Oxovanadium(IV)-salen ion catalyzed H2O2 oxidation of tertiary amines to n-oxides- critical role of acetate ion as external axial ligand

Article abstract of DOI:10.1002/kin.20910

The oxovanadium(IV)-salen ion catalyzed H₂O₂ oxidation of N,N-dimethylaniline forms N-oxide as the product of the reaction. The reaction follows Michaelis-Menten kinetics and the rate of the reaction is accelerated by electron donating groups present in the substrate as well as in the salen ligand. This peculiar substituent effect is accounted for in terms of rate determining bond formation between peroxo bond of the oxidant and the N-atom of the substrate in the transition state. Trichloroacetic acid (TCA) shifts the λ_max value of the oxidant to the red region and catalyzes reaction enormously. The cleavage of N￡O bond by vanadium complex leads to moderate yield of the product. But the percentage yield of the product becomes excellent in the presence of TCA.

Full text of DOI:10.1002/kin.20910

Oxovanadium(IV)-Salen Ion
Catalyzed H O Oxidation
2
2
of Tertiary Amines to
N-Oxides – Critical Role of
Acetate Ion as External
Axial Ligand
ALAGARSAMY MATHAVAN, ARUMUGAM RAMDASS, MOHANRAJ RAMACHANDRAN,2
1
,2
2
SEENIVASAN RAJAGOPAL2
1Department of Chemistry, V.O.Chidambaram College, Tuticorin, 628 008, India
2School of Chemistry, Madurai Kamaraj University, Madurai, 625 021, India
Received 7 October 2014; revised 2 March 2015; accepted 2 March 2015
DOI 10.1002/kin.20910
Published online 24 March 2015 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The oxovanadium(IV)-salen ion catalyzed H2O2 oxidation of N,N-dimethylaniline
forms N-oxide as the product of the reaction. The reaction follows Michaelis–Menten kinetics
and the rate of the reaction is accelerated by electron donating groups present in the substrate
as well as in the salen ligand. This peculiar substituent effect is accounted for in terms of
rate determining bond formation between peroxo bond of the oxidant and the N-atom of
the substrate in the transition state. Trichloroacetic acid (TCA) shifts the λmax value of the
oxidant to the red region and catalyzes reaction enormously. The cleavage of N–O bond by
vanadium complex leads to moderate yield of the product. But the percentage yield of the
product becomes excellent in the presence of TCA. ꢀC 2015 Wiley Periodicals, Inc. Int J Chem
Kinet 47: 315–326, 2015
INTRODUCTION
past three decades after the discovery of vanadate-
dependent enzymes, vanadium haloperoxidases [1–3],
from various sea algae and terrestrial fungi [4]. These
enzymes serve as efficient catalysts for the oxidative
halogenation and oxidation of organic substrates using
H2O2 as the oxidant [5–7]. Several oxo- and dioxo-
vanadium(V) complexes serve as functional models
of haloperoxidases and catalyze oxyhalogenation of
aromatic substrates [4,8,9]. Vanadium(V) complexes
The catalytic role of vanadium in the +4 and +5 oxi-
dation states has received an intense attention in the
Correspondence to: Dr. Seenivasan Rajagopal; e-mail:
rajagopalseenivasan@yahoo.com.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
ꢀ
C
2015 Wiley Periodicals, Inc.

3
16
MATHAVAN ET AL.
with O, N coordination are regarded as bioinorganic
catalysts as prosthetic groups present in the enzymes
vanadate-dependent peroxidases have similar coordi-
nation environment. Apart from their role as cata-
lysts, V(IV)/V(V) complexes have also been tested as
insulin-like agents but recent reports indicate their an-
ticancer activity also [13].
From an environmental and economical stand point,
catalytic oxidation processes are valuable and those
employing atom-efficient oxidants such as H2O2 are
less expensive. This process involving H2O2 as the ox-
idant generating water as the only by-product is partic-
ularly attractive from green chemistry point of view. A
recyclable silica-supported [VO(acac)2]-catalyzed oxi-
dation of tertiary amines to the corresponding N-oxides
with 30% H2O2 is reported [32]. The recent reports
from the laboratory of Crucianelli and co-workers [33]
highlight the importance of catalytic role of V(IV)
complexes on the H2O2 oxidation of simple and con-
jugated olefins and styrene derivatives. Recently, we
have reported a new and highly efficient methodology
for the oxidation of tertiary amines to N-oxides with
aqueous H2O2 in the presence of catalytic amounts of
the catalyst, nanoruthenium (Ru(PVP)/γ -Al2O3 [34].
In this report, we present our results on the cat-
alytic activities of a series of [V(IV)-salen] complexes
ꢁ
In the past three decades, salen (N,N -
bis(saliylidene)ethylenediaminato) and its derivatives
have received attention of many researchers because
of their extensive application in the fields of synthesis
and catalysis [14–16]. Salen ligand binds metal ions
through a tetradentate binding motif, reminiscent
of the porphyrin framework in the heme-based
oxidase enzymes [17–19] but salen derivatives can
be more easily manipulated to create an asymmetric
environment around the active metal site. Metal-salen
complexes form a standard system in coordination
chemistry having great potential as catalysts for oxo
transfer reactions [20,21] and other processes [22].
One of the first vanadyl-catalyzed reactions is the
V(IV)-salen-catalyzed oxidation of organic sulfides
reported by Fujita et al. [23]. Sun et al. [24] also
reported an efficient asymmetric oxidation of sulfides
catalyzed by vanadium-salen systems. Further V(IV)
and V(V) oxo/peroxo species are considered as
important intermediates in the proposed reaction
mechanisms [18,25].
The reaction of oxovanadium(IV)-Schiff base com-
plexes with an organic peroxide leads to the final form
of the active oxovanadium(V) species [25b]. Curci et
al. [26] proposed that in the oxidation of dibutyl sul-
fide with tert-butyl hydroperoxide (TBHP) catalyzed
by VO(acac)2, the catalytically active species is a
vanadium(V) and not a vanadium(IV) species. In the
V(IV)-catalyzed reactions, the possible intermediate is
a metal-oxo, metal-peroxo or bridging oxo complex
of various types [25,27]. The terminal oxo complex
can arise when a single catalytic metal ion abstracts
an oxygen atom from an oxygen source making two-
electron-oxidized metal oxo species [28].
ꢁ
on the H2O2 oxidation of N,N -dimethylanilines. This
reaction is interesting in the sense that the rate of the
reaction is accelerated by the electron-donating groups
present in the substrate as well as in the oxidant and
negative reaction constant (ρ) value is obtained from
the study of substituent effect on the oxidant as well
as on the substrate. This is a very rare reaction where
the formation of the transition state is facilitated by
the introduction of electron-releasing substituent in the
oxidant as well as on the substrate. Further, the title
reaction is catalyzed and the product yield improved
enormously by trichloroacetic acid (TCA).
EXPERIMENTAL
Materials
Salicylaldehyde and substituted salicylaldehydes (5-
methyl, 5-methoxy, 5-chloro, 5-bromo, 5-nitro, 3,5-
ꢁ
dichloro, 3,5-di-tert-butyl), N,N -dimethylaniline, and
ꢁ
four substituted N,N -dimethylanilines (methyl, cyano,
bromo, and carboxylic) were purchased from Aldrich
and used as such. HPLC-grade dichloromethane as the
solvent and 30% H2O2 as the oxidant were used as
received.
The oxidation of tertiary amines may lead to the for-
mation of different products such as N-oxide, demethy-
lated products, and polyanilines [29]. Demethylation
is a biologically important process and oxidation of
N-compounds to N-oxides is an important transforma-
tion from biological and synthetic point of view [30].
N-Oxides find novel applications in inorganic and or-
ganic chemistry as protecting groups, oxidants, cata-
lysts, and ligands [31]. Though many oxidants have
been utilized for the oxidation of tertiary amines to
N-oxides, the selective oxidation is still a challenging
process. In several cases, N-oxides are more active than
their corresponding tertiary amines.
Instrumentation
The UV-vis absorption spectral measurements have
been carried out in a quartz cuvette of 1 cm using
JASCO 530 UV-vis spectrophotometer and Analytik
Jena Specord S 100 diode spectrophotometer with
a constant accumulation of 10 and a constant inte-
gration time of 40 ms over a wavelength range of
International Journal of Chemical Kinetics DOI 10.1002/kin.20910

OXOVANADIUM(IV)-SALEN ION CATALYZED H2O2 OXIDATION OF TERTIARY AMINES
317
2
00–1000 nm. The infrared spectra of the complexes
Product Analysis
A solution containing 4-X-N,N -dimethylanilines (X =
have been recorded in a JASCO FTIR-410 spectropho-
tometer, in solid phase, as KBr pellets. H NMR spectra
were recorded on a Bruker 300 MHz NMR spectrome-
ter with CDCl3 as the solvent. Cyclic voltammograms
ꢁ
1
–
CH , –H, –Br, –COOH, –CN) was reacted with excess
3
H2O2 in the presence of [oxovanadium(IV)(salen)]
complex in CH2Cl2 and the reaction mixture was
stirred for 3–10 h depending on the nature of sub-
strate. The organic product was extracted several times
in chloroform and analyzed in GC. The solvent was
(CVs) were obtained using Sinsil CH work station elec-
trochemical system with a three-electrode system and
a personal computer for data storage and processing.
An Ag/AgCl (saturated KCl)/3M KCl reference elec-
trode, a Pt wire (counter electrode), and glassy carbon
working electrode were employed for electrochemi-
cal studies. EPR spectra were recorded using Bruker
EMX Micro premium X spectrometer at liquid N2 and
at room temperature at X band (9.4 GHz).
1
evaporated and the product was analyzed using H
NMR, ESI–MS, FTIR, and GC techniques. The prod-
uct investigations through the spectral studies reveal
the formation of N-oxides as the product of the reac-
tion. The details of the product analyses are elaborated
in the discussion section.
Synthesis of Salen Ligands and
[
(
Oxovanadium(IV)(Salen)] Complexes
I–VIII)
RESULTS AND DISCUSSION
ꢁ
ꢁ
The structures of five tert-amines and eight synthe-
sized [oxovanadium(IV)(salen)] complexes used in the
present study are shown in Chart 1. All eight [oxovana-
dium(IV)(salen)] complexes (I–VIII) synthesized in
the present study are known compounds and the spec-
tral and electrochemical data collected in Table I are in
good agreement with reported results [37–42].
The salen ligands, 5,5 -(CH3)2-salen, 5,5 -(OCH3)2-
ꢁ
ꢁ
ꢁ
salen, 5,5 -(Cl)2-salen, 5,5 -(Br)2-salen, 5,5 -(NO2)2-
ꢁ
ꢁ
ꢁ
ꢁ
salen, 3,3 ,5,5 -(Cl)4-salen, and 3,3 ,5,5 -(tert-Bu)4-
salen were prepared by condensing the corresponding
substituted salicylaldehyde with ethylene diamine in
ethanol [35] and recrystallized from cold methanol.
The H and C NMR data are given in supporting
information (Tables S-I and S-II) and the data are in
close agreement with the previous reports [36].
1
13
Absorption Spectral Studies
[
Oxovanadium(IV)(salen)] complexes (I–VIII)
were prepared by the literature procedures by the reac-
tion of stoichiometric amounts of the respective ligand
with vanadyl sulfate in alcoholic medium [37–40]. All
the eight [oxovanadium(IV)(salen)] complexes were
characterized by UV-vis, ESI–MS, FTIR, EPR spec-
tral studies, and cyclic voltammetric measurements and
the data are given in Table I. The spectra of some com-
plexes are given in supporting information (Figs. S1–
S21) and the relevant spectral and electrochemical data
in the Tables S-III to S-V.
The [oxovanadium(IV)(salen)] complexes I–V and
VIII are soluble in CH2Cl2 whereas the complexes
VI and VII are soluble in DMSO. These V(IV)-salen
complexes are poorly soluble in CH3CN and alcohols.
The absorption of parent [oxovanadium(IV)(salen)]
complex (I) at 243 and 263 nm in CH2Cl2 (Fig.
S1) corresponds to the ligand centered (LC) transi-
tion and the absorption at 368 nm corresponds to lig-
and to metal charge transfer (LMCT) transition [39a].
This LMCT absorption maximum is sensitive to the
Table I UV-Vis, ESI–MS, FTIR, CV and EPR Data of [Oxovanadium(IV)(Salen)] Complexes
E1/2(V) vs. SCE
ε (LMCT)
(M cm )
ESI–MS
Data
IR data (cm−1) νC=N
νC–O νV=O
EPR Data
−
1
−1
Complexes
λ (nm)
DCM
DMSO
(g )
┴
I
II
243, 263, 368
244, 265, 381
247, 266, 403
244, 261, 379
244, 262, 380
238, 260, 336
272, 383
9860
7752
5219
5845
4510
9262
9262
3812
334
362
394
441
–
–
–
567
1622, 1302, 986
1622, 1298, 962
1630, 1281, 974
1616, 1294, 966
1616, 1294, 966
1601, 1309, 876
1637, 1300, 867
1622, 1333, 974
0.641
0.544
0.567
0.663
0.683
–
0.372
0.302
0.340
0.483
0.486
0.635
0.452
0.216
2.02
1.89
2.00
1.89
1.89
–
III
IV
V
VI
VII
VIII
–
–
1.90
243, 267, 338, 619
0.402
International Journal of Chemical Kinetics DOI 10.1002/kin.20910

3
18
MATHAVAN ET AL.
Chart 1 Structure of [oxovanadium(IV)(salen)] complexes and tert-amines.
Figure 1 (A) UV-vis absorption spectral changes observed on the addition of H2O2 (10 mM) to I (0.1 mM) in CH2Cl2 at
98 K. (B) Changes observed in the d–d band absorption of I (0.5 mM) on the addition of H2O2 (50 mM).
2
nature of substituent present in the salen ligand
Table I). Introduction of 5-OMe group at the benzene
ring (III) shifts the λmax value from 368 nm to 403 nm
red shift) in DCM whereas 5-Cl and 5-Br (IV and V)
at 243, 266, and 302 nm. The rate of decay of ac-
tive species generated in the reaction mixture was fol-
lowed spectrophotometrically from the decrease in ab-
sorbance at appropriate wavelength (Table I).
(
(
groups cause a comparatively less shift from 368 nm
to 379 nm. The shift in the λmax to red region is due to
the stabilization of excited state by electron-donating
groups [43,44]. A substantial change in the absorption
intensity of [V(IV)-salen] complex I is observed on
the addition of H2O2 not only at the LMCT band at
The absorption intensity of the mixture of I and
H2O2 at 367 and 280 nm decreases whereas the ab-
sorbance at 324 and 425 nm increases with isosbestic
points at 300, 345, and 400 nm (Fig. 1). At higher
concentration of I and [H2O2] the intensity of d–d ab-
sorption band decreases with time at 572 nm. These
spectral changes can be taken as evidence for the for-
mation of active species which may be formulated
as vanadium(V)-peroxo complex or vanadium-H2O2
adduct (vide infra) [45]. Similar results were also ob-
370 nm but also at the d–d band at 570 nm. The details
of spectral changes of complex I at 370 and 570 nm at
low and high [H2O2] are shown in Fig. 1.
IV
served for the solution of [V O (sal-his) (acac)] in
Kinetic Studies
methanol on the addition of H2O2 due to the forma-
tion oxoperoxo species [45e]. The formation of active
species is evident from the color change from green to
red [46].
The spectral changes shown in Fig. 2 also reveal
the formation of N-oxide as the product of the reaction
which is evident from the increase of absorbance at
The kinetic studies for the [V(IV)-salen] complex
ꢁ
catalyzed H2O2 oxidation of 4-X-substituted N,N -
dimethylanilines (X = H, –CH3, –CN, –Br, –COOH)
were carried out in CH2Cl2 medium under pseudo-
first order conditions using excess of substrate over
ꢁ
the oxidant. N,N -dimethylaniline has absorption bands
International Journal of Chemical Kinetics DOI 10.1002/kin.20910

OXOVANADIUM(IV)-SALEN ION CATALYZED H2O2 OXIDATION OF TERTIARY AMINES
319
Figure 2 Change of absorbance of III with time in the
Figure 3 Plot of k1 vs [S] for the oxidation of 4-X-N,N-
dimethylanilines with II-H2O2 system at 298 K. (1. X = H,
ꢁ
presence of H2O2 and N,N -dimethylaniline (DMA). [III] =
−
4
−2
−3
1
×10 M, [H2O2] = 1×10 M, [DMA] = 1×10 M.
2
. X = CH3, 3. X = CN, 4. X = Br, 5. X = COOH).
4
40 nm and N-oxide binds to the vanadium(IV) cen-
Based on the Michaelis–Menten formulation the ob-
ter. Similar spectral changes have been reported in the
oxidation of N,N -dimethylanilines using iron-salen +
served rate constant (k1) is given by Eq. (3).
k1 = kobs = k(substrate)/KM + (substrate) (3)
The rearrangement of Eq. (3) leads to Eq. (4)
ꢁ
H2O2 system to form the corresponding N-oxide [43c].
The plot of log Abs vs. time given in the Supporting
information (Fig. S22) shows that there is a decrease in
the absorption intensity (at 381 nm) up to 400 s but the
intensity starts to increase after 400 s for the oxidation
of 1 catalyzed by II + H2O2 system. It indicates that
the oxidation reaction is a reversible process. From the
decrease in absorbance at 381 nm with the time for
the complex II, the rate constant of the reaction is cal-
culated and the rate constant data for the substituted
N,N-dimethylanilines are collected in the Supporting
information (Table S-VI).
1/k1 = 1/k + KM/k[substrate]
(4)
where KM is the Michaelis–Menten constant, k is the
rate constant for the oxidation of substrate. From the
plot of 1/k1 vs. 1/[S] (Fig. S23) using Eq. (4), the values
of KM and k are calculated and given in Tables II and III,
respectively.
The data collected in Table II shows that the rate
constant for the formation of product is sensitive to the
nature of substituents present in the of aryl moiety of
Michaelis–Menten Kinetics
ꢁ
N,N -dimethylanilines. The electron-donating groups
The plot of k1 vs. [substrate] given in Fig. 3 shows that
the rate constant of the reaction increases with increas-
ing the substrate concentration, reaches maximum, and
then attains saturation. At high [substrate], saturation
behavior is observed and the reaction proceeds through
Michaelis–Menten kinetics (Fig. 3).
The rate constant k for the formation of the product
is obtained by using the Michaelis–Menten kinetics,
Eqs. (1) and (2).
3
−1
Table II Rate Constant (k × 10 s ) Values for the
Formation of Product from the Oxidation of
-X-N,N -Dimethylanilines of by [Vanadium(IV)(Salen)]
H2O2 System and the Reaction Constant (ρ) Values
ꢁ
4
+
Complex →
I
II
III
IV
V
ρ
r
Substrate ↓
H
5.0
6.2
0.9
4.0
1.8
6.4
8.5
0.7
1.8
3.0
6.6
7.1
5.6
6.3
2.7
1.9
3.0
2.0 −0.9 0.95
2.4 −0.5 0.97
–
–
–
–
CH3
CN
Br
0.24 0.38 −1.2 0.99
0.95 1.4 −0.5 0.96
Oxidant + Substrate ꢀ Complex
Complex → Product
(1)
(2)
COOH
0.4
0.84 −0.6 0.97
ρ
−0.7 −1.5 −1.2 −1.0 −0.6
–
–
–
–
r
0.99 0.98 0.97 0.99 0.98
International Journal of Chemical Kinetics DOI 10.1002/kin.20910

3
20
MATHAVAN ET AL.
3
Table III Michaelis–Menten Constant, KM × 10 ,
Values
Complex →
Substrate ↓
I
II
III
IV
V
–
–
–
–
–
H 1
3.1
8.6
1.3
1.7
5.8
5.0
8.3
1.7
2.3
4.3
3.4
6.1
2.4
2.0
6.2
5.8
5.9
0.9
1.5
4.2
4.3
7.0
1.3
2.2
1.0
CH 2
3
CN 3
Br 4
COOH 5
Table IV Michaelis–Menten Constant (KM) and Rate
Constant (k) Data for the VI and VII Catalyzed H2O2
ꢁ
Oxidation of 4-X-N,N -Dimethylanilines in DMSO at
298 K
Complex VI
Complex VII
Substrate
↓
ꢁ
Figure 4 Hammett plot for the oxidation of para-X-N,N -
dimethylaniline using II and H2O2.
3
3
3
3
KM × 10 (M) k × 10 KM × 10 M k × 10
CH3
H
CN
Br
COOH
ρ
5.6
2.1
0.7
0.9
1.7
–
5.1
2.1
0.3
0.7
0.6
9.0
6.4
1.0
1.5
9.3
–
6.7
3.5
0.7
3.2
2.3
Here, k0 is the rate constant for the parent com-
pound; k is the rate constant for the para-substituted
compound; σ the substituent constant, depends only
on the specific substituent; and the reaction constants
ρ depend only on the type of reaction. This negative ρ
value indicates that the substrate carries positive charge
in the transition state of reaction. It is important to com-
pare these results with the results observed for the oxi-
−0.6
−0.6
r
–
0.96
–
0.93
present in the para-position accelerates the rate of re-
action and the electron withdrawing groups deceler-
ate the rate of reaction. Similar substituent effect is
observed when the substituents are introduced in the
salen ligand (vide infra).
ꢁ
dation of N,N -dimethylanilines with other oxidants to
get a clue on the mechanism of the reaction. Nam and
coworkers [47] observed a ρ value in the range −2.2
to −3.3 for demethylation reaction with oxoiron(IV)
complexes. But the ρ value is −1.0 for the dioxirane
The complexes VI and VII have better solubility in
DMSO and so the reactions of these complexes with
ꢁ
oxidation of N,N -dimethylanilines to the correspond-
ing N-oxides [48]. The ρ value for the oxidation of
ꢁ
N,N -dimethylanilines have been studied in DMSO.
ꢁ
N,N -dimethylanilines by [Cr(III)-salen]–H2O2 system
The pseudo first-order rate constant (k1) values for
the oxidation of N N-dimethylanilines (1–5) with VII–
H2O2 system are collected in the supporting informa-
tion (Table S-VII). The Michaelis–Menten constant
is in the range of −1.5 to −3.8 [44b] and in the range
of −1.4 to −2.6 for Fe(III)–H2O2 system [43c].
We have also analyzed the kinetic data observed
for the different [V(IV)-salen] complexes carrying
electron-donating and -withdrawing groups in the salen
ꢁ
(KM) and the rate constant (k) values for the H2O2
oxidation reaction using the complexes VI and VII in
DMSO are given in Table IV.
ꢁ
ligand. The electron-donating groups present in 5, 5 -
position of the salen ligand also accelerate the reac-
tion, and the observed ρ values are negative in the
range −0.5 to −1.2. Interestingly, the behavior of the
Substituent Effects
ꢁ
substituents present in the 5, 5 -position in the salen
Application of Hammett equation to the rate constant
data (Fig. 4) gives good correlation and the values of
reaction constant (ρ) in the range of −0.6 to −1.5 are
given in Table II.
ligand of vanadium(IV) complexes is found to be dif-
ferent from the substituent effects observed with other
metal-salen complexes [Fe(III), Cr(III), Mn(III) and
Co(II)] [43,44,49,50].
It is appropriate to quote the similar results reported
recently in the literature. The catalytic potential of
log k/k0 = ρσ
(5)
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OXOVANADIUM(IV)-SALEN ION CATALYZED H2O2 OXIDATION OF TERTIARY AMINES
321
Scheme 1 Mechanism of oxidation of VIV complex by H2O2 to form the protonated VV complex.
vanadyl complexes has been tested using cyclo-octene
and styrene as the substrates and TBHP as the oxidant.
The presence of electron-donating substituents on the
aromatic ring as well as imine bond of salen effec-
tively improves the catalytic activity [45h]. The pres-
ence of two methoxy substituents at the ortho-position
of phenyl groups of vanadium(IV) complexes enhances
the electron density on the ligating atoms of the ligand
through π-donation of the phenyl groups. The metal
center of vanadyl complex is expected to be more
electron-rich and the redox potential of V(V)/V(IV)
shifts in the cathodic direction and the complex be-
comes easier to be oxidized. Thus depending on the
transition state of the reaction the electron-donating
groups present in the ligand of the metal complex may
facilitate the reaction by increasing the electron density
on the metal center. As the substituent effect observed
with the present study is similar to the results observed
by Rayati et al. [45h] we propose a similar transition
state.
Overall the results observed in the present study
seem to be peculiar because the change of the sub-
stituents in the oxidant as well as substrate leads to
similar results and negative (ρ) values are obtained in
both cases. Thus, the formation of transition state is fa-
cilitated by the electron-donating groups present in the
oxidant as well as in the substrate. In order to account
for these interesting and novel results we have proposed
a mechanism involving a transition state, the formation
of which is facilitated by the electron-donating groups
present in the substrate as well as in the oxidant.
complex or in the absence of base, the corresponding
protonated oxocompound [51,52]. This dioxoV(V)
complex further reacts with the oxidant, H2O2 to form
peroxo complex, which is the active species. Scheme 1
shows that the vanadium(IV) complex is oxidized to
vanadium(V) peroxo complex and further converted
into hydroperoxovanadium(V) [53,54].
Proposed Mechanism for the Oxidation
of N,N-Dimethylanilines
On the basis of the observed UV-vis absorption spec-
tral changes, we presume that the active species formed
is hydroperoxovanadium(V) (c) as shown in Schemes 1
and 2. In the first step, oxovanadium(IV) complex
(a) is oxidized to oxovanadium(V) (b) by H O and
2
2
further reaction of H O with (b) forms oxoperox-
2
2
ovanadium(V) complex (c) [45]. This complex (c)
forms oxidant–substrate complex in which the oxy-
gen atom is transferred from the active oxidant species
to the tert-amine to produce the corresponding N-
oxide as the product. It has been established that in
the other H O oxidation reactions catalyzed by oxo-
2
2
vanadium(IV) complex the active species is the species
similar to complex (c). The negative ρ value observed
from the study of substituent effect with the change
of structure of tert-amine indicates that the substrate
carries positive charge in the transition state of the re-
ꢁ
action. The substituents present in the 5, 5 -position
in the salen ligand of vanadium(IV) complex have be-
havior similar to that of the substrate. The bond for-
mation between O–O of the oxidant and N atom of
the substrate is facilitated by more electron density on
the nitrogen center and thus the rate acceleration by
electron-releasing groups in the substrate. Further, the
vanadium-peroxo complex will be more stable with
the increase of electron density on the vanadium cen-
ter, which is possible with the introduction of electron-
releasing group in the salen ligand and thus the negative
ρ value observed with the change of substituent in the
salen ligand also supports the proposed mechanism.
Active Oxidant Species
The parent [oxovanadium(IV)(salen)] complex
has LMCT absorption maximum at 368 nm
−
[
PhO →VO(IV) transition]. Addition of H2O2
to the vanadium(IV) complex leads to a substantial
change in the absorption spectrum (Fig. 1). These spec-
tral changes can be taken as evidence for the formation
of active oxidant species which may be formulated as
peroxovanadium(V) complex or vanadium(IV)-H2O2
adduct (vide infra) [45,46]. The formation of the
active species is also evident from the color change
from green to red [46]. The vanadium(IV) complex
in the presence of H2O2 forms stable dioxoV(V)
Effect of Trichloroacetic Acid
Figure S6 shows the spectral changes during the ad-
dition of TCA to the complex II. In the UV-vis
International Journal of Chemical Kinetics DOI 10.1002/kin.20910

3
22
MATHAVAN ET AL.
ꢁ
Scheme 2 The proposed mechanism for the H2O2 oxidation of 4-X-N,N -dimethylanilines catalyzed by [oxovana-
dium(IV)(salen)] complex.
absorption spectrum, the new broadband appears at
around 615 nm but no peak at the same wavelength in
the absence of TCA. The addition of TCA to the V(IV)
complex also leads to color change from green to dark
blue. Similar spectral changes have also been observed
for the addition of TCA to [oxo(salen)chromium(V)]
ion which is attributed to the binding of carboxylate ion
to the metal center [49d]. The critical role of external
axial ligands in the chirality amplification of metal-
salen complex has recently been highlighted by Fujii
and co-workers [55].
The changes in the absorption spectral data on the
addition of TCA to the [oxovanadium(IV)(salen)] com-
plexes (I–V) in CH2Cl2 (LMCT and d–d band) are
collected in Table V. It is interesting to compare these
Figure 5 UV-vis absorption spectral changes observed on
the addition of H2O2 to I + TCA mixture.
Table V Absorption Spectral Data of Complexes (I–V)
[0.2 mM] in the Absence and Presence of TCA [0.05 M]
λ (nm), λ (nm),
spectral changes observed in the presence of TCA with
those obtained during the reaction of H2O2 with I in
the absence of TCA (Figs. 2 and 5). The UV-vis ab-
ε(M 1cm
−
−1
)
ε(M cm )
−1
−1
Complex
Absence of TCA
Presence of TCA
I
369, 584(400)
381, 594(700)
403, 583(750)
379, 600(660)
380, 601(620)
376, 594(6720)
400, 615(5400)
413, 695(4650)
389, 615(6560)
392, 617(8430)
ꢁ
sorption spectral changes for the oxidation of N,N -
II
III
IV
V
dimethylaniline by I+H2O2 system in the presence
of TCA show that there is a decrease in absorbance
at 595 nm as shown in Fig. 5. The increase in ab-
sorbance change at 500 nm shown in Fig. 5 indicates
International Journal of Chemical Kinetics DOI 10.1002/kin.20910

OXOVANADIUM(IV)-SALEN ION CATALYZED H2O2 OXIDATION OF TERTIARY AMINES
323
the formation of corresponding N-oxide. As the axial
position of V(IV)-salen complex is now occupied by
trichloroacetate ion, N-oxide is not able to bind with
the V(IV)-salen complex [49d].
Table VII Product Yield of N-Oxides Formed from the
Oxovanadium(IV)–Salen-Catalyzed H2O2 Oxidation of
ꢁ
4
-X-N,N -Dimethylanilines in the Presence of TCA
%
Yield in the presence
As indicated above the green-colored complexes
I–V are changed into dark blue by the addition of TCA
but on the addition of oxidant and the substrate the
color changed from blue to pale yellow. The pseudo
first-order rate constant (k1) values are calculated
by measuring the OD at the appropriate wavelength,
λ = 615 nm, for the complex II, with respect to time.
X
% Yield
of TCA
Me
H
CN
Br
COOH
27
14
13
12
10
92
87
81
78
76
ꢁ
The rate for the oxidation of N,N -dimethylanilines us-
ing V(IV)–H2O2 system in the presence of TCA is
almost 10 times higher than in the absence of TCA.
The plot of k1 vs. [substrate] (supporting informa-
tion) shows that the II-catalyzed H2O2 oxidation of
Product Yield
Table VII shows the % of product, the corresponding
N-oxide, and yield obtained in the absence and pres-
ence of TCA. Though the reaction is efficient in the ab-
sence of TCA, the yield is low in all cases. This result
is surprising but requires explanation. Recent report
shows that the vanadium complexes have the excel-
lent ability to cleave the N–O bond of N-oxides [57].
Thus, the major reason for the low yield of N-oxide
formation is the cleavage of N–O bond by the V(IV)
complex. This postulation is also supported by the re-
versibility of N-oxidation of tert-amines by H2O2 in the
presence of V(IV)-salen complex and increase in the
absorbance at 440 nm due to the formation of V(IV)–
N-oxide adduct. Interestingly, the product yield is good
to excellent in the presence of TCA. The yield is more
for amine-carrying electron-donating substituents but
less for electron-withdrawing substituents compared
to the parent compound. This critical role of TCA
makes this V(IV)–salen–H2O2 redox system as an ex-
cellent reagent for the selective oxidation of amines
to N-oxides. Trichloroacetate ion binds on the axial
position of V(IV)–salen complex thereby preventing
the product N-oxide to bind with the catalyst and thus
avoiding the decomposition of N-oxide and improving
the product yield. Thus TCA plays a critical role in this
reaction serving as an axial ligand of V(IV)–salen com-
plex. Fujii et al. recently reported the critical role of
N3 and OCH2CF3 as axial ligands in the Mn(IV)–salen
complex in chirality amplification [55].
ꢁ
N,N -dimethylanilines in the presence of TCA is also
fractional order with respect to substrate and the reac-
tion proceeds through the Michaelis–Menten kinetics.
The double reciprocal plot of 1/k1 vs. 1/[sub] for II-
ꢁ
catalyzed oxidation of N,N -dimethylaniline by H2O2
in the presence of TCA is shown in the supporting
information (Fig. S24).
From the double reciprocal plot the Michaelis–
Menten constant (KM) and the rate constant (k) val-
ues for the formation of product are calculated and
collected in Table VI. Figure S25 shows the Hammett
ꢁ
plot for the oxidation of para-X-N,N -dimethylanilines
using II and H2O2 system in the presence of TCA.
The interesting aspect is that the ρ value (−0.4) is
less negative compared to the ρ value (−1.5) observed
in the absence of TCA. The ρ value is always as-
sociated with the selectivity of the reaction. Thus in
the presence of TCA, the reactivity of [V(IV)-salen]-
ꢁ
catalyzed H2O2 oxidation of N,N -dimethylanilines
is more but selectivity is less. Thus, these reactions
are in accordance with the reactivity–selectivity prin-
ciple (RSP). The operation of RSP has been ob-
served in several other oxidation reactions of or-
ganic substrates catalyzed by [metal-salen] complexes
[49c,56].
The ESI–MS data of reaction mixture prepared us-
ing N,N -dimethylaniline, [vanadium(IV)(salen)] com-
Table VI The Value of Michaelis–Menten Constant
ꢁ
(
KM) and the Rate Constant (k) for the Oxidation of
ꢁ
Para-Substituted N,N -Dimethylanilines by II + H2O2 +
plex I, and H2O2 show a peak with m/z value 138 con-
firming the formation of N-oxide as the product of the
reaction. A sample spectrum is shown in the support-
ing information (Fig. S26). ESI–MS study also reveals
the formation of dimerized product (m/z value 275).
TCA
3
−1
2
Substrate
10 × KM (M
)
10 × k
CH3
H
CN
Br
COOH
10.2
8.9
4.3
9.1
8.7
2.6
2.3
1.2
1.9
1.5
ꢁ
The IR spectrum of N,N -dimethylaniline N-oxide is
shown in the supporting information (Fig. S27) and
the IR data are in close agreement with previous re-
−
1
ports [34,58]. The peak at 944 cm corresponds to
International Journal of Chemical Kinetics DOI 10.1002/kin.20910

3
24
MATHAVAN ET AL.
−
1
N–O stretching mode and the peak at 1347 cm cor-
Cerecetto, H.; Draper, P.; Gonzalez, M.; Piro, O. E.;
Castellano, E. E.; Azqueta, A.; de Cerain, A. L.; Monge-
Vega, A.; Gambino, D. J Inorg Biochem 2005, 99, 443–
451; (d) Shechter, Y.; Goldwaser, I.; Mironchik, M.;
Fridkin, M.; Gefel, D. Coord Chem Rev 2003, 237, 3–
responds to tert-amine. The product was also char-
acterized by NMR spectral technique. The H NMR
spectrum of N-oxide of N,N -dimethylaniline is shown
in Fig. S28 and CDCl3 is used as the solvent. There are
distinct peaks δ at 3.53, 7.18–7.35 and 7.41–7.53 ppm
and these values are in close agreement with our pre-
vious report [34]. This product analysis study demon-
strates that N-oxide is the only product formed under
the present reaction conditions.
1
ꢁ
11; (e) Bastos, A. M. B.; da Silva, J. G.; Maia, P. I. S.;
Deflon, V. M.; Batista, A. A.; Ferreira, A. V. M.; Bo-
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6
. (a) Eady, R. R. Coord Chem Rev 2003, 237, 23–31;
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This work presents a simple method for the efficient
synthesis of N-oxides. The novelty of the present study
is that the [vanadium(IV)(salen)]-catalyzed H2O2 ox-
ꢁ
idation of N,N -dimethylanilines is facilitated by
electron-donating groups present in the substrate as
well as in the oxidant and the negative ρ value is
observed from the study of substituent effect by the
change of substituents in the substrate as well as in
the oxidant. The N-oxide is the major product of the
reaction in the presence of TCA confirmed by UV-
visible, IR, Mass, and NMR spectral studies. In the
rate-controlling step, the active oxidant oxoperoxo-
vanadium(V) ion transfers oxygen to the substrate
through direct oxygen transfer. TCA catalyzes the re-
action through coordination to the V(V) center as the
axial ligand.
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Professor SR thanks UGC, New Delhi for sanctioning UGC-
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and the Management, V. O. C. College, Tuticorin for sanc-
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International Journal of Chemical Kinetics DOI 10.1002/kin.20910

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