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 decrease of dielectric constant did not change kobs. CH3COO–,/NO3–/Cl–/SO42-/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.
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.