Os(VIII)/Ru(III) catalysed oxidation of aspirin drug by a new oxidant, diperiodatoargentate(III) in aqueous alkaline medium: A comparative kinetic study

Article abstract of DOI:10.1016/j.molcata.2005.12.018

The kinetics of Os(VIII) and Ru(III) catalysed oxidation of anti-pyretic drug, aspirin by diperiodatoargentate(III) (DPA) in alkaline medium at 298 K and a constant ionic strength of 0.10 mol dm^-3 was studied spectrophotometrically. The oxidation products in both the cases are 1,4-benzoquinone2-carboxylate ion and Ag(I). The stoichiometry is the same in both the catalysed reactions i.e., [aspirin]:[DPA] = 1:2. The reaction is of first order in Os(VIII)/Ru(III) and [DPA] and has less than unit order in both [ASP] and [alkali]. The oxidation reaction in alkaline medium has been shown to proceed via a Os(VIII)/Ru(III)-aspirin complex, which further reacts with one molecule of DPA in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test, IR, NMR and GC-MS. The reaction constants involved in the different steps of the mechanism are calculated. The catalytic constant (K_c) was also calculated for both catalysed reactions at different temperatures. From the plots of log K _c versus 1/T, values of activation parameters with respect to the catalyst have been evaluated. The activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also determined. It has been observed that the catalytic efficiency for the present reaction is in the order of Os(VIII) > Ru(III). The probable active species of catalyst and oxidant have been identified.

Full text of DOI:10.1016/j.molcata.2005.12.018

Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
Os(VIII)/Ru(III) catalysed oxidation of aspirin drug by a new oxidant,
diperiodatoargentate(III) in aqueous alkaline medium:
A comparative kinetic study
Chanabasayya V. Hiremath, T.S. Kiran,
Sharanappa T. Nandibewoor∗
P.G. Department of Studies in Chemistry, Karnatak University,
Dharwad 580003, India
Received 11 November 2005; received in revised form 3 December 2005; accepted 8 December 2005
Available online 27 January 2006
Abstract
The kinetics of Os(VIII) and Ru(III) catalysed oxidation of anti-pyretic drug, aspirin by diperiodatoargentate(III) (DPA) in alkaline medium
at 298 K and a constant ionic strength of 0.10 mol dm−3 was studied spectrophotometrically. The oxidation products in both the cases are 1,4-
benzoquinone2-carboxylate ion and Ag(I). The stoichiometry is the same in both the catalysed reactions i.e., [aspirin]:[DPA] = 1:2. The reaction
is of first order in Os(VIII)/Ru(III) and [DPA] and has less than unit order in both [ASP] and [alkali]. The oxidation reaction in alkaline medium
has been shown to proceed via a Os(VIII)/Ru(III)–aspirin complex, which further reacts with one molecule of DPA in a rate determining step
followed by other fast steps to give the products. The main products were identified by spot test, IR, NMR and GC–MS. The reaction constants
involved in the different steps of the mechanism are calculated. The catalytic constant (Kc) was also calculated for both catalysed reactions at
different temperatures. From the plots of log Kc versus 1/T, values of activation parameters with respect to the catalyst have been evaluated. The
activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also determined.
It has been observed that the catalytic efficiency for the present reaction is in the order of Os(VIII) > Ru(III). The probable active species of catalyst
and oxidant have been identified.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Aspirin; Os(VIII) catalysis; Ru(III) catalysis; Diperiodatoargentate(III); Oxidation; Kinetics
1. Introduction
of oxidation of various organic substrates. 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 the possible active species of
DPA in alkali and on the other hand they proposed mechanisms
by generalizing the DPA as [Ag(HL)L](x+1)−. However, Kumar
et al. [5] put an effort to give an evidence for the reactive form
of DPA in the large scale of alkaline pH. In the present inves-
tigation, we have obtained the evidence for the reactive species
for DPA in alkaline medium.
In recent years, the use of transition metal ions such as
osmium, ruthenium and iridium, either alone or as binary mix-
tures, as catalysts in various redox processes has attracted
considerable interest [6]. The role of osmium (VIII) as a cat-
alyst in some redox reactions has been reviewed [7]. Although
the mechanism of catalysis depends on the nature of the sub-
strate, the oxidant and experimental conditions, it has been
Aspirin (acetylsalicylic acid) (ASP) is a non-steroidal anal-
gesic, anti-inflammatory and anti-pyretic agent. It is used in
acute conditions such as headache, arthralgia, myalgia and other
cases requiring mild analgesia. Aspirin is widely studied in
medicine and several methods are suggested in literature for
its determination [1].
Diperiodatoargentate(III) (DPA) is a powerful oxidizing
agent in alkaline medium with the reduction potential [2.a]
1.74 V. It is widely used as a volumetric reagent for the determi-
nation of various organic and inorganic species [3]. Jayaprakash
Raoetal. [4]haveusedDPAasanoxidizingagentforthekinetics
∗
Corresponding author. Tel.: +91 836 2770524; fax: +91 836 2747884.
E-mail address: stnandibewoor@yahoo.com (S.T. Nandibewoor).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.12.018

164
C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
shown [8] that metal ions act as catalysts by one of these
different paths such as the formation of complexes with reac-
tants or oxidation of the substrate itself or through the forma-
tion of free radicals. osmium(VIII) and ruthenium(III) catal-
ysis in redox reactions involves several complexes, differ-
ent oxidation states of osmium/ruthenium, etc. The uncatal-
ysed reaction of oxidation of aspirin by DPA has been stud-
ied [9]. We have observed that osmium(VIII) and ruthe-
nium(III) catalyse the oxidation of aspirin by DPA in alka-
line medium in micro amounts. In order to understand the
active species of oxidant and catalyst, and to propose the
appropriate mechanism, the title reaction is investigated in
detail.
the orange crystals separated out and were recrystallised from
water.
The complex was characterized from its U.V. spectrum,
exhibited three peaks at 216, 255 and 362 nm. These spectral
features were identical to those reported earlier for DPA [13].
The magnetic moment study revealed that the complex is dia-
magnetic. The compound prepared was analysed [14] 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 concentra-
tion viz. 1 × 10−4 mol dm−3 of KIO4 was used throughout the
study unless otherwise stated. Thus, the possibility of oxidation
of ASP by periodate was tested and found that there was no sig-
nificant interference due to KIO4 under experimental condition.
Precaution was also taken to avoid the dissolution of O2 and
CO2 in the solution by maintaining inert atmosphere with N2
throughout the study.
2. Experimental
2.1. Materials and reagents
All chemicals used were of reagent grade and double dis-
tilled water was used throughout the work. A solution of aspirin
(M/s. S.S. Antibiotics Pvt. Ltd.,) was prepared by dissolving an
appropriate amount of recrystallised sample in double distilled
water. The purity of ASP sample was checked by comparing its
IR spectrum with literature data and with its m.p.136 ◦C. The
required concentration of ASP was used from its aqueous stock
solution. The osmium (VIII) solution was prepared by dissolv-
ing OsO4 (VIII) oxide (Johnson Matthey) in 0.50 mol dm−3 in
NaOH. The concentration was ascertained [10] by determin-
ing 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 concentration was determined [11] by
EDTA titration. KNO3 and KOH (BDH) were used to maintain
ionic strength and alkalinity of the reaction, respectively. Aque-
ous solution of AgNO3 was used to study the product effect,
2.3. Kinetic measurements
The kinetic measurements were performed on a Varian
CARY 50 Bio UV–vis spectrophotometer. The kinetics was fol-
lowed under pseudo first order condition where [ASP] > [DPA]
at 25 0.1 ◦C, unless specified. The reaction was initiated
by mixing the DPA to ASP solution which also contained
required concentration of KNO3, KOH, catalyst (Ru(III) or
Os(VIII)) and KIO4. The progress of reaction was followed
spectrophotometrically at 360 nm by monitoring decrease in
absorbance due to DPA with the molar absorbancy index, ‘ε’
to be 13900 100 dm3 mol−1 cm−1. It was verified that there
is a negligible interference from other species present in the
reaction mixture at this wavelength.
The pseudo first order rate constants, ‘kobs’, were determined
from the log(absorbance) versus time plots. The plots were lin-
ear up to 85% completion of reaction. The orders for various
species were determined from the slopes of plots of log kobs
versus respective concentration of species except for [DPA]
in which non variation of ‘kobs’ 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
fromtheregressionline, wasperformedwiththeMicrosoftoffice
Excel-2003 programme.
−
Ag(I). A stock standard solution of IO4 was prepared by dis-
solving a known weight of KIO4 (Riedel-de Haen) in hot water
and used after keeping for 24 h. Its concentration was ascertained
iodomettrically [12] at neutral pH maintained using phosphate
buffer. The temperature was maintained constant to within
0.10 ◦C.
2.2. Preparation of DPA
DPA was prepared by oxidizing Ag(I) in presence of KIO4
as described elsewhere [13]: the mixture of 28 g of KOH and
23 g of KIO3 in 100 cm3 of water along with 8.5 g AgNO3
was heated just to boiling and 20 g of K2S2O8 was added
in several lots with stirring then allowed to cool. It was fil-
tered through a medium porosity fritted glass filter and 40 g
of NaOH was added slowly to the filtrate, whereupon a volu-
minous orange precipitate agglomerates. The precipitate is fil-
tered as above and washed three to four times with cold water.
The pure crystals were dissolved in 50 cm3 water and warmed
to 80 ◦C with constant stirring thereby some solid was dis-
solved to give a red solution. The resulting solution was fil-
tered when it was hot and on cooling at room temperature,
3. Results
3.1. Stoichiometry and product analysis
Different sets of reaction mixtures containing varying ratios
of DPA to aspirin in presence constant amount of OH−, KNO3
and catalyst, were kept for 3 h in closed vessel under nitrogen
atmosphere. The remaining concentration of DPA was estimated
by spectrophotometrically at 415 nm. The results indicate that
1:2 stoichiometry for both the catalysts as given in Eq. (1).

C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
165
(1)
The main reaction products were eluted with ether, which
is identified as 1,4-benzoquinone2-carboxylate ion by spot test
[15]. The nature of 1,4-benzoquinone2-carboxylate ion was
confirmed by its IR spectrum which showed a C O stretch-
ing at 1631 cm−1 indicating the presence of C O group at
1,4-benzoquinone moiety, the band at 1580 cm−1 and also at
1362 cm−1 indicating the presence of COO− group. The prod-
uct was also characterized by NMR spectra (CDCl3, δ ppm)
chemical shift at 6.73 (s, 1H, C2-H), 6.80 (s, 1H, C5-H) 7.28 (s,
1H, C6-H). It was further confirmed by its melting point 206 ◦C
(lit. m.p. 205–207 ◦C). Further, 1,4-benzoquinone2-carboxylate
ion was subjected to GC–mass spectral analysis. GC–MS data
was obtained on a 17 A Shimadzu gas chromatograph with a
QP-5050A shimadzu mass spectrometer using the EI ionization
technique. The mass spectrum showed a molecular ion peak
at 152 amu confirming 1,4-benzoquinone2-carboxylate ion. All
other peaks observed in GC–MS can be interpreted in accor-
dance with the observed structure of the product (Fig. 1). Potas-
sium acetate was confirmed by spot test [16]. The formation of
free Ag+ in solution was detected by adding KCl solution to
the reaction mixture, which produced white turbidity due to the
formation of AgCl. It was observed that 1,4-benzoquinone2-
carboxylate ion does not undergo further oxidation under the
present kinetic conditions for both the catalysts.
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
aspirin, IO4−, OH− and catalysts (Ru(III) and Os(VIII)), in turn,
while keeping others constant. The DPA concentration was var-
ied in the range of 1.0 × 10−5 to 1.0 × 10−4 mol dm−3 and the
linearity of the plots of log(absorbance) versus time up to 85%
completion of the reaction indicates a reaction order of unity in
[DPA]. This is also confirmed by varying of [DPA], which did
not result in any change in the pseudo first order rate constants,
kC (Table 1 Ru(III)) (Table 2 Os(VIII)). The aspirin concentra-
tion was varied in the range 1.0 × 10−4 to 1.0 × 10−3 mol dm−3
at 25 ◦C while keeping other reactant concentrations and condi-
tionsconstantforboththecatalysts. ThekC valuesincreasedwith
the increase in concentration of aspirin indicating an apparent
less than unit order dependence on [ASP] (Tables 1 and 2). The
effect of alkali on the reaction has been studied in the range of
0.01–0.10 mol dm−3 at constant concentrations of aspirin, DPA,
catalyst and a constant ionic strength of 0.10 mol dm−3. The rate
constants increased with increasing [alkali] and the order was
found to be less than unity (Table 1 Ru(III)) (Table 2 Os (VIII))
in both the cases.
3.2. Reaction orders
3.3. Effect of [periodate]
As the diperiodatoargentate(III) oxidation of aspirin in alka-
line medium proceeds with a measurable rate in the absence
of Os(VIII) and Ru(III), the catalysed reaction is understood
to occur in parallel paths with contributions from both the
Periodate was varied from 0.5 × 10−5 to 5.0 × 10−5 at
constant [DPA], [ASP], [catalyst] and ionic strength. It was
observed that the rate constants decreased by increasing
Fig. 1. GC–mass spectrum of 1,4-benzoquinone2-carboxylate ion with its molecular ion peak at 152 amu.

166
C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
Table 1
Effect of [DPA], [ASP], [OH−] and [IO4−] on the ruthenium(III) catalysed oxidation of aspirin by DPA(III) in alkaline medium at 25 ◦C, I = 0.10 mol dm−3
[DPA] × 105
[ASP] ×104
[OH−]
(mol dm−3
[IO4] × 105
106 [Ru(III)]
103 kT
104 kU
(s−1
)
kC × 103 (s−1
)
(mol dm−3
)
(mol dm−3
)
)
(mol dm−3
)
(mol dm−3
)
(s−1
)
Found
Calculated
1.0
3.0
5.0
8.0
10
3.0
3.0
3.0
3.0
3.0
1.0
3.0
5.0
8.0
10
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.03
0.05
0.08
0.10
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
3.0
5.0
1.0
1.0
1.0
1.0
1.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
8.0
10
1.81
1.84
1.85
1.83
1.83
1.09
1.81
2.25
2.72
2.98
1.19
1.61
1.81
2.01
2.18
2.15
2.00
1.81
1.40
1.19
0.82
1.32
1.81
2.49
3.02
6.25
6.14
6.24
6.24
6.28
6.01
6.14
6.48
7.08
7.65
5.87
6.09
6.14
6.85
7.24
7.65
6.97
6.14
6.07
6.01
6.14
6.14
6.14
6.14
6.14
1.21
1.19
1.22
1.20
1.20
0.49
1.19
1.61
2.02
2.20
0.60
1.00
1.19
1.32
1.46
1.38
1.30
1.19
0.80
0.59
0.20
0.71
1.19
1.88
2.40
1.21
1.21
1.21
1.21
1.21
0.51
1.21
1.66
2.11
2.32
0.60
1.03
1.21
1.31
1.38
1.39
1.27
1.21
0.79
0.59
0.19
0.72
1.21
1.86
2.43
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
Table 2
Effect of [DPA], [ASP], [OH−] and [IO4−] on the osmium (VIII) catalysed oxidation of aspirin by DPA(III) in alkaline medium at 25 ◦C, I = 0.10 mol dm−3
[DPA] × 105
[ASP] × 104
[OH−]
(mol dm−3
[IO4] × 105
106 [Os(VIII)]
103 kT
104 kU
(s−1
)
kC × 103 (s−1
)
(mol dm−3
)
(mol dm−3
)
)
(mol dm−3
)
(mol dm−3
)
(s−1
)
Found
Calculated
1.0
3.0
5.0
8.0
10
3.0
3.0
3.0
3.0
3.0
1.0
3.0
5.0
8.0
10
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.03
0.05
0.08
0.10
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
3.0
5.0
1.0
1.0
1.0
1.0
1.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
8.0
10
2.11
2.11
2.21
2.18
2.10
1.24
2.11
2.65
3.21
3.61
1.40
1.89
2.11
2.25
2.34
2.42
2.25
2.11
1.66
1.42
0.94
1.54
2.11
3.24
3.96
6.25
6.14
6.24
6.24
6.28
6.01
6.14
6.48
7.08
7.65
5.87
6.09
6.14
6.85
7.24
7.65
6.97
6.14
6.07
6.01
6.14
6.14
6.14
6.14
6.14
1.49
1.50
1.58
1.50
1.48
0.64
1.50
2.00
2.51
2.85
0.83
1.30
1.50
1.60
1.67
1.65
1.55
1.50
1.05
0.82
0.32
0.93
1.50
2.63
3.30
1.49
1.49
1.49
1.49
1.49
0.64
1.49
2.03
2.54
2.78
0.83
1.31
1.49
1.61
1.65
1.66
1.55
1.49
1.05
0.82
0.31
0.92
1.49
2.61
3.34
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0

C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
167
[IO4−] (Table 1 Ru(III)) (Table 2 Os (VIII)) in both the
cases.
versus 1/[OH] and [catalyst]/kC versus [H3IO62−] plots at four
different temperatures. The values are given in Tables 3 and 4.
The activation parameters for the rate determining step were
obtained by the least square method of plot of log k versus 1/T
and are presented in Tables 3 and 4.
3.4. Effect of added products
Initially added products, Ag(I), potassium acetate and 1,4-
benzoquinone2-carboxylate ion did not have any significant
effect on the rate of reaction (for both catalysts).
3.7. Test for free radicals (polymerization study)
The intervention of free radicals was examined as follows,
the reaction mixture, to which a known quantity of acrylonitrile
scavenger had been added initially, was kept in an inert atmo-
sphere for 1 h. Upon diluting the reaction mixture with methanol,
no precipitate resulted, suggesting there is no participation of
free radicals in the reaction for both Ru(III)/Os(VIII) catalysis.
3.5. Effect of ionic strength (I) and dielectric constant of
the medium (D)
The addition of KNO3, to increase the ionic strength of the
reaction, increased the rate of reaction at constant [DPA], [ASP],
√
[OH−] [IO4−] and [catalyst]; the plot of log kobs versus I was
found to be linear with positive slope for Ru(III) and Os(VIII)
catalysis (Fig. 2).
3.8. Effect of [Ru(III)] and [Os(VIII)]
Dielectric constant of the medium, ‘D’ was varied by vary-
ing the t-butyl alcohol and water percentage. The decrease in
dielectric constant of the reaction medium, increases the rate
and the plot of log kC versus 1/D was linear with positive slope
for Ru(III) and Os(VIII) catalysis (Fig. 2).
The [Ru(III)] and [Os(VIII)] concentrations was varied from
1.0 × 10−6 to 1.0 × 10−5 mol dm−3 range, at constant concen-
tration of diperiodatoargentate(III), aspirin, alkali and ionic
strength. The order in [Ru(III)] and [Os(VIII)] was found to be
unity from the linearity of the plots of log kC versus log[Ru(III)]
and log kC versus log[Os(VIII)].
3.6. Effect of temperature (T)
The influence of temperature on the rate of reaction was stud-
ied at 25, 30, 35 and 40 ◦C. The rate constants, (k), of the slow
step of Scheme 1 were obtained from the Slopes and the inter-
cept of the plots of [catalyst]/kC versus 1/[ASP], [catalyst]/kC
3.9. Catalytic activity
It has been pointed out by Moelwyn-Hughes [17] that in
presence of the catalyst, the uncatalysed and catalysed reactions
Fig. 2. Effect of ionic strength and dielectric constant of the medium on Os(VIII) and Ru(III) catalysed oxidation of aspirin by diperiodatoargentate(III) at 25 ◦C.

168
C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
Table 3
Thermodynamic activation parameters for the Ru(III) catalysed oxidation of aspirin by DPA in alkaline medium with respect to the slow step of Scheme 1: (A) effect
of temperature, (B) activation parameters (Scheme 1), (C) effect of temperature to calculate K1, K2 and K3 for the oxidation of Aspirin by diperiodatoargentate(III)
in alkaline medium, (D) thermodynamic quantities using K1, K2 and K3
Temperature (K)
10−2 k (dm3 mol−1 s−1
)
(A)
298
303
308
313
7.62
7.92
8.21
8.46
Parameters
(B)
Values
Ea (kJ mol−1
)
5.2 0.2
3.0 0.2
−180 12
57 2.0
ꢀH# (kJ mol−1
)
ꢀS# (J K−1 mol−1
)
ꢀG# (kJ mol−1
log A
)
3.8 0.2
Temperature (K)
K1 (dm3 mol−1
)
K2 × 104 (mol dm−3
)
K3 × 10−3 (dm3 mol−1
)
(C)
298
303
308
313
0.45 0.01
0.65 0.03
0.85 0.04
1.14 0.05
7.29 0.40
6.83 0.30
5.53 0.16
4.33 0.20
2.50 0.08
3.68 0.10
5.44 0.20
8.40 0.30
Thermodynamic quantities
(D)
Values from K1
Values from K2
Values from K3
ꢀH (k J mol−1
ꢀS (J K−1 mol−1
ꢀG298 (k J mol−1
)
46.8 2.0
−27.5 0.8
−152 10
17.9 0.9
62.3 2.0
274 15
−19.5 0.4
)
)
151
8
1.9 0.1
[DPA] = 3.0 × 10−5; [ASP] = 3.0 × 10−4; [OH−] = 0.05; [Ru(III)] = 5.0 × 10−6 mol dm−3; [IO4] = 1.0 × 10−5 mol dm−3
.
proceed simultaneously, so that
4. Discussion
kT = kU − Kc[catalyst]x
(2)
In the later period of 20th century the kinetics of oxidation
of various organic and inorganic substrates have been studied
by Ag(III) species which may be due to its strong versatile
nature of two elec−trons oxidant. Among the various species of
Here kT is the observed pseudo first-order rate constant in the
presence Ru(III) catalyst; kU, the pseudo first-order rate constant
for the uncatalysed reaction; Kc, the catalytic constant and ‘x’
the order of the reaction with respect to [Ru(III)] or [Os(VIII)].
In the present investigations; x values for the standard run were
found to be unity for Ru(III) and Os(VIII). Then the value of Kc
is calculated using the equation,
Ag(III), Ag(OH)4 diperiodatoargentate(III) and ethylenebis
,
(biguanide), (EBS), silver(III) are of maximum attention to the
researchers due to their relative stability [18]. The stability of
Ag(OH)4− is very sensitive towards traces of dissolved oxygen
andotherimpuritiesinthereactionmediumwhereuponithadnot
drawn much attention. However, the other two forms of Ag(III)
[4,5,19] are considerably stable; the DPA is used in highly alka-
line medium and EBS is used in highly acidic medium.
The literature survey [13] reveals that the water soluble
diperiodatoargentate(III) (DPA) has a formula [Ag(IO6)2]7−
with dsp2 configuration of square planar structure, similar to
diperiodatocopper(III)complexwithtwobidentateligands, peri-
odatetoformaplanarmolecule. Whenthesamemoleculeisused
in alkaline medium, it is unlike to be existed as [Ag(IO6)2]7−
as periodate is known to be in various protonated forms [20]
depending on pH of the solution as given in following multiple
equilibria (4)–(6).
kT − kU
kC
[catalyst]x
Kc =
=
(where, kT − kU = kC)
[catalyst]x
(3)
The values of Kc were evaluated for both the catalysts at dif-
ferent temperatures and found to vary at different temperatures.
Further, plots of log Kc versus 1/T were linear and the values of
energy of activation and other activation parameters with refer-
ence to catalyst were computed. These results are summarized
in Table 5. The value of Kc for Ru(III) is 2.4 × 102 and Os(VIII)
is 3.0 × 102 at 298 K. The values of Kc indicate that Os(VIII) is
most efficient catalyst compared to Ru(III) in the oxidation of
aspirin by DPA in alkaline medium.
H5IO6 ꢀ H4IO6− + H+
(4)

C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
169
Table 4
Thermodynamic activation parameters for the osmium(VIII) catalysed oxidation of Aspirin by DPA in aqueous alkaline medium with respect to the slow step of
Scheme 1: (A) Effect of temperature, (B) activation parameters (Scheme 1), (C) effect of temperature to calculate K1, K2 and K3 for the Os(VIII) catalysed Oxidation
of aspirin by diperiodatoargentate(III) in alkaline medium, (D) thermodynamic quantities using K1, K2 and K3
Temperature (K)
10−2 k (dm3 mol−1 s−1
)
(A)
298
303
308
313
8.84
9.13
9.35
9.68
Parameters
(B)
Values
Ea (kJ mol−1
)
4.6 0.2
ꢀH# (kJ mol−1
)
2.1 0.05
−181 9.0
56 3.4
ꢀS# (J K−1 mol−1
)
ꢀG# (kJ mol−1
log A
)
3.7 0.1
Temperature (K)
K1 (dm3 mol−1
)
K2 × 104 (mol dm−3
)
K3 × 10−3 (dm3 mol−1
)
(C)
298
303
308
313
0.58 0.02
0.92 0.01
1.23 0.04
2.05 0.08
7.79 0.40
6.25 0.30
5.22 0.16
3.83 0.20
2.46 0.08
3.65 0.10
5.20 0.20
7.41 0.30
Thermodynamic quantities
(D)
Values from K1
Values from K2
Values from K3
ꢀH (kJ mol−1
)
62.8
206
2
8
−35.8 0.8
−180 10
17.7 0.9
56.8 2.0
255 15
−19.4 0.4
ꢀS (J K−1 mol−1
ꢀG298 (kJ mol−1
)
)
1.3 0.1
[DPA] = 3.0 × 10−5; [ASP] = 3.0 × 10−4; [OH−] = 0.05; [Os(VIII)] =5.0 × 10−6 mol dm−3; [IO4] = 1.0 × 10−5 mol dm−3
.
H4IO6− ꢀ H3IO62− + H+
H3IO62− ꢀ H2IO63− + H+
(5)
(6)
2−
is in the form H3IO6 or H2IO63−. Hence, DPA could be as
[Ag(H3IO6)2]− or [Ag(H2IO6)2]3− in alkaline medium. There-
fore, under the present condition, diperiodatoargentate(III), may
be depicted as [Ag(H3IO6)2]−. The similar speciation of perio-
date in alkali was proposed [21] for diperiodatonickelate(IV).
Periodic acid (H5IO6) exists in acid medium and also as
H4IO− at pH 7. Thus, under the present alkaline conditions,
6
the main species are expected to be H3IO62−and H2IO63−. At
higher concentrations, periodate also tends to dimerise [2.b].
On contrary, the authors [4] in their recent past studies have
proposed the DPA as [Ag(HL)2]x− in which ‘L’ is a periodate
with uncertain number of protons and ‘HL’ is a protonated peri-
odate of uncertain number of protons. This c−an be ruled out by
considering the alternative form [20] of IO4 at pH > 7 which
4.1. Ru(III) catalysis
It is interesting to identify the probable ruthenium(III) chlo-
ride species in alkaline media. Electronic spectral studies [22]
have confirmed hat ruthenium chloride exists in hydrated form
as [Ru(H2O)5OH]2+. In the present study, it is quite probable
Table 5
Values of catalytic constant (Kc) at different temperatures and activation parameters calculated using Kc values
Temperature (K)
10−2Kc Ru(III)
10−2Kc Os(VIII)
298
303
308
313
2.38
3.30
4.28
5.16
3.00
4.10
5.22
6.28
Ea (kJ mol−1
ꢀH# (kJ mol−1
)
40.1
37.6
−73
59
38.14
35.7
−77
58
)
ꢀS# (J K−1 mol−1
)
ꢀG# (kJ mol−1
log A
)
9.3
9.1
[DPA] = 3.0 × 10−5
;
[ASP] = 3.0 × 10−4
;
[OH−] = 0.05 mol dm−3
;
[IO4] = 1.0 × 10−5 mol dm−3
;
[Os(VIII)] = 5.0 × 10−6 mol dm−3
;
[Ru(III)] = 5.0 ×
10−6 mol dm−3
.

170
C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
Scheme 1. Detailed scheme for the Ru(III) catalysed oxidation of aspirin by alkaline diperiodatoargentate(III).
that the [Ru(III)(OH)x]3−x, the x-value would always be less
than six because there are no definite reports of any hexahy-
droxy ruthenium species. The remainder of the coordination
sphere would be filled by water molecules. Hence, under the
conditions employed, e.g., [OH−] ꢀ [Ru(III)], ruthenium(III)
is mostly present as the hydroxylated species, [Ru(H2O)5OH]2+
[23].
catalyst, ruthenium(III). This intermediate species 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 (C) is given below:
Since, the reaction was enhanced by [OH−], added periodate
retarded the rate and first order dependency in [DPA] and cat-
alyst (either Ru(III) or Os(VIII)) and fractional order in [ASP]
and [OH−], the following Scheme 1 has been proposed by con-
sidering its ASP as anionic form of ASP in alkaline medium
which also explains all other experimental observations.
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 [periodate] (Table 1). It may be expected that
lower Ag(III) periodate species such as monoperiodatoargen-
tate(III)(MPA) is more important active species in the reaction
than the DPA. The inverse fractional order in [H3IO6]2− might
also be due to this reason. In the pre rate determining stage, the
hydroxylated species of Ru(III) combines with a molecule of
anionic form of ASP to give an intermediate complex, which
further reacts with one mole of MPA in a rate determining
step to give intermediate species of aspirin with regeneration of
Spectroscopic evidence for the complex forma-
tion between catalyst and substrate was obtained from
UV–vis spectra of aspirin (3.0 × 10−4), Ru(III) (5.0 × 10−6
,
[OH−] = 0.05 mol dm−3) and mixture of both. A hypsochromic
shift of about 13 nm from 309 nm to 296 nm in the spectra
of DPA was observed and hyperchromicity was observed
at 296 nm. However, the Michelis-Menten plot proved the
complex formation between catalyst and reductant, which
explains less than unit order in [ASP]. The oxygen atom of
carboxyl group of the ASP is involved in the formation of
intermediate, as the molecular order in ASP and the absence
of intervention of free radical redundant such a possibility. The
rate law for the Scheme 1 could be derived as,
−d[DPA]
kK1K2K3[DPA][ASP][OH−][Ru(III)]
Rate =
=
(7)
2−
2−
[H3IO6 ] + K1[OH−][H3IO6 ] + K1K2[OH−] + K1K2K3[OH−][ASP]
dt

C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
171
Rate
k K1K2K3[ASP][OH−][Ru(III)]
= kC = kT − kU
=
(8)
2−
2−
[H3IO6 ] + K1[OH−][H3IO6 ] + K1K2[OH−] + K1K2K3[OH−][ASP]
[DPA]
Eq. (8) can be rearranged to Eq. (9), which is suitable for veri-
fication.
ꢀG, were calculated. These values are also given in Table 3.
A comparison of the latter values with those obtained for the
slow step of the reaction shows that these values mainly refer
to the rate limiting step, supporting the fact that the reaction
before the rate determining step is fairly slow and involves a
high activation energy [24] In the same manner, K2 and K3 values
were calculated at different temperatures and the corresponding
values of thermodynamic quantities are given in Table 3.
2−
2−
]
[Ru(III)]
kC
[H3IO6
]
[H3IO6
=
+
kK1K2K3[ASP][OH−] kK2K3[ASP]
1
1
+
+
(9)
kK3[ASP]
k
The plots of [Ru(III)]/kC versus [H3IO6]2−, 1/[OH−]
and 1/[ASP] were linear (Fig. 4); from the inter-
cepts and slopes of such plots, the reaction con-
stants K1, K2, K3 and k were calculated as (0.45
4.2. Os(VIII) catalysis: mechanism
0.02) dm3 mol−1
,
(7.16 0.10) × 10−4 mol dm−3
,
(2.61
Osmium(VIII) is known to form different complexes
at different OH− [25] concentrations, [OsO4(OH)2]2−
0.14) × 103 dm3 mol−1, (7.4 0.2) × 102 dm3 mol−1 s−1, res-
pectively. These constants were used to calculate the rate
constants and compared with the experimental kC values and
found to be in reasonable agreement with each other (Table 1),
which fortifies the Scheme 1.
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
oxidation increased with increase in [OH−], it is reasonable that
[OsO4(OH)2]2− was operative and its formation is important
in the reaction. To explain the observed orders the following
Scheme 2 is 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 sec-
ond step displacement of a ligand, periodate takes place to give
free periodate which is evidenced by decrease in the rate with
increase in [periodate] (Table 2). It may be expected that lower
Ag(III) periodate species such as MPA is more important in the
reaction than the DPA. The inverse fractional order in [H3IO6]2−
might also be due to this reason. In the pre rate determining
stage, the Os(VIII) species combines with a molecule of anionic
The thermodynamic quantities for the different equilib-
rium steps, in Scheme 1. can be evaluated as follows. The
aspirin and hydroxide ion concentrations (Table 1) were
varied at different temperatures. The plots of [Ru(III)/kC
versus 1/[ASP] (r ≥ 0.9992, S ≤ 0.00131), [Ru(III)]/kC ver-
sus [H3IO62−] (r ≥ 0.9991, S ≤ 0.00132), [Ru(III)/kC versus
1/[OH−] (r ≥ 0.9994, S ≤ 0.00087) should be linear as shown
in Fig. 3. From the slopes and intercepts, the values of K1 are
calculated at different temperatures. A van’t Hoff’s plot was
made for the variation of K1 with temperature [i.e., log K1 ver-
sus 1/T (r ≥ 0.9992, S ≤ 0.1104] and the values of the enthalpy of
reaction ꢀH, entropy of reaction ꢀS and free energy of reaction
Fig. 3. Verification of rate law (6) of Ru(III) catalysed oxidation of aspirin by diperiodatoargentate(III) at 25 ◦C.

172
C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
Scheme 2. Detailed scheme for the Os(VIII) catalysed oxidation of aspirin by alkaline diperiodatoargentate(III).
species of ASP to give an intermediate complex (C), which fur-
ther reacts with one mole of MPA in a rate determining step
to give intermediate species of aspirin and regeneration of cata-
lyst, osmium(VIII). This Intermediate species further reacts with
another molecule of MPA species in further fast step to give the
products as given in Scheme 2.
Spectroscopic evidence for the complex formation
between catalyst and substrate was obtained from UV–vis
spectra of aspirin (5.0 × 10−4), Os(VIII) (5.0 × 10−6
,
[OH−] = 0.05 mol dm−3) and mixture of both. A hypsochromic
shift of about 12 nm from 309 nm to 297 nm in the spectra
of DPA was observed and hypochromicity was observed
at 296 nm. However, the Michelis-Menten plot proved the
complex formation between catalyst and reductant, which
explains less than unit order in [ASP]. The oxygen atom
of carboxyl group of the ASP is involved in the formation
of intermediate, as the molecular order in ASP and the
absence of intervention of free radical redundant such a
possibility. The rate law for the Scheme 2 could be derived
as,
The probable structure of the complex (C) is given below:
−d[DPA]
kK1K2K3[DPA][ASP][OH−][Os(VIII)]
Rate =
=
(10)
(11)
2−
2−
[H3IO6 ] + K1[OH−][H3IO6 ] + K1K2[OH−] + K1K2K3[OH−][ASP]
dt
Rate
kK1K2K3[ASP][OH−][Os(VIII)]
= kC = kT − kU
=
2−
2−
[H3IO6 ] + K1[OH−][H3IO6 ] + K1K2[OH−] + K1K2K3[OH−][ASP]
[DPA]
Eq. (11) can be rearranged to Eq. (12), which is suitable for
verification.
2−
2−
]
[Os(VIII)]
kC
[H3IO6
]
[H3IO6
1
1
=
+
+
+
(12)
k K1K2K3[ASP][OH−] k K2K3[ASP] k K3[ASP]
k

C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
173
Fig. 4. Verification of rate law (9) of Os(VIII) catalysed oxidation of aspirin by diperiodatoargentate(III) at 25 ◦C.
The plots of [Os(VIII)]/kC versus [H3IO6]2−
,
The increase in the rate, with increasing ionic strength, is in
the favor of a reaction between charged species of reactants, as
presented in Schemes 1 and 2 of the proposed mechanism. The
effect of solvent on the reaction rate is described in detail in
the literature [26]. Increasing the content of t-butyl alcohol in
the reaction medium leads to an increase in the rate of reaction
(Fig. 2), which seems to be contrary to the expected interaction
between neutral and anionic species in media of lower relative
permittivity. However, an increase in the rate of reaction with
decreasing relative permittivity may be due to stabilization of the
complex (C) at low relative permittivity, which is less solvated
than DPA at higher relative permittivity because of its larger
size.
1/[OH−] and 1/[ASP] were linear (Fig. 4); from the
intercepts and slopes of such plots, the reaction con-
stants K1, K2, K3 and k were calculated as (0.58
0.02) dm3 mol−1
,
(7.79 0.40) × 10−4 mol dm−3
,
(2.46
0.08) × 103 dm3 mol−1
,
(8.84 0.40) × 102 dm3 mol−1 s−1
,
respectively. These constants were used to calculate the rate
constants and compared with the experimental kC values and
found to be in reasonable agreement with each other, which
fortifies the Scheme 2.
The thermodynamic quantities for the different equilib-
rium steps, in Scheme 2 can be evaluated as follows. The
aspirin and hydroxide ion concentrations (Table 2) were
varied at different temperatures. The plots of [Os(VIII)/kC
versus 1/[ASP] (r ≥ 0.9994, S ≤ 0.00136), [Os(VIII)]/kC ver-
sus [H3IO62−] (r ≥ 0.9990, S ≤ 0.00132), [Os(VIII)/kC versus
1/[OH−] (r ≥ 0.9991, S ≤ 0.00084) should be linear as shown in
Fig. 4. From the slopes and intercepts, the values of K1 are cal-
culated at different temperatures. A van’t Hoff’s plot was made
for the variation of K1 with temperature [i.e., log K1 versus 1/T
(r ≥ 0.9981, S ≤ 0.1101)] and the values of the enthalpy of reac-
tion ꢀH, entropy of reaction ꢀS and free energy of reaction
ꢀG, were calculated. These values are also given in Table 4. A
comparison of the latter values with those obtained for the slow
step of the reaction shows that these values mainly refer to the
rate limiting step, supporting the fact that the reaction before the
rate determining step is fairly slow and involves a high activation
energy [24] In the same manner, K2 and K3 values were calcu-
lated at different temperatures and the corresponding values of
thermodynamic quantities are given in Table 4.
The negative value of ꢀS# suggests that the intermediate
complex is more ordered than the reactants [27]. The observed
modest enthalpy of activation and a higher rate constant for the
slow step indicates that the oxidation presumably occurs via
an inner-sphere mechanism. This conclusion is supported by
earlier observations [28]. The values of ꢀH#, ꢀS#, ꢀG# and
rate constant (k) indicate the order of reactivity of the catalysts
as Ru(III) < Os(VIII) for the oxidation of aspirin by DPA. The
Os(VIII) catalysed reaction, however, is reasonably fast in view
of readiness of Os(VIII) to act across the COO group and the
Ru(III) catalysed reaction is slower probably owing to the less
ability of the Ru(III) to act across the COO group. The acti-
vation parameters evaluated for the catalysed and uncatalysed
reaction explain the catalytic effect on the reaction. The catalyst
Ru(III) or Os(VIII) form the complex (C) with substrate which
enhances the reducing property of the substrate than that with-
out catalyst (Ru(III) or Os(VIII)). Further, the catalyst Ru(III)

174
C.V. Hiremath et al. / Journal of Molecular Catalysis A: Chemical 248 (2006) 163–174
or Os(VIII) modifies the reaction path by lowering the energy
of activation.
[9] T.S. Kiran, S.T. Nandibewoor, J. Chem. Res. (2005) in press.
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35A (1996) 865.
5. Conclusion
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The comparative study of Ru(III) and Os(VIII) catalysed oxi-
dation of aspirin by Diperiodatoargentate(III) was studied. Oxi-
dation products were identified. Among the various species of
Ag(III) in alkaline medium, in earlier reports the diperiodatoar-
gentate(III)wastheactivespecies, whereasdeprotonatedformof
monoperiodatoargentate(III) itself is considered to be the active
species for the title reaction. Active species of Ru(III) is found
to be [Ru(H2O)5OH]2+ and that of Os(VIII) is [OsO4(OH)2]2−
.
Activation parameters were evaluated for both catalysed and
uncatalysed reactions. Catalytic constants and the activation
parameters with reference to catalyst were also computed. Cat-
alytic efficiency is Ru(III) < Os(VIII).
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