Oxidative behavior and relative reactivities of some unsaturated compounds towards hexachloroiridate(IV) in perchloric acid medium

Article abstract of DOI:10.1002/kin.10030

The kinetics of the oxidation of styrene, cinnamic acid, and some of their substituted derivatives by hexachloroiridate(IV) in dimethyl formamide-water mixtures and in the presence of perchloric acid have been investigated. The reactions appear to proceed via the formation of an unstable intermediate 1:1 complex between iridium(IV) and the substrate, followed by the decomposition of the complex in the rate-determining step. Correlation with σ yielded ρ values of -4.0 and -3.5 which suggests the formation of a cationic intermediate in the rate-determining step of the reaction. Subsequent cleavage of the carbon-carbon bond yielded the product aldehydes. Thermodynamic and activation parameters associated with the equilibrium and the rate-determining steps were also evaluated.

Full text of DOI:10.1002/kin.10030

Oxidative Behavior and
Relative Reactivities
of Some Unsaturated
Compounds Towards
Hexachloroiridate(IV)
in Perchloric Acid Medium
KALYAN K. SEN GUPTA, NANDINI BHATTACHARJEE
Department of Chemistry, Jadavpur University, Calcutta 700032, India
Received 10 July 2000; accepted 5 October 2001
DOI 10.1002/kin.10030
ABSTRACT: The kinetics of the oxidation of styrene, cinnamic acid, and some of their substi-
tuted derivatives by hexachloroiridate(IV) in dimethyl formamide–water mixtures and in the
presence of perchloric acid have been investigated. The reactions appear to proceed via the
formation of an unstable intermediate 1:1 complex between iridium(IV) and the substrate, fol-
lowed by the decomposition of the complex in the rate-determining step. Correlation with
yielded values of 4.0 and 3.5 which suggests the formation of a cationic intermediate
in the rate-determining step of the reaction. Subsequent cleavage of the carbon–carbon bond
yielded the product aldehydes. Thermodynamic and activation parameters associated with the
C
equilibrium and the rate-determining steps were also evaluated. 2002 Wiley Periodicals, Inc.
Int J Chem Kinet 34: 411–417, 2002
on the oxidation of organic compounds like ␣-hydroxy
acids [13], aliphatic and aromatic alcohols [14,15],
diols [16,17], and aromatic aldehydes [18] by the oxi-
dant hexachloroiridate(IV). However, there are no lit-
erature data to date concerning the oxidation of unsat-
urated compounds by iridium(IV). The present report
deals with the reactivities of some unsaturated com-
pounds like styrene, cinnamic acid, and their substi-
tuted derivatives towards hexachloroiridate(IV). Since
the substrates are insoluble in water but soluble in
dimethyl formamide (DMF), the reactions have been
carried out in DMF–water mixtures in the presence of
perchloric acid.
INTRODUCTION
The kinetics and mechanism of the oxidation of com-
poundscontainingcarbon–carbondoublebondsbyvar-
ious oxidants have been studied. There are reports
of the oxidation of both styrene and cinnamic acid
by metal ion oxidants in acidic and alkaline media
[1–12]. Considerable work has also been carried out
Correspondence to: Kalyan K. Sen Gupta; e-mail: amigo@cal3.
vsnl.net.in.
Contract grant sponsor: University Grants Commission,
New Delhi.
c
2002 Wiley Periodicals, Inc.

412
SEN GUPTA AND BHATTACHARJEE
EXPERIMENTAL
Reagents
Styrene, trans-cinnamic acid, and their substituted
derivatives (Lancaster) and Na2IrCl6 (Aldrich) were
used. Hexachloroiridate solutions were standardized
spectrophotometrically at the absorption maximum for
2
IrCl6 [19]. All solutions were made in doubly dis-
tilled water. All inorganic materials were of the highest
available purities.
Kinetics and Measurements
The kinetics of the reaction was followed spectropho-
tometrically at 488 nm on a Systronics (India) spec-
2
trophotometer where IrCl6 absorbs to a consider-
ably greater extent than any of the other reactants
or products. The cell compartment of the spec-
trophotometer was kept at constant temperature. The
reactions were performed under pseudo-first-order
conditions with the concentrations of the substrates
in large excess over that of Ir(IV). Requisite volumes
of the temperature-equilibrated solutions of the reac-
tants were mixed and the mixture was immediately
transferred to a cell of path length 1 cm. Pseudo-first-
order rate constants (kobs) were calculated from log A
(A = absorbance) versus “time” plots (Fig. 1). The ab-
sorbance of the reaction mixture initially increases and
then decreases gradually, indicating the formation of
an intermediate complex between the oxidant and the
substrate. For most of the reactions, absorbance plotted
values of Fig. 1 when extrapolated meet the theoreti-
cal t = 0 values, indicating that no other processes oc-
cur before the formation of the intermediate. Duplicate
measurements were reproducible to 3%. The solvent
used was a DMF–water mixture in which the reaction
mixtures remained homogeneous.
Figure 1 Spectrophotometric evidence for intermediate
complex formation between styrene and Ir(IV) at 488 nm.
[IrCl6 ] = 1.15 10 4 mol dm 3, [H+] = 0.5 mol dm
,
,
2
3
2
Temp. = 318 K. [styrene] = (1) 0.5 10 2, (2) 0.7 10
2
(3) 1.0 10 2, (4) 1.2 10 2, and (5) 1.5 10
mol
3
dm
.
A solution of the reaction mixture was kept for 6 h at
roomtemperatureunderapproximatelythesameexper-
imental conditions as were used for the kinetic determi-
nations. After completion of the reactions, the solution
was extracted with chloroform and then the organic
layer was washed with water and dried over anhydrous
MgSO4. It was then evaporated under reduced pressure
to give a residue containing two products as confirmed
by TLC. The products were characterized by column
chromatography as described in an earlier report [20].
Benzaldehyde and glyoxylic acid were identified as the
products of cinnamic acid oxidation. The products of
styrene oxidation were found to be benzaldehyde and
formaldehyde. All the products were characterized as
their 2,4-DNP derivatives and their melting points were
checked against the literature values [21]. The yields
of benzaldehyde were found to be 75–80% while those
of glyoxylic acid and formaldehyde were 5–10%. The
lower yields (<100%) of the products may be due to the
fact that the carbonyl compounds are partially oxidized
to give the corresponding acids. Benzaldehyde was fur-
ther confirmed by 1H and 13C NMR spectroscopy [22].
Under the present experimental conditions there was no
further oxidation of benzaldehyde. Formate and mono-
oxalate ions were also found to be absent in the oxida-
tions of styrene and cinnamic acid respectively.
Product Analysis
Stoichiometric runs with an excess of hexachloroiri-
date(IV)gavea(1[Ir(IV)]/1[substrate])consumption
ratio of 4:1 consistent with Eqs. (1) and (2).
2
XC6H4CH CH2 + 4IrCl6 + 2H2O
= XC6H4CHO + HCHO + 4IrCl6 + 4H+
(X = OMe, Me, Cl)
(1)
3
X0C6H4CH CHCOOH + 4IrCl6 + 2H2O
2
= X0C6H4CHO + OCH COOH
+ 4IrCl6 + 4H+
(2)
3
(X0 = Me, Cl, NO2)

OXIDATIVE BEHAVIOR OF UNSATURATED COMPOUNDS TOWARDS IrCl6 (IV)
413
318 K respectively. The value of kobs is found to be
independent of the initial [Ir(IV)] in each case.
RESULTS AND DISCUSSION
The effect of variation of [substrate] on kobs
was also studied at constant [Ir(IV)] and [H+] of
1.15 10 4 mol dm 3 and 0.5 mol dm 3 respectively,
but at different temperatures (298–328 K). The value
of kobs increases with increase in the substrate con-
centration, and attains a limiting value at much higher
substrate concentrations, indicating the validity of the
rate expression (5). An attempt was made to study the
initial rate of formation alongside the rate of decom-
position of the initial reaction. The two values at each
substrate were found to be different. This is possibly
because the electron is not transferred directly from
the substrate to the oxidant but through the intermedi-
ate formation of the complex. The reactions obey the
following rate expression:
Spectrophotometric Evidence
for Complex Formation
An attempt was made to determine the equilibrium
constant (Ke) of the intermediate complex formed be-
tween styrene and Ir(IV) spectrophotometrically. The
difference in absorbance (1A) of two solutions, both
containing Ir(IV) and perchloric acid in DMF–water
mixture, and one of them furthermore styrene, was
measured. Ke was calculated using the relation (3) [23].
1A/Ca = Ke1εlC0 Ke1A
(3)
where l is the optical path length in centimeters, Ca
and C0 are the concentrations of styrene and Ir(IV)
respectively in mol dm 3, and 1ε is the difference in
the extinction coefficients of the complex and that of
Ir(IV) at 488 nm. The plot of 1A/Ca versus 1A yields a
straight line making an intercept on the y-axis (Fig. 2).
2
2
d[IrCl6
]
kd Ke[IrCl6 ][XC6H4CH CHR]
=
=
dt
1 + Ke[XC6H4CH CHR]
The value of Ke was calculated from the slope of this
plot and was found to be 58 dm3 mol
.
1
(4)
(5)
kd Ke[XC6H4CH CHR]
1 + Ke[XC6H4CH CHR]
kobs
Effect of Reactant Concentrations
The effect of oxidant concentrations on the pseudo-
where
first-order rate constant (kobs) was studied by varying
2
1
d[IrCl6
]
4
the initial [Ir(IV)] in the range (0.5–5.0) 10 mol
kobs
Therefore,
=
=
(6)
(7)
2
dm 3, keeping the [substrate], [H+], and temperature
[IrCl6
]
dt
3
constant at 2.0 10 mol dm 3, 0.5 mol dm 3, and
1
1
1
+
kobs
kd
kd Ke[XC6H4CH CHR]
A linear relation was obtained when 1/kobs was
plotted against 1/[XC6H4CH CHR]. Typical plots are
shown in Fig. 3. From the slope and intercept of each
plot of 1/kobs versus 1/[substrate], the value of Ke (the
equilibrium constant for complex formation) and kd
(the constant for the rate-limiting step) were calculated
at five different temperatures. The value of Ke (57 dm3
mol 1) obtained for the styrene–Ir(IV) complex cal-
culated from the double reciprocal plots of Fig. 3 is
in close agreement with the value calculated from the
slope of the plot of 1A/Ca versus 1A ( Fig. 2).
Effect of H+ Concentration
The effect of variation of [H+] on the rate of the reac-
tion was studied at [substrate], [Ir(IV)], ionic strength,
3
and temperature of 2.0 10
mol dm 3, 1.15
10 mol dm 3, 1.0 mol dm 3, and 318 K respec-
4
tively. The pseudo-first-order rate constant was inde-
Figure 2 Plot of 1ε/Ca vs 1ε for the styrene–iridium(IV)
complex at 318 K.
3
pendent of [H+] in the range 0.2–1.0 mol dm
.

414
SEN GUPTA AND BHATTACHARJEE
[Ir(IV)], [H+], and temperature. Increasing proportions
of DMF in the reaction mixture increased the value of
kobs. Plots of log kobs against the reciprocal of the di-
electric constant (1/D) were linear with positive slopes.
The dielectric constants for the DMF–water mixtures
were calculated from the dielectric constant values for
the pure solvents [20]. The decrease in the rate of the
reaction with increasing proportions of water (i.e., a
more polar medium) may be related to the conversion
of the substrate to a positively charged species. In Eq.
(11), given later, iridium(III) has been designated as
3
IrCl6 . It may be mentioned that the spectral pattern
2
of iridium(III) chloride is different from that of IrCl6
3
solution. Moreover, the IrCl6 ion and its hydrolyzed
products are transparent in the region of maximum ab-
2
sorption of IrCl6 . This is in keeping with the obser-
vations made earlier by Poulsen and Garner [27]. Irid-
ium(III) chloride in aqueous acid medium on further
aquationisknowntogiveIrCl5(H2O)2 , IrCl4(H2O)2
,
and IrCl3(H2O)3 [27,28]. Since the effect of Cl on the
rate is small, as has been mentioned in connection with
the salt effect, the existence of iridium(III) in the form
of aquo complexes cannot be ruled out and may ac-
count for a decrease in charge in the rate-determining
step. While the formation of these aquated species of
iridium(III) seems to explain the observed solvent ef-
fect satisfactorily, the possibility of other factors play-
ing a part remains a matter of speculation.
Figure 3 Variation of pseudo-first-order rate constant with
1
1
substrate concentrations. Plots of kobs
( ) and [cinnamic acid] (N) at different temperatures.
vs [styrene]
1
[IrCl6 ] = 1.15 10 4 mol dm 3, [H+] = 0.5 mol dm
.
2
3
Effect of Salt Concentration
Thermodynamic and Activation Parameters
The enthalpy change (1H0) associated with the equi-
librium step was calculated from the slope of the plot
of log Ke versus 1/T . The corresponding change in
entropy (1S0) was calculated from the relation
The effect of added salts to the reaction mixture was
investigated in the presence of different salts, such as
sodium perchlorate, sodium chloride, and potassium
chloride. The value of kobs increased with increase in
salt concentrations. Plots of kobs against [salt] indicate
that the effect of anions on the pseudo-first-order rate
constants is small. On the other hand, the values of
kobs are higher in KCl than in NaCl indicating that K+
ion has a significant and specific effect on the rate of
the reaction. A possible interpretation of the increase
log Ke = [1S0 (1H0/T )]/2.303R
(8)
The enthalpy of activation (1H=) was obtained from
the slope of the plot of log(kd/T ) versus 1/T , followed
by the evaluation of the entropy of activation (1S=),
using the theory of absolute reaction rates, the equation
being
2
in rate may be that the IrCl6 ion partially associates
with added K+, and the associated species in some way
2
becomes more reactive than the free IrCl6 ion. This
type of salt effect has also been observed in the oxi-
=
1H=/RT
1S /R)/Nh
(9)
2
kd = RT (e
e
dation of alcohols by IrCl6 [14]. Literature evidence
is also available on the K+ ion catalysis of some other
redox reactions [24–26].
where R, N, and h have their usual meanings.
The 1H0, 1S0, 1H=, and 1S= values for the dif-
ferent reactions are recorded in Table I. The enthalpy
of activation is linearly related to the entropy of acti-
vation (r = 0.998) for all the reactions studied which
suggests that the styrenes and cinnamic acids are all
oxidized by the same mechanism. The value of the
Effect of Solvent Polarity
The effect of variation of solvent polarity on the
pseudo-first-order rate constant was also studied for
the oxidations of cinnamic acid and styrene at constant

OXIDATIVE BEHAVIOR OF UNSATURATED COMPOUNDS TOWARDS IrCl6 (IV)
415
Table I Thermodynamic Data Associated with the Complex and Activation Parameters Associated with the Slowest
Steps
1H0
(kJ mol
1S0
(J K mol
1H=
(kJ mol
1S=
(J K mol
1
1
1
1
1
1
Substrate
)
)
)
)
Styrene
p-Methoxystyrene
p-Methylstyrene
m-Methylstyrene
p-Chlorostyrene
m-Chlorostyrene
12
19
16
14
10
8
11
15
13
10
7
2
72
86
78 13
74 13
73
76
71 13
75 13
72
74 13
76
74 13
76
7
7
72
55
60
65
93
108
80
64
70
90
116
128
148
2
64
7
2
4
4
2
2
4
4
2
4
2
4
2
4
4
4
2
2
2
4
4
2
4
4
4
101 13
92 13
78 13
21
19
41
79 13
66 13
28
7
7
7
7
7
Cinnamic acid
p-Methylcinnamic acid
m-Methylcinnamic acid
p-Chlorocinnamic acid
m-Chlorocinnamic acid
m-Nitrocinnamic acid
p-Nitrocinnamic acid
7
7
7
43 13
65 13
123 13
7
7
7
isokinetic temperature [29] was calculated from the
slope of 1S= versus 1H=, and is found to be 409 K.
Since is higher than T , the midpoint of the experi-
mentally used range of temperatures, this indicates that
the reactions are most likely to be enthalpy controlled
[30]. Although not much physical significance should
be attached to the isokinetic temperatures, a linear cor-
relation is usually a necessary condition for the validity
of the Hammett equation. The isokinetic behavior is
also supported by the linear plot of log kd0 versus log kd
(r = 0.999) where kd and kd0 are the constants for the
rate-determining steps at 313 and 318 K respectively.
The kinetics of the oxidation of five ring-substituted
styrenes and six ring-substituted cinnamic acids by
Hammett plots of log kd versus log Ke also exhibited
linear relationships with negative slopes. The values of
kd and Ke for styrene, cinnamic acid, and their substi-
tuted derivatives at 318 K are recorded in Table II.
2
IrCl6 were studied under comparable experimen-
tal conditions in order to determine the effect of sub-
stituents on the rate of the reaction. The kd values were
found to follow the order OMe > Me > H >
NO2 for the substituted styrenes and Me > H >
Cl > NO2 for the cinnamic acids. Correlation of the
kd values with [31] (the Hammett substituent con-
stants) gave values of 4.0 and 3.5 for the styrenes
and cinnamic acids respectively (Fig. 4) (r = 0.994
and 0.992). This suggests that the transition state pos-
sesses a large degree of carbonium ion character re-
sulting from a considerable amount of positive charge
developing at the carbon atom adjacent to the benzene
ring. Earlier studies have shown that values larger
than 3 suggest a fairly large degree of carbonium ion
character in the transition state [9,20]. The addition of
Figure 4 Plots of log kd vs Hammett
values for the
oxidations of substituted (a) styrenes and (b) cinnamic acids
(1, p-methoxystyrene; 2, p-methylstyrene; 3, m-methylsty-
rene; 4, styrene; 5, p-chlorostyrene; 6, m-chlorostyrene;
7, p-methylcinnamic acid; 8, m-methylcinnamic acid;
9, cinnamic acid; 10, p-chlorocinnamic acid; 11, m-chloro-
cinnamic acid; 12, m-nitrocinnamic acid; 13, p-nitrocin-
namic acid).
chlorine to cinnamic acid gave
=
4.01, indicating
a fully developed benzylic carbocation-like transition
state in the slow step of the reaction [32]. The plots of
log Ke versus for the styrenes and cinnamic acids
were found to be linear with positive slopes. The first

416
SEN GUPTA AND BHATTACHARJEE
2
Table II Values of kd and Ke for Styrene, Cinnamic
Acid, and Their Substituted Derivatives at 318 K
˙
XC6H4CH CHR + IrCl6 + H2O
OH
Ke
kd
fast
2
1
1
3
Substrate
(10 dm3 mol
)
(103 s
)
→ XC6H4CH CHR + IrCl6 + H+
(13)
Styrene
p-Methoxystyrene
p-Methylstyrene
m-Methylstyrene
p-Chlorostyrene
m-Chlorostyrene
0.57
0.22
0.27
0.42
1.64
4.36
0.71
0.30
0.45
1.51
4.67
5.01
6.80
5.01
OH OH
33.3
16.7
10.0
0.29
0.13
3.55
2
XC6H4CH CHR + 2IrCl6
OH OH
fast
→ XC6H4CHO + RCHO + 2IrCl6 + 2H+
3
Cinnamic acid
(14)
p-Methylcinnamic acid
m-Methylcinnamic acid
p-Chlorocinnamic acid
m-Chlorocinnamic acid
m-Nitrocinnamic acid
p-Nitrocinnamic acid
15.9
7.94
0.42
0.11
0.02
0.01
The linear reciprocal plots 1/kobs versus 1/[sub-
strate] may also be explained by postulating an alter-
native mechanism where IrCl6 dissociates to give
IrCl5 and Cl followed by the reaction of IrCl5 with
the substrates to give products. However, the addition
of chloride ion does not inhibit the rate of the reaction,
suggesting that IrCl5 is not the active oxidant.
2
The observed order of kd values for the substituted
styrenes and cinnamic acids further supports the forma-
tion of the benzylic carbocation in the rate-determining
step of the reaction. The electron-donating substituents
like OMe and Me that can stabilize a benzylic car-
bocation increase the rate of the reaction (higher kd val-
ues) while electron-withdrawing substituents like Cl
and NO2 decrease the reaction rate.
In earlier reports of the oxidation of the styrenes and
cinnamic acids by hexacyanoferrate [9] and quinolin-
ium dichromate [20], the formation of the benzylic car-
bocation was supported by inverse kinetic isotope ef-
fects which indicate that the benzylic C-atom produced
by the rate-determining step remained sp2 in character
while the terminal C-atom underwent a change in the
state of hybridization from sp2 to sp3.
It is likely that the reactions take place via the
formation of an intermediate 1:1 complex (A) be-
tween the substrate and the oxidant as shown in
Eqs. (10)–(14). There is one literature evidence regard-
ing seven-coordinated iridium(V) complex although
stable seven-coordinated iridium(IV) complexes are
not known [33]. However, the oxidation of a number of
organic compounds by iridium(IV) have been shown to
occur through the intermediate formation of unstable
seven-coordinated complexes between the substrates
and iridium(IV) [14,15]. The oxidations of olefins by
Ag+ and Pd(II) have also been reported to proceed via
initialmetalcomplexationwiththecarbon–carbondou-
ble bond [34–36]. The intermediate breaks down in the
slow step to give the radical ion XC6H4C+H C HR
with the reduction of Ir(VI) to Ir(III). The subsequent
conversion of the radical ion into a glycol takes place
in fast steps [37] [Eqs. (12) and (13)]. The glycol
formed undergoes rapid carbon–carbon bond cleavage
Based on the kinetic information, the following
mechanism is suggested for the oxidation of these un-
2
saturated compounds by IrCl6
.
2
IrCl6
2
.
.
.
in presence of IrCl6 to give the respective aldehy-
Ke
2
des [Eq. (14)]. Earlier reports have established that the
cleavage of 1,2-glycols yields aldehydes as the major
products [38,39]. The observed order with respect to
each reactant and the products obtained corroborate
the suggested mechanism.
XC6H4CH CHR + IrCl6
[XC6H4CH CHR]
fast
(A)
(10)
(R = H for styrene
=
COOH for cinnamic acid)
We thank the reviewers for their helpful comments.
+
kd
3
˙
(A) → XC6H4CH CHR + IrCl6
(11)
(12)
slow
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XC6H4CH CHR + H2O
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