Catalysis and mechanistic study of Ru(III) and Os(VIII) on the oxidation of taurine by BAT in acid and alkaline media: a kinetic modeling

Article abstract of DOI:10.1080/24701556.2020.1799394

The study of oxidation kinetics of drug molecules is of much noteworthy in order to understand their mechanistic chemistry in redox systems. Taurine, being a potent drug molecule, has a wide range of activities in biological system. Therefore, the kinetics of oxidation of taurine (TAU) by bromamine-T (BAT) in HCl medium, catalyzed by Ru(III) and in NaOH medium, catalyzed by Os(VIII) has been investigated at 313 K. The rate law in acidic medium is: rate = k[BAT] _t [TAU]⁰[Ru(III)]^0.95[H⁺]^?0.67. But, it takes the form, rate = k[BAT] _t [TAU]^0.46[Os(VIII)]^0.89[OH^?]^?0.53 in basic medium. Addition of halide ions and p-toluenesulfonamide had no significant effect on the rate in both the cases. Decrease in dielectric constant of medium decreases the reaction rate in both the media. The conjugate acid, CH₃C₆H₄SO₂NHBr (TsNHBr), is assumed to be the oxidizing species in both the media. Equilibrium and decomposition constants have been evaluated in each case.

Full text of DOI:10.1080/24701556.2020.1799394

Inorganic and Nano-Metal Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsrt21
Catalysis and mechanistic study of Ru(III) and
Os(VIII) on the oxidation of taurine by BAT in acid
and alkaline media: a kinetic modeling
Nathegowda Suresha , Kalyan Raj & Malini Subramanya
To cite this article: Nathegowda Suresha , Kalyan Raj & Malini Subramanya (2020):
Catalysis and mechanistic study of Ru(III) and Os(VIII) on the oxidation of taurine by BAT
in acid and alkaline media: a kinetic modeling, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2020.1799394
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INORGANIC AND NANO-METAL CHEMISTRY
https://doi.org/10.1080/24701556.2020.1799394
Catalysis and mechanistic study of Ru(III) and Os(VIII) on the oxidation of
taurine by BAT in acid and alkaline media: a kinetic modeling
a,b
b
c
Nathegowda Suresha , Kalyan Raj , and Malini Subramanya
a
b
Department of Chemistry, Bangalore Institute of Technology, Bangalore, India; Department of Chemistry, B.M.S. College of Engineering,
c
Bangalore, India; Department of Chemistry, Dayananda Sagar College of Engineering, Bangalore, India
ABSTRACT
ARTICLE HISTORY
Received 1 May 2019
Accepted 21 June 2020
The study of oxidation kinetics of drug molecules is of much noteworthy in order to understand
their mechanistic chemistry in redox systems. Taurine, being a potent drug molecule, has a wide
range of activities in biological system. Therefore, the kinetics of oxidation of taurine (TAU) by
KEYWORDS
Oxidation kinetics; taurine;
bromamine-T;
bromamine-T (BAT) in HCl medium, catalyzed by Ru(III) and in NaOH medium, catalyzed by Os(VIII)
0
has been investigated at 313K. The rate law in acidic medium is: rate ¼ k[BAT] [TAU]
t
0
.95 þ ꢀ0.67
0.46
0.89
ꢀ ꢀ0.53
[
Ru(III)] [H ]
. But, it takes the form, rate ¼ k[BAT]
t
[TAU] [Os(VIII)] [OH ]
in basic
Ru(III); Os(VIII)
medium. Addition of halide ions and p-toluenesulfonamide had no significant effect on the rate in
both the cases. Decrease in dielectric constant of medium decreases the reaction rate in both the
3 6 4 2
media. The conjugate acid, CH C H SO NHBr (TsNHBr), is assumed to be the oxidizing species in
both the media. Equilibrium and decomposition constants have been evaluated in each case.
Introduction
particularly in acidic and alkaline media, respectively.
Preliminary experiments revealed that the reactions between
TAU and BAT were found to be ‘very slow’ to be measured
in HCl and NaOH media. Alternatively, it was found that
Ru(III) and Os(VIII) are excellent catalysts for the facile oxi-
dation of TAU by BAT in acidic and alkaline media,
respectively.
In the light of these viewed observations, the present
investigations were undertaken. In this research, we report
the detailed kinetics of TAU oxidation by BAT with Ru(III)
and Os(VIII) catalysis in HCl and NaOH medium, respect-
ively, in order to elucidate the plausible mechanism, to pro-
cure appropriate rate law, and also to appraise the reactivity
of the catalysts.
Haloamines gained importance as both bases and nucleo-
[
1]
philes. They can interact with wide range of functional
groups. Bromamine-T (BAT) is a mild oxidant and one of
the important members of organic haloamines group.
Literature survey has confirmed many of its reactions with
[
2–7]
variety of substrates.
BAT, the bromamine analogue of
chloramine-T, is known to be a better oxidizing agent than
chloro compounds in acidic and alkaline media with two
electron change per mole giving p-toluenesulfonamide or
PTS (CH C H SO NH ) and sodium bromide. According to
3
6
4
2
2
the literature survey, less information is available on kinetics
and mechanistic investigations of oxidation of biologically
important compounds with BAT.
Taurine (2-aminoethenesulfonic acid) is an amino acid
used as antihypertensive, hypoglycemic, and cardiac drug in Experimental
[
8]
pharmaceuticals. Therapeutically also it finds applications
for the treatment of variety of conditions which include car-
Materials
diovascular diseases, epilepsy, seizure disorders, and muscu- Bromamine-T was prepared as reported in the literature.[19]
lar degeneration. Though it finds large number of Its purity was checked by iodometry, UV and IR spectra.
[
9–11]
applications in biological systems,
literature survey An aqueous solution of BAT was standardized iodometri-
reveals no reports on the catalysis and mechanism of oxida- cally and was preserved in brown bottle to prevent any of
tion of this substrate by any of the oxidants viewed from the its photochemical deterioration. Taurine (s.d.fine) was used
kinetic and mechanistic points. So, there was a need for as received and aqueous solutions of desired strength were
understanding the oxidation and mechanism of this sub- prepared whenever required. RuCl3 (Arora-Mathey) and
strate in the presence of catalyst so that the study would OsO4 (Johnson-Mathey) solutions3 were prepared in
3
give useful information on the kinetic and mechanistic 0.05 mol/dm HCl and 0.05 mol/dm NaOH, respectively.
chemistry of this drug in biological systems.
Allowance for the amount of acid/alkali present in the cata-
In recent years, Ru(III) and Os(VIII) have been exten- lyst solution was made, while preparing the reaction mix-
[
12–18]
sively used as catalysts in several redox reactions,
tures for kinetic runs. Heavy water (D O, 99.4%) was
2
CONTACT Kalyan Raj
drkalyanraj40@gmail.com Department of Chemistry, B M S College of Engineering, Bangalore 560019, Karnataka, India.
ß 2020 Taylor & Francis Group, LLC

2
N. SURESHA ET AL.
Figure 2. GC-mass spectrum of sulfoacetic acid with its molecular ion peak
at 140 amu.
Figure 1. GC-mass spectra of p-toluenesulfonamide with its molecular ion peak
at 171 amu.
revealed that one mole of TAU consumed two moles of
BAT in both the media and confirmed by the following stoi-
chiometric equation:
obtained from Bhabha Atomic Research Center, Mumbai,
for studying the solvent isotope effect. The dielectric con-
[
20]
stant (D)
of the reaction mixture was altered by the add-
ition of methanol in varying proportions (v/v). All the other
reagents were of analytical grades of purity. The solutions of
all reagents used in this work were freshly prepared with
doubly distilled water before use.
NH —CH —CH —SO H þ 2TsNBrNa þ 2H O
2
2
2
3
2
þ
ꢀ
!
HOOC—CH2—SO3H þ 2TsNH2 þ 2Na þ 2Br
þ NH3
(
1)
Kinetic procedure
where, Ts ¼ p-CH C H SO –.
3
6
4
2
The reaction mixture was stirred for 24 h at 313 K. After
the completion of reaction (monitored by TLC), the reaction
products were neutralized by NaOH/HCl and extracted with
ether. The products were separated by column chromatog-
raphy on silica gel (60–200 mesh) using gradient elution
All kinetic runs were performed under pseudo-first order
conditions of [TAU] ꢁ [BAT] , at 313± 0.1 K . The reac-
0
0
tions were carried out in glass-stoppered Pyrex boiling tubes
whose outer surfaces were covered black to eliminate the
photochemical effects. For each run, the requisite amount of
substrate, HCl and Ru(III) (in acid medium), substrate,
NaOH and Os(VIII) (in basic medium), and water (to main-
tain total volume constant for all runs) were introduced into
the tube and thermostatted at 313 ± 0.1K for thermal equi-
librium. A measured amount of oxidant (BAT) also thermo-
statted at the same temperature was added rapidly to the
above mixture to initiate the reaction. The mixture was
sporadically shaken to attain uniform concentration. The
progress of the reaction was monitored by pipetting 5 ml of
aliquot into ice cold water (containing known amount of KI
and H SO solutions) at regular intervals of time and titrat-
(from dichloro methane to chloroform) and were detected as
follows. The p-toluenesulfonamide (PTS: TsNH ), reduction
product of BAT, was detected by paper chromatography.
Benzyl alcohol saturated with water was used as the solvent
with 0.5% vanillin in 1% HCl solution as spray reagent
2
[
21]
ꢃ
(
R ¼ 0.905). It was confirmed by its melting point 137–138 C
f
ꢃ
(lit. m.p. 137–140 C) and GC-mass spectrum. The GC–MS
data were obtained on 17A Shimadzu Gas Chromatography
with a QP-5050 mass spectrometer using electron input ion-
izer technique. The mass spectrum showing a molecular ion
peak at 171 amu (Figure 1) clearly confirms PTS. The oxida-
2
4
ing against standard sodium thiosulfate solution using starch tion product of taurine was found to be the sulfoacetic acid in
indicator near the end point to determine unreacted BAT. both the media and was confirmed by GC–MS analysis. The
The reaction was followed for more than two half-lives. The mass spectrum showing a molecular ion peak at 140 amu
0
ꢀ1
pseudo-first order rate constants (k s ) calculated from the (Figure 2) clearly confirms sulfoacetic acid.
linear plots of log[BAT] verses time were reproducible to
±
5%. The regression analysis of experimental data was car-
Results and discussion
ried out using scientific calculator, from which the regres-
sion coefficient ‘r’ was calculated.
In order to find out the effect of concentration of oxidant
on reaction rate, the oxidant concentration was varied at
constant concentration of other ingredients. The plots of
log[BAT] (0.2 ꢂ 10 –2.0 ꢂ 10 mol/dm ) versus time were
Stoichiometry and product analysis
ꢀ
3
ꢀ3
3
Reaction mixtures containing varying proportions of BAT to linear (r > 0.9928) depicting a first-order dependence of
ꢀ
2
substrate in the presence of HCl/NaOH (0.6 ꢂ 10 mol/ reaction rate on [BAT]
for both Ru(III) and Os(VIII) cata-
o
3
ꢀ5
3
0
dm ) and Ru(III)/Os(VIII) (1.93 ꢂ 10 mol/dm ) were equi- lyzed reactions. The pseudo-first order rate constants, k , are
0
librated at 313 K for 48 h. Determination of unreacted BAT listed in Table 1. Further, the values of k are remain

INORGANIC AND NANO-METAL CHEMISTRY
3
þ
ꢀ
Table 1. Effect of the concentration of TAU, BAT, H /OH , and catalyst on
Table 2. Effect of varying dielectric constant of the medium on reaction rate
the rate of reaction at 313 K.
at 313 K.
4
ꢀ1
4
ꢀ1
1
0 k’ (s
)
10 k’ (s )
3
2
2
þ
ꢀ
5
1
0 [BAT]
o
10 [TAU]
o
10 [H /OH ]
10 [Catalyst]
%MeOH
(v/v)
D
3
3
3
3
a
b
(
mol/dm )
(mol/dm )
(mol/dm )
(mol/dm )
Ru(III)
Os(VIII)
Ru(III)
Os(VIII)
0.2
0.5
1.0
1.5
2.0
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
1.0
1.0
1.0
1.0
1.0
1.0
0.2
0.5
1.0
2.0
3.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.2
0.4
0.6
0.8
1.0
0.6
0.6
0.6
0.6
0 6
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
1.93
0.48
0.96
1.93
3.86
5.78
3.72
3.99
3.84
3.81
3.69
3.73
3.61
3.84
3.78
3.69
7.89
4.98
3.84
3.05
2.68
0.79
1.58
3.84
6.81
10.02
5.45
5.21
5.37
5.51
5.45
2.58
3.89
5.37
7.21
8.91
9.51
6.60
5.37
4.65
4.02
1.45
2.85
5.37
9.87
14.3
0
76.73
72.37
67.48
62.71
3.84
3.16
2.56
2.06
5.37
4.37
3.51
2.69
10
20
30
a
ꢀ3
3
ꢀ2
Experimental conditions: [BAT]
o
¼ 1.0 ꢂ 10 mol/dm ; [TAU]
o
¼ 1.0 ꢂ 10
3
ꢀ2
3
ꢀ5
mol/dm ;
[HCl] ¼ 0.6 ꢂ 10 mol/dm ;
[Ru(III)] ¼ 1.93 ꢂ 10 mol/
3
dm ; T ¼ 313 K.
b
ꢀ3
3
3
ꢀ2
Experimental conditions: [BAT]
o
¼ 1.0 ꢂ 10 mol/dm ; [TAU]
o
¼ 1.0 ꢂ 10
3
ꢀ2
ꢀ5
mol/dm ;
[NaOH] ¼ 0.6 ꢂ 10 mol/dm ;
[Os(VIII)] ¼ 1.93 ꢂ 10 mol/
3
dm ; T ¼ 313 K.
The effect of dielectric constant ‘D’ of the medium on
reaction rate was studied by adding MeOH (0–30%, v/v) to
the reaction mixture. A decrease in the rate (Table 2) was
observed with an increase in the MeOH content in both the
cases and the plots of log k versus 1/D were linear
(r > 0.9996) with negative slopes indicating the negative
0
Values in bold letters indicate the variation of concentration of that particular ion–dipole interaction in the rate determining step of
reactant while the concentration of other reactants kept constant.
Ru(III) catalyzed reactions and dipole–dipole interaction in
the rate-limiting step of Os(VIII) catalyzed reactions. The
constant with the variation in [BAT] , confirming the first-
order dependence of rate on [BAT]o in both the cases.
o
dielectric constant values for MeOH–H O mixtures were
2
[22]
taken from the literature.
As the rate was dependent both on [H ] and [OH ],
solvent isotope studies were made in both the media using
Under similar experimental conditions, an increase in
þ
ꢀ
0
[
TAU] did not alter the k values, indicating that the rate
o
was independent of [TAU] (Table 1) in acidic medium cat-
0
0
o
D O. The solvent isotope effect k (H O)/k (D O) was found
2
2
2
0
alyzed by Ru(III). But the values of k found to be increased
to be 1.10 and 1.16 for Ru(III) and Os(VIII) catalyzed reac-
tions, respectively.
with an increase in [TAU] in basic medium catalyzed by
o
0
Os(VIII). The plot of log k
versus log[TAU]
The effect of temperature on the rate of reaction for both
Ru(III) and Os(VIII) catalyzed reactions was studied at dif-
ferent temperatures (303 - 323 K) keeping other experimen-
tal conditions constant. The values of activation parameters
ꢀ
2
ꢀ2
3
(
0.2 ꢂ 10 –3.0 ꢂ 10 mol/dm ) was linear (r ¼ 0.9998) with
a slope of 0.46 indicating a fractional order dependence of
rate on [TAU] (Table 1).
o
The rate was found to be decreased on the variation of
0
were computed from Arrhenius plots of log k versus 1/T
þ
ꢀ
concentration of medium (H /OH ) (Table 1) keeping
(r > 0.9987) for the overall reaction in each case and the
0
other experimental conditions constant. The plots of log k
þ
ꢀ
ꢀ2
ꢀ2
3
results are presented in Table 3.
versus log[H /OH ] (0.2 ꢂ 10 –1.0 ꢂ 10 mol/dm ) were
linear (r > 0.9995) with the slopes of ꢀ0.67 and ꢀ0.53 in
acid and alkaline medium, respectively, indicating a frac- Mechanism and rate law for Ru(III) catalyzed reactions
tional inverse dependence of rate in both the media. An in acidic medium
increase in [catalyst] (Table 1) led to increase the reaction
[
23,24]
0
According to the electronic spectral studies,
the differ-
rate in both the cases and the plots of log k versus log[cata-
ꢀ
5
ꢀ5
3
ent octahedral forms of Ru(III) chloride viz. [RuCl
5
þ
lyst]
(0.48 ꢂ 10 –5.78 ꢂ 10 mol/dm )
were
linear
2
ꢀ
ꢀ
(
H O)] , [RuCl (H O) ] , [RuCl (H O) ], [RuCl (H O) ] ,
2
4
2
þ2
2
3
2
3
2
2
4
(
r > 0.9986) with the slopes of 0.95 and 0.89, respectively,
and [RuCl(H O) ]
do not exist in aqueous solutions.
2
5
for Ru(III) and Os(VIII) catalysts indicating first-order
dependence of rate on [catalyst].
Although, the following equilibrium exist for Ru(III) in
[
25–27]
ꢀ
ꢀ
acidic solutions:
The reaction rate was not affected by Cl or Br ions
ꢀ
2
ꢀ2
3
(
0.4 ꢂ 10 –3.0 ꢂ 10 mol/dm ) showed that no inter halo-
3ꢀ
þ
RuCl ꢄ nH O þ 3HCl ! ½RuCl ꢅ þ nH O þ 3H
3
2
6
2
gen is formed in the reaction in both the cases.
(2)
The effect of ionic strength on reaction rate of both
Ru(III) and Os(VIII) catalysis was studied by varying the
RuCl6ꢅ3ꢀ þ H O $ ½RuCl ðH OÞꢅ þ Clꢀ
2ꢀ
½
(3)
2
5
2
3
concentration of NaClO (0.1–0.5 mol/dm ) at constant con-
4
centration of all other reactants. The added NaClO did not
The above equilibrium has been employed in many redox
4
[
28,29]
affect the reaction rate significantly, indicating the non-ionic reactions catalyzed by Ru(III) in acid medium.
Since
species involvement in the rate-limiting step of both Ru(III) no effect of chloride ion on reaction rate is observed in the
catalyzed and Os(VIII) catalyzed reactions. Hence, no present study, equilibrium (3) does not play any role in the
2ꢀ
attempt was made to keep the ionic strength of the medium reaction. Hence, the complex ion, [RuCl
(H O)]
2
is
5
constant for kinetic runs in both the cases. assumed to be the most likely catalyzing species which reacts

4
N. SURESHA ET AL.
Table 3. Temperature dependence and activation parameters for Ru(III) and Os(VIII) catalyzed and uncatalyzed oxidation of taurine
by BAT in acidic and basic media, respectively.
4
ꢀ1
k’ ꢂ 10 (s
)
Values
Temperature
K)
Activation
parameters
a
b
a
b
(
Ru(III)
Os(VIII)
Ru(III)
Os(VIII)
ꢀ
1
303
308
313
318
323
1.41 (0.08)
2.23 (0.13)
3.84 (0.24)
5.84 (0.40)
9.26 (0.64)
2.27 (0.07)
3.36 (0.11)
5.37 (0.19)
7.90 (0.32)
12.21 (0.55)
E
a
(kJ mol
)
73.8 (82.8)
71.4 (80.2)
ꢀ83.2 (ꢀ78.2)
97.4 (106)
–
65.6 (85.9)
62.9 (83.3)
#
ꢀ1
DH (kJ mol
)
#
ꢀ1
ꢀ1
DS (J K mol
)
ꢀ107 (ꢀ70.0)
#
ꢀ1
DG (kJ mol
)
96.5 (105)
–
–
Values in parenthesis refer to uncatalyzed reactions. Similar experimental conditions as above were employed for uncatalyzed reac-
tions except catalyst.
a
¼ 1.0 ꢂ 10ꢀ3 mol/dm ; [TAU]
3
¼ 1.0 ꢂ 10ꢀ2 mol/dm ; [HCl] ¼ 0.6 ꢂ 10 mol/
3
ꢀ2
Experimental conditions for catalyzed reactions: [BAT]
o
o
3
ꢀ5
3
dm ; [Ru(III)] ¼ 1.93 ꢂ 10 mol/dm .
b
¼ 1.0 ꢂ 10ꢀ3 mol/dm3;
¼ 1.0 ꢂ 10ꢀ2 mol/dm3;
Experimental
conditions
for
catalyzed
reactions:
[BAT]
o
[TAU]
o
[
NaOH] ¼ 0.6 ꢂ 10ꢀ mol/dm ; [Os(VIII)] ¼ 1.93 ꢂ 10 mol/dm .
2
3
ꢀ5
3
The total effective concentration of BAT is
þ
½
BATꢅ ¼ ½TsNH Br ꢅ½TsNHBrꢅ
(4)
ꢀ
t
2
þ
Solving for [TsNH Br ] from step (i) of Scheme 1 and
2
substituting in Equation (4), we get [TsNHBr] as
K ½BATꢅ
1
t
½
TsNHBrꢅ ¼
(5)
þ
½
H ꢅ þ K
1
Step (ii) of Scheme 1 determines the overall rate, so,
Scheme 1. General mechanism in acid medium.
rate ¼ ꢀd½BATꢅ=dt ¼ k2½TsNHBrꢅ½RuðIIIÞꢅ
Substituting for [TsNHBr] from Equation (5) into
(6)
with the oxidant species to form a complex intermedi-
ate (X).
Equation (6), the following rate law can be derived
[
30,31]
The oxidizing species of acidified BAT solutions
are
þ
K k ½BATꢅ ½RuðIIIÞꢅ
1
2
t
TsNHBr, TsNBr , HOBr, and H OBr . Since second-order
2
2
rate ¼
(7)
þ
K1 þ H
½ ꢅ
dependence of rate on [BAT] was not noticed in the pre-
o
sent case, the involvement of TsNBr as oxidizing species
2
Above rate law is consistent with all the experimental
results and can be written as:
can be ruled out. On the other hand, since the added p-tol-
uenesulfonamide (TsNH ) did not alter the reaction rate,
2
þ
1
½H ꢅ
1
HOBr can also be ruled out as oxidizing species. Based on
these kinetic results, TsNHBr is assumed to be the oxidizing
species in the present study and in the presence of acid,
¼
þ
(8)
k= K1k2½RuðIIIÞꢅ k2½RuðIIIÞꢅ
From Equation (8), plot of 1/k versus [H ] (Figure 3)
was found to be linear (r ¼ 0.9981). From the slope and
intercept of the above plot, the values of deprotonation con-
stant K protonation constant K (1/K ), and decomposition
0
þ
þ
TsNHBr further protonates to TsNH Br But an inverse
2
.
þ
fractional order on [H ] suggested the deprotonation of
TsNH Br to regenerate TsNHBr, which is most likely the
þ
2
1,
p
1
active oxidant species involved in the oxidation of taurine.
Based on these facts, a general mechanism (Scheme 1) can
be proposed for the Ru(III) catalyzed oxidation of
taurine by BAT in acid medium to account the
observed kinetics.
constant k2 have been evaluated and found to be
ꢀ
3
3
2
3
3
2
.47 ꢂ 10 mol/dm , 4.1 ꢂ 10 dm /mol, and 69.1 dm /mol/
s, respectively.
The proposed Scheme 1 and derived rate law (7) are sub-
stantiated by the following facts; a negative dielectric effect
has been observed in the present studies indicating the nega-
tive ion–dipole interaction as seen in Scheme 2. For the lim-
iting case of zero angle of approach between two dipoles or
0
In Scheme 1, X and X represent complex intermediate
species whose structures are shown in Scheme 2, where a
detailed mechanistic interpretation of Ru(III) catalyzed
TAU— BAT reaction in acidic medium has shown. In the
0
an ion–dipole system, the plot of log k versus 1/D is lin-
þ
initial fast equilibrium, the cation TsNH Br is deproto-
2
[
32]
ear
with negative slope for reaction between negative ion
nated to form TsNHBr. In the next rate-limiting step,
TsNHBr reacts with the catalyst to form intermediate com-
plex X. In the next fast step, the catalyst in complex X facili-
tates the nucleophilic attack of TAU on bromine of oxidant
and undergoes subsequent hydrolysis to form intermediate
and a dipole or between two dipoles, while a positive ion
[
33]
and dipole interaction
The observed retardation of reaction rate by solvent iso-
tope effect in D O medium is due to the stronger acidity of
results positive slope.
2
þ
[34]
0
complex X with the elimination of NH and regenerating
D
3
O
than hydronium ion.
No effect of added halide
3
0
the catalyst. In the next fast step, complex X reacts with ions on the rate, indicating no inter halogen is formed.
another mole of oxidant to yield the final product sulfoacetic Addition of p-toluenesulfonamide has no significant effect
acid.
Scheme 1 leads to the following rate law:
on the rate, showing that it is not involved in any pre-equi-
librium. The moderate values of DH and DS are favorable
#
#

INORGANIC AND NANO-METAL CHEMISTRY
5
3
Scheme 2. Oxidation of taurine by BAT in acidic medium catalyzed by RuCl .
Scheme 3. General mechanism in basic medium.
oxidant than that without catalyst. Further, the catalyst
modifies the reaction path by lowering the energy
of activation.
Mechanism and rate law for Os(VIII) catalyzed reactions
in basic medium
0
þ
0
Figure 3. Plots of ᭿ 1/k versus [H ] for Equation (8) and ᭡ 1/k versus 1/ The oxidation potential of haloamine–sulfonamide system is
[
TAU] for Equation (18).
pH dependent and it decreases with an increase in pH of
þ
the medium. H OBr does not exist at higher alkaline con-
2
[
36]
centration.
dizing species
An alkaline solution of BAT furnishes the oxi-
[
37]
ꢀ
ꢀ
viz., TsNHBr, TsNBr , HOBr, and OBr .
for electron transfer reaction. The fairly high positive value
#
The alkaline solutions where pH < 11, the most likely react-
of DH shows that the transition state is more solvated. The
#
ive species of BAT is TsNHBr. Further, it has been
high negative value of DS indicate that the intermediate
[
38]
ꢀ
[
35]
reported
that the retarding influence of OH ions on the
complex is more ordered than the reactants.
observed high rate constant for the slow step suggests that
the oxidation presumably occurs via an inner-sphere mech- TsNHBr from TsNBr in base retarding step. In the present
anism. The activation parameters calculated for catalyzed investigations, HOBr as the oxidizing species is ruled out
and uncatalyzed reactions explain the catalytic effect on the because of first-order retardation of rate was not noticed
The
rate of BAT reactions attributed to the formation of
ꢀ
reaction rate. The catalyst, Ru(III), form complex (X) with with the addition of TsNH
to the reaction mixture. Hence,
2
the oxidant and enhances the oxidizing property of the the conjugate free acid TsNHBr is the most probable

6
N. SURESHA ET AL.
Scheme 4. Oxidation of Taurine by BAT in alkaline medium catalyzed by OsO4.
ꢀ
on [OH ] and fractional order on [TAU] , a general mech-
o
Table 4. Values of catalytic constant (K
c
) at different temperatures and activa-
anism shown in Scheme 3 is proposed.
tion parameters calculated using K values.
c
A detailed mechanistic interpretation of Os(VIII) cata-
lyzed oxidation of TAU in NaOH medium is proposed in
K
c
Values
Temperature
K)
Activation
parameters
(
Ru(III)
Os(VIII)
Ru(III)
Os(VIII)
00
Scheme 4, in which the structure of intermediate species X
ꢀ1
303
308
313
318
323
6.89
11.43
16.89
26.83
39.26
60.42
E
a
(kJ mol
)
70.25
68.63
ꢀ10.34
69.32
–
63.87
61.26
///
#
ꢀ1
and X are depicted. In the initial fast equilibrium, the
10.88
18.65
28.19
44.66
DH (kJ mol )
# -1
ꢀ1
ꢀ
DS (J K mol )
ꢀ22.52 anion TsNBr is protonated to form TsNHBr. In the next
#
ꢀ1
DG (kJ mol
)
68.31
–
fast equilibrium, nucleophilic attack by the nitrogen of TAU
–
on electron deficient bromine of the oxidant to form inter-
0
0
00
mediate complex X . In the next slow step, the complex X
coordinates to the central metal of catalyst which activates
0
0
00
oxidizing species of BAT in the present study. A fractional
X by stabilizing the charge on its nitrogen and X under-
order dependence of rate on [TAU] indicates a pre-equilib- goes subsequent hydrolysis to form another intermediate
o
/
//
rium before the rate-limiting step.
complex X with the elimination of NH and regenerating
3
/
//
Osmium is stable in its þ8 oxidation state and exists in the catalyst. In the next fast step, intermediate complex X
[
39,40]
the following equilibria:
reacts with another mole of oxidant to form the final prod-
uct sulfoacetic acid.
Â
Ãꢀ
ꢀ
OsO4 þ OH þ H2O $ OsO4ðOHÞðH2OÞ
(9)
If [BAT] denotes the total concentration of BAT, then,
t
ꢀ
00
Â
Ãꢀ
½BATꢅ ¼ ½TsNBr ꢅ þ ½TsNHBrꢅ þ ½X ꢅ
From steps (i) and (ii) of Scheme 3, we have
(11)
ꢀ
2ꢀ
t
OsO ðOHÞðH OÞ þ OH $ ½OsO ðOH Þꢅ þ H O
4
2
4
2
2
(
10)
0
0
ꢀ
½
X ꢅ½OH ꢅ
ꢀ
2ꢀ
2
ꢀ
The species [OsO (OH)(H O)] and [OsO (OH )]
½TsNBr ꢅ ¼
(12)
(13)
4
2
4
K K ½TAUꢅ½H Oꢅ
5
6
2
with octahedral geometries are likely to form coordination
species with intermediate complex (X ) formed from taurine
and oxidant. But, it is more convenient to postulate that
00
00
½X ꢅ
½
TsNHBrꢅ ¼
K6½TAUꢅ
OsO with tetrahedral geometry can efficiently form com-
4
ꢀ
Substituting for [TsNBr ] and [TsNHBr] from Equations
12) and (13) into Equation (11), solving for [X ], one can
0
0
plex with X . Hence, bearing the preceding discussions in
mind viz., first-order dependence of rate on each of [BAT]
and [Os(VIII)], inverse fractional order dependence of rate
00
(
get,

INORGANIC AND NANO-METAL CHEMISTRY
7
K K ½BATꢅ ½TAUꢅ½H Oꢅ
parameters (Table 4) have been deduced from the linear
0
0
5
6
t
2
½
X ꢅ ¼
(14)
(15)
ꢀ
plots of log Kc versus 1/T (r > 0.9998). Further, for standard
½
OH ꢅ þ K ½H Oꢅ þ K K ½TAUꢅ½H Oꢅ
5
2
5
6
2
0
0
run at 313 K, plots of k versus [Ru(III)] and k versus
From rate-limiting step (iii),
[
Os(VIII)] were found to be linear (r > 0.9986) leading to an
ꢀ
4
ꢀ1
ꢀ4 ꢀ1
0
0
rate ¼ k ½X ꢅ½OsðVIIIÞꢅ
intercept equal to 0.21 ꢂ 10
s
and 0.16 ꢂ 10
s
for
7
Ru(III) and Os(VIII), respectively. These values are highly
comparable with the rate constants obtained experimentally
for the uncatalyzed reactions at 313 K (Table 3). This indi-
cates that both catalyzed and uncatalyzed reactions proceed
in parallel way. Furthermore, the observed kinetic results
indicate that the Ru(III) and Os(VIII) catalyzed reactions
are, respectively, about 16-fold and 28-fold faster than their
corresponding uncatalyzed reactions. The difference in the
reactivity of Ru(III) and Os(VIII) catalysts toward the oxida-
tion of TAU by BAT is due to the electrons present in the
d-orbital. The greater efficiency of osmium to catalyze the
00
Substituting for [X ] from Equation (14) into Equation
15) and solving, we get
(
K5K6k7½BATꢅ ½TAUꢅ½OsðVIIIÞꢅ½H2Oꢅ
t
rate ¼
(16)
ꢀ
½
OH ꢅ þ K ½H Oꢅ þ K K ½TAUꢅ½H Oꢅ
5
2
5
6
2
0
Since rate ¼ k [BAT] , under pseudo-first order condi-
t
tions of [TAU] ꢁ [BAT] , rate law (16) can leads to
o
o
Equation (17)
K5K6k7½TAUꢅ½OsðVIIIÞꢅ½H2Oꢅ
=
k ¼
(17)
ꢀ
½
OH ꢅ þ K ½H Oꢅ þ K K ½TAUꢅ½H Oꢅ
5
2
5
6
2
o
oxidation of TAU by BAT is because of its d electronic
5
The rate law (17) is in good agreement with all the
observed results and it can be rewritten as:
configuration as compared to ruthenium having d elec-
tronic configuration. Thus, the catalytic efficiency of metal
ion decreases as the number of electrons in the d-orbital
ꢀ
1
½OH ꢅ þ K
1
5
[
46]
¼
þ
(18)
increases.
As seen in Schemes 2 and 4 the metal ion
k= K K k ½TAUꢅ½OsðVIIIÞꢅ k7½OsðVIIIÞꢅ
5
6 7
undergoes reduction momentarily when it is attached to oxi-
0
The double reciprocal plot of 1/k versus 1/[TAU] (Figure dant/oxidant–substrate complex and after this the metal ion
) of Equation (18) is linear (r ¼ 0.9830) and from the inter- comes back to its original state. Hence, in the present study
3
cept, the value of decomposition constant (k ) has been eval- the observed trend is based on the d electronic configuration
7
3
uated and found to be 1.8 dm /mol/s. If K is assumed to be of the metal ions.
5
ꢀ
small, the slope ([OH ] þ K )/K K k [TAU][Os(VIII)] of
5
5
ꢀ
6 7
Equation (18) becomes [OH ]/K K k [TAU][Os(VII)].
5
6 7
Conclusion
From this slope, the value of K K is calculated and found
5
6
3
to be 0.80 dm /mol.
The kinetics of oxidation of taurine by BAT in HCl
medium, catalyzed by Ru(III), and in NaOH medium, cata-
lyzed by Os(VIII) has been studied at 313 K. The stoichiom-
etry and oxidation products are same in both the media.
But, their kinetic and mechanistic behaviors are found to be
different under identical set of experimental conditions.
Suitable reaction schemes and kinetic models have been
designed in both the cases. Thermodynamic parameters
have been evaluated for catalyzed and uncatalyzed reactions.
Catalytic constants and activation parameters are also calcu-
lated with reference to Ru(III) and Os(VIII) catalysts. The
observed kinetic results indicate that Ru(III) and Os(VIII)
catalyzed reactions are 16-fold and 28-fold faster than the
uncatalyzed reactions. This difference in the efficiency of
catalysts in catalyzing the TAU-BAT redox system is attrib-
uted to the difference in d electronic configuration of metal
ions. Further, it can be concluded that Ru(III) and Os(VIII)
act as efficient catalysts for the oxidation of taurine in acid
and alkaline media, respectively.
The proposed Scheme 3 and derived rate law (16) are
supported by the following facts; the negative dielectric
effect observed indicates the dipole–dipole interaction in the
0
rate determining step. The solvent isotope effect k (H O)/
2
0
k (D O) > 1 in Os(VIII) catalyzed reaction, is generally cor-
related with greater basicity of OD as compared to
2
ꢀ
ꢀ
[41,42]
OH .
The mechanism is supported by the moderate
values of energy of activation and thermodynamic parame-
ters (Table 3). The fairly high positive values of free energy
of activation and enthalpy of activation indicating the transi-
tion state is more solvated. The high rate constant for the
slow step indicates that the oxidation presumably occurs via
[
43,44]
an inner-sphere mechanism.
The activation parameters
calculated for catalyzed and uncatalyzed reactions explain
the catalytic effect on the reaction rate. The catalyst,
00
Os(VIII), coordinates to the complex (X ) and activates it
by stabilizing the charge on its nitrogen than that with-
out catalyst.
[
45]
It has been observed by Moelwyn-Hughes
that in the
presence of catalyst, the uncatalyzed and catalyzed reactions
proceed simultaneously so that:
Acknowledgments
k1 ¼ k0 þ KC½catalystꢅx
(19) The authors are thankful to BMSCE, Department of Chemistry,
Karnataka, India for providing the facilities. One of the authors (NS) is
Here, k and k are the rate constants for catalyzed and
1
0
grateful to Rajya Vokkaligara Sanhga, and Principal, Bangalore
uncatalyzed reactions, respectively, Kc is catalytic constant Institute of Technology, Karnataka, India for the support.
and x is the order of the reaction with respect to catalyst. In
the present studies, ‘x’ value was found to be unity for both
Ru(III) and Os(VIII) catalysts. From the calculated values of
Disclosure statement
KC at different temperatures, the corresponding activation No potential conflict of interest was reported by the authors.

8
N. SURESHA ET AL.
2
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5. Davforkratova, T. Analytical Chemistry of Ruthenium; Academy
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6. Backhours, J. R.; Doyer, F. D.; Shales, N. Proc. Roy. Soc. 1950,
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