Kinetics and mechanism of O-O bond cleavage in the reaction of [Ru III(edta)(H2O)]- with hydroperoxides in aqueous solution

Article abstract of DOI:10.1039/b803181a

The reactions of [Ru^III(edta)(H₂O)]^- (1) (edta = ethylenediaminetetraacetate) with tert-butylhydroperoxide ( ^tBuOOH) and potassium hydrogenpersulfate (KHSO₅) were studied kinetically as a function of oxidant concentration and temperature (10-30 °C) at a fixed pH of 6.1 using stopped-flow techniques. Kinetic results were analyzed by using global kinetic analysis techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru ^III(edta)(OOR)]^2- intermediate, which subsequently undergoes heterolytic cleavage to form [(edta)Ru^V=O]^-. Since [(edta)Ru^V=O]^- was produced almost quantitatively in the reaction of 1 with the hydroperoxides ^tBuOOH and KHSO ₅, the common mechanism is one of heterolytic scission of the O-O bond. The water soluble and easy to oxidize substrate 2,2′-azobis(3- ethylbenzithiazoline-6-sulfonate (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first order conditions for [ABTS] [hydroperoxide] ? [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS⁺. The detailed suggested mechanism is consistent with the reported rate and activation parameters, and discussed in reference to the results reported for the reaction of [Ru^II(edta)(H ₂O)]^- with H₂O₂.

Full text of DOI:10.1039/b803181a

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Kinetics and mechanism of O–O bond cleavage in the reaction of
[RuIII(edta)(H2O)]− with hydroperoxides in aqueous solution†
Debabrata Chatterjee,*a,b Anindita Sikdar,a Vidya R. Patnam,a Alexander Theodoridisb and Rudi van Eldik*b
Received 25th February 2008, Accepted 2nd May 2008
First published as an Advance Article on the web 13th June 2008
DOI: 10.1039/b803181a
The reactions of [RuIII(edta)(H2O)]− (1) (edta = ethylenediaminetetraacetate) with
tert-butylhydroperoxide (tBuOOH) and potassium hydrogenpersulfate (KHSO5) were studied
kinetically as a function of oxidant concentration and temperature (10–30 ◦C) at a fixed pH of 6.1 using
stopped-flow techniques. Kinetic results were analyzed by using global kinetic analysis techniques. The
reaction was found to consist of two steps involving the rapid formation of a [RuIII(edta)(OOR)]2−
−
intermediate, which subsequently undergoes heterolytic cleavage to form [(edta)RuV O] . Since
=
[(edta)RuV O] was produced almost quantitatively in the reaction of 1 with the hydroperoxides
−
=
tBuOOH and KHSO5, the common mechanism is one of heterolytic scission of the O–O bond. The
water soluble and easy to oxidize substrate 2,2ꢀ-azobis(3-ethylbenzithiazoline-6-sulfonate (ABTS), was
employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first order
conditions for [ABTS] ꢁ [hydroperoxide] ꢁ [1], and were monitored as a function of time for the
formation of the one-electron oxidation product ABTS•+. The detailed suggested mechanism is
consistent with the reported rate and activation parameters, and discussed in reference to the results
reported for the reaction of [RuII(edta)(H2O)]− with H2O2.
oxidants is required in order to understand the mode of O–O
Introduction
bond scission and the general role of catalysis in such systems.
Interaction of metal complexes with oxygen donors assumes
The present investigation is an extension of our earlier work and
great importance in biomimetic approaches toward mimicking the
chemistry of enzymes.1 The reaction of cytochrome P-450 enzymes
involves an examination of the reaction of 1 with two precursor
oxidants potassium hydrogenpersulfate (KHSO5) and tert-butyl
hydroperoxide (tBuOOH). Both KHSO5 and tBuOOH with a non-
symmetrical peroxidic oxygen–oxygen bond are expected to lead
to more favourable heterolytic O–O bond cleavage than found for
H2O2.7
with various oxygen transfer agents (peroxide shunt mechanism)
leads to an enzyme oxidant having characteristics similar to the
oxidant produced via the biochemical pathway.1 We have been
engaged in exploring the potential of Ru–edta (edta = ethylene-
diaminetetraacetate) type of complexes in biochemical processes.2
Catalytic activity of RuIII–edta complexes that mimic enzymatic
hydrocarbon oxidation by cytochrome P-450 under homogeneous
In the present study we also used 2,2ꢀ-azinobis(3-ethylbenz-
thiazoline-6-sulfonate) (ABTS) as a trap for the intermediate oxi-
dizing species.8 We describe herein the kinetic results together with
detailed mechanistic studies of the reactions of [RuIII(edta)(H2O)]−
conditions has also been well established in the literature.3–6
A high-valent [(edta)RuV O] species has been proposed as
−
=
t
with BuOOH and KHSO5, and its subsequent reaction with
a catalytically active intermediate in oxygen transfer processes.
Therefore, studies on the kinetics and mechanism of the reactions
ABTS based on a global kinetic analysis methodology. The
mechanism of the overall catalytic process is discussed in reference
to the data reported for the reaction between [RuIII(edta)(H2O)]−
and H2O2.
of [RuIII(edta)(H2O)]− with various oxygen atom donors leading to
the formation of the active intermediate [(edta)RuV O] species,
−
=
are important as it governs the efficiency of the catalytic cycle.
In previous studies we have investigated the reaction of
hydrogen peroxide with [RuIII(edta)(H2O)]− (1) in water.7 We
have shown that the reaction involves the rapid formation of a
[(edta)RuIII(OOH)]2− intermediate which subsequently undergoes
parallel heterolytic and homolytic cleavage of the O–O bond
Experimental
Materials
K[RuIII(Hedta)Cl]·2H2O was prepared using
a published
procedure9 and characterized by elemental analysis and spec-
tral data, which were in agreement with data reported in the
literature.9 Anal. calculated for K[RuIII(Hedta)Cl]·2H2O: C 24.0,
H 3.42, N 5.59; Found. C 23.8, H 3.45, N 5.63. IR, m/cm−1:
1720 (free-COOH), 1650 (coordinated-COO−). UV-vis in H2O:
V
−
to produce [(edta)Ru ( O)] and [(edta)RuIV(OH)]− in the ratio
=
of approximately 1 : 1. However, further investigation with other
aChemistry Group, Central Mechanical Engineering Research Institute,
Durgapur, 713209, India
kmax/nm (emax/M−1 cm−1): 283 (2800
The complex rapidly hydrolyses to the aqua complex when
dissolved in water, and thus exists predominantly in its most
labile form [RuIII(edta)(H2O)]− (1) in the pH range 4 to 6.10,11
50), 350 sh (680
10).
bInorganic Chemistry, Department of Chemistry and Pharmacy, University
of Erlangen-Nu¨rnberg, Egerlandstrasse 1, 91058, Erlangen, Germany
† Electronic supplementary information (ESI) available: Fig. S1–S3 and
Table S1. See DOI: 10.1039/b803181a
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2,2ꢀ-Azobis(3-ethylbenzithiazoline-6-sulfonate) (ABTS) diammo-
nium salt was purchased from Aldrich and used as received.
Potassium hydrogen persulfate (KHSO5), commercially known
as “oxone”, was obtained from Merck. Solutions of tert-butyl
hydroperoxide (tBuOOH) were prepared by dilution of reagent-
grade tert-butyl hydroperoxide (70% aqueous solution, Lancaster)
and stored at 4 ◦C prior to use. All other chemicals used were
of AR grade, and doubly distilled H2O was used to prepare
t
all solutions. The concentration of KHSO5 and BuOOH was
periodically determined iodometrically.12
Instrumentation
A Perkin-Elmer 240C elemental analyzer was used to collect
microanalytical (C, H, N) data. The UV-vis and IR spectra were
recorded on Perkin Elmer Model Lambda 35 and Model 783
spectrophotometers using KBr pellets, respectively. The kinetics
t
of the reaction of 1 with BuOOH and KHSO5 was studied
Fig. 1 Spectral changes that occur during the reaction of 1 (1.00 ×
10−4 M) with KHSO5 (2.5 × 10−3 M) in water at 25 ◦C and pH = 6.2
(0.2 M acetate buffer). Inset: kinetic trace at 391 nm (black line) and fit
(red line) obtained by using the global kinetic fit routine.
spectrophotometrically using a SF-61SX2 (HI-TECH) stopped-
flow spectrophotometer coupled to a KinetaScan rapid scan
spectral attachment. The solution temperature was maintained to
within 0.1 ◦C using a circulating water bath (JEIO TECH RW-
1025G). There was no change in absorbance during the mixing
time of the instrument, which ruled out the possibility of any fast
step prior to the detected steps. Rate constants were measured
under pseudo-first order conditions of an excess (20–100 times)
t
of precursor oxidant (POH = BuOOH and KHSO5). The pH
of the solutions was measured with a Mettler Delta 350 pH
meter. Acetic acid–acetate buffers were used to adjust the pH of
the experimental solutions. In all cases spectral-kinetic data were
processed by the use of the SPECFIT program. The results of such
fit procedure return both globally optimized rate constant and
predicted spectra of the colored species. Observed rate constants
(kobs) are presented as an average of several kinetic runs (at least
5–8) and were reproducible within 4%.
Results and discussion
Fig. 2 Spectra calculated by the global analysis software package
(SPECFIT) for the reaction between [RuIII(edta)H2O]− and KHSO5 in
Fig. 1.
Kinetics of the reaction of [RuIII(edta)(H2O)]− with KHSO5
KHSO5 is a reasonably strong oxidant (E◦ = 1.82 V)13 which
is readily available and stable in the solid state as a “triple salt”
(2KHSO5, K2SO4, KHSO4). It is mainly used as oxidant in organic
synthesis.14 In the pH range 6.0–8.0 it exists as HSO5−, whereas,
at higher pH (>10.5) it exists as SO52−. Aqueous solutions of
of the reaction. No clean pseudo-first order kinetic traces even at
lower temperature could be observed due to the small absorbance
change associated with the formation of the intermediate in the
wavelength range 300–700 nm as seen in Fig. 2, which shows up
as an apparent induction period in the kinetic trace recorded at
391 nm, where maximum difference in absorbances exists.
−
HSO5 undergo slow decomposition (evolution of dioxygen was
observed) in the pH range 7.0–9.0, whereas at acidic pH (below
1.0) it decomposes to H2O2.
In Fig. 1, the UV-vis spectral changes that occurred upon mixing
aqueous solutions of 1 and KHSO5 in the chamber of the stopped-
flow instrument equipped with a rapid scan spectral attachment
are reported as a function of time. The inset of Fig. 1 displays
a typical kinetic trace at 391 nm generated from the recorded
spectra by using global kinetic analysis software (SPECFIT/32).
The final spectrum of the reaction solution is in good agreement
For this reason, single wavelength measurements at 391 nm
following the induction period were fitted to pseudo-first order
kinetics. The effect of the KHSO5 concentration on the values
of the observed rate constant (kobs) is shown in Fig. 3. The
observed saturation of the values of kobs at higher KHSO5 concen-
tration, suggests a process that involves a rapid pre-equilibrium
step followed by the rate-determining product formation step.
On the basis of a pre-equilibrium approach, the above kinetic
with the spectrum of [RuV(edta)O)]− (kmax = 391 nm, emax
=
V
−
20 M−1 cm−1).5 A closer analysis of the spectra using
results for the formation of [(edta)Ru ( O)] in the reaction of
[(edta)RuIII(H2O)]− with KHSO5 can be accounted for in terms of
the mechanism proposed in Scheme 1.
=
8000
SPECFIT software suggested the formation of an intermediate
(the spectrum of which is shown in Fig. 2) during the initial stage
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Table 1 Rate and activation parameters for the reaction of RuIII(edta)
a
with KHSO5
Temp./◦C k1/s−1
K1/M−1 DH1=/kJ mol−1 DS1=/J K−1 mol−1
11
15
20
25
0.013 0.002 120 28 37
2
−148
8
0.016 0.003 121 26
0.022 0.003 140 31
0.031 0.004 227 44
0.037 0.004 192 37
30.6
a [RuIII] = 1 × 10−4 M, pH = 6.1 (0.2 M acetate buffer).
Fig. 3 Plot of kobs vs. [HSO5−] for the reaction between [Ru(edta)H2O]−
(1.0 × 10−4 M) and KHSO5 at different temperatures and pH = 6.0 (0.2 M
acetate buffer), followed at 391 nm.
Scheme 1
In the proposed mechanism, reaction (1) involves the forma-
tion of the [(edta)RuIII(OOSO3)]3− intermediate in a fast pre-
equilibrium step observed as an induction period in the kinetic
trace. In the subsequent slower step (2), (edta)RuIII(OOSO3)]3−
undergoes rate-determining heterolytic cleavage of the O–O bond
Fig. 4 Spectral changes that occur during the reaction of 1 (1.00 ×
10−4 M) with BuOOH (1.25 × 10−2 M) in water at 25 ◦C and pH =
t
6.2 (0.2 M acetate buffer).
V
−
to generate [(edta)Ru ( O)] and SO42−. The yield for conversion
(Singular Value Decomposition) analysis of the spectral data in
Fig. 4 showed the presence of an intermediate, assumed to be
[RuIII(edta)(OOBut)]2−. The predicted concentration-time profiles
from a typical global fit analysis are shown in Fig. 5. The values
of the pseudo-first order rate constant (kobs) determined from
single wavelength kinetics at 391 nm, following the induction
period for the formation of the intermediate, increased linearly
=
V
−
of [RuIII(edta)(H2O)]− to [(edta)Ru ( O)] was found to be 90%
based on the quoted extinction coefficient at 391 nm5 under the
specified conditions. The following rate-law can be derived for the
=
reactions in Scheme 1 on the basis that the rate-determining step
−
involves the formation of [(edta)RuV O] .
=
kobs = k1K1[HSO5−]/{1 + K1[HSO5−]}
(3)
(4)
−
1/kobs = 1/k1 + 1/k1K1[HSO5
]
The plots of 1/kobs versus 1/[HSO5−] were linear (see ESI,†
Fig. S1), and the values of k1 and K1 calculated from the
intercept and slope at various temperatures are summarized in
Table 1. The values of the activation parameters (DH= and DS=)
for k1, determined from the Eyring plot (see ESI,† Fig. S2)
are also summarized in Table 1. The large negative entropy of
activation may arise from the heterolysis of the O–O bond in
[(edta)RuIII(OOSO3)]3−, which will involve a considerable decrease
in entropy due to the formation of the RuV O species and
=
2−
electrostriction caused by the release of SO4
.
Kinetics of reaction of [RuIII(edta)(H2O)]− with tBuOOH
t
The kinetic behaviour of the reaction of 1 with BuOOH was
Fig. 5 Concentration-time profiles for the reaction of 1 (1.00 × 10−4 M)
found to be similar to that observed for the reaction of 1 with
H2O2.7 Spectral changes with time for the reaction of 1 with
tBuOOH shown in Fig. 4 were analyzed with SPECFIT. The SVD
with BuOOH (1.25 × 10−2 M) in water at 25 ◦C and pH = 6.2 (0.2 M
t
acetate buffer) obtained by using global kinetic fit routine.
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Table 2 Rate and activation parameters for the reaction of RuIII(edta)
(see ESI,† Fig. S3) with increasing tBuOOH concentration. With
V
−
with tBuOOHa
=
the independent information on the spectrum of [(edta)Ru ( O)]
(kmax = 391 nm; e = 8 000 M−1 cm−1) in H2O,8 it was found
Temp./◦C
K2k2/M−1 s−1
DH2=/kJ mol−1
52
DS2=/J K−1 mol−1
−68
V
−
=
that 80% [(edta)Ru ( O)] is produced during the reaction of
1 with tBuOOH under the specified conditions. This suggests that
unlike for H2O2, cleavage of the O–O bond in BuOOH occurs
10
15
20
25
30
0.44 0.01
0.74 0.02
1.32 0.08
1.45 0.06
2.08 0.04
3
9
t
predominantly through a heterolytic pathway that leads to the
V
−
=
formation of [(edta)Ru ( O)] . Considering the existence of an
intermediate as identified by SPECFIT analysis of the observed
spectral changes, suggested to be [RuIII(edta)(OOBut)]2−, the above
kinetic results may be understood in terms of a mechanism
(Scheme 2) similar to that proposed for the reaction of 1 with
H2O2.7
a [RuIII] = 1 × 10−4 M, pH = 6.1 (0.2 M acetate buffer).
t
peroxide8 and BuOOH.15 We have previously used ABTS to
substantiate our mechanistic proposal for the reaction of 1 with
H2O2 and HOBr.16 Preliminary experiments revealed that in the
7
absence of 1, a solution of ABTS in the presence of the primary
t
oxidant, POH (POH = KHSO5 and BuOOH), was stable at
pH 6.1 (0.2 M acetate) for several hours without showing any
formation of ABTS•+. However, addition of 1 to the solution
of ABTS containing PO catalyzed the formation of ABTS•+ as
indicated by the formation of the characteristic 660 nm band in the
spectrum of the reaction mixture. Under the specified conditions
of [ABTS] ꢁ [POH] ꢁ [1], the formation of ABTS•+ obeyed first-
order kinetics with respect to 1 and the observed rate constants
(kꢀobs) were found to be independent of the ABTS concentration.
The values of kꢀobs at various [POH] are summarized in Table S1
(see ESI†). When KHSO5 was used as an oxidant in the oxidation
of ABTS catalyzed by 1, the values of kꢀobs at various [KHSO5] did
not differ appreciably from those obtained directly for the reaction
of 1 with KHSO5 at the corresponding [KHSO5]. Similarly, the
close resemblance of the kꢀobs values for the catalyzed oxidation of
Scheme 2
In the proposed Scheme 2, formation of [(edta)RuIII(OOBut)]2−
takes place in a pre-equilibrium step (5), that accounts for the
concentration-time profiles observed in Fig. 5. In the subse-
quent slower step (6), [(edta)RuIII(OOBut)]2− is converted into
[(edta)Ru ( O)] involving heterolytic cleavage of the O–O bond.
Considering the fact that the value of K2 is not very high, no
kinetic saturation at high [tBuOOH] was observed in the plots
of kobs vs. [tBuOOH] (see ESI,† Fig. S3). This can be accounted
for in terms of the rate expression outlined in eqn (7). The small
intercepts observed in the plots of kobs vs. [tBuOOH] (especially
at lower temperatures) are ascribed to experimental errors and/or
the kinetic interference of the pre-equilibration step (5), especially
at low [tBuOOH].
V
−
=
ABTS by tBuOOH with those obtained from the direct reaction of
−
1 with tBuOOH, suggest that formation of [(edta)RuV O] is the
=
rate controlling step in the catalytic oxidation of ABTS by both
t
the precursor oxidants, KHSO5 and BuOOH. Based on these
results, the mechanism in Scheme 3 is proposed to account for the
kobs = K2k2[tBuOOH]
(7)
formation of ABTS•+ in both the cases.
The values for K2k2 estimated from the slope of the plot of
kobs versus [tBuOOH] at different temperatures are summarized in
Table 2. The values for DH= and DS=, determined from the Eyring
plot constructed from the values of K2k2 at different temperatures,
are also included in Table 2. The reported DH= and DS= values
are composite in nature which include contributions from the pre-
equilibrium step (K2) as well as from heterolytic cleavage of the
O–O bond in the [(edta)RuIII(OOBut]2− intermediate (k2) to form
V
−
=
[(edta)Ru ( O)] as shown in eqn (6) in Scheme 2. The entropy
of activation is less negative than reported for the reaction with
Scheme 3
−
HSO5 in Table 1. This must be partly related to the fact that less
charge creation occurs in reaction (6) as compared to reaction (4).
Considering that the rate-determining step involves the forma-
−
tion of [(edta)RuV O] , the following rate-law can be derived.
=
Kinetics of the reaction of 1 with KHSO5/tBuOOH in the
presence of ABTS
kꢀobs = kꢀKꢀ[POH]/{1 + Kꢀ[POH]}
(11)
In order to consolidate our mechanistic proposals as outlined in
Schemes 1 and 2 for the reaction of 1 with KHSO5 and tBuOOH,
respectively, we have performed the catalytic oxidation of an
easily oxidizable substrate, 2,2ꢀ-azobis(3-ethylbenzithiazoline-6-
sulfonate), ABTS. The use of ABTS as a water-soluble trap for
higher-valent iron-oxo species was reported earlier in studying
the kinetics of the reaction of iron(III) porphyrins with hydrogen
1/kꢀobs = 1/kꢀ + 1/kꢀKꢀ[POH]
(12)
The effect of increasing [POH] on the values of kꢀobs for the
formation of ABTS•+ is shown in Fig. 6. For POH = KHSO5,
the observed saturation of kꢀobs at high [KHSO5] (Fig. 6A) is
in agreement with that expected of a process that involves the
catalytic oxidation of ABTS to ABTS•+ at [oxidant] ꢁ [catalyst],
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V
−
=
of the precursor oxidants (ROOH) to form [(edta)Ru ( O)]
as the reactive oxidizing species in solution. Based on the pKa
values of the ROOH oxidants, the basicity of the ROOH species
is expected to follow the order HSO5− < H2O2 < tBuOOH, which
should also be reflected in the equilibrium constant K for the
formation of the [(edta)RuIII(OOR)]2−/3− intermediates. However,
according to the experimental observations, K is much larger for
t
H2O2 and KHSO5 than for BuOOH, which indicates that the
basicity of the ROOH species seems to be less important than
the steric hindrance caused by the tert-butyl group attached to
the hydroperoxide fragment. On the other hand, the formation
V
−
III
2−/3−
=
of [(edta)Ru ( O)] from [(edta)Ru (OOR)]
will depend on
the ability of this complex to undergo heterolytic cleavage of
the O–O bond expressed by the rate constant k, which in turn
should correlate with the reduction potential of the different
hydroperoxides under consideration, i.e. tBuOOH (E◦ = 1.15 V)17
ꢃ H2O2 (E◦ = 1.78 V) ≤ HSO5 (E◦ = 1.82 V).13 From a
−
Fig. 6 Plots of kꢀobs vs. [POH] for oxidation of ABTS (1 × 10−3 M) by
POH (= KHSO5 (A) and tBuOOH (B)) catalyzed by [Ru(edta)H2O]− (1.0 ×
10−4 M) at 25 ◦C and pH = 6.0 (0.2 M acetate buffer).
combination of the separate trends expected for k and K, and
a comparison of the trend observed in the values of kK at 25 ◦C, it
follows that in addition to the electronic factors considered here,
for which the observed rate constant (kꢀobs) for product formation
should be independent of the substrate concentration. The values
of kꢀ and Kꢀ calculated from the intercept and slope of the plot of
1/kꢀobs versus 1/[KHSO5] are 0.034 0.001 s−1 and 193 6 M−1
at 25 ◦C, respectively. These values are in very close agreement
with the values of k1 (0.031 0.004 s−1 at 25 ◦C) and K1 (227
44 M−1 at 25 ◦C) obtained independently for the reaction of
1 with KHSO5 (see Table 1). These results further substantiate
the validity of the mechanism proposed in Scheme 1. In the
t
−
steric hindrance on BuOOH and HSO5 as compared to H2O2
should further account for the fact that kK is significantly smaller
for tBuOOH and HSO5 as compared to H2O2.
−
Conclusion
In this work a detailed kinetic and mechanistic study of the
reaction between RuIII(edta)(H2O)]− with two hydroperoxides,
tBuOOH and KHSO5, is presented. The findings of this work taken
together with those for the reaction of [RuIII(edta)(H2O)]− with
H2O2,7 strongly suggest that the oxo-transfer from the precursor
oxidant ROOH (H2O2, tBuOOH and KHSO5) to 1 that results in
t
case of BuOOH, non-observance of limiting kꢀobs values at high
[tBuOOH] (Fig. 6B) is typical for the small value of Kꢀ, such
that the observed rate constant (kꢀobs) for oxidation of ABTS to
ABTS•+ does not reach a limiting value at higher [tBuOOH] over
the concentration range studied. The rate-law for the catalytic
oxidation of ABTS in eqn (11) was derived on the basis of the
V
−
=
the formation of the catalytically active [(edta)Ru ( O)] species
should proceed through the formation of [(edta)RuIII(OOR)]2−/3−
t
intermediates (R = H, Bu and SO3−) in a rapid pre-equilibrium
suggested pre-equilibrium and the rate-determining step involving
step, which subsequently undergoes rate-controlling heterolytic
−
the formation of [(edta)RuV O] , can now be simplified to eqn
(13) under such conditions.
=
V
−
=
cleavage of the O–O bond to produce [(edta)Ru ( O)] as a major
product (80–90%). However, in the case of [(edta)RuIII(OOH)]2−
formed in the reaction of 1 with H2O2, the subsequent reaction
kꢀobs = kꢀKꢀ[tBuOOH]
(13)
involves both parallel heterolytic and homolytic cleavage to
The◦close resemblance of the value of kꢀKꢀ (1.52 0.04 M−1 s−1
at 25 C) calculated from the slope of the plot of kꢀobs versus
[tBuOOH] (Fig. 6B) with the value (1.45 0.06 M−1 s−1 at 25 ◦C)
obtained directly (see ESI,† Fig. S3), substantiates the mechanistic
−
produce [(edta)RuV O] and [(edta)RuIV(OH)]− in the ratio of
=
approximately 1 : 1.7 Thus, the tBu and SO3 substituents in
−
[(edta)RuIII(OOR)]2−/3− cause the O–O bond cleavage to clearly
favour heterolysis above homolysis. In this way, the three hydroper-
oxide systems studied so far demonstrate the capability to tune
the fundamental nature of the O–O bond cleavage process, and
underline the advantage of studying such systems in such detail as
reported in this paper.
hypothesis in Scheme 2 for the reaction of 1 with tBuOOH.
−
Since [(edta)RuV O] was obtained as predominant product
=
t
in the reaction of 1 with the precursor oxidants BuOOH and
KHSO5, the common suggested mechanism involves heterolytic
cleavage of the O–O bond. The overall second-order rate constants,
expressed as kK, i.e. the product of the equilibrium constant K
t
for the formation of [(edta)RuIII(OOR)]2−/3− (R = H, Bu and
Acknowledgements
SO3−) and the rate constant k for the subsequent heterolytic
V
−
=
cleavage of the O–O bond to form [(edta)Ru ( O)] , have the
values 26.5
7.0
The authors gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft and DST-DAAD (Grant No.
INT/DAAD/P-130/2005). This work is further supported by a
research grant No. SR/S5/BC-15/2006 from the Department of
Science and Technology, Government of India. DC is thankful to
Dr G. P. Sinha, Director of the Central Mechanical Engineering
Research Institute, for support of this work.
t
0.4 (for H2O2),7 1.45
0.06 (for BuOOH) and
2.3 M−1 s−1 (for KHSO5) at 25 ◦C. Since the overall
reaction is a two step process as outlined in Schemes 1 and
t
2, the ability to form [(edta)RuIII(OOR)]2−/3− (R = H, Bu and
SO3−) in the first reaction step through ligand substitution of
[(edta)RuIII(H2O)]− by ROOH, will partially govern the efficiency
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