Synthesis, characterization and crystal structure determination of a new vanadium(IV) Schiff base complex (VOL2) and investigation of its catalytic activity in the epoxidation of cyclooctene

Article abstract of DOI:10.1016/j.poly.2012.12.008

The vanadium(IV) Schiff base complex VOL₂, L = 2-{(E)-[2-chloroethyl)imino]methyl}-6-methoxy phenol, has been synthesized by the reaction of a methanolic solution of VO(acac)₂ with a methanolic solution of the Schiff base ligand, and

Full text of DOI:10.1016/j.poly.2012.12.008

Polyhedron 51 (2013) 54–60
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Synthesis, characterization and crystal structure determination of a new
vanadium(IV) Schiff base complex (VOL2) and investigation of its catalytic activity
in the epoxidation of cyclooctene
a
b
c
c
Gholamhossein Grivani a, , Vida Tahmasebi , Aliakbar Dehno Khalaji , Karla Fejfarová , Michal Dušek
⇑
a School of Chemistry, Damghan University, Damghan 36715-364, Iran
b Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran
c Institute of Physics ASCR, v.v.i., Na Slovance 2, 182 21 Praha 8, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
The vanadium(IV) Schiff base complex VOL2, L = 2-{(E)-[2-chloroethyl)imino]methyl}-6-methoxy phenol,
has been synthesized by the reaction of a methanolic solution of VO(acac)2 with a methanolic solution of
the Schiff base ligand, and it has been characterized using single-crystal X-ray crystallography, elemental
analysis (CHN) and FT-IR spectroscopy. The crystal structure determination of this complex shows a
deformed tetragonal pyramidal N2O3 coordination sphere of the vanadium center. The Schiff base ligand
acts as a bidentate ligand with the two phenolato oxygen atoms and the two imine nitrogen atoms in
trans positions. Non-classical inter- and intra-molecular hydrogen bonds of the type CAHÁ Á ÁO have been
found in the structure, the latter connecting the monomeric VOL2 units. The catalytic activity of the VOL2
Schiff base complex in the epoxidation of cyclooctene was investigated using different reaction parame-
ters, such as solvent effect, oxidant, alkene/oxidant ratio and the catalyst amount. The results showed
that with a catalytic amount of the VOL2 Schiff base complex and a 1:3 ratio of the cyclooctene/TBHP,
the cyclooctene was effectively and selectively converted into the corresponding epoxide with CHCl3
as the solvent.
Received 11 October 2012
Accepted 4 December 2012
Available online 20 December 2012
Keywords:
Vanadium(IV)
Schiff base
X-ray crystallography
Catalysis
Epoxidation
Ó 2012 Published by Elsevier Ltd.
1. Introduction
activities of their vanadyl complexes in the epoxidation of alkenes
[47,48]. In a continuation of our work in this area, here we describe
Schiff bases are versatile ligands that can bind to metal ions and
form metal complexes with interesting properties [1–11]. These
complexes have high stability, interesting and important properties
in different oxidation states and their coordination chemistry has
been extraordinarily developed in different aspects [12–14]. Re-
cently many researchers have been concentrating on the coordina-
tion chemistry of vanadium complexes [15–18] with well-
established different chemical and biological functions. Vanadium
plays a central role in a variety of biochemical processes, such as
haloperoxidation [19–23], nitrogen fixation [24,25], phosphoryla-
tion [26], glycogen metabolism [27–29], insulin mimicking
[30–33] and in anti-parasitic agents [34]. In addition, high-valent
vanadium complexes have been considered as new versatile cata-
lytic reagents for a wide range of oxidation reactions [35,36], like
oxidation of olefines and alcohols [37–41], benzene/alkylaromatic
compounds [42,43] and sulfides [44–46]. Recently we have charac-
terized two new O, N Schiff base ligands derived from salicylalde-
hyde with 2-bromo ethyl ammonium hydrobromide and 2-chloro
ethyl ammonium hydrochloride, and have investigated the catalytic
the synthesis, characterization, crystal structure determination
and catalytic activity of the VOL2 Schiff base complex (L = 2-
{(E)-[2-chloroethyl)imino]methyl}-6-methoxy phenol) (Scheme 1).
2. Experimental
All reagents and solvents for the synthesis and analysis were
commercially available and purchased from Merck, and were used
as received without further purification. Infrared spectra were
recorded using KBr disks on a FT-IR Perkin Elmer RXI spectropho-
tometer. Elemental analyses were carried out using a Heraeus
CHN–O-Rapid analyzer and the results agreed with the calculated
values. The 1H NMR spectrum of the ligand was measured on a
BRUKER DRX-500 AVANCE spectrometer at 500 MHz. All GC yields
based on the starting materials were obtained by using a Varian
CP-3800 instrument with a silicon-DC 200 column.
2.1. Synthesis of the 2-{(E)-[2-chloroethyl)imino]methyl}-6-methoxy
phenol (L)
⇑
In a 250 ml round bottom flask containing 100 ml of absolute
methanol were added 10 mmol of 3-methoxy-2-hydroxybenzalde-
Corresponding author. Tel./fax: +98 232 5235431.
E-mail address: grivani@du.ac.ir (G. Grivani).
0277-5387/$ - see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.poly.2012.12.008

G. Grivani et al. / Polyhedron 51 (2013) 54–60
55
CH2CH2Cl
O
Cl
N
CH3OH/ NaOH
ClH.H2N
OH
OH
Reflux/ 4 h
L
OMe
OMe
CH2CH2Cl
O
N
Reflux/ 2 h
CH3OH
O
L
V
O
OMe
N
OMe
ClH2CH2C
VOL2
Scheme 1. Preparation procedures for the new bidentate O,N Schiff base ligand L and its vanadyl Schiff base complex.
hyde and 10 mmol of 2-chloroethyl ammonium hydrochloride.
Then 10 mmol of NaOH, dissolved in 5 ml H2O, were added to
the above solution and the contents were refluxed for 4 h. After
cooling to room temperature, the reaction mixture was added to
100 ml of water at 0 °C in a beaker, and then kept in an ice bath
for 1 h. The yellow crystals that formed were filtered off, washed
with slightly cold water and dried in an oven. M.p.: 68 °C. Anal.
Calc. for C10H12ClNO: C, 55.87; H, 5.58; N, 6.53. Found: C, 56.20;
H, 5.62; N, 6.55%. IR (KBr pellet, cmÀ1): 3150–3300 (b, OAH, phe-
nolic), 2780–2870 and 2890–3000 (w, CAH aliphatic and aro-
matic), 1634 (s, C@N), 1471, 1439 (m, C@C), 734 (m, CACl). 1H
NMR (CDCl3, d (ppm)): 13.41 (1H, OAH), 8.36 (1H, HAC@N)),
6.95 (1H, phenylAH), 6.91 (1H, phenylAH), 6.82 (1H, phenylAH),
3.90 (2H, ACH2Cl) (3H, AOMe), 3.78 (2H, ANACH2A).
during the refinement. In this structure no H atoms are bounded
to nitrogen. The isotropic atomic displacement parameters of all
hydrogen atoms were evaluated as 1.2Ueq of the parent atom.
Crystallographic data, details of the data collection and structure
solution and refinements are listed in Table 1.
2.4. General procedure of the epoxidation reaction
In a 10 ml round bottom flask equipped with a magnetic stirring
bar, 0.5 mmol of the alkene was reacted with different amounts of
the oxidant and the vanadyl Schiff base complex in 5 ml of solvent,
by refluxing the reaction mixture. The progress of the reaction was
monitored by GLC.
3. Results and discussion
2.2. Synthesis of VO(L)2
3.1. Synthesis and characterization
In a 100 ml round bottom flask, 5 mmol of L was dissolved in
40 ml of MeOH and to this solution was added 2.5 mmol of
VO(acac)2 dissolved in 20 ml of MeOH. The contents of the flask
were then refluxed with stirring for 2 h. After evaporating the sol-
vent, the remaining reaction mixture was crystallized in a solvent
mixture of MeOH/CHCl3 (1/1, v/v). The greenish crystals that
formed were filtered off, washed with n-hexane and dried in an
oven. M.p.: 194 °C. Anal. Calc. for C20H22Cl2N2O5V: C, 48.10; H,
4.23; N, 5.62. Found: C, 48.78; H, 4.47; N, 5.69% IR (KBr pellet,
cmÀ1): 2780–2870 and 2890–3000 (w, CAH aliphatic and aro-
matic), 1622 (s, C@N), 1540, 1473 (m, C@C), 738 (m, CACl), 984
(s, V@O).
Scheme 1 shows the preparation procedures for the bidentate
Schiff base ligand L and its VOL2 Schiff base complex. The Schiff
Table 1
Crystallographic data and structure refinement for VOL2.
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C20H22Cl2N2O5V1
492.3
monoclinic
P21/n
11.2198 (6)
12.0634 (7)
16.6407 (10)
108.434 (5)
2136.7 (2)
4
6.49
0.557
1
16499
3797
3238
0.035
b (Å)
c (Å)
b (°)
V (Å3)
2.3. X-ray structure determination
Z
l
(mmÀ1
)
A single crystal of dimensions 0.32 mm  0.23 mm  0.11 mm
was chosen for the X-ray diffraction study. Crystallographic
measurements were carried out at 120 mmK with a four circle
CCD diffractometer, Gemini of Oxford diffraction, Ltd., with mir-
Tmin
Tmax
Measured reflections
Independent reflections
Reflection with I > 3
r(I)
ror-collimated Cu K
a radiation (k = 1.54184 Å), using the area
Rint
S
detector Atlas. The crystal structure was solved by direct methods
with the program SIR2002 [49] and refined with the JANA2006 pro-
gram package [50] by the full-matrix least-squares technique on
F2. The molecular structure plots were prepared by ORTEP III [51].
Hydrogen atoms were mostly discernible in the difference Fourier
maps and could be refined to a reasonable geometry. According to
common practice, they were nevertheless kept in ideal positions
1.48
0.027
0.063
272
R[F2 > 2
r
(F2)]
wR(F2)
Parameters
D
D
qmax (e ÅÀ3
qmin (e ÅÀ3
)
)
0.21
À0.20
Crystal size (mm3)
0.32 Â 0.23 Â 0.11

56
G. Grivani et al. / Polyhedron 51 (2013) 54–60
base ligand L was prepared by the simple reaction of OMe-salicyal-
dehyde with an equimolar amount of 2-chloroethyl ammonium
hydrochloride in the presence of NaOH in methanol as a solvent
under reflux conditions. In the subsequent reaction of the prepared
Schiff base ligand L and VO(acac)2 in a molar ratio of 2:1 in meth-
anol under reflux conditions, the vanadyl Schiff base complex was
prepared. The Schiff base ligand L and its vanadyl Schiff base com-
plex were characterized by CHN analysis, FT-IR and 1H NMR spec-
tra. The structure of the vanadium(IV) Schiff base complex, VOL2,
was determined by single crystal X-ray analysis, while the FT-IR
spectra of L and VOL2 provided data regarding the nature of their
functional groups. The FT-IR spectrum of the Schiff base ligand L
shows a broad band in the region 3150–3300 cmÀ1, indicating
the presence of the phenolic OAH group. This band is absent in
the FT-IR spectrum of the vanadium(IV) Schiff base complex
VOL2, indicating the coordination of the ligand via phenolato
group. The fundamental stretching mode of the azomethine moi-
ety, vC@N, for the free ligand appeared at 1634 cmÀ1, and this is
anionic and N,O-bidentate. Vanadium(IV) is coordinated by two
nitrogen and two oxygen atoms of two independent ligands in
the basal plane and by one oxygen atom in the apical position.
The square pyramidal geometry around the vanadium(IV) ion in
this complex is distorted because of the different bond distances
and angles. The largest angular distortion from an ideal
square pyramidal geometry (90 and 180°) occurs in the angles
O1–V1–C2 [110.58(6)°], O1–V1–O4 [112.36(7)°], O2–V1–O4
[137.06(6)°] and N1–V1–N2 [157.03(6)°]. The V@O, VAO and
VAN distances (Table 2) are similar to those found in other vana-
dium(IV) complexes with bidentate Schiff base ligands [47,48].
In this complex, the two ligands are planar and the angle be-
tween the planes of the two VAOACACACAN rings is
24.980(32)°. This complex shows intramolecular CAHÁ Á ÁO hydro-
gen bonding (Fig. 1, Table 3), as well as non-classical CAHÁ Á ÁO
intermolecular hydrogen bonds linking the monomeric units to
each other (Fig. 2, Table 3).
shifted to a lower frequency by 22 cmÀ1, appearing at 1612 cmÀ1
,
3.3. Catalytic activity
in the FT-IR spectrum of the vanadium(IV) Schiff base complex
VOL2. This change is attributed to the involvement of the azome-
thine nitrogen of the Schiff base ligand (L) in coordination to the
vanadium(IV) center. VOL2 complexes generally show vV@O around
860 cmÀ1 for polynuclear linear chain structures (V@OÁ Á ÁV@OÁ Á Á)
and have an orange color, but they show vV@O around 970 cmÀ1
for the monomeric form with a green color in the solution and solid
state [52,53]. Thus the sharp band at 984 cmÀ1 in the FT-IR spec-
trum of the VOL2 Schiff base complex is attributed to the V@O
vibrations, indicating the monomeric form of the L2V@O Schiff base
complex in the solid state, as confirmed by single crystal X-ray dif-
fraction (Fig. 1).
In order to investigate the catalytic activity of the vanadium(IV)
Schiff base complex (VOL2) in epoxidation reactions, cyclooctene
was used as a model substrate and different reaction parameters,
such as solvent, oxidant, alkene/oxidant ratio and the amount of
the catalyst, were optimized. Fig. 3 illustrates the results of the
epoxidation of cyclooctene in the presence of tert-butyl hydroper-
oxide (TBHP) as an oxidant with a catalytic amount of the vana-
dium(IV) Schiff base complex (VOL2) in different solvents. The
trend of the observed solvent effect was CHCl3 > CH3CN > CCl4 > -
CH2Cl2 > MeOH > CH3CN/H2O > THF. It seems that in aprotic sol-
vents, such as CHCl3, CH3CN and CCl4, a high epoxidation yield is
observed. Rayati et al. also investigated the catalytic activities of
two vanadium(IV) Schiff base complexes in the cyclooctene epox-
idation reaction with TBHP using chloroform, acetonitrile and
dichloromethane as solvents and they obtained the highest conver-
sion (94–95%) with chloroform [54]. Martins and co-workers stud-
ied the epoxidation of cyclooctene using a diamine bis(phenolate)
3.2. Crystal and molecular structure of the VOL2
An ORTEP view of the VOL2 Schiff base complex with the atoms
numbering scheme is given in Fig. 1. Table 2 lists selected bond
lengths and angles. In this complex, the Schiff base ligands are
Fig. 1. An ORTEP view of VOL2 showing 50% probability displacement ellipsoids and the atom-numbering. Dashed lines indicate intramolecular CAHÁ Á ÁO hydrogen bonds.

G. Grivani et al. / Polyhedron 51 (2013) 54–60
57
Table 2
dium complexes in the epoxidation of cyclooctene in the presence
of TBHP in CHCl3 is much higher than in the presence of H2O2 in
CH3CN. Thus our results are consistent with the previous observa-
tions. By addition of H2O (as a protic solvent) to CH3CN, the conver-
sion was decreased dramatically (Fig. 3). In another controlled
experiment, in order to further confirm the effect of the coordina-
tion ability of the solvent on the yield of the epoxide, we tested the
influence of the addition of imidazole. As can be seen in Fig. 4, the
yield of the epoxidation of cyclooctene by the vanadium(IV) Schiff
base complex (VOL2) in CHCl3 in the presence of TBHP decreased
significantly on addition of imidazole. Therefore the observed
low conversion in MeOH, CH3CN/H2O and THF in Fig. 3 can be re-
lated to the high coordination ability of these solvents to the metal
center. Thus, as the coordination ability of the solvent is increased,
the activity of the titled complex is decreased. We also tested dif-
ferent reaction media to obtain a suitable oxidant media for the
epoxidation of cyclooctene (Table 4). The results show that a high
conversion is only obtained in the presence of TBHP in CHCl3. This
may be related to the ability of TBHP and the inability of H2O2 and
NaIO4 to mix with the organic substrate phase. Figs 5 and 6 show
respectively the cyclooctene/oxidant ratio and the effect of the
amount of catalyst in the epoxidation reaction of cyclooctene by
the Schiff base complex (VOL2). According to these plots, a 1:3 ratio
of cyclooctene/oxidant and 0.014 mmol of the catalyst can be cho-
sen as the optimal amounts providing the highest epoxide yields.
In our previous researches we investigated the epoxidation of
cyclooctene by vanadium(IV) Schiff base complexes derived from
salicylaldehyde and 2-halo ethyl amine (Br, Cl) [47,48]. We found
that the trend of the solvent effect on the epoxidation of cyclooc-
tene by the related vanadium(IV) Schiff base complexes in the
presence of TBHP, is changed by using 2-chloro ethyl amine instead
Selected bond distances (Å) and angles (°) for VOL2.
V1–O1
V1–O2
V1–O4
V1–N1
V1–N2
N2–C19
O1–V1–O2
O1–V1–N1
O2–V1–O4
O2–V1–N2
O4–V1–N2
V1–O2–C1
V1–N1–C8
V1–N2–C18
C18–N2–C19
O2–C1–C6
O4–C11–C16
1.5950(13)
1.9082(13)
1.9056(12)
2.1019(15)
2.1026(15)
1.470(3)
O2–C1
O4–C11
N1–C8
N1–C9
N2–C18
1.3253(18)
1.3193(19)
1.294(2)
1.471(3)
1.290(2)
110.58(6)
O1–V1–O4
O1–V1–N2
O2–V1–N1
O4–V1–N1
N1–V1–N2
V1–O4–C11
V1–N1–C9
V1–N2–C19
O2–C1–C2
O4–C11–C12
C12–C11–C16
112.36(7)
101.06(6)
86.11(6)
101.91(6)
137.06(6)
85.74(6)
85.38(6)
86.05(6)
157.03(6)
128.26(12)
121.83(10)
119.62(10)
122.83(17)
123.10(17)
118.52(15)
125.85(13)
121.76(13)
122.97(14)
117.08(15)
118.24(17)
118.34(16)
Table 3
Geometric parameters of hydrogen bonds for VOL2.
DAHÁ Á ÁA
Distance (Å)
D–HÁÁÁA (°)
DAH
HÁ Á ÁA
DÁ Á ÁA
C9AH9aÁ Á ÁO1
C9AH9bÁ Á ÁO5
C2AH20aÁ Á ÁO1
C8AH8Á Á ÁO1
0.960
0.960
0.960
0.960
2.528
2.855
2.872
2.509
3.365
3.558
3.499
3.226
145.649
130.793
123.828
131.471
vanadium complex in the presence of the TBHP and H2O2 in CHCl3
and CH3CN [55]. Their results show that the activity of the vana-
Fig. 2. Packing arrangement and hydrogen bonding for VOL2.

58
G. Grivani et al. / Polyhedron 51 (2013) 54–60
Table 4
Epoxidation of cyclooctene in different reaction media by the vanadium Schiff base
complex VOL2.a
Solvent
Oxidant
Time (min)
Conversion (%)
CHCl3
CHCl3
CH3CN/H2O(3:2)
THF
CH3CN
MeOH
CCl4
CH3CN/H2O(3:2)
TBHP
H2O2
NaIO4
H2O2
H2O2
H2O2
H2O2
H2O2
114
114
114
114
114
114
114
114
86
7
13
3
4
no reaction
no reaction
no reaction
a
Reaction conditions: 5 ml solvent, 0.5 mmol cyclooctene, 1.5 mmol oxidant and
0.014 mmol VOL2 complex.
Fig. 3. The catalytic epoxidation of cyclooctene in different solvents in the presence
of TBHP by the vanadyl Schiff base complex VOL2. Reaction conditions: 5 ml solvent,
0.5 mmol cyclooctene, 1.5 mmol TBHP and 0.014 mmol VOL2 complex.
Fig. 5. The effect of the alkene/oxidant ratio in the epoxidation of cyclooctene with
CHCl3 as the solvent by the vanadyl Schiff base complex VOL2. Reaction conditions:
5 ml solvent, 0.5 mmol cyclooctene, 2 mmol, 1.5 mmol, 1 mmol and 0.6 mmol for
1:4, 1:3, 1:2 and 1: 1.2 ratios, respectively TBHP and 0.014 mmol VOL2 complex.
methoxy group on the salicyliden moiety. Thus the variations in
the observed trend for the solvent effect in comparison to the re-
lated trend in Ref. [48] can be attributed to the electron donating
properties of the methoxy group of the salicyliden moiety. The ef-
fect of different pendant groups and different substituted salicyli-
dens in vanadium(IV) Schiff base complexes on the solvent effect
of the cyclooctene epoxidation reaction are under investigation
by our research group. Very recently the mechanistic aspects of
the catalytic epoxidation of alkenes by vanadium(IV) complexes
were reviewed by Conte et al. [56]. Our obtained results on the
epoxidation of cyclooctene by the vanadium(IV) Schiff base com-
plex (VOL2) confirm the proposed mechanism for the epoxidation
of cyclooctene by vanadium(IV) complexes, see Scheme 2 [56].
The formation of peroxyvanadium(V) species in the epoxidation
route for alkenes has been established [57]. This can be examined
by titration of a solution of the vanadium(IV) Schiff base complex
with TBHP. Fig 7 shows the changes of the UV–Vis spectra of a
solution of the vanadium(IV) Schiff base complex (VOL2) during
titration with TBHP. The intensity of the bands at 375 and
285 nm was reduced, along with the gradual appearance of two
new bands at 330 and 265 nm. At the end of the titration the bands
at 375 and 285 nm disappeared. At the same time, the bands
appearing at 610 and 550 nm, due to d–d transitions of [VIVOL2],
slowly disappeared upon addition of TBHP. In agreement with
Fig. 4. The catalytic epoxidation of cyclooctene in CHCl3 in the presence of TBHP by
the vanadyl Schiff base complex VOL2 without (No Im) and with imidazole addition
(With Im). Reaction conditions: 5 ml solvent, 0.5 mmol cyclooctene, 1.5 mmol TBHP
and 0.014 mmol VOL2 complex and 0.014 mmol imidazole in the case of imidazole
addition.
of 2-bromo ethyl amine. The found trend was CCl4 > CHCl3 > CH3-
CN > THF > MeOH > CH3CN/H2O for 2-bromo ethyl amine
and CHCl3 > CCl4 > CH3CN > CH2Cl2 > CH3CN/H2O > THF > MeOH for
2-chloro ethyl amine. In this research we used the substituted sal-
icylaldehyde (3-methoxy salicylaldehyde) and 2-chloro ethyl
amine, and we observed the trend CHCl3 > CH3CN > CCl4 > CH2-
Cl2 > MeOH > CH3CN/H2O > THF for the solvent effect on the epox-
idation of cyclooctene by VOL2 in the presence of TBHP. A
comparison of the observed solvent effect for the titled complex
in this work and the related complex in Ref. [48] can be useful.
The two vanadium(IV) Schiff base complexes are different in the

G. Grivani et al. / Polyhedron 51 (2013) 54–60
59
the literature [58–60], these spectral changes and the presence of
two isosbestic points at 355 and 310 nm suggest the oxidation of
vanadium(IV) and the interaction of the formed vanadium(V) com-
plex with TBHP to give oxoperoxovanadium(V) species. Thus the
active center in the epoxidation of cyclooctene by the vana-
dium(IV) Schiff base complex (VOL2) in the presence of TBHP is
the vanadium(V). The observed trend in Fig. 3 shows that a high
epoxide yield is obtained in solvents with a low coordination abil-
ity. In this case TBHP, containing a hard coordinating atom (O),
links to the hard vanadium(V) center without any restriction by
the solvent molecules. In solvents with high coordination ability,
the solvent molecules compete with TBHP and coordinate to the
vanadium center, thus retarding the coordination of the TBHP,
causing a lower epoxide yield. This is tested by addition of the
highly coordinating molecule imidazole during the epoxidation of
cyclooctene under optimized conditions, where upon the epoxide
yield was lowered on addition of imidazole as a highly coordinat-
ing molecule (Fig. 4). Thus our results agree with this mechanism.
4. Conclusion
In conclusion, we have readily synthesized a new vanadium(IV)
Schiff base complex (VOL2). The catalytic activity of the VOL2 com-
plex was investigated in the epoxidation of cyclooctene. The reac-
tion conditions were optimized and the results showed that the
VOL2 complex can be used as a highly active and selective homoge-
neous catalyst in the epoxidation of cyclooctene. In addition some
mechanistic aspects of the cyclooctene epoxidation by the vana-
dium(IV) Schiff base complex VOL2 were investigated.
Fig. 6. The effect of the amount of the vanadyl Schiff base complex VOL2 in the
epoxidation of cyclooctene with CHCl3 as the solvent in the presence of TBHP.
Reaction conditions: 5 ml solvent, 0.5 mmol cyclooctene, 1.5 mmol TBHP and 0.01,
0.014 and 0.02 mmol VOL2 complex.
(CH3)3COOH
Acknowledgments
OH
We acknowledge Damghan University (DU) and Golestan Uni-
versity (GU) for partial support of this work, and the project
P204/11/809 of the Grant Agency of the Czech Republic.
(CH3)3COOH
L2VVO
L2VV
L2VIV
O
OOC(CH3)3
Appendix A. Supplementary data
CCDC 890891 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
O
Scheme 2. The proposed mechanism for the epoxidation of cyclooctene by VOL2.
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Fig. 7. The UV–Vis spectral changes observed during the titration of the VIVOL2
Schiff base complex with TBHP. The spectra were recorded after successive
additions of 1 l
l of TBHP to 5 ml a ca. 10À4 M solution of VIVOL2 in chloroform.

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