Paramagnetic mononuclear oxovanadium(IV) complex as oxidation catalyst

Article abstract of DOI:10.14233/ajchem.2018.21068

2-Thiophenecarbanicotinic hydrazone is added with equimolar mixture of vanadyl acetyl acetonate in methanol to obtain oxovanadium(IV) complex of 2-thiophenecarbanicotinic hydrazone. Oxovanadium(IV) complex of 2-thiophenecarbanicotinic hydrazone is acted as an effective catalyst in the process. The catalytic reactions were carried under room temperature. The products generated were benzil and furil. The influence of solvent, oxidant and quantity of catalyst has been investigated. Oxovanadium(IV) complex of 2-thiophenecarba-nicotinic hydrazone proves significantly higher catalytic activity towards oxidation of secondary alcohols to ketones. The catalyst was proved to be very effective due to its recovery by simple filteration after completion of the reaction. It was reused several times which suggests that there is no change in the catalytic efficiency. Oxovanadium(IV) complex of 2-thiophenecarbanicotinic hydrazone did not show any leaching during the reaction, confirmed the heterogeneous nature.

Full text of DOI:10.14233/ajchem.2018.21068

Asian Journal of Chemistry; Vol. 30, No. 5 (2018), 972-974
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21068
Paramagnetic Mononuclear Oxovanadium(IV) Complex as Oxidation Catalyst
JYOTHY G. VIJAYAN
Department of Chemistry, Christ University, Bengaluru-560 029, India
Corresponding author: E-mail: jyothy.vijayan@res.christuniversity.in
Received: 28 September 2017;
Accepted: 31 January 2018;
Published online: 29 March 2018;
AJC-18822
2-Thiophenecarbanicotinic hydrazone is added with equimolar mixture of vanadyl acetyl acetonate in methanol to obtain oxovanadium(IV)
complex of 2-thiophenecarbanicotinic hydrazone. Oxovanadium(IV) complex of 2-thiophenecarbanicotinic hydrazone is acted as an
effective catalyst in the process. The catalytic reactions were carried under room temperature. The products generated were benzil and
furil. The influence of solvent, oxidant and quantity of catalyst has been investigated. Oxovanadium(IV) complex of 2-thiophenecarba-
nicotinic hydrazone proves significantly higher catalytic activity towards oxidation of secondary alcohols to ketones. The catalyst was
proved to be very effective due to its recovery by simple filteration after completion of the reaction. It was reused several times which
suggests that there is no change in the catalytic efficiency. Oxovanadium(IV) complex of 2-thiophenecarbanicotinic hydrazone did not
show any leaching during the reaction, confirmed the heterogeneous nature.
Keywords: Oxovanadium(IV) complex, 2-Thiophenecarbanicotinic hydrazone, Catalytic activity, Oxidation reactions.
Oxovanadium complexes are found to be effective catalysts.
As potential catalysts oxovanadium complexes can influence
the yields of products and selectivity in many reactions [9,10].
Due to their catalytic efficiencies, oxovanadium complexes
have been commonly utilized as precatalysts for epoxidation
reactions.Vanadium catalyzed oxidation reactions has attracted
several chemists because of the selectivity, reactivity and stereo-
selectivity [11]. The catalyst showed high stability, reactivity,
and efficiency under various reaction conditions and solvents.
The catalysts can be recycled and reused without showing
much metal leaching.
INTRODUCTION
Depending on tautomerism, hydrazones exhibits chela-
ting behaviour. According to the number and type of the
substituents in the hydrazone framework, the coordination
mode gets influenced. The linkage of hydrazone to a metal
ion depends on many factors which includes tautomerism,
reaction conditions,stability of the complex formed, number
and nature of the substituents on the hydrazone skeleton [1-
3]. Metal complexes containing acid hydrazones are found to
serve as catalysts in several organic transformation reactions
such as polymerization, epoxidation, reduction, oxidation,
alkylation and condensation. Tahseen et al. [4] have found the
catalytic abilities of oxovanadium complexes with hydrazones.
They proposed that the complexes are very effective in the
conversion of benzoin to benzil. One of the characteristic
attribute of oxovanadium is the possibility to tune the electronic
and steric properties of complexes. Hydrazones on coordi-
nation to metal center through the iminol form facilitates the
conjugation and enhances the nonlinearity [5,6]. Cariati et al.
[7] and Naseema et al. [8] have studied the second order non-
linear optical parameters of copper and palladium complexes
of N-salicylidiene-N'-aroylhydrazines. They have proposed the
third order non-linear optical properties of some hydrazones
derived from p-tolyloxyacetohydrazide and have observed that
the compounds exhibit optical limited behaviour.
EXPERIMENTAL
2-Thiophene carboxaldehyde, nicotinic hydrazide, benzil,
furil, methanol, glacial acetic acid, vanadyl acetylacetonate
and acetone were procured from Sigma Aldrich and used
without further purification. 1H NMR spectra were recorded
on Bruker AMX 400 FT-NMR spectrometer with DMSO-d6
as solvent and TMS as internal standard.
Preparation of heterogeneous catalyst 2-thiophene-
carbanicotinic hydrazonato oxovanadium(IV):To a solution
of 2-thiophenecarbanicotinic hydrazone (0.23 g, 1 mmol) in
methanol (20 mL), vanadyl acetylacetonate (0.27 g, 1 mmol)
dissolved in methanol (20 mL) was added. The reaction
mixture was stirred at room temperature with a magnetic stirrer

Vol. 30, No. 5 (2018)
Paramagnetic Mononuclear Oxovanadium(IV) Complex as Oxidation Catalyst 973
for 5 h. The resulting solution was allowed to stand at room
temperature for slow evaporation, a precipitated greenish
brown coloured solid was separated out. It was washed with
methanol and dried under vacuo. Yield: 75 %.
H 7.6
H 7.8
RESULTS AND DISCUSSION
7.9
H
O
H7.6
Physical data of the reactants and synthesized products
are summarized in Table-1.
H7.9
7.9 H
O
H
TABLE-1
PHYSICAL PROPERTIES OF THE
REACTANTS AND OXIDIZED PRODUCTS
7.6
H
7.9
Compound
Benzoin
Benzil
m.f.
C14H12O2
C14H10O2
Colour
White
Pale yellow
m.p. (°C)
134-138
99
7.8 H
H 7.6
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
ppm
Catalytic activity
Oxidation of benzoin and furoin: Benzoin (0.42 g, 2
mmol) or furoin (0.38 g, 2 mmol) was taken in a round bottom
flask (10 mL) and added acetone (5 mL), 2-thiophenecarba-
nicotinic hydrazonato oxovanadium(IV) (0.1 mmol) as catalyst
and KMnO4 (0.38 g, 0.5 mmol) or urea-H2O2 adduct (0.38 g,
4 mmol) as the oxidant . The reaction mixture was stirred at room
temperature and the progress of the reaction was monitored
by using TLC. After completion of the reaction the product
was purified by column chromatography. The resulting solution
was allowed to stand at room temperature for slow evaporation
until a yellow coloured crystals formed. The product was washed
with water, methanol and ethyl acetate and recrystallized from
the solvent used. The product was characterized by 1H NMR
(Figs. 1 and 2). Oxidation of benzoin gave benzil (Scheme-I)
wile furoin oxidized to furil (Scheme-II). The NMR spectra
of benzoin (Fig . 1) and benzil (Fig. 2) have revealed that OH
signal observed at δ 6.0 ppm in the former has disappeared in
the latter implyed that oxidation has taken place. The proton
NMR spectra of furoin (Fig. 3) and furil (Fig. 4) have displayed
that hydroxyl resonance at δ 5.9 observed in the former has
been disappeared in the latter suggesting that oxidation process
has occurred.
Fig .2. 1H NMR spectrum of benzil (product) in DMSO-d6
O
Acetone/KMnO4 or UHP
O
HO
[VOL(OMe)]H2O
O
Benzil
Benzoin
Scheme-I: Catalytic oxidation of benzoin to benzil
O
O
OH
O
O
Acetone/KMnO4 or UHP
[VOL(OMe)]H2O
O
O
O
Furoin
Furil
Scheme-II: Catalytic oxidation of furoin to furil
7.2
H
6.4
H 7.3
6.2
H
H
H
O
H
6.1
6.0
H 6.2
7.7
H
H
O
H
H
7.3
O
8.0
7.4
H
H
H
8.0
H
5.9
O
H 7.4
O
H
7.6
O
9.0
8.5
8.0
7.5
7.0
6.5
6.0
ppm
7.6
H
H
8.0
1
H6.7
Fig. 1. H NMR spectrum of benzoin (reactant) in DMSO-d6
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
ppm
Recycling of the catalyst: The catalyst used in the first
cycle of the reaction was washed with water, ethyl acetate,
and acetone and dried under vacuum and reused again as
catalyst.
Fig .3. 1H NMR spectrum of furoin (reactant) in DMSO-d6

974 Vijayan
Asian J. Chem.
TABLE-3
EFFECT OF SOLVENT IN THE OXIDATION OF ALCOHOLS
Compounds
Acetone
Acetonitrile
Benzene
Reactant
Benzoin
Furoin
Product
Benzil
Furil
Time (h)
3
2.5
Yield (%)
Time (h)
Yield (%)
Time (h)
Yield (%)
94
95
6
6
85
82
8
8
78
80
Reaction conditions: 2 mmol alcohol, 0.5 mmol KMnO4, 5 mL solvent and 0.1mmol catalyst [VOL(OMe)]H2O, room temperature
TABLE-4
EFFECT OF QUANTITY OF THE CATALYST IN THE OXIDATION OF ALCOHOLS
Compounds
Reactant
Benzoin
Furoin
0.01 mmol
0.1 mmol
0.15 mmol
Product
Benzil
Furil
Yield (%)
Time (h)
Yield (%)
Time (h)
3
2.5
Yield (%)
Time (h)
89
89
3
2.5
94
95
95
96
3
2.5
Reaction conditions: 2 mmol alcohol, 0.5 mmol KMnO4, 5 mL acetone solvent and catalyst [VOL(OMe)]H2O, room temperature
volume, quantities of oxidant and substrates constant in
oxidation reactions. It was observed that 0.1 mmol of catalyst
was found to be appropriate (Table-4).
6.8
H
H
8.2
Conclusion
The catalytic efficiency of oxovanadium(IV) complex of
2-thiophenecarbanicotinic hydrazone, the oxidation of benzoin
and furoin were carried out. The experiments were conducted
in the presence of 0.01, 0.10 and 0.15 mmol catalyst in three
different solvents at room temperature. The rate of the reaction
with KMnO4 was found to be better than urea-H2O2 adduct
and no other side products were formed. The advantage of
using oxidants KMnO4 or urea-H2O2 adduct was that both
could be removed from the reaction mixture easily by washing
with water and the catalyst could be reused. The catalyst
continued to be active for repeated cycles. There was no appre-
ciable decrease in the activity of 2-thiophene-carbanicotinic
hydrazonato oxovanadium(IV) catalyst upto five cycles.
O
H
7.6
O
O
H
7.6
O
H
H
6.8
8.2
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
ppm
Fig .4. 1H NMR spectrum of furil (product) in DMSO-d6
ACKNOWLEDGEMENTS
Effect of oxidant: In the present case, KMnO4 or urea-
H2O2 adduct was added as oxidants at room temperature (Table-
2). By comparing the efficiency of these two oxidants, KMnO4
was observed to be more effective than urea-H2O2.
The author is greatful to Christ University, Bangalore,
India for a support.Also acknowledge to SophisticatedAnalytical
Instrumentation Facility, IISc Bangalore, India for NMR analysis.
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