Synthesis and spectral investigations of Mn(II) complexes of pentadentate bis(thiosemicarbazones)

Article abstract of DOI:10.1016/j.saa.2009.11.022

Five Mn(II) complexes of bis(thiosemicarbazones) which are represented as [Mn(H₂Ac4Ph)Cl₂] (1), [Mn(Ac4Ph)H₂O] (2), [Mn(H₂Ac4Cy)Cl₂]·H₂O (3), [Mn(H₂Ac4Et)Cl₂]·3H₂O (4) and [Mn(H₂Ac4Et)(OAc)₂]·3H₂O (5) have been synthesized and characterized by elemental analyses, electronic, infrared and EPR spectral techniques. In all the complexes except [Mn(Ac4Ph)H₂O], the ligands act as pentadentate neutral molecules and coordinate to Mn(II) ion through two thione sulfur atoms, two azomethine nitrogens and the pyridine nitrogen, suggesting a heptacoordination. While in compound [Mn(Ac4Ph)H₂O], the dianionic ligand is coordinated to the metal suggesting six coordination in this case. Magnetic studies indicate the high spin state of Mn(II). Conductivity measurements reveal their non-electrolyte nature. EPR studies indicate five g values for [Mn(Ac4Ph)H₂O] showing zero field splitting.

Full text of DOI:10.1016/j.saa.2009.11.022

Spectrochimica Acta Part A 75 (2010) 585–588
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and spectral investigations of Mn(II) complexes of pentadentate
bis(thiosemicarbazones)
Suja Krishnan, K. Laly, M.R. Prathapachandra Kurup∗
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682022, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Five Mn(II) complexes of bis(thiosemicarbazones) which are represented as [Mn(H2Ac4Ph)Cl2]
Received 28 March 2009
Received in revised form
10 November 2009
(1),
[Mn(Ac4Ph)H2O]
(2),
[Mn(H2Ac4Cy)Cl2]·H2O
(3),
[Mn(H2Ac4Et)Cl2]·3H2O
(4)
and
[Mn(H2Ac4Et)(OAc)2]·3H2O (5) have been synthesized and characterized by elemental analyses,
electronic, infrared and EPR spectral techniques. In all the complexes except [Mn(Ac4Ph)H2O], the
ligands act as pentadentate neutral molecules and coordinate to Mn(II) ion through two thione sulfur
atoms, two azomethine nitrogens and the pyridine nitrogen, suggesting a heptacoordination. While in
compound [Mn(Ac4Ph)H2O], the dianionic ligand is coordinated to the metal suggesting six coordination
in this case. Magnetic studies indicate the high spin state of Mn(II). Conductivity measurements reveal
their non-electrolyte nature. EPR studies indicate five g values for [Mn(Ac4Ph)H2O] showing zero field
splitting.
Accepted 10 November 2009
Keywords:
2,6-Diacetylpyridine
Bis(thiosemicarbazones)
Manganese(II) complexes
EPR studies
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
tumor activity [13,14]. Spectral and biological studies have
been carried out on metal complexes of 2,6-diacetylpyridine
The synthesis and study of coordination complexes with
unusual geometry and coordination number is a challenging task
for the practical chemist. The most important factor in this objec-
tive is probably the design of ligands with an appropriate structural
backbone that can coerce the metal ion into the desired coordi-
nation geometry. Though tridentate ligands with mixed NS donor
points at strategic positions of the donor-framework are common
[1,2], such pentadentate ligands are seldom encountered in coordi-
nation chemistry. The chelating properties of 2,6-diacetylpyridine
bis(thiosemicarbazone), have been investigated and several coor-
dination modes have been found [3–6]. There are reports on
heptacoordinated bis(thiosemicarbazones) of 2,6-diacetylpyridine
[7]. This type of coordination is commonly found in metals like
Sn(IV) [8], Mn(II) [9] and indium(III) [10]. As previous research
[11] show, these ligands tend to form pentagonal bipyramidal
complexes in which the ligand acts as a pentacoordinated chelate
and the two arms (thiosemicarbazide groups) of the ligand have
remained protonated [12].
bis(N4-substituted thiosemicarbazones)[15]. Manganese coordi-
nation chemistry with a diverse range of ligands has much
relevance in biological systems with a number of model man-
ganese complexes. Manganese coordination compounds are also of
growing importance as homogeneous catalysts in oxidation reac-
tions [16,17]. In such studies manganese complexes in different
oxidation states were obtained and their magnetic and spectral
properties were studied in depth. The absence of ligand field
stabilization energy for high spin Mn(II) complexes leads to the
possibility to obtain various coordination geometries and a lower
stability of Mn(II) complexes compared with those of other divalent
3d metals.
In the present article, synthesis and characterization of some
2,6-diacetylpyridine bis(N4-substitutedthiosemicarbazones) and
their Mn(II) complexes are investigated with the help of physico-
chemical techniques.
2. Experimental
Transition metal complexes of bis(thiosemicarbazone) lig-
ands have been investigated as metallodrugs for a number of
years. It has been reported that ␣-diketone and ␣-ketoaldehyde
bis(thiosemicarbazones) and their metal complexes show anti-
2.1. Materials
2,6-Diacetylpyridine (Aldrich), hydrazine hydrate, N4-phenyl
isothiocyanate, N4-cyclohexyl isothiocyanate, N4-ethylthiosemi-
carbazide, manganese(II) chloride tetrahydrate and manganese(II)
acetate tetrahydrate were used as supplied for the preparation of
complexes. Solvents used were methanol, ethanol, dimethylfor-
mamide and chloroform.
∗
Corresponding author. Tel.: +91 484 2862423; fax: +91 484 2575804.
E-mail addresses: mrp@cusat.ac.in, mrp k@yahoo.com
(M.R. Prathapachandra Kurup).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.11.022

586
S. Krishnan et al. / Spectrochimica Acta Part A 75 (2010) 585–588
Table 1
Colors, magnetic susceptibilities and molar conductivities of the Mn(II) complexes.
Compound
Color
␮ (B.M.)
aꢀM
[Mn(H2Ac4Ph)Cl2] (1)
[Mn(Ac4Ph)H2O] (2)
[Mn(H2Ac4Cy)Cl2]·H2O (3)
[Mn(H2Ac4Et)Cl2]·3H2O (4)
[Mn(H2Ac4Et)(OAc)2]·3H2O (5)
pale yellow
yellow
pale yellow
yellow
5.91
5.83
5.84
5.78
6.08
33
29
14
10
20
Scheme 1.
yellow
2.2. Synthesis of bis(thiosemicarbazones)
a
Molar conductivity, 10−3 M DMF at 298 K.
2,6-Diacetylpyridine bis(N4-phenylthiosemicarbazone) (H2Ac-
4Ph)
and
2,6-diacetylpyridine
bis(N4-cyclohexylthiosemi-
N, 15.99 (15.88). [Mn(H2Ac4Et)Cl2]·3H2O (4): C, 33.54 (33.03); H,
5.59 (5.36); N, 18.21 (17.98). [Mn(H2Ac4Et)(OAc)2]·3H2O (5): C,
39.07 (38.51); H, 6.14 (5.95); N, 17.03 (16.55).
carbazone) (H2Ac4Cy)
Step 1. Preparation of N4-phenylthiosemicarbazide/N4-
cyclohexylthiosemicarbazide
2.4. Physical measurements
The thiosemicarbazides were synthesized by stirring equimo-
lar amounts of the corresponding isothiocyanate in ethanol and
hydrazine hydrate in methanol for half an hour. The prod-
uct (N4-phenylthiosemicarbazide/N4-cyclohexylthiosemicarbazide)
formed was filtered, washed with ethanol and ether and dried in
vacuo over P4O10 (Scheme 1).
Elemental analyses of ligands and the complexes were done
on a Vario EL III CHNS analyzer at SAIF, Kochi, India. Infrared
spectra were recorded on
a JASCO FT/IR-4100 type Fourier
Transform Infrared Spectrometer using KBr pellets in the range
4000–400 cm−1. The far IR spectra were recorded using polyethy-
lene pellets in the 500–100 cm−1 region on a Nicolet Magna 550
FTIR instrument at the SAIF, Indian Institute of Technology, Bom-
bay. Electronic spectra were recorded on a Cary 5000 version 1.09
UV–Vis–NIR spectrophotometer from solutions in DMF. The mag-
netic susceptibility measurements were done in the polycrystalline
state at room temperature on a Vibrating Sample Magnetometer
at the Indian Institute of Technology, Roorkee, India. EPR spectra
of complexes in solid state at 298 K and in frozen DMF at 77 K
were recorded on a Varian E-112 spectrometer at X-band, using
TCNE as a marker with 100 kHz modulation frequency and 9.1 GHz
microwave frequency at SAIF, IIT Bombay, India. The molar conduc-
tivities of the complexes in dimethylformamide solutions (10−3 M)
at room temperature were measured using a direct reading con-
ductivity meter.
Step 2. Synthesis of thiosemicarbazone (H2Ac4Ph)/(H2Ac4Cy)/
(H2Ac4Et)
A hot solution of the thiosemicarbazide in 25 ml of ethanol and
2,6-diacetylpyridine in 25 ml of ethanol were mixed in 2:1 ratio
with constant stirring. The above mixture was slowly refluxed for
5 h. After cooling, the compounds obtained as pale yellow solids,
were filtered, washed with ethanol and dried in vacuo over P4O10
(Scheme 2).
Elemental Anal. Found (Calcd.) (%): H2Ac4Ph: C, 59.44 (59.84);
H, 4.75 (5.02); N, 21.87 (21.24). H2Ac4Cy: C, 58.70 (58.32); H, 7.75
(7.45); N, 20.55 (20.70). H2Ac4Et: C, 49.05 (49.29); H, 6.93 (6.34);
N, 26.64 (26.82).
2.3. Synthesis of complexes
3. Results and discussion
[Mn(H2Ac4Ph)Cl2] (1) and [Mn(Ac4Ph)H2O] (2) were prepared
by refluxing 1 mmol each of MnCl2·4H2O/Mn(OAc)2·4H2O in
methanol and H2Ac4Ph in DMF for 4 h. [Mn(H2Ac4Cy)Cl2] H2O
(3) was prepared by refluxing 1 mmol each of methanolic
Condenzation reaction of 2,6-diacetylpyridine and N4-
substituted thiosemicarbazides in
a
molar ratio of 1:2
resulted in ligand systems 2,6-diacetylpyridine bis(N4-
substitutedthiosemicarbazones). All the five Mn(II) complexes
were prepared by the reaction of corresponding ligands with
metal salts in 1:1 ratio. Unfortunately, attempts to grow crys-
tals for structural studies of these metal complexes have failed
to date. While the present work was in progress, a dianionic
Mn(II) complex of H2Ac4Et was reported [18]. Colors, molar
conductivities and magnetic susceptibilities are listed in Table 1.
Conductivity measurements were done in 10−3 DMF solutions
and these values, although indicate extensive dissociation of these
complexes in DMF, are much lower than that observed for 1:1
electrolytes in this solvent. They can, therefore, be considered
solution
form.
of
The
MnCl2·4H2O
and
H2Ac4Cy
in
chloro-
(4) and
complexes
[Mn(H2Ac4Et)Cl2] 3H2O
[Mn(H2Ac4Et)(OAc)2] 3H2O (5) were prepared by refluxing
1 mmol of MnCl2·4H2O/Mn(OAc)2·4H2O in methanol and 1 mmol
of H2Ac4Et in chloroform for 4 h. The compounds formed were
filtered, washed with methanol and ether and dried in vacuo over
P4O10
.
Elemental Anal. Found (Calcd.) (%): [Mn(H2Ac4Ph)Cl2]
(1): C, 46.87 (47.02); H, 4.08 (3.95); N, 16.61 (16.69).
[Mn(Ac4Ph)H2O] (2): C, 52.94 (52.77); H, 4.17 (4.24); N,18.84
(18.73). [Mn(H2Ac4Cy)Cl2]·H2O (3): C, 45.39 (44.73); H, 5.81(6.04);
Scheme 2.

S. Krishnan et al. / Spectrochimica Acta Part A 75 (2010) 585–588
587
Fig. 1. EPR spectra of Mn(II) compounds in DMF at 77 K.
Table 2
Selected IR bands (cm−1) with tentative assignments of manganese(II) complexes.
3.1. Infrared and electronic spectra
The bands assigned in the spectra of ligands and complexes,
because of coordination are illustrated in Table 2. The IR spectra
of the free ligands show bands in the 3300–3000 cm−1 region,
attributable to stretching modes of two NH groups each in the two
arms of the ligand. A considerable downfield shift in compound
2 can be due to shifting of NH protons resulting in enolisation
followed by deprotonation. The pyridine nitrogen coordination is
evidenced by the positive shift of pyridine ring deformation mode
in all the complexes. The ꢁ(C N) bands of ligands are found to be
shifted to lower wavenumbers in the spectra of all complexes sug-
gesting the coordination of the azomethine nitrogen to the metal
[19]. Due to this the ꢁ(N N) band of the spectra enjoy a shift
to higher frequencies. The presence of a new band in the range
450–470 cm−1, assignable to ꢁ(Mn Nazo) confirm coordination of
azomethine nitrogen in these complexes [1].
Compound
ꢁ(C N) ꢁ/ı(C S) ꢁ(Mn N) ꢁ(N N) py(ip)
H2Ac4Ph
[Mn(H2Ac4Ph)Cl2] (1)
[Mn(Ac4Ph)H2O] (2)
1597
1562
1560
1322,808
1320, 802 486
1205, 710 434
–
1028
1047
1095
628
663
676
H2Ac4Cy
[Mn(H2Ac4Cy)Cl2]·H2O (3)
1596
1555
1307, 857
1208, 802 460
–
1013
1068
601
680
H2Ac4Et
1541
1527
1305, 857
1289, 726 427
1293, 806 489
–
1050
1068
1086
639
652
663
[Mn(H2Ac4Et)Cl2]·3H2O (4)
[Mn(H2Ac4Et)(OAc)2]·3H2O (5) 1526
as non-electrolytes in this solvent. Because of the additional
stability of the half filled d shell, Mn(II) generally forms high spin
complexes with an orbitally degenerate 6S ground state term
and the spin only magnetic moment of 5.92 B.M. The magnetic
moments of the present complexes indicate them to be of high spin
type.
The stretching and bending vibrations of the C S group of the
ligands are shown in the 1305–1322 and 807–857 cm−1 regions.
If coordinated in the neutral form, there is no considerable neg-
ative shift in the case of ꢁ(C S) band. But in compound 2 there
is considerable shift in ꢁ/ı(C S) bands and that may be due to
the formation of strong metal-sulfur bonds [20]. Bands observed
at ca 247 cm−1 in the far IR spectra have been assigned to ter-
minal ꢁ(Mn Cl) bands in compounds 1, 3 and 4. Bands at 1526
and 1484 cm−1 correspond to symmetric and asymmetric stretch-
ing vibrations of the acetate group, consistent with the presence of
a monodentate acetate group in the complex 5 [21]. For a d5 com-
Table 3
Electronic spectral assignments (nm) of manganese(II) complexes.
*
*
Compound
␲ → ␲
n → ␲
LMCT
H2Ac4Ph
310
310
311
346
353
351
–
456
452
[Mn(H2Ac4Ph)Cl2] (1)
[Mn(Ac4Ph)·H2O] (2)
6
H2Ac4Cy
[Mn(H2Ac4Cy)Cl2]·H2O (3)
308
310
342
345
–
438
plex, transitions from the A1g ground state are doubly forbidden
and hence electronic transition intensities are found to be very low.
In the present compounds, d–d bands could not be identified due
to their low intensity. However the broad band at ca. 450 nm in
the complexes may be probably due to charge transfer bands. The
electronic spectral assignments are summarized in Table 3
H2Ac4Et
310
311
310
340
343
346
–
444
455
[Mn(H2Ac4Et)Cl2]·3H2O (4)
[Mn(H2Ac4Et)(OAc)2]·3H2O (5)
*
Antibonding orbital.

588
S. Krishnan et al. / Spectrochimica Acta Part A 75 (2010) 585–588
Table 4
EPR spectral data for complexes in polycrystalline state at 298 K and DMF solution at 77 K.
Compound
g values
aA in G
92
Polycrystalline state (298 K).
DMF solution (77 K)
[Mn(H2Ac4Ph)Cl2] (1)
[Mn(Ac4Ph)H2O] (2)
[Mn(H2Ac4Cy)Cl2]·H2O (3)
[Mn(H2Ac4Et)Cl2]·3H2O (4)
[Mn(H2Ac4Et)(OAc)2]·3H2O(5)
2.027
2.005
2.050
2.040
2.070
2.012
5.69, 3.22, 2.65, 2.23, 1.82
1.996
1.992
2.021
96
96
92
a
Expressed in units of cm−1 multiplied by a factor of 10−4
.
splitting. However complexes containing anionic ligand showed
zero-field splitting also.
Acknowledgements
The authors are thankful to the SAIF, Cochin University of Sci-
ence and Technology, Kochi, Kerala, India for elemental analyses.
We are thankful to IIT Bombay, India for EPR spectra and IIT Roor-
kee, India for magnetic studies.
Appendix A. Supplementary data
Fig. 2. EPR spectrum of compound [Mn(Ac4Ph)H2O] (2) in DMF at 77 K.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2009.11.022.
3.2. EPR spectra
EPR spectra of all Mn(II) complexes were recorded in polycrys-
talline state at 298 K and in frozen DMF at 77 K. The values are given
in Table 4. All compounds at 298 K, in polycrystalline state exhibited
a broad signal with a g value at ca 2.027 with no hyperfine split-
tings. Broadening of the signal is due to dipole–dipole interactions
and random orientation of the Mn(II) ions [21].
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