KINETICS AND MECHANISMS OF THE REDOX REACTIONS OF µ-ADIPATO-DI (N,N/ BIS(SALICYLIDENE) ETHYLENEDIAMINATOIRON (III) [(Fe-Salen)2adi] WITH SOME THIOLS

ABSTRACT

The kinetics and mechanisms of the redox reactions of oxidant, m -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 ± 1o C. 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 constants k2 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 [ ] [ ] ˜ ¯ ˆ Á Ë Ê = + - + a b H dt d Fe adi 2 [Fe2adi] [reductant] [ + H ] for Fe2adi–LSH, Fe2adi – GSH, and Fe2adi - BTSH systems where a and b are the intercept and slope respectively, [ ] ( [ ] ) 2 2 Fe adi + = + - a b H dt d [Fe2adi] [USH] 2 for Fe2adi–USH, while the rate equation, was for Fe2adi –BSH system is [ ] b dt d = - Fe adi 2 [Fe2adi] [BSH] [ + H ] 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 dm3 mol-1 s -1 and b = 1.23 x 10-3 dm6 mol-2 s -1 ), USH (a=6.2x10-4 dm3 mol-1 s -1 and b = 6.4 x10-3 dm6 mol-2 s -1 ), GSH (a = 1.9 x 10-4 dm3 mol-1 s -1 and b = 8.18 x 10-2 dm6 mol-2 s -1 ), BTSH (a = 0.51 x 10-5 dm3 mol-1 s -1 and b = 2.0 x 10-3 dm6 mol-2 s -1 ), BSH (b =8.0 x 10-3 dm6 mol-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, DH# and DS# were also determined for the five systems; the values being 89.99kJmol-1 and -30195Jk-1 mol-1 for Fe2adi-LSH system, 20.68kJmol-1 and -197.60Jk-1 mol-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-1 mol-1 for Fe2adi-BSH system. Michaelis-Menten plot of 1/kobs 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 mechanism.

TABLE OF CONTENTS

Title page -- -- -- -- -- -- -- -- -- -- i

Declaration -- -- -- -- -- -- -- -- -- ii

Certification -- -- -- -- -- -- -- -- -- iii

Dedication -- -- -- -- -- -- -- -- -- iv

Acknowledgment -- -- -- -- -- -- -- -- v

Abstract -- -- -- -- -- -- -- -- -- -- vi

Abbreviations -- -- -- -- -- -- -- -- -- vii

Table of contents -- -- -- -- -- -- -- -- ix

List of tables -- -- -- -- -- -- -- -- -- xi

List of figures -- -- -- -- -- -- -- -- -- xii

Abbreviations -- -- -- -- -- -- -- -- -- xiv

CHAPTER ONE

INTRODUCTION

1.0 Introduction -- -- -- -- -- -- -- -- 1

1.2 Methods of Monitoring Reaction Rates-- -- -- -- 1

1.2.1 Conventional method (slow technique) -- -- -- -- 2

1.2.2 Monitoring the rates of fast reactions -- -- -- -- 2

1.3 Theories of Reaction Rate -- -- -- -- -- -- 4

1.3.1 Arrhenius Theory-- -- -- -- -- -- -- 5

1.3.2 Collision Theory -- -- -- -- -- -- -- 5

1.3.3 Theory of Absolute Reaction Rate -- -- -- -- -- 6

1.4 Theories of Electron Transfer Processes -- -- -- 7

1.4.1 Marcus Theory -- -- -- -- -- -- -- -- 7

1.4.2 Electron tunneling theory -- -- -- -- -- -- 8

1.4.3 Frank Condon Principle -- -- -- -- -- -- 9

1.5. Electron Transfer Reactions-- -- -- -- -- -- 11

1.5.1 Classes /Types of Electron Transfer Reactions -- -- -- 12

1.5.1.1 Homonuclear or Isotopic Exchange Reactions-- -- 12

1.5.1.2 Heteronuclear or cross reaction -- -- -- -- -- 13

1.6 Proton-coupled electron transfer (PCET) -- -- -- -- 13

1.7 Mechanism of electron transfer reactions -- -- -- 15

1.7.1 Outer-sphere mechanism -- -- -- -- -- -- 16

1.7.2 Inner-sphere mechanisms -- -- -- -- -- -- 17

1.7.3. Distinction between the Outer-Sphere and Inner-sphere Reactions-- -- -- -- -- -- 19

1.8 Determination of the mechanisms of redox reaction -- -- 20

1.8.1 Identification of binuclear intermediate -- -- -- -- 20

1.8.2 Reactivity Patterns -- -- -- -- -- -- -- 21

1.8.3 kred versus ksub -- -- -- -- -- -- -- -- 23

1.8.4 Effect of added ions -- -- -- -- -- -- -- 23

1.8.5 Activation Parameters -- -- -- -- -- -- -- 24

1.8.6 Product Identification -- -- -- -- -- -- -- 24

1.8.7 Michaelis-Menten Plots -- -- -- -- -- -- 25

1.9 Objectives of the Project -- -- -- -- -- 27

CHAPTER TWO:

2.0 Literature Review -- -- -- -- -- -- 29

2.1 Electron transfer reaction of µ-oxo bridged Fe(III) complexes 29

2.2 Kinetics and mechanism of the reduction of µ-adi-di(N,N/- bis{salicylideneethylenediaminatoiron (III)} by dithionate ion -- 30

2.3 Kinetics and mechanism of electron transfer reactions of thiols (L-cysteine, thiourea, thioglycolic acid, 2-mercaptobenzothiazole and benzyl mercaptan) -- -- -- -- -- -- -- 30

CHAPTER THREE:

3.0 Experimental -- -- -- -- -- -- -- -- 35

Equipment -- -- -- -- -- -- -- -- 35

3.1 Synthesis of complexes -- -- -- -- -- -- -- 35

3.1.1 Ferric hydroxide, [Fe(OH)3] -- -- -- -- -- -- 35

3.1.2 N,N/ bis(Salicylidene)ethylenediamine, (H2Salen) -- 35

3.1.3 µ-oxo-di(N,Ni-bis(salicylidene)ethylenediaminatoiron (III), [(Fe-Salen)2O] -- -- -- -- -- -- -- 36

3.1.4 µ-adipato-di (N,NI-bis (salicylidene) ethylene diaminatoiron (III), [(Fe-Salen)2 adi ] -- -- -- -- 37

3.2 Preparation of reagents -- -- -- -- -- -- 38

3.2.1 Sodium Perchlorate salt (NaClO4) -- -- -- -- -- 38

3.2.2 Perchloric acid (HClO4) -- -- -- -- -- -- 38

3.2.3 Sodium acetate (CH3COONa) -- -- -- -- -- 38

3.2.4 Magnesium Chloride (MgCl2) -- -- -- -- -- 38

3.2.5 L-cysteine (LSH) -- -- -- -- -- -- -- 38

3.2.6 Thiourea (USH) -- -- -- -- -- -- -- -- 38

3.2.7 Thioglycolic acid (GSH) -- -- -- -- -- -- 39

3.2.8 2-mercaptobenzothiazole -- -- -- -- -- -- 39

3.2.9 Benzyl mercaptan -- -- -- -- -- -- -- 39

3.3. Stoichiometric Studies -- -- -- -- -- -- -- 39

3.4. Kinetic measurements -- -- -- -- -- -- -- 40

3.5 Test for free radical -- -- -- -- -- -- -- 40

3.6. Product Analysis -- -- -- -- -- -- -- -- 41

3.7 UV Analysis -- -- -- -- -- -- -- -- 41

3.8.0 UV and IR Analysis -- -- -- -- -- -- -- 41

CHAPTER FOUR:

RESULTS AND DISCUSSION

4.0. Result -- -- -- -- -- -- -- -- -- 42

4.1 Stoichiometry -- -- -- -- -- -- -- -- 42

4.2 Determination of order of reaction -- -- -- -- -- 48

4.3 Effect of hydrogen ion concentration on the rates of reaction -- 65

4.4 The effect of ionic strength -- -- -- -- -- -- 72

4.5 Effect of added ions -- -- -- -- -- -- -- 79

4.6 Effect of dielectric constant -- -- -- -- -- -- 79

4.7 Temperature dependence of rates of reaction -- -- -- 86

4.8 Test for formation of intermediate complex and products -- 94

4.8.1 Michaelis-Menten plots -- -- -- -- -- -- 94

4.8.2 Product Analysis ---- -- -- -- -- -- -- 94

4.8.3 Test for free radicals -- -- -- -- -- -- -- 100

4.8.4 UV and IR Analysis -- -- -- -- -- -- -- 100

4.9 Summary and conclusion -- -- -- -- -- -- 109

References

Appendix