Dioxomolybdenum(VI) complexes of S-methyl-5-bromosalicylidene-N-alkyl substituted thiosemicarbazones: Synthesis, catalase inhibition and antioxidant activities

Article abstract of DOI:10.1016/j.ica.2017.10.007

We synthesized a series of S-methyl-5-bromosalicylidene-N-alkyl substituted thiosemicarbazones and their cis-dioxomolybdenum(VI) complexes having long alkyl chains (propyl, butyl, pentyl, hexyl, and octyl) on thioamide nitrogen. The compounds were characterized by using analytical and spectroscopic methods. The structure of the complex with pentyl group (complex 3) as a representative molecule was determined by X-ray single-crystal diffraction method. The free ligand and their dioxomolybdenum(VI) complexes have been tested for in vitro antioxidant capacity by reduction of copper (II) neocuproine (Cu(II)-Nc) using the CUPRAC (CUPric Reducing Antioxidant Capacity) method. The ligands exhibited more potent in vitro antioxidant capacity than that of the complexes. The obtained trolox equivalent antioxidant capacity (TEAC) value of complex 1 (TEAC_CUPRAC = 0.73) was higher than those of other complexes. Furthermore, the catalase activity and reactive oxygen species (ROS) scavenging ability of the free ligands and their complexes were determined, showing that complex 1 had significant scavenging activity for ROS.

Full text of DOI:10.1016/j.ica.2017.10.007

Inorganica Chimica Acta 469 (2018) 495–502
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Research paper
Dioxomolybdenum(VI) complexes of S-methyl-5-bromosalicylidene-N-
alkyl substituted thiosemicarbazones: Synthesis, catalase inhibition and
antioxidant activities
Songül Eg˘lence a, Musa Sßahin a, , Mustafa Özyürek b, Reßsat Apak b, Bahri Ülküseven a
⇑
a Department of Chemistry, Inorganic Chemistry Division, Istanbul University, 34320 Avcilar, Istanbul, Turkey
b Department of Chemistry, Analytical Chemistry Division, Istanbul University, 34320 Avcilar, Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
We synthesized a series of S-methyl-5-bromosalicylidene-N-alkyl substituted thiosemicarbazones and
their cis-dioxomolybdenum(VI) complexes having long alkyl chains (propyl, butyl, pentyl, hexyl, and
octyl) on thioamide nitrogen. The compounds were characterized by using analytical and spectroscopic
methods. The structure of the complex with pentyl group (complex 3) as a representative molecule
was determined by X-ray single-crystal diffraction method. The free ligand and their dioxomolybde-
num(VI) complexes have been tested for in vitro antioxidant capacity by reduction of copper (II) neocu-
proine (Cu(II)-Nc) using the CUPRAC (CUPric Reducing Antioxidant Capacity) method. The ligands
exhibited more potent in vitro antioxidant capacity than that of the complexes. The obtained trolox
equivalent antioxidant capacity (TEAC) value of complex 1 (TEACCUPRAC = 0.73) was higher than those
of other complexes. Furthermore, the catalase activity and reactive oxygen species (ROS) scavenging abil-
ity of the free ligands and their complexes were determined, showing that complex 1 had significant
scavenging activity for ROS.
Received 13 March 2017
Received in revised form 10 July 2017
Accepted 9 October 2017
Available online 10 October 2017
Keywords:
Thiosemicarbazone
Dioxomolybdenum(VI)
Antioxidant activity
ROS scavenging activity
Catalase activity
Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction
for the enzymes like xanthine oxidase, sulfite oxidase and nitroge-
nase known as molybdoenzymes [20,21]. The molybdenum com-
A considerable amount of research so far provided strong evi-
dence that thiosemicarbazone derivatives and their metal com-
plexes are highly effective in many different biological systems
[1–4]. These studies have revealed that the biological activities of
thiosemicarbazone compounds depend on several parameters such
as the type of metal atom [5], coordination environment [6], the
type of mixed ligand [7] and the substitutions on sulfur or nitrogen
atoms [8].
The coordination behaviours of tridentate thiosemicarbazones
derivated with a carbonyl compound in the presence of various
metal ions such as Ni(II) [9], Cu(II) [10], Mo(VI) [11], U(VI) [12],
Mn(II) [13], Pd(II) [14], Pt(II) [15], Au(III) [16], Ru(II) [17] have been
the subject of many studies. This carbonyl compound is generally a
ketone or aldehyde having extra donor site.
plexes of thiosemicarbazones are potential enzyme catalysts in
which the catalytic capacity is thought to be related to the struc-
tural features of the chelated cis-MoO22+ core [22,23]. In recent
years, there has been an increasing interest in molybdenum chem-
istry due to the fact that it has different oxidation numbers and
potency to give complexes having various coordination environ-
ments and stereostructures [24].
With respect to their biological roles and industrial applica-
tions, thiosemicarbazone analogues substituted with sulfur and
nitrogen are more versatile intermediates than the oxygenated
ones [25]. Vrdoljak et al. [26] investigated the synthesis and char-
acterization of new molybdenum complexes by the reaction of
[MoVIO2(acac)2] or (NH4)2[MoVOCl5] with different N-substituted
pyridoxal thiosemicarbazone ligands, namely pyridoxal 4-
phenylthiosemicarbazone, pyridoxal 4-methylthiosemicarbazone,
and pyridoxal thiosemicarbazone, and found out that molybdenum
is coordinated with a tridentate doubly-deprotonated ligand.
Recently, Ceylan et al. [27] synthesized and characterized cis-diox-
omolybdenum(VI) complexes of a new ONN chelating thiosemicar-
bazidato ligand, S-propyl-N4-phenyl-thiosemicarbazone, and
reported much higher antioxidant capacities (with DPPH and
Even though many transition metal ions have been used in dif-
ferent fields for complexation of thiosemicarbazones, molybdenum
as an oligo element and transition metal has a high importance in a
wide range of biological systems [18,19]. It is a necessary co-factor
⇑
Corresponding author.
E-mail address: musahin@istanbul.edu.tr (M. Sßahin).
https://doi.org/10.1016/j.ica.2017.10.007
0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

496
S. Eg˘lence et al. / Inorganica Chimica Acta 469 (2018) 495–502
CUPRAC methods) for the parent ligand than for its Mo(VI)-
complexes.
recorded on a Bruker Avance-500 model spectrometer relative to
SiMe4 using deuterated chloroform as solvent at 25 2 °C. The X-
ray intensity data were measured on a Bruker D8 VENTURE system
Apart from the antimicrobial activities, thiosemicarbazone
derivatives have potency of inhibition on some enzymes such as
catalase, a common antioxidant enzyme and important antioxidant
defence system found in nearly all living organisms exposed to
oxygen, which catalyzes the decomposition of hydrogen peroxide
to water and oxygen. This enzyme has one of the highest turnover
numbers of all enzymes; one catalase molecule can convert mil-
lions of molecules of hydrogen peroxide to water and oxygen each
second [28,29].
equipped with a multilayer monochromator and a Mo K
a Sealed
tube (k = 0.71073 Å). Spectrophotometric antioxidant capacity,
ROS scavenging activity and enzyme activity measurements of
synthesized compounds were performed by using a Perkin Elmer
Lambda 35 UV–Vis spectrophotometer using a pair of matched
quartz cuvettes of 1 cm thickness.
2.2. Synthesis
Although extensive research has been carried out on aliphatic
substitutions on sulfur atom of thiosemicarbazones, there are a
limited number of studies including long alkyl-chain substitution
on both sulfur and amidic nitrogen atoms [30]. In the current
study, we synthesized a series of S-methyl-5-bromosalicylidene-
thiosemicarbazone derivatives and their cis-dioxomolybdenum
(VI) complexes bearing alkyl chains in different lengths (propyl,
butyl, pentyl, hexyl or octyl) on thioamide region (Fig. 1 and
Fig. 2). The compounds were characterized by elemental analysis,
IR, and 1H NMR spectroscopies. The structure of cis-dioxo-(S-
methyl-N1-5-bromosalicylidene-N4-octyl thiosemicarbazonato)(N,
N’,O)-methanol-molybdenum(VI) (complex 3) was determined by
X-ray single-crystal diffraction method. As biochemical properties,
we evaluated the antioxidant capacity, ROS scavenging activity and
catalase activity of the free ligand and its dioxomolybdenum(VI)
complexes by various in vitro assays.
2.2.1. Synthesis of S-methyl-N1-5-bromosalicylidene-N4-octyl
thiosemicarbazone (L5)
The thiosemicarbazone ligand (L5) was prepared with small
modifications of general methods [31]. Firstly, N4-octyl thiosemi-
carbazide was obtained by reaction of octyl isothiocyanate and
hydrazine hydrate [32]. Colourless solid product was filtered,
recrystallized from ethanol-petroleum ether (1:1), washed with
petroleum ether (b.p. 60–80 °C) and dried in vacuo (yield 80%). In
second step, N4-octylthiosemicarbazide (0.20 g, 1 mmol) and
methyliodide (0.16 g, 1.1 mmol) were refluxed in 10 mL ethanol
for 2 extra hours. Five milliliters of ethanolic solution of 5-bromos-
alicylaldehyde (0.20 g, 1 mmol) were added, and refluxing was
continued for 2 extra hours. The hydroiodide form of the ligand
was neutralized with a sufficient amount of aqueous NaHCO3 solu-
tion (10%, w/w). The separated yellow precipitate was filtered and
washed several times with cold water and ethanol. The ligand was
recrystallized from ethanol and dried under vacuum (yield 82%).
The purity was checked with TLC. Thiosemicarbazones, L1-4, were
prepared by following the same procedure.
2. Experimental section
2.1. Materials and physical measurements
L1: Yellow, m.p. 129–130 °C, yield 90%. Anal. Calc. for C12H16
BrN3OS (330.24 g/mol): C, 43.64; H, 4.88; N, 12.72; S, 9.71. Found:
C, 43.48; H, 4.82; N, 12.86; S, 9.52%. UV–Vis (CHCl3) [kmax (nm),
(dm3 cmꢀ1 molꢀ1)]: 239 (29066), 300 (26366), 311 (25966), 345
(30000). FT-IR (KBr, cmꢀ1): (C@N1)
(OH) 3423; (NH) 3415;
1607;
-
All chemicals used in the study were of reagent grade and were
used without purification. The elemental analyses were performed
on a Thermo-Finnigan Flash EA 1112 Series Elementary Analyzer.
UV–Vis spectra of the compounds were recorded at 10ꢀ5 mol
dmꢀ3 concentration in CHCl3 solution with a Shimadzu 2600 UV–
Vis Spectrometer. Infrared spectra of the complexes were recorded
on a Mattson 1000 FTIR spectrophotometer from the wavenumbers
of 4000–400 cmꢀ1 at room temperature. 1H NMR spectra were
e
t
t
t
t
(C@N2) 1569. 1H NMR (CDCl3): 11.67, 11.37 (s, cis/trans:
3/1, 1H, OH), 8.28, 8.22 (s, syn/anti: 3/1, 1H, CH = N1), 6.79 (d,
1H, a), 7.27–7.21 (m, 2H, b, c), 2.41, 2.33 (s, cis/trans: 1/3, 3H, S-
CH3), 5.51, 4.36 (s, cis/trans:1/3, 1H, N4H), 3.35, 3.19 (q, cis/trans:
3/1, 2H, N4-C1H2), 1.60 (m, 2H, –C2H2–), 0.90 (m, 3H, –C3H3).
L2: Yellow, m.p. 124–125 °C, yield 78%. Anal. Calc. for C13H18
BrN3OS (344.27 g/mol): C, 45.35; H, 5.27; N, 12.21; S, 9.31. Found:
C, 45.45; H, 5.29; N, 12.16; S, 9.30%. UV–Vis (CHCl3) [kmax (nm),
(dm3 cmꢀ1 molꢀ1)]: 240 (29466), 300 (27866), 311 (27566), 345
(31866). FT-IR (KBr, cmꢀ1): (C@N1)
(OH) 3432; (NH) 3407;
1607;
(C@N2) 1561. 1H NMR (CDCl3): 11.66, 11.35 (s, cis/trans:
-
e
t
t
t
t
3/1, 1H, OH), 8.28, 8.23 (s, syn/anti: 1/3, 1H, CH = N1), 6.79 (d,
1H, a), 7.27–7.21 (m, 2H, b, c), 2.42, 2.33 (s, cis/trans: 1/3, 3H, S-
CH3), 5.50, 4.33 (s, cis/trans:1/3, 1H, N4H), 3.40, 3.22 (q, cis/trans:
3/1, 2H, N4-C1H2), 1.56 (m, 2H, –C2H2–), 1.34 (m, 2H, –C3H2–),
0.90 (m, 3H, -C4H3).
Fig. 1. N-alkyl thiosemicarbazones.
L3: Yellow, m.p. 118–119 °C, yield 84%. Anal. Calc. for C14H20
BrN3OS (358.29 g/mol): C, 46.93; H, 5.63; N, 11.73; S, 8.95. Found:
C, 46.68; H, 5.56; N, 11.86; S, 8.85%. UV–Vis (CHCl3) [kmax (nm),
(dm3 cmꢀ1 molꢀ1)]: 240 (27633), 300 (27233), 311 (27166), 345
(31533). FT-IR (KBr, cmꢀ1): (C@N1)
(OH) 3430; (NH) 3407;
1607;
-
e
t
t
t
t
(C@N2) 1561. 1H NMR (CDCl3): 11.56 (s, 1H, OH), 8.26 (s,
1H, CH = N1), 6.80 (d, 1H, a), 7.25–7.23 (m, 2H, b, c), 2.36 (s, 3H,
S-CH3), 4.43 (s, 1H, N4H), 3.41, 3.25 (s, cis/trans: 3/1, 2H, N4-
C1H2), 1.59 (s, 2H, –C2H2–), 1.30 (s, 4H, –C3H2–, –C4H2–), 0.85 (s,
3H, –C5H3).
L4: Yellow, m.p. 108–109 °C, yield 86%. Anal. Calc. for C15H22
BrN3OS (372.32 g/mol): C, 48.39; H, 5.96; N, 11.29; S, 8.61. Found:
C, 48.33; H, 5.82; N, 11.32; S, 8.57%. UV–Vis (CHCl3) [kmax (nm),
(dm3 cmꢀ1 molꢀ1)]: 240 (31233), 299 (30033), 309 (27966), 346
-
e
Fig. 2. Dioxomolybdenum(VI) complexes of N-alkyl thiosemicarbazones.

S. Eg˘lence et al. / Inorganica Chimica Acta 469 (2018) 495–502
497
(29566). FT-IR (KBr, cmꢀ1):
1600;
t
(OH) 3432;
t
(NH) 3407;
t
(C@N1)
(s, 3H, S-CH3), 3.71 (t, 2H, N4-C1H2), 1.71 (s, 2H, –C2H2–), 1.26–
1.18 (m, 6H, –C3H2–, –C4H2–, –C5H2–), 0.82 (m, 3H, –C6H3), 3.42
(s, 3H, –CH3).
t
(C@N2) 1561. 1H NMR (CDCl3): 11.67, 11.37 (s, cis/trans:
3/1, 1H, OH), 8.26, 8.22 (s, syn/anti: 1/3, 1H, CH = N1), 6.78 (d,
1H, a), 7.27–7.21 (m, 2H, b, c), 2.40, 2.33 (s, cis/trans: 1/3, 3H, S-
CH3), 5.45, 4.29 (s, cis/trans:1/3, 1H, N4H), 3.38, 3.21 (q, cis/trans:
3/1, 2H, N4-C1H2), 1.57 (m, 2H, –C2H2–), 1.33–1.24 (m, 6H, –
C3H2–, –C4H2–, –C5H2–), 0.82 (m, 3H, –C6H3).
5: Red, m.p. 173–175 °C, yield 57%. Anal. Calc. for C18H28
-
BrMoN3O4S (558.35 g/mol): C, 38.72; H, 5.05; N, 7.53; S, 5.74.
Found: C, 38.59; H, 5.13; N, 7.82; S, 5.75%. UV–Vis (CHCl3) [kmax
(nm),
(15840), 457 (3420). FT-IR (KBr, cmꢀ1):
1600;
(C@N2) 1538; ts
e
(dm3 cmꢀ1 molꢀ1)]: 239 (19340), 252 (18840), 311
L5: Yellow, m.p. 91–92 °C, yield 82%. Anal. Calc. for C17H26BrN3-
OS (400.37 g/mol): C, 51.00; H, 6.55; N, 10.50; S, 8.01. Found: C,
t
(OH) 3446;
t
(C@N1)
t
, t
as(MoO2) 938, 900. 1H NMR (CDCl3):
51.00; H, 6.65; N, 10.55; S, 7.96%. UV–Vis (CHCl3) [kmax (nm),
(dm3 cmꢀ1 molꢀ1)]: 240 (30000), 300 (29700), 310 (28600), 345
(31633). FT-IR (KBr, cmꢀ1): (C@N1)
(OH) 3435; (NH) 3407;
1607;
e
8.59 (s, 1H, CH = N1), 6.94 (d, 1H, a), 7.52–7.51 (d, 2H, b, c), 2.52
(s, 3H, S-CH3), 3.70 (t, 2H, N4-C1H2), 1.70 (s, 2H, –C2H2–), 1.25–
1.18 (m, 10H, –C3H2–, –C4H2–, –C5H2–, –C6H2–, –C7H2–), 0.81 (t,
3H, –C8H3), 3.41 (s, 3H, –CH3).
t
t
t
t
(C@N2) 1561. 1H NMR (CDCl3): 11.58 (s, 1H, OH), 8.27 (s,
1H, CH = N1), 6.80 (d, 1H, a), 7.30–7.23 (m, 2H, b, c), 2.36 (s, 3H,
S-CH3), 4.30 (s, 1H, N4H), 3.40, 3.24 (s, cis/trans: 3/1, 2H, N4-
C1H2), 1.57 (m, 2H, –C2H2–), 1.28–1.18 (m, 10H, –C3H2–, –C4H2–,
–C5H2–, –C6H2–, –C7H2–), 0.81 (t, 3H, –C8H3).
2.3. Antioxidant capacity
2.3.1. CUPRAC antioxidant capacity
The CUPRAC method, as described by Apak et al. [34], is based
on the reduction of a cupric neocuproine complex (Cu(II)-Nc) by
compounds having antioxidant capacity to the yellow-orange col-
ored cuprous chelate (Cu(I)-Nc). The CUPRAC reaction mixture
consisted of CuCl2ꢁ2H2O (1 mL, 10 mmol dmꢀ3), Nc (1 mL, 7.5
mmol dmꢀ3), pH 7 NH4Ac buffer solution (1 mL, 1.0 mol dmꢀ3), x
mL synthesized compound, and H2O (1.1–x mL) in this order. The
mixture (4.1 mL) was then incubated at 25 °C. After 30 min of incu-
bation, the absorbance at 450 nm was recorded against a reagent
blank. Under the described experimental conditions, the calibra-
tion curves (absorbance versus concentration graphs) of each com-
pound were constructed, and their TEAC coefficients
2.2.2. Synthesis of cis-dioxo-(S-methyl-N1-5-bromosalicylidene-N4-
octyl thiosemicarbazonato)(N,N’,O)-methanol-molybdenum(VI),
(complex 5)
Considering the literature methods [33], S-methyl-N1-5-bro-
mosalicylidene-N4-octyl-thiosemicarbazone (L5), (0.40 g, 1.0
mmol) was dissolved in absolute methanol (4.0 mL) at about 60–
70 °C. The hot solution was treated with 1.0 mL of a methanolic
solution of MoO2(acac)2 (0.36 g,1.1 mmol). The reaction mixture
was stirred at 40 °C for 1 h. The red precipitate was collected by fil-
tration and washed twice with 2–4 mL of cold methanol. Recrystal-
lization of the product from methanol gave the analytical grade
pure compound. Other complexes (complex 1–4) were prepared
by a similar procedure. The air-dried samples of the complexes
were used for all analyses.
ꢀ
ꢁ
eeach compound
;
e
: molar absorptivity were calculated. The experi-
etrolox
mental antioxidant capacity results were performed in triplicate.
1: Red, m.p. 186–187 °C, yield 78%. Anal. Calc. for C13H18
BrMoN3O4S (488.20 g/mol): C, 31.98; H, 3.72; N, 8.61; S, 6.57.
-
2.4. ROS scavenging activity
Found: C, 31.89; H, 3.74; N, 8.75; S, 6.77%. UV–Vis (CHCl3) [kmax
e
(dm3 cmꢀ1 molꢀ1)]: 239 (18820), 253 (18900), 312
2.4.1. Hydroxyl radical scavenging activity
(nm),
(15980), 454 (3480). FT-IR (KBr, cmꢀ1):
t
(OH) 3446;
t
(C@N1)
Hydroxyl radical scavenging (HRS) assay was carried out fol-
lowing the spectrophotometric procedure of Özyürek et al. [35]
To a test tube were added phosphate buffer (1.5 mL, pH 7.0),
sodium salicylate probe (0.5 mL, 10 mmol dmꢀ3), Na2-EDTA (0.25
mL, 20 mmol dmꢀ3), FeCl2 solution (0.25 mL, 20 mmol dmꢀ3),
1600;
t , t
(C@N2) 1538; ts as(MoO2) 946, 915. 1H NMR (CDCl3):
8.61 (s, 1H, CH = N1), 6.94 (d, 1H, a), 7.53–7.51 (d, 2H, b, c), 2.53
(s, 3H, S-CH3), 3.68 (t, 2H, N4-C1H2), 1.60 (m, 2H, –C2H2–), 0.90
(m, 3H, –C3H3), 3.41 (s, 3H, –CH3).
H2O (2-x mL), synthesized compounds (ꢂmL, 1.0 mmol dmꢀ3
)
2: Red, m.p. 192–193 °C, yield 75%. Anal. Calc. for C14H20
-
and H2O2 (0.5 mL, 10 mmol dmꢀ3) rapidly in this order. The mix-
ture (total volume 5.0 mL) was then incubated at 37 °C. After 10
min of incubation, the reaction was stopped with adding catalase
solution (0.5 mL, 268 U mLꢀ1), and then absorbance value of the
0.5 mL final incubation solutions were recorded at 450 nm by
applying the CUPRAC method (5 min later). The percentage of
HRS of the tested compounds (%) were calculated using Eq. (1).
All experimental results were expressed as the mean standard
deviation (S.D.) of triplicate determinations.
BrMoN3O4S (502.23 g/mol): C, 33.48; H, 4.01; N, 8.37; S, 6.38.
Found: C, 33.43; H, 3.89; N, 8.45; S, 6.36%. UV–Vis (CHCl3) [kmax
(nm),
e
(dm3 cmꢀ1 molꢀ1)]: 239 (18520), 252 (18220), 312
(15360), 451 (3420). FT-IR (KBr, cmꢀ1):
t
(OH) 3453;
t
(C@N1)
1592;
t , t
(C@N2) 1538; ts as(MoO2) 938, 907. 1H NMR (CDCl3):
8.63 (s, 1H, CH = N1), 6.94 (d, 1H, a), 7.53–7.51 (d, 2H, b, c), 2.54
(s, 3H, S-CH3), 3.72 (t, 2H, N4-C1H2), 1.71 (m, 2H, –C2H2–), 1.34
(m, 2H, –C3H2–),0.90 (t, 3H, –C4H3), 3.42 (s, 3H, –CH3).
3: Red, m.p. 201–202 °C, yield 71%. Anal. Calc. for C15H22
-
ꢂ
ꢃ
BrMoN3O4S (516.26 g/mol): C, 34.90; H, 4.30; N, 8.14; S, 6.21.
Found: C, 34.85; H, 4.30; N, 8.21; S, 6.21%. UV–Vis (CHCl3) [kmax
A0 ꢀ A
HRS ð%Þ ¼ 100 ꢂ
ð1Þ
A0
(nm),
e
(dm3 cmꢀ1 molꢀ1)]: 239 (18640), 253 (19000), 311
(16060), 454 (3860). FT-IR (KBr, cmꢀ1):
t
(OH) 3438;
t
(C@N1)
where A0 and A are the CUPRAC absorbances of the system in the
absence and presence of synthesized compounds, respectively.
1600;
t , t
(C@N2) 1538; ts as(MoO2) 930, 915. 1H NMR (CDCl3):
8.58 (s, 1H, CH = N1), 6.93 (d, 1H, a), 7.52–7.50 (d, 2H, b, c), 2.52
(s, 3H, S-CH3), 3.70 (t, 2H, N4-C1H2), 1.71 (m, 2H, –C2H2–), 1.32–
1.21 (m, 4H, –C3H2–, –C4H2–),0.85 (t, 3H, –C5H3), 3.41 (s, 3H, –CH3).
2.4.2. Hydrogen peroxide scavenging activity
The hydrogen peroxide scavenging (HPS) activity assay was car-
ried out following the procedure of Özyürek et al. [36]. To three
test tubes were added pH 7.4 phosphate buffer (0.7–x mL), H2O2
(0.4 mL, 1 mM), x mL (0.2 mL) synthesized compounds, CuCl2ꢁ2H2-
O (0.4 mL, 1 mmol dmꢀ3) solution rapidly in this order (reference
solution without scavengers). These mixtures (total volume 1.5
mL) were then incubated at 37 °C. After 30 min of incubation, to
reference and scavenger solution-I was added 0.4 mL H2O and to
4: Red, m.p. 183–184 °C, yield 60%. Anal. Calc. for C16H24
-
BrMoN3O4S (530.28 g/mol): C, 36.24; H, 4.56; N, 7.92; S, 6.05.
Found: C, 35.96; H, 4.57; N, 7.85; S, 6.03%. UV–Vis (CHCl3) [kmax
(nm),
(15680), 456 (3740). FT-IR (KBr, cmꢀ1):
1600;
(C@N2) 1538; ts as(MoO2) 946, 915. 1H NMR (CDCl3):
8.63 (s, 1H, CH = N1), 6.95 (d, 1H, a), 7.52–7.32 (d, 2H, b, c), 2.54
e
(dm3 cmꢀ1 molꢀ1)]: 239 (18980), 253 (18840), 312
t
(OH) 3461; t
(C@N1)
t
, t

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S. Eg˘lence et al. / Inorganica Chimica Acta 469 (2018) 495–502
scavenger solution-II was added 0.4 mL of 268 U mLꢀ1 catalase
solution, and then absorbance value of the 1 mL final solutions
were recorded at 450 nm by applying the CUPRAC method. The
percentage of HPS of the tested compounds (%) were calculated
using Eq. (2):
The chemical shift values of all compounds are consistent with
desired structures and resemble the related molecules in the liter-
ature [39,40]. In the 1H NMR spectra of thiosemicarbazones, the
isomer peaks stem most probably from the syn-anti isomerism
which is not seen in the spectra of dioxomolybdenum(VI) com-
plexes due to hindered rotation of imine (C@N1) double bonds
[22,23]. The 1H NMR chemical shift values confirm the complex
structures in which ligand system coordinates to the molybdenum
center through deprotonated azomethine nitrogen and phenolic
oxygen. In the complex spectra, the signals from the phenolic OH
proton and –NH proton of thiosemicarbazone disappeared as a
consequence of chelate complex formation. Besides, the chemical
shifts of the aromatic protons were recorded at a lower field com-
pared to the free ligands in the values between 6.78–7.53 ppm. The
signal belonging to hydroxyl group of coordinated methanol mole-
cule was not clearly observed in the 1H NMR spectra, however, the
existence of coordinated methanol molecule was proved in the
spectra by seeing the new signal assigned to the methyl group at
3.41 ppm.
ꢂ
ꢃ
A0 ꢀ ðA1 ꢀ A2Þ
HPS ð%Þ ¼ 100 ꢂ
ð2Þ
A0
where A0 is the CUPRAC absorbance of reference H2O2 incubation
solution, A1 and A2 are the CUPRAC absorbances of scavenger-syn-
thesized compound solutions-I and -II, respectively.
2.4.3. Superoxide anion radical scavenging activity
The superoxide anion radical scavenging activity (SRSA) was
determined spectrophotometrically by measuring the formation
of nitroblue tetrazolium chloride (NBT) formazan at 560 nm [37].
To a test tube were added DMSO (completely solubilization of
the formazan is achieved by adding DMSO) (2.0 mL), synthesized
compounds (0.5 mL, 1.0 mmol dmꢀ3), nicotinamide adenine dinu-
cleotide (NADH) (2.0 mL, 0.468 mmol dmꢀ3), and NBT (1.0 mL,
0.3 mmol dmꢀ3) in this order. Reaction was launched by adding
phenazine methosulfate (PMS) solution (1.0 mL, 0.06 mmol
dmꢀ3). The mixture (total volume 6.5 mL) was then incubated at
25 °C. After 5 min of incubation, the absorbance was read at 560
nm against DMSO. The percentage of SRSA of the tested com-
pounds (%) were calculated. The inhibition ratio or SRSA of scav-
engers (%) was calculated using Eq. (3):
3.1. Crystallography
Complex 3 crystallizes in monoclinic crystal system with P 21/c
space group symmetry. Data and structural parameters of selected
orange-crystalline block with 0.03 ꢂ 0.20 ꢂ 0.40 size are presented
in Table 1. In the complex molecule, the thiosemicarbazone ligand
is coordinated to dioxomolybdenum(VI) center via phenolic oxy-
gen, azomethine nitrogen and thioamide nitrogen atoms (Fig. 3).
Considering the bond distance and angle values (Tables 2 and 3),
the coordination sphere of the complex can be described as dis-
torted octahedral geometry. Fourth and fifth coordination sites of
molybdenum atom was occupied by two oxo-oxygens (O1 and
O2) located in equatorial and axial positions of the octahedron.
Methanol molecule is attached in sixth coordination site of molyb-
denum through its oxygen atom. With a nearly Ooxo-Mo-Ooxo angle
ꢂ
ꢃ
A0 ꢀ A
SRSA ð%Þ ¼ 100 ꢂ
ð3Þ
A0
where Ao and A are the absorbances of the incubation reaction mix-
ture in the absence and presence of scavenger-synthesized com-
pound, respectively.
2.5. Enzyme activity
Table 1
Crystal data and structure refinement parameters for complex 3.
2.5.1. Catalase enzyme activity
Parameters
The method described by Bekdeser et al. was employed for the
determination of catalase enzyme activity [38]. The reaction mix-
ture consisted of H2O2 (0.5 mL, 1.0 mmol dmꢀ3), H2O (1.8 mL),
catalase solution (0.1 mL, 3.691 U mLꢀ1), and synthesized com-
pound (0.2 mL, 1.0 mmol dmꢀ3). This mixture (total volume 2.6
mL) was then incubated at 25 °C. After 30 min incubation period,
the optical CUPRAC sensor (Cu(II)-Nc-impregnated membrane)
was taken out and immersed in a test tube consisting of 2.0 mL
of the incubation reaction mixture + 6.2 mL of EtOH. After 30 min
agitation, the colored membrane was taken out and its absorbance
was recorded at 450 nm against that of a blank membrane without
tested sample.
CCDC deposition No.
Color/shape
1497812
Red/rod
C15H22BrMoN3O4S
Chemical formula
Formula weight (g molꢀ1
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell parameters
a, b, c (Å)
)
516.26
307(0)
0.71073
Monoclinic
P 21/c
13.3028(5), 9.5948(4), 15.6931(7)
a
, b,
c
(°)
90, 100.874(2), 90
1967.07(14)
4
Volume (Å3)
Z
Dcalc (g cmꢀ3
)
1.743
Absorption correction
min, Tmax
F(0 0 0)
Multi-scan
0.596, 1.000
1032
T
3. Results and discussion
Crystal size (mm)
Diffractometer/measurement
method
0.030 ꢂ 0.200 ꢂ 0.400
The analytical and spectroscopic data of the ligands and com-
plexes are in accordance with the obtained structures. The pheno-
lic
m(OH) and m(NH) vibrations of the ligands disappeared by
Index ranges h,k,l
ꢀ15 ꢃ h ꢃ 15, ꢀ11ꢃk ꢃ 11, ꢀ18 ꢃ l ꢃ
17
complexation. Additionally, the observed shift to the lower
h Range for data collection (°)
Reflections collected
Independent reflections
Rint
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
2.50 ꢃ h ꢃ 25.00
wavenumber region in the bands assigned to the azomethine
18613
groups, m m
(C@N)1 and (C@N)2, confirm the formation of the com-
3462
0.1450
plex and ONN donor set. The band appearing at 3438–3461 cmꢀ1
region shows the hydroxyl group of methanol molecule coordi-
nated to the dioxomolybdenum center. The characteristic bands
of dioxomolybdenum group were also monitored at 930–946
Full-matrix least-squares on F2
3462/181/233
1.053
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0480, wR2 = 0.1232
R1 = 0.0691, wR2 = 0.1383
cmꢀ1 as asymmetric
t
as(MoO2) and 900–915 cmꢀ1 as symmetric
ts(MoO2) in spectra of complexes.

S. Eg˘lence et al. / Inorganica Chimica Acta 469 (2018) 495–502
3.2. Antioxidant studies
499
3.2.1. Antioxidant properties
Thiosemicarbazones and their metal complexes possess many
bioactivities including antitumor, antifungal, antiviral, antibacte-
rial and antioxidant activities. Various activities of these com-
pounds are based on the position of attachment of
thiosemicarbazone group to the ring as well as the form of
thiosemicarbazone moiety and the nature of the hetero atomic
ring [41]. There were limited studies about the antioxidant capac-
ity and ROS scavenging activity of thiosemicarbazones [42]. The
CUPRAC molar absorption coefficient (e) of the tested compound
divided by that of TR under the same conditions [34] gave the
TEAC-CUPRAC values (Table 4). Amongst the compounds screened
for antioxidant capacity, L1 exhibited the highest antioxidant
capacity. As we compared the thiosemicarbazone ligands and
their complexes with TR, the TEAC coefficients of L1-L5 were
higher than unity (TEACTR = 1.0) and followed the order of L1 >
L3 > L4 > L5 > L2. When testing for correlation at the 5% signifi-
cance level (2-tailed test), Spearman’s rank correlation test did
not yield a statistically significant association between the order
of TEAC values and of other ROS scavenging assays. This is natu-
ral, as the electron donation capability of the tested compounds is
not identical with ROS scavenging ability, considering the differ-
ent mechanisms involved [43]. The complexes 1–5 showed less
antioxidant capacity with lower TEAC values (Table 4). A similar
observation, showing less antioxidant capacity (with respect to
DPPH scavenging) of Cu(II) and Ni(II) complexes of thiosemicar-
bazone ligands than those of the parent compounds, was also
reported in the literature [44], probably due to the coordinative
blocking of electron-donating sites important in electron-transfer
based antioxidant capacity measurement. For dioxomolybdenum
(VI) complexes without OH group in the ring, the TEAC value
was smaller, because electron transfer-based antioxidant capacity
is usually associated with electron donation from the phenolic
hydroxyl groups.
Fig. 3. ORTEP diagram of complex
ellipsoids and the atom-numbering scheme.
3 showing 50% probability displacement
Table 2
Bond lengths (Å) for complex 3.
Mo-O2
Mo-O4
Mo-N3
1.698(4)
1.946(4)
2.229(4)
Mo-O1
Mo-N1
Mo-O3
1.698(4)
2.058(4)
2.367(4)
Table 3
Bond angles (°) for complex 3.
O2-Mo-O1
O1-Mo-O4
O1-Mo-N1
O2-Mo-N3
O4-Mo-N3
O2-Mo-O3
O4-Mo-O3
N3-Mo-O3
105.33(18)
O2-Mo-O4
O2-Mo-N1
O4-Mo-N1
O1-Mo-N3
N1-Mo-N3
O1-Mo-O3
N1-Mo-O3
104.29(18)
95.67(19)
150.06(16)
96.55(16)
71.55(14)
172.44(16)
82.00(14)
98.02(19)
97.94(19)
156.11(16)
81.58(15)
82.17(16)
78.93(15)
76.21(13)
of 105.33°, two oxo groups of dioxomolybdenum moiety lie in cis
position to one another. The bond distance between molybdenum
center and the hydroxy oxygen atom (O3) from methanol is 2.367
Å and shows that the solvent associates with a weak interaction. In
the crystal structure, two dioxomolybdenum complexes connect to
the each other via N2ꢁ ꢁ ꢁH11-O3 hydrogen bond, so forming
together a centrosymmetric dimer (Fig. 4). These dimers are
stacked in the crystal such as independent units (Fig. 5).
3.2.2. Hydroxyl radical scavenging activity
Hydroxyl radicals (ÅOH), produced via Fenton reaction, attack
the salicylate probe to form products that, upon incubation in solu-
tions under acidic conditions, yield a yellow-orange product (Cu(I)-
Nc) with Cu(II)-Nc. Hydroxyl radical scavengers compete with the
Å
probe for the produced OH, and result in decreased chromophore
formation from Cu(II)-Nc. HRS activities of reference (TR), free
ligands and their cis-dioxomolybdenum(VI) complexes (as per-
Fig. 4. Intermolecular interactions of complex 3.

500
S. Eg˘lence et al. / Inorganica Chimica Acta 469 (2018) 495–502
Fig. 5. Crystal motif for complex 3.
Table 4
The TEAC coefficients of the samples with respect to the CUPRAC assay.
Sample
Working range
Calibration equation
Correlation coefficient (r)
TEAC
L1
L2
L3
L4
L5
1
2
3
4
5
4.69 ꢂ 10ꢀ6–1.97 ꢂ 10ꢀ5
5.08 ꢂ 10ꢀ6–1.98 ꢂ 10ꢀ5
4.65 ꢂ 10ꢀ6–1.90 ꢂ 10ꢀ5
4.38 ꢂ 10ꢀ6–1.94 ꢂ 10ꢀ5
4.62 ꢂ 10ꢀ6–1.91 ꢂ 10ꢀ5
3.76 ꢂ 10ꢀ5–7.55 ꢂ 10ꢀ5
3.66 ꢂ 10ꢀ5–7.46 ꢂ 10ꢀ5
3.79 ꢂ 10ꢀ5–7.52 ꢂ 10ꢀ5
3.51 ꢂ 10ꢀ5–6.96 ꢂ 10ꢀ5
3.63 ꢂ 10ꢀ5–7.41 ꢂ 10ꢀ5
A = 2.66 ꢂ 104c ꢀ 0.015
A = 1.97 ꢂ 104c + 0.019
A = 2.59 ꢂ 104c ꢀ 0.010
A = 2.40 ꢂ 104c + 0.024
A = 2.28 ꢂ 104c + 0.005
A = 1.21 ꢂ 104c ꢀ 0.106
A = 1.11 ꢂ 104c ꢀ 0.085
A = 1.07 ꢂ 104c ꢀ 0.087
A = 9.83 ꢂ 103c ꢀ 0.045
A = 9.26 ꢂ 103c ꢀ 0.016
0.9944
0.9976
0.9906
0.9911
0.9954
0.9744
0.9854
0.9832
0.9986
0.9902
1.59 0.03
1.18 0.04
1.55 0.06
1.44 0.06
1.37 0.05
0.73 0.02
0.66 0.03
0.64 0.02
0.59 0.03
0.55 0.01
Table 5
centage inhibition ratio) were assayed by using the above-men-
tioned model [37]. The ascending order of synthesized ligands
towards the HRS activity was: L1 > L2 > L3 > L4 > L5. When testing
for correlation (2-tailed test), Spearman’s rank correlation test
yielded statistically significant association between the order of
HRS values and of other ROS scavenging assays (for example,
HRS-HPS correlation gave R = 1.0 for P = 0, and HRS-SRSA correla-
tion gave R = 0.9 for P = 0.0374), due to the similarity between
the mechanisms of ROS scavenging assays [43]. On the other hand,
the order of HRS activity of the synthesized compounds in terms of
percentage inhibition ratio was: 1 > 2>3 > 4 > 5. The HRS activities
of the dioxomolybdenum(VI) complexes (complex 1 and 2) were
also higher than that of TR.
ROS scavenging activities of the samples, as percentage inhibition ratio.
Scavenger Hydroxyl radical
scavenging
Hydrogen peroxide Superoxide anion
scavenging activity radical scavenging
activity (%)
(%)
activity (%)
L1
L2
L3
L4
L5
1
2
3
4
5
84.56 4.22
79.53 6.01
68.75 20.6
25.76 1.03
13.84 0.69
84.86 3.39
82.30 2.24
81.63 2.44
79.82 3.12
65.46 3.82
33.03 0.74
30.11 0.62
28.95 1.70
24.23 1.21
23.84 0.47
74.38 2.97
69.37 2.77
63.51 1.90
68.62 2.74
53.47 1.07
20.55 0.41
19.41 0.64
16.34 0.36
17.21 0.63
10.36 0.52
55.83 2.23
54.58 2.22
47.08 1.88
53.99 2.15
48.40 1.94
Initial concentrations: 1 ꢂ 10ꢀ4 M.
3.2.3. Hydrogen peroxide scavenging activity
3.2.4. Superoxide anion radical scavenging activity
The HPS activities of synthesized compounds were directly
measured without interference in the presence of Cu(II) catalyst
using the HPS-CUPRAC method [36]. The percentage inhibition
ratio ranged between 13.8% and 84.9% (Table 5). The order of
HPS activity of the synthesized ligands in terms of inhibition ratio
was: L1 > L2 > L3 > L4 > L5. The HPS activities of the ligands were
higher than that of trolox (17.0%). On the other hand, the order
of HPS activity of the synthesized compounds in terms of inhibition
ratio was: 1 > 2 > 4 > 3 > 5. The HPS activities of the complexes
were generally higher than those of the parent ligands (Table 5),
in accordance with the well-known transition metal ion-catalyzed
degradation of hydrogen peroxide.
Various antioxidant compounds are able to inhibit the forma-
tion of blue formazan from NBT [37]. Decrease in absorbance with
antioxidants is measured at 560 nm which is proportional to the
amount of OÅ2ꢀ in the reaction mixture. As shown in Table 5, the
percentage inhibition of OÅ2ꢀ generation by 0.1 mM concentrations
of the synthesized compounds were found between 8.0% and
98.9%. The order of SRSA activity of the synthesized compounds
in terms of percentage inhibition ratio was: L1 > L2 > L4 > L3 >
L5. The order of SRSA had strong positive correlation (using Spear-
man’s rank correlation) with those of other ROS scavenging assays
(HRS and HPS). The SRSA values of the ligands were smaller than

S. Eg˘lence et al. / Inorganica Chimica Acta 469 (2018) 495–502
501
Fig. 6. Catalase enzyme activities of the ligands (L1-L5) and complexes (1–5), U mLꢀ1
.
that of trolox (36.3%). SRSA mainly arises from the reaction affinity
4. Conclusions
of the OÅ2ꢀ with redox-active transition metal ions and their com-
plexes. On the other hand, the order of HPS activity of the synthe-
sized compounds in terms of percentage inhibition ratio was: 1 >
2>4 > 3 > 5. The SRSA activities of the complexes were generally
higher than those of the parent ligands (Table 5), in accordance
with the well-known transition metal ion-catalyzed degradation
of superoxide anion radicals.
We investigated antioxidant, ROS scavenging and catalase
enzyme activities of newly synthesized five ONN-chelating
thiosemicarbazone ligands and their dioxomolybdenum(VI) com-
plexes. The ligands and complexes had relatively high antioxi-
dant potential in comparison with trolox reference; especially
L1 exhibited better antioxidant capacity than the other ligands
and complexes. The results revealed that complex 1 exhibited
high ROS scavenging properties (depending on the type of
ROS) compared to the ligands and also other complexes. Gener-
ally in ROS scavenging tests, the order of antioxidant activity of
the compounds followed the order of L1 > L2 > L3 > L4 > L5, in
accordance with the length of the alkyl side chain on the thioa-
mide-nitrogen atom. This is most probably associated with rad-
ical stabilization, because when a H-atom (or an electron) is
donated from thioamide nitrogen of the compound –N = C(S-
Me)-NH-R as a result of antioxidant action, the length of the
electron-donating alkyl group is an important factor in stabiliz-
ing the emerging radical –N = C(S-Me)-NÅ-R. The more this N-
centered radical is stabilized upon electron loss, the stronger will
be the antioxidative effect (like the antioxidant activity of phe-
nolic compounds via stabilization of the 1-e oxidized semiqui-
3.2.5. Overall evaluation of antioxidant capacity/activity results
Certain copper(II)-thiosemicarbazone complexes may owe their
antitumor and antiinflammatory activity to their antioxidant abil-
ity [45]. Recently, Ceylan et al. [27] reported much higher CUPRAC
antioxidant capacity and DPPH radical scavenging activity values
for S-propyl-4-phenyl-thiosemicarbazone than for its Mo(VI)-com-
plexes, probably due to the deprotonation and coordination of the
2-hydroxyl group on the ligand responsible for antioxidative (i.e.,
phenolic H-atom and/or electron donation) properties. Although
the complexes possessed less CUPRAC antioxidant capacity than
ligands (Table 4), they showed higher antioxidant activity in terms
of ROS scavenging than the parent compounds (Table 5), possibly
due to the enhanced ROS ꢀ degradative power of transition metal
ion complexes.
none
radicals).
The
hyperconjugative
stabilization
of
Thiosemicarbazone complexation with the metal ion may pro-
tect the ligand against enzymatic degradation because of the inert-
ness of certain metal-ligand linkages. The metal chelating power of
thiosemicarbazones can naturally reduce the polarity of the central
metal atom such as Mo(VI) due to charge neutralization (i.e. partial
sharing of metal ion’s positive charge with the ligand) thereby
increasing its bioavailability. Therefore, the activity can be
strengthened by the combined effect of the metal and ligand
[25]. On the other hand, there may be antioxidant capacity-inhibi-
tive combinations as well, especially when the phenolic hydroxyl
and amine functionalities of the parent ligand are coordinated to
the central metal ion, thereby obstructing H-atom donation or
electron donation power.
alkylaminyl radicals occurs through interaction of the unpaired
spin (on nitrogen) with adjacent C-H (or C-C) bonds. Hypercon-
jugation results from overlap between the unpaired electron in a
2p atomic orbital on nitrogen with the occupied rCH bond orbi-
tals on the adjacent methylene group. Although the stability of
alkylaminyl radicals is expected to increase with the number
of alkyl substituents attached to the N-radical center, increasing
the size of the attached alkyl group was shown to lead to less
efficient hyperconjugative stabilization, as calculated from the
corresponding radical stabilization energies [46]. In accordance
with common knowledge, longer alkyl chain (the increase in
molecular size) also resulted in a decrease in biological activity
potential. In summary, it can be said that these thiosemicar-
bazone ligands and dioxomolybdenum(VI) complexes have a rel-
atively high potential in terms of their catalase inhibition and
antioxidant activities.
3.3. Catalase enzyme inhibition activity
As shown in Fig. 6, some of the compounds L4 (from ligands)
and complex 4 (from complexes) caused significant elevation of
catalase activity. Free ligands were generally shown to exhibit
higher CAT activity than their dioxomolybdenum(VI) complexes.
Conflict of interest
The authors declare that they have no conflict of interest.

502
S. Eg˘lence et al. / Inorganica Chimica Acta 469 (2018) 495–502
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Acknowledgement
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Polyhedron 85 (2015) 93–103.
This work was supported by the Scientific Research Projects
Coordination Unit of Istanbul University (Project Number: 19222).
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CCDC 1497812 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
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