Outer-sphere electron transfer reactions involving surfactant-cobalt(iii) complexes and Fe(CN) 6 4- ion

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1 V Outer-sphere electron transfer reactions involving surfactant-cobalt(iii) complexes and Fe(CN) 6 ion 5.1. Introduction There are many reports available on the reaction between Fe(CN) 6 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 ionpairing of the complexes with macrocycle STCA accelerates the FeCN 6 -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.

2 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) 6 in self micelles of the surfactant-complexes. 102

3 The surfactant-cobalt(iii) complexes studied are : cis-[co(en) 2 (C 12 H 25 NH 2 ) 2 ] (ClO 4 ) 3 cis-α-[co(trien)(c 12 H 25 NH 2 ) 2 ](ClO 4 ) 3 cis-[co(bpy) 2 (C 12 H 25 NH 2 ) 2 ](ClO 4 ) 3 3H 2 O cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ](ClO 4 ) 3 3H 2 O In contrast to the reactants mentioned in chapter IV (both with positive charges), this chapter concerns with reactants of oppposite charges Experimental 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 ± C. 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 ± C. The decrease in the absorbance was followed at 421 nm for the complexes containing bipyridine and phenanthroline 103

4 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(CN 6 ) in excess over cobalt(iii) complex. The concentration of Fe(CN 6 ) 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 to moldm -3 region. The ionic strength was maintained at 1.0 mol dm -3 in all runs using NaNO 3. The second-order rate constant, k, for the Fe(CN) 6 reduction of the cobalt(iii) complex defined by d[co(iii)]/dt = k[co(iii)][fe(cn) 6 ] was calculated from the concentration of Fe(CN) 6 and the slope of the log (A t -A α ) versus time plot, which is equal to k [ Fe(CN) 6 ] / 2.303, where A t is the absorbance at time t, A α, 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 A α was measured at times corresponding to 10 half-lives. All the first-order plots were substantially linear for at least five halflives. 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%. 104

5 Nature of the reaction On mixing Fe(CN) 6 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 25 0 C is shown in Fig The reaction is represented as Cobalt(III) complex + Fe(CN) 6 Co aq 2+ + Fe(CN) protonated amines and the rate is given by, rate = k[cobalt(iii) complex] [Fe(CN) 6 ] where k is the second order rate constant. 105

6 5.3. Results and Discussion Effect of variation of initial concentration of surfactant-cobalt(iii) complexes The reduction of surfactant-cobalt(iii) complexes by Fe(CN) 6 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 [Co(en) 2 (DA) 2 ] 3+ + Fe(CN) 6 K IP [Co(en) 2 (DA) 2 ] 3+ ; Fe(CN) 6 [Co(en) 2 (DA) 2 ] 3+ ; Fe(CN) 6 k et [Co(en) 2 (DA) 2 ] 2+ ; Fe(CN) 6 3- [Co(en) 2 (DA) 2 ] 2+ ; Fe(CN) 6 3- Fast Products en : ethylenediamine, DA: dodecylamine 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 106

7 on increasing with increase in the initial concentration of the complex from to 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) 6 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). 107

8 Table 5.1 k mol -1 dm 3 s -1 Oxidizing agent [Complex] 10 4 mol dm K 303K 313K cis-[co(en) 2 (C 12 H 25 NH 2 ) 2 ] cis-α-[co(trien)(c 12 H 25 NH 2 ) 2 ] k 10 2, mol -1 dm 3 s cis-[co(bpy) 2 (C 12 H 25 NH 2 ) 2 ] cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ]

9 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) 6 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 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(CN 6 ) have also been investigated. In the presence of cyclodextrin media the reduction of the surfactant-cobalt(iii) complexes with Fe(CN 6 ) 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 109

10 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) Fe 2+ aq versus Fe(CN) 6 as reductant : A comparison Previous chapter explains the kinetics of reduction of the same surfactantcobalt(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 () 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. 110

11 Table 5.2 Oxidizing agent [β-cd] 10 3, mol dm -3 k 10 3 mol -1 dm 3 s cis-[co(en) 2 (C 12 H 25 NH 2 ) 2 ] cis-[co(trien)(c 12 H 25 NH 2 ) 2 ] cis-[co(bpy) 2 (C 12 H 25 NH 2 ) 2 ] cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ]

12 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 k B /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 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 ) indicating that a common mechanism exists in all the initial concentrations of the complexes studied. 112

13 Table 5.3 Oxidizing agent [Complex] x 10 4 moldm -3 H kjmol -1 S JK cis-[co(en) 2 (C 12 H 25 NH 2 ) 2 ] cis-[co(trien)(c 12 H 25 NH 2 ) 2 ] cis-[co(bpy) 2 (C 12 H 25 NH 2 ) 2 ] cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ] Complex: surfactant-cobalt(iii) complex ion 113

14 Repetitive scan for the reduction of cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ] 3+ by Fe(CN) 6 at C [complex] = 4x10-4 mol dm -3, Fe(CN) 6 = 0.01 mol dm -3 cycle time = 60 s 2 absorbance wavelength(nm) Fig

15 Plot of k against cobalt complex ion under various temperatures viz 298, 303, 308 K. [Fe(CN) 6 ] = 0.01 mol dm -3, µ = 1.0 moldm K 303 K 308 K kx10 2 mol -1 dm 3 S [Cobalt(III) complex]x10 4 moldm -3 (a) cis-[co(en) 2 (C 12 H 25 NH 2 ) 2 ]

16 K 303 K 308 K 12 kx10 2 mol -1 dm 3 S [Cobalt(III) complex]x10 4 moldm -3 (b) cis-[co(trien)(c 12 H 25 NH 2 ) 2 ] 3+ Fig

17 K 303 K 308 K 10 kx10 2 mol -1 dm 3 S [Co(III)]x10 4 M (a) cis-[co(bpy) 2 (C 12 H 25 NH 2 ) 2 ]

18 K 303 K 308 K kx10 2 mol -1 dm 3 S [Co(III)]x10 4 M (b) cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ] 3+ Fig

19 Plot of [β-cd] vs k for (a) cis-[co(en) 2 (C 12 H 25 NH 2 ) 2 ] 3+ (b) cis-[co(trien)(c 12 H 25 NH 2 ) 2 ] k x10 3 mol -1 dm 3 S (a) (b) [[ß-CD]x10 4 moldm -3 Fig

20 Plot of [β-cd] vs k for a) cis-[co(bpy) 2 (C 12 H 25 NH 2 ) 2 ] 3+ b) cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ] 3+ with concentration k x10-3 mol dm -3 S (a) (b) [[ß-CD]x10-4 moldm -3 Fig

21 Isokinetic plot of the activation parameters for the reduction of cis-co(en) 2 (C 12 H 25 NH 2 ) 2 ] 3+ by Fe(CN) H # kjmol S # JK -1 mol -1 Fig

22 Isokinetic plot of the activation parameters for the reduction of cis-[co(trien)(c 12 H 25 NH 2 ) 2 ] 3+ by Fe(CN) H # kjmol S # JK -1 mol -1 Fig

23 Isokinetic plot of the activation parameters for the reduction of cis-[co(bpy) 2 (C 12 H 25 NH 2 ) 2 ] 3+ by Fe(CN) 6 20 H # kjmol S Jmol -1 K -1 Fig

24 Isokinetic plot of the activation parameters for the reduction of cis-[co(phen) 2 (C 12 H 25 NH 2 ) 2 ] 3+ by Fe(CN) H kjmol S Jmol -1 K -1 Fig

25 References 1. P.Rillema, J.F. Endicott, R.C.Patel, J.Am.Chem.Soc., 1972, 94, R.Van Eldik, H.Kelm, Inorg.Chim.Acta., 1982, 73, K. Kustin, I.R.Epstein, J.Chem Soc. Dalton Trans., 1990, R. Larrson, Acta.Chem.Scand., 1967, 21, 257. Hammorstrom, J. Sauvage, J.Am.Chem.Soc., 2002, 124, A.R. Mustafina, V.G. Shtyrin, L.Y. Zakharova, V.V. Skripacheva, R.R.Zairov, S.E. Soloreva, I.S.Antipen and A.I. Konovalov, J.Incl.Phenom.Macrocycl.chem., 2007, 29, A.J.miralles, R.E.Armstrong and A.Haim, J.Am.Chem.Soc., 1977, 99, A.J.Miralles, A.P.Szecsy and A.Haim, Inorg.Chem., 1982, 21, A.A.Holder, T.P.Dasgupta, Inorg.Chim.Acta., 2002, 331, A.P.Szecsy and A.Haim, J.Am.Chem.Soc., 1981, 103, J.J.Jwo, P.L. Gaus and A.Haim, J.Am.Chem.Soc., 1979, 101, M.Martinez, M.A. Pitarque, R.V.Eldik, Inorg.Chim.Acta., 1997, 256, J. K. J. Salem, R. M. Baraka, T. M. Haboush, A. A. S. El Khaldy, Tenside Surfactants Deterg., 2002, 39, Gaswick, D.; Haim,A. J.Am.Chem.Soc., 1971, 93, N.Arulsamy, D.S.Bhole, P.A.Goodson, D.A.Jaeger, V.B.Reddy, Inorg. Chem., 2001,40, Attwood,J.L.; Davies,J.E.D.; MacNicol,D.D.; Vogtle,F.; Lenin,J.M. Comprehensive Supramolecular chemistry Ed, Elsiever Science Ltd, Oxford 1993;Vol F. Cramer, W.Saenger. H.Spatz, J.Am.Chem.Soc., 1967, 89, M.Bender, L.Komiyama, Cyclodextrin Chemistry, Springer-Verlag, Berlin, R.J.Clarke, J.H.Coates, S.F. Lincoln,Adv.Carbohydr.Chem.Biochem., 1988, 46,

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