Synthesis, characterization and crystal structures of new bidentate Schiff base ligand and its vanadium(IV) complex: The catalytic activity of vanadyl complex in epoxidation of alkenes

Article abstract of DOI:10.1016/j.poly.2011.09.011

The Schiff base ligand N-salicylidin-2-bromoethylimine (L) and its vanadium(IV) complex, VOL₂ (1), were synthesized and characterized by using X-ray, CHN, ¹H NMR and FT-IR methods. X-ray analysis shows the Schiff base ligand L acts as a bidentate (O, N) chelating ligand and coordinates via imine nitrogen and phenolato oxygen atoms to the V(IV) center. The coordination geometry around the V(IV) center in 1 is approximately square pyramidal, as indicated by the unequal metal-ligand bond distances and angles, with the basal plane formed by the N₂O₂ donors of the two bidentate Schiff base ligands, the two phenolato O atoms and the two imine N atoms are in the trans position. The coordination sphere of the V(IV) is completed by one oxygen atom in apical position. In the Schiff base ligand, L, there are some classical intramolecular O1-H1...N1 and non-classical intermolecular C9-H9b...O1 hydrogen bonds, while in 1, there are two non-classical intermolecular C7-H7...O3 and C8-H8b...O3 hydrogen bonds. The catalytic activity of 1 in epoxidation of cyclooctene was investigated in different conditions to obtain optimum conditions. The effects of solvent, oxidant, catalyst concentration and alkene/oxidant ratio were studied and the results showed that in CCl₄ in the presence of tert-butylhydroperoxide in 1:3 alkene/oxidant ratio, high epoxide yield was obtained. The epoxidation of alkenes was also carried out in optimized conditions that high catalytic activity and selectivity were obtained.

Full text of DOI:10.1016/j.poly.2011.09.011

Polyhedron 31 (2012) 265–271
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Polyhedron
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Synthesis, characterization and crystal structures of new bidentate Schiff base
ligand and its vanadium(IV) complex: The catalytic activity of vanadyl complex
in epoxidation of alkenes
b
a
c
c
Gholamhossein Grivani a, , Aliakbar Dehno Khalaji , Vida Tahmasebi , Kazuma Gotoh , Hiroyuki Ishida
⇑
a School of Chemistry, Damghan University, Damghan 36715-364, Iran
b Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran
c Department of Chemistry, Faculty of Science, Okayama University, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 May 2011
Accepted 13 September 2011
Available online 22 September 2011
The Schiff base ligand N-salicylidin-2-bromoethylimine (L) and its vanadium(IV) complex, VOL2 (1), were
synthesized and characterized by using X-ray, CHN, 1H NMR and FT-IR methods. X-ray analysis shows the
Schiff base ligand L acts as a bidentate (O, N) chelating ligand and coordinates via imine nitrogen and
phenolato oxygen atoms to the V(IV) center. The coordination geometry around the V(IV) center in 1
is approximately square pyramidal, as indicated by the unequal metal–ligand bond distances and angles,
with the basal plane formed by the N2O2 donors of the two bidentate Schiff base ligands, the two pheno-
lato O atoms and the two imine N atoms are in the trans position. The coordination sphere of the V(IV) is
completed by one oxygen atom in apical position. In the Schiff base ligand, L, there are some classical
intramolecular O1–H1Á Á ÁN1 and non-classical intermolecular C9–H9bÁ Á ÁO1 hydrogen bonds, while in 1,
there are two non-classical intermolecular C7–H7Á Á ÁO3 and C8–H8bÁ Á ÁO3 hydrogen bonds. The catalytic
activity of 1 in epoxidation of cyclooctene was investigated in different conditions to obtain optimum
conditions. The effects of solvent, oxidant, catalyst concentration and alkene/oxidant ratio were studied
and the results showed that in CCl4 in the presence of tert-butylhydroperoxide in 1:3 alkene/oxidant
ratio, high epoxide yield was obtained. The epoxidation of alkenes was also carried out in optimized con-
ditions that high catalytic activity and selectivity were obtained.
Keywords:
Schiff base
Vanadium(IV) complex
Non classical hydrogen bond
Epoxidation
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
We started to synthesize and determine the structural features
of some new bidentate O, N Schiff base ligands and their metal com-
Schiff base ligands played central role as chelating ligands in the
main group and transition metal coordination chemistry [1,2]
because they are capable to bind with transition, non-transition,
lanthanide and actinide metal ions to give complexes with suitable
properties for theoretical studies and/or practical applications
[3–5]. In the last decades the coordination chemistry of transition
metal Schiff base complexes has extraordinary developed in differ-
ent aspect [6–14]. Recently much research have been focused on
the coordination chemistry of vanadium complexes because of its
interesting structural features [15–17], catalytic applications
[18–20], and biological roles in a variety of biochemical processes
such as haloperoxidation [21–23], nitrogen fixation [24], phos-
phorylation [25], glycogen metabolism [26–28] and insulin mim-
icking [29,30].
plexes, and investigate the catalytic and biological activity of them.
In this research we describe the synthesis, characterization and
structural features of the new Schiff base ligand N-salicylidin-2-
bromoethylamine (L) and its vanadium(IV) complex (1) and inves-
tigate the catalytic activity of vanadium Schiff base complex (1) in
epoxidation of alkenes (Scheme 1).
2. Experimental
All reagents and solvents for synthesis and analysis were com-
mercially available and purchased from Merck and used as re-
ceived without further purifications. Infrared spectra were
recorded using KBr disks on a FT-IR PerkinElmer spectrophotome-
ter. Elemental analyses were carried out using a Heraeus CHN–O-
Rapid analyzer and results agreed with calculated values. 1H
NMR spectrum of ligand was measured on a BRUKER DRX-500
AVANCE spectrometer at 500 MHz for the Schiff-base ligand. All
GC yields base on starting materials were obtained by using Varian
CP-3800 instrument with silicon-DC 200 column.
⇑
Corresponding author. Tel./fax: +98 232 5235431.
E-mail address: grivani@du.ac.ir (G. Grivani).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.011

266
G. Grivani et al. / Polyhedron 31 (2012) 265–271
using SHELXL97 [32]. The molecular structure plots were prepared by
OH
OH
Br
using the ORTEP-3 [33]. The C-bound H atoms were placed in geo-
metrically idealized positions (C–H = 0.95 or 0.99 Å) and allowed to
ride on their parent atoms, with Uiso(H) = 1.2Ueq(C). Crystallo-
graphic data and details of the data collection and structure refine-
ment are listed in Table 1.
Methanol
N
+
O
4
Ref /
h
HBr.H2N
H
Br
L
Br
2.4. General procedure for epoxidation of alkenes catalyzed by 1
To a 25 ml round bottom flask equipped with a magnetic stir-
ring bar was added 5 ml solvent, 0.5 mmol of alkene, 1.5 mmol of
tert-butylhydroperoxide, 0.0075 g (0.014 mmol) of 1 and was re-
fluxed. The progress of reaction was monitored by GLC.
O
V
N
O
O
N
VO(acac)2
L
Br
3. Results and discussion
1
3.1. Synthesis
Scheme 1. Preparation procedures of N-salicylidin-2-bromoethylimine (L) and its
vanadium(IV) complex (1).
The Schiff base ligand N-salicylidin-2-bromoethylimine (L) was
synthesized in a simple preparation procedure by condensation
reaction of salicylaldehyde and 2-bromoethyl ammonium hyrobr-
omide in methanol as a solvent in reflux conditions (Scheme 1).
In a subsequent reaction of this prepared Schiff base ligand with
VO(acac)2 in MeOH as a solvent in reflux conditions the greenish
bis(N-salicylidin-2-bromoethylimine)vanadium(IV) Schiff base
complex (1) was prepared (Scheme 1).
2.1. Synthesis of N-salicylidin-2-bromoethylimine (L)
To a 250 ml round bottom flask containing 100 ml absolute eth-
anol were added 10 mmol of salicylaldehyde, 10 mmol of 2-
bromoethyl ammonium hyrobromide and the content was stirred.
In addition, 10 mmol of NaOH was dissolved in 5 ml H2O and was
added to above solution and the content was refluxed for 4 h. After
cooling to room temperature the content was added to 100 ml of
water in 0 °C in a baker and kept it in an ice bath for 2 h. The yellow
crystals were filtered off and washed with slightly cold water and
dried in an oven. Yield: 1.73 g, 76%. M.p. 62 °C. Anal. Calc. for
C9H10BrNO: C, 47.39; H, 4.42; N, 6.14. Found: C, 46.99; H, 4.33;
N, 6.09%. IR (KBr pellet, cmÀ1): 3150–3300, (b, O–H, phenolic),
2840–2900 and 300–3031, (w, C–H aliphatic and aromatic), 1631
(s, C@N), 1579, 1459 (m, C@C), 1334 (m, C–O), 758 (m, C–Br). 1H
NMR (CDCl3, d (ppm)): 13.01(1H: O–H), 8.37 (1H: H–C@N), 6.9
(1H: phenyl–H), 6.99 (1H: phenyl–H), 7.29 (1H: phenyl–H), 7.34
(1H: phenyl–H), 3.65 (2H: –CH2Br), and 4 (2H: N–CH2–).
3.2. Characterization
The Schiff base ligand (L) and its vanadium(IV) complex (1)
were characterized by elemental analysis, FT-IR spectrum and sin-
gle-crystal X-ray analysis. In addition, 1H NMR was also used to
characterization of L. They proved the success of synthesis of L
and 1.
The 1H NMR spectrum of L and its assignment were showed in
Fig. 1, which finely confirmed the structure of the ligand. The two
distinct triplet peaks at 3.65 and 4 ppm were corresponded to pro-
tons (d and c) of –CH2–Br and N–CH2–, respectively. Two peaks at
6.9 and 7.34 ppm (doublets of triplets) and another two peaks at
6.99 and 7.29 ppm (doublets of doublets) were also attributed to
2.2. Synthesis of VO(L)2 (1)
In a 100 ml round bottom flask, 5 mmol of L was dissolved in
40 ml of CHCl3–MeOH (1:1 v/v) and to this solution was added
2.5 mmol of VO(acac)2 (in 10 ml of MeOH) and the content was re-
fluxed with stirring for 2 h. After evaporating of solvent to 1/5 va-
lue, the solution was kept in refrigerator for 1 week. Then the
greenish crystals were filtered off and washed with n-hexane and
dried in an oven. Yield: 0.56 g, 43%. M.p. 103 °C. Anal. Calc. for
Table 1
Crystallographic data and detail of L and 1.
L
1
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C9H10BrNO
228.09
monoclinic
P21/c
11.1638(13)
4.9403(5)
19.382(2)
120.720(4)
918.96(18)
4
456
1.648
4.44
16189
2664
1976
113
0.074
C18H18Br2N2O3V
521.10
monoclinic
P21/c
14.3054(8)
9.1586(5)
18.2918(10)
125.5308(17)
1950.31(18)
4
1028
1.775
4.64
34697
5665
4303
235
0.036
C
18H18Br2N2O3V: C, 41.49; H, 3.48; N, 5.38. Found: C, 41.25; H,
3.31; N, 5.40%. IR (KBr pellet, cmÀ1): 2898–2943, and 2990–3058,
(w, C–H aliphatic and aromatic), 1620 (s, C@N), 1545, 1475 (m,
C@C), 1304 (m, C–O), 758 (m, C–Br).
b (Å)
c (Å)
b (°)
V (Å3)
Z
2.3. X-ray structure determinations
F(000)
Dx
l
(mmÀ1
)
Suitable yellow and green single crystals with the dimensions
0.40 mm  0.36 mm  0.15 and 0.26 mm  0.22 mm  0.09 mm,
respectively, were chosen for X-ray diffraction study. Crystallo-
graphic measurements were done with Rigaku RAXIS-RAPID II dif-
Measured reflections
Independent reflections
Reflection with I > 2
Parameters
Rint
r(I)
fractometer with graphite monochromated MoK
a radiation
R[F2 > 2
wR(F2)
r
(F2)]
0.055
0.157
0.035
0.092
(k = 0.71075 Å) at 180 K. The intensity data were corrected for
absorption using a numerical method [31]. The crystal structures
was solved by direct methods using program SHELXS97 [32] and re-
fined by full-matrix least-squares technique based on F2 with
atomic anisotropic thermal parameters for all non-hydrogen atoms
D
qmax,
(e ÅÀ3
)
1.09, À0.91
1.47, À1.21
min
(D/r)
Crystal size (mm)
<0.001
0.001
max
0.40 Â 0.36 Â 0.15
0.26 Â 0.22 Â 0.09

G. Grivani et al. / Polyhedron 31 (2012) 265–271
267
Fig. 1. The 1H NMR spectrum of the Schiff base ligand (L).
aromatic protons of g, f, e and h, respectively. The chemical shift of
3.3. Description of the crystal structure of L
phenolic proton (a) was strongly shifted to down field and exhib-
ited at 13 ppm due to intramolecular hydrogen bonding between
nitrogen of imine and hydrogen of phenol.
The molecular structure of N-salicylidin-2-bromoethylamine (L)
with the atom-numbering scheme is presented in Fig. 2. The Schiff
base ligand L crystallized in a monoclinic system.
L adopts a trans configuration with respect to the C7@N1 double
bond. Bond lengths and angles of L are given in Table 2. The bond
lengths and angles are within the normal ranges [36]. All bond
lengths and angles discuss here, are slightly different in the com-
plex 1, due to the coordination of L to the central vanadium(IV) ion.
From the FT-IR spectrum, the Schiff base ligand L exhibits a
broad band in the range of 3150–3300 cmÀ1 for the stretching
vibration frequencies of (phenolic) O–H group. Absence of this
band in the FT-IR spectrum of 1 indicates the destruction of this
hydrogen bond followed by the coordination of phenolic oxygen
to vanadium(IV) center after deprotonation. The sharp band
appearing at 1631 cmÀ1 in FT-IR spectrum of L , due to
m(C@N) (azo-
methine), was shifted to lower wave number and appears at
1620 cmÀ1 in FT-IR spectrum of 1, indicating the binding of azome-
thine nitrogen to the vanadium(IV) center. Similarly, decreasing in
C–O vibration frequencies was attributed to coordination of pheno-
late oxygen to vanadium(IV) center.
Complex 1 exhibits a sharp band at 993 cmÀ1 due to m(V@O)
stretching vibration that could be clearly identified the vana-
dium(IV) complex (1). It has been reported that oxovanadium com-
plexes with coordination number of
5
(monomeric five
coordinated V@O complexes) have the m(V@O) about 990 cmÀ1 and
with coordination number of 6 (with polymeric vanadium–oxygen
as (Á Á ÁV@OÁ Á ÁV@OÁ Á Á units) have the m(V@O) lower than about
990 cmÀ1 and this figure provides an available index for discrimi-
nating the coordination number of oxovanadium complexes
[20,34,35]. Thus the sharp band at 993 cmÀ1 finely was attributed
to
m(V@O) of 1 in five coordinated form. These data completely con-
Fig. 2. ORTEP view of L, showing the atom-numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level.
sist with crystal structure of 1.

268
G. Grivani et al. / Polyhedron 31 (2012) 265–271
Table 2
Bond lengths and angles of L.
Br1–C9
O1–C1
C1–C6
C1–C2
C2–C3
C2–C7
O1–C1–C2
O1–C1–C6
C6–C1–C2
C3–C2–C1
C3–C2–C7
C1–C2–C7
C4–C3–C2
1.955(4)
N1–C7
N1–C8
C3–C4
C4–C5
C5–C6
C8–C9
N1–C8–C9
N1–C7–C2
C7–N1–C8
C8–C9–Br1
C4–C5–C6
C5–C4–C3
C5–C6–C1
1.278(5)
1.463(5)
1.385(6)
1.379(7)
1.382(7)
1.509(6)
110.5(3)
122.4(4)
119.2(4)
111.5(3)
121.3(4)
119.3(4)
119.4(4)
1.355(4)
1.398(5)
1.403(5)
1.393(5)
1.462(5)
121.4(3)
118.6(4)
120.0(4)
118.9(4)
120.5(4)
120.6(4)
121.1(4)
Table 3
Hydrogen bonds of L.
Fig. 4. Molecular structure of 1, showing the atom-numbering scheme. Displace-
ment ellipsoids are drawn at the 50% probability level.
D–HÁ Á ÁA
D–H
HÁ Á ÁA
DÁ Á ÁA
D–HÁ Á ÁA
O1–H1Á Á ÁN1
C9–H9bÁ Á ÁO1
0.83
0.99
1.84
2.54
2.589(5)
3.425(6)
150
148
Table 4
Selected bond lengths and angles of 1.
V1–O3
V1–O2
V1–O1
V1–N1
V1–N2
N2–C16
N2–C17
1.5987(16)
1.9038(16)
1.9194(17)
2.1088(19)
2.1095(18)
1.285(3)
Br1–C9
Br2–C18
O1–C1
O2–C10
N1–C7
N1–C8
1.959(3)
1.954(3)
1.318(3)
1.326(3)
1.287(3)
1.479(3)
Ortho position to the imine group of Schiff-base occupied with
the OH is interesting due to the existence of O–HÁ Á ÁN hydrogen
bond and tautomerism between enol-imine and keto-amine forms
[37,38]. In this structure, there is an intramolecular O1–H1Á Á ÁN1
hydrogen bond. Furthermore, there is an intermolecular
C9–H9bÁ Á ÁO1 hydrogen bond (Table 3) between two molecules to
form a centrosymmetric dimeric unit (Fig. 3).
1.471(3)
O3–V1–O2
O3–V1–O1
O2–V1–O1
O3–V1–N1
O2–V1–N1
O1–V1–N1
C7–N1–C8
C7–N1–V1
C8–N1–V1
C16–N2–V1
111.99(8)
110.79(8)
137.21(8)
100.48(8)
84.63(7)
O3–V1–N2
O2–V1–N2
O1–V1–N2
N1–V1–N2
C1–O1–V1
C10–O2–V1
C17–N2–V1
N2–C16–C11
N2–C17–C18
100.02(8)
87.06(7)
86.59(7)
159.49(8)
131.37(15)
132.52(15)
118.85(14)
127.0(2)
3.4. Description of the crystal structure of 1
86.84(7)
The molecular structure of VO(L)2 (1) with the atom-numbering
scheme is presented in Fig. 4. The crystallographic data reveal that
the vanadium(IV) ion is five-coordinated. Two phenolato oxygene
and two imine nitrogen atoms in basal plane with one oxygen
atom in apical position were coordinated to the vanadium(IV) cen-
ter. The two ligands coordinate to the vanadium center in trans
geometry with respect to each other. The geometry around the
vanadium(IV) ion is a distorted square-pyramid, as indicated by
the unequal metal–ligand bond distances and angles: i.e. V1–
O1 = 1.9194(17), V1–N1 = 2.1088(19) and V1–O3 = 1.5987(16) Å
and N1–V1–O1 = 86.84(7)°, N2–V1–O2 = 87.06(7)°, N2–V1–
115.44(19)
125.03(16)
119.38(14)
124.75(16)
112.49(19)
O3 = 100.02(8)° and O2–V1–O3 = 111.99(8)°. All distances and an-
gles (Table 4) in 1 agree well with the same distances in other
vanadium(IV) complexes [7–10]. In consistence with FT-IR spec-
trum of VO(L)2 (1) no polymeric vanadium–oxygen as
Á Á ÁV@OÁ Á ÁV@OÁ Á Á units were observed.
Fig. 3. Part of the crystal structure of L, showing the formation of dinuclear units. Hydrogen bonds are shown as dashed lines.

G. Grivani et al. / Polyhedron 31 (2012) 265–271
269
Table 5
Hydrogen bonds of 1.
D–HÁ Á ÁA
D–H
HÁ Á ÁA
DÁ Á ÁA
D–HÁ Á ÁA
C7–H7Á Á ÁO3
0.95
0.99
2.51
2.43
3.418(3)
3.401(3)
160
167
C8–H8bÁ Á ÁO3
Table 5 and Fig 5 show the hydrogen bonds between the mole-
cules of complex 1. There are two intermolecular hydrogen bonds
C7–H7Á Á ÁO3 and C8–H8bÁ Á ÁO3, forming a zigzag chain along the b
axis.
3.5. Catalytic activity
Fig. 6. The effect of solvents on the conversion of cyclooctene to cyclooctene
epoxide in the presence of TBHP as oxidant by catalytic amount of complex (1).
The catalytic activity of 1 was initially tested for the epoxidation
of cis-cyclooctene as a model substrate. In order to assessment of
suitable reaction conditions to achieve the optimum conditions
for obtaining of maximum yield, we studied the epoxidation of
cyclooctene in different solvents, oxidants and investigated the ef-
fect of catalyst concentration and alkene:oxidant ratio. The results
of the epoxidation of the cyclooctene (0.5 mmol) in different sol-
vents (5 ml) with tert-butylhydroperoxide (1.5 mmol) in the pres-
ence of 1 were illustrated in Fig. 6. The results showed that in the
solvents such as CCl4 and CHCl3 with no coordinating ability to me-
tal center and low dielectric constant, high epoxidation yields were
obtained. On the other hand, in the coordinated solvents, such as
THF, CH3CN, MeOH or H2O with high coordinating ability and high
dielectric constant, low epoxidation yields were observed. The dif-
ference between CCl4 and CHCl3 can be related to the higher boil-
ing point of CCl4 than CHCl3. The epoxidation of cyclooctene in the
presence of H2O2 and NaIO4 in different solvents was also tested
(Table 6). By comparison the results of Fig. 6 and Table 6, it was
clear that in CCl4 in the presence of tert-butylhydroperoxide
(TBHP) high epoxide yield was observed. This may be related to
the ability of TBHP and the inability of the H2O2 and NaIO4 reagents
to mix with the organic substrate phase. Very recently Rahchamani
et al. [39] have studied the synthesis and investigation of the cat-
alytic activities of some oxovanadium(IV) Schiff base complexes in
cyclooctene epoxidation reaction with TBHP and H2O2 in CH3CN.
Their results showed the higher conversion of cyclooctene by the
TBHP than H2O2 with the 58% conversion as the highest conver-
sion. Monfared et al. have investigated the catalytic activities of
Table 6
Epoxidation of cyclooctene in the presence of different oxidants in different
conditions by catalytic amount of vanadium(IV) Schiff base complex (1).a
Solvent
Oxidant
Time (min)
%Conversionb
CH3CN/H2O (3:2)
CCl4
MeOH
CHCl3
THF
CH3CN
CH3CN/H2O (3:2)
CCl4
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
NaIO4
TBHP
114
114
114
114
114
114
114
114
No reaction
No reaction
No reaction
7
3.5
3
13.5
86
a
Reaction conditions: solvent (5 ml), cyclooctene (0.5 mmol), oxidant
(1.5 mmol), vanadium(IV) Schiff base catalyst (0.014 mmol).
b
Based on GLC.
the three oxovanadium hydrazone Schiff base complexes in the
epoxidation of cyclooctene with H2O2 in different solvents [40]
and observed the trend: CH3CN > MeOH > EtOH > THF > (CH3)2-
CO > CHCl3 > MeCOEt > CCl4 > DMF. Rayati et al. [41] have also
investigated the synthesis and catalytic activities of the two oxova-
nadium(IV) Schiff base complexes in cyclooctene epoxidation reac-
tion with TBHP. They were used chloroform, acetonitrile and
dichloromethane as solvent and the highest conversion (94–95%)
was obtained in chloroform. Thus our results are in agreement
with these researches as obtaining the high epoxidation yield in
solvents CCl4 and CHCl3 in the presence of TBHP as oxidant by
Fig. 5. Part of the crystal structure of L, showing the formation of 1D network along the b axis. Hydrogen bonds are shown as dashed lines.

270
G. Grivani et al. / Polyhedron 31 (2012) 265–271
Table 7
Epoxidation of alkenes catalyzed by vanadium(IV) Schiff base complex (1) in the
presence of TBHP at reflux conditions.a
Alkene
Time (min)
%Conversion (%epoxide)b
86 (100)
114
300
37 (100)
87 (100)
50
60
85 (100)
50 (100)
240
Fig. 7. The effect of catalyst concentration on the conversion of cyclooctene to
cyclooctene epoxide in CCl4 in the presence of TBHP as oxidant.
300
300
32 (100)
27 (100)
a
Reaction conditions: CCl4 (5 ml), alkene (0.5 mmol), TBHP (1.5 mmol), vana-
dium(IV) Schiff base catalyst (0.014 mmol).
b
Based on GLC.
100.00%
90.00%
80.00%
70.00%
60.00%
50.00%
OH
O
OH
O
OH
L2VIV
O
L2VVO
L2VV
O
O
40.00%
30.00%
20.00%
1:4
1:3
1:2
1:1.2
10.00%
0.00%
O
Scheme 2. The proposed mechanism for the epoxidation of alkenes by the
oxovanadium complex VOL2 (1) in the presence of TBHP.
0
20
40
60
80
100
120
140
Time(min)
Fig. 8. The effect of alkene/oxidant ratio on the conversion of cyclooctene to
cyclooctene epoxide in CCl4 in four amounts of TBHP as oxidant.
yield strongly. Thus the 1:3 ratio of cyclooctene:TBHP was the best
one because of the highest conversion was obtained in this ratio.
Thus the epoxidation of different alkenes was investigated in
optimized conditions (0.0075 g of catalyst, TBHP as an oxidant,
1:3 ratio of alkene:TBHP and CCl4 as a solvent). The obtained re-
sults were showed in Table 7. As seen from this table cyclic alkenes
were more efficiently converted to their epoxides than linear al-
kenes and all alkenes selectively converted to their epoxides by
the Schiff base complex (1) in CCl4 in the presence of TBHP. In
the case of styrene and 1-methylstyrene by continuation of epoxi-
dation reaction the selectivity of reaction was decreased and by
products (benzaldehyde and acetophenone, respectively) were
observed.
Very recently Conte et al. have reviewed the mechanistic as-
pects of oxovanadium catalyzed oxidations with peroxides [43].
Several mechanisms have been proposed for the oxidation of
alkenes catalyzed by the oxovanadium complexes in the presence
of H2O2 and these mechanisms named as Mimoun- and Sharpless-
type (polar) mechanisms and the (radical) mechanism examined
by Manikandan et al. [43,44]. The proposed mechanism of alkenes
by TBHP with oxovanadium complexes was showed in Scheme 2. It
is established the formation of peroxy-vanadium(V) species in the
the oxovanadium complexes. It seems that the nature of the sol-
vent has the inverse effect on the epoxidation of cyclooctene in
the presence of TBHP respect to the H2O2 by the oxovanadium
Schiff base complexes.
Fig. 7 shows the effect of catalyst concentration in epoxidation
of cyclooctene in CCl4 as a solvent and in the presence of TBHP as
an oxidant. The epoxidation reaction was carried out by using
three amounts of catalyst (0.005, 0.0075 and 0.01 g) with keeping
of other conditions. According to this plot it was clear that in the
case of 0.0075 g (0.014 mmol) of catalyst the highest conversion
(86%) was obtained. The effect of cyclooctene:TBHP ratio on the
catalytic epoxidation of cyclooctene was illustrated in Fig. 8. Four
ratios (1:1.2, 1:2, 1:3 and 1:4) were used in the keeping of the
other conditions (CCl4 as a solvent and 0.0075 g catalyst) as were
used in literature [42]. Increasing the cyclooctene:TBHP ratio from
1:1.2 to 1:2, increased the conversion slowly but increasing this
ratio from 1:2 to 1:3, increased it strongly. Further increasing the
catalyst amount to 1:4 ratio, resulted in decreasing the epoxidation

G. Grivani et al. / Polyhedron 31 (2012) 265–271
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271
epoxidation routes of alkenes with TBHP [43]. The observed sol-
vent effect in Fig. 6 is in agreement with this mechanism. As the
coordinating ability of the solvent increased the linking of the sol-
vent to the vanadium center increased and the formation of the
peroxy-vanadium(V) species prevented, resulted in lowering the
conversion. Based on the high epoxidation yield in the solvents
CCl4 and CHCl3 (that are suitable media for the formation of the
radical) and by consideration of the polar and radical routes for
the oxovanadium catalyzed epoxidation mechanisms, the radical
route cannot be unexpected. Despite the above statements the
mechanistic aspects of the oxovanadium catalyzed epoxidation
with peroxides need to very extended and deep experimental
investigations as proceeding in recent researches. The synthesis
and investigation of the catalytic activities of oxovanadium com-
plexes with ligands derived from (electron donating and electron
withdrawing) substituted salicylaldehyde and alkyl halide ammo-
nium salts are in progress by our research group.
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ligand and its vanadyl complex (1) in two simple preparation pro-
cedures and they were characterized with 1H NMR and FT-IR spec-
tra and CHN and X-ray analysis. The crystallographic data showed
that the vanadium(IV) ion in VOL2 Schiff base complex (1) is five-
coordinated in a distorted square-pyramid geometry. There are
some classical and non-classical inter- and intramolecular hydro-
gen bonds in the crystal structures of Schiff base ligand (L) and
its vanadyl complex (1). The catalytic activity of vanadyl complex
(1) was studied in epoxidation of alkenes. Different (epoxidation
reaction) parameters were investigated and the obtained results
showed that various alkenes can be converted efficiently and selec-
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amount of vanadyl Schiff base complex (1).
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Appendix A. Supplementary data
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CCDC 821461 and 821462 contains the supplementary crystal-
lographic data for L and 1. 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.
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