Synthesis, characterization and pronounced epoxidation activity of cis-dioxo-molybdenum(VI) tridentate Schiff base complexes using tert-butyl hydroperoxide

Article abstract of DOI:10.1016/j.poly.2010.06.024

The synthesis and catalytic performance of novel cis-dioxo-Mo(VI) complexes containing simple ONO tridentate Schiff base ligands in the epoxidation of various olefins using tert-butyl hydroperoxide in desired times with excellent chemo- and stereoselectivity have been described. The study of turnover numbers and the UV-Vis spectra of the Mo complexes in the present epoxidation system indicate well the high efficiency and stability of the catalysts during the reaction. The electron-deficient and bulky groups on the salicylidene ring of the ligand promote the effectiveness of the catalyst.

Full text of DOI:10.1016/j.poly.2010.06.024

Polyhedron 29 (2010) 2703–2709
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and pronounced epoxidation activity of cis-dioxo-
molybdenum(VI) tridentate Schiff base complexes using tert-butyl hydroperoxide
b
Abdolreza Rezaeifard a, , Iran Sheikhshoaie , Niaz Monadi a,b, Mahboubeh Alipour a
*
a Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand, Birjand 97179-414, Iran
b Department of Chemistry, Faculty of Science, Shahid Bahonar University of Kerman, Kerman 76175-133, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and catalytic performance of novel cis-dioxo-Mo(VI) complexes containing simple ONO tri-
dentate Schiff base ligands in the epoxidation of various olefins using tert-butyl hydroperoxide in desired
times with excellent chemo- and stereoselectivity have been described. The study of turnover numbers
and the UV–Vis spectra of the Mo complexes in the present epoxidation system indicate well the high
efficiency and stability of the catalysts during the reaction. The electron-deficient and bulky groups on
the salicylidene ring of the ligand promote the effectiveness of the catalyst.
Received 22 April 2010
Accepted 22 June 2010
Available online 1 July 2010
Keywords:
Synthesis
Epoxidation
Dioxomolybdenum
Schiff base
Peroxide
Ó 2010 Elsevier Ltd. All rights reserved.
Catalyst
1. Introduction
selective epoxidation of olefins, which is a particular challenging
problem in organic chemistry, has been described (Scheme 1). The
High-valent molybdenum complexes have attracted continuing
attention due to their important practical applications as catalysts
in several industrial processes such as ammoxidation of propene,
epoxidation of olefins, etc. [1–7] and as optoelectronic materials
[8–10]. In this context, molybdenum(VI) dioxo-complexes have
been extremely well investigated [11–18], particularly with re-
spect to the catalytic role of transferase enzymes like nitrate reduc-
tase in which their active sites consist of a cis molybdenum dioxo
moiety [19–21]. The ability of molybdenum to form stable com-
plexes with oxygen-, nitrogen- and sulfur-containing ligands led
to the development of molybdenum Schiff base complexes which
are efficient catalysts both in homogeneous and heterogeneous
reactions [22–28]. The activity of these complexes varies markedly
with the type of ligands and coordination sites [29,30]. In continu-
ation of our ongoing research on the development of biomimetic
oxidation reactions [31–36], very recently we have introduced a di-
oxo-Mo(VI) Schiff base complex [MoO2(L1)(CH3OH)] (Scheme 1)
containing a ONO tridentate Schiff base ligand, (L1 = 2-[(2-hydroxy-
propylimino)methyl]phenol) as an efficient and selective catalyst
for oxidation of sulfides to sulfoxides and sulfones [37]. In this report,
the catalytic performance of this simple Mo catalyst in the highly
influence of the electronic and structural requirements of the ligand
on the catalytic activity of the catalyst has also been investigated
(Scheme 1).
2. Experimental
2.1. General remarks
All reagents were used as received. Solvents were purified by
standard methods and dried if necessary. Methanol was distilled
from magnesium methoxide. Acetylacetate and ammonium hepta-
molybdate were purchased from Fluka Company and these re-
agents were used as received. All reactions and workups were
carried out in air.
2.2. Instrumentation
Purity determinations of the products were accomplished by GC
on
a
Shimadzu GC-16A instrument using
a 25m CBP1-S25
(0.32 mm ID, 0.5
lm coating) capillary column. IR spectra were re-
corded on a Perkin–Elmer 780 instrument. UV–Vis spectra were re-
corded on a 160 Shimadzu spectrophotometer. NMR spectra were
recorded on a Brucker Avance DPX 500 MHz instrument. Mass
spectra were recorded on a Shimadzu GC–MS-QP5050A.
* Corresponding author. Tel.: +98 561 2502516; fax: +98 561 2502515.
E-mail address: rrezaeifard@birjand.ac.ir (A. Rezaeifard).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.06.024

2704
A. Rezaeifard et al. / Polyhedron 29 (2010) 2703–2709
1153(m
COphenolic); 1H NMR (500 MHz, DMSO) dppm: 1.12 (d, J = 6.3,
CH3
3H), 3.47 (dd, J1 = 11.8, J2 = 6.3, 1H), 3.57 (dd, J1 = 11.85, J2 = 4.8,
1H), 3.41 (s, 3H), 3.87–3.83 (m, 1H), 6.8 (d, J = 8.9, 1H), 6.94 (dd,
J1 = 8.9, J2 = 3.1, 1H), 7.04 (d, J = 3.0, 1H), 8.45 (s, 1H), 13.00 (s,
1H); 13C NMR (125 MHz, DMSO) dppm: 22.22, 56.42, 66.56, 67.26,
115.75, 118.02, 119.34, 120.01, 152.31, 155.59, 167.06; Anal. Calc.
for C11H15NO3: C, 63.14; H, 7.23; N, 6.69. Found: C, 63.16; H, 7.18;
N, 6.70%. MS: m/z 209 (M+).
H
X
N
O
MeHO Mo
O
O
O
O
Y
2.3.5. MoL3
TBHP, DCE, 80°C
To 0.01 mol H2L3 ligand (2.09 g), was added 0.01 mol dioxo-
molybdenum(VI) acetylacetonate (3.28 g) in 10 mL methanol. The
reaction mixture was stirred under reflux condition for 1 h. After
one day, a yellow precipitate was removed by filtration. By wash-
ing with cooled methanol, recrystallization from methanol and
drying in vacuum, yellow crystals were secured in 80% yield
X, Y=H
MoL1
X=NO2, Y=H MoL2
X=OMe, Y=H MoL3
X, Y=t-Bu
MoL4
Scheme 1. Epoxidation of olefins catalyzed by the MoVI Schiff base complexes
(2.68 g), M.p.: >250 °C. IR (KBr disc, cmÀ1): 1644(
mC@N), 1478(mC@C),
employed in this study.
881 and 916(
m
Mo–O); 1H NMR (500 MHz, DMSO) dppm: 1.26 (d,
J = 6.0, 3H), 3.59–3.52 (dd, 1H), 4.15 (dd, J1 = 13.4, J2 = 3.7, 1H),
3.74 (s, 3H), 4.45–4.41 (m, 1H), 6.82 (d, J = 8.9, 1H), 7.08–7.13
(m, 2H), 8.66 (s, 1H); 13C NMR (125 MHz, DMSO) dppm: 20.73,
56.50, 67.51, 77.96, 116.91, 120.93, 121.45, 122.89, 152.72,
156.92, 164.23; Anal. Calc. for C11H13MoNO5: C, 39.42; H, 3.91; N,
4.18. Found: C, 39.38; H, 3.88; N, 4.2%. UV–Vis: kmax = 346 nm.
2.3. Syntheses
2.3.1. H2L1 and MoL1
The synthesis, characterization and crystal structure have previ-
ously been reported [37].
2.3.2. H2l2
2.3.6. H2l4
The asymmetric tridentate Schiff base H2L2 was obtained by
addition of a solution of 0.01 mol 1-amino-2-propanol (0.75 g) in
methanol (10 ml) to a solution of 0.01 mol 5-nitro salicylaldehyde
(1.67 g) in 10 mL methanol and the reaction mixture heated for
1 h, giving a yellow precipitate. The crude product was recrystal-
lized from CHCl3–hexane (1/4 v/v). Yield: 75% (1.68 g). M.p.:
The asymmetric tridentate Schiff base H2L4 was obtained by
addition of a solution of 0.01 mol 1-amino-2-propanol (0.75 g) in
methanol (10 ml) to a solution of 0.01 mol 3,5-di-tert-butyl salicyl-
aldehyde (2.34 g) in 10 ml methanol. The reaction mixture was
heated at reflux for 1 h, giving a yellow oil. IR (KBr, cmÀ1):
3347(mOH), 1628(mC@N), 1477(mC@C), 1271(m
COphenolic); 1H NMR
162 °C. IR (KBr disc, cmÀ1): 33461(
mOH), 1667(mC@N), 1531(mC@C),
(500 MHz, CDCl3) dppm: 1.75 (d, J = 6.2, 3H), 1.29 (s, 9H), 1.40 (s,
9H), 3.58 (dd, J1 = 12.2, J2 = 7.2, 1H), 3.75 (dd, J1 = 12.04, J2 = 2.9,
1H), 4.27–4.13 (m, 1H), 7.15 (s, 1H), 7.43 (s, 1H), 8.43 (s, 1H),
13.65 (s, 1H); 13C NMR (125 MHz, CDCl3) dppm: 21.28, 29.85,
31.92, 67.67, 67.74, 118.22, 126.50, 127.58, 137.19, 140.64,
158.49, 168.36; Anal. Calc. for C18H29NO2: C, 74.18; H, 10.03; N,
4.81. Found: C, 74.21; H, 9.96; N, 4.80%. MS: m/z 291 (M+).
1265(
m
COphenolic); 1H NMR (500 MHz, DMSO) dppm: 1.19 (d, J = 6.2,
3H), 3.47 (dd, J1 = 12.7 J2 = 7.2, 1H), 3.70 (dd, J1 = 12.7, J2 = 3.0,
1H), 3.90 (m, 1H), 6.58 (d, J = 9.7, 1H), 8.02 (dd, J1 = 9.6, J2 = 3, 1H),
8.43 (d, J = 2.9, 1H), 8.69 (s, 1H), 14.21 (s, 1H); 13C NMR
(125 MHz, DMSO) dppm
130.16, 133.64, 134.36, 168.69, 179.17; Anal. Calc. for
: 21.60, 59.66, 65.46, 114.21, 123.83,
C
10H12N2O4: C, 53.57; H, 5.39; N, 12.49. Found: C, 53.52; H, 5.36;
N, 12.52%. MS: m/z 224(M+).
2.3.7. MoL4
To 0.01 mol H2L3 (2.91 g), was added 0.01 mol dioxo-molybde-
num(VI) acetylacetonate (3.28 g) in 10 ml methanol. The reaction
mixture was stirred under reflux conditions, the solution color
changed to deep yellow and yellow crystals started to precipitate
after 5 min. The reflux was continued for 1 h. The product was fil-
tered and washed with cooled methanol (78% yield, 3.5 g). M.p.:
2.3.3. MoL2
To 0.01 mol H2L2 ligand (2.24 g) was added 0.01 mol dioxo-
molybdenum(VI) acetylacetonate (3.28 g) in 10 ml methanol. The
reaction mixture was stirred under reflux condition for 2 h. After
one day, a yellow precipitate was removed by filtration. By wash-
ing with cooled methanol, recrystallization from methanol and
drying in vacuum, yellow crystals were secured in 62% yield
>250 °C. IR (KBr disc, cmÀ1): 1634(
925(
Mo–O); 1H NMR (500 MHz, DMSO) dppm: 1.26 (s, 9H), 1.36 (s,
9H), 3.17 (d, J = 4.9, 3H), 4.13–4.07 (m, 2H), 4.44 (m, 1H), 7.39 (s,
1H), 7.46 (s, 1H), 8.69 (s, 1H); 13C NMR (125 MHz, DMSO) dppm
mC@N), 1477(mC@C), 910 and
(2.18 g). M.p.: >250 °C. IR (KBr disc, cmÀ1): 1653(
899 and 930(
Mo–O); 1H NMR (500 MHz, DMSO) dppm: 1.26 (d, J = 6,
m
C@N), 1605(mC@C),
m
m
:
3H), 3.67–3.58 (dd, 1H), 4.22–4.11 (dd, 1H), 4.58–4.54 (m, 1H),
6.99 (d, J = 9.2, 1H), 8.23 (dd, J1 = 9.2, J2 = 3, 1H), 8.56 (d, J = 3,
1H), 8.84 (s, 1H); 13C NMR (125 MHz, DMSO) dppm: 20.15, 67.38,
79.08, 120.76, 121.41, 129.70, 130.98, 139.22, 163.53, 167.25; Anal.
Calc. for C10H10MoN2O6: C, 34.30; H, 2.88; N, 8. Found: C, 34.28; H,
2.85; N, 8%. UV–Vis: kmax = 328 nm.
20.75, 30.42, 32.08, 34.78, 35.78, 49.47, 67.42, 77.75, 121.43,
129.01, 129.48, 138.89, 141.33, 159.54, 165.23; Anal. Calc. for
C19H31MoNO5: C, 50.78; H, 6.95; N, 3.12. Found: C, 50.80; H,
6.92; N, 3.10%. UV–Vis: kmax = 344 nm.
2.4. General oxidation procedure
2.3.4. H2l3
The asymmetric tridentate Schiff base H2L3 was obtained by
addition of a solution of 0.01 mol 1-amino-2-propanol (0.75 g) in
methanol (10 ml) to a solution of 0.01 mol 5-methoxy salicylalde-
hyde (1.52 g) in 10 mL methanol. The reaction mixture was heated
for 1 h, giving a yellow precipitate. The crude product was recrys-
tallized from CHCl3–hexane (1/4 v/v). Yield: 81% (1.69 g). M.p.:
To a solution of olefin (0.5 mmol) and MoLx (Scheme 1)
(0.005 mmol) in 5 ml DCE was added TBHP (1 mmol) and the reac-
tion mixture was stirred under air at 80 °C for the required time.
The reaction progress was monitored by GC and the yields of the
products were determined by GC and NMR analysis. Further
purification was achieved by silica chromatography eluting with
n-hexane/ethyl acetate (10/2).
58 °C. IR (KBr disc, cmÀ1): 3193(
mOH), 1645(mC@N), 1521(mC@C),

A. Rezaeifard et al. / Polyhedron 29 (2010) 2703–2709
2705
Table 1
Analytical and selected spectral data of the Mo(VI) complexes.
Selected spectral data
Complex
Elem. Anal. (Calc.)
mC@N (cmÀ1
)
mMo@O (cmÀ1
)
d–CH@N–(ppm)
d(methanol)(ppm)
kmax (nm)
[MoO2L1(CH3OH)]
[MoO2L2]
C, 39.2(39.18), H, 4.45(4.48), N, 4.17(4.15)
C, 34.31(34.30), H, 2.85(2.88), N, 7.92(8)
C, 39.39(39.42), H, 3.93(3.91), N, 4.2(4.18)
C, 50.75(50.78), H, 6.92(6.95), N, 3.2(3.12)
1638
1653
1644
1632
900, 924
899, 930
881, 916
910, 925
8.70
8.84
8.66
8.69
3.18 (J = 5,3H)
–
–
341 (LMCT)
328 (LMCT)
346 (LMCT)
344 (LMCT)
[MoO2L3]
[MoO2L4(CH3OH)]
3.17 (J = 4.9,3H)
3. Results and discussion
3.1. Structural discussion of the cis-dioxo-Mo(VI) complexes
The Schiff base ligands H2L1–H2L4 (Scheme 1) were prepared by
the condensation of 1-amino-2-propanol with a number of salicyl-
aldehydes substituted by different electronic and steric groups (5-
NO2-, 5-OCH3- and 3,5-di-tert-butyl salicylaldehyde) in methanol
and could be isolated by precipitation on the addition of CHCl3–
n-hexane (1/4 v/v). The corresponding Mo complexes (Scheme 1,
MoO2L1–MoO2L4) of these ligands were synthesized by the
reaction of MoO2(acac)2 and the related Schiff base ligands in
methanol. All the new complexes were recrystallized from
methanol and are quite air stable as solids and also in solution.
Analytical and selected spectral data of the Mo complexes are
given in Table 1.
Fig. 1. Electronic spectra of H2L2 (a) and MoL2 (b) in methanol.
In addition to crystal data for MoL1 [37], spectroscopic and
elemental analyses are consistent with a monomeric complex with
a ligand:Mo ratio of 1:1. When the synthesis reactions were per-
formed using two equivalents of the ligands, the same products
were formed as observed with one equivalent of the ligand. The
infrared spectra of the Mo complexes exhibit two strong mMo@O
bands in the regions 880–910 and 916–930 cmÀ1, characteristic
of the symmetric and asymmetric stretching vibrations of cis-
[MoO2]2+ fragments [11–18]. The absence of a broad band around
3354–3193 cmÀ1 in the spectra of the molybdenum complexes
compared to the Schiff base ligands indicates the coordination of
the phenolic oxygen after deprotonation. The characteristic imine
bands in the ligands (1635–1667 cmÀ1) are shifted to lower wave-
numbers after coordination of the azomethine nitrogen to the Mo
center, and appeared at 1632–1653 cmÀ1. A sharp singlet at 8.36–
8.69 ppm, which be rationalized as the azomethine proton of the
ligands, shifted downfield and appeared at 8.66–8.84 ppm, result-
ing from the coordination of the azomethine nitrogen [38,39]. The
appearance of a signal at 3.17–3.18 ppm in the 1H NMR spectra of
[MoO2(L1)(CH3OH)] and [MoO2(L4)(CH3OH)] confirms the coordi-
nation of a methanol molecule to the Mo(VI) center of these com-
plexes [35,37].
in various solvents such as EtOH, MeOH, MeCN, (Me)2CO, CH2Cl2,
CHCl3 and 1,2-C2H4Cl2 (DCE) was carried out in the presence of
1 mol% of the simple MoL1 catalyst. While coordinative solvents
inhibited the reaction completely under mild and reflux condi-
tions, cyclohexene oxidation proceeded in CH2Cl2, CHCl3 and DCE
at various temperatures. Considering the yield and reaction rate,
DCE was found to be the best solvent; however the reaction re-
quired a temperature of 80 °C for completion in the desired time
(Fig. 2). The poorer yields obtained with CH2Cl2 (34%) and CHCl3
(66%) is probably caused by the lower reaction temperature for
their reflux conditions.
Different catalyst/alkene molar ratios have been used in the oxi-
dation of cyclohexene with different reaction times (Fig. 3). It was
observed that oxidation of cyclohexene required 40 min for com-
pletion using 1 mol% of the catalyst and an increase in this ratio
up to 5 mol% did not noticeably affect the reaction rate (Fig. 3).
The examination of various molar ratios of TBHP/alkene in the
catalytic oxidation of cyclohexene in DCE showed that the full oxi-
dation of the starting material was obtained by two equivalents of
The electronic spectra of the Mo(VI) complexes revealed a high
intensity charge transfer band (LMCT) in the region 300–350 nm
and a regular trend between kmax and the electronic requirements
of the complexes was observed. As expected the complex contain-
ing an electron-deficient Schiff base ligand (H2L2) requires higher
energy for the LMCT [38–41]. The electronic spectra of the H2L2 li-
gand (kmax = 346, 360 nm) and the MoL2 complex (kmax = 328 nm)
are shown in Fig. 1.
80oC
100
80
60oC
60
40
20
3.2. Catalytic activity
30oC
0
The oxidation of cyclohexene using tert-butyl hydroperoxide
(TBHP), which did not proceed in the absence of catalyst under
mild and reflux conditions, was used as a model reaction. To find
the optimum reaction conditions, the influence of different factors
that may affect the conversion and selectivity of the reaction was
investigated. A systematic examination of cyclohexene oxidation
0
10
30
40
60
Time/min
Fig. 2. The influence of temperature on the oxidation of cyclohexene using TBHP
catalyzed by MoL1. The reactions were run under air in DCE and the molar ratio of
cyclohexene:TBHP:catalyst was 100:200:1.

2706
A. Rezaeifard et al. / Polyhedron 29 (2010) 2703–2709
100
80
60
40
20
0
sphere of the molybdenum complex (Scheme 2). The Lewis acid-
ity of the Mo center increases the oxidizing power of the peroxo
group and the alkene is subsequently oxygenated by nucleophilic
attack on an electrophilic oxygen atom of the coordinated TBHP
peroxide [44]. Spectroscopic and computational methods applied
to molybdenum dioxo and oxodiperoxo complexes support a
reaction mechanism involving an activation state formed be-
tween the complex, oxidant and olefin (Scheme 2, IV) [45,46].
The excellent stereoselectivity obtained in the oxidation of cis-
stilbene indicates that the closure of the epoxide ring occurs fas-
ter than the rotation around the C–C bond during the oxygen
transfer step from this active oxidizing species to the olefin
(Scheme 2, IV). In this process, in higher conversions, tert-butanol
can compete with TBHP for coordination to the molybdenum cen-
ter, forming a less reactive species (Scheme 2, V) that leads to the
decreasing reaction rate [45]. Similarly, for complex MoL1, the
initial reaction rates were much higher than those observed later
in the reactions (Fig. 5, none). The inhibiting effect of coordina-
tive solvents used in the preliminary experiments may be more
evidence for this suggestion. This was further supported by the
retarding effect of nitrogenous bases on the catalytic activity of
MoL1. The addition of pyridine and imidazole to the reaction mix-
ture containing the MoL1 complex in DCE under the same condi-
tions retarded remarkably the epoxidation of cyclohexene (Fig. 5).
0
0.5
0.7
1
2
3
5
Catalyst mol%
Fig. 3. The influence of catalyst concentration on the oxidation of cyclohexene
using TBHP catalyzed by MoL1. The reactions were run under air in DCE and the
molar ratio of cyclohexene:TBHP:catalyst was 100:200:X.
100
10 min
30 min
45 min
100
80
60
40
20
0
89
89
80
78
60
Presumably, pyridine and in particular strong
p-donor imidazole
55
form an inactive species (Scheme 2, VI) by coordinating to the
molybdenum ion [47–49], making it a sluggish catalyst for activa-
tion of TBHP.
It is noteworthy that the easy preparation method of the
Mo-Schiff base catalyst and advantages of TBHP as an oxidant for
industrial synthesis offer ready scalability. The performance of
the oxidation reaction using 100 mmol norbornene as a substrate
gave 92% of the related epoxide after purification over silica
chromatography eluting with n-hexane/ethyl acetate (10/2).
47
38
1
1.5
2
TBHP/Cyclohexene molar ratio
3.3. Activity and stability of dioxo-Mo(VI) complexes
Fig. 4. The influence of TBHP/alkene molar ratios on the oxidation of cyclohexene
catalyzed by MoL1 in DCE.
The high/excellent yields of the epoxides (66–100%) obtained
using these new catalytic methods in relatively short times
(45 min) display the superior catalytic activity of the present
dioxo-Mo(VI) complexes [22–26]. This was further supported by
the impressive turnover number of the MoL1 catalyst (4925/24 h)
in the oxidation of cyclooctene by TBHP using a 5000:10000:1 M
ratio for the substrate/oxidant/catalyst. More evidences in this
matter have been obtained by monitoring the electronic spectra
of the MoL1 complex in the epoxidation of the less reactive 1-oc-
tene (Fig. 6). The intensity of the characteristic absorption bands
remains approximately unchanged during the hour reaction time,
indicating the high stability of the Mo catalyst under the oxidation
conditions. The initial decrease in the intensity upon addition of
the oxidant may be attributed to the formation of active species
in the oxidation reaction [50–53].
The influence of the electronic and structural requirements of
the Schiff base ligand on the catalytic activity and stability of the
Mo catalyst was investigated. Different electronically and structur-
ally Mo complexes (Scheme 1) were subjected to the epoxidation
of cyclooctene by TBHP using a 10 000:20 000:1 M ratio for the
substrate/oxidant/catalyst. The order of catalytic activity was
found to be MoL4 > MoL2 > MoL1 > MoL3 according to turnover fre-
quency of the catalysts per hour (Fig. 7). As expected, an electron-
withdrawing group in the salicylidene ring (MoL2) increases the
effectiveness of the catalyst, resulting from the elevated Lewis
acidity of the molybdenum centre and therefore the activation of
TBHP (Scheme 2, II), promoting the catalytic activity of epoxidation
[46,54]. However, the introduction of the bulky and electron-rich
tert-Bu substituents on the salicylidene ring near the coordination
TBHP (Fig. 4). Under the optimized conditions (100:200:1 M ratio
for alkene:TBHP:catalyst in DCE at 80 °C), cyclohexene converted
completely within 40 min and 95% of the epoxide product was se-
cured as the sole product.
In order to establish the general applicability of the method,
various olefins were subjected to the oxidation protocol under
the influence of the MoL1 catalyst (Table 2). As summarized in
Table 2, different alkenes are generally excellent substrates for this
catalyst (Table 2, entries 1–11). It led to complete conversion
of cyclohexenes, cyclooctene and norbornene with the formation
of the corresponding epoxides as the sole products. Inspections
of the results in Table 2 indicate several useful features of this
catalytic method. The least reactive aliphatic terminal and non-ter-
minal alkenes were oxidized in desired times in good/excellent
yields and excellent selectivities (Table 2, entries 5, 6). The che-
moselectivity of the procedure was notable. The oxidative hydroxyl
group was tolerated under the influence of this catalytic system
and the corresponding epoxide was obtained in 100% selectivity
(Table 2, entries 7–9). A novel feature of this simple catalytic
method is its excellent stereoselectivity. The complete retention
of configuration in the epoxidation of trans-stilbene and excellent
stereoselectivity in the oxidation of cis-stilbene (>94%) were
obtained (Table 2, entries 10, 11).
According to the suggested mechanism (Scheme 2) [42,43],
the second stage of the process is the interaction between the
olefin and the TBHP molecule, activated in the coordination

A. Rezaeifard et al. / Polyhedron 29 (2010) 2703–2709
2707
Table 2
Epoxidation of olefins using TBHP catalyzed by MoL1 in DCEa.
Entry
Alkene
Conversion %b (isolated yield%)
Productc
dppm (C–H oxiran ring)
Selectivity %b
1
O
O
100 (94)
100 (91)
100 (95)
3.1
100
100
100
2
3
4
2.9
2.9
O
O
100 (96)
2.9
100
5
6
66 (58)
2.5–2.9
2.7
100
100
O
O
100 (90)
OH
OH
7
8
100 (92)
100 (94)
3.1–3.3
2.7
100
100
O
OH
OH
OH
O
9
100 (90)
78d (75)
2.5
4.4
100
94d
OH
O
O
10
Ph
Ph
Ph
Ph
Ph
Ph
3.9
3.9
6d
Ph
Ph
O
O
Ph
68d (60)
100d
11
Ph
a
b
c
The reactions were run for 45 min under air in DCE at 80 °C and the molar ratio of alkene:TBHP:catalyst was 100:200:1.
Conversions and selectivities were determined by GC based on the starting alkene.
All products were identified by their IR, 1H NMR and GC–MS spectral data in comparison with authentic samples.
Determined by 1H NMR.
d
site (MoL4) enhanced remarkably the catalytic performance of the
catalyst. Presumably, partial ligand dissociation would open a
coordination site at the molybdenum centre of the hindered
MoL4 for coordination/activation of the TBHP and additionally en-
hance its Lewis acidity [50,51,55]. Moreover, the higher catalytic
activity of the electron-deficient MoL2 as well as the hindered
MoL4 complex is probably caused by their resistance to formation
4. Conclusion
In summary, a highly efficient epoxidation method using TBHP
activated by simple Mo(VI)-Schiff base complexes in desired times
with excellent chemo- and stereoselectivity has been developed.
To investigate the influence of the electronic and steric demands
of the ligands on the catalytic activity of Mo complexes, three novel
Mo(VI) complexes, MoLx, based on 2-[(2-hydroxypropylimi-
no)methyl]phenol as ONO tridentate Schiff base ligands have been
synthesized. It was observed that electron-poor and bulky groups
on the salicylidene ring of ligand promote the effectiveness of
the catalyst. The relative high TOF and TON obtained in the oxida-
tion of olefins, along with the high percentage of catalyst remain-
ing at the end of the reactions confirms clearly the high activity
and also relative stability of the Mo catalysts towards oxidative
degradation. The applicability of these easily prepared Mo Schiff
base complexes in the epoxidation of diverse olefins with excellent
chemo- and stereoselectivity, and ready scalability, highlights the
novelty of this epoxidation method and makes it more attractive
for applied goals.
of catalytically inactive species, such as a l-oxo dimer (Scheme 2,
III), during the oxidation process, thus enhancing the stability of
the catalysts towards oxidative degradation [56–58]. This sugges-
tion is further supported by high total TONs of the MoL2 and
MoL4 catalysts in the epoxidation of cyclooctene after 24 h
(TON = 9667 and 9732/24 h for MoL2 and MoL4, respectively).
The promising results for the activity and stability of these eas-
ily prepared molybdenum Schiff base complexes during the oxida-
tion reactions, leading to high/excellent yields of products at
reasonably low reaction times, along with excellent chemo- and
stereoselectivity in the epoxidation reactions are strong points of
the present cis-dioxo-Mo(VI) Schiff base complexes as oxidation
catalysts [22–26].

2708
A. Rezaeifard et al. / Polyhedron 29 (2010) 2703–2709
O
5000
4000
3000
2000
1000
0
4020
N
O
Mo
O
O
O
H3C
H
CH3OH
2126
O
(I)
N
O
Mo
Me3COOH
O
O
524
496
Im
Me3COOH
1
Me3COH
4
1
2
3
4
O
Mo
N
MoL3
MoL1
MoL2
MoL4
O
N
O
(II)
N
O
Mo-Complex
Mo
O
(VI)
O
O
O
O
Fig. 7. The TOF of the MoLx complexes in the epoxidation of cyclooctene by TBHP in
DCE at 80 °C after 1 h using 10 000:20 000:1 M ratio for olefin:TBHP:catalyst.
O
H
O
Me3C
O
N
N
O
O
N
Mo
O
N
O
O
O
O
O
Mo
Ph
Ph
(V)
O
O
H
+ (I)
(III)
2
O
Acknowledgements
Me3C
H
Mo
O
Ph
O
Support for this work by Research Council of University of
Birjand and Shahid Bahonar University of Kerman are highly
appreciated.
N
O
3
Mo
O
O
Ph
O
O
H
Me3C
O
94%
Ph
Ph
Ph
Ph
Ph
References
O
(IV)
6%
[1] B.I. Ceylan, Y.D. Kurt, B. Ulkuseven, Rev. Inorg. Chem. 29 (2009) 49.
[2] F.E. Kuhn, A.M. Santos, M. Abrantes, Chem. Rev. 106 (2006) 2455.
[3] H.A. McManus, P.J. Guiry, Chem. Rev. 104 (2004) 4151.
[4] R.K. Grasselli, Catal. Today 49 (1999) 141.
[5] R.J. Cross, P.D. Newman, R.D. Peacock, D. Stirling, J. Mol. Catal. 144 (1999) 273.
[6] R.H. Holm, Coord. Chem. Rev. 100 (1990) 183.
Ph
Scheme 2. Suggested mechanism of cis-stilbene epoxidation with TBHP catalyzed
by the MoLx complexes.
[7] J.T. Spence, Coord. Chem. Rev. 48 (1983) 59.
[8] Z.G. Hu, T. Higashiyama, M. Yoshimura, Y.K. Yap, Y. Mori, T. Sasaki, Jpn. J. Appl.
Phys. Part 2 37 (1998) 1093.
[9] A. Akella, D.A. Keszler, Mater. Res. Bull. 30 (1995) 105.
[10] V.P. Dotsenko, N.P. Efryushina, I.V. Berezovskaya, Mater. Lett. 28 (1996) 517.
[11] S.P. Shylesh, M. Jia, A. Seifert, S. Adappa, S. Ernst, W.R. Thiel, New J. Chem. 33
(2009) 717.
[12] S. Gago, P. Neves, B. Monteiro, M. Pessêgo, A.D. Lopes, A.A. Valente, F.A.
Almeida Paz, M. Pillinger, J. Moreira, C.M. Silva, I.S. Gonçalves, Eur. J. Inorg.
Chem. (2009) 4528.
[13] P. Neves, S. Gago, C.C.L. Pereira, S. Figueiredo, A. Lemos, A.D. Lopes, I.S.
Goncalves, M. Pillinger, C.M. Silva, A.A. Valente, Catal. Lett. 132 (2009) 94.
[14] F.-L. Chai, H.-L. Su, X.-Y. Wang, J.-C. Tao, Inorg. Chim. Acta 362 (2009) 3840.
[15] M. Bagherzadeh, R. Latifi, L. Tahsini, L. Keith Woo, Inorg. Chim. Acta 362 (2009)
3698.
[16] J.C. Alonso, P. Neves, M.J.P. da Silva, S. Quintal, P.D. Vaz, C. Silva, A.A. Valente, P.
Ferreira, M.J. Calhorda, V. Felix, M.G.B. Drew, Organometallics 26 (2007) 5548.
[17] S.K. Maiti, S. Dinda, N. Gharah, R. Bhattacharyya, New J. Chem. 30 (2006) 479.
[18] S.K. Maiti, S. Banerjee, A.K. Mukherjee, K.M. Abdul Malikc, R. Bhattacharyya,
New J. Chem. 29 (2005) 554.
100
80
60
40
20
0
None
Py
Im
0
10
30
40
60
Time/min
[19] M.M. Abu-Omar, A. Loaiza, N. Hontzeas, Chem. Rev. 105 (2005) 2227.
[20] A. Sigel, H. Sigel (Eds.), Metal Ions in Biological Systems 39, Molybdenum and
Tungsten: Their Roles in Biological Processes, Marcel Dekker, New York, 2002.
[21] R. Hille, Chem. Rev. 96 (1996) 2757.
Fig. 5. The influence of nitrogenous donors on the oxidation of cyclohexene using
TBHP catalyzed by MoL1 in DCE at 80 °C.
[22] G. Lyashenko, G. Saischek, M.E. Judmaier, M. Volpe, J. Baumgartner, F. Belaj, V.
Jancik, R. Herbst-Irmer, N.C. Mosch-Zanetti, J. Chem. Soc., Dalton Trans. (2009)
5655.
[23] M. Bagherzadeh, R. Latifi, L. Tahsini, V. Amani, A. Ellern, L. Keith Woo,
Polyhedron 28 (2009) 2517.
[24] S.M. Bruno, S.S. Balula, A.A. Valente, F.A. Almeida Paz, M. Pillinger, C. Sousa, J.
Klinowski, C. Freire, J. Mol. Catal. A: Chem. 270 (2007) 185.
[25] M. Salavati-Niasari, M. Bazarganipour, J. Mol. Catal. A: Chem. 278 (2007) 173.
[26] M. Masteri-Farahani, F. Farzaneh, M. Ghandi, Catal. Commun. 8 (2007) 6.
[27] X.-H. Lu, Q.-H. Xia, H.-J. Zhan, H.-X. Yuan, C.-P. Ye, K.-X. Su, G. Xu, J. Mol. Catal.
A: Chem. 250 (2006) 62.
[28] Q.-H. Xia, H.-Q. Ge, C.-P. Ye, Z.-M. Liu, K.-X. Su, Chem. Rev. 105 (2005) 1603.
[29] K.C. Gupta, A.K. Sutar, C.-C. Lin, Coord. Chem. Rev. 253 (2009) 1926.
[30] K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420.
[31] D. Mohajer, A. Rezaeifard, Tetrahedron Lett. 43 (2002) 1881.
[32] N. Iranpoor, D. Mohajer, A.-R. Rezaeifard, Tetrahedron Lett. 45 (2004) 3811.
[33] A. Rezaeifard, M. Jafarpour, G. Kardan Moghaddam, F. Amini, Bioorg. Med.
Chem. 15 (2007) 3097.
[34] A. Rezaeifard, M. Jafarpour, S. Rayati, R. Shariati, Dyes Pigm. 80 (2009) 80.
[35] A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans, Eur. J. Inorg. Chem.
(2010) 799.
[36] A. Rezaeifard, M. Jafarpour, M.A. Nasseri, R. Haddad, Helv. Chim. Acta 93 (2010)
711.
Fig. 6. UV–Vis spectral changes of the MoL1 complex during the oxidation of 1-
octene using TBHP in DCE at 80 °C.

A. Rezaeifard et al. / Polyhedron 29 (2010) 2703–2709
2709
[37] I. Sheikhshoaie, A. Rezaeifard, N. Monadi, S. Kaafi, Polyhedron 28 (2009) 733.
[38] K. Ambroziak, R. Pelech, E. Milchert, T. Dziembowska, Z. Rozwadowski, J. Mol.
Catal. A: Chem. 211 (2004) 9.
[39] W.E. Hill, N. Attabay, C.A. McAuliffe, F.P. McCullough, S.M. Razzoki, Inorg.
Chim. Acta 35 (1979) 35.
[40] U. Sandbhor, S. Padhe, E. Sinn, Transition Met. Chem. 27 (2002) 681.
[41] M. Gullotti, A. Pasini, G.M. Zanderighi, G. Ciani, A. Sironi, J. Chem. Soc., Dalton
Trans. (1981) 902.
[42] J.M. Sobczak, J.J. Ziolkowski, J. Mol. Catal. 13 (1981) 11.
[43] J.M. Sobczak, J.J. Ziolkowski, Appl. Catal. A: Gen. 248 (2003) 261.
[44] C.D. Nunes, A.A. Valente, M. Pillinger, J. Rocha, I.S. Goncualves, Chem. Eur. J. 9
(2003) 4380.
[48] M. Cindric, N. Strukan, V. Vrdoljak, B. Kamenar, Z. Anorg. Allg. Chem. 630
(2004) 585.
[49] C. Zhang, G. Rheinwald, V. Lozan, B. Wua, P.-G. Lassahn, H. Lang, C. Janiak, Z.
Anorg. Allg. Chem. 628 (2002) 1259.
[50] J.A. Brito, H. Teruel, S. Massou, M. Gomeza, Magn. Reson. Chem. 47 (2009) 573.
[51] A. Hroch, G. Gemmecker, W.R. Thiel, Eur. J. Inorg. Chem. (2000) 1107.
[52] W.R. Thiel, M. Angstl, T. Priermeier, Chem. Ber. 127 (1994) 2373.
[53] W.R. Thiel, M. Angstl, N. Hansen, J. Mol. Catal. A: Chem. 103 (1995) 5.
[54] Y. Sui, X. Zeng, X. Fang, X. Fu, Y. Xiao, L. Chen, M. Li, S. Cheng, J. Mol. Catal. A:
Chem. 270 (2007) 61.
[55] J.A. Brito, M. Gomez, G. Muller, H. Teruel, J.-C. Clinet, E. Dunach, M.A. Maestro,
Eur. J. Inorg. Chem. (2004) 4278.
[45] F.E. Kuhn, M. Groarke, E. Bencze, E. Herdtweck, A. Prazeres, A.M. Santos, M.J.
Calhorda, C.C. Romao, I.S. Goncualves, A.D. Lopes, M. Pillinger, Chem. Eur. J. 8
(2002) 2370.
[46] W.R. Thiel, J. Eppinger, Chem. Eur. J. 3 (1997) 696.
[47] Y.-W. Lin, Y.-P. Tong, C. Yang, Y.-R. Lin, Inorg. Chem. Commun. 12 (2009) 252.
[56] C.J. Whiteoak, G.J. Britovsek, V.C. Gibson, A.J.P. White, J. Chem. Soc., Dalton
Trans. (2009) 2337.
[57] L. Stelzig, S. Kötte, B. Krebs, J. Chem. Soc., Dalton Trans. (1998) 2921.
[58] C.J. Doonan, D.A. Slizys, C.G. Young, J. Am. Chem. Soc. 121 (1999) 6430.