Alteration of electronic effect causes change in rate determining step: Oxovanadium(IV)–salen catalyzed sulfoxidation of phenylmercaptoacetic acids by hydrogen peroxide

Article abstract of DOI:10.1016/j.poly.2019.114172

Sulfoxidation of a series of phenylmercaptoacetic acids (PMAA) by hydrogen peroxide catalysed by oxovanadium(IV)–salen complexes has been carried out spectrophotometrically in 100% acetonitrile medium. The formation and involvement of hydroperoxovanadium(

Full text of DOI:10.1016/j.poly.2019.114172

Polyhedron 175 (2020) 114172
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Alteration of electronic effect causes change in rate determining step:
Oxovanadium(IV)–salen catalyzed sulfoxidation of
phenylmercaptoacetic acids by hydrogen peroxide
b
C. Kavitha a,b, , P. Subramaniam
⇑
a Research Department of Chemistry, Aditanar College of Arts and Science, Tiruchendur 628 216, Tamil Nadu, India
b Manonmaniam Sundaranar University, Abishekapatti, Tirunelveli 627 012, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Sulfoxidation of a series of phenylmercaptoacetic acids (PMAA) by hydrogen peroxide catalysed by oxo-
vanadium(IV)–salen complexes has been carried out spectrophotometrically in 100% acetonitrile med-
ium. The formation and involvement of hydroperoxovanadium(V) as the active oxidizing species has
been evidenced from the appearance of new absorption peak and the initial induction period in the reac-
tion. Introduction of both electron releasing and electron withdrawing substituents in PMAA retard the
rate of sulfoxidation. However, substituents in the salen complex followed a different order: acceleration
of rate by electron releasing substituents and retardation by electron withdrawing substituents.
Hammett correlation yields a non-linear concave downward curve having two straight lines indicating
that the mode of transmission of electronic effect between the substituent and the reaction site is differ-
ent for electron donating and electron withdrawing substituents. The selective oxidation of sulfide to sul-
foxide is confirmed by the formation of phenylsulfinylacetic acid as the sole product. All the observed
results have been explained by proposing two different rate determining steps within a same mecha-
nism: one for electron donating substituents and another for electron withdrawing substituents.
Ó 2019 Elsevier Ltd. All rights reserved.
Received 11 September 2019
Accepted 9 October 2019
Available online 18 October 2019
Keywords:
Sulfoxidation
Oxovanadium(IV)–salen
Phenylmercaptoacetic acid
Downward Hammett plot
Hydroperoxovanadium(V)
1. Introduction
cation or direct oxygen transfer mechanism has been proposed.
Though phenylsulfinylacetic acid (PSAA) has been reported as the
Organo sulfur compounds widely distributed in food were
found to possess antiseptic, antibiotic, antithrombotic and antiox-
idant activities [1–6]. Owing to the possession of these properties
and the generation of biologically and pharmaceutically active
intermediates during sulfoxidation [7,8], the study of oxidation of
sulfides to sulfoxides is of great significance. Selective oxidation
of phenylmercaptoacetic acid (PMAA), a derivative of an organic
sulfide, has been extensively studied using different metals [9–
12], metal complexes [13–16], N-halo compounds [17–23], halo
chromates [24–26], peroxo compounds [27,28], sodium perborate
[29–32] and percarbonate [33]. In these reactions, S2N type mecha-
nism with an electron deficient sulfonium ion intermediate or sin-
gle electron transfer mechanism (SET) involving sulfur radical
sole product in most of the reactions, thiophenol [24,30], a-
hydroxyphenylmercaptoacetic acid [13] and disulfide [9,10] were
also identified as products.
In recent years, transition metal complexes derived from syn-
thetic organic ligands viz., Schiff bases [34,35], porphyrin [36–38]
and phthalocyanine [39] which mimic the reactivity of metallo
enzymes are widely employed either as an oxidant or as a catalyst.
Among them, metal salen complexes have received special atten-
tion due to its simple method of preparation and its versatile cat-
alytic action on oxygen insertion reactions.
Several research groups have used the vanadium metal com-
plexes as catalyst in the oxidation reactions because of their speci-
ficity and stereo selectivity. Oxovanadium(IV)—salen and salophen
complexes have been used as effective catalysts in the oxidation of
sulfoxides [40], tertiary amines [41] and phenol [42]. The oxovana-
dium(IV)-Schiff base complexes have been employed for the oxida-
tion of cyclooctene and styrene [43]. Schiff base complexes of
vanadium(III, IV, V) have been utilized for the electro reduction
of O2 to H2O [44]. Vanadyl porphyrin complexes catalyze the epox-
Abbreviations: PMAA, phenylmercaptoacetic acid; PSAA, phenylsulfinylacetic
acid; EWS, electron withdrawing substituent; EDS, electron donating substituent; r,
correlation coefficient.
⇑
Corresponding author at: Research Department of Chemistry, Aditanar College
of Arts and Science, Tiruchendur 628 216, Tamil Nadu, India.
E-mail address: sivakavetha@gmail.com (C. Kavitha).
https://doi.org/10.1016/j.poly.2019.114172
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

2
C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
idation reaction [45]. Vanadium(IV) tetraphenylporphyrin has
been used as catalyst for the acetylation of alcohols and phenols
[46]. Vanadyl acetylacetonate was used as a catalyst in the epoxi-
dation of allylic alcohol [47,48]. The vanadium complex of a
diacetylmonooxime Schiff base of S-methyl dithiocarbazate has
been used as a catalyst during the oxidation of alcohols in the pres-
ence of t-BuOOH and H2O2 [49].
2.2. Preparation of phenylmercaptoacetic acids
PMAAs were prepared by the condensation of the correspond-
ing thiophenols with chloroacetic acid in alkaline medium [77].
The purity of PMAAs was checked using their melting point with
the literature value and LCMS spectra.
In hydroxylation reaction of aromatic compounds using H2O2 as
the oxidant and V(IV) complex as the catalyst, hydroperoxovana-
dium(V) species has been identified as the oxidizing species
[50,51]. The involvement of hydroperoxovanadium(V) species has
also been reported in the oxidation of phenylsulfinylacetic acid
[40], tertiary amines [41], phenol [42] by H2O2 and V(IV) catalysed
peroxidative oxidation of benzene and mesitylene with N,O-
ligands [52]. Hydroperoxovanadium(V) species can act as either
electrophile [12,40] or radical oxidant [53–55] and was found to
be more stable in bent conformation [56]. Peroxovanadium(V)
complexes have found to possess antitumor [57–63] and insulin
mimetic activities [64–66]. Some of the oxovanadium(IV) com-
plexes of porphyrin derivatives are familiar for their inhibitory
effects on HIV-I(Bal) replication in Hut/CCR5 cell [67]. Oxovana-
dium complexes of cyclam (1,4,8,11-tetraazacyclotetradecane)
and bicyclam exhibit potent anti-HIV properties [68].
In continuation of our recent works on exploring different
mechanistic pathways in sulfoxidation reactions using salen com-
plexes of Fe(III) [69,70], Cr(V) [71–73] and V(IV) [40] and Fe(III)
polypyridyl complexes [74–76], yet another attempt is made on
the sulfoxidation of phenylmercaptoacetic acids (PMAA) catalyzed
by oxovanadium(IV)–salen complexes and the results are reported
in this work. Here, safe, low cost and environment friendly green
oxidant, H2O2 is used to generate hydroperoxovanadium(V) spe-
cies. The overall pathway of the reaction is presented in Scheme 1.
2.3. Synthesis of oxovanadium(IV)–salen complexes
Oxovanadium(IV)–salen complexes (I–V) were synthesized
from the corresponding salen ligands using standard procedure
[78,79] by utilizing the reaction between the ligand and vanadyl
sulfate in alcoholic medium and recrystallized from hot methanol.
The absorption spectral data in acetonitrile medium for the com-
plexes (I–V) are consistent with the literature data [56]. The salen,
5,50-(CH3)2 salen, 5,50-(OCH3)2 salen, 5,50-(Cl)2 salen and 3,30,5,50-
(t-butyl)4 salen ligands were prepared by condensing substituted
salicylaldehydes with ethylene diamine in ethanol medium [80]
and recrystallized from cold methanol.
2.4. Instrumentation
A double beam BL 222 Elico UV–Vis Biospectrophotometer with
an inbuilt thermostat was employed to record the absorption spec-
tra and to follow the kinetics of the reactions. Infrared spectrum of
the product was recorded using KBr pellet on a JASCO FTIR-410
spectrophotometer. LCMS was performed on a HPLC coupled Agi-
lent ion trap mass spectrometer using inestsil-ODS-3V column of
size 4.6 ꢀ 250 mm.
2.5. Kinetic measurements
The kinetic studies were carried out in 100% acetonitrile med-
ium under pseudo first order conditions using at least a tenfold
excess of substrate over H2O2 and the complex. In these reactions
the ratio between the complex and H2O2 is maintained at least
minimum as 1:100. The reactions were initiated quickly by inject-
ing the H2O2 into oxovanadium(IV)–salen-PMAA system at zero
time. The kinetics of the reaction was carried out at different con-
centrations of PMAA, salen complex and H2O2 by keeping other
reaction conditions as constant. The rate of the reaction is moni-
tored by measuring the decay of the active species in the reaction
2. Experimental
2.1. Materials
Salicylaldehyde, 5-methyl, 5-methoxy, 5-chloro and 3,5-di-tert-
butyl salicylaldehydes and vanadyl sulfate were purchased from
Alfa Aeser and used as such. Thiophenols were purchased from
Sigma Aldrich. HPLC-grade acetonitrile (Merck) and 30% H2O2 were
used as received.
Scheme 1. Overall reaction pathway.

C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
3
mixture spectrophotometrically at an appropriate wavelength. The
absorption intensity changes during the reaction between PMAA,
H2O2 and I & IV at different time intervals are depicted in Fig. 1
as a representative case. The recorded overlay clearly indicates
the existence of isobestic point at the kmax of 405 nm for the reac-
tion between PMAA and I (Fig. 1a). The reaction between PMAA
and IV does not show any isobestic point (Fig. 1c). The pseudo first
order rate constants (k1) were calculated from the slope of pseudo
first order linear plots of log OD versus time and the overall rate
constants were calculated using the expression kov = k1/[PMAA]n,
where n is the order. The error in the rate constants is given in
accordance to 95% student’s t-test. Thermodynamic parameters,
with chloroform and dried over anhydrous sodium sulfate. The
chloroform was removed by evaporation and the solid obtained
was analyzed using IR and mass spectral techniques. The observed
FT-IR frequencies (KBr, cmꢁ1) are: 3421
c
str(OH), 2983
str(C@C), 1346 cben (CH2), 1025 cstr (SO) and
ben (Ar-H) (Supporting information Fig. S1), which are in con-
cstr(CH2),
1725
854
cstr(CO), 1525 c
c
sistence with the frequencies of PSAA. The existence of only one
peak eluted at the wavelength of 254 nm and at the retention time
of 5.562 min. with an overall area of 95.5% in the HPLC chro-
matogram clearly shows the existence of single moiety in the prod-
uct. The LC-MS peak of the product at the retention time of
5.604 min. and ionizes in APCI (ꢁ) mode at m/z = 183.1 (Supporting
information Fig. S2) confirm the formation of PSAA as the only
product.
enthalpy of activation (D D
àH) and entropy of activation ( àS) were
evaluated from the linear Eyring’s plots of log kov/T versus 1/T by
least square method.
3. Results
2.6. Product analysis
3.1. Spectral studies
A mixture of PMAA, oxovanadium(IV)–salen and H2O2 in ace-
tonitrile medium was set aside at room temperature for comple-
tion of the reaction. After completion, the solvent was
evaporated under reduced pressure; the residue was extracted
The absorption maximum of oxovanadium(IV)–salen complex
(I) at 363 nm in acetonitrile medium is sensitive to the nature of
the substituent present in the salen moiety. Introduction of meth-
Fig. 1. Change in absorption intensity at different time intervals for the reaction between PMAA, H2O2 and salen. [PMAA] = 1.0 ꢀ 10ꢁ1 M; [H2O2] = 1.0 ꢀ 10ꢁ2 M; [I]
= 1.0 ꢀ 10ꢁ4 M; [IV] = 0.7 ꢀ 10ꢁ4 M; Solvent = 100% CH3CN; Temp. = 30 °C.

4
C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
oxy group at the 5 and 50 positions (III) shifts the kmax value to
397 nm (red shift) while all other groups cause a comparatively
less red shift (Table 1). The shift in the kmax towards red region
by the substituents has been explained by the stabilization of
excited state by the substituents [41,42].
sity begins to increase. It gives evidence that the complex is regen-
erated in the reaction mixture during the course of the reaction. A
similar observation of increase in absorption intensity after a time
gap was noticed in the sulfoxidation of PSAA catalyzed by oxovana-
dium(IV)—salophen [40] and oxovanadium(IV)–salen catalyzed
H2O2 oxidation of tertiary amines [41].
A very little shift in the kmax value (~3 to 5 nm) of oxovanadium
(IV)–salen complex is observed by the addition of PMAA, which is
contrary to that ascertained by Subramaniam et al. in the oxidation
of PSAA [70–72] and PMAA [73] by oxo(salen) complexes where
the kmax shifts appreciably to higher wavelengths. However, they
noticed a very little shift in the kmax value (~3 to 5 nm) by the addi-
tion of PSAA to oxo(salophen)vanadium(IV) complexes [40].
It is interesting to note that by the addition of H2O2 to the com-
plex, a new broad absorption peak appears around 560 nm after a
time interval followed by increase its intensity (Fig. 1b). This
clearly indicates the formation of a new vanadium species as inter-
mediate. Similar formation of new broad absorption peak was
noticed around 560 nm in the asymmetric oxidation of sulfides
to sulfoxides by organic hydro peroxides with Schiff base oxovana-
dium(IV) and (V) complexes [81], at 590 nm in the oxovanadium
(IV)–salen catalyzed H2O2 oxidation of phenol [42] and at
570 nm in the oxovanadium(IV)–salen oxidation of tertiary amines
to N-oxides [41]. In all these reactions, the observation was
explained on the basis of the formation of hydroperoxovanadium
(V) species. The formation and involvement of hydroperoxovana-
dium(V) species as the active intermediate was also ascertained
in the peroxidative oxidation of benzene and mesitylene by vana-
dium(IV) and (V) complexes with N,O-ligands [52], sulfides and
benzoin by polystyrene bound oxidovanadium(IV) and dioxi-
dovanadium(V) [82] and sulfides and styrene by oxovanadium
(IV) complex catalyzed by N,S donor ligands [83]. Coletti et al.
[56] reported evidences for the existence of hydroperoxovana-
dium(V) species using 51V NMR spectra, IR spectra and theoretical
studies during salen and salophen oxovanadium(IV) catalyzed sul-
fide oxidation by H2O2.
Besides, during the initial period of reactions, a well-defined
induction period is witnessed for a distinct time before gradual
decrease in absorbance (Fig. 2). The existence of such induction
period also been noticed during the oxygenation reactions of sul-
fides [84] and cyclohexane [85] catalyzed by cobalt(III)—salen ion
and PSAA by oxovanadium(IV)—salophen [40]. In these cases, exis-
tence of the induction period was explained by the time required
for the slow generation of active species from the oxidant and cat-
alyst. On these basis, the observed spectral change around 560 nm
in the present study is taken as evidence for the formation of
hydroperoxovanadium(V) species.
An interesting observation noted is that the magnitude of
induction period decreases with increase in temperature and con-
centrations of oxovanadium(IV)–salen and H2O2 (Fig. 2). The
induction period is also influenced by the nature of substituents
in the salen moiety. The introduction of electron donating sub-
stituents in the salen moiety (II and III) shortens the induction per-
iod whereas electron withdrawing chloro substituent (IV)
lengthens the induction period. Another observation noted during
the course of the reaction is that after decrease in absorbance of
the reaction mixture up to a particular time, the absorbance inten-
The self-decay of oxovanadium(IV)–salen complex has not
taken place neither with H2O2 alone nor with PMAA which is
inferred from the existence of constant absorbance value at
365 nm at different time intervals. Similarly, neither oxovana-
dium(IV)–salen complex nor H2O2 alone shows no reactivity
towards PMAA and they do so only in combination of all. Thus it
is inferred that the decrease in absorbance of the reaction mixture
at 365 nm with time is due to the reaction between oxovanadium
(IV)–salen complex, H2O2 and PMAA. The absence of any reaction
between the salen complex and PMAA in 100% AN medium not
only rules out the possibility of oxovanadium(IV)–salen complex
as such as an oxidizing agent but also indicates that the interaction
between the salen and H2O2 plays an important role in the cataly-
sis and eliminates the existence of hydroperoxovanadium(V) spe-
cies in the absence of H2O2.
3.2. Kinetic results
To shed more light on the sulfoxidation reaction, the effect of
oxidant on the oxidation of PMAA in the presence of I, II and IV
in acetonitrile medium is studied by varying the concentration of
H2O2 from 2 ꢀ 10ꢁ3 to 1 ꢀ 10ꢁ2 M by keeping all other reaction
conditions as constant. As the concentration of the oxidant
increases, the pseudo first order rate constant increases
(Table S1) and excellent linear pseudo first order plots are observed
with decrease in the induction period. Excellent linear first order
plots indicate that the order with respect to H2O2 is one. The
increase in concentration of salen complex from 1 ꢀ 10ꢁ5 to
10 ꢀ 10ꢁ5 M also increases the k1 value as indicated in Table S1.
This evidently shows that oxovanadium(IV)–salen complex acts
as a catalyst in this reaction.
The dependence of the reaction rate on [PMAA] is studied by
varying the concentration of PMAA which reveals that the rate of
the reaction increases with an increase in [PMAA] without any sat-
uration (Table S2). The non-integral slope value obtained in the
plot of log k1 versus log [PMAA] and constant kov value obtained
at different initial concentrations of PMAA confirm the fractional
order dependence of PMAA. The plots of 1/k1 versus 1/[PMAA]
are linear with an intercept on the rate coordinate showing
Michaelis–Menten type kinetics with respect to PMAA i.e., binding
of PMAA takes place with other reactants in a reversible process
before the rate controlling step. However, the observed non-satu-
ration kinetics in the plot of k1 versus [PMAA] ruled out strong
binding of PMAA either with salen complex or H2O2. This fact is
supported by the high Michaelis–Menten constant values (KM)
obtained from the plot of 1/k1 versus 1/[PMAA] with all complexes,
I–V. The calculated Michaelis–Menten constants (KM) are pre-
sented in the Table 2.
The sulfoxidation reaction was conducted at three different sol-
vents: water, dichloromethane and acetonitrile and found that the
catalytic activity of oxovanadium(IV)–salen complex is more pro-
nounced in the acetonitrile than dichloromethane. The overall rate
constants for the reaction at standard conditions and at different
solvent media are: 39.1 ꢀ 10ꢁ3 for acetonitrile and 14.2 ꢀ 10ꢁ3
for dichloromethane. This is partly visualized by the dielectric con-
stant of the medium. However, the reaction does not occur in high
dielectric constant medium, water and is interpreted by the insta-
bility of the active species, hydroperoxovanadium(V) in the aque-
ous medium which is also confirmed by the absence of
appearance of new peak at 560 nm in the presence of water. The
Table 1
Absorption maxima of oxovanadium(IV)–salen complexes.
Complex
kmax (observed) kmax (literature)
Vanadium oxo salen (I)
363
376
397
373
362
Vanadium-5,50-dimethyl salen (II)
Vanadium-5,50-dimethoxy salen (III)
Vanadium-5,50-dichloro salen (IV)
392
370
386
Vanadium-3,30,5,5-tetra t-butyl salen (V) 380

C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
5
Fig. 2. Existence of induction period at different conditions. [PMAA] = 1.0 ꢀ 10ꢁ1 M; solvent = 100% CH3CN; Temp. = 30 °C.
Table 2
presented in the Tables 3 and 4. The data furnished in these tables
point out that the sulfoxidation reaction is highly sensitive and
alter the rate appreciably by varying the substituents in the phenyl
moiety of PMAA and in the 3,30,5,50-positions of the salen complex.
It is interesting to note that both electron donating and electron
withdrawing substituents in the m- and p-positions of PMAA
decelerate the reaction rate. As the order with respect to PMAA
is found to be fractional, kov values are given in all places. The tem-
perature has shown positive effect on the rate of the reaction.
Michaelis–Menten constants (KM) of PMAA with complexes I to V.
Complex
KM value
I
0.133
0.106
0.136
0.205
0.203
II
III
IV
V
The Hammett q/r analysis are utilized to measure the extent of
inductive electronic effect of the substituents on the rate of reac-
tion. A Hammett correlation for substituted PMAAs using data in
Tables 3 and 4 unveils a deviation from linearity. The Hammett
plot shows a biphasic concave downward curve with two distinct
lines, one for electron donating and another for electron withdraw-
ing substituents with a break point at parent PMAA. The illustra-
tive Hammett plots are shown in Fig. 3. Besides, contrary to
higher reactivity in acetonitrile medium proves without ambiguity
that cationic species is involved in the rate determining step. Sim-
ilar solvent effect was advocated by Mathavan et al. [42] in the
oxovanadium(IV)–salen catalyzed H2O2 oxidation of phenol.
3.3. Substituent effects
typical sulfoxidation reactions, an unusual high positive
for electron donating substituents (EDS) and high negative
q
value
for
q
To evaluate the importance of electronic effect in the sulfoxida-
tion reaction, twelve phenylthioacetic acids and five oxovanadium
(IV)–salen complexes were chosen by varying the position, size
and inductive effect of the substituents. The rate constants for
the sulfoxidation of substituted PMAAs by H2O2 catalyzed by oxo-
vanadium(IV)–salen complexes (I–V) were determined and are
electron withdrawing substituents (EWS) are obtained. The analy-
sis reveals that the magnitude of inductive effect goes on decreas-
ing with increase in temperature (Table 3). Further, from the
results it is concluded that both type of substituents exert almost
same but opposite magnitude of inductive effect with the reaction
Table 3
Overall rate constants along with thermodynamic and Hammett parameters for the reactions of PMAAs with H2O2 catalyzed by I.
n
S. No.
X
103 kov [(Mꢁ1
20 °C
)
sꢁ1
]
D
àH
(kJ molꢁ1
ꢁ
D
àS
)
(JKꢁ1 molꢁ1
)
30 °C
40 °C
1
2
3
4
5
6
7
8
p-OMe
p-OEt
p-t-Bu
p-Me
p-Et
1.32 0.05
1.63 0.05
4.56 0.04
4.88 0.07
5.91 0.10
6.79 0.09
32.6 0.38
21.1 0.20
12.7 0.16
3.82 0.08
3.38 0.04
0.971 0.02
ꢁ 4.52 0.21
0.996
2.93 0.05
3.23 0.11
6.37 0.08
6.86 0.09
7.62 0.10
9.49 0.12
39.0 0.48
25.6 0.29
15.0 0.24
5.82 0.08
4.35 0.07
1.80 0.04
ꢁ 4.02 0.18
0.996
4.33 0.10
5.14 0.05
9.14 0.12
9.84 0.09
10.3 0.20
11.0 0.18
45.5 0.57
32.4 0.36
17.8 0.20
7.29 0.12
5.51 0.07
3.25 0.06
ꢁ 3.59 0.24
0.991
43.2 2.1
41.3 2.0
30.8 0.8
22.8 0.9
19.3 1.2
15.9 1.1
9.71 0.91
14.0 0.8
10.5 1.0
22.9 1.3
16.7 1.1
43.8 1.5
18.0 7.3
22.8 7.0
51.9 2.9
77.2 3.2
87.9 4.3
98.1 4.0
106 3.2
95.0 2.9
111 3.5
78.9 4.6
101 3.8
18.8 5.4
p-iPr
H
p-F
m-OMe
p-Cl
9
10
11
12
p-Br
m-F
qEWS
R
qEDS
5.23 0.31
0.992
4.22 0.25
0.991
3.75 0.23
0.991
R
[X] = 5.0 ꢀ 10ꢁ2 M; [H2O2] = 5.0 ꢀ 10ꢁ3 M; [I] = 5.0 ꢀ 10ꢁ5 M; Solvent = 100% CH3CN.

6
C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
Table 4
Overall rate constants and the Hammett parameters for the reactions of PMAAs with I–V.
n
S. No.
X
103 kov [(Mꢁ1
)
sꢁ1)]
q
III
II
I
IV
V
1
2
3
4
5
6
7
8
p-OMe
p-OEt
P-t-Bu
p-Me
p-Et
35.0 0.15
43.0 0.16
57.0 0.12
74.0 0.16
92.0 0.17
116 0.18
341 0.67
244 0.36
158 0.28
80.0 0.24
67.0 0.18
26.0 0.10
ꢁ3.80 0.22
0.993
9.80 0.15
12.6 0.11
16.1 0.10
19.5 0.11
23.1 0.13
26.5 0.12
89.2 0.57
61.8 0.26
38.4 0.20
21.0 0.20
17.3 0.11
6.80 0.08
ꢁ3.34 0.20
0.993
2.93 0.05
3.23 0.11
6.37 0.08
6.86 0.09
7.62 0.1
9.49 0.12
39.0 0.48
25.6 0.29
15.0 0.24
5.82 0.08
4.35 0.07
1.80 0.04
ꢁ4.02 0.18
0.996
0.852 0.05
1.06 0.11
1.68 0.12
2.08 0.09
2.47 0.13
2.64 0.12
9.22 0.29
6.67 0.16
3.82 0.12
1.86 0.12
1.51 0.18
0.593 0.08
ꢁ3.48 0.20
0.993
0.442 0.15
0.521 0.16
0.821 0.12
1.08 0.11
1.37 0.13
1.65 0.09
6.18 0.29
4.12 0.16
2.58 0.16
1.03 0.12
0.771 0.15
0.292 0.08
ꢁ3.93 0.21
0.994
ꢁ1.36
ꢁ1.37
ꢁ1.31
ꢁ1.30
ꢁ1.29
ꢁ1.31
ꢁ1.26
ꢁ1.27
ꢁ1.29
ꢁ1.35
ꢁ1.38
ꢁ1.39
P-iPr
H
p-F
m-OMe
p-Cl
9
10
11
12
p-Br
m-F
qEWS
r
qEDS
3.84 0.23
0.991
3.55 0.14
0.996
4.22 0.25
0.991
3.82 0.12
0.998
4.35 0.19
0.995
r
[X] = 5.0 ꢀ 10ꢁ2 M; [H2O2] = 5.0 ꢀ 10ꢁ3 M; [salen] = 5.0 ꢀ 10ꢁ5 M; Solvent = 100% CH3CN; Temp = 30 °C; n = order with respect to PMAA.
Fig 3. Hammett plots for the sulfoxidation of PMAAs with II and IV at 30 °C.
center as indicated by their
q
values. The unusual deviation from
stituents enhance the rate of the reaction in the oxygenation of
organic sulfides and sulfoxides with oxo(salen)chromium(V) com-
plexes [93] and in the oxidation of PTAA by oxo(salen)manganese
(V) complexes [16] whereas in the case of sulfoxidation of PTAA by
oxo(salen)chromium(V) complexes [73] and in the oxidation of
PSAA by oxo(salen)chromium(V) complexes [71,72] both electron
releasing and electron withdrawing substituents enhance the sul-
foxidation. Interestingly, it is noted that even though complex V
has four electron donating t-butyl substituents, contrary to the
expectation it has the least reactivity among all other salen com-
plexes. It is worthwhile to mention here that in the reactions of
methionine [92], phenyl methyl sulfides and sulfoxides [93],
organic sulfides in the presence of DMSO [94] and donor ligand
oxides [95], anilines [96] with [oxo(salen)chromium(V)] ions and
oxidation of phenyl methyl sulfides and sulfoxides [97] catalyzed
by iron(III)–salen complexes, the presence of t-butyl group in the
3,30,5,50 positions leads to enormous rate-retardation than other
complexes which was explained by steric effect. In contradiction,
increase in rate by salen complexes having t-butyl group in
3,30,5,50-positions was observed in the polymerization of propylene
[98] and 1,3-butadiene [99] and in the sulfoxidation of PSAA by Cr
(salen) complexes [100]. This behavior has been explained by
proposing stepped conformation for salen complex which facili-
tates the approach of substrate to the reaction center. In the pre-
sent study, the negative deviation observed with complex V is
the Hammett straight line is visualized by a change in the rate
determining step with change in electronic effect of the
substituents.
Similar non-linear downward Hammett plot has been reported
during the mechanistic elucidation of the vanadium catalysed sul-
foxidation of thioanisoles [86], oxidation of aromatic anils by ison-
icotinium dichromate [87], pyridinolysis of methylphenyl
phosphinic chloride [88] and in the reaction between benzoyolala-
nines and pseudomonas fluorescens kynureninase [89]. Such type
of non-linear downward Hammett plot has been interpreted by
the operation of same mechanism involving two different rate
determining steps (RDS) for electron donating and electron with-
drawing substituents [90].
Substituents on the salen complex also affect the outcome of
the reaction. Electron donating substituents in the salen moiety
accelerate the reaction rate while electron withdrawing sub-
stituents decelerate the rate of the reaction (Table 4). The ascer-
tained order of reactivity among the oxovanadium(IV)–salen
complexes is 5,50-dimethoxy > 5,50-dimethyl > parent salen > 5,50-
dichloro > 3,30,5,50-tetra-t-butyl. Same order has been reported by
Coletti et al. [56], Mathavan et al. [41,42], Subramaniam et al.
[40] and Rayati et al. [43] during salen and salophen catalyzed
reactions. In contrast, electron donating substituents in the salen
moiety retard the reaction rate while electron withdrawing sub-

C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
7
explained by the steric effect of bulky t-butyl groups in 3,30-posi-
tions which prevents the formation of the active species and
restricts the approach of PMAA towards the reaction site.
When log kov of substituted salen complex is plotted against Rr
values, a single straight line for both electron donating and elec-
tron withdrawing substituents having almost same reaction con-
stants for all substituted PMAAs is obtained (Fig. 4; Table 4). As
the t-butyl substituent deviates much from the correlation, com-
plex V is not included in the Hammett treatment. The existence
of indistinguishable reaction constant (q) with different substi-
tuted salen complexes for all PMAAs implies that the extent of
transmission of electronic effect is same during the course of the
reaction irrespective of the nature and position of substituents.
3.4. Thermodynamic parameter
Fig. 5. Plot of
D D
àH vs. àS for the reactions of PMAAs and H2O2 catalyzed by I.
The enthalpy of activation (
the linear Eyring’s plot (Fig. S3) ranges from 9.71 0.91 to
D
àH) calculated from the slope of
43.8 1.5 kJ molꢁ1 and the entropy of activation ( àS) calculated
D
[56,86,101–103], whereas peroxo form has been proposed as oxi-
dizing agent during the oxidation of sulfides [104,105], alkanes
[106], other substrates [107] and epoxidation reaction [108].
Besides, Balcells et al. [109] have shown by theoretical calculation
that these two forms can be interconverted to each other as they
have sufficiently close in energy within a span of 5 kcal/mol. The
51V NMR spectral study [110] also proved the existence of rapid
equilibrium among short lived vanadium(V) catalyst precursor
species. In all cases, attempt to isolate the active species was
unsuccessful. So, it is believed that both forms coexist in solution
and are feasible candidates for the active form of the catalyst.
The choice among them, hydroperoxo or peroxo as the active spe-
cies depends on their reactivity with different substrates.
from the intercept ranges from ꢁ18.0 7.3 to ꢁ111 3.5 JKꢁ1
-
molꢁ1. The positive
D
àH values suggest that the reaction is
endothermic in nature and the negative values of
D
àS suggest dis-
order arrangement of reactants over the products in the rate deter-
mining step. The isokinetic temperature calculated from the linear
D
àH versus àS plot (Fig. 5) is found to be 354 K which is above the
D
experimental temperature that indicates the applicability of the
Hammett equation to the title reaction. The linear isokinetic rela-
tion between
D D
àH and àS reveals that there is no change in reac-
tion mechanism with respect to substituents.
4. Discussion
Since hydroperoxo species was suggested as the active oxidiz-
ing species in most of the sulfide oxidation and the sulfur atom
of PMAA in the present case is relatively more electropositive than
the sulfur atom of other sulfides, we strongly believe that
hydroperoxo intermediate would be the most probable oxidizing
species. The formation and involvement of new active species in
the reaction is confirmed by the gradual decrease in intensity of
the characteristic peak of salen with the concomitant formation
of a new peak around 560 nm, subsequent increase in the intensity
of the new peak and existence of an induction period. It has been
proposed that initially H2O2 coordinates with vanadium centre of
oxovanadium(IV)–salen to form a peroxide vanadium(V)–salen
complex (C1). As this intermediate is acidic in nature it is converted
into bona fide oxidizing species (C2) by losing a proton. The acidic
nature of the hydroxyl group coordinated to the vanadium center
is corroborated by Coletti et al. [86]. Analogous scheme for the gen-
eration of active species has been proposed in the oxidation of
cyclooctene [111] and sulfides [86] by oxovanadium(IV) Schiff base
complexes.
4.1. Active oxidizing species
It has been suggested that all reactions involving vanadium(IV)
Schiff bases and hydrogen peroxide proceed via a neutral vana-
dium(V) intermediate as reactive species, which consists of oxo-
vanadium, Schiff base and peroxide units in unimolar
composition. Literature survey shows that the intermediate vana-
dium(V) species exists in different structural forms, mainly as
hydroperoxo and peroxo forms (Fig. 6).
Several researchers have suggested different forms of active
species in the reactions of epoxidation, sulfoxidation and oxidation
of other substrates. Hydroperoxo form has been suggested as the
oxidizing species in the sulfoxidation of sulfides to sulfoxide
An interesting observation noted is that the sulfoxidation pro-
ceeds without induction period if the oxidizing species had been
generated by mixing the salen and H2O2 prior to the addition of
PMAA. Besides, the results demonstrate that the factors which
facilitate the formation of oxidizing species viz., increase in concen-
tration of salen, H2O2, introduction of electron donating sub-
stituents in the salen moiety and raise in temperature have
direct relation in reducing the induction period (Fig. 2). On the
other hand, electron withdrawing substituent in the salen prevents
the easy formation of oxidizing species which is shown by length-
ening of the induction period. All these findings support our
assumption that the initial induction period in the reactions
(Fig. 2) is a consequence of slow formation of active species. It is
noteworthy to mention here that similar induction period has been
observed in the oxygenation reactions of sulfides [84] and cyclo-
Fig. 4. Hammett plot of log kov of salen complexes against
Rr.

8
C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
Fig. 6. Different forms of oxidizing species.
hexane [85] catalyzed by cobalt(III)salen ion and PSAA by oxovana-
dium(IV)salophen [40] and the existence of induction period was
interpreted by the time required for the formation of active reac-
tive species from the oxidant and catalyst.
of the Hammett plot in the region of electron donating substituents
(EDS) is positive, while negative slope is observed in the region of
electron withdrawing substituents (EWS). The existence of non-
linear relationship in the Hammett correlation not only indicates
the mode of transmission of electronic effect of a substituent to
the active species through sulfur atom is different for electron
donating and electron withdrawing substituents, but also empha-
size that the process of oxygen transfer might involve two different
rate determining steps (RDS) within a given mechanistic pathway
or two different mechanisms; different for two types of sub-
stituents [90,116–119].
4.2. Reaction mechanism
Two major mechanistic pathways have been postulated in the
literature for the sulfoxidation reaction catalyzed by vanadium
Schiff base complexes (Scheme 2). First one is the insertion mech-
anism where the reaction proceeds via nucleophilic attack of sulfur
compound on metal complex [86,110,112]. Second type is the
direct oxygen transfer mechanism, where an oxygen atom of -
OAOA group is directly transferred to the sulfur atom without
any coordination between the substrate and oxidizing species
[56,101,109,113]. Single electron transfer mechanism has also
been reported in the oxygenation of PMAAs by oxo(salen)man-
ganese(V) [16], oxidation of organic sulfides with oxo(salen)Cr(V)
[114] and reactions of anilines with H2O2-iron(III)salen [91] on
The existence of linear isokinetic plot [120,121] and Exner’s plot
[16,87] containing both EDS and EWS in the same plot have been
taken as the evidence for the operation of single mechanism in
the reaction series where non-linear Hammett plot is observed.
In the present system excellent correlation (r = 0.994) observed
in the plot of
D D
àH against àS (Fig. 5) eliminates the involvement
of two different mechanisms i.e., change in mechanism on chang-
ing the substituents from electron donating to electron withdraw-
ing. Besides, isobestic point is observed in the overlay spectrum of
the reaction mixture at different time intervals for the reactions
where single intermediate species is involved [122,123]. The pres-
ence of isobestic point in the overlay plot (Fig. 1a) unambiguously
supports the involvement of single intermediate species in the
reaction and eliminates two different intermediate for two types
of substituents. Further, the observed reaction constants of same
magnitude having opposite sign for EDS and EWS (Tables 3 and
4) lead to the conclusion that single intermediate is involved in
both type of substituents.
It is worthwhile to mention here that the isobestic point has a
direct connection with the induction period of a reaction. In the
reaction between PMAA and I, at the reaction conditions where
there is a minimum induction period, a clear isobestic point is
observed (Fig. 1a). Similar isobestic point has been noticed with
the salen complexes having electron donating substituents,
dimethyl (II) and dimethoxy (III). However, no such isobestic point
has been observed with complex IV (Fig. 1c), where the electron
withdrawing chloro substituents appreciably lengthen the induc-
tion period. The electron donating substituents in the salen com-
plex increase the electron density around vanadium which favors
the polarization of the V@O bond and easy generation of active
species, while electron withdrawing substituent disfavor the
V@O bond polarization which in turn delay the formation of active
species and lengthen the induction period.
the basis of straight line obtained by plotting log k2 versus Eox/ERed
.
In the present system single electron transfer mechanism has been
ruled out because of the non-existence of linear plot between log
kov and Eox of PMAAs (Fig. 7).
Generally coordination of hydrogen peroxide to the Lewis acid
transition metal center often activates the non-protonated peroxi-
dic oxygen atom or vanadium towards electrophilic attack on elec-
tron rich center. If oxygen atom activates, the reactive species C2
undergoes a momentary electrophilic attack on the electron rich
sulfur atom followed by coordination of protonated peroxo oxygen
atom of hydroperoxo group with the vanadium leading to the for-
mation of an intermediate C3 which promotes transfer of oxygen
from oxidant to PMAA (Direct oxygen transfer mechanism). Bal-
cells et al. [109] reported the angle value of 176.6° for an SAOAO
linkage in a similar transition state for the vanadium catalyzed oxi-
dation of sulfides by H2O2. The intermediate C3 then disproportion-
ates into C6 and the product PSAA.
In another mechanism, coordination of an electron pair of the
sulfur atom of PMAA with electron poor vanadium atom to form
the intermediate C4 is proposed. Such type of intermediate forma-
tion has been reported earlier in the sulfoxidation reactions cat-
alyzed by vanadium Schiff base complexes [81,86,109,112,115].
The partially electropositive sulfur atom in the intermediate C4
then binds with the electron rich protonated oxygen of hydroper-
oxyl group to generate another intermediate C5, where oxygen
transfer to PMAA takes place from oxidizing species. In the final
step, intermediate C5 disproportionates into products, PSAA and
C6 (Insertion mechanism). Afterward the intermediate C6 react
with another H2O2 to regenerate the reactive species, C2 and water
(Scheme 2).
On the basis of foregoing discussions, it is concluded that the
observed downward curvature in the Hammett plot can be better
explained by a change in RDS in a two step mechanism upon the
substituent change from electron donating to electron withdraw-
ing. This assumption quite coincides with the earlier conclusions
derived to explain the downward Hammett plots by proposing
two different steps of the same mechanism as rate determining
steps; one for EDS and another for EWS. Earlier in the reactions
The data in the Tables 3, 4 and Fig. 3 exhibit that substituted
PMAAs containing either electron donating or electron withdraw-
ing substituents have low reactivity compared to PMAA. The slope

C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
9
Scheme 2. Overall mechanism for sulfoxidation.
of sulfides with pyridinium dichromate [124], b-benzoylalanines
with pseudomonas fluorescens Kynureninase [89], N-methyl-/-
bromoacetanilides with benzylamines [125], cationic oxorhenium
(V) complex with aryl azides [126] and pyridinolysis of methyl

10
C. Kavitha, P. Subramaniam / Polyhedron 175 (2020) 114172
among substituted oxovanadium(IV)–salen complexes is OMe > -
Me > H > Cl > t-butyl. The sulfoxidation of PMAA catalyzed by oxo-
vanadium(IV)–salen complex is fractional order with respect to
[PMAA]. The fractional order dependence of PMAA and high
Michaelis–Menten constants reveal that PMAA coordinates with
active species in a reversible manner and the bonding is weak in
nature. Both electron donating and electron withdrawing sub-
stituents in the PMAA exert retardation effect on rate of reaction.
The Hammett correlation gave downward curvature with negative
slope for EWS and positive slope for EDS. The observed substituent
effect and kinetic results have been justified by proposing two dif-
ferent rate determining steps in a single mechanism i.e., switches
from electrophilic attack of active species on the sulfur atom of
PMAA to the binding of sulfur atom with oxygen atom of hydroper-
oxyl group when the substituents change from electron withdraw-
ing to electron donating nature.
Acknowledgements
Fig. 7. Plot of log kov against Eox of PMAAs with complex, I.
Financial support from University Grants Commission, New
Delhi, India in the form of major research project (F.No. 39-
817/2010(SR) to PS is gratefully acknowledged. CK thanks the
UGC, SERO, Hyderabad (No. TNTK00003661) and Manonmaniam
Sundaranar University for sanctioning permission to avail the ben-
efits of Faculty Development Programme. The authors thank the
Management of Aditanar College of Arts and Science, Tiruchendur
for providing facilities to carry out research.
phenyl phosphinic chloride [88] the downward Hammett plot has
been explained by proposing two different rate determining steps.
As the direct oxygen transfer mechanism involves only one step
and it does not explain the observed Michaelis–Menten kinetics,
we strongly believe that SN2 type insertion mechanism is operating
in this sulfoxidation reaction which explains all the observed
experimental facts. Formation of PSAA as the only product in the
absence of any other oxygen source demonstrates that the oxygen
atom incorporated in the PMAA should be derived from the oxidiz-
ing species which gave additional evidence for the above proposed
mechanism.
In the insertion mechanism it can be seen that electrophilic
attack of the oxidizing species on the sulfur atom of PMAA is pos-
sible only if the sulfur atom has comparable electron density. It is
noted that electron donating groups in the para- and meta-posi-
tions increase the electron density on the sulfur atom thereby facil-
itate the formation of intermediate C4. EDS not only alleviate the
formation of C4 but also stabilize the intermediate C4 by dissipating
the fractional positive charge accumulated on the sulfur atom. The
less electropositive nature of sulfur in C4 along with the stabiliza-
tion of C4 by EDS makes the conversion of C4 to C5 more difficult.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2019.114172.
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