A new oxidovanadium(IV) Schiff base complex containing asymmetric tetradentate ONN′O′ Schiff base ligand: Synthesis, characterization, crystal structure determination, thermal study and catalytic activity

Article abstract of DOI:10.1016/j.cclet.2015.03.014

Abstract After synthesis of an asymmetric tetradentate ONN′O′ Schiff base ligand (H₂L) followed by reaction of the synthesized H₂L with an equimolar mixture of methanolic solutions of the VO(acac)₂, a new oxidovanadium(IV)

Full text of DOI:10.1016/j.cclet.2015.03.014

G Model
CCLET 3243 1–6
Chinese Chemical Letters xxx (2015) xxx–xxx
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2
Original article
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4
5
A new oxidovanadium(IV) Schiff base complex containing asymmetric
tetradentate ONN0O0 Schiff base ligand: Synthesis, characterization,
crystal structure determination, thermal study and catalytic activity
a
b,c
c
Q1 Gholamhossein Grivani a, , Abbase Ghavami , Vaclav Eigner , Michal Dusek ,
*
´
ˇ
6
7
Aliakbar Dehno Khalaji d
8
9
10
11
a School of Chemistry, Damghan University, Damghan 36715-364, Iran
b
´
Department of Solid State Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic
c Institute of Physics ASCR, v.v.i., Na Slovance 2, 182 21 Praha 8, Czech Republic
d Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran
A R T I C L E I N F O
A B S T R A C T
After synthesis of an asymmetric tetradentate ONN0O0 Schiff base ligand (H2L) followed by reaction of the
Article history:
Received 19 October 2014
Received in revised form 28 December 2014
Accepted 9 February 2015
Available online xxx
synthesized H2L with an equimolar mixture of methanolic solutions of the VO(acac)2,
a new
oxidovanadium(IV) Schiff base complex (VOL) was synthesized. The Schiff base ligand and its complex
were characterized by FT-IR and UV–vis spectra and C, H, N analysis. The crystal structure of VOL was also
determined by single crystal X-ray analysis. The VOL complex crystallizes in monoclinic space group
Cc. The Schiff base ligand acts as a tetradentate ligand through its two iminic nitrogens and two phenolic
and acetylacetonate oxygens. Thermogravimetric analysis of the VOL showed that it decomposes in two
steps and converts to mixed vanadium oxides at 477 8C. In addition, thermal decomposition of the VOL
complex in air at 660 8C leads to formation of V2O5 nanoparticles with the average size estimated from
XRD 49 nm. The catalytic activity of the VOL complex was investigated in the epoxidation reaction and
different reaction parameters were optimized. The results showed that the cyclic alkenes were
efficiently converted to the corresponding epoxides, whereas the VOL did not appreciably convert the
linear alkenes.
Keywords:
Oxidovanadium(IV)
Schiff base
Crystal structure
Nanoparticle
Epoxidation
ß 2015 Gholamhossein Grivani. Published by Elsevier B.V. on behalf of Chinese Chemical Society and
Institute of Materia Medica, Chinese Academy of Medical Sciences. All rights reserved.
12
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1. Introduction
insulin mimicking [17–19], nitrogen fixation [20], tumor growth 26
inhibition, and prophylaxis against carcinogenesis [17]. In addition, 27
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24
25
Azomethines, or Schiff base compounds, are ones of the most
widely used organic compounds and played a central role in the
development of coordination chemistry of transition metals [1,2]. In
the area of bioinorganic chemistry, interest in Schiff base complexes
with transition and inner-transition metals has focused on the role
of such complexes in providing useful synthetic models for the
metal-containing sites in metallo-proteins and enzymes [3–11]. In
recent years, the coordination chemistry of Schiff base complexes of
oxidovanadium(IV) and dioxidovanadium(V) with N, O, and S-donor
chelating ligands have received considerable attention. They show
catalytic activity and play vital role in a variety of biochemical
processes such as haloperoxidation [12–15], phosphorylation [16],
high-valent vanadium complexes have been considered as new 28
versatile catalytic reagents for a wide range of oxidation reactions 29
[21,22], like oxidation of olefines and alcohols [23–27], benzene/ 30
alkylaromatic compounds [28,29], and sulfides [30–32]. Thus the 31
design of vanadium(IV) Schiff base complexes containing O, N Schiff 32
base ligands is the center of current interest. Herein we describe the 33
synthesis, characterization, crystal structure determination, thermal 34
study, and catalytic activity of a new oxidovanadium Schiff base 35
complex (VOL) containing tetradentate asymmetric ONN0O0 Schiff 36
base ligand (Scheme 1).
37
2. Experimental
38
All reagents and solvents for synthesis and analysis were 39
commercially available and purchased from Merck and used as 40
*
Corresponding author.
E-mail address: grivani@du.ac.ir (G. Grivani).
received without further purifications.
41
http://dx.doi.org/10.1016/j.cclet.2015.03.014
1001-8417/ß 2015 Gholamhossein Grivani. Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical
Sciences. All rights reserved.
Please cite this article in press as: G. Grivani, et al., A new oxidovanadium(IV) Schiff base complex containing asymmetric tetradentate
ONN0O0 Schiff base ligand: Synthesis, characterization, crystal structure determination, thermal study and catalytic activity, Chin.
Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.03.014

G Model
CCLET 3243 1–6
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G. Grivani et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
was dried in air (Yield0.37 g, 79%). M.p. 270 8C. Anal. Calcd. for
60
61
62
63
(C14H15BrN2O2)VO: C, 42.79; H, 3.36; N, 7.03%. Found: C, 43.03; H,
3.84; N, 7.18%. IR (KBr pellet, cmꢀ1): 1630 and 1605 (s, C = N), 974
(s, V = O).
2.3. X-ray crystallography
64
A
single crystal of the dimensions 0.38 mm ꢁ 0.34 mm ꢁ
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73
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75
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77
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90
0.03 mm of VOL was chosen for X-ray diffraction study. Crystallo-
graphic measurements were done at 120 K with four circle CCD
diffractometer Gemini of Oxford diffraction, Ltd., with mirrors-
˚
a radiation (l = 1.54184 A). The crystal structure
collimated Cu K
was solved by charge flipping with the program SUPERFLIP [34] and
refined with the Jana2006 program package [35] by full-matrix
least-squares technique on F2. The molecular structure plots were
prepared by ORTEP III [36]. Hydrogen atoms were mostly discernible
in difference Fourier maps and could be refined to reasonable
geometry. Crystallographic data and details of the data collection,
structure solution, and refinements are listed in Table 1.
The measured sample was a thin slide. However, refinement of
the indexed shape indicated that probably only part of this sample
was diffracting, because the sample shrank considerably during
the refinement. The sample also contained a faint minor domain
with the same unit cell parameters but with no systematic
orientation with respect to the major domain, which would
indicate twinning.
The difference Fourier map contained smeared maxima around
positions for Vanadium and Bromine. These maxima could be
described by refinement of the 4th order anharmonic ADP for each
atom. However, due to insufficient absorption correction, anhar-
monic ADP refinement may incorrectly identify residues. Keeping
in mind the above mentioned limitations of our sample, we
decided to refine only harmonic ADP (i.e. ellipsoids).
Scheme 1. Preparation procedure of the asymmetric teradentate ONN0O0 Schiff base
ligand (H2L) and its vanadyl complex (VOL).
42
2.1. Synthesis of the H2L
43
44
45
46
47
48
49
50
51
52
53
54
This ligand was prepared in two steps: At first, acacen was
prepared as described in the literature [33]. A solution of 10 mmol
acetylacetone in 50 mL of chloroform was added dropwise to a
solution of 10 mmol ethylenediamine in 50 mL of chloroform in a
250 mL round bottom flask at 30 min with stirring at room
temperature. After stirring for 3 h, the solvent was evaporated
and the resulting solution was reacted with a solution of 10 mmol
of 5-bromosalicylaldehyde in 50 mL of methanol and refluxed for
2 h. After evaporation of the solvent, the yellow precipitate was
dried in air. Yield (1.98 g, 61%). M.p. 77. Anal. Calcd. for
C14H17BrN2O2: C, 51.41; H, 5.08; N, 9.56%. Found: C, 51.70; H,
5.21; N, 9.82%. IR (KBr pellet, cmꢀ1): 1635 and 1613 (s, C = N).
55
2.2. Synthesis of the VOL
3. Results and discussion
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92
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57
58
59
1.2 mmol of the H2L was dissolved in a 50 mL of absolute
methanol and a solution of 1.2 mmol of VO(acac)2 in 50 mL of
absolute methanol was added. The resulting solution was heated
for 1 h at 50 8C. After evaporating the solvent, the green precipitate
3.1. Synthesis and characterization
The scheme shows the preparation procedure of the asymmet-
ric tetradentate ONN0O0 Schiff base ligand of H2L and its
oxidovanadium(IV) Schiff base complex. Reaction of a methanolic
solution of ligand with VO(acac)2 at 50 8C for 1 h resulted in
production of the asymmetric oxidovanadium(IV) Schiff base
complex (VOL). The Schiff base ligand (H2L) and its VOL complex
were characterized by C, H, N analysis, UV–vis, and FT-IR spectra.
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Table 1
The crystal data, data collection and structure refinement of VOL.
Chemical formula
C14H15Br1N2O3V1
Formula weight
390.1
Crystal system
Monoclinic
Cc
Space group
˚
a
7.0332 (4) A
˚
19.1602 (10) A
b
˚
c
11.7322 (6) A
b
V
105.815 (4)8
3
˚
1521.16 (14) A
4
8.63 mmꢀ1
120 K
Z
m
T
Crystal size
0.38 ꢁ 0.34 ꢁ 0.03 mm
Independent reflections
2354
2300
0.158
0.76
Reflections with I > 3s(I)
Tmin
Tmax
Rint
0.035
4086
0.052
0.134
2.29
Measured reflections
R[F2 > 3
wR(F2)
S
s
(F2)]
Reflections
Parameters
Drmax
2354
191
ꢀ3
˚
1.00 e A
ꢀ3
˚
Drmin
ꢀ0.63 e A
Fig. 1. The UV–vis spectra of the H2L and VOL (10ꢀ5 mol/L in CHCl3).
Please cite this article in press as: G. Grivani, et al., A new oxidovanadium(IV) Schiff base complex containing asymmetric tetradentate
ONN0O0 Schiff base ligand: Synthesis, characterization, crystal structure determination, thermal study and catalytic activity, Chin.
Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.03.014

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G. Grivani et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
3
Fig. 3. The TGA profile of the VOL.
[32,37,38], indicating the monomeric five coordination of the VOL 143
as confirmed by crystal structure determination.
144
3.2. X-ray structure
145
Determination of the structure of VOL by single-crystal X-ray 146
crystallography (Fig. 2 and Table 2) shows that the vanadium(IV) 147
center is coordinated by three oxygen and two nitrogen atoms, 148
resulting in a distorted square pyramid complex. The complex VOL 149
is mononuclear, neutral, and crystallizes in monoclinic space group 150
Cc. The Schiff base ligand acts as a tetradentate ligand through its 151
two iminic nitrogens and two phenolic and acetylacetonate 152
oxygens. The selected bond distances and angles of VOL are listed 153
in Table 2. The V1–N8 and V1–N5 bond distances were found to 154
Fig. 2. An ORTEP view of VOL showing atoms labeling and ellipsoid probability of
50%.
10123456789012
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129
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The chemical composition of the H2L and VOL were determined by
the C, H, N analysis and the results confirmed the net chemical
compositions. Fig. 1 shows the UV–vis spectra of the H2L and VOL.
Three bands were seen in the spectrum of the H2L at 241 nm,
259 nm, and 322 nm. The two bands in the higher energy region
(241 nm, 259 nm) were attributed to the
the band at 322 nm was attributed to the n !
p
!
p
* transitions and
p
* transitions. In the
´
˚
be 2.054(7) and 2.046(5) A, respectively, while the V1–O1 and V1– 155
UV–vis spectrum of the VOL complex, these bands shifted to lower
energy regions and appeared at 280 nm, 292 nm, and 387 nm due
to the anionic coordination of the H2L to the vanadium center. In
addition, a new band with very low intensity appeared at 604 nm,
which was attributed to the d–d transition.
´
˚
O16 bond distances were found to be 1.950(6) and 1.924(4) A, 156
respectively, and in agreement with bonds in similar oxidovana- 157
dium(IV) Schiff base complexes [39,40]. The V155O1V bond 158
´
˚
distance (1.597(5) A) is shorter than the other V–O distances, 159
similarly like other V55O double bonds [39,40]. 160
In the FT-IR spectrum of H2L, two distinct bands appeared at
1635 cmꢀ1 and 1613 cmꢀ1 due to the two distinct C55N bonds in
the H2L, related to the vC5N of salicylate and acetylacetonate
moieties, respectively. These bands shifted to the lower wave-
numbers in the FT-IR spectrum of the VOL due to the coordination
of the nitrogen of imines to the vanadyl center, and appeared at
1630 cmꢀ1 and 1605 cmꢀ1, respectively. The observed band at
974 cmꢀ1 in the FT-IR spectrum of the VOL was attributed to the
V55O stretching vibration frequencies in agreement with literature
In this structure, the bond angles around vanadium are from 161
79.1(2)8 for N5–V1–N8 to 151.3(2)8 for O1–V1–N8, which indicates 162
the significant deformation of the square pyramidal coordination 163
geometry [39,40].
164
3.3. Thermal study
165
Fig. 3 shows the TG profile of the VOL. The VOL complex was 166
stable up to 280 8C. After that, it decomposed in two stages by 167
losing Br in the first stage in the temperature range of 301–374 8C 168
Table 2
´
˚
Selected bond distances (A) and angles (8) of VOL.
V1 O1v
1.597 (5)
V1 O1
1.950 (6)
V1 N5
2.046 (5)
1.924 (4)
1.470 (10)
1.283 (8)
1.512 (10)
105.4 (3)
103.0 (3)
88.3 (2)
V1 N8
2.054 (7)
1.334 (9)
1.468 (9)
1.313 (8)
1.333 (9)
108.2 (2)
111.8 (2)
151.3 (2)
79.1 (2)
V1 O16
C4 N5
N5 C6
C7 N8
N8 C9
O1 C2
C6 C7
C15 O16
O1v V1 N5
O1v V1 O16
O1 V1 N8
N5 V1 N8
N8 V1 O16
O1 C2 C3
C3 C4 N5
V1 N5 C4
C4 N5 C6
C6 C7 N8
V1 N8 C9
N8 C9 C10
O1v V1 O1
O1v V1 N8
O1 V1 N5
O1 V1 O16
N5 V1 O16
V1 O1 C2
O1 C2 C17
N5 C4 C18
V1 N5 C6
N5 C6 C7
V1 N8 C7
C7 N8 C9
86.3 (2)
139.7 (2)
126.5 (5)
114.5 (6)
120.3 (7)
116.3 (4)
108.2 (6)
110.8 (4)
120.2 (7)
86.8 (2)
123.9 (7)
121.9 (6)
125.1 (5)
118.6 (6)
108.4 (6)
129.0 (5)
123.8 (7)
Fig. 4. (a) The XRD pattern of the obtained powder from the thermal decomposition
of the VOL complex at 660 8C (b) the XRD pattern of the V2O5 from the Merck
reference.
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ONN0O0 Schiff base ligand: Synthesis, characterization, crystal structure determination, thermal study and catalytic activity, Chin.
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Fig. 5. The effect of solvent on the conversion of the cyclooctene to cyclooctene
epoxide in the presence of TBHP as oxidant by the catalytic amount of VOL in
refluxed conditions.
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174
175
and losing the residual organic moiety in the second stage in the
temperature range of 430–477 8C, resulting in mixed vanadium
oxides. In addition, the VOL complex was taken out in an oven at
660 8C. The obtained powder from this decomposition was
analyzed by XRD-powder diffraction. Based on the resulting
XRD pattern, (Fig. 4) the average crystallite size calculated by using
Scherrer’s formula was found to be around 49 nm.
Fig. 6. The effect of alkene/oxidant ratio on the conversion of cyclooctene to
cyclooctene epoxide in CHCl3 in the presence of VOL in refluxed conditions.
176
3.4. Catalytic activity
conversion did not change appreciably. The epoxidation reaction
was also carried out using three amounts of catalyst (0.01,
0.015 and 0.02 mmol) while keeping other conditions constant
(Fig. 7). According to this plot, it is clear that in the case of
0.015 mmol, the catalytic activity of catalyst is similar to the case
of 0.02 mmol, but it is higher than in the case of 0.01 mmol. Thus
the optimized conditions (0.015 mmol of catalyst, TBHP as an
oxidant, 1:3 ratio of alkene:TBHP and CHCl3 as a solvent) were
chosen, and then the epoxidation of alkenes was tested under these
conditions (Table 4). The cyclic alkenes were efficiently converted
to corresponding epoxides, but the VOL did not appreciably
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In order to assess the catalytic activity of the VOL complex,
cyclooctene was chosen as a model substrate. Different parameters
such as solvent, oxidant, and the amount of oxidant and catalyst
were optimized in the epoxidation of cyclooctene in the presence
of the VOL as a catalyst. The epoxidation of cyclooctene was
investigated in the solvents CHCl3, CH2Cl2, CCl4, CH3CN, THF, and
MeOH in the presence of tert-butylhydroperoxide (TBHP) as an
oxidant and VOL as a catalyst (Fig. 5). High epoxide yields were
obtained in the non-coordinating solvents such as CHCl3, CH2Cl2,
and CCl4. In the coordinating solvents such as CH3CN, THF, and
MeOH, the epoxide yield decreased. Based on the proposed
mechanisms, the coordinating solvents compete with TBHP in
coordination to the vanadium center and retard the coordination of
the TBHP, resulting in decreased epoxide yield [40–42]. The
epoxidation reaction was carried out in three different oxidation
media, TBHP, H2O2 and NaIO4, and the results showed that high
epoxide yield was obtained in the TBHP (Table 3). The effect of
cyclooctene:TBHP ratio on the catalytic epoxidation of cyclooctene
was illustrated in Fig. 6. Three ratios (1:2, 1:3, and 1:4) were used
while the other conditions were kept constant. By increasing the
cyclooctene:TBHP ratio from 1:2 to 1:3, the conversion increased
slowly, whereas by increasing this ratio from 1:3 to 1:4, the
Table 3
The epoxidation of cyclooctene in different oxidation media by the VOL in refluxed
conditionsa
Solvent
Oxidant
Time (min)
%Conversionb
CHCl3
TBHP
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
NaIO4
150
150
150
150
150
150
150
150
81
CHCl3
2
CH3CN
1
1, 2-Dichloroethan
MeOH
2
No reaction
No reaction
No reaction
No reaction
THF
CCl4
CH3CN/H2O
a
Reaction conditions: cis-cyclooctene (0.5 mmol), TBHP (1.5 mmol), H2O2 and
NaIO4 (2.5 mmol), catalyst (0.015 mmol), solvent (5 mL).
Fig. 7. The effect of catalyst amount on the conversion of the cyclooctene to
cyclooctene epoxide in CHCl3 in the presence of TBHP as oxidant and VOL as catalyst
in refluxed conditions.
b
GLC yield based on the starting alkene.
Please cite this article in press as: G. Grivani, et al., A new oxidovanadium(IV) Schiff base complex containing asymmetric tetradentate
ONN0O0 Schiff base ligand: Synthesis, characterization, crystal structure determination, thermal study and catalytic activity, Chin.
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240
242
The epoxidation of alkenes catalyzed by VOL catalyst with TBHP under reflux
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Alkene
% Conversion (% epoxide)b
81 (100)
Time (min)
150
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214
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220
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223
In conclusion, a new asymmetric tetradentate VOL Schiff base
complex was synthesized and characterized by C, H, N analysis,
FT-IR, and UV–vis spectra. The crystal structure of the VOL was
determined by the single crystal X-ray analysis. Thermogravimetric
analysis of the VOL showed that it was stable up to 280 8C, and it
decomposed in two stages. Thermal decomposition of the VOL
complex at 660 8C converts it to V2O5 nanoparticles. In addition, the
catalytic activity of the complex was investigated in epoxidation
reaction, and different reaction parameters were optimized, showing
that it can be active and selective under the optimized conditions.
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G. Grivani, A. Ghavamiand A.D. Khalajiaregrateful to the
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Appendix A. Supplementary data
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Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Center, CCDC No. 987361. Copies of
the data can be obtained free of charge through e-mail (deposit@
ccdc.cam.ac.uk) or web page (http:www.ccdc.cam.ac.uk).
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