New molybdenum(VI) complex with ONS-donor thiosemicarbazone ligand: Preparation, structural characterization, and catalytic applications in olefin epoxidation

Article abstract of DOI:10.1016/j.inoche.2012.10.016

New dioxomolybdenum(VI) complex was prepared by reacting 1-(2,4-dihydroxybenzylidene)-N-methyl-N-phenylthiosemicarbazone (H₂L) as ligand and [MoO₂(acac)₂] in acetonitrile solution. The doubly deprotonated ligand is coordinated to molybdenum through sulfur atom, hydrazinic nitrogen atom and phenolic oxygen atom. The resulting complex with the formula [MoO₂L(CH₃CN)] which contains an acetonitrile molecule in sixth site of coordination, was characterized by elemental analyses, ¹H NMR, IR and electronic spectroscopic studies and single crystal X-ray diffraction analysis. This complex was tested as a catalyst for the homogeneous epoxidation of olefins using tert-butyl hydrogen peroxide (TBHP) as an oxidant. The catalyst shows efficient reactivity in the olefins epoxidation reactions giving high yield and selectivity under atmospheric conditions, in most cases.

Full text of DOI:10.1016/j.inoche.2012.10.016

Inorganic Chemistry Communications 27 (2013) 26–30
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
New molybdenum(VI) complex with ONS-donor thiosemicarbazone ligand:
Preparation, structural characterization, and catalytic applications in olefin epoxidation
b
a
c
Zeinab Moradi-Shoeili a, Davar M. Boghaei a, , Mojtaba Amini , Mojtaba Bagherzadeh , Behrouz Notash
⁎
a
Department of Chemistry, Sharif University of Technology, Azadi Ave., P.O. Box 11155-3516, Tehran, Iran
b
Department of Chemistry, Faculty of Science, University of Maragheh, P.O. Box: 55181-83111731, Maragheh, Iran
c
Chemistry Department, Shahid Beheshti University, G. C., Evin, Tehran 1983963113, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
New dioxomolybdenum(VI) complex was prepared by reacting 1-(2,4-dihydroxybenzylidene)-N-methyl-N-
phenylthiosemicarbazone (H2L) as ligand and [MoO2(acac)2] in acetonitrile solution. The doubly deprotonated li-
gand is coordinated to molybdenum through sulfur atom, hydrazinic nitrogen atom and phenolic oxygen atom.
The resulting complex with the formula [MoO2L(CH3CN)] which contains an acetonitrile molecule in sixth site of
coordination, was characterized by elemental analyses, 1H NMR, IR and electronic spectroscopic studies and single
crystal X-ray diffraction analysis. This complex was tested as a catalyst for the homogeneous epoxidation of olefins
using tert-butyl hydrogen peroxide (TBHP) as an oxidant. The catalyst shows efficient reactivity in the olefins epox-
idation reactions giving high yield and selectivity under atmospheric conditions, in most cases.
© 2012 Elsevier B.V. All rights reserved.
Received 18 August 2012
Accepted 9 October 2012
Available online 18 October 2012
Keywords:
Dioxomolybdenum(VI) complex
Thiosemicarbazone
Olefin epoxidation
The oxygen atom transfer (OAT) between competent centers as a fun-
damental reaction in chemistry and biology, has been extensively investi-
gated in past decades [1,2]. In most biological systems, molybdenum-
containing enzymes, such as sulfite oxidase, dimethyl sulfoxide reductase,
and nitrate reductase, catalyze oxygen atom transfer reactions at the
mononuclear oxomolybdenum active centers [3,4], wherein XO/X func-
tions as donor/acceptor and the oxidation state of molybdenum atom is
changed by two units (Eq. 1).
The chemistry of thiosemicarbazones, containing mixed hard-soft
donor sets in tridentate ligands, oxygen/nitrogen–sulfur containing
chelates, and their related complexes of high valent metal oxo moie-
ties therefore is of current interest as model systems for the active
sites of molybdoenzymes such as xanthine oxidase and sulfite oxidase
[8–10].
Except as being models of the enzymatic active sites, catalytic OAT
properties of monomeric dioxomolybdenum complexes are also im-
portant in industrial processes including epoxidation of olefins in
the homogeneous phases [11]. Epoxidation is one of the typical reac-
tions in organic synthesis that can produce various synthetic products
such as enantioselective drugs, pesticides, epoxy paints, rubber pro-
moters and dyestuffs [12–14].
Here, we report the synthesis, spectral characterization and single
crystal X-ray diffraction studies of dioxomolybdenum(VI) complex of
N(4)-substituted thiosemicarbohydrazone as ONS-donor ligand bind-
ing to the molybdenum through sulfur atom, hydrazinic nitrogen
atom and phenolic oxygen atom (Scheme 1). In this report, we have
also investigated the catalytic behavior of dioxomolybdenum(VI),
[MoO2L(CH3CN)], complex towards the epoxidation of olefins using
tert-butyl hydrogen peroxide (TBHP).
The dioxomolybdenum(VI) complex was obtained by the reaction of
bis(acetylacetonato)dioxomolybdenum(VI) with a stoichiometric amount
of thiosemicarbazone ligand derived from 4-hydroxosalicylaldehyde and
4-methyl-4-phenylthiosemicarbazide, in acetonitrile solution. Brown crys-
tals of complex were isolated from reaction mixture by slow evaporation
of solvent at room temperature (see supporting information). This
complex was found to be fairly soluble in most of the common organic
solvents.
MoVIO2Ln þ X ↔ MoIVOLn þ XO
ð1Þ
Extended X-ray absorption fine structure (EXAFS) spectroscopic
studies have implicated the presence of a sulfur atom, besides oxygen
and nitrogen, at the active sites of oxo-transfer molybdoenzymes [5].
This discovery led to the design, synthesis of mimetic Mo/W-
complexes, which permitted the detailed elucidation of their novel
biological roles, leading to the development of new model complexes
that mimic their actions [6].
Structurally related thiosemicarbazone ligands supplying a tridentate
ONS donor set, have been used as polydentate chelating ligands to pre-
pare complexes containing the MoO22+ core. In addition to the above
mentioned donor groups, molybdenum thiosemicarbazone complexes
have attracted much interest due to their unique advantages in forming
compound types MoO2L or MoOL which possess one or two “open” coor-
dination sites that can be utilized for substrate binding [7].
⁎
Corresponding author. Tel.: +98 21 66165306; fax: +98 21 66012983.
E-mail address: dboghaei@sharif.edu (D.M. Boghaei).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.10.016

Z. Moradi-Shoeili et al. / Inorganic Chemistry Communications 27 (2013) 26–30
27
the770–850 cm−1 region, indicative of intermolecular metal oxygen
interaction is characteristic of the spectra of monomeric complexes
[MoO2L(CH3CN)] [21,22].
OH
OH
OH
The 1H NMR spectrum of the ligand exhibits two phenolic (OH)
proton resonances at 9.89 and 10.08 ppm, respectively. Upon coordi-
nation to Mo atom, one signal disappeared and the second was ob-
served at 10.36, indicating the deprotonation of one phenolic OH in
order to coordinate with the dioxomolybdenum(VI) cation [23]. The
disappearance of the signal at 10.85 ppm in the free ligand upon com-
plexation, suggests that during complexation the thioketone form
undergoes tautomerization to thiol form (Scheme 2) followed by de-
protonation of the SH group [24]. The azomethine C―H signal in
complex is shifted from 8.25 to 8.54 ppm compared to the free ligand.
The down field shift of this signal is attributable to the drainage of the
electron density from the nitrogen atom of the azomethine group to
the Mo atoms [25]. This confirms that the thiosemicarbazone ligand
is bonded to the molybdenum atom through the phenolic oxygen,
thiol sulfur and azomethine nitrogen as in results of IR spectra. The
proton resonance for methyl group of substituted acetonitrile molecule
in DMSO-d6 is found at 2.07 ppm. The ratio of integrated intensities for
proton resonance of ligand and acetonitrile molecule is consistent with
the stoichiometric ratio 1:1. The methyl protons of ligand were shifted
to lower frequencies from 3.58 to 3.45 ppm.
O
O
N
Mo
O
C
N
MoO2(acac)/ CH3CN
Reflux
N
N
N
HN
N
S
S
CH3
H3C
H3C
[MoO2(L)(CH3CN)]
H2L
Scheme 1. Synthesis of [MoO2(L)(CH3CN)].
The formulation is in accordance with the data of elemental analysis
and physicochemical measurements. A single crystal X-ray diffraction
study of [MoO2L(CH3CN)] (1) revealed a six-coordinate monomer in
which the central Mo atom is coordinated by a tridentate ONS-donor
and the sixth coordination site is occupied by an acetonitrile molecule.
The IR spectrum of the complex is in agreement with the
proposed structure shown in Scheme 1. The bands at the region
3450–3320 cm−1 [ν(NH)] and 871 cm−1 [ν(C S)] vanished upon
coordinating with MoO22+ suggesting involvement of sulphur in coordi-
nation [15,16]. The band around 2870–3050 cm−1 which is typical for
ν(O―H) in the ligand [17], decreases in intensity upon complexation,
confirming deprotonation of one phenolic oxygen on coordination to
metal. The sharp band at 1634 cm−1which was assigned to ν(C=N)
has shifted to the 1596 cm−1 range in the spectra of the complex
suggesting coordination of nitrogen to molybdenum which are in
agreement with the literature values [18,19,14]. Two strong absorption
bands in 906 and 939 cm−1 are indicative of symmetrical and
anti-symmetrical stretching vibration of two cis doubly-bonded oxygen
atoms at the molybdenum core [20]. The absence of the band, in
Electronic absorption spectra of the ligand and the complex were
recorded in acetonitrile (Fig. 1). The compounds are found to display
several common absorption maxima in the range 200–350 nm which
assigned to intra-ligand transitions. The complex exhibits one low en-
ergy electronic absorption in the region 400–450 nm which may pos-
sible to assigned as charge transfer transition from HOMO of sulfur
(Pπ-orbital) to the LUMO of molybdenum (dπ-orbital) [26,27]. The
absence of a d–d transition absorption band in the visible region con-
firms the 4d0 electronic configuration of Mo(VI) [28].
Brown crystals of [MoO2L(CH3CN)] were obtained by slow evapo-
ration of CH3CN solution of complex at room temperature. Crystallo-
graphic data and parameters for complex are summarized in Table 1.
This table reveals that [MoO2L(CH3CN)] complex crystallizes in the
triclinic (space group, Pī) crystal system.
The asymmetric unit of the complex consists of one crystallographical-
ly independent Mo(VI) species. An ORTEP drawing of the Mo(VI) center is
presented in Fig. 2. The coordination geometry around molybdenum ion
can be described as distorted octahedral geometry. Molybdenum(VI)
OH
HO
S
N
N
N
H
thioketone form
CH3
OH
HO
SH
N
N
N
thiol form
CH3
Fig. 1. UV–vis spectroscopy of (a) ligand (8×10−5 M), (b) complex [MoO2L(CH3CN)]
Scheme 2. Thiol-thioketone tautomerization in H2L.
(4.3×10−5 M).

28
Z. Moradi-Shoeili et al. / Inorganic Chemistry Communications 27 (2013) 26–30
Table 1
When the oxidation of cyclooctene was performed in the absence
of catalyst, no reaction occurred, whereas the [MoO2L(CH3CN)] /TBHP
oxidizing system was able to oxidize a wide variety of olefins at 80 °C
under aerobic condition and in short reaction times (60 min). The ox-
idations of various olefins were investigated and the details of catalyt-
ic activity with respect to epoxidation of olefines are presented in
Table 2.
Generally, aliphatic substrates show excellent epoxide yields and
selectivities in comparison to aromatic substrates. Cyclohexene was
the most reactive one considering epoxide yield and selectivity
(Table 2, entry 2). The catalytic activity tends to decrease as the olefin
becomes conjugated with phenyl group so that the lowest epoxide
yields were obtained for styrene and indene (Table 2, entries 1, 6).
Moreover the present catalytic system was completely selective to
the epoxidation for all substrates except for styrene with benzalde-
hyde and benzoic acide formed the side products.
In conclusion, the thiosemicarbazone ligand functions as tridentate
chelating ligand which binds such that its ONS-donor set are coordinated
to MoO22+ moiety that is in good agreement with the results obtained by
IR, 1H NMR, electronic spectra of the complex and also single crystal X-ray
diffraction analysis. The epoxidation of olefins to epoxides in present of
[MoO2L(CH3CN)] as catalyst and TBHP as oxidant at 80 °C under atmo-
spheric conditions was described in this paper. In general, easy prepara-
tion, mild reaction condition, high yields of the products, short reaction
time, high selectivity and inexpensive system make this catalytic system
a useful method for epoxidation of olefins.
Crystallographic and structure refinement data for MoO2(L)(CH3CN).
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Crystal size (mm3)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C17H16MoN4O4S
468.35
298(2)
0.71073
Triclinic
Pī
0.35×0.15×0.10
7.3170(15)
8.9178(18)
15.800(3)
98.73(3)
103.27(3)
93.80(3)
986.3(4)
2
γ (°)
Volume (Å3)
Z
Densitycalc (g cm−1
)
1.577
2.32-29.20
472
θ Ranges for data collection (°)
F(000)
Absorption coefficient (mm-1
)
0.800
Index ranges
−10≤h≤10
−12≤k≤12
−21≤l≤21
11086
5274, (0.0320)
5274/0
0.0304/0.0707
0.0401/0.0735
0.991
Data collected
Unique data, (Rint
Parameters/restrains
Final R1/wR2 ( I>2σ(I) )
)
a
a
Final R1/wR2 (all data)
Goodness-of-fit on F2 (S)
Largest diff. peak and hole (e Å−3
)
0.666 , −0.411
ꢄ
ꢂ
ꢃ
ꢅ
a
1=2
ꢀ
ꢁ
ꢀ
ꢁ
2
2
2
2
R1 ¼ ΣjjFoj−jFcjj=ΣjFoj; wR2
¼
Σ
w
Fo2−Fc
=Σw Fo
Acknowledgements
The authors are grateful to the Research Council of Sharif Univer-
sity of Technology for their financial support.
has six-coordination number in which cis-[MoO2]2+ species is coordinat-
ed by doubly deprotonated tridentate ONS-donor thiosemicarbazone
ligand (L) in a meridional fashion. Also the sixth site of distorted octahe-
dral geometry is occupied by one acetonitrile solvent molecule. Selected
bond lengths and angles with their standard deviations for complex
[MoO2L(CH3CN)] is given in Table S1 (see supporting information). Struc-
tural parameters shown in Table S1 are comparable with previously
reported Mo(VI) containing tridentate ONS-donor thiosemicarbazone
ligands [9,24,29].
This complex was tested as a catalyst for the homogeneous epoxida-
tion of olefins using tert-butyl hydrogen peroxide (TBHP) as an oxidant.
We have used optimized condition for homogeneous epoxidation of
olefins in our study (see supporting information) using molybdenum
complex [MoO2L(CH3CN)] as we did in our previous report [30].
Appendix A. Supplementary material
The syntheses and characterization of H2L and [MoO2L(CH3CN)] and
also, catalytic experiment conditions are founded in the supporting
information. A summary of crystal data, experimental details, and refine-
ment results is given in Table S1. The CIF file of crystal structure [MoO2-
L(CH3CN)] has been deposited with the CCDC No. 895788. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2012.10.016.
Fig. 2. The labeled diagram of asymmetric unit of [MoO2L(CH3CN)]. Thermal ellipsoids are at 30% probability level.

Z. Moradi-Shoeili et al. / Inorganic Chemistry Communications 27 (2013) 26–30
29
Table 2
Details of the catalytic oxidation of olefins catalyzed by [MoO2L(CH3CN)].
a
Entry
1
Substrate
Product
Yield (%)b
51(67:27:5)
O
O
O
OH
+
+
2
3
98
81
O
O
4
5
6
93
95
44
OH
CH3
CH3
O
O
CH3
O
7
8
61
72
O
O
a
The molar ratios for using [MoO2L(CH3CN)]:olefin:TBHP are 1:5000:5000.
The GC conversion (%) are measured relative to the starting olefin after 60 min.
b
localization function and catastrophe theory, J. Phys. Chem.
514–522.
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