Synthesis, characterization and crystal structure determination of a new oxovanadium(IV) Schiff base complex: The catalytic activity in the epoxidation of cyclooctene

Article abstract of DOI:10.1016/j.inoche.2011.12.044

A new oxovanadium(IV) Schiff base complex, V^IVOL₂ (1), was prepared by the reaction of a bidentate Schiff base ligand L and VO(acac)₂ (L = N-salicylidin-2-chloroethylimine). The Schiff base ligand, L, and its oxovanadium(I

Full text of DOI:10.1016/j.inoche.2011.12.044

Inorganic Chemistry Communications 18 (2012) 15–20
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization and crystal structure determination of a new
oxovanadium(IV) Schiff base complex: The catalytic activity in the epoxidation
of cyclooctene
b
b
Gholamhossein Grivani a, , Giuseppe Bruno , Hadi Amiri Rudbari ,
⁎
Aliakbar Dehno Khalaji c, Pegah Pourteimouri a
a
School of Chemistry, Damghan University, Damghan, 36715-364, Iran
b
Dipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di Messina, 98166 Messina, Italy
Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran
c
a r t i c l e i n f o
a b s t r a c t
A new oxovanadium(IV) Schiff base complex, VIVOL2 (1), was prepared by the reaction of a bidentate Schiff
base ligand L and VO(acac)2 (L=N-salicylidin-2-chloroethylimine). The Schiff base ligand, L, and its oxova-
nadium(IV) complex, 1, were characterized by the elemental analysis (CHN) and FT-IR spectroscopy. In ad-
dition, 1H-NMR was employed for characterization of the ligand. The crystal structure of 1 was determined
by the single crystal X-ray analysis. The Schiff base ligand L acts as a chelating ligand and coordinates via
one nitrogen and one oxygen atom to the vanadium center. The complex 1 crystallizes in the monoclinic sys-
tem, with space group P21/n, having one symmetry-independent V4+ ion coordinating in an approximately
square pyramidal N2O3 geometry by two imine nitrogen, two phenolato oxygen atoms from two Schiff base
ligands in a square plane and one oxygen atom in an apical position. The catalytic activity of the complex 1
was tested in the epoxidation of cyclooctene. The results showed that the complex 1 was highly active and
selective catalyst in optimized conditions in the epoxidation of cyclooctene.
Article history:
Received 18 October 2011
Accepted 29 December 2011
Available online 12 January 2012
Keywords:
Vanadyl Schiff base complex
Crystal structure
Square pyramidal
Catalytic activity
Epoxidation
© 2012 Elsevier B.V. All rights reserved.
Schiff base compounds, the aldehyde- or ketone analogs in which
the carbonyl group is replaced by an imine or azomethine group, are
considered privileged ligands, because of their simple preparation in
an one-pot condensation of aldehydes (or ketones) and primary
amines in an alcohol solvent. The metal complexes of them have
been extensively investigated for more than a century and employed
in different aspects including biological studies [1–6], magneto chem-
istry [7], non-linear optics [8], photo physical studies [9], catalysis
[10], materials chemistry [11], chemical analysis [12], absorption
and transport of oxygen [13]. In the past two decades the coordina-
tion chemistry of vanadium and specifically the Schiff base complexes
of oxovanadium(IV) and dioxovanadium(V) with N, O and S-donor
chelating ligands has received considerable attention due to its vital
roles in a variety of biochemical processes such as haloperoxidation
[14], phosphorylation[15], insulin mimicking [16–18], nitrogen fixa-
tion[19], tumor growth inhibition and prophylaxis against carcino-
genesis [20].
by transition metal complexes is an area of intense research activity
[21,22]. Recently a large number of publications, described the syn-
thesis, structural study and catalytic activity of oxovanadium Schiff
base complexes in oxidation of various organic substrates [23–25].
Essentially in all these complexes the metal center is surrounded by
N O coordination environment. We started to the synthesis and char-
acterization of a series of new bidentate (NO) Schiff base ligands by
the condensation of alkylammonium halides with various s salicylal-
dehyde derivatives and their metal complexes to investigate the cat-
alytic and biological activities of them. In this research we report the
synthesis, characterization and crystal structure determination of a
new oxovanadium(IV) Schiff base complex containing chloroethyl
chain and its catalytic activity in epoxidation of cyclooctene.
By reaction of salicylaldehyde and 2-chloroethyl ammonium hy-
drochloride in refluxing methanol the Schiff base ligand
L
(Scheme 1) was prepared [26]. The ligand L salts were previously de-
scribed in ref [27]. The new VOL2 Schiff base complex, 1, was also pre-
pared by the simple reaction of VO(acac)2 and L (Scheme 1) in
refluxing methanol [28]. The Schiff base ligand L and its vanadyl com-
plex were characterized by the elemental analysis (CHN) and FT-IR
spectroscopy. The results of CHN analysis confirmed the chemical
composition of L and 1.
The epoxidation of alkenes is one of the most widely studied reac-
tions in organic chemistry since epoxides are key starting materials
for a wide variety of products. The catalytic epoxidation of olefins
The FT-IR spectrum of L shows a sharp band in the 1640 cm−1, at-
tributed to the stretching vibration frequency of imine (\C_N\). In
⁎
Corresponding author. Tel./fax: +98 2325235431.
E-mail address: grivani@du.ac.ir (G. Grivani).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.12.044

16
G. Grivani et al. / Inorganic Chemistry Communications 18 (2012) 15–20
Scheme 1. Preparation procedures of Schiff base ligand, L, and its Oxovanadium(IV) complex, 1.
the FT-IR spectrum of 1, this band shifts to the lower wave number,
1620 cm−1, indicating the coordination of azomethine to the vana-
dium(IV) center. Another characteristic of the FT-IR spectrum of 1 is
the exhibition of a sharp band in the 984 cm−1, corresponding to
the V_O stretching vibration. The appearance of a band in this region
indicates the existence of monomeric five coordinated V_O units in-
stead of the polymeric ⋯V_O⋯V_O⋯ units [29–31]. This is approved
by the single crystal structure determination of the VOL2 (Fig. 1).
The molecular structure of 1 with the atom-numbering scheme is
presented in Fig. 1. The crystallographic data revealed that the vana-
dium (IV) ion was surrounded by two oxygen and two nitrogen
atoms from two Schiff base ligands and one oxygen atom from an
oxo ligand in the five-coordinated geometry. The two ligands coordi-
nate 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 dis-
tances and angles: i.e. V\O1=1.8968(15), V\O2=1.9131(14),
V\N1=2.1079(16), V\N2=2.1085(16) and V\O3=1.5925(15) Å
and O(3)\V\O(1)=112.24(8), O(3)\V\O(2)=111.12(8), O(1)\
V\O(2)=136.63(8), O(3)\V\N(1)=99.64(7), O(1)\V\N(1)=
87.00(7), O(2)\V\N(1)=86.73(7), O(3)\V\N(2)=100.31(7),
O(1)\V\N(2)=84.84(7), O(2)\V\N(2)=86.77(6) and N(1)\
V\N(2)=160.04(7)°. All distances and angles in 1 agree well with
the same distances in other vanadium (IV) complexes [32–35]. Mo-
lecular packing of the 1 can be viewed molecules held together by
non-classical C\H⋯O intermolecular hydrogen bonds (Fig. 2, Table 1).
The catalytic activity of the new vanadyl Schiff base complex VOL2
(1) was tested in the epoxidation of cyclooctene [36]. Different pa-
rameters such as the effect of solvent, oxidant, catalyst amount and
the alkene/oxidant ratio were studied in the epoxidation of cyclooc-
tene. Fig. 3 and Table 2 show the results of the catalytic activity of 1
in the epoxidation of cyclooctene in different solvents in the presence
of tert-butylhydroperoxide (TBHP). As it is seen, the catalytic activity
of 1 in CHCl3 and CCl4, with low coordination ability, is higher than in
MeOH, THF, CH3CN and H2O/CH3CN with high coordination ability.
The different reaction media were used to obtain the suitable oxidant
in the epoxidation of cyclooctene (Table 3). The high yield was
obtained in the presence of TBHP in CHCl3. Then the alkene/oxidant
Fig. 1. Molecule of 1, showing the atom numbering schemes. Displacement ellipsoids are drawn at the 50% probability level.

G. Grivani et al. / Inorganic Chemistry Communications 18 (2012) 15–20
17
Fig. 2. Crystal packing of 1, showing inter-molecular hydrogen bonds (dashed lines).
ratio was also investigated (Fig. 4 and Table 4). Among the 1:4, 1:3
and 1:2 ratios, the 1:3 ratio showed the maximum epoxidation
yield. We also checked the effect of changing the amount of Schiff
base complex 1 on the epoxidation of cyclooctene(Table 5). Three
amounts of 1 (0.009, 0.013 and 0.019 mmol) were used in the epoxi-
dation of cyclooctene in CHCl3 in the presence of TBHP but the maxi-
mum epoxidation yield was made by 0.013 mmol of 1 (Fig. 5). In
conclusion the catalytic activity and selectivity of the vanadyl Schiff
base complex 1 in optimized conditions with TON of 32 (mol of
cycloocten converted by mol of catalyst) is high (84% conversion
and 100% cyclooctene epoxide yield).
Recently the catalytic activities of some five coordinated oxovana-
dium(IV) Schiff base complexes (VOL) with teradentate N2O2 or tri-
dentate NO2 ligands in cyclooctene epoxidation reaction were
investigated [37–39]. The investigation results of Rahchamani et al.
in the catalytic activities of some oxovanadium(IV) Schiff base com-
plexes in cyclooctene epoxidation reaction with TBHP and H2O2 in
CH3CN showed the higher conversion of cyclooctene by the TBHP
than H2O2 with the 58% conversion as the highest conversion [37].
Monfared et al. observed the trend: CH3CN>MeOH>EtOH>
THF>(CH3)2CO>CHCl3 >MeCOEt>CCl4 >DMF in investigation of
the catalytic activities of three oxovanadium hydrazone Schiff base
complexes in epoxidation of the cyclooctene with H2O2 in these sol-
vents [38]. Rayati et al. [39] have also investigated the catalytic activ-
ities of two oxovanadium(IV) Schiff base complexes in cyclooctene
epoxidation reaction with TBHP by using Chloroform, acetonitrile
and dichloromethane as solvent and obtained the highest conversion
(94–95%) in the chloroform. In summary based on the results of
these researches the high epoxidation yield by related oxovanadium
Schiff base complexes in the presence of TBHP is obtained in the sol-
vents CCl4 and CHCl3 that our results consist with them.
trend in the solvent effect of cyclooctene epoxidation reaction [40]:
CCl4 >CHCl3 >CH3CN>CH3CN/H2O>THF>MeOH. But in this re-
search the activity of Schiff base complex 1 in CHCl3 is higher than
in CCl4. Thus it seems that the substitution of the ethyl bromide
chain with ethyl chloride inverses (or decreases) the catalytic activity
of the Schiff base complex 1 in CCl4 respect to the CHCl3.
The mechanistic aspects of the epoxidation reaction of oxovana-
dium complexes were reviewed recently by Conte et al. [41]. The pro-
posed mechanism of epoxidation reaction by oxovanadium (IV) Schiff
base complex 1 in the presence of TBHP was showed in Scheme 2. It is
established that in epoxidation reaction catalyzed by oxovanadium
complexes in the presence of TBHP the active species are the II [41].
The titration curve of Schiff base complex 1 (VOL2) with TBHP was
monitored by electronic absorption spectroscopy. These spectral
90
80
70
60
50
40
30
20
10
0
Recently we have investigated the catalytic activity of the oxova-
nadium(IV) Schiff base complex similar to 1 in that the ethyl chloride
chain was substituted with ethyl bromide and observed the similar
THF
CCl4
MeOH
CHCl3
CH3CN
CH2Cl2
CH3CN/H2O
Table 1
Solvent
Experimental hydrogen bond geometries (Å, °) of 1.
D-H⋯A
D-H
H⋯A
D⋯A
D-H⋯A
Fig. 3. The catalytic epoxidation of cyclooctene in different solvents in the presence of
TBHP by the vanadyl Schiff base complex 1 in the reflux conditions. Reaction condi-
tions: (5 ml) solvent, (0.5 mmol) cyclooctene, (1.5 mmol) oxidant and (0.013 mmol)
complex 1.
C11_H11a⋯O3
C12_H12⋯O3
0.970
0.931
2.441
2.564
3.395
3.446
167.491
158.373

18
G. Grivani et al. / Inorganic Chemistry Communications 18 (2012) 15–20
Table 2
Table 4
The solvent effect on the catalytic epoxidation of cyclooctene in the presence of TBHP
The effect of alkene/oxidant ratio in the epoxidation of cyclooctene in CHCl3 as solvent
by the vanadyl Schiff base complex 1 in the presence of the TBHP in the reflux
conditionsa.
by the vanadyl Schiff base complex 1 in the reflux conditionsa.
Solvent
Time (min)
% Conversion
Run
Solvent
Time
(min)
% conversion
1:2b
CHCl3
CCl4
CH3CN
CH2Cl2
CH3CN/H2O(3:2)
THF
240
240
240
240
240
240
240
84
69
63
17.5
14
11
4
1:3b
1:4b
1
2
3
4
5
6
7
8
9
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
30
60
90
120
150
180
210
240
270
33
55
64
69
72
73
74
75
75
36
58
66
71
77
80
82
83
84
50
68
79
81
–
MeOH
Reaction conditions: (5 ml) solvent, (0.5 mmol) cyclooctene, (1.5 mmol) oxidant and
83
79
76
76
(0.013 mmol) complex 1.
a
Reaction conditions.
Table 3
Reation conditions: (5 ml) solvent, (0.5 mmol) cyclooctene, (2 mmol, 1.5 mmol and
1 mmol for 1:4, 1:3 and 1:2 ratios, respectively) oxidant and (0.013 mmol) complex 1.
Alkene/oxidant ratio.
Epoxidation of cyclooctene in the presence of different oxidants by the vanadyl Schiff
base complex 1 in different conditions by 1.a
a
Reaction conditions.
Solvent
Oxidant
Time (min)
% conversionb
b
Alkene/oxidant ratio.
CH3CN/H2O (3:2)
CCl4
MeOH
CHCl3
THF
CH3CN
CH3CN/H2O (3:2)
CHCl3
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
NaIO4
TBHP
240
240
240
240
240
240
240
240
19
No reaction
7
1
No reaction
No reaction
6
84
Table 5
The effect of the amount of the vanadyl Schiff base complex 1 in the epoxidation of
cyclooctene in CHCl3 as solvent in the presence of the TBHP in reflux conditionsa.
Run
Solvent
Time
(min)
% conversion
0.009b
0.013b
0.019b
a
Reaction conditions: (5 ml) solvent, (0.5 mmol) cyclooctene, oxidant, (1.5 mmol),
and (0.013 mmol) complex 1.
1
2
3
4
5
6
7
8
9
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
30
60
90
120
150
180
210
240
270
26
47
60
67
73
76
79
–
36
58
66
71
77
80
82
83
84
46
68
71
76
79
80
79
–
b
Based on GLC.
changes are presented in Fig. 6. The titration of a solution of the oxo-
vanaduim(IV) Schiff base complex 1 (VOL2) in chloroform (ca.
10−4 M solution) by addition of 5 μl of TBHP resulted in slow decreas-
ing of intensity of the band at 352 nm and gradual appearance of two
new bands in 328 and 305 nm and increasing of the intensities of
these new bands by continuation of titration. At the end of titration
the band at 352 nm was disappeared. At the same time, the bands
appearing at 591 and 529 nm due to the d–d transitions of [VIVOL2]
slowly disappeared upon addition of TBHP. In similar to the results
observed in the literature [42–44], these spectral changes and the
presence of an isosbestic point at 337 nm suggest the oxidation of
VIV and the interaction of the vanadium(V) complex formed with
TBHP, to give oxoperoxovanadium(V) species. In addition the FT-IR
spectrum of the complex at the end of the epoxidation reaction of
cyclooctene in chloroform in the presence of TBHP resulted in disap-
pearing of the band at 984 cm−1 due to VIV_O stretching vibration
frequencies and appearing of a new band at 1018 cm−1 that attribut-
ed to VV_O stretching vibration frequencies due to the formation of
VV_O in species such as II.
–
–
Reaction conditions: (5 ml) solvent, (0.5 mmol)cyclooctene, (1.5 mmol) oxidant and
(0.019, 0.013 and 0.009 mmol) complex 1.
mmol of catalyst.
a
Reaction conditions.
mmol of catalyst.
b
The observed trend in Fig. 3 for solvent effect also agrees with this
mechanism. As the coordination ability of the solvent is increased, the
solvent binding instead of TBHP binding to vanadium center is also
increased and the formation of species III is prevented and retarded
the progress of epoxidation reaction.
90
80
70
60
0.019mmol
50
0.013mmol
40
30
20
10
0
0.0009mmol
0
50
100
150
200
250
300
time(min)
Fig. 4. The effect of alkene/oxidant ratio in the epoxidation of cyclooctene in CHCl3 as
solvent by the vanadyl Schiff base complex 1 in the reflux conditions. Reaction condi-
tions: (5 ml) solvent, (0.5 mmol) cyclooctene, (2 mmol, 1.5 mmol and 1 mmol for
1:4, 1:3 and 1:2 ratios, respectively) oxidant and (0.013 mmol) complex 1.
Fig. 5. The effect of the amounts of the vanadyl Schiff base complex 1 in the epoxida-
tion of cyclooctene in CHCl3 as solvent in reflux conditions. Reaction conditions:
(5 ml) solvent, (0.5 mmol) cyclooctene, (1.5 mmol) oxidant and (0.019, 0.013 and
0.009 mmol) complex 1.

G. Grivani et al. / Inorganic Chemistry Communications 18 (2012) 15–20
19
3
2.5
2
obtained free of charge on application to The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK, fax: +44 1223 336 033, e-mail:
deposit@ccdc.cam.ac.uk or http:www.ccdc.cam.ac.uk.
0.025
0.02
0.015
0.01
0.005
0
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Scheme 2. The proposed mechanism for the epoxidation of alkenes by the oxovanadium complex VOL2(1) in the presence of TBHP.

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5.37; N, 7.48%. FT-IR (KBr pellet, cm−1): v (C_N) 1640 cm−1 1H-NMR(CDCl3;
.
δ, ppm):13.00 (s, 1H: \O\H), 8.39 (s, 1H: \HC_N\), 7.34 (t, 1H: Ph\H),
7.30(d, 1H:Ph\H), 6.99 (d, 1H:Ph\H), 6. 90 (t, 1H: Ph\H), 3.92 (t, 2H:
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crystals were filtered off and washed with n-hexane and dried in an oven.
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18H18N2O3ClV: C, 50.00; H, 4.16; N, 6.48. Found: C, 49.48; H, 4.02; N, 6.27%. FT-
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