Electron transfer reactions of iron(III)-polypyridyl complexes with organic sulfoxides

Article abstract of DOI:10.1016/S0040-4020(03)00509-X

The redox reactions of four iron(III)-polypyridyl complexes with six aryl methyl sulfoxides have been investigated by spectrophotometric technique. The reaction follows clean second order kinetics and proceeds through rate determining electron transfer (ET) from organic sulfoxides to iron(III). The Marcus cross-reaction relation has been applied to obtain the self exchange rate constant for the ArSOR/ArS^·+(O)R couple as 1.3×10⁷M^-1s^-1. The application of Marcus theory to this ET reaction shows that the contribution of inner sphere reorganization energy is 0.4eV. The rate constant and reaction constant values observed with organic sulfoxides are small compared with organic sulfides towards the same oxidant Fe(NN)₃³⁺.

Full text of DOI:10.1016/S0040-4020(03)00509-X

Tetrahedron 59 (2003) 3613–3619
Electron transfer reactions of iron(III)-polypyridyl complexes
with organic sulfoxides
a
Kulandai John Adaikalasamy, Natarajan Sathiyamoorthy Venkataramanan
b
b,
*
and Seenivasan Rajagopal
a
Department of Chemistry, The American College, Madurai 625002, India
School of Chemistry, Madurai Kamaraj University, Palkalai Nagar, Madurai 625021, India
b
Received 13 December 2002; revised 6 March 2003; accepted 27 March 2003
Abstract—The redox reactions of four iron(III)-polypyridyl complexes with six aryl methyl sulfoxides have been investigated by
spectrophotometric technique. The reaction follows clean second order kinetics and proceeds through rate determining electron transfer (ET)
from organic sulfoxides to iron(III). The Marcus cross-reaction relation has been applied to obtain the self exchange rate constant for the
zþ
7
21 21
ArSOR/ArS (O)R couple as 1.3£10 M
s
sphere reorganization energy is 0.4 eV. The rate constant and reaction constant values observed with organic sulfoxides are small compared
. The application of Marcus theory to this ET reaction shows that the contribution of inner
3þ
with organic sulfides towards the same oxidant Fe(NN)
3
. q 2003 Published by Elsevier Science Ltd.
1. Introduction
to transferrin at low pH is of considerable importance in
gaining an understanding of the physiological activity of the
iron transport protein, transferrin. In order to understand the
mechanism of this ET reaction, Fe(III)!Fe(II), porphyrin
and polypyridyl complexes of Fe(III) have been synthesized
as model compounds and used as electron acceptors from
The important role of sulfur compounds and, in particular,
sulfur centered radicals in biological systems is now well
established. As a consequence much attention has been
paid to the chemistry of these species. The electron transfer
1
,2
2
1–24
(
several metal ions to generate sulfide radical cations has
ET) reaction of biologically important organic sulfides with
biological reductants.
Realizing the importance of
organic sulfides and sulfoxides as reductants in biological
systems we have investigated the ET reaction of iron(III)-
polypyridyl complexes with organic sulfides and this ET
reaction is highly sensitive to the structure of aromatic
sulfide as well as the structure of the ligand of the Fe(III)-
3
–14
been extensively studied.
This sulfide radical cation in
the presence of water produces sulfoxide as one of the major
products. On the other hand little attention has been paid to
the next stage of oxidation i.e. one electron oxidation of
organic sulfoxides to the corresponding sulfoxide radical
cation. The formation of a sulfide radical cation as an
intermediate during sulfide oxidation to sulfoxide has been
3
polypyridyl complexes. The experimentally determined
rate constants for this ET reaction agree well with the rate
constants estimated from the Marcus cross-reaction
relationship. In order to understand the role of organic
sulfoxides as electron donors towards Fe(III) complexes, we
have extended the study of ET reactions to several iron(III)-
polypyridyl complexes with six para-substituted phenyl
methyl sulfoxides by spectrophotometric technique and the
observed results are presented in this article.
1
5–18
recently identified by flash photolysis.
As far as we
know, no report is available on the detection of the sulfoxide
radical cation by flash photolysis. However EPR has been
extensively applied for the detection of sulfoxide radical
1
cations. Recently we have studied the oxidation of organic
sulfoxides with powerful one electron oxidants, Ru(III)-
polypyridyl complexes, and detected the formation of the
9
sulfoxide radical cation by conventional flash photolysis.
2. Results
In biological systems the reduction of Fe(III) to Fe(II) by
9,20
1
biological reductants is a well known phenomenon. For
example the detailed study on the reduction of Fe(III) bound
The structure of the ligands and the abbreviations of
iron(III) complexes used in the present study are shown in
Chart 1.
Keywords: iron(III)-polypyridyl complexes; methylphenyl sulfoxides;
Marcus cross-reaction.
The kinetics of ET from six aryl methyl sulfoxides to four
0
3
3
þ
0
*
Corresponding author. Tel.: þ91-0452-2458246; fax: þ91-0452-
iron(III) complexes, Fe(NN) (NN¼2,2 -bipyridine,4,4 -
0
2459139; e-mail: seenirajan@yahoo.com
dimethyl-2,2 -bipyridine,1,10-phenanthroline, and 4,7-
0040–4020/03/$ - see front matter q 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4020(03)00509-X

3614
K. J. Adaikalasamy et al. / Tetrahedron 59 (2003) 3613–3619
Figure 2. Plot of k
1
vs [sulfoxide] for the oxidation of p-XC
in aqueous methanol (50% v/v) at 298 K.; X¼H;
W¼–Me; P¼–OMe; K¼–Cl; B¼–Br; A¼–NO
6 4
H S(O)Me
3
þ
with Fe(phen)
3
2
.
drawn for the kinetic data obtained for the oxidation
3
þ
p-XC H S(O)Me with Fe(phen) shows satisfactory cor-
6
Chart 1. Structure of the ligands and abbreviations of iron(III) complexes.
3þ
4
3
relation (r¼0.92) with sigma values and the r value is 21.0.
3
þ
and Fe(dmphen)3 , respectively, has been
dimethyl-1,10-phenanthroline), Fe(bpy) , Fe(dmbpy)3
,
However the correlation is improved if log k
2
values are
plotted against Brown–Okamoto’s s values and the
3
3
þ
3þ
þ
sample plot is shown in the Figure 3 and r value is
Fe(phen)
3
þ
20.8. The negative r value indicates that the sulfoxides is
studied spectrophotometrically by taking methyl phenyl
sulfoxide MPSO) as the sample substrate. A sample run is
shown in Figure 1.
þ
partially positively charged in the transition state.
þ
oxidation of sulfoxides with other Fe(NN)
the r values are given in Table 2.
The reaction is of total second order, first order in the
oxidant and in the substrate. The first order dependence in
the oxidant is understood from the linear log(A 2A ) vs
Similar linear log k
2
vs s plots were obtained for the
3
þ
complexes and
3
þ
1
t
time plots. The first order dependence in the substrate is
confirmed from the linear k vs [substrate] plot (Fig. 2).
1
3
. Discussion
The rate of the reaction is little affected by the change in
þ
[
H ] but the increase in the ionic strength of the medium
The Fe(III)-polypyridyl complexes are well established one
2
decreases the rate slightly. The increase in the methanol
content of the H O–CH OH mixture favors the reaction
5,26
electron oxidants.
The observation of sulfoxide radical
2
3
9
cation as a transient species in our recent study with
Ru(III)-polypyridyl complexes prompts us to postulate that
the same sulfoxide cation radical would have been formed
during the one electron oxidation of sulfoxides with Fe(III)-
polypyridyl complexes. Further, the observation of isobestic
point at 580 nm for the reduction of Fe(III) to Fe(II) (Fig. 1)
points out that the conversion proceeds neatly without any
intermediate and confirms that the reaction follows simple
kinetics without involving any complex mechanism. These
experimental observations are strongly in favor of ET from
sulfoxide to Fe(III)-complex in the rate determining step.
The additional support for the ET mechanism comes form
(
Table 1). The study of substituent effect with several para-
substituted phenyl methyl sulfoxides shows that the rate of
the reaction is accelerated by electron donating groups and
retarded by electron withdrawing substituents present in the
phenyl ring of PhS(O)Me (Table 2). The Hammett plot
þ
the substituent effect study. The r observed here (20.8) is
just one fourth of the r value (23.2) observed for the
3
þ
oxidation of aryl methyl sulfides. An ET
þ
mechanism involving rate determining ET from sulfide to
Fe(NN)
3
3
þ
low r value obtained with aryl methyl sulfoxides is
3
Fe(NN) has been proposed in our previous study. This
3
þ
understandable as sulfoxides are less powerful nucleophiles
compared to sulfides, because of polar S–O bond.
To account for the above spectral and kinetic results a
mechanism shown in Scheme 1, (Eqs. (1)–(4)) similar to the
2
3
þ
formed from the
Figure 1. Increase in the absorbance of Fe(phen)
3
3
þ
24
23
one postulated, in our previous report, for the reaction of
3
reaction of Fe(phen) (2£10 M) with MPSO (2£10 M) in aqueous
3
þ
methanol (50% v/v) at 298 K at a time interval of 50 s.
Fe(NN) with organic sulfides, has been proposed.
3

K. J. Adaikalasamy et al. / Tetrahedron 59 (2003) 3613–3619
3615
þ
3þ
3
oxidation of C
Table 1. Effect of changing the solvent composition, [H ] and ionic strength on the Fe(NN)
6 5
H S(O)Me at 298 K
4
21 a
)
þ
4
21 b
)
4
21 a
)
%
CH
3
OH
k
1
£10 (s
[H ] (M)
k
1
£10 (s
m (M)
k
1
£10 (s
3
þ
3þ
3þ
3þ
3þ
3þ
Fe(bpy)
3
Fe(phen)
3
Fe(bpy)
3
Fe(phen)
3
Fe(bpy)
3
Fe(phen)
3
1
2
4
5
6
8
0
3.8^0.03
4.2^0.02
6.6^0.04
7.4^0.04
10.6^0.17
13.1^0.23
3.5^0.06
5.0^0.08
7.0^0.01
8.1^0.16
9.9^0.18
14.2^0.17
0.5
0.6
0.7
0.8
0.9
–
3.3^0.05
3.0^0.09
2.7^0.03
2.7^0.03
2.6^0.04
–
4.0^0.02
3.9^0.03
3.8^0.03
3.8^0.03
3.7^0.02
–
0.5
0.6
0.7
0.8
0.9
1.0
5.4^0.04
5.2^0.08
4.3^0.06
3.9^0.06
3.4^0.05
3.3^0.05
6.0^0.16
5.3^0.04
4.8^0.03
4.4^0.03
4.2^0.03
4.0^0.02
0
0
0
0
0
a
23
General conditions: [MPSO]¼2£10 M; [Fe(NN)
3þ
3
24
]¼1£10 M; solvent¼50% (v/v) aqueous methanol [H ]¼0.5 M.
þ
b
23
3þ
24
]¼1£10 M; solvent¼50% (v/v) aqueous methanol [m]¼1.0 M.
General conditions: [MPSO]¼2£10 M; [Fe(NN)
3
2
, M
1
21
s
3þ
Table 2. Second order rate constant (10k
2
) values for the oxidation of p-XC
6
H
4
S(O)Me by Fe(NN)
3
in 50% (v/v) aqueous methanol at 298 K
3
3
þ
3þ
3þ
3þ
3
p-XC
6
H
4
S(O)Me (X¼)
Eox V(SCE)
Fe(phen)
Obs
Fe(bpy)
3
Fe(dmphen)
3
Fe(dmbpy)
Cal
Obs
Cal
Obs
Cal
Obs
Cal
H
1.73
1.62
1.51
1.81
1.81
–
3.0^0.08
6.0^0.18
9.8^0.14
1.4^0.02
1.0^0.01
0.7^0.01
20.80^0.23
0.95
5.0
42.0
361
1.3
1.3
–
2.7^0.02
4.2^0.10
8.6^0.11
1.3^0.09
1.2^0.08
0.7^0.01
20.80^0.19
0.97
4.1
34.9
297
1.1
1.1
–
0.1^0.01
0.48
4.09
34.9
0.12
0.12
–
0.08^0.00
0.41
3.47
29.5
0.10
0.10
–
Me
OMe
Cl
0.3^0.02
0.5^0.03
0.1^0.01
0.1^0.01
0.08^0.0
20.58^0.10
0.97
0.28^0.04
0.36^0.01
0.07^0.01
0.09^0.01
0.06^0.01
20.56^0.09
0.92
Br
NO
2
þ
r
r
3
þ]¼1£10 M.
24
[
Fe(NN)
3
3
þ
þz
Generally the outer sphere oxidants, Fe(NN) , undergo
the radical cation ArS (O)Me. Consequently, the sulfoxide
radical cation may undergo other types of reactions, either
3
reaction with electron donors by second order kinetics with
rate limiting ET to generate an organic radical ion. In the
3
þ
The formation of sulfone as
fragmentation or reaction with Fe(NN)
27–30
competition with back ET.
or solvent in
3
3
3
þ
for back ET is diminished resulting in the long lifetime of
presence of strong oxidants like Fe(NN) , the driving force
the major product helps us to conclude that the major
portion of sulfoxide radical cation is consumed by the
solvent, water, though fragmentation and back ET may be
competing processes. Further, a good agreement between
the experimentally observed second-order rate constants
and the values calculated by Marcus theory also supports the
proposed mechanism (vide infra).
3.1. Application of Marcus cross-reaction relation
3
1
Marcus introduced the concept that the rate constant for
þ
ET from the electron donor (S) to the acceptor (M )
(
reactivity of the redox couples involved, the self-exchange
Eq. (5)), k , could be calculated from the intrinsic
12
þ
force and the thermodynamics for ET between the couples.
þ
rate constants, k11 (M !M) and k (S!S ) at zero driving
22
k12
þ
þ
S þ M ! S þ M
Here M represents Fe(NN) , the oxidant, and S represents
ð5Þ
Figure 3. Hammett plot for the oxidation of p-XC
6
H
4
S(O)Me by
in aqueous methanol (50% v/v) at 298 K; 1¼H; 2¼–Me;
¼–OMe; 4¼–Cl; 5¼–Br; 6¼–NO
3þ
Fe(phen)
3
þ
ArS(O)Me, the substrate undergoing oxidation.
3þ
3
3
2
.
The thermodynamic data for the reaction can be estimated
þ
from the redox potentials of M and S when the couples are
stable enough that k12 can be experimentally measured. The
simple form of Marcus’s cross-reaction relation is given in
Eqs. (6a) and (6b)
1
=2
k12 ¼ ðk k K f Þ
ð6aÞ
ð6bÞ
11
22 12 12
2
2
ln f12 ¼ ½lnðK Þꢀ =½ð4 ln k k Þ=Z ꢀ
12
11 22
Scheme 1.
The self-exchange rate constants, k11 and k22 and the

3
616
K. J. Adaikalasamy et al. / Tetrahedron 59 (2003) 3613–3619
) values and activation parameters for the oxidation of p-XC S(O)Me by Fe(phen)
3
3
þ
Table 3. Second order rate constant (k
at three different temperatures
2
6
H
4
in aqueous methanol (50% v/v)
2
(M
1
21
s
–
21
–
21
2DS (J K mol
21
–
21
No.
p-XC
6
H
4
S(O)Me (X¼)
10k
2
)
DH (kJ mol
)
)
DG (kJ mol
)
2
98 K
308 K
318 K
1
2
3
4
5
H
3.0^0.08
6.0^0.18
1.4^0.02
1.0^0.00
0.7^0.01
6.2^0.20
10.0^0.31
2.5^0.07
2.2^0.07
1.2^0.02
11.0^0.28
22.0^0.41
6.0^0.15
4.1^0.10
2.0^0.06
48.5^1.2
48.5^3.6
57.3^3.8
54.8^1.4
38.9^0.1
91.2^0.9
86.6^2.5
69.0^2.7
79.0^1.0
137^1.0
76.6^0.9
74.9^2.7
79.1^2.9
79.1^0.3
80.7^0.1
Me
Cl
Br
NO
2
3
þ/2þ
equilibrium constant K12 are the principal parameters
determining k . The term Z is the pre-exponential factor
exchange rates observed with [M(NN)3]
(M¼Fe, Ru
2
7
and Os) complexes is accounted for by Sutin et al. and
others. On taking logarithms Eq. (6a) becomes Eq. (8)
1
2
1
1
21 21
which is often taken as 10
M
s
(Table 3).
The more general form of Eqs. (6a) and (6b) including work
terms has successfully been applied to a variety of
inorganic, organic, organometallic and biochemical
log k12 2 0:5 log k11 2 0:5 log f12
¼ 0:5 log k22 þ 0:5 log K12
In Eqs. (6a) and (6b), the value of K12 is calculated from the
ð8Þ
2
7–35
reactions.
3
3
þ
2þ
redox potentials of the couples Fe(NN) /Fe(NN)3 and
þ
Eqs. (6a) and (6b) were derived assuming that ET reactions
are adiabatic which requires the pre-exponential factor in
.S O/.SO using Eqs. (9) and (10).
2
DG#/RT
the expression k ¼Z e
to be a constant and the rate
0
0
1
2
DG ¼ nFðE
þ
2 E 3þ
2þ
Þ
ð9Þ
S O=SO
Fe =Fe
constant to be controlled by the activation barrier. The
fundamental assumption in deriving Eqs. (6a) and (6b) was
that averaging the activation barriers for self-exchange
reactions produced the proper activation barrier for the
0
K12 ¼ expð2DG =RTÞ
ð10Þ
þ
The value of k22 for .S O/.SO couple can be estimated
from an iterative procedure i.e. a value of k22 is guessed and
plugged into Eq. (8) to calculate f. With the calculated value
of log f, a plot of left hand side of Eq. (8) vs log K12 is made
and from mean least squares calculations, the intercept and
slope of such plots are determined. From the intercept a new
estimated value of k22 is obtained and this is then used to
calculate a new log f. The entire iterative process was
repeated until successive estimates of k22 differed by less
3
0
cross-reaction. Recently, it has been established that the
3
6
great majority of ET reactions are non-adiabatic. Non-
adiabatic reaction rate constants are controlled by activation
barriers as well as by the widely varying pre-exponential
factors (Eqs. (7a) and (7b))
2
2s
–
k12 ¼ lHDAl e n exp½2DG =k Tꢀ
ð7aÞ
n
B
where
7
21 21
than 10%. The final result gives k ¼1.3£10 M
s
if we
2
2
–
0
2
DG ¼ ðDG þ lÞ =4l
ð7bÞ
use all four Fe(III) complexes as oxidants and the plot is
shown in Figure 4.
2
In Eqs. (7a) and (7b) lH l is the ET orbital overlap
integral squared and s is the vibronic coupling constant
which is the ratio of hnv to the internal vibrational
DA
This seems to be the first report for the self-exchange rate
constant for the sulfoxide/sulfoxide radical cation couple. It
is pertinent to point out that similar treatment of the
3
0
component of l, l where l is the reorganization energy.
v
0
–
The terms DG and DG are the standard free energy
change and free energy of activation, respectively. Eq. (7b)
is called the Marcus equation. In addition to the vertical
reorganization energy (l) that controls the rate constant for
adiabatic ET, the size of the electronic interaction between
the reactants at the ET transition state (also called the
electronic coupling matrix element, HDA) and the energy
corresponding to the inherent barrier-crossing frequency
3þ
oxidation of organic sulfides with Fe(NN)3 has given
the same value of k , the self exchange rate constant for the
2
2
sulfide/sulfide radical cation couple. After calculating the
value of k , it is then used to get the rate constant for ET,
2
2
3þ
collected in Table 2. These calculated values are in fair
k , from organic sulfoxides to Fe(NN) and the values are
1
2
3
(
adiabatic reactions. Despite the predictions of modern
nv) are important in determining the rate constant for non-
3
6
30,37
theory by Bixon and Jortner, Nelsen et al.
have
applied Eqs. (6a) and (6b) successfully for the calculation of
rate constants, k , for 141 reactions having couples of a
1
2
wide range of structural types. Since Eqs. (6a) and (6b) are
successful in accounting for ET reactions for couples that
have a wide range of structural types including heteroatom
substituted aromatics we have attempted to apply this
equation to the titled reaction to get the self-exchange rate of
S O/.SO couple. The value of self-exchange rate of
þ
Fe(NN) /Fe(NN) couple is known from previous studies
Figure 4. Plot of log k1220.5 (log k11þlog f) vs log K12: X¼sulfoxides vs
Fe(dmphen); W¼sulfoxides vs Fe(dmbpy); K¼sulfoxides vs Fe(bpy):A¼
sulfoxides vs Fe(phen); B¼sulfide vs Fe(phen); P¼Sulfide vs Fe(bpy);
X¼sulfide vs Fe(dmbpy).
.
3
þ
2þ
3
3
2
7
8
21 21
of Sutin et al. as 3.3£10 M
s
. The high self-

K. J. Adaikalasamy et al. / Tetrahedron 59 (2003) 3613–3619
3617
agreement with the experimental values, supporting our
arguments presented above. However, a difference of 1–2
orders of magnitude is observed between the experimental
and calculated values for sulfoxides carrying an electron
donating group on the phenyl ring of ArS(O)Me. It is
important to point out that solvation of the reactants and
transition state play an important role in deciding the
reactivity and course of the reactions particularly when
polar solvents are used in reactions involving charge
development. The presence of electron donating and
withdrawing groups leads to a different solvation energy
1
Figure 5. H NMR spectrum of the reaction solution.
1
3.2. Identity of the product of the reaction using H NMR
0
38,39
DH . Recently it has been established
z
that p-OCH -
3
solv
21
3þ
H NMR to determine the identity of the product. Recently
C H S involves a solvation energy of 42 kJ mol and
z
p-NO C H S 296 kJ mol in CH CN. Thus in order to
The oxidation of MPSO with Fe(bpy) was monitored by
3
6
4
21
1
2
6
5
3
4
2
1
account for the dynamics of ET reactions it is essential to
include the solvation energy in the calculation of driving
force of the reaction. Thus, we presume that if the solvation
energy is also included, the agreement between the
experimental and calculated values will be better. In the
case of p-OCH C H S(O)CH if solvation energy of
Pecoraro et al. used H NMR to identify products in the
vanadium haloperoxidase model complexes oxidation of
organic sulfides. The aromatic protons from MPSO appear
as multiplets centered at 7.58 and 7.68 ppm. With two
3
3
þ
equivalents of Fe(bpy) per equivalent of MPSO NMR
3
6
4
3
spectrum was recorded after 1 h. The spectrum is shown in
Figure 5. After 1 h just one half of the sulfoxide appears to
be oxidized to sulfone. The protons from sulfone appear as
multiplets centered at 8 and at 8.1 ppm. Similar results were
reported by Pecoraro et al. The formation of proton signals
for the sulfone confirms that the sulfone is the major product
of the reaction.
1
6.7 kJ M2 is included in the calculation of rate constant
1
using Marcus equation the agreement between the experi-
mental and calculated value becomes good. The solvation
energies of benzene thiols containing p-Cl and p-Br
substituents do not vary significantly from the parent
3
8,39
thiols.
After establishing successful application of
Marcus cross-reaction relation of the ET reaction between
3
3
þ
Eq. (11) to calculate the value of the reorganization energy,
3.3. Comparison with Fe(NN) 33 1 oxidation of organic
sulfides
Fe(NN) and organic sulfides and sulfoxides we can apply
l, of the reaction.
To compare the nucleophilicity of organic sulfides and
sulfoxides it is useful to compare the kinetic data observed
for the ET reactions of organic sulfides and sulfoxides
0
2
2
ðDG þlÞ =4lRT
k12 ¼ v e
ð11Þ
n
The value of nn is taken as 1.0£10 s21 and DG as
1
1
0
3þ
towards the one electron oxidant Fe(NN)3 . If we compare
the kinetic data observed in the present study with data
3
3
þ
Substitution of the values of k , n and DG , gives the
0
.96 eV for the reaction of Fe(bpy) with PhS(O)CH .
3
0
3
1
2
n
obtained for organic sulfides in our previous report we
value of l as 1.4 eV. This value is the sum of two terms, lo
and l , the outer and inner-sphere reorganization energy.
We have estimated the value of l using Eq. (12) as 1.0 eV.
o
realize that the second order rate constants measured for the
oxidation of organic sulfides are larger by one to three
orders of magnitude compared to the k values observed for
i
2
organic sulfoxides. To know the range of k values the data
2
2
l ¼ ½ðDeÞ =4p1 ꢀ ð1=2r þ 1=2r 2 1=rÞð1=D
obtained for the parent, p-Cl, p-Me and p-OMe phenyl
methyl sulfides and sulfoxides are given in the order 1.4,
o
o
Fe
S
op
1
21
0
.27; 1.01, 0.13; 16.9, 0.42 and 398, 0.86 M2
s
,
2
1=DSÞ
ð12Þ
In Eq. (12) De, Dop and D are the number of electrons
respectively. Further, the reaction constant value obtained
for the oxidation of organic sulfoxides is small compared to
the value calculated with organic sulfides. These rate
constant and reaction constant data prompt us to conclude
that not only the reactivity, but the selectivity is also less
with organic sulfoxides compared to organic sulfides. From
this observation, we presume that the transition state is
similar in both oxidation reactions. These results are in
contrary to our recent observation with oxo(salen)manga-
S
transferred, the optical and static dielectric constant
respectively and 1 is the permittivity of vacuum. The
o
3
3
þ
respectively, and the sum of the radii is r. The values of
radii of Fe(bpy) and MPSO are represented by r and rS,
Fe
4
0
Dop and D are available in the literature and the radii of
S
˚
Fe(bpy) and MPSO are 7 and 4 A, respectively.
3
3
þ
27,28
4
3
From this analysis we understand that the contribution from
the inner-sphere reorganization energy, l to the overall l
nese(V) oxidation of organic sulfides and sulfoxides,
wherein we have postulated an early transition state for the
oxidation of organic sulfides and a late transition state for
sulfoxides based on the r values. We wish to recall that the
oxo(salen)manganese(V) oxidation of organic sulfides and
sulfoxides proceeds through a common mechanism involv-
ing an electrophilic attack of oxidant at the sulfur center of
the substrate. Thus the reactions in the present study
proceeding through an ET mechanism fail to obey the
reactivity–selectivity principle (RSP). To have selectivity
the reactants should orient suitably with definite structure in
i
value is 0.4 eV. It is well known that l value for reactions
i
3
3
þ
negligible. Thus we understand that during ET from
involving M(bpy)
2
complexes (M¼Fe, Ru and Os) is
7
3
3
þ
sulfoxide is significant leading to appreciable l value. This
ArS(O)Me to Fe(NN) the change in the bond length in
i
is understandable because during ET the S–O bond length
may vary substantially leading to appreciable l value. In
recent years attempts have been made to evaluate l value
i
i
4
1
theoretically and for benzyl system the value is ,0.6 eV.

3618
K. J. Adaikalasamy et al. / Tetrahedron 59 (2003) 3613–3619
the transition state. The implicit assumption in the outer-
sphere ET reactions is that little interaction exists between
the electron donor and acceptor in the transition state i.e.
there is no definite structure for the transition state. As the
titled reaction proceeds through outer sphere ET mechanism
the failure of RSP is understandable.
different ratios of oxidant and substrate concentrations.
These studies indicated that one mole of sulfoxide was
consumed per two moles of Fe(NN)3 in accordance with
Eq. (13).
4
4
3þ
2þ
2
FeðNNÞ 33 þ þ ArSðOÞR þ H O ! 2FeðNNÞ
2
3
þ ArSO R þ 2Hþ
ð13Þ
2
4. Experimental
4
.1. Materials
4
.4. Product analysis
0
0
0
The ligands, 2,2 -bipyridine,4,4 -dimethyl-2,2 -bipyri-
dine,1,10-phenanthroline and 4,7-dimethyl-1,10-phenan-
throline and iron(II)-sulfate were obtained from Aldrich
The inorganic product under kinetic conditions was
confirmed to be Fe(NN)3 as the rate of the reaction was
2þ
followed by measuring the increase in the concentration of
0
0
and used as such. Tris(2,2 -bipyridine)iron(II), tris(4,4 -
dimethyl-2,2 -bipyridine)iron(II), tris(1,10-phenanthrol-
0
ine)iron(II) and tris(4,7-dimethyl-1,10-phenanthroline)-
2þ
Fe(NN)3 spectrophotometrically. To identify the organic
product, the solution after the completion of the reaction
was extracted with chloroform and dried with anhydrous
sodium sulfate. After evaporating the solvent, the sample
was subjected to GC analysis and the product was found to
be the corresponding sulfone. This was also confirmed from
3
,25,26
iron(II) were prepared by known procedures.
iron(III) complexes were obtained by the oxidation of
iron(II) complexes with either lead dioxide in 1 M H SO or
ceric ammonium nitrate. The iron(III) complexes were
precipitated as perchlorate salts. The purity of the iron(II)
and iron(III) complexes were checked by IR and UV–Vis
spectra and compared with the literature data. Stock
solutions of the iron(III) complexes were made up in
concentrated H SO or HClO solutions. Such solutions
The
2
4
1
the analysis of the product with H NMR (vide supra).
Acknowledgements
2
4
4
were diluted with aqueous methanol to the desired acid
strength immediately before each kinetic run. MPSO, and
substituted phenyl methyl sulfoxides were synthesized by
established procedures and purity checked by GC. All
other reagents used were of AnalaR grade and solvents
The authors thank Professor C. Srinivasan, Emeritus
Scientist, Department of Materials Science, Madurai
Kamaraj University, for his constant encouragement, and
K. J. A. thanks University Grants Commission, New Delhi
for providing FIP study leave and the Principal and the
Management of The American College, Madurai, for their
support.
4
3,45
(
before use.
methanol and water) were purified by known procedures
4
6
4
.2. Kinetic measurement
The iron(II)-polypyridyl complexes have molar extinction
4
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