Electronic effects of substituents on the oxidation potentials of vanadyl complexes with tetradentate Schiff base ligands derived from 1,2-propylenediamine

Article abstract of DOI:10.1016/j.catcom.2010.02.017

Two tetradentate Schiff base ligands (H₂L^1-2) (H₂L¹ = bis(2-hydroxy-3-methoxy-benzaldehyde)-1,2-propandiimine, H₂L² = bis(2-hydroxy-4-methoxy-acetophenone)-1,2-propandiimine) were prepared

Full text of DOI:10.1016/j.catcom.2010.02.017

Catalysis Communications 11 (2010) 792–796
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Electronic effects of substituents on the oxidation potentials of vanadyl
complexes with tetradentate Schiff base ligands derived from 1,2-propylenediamine
b
a
Saeed Rayati a, , Akbar Ghaemi , Nasrin Sadeghzadeh
*
a Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran, Iran
b Department of Chemistry, Islamic Azad University of Saveh Branch, P.O. Box 39187-366, Saveh, Iran
a r t i c l e i n f o
a b s t r a c t
Two tetradentate Schiff base ligands (H2L1–2) (H2L1 = bis(2-hydroxy-3-methoxy-benzaldehyde)-1,2-pro-
pandiimine, H2L2 = bis(2-hydroxy-4-methoxy-acetophenone)-1,2-propandiimine) were prepared by
reaction of 1,2-propylenediamine and o-hydroxycarbonyls compounds containing a methoxy group
and characterized by elemental analysis, FT-IR, 1H and 13C NMR. The vanadyl complexes were synthe-
sized and characterized. The catalytic potential of these complexes was tested for the oxidation of cyclo-
octene and styrene using tert-butylhydroperoxide (TBHP) as oxidant. It has been shown that the presence
of electron-donating substituents on the aromatic ring as well as the imine bond can effectively improve
the catalytic activity and the product selectivity of catalysts.
Article history:
Received 24 December 2009
Received in revised form 12 February 2010
Accepted 15 February 2010
Available online 19 February 2010
Keywords:
Schiff base
Oxovanadium
Catalyst
Ó 2010 Elsevier B.V. All rights reserved.
Olefins
tert-Butylhydroperoxide
1. Introduction
2. Experimental
The oxidation of hydrocarbons using transition metals Schiff
base complexes has attracted both academic and industrial interest
[1]. They are efficient catalysts in homogeneous and heterogeneous
conditions. The catalytic activity of these complexes depends on
the nature of substituents as well as the metal centre [1–4]. The
interest in vanadium coordination chemistry promoted by the
presence of this element in biological systems [5], and by catalytic
[4], inhibitory [6], medicinal [7,8] and structural [9–11] properties
of its compounds. The potential catalytic abilities of vanadium
compounds have lead to an increasing interest in vanadium coor-
dination chemistry in recent years [2]. Homogeneous catalysts of
oxovanadium(IV) complexes have been shown to induce organic
reactions such as the oxidation of sulfides to sulfoxides and sulf-
ones [12–14], the epoxidation of alkenes [15–18], the hydroxyl-
ation of hydrocarbons [19], and the oxidation of alcohols to
aldehydes and ketones [20,21]. These studies are indicative that
oxovanadium(IV) complexes are potential catalysts to influence
the yield and selectivity in chemical transformation.
2.1. Instruments and reagents
All chemicals were supplied by either Merck or Fluka. Solvents
(ethanol and acetonitrile) were dried and distilled by standard
methods before use [22]. Chemicals were used as received. Infrared
spectra were recorded as KBr pellets using a Unicam Matson 1000
FT-IR. Elemental analyses (C, H, N) were performed using a Heraeus
Elemental Analyzer CHN-O-Rapid (Elemental-Analyze system KBr
pellets, Gmbh, West Germany). 1H and 13C NMR spectra were ob-
tained in CDCl3 solution on a Bruker FT-NMR 250 (250 MHz) spec-
trometer. A varian (AA220) flame atomic absorption spectrometer
was used for vanadium determination. A Metrohm 757 VA Compu-
trace was employed to evaluate electrochemical measurements at
room temperature (25 °C) under nitrogen with 0.1 M tetrabutyl-
ammonium hexafluorophosphate (TBAH) as the supporting elec-
trode. The reaction products of oxidation analyzed by HP Agilent
6890 gas chromatograph equipped with a HP-5 capillary column
(phenyl methyl siloxane 30 m  320
lm  0.25
lm) and flame-
In continuation of our efforts to develop new oxovanadium(IV)-
based homogeneous oxidation catalysts, we report herein the syn-
thesis of two new oxovanadium(IV) Schiff base complexes with
electron donor substituents that exhibited higher potential cata-
lytic activity and product selectivity for the oxidation of cyclooc-
tene and styrene with tert-butylhydroperoxide (TBHP).
ionization detector.
2.2. Synthesis of ligands
2.2.1. Bis(2-hydroxy-3-methoxy benzaldehyde)-1,2-propandiimine
(H2L1)
H2L1 was prepared according to the described procedure [23].
To a stirred ethanolic solution (20 ml) of 1,2-propylenediamine
* Corresponding author. Tel.: +98 21 22850266; fax: +98 21 22853650.
E-mail addresses: srayati@yahoo.com, rayati@kntu.ac.ir (S. Rayati).
1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.02.017

S. Rayati et al. / Catalysis Communications 11 (2010) 792–796
793
(0.074 g, 1 mmol), 2-hydroxy-3-methoxybenzaldehyde (0.308 g,
2 mmol) was added. The bright yellow solution was stirred and
heated to reflux for 1 h. A yellow precipitate was obtained that
was filtered off and washed with diethyl ether. Yield (95%), melting
point 125 °C. Analysis calculated for C19H22N2O4 (342.2): C, 68.68;
H, 6.42; N, 8.18. Found: C, 68.93; H, 6.01; N, 8.72%. Selected FT-IR
have been shown in Fig. 1. The melting points, yields, and elemen-
tal analyses for the complexes are given in Table 1.
2.4. General oxidation procedure
Catalytic experiments were carried out in a 50 ml round bottom
flask fitted with a water condenser. In a typical procedure,
0.032 mmol vanadyl complex was dissolved in 10 ml solvent (chlo-
roform, acetonitrile or dichloromethane). Then 10 mmol alkene
(cyclooctene or styrene) was added to the reaction mixture and
30 mmol TBHP was added. The reaction mixture was refluxed for
6 h. The oxidation products were identified by comparison with
authentic samples (retention times in GC).
data,
t
(cmÀ1): 3423 (O–H), 2923–3015 (C–H), 1630 (C@N), 1561
(C@C), 1031 (C–O). 1H NMR (d): 1.39, 1.41 (d, 3H, NCH2CH(CH3)N),
3.58, 3.65 (d, 2H, NCH2CH(CH3)N), 3.65–3.93 (m, 1H, NCH2-
CH(CH3)N), 4.03, 4.16 (s, 6H, OMe), 6.44–6.90 (m, 6H, ArH), 8.27,
8.31 (s, 2H, HC@N), 13.70, 13.78 (s, 2H, OH). 13C{1H}NMR (d):
20.4 (NCH2CH(CH3)N), 55.9 (NCH2CH(CH3)N), 56.0 (NCH2CH-
(CH3)N), 64.7, 65.5 (OCH3), 113.9, 151.3 (aromatic C), 164.5,
166.4 (C@N).
3. Results and discussion
2.2.2. Bis(2-hydroxy-4-methoxy acetophenone)-1,2-propandiimine
(H2L2)
3.1. Characterization of the ligands and oxovanadium(IV) complexes
To a stirred ethanolic solution (20 ml) of 1,2-propylenediamine
(0.074 g, 1 mmol), 2-hydroxy-4-methoxyacetophenone (0.304 g,
2 mmol) was added. The mixture was stirred and heated to reflux
for 1 h. A yellow precipitate was obtained that was filtered off and
washed with diethyl ether. Yield (90%), melting point 130 °C. Anal-
ysis calculated for C21H26N2O4 (370.2): C, 68.12; H, 7.02; N, 7.56.
3.1.1. IR spectral studies
A practical list of IR spectral data is presented in Table 2. Com-
parison of the spectra of the complexes with the ligands provides
evidence for the coordination mode of ligand in catalysts. The
Schiff base ligands exhibit a broad band around 2969–3430 cmÀ1
region due to the presence of extensive hydrogen bonding between
phenolic hydrogen and azomethine nitrogen atoms. Absence of
this band in the spectra of complexes indicates the breaking of
hydrogen bonding followed by coordination of phenolic oxygen
to the metal ion after deprotonation. A sharp band appearing at
Found: C, 68.36; H, 6.82; N, 7.83%. Selected FT-IR data,
t
(cmÀ1):
3400 (O–H), 2916–3000 (C–H), 1623 (C@N), 1560 (C@C), 1160
(C–O). 1H NMR (d): 1.39, 1.50 (d, 3H, NCH2CH(CH3)N), 2.50, 2.60
(d, 2H, NCH2CH(CH3)N), 3.40–3.89 (m, 1H, NCH2CH(CH3)N), 3.76,
3.81 (s, 6H, O7.02–6.82Me), 6.24–7.70 (m, 6H, ArH), 2.25, 2.35 (s,
6H, (CH3)C@N), 12.75, 12.81 (s, 2H, OH). 13C{1H}NMR (d): 14.24,
1623–1630 cmÀ1 due to
t(C@N) (azomethine), shifts to lower wave
number by 15–16 cmÀ1 and appears at 1607–1623 cmÀ1. This indi-
cates the involvement of azomethine nitrogen in coordination. IR
14.43
(NCH2CH(CH3)N),
54.6
(NCH2CH(CH3)N),
53.7
(NCH2CH(CH3)N), 55.2 (OCH3), 101.9, 168.8 (aromatic C), 170.5,
172.0 (C@N), 20.0 ((CH3)C@N).
spectra of VOL1 shows
t
(V@O) at 969 cmÀ1 and for VOL2 at
984 cmÀ1. Tetradentate Schiff base oxovanadium(IV) complexes
with t
(V@O) around 970 cmÀ1 are in monomeric form. Thus, both
2.3. Preparation of vanadyl complexes (VOLx (x = 1–2))
of VOL1 and VOL2 are assigned to have monomeric structure [24].
The complexes were prepared by a general procedure: the li-
gand, H2L1 (0.34 g, 1 mmol) or H2L2 (0.40 g, 1 mmol) was dissolved
in 30 ml of ethanol. An ethanolic solution of oxobis(pentane-2,4-
dionato)vanadium(IV) (0.265 g, 1 mmol) was added and the reac-
tion mixture was refluxed for 2 h. The colored solution was con-
centrated to yield colored powders. The products washed with
warm ethanol. General structure of oxovanadium(IV) complexes
3.1.2. Electrochemical studies
The electrochemical properties of the VO–Schiff base complexes
(VOL1 and VOL2) were investigated in CH3CN, using TBAH (0.1 M)
as the supporting electrolyte by cyclic voltammetry. Fig. 2 shows
the cyclic voltammograms of 0.01 mmol of VOL1 complex in aceto-
nitrile at various scan rates. Both vanadyl complexes exhibit a
Y
X
X
Y
O
V
O
N
O
N
Complex
R
X, Y
VOL1
H
X: OMe, Y: H
X: H, Y: OMe
VOL2
Me
R
R
Me
Fig. 1. Oxovanadium(IV) complexes used in this study.
Table 1
Physical and analytical data of the complexes.
Compound formula
Formula weight
Yield (%)
Color
Found (calculated)
%C
%H
%N
%V
VOL1
VOL2
VC19H20N2O5
VC21H24N2O5
406.9
432.7
90
85
Green
Green
56.39 (56.08)
58.03 (58.34)
4.61 (4.91)
5.14 (5.55)
6.93 (6.88)
6.79 (6.47)
12.46 (12.51)
11.89 (11.77)

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S. Rayati et al. / Catalysis Communications 11 (2010) 792–796
Table 2
Table 3
IR spectral data of ligands and the vanadium complexes.
Electrochemical data for the oxidation of oxovanadium(IV) complexes in CH3CN.
Eap (V)
Ecp (V)
Compound
Selective IR bands (cmÀ1
)
Complex
E1/2 (V)
DEp (V)
VOL1
VOL2
0.614
0.595
0.504
0.490
0.559
0.545
0.110
0.105
V@O
C@N
H2L1
H2L2
VOL1
VOL2
–
–
969
984
1630
1623
1615
1607
The same result was obtained using VOL2 and in the 3:1 M ratio
(TBHP/cyclooctene), the maximum conversion (99.8%) was
achieved. Another important feature of this catalytic system with
respect to the similar systems [26] is the excellent product selec-
tivity (100%) for epoxide that may be a result of using electron rich
vanadyl Schiff base complexes.
The influence of the solvent nature in the catalytic epoxidation
of cyclooctene by VOL1 and VOL2 has been investigated. Chloro-
form, acetonitrile and dichloromethane were used as solvent and
the highest conversion (98% in VOL1 and 98.8% in VOL2) was ob-
tained in chloroform. It was observed that the catalytic activity
of the vanadyl complexes decreases in order chloroform > acetoni-
trile > dichloromethane. Hundred percentage selectivity for epox-
ide formation was obtained for all solvents. More efficient
oxidation of cyclooctene in chloroform relative to dichloromethane
at reflux condition seems to be due to the higher boiling point of
chloroform. The decreased conversion of cyclooctene in acetoni-
trile with respect to that of chloroform, in spite of the higher b.p
of acetonitrile may be due to the donor ability of acetonitrile which
can compete with the oxidant in coordination to the metal centre
(see the proposed mechanism, Fig. 3).
reversible electrochemical behavior as shown in the cyclic voltam-
mograms of Fig. 2. Redox potentials for two complexes are given in
Table 3. Introduction of electron-donating substituents instead of
electron-withdrawing ones [14] decreases the oxidation potential
of V(IV).
The presence of two methoxy substituents at ortho positions of
VOL1 phenyl groups enhances the electron densities on the ligating
atoms of the ligand through
p-donation to the phenyl groups. It
should be noted that the methoxy substituent enhances the elec-
tron density at the ortho and para positions with respect to the sub-
stituent. Therefore, the ‘‘O” atoms of VOL1 are expected to be more
affected by the p-donation of methoxy groups whereas in the case
of VOL2 the ‘‘N” atoms are the more affected one. Furthermore, in
VOL2, there are two methyl groups at imine moieties of the ligand
as well as the two methoxy residues at the phenyl rings. In addi-
tion to the
may also be considered as a
p-donation from –OMe to the ligand, methyl group
p-donor due the hyperconjugation ef-
fect of this group [25]. Accordingly, the metal centre of VOL2 is ex-
pected to be more electron-rich than that of VOL1 (vide infra) and
the redox potentials of V(V)/V(IV) shifts in the cathodic direction
and the complex become easier to oxidize.
In order to get the best reaction temperature, reaction mixture
was stirred at 40 °C, 50 °C and 60 °C and for both catalysts, a max-
imum of conversion of cyclooctene were obtained at 60 °C (93.6%
for VOL1 and 98.8% in VOL2).
3.2. Catalytic activity studies
3.2.2. Oxidation of styrene
3.2.1. Oxidation of cyclooctene
The oxidation of styrene with VOL1 and VOL2 gave styrene oxide
as dominant product and benzaldehyde as by product. Reaction
temperature, solvent and oxidant/styrene molar ratio have been
optimized. In order to obtain the best oxidant/styrene molar ratio,
three different TBHP/styrene molar ratios (0.5:1, 1:1, 2:1 and 3:1)
were considered while keeping the fixed amount of styrene (1.04 g,
10 mmol) and catalyst (0.032 mmol) in 10 ml of CH3CN for the
reaction at 60 °C. A maximum of 48% conversion for VOL1 (with
87.5% selectivity for epoxide formation) and 88% conversion for
VOL2 (with 99% selectivity for epoxide formation) was obtained
at 3:1 M ratio. Further increment in oxidant concentration hardly
improved the conversion. Table 4 shows product selectivity and
percent conversion of styrene with both catalysts. Oxidation of sty-
rene has been influenced by the nature of solvent. Three different
The oxidation of cyclooctene, catalyzed by VOL1 and VOL2, was
carried out using tert-butylhydroperoxide as an oxidant to give
cyclooctene oxide as the sole product. To achieve the maximum
epoxide yield, several parameters such as, the effect of oxidant
concentration (moles of TBHP per moles of cyclooctene), solvent
and temperature of the reaction have been optimized. Different
TBHP/cyclooctene molar ratios (0.5:1, 1:1, 2:1 and 3:1) were con-
sidered while keeping the fixed amount of cyclooctene (1.102 g,
10 mmol) and catalyst VOL1 (0.013 g, 0.032 mmol) in 10 ml of
CHCl3 at 60 °C. Increasing the TBHP/cyclooctene ratio from 0.5:1
to 3:1 the conversion increases from 32% to 94%, probably due to
fast formation of active intermediate in the presence of higher
amount of oxidant.
40
40
30
A
B
30
20
c
b
a
c
20
b
10
0
a
10
0
-10
-20
-30
-10
-20
-30
0.1
0.3
0.5
0.7
0.9
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
mV
mV
Fig. 2. Cyclic voltammograms of 0.01 mmol VOL1 (A) and VOL2 (B) in acetonitrile containing 0.1 M TBAH as supporting electrolyte. Scan rates (mV/s) are (a) 20, (b) 50 and (c)
100, respectively.

S. Rayati et al. / Catalysis Communications 11 (2010) 792–796
795
TBHP
TBHP TBOH
OH
O
O
V V
V V
VIV
(I)
O
O
t-Bu
(II)
TBOH
OH
OH
OH
V V
V V
V V
O
O
O
O
O
t-Bu
O
t-Bu
t-Bu
Fig. 3. Proposed catalytic cycle for the oxidation of cyclooctene (or styrene) by VOL1 or VOL2.
Table 4
Oxidation of styrene and cyclooctene with TBHP in the presence of VOL1 and VOL2.a
Catalyst
Olefin
Conversion (%)
TONb (Total)
Product selectivity (%)
Epoxide
Benzaldehyde
None
VOL1
VOL2
None
VOL1
VOL2
Styrene
Styrene
Styrene
Cyclooctene
Cyclooctene
Cyclooctene
10
48
88
4
94
99.8
3.1
45
87.5
99
55
12.5
1
150
275
1.2
293
312
100 (cyclooctene oxide)
100 (cyclooctene oxide)
100 (cyclooctene oxide)
a
Solvent: 10 ml; catalyst: 0.032 mmol; styrene and/or cyclooctene: (10 mmol); TBHP: 30 mmol.
TON: (total turnover number) the ratio of the number of moles of product to the number of moles of catalyst.
b
solvents (chloroform, dichloromethane and acetonitrile) were
used. It was observed that the catalytic activity of the vanadyl com-
plexes decreases in order acetonitrile > chloroform > dichloro-
methane. Interestingly, in the case of styrene, acetonitrile is
better than chloroform which may be partly due to the higher
other TBHP coordinate to the V(V)@O species to form tert-buty-
lOOVV–OH intermediate (Fig. 3, II) that is probably the active
oxidizing species, which can react with olefin to produce epoxide.
Titration of oxovanadium(IV) complexes in chloroform solution
with TBHP has been done and the reaction was monitored by UV–
Vis spectroscopy. Upon the addition of TBHP, the weak broad band
due to d–d transition (574 nm for VOL1 and 554 nm for VOL2),
slowly become weaker and finally disappears. This observation
suggests the formation of peroxovanadium(V) species [28] which
is in accord with the previously reported studies by Maurya et al.
[19,29].
The higher conversion and selectivity obtained in the presence
of the title electron rich vanadyl complexes in comparison with
other reports [17,26] suggests that the catalytic activity signifi-
cantly increases with a decrease in the V5+/V4+ potential. Therefore,
it seems that the oxidation of VIV@O to VV@O occur in rate limiting
step.
b.p of acetonitrile (82 °C) and also dielectric constant
(e/
e
0 = 35.94) and dipole moment ( = 3.90 D) which are the highest
l
of all the solvents used. Also, it is proposed that the involvement
of an intermediate with some carbocation character in the catalytic
cycle of styrene cause the observed trend. This assumption is
according to the stability of carbocation of styrene due to the con-
jugation with the phenyl ring in organic reaction. The presence of
solvent such as acetonitrile which can stabilize the cationic centres
increases the rate of reactions bearing intermediates with more
cationic character.
In order to get the best reaction temperature, the reaction mix-
ture was stirred at various temperatures (40, 50, 60, 70 and 80 °C)
and the obtained results show that the maximum conversion was
obtained at 80 °C.
In oxidation of styrene, the benzaldehyde formation is possibly
due to over-oxidation of styrene oxide by nucleophilic attack of
hydroperoxide [30].
The reusability of the catalyst was examined. The catalyst can
be reused four times without a detectable catalyst leaching or a
significant loss of its activity.
4. Conclusions
3.2.3. Proposed mechanism
In the proposed reaction mechanism oxovanadium(IV) oxidized
to oxovanadium(V) (Fig. 3, I) in the presence of TBHP [27] and an-
VOL1 and VOL2 complexes were prepared and characterized.
Oxidation of cyclooctene and styrene with tert-butylhydroperoxide
(TBHP) in the presence of the title complexes shows that the intro-

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S. Rayati et al. / Catalysis Communications 11 (2010) 792–796
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duction of electron-donating substituents on the aromatic rings as
well as the imine bonds of salene type Schiff bases effectively im-
prove their catalytic activity. The catalytic performance of vanadyl
complexes depends on the oxidant-to-olefin molar ratio, the type
of solvent and the reaction temperature. The conversion of cyclooc-
tene to cylcooctene oxide was 98% for VOL1 and 98.8% for VOL2 and
a selectivity of 100% was attained. In the case of styrene, 48–88%
conversion to epoxide and benzaldehyde has been observed.
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We gratefully acknowledge practical support of this study by
K.N. Toosi University of Technology and Islamic Azad University
of Saveh Branch.
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