A new vanadium Schiff base complex as catalyst for oxidation of alcohols

Article abstract of DOI:10.1080/00958972.2010.520316

The monoanionic bidentate Schiff base, N-(phenolyl)-benzaldimine (HL), has been employed to synthesize a new vanadium(IV) complex of general composition [VO(L)₂] (where L = O, N donor of Schiff base). The ligand and complex have been fully characterized by elemental analyses, molar conductance data, FT-IR, ¹H- and ¹³C-NMR, and UV-Vis spectroscopies. Oxidation of alcohols to their corresponding aldehydes and ketones was conducted by this complex catalyst using Oxone as oxidant under biphasic reaction conditions (CH₂Cl₂/H₂O) and tetra-n-butylammonium bromide as phase transfer agent under air at room temperature.

Full text of DOI:10.1080/00958972.2010.520316

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A new vanadium Schiff base complex as
catalyst for oxidation of alcohols
Mojtaba Bagherzadeh a & Mojtaba Amini a
a Department of Chemistry, Sharif University of Technology, PO
Box 11155‐3615, Tehran, Iran
Version of record first published: 13 Oct 2010.
To cite this article: Mojtaba Bagherzadeh & Mojtaba Amini (2010): A new vanadium Schiff base
complex as catalyst for oxidation of alcohols, Journal of Coordination Chemistry, 63:21, 3849-3858
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Journal of Coordination Chemistry
Vol. 63, No. 21, 10 November 2010, 3849–3858
A new vanadium Schiff base complex as catalyst for
oxidation of alcohols
MOJTABA BAGHERZADEH* and MOJTABA AMINI
Department of Chemistry, Sharif University of Technology, PO Box 11155-3615,
Tehran, Iran
(Received 3 May 2010; in final form 19 July 2010)
The monoanionic bidentate Schiff base, N-(phenolyl)-benzaldimine (HL), has been employed
to synthesize a new vanadium(IV) complex of general composition [VO(L)2] (where L ¼ O, N
donor of Schiff base). The ligand and complex have been fully characterized by elemental anal-
yses, molar conductance data, FT-IR, 1H- and 13C-NMR, and UV-Vis spectroscopies.
Oxidation of alcohols to their corresponding aldehydes and ketones was conducted by this
complex catalyst using Oxone as oxidant under biphasic reaction conditions (CH2Cl2/H2O) and
tetra-n-butylammonium bromide as phase transfer agent under air at room temperature.
Keywords: Bidentate Schiff base; Vanadium complex; Catalytic oxidation; Oxone
1. Introduction
Selective oxidation of alcohol to aldehydes and ketones is an important reaction in the
laboratory and industry [1–4]. Traditionally, oxidizing reagents including dichromate
and permanganate have been employed in order to accomplish this transformation
[5, 6]. These reagents are moderately expensive, toxic, corrosive, unstable, without
selectivity, dangerous to prepare, moisture-sensitive and tend to over-oxidize to
carboxylic acids [7–9].
Transition metal complexes as catalysts in organic synthesis is an exciting field of
research, and numerous reactions which are impossible to achieve by conventional
synthetic methods have already been discovered [10–12]. There has been considerable
interest in the chemistry of transition metal complexes of Schiff bases, due to the fact
that Schiff bases stabilize many different metals in various oxidation states, controlling
the performance of metals in useful transformations. Bidentate Schiff bases containing
imine groups which can be used as modulators of structural and electronic properties of
transition metal centers represent an important class of chelating ligands [13, 14].
Vanadium is an essential metal and exhibits great biological activity and potential
medicinal applications when associated with certain metal–protein complexes,
participating in oxygen transport, electron transfer reactions, or the storage of ions
[15–19], creating enormous interest in study of systems containing this metal [20–28].
*Corresponding author. Email: bagherzadehm@sharif.edu
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.520316

3850
M. Bagherzadeh and M. Amini
Oxone, catalyst, n-Bu NBr
2
RR„CO
RR„CHOH
CH Cl /H O, 5 min, r.t.
2
2
2
R, R„= alkyl, allyl, benzyl and H
O
V
N
O
O
catalyst=
N
Scheme 1. The oxidation of alcohols by the [VO(L)2]/Oxone/n-Bu4NBr catalytic system.
However, activity of Schiff-base vanadium(IV) complexes toward catalytic oxidation in
a homogeneous medium is not well documented with only a few vanadium(IV)
Schiff-base complexes reported to be catalytically active toward alcohol oxidation [29].
In this article, we report the synthesis and characterization of the ligand, obtained
from reaction of benzaldehyde with o-aminophenol, and its vanadium complex by
physico-chemical methods and demonstrate the efficient and selective oxidation of
alcohols to their corresponding aldehydes and ketones, using this bidentate Schiff-base
vanadium complex under mild and biphasic conditions (CH2Cl2/H2O) at room
temperature with Oxone as oxidant (scheme 1).
2. Experimental
Chemical materials and solvents were purchased from Chemical companies, Fluka and
Merck.
Elemental analyses (carbon, hydrogen, and nitrogen) were obtained from a Carlo
ERBA Model EA 1108 analyzer. FT-IR spectra were obtained by using a Unicam
Matson 1000 FT-IR spectrophotometer using KBr discs at room temperature. UV-Vis
spectrum was recorded on a CARY 100 Bio VARIAN UV-Vis spectrophotometer in
methanol. Molar conductances of the ligand and complex were determined in methanol
(ca 10ꢀ3 mol Lꢀ1) at room temperature using a Toa CM 405 conductivity meter. The
1H- and 13C-NMR spectra of the free ligand in CDCl3 were obtained using a Bruker
FT-NMR 500 MHz spectrometer. Melting points were taken on a Gallenkamp melting
point apparatus. The reaction products of oxidation were determined and analyzed by a
HP Agilent 6890 gas chromatograph equipped with a HP-5 capillary column (phenyl
methyl siloxane 30 m ꢁ 320 lm ꢁ 0.25 lm) and flame-ionization detector.

Vanadium Schiff base
2.1. Synthesis of N-(phenolyl)-benzaldimine
3851
A solution of o-aminophenol (4.6 mmol, 0.50 g) in absolute EtOH (20 mL) was added to
solution of benzaldehyde (4.6 mmol, 0.49 g) in absolute EtOH (20 mL) and the reaction
mixture was refluxed at 80ꢂC for 2 h. The solution was then evaporated to 15 mL and
cooled to room temperature. The precipitated light yellow product was filtered off,
washed with cooled ethanol and then dried in air. Yield: (87%); IR: 3329, 1623, 1580,
1481 cmꢀ1; 1H-NMR (CDCl3), ꢀ: 8.75 (1H, s), 7.97 (2H, d), 7.54 (3H, m), 7.35 (1H, d),
7.27 (1H, t), 7.09 (1H, d), 6.97 (1H, t). 13C-NMR (CDCl3): 157.58, 152.80, 136.30,
135.95, 132.13, 129.39, 129.32, 129.27, 120.56, 116.33, and 115.47. Elemental Anal.
Calcd for C13H11NO: C, 79.19%; H, 5.58%; N, 7.11%. Found: C, 79.09%; H, 5.54%;
N, 7.02%. UV-Vis (ꢁ(", mol Lꢀ1 cmꢀ1)): 216 (6559), 243 (10,543), 280 (5378), 341
(2612), ÃM (ꢀꢀ1 cm2 molꢀ1): 7.6, and m.p.: 87–89ꢂC.
2.2. Synthesis of [VO(L)2]
The ligand, HL (1.52 mmol) was dissolved in methanol (25 mL) and added to a
methanol solution (20 mL) of [VO(acac)2] (0.76 mmol) and the mixture was gently
refluxed at 65ꢂC for 3 h. After cooling, the resulting brown crystals were filtered and
washed with cooled methanol and then dried in air. Yield: (63.17%). IR: 1595,
983 cmꢀ1, Elemental Anal. Calcd for C26H20N2O3V: C, 67.98%; H, 4.36%; N, 6.1%.
Found: C, 67.81%; H, 4.39%; N, 6.02%. UV-Vis (ꢁ(", mol Lꢀ1 cmꢀ1)): 215, 238, 265,
430, 590, and ÃM (ꢀꢀ1 cm2 molꢀ1): 1.4.
2.3. Typical procedure
In a typical experiment, a solution of Oxone (0.2 mmol) in H2O (5 mL) at room
temperature was added to a solution of alcohol (0.2 mmol), tetra-n-butylammonium
bromide (0.1 mmol), and [VO(L)2] (0.01 mmol) in CH2Cl2 (1 mL), and the biphasic
mixture stirred vigorously. Formation of products and consumption of substrates were
monitored by GC. The identity of products was determined by comparison with
authentic samples using gas–liquid chromatography. Oxidation product yields based on
the oxidant were quantified by comparison with chlorobenzene.
3. Results and discussion
3.1. Complex characterization
A new vanadium(IV) complex, [VO(L)2] (L ¼ the anion of the Schiff base), has been
prepared from [VO(acac)2] (1 mol) and the Schiff base (2 mol) in methanol as shown in
scheme 2. The complex is brown, stable to air and light and soluble in methanol,
ethanol, acetonitrile, methylene chloride, DMF, and DMSO. The analytical data for the
new complex (table 1) agrees very well with the proposed formula.
In order to study the binding mode of the Schiff base to vanadium in the new
complex, IR spectra of the free ligand were compared with spectra of the vanadium
complex. A strong band is observed in the free ligand at 1624 cmꢀ1, characteristic of

3852
M. Bagherzadeh and M. Amini
O N
O
CH OH
3
V
O
[VO(acac) ] + Schiff base
2
N
Scheme 2. Synthesis of complex [VO(L)2].
Table 1. Microanalytical and spectral data for HL and [VO(L)2].
UV-Vis
C, H, N
Molar conductance
ꢁ (nm) "
Ligand/complex Solvent (mol Lꢀ1 cmꢀ1
)
C (%) H (%) N (%)
IR (cmꢀ1
)
ÃM (ꢀꢀ1 cm2 molꢀ1
)
C
13H11NO (HL) C2H5OH
216 (6559) 79.09
5.54
7.02 ꢄ(O–H) 3330
7.6
243 (10,543) (79.19) (5.58) (7.11) ꢄ(C¼N) 1624
FW ¼ 240.26
m.p. ¼ 87–89ꢂC
[VO(L)2]
280 (5378)
341 (2612)
C2H5OH 215, 238, 265 67.81
4.39
6.02 ꢄ(O–H) Absent
1.4
C26H20N2O3V
FW ¼ 458.94
430, 590 (67.98) (4.36) (6.1) ꢄ(C¼N) 1595
ꢄ(V¼O) 983
azomethine (C¼N) [30]. Coordination of the Schiff base to the metal through the
azomethine nitrogen is expected to reduce the electron density in the azomethine and
lower ꢂ(C¼N). In the spectra of the complex, ꢂ(C¼N) showed a negative shift to
1595 cmꢀ1, indicating coordination of the azomethine nitrogen to vanadium (table 1)
[31, 32].
A medium intensity band, at 3330 cmꢀ1 in the free ligand due to ꢂ(OH), was absent in
the complex, indicating deprotonation of the Schiff base prior to coordination [31]. The
complex exhibits characteristic and strong ꢂ(V¼O) at 983 cmꢀ1, indicating that there is no
intermolecular interaction with participation of the oxovanadium moiety [30].
Molar conductivities of ligand and its complex in methanol were 7.6 and
1.4 ꢀꢀ1 cm2 molꢀ1, respectively, and are given in table 1. These results show that the
ligand coordinates to the metal ions to form a neutral coordination compound. Molar
conductances are much smaller than those of 1 : 1 electrolytes [33], thus they are
considered to be non-electrolytes.
Electronic spectra of the ligand and its complex were recorded in EtOH (table 1). The
ligand exhibits intraligand transitions at ꢃ280 nm due to ꢃ ! ꢃ* transitions and at
ꢃ341 nm due to n ! ꢃ* transitions [34]. On complexation, these bands due to the
azomethine are shifted to lower energy, supporting coordination of the imine nitrogen
[34, 35]. In the spectra of the complex, new bands at 430 nm can be attributed to charge
transfer bands (ligand to metal or metal to ligand) [36]. Bands at 590 nm in the complex
come from the d–d transition of vanadium(IV).

Vanadium Schiff base
3853
Table 2. The effect of the nature of solvent/H2O on the oxidation of
benzylalcohol by the [VO(L)2]/Oxone/n-Bu4NBr catalytic system.a
Entry
Solvent
CH2Cl2
CH3Cl
CH3CN
C6H6
CH3COCH3
CCl4
n-hexane
Conversion (%)b
1
2
3
4
5
6
7
91
79
85
51
81
63
47
aThe molar ratios for [VO(L)2] : n-Bu4NBr : benzylalcohol : Oxone are 1 : 10 : 20 : 20.
bConversions (%) are measured after 5 min at room temperature.
Conversion
Selectivity
120
100
80
60
40
20
0
120
100
80
60
40
20
0
0
0.001 0.002 0.003 0.004 0.005 0.006
mmol of catalyst
Figure 1. The effect of the amount of catalyst on oxidation of benzylalcohol by the [VO(L)2]/Oxone/
n-Bu4NBr catalytic system. Reaction condition: benzylalcohol 0.2 mmol, Oxone 0.2 mmol, catalyst:
n-Bu4NBr ¼ 0.1. Selectivity to benzaldehyde ¼ (benzaldehyde%/(benzaldehyde% þ benzoic acid%)) ꢁ 100.
3.2. Catalytic reaction
In order to evaluate catalytic activities of [VO(L)2] for oxidation of alcohols, the
reactions were optimized for oxidation of benzylalcohol through investigation of the
influence of solvent, the amount of catalyst, and the amount of Oxone.
3.2.1. Effect of solvent on the oxidation of benzylalcohol. For finding the best solvent
under biphasic reaction conditions, dichloromethane, chloroform, acetonitrile, benzene,
acetone, carbon tetrachloride, and n-hexane were employed as solvents. Among the
solvents examined, dichloromethane was found to be the best for this protocol (table 2).
3.2.2. Optimization of amount of catalyst. Figure 1 illustrates the effect of amount of
catalyst on the conversion and the selectivity of benzylalcohol oxidation, at room
temperature for 5 min in CH2Cl2/H2O. The conversion of benzylalcohol increases with
the addition of catalyst from 0.001 to 0.005 mmol. When the amount of catalyst is
increased to 0.006 mmol, the selectivity of benzylalcohol oxidation reduces from 98% to

3854
M. Bagherzadeh and M. Amini
Selectivity
Conversion
120
100
80
60
40
20
0
120
100
80
60
40
20
0
0.05
0.1
0.15
mmol of Oxone
0.2
0.25
Figure 2. The effect of the amount of Oxone on oxidation of benzylalcohol by the [VO(L)2]/Oxone/
n-Bu4NBr catalytic system. Reaction condition: benzylalcohol 0.2 mmol, catalyst 0.005 mmol, n-Bu4NBr
0.05 mmol.
81%. The reaction without catalyst proceeds only very slightly. Hence, the amount of
catalyst enhances the reaction rate for selective oxidation of benzylalcohol.
3.2.3. Optimization of amount of Oxone. The amount of Oxone could also affect the
conversion and benzaldehyde selectivity (figure 2). When the amount of Oxone was
increased from 0.05 to 0.2 mmol, the conversion of benzylalcohol increased from 28%
to 91%. With a further increase of Oxone to 0.25 mmol, the selectivity to benzaldehyde
decreases from 98% to 71%, although the conversion of benzylalcohol also increased
from 91% to 100%.
3.2.4. Oxidation of alcohols with Oxone catalyzed by [VO(L)2]. Reactions were
performed at room temperature under air in CH2Cl2/H2O containing [VO(L)2], tetra-
n-butylammonium bromide, substrate, and Oxone in 1 : 10 : 20 : 20 molar ratio,
respectively. The system was applicable for oxidation of a wide variety of primary
and secondary alcohols to corresponding aldehydes and ketones (table 3).
Benzylalcohols were oxidized to give the corresponding aldehydes with 86–98%
conversion (entries 1–8). Substrates having electron-donating and -withdrawing
substituents in the aromatic ring were compatible with this protocol. The change of
nature and site of substituents (methyl, methoxy, chloro, and nitro) in the aromatic ring
of benzylalcohol to probe the electronic effects displayed no regular trends in the
conversion. The catalytic oxidation system can over-oxidize aldehyde product to
carboxylic acid (2% for benzylalcohol oxidation at 5 min, table 3, entry 1). With the
increase in the reaction time up to 30 min, more reduction of selectivity was observed
due to over-oxidation (e.g., 9% benzoic acid is formed in the benzylalcohol oxidation
after 30 min). Aliphatic alcohols and secondary alcohols were less reactive in
comparison to aromatic alcohols (entries 9–19). Secondary alcohols, e.g., diphenyl-
methanol and 1-phenylpropanol, could be converted to the corresponding ketones in
76–80% conversion (entries 9 and 10). Oxidations of borneol and 2-isopropyl-5-methyl-
cyclohexanol require longer reaction time (30 min) to afford the corresponding ketones
(entries 15 and 16).

Vanadium Schiff base
3855
Table 3. Oxidation of alcohols by the [VO(L)2]–Oxone–n-Bu4NBr catalytic system.a
Selectivity to
Conversion (%)b (TON)c aldehyde (%)d
Entry
1
Substrate
Product
91(18.2)
98
97
CH OH
CHO
2
2
3
4
95(19)
CH
CH
3
3
CH OH
CHO
2
96(19.2)
91(18.2)
96
98
OMe
OMe
CHO
CH OH
2
MeO
MeO
CH OH
CHO
2
5
6
98(19.6)
86(17.2)
97
CHO
MeO
MeO
CH OH
2
100
NO
NO
2
2
CH OH
CHO
2
7
8
9
89(17.8)
92(18.4)
80(16)
100
98
CHO
O N
CH OH
O N
2
2
2
Cl
CH OH
Cl
CHO
2
100
OH
O
10
11
76(15.2)
63(12.6)
100
OH
O
100
OH
O
(Continued )

3856
M. Bagherzadeh and M. Amini
Table 3. Continued.
Selectivity to
Conversion (%)b (TON)c aldehyde (%)d
Entry
12
Substrate
Product
68(13.6)
100
CH
CH
3
3
OH
O
13
14
72(14.4)
75(15)
100
100
H C
H C
3
OH
O
3
OH
O
15
16
61e(12.2)
59e(11.8)
100
100
CH
H C
3
CH
H C
3
3
3
CH
CH
3
3
O
HO
CH
CH
3
3
O
OH
H C
CH
3
3
H C
CH
3
3
17
71(14.2)
100
CH
OH
CH
O
3
3
18
19
63(12.6)
61(12.2)
100
100
OH
O
OH
O
aThe molar ratio for [VO(L)2] : n-Bu4NBr : benzylalcohol : Oxone is 1 : 10 : 20 : 20.
bConversions (%) are measured relative to the starting alcohol after 5 min.
cmmol of product/mmol of catalyst.
dSelectivity to benzaldehyde ¼ (aldehyde%/(aldehyde% þ carboxylic acid%)) ꢁ 100.
eThe reactions were run for 30 min at room temperature.
Exact mechanism for the catalytic oxidation of alcohols catalyzed by
oxovanadium(IV) complex in the presence of Oxone is not clear at present. However,
based on the nature of oxidation products, optimization results that indicate the
reaction yield depends on both catalyst and oxidant concentration, and previously

Vanadium Schiff base
3857
O
O
RCH OH
2
IV
–
L
V
2
HSO ,
OH ,
4
2
a
H
R
OH
OH
IV
VL
IV
O
S
L
b
V
O
2
d
HO
O
O
H
2
O
O
H
R
H
R
H
c
–
HSO
5
OH
O
S
V
V
L
O
H
O
OH
2
O
O
R
H
Scheme 3. Proposed mechanism for alcohol oxidation by the [VO(L)2]/Oxone/n-Bu4NBr catalytic system.
reported mechanism [37, 38], a catalytic cycle is proposed in scheme 3. The alcohol can
coordinate to [VOL2], a, to provide the complex b. Then b reacts with Oxone to give a
corresponding intermediate c that can readily undergo intramolecular hydrogen
abstraction to give the intermediate d. Finally, with separation of aldehyde or
ketone, water and hydrogen sulfate of intermediate d and regeneration of catalyst a, the
catalytic cycle is completed.
4. Conclusion
The complex [VO(L)2] has been synthesized and characterized by physico-chemical
methods. We have demonstrated the effectiveness of a vanadium(IV) complex as
catalyst for oxidation of different alcohols to carbonyl compounds with Oxone. Easy
preparation, mild reaction condition, high yields of the products, short reaction time,
no further oxidation to the corresponding carboxylic acids, high selectivity, and an
inexpensive system make this catalytic system a useful method for oxidizing alcohols.
Acknowledgments
Financially support of this study by the Research Council of Sharif University of
Technology is greatly appreciated.

3858
M. Bagherzadeh and M. Amini
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