Mechanistic study on the oxidation of (phenylthio)acetic acids by oxo(salen)manganese(V) complexes and the reactivity-selectivity principle

Article abstract of DOI:10.1016/j.molcata.2007.12.004

A systematic study on the kinetics and mechanism of oxidation of several (para-substituted phenylthio)acetic acids with various substituted oxo(salen)manganese(V) complexes in acetonitrile at 20 °C is presented. The kinetic data indicate that the reaction is second-order overall, first-order each in (phenylthio)acetic acid and oxo(salen)manganese(V) complex. Rate studies with substituted (phenylthio)acetic acids give an excellent Hammett correlation with σ⁺/σ^- constants and the ρ-values are in the range of -0.82 to -1.17 for different oxo(salen)manganese(V) complexes. The log k₂ values observed in the oxidation of each (phenylthio)acetic acid by substituted oxo(salen)manganese(V) complexes correlate with 2σ, giving ρ-values from 0.26 to 0.56. A mechanism involving single electron transfer from the sulfur center of the substrate to the oxo complex in the rate-controlling step is envisaged. Kinetic data were obtained over a temperature range of 15-35 °C and the activation parameters evaluated. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reactions of various (phenylthio)acetic acids with a given oxo(salen)manganese(V) complex and also in various oxo(salen)manganese(V) complexes with a given (phenylthio)acetic acid. Mathematical treatment of the results shows the operation of a valid reactivity-selectivity principle in this redox system.

Full text of DOI:10.1016/j.molcata.2007.12.004

Available online at www.sciencedirect.com
Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
Mechanistic study on the oxidation of (phenylthio)acetic
acids by oxo(salen)manganese(V) complexes and the
reactivity–selectivity principle
Arunachalam Chellamani∗, Paramasivan Sengu
Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627012, India
Received 17 September 2007; received in revised form 7 November 2007; accepted 1 December 2007
Available online 8 December 2007
Abstract
A systematic study on the kinetics and mechanism of oxidation of several (para-substituted phenylthio)acetic acids with various substituted
oxo(salen)manganese(V) complexes in acetonitrile at 20 ◦C is presented. The kinetic data indicate that the reaction is second-order overall, first-
order each in (phenylthio)acetic acid and oxo(salen)manganese(V) complex. Rate studies with substituted (phenylthio)acetic acids give an excellent
Hammett correlation with σ+/σ− constants and the ρ-values are in the range of −0.82 to −1.17 for different oxo(salen)manganese(V) complexes.
The log k2 values observed in the oxidation of each (phenylthio)acetic acid by substituted oxo(salen)manganese(V) complexes correlate with 2σ,
giving ρ-values from 0.26 to 0.56. A mechanism involving single electron transfer from the sulfur center of the substrate to the oxo complex in
the rate-controlling step is envisaged. Kinetic data were obtained over a temperature range of 15–35 ◦C and the activation parameters evaluated.
Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reactions of various (phenylthio)acetic
acids with a given oxo(salen)manganese(V) complex and also in various oxo(salen)manganese(V) complexes with a given (phenylthio)acetic acid.
Mathematical treatment of the results shows the operation of a valid reactivity–selectivity principle in this redox system.
© 2007 Elsevier B.V. All rights reserved.
Keywords: (Phenylthio)acetic acids; Iodosylbenzene; (Salen)MnIII complexes; Catalyzed oxidation; Reaction mechanism
1. Introduction
oxides. High-valent metal-oxo species play important roles in
a large number of synthetic and biological transformations [1].
The preparation of Mn-substituted cytochrome P-450 cam by
Gelb et al. [19] and the suspected involvement of higher valent
oxomanganese species in oxygen evolution in photosynthesis
[20] have encouraged studies of oxygen atom transfer reactions
catalyzed by manganese complexes. Manganese-oxo complexes
of both porphyrin [21] and salen [22] catalysts are implicated in
various oxygen atom transfer reactions of considerable synthetic
importance and have also served as models for intermediates in
heme protein catalyzed oxidations and related reactions. Mn(III)
is also present in active sites of enzymes [23] like superox-
ides dismutase and azide-insensitive catalase which catalyze the
redox reactions of oxygen species.
Selective oxygenation of organic compounds catalyzed by
transition metal ions is one of the most productive and elegant
techniques for the oxo functionalization of organic substrates
[1,2]. Many groups have extensively used the metal–salen com-
plexes (metal = Mn, Cr, Fe, Ru, Co, V and Ti) for the epoxidation
reactions [3,4]. But employment of these catalysts for the oxida-
tion of organic substrates containing heteroatoms like S, N and
P is limited [4–18]. For the oxygenation reactions of organic
substrates the metal-oxo species are generated from transition
metal complexes using dioxygen [5] and a wide variety of oxy-
gen transfer agents [8] including iodosylbenzene, percarboxylic
acids, hydrogen peroxide, sodium perchlorite and activated N-
Though a large variety of oxidants have been used in the
past five decades for the oxidation of biologically important
organic sulfides [24–31], the recent interest is on the oxidation
of organic sulfur compounds using transition metal based oxi-
dants [7–18,32–40]. Oxidation of (phenylthio)acetic acids by
Abbreviation: salen, N,Nꢀ-ethylenebis(salicylideneaminato).
Corresponding author. Tel.: +91 462 2333887; fax: +91 462 2322973.
E-mail address: achellamani@yahoo.co.in (A. Chellamani).
∗
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.12.004

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A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
periodic acid with RuO4 catalyst [41], permanganate [42], potas-
sium peroxydisulfate [43], peroxomonophosphoric acid [44],
potassium peroxydiphosphate [45], Chloramine-T [46], Ce(IV)
[47], pyridinium fluorochromate [48], N-chlorosuccinimide
[49], potassium ferrate [50], Bromamine-T [51] and potassium
hexacyanoferrate(III) [52] have been reported. We have initiated
a systematic study on the oxygenation reactions of organosulfur
compounds with oxometal complexes by taking Cr, Mn and Ru
as metal ions. Recently we reported the mechanism of oxidation
of organic sulfides and sulfoxides with PhIO [8–12], NaOCl
[13–15] and H2O2 [16] catalyzed by metal–salen complexes.
In this paper, we report the kinetics and mechanism of oxida-
tion of (phenylthio)acetic acids with oxo(salen)manganese(V)
complexes 2a–f generated in situ from the corresponding
[MnIII(salen)]+PF6− complexes and PhIO as represented in Eq.
(1). To our knowledge, this seems to be the first report on the
metal-oxo oxygenation of (phenylthio)acetic acids:
ues [43]. The GC and TLC analyses of each (phenylthio)acetic
acid showed the presence of a single entity. Acetonitrile (GR, E.
Merck) was first refluxed over P2O5 for 5 h and then distilled.
Iodosylbenzene was prepared from (diacetoxyiodo)benzene by
a standard method [55].
The [MnIII(salen)]+PF6 complexes 1a–f were synthesized
−
according to a known procedure [3,8–11,13–16]. The IR and
UV–vis spectral studies of all of the complexes were found to be
identical with literature data [3]. The oxo complexes 2a–f were
generated [8–11] in situ by stirring magnetically 0.15 mmol of
finely powdered PhIO in 5 ml of an acetonitrile solution con-
taining 0.015 mmol of the corresponding (salen)MnIII complex
for 5 min under nitrogen, followed by filtration at ice tempera-
ture to remove the undissolved iodosylbenzene. The conversion
of MnIII to MnV may be considered quantitative in the sense
that prolonged stirring causes no change in the initial absorption
spectrum of the MnV solution. Because the oxomanganese(V)
(1)
complex undergoes auto decomposition, the solutions were pre-
pared freshly for each kinetic run.
Reactivity–selectivity principle (RSP) has been tested with
hydration, acid–base catalysis, solvolysis, oxidation, reduction
and other types of reaction [10,25,53,54]. The study of appli-
cability of RSP to biologically relevant oxygen atom transfer
reactions is of current interest. Recently we have reported the
applicability of RSP in the oxidation of organic sulfides and
sulfoxides with PhIO [9–11] and NaOCl [14,15] catalyzed by
(salen)MnIII complexes. Hereinwealsoreportthestudyofappli-
cability of RSP to the present system.
2.2. Kinetic measurements
Reaction mixtures for kinetic runs were prepared by
quickly mixing the solutions of the oxo complex and the
(phenylthio)acetic acid in varying volumes so that in each run
the total volume was 5 ml. The progress of the reaction was fol-
lowedinaPerkinElmerUV–visspectrophotometer(Lambda25)
fitted with a thermostated cell compartment under pseudo-first-
order conditions ([substrate] > [oxo complex]) in acetonitrile at
20 0.1 ◦C by monitoring the decay of the oxo complex at
680 nm [3,8–11,13–15].
2. Experimental
2.1. Materials
(Phenylthio)acetic acid and (substituted phenylthio)acetic
acids were prepared by standard procedure from the cor-
responding thiophenols and chloroacetic acid in alkaline
medium [43]. A different procedure was adopted for obtain-
ing (p-nitrophenylthio)acetic acid from p-chloronitrobenzene
and thioglycollic acid [43]. The physical constants for the
(phenylthio)acetic acids were found to agree with literature val-
The rate constants were computed from the linear least-
squares plots of log(At − A ) versus time, where At is the
∞
absorbance at time t and A is the experimentally determined
∞
infinity point. The first-order rate constants k1(dec) for the auto
decomposition of oxomanganese(V) were determined from the
first-orderplotsupto50–60%ofreaction. Theplotsforthedecay
of the oxo complex in the presence of (phenylthio)acetic acid

A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
85
were linear over 50% of the reaction and the pseudo-first-order
was isolated in 79% yield (Eq. (3)):
rate constants k1(obs) were determined from the disappearance of
the oxo complex up to this extent. The values of k1 were obtained
[5,8–11,13–15] as k1 = k1(obs) − k1(dec). The second-order rate
constants were obtained by the relation k2 = k1/[substrate].
(3)
2.3. Product analysis
The absorption spectrum of the final solution coincided
with that of the original (salen)MnIII complex. The active
species in the present reaction is considered to be the
oxo(salen)manganese(V) complex, as proposed in the earlier
reports [3,8–11,13–15] on the (salen)MnIII catalyzed PhIO
and NaOCl oxygenation reactions. We could not isolate this
oxo(salen)manganese(V) complex and the present spectral data
are similar to those in earlier reports [3,8–11,13–15]. Even
though there is no report on the structural characterization of
the oxo(salen)manganese(V) complex, recent theoretical work
[56] suggested a triplet ground state for this species. How-
ever, Groves et al. [57] and others [58,59] have characterized
oxomanganese(V)–porphyrin complexes. As the isolation of
pure oxomanganese(V) complexes is difficult [3,8–11,13–15]
they were generated in situ for the studies reported here.
The reaction mixture from an actual kinetic run (after keep-
ing it for overnight) was extracted with ether and dried over
anhydrous Na2SO4. The solvent was removed at reduced pres-
sure. TLC analysis (silica gel) of the reaction mixture {solvent
system: n-butanol–water–acetic acid (4:5:1)-upper layer} gave
two spots corresponding to (phenylthio)acetic acid (Rf = 0.84)
and phenylsulfinylacetic acid (Rf = 0.45), respectively, thereby
establishing phenylsulfinylacetic acid as the reaction prod-
uct. The infrared spectrum of the reaction product, separated
from the reaction mixture, gave an intense absorption band at
1050 cm−1 characteristic of SO stretching. The product was dis-
solved in acetonitrile and GC analyses of the samples showed
the presence of phenylsulfinylacetic acid and iodobenzene.
The yield of phenylsulfinylacetic acid (78–98%) depended on
the (phenylthio)acetic acid and oxo complex employed. The
lower yield of phenylsulfinylacetic acid may be attributed to
the competitive self-decomposition of the oxomanganese(V)
complex to the MnIII complex, which is confirmed by the
near quantitative recovery of MnIII complex from the reaction
mixture.
3.2. Kinetics of oxygen atom transfer from
oxo(salen)manganese(V) to (phenylthio)acetic acids
The oxidation proceeds smoothly and the reaction is first-
order in 2a as evidenced by excellent linear plots (r > 0.995)
of log(At − A ) against time and also by the constant pseudo-
∞
first-order rate constants, k1, at different initial concentrations
of 2a (Table 1). The data in Table 1 reveals that the k1 val-
ues for the oxidation of (phenylthio)acetic acid [PTAA] by the
oxo(salen)manganese(V) increases with increase in substrate
concentration. A plot of k1 versus [PTAA] yielded a straight line
2.4. Stoichiometry
The reaction was carried out under the experimental condi-
tions, [2] = 0.0018 M; [PhSCH2COOH] = 0.2 M. The resultant
solution after the completion of the reaction gave phenylsulfiny-
lacetic acid in ca.79% yield and (salen)MnIII complex in ca.96%
yield. Accordingly, the stoichiometry of the reaction can be
represented by the following equation:
(2)
3. Results and discussion
3.1. Generation of oxo(salen)manganese(V) complex
It has been well established that oxo(salen)manganese(V)
complexes 2a–f are formed by stirring clear solutions of
(salen)MnIII complexes 1a–f in acetonitrile with PhIO [3,8–11].
The formation of oxomanganese(V) species is associated with
the following two changes: (i) the light brown color darkens, and
(ii) the characteristic peak of (salen)MnIII at λmax ≈ 350 nm dis-
appears and a new absorption band at λmax ≈ 530 nm appears.
The dark brown solution, on standing, faded to the original light
brown solution within 2–3 h. When the same experiment was
carried out in the presence of (phenylthio)acetic acid, fading
occurred in less than 15–20 min and phenylsulfinylacetic acid
Fig. 1. Plots of k1 vs. [substrate] for the oxidation of (a) PTAA with 2b, (b)
p-BrPTAA with 2a, (c) PTAA with 2e, (d) PTAA with 2a, (e) PTAA with 2c, (f)
p-MePTAA with 2a, and (g) PTAA with 2d.

86
A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
Table 1
Rate constants for the oxidation of (phenylthio)acetic acid by oxo(salen)manganese(V) complexes 2a–f in acetonitrile at 20 ◦Ca
b
c
d
e
[PTAA]0 × 102 (M)
[2]0 × 103 (M)
k1(obs) × 103 (s−1
)
k1(dec) × 104 (s−1
)
k1 × 103 (s−1
)
k2 × 102 (M−1 s−1
)
2a (0.751)f
3.50
3.50
3.50
3.50
3.50
3.50
1.80
5.40
0.83
1.24
1.65
1.80
2.02
3.04
1.80
1.80
1.80
1.80
1.80
2.92 0.03
2.86 0.04
2.83 0.02
2.82 0.07
2.97 0.05
2.87 0.03
1.52 0.04
4.28 0.04
5.94 0.12
6.84 0.03
8.45 0.04
2.43 0.06
2.27 0.05
2.03 0.05
2.04 0.04
2.15 0.02
2.04 0.02
2.04 0.04
2.04 0.04
2.04 0.04
2.04 0.04
2.04 0.04
2.68 0.02
2.63 0.04
2.63 0.02
2.62 0.07
2.76 0.05
2.67 0.04
1.32 0.04
4.08 0.04
5.74 0.12
6.64 0.03
8.25 0.04
7.65 0.07
7.52 0.10
7.51 0.04
7.47 0.19
7.87 0.14
7.62 0.08
7.31 0.20
7.55 0.07
7.97 0.16
7.37 0.03
7.63 0.03
7.20
9.00
10.8
2b (0.739)f
1.80
3.60
5.40
7.20
1.80
1.80
1.80
1.80
0.93 0.01
1.69 0.03
2.51 0.05
3.25 0.10
1.61 0.01
1.61 0.01
1.61 0.01
1.61 0.01
0.77 0.01
1.53 0.03
2.35 0.05
3.09 0.10
4.27 0.05
4.25 0.08
4.35 0.09
4.29 0.14
2c (0.770)f
1.80
3.60
7.20
10.8
1.80
1.80
1.80
1.80
2.83 0.02
5.53 0.01
10.8 0.03
16.2 0.07
2.11 0.02
2.11 0.02
2.11 0.02
2.11 0.02
2.62 0.02
5.32 0.01
10.6 0.03
16.0 0.07
14.6 0.10
14.8 0.02
14.7 0.04
14.8 0.06
2d (0.798)f
1.80
3.60
5.40
7.20
1.80
1.80
1.80
1.80
5.73 0.01
11.5 0.02
16.8 0.04
22.5 0.03
2.20 0.01
2.20 0.01
2.20 0.01
2.20 0.01
5.51 0.01
11.3 0.02
16.6 0.04
22.3 0.03
30.6 0.05
31.4 0.05
30.7 0.07
31.0 0.04
2e (0.747)f
1.80
3.60
5.40
7.20
1.80
1.80
1.80
1.80
1.34 0.03
2.46 0.05
3.64 0.11
4.81 0.12
2.01 0.05
2.01 0.05
2.01 0.05
2.01 0.05
1.14 0.03
2.26 0.05
3.44 0.11
4.61 0.12
6.33 0.14
6.28 0.13
6.37 0.19
6.40 0.16
2f (0.744)f
1.80
3.60
5.40
7.20
1.80
1.80
1.80
1.80
1.26 0.01
2.39 0.01
3.50 0.03
4.52 0.08
1.74 0.01
1.74 0.01
1.74 0.01
1.74 0.01
1.01 0.01
2.22 0.01
3.33 0.03
4.35 0.08
6.03 0.05
6.16 0.03
6.16 0.05
6.04 0.10
a
As determined by a spectrophotometric technique following the disappearance of the oxo complex at 680 nm; the error quoted in k values is the 95% confidence
limit of the Student’s t-test.
b
Estimated from pseudo-first-order plots over 50% reaction.
Estimated from first-order plots over 50–60% reaction in the absence of (phenylthio)acetic acid.
c
d
Obtained as k1 = k1(obs) − k1(dec)
.
e
f
Individual k2 values estimated as k1/[PTAA]0.
Reduction potential values of oxo(salen)MnV complexes taken from Ref. [8].
passing through the origin (Fig. 1) indicating that the reaction is
overall second-order, first-order in each reactant. Similar results
were obtained for the oxidation of substituted (phenylthio)acetic
acids with oxo(salen)manganese(V) complexes 2a–f (Table 1,
Fig. 1). Hence, the rate law can be depicted as in the following
equation:
dation of (phenylthio)acetic acid with oxo(salen)manganese(V)
species 2a at various concentrations of added PyO. The rate
constant values listed in Table 2 indicate that PyO has no appre-
ciable effect on the reaction rate. The constancy of k2 values at
different [PyO] points out that PyO is not binding with oxoman-
ganese(V)species. IfbindingofPyOwereoccurredasinthecase
of oxochromium(V) complexes [60], then changes in absorption
spectrum and reaction rates would have been observed.
The data provided in Table 3 point out that the redox
reaction between oxo(salen)manganese(V) complexes and
(phenylthio)acetic acids is highly sensitive to the introduction
of the substituents in the aryl moiety of (phenylthio)acetic
acids and in the 5,5ꢀ-positions of the salen ligand. The intro-
d[2]
−
= k2[2][PTAA]0
(4)
dt
Addition of pyridine N-oxide (PyO), a donor ligand, to the dark
brown solution of 2a caused no change in the absorption spec-
trum of oxo(salen)manganese(V). The effect of donor ligand on
the reaction rate was determined by measuring k1 for the oxi-

A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
87
Table 2
Effect of pyridine N-oxide on the rate of oxidation of (phenylthio)acetic acid by 2a in acetonitrile at 20 ◦Ca
b
c
d
e
[PTAA]0 × 102 (M)
[2a]0 × 103 (M)
[PyO]0 × 102 (M)
k1(obs) × 103 (s−1
)
k1(dec) × 104 (s−1
)
k1 × 103 (s−1
)
k2 × 102 (M−1 s−1
)
1.80
1.80
1.80
1.80
1.80
1.80
1.80
1.80
1.80
1.80
0.02
0.05
0.10
0.15
0.20
1.62 0.03
1.63 0.03
1.65 0.04
1.65 0.04
1.67 0.02
2.25 0.04
2.21 0.07
2.26 0.10
2.26 0.05
2.18 0.04
1.40 0.03
1.41 0.02
1.42 0.03
1.42 0.04
1.45 0.02
7.75 0.14
7.83 0.13
7.91 0.17
7.91 0.19
8.07 0.09
a
As determined by a spectrophotometric technique following the disappearance of the oxo complex at 680 nm; the error quoted in k values is the 95% confidence
limit of the Student’s t-test.
b
Estimated from pseudo-first-order plots over 50% reaction.
Estimated from first-order plots over 50–60% reaction in the absence of (phenylthio)acetic acid.
c
d
Obtained as k1 = k1(obs) − k1(dec)
.
e
Individual k2 values estimated as k1/[PTAA]0.
Table 3
Second-order rate constants and ρ-values for the reactions of p-XC6H5SCH2COOH with 2a–d in acetonitrile at 20 ◦Ca
No.
X
Eox (V)b
Oxo(salen)manganese(V) complex, k2 × 102 (M−1 s−1
)
ρc
r
2b
2a
2c
2d
1.
2.
3.
4.
5.
6.
7.
8.
OMe
Me
i-Pr
F
H
Cl
1.408
1.445
1.447
1.465
1.466
1.474
1.477
1.535
38.8 0.79
10.9 0.25
8.67 0.11
5.16 0.09
4.28 0.06
2.83 0.04
2.44 0.01
0.33 0.01
56.1 0.54
16.0 0.95
15.4 0.10
7.63 0.09
7.34 0.21
5.61 0.04
4.92 0.01
0.56 0.01
86.7 0.65
27.9 0.22
25.7 0.19
16.2 0.11
14.6 0.08
13.2 0.03
12.2 0.03
1.88 0.01
134 0.36
55.4 0.05
48.3 0.34
32.7 0.28
30.6 0.06
25.0 0.02
24.4 0.04
4.61 0.04
0.256 0.035
0.341 0.031
0.351 0.044
0.392 0.052
0.411 0.048
0.450 0.082
0.477 0.084
0.563 0.090
(0.982)
(0.992)
(0.985)
(0.983)
(0.986)
(0.969)
(0.970)
(0.976)
Br
NO2
ρd
−1.17 0.04
−1.12 0.02
−0.920 0.03
−0.815 0.01
r
(0.997)
(0.999)
(0.997)
(0.999)
a
b
c
d
General conditions: [2]0 = 0.0018 M; [substrate]0 = 0.018 M.
Oxidation potential values of substituted (phenylthio)acetic acids estimated using Marcus equation [8].
The values were obtained by correlating log k2 with 2σp for the reaction of various oxo(salen)manganese(V) complexes with a given (phenylthio)acetic acid.
The values were obtained by correlating log k2 with σ+/σ− for the reaction of various (phenylthio)acetic acids with a given oxo(salen)manganese(V) complex.
duction of substituents in the para-position of the phenyl ring
of (phenylthio)acetic acid alters the rate appreciably; i.e., the
electron-donating substituents accelerate the rate and electron-
withdrawing groups decelerate the rate. It is of interest to find
out if any Hammett type of correlation exists for the oxida-
tion of (phenylthio)acetic acids by oxo(salen)manganese(V).
The reactivity pattern is brought out by two different corre-
lations, one a log k2 versus σ plot and the other with log k2
and σ+/σ−. Since sulfur atom can exhibit the dual character
of both electron-releasing and electron-accepting type, we have
used both σ+and σ− simultaneously in the second correlation.
There are several reactions involving a sulfur center where
both σ+ and σ− were simultaneously employed [25,26,43].
Though a satisfactory correlation is observed with σ values
[43] (r = 0.970, ρ = −1.66 0.17, s = 0.150), an excellent cor-
relation exists when σ+/σ− values [43] are plotted against log k2
(Fig. 2, r = 0.999, ρ = −1.12 0.02, s = 0.027). The negative ρ-
value indicates that the sulfur center of the substrate is more
positively charged in the transition state than it is in the reactant.
The ρ-values for the substituent variation in (phenylthio)acetic
acids for each oxo complex fall in the range of −0.815 to
−1.17 (Table 3). Further, the plot of log k2 against the oxida-
tion potential, Eox, of (phenylthio)acetic acids calculated from
the analysis of the kinetic data in terms of Marcus equation
Fig. 2. Hammett plot for the oxidation of (phenylthio)acetic acids by 2a. The
points are referred to by the same numbers as in Table 3.

88
A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
Fig. 5. Plot of ln k2 vs. reduction potential of oxo(salen)manganese(V) com-
plexes 2a–f.
Fig. 3. Plot of log k2 vs. oxidation potential for the oxidation of (substituted
phenylthio)acetic acids by 2a. The points are referred to by the same numbers
as in Table 3.
plexes for each (phenylthio)acetic acid (Table 3). A plot of ln k2
versus the reduction potential values, E0, of MnV/MnIV couple
[8] is perfectly linear (Table 1, Fig. 5, r = 0.999). To realize the
role of steric effect in this reaction, oxo(salen)manganese(V)
complexes with methyl or phenyl groups in 7 and 7ꢀ positions of
thesalenligand(complexes2eand2f)areusedtostudythekinet-
ics of the oxygenation reaction. Comparison of the kinetic data
forcomplexes2a, 2eand2fintheoxidationof(phenylthio)acetic
acid (Table 1) reveals that the presence of methyl or phenyl
group at the 7,7ꢀ-positions of salen ligand slightly reduces the
rate, as in the sulfide [8,13] and sulfoxide [11,15] oxidations.
Thus, the steric effect observed with MnV complexes is small,
which is contrary to the substantial steric effect noted in the
oxo(salen)CrV complexes oxidation of alkynes [61] and organic
sulfides [17].
[8] is linear (Table 3, Fig. 3, r = 0.999) with a slope of −16.2.
The introduction of a substituent in the salen ligand also alters
the rate of the reaction substantially. It is seen that the electron-
withdrawing substituents at the 5,5ꢀ-positions of the salen ligand
enhance the rate, whereas electron-releasing substituents retard
it (Tables 1 and 3). Hammett correlation of log k2 with 2σp
shows a satisfactory linear relationship with a positive slope of
0.411 0.048 (Fig. 4, r = 0.986, s = 0.071). The ρ-values range
from 0.256 to 0.563 for the substituent variation in the oxo com-
3.3. Mechanism of (phenylthio)acetic acid oxidation with
oxo(salen)manganese(V) complex
In the oxygenation reactions, single oxygen atom donors like
iodosylbenzene, sodium oxychloride, etc., first transfer the oxy-
gen to metal complex and then to the substrate. The catalytic
efficiency of a metal complex depends on factors like nature
of the metal center and the geometry of the complex molecule.
The selection of the metal is important because the complex
derived from it should form an oxo-cation. Oxygen transfer by
hypervalent oxometal species has been proposed to proceed by
electron transfer [6,7], radical addition [62], carbocation forma-
tion [63], metallaoxetane formation [64], or a combination of
these mechanisms [65]. In the present investigation, since we
identified oxo(salen)manganese(V) species as the crucial inter-
mediate for the catalytic oxidation, let us now consider how the
oxygen atom is actually transferred from the oxo species to the
(phenylthio)acetic acid in the rate-limiting step.
Fig. 4. Hammett plot for the oxidation of PTAA by substituted
oxo(salen)manganese(V) complexes.

A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
89
Of the two possible mechanisms for the oxidation of
organosulfur compounds, one involves nucleophilic attack of
sulfur on the metal center (SN2) and the second mechanism
involves single electron transfer (SET) from sulfur to the
oxidant. The oxidants such as peroxybenzoate [28], hydroper-
oxidase [30], Cr(VI) [27], Ce(IV) [36] and oxoruthenium(IV)
complexes [7] oxidize sulfides by a SET mechanism. The
oxidation of sulfides by peroxoanions [25], molybdenum per-
oxypolyoxoanions [37], sulfamyloxaridines [31], phenyliodoso
diacetate [26], pyridinium chlorochromate [29], permanganate
[35] and oxo(salen)manganese(V) [11,13] follow a SN2 mecha-
nism. The oxidation of sulfoxides by oxo(salen)manganese(V)
[11,15] follow a SN2 mechanism. In the present study,
(phenylthio)acetic acids are taken as a simple model system
to comprehend the reactivity and mechanism of the oxometal
complexes towards organic compounds containing heteroatoms.
The formation of phenylsulfinylacetic acid in the absence
of other oxygen sources (i.e., at inert atmosphere and in
non-aqueous solvent) clearly indicates that the oxygen atom
incorporated in the substrate is derived from the oxoman-
ganese(V) ions. The k1 values obtained for different initial
concentrations of (phenylthio)acetic acid (Table 1) and the lin-
ear plots in Fig. 1 show that the kinetics saturation is not
observed even at high concentration of (phenylthio)acetic acid.
This suggests that the oxidation via preliminary coordina-
tion of (phenylthio)acetic acid to oxomanganese(V) can be
ruled out. The results obtained from the influence of elec-
tronic effects on substrate and oxidant throw more light on
the mechanism of oxygen atom transfer. The observed ρ-
value of −1.12 in the present study is comparable to the
ρ-values found for the oxidation of substituted thioanisoles by
tert-butyl p-chloroperoxybenzoate [28] (ρ = −1.68), by singlet
oxygen [24] (␳ = −1.63), by carboxylato-bound chromium(V)
[39] (ρ = −1.19) and by oxo(phosphine)–ruthenium(IV) com-
plexes [7] (ρ = −1.56). For all these cases, SET mechanism
has been proposed. Also SET mechanism has been postu-
Reactions [33,34,38,39] that involve rate-limiting SET from
sulfur to yield radical cation intermediates are known to give
better Hammett correlations when σ+ substituent constants
are used. For the oxidation of thioanisoles by oxo(phosphine)
ruthenium(IV) complexes, Takeuchi and co-workers [7] have
suggested a SET mechanism based on the excellent correlation
between log kx/kH and σ− values. In the potassium peroxy-
disulfate oxidation of (phenylthio)acetic acids [43] a better
correlation of log k2 with σ+/σ− than σ exists and a partial
polar mechanism is proposed. Hence, the observed better cor-
relation of log k2 with σ+/σ− than σ may be taken as a clue
for the operation of a SET mechanism in the present inves-
tigation. This is in accordance with the Hammond postulate
[66] that for a single electron transfer mechanism the transi-
tion state resembles a radical cation and a better correlation
is observed with σ+ values. It is pertinent to mention here
that in the oxidation of organic sulfides with NaOCl catalyzed
by (salen)MnIII complexes [13], a SN2 mechanism is pro-
posed based on the better correlation of log k2 with σ than
σ+/σ−. Further, from the linear log k2 versus Eox plot (Fig. 3)
we can obtain useful information on the mechanism of the
reaction by comparing the present results with the recent obser-
vations made by Goto et al. [40] on sulfoxidation catalyzed
by high-valent intermediates of heme enzymes. Goto et al.
[40] observed a slope of −10.5 when the reaction proceeds
through an electron transfer mechanism and −2.2 in the case
of the reaction proceeding via direct oxygen transfer. Hence,
the observed slope of −16.2 in the present study is in favour
of a mechanism proceeding through single electron transfer.
Based on similar arguments, a direct oxygen transfer mechanism
has been proposed for the oxo(salen)iron [18] (slope = −1.70)
and oxo(salen)manganese [13] (slope = −3.5) oxygenation of
organic sulfides.
The effects of electron-donating and electron-withdrawing
substituents at the 5,5ꢀ-positions of salen ligand on the reac-
tion rate shown in Table 1 can be interpreted as a reflection
of a changing electron density in the oxomanganese(V) func-
tionality. The observed positive ρ-value (0.411) indicates the
build-up of a negative charge on the metal center in the tran-
sition state of the rate-determining step, as would be expected
in the reduction of manganese(V) to manganese(III) [67]. The
observed linear plot of ln k2 versus the reduction potentials E0 of
MnV O complexes (Fig. 5) also supports single electron transfer
in the rate-determining step.
Based on the arguments presented and the kinetic data
obtained, a SET mechanism shown in Scheme 1 is proposed
for the oxidation of (phenylthio)acetic acids by oxo(salen)
manganese(V) complexes. The mechanism envisages the for-
mation of a radical cation in the slow step. The formation of
cation radicals during the oxidation of organic substrates has
been firmly established [36]. The formation of oxametallacyclic
intermediate has been excluded, because the reaction involves
little steric effect when bulky methyl and phenyl groups are
introduced in the 7,7ꢀ-positions (Table 1) [59]. Then the interme-
diate I decomposes to give Mn(III) and phenylsulfinylacetic acid
as the products. The rate acceleration by electron-withdrawing
nitro-substituents on the 5 and 5ꢀ positions of salen and by the
3+
lated in the cytochrome P-450 (ρ = −0.20) and Fe(NN)3
(NN = 2,2ꢀ-bipyridyl or 1,10-phenanthroline) (ρ = −3.21) oxi-
dation of ArSMe [32,38]. Partial polar and polar mechanisms
have been proposed for the oxidation of (phenylthio)acetic
acids with potassium peroxydisulfate [43] (ρ = −0.62) and
potassium peroxydiphosphate [45] (ρ = −0.45), respectively. A
mechanism involving reversible proton abstraction followed
by reversible electron transfer step has been postulated in the
rate studies on substituted phenylsulfanylacetates [52] with
potassium hexacyanoferrate(III) (ρ = 1.31). In the Ce(IV) oxi-
dation of (phenylthio)acetic acids [47], a ρ-value of −1.86
has been obtained and a free radical mechanism is proposed.
In the Bromamine-T [51] oxidation of phenylsulfanylacetate
ions, a ρ-value of −2.42 is obtained. Electrophilic attack
of the BrO− on the phenylsulfanylacetate ions in the rate-
determining step is proposed. Hence, the low or high magnitude
of the ρ-value cannot be taken as evidence for the opera-
tion of a SET or SN2 mechanism in a particular reaction.
According to Miller et al. [30], a decision on reaction mech-
anism simply based on the magnitude of the ρ-value cannot be
reliable.

90
A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
Table 4
Second-order rate constants and activation parameters for the oxidation of p-XC6H5SCH2COOH by 2a in acetonitrile at five temperaturesa
X
k2 × 102 (M−1 s−1
)
ꢀH= (kJ mol−1
)
ꢀS= (J K−1 mol−1
)
288 K
293 K
298 K
303 K
308 K
OMe
Me
i-Pr
H
F
Cl
31.3 0.20
9.00 0.07
8.44 0.04
4.22 0.03
5.06 0.05
3.11 0.01
2.89 0.01
0.39 0.01
56.1 0.54
16.0 0.95
15.4 0.10
7.34 0.21
7.63 0.09
5.61 0.04
4.92 0.01
0.56 0.01
96.1 0.95
26.6 0.21
24.2 0.19
11.7 0.04
13.7 0.09
8.61 0.10
7.72 0.03
0.67 0.02
164 2.69
50.4 0.28
42.3 0.25
24.4 0.03
30.3 0.08
18.9 0.14
17.5 0.14
0.94 0.03
311 3.10
103 0.30
85.6 0.66
49.8 0.12
62.8 0.26
39.3 0.34
35.8 0.20
1.36 0.06
81.1
86.3
80.7
87.9
92.0
90.1
90.3
42.0
26.9
34.3
14.7
33.5
48.5
38.6
38.5
Br
NO2
−145
a
General conditions: [2a]0 = 0.0018 M; [substrate]0 = 0.018 M.
last column shows ρ-values for the substituent variation in oxo
complex for each (phenylthio)acetic acid. The reaction constant
data show that there is a significant variation of reaction con-
stants (selectivity) on varying the nature of the substituent in
either the oxo complex or the (phenylthio)acetic acid. Also, it is
apparent that as the reactivity of either the (phenylthio)acetic
acid or oxo complex decreases the reaction constant value
increases, i.e., there is an inverse relationship between reactivity
and selectivity in both cases.
The applicability of RSP is verified by Exner [54] method.
Accordingly, the rate data reported in Table 3 were subjected to
mathematical treatment using Eqs. (5) and (6):
Scheme 1. Mechanism of [(salen)MnV O]+ oxidation of (phenylthio)acetic
acids.
electron-donating p-methoxy group on (phenylthio)acetic acid
support this formulation.
log kFi = a + b log kSi + εi
(5)
(6)
ꢀ
ꢀ
log kF
−
log kS
i
i
i
i
The oxo(salen)manganese(V) oxidation of (para-substituted
phenylthio)acetic acids was carried out at five different tem-
peratures, and the thermodynamic parameters evaluated using
the Eyring equation are collected along with k2 values in
Table 4. The ꢀH= (42–92 kJ mol−1) and ꢀS= (−145 to
48.5 J K−1 mol−1) values are in favor of SET mechanism. Even
though the correlation between ꢀH= and ꢀS= is not satis-
factory, a plot of log k2 at 15 ◦C and 35 ◦C gives an excellent
correlation (r = 0.994, s = 0.072) implying that all the (substi-
tuted phenylthio)acetic acids follow the same mechanism.
Δ =
N
where kFi and kSi are the second-order rate constants for the
reactions of fast and slow reagents (oxo complexes), respec-
tively, with each (phenylthio)acetic acid, εi is the error of the
log kFi versus log kSi correlation, and Δ is the mean difference.
The values of b and Δ were calculated for all of the six possi-
ble combinations of one fast and one slow reagent (among the
four oxo complexes) with a series of similar substrates (eight
(phenylthio)acetic acids). The results summarized in Table 5
show a valid RSP in all cases as the value of b is less than unity
and Δ is not too small.
3.4. Reactivity–selectivity principle
In a system involving more than one reagent and the same
set of substrates, the existence of a “magic point” [54] in the
log kFi versus log kSi plots is an indication for a strong RSP.
The magic point represents some limiting value of reactivity in
which, for a particular substrate, the reaction rate is indepen-
dent of the reagent and vice versa. In this aspect, the present
The reactivity–selectivity principle (RSP) says that there is
an inverse relationship between the reactivity of a reagent and
its selectivity among a set of similar substrates. The last row of
Table 3 contains reaction constant values for substituent varia-
tion in (phenylthio)acetic acid for each oxo complex while the
Table 5
Results of correlation between log kFi and log kSi according to Eq. (5) for (phenylthio)acetic acidsa
Results
Oxo(salen)manganese(V) complexes (F and S)
2a and 2b
2c and 2b
2c and 2a
2d and 2b
2d and 2a
2d and 2c
r
0.996
0.993
0.998
0.996
0.999
0.999
b
Δ
0.952 0.034
0.226
0.780 0.039
0.546
0.821 0.021
0.320
0.691 0.027
0.842
0.726 0.014
0.616
0.883 0.017
0.296
a
[2]0 = 0.0018 M.

A. Chellamani, P. Sengu / Journal of Molecular Catalysis A: Chemical 283 (2008) 83–92
91
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Acknowledgments
A.C. thanks Manonmaniam Sundaranar University for the
financial assistance from UGC-UAG. P.S. thanks the UGC, New
Delhi and Manonmaniam Sundaranar University, Tirunelveli,
for the award of a fellowship under FIP.

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