Synthesis, potentiometric and antimicrobial studies on metal complexes of isoxazole Schiff bases

Article abstract of DOI:10.1016/j.saa.2007.07.028

The metal complexes of Cu(II), Ni(II) and Co(II) with Schiff bases of 3-(2-hydroxy-3-ethoxybenzylideneamino)-5-methyl isoxazole [HEBMI] and 3-(2-hydroxy-5-nitrobenzylidene amino)-5-methyl isoxazole [HNBMI] which were obtained by the condensation of 3-amino-5-methyl isoxazole with substituted salicylaldehydes have been synthesized. Schiff bases and their complexes have been characterized on the basis of elemental analysis, magnetic moments, molar conductivity, thermal analysis and spectral (IR, UV, NMR and Mass) studies. The spectral data show that these ligands act in a monovalent bidentate fashion, co-ordinating through phenolic oxygen and azomethine nitrogen atoms. Chelates of Co(II), Ni(II) appear to be octahedral and Cu(II) appears to be distorted octahedral. To investigate the relationship between formation constants of binary complexes and antimicrobial activity, the dissociation constants of Schiff bases and stability constants of their binary metal complexes have been determined potentiometrically in aqueous solution at 30 ± 1 °C and at 0.1 M KNO₃ ionic strength and discussed. Antimicrobial activities of the Schiff bases and their complexes were screened. The structure-activity correlation in Schiff bases and their metal(II) complexes are discussed, based on the effect of their stability constants. It is observed that the activity enhances upon complexation and the order of activity is in accordance with stability order of metal ions.

Full text of DOI:10.1016/j.saa.2007.07.028

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 30–35
Synthesis, potentiometric and antimicrobial studies on
metal complexes of isoxazole Schiff bases
Y. Prashanthi, K. Kiranmai, N.J.P. Subhashini, Shivaraj∗
Department of Chemistry, Osmania University, Hyderabad, Andhra Pradesh 500 007, India
Received 5 December 2006; received in revised form 2 July 2007; accepted 10 July 2007
Abstract
The metal complexes of Cu(II), Ni(II) and Co(II) with Schiff bases of 3-(2-hydroxy-3-ethoxybenzylideneamino)-5-methyl isoxazole [HEBMI]
and 3-(2-hydroxy-5-nitrobenzylidene amino)-5-methyl isoxazole [HNBMI] which were obtained by the condensation of 3-amino-5-methyl isox-
azole with substituted salicylaldehydes have been synthesized. Schiff bases and their complexes have been characterized on the basis of elemental
analysis, magnetic moments, molar conductivity, thermal analysis and spectral (IR, UV, NMR and Mass) studies. The spectral data show that
these ligands act in a monovalent bidentate fashion, co-ordinating through phenolic oxygen and azomethine nitrogen atoms. Chelates of Co(II),
Ni(II) appear to be octahedral and Cu(II) appears to be distorted octahedral. To investigate the relationship between formation constants of binary
complexes and antimicrobial activity, the dissociation constants of Schiff bases and stability constants of their binary metal complexes have been
determined potentiometrically in aqueous solution at 30 1 ◦C and at 0.1 M KNO3 ionic strength and discussed. Antimicrobial activities of the
Schiff bases and their complexes were screened. The structure–activity correlation in Schiff bases and their metal(II) complexes are discussed,
based on the effect of their stability constants. It is observed that the activity enhances upon complexation and the order of activity is in accordance
with stability order of metal ions.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Isoxazole Schiff bases; Metal chelates; Formation constants; Biological activity
1. Introduction
that several Schiff base complexes have anti-inflammatory, anti-
pyretic, analgesic, anti-diabetic, anti-bacterial, anti-cancer and
anti-HIV activity [9,10]. The antimicrobial activity of 3-amino-
5-methyl isoxazole Schiff bases with methoxy salicylaldehyde,
naphthaldehyde and their metal complexes was reported earlier
and it was found that these compounds show increased activity
when administered as metal complexes rather than as organic
compounds [11]. In the present investigation we report here the
synthesis, characterization, formation constants and antimicro-
bial studies of Schiff bases HEBMI, HNBMI and their metal
complexes.
A wide range of biological activities of isoxazole derivatives
include pharmacological properties such as hypoglycemic, anal-
gesic, anti-inflammatory, anti-bacterial, anti-cancer, anti-HIV
activity and also agrochemical properties like herbicidal and soil
fungicidal activity and have applications as pesticides and insec-
ticides [1]. Studies of a new kind of chemotherapeutic Schiff
bases are now attracting the attention of biochemists [2]. Schiff
bases derived from an amine and any aldehyde are a class of
compounds which co-ordinate to metal ions via the azomethine
nitrogen [3]. Metal complexes of Schiff bases derived from sub-
stituted salicylaldehydes and various amines have been widely
investigated because of their wide applicability [4–7]. Chelating
ligands containing O and N donor atoms show broad biological
activity and are of special interest because of the variety of ways
in which they are bonded to metal ions [8]. It is well known
2. Experimental
2.1. Synthesis of Schiff bases
Methanolic solution of substituted salicylaldehydes
(0.01 mol) was added dropwise to a solution of 3-amino-
5-methyl isoxazole (0.01 mol) dissolved in methanol. The
mixture was refluxed on water bath for 2 h then cooled to room
temperature and filtered. The reaction was monitored by TLC.
∗
Corresponding author. Tel.: +91 40 27682337.
E-mail address: shivaraj sand@yahoo.co.in ( Shivaraj).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.07.028

Y. Prashanthi et al. / Spectrochimica Acta Part A 70 (2008) 30–35
31
The resulting solution was evaporated under reduced pressure
2.6. Physical measurements
and recrystallized from methanol to get yellow crystals. Yield
was about 80–85%.
1H NMR spectra of the ligands were recorded at 200 and
300 MHz on Varian Gemini Unity Spectrometer using TMS as
internal standard. The EI mass spectra were recorded on a VG
micro mass 7070–H Instrument, ESIMS spectra were recorded
on VG AUTOSPEC mass spectrometer. IR spectra of the ligand
and complexes were recorded using KBr pellets in the range
(4000–400 cm−1) on Perkin-Elmer Infrared model 337. The
conductivity measurements were measured in DMSO solutions
(0.001 M) using Digisun Electronic Digital conductivity meter
of model: DI-909 having a dip-type cell calibrated with KCl
solution. Electronic spectra of metal complexes in DMSO were
recorded on Schimadzu UV-VIS 1601 spectrophotometer. TGA
of complexes were carried on Mettler Toledo Star system in the
temperature range of 0–1000 ◦C. Melting points of the ligands
and decomposition temperature of complexes were determined
on Polmon instrument (model No. MP-96). Magnetic suscep-
tibilities of the complexes were determined on Gouy balance
model 7550 using Hg[Co(NCS)4] as standard. The diamagnetic
corrections of the complexes were computed using Pascal’s con-
stants. The percentage composition of C, H, N for the complexes
and necessary ligands was determined by using microanalytical
techniques on Perkin-Elmer 240C (USA) elemental analyzer.
The percent compositions of metal ions in solid metal complexes
were determined by EDTA titration procedure [13].
2.2. Synthesis of complexes: general method
Hot methanolic solution of ligand (0.01 mol) and hot
methanolic solution of corresponding metal salts (0.005 mol)
were mixed together with constant stirring. The mixture was
refluxed for 2–3 h at 70–80 ◦C on water bath. On cooling, col-
ored solid metal complex was precipitated out. The product was
filtered, washed with cold methanol and dried under vacuum
over P4O10. Purity of the complex was checked by TLC and
melting points.
2.3. Potentiometric reagents and solutions
All the chemicals and solvents used were of Analar grade.
Distilled water was further purified by re-distillation process by
using distillation plant. The double distilled water obtained was
cooled and used for the preparation of the solutions. All metal
ion solutions were prepared from their analytical grade nitrates
and were standardized by reported procedures. A standard 0.1 M
KOH and 0.04 M HNO3 solutions (Merck) were prepared and
standardized by known procedures. Alkali solutions were stored
under a nitrogen atmosphere.
2.7. Potentiometric measurements
2.4. Biological screening
The potentiometric measurements were carried out using a
digital Digisun Electronic model DI-707 pH meter equipped
with a combined glass and calomel electrode. The system was
calibrated to read the hydrogen ion concentration by titration
of HCl solution at 30 1 ◦C and 0.1 M KNO3 ionic strength
and KOH solution according to Gran’s method. The solutions
were titrated in the 2.0–11.0 pH range for the ligands and the
complexes. There was no precipitation within the pH range at
which the titrations performed. The metal ion and ligand ratio
was maintained at 1:5, respectively. The pKa values of Schiff
bases and stability constants of their binary complexes were
determined by Irwing–Rossotti pH metric technique using com-
puter program. The n¯A, n¯ and PL values were calculated by
using standard equation [14].
2.4.1. Anti-bacterial screening
The bacterial sub-cultures Pseudomonas aeurogenosa and
Escherichia coli were obtained form Botany department, O.U.
Theanti-bacterialactionoftheligandHEBMI, HNBMIandtheir
complexes of Co(II), Ni(II) and Cu(II) was checked by paper
disc method [12]. The compounds were dissolved in acetone
(1.0 mg ml−1) and the discs of Whattmann filter paper No.41
having the diameter 4 mm were prepared and soaked in it. These
soaked discs were placed on nutrient agar plates inoculated with
bacteria. These plates were incubated for 36 h at 30 ◦C. The
inhibition zone was observed after 36 h. Acetone was used as a
control and gentamycin as a standard drug.
2.5. Anti-fungal screening
3. Results and discussion
AspergillusnigerandRhizoptoniasolaniwereusedforfungal
test. The anti-fungal activity of the ligands and their complexes
was studied by paper disc method [12]. The fungal strains were
directly mixed with the PDA medium (potato dextrose agar)
and dispersed into the Petri plates. Filter paper discs of 4 mm
diameter were prepared prior to the experiment. These discs
were soaked in acetone in which the test compound was dis-
solved and the acetone was used as a control. The filter paper
discs were placed on nutrient medium mixed with fungal strains.
These Petri dishes were incubated at 35 ◦C for 48 h. The growth
of the fungus was measured by recording the diameter of fungal
mycelia.
On the basis of elemental analysis, the complexes were
assigned to possess the composition shown in Table 1. The
molar conductance measurements of the complexes in DMSO
correspond to non-electrolytes. Thus, the complexes may be
formulated as [M(L)2(H2O)2]. Where M = Cu(II), Co(II), Ni(II).
3.1. Spectral analysis
3.1.1. Mass spectrum of HEBMI (L1)
The mass spectrum of isoxazole 3-ethoxy salicylaldehyde
(Fig. 1) Schiff base showed molecular ion (M+) peak at m/z

32
Y. Prashanthi et al. / Spectrochimica Acta Part A 70 (2008) 30–35
Table 1
Analytical and physical properties
Complex
Emperical formula
Molecular weight
Melting point (◦C)
C
H
N
HEBMI
C13H14N2O2
246
147
190
250
210
245
210
250
250
63.41
52.92
53.36
53.34
53.44
44.63
44.99
44.98
5.69
5.08
5.13
5.12
3.64
3.38
3.40
3.40
11.38
9.49
9.57
[Cu(HEBMI)2(H2O)2]
[Ni(HEBMI)2(H2O)2]
[Co(HEBMI)2(H2O)2]
HNBMI
[Cu(HNBMI)2(H2O)2
[Ni(HNBMI)2(H2O)2]
[Co(HNBMI)2(H2O)2)
[CuC26H30N4O6]
[NiC26H30N4O6]
[CoC26H30N4O6]
C11H9N3O4
[CuC22H20N6O10
[NiC22H20N6O10
589.5
584.7
584.9
247
591.5
586.7
586.9
9.57
17.00
14.20
14.31
14.31
]
]
]
[CoC22H20N6O10
protons H-4ꢀ, 5ꢀ, 6ꢀ appeared as a multiplet at δ 6.81–7.07. 2ꢀ-
OH appeared at δ 12.58, OCH2 of 3ꢀ-OEt appeared at δ 4.15 as
quartet (J = 6.7 Hz) and CH3 of 3ꢀ-OEt appeared as triplet at δ
1.51 (J = 6.7 Hz) base.
3.1.4. 1H NMR of HNBMI (L2)
The 1H NMR, the imine proton appeared as a singlet at δ 9.10.
IsoxazoleH-4appearedat δ 6.12asasinglet and5-CH3 appeared
at δ 2.51 as a singlet. Phenyl protons H-3ꢀ, 4ꢀ, 6ꢀ appeared as
multiple δ 7.10–7.19 and 2ꢀ-OH appeared as a singlet at δ 13.20.
3.1.5. 13C spectrum of HEBMI (L1)
13C spectrum of HEBMI (Fig. 3) has167.5 (CH N), 171.1
(C-3), 96.8 (C-4), 167.1 (C-5), 12.6 (C-5-CH3), 118.5 (C-1ꢀ),
118.5 (C-2ꢀ), 147.6 (C-3ꢀ), 124.5 (C-4ꢀ), 117.5 (C-5ꢀ), 118.8
(C-6ꢀ), 64.7 (–OCH2CH3), 14.7 (–OCH2CH3).
Fig. 1. Mass spectra of L1.
246. The other fragments were observed at m/z 230, 203, 164,
148, 108 and 68.
3.1.6. 13C spectrum of HNBMI (L2)
3.1.2. Mass spectrum of HNBMI (L2)
13C spectrum of HNBMI has 165.5 (CH N), 159.8
(C-3), 94.6 (C-4), 167.3 (C-5), 12.0 (C-5-CH3), 119.6 (C-
1ꢀ), 167.8 (C-2ꢀ), 117.8 (C-3ꢀ), 128.4 (C-4ꢀ), 141.0 (C-5ꢀ),
126.5 (C-6ꢀ).
The mass spectrum of isoxazole 5-nitro salicylaldehyde
Schiff base also exhibits a stable molecular ion (M+) peak at
m/z 247. Other fragments were observed at m/z 230, 202, 156,
141, 119, 68 and 43.
3.1.7. IR data
3.1.3. 1H NMR of HEBMI (L1)
In the H NMR (CDCl3) Fig. 2, the imine proton appeared
as a singlet at δ 8.93. The isoxazole H-4 appeared at δ 6.07
as a singlet and 5-CH3 appeared at δ 2.48 as a singlet. Phenyl
In order to study the binding mode of the Schiff base to the
metal in complexes, the IR spectrum of the free ligand was
compared with the spectra of the complexes. The IR spectra
1
Fig. 2. 1H NMR of L1.
Fig. 3. 13C spectrum of L1.

Y. Prashanthi et al. / Spectrochimica Acta Part A 70 (2008) 30–35
33
Table 2
IR data of complexes
Compound
(OH)
C
N
C–O Coordinated M–N M–O
water
HEBMI
[Cu(HEBMI)2(H2O)2]
[Ni(HEBMI)2(H2O)2] 3371 1599 1292 794
[Co(HEBMI)2(H2O)2]
HNBMI
3376 1620 1249
–
–
458
–
456
–
–
551
552.7
544
–
–
1613 1295 758
–
1605 1297 766
Fig. 4. IR spectra of Ni(II) complex of HEBMI.
3440 1636 1247
–
[Cu(HNBMI)2(H2O)2] 3429 1613 1290 790
[Ni(HNBMI)2(H2O)2] 3394 1624 1290 787
[Co(HNBMI)2(H2O)2] 3394 1624 1290 830
594
–
497
440
462
–
of the ligands show a strong band in the 3200–3400 regions
assigned to the OH group. The disappearance of this band in
the spectra of the complexes [15] indicates the deprotonation
of the hydroxyl group and co-ordination through oxygen. The
band observed at 1636–1620 cm−1 in the ligand is assigned
to azomethine group. The shift of this band in the complexes
towards lower region to the extent of 10–20 cm−1, indicates
co-ordination through the azomethine nitrogen [16,17]. In the
free ligands, bands at 1249 cm−1 due to C–O (phenolic) shift to
higher frequency by 30–50 cm−1 in the complexes, indicating
co-ordination of the phenolic oxygen atom to the metal ion [18].
These facts suggest that the shifts are due to co-ordination of
ligand to the metal atom by the azomethine nitrogen and phe-
nolic oxygen [19]. All the complexes of HEBMI and HNBMI
show a broad diffuse band at 3200–3400 cm−1, and appear-
ance of a band at 795–825 cm−1 at lower frequency region
indicates the presence of water molecules in the co-ordination
sphere. This fact is also supported by the results of elemental
analyses, and TGA of complexes. Two new bands appearing
in the low frequency ranges 515–581 cm−1 and 420–481 cm−1
are assigned to (M–O) and (M–N), respectively [20] (Fig. 4)
(Table 2).
From this observation, it may be concluded that Schiff bases act as mono nega-
tive, bidentateco-ordinatingthroughoxygenofphenolicC–Ogroupandnitrogen
of C N group.
of [Ni(HEBMI)2(H2O)2] complex. Wherein, the metal to ligand
ratio is 1:2 with two water molecules. Thermal study of the com-
plex supported the presence of co-ordinated water molecules
(Fig. 5).
3.1.9. Mass spectrum of [Ni(HNBMI)2(H2O)2]
The fast atom bombardment (FAB) spectrum exhibits molec-
ular ion peak (M + Na) at m/z 609.7, which corresponds to
mass of [Ni(HNBMI)2(H2O)2] complex. Wherein, the metal to
ligand ratio is 1:2 with two water molecules. Thermal study
of the complex supported the presence of co-ordinated water
molecules.
4. Magnetic moment and electronic spectral data of the
ligands and the complexes
4.1. Magnetic measurements
3.1.8. Mass spectrum of [Ni(HEBMI)2(H2O)2]
The fast atom bombardment (FAB) spectrum exhibits molec-
ular ion peak (M + 1) at m/z 585.7, which corresponds to mass
The magnetic moments of the complexes are given in
(Table 3). The magnetic moments of the copper(II) complexes
Fig. 5. Mass spectrum of [Ni(HEBMI)2(H2O)2].

34
Y. Prashanthi et al. / Spectrochimica Acta Part A 70 (2008) 30–35
Table 3
Magnetic moments and electronic spectrum data of the ligands and complexes
Ligand
ν (cm−1
)
v2/v1 10 DQ
B
β
B.M.
–
HEBMI 34,482; 44,247
–
–
–
–
Cu(II)
Co(II)
Ni(II)
11,483
12,135; 21,739; 25,641 1.79
1.87
9,604
9,987
731
711
–
0.75 4.98
0.69 3.21
9,987; 16,534; 24,096
1.65
–
HNBMI 31,847; 38,775
–
–
–
Cu(II)
Co(II)
Ni(II)
13,280; 29,850
14,692; 26,041; 29,411 1.77
9,363; 16,309; 26,315 1.74
1.96
Fig. 6. VISIBLE spectra of Cu(II) of HEBMI.
11,349
9,363
758
969
0.78 5.01
0.94 3.05
4.3. Thermal analysis
The TGA curves show that the initial mass loss occurring
within 136–150 ◦C range is interpreted as loss of moisture and
hydrated water molecules during the chelate drying process, and
the second weight loss within 150–280 ◦C range is due to co-
ordinated water molecules.
are 1.87 and 1.96 B.M., consistent with distorted octahedral
geometry [21]. The magnetic moments of the cobalt(II) com-
plexes are 4.98 and 5.01 B.M., which suggest an octahedral
geometry [22]. And magnetic moments of nickel(II) complexes
are 3.05 and 3.21 B.M., indicating that these complexes are octa-
hedral [23].
4.4. Potentiometric studies
The dissociation constants of HBBMI, HCBMI and stability
constants of their Co(II), Ni(II) and Cu(II) have been reported
earlier. It is found that the order of stability constants was in
accordance with basicity order of ligands [27]. In the present
investigation HEBMI is greater than HNBMI. This is attributed
to more basic nature of HEBMI compound. The stability con-
stants of binary complexes of HEBMI and HNBMI with bivalent
Co(II), Ni(II) and Cu(II). Follow the order w.r.t. ligands M(II)
HEBMI > M(II) HNBMI. This order is in accordance with the
basicity order of ligands. Order of stability with respect to metal
ions is Cu(II) > Ni(II) > Co(II). This is in accordance with the
Irwing–Williams natural order.
4.2. Electronic spectral data
4.5. Biological activity
The electronic spectra of the metal complexes were recorded
in DMSO in the range of 200–1100 nm. The electronic
spectra of Cu(II) complexes exhibit single broad band at
11,483 and 13,280 cm−1 which may be assigned to the
The ligands presented here and their transition metal com-
plexes gave better results against the growth of bacteria and
fungi. It is found that the activity increases upon co-ordination.
The increased activity of the metal chelates can be explained
on the basis of chelation theory [28]. The orbital of each metal
2
transition 2Eg → T2g (D) in distorted octahedral geome-
try [24]. The cobalt(II) complexes exhibit three bands at
12,135–14,6925; 21,739–26,041; 25,641–29411 cm−1 which
4
4
may be assigned to the transitions T1g (F) → T2g (F)
4
4
4
4
(v1); T1g (F) → A2g (F) (v2); and T1g(F) → T1g(P) (v3),
expected for an octahedral structure [25]. The electronic spec-
tra of Ni(II) complexes exhibit three bands at 9369–10,000;
16,300–17,000; 24,096–26,400 cm−1, which may be due to the
3
3
3
3
transitions A2g(F) → T2g(F) (v1); A2g(F) → T1g(F) (v2)
3
3
and A2g(F) → T1g(P) (v3), respectively, which suggests an
octahedral geometry [26] (Fig. 6).
The values of the ligand field splitting energy (Dq), Racah
inter-electronic repulsion parameter (B), nephelauxetic ratio (β)
and ratio v2/v1 for the cobalt(II) and nickel(II) are presented
in Table 3. The assignment of an octahedral geometry to the
complexes is further supported by their v2/v1, which lies in the
range 1.65–1.79.
Fig. 7. E. coli action on Cu(II) of HEBMI.

Y. Prashanthi et al. / Spectrochimica Acta Part A 70 (2008) 30–35
35
Fig. 8. R. oryzae action on Cu(II) of HEBMI and HNBMI.
Table 4
Stability constants and Biological activity of ligands and complexes
Complex
pKa
log k1
log k2
E. coli
P. aeurogenosa
R. oryzae
A. niger
HEBMI
Cu(II)
Ni(II)
8.58
–
–
+
+
+
+
–
++
++
–
–
–
6.45
4.75
4.27
4.78
3.48
3.09
+++
++
++
+++
++
+
++
++
++
Co(II)
HNBMI
Cu(II)
Ni(II)
6.05
–
–
+
+
–
+
+
++
+
–
–
–
6.31
5.20
4.70
4.75
4.50
4.23
+++
++
++
++
++
+++
+++
++
++
Co(II)
Note: Maximum zone of inhibition was represented as +++ (20–35 mm). Medium zone of inhibition as ++ (10–20 mm). Minimum zone of inhibition as + (5–10 mm).
ion is made so as to overlap with the ligand orbital. Increased
activity enhances the lipophilicity of complexes due to delocal-
isation of pi-electrons in the chelate ring [29]. In some cases
increased lipophilicity leads to breakdown of the permeability
barrier of the cell [30,31]. The results of anti-fungal and anti-
bacterial screening, indicate that Cu(II) complexes show more
activity than the other complexes. These results may be due to
higher stability constant of the Cu(II) complexes than the other
complexes (Figs. 7 and 8) (Table 4).
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