Spectroscopy, electrochemistry, and structure of 3d-transition metal complexes of thiosemicarbazones with quinoline core: Evaluation of antimicrobial property

Article abstract of DOI:10.1080/00387010903329383

A series of Co(II), Ni(II), Cu(II), and Zn(II) complexes of quinoline-thiosemicarbazones was prepared. The Schiff base ligands that provide N, O, and S donor atoms for ligation are synthesized by the condensation of 2-hydroxy-3-formylquinoline with substituted thiosemicarbazides in ethanol. The ligands and complexes are characterized by elemental analysis, infrared, ¹H NMR, UV-Vis, fast atom bombardment (FAB) mass spectroscopy, and electron spin resonance (ESR) spectral studies followed by magnetic susceptibility and conductivity measurements. The ligand-to-metal ratio is found to be 1:1 and 2:2 for the complexes of L¹H₂ and LcH₂, respectively. All the complexes are found to have octahedral geometry except [CuL¹H(H₂O)Cl], which exhibits a square pyramidal structure. All the complexes are nonelectrolytic in nature and the electrochemical behavior of complexes is dealt with briefly. Further ligands and complexes were evaluated for their antimicrobial activity against bacteria Escherichia coli and Pseudomonas aeruginosa and fungi Aspergillus niger and Cladosporidium. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00387010903329383

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Spectroscopy, Electrochemistry, and Structure of 3d-
Transition Metal Complexes of Thiosemicarbazones
with Quinoline Core: Evaluation of Antimicrobial
Property
Naveen V. Kulkarni a , Ganesh S. Hegde a , Gurunath S. Kurdekar a , Srinivasa Budagumpi a
M. P. Sathisha a & Vidyanand K. Revankar a
,
a Department of Studies in Chemistry , Karnatak University , Pavatenagar, Dharwad,
Karnataka, India
Published online: 05 Apr 2010.
To cite this article: Naveen V. Kulkarni , Ganesh S. Hegde , Gurunath S. Kurdekar , Srinivasa Budagumpi , M. P. Sathisha
& Vidyanand K. Revankar (2010) Spectroscopy, Electrochemistry, and Structure of 3d-Transition Metal Complexes of
Thiosemicarbazones with Quinoline Core: Evaluation of Antimicrobial Property, Spectroscopy Letters: An International Journal
for Rapid Communication, 43:3, 235-246, DOI: 10.1080/00387010903329383
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Spectroscopy Letters, 43:235–246, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0038-7010 print=1532-2289 online
DOI: 10.1080/00387010903329383
Spectroscopy, Electrochemistry, and
Structure of 3d-Transition Metal Complexes
of Thiosemicarbazones with Quinoline
Core: Evaluation of Antimicrobial Property
Naveen V. Kulkarni,
Ganesh S. Hegde,
ABSTRACT A series of Co(II), Ni(II), Cu(II), and Zn(II) complexes of
quinoline-thiosemicarbazones was prepared. The Schiff base ligands that
provide N, O, and S donor atoms for ligation are synthesized by the conden-
sation of 2-hydroxy-3-formylquinoline with substituted thiosemicarbazides
in ethanol. The ligands and complexes are characterized by elemental
Gurunath S. Kurdekar,
Srinivasa Budagumpi,
M. P. Sathisha,
and Vidyanand K. Revankar
Department of Studies in
Chemistry, Karnatak University,
Pavatenagar, Dharwad,
Karnataka, India
1
analysis, infrared, H NMR, UV-Vis, fast atom bombardment (FAB) mass
spectroscopy, and electron spin resonance (ESR) spectral studies followed
by magnetic susceptibility and conductivity measurements. The ligand-to-
metal ratio is found to be 1:1 and 2:2 for the complexes of L1H2 and L2H2,
respectively. All the complexes are found to have octahedral geometry
except [CuL1H(H2O)Cl], which exhibits a square pyramidal structure. All
the complexes are nonelectrolytic in nature and the electrochemical beha-
vior of complexes is dealt with briefly. Further ligands and complexes were
evaluated for their antimicrobial activity against bacteria Escherichia coli and
Pseudomonas aeruginosa and fungi Aspergillus niger and Cladosporidium.
KEYWORDS antimicrobial activity, electrochemistry, quinoline, thiosemi-
carbazone
INTRODUCTION
Thiosemicarbazones are an important class among the sulfur-containing
compounds, in view of their versatility in the coordination behavior and
pharmacological applications. Thiosemicarbazones and their metal
complexes are well known for their anticarcinogenic, antibacterial, and
antifungal properties.[1,2] It has been shown in the literature that the metal
derivatives are more effective than the free ligands in anticancer activity.
One of the many reasons is that the binding affinity of metals to proteins
or enzymes will change the interaction process with DNA, thereby affecting
DNA replication and cell proliferation.[3] In this context, hybrid chelate
ligands containing mixed functionalities are widely used as ligands for the
isolation of interesting and important metal complexes.[4] In particular,
Received 23 July 2009;
accepted 11 September 2009.
Address correspondence to
Vidyanand K. Revankar, Department
of Studies in Chemistry, Karnatak
University, Pavatenagar,
Dharwad-580 003, Karnataka, India.
E-mail: vkrevankar@rediffmail.com
235

thiosemicarbazone usually coordinates metal ions
through the imine nitrogen and the sulfur atom in a
bidentate mode and can increase the coordination
mode to tridentate with incorporation of additional
donor groups as heterocyclic substituents. The most
popular donor set involves ONS or NNS atoms
displaying versatile coordination behavior like mono-
mers, with one or two ligands attached to the metal
ion, or dimers, in which the metal centers can be
bridged through either the thiolate sulfur or the non-
thiosemicarbazone coligand (i.e., Cl, N3, NCS,
CH3COO, etc.).[5] Biological studies have shown that
the type of substitution influences the antimicrobial
effects and enhancement of the biological activity
of the complexes.[6] Quinoline, the backbone for
most natural products, is used for the design of many
synthetic compounds having diverse pharmacologi-
cal applications such as antibacterial, antifungal,[7]
immunosuppressive,[8] analgesic, vasorelaxing,[9]
antiplasmodial,[10] anticancer,[11] anticonvulsant, and
antihypertensive[12] activities. Further quinoline core
incorporation with appropriate functional groups
can serve as a precursor for the synthesis of potential
ligands. In this context, we have designed new ligand
systems in which 3-hydroxy-3-formyl quinoline was
introduced into the backbone of thiosemicarbazone
or vice versa to further investigate the chemistry of
a new quinoline-thiosemicarbazones and their metal
derivatives and to investigate their potential useful-
ness as therapeutic agents.
measurements were made using a Faraday balance at
room temperature using Hg[Co(SCN)4] as a calibrant.
1
The H NMR spectra were recorded in DMSO-d6 sol-
vent on a Bruker 300-MHz spectrometer (USA) at
room temperature using tetramethylsilane (TMS) as
internal reference. Infrared (IR) spectra were recorded
in KBr matrix using an Impact 410 Nicolet Fourier
transform infrared (FTIR) spectrometer (USA) in the
4000–400 cmꢀ1 range. The electronic spectra of the
complexes were recorded on a Hitachi 150-20 in
the spectrophotometer in range 1000–200 nm. Cyclic
voltammetric studies were performed at room tempe-
rature in DMF under oxygen free condition created by
purging pure nitrogen gas, with a CHI1110A electro-
chemical analyzer (USA) comprising a three-electrode
assembly of glassy carbon working electrode, plati-
num auxiliary electrode, and Ag=AgCl reference elec-
trode. Tetramethylammonium chloride (0.01M) was
used as supporting electrolyte and the instrument
was standardized by a ferrocene=ferrocenium redox
couple. The ESR study of the copper complexes was
carried out on a Varian E-4X-band electron paramag-
netic resonance (EPR) spectrometer (USA), using tet-
racyanoethylene (TCNE) as the g-marker. Fast atom
bombardment (FAB) mass spectra were recorded on
a JEOL SX102=DA-6000 mass spectrometer (Tokyo,
Japan) using argon=xenon (6 kV, 10 mA) as the FAB
gas and m-nitro-benzyl alcohol the as matrix.
Synthesis of Thiosemicarbazones
Thiosemicarbazide (0.01 mol) in ethanol (100 mL)
was treated with 2-hydroxy-3-formyl quinoline
(0.01 mol). The reaction mixture was refluxed for
3–4 hr and the yellow solid separated was filtered,
washed two to three times with ethanol, and dried
(Scheme 1).
MATERIALS AND METHODS
The chemicals used were of reagent grade. Purified
solvents were used for the synthesis of ligands and
complexes. Synthesis of 2-chloro-3-formyl quinoline
was carried out according to the reported method
with slight modification,[13] which on acid hydrolysis
yields 3-hydroxy-3-formyl quinoline. Thiosemicarba-
zides were prepared by the method described in the
literature.[14] The metal chlorides used were in the
hydrated form. C, H, N, and S analysis was carried
out on a Thermo Quest elemental analyzer (Milan,
Italy). Metal and chloride estimations were done fol-
lowing standard procedures. The molar conductivity
measurements in dimethylformamide (DMF) were
made on an ELICO-CM-82 conductivity bridge (EITI
Ltd., Hyderabad, India) with a conductivity cell having
a cell constant 0.51 cmꢀ1. The magnetic susceptibility
Synthesis of Complexes
Ligand (0.001 mol) was taken in 35–40 mL of hot
ethanol. To this, a hot ethanolic solution of metal
SCHEME 1 Reaction pathway representing the preparation of
thiosemicarbazones.
N. V. Kulkarni et al.
236

chloride (0.001 mol) was added drop-wise with
stirring at 60–65ꢁC. After complete addition of metal
salt solution, the reaction mixture was stirred for
another 30–40 min at the same temperature and
refluxed for 3–4 hr in a water bath. The isolated
complexes were filtered in hot condition, washed
with hot ethanol, and dried.
made in the plate. Each plate was inoculated with
150 mL of 48-hr-old cultures and spread evenly on
the plate. After 20 min, the wells were filled with
500 mL of each compound (10 mg=mL in DMF).
Control plates with fluconazole (10 mg=mL) and
DMF were also prepared. All plates were incubated
at 37ꢁC for 48 hr and the diameters of the inhibition
zone were noted in centimeters. The values were
compared with that of fluconazole and the samples
showing significant inhibition were selected for
further calculation of MIC.
Antibiogram Analysis of Compounds
against Bacteria
The MICs of compounds were determined by
assaying at 500- and 250-mg concentrations along
with standard fluconazole at the same concentra-
tions. The cultures were grown for 48 hr and the
zones were compared with that of fluconazole and
percentages of inhibition were calculated.
Media used in the analysis was prepared by adding
10 g of peptone and 10 g of sodium chloride to the
homogeneous mixture of 5 g of yeast extract and
20 g of nutrient agar taken in 1000 mL of distilled
water. Initially, the stock cultures of Escherischia coli
and Pseudomonas aeruginosa were revived by inocu-
lating in broth media and grown at 37ꢁC for 18 hr. The
agar plates of the above media were prepared and
wells were made in the plate. Each plate was inocu-
lated with 150 mL of 18-hr-old cultures and spread
evenly on the plate. After 20 min, the wells were filled
with 100 mL of each compound (10 mg=mL in DMF).
The control plates with gentamycin (10 mg=mL) and
DMF were also prepared. All the plates were incu-
bated at 37ꢁC for 24 hr and the diameters of inhibition
zone were noted in centimeters. The values were
compared with that of gentamycin and the samples
showing significant inhibition were selected for the
further calculation of minimum inhibition concen-
tration (MIC). The MICs of compounds were determ-
ined by assaying at 500 and 250 mg concentrations
along with standard gentamycin at the same concen-
trations. The cultures were grown for 24 hr and the
zones were compared with that of gentamycin and
percentage of inhibition was calculated.
RESULTS AND DISCUSSION
The complexes obtained in the present study were
nonhygroscopic and in the form of amorphous
solids. The metal complexes are easily soluble in
DMSO and DMF and sparingly soluble in ethanol
and methanol, whereas they are insoluble in chlori-
nated hydrocarbons. The metal complexes melt with
decomposition above 300ꢁC. The elemental analysis
data of the ligands and their complexes along with
molar conductivity values are compiled in Table 1.
Molar Conductivity Measurements
The molar conductance values of the complexes
measured at room temperature in DMF solution with
10ꢀ3 mol=dm3 concentration fall in the range 4.6 to
8.9 ohmꢀ1 cm2 molꢀ1, indicating the nonelectrolytic
nature of the complexes.
Antibiogram Analysis of Compounds
against Fungi
IR Spectral Studies
Important IR absorption bands are summarized in
Table 2. The possibility of thione-thiol tautomerism
Potato dextrose agar (PDA) media used in the
analysis was prepared by mixing 20 g of dextrose
to the filtrate obtained by the 250 g of peeled potato
boiled for 20 min, squeezed, and filtered; the volume
was made up to 1000 mL by adding distilled water.
Initially, the stock cultures of Aspergillus niger and
Cladosporidium were revived by inoculating in broth
media and grown at 37ꢁC for 48 hr. The agar plates of
the above media were prepared and wells were
=
=
(HꢀNꢀC S , C NꢀSH) in the ligands has been
ruled out, because there is no band around 2500–
2600 cmꢀ1, which is characteristic of a thiol group.
The amine v(4NH) and hydrazine v(2NH) are
observed around 3300 and 3150 cmꢀ1, respectively.
Coupled vibration among the components of thioa-
mide bands I [v(CN) and b(NH) þ d(CH)], II [v(CN)
and b(CS)], III [v(CS) and v(CS) þ v(CN)], and IV
237
Spectroscopy, Electrochemistry, and Structure

TABLE 1 Elemental Analysis and Molar Conductivity Data
Elemental analysis (%) calculated (found)
Molar cond.
KM (ohmꢀ1 cm2
Compound
L1H2
C
H
N
S
M
Cl
molꢀ1
)
55.38 (55.62) 4.62 (4.93) 21.83 (21.76) 12.30 (12.48)
[CoL1H(H2O)2Cl] ꢂ 2H2O 33.89 (34.02) 4.38 (4.30) 13.16 (13.21) 7.46 (7.54) 13.86 (14.02) 8.12 (8.21)
—
—
4.6
6.8
5.3
8.8
7.3
6.7
7.9
8.9
8.2
5.2
[NiL1H(H2O)2Cl] ꢂ 2H2O 33.92 (34.11) 4.44 (4.51) 13.13 (13.18) 7.53 (7.58) 13.78 (13.98) 8.24 (8.32)
[CuL1H(H2O)Cl]
38.40 (38.86) 3.46 (3.51) 14.93 (14.80) 8.53 (8.48) 16.93 (17.02) 9.33 (9.25)
[ZnL1H(H2O)2Cl] ꢂ H2O 34.82 (35.03) 4.11 (4.23) 13.54 (13.63) 7.73 (7.82) 15.84 (16.12) 8.46 (8.52)
L2H2
59.44 (59.62) 4.02 (4.13) 21.67 (21.76) 9.90 (10.02)
—
—
—
—
—
—
[Co2L22(H2O)4] ꢂ 2H2O
[Ni2L22(H2O)4] ꢂ 2H2O
[Cu2L22(H2O)4]
[Zn2L22(H2O)4] ꢂ H2O
44.03 (44.14) 4.30 (4.38) 16.05 (16.13) 7.30 (7.62) 13.50 (13.83)
44.00 (44.10) 4.36 (4.52) 16.00 (16.32) 7.34 (7.53) 13.43 (13.62)
45.41 (45.56) 4.02 (4.23) 16.50 (16.43) 7.57 (7.73) 15.02 (15.13)
44.21 (44.54) 4.15 (4.23) 16.14 (16.26) 7.38 (7.58) 15.10 (15.27)
[v(CS)] are distributed around 1590, 1490, 1430, and
950 cmꢀ1. The quinoline v(OH) is found around
3050 cmꢀ1 as a weak band that disappears in all com-
plexes, indicating deprotonation and coordination to
metal. This is further supported by a shift of v(CꢀO)
to a higher frequency.[15] Thioamide bands III and IV,
the coordination of azomethine group to the metal
center. The presence of a peak assignable to the
hydrazine ꢀNH in the spectrum of [ZnL1H(H2O)2Cl].
H2O with a slightly different d value than the ligand
suggests the thione form of ligand in the complex. In
case of zinc complex of L2H2, no peak with respect
to hydrazine ꢀNH was observed, suggesting the thiol
form of ligand and ligation of sulfur upon deprotona-
tion. The aromatic protons distributed in the range
6–8 ppm of spectra of free ligands show small shifts
in complexes that are attributable to a variation in
electron density and steric constraints brought about
by chelation. The presence of coordinated water
molecules is evidenced by the peak observed around
3.60ppm in the spectra. It overlaps with the DMSO
peak at 3.50 ppm and broadens the signal. The
numerical NMR data are given in Table 3 and the
spectra of L1H2 and [ZnL1H(H2O)2Cl] ꢂ H2O are given
in Fig. 1.
=
which have major contribution of v(C S), have under-
gone considerable reduction in intensity and shifted
to lower frequency in the complexes of L1H2, indicat-
ing the involvement of thione sulfur in the coordi-
nation. Further, in all other complexes these bands
remain absent, suggesting the thioenolization and
subsequent coordination to the metal, which is sup-
ported by the appearance of a weak band around
600–650 cmꢀ1 due to v(CꢀS) and disappearance of
v(2NꢀH). The band due to v(4NH) remains unaltered
in all the complexes and the broad band observed
around 3400 cmꢀ1 is assigned to v(OH) of water
molecule. The band around 1633–1655 cmꢀ1, which
=
is assigned to v(C N), has shifted to lower energy,
supporting the coordination of azomethine nitrogen.
The low-frequency nonligand bands in the 515–
450 cmꢀ1 and 400–440 cmꢀ1 region are assigned to
v(MꢀN) and v(MꢀS), respectively.
Electronic Spectral Studies
Both ligands exhibit similar electronic transitions
with bands around 260–300 nm, assigned to the
intraligand p ! pꢃ transition, which were almost
unchanged in the spectra of complexes. A broad
band at ꢄ 390 nm with a shoulder on the low energy
side is observed due to n ! pꢃ transition associated
with azomethine linkage. This band suffers a red shift
in all complexes due to participation of azomethine
nitrogen in coordination. The band observed near
415 nm in the complexes with e ꢄ 25,000 l cmꢀ1
molꢀ1 is assigned to S ! M (LMCT).[15a,b] Electronic
spectra of cobalt complexes show d-d transitions
1H NMR Studies
1
In the H NMR spectra of ligands a sharp singlet
observed around 12.06 ppm attributable to the
hydroxy group of quinoline disappeared in the zinc
complexes, indicating the deprotonation followed by
the coordination of oxygen. The azomethine protons
experience the downfield shift in [ZnL1H(H2O)2Cl] ꢂ
H2Oandupfieldshiftin[Zn2L22(H2O)4] ꢂ H2O, indicating
N. V. Kulkarni et al.
238

239

TABLE 3 NMR Spectral Data
1H NMR d ppm
=
Compound
ArꢀH
ꢀHC N
Quinoline OH
Hydrazine NH
RꢀNH
8.60
8.4
ꢀCH3
2.51
2.50
—
H2O
H2L1
7.2–7.68
6.9–7.89
7.2–7.8
6.9–7.8
8.67
8.70
8.85
8.55
12.01
—
11.70
11.6
11.64
—
—
3.60
—
[ZnL1H(H2O)2Cl] ꢂ H2O
H2L2
12.06
—
9.89
8.0
[Zn2L22(H2O)4] ꢂ H2O
—
3.50
4
around 580, 450, and 910 nm assignable to T1g !
octahedral geometry around the Ni ion.[16] In case
of [CuL1H(H2O)Cl] the low energy asymmetric band
observed at 550 nm was assigned to d-d transition.
A similar type of the bands with low energy shoulder
band were assigned for Schiff base Cu(II) complexes
with square pyramidal geometry.[17] [Cu2L22(H2O)4]
exhibits absorptions around 500 and 650 nm with
low e value. Generally Cu(II) ions can adopt square-
planar, square-pyramidal, trigonal-bipyramidal,
4A2g, 4T1g(F) ! T1g(P), and 4T1g(F) ! T2g transitions
with a sharp charge transfer band at 410 nm, which is
inconsistent with the octahedral geometry. In Ni(II)
complexes the lowest energy band observed at
4
4
3
3
910 nm was due to A2g ! T2g (n1) and bands near
3
3
650 and 450 nm were assigned to A2g ! T1g (n2)
3
3
and A2g ! T1g (P) (n3), respectively. Appearance
of these three spin allowed d-d bands assigns
FIGURE 1 1H NMR spectra. (a) 1H spectrum of L1H2. (b) 1H spectrum of L1HZn.
N. V. Kulkarni et al.
240

octahedral, and tetrahedral geometries, which, except
for the first, are generally distorted from the idealized
structures. The d-d spectra shown by these coordi-
nation geometries are distinctive only in the case of
the tetrahedral environment where the absorptions
occur at much lower energies and generally show
well-separated absorption peaks; in case of all other
distorted geometries, the spectrum shows closely
spaced absorption manifolds. Hence, it is difficult to
predict the accurate structure for the complexes only
on the basis of electronic spectral analysis. Zn(II)
complexes show only the absorptions around 400
and 420 nm, which corresponds to intraligand transi-
tions. The electronic spectral data of the ligands and
their complexes are summarized in Table 4 and
the spectra of L2H2 and [Ni2L22(H2O)4] ꢂ 2H2O are given
in Fig. 2.
the coordinating atoms. This type of behavior has
been observed for Schiff base complexes and is
attributed to (a) higher covalancy of CuꢀS compared
to CuꢀO bond and (b) higher spin–orbit coupling
constants for sulfur than for oxygen. Both factors
reduce the spin–orbit contribution of the Cu(II) ion
to the g-tensor, decreasing the g-value.[18b] We
observe here that the complex [Cu2L2(H2O)], which
has two thiolate sulfur atoms bridging between the
two metal centers, possesses lower giso value (2.05)
compared to the complex [Cu2L12Cl(H2O)]
(giso ¼ 2.09), which has only one nonbridging sulfur
coordination. This observation evidences the depen-
dency of EPR parameters on the coordinating atoms.
The presence of two bridging sulfur atoms in the for-
mer complex may lead a maximum reduction in the
spin–orbit contribution of Cu(II) into the g-tensor,
giving a lower g-value for the complex.
EPR Studies
Magnetochemistry
The solid-state X-band EPR spectra of copper
complexes of L1H2 and L2H2 exhibit isotropic intense
broad signals with giso values 2.09 and 2.05, respect-
ively, with no hyperfine splitting in g or g region.
The experimentally determined room temperature
magnetic moments of all the complexes are given in
the Table 4. The copper complexes of L1H2 and L2H2
exhibit the effective magnetic moment (leff) of 1.58
and 1.40 BM at 303 K. Fairly lesser effective magnetic
moment values are indicative of higher covalancy of
Cu-S bond and lower order spin–orbit coupling of
sulfur. The magnetic moment values of Ni(II) com-
plexes give valuable information regarding their
stereochemistry. This is due to favorable changes
occurring in the number of unpaired electrons and
the orbital contribution to the magnetic moments,
when stereochemistry around the Ni(II) ion is chan-
k
?
No half-field absorption, which is an index of
metal–metal interaction, was observed. This type of
spectra was reported earlier for the complexes bear-
ing large organic ligand substituents having con-
siderable covalent character for metal–ligand
bonds.[18a] The low giso values are attributed to the
presence of sulfur coordination in the complexes.
A similar comparison is made between S-bonded
and O-bonded complexes. A decrease of 0.144 in geff
values for S-bonded complexes was observed when
compared with the O-bonded complexes. It is con-
cluded that the EPR parameters are dependent on
3
ged. The octahedral complexes have A2g ground
state and hence the orbital contribution to the
TABLE 4 Electronic Spectra and Magnetic Moment Data
Compound
kmax (nm)
270, 341, 356, 366, 372, 391
leff (B.M)
L1H2
—
4.31
[CoL1H(H2O)2Cl] ꢂ 2H2O
[NiL1H(H2O)2Cl] ꢂ 2H2O
[CuL1H(H2O)Cl]
[ZnL1H(H2O)2Cl] ꢂ H2O
L2H2
321, 333, 348, 401, 450, 580, 912
303, 369, 414, 450, 688, 916
323, 350, 403, 450, 550
280, 353, 401, 420
2.92
1.58
Diamagnetic
—
268, 288, 318, 357, 393
347, 371, 435, 500, 913
321, 336, 414, 450, 680, 916
300, 369, 420, 500, 650
340, 370, 421
[Co2L22(H2O)4] ꢂ 2H2O
[Ni2L22(H2O)4] ꢂ 2H2O
[Cu2L22(H2O)4]
[Zn2L22(H2O)4] ꢂ H2O
4.35
2.81
1.40
Diamagnetic
241
Spectroscopy, Electrochemistry, and Structure

FIGURE 2 The electronic spectra of ligand and complex. (a) Electronic spectrum of L2H2. (b) Electronic spectrum of
[Ni2L22(H2O)4] · 2H2O.
magnetic moment due to orbital degeneracy is
absent. However, octahedral nickel(II) complexes
Mass Spectral Analysis
The elemental and analytical data suggest the
show the effective magnetic moments that are
empirical formula [CuL1H(H2O)Cl] and [Cu2L22(H2O)4]
more than the spin-only value of 2.83 B.M. This is
for both the copper complexes, respectively,
3
because the excited state T2g carries the orbital
which is supported by the FAB mass spectral analy-
magnetic moment. The spin–orbit coupling brings
sis. The peaks at highest m=z value can be assigned
about some mixing of the ground state with the
to the molecular ion by certainty, with the aid of a
excited state, thus forcing some orbital contribution
consistent isotopic pattern. A peak observed at m=z
that increases the magnetic moment for about 10%
378 for the [CuL1H(H2O)Cl] complex is in consistent
than the spin-only value. Therefore, magnetic
with the molecular mass of the complex, represent-
moment values obtained in the present investigation,
ing the monomeric structure. The FAB mass
2.92 and 2.81, indicate an octahedral geometry
spectrum of [Cu2L22(H2O)4] exhibits a molecular ion
around the nickel(II) ion for both complexes.[19]
peak at m=z 841, which evidences the proposed
The cobalt complexes exhibit paramagnetic suscep-
binuclear structure of complex. Apart from this, the
tibility, with corresponding magnetic moment values
mass spectra show some other peaks, which are
lying around 4.30–4.35 BM, which signify the octa-
due to molecular cations of various fragments of
hedral geometry. The subnormal magnetic moment
the complex. By comparing the analytical and
values are accounted for the higher covalancy of
spectral data of cobalt, nickel, and zinc complexes,
Cu-S bond and lower order spin–orbit coupling of
similar structures were assigned for the respective
sulfur.[20]
N. V. Kulkarni et al.
242

series. The proposed structures assigned are repre-
sented in Fig. 3.
value ꢄ0.39 V. The appearance of Cu(III)=Cu(II)
and absence of Cu(II)=Cu(I) couple can be rationa-
lized in terms of flexibility and the size of the coordi-
nation cavity in the complexes and the geometric
requirements and the size of the metal ions in the
different oxidation states. The voltammogram of
complex [CuL1Cl(H2O)] exhibits one irreversible
cathodic (reduction) peak in the range ꢀ0.23 to
ꢀ0.28 V along with quasi-reversible reaction peaks
(with Epa in the range 0.11–0.13 V and Epc 0.18–
0.23) assigned to CuIII=CuII couple, with the Ipc=Ipa
ꢄ 1. A voltammogram of complexes scanned after
keeping the solution overnight and at slower rates
shows a decrease in DEp value, indicating the reversi-
bility of reaction with a slow rate.
Electrochemistry
The cyclic voltammograms (CV) of all ligands and
their complexes were scanned in the potential range
of þ1.0 to ꢀ1.0 V with different scan rates (0.15, 0.1,
and 0.05 V=s). Both the copper complexes exhibit a
redox property, whereas ligands and other com-
plexes do not show any electrochemical response
over the working potential range, indicating that elec-
trochemical activity of the copper complexes is
purely metal based. The voltammograms of the two
copper complexes are illustrated in Fig. 4 and
numerical results are compiled in Table 5. The vol-
tammogram of [Cu2L22(H2O)2] shows a single oxi-
dation peak in the range 0.43–0.46 V in the anodic
potential scan. The cathodic scan exhibits a single
peak in the range 0.32–0.35 V, indicating the
reduction of species. The high value of DEp, separ-
ation between the cathodic and anodic peak
potentials (Epa ꢀ Epc), which is greater than 60 mV,
indicates quasi-reversible redox process[21] assignable
to the Cu(III)=Cu(II) couple with E1=2 [(Epc þ Epa)=2]
Antimicrobial Analysis of
the Compounds
The prepared ligands, Cu(II) and Ni(II) complexes,
were screened for antimicrobial activity against bac-
teria Escherichia coli and Pseudomonas aeruginosa
and fungi Aspergillus niger and Cladosporidium.
The results obtained are graphically represented in
FIGURE 3 Tentative structures assigned for the complexes.
243
Spectroscopy, Electrochemistry, and Structure

FIGURE 5 Graphical representation of antibacterial analysis of
the ligand and complexes. (a) Escherichia coli. (b) Pseudomonas
aeruginosa.
degree of inhibition against both bacteria and
moderate activity against both the fungi. The observa-
tions made reveal that there is enhancement in the
antimicrobial activity of the complexes, which is
represented by the high degree of activity of copper
and nickel complexes compared with their ligands.
Enhancement in the antimicrobial property of thiose-
micarbazones is often related to lipophilicity, which
controls the rate of entry of molecules into the cell.
The lipophilicity can be modified by the coordi-
nation, so that metal complexes become more access-
ible to the microbes, and hence the inhibitory action
FIGURE 4 Cyclic voltammograms of copper complexes with
scan rate 0.1 V=s.
Figs. 5 and 6. From the data it is concluded that the
ligand L1H2 is inactive against fungus Cladospori-
dium, less active against fungus Aspergillus niger
and both the bacteria. The ligand L2H2 shows a high
TABLE 5 Cyclic Voltammetry Results
Complex
Scan rate (V=S)
Epa (V)
Epc (V)
DEp (V)
E1=2 (V)
Ipc=Ipa
[CuL1H(H2O)Cl]
0.15
0.1
0.13
0.12
0.11
0.46
0.44
0.43
0.23 & ꢀ0.28
0.20 & ꢀ0.25
0.18 & ꢀ0.23
0.32
0.11
0.08
0.07
0.14
0.09
0.08
0.18
0.16
0.145
0.39
0.385
0.39
1.08
1.10
1.15
1.30
1.31
1.36
0.05
0.15
0.1
[Cu2L22(H2O)4]
0.33
0.05
0.35
Ep ¼ Epa ꢀ Epc; E1=2 ¼ [Epc þ Epa]=2.
N. V. Kulkarni et al.
244

metal ions. The ligand-to-metal ratio is found to be
1:1 and 2:2 for the complexes of L1H2 and L2H2,
respectively. All the complexes are soluble in
DMSO=DMF and are nonelectrolytic in nature. Only
the copper complexes were found to be electroche-
mically active, exhibiting the quasireversible redox
process. By considering the high antimicrobial pro-
perty of the quinoline derivatives and bioaccessibility
of Cu and Ni ions, the ligands and their respective
copper and nickel complexes were screened against
bacteria Escherichia coli and Pseudomonas aerugi-
nosa and fungi Aspergillus niger and Cladospori-
dium. The compounds were found to exhibit
higher activity against fungi than the bacteria. The
complexes [CuL1H(H2O)Cl] and [Cu2L22(H2O)4]
exhibited prominent activity toward Aspergillus
niger, hence proving their usefulness in the field of
biology as antimicrobial agents. Further, the investi-
gation of the effectiveness of these compounds
toward tumor cells is in progress.
ACKNOWLEDGMENTS
The authors thank the Department of Chemistry
and USIC, Karnatak University, Dharwad, for provid-
ing spectral and analytical facilities. Recording of
FAB mass spectra (CDRI Lucknow) and ESR spectra
(IIT Bombay) are gratefully acknowledged. The
authors (NVK and SMP) thank Karnatak University,
Dharwad, for providing the Nilekani and University
Research Scholarship and SB thanks the University
Grant Commission for providing RFSMS.
FIGURE 6 Graphical representation of antifungal activity of
ligand and complexes. (a) Aspergillus niger. (b) Cladosporidium.
will be more.[22] Further, the complexation with the
metal protects the molecule against enzymatic
degradation, because of the inertness of certain metal
ligand linkages; therefore, the activity can be rein-
forced by the combined effect of the ligand and metal
residue.[23] Studies on the structural and kinetic
properties involved in biological activity of an antitu-
mor drug, cisplatin, reveals that the mechanism of
action can involve binding to the metal in vivo, or
the metal complex may be a vehicle for activation
of the ligand as the inhibitory agent.[24] Moreover,
coordination may lead to a significant reduction of
drug resistance of a cell[25] and enhance the activity
of drug.
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