Ru(II)-DMSO complexes containing aromatic and heterocyclic acid hydrazides: Structure, electrochemistry and biological activity

Article abstract of DOI:10.1016/j.poly.2009.03.023

The reaction of cis-[RuCl₂(DMSO)₄] with a family of aromatic and heterocyclic acid hydrazides yielded new complexes of the general formula trans-[RuCl₂(DMSO)₂(hydrazide)] · nH₂O (n = 0; 1-6; n = 1; 7)

Full text of DOI:10.1016/j.poly.2009.03.023

Polyhedron 28 (2009) 1532–1540
Contents lists available at ScienceDirect
Polyhedron
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Ru(II)–DMSO complexes containing aromatic and heterocyclic acid
hydrazides: Structure, electrochemistry and biological activity
Viswanathan Mahalingam a, Nataraj Chitrapriya a, Matthias Zeller b, Karuppannan Natarajan a,
*
a Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
b Department of Chemistry, Youngstown State University, OH, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of cis-[RuCl2(DMSO)4] with a family of aromatic and heterocyclic acid hydrazides yielded
new complexes of the general formula trans-[RuCl2(DMSO)2(hydrazide)] ꢀ nH2O (n = 0; 1–6; n = 1; 7).
The new complexes have been characterized by IR, UV–Vis and 1H NMR spectroscopic methods. In addi-
tion, the structure of one of the complexes, [RuCl2(DMSO)2(tcah)] ꢀ H2O (tcah = thiophene-2-carboxylic
acid hydrazide), has been determined by single crystal X-ray diffraction. All the studies reveal the neutral
bidentate coordination of the hydrazide ligands through the acyl oxygen and amine nitrogen atoms. The
electron transfer properties of the complexes were studied by cyclic voltammetry and all the complexes
except one show an irreversible/quasi-reversible reduction wave (RuII/RuI) and an uncoupled oxidation
peak (RuIII/ RuII). The preliminary DNA-binding ability of the complexes, studied with herring sperm
DNA, shows the binding of the complexes with DNA with a lesser affinity than classical intercalators.
The complexes have also been screened for their antibacterial activity against five pathogenic bacteria.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 17 November 2008
Accepted 11 March 2009
Available online 31 March 2009
Keywords:
Ru(II)–DMSO complexes
Carboxylic acid hydrazides
Antibacterial activity
Cyclic voltammetry
DNA-binding
1. Introduction
[7–9]. The therapeutic effects have been attributed to the ability
of both isomers to form stable adducts with DNA, cross-links be-
Despite the widespread clinical use, chemotherapy with cis-
platin and its analogues still has several drawbacks, such as a rela-
tively narrow spectrum of activity, severe side-effects and acquired
resistance. These considerations have stimulated the search for new
metal based antitumor drugs [1–3]. Complexes of transition metals
other than platinum may exhibit antitumor activity and toxic side
effects markedly different from that of the Pt drugs for a number
of obvious reasons viz. different chemical behavior, hydrolytic rates
and mechanism(s) of action. Among the several metals that are cur-
rently being investigated for their anticancer activity, ruthenium
occupies a prominent position [4–6]. The ruthenium(II) complexes,
trans-[RuCl2(DMSO)4] and cis-[RuCl2(DMSO)4] have both been
shown to exhibit antiproliferative activity in several experimental
tumors, such as P388 leukemia, Lewis lung carcinoma, B16 mela-
noma and MCa mammary carcinoma. It has been found that both
isomers tend to localize to a higher extent in the tumor than in nor-
mal tissue and show significant antimetastatic activity and lower
toxicity for normal proliferating tissue in comparison to cisplatin
tween adjacent guanines being the dominant ones [10,11]. In spite
of the plethora of biological investigations, both in vitro and in vivo,
the mode of action of Ru–DMSO compounds at a molecular level is
largely not understood, and is of much interest [12–14]. In this re-
gard, many Ru–DMSO complexes have been prepared, incorporat-
ing a variety of ligands, to study the coordination behavior and
DNA-binding activity in order to understand their mode of action
in inhibiting the growth of various tumors [14–17]. During our
methodical investigation on the reaction of cis-[RuCl2(DMSO)4]
with diverse ligand systems, we carried out detailed structural
and biological studies of several heterocyclic hydrazone and
semi-/thiosemicarbazone ligand complexes of cis-[RuCl2(DMSO)4]
[18,19]. In continuance to this, we, after a cautious search, chose
a few aromatic- and heterocyclic carboxylic acid hydrazides to be
used as ligands for the synthesis of new Ru–DMSO complexes, for
they provide various active potential donor sites, namely C@O,
N–H and NH2, and have been shown to posses a wide range of bio-
logical activities [20–22] such as antibacterial activity and tubercu-
lostatic properties [23–25]. Thus, the present paper reports the
reactions of some aromatic and heterocyclic acid hydrazides with
cis-[RuCl2(DMSO)4], the characterization of the products and their
biological activities, such as DNA-binding and antibacterial activity.
The structural drawings of the acid hydrazide ligands along with
the abbreviations and numbering scheme for 1H NMR are given in
Chart 1.
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 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.03.023

V. Mahalingam et al. / Polyhedron 28 (2009) 1532–1540
1533
O
O
2
2
1
NH2
NH2
1
3
3
N
H
N
H
6
6
4
4
5
5
p-Methyl-benzoic acid hydrazide
o-Methyl-benzoic acid hydrazide
(pmbh)
(ombh)
O
O
O
6
8
1
1
9
5
2
3
NH2
4
NH2
NH2
5
4
7
N
H
N
6
N
H
H
6
N
3
10
OH
OH
2
1
3
4
5
2
o-Hydroxy-naphthalene-2-carboxylic
o-Hydroxy-benzoic acid
hydrazide
Isonicotinic acid
hydrazide
(inah)
acid hydrazide
(hnah)
(hbah)
3
O
3
O
4
5
4
5
NH2
2
NH2
N
H
N
H
2
S
O
1
1
Furan-2-carboxylic acid hydrazide
(fcah)
Thiophene-2-carboxylic acid hydrazide
(tcah)
Chart 1. Structural drawings of the acid hydrazide ligands with the numbering scheme for 1H NMR discussion.
in ethanol (30 mL) and heated under reflux for 5–6 h, whereby
the solution turned from pale yellow to reddish brown. After
reducing the content to half the volume and standing for a day,
the complex was obtained as reddish brown precipitate. It was fil-
tered and washed several times with ethanol and finally with
diethyl ether, and dried under fused calcium chloride. Yield:
61 mg; 61%. Elemental Anal. Calc. for C12H22N2S2O3Cl2Ru
(478.42): C, 30.13; H, 4.63; N, 5.86; S, 13.40. Found: C, 30.48; H,
2. Experimental
2.1. Reagents and materials
Analytical grade dimethyl sulfoxide and distilled ethanol were
used for synthetic purposes. Commercial RuCl3 ꢀ 3H2O (Himedia)
was used without further purification. 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 buf-
fer (5 mM Tris–HCl, 50 mM NaCl, pH 7.1) 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. 1.7–
1.8:1, indicating that the DNA was sufficiently free of protein
[26]. The DNA concentration per nucleotide was determined by
absorption spectroscopy using the molar absorption coefficient
(6600 Mꢁ1 cmꢁ1) at 260 nm [27]. Distilled dimethyl sulfoxide
was used for recording electronic spectra of the complexes and
for absorption titrations with DNA. Commercially available TBAP
was properly dried and used as an 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. [28] and the hydrazide ligands were purchased from
Acros organics.
4.42; N, 5.56; S, 13.75%. IR (KBr, cmꢁ1):
(NH) 3140(w); d(NH2) 1556(s); (C@O) 1646(s);
1080(s) (asy, asymmetric; sym, symmetric; s, strong; w, weak).
m
(NH2)(asy+sym) 3399(w);
(S@O)s-bonded
m
m
m
UV/Vis (DMSO) k, nm (e
, molꢁ1 cmꢁ1 L): 277 (9190), 295 (8130).
1H NMR (400 MHz, DMSO-[D6], d ppm): 2.40 (s, 3H, Ar–CH3),
3.22–3.33 (group of peaks, 12H, DMSO), 7.27–7.32 (m, 3H, C3–H,
C4–H and C5–H), 7.43 (d, 1H, J = 7.84 Hz, C6–H), 7.68 (s, 2H, –
NH2), 11.58 (s, 2H, –NH).
2.3.2. [RuCl2(DMSO)2(pmbh)] (2)
This complex was prepared as described in Section 2.3.1 by the
reaction of cis-[RuCl2(DMSO)4] (0.1 g; 0.21 mmol) and the ligand
pmbh (p-methyl-benzoic acid hydrazide) (0.031 g; 0.21 mmol).
Yield: 62 mg; 62%. Elemental Anal. Calc. for C12H22N2S2O3Cl2Ru
(478.42): C, 30.13; H, 4.63; N, 5.86; S, 13.40. Found; C, 30.37; H,
4.79; N, 5.51; S, 13.63%. IR (KBr, cmꢁ1):
(NH) 3141(w); d(NH2) 1597(s); (C@O) 1638(s);
1079(s). UV/Vis (DMSO) k, nm (
, molꢁ1 cmꢁ1 L): 261 (5500), 325
m(NH2)(asy+sym) 3450(w);
m
m
m
(S@O)s-bonded
2.2. Physical measurements
e
(1170). 1H NMR (400 MHz, DMSO-[D6], d ppm): 2.37 (s, 3H, Ar–
CH3), 3.15–3.43 (group of peaks, 12H, DMSO), 7.34 (d, 2H,
J = 7.92 Hz, C3–H and C5–H), 7.63 (s, 2H, –NH2), 7.86 (d, 2H,
J = 8.20 Hz, C2–H and C6–H), 11.64 (s, 1H, –NH).
Infrared spectra were recorded on a Nicolet Avatar Model FT-IR
spectrophotometer, while UV–Vis spectra were measured on a Var-
ian Cary 500 Scan UV–Vis–NIR spectrophotometer. 1H NMR spectra
were acquired on a Varian-Australia AMX-400 instrument. Micro-
analyses were carried out on a Vario EL III Elementar elemental
2.3.3. [RuCl2(DMSO)2(hbah)] (3)
analyzer. Cyclic voltammograms were obtained using
a CHI
This complex was prepared as described in Section 2.3.1 by the
reaction of cis-[RuCl2(DMSO)4] (0.1 g; 0.21 mmol) and the ligand
hbah (2-hydroxy-benzoic acid hydrazide) (0.032 g; 0.21 mmol).
Yield: 67 mg; 67%. Elemental Anal. Calc. for C11H20N2S2O4Cl2Ru
(480.39): C, 27.50; H, 4.20; N, 5.83; S, 13.35. Found: C, 27.22; H,
1120A electrochemical analyzer. A three-electrode system was
employed with a Pt disc working electrode, a Pt wire counter elec-
trode and an Ag/Ag+ reference electrode.
2.3. Syntheses
4.05; N, 5.41; S, 13.54%. IR (KBr, cmꢁ1):
m
(NH2)(asy+sym) 3484(w);
(NH) 3122(w); d(NH2) 1602(s); (C@O) 1634(s);
(S@O)s-bonded 1074(s). UV/Vis (DMSO) k, nm (
, molꢁ1 cmꢁ1 L):
m
(OH) 3300(w);
m
m
e
2.3.1. [RuCl2(DMSO)2(ombh)] (1)
Cis-[RuCl2(DMSO)4] (0.1 g; 0.21 mmol) and the ligand ombh (o-
methyl-benzoic acid hydrazide) (0.031 g; 0.21 mmol) were taken
m
299 (10890), 352 (2720). 1H NMR (400 MHz, DMSO-[D6], d ppm):

1534
V. Mahalingam et al. / Polyhedron 28 (2009) 1532–1540
3.21–3.39 (group of peaks, 12H, DMSO), 6.98–7.05 (m, 2H, C4–H
and C5–H), 7.47 (d, 1H, J = 7.90 Hz, C3–H), 7.50 (s, 2H, –NH2), 8.02
(d, 1H, d, J = 8.00 Hz, C6–H), 10.57 (s, 1H, –OH), 11.74 (s, 1H, –NH).
2.4. Crystallographic structure determination
X-ray diffraction measurements were performed on a Bruker
AXS SMART APEX CCD diffractometer with graphite monochro-
2.3.4. [RuCl2(DMSO)2(hnah)] (4)
matic Mo Ka radiation. The unit cells were determined using SMART
This complex was prepared as described in Section 2.3.1 by the
reaction of cis-[RuCl2(DMSO)4] (0.1 g; 0.21 mmol) and the ligand
hnah (3-hydroxy-naphthalene-2-carboxylic acid hydrazide)
(0.042 g; 0.21 mmol). Yield: 73 mg; 65%. Elemental Anal. Calc. for
C15H24N2S2O4Cl2Ru (532.47): C, 33.83; H, 4.54; N, 5.26; S, 12.04.
Found: C, 33.41; H, 4.72; N, 5.69; S, 12.45%. IR (KBr, cmꢁ1):
[29] and SAINT+ [30], and the data were corrected for absorption
using SADABS [31]. The structure was solved by direct methods and
refinements were carried out by full-matrix least-square tech-
niques. The hydrogen atoms were treated by a mixture of indepen-
dent and constrained refinement. The computer program SHELXTL
6.14 [32] was used for structure solution, refinement and molecu-
lar graphics.
m
(NH2)(asy+sym) 3437(w);
1591(s); (C@O) 1634(s);
k, nm (
, molꢁ1 cmꢁ1 L): 257 (12420), 278 (3640), 352 (1430). 1H
m
(OH) 3284(w);
m(NH) 3088(w); d(NH2)
m
m(S@O)s-bonded 1078(s). UV/Vis (DMSO)
e
2.5. Antimicrobial screening
NMR (400 MHz, DMSO-[D6], d ppm): 3.23–3.39 (group of peaks,
12H, DMSO), 7.33–7.37 (m, 1H, C7–H), 7.52–7.56 (m, 1H, C6–H),
7.59 (s, 1H, C4–H), 7.78 (d, 1H, J = 8.04 Hz, C5–H), 8.05 (d, 1H,
J = 8.41 Hz, C8–H), 8.58 (s, 1H, C1–H), 8.70 (s, 2H, –NH2), 10.83 (s,
1H, –OH), 11.76 (s, 1H, –NH).
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. The agar well diffusion method [33] was em-
ployed for the determination of anti-microbial activities. MICs
(minimum inhibitory concentrations) of the compounds against
test organisms were determined by the broth micro dilution meth-
od. All the tests were performed in duplicate and repeated twice.
Modal values were selected.
2.3.5. [RuCl2(DMSO)2(inah)] (5)
This complex was prepared as described in Section 2.3.1 by the
reaction of cis-[RuCl2(DMSO)4] (0.1 g; 0.21 mmol) and the ligand
inah (isonicotinic acid hydrazide) (0.029 g; 0.21 mmol). Yield:
60 mg; 61%. Elemental Anal. Calc. for C10H19N3S2O3Cl2Ru
(465.38): C, 25.81; H, 4.12; N, 9.03; S, 13.78. Found: C, 25.49; H,
2.5.1. Agar-well diffusion method
The compounds were weighed and dissolved in dimethyl sulf-
oxide (DMSO). The compounds were filter sterilized using a
4.16; N, 9.38; S, 13.52%. IR (KBr, cmꢁ1):
(NH) 3085(w); d(NH2) 1591(s); (C@O) 1644(s);
1080(s). UV/Vis (DMSO) k, nm (
, molꢁ1 cmꢁ1 L): 274 (3280), 352
m
(NH2)(asy+sym) 3430(w);
(S@O)s-bonded
m
m
m
e
0.45 lm membrane filter. Each microorganism was suspended in
(2870). 1H NMR (400 MHz, DMSO-[D6], d ppm): 3.14–3.35 (group
of peaks, 12H, DMSO), 7.85–7.95 (m, 2H, C3–H and C5–H), 8.76–
8.83 (m, 2H, C2–H and C6–H), 9.06 (s, 2H –NH2), 12.30 (s, 1H, –NH).
sterile saline 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.
2.3.6. [RuCl2(DMSO)2(fcah)] (6)
This complex was prepared as described in Section 2.3.1 by the
reaction of cis-[RuCl2(DMSO)4] (0.1 g; 0.21 mmol) and the ligand
fcah (furan-2-carboxylic acid hydrazide) (0.026 g; 0.21 mmol).
Yield: 59 mg; 62%. Elemental Anal. Calc. for C9H18N2S2O4Cl2Ru
(454.36): C, 23.79; H, 3.99; N, 6.17; S, 14.11. Found: C, 23.35; H,
2.5.2. Determination of minimum inhibitory concentration (MIC)
A micro dilution broth susceptibility assay was used as recom-
mended [34] by NCCLS for determination of the MIC. All the tests
were performed in Mueller-Hinton Broth (MHB) supplemented
with Tween 80 detergent (final concentration 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). The plates were incubated
under normal atmospheric conditions at 37 °C for 24 h. The MIC of
Kanamycin and Oxytetracycline were individually determined in
parallel experiments in order to control the sensitivity of the test
organisms. Bacterial growth was indicated by the presence of a
white ‘‘pellet” on the well bottom.
4.21; N, 6.32; S, 14.10%. IR (KBr, cmꢁ1):
(NH) 3138(w); d(NH2) 1601(s); (C@O) 1642(s); m
m
(NH2)(asy+sym) 3463(w);
m
m
(S@O)s-bonded
1093(s). UV/Vis (DMSO) k, nm (e
, molꢁ1 cmꢁ1 L): 261 (11980),
322 (6690). 1H NMR (400 MHz, DMSO-[D6], d ppm): 3.22–3.33
0
(group of peaks, 12H, DMSO), 6.73 (t, 1H, Jbb = 3.36 Hz, J
=
a
0b0
1.80 Hz, C4–H of furan ring), 7.48 (d, 1H, J = 3.20 Hz, C3–H of furan
ring), 7.63 (s, 2H, –NH2), 8.02 (s, 1H, C5–H of furan ring), 11.72 (s,
1H, –NH).
2.3.7. [RuCl2(DMSO)2(tcah)] ꢀ H2O (7)
Cis-[RuCl2(DMSO)4] (0.1 g; 0.21 mmol) and the ligand tcah (thi-
ophene-2-carboxylic acid hydrazide) (0.030 g; 0.21 mmol) were ta-
ken in ethanol (30 mL) and heated under reflux for 5–6 h, whereby
the solution turned from pale yellow to a clear red solution. Slow
evaporation of the solvent after reducing the content to half the
volume yielded red crystals of X-ray quality. The crystals were
washed several times with ethanol and dried under fused calcium
chloride. Yield: 65 mg; 66%. Elemental Anal. Calc. for
C9H20N2S3O4Cl2Ru (488.42): C, 22.13; H, 4.13; N, 5.74; S, 19.69.
Found: C, 22.24; H, 4.37; N, 5.48; S, 19.54%. IR (KBr, cmꢁ1):
3. Results and discussion
The discovery of new anticancer drugs that would be more effi-
cient, with less toxicity and less sensitivity to resistance mecha-
nisms remains a fundamental challenge. Recent exploration of
ruthenium based compounds as anticancer drugs gave us buoy-
ancy to undertake a work in search of new Cl–Ru–DMSO type com-
plexes with hydrazide ligands. The straightforward reaction of the
precursor cis-[RuCl2(DMSO)4] and the respective hydrazide ligand
in an equimolar ratio in ethanol, as portrayed in Scheme 1, resulted
in fair yields of complexes of the type [RuCl2(DMSO)2(hydra-
m(NH2)(asy+sym) 3423(w); m(NH) 3102(w); d(NH2) 1573(s); m(C@O)
1645(s);
m(S@O)s-bonded 1089(s). UV/Vis (DMSO) k, nm (e,
molꢁ1 cmꢁ1 L): 262 (12780), 343 (5620). 1H NMR (400 MHz,
DMSO-[D6], d ppm): 3.21–3.35 (group of peaks, 12H, DMSO), 7.23
0
(t, 1H, Jbb = 4.82 Hz, J = 3.00 Hz, C4–H of thiophene ring), 7.75
a
0b0
(s, 2H, –NH2), 7.97–7.99 (m, 2H, C3–H and C5–H of thiophene ring),
11.78 (s, –NH, 1H).

V. Mahalingam et al. / Polyhedron 28 (2009) 1532–1540
1535
R
C
Cl
Ru
Cl
O
Me2OS
Me2OS
O
C
Ethanol
NH
cis - [RuCl2(dmso)4]
+
R
NH2
OH
N
H
Reflux 6-8 h
N
H2
CH3
O
S
R =
,
,
,
,
,
,
H3C
N
OH
Scheme 1. Reaction scheme showing the preparation of the new Ru(II)–DMSO–hydrazide complexes.
zide)] ꢀ nH2O (n = 0 or 1). All of the complexes were stable to air
and light and were soluble in organic solvents such as DMSO and
DMF. The complexes were analytically pure as their microanalyti-
cal data conform to the proposed molecular formula (data given in
Section 2.3). In all the reactions, the hydrazide ligands act as a
bidentate chelating ligand replacing two DMSO ligands from the
precursor.
zide ligands. At this juncture, a comparison of the IR spectra of the
complexes with that of the precursor cis-[RuCl2(DMSO)4)], which
contains one O-bonded DMSO and three S-bonded DMSO ligands,
was helpful in the unambiguous assignment of the substitution
of the two types of DMSO ligands present in the complex by the
incoming ligand. The new complexes displayed a band around
1090–1080 cmꢁ1, characteristic of
m
(SO)(S-bonded), and no peak
was observed at 915 cmꢁ1, characteristic of
m(SO)(O-bonded)
3.1. IR and electronic spectra
exhibited by the precursor cis-[RuCl2(DMSO)4)], which confirms
the presence of only S-bonded DMSO ligands [39,40]. Thus, it is
apparent that the ligands replace one O-bonded DMSO and one
S-bonded DMSO from the precursor. This is in accordance with
the previous observations [18,19,28] that chelating ligands will re-
place first the weakly held O-bonded DMSO and then the S-bonded
DMSO cis to it.
The IR spectra of the free hydrazide ligands and their corre-
sponding Ru(II) complexes were compared in order to gain preli-
minary information on the coordination of the ligands with the
metal. The principal IR frequencies of the ligands and the corre-
sponding complexes are collected in Table 1. The OH region for
the ligands hbah and hnah and their complexes was not clear-cut
due to possible poor resolution of the peak and was less informa-
tive, but from the NMR studies (discussed in Section 3.2), the
non-participation of phenolic OH group in coordination was con-
The electronic spectra of all the complexes exhibited two or
three peaks in the ultraviolet region. The absorption maximum
and molar extinction coefficients are given in experimental section.
The bands below 300 nm are assignable to intraligand
n ? p* transitions [41,42]. The band above 300 nm in the com-
plexes can be assigned to a d (Ru) ?
* (L) MLCT transition, based
p ?
p* or
firmed. The peak due m(NH2)(asy+sym) in the free ligands was found
to be shifted towards lower wavenumbers in the complexes. Thus,
p
p
it is possible to ascertain the coordination of the nitrogen of the
on bands previously observed for similar Ru(II)–DMSO octahedral
NH2 group to ruthenium [35,36]. Similarly, when m(CO) of the li-
complexes [14,43,44].
gands and the complexes were compared, a significant negative
shift of this band was observed in all the complexes, which corrob-
orates the coordination of the ligand through the carbonyl oxygen
3.2. 1H NMR spectra
[37]. The position of the
in the complexes but slightly broadened when compared to the
free ligands. The presence of the (NH) band in the complexes rules
m
(NH) band remained virtually unaltered
The 1H NMR spectra of the complexes recorded in [D6]-DMSO
comprise neat signals characteristic of diamagnetic complexes.
The group of peaks appearing in the new complexes around ꢃd
3.1–3.4 ppm is due to the methyl resonances of S-bonded DMSO li-
gands. No peaks could be observed at d 2.7 ppm, which indicates
the absence of O-bonded DMSO in the complexes. These observa-
tions and assignments are concurrent with earlier works
[18,19,28]. All the complexes show a singlet between d 11.58 and
12.30 ppm due to the NH proton, suggesting a neutral bidentate
nature of the hydrazide ligands. This decision is in tune with IR
observations and the single crystal structure of 7, to be discussed
in the next section. The methyl resonance of the ligands ombh
and pmbh in complexes 1 and 2 appears as a singlet at d 2.40
and 2.37 ppm, respectively. Complexes 3 and 4 exhibit a singlet
for the OH proton at d 10.57 and 10.83 ppm respectively, confirm-
ing the non-participation of the oxygen in coordination. In the
spectra of the free hydrazide ligands, the NH2 protons appear as
a singlet between d 4.23 and 5.63. This signal has been shifted
downfield in the complexes (d 7.50–7.75 for 1, 2, 3, 6 and 7; d
8.70 for 4 and d 9.06 for 5) indicating the coordination of the
NH2 nitrogen to ruthenium. The distinct downfield shift for 4 and
5 can be explained as follows. In 4, the greater downfield shift is
due to the electron withdrawing naphthalene ring, and in 5 it is
due to the attachment of CO–NH–NH2 at the electron deficient po-
sition (para to N atom) of the pyridine ring. In complexes 2, 4 and 7,
m
out the possibility of enolization of the ligands during coordination
and the broadening of this band in the complexes is an indication
of the coordination of the NH2 group [38]. All the above mentioned
data are in support of the neutral bidentate behavior of the hydra-
Table 1
IR data of the hydrazide ligands and their ruthenium complexes.
Compound
m
(NH2)a cmꢁ1
m
(NH) cmꢁ1
m
(C@O) cmꢁ1
ombh
1
pmbh
2
hbah
3
hnah
4
inah
5
fcah
6
tcah
7
3412
3399
3464
3450
3495
3484
3473
3437
3460
3430
3480
3463
3460
3423
3142
3140
3142
3141
3120
3122
3086
3088
3087
3085
3140
3138
3105
3102
1661
1646
1661
1638
1644
1634
1644
1634
1666
1644
1684
1642
1682
1645
a
(asy + sym).

1536
V. Mahalingam et al. / Polyhedron 28 (2009) 1532–1540
the downfield signal(s) at d 7.86(d), 8.58(s) and 7.97–7.99(m)
respectively in the aromatic region could be assigned unequivo-
cally to proton(s) ortho to C@O, in view of the coordination of
the carbonyl oxygen to ruthenium, which will reduce the electron
density around the proton(s) cis to it. The position and pattern of
the signals of the other protons are as expected (see 1H NMR data
given in Section 2.3).
unchanged forming cis,fac-[RuCl2(DMSO)3(L)]. With chelating li-
gands (LL0), displacement of the more weakly held O-bonded DMSO
occurs first followed by displacement of a DMSO ligand cis to it.
Here, the substitution of two DMSO ligands requires more forcing
conditions and can be accompanied by a rearrangement of the
RuCl2 fragment from cis to trans. Even though, the isomer cis,cis,-
cis-[RuCl2- (DMSO)2(LL0)] is thermodynamically more stable than
the trans,cis,cis isomer, in many cases the kinetic product is isolated
as it precipitates before being transformed into the more stable
cis,cis,cis isomer [46].
3.3. X-ray crystallography of 7
The details concerning the data collection and structure refine-
ment of 7 are summarized in Table 2. Relevant geometrical data
and hydrogen bonding geometries are given in Tables 3 and 4,
respectively. The ORTEP diagram of the complex is shown in
Fig. 1. The complex crystallized from the reaction mixture by the
slow evaporation of the solvent at room temperature as red blocks.
The complex contains a solvated water molecule. The unit cell
dimensions show that the crystals belong to the monoclinic system
with the P21/c space group. The ruthenium atom in the complex is
surrounded by six donor atoms in an octahedral fashion and the
octahedral environment is determined by two sulfur atoms of
the DMSO ligands, two Cl atoms and the ON chelate. The distortion
in the complex from ideal octahedral geometry is due to the cus-
tomary small bite angle of the ON chelate and the bending of chlo-
ride ligands towards the chelate, which is evident from the angles
O1–Ru1–N1 = 78.35(5) and Cl1–Ru1–Cl2 = 169.248(17)°. An inter-
esting feature in the complex is the trans arrangement of the two
Cl ligands, that were mutually cis in the precursor as expected
and observed previously [8,19,45]. The precursor contains three
S-bonded DMSO’s and one O-bonded DMSO and the complete
structural formula can be written as cis,fac-[RuCl2(DMSO)3-
(DMSO)]. The O-bonded DMSO is the most labile ligand in
cis,fac-[RuCl2(DMSO)3(DMSO)] and is selectively replaced by stron-
The mean value (2.3957 Å) of the trans Ru–Cl bond lengths Ru1–
Cl1 = 2.4027(4); Ru1–Cl2 = 2.3887(4) Å is comparable with the Ru–
Cl distance of similar Ru(II) complexes [18,19] and with 2.402(2) Å
in trans-[RuCl2(DMSO)4], and it is shorter than the mean values
found in the three polymorphs of cis-[RuCl2(DMSO)4], which is in
agreement with the greater trans influence of the DMSO operative
in the latter [47]. The Ru–S bond lengths (Ru1–S2 = 2.2244(4) and
Ru1–S3 = 2.2234(4) Å) are intermediate between the shortest pub-
lished value of 2.188(3) Å in [Ru(NH3)5(DMSO)] (PF6) [48] and the
longest published values of 2.360(1) Å in [RuX2(DMSO)4] for X = Br
[49] and 2.347(1) Å for X = Cl [50], and are comparable with our
earlier works [18,19]. The decrease in S–O bond lengths of coordi-
nated DMSO ligands compared to that of free DMSO is also ex-
pected based on previous studies [18,19,51,52]. The complex is
stabilized by intermolecular hydrogen bonding networks. Solvated
water, NH2 and NH act as donors and the O atoms of DMSO and Cl
act as acceptors. The hydrogen bonding distances and angles (Table
3) show that these hydrogen bonds are significant in stabilizing the
complex.
Although, crystal structures are not available for the other com-
plexes, based on the analytical and spectroscopic data, a structure
similar to that of 7 is proposed for complexes 1–6, as shown in
Scheme 1. Since all the complexes replace two DMSO ligands from
the precursor, the rearrangement of the RuCl2 fragment from cis to
trans is believed to be operative in all cases.
ger
r- and p-donors (L), leaving the geometry of the complex
Table 2
Crystal data and refinement parameters for 7.
Table 3
Selected geometrical parameters for 7.
Empirical formula
Formula weight
T (K)
Wavelength (Å)
Crystal color, habit
Crystal dimensions (mm)
Crystal system
Lattice type
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
C9H20Cl2N2O4RuS3
488.42
298(2)
Bond distances (Å)
Ru1–Cl1
Ru1–Cl2
Ru1–N1
Ru1–O1
2.40(4)
2.39(4)
2.13(13)
2.11(11)
Ru1–S2
Ru1–S3
S2–O2
S3–O3
2.22(4)
2.22(4)
1.49(14)
1.48(13)
0.71073
red, block
0.51 ꢂ 0.48 ꢂ 0.40
monoclinic
primitive
Bond angles (°)
O1–Ru1–N1
O1–Ru1–S3
N1–Ru1–S3
O1–Ru1–S2
N1–Ru1–S2
S3–Ru1–S2
O1–Ru1–Cl2
N1–Ru1–Cl2
S3–Ru1–Cl2
S2–Ru1–Cl2
O1–Ru1–Cl1
78.35(5)
177.19(4)
99.71(4)
92.17(3)
170.27(4)
89.84(17)
86.36(4)
86.08(4)
95.56(17)
91.18(16)
86.19(4)
N1–Ru1–Cl1
S3–Ru1–Cl1
S2–Ru1–Cl1
Cl2–Ru1–Cl1
C5–S1–C2
84.83(4)
P21/c
91.63(17)
96.84(17)
169.25(17)
91.53(9)
13.6053(8)
13.3017(7)
10.6671(6)
90
109.2580(10)
90
1822.20(18)
4
O2–S2–C6
O2–S2–C7
C6–S2–C7
O3–S3–C9
O3–S3–C8
106.07(12)
105.49(11)
100.91(14)
108.27(10)
105.52(10)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc. (Mg mꢁ3
h Range (°)
)
1.780
2.20–28.28
1.150
Absorption coefficient (mmꢁ1
)
F[000]
984
Reflections collected/unique reflections
Reflections [I > 2 (I)]
Completeness to hmax = 28.28°
Refinement method
17540/4522
4265
Table 4
r
Hydrogen-bonding distances (Å) and angles (°) for 7.
99.8%
full-matrix least squares on F2
D–HꢀꢀꢀA
N1–H1A–Cl2
N1–H1B –O3
N2–H2–O4
O4–H4B–O2
O4–H4A–Cl1
D–H
HꢀꢀꢀA
DꢀꢀꢀA
D–HꢀꢀꢀA
Data/restraints/parameters
4522/5/209
0.0209
0.0560
0.0159
1.036
R [F2 > 2
wR [F2]
Rint
r
(F2)]
0.87(15)
0.86(15)
0.87(15)
0.83(18)
0.81(18)
2.46(16)
2.07(16)
1.83(16)
1.95(19)
2.41(19)
3.28(15)
2.89(19)
2.69(2)
2.76(2)
3.21(19)
158(18)
159(19)
170(2)
167(3)
168(4)
Goodness-of-fit (GOF) on F2
Largest peak and hole (e Åꢁ3
)
0.416 and ꢁ0.514

V. Mahalingam et al. / Polyhedron 28 (2009) 1532–1540
1537
Fig. 1. ORTEP diagram of 7 with anisotropic displacement ellipsoids at 50% probability. Hydrogen atoms (except for solvate) are omitted for clarity.
3.4. Cyclic voltammetric studies
the affinity of the compound under study and also decides whether
to carry out further studies such as competitive binding studies,
melting, viscosity measurements, cyclic voltammetry and other
methods. The absorption titration was carried out using the
respective complex with a final concentration of ꢃ1 ꢂ 10ꢁ4 M
and varying the concentration of herring sperm DNA. The concen-
tration 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 maintain-
ing a constant total volume (3 ml). The titrations were carried out
until no further change was seen in the spectra. The complex-DNA
mixture was allowed to stand for a uniform time of 5 min to get
consistent and comparable results. The intrinsic binding constant
of the complexes with herring sperm DNA was determined from
the equation,
The electrochemical 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 a supporting electrolyte, and
all the potentials were referenced to the Ag/Ag+ electrode at a scan
rate of 0.1 V/s. The data are given in Table 5. A representative vol-
tammogram is shown in Fig. 2. All the complexes were electroac-
tive in the selected potential window of 1.8 V and displayed
Ru(II)/Ru(I) redox waves with E1/2 values falling in the range
ꢁ0.80–ꢁ1.14 V, that compare well with literature values [53,54].
The peak to peak separation values of this redox process ranges
from 120 to 460 mV and the process is best described as a revers-
ible/quasireversible electron transfer. An uncoupled DMSO based
reduction was found in all of the complexes in the range ꢁ0.45
to ꢁ0.62 V, which is in agreement with the observations made
for a similar type of complexes [19]. With the exception of 5, all
the complexes show an uncoupled oxidation peak in the range
0.50–0.93 V, assignable to a Ru(III)/Ru(II) redox process, and the
irreversibility could be accounted for as a result of short-lived re-
duced species [55,56].
½DNAꢄ=½ea
ꢁ
ef ꢄ ¼ ½DNAꢄ=½eb
ꢁ
ef ꢄ þ 1=Kb½eb
ꢁ
ef
ꢄ
through a plot of [DNA]/[ea
concentration of DNA in the base pairs. The apparent absorption
ꢁ
ef] versus [DNA], where [DNA] is the
3.5. UV–Vis spectra of the complexes in the presence of buffered
herring sperm DNA solution
In any DNA-binding assessment, the first and foremost experi-
ment is absorption titration as it gives preliminary ideas about
Table 5
Cyclic voltammetric data of the new ruthenium complexes.
Complex
Ru(II)/Ru(I)a
E1/2/V( Ep/mV)
Ru(III)/Ru(II)
oxidationb
DMSO based
reductionc
D
1
2
3
4
5
6
7
ꢁ1.14(120)
ꢁ1.11(210)
ꢁ0.89(330)
ꢁ0.89(270)
ꢁ0.80(390)
ꢁ0.83(460)
ꢁ0.97(300)
0.84
0.84
0.50
0.50
–
ꢁ0.60
ꢁ0.62
ꢁ0.58
ꢁ0.56
ꢁ0.45
ꢁ0.56
ꢁ0.45
0.78
0.93
a
Reduction couple.
Irreversible, Epa value.
Irreversible, Epc value.
b
c
Fig. 2. Cyclic voltammetric response of 2 in DMSO.

1538
V. Mahalingam et al. / Polyhedron 28 (2009) 1532–1540
coefficients ea
,
ef and eb correspond to Aobsd/[Ru], the extinction
intrinsic binding constant Kb can be obtained from the ratio of the
slope to the Y-intercept [57].
coefficient for the free ruthenium complex and the extinction coef-
ficient for the ruthenium complex in the fully bound form, respec-
The peak displayed by the unbound complexes in Tris–HCl buf-
fer in the range 240–268 nm, due to an intraligand transition, was
considered for monitoring the change in wavelength and absor-
tively. The slope and Y intercept of the linear fit of [DNA]/[ea ef]
ꢁ
versus [DNA] give 1/[ea
ꢁ
ef] and 1/Kb
[
eb ef], respectively. The
ꢁ
Table 6
Change in UV–Vis spectra of the complexes upon gradual addition of herring sperm DNAa.
Complex
kmax/nm (emax ꢂ 103)
r = 0
Intrinsic binding constant Kb (Mꢁ1
)
r = 1
262(5.6610)
242(15.2944)
244(18.9757)
252(11.2751)
253(23.4741)
253(16.1209)
r = 2
r = 3
r = 4
1
2
3
5
6
7
268(1.5288)
240(11.5948)
245(14.6167)
251(5.5591)
251(18.9348)
251(12.0141)
260(13.1766)
248(20.5792)
243(24.5725)
255(17.4807)
254(30.2577)
255(22.5385)
259(18.5967)
249(25.4005)
242(27.9887)
256(23.5158)
255(35.2151)
256(27.8441)
258(30.5234)
253(34.5092)
241(32.7093)
257(32.5669)
257(44.0462)
259(39.2981)
1.958 ꢂ 103
2.504 ꢂ 103
4.985 ꢂ 103
3.894 ꢂ 103
3.085 ꢂ 103
2.258 ꢂ 103
Concentration of the complexes ꢃ10ꢁ4 M in DMSO + buffer; concentration of DNA = 3.7878 ꢂ 10ꢁ3 in Tris–HCl buffer (pH 7.1); r = [DNA]/[complex].
a
Fig. 3. Absorption bands of 6 on titration with herring sperm DNA.
Table 7
Antibacterial activity data of the hydrazide ligands and their ruthenium complexes.
Compound
Escherichia coli
Shigella sonnei
Staphylococcus aureus
Staphylococcus epidermidis
Klebsiella pneumoniae
WDa
MICb
WD
MIC
WD
MIC
WD
MIC
WD
MIC
ombh
1
pmbh
2
hbah
3
hnah
4
inah
5
fcah
6
tcah
7
Oxytetracycline
Kanamycin
13
16
14
17
22
23
20
20
NA
12
15
22
20
23
30
32
82
75
80
71
50
45
42
52
NT
68
82
52
51
42
15
18
20
24
16
26
NA
26
20
NA
15
20
15
23
24
NA
37
48
70
52
84
43
NT
52
50
NT
69
58
83
68
62
NT
8
13
14
14
16
18
21
20
22
14
22
15
21
15
24
32
40
81
78
77
72
65
52
53
54
72
52
75
58
78
58
7
15
19
13
17
15
20
13
22
18
21
12
16
16
21
44
60
88
72
91
68
69
52
63
58
68
52
75
75
74
54
8
12
13
12
14
12
21
17
20
16
18
13
26
20
24
30
35
86
79
86
81
82
55
68
54
78
65
79
42
58
51
8
9
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. A 100
were used.
lg of both compound and antibiotics
b
MIC, minimum inhibitory concentration. Values given as lg/ml for both compounds and antibiotics.

V. Mahalingam et al. / Polyhedron 28 (2009) 1532–1540
1539
bance as increasing amounts of DNA were added, and the results
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plexes 2, 5, 6 and 7 showed bathochromic shifts, with the
difference between the absorption maximum of the unbound and
fully bound complex being 13, 6, 5 and 8 nm, respectively, while
the other two complexes, 1 and 3, showed a considerable hypso-
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3.6. Antibacterial activity
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