Syntheses, spectroscopic characterization, SOD-like properties and antibacterial activities of dimer copper (II) and nickel (II) complexes based on imine ligands containing 2-aminothiophenol moiety: X-ray crystal structure determination of disulfide Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2018.03.041

A series of new dimer complexes of copper (II) and nickel (II) were designed and synthesized using the Schiff base ligands which was formed by the condensation of 2-aminothiophenol with 2- methoxybenzaldehyde, 3-formylbenzonitrile and 3-bromo-2-hydroxy-5-nitrobenzaldehyde, respectively. The synthesized metallic complexes were characterized by using different physicochemical and spectroscopic methods. The most plausible geometry for the 1:2 complexes appeared to be distorted square-planar or tetrahedral environments. All the synthesized metal complexes are found to be binuclear and confirmed by elemental analyses, magnetic susceptibility measurements and ESR spectroscopy. The Schiff base ligands (HL¹/HL²/H₂L) were coordinated to the metal ions through the ONS/SNN and/or N, S donor atoms. In order to prevent the oxidation of the thiol group during the formation of Schiff bases and its complexes, all of the reactions were carried out under an inert atmosphere of argon. The X-ray structures of the Schiff base ligands showed that in the crystalline form the SH groups were oxidized to produce a disulfide Schiff bases as a new double Schiff base ligands (L¹ _a/L² _a/H₂L_a). The L¹ _b ligand is a bicyclic ring system of N, S-containing heterocyclic. The crystal structures of the double Schiff bases were determined by single crystal X-ray diffraction. The molar conductivity values of the complexes in DMSO implied the presence of non-electrolyte species. The SOD-like activity of Schiff bases and its complexes were investigated by NBT-DMSO assay and IC50 values were evaluated. Their biological properties have also been studied. These complexes were also tested for their in vitro antibacterial screening activities against three bacteria (Streptococcus aureus, Salmonella typhi, and Escherichia coli) comparing with the Schiff base ligands. Most of the complexes have higher antibacterial activities than those of the free Schiff bases, double Schiff bases and the control.

Full text of DOI:10.1016/j.molstruc.2018.03.041

Accepted Manuscript
Syntheses, spectroscopic characterization, SOD-like properties and antibacterial
activities of dimer copper (II) and nickel (II) complexes based on imine ligands
containing 2-aminothiophenol moiety: X-ray crystal structure determination of disulfide
Schiff bases
Sulakshna Bharti, Mukesh Choudhary, Bharti Mohan, S.P. Rawat, S.R. Sharma, K.
Ahmad
PII:
S0022-2860(18)30330-2
10.1016/j.molstruc.2018.03.041
MOLSTR 24980
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 18 September 2017
Revised Date: 8 March 2018
Accepted Date: 12 March 2018
Please cite this article as: S. Bharti, M. Choudhary, B. Mohan, S.P. Rawat, S.R. Sharma, K. Ahmad,
Syntheses, spectroscopic characterization, SOD-like properties and antibacterial activities of dimer
copper (II) and nickel (II) complexes based on imine ligands containing 2-aminothiophenol moiety: X-
ray crystal structure determination of disulfide Schiff bases, Journal of Molecular Structure (2018), doi:
10.1016/j.molstruc.2018.03.041.
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ACCEPTED MANUSCRIPT
Graphical Abstract:
Three new dimer complexes of copper (II) and nickel (II) were designed and successfully synthesized
using the Schiff base ligands which was formed by the condensation of 2-aminothiophenol with 2-
methoxybenzaldehyde, 3-formylbenzonitrile and 3-bromo-2-hydroxy-5-nitrobenzaldehyde, respectively.
The main idea of this paper is the synthesis, characterization and catalytic activity of SOD-like metallic
complexes. All the synthesized metal complexes are found to be binuclear and confirmed by elemental
analyses, magnetic susceptibility measurements and ESR spectroscopy. The X-ray structures of the
Schiff base ligands showed that in the crystalline form the SH groups were oxidized to produce a
disulfide Schiff bases as a new double Schiff base ligands (Fig. 1(a) and 1(b).
(a)
(b)
Fig. 1(a) and 1(b).

ACCEPTED MANUSCRIPT
Syntheses, spectroscopic characterization, SOD-like properties and antibacterial
activities of dimer copper (II) and nickel (II) complexes based on imine ligands
containing 2-aminothiophenol moiety: X-ray crystal structure determination of
disulfide Schiff bases
Sulakshna Bhartia, Mukesh Choudharya*, Bharti Mohana, S. P. Rawatb, S. R. Sharmaa and K. Ahmada
aDepartment of Chemistry, National Institute of Technology Patna, Ashok Raj path, Patna -800005, Bihar,
India.
bDepartment of Chemistry, Govt. Vivekanand P. G. College, Maihar-485 771, Madhya Pradesh, India.
*Corresponding author: mukesh@nitp.ac.in
Abstract:
A series of new dimer complexes of copper (II) and nickel (II) were designed and synthesized using the
Schiff base ligands which was formed by the condensation of 2-aminothiophenol with 2-
methoxybenzaldehyde, 3-formylbenzonitrile and 3-bromo-2-hydroxy-5-nitrobenzaldehyde, respectively. The
synthesized metallic complexes were characterized by using different physicochemical and spectroscopic
methods. The most plausible geometry for the 1:2 complexes appeared to be distorted square-planar or
tetrahedral environments. All the synthesized metal complexes are found to be binuclear and confirmed by
elemental analyses, magnetic susceptibility measurements and ESR spectroscopy. The Schiff base ligands
(HL1/HL2/H2L) were coordinated to the metal ions through the ONS/SNN and / or N, S donor atoms. In
order to prevent the oxidation of the thiol group during the formation of Schiff bases and its complexes, all of
the reactions were carried out under an inert atmosphere of argon. The X-ray structures of the Schiff base
ligands showed that in the crystalline form the SH groups were oxidized to produce a disulfide Schiff bases
as a new double Schiff base ligands (L1 /L2 /H2La). The L1 ligand is a bicyclic ring system of N, S-
a
a
b
containing heterocyclic. The crystal structures of the double Schiff bases were determined by single crystal
X-ray diffraction. The molar conductivity values of the complexes in DMSO implied the presence of non-
electrolyte species. The SOD-like activity of Schiff bases and its complexes were investigated by NBT-
DMSO assay and IC50 values were evaluated. Their biological properties have also been studied. These
complexes were also tested for their in vitro antibacterial screening activities against three bacteria
(Streptococcus aureus, Salmonella typhi, and Escherichia coli) comparing with the Schiff base ligands. Most
of the complexes have higher antibacterial activities than those of the free Schiff bases, double Schiff bases
and the control.
Keywords: Dimer copper (II) and nickel (II) complexes; 2-aminothiophenol moiety; crystal structures
of disulfide Schiff bases; catalytic SOD-like properties; Antibacterial screening activities.
[1]

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1. Introduction
The thiazole units are found in many naturally occurring compounds [1]. Benzothiazoles are used as
neuroprotectors [2] and antioxidants [3]. It consists of a five-membered 1, 3-thiazole ring fused to a benzene
ring [4]. Metal complexes with 2-(2-methoxyphenyl) benzo[d]thiazole as N and S chelating ligands have
attracted considerable attention because of their interesting physicochemical properties and pronounced
biological and pharmacological activities [5-7]. Schiff bases are an important class of organic compounds
[8]. This kind of ligands have significant importance in chemistry, especially in the development of Schiff
base complexes because Schiff base ligands are potentially capable of forming stable complexes with metal
ions[9]. Schiff base ligands with N and S donor atoms in their structures act as good chelating agent for metal
ions [10]. The N and S atoms play a key role in the coordination of metals at the active sites of various
metallo-biomolecules. Coordination of such compounds with metal ions, such as copper, nickel and iron
often enhance their biological activities [11, 12]. Schiff base ligands have proven to be very effective in
constructing supramolecular architectures such as coordination polymers, double helixes and triple helicates
[13]. These complexes have significant contribution in the development of catalysis and enzymatic
reactions, magnetism, molecular architectures and materials chemistry [13, 14]. Schiff base metal complexes
have been interest in coordination chemistry for many years due to their facile synthesis and wide
applications. Interest in coordination chemistry of tridentate Schiff base ligands with N, O and S donating
sites have increased due to the capabilities of forming stable complexes of four, five or six coordinated
[15,16] and have a broad range of antifungal [17], antibacterial [18] anticancer [19] properties. Particular
attention has been devoted to tridentate ONS donor Schiff base ligands containing 2-aminothiophenol
moiety because they can promote chelation and provides extra stability to the metal centres in its 1:1
and/or 1:2 complexes. In all complexes, the Schiff base ligands either behave as tridentate (ONS/SNN) and/
or bidentate-N, S donor ligands. The most plausible geometry for the 1:2 complexes appeared to be a
distorted square planar (four coordinated) or tetrahedral.
Superoxide dismutase is an important antioxidant to control the free radical reactions related to
superoxide generated in biological system [20]. However, its large molecule and short life span in vivo
limit its clinical use. Extensive studies have been carried out to find the suitable SOD-mimics to
substitute it. In order to acts as SOD mimics, a compound should be non-toxic, stable, and easy to
reach its targets and retain high SOD activity in vivo. Synthetic transition metal complexes mimicking
superoxide dismutase, provide models for metalloproteins active sites and lend inside towards the design of
new catalysis. Metal complexes with Schiff base ligands behave as better catalysts for Cu, Zn-SOD enzyme
[21, 22]. This metalloenzyme is a dimeric protein with two identical subunits each containing one Cu (II)
and Zn (II). X-ray crystallographic studies [23] have shown that the Cu (II) and Zn (II) ions exist in distorted
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square planar and tetrahedral coordination environments, respectively. Superoxide dismutase (SOD) is
dedicated to keep the concentration of O-2 in controlled low limits, thus protecting biological molecules from
oxidative damage [24]. Nickel containing superoxide dismutase (Ni-SOD) has been isolated from several
Streptomyces species [25]. The enzymatic activity of Ni-SOD [26] however is the same high level as that of
Cu, Zn-SOD of about 109 M-1S-1per metal center. X-ray crystallographic studies [27, 28] of Ni-SOD revealed
a square planar N2S2NiII in reduced form and a square-pyramidal N3S2NiIII in oxidized form as shown in
Scheme1.The SODs disproportionate the toxic superoxide radical anion (O-2) to molecular oxygen (O2) and
hydrogen peroxide (H2O2) of their redox active metal canters [29] such as Cu, Ni, Fe, Mn. All the SOD’s
including Cu, Zn-SOD and Mn-SOD in mammalian systems, Fe-SOD and Ni-SOD in bacterial systems
employ the two steps “Ping-Pong” mechanism which consist of two diffusion-controlled steps, shown in
Equation (1) and (2) , where ME is the redox active centre of metalloenzymes capable of both oxidizing and
reducing superoxide properties.
-
O2 + [MEII-SOD]
O2 + [MEI-SOD]
H2O2 + [MEII-SOD]
(1)
(2)
-
O2 + [MEI-SOD] + 2H+
In continuation of our previous work on copper (II) and nickel(II) Schiff base complexes [30], in this
article, we reported the syntheses, spectroscopic characterization and catalytic activity of SOD-like
metallic complexes based on imine ligands containing 2-aminothiophenol moiety which can be
considered as possible models of functional SOD mimics. The Schiff bases and its metallic complexes
have been synthesized and tested and have shown antioxidant SOD-like activity in vitro, while the
double Schiff base ligands did not exhibit the SOD activity. These complexes were also tested for their
in vitro antibacterial screening activities against three bacteria (Streptococcus aureus, Salmonella typhi,
and Escherichia coli) comparing with the Schiff bases and double Schiff bases. Most of the complexes
have higher antibacterial activities than those of the free Schiff bases, double Schiff bases and the
control. All the synthesized metal complexes are found to be binuclear and confirmed by elemental analyses,
magnetic susceptibility measurements and ESR spectroscopy. Very few Schiff bases with different structure
have been synthesized and characterized as disulfide bonds reported so far [31, 32]. The oxidized form of
Schiff bases was also characterized by single crystal X-ray analysis. Several attempts were made to develop
the single crystal of the complexes but failed due to the insolubility of the complexes in common organic
solvents. The SOD activities have been measured using alkaline DMSO as a source of superoxide radical
–
–
(O2 ) and NBT (nitro blue tetrazolium) as a (O2 ) scavenger.
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Scheme1. Structure and functional model of Ni-SOD active sites.
2. Experimental
2.1 Materials used for synthesis
All chemicals were commercially available and were used as received. Reagents used for the physical
measurements were of spectroscopic grade and were used without further purification.
2.2. Syntheses
2.2.1. Synthesis of (Z)-2-(2-methoxybenzylideneamino) benzenethiol (HL1)
A solution of 2-methoxybenzaldehyde (10 mmol, 1.36g), in 20 mL of absolute ethanol at 60°C was slowly
added to a solution of 2-aminothiophenol(10 mmol, 1.25g) in 20 mL of absolute ethanol at 60°C and the
mixture stirred for 6h at 60°C under the inert atmosphere of argon. After removal of the solvents, the yellow
precipitate was obtained and washed with methanol. Then, the product was dried in vacuo at room
temperature. The purity was checked by elemental analysis. Isolated yield: 78%. M p.: ~92°C. Anal. Found
for C14H11NOS, HL1 (%): C, 69.98; H, 4.59; N, 5.80. Calcd. (%): C, 70.01; H, 4.62; N, 5.81. FAB- mass
(m/z) Anal. : 241.06, Calcd. : 241
2.2.2 Synthesis of 3-((Z)-(2-mercaptophenylimino) methyl)-benzonitrile (HL2)
2-aminothiophenol (10 mmol, 1.25g) and 3-formylbenzonitrile (10 mmol, 1.31g) were mixed and then fused
together at 80°C under the inert atmosphere of argon with constant stirring for 10h. The mixture was then
cooled, and absolute ethanol (60mL) was added. The mixture was stirred for 15 min and then filtered off. The
filtrate volume reduced to about 10mL and resulting in the formation of a green precipitate. The precipitate
was decanted off and washed twice with methanol (2×10mL). Then, the product was dried in vacuo at room
temperature and stored in a CaCl desiccator. The purity was checked by elemental analysis. Isolated yield:
2
75%. M p.: ~154°C. Anal. Found for C14H10N2S, HL2 (%): C, 70.56; H, 4.23; N, 11.76. Calcd. (%): C,
70.53; H, 4.25; N, 11.78. FAB- mass (m/z) Anal. : 238.31, Calcd. : 238.
[4]

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2.2.3 Synthesis of 2-((E)-(2-methoxyphenylamino) methyl)-6-bromo-4-nitrophenol (H2L)
This ligand was synthesized in the same manner as HL1 and HL2, except that the 3-bromo-2-hydroxy-5-
nitrobenzaldehyde (10 mmol, 2.46g) was used in 20 mL of absolute ethanol at 70°C and heated on a water
bath at 58°C. The filtrate volume reduced to about 10mL and resulting in the formation of a brown red
precipitate. The precipitate was decanted off and washed twice with diethyl ether (2×10mL). Then, the
product was dried in vacuo at room temperature and stored in a CaCl desiccator. Its purity was checked by
2
elemental analysis. Isolated yield: 70%. M p.: ~180°C. Anal. Found for C13H9BrN2O3S, H2L (%): C,
44.21; H, 2.57; N, 7.93. Calcd. (%): C, 44.23; H, 2.58; N, 7.92. FAB- mass (m/z) Anal. : 353.19, Calcd. :
353.
2.2.4 Synthesis of copper dimer complex [Cu2 (L1)2]2+ 1 and [Cu2 (L2)2 (H2O)2] 2
To an ethanolic solution of Cu (OCOCH3)2.H2O (1.0 mmol, 0.199g) was added dropwise to a solution
(10mL) of HL1 (2.0 mmol, 0.592 g) in the presence of triethylamine(3.0mmol, 40µL) as base. The reaction
mixture was heated with constant stirring on a water bath at 70oC for 6-8h. The resulting dark green solution
was evaporated and the green precipitate products were recovered from mother liquor which were filtered,
washed with methanol and stored in a desiccator over CaCl2. Complex 2 was synthesized in the same
manner as complex 1, except that the HL2 (2.0 mmol, 0.476 g) in the presence of triethylamine(3.0mmol,
40µL) as base. Their purity was checked by elemental analysis. Isolated yield (65-68%). Anal. Found for
C28H24Cu2N2O2S21 (%): C, 54.98; H, 3.95; N, 4.58. Calcd. (%): C, 55.00; H, 3.97; N, 4.60.FAB- mass (m/z)
Anal.: 611.72, Calcd.: 612.Anal. .Anal. Found for C28H22 Cu2 N4O2 S2 2(%): C, 52.73; H, 3.48; N, 8.79.
Calcd. (%): C, 52.76; H, 3.51; N, 8.82. FAB- mass (m/z) Anal. : 636, Calcd. : 638.
2.2.5 Synthesis of nickel dimer complex [Ni2(L)2] 3
The H2L (2.0 mmol, 0.592 g) dissolved in ethanol (20 mL) in the presence of triethylamine(3.0mmol, 40µL)
and was reacted with Ni (OAc) 2.4H2O (1.0 mmol, 0.249 g) in constant stirring and reflux(80oC). The dark
red solution was stirred during 5h. The resulting red solution was evaporated and the dark red precipitate
product were recovered from mother liquor which were filtered, washed with methanol and stored in a
desiccator over CaCl2. This complex gives satisfactory elemental analysis. Isolated yield (60%). Anal.
Found for C26 H14 Br2 N4 Ni2 O6S2 3 (%): C, 38.09; H, 1.72; N, 6.83 Calcd. (%): C, 38.11; H, 1.73; N, 6.82.
FAB- mass (m/z) Anal. : 819.74, Calcd. : 819.
2.3. Physical measurement
Elemental analyses (CHN) were performed on an Elementar Vario EL III Carlo Erba 1108 analyzer. FAB
mass spectra were recorded on a JEOL SX 102/DA 6000 mass spectrometer using xenon (6kV, 10mA) as the
FAB gas. The accelerating voltage was 10 kV and the spectra were recorded at room temperature (RT) with
[5]

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m-nitro benzoyl alcohol as the matrix. Magnetic susceptibility measurements of powder samples of
complexes were made on a Gouy balance using a mercury (II) tetrathiocynato cobaltate (II) as calibrating
agent (xg=16.44×10-6c.g.s. units). All the experimental data were corrected for diamagnetic were estimated
from Pascal table and temperature independent paramagnetism (TIP). The molar ion exchange was measured
using a systronics digital conductivity meter (TDS-308) using a 10-3M solution in DMSO.UV-Vis spectra
were recorded at 25°C on a Thermo scientific UV-Vis recording spectrophotometer Evolution-3000 in quartz
cells. Fluorescence spectra were recorded at room temperature on a Horiba Scientific Fluoromax-4
spectrofluorometric in quartz cell. IR spectra were recorded in KBr medium on a Shimadzu IR Affinity -1S
Fourier transform infrared spectrophotometer X-band (~9.4 GHz). The ESR spectra were recorded with a
Varian E-line Century Series. ESR Spectrometer equipped with a dual cavity and operating at X-band of the
100 kHz modulation frequency at room temperature. Tetracyanoethylene (TCNE) was used as field marker
(g=2.00277). The solution was deoxygenated by purging nitrogen gas.
2.4. Scavenger measurements of superoxide radicals
The SOD- like activities of all the complexes was investigated by NBT-DMSO assay. The in vitro SOD
-
activity was measured using alkaline DMSO as a source of superoxide radical (O2 ) and nitroblue tetrazolium
-
chloride (NBT) as (O2 ) scavenger [33]. In general, 400µl sample to be assayed was added to a solution
containing 2.1 ml of 0.2 M potassium phosphate buffer (pH 8.6) and 1ml of 56 µM of alkaline DMSO
solution was added while string. The absorbance was then monitored at 540 nm against a sample prepared
under similar condition except NaOH was absent in DMSO. A unit of superoxide dismutase (SOD) activity
is concentration of complex, which causes 50% inhibition of alkaline DMSO mediated reduction of
nitrobluetetrazolium chloride (NBT).
2.5. Antibacterial activity
The in vitro biological activity of the investigated Schiff base ligands and their metal complexes 1-3 were
tested against three bacteria Streptococcus aureus, Salmonella typhi and Escherichia coli by the paper disc
diffusion method [34] using nutrient agar as the medium and chloramphenicol as the control. DMSO solvent
was used as positive control. Each of the complexes was dissolved in DMSO and the solutions of different
concentrations (10-30mM) were prepared separately. The liquid medium containing the bacterial subcultures
was autoclaved for 20 min at 121°C and at 15 lb pressure before inoculation. The bacteria were then cultured
for 24h at 36°C in an incubator. Nutrient agar was poured into a plate and allowed to solidify. The test
compounds (DMSO solutions) were added drop wise to a 10 mm diameter filter paper disc placed at the
center of each agar plate. The plates were then kept at 5°C for 1 h then transferred to an incubator maintained
[6]

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at 36°C. The width of the growth inhibition zone around the disc was measured after 24 h incubation. All the
experiments were performed in triplicates.
2.6. Crystal structure determination
To unambiguous confirm the structure of double Schiff bases, an X-ray are desired. Single crystal X-ray
diffraction data for double Schiff base ligands were collected at 293(2) and 296(2) K. The X-ray diffraction
measurements of double Schiff bases were collected on CCD detector based diffractometer (SMART,
APEX-II) from Bruker-Nonius CAD-4/MACH3, AXs and CrysalisPro, Oxford diffractometer using graphite
monochromatized Mo-Kα radiation (λ=0.71073Å) for the double Schiff bases from a fine focus sealed tube
radiation source. The crystal orientation, cell refinement and intensity measurements were made using the
program CAD-4PC performing ψ-scan measurements. Multi-scan absorption corrections were applied
empirically to the intensity values (Tmax= 0.7455 and Tmin=0.6844 for L1 , Tmax= 0.5615 and Tmin=0.4411 for
a
L2 and Tmax= 0.9295 and Tmin=0.9185 for H2La) using the program SADABS [35] or SCALE3 ABSPACK
a
algorithm within CRYSALISPRO. The crystal structures were solved by the direct or charge-flipping
methods using programs SHELXT, SIR 2014[36], OLEX2 solve [37] or SUPERFLIP [38] and refined by the
full- matrix least square procedure with SHELXL or CRYSTALS .Geometrical analysis was performed using
SHELXL on F2 or CRYSTALS. All non- hydrogen atoms were refined anisotropically. All the hydrogen
atoms were geometrically fixed and allowed to refine using isotropic thermal parameters. The
crystallographic data of double Schiff base ligands have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication CCDC Nos. 1536404, 1573259 and 1561750 for L1 , L2 and
b
a
H2La, respectively.
3. Results and discussion
3.1 Synthesis and characterization
The composition and molecular formulas of the three dimer complexes with ONS/SNN or N, S donor Schiff
base ligands were determined by elemental analyses. The protocol used for the syntheses for tridentate
Schiff bases, double Schiff base ligands and their dimer complexes 1-3 was given in Scheme 2-4. The most
plausible geometry for the 1:2 complexes appeared to be distorted square-planar or tetrahedral environments.
The isolated products were characterized by various physicochemical and spectroscopic techniques. These
complexes are non-electrolyte in nature. In these metallic complexes, Schiff base ligands acted as a
chelating agent towards the metal ions as a tridentate ligands via azomethine-N atoms, one oxygen/ nitrogen
atoms and CuCu and NiNi metal centers were bridged by the S atoms and thus facilitated the linkage of two
units (Scheme2-4). Similar dimer copper (II) and nickel (II) complexes were reported by some other workers
[39]. Furthermore, the SH groups of two Schiff bases (HL1/HL2/H2L), were oxidized to form double Schiff
[7]

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bases during the formation of crystalline form in the air [40-42]. The suitable single crystals of double Schiff
base ligands were obtained by slow diffusion of diethyl ether into a concentrated solution of Schiff bases in
DMF in the presence of CuCl2.2H2O and bases (K2CO3, K3PO4, Na2CO3 and NaOH) after five days at room
temperature and its structure were determined by X-ray crystallography, whereas suitable single crystals
could not be grown for the metal complexes by different methods. Although, a lot of Schiff bases with
different structures have been synthesized and characterized, however, little attention has been given to the
Schiff bases which include disulfide bonds. We are also surprising to get this new disulfide Schiff bases
which are unique and there are some literature on this kind of crystal structure having disulfide bonds [40-
42]. As shown in Scheme 2-4, a tentative reaction pathway for the formation of dimeric complexes 1-3
was proposed. The complexes are isolated as binuclear and confirmed by elemental analyses, magnetic
susceptibility measurements, IR and ESR spectroscopy.
3.2 Molar conductivity and elemental analysis
The molar conductivity of the complexes (1×10-4 M) was measured in DMSO solution at room temperature.
The molar conductivity values of all complexes were in the range of 1.58-5.83Ω-1cm2mol1, indicating the
non-electrolyte nature of these metallic complexes [43]. The elemental analyses of the complexes indicated
that the absence of counter ions in the formula. The obtained elemental analyses results were in good
agreement with those calculated for the suggested formula of Schiff base ligands and metal complexes. In
complexes 1-3, it has been observed that Schiff base ligands either behaves as tridentate (ONS/SNN) and/ or
bidentate-N, S donor ligands in the 1:2 complexes.
[8]

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CH3
NH2
O
O
SH
N
S
+
O
CuCl2.2H2O
Stirred 14h
CH3
2-aminothiophenol
Refluxed 8h
2-methoxybenzaldehyde
2-(2-methoxyphenyl)benzo[d]thiazole(L1b)
Crystallized form
H3C
EtOH
O
CuCl2.2H2O
Stirred 8h, Na2CO3
H3C
O
N
Stirred 14h, Sulfur powder, -H2S
N
S
CuCl2.2H2O, K2CO3
S
SH
N
O
(Z)-2-(2-methoxybenzylideneamino)benzenethiol(HL1)
H3C
Schiff base formed under atmosphere of Ar
(15E)-N-(2-methoxybenzylidene)-2-
(2-((Z)-2-(2-methoxybenzylideneamino)phenyl)
disulfanyl)benzenamine (L1a)
(C2H5)3N
Cu(OAc)2
N
H3C
S
Cu
O
Cu
O
S
CH3
N
1
Scheme 2. The proposed reaction pathway of the HL1, L1a, L1b ligand and dimer copper complex 1.
[9]

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O
NH2
HS
N
SH
EtOH
+
C
N
Refluxed 8h
C
N
2-aminothiophenol
3-((Z)-(2-mercaptophenylimino)methyl)benzonitrile(HL2)
3-formylbenzonitrile
N
C
Schiff base formed under atmosphere of Ar
N
N
C
Sulfur powder, -H2S
CuCl2.2H2O, K2CO3,
Stirred 10h
S
S
N
Crystallized form
L2a
N
Cu(OAc)2
N
H
S
H2O
Cu
(C2H5)3N
C
O
H
Cu
C
S
N
N
2
Scheme 3. Proposed reaction pathway of the HL2, L2a ligand and dimer copper complex 2.
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NO2
NH2
O
O2N
SH
N
EtOH
HO
+
SH
Refluxed 8h
Br
OH
3-bromo-2-hydroxy-5-nitrobenzaldehyde
2-aminothiophenol
Br
2-((E)-(2-mercaptophenylimino)methyl)-6-bromo-4-nitrophenol(H2L)
Br
Schiff base formed under atmosphere of Ar
(C2H5)3N
Ni(OAc)2
OH
O2N
O2N
N
CuCl2.2H2O,K2CO3,
refluxed 10h, S powder
N
S
Br
Ni
O
S
O
S
Ni
Br
S
N
OH
N
NO2
Br
3
NO2
Crystallized form, H2La
Scheme 4. Proposed reaction pathway of the H2L, H2La ligand and dimer nickel complex 3.
3.3 Description of the molecular structures of double Schiff base ligands
The suitable single crystals of double Schiff base ligands were obtained by slow diffusion of diethyl ether
into a concentrated solution of Schiff bases in DMF in the presence of bases including K2CO3, K3PO4,
Na2CO3 and NaOH and its structure were determined by X-ray crystallography, whereas suitable single
crystals could not be grown for the metal complexes by different methods. The ORTEP structures of double
Schiff bases are shown in Figs.1-3, respectively. Crystallographic data and molecular structure refinement
important inter atomic parameters are given in Table 1. Selected bond angles and lengths for double Schiff
base ligands were given in Table2. The 2-(2-methoxyphenyl) benzo[d]thiazole ligand (L1 ) was crystallized
b
in the centro-symmetric monoclinic system of the space group P21/n, a=7.3276(6)Å, b=12.6798(11) Å, c
=12.7253(11)Å,α =90
̊
, β=96.484 (2),
γ=90̊ and Z=4. Here, the Schiff base, 2-(2-methoxyphenyl)
benzo[d]thiazole (L1 ) act as the bidentate N, S-donor ligand (Scheme2). It is an aromatic heterocyclic
b
compound with the chemical formula C14H11NOS [44]. It consists of a five-membered 1, 3-thiazole ring
fused to a benzene ring. Its derivatives are used in inorganic chemistry for building polydentate ligands [1].
Furthermore, The 2-(2-methoxyphenyl) benzo[d]thiazole ligand are bicyclic ring system and constitute an
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important class of compounds with profound interest to medicinal/ industrial chemists as compounds bearing
the benzothiazolyl moiety[45]. To the best of our knowledge, the application of 2-aminothiophenol for the
synthesis of 2-(2-methoxyphenyl) benzo[d]thiazole ligand and its single crystal X-ray determination crystal
is still rare [45].
The HL2 was crystallized in the monoclinic system of the space group C/2c, a=14.8849(18) Å,
a
b=8.3000(9) Å, c =19.664(2) Å, α =90,̊ β=100.625(9), γ=90̊ and Z=4. The double Schiff base H2La
was crystallized in the monoclinic system of the space group P-1, a=8.2685(12) Å, b=11.7305(16) Å, c
=14.4154(18) Å, α =102.618(7), β=96.008(7), γ=100.367(7) and Z=2. According to the crystal
structures (Fig. 2 and 3), these double Schiff bases were contained disulphide bond. Intramolecular S-
S bond in the Schiff base ligand was observed in crystalline structure obtained by X-ray crystallography and
also its data was obtained by different spectroscopy methods. We are surprising to get this new double Schiff
base structures which is unique and there are some literature on this kind of crystal structures of disulfide
Schiff bases [46]. The structural parameters which have shown in Table1 are in good agreement with other
reported similar compounds [46]. The packing diagram of double Schiff base ligands were shown in Figs.4-
6 along a-axis , respectively and this shows that how the disulfide structure arranged donor sites.
Furthermore, the SH groups of two tridentate Schiff bases were oxidized to form double Schiff bases as a
new class of inorganic compounds with sulfur atoms (Scheme2-4).
3.4 IR spectral investigations
The Schiff base ligands were characterized by single crystal X-ray analysis, FT-IR spectroscopy and
elemental analysis. In order to characterize the binding mode of the Schiff base ligands to the metal ion in the
complexes, the IR spectrum of the free ligands was compared with the spectra of the metal complexes.
Analysis by FT-IR results in the transmittance spectra in the infrared region (400-4000cm-1) and register
bands or signals (Table 3). The IR spectrum of Schiff base ligands exhibits strong bands at 1581-1627cm−1,
which can be attributed to υ(C=N) stretching vibration. The IR spectrum for the ligands confirmed the
presence of imine and the absence of carbonyl and amine functional groups of the starting materials.
Furthermore, the IR spectra data confirms the coordination of Schiff bases through azomethine-N to the
metal atoms by shifting the υ(C=N) stretching frequency of free Schiff base ligands to the higher frequency
in the range of 1624-1632cm-1 in complexes. This shift to higher wavenumber may be attributed to the
increase in bond order of the carbon to the nitrogen link [47]. The participation of phenolic group is deduced
by clarifying the effect of chelation on the υ(C-O) stretching vibration. The shift in υ(C-O) of the phenolic
groups from 1512-1427cm-1 in the free ligands to lower frequencies in the region of 1311-1122cm-1 for
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complexes was observed , indicating the participation of the oxygen atom of phenolic group in coordination
to the metal ions[48].
Furthermore, the spectra of Schiff base ligands showed broad bands in 3051-2994cm-1, attributed to the
starching vibration of the O-H groups. The absence of the vibration peaks of OH in all complexes exhibits
that the OH groups is deprotonated and coordinated[49,50].The weak bands in the regions of 509-543cm-1,
and 540-430cm-1 , in the complexes may be assigned to the υ(M-O) and υ(M-N), respectively[51]. Again,
υ(C-S) in complexes shifted to higher frequencies from 721 to 740-790 cm-1, which is evidence of the M-S
bond [52]. The bands in 1114-1465cm-1 regions are due to the skeletal stretching vibration of C=C backbone
in the benzene rings. Several weak bands observed in the region 3000-2700cm-1 is most likely due to the
aromatic C-H groups. Also, in these compounds the common bands at 750-753 cm-1 were assigned to the C-S
and S-S stretches [53]. The vibrational peak of NO2 in the IR spectrum of H2L is observed in 1597 and 1573
cm-1. The IR spectra of Schiff base ligands and its dimeric complexes were shown in Figs. 7-11. The IR data
of ligand showed that the Schiff bases was coordinated to the metal ions in a tridentate-ONS/ NNS or
bidentate-NS manner.The infrared spectra of ligands and their complexes were agreed with the proposed
molecular structures.
3.5 FAB-mass study
The FAB- mass (m/z) spectra suggested that the present binuclear complexes 1-3 showed dimeric nature. The
Schiff base ligands and their copper (II) and nickel (II) complexes, provide strong evidence for the formation
of these compounds and exhibits peaks at higher molecular weights. Thus, mass spectra of tridentate Schiff
base ligands (HL1/HL2/H2L), double Schiff base ligands (L1 /L2 /H2La) and complexes 1-3 showed the
a
a
highest mass peaks at obs. (calcd.): 241(241), 238(238), 353(354), 485(486), 474(475), 701(704), 611(612),
636(638) and 819(820), respectively. Some representative FAB-mass (m/z) spectra of metallic complexes
are presented in Figs. 12-14, respectively.
3.6 Electronic spectra, fluorescence features and magnetic moment studies
The magnetic susceptibility measurements for binuclear complexes 1-3 were measured in the solid state. The
observed magnetic moments of these complexes are quite close to the value expected for the other copper (II)
and nickel (II) complexes. The value of magnetic moment at room temperature allows to suppose that the
complex 1 and 2 are built up by the antiferromagnetic coupled binuclear cluster (S=1 state) and that the
magnetism of complex 3 are due to the unpaired one-spin system (S=½), with mild intermolecular
interactions. Complex 1 and 2 are paramagnetic in solid state at room temperature as expected from d9
electronic configuration of Cu (II) ion. The binuclear complex 1 and 2 has substantially reduced magnetic
moments 1.51 BM. for 1 and 1.48 B.M. for 2, consistent with proposed dimeric structures, suggesting the
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possibility of spin-coupled system [54]. This behavior is ascribed to a paramagnetic species containing two
antiferromagnetic ally coupled Cu (II) ions. The antiferromagnetic behavior occurs when two copper (II)
coordination spheres connect at axial sites of two square planar units. As expected, room temperature
magnetic moments values of complexes 3 differ markedly from that of the complex 1 and 2. The observed
magnetic moment value of complex 3 was 3.61 B.M., in agreement with an unpaired one spin (S= ½) system.
The UV-Vis spectral data of tridentate Schiff base ligands and its complexes 1-3 were recorded (Fig.15). The
electronic absorption spectrum provides reliable information about the ligand arrangement in transition metal
complexes. The electronic spectra of free tridentate ligands exhibits absorption bands at 430-450 and 370-
390nm, which are assigned to the n-π*transitions of the imine moiety and π -π*transitions of phenyl rings,
respectively. The electronic spectrum of 1 of HL1 exhibited three bands, 450,380 and 268nm, which may be
assigned to d-d, n-π* and π -π*transitions, respectively. The electronic spectra of 2 shows a broad band at
440nm due to d-d transition and at367nm which may be due to n-π* band of the imine. Complex 2 of HL1
has another band cantered at 272nm which is assigned to π-π*transitions of phenyl rings [55, 56]. Complex 3
of H2L shows bands at 450, 380 and 280 nm which may be assigned to d-d, n-π*, and π -π*transitions,
respectively. All these observations make us to suggest the structure of these complexes.
Fluorescent properties (absorption and emission properties) of tridentate Schiff base ligands and its
complexes were investigated at room temperature in DMSO (Fig. 16). The tridentate Schiff base ligands has
two absorption maxima; the first one is between 260-275nm and the second one is between 350-385nm. For
the first absorption maxima appeared in the same region, the second absorption maxima shifted to lower
wavelengths between 410-420nm, presumably due to the complete transfer of the electrons from the ligands
to the metal ion. The extinction coefficients at the shorter absorption maxima are higher than the extinction
coefficients at the longer absorption maxima for ligands and all the complexes. Complexes have one
emission band, which is between 410-480nm. The fluorescence quantum efficiencies of the ligands and the
complexes with a standard of quinine sulfate indicated that the highest quantum yield values were obtained
for all the tridentate Schiff base ligands compared to its complexes [57].
3.7 ESR study
ESR is an ideal, complementary technique to study materials with unpaired electrons. Best evidence for the
proposed structures of the binuclear complexes comes from the magnetic measurements and ESR spectra.
ESR spectra of binuclear complex 1 and 2 were recorded in polycrystalline state and also in 100%DMSO and
at liquid nitrogen temperature (77K). The ESR spectra of 1 and 2 are shown in Figs.17 and 18. Basic spectral
characteristics at both temperatures are the same with slightly better resolution at LNT. Conforming the
magnetic measurements, the polycrystalline ESR spectra of both complexes showed the spectral features of
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an antiferromagnetically coupled binuclear species: (i) The ∆Ms=± ‘half filled ’ signals is observed at 1700G
, (ii) ∆Ms=±1 region shows two broad signals at -2960G and 3200 G, -3080G and 3400 G , respectively ,and
is characteristics of isotropic spectral feature. Interestingly, the EPR spectra of 1 and 2 are similar. For each
complex spectral feature of RT and LNT was also similar. The g = 2 signals for these complexes was broad
and nearly isotropic. This is suggestive of the spin-spin interactions arising due to solid effect [58]. The room
temperature polycrystalline ESR spectra of 1 and 2 exhibits the usual shape line for mononuclear copper (II)
2
complexes with g||> g >2.0023, indicating axial symmetry (dx -y2).
Both complexes exhibits two geff (g||= 2.058±0.003, g =2.268±0.001 with gav= 1.68002±1 for 1 and g||=
2.246±0.001, g =2.268±0.001 with gav= 1.5862±1 for 2). It should be pointed out that these values are quite
close to that of Cu, Zn-SOD (g||= 2.271, g =2.083) [59]. The value of geff and the shape of the ESR signals of
the copper (II) complexes suggest square planar coordination around the Cu (II) ions [60]. The positive
contribution of the geff values over the free electron value (2.0023) indicates an increase in the covalency
between the ligands and the Cu (II) ions in the complexes. The magnetic moment for 1 and 2 are µeff =1.51
and 1.48, respectively at room temperature. The calculated µeff values of the magnetic moments may be due
to the antiferromagnetic interaction between the two electrons in the two adjacent Cu (II) ions. Since the g||
and g values are closer to 2 and g||> g a tetragonal distortion around the Cu (II) ion is suggested. The trend
,
2
2
(g||> g >ge(2.0023) shows that the unpaired electron is localized in the dx -y orbital in the ground state of
Cu(II) and spectra were characteristics of axial symmetry environments (g|| > g > 2.0023) , with an electronic
configuration 3d9, electron spin S=1/2 and nuclear spin I=3/2. The value of g||>2.3 is characteristics of an
ionic environment and g||<2.3 indicates a covalent environment in metal-ligand bonding. As observed in the
investigated spectra, complex 3 show g|| value < 2.3, which is a feature observed in copper (II) complexes
with covalent bonds [61,62]. The hyperfine coupling constant for 1 and 2 was 170G and 172G, respectively.
The exchange coupling interaction between two Cu (II) ions is explained by the Hathaway expression [63].
G = (g|| – 2.0023) / (g – 2.0023)
(4)
According to Hathaway, if the value G is greater than 4 (G> 4.0), the exchange interaction is negligible
whereas when the value of G is less than 4 (G<4.0), a considerable exchange coupling is present in a solid
complex. As observed in the spectra of complexes 1-3, G value was less than 4((3.37 for 1 and 3.65for 2),
indicating considerable exchange interaction in this complex.
3.8 SOD-like activity
The SOD-like activities of tridentate Schiff base ligands and its complexes 1-3 were investigated by NBT-
DMSO assay, and the catalytic activity towards the dismutation of superoxide ion was measured. The IC50
value is considered the concentration equivalent to one unit of SOD activity. The coordination arm with two
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tridentate Schiff base ligands provide a stable and flexible environment to that in the active sites of the
native enzymes, insuring the stable existence of the native enzyme and the stable existence of the complexes.
In this work, the SOD-like activities for the three binuclear complexes were measured. These complexes
exhibit effective catalytic activity towards the dismutation of superoxide anions. The average suppression
-
ratio against O2 increases with increasing concentration in the range of the tested compounds. The SOD
activity data showed that complexes 1-3 exhibited effective SOD activity, which is indicative of their
potential application as an antioxidant. The IC50 values presented by complexes were compared with earlier
reported values for nickel (II) and copper (II) Schiff base complexes and Cu, Zn-SODs (determined by
different research groups) [64-70]. The catalytic activity of Ni-SOD [25], however, is a same high level as
that of Cu, Zn-SOD at about 109(ML-1)-1S-1 per metal center [26].The representative graph showing IC50 data
of the SOD activity of tridentate Schiff base ligands and compounds are presented in Figs.19 and 20. The
results (Table 4) indicated that the prepared compound is more efficient antioxidant than vitamin C, which is
the standard superoxide dismutase [69].
Interestingly, the double Schiff base ligands did not exhibit the SOD activity. Complex 1 exhibits more
-
active scavenging effects against O2 than HL1 and complex 2 of HL2and complex 3 of H2L under the same
conditions. The scavenging effect of the complexes may be ascribed to the chelating function of Schiff base
with metal ions to achieve significant activity of radical scavenging activity [70]. Schiff base complex 1-3
show lower IC50, and exhibits higher SOD activity. The IC50 values for tridentate Schiff base ligands and its
complexes 1-3 are 62, 59, 56, 44, 41, and 39 µmol dm-3, respectively. The activities of these complexes are
similar to those of the other complexes [71, 72]. They are among the most active model compounds but are
somewhat less active than the native enzyme (IC50=0.04µmol dm-3). The kinetic catalytic constant for
complexes 1-3 were 3.75×104, 5.52×104 and 4.12×104(ML-1)-1s-1, respectively. Values of the catalytic rate
constant for superoxide disproportionation clearly showed that these complexes can be used as superoxide
scavengers. The IC50 data of SOD assay along with kinetic catalytic constants of compounds kMCCF =kNBT
[NBT]/ IC50, where kNBT (pH=7.8) =5.94x104M-1S-1[73, 74] are listed in Table 4. Comparison of k
of
MCCF
metal complexes revealed that the geometry around M (II) contributes to modulate SOD activity, with 1 to
being best suited to react with superoxide radical. On the basis of above results and early suggested catalytic
mechanism [75], a possible mechanism for catalytic process is proposed as follows: First a superoxide anion
-
-
(O2 ) is attracted and bounded by M (II) ion with all coordination sites. Second, a superoxide anion (O2 )
gives up its electron and form an oxygen molecule, and then the electrically neutral oxygen molecule leaves.
-
Third, another superoxide anion (O2 ) is attracted and bounded by M (I) ion. Fourth, the superoxide anion
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-
(O2 ) will accept an electron from M (I) ion and two protons from the buffer solution, and then form a H2O2
molecule. Finally, electrically neutral hydrogen peroxide leaves, completing a catalytic cycle.
3.9 Biological screening activity
Antibacterial activities were estimated by MIC (mm) as shown elsewhere [76]. The present complexes were
tested for their in vitro antibacterial activities against three bacteria against three bacteria Streptococcus
aureus, Salmonella typhi and Escherichia coli comparing with tridentate Schiff base ligands. The results
(Table 5) of antibacterial activities of dimeric complexes of together with the results of the Schiff bases and
of the double Schiff bases as comparison are shown in Figs.21 and 22. The Schiff bases shown a wide
spectrum of antibacterial activities against the test organisms and especially, show effective activities against
E.coli. The double Schiff base ligands show modest activities against selected bacterial (Streptococcus
aureus, Salmonella typhi and Escherichia coli). The free ligand HL1a, which contain bicyclic ring system
with N, S donor atoms did not exhibit the growth of test organism. The ligand L1b did not show effective
antibacterial activities against selected test organisms although the ligand contained no O atoms (bidentate N
and S). Any chemotherapeutic agents reduce the growth of microbes by microbial or microstatic mechanism.
The tested compounds 1-3 show good antibacterial activity against microorganisms. On comparing the
antibacterial activities of free Schiff base ligands, double Schiff base ligands and metal complexes. With
these of standard bactericide, it was shown that metal complexes had moderate activity as compared to
standard Chloramphenicol but all the complexes were more active than tridentate Schiff base ligands and
double Schiff base ligands. These complexes show better activity than the known antibiotic with 1 being
more active than 2 and 3. Complexes 1 and 2 have shown excellent antibacterial activity than E.coli
comparable to that to Streptococcus aureus and Salmonella typhi. They inhibit the bacterial growth up to 15-
25% µgml-1 whereas other has poor antibacterial activity [77]. The higher inhibition zone of the metal
complexes than those of free tridentate Schiff bases and double Schiff bases can be explaining based on the
Overtone’s concept and the Chelation theory[78,79]. According to Overtone’s concept of cell permeability,
the lipid membrane that surrounds the cell favours the passage of only lipid-soluble materials due to which
liposolubility is an important factor which controls the antibacterial activity. On chelation, the polarity of the
metal ion will be reduced to a greater extent due to the overlap of the ligand orbital and partial sharing of the
positive charge of the metal ion with donor groups. Further, it increases the delocalization of π-electrons over
the whole chelating ring and enhances the lipophilicity of the complexes. This increased lipophilicity
enhances the penetration of the complexes into lipid membranes and blocking of the metal binding sites in
the enzymes of microorganisms. These complexes also disturb the respiration process of the cell and thus
block the synthesis of proteins which restricts further growth of the microorganism [80, 81].
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Conclusion
In this paper, we reported syntheses, characterization and catalytic activity of SOD-like metallic complexes
of copper (II) and nickel (II) with three new Schiff base ligands containing 2-aminthiophenol moiety.
Elemental analyses showed that all metals reacted with the ligands in a 1:2 molar ratio. The FAB- mass (m/z)
spectra suggested that the present binuclear complexes 1-3 showed dimeric nature. The room temperature
polycrystalline ESR spectra of 1 and 2 exhibits the usual shape line for mononuclear copper (II) complexes
2
with g||> g >2.0023, indicating axial symmetry (dx -y2). The X-ray structures of the Schiff base ligands
showed that in the crystalline form the SH groups were oxidized to produce a disulfide Schiff bases as a new
double Schiff base ligands (L1 /L2 /H2La). The Schiff bases shown a wide spectrum of antibacterial activities
a
a
against the test organisms and especially, show effective activities against E.coli. The double Schiff base
ligands show modest activities against selected bacterial (Streptococcus aureus, Salmonella typhi and
Escherichia coli). Most of the complexes have higher antibacterial activities than those of the free Schiff
bases and the control. These complexes show lower biological activity than the known antibiotic
chloramphenicol with complex 1 being more active than complex 2 and 3. Complex 1 exhibits more active
-
scavenging effects against O2 than HL1 and complex 2 of HL2and complex 3 of H2L under the same
conditions. The double Schiff base ligands did not exhibit the SOD activity
Acknowledgements
Author thanks the Sophisticated Analytical Instrument Facility (SAIF), Single Crystal X-Ray Division, IIT
Chennai and Sophisticated Instrumentation Centre (SIC) IIT Indore for single crystal data collection,
respectively. Author wish to thanks to Head RSIC (SAIF), IIT Mumbai for EPR measurements. The Head
RSIC (SAIF), Central Drug Research Institute, Lucknow is also thankfully acknowledged for providing
analytical and spectral facilities. This work was financially supported from CSIR (Scheme No.
01(2856)/16/EMR-II), New Delhi, India is thankfully acknowledged.
Supplementary material
CCDC 1536404, 1573259 and 1561750, contain the supplementary crystallographic data for double Schiff
base ligands L1b/L2 /H2La, respectively. Copies of this information can be obtained free of charge via
a
http://www.ccdc.cam.ac.uk/conts/retrieving.html/ data request/cif, or from The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK, Fax :( +44)-1223-336033/ Email:
deposit@ccdc.cam.ac.uk.
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[22]

Table 1.Crystal data and structure refinement of ligands (L1b), (L2a) and H2La.
ACCEPTED MANUSCRIPT
L1b
L2a
H2La
Empirical formula
C14 H11 N O S
C28 H18 N4 S2
C26 H14 Br2 N4 O6 S2
Formula weight
Temperature (K)
241.30
293(2)
474.58
296(2)
702.35
296(2)
Wavelength (Å)
Crystal system
0.71073 Å
Monoclinic
P21/n
0.71073 Å
Monoclinic
C/2c
0.71073 Å
Triclinic
P-1
Space group
Unit cell dimensions
a (Å)
7.3276(6) Å
12.6798(11) Å
12.7253(11) Å
90
14.8849(18) Å
8.3000(9) Å
19.664(2) Å
90
8.2685(12) Å
11.7305(16) Å
14.4154(18) Å
102.618(7)
b (Å)
c (Å)
α
(0)
(0)
β
96.484(2)°
100.625(9)
96.008(7)
ᵞ
(0)
90
90
100.367(7)
1327.0(3)
Volume (Å3)
1174.78(17)
2387.7(5)
Dcalc. (Mg/m3)
Z
1.364
4
1.320
4
1.758
2
Absorption coefficient (mm-1)0.256
0.247
984
3.262
696
F (000)
504
Crystal size (mm)
θRange for data collection (°) 2.275 - 25.000
0.30 x 0.25 x 0.20
0.35 x 0.35 x 0.30
2.11 -28.36
0.30 x 0.20 x 0.20
1.46 - 28.32
Limiting index
-8<=h<=8
-15<=k<=15
-15<=l<=15
-19<=h<=19
-10<=k<=11
-24<=l<=26
-10<=h<=10
-15<=k<=14
-18<=l<=19
Reflections collected/unique [Rint] 12602/2071 [0.0288] 12426 / 2797 [0.0596]10649 / 5951 [0.0253]
Completeness to θ
Absorption correction
25.00−100%
Semi-empirical
28.36- 93.5 %
Semi-empirical
from equivalents
0.9295 and 0.9185 0.5615 and 0.4411
Full-matrix
28.32 - 90.1 %
Semi-empirical
from equivalents
from equivalents
0.7455 and 0.6844
Full-matrix
Max. and min. transmission
Refinement method
Full-matrix
least-squares
2071 / 0 / 154
least-squares
2797 / 0 / 159
least-squares
5951 / 0 / 365
Data / restraints / parameters
Goodness-of-fit on F2
1.089
1.112
0.836
Final R indices [I>2 σ(I)]
R1 = 0.0361,
wR2=0.0887
R1 = 0.0548,
wR2 = 0.1061
R1 = 0.0655,
wR2 = 0.1434
R1 = 0.0982,
wR2 = 0.16651750
R1 = 0.0497
wR2 = 0.1316
R1 = 0.1138
wR2 = 0.1750
R indices (all data)
Largest diff. peak and hole (e.Å-3) 0.198 and -0.191
0.692 and -0.334
0.887 and -0.881
1

ACCEPTED MANUSCRIPT
Table 2. Selected bond lengths (Å) and angles (°) for (L1b), (L2a) and H2La.
(L1b)
Bond length C(7)-S(1)
C(13)-O(2) 1.361(2)
Bond angles N(1)-C(7)-C(8)
1.752(2)
C(7)-N(1)
1.301(3)
1.471(3)
115.23(16)
C(7)-C(8)
121.45(18)
N(1)-C(7)-S(1)
(L2a)
Bond length S(2)-S(2)#1
N(1)-C(6)
2.0349(15)
1.415(4)
N(1)-C(7)
1.234(4)
1.133(5)
122.7(3)
C(14)-N(2)
C(7)-N(1)-C(6)
Bond angles C(5)-S(2)-S(2)#1
H2La
104.74(10)
Bond length S(1)-S(2)
C(5)-O(3)
2.062(2)
1.305(6)
103.96(17)
124.8(4)
C(7)-N(2)
1.289(6)
1.415(5)
103.91(17)
124.0(4)
C(19)-N(3)
Bond angles C(18)-S(1)-S(2)
C(7)-N(2)-C(8)
C(9)-S(2)-S(1)
C(20)-N(3)-C(19)
Symmetry transformations used to generate equivalent atoms: #1 -x+2, y,-z+1/2 (L2a).
Table 3. IR characteristic band frequencies (cm-1) of the Schiff base ligands and its metal complexes.
Ligands/
ν(C=N) ν(C-O) ν(C=C) ν(O-H)
ν(M–N) ν(M–O) C–H C-S
Ar-H
Compounds
HL1
1581
1157
1484
-
-
-
2910 755
3051
HL2
H2L
1
1627
1597
1624
-
1464
1482
1460
-
-
-
.
2812 753
2855 750
2918 751
3032
3045
3047
1230
1136
3390
-
-
576
482
1635
1635
-
1572
1565
-
547
552
466
485
2890 749
2872 747
3045
3029
2
3
1222
3385
2

ACCEPTED MANUSCRIPT
Table 4.Superoxide dismutase activity of some copper (II)/nickel (II) complexes.
S. N. Metal complexes
IC50
KMCCF X104(ML-1)-1S-1
Reference
(µmol dm-3)
1.
2.
3.
44
41
39
62
59
56
3.75
This work
This work
This work
This work
This work
This work
[70]
1
2
5.52
4.12
3
4. HL1
5. HL2
-
-
6. H2L
-
7. CuCl2.2H2O
8. Cu, Zn- SODa
9. Cu, Zn- SODb
10. Cu, Zn- SODc
11. Cu, Zn- SODc
12. Vc
0.910
0.15
0.03
0.0026
0.04
852
42
-
-
[69]
-
[69]
-
[70]
-
[70]
-
[71]
13. [Cu(L2)]
7.86
8.50
9.50
12.79
5.28
3.14
8.99
[30]
14. [Cu(L2)].DMF
15. [(L) Cu (µ-CH3COO)2Cu(L)]
16. [(L) Cu (µ-NO3)2Cu(L)]
17. [Cu(HL)2].(ClO4)2
45
[30]
8.26
26
[22]
[22]
63
[66]
18. [Ni (HL) 2].NO3. ClO4. 0.5H2O 106
19. [Cu((µ-CH3COO) (L)2].4H2O 37
[66]
[66]
aNBT assay in the presence of alkaline DMSO: the other species were evaluated in the
xanthine/xanthine oxidase system.
bIC50 was determined by employing xanthine/xanthine oxidase –mediated reduction of NBT.
cIC50 was determined by measuring the inhibition of the photoreduction of NBT.
kMcCF were calculated by K = kNBT × [NBT]/IC50, kNBT (pH 7.8)= 5.94×104 (M L-1)-1s-1 [73,74].
3

ACCEPTED MANUSCRIPT
Table 5.Antibacterial activities of tridentate Schiff bases and its metal complexes 1-3
.
Complexes (mM)
Diameter of inhibition zone (in mm)
Salmonella typhi Streptococcus aureus E.Coli
HL1
10
5
0
7
20
14
16
17
19
9
30
HL2
18
7
10
10
13
15
12
15
19
20
10
14
30
H2L
4
10
0
8
15
20
10
15
14
22
19
30
1
8
10
11
14
28
12
22
32
20
15
24
30
2
10
9
16
24
31
7
20
21
23
13
22
9
30
3
10
13
14
31
18
25
33
16
21
20
30
Chloramphenicol
30
30
35
25
None
DMSO
None
None
Note: Key to interpretation (10-30mM): less than 10mm, inactive; 10-15mm, moderately active; above 15mm,
highly active. DMSO (control) shows not clear inhibition zone. Estimated error: ±1mM.
4

ACCEPTED MANUSCRIPT
Fig.1.ORTEP structure of Schiff base (L1b) with 30% thermal ellipsoid and with atom numbering scheme.
Fig.2.ORTEP structure of double Schiff base L2a with 50% thermal ellipsoid and with atom numbering
scheme.

ACCEPTED MANUSCRIPT
Fig.3. ORTEP structure of double Schiff base H2La with 50% thermal ellipsoid and with atom numbering
scheme.