Synthesis, characterization, and antimicrobial activities of nickel(II) and copper(II) Schiff-base complexes

Article abstract of DOI:10.1080/00958970903296362

Ni(II) and Cu(II) metal complexes of simple unsymmetrical Schiff-base ligands derived from salicylaldehyde/5-methylsalicylaldehyde and ethylenediamine or diaminomaleonitrile (DMN) were synthesized. The ligands and their complexes were characterized by elemental analysis, ¹H NMR, FT IR, and mass spectroscopy. The electronic spectra of the complexes show d-d transitions in the region at 450-600 nm. Electrochemical studies of the complexes reveal that all mononuclear complexes show a one-electron quasi-reversible reduction wave in the cathodic region. ESR spectra of the mononuclear copper(II) complexes show four lines, characteristic of square-planar geometry, with nuclear hyperfine spin 3/2. The copper(II) complexes show a normal room temperature magnetic moment value μ_eff = 1.70-1.74 BM which is close to the spin only value of 1.73 BM. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as catalysts were also carried out. The in vitro antimicrobial activity of the investigated compounds was tested against human pathogenic bacterias such as Staphylococcus aureus, Bacillus subtilis, Klebsiella pneumonia, Pseudomonas aeruginosa and Escherichia coli. The antifungal activity was tested against Candida albicans. Generally, the metal complexes have higher antimicrobial activity than the free ligands.

Full text of DOI:10.1080/00958970903296362

Journal of Coordination Chemistry
Vol. 63, No. 1, 10 January 2010, 136–146
Synthesis, characterization, and antimicrobial activities of
nickel(II) and copper(II) Schiff-base complexes
M. RAJASEKARy, S. SREEDARANz, R. PRABUz, V. NARAYANANz,
R. JEGADEESHx, N. RAAMANx and A. KALILUR RAHIMAN*y
yPG & Research Department of Chemistry, The New College (Autonomous),
Chennai 600 014, Tamil Nadu, India
zDepartment of Inorganic Chemistry, School of Chemical Sciences, University of Madras,
Guindy Campus, Chennai 600 025, Tamil Nadu, India
xCentre for Advanced Studies in Botany, University of Madras, Guindy Campus,
Chennai 600 025, Tamil Nadu, India
(Received 29 January 2009; in final form 8 July 2009)
Ni(II) and Cu(II) metal complexes of simple unsymmetrical Schiff-base ligands derived from
salicylaldehyde/5-methylsalicylaldehyde and ethylenediamine or diaminomaleonitrile (DMN)
were synthesized. The ligands and their complexes were characterized by elemental analysis,
1H NMR, FT IR, and mass spectroscopy. The electronic spectra of the complexes show d–d
transitions in the region at 450–600 nm. Electrochemical studies of the complexes reveal that all
mononuclear complexes show a one-electron quasi-reversible reduction wave in the cathodic
region. ESR spectra of the mononuclear copper(II) complexes show four lines, characteristic
of square-planar geometry, with nuclear hyperfine spin 3/2. The copper(II) complexes show a
normal room temperature magnetic moment value ꢀeff ¼ 1.70–1.74 BM which is close to the
spin only value of 1.73 BM. Kinetic studies on the oxidation of pyrocatechol to o-quinone using
the copper(II) complexes as catalysts were also carried out. The in vitro antimicrobial activity
of the investigated compounds was tested against human pathogenic bacterias such as
Staphylococcus aureus, Bacillus subtilis, Klebsiella pneumonia, Pseudomonas aeruginosa and
Escherichia coli. The antifungal activity was tested against Candida albicans. Generally, the
metal complexes have higher antimicrobial activity than the free ligands.
Keywords: Schiff-base ligands; Cyclic voltammetry; Catecholase activity; Antimicrobial
activity
1. Introduction
Schiff-base ligands derived from the condensation of salicylaldehyde with diamines and
their complexes [1, 2] played an important part in the development of inorganic
chemistry, as widely studied coordination compounds are increasingly important as
biochemical, analytical, and antimicrobial reagents. Tetradentate Schiff-base complexes
of transition metal ions [3–5] have attracted considerable attention as promising
materials due to interesting magnetic exchange and spectroscopic characteristics [6–9].
*Corresponding author. Email: akrahman_tajn@yahoo.co.in
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958970903296362

Copper(II) Schiff-base complexes
137
Complexes of Schiff bases have been used as antibacterial, antifungal, anticancer,
antitubercular, hypertensive, and hypothermic reagents [10–12]. Salen complexes of
transition metals have also been used as biomimetic catalysts for oxygen atom
transfer [13].
Structure–activity relationships of the metal complexes are determined by the
oxidation state of metals, the type and number of donor atoms, and relative disposition
within the ligand [14]. Parashar et al. [15] reported copper(II), cobalt(II), and nickel(II)
complexes of the Schiff-base ligands obtained from 2-substituted anilines, and
salicylaldehyde exhibit good antifungal and antibacterial activity.
Phenolic compounds are universal inhibitors of free-radical processes, retarding both
oxidation and free-radical fragmentation reactions of important biomolecules (lipids,
peptides, carbohydrates, vitamins, etc.) and many phenolic compounds have antibac-
terial activities [16]. In the presence of metal ions these phenolic compounds are believed
to damage the cytoplasmic membrane, which is related to their bactericidal activity [17].
These data provide a good basis for attempts to use simple phenolic ligands in the
syntheses of new bioactive metal complexes.
As part of our continuing efforts to synthesize and characterize transition metal
complexes of simple and inexpensive Schiff bases, in this article we describe the
synthesis, characterization, and antimicrobial activities of Schiff bases and their Ni(II)
and Cu(II) complexes derived from salicylaldehyde/5-methylsalicylaldehyde and
ethylenediamine or diaminomaleonitrile (DMN).
2. Experimental
2.1. Physical measurements
Elemental analyses of the complexes were obtained using a Haereus CHN rapid
analyzer. IR spectra were recorded on a Shimadzu FT-IR 8300 series spectro-
photometer on KBr disks from 4000 to 400 cmꢀ1. 1H NMR spectra were recorded using
a JEOL GSX 400 MHz NMR spectrometer. Atomic absorption spectral data were
recorded using a Varian spectra AA-200 model atomic absorption spectrophotometer.
Mass spectra were obtained on a JEOL DX-303 mass spectrometer. Electronic spectral
studies were carried out on a Hitachi 320 spectrophotometer from 200 to 1100 nm.
Cyclic voltammograms were obtained on a CHI-600A electrochemical analyzer under
oxygen-free conditions using a three-electrode cell in which a glassy carbon electrode
was the working electrode, a saturated Ag/AgCl electrode was the reference electrode,
and platinum wire was the auxiliary electrode. A ferrocene/ferrocenium (1þ) couple
was used as an internal standard and E1/2 of the ferrocene/ferrocenium (Fc/Fcþ)
couple under the experimental condition was 470 mV. Tetra(n-butyl)ammonium
perchlorate (TBAP) was used as the supporting electrolyte. Room temperature
magnetic moments were measured on a PAR vibrating sample magnetometer Model-
155. X-band EPR spectra were recorded at 25ꢁC on a Varian EPR-E 112 spectrometer
using diphenylpicrylhydrazine (DPPH) as the reference. Catalytic oxidation of catechol
to o-quinone by the copper complexes was studied in 10ꢀ3 M dimethylformamide
solutions. The reactions were followed spectrophotometrically with the strongest
absorption band of o-quinone at 390 nm and monitoring the increase in absorbance.

138
M. Rajasekar et al.
A plot of log(A /A1 ꢀ At) versus time was made for each complex and the rate
1
constants for the catalytic oxidations were calculated.
2.2. Materials and safety note
5-Methyl salicylaldehyde [18] was prepared by following the literature method.
Glycerol, boric acid, p-cresol, THF, and hexamethylenetetramine were purchased
from Qualigens and used as received. Analytical grade methanol, ethanol, chloroform,
and dimethylformamide were purchased from Qualigens. TBAP used as supporting
electrolyte in electrochemical measurement was purchased from Fluka and recrystal-
lized from hot methanol. (Caution: TBAP is potentially explosive; perchlorate
complexes are also potentially explosive; and hence, care should be taken in handling
the compounds. However, the small quantities used in this study were not found to
present a hazard.) All other chemicals and solvents were of analytical grade and used
as received.
2.3. Synthesis of L1
5-Methy1 salicylaldehyde (10 mmol, 1.38 g) and ethylenediamine (10 mmol, 0.7 mL)
were added to chloroform (50 mL) and the mixture was refluxed for 18 h. The resulting
yellow solution was cooled to room temperature and the yellow needles were collected
by filtration, washed with hexane, and recrystallized from THF. The product (10 mmol,
1.78 g) obtained was refluxed with salicylaldehyde (10 mmol, 1.04 mL) in ethanol
(50 mL) for 16 h. The resulting yellow solution was cooled in ice to give yellow
precipitate which was filtered, washed with hexane, and dried under vacuum.
Recrystallization from THF gave yellow crystals.
Yield (for monocondensed Schiff base): 0.62 g (34.8%). m.p.: 158ꢁC. Anal. Calcd for
C10H14N2O (%): C, 67.39; H, 7.92; N, 15.72. Found (%): C, 67.12; H, 7.98; N, 15.50.
ESI MS: (m/z) (%) 179.16, Calcd av. m/z 178.28. Yield (L1): 0.4 g (14.2%). m.p.: 128ꢁC–
132ꢁC. Anal. Calcd for C17H18N2O2 (%): C, 71.81; H, 6.43; N, 9.92. Found (%): C,
1
71.72; H, 6.34; N, 9.81. ESI MS: (m/z) (%) 283.20, Calcd av. m/z 282.34. H NMR: ꢁ
(ppm in CDCl3): 2.25 (s, 3H, Ar–CH3), 3.91–3.92 (d, 4H, N–CH2), 6. 82–6.86 (t, 1H),
6.92–6.94 (d, 1H), 7.0 (d, 1H), 7.07–7.09 (d, 1H), 7.21–7.28 (t, 1H), 8.28–8.29 (d, 2H,
–CH¼N), 12.97 (s, Ar–OH).
2.4. Synthesis of L2
A solution of 5-methyl salicylaldehyde (4.70 mmol, 0.64 g) and diaminomaleonitrile
(DMN) (4.65 mmol, 0.503 g) in THF was refluxed for 18 h. The resulting brown
solution was cooled to give brown precipitate which was washed with hexane. The
product (2 mmol, 0.45 g) was refluxed with a solution of salicylaldehyde (2 mmol,
0.210 mL) in THF (50 mL) for 16 h. The resulting brown solution was cooled in ice to
give brown precipitate, which was collected, washed with hexane and dried under
vacuum. Recrystallization from chloroform gave brown solid.
Yield (for monocondensed Schiff base): 0.43 g (40.6%). m.p.: 120ꢁC. Anal. Calcd for
C12H10N4O (%): C, 63.71; H, 4.46; N, 24.77. Found (%): C, 63.53; H, 4.61; N, 24.65.

Copper(II) Schiff-base complexes
139
ESI MS: (m/z) (%) 227.40, Calcd av. m/z 226.24. Yield (L2): 0.17 g (25.8%). m.p.: 210ꢁC
(dec). Anal. Calcd for C19H14N4O2 (%): C, 69.08; H, 4.27; N, 16.96. Found (%): C,
68.96; H, 4.32, N, 16.88. ESI MS: (m/z) (%) 331.41, Calcd av. m/z 330.35. 1H NMR: ꢁ
(ppm in CDCl3): 2.27 (s, 3H, Ar–CH3), 6. 83–6.85 (t, 1H), 6.92–6.94 (d, 1H), 7.0
(d, 1H), 7.15–7.19 (d, 1H), 7.21–7.28 (t, 1H), 8.34–8.46 (d, 2H, –CH¼N), 12.93
(s, Ar–OH).
2.5. Synthesis of metal complexes
Metal complexes were synthesized by mixing 1 : 1 methanolic solution of L1/L2 and the
appropriate metal perchlorate salts. The resulting mixture was refluxed for 8–12 h in a
water bath, then concentrated to ca 15 mL and ca 15 mL of ether was added and the
reaction mixture was cooled to 0ꢁC. The precipitated metal complexes were obtained by
filtration, washed with ether, recrystallized from chloroform, and dried under vacuum.
Ni(II)L1: Yield: 2.14 g (63%). m.p.: 4250ꢁC (dec). Anal. Calcd for C17H16N2O2Ni
(%): C, 60.22; H, 4.76; N, 8.26; Ni, 17.31. Found (%): C, 60.17; H, 4.72; N, 8.22; Ni,
17.28. ESI MS: (m/z) (%) 338.12, Calcd av. m/z 339.02.
Cu(II)L1: Yield: 2.83 g (82%). m.p.: 4250ꢁC (dec). Anal. Calcd for C17H16N2O2Cu
(%): C, 59.38; H, 4.69; N, 8.15; Cu, 18.48. Found (%): C, 59.32, H, 4.65; N, 8.12; Cu,
18.46. ESI MS: (m/z) (%) 344.76, Calcd av. m/z 343.87. EPR: g , 2.29; g , 2.05; A , 153.
jj
?
jj
Ni(II)L2: Yield: 2.53 g (65%). m.p.: 4250ꢁC (dec). Anal. Calcd for C19H12N4O2Ni
(%): C, 58.97; H, 3.12; N, 14.48; Ni, 15.17. Found (%): C, 58.92; H, 3.10; N, 14.43; Ni,
15.14. ESI MS: (m/z 386.20 Mþ, Calcd av. m/z 387.02).
Cu(II)L2: Yield: 3.10 g (79%). m.p.: 4250ꢁC (dec). Anal. Calcd for C19H12N4O2Cu
(%): C, 58.24; H, 3.09; N, 14.30; Cu, 16.22. Found (%): C, 58.12; H, 2.94; N, 14.13; Cu,
16.14. ESI MS: (m/z) (%) 390.63, Calcd av. m/z 391.88. EPR: g , 2.26; g , 2.06; A , 165.
jj
?
jj
3. Results and discussion
Schiff bases have been synthesized by condensation reaction of 5-methy1salicylalde-
hyde/salicylaldehyde and ethylenediamine or DMN (scheme 1); metal complexes
(figure 1) were prepared by using 1 : 1 ratio of ligand and appropriate metal complexes
as per the following known equation:
M
nþ þ H2L ! ML þ 2Hþ
Attempts to grow single crystals of the complexes by diffusion of diethyl ether vapor
into DMF or recrystallization of the complexes from acetonitrile only gave brown/
yellow powder or micro crystals.
3.1. Spectroscopic studies
The FT IR spectrum of 5-methy1salicylaldehyde shows ꢂ(C¼O) at 1642 cmꢀ1. The
ligands as well as 5-methy1salicylaldehyde show bands at 3400 cmꢀ1 corresponding

140
M. Rajasekar et al.
CH3
(a)
OH
CHO
CH3
CH3
OH
H2N
NH2
OH
OH
N
N
CHCl3
C2H5OH
OH
O
N
H2N
(b)
CH3
OH
OH
CH3
CHO
NC
H2N
THF
CH3
CN
NH2
OH
THF
N
N
CN
CN
N
OH
CN
CN
O
H2N
OH
Scheme 1. (a) Synthesis of ligand L1; (b) Synthesis of ligand L2.
CH3
CH3
O
O
N
N
N
N
CN
CN
M
M
O
O
M = Ni or Cu
Figure 1. Metal complexes of nickel and copper.
to ꢂ (–OH). The absence of these peaks in the complexes indicates the absence of OH
due to complexation. Ligands and complexes show a sharp band at 1618–1629 cmꢀ1
due to C¼N group. Formation of this new peak and the disappearance of ꢂ (C¼O) and
ꢂ (–OH) in the complexes indicate Schiff-base condensation [12, 19, 20]. For the
complexes bands at 466–430 cmꢀ1 could be assigned ꢂ (M–O). Other weak bands at
lower frequency could be assigned to ꢂ (M–N) [21]. A strong band at 1260 cmꢀ1 in the
free Schiff bases has been assigned to phenolic C–O stretch. Upon complexation, this
band shifts to higher frequency (ꢂ1300 cmꢀ1) showing coordination through phenolic
oxygen [22].

Copper(II) Schiff-base complexes
Table 1. Electronic spectral data of the complexes.
ꢄmax (nm) (" (Mꢀ1 cmꢀ1)] in methanol
141
Compound
d–d
Charge transfer
Ni(II)L1
Cu(II)L1
Ni(II)L2
Cu(II)L2
457 (5688)
574 (2084)
461 (5704)
583 (1640)
261 (35,348), 328 (55,600), 385 (75,692)
273 (48,840), 363 (12,856)
262 (41,120), 335 (58,480), 381 (84,184)
265 (25,196), 328 (14,036)
CDCl3 was used as solvent for 1H NMR spectra (Supplementary material). A singlet
at 2.2–2.4 ppm is assigned to methyl of the Schiff base. Aromatic protons were observed
at 6.8–7.4 ppm, the aromatic hydroxyl protons at 12.8–13.0 ppm, and two equivalent
imine protons (ꢀCH¼N) as a singlet at 8.2–8.4 ppm, respectively.
Electronic spectra of the complexes were recorded in CH3OH (table 1). The band
below 280 nm is due to intraligand transition (ꢃ–ꢃ*). The band between 325 and 390 nm
is due to LMCT (n–ꢃ*). The nickel(II) complexes exhibit four bands in their electronic
spectra with the band at ꢂ460 nm attributed to the d–d transition. Schiff-base
copper(II) complexes exhibit three bands in their UV-vis region with the band at
ꢂ580 nm attributed to the d–d transition. The electronic spectral values of these
complexes are close to the reported square planar Schiff-base ligands [23–26]. This
shows the existence of complexes in square planar structures. The electronic spectra
of the complexes show a ‘‘red shift’’ in the d–d absorption band upon introducing an
electron-withdrawing group in the side arm.
3.2. ESR spectra
Solid state ESR spectra of the copper(II) complexes were recorded in the X-band region
at room temperature. The ESR spectrum of Cu(II)L2 (Supplementary material) shows
four lines [27] with nuclear hyperfine spin 3/2 due to hyperfine splitting. The observed
g values are less than 2.3 indicating considerable covalent character in the M–L bonds
jj
[28]. The g values of Cu(II)L1 and Cu(II)L2 are 2.29 and 2.26 and g values are 2.05
jj
?
and 2.06, respectively, indicating square-planar geometry. The absence of the half-field
signal at 1600 G, corresponding to DMs ¼ ꢃ2 transition, ruled out Cu–Cu interaction
[29], consistent with mononuclear complexes.
The hyperfine A splitting falls in the range of 150–165 ꢄ 10ꢀ4 cmꢀ1, indicative of an
jj
electron interacting with only one copper nucleus. The relation g 4 g is typical of d9
jj
?
copper(II) complexes in a ground state doublet with the unpaired electron in a dx2ꢀy2
orbital [30–32]. The order g 4 g 4 2 and the ESR parameters coincide with related
systems suggesting the square-planar geometry [33]. In the axial spectra, the g values are
jj
?
related to the exchange interaction term G by the expression
G ¼ ð g ꢀ 2:0023Þ=ð g ꢀ 2:0023Þ
jj
?
According to Hathaway and Tomlinson [34], if G 4 4.0, exchange is negligible because
the local tetragonal axes are aligned parallel or are slightly misaligned. If G 5 4.0,
exchange is considerable and the local tetragonal axes are misaligned. The observed G
values for the complexes are higher than 4.0.

142
M. Rajasekar et al.
Table 2. Electrochemical data of Ni(II) and Cu(II) complexes (reduction at cathodic
potential).
Complexes
Epc (V)
Epa (V)
E
½
(V)
DE (mV)
Cu(II)L1
Cu(II)L2
Ni(II)L1
Ni(II)L2
ꢀ1.16
ꢀ1.10
ꢀ1.28
ꢀ1.16
ꢀ0.88
ꢀ0.90
ꢀ0.94
ꢀ0.86
ꢀ1.02
ꢀ1.00
ꢀ1.11
ꢀ1.01
280
200
340
300
The room temperature (at 298 K) magnetic moments of the complexes were
determined by vibrating sample magnetometer (VSM). The nickel(II) complexes are
diamagnetic in nature due to the square planar geometry [27]. The magnetic moments of
copper(II) complexes show a ꢀeff in the range of 1.70–1.74 BM, close to the spin-only
value of copper(II) [35]. The room temperature magnetic data of the copper(II)
complexes do not provide conclusive evidence for the geometry; however, the electronic
spectral data coupled with magnetic moment values indicate the square planar
geometry for both nickel(II) and copper(II) complexes.
3.3. Electrochemical studies
The electrochemical properties of the complexes were studied by the cyclic voltammetry
in the potential range of 0 to ꢀ2.0 V versus Ag/AgCl using DMF and 10ꢀ1 M TBAP as
supporting electrolyte. The cyclic voltammograms of Ni(II)L2 and Cu(II)L2 are shown
in Supplementary material (data are summarized in table 2).
3.3.1. Reduction at negative potential. All the complexes show one quasi-reversible
reduction wave at negative potential. DE increases with increasing scan rate and is
always greater than 60 mV. Controlled potential electrolysis carried out at 100 mV more
negative than the reduction wave consumes one electron per molecule [36].
The Ni(II) complexes show a well-defined quasi-reversible reduction peak at ꢀ0.96 to
ꢀ1.16 V which correspond to NiII
NiI [37–40].
ꢀꢀꢀꢀꢀꢀ*
)ꢀꢀꢀꢀꢀꢀ
The Cu(II) complexes show a well-defined quasi-reversible reduction peak at ꢀ0.96
to ꢀ1.12 V which corresponds to CuII
CuI. The electrochemical behavior of
ꢀꢀꢀꢀꢀꢀ*
)ꢀꢀꢀꢀꢀꢀ
the copper complexes agree with that observed for other neutral copper(II) Schiff-base
complexes [41]. Complexes of L2 are reduced at lower negative potential than L1 due to
the presence of two electron-withdrawing groups in the side arm.
3.3.2. Oxidation at positive potential. The cyclic voltammogram for Ni(II) complexes
recorded at anodic potential in the range 0 to þ1.6 V shows one quasi-reversible
oxidation wave at a positive potential (þ1.21 to þ1.36 V) (Supplementary material).
The E values of oxidation correspond to reported values [39, 40]. Controlled potential
½
electrolysis carried out at 100 mV more negative than the reduction wave indicates
consumption of one electron per molecule. Thus, the one electron process occurred at
the electrode surface is
NiII
NiIII
ꢀꢀꢀꢀꢀꢀ*
)ꢀꢀꢀꢀꢀꢀ

Copper(II) Schiff-base complexes
143
3.4. Kinetic studies of oxidation of pyrocatechol (catecholase activity)
In most of the catecholase activity studies pyrocatechol has been employed as the
substrate. The product o-quinone is stable and shows a strong absorption at
ꢄmax ¼ 390 nm [12, 36, 42]. Thus, the activities and reaction rates, respectively, can be
determined using electronic spectroscopy by following the appearance of the absorption
of o-quinone.
Catalytic activity of the copper(II) complexes was carried out using pyrocatechol
as the model substrate for identification of the functional models for metalloenzymes
[43, 44]. The oxidation of catechols (i.e., o-diphenols) to the corresponding quinones is
known as catecholase activity (scheme 2). Solutions (10ꢀ3 mol dmꢀ3) of complexes in
dimethylformamide were treated with 100 equivalents of pyrocatechol in the presence
of air. The course of the reaction was followed spectrophotometrically at 390 nm for
45 min at time intervals of 5 min. The slope was determined by the method of initial
rates by monitoring the growth of the 390 nm band of the product o-quinone. A linear
relationship for initial rate and the complex concentration obtained for the copper(II)
complexes shows a first-order dependence on the complex concentration for
the systems.
Plots of log(A /A1 ꢀ At) versus time for catecholase activity of the copper(II)
1
complexes are shown in figure 2. The observed kinetics show a first-order dependence
on complex concentration. Initial rate constant values show that Cu(II)L2 has higher
activity (7.4 ꢄ 10ꢀ3 minꢀ1) than Cu(II)L1 (5.1 ꢄ 10ꢀ3 minꢀ1). Reactivity of the
complexes differs significantly by introducing electron-withdrawing group on the
side arm.
O
O
OH
OH
1/2 O2
+
H2O
+
Scheme 2. Oxidation of catechol.
0.16
0.14
0.12
0.1
b
a
0.08
0.06
0.04
0.02
0
0
10
20
30
40
50
Time (min)
Figure 2. Catecholase activity of (a) Cu(II)L1 and (b) Cu(II)L2.

144
M. Rajasekar et al.
Structural features and electrochemical properties are important factors in
determining the catalytic activity of complexes. If the reduction potential is too
negative, the complex has decreased the catalytic activity due to a more difficult
reduction to metal(I). Literature reports [45, 46] also show that complexes containing
electron-withdrawing groups have higher catalytic activity than complexes with
electron-donating substituents.
3.5. Antifungal and antimicrobial activities
Antifungal and antibacterial activities of the complexes were tested by the well diffusion
method using Sabouraud dextrose agar and Muller Hinton agar [47]. The zone of
inhibition was recorded on the completion of the incubation and the mean diameter
for each complex at 100 mg mLꢀ1 was recorded. The zone of inhibition was recorded in
millimeters (mm). Stock solutions of tested compounds were prepared in dimethyl
sulfoxide. The diameters of the zone of inhibition produced by the compounds were
compared with the standard antibiotics Streptomycin 10 mg per well and Azithromycin
15 mg per well (for Candida albicans). Each experiment was repeated three times to
minimize the error.
3.5.1. Inoculum preparation. Fresh bacterial cultures were used for the antibacterial
susceptibility test. Inoculate five ATCC colonies of the strains to Tryptic soy or Brain
Heart Infusion broth incubate at 37ꢁC for 22–24 h. The turbidity was adjusted with
sterile broth to correspond to the 0.5 Mc Farland standard; standard inoculum of
the microorganism of 1.5 ꢄ 106 colony forming units CFU mLꢀ1 diluted 1 : 100 gives
suspension of turbidity equal to a Mc Farland standard 0.5. The turbidity was adjusted
to match a Mc Farland 0.5 barium sulfate, prepared by adding 0.5 mL of 1.175% w/v
(0.048 m) hydrate (BaCl2 ꢅ 2H2O) to 99.5 mL 0f 1% w/v (0.36) sulfuric acid.
3.5.2. Antifungal activity. We have evaluated the antifungal activity of the ligands
and complexes against C. albicans. The screening data are reported in table 3.
Table 3. Antibacterial and antifungal activity data of Schiff-base ligands and their complexes.
Representation zone of inhibition (diameter in mm)
Antibacterial activity
(100 mg mLꢀ1
Antifungal activity
(100 mg mLꢀ1
)
)
Sample
S.a
B.s
K.p
P.a
E.c
C.a
L1
–
–
–
–
–
–
–
–
L2
12.0 ꢃ 0.6
12 ꢃ 1
14 ꢃ 2
17 ꢃ 2
16 ꢃ 1
–
16 ꢃ 2
–
14.0 ꢃ 0.6
11.0 ꢃ 0.6
9
14 ꢃ 1
14 ꢃ 2
10 ꢃ 1
16 ꢃ 2
18 ꢃ 2
Ni(II)L1
Cu(II)L1
Ni(II)L2
Cu(II)L2
13 ꢃ 2
12.0 ꢃ 0.5
16 ꢃ 1
15 ꢃ 1
14 ꢃ 1
12.0 ꢃ 0.6
15 ꢃ 1
14.0 ꢃ 0.6
18 ꢃ 2
18 ꢃ 0.6
–
14.0 ꢃ 0.6
S.a ¼ Staphylococcus aureus (ATCC 12600); B.s ¼ Bacillus subtilis (ATCC 6633); K.p ¼ Klebsiella pneumonia (ATCC 13883);
P.a ¼ Pseudomonas aeruginosa (ATCC 10145); E.c ¼ Escherichia coli (ATCC 11775); and C.a ¼ Candida albicans. The values
are the arithmetic mean of zone of inhibition ꢃ standard deviation.

Copper(II) Schiff-base complexes
145
Generally, Ni(II) and Cu(II) complexes show higher activity than their corresponding
ligands and complexes of L2 show higher activity than complexes of L1.
3.5.3. Antibacterial activity. The Schiff-base ligands and their complexes were
subjected to antibacterial activity against Pseudomonas aeruginosa (ATCC 10145),
Staphylococcus aureus (ATCC 12600), Bacillus subtilis (ATCC 6633), Klebsiella
pneumonia (ATCC 13883), and Escherichia coli (ATCC 11775). The screening results
are shown in table 3. The activity reported by the present complexes is comparable with
salicylaldehyde derived Schiff-base complexes [48]. Complexes containing electron-
withdrawing group show higher activity. The complexes show higher activities against
S. aureus, P. aeroginosa, K. pneumonia, B. cearus, and E. coli. [49]. Further studies are
needed to understand the functions responsible for antifungal and antibacterial
activities of the complexes.
4. Conclusion
Simple Schiff-base complexes have been synthesized and their coordination chemistry
and antibacterial activity are investigated. Electronic spectra of Ni(II) and Cu(II)
complexes indicate square planar geometry; cyclic voltammetry exhibit one electron
quasi-reversible process. The copper complexes show higher catalytic activity on
introducing an electron-withdrawing group. All the complexes show antimicrobial
activity and the presence of an electron-withdrawing group enhances the antimicrobial
activities. The observed variation in the activity of the metal complexes and their
corresponding ligands across the various classes of organisms studied may be
attributable to the differences in cell wall and/or membrane construction. All these
studies of the complexes agree well with established trends.
Acknowledgements
The authors express their sincere thanks to the Management, Principal, and Prof. A.G.
Md. S. Oolvi, HOD, PG, and Research Department of Chemistry, The New College,
Chennai, for providing research facilities. We also thank Dr S. Sriman Narayanan,
Professor, Department of Analytical Chemistry, University of Madras, Chennai, for
providing cyclic voltammetry facility.
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