Electrophilic and nucleophilic pathways in ligand oxide mediated reactions of phenylsulfinylacetic acids with oxo(salen)chromium(V) complexes

Article abstract of DOI:10.1016/j.poly.2016.05.012

The mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAA) by oxo(salen)Cr(V)⁺ ion in the presence of ligand oxides has been studied spectrophotometrically in acetonitrile medium. Addition of ligand oxides (LO) causes a red shift in the λ_max values of oxo(salen) complexes and an increase in absorbance with the concentration of LO along with a clear isobestic point. The reaction shows first-order dependence on oxo(salen)-chromium(V)⁺ ion and fractional-order dependence on PSAA and ligand oxide. Michaelis-Menten kinetics without kinetic saturation was observed for the reaction. The order of reactivity among the ligand oxides is picoline N-oxide > pyridine N-oxide > triphenylphosphine oxide. The low catalytic activity of TPPO was rationalized. Both electron-withdrawing and electron-donating substituents in the phenyl ring of PSAA facilitate the reaction rate. The Hammett plots are non-linear upward type with negative ρ value for electron-donating substituents, (ρ^- = -0.740 to -4.10) and positive ρ value for electron-withdrawing substituents (ρ⁺ = +0.057 to +0.886). Non-linear Hammett plot is explained by two possible mechanistic scenarios, electrophilic and nucleophilic attack of oxo(salen)chromium(V)⁺-LO adduct on PSAA as the substituent in PSAA is changed from electron-donating to electron-withdrawing. The linearity in the log k vs. E_ox plot confirms single-electron transfer (SET) mechanism for PSAAs with electron-donating substituents.

Full text of DOI:10.1016/j.poly.2016.05.012

Accepted Manuscript
Electrophilic and nucleophilic pathways in ligand oxide mediated reactions of
phenylsulfinylacetic acids with oxo(salen)chromium(V) complexes
P. Subramaniam, S. Sugirtha Devi, S. Anbarasan
PII:
S0277-5387(16)30154-1
DOI:
http://dx.doi.org/10.1016/j.poly.2016.05.012
POLY 11985
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
31 January 2016
5 May 2016
Please cite this article as: P. Subramaniam, S. Sugirtha Devi, S. Anbarasan, Electrophilic and nucleophilic pathways
in ligand oxide mediated reactions of phenylsulfinylacetic acids with oxo(salen)chromium(V) complexes,
Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.05.012
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Electrophilic and nucleophilic pathways in ligand oxide mediated reactions
of phenylsulfinylacetic acids with oxo(salen)chromium(V) complexes
P. Subramaniama*, S. Sugirtha Devib, S. Anbarasana
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
The mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAA) by
oxo(salen)Cr(V)+ ion the presence of ligand oxides has been studied spectrophotometrically
in acetonitrile medium. Addition of ligand oxides (LO) causes a red shift in the λmax values of
oxo(salen) complexes and an increase in absorbance with the concentration of LO along with
a clear isobestic point. The reaction shows first-order dependence on oxo(salen)-
chromium(V)+ ion and fractional-order dependence on PSAA and ligand oxide. Michaelis-
Menten kinetics without kinetic saturation was observed for the reaction. The order of
reactivity among the ligand oxides is picoline N-oxide > pyridine N-oxide >
triphenylphosphine oxide. The low catalytic activity of TPPO was rationalized. Both
electron-withdrawing and electron-donating substituents in the phenyl ring of PSAA facilitate
the reaction rate. The Hammett plots are non-linear upward type with negative ρ value for
electron-donating substituents, (ρ- = -0.740 to -4.10) and positive ρ value for electron-
withdrawing substituents (ρ+ = +0.057 to +0.886). Non-linear Hammett plot is explained by
two possible mechanistic scenarios, electrophilic and nucleophilic attack of
oxo(salen)chromium(V)+-LO adduct on PSAA as the substituent in PSAA is changed from
electron-donating to electron-withdrawing. The linearity in the log k vs. Eox plot confirms
single-electron transfer (SET) mechanism for PSAAs with electron-donating substituents.
________
Abbreviations: PSAA, phenylsulfinylacetic acid; salen, N,N’-bis(salicylidene)-
ethylenediaminato; LO, ligand oxide; TPPO, triphenylphosphine oxide; PyO, pyridine N-
oxide; PicNO, picoline N-oxide; OD, optical density; SET, single electron-transfer.
*Corresponding author. Tel.: +91 9443500381
E-mail addresses: subramaniam.perumal@gmail.com (P. Subramaniam),
sugirthadevi.s@gmail.com (S. Sugirtha Devi), sanbarasan62@gmail.com (S. Anbarasan).
Keywords: Oxo(salen)chromium(V); Phenylsulfinylacetic acid; Single electron-transfer; Non-
linear Hammett plot; Michaelis-Menten kinetics.
1

1. Introduction
Oxygen transfer is so common and important in biological systems as well as in
chemical synthesis. Because of the easy preparation, reasonable stabilities, biological
activities etc., salen ligand [salen = 1,6-bis(2-hydroxyphenyl)-2,5-diazahexa-1,5-diene], its
derivatives and their associated metal complexes are widely used as potential catalysts in oxo
transfer reactions [1-5] and many other processes [6-11]. They also mimic enzymatic
reactions [12,13]. Further, the applications of N-oxide derivatives have received considerable
attention due to their usefulness as synthetic intermediates and biological importance.
Pyridine N-oxide derivatives represent a new class of anti-HIV compounds and oxide part of
the pyridine moiety proved to be indispensable for anti-coronavirus activity. The potential
and virus specificity of the pyridine N-oxides were shown to vary depending on the nature
and location of the substituents in the molecule [14]. Heterocyclic N-oxides are used as
protecting groups, auxiliary agents, oxidants, ligands in metal complexes and catalysts [15]
besides bio-reductive drugs [16]. Hence the study of the role and action of N-oxides on the
activity of metallo-enzymes are of biological importance.
Over a past few decades, a variety of effects have been reported with different
additives for various reactions [17-22] and different explanations have been given for these
effects. Although, a large number of reports on the reactivity of oxo metal complexes with
nitrogen bases as additives [23-27] have appeared in the literature only little interest has been
paid by using ligand oxides as additives in these reactions. Even in majority of cases where
ligand oxides are used as additive, no effect is observed in epoxidation [28] and sulfoxidation
reactions [29-33]. Further, there was no detailed mechanistic study on the oxidation of
phenylsulfinylacetic acid (PSAA) except our recent publications on the proximal effects of
nitrogen bases with oxo(salen)Cr(V)+ ion [34] and Cr(VI) oxidation of PSAA [35] mediated
by cetyltrimethylammonium bromide [36], oxalic acid [37] and picolinic acid [38]], though
PSAA is a versatile synthetic agent used for the synthesis of antibiotics [39] and various
organic compounds [40-44]. Out of our much interest on the oxidation studies of sulfur
compounds especially with PSAA in the presence of additives and a significant effect
observed with ligand oxides, detailed kinetic investigation on the reactions between PSAAs
and oxo(salen)chromium(V) complexes in the presence of triphenylphosphine oxide (TPPO),
pyridine N-oxide (PyO) and picoline N-oxide (PicNO) has been carried out. The following
scheme (Scheme 1) summarises the work described in this paper.
2

Ia : X = H; Ib : X = CH3; Ic : X = Cl
PSAA : Y = p-OMe, p-OEt, p-Me, m-Me, H, p-F, p-Cl, m-F, m-Cl, p-NO2
Scheme 1. Oxidative decarboxylation of PSAA by oxo(salen)Cr(V)+ ion.
2. Experimental
2.1. Preparation of (salen)chromium(III) chloride
(Salen)chromium(III) chloride was synthesized by the established procedure [45].
Chromium(III) chloride hexahydrate (0.01 M, 2.5 g) dissolved in minimum amount of
distilled water was reduced to chromium(II) chloride by Zn/Hg under nitrogen atmosphere.
The blue colour solution of chromium(II) chloride formed was added in drops to a suspension
of salen ligand (0.008 M, 2.15 g) in 30 cc of acetone under nitrogen atmosphere. After
exposing the resulting dark brown solution in air, it was heated to reflux for 1 hour. The wet
residue obtained after the removal of solvent was stirred with water for 2 hours. The
(salen)chromium(III) chloride formed was filtered, washed with dichloromethane and
saturated NH4Cl followed by aqueous sodium chloride. By adopting a similar procedure
(5,5ʹ-dichlorosalen)Cr(III) chloride and (5,5ʹ-dimethylsalen)Cr(III) chloride were prepared.
The (salen)chromium(III) chloride complexes were recrystallized from methanol-water
ʹ
mixture. The salen, 5,5 -dichloro and 5,5 -dimethyl substituted salen ligands were prepared
ʹ
by refluxing the corresponding salicylaldehyde and ethylenediamine in ethanol.
2.2. Preparation of (salen)chromium(III) hexafluorophosphate
In order to get better solubility of Cr(III) complexes, (salen)chromium(III) chloride
was converted into (salen)chromium(III) hexafluorophosphate by crystallizing from an
equimolar mixture of (salen)chromium(III) chloride in methanol-water and ammonium
hexafluorophosphate in minimum amount of water [46]. The dried samples were stored in the
o
-
refrigerator at 0 C. As the concentrations of oxo(salen)chromium(V)+ PF6 complexes were
3

-
determined on the basis of concentration of (salen)chromium(III)+ PF6 , their purity were
confirmed by elemental micro analyses and characterized by UV-vis. spectroscopy
(Supporting information Fig. S1). The same λmax values were reported for these complexes by
Premsingh et al. [47].
-
(Salen)Cr(III)+ PF6 , Anal. Calc. for C16 H14 O2 N2 Cr PF6: C, 41.48; H, 3.05; N, 6.05.
Found: C, 41.35; H, 3.12; N, 5.95 %. UV-vis (λmax): 228, 285, 317, 360, 415 nm.
-
5,5′-(Me2salen)Cr(III)+ PF6 , Anal. Cal. for C18 H18 O2 N2 Cr PF6: C, 44.01; H, 3.69; N, 5.70.
Found: C, 44.21; H, 3.75; N, 5.60 %. UV-vis (λmax): 227, 286, 328, 429 nm.
-
5,5′-(Cl2salen)Cr(III)+ PF6 , Anal. Cal. for C16 H12 O2 N2 Cl2 Cr PF6: C, 36.11; H, 2.27; N,
5.27. Found: C, 36.25; H, 2.21; N, 5.23 %. UV-vis (λmax): 228, 288, 325, 427 nm.
-
2.3. Synthesis of oxo(salen)chromium(V)+ PF6 complexes
A slight excess of iodosobenzene (0.0051 M, 0.06 g) was added to
(salen)chromium(III) hexafluorophosphate (0.005 M, 0.116 g) dissolved in 50 ml of
acetonitrile. The solution was stirred for 30 minutes and then filtered to remove the unreacted
iodosobenzene. The colour of the solution turned from red orange to dark green brown. The
dark green solution of oxo(salen)chromium(V)+ ion was used for the kinetic study after
-
subsequent dilution. Following the above procedure, other substituted oxo(salen)Cr(V)+ PF6
complexes (Ib & Ic) were prepared and characterised by their UV-vis absorption spectra
which are identical with the previous report [47].
2.4. Preparation of phenylsulfinylacetic acids
Phenylmercaptoacetic acids and its substituted acids were prepared using established
procedure [48,49] by refluxing the corresponding thiophenols with chloroacetic acid in
alkaline medium for 5-6 hrs. Phenylmercaptoacetic acids were converted into corresponding
phenylsulfinylacetic acids by controlled oxidation with equimolar hydrogen peroxide [50]
and were recrystallised with suitable solvent mixture. The purity of PSAAs was checked by
elemental micro analyses, melting point and LC-MS analysis. The LC-MS spectrograms are
recorded in APCI (-) mode and are shown in the Supporting information Fig. S2. The melting
points determined for different PSAAs are found to agree with those reported in the literature
[48,50].
o
PSAAa: Mp.: 115-117 C. Mass Spec.: Calc. 183 amu. Found: 183.1 amu. Anal. Calc.: C,
52.17; H, 4.35; Found: C, 52.08; H, 4.30%.
4

o
p-OMe PSAAa: Mp: 104 C. Mass Spec.: Calc.: 214 amu. Found: 213.1 amu. Anal. Calc.: C,
50.47; H, 4.67; Found: C, 50.58; H, 4.70%.
o
p-OEt PSAAa Mp: 124-125 C. Mass Spec.: Calc.: 228 amu. Found: 226.9 amu. Anal. Calc.:
C, 52.63; H, 5.26; Found: C, 52.58; H, 5.30%.
o
p-Me PSAAb Mp: 104 C. Mass Spec.: Calc.: 198 amu. Found: 197.2 amu. Anal. Calc.: C,
54.55; H, 5.05; Found: C, 54.60; H, 5.09%.
o
m-Me PSAAb Mp: 88 C. Mass Spec.: Calc.: 198 amu. Found: 197.0 amu. Anal. Calc.: C,
54.55; H, 5.05; Found: C, 54.48; H, 5.01%.
o
p-F PSAAa Mp: 122 C. Mass Spec.: Calc.: 202 amu. Found: 200.9 amu. Anal. Calc.: C,
47.52; H, 3.47; Found: C, 47.48; H, 3.42%.
o
p-Cl PSAAa Mp: 127-128 C. Mass Spec.: Calc.: 218.5 amu. Found: 217.3 amu. Anal. Calc.:
C, 43.94; H, 3.20; Found: C, 44.01; H, 3.22%.
o
m-F PSAAa Mp: 103 C. Mass Spec.: Calc.: 202 amu. Found: 201.1 amu. Anal. Calc.: C,
47.52; H, 3.4; Found: C, 47.55; H, 3.46%.
o
m-Cl PSAAa Mp: 109 C. Mass Spec.: Calc.: 218.5 amu. Found: 217.7 amu. Anal. Calc.: C,
43.94; H, 3.20; Found: C, 43.89; H, 3.16%.
p-NO2 PSAAc Mp: 167 oC. Anal. Calc.: C, 41.92; H, 3.06; Found: C, 41.97; H, 3.10%.
a
Recrystallizing solvent: ethyl acetate-benzene (1:1); chloroform-petroleum ether (1:1);
b
caqueous acetone (1:1).
Salicylaldehyde and various substituted salicylaldehydes (Alfa Aeser), CrCl3.6H2O
(Sigma Aldrich), pyridine N-oxide (SIGMA-ALDRICH), triphenylphosphine oxide (Alfa
Aesar), picoline N-oxide (SIGMA-ALDRICH), thiophenol (S D fine) and substituted
thiophenols (SIGMA ALDRICH) were purchased and used as such. H2O2 (GR, Merck) and
acetonitrile (HPLC grade, Merck) were used as received.
2.5. Stoichiometry and product analysis
The stoichiometry of the reaction between O=CrV(salen) complex and PSAA was
found to be 1:1 from the study of mixing different ratios of concentration of oxo(salen) and
PSAA. The reaction mixtures containing equivalent moles of PSAA (0.6 mM) and
oxo(salen)chromium(V)+ ion (0.6 mM) in acetonitrile in the presence of ligand oxides
(kinetic run concentration) were kept aside for two days at room temperature for completion
of the reaction which is shown from the reappearance of brown colour of (salen)chromium
(III) complex from the deep green colour. After the completion of the reaction, the solvent
5

was removed in vaccum; the residue was extracted with diethyl ether and dried over
anhydrous sodium sulfate. The organic product formed during the reaction was characterised
as methyl phenyl sulfone by LC-MS (Supporting information Fig. S3), GC-MS (Supporting
information Fig. S4) and IR (Supporting information Fig.S5) spectral methods.
2.6. Cyclic voltammetry
Cyclic voltammetric (CV) studies were performed using CH instrument model CHI
650C in acetonitrile medium containing 0.1 M tetrabutyl ammonium perchlorate as the
supporting electrolyte. The voltammetry utilizes a three electrode configuration. The glassy
carbon electrode is the working electrode at which the electrolysis takes place. The current
required to sustain electrolysis is provided by the counter electrode namely platinum
electrode that is directly placed into the solution. The reference electrode is Ag/AgCl
electrode. Cyclic voltammograms are obtained by measuring the current at the working
electrode during the potential change. Cyclic voltammograms were recorded at the sweep rate
of 0.05 V/s in the scan range of +1.4 V to -0.4 V for oxo(salen) chromium(V)+ ions and at the
sweep rate of 0.1 V/s in the scan range of -1 V to +1.4 V for PSAAs.
3. Results and discussion
The complexes Ia, Ib and Ic have an absorption maximum (λmax) at 560 nm, 557 nm
and 584 nm respectively in 100 % acetonitrile medium. Additions of ligand oxide/PSAA to
the complex shift the λmax appreciably to higher wavelength along with an immediate colour
change to emerald green.
Table 1
λ
-
max values of oxo(salen)chromium(V)+ PF6 complexes in the presence of PSAA and ligand
oxides.
λ
max (nm) in the presence of additives
Complex
PSAA
a
TPPO
b
PyO
b
PicNO
b
Without
additive
c
c
c
560
557
584
570
613
633
595
621
612
635
597
630
612
627
598
617
Ia
Ib
584
622
640
627
627
627
627
622
Ic
6

aoxo(salen) + PSAA.
c
boxo(salen) + LO. oxo(salen) + PSAA + LO.
The shift in the λmax is in the range of 13 nm - 78 nm and the observed λmax values in the
presence and absence of additives are recorded in Table 1. The shift in the λmax value on
adding ligand oxide/PSAA to the oxo(salen) complex indicates binding of LO/PSAA with
oxo(salen)chromium(V)+ to form adduct.
3.1. Determination of binding constants between complex and ligand oxide
The binding constants for the formation of adduct between ligand oxides viz. TPPO,
PyO, PicNO and oxo(salen)chromium(V)+ ions (Ia-Ic) were determined by absorption
spectroscopy. Absorption spectral titration experiments were carried out by keeping the
concentration of oxo(salen)chromium(V)+ complex constant (5 × 10-4 M) and varying the
ligand oxide concentrations by maintaining the total volume of the reaction mixture as
constant. Increase in concentrations of ligand oxide to the oxo(salen) complex showed both
hyperchromicity and red shift in the absorption maxima for all the three complexes. The
representative absorption spectral changes observed during the titration processes are shown
in Fig. 1. The spectral titration curves of the oxo(salen) complexes with ligand oxides show
the existence of isobestic point. This confirms the formation of 1:1 adduct between ligand
oxide and the oxo(salen) complex in a reversible process.
Fig. 1. Absorption changes of Ia and Ic during spectral titration with ligand oxides.
7

The absorbance reading (OD) of complexes at different ligand oxide concentrations in
the maximum absorption was recorded and the binding constant (Kf) for the adduct formation
was evaluated using the modified Benesi-Hildebrand equation (Eq.1) and plot [51,52]. The
calculated binding constant values are recorded in Table 2.
[oxo(salen)] [LO]
∆(OD)
[oxo(salen)] + [LO]
1
=
+
(1)
∆ε
K f ∆ε
Table 2
Binding constant values of Ia-Ic with ligand oxides.
Binding constant (M-1)
Complex
TPPO
PyO
PicNO
49.0
40.8
129
1817
985
747
Ia
Ib
Ic
78.6
1892
3160
The binding constant value depends on the nature of the ligand oxide as well as the
nature of the substituent in the salen ligand. Electron-donating methyl substituent in the 5 and
5′-positions of the salen complex shows low binding constant while the electron-withdrawing
chloro substituent has high binding constant value compared with the unsubstituted salen
complex. Among the three ligand oxides used in the study, TPPO has the least binding
constant. The low binding constant values with TPPO is expected because of its bulky nature.
Thus the observed trend of binding constant indicates the involvement of a substantial steric
inhibition during adduct formation between ligand oxides and oxo(salen) complexes. The
enormous shift along with increase in absorbance in the λmax value of
oxo(salen)chromium(V)+ by the addition of ligand oxides and the observed large binding
constant values support the strong binding of ligand oxides with the metal centre resulting in
the formation of LO → CrVoxo(salen) adduct. This was supported by the work of Kochi and
co-workers [28,45] who have isolated the oxo(salen)chromium(V)+-PyO adduct and reported
their X-ray analysis data. Though they isolated the adduct of sterically hindered oxo(salen),
dichlorotetramethyloxo(salen) with PyO, they were unable to isolate adduct with other LOs
and salen complexes.
3.2. Kinetic study
8

The kinetic study for the oxidative decarboxylation of PSAAs with oxo(salen)Cr(V)+
ions (Ia-Ic) in the presence of ligand oxides were carried out spectrophotometrically at 303
K under pseudo-first-order conditions in 100 % acetonitrile medium using excess of PSAA
over oxo(salen)Cr(V)+ ion concentration in the ratio of at least 100:1. The concentration of
oxo(salen)Cr(V)+ ion was maintained at 5 × 10-4 M unless or otherwise specified and the
concentration of substrate PSAA was maintained in the range of 0.05 M to 0.4 M. The
concentration of ligand oxides was maintained in the range of 0.1 × 10-4 M to 100 × 10-4 M.
All the reactions were initiated by adding required amount of oxo(salen) complex to a
thermally equilibrated mixture of PSAA and LO in acetonitrile in such a way that in each run
the total volume was 3 ml. The kinetics of the reaction was followed by measuring the decay
in absorbance of the oxo(salen)chromium(V)+-LO-PSAA adduct with time at the appropriate
wavelength (Table 1). The representative kinetic run showing the decrease in the absorbance
value of oxo(salen)chromium(V)+-LO-PSAA adduct during its reaction with PSAA in the
presence of ligand oxide is shown in Fig. 2.
Fig. 2. The absorption spectral change with time for the reaction between PSAA and Ic in the
presence of TPPO. [PSAA] = 2 × 10-1 M; [Ic] = 5 × 10-4 M; [TPPO] = 1 × 10-2 M.
3.3. Dependence of reaction rate on reactants
9

The dependence of the reaction rate on PSAA in the presence of three ligand oxides
namely TPPO, PyO, PicNO is studied by measuring the rate at different [PSAA] in the range
from 0.05 M to 0.4 M. The rate constants calculated from the pseudo-first-order plots at
different concentrations of PSAA are recorded in supporting information Table S1 which
indicates that an increase in PSAA concentration increases the k1. However, the non-integral
slope value observed in the plots of log k1 vs. log [PSAA] with different ligand oxides
supports the fractional-order dependence of PSAA. The order observed with PSAA from the
double logarithmic plot of k1 and [PSAA] for different combinations are given in supporting
information Table S2. Again the plots of k1 against [PSAA] (Fig. 3) and 1/k1 against
1/[PSAA] are found to be linear with specific intercept in the y-axis confirm the fractional
order dependence on PSAA.
Fig. 3. Plots of k1 vs. [PSAA] for the reactions. a, Ia with TPPO; b, Ia with PyO; c, Ia with
PicNO; d, Ib with TPPO; e, Ib with PyO; f, Ib with PicNO; g, Ic with TPPO; h, Ic with
PyO; i, Ic with PicNO. General conditions are as in Table 3.
The observed linear pseudo-first-order plots and the constant pseudo-first-order rate
constants calculated at different initial concentrations of oxo(salen)Cr(V)+ ions (Supporting
information Table S1) show the first-order kinetics of the reaction with respect to
oxo(salen)Cr(V)+. The observed trend of constant k1 with varying [oxo(salen)Cr(V)+] is
contradictory with that observed for the reactions in the absence of ligand oxides and in the
presence of nitrogen bases [34] where decrease in k1 with concentration is noted. This
indicates that oxo(salen) dimer formation is prevented by ligand oxides.
10

From the supporting information Table S1 it is inferred that the rate of the reaction is
influenced by the nature of the substituents in the salen ligand. Electron-withdrawing chloro
substituent in 5,5′-positions of oxo(salen) complex increases the rate constant of the reaction
while electron-releasing methyl substituent in the 5,5′-positions slightly reduces the reaction
rate except with TPPO (where rate acceleration is observed) when compared to that of the
unsubstituted oxo(salen) complex. Thus the observed order of reactivity among the
oxo(salen) chromium(V)+ complexes in the presence of ligand oxide is oxo(5,5′-(CH3)2salen)
< unsubstituted < oxo(5,5′-Cl2salen).
Further, the overall rate constant (kov) calculated by dividing the pseudo-first-order
rate constant by [PSAA]order appears to be a constant for a particular oxo complex
(Supporting information Table S1). From the above results and observation, it is concluded
that the order with respect to PSAA is fractional and the reactions of PSAA with
oxo(salen)Cr(V)+ ion proceed through h Michaelis-Menten kinetics in the presence of ligand
oxides. The Michaelis-Menten constants (KM) calculated from the slope and intercept values
of the linear 1/k1 vs. 1/ [PSAA] plots are given in Table S3. The data in Table S3 indicate that
KM value is not sensitive to the change of substituents in the salen ligand and the nature of
ligand oxides. The increase in pseudo-first-order rate constant with increase in [PSAA]
without saturation even at high PSAA concentrations and relatively high value of KM prove
that the binding of PSAA with the active species of oxo(salen)Cr(V)+ ion is weak in nature.
3.4. Effect of ligand oxides on the reaction rate
The influence of ligand oxides on the reaction rate is studied by varying the
concentrations of TPPO, PyO and PicNO over the range of 0.50 x 10-3 M and 100 x 10-3 M.
The reactions could not be carried out at higher concentrations as they are too fast to be
measured or the oxo(salen) complex undergoes some sort of decomposition which is shown
by decrease in absorbance. The calculated pseudo-first-order and overall rate constant values
at various ligand oxide concentrations for different combinations of oxo(salen)Cr(V)+ ions
and ligand oxides are presented in Supporting information Table S4.
It is inferred from the data that all the ligand oxides studied have strong influence on
rate acceleration with all the three oxo(salen) complexes. The observed order of reactivity
among the ligand oxides is PicNO > PyO > TPPO. One interesting observation noted in the
ligand oxide catalysed reactions is the saturation kinetics observed in TPPO with complex Ib.
Though TPPO shows catalytic effect with complexes Ia and Ic and the reactions of complex
Ib get accelerated by PyO and PicNO without any saturation, the reaction of PSAA with
11

complex Ib and TPPO attained a saturation in rate constant even from the lowest
concentration.
Linear plots of log k1 vs. log [LO] with non-integral slope values for the data in
supporting information Table S4 indicate the fractional-order dependence and operation of
Michaelis-Menten kinetics with respect to ligand oxide. Michaelis-Menten constants
calculated from the double reciprocal plots of k1 and [LO] are tabulated in supporting
information Table S3. The appreciable low Michaelis-Menten constants clearly show that
binding between LOs and complexes are strong in nature.
3.5. Substituent effects
In order to study the effect of substituents in the oxidative decarboxylation, the
reactivity of ten para- and meta-substituted PSAAs with oxo(salen)Cr(V)+ ions (Ia-Ic) were
studied in the presence of TPPO, PyO and PicNO. The overall rate constants calculated for
the reactions in the presence of ligand oxides are tabulated in Table 3. The kinetic data in the
table indicate that the reactions in the presence of ligand oxides are highly sensitive to the
nature of the substituents in the aryl moiety of PSAA and in the phenolic part of the salen
ligand. Both electron-withdrawing and electron-donating substituents in the phenyl ring of
PSAA increase the rate of the reaction. In order to confirm the extent of charge separation in
the transition state, the rate constant values were analysed in terms of Hammett σ values. V-
shaped Hammett plots with a minimum for unsubstituted PSAA are seen with all ligand
oxides and oxo(salen) complexes. The Hammett plots are found to have positive ρ value for
electron-withdrawing substituents (ρ+ = 0.057 to 0.886) and negative ρ value for electron-
releasing substituents (ρ- = -0.740 to -4.10). The representative Hammett plots are shown in
Fig.4. Regarding the substituent effect in salen complex, the electron-withdrawing chloro
substituent in the 5,5ʹ positions of the salen ligand (Ic) enhances the rate enormously while
the electron-donating methyl substituent (Ib) decreases the rate which is slightly lower than
the unsubstituted oxo(salen) complex (Ia). This is in accordance with the low binding
constant value observed in 5-Me substituted oxo(salen) complex with the ligand oxides.
12

Table 3
Overall rate constants for the oxidative decarboxylation of PSAAs by Ia - Ic in the presence of ligand oxides.
10 4 kov, (M-1)n s-1
(Ia)
(Ib)
(Ic)
X-PSAA
TPPO
PyO
PicNO
TPPO
PyO
PicNO
TPPO
PyO
aPicNO
p-OMe
p-OEt
p-Me
m-Me
H
49.1 0.83 82.9 2.5
76.3 0.30 57.8 0.24 79.6 0.18 65.9 0.67 109 8.9
119 9.7
105 7.5
244 3.1
148 14
110 1.0
65.0 1.2
48.1 2.4
36.0 1.3
79.8 0.61 56.6 1.9
43.4 2.0
60.0 0.73 54.1 1.1
86.7 2.7
14.4 0.25 60.2 1.6
8.37 0.15 50.0 2.3
7.43 0.40 51.7 1.2
56.5 0.59 28.0 1.0
48.5 1.5
50.7 1.2
40.8 4.4
31.4 0.77 90.0 2.7
17.6 0.49 79.0 6.8
46.2 0.63 15.4 0.27 42.2 1.9
30.0 1.6
14.7 0.51 35.2 0.48 27.4 0.81 16.4 0.49 73.4 3.1
p-F
8.69 0.18 50.3 0.50 30.4 0.28 15.2 0.16 37.0 0.14 29.2 0.15 17.1 0.42 80.5 0.39 50.8 1.1
p-Cl
11.4 1.7
12.9 0.77 53.6 0.76 33.4 1.9
14.6 0.23 54.3 0.16 34.5 0.13 20.0 0.47 40.2 3.3
52.1 2.6
32.4 0.34 17.8 0.45 38.5 3.2
29.9 1.1
32.8 0.58
21.2 0.28 95.2 0.51 53.1 1.3
m-F
25.9 0.48 41.0 1.8
27.0 1.1
130 4.9
52.9 0.75
54.0 0 .28
58..6 6.8
m-Cl
p-NO2
34.3 0.43 33.1 0.75 131 0.48
47.8 1.6 76.7 1.4 172 11
25.8 2.5
-3.12 0.52 -1.16 0.15
( 0.961) ( 0.985)
56.1 2.0
49.8 1.2
51.2 1.6
52.5 2.9
ρ- =
(r =)
-1.24 0.26
(0.939)
-2.27 0.29 -1.15 0.20 -1.23 0.19 -4.10 0.60 -0.740 0.10 -2.42 0.28
(0.976) ( 0.956) ( 0.965) ( 0.980) ( 0.975) ( 0.981)
ρ +=
(r =)
0.655 0.03 0.057 0.01 0.308 0.04 0.751 0.12 0.214 0.02 0.303 0.03 0.886 0.07 0.491 0.07 0.084 0.01
(0.997) (0.934) ( 0.972) ( 0.966) ( 0.982) ( 0.981) (0.989) (0.961 ) ( 0.985)
[TPPO] = 1.0 × 10-2 M; [PyO] = 5.0 × 10-3 M; [PicNO] = 1.0 × 10-3 M . [Ia] = [Ib] = [Ic] = 5.0 × 10-4 M.
aFor (Ic) [PicNO] = 5.0 × 10-4 M; Temp. = 303 K ; solvent = 100 % CH3CN; n = order w.r.t. PSAA.
13

Fig. 4. Hammett plots for the substituent variation in PSAA in the presence of ligand oxides.
3.6. Effect of temperature and activation parameters
In order to calculate the thermodynamic parameters, the kinetics of the reaction
between PSAA and Ia is followed at three different temperatures in the range 298 K - 313 K
in the presence of added ligand oxides. The overall rate constants and the thermodynamic
parameters calculated using Eyring’s plot of log (kov/T) against 1/T (Supporting information
Fig. S6) are given in Supporting information Table S5. The obtained positive value of
14

enthalpy of activation, ∆‡H reflects endothermic nature of the reaction. The negative entropy
of activation, ∆‡S values indicate the involvement of highly structured transition state in the
mechanism. The observed constant ∆‡H (66.0 to 77.2 kJ mol-1) and ∆‡S (-54.6 to -81.0 JK-1
mol-1) values in the presence of different ligand oxides not only indicate the operation of
same mechanism with all ligand oxides and also proves that the reactions are neither enthalpy
controlled nor entropy controlled.
3.7. Discussion
The observed spectral changes involving enormous red shift in λmax value, significant
increase in absorbance value and the existence of an isobestic point in the absorption spectral
titrations with different concentrations of LOs (Fig. 1) along with the visible colour change
during the addition of LO to oxo(salen)Cr(V)+ ion unambiguously confirm the formation of
1:1 adduct between LO and oxo(salen) complex in a reversible process. Strong binding of
LOs with the complex during adduct formation is confirmed from the larger binding constant
values (Table 2) and smaller Michaelis-Menten constants observed with LOs (Table S3). In
many salen mediated oxidation reactions, the LO-salen adduct formed during the course of
the reaction has been proposed as the active oxidizing species and it is proved to be a more
powerful oxidant than the salen complex itself. The catalytic activity shown by the added
ligand oxides is 2-70 times (Table S4) greater than that in its absence. This confirms the
involvement of oxo(salen)Cr(V)+-LO adduct as the active reactive species in the present
reaction series also.
Ligand oxides are reported to co-ordinate with the Cr centre through the oxygen atom
in the apical centre to complete the octahedral co-ordination which weakens the Cr=O bond,
increasing the bond length by donating electron density into the Cr=O anti bonding orbital
[53,54]. The rate enhancement observed in the presence of N-oxides is in line with the
increase in rate observed in the sulfoxidation [55,56] and epoxidation reactions by
oxo(salen)Cr(V)+ [57-60] and oxo(salen)Mn(V) [61] complexes. Additive like
4-phenylpyridine N-oxide (4-PPNO) facilitates faster reaction rates, higher epoxide yields
and improved enantio-selectivity [61-67] in the Mn(III) salen mediated reactions. On the
other hand in the epoxidation of olefins [53] and in the oxidation of aryl methyl sulfides
[29,68], aryl phenyl sulfides [30,69], sulfoxides [31,32] and arylthioacetic acids [33] by
oxo(salen)manganese(V) complexes, the added LOs have no appreciable effect on the
reaction rate.
15

From the kinetic data in Tables 3, S1 and S2 it is shown that the reactivity is more
pronounced in the presence of added PyO and PicNO while it is not so with added TPPO.
This is a striking contrast to the order of catalysis observed with LOs in the oxygenation of
organic sulfides and sulfoxides by oxo(salen)Cr(V)+ ion [55,56] where highest reactivity is
observed with TPPO. The low binding constant values and least reactivity observed with
TPPO among the LOs in this reaction series may be attributed not only to the steric hindrance
created by TPPO during the binding with chromium centre [70] but also due to decrease in
electron density on the oxygen atom of TPPO as a result of the presence of three electron-
withdrawing phenyl groups. These effects decelerate the formation of oxo(salen)-TPPO
reactive adduct species. Earlier studies [28,45] revealed that TPPO adduct with
oxo(salen)Cr(V)+ ions are less stable than PyO adduct. In the computational assessment of
donor ligand effect on the product distribution in the manganese(salen) catalysed epoxidation
of olefins, Jacobsen et al. [71] have shown that PyO forms a stronger bond with the transition
metal centre than TPPO. Gilheany et al. [17] reported that bulky ligands are less effective
donor ligands and their rate of donor ligation is lower than expected. The binding constant
values in Table 2 reveal that the PyO forms stronger adduct with oxo(salen)Cr(V)+ ions. The
unusual high binding constants of PyO has been attributed to the dual nature of the N-oxide
group which acts both as an electron-donor and an electron-acceptor [72,73].
The observed higher reactivity in the presence of PicNO than with PyO may be due to
the existence of methyl group in the para-position of the pyridine ring which increases the
electron density on oxygen of PicNO. Though the binding constant between oxo(salen) and
PyO is higher than with PicNO the presence of methyl group in PicNO favours the easy
formation of oxo(salen)-PicNO adduct which is the active species necessary for a reaction to
proceed. These findings together with lower reactivity in TPPO confirm the existence of
correlation between the order of catalytic activity and the donor ability of the LOs, suggesting
the importance of electronic effect.
On the basis of the foregoing argument it is concluded that the first step in the
mechanism (Scheme 2) is the formation of 1:1 adduct (C1) between oxo(salen) complex and
LO in a reversible step which is the active oxidant. From the fractional-order dependence on
PSAA and the observed high Michaelis-Menten constants, it is assumed that in the second
step of the mechanism PSAA binds in a weak manner with C1 to form a ternary complex
(C2/C3) in a reversible step. The spectral evidence for the formation of C2/C3 is the change in
the absorbance and λmax value (Table 1) of the oxo(salen) complex-LO adduct by the addition
of PSAA. From the density functional theory (DFT) it was calculated that the co-ordination
16

of donor ligands with oxo(salen)Cr(V)+ ions lowers the energy of activation [12,13] during
the addition of alkene to oxo(salen)Cr(V)+ which explains the increase in rate noted in the
epoxidation reactions with donor ligand as additive.
Further, the axial ligands with strong binding ability have a profound effect on the
conformation of the basal salen ligand. The metal centre, which is positioned above the plane
of the four basal hetero atoms in oxo salen [45], moves into the plane and the salen ligand is
slightly distorted towards a step like transoid conformation when donor ligand is coordinated
[74] which favours the attack of a substrate. Bahraoui et al. [75] have shown experimentally
that axial coordination of N-oxide ligand to the oxo(salen)-manganese(V) vomplex raises the
catalytic activity and the oxygen transfer dramatically. Thus it is proposed that one of the
reasons for the observed significant rate acceleration in the presence of ligand oxides is the
favourable condition for the binding of PSAA with oxo oxygen of the salen moiety.
The Hammett correlations of reactivity of different para- and meta-substituted
derivatives of PSAA with complexes Ia–Ic in the presence of LOs reveal V-shaped plots
(Fig. 4). Similar non-linear Hammett plots have been observed in the oxidation of substituted
benzaldehyde by quinolinium chlorochromate [76], thioether substrates by manganese(V)
corrolazine complex [77], trans cinnamic acid by pyridinium chlorochromate [78] and by
chloramine T [79], styrene derivatives by quaternary ammonium permanganate [80],
sulfoxidation by titanium complex [81] and H-atom transfer reaction in p-substituted 2,6-di(t-
Bu)phenol by manganeseV(imido)(corrole) complex [82] etc. Such type of non-linear upward
Hammett plots have been interpreted by a distinct change in reaction mechanism or free
radical mechanism [77,82,83].
In the oxidation of alcohols by ruthenate and perruthenate ions [84] the appearance of
concave upward Hammett plot was explained by the involvement of free radical like
transition state. In the Cr(VI) oxidation of benzylamines [85] the observed U-shaped
Hammett plot was indicated by a change in the relative importance of bond formation and
bond fission in the transition state. Because of the competition between the rate of complex
formation by electron-releasing groups and its rate of decomposition by electron-withdrawing
groups, V-shaped Hammett plots are seen in the oxidation of benzaldehydes by quinolinium
chlorochromate [76]. V-shaped Hammett plots observed in manganese(V) oxo(corrolazine)
[77] and manganeseV(imido)(corrole) [82] complexes were rationalized based upon a change
17

in mechanism that hinges the complexes to function as either an electrophile or nucleophile.
In the present system too the V-shaped Hammett plot is indicative of a change in mechanism.
In Fig. 4 electron-donating substituents show a good linear correlation with an
expected negative slope (ρ-) which can be successfully explained by the mechanism involving
electrophilic attack of the oxo(salen)chromium(V)+-LO adduct (C1) on the sulfur centre of
PSAA. The observed negative reaction constant (ρ-) indicates the accumulation of positive
charge at the sulfur centre while the magnitude of ρ- indicates the extent of charge
development on the sulfur centre in the transition state. The appreciably high ρ- values
obtained indicate that electron-releasing substituents in PSAA create more positive charge on
the sulfur atom and thus increase the nucleophilicity of PSAA. Another reason for the high ρ
value may be due to significant increase in electrophilic character of the ligand oxide
coordinated octahedral oxo(salen)chromium(V)+ adduct. Also the DFT studies on
sulfoxidation by oxo(salen)Cr(V)+ ions made by Venkataramanan et al. [56] indicated an
increase in negative charge over chromium atom in the transition state. A negative ρ value
was already reported for a series of sulfoxidation reactions with oxo(salen)Cr(V)+ ion [55].
An interesting observation noted in the present study is that the log k2 values are linearly
related with the oxidation potential of various substituted PSAAs when electron-donating
groups are present, while linearity is not observed with electron-withdrawing substituents.
The plots of log k2 vs. Eox are shown in Fig. 5.
Fig. 5. Log kov vs. Eox plot for the reaction of PSAA with Ic in the presence of ligand oxides.
The numbers are in the order p-OMe, p-OEt, p-Me, m-Me, H, p-F, p-Cl, m-F m-Cl and p-
NO2.
18

Linear correlation is expected between log k2 and Eox/Ered for a single-electron transfer
process (SET). The observed linear plots of log kov vs. Eox along with negative ρ value
obtained in the Hammett correlation for electron-releasing substituents give direct evidence
for the electrophilic attack of the oxo complex on PSAA involving single-electron transfer
from the sulfur atom to the oxidant species. This step involves the formation of sulphur cation
radical and this would be the slow rate determining step. Such type of sulphur cation radical
formation in a rate determining step is firmly estabilished in literature. Sulphur cation radical
then undergoes several fast steps including oxygen transfer (OT) from the oxo function of the
complex to PSAA and decarboxylation from PSAA moiety by α, β- cleavage leading to the
formation of the products, methyl phenyl sulfone and CO2 for PSAAs having electron-
releasing groups. It is pertinent to mention here that on the basis of the observed linear
correlation of log k2 and Ered, SET mechanism has been proposed for the sulfoxidation of
PTAA [33] and sulphides [31,68] by oxo(salen)Mn(V) complexes, and oxidation of anilines
by oxo(salen)Cr(V)+ [47] and oxo(salen)iron(IV) [23] complexes.
The observed positive reaction constant (ρ+) for electron withdrawing substituents in
the Hammett plot indicates a change in mechanism for PSAAs with such substituents. A
possible explanation for the increase in rate by electron-withdrawing substituents comes from
the existence of quinonoid type resonance structures shown in Fig. 6 which may be partly
stabilized by electron-withdrawing substituents.
Fig. 6. Resonance structures for electron-withdrawing substituents.
The evidence for the contribution of a similar quinonoid type resonance form in
organic sulfur compounds containing electron-withdrawing substituents is recently advocated
by Nakazawa et al. [86] and Neu et al. [77]. As a result of generation of partial positive
charge on the sulfur atom, PSAAs with electron-withdrawing groups become electrophilic in
character. In such cases, the hexa coordinated oxo(salen)-LO adducts which behave as
electrophiles with PSAAs containing electron-donating substituents now act as nucleophiles
and facilitate the formation of intermediate C3 in a slow reversible rate determining step
which then undergoes decarboxylation and decomposition to give [(salen)Cr(III)-LO]+, CO2
and sulfone as the products in a fast step. Such type of sulfonium ion formation between
19

oxo(salen)Cr(V)+ ion and sulfur compounds in a slow step has been advocated earlier in
many sulfoxidation reactions of sulfides and sulfoxides [55,56,87].
The quinonoid type resonance structure of PSAA containing electron withdrawing
substituents can enhance the rate of S-O bond formation in the intermediate (C3) followed by
transfer of oxygen to PSAA. These are facilitated further more with the increase in electron-
withdrawing power of the substituents and accounts for the observed positive ρ value with
electron withdrawing substituents in PSAA. In contrast, the electron-donating substituents
exhibit negligible contribution for the formation of C3 because of their high electron density
on sulfur atom. Thus the observed substituent effect and V-shaped Hammett plot are
rationalized on the basis of change in mechanism that hinges the oxo(salen)Cr(V)+ ion to
function either as an electrophile or nucleophile depending upon the nature of substituents in
PSAA. The two possible mechanistic scenarios, electrophilic and nucleophilic attack of
oxo(salen)-chromium(V) with PSAA are shown in Scheme 2.
The liberationof CO2 during the course of the reaction was confirmed using the
method reported by Crossno et al. [88]. IR, LC-MS and GC-MS spectra confirm the
formation.of methyl phenyl sulfone. The IR spectrum of the product is found to have
characteristic SO2 stretching frequencies at 1148 cm-1 and 1290 cm-1. The GC-MS analysis
of the product indicates a peak corresponding to m/z = 156 which authenticate the formation
of methyl phenyl sulfone as the only product under the experimental conditions. The peak
eluted in LC-MS at a retention time of 1.87 minutes ionizes in APCI (+) mode corresponds to
a mass of 157 also conforms that methyl phenyl sulfone is the only product formed in the
reaction. The absorption spectrum of the inorganic product formed during the reaction is
identical with that of CrIII(salen) spectrum.
20

Scheme 2. Possible electrophilic and nucleophilic pathways.
In order to gain additional evidence for the operation of different mechanisms in
different substituted PSAAs and to test the applicability of reactivity-selectivity principle
(RSP) in the present system, the k2 values (Table 3) for the reactions of different substituted
PSAAs with oxo(salen)Cr(V)+ ions (Ia-Ic) in the presence of ligand oxides are subjected to a
mathematical treatment formulated by Exner [89] using equation 2.
21

log kFi = a + b log kSi + εi
(2)
In the present system no linear correlation is observed between logarithms of rate
constants of fast reaction (kFi) versus slow reaction (kSi). The observed results not only show
the non-applicability of RSP in the presence of ligand oxides but also confirm the operation
of different mechanisms in different substituted PSAAs as proposed in Scheme 2 for
electron-donating and electron-withdrawing substituents.
4. Conclusions
Since decarboxylation process play an important role in different areas of organic
synthesis as well as in biochemical reactions and N-oxides of heterocyclic amines play
important role in the reactions leading to the modification of enzymes and biological
reductive systems, the oxidative decarboxylation of several phenylsulfinylacetic acids by
oxo(salen)Cr(V)+ ions in the presence of ligand oxides have been investigated in acetonitrile.
The effect of additives like ligand oxides on the reactivity showed an accelerating effect. The
order of catalytic activity of ligand oxides, PicNO > PyO > TPPO has been explained on the
basis of electron density on oxygen and relative ability of binding of LO with
oxo(salen)Cr(V)+ ion. The observed nonlinear substituent effect in the Hammett plot has been
rationalized by change in mechanism from single electron transfer to SN2 type reaction upon
changing the substituents in PSAA from electron-donating to electron withdrawing. Thus the
study of reactivity of oxo(salen)Cr(V)+ ion in the presence of different N-oxides estabilish a
more efficient biomimetic catalyst system for the catalytic oxidative decarboxylation of
aromatic acids.
Acknowledgement
Financial support from UGC, New Delhi in the form of a major research project
(F.No.39-817/2010(SR) to PS is gratefully acknowledged. SSD is thankful to UGC, SERO
(No.F.ETFTNMS045), Hyderabad and Manonmaniam Sundaranar University, Tirunelveli for
awarding fellowship under FDP. The authors are extremely thankful to the management of
Aditanar College of Arts and Science, Tiruchendur for providing facilities
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26

Oxidative decarboxylation of PSAAs by oxo(salen)chromium(V) complexes in the presence
of ligand oxides proceeds through Michaelis-Menten kinetics without kinetic saturation. The
order of reactivity among the three ligand oxides is PicNO > PyO > TPPO. The observed
non-linearity in the Hammett plot is due to change in reaction mechanism.
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28