Synthesis, electrochemical and antimicrobial studies of mono and binuclear iron(III) and oxovanadium(IV) complexes of [ONO] donor tridentate Schiff-base ligands

Article abstract of DOI:10.1080/00958972.2010.525703

Tridentate Schiff bases (H₂L¹ or H₂L ²) were derived from condensation of acetylacetone and 2-aminophenol or 2-aminobenzoic acid. Binuclear square pyramidal complexes of the type [M ₂(L¹)

Full text of DOI:10.1080/00958972.2010.525703

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Synthesis, electrochemical and
antimicrobial studies of mono and
binuclear iron(III) and oxovanadium(IV)
complexes of [ONO] donor tridentate
Schiff-base ligands
Chira R. Bhattacharjee a , Pankaj Goswami b & Mahuya Sengupta c
a Department of Chemistry, Assam University, Silchar 788011,
Assam, India
b Department of Chemistry, Silchar Polytechnic, Silchar 788015,
Assam, India
c Department of Biotechnology, Assam University, Silchar 788011,
Assam, India
Published online: 21 Oct 2010.
To cite this article: Chira R. Bhattacharjee , Pankaj Goswami & Mahuya Sengupta (2010) Synthesis,
electrochemical and antimicrobial studies of mono and binuclear iron(III) and oxovanadium(IV)
complexes of [ONO] donor tridentate Schiff-base ligands, Journal of Coordination Chemistry, 63:22,
3969-3980, DOI: 10.1080/00958972.2010.525703
To link to this article: http://dx.doi.org/10.1080/00958972.2010.525703
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Journal of Coordination Chemistry
Vol. 63, No. 22, 20 November 2010, 3969–3980
Synthesis, electrochemical and antimicrobial studies of mono
and binuclear iron(III) and oxovanadium(IV) complexes of
[ONO] donor tridentate Schiff-base ligands
CHIRA R. BHATTACHARJEE*y, PANKAJ GOSWAMIz and
MAHUYA SENGUPTAx
yDepartment of Chemistry, Assam University, Silchar 788011, Assam, India
zDepartment of Chemistry, Silchar Polytechnic, Silchar 788015, Assam, India
xDepartment of Biotechnology, Assam University, Silchar 788011, Assam, India
(Received 20 April 2010; in final form 26 August 2010)
Tridentate Schiff bases (H2L1 or H2L2) were derived from condensation of acetylacetone and
2-aminophenol or 2-aminobenzoic acid. Binuclear square pyramidal complexes of the type
[M2(L1)2] ꢀ nH2O (M ¼ Fe–Cl, n ¼ 0; M ¼ VO, n ¼ 1) were accessed from interaction of H2L1
with anhydrous FeCl3 and VOSO4 ꢀ 5H2O, respectively. A similar reaction with H2L2, however,
produced mononuclear complexes [ML2(H2O)x] ꢀ nH2O (M¼Fe–Cl, x ¼ 0, n ¼ 0; M¼VO, x ¼ 1,
n ¼ 1). The compounds were characterized using elemental analysis, FT-IR, UV-Vis, and NMR
(for ligand only), and mass spectroscopies and solution electrical conductivity studies. Magnetic
susceptibility measurements suggest antiferromagnetic exchange in binuclear Fe(III) and
VO(IV) complexes. Thermo gravimetric analysis (TGA) provided unambiguous evidence for
the presence of coordinated as well as lattice water in [VOL2(H2O)] ꢀ H2O. Cyclic voltammetric
studies showed well-defined redox processes corresponding to Fe(III)/Fe(II) and
VO(V)/VO(IV). In vitro antimicrobial activities of the compounds were investigated against
Klebsiella pneumoniae, Staphylococcus aureus, Pseudomonas aeroginosa, Escherichia coli,
Bacillus subtilis, and Proteus vulgaris. H2L1 and its binuclear complexes exhibited pronounced
activity against all the microorganisms tested.
Keywords: Schiff bases; Acetylacetone; Oxovanadium(IV) complexes; Iron(III) complexes;
Antimicrobial activity
1. Introduction
Metal-Schiff base complexes have synthetic proclivity, structural diversity, and
potential application in pharmacology and catalysis [1]. Design, synthesis, and
characterization of Schiff bases and complexation with transition metals are focus of
current research in coordination chemistry. Dinuclear complexes which serve as models
mimicking metallobiosites are of particular interest [2, 3]. Potential applications
like separation materials, catalysis precursors, models of the catalyze enzymes [4–7], and
interesting structures [8–10] have spurred research in this field. Amino acid based Schiff
bases and their first row transition metal complexes were reported to exhibit fungicidal,
*Corresponding author. Email: crbhattacharjee@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.525703

3970
C.R. Bhattacharjee et al.
bactericidal, antiviral, and antitubercular activity [11–13]. Use of iron-Schiff base
complexes in different catalytic reactions has been reported in recent work [14–16].
Schiff bases derived from 2,4-pentanedione/1-phenyl-1,3-butanedione and ethylenedia-
mine have been reported [17]. The development of ꢀ-diketone-based Schiff base and
their transition metal complexes as antimicrobial agents is very attractive. Complexes
of vanadium(IV/V) with tridentate [ONO] donor Schiff-base ligands have been
reviewed [18]. Structural and functional models for vanadium-dependent enzymes
have also stimulated the coordination chemistry of the metal [19, 20]; ꢀ-diketone-based
dibasic tridentate [ONO] donor Schiff-base ligands form vanadium(IV/V) complexes
under anaerobic conditions [21–23]. Recently, we reported the synthesis and reactivity
of [Fe(acacen)(H2O)2]NO3 incorporating a quadridentate Schiff base [24].
Herein, we report the synthesis and structural characterization of Fe(III) and VO(IV)
complexes with tridentate [ONO] donor Schiff bases derived from acetylacetone and
2-aminophenol or 2-aminobenzoic acid. Magnetic, thermal, and electrochemical
behavior of the complexes are also studied. In-vitro antimicrobial activities of these
compounds against Klebsiella pneumoniae, Staphylococcus aureus, Pseudomonas
aeroginosa, Escherichia coli, Bacillus subtilis, and Proteus vulgaris are also incorporated.
2. Experimental
2.1. Materials
Reagent grade and HPLC grade solvents and chemicals were used. Acetylacetone,
ethanol, methanol, and other solvents were distilled prior to use. In this study,
2-aminophenol and 2-aminobenzoic acid were obtained from Qualigens Fine Chemicals
and recrystallized before use. Anhydrous ferric chloride and vanadyl sulfate
pentahydrate were obtained from E. Merck India Ltd. and used as received.
2.2. Measurements
Microanalytical (C, H, and N) data were obtained with a Perkin Elmer Model 240C
elemental analyzer. Infrared spectra were obtained using KBr pellets on a Spectrum BX
series FT-IR spectrophotometer from 400–4000 cmꢁ1. Electronic spectra were recorded
in DCM on a Shimadzu 1600-PC UV-Vis spectrophotometer from 200 to 800 nm.
Nuclear magnetic resonance spectra (1H and 13C) were acquired from a Bruker
Advance 300 MHz FT-NMR Spectrometer using CDCl3 as solvent and TMS as
internal standard. Mass spectra were recorded on a Jeol SX-102 spectrometer with fast
atom bombardment. Molar conductances of the complexes were determined in DMSO
(ca 10ꢁ3 mol Lꢁ1) at room temperature using a Toa CM 405 conductivity meter.
Magnetic susceptibilities of the complexes were measured at room temperature on a
Lakeshore 7407 Vibrating Sample Magnetometer from ꢁ20,000 to 20,000 gauss.
Thermal investigation of the sample was recorded on a PYRIS DIAMOND thermal
analyzer under dynamic flow of nitrogen (100 mL minꢁ1) and heating rate of
10ꢂC minꢁ1 from ambient temperature to 1000ꢂC. The number of decomposition
steps was identified with DTG. Electrochemical behaviors of the complexes were
investigated by cyclic voltammetry in a CHI 660C Electrochemical Workstation in

Tridentate Schiff base
3971
dicholoromethane versus SCE at room temperature in the potential range ꢁ1.0 to 1.0 V.
Antimicrobial activities of the compounds were evaluated by Kirby–Bauer disc
diffusion method using ethanol as solvent as well as control and tetracycline as
standard drug.
3. Synthesis
3.1. Schiff base (H2L1)
The ligand was prepared from condensation of acetylacetone and 2-aminophenol in 1 : 1
molar ratio. Also, 2-aminophenol (10 mmol, 1.09 g) was dissolved in dry ethanol and
then added to an ethanolic solution of acetylacetone (10 mmol, 1.00 g) containing a few
drops of acetic acid and the mixture was refluxed for 4 h. The solvent was then removed
on a rotary evaporator and the residue crystallized at room temperature. The yellow
crystals so obtained were recrystallized from methanol. Yield 69% and melting point
188ꢂC. Elemental Anal. (%) Calcd C (69.11), H (6.81), and N (7.32). Found C (68.92),
H (6.84), and N (7.55). FT-IR (KBr pellets, ꢁ cmꢁ1) 1166 (C–O), 1351 (C–O), 1614
(C¼N), and 3451 (O–H). UV-Vis (CH2Cl2, ꢂmax nm, " (mol L
)
ꢁ1 ꢁ1 cmꢁ1) 235 (11,460),
and 312 (32,320). 1H-NMR (300 MHz, ꢃ ppm, from TMS in CDCl3) 11.72 (s, 1H, OH),
9.01 (s, 1H, OH), 6.82–7.19 (m, 4H, ring proton), 5.17 (s, 1H, –CH¼C5), 2.06 (s, 3H,
CH3), and 1.84 (s, 3H, CH3). 13C-NMR (300 MHz, ꢃ ppm, from TMS in CDCl3) 19.61,
28.61, 97.35, 117.26, 120.13, 125.37, 128.00, 128.74, 152.44, 163.46, and 196.23. MS
(m/z) 191.9[M]þ, 173.9, and 133.8.
3.2. Schiff base (H2L2)
The ligand was prepared by a procedure similar to that of H2L1 using 2-aminobenzoic
acid (10 mmol, 1.37 g) instead of 2-aminophenol. Yield 66% and melting point 148ꢂC.
Elemental Anal. (%) Calcd C (65.75), H (5.93), and N (6.39). Found C (65.94),
H (5.74), and N (6.55). FT-IR (KBr pellets, ꢁ cmꢁ1) 1166 (C–O), 1215 (C–O), 1599
(C¼N), and 3414 (O–H). UV-Vis (CH2Cl2, ꢂmax nm, " (mol L
)
ꢁ1 ꢁ1 cmꢁ1) 247 (20,060),
and 341 (15,300). 1H-NMR (300 MHz, ꢃ ppm, from TMS in CDCl3) 12.97 (s, 1H, OH),
6.63–8.04 (m, 4H, ring proton), 5.26 (s, 1H, –CH¼C5), 1.67 (s, 3H, CH3), and 1.25
(s, 3H, CH3). 13C-NMR (300 MHz, ꢃ ppm, from TMS in CDCl3) 20.54, 24.86, 99.95,
116.38, 124.60, 125.27, 132.09, 135.80, 151.04, 162.82, 168.50, and 196.65. MS (m/z)
219.9[M]þ, 201.9, and 161.8.
3.3. Synthesis of iron(III) complex, [Fe2(L1)2Cl2]
H2L1 (1 mmol, 0.191 g) dissolved in ethanol (20 cm3) was added to an ethanol solution
(20 cm3) of anhydrous FeCl3 (1 mmol, 0.162 g) and the mixture was refluxed for 3 h.
On standing overnight, pale brown complex precipitated was filtered off, washed
with cold ethanol, recrystallized from methanol, and dried in air. Yield 58% and
melting point 4300ꢂC. Elemental Anal. (%) Calcd C (47.05), H (3.92), and N (4.99).
Found C (47.31), H (3.88), and N (5.16). FT-IR (KBr pellets, ꢁ cmꢁ1) 443 (Fe–O),

3972
C.R. Bhattacharjee et al.
546 (Fe–N), 1218 (C–O), 1315 (C–O), and 1604 (C¼N). UV-Vis (CH2Cl2, ꢂmax nm,
" (mol L
ꢁ1 ꢁ1 cmꢁ1) 250 (28,200), 315 (17,530), 364 (20,340), 441 (16,550), 462 (16,210),
and 717 (440). MS (FAB, m/z) 561[M]þ, 489, and 281.
)
1
3.4. Synthesis of oxovanadium(IV) complex, [(VO)2(L )2] H2O
.
The complex was prepared by a procedure similar to that of [Fe2(L1)2Cl2] with
VOSO4 ꢀ 5H2O (1 mmol, 0.253 g) instead of anhydrous FeCl3. The green complex was
obtained in 63% yield with melting point 4300ꢂC. Elemental Anal. (%) Calcd
C (49.81), H (4.52), and N (5.28). Found C (49.63), H (4.78), and N (5.06). FT-IR (KBr
pellets, ꢁ cmꢁ1) 460 (V–O), 530 (V–N), 966 (V¼O), 1212 (C–O), 1319 (C–O), 1596
(C¼N), and 3403 (O–H). UV-Vis (CH2Cl2, ꢂmax nm, " (mol L
and 410 (7250). MS (FAB, m/z) 529[M]þ, 512, and 254.
)
ꢁ1 ꢁ1 cmꢁ1) 231 (18,500),
3.5. Synthesis of iron(III) complex, [Fe(L2)Cl]
The complex was prepared by a procedure similar to that of [Fe2(L1)2Cl2] using H2L2
(1 mmol, 0.219 g) instead of H2L1. Yield 61% of brown product with melting point
122ꢂC. Elemental Anal. (%) Calcd C (46.67), H (3.56), and N (4.53). Found C (46.93),
H (3.68), and N (4.36). FT-IR (KBr pellets, ꢁ cmꢁ1) 483 (Fe–O), 528 (Fe–N), 1255
(C–O), 1301 (C–O), and 1578 (C¼N). UV-Vis (CH2Cl2, ꢂmax nm, " (mol L
) )
ꢁ1 ꢁ1 cmꢁ1
230 (20,650), 250 (28,200), 247 (18,250), 334 (10,000), and 493 (250). MS (FAB, m/z)
309[M]þ.
2
3.6. Synthesis of [VO(L )(H2O)] H2O
.
The complex was prepared by a procedure similar to that of [(VO)2(L1)2] ꢀ H2O using
H2L2 (1 mmol, 0.219 g) instead of H2L1. Yield 56% of pale green product decomposing
at 140ꢂC. Elemental Anal. (%) Calcd C (45.00), H (4.68), and N (4.37). Found C
(45.36), H (4.48), and N (4.53). FT-IR (KBr pellets, ꢁ cmꢁ1) 488 (V–O), 539 (V–N), 656
(ꢄwaggH2O), 906 (ꢄrockH2O), 976 (V¼O), 1242 (C–O), 1303 (C–O), 1586 (C¼N), and
3364 (O–H). UV-Vis (CH2Cl2, ꢂmax nm, " (mol L )
ꢁ1 ꢁ1 cmꢁ1) 254 (29,400), 276 (28,400),
299 (35,400), and 493 (200). MS (FAB, m/z) 320[M]þ and 284.
4. Results and discussion
4.1. Chemistry
A condensation reaction occurred between acetylacetone and 2-aminophenol or
2-aminobenzoic acid in 1 : 1 molar ratio yielding yellow H2L1 or H2L2 in weakly
acidic medium (scheme 1). Acetylacetone mostly exists in enol form at room
temperature and under mild acidic conditions offers only one carbonyl for condensa-
tion with amines. This reactivity trend is typical of ꢀ-diketone chemistry. Formation
1
of Schiff bases was confirmed by FT-IR, UV-Vis, H–NMR, and 13C-NMR, and mass
spectroscopies. Iron(III) and oxovanadium(IV) complexes were prepared in good yield

Tridentate Schiff base
3973
H3C
H3C
H3C
1:1, H+
OH
O
O
N
+
+ H2O
Reflux, 4h
OH
H2N
OH
H3C
Schiff base (H2L1)
2-Aminophenol
Acetylacetone
H3C
H3C
COOH
O
N
1:1, H+
+
+ H2O
Reflux, 4h
O
OH
H2N
COOH
H3C
H3C
Schiff base (H2L2)
Acetylacetone
2-Aminobenzoic acid
Scheme 1. Synthesis of Schiff bases.
by reaction of the Schiff-base ligands with anhydrous ferric chloride or vanadyl sulfate
pentahydrate in 1 : 1 molar ratio (scheme 2).
The compounds are soluble in polar solvents (ethanol, methanol, chloroform,
dichloromethane, and dimethylsulfoxide) but insoluble in non-polar solvents. Based on
physical studies, a distorted square pyramidal binuclear structure involving two
tridentate [ONO] donors with the phenoxide group of each ligand bridging the metal
centers has been proposed for complexes with H2L1. Five-coordinate square pyramidal
dimeric copper(II) and vanadyl(IV) complexes of H2L1 have been reported [25]. [NO2]
donor Schiff base derived from 2-hydroxy-5-methylacetophenone and glycine and its
complexes with a number of divalent metal ions including Fe(II) [26] and mononuclear
ion-pair complexes of Co(II) and Fe(II) with [NO2] donor salicylidenglycine Schiff base
have recently been reported [27]. However, a distorted mononuclear structure with Clꢁ
occupying the fourth site in Fe(III) complex and a square pyramidal structure with
aquo occupying the fifth site for VO(IV) have been proposed for complexes with H2L2.
Formation of both dinuclear and mononuclear complexes with dibasic tridentate
[ONO] Schiff-base ligands can be attributed to the difference in the stereochemical
disposition of OH in the aromatic ring. This aspect can be exploited further to access
newer mono or dinuclear complexes.
4.2. IR spectra
The free ligands show stretching modes attributed to C¼N, C–O, and O–H in the range
1599–1614, 1166–1351, and 3414–3451 cmꢁ1, respectively. On complexation, the C¼N
band shifted to lower wavenumber, indicating coordination through the azomethine
nitrogen to the metal [28]. Bands due to ꢁ(O–H) were absent in the complexes,
indicating coordination through deprotonated ligand. Coordination of oxygen and
azomethine nitrogen in the complexes is further supported by the appearance of

3974
C.R. Bhattacharjee et al.
CH3
CH3
H3C
Cl
H3C
H3C
O
O
N
N
O
OH
N
Fe
Fe
Reflux, 3h
FeCl3
+
O
EtOH
Cl
OH
H3C
H2L1
[Fe2(L1)2Cl2]
CH3
H3C
O
H3C
O
O
O
N
N
OH
Reflux, 3h
N
V
V
+ VOSO4 5H2O
H2O
MeOH
O
OH
O
CH3
H3C
H3C
H2L1
[(VO)2(L1)2] H2O
H3C
H3C
COOH
C
N
O
N
Reflux, 3h
+
FeCl3
Fe
O
EtOH
OH
O
Cl
H3C
H3C
H2L2
[Fe(L2)Cl]
H3C
H3C
H3C
COOH
N
O
C
O
Reflux, 3h
MeOH
N
H2O
+
VOSO4 5H2O
V
O
OH
O
OH2
H3C
H2L2
[VO(L2)(H2O)] H2O
Scheme 2. Synthesis of complexes.
ꢁ(M–O) and ꢁ(M–N) in the range 443–488 and 528–546 cmꢁ1, respectively [29].
The oxovanadium(IV) complexes showed a strong band at ca 970 cmꢁ1 attributed to
stretching of the vanadyl (V¼O) with linear chain V¼O ꢀ ꢀ ꢀ V¼O interaction being
absent [30]. Spectra of both mono and dinuclear oxovanadium(IV) complexes show
OH mode from lattice water. Additional bands at 906 and at 656 cmꢁ1 in the
mononuclear oxovanadium(IV) complex attributable to rocking and wagging modes
of coordinated water attest to the presence of water [31].

Tridentate Schiff base
3975
4.3. Electronic spectra
Electronic spectra of the ligands and complexes in dichloromethane exhibited high
intensity bands at 235–341 nm attributed to intraligand ꢅ ! ꢅ* and n ! ꢅ* transitions
[32]. Lower energy bands (364–462 nm) in complexes are due to ligand-to-metal
charge transfer (LMCT) transition [33, 34]. Low intensity bands at 717 nm
(" ¼ 440 (mol L
)
ꢁ1 ꢁ1 cmꢁ1) in binuclear Fe(III) complex and at 493 nm (" ¼ 250
(mol L
)
ꢁ1 ꢁ1 cmꢁ1) in mononuclear Fe(III) complex arises from partly allowed
6A1 ! excited state d–d transitions, confirming high spin iron(III) [35]. A very weak
band at ca 490 nm (" ¼ 200 (mol L
)
ꢁ1 ꢁ1 cmꢁ1) in the mononuclear vanadyl complex is
2
2
presumably due to B2 ! E transition [35].
4.4. NMR spectra
1H-NMR spectra of H2L1 and H2L2 revealed a singlet at 11.72 and 12.97 ppm,
respectively, for the proton of the hydroxyl. Multiplets in the range 6.63–8.04 ppm are
attributable to protons of benzene. Sharp singlets at 5.17, 2.06, and 1.84 ppm in H2L1
and 5.26, 1.67, and 1.25 ppm in H2L2 are due to olefinic proton (–CH¼C5) and
magnetically non-equivalent methyl protons (–CH3), respectively.
The 13C-NMR spectra of the ligands showed resonances concordant with magnet-
ically non-equivalent carbons. The azomethine carbon is at 163 ppm in both ligands.
Resonances at 19.61–28.61 ppm are due to carbons of the magnetically non-equivalent
methyls. Carbons of benzene showed resonances from 116 to 152 ppm; –CH¼ is at
97.35 and 99.95 ppm in H2L1 and H2L2, respectively. Resonances around 196 ppm in
both ligands are due to the carbon of ¼C–OH group, while carbon of –COOH group
of H2L2 was at 168.50 ppm.
4.5. Mass spectra
Mass spectra of the compounds were recorded in FABþ ionization mode. The
molecular ion peak of H2L1 and H2L2 appeared at m/z 191 and 219, respectively, along
with other assignable fragments. The molecular formula Fe2C22H22N2O4Cl2,
V2C22H24N2O7, FeC12H11NO3Cl, and VC12H15NO6 suggested for the complexes were
confirmed by observed peak at m/z 561 (ca 561), 529 (ca 530), 309 (ca 308.5), and
320 (ca 320), respectively. The peak at m/z 284 observed for the complex
[VO(L2)(H2O)] ꢀ H2O is attributed to [M ꢁ 2H2O]þ species arising from the loss of
coordinated as well as lattice water. In addition to [M]þ peak, the binuclear complex of
Fe(III) showed appearance of [M þ 2]þ as well as [M þ 4]þ peak. In mononuclear
Fe(III) complex, [M þ 2]þ was observed along with molecular ion peak. The isotopic
peak was observed further in all the fragments containing chlorine. The probable
fragmentation patterns of the ligands and binuclear Fe(III) and VO(IV) complexes are
provided in ‘‘Supplementary material’’ (schemes 1S and 2S).
4.6. Molar conductance
Molar conductances of [Fe2(L1)2Cl2], [(VO)2(L1)2] ꢀ H2O, [Fe(L2)Cl], and
[VO(L2)(H2O)] ꢀ H2O in DMSO (ca 10ꢁ3mol Lꢁ1) were found to be 13.1, 9.2, 11.6,
and 9.8 ꢀꢁ1 cm2 molꢁ1, respectively, suggesting non-electrolytes [36].

3976
C.R. Bhattacharjee et al.
Figure 1. Field dependence of magnetization of (a) [Fe2(L1)2Cl2] and (b) [VO(L2)(H2O)] ꢀ H2O.
4.7. Magnetic susceptibility
Room temperature magnetic susceptibilities of [Fe2(L1)2Cl2] and [(VO)2(L1)2] ꢀ H2O
were conducted using a Vibrating Sample Magnetometer. The magnetizations versus
magnetic field (figure 1) were examined from ꢁ20,000 to 20,000 gauss. The parameters
extracted from the hysteresis loop are the saturation magnetization (Ms), the remanence
(Mr), the coercivity (Hc), the squareness ratio (SQR), and the initial slope (table 1).
The effective magnetic moments of the complexes are then calculated from the initial
slope of the hysteresis using the relation
À
Á
ꢆeff ¼ 2:827 ꢃ ꢄ ꢇ0M ꢃ T , ꢇM0 ¼ ꢇM ꢁ ꢇdia
where ꢇdia is diamagnetic correction. The diamagnetic corrections are calculated by
summing the contributions from the atoms, ions, and bonds of the molecule. These
group contributions are known as Pascal’s constants.
The magnetic moment values of [Fe2(L1)2Cl2] and [(VO)2(L1)2] ꢀ H2O at room
temperature were 5.36 and 1.13 B.M., respectively, supporting a high spin, strongly

Tridentate Schiff base
Table 1. Magnetic behavior of the complexes.
3977
Compounds
Ms (ꢃ10ꢁ3
)
Mr (ꢃ10ꢁ3
)
Hc
SQR
Initial slope (ꢃ10ꢁ6
)
meff (B.M.)
[Fe2(L1)2Cl2]
434.851
14.186
11.552
1.828
238.51
245.32
0.02656
0.12891
19.167
0.607
5.36
1.13
[(VO)2(L1)2] ꢀ H2O
Table 2. TGA and DTG data of [VO(L2)(H2O)] ꢀ H2O.
Mass loss (%)
Thermo
gravimetrical
plateau (ꢂC)
DTG
(ꢂC)
Experiment
Calculated
Process
40–128
128–321
48
172
4.29
31.43
5.59
–
Loss of lattice water
Loss of coordinated water and partial
decomposition of the ligand
321–435
435–928
365
458
11.71
44.01
–
–
Partial decomposition of the ligand
Final decomposition of the ligand
Table 3. DTA data of [VO(L2)(H2O)] ꢀ H2O.
Temperature range (ꢂC)
DTA peaks (ꢂC)
DH (J gꢁ1
)
Process
110–190
414–608
139 endo
464 exo
158.12
1950.20
Dehydration
Decomposition
antiferromagnetically coupled configuration for the Fe(III) and VO(IV) complexes.
Such subnormal magnetic moments due to antiferromagnetic interaction in binuclear
complexes are well documented [37]. Typical magnetic moment values of 5.83 and
1.79 B.M. are in conformity with high spin mononuclear Fe(III) and VO(IV) complexes.
Moderate saturation magnetization and coercivity values of the complexes may qualify
the compounds as intermediate magnetic materials.
4.8. Thermogravimetric analysis
Thermal investigation of [VO(L2)(H2O)] ꢀ H2O was carried out from ambient temper-
ature to 1000ꢂC at a heating rate of 10ꢂC minꢁ1. The decomposition stages have been
rationalized (table 2) and corresponding curves are provided in ‘‘Supplementary
material’’ (figures 1S and 2S). The DTA data of the complex are presented in table 3.
The complex underwent decomposition in four stages. The first (40–128ꢂC) with DTG
peak at 48ꢂC corresponds to release of lattice water [38]. The second stage (128–321ꢂC)
with DTG peak at 172ꢂC corresponds to loss of coordinated water [39] and partial
decomposition of the ligand. The dehydration step is associated with endothermic
change in 110–190ꢂC with DTA peak at 139ꢂC. The exothermic change in 414–608ꢂC
with DTA peak at 464ꢂC is associated with decomposition of the ligand.
4.9. Electrochemical behavior
Electrochemical behaviors of the complexes were studied by cyclic voltammetry at
room temperature in 10ꢁ3 mol Lꢁ1 dichloromethane solution containing 0.1 mol Lꢁ1

3978
C.R. Bhattacharjee et al.
Table 4. Electrochemical data of the complexes.
a
Ep (V)
c
Ep (V)
Compound
DE (V)
E1/2 (V)
[Fe2(L1)2Cl2]
ꢁ0.183
ꢁ0.468
ꢁ0.311
ꢁ0.357
ꢁ0.738
ꢁ0.525
ꢁ0.688
ꢁ0.412
0.555
0.057
0.377
0.055
ꢁ0.460
ꢁ0.496
ꢁ0.499
ꢁ0.384
[(VO)2(L1)2] ꢀ H2O
[Fe(L2)Cl]
[VO(L2)(H2O)] ꢀ H2O
Table 5. Antimicrobial activities of the ligands and their complexes.
Microorganism (inhibition zonea (mm))
Compound
K. pneumoniae
S. aureus
P. aeroginosa
E. coli
B. subtilis P. vulgaris
Control
Tetracyclin
N.O.
N.O.
N.O.
19.0 ꢆ 1.0
7.7 ꢆ 1.2
N.O.
N.O.
N.O.
N.O.
24.7 ꢆ 0.6
8.3 ꢆ 0.6
N.O.
14.3 ꢆ 1.2
6.3 ꢆ 0.6
11.7 ꢆ 0.6
N.O.
16.3 ꢆ 0.6
7.7 ꢆ 0.6
6.7 ꢆ 0.6
10.0 ꢆ 1.0
N.O.
20.7 ꢆ 0.6
10.0 ꢆ 1.0
N.O.
13.7 ꢆ 0.6
6.3 ꢆ 0.6
12.0 ꢆ 1.0
N.O.
22.7 ꢆ 1.2 18.0 ꢆ 0.0
14.0 ꢆ 1.0 12.7 ꢆ 1.2
10.7 ꢆ 1.2 12.0 ꢆ 0.0
13.0 ꢆ 1.0 10.3 ꢆ 0.6
H2L1
H2L2
[(VO)2(L1)2] ꢀ H2O
[VO(L2)(H2O)] ꢀ H2O
[Fe2(L1)2Cl2]
[Fe(L2)Cl]
11.0 ꢆ 0.0
N.O.
10.3 ꢆ 0.6
N.O.
N.O.
N.O.
8.3 ꢆ 0.6
12.3 ꢆ 0.6 11.0 ꢆ 1.0
5.3 ꢆ 0.6
7.7 ꢆ 1.2 N.O.
N.O., not observed.
aIncluding disc diameter 4 mm.
tetrabutyl ammonium perchlorate as supporting electrolyte from ꢁ1.2 to 1.2 V versus
SCE at a scan rate of 100 mV sꢁ1. The electrochemical data are presented in table 4
and corresponding voltammograms are provided in ‘‘Supplementary material’’
(figure 3Sa–Sd). The oxovanadium(IV) complexes exhibit reversible one electron
oxidation behavior (DE ꢅ 59 mV) assignable to VO(V)/VO(IV) couple [40]. The
iron(III) complexes display a quasireversible one electron wave with DE 4 100 mV,
assignable to Fe(III)/Fe(II) redox couple. The half-wave potentials (E ) for the
½
complexes are negative and small, indicating ready redox susceptibility of the
compounds. These features render the complexes as valuable catalysts for redox
reactions [41]. The free ligands did not show any responses in the potential range
of ꢁ1.2 to 1.2 V under similar experimental conditions, implying their redox innocent
character.
4.10. Antimicrobial activity
The ligands and their complexes were assayed for in vitro antimicrobial activities against
K. pneumoniae, S. aureus, P. aeroginosa, E. coli, B. subtilis, and P. vulgaris at
50 mg mLꢁ1 using ethanol as solvent as well as control and tetracyclin as standard drug.
The antibacterial activity was evaluated by measuring the zone of inhibition in mm.
Ethanol had no inhibitory effect on the bacteria in the concentration studied. The
screening data (table 5) revealed that the ligand H2L1 and its complexes showed
moderate inhibitory activity against all the tested microorganisms. The complexes
showed high antibacterial activity against K. pneumoniae, S. aureus, P. aeroginosa,

Tridentate Schiff base
3979
Table 6. Minimum inhibitory concentrations (MICs) of the ligands and their complexes.
MIC (mg mLꢁ1
)
Compound
K. pneumoniae
S. aureus
P. aeroginosa
E. coli
B. subtilis
P. vulgaris
H2L1
36
42
28
–
36
44
39
–
34
48
38
–
43
–
32
–
32
–
41
43
30
–
38
46
36
39
32
–
30
–
40
–
28
42
34
–
H2L2
[(VO)2(L1)2] ꢀ H2O
[VO(L2)(H2O)] ꢀ H2O
[Fe2(L1)2Cl2]
[Fe(L2)Cl]
and P. vulgaris. Such an enhancement in the activity of metal complexes against certain
specific microorganisms may be explained on the basis of Overtone’s concept [42] and
Tweedy’s chelation theory [43]. The Schiff base H2L2 and its complexes were active
against some bacterial strains. The minimum inhibitory concentrations (MIC) were
determined by broth microdilution method [44]. The observed MIC values in mg mLꢁ1
are reported in table 6, showing minimum inhibitory concentrations between
28–48 mg mLꢁ1
. The highest zone of inhibition (14.3 mm) was exhibited by
[(VO)2(L1)2] ꢀ H2O against P. vulgaris. The least MIC (28 mg mLꢁ1) was also exhibited
by the same compound against K. pneumoniae and P. vulgaris.
5. Conclusion
Complexation of [ONO] donor tridentate Schiff bases with Fe(III) and VO(IV)
has been acomplished. Both mono and binuclear complexes have been prepared.
Besides, the compounds were characterized by spectral, magnetic, thermal, and
electrochemical properties. The magnetic susceptibilities suggest antiferromagnetic
interactions between the metal ions in the binuclear complexes. The VO(IV) complexes
showed reversible redox behavior while the Fe(III) complexes showed quasireversible
redox behavior. In vitro antimicrobial activity studies against K. pneumoniae, S. aureus,
P. aeroginosa, E. coli, B. subtilis, and P. vulgaris revealed that H2L1 derived from
2-aminophenol exhibited maximum inhibitory activity against E. coli while its
oxovanadium complex showed promising inhibitory effect against P. vulgaris. The
synthetic strategy may serve as a paradigm for accessing new mononuclear and
binuclear Schiff-base complexes. The pendent phenolic-OH group on the amine of the
dibasic tridentate ligand, L1, bridges two Fe(III) centers to form [Fe2O2] dinuclear core
with a chloride on each Fe(III) compensating the overall charge. VOSO4 ꢀ 5H2O readily
forms the spontaneously charge-balanced [(VO)2O2] core. For L2, the carboxylato
group on the amine is stereochemically not poised for bridging structures leading
to mononuclear complexes with aquo or chloro as ancillary ligands. The complexes
were all obtained as coordinatively unsaturated species. The possibilities of solvent
molecules coordinating the vacant sites were ruled out from elemental analysis and
FT-IR spectroscopy. The complexes reported herein may be used as catalysts in various
redox reactions.

3980
C.R. Bhattacharjee et al.
Acknowledgments
The authors express sincere thanks to SAIF, NEHU, Shillong; SAIF, CDRI, Lucknow;
SAIF, STIC, Kochi, and CIF, IIT, Guwahati, India, for providing analytical and
spectral results.
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