Micellar catalysis on the electron transfer reactions of iron(III)-polypyridyl complexes with organic sulfides - Importance of hydrophobic interactions

Article abstract of DOI:10.1039/b509761d

The oxidation of organic sulfides with iron(iii)-polypyridyl complexes [Fe(NN)₃]³⁺ proceeds through an electron transfer mechanism and an increase in the methanol content in the methanol-water mixture favors the reaction. The reactio

Full text of DOI:10.1039/b509761d

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Micellar catalysis on the electron transfer reactions of iron(III)-polypyridyl
complexes with organic sulfides—importance of hydrophobic interactions†‡
a
b
a
a
a
a
S. Balakumar, P. Thanasekaran, E. Rajkumar, K. John Adaikalasamy, S. Rajagopal,* R. Ramaraj,
a
c
b
T. Rajendran, B. Manimaran and K.-L. Lu
Received 11th June 2005, Accepted 17th November 2005
First published as an Advance Article on the web 12th December 2005
DOI: 10.1039/b509761d
3
+
The oxidation of organic sulfides with iron(III)-polypyridyl complexes [Fe(NN) ] proceeds through an
3
electron transfer mechanism and an increase in the methanol content in the methanol–water mixture
favors the reaction. The reaction is catalyzed by both the anionic surfactant, sodium dodecyl sulfate
(
SDS) and the cationic surfactant, cetyltrimethylammonium bromide (CTAB). The micellar catalysis in
the presence of SDS is accounted for in terms of strong binding of the cationic oxidant with the anionic
surfactant and the development of positive charge on sulfur center of substrate in the transition state.
3
+
The micellar catalysis observed on the reaction involving a trication, [Fe(NN)
3
] , in the presence of
CTAB indicates the importance of hydrophobic interaction between the micelle and hydrophobic ligand
3
+
of [Fe(NN) ] . The micellar catalysis is explained in terms of a pseudophase ion exchange model.
3
Introduction
As far as we know, no attempt has been made so far to study
the influence of micelles on the oxidation of organic sulfides with
The chemical reactivities exhibited by molecules and ions orga-
nized around mesoscopic assemblies such as micelles are often very
3+
cationic oxidants. In recent reports we have established that Fe
3+
and Ru ions undergo efficient electron transfer reactions with
1
–4
different from their reactivities in homogeneous solution. The
use of surfactants instead of organic solvents gains importance
from a green chemistry point of view. Anionic micelles inhibit
the reactions of nonionic organic substrates with anions by
incorporating the substrate and (at least partially) excluding the
10–13
organic sulfur compounds.
It is convenient to use these metal
ions carrying the ligands phenanthroline and alkyl substituted 2,
ꢀ
2
-bipyridine (NN) as the reactants, because both the electrostatic
and hydrophobic interactions can be varied substantially using
3+
14,15
M(NN)
3
complexes.
By changing the structure of the ligands
1,2
anions. On the contrary, cationic micelles incorporate both
nonionic and anionic reagents and increase rates of anionic
nucleophilic reactions. Though a broad range of organic reactions
have been studied in micellar media, little attention has been paid
so far to the use of organic sulfides as substrates except for the
2+
ꢀ
ꢀ
in [Ru(NN)
3
]
(NN = 4,4 -dialkyl substituted 2,2 -bipyridine or
phenyl substituted 1,10-phenanthroline) as well as the quenchers,
we have proved that the photoinduced electron transfer reactions
of Ru(II)-polypyridyl complexes with phenolate ions are highly
influenced by the hydrophobic interactions of the reactants with
5–8
9
recent reports from the laboratories of Bunton and Richardson.
Both the anionic and cationic surfactants inhibit the periodate
16,17
the anionic and cationic surfactants.
Based on the kinetic and spectral studies of the redox re-
−
−
−
(
IO ), peroxomonosulfate (HSO ), and percarbonate (HCO )
3+
4
5
4
action between iron(III)-polypyridyl complexes, [Fe(NN)3] and
oxidation of dialkyl and aryl methyl sulfides, the inhibition being
organic sulfides we have proposed a mechanism involving an
5–9
more pronounced in the presence of cationic surfactants. The
authors explained the enormous inhibition in the presence of
cationic micelle with the postulation that the build-up of positive
charge on the sulfur center of the substrate in the transition state
leads to the electrostatic repulsion from the cationic surface of
the micelle. If the charge development on the transition state has
an enormous effect on the rate of the reaction then the reaction
between a cationic oxidant and sulfide is expected to be favored by
the negatively charged surface provided by the anionic surfactant.
3+
electron transfer (ET) from sulfide to [Fe(NN)
3
]
(Scheme 1).
Several reductants, particularly sulfur compounds, present in
the biological system undergo an electron transfer reaction with
Fe(III). Electron transfer within a protein matrix is critical to
the function of a wide range of biological processes. The work
carried out in a homogeneous aqueous system is far from being a
realistic representation of the complexity of the heterogeneous
a
School of Chemistry, Madurai Kamaraj University, Madurai, 625 021, India
b
Institute of Chemistry, Academia Sinica, Taipei, 115, Taiwan
c
Department of Chemistry, Pondicherry University, Pondicherry, India
†
Electronic supplementary information (ESI) available: Fig. S1–5. See
DOI: 10.1039/b509761d
Abbreviations used: MPS: methyl phenyl sulfide; MPMS: p-
‡
methylphenyl methyl sulfide; CPMS: p-chlorophenyl methyl sulfide; EPS:
ethyl phenyl sulfide; IPPS: isopropyl phenyl sulfide; DES: diethyl sulfide;
DIPS: diisopropyl sulfide; DBS: dibutyl sulfide; DTBS: di-tert-butyl
sulfide.
3+
3
Scheme 1 ET from organic sulfides to [Fe(NN) ] complex in the absence
of micelles.
3
52 | Org. Biomol. Chem., 2006, 4, 352–358
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3
+
nature inherent to biological systems. In order to understand
the efficiency of the redox reaction between Fe(III) and the
thioether moiety present in biochemical systems it is useful to
study the reaction in model systems. In the biological process, both
electrostatic and hydrophobic interactions influence the reactivity
and micelles are very good model systems to understand the role of
3
Table 1 Effects of changing the solvent composition of the [Fe(NN) ]
oxidation of MPS and DES at 298 K
3
−1
3
−1
MPS k
1
× 10 /s
DES k
1
× 10 /s
3
+a
3+b
3+a
3+b
CH
3
OH (%) [Fe(bpy)
3
]
[Fe(phen)
3
]
[Fe(bpy)
3
]
3
[Fe(phen) ]
1
2
30
40
0
0
—
—
2.96 ± 0.35 2.82 ± 0.12
3.46 ± 0.36 6.23 ± 0.23
4.40 ± 0.69 8.14 ± 0.34
5.01 ± 0.57 8.95 ± 0.42
5.73 ± 0.43 10.1 ± 1.12
5.80 ± 0.67 10.5 ± 1.26
5.82 ± 0.70 10.0 ± 0.97
5.85 ± 0.71 10.1 ± 1.20
18–20
these interactions in the biologically important process.
Now
1.61 ± 0.09 1.34 ± 0.08
2.19 ± 0.21 2.25 ± 0.12
2.06 ± 0.17 2.81 ± 0.18
2.21 ± 0.09 5.38 ± 0.38
4.18 ± 0.32 7.57 ± 1.12
6.10 ± 0.66 8.64 ± 1.04
we have investigated the effect of anionic and cationic surfactants
3
+
on the ET reaction between [Fe(NN)
3
]
and organic sulfides
5
6
7
0
0
0
by spectrophotometric techniques to get an idea of the role of
electrostatic and hydrophobic interactions in biological systems.
Interestingly both the surfactants catalyze the ET reaction and
the observed results are analyzed in terms of pseudo phase ion
exchange model of micelles and presented in this report.
80
—
—
a
3+
−4
−3
b
For [Fe(bpy)
3
]
= 1 × 10 M; [MPS] = [DES] = 1.5 × 10 M. For
3+
−
5
−
4
[
Fe(phen)
3
]
= 5 × 10 M;[MPS] = [DES] = 7.5 × 10 M.
3
+
Results and discussion
The rate of ET from sulfide to [Fe(NN)3] is favored with an
increase in the methanol content of the medium and k values at
different solvent compositions (H O–CH OH) are given in Table 1.
1
The structure of the ligands and the abbreviations of the iron(III)
complexes used in the present study are shown in Chart 1.
2
3
Thus, the reaction is facilitated with a decrease in the polarity of
the medium. However, it should be mentioned that in the case of
3
+
3+
oxidation of DES by [Fe(bpy)
attains saturation at 50% CH
CH OH content of the medium has no effect on the rate of the
reaction.
3
]
and [Fe(phen)
3
]
the k value
1
3
OH and further increase in the
3
Kinetics of the reaction in the presence of surfactants
Because of the low solubility of sulfides in aqueous media, all the
reactions in the presence of anionic and cationic surfactants have
been carried out in 1% CH
3
OH–99% H O (v/v). For comparison
2
the reaction in the micellar free medium has also been studied in
a similar solvent system. Bunton and co-workers have also used
similar solvent system (1% CH
3
CN–99% H
2
O (v/v)) to study the
−
−
micellar effect on the IO , and HSO oxidation of organic sulfides.
4
5
As the stability of iron(III) complexes used in the present study is
more at higher acid concentration, all reactions have been carried
out using either 0.5 M HClO
have used H SO to maintain [H ] in the presence of CTAB because
creates solubility problem in the presence of
4
(SDS) or 0.64 M H
2
SO (CTAB). We
4
+
2
4
addition of HClO
CTAB.
4
3
+
Effect of SDS on the [Fe(NN)
3
] oxidation of organic sulfides
3
+
3
Chart 1 Structure of ligands of [Fe(NN) ] , organic sulfides and
surfactants.
In the presence of SDS also the reaction is of total second order,
3
+
first order each in [Fe(NN)3] and sulfide (Fig. 1). The change of
rate constant with the change of [SDS] at constant ionic strength
for different sulfides is shown in Fig. 2 and in the ESI (Fig. S1
and S2). These results show that the reaction is catalyzed by the
Kinetics of the reaction in the absence of surfactants
anionic surfactant. The k value increases with the increase in
1
3
+
The reaction of [Fe(NN)
3
]
with aryl methyl, alkyl phenyl and
[surfactant], but further increase in [surfactant] leads to saturation.
These are expected results as one of the reactants is a cation and
the other is a neutral molecule and hence both the reactants
bind to the anionic surfactant by columbic and hydrophobic
interactions, respectively. Further this reaction proceeds through
an electron transfer mechanism and a positive charge develops
on the S center in the transition state. It is well known that
the neighboring carboxylate anion present in the organic sulfides
stabilizes the sulfide cation radical in the ET reactions of organic
dialkyl sulfides follows simple second order kinetics—first order
each in the iron(III) complex and the sulfide. Based on the
substituent and solvent effects on these redox reactions, and
from the successful application of the Marcus theory of electron
transfer to this reaction, a mechanism (Scheme 1) involving ET
from sulfide to [Fe(NN) ] in the rate determining step has been
postulated. This mechanism is very similar to the one proposed
for the electrochemical and cytochrome P-450 oxidation of organic
3
+
3
10
21,22
23
sulfides.
sulfides thereby favoring the reaction. Thus, in the presence of
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3
+
and [Fe(NN)
3
]
for binding with SDS. Since the concentration of
+
3+
H is very high compared to [Fe(NN) ] , the binding constant of
3
3
+
[
Fe(NN)
absence of H . When SDS is added to [Fe(NN)
max to the tune of 10 nm is observed and a substantial increase
3
] with SDS will be less comparable to the value in the
+
3+
3
]
a shift in the
k
in the absorbance with the increase in [SDS] is noticed (Fig. 3).
These absorbance changes with [SDS] have been used to estimate
3
+
the binding constant of [Fe(NN)
3
]
with SDS and the KFe values
3
+
3+
3+
for [Fe(bpy)
3
] , [Fe(dmbpy)
3
]
and [Fe(phen) ] are 400, 450 and
000 M respectively (details are given in the ESI).
3
−
1
1
Fig. 1 Plot of k
= CTAB].
1
versus [sulfide] in the presence of surfactants [᭹ = SDS,
ꢀ
3+
Fig. 3 The change of absorbance of [Fe(phen)
3
] with [SDS].
3
+
With the reasonable assumption that [Fe(NN)
3
]
ion is asso-
ciated with the anionic micelle and that the substrate distributes
between the aqueous and micellar phases and the reaction occurs
in the aqueous as well as in the micellar pseudo phases, the ET
reaction in the presence of SDS can be explained by Scheme 2.
The subscripts M and W stand for micellar and aqueous phases,
respectively. According to Scheme 2, the Berezin expression for a
second order rate constant for the micellar effect on the reaction
Fig. 2 Variation of rate constant with [SDS] for the reaction of
3+
[
3
Fe(bpy) ] with aryl sulfides.
3
+
ꢀ
between [Fe(NN)
eqn (1).
3
]
and RSR can be given in the form of
25
SDS the sulfate head group can stabilize the positive charge on S in
the transition state. All these favorable aspects will account for the
micellar catalysis observed here in the presence of SDS. But these
results observed in the presence of SDS are in striking contrast
k = k
m
P
s
P
Fe
C
m
V + k
W
(1 − C
m
V)/
[
1+ (P
s
− 1)C V] [1 + ((PFe − 1) C
m
m
V]
(1)
−
−
−
to those observed in the IO , HSO and HCO4 oxidation of
In eqn (1), P
micelles and water for RSR and [Fe(NN)
s
and PFe represent the partition coefficients between
4
5
5–7,9
ꢀ
3+
organic sulfides wherein micellar inhibition is noticed.
On the
3
] , respectively, k and
m
other hand micellar catalysis has been observed by us on the
Cr(VI) oxidation of dialkyl sulfides in the presence of SDS and
k
w
are the rate constants in the micellar and aqueous phases,
respectively, V is the partial molar volume of the micelles and C
is the concentration of the micellized surfactant (C
= ([surfac-
tant] − cmc)/N) where N is the aggregation number. The volume
fraction of the micelle, C V is small at all [surfactant] under the
m
24
the reaction proceeds through ET mechanism.
m
5–8
Bunton and co-workers, from the measurement of binding
constants by spectral techniques, have established that organic
sulfides bind efficiently with anionic as well as cationic surfactants
m
−
1
and the binding constants are in the range of 80–340 M .
The spectral study carried out in our laboratory also provides
similar values for the binding constants of the substrates with the
3
+
anionic and cationic micelles. [Fe(NN) ] , the cationic reactant,
3
will associate strongly with the anionic surfactant and the binding
constants may be high. Since the reaction has been carried out in
the presence of 0.5 M H , there will be competition between H
3+
Scheme 2 ET from organic sulfides to [Fe(NN)3] complex in the
presence of micelle.
+
+
3
54 | Org. Biomol. Chem., 2006, 4, 352–358
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present experimental conditions. The partition coefficients of both
reactants P and PFe are also much larger than unity because of
s
ꢀ
3+
large binding constants of RSR and [Fe(NN)
can be simplified in to eqn (2).
3
] . Hence eqn (1)
k = k
where K
m
K
s
K
Fe
C
m
+ k
w
(1 + K
S
C
m
)(1 + KFe
C
m
)
(2)
s
3
and KFe are binding constants of sulfides and
+
[
Fe(NN)
3
] , respectively. The binding constants and the rate
) are defined by eqn (3–5).
constants in the micelle (k
m
K
s
= V(P
s
− 1)
(3)
(4)
K
Fe = V (PFe − 1)
m
2
k
m
= k /V
(5)
Since in eqn (2) all terms are known except k
m
and KFe, they can
be calculated by the best fit method. The values of k
m
calculated
at different [SDS] are collected in Table 2. It is interesting to point
Fig. 4 Plot of calculated and experimentally observed rate constant values
versus [SDS] for the reaction of [Fe(phen) ] with MPS in the presence of
3
SDS.
3
+
out that the binding constant of [Fe(phen)
3
]
with SDS from the
3+
−
1
spectral technique is 700 M (Fig. 3) which is in fair agreement
with the value obtained from the kinetic data supporting our
arguments for the observed micellar catalysis.
rate enhancement observed in micellar solution is entirely due
to an increase of the reactant concentration in the micellar pseudo
To compare the second order rate constants in water, k , with
w
the second order rate constant in the micellar phase, the volume
of micellar phase must be known. Second order rate constants
1,30,31
phase.
m
2
−1 −1
The data in Table 1 show that when the percentage of methanol is
changed from 20 to 70 the pseudo first order rate constant increases
in the micellar phase with same units, k , M s , are given by
eqn (5), where V is the molar volume of the reactive region at
−
3
−1
3+
−
1
from 1.6 to 6.1 × 10
s
with [Fe(bpy) ] as the oxidant and the
3
the micellar surface and V = 0.37 M on the basis of earlier
−
3
−1
3+
5,6,26,27
increase is from 1.4 to 8.6 × 10
s
in the case of [Fe(phen) ]
3
reports. To compare the rate constants in the micellar phase
m
2
with MPS as the substrate (Table 1). This observation shows that
polarity changes due to the introduction of surfactant is one of
the reasons for the micellar catalysis. For the Stern layer, a polarity
(
k ) with the corresponding values in aqueous phase (k
w
), the
m
values of the ratio (k /k
of k2 for reactions in the micellar Stern layer are close to those
in water for all [Fe(NN)
based on the conclusion that k2 is close to k
w
) are collected in Table 2. The values
2
m
32
3
+
comparable to that of ethanol has been established.
3
] . Thus we explain the observed results
m
Though the observed micellar catalysis has been explained
based on the Scheme 2 it is important to point out the following
interesting observations:
w
. There are many
examples of reactions for which second order rate constants in
micelles are similar to those in water and this generalization can
28,29
(i) Among all aliphatic and aromatic sulfides used, DES shows
enhanced catalysis. One of the explanations offered for the micellar
catalysis is the stabilization of positive charge developed on
the sulfur center in the transition state. With aromatic sulfides,
resonance stabilization of positive charge on the sulfur center
through conjugation with neighboring benzene ring is possible
be used to predict overall rate enhancements.
In order to assess
the success of Scheme 2 to the reaction, we applied eqn (2) to
calculate the rate constant of the reaction at different [SDS]. The
values estimated from eqn (2) and the experimentally observed
values are shown in Fig. 4. The close agreement between the
experimental and calculated values proves the success of the model
to the titled reaction. Therefore, it can be concluded that the
18
which is absent in dialkyl sulfides. Thus, the stabilization of the
positive charge on the sulfur center with the anionic head group of
SDS may be more felt with dialkyl sulfides compared to aromatic
sulfides. Among the dialkyl sulfides chosen for the present study,
DES is less bulky. Thus the steric effect seems to play a role in the
case of sulfides carrying isopropyl and tert-butyl groups. These
two aspects favor the reaction of DES in SDS leading to enormous
micellar catalysis. Interestingly, in the case of DES there is slight
rate retardation at low [SDS]. (ii) The kinetic data given in the
m
Table 2 Second order rate constants (k ) in the micellar pseudo phase
3+
m
2
(
SDS) and k /k
w
values for the [Fe(NN)
3
]
6 5
oxidation of ArSMe, C H SR
2
and R S at 298 K
3+
3+
3+
[
Fe(bpy)
3
]
[Fe(dmbpy)
3
]
3
[Fe(phen) ]
m
2
m
2
m
2
Sulfide
k
m
k /k
w
k
m
k /k
w
k
m
k /k
w
3
+
ESI (Fig. S1 and S2) show that, compared to [Fe(bpy) ] , the
3
C
6
H
5
SMe
p-MeC
p-ClC
0.70
14.6
0.65
0.90
0.28
1.05
1.05
0.42
0.12
0.37
0.64
0.50
0.52
0.54
0.35
1.20
0.22
0.50
0.12
0.48
0.24
0.16
0.15
0.39
0.04
0.17
0.03
0.40
0.17
0.85
0.47
0.96
1.00
0.56
0.47
0.58
1.15
5.80
1.25
1.25
0.33
0.48
0.17
0.90
0.61
0.49
3
+
6
H
H SMe
4
4
SMe
reaction of [Fe(phen)3] with sulfides is favored in SDS. Though
both complexes have a similar size, phen is more hydrophobic
compared to bpy. Thus hydrophobicity favors more efficient
6
C
C
DES
DIPS
DBS
DTBS
6
H
H
5
SEt
SPr
6
5
3
+
3+
1055 0.26
binding of [Fe(phen)
3
]
with SDS compared to [Fe(bpy) ]
3
−
1
1.55
1.07
0.14
0.81
0.37
0.24
and the binding constants are 1000 and 400 M , respectively.
This is supported by the results observed on the solvent effect
(
cf. Table 1).
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3
+
Reaction in the presence of the cationic surfactant, CTAB
to overcome the columbic repulsion between [Fe(NN) ] and the
3
cationic micelle. Similar results have been observed earlier in the
The micellar catalysis for the reaction of a cationic reactant,
photochemical and electrochemical ET reactions of polypyridyl
3
+
[
Fe(NN)
3
] , with organic sulfides in the presence of anionic
16,33
complexes Ru(II) and Co(II), respectively.
The observed micel-
surfactant is expected. When the same reaction is carried out in the
presence of cationic surfactant, if the columbic interaction is the
major factor, micellar inhibition is expected. Interestingly Blasko
3+
lar catalysis indicates that [Fe(NN) ] binds with CTAB. To check
this, we have recorded the absorption spectrum of [Fe(phen)
3
3+
3
] in
the presence of different [CTAB] and the spectra are shown in the
5
,6
9
et al. and Yao and Richardson observed enormous micellar
3+
ESI (Fig. S5). The increase in absorbance of [Fe(phen) ] with an
3
−
−
5
inhibition even for the reaction of anionic oxidant (IO , HSO and
4
increase in [CTAB] indicates binding of oxidant with CTAB and
−
HCO ) with organic sulfides in the presence of cationic surfactant.
−1
4
binding constant is 50 M (see ESI for details).
In order to check whether columbic repulsion (charge effect) is
the major force that decides the effect of micellar system on the
The micellar catalysis observed in the presence of CTAB can be
discussed in terms of Scheme 2. Thus the arguments presented for
the redox reaction in the presence of SDS, can also be extended
to the analysis of results observed in the presence of CTAB. Here
again we applied eqn (2) to calculate the rate constant at different
3
+
titled reaction we have studied the ET reaction of [Fe(NN) ] with
3
organic sulfides in the presence of CTAB. To our surprise micellar
catalysis is observed in the presence of CTAB and the interesting
results are presented in this section. The kinetic data obtained for
[
CTAB] and the values are shown in Fig. 6 along with the
3
+
the reaction of [Fe(NN)
3
]
with several sulfides in the presence
experimentally observed values. The close agreement between the
of CTAB are shown in Fig. 5 and the ESI (Fig. S3 and S4). The
kinetic data provided in Fig. 5, and ESI (Fig. S3 and S4) at various
experimental and calculated values supports the arguments. The
3+
binding constants of [Fe(NN) ] with CTAB calculated from the
3
3
+
[
CTAB] show that the ET reaction between [Fe(NN)
3
]
and
kinetic data are close to the values estimated from the spectral data
organic sulfides is substantially catalyzed by cationic surfactant
also. These results are surprising since one of the reactants carries
triple positive charge and it is expected that the cationic reactant is
expelled from the surface of the cationic micelle. Organic sulfides
32
and similar to the values reported by Pelizzetti et al. for iron(III)
complexes. The binding constant value indicates that a significant
portion of the reaction takes place in the micellar phase. Thus
the analysis of the kinetic data in terms of a pseudo phase model
and the calculated binding constants satisfactorily account for
the observed micellar catalysis of the reaction in the presence of
5–7
bind more efficiently with cationic micelles than anionic micelles.
The inference from this experimental observation is that some
3
+
other interaction is operating between [Fe(NN)
3
]
and the cationic
34
m
2
CTAB. The values of k
m
and k are collected in Table 3. These
micelle which offsets the columbic repulsion between the reactant
and the micelle. From a comparison of the catalytic behavior of
cationic surfactants with that of anionic surfactants, a similarity in
their roles can be realized. In both cases, the addition of surfactant
catalyzes the ET reaction but further increase in [surfactant] leads
to the maximum rate resulting in the saturation kinetics. The
catalysis of the reaction between a cation and a neutral molecule by
the cationic micelle indicates that here also the major part of the re-
action takes place in the Stern layer. These results demonstrate the
importance of hydrophobic interactions in the binding of charged
metal complexes to micelles. In the positively charged CTAB
micellar surface, the hydrophobic interactions of the ligands of
data indicate that in the presence of CTAB also the rate constants
in the micellar phase are similar to those in aqueous phase.
3
+
[
Fe(NN) ] complexes with the micelles are apparently sufficient
3
Fig. 6 Plot of calculated and experimentally observed rate constant values
3+
versus [CTAB] for the reaction of [Fe(phen)
3
]
with MPS in the presence
of CTAB.
−
−
Comparison with [Cr(V)(ehba)
oxidation of organic sulfides
2
] and percarbonate ion (HCO )
4
3
5
9
Recent results observed by us and by Yao and Richardson
−
Fig. 5 The change of rate constant with [CTAB] on the rate constant for
on the [Cr(V)(ehba)
2
]
(ehba = 2-ethyl-2-hydroxy butyric acid)
3+
−
the reaction of [Fe(bpy)
3
]
with alkyl aryl sulfides.
and percarbonate ion, HCO , oxidation of organic sulfides,
4
3
56 | Org. Biomol. Chem., 2006, 4, 352–358
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Table 3 Second order rate constants in the micellar pseudo phase (k
m
)
sulfate (SDS) and cetyltrimethylammonium bromide (CTAB)
were purchased from Aldrich and purified before use. All other
reagents used were of AnalaR grade and the solvents (methanol
m
w
and ratio of rate constants (k /k ) at 298 k in CTAB
2
3
+
3+
m
[
Fe(bpy)
3
]
3
[Fe(phen) ]
40
and water) were purified by known procedures before use.
m
2
Sulfide
k
m
k /k
w
k
m
k /k
w
2
Kinetic measurements
C
6
H
5
SMe
p-MeC
p-ClC
0.90
0.70
0.45
0.50
0.35
0.75
0.72
0.65
0.18
0.47
0.30
0.35
0.29
0.67
0.26
0.86
0.34
0.74
0.52
16.5
0.27
0.38
0.35
1.20
0.12
0.50
0.18
0.22
0.50
0.20
0.19
0.53
0.20
0.10
0.17
0.31
The iron(II)-polypyridyl complexes have molar extinction coeffi-
6
H
4
SMe
4
−1
−1
6
H
4
SMe
cients on the order of 1 × 10 M cm in the wavelength region
10–530 nm, [Fe(bpy) (522 nm), [Fe(phen) (510 nm) and
(529 nm) while the corresponding Fe(III) com-
C
C
DES
DIPS
DBS
DTBS
6
H
H
5
SEt
SPr
2+
2+
5
3
]
3
]
6
5
2
+
[
Fe(dmbpy)
3
]
41
plexes are practically transparent at this wavelength region. The
3+
kinetics of [Fe(NN) ] oxidation of organic sulfides were followed
3
spectrophotometrically under pseudo first order conditions (a
minimum of 15 fold excess of substrate over the oxidant) in the
presence and absence of surfactants at 298 K by measuring the
respectively show that both reactions are inhibited by anionic
micelles, but the reaction of [Cr(V)(ehba)
−
2+
41
2
] with organic sulfides
increase in absorbance of [Fe(NN) ] with time. A sample kinetic
3
is catalyzed by cationic micelles of CTAB. On the other hand
run is shown in Fig. 7. The absorption spectral studies were carried
out on a JASCO model 7800 spectrophotometer. The plots of log
−
the oxidation of organic sulfides with HCO and other peroxo-
4
−
−
oxidants IO and HSO are also enormously inhibited by cationic
(A
constant, k
the details are given in our previous reports.
runs showed that the rate constants were reproducible to within
± 5%. Here, A is the final absorbance and A is the absorbance at
time t. The second order rate constant, k , values were obtained
from the equation, k /[substrate].
= k
∝
− A
t
) versus time were linear and the pseudo first order rate
4
5
micelles. Thus the results observed in the present study seem to be
novel as far as the oxidation of organic sulfides is concerned. It is
interesting to recall that both reactants, Fe(III) and thioethers, are
biologically important. As both reactants contain hydrophobic
alkyl and aryl groups, one of the reasons for the micellar catalysis,
particularly in the presence of cationic micelle, is the significant
contribution of hydrophobic interaction. Comparison of the
present results with the previous reports point out that under
certain conditions, hydrophobic interaction will offset columbic
repulsion and micellar catalysis will be observed even if the system
involves two like charges.
1
values were calculated by least squares method and
10,11
Duplicate kinetic
∝
t
2
2
1
Conclusion
The micellar enhancement observed for the reaction of a triply
3
+
charged cation, [Fe(NN) ] , in the presence of cationic surfactant
3
is novel and this study gives a clue on the importance of
hydrophobic interaction on the ET reaction taking place within
3
+
2+
a protein matrix where Fe /Fe is involved as one of the redox
couples. These results show that the micellar system is an excellent
reaction medium and can be superior to organic co-solvents in the
oxidation of organic sulfides by Fe(III). As the major product of
the reaction is the organic sulfoxide, this method seems to be a
good for the preparation of sulfoxides in aqueous system. Organic
sulfoxides are interesting substrates and find extensive applications
as starting materials for a variety of important organic molecules.
2+
Fig. 7 Increase in the absorbance of [Fe(phen)
3
]
with time for the
3+
reaction of Fe(phen)
3
with methyl phenyl sulfide.
Estimation of binding constants, K
3
+
The binding constants of [Fe(NN)
3
]
with SDS and CTAB
Experimental section
micelles, K, were determined by spectrophotometric titration. The
3
+
change in absorbance, A of the [Fe(NN) ] with the incremental
3
Materials
addition of surfactant was measured at the corresponding kmax
ꢀ
ꢀ
ꢀ
Tris(2,2 -bipyridine)iron(II),
tris(4,4 -dimethyl-2,2 -bipyridine)-
value (600 nm). The binding constants were then evaluated using
6
iron(II) and tris(1,10-phenanthroline)iron(II) were prepared by
the following eqn (6).
36
known procedures. Iron(III) complexes were obtained by the
oxidation of iron(II) complexes with PbO in 1 M H SO . The
(
A
obs − A
w
)/[D
n
] = A
M
K − Aobs
K
(6)
2
2
4
iron(III) complexes were precipitated as perchlorate salts. Aryl
methyl and alkyl phenyl sulfides were synthesized by published
where Aobs is the observed absorbance and subscripts W and M
denote aqueous and micellar media. A sample spectrum with the
37–39
3+
procedures and purity checked by spectral techniques.
All
increase in OD of [Fe(phen) ] with the increase in [SDS] is shown
3
dialkyl sulfides used in the present study were obtained from
Aldrich and used as such. The surfactants sodium dodecyl
in Fig. 3 and for CTAB the spectral changes are shown in the
ESI (Fig. S5). The plot of (Aobs − A )/[D ] versus Aobs is linear
w
n
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and the slope gives the K value. A similar method was used by
Bunton et al. for the estimation of binding constants of organic
sulfides.
Torri, Electroorganic Synthesis, Part 1: Oxidation, VCH, Weinheim,
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