Kinetics and mechanism of the reaction of dichlorotetraaquaruthenium(III) and thiols

Article abstract of DOI:10.1071/CH11352

The formation of an intermediate ruthenium(III) thiolate complex by the interaction of thiols, RSH (R=glutathione and L-cysteine) and dichlorotetraaquaruthenium(III), [Ru^IIICl₂(H ₂O)₄]⁺, is reported in the temperature range 25-40°C. The kinetics and mechanism of formation of the intermediate complex were studied as a function of [Ru^IIICl₂(H ₂O)₄]⁺, [RSH], pH, ionic strength and temperature. Reduction of the intermediate complex takes place slowly and results in the corresponding disulfides RSSR and [Ru^IICl ₂(H₂O)₄]⁺. The results are interpreted in terms of a mechanism involving a rate-determining inner-sphere one-electron transfer from RSH to the oxidant used in the present investigation and a comparison of rate and equilibrium constants is presented with activation parameters.

Full text of DOI:10.1071/CH11352

CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 113–120
Full Paper
http://dx.doi.org/10.1071/CH11352
Kinetics and Mechanism of the Reaction
of Dichlorotetraaquaruthenium(III) and Thiols
A
,
C
B
A
Suprava Nayak,
Gouri Sankhar Brahma, and K. Venugopal Reddy
A
Department of Chemistry, Osmania University, Hyderabad-500 007, India.
B
Faculty of Science and Technology, The ICFAI University, Dehradun-248197, India.
C
Corresponding author. Email: suprava7107@gmail.com
The formation of an intermediate ruthenium(III) thiolate complex by the interaction of thiols, RSH (R ¼ glutathione and
III
L-cysteine) and dichlorotetraaquaruthenium(III), [Ru Cl (H O) ] , is reported in the temperature range 25–408C. The
þ
4
2
2
III
kinetics and mechanism of formation of the intermediate complex were studied as a function of [Ru Cl (H O) ] , [RSH],
þ
2
2
4
pH, ionic strength and temperature. Reduction of the intermediate complex takes place slowly and results in the
II
corresponding disulfides RSSR and [Ru Cl (H O) ] . The results are interpreted in terms of a mechanism involving a
þ
2
2
4
rate-determining inner-sphere one-electron transfer from RSH to the oxidant used in the present investigation and a
comparison of rate and equilibrium constants is presented with activation parameters.
Manuscript received: 26 August 2011.
Manuscript accepted: 18 October 2011.
Published online: 25 November 2011.
Introduction
In recent years, much attention has been given to the role of Ru
complexes in biological processes owing to their very promising
group. CySH, one of the simplest biological thiols, exists widely
inside and outside cells and it behaves as a one-electron donor in
oxidation processes either by metal ions or by non-metallic
oxidants to form the corresponding sulfinic acid or disulfide,
depending on the oxidative capability of the oxidant,
while GSH is a tripeptide (g-L-glutamyl-L-cysteinyl-glycine)
containing the core amino acid CySH and is the most common
[17]
cellular non-protein thiol. In cells, it exists predominately in
the reduced form (GSH) at concentrations of 0.1–10 mM and is
III
[
1,2]
anticancer activity and as a better alternative to platinum.
III
[14–16]
Till now, few Ru complexes and their analogues that possess
potential properties in preventing the formation and growth of
[
3–5]
tested tumours have entered clinical trials.
Though the mode
III
of action of these new Ru complexes is not fully clarified yet, it
was suggested that various metabolic transformations take place
post-injection that commonly involve ligand substitution and
redox reactions occurring in the blood or in the cell cytoplasm.
Reducing agents such as glutathione (GSH) in the cell and
ascorbic acid in the blood have the propensity to reduce com-
plexes and this process needs to be taken into account because it
readily oxidized to the disulfide (GSSG, E8) ꢀ0.246 V versus
[
18]
normal hydrogen electrode. Also, understanding the mecha-
nism of the oxidation of RSH by metal complexes may be
beneficial in developing rapid and reliable sensing techniques
[
19–21]
for the detection of biological thiols.
[
6,7]
can influence their anticancer activity.
In the present investigation, we sought to address the
following points: (i) elucidation of plausible mechanisms and
deduction of appropriate rate laws; (ii) detection of various
reactive species and reaction products; (iii) understanding of the
redox chemistry of thiols in the presence of the ruthenium
metal ion; and (vi) comparison of the reactivity of GSH with
To gain some knowledge about their mechanisms of
action, we undertook studies on the oxidation kinetics of
RSH (R ¼ GSH, L-cysteine (CySH) (Fig. 1)), the most
important a-aminothiols in the biological domain, using
III
þ
Ru Cl (H O) ] as oxidant. Taqui Khan et al. have reported
2 2 4
[8]
[
III
the kinetics and spectroscopic study of the oxidation of
CySH towards the Ru centre.
III
L-ascorbic acid by [Ru Cl (H O) ] . Though proton-coupled
þ
2
2
4
III
electron transfer is usually exhibited by Ru complexes while
undergoing redox reactions with various reductants, the course
Results and Discussion
III
of electron transfer undergone by chloro complexes of Ru is
Stoichiometry and Reaction Products
The stoichiometry of the reaction shows that one mole of
a subject of great interest owing to its abnormal speciation
[
9]
III
substrate reacts with one mole of Ru . In order to establish
the stoichiometry of the redox reaction, a 1:2 ratio molar mixture
behaviour in aqueous acidic medium.
Thiol-containing compounds play important roles in bio-
chemical processes, including the maintenance of cellular redox
III
of Ru complex and RSH at pH 2.0 was left overnight at room
[
10–12]
potentials, regulating metabolism and gene expression
and protecting cells against oxidative stress and heavy metal
temperature under anaerobic conditions i.e. by passing nitrogen
gas of high purity through the reaction mixture. The unreacted
[
13]
[22]
toxicity. In the latter case, GSH scavenges and sequesters
heavy metal ions by coordinating them through its sulfhydryl
RSH was estimated iodometrically.
1:1 stoichiometry.
The result confirmed a
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

1
14
S. Nayak, G. S. Brahma, and K. V. Reddy
2
O
OH
1
O
OH
NH
O
OH
H
NH
HS
H3ϩN
3
H3ϩN
O
O
SH
4
L-cysteine
Glutathione
Fig. 1. Structure of reductants (RSH).
K2
Ð
ꢀ
2ꢀ
þ Hþ
The reaction products were identified as the disulfide (RSSR)
HA
A
ð3Þ
of RSH (i.e. GSSG for oxidized glutathione, and L-cystine
disulfide for L-cysteine). The product of GSH was identified
by comparison with the UV-vis absorption spectrum (see Fig. S1
The pK and pK for GSH at 258C and I ¼ 0.5 M have been
1
2
[
24]
reported as 2.05 and 3.40 respectively.
CySH behaves as a
in the Supplementary Material) of the pure glutathione oxidized
(
monoprotic acid (HA) and its COOH dissociation takes place in
the following manner:
III
GSSG) in aqueous solution (here the Ru to GSH ratio was
kept at 1:1). The product of CySH was identified as L-cystine
qualitatively by melting point measurements (mp ,2608C) and
its IR spectrum (see Fig. S2 in the Supplementary Material). The
K3
Ð
ꢀ
HA
A þ Hþ
ð4Þ
ꢀ
1
infrared spectrum (in KBr) displayed a peak at 453 cm as
[
25]
ꢀ
1
[23a]
with pK ¼ 1.9 at 258C and I ¼ 1.0 M. The proton abstraction
opposed to 460 cm characteristic of a disulfide bond.
The
other peaks at 1338, 1406 and 1592–1656 cm were assigned to
3
ꢀ1
from N and S of GSH and CySH is only evident at pH $ 7.0. The
dissociation constants of GSH and CySH obtained in the present
ꢀ
the cystine CꢀC, ꢀCOO , and C=O group stretching frequen-
[
23b]
study directly by potentiometric titration at different tempera-
3
cies respectively.
the oxidation products.
This confirmed the presence of cystine in
tures at I ¼ 0.5 M were: K ꢃ 10 : 8.91 (258C), 12.20 (308C),
1
3
1
3
(
5.97 (358C) and 21.32 (408C) for GSH (H A) and K ꢃ 10 :
The overall stoichiometry of oxidation of RSH can be
represented by the equation
2
3
.63 (258C), 4.86 (308C), 6.52 (358C) and 8.73 (408C) for CySH
HA). This is in agreement with reported values.
[
24,25]
III
þ
II
þ
4
2
½Ru Cl2ðH2OÞ ꢁ þ 2RSH Ð 2½Ru Cl2ðH2OÞ ꢁ
4
III
1
Interaction of RSH with [Ru Cl (H O) ]
þ
2
2
4
þ RSSR þ 2H
ð1Þ
Preliminary Observations
The absorption spectrum (dark amber colour) of freshly
III
prepared Ru solution shows an intense band at 376 nm, which
III
Species of Ru Complex
[
9]
It is mentioned in the literature that in the pH range 0.4–2.0,
RuCl ꢂ 3H O coordinates with water molecules and exists as
can be assigned to the ligand to metal charge transfer (LMCT)
ꢀ
III
III
þ
band Cl - Ru . The reaction of RSH and [Ru Cl (H O) ]
4
3
2
2
2
ꢀ
þ
4
four major species, viz. [RuCl (H O) ] , [RuCl (H O) ] ,
at pH ,2.0 was indicated by an instantaneous change in colour
of the reaction mixture, from dark amber to yellowish brown,
and then to a colourless solution with consequent and progres-
sive decrease in absorbance at 376 nm (see Fig S3 in the
Supplementary Material). The initial rapid change in colour
may be attributed to outer-sphere complex formation between
4
2
2
2
2
2
ꢀ
RuCl (H O)] and [RuCl(H O) ] . Out of these four species,
5 2 2
2þ
[
5
III
III
þ
Ru exclusively exists as Ru Cl (H O) , the most stable
4
2
2
species, at pH ,2.0. In the present study, all the reactions were
carried out at pH ,2.0, in order to ensure no interference from
the other aquaruthenium(III) species.
III
þ
Ru Cl (H O) ] and RSH, whose kinetics of formation were
2 2 4
[
pK of RSH
monitored at 376 nm using stopped-flow spectrophotometer,
and the consequent decrease of absorbance is attributed to the
GSH exists in a fully protonated form and it has four dissociable
protons: the L-glutamyl carboxylic acid group, glycyl carboxylic
acid group, sulfhydryl group and ammonium group, as indicated
by 1–4 in Fig. 1. At the studied pH, fully protonated glutathione
III
þ
II
reduction of [Ru Cl (H O) ] to the [Ru Cl (H O) ] spe-
2þ
2
2
4
2
2
4
cies by RSH, which was studied by conventional UV-vis
spectrophotometry. No such significant colour change was
III
noticed on mixing Ru complex with CySH.
behaves as a diprotic acid (H A) with the ionization of the
2
L-glutamyl carboxylic acid group (K ) and the glycyl carboxylic
1
acid group (K ) at 258C and iconic strength, I ¼ 1.0 M. Disso-
Rate of Complexation
2
ciation of GSH occurs through the following equilibrium steps
(hereafter the GSH species are written as H A, HA and A and
The absorbance versus time curve for the complexation
III
reaction between RSH and Ru at an appropriate time scale
displayed monophasic kinetics with a single exponential
curvature. The monophasic reaction progress in the stopped-
flow time scale remaining unaffected for a substantial time
indicates that the subsequent electron transfer within the
ꢀ
2ꢀ
2
ꢀ
CySH species are written as HA and A in equations and
schemes):
K1
Ð
ꢀ
þ
H2A
HA þ H
ð2Þ

Kinetics and Reaction Mechanism of Ru and Thiols
115
1
1
40
20
A
Table 1. Rate constants (kobs) and activation parameters for the
III
formation of the intermediate complex between [Ru Cl
1
2
2 2 4
(H O) ]
and glutathione (GSH) at pH 2.0 ± 0.05 and ionic strength 0.5 M
III
B
C
1
[
Ru
]
[GSH]
ꢀ3
T
Temperature
k
obs
ꢀ1
k
1
ꢀ4
3
ꢀ1 ꢀ1
100
[
10 M]
[10 M]
[8C]
[s
]
[10 M
s ]
2
2
2
2
2
2
2
4
6
8
1
2
2
2
2
4
25
25
25
25
25
25
25
25
25
25
25
30
35
40
14.78 ꢄ 1.9
28.72 ꢄ 2.2
41.36 ꢄ 0.9
57.26 ꢄ 1.2
71.35 ꢄ 3.8
88.11 ꢄ 6.3
104.3 ꢄ 7.3
27.86 ꢄ 1.9
28.94 ꢄ 0.8
27.75 ꢄ 2.1
28.55 ꢄ 0.9
20.01 ꢄ 1.2
26.73 ꢄ 1.7
36.81 ꢄ 2.6
15.68
8
6
0
0
15.24
14.63
15.19
15.14
15.58
15.81
14.78
15.35
14.72
15.14
18.21
21.73
27.03
6
8
10
12
14
4
40
4
2
0
4
0
4
2
0
2
0
0.005
0.01
0.015
2
[
RSH] [M]
T
A
B
#
ꢀ1
#
ꢀ1
ꢀ1
DH 7.6 kJ mol ; DS ꢀ64.75 J K mol
.
T
Fig. 2. Dependence of kobs on [RSH] for the formation of complex
Observed rate constant.
III
between [Ru Cl
þ
ꢀ4
C
2
(H
2
O)
4
]
(2.0 ꢃ 10 M) and RSH at 258C, pH
Second order complex formation rate constant.
2
.0 ꢄ 0.01. RSH ¼ Glutathione (GSH) (r) and L-cysteine (CySH) (’).
III
Table 3. pH dependence for intermediate (Ru ]GSH) complex
A
Table 2. Rate constants (kobs) and activation parameters for the
III
), [Ru Cl
1
24
formation of at 258C, I 0.5 M (NaClO
4
2
(H
2
O)
4
]
2 3 10
M
III
formation of the intermediate complex between [Ru Cl
1
2
(H O)
2
4
]
A
A] 2 3 10
23
and [H
3
2
M
and cysteine (CySH) at pH 1.99 ± 0.02 and ionic strength 0.5 M
ꢀ1 ꢀ1
k
1
¼ (15.10 ꢄ 0.7) ꢃ 10 M
s
(calculated from slope of Eqn 6)
III
B
1
[
[
Ru
]
[CySH]
ꢀ3
T
Temperature
k
obs
ꢀ1
k
B
ꢀ1
C
1
3 ꢀ1 ꢀ1
[10 M s ]
pH
k
obs [s
]
k
ꢀ4
10 M]
3 ꢀ1 ꢀ1
[10 M
[10 M]
[8C]
[s
]
s ]
1
1
1
2
2
.49
6.61 ꢄ 0.5
10.20 ꢄ 1.1
14.52 ꢄ 1.2
19.52 ꢄ 1.5
23.87 ꢄ 1.9
15.28
15.74
15.41
15.78
16.37
2
2
2
2
2
4
6
8
1
2
2
2
2
4
25
25
25
25
25
25
25
25
25
30
35
40
16.23 ꢄ 0.9
34.16 ꢄ 1.5
52.12 ꢄ 3.6
71.26 ꢄ 4.2
90.01 ꢄ 3.8
17.35 ꢄ 1.4
16.95 ꢄ 0.8
17.41 ꢄ 1.1
17.11 ꢄ 1.3
22.37 ꢄ 1.8
30.46 ꢄ 2.3
40.93 ꢄ 2.8
30.46
.73
32.06
32.61
33.44
33.79
32.57
31.82
32.68
32.12
34.19
38.58
43.91
.99
.26
.48
6
8
10
2
A
B
Undissociated species of GSH.
2
Observed rate constant.
2
C
Second order complex formation rate constant.
0
2
2
2
III
Table 4. pH dependence for the intermediate (Ru ]CySH) complex
formation at 258C, I 0.5 M NaClO
2
1
24
4
, [RuCl (H
2
2
O)
4
] 2 3 10 M and
A
[HA] 2 3 10
23
A
B
#
ꢀ1
#
ꢀ1
ꢀ1
M
DH 11.3 kJ mol ; DS ꢀ94.35 J K mol
.
3
ꢀ1 ꢀ1
k
1
¼ (34.35 ꢄ 1.1) ꢃ 10 M
s
(calculated from slope of Eqn 7)
Second order complex formation rate constant.
B
ꢀ1
C
1
3 ꢀ1 ꢀ1
[10 M s ]
pH
k
obs [s
]
k
1
1
1
2
2
.52
7.17 ꢄ 0.5
11.20 ꢄ 1.1
16.48 ꢄ 1.2
23.75 ꢄ 1.4
31.27 ꢄ 1.9
33.41
32.29
31.46
31.14
30.22
complex is sufficiently slow to be discarded in the time scale of
the formation of the precursor complex. Such precursor complex
formation before electron transfer between ruthenium(III) sub-
strates and the reductants CySH and GSH has been reported
.76
.99
.23
.47
[
26–28]
earlier.
The values of the observed rate constant for the formation of
the precursor complex as a function of [RSH] , [complex] and
A
B
Undissociated species of CySH.
Observed rate constant.
T
C
Second order complex formation rate constant.
temperature at I ¼ 0.5 M are given in Tables 1 and 2. The
pseudo-first-order rate constants k are independent of the
obs
III
concentration of [Ru Cl (H O) ] and increased linearly with
þ
2
2
4
[
28,29]
increasing RSH concentration, and exhibited near zero intercept
see Fig. 2) indicating that neither a reverse nor a parallel
agrees with that reported earlier.
The pH dependence of the
(
complex formation reaction was studied in the pH range
1.50–2.50 (see Tables 3 and 4) and the rate of complex formation
was found to increase with pH, showing inverse dependence on
reaction contributes significantly to the observed kinetics. This
behaviour is due to first-order dependence of rate on [complex]
and [substrate]. The nature of graphs found in the present study
þ
[H ]. The mechanism for the initial formation of the precursor

1
16
S. Nayak, G. S. Brahma, and K. V. Reddy
outer-sphere complex for GSH is represented by the following
scheme.
Table 5. Rate constants for the electron transfer between
III
1
[Ru Cl
complex at pH 1.99 ± 0.06, temperature 25.0 ± 0.18C, ionic strength
.5 M
2
(H
2
O)
4
]
and glutathione (GSH) within the outer-sphere
0
þ
ꢀ
k1
III
½
Ru Cl2ðH2OÞ ꢁ þ HA ꢀ! Complex I
4
III
A
obs
B
2
[
Ru
]
[GSH]
ꢀ3
k
k
ꢀ4
ꢀ2 ꢀ1
2 ꢀ1 ꢀ1
[10 M
[
10 M]
[10 M]
[10
s
]
s
]
The rate expression forthe reaction interms ofkobs iswritten as
2
2
2
2
2
2
2
1
3
4
5
2
4
1.10 ꢄ 0.06
1.88 ꢄ 0.05
3.06 ꢄ 0.09
3.97 ꢄ 0.06
4.81 ꢄ 0.09
5.76 ꢄ 0.12
6.51 ꢄ 0.16
0.65 ꢄ 0.04
2.57 ꢄ 0.07
3.62 ꢄ 0.05
4.49 ꢄ 0.04
0.55
þ
kobs ¼ k1K1½H2Aꢁ =½H ꢁ þ K1
ð5Þ
0.94
1.53
1.98
2.40
2.88
3.25
0.65
0.86
0.91
0.90
T
6
8
where K1 is the dissociation constant of H A as given in
2
10
12
14
4
Eqn 2, and
ꢀ
½
H2Aꢁ ¼ ½HA ꢁ þ ½H2Aꢁ
T
e
e
4
4
where the subscript ‘T’ represents the total concentration of
GSH and the subscript ‘e’ denotes the concentration at
equilibrium.
4
A
B
Observed rate constant.
Double reciprocal fit to Eqn 5 yields
Second order reduction rate constant.
ꢀ1
obs
þ
k
¼ 1=k1½H2Aꢁ þ ½H ꢁ=k1K1½H2Aꢁ
ð6Þ
T
T
III
from the reaction of L-ascorbic acid with [Ru Cl (H O) ] was
þ
2
2
4
A similar mechanism is predicted for the reaction of
[8]
reported by Taquikhan et al. and the formation of thiolatopen-
taammineruthenium(III) complex from the reaction between
III
þ
Ru Cl (H O) ] with CySH, for which the equation can be
2 2 4
[
written as
[30]
Cl(NH ) Ru]Cl and GSH was reported by Kuehn et al. and
[
Fracsca et al.
3 5 2
[
31]
ꢀ1
obs
þ
k
¼ 1=k1½HAꢁ þ ½H ꢁ=k1K3½HAꢁ
ð7Þ
T
T
Equilibrium Studies for the Formation of the Ru–RSH
Complex
where K3 is the first dissociation constant value of CySH
(or HA) (Eqn 4).
The first and second dissociation constants of GSH and
CySH determined at 258C (I ¼ 0.5 M) are reported in the previ-
Substituting the values of K and K in the slope of the plot of
1
3
ꢀ
1
obs
þ
k
versus [H ] in Eqns 6 and 7 yields the second-order rate
3
ous section. The equilibrium constants (K ) for outer-sphere
eq
constants k at 258C to be (15.10ꢄ 0.7)ꢃ 10 and (34.35ꢄ 1.1) ꢃ
III
Ru –GSH and Ru –CySH intermediate complexes were
III
1
3
0 M
ꢀ1 ꢀ1
1 s for GSH and CySH respectively. The rate constants
k1 computedusingthe respectivedissociationconstantsKinEqn 5
[32]
directly calculated by spectrophotometric methods at differ-
ent temperatures by fitting to the following equation:
are fairly constant with respect to variation of reductants, pH and
III
þ
3
[
Ru Cl (H O) ] , with an average value of (15.21ꢄ 0.32) ꢃ 10
l
l
max
l
2
ꢀ1
2
4
DA ¼ DA
ꢀ DA =Keq½Lꢁ
ð8Þ
ꢀ
1
3
ꢀ1 ꢀ1
eq
M for GSH and (32.39ꢄ 0.72) ꢃ 10 M
Values of k obtained using both methods are in excellent
s
s
for CySH.
1
l
where DA is the absolute value of the difference in absorbance
between solutions containing Ru complex and ligand L (where
agreement with each other. These results indicate that there is
III
III
no significant reaction of [Ru Cl (H O) ] with undissociated
þ
2
2
4
III
L ¼ GSH or CySH) and solutions containing Ru alone at the
species of RSH (i.e. H A for GSH or HA for CySH) under the
2
same concentration. [L] is the equilibrium concentration of the
eq
present experimental conditions. A comparative value of k (in
1
l
l
ꢀ
1
ꢀ1
[28]
) of 332 at pH 5.0, 170 at pH 5.5 and 271 ꢄ 10 at
ligand and DA
Values of equilibrium constants obtained by least-square fitting
is the maximal DA obtainable at high [L] .
max eq
M
s
[
29]
III
pH 6.0
has been reported for the reaction of [Ru (edta)
ꢀ
5
ꢀ
H O)] with cysteine and other thio-amino acids. Formation of
2
of absorbance data to Eqn 8 are: Keq ꢃ 10 : 113 (258C), 168
(
ꢀ1
III
(308C), 211 (358C) and 242 (408C) M for Ru –GSH, and
III
Ru –RSH precursor complexes is largely guided by the steric
effect, as evident from the values of the formation rate constant
with GSH and CySH. Kinetic studies indicate the fact that
precursor complex formation involving GSH is relatively hin-
dered owing to the bulkiness of the tripeptide, whereas aqua
ligand substitution is facilitated in the case of CySH as the
incoming ligand. Activation parameters for the formation of the
precursor complexes are given in Tables 1 and 2. Low positive
values of the enthalpy of activation together with negative values
of the entropy of activation are compatible with an associative
ꢀ
5
K ꢃ 10 : 331 (258C), 414 (308C), 519 (358C) and 624 (408C)
eq
ꢀ
1
III
for Ru –CySH.
M
III
þ
4
Reduction of [Ru Cl (H O) ] by RSH
2
2
In order to explore possible pathways for electron transfer
III
from R–SH to the Ru metal ion in the respective precursor
complexes, a series of kinetic experiments were performed by
III
varying the initial concentration of [RSH] and [Ru ] individu-
ally (see Tables 5 and 6) at constant ionic strength and tempera-
ture. The dependence of first-order rate constants, kobs, on
mode of activation, i.e. (I or A) mechanism, with the formation
a
of dichloroglutathionatodiaquaruthenium(III) and dichlorocys-
teinatodiaquaruthenium(III). As we detected the intermediate
complex formation kinetically, no detailed structural information
could be obtained on the proposed intermediate complex. Similar
formation of dichloroascorbatodiaquaruthenium(III) complex
[RSH] at a given temperature and I ¼ 0.5 M yielded a straight
T
III
line (Fig. 3). Similar linear variation of k with [Ru ] at a
obs
given [RSH] (Fig. 4) indicates the rate of reduction is first order
T
III
both with respect to reductants as well as to the Ru complex.
III
Hence, the electron transfer from the reductants to the Ru

Kinetics and Reaction Mechanism of Ru and Thiols
117
7
6
5
4
3
2
1
0
Table 6. Rate constants for the electron transfer between
1
III
Ru Cl
[
2 2 4
(H O) ] and cysteine (CySH) within the outer-sphere com-
plex at pH 2.01 ± 0.06, temperature 25.0 ± 0.18C, ionic strength 0.5 M
III
A
obs
B
[
Ru
]
[L-cysteine]
ꢀ3
k
k
2
ꢀ4
ꢀ2 ꢀ1
2
ꢀ1 ꢀ1
[
10 M]
[10 M]
[10
s
]
[10 M
s ]
2
2
2
2
2
2
2
1
3
4
5
2
4
1.51 ꢄ 0.11
2.66 ꢄ 0.12
3.77 ꢄ 0.09
4.64 ꢄ 0.21
5.66 ꢄ 0.36
6.52 ꢄ 0.15
7.74 ꢄ 0.22
1.35 ꢄ 0.08
3.72 ꢄ 0.13
4.93 ꢄ 0.11
5.91 ꢄ 0.23
0.75
1.33
1.88
2.32
2.83
3.26
3.87
1.35
1.24
1.23
1.18
6
8
10
12
14
4
4
4
0.00
2.00 ϫ10Ϫ4
4.00 ϫ10Ϫ4
6.00 ϫ10Ϫ4
4
III
A
B
[Ru ] [M]
Observed rate constant.
T
Second order reduction rate constant.
III
Fig. 4. Plots of kobs versus [Ru Cl
III
Ru Cl
þ
T
for the reduction of
2
(H
2
O)
4
]
[
2
(H
2
O)
4
]
by RSH at constant [RSH] ¼ 0.004 M, 258C, pH
9
8
7
6
5
4
3
2
1
0
2
.0 ꢄ 0.01. RSH ¼ glutathione (r) and cysteine (’).
2
Table 7. pH and temperature dependence for reduction of
1
1
III
Ru Cl
[
2
(H
2 4
O)
III
]
by glutathione (GSH) at ionic strength 0.5 M,
23
24
] 2.0 3 10 , [GSH]
[
Ru Cl
2
(H
2
O)
4
T
2.0 3 10
M
A
ꢀ2 ꢀ1
B
2
2 ꢀ1 ꢀ1
[10 M s ]
pH
Temperature [8C]
k
obs [10
s
]
k
1
.41
.61
.79
.99
.21
.42
.63
25
25
25
25
25
25
25
30
35
40
0.28
0.14
0.28
0.41
0.54
1.12
1.77
2.35
0.75
1.05
1.46
1
1
1
2
2
2
0.57
0.83
1.09
2.24
3.54
4.71
1.51
2.11
2.93
2.00
.00
2
2.00
0
0.005
0.01
0.015
A
Observed rate constant.
[
RSH] [M]
T
B
#
Second order reduction rate constant. Activation parameters DH ¼ 9.6 kJ
ꢀ1
#
ꢀ1
ꢀ1
.
mol ; DS ¼ ꢀ34.8 J K mol
III
Fig. 3. Plots of kobs versus [RSH] for the reduction of [Ru Cl (H O) ]
T 2 2 4
þ
ꢀ
4
(
2.0 ꢃ 10 M) by RSH at 258C, pH 2.0 ꢄ 0.01. RSH ¼ glutathione (¢) and
cysteine (’).
III
of Ru complex, in a very fast step. A plausible mechanism can
be proposed for these reactions:
complex follows overall second-order kinetics. The dependence
þ
K1
Ð
of kobs on [H ] ion concentration was investigated over the pH
range 1.50 to 2.6 at constant [RSH] (see Tables 7 and 8). The
ꢀ
HA þ Hþ
H2A
ð9Þ
T
III
rate of reduction of [Ru Cl (H O) ] increases with increasing
þ
2
2
4
½Ru Cl2ðH2OÞ ꢁ þ 2HA ðor 2AꢀÞ
III
þ
ꢀ
2
4
pH, indicating that the deprotonated protolytic species are much
more reactive than the protonated ones. There was a rapid and
constant variation in kobs similar to that observed in many of the
Keq
III
2½Ru Cl2ðH2OÞ ðHA ðor AÞÞꢁ þ 2H2O
Ð
ð10Þ
3
[
15,16,27]
oxidation studies of thiol-containing moieties.
The
III
2½Ru Cl ðH OÞ ðHA ðor AÞÞꢁ
III
2
2
3
mechanism proposed for the reduction of the Ru complex in
the said pH range, therefore, involves the formation of mixed-
k
II
ꢅ
ꢅ
ꢀ
! 2½Ru Cl2ðH2OÞ ꢁ þ 2H2O þ 2HA ðor 2A Þ ð11Þ
III
ligand complexes in the pre-equilibrium steps. The Ru com-
2
II
plex is reduced to Ru species in the rate-determining step
III
2½Ru Cl2ðH2OÞ ꢁ þ 2HA ðor 2A Þ
þ
ꢅ
ꢅ
III
Eqn 11) by one-electron transfer from RSH to the Ru complex
4
(
ꢅ
fast
in an inner-sphere manner, producing RS radicals. In the
subsequent and kinetically inconsequential step (Eqn 12), the
RS radical species rapidly transforms into RSSR (oxidized
form of RSH) and this radical also reacts with another molecule
II
þ
ꢀ
! 2½Ru Cl2ðH2OÞ ꢁ þ 2H þ 2HA ðor 2AÞ
ð12Þ
2
ꢅ
fast
HA ðor 2A Þ ꢀ! HA ꢀ AH=A ꢀ A
ꢅ
ꢅ
2

1
18
S. Nayak, G. S. Brahma, and K. V. Reddy
3
.5
Table 8. pH and temperature dependence for reduction of
1
1
III
Ru Cl
[
2
(H
2
O)
4
]
by cysteine (CySH) at ionic strength 0.5 M,
23
III
Ru Cl
24
] 5 2.0 3 10 M and [CySH]
3
[
2
(H
2
O)
4
T
2.0 3 10
M
A
ꢀ2 ꢀ1
B
2
2
[10 M
ꢀ1 ꢀ1
2
pH
Temperature [8C]
k
obs [10
s
]
k
s
]
2.5
2
1
1
1
2
2
2
2
2
2
2
.41
.61
.79
.00
.21
.42
.63
.00
.00
.00
25
25
25
25
25
25
25
30
35
40
0.51
0.25
0.46
0.70
0.91
1.45
2.28
3.17
1.34
1.86
2.57
0.93
1.39
1.82
2.89
4.57
6.34
2.68
3.73
5.15
1.5
1
0.5
0
A
B
Observed rate constant.
0
100
200
300
400
500
#
Second order reduction rate constant. Activation parameters DH ¼ 11.13
/[H ] [MϪ1]
ϩ
1
ꢀ1
#
ꢀ1
ꢀ1
.
kJ mol ; DS ¼ ꢀ48.5 J K mol
þ
Fig. 5. Plots of second-order rate constant k
III
reduction of [Ru Cl
2
versus [1/H ] for the
] by RSH at constant [complex] ¼ 2.0 ꢃ
ꢀ3
2
(H
2 4
O)
ꢀ4
1
0
M, [RSH]
T
¼ 2.0 ꢃ 10 M at 258C, I ¼ 0.5 M RSH ¼ CySH (r)
The rate law describing the oxidation of GSH may be written
in the form
and GSH (’).
d½H2Aꢁ
dt
III
¼ k½Ru Cl2ðH2OÞ ðHAÞꢁ
ꢀ
ð13Þ
3
Table 9. Ionic strength (I) dependence for the oxidation of thiol (RSH)
1
III
RSH 5 glutathione, GSH and cysteine, CySH) by [Ru Cl
(
2
(H
2
O)
4
]
at
III
þ
III
258C, pH 2.00, [Ru Cl (H O) ] 5 4 3 10 M and [RSH] 5 4 3 10
24
23
K1½H2Aꢁ½Ru Cl2ðH2OÞ ꢁ
M
III
where ½Ru Cl2ðH2OÞ ðHA=AÞꢁ ¼
4
2
2
4
3
þ
½
H ꢁ
A
2
2
[10 M
ꢀ1 ꢀ1
A
2
2 ꢀ1 ꢀ1
[10 M s ]
I [M]
k
s
]
k
ð14Þ
(
for GSH)
(for CySH)
0
0
0
0
.2
.4
.5
.6
0.92
0.81
0.75
0.70
0.64
1.28
1.11
1.03
0.97
0.89
Hence
ꢀ
d½H2Aꢁ
kK1Keq
k2 ¼
¼
ð15Þ
ð16Þ
þ
dt½H2Aꢁ½oxidantꢁ
½H ꢁ
0.8
A
Second order reduction rate constant.
or
0
þ
0
2
k2 ¼ k K1=½H ꢁ; where k ¼ kKeq
2
where B is a constant that increases with increasing size of the
ion; Z and Z are the charges on the two reactant species and k
0
is the rate constant at infinite dilution. A is also a constant and
given by
A
B
Similar sets of equations can be derived for the oxidation of
CySH by replacing H A by HA.
2
The plot of second-order rate constants, k , against inverse
2
hydrogen ion concentration yields a straight line (see Fig. 5) and
suggests that the neutral forms of the reductants are not signifi-
cantly active in the said pH range. The rate constants k
A ¼ ð1:82 ꢃ 10ꢀ Þ=ðDTÞ
6
3=2
calculated are 5.66 ꢃ 10ꢀ and 6.07 ꢃ 10ꢀ6
6
s for oxidation
ꢀ1
where D is the dielectric constant of the medium and T is the
absolute temperature. At 258C, Eqn 17 reduces to
of RSH and CysH respectively.
Effect of Ionic Strength
1
=2
1
:02 ZAZBI
In order to investigate the effect of ionic strength, kinetic runs
were performed at low acidity with incremental addition of the
supporting electrolyte NaClO4 in the ionic strength range
log k2 ¼ log k0 þ
as A ¼ 0.509 and B ¼ 1.
ð18Þ
1=2
1 þ I
0
.2–0.8 M. Values of the second-order rate constants obtained
with variation of ionic strength are listed in Table 9. It was
observed that the rate constant decreases with increase in ionic
strength. The second-order rate constant obeys Eqn 17, from
1/2
1/2 ꢀ1
The plots of log k as a function of I (1 þ I
)
exhibited a
2
slope of unit (1) positive slope in each case (for both GSH and
CySH), reconciling with the fact that interaction takes place
between a positively charged and a negatively charged species.
The ionic strength dependence firmly agrees with the charges on
the interacting species determined in the rate-determining step
[
33]
Bronsted–Bjerrum–Christiansen,
p
ZAZBA I
p
2
log k2 ¼ log k0 þ
ð17Þ
1
þ B I
(see Eqn 11).

Kinetics and Reaction Mechanism of Ru and Thiols
119
Effect of Temperature
Supplementary Material
The kinetic data on temperature dependence of the rate of
Figures S1, S2 and S3 are spectrums which are available on the
Journal’s website.
III
reduction of [Ru Cl (H O) ] by H A and HA are given in
þ
2
2
4
2
Tables 7 and 8. The activation parameters corresponding to
these are listed in the footnote of the corresponding tables. The
low positive values of activation enthalpy together with moder-
ately negative values of activation entropy reveal the fact that
electron transfer takes place within a pre-formed associative
complex. Similar observations have been made in a number of
electron-transfer reactions involving the thiol moiety where the
Acknowledgements
This work was carried out with financial assistance from the University
Grant Commission (UGC), New Delhi through a Dr D. S. Kothari post-
doctoral fellowship (scheme no.F.4–2/2006(BSR)/13–47/2007(BSR)). The
award of a post-doctoral fellowship to SN is gratefully acknowledged.
[
34–36]
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