A. Patra et al. / Polyhedron 30 (2011) 2783–2789
2785
direct methods and subsequent Fourier analyses, and refined by
the full-matrix least-squares method based on F2 with all observed
reflections [21]. The contribution of hydrogen atoms at calculated
while that of 2 was 61 Ko molꢀ1 cmꢀ1 at 300 K. These values sug-
gest that all the complexes (1 and 2) exist as non-electrolytes. The
observation of the non-electrolytic nature of the complexes 1a–d
are comparable with the reported neutral [Ni(L)(ONO2)2] deriva-
tive [26].
positions (except those of the aqua ligand in 2 detected in the
DF
map) were included in the final cycles of refinement. All the calcu-
lations were performed using WinGX System, Ver. 1.80.05 [22].
3.2. Structure of complexes 1d and 2
2.5. DNA binding experiments
The ORTEP views of 1d and 2, with the atom numbering
schemes, are illustrated in Figs. 1 and 2, respectively, and a selec-
tion of bond distances and angles are listed in Table 2.
In the mononuclear complex 1d the organic moiety L behaves as
a tetradentate ligand, chelating the metal in such a way as to locate
the pyridine rings in trans positions, and the metal ion completes
the highly distorted octahedral coordination sphere with two cis
located isothiocyanato anions. The configuration is similar to the
complexes [Fe(L)Cl2] and [Ni(L)Cl2] reported by these laboratories
[19].
The Zn–N(py) and Zn–NCS bond lengths are comparable within
their e.s.d.’s (mean values 2.142(5) and 2.040(5) Å, respectively),
while the Zn–S distances differ by ca. 0.02 Å, being of 2.6500(18)
and 2.6990(17) Å. The bond angles subtended at the metal deviate
considerably from the ideal octahedral values and the N(1)–Zn–
N(2) and N(2)–Zn–S(2) angles of 161.28(17)° and 77.57(13)°,
respectively show the highest deviation. In fact the pyridine rings
of the ligand are bent with respect to the S2N2 plane, forming a
dihedral angle of 82.3(1)°, induced by ligand constraints upon
coordination, while the dihedral angle made by the trans located
pyridine mean planes is 52.2(2)°.
Tris–HCl buffer solution was used in all the experiments involv-
ing CT-DNA. This Tris–HCl buffer (pH 7.2) 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 approximately 1.9 [23]. The
concentration of DNA was estimated with the help of the extinc-
tion coefficient (6600 Mꢀ1 cmꢀ1) of the DNA solution at 260 nm
[24] and the stock solution of DNA was always stored at 4 °C.
The stock solution of the zinc(II) complex was prepared by adding
2 mL DMSO for dissolving and it was diluted with Tris–HCl buffer
to get the required concentration for all the experiments. The
absorption spectral titration experiment was performed by keep-
ing the concentration of the zinc(II) complex constant and varying
the CT-DNA concentration. To eliminate the absorbance of DNA
itself, an equal solution of CT-DNA was added to the reference
solution.
In the ethidium bromide (EB) fluorescence displacement exper-
iment, 5 l
L of the EB Tris–HCl solution (1 mmol Lꢀ1) was added to
1 mL of the DNA solution (at saturated binding levels) [25], stored
in the dark for 2 h. Then a solution of the zinc(II) complex was
titrated into the DNA/EB mixture and diluted in Tris–HCl buffer
to 5 mL to get a solution with the appropriate complex/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 with the emission
wavelength of 610 nm (kex = 522 nm) in the Fluorimeter (Hitachi-
2000).
The Zn–N–C coordination bond angles involving the thiocya-
nate are slightly different, with values of 159.4(5)° and 165.7(5)°,
the difference might likely be adopted to favor sulfurꢃ ꢃ ꢃ
tions. In fact the crystal packing of complex 1d shows a 1D poly-
interactions along the a-axis
p interac-
meric arrangement built by S(4)ꢃ ꢃ ꢃ
p
(Fig. 3). Labeling Cg as the centroid of the pyridine N(1) at ꢀ1 + x,
y, z, the Sꢃ ꢃ ꢃCg distance is 3.902(4) Å, with a C(17–S(4)ꢃ ꢃ ꢃCg angle
of 108.0(2)°.
The solid state structure of complex 2 demonstrates that the
organic moiety L behaves as bridging ligand between two zinc ions,
3. Results and discussion
3.1. Synthesis and characterization
The organic ligand 1,3-bis(2-pyridylmethylthio)propane (L) was
prepared by the reaction of the propane dithiol compound with
2-picolyl chloride in the presence of sodium ethoxide. In the com-
plexes, the organic molecule L acts as a tetradentate neutral ligand
through N2S2 donor centers. The complexes 1a–d were obtained in
good yield from the reaction of zinc(II) acetate with an equimolar
amount of the organic moiety in the ethanol medium under reflux-
ing conditions, followed by the addition of an ethanol–water solu-
tion of sodium chloride, sodium cyanate or potassium thiocyanate
to the reaction mixture under cold conditions (viz. Scheme 1). In
this context it is worth mentioning that complex 1a could not be
obtained by using zinc(II) chloride in place of the acetate salt in
the first step, and no significant reaction of L with zinc(II) nitrate
was observed in this study. On the contrary, complex [Zn(L)(ClO4)
(H2O)](ClO4) (2), was obtained only when zinc(II) perchlorate was
allowed to react with L. The crystal structure of 2 shows the ligand
L acting as a bridging ligand, giving origin to a 1D coordination
polymer.
The neutral complexes 1a–d are soluble in DMF, but are only
sparingly soluble in acetonitrile and methanol, while the mono-
cationic complex 2 is soluble both in acetonitrile and DMF. The
conductivity measurements in DMF showed that the conductance
Fig. 1. An ORTEP diagram of the complex [Zn(L)(SCN)2] (1d) with the atom
numbering scheme.
of the complexes 1a–d were in the range 32–46 Ko molꢀ1 cmꢀ1
,
Patra, Animesh
