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±0.1^oC. 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 decrease of dielectric constant did not change k_obs. CH₃COO^–_,/NO₃^–/Cl^–/SO₄^2-/K⁺ and Mg²⁺,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 Fe^{3+ 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 process^13-16.

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.