Importance of ground state stabilization in the oxovanadium(IV)-salophen mediated reactions of phenylsulfinylacetic acids by hydrogen peroxide – Non-linear Hammett correlation

Article abstract of DOI:10.1016/j.poly.2016.06.038

A systematic study on the oxidative decarboxylation of a series of phenylsulfinylacetic acids (PSAA) by hydrogen peroxide with four oxovanadium(IV)-salophen catalysts in 100% acetonitrile medium is presented. The hydroperoxovanadium(V)-salophen generated from the reaction mixture is identified as the bonafide active oxidizing species. Introduction of electron donating groups (EDG) in the oxovanadium(IV)-salophen catalyst and electron withdrawing groups (EWG) in PSAA enhances the reactivity, whereas EWG in the catalyst and EDG in PSAA have a retarding effect on the reaction. A Hammett correlation displays a non-linear downward curvature, which consists of two intersecting straight lines and the ρ value shifts from small positive to moderately high as the substituents change from EWG to EDG. The importance of the ground state stabilization of PSAA is inferred from a linear Yukawa–Tsuno plot. Based on the observed substituent effects and the spectral changes, a mechanism involving electrophilic attack of PSAA on the nucleophilic peroxo oxygen atom of the vanadium complex in the rate determining step followed by oxygen atom transfer is proposed.

Full text of DOI:10.1016/j.poly.2016.06.038

Polyhedron 117 (2016) 496–503
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Importance of ground state stabilization in the
oxovanadium(IV)-salophen mediated reactions of phenylsulfinylacetic
acids by hydrogen peroxide – Non-linear Hammett correlation
b
b
P. Subramaniam a, , R. Jeevi Esther Rathnakumari , J. Janet Sylvia Jaba Rose
⇑
a Research Department of Chemistry, Aditanar College of Arts and Science, Tiruchendur 628 216, Tamil Nadu, India
b Department of Chemistry, Nazareth Margoschis College, Nazareth 628 617, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A systematic study on the oxidative decarboxylation of a series of phenylsulfinylacetic acids (PSAA) by
hydrogen peroxide with four oxovanadium(IV)-salophen catalysts in 100% acetonitrile medium is pre-
sented. The hydroperoxovanadium(V)-salophen generated from the reaction mixture is identified as
the bonafide active oxidizing species. Introduction of electron donating groups (EDG) in the oxovana-
dium(IV)-salophen catalyst and electron withdrawing groups (EWG) in PSAA enhances the reactivity,
whereas EWG in the catalyst and EDG in PSAA have a retarding effect on the reaction. A Hammett corre-
lation displays a non-linear downward curvature, which consists of two intersecting straight lines and
Received 2 May 2016
Accepted 14 June 2016
Available online 4 July 2016
Keywords:
Oxovanadium(IV)-salophen complex
Phenylsulfinylacetic acid
Non-linear Hammett
the
q value shifts from small positive to moderately high as the substituents change from EWG to
EDG. The importance of the ground state stabilization of PSAA is inferred from a linear Yukawa–Tsuno
plot. Based on the observed substituent effects and the spectral changes, a mechanism involving elec-
trophilic attack of PSAA on the nucleophilic peroxo oxygen atom of the vanadium complex in the rate
determining step followed by oxygen atom transfer is proposed.
Ground state stabilization
Oxygen atom transfer
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
ligands have been used in medicinal studies as models for superox-
ide dismutase [8,9]. While manganese-salophen/salen derivatives
The coordination chemistry of vanadium is of current interest
because of its existence in abiotic as well as biotic systems. The
presence of vanadium in biological systems, its insulin enhancing
action [1] and anticancer activity [2] has driven a considerable
amount of research. Several Schiff base vanadium complexes have
been used as insulin enhancing agents, oral insulin substitutes for
the treatment of diabetes and for the treatment of obesity and
hypertension [3]. Schiff base complexes formed between oxovana-
dium(IV)/dioxovanadium(V) and chelating ligands with hetero
atoms are found to be involved in a variety of biochemical and
medicinal processes, such as haloperoxidation [4], phosphorylation
[5], nitrogen fixation [6], tumor growth inhibition and prophylaxis
against carcinogenesis [7]. Metal complexes of salen and salophen
display cyto-protective features in fibroblast cultures via hydrogen
peroxide scavenging [10], oxovanadium(IV)-salophen complexes
inhibit the growth of AGS gastric cell lines [11]. Certain metal-
salophen complexes have been designed for their application as
functional materials [12].
Recent advances have shown vanadium complexes to be effec-
tive catalysts for the activation of peroxides by virtue of vanadium
in terms of stereoselectivity, reactivity and specificity [13,14]. The
capability of these complexes to form metalloperoxo species,
which in turn effectively transfer an oxygen atom to reductants
with a high degree of selectivity, made them synthetically useful
for obtaining valuable molecules. Oxovanadium-Schiff base com-
plexes have been reported as effective catalysts in the oxidation
of sulfides [15,16], alcohols [13], phenols [17], tertiary amines to
N-oxides [18], epoxidation of olefins [19] and hydroxylation of
phenols [20,21]. Salophen, a tetradentate Schiff base derived from
1,2-phenylenediamine and salicylaldehyde, and its derivatives are
able to stabilize different metal ions in various oxidation states and
control a variety of catalytic transformations [22].
Abbreviations: PSAA, phenylsulfinylacetic acid; EWG, electron withdrawing
group; EDG, electron donating group; GS, ground state; OD, optical density;
r, correlation coefficient.
⇑
Corresponding author.
E-mail addresses: subramaniam.perumal@gmail.com (P. Subramaniam),
jeeviesther@gmail.com (R. Jeevi Esther Rathnakumari), janetjebaraj@gmail.com
(J. Janet Sylvia Jaba Rose).
Oxidative decarboxylation plays an important role in biological
systems, organic synthesis and drug metabolism. In humans,
http://dx.doi.org/10.1016/j.poly.2016.06.038
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

P. Subramaniam et al. / Polyhedron 117 (2016) 496–503
497
oxidative decarboxylation continually produces carbon dioxide in
2. Experimental
all live tissues during metabolism. Oxidative decarboxylation, in
conjunction with electron transport and oxidative phosphoryla-
tion, takes place in the mitochondrial membrane and provides
the basis of human cell respiration [23]. Anti-inflammatory drugs
such as indomethacin and ibuprofen are decarboxylated during
drug metabolism by cytochrome p-450 in vivo and the carbon
dioxide released was found to reduce pain [24,25]. Enantiopure
sulfoxides constitute a class of versatile chiral controllers and use-
ful synthons in asymmetric synthesis and the pharmaceutical
industry [26]. Further, sulfur substituted amino acids, vitamins,
drugs and other xenobiotics are often involved in metabolism/-
catabolism [27]. The chemistry of sulfones has been explored due
to their therapeutic activities like antimicrobial [28], anticancer
[29], anti-HIV [30], antimalarial [31] and anti-inflammatory [32].
Additionally, several drug molecules used for the treatment of
leprosy, dermatitis herpetiformis [33] and tuberculosis [34] are
found to contain the sulfone moiety. Hence the oxidation of
organic sulfur compounds has been the subject of extensive studies
in recent years. Phenylsulfinylacetic acid (PSAA), a sulfoxide con-
taining acid group, is an ambidentate ligand and with its three
donor atoms acts as a good chelating agent.
2.1. Synthesis of the oxovanadium(IV)-salophen complexes
The synthesis of the oxovanadium(IV)-salophen complexes was
accomplished by a procedure slightly different from that reported
in the literature [46]. To a hot methanolic solution (50 cc) of vanadyl
sulfate (VOSO4ꢀ5H2O; 0.25 g, 1 mM), the appropriate salophen
ligand (1 mM) was added with stirring. The mixture was refluxed
for one hour and cooled to room temperature. The green crystals
separated were filtered, washed with diethyl ether and dried.
Recrystallization was carried out from pure hot methanol. The
absorption spectral data for the complexes (I to IV) are consistent
with the literature data [11,44]. Their absorption maxima are 396
(I), 440 (II), 416 (III) and 410 nm (IV). The salophen ligands were
prepared by a procedure similar to that reported in the literature
[44]. Two equivalents of salicylaldehyde were dissolved in a mini-
mum volume of boiling methanol (20 ml) and it was added drop-
wise to one equivalent of 1,2-benzenediamine in 5 ml methanol.
The solution was refluxed for one hour and then cooled to room tem-
perature. The yellow solid thus obtained was filtered, washed with
methanol, diethyl ether and dried. All the compounds gave UV–Vis
spectra consistent with their structures and literature data [47].
Recently, systematic attempts have been made with PSAA in
this laboratory to study the mechanism and substituent effects
during co-oxidation with oxalic acid by Cr(VI) [35], oxidative
decarboxylation by Cr(VI) [36] and its reaction in the presence of
cetyltrimethylammonium bromide [37] and picolinic acid [38],
electron transfer reactions with iron(III)-polypyridyl complexes
[39] and the catalytic effect of nitrogen bases [40] and ligand oxi-
des [41] in the reactions with oxo(salen)chromium(V) complexes.
Although vanadium(IV)-salen systems have been used as efficient
catalysts in the asymmetric oxidation of organic sulfides by Fujita
et al. [42] and Sun et al. [43], no detailed kinetic study has been
reported so far in the literature on organic sulfur compounds by
oxovanadium(IV)-salophen complexes. Nevertheless, Coletti et al.
[44] used several salophen oxovanadium complexes as catalysts
during the selective oxidation of sulfides to sulfoxides. Recently
Sankareswari et al. [11] have synthesized eight salophen ligands
and their oxovanadium(IV) complexes having different sub-
stituents and characterized these complexes by UV–Vis, FT-IR,
EPR, ESI-MS, 1H and 13C spectral methods. They also reported their
interaction with bovine serum albumin and cytotoxicity against
cancer. The synthesis and X-ray structural characterization of such
complexes were reported even earlier by Weberski Jr. et al. [45]. All
these facts paved the way to explore the catalytic activity of oxo-
vanadium(IV)-salophen complexes in the reactions of PSAAs with
H2O2 and the results obtained on the mechanistic and substituent
effects are discussed in this paper. The overall scheme of the reac-
tion is as shown below.
2.2. Preparation of phenylsulfinylacetic acids
Phenylsulfinylacetic acid and its meta- and para-substituted
acids were prepared from the corresponding phenylmercap-
toacetic acids by controlled oxidation with an equimolar amount
of hydrogen peroxide [35]. PSAA and the p-chloro, m-chloro,
p-bromo, p-fluro, p-methoxy and p-ethoxy PSAAs were purified
by recrystallization from 1:1 benzene and ethyl acetate solvent
mixture, whereas p-methyl PSAA was recrystallized with chloro-
form and PET ether. The purity of the PSAAs was checked by melt-
ing point [35] and LC–MS. The recrystallized PSAAs were kept
inside a vacuum desiccator in order to prevent their decomposition
with moisture in the atmosphere. The phenylmercaptoacetic acids
were prepared by the condensation of the corresponding thiophe-
nols with chloroacetic acid in alkaline medium [35].
Salicylaldehyde, 5-methyl, 5-methoxy and 5-chloro salicylalde-
hydes (Alfa Aeser), VOSO4ꢀ5H2O (Sigma–Aldrich), thiophenol (SD
fine) and the substituted thiophenols (Sigma–Aldrich) were pur-
chased and used as such. H2O2 (GR, Merck) and acetonitrile (HPLC
grade, Merck) were used as received.
2.3. Kinetic studies
A double beam BL 222 Elico UV–Vis bio spectrophotometer with
an inbuilt thermostat was employed to record the absorption

498
P. Subramaniam et al. / Polyhedron 117 (2016) 496–503
spectra, measure the absorbance and to follow the kinetics of the
reactions. The kinetic study for the oxidative decarboxylation of
PSAA and substituted PSAAs by H2O2 in the presence of oxovana-
dium(IV)-salophen complexes was carried out in 100% acetonitrile
medium under pseudo first order conditions with an excess PSAA
concentration over the H2O2 and complex concentrations. The
reactions were started by quickly injecting H2O2 into the reaction
mixture containing PSAA and the oxovanadium(IV)-salophen com-
plex at zero time, in a quartz cuvette. The rate of the reaction was
measured by following the decay of the absorbance of the
hydroperoxovanadium(V)-salophen complex with time at the
appropriate wavelength. Neither the oxovanadium(IV)-salophen
complex nor hydrogen peroxide efficiently oxidizes the PSAA
alone, they do so only in combination in acetonitrile medium.
The pseudo first order rate constants were calculated from the
slope of linear plots of log OD versus time. The second order rate
constants were calculated by dividing the pseudo first order rate
constants with the concentration of the substrate. The error in
the rate constants was calculated according to 95% of the student’s
t-test. The thermodynamic parameters, the enthalpy of activation
unit slope values obtained from the plots of log k1 versus log
[PSAA], the observed linear plots between k1 and [PSAA] and the
constant second order rate constants at different [PSAA] for all
the four oxovanadium(IV)-salophen complexes under pseudo first
order conditions confirm the first order dependence of the reaction
on PSAA.
Although the reaction is found to exhibit first order dependence
on all the four oxovanadium(IV)-salophen complexes, as evidenced
from excellent linear log OD versus time plots, the pseudo first
order rate constants are found to decrease with the increase in con-
centration of complexes I to IV. In contradiction, it is observed that
at lower concentrations of H2O2 there is a significant increase in
the pseudo first order rate constants, while at higher H2O2 concen-
trations, beyond a particular concentration the reaction rate begins
to decrease for all the complexes I to IV. The variation of the rate
constant with the concentrations of vanadium complex and H2O2
is shown in Table 2. Among the four oxovanadium(IV)-salophen
complexes, the methoxy (II) and methyl (III) oxovanadium(IV)-sal-
ophen complexes are found to be more reactive than the parent
complex (I) and the chlorooxovanadium(IV)-salophen complex
(IV) shows the least reactivity.
(D D
àH) and entropy of activation ( àS), were evaluated from a linear
Eyring’s plot of log (k2/T) versus 1/T by the least square method.
3.1. Activation parameters and linear free energy relationship
2.4. Product analysis
The substituent effect and linear free energy relationship were
studied using several para- and meta-substituted PSAAs with
H2O2 in the presence of complexes I, II and IV at 30 °C and also
at three different temperatures with complex I to understand the
nature of the transition state, rate determining step and the extent
of charge transfer. It has been noted that the temperature has a
positive effect on rate only in a limited range of temperatures.
The reaction rate is too slow to be measured at low temperatures,
while at high temperatures the rate decreases. The observed
decrease in rate at higher temperatures beyond a particular tem-
perature may be due to the decomposition of H2O2. The thermody-
A solution containing 3 mM of PSAA, 0.3 mM of oxovanadium
(IV)-salophen complex and 3 mM of H2O2 in 10 ml CH3CN was stir-
red at room temperature until the completion of the reaction. The
reaction mixture was then evaporated to dryness and the solid
obtained was extracted with chloroform to recover the organic
product. The chloroform extract was dried over sodium sulfate
and the solvent was removed by evaporation. The product
obtained was analyzed using IR and mass spectral techniques.
Strong bands at 1148 and 1290 cmꢁ1, characteristic of symmetric
and asymmetric stretching vibrations respectively of the >SO2
group, were observed in the IR spectrum (Supporting information
Fig. S1). The peak eluted in the LC–MS at a retention time of
1.829 min, ionizing in the APCI (+) mode at a mass of 157 (Support-
ing information Fig. S2) and the parent peak at M/Z = 156 in the
GC–MS (Supporting information Fig. S3) clearly show that methyl
phenyl sulfone is the only product of the reaction.
namic parameters
D D
àH and àS, evaluated from the slope and
intercept of the linear Eyring’s plots, are shown in Table 3. The
entropies of activation are found to be highly negative, while the
enthalpies of activation have relatively small positive values. The
positive
nature. The correlation between
D
àH values suggest that the reaction is endothermic in
D
àH and àS for the various sub-
D
stituted PSAAs is found to be linear (Supporting Information
Fig. S4). The isokinetic temperature calculated from the linear
3. Results
D D
àH versus àS plot is found to be 326 K, which is above the exper-
imental temperature and indicates that the application of the
Hammett equation to the title reaction is valid. The linear isoki-
netic relation between
the same mechanism.
The observed kinetic data in Table 3 show that the reaction is
sensitive to the change of substituents in the phenyl ring of PSAA
and the salophen moiety of the complex. It is observed that elec-
tron donating groups (EDG) in PSAA retard the rate of reaction
while electron withdrawing groups (EWG) accelerate the rate.
The analysis of the kinetic data and the plot of log k2 versus the
The kinetics of the reaction were measured out at different ini-
tial concentrations of the reactants, oxovanadium(IV)-salophen,
PSAA and H2O2, keeping the other reaction conditions as constant.
The pseudo first order rate constants, evaluated from the linear
portions of decrease in OD at different [PSAA], show a linear
increase with concentration. However, the second order rate con-
stants remain the same for all concentrations. The calculated val-
ues of the pseudo first order and second order rate constants as a
function of concentration of PSAA are presented in Table 1. The
D D
àH and àS reveals that all PSAAs follow
Table 1
Dependence of the pseudo first order and second order rate constants on [PSAA].
102 [PSAA] (M)
I
II
III
IV
103 k1 (sꢁ1
)
102 k2 (Mꢁ1 sꢁ1
)
103 k1 (sꢁ1
)
102 k2 (Mꢁ1 sꢁ1
)
103 k1 (sꢁ1
)
102 k2 (Mꢁ1 sꢁ1
)
103 k1 (sꢁ1
)
102 k2 (Mꢁ1 sꢁ1
)
3.0
5.0
7.0
9.0
11.0
0.550 0.02
0.841 0.01
1.39 0.03
1.84 0.02
2.30 0.05
1.83 0.67
1.68 0.20
1.98 0.43
2.04 0.22
2.09 0.45
6.76 0.05
12.3 0.08
18.6 0.02
23.3 0.01
29.8 0.11
22.5 1.7
24.6 1.6
30.7 0.29
25.9 0.11
27.1 1.0
4.15 0.03
7.62 0.02
10.6 0.04
13.1 0.06
16.3 0.01
13.8 1.0
0.119 0.01
0.152 0.01
0.210 0.01
0.291 0.02
0.341 0.02
0.397 0.33
0.304 0.20
0.300 0.14
0.322 0.22
0.310 0.18
15.2 0.40
15.1 0.57
14.6 0.67
14.8 0.09
Complex = 1.5 ꢂ 10ꢁ4 M; [H2O2] = 3.0 ꢂ 10ꢁ3 M; T = 30 °C; solvent = 100% CH3CN.

P. Subramaniam et al. / Polyhedron 117 (2016) 496–503
499
Table 2
Dependence of the pseudo first order rate constant on oxovanadium(IV)-salophen [I–IV]a and [H2O2].b
104 [complex] (M)
103 [H2O2] (M)
103 k1 (sꢁ1
)
I
II
III
IV
0.25
0.50
0.75
1.0
1.5
3.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3.0
3.0
3.0
3.0
3.0
3.0
1.0
3.0
4.0
5.0
7.0
9.0
11.0
13.0
1.58 0.04
1.24 0.02
1.00 0.01
0.83 0.09
0.55 0.02
0.17 0.03
1.01 0.01
1.84 0.08
2.83 0.06
5.58 0.09
8.69 0.15
13.0 0.10
11.5 0.20
8.10 0.17
20.9 0.12
16.6 0.02
12.1 0.06
9.61 0.15
6.76 0.05
3.81 0.02
14.1 0.06
23.3 0.01
29.8 0.53
40.9 0.21
52.3 0.09
61.8 0.32
59.3 0.11
48.1 0.20
13.9 0.80
11.1 0.21
8.18 0.04
6.02 0.13
4.15 0.03
1.49 0.01
7.59 0.07
13.1 0.06
20.9 0.51
28.5 0.15
24.4 0.09
45.2 0.61
40.1 0.08
32.3 0.12
0.310 0.02
0.235 0.03
0.154 0.01
0.138 0.03
0.119 0.02
–
0.203 0.01
0.291 0.04
0.510 0.02
0.809 0.06
1.09 0.01
1.18 0.07
1.11 0.05
1.02 0.03
a
[PSAA] = 3.0 ꢂ 10ꢁ2 M.
b
[PSAA] = 9.0 ꢂ 10ꢁ2 M, T = 30 °C, Solvent = 100% CH3CN.
Table 3
Second order rate constants (k2), thermodynamic and Hammett parameters for the reactions of PSAAs with H2O2 catalyzed by oxovanadium(IV)-salophen complexes.
Y
I
D
àH (kJ molꢁ1
)
ꢁ
D
àS (JKꢁ1 molꢁ1
)
II
IV
q
r
102 k2 (Mꢁ1 sꢁ1
)
102 k2 (Mꢁ1 sꢁ1
)
25 °C
30 °C
35 °C
30 °C
30 °C
p-Cl
m-Cl
p-F
p-Br
H
7.98 0.15
8.98 0.48
7.46 0.39
8.20 0.18
7.28 0.19
4.79 0.11
1.40 0.06
3.89 0.09
0.239 0.03
0.982
10.6 0.23
12.6 0.31
8.91 0.35
11.3 0.41
8.70 0.16
5.05 0.09
2.82 0.02
4.29 0.10
0.451 0.11
0.987
12.9 0.29
15.8 0.47
11.5 0.25
13.4 0.34
10.6 0.12
7.50 0.22
3.55 0.11
6.64 0.24
0.446 0.03
0.986
33.7 3.1
40.9 2.8
29.7 1.5
35.0 1.2
25.8 1.7
30.6 2.8
68.4 2.1
37.2 3.0
153 6.8
128 8.1
167 7.3
148 5.3
180 12
168 9.2
50.2 8.5
148 7.8
91.2 0.31
99.9 0.50
87.1 0.44
95.5 0.40
79.4 0.10
50.1 0.21
20.0 0.35
35.5 1.3
0.247 0.03
0.970
1.79 0.08
1.89 0.12
1.71 0.03
1.81 0.07
1.60 0.09
1.12 0.03
0.591 0.06
0.901 0.09
0.180 0.02
0.985
ꢁ1.74 0.09
ꢁ1.72 0.14
ꢁ1.76 0.10
ꢁ1.72 0.11
ꢁ1.70 0.10
ꢁ1.66 0.12
ꢁ1.53 0.08
ꢁ1.60 0.06
0.999
0.999
0.998
0.999
0.999
0.997
0.999
0.999
p-Me
p-OMe
p-OEt
q+EWG
r
q+EDG
r
1.12 0.07
0.998
1.30 0.04
0.998
0.853 0.07
0.999
1.40 0.05
0.988
1.01 0.04
0.995
[I = II = IV] = 1.0 ꢂ 10ꢁ4 M, [PSAA] = 5.0 ꢂ 10ꢁ2 M, [H2O2] = 5.0 ꢂ 10ꢁ3 M, solvent = 100% CH3CN.
Fig. 1. Hammett plot for the reactions between the PSAAs and H2O2 in the presence of I and II. [I] = [II] = 1.0 ꢂ 10ꢁ4 M, [PSAA] = 5.0 ꢂ 10ꢁ2 M, [H2O2] = 5.0 ꢂ 10ꢁ3 M,
solvent = 100% CH3CN, T = 30 °C.
Hammett’s
r
substituent constants show non-linear behavior. The
withdrawing substituents, which are indicated by rather small
positive values (0.180–0.451). A similar trend of the substituent
effect has already been reported in the alkaline hydrolysis of
o-arylthionobenzoates [48], pyridinolysis of substituted 2,4-
dinitrophenyl benzoates [49] and in the reactions of nucleophiles
Hammett plots (Fig. 1) exhibit distinct curvature with two inter-
secting linear portions. The data in Table 3 and Fig. 1 reveal that
the reaction is fairly susceptible to electron releasing substituents
q
with positive
q values (0.853–1.40) and less susceptible to electron

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P. Subramaniam et al. / Polyhedron 117 (2016) 496–503
with aryl benzoates [50]. In all these cases the Hammett parameter
changes from a large positive value to a small one as the sub-
explained on the basis of the formation of hydroperoxovanadium
(V) species. Further, a decrease in the intensity of the weak broad
band was observed at 590 nm in the oxovanadium(IV)-salen cat-
alyzed hydrogen peroxide oxidation of phenols [17] and at
570 nm in the oxovanadium(IV)-salen oxidation of tertiary amines
to N-oxides [18] at higher concentrations of H2O2. In these cases,
hydroperoxo species were identified as the active species. On this
basis, the observed spectral changes are considered as evidence
for the formation of hydroperoxovanadium(V) species in the reac-
tion. The formation and involvement of peroxovanadium(V) species
as an active intermediate was also confirmed in the peroxidative
oxidation of benzene and mesitylene by vanadium(IV) and (V) com-
plexes with a nitrogen and oxygen containing ligand catalyst [51],
sulfides and benzoin by polystyrene bound oxidovanadium(IV)
and dioxidovanadium(V) [52], sulfides and styrene by an oxovana-
dium(IV) complex catalyzed by N,S donor ligands [53] and oxo-
vanadium(IV)-salen catalyzed H2O2 oxidation of phenols to
catechol [17] and tertiary amines to N-oxides [18]. The existence
of hydroperoxovanadium(V) species in the reactions involving sal-
ophen and salen oxovanadium(IV) complexes was confirmed by IR,
51V NMR and theoretical studies by Coletti et al. [44].
q
stituent changes from an EDG to a strong EWG, and the observed
non-linearity is explained on the basis of ground state stabilization
of the substrates possessing an EDG. Another interesting observa-
tion noted in the Hammett correlation is the anomalous behavior
of the methoxy substituent which shows a significant positive
deviation from the rest of the substituents.
The effect of the substituents in the salophen ligand of the oxo-
vanadium(IV)-salophen complex reveals that the reaction rate is
very much enhanced by the electron releasing methoxy group
and strongly retarded when the electron withdrawing chloro group
is introduced. The Hammett plots obtained by plotting log k2 ver-
sus 2
straight line for both electron releasing and electron withdrawing
substituents, with high negative
values ranging between ꢁ1.53
and ꢁ1.76. From Table 3 it has been observed that the values of
the reaction constant ( ) are almost constant for all PSAAs, imply-
r of substituted complexes with different PSAAs show a single
q
q
ing that the extent of transmission of the electronic effect is the
same in every case.
Another prominent observation noted in the pseudo first order
plots during the kinetic runs with complex I was an initial well
defined induction period for a definite period of time, followed
by gradual decrease in absorbance. The induction period and initial
time taken for the decrease in OD were found to decrease with
increase in temperature and concentrations of the complex and
hydrogen peroxide (Fig. 3). It is suggested that the induction period
is due to slow generation of the active oxidizing species, hydroper-
oxovanadium(V), during the course of the reaction. The observed
decrease in the induction period with temperature may be due
to the easy formation of hydroperoxo species at high temperatures.
However, when substituents are introduced into complex I, they
do not show any appreciable induction period which indicates
the instantaneous generation of peroxo species during the reac-
tions with II–IV. A similar induction period observed during the
oxygenation reactions of cyclohexane [54] and sulfides [55], cat-
alyzed by cobalt(III)-salen ion, was explained on the basis of a
delay in the generation of the active species [Co(III)-salen-OOH].
4. Discussion
4.1. Active species
Oxovanadium(IV)-salophen (I) has an absorption maximum at
396 nm, which arises due to its ligand to metal charge transfer.
This peak is slightly affected by the addition of H2O2 after a certain
period of time and shows a blue shift, with an absorption maxi-
mum of 393 nm, which indicates a change in the species. Coletti
et al. [44] have prepared the oxovanadium(V)-salophen complex
and reported the absorption maximum of the complex as
393 nm. This clearly indicates that in the present study the oxo-
vanadium(IV)-salophen complex is oxidized to the oxovanadium
(V)-salophen complex with H2O2. In addition, it is interesting to
note that addition of H2O2 at a higher concentration to the complex
shows an increase in intensity of a new broad absorption peak
around 610 nm after a time interval, followed by a decrease during
the course of time, which depends upon the experimental condi-
tions (Fig. 2). This observation demonstrates the formation of a
new intermediate vanadium species in the reaction mixture.
A similar observation, noted around 560 nm in the asymmetric
oxidation of sulfides to sulfoxides by Schiff base-oxovanadium(IV)
and (V) complexes with organic hydro peroxides [46] was
4.2. Mechanism
The existence of an induction period and an increase in the
absorbance at 610 nm in the present study are rationalized by
Fig. 2. Absorption spectral changes of complex I with time on addition of H2O2. [I] = 5.0 ꢂ 10ꢁ4 M, [H2O2] = 5.0 ꢂ 10ꢁ3 M.

P. Subramaniam et al. / Polyhedron 117 (2016) 496–503
501
C2 polarizes the sulfoxide group of PSAA, rendering the sulfur as
electrophilic in nature. The biphilic nature of the organic sulfox-
ides, electrophilic or nucleophilic, is well established and it
depends on the nature of the oxidant involved in the reaction
[58,59].
The behavior of sulfoxide in a reaction is ascertained from a
study of substituent effects. Negative
foxides [60,61] and positive values for electrophilic sulfoxides
[58,62] have been reported in reactions with different substituents.
From the observed positive values for both electron releasing and
q values for nucleophilic sul-
q
q
electron withdrawing groups of PSAA in the Hammett correlation
of the substituent effect, it is concluded that PSAA behaves as elec-
trophile. The behavior of sulfoxide as an electrophile is well estab-
lished in the oxidation reactions of sulfoxides with fluorenone
carbonyl oxide [62] and permanganate [58] where nucleophilic
attack of the oxidant on the sulfoxide in the rate determining step
has been proposed. On this basis, an electrophilic attack of the sul-
fur atom of PSAA on the peroxo nucleophilic oxygen atom leading
to the formation of the transition state (C4) in a slow rate deter-
mining step (RDS) is proposed for the reaction. Based on the above
discussions, the following mechanism (Scheme 1) involving the
catalytic cycle of oxovanadium(V)-salophen is proposed for the
oxidative decarboxylation of PSAA by H2O2.
The proposed RDS is consistent with the observed substituent
effects, i.e. the increase in rate with EDG in the complex and
EWG in PSAA and also the decrease in rate by EWG in the complex
and EDG in PSAA. The increase in rate with EDG in the complex is
due to the increase in nucleophilicity of the complex. A similar rate
acceleration was observed by Mathavan et al. [18] in the oxovana-
dium(IV)-salen catalyzed oxidation of tertiary amines by H2O2.
EWG in the phenyl ring of PSAA makes the sulfur atom more elec-
tropositive, thereby facilitating the electrophilic approach of PSAA
towards the nucleophilic peroxo oxygen atom. On the other hand,
EDG in the phenyl ring of PSAA makes the sulfur atom less elec-
tropositive and thus slows down the electrophilic attack of PSAA
on the nucleophilic oxygen atom. The intermediate (C4) then
undergoes fast internal oxygen atom transfer, leading to the forma-
tion of a methyl phenyl sulfone with the regeneration of the oxo-
vanadium(V) complex. The observed increase in OD in the
pseudo first order plots after a certain period of time (Fig. 3) is
interpreted by the regeneration of the oxovanadium(V)-salophen
Fig. 3. Pseudo first order plots for the reactions of PSAA with complex I at different
[H2O2]. [I] 1.5 ꢂ 10ꢁ4 M, [PSAA] = 9.0 ꢂ 10ꢁ2 M.
the slow generation of the active oxidizing species, hydroperoxo-
vanadium(V)-salophen, from the vanadium(IV) complex and
H2O2. It is proposed that the oxovanadium(IV)-salophen complex
is first oxidized to oxovanadium(V)-salophen (C1) which further
reacts with excess of hydrogen peroxide to form the active species,
hydroperoxovanadium(V)-salophen (C2). The formation of the
hydroperoxo species in the reaction is ascertained by the occur-
rence of the reaction only in the presence of H2O2 and the salophen
complex together. An analogous scheme for the generation of
active species has been proposed in the epoxidation of cyclooctene
by an oxovanadium(IV)-Schiff base complex [56]. In the hydroxyla-
tion reactions of aromatic compounds by H2O2 using a vanadium
(IV) based catalyst [57], it has been shown that the vanadium(IV)
complex first interacts with H2O2 and rapidly transforms into a
stable vanadium(V) species, which further reacts with H2O2 to
yield a peroxovanadium(V) intermediate species. As the hydroper-
oxo species is acidic in nature, it is transformed into the interme-
diate (C3) by the release of a proton. The acid behavior of the
hydroxyl group coordinated to the vanadium center is confirmed
by Coletti et al. [44] from its pKa value. The proton released from
Scheme 1. Catalytic cycle of the oxovanadium(V)-salophen complex during the oxidative decarboxylation of PSAA.

502
P. Subramaniam et al. / Polyhedron 117 (2016) 496–503
complex. In the epoxidation reaction of alkenes catalyzed by oxo-
vanadium(IV)-Schiff base complexes with organic peroxides, it
has been shown that the final form of the complex is oxovana-
dium(V) [63]. A similar nucleophilic oxygen transfer from the
active hydroperoxide to electropositive sulfur atom has been pro-
posed in the vanadium-Schiff base catalyzed sulfoxidation [64].
The formation of the methyl phenyl sulfone in the absence of
any other oxygen sources clearly demonstrates that the oxygen
atom incorporated in PSAA is derived from the hydroperoxovana-
dium(V)-salophen species.
In the reactions of H2O2 catalyzed by oxovanadium salophen
and salen complexes it has been shown that the oxidation of the
substrate and decomposition of H2O2 take place simultaneously
[44,65]. Thus the observed decrease in reaction rate with the
increase in concentration of the oxovanadium(IV)-salophen com-
plexes (Table 2) can be interpreted by the fast decomposition of
hydrogen peroxide. The observed decrease in the reaction rate at
high concentrations of hydrogen peroxide in all four complexes
(Table 2) may be visualized by the conversion of the vanadium
complex into the inorganic peroxovanadate (Fig. 4). The existence
of diperoxo vanadates at high concentrations of H2O2 has been
shown in the reactions of vanadium(IV) base catalyzed oxidations
[66,67] and proven by 51V NMR studies [68].
Fig. 6. Yukawa–Tsuno plots for the reactions of PSAAs with H2O2 and the
complexes. a = II at 30 °C, b = I at 35 °C, c = I at 30 °C.
containing EWG by the decrease in the resonance stabilization of
the GS of the esters.
To examine and ascertain the validity of the above argument,
the rate data have been treated with the Yukawa–Tsuno equation
[75–77].
4.3. Interpretation of the non-linear Hammett plot
The existence of distinct curvature with two intersecting linear
portions in a non-linear Hammett plot has been traditionally inter-
preted as a change in the reaction mechanism or a change in the
RDS within a given mechanism [69,70]. However in the present
study the apparent downward curvature in the Hammett plots
has been ascribed to stabilization of the ground state of PSAA
through a resonance interaction on changing the substituents from
electron withdrawing to electron releasing. This argument is sup-
ported by the significant negative deviation observed in the Ham-
logðkX=kHÞ ¼
q
½
ro þ rðrþ
ꢁ
roÞꢃ
The term (r+
–r
o) is the resonance substituent constant, while
the ‘r’ value is a parameter characteristic of a reaction representing
the extent of the resonance contribution. Interestingly the
Yukawa–Tsuno plots (Fig. 6) exhibit a good linearity for the reac-
tions under study and the ‘r’ values obtained (0.877–0.920) suggest
that the resonance interaction is relatively significant. The
observed linear Yukawa–Tsuno plots not only confirm a common
mechanism for all PSAAs but also prove that the ground state sta-
bilization of the PSAAs through a resonance interaction is the cause
for the non-linearity in the Hammett plots. Um Ik-Hwan and
coworkers [73,78] have reported such type of linear Yukawa–
Tsuno plots for non-linear Hammett plots and explained this on
the basis of ground state stabilization of the substrate.
mett plot as the substituent becomes
a stronger EDG. The
resonance structures as a result of the interaction between the
electron donating substituent and the thionyl functionality are
represented in Fig. 5.
The presence of such resonance structures in substances with
electron releasing groups stabilizes the GS and causes a decrease
in reactivity. Accordingly, one can expect a large
q value for EDG
compared with EWG. In fact the value decreases considerably
q
from 1.30 for the reactions with EDG to 0.451 for those with
EWG. Thus the deviation from the Hammett correlation is taken
as evidence for ground state stabilization by EDG. The observed
non-linear Hammett plots in the aminolysis reactions of substi-
tuted phenyl-2-methoxy benzoates [71], 4-pyridyl substituted
benzoates [72] and 2-pyridyl substituted benzoates [73] have been
interpreted by ground state stabilization of the substrates. Neuvo-
nen et al. [74] have explained the enhanced reactivity of esters
5. Conclusion
In the oxidative decarboxylation of phenylsulfinylacetic acids to
methyl phenyl sulfones by H2O2 and catalyzed by oxovanadium
(IV)-salophen complexes, hydroperoxovanadium(V)-salophen has
been identified as the active species. The observed order of reactiv-
ity, OMe > Me > H > Cl, among substituted oxovanadium(IV)-salo-
phen complexes has been explained on the basis of electronic
effects. A mechanism involving electrophilic attack of PSAA on
the peroxo nucleophilic oxygen atom of the active species formed
between the oxidant and catalyst, followed by fast oxygen atom
transfer to PSAA has been proposed for the reaction. The Hammett
correlation of the kinetic data of the substituted PSAAs gave a
downward curvature with small positive
q values for EWG and
Fig. 4. Structure of peroxovanadate.
moderately high positive values for EDG, while better linearity
q
Fig. 5. Ground state stabilization of PSAA.

P. Subramaniam et al. / Polyhedron 117 (2016) 496–503
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major research project (F. No. 39-817/2010(SR)) to PS is gratefully
acknowledged. RJER is thankful to UGC, SERO, Hyderabad
(No. F.ETFTNMS181) and Manonmaniam Sundaranar University,
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