A new oxovanadium(IV) complex containing an O,N-bidentate Schiff base ligand: Synthesis at ambient temperature, characterization, crystal structure and catalytic performance in selective oxidation of sulfides to sulfones using H2O2 under solvent-free conditions

Article abstract of DOI:10.1016/j.molstruc.2015.08.060

A new bidentate ON Schiff base ligand, HL, was synthesized by simple condensation reaction of isopropylamine and salicylaldehyde. Then by reaction of HL and VO(acac)₂ in the ratio of 2:1 at ambient temperature, a new oxovanadium(IV) Schiff base complex, VOL₂, was synthesized. The Schiff base ligand and its oxovanadium(IV) complex were characterized by elemental analyses, FT-IR, ¹H NMR, ¹³C NMR and UV-visible spectroscopies. The crystal structure of oxovanadium(IV) complex, VOL₂, was also determined by single crystal X-ray analysis. The vanadium center in this structure is coordinated to two bidentate Schiff base ligands with the two nitrogen and two phenolate oxygen atoms in equatorial positions and one oxo oxygen in the axial position to complete the distorted trigonal bipyramidal N₂O₃ coordination sphere. Catalytic performance of the VOL₂ complex was studied in the selective oxidation of thioanisole with the green oxidant 35% aqueous H₂O₂ under solvent-free conditions and under organic solvents (EtOH, CHCl₃, CH₂Cl₂, DMF, CH₃CN, EtOAc) as a model. Due to better catalytic performance of the VOL₂ complex under solvent-free conditions, this complex used for the oxidation of the different sulfides to the corresponding sulfones under solvent-free conditions. The use of hydrogen peroxide as oxidant and the absence of solvent makes these reactions interesting from environmental and economic points of view.

Full text of DOI:10.1016/j.molstruc.2015.08.060

Accepted Manuscript
A new oxovanadium(IV) complex containing an O,N-bidentate Schiff base ligand:
Synthesis at ambient temperature, characterization, crystal structure and catalytic
performance in selective oxidation of sulfides to sulfones using H O under solvent-
2
2
free conditions
Saeid Menati, Hadi Amiri Rudbari, Mahsa Khorshidifard, Fariba Jalilian
PII:
S0022-2860(15)30247-7
10.1016/j.molstruc.2015.08.060
MOLSTR 21792
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 7 July 2015
Revised Date: 20 August 2015
Accepted Date: 25 August 2015
Please cite this article as: S. Menati, H.A. Rudbari, M. Khorshidifard, F. Jalilian, A new oxovanadium(IV)
complex containing an O,N-bidentate Schiff base ligand: Synthesis at ambient temperature,
characterization, crystal structure and catalytic performance in selective oxidation of sulfides to sulfones
using H O under solvent-free conditions, Journal of Molecular Structure (2015), doi: 10.1016/
2
2
j.molstruc.2015.08.060.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
A new oxovanadium(IV) complex containing an O,N-bidentate Schiff base
ligand: Synthesis at ambient temperature, characterization, crystal structure
and catalytic performance in selective oxidation of sulfides to sulfones using
H O under solvent-free conditions
2
2
a
b,
b
b
Saeid Menati , Hadi Amiri Rudbari
*, Mahsa Khorshidifard , Fariba Jalilian
a
Department of Chemistry, Khorramabad Branch, Islamic Azad University, Khorramabad, Iran.
Faculty of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.
b
*
Corresponding author. Tel.: +98 31 37934918.
E-mail address: h.a.rudbari@sci.ui.ac.ir; hamiri1358@gmail.com (H. Amiri Rudbari)

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Abstract
A new bidentate ON Schiff base ligand, HL, was synthesized by simple condensation reaction of
isopropylamine and salicylaldehyde. Then by reaction of HL and VO(acac) in the ratio of 2:1 at
2
ambient temperature, a new oxovanadium(IV) Schiff base complex, VOL , was synthesized. The
2
Schiff base ligand and its oxovanadium(IV) complex were characterized by elemental analyses,
1
13
FT-IR, H NMR, C NMR and UV–visible spectroscopies. The crystal structure of
oxovanadium(IV) complex, VOL , was also determined by single crystal X-ray analysis. The
2
vanadium center in this structure is coordinated to two bidentate Schiff base ligands with the two
nitrogen and two phenolate oxygen atoms in equatorial positions and one oxo oxygen in the axial
position to complete the distorted trigonal bipyramidal N O coordination sphere.
2
3
Catalytic performance of the VOL complex was studied in the selective oxidation of thioanisole
2
with the green oxidant 35% aqueous H O under solvent-free conditions and under organic
2
2
solvents (EtOH, CHCl , CH Cl , DMF, CH CN, EtOAc) as a model. Due to better catalytic
3
2
2
3
performance of the VOL complex under solvent-free conditions, this complex used for the
2
oxidation of the different sulfides to the corresponding sulfones under solvent-free conditions.
The use of hydrogen peroxide as oxidant and the absence of solvent makes these reactions
interesting from environmental and economic points of view.
Keywords
Schiff base, Oxovanadium(IV) complex, Oxidation of sulfide, H O , Solvent free
2
2

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Introduction
It is well-known that various self-assemblies are formed in the coordination chemistry of
vanadium, which exhibits a variety of oxidation states [1,2]. From a structural standpoint, the
formations of these kinds of molecular assemblies are determined by the preference of the
specific coordination geometries around the vanadium atoms [3]. There have been various
investigations of the tetravalent vanadium complexes that have strong vanadium oxygen double
bonds [4,5]. The oxovanadium(IV) complexes are used in many scientific fields, for example,
catalytic chemistry [6-12], bioinorganic chemistry [13], electrochemistry [14,15], and molecule-
based magnetism [16-23].
For preparation of oxovanadium(IV) complexes, [VO(acac) ] serves as a good precursor and
2
undergoes ligand exchange reaction where one or both acetylacetonato groups can easily be
exchanged with organic ligands having coordinating atoms of different potentialities. Usually
oxovanadium(IV) complexes form when [VO(acac) ] reacts with ligands under anhydrous
2
conditions. Under aerobic conditions most vanadium complexes stabilize in their highest
oxidation state (i.e. V). Factors such as nature of ligands, solvents, pH of the reaction and
reaction medium etc. have, however, great influence on the stoichiometry and nature of the
resulting complexes. Vanadium(IV) complexes obtained by this process, usually, contain only
2
+
one oxygen and stabilize as [VO] , while the oxidized complexes are associated with one or two
3
+
+
4+
oxygen atoms and stabilize with [VO] , [VO ] or [V O ] cores.
2
2
3
In contrast to many other metals, the vanadium atom can adopt a variety of different coordination
numbers. It generally shows no significant preferences for five-, six-, or seven-coordination; the
actual geometry is often highly ligand-dependent. Even well-known vanadium compounds are
often poorly described with respect to their stability and structure in aqueous solution [24].

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Five-coordinate geometries for transition metal ions such as vanadium are generally found to be
between square pyramidal (SP) and trigonal bipyramidal (TBP). Standard examples of each kind
exist for vanadium centers with, for example, many vanadyl porphyrin complexes adopting a
square pyramidal geometry [25] and triethanolamine-bound structures [26] with a trigonal
bipyramidal geometry. Examination of the five-coordinate geometry of the metal center in detail
is important because the ligands are likely to impact the specific structure at the metal ion.
Square pyramidal (SP) geometry is more common in small-molecule vanadium compounds. In
contrast, the trigonal bipyramidal (TBP) geometry is more common for vanadium complexes in
protein structures. Because the vanadium complexes in the protein are in the geometry that is less
prevalent (and therefore presumably less stable).
Indeed, Addison et al. have used a simple index parameter, tau (τ), to calculate any potential
distortion, where τ = (β – α )/60 [27]. In a simple ideal square pyramidal geometry where B, C,
D, E, and M are coplanar, the two largest basal angles α and β (Scheme 1) are both 180°, which
leads to the value τ = 0 for square pyramidal. In contrast, for the ideal trigonal bipyramidal
geometry, angles α and β, as shown in Scheme 1, lead to τ = (180–120)/60 = 1. To determine τ
for a given complex, A is designated so that it is not one of the four ligands that define the two
largest possible basal angles. Therefore, any structure that does not have the ideal square
pyramidal or trigonal bipyramidal geometry will ideally have a τ value between 0 and 1.
Monomeric oxovanadium(IV) complexes of the type [VOL ] are generally formed when
2
2
monobasic bidentate ligands (HL) react with [VO(acac) ] in dry solvents. In continuation of our
research works on Schiff bases ligands and complexes [28-36], in this study a new

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unsymmetrical Schiff base ligand (HL) derived from condensation of isopropylamine with
salicylaldehyde (Scheme 2) and its V(IV) complex (VOL ) have been synthesized (Scheme 3).
2
The crystal structure of the VOL was determine d by the single-crystal X-ray analysis. Unlike
2
the usual reports about five-coordinate vanadium(IV) complexes that adopt a square pyramidal
geometry, the geometry about the our synthesized oxovanadium(IV) Schiff base complex,
VOL , is best described as distorted trigonal bipyramidal. After synthesis and characterization of
2
vanadium(IV) complex, we used it in oxidation of sulfides with hydrogen peroxide as a green
oxidant. This process is clean, safe, selective, simple and cost-effective for the oxidation of
sulfides to the corresponding sulfones.
Experimental section
Materials and general methods
All reagents and solvents for synthesis and analysis were commercially available and used as
received without further purifications. . Fourier transform infrared (FT-IR) spectra were recorded
on a JASCO, FT/IR-6300 spectrometer in KBr pellets. About 1 mg of sample and 200 mg of
KBr powder were ground together and hydraulically pressed into a small pellet (~13 mm in
diam. and ~1 mm thick). The palletized KBr samples were dried in an oven at 100 C for 2 h prior
to analysis to minimize interferences from absorbed water. The sample was scanned from 4000–
-1
4
00 cm .
1
13
H and C NMR spectra for Schiff base ligand were recorded on a Bruker Avance 400
spectrometer using CDCl as solvent and TMS as reference. The elemental analysis was
3
performed on Leco, CHNS-932 and Perkin-Elmer 7300 DV elemental analyzers. UV–vis spectra
were recorded on JASCOV -670 UV–vis spectrophotometer. The oxidation products were

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quantitatively analyzed by gas chromatography (GC) on a Varian CP-3800 instrument using a
Varian CP-3800 gas chromatograph using capillary column CP-Wax 52 CB and/or CP-Sil 8 CB
and FID detector. Analysis parameters were as follows: initial temperature = 60 °C, initial time =
3
2
min, ramp = 10 °C/min, final temperature = 200 °C, final time = 5 min, detector temperature =
00 °C.
Single crystal diffraction studies
X-ray data for VOL was collected on a STOE IPDS-II diffractometer with graphite
monochromated Mo-Kα radiation. For VOL , a green crystal was chosen using a polarizing
2
2
microscope and they were mounted on a glass fiber which was used for data collection. Data was
collected at a temperature of 298(2) in a series of ω scans in 1° oscillations and integrated using
the Stöe X-AREA [37] software package. A numerical absorption correction was applied using
the X-RED [38] and X-SHAPE [39] software’s. The data were corrected for Lorentz and
Polarizing effects. The structures were solved by direct methods using SIR2004 [40]. The non-
2
hydrogen atoms were refined anisotropically by the full-matrix least-squares method on F using
SHELXL [41]. Hydrogen atoms attached to carbon were located at their idealized positions using
appropriate HFIX instructions in SHELXL (43 for hydrogens of aromatic and CH groups and 93
for hydrogens of CH groups) and included in subsequent refinement cycles in riding-motion
3
approximation with isotropic thermal displacements parameters (U ) fixed at 1.2 times U of
iso
eq
the carbon atom to which they are attached. Crystallographic data for VOL are listed in Table 1.
2
Selected bond distances and angles are summarized in Table 2.
Synthesis of Schiff-base ligand (HL)

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Isopropylamine (10 mmol) was dissolved in 30 ml of absolute methanol or chloroform and added
slowly to a stirred solution salicylaldehyde (10 mmol) in 30 ml of absolute methanol or
chloroform in ambient temperature. The color immediately changed to yellow. The mixture was
then stirred for 2h in ambient temperature. The reaction mixture was allowed to stand for slow
evaporation of the solvent until the formation of yellow oil. Anal. calc. for C H NO: C: 73.59,
1
0
13
1
H: 8.03, N: 8.58. Found: C: 73.66, H: 7.99, N: 8.64. HNMR (CDCl
3
, 400 MHz, 298 K) δ/ppm:
1
3.73 (1H, s, OH); 8.36 (s, 1H, N=CH); 7.32 (d of t, 1H, H ), 7.25 (d of d, 1H, H ), 6.99 (d, 1H,
b
d
1
3
H ), 6.89 (d of t, 1H, H ), 3.56 (septet, 1H, CH(CH ) ), 1.32 (d, 6H, CH ). CNMR (CDCl , 400
a
c
3 2
3
3
MHz, 298 K) δ/ppm: 162.09 (CH=N), 161.34, 131.98, 131.08, 118.87, 118.41, 116.95 (Ar
3 2 3 2
carbons), 60.00 (CH(CH ) ), 24.20 (CH(CH ) .
Preparation of VOL2
A MeOH solution (20 ml) of isopropylamine (10 mmol) was added dropwise to a MeOH
solution (20 ml) of the salicylaldehyde (10 mmol). The yellow, air stable solution was stirred for
2
h in ambient temperature. Then a solution of triethylamine (15 mmol) in absolute MeOH (5 ml)
was added to the solution. The mixture stirred for 10 min, and then a solution of VO(acac) (5
mmol) in absolute MeOH (20 ml) was added dropwise. Upon addition, immediate precipitation
of VOL complex occurs. The resulting solution was stirred for 2h at ambient temperature. The
2
2
green powder was isolated from the solution and purified by washing with cold methanol.
Appropriate single crystals for X-ray crystallography were obtained directly from the reaction
mixture. The typical yield was 89%. Anal. calc. for C H N O V: C: 61.38, H: 6.18, N: 7.16.
2
0
24
2
3
Found: C: 61.31, H: 6.21, N: 7.19.

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Catalytic activity and optimization of the reaction conditions
Oxidation of thioanisole with H O catalyzed by VOL Schiff base complexes
2
2
2
In this work to find the optimized conditions, we investigated the oxidation of thioanisole as a
model substrate using hydrogen peroxide (Scheme 4). The effect of solvent was studied for the
thioanisole oxidation. Generally, oxidation of thioanisole gives sulfoxide and sulfone. As shown
in Figure 7, under solvent free condition, the sulfone is the only product and the conversion is
higher than use of organic solvents.
General procedure for oxidation of thioanisole
The catalytic reactions were carried out in a glass tube containing a magnetic stirrer bar.
Hydrogen peroxide (1.5 mmol, 30%) was added to thioanisole (1 mmol) containing 0.01 mmol
of VOL complex and 3 ml of solvent (in free-solvent conditions, we don’t use of solvent). The
2
reaction mixture was stirred at 45 °C about 45 min with a magnetic stirrer bar. The reaction
progress was monitored by TLC, and the yield of products was determined by GC analysis using
toluene as the internal standard. After 45 min, ethyl acetate, 3 × 2 mL was added, and the organic
phase was extracted followed by evaporation of solvent. Higher purification of products was
achieved by chromatography over silica gel (eluent: n-hexane/ethyl acetate 70:30 for sulfone
isolation).
Procedure for the catalytic oxidation of sulfides under solvent free conditions
To a mixture of the sulfide (1 mmol) and 30% H O (1.5 mmol), the catalyst (0.01 mmol) was
2
2
added and the mixture was stirred at 45 °C for a specified time. The progress of the reaction was
monitored by TLC (petroleum ether/ethylacetate 8:3) and GC. After completion of the reaction,

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the product was extracted with ethyl acetate and the catalyst was separated by filtration. The
combined organics were washed with brine (5 ml) and dried over anhydrous Na SO . Further
2
4
purification was achieved by short-column chromatography on silica gel with EtOAc/n-hexane
as the eluent.
Results and discussion
The procedure synthesis of the Schiff base ligand, HL, reported in Scheme 2. The ligand
obtained by the self-condensation reaction between isopropylamine and salicylaldehyde in
methanol or chloroform as solvent at the ambient temperature. The Schiff base ligand is yellow
oil, stable in air and moderately soluble in the most common organic solvents Such as MeOH,
EtOH, CHCl , DMSO, DMF. Vanadium(VI) Schiff-base complex, VOL , was obtained by
3
2
treating VO(acac) with two molar equivalents of the ligand in the presence of triethylamine in
2
room temperature (Scheme 3).
Characteristic IR bands of the ligand and V(IV) complex are given in Table 3. In the FT-IR
-1
spectra of the ligand and V(IV) complex, a sharp band appeared at 1630 and 1638 cm , which is
attributed to the C=N vibrations of the imine group [28,36].
-1
Monomeric five coordinated vanadyl complex shows a band around 970 cm whereas in
polymeric six coordinated vanadyl complexes with linear (polymeric) …V=O…V=O… units
-1
this band shifts to lower wave numbers and appears around 860 cm [28, 42-44]. Thus the sharp
-1
band at 949 cm in the FT-IR spectrum of the vanadium(IV) Schiff base complex, VOL , is
2
attributed to V=O vibrations, indicating the monomeric form of the L V=O Schiff base complex
2
in the solid state, as confirmed by the single crystal X-ray diffraction. The presence of several

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-1
medium intensity bands between 3060 and 2870 cm in the ligand as well as in the V(IV)
complex suggests the existence of C–H stretching vibrations of aliphatic and aromatic protons.
1
13
1
13
Schiff base ligand, HL, was studied by H and C NMR experiments. The H and C NMR
were run immediately after solution in CDCl and gave the expected simple spectra, indicating
3
1
the integrity of the schiff base ligand. Fig. 1 shows the H NMR spectrum of HL. In agreement
with the structure of the HL, two singlet peaks with an integration of 1: 1 are seen at 13.73 and
8
.36 ppm, due to the phenolic (—OHphenolic) and iminic (Himine—C=N) protons, respectively [36].
1
H NMR spectrum of Schiff base ligand (Fig. 1) shows the methyl and methine protons as
doublet and septet at 1.32 and 3.56 ppm, respectively. The reason for splitting of —CH and —
CH peaks are coupling of hydrogens of —CH and —CH with together.
3
3
1
3
The CNMR spectrum shows 9 signals for schiff base ligand (HL). The peak at 162.09 ppm,
assignable to the imine carbon atoms, confirms the presence of the Schiff base in the complex
[36].
Electronic spectra of the free schiff base ligand, HL, and Vanadium(IV) complex, VOL , were
2
recorded in a MeOH solution. The absorption spectra are shown in Fig. 2. The spectra of HL
and VOL
Investigation of the electronic spectrum of Schiff base ligand exhibits five main peaks at 216,
52, 277, 314 and 398 nm (Fig. 2). The first, second and third peaks are attributed to the benzene
2
show five absorption bands in the UV–Vis region.
2
π→π* transitions. The azomethine π→π* and n→π* transitions are viewed at 314 nm
and 398 nm, respectively [36,45].
In the V(IV) complex, there are five bands in the region of 200–800 nm at 215, 257, 276,
3
03 and 395 due to the intra-ligand (π→π* and n→π*) and LMCT (O(p) →V(d)) transitions.

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Recall that this is in the range usually observed for azomethine n→π* and benzene π→π*
transitions and LMCT complexes [46, 47].
Crystal structure of VOL
2
An ORTEP view of VOL
2
with the atom-numbering scheme is presented in Fig. 3 and the
crystallographic data and selected bond lengths and angles are collected in Tables 1 and 2. The
X-ray structural study reveals that the complex crystallizes in the monoclinic system with P21
space group.
In this complex, the oxovanadium(IV) unit is coordinated to two bidentate Schiff base ligands
with the two nitrogen and two phenolate oxygen atoms and one oxygen atom to complete the
N O coordination sphere. The V—O distances of 1.901(4) and 1.914(3) Å and the V—N bond
2
3
lengths of 2.088(4) and 2.087(4) Å are similar to those seen in related complexes [28,48,49]. The
C=N bond distances are 1.296(6) Å (N1=C7) and 1.280(6) Å (N2=C17), which are consistent
with a slight elongation of the C—N double bond when coordinated to a metal center [28].
The structural chemistry of small vanadium(IV) molecules is well-established, and, as with
vanadium(V), the preponderance of five-coordinate vanadium(IV) complexes adopt a square
pyramidal or distorted square pyramidal geometry. To quantitatively compare these structures in
terms of their relative geometries about the vanadium atom, we have used the angular structural
parameter, τ, which is defined in Scheme 1 [27]. For purely square pyramidal complexes τ is
zero. Moving two trans ligands out of the equatorial square plane results in a distortion toward a
“trigonal bipyramid” for which τ approaches 1 (τ=1 for a perfect trigonal bipyramid). It should
be noted that the symmetry change shown in Scheme 1 is C to C if all equatorial ligands are
4
ν
2ν
equivalent. As reported in other papers [50,51], we can describe complexes with 0 ≤ τ ≤ 0.5 as

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distorted square pyramids and with 0.5 ≤ τ ≤1 as distorted trigonal bipyramids. A search of the
Cambridge Structural Database for monomeric five-coordinate vanadyl complexes with O/N
donors yields more than 100 structures. A few of them have distorted trigonal bipyramids
structure, while most of them have distorted square pyramidal geometry. The paucity of trigonal
bipyramids structures for vanadium(IV) suggests that there are electronic and/or steric
requirements that preclude the formation of the trigonal bipyramids geometry.
According to the above description, the geometry about the our synthesized oxovanadium(IV)
2
Schiff base complex, VOL , is best described as distorted trigonal bipyramidal with τ = 0.58
unlike the usual reports about five-coordinate vanadium(IV) complexes that adopt a square
pyramidal geometry. The two bidentate Schiff base ligands in complex are oriented trans to each
other, with the oxo ligand in the “axial” position (axial direction defined as being along the
highest rotation axis) and the imine nitrogens and the phenolate oxygens forming the
“equatorial” plane. The β-angle N1—V(1)—N2 (see Scheme 1) is 163.76(17)° and the α-angle
O1—V(1)—O2 is 129.24(18)°. Thus, the phenolate oxygens are displaced out of the “equatorial”
plane away from the axial oxo ligand. The V—N and V—O distances (Table 2) are similar to
those found in other V(IV) complexes with Schiff base ligands [ 28,52,53 ].
A search for salicylidenimine oxovanadium(IV) complexes with the Cambridge Structural
Database System 2014 showed that with going from non-bulky to bulky group on nitrogen atom
of imine group, the τ parameter don’t change in regular way (Fig. 4) [54-57]. This means that
increase in size of R substituent of iminic nitrogen atoms don’t have a direct relationship with τ
parameter or in other words we can say changing τ parameter depends on a delicate balance of
the steric and electronic factors. For example in similar complexes (row 5 and 6 in Fig. 4), only
with replacement of H with Cl (R′ group), the geometry of complex changed from distorted

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square pyramids (τ=0.44) to distorted trigonal bipyramidal (τ=0.62). This suggests an electronic
preference for distorted trigonal bipyramidal geometry.
As depicted in Fig. 5, in the crystal packing of VOL , four non-classical intramolecular hydrogen
2
bonds and two non-classical intermolecular hydrogen bonds of the type C—H…O are formed
between the C—H in the CH and CH groups as donors and oxygen in the vandyl moiety as
2
acceptor.
2
Also, in the lattice, the monomeric VOL moieties are linked through C–H···π stacking
interactions (Fig. 6). C–H…π edge-to-face interactions are present between hydrogens of CH3
groups with H…π distance of 2.90 Å for C(3)—H(3)…Cg(1), 2.94 Å for C(10)—
H(10B)…Cg(2), 2.66 Å for C(10)—H(10C)…Cg(1) and 2.97 Å for C(19)—H(19B)…Cg(2)
[Cg(1) is centroid for V(1)-O(1)-C(1)-C(6)-C(7)-N(1) ring and Cg(2) is centroid for V(1)-O(2)-
C(11)-C(16)-C(17)-N(2) ring].
Catalytic activity
Organosulfur compounds, such as sulfoxides and sulfones, are useful synthetic reagents in
organic chemistry. Sulfones represent an important class of the compounds due to their
properties and reactivity. The strong inductive effect of the sulfone moiety and the broad
possibilities of the subsequent transformations make them very attractive in the field of
asymmetric organocatalysis [58]. Numerous sulfones and their derivatives exhibit different
biological activities and have been a subject of an extensive investigation [59-62]. Phenyl- and
heteroaryl-substituted sulfones are important reactants in the Julia-type olefination reaction [63];
in addition, the Grignard reagents of sulfones are useful intermediates for the synthesis of
variously functionalized sulfones [64].

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For the oxidation of sulfides to sulfones, a range of oxidants have been investigated. For
example, stoichiometric oxidation of sulfides by sodium percarbonate, sodium perborate, H IO ,
5
6
and the other unclean oxidant was studied [65–67]. Unfortunately, most of these reagents are not
satisfactory for medium- to large-scale synthesis because of the low content of effective oxygen,
toxic oxidants and solvents, the formation of environmentally unfavorable co-products, and high
cost. Obviously, current demand for environmentally friendly processes requires the
development of green oxidation methods that employ clean oxidants, such as hydrogen peroxide.
During recent years, aqueous hydrogen peroxide has been one of the most attractive “green
oxidants” for its environmentally benign characteristics, only producing harmless water as by-
product, safety in storage and operation, cheapness, readily available and the high effective
oxygen content [68-71]. Also solid-phase-assisted reaction systems without organic solvents
have been increasingly attracted as environmentally benign organic reaction systems [36,72].
In this work, initially the catalytic performance of VOL schiff base complex in the oxidation of
2
sulfides was investigated with the oxidation of thioanisole (1 mmol) by 30 % H O (1.5 mmol) in
2
2
the presence of 0.01 mmol of VOL
2
at 45 °C under free-solvent conditions and several organic
Cl , DMF, CH CN, EtOAc) for 45 min. The results are
solvents (Solvents: EtOH, CHCl , CH
3
2
2
3
illustrated in Fig. 7. Under reaction conditions the presence of catalyst for conversion of
thioanisole into the corresponding sulfone and/or sulfoxide was crucial. As depicted in Fig. 7,
under solvent-free conditions and in presence of DMF and EtOAc as solvent, the only product is
sulfone. But the maximum conversion (>98) was obtained under solvent-free conditions. So, our
efforts were concentrated on the best conditions for producing the sulfone.
Optimum reaction conditions were obtained by varying amounts of hydrogen peroxide. From
2 2
Fig. 8, the dependence of selectivity pattern to the different amounts of H O is evident. The best

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amount of H
2
O
2
for higher yields of sulfone was 1.5 mmol. From these results, the amounts of
hydrogen peroxide play an important role in selective oxidation of thioanisole into the
corresponding sulfone. After optimization of the reaction conditions, to expand the scope of this
catalytic system, the oxidation of various sulfides were examined (Table 4). Different sulfides
such as phenyl alkyl-sulfides, dialkyl-sulfides and unsaturated sulfides were converted into the
corresponding sulfones in the presence of VOL as catalyst. Sulfide with C=C double bond
2
(Table 1, entry 5) was selectively oxidized into its corresponding sulfone without affecting these
reactive functional groups.
Conclusion
In conclusion, we have investigated the synthesis and characterization of a new bidentate ON
Schiff base ligand, HL, and its monomeric oxovanadium(IV) Schiff base complex, VOL . The
2
crystal structure of the VOL was determined by the single crystal X-ray analysis. The
2
coordination geometry around the vanadium center can be described as a distorted trigonal
bipyramidal formed by two phenolato oxygen and two imine nitrogen atoms of the two Schiff
base ligands in the equatorial position and oxo oxygen atom coordinated in the axial position.
Unlike the usual reports about five-coordinate vanadium(IV) complexes that adopt a square
pyramidal geometry, the geometry about the our synthesized oxovanadium(IV) Schiff base
complex, VOL , is best described as distorted trigonal bipyramidal with τ = 0.58. Due to better
2
catalytic performance of the VOL complex under solvent-free conditions, this complex used for
2
the oxidation of the different sulfides to the corresponding sulfones under solvent-free
conditions.

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Acknowledgement
Support for this research by the University of Isfahan and Islamic Azad University (Shahrekord
Branch) is acknowledged.
Appendix A. Supplementary data
CCDC 1410577 contains the supplementary crystallographic data for VOL . Copies of the data
2
can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB21EZ,
UK (Fax: +44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).
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A
B
β
β
B
E
D
M
A
M
C
α
E
D
C
α
square pyramidal
α = β = 180°
τ = 0
trigonal bipyrimidal
α = 120° ; β = 180°
τ = 1
β - α
τ =
6
0
Scheme 1. The distortion parameter τ is defined in terms of the angles α and β as shown. τ = (β -
α )/60, such that in the ideal square pyramid on the left, τ = 0, and in the ideal trigonal bipyramid
on the right, τ = 1.

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OH
OH
CHO
NH2
MeOH or CHCl3
+
CH3
CH3
N
Ambient temperature
H C
3
H
H
HL
CH3
Scheme 2. Synthesis of ligand (HL)
O
V
OH
VO(acac) , NEt
O
N
2
3
N
N
Ambient temperature
O
Scheme 3. Synthetic routes for the preparation of VOL complex
2

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O
O
S
cat. VOL2
S
S
1
.5 eq. H O (30% eq.)
2
2
O
+
45 °C , 45 min
Scheme 4. Oxidation of thioanisole with hydrogen peroxide in the presence of VOL Schiff base
2
complex as catalyst under solvent-free conditions and under organic solvents (EtOH, CHCl3,
CH Cl , DMF, CH CN, EtOAc)
2
2
3

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1
Fig. 1. H NMR spectrum of Schiff base ligand (HL). A star mark in black color represents peak
for CDCl3.

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Fig. 2. UV-Visible spectra of the Schiff base Ligand (HL) and V(IV) complex (VOL ) in MeOH
2
at 25 °C.

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Fig. 3. ORTEP representation of VOL . Displacement ellipsoids are drawn at the 30%
2
probability level and H atoms are shown as small spheres of arbitrary radii.
O
V
R
H
CH3
CH
R′
α
β
τ
Ref.
R
H
H
H
H
H
139.2 152.3 0.22 54
129.6 162.5 0.55 55
130.4 162.0 0.53 56
129.2 163.7 0.58 This paper
133.3 159.6 0.44 57
R'
O
N
N
O
2 2 3
(CH CH )
CH(CH3)2
CH(CH )(C H )
CH(CH )(C H ) Cl 127.5 164.5 0.62 57
R'
3
6
5
R
3
6
5
Fig. 4.

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D—H...A
D—H
0.98
0.93
0.98
0.98
0.98
H...A D...A
D—H...A
3.017(7) 109
3.372(6) 147
Symmetry code
x,y,z
2-x,1/2+y,1-z
x,y,z
x,y,z
x,y,z
C(8)—H(8)...O(2)
C(17)—H(17)...O(3)
C(18)—H(18)...O(1)
C(8)—H(8)...O(3)
C(18)—H(18)...O(3)
2.55
2.56
2.55
2.93
2.93
Intra
Inter
Intra
Intra
Intra
3.046(6) 111
3.339(8) 106
3.281(8) 102
Fig.5. Intramolecular and intermolecular hydrogen bond geometries (Å,°).