Synthesis, spectroscopic (IR, electronic, FAB-mass, and PXRD), magnetic, and antimicrobial studies of new iron(III) complexes containing Schiff bases and substituted benzoxazole ligands

Article abstract of DOI:10.1080/00958972.2011.594886

Mixed ligand complexes of iron(III), [Fe(sb)₂(py)Cl] 2H ₂O (1-9) [where sbH = Schiff bases (derived from condensation of 2-aminopyridine (sapH), 2-aminophenol (saphH), o-toluidine (o-smabH), aminobenzene (sabH), p-toluidine (p-smabH), 3-nitroaniline (snabH), and anthranilic acid (saaH) with salicylaldehyde and substituted (mercapto-)benzimidazole (mbzH), {2-(o-hydroxyphenyl)}benzoxazole, (pboxH)], have been synthesized by the interactions of iron(III) chloride with corresponding ligands in 1 : 2 molar ratio in refluxing pyridine. These complexes have been characterized by elemental analyses, melting points, spectral, and magnetic studies. Powder X-ray diffraction studies of some representative complexes are also reported herein. The antibacterial and antifungal activities of the free ligands and their iron(III) complexes were found invitro. The complexes showed good antibacterial and antifungal effect to some bacteria and fungi. Two standard antibiotics (chloromphenicol and terbinafine) were used for comparison with these complexes.

Full text of DOI:10.1080/00958972.2011.594886

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Synthesis, spectroscopic (IR,
electronic, FAB-mass, and PXRD),
magnetic, and antimicrobial
studies of new iron(III) complexes
containing Schiff bases and substituted
benzoxazole ligands
Raj K. Dubey a , Uma K. Dubey a & Sharad K. Mishra a
a Synthetic Inorganic and Metallo-organic Research Laboratory,
Department of Chemistry , University of Allahabad ,
Allahabad-211 002, India
Published online: 05 Jul 2011.
To cite this article: Raj K. Dubey , Uma K. Dubey & Sharad K. Mishra (2011) Synthesis,
spectroscopic (IR, electronic, FAB-mass, and PXRD), magnetic, and antimicrobial studies of
new iron(III) complexes containing Schiff bases and substituted benzoxazole ligands, Journal of
Coordination Chemistry, 64:13, 2292-2301, DOI: 10.1080/00958972.2011.594886
To link to this article: http://dx.doi.org/10.1080/00958972.2011.594886
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Journal of Coordination Chemistry
Vol. 64, No. 13, 10 July 2011, 2292–2301
Synthesis, spectroscopic (IR, electronic, FAB-mass, and
PXRD), magnetic, and antimicrobial studies of new iron(III)
complexes containing Schiff bases and
substituted benzoxazole ligands
RAJ K. DUBEY*, UMA K. DUBEY and SHARAD K. MISHRA
Synthetic Inorganic and Metallo-organic Research Laboratory, Department of Chemistry,
University of Allahabad, Allahabad-211 002, India
(Received 7 March 2011; in final form 9 May 2011)
Mixed ligand complexes of iron(III), [Fe(sb)2(py)Cl] ꢀ 2H2O (1–9) [where sbH ¼ Schiff bases
(derived from condensation of 2-aminopyridine (sapH), 2-aminophenol (saphH), o-toluidine
(o-smabH), aminobenzene (sabH), p-toluidine (p-smabH), 3-nitroaniline (snabH), and
anthranilic acid (saaH) with salicylaldehyde and substituted (mercapto-)benzimidazole
(mbzH), {2-(o-hydroxyphenyl)}benzoxazole, (pboxH)], have been synthesized by the interac-
tions of iron(III) chloride with corresponding ligands in 1 : 2 molar ratio in refluxing pyridine.
These complexes have been characterized by elemental analyses, melting points, spectral, and
magnetic studies. Powder X-ray diffraction studies of some representative complexes are also
reported herein. The antibacterial and antifungal activities of the free ligands and their iron(III)
complexes were found in vitro. The complexes showed good antibacterial and antifungal effect
to some bacteria and fungi. Two standard antibiotics (chloromphenicol and terbinafine) were
used for comparison with these complexes.
Keywords: Schiff bases; Iron(III) complexes; UV-Vis; IR; FAB-MS; Magnetic studies; PXRD;
Biological activity
1. Introduction
Metal complexes of Schiff bases have been studied for the multidenticity of these
ligands and their pharmacological and antitumor activities [1, 2]. Design, synthesis, and
characterization of iron complexes with Schiff bases play a role in the coordination
chemistry of iron due to their importance as synthetic models for iron-containing
enzymes and oxidation catalysts [3]. Our recent investigations on Schiff base–mixed-
ligand complexes of ‘‘3d’’ transition metals [4–13] prompted us to synthesize and
characterize new mixed ligand complexes of iron(III) due to their biological activities
[14, 15] and also in preparation of drugs [16]. Transition metal complexes of Schiff bases
and substituted benzimidazole/benzoxazole have attracted attention due to their
*Corresponding author. Email: rajalkoxy@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
DOI: 10.1080/00958972.2011.594886

Iron(III) Schiff bases
2293
significant role in enhancing fungicidal and agrochemical activities [17]. A number of
mononuclear iron(III) complexes of nitrogen, carboxylate, and phenolate donors have
been synthesized as functional models to explore their dioxygenase activity [18]. Such
mixed-ligand complexes have potential applications in photoluminescence, catalysis,
magnetism, molecular architecture, materials chemistry, and therapeutic effects [19, 20].
In continuation to our interest on synthetic and structural characterization of some
mixed ligand complexes of transition metals, we report herein the synthesis,
characterization (IR, UV-Vis, FAB-mass), powder X-ray diffraction (PXRD), and
magnetic values as well as biological studies of iron(III) complexes.
2. Experimental
All the chemicals and solvents used in all preparative and analytical works were of A.R.
(BDH or Fluka) grade. Solvents were dried by standard procedures [21]. Iron was
determined by atomic absorption spectroscopy, GBC-932 AA and chloride present in
the complexes was estimated by Volhard’s method [22]. Elemental analyses (C, H, and
N) were carried out on a Heraceous Carlo Erba 1108. IR (4000–200 cmꢁ1) and
electronic spectra were recorded on Perkin Elmer grating and Pye-Unicam model 557/
SP 8-100 spectrophotometers. Magnetic susceptibilities were measured on a Guoy-
balance using Hg[Co(NCS)4] as the standard. FAB-mass spectra of iron(III) complexes
were recorded on JEOLSX 102/DA-6000 mass spectrometer/data system using argon/
xenon (6 kV, 10 mA) as the FAB-gas. PXRD of complexes were recorded on a Rigaku
˚
model D/Max-2200 PC using Cu-Ka1 radiation (ꢀ ¼ 1.5406 A).
2.1. Synthesis
2.1.1. Preparation of ligands. The Schiff bases (sapH, o-smabH, p-smabH, sabH,
saphH, snabH, and saaH) were synthesized by refluxing (ꢂ4–5 h) a mixture of an
equimolar amount of salicylaldehyde with corresponding amines (such as 2-aminopyr-
idine, o-toluidine, p-toluidine, aniline, 2-aminophenol, 3-nitroaniline, and anthranilic
acid) in methanol on a water bath. The resulting products were recrystallized using
THF–C6H6 mixture. 2-(o-hydroxyphenyl)-benzoxazole was prepared by standard
literature procedure [23]. The general structures of the ligands are given in figure 1.
.
[Fe(sb)2(py)(Cl)] 2H2O. The
2.1.2. Preparation
of
iron(III)
complex
[Fe(sb)2(py)(Cl)] ꢀ 2H2O has been prepared by the interaction of a mixture of iron(III)
chloride hexahydrate (1.32 g, 4.9 mmol) with sapH (1.94 g, 9.8 mmol) in 1 : 2 molar ratio
in freshly distilled boiling pyridine (ꢂ50 mL), immediately followed by the addition of
zinc dust (ꢂ1 g). The mixture was refluxed for ꢂ1 h and then filtered while hot and the
filtrate was allowed to attain room temperature. The filtrate was treated with a few
milliliters of water (5 mL), stirred thoroughly, and filtered to obtain
[Fe(sap)2(py)Cl] ꢀ 2H2O. Yield: 2.127 g, 72%.

2294
R.K. Dubey et al.
X
Y
OH
OH
H
C
N
z
N
C
N
C
H
N
X
H
O-H
sapH
X = COOH (saaH); OH (saphH);
X = H (sabH); CH3 (o-smabH);
Y = NO2 (snabH); Z = CH3 (p-smabH)
O
N
C
N
SH
N
O
H
H
(Hopbox)
(mbzH)
Figure 1. General structure of the ligands.
Table 1. Synthetic, analytical and magnetic data of iron(III) complexes containing Schiff bases and
substituted benzoxazole.
% Analysis found (Calcd)
S.
No.
ꢁeff
(B.M.)
Reactants (g, mmol)
Product (g, % yield)
C
H
N
Fe
Cl
1
2
3
4
5
6
7
8
9
FeCl3 ꢀ 6H2O
þ
sapH
(1.94, 9.8)
[Fe(sap)2(py)Cl] ꢀ 2H2O (1)
57.25
4.26
11.40
9.01
5.60
5.81
5.76
5.80
5.79
5.70
5.85
5.75
5.76
5.80
(1.32, 4.9)
FeCl3 ꢀ 6H2O
(1.08, 4.0)
(2.127, 72)
(57.96) (4.52) (11.65) (9.9) (5.89)
5.06 6.18 8.68 5.40
(63.21) (5.30) (6.70) (8.90) (5.65)
p-smabH [Fe(smab)2(py)Cl] ꢀ 2H2O (3) 62.95 5.21 6.29 9.75 5.42
(1.732, 8.2) (1.8, 70) (63.21) (5.30) (6.70) (8.90) (5.65)
sabH 4.36 6.80 9.16 5.95
[Fe(sab)2(py)Cl] ꢀ 2H2O] (4) 61.80
(1.58, 7.6) (1.547, 68) (62.16) (4.88) (7.01) (9.32) (5.92)
saphH [Fe(saph)2(py)Cl] ꢀ 2H2O] (5) 59.15 4.27 6.31 8.98 5.45
(1.66, 7.8) (1.74, 71) (59.01) (4.63) (6.65) (8.85) (5.62)
snabH 3.41 9.20 8.26 5.17
(54.04) (3.95) (10.16) (8.10) (5.14)
58.07 4.08 6.25 8.02 5.25
(57.69) (4.25) (6.11) (8.13) (5.16)
3.78 6.87 8.32 5.41
(59.43) (4.02) (6.70) (8.91) (5.66)
[Fe(mbz)2(py)Cl] ꢀ 2H2O (9) 45.56 3.22 13.34 10.06 6.92
(1.98, 82) (45.20) (3.79) (13.87) (10.37) (7.07)
þ
o-smabH [Fe(smab)2(py)Cl] ꢀ 2H2O (2) 62.62
(1.69, 8.0) (1.88, 75)
FeCl3 ꢀ 6H2O
(1.107, 4.1)
FeCl3 ꢀ 6H2O
(1.026, 3.8)
FeCl3 ꢀ 6H2O
(1.05, 3.9)
þ
þ
þ
FeCl3 ꢀ 6H2O
þ
[Fe(snab)2(py)Cl] ꢀ 2H2O (6) 53.56
(0.967, 3.58) (1.73, 7.16)
(1.6, 65)
[Fe(Saa)(Py)Cl] ꢀ 2H2O (7)
(1.71, 78)
FeCl3 ꢀ 6H2O
(0.864, 0.32)
FeCl3 ꢀ 6H2O
(0.648, 2.4)
FeCl3 ꢀ 6H2O
(1.297, 4.8)
þ
saaH
(1.54, 6.4)
þ
pboxH
[Fe(Pbox)(Py)Cl] ꢀ 2H2O (8) 58.85
(1.01, 4.8)
(1.20, 20)
þ
mbzH
(1.44, 9.6)
Similar procedure was adopted for the preparation of other iron(III) complexes.
All the synthetic details are collected in table 1. The synthesis and structures of the
representative complexes are given in scheme 1.
2.2. Antimicrobial evaluation
Most of the synthesized complexes and ligands were screened for their in vitro
antimicrobial activities against three human pathogenic bacterial species

Iron(III) Schiff bases
2295
y
x
H
z
+
N
H2
C
O
OH
y
x
H
C
N
z
OH
.
(i) Hot pyridene
FeCl3 6H2 O
1:2
(ii) ~ 1g. Zinc dust
Δ
-3h
(iii)
y
x
H
C
N
z
O
Fe
Cl
N
O
N
C
Z
X
Y
H
[X=H for (sab); CH for (o-smab); Y=NO for (snab); Z=CH for (p-smab)]
3
2
3
Scheme 1. Synthesis and structure of iron(III) complexes with Schiff bases.
[Staphylococcus aureus (ATCC 9144) (Gþ), Bacillus subtilis (ATCC 6051) (Gþ), and
Escherichia coli (ATCC 9037) (Gþ) and two plant fungal species [Aspergillus niger
(ATCC 9029) and Penicillium chrysogenum (ATCC 10106)] by using the well disc
diffusion method [23]. Chloromphenicol and terbinafine were used as standard drugs
for comparison. The compounds were dissolved in DMF to get 200 mg mLꢁ1 solutions.
Further progressive double dilution was performed to obtain the required concentra-
tions of 100 and 50 mg mLꢁ1. About 0.5 mL (containing 107 microorganisms per
milliliter) of investigated microorganism was added to a sterile nutrient agar (for
bacteria)/dextrose agar (for fungi) medium just before solidification, then poured onto
sterile petri dishes (9 cm in diameter) and left to solidify. Using a sterile cork borer
(6 mm in diameter), three holes (wells) were made in each disc and then 1 mL of tested
compound dissolved in DMF (50, 100, and 200 mg mLꢁ1) was poured into these holes.

2296
R.K. Dubey et al.
Table 2. Physical properties and electronic spectral data of iron(III) complexes.
Transition (cmꢁ1
)
M.p.
6
4
6
4
6
4
Compound
Physical state
(ꢃC) A1g ! T1g(G) A1g ! T2g(G) A1g ! Eg(G)
C.T. Bands
1
2
3
4
5
6
7
8
9
Light brown solid
Brown solid
Brown solid
Blackish brown
Chocolaty solid
Dark brown solid
Brown solid
300b
240a
260a
200a
325b
290a
320b
300b
12,360
12,072
12,796
12,836
12,332
12,560
12,140
12,800
12,230
19,986
20,260
20,520
20,775
20,230
19,896
20,350
20,440
20,760
25,560
26,770
25,240
25,768
25,950
25,470
25,650
25,372
25,452
30,550 34,560
29,290
29,450 34,056
30,560
30,772 34,210
34,280
–
–
–
30,320 34,427
29,980 34,561
Brown solid
Pinkish brown solid 325b
29,210
–
aDecomposed.
bNeither melt nor decomposed to the measured temperature.
Finally the dishes were incubated at 37ꢃC for 24 h for bacteria and at 30ꢃC for 72 h for
fungi, where clear inhibition zones were detected around each hole. Inhibitory activity
was measured (in mm) as the diameter of the inhibition zones. A blank containing only
DMF showed no inhibition on organisms in a preliminary test.
3. Results and discussion
The new complexes have been synthesized by the interactions of FeCl3 ꢀ 6H2O with
salicylidene-2-aminopyridine (sapH), salicylidene-2-methyl-1-aminobenzene (o-smabH),
salicylidene-4-methyl-1-amino-benzene
(p-smabH),
salicylidene-1-aminobenzene
(sabH), salicylidene-2-aminophenol (saphH), salicylidene-3-nitroaniline (snabH), sali-
cylidene-anthranilic acid (saaH), 2-(o-hydroxyphenyl)-benzoxazole (pboxH), and
2-mercaptobenzimidazole (mbzH) in pyridine. The structures of the iron(III) complexes
are given in scheme 1; the syntheses are represented by the following general equation:
hot pyridine
FeCl3 ꢀ 6H2O þ sbH ꢁꢁꢁꢁꢁꢁꢁ! ½FeðsbÞ2ðpyÞClꢄ ꢀ 2H2O:
ꢂ1 g zinc dust
All these complexes are colored solids (table 2), purified by washing with ethanol and
drying over fused CaCl2. The complexes were soluble in hot pyridine, DMSO, and
DMF but insoluble in water, carbon tetrachloride, and benzene; purity of the
complexes was checked by TLC. The complexes are thermally stable and do not melt or
decompose up to 240ꢃC. The synthetic and analytical data of these complexes are
collected in table 1. Elemental analyses are in agreement with the values calculated for
molecular formulae assigned to these complexes; suggested structures of the complexes
are shown in figure 2.
3.1. IR spectral studies
IR absorption bands (table 3) due to ꢂC¼N at 1655–1618 cmꢁ1 in the Schiff bases and
substituted benzimidazole/benzoxazole [6] shifted to lower frequency, 1686–1595 cmꢁ1
.

Iron(III) Schiff bases
2297
(a)
(b)
Figure 2. Proposed structures of [Fe(sb)2(py)Cl] ꢀ 2H2O (a) (1–8) and (b) (9) complexes.
Table 3. Characteristic IR frequencies (cmꢁ1) of 1–9.
ꢂ(C–O)
Phenolic
ꢂC¼N
Pyridine
Complexes
ꢂFe–N
560
558
552
545
539
577
542
558
585
ꢂFe–O
442
463
460
458
445
456
459
453
–
ꢂFe–S
ꢂC¼N
1602
1597
1608
1596
1595
1617
1686
1605
1599
ꢂCOO
ꢁ
ꢂNO2
ꢂOH
1
2
3
4
5
6
7
8
9
–
–
1276
1278
1281
1280
1289
1275
–
–
–
–
–
–
–
1449(S)
1067(M)
1479(S)
1040(M)
1468(S)
1065(M)
1450(S)
1034(M)
1472(S)
1040(M)
1465(S)
1047(M)
1452(S)
1034(M)
1480(S)
1040(M)
1400(S)
1075(M)
–
3400
858
3443
838
3428
820
3422
827
3430
820
3447
827
3424
836
3445
816
–
–
–
–
–
–
–
–
1330(S)
–
1396
–
–
–
–
1279
–
–
–
402
3445
841
Such a shift on complexation suggests coordination via azomethine [24]. The
characteristic ꢂ(C–O) (phenolic) stretching frequencies, at 1275–1265 cmꢁ1 in all the
free ligands, shifted to higher frequency at 1289–1275 cmꢁ1, showing chelation of
ligands with the metal through the phenolic oxygen [9]. The free COOH group of saaH
at ꢂ1675 cmꢁ1 is shifted to ꢂ1577 cmꢁ1 being consistent with unidentate carboxylate
coordination [21].
In free mbzH, IR peak at ꢂ3300 cmꢁ1 (–NH of the imidazole group) remains
unchanged in the complex [17], indicating that –NH of the imidazole ring does not
coordinate to metal. However, ꢂ(C¼N) (imidazole-tertiary nitrogen) occurring at
ꢂ1615 cmꢁ1 in the complexes indicated coordination of ligand to iron via imidazole
tertiary nitrogen [25]. Pyridine ring vibrations were observed at 1460 and 1065 cmꢁ1 in
the iron(III) complexes, supporting coordination of pyridine [26]. A broad band in all
the complexes at ꢂ3400 cmꢁ1 is due to ꢂOH of lattice water. Absence of ꢃH2O and

2298
R.K. Dubey et al.
ꢄH2O, for coordinated water, confirms the presence of lattice water [27]. Beside these
frequencies, all the iron(III) complexes showed characteristic IR bands at 480–450, 580–
530, and 404 cmꢁ1, which can be attributed to ꢂ(Fe–O), ꢂ(Fe–N), and ꢂ(Fe–S) bands,
respectively [28, 29]. The band at 360 cmꢁ1 has been assigned for terminal ꢂ(Fe–Cl) [5].
3.2. Electronic spectral studies
The electronic spectra of iron(III) complexes (table 2) were measured in DMSO. The
complexes exhibit three bands in the regions 12,475 ꢅ 360, 20,325 ꢅ 450, and 25,693 ꢅ
6
4
6
4
6
4
260 cmꢁ1, assigned as A1g ! T1g(G), A1g ! T2g(G), and A1g ! Eg(G) transitions
expected for an octahedral iron(III) complexes [30–32].
3.3. Magnetic studies
The magnetic moments of iron(III) complexes (1–9) containing Schiff bases and
substituted benzimidazole/benzoxazole (table 1) at room temperature were in the
range ꢁeff ¼ 5.75–5.88 B.M., indicating high-spin paramagnetic iron(III) in ‘‘Oh’’
geometry [33–35].
3.4. FAB-mass spectral studies
The FAB-mass spectrum [5, 8] of [Fe(saph)2(py)Cl] ꢀ 2H2O (5) and
[Fe(mbz)2(py)Cl] ꢀ 2H2O (9) showed molecular ion peaks at m/z ¼ 594 and 468 (figures
3 and 4, Supplementary material), respectively, which correspond to the molecular mass
of 5 and 9 without 2H2O molecules, support for their monomeric nature. Besides these
peaks other important fragmentation ion peaks were observed, which are indicative for
the fragmentation of ligand from the complexes by the formation of radical cations.
The fragmentation patterns showing structural information are represented in schemes
2 and 3 (Supplementary material).
3.5. PXRD studies
XRD studies of [Fe(smab)2(py)Cl] ꢀ 2H2O (2), [Fe(saph)2(py)Cl] ꢀ 2H2O (5), and
[Fe(mbz)2(py)Cl] ꢀ 2H2O (9) were made with the powder pattern method using Cu-Ka
˚
radiation with ꢀ ¼ 1.5406 A. The PXRD pattern was recorded in the 2ꢅ range between
15.239 and 81.554 with a step size of 0.0167. The PXRD diffractograms of iron(III)
complexes exhibit almost similar, amorphous nature. The PXRD of these complexes are
given in figures 5, 6, and 7 (Supplementary material).
3.6. Antimicrobial activity
The antimicrobial activities of some of the complexes have been carried out at 50, 100,
and 200 mg mLꢁ1 against three bacteria and two fungi. The inhibitory effects of these
complexes are given in table 4. A chloromphenicol (antibacterial drug) and terbinafine
(antifungal drug) were used as standard drugs for comparison. The results obtained by

Iron(III) Schiff bases
2299

2300
R.K. Dubey et al.
disc diffusion indicate that coordination compounds containing Schiff bases,
substituted benzoxazole/mercaptoimidazole as well as coordinated pyridine have
enhanced activity compared to the ligands (Schiff bases/imidazole). Comparing with
chloromphenicol (standard antibacterial drug) and terbinafine (standard antifungal
drug) the following results can be obtained: (i) all the complexes have higher or equal
activity against all organisms than free ligands. (ii) All Schiff bases possess pronounced
antimicrobial effect against all tested fungi and Gram-positive bacteria in comparison
to antibiotics (terbinafine and chloromphenicol) used. (iii) The tested complexes have
greater activity than standard antibiotics (terbinafine and chloromphenicol) against
tested fungi and Gram-positive bacteria except 9. (iv) Complexes 4 and 5 are better
antimicrobial agents than chloromphenicol and terbinafine against all tested organisms.
4. Conclusions
[Fe(sb)2(py)(Cl)] ꢀ 2H2O (1–8) have been synthesized using bidentate Schiff bases and
substituted benzoxazole having ‘‘O’’ and ‘‘N’’ donors, whereas 9 bears ‘‘S’’ instead of
‘‘O’’. The geometry is octahedral in all the iron(III) complexes.
[Fe(saph)2(py)(Cl)] ꢀ 2H2O (5) and [Fe(mbz)2(py)(Cl)] ꢀ 2H2O (9) show molecular ion
peaks corresponding to their formulation and other important peaks due to successive
degradation of the monomer. Formulae of the prepared complexes have also been
suggested by elemental analysis, electronic, IR, NMR (1H, 13C) spectral studies as well
as magnetic values. PXRD patterns reveal that the complexes are amorphous.
Antimicrobial studies indicate that 4 and 5 are more potent against three Gram-
positive bacteria and two fungi, compared with literature values [31].
Acknowledgments
The authors are grateful to RSIC IIT, Mumbai, for providing FTIR and magnetic
facility. CDRI, Lucknow is acknowledged for elemental analysis, FAB-mass spectral
studies as well as for antimicrobial activity measurements. Nanophosphor Application
Center, University of Allahabad is also acknowledged for PXRD studies.
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