Crystal structure, electrochemistry, and catalytic studies of a series of new oxidovanadium(IV) Schiff-base complexes derived from 1,2-diphenyl-1,2- ethylenediamine

Article abstract of DOI:10.1080/00958972.2012.662275

New derivatives of N₂O₂ tetradentate Schiff bases, from condensation of meso-1,2-diphenyl-1,2-ethylenediamine and salicylaldehyde derivatives (X-salicylaldehyde; X=3-OMe, 4-OMe, 5-OMe, 6-OMe, 5-Cl, 5-Me), and oxidovanadium(IV) complexes were synthesized and characterized by ¹H NMR, UV-Vis, IR spectroscopy, and elemental analysis. Crystal structure of 5-OMe; H₂L³and two of the complexes (VOL² and VOL³) were also obtained. In the crystals, the molecule of H ₂L³ is C_i symmetrical, as it occupies the special position on the center of symmetry; its conformation is partially determined by classical intramolecular O-H ... N hydrogen bonds. The complexes have monomeric structures with a distorted square pyramid of vanadium, with the oxo ligand in the apical position. Cyclic voltammetry studies show quasi-reversible V^IV/V^V redox for which the presence of electron-withdrawing groups on salicylaldehyde derivatives shifts the E ⁰ to more positive values. The complexes were used as catalysts for selective epoxidation of cyclooctene with tert-butylhydroperoxide as oxidant, in various solvents and reaction conditions. High catalytic activities and excellent selectivity was found. The catalytic activity of the complexes increased increasing E⁰, a consequence of the presence of electronegative substituents. This epoxidation process with the new catalysts was also studied under solvent-free condition and excellent reactivity was observed.

Full text of DOI:10.1080/00958972.2012.662275

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Crystal structure, electrochemistry,
and catalytic studies of a series
of new oxidovanadium(IV) Schiff-
base complexes derived from 1,2-
diphenyl-1,2-ethylenediamine
b
a
a
Abolfazl Ghaffari , Mahdi Behzad , Grzegorz Dutkiewicz
,
b
a
Maciej Kubicki & Mehdi Salehi
a
Department of Chemistry , College of Sciences, The University
of Semnan , Semnan , Iran
b
Department of Chemistry , Adam Mickiewicz University,
Grunwaldzka 6 , 60-780 Poznan , Poland
Published online: 21 Feb 2012.
To cite this article: Abolfazl Ghaffari , Mahdi Behzad , Grzegorz Dutkiewicz , Maciej Kubicki &
Mehdi Salehi (2012) Crystal structure, electrochemistry, and catalytic studies of a series of new
oxidovanadium(IV) Schiff-base complexes derived from 1,2-diphenyl-1,2-ethylenediamine, Journal
of Coordination Chemistry, 65:5, 840-855, DOI: 10.1080/00958972.2012.662275
To link to this article: http://dx.doi.org/10.1080/00958972.2012.662275
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Journal of Coordination Chemistry
Vol. 65, No. 5, 10 March 2012, 840–855
Crystal structure, electrochemistry, and catalytic studies
of a series of new oxidovanadium(IV) Schiff-base complexes
derived from 1,2-diphenyl-1,2-ethylenediamine
ABOLFAZL GHAFFARIy, MAHDI BEHZAD*y, GRZEGORZ DUTKIEWICZz,
MACIEJ KUBICKIz and MEHDI SALEHIy
yDepartment of Chemistry, College of Sciences, The University of Semnan, Semnan, Iran
zDepartment of Chemistry, Adam Mickiewicz University, Grunwaldzka 6,
6
0-780 Poznan, Poland
(
Received 3 December 2011; in final form 2 January 2012)
New derivatives of N
2
O
2
tetradentate Schiff bases, from condensation of meso-1,2-diphenyl-
,2-ethylenediamine and salicylaldehyde derivatives (X-salicylaldehyde; X ¼ 3-OMe, 4-OMe,
-OMe, 6-OMe, 5-Cl, 5-Me), and oxidovanadium(IV) complexes were synthesized and charac-
1
5
1
terized by H NMR, UV-Vis, IR spectroscopy, and elemental analysis. Crystal structure of
3
L and two of the complexes (VOL and VOL ) were also obtained. In the crystals,
2
3
5-OMe; H
the molecule of H
2
3
2
i
L is C symmetrical, as it occupies the special position on the center of
symmetry; its conformation is partially determined by classical intramolecular O–Hꢀ ꢀ ꢀN
hydrogen bonds. The complexes have monomeric structures with a distorted square pyramid of
vanadium, with the oxo ligand in the apical position. Cyclic voltammetry studies show
IV
V
quasi-reversible V /V redox for which the presence of electron-withdrawing groups on
ꢁ
salicylaldehyde derivatives shifts the E to more positive values. The complexes were used as
catalysts for selective epoxidation of cyclooctene with tert-butylhydroperoxide as oxidant,
in various solvents and reaction conditions. High catalytic activities and excellent selectivity
ꢁ
was found. The catalytic activity of the complexes increased increasing E , a consequence of the
presence of electronegative substituents. This epoxidation process with the new catalysts was
also studied under solvent-free condition and excellent reactivity was observed.
Keywords: Schiff base; Tetradentate; Oxidovanadium(IV); Epoxidation
1. Introduction
The chemistry of vanadium complexes, especially oxidovanadium(IV), has attracted
particular interest [1–10] for their biological, medicinal, synthetic, and catalytic
activities [11–15]. Vanadium compounds are potential oral drugs for diabetic patients
[
disadvantages of the use of inorganic salts of this element in medical treatments, mostly
16] and numerous investigations are carried out in this regard [17, 18]. Due to the
IV
neutral complexes of vanadium (V O), chelated to organic ligands, have been studied
[19, 20]. Another reason for the study of vanadium containing complexes is their ability
to catalyze important reactions. Of particular interest is the use of tetradentate
*
Corresponding author. Email: mbehzad@semnan.ac.ir
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.662275

Oxidovanadium(IV) Schiff base
841
Schiff-base complexes of oxidovanadium(IV) in oxidation catalysis [21–24].
Coordination compounds of vanadium are potential candidates for catalytic purposes
thanks to the several oxidation states of this element, as well as a variety of
coordination numbers, readily accessible high oxidation states, and a high affinity
toward oxygen. The epoxidation of olefinic compounds is an important reaction, since
it results in a variety of useful synthetic and industrially important intermediates.
Schiff-base complexes of oxidovanadium(IV) are among the most widely employed
catalysts for aerobic and non-aerobic alkene epoxidation. Molecular oxygen, hydrogen
peroxide, and tert-butylhydroperoxide (TBHP) are used in such studies. Molecular
oxygen is the greenest but low reaction rates limit its application. Others and we have
found that TBHP is a useful oxidizing agent. Hence, and in continuation of our
previous studies, we herein report the synthesis, characterization, crystal structure, and
catalytic performance of a series of oxidovanadium(IV) complexes of Schiff-base
ligands derived from condensation 1,2-diphenyl-1,2-ethylenediamine with salicylalde-
hyde derivatives. The new complexes were tested as catalysts in the epoxidation of
cyclooctene with TBHP under various conditions to find the optimum operating
conditions. Our results show that these complexes are highly efficient catalysts with
high turnover numbers (TON) and excellent selectivity toward cycloocteneoxide.
2
2
. Experimental
.1. Materials and instruments
All chemicals were of the highest purity and used as received. Cyclooctene and
cycloocteneoxide were purchased from Aldrich and other chemicals were purchased
from Merck. All syntheses and purifications were conducted in aerobic conditions.
1
[
,2-Diphenyl-1,2-ethylenediamine was synthesized following the literature procedure
0
25]. N,N -bis(salicylidene)-1,2-diphenyl-1,2-ethylenediamine (H L ) was synthesized
2
7
according to the literature procedure [24]. New tetradentate Schiff bases were
7
synthesized following similar procedure for the synthesis of H L . Melting points
2
1
were obtained on a Thermoscientific 9100 apparatus. H NMR spectra were recorded
on a 500 MHz Bruker FT-NMR spectrometer using CDCl and DMSO-d ; chemical
3
6
shifts (ꢀ) are given in ppm. Infrared (IR) spectra were obtained as KBr pellets using a
Bruker FT-IR instrument and UV-Vis spectra were obtained on a Shimadzu
UV-1650PC spectrophotometer. Elemental analyses (C, H, and N) were performed
using a Heraeus Elemental Analyzer CHN-O-Rapid (Elemental-Analysesysteme,
Gmbh, West Germany). A Metrohm 757 VA computerace instrument was employed
ꢁ
to obtain cyclic voltammograms in acetonitrile at room temperature (25 C) using
0
–
.1 mol L tetra-n-butylammonium hexafluorophosphate (TBAHFP) solution as sup-
1
porting electrolyte. A platinum electrode was used as the auxiliary electrode and an
Ag/AgCl electrode as the working electrode. Gas chromatography (GC) analyses were
carried out on a GC-17A Shimadzu instrument.
2
.2. Typical procedure for the synthesis of the ligands
The ligands were synthesized by condensation of 1,2-diphenyl-1,2-ethylenediamine with
different salicylaldehyde derivatives in 1 : 2 ratio in ethanol. In all cases, analytically

8
42
A. Ghaffari et al.
H N
NH2
R3
R1
R4
R4
R3
R1
2
R3
R1
R4
R2
OH
N
HO
N
R2
Ethanol
R2
OH
O
+
R3
R1
R4
R4
R3
R1
O
R2
O
N
O
R2
V
VO(acac)2
Ethanol
N
Ligand/Complex
R
1
R
2
R
3
R
4
Ligand/Complex
R
1
R
2
R
3
R
4
1
1
2
3
4
5
5
6
7
H
2
H
2
H
2
H
2
L /VOL
H
H
H
H
OCH
3
H
2
H
2
H
2
L /VOL
H
H
H
Cl
CH
H
H
H
H
H
H
H
2
6
L /VOL
H
OCH
3
H
L /VOL
3
3
7
L /VOL
H
OCH
3
H
H
L /VOL
4
L /VOL
OCH
3
H
H
H
Scheme 1. Typical procedure for the synthesis of the ligands and oxidovanadium(IV) complexes.
pure materials were obtained by recrystallization from ethanol. This procedure is shown
6
in scheme 1. The target ligands in all cases were yellow except H L which was white.
2
2
(
.2.1. Preparation of [N,N9-bis(3-methoxysalicylidene)-1,2-diphenyl-1,2-ethylenediamine]
1
H L ). To a vigorously stirred solution of 1,2-diphenyl-1,2-ethylenediamine (0.32 g,
2
1.5 mmol) in 15 mL of absolute ethanol was added a solution of 3-methoxysalicylalde-
hyde (0.46 g, 3 mmol) in 10 mL of ethanol. The reaction mixture was stirred for 30 min
and then refluxed for 1 h. The reaction mixture was then cooled to room temperature
and left undisturbed overnight. The yellow precipitate was filtered off and washed with
1
0 mL of ethanol and air dried. Recrystallization from ethanol gave analytically pure
material.
Other ligands were synthesized following the same procedure. Spectroscopic and
analytical data are collected in table 1 to confirm the synthesis of the ligands.
2.2.2. Typical procedure for the synthesis of the oxidovanadium(IV) complexes.
Oxidovanadium(IV) complexes were synthesized by equimolar reaction of
bis(acetylacetonato)oxovanadium(IV) with the tetradentate Schiff-base ligands in
ethanol. In all cases, green microcrystalline products were collected by filtration
and air dried. Recrystallization from DMSO or mixture of DMSO/methanol gave

Oxidovanadium(IV) Schiff base
843

8
44
A. Ghaffari et al.
analytically pure complexes. Suitable crystals for X-ray crystallography were obtained
2
3
for VOL and VOL and the crystal structure of these complexes were analyzed. Table 2
contains analytical and spectroscopic data for the complexes.
2
.3. X-ray crystallography
Diffraction data were collected at room temperature by the !-scan technique on an
Oxford Diffraction Xcalibur four-circle diffractometer with Eos CCD detector,
˚
equipped with graphite-monochromated Mo-Ka radiation source (ꢂ ¼ 0.71073 A).
The data were corrected for Lorentz polarization as well as for absorption effects [26].
3
Precise unit-cell parameters were determined by a least-squares fit of 1658 (H L ), 2935
2
3
[VOL ]. Sol), and 5171 ([VOL ]. Sol) reflections of highest intensity. The structures
2
(
were solved with SIR92 [27] and refined with the full-matrix least-squares procedure on
2
F by SHELXL-97 [28]. The scattering factors incorporated in SHELXL-97 were used.
P
2
2 2
–1
2
2
2
The function
2
w(|F | – |F | ) was minimized, with w ¼ [ꢃ (F ) þ (AP) þ BP]
o
2
c
c
o
(
P ¼ [Max(F , 0) þ 2F ]/3). The final values of A and B are listed in table 3. All
o
non-hydrogen atoms were refined anisotropically. Hydrogen atoms in the complexes
were inserted in idealized positions and refined riding with the atoms to which they were
bonded, while in the ligand they were found in difference Fourier maps and freely
3
2
refined. Solvent molecules in ([VOL ]. Sol) (CH OH) and in ([VOL ]. Sol) (DMSO)
3
experience orientation disorder and were treated accordingly with weak constraints
applied to their geometry and thermal parameters. The large residual density in the
2
structure of ([VOL ]. Sol) was interpreted as disordered water, but attempts to find the
hydrogen atoms of this molecule gave no results. Relevant crystal data are listed in
table 3, together with refinement details.
3
3
. Results and discussion
.1. Syntheses
The reaction between 1,2-diphenyl-1,2-diaminopropane with different salicylaldehyde
1
derivatives gave the tetradentate N O type Schiff bases (H L ) in good yields.
–7
2
2
2
Reaction between the ligands with bis(acetylacetonato)oxidovanadium(IV) resulted in
the target complexes. The green color of the precipitated complexes indicated that the
complexes were in their monomeric form [23]. The procedure for synthesis of the
ligands and complexes are summarized in scheme 1.
3
.2. Spectroscopic characterization of the ligands and complexes
1
Common spectroscopic methods such as H NMR, IR, and UV-Vis as well as elemental
3
analyses support the synthesis of the ligands and complexes. Crystal structures of H L ,
2
2
3
VOL , and VOL are also characterized. Cyclic voltammetry further supports the
preparation of VO(IV) complexes. In IR spectra of the ligands, the strong band around
–
1
1
625 cm is indicative of the formation of azomethine (C¼N). The disappearance of
the C¼O stretching bands of the corresponding salicylaldehydes is also a good sign of

Oxidovanadium(IV) Schiff base
845

8
46
A. Ghaffari et al.
Table 3. Crystal data, data collection, and structure refinement.
3
3
VOL 0.5(CH
2
Compound
H
2
L
3
OH) 0.5(H
2
O)
VOL ꢀ DMSO
Empirical formula
Formula weight
Crystal system
Space group
C
30
H
28
N
2
O
4
C
569.49
30.5
H
28
N
2
O
6
V
32 32 2 6
C H N O SV
480.54
Monoclinic
P2 /n
623.60
Monoclinic
P2 /c
Triclinic
P-1
1
1
˚
Unit cell dimensions (A,
ꢁ
)
a
b
c
ꢅ
ꢆ
ꢇ
12.5288(12)
5.9928(4)
17.3311(13)
90
100.304(10)
90
1280.28(18), 2
1.25
0.083
8.9728(6)
12.1166(5)
14.3049(6)
72.688(4)
80.749(4)
82.076(4)
1458.76(13), 2
1.30
0.384
27.7787(16)
6.5960(3)
17.3846(9)
90
106.362(6)
90
3056.4(3), 4
1.35
0.439
3
˚
Volume (A ), Z
Calculated density (g cm
–3
)
Absorption coefficient (mm
ꢂ1
)
F(000)
ꢈ range for data collection ( )
Limiting indices
508
592
2.9–25.0
ꢂ10 ꢃ h ꢃ 10;
ꢂ14 ꢃ k ꢃ 14;
ꢂ17 ꢃ l ꢃ 16
1300
3.1–25.0
ꢂ33 ꢃ h ꢃ 33;
ꢂ7 ꢃ k ꢃ 7;
ꢂ20 ꢃ l ꢃ 20
ꢁ
3.3–28.6
ꢂ16 ꢃ h ꢃ 7;
ꢂ4 ꢃ k ꢃ 8;
–
20 ꢃ l ꢃ 22
Reflections
Collected
4983
8709
37,790
5364 (0.065)
3898
Unique (Rint
)
2724 (0.013)
1916
209
5043 (0.019)
3933
368
With I 4 2ꢃ(I)
Number of parameters
Weighting scheme
392
A
B
0.0427
0.1855
1.03
0.0400
1.8950
1.18
0.1037
6.0300
1.07
2
Goodness-of-fit on F
Final R indices [I 4 2ꢃ(I)]
R indices (all data)
R
1
¼ 0.042,
wR
¼ 0.097
¼ 0.065,
wR
¼ 0.110
0.14 and ꢂ0.13
R
1
¼ 0.061,
wR
¼ 0.146
¼ 0.078,
wR
¼ 0.153
0.80 and ꢂ0.36
R
1
¼ 0.080,
wR
¼ 0.201
¼ 0.112,
wR
¼ 0.2183
1.04 and –1.17
2
2
2
R
1
R
1
R
1
2
2
2
Largest difference peak
)
–
3
˚
and hole (e A
formation of the iminic bonds. In IR spectra of the VO(IV) complexes, these bands are
shifted to lower wavenumbers, indicative of coordination of C¼N, through nitrogen.
The ꢁ of the ligands is also absent in the complexes, indicative of coordination
through the phenolic oxygen atoms of the ligands. IR spectra of the monomeric form of
O–H
–
1
VO(IV) Schiff-base complexes contain a band at 960–990 cm due to V¼O stretching,
–
1
while the polymeric form (V¼O ꢀ ꢀ ꢀ V¼Oꢀ ꢀ ꢀ) contains a band at 850–880 cm .
–
IR spectra of the complexes in this study contain a band at 980 cm which is a sign
1
1
of their monomeric form. H NMR data of the ligands and the corresponding
assignments are summarized in table 1. The appearance of a broad signal at 13 ppm
(
phenolic O–H), a sharp band at 9 ppm (HC¼N), multiplets at 6–8 ppm (H ),
Ar
and signals due to aliphatic C–H of both diamine and salicylaldehydes confirm the
synthesis of the ligands. In the UV-Vis spectra of the complexes, the presence of a band
–
1 –1
–1
at 600 nm with ꢄ of about 150 (mol L ) cm is due to the d ! d transitions in VO(IV)
1
d system.

Oxidovanadium(IV) Schiff base
Table 4. Selected geometrical parameters.
847
3
VOL ]. solv
2
[VOL ]. solv
[
V1–O1
V1–O12
V1–O30
V1–N8
V1–N26
V1–N8–C1–C2
V1–N26–C19–C20
1.599(3)
1.915(3)
1.919(3)
2.072(3)
2.069(3)
164.1(3)
–92.0(3)
1.595(3)
1.915(4)
1.919(3)
2.043(3)
2.057(4)
–91.2(4)
139.3(3)
3
2
Figure 1. Perspective view of H L ; the ellipsoids are drawn at 50% probability level, hydrogen atoms are
shown as spheres of arbitrary radii. Intramolecular hydrogen bonds are shown as dashed lines. The unlabeled
half of the molecule is related to the symmetry operation –x, 2 – y, 1 – z.
3.3. Description of the crystal structures
Selected bond lengths and angles for the structures are listed in table 4. Figure 1 shows
3
the perspective view of H L . The molecule is C symmetrical, the middle point of the
2
i
central C–C bond lies at the center of symmetry. There are classical intramolecular
N–H ꢀ ꢀ ꢀ O hydrogen bonds between the hydrogen atoms of OH and nitrogen atoms
(
figure 1, table 5), and these hydrogen bonds make the plane of the salicylidene phenyl
ring almost coplanar with the plane of the C–N¼C–C chain; the appropriate dihedral
ꢁ
angle is only 2.02(15) . The dihedral angle between the planes defined by the rings of the
3
salicylidene fragment and the phenyl substituent in H L is 46.38(6) , compared to just
ꢁ
2
2
3.8 in the meso isomer [29]. The molecules are arranged in undulating infinite layers
along the b-axis by weak intermolecular C–Hꢀ ꢀ ꢀO interactions (figure 2). Figures 3
3
2
and 4 show perspective views of the structures [VOL ] and [VOL ], respectively. In both
complexes the tetradentate Schiff-base ligand coordinates vanadium in four equatorial
positions, and the geometry around each vanadium can be described as a distorted
square pyramid with the oxo in the apical position. The vanadium is displaced from the
˚
N(26)N(8)O(12)O(30) coordination plane toward the apical oxo by 0.580(5) A in
3
2
˚
VOL ] and by 0.584(5) A in [VOL ]. The two phenyl groups on C19 and C1 at the N–N
[

8
48
A. Ghaffari et al.
˚
Table 5. Hydrogen-bond data (A, ).
ꢁ
D–H ꢀꢀꢀ A
D–H
H ꢀꢀꢀ A
D ꢀꢀ ꢀA
DHA
3
L ]
[
H
2
O12–H12 ꢀꢀꢀ N8
1.00(2)
1.71(2)
2.6141(16)
149(2)
3
[
VOL ]. solv
i
C1–H1 ꢀꢀꢀ O12
0.98
0.93
0.93
2.40
2.46
2.50
3.269(5)
3.236(5)
3.420(5)
147
140
172
C25–H25 ꢀꢀꢀ O1
ii
C32–H32 ꢀꢀꢀ O34
2
VOL ]. solv
[
iii
C1–H1 ꢀꢀꢀ O1
C7–H7 ꢀꢀꢀ O1
C9–H9 ꢀꢀꢀ O1
0.98
0.93
0.93
0.96
0.93
2.56
2.18
2.27
2.53
2.49
3.442(5)
3.071(7)
3.163(6)
3.418(18)
3.305(6)
150
159
161
153
147
iii
C34–H34C ꢀꢀꢀ O1S
iv
C35–H35 ꢀꢀꢀ O1
i
ii
iii
iv
Symmetry codes: –x, –y, 1 – z; –x, –1 – y, 2 – z; x, 1 þ y, z; x, –1/2 – y, 1/2 þ z.
3
L along the [10] direction. The dashed lines denote hydrogen bonds.
Figure 2. The crystal packing of H
2
3
Figure 3. Perspective view of [VOL ]; the ellipsoids are drawn at 50% probability level, hydrogen atoms are
shown as spheres of arbitrary radii. Solvent molecules are omitted for clarity.

Oxidovanadium(IV) Schiff base
849
2
Figure 4. Perspective view of [VOL ]; the ellipsoids are drawn at 50% probability level, hydrogen atoms are
shown as spheres of arbitrary radii. Solvent molecule is omitted for clarity.
chelate take a different orientation. One phenyl is axial and the other equatorial. The
axial group is almost parallel to the apical V¼O with a slight deviation caused by
repulsion with the adjacent equatorial phenyl. The five-membered N–N chelate rings
adopt a distorted gauche conformation with the phenyl groups in axial and equatorial
ꢁ
ꢁ
orientations. The V–N–C–C (phenyl) torsion angles are 164.1(3) and –92.0(3) in
[
3
VOL ] and 139.3(3) and –91.2(4) in [VOL ]. The average V–N and V–O bond
ꢁ ꢁ
2
distances of the complexes are in the range observed for many other vanadium Schiff-
base complexes [21, 30, 31]. The VO-complexes are arranged in layers parallel to the
(
010) planes of their unit cells (figures 5 and 6). Within these layers the complexes are
3
held together by C–H ꢀ ꢀ ꢀ O hydrogen bonds (cf. table 5), and in the case of [VOL ] by
weak ꢉ–ꢉ-stacking interactions between phenyls (the centroid-to-centroid distances are
˚
ca 3.95 A; the interplanar separation is around 3.5 A).
˚
3
.4. Electrochemical studies
–
3
–1
Cyclic voltammograms of 10 mol L solutions of these tetradentate Schiff-base
–
complexes were recorded in DMSO containing 0.1 mol L tetra-n-butylammonium
1
hexafluorophosphate (TBAHPF) as supporting electrolyte at a platinum working
3
electrode. Figure 7 shows the voltammogram of VOL and other voltammetry data are

8
50
A. Ghaffari et al.
3
Figure 5. The crystal packing of [VOL ] as seen along the [10] direction.
2
Figure 6. The crystal packing of [VOL ] as seen along the [10] direction. The dashed lines denote hydrogen
bonds.
IV
V
collected in table 6. These data are consistent with the quasi-reversible V /V redox
process. As the electron-withdrawing nature of the substituents on the ancillary ligand
IV
V
system increases, increase in the E1/2 of the V /V redox process is observed. Our
previous studies show that the E1/2 is a major factor which influences the catalytic
activity of oxidovanadium(IV) complexes.

Oxidovanadium(IV) Schiff base
851
6
4
2
0
2
4
–
–
0
.1
0.3
0.5
0.7
0.9
E(V vs. Ag/AgCl)
–
3
–1
Figure 7. Cyclic voltammogram of 10 mol L solution of VOL in DMSO containing 0.1 mol L
3
–1
–1
TBAHFP and scan rate 50 mV s
.
Table 6. Electrochemical data for the complexes.
a
p
c
p
Complex
E
(mV)
E
(mV)
E
1/2 (mV)
DE (mV)
1
VOL
VOL
VOL
VOL
VOL
VOL
550
568
451
586
626
538
390
408
349
432
518
402
470
488
401
509
572
470
160
160
102
154
108
136
2
3
4
5
6
–
3
–1
–1
Electrochemical data of 10 mol L solution of the complexes were obtained using 0.1 mol L TBAHFP solution as
supporting electrolyte.
3.5. Catalytic studies
These oxidovanadium(IV) Schiff-base complexes were used as catalysts for oxidation of
cyclooctene using TBHP as oxidant in different solvents and various reaction
conditions. The progress of the reaction was monitored by GC in 30 min intervals.
In the absence of the complexes, no or very little oxidation products were observed.
Oxidation of cyclooctene gave cycloocteneoxide as the sole product of the reaction. The
retention times for the starting materials and the products were determined by
comparison with authentic samples. The conversion percent (%) and the TON were
calculated by the following equations, in which Cinitial and Cfinal are initial and final
concentration of the substrate, respectively, and [Q] is the concentration of the catalyst.
Cinitial ꢂ Cfinal
%
Conversion¼
Turnover ¼
ꢄ 100,
Cinitial
%
Conversion ꢀ C
initial
:
½
Qꢅ

8
52
A. Ghaffari et al.
7
Figure 8. The results of the optimization of solvents for the epoxidation of cyclooctene with VOL .
In a typical experiment, 15 mmol of the oxidovanadium catalyst was dissolved in
0 mL of freshly distilled acetonitrile and then 15 mmol of cyclooctene and 30 mmol of
2
TBHP were added. The reaction mixture was refluxed while being stirred and the
reaction progress was monitored at 30 min intervals. Various reaction conditions were
optimized. The optimized reaction time was obtained by following the reaction progress
at 30 min intervals up to 27 h; 24 h was the optimized reaction time. Figure 8 shows the
ꢁ
results of the oxidation of cyclooctene in different solvents at 40 C. Acetonitrile was the
best solvent for this study. The reaction was performed in acetonitrile at reaction
ꢁ
ꢁ
ꢁ
ꢁ
temperatures ranging from 50 C to 80 C (10 C intervals) and 80 C was found to be the
optimized reaction temperature. The amounts of catalysts were also optimized to
1
the results of the epoxidation of cyclooctene under optimized conditions for VOL
5 mmol and the oxidant to substrate ratio was also optimized to 2 : 1. Figure 9 shows
1–7
;
VOL with electronegative Cl on the Schiff base has the highest conversion percent
5
6
while the electron donating methyl substituted complex, VOL , has the lowest catalytic
activity. It should, of course, be mentioned that the electronegativity of the substituents
is not the only factor affecting such catalytic ability and other electronic and steric
factors play important roles.
3
.6. Solvent-free catalytic studies
To the best of our knowledge, all previous catalytic studies on the epoxidation of
alkenes with tetradentate Schiff-base complexes of oxidovanadium(IV) are performed
in solvents. Acetonitrile is a common solvent used in such experiments. Our previous
studies, as well as the present study, show that the type of the solvent and its amount
has great influence on the conversion percent and TON. Therefore, we decided to study
the solvent-free oxidation of cyclooctene with these complexes. Typically, a mixture of
15 mmol of the catalysts, 15 mmol of cyclooctene, and 30 mmol of TBHP were refluxed
and the progress of the reaction monitored every 20 min by GC. As seen from table 7,
cyclooctene was converted in very good yields to cyclooctene epoxide in less than 2 h.

Oxidovanadium(IV) Schiff base
853
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
6
2
7
4
1
3
5
VOLx (x= 1–7)
1
–7
.
Figure 9. The results of the catalytic epoxidation of cyclooctene with VOL
1
–7
Table 7. Comparison of the catalytic ability of the VOL
conditions in their optimized conditions.
complexes in acetonitrile and solvent-free
Acetonitrile
Product (%)
Solvent free
Product (%)
O
O
Complex Conversion (%) Turnover
Conversion (%) Turnover
1
VOL
VOL
VOL
VOL
VOL
VOL
VOL
56.03
40.97
65.15
48.14
66.87
41.92
48.04
560
410
652
481
669
419
480
100
100
100
100
100
100
100
71.39
63.06
80.58
76.79
86.40
55.87
81.63
714
631
806
768
864
559
816
100
100
100
100
100
100
100
2
3
4
5
6
7
These results are comparable with previous studies in organic solvents. Moderate
conversion percent and long reaction times are usually among the disadvantages of such
experiments. In this study, tetradentate Schiff-base complexes of oxidovanadium(IV)
catalyze the epoxidation of cyclooctene in excellent yields and selectivity. The
appearance of cycloocteneoxide as the sole product of the reaction is of great
importance. Besides, the reaction of epoxidation is almost complete in less than 2 h.
Energy saving is another strong point in these solvent-free experiments. The reactions
are performed at the boiling point of cyclooctene which is less than the reaction
temperatures in acetonitrile. The trends in the catalytic ability of the complexes are
similar to our solvent based experiments. The results for epoxidation of cyclooctene

8
54
A. Ghaffari et al.
x
Figure 10. The results for the solvent-free epoxidation of cyclooctene with VOL complexes and TBHP.
1
–7
with VOL are shown in figure 10. A comparison between the ability of the catalysts
in both solvent free and with solvent conditions is presented in table 7. The catalysts are
more effective in solvent-free conditions. We deliberately obtained the conversion
percentages in 2 h. At longer reaction times, all of the catalysts would probably convert
cyclooctene to its epoxide quantitatively, and we would not be able to compare the
7
catalysts with each other. VOL gave more than 95% epoxidation product after 3 h.
Supplementary material
3
2
3
CCDC 854918 (H L ), 854919 ([VOL ]. Sol), and 854920 ([(VOL )]. Sol) contain the
2
supplementary crystallographic data for this article. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif or E-mail: deposit@ccdc.cam.ac.uk
Acknowledgments
We acknowledge the office of gifted student at the University of Semnan for the
financial support.
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