Spectral characterization of iron(III) complexes of 2-benzoylpyridine N(4)-substituted thiosemicarbazones

Article abstract of DOI:10.1016/j.saa.2005.07.067

Three iron(III) complexes (1-3) of 2-benzoylpyridine N(4)-phenyl thiosemicarbazone (HL¹) and one iron(III) complex (4) of 2-benzoylpyridine N(4)-cyclohexyl thiosemicarbazone (HL²) were synthesized and characterized by means of different physicochemical techniques viz., molar conductivity measurements, magnetic susceptibility studies and electronic, infrared and EPR spectral studies. The analytical data and the molar conductance measurements of the complexes reveal that two molecules of the ligand and the anion are coordinated to the metal atom in all the four complexes. The magnetic moments of the complexes suggest that they are of low spin. From the infrared spectra of the ligands and the complexes it is confirmed that the ligands coordinate to iron(III) as an anion coordinating via the azomethine nitrogen, pyridyl nitrogen, and the thiolate sulphur. The EPR spectra of the complexes in the polycrystalline state at 298 and 110 K and in DMF solution at 110 K were recorded and all the spectra show three g values indicating that these complexes have rhombic distortion. All the iron(III) complexes in DMF solution at 110 K have similar anisotropic spectra with almost the same g_av values, indicating that the bonding in all the complexes is similar and is unaffected by the coordination of the anion.

Full text of DOI:10.1016/j.saa.2005.07.067

Spectrochimica Acta Part A 64 (2006) 637–641
Spectral characterization of iron(III) complexes of 2-benzoylpyridine
N(4)-substituted thiosemicarbazones
∗
Marthakutty Joseph, Anandaram Sreekanth, V. Suni, M. R. Prathapachandra Kurup
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India
Received 11 April 2005; accepted 29 July 2005
Abstract
1
Three iron(III) complexes (1–3) of 2-benzoylpyridine N(4)-phenyl thiosemicarbazone (HL ) and one iron(III) complex (4) of 2-benzoylpyridine
2
N(4)-cyclohexyl thiosemicarbazone (HL ) were synthesized and characterized by means of different physicochemical techniques viz., molar
conductivity measurements, magnetic susceptibility studies and electronic, infrared and EPR spectral studies. The analytical data and the molar
conductance measurements of the complexes reveal that two molecules of the ligand and the anion are coordinated to the metal atom in all the four
complexes. The magnetic moments of the complexes suggest that they are of low spin. From the infrared spectra of the ligands and the complexes it
is confirmed that the ligands coordinate to iron(III) as an anion coordinating via the azomethine nitrogen, pyridyl nitrogen, and the thiolate sulphur.
The EPR spectra of the complexes in the polycrystalline state at 298 and 110 K and in DMF solution at 110 K were recorded and all the spectra
show three g values indicating that these complexes have rhombic distortion. All the iron(III) complexes in DMF solution at 110 K have similar
anisotropic spectra with almost the same gav values, indicating that the bonding in all the complexes is similar and is unaffected by the coordination
of the anion.
©
2005 Elsevier B.V. All rights reserved.
Keywords: 2-Benzoylpyridine; EPR spectra; Infrared spectra; Solid state reflectance spectra; Thiosemicarbazone
1
. Introduction
complexes are also reported [10]. As a part of our study of
NNS donor ligands and their metal complexes [11–13], we now
report the spectral characterization of the iron(III) complexes
of two N(4)-substituted thiosemicarbazones derived from 2-
benzoylpyridine.
Thiosemicarbazones and their metal complexes exhibit a
wide range of biological applications. Owing to the interest
they generate in pharmacology [1,2], thiosemicarbazones and
their metal complexes have been extensively studied. The
biological activity of thiosemicarbazones results from their
ability to form chelates with metal ions. Thiosemicarbazones
exist in the tautomeric thione (A) and thiol (B) forms (Fig. 1).
The thione form acts as a neutral bidentate ligand while the
thiol form deprotonates and acts as an anionic ligand [3].
There are some reports on the studies of N(4)-substituted
2. Experimental
2.1. Physical measurments
Carbon, hydrogen and nitrogen analyses of the ligands and
the complexes were done on a Heraeus elemental analyzer at
CDRI, Lucknow, India. The IR spectra were recorded on a Shi-
madzu DR 8001 series FTIR instrument using KBr pellets in the
2
-benzoylpyridine thiosemicarbazones and their metal com-
plexes [4–6]. We have reported spectral and structural studies
of copper(II) and iron(III) complexes of 2-benzoylpyridine
N(4), N(4)-(butane-1,4-diyl) thiosemicarbazone [7,8]. Spectral
characterization and crystal structure of 2-benzoylpyridine
N(4)-cyclohexyl thiosemicarbazone [9] and its copper(II)
−1
range 400–4000 cm and far IR spectra were recorded in the
−1
range 50–500 cm on a Nicolet Magna 550 FT-IR spectrom-
eter using polyethylene pellets at RSIC, IIT, Bombay, India.
The solid state electronic spectra of the ligands and their com-
plexes were recorded in the 250–900 nm range in a diffused
reflectance mode using MgO matrix on Ocean Optics SD2000
Fiber Optic Spectrometer. Electronic spectra of the ligands
and the complexes in DMF solution were recorded on Shi-
∗
Corresponding author. Tel.: +91 484 2575804; fax: +91 484 2577595.
E-mail address: mrp@cusat.ac.in (M.R.P. Kurup).
1
386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.07.067

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38
M. Joseph et al. / Spectrochimica Acta Part A 64 (2006) 637–641
2.3. Synthesis of complexes
Ethanolic solutions of ferric nitrate Fe(NO3)3·9H2O
1
(
1 mmol) and HL (2 mmol) were mixed and refluxed for 8 h to
1
prepare [Fe(L )2NO3]·2H2O (1). The brown shining solids sep-
arated on keeping the solution for 2 days were filtered, washed
with hot water, hot ethanol and ether and dried over P4O10 in
1
2
vacuo. [Fe(L )2Cl] (2) and [Fe(L )2NO3]·C2H OH (4) were
5
Fig. 1. Thione–thiol tautomers of thiosemicarbazones.
prepared by similar methods using the appropriate thiosemi-
carbazone and ferric chloride and ferric nitrate, respectively.
Compound 3, [Fe(L )2NCS] was prepared by a metathetical dis-
1
madzu 160A UV–Visible spectrophotometer. The EPR spectra
of the complexes were recorded on a Varian E-112 Spectrom-
eter using TCNE as the standard and on a Bruker ESP300
X-band CW EPR spectrometer operating at 9.52 MHz equipped
with a liquid nitrogen cryostat with the modulation amplitude
of 0.05 mT and 100 kHz modulation frequency, and the field
was calibrated by using 2,2-diphenyl-1-picrylhydrazyl (DPPH)
with a g value of 2.0036. The magnetic susceptibility mea-
surements were carried out at Indian Institute of Technology,
Roorkee, India at room temperature in the polycrystalline state
on a PAR model 155 Vibrating Sample Magnetometer at 5 K
Oersted field strength. The molar conductances of the com-
placement of the chloride anion of the ferric chloride complex
by the addition of 1 mmol methanolic solution of KCNS to the
reaction mixture containing HL (2 mmol) and ferric chloride
1
(1 mmol) in methanol. The resulting solution was refluxed for
4
h and kept overnight. The black crystals, which separated were
filtered, washed with methanol and ether and dried over P4O10
in vacuo.
3. Results and discussion
3.1. Analytical measurements
−
3
plexes of the solutions in dimethylformamide (10 M) at room
temperature were measured using a direct reading conductivity
meter.
The colors, stoichiometries, elemental analyses, and mag-
netic moments of the Fe(III) complexes are given in Table 1.
The analytical data calculated for the four iron(III) complexes
reveal the 1:2:1 stoichiometry for iron, thiosemicarbazone and
gegenion. All the iron(III) complexes are found to be insolu-
ble in methanol, ethanol, acetone and chloroform, but soluble in
dimethylformamide and in dimethylsulphoxide. The molar con-
2
.2. Synthesis of ligands
-Benzoylpyridine N(4)-phenylthiosemicarbazone (HL1)
2
−
3
ductivity measurements of all the complexes in DMF (10
M
was synthesized as follows [14]. Phenyl isothiocyanate 5.9 ml
−1
2
−1
solution) have values in the range 30–50 ohm cm mol ,
(
(
50 mmol) in 50 ml methanol and hydrazine hydrate 2.4 ml
50 mmol) in 50 ml methanol were mixed with constant stir-
which are consistent with their non-electrolytic nature [15]. This
indicates that the anions are associated in the first coordination
sphere of Fe(III).
ring. The resulting solution was kept in stirred condition for
.5 h. The white product, N-phenylthiosemicarbazide formed
0
was washed with methanol, and dried. 8.35 g (50 mmol) of N-
phenylthiosemicarbazide was dissolved in 100 ml methanol. To
this solution, a methanolic solution of 2-benzoylpyridine, 9.16 g
3
.2. Magnetic susceptibilities
High spin Fe(III) complexes in general, have magnetic
(
50 mmol) and two or three drops of acetic acid were added and
moments at room temperature very close to the spin only value of
refluxed for 5 h. The dark brown crystals, separated on cooling,
were washed with methanol, recrystallized from ethanol and
6
5.9 BMbecausethegroundstate(derivedfromthe Sstateoffree
◦
ion) has no orbital angular momentum and there is no effective
mechanism for introducing any coupling with the excited state.
dried over P4O10 in vacuo m.p. 135 C.
2
-Benzoylpyridine
N(4)-cyclohexylthiosemicarbazone
HL ) were synthesized as reported previously [9]. The
molecular structures of the ligands are shown in Fig. 2.
The low-spin complexes with t5 configurations, usually have
2
(
2g
considerable orbital contribution to their magnetic moments and
at room temperature the magnetic moment value is ∼2.3 BM
[16]. The magnetic moments of the synthesized iron(III) com-
plexes in the polycrystalline state at room temperature are found
to be in the range of 1.98–2.49 BM. These µeff values fall in the
range of those of low-spin iron(III) complexes [17].
3.3. Infrared spectra
The tentative infrared spectral assignments of the ligands
1
2
HL and HL and their Fe(III) complexes are given in Table 2.
1
2
The ν(C N) bands of HL and HL are found at 1591 and
1582 cm , respectively. These strong bands are shifted after
1
2
−1
Fig. 2. Molecular structures of the ligands HL and HL .

M. Joseph et al. / Spectrochimica Acta Part A 64 (2006) 637–641
639
Table 1
Analytical data, color and magnetic moments of Fe(III) complexes
a
Compound
Empirical formula
Color
Analytical data, found (calculated) (%)
µ (BM) at 300 K
Λ M (in DMF)
C
H
N
1
[
[
[
[
Fe(L )2NO3]·2H2O (1)
C38H34N9S2O5Fe
C38H30N8S2ClFe
C39H30N9S3Fe
Brown
Blue
Black
Brown
56.57 (55.89)
60.91 (60.53)
60.01 (60.32)
56.95 (57.29)
4.41 (4.16)
4.11 (3.98)
4.09 (3.87)
5.62 (5.73)
15.07 (15.44)
14.60 (14.86)
16.04 (16.24)
15.16 (15.04)
2.2
35.6
31.2
42.6
49.9
1
Fe(L )2Cl] (2)
2.49
1.98
2.0
1
Fe(L )2NCS] (3)
2
Fe(L )2NO3].C2H5OH (4)
C40H48N9S2O4Fe
a
−1
2
−1
−3
Molar conductivity in ohm cm mol at 298 K using 10 M solution.
Table 2
IR spectral assignments (cm ) of HL and HL and its Fe(III) complexes
−1
1
2
Compound
ν(C N)
ν(N N)
ν(C S)
ρ(py)
ν(Fe N)
ν(Fe N )py
ν(Fe S)
HL1
1591 s
1546 s
1566 s
1562 s
1582
1102 s
1125 m
1123 s
1118 m
1118 s
1153 m
835
622 m
665 m
659 m
638 w
607 m
641 w
–
–
–
1
a
[
[
[
Fe(L )2NO3]·2H2O (1)
752 m
756 s
766 s
833 m
744 m
492 s
491 s
495 s
–
363 s
368 s
370 s
–
442 s
455 s
451 s
–
1
Fe(L )2Cl (2)
1
Fe(L )2NCS] (3)
2
HL
2
b
[
Fe(L )2NO3]·C2H5OH (4)
1546 s
482 s
368 s
454 s
a
−1
−1
ν(NO3): 1492, 1384 cm
ν(NO3): 1439, 1319 cm
.
.
b
coordination towards lower energies by ca. 30 cm 1, indicating
−
water. Similarly, the bands at 1492 and 1384 cm in the spec-
−1
2
coordination via azomethine nitrogen [18]. The ν(N N) bands
of the thiosemicarbazones are found at 1102 and 1118 cm
trumof[Fe(L )2NO3]·C2H OHconfirmtheterminalunidentate
5
−
1
.
coordination of the nitrate ion to Fe(III) in the complex.
The increase in frequency of this band in the spectra of the com-
plexes is an evidence for the enolization of the ligand and the
coordination via the azomethine nitrogen. The bands observed at
3
.4. Electronic spectra
−
1
1
2
8
35 and 833 cm for HL and HL , respectively, are assigned
The electronic absorption bands recorded in the polycrys-
to ν(C S). These bands are found to be shifted to low energies in
the spectra of the complexes, indicating coordination via the thi-
olate sulfur [3,11]. The out-of-plane pyridine ring deformation
talline state of the iron(III) complexes are given in Table 3.
The bands observed at ca. 277 and 335 nm can be assigned to
*
*
the ␲ → ␲ and n → ␲ transitions of the thiosemicarbazones.
−
1
*
*
modes of the free ligands at 622 and 607 cm are found to be
shifted to higher energies in the spectra of the complexes indicat-
ing coordination via the nitrogen atom of the pyridine ring [19].
The coordination positions of the thiosemicarbazones in the
iron(III) complexes are confirmed by assigning the strong bands
observed in the far IR spectra of the complexes as suggested by
Usually n → ␲ transitions occur at a lower energy than ␲ → ␲
transitions. Two broad absorption bands are present in the region
*
6
56–400 nm. They are charge transfer transitions due to d → ␲
metal-to-ligand and S → Fe(III) electronic transitions [24–26].
Because of the greater oxidizing power of Fe(III), ligand–metal
charge transfer bands often obscure the very low intensity, spin
forbidden, d–d absorption bands [27]. In the spectra of iron(III)
complexes, the shoulders that appeared at ca. 890 nm are due to
d → d transitions.
−
1
Nakamoto [20]. The strong bands observed at 363–370 cm are
assigned to ν(Fe N) of the pyridine ring. Another ν(Fe N) band
−
1
is observed at 482–495 cm in the spectra of the Fe(III) com-
plexes indicating ν(Fe N) of the azomethine group. Again the
−
1
strong bands observed in the region 442–455 cm are assigned
3.5. Electron paramagnetic resonance spectra
to the ν(Fe S) bond. The iron–chlorine stretching band is nor-
−
1
mally observed between 300 and 200 cm [21]. A strong band
The X-band CW-EPR spectra of the iron(III) complexes in
−1
at 255 cm in the chloro complex confirms the coordination of
the chloride ion to Fe(III).
the polycrystalline state were recorded at temperatures 110 and
98 K and in DMF solution at 110 (Fig. 3). The EPR spec-
2
1
The IR spectrum of [Fe(L )2NO3]·2H2O, which exhibits two
−
1
bands at 1439 and 1319 cm corresponds to the NO stretch-
ing bands of the nitrate ion. The separation of these bands by
Table 3
Electronic spectral data (nm) of Fe(III) complexes
−1
1
20 cm indicates the unidentate nature of the nitrate ion in
*
*
Compound
d–d
Charge transfer
n → ␲
␲ → ␲
the complex [22]. According to Stefov et al. [23] coordinated
−
1
1
water should exhibit frequencies at 825, 575 and 500 cm
.
[Fe(L ) NO ]·2H O (1)
875 sh
887 sh
884 sh
890 sh
656, 419
655, 417
652, 408
650, 402
332
337
330
335
278
278
280
276
2
3
2
1
The absence of spectral bands in these regions in the spec-
[Fe(L )2Cl] (2)
1
[
Fe(L )2NCS (3)
trum of this complex indicates that the water molecules in
2
Fe(L )2NO3·C2H5OH (4)
1
[
Fe(L )2NO3]·2H2O are not coordinated, but exist as lattice

6
40
M. Joseph et al. / Spectrochimica Acta Part A 64 (2006) 637–641
netic moment values. The anisotropic character with the three
g values due to rhombic distortion is not uncommon for spin-
paired iron(III) since this behavior has been reported for Schiff
base as well as porphyrin Fe(III) complexes. The small deviation
of the anisotropic g value from 2.0 suggests that the electronic
2
2
1
structure of the ground state is (dxz) , (dyz) , (dxy) [8,32].
At 110 K, all the iron(III) complexes in DMF solution have
similar anisotropic spectra with almost the same gav values, indi-
cating that the bonding in all the complexes is similar and is
unaffected by the coordination of the anion.
Based on the physicochemical studies, all the iron(III) com-
plexes are assigned either a capped octahedral or a pentago-
nal bipyramidal geometry [33]. EPR data also suggest that the
iron(III) complexes are of low spin with the association of anions
in the first coordination sphere of iron(III) in the solid state. In
1
the capped octahedral structure two molecules of the ligand L
2
or L can occupy the six corners of an octahedron and an addi-
tional position is occupied by an anion at one triangular face of
this octahedron. In an alternate pentagonal bipyramidal struc-
ture [33], two molecules of the L or L can occupy two axial
positions and four positions in the equatorial position plane, an
additional fifth position in the equatorial plane being occupied
by the anion.
The complexes prepared from 2-acetylpyridine thiosemicar-
bazones and FeCl3 given in earlier reports were found to exist
as an ion pair consisting of a low-spin iron cation center and
high spin tetrachloroferrate anion with the composition [FeL2]
Fig. 3. X-band CW-EPR spectra of compounds 1 (top) and 2 (bottom) in DMF
measured at 110 K. Experimental spectrum (Exp.), simulated spectrum (Sim.).
Small signal on top is DPPH.
1
2
tral parameters are presented in Table 4. The spin Hamiltonian
parameters of the complexes were obtained after the spectral
simulations performed by EasySpin package [28]. The EPR
5
spectrum of the d iron(III) is expressed by the spin Hamiltonian
[
29,30]:
ˆ
FeCl . The presence of high spin in these complexes is proved
by an EPR signal at ca. 4.3 [8,26]. However, the absence of an
EPR signal at 4.3 in the present complex 2 indicates exclusive
presence of spin-paired iron(III) in conformitywiththe magnetic
moment measurements.
H = β[gxHx · Sx + gyHy · Sy + gzHz · Sz] with
4
1
S = 2 and g = 2.00
Spectra of all complexes recorded at room temperature in
polycrystalline state as well as a solution in DMF show three
g values indicating rhombic distortion. There is little differ-
ence in the spectra obtained in frozen solution at 110 K and
in the solid state indicating that the iron(III) centres do not
undergo alteration in solution. The observed anisotropic char-
acter with three g values due to rhombic distortion is common
for spin-paired iron(III) complexes [31]. The g values confirm
the low–spin character of iron(III) as confirmed from the mag-
Acknowledgments
TheauthorsareindebtedtotheRegionalSophisticatedInstru-
mentation Centre, CDRI, Lucknow, India and RSIC, IIT, Bom-
bay, for analytical measurements. M. Joseph would like to thank
Kerala State Council for Science, Technology and Environment,
Government of Kerala for financial assistance.
Table 4
EPR spectral parameters of Fe(III) complexes
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K)
g2
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