An insight into the potent antioxidant activity of a dithiocarbohydrazone appended cis-dioxidomolybdenum (VI) complexes

Article abstract of DOI:10.1002/aoc.5762

In search of antioxidants with enriched potency, the present study focus on the design and synthesis of a dithiocarbohydrazone, H₃TCL derived from thiocarbohydrazide and 3-ethoxysalicylaldehyde and its coordination complexes with molybdenum, viz, [MoO₂(HTCL)D] (1–2) (where D = methanol (1), DMSO (2)) and [MoO₂(HTCL)D]·DMF (where D = H₂O (3)). The synthesized compounds were characterised by elemental analysis, spectroscopic techniques (FT-IR, UV–vis and ¹H-NMR), conductivity measurements and cyclic voltammetry. Moreover the solid state structures of all the three complexes were established by single crystal X-ray diffraction analysis as mononuclear neutral species in which the molybdenum centre assumes a distorted octahedral geometry. The dithiocarbohydrazone binds to the molybdenum centre through its phenolate oxygen, O(1), azomethine nitrogen, N(1) and thioenolate sulfur, S(1) in a dianionic tridentate mode. The assessment of intermolecular contacts in the crystal arrangement was quantified using Hirshfeld surface analysis. Further the antioxidant potential of the dithiocarbohydrazone, H₃TCL and its molybdenum complexes 1–3 were evaluated using 1,1-diphenyl-2-picrylhydrazyl(DPPH), 2,2′-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) and total antioxidant assays. The antioxidant activities were then compared with standard antioxidant, L-ascorbic acid. The antioxidant potential of the synthesized compounds were then validated by molecular docking studies. Molecular modelling study was achieved to evaluate the recognition of target compound at the binding pocket of the human antioxidant enzyme, 3MNG. The docking results showed that the complexes selectively bond to the vital amino acids present in the binding pocket of the target enzyme, 3MNG.

Full text of DOI:10.1002/aoc.5762

Received: 10 February 2020
DOI: 10.1002/aoc.5762
Revised: 26 March 2020
Accepted: 6 April 2020
F U L L P A P E R
An insight into the potent antioxidant activity of a
dithiocarbohydrazone appended cis-dioxidomolybdenum
(VI) complexes
T.M. Asha1 | M.R. Prathapachandra Kurup1,2
1Department of Applied Chemistry,
In search of antioxidants with enriched potency, the present study focus on
the design and synthesis of a dithiocarbohydrazone, H3TCL derived from
thiocarbohydrazide and 3-ethoxysalicylaldehyde and its coordination com-
plexes with molybdenum, viz, [MoO2(HTCL)D] (1–2) (where D = methanol
(1), DMSO (2)) and [MoO2(HTCL)D]·DMF (where D = H2O (3)). The synthe-
sized compounds were characterised by elemental analysis, spectroscopic tech-
Cochin University of Science and
Technology, Kochi, Kerala, 682 022, India
2Department of Chemistry, School of
Physical Sciences, Central University of
Kerala, Tejaswini Hills, Periye, Kasargode,
Kerala, 671 320, India
Correspondence
1
niques (FT-IR, UV–vis and H-NMR), conductivity measurements and cyclic
M. R. Prathapachandra Kurup,
Department of Applied Chemistry, Cochin
University of Science and Technology,
Kochi 682 022, Kerala, India.
Email: mrpcusat@gmail.com; mrp@
cukerala.ac.in
voltammetry. Moreover the solid state structures of all the three complexes
were established by single crystal X-ray diffraction analysis as mononuclear
neutral species in which the molybdenum centre assumes a distorted octahe-
dral geometry. The dithiocarbohydrazone binds to the molybdenum centre
through its phenolate oxygen, O(1), azomethine nitrogen, N(1) and thioenolate
sulfur, S(1) in a dianionic tridentate mode. The assessment of intermolecular
contacts in the crystal arrangement was quantified using Hirshfeld surface
analysis. Further the antioxidant potential of the dithiocarbohydrazone,
H3TCL and its molybdenum complexes 1–3 were evaluated using
1,1-diphenyl-2-picrylhydrazyl(DPPH),
2,20-azinobis-3-ethylbenzothiazoline-
6-sulfonic acid (ABTS) and total antioxidant assays. The antioxidant activities
were then compared with standard antioxidant, L-ascorbic acid. The antioxi-
dant potential of the synthesized compounds were then validated by molecular
docking studies. Molecular modelling study was achieved to evaluate the rec-
ognition of target compound at the binding pocket of the human antioxidant
enzyme, 3MNG. The docking results showed that the complexes selectively
bond to the vital amino acids present in the binding pocket of the target
enzyme, 3MNG.
K E Y W O R D S
antioxidant assay-, dithiocarbohydrazone; Hirshfeld surface analysis, L -ascorbic acid, molecular
docking, X-ray diffraction analysis
Appl Organomet Chem. 2020;e5762.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5762

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ASHA AND PRATHAPACHANDRA KURUP
1 | INTRODUCTION
2 | EXPERIMENTAL SECTION
Thiocarbohydrazones are an elite class of Schiff bases
that are formed by the condensation reaction of
thiocarbohydrazide with aldehydes or ketones. Since
both the hydrazine groups of the thiocarbohydrazide are
very reactive, they serve as favourable units for the syn-
thesis of polyfunctional organic compounds, mostly bis-
derivatives with the carbonyl compounds.[1] They are
extensively used in medicinal and pharmaceutical fields
owing to their ability to exhibit biological activities like
antibacterial, antifungal, anticancer[2] etc. Due to the
availability of several potential donor sites, they find
application in the self-assemblage of tetranuclear molecu-
lar square structures, in precise, mixed valence iron(II)/
(III), Zn (II), Cd (II) and Ni (II) clusters.[3]
Molybdenum is the only second row transition
metal that exist in a varied range of metalloenzymes
and parades biological importance in various living
organisms including fungi, bacteria and algae.[4] Due
to the ability of molybdenum to chelate with O, N and
S donor ligands, a number of molybdenum complexes
with various ONO and ONS donor organic ligands
have been synthesized.[5–7] Moreover the biological
activities of such complexes were also been investi-
gated. For example, molybdenum complexes derived
from ONO donor aroylhydrazones were studied for
their in-vitro cytotoxicity and antibacterial activities.[6]
Similarly molybdenum complexes of an ONS donor
thiosemicarbazone were demonstrated to act as models
for active site of oxo-transfer molybdoenzymes.[7]
2.1 | Materials and methods
Reagents used for the synthesis are dithiocarbohydrazine
(Alfa Aesar), 3-ethoxysalicylaldehdye (Alfa Aesar) and
[MoO2(acac)2] (Sigma Aldrich). All other chemicals used
were reagent grade, available commercially and used as
received. Solvents were purchased from spectrochem and
distilled prior to use.
Elemental analyses (CHNS) of the synthesized
dithiocarbohydrazone, H3TCL and its molybdenum
complexes, 1–3 were executed on a Vario EL III CHNS
analyzer after drying the samples. Molar conductivity
measurements were performed in a 10−3 M solution of
the molybdenum complexes in DMF at room tempera-
ture using a Systronic model 303 direct reading conduc-
tivity meter. FT-IR spectra were recorded using KBr
pellets in the 4000–400 cm−1 range on a JASCO FT-IR
5300 spectrometer. Electronic spectra of all the
synthesised compounds were recorded in UV grade
DMF on Thermo Scientific Evolution 220 model UV–vis
spectrophotometer in the 200–900 nm range at ambient
temperature. The 1H-NMR spectra were recorded in
DMSO-d6 solvent on a Bruker AMX 400 FT-NMR spec-
trometer at 400 MHz and tetramethylsilane (TMS) as
internal standard. Redox properties of the compounds
were studied on CH 6017 B instrument. The measure-
ments were carried out using a three-electrode cell in
which a Pt electrode, saturated Ag/AgCl and platinum
wire are used as the working, reference and auxiliary
electrodes respectively. Tetraethylammonium perchlo-
rate (TEAP) was used as supporting electrolyte for the
electrochemical work and DMSO was chosen as the sol-
vent of analysis.
Antioxidants act as important neutraceuticals in view
of their various health benefits and find application in
food chemistry[8] due to their diverse roles in reducing
toxic effect of oxidative stress. Even though the various
biological activities of molybdenum complexes of differ-
ent Schiff bases have been investigated,[6,7,9,10] their anti-
oxidant activities seem to be much less explored.[11] In
view of these findings, the present study focuses on the
design and synthesis of a new dithiocarbohydrazone,
2.2 | Synthesis
2.2.1 | Synthesis of dithiocarbohydrazone,
H3TCL
H3TCL
derived
from
thiocarbohydrazide
and
3-ethoxysalicylaldehyde (Scheme 1) as well as its
dioxidomolybdenum (VI) complexes, 1–3 and their appli-
cation in free radical scavenging.
The
bis(3-ethoxysalicylidene)thiocarbohydrazone
(H3TCL) was prepared by the condensation reaction of
thiocarbohydrazide with 3-ethoxysalicylaldehydes in a
1:2 molar ratio (Scheme 2).
To
a solution of thiocarbohydrazide (0.265 g,
2.5 mmol) in methanol was added a methanolic solution
of 3-ethoxysalicylaldehyde (1.610 g, 5 mmol). The
resulting solution was refluxed with stirring for 3 hr in
presence of few drops of glacial acetic acid. The resulting
pale yellow solid thus formed was filtered off, washed
with methanol and dried in air.
SCHEME 1 Structure of the dithiocarbohydrazone ligand,
H3TCL

ASHA AND PRATHAPACHANDRA KURUP
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SCHEME 2 Synthetic pathway for the
dithiocarbohydrazone, H3TCL and its
dioxidomolybdenum (VI) complexes 1–3
Yield: 0.3420 g, 85%. Anal. Cal. for C19H22N4O4S
of the molybdenum precursor, [MoO2(acac)2] (0.326 g,
1 mmol) and the resulting dark red coloured solution was
refluxed with stirring for 5 hr. The dark red coloured
product thus formed was filtered, washed with methanol
and recrystallized in methanol to afford dark red block
shaped crystals of complex 1.
(402.47 g mol−1): C, 56.70; H, 5.51; N, 13.92; S, 7.97. Fou-
nd: C, 56.59; H, 5.49; N, 13.97; S, 7.99. FT-IR (KBr) νmax
,
cm−1: ν(O–H) 3414, ν(N–H) 3197, ν(C=N) 1615, 1534,
1
ν(C=S) 1370, ν(C–O) 1218. H NMR (400 MHz, DMSO-
d6, Me4Si, ppm) δ: 12.072, 11.875 (s, 2H, OH), 11.618,
9.142 (s, 2H, N–NH), 8.754, 8.546 (s, 2H, CH=N),
6.825–7.059 (m, 6H, aromatic), 4.065 (q, 4H, J = 6.4 Hz,
Yield: 0.3142 g, 56%. Molar conductance (10−3
M
DMF): 10 Ω−1 cm2 mol−1. Anal. Cal. for C20H24MoN3O7S
(560.43 g mol−1): C, 42.86; H, 4.32; N, 10.00; S, 5.72.
Found: C, 42.81; H, 4.34; N, 10.01; S, 5.75. FT-IR (KBr)
OCH2), 1.364 (t, 3H, J = 6.8 Hz CH3). UV/vis (DMF) λmax
,
nm (ε, L mol−1 cm−1): 327 (45910), 346 (45170),
363 (39290).
ν
max, cm−1: ν(O–H) 3410, ν(N–H) 3196, ν(C=N) 1595,
ν(C–S) 1016, νsym (cis-MoO2) 936, νasym (cis-MoO2) 896.
1H NMR (400 MHz, DMSO-d6, Me4Si, ppm) δ: 11.921 (s,
1H, OH), 10.440 (s, 2H, N–NH), 8.364, 8.308 (s, 2H,
CH=N), 6.811–8.284 (m, 6H, aromatic), 4.071 (q, 4H,
J = 7.2 Hz, OCH2), 1.357 (t, 3H, J = 6.8 Hz, CH3). UV/vis
(DMF) λmax, nm (ε, L mol−1 cm−1): 326 (36950), 348
(34470), 364 (30630), 421 (3000).
2.2.2 | Synthesis of molybdenum
complexes 1–3
A similar synthetic strategy was adopted for the synthesis
of dioxomolybdenum(VI) complexes (Scheme 2). The
reaction
of
[MoO2(acac)2]
with
the
dithiocarbohydrazone, H3TCL in a 1:1 ratio in presence
of donor solvents D (D = methanol (1), DMSO (2) and
DMF (3)) afforded complexes 1–3. The synthetic proce-
dures are discussed below:
Synthesis of the molybdenum complex, [MoO2(HTCL)
(DMSO)] (2)
Methanolic solution of dithiocarbohydrazone, H3TCL
(0.402 g, 1 mmol) was added to a methanolic solution of
the molybdenum precursor, [MoO2(acac)2] (0.326 g,
1 mmol) and the resulting dark orange coloured solution
was refluxed with stirring for 4 hr in presence of few
drops of DMSO. The resulting dark orange coloured solu-
tion was kept aside for evaporation. The dark orange
Synthesis of the molybdenum complex, [MoO2(HTCL)
(MeOH)] (1)
Methanolic solution of dithiocarbohydrazone, H3TCL
(0.402 g, 1 mmol) was mixed with a methanolic solution

4 of 14
ASHA AND PRATHAPACHANDRA KURUP
needle shaped crystals of complex 2 thus formed after a
week was separated and dried in air.
The structure was solved by direct methods using
SHELXS-97 and refined by full matrix least-squares
refinement on F2 using SHELXL- 2014/7 and SHELXL-
2018/7[14] on a WinGX software package.[15] The aniso-
tropic refinements were performed for all non-hydrogen
atoms and all H atoms on C atoms were placed in calcu-
lated positions, guided by difference maps, with C–H
bond distances of 0.93–0.96 Å. H atoms were assigned as
Uiso = 1.2 Ueq (1.5 for Me). The hydrogen atoms, H(3A)
and H(30) attached to nitrogen, N(3) and O(3) respec-
tively of all the complexes were located from difference
maps and their distances were restrained using DFIX
instructions with the distance restraints of N–
H = 0.88 0.01 Å and O–H = 0.86 0.01 Å respectively.
Similarly the hydrogen atoms, H(7A) and H(7B) attached
to the oxygen atom O(7) of the coordinated water mole-
cule in 3 were located from the difference map and their
distances and angles were restrained using DFIX and
DANG instructions with distance restraints of O–
H = 0.86 0.01 Å and H···H = 1.36 0.02 Å, followed
by refinement of their displacement parameters. Reflec-
tions, (1 0 0) and (1 1 0) of complex 1, (0 2 1) and (0 2 0)
of complex 2; (1 1 0) and (1 0 0) of complex 3 were omit-
ted owing to bad agreement. The molecular and crystal
structures were plotted using ORTEP 3[15] and DIA-
MOND version 3.2 g.[16]
Yield: 0.3642 g, 60%. Molar conductance (10−3
M
DMF): 7 Ω−1 cm2 mol−1. Anal. Cal. for C21H26MoN4O7S2
(606.52 g mol−1): C, 41.58; H, 4.32; N, 9.24; S, 10.57.
Found: C, 41.53; H, 4.31; N, 9.29; S, 10.59. FT-IR (KBr)
ν
max, cm−1: ν(O–H) 3410, ν(N–H) 3189, ν(C=N) 1602,
ν(C–S) 1024, νsym (cis-MoO2) 929, νasym (cis-MoO2) 902.
1H NMR (400 MHz, DMSO-d6, Me4Si, ppm) δ: 11.921 (s,
1H, OH), 10.484 (s, 2H, N–NH), 8.635, 8.307 (s, 2H,
CH=N), 6.811–7.826 (m, 6H, aromatic), 4.055 (q, 4H,
J = 6.4 Hz, OCH2), 1.359 (t, 3H, J = 6.8 Hz, CH3). UV/vis
(DMF) λmax, nm (ε, L mol−1 cm−1): 328 (34750), 348
(35320), 364 (32250), 421 (4620).
Synthesis of the molybdenum complex, [MoO2(HTCL)
(H2O)]·DMF (3)
To a solution of dithiocarbohydrazone, H3TCL (0.402 g,
1 mmol) in methanol was added the methanolic solution
of the molybdenum precursor, [MoO2(acac)2] (0.326 g,
1 mmol) and the resulting dark orange coloured solution
was refluxed with stirring in presence of few drops of
DMF for 4 hr. The resulting dark orange coloured solu-
tion was kept aside for evaporation. The dark orange nee-
dle shaped crystals of complex 3 thus formed after a week
was separated and dried in air.
Yield: 0.3410 g, 55%. Molar conductance (10−3
M
DMF): 11 Ω−1 cm2 mol−1. Anal. Cal. for C22H29MoN5O8S
(619.50 g mol−1): C, 42.65; H, 4.72; N, 11.30; S, 5.18.
Found: C, 42.67; H, 4.76; N, 11.32; S, 5.20. FT-IR (KBr)
2.4 | Hirshfeld surface analysis
ν
max, cm−1: ν(O–H) 3377, ν(N–H) 3192, ν(C=N) 1595,
The various intermolecular interactions present in the
crystal structures of the molybdenum complexes were
further quantified using crystal explorer 17.5 software.[17]
The Hirshfeld surface calculations were carried out with
the aid of the CIF files of the crystals. The intermolecular
contacts present in the crystals were mapped using nor-
malized contact distance, dnorm which is defined in terms
of di, de and van der Waals radii (rvdW) of atoms by Equa-
tion 1.
ν(C–S) 1024, νsym (cis-MoO2) 923, νasym (cis-MoO2) 906.
1H NMR (400 MHz, DMSO-d6, Me4Si, ppm) δ: 11.921 (s,
1H, OH), 10.443 (s, 2H, N–NH), 8.635, 8.307 (s, 2H,
CH=N), 6.811–7.826 (m, 6H, aromatic), 4.073 (q, 4H,
J = 6.8 Hz, OCH2), 1.359 (t, 3H, J = 6.8 Hz, CH3). UV/vis
(DMF) λmax, nm (ε, L mol−1 cm−1): 328 (29880), 348
(32090), 364 (27770), 421 (2260).
di −rvi dW de −rvedW
2.3 | X-ray crystallography
dnorm
=
+
ð1Þ
rvi dW
rvedW
For X-ray diffraction measurements, single crystals of the
Mo complexes were mounted on a Bruker SMART
APEXII CCD diffractometer, equipped with a graphite
crystal, incident-beam monochromator and a fine focus
sealed tube with Mo Kα (λ = 0.71073 Å) radiation as the
X-ray source. The unit cell dimensions were measured
and the data collection was performed. The programs
SAINT and XPREP were used for data reduction and
APEX2 and SAINT were used for cell refinement.[12]
Absorption corrections were carried out using SADABS
based on Laue symmetry using equivalent reflections.[13]
Where, di and de represents the distance from the
Hirshfeld surface to the nearest nucleus inside and out-
side the surface. This equation was visualized by
employing a red-white-blue colour code. Red colour rep-
resents intermolecular contacts closer than the sum of
van der Waals radii whereas the blue colour represents
the regions devoid of close contacts. White colour dis-
plays the contacts around the sum of van der Waals radii.
Similarly the set of points in 2D space portrays the overall

ASHA AND PRATHAPACHANDRA KURUP
5 of 14
contribution of intermolecular interactions using finger
print plots.[18]
L-Ascorbic acid was used as the standard and all the
experiments were carried out in triplicate.
2.5 | Antioxidant activity
2.5.2 | 2–20- Azinobis-
3-ethylbenzothiazoline-6-sulfonic acid
(ABTS) assay
2.5.1 | 2, 20-Diphenyl-1-picrylhydrazyl
(DPPH) radical assay
ABTS assay serve as a well-known method for the indi-
rect determination of the antioxidant capacity of a com-
pound[20] ABTS.+ is a rather stable radical that has an
strong absorption around 750 nm, but in presence of per-
oxy radicals, it undergoes oxidation to its metastable radi-
cal form. When the ABTS radical is scavenged by an
antioxidant, the absorption decreases with the disappear-
ance of the blue/green colour of this radical (Figure 2).
The ABTS radical scavenging abilities of the
dithiocarbohydrazone and its molybdenum complexes
were studied by the procedure reported by Tuberoso et
al.[21] The working solution was prepared by mixing two
stock solutions of 7 mM ABTS solution and 2.4 mM
potassium persulfate solution in equal amounts (1:1) and
the solution was allowed to react in the dark for 12 hr at
room temperature. The resulting solution was further
diluted by mixing 1 ml ABTS+ solution to obtain an
DPPH is a stable nitrogen based organic free radical[19]
with a characteristic absorption band at 528 nm. It pos-
sesses a violet colour (Figure 1) in its free radical form,
which changes to yellow after accepting either an elec-
tron or hydrogen radical from a compound generally reg-
arded as an antioxidant or free radical scavenger to form
the corresponding stable diamagnetic molecule. Hence
DPPH assay[19] is as an easy and beneficial method for
evaluating the free radical scavenging capacity of biologi-
cal samples as well as organic compounds. The decrease
in the absorbance of the sample at 517 nm is measured
spectrophotometrically. Figure 1 represents the changes
taking place during the interaction of DPPH free radical
with the antioxidant compound.
The DPPH radical scavenging abilities of the
dithiocarbohydrazone and its molybdenum complexes
were studied by a modifying the procedure reported by
Brand-Williams et al.[19] 1 ml of variable concentration
(10–100 μM) of the compound under study in DMF was
added to 1 ml methanolic solution of DPPH radical
(0.5 μM) and the final volume was made to 4 ml with
absorbance of 0.706
0.001 units at 734 nm using the
spectrophotometer. Test samples (1 ml) were allowed to
react with 1 ml of the ABTS.+ solution, followed by the
absorbance reading at 734 nm after 7 min using the spec-
trophotometer. L-ascorbic acid was used as the reference
standard and the control was prepared by adding the
same volume of reagent without any test compound and
95% methanol. All tests and analyses were run in tripli-
cate and the results obtained were averaged. The percent-
age inhibition was calculated as ABTS radical scavenging
activity using Equation 2.
ꢀ
methanol. The solution was then incubated at 37 C for
30 min and the decrease in the absorbance of DPPH was
measured at 517 nm. The same experiment with solution
devoid of thecompound under study was used as the con-
trol and the percentage inhibition of the compound was
calculated using the Equation 2.
A0 −A1
%Inhibition =
× 100
ð2Þ
A0
2.5.3 | Total antioxidant assay
Where A0 is the absorbance of the control and A1 is
the absorbance of the sample. Then the conc. (μM) was
plotted against percentage inhibition and the IC50 values
of the dithiocarbohydrazone and its molybdenum com-
plexes were obtained as the concentration at which the
absorbance of the DPPH solution reduces to 50%.
Total antioxidant capacity (TAC) assay by phosphor-
molybdenum method is based on the reduction of Mo (VI)
by the sample analyte and subsequent formation of a green
phosphate-Mo(V) complex (Figure 3) at acidic pH.
The
total
antioxidant
capacity
of
the
dithiocarbohydrazone and its molybdenum complexes
FIGURE 1 The changes taking place
during the interaction of DPPH radical with the
free radical scavenger

6 of 14
ASHA AND PRATHAPACHANDRA KURUP
FIGURE 2 The changes
taking place during the
interaction of ABTS radical with
the free radical scavenger
FIGURE 3 The colour change taking place
during the interaction of phosphomolybdenum
reagent with the free radical scavengers
were evaluated by the phosphor-molybdenum method,
according to the procedure described by Prieto et al.[22]
100 μL of sample was combined with 3 ml of reagent
solution (0.6 M sulfuric acid, 28 mM sodium phosphate
and 4 mM ammonium molybdate). The tubes containing
the reaction solution were incubated at 95 ꢀC for 90 min.
Then, the absorbance of the solution was measured at
695 nm using a UV–vis. Spectrophotometer against blank
after cooling to room temperature. The total antioxidant
activity was expressed as the number of gram equivalent
of ascorbic acid. The calibration curve was prepared by
mixing L- ascorbic acid with methanol.
3.2 | Spectral characterization
3.2.1 | Vibrational spectra
The FT-IR spectrum of the dithiocarbohydrazone,
H3TCL exhibits bands at 3290 and 3155 cm−1
corresponding to the ν(O–H) and ν(N–H) stretching
vibrations respectively.[7][,3(e,2(d)]) Presence of the bands
(ν(O–H) = 3377–3410 cm−1 and ν(N–H) = 3189–
3196 cm−1) in the spectra of the molybdenum com-
plexes 1–3 indicate that one of the OH and NH groups
are still retained in each molybdenum complex without
coordinating to the metal centre. Weak bands at
1609 and 1585 cm−1 in the spectrum of the
dithiocarbohydrazone is attributed to the ν(C=N)
stretch, one of which undergoes a shift to lower fre-
quency (1595–1602 cm−1) in the spectra of molybde-
num complexes designating the coordination of
azomethine nitrogen to the metal centre.[7][,2(d]) This
leads to the appearance of a new band at 620–
640 cm−1 in the spectra of molybdenum complexes
ascribed to the ν (Mo–N) stretch. The band appearing
at 1386 cm−1 for the ligand matches to the stretching
vibration of NH–C=S group and the consequent disap-
pearance of this band in the spectra of the complexes
signposts the tautomerisation of this functionality and
the subsequent coordination of the thiol sulfur to the
metal after its deprotonation. Moreover this gives rise
to a new band in the spectra of the complexes in the
range 1016–1027 cm−1 signalling the ν(C-S) stretching
vibration. Whereas the bands at 1151 cm−1 and
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
Dithiocarbohydrazone, H3TCL and its molybdenum com-
plexes 1–3 were isolated in good to excellent yields and
characterized by elemental analysis as well as various
physicochemical techniques. The dithiocarbohydrazone
was obtained as white solid, whereas all the molybdenum
complexes were obtained as dark orange crystalline
solids. Both the dithiocarbohydrazone and its complexes
were air stable at room temperature. They were found to
be completely soluble in DMSO, DMF and acetonitrile;
partially soluble in methanol and ethanol and completely
insoluble in CCl4, CHCl3 and water. The ligand and the
complexes portrayed excellent free radical scavenging
activities.

ASHA AND PRATHAPACHANDRA KURUP
7 of 14
1104–1190 cm−1 in the spectra of the ligand and the
complexes respectively corresponds to the ν(N–N)
vibration.
Finally the spectra of the molybdenum complexes
exhibit two bands in the ranges 923–936 cm−1 and 896–
906 cm−1 assigning to the symmetric and antisymmetric
stretching vibrations[23] of the [MoO2]2+ core. The results
of the FT-IR spectral assignments are provided in
Table S1.
molybdenum ion during complexation. The aromatic
protons of both the dithiocarbohydrazone and its com-
plexes were observed at the expected region of 6.811–
8.284 ppm. The OCH2 and CH3 protons were observed in
the region 4.055–4.073 ppm and 1.357–1.364 ppm respec-
tively (Table S3). 1H-NMR spectra of the
dithiocarbohydrazone, H3TCL and its molybdenum com-
plexes are displayed Figures S1-S3.
3.3 | Electrochemical studies
3.2.2 | Electronic spectra
The electrochemical studies of the dithiocarbohydrazone
and its molybdenum complexes were evaluated by cyclic
voltammetry in 10−5 M DMSO solution of the com-
pounds. The results are summarized in Table 1.
The electronic spectra of the dithiocarbohydrazone,
H3TCL and its molybdenum complexes 1–3 were
recorded in DMF at room temperature and using 1 x
10−5 M solution. The spectrum of dithiocarbohydrazone
exhibits three absorptions at λ = 327, 346 and 363 nm
assigned to π ! π* and n ! π* transitions of phenyl rings
of the salicylaldehyde part and C=S moiety respec-
tively.[7] The electronic spectra of the molybdenum com-
plexes are almost similar proposing that they have
principally similar structure. In addition to the bands
corresponding to π ! π* and n ! π* transitions, they
exhibit bands in the range 414–421 nm attributed to the
charge transfer transition (LMCT) from the highest occu-
pied molecular orbital of the dithiocarbohydrazone to the
lowest unoccupied molecular orbital of the molybdenum
ion.[6,7] The dearth of absorption in the visible region
arising as a result of d-d transitions confirms the 4d0 elec-
tronic configuration of the central metal ion, Mo (VI).[6,7]
The results of electronic spectral assignments of the syn-
thesized compounds are given in Table S2.
The
cyclic
voltammograms
of
the
dithiocarbohydrazone show irreversible oxidative and
reductive peaks at 0.311 and − 0.510 V respectively. Both
the complexes 1 and 3 exhibited irreversible reductive
peak at −0.512 V which may be due to the redox behav-
iour of the dithiocarbohydrazone and do not account for
the molybdenum centre. On the other hand, the irrevers-
ible reductive responses exhibited by the complexes 2 and
3 at −0.533 and − 0.294 V can be ascribed to the Mo (VI)/
Mo(V) process.[24,6.7]
3.4 | Crystal structure description
3.4.1 | Collective features of the
molybdenum complexes, 1–3
Diffraction quality single crystals of the complexes 1–3
obtained were subjected to single crystal X-ray diffraction
analysis. The ORTEP plots of the complexes are displayed
in Figure 4. Table S4 summarizes the crystal refinement
details of the complexes and the important bond lengths
and bond angles are collated in Table S5.
1
3.2.3 | H-NMR spectra
The 1H-NMR spectra of the dithiocarbohydrazone and its
molybdenum complexes were recorded in DMSO. The
1H-NMR spectrum of the dithiocarbohydrazone, H3TCL
exhibits two OHphenolic protons at δ = 12.072 and
11.875 ppm, whereas the spectra of complexes adopt only
one OHphenolic signal at 11.921 ppm since the other OH
proton is involved in bonding with molybdenum ion.
Although the ligand exhibits two NH signals at 11.618
and 9.142 ppm, the spectra of the complexes 1–3 display
only one signal at 10.440, 10.484 and 10.443 ppm respec-
tively due to the involvement of the other NH proton in
the enolisation and deprotonation with subsequent coor-
dination to molybdenum ion. Likewise the spectrum of
the dithiocarbohydrazone displays two azomethine pro-
ton signals at 8.754 and 8.546 ppm, one of which
undergoes downfield shift on coordination with
TABLE 1
Results of the cyclic voltammetric analysis of the
dithiocarbohydrazone, H3TCL and its molybdenum complexes 1–3
Compound
H3TCL
E
pc [V][a]
0.311, −0.510
−0.512
Complex 1
Complex 2
Complex 3
−0.533
−0.294, −0.512
[a]Solvent: DMSO, Working electrode: Pt rod, Auxiliary electrode: Pt
wire, Reference electrode: Ag/AgCl, Supporting electrolyte: 0.1 M
TEAP, Scan rate: −100 mV/s. Epc is the cathodic potential.

8 of 14
ASHA AND PRATHAPACHANDRA KURUP
oxygen atom, O(7) from the coordinated methanol mole-
cule, DMSO molecule and H2O molecule of the respec-
tive complexes 1–3 bond to the sixth coordination sphere
of the complex accomplishing the expected prerequisite
of molybdenum hexacoordination.[5–7]
Analogous to other reported cis-dioxidomolybdenum
(VI) complexes, the molybdenum atom in all the three
complexes are shifted from the centre of the octahedron
by 0.2 Å towards the terminal oxido oxygen, O(4).[5–7]
The two oxido oxygen atoms, O(4) and O(5) arecis posi-
tioned to each other and the Mo=O distances of the
corresponding oxido oxygen atoms fall in the usual range
of 1.693(2) and 1.689(2) Å for complex 1; 1.680(4) and
1.702(4) Å for complex 2 and 1.709(2) and 1.670(3) Å for
complex 3. Likewise the Mo–S bond lengths for the com-
plexes were also observed in the expected range of 2.4314
(9)-2.4393(10) Å. The Mo–N bond lengths for the com-
plexes, 2.277(2) Å for complex 1, 2.256(5) Å for complex 2
and 2.242(3) Å for complex 3 were observed to be rather
long in consequence of the trans effect of the terminal
oxido ligand, O(5). Regardless of this, all the other molyb-
denum-ligand bond lengths well fit with the bond length
range for other reported cis-dioxidomolybdenum
complexes.[6,7]
The cis bond angles observed for the complexes range
from 81.92(9)-83.33(17) º for N(1)–Mo(1)–O(1), 76.17(7)-
76.31(7)º for N(1)–Mo(1)–S(1), 87.91(8)-89.55(9) º for S
(1)–Mo(1)–O(5) and 104.65(19)-107.32(11) º for O(1)–Mo
(1)–O(5), whereas the trans angles range from 151.94(7)-
154.25(12) º for O(1)–Mo(1)–S(1), 157.79(19)-159.65(11) º
for N(1)–Mo(1)–O(5) and 168.42(11)-170.85(17) º for O
(6)–Mo(1)–O(7). Incorporation of molybdenum into a five
membered ring accounts for these distortions to a greater
extent.
From the ring puckering analysis and least square
plane calculations,[25] it is evident that the five membered
chelate ring, Cg(1) comprising the Mo atom, Mo(1)/S(1)/
C(8)/N(1)/N(2) in complexes 1–3 are puckered with
puckering amplitudes of Q2 = 0.1442(19), 0.303(3) and
0.242(2) Å and ϕ = 11.0(11)ꢀ, 7.1(9)ꢀ and 197.1(9)ꢀ
respectively. Similarly the six membered chelate ring, Cg
(2) comprising the Mo atom, Mo(1)/O(1)/C(1)/C(6)/C(7)/
N(2) of complex 1 is puckered with puckering amplitude
Q2 = 0.260(2) Å and ϕ = 204.0(7)ꢀ.
FIGURE 4 The ORTEP plots of complexes 1–3 (a-c) with
atom numbering scheme for all non-hydrogen atoms.
(Displacement ellipsoid drawn at 50% probability)
Complexes 1 and 3 got crystallized in monoclinic
P21/c space group whereas complex 2 in orthorhombic
Pbca space group. In complex 3, a DMF molecule is pre-
sent without coordination to the central metal, molybde-
num. In all the complexes, the dithiocarbohydrazone
ligand, H3TCL performs as an ONS donor tridentate
ligand despite being a potential pentadentate ligand. The
molybdenum centre in all the three complexes portrays a
distorted octahedral geometry where it coordinates with
the three donor atoms of the ligand viz. the thioenolic
sulfur, S(1); azomethine nitrogen, N(1) and phenolic oxy-
gen, O(1). Further the molybdenum centre is coordinated
by two oxido oxygen atoms, O(5) and O(6). Finally the
3.4.2 | Supramolecular aspects
The supramolecular architectures of all the complexes
were driven by several non-covalent interactions includ-
ing hydrogen bonding, π···π and C–H···π interactions.
The crystal structure of complex 1 is stabilized by
classical intra- and intermolecular hydrogen bonding

ASHA AND PRATHAPACHANDRA KURUP
9 of 14
interactions. The phenolic proton, H(30) and amino pro-
ton, (H3A) attached to the phenolic oxygen, O(3) and
amino nitrogen, N(3) respectively are involved in intra
and inter-molecular hydrogen bonding interaction with
the azomethine nitrogen, N(4), O(3)–H(30)···N(4) and
oxido oxygen, O(6), N(3)–H(3A)···O(6) [x, 1/2-y,
−1/2 + z] respectively (Figure S4).
In addition to that, the crystal structure is further sta-
bilized by π···π and C–H···π interactions where the π
cloud of the phenolic ring [C(1)/C(2)/C(3)/C(4)/C(5)/C
(6)] bonded to molybdenum atom, Cg(3) of one molecule
is involved in π···π interaction with the π cloud of the
similar phenolic ring of the adjacent molecule with a cen-
troid-centroid distance of 3.669(2) Å (Figure S4). Simi-
larly a C–H···π interaction exist between the ethoxy
proton, H(17B) of the phenolic ring coordinated to
molybdenum with the π cloud of the phenolic ring that is
not coordinated to the Mo centre, Cg(4) with a centroid-
centroid distance of 3.756(5) Å (Figure S4).
Similarly complex 2 also exhibits the same two classi-
cal hydrogen bonding interactions exhibited by complex
1, viz. O(3)–H(30)···N(4) and N(3)–H(3A)···O(5) [1/2 + x,
y, −1/2-z]. In addition to that the crystal structure con-
sists of two more non-classical intermolecular hydrogen
bonding interactions where the ethoxy proton of the phe-
nolic ring that is not coordinated to the Mo centre is
coordinated with the oxido oxygen atom, O(6) of the
adjacent molecule, C(19)–H(19C)···O(6), [2-x, 1-y, 1-z]
and the methoxy proton of the coordinated DMSO mole-
cule, H(21B) is hydrogen bonded to the phenolic oxygen
atom, O(3) of the adjacent molecule, C(21)–H(21B)···O
(3), [1-x, 1-y, 1-z] (Figure S5).
Complex 3 exhibits two classical hydrogen bonding
interactions similar to complexes 1 and 2 where the
phenolic proton, H(3A) attached to the phenolic oxy-
gen, O(3) is involved in intramolecular hydrogen bond-
ing interaction with the azomethine nitrogen, N(4), O
(3)–H(3A)···N(4). Unlike complexes 1 and 2, the amino
proton, (H3N) is involved in intermolecular hydrogen
bonding interaction with the oxygen atom, O(8) of the
uncoordinated DMF molecule, N(3)–H(3 N)···O(8) [1-x,
1-y, 1-z] rather than the oxygen atom, O(7) of
the coordinated water molecule. The methoxy protons,
H(21B) and H(21C) of the uncoordinated DMF
molecule are engaged in intra- and intermolecular
hydrogen bonding interaction with ethoxy oxygen
atom, O(2), C(21)–H(21B)···O(2) and phenoxy oxygen
atom, O(3), C(21)–H(21B)···O(3) [1-x, 1-y, 1-z] respec-
tively (Figure S6).
molybdenum complexes are summarized in Tables S6, S7
and S8 respectively.
3.5 | Hirshfeld surface analysis
The visualization and quantification of the close contacts
present in the crystal structures of all the three complexes
were assured with the aid of Crystal Explorer 17.5 soft-
ware. The Hirshfeld surface and the associated finger-
print plots of the complexes were computed.
Figure 5 portrays the Hirshfeld surfaces of the com-
plexes 1–3 mapped over dnorm
, shape index and
curvedness. The dnorm surfaces (ranged between −0.5871
to 1.3351 Å for complex 1, −0.4479 to 1.5473 Å for com-
plex 2 and − 0.5963 to 1.5387 Å for complex 3) are trans-
parently shown to visualize the moieties around which it
was calculated. The bright red spots present in the dnorm
surface[25] signifies the hydrogen bond centres, whereas
the pale coloured regions signpost the weak and long
range intermolecular interactions. Similarly the red spots
in shape index indicate the hydrogen bonding interac-
tions and the complimentary hydrogen bonding interac-
tions are displayed as blue spots. In addition to that the
π–π and C–H···π interactions appear as ‘bow-tie’ patterns
in the shape index plot,[26] however these interactions
appear as flat regions in the curvedness plot.
The two dimensional fingerprint plots displayed in
the range 1.0–2.4 Å with the de (the closest external) and
di (the closest internal) distance scales displayed on graph
axes showcases particular atom-pair contacts and aids the
parting of contributions from different interaction types
that overlap in the full fingerprint plot.[26,27] The finger-
print plots of the complexes are provided in Figure S7.
H···H interactions are the most recurrent interactions
in the complexes encompassing about 44, 47.4 and 43.5%
of the total Hirshfeld surface of the complexes 1, 2and 3
respectively ascribed to the abundance of hydrogen on
the molecular surface. These interactions appear as two
end points that points towards the origin. Whereas the
second largest interaction is the O···H interaction which
appear as two distinct spikes of almost equal length in
the 2D fingerprint plots contributing to 22, 25 and 29.4%
of the total Hirshfeld surface of the complexes 1, 2 and 3
respectively. Similarly the C···H interactions appear as
two symmetrical wings on left and right side and corre-
sponds to 14, 11.4 and 13.2% of the total Hirshfeld surface
of the complexes 1, 2 and 3 respectively. Other weak
interactions like S···H (4.1, 4.5 and 4.2% of the total
Hirshfeld surface for complexes 1, 2 and 3 respectively),
N···H (5.7, 3.1 and 3.9% of the total Hirshfeld surface for
complexes 1, 2 and 3 respectively) and C···C (3.5% for
complex 1) are characterized by their small space
Crystal structure of complexes 2 and 3 do not exhibit
any π–π or C–H···π interactions. The interaction parame-
ters for the various hydrogen bonding, π–π and C–H···π
interactions present in the crystal structures of the

10 of 14
ASHA AND PRATHAPACHANDRA KURUP
FIGURE 5 Hirshfeld surface of complexes 1–3 mapped with (a) dnorm, (b) shape index and (c) curvedness
occupations in the fingerprint plot. Relative contributions
from all major interactions to the Hirshfeld surface are
detailed in Figure 6.
Hirshfeld surface analysis also assists in envisioning
the prominent hydrogen bonding interactions present in
the complexes. Important hydrogen bonding interactions
of complex 1 are portrayed in Figure S8.
DPPH, ABTS and total antioxidant assays with respect to
the standard antioxidant, L ascorbic acid.
In the DPPH assay the dithiocarbohydrazone, H3TCL
and its molybdenum complexes displayed good antioxi-
dant capacity by reducing the stable violet coloured
DPPH radical to the yellow coloured diphenyl picryl
hydrazine.
The radical scavenging activities of the compounds
were studied at different concentrations (10–100 μM) of
the compounds under study and the activity followed a
dose dependent manner. The results of the analysis was
then expressed as their IC50 values (Figure 7). The molyb-
denum complexes exhibited lower IC50 values and higher
activity compared to the dithiocarbohydrazone ligand.
The standard, L-ascorbic acid failed to produce 50% inhi-
bition at the studied concentration range. Therefore the
DPPH radical scavenging ability of the compounds
followed the order Ascorbic acid< H3TCL < Complex
1 < Complex 3 < Complex 2. The expected mechanism
for the DPPH assay of these compounds are exhibited in
Scheme 3.
3.6 | Antioxidant activity studies
The antioxidant activity of the dithiocarbohydrazone,
H3TSL and its molybdenum complexes were evaluated by
A possible mechanism for the antioxidant activity of
the synthesized compounds are being discussed in
Scheme 3 by considering dithiocarbohydrazone as the
representative example. Generally the antioxidant activ-
ity of a compound depends on the number of hydrogen
atoms available for donation to the DPPH radical. In the
case of the dithiocarbohydrazone, the acidic hydrogen
(responsible for thioketo-thioenol tautomerism in
FIGURE 6 Percentage contributions to the Hirshfeld surface
area for the various close intermolecular contacts for molecules 1–3

ASHA AND PRATHAPACHANDRA KURUP
11 of 14
FIGURE 7 The results of DPPH antioxidant assay of the dithiocarbohydrazone and its molybdenum complexes
H3TCL) shown in red colour stabilises the DPPH radical
resulting in the formation of a new radical (radical A)
which is resonance stabilized. But when it comes to the
case of molybdenum complexes, the acidity of the hydro-
gen involved in the scavenging of DPPH radical increases
due to the electron withdrawing effect of the metal,
molybdenum which exist in its highest possible oxidation
state (+6), thereby increasing the radical scavenging abil-
ity of the resultant complexes. Moreover the
uncoordinated phenolic proton can also contribute to the
antioxidant activity. But the role of coordinated solvent
molecules towards antioxidant activity is still unknown.
The radical scavenging ability of the tested com-
pounds towards the ABTS radical cation characteristi-
cally showed that even though the tested compounds
were less efficient antioxidants compared to the standard
antioxidant, L-ascorbic acid, the molybdenum complexes
displayed higher activity compared to the free dit-
hiocarbohydrzone, H3TCL (Figure 8(a)). The activity
followed the order L-ascorbic acid> complex 3 > complex
1 > complex 2 > H3TCL.
The
total
antioxidant
capacity
of
the
dithiocarbohydrazone and its molybdenum complexes
evaluated by the phosphor-molybdenum method also
showed that these compounds exhibit antioxidant activi-
ties comparable to that of the standard antioxidant, L-
ascorbic acid (Figure 8(b)). Among the tested com-
pounds, complex 1 exhibited the maximum AEAC
(Ascorbic acid equivalent antioxidant capacity). The
activity followed the order, Complex 2 < H3TCL< Complex
3 < Complex 1.
Table 2 summarises the results of the various antioxi-
dant assays.
To conclude, the dithiocarbohydrazone and its molyb-
denum complexes displayed good antioxidant capacities
where the former exhibited much lesser activity com-
pared to the latter which can be attributed to the chela-
tion of the ligand with the molybdenyl cation.

12 of 14
ASHA AND PRATHAPACHANDRA KURUP
SCHEME 3 Possible mechanism of
DPPH radical scavenging by the
thiocarbohydrazone ligand and its
molybdenum complexes
FIGURE 8 The results of (a) ABTS antioxidant assay and (b) total antioxidant assay of the dithiocarbohydrazone and its molybdenum
complexes

ASHA AND PRATHAPACHANDRA KURUP
13 of 14
TABLE 2
Results of various antioxidant assays
award of a Senior Research Fellowship (09/239(0510)/
2015-EMR-I). The authors are thankful to the Sophisti-
cated Analytical Instrumentation Facility, Cochin Uni-
versity of Science and Technology, Kochi, India for
elemental analysis & single crystal X-ray diffraction mea-
surements and Octagon Scientific Services Private Ltd.,
Total
DPPH
ABTS
antioxidant
assay
(AEAC)[a]
(μg)
antioxidant
assay (IC50
antioxidant
assay (IC50
Compound values (μM)) values (μL))
1
Tiruchirapally, Tamil Nadu, India for H NMR spectral
analysis.
H3TCL
44.01
39.76
26.82
28.86
-
135
12.91
19.17
8.86
19.04
-
Complex 1
Complex 2
Complex 3
23.91
113
APPENDIX A. SUPPLEMENTARY DATA
CCDC 1972016, 1972017 and 1972018 contain the supple-
mentary crystallographic data for complexes 1, 2 and 3
respectively. These data can be obtained free of charge
viawww.ccdc.cam.ac.uk/conts/retrieving.html or from
the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 IEZ, UK; fax: (+44) 1223–336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
16.49
7.58
L-ascorbic
acid
[a]Ascorbic acid equivalent antioxidant capacity = antioxidant
capacity of the compound equivalent to that of 1 g ascorbic acid.
4 | CONCLUSION
ORCID
In the present work, we have included studies on a new
dithiocarbohydrazone and its three molybdenum com-
plexes. These complexes exhibit a 1:1 metal ligand stoi-
chiometry from the analytical data. All complexes were
found to be non-electrolytes in DMF solution and
exhibited an irreversible redox behaviour as revealed by
the cyclic voltammetric studies. The spectral characterisa-
tion along with the single crystal X-ray diffraction analy-
sis revealed that the dithiocarbohydrazone binds with Mo
ion in a tridentate fashion through nitrogen atom of the
azomethine and oxygen atoms of hydroxyl group of the
3-ethoxysalicylaldehyde besides the thioenolate sulfur
from the dithiocarbohydrazide moiety in all the com-
plexes. The evaluation of antioxidant activities of the
dithiocarbohydrazone and its complexes by various anti-
oxidant assays disclosed that the metal complexes
exhibited higher free radical scavenging abilities com-
pared to the free dithiocarbohydrazone ligand. The Mo
complexes are capable of donating electron or hydrogen
atom and subsequently react with free radicals or termi-
nate chain reactions in a dose-dependent pattern. Further
these observations were validated with the aid of molecu-
lar docking approaches (provided in the supplementary
information, Figure S9 and Table S9) which revealed that
the antioxidant activity exhibited by the compounds are
directly dependent on the extend of interaction of them
with the amino acid residues present in the active site of
the human antioxidant enzyme, 3MNG. Greater the
number of conventional hydrogen bonding interactions
between the two, greater will be the binding constant
value and hence greater will be the antioxidant activity.
M.R. Prathapachandra Kurup https://orcid.org/0000-
0002-9434-890X
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How to cite this article: Asha TM,
Prathapachandra Kurup MR. An insight into the
potent antioxidant activity of a
dithiocarbohydrazone appended cis-
dioxidomolybdenum (VI) complexes. Appl
Organomet Chem. 2020;e5762. https://doi.org/10.
1002/aoc.5762