Catalysis and mechanistic studies of ruthenium and osmium on synthesis of anthranilic acids

Article abstract of DOI:10.1002/aoc.1685

Ruthenium, osmium and ruthenium + osmium catalyzed synthetic methodology was developed for the synthesis of anthranilic acids from indoles in good to excellent yields using bromamine-B in alkaline acetonitrile-water (1:1) at 313 K. Detailed catalysis studies of ruthenium, osmium and the mixture of both were carried out for the synthetic reactions. The positive synergistic catalytic activity of Ru(III) + Os(VIII) was observed to a large extent with the activity greater than the sum of their separate catalytic activities. Detailed kinetic and mechanistic investigations for each catalyzed reactions were carried out. The kinetic pattern and mechanistic picture of each catalyzed reaction were found to be different for each catalyst and to obey the underlying rate laws: rate = k[BAB]_t[Indole][Ru(III)]^x[OH^-] ^y rate = k[BAB]_t[Indole][Os(VIII)][OH^-] ^y rate = k[BAB]_t[Indole]^o[Ru(III) + Os(VIII)][OH^-]^y where, x, y < 1. The reactions were studied at different temperatures and the activation parameters were evaluated for each catalyzed reaction. Under the identical set of experimental conditions, the kinetics of all the three catalyzed reactions were compared with uncatalyzed reactions, revealing that the catalyzed reactions were 6- to 42-fold faster. The catalytic efficiency of aforementioned catalysts followed the order: Ru(III) + Os(VIII) > Os(VIII) > Ru(III). This trend may be attributed to the different d-electronic configuration of the catalysts. The proposed mechanisms and the rigorous kinetic models derived give results that fit well with the experimental data in each catalyzed reaction. Copyright

Full text of DOI:10.1002/aoc.1685

Full Paper
Received: 28 January 2010
Revised: 4 April 2010
Accepted: 28 April 2010
Published online in Wiley Online Library: 24 August 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1685
Catalysis and mechanistic studies of ruthenium
and osmium on synthesis of anthranilic acids
a
a∗
a
P. Karthikeyan , Rajenahally V. Jagadeesh , Y. Sree Sandhya ,
b
a
a
a
Puttaswamy , P. Nithya , S. Senthil Kumar and P.R. Bhagat
Ruthenium, osmium and ruthenium + osmium catalyzed synthetic methodology was developed for the synthesis of anthranilic
acids from indoles in good to excellent yields using bromamine-B in alkaline acetonitrile–water (1 : 1) at 313 K. Detailed
catalysis studies of ruthenium, osmium and the mixture of both were carried out for the synthetic reactions. The positive
synergistic catalytic activity of Ru(III) + Os(VIII) was observed to a large extent with the activity greater than the sum of their
separate catalytic activities. Detailed kinetic and mechanistic investigations for each catalyzed reactions were carried out. The
kinetic pattern and mechanistic picture of each catalyzed reaction were found to be different for each catalyst and to obey the
underlying rate laws:
x
− y
rate = k[BAB]t[Indole][Ru(III)] [OH ]
−
y
rate = k[BAB]t[Indole][Os(VIII)][OH ]
o
− y
rate = k[BAB]t[Indole] [Ru(III) + Os(VIII)][OH ]
where, x, y < 1. The reactions were studied at different temperatures and the activation parameters were evaluated for each
catalyzed reaction. Under the identical set of experimental conditions, the kinetics of all the three catalyzed reactions were
compared with uncatalyzed reactions, revealing that the catalyzed reactions were 6- to 42-fold faster. The catalytic efficiency of
aforementioned catalysts followed the order: Ru(III) + Os(VIII) > Os(VIII) > Ru(III). This trend may be attributed to the different
d-electronic configuration of the catalysts. The proposed mechanisms and the rigorous kinetic models derived give results that
fit well with the experimental data in each catalyzed reaction. Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: Ru(III)/Os(VIII)/Ru(III) + Os(VIII) catalysis; anthranilic acids; indoles; reaction mechanism; bromamine-B
Introduction
Bromamine-B (C6H5SO2NBrNa.1.5H2O or PhSO2NBrNa or BAB),
the bromine derivative of sulfonamide, is gaining importance
as an oxidant and reagent for the synthesis of a variety of or-
Theimportanceofanthranilicacidsresidesintheirwellestablished
anti-inflammatoryactivityandtheirpotentialusesasintermediates
for the production of dyes, pigments and saccharin. Anthranilic
acid and its ester derivatives are used in preparing perfumes,
pharmaceuticalsandUV-absorberaswellascorrosioninhibitorsfor
metals and mould inhibitors in soy sauce. Thus, many derivatives
of anthranilic acids have been synthesized with the objective
ganic molecules. Although the mechanistic aspects of many of
haloamine reactions have been well documented,[
9–12]
similar
studies on bromine analogu are sparse. In view of these facts,
there is a considerable scope for the study with BAB to achieve a
better insight into the speciation of BAB reaction models and to
understand its redox chemistry in solutions.
[
1–4]
of discovering new pharmacological agents.
Because of the
In recent times, studies on the use of transition metal ions as
versatile properties and pharmaceutical applications of anthranilic
acids, synthesis of anthranilic acids is an important task for
exploring convenient and efficient methodologies. There are
several reports available in the literature about the synthesis of
catalysts in many synthetic and redox reactions have been gaining
interest.[
13–15]
The catalytic activities are due to the existence of
variable oxidation states, as a consequence of partially filled d or
f orbitals. Their ability to form both σ and π bonds with other
moieties or ligands is one of the chief facts for imparting catalytic
[
4,5]
anthranilic acids with different approaches.
However, there is
stillaneedtointroducesaferandmoreefficientreagentsincluding
favorable methods for the synthesis of anthranilic acids.
Syntheticmethodology, asthebuildingblockoforganicsynthe-
sis, continuously seeks new reagents, better reaction conditions
and more efficient and selective methods. In this regard, a large
group of compounds entitled sodium N-haloarenesulfonamidates
∗
Correspondence to: Rajenahally V. Jagadeesh, Division of Organic Chemistry,
School of Advanced Sciences, VIT University, Vellore-632014, India.
E-mail: rvjdeesh@yahoo.com
organic haloamines) are widely used in fine organic synthesis.[6,7]
a Division of Organic Chemistry, School of Advanced Sciences, VIT University,
(
Vellore-632014, India
N-haloamines act as good oxidants and reagents both in alkaline
and acidic media, and have been widely used for the oxida-
tion and synthesis of variety of organic and bio molecules.[7–9]
b Department of Chemistry, Bangalore University, Central College Campus,
Bangalore-560001, India
Appl. Organometal. Chem. 2011, 25, 34–46
Copyright ꢀc 2010 John Wiley & Sons, Ltd.

Catalysis and mechanistic studies of ruthenium and osmium
R
R
COOH
NH2
M stock solutions of RuCl3 (Merck) was prepared in 0.01 mol dm3
HCl in order to avoid the hydrolysis of RuCl3 in water. The 0.1
M OsO4 (Merck) stock solution was prepared in 0.01 mol dm
NaOH to retain the stable oxidation state (+8) of Os without any
formation of aquo complex with water. Allowances were made for
theamountofNaOHandHClpresentinthecatalystsolutionswhile
preparing solutions for kinetic runs. Solvent isotope studies were
Bromamine-B/Catalyst
−
3
Acetonitrile:water(1:1)
NaOH, 313 K
N
H
(
Catalyst = Ruthenium or Osmium or Ruthenium + Osmium
Scheme 1. Catalyzed synthesis of anthranilic acids.
made with D O (99.4%) supplied by Bhabha Atomic Research
2
Center, Mumbai, India. Reagent-grade chemicals and double-
distilled water were used throughout. Bromamine-B was prepared
by the known procedure.[
properties to transition metals as well as their complexes. Most
of the d-block elements show characteristic interligand migration
reactions and such a process forms one of the most important
types of reactions in homogeneous catalysis. Ruthenium (III)
chloride [Ru(III)] and Os(VIII) oxide [Os(VIII)] have been widely
used as homogeneous catalysts in various redox reactions.[
The catalytic effect of Ru(III) and Os(VIII) on various reactions
is well known, but sparse information is available about Ru(III)
16]
Preparation of Bromamine-B
Bromamine-B was prepared[16] by the partial debromination
15–19]
of dibromamine-B (DBB), which was obtained as follows. Pure
chlorine was bubbled through an aqueous solution of chloramine-
B (30 g in 560 ml of water) and liquid bromine (6 ml) was added
drop-wise with constant stirring. The resulting yellow precipitate
of dibromamine-B (DBB) was thoroughly washed with water,
filtered under suction and dried in a desiccator. Dibromamine-
+
Os(VIII) mixed catalyst. Hence in the present work, we used
Ru(III) and Os(VIII), both alone and in mixtures as a catalyst.
The main objective of the present study is the development of
catalytic methodology for the synthesis of anthranilic acids and
the exploration of catalytic efficiency and mechanism of Ru(III),
Os(VIII) and Ru(III) + Os(VIII) mixture and also synergism of Ru(III)
B (31.5 g) was digested in batches with constant stirring in
−3
5
0 ml of 4 mol dm NaOH. The mass obtained was cooled in
ice, filtered under suction and the product (BAB) was dried
over anhydrous calcium chloride. The purity of bromamine-
B was assayed iodometrically to determine the active halogen
content. Aqueous solutions of the oxidant were standardized by
iodometric procedure and preserved in brown bottles to prevent
photochemical deterioration. Bromamine-B was confirmed by
mass spectral analysis.
+
Os(VIII) catalyst on the title reaction.
The wide range of applications of anthranilic acids and the
usefulness of transition metals catalysis in organic reactions
instigatedustocarryoutthetitlereactiontodevelopaprotocolfor
the synthesis of anthranilic acids from indoles with an interest in
studying the reaction mechanism and kinetics. By keeping above
points in mind, we report herein a new and simple method for the
preparation of anthranilic acids from indoles using bromamine-B
and ruthenium/osmium catalyst (Scheme 1).
Reaction Stoichiometry
Reaction mixtures containing varying ratios of BAB to indoles in
−
3
−3
−6
thepresenceof2.0×10 mol dm NaOHand2.0×10 catalyst
were equilibrated at 313 K for 24 h. Determination of unreacted
BAB in reaction mixture showed that 1 mole of indole consumed
Experimental
Materials
3
moles of BAB in all the three catalyzed reactions, confirming the
Melting points were determined on X-4 apparatus and are
uncorrected. IR spectra were obtained using a Shimadzu FTIR-
stoichiometry in Scheme 2.
8
1
900 spectrometer. Mass spectrometry data were obtained on a
7A Shimadzu gas chromatograph with a QP-5050A Shimadzu
Synthesis of Anthranilic Acids
mass spectrometer. The mass spectrum was obtained using
the electron impact ionization technique. Indole and substituted
indoles (Lancaster and Sigma Aldrich), were of acceptable grade
of purity and used as received. Aqueous solutions of the
indoles were prepared by dissolution in aqueous acetonitrile (1 : 1
water–acetonitrile) and were employed for the kinetic study. A 0.1
To a stirred solution of indoles (10 mmol), bromamine-B (30 mmol)
in alkaline (20 mmol NaOH) acetonitrile–water (1 : 1) mixture
(20 ml), catalyst (2 mmol) was added and the mixture was heated
at 313 K for 3–6 h. The reaction progress was monitored by
TLC. After completion of the reaction, the reduction product of
bromamine-B, benzenesulfonamide (PhSO2NH2) was extracted
O
R
Catalyst (2 × 10-6 mol dm-3
)
+
3
S
N
Br Na
+
3
NaOH (2 × 10-3 mol dm-3
)
N
O
(Indole)
(BAB)
H
O
S
R
COOH
+
NH
2
+ 3 NaBr + CO
2
(1)
O
NH
2
(Anthranilic acid)
(BSA)
Scheme 2. Stoichiometic equation.
Appl. Organometal. Chem. 2011, 25, 34–46
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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P. Karthikeyan et al.
Table 1. Synthesis of anthranilic acids from indoles
Reaction time
(hr. minutes)
◦
Entry
1
Sustrate
Product
Yeild (%)
95
mp ( C)
5.00
4.30
4.20
5.30
4.0
145 (146–148)
177 (178–179)
237 (235–236)
213 (211–216)
171 (172)
2
3
4
5
6
96
96
94
96
95
5.30
210 (210)
7
8
5.35
5.30
92
92
335 (336)
180 (183–184)
mp given in paranthesis refer to authetinc samples
with ethyl acetate, identified by TLC and confirmed by mass
spectral analysis. The aqueous part of the reaction mixture was
neutralized with acid, the solvent was evaporated under reduced
pressure and the residue was dissolved in dichloromethane. The
dichloromethane layer was washed twice with water and dried
over sodium sulfate. The solvent was evaporated under reduced
pressure and the residue thus obtained was purified by passing
through a shortsilica gel column using dichloromethane as eluent.
Evaporation of the solvent yielded anthranilic acids around 95%
yield. Alternatively, after extracting out benzenesulfonamide, the
aqueous layer was neutralized with acid and the anthranilic acids
are estimated as their zinc anthranilates. The procedure for the
estimation is as follows: anthranilate was precipitated as its zinc
by comparison with authentic samples. Further, the compounds
were confirmed by mass spectral analysis.
Kinetic Measurements
The detailed kinetic experiments were made with respect to
indole as model compounds. The reactions were carried out under
pseudo first-order conditions with a known excess of [Indole]o
over [BAB]o at 313 K. The reaction was carried out in stoppered
Pyrex boiling tubes whose outer surfaces were coated black to
eliminate photochemical effects. For each run, requisite amounts
of solutions of indole, NaOH, catalyst and aqueous acetonitrile
(1 : 1 water–acetonitrile; to keep the total volume constant for all
runs) were introduced in to the tube and thermostated at 313 K
until thermal equilibrium was attained. A measured amount of
BAB solution, also thermostated at the same temperature, was
rapidly added with stirring to the mixture in the tube. The progress
of the reaction was monitored by the iodometric determination
of unreacted BAB in aliquots (5 ml each) of the reaction mixture
salt.[ Tothereactionmixture, acalculatedvolumeof 1 mol dm
20]
−3
HCl was added, followed by 10 ml of pH 5.0 buffer and 10 ml of
1
1
% zinc sulfate. The precipitate formed was filtered, dried at
05–110 C and weighed, and the recovery of (C7H6O2N)2 Zn was
◦
found be around 95%. The reaction times and yields are given in
Table 1. The products were identified by TLC and melting point
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 34–46

Catalysis and mechanistic studies of ruthenium and osmium
withdrawn at different intervals of time. The course of the reaction
was studied for at least two half-lives. The pseudo first-order rate
constants (k ) calculated from the linear plots of log [BAB] vs time
were reproducible within ± 5%.
Table 2. Effect of varying reactant concentrations on the reaction
rate at 313 K
ꢁ
4
ꢁ
−1
10 k (s )
4
3
1
(
0 [BAB]o
10 [Indole]o
Ru(III)
+ Os(VIII)
mol dm 3)
−
(mol dm
−3
)
Ru(III)
Os(VIII)
Results
0
.2
1.0
1.0
1.0
1.0
1.0
0.2
0.5
1.0
2.0
4.0
3.30
3.19
3.21
3.28
3.16
0.81
1.60
3.21
6.50
13.0
6.75
6.67
6.70
6.69
6.77
1.42
3.32
6.70
14.0
27.5
21.8
21.9
21.7
22.0
21.5
21.2
21.9
21.7
21.4
22.0
Synthesis of anthranilic acid and substituted anthranilic acids was
achieved using catalytic amounts of ruthenium, osmium and a
0.5
1.0
2.0
4.0
1.0
1.0
1.0
1.0
1.0
◦
mixtureofbothinacetonitrile–water(1 : 1)at40 Cbybromamine-
B with 1 : 3 indole–bromamine-B ratio in the presence of alkali. The
products and the yields were summarized in Table 1. In general
substrates containing electron-donating moieties were found to
be slightly more reactive and required shorter reaction times
compared to substrates containing electron-withdrawing groups.
The synthesis of anthranilic acids from indoles proceeds with
the formation of isatins as the intermediates. Indoles first utilize
−
3
−3
−6
−3
[
NaOH] = 2.0 × 10 mol dm ; [catalyst] = 2.0 × 10 mol dm
;
2
moles of bromamine-B to form isatins. Thus formed isatins
−3
I = 0.3 mol dm [in the case of Os(VIII) and Ru(III) + Os(VIII) catalysis].
consume another mole of bromamine-B to yield the ultimate and
desired compounds, anthranilic acids. The detailed mechanisms
for reactions are shown in Schemes 4, 6 and 8.
The reactions were studied in various solvents (acetonitrile, 1, 2-
dichloromethane, ethanol and acetonitrile–water (1 : 1) mixture).
The mixture of acetonitrile–water (1 : 1) was found to be the best
solvent system, perhaps due to the dielectric constant and the
solubility of the reactants. Indoles are not very soluble in water,
but markedly soluble in acetonitrile–water mixture. Moreover the
organic haloamines furnish different species better in aqueous
(water) medium rather than in organic medium. For all these
reasons acetonitrile–water with a 1 : 1 ratio system was the better
choice of solvent system in the present synthetic study. The
reactions were found to be highly dependent upon the pH of
the system. To evaluate the effect of pH, the reactions were
carried out under similar experimental conditions at different pHs
using NaOH. At neutral pH, the oxidation reactions were found
to be very slow. Reaction rates increase with increasing in pH
(addition of NaOH). This behavior of the reaction is attributed
to the dissociation of bromamine-B in aqueous medium by
furnishing different oxidizing species. This behavior of dissociation
of bromamine-B is well explained in the Discussion. Because of the
increase rate in the presence of NaOH, the reactions were carried
out at 2.0 × 10 mol dm NaOH. The kinetics of the oxidative
conversion of indole by BAB was investigated at several initial
concentrations of the reactants in NaOH medium in presence of
Ru(III), Os(VIII) and Ru(III) + Os(VIII) mixture catalysts at 313 K under
identical experimental conditions.
ꢁ
Figure 1. Plots of log k versus log[Indole].
−
3
−3
Effect of Varying NaOH and Catalyst Concentrations
on the Rate
The kinetic experiments showed that the rate is unaffected with
aqueous acetonitrile markedly. The reactions were carried out
with different amounts of acetonitrile and it was found that the
rate is not affected significantly by acetonitrile. However, rate is
significantly affected by alkali. The rate of the reaction increased
with increase in [NaOH] in all cases (Table 3). The log–log plots
of rate vs [NaOH] (Fig. 2) showed that the orders in [NaOH] were
Effect of Varying Reactant Concentrations on the Rate
With the indole in excess at constant [Indole]o, [NaOH], [catalyst]
and temperature, plots of log [BAB] vs time were linear, indicating
a first-order dependence of the rate on [BAB]o in all the three
ꢁ
catalyzed reactions. The pseudo first-order rate constants (k ) are
ꢁ
less than unity, suggesting a fractional-order dependence on
OH ] in all three catalyzed reactions. The reaction rate increased
recorded in Table 2. Further these k values were unaffected by
−
[
the variation of [BAB]o, confirming the first-order dependence of
the rate on [BAB]o. Under the same experimental conditions, the
rate of the reaction increased with increase in [Indole]o for Ru(III)
with increase in [catalyst] in all the cases (Table 3). The order
with respect to [Ru(III)] was found to be fractional but it was
found to be unity for [Os(VIII)] and [Ru(III) + Os(VIII)] catalysts.
This clearly indicates a fractional-order dependence on [Ru(III)],
and first-order dependence on [Os(VIII)] and [Ru(III) + Os(VIII)]
ꢁ
and Os(VIII)-catalyzed reactions and plots of log k vs log [Indole]
(Fig. 1) were found to be linear with unit slopes, indicating first-
order dependence on [Ru(III)] and [Os(VIII)] catalysts. However, the
order with respect to Ru(III) + Os(VIII) catalyst was found to be
zero order. These results are given in Table 2.
ꢁ
catalysts. The plots of log k vs log [catalyst] are presented in
Fig. 3.
Appl. Organometal. Chem. 2011, 25, 34–46
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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P. Karthikeyan et al.
case of Ru(III). However, the rate of the reaction increased with
increasing ionic strength in the case of Os(VIII) catalysis and the
rate decreased in the case of Ru(III) + Os(VIII) catalysis. Plots of log
Table 3. Effect of varying NaOH and catalyst concentrations on the
reaction rate at 313 K
4
ꢁ
−1
ꢁ
1/2
1
0 k (s
)
k vs I were found to be linear with slopes +0.50 and −1.60 in
Os(VIII) and Ru(III) + Os(VIII) catalysis respectively (Table 4). Hence,
the ionic strength of the medium was maintained at a constant
3
6
1
(
0 [NaOH]
10 [catalyst]
Ru(III)
+ Os(VIII)
−
3
−3
mol dm
)
(mol dm
)
Ru(III)
Os(VIII)
−
3
concentration of 0.30 mol dm of NaClO4 in the case of Os(VIII)
0.5
1.0
2.0
4.0
6.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
2.0
4.0
8.0
1.5
2.02
3.21
5.00
6.96
1.1
3.32
4.80
6.70
9.00
11.4
1.72
3.26
6.70
14.3
28.5
10
ta hn ed r Re au c( ItI iI o) n+. Os(VIII) catalysis for kinetic runs in order to swamp
13.5
21.7
31.6
40.0
5.00
10.0
21.7
43.0
88.0
Effect of Solvent Isotope on the Rate
Studies of the reaction rate in D2O medium for Ru(III), Os(VIII) and
1.71
3.21
6.38
10.0
ꢁ
Ru(III) + Os(VIII) catalyzed reactions revealed that k (H2O) was
−4
−4
−4
ꢁ
equal to 3.20 × 10 , 6.70 × 10 and 21.7 × 10 and k (D2O)
−4
−4
−4
=
4.50 × 10 , 8.45 × 10 and 29.1 × 10 , respectively. The
ꢁ
ꢁ
solvent isotope effect, k (H2O)/K (D2O) was found to be 0.71,0.79
and 0.75 for the three catalyzed reactions. These results are given
Table 5.
−
4
−3
−3
−3
[
BAB]o = 1.0 × 10 mol dm ; [Indole]o = 1.0 × 10 mol dm
;
−3
I = 0.3 mol dm [in the case of Os(VIII) and Ru(III) + Os(VIII) catalysis].
Effect of Varying Temperature on the Rate
Effect of Varying Ionic Strength of the Medium on the Rate
The reactions were studied at different temperatures (303–323 K),
keeping other experimental conditions constant. From the linear
Arrhenius plots of log k vs 1/T, values of activation parameters
An increase in ionic strength of a reaction system by addition
of NaClO4 showed a negligible effect on the reaction rate in the
ꢁ
ꢁ
Figure 2. Plots of logk versus log[NaOH].
ꢁ
Figure 3. Plots of logk versus log[catalyst].
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 34–46

Catalysis and mechanistic studies of ruthenium and osmium
is not involved in any step prior to the rate-limiting step in the
schemes proposed.
Table 4. Effect of varying ionic strength of the medium on the
reaction rate at 313 K
4
ꢁ
−1
1
0 k (s )
Effect of Varying Halide Ion Concentrations on the Rate
Ionic strength (I)
Ru(III)
+ Os(VIII)
− −
The rate remained unchanged with the addition of Cl or Br ions
−3
(
mol dm
)
Ru(III)
Os(VIII)
−
2
−2
−3
in the form of NaCl and NaBr (1.0 × 10 to 8.0 × 10 mol dm ).
0.1
0.2
0.3
0.4
0.5
3.28
3.19
3.21
3.30
3.17
5.00
6.00
6.7
48.9
28.8
21.7
14.8
11.2
These results showed that halide ions play no role in the reaction.
Test for Free Radicals
7.60
8.70
The addition of the reaction mixtures to the aqueous acrylaminde
monomer solution did not initiate polymerization indicating the
absence of in situ formation of free radical species in the reaction
sequence. The control experiments were also performed under
the same reaction conditions but without the oxidant, BAB.
BAB]o = 1.0 × 10− mol dm ; [Indole]o = 1.0 × 10 mol dm
4
−3
−3
−3
−6
−3
;
[
[
NaOH] = 2.0 × 10−3 mol dm ; [catalyst] = 2.0 × 10 mol dm
−3
.
Table 5. Effect of solvent isotope on the rate at 313 K
Discussion
ꢁ
ꢁ
k (H2O)/K
(D2O)
ꢁ
ꢁ
k (Water)
k (D2O)
Reactive Species of Bromamine-B
−
−
−
4
4
4
−1
−1
−1
−4 −1
[18]
[19]
Ru(III)
3.20 × 10
6.70 × 10
21.7 × 10
s
s
s
4.50 × 10
8.45 × 10
29.1 × 10
s
0.71
0.79
0.75
InvestigationsofMorriset al., BishopandJennings, Prydeand
−4 −1
[20]
[21]
Os(VIII)
s
Soper and Hardy and Johnston have shown the existence of
similar equilibria of N-metallo-N-arylhalosulfonamides in aqueous
media. Bromamine-B, like chloramine-T, acts as an oxidizing agent
in both acidic and alkaline solutions. Depending on the pH, BAB
exhibits different equilibria in aqueous solutions.
solutions of BAB, PhSO2NBr2 does not exist, and the possible
oxidizing species are PhSO2NBr and OBr anions, which could
be transformed into the more reactive species PhSO2NHBr and
HOBr during the course of the reaction in alkali-retarding steps.
Several workers have observed the retarding effect of OH ions
on the rate of haloamines with a number of substrates
−4 −1
Ru(III) + Os(VIII)
s
BAB]o = 1.0 × 10− mol dm ; [Indole]o = 1.0 × 10 mol dm
4
−3
−3
−6
−3
−3
;
;
[
[
−3
−3
NaOH] = 2.0 × 10 mol dm ; [catalyst] = 2.0 × 10 mol dm
[
18–21]
−3
In alkaline
I = 0.3 mol dm [in the case of Os(VIII) and Ru(III) + Os(VIII) catalysis].
[
8]
−
−
Table 6. Temperature dependence and values of composite activa-
tion parameters for the synthesis of anthranilic acid by BAB in the
presence and absence of catalyst
−
[22–27]
and
have suggested that the reactivity of weakly alkaline solutions
of haloamines is due to the formation of the conjugate acid
4
ꢁ
−1
1
0 k (s )
Ru(III)
−
−
PhSO2NHBr from PhSO2NBr in the OH retarding step. However,
Temperature (K)
Ru(III)
Os(VIII)
+ Os(VIII) Uncatalyzed
−
in the present investigations, the OH ions increase the rate of the
−
3
3
3
3
3
03
08
13
18
23
1.31
2.01
3.34
5.01
6.70
14.0
17.8
21.7
26.4
33.5
29.7
26.7
90.0
0.18
0.32
0.52
0.90
1.58
reaction, clearly indicating PhSO2NBr as the reactive oxidizing
species. In our earlier work[
28,29]
the positive influence of OH ion
−
3.21
on the rate of haloamine reactions with a number of substrates has
−
5.26
10.0
been observed and RNX (R = Ts or PhSO2; X = Cl or Br) suggested
7.01
13.4
52.3
49.8
92.6
as the reactive oxidizing species. In the present investigations, the
Ea (kJ mol 1)
−
70.3
85.4
−
rate of the reaction accelerated by OH ions clearly indicates that
ꢂ
=
−1
−1
−
ꢀ
ꢀ
ꢀ
H
(kJ mol
)
)
67.4
82.8
98.4
the anion PhSO2NBr is the most likely reactive species involved
ꢂ
=
=
G
ꢂ
(kJ mol
−1
94.6
in the synthesis of anthranilic acids from indoles by BAB in all three
catalyzed reactions.
−1
S
(J K
mol )
−80.0
8.62
−141
5.85
−206
2.45
−52.7
10.4
log A
−
4
−3
−3
−3
;
Mechanism and Rate Law of Ru(III) Catalysis
[
[
BAB]o = 1.0 × 10 mol dm ; [Indole]o = 1.0 × 10 mol dm
−3
−3
−6
NaOH] = 2.0 × 10 mol dm ; [catalyst] = 2.0 × 10 mol
Ru(III) chloride acts as catalyst in many organic reactions,
−3
−3
dm ; I = 0.3 mol dm [in the case of Os(VIII) and Ru(III) + Os(VIII)
[
30–33]
particularly in an alkaline medium.
Under the experimental
catalysis].
−
−
conditions [OH ] ꢃ [Ru(III)] and owing to the fact that [OH ]
increases the rate, ruthenium (III) is predominantly present as the
2
+
hydroxylated species [Ru(H2O)5OH] and its formation is shown
in the following equilibrium:
ꢂ= ꢂ=
ꢂ=
(
Ea, ꢀH , ꢀG , ꢀS and log A) for the composite reaction were
deduced. All these data are summarized in Table 6.
3
+
−
[Ru(H2O)5(OH)]2+ + H2O
(2)
[
Ru(H O)6] + OH
2
Effect of Varying Benzenesulfonamide Concentrations
on the Rate
Similar equilibria have been reported between Ru(III) catalyzed
reactions in alkaline medium.
The existence of a complex between the catalyst and BAB was
evidenced from the UV–vis spectra of both Ru(III) and Ru(III)–BAB
−
3
[31,33]
Addition of benzenesulfonamide (BSA; 1.0 × 10
to 8.0 ×
−
3
−3
1
0
mol dm ), a reduction product of BAB, to the reaction
mixture did not affect the rate significantly. This indicates that BSA
Appl. Organometal. Chem. 2011, 25, 34–46
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

P. Karthikeyan et al.
ꢁ
mixture. Absorption maxima of Ru(III) in HCl medium, BAB and
Ru(III)–BAB in aqueous alkaline acetonitrile medium appear at
Here, X and X are the intermediate complexes whose
structuresareillustratedinScheme 4,whereadetailedmechanistic
interpretation of indole-BAB reaction in presence Ru(III) has been
proposed.
3
52, 223 and 338 nm respectively. A hypsochromic shift of 14 nm
from 352 to 338 nm of Ru(III) indicates the formation of complex
between Ru(III) and BAB.
If [BAB] represents the total effective concentration of BAB,
t
Further, for a general equilibrium (3)
then
−
[
BAB]t = [PhSO2NHBr] + [PhSO2NBr ] + [X]
(5)
K
(3)
Catalyst + nS
(Catalyst Sn)
−
By substituting for [PhSO2NHBr] and [PhSO2NBr ] into equa-
tion (5) and solving for [X] we get:
between two metal species, M and MSn having different extinction
coefficients, Ardon[ has derived the following equation (4):
34]
−
K1K2[BAB]t[OH ][Ru(III)]
1
1
1
1
[X] =
(6)
−
−
=
(
)
+
(4)
[H O] + K [OH ] + K K [OH ][Ru(III)]
[S]n (ꢀE)K[Catalyst]Total
2
1
1 2
ꢀA
(ꢀE)[Catalyst]Total
where K is the formation constant of the complex, [S] is the
concentration of BAB, ꢀE is the difference in extinction co-
efficient between two metal species, [Catalyst]Total is the total
concentration of catalyst species and ꢀA is the absorbance differ-
ence of solution containing with and without BAB. Equation (4) is
valid provided that [BAB] is much greater than [Catalyst]Total. Ac-
From the slow and rate-determining step of Scheme 3, the rate of
reaction is given by
Rate = −d[BAB]/dt = k3[Indole][X]
(7)
2
Onsubstitutingthevalueof[X]fromequation (6) intoequation (7),
one obtains the rate law (8):
cording to equation (4), a plot of 1/ꢀA vs 1/[BAB] or 1/[BAB]
should be linear with an intercept in the case of 1 : 1 or
1
: 2 type of complex formation between M and S. The ra-
−
tio of intercept to slope of this linear plot gives the value
of K.
Ruthenium (III) in aqueous alkaline acetonitrile medium con-
taining BAB showed an absorption peak at 338 nm (λmax for the
complex). The complex formation studies were made at this λmax
of 338 nm. In a set of experiments, the solutions were prepared by
−d[BAB]
K1K2k3[BAB]t[Indole][OH ][Ru(III)]
Rate =
=
−
−
dt
[H O] + K [OH ] + K K [OH ][Ru(III)]
2
1
1 2
(
8)
Rate law (8) is in complete agreement with experimental data.
Hence the proposed mechanism is valid.
Since rate = k [BAB]t, rate law (8) can be transformed into the
−
−
4
−3
−3
−3
takingdifferentamountsofBAB(2.0×10 −4.0×10 mol dm
)
ꢁ
6
at constant amounts of RuCl3 (2.0 × 10 mol dm ) and NaOH
−
2
−3
following equations (9)–(12):
(
2.0 × 10 mol dm ) in aqueous acetonitrile at 313 K. The
absorbance of these solutions was measured at 338 nm. The
absorbance of the solution in the absence of BAB was also mea-
suredatthesamewavelength.Thedifferenceoftheseabsorbances
−
K1K2k3[Indole][OH ][Ru(III)]
kꢁ =
(9)
(10)
(11)
(12)
−
−
[H O] + K [OH ] + K K [OH ][Ru(III)]
2
1
1 2
(with and without BAB) gives the differential absorbance, ꢀA. A
1
2
[H O]
=
plot of 1/ꢀA vs 1/[BAB] was linear with an intercept suggesting
the formation of a 1 : 1 complex between BAB and Ru(III) cat-
alyst. Similar behavior for the formation of complex has been
ꢁ
−
k
1 2 3
K K k [Indole][OH ][Ru(III)]
1
1
+
+
K2k3[Indole][Ru(III)]
[H2O]
k3[Indole]
[
34,35]
reported in earlier works.
Further, the plot of log (1/ꢀA) vs
1
1
log (1/[BAB]) was also linear (r = 0.9801). From the slope and
=
{
−
}
ꢁ
k
[OH ] K1K2k3[Indole][Ru(III)]
intercept of the plot 1/ꢀA vs 1/[BAB], the value of the forma-
1
1
tion constant, K, of the complex was deduced and found to be
6
+
{
+
k3[Indole]}
2
.06 × 10 .
K2k3[Indole][Ru(III)]
[H2O]
In view of the above observations, a general mechanism
1
1
1
=
{
+
}
ꢁ
−
(
Scheme 3) is proposed for the Ru(III)-catalyzed synthesis of
k
[Ru(III)] K K k [Indole][OH ] K k [Indole]
1
2
3
2 3
anthranilic acids from indoles by BAB to account for the observed
experimental results.
1
+
k3[Indole]
K1
−
PhSO NHBr + OH
−
2
PhSO NBr + H O
(i) fast
2
2
K2
−
PhSO NBr + Ru(III)
X
2
(ii) fast
k3
X + Indole
X′
(iii) slow and rds
(iv) fast
k4
−
X′ + 2 PhSO NBr
Products
2
Scheme 3. Ru(III)-catalyzed mechanistic scheme.
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 34–46

Catalysis and mechanistic studies of ruthenium and osmium
Br
N
S
R
COOH
H
O
O
3
+
2
[
Ru(H O) ]
2 6
NH
2
−
−
−
OH
OH
Bromamine-B
O
+
[
Ru(H O) OH]
2
5
R
−
−
−
OH
OH
PhSO NH
N
O
2
2
H
Br
N
S
O
O
Bromamine-B
O
S
N
Br
O
R
2
+
[
Ru(H O) OH]
2 4
(
X)
N
H
Scheme 4. Ru(III)-catalyzed mechanism for the synthesis of anthranilic acids.
ꢁ
−
A plot of 1/k vs 1/[OH ] from equation (11) yields
It was felt reasonable to compare the reactivity of BAB
towards indole in the absence of Ru(III) catalyst under identical
experimentalconditionsinordertoevaluatethecatalyticefficiency
of Ru(III). The reactions were carried out at different temperatures
(303–323 K) and from the plot of log k vs 1/T (Fig. 4), activation
parameters were also evaluated for the uncatalyzed reactions
[
H2O]
Slope =
and Intercept
1
K1K2k3[Indole][Ru(III)]
ꢁ
1
=
+
k3[Indole]
K2k3[Indole][Ru(III)]
(Table 6). However, the Ru(III)-catalyzed reactions were found to
ꢁ
Similarly, a plot of 1/k vs 1/[Os(VIII)] from equation (12) yields
be about 6 times faster than uncatalyzed reactions. The activation
parameters evaluated for the catalyzed and uncatalyzed reactions
explain the catalytic effect on the reaction. The catalyst Ru(III)
forms a complex (X) with the BAB, which increases the reactivity
of BAB.
[H2O]
1
Slope =
+
−
K1K2k3[Indole][OH ]
K2k3[Indole]
1
and Intercept =
The general equation relating for uncatalyzed and catalyzed
reactions has been derived by Moelwyn and Hughes[ as:
k3[Indole]
38]
Therefore from the slopes and intercepts of equations (11)
and (12), the values of equilibrium constants K1 and K2 and
decomposition constant k3 were calculated for the standard
run with [BAB]o = 1.0 × 10 mol dm ; [Indole] = 1.0 ×
x
k1 = ko + KC[catalyst]
(13)
−
4
−3
Here k1 is the observed pseudo first-order rate constant obtained
in the presence of Ru(III) catalyst, ko is the pseudo first-order
constant for the uncatalyzed reaction, KC is the catalytic constant
and x is the order of the reaction with respect to [Ru(III)]. In the
present investigations, x was found to be 0.8. Then the value
of KC was calculated using the equation (13).The values of KC
were evaluated at different temperatures (303–323 K) and KC was
found to vary with temperature. Further, a plot of log KC vs 1/T
(Fig. 5) was linear and the values of energy of activation and other
activation parameters for the Ru(III) catalyst were computed and
are summarized in Table 7.
The proposed mechanism is supported by the observed
moderatevaluesofenergyofactivationandotherthermodynamic
parameters. The high positive values of the free energy of
activation and of the enthalpy of activation suggest that the
transition state is highly solvated, while fairly high negative
entropy of activation (Table 6) indicates the formation of a rigid
associated transition state. Variation of ionic strength of the
−
3
−3
−3
−3
1
2
0
mol dm ; [NaOH] = 2.0 × 10 mol dm
[Ru(III)] =
−
6
.0 × 10 mol dm-3 at 313 K. The values obtained were
−
2 5 3
−1
−1
k3 = 6.14 × 10
s
; K2 = 8.00 × 10 dm mol ; K1 =
−
3
3
−1
7
.89 × 10 dm mol
.
The proposed Scheme 3 and the derived rate law (8) are
substantiated by the experimental observations discussed below.
+
−
For a reaction involving a fast pre-equilibrium H or OH
ion transfer, the rate increases in D2O since D3O and OD are
–3 times stronger acids and stronger bases,
than H3O and OH ions. In the present studies the observed
solvent isotope effect of k (H2O)/k (D2O) < 1 is due to the greater
basicity of OD compared twith OH . This isotope effect observed
in the present case confirms to the above fact. However, the
+
−
[
36,37]
2
respectively,
+
−
ꢁ
ꢁ
−
−
ꢁ
ꢁ
magnitude of increase of rate in D2O is small [k (H2O)/k (D2O) =
0
.71] compared with the expected value of 2–3 times greater,
which can be attributed to the fractional-order dependence of
−
rate on [OH ].
Appl. Organometal. Chem. 2011, 25, 34–46
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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P. Karthikeyan et al.
ꢁ
Figure 4. Plots of logk versus 1/T.
Figure 5. Plots of logKC vs. 1/T.
−
2−
medium did not alter the reaction rate, which indicates that
the noninvolvement of ionic species in the rate-determining step.
Addition of the reduction product of BAB, BSA, does not influence
the rate, showing that it is not involved in pre-equilibrium.
Addition of halide ions has no effect on the rate, indicating
that there is no role for halide ions in the reaction. All these
observations also confirm the proposed mechanism and derived
rate law.
The complexes [OsO4(OH)(H2O)] and [OsO4(OH)2] , which can
2
−
be reduced to [OsO2(OH)4] , with octahedral geometries are less
likely to form species of higher coordination. It is more realistic to
postulate that OsO4, which has tetrahedral geometry, as the active
catalyst species can effectively form a complex.
The existence of a complex between indole and Os(VIII)
was evidenced from the UV–vis spectra of indole, Os(VIII) and
indole–Os(VIII) mixture. Absorption maxima in aqueous alkaline
acetonitrile medium appeared at 286 nm for indole, 319 nm for
Os(VIII) and 300 nm for a mixture of both. A bathochromic shift of
Mechanism and Rate Law of Os (VIII) Catalysis
1
4 nm from 286 to 300 nm of indole suggests that complexation
occurred between indole and Os(VIII). According to equation (4) a
plot of 1/ꢀA vs 1/[Indole] with an intercept suggests the formation
of 1 : 1 complex between Os(VIII) and indole. Further, the plot of
log 1/ꢀA vs log 1/[Indole] was also linear (r = 9898). From the
slope and intercept of the plot 1/ꢀA vs 1/[Indole], the value of
It has been shown that osmium has a stable +8 oxidation
state[
14,39,40]
and exists in the following equilibria in alkaline
solutions:
2
−
−
(14)
(15)
formation constant, K, of the complex was found to be 8.23 × 10 .
OsO + OH + H O
[OsO4 (OH) H2O]
4
2
−
Based on the experimental results, it is likely that PhSO2NBr
−
−
2−
[
OsO (OH) H O] + OH
[OsO4 (OH)2] + H2O
4
2
itself also acts as the reactive oxidant species in the present case.
wileyonlinelibrary.com/journal/aoc
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 34–46

Catalysis and mechanistic studies of ruthenium and osmium
into equations (20) and (21):
Table 7. Values of catalytic constant (KC) at different temperatures
and activation parameters calculated using KC values
−
K5K6k7[Indone][OH ][Os(VIII)]
kꢁ =
(20)
(21)
KC × 10
−
[
H2O] + K5[OH ]
Ru(III)
1
[H2O]
Temperature (K)
Ru(III)
Os(VIII)
+ Os(VIII)
=
ꢁ
−
k
K K k [Indole][Os(VIII)][OH ]
5 6 7
3
3
3
3
3
03
08
13
18
23
5.65
8.45
13.4
21.8
54.3
88
31.6
46.9
62.8
91
138
172
1
+
K k [Indole][Os(VIII)]
6
7
211
ꢁ
−
255
A plot of 1/k vs 1/[OH ] according to equation (21), with
other experimental conditions held constant, was found to be
linear (r > 0.9899). K5 and K6k7 values were calculated from
the slope and intercept of such a plot and was found to be
118
319
Ea (kJ mol 1)
−
50.6
47.3
70.2
−52.3
8.98
41.8
38.1
62.2
−45.7
5.86
ꢂ
=
−1
−1
ꢀ
ꢀ
ꢀ
H
(kJ mol
)
)
85.1
93.4
−70.8
13.1
ꢂ
=
=
3
−5
3
−1 −1
G
ꢂ
(kJ mol
−1
2
[
[
.67 × 10 and 2.0 × 10 dm mol
s
for the standard run,
−1
−4
−3
−3
−3
−3
S
(J K
mol )
BAB]o = 1.0×10 mol dm ; [Indole]o = 1.0×10 mol dm
;
log A
−6
−3
Os(VIII)] = 2.0 × 10 mol dm ; and I = 0.3 mol dm at 313 K.
BAB]o = 1.0 × 10− mol dm ; [Indole]o = 1.0 × 10 mol dm
4
−3
−3
−3
−6
−3
−3
Scheme 5 and rate law (19) can explain the following observed
experimental results.
[
[
;
;
−3
NaOH] = 2.0 × 10 mol dm ; [catalyst] = 2.0 × 10 mol dm
−
3
I = 0.3 mol dm [in the case of Os(VIII) and Ru(III) + Os(VIII) catalysis].
The ionic strength (I) effect on the reaction rates was described
according to the theory of Bronsted and Bjerrum,[
41]
which
postulates the reaction through the formation of an activated
complex. According to this theory, the effect of ionic strength
on the rate for a reaction involving two ions is given by the
relationship
Considering the above facts and all the observed experimental
data, the following reaction, Scheme 5, can be suggested for
Os(VIII) catalyzed synthesis of anthranilic acids from indoles by
BAB in alkaline medium:
Here, X and X are the intermediate complexes whose struc-
tures are given in Scheme 6, wherein a detailed mechanistic
pathway for Os(VIII) catalysis reaction is illustrated. The total
effective concentration of BAB is given by:
ꢁ
1/2
log k = log ko + 1.02ZAZBI
(22)
ꢁꢁ
ꢁꢁꢁ
Here ZA and ZB are the valency of the ions A and B, and k and
ko are the rate constants in the presence and absence of the
ꢁ
1/2
added electrolyte, respectively. A plot of log k against I should
be linear with a slope of 1.02 ZAZB. If ZA and ZB have similar
signs, the quantity ZAZB is positive and the rate increases with
the ionic strength, having a positive slope, while if the ions have
dissimilar charges, the quantity ZAZB is negative and the rate will
decrease with the increase in ionic strength, having a negative
slope. In the present case, a primary salt effect was observed
as the rate increased with the increase in ionic strength of the
medium,[ supporting the involvement of ions of same sign in
[
BAB]t = [PhSO2NHBr] + [PhSO2NBr−]
(16)
From step (ii) of Scheme 5,
ꢁ
ꢁ
[
X ] = K6[Indole][Os(VIII)]
(17)
44]
From the slow and rate determining step (iii) of Scheme 5, the rate
of reaction is given by:
ꢁ
the rate-limiting step (Scheme 6). The Debye–Huckel plot (log k
1
/2
against I ) gave a straight line with a slope of 0.6. In the present
system, a two negative ions (Z = −1 and Z = −1) were involved
ꢁꢁ
−
A
B
Rate = −d[BAB]t/dt = k7[X ][PhSO2NBr ]
(18)
in the rate-determining step and the expected slope of +1 was
not found. This may be due to the fact that the ionic strength
employed was beyond the formal Debye–Huckel limiting range.
Alternatively, therecouldbeformationofionpairsinconcentrated
−
ꢁꢁ
On substituting for [PhSO2NBr ] and [X ] into equation (18), the
rate law (19) was obtained:
−
[44]
K5K6k7[BAB]t[Indole][OH ]Os(VIII)]
solutions, as suggested by Bjerrum.
Rate =
(19)
H2O + K5[OH−]
The reactivity of BAB towards indole in the absence of Os(VIII)
catalyst was compared with the Os(VIII) catalyzed reaction under
identical experimental conditions. Rate constants revealed that
the Os(VIII) catalyzed reactions were about 13-fold faster than
[
The rate law (19) is in agreement with the experimentally observed
ꢁ
results. Since rate = k [BAB]t, equation (19) can be transformed
K5
−
PhSO NHBr + OH
−
2
PhSO NBr + H O
(i) fast
(ii) fast
2
2
K6
Indole + Os(VIII)
X″
k7
−
X″ + PhSO NBr
(iii) slow and rds
(iv) fast
2
X″′
k8
−
X″′ + PhSO NBr
Products
2
Scheme 5. Os(VIII)-catalyzed mechanistic scheme.
Appl. Organometal. Chem. 2011, 25, 34–46
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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P. Karthikeyan et al.
R
COOH
NH2
O
Bromamine-B
R
O
Os
O
O
O
O
PhSO NH
2
N
H
2
/
//
(
X )
R
R
N
H
N
2
O
O
O
O
Os
O
S
O
N
//
(
X )
Br
−
−
−OH
OH
2
O
S
O
H
N
Br
Scheme 6. Os(VIII)-catalyzed mechanism for the synthesis of anthranilic acids.
uncatalyzed reactions (Table 6). The values of KC were determined
at different temperatures and, from a plot of log KC vs 1/T, the
valuesofactivationparametersforOs(VIII)catalystwerecomputed
From the above preceding discussions, Scheme 7 has been pro-
posed to account for the observed kinetic results. In this scheme,
IV
V
X
and X represent the complex intermediate species. Scheme 8
(Table 7). The negligible effect of the addition of BSA and halide
depicts the mode of indole–BAB reaction in the presence of Ru(III)
ions on the rate of the reaction and also activation parameters was
in good agreement with the mechanism proposed and the rate
law derived.
+ Os(VIII) mixture. The total effective concentration of BAB is
[
BAB]t = [PhSO2NHBr] + [PhSO2NBr−]
(23)
From the slow step of Scheme 7,
Mechanism and Rate Law of Ru(III) + Os (VIII) Catalysis
−
rate = −d[BAB]t = k10[PhSO2NBr ][Ru(III) + Os(VIII)]
(24)
−
The kinetic data suggested that PhSO2NBr is also the reactive
−
By substituting for [PhSO2NBr ] into equation (24) the following
rate law is obtained:
species of BAB in Ru(III) + Os(VIII) catalyzed reaction. UV–visible
spectral studies revealed that there is transient existence of
a complex between Ru(III) and Os(VIII) in aqueous alkaline
acetonitrile medium. Absorption maxima in aqueous alkaline
acetonitrile medium appeared at 352 nm for Ru(III), 319 nm for
Os(VIII) and 336 nm for a mixture of both. This clearly indicates the
formation of a transient complex between Ru(III) and Os(VIII) and
this transient complex [Ru(III) + Os(VIII)] acts as reactive catalyst
species in this case.
−
−
d[BAB]t
dt
K9k10[BAB]t[Ru(III) + Os(VIII)][OH ]
rate =
=
(25)
−
[H O] + K [OH ]
2
9
Rate law (25) is in good agreement with the experimental data.
Since rate = k [BAB]t, equation (25) can be transformed into
ꢁ
equations (26) and (27):
−
K9k10[Ru(III) + Os(VIII)][OH ]
The existence of other complex between Ru(III) + Os(VIII) and
kꢁ =
(26)
(27)
−
BABwasalsoevidencedfromtheUV–visspectraofbothBAB,Ru(III)
[H2O] + K9[OH ]
+
Os(VIII) and a mixture of both in alkaline medium. Absorption
maxima in aqueous alkaline acetonitrile medium appeared at
23 nm for BAB, 336 nm for Os(VIII) and 245 nm for a mixture of
1
[H2O]
=
ꢁ
−
k
K k [Ru(III + Os(VIII)][OH ]
9
10
2
1
+
both.
k9[Ru(III) + Os(VIII)]
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Appl. Organometal. Chem. 2011, 25, 34–46

Catalysis and mechanistic studies of ruthenium and osmium
K9
−
−
PhSO NHBr + OH
PhSO NBr + H O
2 2
2
(i) fast
(ii) slow and rds
(iii) fast
k10
−
XIV
PhSO NBr + [Ru(III) + Os(VIII)]
2
k11
XIV + Indole
XV
k12
XV + PhSO NBr
−
2
Products
(iv) fast
Scheme 7. Ru(III)+Os(VIII)-catalyzed mechanistic scheme.
2
+
OsO + [Ru(H O) OH]
Br
4
2
5
R
COOH
NH2
H
O
N
S
O
O
O
Os
O
O
Bromamine-B
R
O
O
+
2
[
Ru(H O) OH)]
−OH− OH−
2
4
PhSO NH
2
2
N
H
Br
N
O
S O
Bromamine-B
O
S
O
N
Br
R
O
O
Os
N
H
O
O
2
+
[
Ru(H O) OH]
2 4
IV
(
X )
Scheme 8. Ru(III) + Os(VIII)-catalyzed mechanism for the synthesis of anthranilic acids.
ꢁ
−
Based on equation (27), a plot of 1/k vs 1/[OH ] at constant
[
catalyzed reaction under identical experimental conditions. Rate
constants revealed that the Ru(III) + Os(VIII) catalyzed reactions
are about 42-fold faster than uncatalyzed reactions (Table 6). The
values of KC were determined at different temperatures and from
a plot of log KC vs 1/T; values of activation parameters for Ru(III)
+ Os(VIII) catalyst were computed (Table 7). The negligible effect
on the addition of BSA and halide ions on the rate of the reaction
and also activation parameters was in good agreement with the
mechanism proposed and the rate law derived.
BAB]o, [Indole]o, [Ru(III) + Os(VIII)] ionic strength and temperature
was found to be linear (r = 0.9899). From the values of slope
and intercept of such a plot, the equilibrium constant (K9) and
decomposition constant (k10) were calculated and found to be
4
3
−1
−2 −1
s respectively.
2
.35 × 10 dm mol and 1.26 × 10
The proposed mechanism and the derived rate law were
substantiated by the following experimental facts. The ionic
strength (I) effect on the reaction rates was studied and a primary
salt effect was observed as the rate decreases with increase in
The kinetics of all the three catalyzed reactions was compared
withuncatalyzedreactions, undersimilarexperimentalconditions,
and it was found that the catalyzed reactions were 6- to 42-fold
faster. For the catalyzed reactions, it is seen from the Table 6 that
the activation energy was highest for the slowest reaction and
vice-versa. From the inspection of rate constants and the values
of energy of activation (Table 6), the relative reactivity of these
catalysts was in the order: Ru(III) + Os(VIII) > Os(VIII) > Ru(III). The
reactivity was faster in the case of Os(VIII) compared with Ru(III).
This may be attributed to the d-electronic configuration of the
[
41]
ionic strength of the medium, supporting the involvement of
ions of opposite sign in the rate-determining step (Scheme 8). The
ꢁ
1/2
Debye–Huckel plot (log k against I ) gave straight line with
slope of −1.60. In the present system, a negatively charge ion and
two positively charged ions were involved in the rate-determining
step (Scheme 8) and the expected slope of −2 was not found.
This may be due to the fact that the ionic strength employed was
beyond the formal Debye–Huckel limiting range. Alternatively,
there could be formation of ion pairs in concentrated solutions, as
[
44]
o
suggested by Bjerrum.
metal ions. Osmium having d electronic configuration has greater
The reactions were also studied in absence of Ru(III) + Os(VIII)
catalyticefficiencytooxidizethesubstratecomparedtotheRu(IIII),
which is of d electronic configuration. The reactivity in the case of
5
catalyst and rates were compared with that of Ru(III) + Os(VIII)
Appl. Organometal. Chem. 2011, 25, 34–46
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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P. Karthikeyan et al.
Ru(III) + Os(VIII) catalysis was maximum. This trend may be due to
the formation of a transient complex between Ru(III) and Os(VIII).
Ru(III) effectively forms a complex with Os(VIII) which increases the
catalytic property of Os(VIII) and thereby the catalytic effect of the
mixture of these is greater when compared with the individual
catalysts and thus the positive synergistic catalytic activity of
Ru(III) + Os(VIII) mixture. Therefore the observed reactivity in the
present study follows the order: Ru(III) + Os(VIII)> Os(VIII)> Ru(III).
Consequently, we can conclude that the synergistic effect of Ru(III)
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[
Ru(III), Os(VIII) and a mixture of both catalyzed synthesis of
anthranilic acids from indoles has been performed efficiently
using bromamine-B. The aforementioned metal ions were found
to be efficient catalysts for the facile synthesis of anthranilic acids
in aqueous alkaline acetonitrile medium. Ru(III) + Os(VIII) serves as
an efficient catalyst when compared with the individual catalysts
and shows positive synergetic catalytic activity. The stoichiometry
and anthranilic acids products of Ru(III), Os(VIII) and Ru(III) +
Os(VIII) catalysis were same but their observed kinetic patterns
and mechanistic aspects were different, obeying the underlying
rate law:
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7
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x
− y
rate = k[BAB]t[Indole][Ru(III)] [OH ]
[
[
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[
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catalysts, under identical experimental conditions, was found be
in the order: Ru(III) + Os(VIII) > Os(VIII) > Ru(III). Further, under
identical experimental conditions, the kinetics of all the three
catalyzed reactions were compared with uncatalyzed reactions
andwerefoundtoproceed6-to42-foldfasterthantheuncatalyzed
reactions. Based on the observed experimental results, detailed
mechanistic interpretations and the related kinetic modelings
were worked out for each catalyst. It can be concluded that Ru(III),
Os(VIII)andRu(III)+Os(VIII)actasefficientcatalystsinthesynthesis
of anthranilic acids from indoles brought about by BAB in alkaline
medium and predominantly the synergism of Ru(III) + Os(VIII)
mixed catalyst is prodigious.
[
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Supporting information
Supporting information may be found in the online version of this
article.
[
[
[
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Appl. Organometal. Chem. 2011, 25, 34–46