Modulation of catalytic activity by ligand oxides in the sulfoxidation of phenylmercaptoacetic acids by oxo(salen)chromium(V) complexes

Article abstract of DOI:10.1016/j.poly.2016.08.013

Mechanism of sulfoxidation of eleven para-substituted phenyl mercaptoacetic acids (PMAAs) by three oxo(salen)chromium(V)⁺PF₆^?complexes in the presence of different ligand oxides (LOs) such as triphenylphosphine oxide, pyridine N-oxide and 4-picoline N-oxide have been studied spectrophotometrically in 100% acetonitrile medium. Spectral and kinetic profiles establish the formation of adduct, O[dbnd]Cr(V)(salen)⁺-LO as the reactive intermediate in the catalytic cycle. The rate of sulfoxidation is found to be enhanced significantly by the addition of LOs and introduction of substituent in PMAA and salen complex. Both electron releasing and electron withdrawing substituents in the substrate and oxidant facilitate the rate of sulfoxidation. Correlation with Hammett constants yields a non-linear concave upward curve. Based on the experimental results and substituent effects two different mechanisms, a direct oxygen atom transfer (DOT) for PMAAs with electron withdrawing substituents and a single electron transfer for PMAAs with electron donating substituents have been postulated.

Full text of DOI:10.1016/j.poly.2016.08.013

Accepted Manuscript
Modulation of catalytic activity by ligand oxides in the sulfoxidation of phe-
nylmercaptoacetic acids by oxo(salen)chromium(V) complexes
P. Subramaniam, S. Anbarasan, S. Sugirtha Devi, A. Ramdass
PII:
S0277-5387(16)30372-2
DOI:
http://dx.doi.org/10.1016/j.poly.2016.08.013
POLY 12150
Reference:
Polyhedron
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Revised Date:
Accepted Date:
15 June 2016
7 August 2016
9 August 2016
Please cite this article as: P. Subramaniam, S. Anbarasan, S. Sugirtha Devi, A. Ramdass, Modulation of catalytic
activity by ligand oxides in the sulfoxidation of phenylmercaptoacetic acids by oxo(salen)chromium(V) complexes,
Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.08.013
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Modulation of catalytic activity by ligand oxides in the sulfoxidation of
phenylmercaptoacetic acids by oxo(salen)chromium(V) complexes
P. Subramaniama,*, S. Anbarasana, S. Sugirtha Devib, A. Ramdassa
aResearch Department of Chemistry, Aditanar College of Arts and Science, Tiruchendur
628 216, Tamil Nadu, India
bDepartment of Chemistry, Kamaraj College, Thoothukudi 628 003, Tamil Nadu, India
A B S T R A C T
Mechanism of sulfoxidation of eleven para-substituted phenyl mercaptoacetic acids
−
(PMAAs) by three oxo(salen)chromium(V)+ PF6 complexes in the presence of different
ligand oxides (LOs) such as triphenylphosphine oxide, pyridine N-oxide and 4-picoline
N-oxide have been studied spectrophotometrically in 100 % acetonitrile medium. Spectral
and kinetic profiles establish the formation of adduct, O=Cr(V)(salen)+-LO as the reactive
intermediate in the catalytic cycle. The rate of sulfoxidation is found to be enhanced
significantly by the addition of LOs and introduction of substituent in PMAA and salen
complex. Both electron-releasing and electron-withdrawing substituents in the substrate and
oxidant facilitate the rate of sulfoxidation. Correlation with Hammett constants yields a non-
linear concave upward curve. Based on the experimental results and substituent effects two
different mechanisms, a direct oxygen atom transfer (DOT) for PMAAs with electron
withdrawing substituents and single electron transfer for PMAAs with electron donating
substituents have been postulated.
________
Abbreviations: PMAA, phenylmercaptoacetic acid; PSAA, phenylsulfinylacetic acid;
salen,
N,N’-bis(salicylidene)-ethylenediaminato;
LO,
ligand
oxide;
TPPO,
triphenylphosphine oxide; PyO, pyridine N-oxide; PicNO, picoline N-oxide; SET, single
electron transfer
*Corresponding author. Tel.: +91 9443500381
E-mail addresses: subramaniam.perumal@gmail.com (P. Subramaniam),
sanbarasan62@gmail.com (S. Anbarasan), sugirthadevi.s@gmail.com (S. Sugirtha Devi),
ramdasschem@gmail.com (A. Ramdass).
Keywords: Oxo(salen)chromium(V); Phenylmercaptoacetic acid; Sulfoxidation; Non-linear
Hammett plot; Ligand oxide.
1

1. Introduction
Sulfur is an essential element to life and a key component in fats, body fluids, bones
and proteins [1,2]. The study on the oxidation of organic sulfide moiety is important because
of the central role of sulfides in living organisms and their ability to act as antioxidant. The
antioxidant activity of sulfides could be ascribed partly to their ease of oxidation to form
sulfoxide [3]. The sulfide moiety in biological systems is more prone to oxidation and such
sulfide oxidation has received a detailed examination to understand the underlying
mechanism [4−7].
Selective oxygenation of sulfide to sulfoxide is one of the most interesting routes for
the production of synthetic intermediates in the construction of biologically and
pharmaceutically active molecules [8,9]. Phenylmercaptoacetic acid (PMAA) and its
derivatives are one among organic sulfides widely used in the fields of medicine [10] and
biology [11]. Selective oxidation of PMAA and substituted PMAAs by metals [12−15], metal
complexes [16−19], N-halo compounds [20−26], halo chromates [27−29], peroxocompounds
[30−32], sodium perborate [33−36] and percarbonate [37] have been extensively studied. In
2
these reactions SN type mechanism with the formation of electron deficient sulfonium ion
intermediate or single electron transfer mechanism (SET) with the formation of sulfur radical
cation intermediate has been proposed. Though phenylsulfinylacetic acid was reported as the
sole product in most of the reactions, thiophenol [28,36], α-hydroxyphenylmercaptoacetic
acid [19] and disulfide [12,14] were also identified as products in some cases.
Oxidation of organic sulfur compounds such as sulfides, sulfoxides and thioacids etc.,
using transition metal complexes either as an oxidant or as a catalyst is an area of current
interest. In this way, synthetic organic ligands viz. Schiff’s bases [38,39], porphyrin [40−42]
and phthalocyanine [43] have been recently employed in the development of synthetic metal
complexes in order to mimic the reactivity of metallo enzymes. In the past three decades,
relatively long lived chromium(V) intermediates formed with various chelating agents have
been detected both in vivo and in vitro. Since Cr(V) intermediates are more labile and
reactive these are considered to be the key species in Cr(VI) carcinogenesis. Of the
biomimetic Cr(V) complexes studied, the chromium(V)-salen complex chosen for the present
study mimics Cr(V)-peptide complexes that were formed during intracellular reoxidation of
Cr(III)-peptide complexes [44].
2

Several donor ligands have been employed in porphyrin and salen catalysed reactions
to enhance the turnover rates and product selectivity [45,46]. The added donor ligands
directly coordinate with metal centre of the porphyrin and salen complexes thereby enhancing
their oxidizing ability by decreasing the oxidation potential of the metal complexes [47]. In
recent years we have made systematic attempts to explore different mechanistic pathways in
the oxidation of organic sulfur compounds using Cr(VI) [48−51], Fe(III) complexes [52−55],
Cr(V) complexes [56,57] and V(IV) complexes [58] under different conditions. In
continuation of our progress, we carried out a systematic study on the oxidation of eleven
para substituted PMAAs with three [oxo(salen)chromium(V)]+ ions (Ia-Ic) in the presence of
three donor ligand oxides viz. triphenylphosphine oxide (TPPO), picoline N-oxide (PicNO)
and pyridine N-oxide (PyO) (Chart 1) and the results are discussed in this report.
−
Chart 1. Structures of phenylmercaptoacetic acids (1-11), [oxo(salen)chromium(V)]+PF6
(Ia-Ic) and ligand oxides.
2. Experimental
2.1. Materials
Salicylaldehyde, 5-methyl and 5-chloro salicylaldehydes and triphenylphosphine
oxide were purchased from Alfa Aeser and used as such. The chemicals CrCl3·6H2O,
pyridine N-oxide, picoline N-oxide and substituted thiophenols were purchased from Sigma-
Aldrich. Pyridine and thiophenol were purchased from SD fine. HPLC-grade acetonitrile
(Merck) was used as the solvent without further purification.
3

2.2. Preparation of phenylmercaptoacetic acids
PMAAs were prepared by established procedure by the condensation of the
corresponding thiophenols with chloroacetic acid in alkaline medium [59]. The
p-nitrophenylmercaptoacetic acid was prepared by a different procedure as advocated by
Srinivasan et al. from p-chloronitrobenzene and thioglycollic acid [60]. The purity of PMAAs
was checked by determining their melting points and comparing them with literature values
[60]. Iodosobenzene was prepared from (diacetoxyiodo)benzene by
procedure [61].
a
standard
2.3. Preparation of (salen)chromium(III) chloride complexes
Chromium(III) chloride hexahydrate (2.5 g, 0.01 M) dissolved in distilled water was
reduced to chromium(II) chloride by zinc amalgam under deaerated condition by passing
nitrogen gas. The blue solution of chromous chloride formed was added to a suspension of
salen ligand (2.15 g, 0.008 M) in acetone under nitrogen atmosphere. The dark-brown
solution was allowed to reflux in the presence of air for 2 h. The solvent was removed in
vacuum, the residue was suspended in water and allowed to stir for 2 h in the presence of air.
The yellowish-brown material was collected by filtration, redissolved in CH2Cl2 (60 mL) and
washed with saturated NH4Cl and then with aqueous NaCl. The organic phase was separated
and dried over Na2SO4 and concentrated under reduced pressure to afford the
(salen)chromium(III) chloride complex.
2.4. Preparation of (salen)chromium(III) hexafluorophosphate complexes
Equimolar solutions of (salen)chromium(III) chloride in methanol-water mixture and
ammonium hexafluorophosphate in minimum amount of water were mixed together with
stirring for 4 hours and kept aside. The formed orange crystal after 2 days were filtered
through pump, washed with water, diethyl ether and dried in vacuum.
2.5. Preparation of [oxo(salen)chromium(V)]+ ions
The solutions of [oxo(salen)chromium(V)]+ (Ia), [oxo(5,5’-dimethylsalen)-
-
chromium(V)]+ (Ib) and [oxo(5,5’-dichlorosalen)chromium(V)]+ (Ic) ions with PF6
counterion were prepared by the oxidation of corresponding orange coloured
4

-
[CrIII(salen)]+PF6 complexes with slight excess of iodosobenzene [62] in acetonitrile. The
dark green coloured solutions of [oxo(salen)chromium(V)]+ ions formed were filtered to
remove the excess of iodosobenzene and used for the kinetic study after dilution. The purity
of the CrIII(salen)]+PF6 and [oxo(salen)chromium(V)]+ ions were checked by comparing
-
their absorption spectra with the previous reports [63,64].
2.6. Kinetic measurements
The kinetic study of PMAA and substituted PMAAs with [oxo(salen)chromium(V)]+
ions in the presence of TPPO, PyO and PicNO was carried out in acetonitrile medium under
pseudo-first order conditions with excess of PMAA concentration over the
[oxo(salen)chromium(V)]+ ion. The reactions were started by quickly injecting
[oxo(salen)chromium(V)]+ ion in to a solution of PMAA and LO in varying concentrations in
such a way that in each run the total volume was maintained as 5 cc. The reaction was
followed by measuring the decrease in absorbance of oxo(salen)Cr(V)–LOs adduct at the
appropriate wavelength. To follow the progress of the reaction and to record the absorption
spectra of [oxo(salen)chromium(V)]+ ions and the adduct formed between
[oxo(salen)chromium(V)]+ ion and LO, double beam BL 222 Elico UV–vis bio
spectrophotometer was employed.
Fig. 1. The absorption spectral change for the reaction between PMAA and Ia in the presence
of TPPO. [PMAA] = 1 × 10-1 M; [Ia] = 5 × 10-4 M; [TPPO] = 5 × 10-3 M; temp. = 303 K;
solvent = 100% CH3CN.
5

The representative plot showing decrease in absorbance with time for the reaction
between PMAA and Ia in the presence of TPPO is shown in the Figure 1. The pseudo-first
order rate constants were calculated from the slope of linear plots of log OD vs. time and the
overall rate constants were obtained by using the expression kov = k1 / [PMAA]n, where n is
the order with respect to PMAA. The error in the rate constants are given in accordance to
95 % student’s t-test.
2.7. Binding constant
The binding constant values for the adduct formed between the complexes (Ia-Ic) and
LOs were determined by spectral titration method using UV-vis spectroscopy. Typically 5.0
× 10-4 M solution of the [oxo(salen)Cr(V)]+ ion in CH3CN was titrated against successive
addition of a standard solution of the LO without changing the total volume. Upon increase in
[LO], the absorption spectrum of the [oxo(salen)Cr(V)]+ ion showed increase in λmax value
along with the enhancement in the extinction coefficient. Similar results were obtained for all
LOs and complexes.
Fig. 2. Absorption spectral changes during the spectral titration between Ib and PyO.
The Figure 2 shows the absorption spectral changes observed during the spectral
titration of Ib with PyO. The existence of isobestic point in the spectral titration curves of
[oxo(salen)Cr(V)]+ ions with all LOs provides a direct proof for the binding of one equivalent
6

of LO with the [oxo(salen)Cr(V)]+ ion in a reversible process. From the absorbance at
different concentrations of LOs at the appropriate λmax, the binding constant values were
calculated using the modified Bensi-Hildebrand equation (1).
where ∆(OD) and ∆ε are the difference in the absorption intensity and molar extinction
coefficient of the complex in the presence and absence of LO. From the ratio of the slope to
y-intercept in the linear plot of [complex] [LO] / ∆(OD) vs. [complex] + [LO], the binding
constant, Kf can be calculated and the values are given in Table S1.
2.8. Product analysis
A mixture of PMAA (2 mM), [oxo(salen)chromium(V)]+ ion (2 mM) and PyO (0.2
mM) in 100% CH3CN was stirred well till the completion of the reaction. The solvent was
evaporated under reduced pressure. The residue was extracted with chloroform and dried
over anhydrous sodium sulfate. After the removal of chloroform under reduced pressure, the
residue was analyzed by FT-IR spectroscopy and LC-MS technique. The IR spectrum of
product was found to be similar to that of phenylsulfinylacetic acid (PSAA) and the observed
FT-IR (KBr, cm-1) frequencies are: 3421 νstr (OH), 2983 νstr (CH2), 1725 νstr (CO), 1525 νstr
(C=C), 1346 νben (CH2), 1025 νstr (SO) and 854 νben (Ar-H) (Fig. S1). The HPLC
chromatogram has only one major peak. The peak eluted at wavelength of 254 nm and at the
retention time of 5.562 min. has an overall area of 95.5 %. This clearly shows that the
product contains only one component. The peak eluted in LC-MS of the product at the
retention time of 5.604 min. ionizes in APCI (-) mode at m/z = 183.1 (Fig. S2) confirmed the
formation of PSAA as the only product. The UV-vis spectrum of the reaction mixture after
completion of the reaction was identical with that of [(salen)Cr(III)]+ ion. These observations
confirm that PMAA and [oxo(salen)chromium(V)]+ are converted into PSAA and
[(salen)Cr(III)]+ respectively and are the only products of the reaction.
3. Results
3.1. Spectral studies
The electronic spectrum of [oxo(salen)chromium(V)]+ ion (Ia-Ic) in acetonitrile
medium has absorption maximum at 560, 557 and 584 nm respectively corresponding to d-π*
7

charge-transfer transition [65]. Addition of ligand oxides such as TPPO, PicNO and PyO to
the solution of [oxo(salen)chromium(V)]+ causes an immediate colour change from dark
green-black to emerald green, a significant red shift in the λmax and increase in absorbance in
the absorption maxima. Moreover, upon successive addition of incremental amounts of donor
ligands to the complexes, an increase in OD is observed. This shift in λmax value depends
upon the nature of LO and nature of substituent in the complex and is found to be in the range
of 46 - 85 nm. The observed λmax value of complexes (Ia-Ic) in the absence and presence of
LOs are listed in the Table 1. Maximum red shift is observed for the complex Ib with PicNO
and a minimum red shift is noticed in Ic with PicNO and PyO. Similar shift of λmax value for
the complexes with LO has been observed by Samsel et al. [66], Venkataramanan et al. [67]
and Srinivasan and Kochi [68]. They explained the spectral changes on the basis of the
formation of adduct between [oxo(salen)Cr(V)]+ and LO. Contradictorily, addition of donor
ligand during the oxidation of organic sulfides by sodium hypochlorite catalyzed by
(salen)Mn(III) [69] and the oxidation of PMAAs by oxo(salen)Mn(V) complexes [16] no
change in the absorption spectrum of the complex was noted.
Table 1. Absorption maxima of complexes in the presence and absence of ligand oxides
λmax (nm)
Complex
Without additive
TPPO PicNO PyO
560
557
584
627
640
645
608
642
630
612
620
630
Ia
Ib
Ic
3.2. Kinetic results
The sulfoxidation reaction is first-order with respect to [oxo(salen)chromium(V)]+ ion
as evidenced from the linearity of log OD vs. time plots (Fig. S3) and the constant value of
pseudo-first order rate constants calculated at various concentrations of the complex (Table
S2). Though the pseudo-first order rate constants (k1) increase with increase of PMAA
concentration (Table S2 and Fig. S4), the second order rate constants evaluated using the
expression k2 = k1/[PMAA] are not constant. This along with fractional slope values (Table
S3) obtained from the plots of log k1 vs. log [PMAA] (Fig. S5) indicate a fractional order
8

dependence on PMAA. The operation of Michaelis-Menten kinetics [70] is confirmed from
the excellent linear plots of 1/k1 vs. 1/[PMAA] having positive slopes without passing
through the origin (Fig. S6). The Michaelis-Menten constants (KM) calculated from the slope
and intercept values of the Michaelis-Menten plots are listed in Table S4. The observed
constant Michaelis-Menten value for each LO with different complexes shows that the
binding of PMAA with [oxo(salen)chromium(V)]+-LO adduct is independent of the nature of
the substituents in the complex. The non-saturation obtained at high concentrations of PMAA
and the observed high KM values indicate that the binding of PMAA is weak in nature. The
weak binding of PMAA with complex is further evidenced from the non occurrence of any
spectral change upon the addition of PMAA either to the complex alone or along with LOs.
Similar kinetic results have been noticed for all PMAAs with all complexes.
3.3. Effect of donor ligand oxides
In order to find the effect of donor ligands on sulfoxidation, the reaction was carried
out in the presence of three donor ligand oxides viz. TPPO, PicNO and PyO. Addition of
these donor ligands show that the rate constant increases with increase of LO concentration,
reaches maximum and then attains saturation at higher concentration of LO. The rate
constants at different concentrations of LOs are given in Table 2. The kinetic data show that
the rate of sulfoxidation in the presence of LO is about 50-150 times higher than its absence.
The order of reactivity among the LOs is PyO > PicNO > TPPO and more specifically the
rate is about one and half times higher in the presence PicNO and about two and half times
higher in the presence of PyO than that in the presence of TPPO. The highest reactivity in the
presence of PyO is due to the easy formation of the oxidizing species, O=Cr(salen)-PyO (C1)
as evidenced from its higher binding constant value. The strong binding of PyO weakens the
Cr-O oxo bond assisting the rate enhancement enormously. The observed least reactivity in
the presence of TPPO is due to the least binding of TPPO with the complex.
Gilheany and co-workers [71] have shown that the rate of ligation of bulky ligands
with [oxo(salen)chromium(V)]+ is lower than expected which is substantiated by lower
reduction potential of O=Cr(V)-TPPO adduct than O=Cr(V)-PyO [44]. Similar acceleration
of rate was observed in the oxidation of olefins [66] and organic sulfides [67,72] by
oxo(salen)chromium(V) complexes and in (salen)Mn(III) catalysed epoxidation of olefins
[73].
9

Table 2. Effect of ligand oxide concentrations on the reaction rate
TPPO
103 kov
((M-1)n s-1) (s-1)
PicNO
103 kov
((M-1)n s-1) (s-1)
PyO
104[LO]
M
103 k1
(s-1)
103 k1
103 k1
103 kov
((M-1)n s-1)
aIa
0
0.13 0.01 0.13 0.01
0.13 0.01 0.13 0.01
1.27 0.01 3.26 0.13
1.97 0.02 5.64 0.11
2.42 0.20 6.94 0.57
2.48 0.14 7.11 0.40
0.13 0.01
0.13 0.01
1.0
3.0
5.0
10
30
50
100
-
-
-
-
-
-
1.35 0.11
4.27 0.34
2.16 0.14
6.83 0.44
2.88 0.21
9.12 0.67
1.45 0.05 5.38 0.18
2.15 0.11 7.98 0.41
2.42 0.06 8.98 0.22
2.50 0.02 9.28 0.07
3.23 0.18
10.2 0.57
-
-
-
-
-
-
-
-
-
-
-
-
Ib
0
0.21 0.01 0.41 0.02
0.21 0.01 0.41 0.02
3.26 0.07 11.8 0.25
4.04 0.02 14.6 0.07
5.00 0.04 18.1 0.14
6.12 0.19 22.1 0.67
6.18 0.08 24.5 0.29
0.21 0.01
0.41 0.02
1.0
3.0
5.0
10
30
50
100
-
-
-
-
-
-
2.86 0.05
16.1 0.28
3.79 0.07
26.9 0.50
4.68 0.11
26.3 0.61
2.22 0.02 11.8 0.11
2.51 0.12 13.4 0.64
3.64 0.11 19.4 0.58
3.91 0.14 20.8 0.74
5.10 0.15
28.6 0.84
-
-
-
-
-
-
-
-
-
-
Ic
0
0.28 0.01 0.57 0.02
0.28 0.01 0.57 0.02
5.10 0.09 25.4 0.45
8.43 0.07 42.0 0.35
11.0 0.03 54.6 0.15
11.2 0.14 55.8 0.70
0.28 0.01
0.57 0.02
1.0
3.0
5.0
10
30
50
100
-
-
-
-
-
-
6.32 0.12
38.4 0.72
9.84 0.21
57.1 1.22
13.7 0.18
82.7 1.09
4.87 0.09 28.5 0.52
5.52 0.06 35.0 0.38
6.26 0.05 36.9 0.29
6.39 0.07 38.4 0.42
14.3 0.12
86.0 0.72
-
-
-
-
-
-
-
-
-
-
-
-
[Ia] = [Ib] = [Ic] = 5.0 × 10-4 M; Temp. = 303 K; [PMAA] = 5.0 × 10-2 M; a[PMAA] = 0.1 M;
solvent = 100 % CH3CN; n = order w.r.t. PMAA.
10

The binding of oxygen of LO to the metal atom followed by weakening of M=O bond
in the complex which facilitate the oxygen atom transfer to reactive center of the substrate
was given as the reason for the enhancement of the reaction rate as well as product yield in
the presence of donor ligand. In contrast, an opposite effect is noticed in the oxidation of
aromatic amines [64] and phenols [44] with the oxo(salen)chromium(V) complexes owing to
the decrease in the reduction potential of [oxo(salen)chromium(V)]+ ion in the presence of
LO. However, in the (salen)Mn(III) catalysed oxidation of organic sulfides by sodium
hypochlorite [69], sodium metaperiodate [74] and sodium hypochlorite [75], in the oxidation
of organic sulfoxides catalyzed by sodium hypochlorite [76] and in the oxygen atom transfer
reaction of oxo(salen)manganese(V) complexes with PMAA [16] addition of ligand oxides
has no appreciable effects on the rate of sulfoxidation reactions.
As the reaction was too fast to measure in certain cases and the decomposition of the
complexes were found to be appreciable at higher concentration of LOs as shown by decrease
in absorption with time, reactions were conducted in the moderate range of concentration
mentioned in the Table 2. The kinetic evidence for the formation of O=Cr(V)−LO adduct is
shown from the observed Michaelis-Menten kinetics with LOs (Fig. S6). The reversible
formation of 1:1 adduct with strong binding nature is evidenced from the saturation kinetics
observed in the plot of k1 vs. [LO] (Fig. 3) and the low Michaelis-Menten constant (KM)
values (Table S4).
Fig. 3. Plot of k1 vs. [PicNO] for the oxidation of PMAA by Ia-Ic at 303 K.
11

3.4. Effect of substituents
In order to probe the electronic sensitivity between the substituent and the reaction
centre, three [oxo(salen)chromium(V)]+ complexes and eleven PMAAs having different
electron releasing and electron withdrawing substituents in the para positions were studied
and the relevant kinetic data are listed in the Table 3. Interestingly both electron-releasing
and electron-withdrawing substituents in the complex and PMAA enhance the rate of
sulfoxidation appreciably. Insight in to the nature of the transition state was sought from the
structure-reactivity correlation of Hammett [77−79]. In the Hammett correlation two
intersecting linear lines with opposite slopes, a positive slope for electron withdrawing
substituents and a negative slope for electron donating substituents are obtained. The
unsubstituted PMAA has a minimum reactivity in the plot. Similar non-linear Hammett
correlations have been reported in the oxidative decarboxylation of phenylsulphinylacetic
acids by [oxo(salen)chromium(V)]+ complexes in the presence of nitrogen bases [56] and
iron(III)polypyridyl complexes [55], oxidation of substituted trans-cinnamic acids by
pyridinium chlorochromate [80], chloramine-T [81] and acid bromate [82], oxygen-atom
transfer reaction of axially ligated Mn(V)-oxo complexes [83] and stereoselective catalytic
sulfoxidations mediated by titanium and zirconium trialkylanolamine complexes [84].
A high negative ρ value (-3.33 to -5.44) for electron donating substituents and
comparatively low positive ρ value (2.07 to 2.55) for electron withdrawing substituents have
been noticed (Table 3 and Fig. 4). The high magnitude of ρ value shows a significant charge
separation in the transition state and a charge transfer between the reaction centre and
substituents. Similar high ρ- values have been reported for the oxidation of PMAAs by N-
3+
bromophthalimide (-3.22) [20], chlorosaccharin (-3.12) [21], Cr(phen)3 (-4.28) [17] and
chloramine T (-2.46) [85]. On the other hand, comparatively low ρ- values are obtained for
the oxidation of PMAAs by oxo(salen)manganese(V) complexes (-1.12) [16], chromium(VI)
catalysed by EDTA (-1.50) [13], peroxomonophosphoric acid (-0.996) [30] and potassium
peroxodiphosphate (-0.45) [32]. Though the magnitude of ρ- depends on the nature of LO and
complex, the magnitude of ρ+ is independent of LO as well as complex (Table 3).
12

Table 3. Overall rate constants for the oxidation of X-PMAA by Ia–Ic in presence of ligand oxides
S.No X
102 kov ((M-1)ns-1)
Ia
Ib
Ic
TPPO
PicNO
PyO
TPPO
PicNO
PyO
TPPO
PicNO
PyO
1
OMe 25.02 1.08 30.2 0.28 31.4 0.75 11.0 0.07 22.2 0.75 44.8 0.32 61.6 2.1
55.4 0.45 184 1.6
52.6 0.39 171 1.9
2
OEt 17.02 1.0
t-Bu 10.1 0.34
26.8 0.31 27.8 0.58 9.32 0.65 20.4 0.13 41.2 0.56 49.0 1.3
3
17.2 0.12 19.8 0.23 7.27 0.25 10.7 0.11 23.0 0.21 20.9 0.66 42.1 0.17 44.6 0.32
14.8 0.09 16.6 0.31 5.80 0.21 9.14 0.06 18.6 0.14 16.9 0.39 28.3 0.12 40.6 0.23
8.84 0.06 9.36 0.12 5.58 0.09 7.40 0.05 16.8 0.15 15.4 0.43 21.0 0.13 39.8 0.12
8.2 0.04 10.5 0.16 5.21 0.22 7.96 0.04 18.1 0.12 14.1 0.51 24.3 0.17 37.6 0.22
1.56 0.03 2.62 0.02 1.34 0.08 1.82 0.03 2.76 0.07 3.73 0.15 5.44 0.22 8.22 0.13
1.84 0.03 2.88 0.06 1.68 0.05 2.80 0.02 3.92 0.08 4.42 0.11 8.29 0.06 11.6 0.04
4
Me
Et
iPr
H
7.13 0.61
4.47 0.11
4.81 0.23
0.83 0.04
0.96 0.03
2.36 0.07
2.96 0.03
5
6
7
8
F
9
Cl
Br
5.98 0.06 7.16 0.13 5.95 0.15 7.6 0.05
5.82 0.07 9.68 0.08 6.28 0.12 7.9 0.04
7.7 0.06 11.3 0.12 18.7 0.09 20.2 0.07
11.7 0.09 11.8 0.18 19.5 0.12 21.2 0.13
10
11
NO2 39.1 0.87
98.4 0.96 202 1.65 60.8 6.8 128.4 1.9 256 2.5
254 9.6
268.5 2.8 561 4.5
ρ+
r
ρ-
2.18 0.08 2.33 0.09 2.48 0.08 2.11 0.18 2.33 0.08 2.55 0.17 2.39 0.05 2.07 0.02 2.37 0.15
0.998
0.998
-
0.998
-
0.988
-
0.997
-
0.995
-
0.999
-
0.999
-
0.994
-
-5.44 0.19
5.19 0.27 4.11 0.18 3.33 0.23 4.06 0.13 4.48 0.28 4.30 0.26 3.54 0.22 5.01 0.56
0.993 0.996 0.988 0.998 0.990 0.991 0.990 0.970
r
0.997
[Ia] = [Ib] = [Ic] = 5.0 × 10-4 M; [TPPO] = 5.0 × 10-3 M; [PicNO] = [PyO] = 5.0 × 10-4 M; n = order w.r.t. PMAA; solvent = 100 % CH3CN.
13

Fig. 4. Hammett plots for the sulfoxidation of X-PMAA by Ia and Ic in the presence of LO.
3.5. Effect of temperature
The oxidation of PMAA by complexes Ia-Ic was carried out at three different
temperatures in the presence of LOs. The thermodynamic parameters, enthalpy of activation
(∆‡H) and entropy of activation (∆‡S) were calculated from the Eyring’s plot of log (kov/T)
against 1/T (Fig. S7) and are listed in the Table S5. The enthalpy of activation (∆‡H) ranges
from 33.9 0.24 to 57.2 0.46 kJ mol-1 and the entropy of activation (∆‡S) ranges from -
111.5 3.5 to -172.8 8.5 JK-1 mol-1. The negative ∆‡S values indicate that the transition state
is highly ordered compared to that in the ground state. In each complex PyO has the
minimum enthalpy of activation and maximum entropy of activation and TPPO has the
maximum enthalpy of activation and minimum entropy of activation (Table S5). As the
binding of PyO is maximum, in the transition state the number of orderly O=Cr(V)−PyO
adduct species is more than the other two LO-adduct species. Consequently, a higher
negative value of entropy of activation with PyO is observed.
4. Discussion
The observed color change, substantial red shift in the λmax and increase in OD of the
complexes upon addition of LOs can be taken as evidence for the formation of 1:1 adduct
(C1) between the [(salen)CrV=O]+ ion and LO prior to the rate determining step. The
evaluated binding constant values from spectral titration experiments (Table S1) also confirm
15

the adduct formation in the present study. Samsel et al. [66] have isolated the adduct formed
between [(salen)CrV=O]+ ion and PyO and showed from the ORTEP diagram that the LO
occupies the vacant apical position in [(salen)CrV=O]+, completing the octahedral
coordination of the chromium centre. During the adduct formation it was found that the Cr
atom is pulled back from the plane of ligand oxygen and nitrogen atoms by 0.27 Å and
lengthened of the Cr−O bond by 0.03 – 0.04 Å.
The observed order of binding constant values among LOs is PyO > PicNO > TPPO
while among the complexes it is Ic > Ia > Ib. From the magnitude of the binding constants it
is inferred that the binding constant values are affected not only by the nature of substituents
at 5 and 5′ positions of the salen ligands but also by the nature of the donor ligand oxides.
The strong binding of (5,5′-dichlorosalen)chromium(V) ion (Ic) with LOs is intercepted by
the decrease in electron density on chromium atom due to the presence of two electron
attracting chlorine atoms. The low binding of Ib is due to the presence of electron donating
methyl group at 5,5′-positions which prevents the attack of LOs. Among the LOs, PyO has
maximum binding constant values as the O=Cr(V)(salen)-PyO adduct is comparatively more
stable [68]. The low binding constant value for TPPO with oxo(salen)chromium(V)
complexes is not only due to the steric effect of three bulky phenyl groups but also due to the
decrease in electron density on the oxygen atom of TPPO by phenyl groups. The weak
binding of TPPO and low stability of its adduct with the oxo(salen)chromium(V) complex
was already shown by Kochi et al. [68] in the sulfoxidation by means of the spectral changes
and Dalton et al. [86] in the asymmetric alkene epoxidation based on the steric hindrance of
bulky TPPO.
It is pertinent to note that in the oxidation of olefins [69], organic sulfides [67] and
sulfoxides [72] by Cr(V), the adduct formed between [(salen)Cr(V)=O]+ ion and LO has been
proposed as the active oxidizing species. The involvement of adduct (C1) as the active
oxidizing species in the present case is unambiguously proved from the above discussions
along with the rate acceleration observed with LOs. The absence of any spectral change
during the addition of PMAAs to the reaction mixture eliminates its adduct formation with
other ingredients. The absence of its binding is also confirmed from the observed high
Michaelis-Menten constant values (Table S4).
16

Fig. 5. Plot of log kov vs Eox of PMAAs.
The observed substituent effect paved way to postulate a mechanism shown in
Scheme 1. The non-linear Hammett behaviour in the current study can be visualized by two
concurrent pathways having complementary electron demand or by two distinct rate
controlling steps [87]. The observed linearity in the plot of log kov against Eox potential of
PMAAs (Fig. 5) with excellent correlation (r = 0.989-0.996) for electron-donating
substituents and non-linearity for electron withdrawing substituents in PMAA not only rule
out the operation of single electron transfer (SET) mechanism for the electron withdrawing
substituents but also establish different mechanisms for electron donating and withdrawing
substituents. It is worthwhile to mention here that based on the linear correlation observed
between log k2 and Eox/red, a SET mechanism has been proposed for the oxidation of organic
sulfides by oxo(salen)chromium(V) complexes [88], anilines by oxo(salen)chromium(V) [64]
and H2O2 catalyzed by iron(III)-salen complexes [89] and phenols with oxovanadium(IV)-
salen catalyzed H2O2 [90].
Further, operation of same mechanism involving two different rate determining steps
(RDS) for electron donating and withdrawing substituents is ignored in the current study, as
the Hammett plots do not show downward curvature [87]. For electron donating substituents,
a more reasonable mechanism involves a single electron transfer from the PMAA to the
reactive oxidizing species (C1) to form sulfide cation radical (C2) in a slow reversible rate
determining step. The formation of sulfide cation radical has been well established in the
17

oxidation of organic sulfides by iron(III)-salen chloride [53], cobalt(III)-salen ion [91], Cr(V)
[72,92] and oxo(salen)manganese(V) complexes [16]. The negative ρ value obtained for
electron donating substituents in the Hammett plots also confirms the development of
positive charge on the sulfur centre of PMAA at the transition state. The easy formation of
intermediate (C2) is proved by enhancement of rate by means of increasing electron density
around the sulfur centre and thereby increasing the nucleophilicity of PMAA. The electron
releasing substituents not only increase the nucleophilicity of PMAA but also stabilize the
intermediate C2 through resonance. The SET mechanism is further ascertained by
enhancement of rate in the presence of electron withdrawing substituent at 5,5′-positions of
the [(salen)CrV=O]+ there by making it more electrophilic.
Scheme 1. Catalytic cycle for the oxo(salan)chromium(V)+ ion
Similar rate enhancement has been noticed for the oxidative decarboxylation of PSAA
by [oxo(salen)chromium(V)]+ ion in the presence of nitrogen bases [56] and ligand oxides
18

[57] and for the oxidation of thioethers by manganese(V) complex, MnV(O)(TBP8C2) [83]
where SET mechanism has been proposed. The sulfide cation radical (C2) then undergoes
rapid oxygen atom transfer from the oxo(salen)chromium(IV) species to PMAA through the
intermediate (C3). The products formed are phenylsulfinylacetic acid and the [(salen)Cr(III)]+
complex. The formation of [(salen)Cr(III)]+ ion as one of the products is evidenced from the
change of colour from emerald-green to orange and the formation of new peaks at the λmax
values of 228, 285, 305, 415 nm at the end of the reaction.
If similar SET mechanism is operating in PMAAs having electron-withdrawing
substituents, the rate should be decelerated as the electron withdrawing substituents diminish
the electron density around sulfur centre of the PMAA. But, the observed rate enhancement,
positive Hammett ρ value and non linearity in the plot of log kov vs. Eox for electron-
withdrawing substituents clearly demonstrate that these PMAAs do not follow SET path way
and the S centre of the PMAA behaves as an electron deficient centre. So, it is conveniently
assumed that these substrates follow an electrophilic path in which
[oxo(salen)chromium(V)]+ ion acts as a nucleophile. Consequently, the rate determination
step for this path is proposed as the formation of ternary complex (C4) by nucleophilic attack
of the active species (C1) on PMAA in a reversible manner. The electrophilicity of these
PMAAs is due to the presence of phenyl ring with electron withdrawing substituents on one
side of the sulfur atom and electron withdrawing carboxyl group on the other side. Electron-
withdrawing substituents disseminate the electron density on the sulfur centre and also
stabilize C4 by resonance interaction. The formation of ternary complex (C4) in the rate-
limiting step is further evidenced from enhancement of rate by electron donating substituents
at 5,5′-positions of the [(salen)CrV=O]+ ion. In the ternary complex (C4) direct oxygen atom
transfer takes place from oxo(salen)chromium(V) complex to PMAA with the formation of
products in a fast step. Such type of mechanism involving nucleophilic attack of the oxidant
in the rate determining step during sulfoxidation reaction has been proposed by Curci et al.
[93] and Sawaki et al. [94]. Venkataramanan et al. [72] have shown through density
functional theory (DFT) that during the formation of ternary complex between donor ligand
oxide (DMSO), [oxo(salen)chromium(V)] complex and PhSMe (MPS), the substrate
approaches [oxo(salen)chromium(V)] complex from the direction of oxygen atom in a
perpendicular direction of the salen plane.
19

It is pertinent to recall that in most of the sulfide and sulfoxide reactions with
oxo(salen)chromium(V) [67,95,96] and oxo(salen)manganese(V) [16,69,76] complexes,
SN2 type electrophilic attack of salen complex on substrate was proposed on the basis of the
observed negative ρ reaction constant. Similar electrophilic attack of salen complex on
alkenes was also proposed during epoxidation reactions based on the observed negative ρ
values [66]. Again in these reactions an excellent linear correlation is observed with Hammett
σ constants for both electron releasing and electron withdrawing substituents. The observed
anomalous behaviour of ‘V’ shaped Hammett correlation and nucleophilic attack of
[oxo(salen)chromium(V)]+ during sulfoxidation of PMAA in the present study compared to
sulfides and sulfoxides may be due to the existence of through-space interaction between the
carboxyl group and sulfur atom in PMAA. Similar through-space interaction has been
advocated earlier in PMAA to explain greater dependence of pKa values on the nature of
substituents [97] and the low metal-ligand covalency in complexes with lanthanides [18].
5. Conclusions
The oxygenation of a series of p-substituted phenylmercaptoacetic acids by
oxo(salen)chromium(V) complexes in the presence of different ligand oxide such as TPPO,
PicNO and PyO has been studied in acetonitrile medium. Spectral and kinetic data reveal that
the reactive species in this oxidation is the adduct formed between [(salen)Cr(V)=O]+ ion and
LO. Addition of ligand oxide to the solution of the complex causes a significant red shift and
increase in absorption in the absorption maxima. The observed catalytic activity among the
various LOs used in the study is PyO > PicNO > TPPO. The weakening of Cr=O bond as a
result of binding of LO with metal centre of [(salen)Cr(V)=O]+ is given as explanation for
enhancement of rate. The novelty of the present work between [oxo(salen)chromium(V)]+
and PMAAs is that the reaction is facilitated not only by electron donors but also by electron
acceptors in the para- position of the PMAA as well as 5,5′-positions of salen moiety of the
oxidant. A non-linear concave Hammett plots with high reactivity constant values have been
observed. Two different mechanistic pathways, a SET mechanism for electron donating
substituents and direct oxygen transfer mechanism for electron withdrawing substituents have
been proposed for the reaction. Phenylsulfinylacetic acid is the only product formed in the
reaction.
20

Acknowledgement
PS thanks the UGC, New Delhi for sanctioning major research project (F.No.39-
817/2010(SR). SA thanks the UGC, New Delhi (SERO, F.No.TNMS005) and Manonmaniam
Sundaranar University, Tirunelveli to avail the benefits of Faculty Development Programme
(FDP). The authors thank the Management, Aditanar College of Arts and Science,
Tiruchendur for providing research facilities. They also thank Prof. S. Rajagopal, School of
Chemistry, Madurai Kamaraj University, Madurai for helpful discussions and continuous
encouragement.
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Graphical Abstract
The presence of triphenylphosphine oxide, pyridine N-oxide and picoline N-oxide in the
oxidation of phenylmercaptoacetic acid by [oxo(salen)chromium(V)]+ causes rate
acceleration. Substituent effects and non-linear Hammett correlation paved way to propose a
single electron transfer mechanism for electron donors and direct oxygen atom transfer
mechanism for electron acceptors.
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