Synthesis, structural studies and catalytic activity of a series of dioxidomolybdenum(VI)-thiosemicarbazone complexes

Article abstract of DOI:10.1016/j.ica.2018.01.023

Reaction of the thiosemicarbazone ligands, [4-(p-bromophenyl)thiosemicarbazone of salicylaldehyde (H₂L¹), 4-(p-X-phenyl)thiosemicarbazone of o-vanillin {X = F (H₂L²), X = Cl (H₂L³) and X = OMe (H₂L⁴)}, 4-(p-bromophenyl)thiosemicarbazone of 5-bromosalicylaldehyde (H₂L⁵), and 4-(p-chlorophenyl)thiosemicarbazone of o-hydroxynaphthaldehyde (H₂L⁶)] with [MoO₂(acac)₂] afforded a series of new oxidomolybdenum(VI) complexes [Mo^(VI)O₂L^1–6(solv)] (1–6) {where solv (solvent) = DMSO (1, 3, 5 & 6) and H₂O (2 & 4)}. The molecular structures of 2 and 3 were determined by X-ray crystallography, demonstrating the dibasic tridentate behavior of ligands. The cyclic voltammogram pattern is similar for 1–6, which includes two irreversible reduction processes within the potential window ?0.71 to ?0.66 V and ?0.92 to ?0.85 V corresponding to the metal centered reduction from Mo^(VI)/Mo^(V) and Mo^(V)/Mo^(IV) respectively. Catalytic potential of 1–6 was tested for the oxidation of styrene and cyclohexene. The effect of various parameters such as the amount of catalyst, oxidant, NaHCO₃, and solvent was checked to optimize the conditions for the best performance of the catalyst. 100% product selectivity for the formation of cyclohexene oxide from cyclohexene and ～98–99% product selectivity for the oxidation of styrene to styrene oxide was observed.

Full text of DOI:10.1016/j.ica.2018.01.023

Inorganica Chimica Acta 474 (2018) 134–143
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis, structural studies and catalytic activity of a series of
dioxidomolybdenum(VI)-thiosemicarbazone complexes
Satabdi Roy a,b, Saswati a, Sudhir Lima a, Sarita Dhaka c,d, Mannar R. Maurya c, Rama Acharyya a,
Cassandra Eagle e, Rupam Dinda a,
⇑
a Department of Chemistry, National Institute of Technology, Rourkela 769008, Odisha, India
b Department of Chemistry, Indian Institute of Technology, Kanpur 208016, Uttar Pradesh, India
c Department of Chemistry, Indian Institute of Technology, Roorkee 247667, India
d Central Scientific Instruments Organisation (CSIR-CSIO), Sector 30 C, Chandigarh 160030, India
e Department of Chemistry, East Tennessee State University, TN 37614, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of the thiosemicarbazone ligands, [4-(p-bromophenyl)thiosemicarbazone of salicylaldehyde
(H2L1), 4-(p-X-phenyl)thiosemicarbazone of o-vanillin {X = F (H2L2), X = Cl (H2L3) and X = OMe (H2L4)},
4-(p-bromophenyl)thiosemicarbazone of 5-bromosalicylaldehyde (H2L5), and 4-(p-chlorophenyl)
thiosemicarbazone of o-hydroxynaphthaldehyde (H2L6)] with [MoO2(acac)2] afforded a series of new
oxidomolybdenum(VI) complexes [Mo(VI)O2L1–6(solv)] (1–6) {where solv (solvent) = DMSO (1, 3, 5 & 6)
and H2O (2 & 4)}. The molecular structures of 2 and 3 were determined by X-ray crystallography, demon-
strating the dibasic tridentate behavior of ligands. The cyclic voltammogram pattern is similar for 1–6,
which includes two irreversible reduction processes within the potential window À0.71 to À0.66 V
and À0.92 to À0.85 V corresponding to the metal centered reduction from Mo(VI)/Mo(V) and
Mo(V)/Mo(IV) respectively. Catalytic potential of 1–6 was tested for the oxidation of styrene and cyclohex-
ene. The effect of various parameters such as the amount of catalyst, oxidant, NaHCO3, and solvent was
checked to optimize the conditions for the best performance of the catalyst. 100% product selectivity for
the formation of cyclohexene oxide from cyclohexene and ꢀ98–99% product selectivity for the oxidation
of styrene to styrene oxide was observed.
Received 24 October 2017
Received in revised form 19 January 2018
Accepted 23 January 2018
Keywords:
Thiosemicarbazone
Oxidomolybdenum(VI)
X-ray crystal structure
Epoxidation catalysts
Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction
to aldehydes over alcohols [39] and have also demonstrated
chemoselective deoxygenation of sulfoxides to sulfides in excellent
The transition element molybdenum (Mo) is of essential impor-
tance for nearly all biological systems as it is required by enzymes
catalyzing diverse key reactions in the global carbon, sulfur and
nitrogen metabolism [1]. It occurs in a wide range of metalloen-
zymes in bacteria, fungi, algae, plants, and animals where it forms
part of the active sites of these enzymes [2].
Molybdenum complexes are known for their catalytic applica-
tions in the oxidation [3–19] and oxidative bromination of organic
substrates [20–23], and the oxidation of sulfides [24–27]. Mo(VI)
complexes are also known to be very active as epoxidation cata-
lysts, generally in a solution of an organic solvent, especially for
neutral complexes [28–30], much as evidenced by a number of
publications [11,16,18,24,31–38]. Oxido-peroxido Mo complexes
have been known to exhibit selective catalytic oxidation of alkanes
yields [25]. The catalytic activity of molybdenum complexes are
sensitive to the donor/acceptor ability of the ligand and to steric
and strain factors.
In addition, thiosemicarbazones are versatile ligands which can
coordinate in both their neutral and anionic form [40,41]. Transi-
tion metal complexes derived from thiosemicarbazones have
shown potent catalytic activity such as oxidation of benzyl alcohol
and cyclohexanol [42,43], catalysts for the aryl–aryl coupling reac-
tion [44], reduction and condensation of nitriles [45] and the
Suzuki–Miyaura coupling reaction [46]; the catalytic efficiency
being dependent on the substituents on the thiosemicarbazone
ligand core.
In continuation with our previous studies on the catalytic activ-
ity of dioxidomolybdenum(VI) complexes of O, N donating ligands
[22,47,48], herein we have tried to explore the catalytic activity of
complexes derived from ligands containing ONS donor atoms for
epoxide generation. Epoxides are important pharmacophores in
⇑
Corresponding author.
E-mail address: rupamdinda@nitrkl.ac.in (R. Dinda).
https://doi.org/10.1016/j.ica.2018.01.023
0020-1693/Ó 2018 Elsevier B.V. All rights reserved.

S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
135
bioactive natural products, such as dynemicin and fumagillin
[49,50] and are readily obtained from alkenes by the use of strong
organic oxidants (m-CPBA, NaClO4) [51] or smoother oxidants
(THBP, H2O2) [28,29,52] with the assistance of metal-based cata-
lysts (Fe, Mn, Re, Mo, V, W) [28,29,52–54]. Among these metals,
molybdenum garnered much attention.
mixture was refluxed for 2 h and filtered. Dark brown colored
microcrystalline residues were obtained in each case, which was
recrystallized from DMSO to obtain complexes [Mo(VI)O2L1–6(solv)]
(1–6).
2.2.1. [Mo(VI)O2L1(DMSO)] (1)
In this work, we have described the synthesis of a series of
cis–dioxidomolybdenum(VI) [Mo(VI)O2L1/5/6(DMSO)] (1, 5 & 6),
[Mo(VI)O2L2/4(H2O)] (2 & 4), and [Mo(VI)O2L3(DMSO)]4Á2DMSO (3)
complexes using tridentate ONS donating thiosemicarbazone
ligands. The complexes have been fully characterized by
spectroscopic methods and their redox behavior studied by cyclic
voltammetry. Molecular structures of 2 and 3 have been deter-
mined by X–ray crystallography. The role of the synthesized Mo(VI)
complexes (1–6) as catalysts for the epoxidation of styrene and
cyclohexene have also been studied, considering the importance
of epoxides as precious precursors in organic synthesis.
Yield: 0.40 g (73%). Anal. calc. for C16H16BrMoN3O4S2: C, 34.67;
H, 2.91; N, 7.58. Found: C, 34.65; H, 2.90; N, 7.60. 13C NMR
(DMSO d6, 100 MHz) d: 161.55, 159.41, 155.60, 152.56, 147.76,
140.38, 134.74, 134.57, 131.83, 122.02, 121.50, 121.12, 118.58,
114.42, 50.34. ESI-MS (CH3CN): m/z 550.44 [MÀ4H]+, 476.74
[MÀDMSO]+.
2.2.2. [Mo(VI)O2L2(H2O)] (2)
Yield: 0.31 g (68%). Anal. calc. for C15H14FMoN3O5S: C, 38.89; H,
3.05; N, 9.07. Found: C, 38.87; H, 3.02; N, 9.10. 13C NMR (DMSO d6,
100 MHz) d: 172.74, 163.51, 155.51, 147.67, 146.86, 143.83,
143.74, 137.75, 134.33, 127.54, 125.59, 124.76, 120.68, 118.47,
51.56. ESI-MS (CH3CN): m/z 463.29 [M]+.
2. Experimental
2.2.3. [Mo(VI)O2L3(DMSO)]4Á2DMSO (3)
2.1. Materials and methods
Yield: 1.20 g (52%). Anal. calc. for C72H84Cl4Mo4N12O22S10: C,
37.34; H, 3.66; N, 7.26. Found: C, 37.37; H, 3.68; N, 7.25. 13C
NMR (DMSO d6, 100 MHz) d: 169.49, 167.35, 162.37, 158.23,
155.06, 148.98, 146.77, 144.58, 140.30, 137.67, 135.53, 131.47,
126.46, 120.96, 53.35, 48.27. ESI-MS (CH3CN): m/z 2159.67
[MÀ2DMSO]+.
All chemicals were purchased from commercial sources and
used without further purification. Reagent grade solvents were
dried and distilled prior to use. [Mo(VI)O2(acac)2] was prepared as
described in the literature [55]. The thiosemicarbazides were pre-
pared from distilled substituted aniline by a known method
reported earlier [56]. The ligands 4-(p-bromophenyl)thiosemicar-
bazone of salicylaldehyde (H2L1), 4-(p-X-phenyl)thiosemicar-
bazone of o–vanillin {where X = F (H2L2) and X = Cl (H2L3) and
X = OMe (H2L4)}, 4-(p-bromophenyl)thiosemicarbazone of 5–bro-
mosalicylaldehyde (H2L5), and 4–(p-chlorophenyl)thiosemicar-
bazone of o-hydroxynaphthaldehyde (H2L6) were prepared by
reported methods [41,57,58]. Elemental analyses were performed
on a Vario ELcube CHNS Elemental analyzer. IR spectra were
recorded on a Perkin–Elmer Spectrum RXI spectrometer. 1H and
13C NMR spectra were recorded with a Bruker Ultrashield 400
MHz spectrometer using SiMe4 as an internal standard. Electronic
spectra were recorded on a Lamda25, PerkinElmer spectropho-
tometer. Electrochemical data were collected using PAR electro-
chemical analyzer and a PC–controlled potentiostat/galvanostat
(PAR 273A) at 298 K in a dry nitrogen atmosphere, using CH3CN
as the solvent medium. The complex concentration used for cyclic
voltammetry was 10À3 M. Cyclic voltammetry experiments were
carried out with glassy carbon working electrode, platinum auxil-
iary electrode and Ag/AgCl electrode. The starting potential used
for recording the cyclic voltammogram was 0.0 V. Commercially
available TEAP (tetraethyl ammonium perchlorate) was dried and
used as a supporting electrolyte (0.1 M) for recording cyclic
voltammograms of the complexes. ESI-MS was recorded on the
SQ–300 MS instrument operating in ESI mode, employing complex
concentration of 100 pmol/microliter. The capillary exit voltage
2.2.4. [Mo(VI)O2L4(H2O)] (4)
Yield: 0.31 g (67%). Anal. calc. for C16H17MoN3O6S: C, 40.43; H,
3.60; N, 8.84. Found: C, 40.45; H, 3.62; N, 8.81. 13C NMR (DMSO d6,
100 MHz) d: 176.34, 172.45, 167.53, 160.43, 158.76, 150.88,
144.14, 142.46, 140.21, 135.54, 129.23, 123.52, 119.74, 113.45,
56.13, 50.24. ESI-MS (CH3CN): m/z 475.40 [M]+.
2.2.5. [Mo(VI)O2L5(DMSO)] (5)
Yield: 0.43 g (68%). Anal. calc. for C16H15Br2MoN3O4S2: C, 30.35;
H, 2.39; N, 6.64. Found: C, 30.38; H, 2.40; N, 6.65. 13C NMR
(DMSO d6, 100 MHz) d: 168.63, 159.45, 154.72, 151.35, 144.74,
141.33, 135.56, 132.35, 130.54, 128.01, 123.65, 120.64, 117.87,
112.53, 52.67. ESI-MS (CH3CN): m/z 633.80 [M]+.
2.2.6. [Mo(VI)O2L6(DMSO)] (6)
Yield: 0.39 g (71%). Anal. calc. for C20H18ClMoN3O4S2: C, 42.90;
H, 3.24; N, 7.50. Found: C, 42.92; H, 3.22; N, 7.51. 13C NMR
(DMSO d6, 100 MHz) d: 168.53, 167.12, 165.53, 158.80, 156.35,
153.65, 146.34, 143.67, 140.64, 139.12, 136.57, 130.76, 126.36,
122.56, 120.65, 117.87, 113.77, 112.63, 56.32. ESI-MS (CH3CN):
m/z 557.30 [MÀ2H]+.
2.3. X-ray crystallography
Single crystals of complexes were mounted on, Rigaku XtaLAB
mini diffractometer (2 and 3), equipped with a graphite monochro-
⁰
was 120 V and the drying gas temperature was 300 C. The identity
of the catalytic products was confirmed using a GC–MS model Per-
kin–Elmer, Clarus 500 by comparing the fragments of each product
with the library available. The percent conversion of the substrate
and the selectivity of the products were calculated from GC data
using the formulae presented elsewhere [13].
mator and a Mo K
a source (k = 0.71073 Å). Crystallographic data
and details of refinement of 2 and 3 are given in Table 1. The unit
cell dimensions and intensity data were measured at 293 K for 2
and 3. The data were integrated and scaled using SAINT, SADABS
within the APEX2 software package by Bruker [59]. Absorption cor-
rections were applied using SADABS [60] and the structures were
solved by direct methods using the program SHELXS-97 [61] and
refined using least squares with the SHELXL-97 [61] software pro-
gram. Hydrogen atoms were placed in geometrically idealized
positions and constrained to ride on their parent atoms with C–H
distances in the range 0.95–1.00 Å. The non–hydrogen atoms were
refined anisotropically.
2.2. Synthesis of dioxidomolybdenum(VI) complexes [Mo(VI)O2L1–6(solv)]
(1–6)
The synthetic procedure for complex preparation is as reported
previously [41,57]. To a solution of ligand, H2L1–6 (1.0 mmol), 30
mL of ethanol, Mo(VI)O2(acac)2 (1.0 mmol) was added. The resulting

136
S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
O(1)
Table 1
*
Crystal data and refinement details of [Mo(VI)O2L2,3(solv)] (2 and 3).
OH
X
Complex
1
solv
X
Y
Ligand
S(1)
C(7)
*
Compound
2
3
*
C
N
S
H2L1
N(2)
H
DMSO
H2O
N(1)
Br
H
N
C
Formula
M
C
15H14FMoN3O5S
C72H84Cl4Mo4N12O22S10
2315.67
H
N(3)
C(8)
463.29
N
Y
H2L2
H2L3
H2L4
F
2
3
4
H
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P 21/c
15.082(15)
13.711(13)
10.399(10)
90
Monoclinic
C 2/c
22.871(13)
17.611(10)
11.323(8)
90
I
Cl
o-OCH3
DMSO
H2O
O
O
OCH3
O
N
MoVI
p-Br
H2L5
H2L6
DMSO
DMSO
Br
Cl
5
6
S
a
(°)
b (°)
93.938(7)
90
2145.3(4)
4
1.434
928
92.129(7)
90
4557.3(5)
2
1.688
2344
solv
c
(°)
Ph
II
V (Å3)
Z
Dcalc (Mg.m–3
F(0 0 0)
)
Chart 1. Representation of ligands (I) and dioxidomolybdenum complexes (II),
[Mo(VI)O2L1–6(solv)] (1–6); * = donor sites.
l(Mo–K
a
) (mm–1
)
0.743
0.959
max./min. trans.
2h(max) (°)
0.8778/0.8189
27.48
0.8313/0.8313
27.48
analyses, spectroscopic characteristics (IR, electronic, 1H and 13C
NMR), ESI-MS, and single crystal X-ray diffraction studies of 2
and 3. All the complexes are highly soluble in aprotic solvents,
viz. DMF or DMSO and are sparingly soluble in CH3OH, CH3CN,
and CHCl3.
Reflections collected/
unique
4912/4912 [R(int) =
0.0293]
R1 = 0.0332, wR2 =
0.0973
R1 = 0.0383, wR2 =
0.1003
23,695/5226 [R(int) =
0.0226]
R1 = 0.0214, wR2 =
0.0548
R1 = 0.0233, wR2 =
0.0557
R1[I > 2
r
(I)]
wR2 [all data]
S [goodness of fit]
min./max. res. (e.Å–3
CCDC deposition
number
1.076
0.720/–0.459
1408558
1.052
0.277/–0.532
1408559
)
3.2. Spectral characteristics
3.2.1. IR spectroscopy
The infrared spectra of complexes 1–6 exhibits two strong
absorptions in the range 933–887 cm–1, besides typical ligand
vibrations, which are attributed to the symmetric and antisymmet-
2.4. Catalytic reactions of [Mo(VI)O2L1–6(solv)] (1–6)
ric
firming the formation of dioxidomolybdenum(VI) complexes. The
m
(Mo@O) vibrations of the C2v cis-MoO22+ groups [33], thus con-
2.4.1. Oxidation of styrene
sharp band in the range 780–792 cmÀ1 due to
m(C(8)–S(1)) stretch-
In a typical oxidation reaction, styrene (0.520 g, 5 mmol), aque-
ous 30% H2O2 (2.27 g, 20 mmol), catalyst (0.0010 g, 1.8 Â 10À6
mmol) and NaHCO3 (0.126 g, 1.5 mmol) were mixed in 5 mL of
CH3CN. The reaction mixture was heated at 60 °C for 4 h. The pro-
gress of the reaction was monitored by withdrawing samples at
different time intervals and extracted with n-hexane, which were
then analyzed quantitatively using gas chromatography. The effect
of various parameters such as the amount of catalyst, oxidant,
NaHCO3, and solvent were checked to optimize the conditions for
the best performance of the catalyst. The identity of the products
was confirmed by GC mass.
ing in the ligand is lowered by 10–15 cmÀ1 upon complexation,
indicating participation of the thione sulfur in coordination. The
infrared spectra of all the complexes are mostly similar. The
ligands exhibit bands due to m(–C(1)O(1)–H) moiety in the 3357–
3300 cm1 region, however except complexes 2 and 4 (having coor-
dinated H2O molecule to the Mo center) other complexes do not
exhibit bands in the region beyond 3300 cmÀ1. Thus it reveals that
the ligands coordinate to the metal centre in their phenolate form.
The detailed IR data has been tabulated in Table 2.
3.2.2. UV–vis spectroscopy
2.4.2. Oxidation of cyclohexene
The electronic spectra of complexes 1–6 were recorded in
CH3CN and are listed in Table 3. The representative UV–vis spec-
trum of [Mo(VI)O2L2(H2O)] (2) is shown in Fig. 1. Complexes 1–6
display medium intensity band in 442–417 nm region and two
strong absorptions in 346–243 nm range, for the LMCT and intrali-
gand transitions, respectively [62,63]. As compared to the ligands,
the complexes have an extra peak in 442–417 nm region, which
indicates coordination of the ligand upon complexation. We have
assigned the LMCT signals of metal based transitions on the basis
of previous reports by our group [47,48,64,65]. Therefore we did
not go for the theoretical study.
Aqueous 30% H2O2 (1.70 g, 15 mmol), cyclohexene (0.41 g, 5
mmol), NaHCO3 (0.126 g, 1.5 mmol) and catalyst (0.0010 g, 1.8 Â
10À6 mmol) were mixed in CH3CN (5 mL) and the reaction mixture
was heated at 60 °C with continuous stirring in an oil bath for 4 h.
The reaction was monitored by withdrawing small aliquots of the
reaction mixture at every 60 min and analyzing them quantita-
tively by gas chromatography. The identities of the products were
confirmed as mentioned above. The effects of various parameters,
such as amounts of oxidant, amount of sodium bicarbonate, cata-
lyst, temperature, and solvent were studied to see their effect on
the conversion.
3.2.3. NMR spectroscopy
3. Result and discussion
The NMR spectra (1H and 13C) of the dioxidomolybdenum(VI)
complexes 1–6 were recorded using DMSO d6. The 1H NMR
spectra exhibit two singlets in the range 9.83–8.56 ppm due to
NH (–C(8)–N(3)H) and CH (–N(1)@C(7)–H) groups. Signals for
aromatic protons were found as multiplets in 7.78–6.56 ppm. The
detailed 1H NMR data of dioxidomolybdenum(VI) complexes
are summarized in Table 4. In complexes 2 and 4 the peak for
the H2O molecule overlapped with the signal for the presence
of H2O in the solvent molecule. The representative 1H NMR of
[Mo(VI)O2L1(DMSO)] (1) is shown in Fig. S1.
3.1. Synthesis
The ligands H2L1–6 (I, Chart 1) and complexes [Mo(VI)O2L1–6
(solv)] {where solv = DMSO (1, 3, 5 & 6) and H2O (2 & 4)}
(II, Chart 1) used in this work have been synthesized by refluxing
an equimolar ratio of [Mo(VI)O2(acac)2] and ligands in an ethanolic
medium, followed by recrystallisation of the product in DMSO.
Proposed structures of these complexes are based on elemental

S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
137
Table 2
IR spectral data (in cmÀ1) of [Mo(VI)O2L1/5/6(DMSO)] (1, 5 & 6), [Mo(VI)O2L2/4(H2O)] (2 & 4), and [Mo(VI)O2L3(DMSO)]4Á2DMSO (3).
Complexes
m(H2O)
m
(N(3)–H)
m
(C(7)–H)
m
(C@C)
m
(C(7)@N(1))
m
(Mo@O)
m(C(8)–S(1))
1
2
3
4
5
6
–
3271
3285
3278
3299
3211
3282
3018
2968
3015
2978
2915
3013
1598 s
1603 s
1592 s
1591 s
1594 s
1589 s
1504 m
1516 m
1521 m
1517 m
1554 m
1512 m
920 s, 896 s
931 s, 887 s
916 s, 893 s
925 s, 895 s
938 s, 892 s
933 s, 887 s
760 s
738 s
778 s
735 s
767 s
766 s
3395
–
3401
–
–
3.3. ESI-MS
Table 3
Electronic spectral and Cyclic voltammetrica results for complexes 1–6.
The ESI-MS data are summarized in the experimental section.
Figs. S2 and S3 depict the representative ESI-MS of 1 and 4
respectively.
Complex
kmax/nm(
e
/dm3 mol–1 cm–1
)
Potentials (V) versus Ag/AgCl
Mo(VI)/Mo(V)
Epc
Mo(V)/Mo(IV)
Epc
1
2
3
4
5
6
268(17,072), 341(14,323),
433(3370)
263(16,341), 335(12,543),
417(2789)
264(17,534), 346(13,535),
436(5422)
274(16,432), 328(14,212),
442(3856)
270 (16,322), 343(13,648),
439(3922)
243(16,429), 338(13,768),
437(3425)
À0.66
À0.71
À0.70
À0.68
À0.66
À0.70
À0.87
À0.92
À0.92
À0.85
À0.87
À0.88
3.4. Electrochemical properties
The electrochemical properties of 1–6 were examined in CH3CN
solution (0.1 M TEAP) by cyclic voltammetry using a glassy carbon
working electrode, platinum auxiliary electrode, and Ag/AgCl as
the reference electrode. The potential data are listed in Table 2
and Fig.
2
depicts the representative voltammogram of
[Mo(VI)O2L2(H2O)] (2). The voltammogram pattern is similar for
1–6, which includes two irreversible reduction processes within
the potential window À0.71 to À0.66 V and À0.92 to À0.85 V cor-
responding to metal centered reduction from Mo(VI)/Mo(V) and
Mo(V)/Mo(IV) respectively [66–68]. Similar cyclic voltammogram
pattern was observed in our previous reports on dioxidomolybde-
num(VI) complexes [22,47,48]. We had varied the ligands to see
their effect, if any, on their redox properties, however no signifi-
cant changes in the values could be observed.
a
In CH3CN at a scan rate 100 mV/s. Epc is cathodic peak potentials vs. Ag/AgCl.
1.2
263
335
1.0
3.5. Description of X–ray structure of dioxidomolybdenum(VI)
complexes 2 and 3
0.8
0.6
Although the preliminary characterization data (elemental
analysis, 1H and 13C NMR and IR) indicated the presence of the
ligand, two oxido groups and solvent in the complexes 1–6, they
could not point to any definite stereochemistry of the complexes,
or the coordination mode of H2L1À6. For an unambiguous charac-
terization of these complexes, structures of 2 and 3 were deter-
mined by X-ray crystallography. The X–ray structures show that
all the indications given by the 1H NMR spectra are indeed correct.
The molecular structure and the atom numbering scheme for 2 and
3 are shown in Figs. 3 and 4, respectively. The relevant bond dis-
tances and angles are collected in Table 5. The coordination geom-
etry around the molybdenum(VI) atom in 2 and 3 reveals a
distorted octahedral environment with NO4S coordination sphere
(Chart 1). Complex 3 contains two DMSO molecules as solvent of
crystallization. Each ligand molecule behaves as a dianionic triden-
tate one and is bonded to the metal centre through the phenolate
0.4
417
0.2
0.0
300
400
500
600
Wavelength(nm)
Fig. 1. UV–vis spectrum of [Mo(VI)O2L2(H2O)] (2) in CH3CN.
Table 4
1H NMR data of [Mo(VI)O2L1/5/6(DMSO)] (1, 5 & 6), [Mo(VI)O2L2/4(H2O)] (2 & 4), and [Mo(VI)O2L3(DMSO)]4Á2DMSO (3).
Complexes
C(8)–N(3)H
N(1)@C(7)H
Ar-H
OCH3
DMSO
1
2
3
4
9.83 (s, 1H)
9.72 (s, 1H)
9.81 (s, 1H)
9.56 (s, 1H)
8.82 (s, 1H)
8.75 (s, 1H)
8.79 (s, 1H)
8.70 (s, 1H)
7.74–6.90 (m, 8H)
7.77–6.94 (m, 7H)
7.78–6.97 (m, 7H)
7.64–6.85 (m, 7H)
–
2.54 (s, 6H)
–
2.54 (s, 6H)
–
3.78 (s, 3H)
3.79 (s, 3H)
3.78 (s, 3H)
3.72 (s, 3H)
–
5
6
9.34 (s, 1H)
9.34 (s, 1H)
8.76 (s, 1H)
8.56 (s, 1H)
7.63–6.56 (m, 7H)
7.35–6.65 (m, 10H)
2.53 (s, 6H)
2.53 (s, 6H)
–

138
S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
Table 5
0.000008
0.000006
0.000004
0.000002
0.000000
-0.000002
-0.000004
-0.000006
-0.000008
-0.000010
-0.000012
Selected geometric parameters (Å, °) for 2 and 3.
2
3
Bond lengths
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–N(1)
Mo(1)–S(1)
1.926(2)
1.706(2)
1.690(2)
2.342(2)
2.266(2)
2.426(7)
1.929(1)
1.703(1)
1.707 (1)
2.314(1)
2.275(1)
2.438(4)
0.000005
0.000004
0.000003
0.000002
0.000001
0.000000
Bond angles
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(1)
O(3)–Mo(1)–O(1)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–N(1)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–O(4)
O(3)–Mo(1)–O(4)
O(1)–Mo(1)–O(4)
N(1)–Mo(1)–O(4)
O(2)–Mo(1)–S(1)
O(3)–Mo(1)–S(1)
O(1)–Mo(1)–S(1)
N(1)–Mo(1)–S(1)
O(4)–Mo(1)–S(1)
105.41(1)
100.16(9)
104.05(9)
90.79(9)
160.30(1)
83.52(7)
168.07(9)
86.42(1)
77.90(8)
77.31(8)
96.61(8)
91.06(8)
153.42(6)
75.74(5)
81.42(6)
104.95(6)
98.15(6)
107.68(5)
91.12(5)
158.93(5)
82.85(5)
169.92(5)
85.08(5)
77.48(5)
79.38(4)
97.32(4)
88.79(4)
153.53(4)
75.49(3)
83.66(3)
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
V vs Ag/AgCl
-1.5
-1.0
-0.5
0.0
0.5
1.0
V vs Ag/AgCl
Fig. 2. Cyclic voltammogram of [Mo(VI)O2L2(H2O)] (2) in CH3CN (Inset: metal
centered reduction from Mo(VI)/Mo(V) and Mo(V)/Mo(IV) in the cathodic region).
coordinated to the Mo(1) center. Also, in our previous reports
[57,69], the presence of the enethiol form was predominant in
the thiosemicarbazone complexes.
Mo–O(2) and Mo–O(3) bond distances of the [MoO2]2– group
are unexceptional [62,70] and almost equal 1.707(1)–1.690(2) Å
for complexes 2 and 3. The bond between Mo and the azomethine
nitrogen in the complexes are within the range of 2.275(1)–2.266
(2) Å, which is comparatively longer than other Mo–N single
bonds. This is due to the trans effect generated by the oxido group
trans to the Mo–N bond [63,70]. The other Mo–hetero atom (O and S)
distances are normal, as observed in other structurally character-
ized complexes of molybdenum containing these bonds [71]. In
complexes 2 and 3, [MoO2L2,3(solv)], a solvent molecule completes
the distorted octahedral coordination sphere, which lies trans to
the other oxido group O(2). The Mo–O(solvent) bond in the range
2.342(2)–2.314(1) Å is significantly longer than the other Mo–O
bonds (1.929(1)–1.690(2) Å) indicating that the solvent molecule
is weakly bonded to the [MoO2]2– core.
Fig. 3. ORTEP diagram of [Mo(VI)O2L2(H2O)] (2) with atom labeling scheme.
3.6. Catalytic activity studies of 1–6
3.6.1. Oxidation of styrene
There are many reports of oxidation of styrene catalyzed by
molybdenum complexes, where H2O2 has been used as an oxidant
in the presence of NaHCO3 [72]. Aqueous 30% H2O2 as the sole oxi-
dant in presence of dioxidomolybdenum(VI) complexes as catalyst
gave poor catalytic efficiency, whereas the addition of NaHCO3 as
an additive activated the catalytic oxidation process and gave
two products, namely styrene oxide (major) and phenyl acetalde-
hyde (minor) (Scheme 1). Such NaHCO3 assisted oxidation of
Fig. 4. ORTEP diagram of asymmetric unit of [Mo(VI)O2L3(DMSO)]4Á2DMSO (3) with
atom labeling scheme.
oxygen O(1), thiolate sulfur S(1) and the imine nitrogen N(1)
forming a five membered and a six membered chelate ring with
S(1)–Mo(1)–N(1) and O(1)–Mo(1)–N(1) bite angles of 75.74(5)°–
75.49(3)° and 83.52(7)°–82.05(2)° respectively. In all the com-
plexes, one of the two oxido groups, O(3) is located trans to the
imine nitrogen N(1) and the other oxido group O(2) is located in
the axial plane along with the solvent molecule, H2O (2) and DMSO
(3). The bond length values examined in the X-ray data give an
indication for the presence of the enethiol form of the ligand
O
Catalyst
H2O2, NaHCO3
O
Scheme 1. Oxidation of styrene with H2O2 and NaHCO3.

S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
139
styrene has been reported in the literature and gave styrene oxide
selectively [73–75].
and the reaction was carried out at 60 °C. As illustrated in entries
No. 2, 6 and 7 of Table S1, the percent conversion of styrene
improved from 77.7% to 92.5% on increasing the styrene to oxidant
ratio from 1:3 to 1:4 but on further increment of ratio (1:5) there is
no remarkable improvement in conversion, suggesting that 1:4
(styrene:H2O2) molar ratio is sufficient enough to perform the
reaction with good conversion. Similarly, for three different
amounts viz. 0.0005 (1.0 Â 10–6 mmol), 0.0010 (1.8 Â 10–6 mmol)
and 0.0015 g (2.7 Â 10–6 mmol) of catalyst, at styrene to H2O2
molar ratio of 1:4 under above reaction conditions, 79.0%, 92.5%
and 93.4% conversion was observed. The increase of catalyst
[Mo(VI)O2L1(DMSO)] (1) was considered as a representative cat-
alyst to optimize the reaction conditions for the maximum oxida-
tion of styrene, by studying five different parameters viz. the effect
of amount of oxidant, amount of catalyst, amount of NaHCO3, sol-
vent and temperature of the reaction mixture in detail. The effect
of oxidant was studied considering styrene:aqueous 30% H2O2 in
the molar ratios of 1:3, 1:4 and 1:5, where the mixture of styrene
(0.520 g, 5 mmol), catalyst (0.0010 g, 1.8 Â 10–6 mmol), NaHCO3
(0.126 g, 1.5 mmol) and oxidant were taken in 5 mL of CH3CN
Fig. 5. (a) Effect of oxidant amount on the oxidation of styrene. Reaction condition: styrene (0.520 g, 5 mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g, 1.8 Â 10–6 mmol),
NaHCO3 (0.126 g, 1.5 mmol), temperature 60 °C and acetonitrile (5 mL). (b) Effect of catalyst [Mo(VI)O2L1(DMSO)] (1) amount on the oxidation of styrene. Reaction condition:
styrene (0.520 g, 5 mmol), 30% H2O2 (2.27 g, 20 mmol), NaHCO3 (0.126 g, 1.5 mmol), temperature 60 °C and acetonitrile (5 mL). (c) Effect of NaHCO3 amount on the oxidation
of styrene. Reaction condition: styrene (0.520 g, 5 mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g, 1.8 Â 10–6 mmol), 30% H2O2 (2.27 g, 20 mmol), temperature 60 °C and
acetonitrile (5 mL). (d) Effect of temperature on the oxidation of styrene. Reaction conditions: styrene (0.520 g, 5 mmol), 30% H2O2 (2.27 g, 20 mmol), NaHCO3 (0.126 g, 1.5
mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g, 1.8 Â 10–6 mmol) and acetonitrile (5 mL). (e) Effect of solvent amount on the oxidation of styrene. Reaction conditions:
styrene (0.520 g, 5 mmol), 30% H2O2 (2.27 g, 20 mmol), NaHCO3 (0.126 g, 1.5 mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g, 1.8 Â 10–6 mmol) and temperature 60 °C. (f)
Plot showing conversion of all metal complexes.

140
S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
amount from 0.0010 to 0.0015 g shows very minor improvement
in the conversion (entries No. 1, 2 and 3 of Table S1). The amount
of NaHCO3 has played a significant role in the conversion of styrene
and best conversion was obtained with 1.5 mmol of NaHCO3
(entries No. 2, 4 and 5 in Table S1). There was no conversion of sub-
strate in the absence of NaHCO3. Among three selected tempera-
tures of 40, 50 and 60 °C, running the reaction at 60 °C for the
following fixed operating condition of styrene (0.52 g, 5 mmol),
H2O2 (2.27 g, 20 mmol), [Mo(VI)O2L1(DMSO)] (1) (0.0010 g,
1.8 Â 10–6 mmol), NaHCO3 (0.126, 1.5 mmol) and CH3CN (5 mL),
gave best conversion (entries No. 2, 10 and 11 of Table S1). Varia-
tion in the volume of CH3CN (5, 7 and 10 mL) was also studied
(entries No. 2, 8 and 9 of Table S1) and it was observed that 5 mL
of CH3CN was sufficient enough to get good transformation of styr-
ene while running the reaction at 60 °C under above conditions.
Thus, the best reaction conditions as concluded above (i.e. styrene
0.52 g, 5 mmol, H2O2 2.27 g, 20 mmol, [Mo(VI)O2L1(DMSO)] (1)
0.0010 g, 1.8 Â 10–6 mmol, NaHCO3 0.126, 1.5 mmol, CH3CN 5 mL
and reaction temperature 60 °C) were further applied for the max-
imum transformation of styrene. Table S1 and Fig. 5 summarize all
the conditions and conversions obtained in this work.
3.6.2. Oxidation of cyclohexene
Oxidation of cyclohexene, catalyzed by dioxidomolybdenum
(VI) complexes using aqueous 30% H2O2 as oxidant in the presence
of NaHCO3 gave one product, namely cyclohexene oxide
(Scheme 2).
[Mo(VI)O2L1(DMSO)] (1) was again considered as a representa-
tive catalyst for the oxidation of cyclohexene by H2O2 in presence
of NaHCO3. In order to achieve optimum reaction conditions, the
effect of catalyst considering three different amounts viz. 0.0005 g
(1.0 Â 10–6 mmol), 0.0010 g (1.8 Â 10–6 mmol) and 0.0015 g
(2.7 Â 10–6 mmol) as a function of time was studied for the fixed
amount of cyclohexene (0.41 g, 5 mmol), NaHCO3 (0.126 g, 1.5
mmol) and 30% H2O2 (1.70 g, 15 mmol) in 5 mL of CH3CN and reac-
tion was carried out at 60 °C. The results are illustrated in Fig. 7(a)
(entries No. 1, 2 and 3 of Table S2). It is evident from the plot that
0.0005 g (1.0 Â 10–6 mmol) gave 81.0% conversion of the substrate,
whereas 0.0010 g (1.8 Â 10–6 mmol) and 0.0015 g (2.7 Â 10–6
mmol) of catalyst gave 93.0% and 95.0% respectively. Out of these
0.0010 g (1.8 Â 10–6 mmol) was the best one to obtain a maximum
of 93.0% conversion of cyclohexene. For the effect of oxidant three
different cyclohexene to aqueous 30% H2O2 molar ratios viz. 1:2,
1:3 and 1:4 were considered for the fixed amount of cyclohexene
(0.41 g, 5 mmol), NaHCO3 (0.126 g, 1.5 mmol) and catalyst
(0.0010 g, 1.8 Â 10–6 mmol) in 5 mL of CH3CN and reaction was
carried out at 60 °C. As presented in entries No. 2, 6 and 7 of
Table S2 and Fig. 7(b), a maximum of 63.6% conversion was
obtained at a cyclohexene to H2O2 molar ratio of 1:2 in 4 h of reac-
tion time. This conversion reached 93.0% on increasing this ratio to
1:3 while 1:4 ratios improve the conversion slightly (99.0%). The
best conditions as found above were cyclohexene (0.41 g, 5 mmol),
30% H2O2 (1.70 g, 15 mmol), NaHCO3 (0.126 g, 1.5 mmol) in 5 mL
of CH3CN. We have also optimized the amount of NaHCO3 (entries
No. 2, 4 and 5), the amount of solvent (acetonitrile, entries No. 2, 8
and 9) and temperature (entries No. 2, 10 and 11) of the reaction
and found that 1.5 mmol of NaHCO3, 5 mL of solvent and 60 °C
reaction temperature were sufficient enough to obtain 93.0% con-
version under above reaction conditions. Table S2 and Fig. 7 sum-
marize all the conditions and the conversions obtained under
several selected conditions. No conversion of the substrate in the
absence of NaHCO3 was obtained. Blank reaction under above reac-
tion conditions gave 32.0% conversion.
[Mo(VI)O2L1(DMSO)] (1) as catalyst under the optimized reac-
tion conditions (entry No. 2 of Table S1) has been analyzed as a
function of time and is presented in Fig. 6 and Table 6. Thus, under
optimized reaction conditions, the selectivity of the two oxidation
products varies in the order: styrene oxide (98.6%) > phenyl
acetaldehyde (1.4%). Complexes [Mo(VI)O2L1–6(solv)] (1–6) cause
the same order of the selectivity of products with an almost iden-
tical conversion. Blank reaction under above reaction conditions
gave 30.0% conversion.
100
80
60
40
20
0
100
80
60
40
20
0
% Conversion
Styrene Oxide
Phenylacetaldehyde
50
100
150
200
250
Catalyst
O
Time/ min
H2O2, NaHCO3
Fig. 6. Plot showing conversion and product selectivity as a function of time in the
optimized reaction condition.
Scheme 2. Oxidation of cyclohexene with H2O2 and NaHCO3.
Table 6
Oxidation of styrene, TOF and product selectivity using 1–6 as catalyst.
a
Catalyst
TOF [hÀ1
]
Conv. [%]
Selectivity [%]b
phac
so
[Mo(VI)O2L1(DMSO)] (1)
[Mo(VI)O2L2(H2O)] (2)
[Mo(VI)O2L3(DMSO)]4Á2DMSO (3)
[Mo(VI)O2L4(H2O)] (4)
[Mo(VI)O2L5(DMSO)] (5)
[Mo(VI)O2L6(DMSO)] (6)
Blank reaction
642
600
676
586
651
647
–
92.5
96.0
92.0
94.0
93.8
93.0
30.0
1.4
1.3
0.8
0.9
1.1
0.6
3.0
98.6
98.7
99.2
99.1
98.9
99.4
97.0
a
TOF values calculated at 4 h of reaction time.
so: styrene oxide; phac: phenyl acetaldehyde.
b

S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
141
Fig. 7. (a) Effect of catalyst [Mo(VI)O2L1(DMSO)] (1) amount on the oxidation of cyclohexene. Reaction conditions: cyclohexene (0.410 g, 5 mmol), 30% H2O2 (1.70 g, 15 mmol),
NaHCO3 (0.126 g, 1.5 mmol), temperature 60 °C and acetonitrile (5 mL). (b) Effect of oxidant amount on the oxidation of cyclohexene. Reaction condition: cyclohexene (0.410
g, 5 mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g, 1.8 Â 10–6 mmol), NaHCO3 (0.126 g, 1.5 mmol), temperature 60 °C and acetonitrile (5 mL). (c) Effect of NaHCO3 amount
on the oxidation of cyclohexene. Reaction condition: cyclohexene (0.410 g, 5 mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g, 1.8 Â 10–6 mmol), 30% H2O2 (1.70 g, 15
mmol), temperature 60 °C and acetonitrile (5 mL). (d) Effect of solvent amount on the oxidation of cyclohexene. Reaction conditions: cyclohexene (0.410 g, 5 mmol), 30% H2O2
(1.70 g, 15 mmol), NaHCO3 (0.126 g, 1.5 mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g, 1.8 Â 10–6 mmol) and temperature 60 °C. (e) Effect of temperature on the oxidation
of cyclohexene. Reaction conditions: cyclohexene (0.410 g, 5 mmol), 30% H2O2 (1.70 g, 15 mmol), NaHCO3 (0.126 g, 1.5 mmol), catalyst [Mo(VI)O2L1(DMSO)] (1) (0.0010 g,
1.8 Â 10–6 mmol) and acetonitrile (5 mL). (f) Plot showing conversion of all metal complexes on the oxidation of cyclohexene.
The oxidation of cyclohexene, TOF and selectivity of cyclohex-
ene oxide using different dioxidomolybdenum(VI) complexes
(1–6) as catalyst is given in Table 7.
styrene oxide was found to be comparable to many related
dioxidomolybdenum(VI) complexes [23,47,60,77–83].
The possible side products for oxidation of cyclohexene could
be 2-cyclohexene-1-ol, 2-cyclohexene-1-one and cyclohexane-
1,2-diol [76]. The catalytic potential of our synthesized complexes
in terms of selectivity towards epoxydation of cyclohexene and
3.6.3. Reactivity of complexes with H2O2
Dioxidomolybdenum(VI) complexes (1–6) react with H2O2 to
give the corresponding oxido–peroxido complexes. The generation
of such species has been established in DMSO by electronic

142
S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
Table 7
Oxidation of cyclohexene, TOF and selectivity of cyclohexene oxide using 1–6 as
catalyst.
O
N
O
-
HCO4
Mo
O
O
S
a
Catalyst
TOF [hÀ1
]
Conv. [%]
Selectivity [%]
[Mo(VI)O2L1(DMSO)] (1)
[Mo(VI)O2L2(H2O)] (2)
[Mo(VI)O2L3(DMSO)]4Á2DMSO (3)
[Mo(VI)O2L4(H2O)] (4)
[Mo(VI)O2L5(DMSO)] (5)
[Mo(VI)O2L6(DMSO)] (6)
Blank reaction
646
579
705
593
664
670
–
93.0
92.7
96.0
95.0
95.6
96.4
32.0
100
100
100
100
100
100
100
N
O
N
O
O
Mo
O
O
Mo
O
S
S
OH
O
O
C
O
O
a
TOF values calculated at 4 h of reaction time.
-
HCO3
absorption spectroscopy. In
a typical reaction, 20 mL of ca.
Scheme 3. Proposed catalytic mechanism for the oxidation of alkenes.
3.6 Â 10–5 M solution of [Mo(VI)O2L1(DMSO)] (1) was treated with
one drop of 30% aqueous H2O2 (0.260 g, 2.3 mmol) dissolved in
5 mL of DMSO and the resultant spectroscopic changes are
presented in Fig. 8. The intensity of bands at 312 and 343 nm
increases, while 267 and 420 nm decreases considerably. These
changes indicate the interaction of [Mo(VI)O2L1(DMSO)] (1) with
H2O2 and formation of oxido–peroxido species [22].
the formation of a oxido-peroxido species. The same has been con-
firmed in a study reported by Maurya et al. [94], wherein the
molybdenum peroxide species had been successfully isolated.
The percent conversion values indicate that the synthesized
complexes (1–6) have shown better activity than some of the pre-
vious reports on Mo(VI) epoxidation catalysts, such as those of Mo
(VI) hydroxamates and pyrazole based aryloxide ligands [89,95].
There is a marginal increase in the percent conversion values of
epoxidation of cyclohexene as compared to our previous catalytic
report on Mo(VI) complexes of Schiff bases [47].
Though no direct correlation of the substituent effect on the cat-
alytic efficacy could be observed, however, both the percent con-
version to cyclohexene oxide from cyclohexene and the percent
selectivity for oxidation of styrene to styrene oxide were found
to be marginally higher for the chloro substituted complexes (3
and 6).
During catalytic action, NaHCO3 and H2O2 jointly affect the
reactivity of 1. To understand the mechanism, a mixture of NaHCO3
(0.13 mmol) and 30% H2O2 (0.58 mmol) dissolved in 5 mL of DMSO
was treated with 20 mL of 3.6 Â 10–5 M solution of 1. Fig. S4 of the
Supporting Information represents the very similar spectral
changes as shown by H2O2 alone. The oxido–peroxido species is
formed immediately on addition of only few drops of a mixture
of NaHCO3 and H2O2. It has been accounted that HCO–4 (peroxy-
monocarbonate) species generates quickly on the reaction of
NaHCO3 and H2O2 [72–75], and then this species reacts with diox-
idomolybdenum(VI) complex to give oxido-peroxidomolybdenum
(VI) intermediate instantly as compared to H2O2 alone. Exact
mechanism for the oxidation of olefins by oxidomolybdenum com-
plexes in presence of H2O2 is not clear at present. However, the for-
mation of relevant oxido-peroxido complexes has been reported by
us [47,48,84] and some other groups in the past [74,85–90]. Alter-
native plausible mechanisms of epoxidation have also been theo-
retically proved, wherein the peroxido complexes were
established to be less active than the oxido complexes [91–93].
Although no attempts to isolate appropriate intermediates for
our system have been performed, based on the oxidation products
obtained and experiments carried out above, a reaction pathway
including oxido-peroxido intermediates can be proposed as shown
in Scheme 3. The changes in the electronic absorption spectra
(Fig. 8) indicate the interaction of [MoO2L1(DMSO)] with H2O2 with
4. Conclusion
In this work, six hexacoordinated molybdenum(VI) thiosemi-
carbazone complexes (1–6) have been prepared and characterized
by spectroscopic and electrochemical techniques. The molecular
structures of two of the synthesized complexes (2 and 3) have been
successfully solved. The catalytic activity (oxidation of styrene and
cyclohexene) of Mo(VI) complexes (1–6) have been successfully
evaluated. In all the cases, the percent conversion is increased sig-
nificantly (>92%) in the presence of the catalysts. Moreover, there
is 100% product selectivity for the formation of cyclohexene oxide
from cyclohexene and ꢀ98–99% product selectivity for the oxida-
tion of styrene to styrene oxide. In view of the above results, it
can be concluded that molybdenum complexes derived from
thiosemicarbazone ligands have the potential to stimulate the
research for the synthesis of a better catalyst.
0.9
343
267
Acknowledgements
312
0.6
The authors thank Council of Scientific and Industrial Research,
New Delhi, Government of India [Grant No. 01(2735)/13/EMR-II
(R.D.)] and Department of Science and Technology, Govt. of India
[Grant No. SR/FT/CS-010/2009 (R.A.)] for funding this research.
Saswati gratefully acknowledges the Council of Scientific and
Industrial Research, New Delhi, India, for the award of senior
research fellowship Grant 9/983(0012)2k13-EMR-I.
0.3
420
0.0
300
400
500
Wavelength/nm
Appendix A. Supplementary data
Fig. 8. UV–vis spectral changes observed during titration of complex 1 with H2O2.
The spectra were recorded after successive additions of
1 drop of 30% H2O2
(2.3 mmol) dissolved in 5 mL of DMSO to 20 mL of 3.6 Â 10–5 M solution of complex
1 in DMSO.
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.ica.2018.01.023.

S. Roy et al. / Inorganica Chimica Acta 474 (2018) 134–143
143
[51] A. Corma, I. Dominguez, A. Domenech, V. Fornes, C.J. Gomez-Garcia, T.
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