v outer-sphere electron transfer reactions involving...

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.


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


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%.


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.


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


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).


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


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


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.


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


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.


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


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)


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+


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


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+


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


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


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


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


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


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


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


125

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