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 oxidant, -adipato-di(N,N^/-bis(salicylidene)ethylenediaminatoiron(III),[(Fe-salen)₂adi],hereafter denoted as Fe₂adi, with the thiols L-cysteine (LSH), thiourea (USH), thioglycolicacid(GSH),2–mercaptobenzothiazole  (BTSH)   and benzylmercaptan (BSH) have been studied spectrophotometrically at 495 nm in aqueous perchloric acid, I=0.01mol dm^-3  (NaClO₄) and at 29 1^oC. The stoichiometry of 1:1 was obtained for Fe₂adi – LSH, Fe₂adi – GSH and Fe₂adi –BTSH systems while 1:2 was obtained for Fe₂adi – USH and Fe₂adi–BSH systems. Under pseudo – first order conditions of a large excess of the reductants, pseudo–first order rate constants increased with increase in concentrations of the thiols (LSH, USH, GSH and BTSH), but decrease in pseudo – first order rate constant was observed as the concentration of BSH  increased. The second order rate constantsk₂ were fairly same for all the five systems with values.  0.009 0.003 dm³ mol^-1 s^-1, 0.063 0.01dm³ mol^-1 s^-1, 0.038 0.005 dm³ mol^-1 s^-1, 0.011 0.001 dm³ mol^-1 s^-1. and. 0.0015 0.0035 dm³ mol^-1 s^-1 for LSH, USH, GSH, BTSH and BSH respectively. The rates of reaction were directly dependent on acid concentrations for the five systems. The overall rate equation for the reactions can be given as  [Fe₂adi] [reductant] [ ] for Fe₂adi–LSH, Fe₂adi – GSH, and Fe₂adi – BTSH systems where a and b are the intercept and slope respectively, [Fe₂adi] [USH]² for Fe₂adi–USH,  while the rate equation,  was for Fe₂adi –BSH system is [Fe₂adi] [BSH] [ ]². The values of a and b obtained for the reaction of Fe₂adi with the thiols are given as LSH (a = 0.3 x 10^-4 dm³mol^-1 s^-1 and b = 1.23 x 10^-3 dm⁶ mol^-2 s^-1), USH (a=6.2×10^-4 dm³mol^-1s^-1 and b = 6.4 x10^-3 dm⁶mol^-2s^-1),  GSH (a = 1.9 x 10^-4 dm³mol^-1s^-1 and b = 8.18 x 10^-2 dm⁶mol^-2 s^-1), BTSH (a = 0.51 x 10^-5 dm³mol^-1s^-1 and b =  2.0 x 10^-3 dm⁶mol^-2s^-1), BSH (b =8.0 x 10^-3 dm⁶mol^-2 s^-1). The rates of reactions was observed to decrease with the increase in ionic strength of the medium for the five systems under study. Addition of magnesium and acetate ions in small amount did not affect the rates of reactions for the redox reactions of Fe₂adi with LSH, USH, GSH and BTSH, but decrease in rates of reaction was observed for that of BSH.  The rates of reaction were not affected by the decrease in dielectric constant D for the reaction of Fe₂adi with LSH, GSH and BTSH but it was enhanced for USH and BSH under the same conditions. Furthermore, the activation parameters, H^# and S^# were also determined for the five systems; the values being 89.99kJmol^-1and -30195Jk^-1 mol^-1for Fe₂adi-LSH system, 20.68kJmol^-1 and -197.60Jk^-1mol^-1 for Fe₂adi-USH system, 27.38kJmol^-1 and-303.48JK^-1 mol^-1 for Fe₂adi-BTSH system and 12.83kJmol^-1 and -313.82JK^-1mol^-1 for Fe₂adi-BSH system. Michaelis-Menten plot of 1/k_obs versus 1/[reductants] were linear with intercepts for the redox reactions of Fe₂adi with LSH, GSH, BTSH, USH and BSH. On the basis of the results obtained above, the reactions have been proposed to follow the inner-sphere m

CHAPTER ONE

• INTRODUCTION

The electron transfer reactions of binuclear iron (III) complexes have attracted a lot of interest in recent time due to their application as models for the investigation of the physiological role played by iron in biochemical processes ^2, such as hemerythrin ^2,3,4.6 and ferric porphyrin^{7,27,28 47}. Previously, the dynamics of electron transfer reactions of dinuclear oxo bindged iron(III) complexes of the form [Fe₂O]⁴⁺ with ascorbic acid ⁴,   mercapto acetic acid⁵ and mercaptoethylamine ⁶ have been investigated. Most of these reactions followed outer sphere electron transfer route with intervening ion-pair complexes and free radicals..

The behaviour of transition metal ions with respect to their electron transfer and the roles played by bridging ligands in the course of redox reaction formed the bed rock of this study. ^37,39 The main advantage of this research is that the results provide additional insight into the complexities attending reactions of bridged iron(III) complexes and the extent of influence of the bridging ligand on the rate of electron transfer. It is therefore hoped that this research will enhance the knowledge of the kinetics and mechanisms of electron transfer reactions of binuclear iron (III) complexes and other transition metal complexes with these set of thiols.

1.2     Methods of Monitoring Reaction Rates