Low-spin manganese(II) and high-spin manganese(III) complexes derived from disalicylaldehyde oxaloyldihydrazone: Synthesis, spectral characterization and electrochemical studies

Article abstract of DOI:10.1016/j.molstruc.2017.09.052

Low-spin manganese(II) complexes [Mn^II(H₂slox)].H₂O (1), [Mn^II(H₂slox)(SL)] (where SL (secondary ligand) = pyridine (py, 2), 2-picoline (2-pic, 3), 3-picoline (3-pic, 4), and 4-picoline (4-pic, 5) and high-spin manganese(III) complex Na(H₂O)₄[Mn^III(slox)(H₂O)₂].2.5H₂O have been synthesized from disalicyaldehyde oxaloyldihydrazone in methanolic – water medium. The composition of complexes has been established by elemental analyses and thermoanalytical data. The structures of the complexes have been discussed on the basis of data obtained from molar conductance, UV visible, ¹H NMR, infrared spectra, magnetic moment and electron paramagnetic resonance spectroscopic studies. Conductivity measurements in DMF suggest that the complexes (1–5) are non-electrolyte while the complex (6) is 1:1 electrolyte. The electronic spectral studies and magnetic moment data suggest five – coordinate square pyramidal structure for the complexes (2–5) and square planar geometry for manganese(II) in complex (1). In complex (6), both sodium and manganese(III) have six coordinate octahedral geometry. IR spectral studies reveal that the dihydrazone coordinates to the manganese centre in keto form in complexes (1–5) and in enol form in complex (6). In all complexes, the ligand is present in anti-cis configuration. Magnetic moment and EPR studies indicate manganese in +2 oxidation state in complexes (1–5), with low-spin square planar complex (1) and square pyramidal stereochemistries complexes (2–5) while in +3 oxidation state in high-spin distorted octahedral stereochemistry in complex (6). The complex (1) involves significant metal – metal interaction in the solid state. All of the complexes show only one metal centred electron transfer reaction in DMF solution in cyclic voltammetric studies. The complexes (1–5) involve Mn^II→Mn^I redox reaction while the complex (6) involves Mn^III→Mn^II redox reaction, respectively.

Full text of DOI:10.1016/j.molstruc.2017.09.052

Journal of Molecular Structure 1151 (2018) 343e352
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Low-spin manganese(II) and high-spin manganese(III) complexes
derived from disalicylaldehyde oxaloyldihydrazone: Synthesis,
spectral characterization and electrochemical studies
Ibanphylla Syiemlieh a, Arvind Kumar b, Sunshine D. Kurbah a, Arjune K. De c,
,
*
Ram A. Lal a
a Department of Chemistry, Centre for Advanced Studies, North-Eastern Hill University, Shillong, 793022, India
b Department of Chemistry, Faculty of Science and Technology, The University of Wes- Indies, St. Augustine, Trinidad and Tobago
c Department of Science and Humanities, Tripura Institute of Technology, Narsingarh, 799009, Tripura, India
a r t i c l e i n f o
a b s t r a c t
Low-spin manganese(II) complexes [MnII(H2slox)].H2O (1), [MnII(H2slox)(SL)] (where SL (secondary
ligand) ¼ pyridine (py, 2), 2-picoline (2-pic, 3), 3-picoline (3-pic, 4), and 4-picoline (4-pic, 5) and high-
spin manganese(III) complex Na(H2O)4[MnIII(slox)(H2O)2].2.5H2O have been synthesized from dis-
alicyaldehyde oxaloyldihydrazone in methanolic e water medium. The composition of complexes has
been established by elemental analyses and thermoanalytical data. The structures of the complexes have
been discussed on the basis of data obtained from molar conductance, UV visible, 1H NMR, infrared
spectra, magnetic moment and electron paramagnetic resonance spectroscopic studies. Conductivity
measurements in DMF suggest that the complexes (1e5) are non-electrolyte while the complex (6) is 1:1
electrolyte. The electronic spectral studies and magnetic moment data suggest five e coordinate square
pyramidal structure for the complexes (2e5) and square planar geometry for manganese(II) in complex
(1). In complex (6), both sodium and manganese(III) have six coordinate octahedral geometry. IR spectral
studies reveal that the dihydrazone coordinates to the manganese centre in keto form in complexes (1
e5) and in enol form in complex (6). In all complexes, the ligand is present in anti-cis configuration.
Magnetic moment and EPR studies indicate manganese in þ2 oxidation state in complexes (1e5), with
low-spin square planar complex (1) and square pyramidal stereochemistries complexes (2e5) while
in þ3 oxidation state in high-spin distorted octahedral stereochemistry in complex (6). The complex (1)
involves significant metal e metal interaction in the solid state. All of the complexes show only one
metal centred electron transfer reaction in DMF solution in cyclic voltammetric studies. The complexes (1
e5) involve MnII/MnI redox reaction while the complex (6) involves MnIII/MnII redox reaction,
respectively.
Article history:
Received 9 May 2017
Received in revised form
16 September 2017
Accepted 18 September 2017
Available online 19 September 2017
Keywords:
ESR
Oxaloyldihydrazone
Redox activity
Low-spin manganese(II)
High-spin manganese(III)
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
in manganese superoxide dismutase. The divalent high-spin man-
ganese complexes are quite common in the field of coordination
Manganese is an essential trace element occurring in the bio-
logical systems [1,2], and plays a significant role in electron and
oxygen transfer processes in nature with iron and copper. Although
manganese occurs in some of the enzymes as Mn(II), in other
biological systems, it occurs in higher oxidation states, such as
Mn(III) and Mn(IV) like oxygen evolving complex [3] and as Mn(III)
chemistry. They do not have strong stereo - chemical preferences as
it is controlled by the nature of the ligand and hence its complexes
can exist in square planar, tetrahedral, trigonal bipyramidal, square
pyramidal, octahedral and pentagonal bipyramidal stereochemis-
tries etc. Although high-spin manganese(II) complexes have been
extensively studied but low-spin manganese complexes are very
few in number. Manganese(II), has very high spin-pairing energy
amongst 3d metal ions. Hence, ligands with very strong ligand
fields only can induce low-spin character on manganese ion.
Cyano- [4,5], phosphine- [6], dithiochelate ligands [7], and oxime
* Corresponding author.
E-mail address: ralal@rediffmail.com (R.A. Lal).
https://doi.org/10.1016/j.molstruc.2017.09.052
0022-2860/© 2017 Elsevier B.V. All rights reserved.

344
I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
ligands [8]. Goswami and co-workers [9] and others [10,11] have
recently reported low-spin manganese complexes. The man-
ganese(II) complexes in low oxidation states are commonly 6 -
coordinate octahedral. Further, four coordinate Mn(II) or Mn(III)
complexes in high-spin state are known to be present in distorted
square planar or tetrahedral geometries but these geometries are
uncommon for low-spin manganese(II) or manganese(III)
complexes.
The hydrazones are multidentate molecules and find application
in many fields such as pharmacology [12,13], analytical chemistry
[14,15] and technology [16,17] etc. Such molecules are capable of
exhibiting keto-enol tautomerism.
400 MHz in DMSO-d6. Tetramethyl silane (TMS) was used as an
internal standard. The ESR spectra of the complexes were recorded
in powder form as well as in DMSO glass at room temperature and
LNT at X-band frequency. The electron transfer properties of the
complexes were studied by cyclic voltammetry using CH Instru-
ment Electrochemical Analyzer under nitrogen atmosphere. The
electrolytic cell comprises of 3 electrodes, the working electrode
was Pt disk while the reference electrode and auxiliary electrode
were Ag/AgCl (3 M KCl) separated from the sample solution by a salt
bridge. 0.1 M Tetra-n-butyl ammonium perchlorate (TBAP) was
used as the supporting electrolyte.
The polyfunctional dihydrazone used in the present study has
been derived from condensation of oxaloyldihydrazine with sali-
cyaldehyde. The two dihydrazone grouping are directly bonded to
one another and hence give rise to better multidentate ligand than
monohydrazones. It possesses as many as eight oxygen and nitro-
gen donor atoms offering several alternate modes of bonding and is
capable of giving monometallic, homobimetallic, heterometallic,
homomultimetallic and heteromultimetallic complexes.
It is evident from the literature survey that although work on
metal complexes of monohydrazide based ligands and their Schiff
bases has been carried out in some detail [18e20], those on metal
complexes of dihydrazones are quite meagre [21e23]. Moreover,
the work on manganese(II) and manganese(III) complexes with
dihydrazones in low spin state is almost non-existent. Hence, in
view of the polyfunctional nature of disalicyaldehyde oxaloyldi-
hydrazone and its possibility to offer several alternate bonding
mode in the complexes and meagre amount of work on the man-
ganese(II) and manganese(III) complexes of dihydrazones, we have
decided to explore the possibility of coordination of dihydrazone to
manganese(II) under different experimental conditions and isolate
the resulting complexes and characterize them in solid and solution
state. The electron transfer reactions of the complexes have also
been studied by cyclic voltammetry.
2.2. Preparation of disalicyaldehyde oxaloyldihydrazone
Oxaloyldihydrazine was prepared by reacting diethyloxalate
(20 mL) and hydrazine hydrate (13 mL) in 1:2 M ratios under stir-
ring for 30 min. The product thus isolated was recrystallized from
H2O. (Yield: 95%)
Disalicylaldehyde oxaloyldihydrazone (H4slox) was prepared by
condensing oxaloyldihydrazine (3 g, 25.42 mmol), in hot H2O
(100 mL) with salicylaldehyde (6.20 mL) in ethanol (20 mL) over
hot plate at 70 ꢁC with constant stirring for 1 h. The light yellow
precipitate thus obtained was washed with hot H2O-ethanol,
ethanol and finally with ether and dried over anhydrous CaCl2.
(Yield: 93%)
2.3. Synthesis of [Mn(H2slox)].H2O (1)
MnSO4$H2O (1 g, 5.91 mmol) was dissolved in H2O (60 mL). This
solution was added to a solution of H4slox (0.64 g, 1.963 mmol) in
MeOH (20 mL) and it was stirred vigorously for ½ hour to get a
homogenous suspension. The reaction mixture was then refluxed
for 6 h and filtered. The residue was washed with H2O-MeOH,
MeOH and finally with ether and dried in an electronic oven. (Yield:
90%)
Synthesis of [Mn(H2slox)(A)] [A ¼ pyridine (2),
picoline (4), -picoline (5)]
The complexes (2e5) were also prepared by following essen-
tially the above method in which either pyridine (2) or -picoline
(3) or -picoline (4) or -picoline (5) was added into the reaction
a-picoline (3), b-
2. Experimental section
g
All reagents and chemicals were E-Merck or equivalent grade,
and all solvents were used as received.
a
b
g
mixture consisting of MnSO4$H2O and H4slox maintaining
MnSO4$H2O:H4slox:A molar ratio at 1:1:4 and refluxed the reaction
mixture for 6 h (complex 2) while stirring the reaction mixture at
room temperature for 6 h (complex 3e5). (Yield: 80% (2), 75% (3),
82% (4), 80% (5))
2.1. Physical measurements
Manganese [24] was determined by standard literature
procedure.
Carbon, hydrogen and nitrogen analyses were carried out on a
Perkin-Elmer 2400 CHNS/O Analyzer. The molar conductance of the
complexes at 10ꢀ3 M dilution in DMF solution were measured on a
Direct Reading Conductivitymeter-303 with a dip type conductivity
cell at room temperature. The loss of weight of the complexes was
determined by heating the compounds at 110 ꢁC, 180 ꢁC and 220 ꢁC
for 4 h in an electronic oven. The thermogravimetric analyses of the
complexes were carried out on the perkin-Elmer STA 6000
(Simultaneous Thermal Analyzer) model in a ceramic crucible un-
der dynamic nitrogen atmosphere. The heating rate of the com-
plexes was maintained at 20 ꢁC/minꢀ1. The DTA standard used in
the experiment is Pt 10% Rh. Infrared spectra in the range
4000e400/500 cmꢀ1 were measured on Perkin-Elmer, model 983
spectrophotometer with samples investigated as KBr discs. Room
temperature magnetic susceptibility measurements were made on
Sherwood Magnetic Susceptibility Balance MSB-Auto. The elec-
tronic spectra of the complexes at 10ꢀ5 M in DMF solution were
measured from 200 to 1100 nm on a Perkin Elmer Lamda 25 UV/Vis
Spectrophotometer at room temperature. 1H and 13C Nuclear
Magnetic Resonance spectra were recorded on a Bruker Avance II-
2.4. Synthesis of Na(H2O)4[Mn(slox)(H2O)2].2.5H2O (6)
This complex was also prepared by following essentially the
above method and adding Na2CO3 to the reaction mixture obtained
by
mixing
MnSO4$H2O
and
H4slox
maintaining
MnSO4$H2O:H4slox:Na2CO3 molar ratio at 1:1:3.(Yield: 68%)
3. Results and discussion
Synthesis of the complex (1) was achieved by reacting
MnSO4$H2O with ligand (H4slox) in aqueous methanol medium
under reflux (complex 1) for 6 h maintaining MnSO4$H2O and
ligand molar ratio at either 3:1 or 2:1 or 1:1. On the other hand, the
complex (2) was achieved by reacting MnSO4$H2O, H4slox and
pyridine (complex 2) under reflux for
MnSO4$H2O:H4slox:pyridine molar ratio at 1:1:4. But, the com-
plexes [Mn(H2slox)( -pic)2] (3), [Mn(H2slox)( -pic)] (4) and
[Mn(H2slox)( -pic)] (5) were obtained when the MnSO4$H2O,
H4slox and secondary ligand (SL) ligand were stirred at room
6 h maintaining
a
b
g

I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
345
temperature for 6 h maintaining MnSO4$H2O:H4slox:SL at 1:1:4 M
-picoline (3), -picoline (4), -picoline (5)). The com-
arm of a branched tube (branched tube method). A mixture of
acetonitrile and DMSO (15:85 v/v, 15 mL) was carefully added to fill
the arms, the tube was sealed and the reagents containing arm
immersed in an oil bath at 60 ꢁC, while the other arm was kept at
ambient temperature. Unfortunately, in all our efforts, only amor-
phous compounds precipitated which prevented analysis of the
complexes by X-ray crystallography.
ratio (SL ¼
a
b
g
plex Na(H2O)4[MnIII(slox)(H2O)2].2.5H2O (6) was obtained when a
mixture of MnSO4$H2O, H4slox and Na2CO3 was refluxed for 6 h
maintaining MnSO4$H2O:H4slox:Na2CO3 in a 1:1:3 M ratio. It is
imperative to mention that under the basic condition of Na2CO3 the
aerial oxidation of Mn(II) occurs to Mn(III) to give the complex
Na(H2O)4[MnIII(slox)(H2O)2].2.5H2O.
The complexes with their colour, decomposition point,
elemental analyses and magnetic moments and molar conductance
are set out in Table 1.
3.1. Molar conductance
The molar conductivity (Table 1) of the complexes (1e5) falls in
the region 0.9e2.5
U
ꢀ1cm2molꢀ1 at 10ꢀ3 M dilution in DMF. This
On the basis of elemental analyses and thermoanalytical data,
the compositions of the complexes have been established to be
value suggested that the complexes are non-electrolyte in this
medium [25]. On the other hand, the complex (6) showed molar
[MnII(H2slox)].H2O (1), [MnII(H2slox)(A)] [A ¼ py (2),
a-pic (3), b-pic
conductance value equal to 75 U
ꢀ1cm2molꢀ1 in DMF revealing that
(4), g-pic (5)] respectively. But, the composition of the complex (6)
was found to be Na(H2O)4[MnIII(slox)(H2O)2].2.5H2O. All these
complexes decompose above 300 ꢁC without melting. This in-
dicates that metal-ligand bonds have comparatively higher ionic
character. The complex (1) showed weight loss corresponding to
one water molecule at 110 ꢁC suggesting its presence in the lattice
structure of the complex while complexes (2e5) showed no weight
loss at 110 ꢁC ruling out the possibility of presence of water
molecule in their lattice. Further, complexes (1e5) did not show any
weight loss at 180 ꢁC ruling out the possibility of coordination of
water molecule to the metal centre. But the complexes (2) to (5)
showed weight loss at 220 ꢁC corresponding to one pyridine, 2-
picoline, 3-picoline and 4-picoline molecule, respectively. The
expulsion of these donor molecules at such a high temperature
indicates that they are coordinated to the metal centre. But the
complex (6) showed weight loss corresponding to 2.5 water mol-
ecules at 110 ꢁC while six water molecules at 180 ꢁC. The loss of 2.5
water molecule at 110 ꢁC indicate their presence in the lattice
structure of the complex while six water molecule at 180 ꢁC reflects
their presence in the first coordination sphere at around metal
centres.
An effort was taken up to crystallize the complexes in various
solvent systems under different experimental conditions. Both
saturated and dilute solutions of the complexes in various solvent
systems were kept for 1 and 2 months under observation at
ambient temperature to grow crystals. In order to promote crystal
growth, the solutions were also gently evaporated at 40 ꢁC, 50 ꢁC
and 60 ꢁC in hot air electronic oven. An effort was too made to grow
crystal from the reaction mixture when solution of the metal salts
was layered with a solution containing the ligand in methanol.
Again the metal salt solutions mixed with the ligand in solution in
DMSO and DMF were also layered with diethyl ether and resulting
solution in a small beaker was kept in a bigger beaker containing n-
hexane. Further, the metal salt and ligand were placed in the main
this complex is 1:1 electrolyte in this medium [25].
3.2. Thermal studies
All of the complexes show loss of surface water molecules. After
loss of surface water molecules, first significant loss of weight
commences in complex (1) at 194 ꢁC which is completed at 268 ꢁC.
The loss of weight corresponds to loss of one H2O molecule (exp:
4.5%, theo: 4.53%) in this temperature range (Fig. S1). The loss of
one H2O molecule in such a high temperature range may indicate
its presence either in the first coordination sphere around the metal
centre or in the lattice in strongly hydrogen bonded form. When
the vapours evolved in this high temperature range were passed
through a trap containing anhydrous CuSO4, it turned out to be
blue. The loss of water molecule in such a high temperature range
in TGA studies may, most probably be due to hydrogen bond
network that permeates the lattice [26]. The hydrogen bonding
may be between the lattice e held water molecules and coordi-
nated ligand to manganese(II) centre. When there is a loss of water
molecule in such a high temperature range in TGA, caution must be
exercised in the classification of water as a lattice held or coordi-
nated on the basis of TGA alone, unless confirmed by X-ray crys-
tallographic studies. However, on the basis of loss of mass at 130 ꢁC,
we assumed that one water molecule is present in the lattice of the
complex by hydrogen bonding with the bonded dihydrazone
ligand, but this still remains tentative unless confirmed by X-ray
crystallography. The complex shows insignificant weight loss in the
temperature range 268e305 ꢁC. After 305 ꢁC, significant and sharp
weight loss occurs which continues upto 350 ꢁC. The weight loss in
this temperature range is equal to 71.73% which corresponds to loss
of one ligand molecule devoid of 2 oxygen atoms, (theo: 72.54%)
which combine with one atom of manganese to give MnO2. MnO2 is
obtained at 350 ꢁC whose mass is 21.52% (theo: 21.91%). After this,
Table 1
Complex, colour, decomposition point and molar conductance for manganese(II) and (III) complexes derived from disalicyaldehyde oxaloyldihydrazone (H4slox).
Sl.
No.
Complex and colour
M.P/D.P
(ꢁC)
Elemental Analysis Found (calcd.) %
Magnetic Moment
meff(BM)
Molar Conductance
ꢀ1cm2molꢀ1
U
Mn
C
H
N
1
2
3
4
5
6
[Mn(H2slox)]$H2O Light yellow
˃305
˃295
˃298
˃300
˃300
13.58
(13.83)
12.32
(11.99)
11.37
(11.63)
12.00
(11.63)
11.29
(11.63)
10.37 (9.92) 35.21
(34.73)
48.83
(48.38)
55.48
(55.03)
56.41
(55.94)
55.48
(55.94)
56.32
(55.94)
3.58
(3.55)
3.78
(3.74)
4.08
(4.05)
4.02
(4.05)
4.02
(4.05)
4.96
14.53
(14.10)
14.92
(15.28)
14.12
(14.83)
14.51
(14.83)
15.29
(14.83)
10.03
0.86
2.01
2.09
2.07
1.83
4.95
0.9
1.7
2.2
2.5
1.8
75
[Mn(H2slox)(py)] Yellowish brown
[Mn(H2slox)(
[Mn(H2slox)(
[Mn(H2slox)(
a-pic)] Yellowish brown
b-pic)] Yellowish brown
g-pic)] Yellowish brown
Na(H2O)4$[Mn(slox)(H2O)2]$2.5H2O Dark ˃300
brown
(4.92)
(10.13)

346
I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
slow mass loss occurs along with the evaporation of the resulting
product under reducing atmosphere of dinitrogen. At the temper-
ature of about 800 ꢁC, the mass vanishes. It is imperative to
mention that the fact that the residue obtained is solely due to
MnO2 only, has been confirmed by recording its IR spectra (Fig. S2)
which matches with that of the authentic sample. The decompo-
sition behaviour of the remaining complexes seems to be almost
same except the complex (6). After initial loss of surface solvent
molecules, a significant weight loss step occurs in the temperature
ranges 285e363 ꢁC, 290e365 ꢁC, 297e368 ꢁC and 300e370 ꢁC in
the complexes (2) (Fig. S3), (3), (4)and (5), respectively. The weight
losses in these temperature ranges are 70.25%, 77.25%, 77.61%,
71.64%, respectively, which correspond to the loss of ligand mole-
cule devoid of four oxygen atoms (2), two oxygen atoms (3, 4) and
four oxygen atoms (5), respectively together with loss of one
The low-spin d5system possesses one unpaired electron, hence
should show magnetic moment values around 1.73 mB. However,
due to orbital contribution, the magnetic moment increases [29],
and has been reported to span the range 1.7e2.30 mB [28]. The
experimentally observed values of the magnetic moment for the
complexes (2e5) suggest that they are all low-spin at room tem-
perature with orbital contribution to some extent.
On the other hand, the magnetic moment value of the complex
(1) is 0.86 mB. This value is consistent with low-spin character of the
complex. The magnetic moment value of the complex should be
around the value for one unpaired electron, however, when orbital
contribution is taken into consideration, the mB value for complex
(1) should fall around 2.18 mB [29]. This value suggests considerable
magnetic exchange interaction within the structural unit of the
complex. The magnetic moment of the complex (1) is considerably
reduced as compared to that expected on the no interaction basis.
Such a decrease in mB value might occur either due to super ex-
change via overlap of the metal orbitals with the orbitals of the
bridging oxygen atoms of phenolate group [29] or due to direct
overlap of metal orbitals of one structural unit with the metal or-
bitals of other structural unit. The lower value of magnetic moment
of complex (1) reveals a square planar environment in which metal
has the ground term configuration 4A1g(b22ge2ga11g).
molecule of pyridine or substituted pyridines (theo: 71.22% (2),
a-
picoline (theo: 78.57%), -picoline (theo: 78.59%), and -picoline
b
g
(theo: 72.04%) in each of the complexes, respectively [27]. The re-
sidual products so obtained have been cheracterized by IR spec-
troscopy. The IR spectra (Fig. S4) of the residues in complexes
(2)e(5) match with an authentic sample of MnO2 which confirms
that the residual products in these complexes are MnO2. However,
experimental weight losses in complexes (2) and (5) were found to
be 23.68% and 23.13%, respectively, which was higher than that of
MnO2 (theo: 18.27% in complex (2) and theo: 17.76% in complex
(5)). This indicated that in these complexes, the end products
contain some carbonaceous material which gets oxidized at higher
temperature only. But the formation of MnO2 is not indicated at any
stage distinctly as there is a continuous weight loss after 363 and
370 ꢁC until the weight vanishes at 900 ꢁC a temperature at which
the heating process comes to stop. On the other hand, the end
products obtained in complexes (3) and (4), corresponded to MnO2
(exp: 18.25%, theo: 17.76%) for complex (3), and (exp: 17.37%, theo:
17.76%) for complex (4), respectively. After 365 and 368 ꢁC, the
oxides lose weight slowly and slowly until and unless at tempera-
tures of ~900 ꢁC, the weight vanishes completely. The complex
Na(H2O)4[MnIII(slox)(H2O)2].2.5H2O shows completely different
decomposition behaviour as compared to the remaining manga-
nese complexes (Fig. S3). The complex decomposes in three major
steps in the temperature ranges 50e100 ꢁC, 100e300 ꢁC and 300 ꢁC
onwards, respectively. A mass loss of about 8.19% is observed in the
temperature range 45e100 ꢁC which corresponds to loss of 2.5H2O
molecules (theo: 8.19%). The loss of these water molecules in this
temperature range indicates their presence in the lattice structure
of the complex. Another mass loss commences at 100 ꢁC and con-
tinues upto 300 ꢁC. The mass loss in this temperature range is equal
to 19.12% suggesting loss of 6H2O molecules (theo: 19.50%). The loss
of these six H2O molecules in the temperature range 100e300 ꢁC
reveals their presence in the first coordination sphere around the
metal centre. After 300 ꢁC, the decomposition of dihydrazone
ligand starts which continues upto 900 ꢁC. In the temperature
range 300e900 ꢁC, the mass loss is 45.85% which corresponds to
loss of ligand molecule devoid of four oxygen atoms (46.75%). The
residue obtained corresponds to NaMnO4 (exp: 27.05%, theo:
25.63%) which has been confirmed by recording its IR spectrum
which matches with that of an authentic sample of NaMnO4
(Fig. S6). It is imperative to mention that complex (6) does not show
its stability at any stage of it decomposition once the process has
commenced.
Manganese(II) has the highest spin-pairing energy [30] among
bivalent 3d metal ions and therefore only very strong-field ligands
can induce low-spin character. Hence, low-spin manganese com-
plexes are relatively uncommon [31]. A significant finding of our
work shows that hydrazonato O, N coordination can be strong
enough and effective in sustaining spin pairing.
The magnetic moment value for the complex (6) is 4.95 mB
,
which falls in the range expected for high-spin Mn(III) complexes
with ground state configuration (t32g eg1, S ¼ 2). This value dismisses
the possibility of any metal-metal interaction in the structural unit
of the complex.
3.4. UV visible spectra
The electronic spectral bands for dihydrazone and metal com-
plexes have been listed in Table 2 along with their molar extinction
coefficients.
The free dihydrazone ligand H4slox shows two absorption bands
at 304 and 340 nm. The first band at 304 nm is attributed to
transition while the band at 340 nm is attributed to n/
p/p*
* tran-
p
sition. The band at 340 nm is characteristic of salicylalamine part of
the ligand. This band has been reported in many mono-
acylhydrazones [18,19].
In addition to the ligand bands, the complexes possess an
additional band in the region 405e430 nm. The molar extinction
coefficient of this band lies in the region 1525-
2500 molꢀ1dm3cmꢀ1. This band has much higher value of the
molar extinction coefficients than the usual values for d-d transi-
tion bands. This band may be assigned to ligand-to-metal charge-
transfer transition arising from transfer of charge, most probably,
from phenolate oxygen atoms to Mn(II) centre. The d-d transitions
in the Mn(II) complexes are both spin as well as laporte forbidden
[30], and hence, they have very weak intensity. However, the band
in the region 405e430 nm may have contribution from the d-
d transitions centred on the Mn(II) metal centre (Fig. S7). It is
imperative to mention that the crystal field of any symmetry is
unable to split the Mn(II) d5 high-spin configuration having a 6S
ground term. Usually high-spin Mn(II) complexes show d-d transi-
tion rarely.
3.3. Magnetic moment
The complexes (1e5) have magnetic moment values (Table 1) in
the range 0.86e2.09 B M at 298 K. This shows that the complexes
have low spin character and idealized t52 (MnII) configuration [28].
The complex (1) shows a broad spectrum in the region
385e420 nm with a shoulder at 410 nm. Its molar extinction co-
efficient is 1797 U
ꢀ1cm2molꢀ1 while the band at 400 nm is assigned

I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
347
Table 2
Electronic spectral bands and EPR data for manganese(II) and (III) complexes derived from disalicyaldehyde oxaloyldihydrazone (H4slox).
Sl. No.
Complex
Electronic spectral band
EPR parameters
DMF solution lmax(εmax) nm(dm3molꢀ1cmꢀ1
)
Temp.
Solid/Solution
g-Value
Amn (G)
1
[Mn(H2slox)]$H2O
306 (15725),
344 (19975),
410 (1797)
RT
LNT
RT
Solid
Solid
DMSO
DMSO
2.007
2.007
g1/2 ¼ 1.994
g1 ¼ 2.622
g3/2 ¼ 4.168
1.868
2.013
2.013 (2.007)
2.007
g1 ¼ 2.121
g2 ¼ 2.058
g3 ¼ 2.004
2.007
e
e
84
e
e
78
e
e
e
e
e
e
76
e
e
e
e
e
e
80
e
e
e
e
e
e
e
80
e
e
e
e
e
e
e
84
e
e
e
e
LNT
2
3
4
[Mn(H2slox)(py)]
306 (18240),
344 (23920),
412 (1535)
RT
LNT
RT
Solid
Solid
DMSO
DMSO
LNT
[Mn(H2slox)(
[Mn(H2slox)(
a
-pic)]
-pic)]
307 (20780),
345 (28750),
418 (1525)
RT
LNT
RT
Solid
Solid
DMSO
DMSO
1.997
1.997
g1/2 ¼ 2.025
g3/2 ¼ 4.198
g1 ¼ 2.120
g2 ¼ 2.041
g3 ¼ 1.919
1.994
LNT
b
306 (27060),
344 (34560),
405 (2100)
RT
LNT
RT
Solid
Solid
DMSO
DMSO
1.994
g1/2 ¼ 2.025
g3/2 ¼ 4.168
g1 ¼ 2.118
g2 ¼ 2.038
g3 ¼ 1.915
1.994
LNT
5
[Mn(H2slox)(
g-pic)]
306 (18900), 344 (23900), 412 (2500)
RT
LNT
RT
Solid
Solid
DMSO
DMSO
1.994
1.982
g1/2 ¼ 1.994
g3/2 ¼ 4.221
g1 ¼ 2.132
g2 ¼ 2.041
g3 ¼ 1.918
2.007
2.006
2.006
feature less
2.006
LNT
6
7
Na(H2O)4$[Mn(slox)(H2O)2]$2.5H2O
306 (9450),
354 (8320),
370 (5820),
430 (4060),
470
RT
LNT
RT
Solid
Solid
DMSO
DMSO
LNT
H4slox
304 (13130),
341 (16990)
e
e
e
e
to ligand-to-metal charge transfer transition, the shoulder at
410 nm is assigned to d-d transition from the partially spin-paired
ground term.
(Fig. S9) as Nujol mulls and appears as a shoulder on the main peak
at 430 nm which maximizes in the ultraviolet region. The peak at
470 nm seems to be a combination of metal-to-ligand charge-
transfer and d-d transitions. As might be expected, the spectral
feature of this complex is very similar to those reported in the
literature [30]. It, therefore, seems plausible to propose that the low
energy shoulder at 470 nm is d-d transition. In octahedral notation,
this would correspond to 5Eg/5T2g or its components in one or
another lower symmetry group [30]. The appearance of only one
shoulder in this region is, probably, due to greater absorption in the
UV for the complex. The spectra possessed the same features as that
in the solution state except that shoulder was more pronounced
and shifted slightly to longer wavelength at 490 nm.
4A1g(b22ge1ga11g) to the b11g (dxy) orbital i.e. (b2g 2e1ga11g)/b22ga11gb11g
This reveals a square-planar environment for Mn (II) centre.
The high-spin Mn(III) complex (6) shows a broad band centred
at 430 nm in addition to ligand band at 304 and 354 nm (Fig. S8).
The band at 430 nm may be attributed to ligand to metal charge
transfer transition because molar extinction coefficient of this band
is 8060 molꢀ1dm3cmꢀ1. As this band is very broad in character,
hence it is attributed to mask any weak band due to d-d transition
centred on the Mn(III) metal. It is worth noting that the band in this
region occurs in the range 405e412 nm in the complexes (2) to (5)
while in the present complex occurs at 430 nm. This indicates the
presence of extended conjugation in the ligand backbone in the
complexed state resulted from enolization of the ligand. Because of
the presence of the extended conjugation in the complex (6), the
ligand band at 340 nm is also red shifted by 15 nm and appears at
354 nm characteristic of bonding of dihydrazone to the metal
centre in the enol form.
3.5. Electron paramagnetic resonance spectroscopy
The X-band EPR spectra for the manganese(II) complexes have
been recorded at RT and LNT in powder as well as in CH3CN - DMSO
solution. The various EPR parameters for the complexes have been
set out in Table 2. EPR spectra for some of the complexes have been
given in (Figs. S10e12). The complexes (2) to (5) show isotropic
spectrum in polycrystalline form at RT as well as at LNT. The gav
Further, the complex (6) displays a broad absorption maxima
with a shoulder at 470 nm in the UV visible region in the solid state

348
I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
values for the complexes (2) to (5) fall in the regions 1.994e2.013
and 1.982e2.013 respectively at RT and LNT, in polycrystalline form.
One remarkable aspect of ESR spectra is that gav values for the
complexes in the polycrystalline form at RT and LNT are almost
same. This reveals that the complexes have almost similar stereo-
chemistry at RT as well as LNT in polycrystalline state.
d
-OH,
ligand exists in staggered configuration. The signals at
and 11.09 ppm disappear on complexation. This reveals bonding
d
-NH and
d
-CH]N signals as singlet suggests that the free
d
12.78 ppm
d
of phenolate oxygen via deprotonation and carbonyl oxygen via
enolization cum deprotonation to the metal centre. The most
remarkable feature of 1H NMR spectrum of complex is the
The manganese(II) complexes (2) to (5) show EPR signal in
DMSO solution at RT, one signal is strong and appears at
1.994e2.025 while the other signal is weak and appears in the
region 4.168e4.221. The signal in the region 4.168e4.221 disap-
pears at LNT and only the signal in the region 1.868e2.007 remains
persistently and becomes strong in intensity. The CH3CN-DMSO
glass of the complexes at LNT gives a well resolved spectrum with
six hyperfine lines [55Mn, I ¼ 5/2] with metal hyperfine coupling
constant in the region 76e84 G. A value spans 75e100 G, the
90e100 G domain being more frequented [32]. Thus the hyperfine
coupling constants for the present manganese(II) complexes fall in
appearance of two signals at
sponding to -CH]N protons as compared to
8.89 ppm in free dihydrazone ligand. The azomethine proton
signal shifts on an average to upfield by 0.19 ppm which indicates
that the azomethine nitrogen atom is bonded to metal atom and
that the phenyl ring electron density flows to Mn through azo-
methine nitrogen atom. The splitting of d-CH]N- proton signals
suggests bonding of dihydrazone to the metal centre in anti-cis
configuration as compared to that of dihydrazone which exist in
d
8.31 ppm and
d
9.10 ppm corre-
d
a
singlet at
d
staggered configuration in free state. The signal at
attributed to axial azomethine proton while the signal at
to equatorial proton.
d
8.31 ppm is
9.10 ppm
d
the lower limit of the reported ranges. The strong dp-pp metal-
ligand interactions might be responsible for these lower values of
the experimentally observed hyperfine coupling constants. The
hyperfine split signals in these complexes are more intense than
those in the high-spin manganese(II) complexes, a feature charac-
teristic of low-spin manganese (II) complexes.
3.7. IR spectra
Some structurally significant IR bands for the free dihydrazone
and the metal complexes have been set out in Table 3.
The complexes (2) to (5) at room temperature exhibit an intense
feature centred in the region 1.994e2.025 and a weak feature in the
region 4.168e4.221 in CH3CN-DMSO glass. When the complex is
cooled, the signal in the region 4.168e4.221 further weakens in
A peak to peak comparison of the IR spectra of the complexes
with that of the free dihydrazone shows that the complexes (1)e(5)
possess ligand in the keto form while the complex (6) contains
ligand in the enol form. This is evident from the fact that the
intensity and finally vanishes at 77 K. The fact that the
D
Ms ¼ 2
n
(OH þ NH) bands occurring in the region 3206-3435 cmꢀ1 in the
forbidden transition appears at RT indicates that the ground state at
RT is not a pure doublet pair state. Instead, it is a mixture of doublet
and quartet state [33]. As the low-field signal disappears, at low
temperature, it suggests that the complex has a pure doublet
ground state at low temperature. The weak signal in the region
4.168e4.221 in the complexes (2) to (5) may be related with a
resonance with the excited manifold arising from the interaction
between the quartet excited state of the complexes and doublet
ground state of the other.
ligand are either slightly changed in their position or remain almost
upshifted [23]. Further, the carbonyl bands appearing at
1669 cmꢀ1in the free dihydrazone also remains almost unshifted in
position in complexes. This dismisses the possibility of coordina-
tion of NH and >C]O groups to the metal centre. However, their
position does not rule out the possibility of involvement of NH and
>C]O groups in the H-bonding in metal complexes asin free
dihydrazone ligands. The bands at 1620 (s); 1603 (s); 1536 (s); and
1276 (m); 1262 (s) in free ligands attributed to
n(C]N); Amide
A poorly resolved rhombic spectrum is obtained in DMSO glass
at 77 K for the complex (2) while fully resolved rhombic spectra in
the complexes (3)e(5). The three g-components for the spectra of
the complex further split due to hyperfine coupling to give complex
spectral features characteristic of the low-spin state of man-
ganese(II) complexes. In contrasts, the high-spin manganese(II)
complexes show only very broad room temperature spectra
spreading over g ¼ 2e5 and very complex low temperature (77 K)
spectra over a very wide range due to zero field splitting in the
distorted metal environment.
The ESR features of the complex (6) are essentially different
from those of the other complexes. This complex has similar
asymmetrical isotropic spectra at RT and LNT in polycrystalline
form as well as at LNT in DMSO glass. The average gav value is 2.006.
This shows that under these conditions, the complex has similar
stereochemistry. On the other hand, the complex has a featureless
spectrum at RT in DMSO solution. All the features of the EPR spectra
reveal that the complex is a high spin Mn (III) complex.
II þ (CeO)(phenolic) and (CeO) (phenolic) also retain their po-
n
b
sition in the complexes. They are involved in hydrogen bonding as
H > C]N—HeOe. It appears that in these complexes the bonding of
>C]N and phenolic OeO occur to the metal centre accompanied by
breakdown of H e bond [21]. The strength of the bond H > C]
N/MneOe in complexes is almost same as that in the hydrogen
bonding in uncoordinated free dihydrazone. On examining the
spectra of the ligand and its complexes below 600 cmꢀ1, a new
weak band in the region 486-503 cmꢀ1 is assigned to
O)(phenolate). The
complexation reaveling the bonding of the ligand to the metal
centre in enol form. This is also corroborated by the fact that the
n(M-
n
C ¼ O band in the complex (6) disappears on
strong ligand at 1536 cmꢀ1 due to amide II þ
n(CeO)(phenolic)
becomes stronger and broader in nature as compared to that in the
free ligand. This indicates the origin of this band due to new -NCOe
group [34] produced as a result of enolization of ligand. Further, in
the complex (6), an additional new weak band is observed at
462 cmꢀ1 in the low frequency region which shows bonding be-
tween enolate oxygen atoms and sodium metal centre [35]. It is
3.6. 1H NMR spectra
imperative to mention that
complexes which reveals the coordination of dihydrazone to the
n
C ¼ N band appear as a doublet in all
The 1H NMR spectrum of free dihydrazone and Na
[Mn(slox)(H2O)6]2$5H2O (6) were recorded in DMSO d6 solution
because of solubility regions.
metal centre in anti-cis configuration.
3.8. Cyclic voltammetry
1H NMR spectrum of ligand shows signals at
d
12.78,
8.89 ppm respectively. These signals are assigned to
-CH]N- protons, respectively. triplet in the region
d
11.09 and
d
d-OH, -NH,
d
The redox properties of the complexes have been studied by
cyclic voltammetry (CV) using a platinum working electrode in
DMF under dinitrogen atmosphere with 0.1 M TBAP as a supporting
d
A
6.50e8.00 ppm is attributed to phenyl protons. The appearance of

I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
349
Table 3
Structurally significant IR spectral bands of disalicyaldehyde oxaloyldihydrazone (H4slox) and it manganese(II) and (III) complexes.
Sl. No.
1
Complex
n
(OH þ NH)
n
(C]O)
n
(C]N)
Amide II þ
n(CeO) (phenolic)
b
(CeO) phenolic
n(M-O)
[Mn(H2slox)]$H2O
3427(s)
3278(s)
3202(s)
3432(s)
3278(s)
3206(s)
3394(s)
3275(s)
3206(s)
3278(s)
3206(s)
3435(s)
3280(s)
3207(s)
3435(s)
1667(s)
1668(s)
1668 (s)
1621(s)
1603(s)
1536 (s)
1275(s)
1262(s)
486
(w)
2
3
[Mn(H2slox)(py)]
1620 (ssh)
1600(s)
1535 (s)
1536 (s)
1275(w)
1262(s)
485 (w)
502 (w)
[Mn(H2slox)(
a
-pic)]
1620 (ssh)
1603 (s)
1278 (w)
1265 (s)
4
5
[Mn(H2slox)(
b
-pic)]
-pic)]
1667(s)
1668(s)
1619(s)
1603(s)
1619(s)
1603(s)
1535 (s)
1535 (s)
1275(w)
1262(s)
1275(w)
1262(s)
487 (w)
488 (w)
[Mn(H2slox)(
g
6
Na(H2O)4$[Mn(slox)(H2O)2]$2.5H2O
e
1624(s)
1608(s)
1620(s)
1603(s)
1536 (s)
1536 (s)
1278(s)
503 (w)
H4slox
3434(s)
3278(s)
3207(s)
3150(s)
1669(s)
1276(m)
1262(s)
e
Table 4
Electrochemical parameters for manganese(II) and (III) complexes derived from disalicyaldehyde oxaloyldihydrazone (H4slox) (Potential vs Ag/AgCl) at a scan rate of 100 mV/s.
Sl. No.
1
Compound
Reduction Epc(V)
Oxidation Epa(V)
DE
[Mn(H2slox)]$H2O
e
þ0.54
þ0.28
e
290
e
ꢀ0.01
ꢀ0.85
ꢀ1.40
þ0.26
ꢀ0.57
ꢀ1.20
þ0.10
ꢀ0.56
ꢀ1.27
þ0.20
ꢀ0.42
ꢀ1.06
e
400
ꢀ1.00
þ0.50
e
2
3
4
[Mn(H2slox)(py)]
240
e
ꢀ1.00
þ0.40
e
200
300
e
[Mn(H2slox)(
[Mn(H2slox)(
a
-pic)]
-pic)]
ꢀ1.21
þ0.69
e
60
410
e
b
ꢀ0.58
ꢀ1.20
þ0.75
e
e
e
5
[Mn(H2slox)(
g
-pic)]
e
e
ꢀ0.58
e
e
þ0.53
ꢀ0.90
þ0.68
þ0.33
þ0.10
þ0.52
ꢀ0.95
e
ꢀ1.11
e
210
e
6
7
Na(H2O)4$[Mn(slox)(H2O)2]$2.5H2O
e
e
ꢀ0.13
e
230
e
H4slox
ꢀ1.32
370
electrolyte at several scan rates. The data have been presented in
Table 4 at scan rate 100 mV/s.
(ꢀ1.06 V) to (ꢀ1.27 V) and oxidative wave in the region (ꢀ1.00) to
(ꢀ1.20) appear very near to the reductive wave at ꢀ1.32 V and
oxidative wave at ꢀ0.95 V in the free ligand, respectively. Hence,
the reductive and oxidative waves in the regions (ꢀ1.06 V) to
(ꢀ1.27 V) and (ꢀ1.00 V) to (ꢀ1.21 V), respectively, are very close to
the reductive and oxidative waves at (ꢀ1.32 V) and (ꢀ0.95 V) in the
uncoordinated ligand. Hence, these waves in the metal complexes
arise due to consistently ligand centred electron transfer reactions.
The complexes (2) to (5) consistently show an irreversible reductive
wave in the region (ꢀ0.42 V) to (ꢀ0.58 V). This reductive wave does
not have its counterpart in the anodic scan either in the free ligand
or in the complexes. This is consistent with the reduction product
undergoing chemical reactions over the longer time scales. This
suggests that the species corresponding to this wave are unstable in
DMF and do not survive long in the DMF solution either in the free
ligand or in the metal complexes. Hence the reductive wave in the
region (ꢀ0.42 V) to (ꢀ0.58 V) corresponds to generation of a ligand
based phenoxyl radical [36]. Hence, this irreversible wave is also
The complexes (2) to (4) display three reductive waves in the
regions (-0.10 V) to (þ0.26 V), (ꢀ0.42 V) to (ꢀ0.57 V), (ꢀ1.06 V) to
(ꢀ1.27 V) and two oxidative waves in the regions (þ0.40 V) to
(þ0.69 V) and (ꢀ1.00 V) to (ꢀ1.21 V), respectively. Apart from these
oxidative waves, the complex (4) shows another additional oxida-
tive wave at ꢀ0.58 V. On the other hand, the complex (1) also shows
three reductive waves at ꢀ0.01 V, ꢀ0.85 V and ꢀ1.40 V and also
three oxidative waves at þ0.54 V, þ0.28 V and ꢀ1.00 V, respectively
(Fig. S13). But only two reductive peaks at ꢀ0.50 V and ꢀ1.13 V are
observed in the complex (5) while three oxidative peaks
at þ0.75 V, þ0.53 V and ꢀ0.92 V, respectively (Fig. S14). It may be
worthwhile to examine the redox potential of the free ligand viz.
H4slox. It displays one reductive wave at ꢀ1.32 V and two oxidative
waves at þ0.52 V and ꢀ0.95 V, respectively. A comparison of the
nature of the voltammograms of the complexes with that of the
free dihydrazone confirms that the reductive wave in the region

350
I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
Fig. 2. Tentatively proposed structure of complex [Mn(H2slox)(A)] (A ¼ py (2), 2-pic
Fig. 1. Tentatively proposed structure of complex [Mn(H2slox)].H2O (1).
(3), 3-pic (4), 4-pic (5)).
considered to arise from ligand centred electron transfer reaction.
This wave does not have its counterpart in the anodic scan either in
the free ligand or in the complexes. It is imperative to mention that
with the highly negatively charged dihydrazone ligand bonded to
the metal centre, it is expected to help make the reduction of the
metal centres unfavourable, leading to quite negative Epc values
[37]. Apart from these waves, the complexes (2) to (4) invariably
show a new reductive wave in the region (-0.10 V) to (þ0.26 V) and
the corresponding oxidative wave in the region (þ0.40 V) to
(þ0.69 V). This wave is assigned to metal centred electron transfer
reaction. It is worth noting that the uncoordinated ligand shows an
oxidative wave at þ0.52 V which is, most probably, due to oxidation
of >C]N group [38]. This wave appears to be merged with the
metal centred oxidative wave in the region þ0.40 V to þ0.69 V in
the complexes. The difference between reductive and oxidative
waves falls in the region 290e470 mV which is much more than
one electron uncomplicated electron transfer reaction. This reveals
that the metal centred electron transfer reaction is either quasi-
reversible or irreversible. The large peak separation originates,
most probably, from a slow heterogeneous one electron exchange
rather than from intervening homogeneous reaction [39]. The
electron transfer reaction may be represented as below
counterpart in the cathodic scan. It appears that the species pro-
duced corresponding to this wave is unstable and does not survive
long in DMF and reverts back to original species. This wave may be
attributed to generation of species corresponding to oxidation of
˃C]N- group in the complex.
The complex [Mn(H2slox)(g-pic)] (5) shows only two reductive
waves at ꢀ0.60 V and ꢀ1.13 V respectively. The reductive at ꢀ0.60 V
does not have its oxidative counterpart in the reverse scan. This
suggests that the species produced corresponding to this reductive
wave are unstable and do not survive long in the DMF medium and
revert to the original species. On the other hand, the reductive
at ꢀ1.13 V has its oxidative counterpart at ꢀ0.91 V. The
DE value for
this redox couple is 220 mV which reveals its quasi-reversible na-
ture. The complex shows two more oxidative waves at þ0.75 V
and þ0.53 V, respectively. The oxidative wave at þ0.53 V is very
near to the oxidative wave at þ0.52 V in the ligand. Hence, this
wave is assigned to the electron transfer reaction centred on the
ligand. The only oxidative wave left is at þ0.75 V which may be
attributed to metal centred electron transfer reactions.
This complex (6) shows only one reductive wave at ꢀ0.31 V in
the forward scan while three oxidative waves at þ0.78 V, þ0.34 V
and þ0.10 V in the reverse scan, respectively (Fig. S15). The
reductive wave at ꢀ0.31 V corresponds to the oxidative wave
h
i
h
iꢀ
þe
ꢀe
II
ðH2sloxÞMnIðAÞ
*
at þ0.10 V. The
DE value for this redox couple is 230 mV, which
ðH2sloxÞMn ðAÞ )
suggests its quasi-reversible nature. The other two oxidation waves
at þ0.78 V and þ0.34 V do not have their counterparts in the for-
ward scan (cathodic scan). Hence, these waves may be attributed to
ligand centred electron transfer reactions. As these oxidative waves
do not have their counterparts in the cathodic scan, the species
produced corresponding to them do not survive long in the DMF
solution and revert back to their original species.
hꢀ
ꢁ
i
2ꢀ
þe
)
H2sloxꢀ MnIðAÞ
*
ꢀe
In the complex (1), the reductive waves at ꢀ0.85 V and ꢀ1.42 V
along with it oxidative counterpart at ꢀ1.02 V may be attributed to
ligand centred electron transfer reactions. Their origin may be
explained in the same way as in the complexes (2) to (4). The
reductive wave at ꢀ0.03 V and its oxidative counterpart at þ0.31 V
may be related to the electron transfer reaction centred on the
On the basis of various physico e chemical and spectroscopic
studies the complexes have, tentatively, been proposed to have the
following structures (Figs. 1e3).
metal centre. The
DE value for this redox couple is 340 mV which
indicates its either quasi-reversible or irreversible nature. The
oxidative wave at þ0.54 V in the anodic scan does not have its

I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
pp. 527e557.
351
[2] S.J. Smith, K.S. Hadler, G.R. Hanson, G. Schenk, N. Mitic, Binuclear Mn-
Dependent enzymes, in: G.R. Hanson, L.J. Berliner (Eds.), Biological Magnetic
Resonance: High Resolution EPR: Applications to Metalloenzymes and Metals
in Medicine, vol. 29, Springer, NewYork, 2009.
[3] V.J. Leveque, M.E. Stroupe, J.R. Lepock, D.E. Cabelli, J.A. Tainer, H.S. Nick,
D.N. Silverman, Biochemistry 39 (2000) 7131.
[4] N.G. Connelly, K.A. Hassard, B.J. Dunne, A.G. Orpen, S.J. Raven, G.A. Carriedo,
V. Riera, J. Chem. Soc. Dalton Trans. 26 (1988) 1623.
[5] P.T. Manoharan, H.B. Gray, Chem. Commun. (1965) 324.
[6] S. Scheuermayer, F. Tuna, M. Bodensteiner, M. Scheer, R.A. Layfield, Chem.
Commun. 48 (2012) 8087.
[7] A. Jezierski, J. Mol. Struct. 115 (1984) 11.
[8] (a) P. Basu, A. Chakravorty, Inorg. Chem. 31 (1992) 4980;
(b) P. Basu, A. Chakravorty, J. Chem. Soc. Chem. Commun. 11 (1992) 809;
(c) S. Ganguly, S. Karmakar, C.K. Pal, A. Chakraborty, Inorg. Chem. 38 (1999)
5984;
(d) S. Chattopadhyay, P. Basu, S. Pal, A. Chakravorty, J. Chem. Soc. Dalton Trans.
29 (1990) 3329.
[9] A. Saha, P. Majumdar, S. Goswami, J. Chem. Soc. Dalton Trans. (2000) 1703.
[10] (a) A.P. Forshaw, J.M. Smith, A. Ozarowski, J. Krzystek, D. Smirnov,
S.A. Zvyagin, T.D. Harris, H.I. Karunadasa, J.M. Zadrozny, A. Schegg,
K. Holldack, T.A. Jackson, A. Alamiri, D.M. Barnes, J. Telser, Inorg. Chem. 52
(2013) 144;
(b) E.A. Osorio, K. Moth-Poulsen, H.S.J. Zant, J. Paaske, P. Hedegard,
K. Flensberg, J. Bendix, T. Bjornholm, Nano Lett. 10 (2010) 105;
(c) F.A. Cotton, G. Wilkinson, P.L. Gaus, Basic Inorganic Chemistry, second ed.s.,
John Wiley and Sons, 1987;
(d) S. Amabilino, R.J. Deeth, Inorg. Chem. 55 (2016).
[11] (a) D. Basumatry, R.A. Lal, A. Kumar, J. Mol. Struct. 1092 (2015) 122;
(b) R.A. Lal, D. Basumatary, A.K. De, A. Kumar, Transit. Mater. Chem. 32 (2007)
481.
[12] (a) B.D. Wang, Z.Y. Yang, P. Crewdson, D.Q. Wang, J. Inorg. Biochem. 101
(2007) 1492;
Fig. 3. Tentatively proposed structure of complex Na(H2O)4[Mn(slox)(H2O)2].2.5H2O
(6).
(b) J. Patole, W. Sandbhor, S. Padhye, D.N. Deobagkar, C.E. Anson, A. Powell,
Bioorg. Med. Lett. 13 (2003) 51;
(c) C.D. Fan, H. Su, J. Zhao, B.X. Zhao, S.L. Zhang, J.Y. Miao, Eur. J. Med. Chem. 45
(2010) 1438;
(d) S. Pasayat, S.P. Dash Saswati, P.K. Majhi, Y.P. Patil, M. Nethaji, H.R. Dash,
S. Das, R. Dinda, Polyhedron 38 (2012) 198.
4. Conclusion
[13] (a) S. Rollas, N. Gulerman, H. Erdeniz, II Farm. 57 (2002) 171;
(b) J.V. Ragavendran, D. Sriram, S.K. Patel, I.V. Reddy, N. Bharathwajan,
J. Stables, P. Yogeeswari, Eur. J. Med. Chem. 42 (2007) 196.
[14] (a) P. Liang, Y. Liu, L. Guo, J. Anal, At. Spectrom. 19 (2004) 1006;
(b) Y.C. Sun, J.Y. Yang, S.R. Tzeng, Analyst 124 (1999) 421.
[15] (a) D.B. Kiewicz, J. Siepak, Anal. Chim. Acta 353 (1997) 85;
(b) R.J. Micic, R.M. Simonovic, B.B. Petkovic, Anal. Sci. 22 (2006) 793.
[16] V. Getautis, J.V. Grazulevicius, M. Daskeviciene, T. Malinauskas, V. Gaidelis,
V. Jankauskas, Z. Tokarski, J. Photochem. Photobiol. A Chem. 180 (2006) 23.
[17] (a) A. Bieliauskas, V. Getautis, V. Martynaitis, V. Jankauskas, E. Kamarauskas,
S. Krikstolaityte, A. Sackus, Synth. Met. 179 (2013) 27;
(b) R. Lygaitis, V. Getautis, J.V. Grazulevicius, Chem. Soc. Rev. 37 (2008) 770.
[18] R.L. Dutta, M.M. Hossain, J. Sci. Ind. Res. 44 (1985) 635.
[19] (a) M.R. Bermejo, R. Pedrido, A.M.G. Noya, M.J. Romero, M. Vazquez, L. Sorace,
New. J. Chem. 27 (2003) 1753;
Five new low-spin manganese(II) complexes and one high-spin
manganese(III) complex have been prepared and reported in the
present paper. In all complexes dihydrazone ligand is bonded to
metal centre in anti-cis configuration. In the complexes (1)e(5) the
ligand is present in the keto formwhile in complex (6) in enol form.
The dihydrazone is bonded to metal centre as dibasic tetradentate
ligand through phenolate oxygen and azomethine nitrogen atom in
the complex (1)e(5) while as tetrabasic hexadentate ligand in enol
form through phenolate oxygen atom and azomethine nitrogen
atoms to one metal centre and enolate oxygen atoms to second
metal atom. The MnII in complex (1) is square planar while in
complex (2) to (5) is five e coordinate square pyramidal. In complex
(6), both the metal centres have distorted octahedral stereochem-
istry. The complex (1) involves metal-metal interaction while no
such interaction is present in the remaining complexes. All of the
complexes show only one metal centred electron transfer reaction
in DMF solution in cyclic voltammetric studies. The tentative
structures for the complexes have been proposed.
(b) S. Pal, S. Pal, J. Chem. Soc. Dalton Trans. (2002) 2102.
[20] (a) G.A. Achazmi, A.A. Elmy, J. Coord. Chem. 62 (2009) 37;
(b) R. Dinda, P. Sengupta, S. Ghosh, W.S. Sheldrick, Eur. J. Inorg. Chem. (2003)
363.
[21] (a) S. Gupta, M.V. Kirillova, M. Fatima, C.G. da Silva, A.J.L. Pombiero,
A.M. Kirillov, Inorg. Chem. 52 (2013) 8601;
(b) H.H. Monfared, R. Bikas, P.M. Anarjan, A.J. Blake, V. Lippolis, N.B. Arslan,
C. Kazak, Polyhedron 69 (2014) 90;
(c) N.A. Lalami, H.H. Monfared, H. Noei, P. Mayer, Transit. Mater. Chem. 36
(2011) 669;
(d) S. Gopinathan, S.S. Tavale, V.G. Puranak, M.P. Degaonkar, Bull. Chem. Soc.
Jpn. 41 (1994) 1797;
Acknowledgement
(e) B. Ji, Q. Du, K. Ding, Y. Li, Z. Zhou, Polyhedron 15 (1996) 403;
(f) A. Bonardi, S. Ianelli, C. Pelizzi, G. Pelizzi, C. Solinas, Inorg. Chim. Acta 232
(1995) 211;
(g) R.A. Lal, S. Choudhury, A. Ahmed, R. Borthakur, M. Asthana, A. Kumar,
Spectrochim. Acta A 75 (2010) 212;
(h) R.A. Lal, M. Chakraborty, O.B. Chanu, S. Choudhury, R. Borthakur,
S. Copperfield, A. Kumar, J. Coord. Chem. 63 (2010) 1239;
(i) R. Borthakur, A. Kumar, R.A. Lal, Spectrochim. Acta A Mol. Biomol. Spec-
trosc. 149 (2015) 621;
Authors are thankful to the Head, SAIF, IIT, Bombay, for recording
EPR Spectra. Two of the authors I. Syiemlieh and S. D. Kurbah also
would like to thank UGC, New Delhi for the award of NFHE
Fellowship.
Appendix A. Supplementary data
(j) R. Borthakur, U. Thapa, M. Asthana, S. Mitra, K. Ismail, R.A. Lal,
J. Photochem. Photobiol. A Chem. 301 (2015) 6;
(k) A. Ahmed, R.A. Lal, J. Mol. Struct. 1048 (2013) 321.
[22] (a) H.H. Monfared, N.A. Lalami, A. Pazio, K. Wozniak, C. Janiak, Inorg. Chim.
Acta 406 (2013) 241;
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.molstruc.2017.09.052.
References
(b) C. Das, P. Adak, S. Mondal, R. Sekiya, R. Kuroda, S.I. Gorelsky,
S.K. Chattopadhyay, Inorg. Chem. 53 (2014) 11426;
(c) T. Sedaghat, M. Aminian, G. Bruno, H.A. Rudbari, J. Organomet. Chem. 737
(2013) 26;
[1] D.W. Foder, J. Hwang, J.E. Permar-Halun, in: A. Siegel, H. Siegel (Eds.), Metal
Ions in Biological Process, Marcel Dekker, Basel, Switzerland, 2000,

352
I. Syiemlieh et al. / Journal of Molecular Structure 1151 (2018) 343e352
(d) P. Paciorek, J. Szklarzewicz, A. Jasinska, B. Trzewik, W. Nitek,
137;
M. Hodorowicz, Polyhedron 87 (2015) 226;
(e) R.T. Sedaghat, L. Tahmadu, H. Motamashi, R.R. Martinez, D.M. Morales,
J. Organomet. Chem. 737 (2013) 712;
(b) J. Fujita, K. Nakamoto, M. Kobayashi, J. Am. Chem. Soc. 78 (1956) 3963.
[27] A.V. Nikolaev, V.A. Logvinenko, L.I. Myachina, Thermal Analysis, vol. 2, Aca-
demic Press, New York, 1969.
(f) J.D. Ranford, J.J. Vittal, Y.M. Wang, Inorg. Chem. 37 (1998) 1226;
(g) L. Zhao, V. Niel, L.K. Thompson, Z. Xu, V.A. Milway, R.G. Harvey, D.O. Miller,
C. Willson, M. Leech, J.A.K. Howard, S.L. Heath, J. Chem. Soc. Dalton Trans.
(2004) 1446;
(h) S. Roy, T.N. Mandal, A.K. Barik, S. Pal, R.J. Butcher, M.S. Fallah, J. Tercero,
S.K. Kar, Dalton Trans. (2007) 1229;
[28] B.N. Figgis, Trans. Faraday Soc. 57 (1961) 204.
[29] U. Knof, T. Weyhermuller, T. Wolter, K. Wieghardt, J. Chem. Soc. Chem.
Commun. (1993) 726.
[30] A.B.P. Lever, Inorganic Electronic Spectrosocpy, second ed.s., Elsevier, New
York, 1984, p. 75.
[31] S. Karmakar, S.B. Choudhury, A. Chakravorty, Inorg. Chem. 33 (1994) 6148.
[32] P. Basu, S. Pal, A. Chakravorty, Inorg. Chem. 27 (1988) 1848.
[33] E. Wasserman, L.C. Synder, W.A. Yager, J. Chem. Phys. 41 (1964) 1763.
[34] M. Mashima, Bull. Chem. Soc. Jpn. 35 (1962) 423.
[35] G.C. Percy, D.A. Thornton, J. Inorg. Nucl. Chem. 34 (1972) 3369.
[36] (a) L. Benisvy, E. Bill, A.J. Blake, D. Collison, E.S. Davies, C.D. Garner, G. McArdle,
E.J.L. McInnes, J. McMaster, S.H.K. Ross, C. Wilson, Dalton Trans. (2006) 258;
(b) D. Parker, E.S. Davies, C. Wilson, J. McMaster, Inorg. Chim. Acta 360 (2007)
203.
[37] S.A. Cameron, S. Brooker, Inorg. Chem. 50 (2011) 3697.
[38] (a) H. Okawa, J. Nishio, M. Ohba, M. Todokoro, N. Matsumoto, M. Koikawa,
S. Kida, D.E. Fenton, Inorg. Chem. 32 (1993) 2949;
(b) M. Koikawa, H. Okawa, J. Chem. Soc. Dalton Trans. (1988) 641.
[39] C.N. Verani, E. Rentschler, T. Weyhermuller, E. Bill, P. Chaudhuri, J. Chem. Soc.
Dalton Trans. (2000) 251.
(i) H.H. Monfared, A. Farrokhi, S. Alavi, P. Mayer, Trans. Mater. Chem. 38
(2013) 267.
[23] (a) X.M. Zhang, X.Z. You, Polyhedron 15 (1996) 1793;
(b) A. Bacchi, A. Bonardi, M. Carcelli, P. Mazza, P. Pelagatti, C. Pelizzi, G. Pelizzi,
C. Solinas, F. Zani, J. Inorg. Biochem. 69 (1998) 101;
(c) H. Yin, J. Cui, Y. Qiao, Polyhedron 27 (2008) 2157;
(d) T. Sedaghat, Z. Shokohi-pour, J. Coord. Chem. 62 (2009) 3837;
(e) T. Sedaghat, M. Naseh, H.R. Khavasi, H. Motamedi, Polyhedron 33 (2012)
435.
[24] A.I. Vogel, A Text Book of Quantitative Inorganic Analysis Including Elemen-
tary Instrumentation Analysis, fifth ed.s., ELBS and Longman, London, 1989,
p. 199, 367.
[25] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
[26] (a) A.C. Fabretti, C.G. Franchini, C.P. Preti, G. Tosi, Can. J. Chem. 19A (1980)