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DOI: 10.1039/C5RA02829A (Paper) RSC Adv., 2015, 5, 48079-48085

A comparative study on the electron transfer reaction (ETR) of surfactant cobalt(III) complexes of aliphatic/aromatic ligands in micro heterogeneous media: a thermodynamic approach†

Kannan Sugumara, Karuppiah Nagaraj*a, Krishnan Senthil Muruganb, Pilavadi Thangamuniyandic, Subramanian Sakthinathana and Pakkiri Vijayakumara
aSchool of Chemistry, Bharathidasan University, Tiruchirapalli 620024, Tamilnadu, India. E-mail: naturalnagaraj@gmail.com; Tel: +91 9677836849
bPost Graduate and Research Department of Chemistry, Vivekananda College, Tiruvedakam West, Madurai 625234, Tamilnadu, India
cDepartment of Polymer Science, University of Madras, Guindy Campus, Chennai 600 025, Tamilnadu, India

Received 13th February 2015 , Accepted 12th May 2015

First published on 14th May 2015

Abstract

The kinetics of the electron transfer reaction (ETR) between the surfactant cobalt(III) complex ions, cis-[Co(ip)2(C12H25NH2)2](ClO4)3, cis-[Co(dpq)2(C12H25NH2)2](ClO4)3 and cis-[Co(dpqc)2(C12H25NH2)2](ClO4)3 (ip = imidazo[4,5-f][1,10]phenanthroline, dpq = dipyrido[3,2-d:2′-3′-f]quinoxaline, dpqc = dipyrido[3,2-a:2′,4′-c](6,7,8,9-tetrahydro)phenazine, C12H25NH2 = dodecylamine) and Fe2+ ions in micelles as well as β-cyclodextrin (β-CD) were studied at different temperatures by a spectrophotometric method under pseudo first order conditions with an excess of reductant. The results from surfactant complexes containing aromatic ligands, which have a higher ETR than that of aliphatic ligands due to the results obtained, have been explained based on the hydrophobic effect. Experimentally the reactions were found to be second order and the electron transfer is postulated as outer sphere. The rate constant increases with increase in the concentration of micelles but the inclusion of the long aliphatic chain of the surfactant cobalt(III) complexes into β-cyclodextrin decreases the rate of the reaction. Thermodynamic parameters were also evaluated.

Introduction

Outer-sphere electron transfer involving transition metal complexes plays an essential role both in vivo1 and in the operation of molecular scale devices such as molecular wires and logic gates.2 Surface-active materials are the major building blocks of many physical, chemical, and biological systems. They have been introduced into several commercial products such as antiseptic agents and germicides,3 and have also found a wide range of applications in diverse areas such as mining, petroleum, and pharmaceutical industries. It has been observed that redox reactions in micellar media can be influenced by hydrophobic and electrostatic forces, and for a given set of reactions, the observed rate depends on the extent of the association between the reactants and micellar aggregates.4 Electron transfer reactions of cobalt(III) complexes are very well known because the kinetics of the reduction of octahedral cobalt(III) complexes are mostly free from complications that arise from reversible electron transfer, aquation, substitution and isomerization reactions. Octahedral cobalt(III) complexes, which are substitutionally inert, are ideal for theoretical and experimental studies. Surfactant cobalt(III) complexes have received a sustained high level of attention for the last few years due to their relevance to various redox processes in biological systems, and their potential as anthelmintic and antibiotic agents.5 Numerous studies have addressed the dependence of electron transfer on different environments, including micelles,6 vesicles7 and DNA.8 Redox processes occurring in biological systems are controlled both by the specific geometry of the inner coordination sphere, which mainly controls the operation potential of the metal center, and by the hydrophobic effect offered by pseudo biological interfaces. Electron transfer in these restricted geometry systems attracts a great deal of interest9,10 because of its potential to prolong the lifetime of charge-transfer states, which is a goal of electron-transfer studies that aim to utilize solar energy.11 Majumdar and Mahapatra12 studied electron transfer reactions between cobalt(III) and iron(II) complexes in a cationic micellar medium (N-cetyl-N,N,N trimethyl ammonium bromide) and also in a reverse micellar medium. The rate of the reaction was found to mainly depend on the inter-phase environment of the micelles and reverse micelles. The micelle-forming properties and electron transfer reactions of many surfactant metal complexes have been studied in our laboratory.13–16 Recently, we reported on the outer-sphere electron transfer reactions between cis-[Co(en)2(C12H25NH2)2](ClO4)3, cis-[Co(trien)(C12H25NH2)2](ClO4)3 and iron(II) in micelles formed by these complexes themselves.17,18 In all these surfactant metal complexes a coordination complex containing a central metal ion with surrounding ligands coordinated to the metal acts as a surfactant. In recent times there have been some reports from various research groups on surfactant metal complexes of various natures and their micelle forming properties.19–22 In all these surfactant metal complexes, the metal complex part containing the central metal ion with its primary coordination sphere acts as the head group and the hydrophobic part of one or more ligands acts as the tail part. Cyclodextrins (CD) are cyclic polysugars that are composed of glucose units linked by 1-4α glycoside bonds.23–26 Their hydrophilic (water soluble) outer surface and (water insoluble) hydrophobic cavities provide different environments, which change the photophysicochemical properties of guest molecules. When a substrate comes into a constrained cyclodextrin cavity, the polarity of the guest molecule is rearranged and its selectivity and photoreactivity are also modified.27,28 The effects of cyclodextrin inclusion on the kinetics and mechanism of ligand substitution29,30 and electron transfer reactions of transition metal complexes in aqueous solution31,32 have received considerable attention in recent years. The inclusion complexation of these host–guest systems occurs through various weak interactions such as hydrogen bonding, van der Waals, electrostatic or hydrophobic interactions.33

In this study, we report a further investigation on the effect of increasing the hydrophobicity of complexes on the kinetics of outer-sphere electron transfer between some surfactant cobalt(III) complexes containing polypyridyl ligands and iron(II) in aqueous media. As these complexes themselves form micelles we conducted the reactions in micelles created by the surfactant metal complex molecules themselves. We also studied the inclusion effect of β-cyclodextrin on these reactions. The surfactant complex ions used in the present study are cis-[Co(ip)2(C12H25NH2)2]3+, cis-[Co(dpq)2(C12H25NH2)2]3+ and cis-[Co(dpqc)2(C12H25NH2)2]3+ (ip = imidazo[4,5-f][1,10]phenanthroline, dpq = dipyrido[3,2-d:2′-3′-f]quinoxaline, dpqc = dipyrido[3,2-a:2′,4′-c](6,7,8,9-tetrahydro)phenazine, C12H25NH2 = dodecylamine).

Experimental

Materials and methods

Electrolytic grade iron powder (Loba Chemie) and perchloric acid (Loba Chemie) were used to prepare iron(II) perchlorate and to maintain an acidic medium. β-Cyclodextrin (β-CD) was purchased from Sigma-Aldrich Chemical Co. (Bangalore, India) and was used as received. To prepare buffer solutions, anhydrous dibasic sodium phosphate (Na2HPO4) and sodium dihydrogen orthophosphate (NaH2PO4·2H2O) were used. All solvents used were of analytical grade.

Preparation of oxidant/reductant

The surfactant cobalt(III) complexes, cis-[Co(ip)2(C12H25NH2)2](ClO4)3, cis-[Co(dpq)2(C12H25NH2)2](ClO4)3 and cis-[Co(dpqc)2(C12H25NH2)2](ClO4)3 were used as oxidants and were prepared as reported by us earlier.34 A stock solution of Fe(ClO4)2 was prepared by dissolving pure iron powder in a slight excess of perchloric acid. The concentration of Fe2+ ions was determined by a method similar to that reported in the literature35 and the ionic strength of the solution was adjusted by the addition of sodium perchlorate solution.

Kinetic measurements

The rate of the reaction was measured spectrophotometrically using a Shimadzu-1800 UV-visible spectrophotometer equipped with a water Peltier system (PCB 150). The temperature was controlled within ±0.01 °C. A solution containing the desired concentration of β-cyclodextrin and surfactant cobalt(III) complex 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 Fe2+ was added anaerobically using the syringe. The reaction was followed by measuring the absorption of the surfactant cobalt(III) complex with time. The decrease in the absorbance of the complexes was followed at 470 nm. All kinetic measurements were performed under pseudo-first order conditions with Fe2+ ions in excess over the cobalt(III) complex. The concentration of Fe(ClO4)2 used was in the 0.1 mol dm−3 region. The ionic strength was maintained at 1.0 mol dm−3 and the concentration of the cobalt(III) complexes was always chosen in a region above their CMC values.34 The second-order rate constant, k, for the Fe2+ ion reduction of the cobalt(III) complex as defined by d[Co(III)]/dt = k[Co(III)][Fe2+] was calculated from the concentration of iron(II) and the slope of the pseudo first order plot of log(At − A∞) versus time, which is equal to −k[Fe2+]/2.303, where At is the absorbance at time t, A∞ is the absorbance after all the cobalt(III) complex has been reduced to cobalt(II), and k is the second-order rate constant. Usually the value of A∞ is measured at times corresponding to ten half-lives. All the first-order plots were substantially linear for at least five half-lives, with a correlation coefficient of >0.999. Each rate constant reported is the average result of triplicate runs. Rate constants obtained from successive half-life values within a single run agreed within ±5%. No trend indicative of systematic error was noted, and the average values did not differ significantly from those obtained from the least-squares treatment of the logarithmic plots of absorbance difference against reaction time.

Stoichiometry

The stoichiometry of the reaction was determined by estimating the amount of iron(III) and cobalt(II) present in the product mixture. Iron(III) was determined spectrophotometrically by Kitson's method36 and Co(II) was determined as [CoCl4]2− at 690 nm in an excess of HCl.37 The ratio of Fe(III) to Co(II) was found to be 1[thin space (1/6-em)]:[thin space (1/6-em)]1 in the reactions studied, indicating a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometry.

Results and discussion

Electron-transfer kinetics

The reduction of the surfactant cobalt(III) complexes, cis-[Co(LL)2(DA)2]3+ by Fe2+, proceeds according to the overall reaction as indicated below:
cis-[Co(LL)2(C12H25NH2)2]3+ + Fe2+ → Coaq2+ + Fe3+ + 2LL + 2C12H25NH2
where LL = ip, dpq and dpqc, and the rate is given by,
Rate = k[cobalt(III) complex] [Fe2+]
where k is the second order rate constant.

This reaction is postulated as outer sphere in comparison to such type of reactions in the literature, involving ordinary low primary amine coordinated cobalt(III) complexes that are similar to the surfactant cobalt(III) complexes of the present study. The complexes of the present study are inert to substitution due to the non-availability of a co-ordination site for the inner-sphere precursor complex. Only a non-bridging intermediate is expected from such reactions, which is inert to substitution. Already there are literature reports38–41 on similar types of complexes that support only the outer-sphere redox pathway. The most favorable mechanism for this outer-sphere electron transfer process consists of three elementary steps (Scheme 1): ion pair formation (kip), electron transfer (ket), and product successor dissociation.


image file: c5ra02829a-s1.tif
Scheme 1 General mechanism for the electron-transfer reaction.

Influence of micelles on electron transfer

The kinetics of the ETR between the surfactant cobalt(III) complexes of the present study were obtained from self micelles formed by the surfactant cobalt(III) complex molecules. The outer-sphere electron transfer proceeds with a second order reaction and second order rate constants were measured from the reactions between the surfactant cobalt(III) complexes and Fe2+ ions at various temperatures and these rate constants are shown in the tables (Tables 1, S1 and S2†) and the plots of k against various initial concentrations of the surfactant cobalt(III) complexes at 298, 303, 308, 313, 318 and 323 K in aqueous solution are shown in Fig. 1 and S1 and S2.† As seen from these tables the rate constant for each reaction increases with the increasing initial concentrations of the surfactant cobalt(III) complexes. Because these concentrations are higher34 than the critical micelle concentration (3 × 10−4 mol dm3 s−1) values of the surfactant complexes, all the rate constant values correspond to the rate constant values in self-micelles formed from these metal complex molecules (attempts to study the kinetics of the same reaction below the CMC of the surfactant cobalt(III) complexes were unsuccessful because the reaction was too slow and no change in the absorbance was observed with time). Hence, the increase in rate constant with increase in concentration of the complexes can be attributed to the aggregation of the metal complex in its own self-micelles. At high initial concentrations of the surfactant cobalt(III) complex, the presence of a higher number of micelles was expected.
Table 1 Second-order rate constants for the reduction of the cobalt(III) complex ion by Fe2+ in aqueous solutions under various temperatures. cis-[Co(ip)2(C12H25NH2)2](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3
[Complex] × 104, mol dm−3 k × 102, dm3 mol−1 s−1
298 K 303 K 308 K 313 K 318 K 323 K
3.0 0.9 1.1 1.7 2.2 2.5 2.6
4.0 1.1 1.3 1.8 2.2 2.7 3.0
5.0 1.4 2.1 2.3 2.7 2.9 3.3
6.0 2.1 3.0 3.2 3.4 3.7 4.1
7.0 3.0 3.3 3.7 4.1 4.3 4.8


image file: c5ra02829a-f1.tif
Fig. 1 Plot of k against the complex ion for cis-[Co(ip)2(C12H25NH2)2](ClO4)3 under various temperatures; cis-[Co(ip)2(C12H25NH2)2](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3.

As seen from the tables (Tables 1, S1 and S2†), the rate constant values of the surfactant cobalt(III) complexes of the present study are very much different from each another. This difference in the rate constant values between the surfactant cobalt(III) complexes of the present study are explained as follows: the rate constant value of the surfactant cobalt(III)-imidazo[4,5-f][1,10]phenanthroline complex is lower than that of the corresponding surfactant cobalt(III)-dipyrido[3,2-a:2′,4′-c](6,7,8,9-tetrahydro)phenazine complex at all the initial concentrations studied. Due to the high hydrophobicity of dpqc containing complexes, the number of micelles formed from these complex molecules are more compared to that of imidazo[4,5-f][1,10]phenanthroline containing complexes at the same concentration values. Hence, dipyrido[3,2-a:2′,4′-c](6,7,8,9-tetrahydro)phenazine containing complexes enhance the overall rate of the reaction. Among these modified phenanthroline surfactant cobalt(III) complexes containing imidazo[4,5-f][1,10]phenanthroline, dipyrido[3,2-d:2′-3′-f]quinoxaline and dipyrido[3,2-a:2′,4′-c]phenazine ligands, the rate constant values of the dipyrido[3,2-a:2′,4′-c](6,7,8,9-tetrahydro)phenazine complex is the highest due to the higher hydrophobicity of the dipyrido[3,2-a:2′,4′-c](6,7,8,9-tetrahydro)phenazine ligand. The rate constant values of surfactant cobalt(III)-modified phenanthroline complexes are higher. This is due to the fact that the extended aromaticity of modified phenanthroline ligands makes aggregation increment, which increases the capacity of these complexes to form micelles than that of the previously reported17,18,41 surfactant cobalt(III) complexes at all the initial concentrations studied.

Effect of cyclodextrin (β-CD) on electron transfer

Cyclodextrins are naturally occurring receptors, which can alter the physical properties and chemical reactivities of guest molecules.42 It is known that the binding of many simple phenyl derivatives is stronger to β-cyclodextrin25 and this trend depends on the cavity sizes of the cyclodextrins, which increase as α-CD < β-CD < γ-CD.43 β-Cyclodextrins have the ability to form complexes with host molecules, and complexes form when suitable hydrophobic molecules displace water from their cavities.44 The effects of cyclodextrin inclusion on the kinetics and mechanism of ligand substitution45,46 and electron transfer reactions of transition metal complexes in aqueous solution47,48 have received considerable attention in recent years. The effects of the presence of cyclodextrin in the medium on the kinetics of the ETR between the surfactant cobalt(III) complexes of the present study and Fe2+ ions have been investigated. In cyclodextrin media, the reduction of the surfactant cobalt(III) complexes with Fe2+ ion also proceeds with a second order reaction and the resulting k values are listed in the tables (Tables 2, S3 and S4†) and the plot of k against various concentrations of β-CD are shown in Fig. 2 and S3 and S4.† As seen from these tables, the addition of increasing concentrations of cyclodextrin to the medium resulted in significant decrease in the second order rate constant. It is well-known that β-cyclodextrin is a good micelle structure breaker. Because the long aliphatic chains of surfactants can be included into the cavities of (Scheme 2) cyclodextrin, the formation of micelles is difficult, which leads to an increase in the CMC values of surfactants in the presence of cyclodextrin. Hence, in our case the decrease of rate constant with increase in the concentration of cyclodextrin in the media can be attributed to the inclusion of the long aliphatic chain that is present in one of the ligands into cyclodextrin. This effect of cyclodextrin in the media also supports the earlier conclusion on the effect of the initial concentration of the surfactant cobalt(III) complexes on the second order rate constant.
Table 2 Second-order rate constants for the reduction of the cobalt(III) complex ion by Fe2+ in the presence of β-cyclodextrin under various temperatures. cis-[Co(ip)2(C12H25NH2)2](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3
[β-Cyclodextrin] × 104, mol dm−3 k × 102, dm3 mol−1 s−1
298 K 303 K 308 K 313 K 318 K 323 K
1.4 6.6 6.9 7.9 8.6 11.5 12.5
1.6 6.3 6.4 6.8 7.8 11.1 12.1
1.8 6.0 6.2 6.4 7.3 10.7 11.6
2.0 5.7 5.8 5.9 6.9 10.4 11.2
2.2 5.4 5.5 5.7 6.4 9.8 10.8
2.4 4.1 4.5 4.9 6.0 9.1 10.1
2.6 3.8 3.9 4.3 5.1 8.6 9.4


image file: c5ra02829a-f2.tif
Fig. 2 Plot of k against [β-CD] for cis-[Co(ip)2(C12H25NH2)2](ClO4)3 at various temperatures; cis-[Co(ip)2(C12H25NH2)2](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3.

image file: c5ra02829a-s2.tif
Scheme 2 Host–guest complex between the surfactant complexes and β-cyclodextrin.

Activation parameters and isokinetic plots

The effect of temperature on rate was studied at six different temperatures for each micelle concentration and β-cyclodextrin in order to obtain the activation parameters for the reaction between the surfactant cobalt(III) complexes with Fe2+. From the transition state theory,49 by making use of the Eyring equation, the values of ΔS# and ΔH# were determined by plotting ln(k/T) versus 1/T and the plots are shown in Fig. 3, 4 and S5–S8.†
ln[thin space (1/6-em)]k/T = ln[thin space (1/6-em)]kB/h + ΔS#/R − ΔH#/RT
The ΔS# and ΔH# values obtained are shown in Table 3 and SI5–SI9.† As seen from these tables, the values of ΔH# are positive for all the reactions (in micelles and β-cyclodextrin), indicating that the formation of the activated complex is endothermic. The ΔH# values decrease with the increasing initial concentration of the surfactant cobalt(III) complexes, and this is due to the presence of more micelles at higher initial concentrations of the surfactant cobalt(III) complexes, which facilitates the reaction. In all the media, the ΔS# values are found to be negative in direction in all the concentrations of complex used, which is indicative of a more ordered structure of the transition state, i.e. a compact ion pair transition state (Scheme 1) leads to more attraction of the surrounding solvent molecules around the positive and negative charges on the ion pair, which results in the loss of freedom of movement of the solvent molecules in the transition state. In order to check for any change of mechanism that occurs during the ETR, isokinetic plots (ΔS# versus ΔH#) for the electron transfer reactions of the surfactant cobalt(III) complexes were obtained. As seen from Fig. 5, 6 and S9–S12† straight lines were obtained for all the isokinetic plots of complexes, which indicate that a common mechanism exists in all the initial concentrations of each complex studied (Table 4).
image file: c5ra02829a-f3.tif
Fig. 3 Eyring plot for cis-[Co(ip)2(C12H25NH2)2](ClO4)3 in aqueous medium. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.

image file: c5ra02829a-f4.tif
Fig. 4 Eyring plot for cis-[Co(ip)2(C12H25NH2)2](ClO4)3 in [β-CD] medium. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3, [β-CD] = 1.8 × 10−3.
Table 3 Activation parameters for the reduction of cis-[Co(ip)2(C12H25NH2)2](ClO4)3, μ = 1.0 mol dm−3 in micellar medium
[Complex] × 104, mol dm−3 ΔH‡ kJ mol−1 ΔS‡ J K−1
3.0 1.15 −203.6
4.0 1.48 −190.8
5.0 2.32 −175.4
6.0 4.34 −125.1
7.0 4.42 −121.9


image file: c5ra02829a-f5.tif
Fig. 5 Isokinetic plot of the activation parameters for the reduction of cis-[Co(ip)2(C12H25NH2)2](ClO4)3 by ion(II) in aqueous solutions. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.

image file: c5ra02829a-f6.tif
Fig. 6 Isokinetic plot of the activation parameters for the reduction of cis-[Co(ip)2(C12H25NH2)2](ClO4)3 by ion(II) in β-cyclodextrin medium. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.
Table 4 Activation parameters for the reduction of cis-[Co(ip)2(C12H25NH2)2](ClO4)3, μ = 1.0 mol dm−3 in β-CD medium
[β-Cyclodextrin] × 103, mol dm−3 ΔH‡ kJ mol−1 ΔS‡ J K−1
1.4 2.36 −164.8
1.6 2.5 −162.4
1.8 2.54 −161.1
2.0 2.62 −159.5
2.2 2.64 −158.9
2.4 3.49 −138.3
2.6 3.62 −135.7

Conclusion

The present work explains the outer-sphere electron transfer between cis-[Co(ip)2(C12H25NH2)2]3+, cis-[Co(dpq)2(C12H25NH2)2]3+, cis-[Co(dpqc)2(C12H25NH2)2]3+ and Fe2+ in micelles as well as β-CD and the kinetics of the outer-sphere ETR between the complexes and Fe2+ ions in self micelles formed by our surfactant cobalt(III) complex molecules. The rate constant increases with increasing concentrations of the complexes due to the fact that the concentration ranges are higher than the critical micelle concentrations (CMC). Upon increasing the concentration of the complexes, the number of micelles present in the medium also increases, which leads to a higher rate and lower activation energy. β-CD is a good structure breaker of micelles. The second order rate constant decreases with an increase in concentration of β-CD because the long aliphatic chains of surfactants can be included into the cavities of cyclodextrin, and this makes the formation of micelles difficult. This is attributed to the inclusion of the long aliphatic chains present in one of the ligands into cyclodextrin. Activation parameters were calculated using the transition state theory. On comparing the previous reports of surfactant complexes containing aliphatic ligands, it was observed that their ETR is lower than that of the present complexes which contain modified phenanthroline and this extends the aromaticity of the ligands.

Acknowledgements

We are grateful to the UGC-SAP and DST-FIST programmes of the Department of Chemistry, Bharathidasan University, and UGC-SAP RFSMS Scholarship sanctioned to one of the authors, K. Nagaraj, by University Grants Commission (UGC), New Delhi. Financial assistance from the CSIR (Grant no. 01(2461)/11/EMR-II), DST (Grant no. SR/S1/IC-13/2009) and UGC (Grant no. 41-223/2012(SR)) sanctioned to S. Arunachalam are also gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02829a
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