Ruthenium(II/III) complexes of redox non-innocent bis(thiosemicarbazone) ligands: Synthesis, X-ray crystal structures, electrochemical, DNA binding and DFT studies

Article abstract of DOI:10.1016/j.poly.2017.04.013

Six tetradentate bis-thiosemicarbazone ligands formed by condensation of pyruvaldehyde with 4-(aryl) thiosmicarbazides and their Ru(II) complexes are reported. The complexes are of formula [Ru(PPh₃)₂(L)], where L is the dianionic form of the bis-thiosemicarbazone ligands. X-ray crystal structures of two of the complexes suggest that the thiosmicarbazone ligands act in a tetradentate fashion occupying the basal plane of the Ru(II) coordination octahedron and each of the two thiosemicarbazone moiety forms five membered chelate ring involving deprotonated thiolato sulfur and the imine nitrogen atoms; the two triphenylphosphine ligands are trans to each other. The complexes undergo Ru(II)/Ru(III) oxidation at a relatively low potential of 0.05–0.12?V versus Ag/AgCl. One of the Ru(II) complex has been chemically oxidized to the corresponding Ru(III) complex [Ru(PPh₃)₂(L)]PF₆ using ferrocenium hexafluorophosphate as an oxidizing agent. E.P.R. spectra of the Ru(III) complex and DFT calculations on the Ru(III) as well as two of the Ru(II) complexes suggest that there is extensive mixing between the ligand and metal valence orbitals. The DNA-binding properties of the Ru(II) complexes were studied by both UV–Vis spectroscopy and fluorescence quenching of ethidium bromide-DNA complex. The complexes showed strong DNA-binding ability with K_app of the order of 10⁶?M^?1.

Full text of DOI:10.1016/j.poly.2017.04.013

Accepted Manuscript
Ruthenium(II/III) complexes of redox non-innocent bis(thiosemicarbazone) li-
gands: Synthesis, X-ray crystal structures, electrochemical, DNA binding and
DFT studies
Bipinbihari Ghosh, Piyali Adak, Subhendu Naskar, Bholanath Pakhira, Partha
Mitra, Shyamal Kumar Chattopadhyay
PII:
S0277-5387(17)30282-6
DOI:
http://dx.doi.org/10.1016/j.poly.2017.04.013
POLY 12591
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
21 December 2016
11 April 2017
Please cite this article as: B. Ghosh, P. Adak, S. Naskar, B. Pakhira, P. Mitra, S.K. Chattopadhyay, Ruthenium(II/
III) complexes of redox non-innocent bis(thiosemicarbazone) ligands: Synthesis, X-ray crystal structures,
electrochemical, DNA binding and DFT studies, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.
2
017.04.013
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Ruthenium(II/III) complexes of redox non-innocent bis(thiosemicarbazone)
ligands: Synthesis, X-ray crystal structures, electrochemical, DNA binding
and DFT studies
a
Bipinbihari Ghosh, Piyali Adak, Subhendu Naskar, Bholanath Pakhira, Partha Mitra,
a
b
a
c
a
Shyamal Kumar Chattopadhyay*
a
Department of Chemistry, Indian Institute of Engineering Science and Technolgy, Shibpur,
Howrah 711103, India
b
Department of Applied Chemistry, Birla Institute of Technology, Mesra, Ranchi 835215,
Jharkhand, India
c
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata 700032, India
_
*
___________________
Corresponding Author.
E mail address: shch20@hotmail.com (S.K. Chattopadhyay).

Abstract
Six teradentate thiosemicarbazone ligands formed by condensation of pyruvaldehyde with 4-
(
[
aryl) thiosmicarbazones and their Ru(II) complexes are reported. The complexes are of formula
Ru(PPh (L)], where L is the dianionic form of the thiosemicarbazone ligands. X-ray crystal
)
3 2
structures of two of the complexes suggest that the thiosmicarbazone ligands act in a tetradentate
fashion occupying the basal plane of the Ru(II) coordination octahedron and each of the
thiosemicarbazone moiety forms five membered chelate ring involving deprotonated thiolato
sulfur and the imine nitrogen atoms; the two triphenylphosphine ligands are trans to each other.
The complexes undergo Ru(II)/Ru(III) oxidation at a relatively low potential of 0.05-0.12 V
versus Ag/AgCl. One of the Ru(II) complex has been chemically oxidized to the corresponding
Ru(III) complex [Ru(PPh
3
)
2
(L)]PF using ferrocenium hexafluorophophate as an oxidizing
6
agent. E.P.R. spectra of the Ru(III) complex and DFT calculations on the Ru(III) as well as two
of the Ru(II) complexes suggest that there is extensive mixing between the ligand and metal
valence orbitals. The DNA-binding properties of the Ru(II) complexes were studied by both Uv-
vis spectroscopy and fluorescence quenching of ethidium bromide-DNA complex. The
6
−1
complexes showed strong DNA-binding ability with Kapp of the order of 10 M .
Keywords: Ruthenium complexes, thiosemicarbazone, pyruvaldehyde, X-ray crystal structure,
DNA-binding
2

1
. Introduction
The synthesis of coordination compounds of ruthenium(II) containing
thiosemicarbazones has attracted widespread attention of chemists as well as biologists, mainly
due to their ability to form complexes with unusual stereochemistry [1], uncommon co-
ordination number [2,3], interesting electronic structures [4,5] along with their wide range of
biological activities, such as antiviral [6,7], antifungal [8], antibacterial [9,10], antitumor [11,12],
anticarcinogenic [13,14], antioxidant [15] activities, insulin mimetic effects [16] and DNA
binding
abilities
[17,18].
Thiosemicarbazides,
thiosemicarbazones
and
related
isothiosemicarbazides have emerged as relatively new class of non-innocent ligands forming
complexes with varied electronic properties, e.g. the formation of redox series [19–25], extensive
mixing between the metal and ligand orbitals [26], stabilization of both reduced and oxidized
ligand radicals [27–32], and high second order hyperpolarizability etc. [33–35]. The various
coordination modes that are found in neutral or anionic forms of thiosemicarbazone ligands have
been tabulated in a recent review [36].
Studies of small molecules, which react at specific sites along a DNA strand as reactive
models for protein–nucleic acid interactions, provide routes towards rational drug design as well
as means to develop sensitive chemical probes for DNA [37,38]. A number of ruthenium
complexes have been used as probes of DNA structure in solution, as agents for mediation of
strand scission of duplex DNA and as chemotherapeutic agents [39–41]. In the burgeoning area
of DNA intercalation studies, ruthenium metal intercalators have been imperative since their
redox and photophysical properties make it possible to utilize multiple techniques to study
intercalation processes [42–46]. The ligands in the metal complexes play a major role in their
binding to DNA. Large planar ligands promote intercalative binding of the metal complexes to
DNA [47,48], whereas the central metal ion plays a crucial role in the cleavage of DNA. Thus
varying substituents in the intercalative ligand can create some interesting differences in the
spatial configuration and the electron density distribution of the complexes, which will result in
differences in the spectral properties and the DNA binding behaviour of the complexes, that will
be helpful to clearly understand the binding mechanism of Ru(II) complexes to DNA [49].
It is also interesting to note that Ru(II) complexes containing a α-diimine fragment,
coordinated to the metal center are of topical interest because of their numerous photophysical,
catalytic and redox activities [50]. Ligands containing α-diimine fragments are redox active and
3

R
can exist in three different electronic states, neutral diimine (L R′
2
), monoanionic diimine anion
R
•−
radical (L R′
2
), and dianionic diimide, as illustrated in Scheme 1. Such type of ligands when
coordinated with electron rich metal ions such as Ru(II), readily accepts an electron to its low-
lying lowest unoccupied molecular orbital (LUMO) thereby undergoing a one-electron reduction
to form a diimine anion radical, resulting in the delocalization of the metal electron density over
the conjugated diimine fragment [51]. Ru(II) complexes of bis(thiosemicarbazone) of
pyruvaldehyde (LH ) that are being reported here also contain a α-diimine fragment, so they too
2
are expected to possess rich redox behavior and strong absorption in the visible spectral region,
and possibly intimate mixing of metal and ligand orbitals.
In this paper we report six Ru(II) and one Ru(III) complexes of bis(thiosemicarbazone)
Schiff base ligands of pyruvaldehyde (LH
2
) formed by condensation of pyruvaldehyde with 4-
(
aryl) thiosemicarbazones (Scheme 2). The X-ray crystal structures of two of the Ru(II)
complexes are also reported here. Spectroscopic and redox behaviors of the structurally
characterized Ru(II/III) complexes are investigated by cyclic voltammetry and UV-vis
spectroscopy in details. DNA binding properties of the Ru(II) complexes studied by using Uv-vis
absorption and luminescence spectra.
R
R
N
R
N
/
/
/
R
R
NH
R
R
+
e
e
+e
-e
-
/
R
/
/
R
NH
R
N
R
N
R
Neutral diimine (L R/
R
2
)
Monoanionic diimine
R
Dianionic diimide
/
anion radical(L R
2
)
Scheme 1
2
2
4
. Experimental Section
.1 Materials and methods
-(aryl) thiosemicarbazides were synthesized following a published procedure [52].
Pyruvaldehyde and triethylamine were purchased from Aldrich and E. Merck respectively.
All other chemicals and solvents were of reagent grade and used as such. Ru(PPh Cl was
3
)
3
2
prepared following a published procedure [53]. Solvents for spectroscopic and cyclic
voltammetry studies were of HPLC grade obtained from Merck or Aldrich. Elemental
4

analyses were performed on a Perkin–Elmer 2400 C, H, N analyzer. Infrared spectra were
recorded as KBr pellets on a JASCO FT-IR-460 spectrophotometer. UV–Vis spectra were
recorded using a JASCO V-530 spectrophotometer. Electrochemical data of 1-2 mM CH CN
3
solutions of the complexes were collected using a CH1106A potentiostat. A three electrode
configuration, consisting of a glassy carbon working electrode and a Pt-wire auxiliary
electrode, Ag/AgCl, KCl(sat) reference electrode and TEAP as supporting electrolyte, were
0
used. The potentials were calibrated against the ferrocene/ferrocenium couple (E = 0.44 V).
1
H NMR spectra were recorded on a Bruker AVANCE DPX 400 MHz spectrometer using,
Si(CH
00 instrument. Fluorescence spectra at room temperature were recorded using PTI made
QuantaMaster40 spectro-fluoremeter.
.2. Synthesis of the Schiff-base ligands (L H
3
)
4
as internal standard. ESI-MS spectra of the samples were recorded on JEOL JMS
6
1
6
2
2
–L H
2
):
1
6
All the six Schiff base ligands (L H
2
–L H
2
) used in this study were prepared following a
similar procedure (Scheme-2). A solution of pyruvaldehyde (2 mmol) in methanol (10 mL) was
added slowly over a period of 1hr, with continuous stirring, to a solution of appropriate 4-(aryl)
thiosemicarbazide (4 mmol) in methanol (30 mL). To this solution 2-3 drops of conc. HCl was
added and the stirring was continued for another 3 hours at room temperature; during this period
a light yellow precipitate of the desired ligand (LH
2
) was obtained in good yield. The residue was
isolated by filtration washed with cold methanol and diethyl ether and dried over fused CaCl
2
.
R
R
R
H C
3
H
H
H
H C
3
H
H
(
1:2), MeOH
Stirr, 4hrs.
H N N
2
N N
N N
N
N
N
O
O
S
S
S
H
H
H
1
R=Me for L H
R=H for L H
R=F for L H
R=Cl for L H
R=Br for L H
R=OMe for L H2
2
2
2
3
2
4
2
5
2
6
Scheme 2. Schematic representation of the synthesis of the bis(thiosemicarbazone) ligands.
1
L H
20.88) %. ESI-MS, m/z: 399 [M+H] ; H NMR (DMSO-d6) (δ ppm): 12.06 (1H, s), 10.71 (1H,
s), 10.14 (1H, s), 9.96 (1H, s), 7.84(1H, s), 7.38 (4H, m), 7.15 (4H, m), 2.31(3H, s), 2.28 (6H, s);
2
: Yield: 70%. Anal. Calc. for C19
H
1
22
N
6
S (Found): C, 57.29 (57.12); H, 5.53 (5.41); N, 21.1
2
+
(
−
1
Selected IR bands (cm ): 3300νN-H, 1590νC=N, 816νC=S
.
5

2
L H
2
: Yield: 73%. Anal. Calc. for C17
H
18
N
6
2
S (Found): C, 55.14 (54.95); H, 4.86 (4.63); N,
+
1
2
2.7(22.59)%. ESI-MS, m/z: 371 [M+H] ; H NMR (DMSO-d6) (δppm): 12.12 (1H, s), 10.78
(
(
1H, s), 10.23 (1H, s), 10.04 (1H, s), 7.85 (1H,s), 7.55 (4H, m), 7.38 (4H, m), 7.22 (2H, m), 2.30
−
1
3H, s); Selected IR bands (cm ): 3299νN-H, 1597νC=N, 751νC=S
.
3
L H
2
: Yield: 75%. Anal. Calc. for C17
H
16
N
6
S
2
2
F (Found): C, 50.25 (50.13); H, 3.94 (3.76); N,
1
2
1
3
0.69 (20.58) %; H NMR (DMSO-d6) (δppm): 12.37 (1H, s), 11.01 (1H, s), 10.44 (1H, s),
−
1
0.24 (1H, s), 8.04 (1H, s), 7.71 (4H, m), 7.39 (4H, m), 2.49 (3H, s); Selected IR bands (cm ):
306νN-H, 1605νC=N, 828νC=S
.
4
L H
2
: Yield: 72%. Anal. Calc. for C17
H
16
N
6
S
2
Cl (Found): C, 46.47 (46.32); H, 3.64 (3.45); N,
2
+
1
1
9.13(19.01) %. ESI-MS, m/z: 462 [M+Na] ; H NMR (DMSO-d6) (δppm): 12.38 (1H, s), 11.07
(
1H, s), 10.48 (1H, s), 10.25 (1H, s), 8.05 (1H, s), 7.78 (4H, m), 7.61 (4H, m), 2.45 (3H, s);
−
1
Selected IR bands (cm ): 3309νN-H, 1604νC=N, 829νC=S
.
5
L H
2
: Yield: 75%. Anal. Calc. for C17
H
16
N
6
S
2
Br (Found): C, 38.64 (38.53); H, 3.03 (2.97); N,
2
1
1
1
1
5.91 (15.79) %; H NMR (DMSO-d6) (δppm): 12.46 (1H, s), 11.13 (1H, s), 10.55 (1H, s),
−
1
0.25 (1H, s), 8.05 (1H, s), 7.73 (8H, m), 2.49 (3H, s); Selected IR bands (cm ): 3283νN-H
,
589νC=N, 821νC=S
.
6
L H
2
: Yield: 72%. Anal. Calc. for C19
H
22
N
6
2
O S
2
(Found): C, 53.02 (52.94); H, 5.12 (5.05); N,
1
1
1
1
2
2
9.53(19.42) %; H NMR (DMSO-d6) (δppm): H NMR (DMSO-d6) (δppm): 12.26 (1H, s),
0.9 (1H, s), 10.35 (1H, s), 10.15 (1H, s), 8.05 (1H, s), 7.58 (4H, m), 7.12 (4H, m), 3.95 (6H, s),
−
1
.49 (6H, s); Selected IR bands (cm ): 3274νN-H, 1596νC=N, 829νC=S
.
.3. Syntheses of the Ru(II) complexes (1-7)
1
2
3
4
The complexes [Ru(PPh
3
)
2
L ]·MeCN (1), [Ru(PPh
3
)
2
L ](2), [Ru(PPh
3
)
2
L ](3), [Ru(PPh
3
) L ](4),
2
5
6
[
Ru(PPh
3
)
2
L ](5), [Ru(PPh
3
)
2
L ](6) were synthesized following a similar procedure. Details
1
method for the [Ru(PPh
3
)
2
L ](1) is given below as a representative example.
5
[
[
Ru(PPh
3
)
2
L ]PF (7) was synthesized following a procedure similar to that reported in literature
6
54], detailed method is also given below.
1
2
.3.1. [Ru(PPh
3
)
2
L ] ·MeCN (1):
1
To a acetonitrile solution (40 mL) of the L H
2
(103.2 mg, 0.25 mmol), Et
3
N (50 mg, 0.5 mmol)
was added, followed by the addition of solid [Ru(PPh
3
)
3
Cl ] (240 mg, 0.25 mmol) and the
2
reaction mixture was heated to reflux with continuous stirring for 12 hours. The solution was
allowed to cool to room temperature, filtered and then evaporated on a rotary evaporator. The
6

residue was washed successively with water, a small amount of CH CN and then diethyl ether;
3
the entire crude product was then purified by column chromatography over silica gel. An intense
yellowish green band was eluted with chloroform. On evaporation of the solvent and
recrystallization from 1:1 chloroform–acetonitrile solution dark yellow crystals were obtained.
Yield: 69 %. Anal. Calc. for C57
H
53
N
7
P
2
RuS (Found): C, 64.39 (64.21); H, 5.02 (4.95); N, 9.22
2
+
−1
−1
(
9.15)%. ESI-MS, m/z: 1022[(M+H)] . UV−vis [MeCN; λmax, nm (ε, M cm )]: 256 (40849),
1
2
8
6
90 (18987), 332 (10728), 430 (23534), 813 (510). H-NMR (DMSO-d
6
): δ (ppm): 8.58 (s, 1H),
.31 (s, 1H), 7.45 (m, 10H), 7.39 (m, 5H), 7.17(m, 19H), 6.97 (m, 2H), 6.88 (m, 3H), 2.54(s,
−
1
H), 2.5(s, 3H). Selected IR bands (cm ): 3392νN-H, 1593νC=N, 733νC-S
.
2
3
4
The other five Ru(II) complexes [Ru(PPh
3
)
2
L ](2), [Ru(PPh
3
)
2
L ](3), [Ru(PPh
3
) L ](4),
2
5
6
[
Ru(PPh
3
)
2
L ](5), [Ru(PPh
3
) L ](6) were synthesized using similar procedure as that of 1, using
2
appropriate ligands.
2
2
.3.2. [Ru(PPh
3 2
) L ](2):
Yield: 65 %. Anal. Calc. for C53
H
46
N
6
S
2
2
P Ru (Found): C, 64.05 (63.91); H, 4.63 (4.57); N, 8.46
+
−1
−1
(
8.37)%. ESI-MS, m/z: 994[(M+H)] . UV−vis [MeCN; λmax, nm (ε, M cm )]: 255 (38551),
1
2
8
2
2
93 (19303), 331 (10294), 426 (25264), 819 (601). H-NMR (DMSO-d
6
): δ (ppm): 8.65 (s, 1H),
.41 (s, 1H), 7.63 (m, 3H), 7.56 (m, 2H), 7.46 (m, 11H), 7.41 (m, 3H), 7.21 (m, 18H), 7.07 (m,
−
1
H), 6.86 (m, 2H), 2.07 (s, 3H). Selected IR bands (cm ): 3403νN-H, 1596νC=N, 743νC-S
.
3
L ](3):
.3.3. [Ru(PPh
3
)
2
Yield: 66 %. Anal. Calc. for C53
H
44
N
6
S
2
F
2
P
2
Ru (Found): C, 61.81 (61.69); H, 4.28 (4.15); N,
+
−1
−1
8
.16 (7.99)%. ESI-MS, m/z: 1030[(M+H)] . UV−vis [MeCN; λmax, nm (ε, M cm )]: 252
1
(
(
35838), 293 (18731), 332 (9860), 423 (22467), 813 (548). H-NMR (DMSO-d
6
): δ (ppm): 8.68
s, 1H), 8.46 (s, 1H), 7.46 (m, 10H), 7.41 (m, 5H), 7.21 (m, 20H), 6.92 (m, 4H), 2.5(s, 3H).
−
1
Selected IR bands (cm ): 3401νN-H, 1594νC=N, 744νC-S
.
4
L ](4):
2
.3.4. [Ru(PPh
3
)
2
Yield: 64 %. Anal. Calc. for C53
H
44
6
N S
2
Cl
2
P
2
Ru (Found): C, 59.89 (59.73); H, 4.14 (3.98); N,
+
−1
−1
7
.91 (7.82)%. ESI-MS, m/z: 1064[(M+2H)] . UV−vis [MeCN; λmax, nm (ε, M cm )]: 262
1
(
(
(
44503), 294 (24358), 330 (12911), 425 (28731), 819 (621). H-NMR (DMSO-d
6
): δ (ppm): 8.78
s, 1H), 8.57 (s, 1H), 7.44 (m, 9H), 7.4 (s, 1H), 7.39 (m, 4H), 7.25 (m, 4H), 7.19 (m, 19H), 7.1
−
1
m, 2H), 2.5(m, 3H). Selected IR bands (cm ): 3403νN-H, 1593νC=N, 742νC-S
.
7

5
2
.3.5. [Ru(PPh
Yield: 69 %. Anal. Calc. for C53
.3 (7.25)%. ESI-MS, m/z: 1152[(M+H)] . UV−vis [MeCN; λmax, nm (ε, M cm )]: 263
3 2
) L ](5):
H
44
N
6
S
2
Br
2
P Ru (Found): C, 55.26 (55.11); H, 3.82 (3.78); N,
2
+
−1
−1
7
1
(
(
(
45725), 294 (25659), 330 (13639), 426 (30178), 817 (749). H-NMR (DMSO-d
6
): δ (ppm): 8.78
s, 1H), 8.57 (s, 1H), 7.47 (m, 10H), 7.4 (m, 4H), 7.36 (s, 1H), 7.28 (m, 3H), 7.25 (m, 4H), 7.19
−
1
m, 14H), 7.13 (m, 3H), 2.5(m, 3H). Selected IR bands (cm ): 3395νN-H, 1594νC=N, 743νC-S
.
6
L ](6):
2
.3.6. [Ru(PPh
3
)
2
Yield: 67 %. Anal. Calc. for C55
H
50
N
6
O
2
S
2
P
2
Ru (Found): C, 62.68 (62.51); H, 4.75 (4.58); N,
+
−1
−1
7
.98 (7.83)%. ESI-MS, m/z: 1054[(M+H)] . UV−vis [MeCN; λmax, nm (ε, M cm )]: 256
1
(
(
(
39044), 296 (19134), 336 (9466), 433 (22705), 791 (616). H-NMR (DMSO-d
6
): δ (ppm): 8.62
s, 1H), 8.48 (s, 1H), 7.43 (m, 8H), 7.39 (m, 6H), 7.25 (s, 1H), 7.2 (m, 18H), 7.09 (m, 3H), 6.68
−
1
m, 3H), 3.69 (m, 6H), 2.5(s, 3H). Selected IR bands (cm ): 3393νN-H, 1596νC=N, 747νC-S
.
5
L ]PF
2
.3.7. [Ru(PPh
3
)
2
6
(7):
A batch of complex 5 (0.57 g, 0.5 mmol) was dissolved in acetonitrile (30 mL), to obtain a
yellowish green solution, in a 100 mL round-bottom flask. Now an acetonitrile solution (5 mL)
of ferrocenium hexafluorophosphate (0.66 g, 2 mmol) was added drop wise to the same solution.
The mixture was stirred at room temperature for 2h to obtain a dark red solution of complex 7.
The reaction mixture was then evaporated on a rotary evaporator. The residue was washed
successively with hexane and diethyl ether; the entire crude product was then purified by column
chromatography over silica gel. An intense red band was eluted with chloroform–acetonitrile
mixture (9:1). Evaporation of the solvent and recrystallization from 1:1 chlorloform–acetonitrile
solution leads to a deep red crystalline product. The crystalline residue was isolated by filtration,
washed with hexane and diethyl ether and dried in air.
Yield: 58 %. Anal. Calc. for C53
H
44
N
6
S
2
F
6
Br
2
P
3
Ru (Found): C, 49.08 (48.81); H, 3.42 (3.34); N,
−
1
−1
6
.48 (6.35)%. UV−vis [MeCN; λmax, nm (ε, M cm )]: 265 (45493), 297 (31330), 465 (25518),
−
1
8
10 (1410), 910 (1484). Selected IR bands (cm ): 3385νN-H, 1588νC=N, 748νC-S, 845νPF6. Molar
−
1
2
−1
conductivity (CH
3
CN): Λ = 127 Ω cm M .
M
2
.4. X-ray crystallography
Single crystal X-ray data were collected for 1 and 5 at 293(2) K, on a Bruker AXS SMART
APEX-II CCD area detector diffractometer using graphite monochromated Mo-Kα radiation.
Data were corrected for absorption effects using the numerical method SADABS [55], solved by
8

2
direct methods and refined by full matrix least squares on F using SHELX-97 [56]. All non-
hydrogen atoms were refined with anisotropic displacement parameters. A summary of the
crystallographic data is presented in Table 1. Important bond distances and bond angles are
collected in Table 2. Crystallographic data has been deposited at CCDC with deposition numbers
1
523852 and 1523853 for complexes 1 and 5 respectively.
Table 1. Crystal data and structure refinement for 1 and 5
Compounds
Formula
Formula weight
Wavelength (Å)
Temperature
Crystal system
Space group
a (Å)
b (Å)
c(Å)
α(°)
β(°)
1
5
C
57
H
53
N
7
P
2
RuS
2
C
53
H
44Br
2
6
N P
2
RuS
2
1063.19
0.71073
293(2)
triclinic
P-1
1151.89
0.71073
150(2)
Orthorhombic
P2
9.278(2)
21.795(5)
24.188(5)
90.00
1
2
1 1
2
9.277(3)
11.903(3)
24.893(7)
93.610(8)
93.704(9)
90.00
γ(°)
112.575(8)
1.400, 2
0.504
90.00
1.564, 4
2.149
−
3
Density (calculated) (g cm ), Z
µ(mm )
−
1
F(000)
1100
2320
Reflections collected: total, unique
Max. and min. Transmission
Observed data [ I>2σ(I)]
R(int)
24407, 7024
0.941, 0.919
4668
42875, 8594
0.733, 0.704
6969
0.1357
0.1163
R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
Data/restraints/parameters
Goodness-of-fit (GOF) on F
Largest difference in peak and hole (e Å ) −1.671, 1.776
0.0924, 0.2309
0.1337, 0.2631
7024/0/626
0.999
0.0573, 0.1362
0.0777, 0.1477
8594/0/596
0.942
2
−
3
−0.849, 1.069
2
.5. Computational Details
In order to understand the electronic structure of the synthesized complex 1, 5 and 7, we have
performed DFT calculations with the Gaussian 03 package (revision B.04) [57]. For the
9

visualization of molecular orbitals Gauss View software was used. Geometry optimizations,
single point calculations, and population analysis of the molecular orbitals were carried out at the
density functional theory (DFT) level with B3LYP hybrid exchange functional [58,59]. For H, C,
N, and O atoms 6-31+G* basis set were used [60]. The LANL2DZ [61] basis set and LANL2DZ
pseudopotentials of Hay and Wadt [62] were used for the Ru atom. The initial geometries of the
complexes were taken from crystal structures and single point energy calculated after
optimization in acetonitrile solvent using CPCM model [62,63]. For complex 7 geometry was
optimized starting with the optimized geometry of complex 5.
2
.6. Spectroscopic studies on DNA interaction
2
.6.1. Electronic absorption spectra
All the experiments involving the interaction of the complexes (1-6) with CT-DNA were
carried out in Tris buffer (5 mM, pH 7.4). A solution of CT-DNA in the buffer gave a ratio of
UV absorbance at 260 and 280 nm of about 1.8–1.9:1, indicating that the DNA was sufficiently
free from protein [64]. The DNA concentration per nucleotide and polynucleotide concentration
were determined by absorption spectroscopy using the molar extinction coefficient (6600
−
1
−1
M cm ) at 260 nm [65]. The intrinsic binding constants K
complexes with DNA have been calculated from the absorption spectral titration data. The
intrinsic binding constant K was determined from the following equation [66],
DNA]/(ε −ε ) = [DNA]/(ε −ε ) + 1/K (ε −ε
Where [DNA] is the concentration of DNA in base pairs, the apparent absorption coefficient ε
and ε correspond to Aobs/[M], the extinction coefficient of the free compound and the
extinction coefficient of the compound when fully bound to DNA, respectively. Plot of
DNA]/(ε −ε ) vs. [DNA] gave a straight line with a slope of 1/(ε −ε ) and an intercept of 1/
(ε −ε ) and K was determined from the ratio of the slope to intercept.
.6.2. Fluorescence spectra
b
for the interaction of these Ru(II)
b
[
a
f
b
f
b
b
f
)
(1)
a
,
ε
f
b
[
a
f
b
f
K
b
b
f
b
2
The emission spectra were recorded using PTI made QuantaMaster40 spectro-
fluoremeter. To study the competitive binding of complexes with ethidium bromide, EB was
excited at 350 nm, in the presence of CT-DNA alone, as well as in the presence of the complexes
0
(
1-6). DNA was pretreated with EB in the ratio [DNA]/[EB] = 1 at 37 C. Fluorescence
quenching experiments with Ru(II) complexes were carried out by consecutive additions of
−
4
aliquots of a 2×10 M stock solution of a complex to the solution containing the mixture
1
0

EB/CT–DNA in Tris-buffered solution. Before the addition of quencher the EB/CT–DNA
solutions were incubated at 4 hours.
2
.7. Electron Paramagnetic Resonance (EPR) spectra
EPR spectrum at X-band of complex 7 was measured in frozen glass MeCN solution at
liquid nitrogen temperature (77 K) using a JEOL JES-FA200 ESR Spectrometer at Sophisticated
Analytical Instrument Facility (SAIF), IIT Mumbai.
3
. Results and discussions
3
.1. Synthesis
1
6
The bis(thiosemicarbazone) ligands L H
2
–L H
2
were prepared by condensation of appropriate
4
-(phenyl) thiosemicarabzide with methyl glyoxal in 2:1 molar ratio in presence of catalytic
amount of HCl. The synthesis of Ru(II) complexes 1-6 were achieved by refluxing
equimolecular amounts of the corresponding bis(thiosemicarbazone) ligand with Ru(PPh Cl
3
)
3
2
and triethylamine in acetonitrile solution for 12 hours, followed by evaporation of the solvent on
a rotary evaporator and purification of the crude product by silica gel column chromatography
using chloroform as the eluent. The compounds were finally recrystallized from chloroform-
acetonitrile (1:1) mixture. Complex 7 was synthesized via chemical oxidation of 5 by excess of
Ferrocenium hexafluorophosphate. Crystalline products of complex 7 were obtained after
purification of the crude product through silica gel column chromatography. The Ru(II)
complexes (1-6) and Ru(III) complex 7 were found to be soluble in most of the organic solvents
such as acetonitrile, dichloromethane, methanol, and N,N-dimethylformamide, but they were less
soluble in water. All the compounds have been characterized by their elemental (C, H, and N)
1
analyses, ESI-MS, IR, UV-vis, HNMR spectroscopic measurements, and the results are given in
the Experimental Section. Single crystals suitable for X-ray crystallography of 1 and 5 were
obtained from the slow evaporation of 1:1 chloroform–acetonitrile mixture.
1
1

R
R
H
3
C
H
R
R
H C
3
H
H
H
N N
N N
S
N N
SH
N N
HS
N
N
N
N
S
H
H
H
H
Dithione form
Dithiol form
Ru(PPh ) Cl
2
3
3
MeCN/Reflux
2 hrs
1
P
R
R
P
H
3
C
H
N N
R
R
H
3
C
H
N N
PF6
[Fe(C
5 5 2 6
H ) ]PF
N N
N N
N
N
Ru
P
MeCN/Stirr, 2 hrs.
N
N
Ru
P
H
S
S
H
H
S
S
H
R=Me for 1
R=H for 2
R=F for 3
R=Cl for 4
R=Br for 5
R=Br for 7
R=OMe for 6
Scheme 3: Schematic representation for the synthesis of the Ru(II/III) complexes.
3
.2. Description of X-ray crystal structure
The ORTEP diagrams and the atom numbering schemes for the X-ray crystal structures of 1 and
are shown in Figures 1 and 2 respectively. In both the complexes Ru(II) resides in a distorted
5
octahedral environment with two tripheylphosphine ligands occupying the trans positions and
the tetradentate bis(thiosemicarbazone) ligand occupying the basal plane. Both the
thiosemicarbazone moieties from five membered chelate rings, in contrast to the four membered
chelate rings that is found in [Ru(PPh
3
)
2
(N^S) ] type of complexes, where N^S is a
2
monodeprotonated thiosemicarbazone of benzaldehyde or thiophen-2-aldehyde [5,67]. All the
metal ligand distances are in the normal range and they follow the order Ru–N < Ru–P < Ru–S.
The C–S bond (C7–S1 and C10–S2) lengths lie in the range 1.73 (1)-1.76(1) Å, which indicates
single bond character of the bond. The thioamide C–N bond (C7–N2 and C10–N5) distances at
1
.31(1)-1.33(1)Å in 1 and 1.30(1)-1.35(1)Å in 5, the N–N bond (N2–N3 and N4–N5) distances
at 1.35(1)-1.36(1) Å in 1 and 1.34-1.40Å in 5, the imine C–N (C8–N3 and C9–N4) distances at
.32-1.33Å and the (CH )C–C(H) bond (C8–C9) distance at 1.41(2)-1.42 (1)Å (compared to C–
CH bond (C8–C53) distance at 1.49-1.50Å) clearly indicate that there is extensive
1
3
3
delocalization in the ligand backbone involving the bis(thiosemicarbazone) moiety.
Fig. 1.
1
2

Fig. 2.
Table 2. Selected bond distances (Å) and angles (º) for 1 and 5.
1
5
Bond distances
Ru1-N3
Ru1-N4
Ru1-P1
Ru1-P2
Ru1-S1
Ru1-S2
2.011(7)
1.994(8)
2.362(3)
2.362(3)
2.408(3)
2.401(3)
Ru1-N3
Ru1-N4
Ru1-P1
Ru1-P2
Ru1-S1
Ru1-S2
1.997(7)
1.993(5)
2.381(2)
2.363(2)
2.406(2)
2.399(2)
Bond angles
N4-Ru1-N3
N4-Ru1-P2
N3-Ru1-P2
N4-Ru1-P1
N3-Ru1-P1
P2-Ru1-P1
N4-Ru1-S2
N3-Ru1-S2
P2-Ru1-S2
P1-Ru1-S2
N4-Ru1-S1
N3-Ru1-S1
P2-Ru1-S1
P1-Ru1-S1
S2-Ru1-S1
79.9(3)
96.6(2)
92.4(2)
95.5(2)
91.4(2)
N4-Ru1-N3
N4-Ru1-P2
N3-Ru1-P2
N4-Ru1-P1
N3-Ru1-P1
P2-Ru1-P1
N4-Ru1-S2
N3-Ru1-S2
P2-Ru1-S2
P1-Ru1-S2
N4-Ru1-S1
N3-Ru1-S1
P2-Ru1-S1
P1-Ru1-S1
S2-Ru1-S1
79.9(2)
94.74(19)
90.7(2)
95.34(19)
99.4(2)
166.85(7)
80.59(18)
160.47(18)
90.53(7)
82.83(7)
160.63(18)
80.75(18)
85.90(7)
87.38(7)
118.78(7)
167.76(10)
80.2(2)
159.9(2)
87.05(9)
93.37(9)
160.5(2)
80.6(2)
85.23(9)
83.90(9)
119.36(9)
1
.3. H NMR Spectra
3
1
1
6
H NMR spectra for the ligands (L H
2
-L H
2
) and the complexes (1−6) were recorded in DMSO-
6
d solutions at room temperature to confirm the molecular structures of the complexes in
solution, and the chemical shift values are given in the Experimental Section while the spectra
are shown in Fig. S1-S6 (ligands) and Fig. S13-S18 (complexes). In the spectra of free ligands
two N-H protons of the thioamide groups are observed at δ12.0-12.5 ppm and 10.5-11.0 ppm.
Disappearance of these two peaks on complexation suggests that the ligand coordinate to the
metal through thiolate form. The two N-H protons adjacent to the phenyl ring are observed at
δ9.96-10.48 ppm in the ligands, which undergoes up field shift in the Ru(II) complexes (1-6)
1
3

where they are observed at 8.6-8.8 ppm and 8.3-8.6 ppm. Aldimine proton of the ligands
observed at around 7.84-8.05 ppm but this proton could not be separately identified from the
aromatic protons in the complexes. The aldimine proton, along with the aromatic protons (total
3
9 protons for all the complexes except 2 which has 41 protons in this region), were detected at
.7–7.5 ppm region. Methyl protons (4-(CH )Ph) of complex 1 and methoxy protons (4-
)Ph) of complex 6, appeared as a singlets at δ2.54 ppm and δ3.69 ppm respectively. In all
the Ru(II) complexes (1-6) methyl protons of pyruvaldehyde group were observed at δ 2.5 ppm.
.4. IR Spectra
6
3
(
OCH
3
3
Assignments of all IR spectral bands to specific vibrations have not been attempted. However,
from the spectral data the following conclusions have been drawn. In the spectra of free ligands,
−
1
a strong band is observed at around 1590–1600 cm which is assigned to ν(C=N) stretching
−
1
vibration [68]. In the spectra of complexes, this band undergoes only small shift by 3-5 cm ;
such small shift of the ν(C=N) band on coordination was reported earlier [69,1]. A sharp band
−
1
which appeared at around 3270–3350 cm due to N–H stretching vibration in the
bis(thiosemicarbazone) ligands disappeared on complexation which acts as evidence for the
enethiolization of the ligand during complexatation. The ν(C=S) of the free ligands observed at
−
1
around 800-830 cm , undergoes a red shift in the complexes, confirming the coordination via
the thiolate sulfur [70,71]. The Ph-NH- stretching frequency of the complexes appeared at
-1
−1
around 3400 cm . In the Ru(III) complex 7 one additional peak is found at 845 cm , which is
−
due to the presence of PF
6
as a counter ion.
3
.5. Conductivity study
−
4
To justify the composition of complex 7 in solution, a molar conductivity study of the 10 M
acetonitrile solution has been performed. The conductance value at 298 K for complex 7 is 127
−
1
2
−1
Ω cm M . This value suggests that complex 7 behaves as a 1:1 electrolyte, as expected if one
−
PF
6
ion present outside the coordination sphere.
3
.6. Electronic Spectra
Electronic spectra of the complexes were recorded in acetonitrile solution and data are displayed
in Table 3. Each Ru(II) complex shows intense absorption in the visible region at 420-440 nm
and a shoulder at around 330 nm that are probably due to the allowed MLCT transitions from
filled dπ-orbitals of Ru(II) to the π*-orbitals of the diimine fragment of the ligand. A weak
transition at 790-820 nm is probably due to formally forbidden transition between singlet ground
1
4

3
state and the lowest MLCT state. A very strong band at about 250 nm and a strong shoulder at
about 290 nm are probably due to intra ligand transitions. In the Ru(III) complex 7 there is a
strong LMCT transition at 465 nm; two more transitions at 810 and 910 nm with intensity much
weaker than the 465 nm transition are also tentatively assigned as LMCT transitions as their
−
1
−1
molar extinction coefficients are around 1400-1500 M cm .
Fig. 3.
Table 3. Electronic spectral data for the complexes
−
1
−1
Complex
λ/nm (ε/M cm )
1
2
3
4
5
6
7
256 (40849), 290 (sh,18987), 332 (sh,10728), 430 (23534), 813 (510)
255 (38551), 293 (sh,19303), 331 (sh,10294), 426 (25264), 819 (601)
252 (35838), 293 (sh,18731), 332 (sh,9860), 423 (22467), 813 (548)
262 (44503), 294 (sh,24358), 330 (sh,12911), 425 (28731), 819 (621)
263 (45725), 294 (sh,25659), 330 (sh,13639), 426 (30178), 817 (749)
256 (39044), 296 (sh,19134), 336 (sh,9466), 433 (22705), 791 (616)
265 (45493), 297 (sh, 31330), 465 (25518), 810 (1410), 910 (1484)
3
.7. Electrochemical studies
Electrochemical studies were performed to examine the redox properties of Ru(II) complexes (1-
6
) and Ru(III) complex 7 in acetonitrile solutions at 298K (Fig. 4). All the complexes show an
0
oxidative response on the anodic side of Ag/AgCl reference electrode and the E value lie
between 0.05 and 0.15 V. The oxidative couple is quasi-reversible with peak to peak separation
(
∆E
tentatively assign this oxidative couple to Ru(II)/Ru(III) oxidation. Another oxidative couple is
observed at 0.72-0.85 V, with peak to peak separation (∆E ) of 70-90 mV; in differential pulse
p
) of 60-70 mV. The anodic peak current is almost equal to cathodic peak current. We
p
voltammetry experiments this couple shows sign of splitting into two waves and we tentatively
assign this due to overlapping Ru(III)/Ru(IV) and ligand oxidation. In the Ru(III) complex 7
these two potentials are slightly shifted towards the lower potential than corresponding Ru(II)
complex 5. On the reductive side two overlapping irreversible/quasi-reversible responses are
probably due to the addition of electron to the diimine fragment of bis(thiosemicarbazone)
ligand.
1
5

A bulk electrolysis was performed at 295 K of an acetonitrile solution of complex 5 at
.35 V (vs Ag/AgCl), using a glassy carbon working electrode and a platinum counter electrode.
0
The one-electron nature of this oxidation has been established by coulometry.
Fig. 4.
[a]
Table 4. Cyclic voltammetry data of complexes 1-7 in MeCN solutions.
0
Complex
Oxidative side
E /V (∆E /mV) values
Reductive side E /V
0
[b]
E
pc
E
pa
E
pc
p
1
2
3
4
5
6
7
0.05(68), 0.74(86)
0.12(68), 0.81(73)
0.12(59), 0.79(76)
0.13(67), 0.83(83)
0.15(69), 0.85(81)
0.06(63), 0.72(81)
0.09(64), 0.77(99)
-1.67
-1.63
-1.59
-1.61
-1.57
-1.68
-1.64
-1.58
-1.56
-1.54
-1.56
-1.53
-1.62
-1.54
-1.76
-1.73
-1.71
-1.73
-1.70
-1.80
[
a] Scan rate is 100 mV/s. [b] Potentials are referred against Ag/AgCl, KCl electrode.
3
.8. Electrochemical and chemical oxidation
The electronic spectra of both 5 and electrochemically generated monocation [5][PF
6
], recorded
°
at 25 C, are shown in Fig. 5. The UV-vis spectrum monitored during the spectroelectrochemical
+
oxidation shows a clean conversion of 5 to 5 . In acetonitrile the parent complex 5 displays three
absorbance maxima, 295 nm, 425 nm and a broad band 800-950 nm. Upon oxidation of 5,
intensity of MLCT band at 425 nm gradually decreases with appearance of a new and more
intense band at ~40 nm longer wavelength. The intensity of the broad band at ~900 nm also
increases and three newly generated bands are observed at around 1150 nm, 1410 nm and 1905
nm. The generation of ruthenium(III) species by the chemical oxidation of the ruthenium(II)
complex with cerium(IV) in acetonitrile has also been studied spectrophotometrically (Fig. 6).
The spectral nature is almost similar to that observed during spectroelectrochemical oxidation.
The spectral changes observed during the redox titration of 5 shows that the intensity of the
MLCT band at 425 nm diminishes gradually along with the growth of a new band around 460
nm, successive absorption curves pass through an isosbestic point at 450 nm. Also, a broad band
is observed at 900-950 nm.
Fig. 5.
1
6

Fig. 6.
3
.9. E.P.R spectra
The e.p.r. spectrum of the Ru(III) complex 7 recorded at 77 K in MeCN solution shows rhombic
pattern (Fig. 7 and Table 5), characteristic of Ru(III). However, the ꢀg (ꢀg = gmax − gmin) value is
only 0.10, meaning weak rhombic distortion. It has been observed for genuine Ru(III)
3
+
3+
complexes like [Ru(bpy)
3
] (g
1
= 2.64, g
2
= 2.64 and g
3
= 1.1), [Ru(en)
3
] (g
1
= 2.64, g
2
= 2.64
= 2.16
3
−
and g
and g
3
3
= 0.33), [Ru(ox)
3
] (g
1
= 2.30, g
2
= 2.02 and g
3
= 1.76), [Ru(acac)
3
] (g
1
= 2.45, g
2
= 1.45), the spread of g values as measured by ꢀg is much larger. However, with thiolate
and Ru(dtp)
or phenolate coordination the ꢀg value decreases appreciably (e.g. for Ru(sacsac)
3
3
the ꢀg values are 0.11 and 0.10 respectively; sacsac = dithioacetyl acetone, dtp =
dithiophosphate) [72,73] because of the appreciable covalence in such complexes which leads to
unpaired electron being considerably delocalized onto the ligands. Thus the e.p.r. g values of
complex 7 suggest that the unpaired electron must be appreciably delocalized over the
thiosemicarbazone ligand framework. This is also supported by DFT calculations which show
that the SOMO has only 12.32% contribution from the Ru-atom (Fig. 8). DFT calculations of
complexes 1 and 5 also indicate that HOMO to HOMO-3 contain substantial amount of ligand
contributions (Figs. S28 and S29).
Fig. 7.
Fig. 8.
Table 5. The g values of complex 7
Complex
7
g
1
2.10
2.07
g
g
g
2
2.00
3
2.06
av
4
.1. DNA binding study
.1.1. Absorption spectral studies
4
An absorption titration experiment was carried out to study the DNA binding property of the new
Ru(II) bis(thiosemicarbazone) complexes (1-6) in tris-buffer solution (pH 7.4) with CT-DNA.
1
7

−
5
The absorption spectra of complexes (1-6) at constant concentration (2×10 M) in the presence
of different concentrations of CT-DNA are given in Fig. 9. The binding behaviour of all the
Ru(II) complexes with CT-DNA is quite similar. The absorption spectrum of the complexes (1-
6
) mainly consist of two resolved bands, intra ligand (IL) band at 250-300 nm and metal to
ligand charge transfer (MLCT) transitions at 420-440 nm. As the DNA concentration is
increased, the hyperchromism with a small red shift of ~5-10 nm was observed in the IL-band,
while the MLCT band at 420-430 nm showed hypochromism with negligible shifts in the
absorption maxima. This led to a spectral change with the isosbestic point at 290 nm for all the
complexes (1-6). In order to compare the DNA-binding affinities of the complexes
quantitatively, their intrinsic binding constants (K
changes in the absorbance in the IL band at the corresponding λmax(260 nm) with increasing
concentration of DNA. Intrinsic binding constant (K ) value is given by the ratio of slope to the
intercept in plots of [DNA]/(ε − ε ) versus [DNA] (insets in Fig. 8) according to the Eq.(1). The
DNA binding constant K values of the complexes are found to vary in the narrow range
.29×10 −6.74×10 M (Table 6). From the binding constant values, it is inferred that all the
b
) to DNA are obtained from monitoring the
b
a
f
b
5
5
−1
3
Ru(II) bis(thiosemicarbazone) complexes(1-6) exhibit almost similar binding efficiency with
CT-DNA.
Fig. 9.
4
.1.2. Fluorescence spectral study
In order to investigate the interaction mode between the complexes and CT-DNA, a competitive
binding experiment of the complexes with ethidium bromide (EB) was carried out using
fluorescence spcetroscopy. The fluorescence spectral technique is a potent method to study the
metal interaction with DNA. EB is a planar cationic dye that is widely used as a sensitive
fluorescence probe for native DNA. EB shows intense fluorescence in the presence of DNA due
to its strong intercalation between the adjacent DNA base pairs. The fluorescence intensity of EB
bound to CT-DNA decreases if the quencher intercalates into DNA, as it leads to a decrease in
the binding sites of DNA available for EB. The method therefore provides indirect evidence for
an intercalative binding mode. The extent of fluorescence quenching may also be used to
determine the extent of binding between the quencher and DNA. The emission spectra of EB
bound to CT-DNA in the absence and presence of Ru(II) complexes(1-6) are shown in Fig. 10.
1
8

In the emission spectra, the binding behaviour of complexes 1–6 is quite similar. On
addition of the complex to DNA, pretreated with EB, the spectrum shows no significant changes
in shape or wavelength. It is obvious that there is a reduction in the fluorescence intensity at
λ
em~600 nm as the concentration of the Ru(II) complexes increases. This clearly indicates that
some EB molecules are displaced from their DNA binding sites and replaced by the complexes.
The reduction of the emission intensity is also a measure of the DNA binding propensity of the
complexes and the stacking interaction (intercalation) between the adjacent DNA base pairs [74].
0
The quenching constant K was determined from the classical Stern–Volmer equation [75]. I /I =
0
1
+ KSV [Q] where I and I are the fluorescence intensities in the absence and presence of
complexes, respectively. KSV is the linear Stern–Volmer quenching constant, [Q] is the
concentration of the Ru(II) (quencher) complex [75]. The fluorescence quenching curve of EB
bound to CT-DNA by Ru(II) complexes illustrates that the quenching of EB bound to DNA by
complexes is in good agreement with the linear curve of Stern–Volmer equation. From the slope
0
of plot of I /I versus [complex] (insets in Fig. 10), the KSV value for the Ru(II) complexes (1-6)
4
4
was found to vary very little, lying in the range 7·65×10 −9.62×10 (Table 6), indicating a strong
affinity of the Ru(II) complexes to CT–DNA.
The apparent binding constant (Kapp) value is determined from the equation
Kapp × [Complex]50 = KEB × [EB]
where Kapp is the apparent binding constant of the complex studied, [complex] is the
concentration of the complex at 50% quenching of DNA-bound ethidium bromide emission
7
−1
intensity, KEB is the binding constant of ethidium bromide (KEB= 1.0 × 10 M ), and [EB] is the
concentration of ethidium bromide (5µM) [74]. The Kapp value for the Ru(II) complexes(1-6) was
6
6
−1
found to be very similar lying between the shallow range of 8.00×10 −9.97×10 M (Table 6),
indicating a strong affinity of the Ru(II) complexes to CT–DNA.
Fig. 10.
Table 6. Binding constants for the interaction of the Ru(II) complexes with CT-DNA
Complex
From Absorption Spectra From Emission Spectra From Emission Spectra
−
1
−1
−1
K
b
(M )
K
SV (M )
K
app (M )
5
4
6
1
2
3
4.37×10
6.74×10
5.08×10
7.65×10
8.14×10
7.83×10
8.00×10
8.43×10
8.06×10
5
5
4
4
6
6
1
9

5
5
5
4
4
4
6
6
6
4
5
6
3.37×10
5.21×10
3.29×10
9.16×10
9.62×10
8.15×10
8.93×10
9.97×10
8.57×10
In Table S1 (supplementary data) we present a collection of DNA binding data (K
b
) of some of
the Ru(II) complexes of thiosemicarbazones, thiosemicarbazides and semicarbazone ligands
containing two triphenylphosphine as ancillary ligands. It is evident from that table that most of
4
5
−1
the reported complexes have binding constant values lying in the 10 to 10 M range, with the
6
−1
exception of one complex, which has binding constant in the range of 10 M . Thus our
complexes have binding constant values which are comparable to the best values reported in the
literature.
5
. Conclusions
In this work we have shown that Ru(II) complexes of planar tetradentate bis(thiosemicarbazone)
ligands show interesting electronic structures and redox behaviour. In these complexes both the
thiosemicarbazone moieties coordinate forming five membered chelate rings compared to four
membered chelate rings found in closely similar mono(thiosemicarbazone) complexes
[
Ru(PPh
3
)
2
(N^S) ], where N^S is a thiosemicarbazone of benzaldehyde or thiophen-2-aldehyde.
2
The X-ray crystal structures of two of the complexes show there is extensive delocalization over
the planar tetradentate ligand backbone. DFT calculations supports there is extensive mixing of
the metal and ligand valence orbitals and most of the frontier orbitals of the complexes have
minority metal contribution. The complexes undergo easy Ru(II)/Ru(III) and Ru(III)/Ru(IV)
oxidation and lignad based reductions. The formally Ru(III) complex has been isolated in one
case by chemical oxidation and the e.p.r. spectrum of the complex suggests that the unpaired
electron is substantially delocalized onto the ligand backbone. The complexes also show
appreciable DNA-binding efficacy and the fluorescence quenching experiments of DNA-EB
complex with the Ru(II) complexes suggest that the binding mode is probably by intercalation.
Acknowledgements
B.G. thanks Indian Institute of Engineering Science and Technology (IIEST) for an Institute
fellowship. PA thanks UGC for Senior Research Fellowship. SKC acknowledges the All India
Council for Technical Education (AICTE) for funding the purchase of a CH1106A potentiostat.
2
0

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2
6

1
Fig. 1. ORTEP diagram (at 30% probability level) of [Ru(PPh ) L ]·MeCN (1). The solvent
3
2
molecule is omitted for clarity of the picture.

5
Fig. 2. ORTEP diagram (at 30% probability level) of [Ru(PPh ) L ](5).
3
2

4
4
4
4
4
5
4
3
2
1
x10
x10
x10
x10
x10
3
2
1
x10
x10
1
2
3
4
5
6
7
3
0
6
00
800
1000
Wavelength(nm)
0
4
00
600
800
1000
Wavelength(nm)
−
5
Fig. 3. Electronic spectra of equimolar (2×10 M) solutions of 1-7, recorded in acetonitrile
solution at room temperature (298 K).