Coordination behavior of 3,4-bis(2-pyridylmethylthio)toluene with copper(II) ions: Synthesis, structural characterization and reactivity, and DNA binding study of the dinuclear copper(II) complex

Article abstract of DOI:10.1016/j.poly.2010.08.005

On reaction of different copper(II) salts with 3,4-bis(2-pyridylmethylthio) toluene (L) having neutral tetradentate NSSN donor set in different chemical environments, two mononuclear copper(II), one dinuclear copper(I) and one dinuclear copper(II) complex

Full text of DOI:10.1016/j.poly.2010.08.005

Polyhedron 29 (2010) 3157–3163
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Coordination behavior of 3,4-bis(2-pyridylmethylthio)toluene with copper(II)
ions: Synthesis, structural characterization and reactivity, and DNA binding study
of the dinuclear copper(II) complex
Sandipan Sarkar a, Supriti Sen a, Sourav Dey a, Ennio Zangrando b, Pabitra Chattopadhyay a,
⇑
a Department of Chemistry, Burdwan University, Golapbag, Burdwan-713 104, India
b Dipartimento di Scienze Chimiche, Via Licio Giorgieri 1, 34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
On reaction of different copper(II) salts with 3,4-bis(2-pyridylmethylthio)toluene (L) having neutral tet-
radentate NSSN donor set in different chemical environments, two mononuclear copper(II), one dinuclear
copper(I) and one dinuclear copper(II) complexes, formulated as [CuII(L)(H2O)2](NO3)2 (1), [CuII(pic)2] (2),
[CuI2(L)2](ClO4)2 (3) and [CuII2(L)2Cl2](ClO4)2 (4), respectively, were isolated in pure form [where
pic = picolinate]. All the complexes were characterized by physicochemical and spectroscopic methods.
The product of the reactions are dependent on the counter anion of copper(II) salts used as reactant
and on the reaction medium. Complexes 1 and 4 were obtained with nitrate and perchlorate copper(II)
salts, respectively. On the other hand, C–S bond cleavage was observed in the reaction of L with copper(II)
chloride to form in situ picolinic acid and complex 2. Dinuclear complexes 3 and 4 were separated out
when copper(II) perchlorate was allowed to react with L in methanol and in acetonitrile, respectively,
under aerobic condition. The X-ray diffraction analysis of the dinuclear complex 3 shows a highly dis-
torted tetrahedral geometry about each copper ion. Complex 4 is converted to 3 in acetonitrile in pres-
ence of catechol. The spectral study of complex 4 with calf thymus DNA is indicative of a groove
binding mode interaction.
Received 8 June 2010
Accepted 7 August 2010
Available online 18 August 2010
Keywords:
Copper(II) complex
Crystal structure
DNA binding
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
copper(II) salts carried out in different media, two mononuclear
copper(II), one dinuclear copper(I) and one dinuclear copper(II)
The coordination chemistry of dinuclear copper(I/II) complexes
has received increased attention over the last decades [1–5]. This
is mainly due to the potential application of these complexes their
potential applications in metallosupramolecular assemblies [6–10],
photosensitization reactions [5,6], bioinorganic chemistry [11],
medicine [12] and catalysis [13]. Many efforts have been devoted
to the design and synthesis of new multidentate ligands that are
able to address the coordination geometry and the properties of
copper(I/II) complexes [14–17]. With flexible polydentate ligands,
the competition between bridging and chelating coordination
mode is an important factor towards the synthesis of mono, di,
and polynuclear metal complexes [18,19].
complexes formulated as [CuII(L)(H2O)2](NO3)2 (1), [CuII(pic)2]
(2), [CuI2(L)2](ClO4)2 (3), and [CuII2(L)2Cl2](ClO4)2 (4), respectively,
were obtained [where pic = picolinate]. It has been observed that
the counter anion of copper(II) salt used as reactant and the reac-
tion medium govern the product of the reactions. The complexes
have been characterized by physicochemical and spectroscopic
tools along with X-ray structural analysis of 3. In continuation of
our interest [23], DNA binding study of dinuclear copper(II) com-
plex 4 has been performed spectroscopically.
2. Experimental
Considering the above facts and in continuation of our research
efforts [20–22] to explore the reactivity of different organic moie-
ties with nitrogen–sulfur donor centres with transition metal ions,
we report here on the coordination behavior of a newly designed
3,4-bis(2-pyridylmethylthio)toluene ligand (L) with copper(II) ions
in different chemical environments. The reaction of L with various
2.1. Materials and physical measurements
All chemicals and reagents were obtained from commercial
sources and used as received. Solvents were distilled from an
appropriate drying agent.
The elemental (C, H, N) analyses were performed on a Perkin–
Elmer model 2400 elemental analyzer. Copper analysis was carried
out by Varian atomic absorption spectrophotometer (AAS) model-
AA55B, GTA using graphite furnace. Electronic absorption spectra
⇑
Corresponding author. Tel.: +91 342 2558339; fax: +91 342 2530452.
E-mail address: pabitracc@yahoo.com (P. Chattopadhyay).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.08.005

3158
S. Sarkar et al. / Polyhedron 29 (2010) 3157–3163
were recorded on a JASCO UV–Vis/NIR spectrophotometer model
V-570. IR spectra (KBr discs, 4000–300 cmꢀ1) were recorded using
a Perkin–Elmer FTIR model RX1 spectrometer. The 1H NMR spectra
were obtained on a Bruker AC300 spectrometer with chemical
shifts reported relative to the residual solvent resonance of CDCl3.
The room temperature magnetic susceptibility measurements
were performed by using a vibrating sample magnetometer PAR
155 model. Molar conductances (KM) were measured in a systron-
ics conductivity meter 304 model using ꢁ10ꢀ3 mol Lꢀ1 solutions in
appropriate organic solvents. Thermogravimetric analysis was per-
formed on a Perkin–Elmer Diamond TG/DTA Thermal Analyzer
from room temperature to 600 °C at a heating rate of 20 °C minꢀ1
using platinum crucibles. The fluorescence spectra of EB bound
to DNA were obtained in the Hitachi-2000 fluorimeter.
copper(II) perchlorate hexahydrate (1.0 mmol) in methanol was
added and the reaction mixture was stirred for 4 h at room temper-
ature. The green colored filtrate was kept at undisturbed condition
in a beaker. After few days colorless crystals suitable for X-ray dif-
fraction were collected. Yield: 84–85%.
[CuI2(L)2](ClO4)2 (3): Anal Calc. for C38H36Cl2Cu2N4O8S4: C,
45.51; H, 3.62; N, 5.59, Cu, 12.67. Found: C, 45.60; H, 3.54; N,
5.51, Cu, 12.59%. IR (cmꢀ1):
m
C @N, 1478;
, B.M.): 1.45 per Cu atom. Conductivity
K
o, ohmꢀ1 cm2 molꢀ1) in DMF: 149.
mC–S, 760, mClO , 1088 and
4
622. Magnetic moment (
l
(
2.6. Preparation of [CuII2(L)2Cl2](ClO4)2 (4)
Complex 4 was synthesized following the same procedure de-
scribed for 3 by taking copper(II) perchlorate hexahydrate and or-
ganic compound (L) in equimolar ratio but the reaction was carried
out in acetonitrile instead of methanol followed by addition of
aqueous solution of sodium chloride (0.585 g) to the reaction mix-
ture. The resulting mixture was filtered and kept undisturbed for
several days. After few days crystalline compound was obtained
from this filtrate. Yield: 75–76%.
2.2. Preparation of 3,4-bis(2-pyridylmethylthio)toluene (L)
The organic moiety, 3,4-bis(2-pyridylmethylthio)toluene (L)
was prepared following our earlier report [24]. An ethanolic solu-
tion of 2-picolyl chloride, hydrochloride (1.64 g, 10.0 mmol) was
added to 3,4-toluenedithiol (0.78 g, 5.0 mmol) in dry ethanol con-
taining sodium ethoxide (0.46 g, 20.0 mmol) at low temperature
(0–5 °C). Then the mixture was allowed to stir at room tempera-
ture for 0.5 h and then refluxed for 2 h. The mixture was cooled
to room temperature, water was added and finally the ethanol
was evaporated off by rotary evaporator. The product was ex-
tracted into dichloromethane and dried by using Na2SO4. Finally
the product 3,4-bis(2-pyridylmethylthio)toluene was obtained as
a yellow oil by removing the dichloromethane by rotary evapora-
tor. The compound was characterized by 1H NMR spectroscopy
using CDCl3. Yield: 82–85%. 1H NMR (d, in CDCl3) ppm: 2.41(s,
3H), 4.39(s, 4H), 6.78(d, 1H, j = 7.2), 6.83(s, 1H), 7.03(d, 1H, j =
7.1), 7.27–7.31(m, 4H), 7.79–7.82(m, 2H) and 8.69(d, 2H, j = 5.1).
[Cu2(L)2Cl2](ClO4)2 (4): Anal Calc. for C38H36Cl4Cu2N4O8S4: C,
42.50; H, 3.38; N, 5.22, Cu, 11.83. Found: C, 42.58; H, 3.34; N,
5.29, Cu, 11.87%. IR (cmꢀ1):
m
C @N, 1478;
, B.M.): 1.45 per Cu atom. Conductivity
K
o, ohmꢀ1 cm2 molꢀ1) in DMF: 156.
mC–S, 760, mClO , 1088 and
4
622. Magnetic moment (
l
(
2.7. DNA binding experiments
The Tris–HCl buffer solution (pH 8.04), used in all the experi-
ments involving CT-DNA, was prepared using deionized and soni-
cated HPLC grade water (Merck). The purity of CT-DNA was
verified by UV absorbance [25], while the concentration of DNA
was ensured by verifying the extinction coefficient of DNA solution
2.3. Preparation of [CuII(L)(H2O)2](NO3)2 (1)
at 260 nm (e
260) of 6600 L molꢀ1 cmꢀ1 [26]. Stock solution of DNA
was always stored at 4 °C and used within 4 days. Stock solution of
the copper(II) complex was prepared by dissolving the complex in
DMSO and suitably diluted with Tris–HCl buffer to the required
concentration for all the experiments. Absorption spectral titration
was performed by keeping constant the concentration of the cop-
per(II) complex, while varying that of CT-DNA. To detach the absor-
bance of DNA itself, equal solution of CT-DNA was added both to
the copper(II) complex solution and to the reference solution.
In the ethidium bromide (EB) fluorescence displacement exper-
To a methanolic solution of 3,4-bis(2-pyridylmethylthio)tolu-
ene (L) (0.339 g), copper(II) nitrate trihydrate (0. 242 g) in metha-
nol was added dropwise and the resulting mixture was stirred for
4 h at ambient temperature. The green colored complex was pre-
cipitated out, filtered and washed with water, cold methanol and
dried in vacuo. Yield: 80–82%.
[Cu(L)(H2O)2](NO3)2 (1): Anal Calc. for C19H22CuN4O8S2: C,
40.60; H, 3.95; N, 9.97, Cu, 11.31. Found: C, 39.92; H, 4.09; N,
10.06, Cu, 11.45%. IR (cmꢀ1): mC @N, 1476;
m
C–S, 758; mNO , 1379.
iment, 5 l
L of the EB Tris–HCl solution (1 mmol Lꢀ1) was added to
3
Magnetic moment (
l, B.M.): 1.81. Conductivity (
K
o, ohmꢀ1 cm2
1 mL of DNA solution (at saturated binding levels) [27], stored in
the dark for 2 h. Then the solution of the copper(II) complex was
titrated into the DNA/EB mixture and diluted in Tris–HCl buffer
to 5 mL to get the solution with the appropriate copper(II) com-
plex/CT-DNA molar ratio. Before measurements, the mixture was
shaken up and incubated at room temperature for 30 min. The
fluorescence spectra of EB bound to DNA, obtained at an emission
wavelength of 522 nm, were followed at the Hitachi-2000
fluorimeter.
molꢀ1) in acetonitrile: 245.
2.4. Preparation of [CuII(pic)2] (2)
Complex 2 was synthesized following a similar procedure as
stated in case of 1, by using copper(II) chloride in place of cop-
per(II) nitrate in equimolar ratio to the organic ligand L. The deep
green colored complex was precipitated out and the solid complex
was filtered, washed with water, cold methanol and finally dried in
vacuo. Yield: 76–78%.
2.8. X-ray crystal structure analysis
[Cu(pic)2] (2): Anal Calc. for C12H8CuN2O4: C, 46.83; H, 2.62; N,
9.10, Cu, 20.64. Found: C, 46.36; H, 2.72; N, 8.94, Cu, 20.54%. IR
The intensity data on a suitable crystal of 3 were collected at
173 K on a Stoe Mark II-Image Plate Diffraction System [28]
(cmꢀ1):
m
C@N, 1476. Magnetic moment (
l, B.M.): 1.81. Conductivity
(K
o, ohmꢀ1 cm2 molꢀ1) in acetonitrile: 105.
equipped with a two-circle goniometer using Mo K
monochromated radiation. Image plate distance 100 mm,
tion scans 0–180° at = 0°, step = 1.0°, exposures of 3 min
a
graphite
x
rota-
2.5. Preparation of [CuI2(L)2](ClO4)2 (3)
u
Dx
per image, 2h range 2.29–59.53°, dmin–dmax = 17.779–0.716 Å. The
structure was solved by direct methods, the refinement and all fur-
ther calculations were carried out using SHELX-97 [29]. The H-atoms
were included at calculated positions and treated as riding atoms
Complex 3 was synthesized using copper(II) perchlorate hexa-
hydrate and organic compounds in equimolar ratio. To a methano-
lic solution of the ligand L (0.338 g, 1.0 mmol), a solution of

S. Sarkar et al. / Polyhedron 29 (2010) 3157–3163
3159
using SHELXL default parameters. The non-H atoms were refined
anisotropically, using weighted full-matrix least-squares on F2. An
absorption correction was applied using the MULscanABS routine
in PLATON [30]; transmission factors: Tmin/Tmax = 0.675/0.865. The
crystallographic data and the ORTEP [31] drawing of the molecular
structure with atom numbering scheme are illustrated in Table 1
and Fig. 1 respectively.
3. Results and discussion
3.1. Ligands and complexes
The ligand 3,4-bis(2-pyridylmethylthio)toluene (L) was synthe-
sized by refluxing the reaction mixture of toluene-3,4-dithiol with
2-picolyl chloride in dry ethanol medium in presence of sodium
ethoxide. The organic product was extracted into dichloromethane
and the pure compound was obtained as yellow viscous liquid by
evaporating the organic solvent. This organic product was charac-
terized using spectroscopic tools. The 1HNMR spectrum of this vis-
cous liquid in CDCl3 showed the characteristic peaks of the
protons. The signals of the three aromatic dithiol toluene protons
of L are at different d values, 6.78 (d, 1H), 6.83(s, 1H), 7.03(d, 1H)
due to the methyl group. But a singlet signal for the four protons
of the two methylene groups was observed at d 4.39. One doublet
signal in the lowest region (d = 8.69) is attributable to the protons
in ortho position of the two pyridine rings.
Fig. 1. ORTEP view (50% probability ellipsoid) of only the cation of the complex 3
with atom labeling scheme of crystallographic independent atoms.
atoms, being border line donors, formerly coordinate the metal
ion by substituting two monodentate ligands, the soft thioether-S
donors in the second step, cannot substitute two other monoden-
tate ligands from the coordination sphere due to the less bonding
affinity of soft thioether-S with borderline copper(II) in the pres-
ence of chlorides. Thus in case of chloride salt the process of L coor-
dination is too slow. By this time, substituted chloride ion/aquo
molecule might attack the methylene centre, being this group
slightly positive due to the bonding of pyridinic-N to copper(II)
ion and as a result, the C–S bond cleavage is taking place. But, in
case of nitrate salt, the rate of coordination process is little bit fas-
ter than that in chloride and the nitrate ion is not good nucleophile
as chloride. Consequently, the C–S bond cleavage was not observed
and the formation of the expected mononuclear copper(II) complex
of L can occur.
Colorless dinuclear copper(I) complex 3 was obtained upon
reaction of L with copper(II) perchlorate hexahydrate at ambient
temperature and under aerobic conditions in methanol, while
green colored dinuclear copper(II) complex 4 was isolated in good
yield under same conditions in acetonitrile medium after the addi-
tion of chloride ions to the reaction medium. Here, the variation of
the reaction medium from methanol to acetonitrile revealed to be
crucial since complex 3 did not form in the latter solvent. Thus
methanol favors the reduction of copper(II) to copper(I), a reaction
usually not observed at aerobic condition. The process for the syn-
thesis of 3 is slow but faster than that observed with CuCl2 salt
where C–S bond cleavage was observed. However the addition of
chloride ions in the reaction medium of 3 and 4 did not induce
C–S bond cleavage of L.
The copper(I/II) complexes [CuII(L)(H2O)2](NO3)2 (1), [CuII(pic)2]
(2), [CuI2(L)2](ClO4)2 (3) and [CuII2(L)2Cl2](ClO4)2 (4) were obtained
in good yield from the reaction mixture of 3,4-bis(2-pyridylmeth-
ylthio)toluene (L) with the correspondent CuIIX2 salt (X = nitrate,
chloride and perchlorate) in equimolar ratio and in different sol-
vents as indicated in Scheme 1. Complex 1 was obtained by react-
ing L with copper(II) nitrate in methanol medium, whereas 2 was
isolated from the in situ formed picolinate through the C–S bond
cleavage of L during the reaction with cupric chloride. Complex 2
has been already reported and it was obtained by oxidative dealky-
lation of diethyl 2-pyridylmethyl-phosphonate and diethyl 2-quin-
olylmethylphosphonate (2-qmpe) ligands when reacting with
CuCl2 [32]. Here and in the cited ref. [31] the counter anion of
the reactant copper(II) salt plays a key role to help different
products.
The present observations might be explained on the basis of
HSAB principle [33] And it might be taken into account that chlo-
rides are more strongly bound in the coordination sphere of cop-
per(II) with respect to nitrate ions. Assuming that the pyridinic-N
Table 1
Crystal data and details of refinement data for complex 3.
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C38H36Cl2Cu2N4O8S4
1002.99
orthorhombic
F dd2
19.272(2)
36.886(4)
11.4258(17)
8122.2(17)
8
1.640
4096
2.1–25.7
1.442
10198
The purity and formulation of all the complexes were checked
by elemental analyses. The characterization of complex 2 was per-
formed by elemental analyses and X-ray crystallography, the latter
corresponding to the structure already reported [32]. Complex 1 is
soluble either in acetonitrile and DMF, while both dinuclear cop-
per(I/II) complexes 3 and 4 are soluble in DMF only. At room tem-
b (Å)
c (Å)
V (Å3)
perature the magnetic moment (l) of 1 is 1.80 B.M., that of 4 is
Z
1.44 B.M. per copper atom. The conductivity measurement of com-
qcalc (g/cm3)
F(0 0 0)
plex 1 in acetonitrile shows a conductance of 145
the values of 3 and 4 in DMF are 145 and 150
K
K
o molꢀ1 cmꢀ1
,
,
o molꢀ1 cmꢀ1
h range(deg)
l
(Mo Ka )
) (mmꢀ1
respectively, at 300 K, suggesting that complexes 1, 3 and 4 behave
in solution as 1:2 electrolytes.
Collected reflections
Independent reflections
3822
Observed data [I > 2.0
R1 [I > 2.0 (I)]
wR2 [I > 2.0 (I)]
r
(I)]
2537
0.0506
0.1096
r
3.2. Thermal analysis
r
Goodness-of-fit
0.888
0.637, ꢀ0.477
The thermal behavior of the complexes 1 and 4 was followed up
to 650 °C in a flowing atmosphere of dinitrogen. For complex 1,
Residuals (e Åꢀ3
)

3160
S. Sarkar et al. / Polyhedron 29 (2010) 3157–3163
Cl
SH
SH
S
S
N
NaOEt/EtOH
Reflux
N
+
N
L
[CuII(L)(H2O)2](NO3)2
1
Cu(NO3)2,3H2O
MeOH / Stir
CuCl2,2H2O
Cu(ClO4)2,6H2O
NaCl
[CuII(pic)2]
Mixture
[CuII (L)2Cl2](ClO4)2
L
2
MeOH / Stir
MeCN / Stir
4
2
Cu(ClO4)2,6H2O
MeOH / Stir
Catechol, MeCN/ Stir
[CuI2(L)2](ClO4)2
3
MeOH/Stir
Scheme 1. Ligand structure and synthetic route of the complexes.
TGA studies indicate that the first weight loss (7.1%) in the temper-
ature range of 84–105 °C is attributed to the removal of two coor-
dinated water molecules (Calc. 6.4%) [34]. Stable framework of
molecular formula has formed in the temperature range of 105–
312 °C with the weight of complex hardly loss. The second stage
has weight loss of 62.59% (calc. 62.92%) in the range of 312–
400 °C, corresponding to the loss of organic moiety. Further heat-
ing, the mass remains unchanged with a residual mass of 14.45%
corresponding to CuO percentage (Calc. 14.15%).
The complex 4 is stable up to 230 °C and then begins to decom-
pose. The TG curve indicates that the complex shows three-stage
decomposition process. The first one occurs in the temperature
range of 230–280 °C while the second stage is sharp endothermic
process in the range of 280–336 °C. The third and final stage leads
to the formation of cupric oxide upto 550 °C with the total mass
loss of 84.48% agrees well with the calculated mass loss of
85.19% and then the mass loss remains unchanged.
3.4. Structure description of Cu(I) complex 3
A perspective view of the molecular structure of complex 3 with
the atom numbering schemes is shown in Fig. 1, while selected
bond lengths and angles are reported in Table 2. The dinuclear
complex 3 resides about a two-fold axis and the crystallographic
independent Cu(I) ion presents an irregular pseudo-tetrahedral
geometry arising from the restricted bite angles of the chelating li-
gand (Table 2) that bridges the two metal ions in such a way that
the thioether-S atoms and one pyridyl-N donors chelate one cop-
per ion, the other pyridyl-N donor being connected to the other
metal.
The Cu–S bond distances are almost comparable in length
(2.302(2), 2.355(2) Å), while the Cu–N ones differ by ca. 0.13 Å
(2.107(5) 1.974(5) Å), and the shorter distance refers to the py ring
that appears conformationally more free (Cu–N2). This feature,
indicative of a strain of L upon coordination, is also reflected by
the coordination bond angles that are close to 90° between the
chelating donors, and in the range 113.26(15)–131.16(15)° for
the angles involving N2. The Cu–Cu separation is of 4.331 Å ex-
cludes any interaction between the metals.
3.3. Conversion of complex 4 to complex 3
By stirring for 12 h, the green colored methanolic solution of
complex 4 became fade, and after several days the colorless crys-
talline complex 3 was obtained. On the other hand, in acetonitrile
the green solution of 4 changed to almost colorless within 1–2 h
after the addition of a catalytic amount of catechol, obtaining the
colorless precipitate of copper(I) complex 3 as indicated in the fol-
lowing equation.
Two mononuclear copper(II) complexes have been reported
containing a py-C-S-(CH2)2-S-C-py fragment, namely the tetrache-
lating ligand [CuI(1,2-bis(20-quinolylmethylthio)ethane)2] [35] and
[CuII(1,2-bis(2-pyridylmethylthio)ethane)2Cl] [20]. Although the S-
(CH2)2-S moiety is expected to be more flexible than the S-toluene-
S chelating fragment of 3, the Cu–S bond distances are slightly
Stir in MeOH
½CuIIðLÞ Cl ꢂðClO Þ ð4Þ
½CuI ðLÞ ꢂðClO Þ ð3Þ
ƒƒƒƒƒƒƒƒƒƒƒƒƒ!
2
4
4
2
2
2
2
2
2
or Chatechol=CH3CN=Stir
Table 2
The above observations indicate that the solvent methanol
plays a key role for the reduction of copper(II) to copper(I) species
in presence of the ligand L. It is worth noting that the reduction
process was observed in aerobic condition and not under inert
atmosphere. In the control experiment the solvent methanol did
not convert copper(II) perchlorate into its corresponding copper(I)
species in absence of 3,4-bis(2-pyridylmethylthio)toluene. There-
fore the reduction of the metal is ligand assisted, similarly to that
already reported [35].
Selected coordination bond distances (Å) and angles (o) for 3.
Bond lengths
Cu–S(1)
Cu–S(2)
2.302(2)
2.355(2)
Cu–N(1)
Cu–N(2i)
2.107(5)
1.974(5)
Bond angles
S(2)–Cu–(1)
S(1)–Cu–(1)
S(1)–Cu–(2)
86.35(16)
92.83(14)
92.02(7)
S(2)–Cu–N(2i)
N(1)–Cu–N(2i)
S(1)–Cu–sN(2i)
113.26(15)
128.1(2)
131.16(15)
Symmetry code (i) ꢀx + 1/2, ꢀy + 1/2, z.

S. Sarkar et al. / Polyhedron 29 (2010) 3157–3163
3161
longer (mean value 2.372 and 2.436 Å, respectively) while the Cu–
N ones (mean values 1.988, 2.000 Å) are close to the short value ob-
served in 3 (Cu–N2). Moreover copper(II) complexes containing
1,2-bis(benzimidazol-2-ylmethylthio)benzene [36] and 3,4-bis(2-
aminoethylthio)toluene [37] have been also reported. The coordi-
nation geometry around the metal is trigonal bipyramidal in the
former case, and distorted octahedral and distorted square pyrami-
dal in the two independent ions of other, while N donors are al-
ways located in trans positions. In these cases the Cu-S bond
distances are even longer (range 2.445–2.609 Å) than those mea-
sured in 3, indicative of steric constraints inside the tetrachelating
ligand upon coordination.
3.5. Spectral studies
In the IR spectrum of 1 a broad band around at 3480 cmꢀ1 and
Fig. 2. Electronic spectral titration of complex 4 with CT-DNA at 266 nm in Tris–HCl
buffer; [complex] = 1.3 ꢃ 10ꢀ5; [DNA]: (a) 0.0, (b) 2.0 ꢃ 10ꢀ6, (c) 4.0 ꢃ 10ꢀ6, (d)
6.0 ꢃ 10ꢀ6, (e) 8.0 ꢃ 10ꢀ6, (f) 1.0 ꢃ 10ꢀ5 mol Lꢀ1. Arrow indicates the increase of
DNA concentration.
an intense band centered at 1380 cmꢀ1 are attributable to the coor-
dinated water and to the to mNO strectching frequency, respec-
3
tively. In the infrared spectra of 3 and 4 a strong absorption at
ca. 1080 cmꢀ1 along with
a weak absorption band at ca.
624 cmꢀ1 are due to the symmetric and asymmetric stretching fre-
quency, respectively, for perchlorate ion. In the IR spectra of all the
complexes, bands at 1469–1478 cmꢀ1 for asymmetric mC@N and
758–761 cmꢀ1 for mC–S stretching were detected. Differently from
the spectra of the other complexes, it is noteworthy the intense
complex, respectively, and ea the copper(II) complex extinction
coefficient during each addition of DNA. The [DNA]/(ea
ꢀ
ef) plot
against [DNA] gave a linear relationship (Fig. 3), from the slope of
which the intrinsic binding constant Kb for the complex 4 was cal-
culated to be 2.21 ꢃ 105 Mꢀ1 (R = 0.99338 for five points). This va-
lue is comparable well with that of the well-established groove
binding rather than classical intercalator agent [44].
ꢀ
band at 1645 cmꢀ1 attributable to mCOO observed in the IR spec-
trum of 2.
The electronic absorption spectra of complexes 1, 3 and 4 were
recorded at room temperature using DMF as solvent and the data
are tabulated in Table 3. Each complex shows an absorption in
the range from 360 to 450 nm, which is not observed for the corre-
sponding free ligand. This suggests that the absorption bands of all
complexes are assignable to the LMCT transition from the copper
center to the pyridyl-N of organic moiety [38]. Transition at high
Ethidium bromide (EB) fluorescence displacement experiments
were also performed in order to investigate the interaction mode of
4 with CT-DNA. EB has been used as probe the interaction of the
complex with DNA. In fact the EB fluorescence intensity will be en-
hanced in the presence of DNA because of its intercalation into the
helix, and it is quenched by the addition of another molecule that
displaces EB from DNA [45]. Here, a significant decrease of the fluo-
rescence intensity of EB bound to DNA at 522 nm was recorded by
increasing the concentration of 4 (Fig. 4).
This observation of EB fluorescence quenching leads us to de-
duce that the copper(II) complex may interact with DNA through
the groove binding mode, releasing some EB molecules from the
EB-DNA system [46]. The quenching of EB bound to DNA by the
copper(II) complex is in agreement with the linear Stern–Volmer
equation:
energy region corresponds to intramolecular
p ? p* and n ? p*
transitions [39]. The characteristic d–d absorption band have been
also observed at ca. 650 nm for 1 and ca. 682 nm for 4. For complex
3 no broad d-d absorption band was observed in the range 600–
680 nm, due to the tetrahedral geometry of copper (I) present also
in solution for this complex [40,41].
3.6. DNA binding studies
I0=I ¼ 1 þ Ksv½complexꢂ
The binding mode of complex 4 with CT-DNA was examined by
using absorption and emission spectra. The absorption spectra of
the copper(II) complex 4, were recorded during the titration with
calf thymas DNA. As shown in Fig. 2, the titration indicated a
significant hyperchromism effect of the absorption spectra, sug-
gesting that there exists a interaction between the copper(II) com-
plex and DNA. From this spectral change it can be said that the
binding mode of copper(II) complex is groove binding [42].
The spectral titration data allows determining the intrinsic
binding constant Kb of 4 with CT-DNA by using the following equa-
tion [43]:
where I0 and I represent the fluorescence intensities in the absence
and presence of quencher, respectively. Ksv is the linear Stern–Vol-
mer quenching constant and [complex] the concentration of the
½DNAꢂ=ðea
ꢀ ef Þ ¼ ½DNAꢂ=ðeb ꢀ ef Þ þ 1=½Kbðeb ꢀ ef Þꢂ
where [DNA] is the DNA concentration, ef and eb represent the
extinction coefficients for the free and fully bound copper(II)
Table 3
UV–Vis spectral data in DMF.
Compds
k nm (e) (e )
, dm3 molꢀ1 cmꢀ1
1
3
4
273 (s, 8,442), 362 (s, 3,076), 650 (br, 238)
263 (s, 8,302), 378 (s, 3,045),
266(s, 8,532), 364 (s, 3,474), 682 (br, 218),
Fig. 3. Plot of [DNA]/(ea
complex 4 in Tris–HCl buffer.
ꢀ ef) vs. [DNA] for the absorption titration of CT-DNA with
s for strong; br for broad.

3162
S. Sarkar et al. / Polyhedron 29 (2010) 3157–3163
5. Supporting data
Crystallographic data for compound 3 have been deposited with
the Cambridge Crystallographic Data Centre, CCDC No. 778243.
Copies of this information is 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 Department of Science and Technology
(DST), New Delhi is gratefully acknowledged. E. Zangrando thanks
MIUR-Rome, PRIN N° 2007HMTJWP_002 for financial support.
Fig. 4. Emission spectra of the CT-DNA–EB system in Tris–HCl buffer upon the
References
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