyclic voltammograms of the complexes.
2. Experimental
2.1. Reagents and materials
2.3. Syntheses
Analytical or chemically pure grade chemicals were used for the
preparation of ligands. RuCl3 ꢀ 3H2O purchased from Himedia was
used as supplied. Protein free herring sperm ds DNA obtained from
SRL chemicals was stored at 0–4 °C. All the spectroscopic DNA
titrations were carried out in Tris–HCl buffer [0.197 g (5 mmol)
of Tris–HCl and 0.73 g (50 mmol) NaCl] are dissolved in double dis-
tilled water and the pH was adjusted to 7.1 using 1 mM NaOH solu-
tion before making up to 250 ml) at room temperature. A solution
of herring sperm DNA (0.1 g) in Tris–HCl buffer (10 ml) gave a ratio
of UV absorbance at 260 and 280 nm of ca. 1.8–1.9:1 indicating
that the DNA was sufficiently free of protein [25]. The DNA concen-
tration per nucleotide was determined by absorption spectroscopy
using the molar absorption coefficient (6600 Mꢁ1 cmꢁ1) at 260 nm
[26]. Distilled dmso was used for the preparation of the starting
complex and for the preparation of solutions of complexes for
DNA-binding studies. Purified dry methanol was used to record
the electronic spectra of the complexes. Commercially available
TBAP was properly dried and used as supporting electrolyte for
recording cyclic voltammograms of the complexes. The starting
complex cis-[RuCl2(dmso)4] was prepared according to the method
reported by Evans et al. [27] and the ligands were prepared by
refluxing an ethanolic solution of the aldehyde or ketone with thi-
osemicarbazide as described in the literature [28].
2.3.1. Synthesis of cis-[RuCl2(dmso)4]
RuCl3 ꢀ 3H2O (1 g) was refluxed in dmso (5 ml) for 5 min. The
solution rapidly turned orange brown and the volume was reduced
to half using rotary evaporator. When acetone (20 ml) was added
to the solution a yellow precipitate separated out. The precipitate
was washed with acetone and ether and dried in vacuum.
2.3.2. Synthesis of ligands
An ethanolic solution of the corresponding aldehyde or ketone
(0.01 mol) with thiosemicarbazide (0.01 mol) was heated under re-
flux for 4–5 h. Then the reaction mixture was cooled to get the
respective thiosemicarbazone as precipitate. The precipitate was
washed with ethanol and dried.
2.3.3. [RuCl2(dmso)2(petsc)] (1)
cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) and the ligand petsc
(0.040 g; 0.21 mmol) were heated under reflux in ethanol (30 ml)
for 4 h during which a brownish solid separated. The solvent was
evaporated on a rotary evaporator and the product was dried un-
der vacuum. Finally, the product was washed with ether several
times to get shiny brown crystalline complex. Yield, 73 mg; 67%.
Elemental Anal. Calc. for C13H23N3S3O2Cl2Ru (521.53): C, 29.93;
H, 4.44; N, 8.06; S, 18.44. Found: C, 29.68; H, 4.12; N, 8.31; S,
18.17%. IR (KBr, cmꢁ1):
3303(w); (NH), 3128(w);
asymmetric; sym, symmetric; s, strong; w, weak). UV/Vis (dmso)
k, nm (
, molꢁ1 cmꢁ1 l): 270 (9780), 294 (6620). 1H NMR ([D6]-
m(NH2 (asy)), 3381(w);
m(NH2 (sym)),
2.2. Physical measurements
m
m
(C@N), 1625 (s); (C@S), 736(w) (asy,
m
A Nicolet Avatar Model FT-IR spectrophotometer was used to re-
cord the IR spectra (4000–400 cmꢁ1) of the free ligands and the com-
plexes. Electronic spectral measurements (800–250 nm) and
absorption titrations (DNA binding) were carried out with a Systron-
ics 119 UV–Vis spectrophotometer. 1H NMR spectra were recorded
e
dmso, d ppm): 2.38 (s, H3C–C@N, 3H), 3.2–3.5 (group of peaks,
dmso, 12H), 3.64 (s, NH2, 2H), 6.98–7.27 (multiplet, aromatic pro-
tons, 5H), 11.23 (s, –NH, 1H).
X
R
C
N
H
N
C
R'
NH2
Ligand
L1
R
C6H5
R'
CH3
CH3
H
X
S
S
S
S
S
S
S
O
Name
Abbreviation
petsc
1-Phenyl-ethanone thiosemicarbazone
L2
p-C6H4CH3
C4H3S
1-p-Tolyl-ethanone thiosemicarbazone
tetsc
L3
Thiophene-2-carbaldehyde
thiosemicarbazone
Furan-2-carbaldehyde
thiosemicarbazone
4-Methoxybenzaldehyde
thiosemicarbazone
1-(4-Methoxy-phenyl)-ethanone
thiosemicarbazone
tctsc
L4
C4H3O
H
fctsc
L5
p- C6H4 OCH3
H
mbtsc
mptsc
cbtsc
L6
p- C6H4 OCH3 CH3
L7
p- C6H4 Cl
CH3
H
2-Chlorobenzaldehyde
thiosemicarbazone
Benzaldehyde semicarbazone
L8
C6H5
bzsc
Chart 1. General structure of the ligands.

V. Mahalingam et al. / Polyhedron 27 (2008) 2743–2750
2745
2.3.4. [RuCl2(dmso)2(tetsc)] (2)
2.3.9. [RuCl2(dmso)2(cbtsc)] (7)
The complex was prepared as described in Section 2.3.3. by the
reaction of cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) with the ligand
tetsc (0.043 g; 0.21 mmol). Yield: 72 mg; 64%. Elemental Anal. Calc.
for C14H25N3S3O2Cl2Ru (535.56): C, 31.39; H, 4.70; N, 7.84; S, 17.96.
The complex was prepared as described in Section 2.3.3. by the
reaction of cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) with the ligand
cbtsc (0.045 g; 0.21 mmol). Yield: 86 mg; 74%. Elemental Anal. Calc.
for C13H22N3S3O2Cl3Ru (555.98): C, 28.08; H, 3.98; N, 7.55; S, 17.30.
Found: C, 31.48; H, 4.47; N, 7.59; S, 17.77%. IR (KBr, cmꢁ1):
m
(NH2
(C@S),
740(w) (asy, asymmetric; sym, symmetric; s, strong; w, weak). UV/
Vis (dmso) k, nm (
, molꢁ1 cmꢁ1 L): 272 (9840), 291(7700). 1H NMR
Found: C, 28.24; H, 3.77; N, 7.38; S, 17.54%. IR (KBr, cmꢁ1):
m
(NH2
(C@N),
(C@S), 680(w) (asy, asymmetric; sym, symmetric; s,
(asy + sym)), 3434(b,w);
m
(NH), 3147(w);
m
(C@N), 1625(s);
m
(asy)), 3360(w);
m
(NH2 (sym)), 3326(w);
m
(NH), 3124(w);
m
1625(s);
m
e
strong; w, weak). UV/Vis (dmso) k, nm (e
, molꢁ1 cmꢁ1 L): 281
([D6]-dmso, d ppm): 1.64 (s, –CH3, 3H), 2.45 (s, H3C–C@N, 3H), 3.1–
3.4 (group of peaks, dmso, 12H), 3.62 (s, –NH2, 2H), 6.7–7.3 (mul-
tiplet, aromatic protons, 4H), 10.87 (s, –NH, 1H).
(13780), 312 (8620). 1H NMR ([D6]-dmso, d ppm):, 2.48 (s, H3C–
C@N, 3H), 3.1–3.4 (group of peaks, dmso, 12H), 3.74 (s, –NH2,
2H), 7.4–7.8 (multiplet, aromatic protons, 4H), 10.96 (s, –NH, 1H).
2.3.5. [RuCl2(dmso)2(tctsc)] (3)
2.3.10. [RuCl2(dmso)2(bzsc)] (8)
The complex was prepared as described in Section 2.3.3. by the
reaction of cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) with the ligand
tctsc (0.039 g; 0.21 mmol). Yield: 67 mg; 62%. Elemental Anal. Calc.
for C10H19N3S4O2Cl2Ru (513.53): C, 23.38; H, 3.72; N, 8.18; S, 24.97.
cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) and the ligand bzsc
(0.034 g; 0.21 mmol) were heated under reflux in ethanol (30 ml)
for 6 h. Slow evaporation of the solvent after reduction to 50% gave
orange crystals suitable for single crystal XRD studies. Yield:
Found: C, 23.29; H, 3.95; N, 8.39; S, 24.76%. IR (KBr, cmꢁ1):
m
(NH2
(C@S),
720(w) (asy, asymmetric; sym, symmetric; s, strong; w, weak). UV/
Vis (dmso) k, nm (
, molꢁ1 cmꢁ1 L): 295 (15230), 314 (11530). 1H
74 mg; 72%. Elemental Anal. Calc. for
(491.440): C, 29.32; H, 4.29; N, 8.55; S, 13.04. Found: C, 29.54; H,
4.08; N, 8.71; S, 12.88%. IR (KBr, cmꢁ1):
(NH2 (asy)), 3325(w);
(NH), 3148(w); (C@N), 1625(s);
(C@O), 1665(s) (asy, asymmetric; sym, symmetric; s, strong; w,
C12H21N3S2O3Cl2Ru
(asy + sym)), 3380(b,w); m(NH), 3142(w); m(C@N), 1629(s); m
m
e
m
m
(NH2 (sym)), 3252(w);
m
m
NMR ([D6]-dmso, d ppm): 3.2–3.4 (group of peaks, dmso, 12H),
4.22 (s, –NH2, 2H), 7.05–7.35 (multiplet, aromatic protons, 3H),
8.17 (s, H–C@N, 1H), 11.24 (s, –NH, 1H).
weak). UV/Vis (dmso) k, nm (
e
, molꢁ1 cmꢁ1 L): 276 (10780), 308
(7620). 1H NMR ([D6]-dmso, d ppm): 3.1–3.4 (group of peaks,
dmso, 12H), 3.82 (s, –NH2, 2H), 7.5–7.9 (multiplet, aromatic pro-
tons, 5H), 8.35 (s, –CH@N, 1H), 11.15 (s, –NH, 1H).
2.3.6. [RuCl2(dmso)2(fctsc)] (4)
The complex was prepared as described in Section 2.3.3. by the
reaction of cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) with the ligand
fctsc (0.035 g; 0.21 mmol). Yield: 70 mg; 64%. Elemental Anal. Calc.
for C10H19N3S3O3Cl2Ru (497.46): C, 24.14; H, 3.84; N, 8.44; S, 19.33.
2.4. Crystallographic structure determination
X-ray diffraction measurements were performed on a Nonius
Kappa CCD (with Oxford Cryostream) diffractometer with graphite
monochromatized Mo Ka radiation. The structure was solved by
direct methods and refinements were carried out by full-matrix
least-square techniques. The hydrogen atoms were treated by a
mixture of independent and constrained refinement. The following
computer programs were used: structure solution SIR-97 [29],
refinement SHELXL-97 [30], molecular diagrams, ORTEP-3 [31] for
windows.
Found: C, 24.33; H, 3.72; N, 8.69; S, 19.45%. IR (KBr, cmꢁ1):
m
(NH2
(C@S),
759(w) (asy, asymmetric; sym, symmetric; s, strong; w, weak). UV/
Vis (dmso) k, nm (
, molꢁ1 cmꢁ1 L): 287 (16930), 302 (16510). 1H
(asy + sym)), 3410(b,w);
m
(NH), 3138(w);
m
(C@N), 1618(s);
m
e
NMR ([D6]-dmso, d ppm): 3.2–3.4 (group of peaks, dmso, 12H),
3.75 (s, –NH2, 2H), 6.6–7.8 (multiplet, aromatic protons, 3H), 8.09
(s, H–C@N, 1H), 11.00 (s, –NH, 1H).
2.3.7. [RuCl2(dmso)2(mbtsc)] (5)
The complex was prepared as described in Section 2.3.3. by the
reaction of cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) with the ligand
mbtsc (0.044 g; 0.21 mmol). Yield: 74 mg; 66%. Elemental Anal.
Calc. for C13H23N3S3O3Cl2Ru (537.53): C, 29.04; H, 4.31; N, 7.81; S,
17.89. Found: C, 28.81; H, 4.16; N, 7.59; S, 18.02%. IR (KBr, cmꢁ1):
2.5. Antimicrobial screening
The bacterial stains, Escherichia coli NCIM 2831, Staphylococcus
aureus NCIM 2492, Staphylococcus epidermidis NCIM 2493, Klebsi-
ella pneumoniae NCIM 5082 and Shigella sonnei MTCC 2957 were
used in the study. Agar-well diffusion method [32] was employed
for the determination of antimicrobial activities. Minimum inhibi-
tory concentrations (MICs) of the compounds against test organ-
isms were determined by broth micro dilution method. All the
tests were performed in duplicate and repeated twice. Modal val-
ues were selected.
m
m
(NH2 (asy)), 3411(w);
(C@N), 1602(s); (C@S), 680(w) (asy, asymmetric; sym, symmet-
, molꢁ1 cmꢁ1 L): 287
m(NH2 (sym)), 3292(w); m(NH), 3148(w);
m
ric; s, strong; w, weak). UV/Vis (dmso) k, nm (
e
(18680). 1H NMR ([D6]-dmso, d ppm): 3.0–3.4 (group of peaks,
dmso, 12H), 3.54 (s, –NH2, 2H), 3.71 (s, –OCH3, 3H), 7.1–7.9 (mul-
tiplet, aromatic protons, 4H), 8.27 (s, H–C@N, 1H), 11.29 (s, –NH,
1H).
2.5.1. Agar-well diffusion method
2.3.8. [RuCl2(dmso)2(mptsc)] (6)
Compounds were weighed and dissolved in dimethyl sulphox-
The complex was prepared as described in Section 2.3.3. by the
reaction of cis-[RuCl2(dmso)4] (0.1 g; 0.21 mmol) with the ligand
mptsc (0.047 g; 0.21 mmol). Yield: 82 mg; 71%. Elemental Anal.
Calc. for C14H25N3S3O3Cl2Ru (551.56): C, 30.48; H, 4.56; N, 7.61; S,
17.44. Found: C, 30.25; H, 4.41; N, 7.39; S, 11.11%. IR (KBr, cmꢁ1):
ide (dmso). Compounds were filter sterilized using a 0.45 lm
membrane filter. Each microorganism was suspended in sterile sal-
ine and diluted at ca.106 colony forming units (cfu/mL). They were
flood inoculated onto the surface of Mueller Hinton Agar (MHA).
The wells (8 mm in diameter) were cut from the agar and 0.1 mL
of solution was delivered into them. After incubation for 24 h at
37 °C, all plates were examined for any zones of growth inhibition,
and the diameter of these zones were measured in millimeters.
m
m
(NH2 (asy)), 3405(w);
(C@N), 1598(s); (C@S), 696(w) (asy, asymmetric; sym, symmet-
, molꢁ1 cmꢁ1 L): 277
m(NH2 (sym)) 3353(w); m(NH), 3156(w);
m
ric; s, strong; w, weak). UV/Vis (dmso) k, nm (
e
(7980). 1H NMR ([D6]-dmso, d ppm): 2.45 (s, H3C–C@N, 3H), 3.1–
3.6 (group of peaks, dmso, 12H), 3.81 (s, –NH2, 2H), 3.87 (s,
–OCH3, 3H), 7.0–7.3 (multiplet, aromatic protons, 4H), 11.14 (s,
–NH, 1H).
2.5.2. Determination of minimum inhibitory concentration (MIC)
A microdilution broth susceptibility assay was used as recom-
mended [33] by NCCLS for determination of MIC. All the tests were

2746
V. Mahalingam et al. / Polyhedron 27 (2008) 2743–2750
performed in Mueller–Hinton Broth (MHB) supplemented with
Tween 80 detergent (final concentration of 0.5% (v/v)). Bacterial
stains were cultured overnight at 37 °C in MHA. Test strains were
suspended in MHB to give a final density of 5 ꢂ 105 cfu/ml and
these were confirmed by viable counts. Geometric dilutions of
the compounds were prepared in a 96-well microtiter plate,
including one growth control (MHB + Tween 80) and one sterility
control (MHB + Tween 80 + Compound). Plates were incubated un-
der normal atmospheric conditions at 37 °C for 24 h. The MIC of
kanamycin and oxytetracycline was individually determined in
parallel experiments in order to control the sensitivity of the test
organisms. The bacterial growth was indicated by the presence of
a white ‘‘pellet” on the well bottom.
of thione-thiol tautometism in the ligands is ruled out because no
bands were observed in the region near 2570 cmꢁ1, which is char-
acteristic of m(C–SH) [34]. This corroborates that the ligands are in
thione form in the solid state. In the IR spectra of the ligand L8, be-
sides other characteristic bands, a strong absorption at 1649 cmꢁ1
was found and tentatively assigned as due to
spectra of the complexes, a considerable decrease (5–10 cmꢁ1
for 5 and 6 and increase (5–45 cmꢁ1) for all other complexes in
the (C@N) frequency shows the coordination of the azomethine
nitrogen. The bands due to (NH2) and (NH) almost remain prac-
m(C@O). In the IR
)
m
m
m
tically unaltered supporting the non-coordination of NH2 nitrogen
and the presence of thione form of the ligand after complex forma-
tion. A small negative shift of 10–40 cmꢁ1 in
m
(C@S) in complexes
1–7 shows the neutral coordination of thioamide sulfur. Similarly,
the positive shift of
(C@O) to 1666 cmꢁ1 in complex 8, suggests
the neutral coordination of amide oxygen to ruthenium. The peak
at 915 cmꢁ1, characteristic of
(SO)(O-bonded) exhibited by the
m
3. Results and discussion
m
As the onset of this investigation, the synthesis of thiosemicar-
bazones was carried out as described in Section 2. On analyzing the
ligands for characterization through IR spectroscopy, a sharp peak
precursor cis-[RuCl2(dmso)4)] is absent in the new complexes and
thus it is perceived that the ligands act as neutral bidentate donors
replacing one O-bonded dmso and one S-bonded dmso from the
precursor, which is in accord with a previous study [27].
at 1649 cmꢁ1 which is characteristic of
m(C@O) was observed for
the ligand (L8) synthesized by condensing benzaldehyde and thio-
semicarbazide Based on this observation, the ligand was supposed
to be benzaldehyde semicarbazone rather than the expected benz-
aldehyde thiosemicarbazone. The reactions of the ligands (L1–L7)
with the starting complex in 1:1 mole ratio in ethanol yielded com-
plexes of the type [RuCl2(dmso)2(L)] (L = thiosemicarbazones) and
that of L8 resulted in a complex of the formula [RuCl2(dm-
so)2(bzsc)] (bzsc = benzaldehyde semicarbazone) which is evident
from its single crystal XRD data (presented in Section 3.3.). Thus,
it becomes manifest that the reaction of benzaldehyde with thio-
semicarbazide in ethanol has lead to the formation of benzalde-
hyde semicarbazone through the conversion of >C@S (of the
expected benzaldehyde thiosemicarbazones) to >C@O. Therefore,
it is presumed that adventitious water in ethanol should have ef-
fected the transformation in situ after the formation of thiosemi-
carbazone as proposed in the tentative mechanism (Scheme 1).
The electronic spectra of all the complexes were identical show-
ing weak absorptions in the ultraviolet region. The absorption
maximum and molar extinction coefficients are given in Section
2. The bands are assignable to transitions within the ligand orbitals
*
*
(intraligand
p ? p or n ? p ) [35,36].
3.2. 1H NMR spectra
The 1H NMR spectra of the complexes recorded in [D6]-dmso
exhibited narrow signals. Comparison of the spectra of the precur-
sor, cis-[RuCl2(dmso)4] with that of the new complexes seemed
inevitable in order to predict the nature of bonding mode of dmso
in the new complexes. Out of the six singlets appeared in the 1H
NMR spectra of cis-[RuCl2(dmso)4] at d 3.50, 3.48, 3.43, 3.32, 2.72
and 2.66 ppm due to six sets of chemically different protons, the
peak at d 2.66 ppm has been assigned to free dmso which resulted
from the 10% dissociation of O-bonded dmso. Similarly the peak at
d 2.72 ppm has been attributed to the O-bonded dmso since the
methyl resonance is close to that of free dmso (d 2.50). The other
peaks appearing in a group have been assigned to S-bonded dmso
and the integration of these peaks had not been possible owing to
their proximity and their sharing the same broad base line [27].
Thus it becomes obvious that the group of peaks appearing in
the new complexes around ꢃd 3.0–3.6 ppm should be attributed
to S-bonded dmso resonances. The lack of any peak at d 2.7 ppm
for any of the new complexes implied the absence of O-bonded
dmso in the complexes and supported the same point inferred
from the IR spectra and evidenced by the crystal structure of 8.
The resonance around d 2.5 ppm in all the complexes is due to
the solvent. The Ar-methyl protons in 2 resonated at d 1.64 ppm.
The NH2 protons in all the complexes gave single resonance peak
in the range d 3.54–4.22. The protons of CH3–C@N in 1, 2, 6 and
7 appeared as a singlet in the range at d 2.38–2.48. The singlets ob-
served at d 3.71 (complex 5) and d 3.87 (complex 6) are due to
methoxy protons. The protons of the methyl group in 3 and 4 gave
resonance at d 2.2 ppm. The aromatic/heterocyclic ring protons
were observed in the range 6.6–7.9 ppm as multiplets. The azome-
thine (HC@N) proton in 3, 4, 5 and 8 resonated in the range d 8.09–
8.35 ppm. The appearance of a singlet in all the complexes around
d 10.87–11.23 ppm is due to NH proton which is matching with the
IR observation that the chelating ligand coordinates in its keto
form to the metal. From the foregoing discussions, it can be estab-
lished that the semicarbazone/thiosemicarbazone ligands behave
as neutral bidentate donors replacing one O-bonded dmso and
one S-bonded dmso from the precursor.
3.1. IR and UV–Vis spectra
The prominent IR regions of interest pertaining to the com-
plexes are furnished in Section 2. The spectra of the free thiosem-
icarbazone ligands (L1–L7) show peaks of concern around 3400–
3250 (two peaks), 3160–3120 and 1615–1585 cmꢁ1 attributed to
the symmetric and asymmetric stretching of NH2,
m(NH) and
m
(C@N), respectively. The weak to medium intensity band around
782–716 cmꢁ1 in the ligands are assigned to
m
(C@S). The question
H
H
C
H
O H
C
N
SH
N
S
N
C
N
C
H
NH2
H
NH2
HO
H
H
C
C
N
H
N
O
SH
N
C
N
C
-
-
H+,
- SH
NH2
H
NH2
O
H
-
H+ + SH
H2S
Scheme 1. Tentative mechanism for the conversion of >C@S to >C@O.

V. Mahalingam et al. / Polyhedron 27 (2008) 2743–2750
2747
Table 1
Crystallographic data for 8
species, and is meso, with potential mirror symmetry. We thus as-
sume that the compound crystallizes as a conglomerate, with indi-
vidual crystals of each hand in the macroscopic sample.
Chemical formula
Formula weight
Crystal color, habit
Crystal dimensions (mm3)
Crystal system
Space group
a (Å)
C12H21Cl2N3O3RuS2
491.41
orange-yellow, fragment
0.23 ꢂ 0.15 ꢂ 0.12
orthorhombic
P212121
9.1078(12)
12.729(2)
16.318(3)
1891.8(5)
4
1.725
1.346
115
35798
4316
4035
992
27.5
The details on data collection, refinement and unit cell parame-
ters are collected in Table 1. Fig. 1 shows the ORTEP drawing of 8
and Table 2 presents the selected geometrical parameters that
are essential for the discussion. The molecular structure of 8 shows
that the coordination sphere around Ru(II) is S2Cl2NO. The coordi-
nation polyhedron of the ruthenium ion can be described as dis-
torted octahedron. The distortion in the complex from ideal
octahedral geometry is due to the small bite angle of the ON che-
late of semicarbazone ligand and the bending of chloride ligands
towards the chelate which is evident from the angles O1–Ru1–
N1 = 77.23(13), Cl1–Ru1–Cl2 = 171.62(4)°.
b (Å)
c (Å)
V (Å3)
Z
DCalc (Mg mꢁ3
)
l
(mmꢁ1) (Mo K
T (K)
a
)
Reflections collected
Unique reflections
Reflections [I P 2
F(000)
hmax (°)
The average Ru–Cl bond lengths Ru1–Cl1 = 2.4154(12); Ru1–
Cl2 = 2.3960(12) Å are slightly longer than found in [RuCl2(dm-
so)2(5-nitro-2-furaldehyde semicarbazone)] [Ru1–Cl12 = 2.386
(4); Ru1–Cl11 = 2.394(4) Å] [8]. This length is shorter than that
r
(I)]
Rint
0.054
0.037
0.087
1.056
found in cis-[RuCl2(dmso)4] (average Ru–Cl = 2.42 Å) and compara-
R [F2 P 2
wR [F2]
r
(F2)]
0
ble with that found in the trans isomer [Ru–Cl = 2.402(2) ÅA] which
is in agreement with the greater trans influence of dmso that weak-
ens the Ru–Cl bond in cis-[RuCl2(dmso)4] [40]. Similarly, the two
Ru–S bond lengths [Ru1–S1 = 2.2054(12); Ru1–S2 = 2.2436(11) Å]
Goodness of fit on F2
0
Largest peak and hole (e ÅAꢁ3
)
1.52 and ꢁ1.03
are significantly shorter than the four equal Ru–S bonds
0
[2.352(2) ÅA] in trans-[RuCl2(dmso)4] and mean Ru–S bond length
of 2.25(1) Å in [Ru(dmso)6]2+ [41] which shows the diminished
3.3. X-ray crystallography of [RuCl2(dmso)2(bzsc)] (8)
competition between two dmso ligands for
p-back donation in 8.
This stronger Ru2+–S bonds in comparison with the Ru3+–S bonds
The structural study of this complex gains importance because
the synthesis of this complex attempted previously by Vieites et al.
by reacting trans-[RuCl2(dmso)4] with benzaldehyde semicarba-
zone in methanol has not afforded any isolable complex and the
characterization was limited only to the 1H NMR spectra of the
reaction mixture [37]. Unlike their synthesis, in our case, X-ray
quality single crystals of the complex could be obtained by follow-
ing the simple procedure given in Section 2.3.8. The complex crys-
tallizes in the orthorhombic space group P212121. The compound
forms enantiopure crystals, as evidenced by the space group and
the value of Flack parameter, ꢁ0.01(4) [38] and Hooft parameter,
0.008(11) [39]. However the complex itself does not contain chiral
in similar Ru(III) complexes{Ru1–S1 = 1.461(3) Å in [(dmso)2H-
0
[trans-Ru(dmso)2Cl4]; Ru–S2 = 2.330(2), Ru–S3 = 2.351(1) ÅA in
mer-[Ru(dmso)3Cl3] and Ru1–S1 = 2.295(2) Å in [pyH]trans-[RuCl4-
(dmso)(py)]} [9,11] is in accordance with the better donation of
d-electron density of less positive Ru(II) to sulfur atoms of dmso
than by Ru(III) centre.
The complex is stabilized by intermolecular and intramolecular
hydrogen bonds. The hydrogen bonding geometries for the com-
plex is given in Table 3. The C–Hꢀ ꢀ ꢀO (azomethine carbon, C2 is
hydrogen bonded to the oxygen atom of the dmso) type intramo-
lecular hydrogen bonds exhibited by the complex is noteworthy.
Fig. 1. ORTEP style drawing of 8 with 50% probability of thermal ellipsoids. Hydrogen atoms are omitted for clarity.

2748
V. Mahalingam et al. / Polyhedron 27 (2008) 2743–2750
Table 2
Selected geometrical parameters for 8
Bond distances (Å)
Ru1–N1
Ru1–O1
Ru1–S1
S1–O2
2.123(4)
2.100(3)
2.2054(12)
1.484(3)
Ru1–S2
Ru1–Cl1
Ru1–Cl2
S2–O3
2.2436(11)
2.4154(12)
2.3960(12)
1.490(4)
Bond angles (°)
O1–Ru1–N1
O1–Ru1–S1
N1–Ru1–S1
O1–Ru1–S2
N1–Ru1–S2
S1–Ru1–S2
O1–Ru1–Cl2
N1–Ru1–Cl2
C11–S2–C12
C9–S1–C10
77.23(13)
175.83(9)
98.60(10)
91.14(9)
167.93(10)
93.03(4)
85.65(9)
86.05(11)
99.7(3)
S1–Ru1–Cl2
S2–Ru1–Cl2
94.10(5)
89.94(5)
O1–Ru1–Cl1
N1–Ru1–Cl1
S1–Ru1–Cl1
S2–Ru1–Cl1
Cl2–Ru–Cl1
C9–S1–O2
C10–S1–O2
C11–S2–O3
C12–S2–O3
87.79(9)
87.41(11)
92.07(5)
95.41(4)
171.62(4)
106.6(3)
104.3(2)
107.4(2)
106.6(3)
99.9(3)
Fig. 2. Cyclic voltammogram of 6.
Table 3
plexes 1 and 4, the cathodic counterpart of this couple is ill-
defined) is ascribed to the oxidation of a slowly dissociated chloride
ion [9,42]. This dissociation in turn accompanies Ru(II)/Ru(I) reduc-
tion occurring in the range ꢁ0.66 to ꢁ1.02 V and this range is con-
current with literature values [43,44]. Further dissociation of
chloride has not been evidenced previously [9] for trans-[Ru(dm-
so)2Cl4]2ꢁ ion and thus the same is expected for the new complexes
and also substantiated by the lack of any other reduction wave at
more negative potentials than that for Ru(II)/Ru(I) reduction pro-
cess. Also the dissociation of two chloride ions is unlikely since it
will lead to the formation of Ru(0) species which is highly unstable.
The other irreversible reduction wave in the range ꢁ0.18 to ꢁ0.40 V
for 2–8 and at ꢁ0.75 V for 1 could not be thought of semicarbazone/
thiosemicarbazones-based reduction for two reasons. Firstly, these
ligand-based reductions occur at more negative potentials, almost
at the negative-end of the chosen potential window here. The other
reason is the observance of a similar peak at ꢁ0.15 V in the cyclic
voltammogram of the precursor, cis-[RuCl2(dmso)4] obtained under
identical experimental conditions. Thus, this irreversible reduction
may be attributed to coordinated dmso. This reduction potential is
affected in the complexes due to coordinated semicarbazone/thio-
semicarbazones. Nevertheless no correlation could be made out
for this with the electronic nature of the ligands.
Hydrogen-bonding distances (Å) and angles(°) for 8
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D–Hꢀ ꢀ ꢀA
N2–H2Nꢀ ꢀ ꢀCl1i
N3–H31Nꢀ ꢀ ꢀO3ii
N3–H32Nꢀ ꢀ ꢀCl1i
C2–H2ꢀ ꢀ ꢀO2
0.89(6)
0.95(6)
0.80(6)
0.95
2.49(6)
2.03(6)
2.55(6)
2.14
3.281(4)
2.937(5)
3.256(4)
2.962(6)
149(5)
160(5)
147(5)
143.5
Symmetry codes (i) 1 ꢁ x, 1/2 + y, 1/2 ꢁ z and (ii) 1/2 + x, 3/2 ꢁ y, ꢁz.
Table 4
Cyclic voltammetric data of the complexes in dmso
Complex Ru(II)/Ru(I)a E1/2/V
Ep/mV)
(Cl)/(Clꢁ)b E1/2/V
Ep/mV)
dmso based
(
D
(D
reductiond
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
ꢁ1.02 (350)
ꢁ0.70 (200)
ꢁ0.66 (360)
ꢁ0.72 (240)
ꢁ0.73 (300)
ꢁ0.72 (240)
ꢁ0.75 (140)
ꢁ0.76 (160)
0.90c
1.06 (300)
1.07 (340)
0.90c
1.08 (400)
1.08 (360)
1.05 (300)
1.09 (340)
ꢁ0.75
ꢁ0.20
ꢁ0.20
ꢁ0.28
ꢁ0.20
ꢁ0.24
ꢁ0.40
ꢁ0.24
a
b
c
Reduction couple.
Oxidation couple.
Irreversible, Epa value.
Irreversible, Epc value.
d
3.5. UV–Vis spectra of the complexes in the presence of buffered
herring sperm DNA solution
By analogy of the structure of 8, a similar structure is proposed
In general, metal complexes can bind to DNA via both covalent
and non-covalent interactions [45–47]. These different modes of
binding can be monitored by following the changes in the wave-
length and intensity of absorption of a particular peak exhibited
by the unbound complex. A decrease in absorbance (hypochro-
mism) or an increase in absorbance (hyperchromicity) upon addi-
tion of DNA to a compound in solution is indicative of an
interaction between these molecules [48].
In a typical experiment, the absorption titration was carried out
with a complex of final concentration ꢃ1 ꢂ 10ꢁ4 M and varying the
concentration of herring sperm DNA. The concentration of the DNA
stock solution was calculated to be 3.7878 ꢂ 10ꢁ3 M. The addition
of DNA to the complex was achieved by adding buffered DNA (in
different mixing ratios (r = [DNA]/[Ru-complex]) to the complex
(0.3 mL) while maintaining the total volume a constant (3 mL).
All the complexes showed single peak in the Tris–HCl buffer in
the range 264–329 nm due to intraligand transitions. The change
in wavelength and absorbance of this peak in the unbound com-
plex was monitored as increasing amounts of DNA were added
for other thiosemicarbzone complexes 1–7, whose spectral charac-
terization supports the neutral bidentate coordination of the SN
chelate to ruthenium.
3.4. Cyclic voltammetric studies
The electron transfer properties of the new Ru(II) complexes
were examined by cyclic voltammetry with dmso solutions
(ꢃ1 ꢂ 10ꢁ3 M) using 0.1 M TBAP as supporting electrolyte. The re-
dox potentials measured relative to Ag/AgCl electrode at a scan rate
of 0.1 V/s are reported in Table 4. All the complexes were electroac-
tive in the sweep range 1.5 V and displayed well-defined one-elec-
tron quasi-reversible coupled redox wave on either side of the
reference electrode besides an irreversible peak at less negative
potentials with respect to the coupled redox wave at the negative
side of Ag/AgCl electrode. A prototype showing all the voltammetric
features is shown Fig. 2. The oxidative couple exhibited by the com-
plexes with formal potentials in the range 1.05–1.09 V (in com-

V. Mahalingam et al. / Polyhedron 27 (2008) 2743–2750
2749
Table 5
Change in UV–Vis spectra of (1)–(8) upon the gradual addition of herring sperm DNAa
Complex
r = 0
r = 1
r = 2
r = 3
kmax (nm)
emax ꢂ 103
kmax (nm)
emax ꢂ 103
kmax (nm)
emax ꢂ 103
kmax (nm)
emax ꢂ 103
1
2
3
4
5
6
7
8
264
264
273
302
283
276
307
257
18.341
16.076
6.138
11.089
10.610
8.557
264
267
270
304
283
273
309
305
23.348
22.432
9.560
11.935
9.682
11.082
17.828
7.867
264
269
265
305
283
270
312
311
25.040
24.970
17.265
13.204
9.682
12.340
18.251
10.033
264
270
264
306
283
268
313
324
25.994
25.816
19.400
13.838
9.682
14.440
18.674
10.033
16.559
4.483
a
Concentration of the complexes ꢃ10ꢁ4 M in dmso; concentration of DNA = 3.7878 ꢂ 10ꢁ3 in Tris–HCl buffer (pH 7.1); r = [DNA]/[complex].
Table 6
Antibacterial activity data of the complexes
Compound
Escherichia coli
Shigella sonnei
Klebsiella pneumoniae
Staphylococcus epidermidis
Staphylococcus aureus
WDa
MICb
WD
MIC
WD
MIC
WD
MIC
WD
MIC
2
3
4
6
7
8
18
NA
14
18
NA
15
30
32
56
NT
64
58
NT
62
15
18
12
NA
13
17
NA
12
37
48
78
NT
69
58
NT
74
8
NA
NA
14
NA
NA
12
30
NT
NT
55
NT
NT
58
8
NA
10
NA
NA
12
14
44
60
NT
61
NT
NT
63
58
8
NA
10
NA
NA
12
13
32
40
NT
82
NT
NT
77
63
7
Oxytetracycline
Kanamycin
9
35
10
11
10
NA – not active; NT – not tested.
a
WD, disc diffusion method as recommended by NCCLS. Diameter of zone of inhibition (mm) including well diameter 8 mm. One hundred micrograms of both compound
and antibiotics were used.
b
MIC, minimum inhibitory concentration. Values given as lg/ml for both compounds and antibiotics.
and the results are recorded in Table 5. Except 1 and 5, all the other
complexes show small red shift or blue shift with a considerable
hypochromism. These changes are indicative of interaction of the
complexes with DNA [49].
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033, or e-mail: deposit@ccdc.cam.ac.uk.
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