Transition metal complexes of thiosemicarbazones with quinoxaline hub: An emphasis on antidiabetic property

Article abstract of DOI:10.1007/s00044-011-9576-6

New transition metal complexes of quinoxaline-thiosemicarbazone ligands were prepared and characterised by spectroanalytical techniques. The ligands L¹H₂ and L²H₂ were obtained by the reaction of quinoxaline-2.3(1,4H)-dione with methyl and phenyl thiosemicarbazide, respectively. All the complexes are found to be monomeric in nature and have tetrahedral geometry. The copper complexes have shown redox responses in the applied voltage range, whereas the ligands and other complexes are electrochemically innocent. The ligands, copper and zinc complexes are explored for antidiabetic activity in the diabetes-induced Wister rats. Evaluation of antidiabetic activity was done by blood-glucose test and oral glucose tolerance test; few compounds have exhibited significant antidiabetic activity and posses low toxicity with a high safety profile. Springer Science+Business Media, LLC 2011.

Full text of DOI:10.1007/s00044-011-9576-6

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2012) 21:663–671
DOI 10.1007/s00044-011-9576-6
ORIGINAL RESEARCH
Transition metal complexes of thiosemicarbazones
with quinoxaline hub: an emphasis on antidiabetic property
•
Naveen V. Kulkarni Vidyanand K. Revankar
•
•
B. N. Kirasur Mallinath H. Hugar
Received: 5 May 2010 / Accepted: 17 January 2011 / Published online: 30 January 2011
Ó Springer Science+Business Media, LLC 2011
Abstract New transition metal complexes of quinoxa-
line–thiosemicarbazone ligands were prepared and char-
acterised by spectroanalytical techniques. The ligands
L1H2 and L2H2 were obtained by the reaction of quinox-
aline-2.3(1,4H)-dione with methyl and phenyl thiosemi-
carbazide, respectively. All the complexes are found to be
monomeric in nature and have tetrahedral geometry. The
copper complexes have shown redox responses in the
applied voltage range, whereas the ligands and other
complexes are electrochemically innocent. The ligands,
copper and zinc complexes are explored for antidiabetic
activity in the diabetes-induced Wister rats. Evaluation of
antidiabetic activity was done by blood-glucose test and
oral glucose tolerance test; few compounds have exhibited
significant antidiabetic activity and posses low toxicity
with a high safety profile.
Introduction
Diabetes Mellitus is a condition characterized by abnormal
glucose levels with a tendency to hyperglycemia, due to a
relative or absolute deficiency of insulin. It is caused by
dysfunction of glucose homeostasis, afflicts over 140 mil-
lion people worldwide and can lead to serious complica-
tions, such as retinopathy, nephropathy and neuropathy.
Amongst them, insulin-dependent diabetes mellitus (IDDM)
type 1, characterized by hyperglycemia due to absolute
deficiency of insulin, requires the daily subcutaneous injec-
tions of insulin. On the other hand, non-insulin-dependent
diabetes mellitus (NIDDM) type 2 can be treated with
several drugs such as sulfonylureas, nateglinide, biguanides
and pioglitazone etc. Though several types of therapeutics
are developed for the treatment of type 2 diabetes, they
have limited efficacy and tolerability and occasionally
cause severe side effects (Moller, 2001).
Keywords Quinoxaline ꢀ Thiosemicarbazone ꢀ
Antidiabetic agent ꢀ Blood-glucose test ꢀ
Oral glucose tolerance test
In this regard, the approaches to synthesize better anti-
diabetic drugs are of immense interest. Quinoxaline
derivatives have large applications in biochemistry and
pharmacology as antimicrobial (Badran et al., 2003) anti-
neoplastic agents and tumour-specific cytotoxins (Das
et al., 2009) and have exhibited significant in vitro activi-
ties against the human melanoma cell line A375, 110
(Deleuze-Masquefa et al., 2009). Mayer and Taberner
(2002) reported the utility of quinoxaline derivatives as
blood glucose level reducing agents and insulinomimetic
agent in mice. A series of 3-anilino-quinoxalinones has
been identified as a new class of glycogen phosphorylase
inhibitors by Dudash et al. (2005). They have made some
incorporation to the quinoxaline scaffold with various ali-
phatic and aromatic compounds to produce novel ana-
logues, some of which are 25 times more potent than the
lead compound. Reddy Shastry et al. (1989) have prepared
N. V. Kulkarni ꢀ V. K. Revankar (&)
Department of Studies in Chemistry, Karnatak University,
Pavate Nagar, Dharwad 580 003, Karnataka, India
e-mail: vkrevankar@rediffmail.com
B. N. Kirasur
Basaveshwar Science College, Bagalkot 587 101,
Karnataka, India
M. H. Hugar
Luqman College of Pharmacy, Gulbarga 585 102,
Karnataka, India
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Med Chem Res (2012) 21:663–671
the some N-arylcarbamoyl and arylthiocarbamoyl hydraz-
inoquinoxaline and evaluated their hyperglycemic activity
in glucose primed rats. The compounds have shown a
temperate order hypoglycemic activity (Reddy Shastry
et al., 1989). These reports have explored antidiabetic
strategy of quinoxalines and hence motivated researchers
to synthesize various quinoxaline derivatives, which can
serve as better antidiabetic agents for the future. Recently,
Cu(II) and Zn(II) ions and their complexes have been found
to exhibit in vitro insulinomimetic activity and in vivo anti-
diabetic (both type-1 and 2) effects in animals (Yasumatsu
et al., 2007; Sakurai et al., 2006). The complexation of
these ions with ligands having antidiabetic molecules as
backbone has been found to exhibit many fold enhancement
in the activity (Yoshikawa et al., 2005, 2009).
with conductivity cell having cell constant 0.51 cm-1. The
magnetic susceptibility measurements were made using
Faraday balance at room temperature using Hg[Co(SCN)4]
as calibrant. The 1H NMR spectra were recorded in
DMSO-d6 solvent on Bruker-300 MHz spectrometer at
room temperature using TMS as internal reference. IR
spectra were recorded in KBr matrix using an Impact-410
Nicolet (USA) FT-IR spectrometer in 4000–400 cm-1
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 temperature in DMF under oxygen free
condition created by purging pure nitrogen gas, with
CHI1110A Electrochemical analyzer (USA) comprising
three electrode assembly of glassy carbon working elec-
trode, platinum auxiliary electrode and Ag/AgCl reference
electrode. Tetramethylammoniumchloride (0.01 M) was
used as supporting electrolyte and instrument was stan-
dardized by ferrocene/ferrocenium redox couple. The ESR
study of the copper complexes was carried out on a Varian
E-4X-band EPR spectrometer, using TCNE as the g-mar-
ker. FAB mass spectra were recorded from JEOL SX
102/DA-6000 mass spectrometer using Argon/Xenon
(6 kV, 10 mA) as the FAB gas and m-nitrobenzyl alcohol
as matrix. TG and DTA measurements of the complexes
were recorded in nitrogen atmosphere on Universal V2 4F
TA instrument at the heating rate of 10°C/min and scan
range of 25–800°C.
In this research study, quinoxaline-2,3-dione is selected
as precursor and incorporated with thiosemicarbazide
scaffolds to aid the coordinating and antidiabetic strategy.
The synthesized SNO donor ligands are treated with Co(II),
Ni(II), Cu(II) and Zn(II) ions to form the complexes and
structural features of the compounds are explored. The
ligands as well as Cu(II) and Zn(II) complexes are tested
for the utility as antidiabetic drugs.
Experimental
Chemistry
Reagents and apparatus
Chemistry
Reagent grade chemicals were used for the preparation of
precursors and purified solvents were used for the synthesis
of ligands and complexes. Synthesis of quinoxaline 2,3-
(1H,4H)-dione (Philips, 1928) and thiosemicarbazides (Sen
and Gupta, 1962) was done according to the literature with
slight modifications. The metal chlorides (CoCl2ꢀ6H2O,
NiCl2ꢀ6H2O, CuCl2ꢀ2H2O and ZnCl2) were used for the
complex formation. C, H, N and S analysis was carried
out on thermo quest elemental analyzer. Metal and chlo-
ride estimations were done by the standard procedures
(Vogel, 1961). The molar conductivity measurements in
DMF were made on ELICO-CM-82 conductivity bridge
Synthesis
Synthesis of thiosemicarbazone ligands
0.1 M of thiosemicarbazide [(methyl thiosemicarbazide,
1.05 g) (phenyl thiosemicarbazide, 1.66 g)] in 100 ml
methanol was treated with quinoxaline 2,3-(1,4H)-dione
(1.62 g, 0.1 M). The reaction mixture was refluxed for
3–4 h and the dirty/greenish yellow solid separated was
filtered, washed 2–3 times with ethanol and dried
(Fig. 1).
Fig. 1 Schematic
representation of preparation of
ligands
H
N
O
O
H
H
NH
Methanol
Reflux
N
N
+
H
H
H2N
R
HN
N
N
N
R
N
H
S
O
S
For L1H2, R = CH3 and for L2H2, R= C6H5
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Med Chem Res (2012) 21:663–671
665
[Note: The preparation of ligand L2H2 has been reported
earlier with different synthetic route (Reddy Shastry et al.,
1989)].
different groups. Initially all test samples were adminis-
tered with 12.5 mg/kg body weight, if all the animals
survived with this dose, then the samples were tested at
higher dose range viz., 25, 50, 100 and 200 mg/kg and if
the test samples caused 100% death at this dose the lower
dose range was treated as LD50 dose. Finally, the
administered dose 150 mg/kg is fixed for all the
compounds.
Synthesis of complexes
A methanolic solution (100 cm3) of metal(II) chloride
[CoCl2ꢀ6H2O (0.237 g, 0.01 M), NiCl2ꢀ6H2O (0.237 g,
0.01 M), CuCl2ꢀ2H2O (0.170 g, 0.01 M) and ZnCl2
(0.135 g, 0.01 M)] was added with stirring to an ethanolic
solution of the ligand [L1H2 (0.267 g, 0.01 M), L2H2
(0.329 g, 0.01 M)] and refluxed at water bath temperature
for 3–4 h. So obtained solid complex was separated by
filtration under suction, washed with hot ethanol and dried
in vacuo.
Alloxan-induced diabetes
Diabetes was induced by a single intra peritoneal injection
of 150 mg/kg body weight of alloxan monohydrate (S.D.
Fine Chem. Ltd., Mumbai.) in 0.9% NaCl saline (O Neil
et al., 2001). After 72 h of alloxan injection, blood glucose
was measured by glucometer. The diabetic rats (glucose
level [ 300 mg/dl) were separated and used for the study
(Wright, 1951).
Pharmacology
Animals for the investigation
Glibenclamide (standard drug antidiabetic used in this
study) in its pure form was obtained from Sun Pharma-
ceuticals Ltd., Andheri (East), Mumbai, India, made as an
aqueous solution and was injected daily for 9 days (as a
600 lg/kg body weight) using an intra gastric tube (Breslin
et al., 2003).
Male Wister rats weighing about 180–200 g were used in
this analysis with prior permission from institutional ani-
mal ethics committee (IAEC). Animal studies were per-
formed as per the rules and regulations of CPCSEA. The
animals were acclimatized to the experimental room hav-
ing normal temperature (23 2°C), controlled humidity
conditions and 12:12 h light and dark cycle. The Wister
rats were housed in sterile Plexiglas transparent cages
containing sterile paddy husk as bedding material with
maximum of four animals in each cage. The rats were fed
on autoclaved standard rat food pellets and water
ad libitum.
Methodology
The weighed animals were divided into nine groups of six
animals each. Group 1 served as normal healthy control
and received food (standard pellet rodent diet) and vehicle
only. Group 2 served as diabetic control group. Group 3
served as standard control group and received glibencla-
mide (600 lg/kg body weight) in the form of aqueous
suspension. The volume of suspension was 1 ml/100 gm
body weight of rat. Groups 4–9 received the prepared
compounds (150 mg/kg body weight). The different
groups received the drug before the food by intragastric
tube. In this study the doses (glibenclamide, ligands,
copper and zinc complexes) were administered once a day
for 9 days.
Acute toxicity test (Mishra et al., 1973)
During the administration of a chemical substance to the
biological system, different types of interactions can
occur and a series of dose-related responses result. In
most cases these responses are desired and useful, but
there are a number of other effects which are not
advantageous. These may or may not be harmful to the
biological system. Hence in the pharmacological evalua-
tion of any synthesized or isolated compound, it is cus-
tomary to carryout acute toxicity study to determine the
safe effective dose of the novel compound. Acute toxicity
is involved in estimation of LD50 (the dose which has
proved to be lethal (causing death) to 50% of the tested
group of animals).
Collection of blood and estimation of blood glucose
levels
At the end of the treatment period, two drops of blood was
collected from rat tail vein after 16 h fasting. The pulsatum
gluco-strips (stored in refrigerator) taken out from the
sachet and glucometer is calibrated to 660 units according
to the specification mentioned in the strips. The blood
removed from the rat tail vein was immediately spread on
the marked end of the strip. The strip was inserted in the
glucometer and the blood glucose level, reading was
recorded.
Wister rats weighing between 180 and 200 g were
starved for 18 h before the experiment. The animals were
divided into the group of eight each, after recording their
body weight. The test sample solutions of suitable con-
centration in 1% gum acacia were administered orally in
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Med Chem Res (2012) 21:663–671
Oral glucose tolerance test (OGTT) (Nawrocka, 1996)
Molar conductivity results
Animals were fasted for 12 h divided into nine groups of
six rats each. Group 1 was served as normal healthy control
and received food (standard pellet rodent diet) and vehicle
only. Group 2 served as diabetic control group. Group 3
served as standard control group and received glibencla-
mide (600 lg/kg body weight) in the form of aqueous
suspension. The volume of suspension was 1 ml/100 gm of
rat. Groups 4–9 received ligands and their copper and zinc
complexes (150 mg/kg body weight) as a fine tween-80
suspension. After 30 min, the rats of all groups were orally
given with 400 mg/kg body weight of glucose. Blood
samples were collected from the rat tail vein just before
glucose administration and at 30, 60 and 90 min after
glucose loading. Blood levels were measured immediately
by using glucometer.
The molar conductance values of the complexes measured
at room temperature in DMSO solution with 10-3 mol/dm3
concentration fall in the range 14.3–18.7/ohm cm2/mol
indicating the non-electrolytic nature of the complexes
(Geary, 1971). Comparatively high values are due to the
solvation effect of DMSO, which replaces anion from the
coordination sphere.
IR spectral studies
The important IR spectral bands of ligands and corre-
sponding complexes along with assignments are presented
in Table 2.
The absence of a band in the region 2500–2600 cm-1
,
which is characteristic of thiol group [v(C-SH)], suggest
the stable thione form of the ligands and hence decline the
thione–thiol tautomerism (H–N–C=S , [ C=N–SH) in
the present set of thiosemicarbazone ligands. The thio-
amidic coupled vibrations, I [v(CN) and v(NH) ? d(CH)],
II [v(CN) and v(CS)], III [v(CS) and v(CS) ? v(CN)] and
IV[v(CS)] are observed around 1600, 1545, 1460 and
940 cm-1 confirm the thioketo form of ligands (Kulkarni
et al., 2010). The v(C=O) of quinoxaline ring was observed
as a sharp, intense band at *1675 cm-1 and the stretching
vibrations of azomethine functionality v(C=N) were
observed near 1645 cm-1. The quinoxaline ring v(NH) and
hydrazine v(2NH) were observed around 3300 cm-1 as
broad intense band.
Statistical analysis
Values are expressed as mean SEM, statistical differ-
ence between means were determined by performing one-
way ANOVA followed by Dunnett’s test. P \ 0.05 was
considered as significant difference in this study.
Results and discussion
The transition metal complexes prepared in this course
were non-hygroscopic (stable at the room temperature) and
in the form of amorphous solids. These are soluble easily in
DMSO, DMF and sparingly in ethanol and methanol,
whereas they are insoluble in chlorinated hydrocarbons.
The elemental analysis data of the ligands and their com-
plexes along with molar conductivity values are compiled
in Table 1.
In all the complexes, the band due to v(C=O) has not
observed due to the coordination of oxygen through enol
mode. A new band appeared around 1260 cm-1 attribut-
1
able to v(C–O) confirms the same. But in the H NMR
studies no peak which can be assigned to –OH (enol form)
is observed for the zinc complexes due the coordination of
oxygen after deprotonation. The band due to v(C=N) has
Table 1 Chemical composition and molar conductivity data of compounds
Compounds Colour
Yield in
percentage
Elemental analysis (%) calculated (found)
Molar conductance
(mho cm2/mol)
C
H
N
S
M
–
L1H2
Dirty yellow
82
80
82
48.19 (48.28) 4.41 (4.52) 28.12 (28.05) 12.83 (12.89)
–
[CoL1(H2O)] Dark brown
[NiL1(H2O)] Dark brown
[CuL1(H2O)] Greenish black 81
[ZnL1(H2O)] Yellow
79
L2H2
Greenish yellow 83
[CoL2(H2O)] Brown
[NiL2(H2O)] Dark brown
[CuL2(H2O)] Black
38.21 (38.32) 3.50 (3.57) 22.29 (22.33) 10.19 (10.23) 18.63 (18.58) 12.1
38.27 (38.24) 3.50 (3.58) 22.32 (22.39) 10.20 (10.26) 18.66 (18.71) 14.2
37.68 (37.72) 3.44 (3.37) 21.94 (22.01) 10.03 (10.14) 19.90 (19.96) 13.2
37.38 (37.84) 3.42 (3.49) 21.80 921.75) 9.96 (10.03) 20.40 (20.32) 11.3
58.06 (57.98) 3.87 (3.93) 22.58 (22.67) 10.82 (11.04)
46.51 (46.43) 3.10 (3.18) 18.08 (18.15) 8.26 (8.32)
46.57 (46.65) 3.10 (3.04) 18.11 (18.18) 8.27 (8.18)
45.85 (45.96) 3.06 (3.14) 17.87 (17.95) 8.17 (8.26)
45.74 (45.65) 3.04 (3.04) 17.78 (17.86) 8.13 (8.22)
–
–
84
80
82
15.21 (15.30) 13.2
15.13 (15.21) 12.7
16.25 (16.37) 13.6
16.64 (16.53) 13.5
[ZnL2(H2O)] Greenish yellow 83
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Med Chem Res (2012) 21:663–671
667
Table 2 Infrared spectral data of ligands and complexes in cm-1
Compounds
m(OH)
water
m(N–H)
m(C=O)
m(C=N)
Thioamide bands
m(C–O)
m(C–S)
m(M–N)
m(M–S)
I
II
III
IV
L1H2
–
3350s
3379b
3314b
3300b
3300b
3376b
3376b
3300b
3288b
3326b
1674s
1644s
1631s
1636s
1640m
1636s
1645s
1640s
1641s
1652s
1650m
1599s
1589s
1605s
1600s
1598s
1602s
1599s
1601s
1605s
1615s
1540s
1536s
1518m
1551s
1546s
1542s
1540s
1546s
1517m
1517m
1460s
939m
–
–
–
–
[CoL1(H2O)]
[NiL1(H2O)]
[CuL1(H2O)]
[ZnL1(H2O)]
L2H2
[CoL2(H2O)]
[NiL2(H2O)]
[CuL2(H2O)]
[ZnL2(H2O)]
3400b
3432b
3422b
3445b
–
–
1403w
1390m
1411m
1411m
1470s
–
1265
1264
1265
1263
–
751s
754s
766s
753s
–
611m
629m
599s
602m
–
450m
440m
437m
413s
–
–
–
–
–
–
–
1676s
944m
3419b
3418b
3407b
3449b
–
–
–
–
1410w
1433w
1430m
1401s
–
–
–
–
1268
1267
1264
1260
753s
753s
759m
725m
601m
601m
602m
617m
430m
406s
420m
470s
s sharp; m medium; b broad; w weak
been shifted to the lower frequency side in all the com-
plexes, owing to the coordination of azomethine nitrogen.
The thioamide bands having major contribution from the
m(C=S) group (thioamide bands IV and III, respectively)
have disappeared or experienced a fall in intensity and
frequency upon complexation suggesting the thioenolisa-
tion and subsequent sulphur coordination. It is further
supported by the strong signal at 760 cm-1 attributable to
v(C–S).
ligand has been disappeared on complexation due to
the thioenolisation. No peak corresponding to –SH was
observed in the spectrum indicating the sulphur coordina-
tion through deprotonation. Amine proton resonating at
7.98 ppm, shift to the low field region and the methyl
protons experience a deshielding upon complexation and
shift to 4.97 ppm. The aromatic protons observed in the
region 7.24–6.92 ppm show small shifts in complex, due to
variation in electron density and steric constraints brought
about by chelation. Water coordination is evidenced by the
broad peak at 3.70 ppm (Table 3).
In ligand L2H2, sharp singlets observed at 10.29, 9.81,
9.00 and 8.03 ppm are assigned to ring NHs, hydrazine NH
and amine NH, respectively. Upon complexation peaks at
10.29 and 9.00 ppm were disappeared due to the coordi-
nation of oxygen and sulphur via deprotonation. Aromatic
protons are distributed in the region 7.99–6.89 ppm and
suffer a small variation in the resonance frequency upon
complexation. The coordinated water resonates at
3.50 ppm as broad peak.
The absence of v(S–H) in the complexes suggests the
coordination of sulphur through deprotonation. The bands
due to v(NH) are broadened in all the complexes due to the
overlapping of v(OH) bands of coordinated water mole-
cule. The low frequency non-ligand bands in the
600–625 cm-1 and 400–450 cm-1 region are assigned to
v(M–N) and v(M–S), respectively.
1H NMR studies
The ligand L1H2 displays two sharp singlets at 11.91 and
10.71 ppm attributed to the two ring NHs of quinoxaline
core. The first signal was disappeared on complexation
with Zn(II) ion, indicating the enolisation and subsequent
coordination of oxygen via deprotonation. Whereas the
second signal experiences a significant up field upon
complexation. Hydrazine NH observed at 8.89 ppm in
Electronic spectral studies
The intense peaks in the range 250–275 nm observed in both
ligands are due to intra ligand p–p* transitions. These bands
remain almost unchanged in the spectra of complexes. The
Table 3 NMR spectral data
Compounds
1H NMR d (ppm)
Quinoxaline–NH
Hydrazine–NH
R–NH
Aromatic protons
–CH3
Coordinated
H2O
L1H2
[ZnL1(H2O)]
L2H2
11.91 and 10.71
8.19
8.89
–
7.98
7.51
8.03
8.09
7.24–6.92
7.44–7.10
7.99–6.89
7.95–6.95
3.50
4.97
–
–
3.70
–
10.29 and 9.81
9.84
9.00
–
[ZnL2(H2O)]
–
3.50
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Med Chem Res (2012) 21:663–671
ligands also show a broad band at 325 nm with a shoulder on
low energy side, due to n–p* transition associated with
azomethine linkage (Lever, 1968). In complexes this band
experiences bathochromic shift due to the coordination of
azomethine nitrogen. The broad peak centred at 365 nm in
the ligand is assigned to n-p* of thioamide chromophore, it
suffers blue shift in complexes due to thioenolisation (Naik
et al., 2002a, b). The electronic spectra of the copper and
nickel complexes are virtually identical. The band observed
at 450 nm is assigned to S ? M charge transfer (LMCT)
transition (e * 20000/lcm/M) (Naik et al., 2002a, b). The
d–d transitions are observed around 820 nm as low intensity
hump (e * 100/lcm/M). Cobalt complexes exhibit asym-
metric broad peak around 600–700 nm (e * 200/lcm/M)
corresponding to the d–d transition (Bailar et al., 1975). The
broad intense peak with high extinction coefficient observed
around 420 nm is attributed to LMCT transition. The zinc
complexes display the peaks around 400 nm attributed to
LMCT along with the intra-ligand transitions.
and 2.05, respectively, in the solid state X-band EPR
spectral analysis. Broad EPR spectra of this kind were
reported earlier for the complexes having large organic
ligand substituents with different donor atoms having
considerable covalent character for metal–ligand bonds
(Kulkarni et al., 2010; Seleem et al., 2005).
FAB mass spectral analysis
The copper complexes of L1H2 and L2H2 exhibit intense
peaks at m/z 319 and 392, respectively, which is in line with
the molecular formula [ML(H2O)] assigned for the com-
plexes. Along with the molecular ion peak, spectra exhibit
various other peaks with different intensity and m/z value,
which are attributed to molecular cations of various frag-
ments of complexes. The proposed structures of complexes
are given in Fig. 2.
Thermal studies
Magnetic properties
To determine the thermal stability of complexes and their
decomposition pattern TG and DTA studies were carried
out. The complexes were analysed in the nitrogen atmo-
sphere and the heating rate of 10°C/min is maintained.
The studies reveal that both the complexes [CuL1(H2O)]
and [CuL2(H2O)] decompose in two steps. The first step of
disintegration of [CuL1(H2O)] corresponds to the loss of
one water molecule with reduction of *5.72% total mass
in the range 75–120°C, the corresponding differential peak
(DTA) suggests the endothermic nature of the process.
Second stage of mass loss of 14.76% at the range
250–380°C is ascribed to the ligand decomposition. The
final product was found to be stable metal oxide, as sug-
gested by the plateau region in the thermogram.
[CuL2(H2O)] exhibits a similar pattern of thermal decom-
position with 4.97% weight loss at the range 60–110°C
representing the endothermic process of loss of one water
molecule. The decomposition of 7.25 and 37.2% in the
The room temperature magnetic moment values of nickel
and cobalt complexes were found to be 2.80, 2.81, 4.18 and
4.21 BM for [NiL1(H2O)], [CoL1(H2O)], [NiL2(H2O)] and
[CoL1(H2O)], respectively, suggesting the four coordi-
nated, tetrahedral geometry (Bailar et al., 1975; Dutta and
Syamal, 1993). Whereas copper complexes [CuL1(H2O)]
and [CuL2(H2O)] exhibit the magnetic moment values
(leff) 1.52 and 1.56 BM, respectively (Table 4). Fairly
lower magnetic moment values of copper complexes are
attributed to the higher covalancy of S–Cu bond and lower
orbital contribution of sulphur (Kulkarni et al., 2010).
EPR spectral analysis
The copper complexes [CuL1(H2O)] and [CuL2(H2O)]
exhibit isotropic intense broad signals with giso values 2.09
Table 4 The electronic spectra and magnetic moment data
Compounds
kmax (nm)
leff (B.M)
L1H2
260, 278, 325, 368
–
NH
[CoL1(H2O)]
[NiL1(H2O)]
[CuL1(H2O)]
[ZnL1(H2O)]
L2H2
[CoL2(H2O)]
[NiL2(H2O)]
[CuL2(H2O)]
[ZnL2(H2O)]
248, 336, 426, 620, 675
347, 371, 435, 480, 830
304, 369, 465, 826
4.18
H
N
N
N
2.80
N
R
1.52
M
O
S
300, 338, 375, 422
Diamagnetic
–
248, 275, 318, 358, 368
250, 348, 422, 630, 680
321, 352, 375, 420, 460, 824
311, 342, 363, 468, 825
340, 372, 421
OH2
4.21
Where M = Co, Ni, Cu and Zn
For L1H2, R = CH3 and for L2H2, R= C6H5
2.81
1.56
Diamagnetic
Fig. 2 Proposed structure of complexes
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Med Chem Res (2012) 21:663–671
669
range 150–250 and 250–500°C, respectively, corresponds
to the ligand disintegration. The plateau region at the
higher temperature range indicates the stable metal oxide
formation.
to the couple Cu(II)/Cu(III). It shows oxidation peak at
0.24, 0.20 and 0.17 V (for the scan rates 0.15, 0.1 and
0.05 V/s, respectively) and corresponding reduction peak
at 0.04, -0.01 and -0.01 V, respectively.
The analysis reveals that both the copper complexes
exhibit quasi reversible redox processes (the separation
between cathodic and anodic peak potentials (DEp =
Epa-Epc) for the redox couple Cu(II)/Cu(III) is greater
than 60 mV, the value of peak potentials depends on scan
rate and values of peak current ratio (Ipc/Ipa) are almost
constant but not unity) (Naik et al., 2002a, b).
Cyclic voltammetry studies
The complexes, as a solution of DMSO were scanned in
potential range of -1 to 1 V in nitrogen atmosphere. In this
case of investigation, only the copper complexes exhibit
redox behaviour in the applied potential range (Table 5).
Both the ligands were found to be electrochemically
innocent indicating that the redox behaviour of copper
complexes is purely metal based. The other complexes of
both series did not show any responses in the applied
potential range. The typical cyclic voltammograms of
[CuL1(H2O)] and [CuL2(H2O)] are shown in Fig. 3.
Acute toxicity study
In acute toxicity study all the tested compounds did not
show significant toxic effects and found to be safe at the
tested dose level of 1500 mg/kg body weight of Wister rat.
[CuL1(H2O)] shows one oxidation peak at 0.49, 0.45 and
0.41 V with the varying scan rates of 0.15, 0.1 and 0.05 V/
s, respectively, during the anodic potential scan and in the
reverse scan (cathodic scan) it exhibits a reduction peak at
-0.25, -0.20 and -0.13 V, respectively. Similarly
[CuL2(H2O)] also exhibits redox responses corresponding
Blood glucose level test
The results of blood glucose level test are shown in
Table 6. All the tested compounds have shown good
activity in the reduction of blood glucose level as compared
with the diabetic control rats. The complex [ZnL1(H2O)]
and ligand L2H2 have showed significant reduction in
blood glucose level, as efficient as glibenclamide, a stan-
dard drug used.
Table 5 Cyclic voltammetry results
Complex
Scan rate Epa Epc
(V/S) (V) (V)
DEp E1/2
Ipc/Ipa
(V)a (V)b
OGTT
[CuL1(H2O)] 0.15
0.49 -0.25 0.74 0.12
0.78
0.1
0.45 -0.20 0.65 0.125 0.82
The results of OGTT are shown in Table 6. At the 30 min
interval after the oral administration of glucose, the
compounds [ZnL1(H2O)] and [CuL2(H2O)] have shown
good activity and have reduced the blood glucose level
notably as compared to the glibenclamide. At the 90 min
interval, the compounds [CuL1(H2O)], [ZnL1(H2O)] and
[CuL2(H2O)] have shown significant reduction in blood
glucose level.
0.05
[CuL2(H2O)] 0.15
0.41 -0.13 0.54 0.14
0.88
0.82
0.86
0.91
0.24 0.04
0.20 0.01
0.20 0.14
0.19 0.11
0.1
0.05
0.17 -0.01 0.18 0.08
a
DEp = Epa - Epc
b
E1/2 = [Epc ? Epa]/2
Fig. 3 Voltammograms of [CuL1(H2O)] and [CuL2(H2O)] at the scan rate 0.1 V/s
123

670
Med Chem Res (2012) 21:663–671
Table 6 Effect of ligands and complexes on blood glucose level of alloxan-induced diabetic Wister rats after prolonged treatment and OGTT in
normal Wister rats
Treatment and groups
Blood glucose level in mg/dl
OGTT in non-diabetic rats
Basal
9th day
Fasting
30 min
90 min
Control (normal saline)
Diabetic control (alloxan)
Glibenclamide (std drug)
L1H2
[CuL1(H2O)]
[ZnL1(H2O)]
L2H2
[CuL2(H2O)]
[ZnL2(H2O)]
76.32 1.42
326.6 3.20
332.3 4.80
323.0 6.82
324.7 6.12
325.0 7.68
312.70 8.2
320.0 7.58
356.0 10.58
78.22 1.02
342.42 1.80
142.4 2.60
332.0 22.6
335.6 12.30
188.8 22.8**
182.2 16.4**
318.04 10.8
333.6 18.2
80.12 1.60
–
188.12 4.22
–
136.62 2.80
–
79.80 3.22
82.26 2.58
76.17 2.60
76.22 2.58
72.40 1.82
75.60 2.58
80.53 2.40
176.8 3.02
180.8 4.44
178.60 3.42
156.6 2.20
176.6 2.32
156.60 10.38
182.8 3.09
102.8 2.30***
120.6 2.33
110.8 3.72***
106.3 2.18***
122.8 3.30
118.0 2.44***
124.70 14.48
One-way ANOVA followed by Dunnet’s ‘t’ test. Values are expressed as mean SEM. P [ 0.05 is considered as non-significant. P \ 0.05 is
considered as significant
** P \ 0.01 as compared to diabetic control group
*** P \ 0.001 as compared to diabetic control group
According to the earlier studies, drugs cause antidiabetic
effect by promoting regeneration of b cells or by protecting
the cells in pancreas from destruction, by restricting glu-
cose load as well as by promoting unrestricted endogenous
insulin action or they effect b cells to release insulin and
activate the insulin receptors to absorb the blood sugar
(e.g., sulphonylureas) (Fosset et al., 1988). The comparable
effect of prepared compounds with glibenclamide suggests
the similar mode of action. Since alloxan permanently
destroys the pancreatic b cells and the compounds have
lowered the blood glucose level in alloxanised rats to sig-
nificant level, indicating that the compounds possess extra
pancreatic effects. The compounds showed beneficial
effects on blood glucose and hyperlipidemia associated
with diabetes, thus they could serve as good adjuvant to
other oral hypoglycemic agents and seems to be promising
for the development of medicines for diabetes mellitus.
Further, detailed studies are necessary to find out the active
constituents, which are responsible for the said activity and
its exact mechanism of action.
done by blood-glucose test and OGTT. The compounds
[ZnL1(H2O)] and L2H2 have showed notable reduction in
blood glucose level. The complexes [CuL1(H2O)],
[ZnL1(H2O)] and [CuL2(H2O)] have exhibited promising
activity in OGTT and posses low toxicity with a high safety
profile.
Acknowledgments The authors thank Department of Chemistry
and USIC, Karnatak University, Dharwad for the spectral facility.
Recording of FAB-mass spectra (CDRI Lucknow) are gratefully
acknowledged. Further, the author (Naveen V. Kulkarni) thank Kar-
natak University, Dharwad for providing Nilekani fellowship.
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