Ru(III)-catalyzed oxidation of cysteine hydrochloride by methylene blue in acidic medium; synergetic effect of Cu(II): A kinetic study

Article abstract of DOI:10.1002/kin.20292

Cysteine hydrochloride and methylene blue (MB) interact in a molar ratio of 2:1 in acidic medium forming cystine and dihydromethylene blue, and the reaction is catalyzed by Ru(III). At low concentrations (ca. 2.0 × 10 ^-8 M), Cu(II) does not catalyze the reaction significantly but at this concentration level the catalytic activity of Ru(III) is found to be augmented by the addition of Cu(II) and the kinetics of Ru(III)-catalyzed reaction has been studied in the absence and in the presence of externally added Cu(II). The reaction follows a half-order kinetics in MB that increases to 3/4 on increasing [MB] beyond 1.5 × 10^-5 M in the Ru-catalyzed reaction, In the Ru-Cu catalyzed reaction; the order in MB is 3/4 even at lower concentrations of MB. The order in cysteine is unity. The rate decreases on increasing [MB] in both cases but attains a limiting value at higher concentrations of MB (ca. >2.0 × 10^-5 M) in the presence of Ru(III) alone. The rate increases on increasing [H⁺) and in Ru-catalyzed reaction, an optimum is noticed. The rate increases linearly with increasing [Ru(III)], but equilibration of the catalyst with other ingredients of the reaction system decreases the rate. The FTIR spectra of the reaction system exhibit time-dependent changes in the stretching as well as bending modes of - SH group. The synergetic effect of Cu(II) has been attributed to its ligation with cysteine and its subsequent interaction with Ru(II) produced in situ in the system. 2008 Wiley Periodicals, Inc.

Full text of DOI:10.1002/kin.20292

Ru(III)-Catalyzed Oxidation
of Cysteine Hydrochloride
by Methylene Blue in Acidic
Medium; Synergetic Effect
of Cu(II): A Kinetic Study
R. CHATURVEDI, K. K. MISHRA
Department of Chemistry, Rani Durgavati University, Jabalpur 482 001, India
Received 18 January 2007; revised 28 August 2007; accepted 7 September 2007
DOI 10.1002/kin.20292
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Cysteine hydrochloride and methylene blue (MB) interact in a molar ratio of 2:1 in
acidic medium forming cystine and dihydromethylene blue, and the reaction is catalyzed by
Ru(III). At low concentrations (ca. 2.0 × 10−8 M), Cu(II) does not catalyze the reaction signifi-
cantly but at this concentration level the catalytic activity of Ru(III) is found to be augmented
by the addition of Cu(II) and the kinetics of Ru(III)-catalyzed reaction has been studied in
the absence and in the presence of externally added Cu(II). The reaction follows a half-order
3
kinetics in MB that increases to on increasing [MB] beyond 1.5 × 10−5 M in the Ru-catalyzed
4
3
4
reaction. In the Ru–Cu catalyzed reaction; the order in MB is even at lower concentrations
of MB. The order in cysteine is unity. The rate decreases on increasing [MB] in both cases but
attains a limiting value at higher concentrations of MB (ca. >2.0 × 10−5 M) in the presence of
Ru(III) alone. The rate increases on increasing [H+] and in Ru-catalyzed reaction, an optimum
is noticed. The rate increases linearly with increasing [Ru(III)], but equilibration of the catalyst
with other ingredients of the reaction system decreases the rate. The FTIR spectra of the re-
action system exhibit time-dependent changes in the stretching as well as bending modes of
SH group. The synergetic effect of Cu(II) has been attributed to its ligation with cysteine and
its subsequent interaction with Ru(II) produced in situ in the system. ꢀ 2008 Wiley Periodicals,
C
Inc. Int J Chem Kinet 40: 145–150, 2008
pects [1–3]. For example, during simulation of the
functions of cytochrome P-450 enzyme with transition-
metal complexes [4], the ruthenium-catalyzed oxi-
dations of amines [5], amides, and β-lactams with
peroxides occur efficiently. In some cases, the use of
transition metal ions, such as osmium [6], iridium [7],
palladium [8] either alone or as binary mixtures as cat-
alyst, has been studied kinetically. Among the platinum
metals, Os as osmium tetroxide has been extensively
INTRODUCTION
In recent years, the oxidative transformations of sub-
strates such as ternary amines catalyzed by ruthenium
have found importance in enzymatic and synthetic as-
Correspondence to: K. K. Mishra; e-mail: ranu jbp@rediffmail.
com, ranu.chem@gmail.com.
c
ꢀ
2008 Wiley Periodicals, Inc.

146
CHATURVEDI AND MISHRA
used as a catalyst but because of its toxic nature [9], it
has largely been replaced by nontoxic and less probed
ruthenium compounds [10–12]. Recently, the binding
of cysteine and glutathione to Ru(II) and Ru(III) cen-
ters has been studied kinetically and the product re-
activities have been compared [13]. With this view,
we have investigated ruthenium-catalyzed oxidation of
cysteine, an important bioactive molecule with model
electron receptor, methylene blue, in acidic medium. It
has been observed that Cu(II) exerts a synergetic effect
on the catalytic activity of Ru(III), and thus the kinetic
features of Ru-catalyzed and Ru–Cu catalyzed reac-
tions have been compared. It is pertinent to mention
here that the kinetic features reported earlier for Cu(II)-
catalyzed oxidation of cysteine [14] differ appreciably
under aerobic and anaerobic conditions, whereas an
identical behavior was noticed in the present investi-
gations. This unequivocally highlights the specific role
of metal ions acting as catalysts for these reactions.
of the reaction mixture and those of the known sam-
ples of the disulfide and dihydromethylene blue (leuco
base) were recorded on Shimadzu FTIR spectrometer,
model no. 8400 S using CCl4 as a solvent.
The reaction vessels (Pyrex, England) were coated
black from outside to preclude the photochemical ef-
fect. The flasks, thoroughly cleaned with chromic acid
and potassium bicarbonate, were steam washed and
dried before use. The ionic strength of the system was
maintained at a constant level by adding the requisite
amount of KCl to the reaction system.
STOICHIOMETRY AND PRODUCT
ANALYSIS
Cysteine and methylene blue are found to interact in a
molar ratio of 2:1, and the empirical equation for the
reaction is given as
2RSH + MB → RSSR + H2MB
The formation of the disulfide and leucobase was con-
firmed spectrophotometrically by recording the ab-
sorption spectra of the reaction mixture after comple-
tion of the reaction. The absorption maxima, obtained
at 276 and 388 nm in the absorption spectrum of the
reaction mixture recorded after completion of the re-
action, were found to be in good agreement with those
found for the samples of cystine and leuco-MB respec-
tively, prepared by the known methods [15]. These
findings were further confirmed by recording FTIR
spectra of the known samples and those obtained for
the reaction mixture in CCl4.
EXPERIMENTAL
The solutions of L-cysteine hydrochloride (Cys, RSH)
and methylene blue (MB) were prepared by dis-
solving an exactly weighed quantity of the samples
supplied by Riedel Dehaen AG (Seelze, Hannover,
Germany) and E. Merck AG (Darmstadt, Germany),
respectively, in double-distilled water. The solution
of the catalyst Ru(III) was prepared by dissolving
1 g of RuCl3·3H2O (Johnson Matthey Chemicals Ltd.,
London) in 250 mL of 1 M HCI solution. This solution
after requisite dilution was used in the reaction system.
All other reagents, such as copper sulfate, hydrochloric
acid, potassium chloride etc. used in these investiga-
tions, were either E. Merck GR or BDH AnalaR grade
samples. Double distilled water was used throughout
these investigations.
As mentioned, the reaction showed identical behav-
ior under aerobic and anaerobic conditions and thus,
the kinetic runs were made under aerobic conditions.
The kinetics of the reaction was followed spectropho-
tometrically by measuring the absorbance of MB and
thereby calculating the depletion in concentration of
MB with time. For this purpose, a Thermo Spectronic
UNICAM UV-500 spectrophotometer set at 664 nm
was used since MB shows maximum absorbance at
664 nm in aqueous medium (εmax = 6.76 × 104 M−1
cm−1). The spectral interference due to leuco base and
the disulfide is ruled out because these species ab-
sorb strongly in the UV region of the spectrum. MB
is found to obey Beer’s law in aqueous medium in the
concentration range employed presently, and the spec-
trophotometer was thus calibrated. The FTIR spectra
RESULTS
The kinetic runs were followed in the presence
of Ru(III) (ca. 2.0 × 10−5 M) and Cu(II) (ca.
2.0 × 10−8 M) added independently to the system as
well as by adding them together. It was found that
Cu(II) does not show an appreciable catalytic activ-
ity at this concentration level. The reaction is cat-
alyzed by Ru(III), and the activity of this metal ion
increases when Cu(II) ions (ca. 2.0 × 10−8 M) are
added along with Ru(III). This is highlighted by the
comparative kinetic data given in Table I. It is seen
from these observations that the rate of reaction in-
significantly increases on adding Cu(II) alone. Fur-
thermore, the extent of reaction increases by about
14% when Cu(II) is added along with Ru(III) to the
system, but the rate constant (k3/4) increases by about
28%. This suggests that Cu(II) exerts some syner-
getic effect on Ru(III). The kinetics of the reaction
was investigated at different initial concentrations of
International Journal of Chemical Kinetics DOI 10.1002/kin

Ru(III)-CATALYZED OXIDATION OF CYSTEINE HYDROCHLORIDE BY METHYLENE BLUE
147
Table I Comparative Kinetic Data
Table III Rate Constants at Different [MB]
Time
(min)
Metal
Ion Added
k
× 105
[MB] × 105
k
1/2× 104
k
× 105
k
3/4
× 105
3/4
3/4
% Reaction (M1/4 s−1
)
(M)
(M1/2 s−1
)
(M1/4 s−1
)
(M1/4 s−1
)
35
35
35
Nil
Cu(II)
Ru(III)
3.0
12.5
83.9
95.4
–
0.5
1.0
1.5
2.0
2.5
3.0
2.9
2.7
2.6
–
–
–
–
–
–
5.2
4.5
4.5
11.7
8.7
6.9
6.6
5.6
4.6
(I)
(II)
0.38 (10.65%)
5.13 (2.26%)
6.59 (1.99%)
(III) 35 Ru(III) + Cu(II)
Solution conditions:
[Cys] = 2.0 × 10−3 M, [MB] = 2.0 × 10−5 M , Temp. = 35◦C.
(I): [Cu(II)] = 2.0 × 10−8 M; (II): [Ru(III)] = 2.0 × 10−5 M;
(III): [Ru(III)] = 2.0 × 10−5 M, [Cu(II)] = 2.0 × 10−8 M.
The coefficient of variance (C.V.) of the rate constants is given
in the parentheses.
Solution conditions:
[Cys] = 2.0 × 10−3 M, [HCl] = 7.0 × 10−3 M, Temperature =
35◦C, µ = 7.0 × 10−3 M.
(A) [Ru(III)] = 2.0 × 10−5 M; (B) [Ru(III)] = 2.0 × 10−5 M,
[Cu(II)] = 2.0 × 10−8 M.
cysteine (Table II). A plot of log [cys] against log k3/4
gives a straight line with a slope nearly equal to unity in
both cases; which shows that order in cysteine is unity.
The first-order dependence on the substrate was fur-
ther verified by the initial rate measurement method.
The Ru-catalyzed reaction (specified as A in the re-
spective tables) shows a half-order kinetics in MB at
strength. The dielectric constant of the medium was
varied by adding different volumes of acetone to the re-
action mixture. The rate constant decreases on adding
acetone to the reaction system (∼5%v/v), but it re-
mains unchanged on subsequent additions. The rate
constant increases linearly on increasing the concen-
tration of Ru(III) in both cases. The reaction shows a
second-order kinetics in MB at lower concentrations of
the catalyst (ca. <1.0 × 10−5 M), but this aspect could
not be investigated in detail because the runs become
erratic under these conditions. The double log plot be-
tween k3/4 and [Ru(III)] gives a straight line with a
slope of 1.0. The rate slightly increases on increasing
[Cu(II)] and subsequently attains a limiting value in
the case of Ru–Cu catalyzed reaction. The rate con-
stant decreases on increasing the time of equilibration
of Ru(III) with other ingredients of the reaction system
(Table IV). The additions of cystine and dihydromethy-
lene blue result in a change in the order in MB from
1
3
low [MB] but the order changes from to on in-
creasing [MB] beyond 1.5 × 10−5 M. Th2e rate4slightly
retards on increasing [MB], but it gradually attains
a limiting value at higher concentrations of MB. In
Ru–Cu catalyzed reaction (represented as B), k3/4 de-
creases with increasing [MB] (Table III). The order in
MB was also confirmed by van’t Hoff’s differential
method. It is observed that the rate of Ru-catalyzed
reaction increases on increasing [H+], but after attain-
ing a maximum value the rate gradually decreases. In
the Ru–Cu catalyzed reaction, the rate increases lin-
early on increasing [H+]. The rate increases slightly
on increasing the ionic strength of the reaction sys-
tem and attains a limiting value on adding KCl, NaCl,
and KNO3. The possibility of the participation of trace
metal ion impurities such as Cu(II) under these con-
ditions seems remote because in Ru–Cu catalyzed re-
action, the rate is not affected on varying the ionic
3
4
to unity, but the first-order rate constant remains un-
changed on subsequent variations in the concentration
of these products. To account for the kinetic influence
of Sn–HCl couple, used to reduce MB, identical blank
runs were made by adding tin, dissolved in HCl to the
reaction system. The activation parameters were eval-
uated by making the runs at different temperatures.
Table II Rate Constants at Different [Cys]
3
The order of reaction increases from to unity on de-
[Cys] × 103 k × 104
k
× 105 [Cys] × 103
k
3/4
(M1/4 s−1
)
× 105
creasing the temperature below 308 4K. ꢀH*, ꢀS*,
1
3/4
(M)
(s−1
)
(M1/4 s−1
)
(M)
and ꢀG* for the reaction were evaluated from the
3
4
1.00
1.50
1.75
2.00
2.50
3.00
7.3
–
–
–
–
–
1.00
1.50
2.00
2.50
3.00
3.50
3.6
5.0
6.6
7.9
8.8
Arrhenius plots when the order is and were found to
be 32.6 kJ, −235.3 J K−1, 105.0 kJ mol−1 and 51.7 kJ,
−232.2 J K−1, and 123.4 kJ mol−1 for Ru-catalyzed
and Ru–Cu catalyzed reactions, respectively.
3.7
4.1
5.2
6.1
7.1
–
11.1
Solution conditions:
DISCUSSION
[MB] = 2.0 × 10−5 M, [HCl] = 7.0 × 10−3 M, Temperature =
35◦C, µ = 7.0 × 10−3 M.
(A) [Ru(III)] = 2.0 × 10−5 M; (B) [Ru(III)] = 2.0 × 10−5 M,
[Cu(II)] = 2.0 × 10−8 M.
It is mentioned in the literature that in the pH
range 0.4–2.0, RuCl3·3H2O coordinates with water
International Journal of Chemical Kinetics DOI 10.1002/kin

148
CHATURVEDI AND MISHRA
Table IV Rate Constants at Different Times of
Equilibration
Table V Rate Constants at Different [Cys] After
Equilibration of Ru(III) for 30 Min with Other Ingredients
of the Reaction System
Time of Equilibration
of Ru(III) (in min)
k
× 105
k
× 105
3/4
3/4
[Cys] × 103 (M)
k3/4 × 105 (M1/4 s−1
)
(M1/4 s−1
)
(M1/4 s−1
)
0
5.2
4.9
4.5
4.0
6.6
5.4
5.6
5.2
1.50
1.75
2.00
2.50
3.00
3.3
3.4
4.3
4.4
5.6
10
20
30
Solution conditions:
[Cys] = 2.0 × 10−3 M, [HCl] = 7.0 × 10−3 M, Temperature =
[MB] = 2.0 × 10−5 M, Temperature = 35◦C, [Ru(III)] =
2.0 × 10−5 M, µ = 7.0 × 10−3 M, [HCl] = 7.0 × 10−3 M.
35◦C, µ = 7.0 × 10−3 M.
(A) [Ru(III)] = 2.0 × 10−5 M; (B) [Ru(III)] = 2.0 × 10−5 M,
[Cu(II)] = 2.0 × 10−8 M.
concentrations (ca. 2.0 × 10−8 M) does not show an
appreciable catalytic activity in the system, and thus it
seems that cysteine preferentially interacts with Ru(III)
under these conditions. It has already been shown that
Ru(III) on equilibration with cysteine retards the rate
that indicates that metal ion–substrate interaction is
time dependent. This was verified by studying the ef-
fect of variation of [cysteine] and [Ru(III)] after equili-
bration of [Ru(III)] with the ingredients of the reaction
system for 30 min (Tables V and VI) and also by record-
ing FTIR spectra of cysteine, in presence of Ru(III) at
different time intervals. On equilibrating cysteine with
Ru(III) for 10–30 min, stretching bands at 3635, 2985,
2892 cm−1 appear along with a new stretching vibra-
tional band at 2335 cm−1. The transmittance of the
new band as well as that of the bending vibrational
modes at about 600 cm−1 decreases gradually to attain
a constant value. Incidentally, the referred stretching
vibrational bands are not found on equilibrating cys-
teine with Cu(II). This again indicates that cysteine
preferably coordinates with Ru(III) under these condi-
tions and the interaction of cysteine with Ru(III) is time
dependent and perhaps the resulting reactive specie has
a distorted geometry.
molecules and exists as four major species, viz.,
[RuCl4(H2O)2]−, [RuCl2(H2O)4]+ , [RuCl5(H2O)]2−
,
and [RuCl(H2O)5]2+ besides RuCl36− [16–19]. Out of
these four species, [RuCl2(H2O)4]+ is stabilized in its
hydrolyzed from [RuCl4(H2O)3OH] according to the
equilibrium given below:
+
ꢁ
ꢂ
[RuCl · (H O) ] + H O
2
2
2
4
[RuCl2 (H2O)3 OH] + H3O+
(1)
In the present investigations, in Ru(III)-catalyzed
reaction, an optimum of rate constant with regard to
[H+] has beennoticed. Thus, onthebasis of thereferred
equilibrium (Eq. (1)), [RuCl2(H2O)3OH] seems to be
the reactive species. This argument is justified in the
light of the fact that the redox potential of MB increases
from +0.01 to + 0.5 V on decreasing the pH from 7.0
to 1.0 [20] and obviously, H+ ions seem to participate
in two diverse processes leading to the optimum.
In the Ru–Cu catalyzed reaction, the rate con-
tinuously increases on increasing [H+] and thus
[RuCl2(H2O)4]+ is likely to be the reactive species.
For the sake of brevity, these two species, viz.
[RuCl2(H2O)3OH] and [RuCl2(H2O)4]+ and those
with +2 oxidation state of ruthenium, have been sim-
ply represented as Ru(III) and Ru(II), respectively, in
the foregoing discussion.
The existence of reactive species eluding an easy
stereochemical interpretation has already been pointed
out by Taylor and coworkers, and these species have
Methylene blue exists in the protonated form in
acidic medium [21], and cysteine is known to form
complexes with metal ions [22]. Furthermore, Cu(II)
and Ru(III) ions have been used earlier as catalysts for
the oxidation of sulfhydryl compounds [23] and here
too, a complex Ru(III)SR is presumed to be formed be-
tween Ru(III) (coordination number six) and cysteine.
It may be pointed out that according to the Irving–
Williams series, the coordination ability of Cu(II) with
cysteine seems to be much higher than that of Ru(III)
species, but the kinetic results reveal that Cu(II) at low
Table VI Rate Constants at Different [Ru(III)] After
Equilibration for 30 Min
[Ru(III)] × 105 (M)
k3/4 × 105 (M1/4 s−1
)
1.0
1.5
2.0
2.5
3.0
2.0
3.2
3.9
5.1
6.4
[Cys] = 2.0 × 10−3 M, Temperature = 35◦C, [MB] = 2.0 ×
10−5 M, µ = 7.0 × 10−3 M, [HCl] = 7.0 × 10−3 M.
International Journal of Chemical Kinetics DOI 10.1002/kin

Ru(III)-CATALYZED OXIDATION OF CYSTEINE HYDROCHLORIDE BY METHYLENE BLUE
149
been termed as activated intermediates [24]. Thus,
Thus,
[Ru(II)] =
K1
MB + H+
ꢀ MBH+
(2)
(3)
k1k4[Ru(III)SR]
−1 (k1k4)1/2 + k2[MBH+]
(13)
ꢁ
k
RSH + Ru(III) K2ꢀ Ru(III)SR + H+
ꢁ
On substituting [MBH+] = K1[MB][H+] and [Ru(III)
SR] = K2[Ru(III)] [RSH]/H+], the rate of reaction is
given as
The complex Ru(III)SR may facilitate electron transfer
to form transient Ru(II) and RS• radical. Ru(II) may
subsequently react with cysteine and protonated MB
to produce HM• (half reduced MB radical) which may
give the end products. Thus,
d[MB]
k1k2k4K1K2[RSH][MB][Ru(III)]
−
=
(14)
k
−1 (k1k4)1/2 + k2K1[MB][H+]
dt
k1
Ru(III)SR
ꢀ Ru(II) + RS•
(4)
Equation (14), thus, predicts a fractional-order
kinetics in methylene blue and first-order kinetics
in Ru(III) and cysteine as has been noticed
experimentally.
ꢁ
k
−1
k2
Ru(II) + MBH+ −→ Ru(III) + HM• + H+ (5)
k3
RSH + HM• −→ RS• + H2M
(6)
It seems that Cu(II) when added to the system co-
ordinates with cysteine as reported in the literature
whereas cysteine interacts with Ru(III) leading to its
reduction to Ru(II) and simultaneous coordination of
the substrate with this metal ion. The labile complex
Cu(II) SR oxidizes Ru(II) produced in situ in the sys-
tem and gives Cu(I). This view is supported by the fact
that a new stretching vibrational band for the Cu–cys
system appears at 2934 cm−1 after equilibration for
10 min and it subsequently disappears. Thus,
k4
2RS• −→ RSSR
fast
(7)
It is pertinent to mention here that the oxidation of
thiols to disulfide has been widely reported to involve
RS• radical [25], whereas the formation of HM• radical
(analogous to semiquinone radicals [26]) by the action
of reducing substrates, such as ascorbic acid on MB,
has been reported in the literature [27,28]. The partic-
ipation of free radicals in the reaction system has been
qualitatively confirmed by the positive polymerization
test with acrylonitrile [29].
K3
CuII + RSH
ꢀ CuII SR + H+
(15)
(16)
ꢁ
The rate of reaction for the above scheme would be
given by the expression
k1ꢁ
RuIII + RSH
ꢀ RuII SR + H+
ꢁꢁ
k−1
k2ꢁ
d[MB]
RuIISR −→ RuII + RS•
(17)
(18)
−
= k2Ru[II][MBH+]
(8)
dt
k3ꢁ
On presuming steady state for Ru(II), RS•, and HM•
RuII + CuIISR −→ RuIII SR + CuI
RuII + MBH+ −→ RuIII + HM• + H+ (19)
radicals
k4ꢁ
k2[Ru(II)][MBH+]
[HM•] =
(9)
k3[RSH]
Cu(I) may in turn be oxidized by HM• radical in acidic
medium to produce the end products as shown below:
[RS•] =
−k−1[Ru(II)]
k2 [Ru(II)]2+4k4(k1[Ru(III)SR]−k2[Ru(II)][MBH+])}1/2
{
−1
k5ꢁ
2k4
Cu I + HM• + H+ −→ Cu II + H2M (20)
(10)
fast
2RS• −→ RSSR
(21)
On neglecting the negative quantities
−k2 [Ru(II)]2+4k4(k1[Ru(III)SR]−k2[Ru(II)][MBH+])}1/2
{
On presuming step (19) as the rate-determining step
and applying steady state for Ru(II) and Ru(II)SR; the
rate of reaction is given as
[RS•] =
−1
2k4
(11)
Since the term involving [Ru(II)] will be comparatively
smaller, Eq. (11) gives
d[MB]
k1ꢁ k2ꢁ k4ꢁ [Ru (III)][RSH][MBH+]
(k−ꢁ 1[H+] + k2ꢁ )(k3ꢁ [Cu (II) SR] + k4ꢁ )
−
=
(22)
dt
(k1k4[Ru(III)SR])1/2
[RS•] =
(12)
On substituting the values of [Cu(II) SR] and MBH+,
k4
International Journal of Chemical Kinetics DOI 10.1002/kin

150
CHATURVEDI AND MISHRA
the rate expression is
BIBLIOGRAPHY
d[MB]
dt
1. Murahashi, S. I.; Komiyo, N.; Terai, H.; Nakae, T. J Am
Chem Soc 2003, 125, 15312.
−
=
k1ꢁ k2ꢁ k4ꢁ K1[Ru (III)][RSH][MB][H+]2
2. Vasak, M. J Trace Elem Med Biol 2005, 19(1), 13.
3. Frasca, D. R. Article Boston College 2000, 2528.
4. Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidation
of Organic Compounds; Academic Press: New York,
1981; p. 355.
(23)
kꢁ [H+] + kꢁ kꢁ K [Cu (II)][RSH] + kꢁ K [MB][H+]2
(
2)(
)
3
1
−1
3
4
5. Murahashi, S. I. Angew Chem, Int Ed Engl 1995, 34,
2443.
6. Perumal, S.; Alagumalai, S.; Selvaraj, S.; Arumugam,
N. Tetrahedron 1986, 42, 610.
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Singh, M. J Phy Org Chem 2004, 17, 249.
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Singh, B. J Mol Catal 2003, 197, 91.
Since [Cu(II)] is very small, Eq. (23) explains a first-
order kinetics in Ru(III) and cysteine, a near zero order
in Cu(II) and a fractional order in MB. The rate ex-
pression also predicts a complex dependence of rate
on [H+], and in fact the relation between the rate of
oxidation of thiols and [H+] is generally found to be
highly complex [30].
It has been observed that the rate decreases on equi-
librating Ru(III) with other ingredients of the reaction
system, and the metal ion-substrate interaction is time
dependent. It seems that the equilibration of metal ion
with the substrate results in the formation of a less
labile complex, and consequently the concentration
of the complex Ru(III)SR decreases in the reaction
system resulting in a decrease in rate. It may, how-
ever, be emphasized that the metal–substrate interac-
tion in these reaction systems eludes an easy interpre-
tation, and the kinetic probe in this direction is being
planned.
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CONCLUSIONS
The kinetic studies reported presently indicate that the
rate of reaction decreases on equilibration of Ru(III)
with cysteine, which suggests that the metal ion–
substrate interaction is time dependent. It is further
confirmed by the gradual decrease in transmittance of
the stretching mode at 2335 cm−1 and the bending vi-
brational mode at about 600 cm−1 on equilibrating cys-
teine with Ru(III) for 10–30 min, which subsequently
attains a constant value. This suggests that perhaps
the reactive specie has a distorted geometry. The cat-
alytic activity of Ru(III) is appreciably increased in the
presence of Cu(II) in the reaction system, and such a
behavior in these systems has been noticed for the first
time, which promises to be an exciting aspect of metal
ion catalysis.
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McGraw-Hill: New York, 1960; p. 611.
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London, 1961; p. 11, 99.
The instrumental facility provided by the Department of Sci-
ence and Technology, Government of India under the FIST
program to the Department of Chemistry, Rani Durgavati
University, Jabalpur, is gratefully acknowledged.
International Journal of Chemical Kinetics DOI 10.1002/kin