KINETICS AND MECHANISMS OF THE ELECTRON TRANSFER REACTIONS OF THE µ-OXO- BRIDGED IRON(III) COMPLEX Na4[(FeEDTA)2O].12H2O WITH SOME THIOLS

ABSTRACT

The kinetics and mechanisms of the redox reactions of   µ-oxo-bridged iron(III) complex ion Na4[(FeEDTA)2 O].12H2O denoted as Fe2 O4+, with the thiols-2-mercaptobenzothiazole(MBSH), 2- mercaptophenol(PhSH), 2-mercaptoacetic acid (MSH), and l-cysteine (RSH) have been investigated in  aqueous perchloric   acid medium at [H+]=1×10-4  mol dm-,3,I=0.05mol dm-3(NaClO4) and at T =27.0±0.1oC. The reactions were monitored under pseudo-first order condition .The rate of reaction

was first-order in reductant and oxidant for all the systems giving overall second –order reactions. The inorganic and organic products of the reaction between Fe2O4+  -MBSH, PhSH, MSH and RSH and oxidants were found to be Fe(II) ions   and disulphides respectively. The stoichiometries of Fe2O4+   -MBSH, PhSH, MSH, and RSH was determined by mole ratio method and was found to be 1:2 for all the systems. The reactions of the thiols (MBSH.PhSH,MSH and RSH)  had an inverse dependence on hydrogen ion concentration ,and so the general rate law can be given as follows

Changes in ionic strength of the reaction medium had a negative effect on the rate of reaction of Fe2O4+ – MBSH and RSH and positive effect in the reaction of Fe2O4+ – PhSH and MSH. Reduction of Fe2O4+  by MBSH, PhSH, MSh and RSH showed no dependence on dielectric constant because

–      –           2-     +                     2+

decrease of dielectric constant did not change kobs. CH3COO-,/NO3 /Cl /SO4   /K and Mg ,were used

to determine the effect of catalysis on Fe2O4+-MBSH,PhSH,MSH and RSH reactions and there was decrease in catalysis. The effect of temperature on the rate of reduction of Fe2O4+  with reductants was studied and was found to have negative entropy which confirmed the formation of binuclear complexes at the activated complex. The results of the study indicate that the reactions of Fe2O4+ and thiols probably occur by the outer-sphere mechanism.

CHAPTER ONE

1.0 Introduction

There has been a great deal of interest in the chemistry of oxo-bridged complexes of Fe3+

1,2,3,4,5,6,7,8. This most probably stems from the fact that structures of these complexes are closely related to biological systems such as the protein hemerythrin and ferriporphyrin6. It is well known that on account of the presence of sulfyhdryl groups in thiols, they possess marked reducing properties and are readily oxidized to disulphide7. Sulfydydryl  compounds have also been utilized in identifying low molecular   weight cellular metabolites which could serve as detoxifying agents against metal poisoning5. Many metal complexes of thiols had been synthesized and found promising in metal chelation therapy. Also reports abound regarding the role played by RSH/RSSR couples in mediating redox potentials at biological sites3.

Kinetic data has been published on the oxidation of β-mercaptoacetic acid by enH2[(FeHEDTA)20].6H203, oxidation of β-mercaptoacetic acid by trioxoiodate(v)11 , reduction of iron(III) complex,enH2[(FeHEDTA)2O].6H2O by thiosulphate ions in acid medium12. These reactions produced no detectable stable intermediates and were rationalized on the basis of

outer-sphere  electron  transfer  mechanism.  In  the  reaction  of  tetraoxoiodate  (VII)  and  L- cysteine, the reaction was shown to have direct acid dependence and negative Bronsted- Debye salt effect11.

Bioinorganic processes have exposed the inorganic reaction mechanists to the outer fringes of catalysis, to the fact that the main criterion for most catalytic action is a site which has similar electronic characteristics to those of the active site of  the  catalyst for the  reaction under consideration. Consequent on this, prerequisites establishing the structure of the reactant and the product(s); the coordination sphere of the complex being robust and making sure that the only reactive site is the one pertinent to the elementary reaction under investigation should be

pursued6. Also the reaction being studied must be stoichiometric and as simple as possible, in order that the maximum mechanistic information can be obtained from it.

The role played by ligands in electron transfer reactions cannot be overlooked since ligand substitution as well as electron transfer attend most redox reactions. This phenomenon influences the reactivity of a particular metal as well as its stability in any oxidation state. These are factors that are dependent on the free energy change for such intramolecular electron transfer process13-16.

Favourable change in free energy and activation energy in a redox process lead to a spontaneous reaction and results in change in oxidation state of at least two of the reactants. Mechanistically, these reactions follow one of the pathways, inner-or outer-sphere, although some other complex reactions operate by simultaneous inner-and outer-sphere mechanisms.17,

18 These facts make it imperative that the inorganic reaction mechanist who has to research in

varied areas as bioinorganic, coordination, organometallic and synthetic inorganic chemistry, becomes abreast with the diverse nature of his work and not become polarized towards one area of chemistry.

Existing literatures on the redox reaction of Na4[(FeEDTA]2O.12H2O with 2- mercaptobenzothiazole, mercaptophenol, mercaptoacetic acid and L-cysteine is adequate. Adequate knowledge of the redox parameters of the oxo-bridged Na4[(FeEDTA]2O.12H20 with thiols is essential, consequently, the behaviour of this complex with thiols,  form the bedrock of this study.

1.1 Rate Monitoring Techniques

The  method  selected  to  monitor  the  concentrations  of  reactants  and  products  and  their variations with time depends on the substances involved and the rapidity with which they change3. Many reactions reach thermodynamic equilibrium over periods of minutes or hours but some reactions reach equilibrium in fractions of a second. Under special conditions modern techniques are capable of studying reactions that are complete within a few femtoseconds (I fs

=10-15s). A particular technique chosen depends on how fast or how slow the reaction is11.

Spectrophotometry, the measurement of the intensity of absorption in a particular spectral region, is widely applicable, and is especially useful when one substance (and only one) in the reaction mixture has a strong characteristic absorption in a conveniently accessible region of the spectrum. For example, the reaction

H2(g)   + Br2(g)                                    2HBr(g)———————————————————(1.1)

can be followed by measuring the absorption of visible light by bromine.

If a reaction changes the number or type of ions present in a solution, then it may be followed by monitoring the conductivity of the solution. Reactions that change the concentration of hydrogen ions may be studied by monitoring the pH of the solution with a glass electrode. Other methods of determining composition include titration, mass spectrometry, gas chromatography, and magnetic resonance. Polarimetry, the observation of the optical activity of a reaction mixture, is occasionally applicable3.

1.1.1 Conventional Methods

These methods involve the measurement of the concentration or any physical property of one or more of the reactants or products as a function of time. For instance, in some reactions,

absorbance of any of the reactants or products could be measured and related directly to the concentration19.

1.1.2 Monitoring Rates of Fast Reactions

Sufficient strides have been made in the development of techniques for the measurement of fast  reaction rates. Such  techniques are of  two  main  types.  The  first  employs  the  same principles as are used for slow reactions, the methods being modified to make them suitable for more rapid reactions. The second type is of a different character and involves special principles like temperature-jump20.

The reasons why conventional techniques lead to difficulties for rapid reactions are as follows: (1) The time that it usually takes to mix the reactant or to bring  them to  a  specified

temperature,  may  be  significant  in  comparison  with  the  t½  of  the  reaction.  An appreciable error therefore will be made, since the initial time cannot be determined accurately.

(2)  The time that it takes to make a measurement of concentration is significant compared with the t½.

Real-Time Analysis

In a real-time analysis the composition of a system is analyzed while the reaction is in progress, either by direct spectroscopic observation of the reaction mixture or by withdrawing a small sample and analyzing it21.

Quenching Method

In the quenching method, the reaction is stopped after it has been allowed to proceed for a certain time, and the composition is analyzed at leisure. The quenching (of the entire

mixture or of a sample drawn from it) can be achieved either by cooling suddenly, by adding the mixture to a large amount of solvent, or by rapid neutralization of an acid reagent. This method is suitable only for reactions that are slow enough for there to be little reaction during the time it takes to quench the mixture21.

Flow Method

In the flow method, the reactants are mixed as they flow together in a chamber. The reaction continues as the thoroughly mixed solutions flow through the outlet tube, and different points along the tube correspond to different times after the start of the reaction. Therefore, spectroscopic observation of the composition at different positions along the tube is equivalent to the observation of the composition of the reaction mixture at different times after mixing. The disadvantage of conventional flow techniques is that a large volume of reactant solution is necessary because the mixture must flow continuously through the apparatus. This disadvantage is particularly important for reactions that take place very rapidly, because to spread the reaction over an appreciable length of tube the flow must be rapid21.

Stopped-Flow Technique

The stopped-flow technique avoids the disadvantage encountered in flow method. The two solutions are mixed very rapidly by injecting them into a mixing chamber designed to ensure that the flow is turbulent and that complete mixing occurs very rapidly. Behind the reaction chamber there is an observation cell fitted with a plunger that moves back as the liquids flood in, but which comes up to a stop after a certain volume has been admitted. The filling of that chamber corresponds to the sudden reaction of an initial sample of that reaction mixture. The reaction then continues in the thoroughly mixed solution and is monitored spectrophotometrically.  Because  only  a  small,  single  charge  of  the  reaction  chamber  is prepared the technique is much more economical than the flow method. The suitability of the

stopped-flow technique to the study of small samples means that it is appropriate for biochemical reactions, and it has been widely used to study the kinetics of enzyme action21.

Flash Photolysis

In flash photolysis, the gaseous or liquid sample is exposed to a brief photolytic flash of light, and then the contents of the reaction chamber are monitored spectrophotometrically. Although discharge lamps can be used for flashes of about 10-5sec duration, most work is now done with lasers, which can be used to generate nanosecond flashes routinely, picoseconds flashes quite readily, and flashes as brief as a few femtoseconds in special arrangements. Both emission and  absorption  spectroscopy  may  be  used  to  monitor  the  reaction,  and  the  spectra  are observed electronically or photographically at a series of times following the flash21.

Relaxation Methods

The limitation of the stopped- flow method is the dead time during which the enzyme and substrate are mixed22. Relaxation method overcomes the mixing problems associated with the flow method. The term ‘relaxation’ denoted the return  of the system to equilibrium. In  its application to chemical kinetics the term indicates that some externally applied influence has shifted the equilibrium position of a reaction normally very quickly, and the reaction relaxes into the new equilibrium position23.

One of the most important relaxation techniques uses a temperature jump. The equilibrium is changed by causing a sudden change of temperature and the concentration are monitored as a function of time. One way of raising the temperature is to discharge electric current through a sample which has been made by conducting the addition of ions. With a suitable choice of condensers, temperature jump of between 5 and 10 K can be achieved18. The recorded data

enables the number of intermediates to be deduced and the various rate constant calculated from the relation times22.

1.1.3 Resonance Techniques

Rates of reactions could be monitored using the nuclear magnetic resonance (nmr) technique. Resonance absorption line is related to the t½ of the nucleus in a given spin state. For cases where electron spin resonance is the method of choice, resonance absorption is related to the life-time of paramagnetic species in a given energy state. If the life-time of these states is shortened by say a chemical interaction, it results into line broadening ‘H nmr. Line broadening has been used to measure the rate of exchange of various mono-and bidentate nitrogen and

oxygen donor ligands coordinated to Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)3.

1.2 Theoretical considerations in electron transfer processes

1.2.1 Franck-Condon Principle

Frank-Condon principle states that electronic transitions are virtually instantaneous in comparison with atomic rearrangement24. In other words, valence, unless they posses sufficient geometrical similarity to reduce to a minimum the energy transfer required by the simultaneous and instantaneous conversion of an ion of one valence to another25.  Electron transitions are rapid  compared  with  nuclear  motions  and  electron  transfer  occurs  without  significant movement of the atoms. Since electron transfer reactions involve bond-breaking and formation, Frank-Condon principle must of necessity come into play. However, since the atomic distances between ligands and the metal ions alter the oxidation state of the metal ion, reorganization of the metal-ligand distances for the reactions and products occurs before electron transfer takes place 26. The sequence of event is represented as follows

2+               + N3+

approach and reorganisation                  M2+

separation and reorganisation                Mo

+ N2+

Where subscripts m and n = equilibrium configurations of the coordination shell for metals M2+

and N3+ respectively, subscripts o = intermediate configuration3.

Electron transfer can only take place when ions approach each other. When the electron transfer step is very rapid, the overall rate is that at which the ions diffuse together to form an ion-pair. Reactions of this type studied by temperature-jump techniques had rates of the order of magnitude of the diffusion-limited values27.

Franck-Condon principle presupposes that electron transfer takes place with the nuclei of the oxidant  and  reductant  virtually  stationary.  The reorganization  undergone  by  the  reactants before electron transfer occurs in such a way that their transition state energies become almost identical and energy change on electron transfer is minimized.

The total change in energy involved in the process can then be represented by equation (1.3)28.

o

ΔG# = ΔG #

+ ΔGi#

+ ΔG #———————————————————————-(1.2)

t

ΔG #

= association free energy

ΔGi# = inner-sphere reorganization energy

o

ΔG #

= outer-sphere reorganization energy

1.2.2 The Electron Tunneling Hypothesis

Considerable insight into the electron transfer process in solution is given by the electron tunneling theory developed by Weiss and by Marcus, Zwolinski, and Eyring. The electron can transfer at distances considerably greater than would correspond to actual collision of the reactants23. The implication is that external value for the specific rate constant as a function of the distance of approach is used to determine the most stable activated complex. This maximization is necessary to find the best distance of approach for the probability of electron penetration consistent with the smallest energy of activation29.

Theoretical considerations based on above views resulted in the relationship known as the

*              *

electron transmission coefficient   k’   which takes the form of the transition state theory of

chemical kinetics28.

k  = KT  kI exp ( -∆Gr     ∆Gr )

h                 RT      RT—————————————————————-(1.3)

k’ = electron transmission coefficient k = rate constant

K = Boltzmann constant

T = absolute temperature

* = activation energy

Gr* = hydration energy for inner coordination shell arrangement

R = gas constant.

The value of the transmission coefficient is less than unity and increases as the exchanging partners come close together. Electrostatic repulsion ensures that activation energy also increases hence the rate of reaction tends to decrease. However, at an optimum distance a

maximum exchange rate is obtained. The electron tunneling is viewed to be involved most electron transfer reactions but might not be the rate determining step in most cases30,31.

1.3 Electron Transfer Reactions

These are redox reactions in which two species come together and electron passes from one to the other. In some instances there is an accompanying change in the coordination shell of one or both of the reactants. Usually, the two complexes are such that, the reaction involves no chemical change. Such a reaction is called a redox (oxidation-reduction) reaction. Redox reaction or electron transfer reactions can be classified into two broad classes, namely homonuclear (isotopic) exchange reactions and chemical or cross reactions. The class into which a particular redox reaction falls is dependent on the thermodynamic parameters involved. The stability of the oxidation state of a metal and therefore the most stable oxidation state varies with the surrounding ligands26.

Redox reactions are usually studied in aqueous system since most metal ions are inert in non- aqueous solution. The oxidation states which are well known for example Ti(III), Cr(III) and Fe(III) are so well known simply because of their stability in the presence of water. The reason why a particular oxidation state is stable may be either thermodynamic when a change in oxidation state may be associated with an unfavourable free energy change or kinetic, when the energy of activation for the intramolecular ligand-metal redox reaction may be large26.

Oxidation of a particular species involves electron loss and reduction involves electron gain, implying that the rate at which a redox reaction occurs is qualitatively related to the redox potential. Each ion in aqueous media has its standard electrode potential, Eo, measured in volts which is determined in comparison to the standard hydrogen electrode which is assigned zero potential. The electrode potential of an ion gives an indication of its readiness to be oxidized or

reduced  by  another  ion.  Hence,  ions  with  higher  negative  values  of  standard  reduction potentials are good reducing agents while those with less negative values or that has positive values function as good oxidizing agents.

Therefore, for two ions involved in a redox reaction, the oxidant is the ion of lower negative value of reduction (electrode) potential while the ion of higher negative reduction potential acts as the reductant. Generally, systems with higher electrode potentials are reduced by systems with lower electrode potentials. This, however, assumes that the entropy terms for the redox reaction are favourable or negligible. Also the electronic configuration of a metal ion is an important factor in determining the stability of a particular oxidation state and hence governs the redox potentials.

1.3.1 Homonuclear or Isotopic Exchange

Isotopic exchange involve only electron transfer between different oxidation states of the same metal in a constant environment24, for example

Fe2+(aq) + F*e3+(aq)→Fe3+(aq)+F*e2+aq.    —————————————(1.4)

2+             *                  2+

3+         *                                    2+

Fe(phen)3

+ Fe (phen)3

→Fe(phen)3

+Fe (phen)           3

————     (1.5)

In  such a reaction  the isotope distribution  tend  towards equilibrium as a  result  of transfers of isotopically different atoms or groups.

1.3.2 Heteronuclear or Cross Reaction

This class of reactions involves electron transfer between different metal ions centres. The products are chemically distinct from the reactants and the overall free energy change (∆Go) is not equal to zero. In most cases, ∆Go is less than zero. The reaction can be complementary if oxidant and reductant undergo equal changes in oxidation states (stoichiometry is 1:1 as in eqn 1.15.

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