Vanadium (IV) complexes with Schiff base ligands derived from 2,3-diaminopyridine as catalyst for the oxidation of sulfides to sulfoxides with H2O2

Article abstract of DOI:10.1007/s13738-018-1480-2

Sulfoxides are substances used in the synthesis of valuable complexes and as drugs in medicine. Sulfides were selectively oxidized to the corresponding sulfoxides in proper yields with (H₂O₂) hydrogen peroxide applying a vanadium (IV) Schiff base complex in the role of a catalyst in glacial acetic acid in the role of solvent beneath mild conditions. For the conversion of sulfides to sulfoxides of various catalysts are applied. It must be noted that in our previous article, the vanadyl complexes (VOY¹) synthesized were applied as a catalyst in the epoxidation of styrene (Zabardasti and Shangaie, J Iran Chem Soc 13:1875–1886, 2016) but in the new work, vanadium (IV) complexes with Schiff base ligands derived from 2,3-diaminopyridine were used to as catalyst for the oxidation of sulfides to sulfoxides with H₂O₂. To the most of our information, there is not any literature description on the selective oxidation of sulfides to sulfoxides by means of a vanadium (IV) Schiff base complex with N, O donor ligand derived from 2,3-diaminopyridine catalyst beneath these conditions. Dimethyl sulfide was chosen as a pattern substrate for optimization experiments. Oxidation of sulfides was functioned at 25?°C temperature in the attendance of a catalytic quantity of the vanadium (IV) complex or (VOY¹) utilizing 20% H₂O₂ in the role of the oxidant, Scheme 1 and glacial acetic acid in the role of the solvent.

Full text of DOI:10.1007/s13738-018-1480-2

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1480-2
ORIGINAL PAPER
Vanadium (IV) complexes with Schiff base ligands derived
from 2,3-diaminopyridine as catalyst for the oxidation of sulfides
to sulfoxides with H2O2
Abedien Zabardasti1 · Sayed Asad Shangaie1
Received: 8 March 2018 / Accepted: 23 August 2018
©
Iranian Chemical Society 2018
Abstract
Sulfoxides are substances used in the synthesis of valuable complexes and as drugs in medicine. Sulfides were selectively
oxidized to the corresponding sulfoxides in proper yields with (H O ) hydrogen peroxide applying a vanadium (IV) Schiff
2
2
base complex in the role of a catalyst in glacial acetic acid in the role of solvent beneath mild conditions. For the conversion
of sulfides to sulfoxides of various catalysts are applied. It must be noted that in our previous article, the vanadyl complexes
1
(
VOY ) synthesized were applied as a catalyst in the epoxidation of styrene (Zabardasti and Shangaie, J Iran Chem Soc
1
3:1875–1886, 2016) but in the new work, vanadium (IV) complexes with Schiff base ligands derived from 2,3-diamino-
pyridine were used to as catalyst for the oxidation of sulfides to sulfoxides with H O . To the most of our information, there
2
2
is not any literature description on the selective oxidation of sulfides to sulfoxides by means of a vanadium (IV) Schiff base
complex with N, O donor ligand derived from 2,3-diaminopyridine catalyst beneath these conditions. Dimethyl sulfide was
chosen as a pattern substrate for optimization experiments. Oxidation of sulfides was functioned at 25 °C temperature in the
1
attendance of a catalytic quantity of the vanadium (IV) complex or (VOY ) utilizing 20% H O in the role of the oxidant,
2
2
Scheme 1 and glacial acetic acid in the role of the solvent.
2
+
Keywords Sulfide · Hydrogen peroxide · Selective oxidation · Catalyst · VO Schiff base complex · 2,3-Diaminopyridine ·
Sulfoxide
Introduction
biological materials. Additionally, complexes of transition
metals which include of salicylaldehyde and diamine have
gained substantial attentiveness. Complexes with N, O donor
ligands have been planned for their dioxygen insight and
oxidative catalysis. The consequences of tetradentate Schiff
base obtained from 2-hydroxybenzaldehyde and diamine
are the topic of a lot of investigation [1–4]. In the last era
metallated Schiff bases have significant attention because
of their broad application and in addition simply forma-
tion by the condensation between imines and aldehydes. In
1986, reported on the asymmetric oxidation of sulfides into
the corresponding sulfoxides using organic hydroperoxides
catalyzed by optically active Schiff base oxovanadium (IV)
complexes. The complexes used for the asymmetric catalytic
reactions involved tetradentate Schiff base ligands derived
from optically active 1,2-diamines and salicylaldehyde. Such
complexes are usually green and in the solid state have a
monomeric five-coordinate square pyramidal structure.
However, a few orange oxovanadium(IV) complexes have
also been reported. The advantage of vanadium complexes
Schiff bases including N, O donor atom sets are renowned
to match with different metal ions and this has fascinated
lots of inventors. The Schiff base property of complex is
affected on the catalytic ability of vanadium oxide. Not
only the Schiff bases perform an important role in coor-
dination chemistry, but also, their complexes have enough
potentials for property of catalyst. Recently, one of the
important ligand synthesis systems is tetradentate Schiff
base from typical of N, O, especially, in relationship with
unsymmetric and selective catalysis. By reason of coordina-
tion facility of N, O Schiff bases their transition metal com-
plexes widely applied in the role of catalyst in production of
*
Abedien Zabardasti
zabardasti.a@lu.ac.ir
1
Department of Chemistry, Lorestan University, Khoramabad,
Iran
Vol.:(0
1
123456789)
3

Journal of the Iranian Chemical Society
as catalysts in comparison to other intermediate elements is
that their CV curves are reversible and play a better catalytic
role in reactions [5].
In recent years, dimethyl sulfoxide has been used to treat
various diseases tested. Chiral sufoxides find application in
specific drugs such as esomeparasole and armodafinil. In
addition, sulfoxides in pharmaceuticals known as an aprotic
polar solvent. DMSO is chiefly applied as a solvent in the
laboratory. The rising emphasis and functions of sulfoxides
have interested investigation into novel techniques for their
synthesis. Organic sulfoxides are useful for the manufacture
of an assortment of chemically and biologically effective
molecules containing medicinal intermediates for example
anti-painful (proton pump preventers), bladder diseases [6]
anti-atherosclerosis, uncontaminated, syndrome palms and
soles, antifungal [7] anti-hypertensive [8] and cardiotonic
intermediaries [9] in addition to psychotropics [10] and
vasodilators [11].
Fig. 1 Structural representation of the Schiff base complex.1 or VOY1
and continue with crystallization in ethanol and drying at
50 °C. Then, the resulting crystals were prepared for X-ray
diffraction.
The oxidation of sulfides to sulfoxides is an uncompli-
cated artificial manner. There are numerous substances and
oxidative processes accessible for this modification. Conse-
quently, checking the reaction conditions, that is tempera-
ture, time and the essential quantity of oxidants, performs
a significant function to prevent constructing oxidative side
results, on the other hand, these necessities are frequently
difficult to seem. Therefore, there is yet substantial attention
in the improvement of selective oxidants for this modifica-
tion [12–17].
X‑ray crystallography
1
Crystals of complex.1 (VOY ) appropriate for X-ray diffrac-
tion examination were acquired by crystallization from etha-
nol. For completely the complexes, the unit cell elements
and the strength statistics were attained on a Bruker-Nonius
SMART APEX CCD filtered crystal X-ray diffractometer,
arranged with a graphite monochromator and a Mo Ka fine-
focus closed pipe (k=0.71073 Å) operated at 2.0 kW. The
indicator was placed at a distance of 6.0 cm from the unit
cell. Information were brought together at 298 K with an
experiment length bond of 0.3 in x and an occurrence time
of 5 s/frame. The SMART software was beneficial for pro-
grams attainment and the SAINT-PLUS software was useful
for programs extraction [25]. The SADABS software [26]
was valueble for the intensity modification. The complex
were identified by application of straightforward techniques
Whereas the hydrogen peroxide oxidation of sulfides
to sulfoxides catalyzed by means of many transition metal
schiff base complexes has been described [18–21], however,
the oxidation catalyzed via vanadium (IV) has obtained
fewer consideration. H O is investigated as a perfect ‘green’
2
2
oxidant because of its intensity and nonexistence of poison-
ous by-yields [22, 23].
In perpetuation of our effort on the discriminating oxi-
dation of hydrocarbons, we describe a mainly appropriate
and selective oxidation of sulfides to the corresponding sul-
2
+
2
foxides consuming H O catalyzed with VO Schiff base
and purified on (F ) by top matrix least-squares techniques.
2
2
1
1
complex.1 (VOY ), beneath mild conditions (Fig. 1).
Our investigation to appropriate single crystal of (VOY )
for X-ray diffraction calculation have been successful. All
non-hydrogen atoms were refined with anisotropic thermal
parameters. Atom scattering factors were taken from the
International Tables for Crystallography. Crystal data and
structure refinement details:
Preparation of the complex
The complex was designed by means of a process alike
to that applied by Karmaker et al. [24]. 3 mmol of the
Empirical formula, C H N O F V; Formula weight,
1
9
11
3
3 2
VO(acac) which was dissolved in 30–45 ml of warmed
417.92 g; Crystal color, green; Crystal dimensions (mm),
2
3
ethanol, added to an ethanolic assortment of 3 mmol of
0.42, 0.35, 0.04; Crystal system, monoclinic; V (Å ),
′
the N,N -bis (5-fluoro-2-hydroxybenzaldehyde) pyridine
1795(4); Space group, P2(1)/c; F(000), 2625; Maximum and
1
−3
Schiff base (H Y ) in the 1:1 molar relative amount. The
minimum peak in difference map (e Å ), 1.65 and −1.22;
2
2
color of the mixture altered in a few seconds. The reaction
mixture was refluxed for 4 h. The production was washed
several times using distilled water, afterwards by ethanol
Goodness-of-fit on F , 1.12.
An illustrative crystal and optimized configuration of
1
complex 1 (VOY ) are showed in Figs. 2 and 3. It should
1
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Journal of the Iranian Chemical Society
1
Fig. 2 Crystal structure of (VOY ). Displacement ellipsoids are drawn at the 20% probability level
monomeric and the configuration close by each vanadium
atom is a deformed square-pyamid with the oxo ligand in
the axial locality.
The entire cis angles are in the situations
1
5
1
1
7.508(15)–88.336(15)° for VOY crystal. The N (2)–V–O(3)
68.401(14)° is the maximum, whereas N(2)–V–O(1),
34.471(16)° is the minimum trans angle.
1
The corresponding bond distances in the crystal (VOY )
for situation of vanadium atom are 1.537(6), 1.765(5),
1
.798(4) Å for V–O(1), V–O(2) and V–O(3), respectively,
and 2.342(6), 2.267(7)Å for V–N(1) and V–N(2), respec-
tively. There is isolated oxygen atom attached to the vana-
dum atom to complete it one axial location (V=O).
The V–O (2) and V–O (3) bonds (1.765 and 1.798 Å) are
longer than of V–O (1) bond (1.537 Å). As forecast, due to
π-back bonding, the V–O bond distance (V–O (1)=1.537
Å) containing vanadyl oxygen is smaller than the V–O(2),
Fig. 3 Representative optimized structure of VOY1 at MP2/6-
11++G(2d,2p)
3
(
1.765 Å) and V–O(3), (1.798 Å) and V–N(1), (2.342 Å)
be noted that in former article of the vanadyl complexes
and V–N(2), (2.267 Å). The vanadium(IV) atom is (0.006
Å) outside plane of the plane described by O(2), O(3), N(1)
and N(2).
1
(
VOY ) synthesized was used as a catalyst in the epoxida-
tion of styrene [27].
Entirely non-hydrogen atoms were refined anisotropically.
Transform ellipsoids are presented at the 20% possibility
uniform. The statistics assortment and modification perfor-
mances are review in Table 1. Chosen angles and bond dis-
tances are presented in Table 2.
IR‑spectral studies and Electronic spectra (Uv–vis)
A practical list of IR spectral data is presented in Tables 3, 4.
By comparing the IR spectrum of dimethyl sulfide and dime-
thyl sulfoxide, it can be seen that an extensive peak arose at
1
The bright green crystal of complex.1 (VOY ) (0.35,
0
2
.04, 0.42 mm) was applied in the diffraction valuation at
°
−1
98 K. This compound was recrystallized from CHCl . The
almost 1050 cm in the dimethyl sulfoxide (Fig. 4). This
3
1
IR spectrum of crystal (VOY ) displayed the stretching band
peak is allocated to the S=O streching vibrations.
−
1
(V=O) at 977 cm , which is demonstrating of a monomeric
The Schiff base ligand show an extensive band almost at
−
1
arrangement.
3360 cm due to hydroxy O–H vibration. A strong band for
−
1
The complex was confirmed by X-ray diffraction and
the consequences designed whole confirmation that it is
Schiff base at almost 1285 cm is allocated to OH in-plane
bending vibration.
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3

Journal of the Iranian Chemical Society
The phenolic streching vibrations (C–O), that arose
catalyst in the epoxidation of styrene [27] but the new work
on the conversion of sulfides to sulfoxides use of vanadyl
−
1
at 1295 cm in the Schiff base, undertake a alteration
−
1
1
to greater frequence (28 cm ) in the complex. This
alteration approves the share of oxygen in the (M–O)
bond. Additionally, an extensive peak arised at almost
complex (VOY ). In recent years, various catalysts ([VO
(sal-oaba)(H O)]-Y, [Cu(sal-oaba)(H O)]-Y and [Ni(sal-
2
2
oaba)(H O) ]-Y) have been used to convert sulfides to sul-
2
3
−
1
1
640 cm in the isolated ligand and their complex. This
foxides with conversion (88.7%), [28].
peak is allocated to the imine streching vibrations (C=N).
An alteration to lesser frequency afterward complexation
suggesting to matching of nitrogen in (C=N) to metal ion
and consequently this is anticipated to decrease the elec-
tron density in the (C=N) bond.
To the best of our knowledge, there is no literature report
on the selective oxidation of sulfides to sulfoxides by means
2
+
of a (VO Schiff base complex with N, O donor ligand
derived from 2,3-diaminopyridine) catalyst under these
conditions. Oxidation of sulfides was functioned at room
temperature in the attendance of a catalytic amount of the
In the short frequncy section the peak detected in the
−
1
1
complex in the Section 480 and 585 cm is allocated to
ν(M–O) and ν(M–N), respectively, proffer that the metal
ion is attached to the ligand via the imino nitrogen (C=N)
and phenolic oxygen(C–O). In brief, the green compound
vanadium (IV) complex or (VOY ) utilizing 20% H O in
2
2
the role of the oxidant, Scheme 1 and glacial acetic acid in
the role of the solvent.
Sulfoxides are generally prepared by oxidation of sulfides
[29]. Diaryl sulfoxides can be designed by Fridel-Crafts
arylation of sulfur oxide applying acid catalyst. A particular
experimental method is as follows:
1
− 1
VOY whose ν(V = O) amount is at about 977 cm , is
monomeric penta-coordinated to display square-pyramidal
geometries with the oxo ligand in the axial arrangement.
It looks electron-withdrawing effect of Schiff bases
with: (R=F>H) replace on the metal core are convinced
concluded π-accepting properties of imin (C = N) bond.
Whereas the electron density of the metal core reduces by
electron-withdrawing units as robust π-acceptors, it enhances
by decreasing π-accepting property of electron giving Schiff
base by way of imin (C = N) bond. Confirmation for this
reality is likeness of imin (C=N) bond energy diminishing
upon complex creation of Schiff bases, in another situation it
has to be anticipated further declining. Our proposition, the
electron-withdrawing effects of Schiff bases to inner metal
have to be shifted through π bond not σ bond.
1
To the complex 1 (VOY ), (1 mmol) in glacial acetic acid
was inserted the sulfide (2 mmol), then hydrogen peroxide
(8 mmol, 20%) was inserted gradually. The reaction mixture
was stirred at room temperature until TLC displayed the
reaction was finished. The consequent solution was neutral-
ized with aqueous NaOH (6 M) and the results was extorted
with CH Cl . The organic level was dehydrated over anhy-
2
2
drous Na SO and then focused under reduced pressure
2
4
to yield a residue, which was passed through a small pad
of silica gel by means of ethyl acetate and hexane (1:1) as
eluent to provide analytically pure product. The produce
remainder was then separated with ether (45–65 °C) and
the activity and selectivity data were studied and measured
by GC. Finally, sulfoxide and sulfone products were detected
by GC. Dimethyl sulfide was chosen as a pattern substrate
for optimization experiments.
The electronic bands of the ligand show binary chief
peaks at 309 nm. The initial peak in the Schiff base are
ascribed to ꢀ → ꢀ ∗transition. Additional peak in the spectra
of the Schiff base at (around 420 nm) is allocated to n → ꢀ ∗
transition. This peak is transferred to lengthier wave length
The oxidation happen to afford dimethyl sulfoxide in
1
(
about 20 nm) accompanied by enlarging in intensity. This
82% yield at the attendance of VOY and 8 equiv of 20%
alteration perhaps ascribed to the gift of the isolated pairs of
H O in 32 min, Table 3, item (1) The great extra of hydro-
2
2
the nitrogen atoms of the ligands to the metal ion (N→M).
gen peroxide needed was a result of its disintegration in the
1
2+
The electronic bands of vanadyl compound, [VOY ]
attendance of the VO catalyst. The oxygen released dur-
in DMF solution displayed two peaks in the range
ing decomposition played little role in the oxidation of the
sulfides. The oxidation take place, although in minor yield,
by easily bubbling molecular oxygen among the reaction
assortment under alike reaction conditions, Table 3, item (2)
The oxidation of dimethyl sulfide in numerous solvents was
investigated, Table 5, items 3–5. Between the solvents stud-
ied, glacial acetic acid was the greatest effective, Table 3,
item 2.
7
54−733 nm. The ground state of V (IV) are d and earli-
xy
est motivated dublet levels in the order of enhancing energy
2
2
(
d , d ) and (d
). Therefore, two peaks conforming to
xz yz
x −y
2
2
d →d −d and d →d are imaginable. These transi-
x −y
xy
xz
yz
xy
tions are in conformity with the arrangement of a square
pyramid configuration.
Catalytic activity research
On the other hand, the reaction was accomplished at
various temperatures under the identical conditions. How-
ever, the oxidation was finished after 32 min at raised tem-
perature, the selectivity diminished considerably, Table 3,
items 6–8. Additional, when the oxidation of dimethyl
For the conversion of sulfides to sulfoxides of various cata-
lysts are applied. It must be noted that in previous work of
1
the vanadyl complexes synthesized (VOY ) was applied as a
1
3

Journal of the Iranian Chemical Society
Scheme 1 Selective oxida-
O
S
1
1
mmol VOY
O
S
S
tion of sulfides to the match-
ing sulfoxides applying H O
R1
2
2
R2
+
R1
R2
2
+
8 eq 20% H2O2
catalyzed by VO Schiff base
R1
R2
1
complex (VOY )
O
R1 , R2= methyl, ethyl, propyl, vinyl, phenyl
Table 3 Table unscaled vibrational frequencies (cm−1) of sulfides
Table 1 Crystallographic and structure refinements data for complex
1
(
VOY )
Compound
vC–S
vC–H
vC–C
vC=C
Complex
VOY1
Me–S–Me
1010
1005
1000
995
1043
1100
1120
1135
2950, 1375
2850, 1375
2820, 1375
2810, 1375
3125, 1450
3200, 1425
3215, 1375
3220, 1420
2800, 1400
–
–
–
–
–
1650
1660
1450
1490
–
Empirical formula
Formula weight
Crystal color
Crystal dimensions (mm)
Crystal system
a (Å)
C H N O F V
1
9
11
3
3 2
CH –CH –S–CH
950
995
990
–
–
–
3
2
3
417.92
Green
(
CH –CH ) S
3
2 2
CH –CH –CH –S–CH
3
2
2
3
0.42, 0.35, 0.04
Monoclinic
12.958(2)
7.643(2)
15.234(10)
90
CH =CH–S–CH
2
3
(
CH =CH) S
2
2
Ph–S–CH3
Ph–S–Ph
b (Å)
c (Å)
α (°)
–
980
9
90
β (°)
γ (°)
V (Å )
Space group
Z
107.825(4)
90
1795(4)
P2(1)/c
S
S
995
2910, 1300
990
–
3
4
Temperature
Tmax, Tmin
F(000)
298
0.654, 0.201
2625
sulfide was accomplished by means of various concentra-
tions of hydrogen peroxide (H O ), the reaction did not
2
2
reach achievement when applying fewer than 8 mmol of
hydrogen peroxide (H O ). Although, when a greater con-
Index ranges
−16≤h≤16
−
−
17≤k≤17
20≤l≤20
2
2
centration was used, neither the conversion nor selectivity
of the reaction was enhanced, Table 3, items 9–11. The
oxidation take place in inadequate yield when the reac-
tion was accomplished under identical reaction conditions,
however, in the nonexistence of complex 1, Table 3, item
Data/restraints/parameters
R1; Rw
Rint
3595/0/242
0.045, 0.145
0.081
Maximum and minimum peak in difference map
1.65 and −1.22
−
3
(
e Å
)
12 or when applying other VO salts, for example VO(acac)
2
2
Goodness-of-fit on F
1.12
as the catalyst, Table 3, item 13.
The oxidation of other sulfides such as ethyl methyl, die-
thyl, methyl propyl, methyl vinyl, divinyl, phenyl methyl,
diphenyl and cyclic sulfides were then examined using
the optimized reaction conditions, Table 4. For aliphatic
sulfides, the conversion (98%) is greater than unsaturated,
aromatic and cyclic sulfides (less than 93%). In the case
of vinyl sulfides, Table 4, items 5 and 6, no oxidation was
observed at the C–H bonds. Similarly, the carbon–carbon
double bonds in vinyl sulfides remained intact during the
oxidation, Table 4, items 5 and 6. This procedure can also
be applied to the oxidation of cyclic sulfides, Table 6, items
Table 2 Selected bond distances (Å) and bond angles (°) for complex
1
(
VOY )
Bond distances (Å) Bond angles (°)
V–O(1)
V–O(2)
V–O(3)
V–N(1)
V–N(2)
1.537(6)
1.765(5)
1.798(4)
2.342(6)
2.267(7)
O(1)–V–O(2)
O(1)–V–O(3)
O(2)–V–N(2)
O(3)–V–N(1)
N(1)–V–N(2)
O(3)–V–N(2) 168.401(14)
O(1)–V–N(1) 137.193(14)
O(1)–V–N(2) 134.471(16)
O(2)–V–N(1) 165.298(14)
57.508(15)
57.128(14)
76.96(14)
80.064(16)
88.336(15)
9
and 10. The deactivation of C–H and C–C bonds has also
been proven by manganese complexes [Mn(III)] [23]. As can
be seen, the selectivity of converting sulfides to sulfoxides
for aliphatic and aromatic compounds with similar condi-
tions is slightly different.
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Journal of the Iranian Chemical Society
Table 4 Table unscaled
vibrational frequencies (cm−1)
of sulfoxides
vC–S
vC–H
vC–C
vC=C
vS=O
vV=O
Compound
(
(
(
(
(
(
(
(
Me) S=O
890
885
880
875
1049
1105
1128
1139
994
3050, 1375
3020, 1375
3010, 1375
3007, 1375
3135, 1375
3210, 1430
3220, 1375
3225, 1415
2815, 1410
–
–
–
–
–
1655
1670
1510
1506
–
1050
1048
1035
1030
1045
1042
1055
1058
1040
–
–
–
–
–
–
–
–
–
2
C H )(CH )S=O
930
1000
995
–
–
–
2
5
3
CH -CH ) S=O
3
2 2
C H )(CH )S=O
3
7
3
CH =CH)(CH )S=O
2
3
CH =CH) S=O
2
2
Ph)(CH )S=O
3
Ph) S=O
–
994
2
O
S
O
S
1005 (68)
–
2930, 1325
3010, 1425
995 (310)
1124(56)
–
1045 (51)
–
–
Catalyst (VOY1)
1602(87)
977 (67)
high, so the oxidation method meets the needs of contempo-
rary green chemistry and is suitable for practical synthesis.
The redox behaviors of totally compounds were studied
in DMF solution by tetra butyl ammonium hexa fluoro phos-
phate [TBAH] in the function of the supporting electrod.
The redox potentials are extracted by reference the Ag/AgCl.
Nearly, totally the complexes with N, O giver Schiff bases
obtained from salen and salophen showed irreversible reduc-
tion in the region of 0.5–1.2 v [31]. When these quantities
are assessed with the fresh complexes, it has been realized
that with the attachment of pyridine ring in the Schiff base
structure, its electron withdrawing character was improved.
Subsequently, declines the electron denseness in the region
of the metal core, perhaps by reason of a diminutive lived
reduced situation of the metal ion or because of oxidative
degradation of the Schiff bases.
The spectral variations studied (as presented in Fig. 5)
upon addition of H O to the solution of catalyst (vanadium
Fig. 4 IR spectrum of dimethyl sulfide and dimethyl sulfoxide
2
2
(
IV) Schiff base complex), in acetic acid, is credited to the
The kinetic studies display that the oxidation of sulfide
is a second order reaction and is not acid catalyzed, so the
reaction times remained constant as a result of increasing
the acidities of the medium by the addition of glacial acetic
acid. However, experiments demonstrate that dry hydrogen
peroxide in glacial acetic acid does not generate per acetic
acid on standing. Further, kinetic study with per acetic acid
under identical reaction conditions reveal that the oxida-
tion of sulfides is very fast; too fast to follow by titrimetry.
Although the precise mechanism of this transformation is
still uncertain, the oxidation probably involves the electro-
philic attach of the peroxide oxygen on the sulfide sulfur
atom [30]. Moreover, this oxidation system is clean, safe
and operationally simple and the yields of the byproducts are
construction of a reactive vanadium (V)-oxo intermediate.
The spectral structures observed former for vanadium (V)-
oxo complexes are similar to those detected in the contem-
porary situation (Fig. 5), [32, 33].
At the potential (0.812 v) the oxidation of the vanadium
(IV) to vanadium (V) takes place as a reversible one-electron
transfer. An explanatory cyclic voltammogram of complex
1
VOY is presented in Fig. 6. Catalyst plays a major role in
the reduction–oxidation pathway.
E = 0.812
Ox : V(IV)OY �����������������������������→�� V(V)OY + e
1
1
(1)
E = 0.725
Red :V(V)OY + e �����������������������������→�� V(IV)OY
1
1
(
2)
1
3

Journal of the Iranian Chemical Society
Table 5 Oxidation of dimethyl
Item Complex.1 (mmol) H O (mmol) Solvent (2 ml) Time (min) T(°C) Conver- Sulfoxide
2
2
sulfide
sion
%)
selectivity
a
(
(%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
1
8
–
8
8
8
8
8
8
3
6
CH COOH
32
500
24
35
195
23
12
32
32
32
25
25
25
25
25
47
56
74
25
25
25
25
25
95
4
82
98
59
56
64
61
45
35
75
68
62
98
98
3
b
CH COOH
3
CH OH
59
95
85
95
95
95
28
45
95
39
52
3
CH CH OH
3
2
CH CN
3
CH COOH
3
CH COOH
3
CH COOH
3
1
1
1
CH COOH
3
0
1
2
3
CH COOH
3
12
8
8
CH COOH
32
32
32
3
Without catalyst
VO (acac)2
CH COOH
3
b
CH COOH
3
a
Separated yields
b
The reaction was carried out under an atmosphere of O as a substitute of in the existence of 20% H O
2
2
2
From the data CV, the following mechanism are expected.
H
The results display that the anodic peak potential (Epa)
agreeing to the intramolecular reductive bond of the imine
units differs as would be anticipated from the electronic
+
1
V(IV)OY + H O ���������→�� V(V)OY + HO
1
⋅
2
2
2
+
′
H
influences of the replacements at locations 5 and 5 . Hence,
1
V(V)OY + HO ���������→�� V(IV)OY + H O
⋅
1
2
2
2
(
(
E ) progresses further positive corresponding to the order:
pa
R=F>H).
Great amount oxidation of dimethyl sulfide (10 mmol)
was also examined and the consequences displayed that oxi-
dation occur in proper yield.
Conclusion
Table 6 Oxidation of sulfides to sulfoxides with H O catalyzed by
2
2
1
complex.1 (VOY )
Oxidation of sulfides was functioned at room temperature
in the attendance of a catalytic amount of the vanadium
Item Substrate
Time (min) Conver- Sul-
1
sion (%) foxide
(IV) complex or (VOY ) utilizing 20% H O in the role of
2
2
b,c
(
%)
the oxidant, Scheme 1 and glacial acetic acid in the role
1
2
3
4
5
6
7
8
9
Me–S–Me
15
18
24
7
22
12
25
22
12
98
98
98
98
93
93
92
92
88
82
91
87
95
94
80
91
89
84
CH –CH –S–CH
3
2
3
(CH –CH ) S
3
2 2
CH –CH –CH -S-CH
3
3
2
2
CH =CH–S–CH
2
3
(CH =CH) S
2
2
Ph–S–CH3
Ph–S–Ph
S
1
0
S
15
90
83
a
2+
2
mmol of substrate at room temperature with 1 mmol of VO cata-
lyst 1, and 8 mmol of 20% aqueous H O , in glacial acetic acid (2 ml)
2
2
Fig. 5 Spectral changes associated with the formation of a possible
b
c
Separated yields
The products were established by comparison of physical and spec-
1
V-oxo intermediate species: a absorption spectrum of VOY in acetic
acid; b spectrum of the resulting solution obtained after addition of
troscopic properties with authentic compounds
20% H O at room temperature
2
2
1
3

Journal of the Iranian Chemical Society
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1
3