Electron transfer reactions of organic sulfoxides with photochemically generated ruthenium(III)-polypyridyl complexes

Article abstract of DOI:10.1016/j.tet.2003.12.009

The electron transfer (ET) reaction of aryl methyl sulfoxides with ruthenium(III)-polypyridine complexes is sensitive to the change of substituent in the aryl moiety of ArS(O)CH₃ and ligand of Ru(III) complex. The detection of sulfoxide radical cation as the transient by conventional flash photolysis confirms ET in the rate-controlling step. The successful application of Marcus cross relation of ET leads to the evaluation of self-exchange rate constant of ArS⁺(O)CH₃/ArS(O)CH₃ as 4.0×10⁵M^-1s^-1 similar to organic sulfides. Comparison with the reactivity of iron(III)-polypyridyl complexes points out that both reactivity and ρ values are higher with Ru(III) complexes.

Full text of DOI:10.1016/j.tet.2003.12.009

Tetrahedron 60 (2004) 1921–1929
Electron transfer reactions of organic sulfoxides with
photochemically generated ruthenium(III)–
polypyridyl complexes
a
Muniyandi Ganesan, Veluchamy Kamaraj Sivasubramanian, Seenivasan Rajagopal
a
b,*
b,
*
and Ramasamy Ramaraj
a
Department of Chemistry, Vivekananda College, Thiruvedagam West, Madurai 625 217, India
School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
b
Received 22 August 2003; revised 7 November 2003; accepted 4 December 2003
Abstract—The electron transfer (ET) reaction of aryl methyl sulfoxides with ruthenium(III)–polypyridine complexes is sensitive to the
change of substituent in the aryl moiety of ArS(O)CH and ligand of Ru(III) complex. The detection of sulfoxide radical cation as the
transient by conventional flash photolysis confirms ET in the rate-controlling step. The successful application of Marcus cross relation of ET
3
þ
5
21 21
leads to the evaluation of self-exchange rate constant of ArS (O)CH /ArS(O)CH
3
3
as 4.0£10 M
s
similar to organic sulfides. Comparison
with the reactivity of iron(III)–polypyridyl complexes points out that both reactivity and r values are higher with Ru(III) complexes.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
The sulfoxide functional group is involved in different
– 3
1
biological activities
considerable importance as synthetic intermediates and
and chiral sulfoxides are of
4
ð2Þ
5
ligands for asymmetric synthesis. Organic sulfoxides are
biphilic substrates and they may act as either electrophiles
or nucleophiles depending on the nature of oxidant.
The presence of polarized S–O bond and weakening of the
S–O bond by protonation make sulfoxide a weaker
nucleophile compared to organic sulfides particularly at
1
,2,5
The
X-ray spectroscopy and MO calculations indicate that the
sulfoxide may be represented as a resonance hybrid of two
cannonical forms, Eq. 1.
þ
high [H ]. When these substrates, organic sulfides and
1
,3
sulfoxides, undergo redox reactions with electrophiles or
electron transfer (ET) reactions with electron acceptors, it is
expected that sulfoxides are less reactive and reaction
constant (r) values are always smaller with aryl methyl
1
,2,6–10
sulfoxides compared to the corresponding sulfides.
To get an idea of the reactivity of these biologically
important substrates, organic sulfides and sulfoxides, we
have initiated a systematic study on the kinetics and
mechanism of oxidation of these substrates with
ð1Þ
1
1–15
several electrophilic oxo(salen)metal complexes
6,17
iron(III)–polypyridyl complexes.
and
1
Both organic sulfides and sulfoxides behave as nucleophiles
1
˚
The S–O bond distance of 1.492 A is lengthened in the
presence of proton and in this case single bond becomes
more important, reducing the bond order. The protonated
1,12
14,15
towards oxo(salen)manganese(V)
and oxo(salen)iron
complexes. With oxo(salen)iron complexes the reactivity
and reaction constant values are smaller for sulfoxides
compared to sulfides. On the other hand, with oxo(salen)-
manganese complexes, though the reactivity of sulfoxides is
slightly less, interestingly the r value is higher with
sulfoxides compared to sulfides. This is explained in terms
of the Hammond postulate that sulfides proceed via the
3
sulfoxide may be represented as given in Eq. 2.
Keywords: Electron transfer reactions; Organic sulfoxides;
Ruthenium(III)–polypyridyl complexes.
*
Corresponding authors. Tel.: þ91-452-2458246; fax: þ91-452-2459139;
e-mail address: seenirajan@yahoo.com
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.12.009

1922
M. Ganesan et al. / Tetrahedron 60 (2004) 1921–1929
3
þ
complexes (I–V) have been studied by
reaction with an early transition state and sulfoxides with a
late transition state. Organic sulfoxides behave differently
with oxo(salen)chromium(V) complexes and they act as a
donor ligand and bind with metal center with appreciable
[Ru(NN)3]
spectrophotometric technique under pseudo first-order
condition by monitoring the increase in the absorbance of
Ru(II) complex formed during the course of reaction. A
sample run to show the change in the O.D. with time is given
in Figure 1.
2
1 13b
binding constant (K¼100 M ). These interesting obser-
7
–19
vations obtained in this laboratory and by other workers
led us to conclude that the presence of polarized S–O bond
in sulfoxides makes these substrates behave differently from
sulfides depending on the nature of the oxidant. Recently,
we have reported our results on the ET reactions of organic
sulfides with powerful one electron oxidants, Ru(III)–
polypyridyl complexes, generated photochemically from
2
0
Ru(II) complexes using molecular oxygen as oxidant.
Because of the unique behavior of organic sulfoxides with
the different metal ions, we wanted to check the reactivity of
organic sulfoxides towards these Ru(III) complexes (see
Chart 1 for the structure of Ru(III) complexes (I–V)). The
study of the reactivity of Ru(III) ion with organic sulfoxides
is significant because several Ru(III)–sulfoxide complexes
2
1
have recently been tried as anti-tumor agents. Though
organic sulfoxides have less reactivity (reactivity differs by
more than two orders) as electron donors towards Ru(III)
complexes (I–V), a comparable r value is obtained
compared to organic sulfides. The ET nature of the titled
reaction is confirmed from the transient absorption spectrum
of sulfoxide radical cation recorded using conventional flash
photolysis technique. Since the self-exchange rates for the
Figure 1. The absorption spectral changes for the reaction between
[Ru(bpy)3] and methyl phenyl sulfoxide at 20 s time intervals.
3
þ
3
þ
2þ
redox couple Ru /Ru and the redox potentials of the
reactants used in the titled reaction are available we utilized
this ET reaction to estimate the self-exchange rate of
2
þ
reaction between [Ru(bpy)3] and methyl phenyl sulfoxide
The linear log OD of [Ru(bpy) ] , formed from the
3
þ
3þ
(MPSO), versus time plot and constant k value at different
ArS (O)Me/ArS(O)Me. The interesting spectral, kinetic
and mechanistic details of the ET reaction of organic
sulfoxides with five Ru(III)–polypyridyl complexes (I–V)
are presented in this article.
1
3
þ
concentration of [Ru(NN)3] indicate that the reaction is
first-order in Ru(III). The kinetic data observed at different
[sulfoxide] but at constant [Ru(III)] of complexes (I–V)
show that the reaction is first-order in sulfoxide also (Fig. 2).
3
]
þ
.
Chart 1. Structures of NN in [Ru(NN)
3
Figure 2. Plots of k
sulfoxides by [Ru(dtbpy)
1
versus [sulfoxide] for the oxidation of aryl methyl
3
þ
in aqueous CH
numbers refer to the sulfoxides given in Table 2.
3
]
3
CN (50% v/v) at 298 K. The
2
. Results and discussion
3
þ
The structure of ligands and abbreviation of [Ru(NN)3]
complexes used in the present study are shown in Chart 1.
In order to understand the influence of other parameters on
the rate of the reaction, the kinetics of the reaction has been
followed at different solvent composition and the results are
collected in Table 1.
The kinetics of oxidation of six organic sulfoxides with five

M. Ganesan et al. / Tetrahedron 60 (2004) 1921–1929
Table 1. Effect of varying solvent composition on the reaction of
1923
3þ
[
Ru(NN)
3
]
with methyl phenyl sulfoxide (MPSO) at 298 K
Solvent composition,
CH CN–H O (v/v)
MPSO
3
2
3
þ
21
3þ
3
[Ru(phen) ] , k
[
Ru(bpy)
21
3
]
s
, k
)
2
2
21 21
(
M
(M
s
)
8
7
6
5
4
2
0:20
0:30
0:40
0:50
0:60
0:80
1.26
1.70
1.94
2.37
3.71
5.63
1.37
r
1.74
2.00
2.45
3.49
4.92
The increase in water content in the CH CN–water
3
mixture favors the reaction which may be attributed
to charge development on the substrate in the transition
state. The reaction is highly sensitive to the change of
substituent in the phenyl ring of X–C H S(O)Me and the
6
4
r
r
r
observed data are collected in Table 2. The data given in
Table 2 show that introduction of electron-donating
groups in the phenyl ring of C H S(O)Me facilitates the
6
5
reaction and electron-withdrawing groups inhibit the
2
2
reaction. Application of the Hammett equation for
the analysis of kinetic data points out that a good correlation
is obtained between log k and Hammett s values (Fig. 3)
2
and the reaction constant values (r) are also collected in
Table 2.
Though there is a substantial variation in the reactivity with
the change of Ru(III) complex, surprisingly the r value
remains almost constant (Table 2). These results support the
postulation of ET in the rate-determining step. To check the
importance of resonance interaction of the substitutent with
the reaction site we tried the correlation of log k with
2
þ
23
Brown–Okamoto’s s values but there is no improve-
þ
worthwhile to recall that better correlation with s and low
ment in the correlation (r ¼20.65, r¼20.941). It is
þ
value have been taken as kinetic evidence for the
þ
operation of ET mechanism in the oxidation of organic
r
2
4
sulfoxides. As Ru(III) is a well established one electron
oxidant and the formation of sulfoxide radical cation is
confirmed from flash photolysis study (vide infra), the
þ
postulation of better correlation of log k with s is not a
2
necessary condition for the operation of an ET
mechanism.
r
3
þ
The ET reaction of [Ru(NN)3] with organic sulfoxides
has been studied in four different temperatures and the
‡
‡
enthalpy (DH ) and entropy (DS ) of activation evaluated
from kinetic data. The rate constants obtained at four
temperatures and the thermodynamic data are collected in
‡
‡
Table 3. The values of DG calculated from these DH and
‡
‡
‡
‡
DS values using the equation DG ¼DH 2TDS have also
‡
‡
been given in Table 2. The values of DH and DS collected
in Table 3 show that the change in DH with the change of
‡
substituent in the aryl moiety of ArS(O)Me is mainly
responsible for the change in the reactivity because there is
‡
little change in DS values, i.e., the reaction is enthalpy
‡
controlled. Further, the values of DG collected in Table 2
are in accordance with the reactivity of different sulfoxides
conforming the operation of an ET mechanism for all
substrates.

1924
M. Ganesan et al. / Tetrahedron 60 (2004) 1921–1929
experiment is designed as follows: the reaction mixture
2
þ
consisting of [Ru(phen)3] and methyl phenyl sulfoxide
MPSO), taken in aqueous CH CN (50% v/v) and 4.5 M
H SO , is purged with molecular oxygen for 20 min.
(
3
2
4
Then the reaction mixture is irradiated with flash of light
and the absorption spectrum of the reaction solution is
monitored at different time intervals. The spectra shown in
Figure 4 show two strong absorptions at 300–350 and
500–550 nm. Comparing these data with the spectral data
obtained for the sulfide radical cation by Baciocchi et al.,
2
5,26
2
7
28
20
Yokoi et al., and others and from our recent study we feel
it reasonable that the peaks at 300–350 and 500–550 nm may
be attributed to the sulfoxide radical cation. The absorption
spectrum and the lifetime of this transient are not affected by
the presence of oxygen. To get a clear picture of the different
transients formed during the course of the reaction, a detailed
study using a facility with high time resolution is required. At
thisstagewetakethepeaksat300–350and500–550 nmtobe
due to the formation of sulfoxide radical cation.
Figure 3. Hammett plot for the oxidation of aryl methyl sulfoxides by
Thus from the conventional flash photolysis study we are able
to record the absorption spectrum of the transient, sulfoxide
radical cation which supports the postulation of ET mechan-
ism for the titled reaction. However, it is important to mention
that though we are able to get spectral evidence for the
formation of sulfoxide radical cation the bands are weak
compared to the transients formed from organic sulfide. The
transient spectrum of sulfide radical cation recorded under
similar reaction conditions is shown in Figure 5.
3
þ
in aqueous CH
to the sulfoxides given in Table 2.
[
Ru(dtbpy)
3
]
3
CN (50% v/v) at 298 K. The numbers refer
2
.1. Formation of sulfoxide radical cation
3
þ
Ruthenium(III) complexes (I–V), [Ru(NN)3] , are one-
electron oxidants and the proposal of ET mechanism can be
confirmed if the short lived transient radical, formed during
the course of the reaction, is identified by time resolved
technique. In the titled reaction, the progress of the reaction
is followed by monitoring the increase in the concentration
of Ru(II) complex formed as the product which supports the
ET nature of the reaction. To confirm it further we have
decided to record the transient absorption spectrum of
sulfoxide radical cation. Though the transient absorption
spectrum of sulfide radical cation has been recorded
previously by generating them using pulse radiolysis and
In the present study, it is possible to record the transient
spectrum of sulfoxide radical cation with millisecond (ms)
þ
The stabilization of this type of radical ion at high [H ] has
time resolution because the species is stable at high [H ].
þ
2
9
already been established.
It is known that molecular oxygen reacts with the excited
state Ru(II) complex, [Ru(NN) ] to produce Ru(III) as
3
p
2þ
shown in Eqs. 3–7 (vide supra) Scheme 1.
2
3–25
flash photolysis techniques by many workers,
as far as
we know no attempt has been made to record the absorption
spectrum of the sulfoxide radical cation. This is under-
standable because X-ray spectroscopy, MO calculations and
other physical measurements show that the S–O band in
organic sulfoxide is polarized with a net positive charge
2
þ
p
2þ
½
RuðNNÞ3Š þ hn ! ½RuðNNÞ Š
ð3Þ
ð4Þ
3
p
RuðNNÞ3Š2þ þ O ! ½RuðNNÞ · · ·O2Š
½RuðNNÞ þ· · ·O2Š ! ½RuðNNÞ3Š3þ· · ·O22
2þ
p
½
2
3
Electron
2
p
z
3
,5
Š
ð5Þ
3
localized on the sulfur atom (Eq. 1). Thus compared to
sulfide it will be difficult to remove an electron from the
positively charged sulfur of organic sulfoxides. In order to
ascertain the formation of the sulfoxide radical cation we
attempted the flash photolysis study of the reaction between
Transfer
3
3
þ
z2
2
þ
3þ
z
2
½
2
RuðNNÞ · · ·O Š þ H ! ½RuðNNÞ Š þ HO
ð6Þ
ð7Þ
3
z
2
HO ! H O þ O
2
2
2
3
þ
[
Ru(NN)3]
complex and organic sulfoxides. The
Scheme 1.
3
]
þ
in aqueous acetonitrile (50% (v/v)) at four different
Table 3. Second order rate constants for the oxidation of sulfoxides (p-XC
6
H
temperatures and activation parameters, enthalpy (DH ) and entropy (DS ) of activation
4
SOCH
3
) by [Ru(phen)
3
#
#
2
(M
1
21
s
#
DH (kcal mol
21
#
21
21
2DS (cal K mol )
No.
X¼
k
2
)
)
2
93 K
298 K
303 K
313 K
1
2
3
4
5
6
OMe
Me
H
Br
Cl
4.49
3.74
2.25
0.66
0.75
0.24
5.18
4.30
2.86
0.80
0.96
0.34
6.06
5.08
3.73
1.01
1.16
0.46
7.92
6.76
4.89
1.48
1.56
0.68
4.5
4.8
6.4
5.8
6.6
8.5
40.1
39.7
35.1
39.3
36.7
32.3
NO
2

M. Ganesan et al. / Tetrahedron 60 (2004) 1921–1929
1925
.2. Mechanism of the [Ru(NN)3]3 oxidation of organic
sulfoxides
1
2
The formation of Ru(II) as one of the products of the
reaction and sulfoxide radical cation as a transient are
strongly in favor of ET from sulfoxide to Ru(III) in the rate
determining step. The additional support for the ET
mechanism comes from the substituent effect study, i.e.,
electron-donating substituents facilitate the reaction and
electron-withdrawing groups retard the rate of reaction. The
reaction constant (r) value is around 21.20 (Table 2). To
account for the above spectral and kinetic results the
mechanism shown in Scheme 2 has been proposed.
Equations 8–12 are given in Scheme 2.
ArSðOÞMe þ ½RuðNNÞ3Š ! ArSþzðOÞMe þ ½RuðNNÞ3Š2þ
3
þ
k
ð8Þ
ð9Þ
ð10Þ
ð11Þ
ð12Þ
Figure 4. The absorption spectra of transients at different time intervals
3
þ
formed from the reaction of [Ru(phen)
aqueous CH
experiment.
3
]
CN (50% v/v) solution at 298 K by flash photolysis
with MPSO in oxygen-saturated
3
Scheme 2.
3
þ
Generally the outer sphere oxidants, [Ru(NN)3] , undergo
reaction with electron donors by second order kinetics with
rate limiting ET to generate organic radical ion. In the
3
þ
presence of strong oxidants like [Ru(NN)3] , the driving
force for back ET is diminished resulting in the long
lifetimes of the radical cation. Consequently, the sulfoxide
radical cation may undergo other types of reaction, either
3
þ
fragmentation or reaction with [Ru(NN) ] or solvent in
3
2
5,30
competition with back ET. Jenks and co-workers have
established that the photolysis of aromatic sulfoxides leads
to a-cleavage as the predominant primary photochemical
3
1
process. The formation of sulfone as the major product of
the reaction 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. The formation of sulfone from sulfoxide radical
cation may be shown as three-step process (Eqs. 9–12).
Further, a good agreement between the experimentally
observedsecond-orderrateconstantsandthevaluescalculated
by Marcus theory (vide infra) also support the proposed
Figure 5. The absorption spectra of transients at different time intervals
þ
with MPS in oxygen-saturated
3
]
formed from the reaction of [Ru(bpy)
aqueous CH CN (50% v/v) at 298 K.
3
3
3þ
mechanism (Scheme 2). The reaction between [Ru(NN)3]
and ArS(O)(OH)Me is fast and leads to the formation ofRu(II)
p
2þ
is supported from the peak observed at 600 nm in the
It is important to mention that apart from ET quenching,
p
complex in the excited state ( [Ru(NN) ] ) (Eq. 11) which
3
2þ
ism to produce singlet oxygen. As the present reaction is
[
Ru(NN)3] is also quenched by energy transfer mechan-
transient spectrum (Fig. 4). However, it is well known that
when [Ru(bpy)3]
3
þ
ArS(O)Me, the role of O2 is not discussed here. The
3þ
powerful electron donors the product [Ru(bpy) ]
concerned with the reaction of [Ru(NN) ]
3
with
captures electrons from radicals or
is
1
2þ
The mechanism proposed
3
1
32–35
oxygenation of organic sulfur compounds with O has been
2
formed in the excited state.
here is similar to the one postulated for the [Ru(NN) ]
20
oxidation of organic sulfides.
þ
1
3þ
extensively studied. It is known that at high [H ] both O
3
2
3
þ
and [Ru(NN)3] contribute equally for the oxidation of
organic substrates. The H O oxidation of organic sulf-
oxides with H O has been extensively studied but the
2
2
2
2
2.3. Application of Marcus cross-reaction relation
reaction is slow. Thus the major part of the redox reaction
3
þ
takes place between [Ru(NN) ] and ArS(O)Me particu-
3
In the past half-century, great progress has been achieved in
understanding both the kinetics and thermodynamics of ET
þ
larly at [H ].

1926
M. Ganesan et al. / Tetrahedron 60 (2004) 1921–1929
reactions (Eq. 13) by applying the Marcus cross-relation.
red þ OX O OX þ red ð13Þ
For ET reactions, like Eq. 13, the value of k12 is given by
In Eqs. 15a and 15b the value of K12 is calculated from the
3
þ
2þ
redox potentials of the couples [Ru(NN) ] /[Ru(NN)3]
3
1
2
1
2
þ
and SO /SO using Eqs. 17 and 18.
0
0
SO =SO
0
DG ¼ nFðE
þ
2 E 3þ
Ru =Ru
2þ
Þ
ð17Þ
ð18Þ
activated complex theory as Eq. 14
o
#
K12 ¼ expð2DG =RTÞ
On taking logarithm Eq. 15a becomes Eq. 18.
k12 ¼ Z12 expð2DG =RTÞ
ð14Þ
Where Z12 is the preexponential term and DG is the free
12
‡
12
energy of activation. According to Marcus theory, reactions
like Eq. 13 can be described using the cross-relation given in
Eqs. 15a and 15b.
log k12 2 0:5 log k11 2 0:5 log f12
¼
0:5 log k22 þ 0:5 log K
ð19Þ
12
1
=2
k12 ¼ ðk k K f Þ
ð15aÞ
ð15bÞ
11
22 12 12
þ
The value of k22 for SO /SO can be estimated from an
iterative procedure, i.e., a value of k22 is guessed and
plugged into Eq. 16 to calculate f. With the calculated value
of log f, plots of left hand side of Eq. 16 versus log K12 are
made and from mean least-square 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
2
2
ln f12 ¼ ½lnðK ފ =½4 ln k k =Z Š
12
11 22
The self-exchange rate constants, k11 and k22 and the
equilibrium constant K12 are the principal parameters
determining k . The term Z is the preexponential factor
1
2
1
1
21 21
which is often taken as 10
M
s . The more general
form of Eqs. 15a and 15b including work terms, has
successfully been applied to a variety of inorganic, organic,
3
3,36– 42
organometallic and biochemical reactions.
The
5
21 21
than 10%. The final results give k ¼4.0£10 M
s
if we
use all five Ru(III) complexes (I–V) as the oxidant and a
2
2
fundamental assumption in deriving Eqs. 15a and 15b was
that averaging the activation barriers for self-exchange
reactions produced the proper activation barrier for the
sample plot is shown in Figure 6.
3
0
cross-reactions. Recently, it has been established that the
3
4
great majority of ET reactions are non-adiabatic. The rate
constant of non-adiabatic ET is given by Eq. 16.
"
#
ꢀ
ꢁ
o
2
1=2
ðDG þ lÞ
3
2
2
k12 4p =h lk T V exp 2
ð16Þ
B
4
lk T
B
In Eq. 16, V is the electronic coupling matrix element, l,
o
the reorganization energy and DG , the free energy
change for ET. The reorganization energy of ET(l) is the
energy required to structurally reorganize the donor,
acceptor and their solvation spheres upon ET. Despite the
4
3
predictions of modern theory by Bixon and Jortner,
3
0,44
Nelson et al., have applied Eqs. 15a and 15b
successfully for the calculation of rate constants, k , for
1
2
1
41 reactions having couples of a wide range of structural
types. Since Eqs. 15a and 15b is successful in accounting
for ET reactions for couples having a wide range of
structural types including heteroatom-substituted aro-
matics we have applied this equation to the titled reaction
12
Figure 6. Plot of log k1220.5(log k11þlog f)) versus log K . The numbers
3þ
3þ
refer to the sulfoxides given in Table 2. [Ru(bpy)
3
3
]
and [Ru(phen)
3
]
(V).
þ
3þ
3þ
þ
(X), [Ru(dmbpy)
3
]
(O) [Ru(dtpby)
3
]
(B) and [Ru(dpphen)
3
]
and to get the self-exchange rate of SO /SO. The value of
3
þ
2þ
self-exchange rate of [Ru(NN)3] /[Ru(NN) ] is known
3
3
7
8
21 21
from previous studies of Sutin et al. as 4£10 M
s
.
It is pertinent to point out that similar treatment to oxidation
of organic sulfides has given the same value of k22 for
3
þ/2þ
The high self-exchange rates observed with [M(NN) ]
3
2
0
(
et al. and others.
strong mixing of metal d and ligand p orbitals so that
significant metal electron density exists on the ligands. The
back-bonding interaction has the effect of shortening the
metal(II)–nitrogen bonds so that inner-shell reorganization
M¼Fe, Ru and Os) complexes is accounted for by Sutin
sulfides. After calculating the value of k , it is then used
22
3
7,42c
In the divalent state there is relatively
to get the values of rate constant for ET, k , from organic
12
p
3þ
sulfoxides to [Ru(NN)
]
and the values are collected in
3
Table 2. These calculated values are in fair agreement with
the experimental values, supporting our arguments pre-
sented above. A difference of 1–2 orders is observed
between the experimental and the calculated values for
sulfoxides carrying electron-donating groups in the phenyl
ring. The solvation of the reactants and transients plays an
important role in deciding the reactivity and course of the
reaction particularly when polar solvents are used in
reactions involving charge development. The presence of
electron-donating and withdrawing groups leads to different
solvation energy. Recently it has been established that
3
7
barrier to ET is lowered. Sutin and co-workers pointed out
3
þ
another reason for large self-exchange rates of [M(NN) ]
3
.
0
The space-filling models of the tris(2,2 -bipyridine) or
tris(1,10-phenanthroline) metal complexes show the favor-
able ‘stacked’ overlap of a pyridine ring in one complex
with one pyridine in another complex. This contact may
offer an especially facile pathway for the self-exchange
reaction.

M. Ganesan et al. / Tetrahedron 60 (2004) 1921–1929
1927
z
21
p-OCH C H S involves a solvation energy of 42 kJ mol
3
Further it is important to note the difference in self-
exchange rate constant obtained for these organic sulfur
6 4
z
21
45
and p-NO C H S 296 kJ mol
2
in CH CN. Thus, in
3
6
5
þ
at 0.5 mH
order to account for the dynamics of ET reactions it is
essential to include the solvation energy in the calculation.
Thus we presume that if the solvation energy was also
included the agreement between the experimental and
calculated values would be better.
compounds at different [H ]. The self-exchange rate is
7
21 21
þ
at 4.5 [H ]. Thus at high [H ] the self-
1.0£10 M
s
but the value is
5
21 21
þ
þ
exchange rate is small because the radical cation is more
4.0£10 M
s
þ
stabilized at high [H ].
3
1
2
.4. Comparison of the reactivity of [Ru(NN) ] with
3
In conclusion the ET reaction between organic sulfoxides
and Ru(III) complexes is established by observation of
absorption spectrum of transient sulfoxide radical cation at
3
1
[
Fe(NN)3]
þ
Recently, iron(III)–polypyridyl complexes have been used
in this laboratory as one-electron oxidants for the oxidation
high [H ]. The Marcus cross relation is applied successfully
to estimate the self-exchange rate constant of sulfoxide
radical cation/sulfoxide redox couple. This conclusion on
the metal ion oxidation of biologically important organic
sulfides and sulfoxides is of importance because the
oxidation of proteins and lipids by a single ET process has
1
6,17
of organic sulfides and sulfoxides.
Compared to Fe(III),
Ru(III) complexes are better electron acceptors and undergo
facile one-ET reactions with a variety of organic, inorganic
and biological substrates. The reduction potentials of
3
þ
3þ
49,50
[
Ru(bpy)3] and [Fe(bpy)3] are 1.02 and 0.76 V versus
been associated with the onset of several diseases.
SCE, respectively. It was interesting to compare the
reactivity of these metal–polypyridyl complexes towards
the same substrate, organic sulfoxides. The second-order
3. Experimental
3.1. Materials
3
þ
rate constants for the ET reaction of [Ru(bpy) ] and
3
3
þ
2
and 0.27 M
[
Fe(bpy)3] with MPSO under similar conditions are 2.3
1
21
o
s
, respectively. The DG values for the
þ
3
3þ
2þ
0
0
4,4 -dimethyl-2,2 -dipyridine (dmbpy), 4,4 -di-tert-2,2 -
reactions of [Ru(bpy)3] and [Fe(bpy)3] with MPSO are
.70 and 0.90 eV, respectively. This comparison points out
The [Ru(NN) ] complexes (NN¼2,2 -bipyridine (bpy),
3
0
0
bipyridine (dtbpy), 1,10-phenanthroline (phen) and 4,7-
0
0
that the reactivity of these metal complexes is in accordance
with the driving force of the reaction. The reaction constant
diphenyl-1,10-phenanthroline (dpphen)) were synthesized
51–53
þ
Fe(bpy)3] with X–C H S(O)Me are 21.2 and 20.8,
3þ
(
[
r ) values for the reaction of [Ru(bpy) ]
3
and
by known procedures.
for the titled reaction were prepared by adopting the
The Ru(III) complexes needed
3
þ
6
4
5
4,55
respectively. These rate (k) and reaction constant (r) data
point out that for Ru(III) complexes the rate as well as the
r values are higher compared to those of Fe(III)
complexes.
procedure described below.
2
The steady-state photolysis
of [Ru(NN)3] in 4.5 M H SO using a 500 W tungsten–
þ
2
4
halogen lamp led to the formation of corresponding Ru(III)
complex. The light beam was made parallel by using a
planoconvex lens. The infrared (IR) and ultraviolet (UV)
radiations were cut-off by passing the light beam through a
5 cm quartz cell filled with water and pyrex glass filter. It
was observed that the color of the solution readily changed
from orange-yellow to green during irradiation. The
This comparison leads us to conclude that reactions
proceeding through an ET mechanism do not obey the
reactivity–selectivity principle (RSP). It is interesting to
recall that though aromatic sulfides have more reactivity by
two orders magnitude compared to sulfoxides towards the
3
þ
formation of [Ru(NN)3] complexes was confirmed by
recording the absorption spectrum of the irradiated solution.
3
þ
same oxidant [Ru(NN)3] , they have a similar r value. On
the other hand, these substrates have r values of ,3.2 and
3þ
[Ru(NN)3] complexes showed peaks around 420–430
and 650–670 nm.
3
þ
.8, respectively, towards [Fe(NN) ] . All these results
3
0
strengthen our conclusion that though all reactions proceed
through a similar ET mechanism, RSP is not successful for
these ET reactions. To provide an explanation for this
behavior, it is useful to take at the nature of the transition
state of this ET reaction. In order to achieve selectivity in a
chemical reaction the transition state should have definite
Methyl phenyl sulfoxide and methyl p-tolyl sulfoxide
(Aldrich) were used as received. The other aryl methyl
sulfoxides used in this study were prepared from the
corresponding sulfides according to the literature pro-
5
6,57
cedures.
All the sulfoxides were purified by vacuum
4
6,47
structure.
For the reactions proceeding through outer-
distillation/recrystallization from suitable solvents. The
physical constants of the sulfoxides were found to agree
with literature values. The GC analyses of each sulfoxide
showed the presence of a single entity, and no impurity peak
sphere ET mechanism the implicit assumption is that little
interaction exists between the electron donor and acceptor
4
8–50
in the transition state.
On the other hand, for the SN2
1
transition state the most dominant interaction exists between
the donor highest occupied molecular orbital (HOMO) and
the acceptor lowest unoccupied molecular orbital (LUMO).
It is interesting to mention that reactions of organic sulfides
and sulfoxides with oxo(salen)metal complexes obey RSP
and these reactions proceed through the nucleophilic attack
of sulfur of the substrate on the oxygen center of the oxidant.
The failure of RSP in the ET reactions led us to presume that
these reactions might be of the unbound, outer-sphere type
involving weakly bound transition states.
appeared in the H NMR spectra. Acetonitrile (GR,
E. Merck) was refluxed over P O for 5 h and then
2
5
5
8
distilled. All other reagents were of AnalaR grade or
used after purification. The kinetic study of the reaction was
performed after confirming the purity of the reactants and
solvents used in the system.
3.2. Conventional flash kinetic spectrometer
Flash photolysis experiments were carried out using an

1928
M. Ganesan et al. / Tetrahedron 60 (2004) 1921–1929
applied photophysics UK KN-020 series model flash kinetic
spectrometer. It consists of two LR-16, xenon filled flash
lamps with an electrode separation of 140 mm, an overall
length of 205 mm and a bore diameter of 10 mm. The
sample cell was made of quartz with an outer jacket. The
cell has an inner diameter of 10 mm and an optical path
length of 105 mm. The cell was placed between the flash
lamps in the middle and the lamps were fired at 10 kV using
a 200 J, 1 mF capacitor bank. A 12 V, 100 W quartz-iodine
lamp (Phillips, Holland) operated by a power supply unit
References and notes
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(
Model 415/01) was used as the light source for the
monitoring beam. The detecting system consisted of a
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(
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3
.3. Kinetic measurements
3þ
The absorption spectra of [Ru(NN)3] and [Ru(NN)3]
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7
2
þ
complexes were obtained using a JASCO model 7800 UV–
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3
þ
following the increase in absorbance of [Ru(NN)3]
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2
þ
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3
pseudo first-order condition. The pseudo first-order rate
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1
9. Murray, R. W.; Jeyaraman, R.; Pillay, M. K. J. Org. Chem.
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(
s). The second-order rate constant (k ) is evaluated from the
2
11. (a) Chellamani, A.; Alhaji, N. M. I.; Rajagopal, S.; Sevvel, R.;
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2
1
‡
–
the enthalpy (DH ) and entropy (DS ) of activation, were
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2
12. Chellamani, A.; Kulandaipandi, P.; Rajagopal, S. J. Org.
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3
.4. Product analysis
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A sample of 0.5 mM substrate (ArSOMe) was added to a
3
þ
complex in 5 ml of
aqueous CH CN (50% v/v). The solution was stirred at
0
.5 mM solution of [Ru(NN)3]
3
2
98 K for ,90 min or more depending upon the nature of
sulfoxide and complex. The products of the reaction were
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(
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21
characteristic region 1100–1150 cm . The GC analysis
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(
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2
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Acknowledgements
The authors thank Professor P. Natarajan and
Dr. P. Ramamurthy, National Centre for Ultrafast Processes,
University of Madras for their help to record transient
spectrum using flash photolysis technique.

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