Synthesis, characterization, potentiometry, and antimicrobial studies of transition metal complexes of a tridentate ligand

Article abstract of DOI:10.1080/00958972.2010.529136

Complexes of Cu(II), Ni(II), Co(II), Mn(II), and Fe(III) with the tridentate Schiff base, 4-hydroxy-3(1-{2-(benzylideneamino)-phenylimino}-ethyl)- 6-methyl-2H-pyran-2-one (HL) derived from 3-acetyl-6-methyl-(2H)-pyran-2,4(3H)- dione (dehydroacetic acid or DHA), o-phenylenediamine, and benzaldehyde were characterized by elemental analysis, molar conductivity, magnetic susceptibility, thermal analysis, X-ray diffraction, IR, ¹H-NMR, UV-Vis spectroscopy, and mass spectra. From analytical data, the stiochiometry of the complexes was found to be 1 : 2 (metal : ligand) with octahedral geometry. The molar conductance values suggest nonelectrolytes. X-ray diffraction data suggest monoclinic crystal systems. IR spectral data suggest that the ligand is dibasic tridentate with ONN donors. To investigate the relationship between formation constants of metal complexes and antimicrobial activity, the dissociation constants of Schiff base and stability constants of its binary metal complexes have been determined potentiometrically in THF-water (60 : 40) at 30 1C and at 0.1 mol L^-1 NaClO₄ ionic strength. The potentiometric titrations suggest 1 : 1 and 1 : 2 complexation. Antibacterial and antifungal activities invitro were performed against Staphylococcus aureus, Escherichia coli, Aspergillus niger, and Trichoderma with determination of minimum inhibitory concentrations of ligand and metal complexes. The structure-activity correlation based on stability constants of metal complexes is discussed. Activity enhances upon complexation and the order of activity is in accord with the stability order of metal ions.

Full text of DOI:10.1080/00958972.2010.529136

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Synthesis, characterization,
potentiometry, and antimicrobial
studies of transition metal complexes
of a tridentate ligand
Sarika Manohar Jadhav a , Vinod Ambadas Shelke a , Achut
Sadashiv Munde b , Sunil Govind Shankarwar a , Vaishali Ramrao
Patharkar a & Trimbak Kamaji Chondhekar a
a Department of Chemistry , Dr. Babasaheb Ambedkar Marathwada
University , Aurangabad 431004, Maharashtra, India
b Department of Chemistry , Milind College of Science ,
Aurangabad 431004, Maharashtra, India
Published online: 20 Nov 2010.
To cite this article: Sarika Manohar Jadhav , Vinod Ambadas Shelke , Achut Sadashiv Munde ,
Sunil Govind Shankarwar , Vaishali Ramrao Patharkar & Trimbak Kamaji Chondhekar (2010)
Synthesis, characterization, potentiometry, and antimicrobial studies of transition metal
complexes of a tridentate ligand, Journal of Coordination Chemistry, 63:23, 4153-4164, DOI:
10.1080/00958972.2010.529136
To link to this article: http://dx.doi.org/10.1080/00958972.2010.529136
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Journal of Coordination Chemistry
Vol. 63, No. 23, 10 December 2010, 4153–4164
Synthesis, characterization, potentiometry, and antimicrobial
studies of transition metal complexes of a tridentate ligand
SARIKA MANOHAR JADHAVy, VINOD AMBADAS SHELKEy, ACHUT
SADASHIV MUNDEz, SUNIL GOVIND SHANKARWARy, VAISHALI
RAMRAO PATHARKARy and TRIMBAK KAMAJI CHONDHEKAR*y
yDepartment of Chemistry, Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad 431004, Maharashtra, India
zDepartment of Chemistry, Milind College of Science, Aurangabad 431004,
Maharashtra, India
(Received 27 February 2010; in final form 31 August 2010)
Complexes of Cu(II), Ni(II), Co(II), Mn(II), and Fe(III) with the tridentate Schiff base,
4-hydroxy-3(1-{2-(benzylideneamino)-phenylimino}-ethyl)-6-methyl-2H-pyran-2-one
(HL)
derived from 3-acetyl-6-methyl-(2H)-pyran-2,4(3H)-dione (dehydroacetic acid or DHA),
o-phenylenediamine, and benzaldehyde were characterized by elemental analysis, molar
conductivity, magnetic susceptibility, thermal analysis, X-ray diffraction, IR, 1H-NMR,
UV-Vis spectroscopy, and mass spectra. From analytical data, the stiochiometry of the
complexes was found to be 1 : 2 (metal : ligand) with octahedral geometry. The molar
conductance values suggest nonelectrolytes. X-ray diffraction data suggest monoclinic crystal
systems. IR spectral data suggest that the ligand is dibasic tridentate with ONN donors. To
investigate the relationship between formation constants of metal complexes and antimicrobial
activity, the dissociation constants of Schiff base and stability constants of its binary metal
complexes have been determined potentiometrically in THF–water (60 : 40) at 30 ꢀ 1ꢁC and at
0.1 mol Lꢂ1 NaClO4 ionic strength. The potentiometric titrations suggest 1 : 1 and 1 : 2
complexation. Antibacterial and antifungal activities in vitro were performed against
Staphylococcus aureus, Escherichia coli, Aspergillus niger, and Trichoderma with determination
of minimum inhibitory concentrations of ligand and metal complexes. The structure–activity
correlation based on stability constants of metal complexes is discussed. Activity enhances upon
complexation and the order of activity is in accord with the stability order of metal ions.
Keywords: Tridentate ligand; Potentiometry; Antimicrobial activity; MIC; Metal complexes
1. Introduction
Heterocyclic aromatic compounds like benzimidazoles and benzothiazoles possess
physiological activities with fungicidal, insecticidal, antimicrobial, and anesthetic
properties. Dehydroacetic acid (DHA) is a heterocyclic compound which is a potential
*Corresponding author. Email: tkcchem@gmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.529136

4154
S.M. Jadhav et al.
chelating agent [1] and shows promising fungicidal and antibacterial activities [2, 3]. It
is also a versatile starting material for the synthesis of a wide variety of heterocyclic ring
systems [4] which show enhanced biological activities. Schiff bases and their metal
complexes exhibit a wide range of biological activities and structures. In view of the
importance of DHA and its metal complexes, it is worthwhile to synthesize Schiff bases
of DHA and their metal complexes. Little attention has been given to tridentate Schiff
bases of DHA containing ONN donors. In continuation of our study [5–8], we report
the synthesis of a tridentate Schiff base formed by condensation of DHA,
o-phenylenediamine, and benzaldehyde. Complexes of Cu(II), Ni(II), Co(II), Mn(II),
and Fe(III) with this ligand have been prepared and characterized by physicochemical
methods.
2. Experimental
2.1. Materials and methods
DHA (purity ꢃ 99%) was purchased from E. Merck and used as supplied.
o-Phenylenediamine and benzaldehyde of AR grade used for synthesis of ligand were
obtained from Acros Organics and Sisco, respectively. AR grade metal nitrates of S.D.
Fine were used for complex preparation. Spectral grade solvents were used for spectral
measurements. The carbon, hydrogen, and nitrogen contents were determined on a
Perkin Elmer (2400) CHNS analyzer. IR spectra (4000–400 cmꢂ1) were recorded on a
Jasco FT-IR-4100 spectrometer using KBr pellets. 1H-NMR spectra of the ligand were
measured in CDCl3 using TMS as an internal standard. The TG/DT analyses were
recorded on a Perkin Elmer TA/SDT-2960 at 10 degrees minꢂ1. X-ray diffraction
(XRD) was recorded on a Philips 3701 powder diffractometer employing Cu-Kꢀ
˚
radiation (ꢁ ¼ 1.541 A). The UV-Vis spectra of the complexes were recorded on a Jasco
UV-530 spectrophotometer. Magnetic susceptibility measurements of the metal
complexes were done on a Gouy balance at room temperature using Hg [Co(SCN)4]
as a calibrant. Molar conductances were measured on an Elico CM-180 conductometer
using 1 mmol Lꢂ1 solutions in DMSO. Elico digital pH meter (model LI-127) equipped
with a CL-51B combined electrode was used for pH measurements, calibrated against
standard buffers (pH 4.02 and 9.18) before measurements. The pH readings were
corrected for organic–aqueous media. Titrations were performed in a double-walled
glass cell in nitrogen at ionic strength of 0.1 mol Lꢂ1 (NaClO4); solutions were
titrated pH-metrically against (0.2N) NaOH. The system used for potentiometric
measurements and calculations has been described previously [9]. Potentiometric
titrations were carried out at 30 ꢀ 1ꢁC. THF used in this study obtained from E. Merck
was further distilled by known literature method [10]. THF : water (60 : 40) solutions
were used as solvent for potentiometric studies. All metal ion solutions were
prepared from their AR grade metal chlorides and standardized by known literature
methods. A standard 0.2N NaOH solution (E. Merck) was used for titrations. Standard
solutions of (AR grade), HClO4, and NaClO4 (1.0 mol Lꢂ1) were prepared and
standardized by known methods [10]. The ligand solution (0.1 mol Lꢂ1) was prepared
in distilled THF.

Transition metal complexes of a tridentate ligand
4155
Scheme 1. Synthesis of ligand.
2.2. Preparation of ligand
2.2.1. Step I. The ligand was prepared by modification of reported methods [11–13]
(scheme 1). In a 50 mL solution of 0.001 mol (0.168 g) of DHA [3-acetyl-6-methyl-(2H)-
pyran-2,4(3H)-dione], 0.001 mol (0.108 g) of o-phenylenediamine was refluxed in super
dry ethanol for 3 h, then cooled to room temperature. On cooling, white mono-Schiff
base precipitated in 80% yield.
2.2.2. Step II. The intermediate (0.001 mol; 0.258 g) was then refluxed with 0.001 mol
(0.10 mL) of benzaldehyde in super dry ethanol for 6 h. The precipitate thus formed was
filtered, dried in vacuum over CaCl2, and recrystallized from ethanol (yield 72%).
2.3. Preparation of complexes
To a hot methanolic solution (25 mL) of ligand (0.02 mol), methanolic solution (25 mL)
of metal nitrate (0.01 mol) was added with constant stirring. The pH was adjusted
to 7.5–8.5 by adding 10% alcoholic ammonia solution and refluxed for 3 h. The
precipitated metal complex was filtered off while hot and washed with hot methanol,
petroleum ether, ethyl acetate, and dried over calcium chloride in a vacuum desiccator
(yield 65%).

4156
S.M. Jadhav et al.
Table 1. Antibacterial activity of compounds (diameter of inhibition zone in mm) and antifungal activity
weight (mg) (% inhibition) of the ligand and complexes.
Antibacterial activity (mg mLꢂ1
E. coli Staphylococcus aureus
0.5 0.5
40 42
)
Antifungal activity (mg mLꢂ1
)
A. niger
Trichoderma
0.5
65
Compound
1
1
0.5
1
1
Ciprofloxin/control
HL
Cu(L)2
Ni(L)2
Co(L)2
45
17
29
16
22
27
24
44
11
25
18
20
23
22
75
65(13)
42(44)
45(40)
55(31)
54(28)
53(29)
72
60
27(55)
20(67)
22(63)
24(60)
27(55)
25(58)
15
27
10
20
25
22
09
18
13
14
16
15
60(17)
10(86)
13(82)
18(75)
22(69)
20(72)
45(31)
30(54)
32(51)
36(45)
45(31)
40(38)
Mn(L)2
Fe(L)2
2.4. Antimicrobial activity
Antibacterial activity of free ligand, its metal complexes, the metal salts, and control
(DMSO) was tested in vitro against Gram-positive bacteria (S. aureus) and Gram-
negative bacteria (Escherichia coli) by paper disc method [8]. Sterile (10 mm diameter)
Whatman no. 42 paper discs were soaked in different concentrations of ligand/
complexes (0.5 and 1 mg Lꢂ1) in DMF, dried, and then placed on nutrient agar plates.
The plates were then incubated for 24 h at 37ꢁC and the inhibition zone around each
disc was measured. The results obtained were compared with Ciprofloxin. Three
replicates were taken and average value is given in table 1.
The free ligand, its metal complexes, metal salts, and control (DMSO) were screened
for antifungal activity against the fungi Aspergillus niger and Trichoderma at 0.5 and
1 mg Lꢂ1 by mycelial dry weight (MDW) method [14]. The cultures of fungi were
purified by single spore isolation technique. The glucose nitrate (GN) medium was used
for growth of fungi. The mycelial biomass was then dried along with filter paper in an
oven at 65 ꢀ 5ꢁC to constant weight, cooled, and finally weighed. The MDW was
obtained by subtracting the weight of mycelium-free filter paper from final dry weight
[15]. Three replicates of each treatment were repeated in all experiments. The MDW
was corrected each time by subtracting the dry weight obtained from incubated
flask under similar experimental conditions. The yields of MDW (mg) are presented
in table 1. The percentage error was found to be ꢀ 0.01. The percent decrease in MDW
to the test compound in each case was calculated and tabulated in terms of average
percentage inhibition. The results indicate that the ligand and its metal complexes
arrested the growth of fungi.
2.5. Minimum inhibitory concentration
Minimum inhibitory concentration (MIC) is the lowest concentration of an antimicro-
bial that inhibits visible fungal growth [16–18]. The MIC of fungi was determined by
using MDW method by preparing different concentrations of test solutions (0.5, 0.75, 1,
and 1.25 mg mLꢂ1). GN medium (50 mL) was taken in clean conical flasks and wrapped
with paper and sterilized in an autoclave at 121ꢁC for 30 min. Different concentrations

Transition metal complexes of a tridentate ligand
4157
Table 2. MIC (mg mLꢂ1) of the ligand and metal complexes against the bacterial and fungal inhibition
zones.
Compound
E. coli
S. aureus
A. niger
Trichoderma
Ciprofloxin/control
HL
Cu(L)2
Ni(L)2
Co(L)2
40(0.25)
15(0.5)
27(0.5)
10(0.5)
20(0.5)
22(0.5)
25(0.75)
42(0.25)
09(0.5)
18(0.5)
13(0.5)
14(0.5)
15(0.5)
16(0.75)
50(1.5)
20(1.5)
05(0.75)
07(1)
11(1)
10(0.75)
05(1.25)
35(1.25)
07(1.25)
04(0.75)
07(1)
08(1)
09(1)
11(1)
Fe(L)2
Mn(L)2
of the ligand and metal complexes as prepared above were added in the conical flasks
in which target fungi A. niger and Trichoderma were added. The conical flasks were
incubated immediately at 37ꢁC for 7 days. The MDW was obtained and the MIC values
calculated are depicted in table 2.
The MIC of ligand and its metal complexes against Gram-positive and Gram-
negative bacteria (S. aureus and E. coli) were determined by paper disc method in
nutrient agar media. The compounds were tested at concentrations 0.25, 0.5, 0.75, 1,
and 1.25 mg mLꢂ1 in DMSO. All experiments were conducted in triplicate and the same
MIC value was obtained. These MIC values are compared with Ciprofloxin as standard
in table 2.
3. Results and discussion
3.1. Elemental analysis
The analytical data of ligand and metal complexes are given in table 3. The elemental
analyses show 1 : 2 (metal : ligand) stoichiometry for the solid complexes, corresponding
with [ML2], where M ¼ Cu(II), Ni(II), Co(II), Mn(II), and Fe(III). The magnetic
susceptibilities of all complexes at room temperature were consistent with octahedral
geometry. The metal complex solutions in DMSO show low conductances which
support their nonelectrolyte nature.
3.2. 1H-NMR spectra
1H-NMR spectra of the ligand in CDCl3 show signals at ꢂ 2.11 (s, C6–CH3), ꢂ 2.54 (s,
3H, N ¼ C-CH3), ꢂ 5.83 (s, 1H, C5–H), 6.9–7.9 (m, 9H, phenyl), ꢂ 8.71 (s, 1H, N¼C–H),
and ꢂ 15.86 (s, 1H, enolic OH of DHA moiety). The structure of the ligand is shown in
figure 1.
3.3. FT-IR spectra
The IR spectrum of free ligand shows bands at 3060–3300, 1698, 1657, 1354, and
1212 cmꢂ1 assignable to intramolecular hydrogen-bonded ꢃ(OH), lactone carbonyl

4158
S.M. Jadhav et al.
Table 3. Analytical and physical data of the ligand and its metal complexes.
Contents (found/calcd) (%)
Molecular
formula
Compound (formula weight) Color point (ꢁC)
Melting
ÃM
(ꢀꢂ1cm2 molꢂ1
)
C
H
N
M
HL
C
21H18N2O3
346.379
Yellow
162
280
285
200
264
270
72.80/72.81 5.20/5.40 8.00/8.10 –
–
Cu(L)2
Ni(L)2
Co(L)2
Fe(L)2
Mn(L)2
C42H34N4O6Cu Green
754.31
C42H34N4O6Ni Yellow
749.454
C42H34N4O6Co Brown
749.697
C42H34N4O6Fe Brown
746.61
66.90/66.87 4.50/4.60 7.50/7.42 8.50/8.42
67.40/67.31 4.60/4.57 7.50/7.47 8.50/8.98
67.60/67.56 4.60/4.50 7.55/7.48 7.50/7.39
67.60/67.56 4.60/4.40 7.55/7.45 7.50/7.40
67.10/67.65 4.60/4.39 7.60/7.50 9.20/9.00
12.02
25.05
15.03
11.02
10.02
C42H34N4O6Mn Brown
745.70
Figure 1. Resonance structure of ligand.
ꢃ(C ¼ O), methyl azomethine ꢃ(C ¼ N), arylazomethine ꢃ(C–N), and phenolic ꢃ(C–O)
stretching modes, respectively [19, 20]. In IR spectra of metal complexes, the absence of
a weak broad band at 3060–3300 cmꢂ1 indicates deprotonation of the intramolecular
hydrogen-bonded OH on complexation. This is supported by shift to higher energy in
phenolic ꢃ(C–O) [14]. The ꢃ(C ¼ N) shifts to lower value with respect to free ligand
denoting that nitrogen of azomethine is coordinated to metal, supported by shift to
higher energy in ꢃ(C–N). IR spectra of metal complexes show bands at 457–540 cmꢂ1
which can be assigned to ꢃ(M–O) and ꢃ(M–N), respectively [21]. From the above facts,
it is evident that coordination takes place via azomethine nitrogen and phenolic OH of
the ligand.
3.4. Electronic absorption spectra and magnetic studies
Electronic spectra of Cu(II) complex in DMSO show bands (" in L molꢂ1 cmꢂ1) at
2
2
13,513(740), 16,339(612), and 25,641(366) cmꢂ1 assignable to a Eg ! T2g transition

Transition metal complexes of a tridentate ligand
Table 4. Ligand field parameter of the complexes.
4159
Ligand field
splitting energy
Racah interelectronic
repulsion parameter
Covalent
factor (ꢄ)
LFSE
(kcal molꢂ1
)
Complex
(Dq cmꢂ1
)
(B cmꢂ1
)
ꢄ (%)
ꢃ2/ꢃ1
Cu(L)2
Ni(L)2
Co(L)2
Fe(L)2
Mn(L)2
1351.3
936.32
1016.26
126,582
1385.042
96.06
843.57
913.97
788.63
648.04
–
–
–
38.512
26.685
28.963
36.075
39.473
0.8103
0.9412
0.7769
0.6743
23.40
6.23
22.30
32.56
1.584
1.832
1.496
1.760
and charge transfer. Electronic spectral data coupled with observed magnetic moment
of 1.84 B.M. suggests octahedral geometry [22]. Ni(II) complex displays bands at
3
3
12,562(1068), 17,482(674), and 25,641(386) cmꢂ1 assignable to A2g ! T2g(F),
3A2g ! T1g(F), and A2 g! T1g(P), respectively. These electronic transitions along
with magnetic moment 2.92 B.M. suggest octahedral geometry for Ni(II) complex
[23, 24]. The Co(II) complex shows three bands at 10,162(984), 18,621(537), and
3
3
3
4
4
4
4
4
25,575(391) cmꢂ1 assignable to T1g(F) ! T1g(P), T1g(F) ! A2g, and T1g(F) !
4T1g(P) transitions, respectively. These transitions and observed magnetic moment of
4.7 B.M. indicate high spin octahedral complex [25, 26]. Fe(III) complex exhibits bands
at 12,658(790), 18,940(528), and 30,864(324) cmꢂ1 assignable to A1g ! T1(D),
6
4
6A1g ! T1, and A1g ! T2g, respectively. The observed magnetic moment of
5.85 B.M along with electronic transitions corresponds to octahedral geometry
[27–30]. For Mn(II) complex, the observed magnetic moment of 5.7 B.M. and the
spectral bands at 16,611(722), 24,390(410), and 26,881(372) cmꢂ1 assignable to
4
6
4
6A1g ! T1(G), A1g ! T2g(G), and A1g ! Eg, respectively, indicate octahedral
geometry [31, 32]. The calculated values of ligand field splitting energy (10 Dq),
Racah interelectronic repulsion parameter (B), covalent factor (ꢄ), ratio ꢃ2/ꢃ1, and
ligand field stabilization energy (LFSE), given in table 4, support the proposed
geometry for all the complexes.
4
6
4
6
4
3.5. XRD study
The X-ray diffractograms of the metal complexes were scanned from 5ꢁ to 100ꢁ at
˚
wavelength 1.543 A. The diffractogram and associated data depict 2ꢅ values for each
peak, relative intensity, and interplanar spacing (d-values). The diffractogram of Ni(II)
complex shows 12 reflections with maxima at 2ꢅ ¼ 23.07ꢁ corresponding to a d-value of
˚
3.855 A. The diffractogram of Co(II) complex shows nine reflections with maxima at
ꢁ
˚
2ꢅ ¼ 23.16 corresponding to d-value of 3.83 A, whereas the diffractogram of Mn(II)
complex shows 26 reflections with maxima at 2ꢅ ¼ 89.95ꢁ corresponding to d-value of
˚
1.089 A. The XRD pattern of these complexes with respect to major peaks having
relative intensity greater than 10% have been indexed by using a computer program
[33]. The above indexing method also yields Miller indices (hkl), unit cell parameters,
and unit cell volume. The unit cell of Ni(II) complex yielded values of lattice constants,
3
˚
˚
a ¼ 10.053, b ¼ 8.571, and c ¼ 9.830 A and unit cell volume, V ¼ 394.253 (A) ; Co(II),
3
˚
˚
˚
˚
a ¼ 16.038 A, b ¼ 4.285 A, c ¼ 10.692 A and unit cell volume, V ¼ 734.170 (A) ; Mn(II),

4160
S.M. Jadhav et al.
3
˚
˚
˚
˚
a ¼ 14.223 A, b ¼ 5.196 A, c ¼ 7.754 A and unit cell volume, V ¼ 568.562 (A) . The
condition a ¼ b ¼ c and ꢀ ¼ ꢆ ¼ 90ꢁ ¼ ꢄ required for the compounds to be monoclinic
were tested and found to be satisfactory for Ni(II), Co(II), and Mn(II) complexes.
Hence, it can be concluded that these metal complexes show monoclinic crystal system.
Experimental density values of the complexes were determined by using specific gravity
method [34] and found to be 1.877, 1.021, and 2.326 gcmꢂ3 for Ni(II), Co(II), and
Mn(II) complexes, respectively.
3.6. Thermogravimetric studies
The Cu(II), Ni(II), and Co(II) complexes were chosen for thermal study. The TG/DT
analyses were carried out from ambient temperature to 1000ꢁC in nitrogen using
ꢀ-Al2O3 as reference. The TG/DTA curves of these complexes show no weight loss up
to 270ꢁC, indicating the absence of coordinated water [35]. In the copper complex, the
first decomposition step at 290–390ꢁC with mass loss (calculated ¼ 29.70%;
TG ¼ 30.80%) as an exothermic peak DTmax ¼ 337ꢁC in DTA attributed to decompo-
sition of noncoordinated part of ligand. The second step from 460ꢁC to 875ꢁC
corresponds to loss of the coordinated part of the ligand (calculated ¼ 52.30%;
TG ¼ 52.30%) and finally, stable metal oxide CuO (calculated ¼ 10.47%; TG ¼ 9.20%).
In the TG curve of nickel complex, the first step from 280ꢁC to 495ꢁC (calcu-
lated ¼ 39.60%; TG ¼ 40.70%) corresponds to decomposition of noncoordinated
ligand. In DTA, an endothermic peak at 400ꢁC was obtained. Second step at 425–
800ꢁC (calculated ¼ 29.59%; TG ¼ 30%) is due to coordinated ligand. A broad
exothermic peak in DTA was obtained at 518.67ꢁC. Finally, stable nickel oxide was
formed (calculated ¼ 9.5%; TG ¼ 9.96%). For cobalt complex, the first decomposition
takes place from 280ꢁC to 370ꢁC (calculated ¼ 54.00%; TG ¼ 53.65%) due to
decomposition of noncoordinated ligand with sharp exotherm at 340ꢁC in the DTA
curve. Second step from 380ꢁC to 930ꢁC (calculated ¼ 31.20%; TG ¼ 30.94%) indicates
decomposition of coordinated ligand. The residues at the end of the decomposition
corresponds to CoO (calculated ¼ 9.70%; TG ¼ 9.99%).
3.7. Potentiometry
Calvin–Bjerrum pH titration technique as modified by Irving and Rossotti [36] was
used to determine proton–ligand and metal–ligand stability constants in THF–water
ꢁ
ꢁ
ꢁ
mixture (60% v/v) at 30 ꢀ 1 C. Irving and Rossotti method was used to calculate nA, n,
pK, and pL values from the pH titration curves. Protonation takes place in the initial
stages of titrations because of the presence of azomethine nitrogen. The pK1 and pK2
ꢁ
were determined at nA ¼ 1.5 and 0.5, respectively. The first stability constant pK1,
therefore, refers to the imine nitrogen. However, there are two imine nitrogens present
in the ligand and only one pK value is displayed, suggesting that one is not involved in
protonation. This may be attributed to intramolecular hydrogen bonding of nitrogen
Nb (figure 1) which undergoes chelation with enolic OH of DHA and hence does not
take part in protonation and deprotonation. Therefore, the pK1 value (2.93) shown by
the ligand is due to the protonation of Na. The second pK observed at 9.83 in the ligand
is due to dissociation of enolic proton which is sufficiently acidic due to the resonance
effect. All the metals form 1 : 1 and 1 : 2 chelates with this ligand. The 1 : 1 and 1 : 2

Transition metal complexes of a tridentate ligand
4161
chelates are formed in a simultaneous process in all the systems. Hence, the method of
least squares has been used for getting accurate values of log k1 and log k2. The values of
log k1, log k2, and log ꢄ for Cu(II) are 10.05, 9.61, and 19.67; Ni(II), 10.02, 8.79, and
18.82; Co(II), 9.83, 3.89, and 13.72; Fe(III), 9.15, 3.38, and 12.54; and Mn(II), 9.09,
3.50, and 12.40, respectively.
Coordination of metal ion with ligand takes place via protonated nitrogen and
oxygen of the enolic group (figure 2). The order of stability constants is
Cu 4 Ni 4 Co 4 Fe 4 Mn which is in agreement with Irving–Williams order [36].
3.8. Antimicrobial activity
The Schiff base HL and its metal complexes were evaluated for antimicrobial activity
against one Gram-positive bacteria (S. aureus), a Gram-negative bacteria (E. coli) and
fungi A. niger and Trichoderma. The antimicrobial activities are presented in table 1,
showing that the free ligand and all metal complexes exhibit antibacterial activity
against both strains. Metal complexes exhibit higher activity than free ligand under
identical experimental conditions. Antimicrobial inhibitions were compared with the
activity of Ciprofloxin as a standard and all compounds were less than the standard.
The metal salts used for synthesis of complexes exhibit negligibly small antimicrobial
activities [37–40]. Enhancement of activity of ligand after chelation can be explained on
the basis of Overtones and Tweedy’s concepts [41]. Inhibition was found to increase
with increasing concentration of metal complex. The results showed that the Cu(II)
complex exhibits higher activity against each class of organism. The activity is related to
the nature and structure of the complexes [42, 43]. The higher activity of copper
complex may be attributed to its higher stability constants. The ferric and manganese
complexes show activity comparable to the copper complex. However, nickel complex
shows low activity against E. coli as compared to S. aureus. All the reported complexes
and ligand show more activity against Gram-negative E. coli than Gram-positive
S. aureus.
Figure 2. Structures of metal complexes.

4162
S.M. Jadhav et al.
Antibacterial activity can be ordered as Cu(L)2 4 Mn(L)2 4 Fe(L)2 4
Co(L)2 4 Ni(L)2, similar to earlier observations [44]. These complexes do not show
strong concentration dependence of antimicrobial activity as compared to the
antifungal activities of the same complexes. The relation between chelation and toxicity
is very complex, expected to be a function of steric, electronic, and pharmakinetic
factors along with mechanistic pathways [44]. Stability constants, solubility, particle
size, size of metal ion, and magnetic moments may also be responsible for the
antimicrobial activity of the complexes [45, 46]. Attempts to show a relationship
between stability constants and antimicrobial activity of the complexes proved futile.
Investigation of antifungal activity of the ligand and its metal complexes revealed
that all the metal chelates are more toxic than the ligand (table 1). The antifungal
activity is enhanced several times on being coordinated with metal. Antifungal activity
of these complexes increases as the stability of the complex increases. The activities of
these complexes follow the order Cu 4 Ni 4 Co 4 Fe 4 Mn which is exactly same as
the order of stability constants of these complexes. Comparison of activities shows that
the copper complex is more active than the ligand against A. niger. Activity of ligand
against Trichoderma increases after chelation; however, the extent of increase is less
than that of A. niger.
3.9. Minimum inhibitory concentration
The inhibitory potential of the compounds were further investigated by determining
the MIC values. The observed inhibition of growth (mm) and MIC (mg mLꢂ1) are
presented in table 2.
The copper complex is most potent against both bacterial strains compared to other
metal complexes. Copper complex showed a MIC of 0.5 mg mLꢂ1 against E. coli and
S. aureus with the highest zone of inhibition at 27 and 18 mm. The Fe(III), Co(II), and
Ni(II) complexes showed zones of inhibition at 22, 20, and 10 mm, respectively, for
MIC 0.5 mg mLꢂ1 against E. coli and 15, 14, and 13 mm zones against S. aureus. Mn(II)
complex shows higher MIC of 0.75 mg mLꢂ1 with zone of inhibitions of 22 and 15 mm
against the two bacterial strains. However, the MIC shown by all the compounds is
much higher than that of Ciprofloxin but lower than the free ligand.
For antifungal activity, the observed MDW (mg) and MIC (mg mLꢂ1) are presented
in table 2. Copper complex has significant growth inhibition of both the fungal strains
and lowest MDW of 5 and 4 mg with an MIC of 0.75 mg mLꢂ1 against A. niger and
Trichoderma indicating its strong antifungal activity. All other complexes showed an
MIC of 1.0 mg mLꢂ1 against both fungal species. The order of activity in terms of their
MIC values is Cu 4 Ni 4 Co 4 Fe 4 Mn against A. niger and Trichoderma. MIC of all
the complexes is less than the ligand and control DMF.
4. Conclusion
On the basis of physicochemical and spectral data discussed above, octahedral
geometry for Cu(II), Ni(II), Co(II), Mn(II), and Fe(III) complexes is proposed. IR
spectra show the ligand as a dibasic, ONN tridentate, coordinating via phenolic oxygen

Transition metal complexes of a tridentate ligand
4163
and imino nitrogen. Thermal study reveals that the complexes are thermally stable.
XRD study suggests monoclinic crystal system for Co(II), Ni(II), and Mn(II)
complexes. The ligand and its metal complexes are biologically active; antibacterial
activity shows that the copper complex is more biologically active. Antifungal activity
of these complexes increases in the order of increase in stability constants of metal
complexes. The same trend has been found for MIC.
Acknowledgments
The authors thank the Head of the Department of Chemistry, Dr. Babasaheb
Ambedkar Marathwada University, for providing necessary facilities during the entire
research work.
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