Mechanistic Aspects of Os(VIII)/Ru(III) Catalysed Oxidationof L-phenylalanine by Ag(III) Periodate Complex in AqueousAlkaline Medium
Shekappa D. Lamani Æ Jyothi C. Abbar ÆSharanappa T. Nandibewoor
Received: 12 July 2009 / Accepted: 18 August 2009 / Published online: 10 September 2009
� Springer Science+Business Media, LLC 2009
Abstract The kinetics of oxidation of ruthenium(III)
(Ru(III)) and osmium(VIII) (Os(VIII)) catalysed oxidation
of L-phenylalanine (L-Pal) by diperiodatoargentate(III)
(DPA) in aqueous alkaline medium at 27 �C and a constant
ionic strength of 0.25 mol dm-3 was studied spectropho-
tometrically. The involvement of free radicals was
observed in the reactions. The reaction between DPA
and L-Pal in alkaline medium exhibits stoichiometry as
[L-Pal]:[DPA] = 1:1. The reaction is of first order in
[Os(VIII)], [Ru(III)] and [DPA] and has negative fractional
order in [IO4-]. It has less than unit order in [L-Pal] and
[OH-]. However, the order in [L-Pal] and [OH-] changes
from first order to zero order as their concentrations
increase. The main oxidation products were identified by
spot test and spectral studies. The probable mechanisms
were proposed and discussed. The catalytic constant (Kc)
was also calculated for Os(VIII) and Ru(III) catalysis at
different temperatures. The activation parameters with
respect to slow step of the mechanisms were computed and
discussed and thermodynamic quantities were also calcu-
lated. It has been observed that the catalytic efficiency for
the present reaction is in the order of Os(VIII) [ Ru(III).
The active species of catalyst and oxidant have been
identified.
Keywords Kinetics � Oxidation � L-phenylalanine �Diperiodatoargantate(III) � Osmium(VIII) catalysis �Ruthenium(III) catalysis
1 Introduction
The oxidation of amino acids is of interest as the oxidation
products differ for different oxidants [1, 2]. Thus, the study
of amino acids becomes important because of their bio-
logical significance and selectivity towards the oxidants.
Amino acids have been oxidized by variety of oxidizing
agents [3]. L-phenylalanine (L-Pal) is an essential amino
acid. It forms active sites of enzyme and helps in main-
taining their proper confirmation by keeping them in proper
ionic states. So oxidation of L-Pal may help in under-
standing some aspects of enzyme kinetics. L-phenylalanine
supplementation helps in the suppression of the pain and
aid weight loss through the suppression of appetite. It is
converted in to tyrosine, which is precursor to dopamine
levels, have a definite function in sexual desire.
Diperiodatoargentate(III) (DPA) is a powerful oxidising
agent in alkaline medium with the reduction potential [4]
1.74 V. It is widely used as a volumetric reagent for the
determination of various organic and inorganic species [5].
Jayaprakash Rao and other researchers have studied DPA
as an oxidizing agent for the kinetics of oxidation of some
organic substrates [6]. They normally found that order with
respect to both oxidant and substrate was unity and [OH-]
was found to enhance the rate of reaction. It was also
observed that they did not arrive at the possible active
species of DPA in alkali, and on the other hand they pro-
posed mechanisms by generalizing the DPA as
[Ag(HL)L](x?1). However, Kumar et al. [7, 8] put an effort
to give an evidence for the reactive form of DPA in large
scale of alkaline pH. Ag(III) complexes can be stabilized in
alkaline medium by periodate or tellurate ions [9, 10].
When the silver(III) species are involved, it would be
interesting to know which of the species is the active
oxidant. In the present investigation, we have obtained the
S. D. Lamani � J. C. Abbar � S. T. Nandibewoor (&)
Department of Chemistry, Karnatak University,
Dharwad 580 003, India
e-mail: [email protected]
123
Catal Lett (2009) 133:142–155
DOI 10.1007/s10562-009-0134-5
evidence for the reactive species for DPA in alkaline
medium.
Transition metals are known to catalyse many oxida-
tion–reduction reactions since they involve multiple oxi-
dation states. In recent years the use of transition metal ions
such as osmium, ruthenium, palladium, manganese, chro-
mium, iridium, either alone or as binary mixtures, as cat-
alysts in various redox processes has attracted considerable
interest [11]. Although the mechanism of catalysis depends
on the nature of the substrate, the oxidant and experimental
conditions, it has been shown that metal ions act as cata-
lysts by one of these different paths such as the formation
of complexes with reactants or oxidation of the substrate
itself or through the formation of free radicals. Osmium
(VIII) (Os(VIII)) and ruthenium(III) (Ru(III)) catalysis in
redox reactions involves different degrees of complexity,
due to the formation of different intermediate complexes
and different oxidation states of osmium/ruthenium, etc.
The uncatalysed reaction of oxidation of L-Pal by DPA has
been studied [12]. We have observed that Os(VIII) and
Ru(III) catalyzes the oxidation of L-Pal by DPA in alkaline
medium in micro amounts. In order to understand the
active species of oxidant and catalyst, and to propose the
appropriate mechanism, the title reaction has been inves-
tigated in detail. An understanding of the mechanism
allows the chemistry to be interpreted, understood and
predicted.
2 Experimental Section
2.1 Chemicals and Solutions
All reagents were of analytical reagent grade and Millipore
water was used throughout the work. A solution of
L-phenylalanine (HiMedia Laboratories) was prepared by
dissolving an appropriate amount of recrystallised sample
in Millipore water. The purity of L-Pal was checked by its
m.p. 273 �C [Lit.m.p.275 �C]. The IR spectrum agreed
with literature. The required concentration of L-Pal was
used from its stock solution. The Os(VIII) solution was
prepared by dissolving OsO4 (Jonshon Matthey) in
0.50 mol dm-3 in NaOH. The concentration was ascer-
tained [13] by determining the unreacted [Fe(CN)6]4- with
standard Ce(IV) solution in an acidic medium. A standard
stock solution of Ru(III) was prepared by dissolving RuCl3(S.D. Fine Chemicals) in 0.20 mol dm-3 HCl. The con-
centration was determined [14, 15] by EDTA titration.
KNO3 and KOH (BDH) were used to maintain the ionic
strength and alkalinity of the reaction, respectively. An
aqueous solution of AgNO3 was used to study the product
effect, Ag(I). A stock solution of IO4- was prepared by
dissolving a known weight of KIO4 (Riedel-de-Hean) in
hot water and used after keeping for 24 h to attain the
equilibrium. Its concentration was ascertained iodometri-
cally [16], at neutral pH maintained using phosphate buf-
fer. The pH of the medium in the solution was measured by
ELICO (L1613) pH meter.
2.2 Instruments Used
(a) For kinetic measurements, a CARY 50 Bio UV–vis
Spectrophotometer (Varian, Victoria-3170, Australia)
was used.
(b) GC-mass data were obtained on a 17A Shimadzu gas
chromatography with a QP-5050A Shimadzu mass
spectrometer
(c) Nicollet 5700-FT-IR spectrometer (Thermo, USA.)
2.3 Preparation of DPA
DPA was prepared by oxidizing Ag(I) in presence of KIO4
as described elsewhere [17]. The complex was character-
ized from its UV spectrum, which exhibited three peaks at
216, 255 and 362 nm. These spectral features were iden-
tical to those reported earlier for DPA [18]. The magnetic
moment study revealed that the complex was diamagnetic.
The compound prepared was analyzed [19] for silver and
periodate by acidifying a solution of the material with HCl,
recovering and weighing the AgCl for Ag and titrating the
iodine liberated when excess KI was added to the filtrate
for IO4-. The aqueous solution of DPA was used for the
required [DPA] in the reaction mixture. During the kinetics
a constant concentration viz. 1.0 9 10-5 mol dm-3 of
KIO4 was used throughout the study unless otherwise sta-
ted. Thus, the possibility of oxidation of L-Pal by periodate
was tested and found that there was no significant inter-
ference due to KIO4 under experimental condition. The
total concentrations of periodate and OH- was calculated
by considering the amount present in the DPA solution and
that additionally added. Kinetic runs were also carried out
in N2 atmosphere in order to understand the effect of dis-
solved oxygen on the rate of the reaction. No significant
difference in the results was obtained under a N2 atmo-
sphere and in the presence of air. In view of the ubiquitous
contamination of carbonate in the basic medium, the effect
of carbonate was also studied. Added carbonate had no
effect on the reaction rates.
2.4 Instrumentation and Kinetic Measurements
All kinetics measurements were performed on a Varian
CARY 50 Bio UV–vis Spectrophotometer. The kinetics
was followed under pseudo-first order condition where
[L-Pal] [ [DPA] in catalysed reactions at 27.0 ± 0.1 �C,
Mechanistic Aspects of Os(VIII)/Ru(III) 143
123
unless specified. The reaction was initiated by mixing DPA
with the L-Pal solution which also contained required
concentrations of Os(VIII) or Ru(III), KNO3, KOH and
KIO4. The progress of the reaction was monitored spec-
trophotometrically at 360 nm (i.e., decrease in absorbance
due to DPA with the molar absorbancy index, ‘e’ to be
13,900 ± 100 dm3 mol-1 cm-1, literature value being
13,892 dm3 mol-1 cm-1, in both catalysed reactions),
which is the maximum absorption wavelength of DPA. The
spectral changes during the chemical reaction for the
standard condition at 300 K are given in Fig. 1. It was
verified that there was almost no interference from other
species in the reaction mixture at this wavelength.
The pseudo-first order rate constants, ‘kC’, were
determined from the log (absorbance) versus time plots.
The plots were linear up to 85% completion of reaction
under the range of [OH-] used. The orders for various
species were determined from the slopes of plots of log kC
versus respective concentration of species except for
[DPA] in which non-variation of ‘kC’ was observed as
expected to the reaction condition. The rate constants
were reproducible to within ±5%. Regression analysis of
experimental data to obtain regression coefficient r and
the standard deviation S, of points from the regression
line, was performed with the Microsoft office Excel-2003
programme.
3 Results
3.1 Stoichiometry and Product Analysis
Different sets of reaction mixtures containing varying
ratios of DPA to L-Pal in the presence of constant amount
of OH-, KNO3 and KIO4 and Os(VIII)/Ru(III) catalyst
were kept for 2 h in a closed vessel under nitrogen atmo-
sphere. The remaining concentration of DPA was estimated
by spectrophotometrically at 360 nm. The results indicated
a 1:1 stoichiometry as given in Eq. 1.
The main oxidation products were identified as aldehyde
(phenylacetaldehyde), by a spot test [19]. Ammonia was
identified by Nessler’s reagent and CO2 was qualitatively
detected by bubbling N2 gas through the acidified reaction
mixture and passing a liberated gas through a tube con-
taining limewater. The aldehyde was confirmed by pre-
paring its 2, 4-DNP derivative. The nature of phenyl
acetaldehyde was confirmed by its IR spectrum which
showed a C=O at 1,630 cm-1 indicating the presence of
aldehydic C=O and C–H stretching at 1,745 cm-1. Further,
phenylacetaldehyde was subjected to GC-mass spectral
analysis. GC-mass data were obtained on a 17A shimadzu
gas chromatography with a QP-5050A Shimadzu mass
spectrometer using the EI ionization technique. The mass
spectrum showed a molecular ion peak at 119 amu con-
firming the presence of phenylacetaldehyde product
(Fig. 2). All other peaks observed in GC–MS can be
interpreted in accordance with the observed structure of
phenylacetaldehyde. The formation of Ag? in solution was
detected by adding KCl solution to the reaction mixture,
CH2 CHO[Ag(H2IO6)(H2O)2]+ + H2IO6
(1)
3-
+ + NH4 Ag(I)+
Ru(III)/Os(VIII)CH2
NH2
COOCH
CO2 H2O+
Fig. 1 Spectroscopic changes occurring in the Os(VIII) catalysed
oxidation of L-phenylalanine by diperiodatoargentate(III) at 27 �C,
[DPA] = 5.0 9 10-5, [L-Pal] = 5.0 9 10-4, [Os(VIII)] = 8.0 9
10-7, [OH-] = 0.05 and I = 0.25 mol dm-3 with scanning time of:
(1) 0.5, (2) 1.0, (3) 1.5, (4) 2.0, (5) 2.5 and (6) 3.0 minFig. 2 GC-mass spectrum of phenylacetaldehyde with its molecular
ion peak at 119 amu
144 S. D. Lamani et al.
123
which produced white turbidity due to formation of AgCl.
It was observed that phenyl acetaldehyde did not undergo
further oxidation under the present kinetic conditions.
3.2 Reaction Orders
As the diperiodatoargantate(III) oxidation of L-phenylala-
nine in alkaline medium proceeds with a measurable rate in
the absence of ruthenium(III) or osmium(VIII), the cata-
lysed reaction is understood to occur in parallel paths with
contributions from both the catalysed and uncatalysed
paths. Thus the total rate constant (kT) is equal to the sum
of the rate constants of the catalysed (kC) and uncatalysed
(kU) reactions, so kC = kT - kU. Hence, the reaction orders
have been determined from the slopes of log kC versus log
(concentration) plots by varying the concentrations of
L-Pal, alkali, periodate and Ru(III) or Os(VIII) catalyst in
turn while keeping all other concentrations and conditions
constant.
3.3 Evaluation of Pseudo-first Order Rate Constants
The oxidant [DPA] was varied in the range of 1.0 9 10-5
to 1.0 9 10-4 mol dm-3 at 27 �C for both the catalysts
while keeping other reactant concentrations and condi-
tions constant. The fairly constant pseudo-first order rate
constants, kC, indicate that the order with respect to
[DPA] was unity (Tables 1 and 2). This was also con-
firmed by linearity of the plots of log (absorbance) ver-
sus time (r [ 0.999, S B 0.04) upto 85% completion of
the reaction.
3.4 The Dependence of Rate on the Concentration
of L-phenylalanine
The effect of L-Pal was studied in the range of
3.0 9 10-4 to 3.0 9 10-3 mol dm-3 at 27 �C while
keeping other reactant concentrations conditions constant
in the presence of both the catalysts. The kC values
Table 1 Effect of [DPA], [L-Pal], [IO4], [OH-] and [Ru(III)] on the ruthenium(III) catalysed oxidation of L-phenylalanine by DPA in aqueous
alkaline medium at 25 �C, I = 0.25 mol dm-3
[DPA] 9 105
(mol dm-3)
[L-pal] 9 104
(mol dm-3)
[IO4] 9 105
(mol dm-3)
[OH-] 9 102
(mol dm-3)
[Ru(III)] 9 106
(mol dm-3)
kT 9 102
(s-1)
kU 9 103
(s-1)
kC 9 102 (s-1)
Found Calculated
1.0 5.0 1.0 5.0 5.0 1.8 4.6 1.3 1.8
3.0 5.0 1.0 5.0 5.0 1.7 4.5 1.3 1.8
5.0 5.0 1.0 5.0 5.0 1.8 4.7 1.3 1.8
8.0 5.0 1.0 5.0 5.0 1.7 4.6 1.3 1.8
10.0 5.0 1.0 5.0 5.0 1.9 4.4 1.5 1.8
5.0 3.0 1.0 5.0 5.0 1.3 4.0 0.95 0.89
5.0 5.0 1.0 5.0 5.0 1.8 4.6 1.3 1.4
5.0 8.0 1.0 5.0 5.0 2.6 5.1 2.1 2.1
5.0 10.0 1.0 5.0 5.0 3.5 6.0 2.8 2.9
5.0 20.0 1.0 5.0 5.0 5.2 7.0 4.1 4.1
5.0 30.0 1.0 5.0 5.0 6.4 8.0 5.5 5.3
5.0 5.0 0.5 5.0 5.0 2.4 6.9 1.7 1.6
5.0 5.0 0.8 5.0 5.0 2.0 5.6 1.5 1.5
5.0 5.0 1.0 5.0 5.0 1.8 4.6 1.3 1.4
5.0 5.0 3.0 5.0 5.0 1.2 3.5 0.85 0.94
5.0 5.0 5.0 5.0 5.0 0.98 2.7 0.71 0.70
5.0 5.0 1.0 1.0 5.0 0.96 2.0 0.76 0.74
5.0 5.0 1.0 3.0 5.0 1.6 4.0 1.2 1.2
5.0 5.0 1.0 5.0 5.0 1.8 4.6 1.3 1.4
5.0 5.0 1.0 8.0 5.0 2.1 5.1 1.6 1.6
5.0 5.0 1.0 10.0 5.0 2.4 5.5 1.8 1.7
5.0 5.0 1.0 5.0 0.1 0.99 4.6 0.53 0.56
5.0 5.0 1.0 5.0 0.3 1.2 4.6 0.75 0.8
5.0 5.0 1.0 5.0 0.5 1.8 4.6 1.3 1.4
5.0 5.0 1.0 5.0 0.8 3.7 4.6 3.2 3.5
5.0 5.0 1.0 5.0 1.0 4.6 4.6 4.1 4.2
Mechanistic Aspects of Os(VIII)/Ru(III) 145
123
increased with increase in [L-Pal]. The order with respect
to [L-Pal] was less than unity (Ru(III) catalysed Table 1
and Os(VIII) catalysed Table 2) (r C 0.9847, S B 0.006).
However the order in [L-Pal] changes from first to zero
order as [L-Pal] varies.
3.5 The Dependence of Rate on the Concentration
of OH-
The effect of alkali was studied in the range of 0.01–
0.10 mol dm-3 at 27 �C and constant concentrations of
DPA, L-Pal, IO4-, KNO3 and Ru(III)/Os(VIII) catalysts.
The rate constants increased with increase in [alkali] and
the order was found to be less than unity (Ru(III) catalysed
Table 1 and Os(VIII) catalysed Table 2) (r C 0.9846,
S B 0.0045). The order in [OH-] also changes from first to
zero order as [OH-] varies.
3.6 The Dependence of Rate on the Concentration
of IO4-
The effect of periodate was studied in the range of
5.0 9 10-6 to 5.0 9 10-5 mol dm-3 at 27 �C, constant
concentrations of DPA, L-Pal, OH-, KNO3 and Ru(III)/
Os(VIII) catalysts. The experimental results indicated that
kC values decreased with increase in the [IO4-]. The order
with respect to IO4- was negative fractional (Table 1)
(r C 0.9982, S B 0.003).
3.7 Effect of [Ru(III)] or [Os(VIII)]
The ruthenium(III) concentrations were varied from
1.0 9 10-6 to 1.0 9 10-5 mol dm-3 and osmium(VIII)
were varied from 5 9 10-7 to 5 9 10-6 mol dm-3 at
27 �C and at constant concentrations of DPA, L-Pal, alkali
and ionic strength. The order in [Ru(III)] and [Os(VIII)]
Table 2 Effect of [DPA], [L-Pal], [IO4], [OH-] and [Os(VIII)] on the osmium(VIII) catalysed oxidation of L-phenylalanine by DPA in alkaline
medium at 25 �C, I = 0.25 mol dm-3
[DPA] 9 105
(mol dm-3)
[L-Pal] 9 104
(mol dm-3)
[IO4] 9 105
(mol dm-3)
[OH-] 9 102
(mol dm-3)
[Os(VIII)] 9 107
(mol dm-3)
kT 9 102
(s-1)
kU 9 103
(s-1)
kC 9 102 (s-1)
Found Calculated
1.0 5.0 1.0 5.0 8.0 2.9 4.6 2.5 2.2
3.0 5.0 1.0 5.0 8.0 2.8 4.5 2.4 2.2
5.0 5.0 1.0 5.0 8.0 2.8 4.7 2.5 2.2
8.0 5.0 1.0 5.0 8.0 2.8 4.6 2.3 2.2
10.0 5.0 1.0 5.0 8.0 2.9 4.4 2.5 2.2
5.0 3.0 1.0 5.0 8.0 1.9 4.0 1.5 1.5
5.0 5.0 1.0 5.0 8.0 2.8 4.6 2.3 2.2
5.0 8.0 1.0 5.0 8.0 3.5 5.1 3.0 3.1
5.0 10.0 1.0 5.0 8.0 4.1 6.0 3.4 3.5
5.0 20.0 1.0 5.0 8.0 5.9 7.0 5.1 4.9
5.0 30.0 1.0 5.0 8.0 6.8 8.0 6.0 5.7
5.0 5.0 0.5 5.0 8.0 3.8 6.9 3.1 3.1
5.0 5.0 0.8 5.0 8.0 3.2 5.6 2.6 2.6
5.0 5.0 1.0 5.0 8.0 2.8 4.6 2.3 2.2
5.0 5.0 3.0 5.0 8.0 1.3 3.5 0.98 0.98
5.0 5.0 5.0 5.0 8.0 0.96 2.7 0.69 0.69
5.0 5.0 1.0 1.0 8.0 0.97 2.0 0.71 0.70
5.0 5.0 1.0 3.0 8.0 2.1 4.0 1.6 1.6
5.0 5.0 1.0 5.0 8.0 2.8 4.6 2.3 2.2
5.0 5.0 1.0 8.0 8.0 3.3 5.1 2.7 2.8
5.0 5.0 1.0 10.0 8.0 3.6 5.5 3.0 3.1
5.0 5.0 1.0 5.0 5.0 1.5 4.6 1.0 1.1
5.0 5.0 1.0 5.0 8.0 2.8 4.6 2.3 2.2
5.0 5.0 1.0 5.0 10 3.6 4.6 3.1 3.5
5.0 5.0 1.0 5.0 30 8.3 4.6 8.3 8.5
5.0 5.0 1.0 5.0 50 13.1 4.6 12.6 12.7
146 S. D. Lamani et al.
123
was found to be unity from the linearity of the plots of kC
versus [Ru(III)] and kC vs. [Os(VIII)].
3.8 The Dependence of Rate on Ionic Strength (I) and
Dielectric Constant (D)
The addition of KNO3, to increase the ionic strength of the
reaction, increased the rate of reaction at constant [DPA],
[L-Pal], [OH-], [IO4-] and Ru(III) in (Fig. 3) and Os(VIII)
in (Fig. 4). The plots of log kC versus HI was found to be
linear with positive slope.
The dielectric constant of the medium, ‘D’ was varied
by varying the t-butyl alcohol and water percentage. The D
values were calculated from the equation: D = Dw
Vw ? DB VB, where Dw and DB are dielectric constants of
pure water and t-butyl alcohol, respectively, in the total
mixture. The decrease in the dielectric constant of the
reaction medium increases the rate and the plot of log kC
versus 1/D was found to be linear with negative slope for
Ru(III) (Fig. 3) and for Os(VIII) (Fig. 4).
3.9 Effect of Initially Added Products
Initially added products Ag(I) and aldehyde did not have
any significant effect on the rate of reaction.
3.10 Polymerization Study
The intervention of the free radicals was examined as
follows. The reaction mixture, to which known quantity of
acrylonitrile scavenger has been added initially, was kept
in an inert atmosphere for 6 h. Upon diluting the reaction
mixture with methanol, precipitate resulted, suggesting
there is participation of free radicals in the reaction.
3.11 Effect of Temperature
The influence of temperature on the rate of reaction was
studied at 22, 27, 32 and 37 �C. The rate constants, (kC), of
the slow step of Schemes 1 and 2 was obtained from the
slopes and the intercepts of the plots of [Ru(III)]/kC or
[Os(VIII)]/kC vs. 1/[L-pal] at four different temperatures.
The values are given in Tables 1 and 2. The activation
parameters for the rate determining step were obtained by
the least square method of plot of log kC vs. 1/T and are
presented in Tables 3 and 4.
3.12 Catalytic Activity
It has been pointed out by Moelwyn-Hughes [20] that in
the presence of catalyst, the uncatalysed and catalysed
reactions proceed simultaneously, so that,
kT ¼ kU þ KC½catalyst]x ð2Þ
Here kT is the observed pseudo-first order rate constant in
the presence of Ru(III)/Os(VIII) catalyst, kU, the pseudo
first-order rate constant for the uncatalysed, KC the
catalytic constant and ‘x’ the order of the reaction with
respect to Ru(III)/Os(VIII). In the present investigations, x
values for the standard run were found to be unity for
Ru(III)/Os(VIII). Then the value of KC is calculated using
the equation,
KC ¼kT � kU
½catalyst]x ¼kC
½catalyst]x where; kT � kU ¼ kCð Þ
ð3Þ
The values of KC were evaluated for both the catalysts at
different temperatures. Further, plots of log KC versus 1/T
were linear and the values of energy of activation and other
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.2 0.4 0.6 0.8
0
0.5
1
1.5
2
2.5
3
1.21.41.61.8
4 +
logk
C
I1/2
4 +
logk
C
1/D X 10-2
Fig. 3 Effect of ionic strength and dielectric constant of the medium
on Ru(III)-catalysed oxidation of L-phenylalanine by diperiodatoarg-
entate(III) at 27 �C
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.2 0.4 0.6 0.8
0
0.8
1.6
2.4
3.2
1.21.41.61.8
4 +
logk
C
1/D X 10-2
I1/2
4 +
logk
C
Fig. 4 Effect of ionic strength and dielectric constant of the Os(VIII)
catalysed oxidation of L-phenylalanine by diperiodatoargentate(III) at
27 �C
Mechanistic Aspects of Os(VIII)/Ru(III) 147
123
activation parameters with reference to catalyst were
computed. These results are summarized in Table 5. The
value of KC for Ru(III) is 2.7 9 103 at 300 K and the
values of KC for Os(VIII) is 2.9 9 104 at 300 K.
4 Discussion
Due to strong versatile nature of two electrons oxidant, the
kinetics of oxidation of various organic and inorganic
substrates has been studied by Ag(III) species. The litera-
ture survey [17] reveals that the water soluble diperiodat-
oargentate(III) (DPA) has a formula [Ag(IO6)2]7- with
dsp2 configuration of square planar structure, similar to
diperiodatocopper(III) complex with two bidentate ligands,
periodate to form a planar molecule. In the alkaline med-
ium, the dissociative equilibria (4–6) of the IO4- were
detected and the corresponding equilibrium constants were
determined at 298.2 K by Aveston [21].
2IO�4 þ 2OH� � H2I2O4�10 logb1 ¼ 15:05 ð4Þ
IO�4 þ OH� þ H2O� H3IO2�6 logb2 ¼ 6:21 ð5Þ
IO�4 þ 2OH� � H2IO3�6 logb3 ¼ 8:67 ð6Þ
The distribution of all species of periodate in aqueous
alkaline solution can be calculated from equilibria (4–6). In
the [OH-] range used in this work the amount of dimer
(H2I2O104-) and IO4
- species of periodate can be neglected.
The main species of periodate are H2IO63- and H3IO6
2-,
which are consistent with the result calculated from
Crouthamel’s data [22]. Hence DPA could be as [Ag(H3
IO6)2]- or [Ag(H2IO6)2]3- in alkaline medium. Therefore,
under the present condition, diperiodatoargentate(III), may
be depicted as [Ag(H3IO6)2]-. The similar speciation of
H+ + OH-fast
H2O
CH2CH
NH2
Ag(OH)++ CH2CHO
NH3 Ag(I)+ +fast
.
[Ag(H3IO6)2]- + OH-
K1
[Ag(H3IO6)(H2IO6)]2- + H2O
HCO3-CO2 OH-+
fast
[Ag(H3IO6)(H2IO6)]2- 2H2O+
K2[Ag(H2IO6)(H2O)2] [H3IO6]
2-+
[OsO4(OH)2]2-+CH2 CH
NH2
COOComplex(C2)
K4
CO2 H++ ++ [OsO4(OH)2]2-H2O +
k2
+++Complex(C2) [Ag(H2IO6)(H2O)2] H2IO63-Ag(OH)+
slow
CH2 CH
NH2
Scheme 2
H+ + OH-fast
H2O
CH2CH
NH2
Ag(OH)++ CH2CHO
NH3 Ag(I)+ +fast
.
+CH2 CH
NH2
COO[Ru(H2O)5OH]2+
Complex(C1)K3
fastHCO3
-CO2 OH-+
CH2 CH
NH2
k+++Complex(C1) [Ag(H2IO6)(H2O)2]
[Ru(H2O)5(OH)]2+
H2IO63-Ag(OH)+
H++ ++
slow
CO2 H2O+
1
Scheme 1
148 S. D. Lamani et al.
123
periodate in alkali was proposed [23] for diperioda-
tonickelate(IV).
It is known that L-phenylalanine exists in the form of
Zwitterion [24] in aqueous alkaline medium. In highly
acidic medium, it exists in the protonated form, where as in
highly basic medium it is in the fully deprotonated form
[24] as
C6H5 � CH2 � NH2 � CH� COO�
4.1 Mechanism of Ru(III) Catalysed Reaction
It is interesting to identify the probable ruthenium(III)
chloride species in alkaline media. The added chloride had
no effect on the rate of reaction. Electronic spectral studies
[23] have confirmed that ruthenium chloride exists in the
hydrated form as [Ru(H2O)5OH]2?. In the present study (at
pH 13.3), it is quite probable that for [Ru(III)(OH)x]3-x, the
Table 4 Thermodynamic activation parameters for the osmium(VIII)
catalysed oxidation of L-phenylalanine by DPA in aqueous alkaline
medium with respect to the slow step of Scheme 1
Temperature (K) k2 9 104 dm3 mol-1 s-1
(a)Effect of temperature
295 2.2
300 2.4
305 2.6
310 2.9
Parameter Value
(b) Activation parameters
Ea (kJ mol-1) 13.0 ± 2.5
DH# (kJ mol-1) 10.5 ± 1.5
DS# (JK-1 mol-1) -107 ± 2.0
DG# (kJ mol-1) 42.6 ± 3.0
log A 7.6 ± 0.3
Temperature (K) K1
(dm3 mol-1)
K2 9 104
(mol dm-3)
K4 9 10-2
(dm3 mol-1)
(c) Effect of temperature on K1, K2 and K4 for the osmium(VIII)
catalysed oxidation of L-phenylalanine by diperiodatoargentate(III)
in aqueous alkaline medium
295 0.4 5.5 6.6
300 0.6 2.3 7.5
305 1.1 1.8 8.6
310 1.8 1.0 9.8
Thermodynamic
quantities
Values
from K1
Values
from K2
Values
from K4
(d) Thermodynamic quantities using K1, K2 and K3
DH (kJ mol-1) 78 -81 20
DS (JK-1 mol-1) 259 -340 121
DG (kJ mol-1) 2.2 21.5 -16.8
Table 5 Values of catalytic constant (KC) at different temperatures
and activation parameters calculated using KC values for Ru(III)/
Os(VIII) catalysed reaction
Temperature (K) KC 9 10-3 Ru(III) KC 9 10-4 Os(VIII)
295 1.9 2.0
300 2.7 2.9
305 4.0 3.4
310 5.3 4.9
Ea (kJ mol-1) 51.3 42.4
DH# (kJ mol-1) 48.8 40.0
DS# (JK-1 mol-1) 14.5 -25.5
DG# (kJ mol-1) 52.1 47.7
log A 12.4 11.8
Table 3 Thermodynamic activation parameters for the ruthe-
nium(III) catalysed oxidation of L-phenylalanine by DPA in aqueous
alkaline medium with respect to the slow step of Scheme 1
Temperature (K) k1 9 104 dm3 mol-1 s-1
(a) Effect of temperature
295 2.2
300 3.1
305 4.0
310 4.8
Parameter Value
(b) Activation parameters
Ea (kJ mol-1) 38.8 ± 2.5
DH# (kJ mol-1) 36.8 ± 1.5
DS# (JK-1 mol-1) -38.5 ± 2.0
DG# (kJ mol-1) 47.5 ± 3.0
log A 11.2 ± 0.3
Temperature (K) K1
(dm3 mol-1)
K2 9 104
(mol dm-3)
K3 9 10-2
(dm3 mol-1)
(c) Effect of temperature on K1, K2 and K3 for the ruthenium(III)
catalysed oxidation of L-phenylalanine by diperiodatoargentate(III)
in aqueous alkaline medium
295 0.7 8.4 1.2
300 0.9 6.4 2.7
305 1.3 3.9 3.7
310 2.2 2.4 4.6
Thermodynamic
quantities
Values
from K1
Values
from K2
Values
from K3
(d) Thermodynamic quantities using K1, K2 and K3
DH (kJ mol-1) 55 -64 66
DS (JK-1 mol-1) 185 -277 265
DG (kJ mol-1) -2.0 19.5 -14.1
Mechanistic Aspects of Os(VIII)/Ru(III) 149
123
x valves would always be less than six because there are no
definite reports of any hexahydroxy ruthenium species. The
remainder of the coordination sphere would be filled by
water molecules. Hence, the condition employed, e.g.,
[OH-] � [Ru(III)], ruthenium(III) is mostly present as the
monomerichydroxylated species, [Ru(H2O)5OH]2? [25].
In the prior equilibrium step 1, the OH- deprotonates
the DPA to give a deprotonated diperiodatoargentate(III);
in the second step displacement of a ligand, periodate takes
place to give free periodate which is evidenced by decrease
in the rate with increase in [IO4-] (Table 1). It may be
expected that lower Ag(III) periodate species such as MPA
is more important active species in the reaction than DPA.
The inverse fractional order in [H3IO62-] might also be due
to this reason. In the pre rate determining stage, the
hydroxylated species of Ru(III) combines with a molecule
of L-Pal to give an intermediate complex, which further
reacts with one mole of MPA in rate determining step to
give a free radical derived from L-Pal with the regeneration
of the ruthenium(III) catalyst. This free radicals from the
L-Pal further reacts with another molecule of MPA species
in a fast step to give the products as given in Scheme 1.
The probable structure of the complex (C1) is given
below;
Spectroscopic evidence for the complex formation
between Ru(III) and L-Pal was obtained from UV–vis
spectra of L-Pal (5.0 9 10-4), Ru(VIII) (5.0 9 10-6,
[OH-] = 0.05 mol dm-3) and a mixture of both. A
hypsochromic shift of about 5 nm from 234 to 229 nm
in the spectra of Ru(III) was observed. The Michaelis–
Menten plot proved the complex formation between
catalyst and substrate, which explains less than unit order
in [L-Pal]. Such a complex between a catalyst and
substrate has also been observed in other studies [26].
From Scheme 1, the rate law (8) can be derived.
Equation 8 can be rearranged to Eq. 9, which is suitable for
verification.
½RuðIIIÞ�kC
¼ ½H3IO2�6 �
k1K1K2K3½L-Pal] [OH�� þ½H3IO2�
6 �k1K2K3½L-Pal]
þ 1
k1K3½L-Pal]þ ½H3IO2�
6 �k1K1K2½OH�� þ
½H3IO2�6 �
k1K2
þ 1
k1
ð9Þ
According to Eq. 9, other conditions being constant, plots
of [Ru(III)]/kC vs. 1/[L-Pal], 1/[OH-] and [H3IO62-] should
be linear and are found to be (Fig. 5a–c). The slopes and
intercepts of such plots, lead to the values of K1, K2, K3 and
k1 (Table 3) calculated as (0.93 ± 0.05) dm3 mol-1, (6.4
± 0.2) 9 10-4 mol dm-3, (2.7 ± 0.1) 9 102 dm3 mol-1,
(3.1 ± 0.2) 9 104 dm3 mol-1 s-1. The value of K1 is in
good agreement with the literature [27] and the value of K2 is
good agreement with the literature [28]. Using these
constants, the rate constants were calculated and compared
with the experimental kC values. There was a reasonable
agreement with each other (Table 1), which fortifies the
proposed mechanism (Scheme 1).
The increase in the rate, with increasing ionic strength,
is in favor of a reaction between charged species of reac-
tants, as presented in Scheme 1 of the proposed mecha-
nism. The effect of solvent on the reaction rate has been
described in detail in the literature [29]. In the present
study, the plot of log kC vs. 1/D is linear with negative
slope (Fig. 3) which seems to be contrary to the expected
reaction between neutral and anionic species in media of
lower relative permittivity. However, an increase in the rate
2+
CH 2CH
NH2
C
O
ORu
OH2
OH2
OH2
OH2
OH2
OH
Rate =�d½DPA�
dt¼ k1K1K2K3½AgðIIIÞ�½L�Pal�½OH
��½RuðIIIÞ�½H3IO2�
6 � þ K1½OH��½H3IO2�6 � þ K1K2½OH�� þ K3½H3IO2�
6 �½L-Pal]
þ K1K3½L-Pal][OH��½H3IO2�6 � þ K1K2K3½OH��½L-Pal]
ð7Þ
Rate
½DPA� ¼ kC ¼ kT � kU ¼k1K1K2K3½L�Pal�½OH
��½RuðIIIÞ�½H3IO2�
6 � þ K1½OH��½H3IO2�6 � þ K1K2½OH�� þ K3½H3IO2�
6 �½L-Pal]
þ K1K3½L-Pal][OH��½H3IO2�6 � þ K1K2K3½OH��½L-Pal]
ð8Þ
150 S. D. Lamani et al.
123
of the reaction with increasing relative permittivity may be
due to stabilization of the complex at high relative
permittivity.
The thermodynamic quantities for the different equi-
librium steps, in Scheme 1 can be evaluated as follows.
The [L-Pal] and [OH-] (Table 1) were varied at four dif-
ferent temperatures. The plots of [Ru(III)]/kC vs. 1/[L-Pal],
[Ru(III)]/kC vs. 1/[OH-] and [Ru(III)]/kC vs. [H3IO62]
should be linear as in Fig. 5a–c. From the slopes and
intercepts, the values of K1, K2 and K3 were calculated at
different temperatures. A van’t Hoff’s plot was made for
the variation of K1, K2 and K3 with temperature (log K1 vs.
1/T, log K2 vs. 1/T and log K3 vs. 1/T). The values of
enthalpy of reaction DH, entropy of reaction DS and free
energy of reaction DG were calculated for the first, second
and third equilibrium steps. These values are given in
Table 3. A comparison of the DH value (55.27 kJ mol-1)
from K1 with that of DH# (36.35 kJ mol-1) of rate limiting
step supports that the reaction before the rate determining
step is fairly fast as it involves low activation energy [30].
A negative value of DS# (-37.4 JK-1 mol-1) suggests that
intermediate complex is more ordered than the reactants
[31].
4.2 Mechanism of Osmium(VIII) Catalysis
Osmium(VIII) is known to form different complexes
at different OH- concentrations, [OsO4(OH)2]2- and
[OsO5(OH)]3-. At higher concentration of OH-, [OsO5
(OH)]3- is significant. At lower concentrations of OH-, as
employed in the present study and since the rate of oxi-
dation increased with increase in [OH-], it is reasonable
that [OsO4(OH)2]2- was operative and its formation is
important in the reaction. Added periodate retarded the
rate. First order dependency in [DPA] and catalyst
(Os(VIII)) and fractional order in [L-Pal] and [OH-] was
observed. To explain the observed orders the following
Scheme 2 has been proposed for osmium(VIII) catalysed
reaction
In the prior equilibrium step 1, the OH- deprotonates
the DPA to give a deprotonated diperiodatoargentate(III);
in the second step displacement of a ligand, periodate takes
place to give free periodate which is evidenced by decrease
in the rate with increase in [IO4-] (Table 5). It may be
expected that lower Ag(III) periodate species such as MPA
is more important active species in the reaction than DPA.
The inverse fractional order in [H3IO62-] might also be due
to this reason. In the pre rate determining stage, the
hydroxylated species of Os(VIII) combines with a mole-
cule of L-Pal to give an intermediate complex, which
further reacts with one mole of MPA in rate determining
step to give a Free radicle of derived from L-Pal with the
regeneration of the osmium(VIII) catalyst. This free radi-
cals from the L-Pal further reacts with another molecule of
MPA species in a fast step to give the products as given in
Scheme 2.
0
2
4
6
8
10
12
0 2 4 6 8 10 12
[Ru(
III)
]/kC
X 1
0-4
mol
dm
-3 s
1/[OH-] X 10-2 dm3 mol-1
295K
300K
305K
310K
0123456789
10
0 1 2 3 4
1/[L-Pal] X 103 dm3 mol-1
[Ru(
III)
]/kC
X 1
0-4 m
ol d
m-3
s
[Ru(
III)
]/kC
X 1
0-4 m
ol d
m-3
s
295K
300K305K310K
0
2
4
6
8
0 2 4 6
[H3IO62- ] X 10 -5 mol dm-3
(b)
(a) (c)Fig. 5 Verification of rate law
(8) of Ru(III) catalysed
oxidation of L-phenylalanine by
diperiodatoargentate(III).
a Plots of [Ru(III)]/kC vs.
1/[L-Pal] at four different
temperatures (conditions as in
Table 1). b Plots of [Ru(III)]/kC
vs. 1/[OH-] at four different
temperatures (conditions as in
Table 1). c Plots of [Ru(III)]/kC
vs. [H3IO62-] at 27 �C
(conditions as in Table 1)
Mechanistic Aspects of Os(VIII)/Ru(III) 151
123
The probable structure of the complex (C2) is given
below;
Spectroscopic evidence for the complex formation
between Os(VIII) and L-pal was obtained from UV–vis
spectra of L-Pal (5.0 9 10-4), Os(VIII) (8.0 9 10-7,
[OH-] = 0.05 mol dm-3) and a mixture of both. A hyp-
sochromic shift of about 6 nm from 240 to 234 nm in the
spectra of Os(VIII) to mixture of Os(VIII) and L-Pal was
observed. The Michaelis–Menten plot proved the complex
formation between catalyst and substrate, which explains
less than unit order in [L-Pal]. Such a complex between a
catalyst and substrate has also been observed in other
studies [32].
From Scheme 2, the rate law (11) can be derived.
Equation 11 can be rearranged to Eq. 12, which is suitable
for verification.
½OsðVIIIÞ�kC
¼ ½H3IO2�6 �
k2K1K2K4½L-Pal][OH��
þ ½H3IO2�6 �
k2K2K4½L-Pal]þ 1
k2K4½L-Pal]
þ ½H3IO2�6 �
k2K1K2½OH�� þ½H3IO2�
6 �k2K2
þ 1
k2
ð12Þ
According to Eq. 12, other conditions being constant,
plots of [Os(VIII)]/kC vs. 1/[L-Pal], 1/[OH-] and [H3IO62-]
should be linear and are found to be so as in Fig. 6a–c.
The slopes and intercepts of such plots lead to the
values of K1, K2, K4 and k2 (Table 2) calculated as
(0.66 ± 0.04) dm3 mol-1 (2.3 ± 0.1) 9 10-4 mol dm-3,
(7.5 ± 0.2) 9 102 dm3 mol-1 and (2.4 ± 0.1) 9 105 dm3
mol-1 s-1, respectively. The value of K1 and K2 is in good
agreement with the literature [12]. Using these constants,
the rate constants were calculated and compared with the
experimental kC values. There was a reasonable agreement
with each other (Table 2), which fortifies the proposed
mechanism (Scheme 2).
The thermodynamic quantities for the different equi-
librium steps, in Scheme 2 can be evaluated as follows.
The [L-Pal] and [OH-] (Table 5) were varied at four
different temperatures. The plots of [Os(VIII)]/kC vs. 1/
[L-Pal], [Os(VIII)]/kC vs. 1/[OH-] and [Os(VIII)]/kC vs.
[H3IO62] should be linear. From the slopes and intercepts,
the values of K1, K2 and K4 were calculated at different
temperatures. A van’t Hoff’s plot was made for the varia-
tion of K1, K2 and K4 with temperature (log K1 vs. 1/T, log
K2 vs. 1/T and log K4 vs. 1/T). The values of enthalpy of
reaction DH, entropy of reaction DS and free energy of
reaction DG were calculated for the first, second and third
equilibrium steps. These values are given in Table 2. A
comparison of the DH value (78.4 kJ mol-1) from K1 with
that of DH# (10.5 kJ mol-1) of rate limiting step supports
that the reaction before the rate determining step is fairly
fast as it involves low activation energy [30].
The increase in the rate, with increasing ionic strength, is
in favor of a reaction between charged species of reactants, as
presented in Scheme 2 of the proposed mechanism. The
effect of solvent on the reaction rate is described in detail in
the literature [29]. For the limiting case of zero angle
approach between two dipoles or anion–dipole system, Amis
[29] has shown that a plot of log KC vs. 1/D gives a straight
line (Fig. 4), with a negative slopes for a reaction between
negative ion and a dipole or two dipoles and with a positive
slopes for a positive ion and dipole interaction. In the present
study, the plot observed had a negative slope, which is in the
right direction as shown in Scheme 2. The complex involved
in both the catalyst is expected to lead to negative entropy of
Os
O
O
O
O
O
O
H
H
CH2 CH
NH2
C
O
O
3-
Rate ¼ �d½DPA�dt
¼ k2K1K2K4½AgðIIIÞ�½L�Pal�½OH��½OsðVIIIÞ�
½H3IO2�6 � þ K1½OH��½H3IO2�
6 � þ K1K2½OH�� þ K4½H3IO2�6 �½L-Pal]
þ K1K4½L-Pal][OH��½H3IO2�6 � þ K1K2K4½OH��½L-Pal]
ð10Þ
Rate
½DPA]¼ kC ¼ kT � kU ¼
k2K1K2K4½L�Pal�½OH��½OsðVIIIÞ�
½H3IO2�6 � þ K1½OH��½H3IO2�
6 � þ K1K2½OH�� þ K4½H3IO2�6 �½L-Pal]
þ K1K4½L-Pal][OH��½H3IO2�6 � þ K1K2K4½OH��½L-Pal]
ð11Þ
152 S. D. Lamani et al.
123
activation and this is found to be the case. The negative value
of DS# (-107 JK-1 mol-1) suggests that intermediate com-
plex is more ordered than the reactants [31]. The observed
higher rate constant for the slow step indicate that the oxi-
dation presumably occurs via an inner-sphere mechanism.
This conclusion is supported by earlier observation [33].
The values of DS#, DG# and the rate constant (k) indicate
that the order of reactivity of the catalysts is Ru(III) \Os(VIII) for the oxidation of L-phenylalanine by DPA. The
Os(VIII)-catalysed reaction, however, is reasonably fast in
view of readiness of Os(VIII) to act across the double bond
and the Ru(III)-catalysed reaction is slower, probably owing
to the less ability of the Ru(III) to act across the double bond.
The activation parameters evaluated for the catalysed and
uncatalysed reactions explain the catalytic effect on the
reaction. The catalyst Ru(III) or Os(VIII) forms a complex
(C) with the substrate, which enhance the reducing property
of the substrate over that without that catalyst (Ru(III) or
Os(VIII)). Further, the catalyst Ru(III) or Os(VIII) modifies
the reaction path by lowering the energy of activation.
5 Conclusion
The comparative study of ruthenium(III) and osmium(VIII)
catalysed oxidation of L-Pal by diperiodatoargantate(III)
was studied. Oxidation products were identified. Among
the various species of Ag(III) in alkaline medium,
[Ag(H2IO6)(H2O)2] is considered to be the active species
for the title reaction. Active species of Ru(III) is found
to be [Ru(H2O)5(OH)2]2? and that for Os(VIII) is [OsO4
(OH)2]2-. Activation parameters were evaluated for both
uncatalysed and catalysed reactions with respect to slow
step of reactions Schemes. Catalytic constants and activa-
tion parameters with respect to catalyst were also com-
puted. The catalytic efficiency is Ru(III) \ Os(VIII).
Appendix
According to Scheme 1
Rate ¼ �d½DPA�dt
¼ k1½C1�½Ag(H2IO6ÞðH2O)2�
¼ k1K1K2K3½DPA�½OH��½L�Pal�½RuðIIIÞ�
½H3IO2�6 �
ðA:1Þ
The total concentration of DPA is given by (where T and f
stands for total and free)
½DPA�T ¼ ½DPA�f + [Ag(H2IO6ÞðH3IO6Þ�2�
+ [Ag(H2IO6ÞðH2O)2�
¼ ½DPA�f þ K1½OH��½DPA� þ K2½AgðH2IO6ÞðH3IO6Þ�2�
½H3IO2�6 �
¼ ½DPA�f þ K1½OH��½DPA� þ K1K2½OH��½H3IO2�
6 �
¼ ½DPA�f 1þ K1½OH�� þ K1K2½OH��½H3IO2�
6 �
� �
¼ ½DPA�f½H3IO2�
6 � þ K1½OH��½H3IO2�6 � þ K1K2½OH��
½H3IO2�6 �
� �
0
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12
295K
300K
305K
310K
[Os(
VII
I)]/
kC X
10-5
mol
dm
-3 s
1/[OH-] X 10-2 dm3 mol-1
0
1
2
3
4
5
6
7
8
0 1 2 3 4
1/[L-Pal] X 103 dm3 mo-1
[Os(
VII
I)]/
kC X
10-5
mol
dm-3
s
295K
300K305K
310K
0
4
8
12
16
0 1 2 3 4 5 6
[Os(
VII
I)]/
kC X
10-5
mol
dm-3
s
[H3IO 62-
] X 10-5
mol dm-3
(a)
(b)
(c)Fig. 6 Verification of rate law
(8) of Os(VIII) catalysed
oxidation of L-phenylalanine by
diperiodatoargentate(III).
a Plots of [Os(VIII)]/kC vs.
1/[L-Pal] at four different
temperatures (conditions as in
Table 2). b Plots of [Os(VIII)]/
kC vs. 1/[OH-] at four different
temperatures (conditions as in
Table 2). c Plots of [Os(VIII)]/
kC vs. [H3IO62-] at 27 �C
(conditions as in Table 2)
Mechanistic Aspects of Os(VIII)/Ru(III) 153
123
Therefore
½DPA�f ¼½DPA�½H3IO2�
6 �½H3IO2�
6 � þ K1½OH��½H3IO2�6 � þ K1K2½OH��
ðA:2Þ
Similarly,
½L-Pal]T ¼½L-Pal]f + [C1�¼½L-Pal]f þ K3½L-Pal][Ru(III)]
¼½L-Pal]ff1þ K3½Ru(III)]g
½L-Pal]f ¼½L�Pal�T
1þ K3½Ru(III)]
In view of the low concentration of [Ru(III)] used
½L-Pal]T ¼ ½L-Pal]f ðA:3Þ
Similarly
½Ru(III)]T ¼½Ru(III)]f + [C1�¼½Ru(III)]f þ K3½L-Pal][Ru(III)]
¼½Ru(III)]ff1þ K3½L-Pal]g
½Ru(III)]f ¼½RuðIIIÞ�T
1þ K3½L-Pal]
ðA:4Þ
Similarly
½OH��T ¼ ½OH��f + [Ag(H3IO6ÞðH2IO6Þ�2�
+ [Ag(H3IO6ÞðH2O2Þ�
¼ ½OH�� þ K1½DPA�½OH�� þ K1K2½DPA�½OH��½H3IO2�
6 �
In view of low concentration of [DPA] and [H3IO62-]
½OH��T ¼ ½OH��f ðA:5Þ
Substituting the Eqs. A.2, A.3, A.4 and A.5 in Eq. A.1 and
omitting the T and f subscripts, we get
Similarly, rate law was derived for osmium(VIII) catalysed
reaction.
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(1982) Int J Chem Kinet 14:1183
4. Sethuram B (2003) Some aspects of electron—transfer reactions
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Rate ¼ �d½DPA�dt
¼ k1K1K2K3½AgðIIIÞ�½L�Pal�½OH��½RuðIIIÞ�
½H3IO2�6 � þ K1½OH��½H3IO2�
6 � þ K1K2½OH�� þ K3½H3IO2�6 �½L-Pal]
þ K1K3½L-Pal][OH��½H3IO2�6 � þ K1K2K3½OH��½L-Pal]
ðA:6Þ
Rate
½DPA� ¼ kC ¼ kT � kU ¼k1K1K2K3½L�Pal�½OH
��½RuðIIIÞ�½H3IO2�
6 � þ K1½OH��½H3IO2�6 � þ K1K2½OH�� þ K3½H3IO2�
6 �½L-Pal]
þ K1K3½L-Pal][OH��½H3IO2�6 � þ K1K2K3½OH��½L-Pal]
ðA:7Þ
154 S. D. Lamani et al.
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Mechanistic Aspects of Os(VIII)/Ru(III) 155
123
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