Cyclometalated rhodium(III) complexes bearing dithiocarbamate derivative: Synthesis, characterization, interaction with DNA and biological study

Article abstract of DOI:10.1016/j.poly.2013.11.028

Reaction of three different dithiocarbamates (4-MePipzcdtH, L¹H; MorphcdtH, L²H and 4-BzPipercdtH, L³H) with [Rh(2-C ₆H₄py)₂Cl]₂.1/4CH₂Cl ₂ afforded a class

Full text of DOI:10.1016/j.poly.2013.11.028

Accepted Manuscript
Cyclometalated rhodium(III) complexes bearing dithiocarbamate derivative:
synthesis, characterization, interaction with DNA and biological study
Titas Mukherjee, Buddhadeb Sen, Animesh Patra, Snehasis Banerjee, Geeta
Hundal, Pabitra Chattopadhyay
PII:
S0277-5387(13)00787-0
DOI:
http://dx.doi.org/10.1016/j.poly.2013.11.028
POLY 10441
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
31 August 2013
22 November 2013
Please cite this article as: T. Mukherjee, B. Sen, A. Patra, S. Banerjee, G. Hundal, P. Chattopadhyay, Cyclometalated
rhodium(III) complexes bearing dithiocarbamate derivative: synthesis, characterization, interaction with DNA and
biological study, Polyhedron (2013), doi: http://dx.doi.org/10.1016/j.poly.2013.11.028
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Cyclometalated rhodium(III) complexes bearing dithiocarbamate derivative: synthesis,
characterization, interaction with DNA and biological study
a
Titas Mukherjee , Buddhadeb Sen, Animesh Patra, Snehasis Banerjee , Geeta Hundal ,
a
a
b
c
a
Pabitra Chattopadhyay *
a
Department of Chemistry, Burdwan University, Golapbag, Burdwan-713104, India
b
Govt. College Of Engineering and Leather Technology, Salt Lake Sector-III, Kolkata 98
c
Department of Chemistry, Guru Nanak Dev University, Amritsar-143005, India
1
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Abstract
1
2
Reaction of three different dithiocarbamates (4-MePipzcdtH, L H; MorphcdtH, L H and 4-
3
BzPipercdtH, L H) with [Rh(2-C
6
H
4
py)
2
Cl]
2
.1/4CH
2
Cl
2
afforded a class of rhodium(III)
III
complexes of the type [Rh (2-C
6
H
4
py)
2
(L)]. The complexes were fully characterized by several
1
spectroscopic tools along with a detailed structural characterization of [Rh(2-C
6
H py)2(L )] (1)
4
by single crystal X-ray diffraction. Structural analysis of 1 showed a distorted octahedron in
which both of the 2-phenylpyridyl nitrogens are in axial positions, trans to one another and the
sulfur atoms are opposite to the phenyl rings. Electrochemical analysis by cyclic voltammetry
reveals irreversible redox behavior of the rhodium centre in 1, 2 and 3. Their DNA binding
ability have been also evaluated from the absorption spectral study as well as fluorescence
quenching properties, suggesting the intercalative interaction of the complexes with CT-DNA
due to the stacking between the aromatic chromophore and the base pairs of DNA. Antibacterial
activity of complexes has also been studied by agar disc diffusion method against some species
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of pathogenic bacteria (Escherichia coli, Vibrio cholerae, Streptococcus pneumonia and Bacillus
cereus).
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Keywords : Cyclometalated rhodium(III) complex; dithiocarbamates, crystal structure, DFT,
DNA binding, antimicrobial study
*Corresponding author: E-mail: pabitracc@yahoo.com
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1. Introduction
Rhodium(III) complexes are the subject of current research activity in the interaction of
complexes with biomolecules as well as the rhodium catalyst is able to fulfill its role over the
other conventional catalysts due to the capability of the metal to change its coordination number
from six to four and also the oxidation state from Rh(III) to Rh(I). This change appears as a
chemically irreversible two-electron reduction involving ligand loss from octahedral Rh(III) to
form square planar Rh(I) complexes. Loss of the ligand depends on the nature of the ligands
present in mixed ligand systems which allow one to tune the electrochemical potential and affect
the reactivity of the rhodium metal center [1]. The discovery of the catalytic properties of
Wilkinson’s catalyst, viz. [RhCl(PPh ) ] naturally brought about a widespread search for other
3
3
rhodium phosphines with catalytic activity [2,3]. Further, octahedral diimine rhodium(III)
complexes are of interest as they have been used in the process of photochemical reduction of
H
2
O to H
2
[4].
-
The dithiocarbamates (R
2
NCS ) have been considered as versatile ligands for bonding to
2
transition as well as main group metal ions [5-17], and got an enormous attention because of their
importance in several fields such as the chemical industry, biology and biochemistry [18-21]. The
nature of the heterocycle attached to dithiocarbamate fragment appears crucial so as to vary the
electron properties of these ligands and thus to control the potential pharmacological attributes as
well as the catalytic efficiency of the metal complexes [22]. Coordination complexes of
platinoids with dithiocarbamato ligands are known in the literature [9-13] and also palladium(II)
and platinum(II) complexes of dithiocarbamato groups together with mono- or diamine ligands
[14-17]. But to the best of our knowledge so far, report of rhodium(III) cyclometalated
complexes bearing dithiocarbamate derivative is still unexplored.
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The binding interactions of these complexes with DNA have also been studied
systematically to explore the biological activity of the new complexes as we know the fact of the
activity of cisplatin by coordination to DNA [23,24]. And from thorough pharmacological
mechanistic studies it is also known that small molecules interact with DNA via electrostatic
forces, groove binding, or intercalation [25], and their effectiveness depends on the mode and
affinity of the binding [26]. Intercalation is one of the most important among these interactions.
Therefore, the search for drugs that show intercalative binding to DNA has been an active
research area for the past several decades [27]. Moreover, although rhodium metal is not bio-
essential element but its compounds have useful applications in the biological field [28-31] and
have significant pharmacological effects through the interaction with DNA [32].
Encouraged by the advantages of the facts stated above, we isolated a new series of
cyclometalated rhodium(III) complexes bearing dithiocarbamate derivatives by a high yield
synthetic pathway under mild reaction conditions. The present report deals with the chemistry of
III
these [Rh (2-C
6
H
4
py) (SS)] complexes, where SS = 4-MePipzcdtH, MorphcdtH, 4-BzPipercdtH
2
with special reference to their formation, structural characterization and electrochemical
behavior. The binding interactions of these complexes with calf thymus-DNA (CT-DNA) have
also been studied systematically to explore the mode of biological activity as part of our
continuing interest [12,33]. In addition, antibacterial activity of the complexes (1, 2 and 3)
against some pathogenic bacteria, namely Escherichia coli, Vibrio cholerae, Streptococcus
pneumonia and Bacillus cereus has also been studied by agar disc diffusion method.
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2. Experimental
2.1. Materials and physical measurements
Rhodium trichloride, 2-phenylpyridine (2-C
6
H
4
py), morpholine and 4-benzylpiperidine
py) Cl] .1/4CH Cl
(Aldrich) were purchased and used without further purification. [Rh(2-C
6
H
4
2
2
2
2
was prepared following the reported procedure [34]. 4-Methylpiperazine (Aldrich) has been dried
by refluxing over NaOH beads, the colorless liquid obtained after distillation and stored over
NaOH beads. Solvents used for spectroscopic studies and for synthesis were purified and dried
by standard procedures before used. The organic moieties, 4-methylpiperazine-l-carbodithioic
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
1
2
acid (4-MePipzcdtH, L H), morpholine-4-carbodithioic acid (MorphcdtH, L H) and 4-benzyl-
3
piperidine-l-carbodithioic acid (4-BzPipercdtH, L H) were obtained as solid products following
the reported procedure [12].
The Fourier transform infrared spectra of the ligand and the complexes were recorded on a
-1
Perkin-Elmer FTIR model RX1 spectrometer using KBr pellet in the range 4000 - 300 cm . The
solution phase electronic spectra were recorded on an JASCO UV–Vis/NIRspectrophotometer
model V-570 in the range 200-1100 nm. Elemental analyses were carried out on a Perkin-Elmer
2400 series-II CHNS Analyzer. The fluorescence spectra complex bound to DNA were obtained
at an excitation wavelength of 522 nm in the Fluorimeter (Hitachi-2000). Mass spectra of 1, 2
TM
and 3 were recorded on Micromass Q-Tof micro . NMR spectrum of the ligands and complexes
has been recorded on Bruker DPX-300. Solution conductivity and redox potentials were
measured using Systronics Conductivity Meter 304 model and CHI620D potentiometer in DMF
-3
-1
at complex concentration of ~10 mol L . Viscosity experiments were conducted on an
o
Ostwald’s viscometer, immersed in a thermostated water-bath maintained to 25 C.
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2.2. Syntheses of [Rh(2-C
6
H
4
py)
2
(L )] (1) [Rh(2-C
6
H
4
py)
2
(L )] (2) and [Rh(2-C
6
H
4
py)
2
(L )] (3)
The complexes have been synthesized following a common procedure stated as below. The
1
2
ligand, L H (89.4 mg, 0.508mmol) for complex 1, L H (83.8 mg, 0.508 mmol) for complex 2 or
3
L H (127.0 mg, 0.508 mmol) for complex 3 was dissolved in DMSO-MeCN (v/v 1:1) solvent
mixture and to this ligand solution dropwise MeCN solution of [Rh(2-C
6
H
4
py)
2
Cl]
2
.1/4CH
2
Cl
2
(468mg, 0.5mmol) was added. The mixture was then refluxed in nitrogen atmosphere for 12 h
and the color changed from faded yellow to orange. On slow evaporation of this solution orange
coloured microcrystalline solid appeared, which was purified by extracting the orange band in
column chromatography using MeCN as an eluant. Needle shaped crystals of [Rh(2-
1
C
6
H
4
py)
2
(L )] suitable for X-ray diffraction study were grown from this solution on evaporation
at ambient temperature.
1
Rh(2-C
6
H
4
py)
2
(L )] (1): [C28
H
27
N
4
RhS ]; Yield: 85 %. Anal. Calc.: C, 57.33; H, 4.64; N,
2
-1
9.55; Anal. Found: C, 57.21; H, 4.58; N, 9.32; IR (cm ): νC=N, 1495; νa(SCS), 1005, 996; ESI-MS
+
+
(m/z): [M+Na ], 609.576(25% abundance); [M+H ] 587.588 (69 % abundance). Conductivity
-1
-1
1
(Λo, M cm ) in DMF: 130 ; H NMR (δ, ppm in dmso-d
of S C-N(CH ); 3.20 (m, 4H of -N(CH ); protons of 2-C
2H), C (7.43, m, 2H), C (7.68, d, 2H).
; Yield: 80 %. Anal. Calc.: C, 56.54; H, 4.22; N,
6
): 4.36 (m, 3H of N-CH
3
); 3.82 (m, 4H
2
2
)
2
2
)
2
6
H
4
py: C (8.84, d, 2H), C
1
2
(7.27, m,
3
(8.01, d, 2H), C
4
(7.69, d, 2H), C
5
6
2
[Rh(2-C
6
H
4
py)
2
(L )] (2): C27
H
24
N
3
ORhS
2
-
1
7.33. Anal. Found: C, 56.52; H, 4.06 N, 7.29; IR (cm ): νC=N, 1485; νa(SCS), 1030, 1014; ESI-MS
+
+
(m/z): [M+Na ], 596.528 (20 % abundance); [M+H ], 574.538 (64 % abundance). Conductivity
-1
-1
1
(Λo, M cm ) in DMF: 127. H NMR (δ, ppm in dmso-d
(m, 4H of O(CH ); protons of 2-C py: C (8.84, d, 2H), C
(7.69, d, 2H), C (7.43, m, 2H), C
6
): 3.86 (m, 4H of S
2
C-N(CH
2
)
2
); 3.66
2
)
2
6
H
4
1
2
(7.27, m, 2H), C
3
(8.01, d, 2H),
C
4
5
6
(7.68, d, 2H).
6

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[Rh(2-C
6
H
4
py)
2
(L )] (3): C35
H
32
N
3
RhS ; Yield: 73 %. Anal. Calc.: C, 63.53; H, 4.87; N,
2
-1
6.35. Anal. Found: C, 63.44; H, 4.79; N, 6.02; IR (cm ): νC=N, 1505; νa(SCS),1035, 1020; ESI-MS
+
+
(m/z): [M+Na ], 684.682 (18 % abundance); [M+H ], 662.688 (42 % abundance). Conductivity
-1
-1
1
(Λo, M cm ) in DMF: 132. H NMR (δ, ppm in dmso-d
4H of S C-N(CH ); 3.21 (t, 2H of CH ); 2.23 (m, 4H of CH
2H), C (7.27, m, 2H), C (8.01, d, 2H), C (7.69, d, 2H), C (7.43, m, 2H), C
6
): 7.29-7.32 (m, 5H of C
); protons of 2-C py: C
(7.68, d, 2H).
6
H
5
); 3.94 (m,
2
2
)
2
2
2
6
H
4
1
(8.84, d,
2
3
4
5
6
2.3. X-Ray crystallography
X-ray data of the suitable crystal of complex 1 were collected on a Bruker’s Apex-II CCD
diffractometer using MoKα (λ = 0.71069). The data were corrected for Lorentz and polarization
effects and empirical absorption corrections were applied using SADABS from Bruker. A total of
13691 reflections were measured out of which 4012 were independent and 2299 were observed
[I > 2σ(I)] for theta (θ) 32°. The structure was solved by direct methods using SIR-92 and refined
2
by full-matrix least squares refinement methods based on F , using SHELX-97 [35]. The two fold
axis passes through the metal ion, nitrogens of the piprazine ring and their substituent carbon
atoms. Therefore the asymmetric unit contains half the molecule. All non-hydrogen atoms were
refined anisotropically. The refinement showed rotational disorder in the piprazine ring which
could be resolved by splitting the two unique carbon atoms into two components and refining
their sof and thermal parameters as free variables with restraints over the bond distances. All
hydrogen atoms were fixed geometrically with their Uiso values 1.2 times of the phenylene and
methylene carbons and 1.5 times of the methyl carbons. All calculations were performed using
Wingx package [36, 37]. Important crystallographic parameters are given in Table 1.
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2.4. Theoretical calculation
To clarify the configurations and energy level of the complexes 1, 2 and 3, DFT calculations
were carried out in G09W program using B3LYP/6-31G(d) calculation and correlation function
as implemented in the Gaussian program package Gaussian 09. Thermal contribution to the
energetic properties was considered at 298.15 K and one atmosphere pressure.
2.5. DNA binding experiments
All the experiments involving CT-DNA were studied by spectroelectronic titration and
fluorescence quenching technique by using ethidium bromide (EB) as a DNA scavenger and
performed the experiment as our previously standardized method [38].
Tris–HCl buffer solution was used in all the experiments involving CT-DNA. This tris–HCl
buffer (pH 7.9) was prepared using deionized and sonicated HPLC grade water (Merck). The CT-
DNA used in the experiments was sufficiently free from protein as the ratio of UV absorbance of
the solutions of DNA in tris–HCl at 260 and 280 nm (A260/A280) was almost ~1.9. The
concentration of DNA was determined with the help of the extinction coefficient of DNA
-1
-1
solution at 260 nm (ε260 of 6600 L mol cm ) [38]. Stock solution of DNA was always stored at
o
4 C and used within four days. Concentrated stock solution of the complex 1 was prepared by
dissolving the compound in DMSO and suitably diluted with tris–HCl buffer to the required
concentration for all the experiments. Absorption spectral titration experiment was performed by
keeping constant the concentration of the complex 1 and varying the CT-DNA concentration. To
eliminate the absorbance of DNA itself, equal solution of CT-DNA was added both to the
complex 1 solution and to the reference solution.
In the ethidium bromide (EB) fluorescence displacement experiment, 5 µL of the EB tris–
-1
HCl solution (1.0 mmol.L ) was added to 1.0 mL of DNA solution (at saturated binding levels),
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stored in the dark for 2.0 h. Then the solution of the compound was titrated into the DNA/EB
mixture and diluted in tris–HCl buffer to 5.0 mL to get the solution with the appropriate complex
1/CT-DNA mole ratio. Before measurements, the mixture was shaken up and incubated at room
temperature for 30 min. The fluorescence spectra of EB bound to DNA were obtained at an
excited wavelength of 522 nm in the Fluorimeter (Hitachi-2000). The interaction of the complex
1 with calf thymus DNA (CT-DNA) has been investigated by using absorption and emission
spectra.
2.6. Antimicrobial screening
The biological activities of free dithiocarbamic acids and the rhodium(III) derivatives of
dithiocarbamates (1, 2 and 3) have been studied for their antibacterial activities by agar well
diffusion method [39-41]. The antibacterial activities were done at 100 µg/mL concentration of
different compounds in DMF solvent by using three pathogenic gram negative bacteria
(Escherichia coli, Vibrio cholerae, Streptococcus pneumoniae) and one gram positive pathogenic
bacteria (Bacillus cereus). DMF was used as a negative control. The Petri dishes were incubated
at 37 °C for 24 h. After incubation plates were observed for the growth of inhibition zones. The
diameter of the zone of inhibition was measured in mm.
3. Results and Discussion
3.1. Synthesis and characterization of complexes
1
2
3
The bidentate sulphur ligands (L H, L H, and L H) were synthesized by the reaction between
1
carbon disulfide with different amines in ethanol, and later characterized by FTIR and H NMR.
Treatment of these ligands with [Rh(2-C
6
H
4
py)
2
Cl]
2
.1/4CH
2
Cl at refluxing condition in DMSO-
2
MeCN (v/v 1:1) solvent mixture having nitrogen atmosphere resulted in cleavage of the chloro
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bridge and led to formation of mononuclear rhodium(III) complexes of general formula of
III
[Rh (2-C
6
H
4
py) (L)] which were obtained from the column chromatography using acetonitrile
2
as orange colored microcrystalline solid on evaporation. Here, the dithiocarbamates behaves as
bidentate monobasic ligands (see Scheme 1). The complexes (1-3) are sparingly soluble in
common organic solvents except hexane but fairly soluble in DMF and DMSO, and are stable in
both the solid state and solution in air. The molar conductivity of freshly prepared solution (~1 x
-3
-1
-1
-1
-1
-1
10 M concentration) of 1 (Λ
M
= 130 M cm ), 2 (Λ
M
= 127 M cm ) and 3 (Λ = 132 M
M
-1
cm ) in DMF are fairly consistent with a non-electrolyte, respectively. The complexes are
diamagnetic in nature. The formulations of the complexes have been confirmed by spectroscopic
methods and elemental analyses.
3.2. Structural description of complex 1
1
An ORTEP view of the complex [Rh(2-C
6
H
4
py)
2
(L )] (1) with atom labeling scheme is
illustrated in Fig.1, and a selection of bond distances and angles is listed in Table 2. The
structural analysis evidenced that the complex resides on a C 2/c site in the monoclinic crystal
system. The crystal structure of 1 shows a distorted octahedron in which both of the 2-
phenylpyridyl nitrogens are in axial positions, trans to one another and the sulfur atoms are
opposite to the phenyl rings. As would be expected, both Rh-C σ-bonds are equal in length
(1.994(4) Å) and significantly shorter than the Rh-N dative bond lengths of 2.039(3) Å due to
the Rh-C σ-bonds increasing electron density on the metal center. The bond distance of Rh-C in 1
is comparable with the previously reported Rh-C bonds in cyclometalated complex (1.996(9) Å)
but the bond Rh-N is slightly longer than those (1.987(7) Å) [42], but both are comparable with
the reported values [43]. Both Rh–S distances are also equal in length (2.4854(11) Å), and these
1
0

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are longer than those in [Rh(Et
2
NCS
2
)
3
] (2.364(3) Å) [44] due to trans influence of the strong σ-
donating carbon atoms of the phenyl groups and shorter than those in similar type complex
[{Rh(Bu2-C
6
H
4
py)
2
}
2
{S
2
P(OMe)
2
}] (2.548(2) Å) due to attachment of sulphur atoms to carbon
o
to of more electronegativity than phosphorous, for this reason, the S-Rh-S angle of 71.00(5) is
o
smaller compared to the observed value of 79.40(1) in the previous report [43].
3.3. IR Spectra
-1
-1
The IR spectrum of the ligands display an intense stretch at 2850 cm and 1445-1430 cm
1
-1
correspond to γC-H of N-Me and γC=N respectively. The γC-H of N-Me of L H for 1 (2910cm ) is blue
-1
shifted than the γC-H of N-Me of the free ligand (2850 cm ) indicating metal ligand coordination.
-l
The band around 1590 cm indicates a double bond character of C-N bond in the ligand frame,
which is confirmed from the bond length of the X-ray structure. This fact could be attributed to
the electron releasing ability of the heterocyclic group towards the sulphur atoms, a feature that
induces an electron delocalization over the carbon-nitrogen bond and the CS
2
fragment. This is
-l
shown by the νC=N shift to higher energies (ca. 1510-1465 cm ) with respect to the free acids (ca.
-1
1445-1430 cm ), and these bands lie in between the stretching frequencies expected for a double
-l
-1
C=N (1610-1690 cm ) and single C-N bond (1250-1350 cm ). The blue-shift of the C=N
stretching frequency on going from the free acids to their metal complexes gives support to the
typical bidentate character [45] of the carbodithioic acid ligands. Two bands in the region of
-l
-l
1040-965 cm (separated by less than 20 cm ) assignable to the νa(SCS) and one band for the vs(SCS)
-l
stretch in the region 705-675cm of the complexes suggest the unsymmetrical chelating bidentate
mode of coordination to rhodium(III) ion [46]. The stretchings due to νCOC (asym and sym), νN-Me
and νCCC (asym and sym) remain unchanged in the spectra of the complexes and in the free
1
1

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ligands. This observation helps to exclude any coordination to the metals via nitrogen and oxygen
donors.
3.3 Electronic Spectra
The electronic spectra of 1, 2 and 3 in DMF were shown in Fig 2. The spectral data have
been tabulated in Table 3. Complexes 1, 2 and 3 display a lower energy band at 675nm, 672nm
and 675nm, respectively with low extinction coefficient values that correspond to the d–d
transition. The higher energy band at 360 nm for all three complexes with high extinction
coefficient values are due to the coordinated carbon atom from pipyridine moiety, C(σ)-Rh(III)
charge transfer (LMCT) transition. The other higher energy intense transitions at 374 nm and 360
nm are due to the n→π* and π→π* charge transfer transitions.
3.5. Redox studies
The cyclic voltammograms (CV) of the complexes 1, 2 and 3 were recorded in DMF solvent
+
at room temperature. Three electrode cell set up such as platinum, Ag/Ag (non-aquous) and a
platinum wire as a working, reference and auxiliary electrode respectively have been used for
measurements. The cyclic voltammograms of all the three complexes 1, 2 and 3 have been shown
in Fig. 3 and the electrochemical data have been tabulated in Table 4. The complexes exhibit an
+
irreversible reductive response at E1/2 value ≈ -0.697 V to -0.767 V versus Ag/Ag (non-aqueous)
3
+
+
corresponds to Rh /Rh couple. Small differences in the ∆E values 652 mV, 658mV, 654 mV
p
for 1, 2 and 3 respectively) have been observed that increases in the order 2> 3>1. This indicates
that the ease of reduction from Rh(III) to Rh(I) with respect to ligand electronic environment is
supposed to be much more in case of complex 1 and least in complex 2. The trend of reduction
1
2

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298
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303
304
potential values followed can be explained by the availability of the electrons on the donor atoms
of the dithiocarbamate ligands. The electron donating capacity through σ bond of the six
membered heterocyclic ring increases in the order 2< 3<1 owing to the presence of different
substituents at the heteroatom i.e. highly electronegative O atom (2), -R effect of benzylic group
(3), +I effect of Me group (1) and so the trend of reduction potential followed as such, which is
further supported by theoretical calculation obtained from DFT study (viz. supporting
information).
1
(L )]
3.6. DNA binding study of [Rh(2-C
6
H
4
py)
2
Absorption spectral study: Electronic absorption spectroscopy is an effective method to
examine the binding modes of complex 1 with DNA. In general, binding of the compound to the
DNA helix is testified by an increase of the CT band complex 1 due to the involvement of strong
intercalative interactions between an aromatic chromophore of compound and the base pairs of
DNA [47-49]. The absorption spectra of complex 1 in the absence and presence of CT-DNA is
given in Fig. 4. The extent of the hyperchromism in the absorption band is generally consistent
with the strength of intercalative binding/interaction [50,51]. Fig. 5 indicates that the complex 1
5
-1
interacts strongly with CT-DNA (K
b
= 1.54 x10 M ), and the observed spectral changes may be
rationalized in terms of intercalative binding [52]. In order to further illustrate the binding
strength of the complex 1 with CT-DNA, the intrinsic binding constant K
b
was determined from
(1)
the spectral titration data using the following equation [53]:
[DNA]/(ε
where [DNA] is the concentration of DNA, ε
respectively, for the free complex 1, for each addition of DNA to the complex 1 and for the
complex 1 in the fully bound form. A plot of [DNA]/(ε –ε ) versus [DNA], gives K , the intrinsic
a
–ε
f
) = [DNA]/(ε
b
–ε
f
) + 1/[K
b
(ε
b
–ε
f
)]
f
, ε
a
and ε
b
correspond to the extinction coefficient,
a
f
b
1
3

305
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312
313
314
315
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321
322
323
324
325
326
327
328
binding constant as the ratio of slope to the intercept. From the [DNA]/( ε
a
–ε
f
) versus [DNA] plot
5
-1
(Fig. 5), the binding constant K for complex 1 was estimated to be 1.54 x 10 M (R = 0.99746
b
for five points), indicating a strong binding of the complex 1 with CT-DNA.
Fluorescence quenching technique: Fluorescence intensity of EB bound to DNA at 612
nm shows a decreasing trend with the increasing concentration of the compound. The quenching
of EB bound to DNA by the compound is in agreement with the linear Stern–Volmer equation
[54]:
I
0
/I = 1 + Ksv [Q]
and I represent the fluorescence intensities in the absence and presence of quencher,
respectively. Ksv is a linear Stern–Volmer quenching constant, Q is the concentration of
(2)
where I
0
quencher. In the quenching plot in Fig. 7 of I
0
/I versus complex 1 Ksv value is given by the ratio
4
of the slope to intercept. The Ksv value for the complex 1 is 0.87 x 10 (R = 0.98873 for five
points), suggesting a strong affinity of 1 to CT-DNA.
Number of binding sites: Flurorescence quenching data were used to determine the binding
sites (n) for the compound 1 with CT-DNA. Fig. 6 shows the fluroscence spectra of EB-DNA in
the presence of different concentrations of compound 1. It can be seen that the fluroscence
intensity at 612 nm was used to estimate Ksv and n.
If it is assumed that there are similar and independent binding sites in EB-DNA, the
relationship between the fluroscence intensity and the quencher medium can be deduced from the
following Eq. (3):
n
nQ + B → Q ….B
(3)
where B is the flurophore, Q is the quencher, [nQ + B] is the postulated complex between the
flurophore and n molecules of the quencher [47]. The constant K is given by Eq. (4):
n
K = [Q
n
….B]/[Q] .[B]
(4)
1
4

329
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347
348
349
350
351
352
If the overall amount of biomolecules ( bound or unbound with the quencher) is B
[Q …B]+ [B], where [B] is the concentration of unbound biomolecules, and the relationship
between the fluorescence intensity and the unbound biomolecule as [B]/[B ] = I/I , that is:
log[(I -I)/I] = logK + nlog[Q] (5)
Where (n) is the number of binding site of compound complex 1 with CT-DNA, which can be
determined from the slope of log[(I -I)/I] versus log[Q],as shown in the Fig. 8. The calculated
o
, then [B
o
] =
n
o
o
o
o
value of the number of binding sites (n) is 1.10 (R= 0.99869 for five points). The value of (n)
approximately equals 1, and thus indicates the existence of one binding site in DNA for
compound 1.
Viscosity Measurement: To further clarify the nature of interaction between complex 1 and
CT DNA, viscosity measurements were carried out. Upon binding, a DNA intercalator causes an
increase in the viscosity of the DNA double helix due to its insertion between the DNA base pairs
and consequently to the lengthening of the DNA double helix. In contrast, a partial and/or
nonclassical intercalation could bend (or kink) the DNA helix, reducing the effective length and
its viscosity [55]. The method is generally considered the least unambiguous to probe the mode
of binding of a compound to DNA. The effect of 1 on viscosity of CT DNA is shown in Fig. 9.
The viscosity of DNA increased dramatically upon addition of complex 1 and is nearly linear
2
(R = 0.99621 for nine points). These results strongly indicate that the complex 1 deeply into the
DNA base pairs in intercalative fashion.
3.7. Antibacterial activity
Antibacterial activity of the dithicarbamic acids (HL) and the corresponding complexes are
tabulated in Table 5. Comparisons of the biological activity of the dithicarbamates and their
rhodium(III) derivatives with the standard antibiotics, chloramphenicol at different
1
5

353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
concentrations have been carried out taking usual precautions. From this study, it is inferred that
all the rhodium(III) complexes have higher activity than the ligand only, but little less efficient
than the antibiotics. The increased activity may be due to the increase of the delocalization of π-
electrons over the whole chelate ring imparts the increased lipophilic character to the metal
complexes. This higher lipophilicity of the complexes facilitates the penetration ability with a
greater extent into the bacterial cell membranes, and as result it perturbs the respiration process of
the bacteria and diminish the further growth of the microorganisms.
4. Conclusion
Three complexes of diimine dithiocarbamate mixed ligand framework Rh(2-
1
2
3
C
6
H
4
py)
2
(L )](1), Rh(2-C
6
H
4
py)
2
(L )](2), Rh(2-C
6
H
4
py) (L )] (3) have been synthesized and
2
characterized by means of solid and solution phase spectroscopic studies including the X-ray
structure of 1. With the knowledge gained from the present study, attempts are now underway to
bind these ligands in the C,N,S-coordination fashion to iridium and other metal ions having
octahedral geometry. The present study of interaction with CT-DNA shows that these
cyclometalated rhodium(III) complexes having dithiocarbamate moieties are good intercalative
binding to with CT-DNA with an adequate number of coordination sites and this strongly binding
ability of the complexes as intercalator encourage to develop these materials as good anticancer
candidates. From the antibacterial studies it is found that all the metal complexes have higher
activities than the free dithiocarbamic acids (LH) against four pathogenic bacteria (Escherichia
coli, Vibrio cholerae, Streptococcus pneumonia and Bacillus cereus, among these three complex
2 has more antibacterial effect.
1
6

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384
385
386
387
388
389
Supporting material
Crystallographic data for complex 1 have been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 932588. Copies of this information are available on request at free of
charge from CCDC, 12 Union Road, Cambridge, CB21EZ, UK (fax: +44-1223-336-033; e-mail:
deposit@ccdc.ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgements
Financial support from the Council of Scientific and Industrial Research (CSIR), New Delhi,
India is gratefully acknowledged.
390
391
392
393
394
395
396
397
398
399
400
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9

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451
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457
458
459
460
461
462
463
464
465
466
467
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and refs. therein.
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Leung, J. Organomet. Chem. 689 (2004) 2401
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468
469
470
471
2
0

472
473
474
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476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
Figures’ Legend
1
Fig. 1. ORTEP view of the complex [Rh(2-C
6
H
4
py) (L )] (1) with atom labeling scheme
2
(excluded H for clarity). (Symmetry codes: (i) -x,y,z; (ii) x,-y,-z; (iii) -x,-y,-z).
Fig. 2. Electronic absorption spectra of 1, 2 and 3 in DMF
Fig. 3. Cyclic voltammograms (scan rate 50 mV/s) of 1, 2 and 3 in DMF solution of 0.1 M
TBAP, using platinum working electrode.
Fig. 4. Electronic spectral titration of complex 1 with CT-DNA at 267nm in tris-HCl buffer;
-
4
-6
-6
-6
[Compound] = 1.09 x 10 ; [DNA]: (a) 0.0, (b) 1.25 x 10 , (c) 2.50 x 10 , (d) 3.75 x 10 ,
-6 ,
-6
-1
(e) 5.00 x 10 (f) 6.25 x 10 mol.L . Arrow indicates the increase of DNA concentration.
Fig. 5. Plot of [DNA]/(ε
a
–ε ) versus [DNA] for the absorption of CT-DNA with the complex 1 in
f
tris-HCl buffer
Fig. 6. Emission spectra of the CT-DNA-EB system in tris–HCl buffer upon the titration of the
-4
-1
-6
-1
compound complex 1. Kex = 522 nm; [EB] =0.96 x 10 molL ; [DNA] = 9.9 x 10 molL ;
-5
-5
-5
-5 ,
[Compound]: (a) 0.0, (b) 1.36 x 10 , (c) 2.72 x 10 , (d) 4.08 x 10 , (e) 5.44 x 10
-5
-1
(f) 6.80 x 10 molL . Arrow indicates the increase of compound concentration.
Fig. 7. Emission spectra of the CT-DNA-EB system in Tris-HCl buffer upon titration with
-5
-1
-5
complex 1c. λex = 522 nm; [EB] = 9.6×10 mol L , [DNA] = 1.25×10 ; [Complex]: (a) 0.0,
-5
-5
-5
-5
-1
(b) 1.25×10 , (c) 2.5×10 ,(d) 3.75×10 , (e) 5.00×10 , mol L . The arrow denotes the
gradual increase of complex concentration. Inset: plot of I /I vs. [complex] of 1c; Ksv = 0.82 ×
0
4
10 (R = 0.99965, n = 5 points).
Fig. 7. Plot of I /I versus complex 1 for the titration of CT-DNA–EB system with complex 1
0
using spectrofluorimeter; linear Stern–Volmer quenching constant (Ksv) for complex 1 =0.87
4
x 10 ; (R = 0.98873 for five points).
2
1

495
496
497
498
Fig. 8. The linear plot shows log[(I -I)/I] versus log[Q], where R = 0.99869 for five points
o
Fig. 9. Effect of increase of amount of 1 on the relative viscosity of CT DNA in tris-HCl having
50 mM NaCl buffer.
Scheme 1. Synthetic method of rhodium(III) complexes
2
2

499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
Table 1. Crystallographic data for complex 1
Empirical Formula
Identification code
C28 H27 N4 Rh S2
shelxl
Fw
586.59
Crystal system
Space group
a, Å
Monoclinic
C 2/c
16.683(5)
17.840(4)
10.251(5)
126.223(5)
b, Å
c, Å
β, deg
3
V, Å
2461.3(15)
Z
4
–
3
D
calcd. (g cm )
1.583
0.889
1200
–
1
µ (MoK
F(000)
θ range, deg
α
), mm
1.90 -31.84
13691
No. of reflns collcd
No. of independent reflns
4012
0.0544
2299
357
R
int
No. of reflns (I > 2σ(I))
No. of refined paramaters
2
Goodness-of-fit (F )
1.034
[a]
R1, wR2 (I >2σ(I))
0.0473, 0.1041
R1 = 0.1053, .1342
R indices (all data)
2
3

523
Table 2. Coordination Bond lengths [Å] and angles [°] for complex 1
5
24
25
26
Bond length (Å)
5
Rh(1) - C(11)
Rh(1) - N(1)
Rh(1) - S(1)
1.994(4)
2.039(3)
Rh(1) - C(11)#1
Rh(1) - N(1)#1
1.994(4)
5
2.039(3)
2.4855(1 512 )7
28
2.4854(11) Rh(1) - S(1)#1
o
Bond angle ( )
5
C(11)-Rh(1)-N(1)
C(11)-Rh(1)-N(1)#1
N(1)-Rh(1)-S(1)
80.30(13) C(11)#1-Rh(1)-N(1)#1 80.29(13)
529
93.30(13) C(11)#1-Rh(1)-N(1)
170.86(11) N(1)-Rh(1)-N(1)#1
100.14(11) C(11)#1-Rh(1)-S(1)#1
93.30(13)
171.09(1 573 )0
C(11)-Rh(1)-S(1)
C (11)#1-Rh(1)-S(1)
N(1)#1-Rh(1)-S(1)
S(1)-Rh(1)-S(1)#1
100.15(11)
31
5
90.15(9)
97.12(9)
71.00(5)
N(1)#1-Rh(1)-S(1)#1
N(1)-Rh(1)-S(1)#1
C(11)-Rh(1)-C(11)#1
90.15(9)
5
32
97.11(9)
88.8(2) 533
534
535
536
537
Symmetry transformations used to generate equivalent atoms: #1 -x+1, y, -z+3/2
Table 3. Electronic absorption spectral data
-5
3
-1
-1 a
Compound
λmax(nm) (10 , ε/dm mol cm )
1
360(7845),
75(160)
360(7900),
72(210)
360(7800),
75(200)
374sh(6780),
375sh(6680),
382sh(6600),
385(6840),
390(6700),
390(6640),
6
2
6
3
6
a
538
in DMF solvent
2
4

a
539
Table 4. Electrochemical data for the complexes 1, 2 and 3.
Complex
E
pc(V)
E
pa(V)
∆E (mV)
E1/2(V)
p
652
658
1
2
-1.023
-1.096
-0.371
-0.438
-0.697
-0.767
3
-1.077
-0.413
654
-0.744
a
540
541
542
Potentials versus non-aqueous Ag/Ag+ reference electrode, scan rate 50 mV/s, supporting
electrolyte: tetra-N-butylammonium perchlorate (0.1 M).
543
5
5
44
45
Table 5. Antibacterial data of free dithiocarbamic acids (LH) and rhodium(III) complexes (1, 2
and 3) (100 µg/ ml)
Compound for
Treatment
Inhibition zone in mm
V.cholerae S.pneumoniae B. cereus
E. coli
1
L H
05
05
04
11
16
12
05
04
07
14
17
14
04
08
06
17
20
18
03
04
03
06
08
07
2
L H
3
L
1
2
3
Chloramphenicol
DMF
22
0
29
0
24
0
09
0
5
46
47
5
548
2
5

550
552
554
556
558
560
562
564
565
566
567
569
571
573
575
577
579
581
583
585
586
587
588
1
Fig. 1. ORTEP view of the complex [Rh(2-C
6
H
4
py) (L )] (1) with atom labeling scheme
2
(excluded H for clarity). (Symmetry codes: (i) -x,y,z; (ii) x,-y,-z; (iii) -x,-y,-z).
4
-----(1)
-----(2)
----(3)
3
2
1
0
-
350
400
450
500
Wavelength(nm)
Fig. 2. Electronic absorption spectra of 1, 2 and 3 in DMF
2
6

590
592
594
596
598
600
602
604
606
607
608
609
611
613
615
617
619
621
623
625
626
627
628
1
0
8
6
4
2
0
2
3
1 2
-
-
1.5 -1.2 -0.9 -0.6 -0.3
0.0
E (V)
Fig. 3. Cyclic voltammograms (scan rate 50 mV/s) of 1, 2 and 3 in DMF solution with 0.1 M
TBAP, using platinum working electrode.
0.4
0.3
0.2
0.1
0.0
f
a
3
00
400
500
λ(nm)
600
700
Fig. 4. Electronic spectral titration of complex 1 with CT-DNA at 267nm in tris-HCl buffer;
-4
-6
-6
-6
[Compound] = 1.09 x 10 ; [DNA]: (a) 0.0, (b) 1.25 x 10 , (c) 2.50 x 10 , (d) 3.75 x 10 ,
-
6 ,
-6
(e) 5.00 x 10 (f) 6.25 x 10 mol.L-1. Arrow indicates the increase of DNA concentration.
2
7

630
632
634
636
638
640
642
644
645
646
647
649
651
653
655
657
659
661
663
664
665
666
667
668
9
.0
.5
8
8.0
7.5
7.0
6.5
6.0
5.5
5.0
1
2
3
4
5
6
7
Fig. 5. Plot of [DNA]/(ε
tris-HCl buffer
a
–ε ) versus [DNA] for the absorption of CT-DNA with the complex 1 in
f
1
200
a
f
1
000
800
600
400
200
5
60
600
640
680
720
760
λ (nm)
Fig. 6. Emission spectra of the CT-DNA-EB system in tris–HCl buffer upon the titration of the
-4
-1
-6
-1
compound complex 1. Kex = 522 nm; [EB] =0.96 x 10 molL ; [DNA] = 9.9 x 10 molL ;
-5
-5
-5
-5 ,
[Compound]: (a) 0.0, (b) 1.36 x 10 , (c) 2.72 x 10 , (d) 4.08 x 10 , (e) 5.44 x 10 (f)
-5
-1
6.80 x 10 molL . Arrow indicates the increase of compound concentration.
2
8

669
671
673
675
677
679
681
683
685
686
687
688
689
691
693
695
697
699
701
703
705
706
707
708
1
.7
.6
.5
.4
.3
1.2
.1
.0
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
5
[
Complex 1] x 10
Fig. 7. Plot of I /I versus complex 1 for the titration of CT-DNA–EB system with complex 1
0
using spectrofluorimeter; linear Stern–Volmer quenching constant (Ksv) for complex 1 =
4
0.87 x 10 ; (R = 0.98873 for five points).
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-
4.0
-4.2
-4.4
-4.6
-4.8
log[Complex 1]
Fig. 8. The linear plot shows log[(I -I)/I] versus log[Q],where R = 0.99869 for five points
o
2
9