Spectral and mechanistic investigation of the osmium(VIII) catalyzed oxidation of diclofenac sodium by the copper(III) periodate complex in aqueous alkaline medium

Article abstract of DOI:10.1007/s10953-010-9587-9

The kinetics of the osmium(VIII) (Os(VIII)) catalyzed oxidation of diclofenac sodium (DFS) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium has been studied spectrophotometrically at a constant ionic strength of 1.0 mol·dm^-3. The reaction showed first order kinetics in [Os(VIII)] and [DPC] and less than unit order with respect to [DFS] and [alkali]. The rate decreased with increase in [periodate]. The reaction between DFS and DPC in alkaline medium exhibits 1:2 [DFS]:[DPC] stoichiometry. However, the order in [DFS] and [OH^-] changes from first order to zero order as their concentration increases. Changes in the ionic strength and dielectric constant did not affect the rate of reaction. The oxidation products were identified by LC-ESI-MS, NMR, and IR spectroscopic studies. A possible mechanism is proposed. The reaction constants involved in the different steps of the mechanism were calculated. The catalytic constant (K _C) was also calculated for Os(VIII) catalysis at the studied temperatures. From plots of log∈₁₀ K _C versus 1/T, values of activation parameters have been evaluated with respect to the catalytic reaction. The activation parameters with respect to the slow step of the mechanism were computed and discussed, and thermodynamic quantities were also determined. The active osmium(VIII) and copper(III) periodate species have been identified.

Full text of DOI:10.1007/s10953-010-9587-9

J Solution Chem (2010) 39: 1291–1310
DOI 10.1007/s10953-010-9587-9
Spectral and Mechanistic Investigation
of the Osmium(VIII) Catalyzed Oxidation
of Diclofenac Sodium by the Copper(III)
Periodate Complex in Aqueous Alkaline Medium
Shekappa D. Lamani · Praveen N. Naik ·
Sharanappa T. Nandibewoor
Received: 29 November 2009 / Accepted: 8 April 2010 / Published online: 23 September 2010
©
Springer Science+Business Media, LLC 2010
Abstract The kinetics of the osmium(VIII) (Os(VIII)) catalyzed oxidation of diclofenac
sodium (DFS) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium has been
−
3
studied spectrophotometrically at a constant ionic strength of 1.0 mol·dm . The reaction
showed first order kinetics in [Os(VIII)] and [DPC] and less than unit order with respect to
[DFS] and [alkali]. The rate decreased with increase in [periodate]. The reaction between
DFS and DPC in alkaline medium exhibits 1:2 [DFS]:[DPC] stoichiometry. However, the
−
order in [DFS] and [OH ] changes from first order to zero order as their concentration
increases. Changes in the ionic strength and dielectric constant did not affect the rate of re-
action. The oxidation products were identified by LC-ESI-MS, NMR, and IR spectroscopic
studies. A possible mechanism is proposed. The reaction constants involved in the different
steps of the mechanism were calculated. The catalytic constant (KC) was also calculated
for Os(VIII) catalysis at the studied temperatures. From plots of log10 KC versus 1/T , val-
ues of activation parameters have been evaluated with respect to the catalytic reaction. The
activation parameters with respect to the slow step of the mechanism were computed and
discussed, and thermodynamic quantities were also determined. The active osmium(VIII)
and copper(III) periodate species have been identified.
Keywords Kinetics · Oxidation · Copper(III) · Diclofenac sodium · Os(VIII) catalyst
1
Introduction
Periodate and tellurate complexes of copper in its trivalent state have been extensively used
in the analysis of some organic compounds [1–3]. Copper(III) was shown to be an intermedi-
ate in the copper(II) catalyzed oxidation of amino acids by peroxydisulphate [4]. Oxidation
reactions usually involve the copper(II)/copper(I) couple and such aspects have been de-
tailed in different reviews [5, 6]. The use of diperiodatocuprate(III) (DPC) as an oxidant in
alkaline medium is new and restricted to a few cases due to the fact of its limited solubility
S.D. Lamani · P.N. Naik · S.T. Nandibewoor (ꢀ)
P.G. Department of Studies in Chemistry, Karnatak University, Dharwad 580003, India
e-mail: stnandibewoor@yahoo.com

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J Solution Chem (2010) 39: 1291–1310
and stability in aqueous media. DPC is a versatile one-electron oxidant for various organic
compounds in alkaline medium and its use as an analytical reagent is now well recognized.
Copper complexes have occupied a major place in oxidation chemistry due to their abun-
dance and relevance in biological chemistry [7–9]. Copper(III) is involved in many biolog-
ical electron transfer reactions [10]. When the copper(III) periodate complex is the oxidant
and multiple equilibria between different copper(III) species are involved, then it will be
interesting to determine which of the possible species is the active oxidant.
Diclofenac, [o-(2,6-dichlorophenyl)amino]phenyl] acetic acid (DFS), belongs to a class
of drugs called nonsteroidal anti-inflammatory drugs (NSAIDs). In pharmacological studies,
diclofenac has shown anti-inflammatory, analgesic, and antipyretic activity [11]. As with
other NSAIDs, its mode of action is not known; however its ability to inhibit prostaglandin
synthesis may be involved in its anti-inflammatory activity, as well as contributing to its
efficiency in relieving pain related to inflammation and primary dysmenorrhea. Diclofenac
is used in treating osteoarthritis, rheumatoid arthritis, and ankylosing spondylitis [12, 13].
Due to the low solubility of the acidic form, it is commercially available as its sodium salt.
Transition metals are known to catalyze many oxidation/reduction reactions since some
of them involve multiple oxidation states. In recent years, transition metal ions such as those
of osmium, ruthenium and iridium, either alone or as binary mixtures, have attracted con-
siderable interest as catalysts in various redox processes [14]. The role of osmium(VIII) as
a catalyst in some redox reactions has been reviewed [15, 16]. Although the mechanism of
catalysis depends on the nature of the substrate, the oxidant and the experimental conditions,
it has been shown [17] that metal ions can act as catalysts by several different paths such as
the formation of complexes with reactants or oxidation of the substrate itself, or through the
formation of free radicals. In an earlier report [18] it was observed that Os(VIII) formed a
complex with a substrate, which was then oxidized by the oxidant to form an Os(VII) inter-
mediate, followed by the rapid reaction of Os(VII) with one more mole of oxidant to regen-
erate the Os(VIII). In another report [19] it was observed that an oxidant–substrate complex
reacted with Os(VIII) to form Os(VI) which then reacted with the oxidant in a fast step to
regenerate Os(VIII). In another report [20] it was observed that Os(VIII) formed a complex
with the substrate that was oxidized by the oxidant with the regeneration of Os(VIII). Hence,
understanding the role of Os(VIII) in catalyzed reactions is important.
Osmium(VIII) catalysis in redox reactions can involve several complexes, different ox-
idation states of osmium, etc. The uncatalyzed oxidation of DFS by DPC has been stud-
ied [21]. We have observed that osmium(VIII) catalyzes the oxidation of DFS by DPC in
alkaline medium. In order to understand the active species of oxidant and catalyst, and to
propose an appropriate mechanism that may contribute to understanding its mode of action
in biological systems, the title reaction is investigated here in detail. An understanding of
the mechanism allows the chemistry to be interpreted and hence understood and predicted.
2
Experimental
2.1 Materials and Reagents
All chemicals used were of reagent grade and millipore water was used throughout this
work. The copper(III) periodate complex was prepared [22, 23] and standardized by a stan-
dard procedure [24]. The UV-vis spectrum had a maximum absorption at 415 nm which
verified the presence of copper(III). The diclofenac sodium (IP) concentration was checked
potentiometrically. The required alkalinity and ionic strength were maintained with KOH

J Solution Chem (2010) 39: 1291–1310
1293
(
BDH) and KNO3 (Analar), respectively. The periodate solution was prepared and stan-
dardized iodometrically [25] at neutral pH using a phosphate buffer. An aqueous solution of
copper sulfate (BDH) was used to study the effect of the product Cu(II). The temperature
was maintained to within ± 0.10 °C.
2.2 Instruments Used
For kinetic measurements, a CARY 50 Bio UV-vis spectrophotometer (Varian, Victoria-
170, Australia) was used. For product analysis, a LC-ESI-MS (liquid chromatography
3
electrospray ionization tandem mass spectrometry; Hewlett-Packard GmbH, Waldbronn,
1
Germany), a Nicollet 5700 FT-IR spectrometer (Thermo, USA), and a 300 MHz H NMR
spectrometer (Brucker, Switzerland) were used.
2.3 Kinetic Studies
The oxidation of diclofenac sodium by diperiodatocuprate(III) was followed under pseudo-
first order rate conditions where [DFS] ꢀ [DPC] at 25 ± 0.1 °C, unless otherwise stated.
The reaction was initiated by mixing the DPC with a DFS solution that also contained the
required concentrations of KNO3, KOH, KIO4, and Os(VIII) catalyst. The reaction progress
was followed spectrophotometrically at 415 nm by monitoring the decrease in absorbance
3
−1
−1
due to DPC using the molar absorbency index ‘ε’ = 6231 ± 100 dm ·mol ·cm . It was
verified that there is negligible interference at this wavelength from other species present in
the reaction mixture.
The pseudo-first order rate constants, ‘kC’, were determined from plots of log10 (ab-
sorbance) versus time. The plots were linear up to 80% completion of the reaction over
−
−
the [OH ] range used. During the kinetics measurements a constant concentration of
1
5
−3
.0 × 10 mol·dm of KIO4 was used unless otherwise stated. Because periodate is
present in excess in the DPC solutions, the possibility of direct oxidation of DFS by pe-
riodate at 25 °C was tested in this alkaline medium. The progress of the reaction was fol-
lowed iodometrically. However, it was found that there was no significant reaction under
the experimental conditions employed compared to the oxidation of DFS by DPC. The total
−
concentrations of periodate and OH were calculated by considering the amount present in
the DPC solution along with those additionally added. Kinetics runs were also carried out
under an N2 atmosphere in order to detect a possible effect of dissolved oxygen on the rate
of reaction. No significant differences were obtained between those obtained under a N2
atmosphere and in the presence of air. In view of the ubiquitous contamination by carbonate
in basic media, the effect of carbonate was also studied. Added carbonate had no effect on
the reaction rates. The spectral changes during the reaction are shown in Fig. 1. It is evident
from the figure that the concentration of DPC decreases as seen by the changes at 415 nm.
2.4 Stoichiometry and Product Analysis
Different sets of reaction mixtures containing varying ratios of DPC to DFS in preset con-
−
stant amounts of OH , KNO3 and Os(VIII) were kept for 4 h in a closed vessel under the
inert atmosphere. The DPC concentration remaining after that interval was estimated by
spectrophotometry at 415 nm. The results indicated a 1:2 reaction stoichiometry as shown

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J Solution Chem (2010) 39: 1291–1310
Fig. 1 Spectroscopic changes
occurring in the Os(VIII)
catalyzed oxidation of DFS by
DPC at 25 °C:
−
5
−3
,
[
[
[
[
DPC] = 5.0 × 10 mol·dm
−
4
−3
,
DFS] = 5.0 × 10 mol·dm
−
−3
OH ] = 0.50 mol·dm
;
−
7
−3
,
Os(VIII)] = 8.0×10 mol·dm
−
3
and I = 1.0 mol·dm with
scanning time intervals of
0.5 min
in Eq. 1.
(1)
The product, [2-(2,6-dicloro-phynylamino)-phenyl]-methanol, was identified by LC-ESI-
MS. This LC-ESI-MS analysis was carried out using a reverse-phase high performance liq-
uid chromatography (HPLC) system with a Phenomenes C-18 column, UV/visible detector,
and mass analyzer in series. A 12 μL sample of acidified reaction mixture was injected. The
mobile phase consisted of 10 mmol·dm− ammonium acetate at pH = 3.0 (eluent A) and
3
−
1
acetic acid (eluent B) at a flow rates of 1 mL·min . Gradient elution was run to separate
the substrate and reaction products. The LC-ESI-MS analysis indicated the presence of two
main products with molecular ions at m/z = 273 (yield ca. 90%) and 598 (yield ca. 5%),
respectively, see Fig. 2. The molecular ion of diclofenac sodium occurs at m/z = 299. The
m/z = 598 thus corresponds to a dimer product of DFS. The product was further confirmed
−
1
by its characteristic IR spectrum (Fig. 3). The absence of a sharp band (peak) at 1695 cm
(
due to the acidic carbonyl in DFS) confirms the nature of the product. Further, the secondary
−
1
1
amine (–NH) group observed around 3387 cm in DFS is retained in the product. The H-
NMR spectrum (in DMSO) shows that of the acidic –OH band disappeared and an alcoholic
hydroxyl proton triplet band appears at 6.859 ppm. Further, the secondary amine (–NH)
proton singlet band at around 3.69 ppm and other aromatic proton bands in DFS remain in
the product, which disappear on D2O exchange, confirms the formation of [2-(2,6-dicloro-
phynylamino)-phenyl]-methanol (Fig. 4). All of these observations proved the formation of
[2-(2,6-dicloro-phynylamino)-phenyl]-methanol as the major product. The dimer structure
of DFS is

J Solution Chem (2010) 39: 1291–1310
1295
Fig. 2 Chromatogram and LC-ESI-MS spectra of the product of the Os(VIII) catalyzed oxidation of di-
clofenac sodium by DPC
Regression analysis of the experimental data to obtain the regression coefficient r and
standard deviation S, of points from the regression line, was performed using Microsoft
Excel-2003.

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J Solution Chem (2010) 39: 1291–1310
Fig. 3 FT-IR spectra of the product of Os(VIII) catalyzed oxidation of diclofenac sodium by DPC
3
Results and Discussion
3.1 Reaction Orders
Inasmuch as the diperiodatocuprate(III) (DPC) oxidation of diclofenac sodium (DFS) in
alkaline medium proceeds at a measurable rate in the absence of Os(VIII), the catalyzed
reaction is thus understood to occur by two parallel paths with contributions from both cat-
alyzed and uncatalyzed paths. Therefore, the total rate constant kT is equal to the sum of the
rate constants of the catalyzed kC and uncatalyzed kU reactions, so kC = kT − kU. Hence,
the reaction orders were determined from the slopes of log kC versus log (concentra-
1
0
10
tion) plots by varying the concentrations of diclofenac sodium, osmium(VIII), alkali and
periodate in turn, while keeping all other concentrations and conditions constant.
3.2 Effect of the Concentration of Diperiodatocuprate(III)
−
5
The diperiodatocuprate(III) concentration was varied in the range 3.0 × 10 to 3.0 ×
−
−
4
4
−3
1
1
0
0
mol·dm while the other concentrations were fixed: diclofenac sodium at 5.0 ×
−3
−5
−
−3
−7
−3
mol·dm , periodate at 1.0×10 mol·dm , Os(VIII) at 8.0×10 mol·dm , and at
3
constant ionic strength of 1.0 mol·dm . All kinetic runs exhibited identical characteristics.
The linearity of log10 (abs) versus time plots (Fig. 5) up to 80% completion of reaction for
different concentrations of diperiodatocuprate(III), and the fairly constant kC values, indi-
cate that the reaction order with respect to the diperiodatocuprate(III) concentration is unity
(Table 1).

J Solution Chem (2010) 39: 1291–1310
1297
Fig. 4 NMR spectra of the product of Os(VIII) catalyzed oxidation of diclofenac sodium by DPC
Fig. 5 First order plots for the
Os(VIII) catalyzed oxidation of
DFS by DPC in aqueous alkaline
5
medium at 25 °C; [DPC] × 10 :
(
1) 3.0, (2) 5.0, (3) 8.0 (4) 10.0
−
3
and (5) 30.0 mol·dm

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J Solution Chem (2010) 39: 1291–1310
1299

1300
J Solution Chem (2010) 39: 1291–1310
3.3 Effect of the Concentration of Diclofenac Sodium
The effect of diclofenac sodium on the rate of reaction was studied at constant concentrations
−
−3
3
of alkali, DPC, periodate and Os(VIII) at a constant total ionic strength of 1.0 mol·dm . The
−
4
−3
concentration of diclofenac sodium was varied from 3.0×10 to 3.0×10 mol·dm . The
kC values increased with increasing concentration of diclofenac sodium. The reaction order
with respect to diclofenac sodium was found to be less than unity (r ≥ 0.9466, S ≤ 0.008)
(
Table 1). However, the order in [DFS] changes from first to zero order as [DFS] is varied.
3.4 Effect of the Concentration of Alkali
−
3
The effect of alkali on the reaction has been studied in the range of 0.1 to 1.0 mol·dm
at constant concentrations of diclofenac sodium, diperiodatocuprate(III) and periodate at a
−
3
constant total ionic strength of 1.0 mol·dm in the presence of the Os(VIII) catalyst. The
rate constants increased with increasing [alkali] and the reaction order was found to be less
than unity (r ≥ 0.9767, S ≤ 0.006) (Table 1). As in the case of DFS, the reaction order in
−
[
alkali] changes from first to zero order as [OH ] is varied.
3.5 Effect of the Concentration of Periodate
The effect of increasing concentration of periodate was studied by varying the periodate
−
6
−5
−3
concentration from 5.0×10 to 5.0×10 mol·dm while keeping all other reactant con-
centrations and the ionic strength fixed. It was found that the added periodate had a retarding
effect on the rate of reaction. The reaction order with respect to periodate concentration was
negative and less than unity (r ≥ 0.9723, S ≤ 0.005) as shown in Table 1.
3.6 Effect of Initially Added Products
Externally added products [2-(2,6-dichlorophenylamino)-phenyl]-methanol and copper(II)
sulfate did not have any significant effect on the rate of the reaction.
3.7 Effect of Ionic Strength (I) and Dielectric Constant (D) of the Medium
The addition of KNO3, at constant concentrations of diperiodatocuprate(III), diclofenac
sodium, alkali, periodate and Os(VIII), was used to vary the ionic strength, and t-butyl al-
cohol was used to vary the dielectric constant of the reaction medium. No significant effect
was observed on the rate of reaction.
Under the experimental conditions, the rate law constant is given by kC = 0.020.
3.8 Effect of the Concentration of Os(VIII)
−
7
−6
−3
The [Os(VIII)] concentrations was varied from 5.0 × 10 to 5.0 × 10 mol·dm , at con-
stant concentrations of diperiodatocuprate(III), diclofenac sodium, alkali and periodate at a
constant ionic strength of 1.0 mol·dm . The reaction order in [Os(VIII)] was found to be
unity based on the linearity of the plot of kC versus [Os(VIII)].
−
3

J Solution Chem (2010) 39: 1291–1310
1301
3.9 Polymerization Study
The possible intervention of free radicals in the reaction was examined as follows. The
reaction mixture, to which a known quantity of acrylonitrile monomer was initially added,
was kept for 2 h in an inert atmosphere. On diluting the reaction mixture with methanol, a
white precipitate was formed indicating the formation of a polymer by intervention of free
radicals in the reaction. Blank experiments with either DPC or DFS alone with acrylonitrile
did not produce any polymerization under the same conditions as those used for the reaction
mixture. Initially added acrylonitrile decreases the rate of reaction indicating free radical
intervention, which was also found to be the case in earlier work [26, 27].
4
Effect of Temperature
The kinetics were studied at four different temperatures with varying concentrations of di-
clofenac sodium, alkali, periodate and Os(VIII), keeping other conditions constant. The
rate constants were found to increase with increasing temperature. The rate constants (k)
of the slow step of Scheme 1 were obtained from the slopes and intercepts of plots of
[
Os(VIII)]/kC versus 1/[DFS] at the four different temperatures and their values were used
to calculate activation parameters. The energy of activation corresponding to these constants
was evaluated from the Arrhenius plot of log k versus 1/T (r ≥ 0.8011), and other activa-
1
0
tion parameters were also obtained as reported in Tables 2a, 2b, 2c.
4.1 Catalytic Activity
It has been pointed out by Moelwyn-Hughes [28] that in presence of the Os(VIII) catalyst,
the uncatalyzed and catalyzed reactions proceeds simultaneously so that
x
kT = kU + KC[Os(VIII)]
(2)
Here, kT is the observed pseudo-first order rate constant in the presence Os(VIII), kU the
pseudo-first order rate constant for the uncatalyzed reaction, KC is the catalytic constant,
and ‘x’ is the order of the reaction with respect to [Os(VIII)]. In the present investigations,
the x value for the standard run was found to be unity. Then, the value of KC was calculated
using Eq. 4 as follows
kT − kU
kC
KC =
=
(4)
x
[Os(VIII)]
[Os(VIII)]
where kT − kU = kC. The values of KC were evaluated at the four different temperatures and
found to vary with temperature. Furthermore, plots of log KC versus 1/T were linear and
1
0
the values of the energy of activation and other activation parameters, with reference to the
catalyst, were computed. These results are summarized in Table 3.
The water soluble copper(III) periodate complex is reported [29] to be [Cu(HIO6)2
(
OH)2]7 . However, in aqueous alkaline medium and at the high pHs employed in the study,
−
4
−
periodate is unlikely to exist as HIO (as present in the complex) as is evident from its
6
involvement in multiple dissociation equilibria [30], Eqs. 4–6, that depend on the pH of the
solution:
−
+
H5IO6 ꢀ H4IO + H
(4)
(5)
6
H4IO ꢀ H3IO + H+
−
2−
6
6

1302
J Solution Chem (2010) 39: 1291–1310
Scheme 1 Detailed scheme for the Os(VIII) catalyzed oxidation of DFS by diperiodatecuprate(III) in aque-
ous alkaline medium
H3IO ꢀ H2IO6 + H+
2
−
3−
(6)
6
−
Periodic acid exists in acidic media as both H5IO6 and H4IO6 around pH = 7. However,
2
6
−
under the employed in alkaline conditions, the main species are expected to be H3IO
and H2IO3 . At higher concentrations, periodate also tends to dimerize [31]. Hence, at the
−
6
pHs employed in this study, the soluble copper(III) periodate complex exists as diperi-
3
−
odatocuprate(III), [Cu(H3IO6)2(OH)2] , a conclusion that is also supported by earlier
work [32, 33].
−
Osmium(VIII) is known to form different complexes at different OH [34] concentra-
tions, [OsO4(OH)2]2 and [OsO5(OH)] . At higher OH concentrations, [OsO5(OH)]
becomes significant. At lower concentrations of OH , as employed in the present study, and
because the rate of oxidation increased with increasing [OH ], it is reasonable to assume
that [OsO4(OH)2]2 was operative and that its formation is important in the reaction [34].
−
3−
−
3−
−
−
−

J Solution Chem (2010) 39: 1291–1310
1303
Table 2a Activation parameters
Effect of temperature
Temperature (K)
for the Os(VIII) catalyzed
oxidation of DFS by DPC in
aqueous alkaline medium with
respect to the slow step of
Scheme 1
k × 10 5 dm ·mol ·s
−
3
−1 −1
288
298
308
318
1.3
2.3
3.5
4.3
Activation parameters (Scheme 1)
Parameters
Values
−
1
Ea (kJ·mol
)
30.7 ± 1.0
28.2 ± 1.4
−48.9 ± 6.0
42.3 ± 4.0
10.7 ± 0.2
#
−1
ꢀ
ꢀ
ꢀ
H
(kJ·mol
)
#
−1
−1
)
S
(J·K ·mol
#
−1
G (kJ·mol
)
log
A
0
1
Table 2b Effect of temperature
on the calculated K , K and K
3
values for the Os(VIII) catalyzed
oxidation of DFS by DPC in
alkaline medium
5
−3
Temperature (K)
K
K × 10
K × 10
1
2
3
1
2
3
−1
−3
3
−1
(dm ·mol
)
(mol·dm
)
(dm ·mol
)
288
298
308
318
5.8
4.8
3.6
2.4
0.28
0.4
3.0
2.0
1.4
0.9
0.66
0.9
Table 2c Thermodynamic
Thermodynamic
quantities
Values from
Values from
Values from
quantities derived from K , K
1
2
and K
K
K
K
3
3
1
2
−
1
ꢀ
ꢀ
ꢀ
H (kJ·mol
)
−1
−22 ± 1
31.2 ± 2.6
4.7 ± 8
−30.5 ± 2.2
−39 ± 3.0
−
1
S (J·K ·mol
)
−63 ± 5
−
1
G (kJ·mol
)
−3.2 ± 0.1
30.4 ± 1.2
−18.8 ± 1.0
In an earlier report [18], it was concluded in the Os(VIII) catalyzed reaction, in view of
its less than unit order in substrate, and unit order in Os(VIII) and oxidant, that Os(VIII)
is regenerated through formation of a Os(VII). In another study [19], it was concluded that
Os(VIII) was regenerated by Os(VI) intervention in view of unit orders in osmium, substrate
and oxidant. In another report [20] it was observed that Os(VIII) formed a complex with the
substrate, which was oxidized by the oxidant with regeneration of the catalyst. Hence, the
study of behavior of Os(VIII) in catalyzed reaction becomes significant. To explain the ob-
served orders, the following Scheme 1 is proposed for our osmium(VIII) catalyzed reaction.
The reaction between the diperiodatocuprate(III) complex and diclofenac sodium in alka-
line medium has a stoichiometry of 1:2 (DFS:DPC), with a first order reaction dependences
−
on [DPC] and Os(VIII), less then unit apparent orders in [DFS] and [OH ], and a nega-
tive fractional order in [periodate]. No effect of added products was observed. Based on
these experimental results, a mechanism is proposed for which all the observed reaction or-
−
−
ders in [oxidant], [reductant], [OH ], [IO ], and Os(VIII) may be well accommodated. Lis-
4

1304
J Solution Chem (2010) 39: 1291–1310
Table 3 Values of the catalytic
× 10 2
−
Temperature (K)
K
constant (K ) at different
C
C
temperatures and activation
parameters calculated using
288
298
1.2
1.9
3.8
4.7
the K values
C
308
18
3
−
1
Ea (kJ·mol
)
34.7
32.2
−169
82
#
−1
ꢀ
ꢀ
ꢀ
H
(kJ·mol
)
#
−1
−1
)
S
(J·K ·mol
#
−1
G (kJ·mol
)
log
A
0
4.4
1
ter [35] proposed that three forms of copper(III) periodate occur in alkaline medium, diperi-
odatocuprate(III) (DPC), monoperiodatocuprate(III) (MPC), and tetrahydroxycuprate(III).
Hence, in the present study, DPC and MPC are considered to be the active forms of the
−
copper(III) periodate complex. In view of the inverse fractional order in [IO ], it may be
4
expected that a lower periodate complex such as MPC is more important than DPC in the
reaction. The increase in reaction rate with increasing alkali concentration, and decrease
in the rate with increasing periodate concentration, suggest that equilibria of different cop-
per(III) periodate complexes may be involved as shown in Eqs. 7 and 8:
K1
[
Cu(H2IO6)(H3IO6)] − + OH ꢁ [Cu(H2IO6)2] + H2O
2
−
3−
(7)
K2
[
Cu(H2IO6)2] − + 2H2O ꢁ [Cu(H2IO6)(H2O)2] + [H2IO6]
3
3−
(8)
Anionic species of diclofenac sodium react with osmium(VIII) species to form a complex
C), which then reacts with one mole of MPC in a slow step to give the free radical of
(
diclofenac and a Cu(II) species with regeneration of the catalyst osmium(VIII). Furthermore,
this free radical of diclofenac reacts with one more mole of MPC in a fast step to yield the
products [2-(2,6-dichlorophenylamino)-phenyl]-methanol, Cu(II) and periodate as given in
Scheme 1.
The probable structure of complex (C) is
Spectroscopic evidence for complex formation between the catalyst and substrate was
−
4
−3
obtained my measuring the UV-vis spectra of DFS (5.0 × 10 mol·dm ), Os(VIII) (8.0 ×
−
7
−3
−
−3
1
0
mol·dm ), [OH ] = 0.5 (mol·dm ), mixtures. A bathochromic shift of about 4 nm
was observed from 278 to 282 nm in the spectra of DFS. Attempts to separate and isolate the
complex were not successful. A Lineweaver-Burk plot also proved that complex formation

J Solution Chem (2010) 39: 1291–1310
1305
occurred between the catalyst and DFS, which explains the less than unit order concentration
dependence on DFS. The rate law for Scheme 1 can then be derived as
d[DPC]
rate = −
dt
−
kK1K2K3[DPC][DFS][OH ][Os(VIII)]
=
2
6
−
−
2−
6
−
−
[
+
H3IO ] + K1[OH ][H3IO ] + K1K2[OH ] + K1K2K3[OH ][DFS]
K3[H3IO ][DFS] + K1K3[DFS][OH ][H3IO ]
2
6
−
−
2−
6
(9)
rate
=
=
kC = kT − KU
[
DPC]
−
kK1K2K3[DFS][OH ][Os(VIII)]
2
6
−
−
2−
6
−
−
[
H3IO ] + K1[OH ][H3IO ] + K1K2[OH ] + K1K2K3[OH ][DFS]
2
−
−
2−
6
+
K3[H3IO ][DFS] + K1K3[DFS][OH ][H3IO ]
(10)
6
which explains all the observed kinetic orders on the concentrations of the different species.
The rate law Eq. 10 can be rearranged into the following form that is suitable for verifi-
cation.
2
−
−
2−
2−
2−
[
Os(VIII)]
[H IO ]
[H IO ]
[H IO ]
[H IO ]
3
6
3
6
3
6
3
6
=
+
+
+
−
kC
kK K K [OH ][DFS] kK2K3[DFS] kK K [OH ]
kK2
1
2
3
1
2
1
1
+
+ k
(11)
kK3[DFS]
−
According to Eq. 11, plots of [Os(VIII)]/kC versus 1/[OH ] (r ≥ 0.986), of [Os(VIII)]/
2
−
kC versus 1/[DFS] (r ≥ 0.9917), and of [Os(VIII)]/kC versus [H3IO ] (r ≥ 0.9923) were
6
linear (Figs. 6a, b and c). From the slopes and intercepts of such plots, the reaction constants
3
−1
−5
−3
K1, K2, K3 and k were calculated as (4.8± 0.1) dm ·mol , (0.4± 0.02)×10 mol·dm
,
3
3
−1
5
3
−1 −1
(
2.0± 0.1)×10 dm ·mol and (13.2± 0.5)×10 dm ·mol ·s respectively. The values
of K1 are in good agreement with a literature value [36]. Using these constants, the rate
constants under different conditions were regenerated (Table 1).
The thermodynamic quantities for the first, second and third equilibrium steps of
2
6
−
−
Scheme 1 can be evaluated as follows. The [H3IO ], [DFS] and [OH ] (as in Table 1)
were varied at the four different temperatures. The plots of [Os(VIII)]/kC versus 1/[DFS],
−
2−
[
Os(VIII)]/kC versus 1/[OH ], and [Os(VIII)]/kC versus [H3IO ] were all linear (Figs. 6a,
6
b and c). From their slopes and intercepts, the values of K1, K2 and K3 were calculated
at different temperatures and these values are given in Table 2. The van’t Hoff’s plots
were made for the variation of K1, K2 and K3 with temperature: log K1 versus 1/T
1
0
(
1
r ≥ 0.8494, S ≤ 0.004), log K2 versus 1/T (r ≥ 0.9339, S ≤ 0.007) and log K3 versus
1
0
10
/T (r ≥ 0.8322, S ≤ 0.008), and the values of enthalpy of reaction ꢀH, entropy of reac-
tion ꢀS and Gibbs energy of reaction ꢀG were calculated for all three equilibrium steps.
−
1
These values are given in Table 2. A comparison of the ꢀH value (31.2 ± 2.2 kJ·mol
)
#
−1
for second equilibrium step of Scheme 1 with that of ꢀH (28.2 ± 1.4 kJ·mol ) of the rate
limiting step supports that the conclusion that the second step of Scheme 1 is fairly slow
#
#
since it involves a high activation energy. The modest ꢀH and ꢀS values are favorable
for an electron transfer processes. The negative value of ꢀS indicates that the intermediate
#
complex (C) is more ordered than the reactants [37].

1306
J Solution Chem (2010) 39: 1291–1310
Fig. 6 (a) Plots of
Os(VIII)]/k against 1/[DFS]
[
C
at four different temperatures
(conditions as in Table 1);
(
b) plots of [Os(VIII)]/k
C
−
against 1/[OH ] at four different
temperatures (conditions as in
Table 1); (c) plot of
3
−
[Os(VIII)]/k versus [H IO
]
C
2
6
at 298 K (conditions as in
Table 1)
#
The value of ꢀS is within the range for radical reactions, which are ascribed [38] to
the nature of electron pairing and unpairing processes and to the loss of degrees of free-
dom formerly available to the reactants upon the formation of a rigid transition state. The
negligible effects from ionic strength and dielectric constant changes are in the right direc-
tion as shown in Scheme 1. The observed modest enthalpy of activation and the relatively
low entropy of activation, as well as a higher rate constant for the slow step, indicate that
the oxidation probably occurs via an inner-sphere mechanism. This conclusion is supported
by earlier observations [39]. The activation parameters evaluated for the catalyzed and un-

J Solution Chem (2010) 39: 1291–1310
1307
catalyzed reaction explain the catalytic effect on the reaction. The Os(VIII) catalyst forms
complex (C) with the substrate, which enhances the reducing property of the substrate over
that found in the absence of catalyst. Furthermore, Os(VIII) modifies the reaction path by
lowering the activation energy.
5
Conclusion
The Os(VIII) catalyzed oxidation of DFS by DPC was studied. Oxidation products were
identified. Among the various species of DPC present in alkaline medium, monoperioda-
tocuprate(III) (MPC) is considered as the active species for the title reaction. The active
species of Os(VIII) is [OsO4(OH)2]2 . Activation parameters were evaluated for various
steps of the proposed mechanism. Catalytic constants and activation parameters with refer-
ence to the catalyst were also computed.
−
Acknowledgements The authors thank the CSIR, New Delhi, for the sanction of research grant [No.
01(2184)/07/EMR-II dated 26-11-2007].
Appendix
According to Scheme 1,
−
d[DPC]
Rate =
= −k[C][Cu(H2IO6)(H2O)2]
dt
−
kK1K2K3[DFS]f[Os(VIII)]f[DPC] [OH ]
f
=
(11)
2
−
[
H3IO ]
6
The total concentration of DPC [DPC] is given with subscripts T and f (for total and free,
respectively).
2
−
[
DPC]T = [DPC]f + [Cu(H2IO6)(H3IO6) ] + [Cu(H2IO6)(H2O)2]
−
K1K2[DPC]f[OH ]
−
=
=
=
[DPC]f + K1[OH ][DPC]f +
2
6
−
[
H3IO ]
ꢀ
ꢁ
−
K1K2[OH ]
−
[DPC]f 1 + K1[OH ] +
2
−
[
H3IO ]
6
ꢀ[
ꢁ
2
6
−
−
2−
−
H3IO ] + K1[OH ][H3IO ] + K1K2[OH ]
6
[DPC]f
(12)
(13)
2
6
−
[
H3IO ]
Therefore
Similarly,
2
−
[
DPC]T[H3IO ]
6
[DPC]f =
2
6
−
−
2−
−
[
H3IO ] + K1[OH ][H3IO ] + K1K2[OH ]
6
[
DFS] = [DFS]f + [complex(c)]
T
=
=
[DFS]f + K3[DFS][Os(VIII)]
[DFS]f{1 + K3[Os(VIII)]}

1308
J Solution Chem (2010) 39: 1291–1310
and therefore
[
DFS]T
+ K3[Os(VIII)]
Because the concentrations of osmium(VIII) used are very small,
[
DFS]f =
(14)
1
∼
[
DFS] = [DFS]
(15)
T
f
[
Os(VIII)] = [Os(VIII)] + [complex(C)]
T
f
=
=
[Os(VIII)] + K3[DFS][Os(VIII)]
f
f
[Os(VIII)] {1 + K3[DFS]}
(16)
(17)
f
and thus
[
Os(VIII)]T
[Os(VIII)]f =
1
+ K3[DFS]
Similarly,
−
−
2−
[
OH ]T = [OH ]f + [Cu(H2IO6)(H3IO6) ] + [Cu(H3IO6)(H2O)2]
−
K1K2[OH ]f[DPC]
−
−
=
[OH ]f + K1[OH ]f[DPC] +
(18)
−
[
H3IO ]
6
and thus
−
2−
[
OH ]T[H3IO ]
−
6
[
OH ]f =
(19)
(20)
2
6
−
2−
[
H3IO ] + K1[H3IO ][DPC] + K1K2[DPC]
In view of low concentrations of DPC and H3IO used,
6
2
6
−
−
∼
−
[
OH ]T = [OH ]f
Substituting Eqs. 13, 15, 17 and 20 in Eq. 11 and omitting the T and f subscripts we get
d[DPC]
rate = −
dt
−
kK1K2K3[DPC][DFS][OH ][Os(VIII)]
=
2
6
−
−
2−
6
−
−
[
+
H3IO ] + K1[OH ][H3IO ] + K1K2[OH ] + K1K2K3[OH ][DFS]
K3[H3IO ][DFS] + K1K3[DFS][OH ][H3IO ]
2
6
−
−
2−
6
(21)
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