Electron transfer reactions of tris(polypyridine)ruthenium(III) complexes with organic sulfides: Importance of hydrophobic interaction

Article abstract of DOI:10.1016/j.tet.2005.02.051

Ruthenium(III)-polypyridyl complexes, generated from the photochemical oxidation of Ru(II) complexes with molecular oxygen, undergo facile electron transfer reaction with dialkyl and aryl methyl sulfides. The rate controlling electron transfer process is

Full text of DOI:10.1016/j.tet.2005.02.051

Tetrahedron 61 (2005) 4863–4871
Electron transfer reactions of tris(polypyridine)ruthenium(III)
complexes with organic sulfides:
importance of hydrophobic interaction
a
Muniyandi Ganesan, Veluchamy K. Sivasubramanian, Thangamuthu Rajendran,
a
a
b
Kalaiyar Swarnalatha, Seenivasan Rajagopal
b,
b,
and Ramasamy Ramaraj
* *
a
Department of Chemistry, Vivekananda College, Thiruvedakam West, Madurai 625 217, India
b
School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
Received 1 July 2004; revised 31 January 2005; accepted 18 February 2005
Available online 8 April 2005
Abstract—Ruthenium(III)–polypyridyl complexes, generated from the photochemical oxidation of Ru(II) complexes with molecular
oxygen, undergo facile electron transfer reaction with dialkyl and aryl methyl sulfides. The rate controlling electron transfer process is
confirmed from the absorption spectrum of the transient sulfide radical cation. The spectrophotometric kinetic study shows that the reaction is
of total second order, first order in Ru(III) complex and in the organic sulfide. The reaction rate is susceptible to the change of ligand in
3C
[
q 2005 Elsevier Ltd. All rights reserved.
Ru(NN)
3
]
and the structure of organic sulfide.
1
. Introduction
in many oxidation reactions, the formation of sulfide radical
cation as the intermediate has only been confirmed in a few
5
,7,32–34
Organic sulfides serve an important function in many
materials such as coal, organic polymers, biological
macromolecules and xenobiotics. They are highly
studies
C$
since the lifetime of sulfide radical cation,
KSK , is short. Recently, Baciocchi, Steenken and others
1
recorded the absorption spectrum of the sulfide radical
5
,7,32–34
cation. The sulfide radical cation can undergo
deprotonation at a C –H bond, C–S fragmentation,
susceptible to oxidation and different pathways have been
2
–4
established depending on the oxidizing species. Among
these pathways the sulfoxidation of organic sulfide
proceeding via sulfide radical cation is a subject of current
a
5
,7,11–14,18
oxidation, aromatic substitution and dimerisation.
After the formation of sulfide radical cation, one of the
major products is the sulfoxide, S /OSO. Though
the formation of OS as the transient has been proposed
for the oxidation of organic sulfides, the mechanism for the
5
–10
C$
interest.
In these oxidation reactions suitable oxidants
C$
can remove an electron from a lone pair on the sulfur atom
to produce radical cations, important intermediates in a
great number of chemical processes, extending from those
C$
conversion of OS /OSO is not yet clearly established,
though the process is very important from a biotechno-
1
1–18
of industrial importance to biological systems.
formation of sulfide radical cation as intermediate has been
The
3
logical point of view. Thus, a mechanistic understanding of
1
proposed in electrochemical oxidation, in chemical
9
$C
the processes which lead to the conversion of OS into
8
20
21
oxidation with Fe(III), Ce(IV), Cr(VI), Cr(V),
6
OS/O is important.
2
2
9
23,24
7a,25
Mn(III), Ru(IV), cytochrome P-450,
peroxidase,
and in the irradiation
of the charge-transfer complex of sulfides with electron
2
,3,18,26–28
in photosensitised oxidation
In our recent study we have proposed an ET mechanism for
3C
the iron(III)–polypyridine complexes, [Fe(NN)3] , oxi-
dation of organic sulfides based on the successful appli-
8
2
9–31
acceptors.
cation of Marcus theory of ET to the reaction.
3C
Ruthenium(III)–polypyridine complexes, [Ru(NN) ]
Though electron transfer (ET) from the aromatic sulfide to
the oxidant as the rate-determining step has been proposed
,
3
3C
are better oxidants than [Fe(NN)3] (vide infra) and they
undergo efficient ET reactions with a number of organic and
Keywords: Ruthenium(III)–polypyridyl complexes; Organic sulfides;
Electron transfer reactions.
Corresponding authors. Tel.: C91 452 2458246;
e-mail: seenirajan@yahoo.com
35–39
inorganic electron donors.
between Fe(III) and Ru(III) complexes, the study of redox
Apart from the similarity
*
3C
reaction of [Ru(NN)3] complexes with organic sulfides is
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.02.051

4864
M. Ganesan et al. / Tetrahedron 61 (2005) 4863–4871
important as Ru(III) complexes carrying organic sulfoxides
as ligands are now used as drugs.
experimental conditions have been already published by
us.
4
0–45
50
To confirm the formation of sulfide radical cation as
intermediate of the reaction, we have followed the reaction
by flash photolysis technique from which we are able to get
the transient absorption spectrum supporting the formation
The kinetics of oxidation of 15 organic sulfides with five
[Ru(NN)3]
3
C
complexes have been studied by spectro-
photometric techniques under pseudo first-order conditions.
The progress of the reaction has been followed by
measuring the increase in absorbance (OD) of Ru(II) ion
formed as the product of the reaction and a sample kinetic
run is shown in Figure 1. The linear log OD vs time plot
C$
þ
of KS K and ½OS : $S!ꢀ species during the course of
3
reaction. The results of the kinetic and spectral studies on
2
3
C
the reaction of five [Ru(NN)3]
complexes with 15
aliphatic and aromatic sulfides are presented in this report.
Though several chemical and electrochemical methods are
available for the generation of Ru(III) from Ru(II)
complexes, in the present study Ru(III) complexes have
been generated using visible light irradiation of
(Figure not shown) and constant k value at different
1
3
C
concentration of [Ru(NN)3] indicate that the reaction is
first-order in Ru(III). The kinetic data observed at different
[sulfide] but at constant [Ru(III)] show that the reaction is
first-order in sulfide also (Fig. 2). 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.
Generally the increase in water content favours the reaction
when charge development takes place in the transition state
of reaction. As the titled reaction involves electron transfer
from sulfide to Ru(III), positive charge is developed on the
sulfur centre of the substrate in the transition state. Thus the
solvent effect supports the formulation of electron transfer
in the rate-controlling step. The behaviour of
2
C
46–49
[
Ru(NN)3] in the presence of molecular oxygen.
2
. Results and discussion
3
C
The structures of ligands of [Ru(NN)3]
and organic
sulfides used in the present study are shown in Chart 1. It is
important to mention here that the major oxidized organic
product of the reaction is the corresponding sulfoxide and
the percentage of sulfoxide formed is in the range 60–90%
depending on the nature of sulfide and the Ru(III) complex.
The details of percentage of sulfoxide formed and product
analysis are given in Section 4. The percentage of sulfone
formed from this reaction is negligible and the oxidation of
sulfoxide to sulfone is slow. The kinetic and mechanistic
details of the redox reaction between these Ru(III)
complexes and organic sulfoxides under the present
3
C
[Ru(dpphen)
]
towards solvent effect seems to be reverse
3
to other Ru(III) complexes which may be attributed to
greater hydrophobicity of the ligand (vide infra). The
reaction is sensitive to the change of substituent in the
phenyl ring of X–C H SMe and nature of alkyl group in R S
6 4 2
3C
as well as to the nature of the ligand of [Ru(NN)3] and the
Figure 1. The absorption spectral changes for the reaction between
K5 K4
(5!10 M) and methyl phenyl sulfide (5!10 M) at
3
C
[
Ru(dmbpy)
3
]
different time intervals (20 s).
3
C
Chart 1. Structures of NN in [Ru(NN)
3
]
and organic sulfides.

M. Ganesan et al. / Tetrahedron 61 (2005) 4863–4871
4865
2.1. Steric effect
The k values obtained for dialkyl sulfides (Table 2b) show
2
that the reaction is influenced predominantly by steric rather
than polar effects. When we change the nature of the alkyl
group from ethyl to tert-butyl the rate constant decreases
with an increase in the bulkiness of the substrate. If the
reaction was controlled by the polar effect, the reverse
would have been the trend. It is relevant to recall that the
*
Taft’s s values become more negative when we move from
ethyl to tert-butyl. Thus if the polar effect is predominant,
substitution of ethyl by tert-butyl would facilitate the
reaction which is not observed. It is to be remembered that a
negative r value was obtained when the kinetic data for aryl
methyl sulfides were analyzed in terms of the Hammett
equation. Thus, realising the importance of steric effect in
this reaction, the k values obtained for six dialkyl sulfides
2
are treated with Taft’s steric energy relationship, log kZ
log k Cd E where E is the steric substituent constant and
o
s
s
5
2
d, the susceptibility of reaction to the steric effect. The
values of d obtained for all five Ru(III) complexes are given
in Table 2b and the d value is sensitive to the bulkiness of
3
C
3C
the ligands. For [Ru(bpy)3]
and [Ru(phen)3]
com-
plexes the d value is w0.25 and for complexes containing
bulky ligands it is in the range of 0.50–0.58. But the
Figure 2. Plot of k
1
vs [sulfide] for the oxidation of aryl methyl sulfides by
in aqueous CH CN (50% v/v) at 298 K. The numbers
3C
behaviour of [Ru(dpphen)3] seems to be exceptional and
3
C
[Ru(dmbpy)
refer to the sulfides given in Table 2a.
3
]
3
the d value for the reaction of this complex is 0.29. The
exceptional behaviour of [Ru(dpphen)3]
3
C
deserves
explanation. It seems to be reasonable to consider the
importance of hydrophobic interactions here since
observed data are collected in Table 2a and b. The data
given in Table 2a show that introduction of electron-
donating groups in the phenyl ring of C H SMe facilitates
3
C
[Ru(dpphen)
]
contains a larger number of phenyl
6
5
3
the reaction and electron-withdrawing groups inhibit the
groups. Though the presence of such bulky groups may
entail a greater steric effect, the greater hydrophobicity of
these ligands may offset to some extent the steric effect. The
hydrophobic interaction may bring the reactants closer and
facilitate the reaction. It has been recently established by us
5
1
reaction. Application of 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 2a. The r value is slightly sensitive to the structure of
that hydrophobic interactions may overcome electrostatic
3
C
2C
the ligand of [Ru(NN)3] and varies from K1.0 to K1.2. It
is interesting to compare these kinetic results with those
observed for the oxidation of organic sulfides with
interactions when excited state [Ru(dpphen)
the reagent for electron transfer reaction with organic
3
]
is used as
5
3
substrates. The results observed here point out that
hydrophobic interaction may lead to less steric effect.
Thus the present system seems to be one of the rare
examples where hydrophobic interaction offsets the steric
effect thereby facilitating ET from the electron donor to
acceptor. Thus the low d value (w0.29) observed with
3
C 8
3C
[
are more than two orders higher than the values obtained for
Fe(NN)3] . The k values observed for [Ru(NN) ]
2 3
3
C
[
Fe(NN)3]
(Table 2a). These results are expected since
the reduction potentials of [Ru(NN) ] (1.0 V vs SCE) are
3
C
3
3
higher than those of [Fe(NN)3] (0.8 V vs SCE). When we
C
3
C
compare the reaction constant (r) values, the r value for
[
[Ru(dpphen)3]
may be attributed to the importance of
3C
Fe(bpy)3] is K3.0 which is high compared to the r value
hydrophobic interaction here. A similar explanation may be
extended to the exceptional solvent effect observed with this
complex (vide supra). Since the correlation is good with the
E values, we tried multiple correlation with s* and E but
3
C
of K1.0 obtained for [Ru(bpy)3] . Thus the trends in
reactivity and r values are as expected on the basis of the
reactivity–selectivity principle (RSP).
s
s
3
C
Table 1. Effect of varying the solvent composition on the reaction of [Ru(NN)
3
]
with sulfides at 298 K
3
C
3C
3C
Solvent composition
CH CN/H O (v/v)
[Ru(bpy)
3
]
s
, k
)
2
,
[Ru(phen)
K1 K1
3
]
, k
2
,
[Ru(dpphen)
K1 K1
3
]
, k
2
K1 K1
3
2
(M
(M
s
)
(M
s
)
MPS
DES
MPS
MPS
8
7
6
5
4
2
0:20
0:30
0:40
0:50
0:60
0:80
475
533
590
653
746
918
320
515
532
543
611
705
533
656
705
759
860
989
1115
978
686
576
559
233
3
]
C
K5
K6
C
(
Reaction conditions: [Ru(NN)
3
Z5!10 M, [MPS]/[DES]Z4!10 M and [H ]Z4.5 M).

4866
M. Ganesan et al. / Tetrahedron 61 (2005) 4863–4871
K1 K1
s
3C
Table 2a. Second order rate constant (k
2
, M
) values for the oxidation of p-XC
6
H
4
SMe by [Ru(NN)
3
]
in aqueous CH
3
CN (50% v/v) at 298 K
K1 K1
s
Sl. no.
p-XC
6
H
4
SMe; XZ
Oxidation potential,
o
E vs SCE(V)
k
2
, M
3C
3C
3C
[
(
Ru(bpy)
a
3
]
(1.02 V)
[Ru(dmbpy)
(0.86 V) (II)
3
]
3
[Ru(dtbpy) ]
b
(0.87 V) (III)
b
I)
c
c
c
1
2
3
4
5
6
7
8
9
Me
H
F
Br
Cl
COOH
COCH
CN
NO
1.24
1.34
1.35
1.41
1.37
1.51
1.54
1.61
1.70
938G25 (16.9)
655G12 (1.40)
481G13
10.9G0.44 (0.38)
2.1G0.08 (0.17)
1.44G0.07
2.85G0.11
1.79G0.05
1.23G0.04
0.76G0.04
0.79G0.04
0.54G0.02
0.39G0.02
0.28G0.01
0.24G0.01
rZ0.985
c
320G15
0.95G0.03
c
c
c
401G13 (1.01)
179G11 (1.18)
138G10
0.93G0.05 (0.14)
0.50G0.02
0.45G0.03
3
127G7.5
116G5
rZ0.993
0.36G0.01
0.25G0.01
rZ0.990
2
rZK1.04G0.08
rZK1.16G0.05
rZK1.14G0.06
o
Oxidation potential, E vs
SCE(V)
K1 K1
s
Sl. no.
p-XC
6
H
4
SMe; XZ
k
2
, M
3
]
C
3C
[
(
Ru(phen)
a
3
(1.02 V)
[Ru(dpphen)
a
(V)
3
]
(0.96 V)
IV)
c
1
2
3
4
5
6
7
8
9
Me
H
F
Br
Cl
COOH
COCH
CN
NO
1.24
1.34
1.35
1.41
1.37
1.51
1.54
1.61
1.70
1017G36 (51.5)
759G30 (7.3)
746G22
583G23
438G17
324G11
299G13
171G8.2
149G7.5
118G6.5
101G4.7
rZ0.991
c
642G23
446G13
428G21 (3.9)
c
c
280G11 (2.86)
3
206G9.3
176G4.9
127G4.6
rZ0.997
2
rZK0.97G0.03
rZK0.98G0.05
3
C
K5
a
K6
b
K3
C
Reaction conditions: [Ru(NN)
c
2
3
]
Z5!10 M. [Sulfide]Z10 M. [Sulfide]Z10 M and [H ]Z4.5 M).
K1 K1
s
3C
k
, M
values for the [Fe(NN)
3
]
oxidation of p-XC
6
H
4
SMe collected from Ref. 8.
$
2
the correlation was only slightly improved. This analysis
indicates the importance of the steric effect in this reaction.
2HO /H O þ O
(5)
2
2
2
It is pertinent to mention that the reaction of H O with
2
2
1
4
organic sulfides is slow. The spectra given in Figure 4
shows two strong absorptions at 300–350 nm and around
2
.2. Formation of sulfide radical cation
5
,7,32
3
C
510–520 nm. Steenken and others
have assigned the
In this work we propose that the reaction of [Ru(NN)3]
bands at 300–350 and 500–520 nm to the radical cation of
sulfide in the monomeric form which are produced by
electron-transfer reaction of organic sulfides with
with organic sulfide produces sulfide radical cation. This
proposal gets support from the absorption spectrum of the
transient, sulfide radical cation, recorded using conventional
flash photolysis technique (Fig. 4). The experiment was
designed as follows. The reaction mixture consisting of
K$
4
2C
SO =Tl . From the spectrum of the transient shown in
Figure 4 we understand that in addition to the two bands
indicated above, other bands appeared at 480 nm (shoulder)
and 600–700 nm. In the present study our aim is to confirm
the formation of sulfide radical cation as one of the
important intermediates of the titled reaction. To establish
this, we have recorded the spectrum of the transient of the
reaction. The peak at 470 nm was observed recently by
2
C
[
Ru(bpy)3]
and methyl phenyl sulfide (MPS), taken in
aqueous CH CN (50% v/v), was purged with molecular
3
oxygen for 20 min. Then the reaction mixture was irradiated
with flash of light and the absorption spectrum of the
reaction solution was monitored at different time intervals.
2
C
The irradiation of [Ru(NN)3]
in the presence of O leads
2
32
Sawaki and co-workers and they assigned the peak at
to the formation of Ru(III) which is formed due to the
oxidative quenching of *[Ru(NN) ]
oxygen particularly at high [H ] as per reactions shown
in Eqs. 1–5.
2
C
around 460–500 nm to s-type complex of the sulfur–sulfur
two-centre three electron bonded dimers (i.e., s* type). The
peak at 600–700 nm disappears with time. It is known that
Ru(III) ion has the absorption maximum at 600–700 nm.
Thus these spectral changes show that during the course of
the reaction Ru(III) disappears and other products are
formed. It is interesting to note that with the increase of time
the peak at 310 nm disappears and a new peak at 340 nm
with molecular
3
C
hn
2C
2C
½
RuðNNÞ3ꢀ ꢁꢁꢁ/ꢁ ꢂ½RuðNNÞ3ꢀ
(1)
(2)
2þ
3
2þ
3
ꢂ½RuðNNÞ3ꢀ þ O /½RuðNNÞ /O ꢀꢂ
2
2
5
4
appears. In a recent paper Bobrowski et al, have indicated
that the absorption band with lmaxZ340 nm represents a
Electron transfer
2
3
þ
ꢁꢁꢁ/ꢁ ½RuðNNÞ 33 þ /O K2 $
½
½
RuðNNÞ /O ꢀ ꢂ
ꢀ
(3)
(4)
_
hydroxy–sulfuranyl radicalKSKOH. In the present study,
2
þ$
Ar S ,ðOHÞCH is formed at the expense of ArSCH (Eq. 8,
3
3
Scheme 1). Thus the appearance of peak at 340 nm may be
3
3
þ /O K2 $
þ
3þ
$
RuðNNÞ
ꢀ þ H /½RuðNNÞ ꢀ þ HO2
3

M. Ganesan et al. / Tetrahedron 61 (2005) 4863–4871
4867
þ$
short lived species ArSCH and dimeric radical cation
3
C
OS rS! by conventional flash photolysis technique with
ms time resolution because these species appear to be more
C
55
stable at high [H ]. This appears to be the first report for
the detection of these species using conventional flash
photolysis technique. Similar transient absorption spectrum
3
C
was obtained when [Ru(phen) ]
[
was used instead of
Ru(bpy)3] . It has been well established that sulfide
3
3C
radical cation derived from dialkyl sulfide forms sulfide
cation dimer more efficiently compared to aromatic
1
8,32
sulfides.
In order to check this, we have recorded the
transient absorption spectra using diethyl and di-propyl
sulfides also and the transient spectra obtained at different
3
C
time intervals for the reaction between [Ru(bpy)3]
and
diethyl sulfide are shown in Figure 5. Here the peak for the
dimer at 480 nm is obtained confirming the earlier
postulations.
1
8,56
Figure 3. Hammett plot for the oxidation of aryl methyl sulfides by
3
C
[Ru(phen)
to the sulfides given in Table 2a.
3
]
3
in aqueous CH CN (50% v/v) at 298 K. The numbers refer
2.3. Cyclic voltammetry
0
Though the E values of all five Ru(III) complexes used in
5
7
attributed to Ar S ,ðOHÞCH . As the present reaction has
3
the present study are available in the literature, because of
the different reaction conditions used here, the redox
potentials were measured using cyclic voltammetry tech-
nique (details are given in Section 4). The redox potentials
of Ru(III) complexes and organic sulfides are given in
Table 2a. The redox potential values measured in the
present study are in close agreement with the reported
C
C
been studied at high [H ], the presence of high [H ]
facilitates the following processes (Eqs. 6 and 7).
(
(
6)
7)
1
0,39,57
values.
3C
.4. Mechanism of the [Ru(NN)3] oxidation of organic
2
sulfides
C
The formation of intermolecularly OS rS! bonded
dimeric radical cation is confirmed from the peak at
Ruthenium(III) complexes used in the present study are
well-known one-electron oxidants.
3
5–39
4
80 nm. In the present study we are able to detect the
The progress of the
K1 K1
s
3C
Table 2b. Second order rate constant (k
2
, M
) values for the oxidation of R
2
S by [RuL
3
]
in aqueous CH
3
CN (50% v/v) at 298 K
K1 K1
s
Sl. no.
Dialkyl sulfides R
2
S
Oxidation potential,
o
E vs SCE(V)
k
2
, M
3
C
3C
3C
[
Ru(bpy)
3
]
(
(1.02 V)
[Ru(dmbpy)
3
]
(0.86 V) (II)
3
[Ru(dtbpy) ]
a
b
b
I)
(0.87 V) (III)
1
2
3
4
5
6
DES
DPS
DIPS
DBS
DSBS
DTBS
1.46
1.44
1.44
1.46
—
569G15
501G18
443G10
456G15
381G13
231G10
2.18G0.09
1.60G0.06
1.50G0.05
1.14G0.06
1.53G0.07
0.87G0.03
0.28G0.02
0.58G0.05
1.13G0.05
0.85G0.07
1.18G0.06
0.65G0.03
0.28G0.02
0.50G0.04
1.46
d
0.26G0.02
K1 K1
s
Sl. no.
Dialkyl sulfides R
2
S
Oxidation potential,
o
E vs SCE(V)
k
2
, M
3
C
3C
[
Ru(phen)
3
]
IV)
(1.02 V)
[Ru(dpphen)
3
]
(V)
(0.96 V)
a
a
(
1
2
3
4
5
6
DES
DPS
DIPS
DBS
DSBS
DTBS
1.46
1.44
1.44
1.46
—
646G26
573G20
504G19
532G16
417G15
294G15
0.23G0.01
484G16
434G17
371G12
397G13
248G11
189G6.7
0.29G0.03
1.46
d
3C
K5
Reaction conditions: [Ru(NN)
3
]
Z5!10 M. DESZdiethyl sulfide, DPSZdipropyl sulfide, DIPSZdiisopropyl sulfide, DBSZdibutyl sulfide, DSBSZ
di-secondary butyl sulfide, DTBSZdi-tertiary butyl sulfide.
a
K6
[
[
Sulfide]Z10 M.
b
K3
C
Sulfide]Z10 M and [H ]Z4.5 M.

4868
M. Ganesan et al. / Tetrahedron 61 (2005) 4863–4871
reaction has been followed with an increase in OD at
50 nm which is the characteristic absorption of Ru(II)
4
complex (Fig. 1). Thus, Ru(III) is reduced to Ru(II) in the
rate controlling step. Further, the formation of sulfide
radical cation as a transient is confirmed from its absorption
spectrum (Figs. 4 and 5). These experimental observations
are strongly in favour of electron transfer (ET) from sulfide
to Ru(III) in the rate determining step. The additional
support for the ET mechanism comes from the substituents
effect study. The reaction constant (r) value is around K1.0.
IV
This r value is close to the value reported for the Ru ZO
oxidation of organic sulfides wherein ET mechanism has
been proposed.
9
The formation of sulfoxide as the major product of the
reaction helps us to conclude that the major portion of
sulfide radical cation is consumed by the solvent, water or
superoxide ion formed due to the oxidative quenching of the
excited state Ru(II) ion with molecular oxygen, though
fragmentation and back ET may be competing processes.
The formation of sulfoxide from sulfide radical cation may
be shown as a three step process (Eqs. 9–11). Similar
formulation for the conversion of Ar S$C Me to ArS(O)Me
has been proposed in the polyoxometalate catalysed tert-
Figure 4. The absorption spectra of transients at different time intervals
3
C
formed from the reaction of [Ru(bpy)
aqueous CH
experiment.
3
]
with MPS in oxygen-saturated
CN (50% v/v) solution at 298 K by flash photolysis
3
5
8
butylhydroperoxide oxidation of organic sulfides.
3. Conclusion
3
C
The oxidation of 15 organic sulfides with five [Ru(NN)3]
ions proceeds through rate determining electron trans-
fer(ET) from the substrate to the oxidant. The ET nature of
the reaction is confirmed by the identification of the sulfide
cation radical using flash photolysis technique and for-
mation of Ru(II) ion as the product of the reaction. The
metal complex containing dpphen ligand behaves differ-
ently from other complexes indicating the importance of
hydrophobic interaction in this ET reaction. This study
highlights the importance of Ru(III) ions as suitable
reagents to have ET reactions with biologically important
organic sulfides.
Scheme 1.
4. Experimental
4
.1. Materials
2
C
0
bpy), 4,4 -dimethyl-2,2 -bipyridine (dmbpy), 4,4 -di-tert-
The [Ru(NN) ] complexes (where NNZ2,2 -bipyridine
3
0
0
0
(
0
butyl-2,2 -bipyridine (dtbpy), 1,10-phenanthroline (phen)
and 4,7-diphenyl-1,10-phenanthroline (dpphen) were syn-
thesised by known procedures
5
2,57,59–63
and the purity was
confirmed by absorption and emission spectra of the Ru(II)
complexes. Our group has already used these complexes for
5
3,64,65
photochemical studies.
The steady-state photolysis of
in 4.5 M H SO using a 500 W tungsten–
2
C
[Ru(NN)3]
halogen lamp led to the formation of corresponding Ru(III)
2
4
4
6–49
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 the
5 cm quartz cell filled with water and pyrex glass filter. It
was observed that the colour of the solution readily changed
from orange-yellow to green during irradiation. The
Figure 5. The absorption spectra of transients at different time intervals
3
C
formed in the electron-transfer oxidation of R
2
S with [Ru(bpy)
3
]
in
CN (50% v/v) solution at 298 K by flash
oxygen-saturated aqueous CH
photolysis experiment.
3

M. Ganesan et al. / Tetrahedron 61 (2005) 4863–4871
4869
4.3. Conventional flash kinetic spectrometer
Flash photolysis experiments were carried out using an
Applied Photophysics Ltd, KN-020 model flash kinetic
spectrometer. The cell solution containing an oxygenated
2
C
aqueous CH CN (50% v/v) solution of [Ru(NN) ]
and
sulfide was subjected to flash photolysis. The reaction
3
3
3
C
between the photogenerated [Ru(NN)3] and sulfide was
followed at different wavelengths. The output signal from
the PMT was fed into an International Electronics India
digital storage oscilloscope and hard copies of the traces
were obtained using a printer.
4.4. Kinetic measurements
A JASCO UV–Vis Spectrophotometer (Model 7800) was
2
C
employed to record the absorption spectra of [Ru(NN) ]
3
3
and [Ru(NN)3] complexes used in the present study and
C
3
C
to follow the electron transfer reactions of [Ru(NN)3]
complexes with organic sulfides. The kinetic study was
carried out in aqueous CH CN (50%, v/v) under pseudo
3
first-order condition. The progress of the reaction was
monitored by following the increase in absorbance of
2C
[
2
Ru(NN) ]
3
98 K.
(lmaxZ450 nm) at definite time intervals at
2
C
39
Figure 6. The absorption spectra of (—) [Ru(phen)
[Ru(phen)
3
]
and (– – –)
3
C
3
]
3
complexes in 50% CH CN.
The pseudo first-order rate constant (k ) for each kinetic run
1
was evaluated from the slope of linear plot of log OD vs
time by the method of least squares. The linearity of each fit
is confirmed from the values of correlation co-efficient (r)
and standard deviation(s). The second-order rate constant
(k ) is evaluated from the relation k Zk /[sulfide].
3
C
formation of [Ru(NN)3]
complexes was confirmed by
recording the absorption spectrum of the irradiated solution.
3
C
[
Ru(NN)3]
complexes showed peaks around 420–430
7,66,67
and 650–670 nm. and the absorption spectrum of
Ru(phen)3] is shown in Figure 6.
4
2
2
1
3
C
[
4.5. Product analysis
para-Methyl phenyl thiol was obtained from Aldrich and
6
8
converted to sulfide by the known procedure. p-Bromo,
p-cyano, p-fluoro and p-nitrophenyl methyl sulfides and
dialkyl sulfides purchased from Aldrich were used as
received. The other sulfides used in the present investigation
In a typical experiment, 0.5 mM of substrate (ArSMe) was
C
3
added to a 0.5 mM solution of [Ru(NN)3]
5
complex in
ml of aqueous CH CN (50% v/v). The solution was stirred
3
6
,21,26,69–71
at 298 K for w1 h depending upon the nature of sulfide and
Ru(III) complex. The products of the reaction were
extracted with chloroform and dried and the solvent was
removed. Then the resulting residue was analysed by IR
spectroscopy and GC. The IR spectrum of the product
were prepared and purified by reported procedures.
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 reagents and
solvents used in the system.
(
sulfoxide) was found to have stretching frequency in the
K1
characteristic region 1070–1030 cm . The GC analysis
(Model GC17A Shimadzu) of the product also indicated the
formation of sulfoxide as the only product under the present
experimental conditions. Sulfides and sulfoxides were
identified by their characteristic retention times and also
by co-injuction with authentic samples. The details of the
4
.2. Electrochemical measurements
The oxidation potentials of sulfides and the reduction
3
potentials of [Ru(NN)3]
using EG and G 273A Princeton Applied Research
Potentiostat/Galvanosat equipped with a X–Y-t-recorder.
The stock solutions of the metal complexes and sulfides for
the electrochemical studies were prepared in aqueous
C
complexes were determined
3
C
Table 3. Percentage of sulfoxide formed by the [Ru(NN)
aryl methyl sulfides
3
]
oxidation of
CH CN (50% v/v). The supporting electrolyte was 0.1 M
3
NaClO . A two compartment three electrode cell was used
4
6 4
p-XC H SMe; XZ
Percentage of sulfoxide formed
3C
3C
[Ru(bpy)3]
[Ru(phen)3]
to record the cyclic voltammograms. A glassy carbon
H
82
88
94
65
61
69
77
82
63
60
2
2
working electrode (areaZ0.07 cm ), 1 cm platinum plate
counter electrode and a standard calomel reference
electrode (SCE) were used in the electrochemical experi-
ments. Cyclic voltammograms were recorded after purging
the solution with nitrogen gas for 30 min.
Me
OMe
Cl
COOH
Conditions: refer Section 4.

4870
M. Ganesan et al. / Tetrahedron 61 (2005) 4863–4871
3
C
percentage of sulfoxide formed using [Ru(bpy) ]
3
[
and
14. Oae, S. In Organic Sulfur Chemistry; Structure and
Mechanism; CRC: Boca Raton, 1991.
3
C
Ru(phen)3] as oxidants are given in the Table 3.
1
5. (a) Garcia, H.; Iborra, S.; Miranda, M. A.; Primo, J. New
J. Chem. 1989, 13, 805. (b) Empling, G. A.; Wang, G.
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Acknowledgements
S.R. and R.R. thank the Department of Science and
Technology (DST) and University Grants Commission
1
6. Kamata, M.; Murakami, Y.; Tamagawa, Y.; Kato, M.;
Hasegawa, E. Tetrahedron 1994, 50, 1282–1286.
(
UGC), New Delhi for partial financial support. M.G. and
1
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references cited therein.
V.K.S. thank UGC and the management and Principal of
Vivekananda College, Thiruvedakam West for sanctioning
FIP study leave. The authors also thank Prof. P. Natarajan
and Dr. P. Ramamurthy, National Centre for Ultrafast
Processes, University of Madras, Chennai for their help with
recording transient spectra using flash photolysis technique.
1
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