Nickel(II) complexes with 2-(pyridin-3-ylmethylsulfanyl)phenylamine and halide/pseudohalides: Synthesis, structural characterisation, interaction with CT-DNA and bovine serum albumin, and antibacterial activity

Article abstract of DOI:10.1016/j.poly.2012.12.031

A new series of hexacoordinated octahedral nickel(II) complexes of 2-(pyridin-3-ylmethylsulfanyl)phenylamine (L) formulated as [Ni(L) ₄(X)₂] (1-4) [where X = Cl^- (1); NCO ^- (2); N₃^- (3) and

Full text of DOI:10.1016/j.poly.2012.12.031

Polyhedron 51 (2013) 156–163
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Nickel(II) complexes with 2-(pyridin-3-ylmethylsulfanyl)phenylamine
and halide/pseudohalides: Synthesis, structural characterisation, interaction
with CT-DNA and bovine serum albumin, and antibacterial activity
Animesh Patra a, Buddhadeb Sen a, Sandipan Sarkar b, Akhil Pandey c, Ennio Zangrando d,
Pabitra Chattopadhyay a,
⇑
a Department of Chemistry, Burdwan University, Golapbag, Burdwan 713104, India
b Department of Applied Science and Humanities, Durgapur Institute of Advanced Technology & Management, Rajbandh, Durgapur 713212, India
c Department of Microbiology, Midnapore College, Midnapore 721101, India
d 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:
A new series of hexacoordinated octahedral nickel(II) complexes of 2-(pyridꢀin-3-ylmethylsulfanyl)phenyl-
amine (L) formulated as [Ni(L)4(X)2] (1–4) [where X = Clꢀ (1); NCOꢀ (2); N3 (3) and NCSꢀ (4)] has been
synthesised and characterised by physicochemical, spectroscopic tools. Details of structural study of
complex 1 using single crystal X-ray crystallography showed that distorted tetragonal environment
around nickel(II) ion has been satisfied by four pyridinic-N donors of four organic moieties (L) and two
chloride ions. All the complexes are redox active and the electrochemical study of the complexes showed
only cathodic NiII/NiI redox couples in the range of ꢀ0.61 to ꢀ695 V versus Ag/AgCl. Interactions of 1
towards calf thymus-DNA by spectroscopic, viscosity-measurement and electrochemical study and
towards bovine serum albumin (BSA) with the help of absorption and fluorescence spectroscopy were
examined. Antibacterial activity of the complexes (1–4) studied by agar disc diffusion method showed
the comparable inhibition activity of the nickel(II) complexes against some pathogenic bacteria namely
Escherichia coli, Vibrio cholerae, Streptococcus pneumonia, Shigella sp. and Bacillus cereus.
Received 4 December 2012
Accepted 31 December 2012
Available online 9 January 2013
Keywords:
Nickel(II) complex
Crystal structure
DNA and BSA binding
Antibacterial activity
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Aspart of our continuous intereston nitrogen–sulfur polydentate
chelators [10,11], here we report an account of nickel(II) complexes
A considerable amount of attention is currently being shown in
the synthesis of geometrically distorted nickel(II) complexes with
mixed nitrogen and sulfur donor sets [1–4]. Nickel is present in
the active sites of several important classes of metalloproteins, as
either a homodinuclear or a heterodinuclear species. The active site
of 2-mercaptoethanol-inhibited urease [5] contains two Ni centres
bridged by thiolate donors, while thiolate bridging between Ni and
Fe centres is present in the Ni/Fe hydrogenases [6–9]. The nickel
coordination sphere in both of these metalloenzyme systems con-
tains N and S donor atoms in unusual 5- or 6-coordinate arrange-
ments with significant distortions from regular geometry. These
distorted configurations often give rise to Ni centres with revers-
ible Ni(II)/Ni(I) and Ni(III)/Ni(II) couples and low Ni(III)/Ni(II) redox
potentials, characteristics which are crucial to the activity of the
enzymes. These unusual structural and electronic features have
led to increased interest in the synthesis of Ni (II) complexes with
mixed N,S donating chelates as structural and spectroscopic mod-
els of the active sites.
of 2-(pyridin-3-ylmethylsulfanyl)-phenylamine (L) (vide Scheme 1).
Four hexacoordinated octahedral nickel(II) complexes of 2-(pyri-
din-3-ylmethylsulfanyl)-phenylamine (L) formulated as [Ni(L)4(X)2]
(1–4) [where X = Clꢀ (1); NCOꢀ (2); N3ꢀ (3) and NCSꢀ (4)] were iso-
lated using different the nickel(II) salts used as reactant, and charac-
terised by physicochemical and spectroscopic tools along with the
detailed structural characterisations of 1 by X-ray crystallography.
The DNA and protein binding study of the nickel(II) complex (1)
has been performed spectroscopically. Antibacterial activity of com-
plexes (1–4) studied by agar disc diffusion method showed the com-
parable inhibition activity of the nickel(II) complexes against some
pathogenic bacteria namely Escherichia coli, Vibrio cholerae, Strepto-
coccus pneumonia, Shigella sp. and Bacillus cereus.
2. Experimental
2.1. Materials and physical measurements
All chemicals and reagents were obtained from commercial
sources and used as received, unless otherwise stated. Solvents
were distilled from an appropriate drying agent. The elemental
⇑
Corresponding author.
E-mail address: pabitracc@yahoo.com (P. Chattopadhyay).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.12.031

A. Patra et al. / Polyhedron 51 (2013) 156–163
157
NiCl2, 6H2O
added and stirring was continued for another 1 h. The volume of
the solution was reduced at room temperature by slow evapora-
tion. The product was collected by washing with cold methanol
and water; and dried. The pure crystallised product was obtained
from methanol.
Complex 1 was also prepared by refluxing the mixture of 2-
(pyridin-3-ylmethylsulfanyl)-phenylamine (L) (864.0 mg, 4.0 mmol)
and nickel(II) chloride, hexahydrate (238.0 mg, 1.0 mmol) in meth-
anol for 4 h. The product was collected by filtration and washing
with cold methanol and water, and dried.
[Ni(L)4Cl2]
MeOH, Reflux
1
S
N
NH2
NaCl / Stir
Ni(OAc)2, 4H2O
MeOH, Reflux
NaNCO
Mixture
L
[Ni(L)4(NCO)2]
Stir
2
Stir
NaN3 / Stir
NaSCN
[Ni(L)4(N3)2]
[Ni(L)4(SCN)2]
Complex 1: [Ni(L)4(Cl)2]: C48H48N8S4Ni1Cl2: Anal. Calc: C, 57.83;
H, 4.74; N, 11.16; Ni, 5.82. Found: C, 57.94; H, 4.82; N, 11.26; Ni,
3
4
5.89%. IR (cmꢀ1):
mC@N, 1478; mC–S, 752. Magnetic moment (l,
Scheme 1. Synthetic strategy of the complexes.
B.M.): 3.10. Conductivity (Ko, ohmꢀ1 cm2 molꢀ1) in DMF: 44. Yield
80–85%.
(C, H, N) analyses were performed on a Perkin Elmer model 2400
elemental analyzer. Nickel analysis was carried out by Varian
atomic absorption spectrophotometer (AAS) model-AA55B, GTA
using graphite furnace. Electronic absorption spectra were re-
corded on a JASCO UV–Vis/NIR spectrophotometer model V-570.
IR spectra (KBr discs, 4000 to 300 cmꢀ1) were recorded using a Per-
kin-Elmer FTIR model RX1 spectrometer. The room temperature
magnetic susceptibility measurements were performed by using
a vibrating sample magnetometer PAR 155 model. Molar conduc-
tances (KM) were measured in a systronics conductivity meter
304 model using ꢁ10ꢀ3 mol Lꢀ1 solutions in appropriate organic
solvents. Electrochemical measurements were performed using
computer-controlled CH-Instruments (Model No. – CHI620D). All
measurements were carried out under nitrogen environment at
298 K with reference to SCE electrode in dimethyl sulfoxide using
[n-Bu4N]ClO4 as supporting electrolyte. The fluorescence spectra
of EB bound to DNA were recorded in the Fluorimeter (Hitachi-
4500).
Complex 2: [Ni(L)4(NCO)2]: C50H48N10Ni1S4O2: Anal. Calc: C,
59.54; H, 4.72; N, 13.82; Ni, 5.74. Found: C, 59.60; H,4.76; N,
13.90; Ni, 5.82%. IR (cmꢀ1):
m
C@N, 1480;
, B.M.): 3.06. Conductivity (Ko, ohmꢀ1 cm2 molꢀ1
in DMF: 42. Yield 75–80%.
mC–S, 756, mNCO, 2184. Mag-
netic moment (
l
)
Complex 3: [Ni(L)4(N3)2]: C48 H48 N14S4Ni1: Anal. Calc.: C, 57.26;
H, 4.70; N, 19.42; Ni, 5.78. Found: C, 57.22; H, 4.76; N, 19.47; Ni,
5.82. IR (cmꢀ1):
m
C@N, 1479;
, B.M.): 3.08. Conductivity (Ko, ohmꢀ1 cm2 molꢀ1) in DMF: 48.
Yield 80–85%.
mC–S, 750, mN3, 2049. Magnetic moment
(l
Complex 4: [Ni(L)4(SCN)2]: C50H48N10S6Ni1: Anal. Calc: C, 57.72;
H, 4.64; N, 13.41; Ni, 5.58. Found: C, 57.77; H, 4.62; N, 13.47; Ni,
5.64%. IR (cmꢀ1):
m
C@N, 1478;
mC–S, 754, mNCS, 2081. Magnetic mo-
ment (
l
, B.M.): 3.09. Conductivity (Ko, ohmꢀ1 cm2 molꢀ1) in
DMF: 40. Yield 75–85%.
2.4. X-ray crystal structure analysis
Crystal data and details of data collection and refinement for
complex 1 was summarised in Table 1. Suitable single crystals for
X-ray diffraction analysis of 1 were grown at ambient temperature
by slow evaporation of a methanolic solution. Diffraction data of 1
was collected at room temperature on a Nonius DIP-1030H system,
2.2. Preparation of 2-(pyridin-3-ylmethylsulfanyl)phenylamine (L)
An ethanolic solution of 3-chloromethylpyridine, hydrochloride
(3.28 g, 20 mmol) was added to 2-aminobenzenethiol (2.5 g,
20 mmol) in dry ethanol containing sodium ethoxide which is pre-
pared by dissolving sodium (1.0 g, 43.4 mmol) in dry ethanol
(25 mL) under cold conditions (0–5 °C). Then this mixture was al-
lowed to stir at room temperature for 0.5 h and then it was re-
fluxed for 3 h. The mixture was cooled to room temperature and
then it was concentrated by rotary evaporation and extracted into
dichloromethane (2 ꢂ 50 mL). The combined organic phases were
washed with H2O and then dried by anhydrous MgSO4, and the
solvent CH2Cl2 was removed by rotary evaporation. The product,
by using Mo Ka radiation (k = 0.71073 Å). Cell refinement, indexing
and scaling of the data set were performed using programs DENZO and
SCALEPACK [12]. The structure was solved by direct methods and subse-
quent Fourier analyses and refined by the full-matrix least-squares
method based on F2 with all observed reflections [13]. The contribu-
tion of hydrogen atoms at calculated positions were included in final
cycles of refinement. All the calculations were performed using the
WINGX System, Ver. 1.80.05 [14].
2-(pyridin-3-ylmethylsulfanyl)phenylamine was obtained as
a
2.5. DNA binding experiments
brownish yellow oil (3.1 g, 72%), which was subsequently purified
by vacuum distillation for spectroscopic characterisation.
1H NMR (d, in CDCl3): 8.52 (s, 1H on py), 8.48 (d, 1H on py), 7.58
(d, 1H on py), 7.19 (t, 1H on py), 6.95 (m, 2H), 6.55 (m, 2H), 4.31
(broad, NH2) and 3.99 (s, 2H). MS-EI+: m/e, 216 (corresponds to
M+).
Tris–HCl buffer (pH 7.0) solution prepared using deionised and
sonicated HPLC grade water (Merck) was used in all the experi-
ments involving CT-DNA. 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 ꢃ1.9 [15]. The concentration of DNA was determined
with the help of the extinction coefficient of DNA solution [16].
Absorption spectral titration experiment was performed by keep-
ing constant the concentration of the nickel(II) complex and vary-
ing the CT-DNA concentration.
2.3. Preparation of [Ni(L)4(X)2] complexes (1–4)
The complexes were synthesised following a common proce-
dure as described below. To a methanolic solution of nickel(II) ace-
tate, tetrahydrate (249.0 mg, 1.0 mmol) was added to the solution
of the organic compound (L) (864.0 mg, 4.0 mmol) in methanol
(10 mL) in stirring condition at room temperature. The resulting
mixture was refluxed for 3 h. To this solution aqueous solution of
sodium chloride (117.0 mg, 2.0 mmol) (for 1), sodium cyanate
(130.0 mg, 2.0 mmol) (for 2), sodium azide (130.0 mg, 2.0 mmol)
(for 3) and sodium thiocyanate (162.0 mg, 2.0 mmol) (for 4) was
In the ethidium bromide (EB) fluorescence displacement exper-
iment, 5 l
L of the EB tris–HCl solution (1.0 mmol Lꢀ1) was added to
1.0 mL of DNA solution (at saturated binding levels) [17], stored in
the dark for 2.0 h. Then the solution of the nickel(II) complex was
titrated into the DNA/EB mixture and diluted in tris–HCl buffer to
5.0 mL to get the solution with the appropriate Ni(II) complex/CT-
DNA mole ratio. Before measurements, the mixture was shaken up

158
A. Patra et al. / Polyhedron 51 (2013) 156–163
Table 1
bated at 37 °C for 24 h. After incubation plates were observed for
the growth of inhibition zones. The diameter of the zone of inhibi-
Crystal data and details of refinement data for complex 1.
tion was measured in millimeters.
Complex 1
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C48H48Cl2N8NiS4
3. Results and discussion
994.79
tetragonal
I4
13.924(3)
13.924(3)
12.523(3)
90.00
90.00
90.00
2427.9(9)
2
1.640
1036
2.19–29.60
0.724
14493
293(2)
3.1. Synthesis and characterisation
b (Å)
c (Å)
The organic compound, 2-(pyridin-3-ylmethylsulfanyl)-phenylamine
(L) was synthesised by the reaction of 2-aminothiophenol and
3-chloromethylpyridine in presence of sodium ethoxide. The
product obtained by this procedure even before distillation is of
a high purity enough for the subsequent synthetic steps. The
identity and purity were verified using spectroscopic tools. The
complexes (1–4) were obtained in good yield from the reaction
of the nickel(II) acetate, tetrahydrate with four equimolar amount
of organic moiety in the methanol medium followed by the
addition of respective aqueous solution of chloride/pseudoha-
lides to the reaction mixture. Complex 1 can also be prepared
directly by refluxing the mixed solution of nickel(II) chloride
and L in 1:4 mol ratio.
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
qcalc (g/cm3)
F(000)
h Range (°)
l
(Mo Ka )
) (mmꢀ1
Independent reflections
Temperature (K)
Observed data [I > 2.0
R1
r
(I)]
2892
0.0370, 0.0518
0.0983, 0.1074
0.948
wR2
Goodness-of-fit (GOF)
In these complexes (1–4), the ligand (L) acts as a monodentate
neutral N donor ligand since only the pyridinic-N atom coordinates
with central nickel(II) ion keeping amine and sulfur atom uncoor-
dinated, though L has three donor (NNS) centres. All the complexes
are soluble in common organic solvents and are polymeric in nat-
ure through H-bonding. Conductivity measurement of the com-
plexes in dimethyl sulfoxide showed conductance values in range
of 40–48 (Ko, ohmꢀ1 cm2 molꢀ1) which suggests that all complexes
and incubated at room temperature for 30 min. The fluorescence
spectra of EB bound to DNA were obtained at an emission wave-
length of 522 nm in the Fluorimeter (Hitachi-4500).
To adjudge the binding mode (groove/intercalative) of 1 with
DNA, the well known method using Ostwald’s viscometer was
used. Titrations were performed by introducing nickel(II) complex
(1) (0.5–3.5 lM) into a CT-DNA solution (5.0 lM) present in the
viscometer. From the observed flow time of CT-DNA-containing
solution corrected from the flow time of buffer alone (t0),
are non-electrolytes. The magnetic moments (l) at room tempera-
ture of these complexes are 3.10, 3.06, 3.08 and 3.09 which indi-
cate all complexes are high spin distorted octahedral geometry.
g
= t ꢀ t0, the calculated viscosity values of the solutions were used
1/3
for plotting the (
g
/g0)
versus the ratio of the concentration of 1
3.2. Structure of complex 1
and CT-DNA, where
g
is the viscosity of CT-DNA in the presence
of the compound and g0 is the viscosity of CT-DNA alone.
The crystal structure of [Ni(L)4Cl2] is shown in Fig. 1 together
with the atomic labelling scheme for all of the atoms except the
hydrogens. The crystallographic data and details of the structure
determinations are given in Table 1 while selected bond lengths
and bond angles appear in Table 2.
2.6. Protein (bovine serum albumin) binding experiments
The sample was placed in quartz cuvettes of 1 cm optical path.
In these experiments, 3 mL of BSA solution were poured into the
cell. The samples were carefully degassed using pure nitrogen
gas for 15 min. Emission spectra were recorded after addition of
the appropriate concentration of the nickel(II) complexes in the
same buffer. The samples were excited at 280 nm.
The complex 1 exists as an octahedral mononuclear structure.
Here, the nickel(II) ion is located on a fourfold axis with a symme-
try plane and exist as a distorted tetragonal environment with li-
gand L and two trans chloride anions. The ligand L only acts as a
monodentate fashion through the pyridine nitrogen atom, while
the amine group remains uncoordinated like a tail. In the equato-
rial plane, each nickel(II) ion is coordinated by four pyridinic nitro-
gen atoms and two chloride anions in axial of the octahedron in
which the bond lengths of Ni(II)–N(1), Ni(II)–Cl(1) and Ni(II)–
Cl(2) are (2.13), (2.44) and (2.45) Å, respectively. This bond length
also indicates that the high electron density of four pyridinic-N
atom form strong bond with nickel-atom but two trans Clꢀ atom
have comparatively lower electron density form weak bond with
nickel-atom and two Ni–Cl distances are comparable. It is noted
that the uncoordinated amine groups are connected as a 2D layer
structure through intermolecular N–Hꢄ ꢄ ꢄN–H hydrogen bonding
interactions (Fig. 2), in which the bond distance of N–N is (3.12 Å).
Crystal packing of the complex 1 shows a polynuclear arrange-
ment through the interaction between chloride atom and amino
groups (Fig. 3), down axis showing N(2)–Cl(1) distances are of
3.64 Å.
2.7. In vitro antibacterial assay
The biological activities of synthesised ligand (L) and its nicke-
l(II) complexes have been studied for their antibacterial activities
by agar well diffusion method [18–21]. The antibacterial activities
were done at 100 lg/mL concentrations of different compounds in
DMF solvent by using four pathogenic gram negative bacteria
(E. coli, V. cholerae, S. pneumoniae, Shigella sp.) and one gram posi-
tive pathogenic bacteria (B. cereus). Stock cultures of the test bac-
terial species were maintained on Nutrient Agar media by sub
culturing in slants. The media were prepared by adding beef ex-
tract 3 g, peptone 5 g, agar 15 g, distilled water 1000 mL and the
pH was adjusted at 7.2. Media was sterilised in the autoclave at
15 lb pressure for 20 min. 20 mL of media was poured in each Petri
dish and allowed to solidify. After solidification, nutrient agar
plates were swabbed by sterile cotton swabs with 12 h old
0.1 mL broth culture of respective bacteria. The wells were bored
with cork-borer and the agar plugs were removed. Then solution
of ligand and its nickel(II) complexes were added to the agar wells.
DMF was used as a negative control. The Petri dishes were incu-
3.3. Spectral studies
The infrared spectral data of all the complexes exhibit charac-
teristic strong to medium intensity band in the region of 1468–

A. Patra et al. / Polyhedron 51 (2013) 156–163
159
The electronic absorption spectra of the complexes 1 and 2–4
were recorded at room temperature using DMF as solvent and
the data are tabulated in Table 3. Each complex shows an absorp-
tion ranging from 350 to 450 nm, which was not observed for the
corresponding free ligand. It suggests that the absorption bands of
all complexes are assignable to the LMCT transition from the pyrid-
inic-N of organic moiety to the Nickel centre. All the spectra of
complexes bands lower than 400 nm are due to intramolecular
p
?
p⁄ and n ? p⁄ transitions for the aromatic ring. In octahedral
nickel(II) complexes, three spin allowed transitions are expected
from the energy level diagram for d8 ions due to 3A2g ? 3T1g (P),
3A2g ? 3T1g(F), 3A2g ? 3T2g transitions, which are observed at low
to high wavelengths, respectively. The bands at 424 and 687 nm,
which may be assigned to 3A2g ? 3T1g(P) and 3A2g ? 3T1g(F) transi-
tion, respectively. Again the intensity of the peak at around 850 nm
observed due to the 3A2g ? 3T2g transition. These observations sug-
gest an octahedral geometry of the nickel(II) ion.
Fig. 1. An ORTEP diagram of Ni complex 1 located on a fourfold axis.
3.4. Electrochemistry
The electrochemical properties of the complexes (1–4) were
examined by cyclic voltammeter using a Pt-disk working electrode
and a Pt-wire auxiliary electrode in dry dimethylformamide using
0.1 M [n-Bu4N]ClO4 as the supporting electrolyte. The voltammet-
ric parameters were studied in the scan rate interval 50–
400 mV sꢀ1. From the voltammetric data given in Table 3, it was
observed that that a quasi-reversible voltammogram correspond-
ing to NiII/NiI redox couples was obtained in every cyclic voltam-
mogram of the [Ni(L)X2]. The E1/2 values for this NiII/NiI redox
couples were in the range of ꢀ0.61 to ꢀ0.695 V versus Ag/AgCl
and the ratio between the cathodic peak current and the square
root of the scan rate is approximately constant. The fact of the
Table 2
Selected bond distances (Å) and bond angles (°) for 1.
Bond lengths (Å)
Bond angles (°)
Ni–N(1)
Ni–Cl(1)
Ni–Cl(2)
2.135(2)
2.4412(17)
2.4593(14)
N(1)–Ni–N(1)
N(1)–Ni–N(1)
N(1)–Ni–Cl(1)
N(1)–Ni–Cl(2)
Cl(1)–Ni–Cl(2)
89.979(3)
177.82(13)
90.09(7)
88.91(7)
180.0
1472 and 758–761 cmꢀ1, which are assigned to
m
C@N stretching and
mC–S, respectively. A broad band around at 3421 cmꢀ1 attributable
to amide group and an intense band centred at 2184 cmꢀ1 assign-
able to mNCO (2a), 2049 cmꢀ1 assignable to mN3 (2b) and an intense
band centred at 2084 cmꢀ1 assignable to mNCS (2c). The observa-
tions support the presence of the ligand frame coordinated to nick-
el(II) centre through pyridinic-N and uncoordinated thioether-S as
highest
DE value (200 mV) for complex 1 and the lowest DE value
(170 mV) for complex 4 indicates that the [NiI(L)4Cl2] state is com-
paratively more stable than the [NiI(L)4(NCS)2]. This is due to the
fact of the existence of the coligand (chloride/cyanate/azide/isothi-
ocyanate) with L in the coordination sphere of the complexes (1–4)
m
C–S was observed in the range of 780–790 cmꢀ1 which was gener-
and the
DE values (170–200 mV) are in the accordance with the
hardness of the coligands. Eventually, the presence of compara-
tively hard isothiocyanate donor ligand in complex 4 like to return
to the [NiII(L)4(N3)2] state from its corresponding [NiI(L)4(N3)2]
state as usual. The peak potential shows a small dependence on
the scan rate. The ratio ipc to ipa is close to unity. From these data,
it can be deduced that the redox couple is related to a quasi-revers-
ible one-electron transfer process controlled by diffusion.
ally observed in free ligand [11].
3.5. DNA-binding studies
The binding interaction of the nickel(II) complex 1 with calf thy-
mus DNA (CT-DNA) has been investigated with the help of spectro-
scopic, viscosity-measurements and electrochemical study. This
study showed that the complex 1 interact with CT-DNA in groove
binding mode of interaction. As a result, this study was performed
taking only complex 1 which is structurally very close to com-
plexes 2–4.
3.5.1. Spectrophotometric study
Electronic absorption spectroscopy is an effective method to
examine the binding modes of metal complexes with DNA. In gen-
eral, binding of the nickel(II) complex to the CT-DNA helix is exam-
ined by an increase of the absorption band (c.a. 264 nm) of
nickel(II) complex. This trend of increase of absorbance indicates
that there is the involvement of strong interactions between com-
plex and the base pairs of DNA [22]. The absorption spectra of the
nickel(II) complex 1 in the absence and presence of CT-DNA are gi-
ven in (Fig. 4). The extent of the hyperchromism in the charge
Fig. 2. Packing of complex 1 down axis c showing the H-bonding scheme involving
la amino groups (N–N distance 3.12 Å). These connections lead to 2D layered
structure.

160
A. Patra et al. / Polyhedron 51 (2013) 156–163
Fig. 3. Packing of complex 1 down axis a showing the interaction between Cl1 and amine groups NH2.
Table 3
UV–Vis spectral and electrochemical data.
a
Compound
k, nm (
e) (e
, dm3 molꢀ1 cmꢀ1
)
Electrochemical dataa,b
Epa (V)
Epc (V)
E1/2, V (
D
E, mV)
ipc/ipa
1
2
3
4
234(22258), 265(2853), 307(139), 422(100)
2 63(11654), 309(7097), 422(951), 686(366)
263(10172), 310(6539), 424(435), 687(172)
264(16388), 310(10388), 422(1284), 686(431)
ꢀ0.51
ꢀ0.57
ꢀ0.54
ꢀ0.61
ꢀ0.71
ꢀ0.75
ꢀ0.73
ꢀ0.78
ꢀ0.61 (200)
ꢀ0.66 (180)
ꢀ0.635 (190)
ꢀ0.695 (170)
1.07
1.02
1.05
1.01
a
In DMF.
b
Scan rate of 100 mV sꢀ1
.
transfer band is generally consistent with the strength of interac-
tion [23–25]. In order to further illustrate the binding strength of
the nickel(II) complex with CT-DNA, the intrinsic binding constant
Kb was determined from the spectral titration data using the fol-
lowing equation [26]:
0.5
f
0.4
0.3
0.2
0.1
0.0
½DNAꢅ=ðea
ꢀ
ef Þ ¼ ½DNAꢅ=ðea
ꢀ
ef Þ þ 1=½Kbðea
ꢀ
ef Þꢅ
where [DNA] is the concentration of DNA, ef
,
e
a and
eb correspond to
the extinction coefficient, respectively, for the free nickel(II) com-
plex, for each addition of DNA to the nickel(II) complex and for
the nickel(II) complex in the fully bound form. From the [DNA]/
a
(
ea ef) versus [DNA] plot (Fig. 5), the binding constant Kb for the
ꢀ
nickel(II) complex 1 was estimated to be 1.65 ꢂ 105 Mꢀ1 (R = 0.99135
for four points) indicating in terms of groove binding [27].
3.5.2. Spectroflurimetric study
Fluorescence intensity of EB bound to CT-DNA at excitation
wavelength of 522 nm shows a decreasing trend with the increas-
ing concentration of the complex 1 (Fig. 6). The quenching of EB
bound to DNA by the complex 1 is in agreement with the linear
Stern–Volmer equation [28]
250
260
270
280
290
300
Wavelength (nm)
I0=I ¼ 1 þ Ksv½Qꢅ
Fig. 4. Electronic spectral titration of complex 1 with CT-DNA at 266 nm in tris–HCl
buffer. Arrow indicates the direction of change upon the increase of DNA
concentration.
where, I0 and I represent the fluorescence intensities in absence and
presence of quencher, respectively. Ksv is a linear Stern–Volmer
quenching constant, Q is the concentration of quencher. The Ksv va-
lue calculated from the plot (Fig. 7) of I0/I versus [complex] for the
complex 1 is 3.12 ꢂ 104 (R = 0.99529 for four points), suggesting a
strong affinity of the complex 1 to CT-DNA.
K and n is the binding constant and binding site of complex 1 to
CT-DNA, respectively. The number of binding sites (n) determined
from the intercept of log[(I0 ꢀ I)/I] versus log[Q] is 0.92 which indi-
cates less association of the complex 1 to the number of DNA bases,
also suggesting strong affinity of the complex 1 through surface or
groove binding.
Number of binding sites can be calculated from fluorescence
titration data using the following equation [29]
log½ðI0 ꢀ IÞ=Iꢅ ¼ logK þ nlog½Qꢅ

A. Patra et al. / Polyhedron 51 (2013) 156–163
161
2.4
2.2
2.0
1.8
1.6
1.4
1
2
3
4
5
[DNA]×10-6
Fig. 5. Plot of [DNA]/(eaꢀef) vs. [DNA] for the absorption titration of CT-DNA with
the nickel (II) complex 1 in Tris–HCl buffer.
Fig. 8. Cyclic voltammograms of complex 1 in tris–HCl buffer in the absence (a) and
presence (b) of CT-DNA.
v .
= 1 V sꢀ1
5000
4000
3000
2000
1000
0
of the DNA solution by adding the complex. This study suggests
that the binding mode is groove binding which is in support of
the above results obtained in spectroscopic study. otherwise if it
be intercalative candidate, then the change of the relative viscosity
of the DNA solution was observed because intercalation leads to an
increase in the DNA viscosity by lengthening the DNA helix, or the
non-classical intercalation could bend (or kink) the DNA helix and
reduce its effective length and, concomitantly, its viscosity [30,31].
a
e
3.5.4. Electrochemical study
Electrochemical investigations is useful technique to analyse
metal–DNA interactions over spectroscopic methods [32,33]. The
binding nature of the nickel(II) complex 1 with DNA, has been
shown in Fig. 8. Cyclic voltammograms of the nickel(II) complex
1 in the absence and presence of CT-DNA is exhibited significant
shifts in the anodic and cathodic peak potentials followed by de-
crease in both peak currents, indicating the interaction existing be-
tween the nickel(II) complex and CT-DNA. Equilibrium binding
constants KR/K0 can be calculated by using the shift value of the
560
580
600
620
640
660
680
700
Wavelength (nm)
Fig. 6. Emission spectra of the CT-DNA–EB system in tris–HCl buffer upon the
titration of the complex 1. kex = 522 nm. Arrow shows the intensity change upon the
increase of the complex concentration.
formal potential (
equation [34]:
D
E0) of Ni(II)/Ni(I) according to the following
E0 ¼ E0 ꢀ E0 ¼ 0:0591logðKR=K0Þ
2.0
1.8
1.6
1.4
1.2
1.0
D
b
f
where Eb0and Ef0 are the formal potentials of the bound and free
complex forms, respectively, and KR and K0 are the corresponding
binding constants for the binding of reduction and oxidation species
to DNA, respectively. Ratio of equilibrium binding constants, KR/K0
is calculated to be 2.17 which indicate the binding of DNA with
the reduced form of the complex 1 over its oxidised form.
3.6. Protein (bovine serum albumin) binding experiments
3.6.1. Absorption characteristics of BSA–nickel(II) complex 1
The absorption spectra of BSA in the absence and presence of
Ni(II) complex 1 (other complexes give same binding interaction)
were studied at different concentrations (Fig. 9). From this study
we observed that absorption of BSA increases regularly upon
increasing the concentration of the complex. It is may be due to
the adsorption of BSA on the surface of the complex. From these
data the apparent association constant (Kapp) determined of the
complexes with BSA has been determined using the following
equation [27]:
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
[Complex]×10-5
Fig. 7. Plot of I0/I vs. [complex] for the titration of nickel (II) complex 1 with CT-
DNA–EB system in tris–HCl buffer.
3.5.3. Viscosity technique
From the experiment on the viscosity measurements study, it
was observed that there is almost no effect on the relative viscosity
1=ðAobs ꢀ A0Þ ¼ 1=ðAc ꢀ A0Þ þ 1=KappðAc ꢀ A0Þ½compꢅ

162
A. Patra et al. / Polyhedron 51 (2013) 156–163
1.6
1.4
1.2
1.0
0.8
0.6
0.4
3.7. Antibacterial activity
5
4
3
2
1
0
Antibacterial activity of the lignd and corresponding complexes
are recorded in Table s1. From the antibacterial studies it is in-
ferred that all the complexes have higher activity than the organic
moiety (L). The increased activity of the metal chelates can be ex-
plained based on the oxidation state of the metal ion, overtone
concept and the Tweedy chelation theory. It is observed that, in a
complex, the positive charge of the metal is partially shared with
the donor atoms present in the ligands, further it increases the
e
a
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
1/[Complex] ×10-4
delocalisation of p-electrons over the whole chelate ring and lipo-
philic character of the metal complexes also increases. This in-
creased lipophilicity enhances the penetration of complexes into
the lipid layer of the bacterial cell membranes and blocks the metal
binding sites in enzymes of microorganisms. These complexes also
disturb the respiration process of the cell and thus block the syn-
thesis of proteins, which restricts further growth of the microor-
ganisms. The complex 1 act as a higher activity than ligand and
other complexes due to higher lipophilicity.
250
300
350
400
450
Wavelength (nm)
Fig. 9. Electronic spectral titration of complex 1 with BSA at 266 nm in tris–HCl
buffer. Arrow indicates the direction of change upon the increase of BSA
concentration.
4. Conclusion
Four new mononuclear nickel(II) complexes with a new organic
moiety, 2-(pyridin-3-ylmethylsulfanyl)phenylamine (L) and halide/
pseudohalides (1–4) have been synthesised and characterised by
spectroscopic and electrochemical techniques along with the de-
tailed structural characterisation of complex 1 by single crystal
X-ray diffraction. Here, L behaves as a monodentate ligand though
it has tridentate NNS donor centres. The crystallographic analysis
of 1 indicates that the nickel(II) ion is in octahedral geometry with
two chloride ions occupying trans position and four pyridinic-N of
the ligand, L. Abundant intermolecular N–Hꢄ ꢄ ꢄN–H hydrogen
bonding the complex 1 connected as a 2D layer framework and
shows polynuclear arrangement through the interaction between
chloride atom and amine groups. The electrochemical study of
the complexes showed a quasi-reversible one-electron transfer
process for cathodic NiII/NiI redox couples in the range of ꢀ0.61
to ꢀ695 V versus Ag/AgCl. The spectroscopic study of interaction
of 1 with CT-DNA showed the groove binding mode of complex 1
and it is also in accordance with the unchanged values of the vis-
cosity of the DNA solution upon addition of complex 1. The absorp-
tion as well as fluorescence spectroscopy tools are used to study
these interactions, proving the formation of a ground state BSA–
[Ni(L)4Cl2] complex. From the antibacterial studies it could be in-
ferred that metal complexes have higher activity than ligand due
to chelation. All four nickel(II) complexes have higher antibacterial
activity than ligand L against five pathogenic bacteria (E. coli, V.
cholerae, S. pneumonia, Shigella sp. and B. cereus).
3500
3.0
a
3000
2.5
2.0
2500
1.5
2000
1500
1000
500
0
e
1.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
[Complex]×10-6
300
325
350
375
400
425
Wavelength (nm)
Fig. 10. Fluorescence quenching of BSA in the presence of various concentrations of
the complex 1, [complex] = 0, 1, 2, 3 and 4 ꢂ 6.35 ꢂ 10ꢀ6 M.
where, Aobs is the observed absorbance of the solution containing
different concentrations of the complex at 280 nm, A0 and Ac are
the absorbances of BSA and the complex at 280 nm, respectively,
with a concentration of complex and Kapp represents the apparent
association constant. The enhancement of absorbance at 280 nm
was due to absorption of the surface complex, based on the linear
relationship between 1/(Aobs ꢀ A0) versus reciprocal concentration
of the complex with a slope equal to 1/Kapp(Ac ꢀ A0) and an inter-
cept equal to 1/(Ac ꢀ A0) (inset Fig. 9). The value of the apparent
Acknowledgements
Financial supports from Department of Science and Technology
(DST), New Delhi and Council of Scientific and Industrial Research
(CSIR), New Delhi, India are gratefully acknowledged. E. Zangrando
thanks MIUR-Rome, PRIN 2007HMTJWP_002 for financial support.
association constant (Kapp
)
determined from this plot is
2.37 ꢂ 104 Mꢀ1 (R = 0.99459 for four points).
3.6.2. Fluorescence quenching of BSA by complex 1
Appendix A. Supplementary material
The fluorescence emission spectrum of BSA were studied with
increasing the concentration of the complex and represented in
Fig. 10. With the addition of complex BSA fluorescence emission
is quenched. The fluorescence quenching is described by the
Stern–Volmer relation [28] described above. A linear plot (inset,
Fig. 10) between I0/I against [complex] was obtained and from
the slope we calculated the KSV as 4.51 ꢂ 104 (R = 0.99275 for four
points).
CCDC 885271 contains the supplementary crystallographic data
for complex 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.12.031.

A. Patra et al. / Polyhedron 51 (2013) 156–163
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