-
V Outer-sphere electron transfer reactions involving
surfactant-cobalt(III) complexes and Fe(CN)64- ion
5.1. Introduction
There are many reports available on the reaction between
Fe(CN)64- and
metal complexes [1-4]. A.R.Mustafina et al. [5] have studied the
outer-sphere
association of p-sulfanotothiacalix[4]arene with some
cobalt(III) complexes. The ion-
pairing of the complexes with macrocycle STCA accelerates the
FeCN64--cobalt(III)
electron transfer reactions. A.J.Miralles et al. [6,7] have
reported the outer-sphere
reductions of pyridinepentamminecobalt(III) and
pyridinepentammineruthenium(III) by hexacyanoferrate(II). They
have discussed
the mechanisms of these reactions on the basis of Marcus’
equation, electrostatic
effects and orbital considerations.
A.A.Holder [8] has done work on the kinetics and mechanism of
the
reduction of the molybdatopentamminecobalt(III) ion by aqueous
sulfite and
aqueous potassium hexacyanoferrate(II). The mechanism of the
reaction has been
confirmed as outer-sphere mechanism. A.P.Szecsy and A.Haim [9]
have studied
the intramolecular electron transfer between
pentacyanoferrate(II) and
pentammine cobalt(III) complexes containing imidazole and its
conjugate base.
They have proposed that the mechanism of the reaction have gone
through the
imidazolate bridge.
-
Chapter V Outer………..Fe(CN)64-anion
102
Jing-Jer Jwo et al. [10] have worked on the intramolecular
electron transfer
between pentammine cobalt(III) mediated by various
4,4’-bipyridines and
pentacyanoferrate(II). It has been suggested that the
conjugation between the two
pyridine rings is essential for electron transfer mediated by
the ligand. When the
two rings are separated by each other by insulating methylene
groups, electron
transfer through the ligand is precluded but ligands that permit
close approach of
the metal centres lead to intramolecular, outer-sphere electron
transfer reaction.
M.Martinez et al [11] have studied the outer-sphere reactions
of
(N)5 – macrocyclic cobalt(III) complexes.
J.K.J.Salem et al.[12] have studied the effects of anionic
micelles on the
oxidation of phenylhydrazine by hexacyanoferrate(III) in aqueous
urea solutions.
The abstraction of an electron from phenyl hydrazine by
[Fe(CN)6]3- is impeded
by anionic micelle-forming agents (for example, SDS). The extent
of the
inhibition depends on the concentration of the substrate in the
proximity of the
micellar surface. Thus, addition of urea causes a displacement
of the substrate
from the Stern layer, and leads to a further retardation of the
oxidation reaction.
This chapter deals with the kinetics of the outer-sphere
electron transfer reaction
between the same cobalt(III)-surfactant complexes mentioned in
Chapter IV and
Fe(CN)64- in self micelles of the surfactant-complexes.
-
Chapter V Outer………..Fe(CN)64-anion
103
The surfactant-cobalt(III) complexes studied are :
cis-[Co(en)2(C12H25NH2)2] (ClO4)3
cis-α-[Co(trien)(C12H25NH2)2](ClO4)3
cis-[Co(bpy)2(C12H25NH2)2](ClO4)3 ּ3H2O
cis-[Co(phen)2(C12H25NH2)2](ClO4)3 ּ3H2O
In contrast to the reactants mentioned in chapter IV (both with
positive charges),
this chapter concerns with reactants of oppposite charges.
5.2. Experimental
5.2.1. Kinetic Measurements
The rate of the reaction was measured spectrophotometrically
using a
Varian cary 500 scan UV-Vis-NIR spectrophotometer equipped with
the water
peltier system (PCB 150). The temperature was controlled within
±0.010C. A
solution containinig the desired concentration of potassium
ferrocyanide, sodium
nitrate and disodium salt of ethylenediamine teraacetic acid
(Sigma Aldrich and
Merck) in oxygen free water was placed in a 1 cm cell which was
then covered
with a serum cap fitted with a syringe needle. This cell was
placed in a
thermostated compartment in the spectrophotometer and then the
solution
containing the surfactant-cobalt(III) complex was added
anaerobically using the
syringe. The kinetics was followed on a varian cary 500 Scan
Uv-Vis-NIR
spectrophotometer equipped with water peltier system (PCB 150).
The
temperature was controlled within ±0.01 0C. The decrease in the
absorbance was
followed at 421 nm for the complexes containing bipyridine and
phenanthroline
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Chapter V Outer………..Fe(CN)64-anion
104
ligands and at 423 nm for the complexes containing
ethylenediamine and
triethylenetetramine as ligands,. All kinetic measurements were
performed under
pseudo-first order conditions with the Fe(CN6)4- in excess over
cobalt(III)
complex. The concentration of Fe(CN6)4- used was 0.01 mol dm-3
and the
concentration of surfacatant-cobalt(III) complex was always
chosen typically
above their CMC values in the 3×10-4 to 7×10-4 moldm-3 region.
The ionic strength
was maintained at 1.0 mol dm-3 in all runs using NaNO3. The
second-order rate
constant, k, for the Fe(CN)64- reduction of the cobalt(III)
complex defined by
– d[Co(III)]/dt = k[Co(III)][Fe(CN)64-] was calculated from the
concentration of
Fe(CN)64- and the slope of the log (At -Aα) versus time plot,
which is equal to – k
[ Fe(CN)64-] / 2.303, where At is the absorbance at time t, Aα ,
the absorbance after
all the cobalt(III) complex has been reduced to cobalt(II), and
k, the second order
rate constant. Usually the value of Aα was measured at times
corresponding to 10
half-lives. All the first-order plots were substantially linear
for at least five half-
lives. Each rate constant reported was the average result of
triplicate runs. Rate
constants obtained from successive half-life values within a
single run agreed to
within ±5%.
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Chapter V Outer………..Fe(CN)64-anion
105
5.2.2. Nature of the reaction
On mixing Fe(CN)64- and surfactant–cobalt(III) complex in
aqueous
solution a precipitate was formed and therefore, homogeneous
kinetic
measurements were precluded. When disodium salt of
ethylenediamine teraacetic
acid was present in the solution to sequester the cobalt(II), no
precipitate was
formed during the reaction and therefore all the experiments
were carried out in
the presence of disodium salt of ethylenediamine teraacetic acid
[13]. Disodium
salt of ethylenediamine teraacetic acid acted as a sequestering
agent to remove
cobalt(II) and prevented the precipitation of cobalt(II) ion as
a hexacyanoferrate
salt. A repetitive scan of the spectrum during the reaction time
at 250 C is shown
in Fig. 5.1.
The reaction is represented as Cobalt(III) complex + Fe(CN)64- →
Coaq2+ + Fe(CN)63- + protonated amines
and the rate is given by ,
rate = k[Cobalt(III) complex] [Fe(CN)64-]
where k is the second order rate constant.
-
Chapter V Outer………..Fe(CN)64-anion
106
5.3. Results and Discussion 5.3.1. Effect of variation of
initial concentration of surfactant-cobalt(III) complexes
The reduction of surfactant-cobalt(III) complexes by Fe(CN)64-
is
postulated as outer-sphere in comparison to such type of
reactions in the literature
[14] involving ordinary lower primary amine coordinated
cobalt(III) complexes
similar to our surfactant-cobalt(III) complexes. Accordingly the
mechanism which
consists of three elementary steps are delineated in Scheme
1
Scheme 1
The observed second order rate constants ks’, are given in Table
5.1 under various
initial concentrations of the surfactant–cobalt(III) complexes,
at 298, 303, 308 K
in aqueous solution. As seen from this Table the rate constant
of the reaction goes
[Co(en)2(DA)2]3+ + Fe(CN)64- [Co(en)2(DA)2]3+ ; Fe(CN)6
4-KIP
[Co(en)2(DA)2]3+ ; Fe(CN)6 4-ket
Products
[Co(en)2(DA)2]2+ ; Fe(CN)6 3-
[Co(en)2(DA)2]2+ ; Fe(CN)6 3- Fast
en : ethylenediamine, DA: dodecylamine
-
Chapter V Outer………..Fe(CN)64-anion
107
on increasing with increase in the initial concentration of the
complex from 3×10-4
to 7×10-4 mol dm-3. As this concentration range is very much
higher than the
critical milcelle concentration values [15] of these surfactant
complexes all these
rate constant values correspond to the rate constant values in
self-micelles formed
from these metal complex molecules themselves. Attempts to
perform the kinetics
of the same reaction at below the cmc of the
surfactant-cobalt(III) complexes were
unsuccessful as the reaction was so slow that there was no
change in the
absorbance was noted with time. So it is concluded that the rate
constants reported
in the present work correspond only to the reaction between
micellized cobalt(III)
complex and Fe(CN)64- and because of this such a peculiar
behaviour of
dependence of second order rate constant on the initial
concentration of one of the
reactants has been observed (Fig. 5.2 to 5.3).
-
Chapter V Outer………..Fe(CN)64-anion
108
Table 5.1
k mol-1 dm3s-1 Oxidizing agent [Complex] × 104 mol dm-3 298K
303K 313K 3 0.9 3.2 5.9 4 1.5 6.8 7.6 cis-[Co(en)2(C12H25NH2)2]3+ 5
2.6 8.3 11.9 6 4.5 10.5 14.0 7 7.3 11.3 15.6 3 2.8 7.8 10.2 4 3.9
9.0 11.0 cis-α-[Co(trien)(C12H25NH2)2]3+ 5 4.3 10.0 12.3 6 5.3 11.0
13.0 7 6.4 12.1 15.2 k×102, mol-1 dm3s-1 3 2.0 2.5 3.5 4 2.9 3.9
5.0 cis-[Co(bpy)2(C12H25NH2)2]3+ 5 3.8 4.8 6.0 6 6.0 7.2 7.8 7 7.6
8.5 12.5 3 1.8 2.0 2.7 4 2.5 3.1 3.8 cis-[Co(phen)2(C12H25NH2)2]3+
5 3.6 4.0 4.5 6 4.5 5.0 5.7 7 6.0 6.5 7.2
-
Chapter V Outer………..Fe(CN)64-anion
109
5.3.2. Effect of Nonbridging Ligand
The results for the effect of non-bridging ligand on the
reduction of the
surfactant-cobalt(III) complexes with Fe(CN)64- indicate that
the same trend as
observed in the case of Fe(II) ion as the oxidant (Chapter IV),
So the explanation
offered in chapter IV holds good here also.
5.3.3. Effect of β- Cyclodextrin
The cyclodextrins are naturally occurring receptors which can
alter the
physical properties and chemical reactivities of guest molecules
[16-18]. The
effects of presence of cyclodextrin in the medium on the
kinetics of the same
electron transfer reactions between the surfactant-cobalt(III)
complexes and
Fe(CN6)4- have also been investigated. In the presence of
cyclodextrin media the
reduction of the surfactant-cobalt(III) complexes with Fe(CN6)4-
proceed with
second order reaction and the results are listed in the Table
5.2. As seen from this
Table addition of increasing concentrations of cyclodextrin has
resulted in
significant decrease in the second order rate constant.
β-cyclodextrin breaks the
nature of the long aliphatic chains of surfactants which can be
included into the
cavities of cyclodextrin. The presence of long aliphatic chains
only favour the
formation of micelles which is inside the cavity will be
difficult leading to
increase of CMC values of surfactants in presence of
cyclodextrin. So in the
-
Chapter V Outer………..Fe(CN)64-anion
110
present study the decrease of rate constant with increase in the
concentration of
cyclodextrin in the media can be attributed to the inclusion of
long aliphatic chain
present in one of the ligands into cyclodextrin which ultimately
decreases the
micelle formation of the surfactant complexes leading to
lowering of rate constant.
This effect of presence of cyclodextrin in the media also
supports the earlier
conclusion on the effect of initial concentration of our
complexes on rate constant
(Fig. 5.4 & 5.5).
5.3.4. Fe2+aq versus Fe(CN)64- as reductant : A comparison
Previous chapter explains the kinetics of reduction of the same
surfactant-
cobalt(III) complexes by iron(II) ion in self-micelles. On
comparing the rate for these
reactions with the results obtained in the present study, the
rate constants for the reactions
with ferrocyanide ion is greater by one order of magnitude. This
may be attributed to the
negative charge (4-) present in the reductant molecules which
can be attracted towards
the self-micelles of surfactant-cobalt(III) complexes containing
a sheath of negative
charges on the surfaces of micelles, whose effect increases with
the increase in the initial
concentration of surfactant-cobalt(III) complexes.
-
Chapter V Outer………..Fe(CN)64-anion
111
Table 5.2
Oxidizing agent [β-CD]× 103 , mol dm-3 k× 103 mol-1 dm3s-1 2 9.9
4 5.2 cis-[Co(en)2(C12H25NH2)2]3+ 6 4.8 8 3.5 2 6.6 4 4.3
cis-[Co(trien)(C12H25NH2)2]3+ 6 3.8 8 3.2 2 5.1 4 3.6
cis-[Co(bpy)2(C12H25NH2)2]3+ 6 3.1 8 2.4 2 3.5 4 3.0
cis-[Co(phen)2(C12H25NH2)2]3+ 6 2.7 8 2.2
-
Chapter V Outer………..Fe(CN)64-anion
112
5.3.5. Activation parameters (∆S≠ and ∆H≠)
To obtain information about the energetics of a reaction, the
rate constant is
determined with the effect of temperature. the mechanism of the
reaction is deduced
indirectly from activation parameters. The effect of temperature
on rate was studied at
three different temperatures for each initial concentration of
the surfactant-cobalt(III)
complexes (Table5.3) viz., 298, 303 and 308 K in order to obtain
the activation
parameters for the reaction.
Using the Eyring equation shown below the values of ∆S≠ and ∆H≠
were
determined by plotting ln(k/T) vs 1/T.
ln k/T = ln kB/h + ∆S≠ / R - ∆H≠ /RT
The results are shown in Table 5.3. Though we expected an
increase of entropy in the
transition state due to charge neutralization process (union of
a positive charged oxidant
and negatively charged reductant) our ∆S≠ values reveal that the
entropy has decreased
(with a slight increase at lower concentration in the case of
bpy complex). This may be
due to released hydration water, on union of the reactants,
still binding on the Stern-layer
of the micellar surface.
5.3.6. Isokinetic Plots
The graphs plotted between enthalpy of activation versus entropy
of activation
values for the series of initial concentration of the complexes
give straight lines
(Fig. 5.6-5.9) indicating that a common mechanism exists in all
the initial concentrations
of the complexes studied.
-
Chapter V Outer………..Fe(CN)64-anion
113
Table 5.3
Complex: surfactant-cobalt(III) complex ion
Oxidizing agent [Complex] x 104 moldm-3 ∆H≠ kJmol-1 ∆S≠ JK-1 3
190.6 570 4 157.0 461 cis-[Co(en)2(C12H25NH2)2]3+ 5 119.3 338 6
82.4 218 7 54.9 130 3 78 204.8 4 74 192.4
cis-[Co(trien)(C12H25NH2)2]3+ 5 68 173.7 6 56 128.0 7 54 120.5 3
22.0 9.41 4 21.2 10.0 cis-[Co(bpy)2(C12H25NH2)2]3+ 5 17.8 0.68 6
12.4 -13.6 7 10.4 -18.4 3 17 -8.0 4 16 -11.0
cis-[Co(phen)2(C12H25NH2)2]3+ 5 9 -27.0 6 8 -28.9 7 7 -32.1
-
Chapter V Outer………..Fe(CN)64-anion
114
Repetitive scan for the reduction of
cis-[Co(phen)2(C12H25NH2)2]3+ by Fe(CN)64- at 25.00C [complex] =
4x10-4 mol dm-3 , Fe(CN)64- = 0.01 mol dm-3 cycle time = 60 s
Fig. 5.1
400 450 500 550
1
2ab
sorb
ance
wavelength(nm)
-
Chapter V Outer………..Fe(CN)64-anion
115
Plot of k against cobalt complex ion under various temperatures
viz 298, 303 , 308 K. [Fe(CN)64-] = 0.01 mol dm-3 , µ = 1.0
moldm-3
2 3 4 5 6 70
2
4
6
8
10
12
14
kx10
2 mol
-1dm
3 S-1
[Cobalt(III) complex]x104 moldm-3
298 K 303 K 308 K
(a) cis-[Co(en)2(C12H25NH2)2]3+
-
Chapter V Outer………..Fe(CN)64-anion
116
3 4 5 6 72
4
6
8
10
12
14
16kx
102 m
ol-1dm
3 S-1
[Cobalt(III) complex]x104 moldm-3
298 K 303 K 308 K
(b) cis-[Co(trien)(C12H25NH2)2]3+
Fig. 5.2
-
Chapter V Outer………..Fe(CN)64-anion
117
3 4 5 6 7
2
4
6
8
10
12
14kx
102 m
ol-1dm
3 S-1
[Co(III)]x104M
298 K 303 K 308 K
(a) cis-[Co(bpy)2(C12H25NH2)2]3+
-
Chapter V Outer………..Fe(CN)64-anion
118
3 4 5 6 7
2
4
6
8kx
102 m
ol-1dm
3 S-1
[Co(III)]x104M
298 K 303 K 308 K
(b) cis-[Co(phen)2(C12H25NH2)2]3+
Fig. 5.3
-
Chapter V Outer………..Fe(CN)64-anion
119
Plot of [β-CD] vs k for (a) cis-[Co(en)2(C12H25NH2)2]3+ (b)
cis-[Co(trien)(C12H25NH2)2]3+
2 4 6 82
4
6
8
10
k x1
03 m
ol-1 d
m3 S
-1
[[ß-CD]x104 moldm-3
(b)
(a)
Fig.5.4
-
Chapter V Outer………..Fe(CN)64-anion
120
Plot of [β-CD] vs k for a) cis-[Co(bpy)2(C12H25NH2)2]3+ b)
cis-[Co(phen)2(C12H25NH2)2]3+ with concentration
2 3 4 5 6 7 82.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
(b)
(a)
k x1
0-3 m
ol d
m-3S-
1
[[ß-CD]x10-4 moldm-3
Fig.5.5
-
Chapter V Outer………..Fe(CN)64-anion
121
Isokinetic plot of the activation parameters for the reduction
of cis-Co(en)2(C12H25NH2)2]3+ by Fe(CN)64-
50 100 150 200
100
200
300
400
500
600
Fig. 5.6
∆H# k
Jmol
-1
∆S# JK-1 mol-1
-
Chapter V Outer………..Fe(CN)64-anion
122
Isokinetic plot of the activation parameters for the reduction
of cis-[Co(trien)(C12H25NH2)2]3+ by Fe(CN)64-
55 60 65 70 75 80
120
140
160
180
200
220
Fig. 5.7
∆H# k
Jmol
-1
∆S# JK-1 mol-1
-
Chapter V Outer………..Fe(CN)64-anion
123
Isokinetic plot of the activation parameters for the reduction
of cis-[Co(bpy)2(C12H25NH2)2]3+ by Fe(CN)64-
Fig.5.8
-18 -12 -6 0 6 12
10
15
20
∆S≠ Jmol-1K-1
∆H# k
Jmol
-1
-
Chapter V Outer………..Fe(CN)64-anion
124
Isokinetic plot of the activation parameters for the reduction
of cis-[Co(phen)2(C12H25NH2)2]3+ by Fe(CN)64-
Fig. 5.9
-35 -30 -25 -20 -15 -10 -56
9
12
15
18
∆S≠ Jmol-1K-1
∆H≠ k
Jmol
-1
-
Chapter V Outer………..Fe(CN)64-anion
125
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