Synthesis, structural characterization, and catalytic reactivity of a new molybdenum(VI) complex containing 1,3,4-thiadiazole derivative as a tridentate NNO donor ligand

Article abstract of DOI:10.1080/00958972.2014.993321

A new cis-dioxo molybdenum(VI) complex was obtained by reaction of 2,4-dihydroxybenzylidene(5-N,N-methylphenylamino-1,3,4-thiadiazol-2-yl)hydrazone as ligand and [MoO₂(acac)₂] in methanol and was characterized by elemental analyses,

Full text of DOI:10.1080/00958972.2014.993321

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Synthesis, structural characterization,
and catalytic reactivity of a new
molybdenum(VI) complex containing
1,3,4-thiadiazole derivative as a
tridentate NNO donor ligand
Zeinab Moradi-Shoeiliab, Maryam Zareac, Mojtaba Bagherzadeha,
Maciej Kubickid & Davar M. Boghaeia
a Department of Chemistry, Sharif University of Technology,
Tehran, Iran
b Faculty of Sciences, Department of Chemistry, University of
Click for updates
Guilan, Rasht, Iran
c Materials Science & Engineering Department, Golpayegan
University of Technology, Golpayegan, Iran
d Faculty of Chemistry, Adam Mickiewicz University, Poznan,
Poland
Accepted author version posted online: 01 Dec 2014.Published
online: 02 Jan 2015.
To cite this article: Zeinab Moradi-Shoeili, Maryam Zare, Mojtaba Bagherzadeh, Maciej Kubicki &
Davar M. Boghaei (2015) Synthesis, structural characterization, and catalytic reactivity of a new
molybdenum(VI) complex containing 1,3,4-thiadiazole derivative as a tridentate NNO donor ligand,
Journal of Coordination Chemistry, 68:3, 548-559, DOI: 10.1080/00958972.2014.993321
To link to this article: http://dx.doi.org/10.1080/00958972.2014.993321
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Journal of Coordination Chemistry, 2015
Vol. 68, No. 3, 548–559, http://dx.doi.org/10.1080/00958972.2014.993321
Synthesis, structural characterization, and catalytic reactivity
of a new molybdenum(VI) complex containing
1,3,4-thiadiazole derivative as a tridentate NNO donor ligand
ZEINAB MORADI-SHOEILI†‡, MARYAM ZARE†§, MOJTABA BAGHERZADEH†,
MACIEJ KUBICKI¶ and DAVAR M. BOGHAEI*†
†Department of Chemistry, Sharif University of Technology, Tehran, Iran
‡Faculty of Sciences, Department of Chemistry, University of Guilan, Rasht, Iran
§Materials Science & Engineering Department, Golpayegan University of Technology, Golpayegan,
Iran
¶Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland
(Received 16 December 2013; accepted 10 November 2014)
A new cis-dioxo molybdenum(VI) complex was obtained by reaction of 2,4-dihydroxybenzylidene
(5-N,N-methylphenylamino-1,3,4-thiadiazol-2-yl)hydrazone as ligand and [MoO2(acac)2] in metha-
1
nol and was characterized by elemental analyses, H NMR, IR, and electronic spectroscopic studies.
The complex was also analyzed by single-crystal X-ray diffraction. The structure determination
revealed a distorted octahedral coordination geometry around molybdenum in which the tridentate
NNO donor (L2–) is bonded to [MoO2]2+ through phenolic oxygen, hydrazinic nitrogen, and thiadia-
zole nitrogen. The sixth coordination site is occupied by a weakly bonded methanol. The complex
was tested as a catalyst for homogeneous epoxidation of olefins using tert-butyl hydrogen peroxide
as an oxidant. In the homogeneous catalytic system, the reactions are efficiently carried out with
high yields and selectivity.
Keywords: Molybdenum(VI) complex; Thiadiazole; Olefin epoxidation
*Corresponding author. Email: dboghaei@sharif.edu
© 2014 Taylor & Francis

Molybdenum(VI) complex
549
1. Introduction
Oxomolybdenum(VI) complexes attract interest because of their relevance to both
biological and industrial systems [1]. It is well known that the biological redox process of
molybdoenzymes involves the transfer or abstraction of oxygen to or from generalized
substrate X or XO, respectively [2]. To demonstrate the viability of catalytic oxo transfer
reactions high-valent oxo-molybdenum complexes have been examined because of their
possible relationship to redox-active molybdoenzymes [3, 4].
Catalytic oxo transfer properties of monomeric dioxomolybdenum complexes are also
important in industrial processes such as the epoxidation of olefins [5, 6]. Subsequently,
various ligands with different donor properties have been used to produce dioxomolybde-
num complexes which serve as catalysts in biological and industrial processes [3, 7].
Antimicrobial, antiviral, anti-inflammatory, and antitumor activities of 1,3,4-thiadiazole
derivatives, which belong to an important group of heterocyclic bioactive compounds, and
their metal complexes, have inspired numerous investigations concerning their synthesis,
characterization, and bioinorganic relevance [8–13].
This report thus describes preparation and characterization of the new cis-dioxo molybdenum
(VI) complex of a 1,3,4-thiadiazole derivative, [MoO2(L)(CH3OH)] which crystallizes with
weakly bound CH3OH and disordered water, in which L2– supplies a tridentate NNO donor set
to molybdenum (scheme 1). In addition, the catalytic behavior of dioxomolybdenum(VI) com-
plex, [MoO2(L)(CH3OH)], toward the epoxidation of olefins using tert-butyl hydrogen peroxide
(TBHP) have been also investigated. The catalyst shows excellent catalytic efficiency in epoxi-
dation reactions with various olefinic compounds including linear and cyclic alkenes.
2. Experimental
2.1. Materials and methods
All reagents employed were purchased commercially and used without purification.
4-Methyl-4-phenyl-3-thiosemicarbazide [14] and bis(acetylacetonato)dioxomolybdenum
OH
OH
O
OH
O
O
N
Mo
N
MoO2(acac)2 /MeOH
Reflux
CH3
O
H
N
HN
N
N
N
N
S
S
N
N
CH3
CH3
Scheme 1. Synthesis of [MoO2(L)(CH3OH)].

550
Z. Moradi-Shoeili et al.
(VI), [MoO2(acac)2] [15] were prepared as described. Elemental analyses (C, H, and N)
were performed on a Perkin Elmer 2400 CHNS/O elemental analyzer. Infrared spectra
(4000–400 cm-1) of solid samples were taken in KBr pellets using a Unicam Matson 1000
FT-IR spectrophotometer. NMR spectra were obtained on a Bruker 500-DRX Avance Spec-
trometer in DMSO-d6. UV–vis spectra were recorded with a CARY 100 Bio VARIAN
UV–vis spectrophotometer using quartz cells having 1.0 cm path lengths. Melting points of
the products were monitored on a BUCHI Melting Point Model B-540.
2.2. Synthesis of H2L
Solution of 2,4-dihydroxybenzaldehyde (0.138 g, 1 mM) in 20 mL methanol was added to
a 20 mL 4-methyl-4-phenyl-3-thiosemicarbazide (0.091 g, 1 mM) methanolic solution.
Three drops of acetic acid were added. The reaction mixture was refluxed with vigorous
stirring for 8 h. A precipitate was formed when the solution was allowed to cool at room
temperature overnight. The orange–yellow precipitate was filtered, washed with cold metha-
nol, and dried in air. Yield: 0.156 g (46%). M.p.: 204–205 °C. Anal. Calcd for
C16H15N5O2S (%): C, 56.29; H, 4.43; N, 20.51. Found: C, 56.38; H, 4.63; N, 19.81.
2.3. Synthesis of [MoO2(L)(CH3OH)]
A solution of H2L (0.172 g, 0.5 mM) dissolved in 5 mL of methanol was added to a solu-
tion of [MoO2(acac)2] (0.164 g, 0.5 mM) in 5 mL methanol. The reaction mixture was then
refluxed for 4 h and the solution was allowed to stand at room temperature. Brown crystals
were formed after slow evaporation for one week. The product was filtered, washed with
ether, and dried in air. Yield: 0.080 g (32%). M.p.: 165 °C. Anal. Calcd for
C17H17N5O5SMo (%): C, 40.89; H, 3.43; N, 14.02. Found: C, 40.05; H, 3.26; N, 13.87.
2.4. Crystal structure determination and refinement
Diffraction data were collected at room temperature, by the ω-scan technique, on an Oxford
Diffraction SuperNova diffractometer (Atlas detector) with mirror-monochromated CuKα
radiation (λ = 1.54178 Å). The data were corrected for Lorentz-polarization and empirical
absorption correction (multi-scan) using spherical harmonics, implemented in SCALE3
ABSPACK scaling algorithm [16]. Accurate unit-cell parameters were determined by a least-
squares fit of 27,523 reflections of highest intensity, chosen from the whole experiment. The
structure was solved with SIR92 [17] and refined with the full-matrix least-squares procedure
on F2 by SHELXL97 [18]. Scattering factors incorporated in SHELXL97 were used. All
non-hydrogen atoms (with the exception of disordered water, vide infra) were refined aniso-
tropically. Hydrogens involved in hydrogen bonds were found in difference Fourier maps.
All other hydrogens were put in idealized positions, and refined as riding, with their isotropic
thermal parameters set at 1.2 (1.5 for methyl groups) times Ueq’s of appropriate carrier
atoms. The relatively large residual electron density near the twofold axis (at 0, y ,1/4) was
interpreted as disordered C2-symmetrical water. The oxygen of this molecule was refined
isotropically, position of hydrogens was calculated on the basis of potential hydrogen
bonding; the O–H and H⋯N distances were restrained at 0.90(1) and 2.20(2) Å, respec-
tively (O⋯N distance is 3.010(8) Å). Relevant crystal data are listed in table 1, together with
refinement details.

Molybdenum(VI) complex
551
Table 1. Crystal data, data collection, and structure refinement.
Formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
(C17H17MoN5O5S)2·CH3OH·1/4(H2O)
1035.28
Monoclinic
C2/c
27.1334(3)
12.0889(2)
27.5632(3)
90
β (°)
108.951(2)
90
8551.0(3)
8
1.61
4196
γ (°)
V (Å3)
Z
dx (g cm−3
F (0 0 0)
)
μ (mm−1
)
6.31
Θ range (°)
hkl range
3.39–75.61
−33 < h < 29
−15 < k < 14
−34 < l < 34
Reflections
collected
unique (Rint
40,612
8717 (0.041)
8233
0.033
0.091
0.035
0.093
1.04
0.84/−0.57
)
with I > 2σ(I)
R(F) [I > 2σ(I)]
wR(F2) [I > 2σ(I)]
R(F) [all data]
wR(F2) [all data]
Goodness-of-fit
Max/min Δρ (e Å−3
)
2.5. Catalytic experiments
To a solution of olefin (1 mM), chlorobenzene (1 mM) as internal standard, and [MoO2(L)
(CH3OH)] (0.003 mM) in 1,2-dichloroethane (1 mL) was added TBHP (1 mM) as oxidant.
The mixture was vigorously stirred at reflux under air for the required time, and the course
of the reaction was monitored using a gas chromatograph. Assignments of the products
were made by comparison with authentic samples. All reactions were run at least in
duplicate.
3. Results and discussion
3.1. Synthesis and spectral analysis of [MoO2(L)(CH3OH)]
H2L has been synthesized following the same procedure as for 1-(2,4-dihydroxybenzylid-
ene)-N-methyl-N-phenylthiosemicarbazone [19], except that the reaction was performed in
the presence of acetic acid and the reaction mixture was refluxed for 8 h. The formation of
the product, (2,4-dihydroxybenzylidene(5-N,N-methylphenylamino-1,3,4-thiadiazol-2-yl)
hydrazone), via self-cyclization of thiosemicarbazone (scheme 2) may be rationalized as
explained by J.P. Scovill et al. [20, 21]. The dioxomolybdenum(VI) complex, [MoO2(L)
(CH3OH)], was obtained by reaction of bis(acetylacetonato)dioxomolybdenum(VI) with a
stoichiometric amount of H2L in refluxing methanol. Brown crystals of the complex were
isolated from the reaction mixture by slow evaporation of solvent at room temperature.
The chemical formula is in accord with the data of elemental analysis and physicochemical

552
Z. Moradi-Shoeili et al.
SH
NR2R3
H
S
S
S
S
reflux
H+
R1HC=NHNC
NH
R1CH=NN=CNHNHCNR2R3
R1CH=NNHCNR2R3
N
HO
OH
R1:
R2:
R3:
H3C
- H2S
HO
OH
HO
OH
HN
N
N
S
N
CH3
CH3
N
N
N
N
N
S
N
H
(B)
(A)
Scheme 2. Ligand synthesis and tautomer forms.
measurements. A single-crystal X-ray diffraction study of [MoO2(L)(CH3OH)] shows it to
be a six-coordinate monomeric complex in which the central Mo is coordinated by a
tridentate NNO-donor ligand and the sixth coordination site is occupied by a methanol.
The characteristic IR frequencies of the complex confirmed the proposed molecular for-
mula shown in scheme 1. The complex shows strong absorption bands at 916 and
887 cm−1 which can be assigned to symmetric and anti-symmetric stretches of [MoO2]2+,
respectively [22]. The sharp band at 1628 cm−1, assigned to ν(C=N) in the free ligand, has
a bathochromic shift to 1605 cm−1 in the spectrum of the complex suggesting coordination
of azomethine nitrogen to molybdenum [23–25]. The ν(N–N) of the hydrazone is found at
1112 cm−1. The increase in the strength of this band in the spectrum of the complex again
confirms coordination via azomethine nitrogen [26]. IR spectrum of the complex shows a
sharp band at 1550 cm−1 due to the newly formed (N=C) bond, also indicating that
thiadiazole nitrogen is coordinated. A slight shift with decreasing intensity of the (O–H)
band at 3200 cm−1 [27] further agreed with coordination to the central metal ion through
deprotonated phenolic oxygen.
The 1H NMR spectrum of the ligand exhibits phenolic OH resonances at 9.86 and
10.86 ppm. Upon coordination to Mo, one signal disappeared and the second was observed
at 10.36, indicating deprotonation of one phenolic OH and coordination [28]. The hydra-
zone NH in the ligand at δ = 9.86 ppm is absent in the complex, suggesting tautomerization
of ligand to form B in scheme 2 consistent with deprotonation of the amide NH in the thia-
diazole ring [29]. The azomethine C–H signal in ligand at 8.25 ppm shifted to 8.51 ppm in
the complex. The downfield shift of this signal might be attributed to removal of electron
density from the nitrogen of the azomethine to the Mo [30]. The methyl protons of ligand
1
were observed at δ = 3.34 ppm. The H NMR data for ligand and complex are consistent
with interpretation of the IR spectra.
Electronic absorption spectra of the ligand and complex were recorded in 10−4 M DMF
solution between 250 and 700 nm (figure 1). The absence of a d–d transition absorption
band in the visible region confirms the 4d0 electronic configuration of Mo(VI). The
compounds display several common absorption maxima at 200–350 nm assigned to

Molybdenum(VI) complex
553
Figure 1. UV–vis spectroscopy of (a) ligand (10−4 M) and (b) [MoO2L(CH3CN)] (10−4 M) in DMF.
intra-ligand transitions. Electronic absorption of the complex exhibits one shoulder at
500 nm (in the region 450–550 nm) which may be assigned as ligand to metal charge
transfer transition.
3.2. Description of the crystal structure of [MoO2(L)(CH3OH)]
The asymmetric unit of the crystal consists of two symmetry-independent, but structur-
ally very similar, Mo(VI) species denoted as A and B, one uncoordinated methanol and
one disordered water (at special position, on the twofold axis). Both Mo(VI) ions have
similar six coordination in which cis-[MoO2]2+ species are meridionally coordinated by
doubly deprotonated L2− via ONN donors, phen-2-ol-oxygen, imine- and thiadiazole-1-
nitrogens, forming six- and five-membered chelate rings around cis-[MoO2]2+. The sixth
site is occupied by oxygen from methanol. The molecular geometry and atom numbering
of one of the structurally similar [MoO2L(CH3OH)] molecules is presented in figure 2.
Selected bond lengths and angles with their standard deviations for [MoO2(L)(CH3OH)]
are given in table 2.
Figure 2. One of the symmetry independent molecules of [MoO2(L)(CH3OH)]; ellipsoids are drawn at the 50%
probability level with hydrogens shown as spheres of arbitrary radii.

554
Z. Moradi-Shoeili et al.
Table 2. Selected bond distances (Å) and
angles (°) for [MoO2(L)(CH3OH)]; numerical
values for B are given in square brackets.
Bond distance
Mo1–O19
Mo1–O25
Mo1–O26
Mo1–O27
Mo1–N3
Mo1–N15
N3–C2
N4–C5
N14–C2
N15–C16
1.922 (2) [1.935(2)]
1.675(2) [1.703(2)]
1.703 (2) [1.698(12)]
2.427(2) [2.380(2)]
2.086(2) [2.067(2)]
2.246(2) [2.236(2)]
1.346(3) [1.351(3)]
1.291(3) [1.293(3)]
1.310(3) [1.310(3)]
1.292(3) [1.298(3)]
Bond angle
O19–Mo1–O25
O19–Mo1–O26
O19–Mo1–O27
O19–Mo1–N15
O25–Mo1–O26
O25–Mo1–O27
O25–Mo1–N3
O25–Mo1–N15
O26–Mo1–O27
O27–Mo1–N3
O27–Mo1–N15
N3–Mo1–N15
100.99(1) [99.23(8)]
106.29(1) [105.90(8)]
78.24(7) [77.25(7)]
81.99(7) [81.54(7)]
106.29(1) [105.58(9)]
169.33(9) [170.93(8)]
100.10(1) [101.69(9)]
94.86(1) [94.96(8)]
84.02(8) [83.47(8)]
76.32(8) [78.00(7)]
74.47(7) [76.32(8)]
70.91(7) [70.68(7)]
As both complexes are structurally very similar, we will discuss them jointly giving the
appropriate numerical values for B in square brackets. The coordination geometry around
molybdenum can be described as distorted octahedral. For example, the cis angles range
from 70.91(7)° [70.68(7)°] for N(3A)-Mo(1A)-N(15A) to 106.29(1)° [105.58(9)°] for O
(25)–Mo(1A)–O(26).
Bond lengths N14–C2 and N3–C2, 1.310(3) [1.310(3)] and 1.346(3) [1.351(3)] Å,
respectively, indicate partial double-bond character, while N4–C5 and N15–C16 are pure
double-bonds with 1.291(4) [1.293(3)] and 1.292(3) [1.298(3)] Å, respectively [31–34].
This suggests that during complexation the ligand undergoes tautomerization to the B
tautomeric form shown in scheme 2.
The Mo–O25 and Mo–O26 bond lengths, 1.675(2) [1.703(2)] and 1.703(2) [1.698(12)]
Å, respectively, are mostly as expected, indicating they are double-bonds [35–37]. The cor-
responding O=M=O angle 106.29(1) [105.58(9)] is significantly larger than 90° because of
the repulsion between the π-electrons of the terminal oxos. This event is accompanied by a
contraction of the opposite angle N3–Mo–N15, 70.91(7) [70.68(7)], in accord to literature
data for cis-dioxo molybdenum(VI) complexes [24, 34]. In the coordination sphere of
molybdenum, the distances of 2.086(2) [2.067(2)] Å to thiadiazole-1-nitrogen N3 and of
1.922(2) [1.935(2)] Å to the phenolate oxygen O19 represent rather normal single-bond val-
ues [25, 27, 29, 38], whereas the bonds from molybdenum to the azomethine nitrogen N15,
2.246(2) [2.236(2)] Å and to O27 of the solvent molecule, 2.427(2) [2.380(2)] Å are longer
than the normal single-bond length due to the strong trans influence of the essentially dou-
bly bonded terminal oxo ligands O25 and O26 [23, 39–41]. Other structural parameters for
ligand framework are comparable with previously reported Mo(VI) complexes, containing
tridentate ONN-donor ligands [29, 42–44].

Molybdenum(VI) complex
555
Crystal packing diagram of [MoO2(L)(CH3OH)] is shown in figure 3. As shown in this
figure, there are several O–H···O, O–H···N, and C–H···O hydrogen bonds which stabilize
the structure of the complex (table 3). Only in intermolecular interactions are two symme-
try-independent molecules essentially different. The principal structural motif is the infinite
chains along the b-direction of alternating A and B complexes connected by O27–H···N14
hydrogen bonds. The chains are connected by hydrogen bridges through solvent methanol
and by direct hydrogen bonds between symmetry-related complex molecules. Two
molecules A and two methanols are connected into centrosymmetric tetramers by
O22–H···O1S–H···N4, while B molecules are connected into centrosymmetric dimers by
O22–H···O25.
3.3. Homogeneous catalytic epoxidation of olefins
In order to find the optimized conditions for oxidation of alkenes with TBHP catalyzed by
[MoO2(L)(CH3OH)], cyclooctene is used as the model substrate. The molar ratio 3 : 1000 :
1000 of the catalyst : cyclooctene : TBHP is the optimum molar ratio for the catalytic system.
The effect of various solvents on the progress of the reactions was investigated on catalytic
epoxidation of cyclooctene (table 4). Efficiency of the catalyst in different solvents decreases
in the order dichloroethane > chloroform > dichloromethane > acetonitrile > methanol.
Figure 3. A fragment of the crystal packing of [MoO2(L)(CH3OH)] as seen along the b-direction (along ~ABAB~
chains). Hydrogen bonds are shown as dashed lines; one can realize the different modes of connecting A and B
(cf. text). Color codes: green – molecule A, blue – molecule B, red – methanol, yellow – water (see http://dx.doi.
org/10.1080/00958972.2014.993321 for color version).

556
Z. Moradi-Shoeili et al.
Table 3. Geometry of the hydrogen bonds (Å, °).
D–H⋯A
D–H
H···A
D···A
D–H···A
O1S–H1S⋯N4Ai
1.00
0.87
0.91
0.82
0.87
0.93
0.90
1.87
1.80
1.86
1.97
1.91
2.38
2.21
2.835(3)
2.652(3)
2.764(3)
2.786(3)
2.765(3)
3.271(3)
3.010(8)
161
169
174
169
169
161
148
O22A–H22A⋯O1S
O22B–H22B⋯O25Bii
O27A–H27A⋯N14B
O27B–H27B⋯N14Aiii
C16B–H16B⋯O26A
O1 W–H1 W⋯N4B
Symmetry operations: i = 1/2 − x, 1/2 − y, −z; ii = –x, 1 − y, −z; iii = 1/2 − x, 1/2 + y, 1/2 − z.
Apparently, highly coordinating solvents such as CH3OH cause a significant decrease in the
catalytic activity, since they compete with TBHP to bind to molybdenum. The best solvent
was 1,2-dicholoroethane. In the next step, the effect of the catalyst amount on the conversion
and selectivity is illustrated in table 4. Five different amounts of catalyst, 0.005, 0.004,
0.003, 0.002, 0.001 mM, were examined while keeping a fixed amount of cyclooctene
(1 mM) and TBHP (1 mM). The results show that decreasing of the catalyst from 0.005 to
0.003 mM does not have an effect on the percent of conversion under the reaction condition
in this study (table 4, entries 5 and 6), but further decreasing the catalyst amount to
0.001 mM causes conversion of cyclooctene to 84%.
Table 4. Optimized reaction condition in oxidation of cyclooctene with TBHP catalyzed by
[MoO2(L)(CH3OH)]a.
Entry
Catalyst (mM)
Solvent
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
7
8
0.005
0.005
0.005
0.005
0.005
0.004
0.003
0.002
0.001
CH3CN
CH2Cl2
CH3OH
CHCl3
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
55
83
38
96
99
99
99
96
84
100
95
99
99
99
99
99
99
99
aReaction condition: 1 mM cyclooctene, 1 mM TBHP.
The reactions were run in reflux for 30 min.
Table 5. [MoO2(L)(CH3OH)] catalyzed epoxidation of different olefins by TBHPa.
Entry
Substrate
Conversion %b
Time (h)
Selectivity to epoxide %b
1
2
3
4
5
6
7
cyclohexene
cyclooctene
styrene
98
99
96
90
85
92
95
1
0.5
4
3
3
>99
99
6 (72c, 22d)
>99
1-octene
1-heptene
trans-stilbene
cis-stilbene
>99
3
2
89e (11)c
65f (35)c
aReaction condition: The molar ratios for catalyst : alkene : TBHP are 3 : 1000 : 1000. The reactions were run in reflux and in
1 mL dichloroethane. bGC yield based on starting alkene. The by-product is: cbenzaldehyde, dbenzoic acid, etrans-stilbene
f
epoxide, cis-stilbene epoxide.

Molybdenum(VI) complex
557
HO
N
S
N
N
O
N
N
CH3
Mo
O
solvent
O
, tert-BuOH
O
N
HO
S
N
N
N
HO
N
S
N
N
O
N
N
CH3
Mo
O
O
N
O
CH3
Mo
O
O
H
O
O
H
BuO
BuO
Scheme 3. Proposed mechanism for oxidation of alkenes catalyzed by [MoO2(L)(CH3OH)] in the presence of
TBHP.
In order to establish the general applicability of the method, various alkenes were
subjected to the oxidation protocol with [MoO2(L)(CH3OH)] (table 5). The most reactive
cyclic alkenes were oxidized in high yield and excellent selectivity. The data for oxygena-
tion of cyclohexene show no product from allylic attack, indicating that catalytic reaction
occurs via non-radical processes [45]. High epoxide yields and selectivities were observed
for aliphatic terminal alkenes such as 1-octene and n-heptene. The epoxidation of styrene
with TBHP gave styrene oxide, benzaldehyde, and benzoic acid. Styrene oxide has been
obtained only in poor yield, while the yield of benzaldehyde is the highest. Additionally,
the epoxidation reaction is stereoselective. Epoxidation of trans-stilbene led to the trans
epoxide, while epoxidation of cis-stilbene under similar conditions gave exclusively the cis
epoxide.
Based on the results of this study, we suggest a general pathway (scheme 3), as proposed
by Thiel [46], for this catalytic reaction, involving transfer of one TBHP proton to one oxo
group of dioxo complex and coordination of TBHP anion to the Lewis acidic metal center.
The six-coordinate species is subsequently activated for oxygen transfer to the olefin.
Recently, we reported the structures and catalytic activities of complexes of the form
[MoO2Lʹ(solv)] (solv = C2H5OH, CH3OH and Lʹ = ONO donor Schiff-base ligand)
[47–49]. The complexes catalyzed epoxidation of olefins using TBHP. The comparison of
the catalytic activity of these complexes and [MoO2(L)(CH3OH)] presented here indicates
that replacing an oxygen donor with a nitrogen donor in the structure of chelating ligand
does not significantly affect the catalytic activity of the complexes in epoxidation.

558
Z. Moradi-Shoeili et al.
4. Conclusion
[MoO2(L)(CH3OH)] was synthesized by reaction of a new Schiff base having a 1,3,4-thia-
diazole moiety with [MoO2(acac)2]. Crystallographic investigation reveals that the ligand is
tridentate via phenolic oxygen, azomethine nitrogen, and thiadiazole nitrogen. The sixth site
of distorted octahedral geometry of the complex is occupied by methanol. [MoO2(L)
(CH3OH)] is employed as a catalyst for the homogeneous epoxidation of olefins using
TBHP. The complex catalyzed epoxidation of olefins to afford the corresponding epoxides
in high yield and selectivity for aliphatic terminal alkenes and cyclic alkenes.
Supplementary material
CCDC 932253 contains supplementary crystallographic data for this paper. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by e-mailing
data_request@ccdc.cam.ac.uk, or by contacting the Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge CB2 1EZ, UK.
Acknowledgement
The authors are grateful to the Research Council of Sharif University of Technology for
their financial support.
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