ABSTRACT
The kinetics of the oxidation of aminocarboxylatocobaltate(II) complexes (hereafter, [Co(II)EDTA]2- and [Co(II)HEDTA(OH2)]-) by hypochlorite (ClO-) and silver-catalysed persulphate ions (S2O82-) at 300 K; 520 nm and 299 K; 525 nm (temperature and λmax) respectively was studied in aqueous acidic medium under pseudo-first order conditions. For the reaction of complexes with persulphate ion, the concentrations were: I = 0.5 mol dm-3 (NaNO3), [H+] = 1.0 × 10-2 mol dm-3 (HNO3) and [Ag+] = 1.0 × 10-2 mol dm-3 (AgNO3). While I = 0.5 mol dm-3 and 0.2 mol dm-3 (NaNO3) and [H+] = 4.0 × 10-2 mol dm-3 and 1.0 × 10-2 mol dm-3 (HNO3) are for the reaction of hypochlorite with [Co(II)EDTA]2- and [Co(II)HEDTA(OH2)]- complexes respectively. The stoichiometry obtained was 2:1 for the oxidation of complexes by persulphate ion and 1:1 for the reaction of complexes with hypochlorite ion. First order kinetics in the concentration of S2O82- and ClO- were observed for all the reactions except the reaction of [Co(II)HEDTA(OH2)]- with persulphate ion, where zero order with respect to concentration of S2O82- was obtained. The reactions of complexes with 𝑆2𝑂82−were independent of [H+] but dependent on [Ag+] while with ClO- the reaction rates were inversely dependent on [H+]. The reactions conform to the following rate equations:
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐸𝐷𝑇𝐴−] 𝑑𝑡=𝑎[𝐴𝑔+][𝐶𝑜(𝐼𝐼)𝐸𝐷𝑇𝐴2−][𝑆2𝑂82−]
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐻𝐸𝐷𝑇𝐴] 𝑑𝑡=𝑏[𝐴𝑔+][𝐶𝑜(𝐼𝐼)𝐻𝐸𝐷𝑇𝐴−]
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐸𝐷𝑇𝐴−]𝑑𝑡=𝑐[𝐻+]−1[𝐶𝑜(𝐼𝐼)𝐸𝐷𝑇𝐴2−][𝐶𝑙𝑂−]
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐻𝐸𝐷𝑇𝐴]𝑑𝑡= 𝑑[𝐻+]−1[𝐶𝑜(𝐼𝐼)𝐻𝐸𝐷𝑇𝐴−][𝐶𝑙𝑂−]
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where a = 5.90 × 10-2 dm3 mol-1 s-1, b = 5.4× 10-2 dm3 mol-1 s-1, c = 1.23 × 10-3 dm3 mol-1 s-1 , d = 2.28 × 10-4 dm3 mol-1 s-1. Zero salt effects for the reaction of complexes with S2O82- and positive salt effects with ClO- were observed. The thermodynamic parameters obtained from the temperature dependence studies showed that the activated complexes are ordered for the oxidation of [Co(II)EDTA]2- (ΔH* = +28.67 kJ mol-1, ΔS* = -170.72 J K-1 mol-1) and [Co(II)HEDTA(OH2)]- (ΔH* = +53.12 kJmol-1, ΔS* = -113.65 J K-1 mol-1) by persulphate ion. The free radical species were detected only in the reaction of complexes with persulphate ion. Addition of ions affected the rates for all the reactions except for the reaction of [Co(II)HEDTA(OH2)]- with persulphate ion, where both added cations and anions had no effect on the reaction rate. Moreover, Michaelis – Menten plot of 1/kobs versus 1/[oxidant] had zero intercept for all the reaction systems except for the oxidation of [Co(II)HEDTA(OH2)]- by persulphate ion. Based on the above observations, an outer-sphere mechanism is proposed as the plausible mechanism for all the reactions except for the reaction of [Co(II)HEDTA(OH2)]- with persulphate ion, where an inner-sphere mechanism is proposed.
TABLE OF CONTENTS
Page
Cover Page i
Fly Leaf ii
Title Page iii
Declaration iv
Certification v
Dedication vi
Acknowledgement vii
Table of Contents viii
List of Tables xii
List of Figures xiv
Abbreviations xvii
Abstract xviii
CHAPTER ONE
1.0 INTRODUCTION 1
1.1 Statement of Research Problem 2
1.2 Justification of the Research 3 1.3 Aim and Objectives 4 1.3.1 Aim 4
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1.3.2 Objectives 4
CHAPTER TWO
2.0 LITERATURE REVIEW 5
2.1 Aminocarboxylatocobaltate(II) Complexes 5
2.2 Reactions of Persulphate Ion (S2O82-) 8 2.3 Reactions of Hypochlorite Ion (ClO-) 9
CHAPTER THREE
3.0 MATERIALS AND METHODS 12
3.1 Materials 12
3.2 Methods 12
3.2.1 Preparation of [Co(II)EDTA]2- complex 12
3.2.2 Preparation of [Co(II)HEDTAOH2]- complex 13
3.2.3 Preparation of stock solution of sodium nitrate 13
3.2.4 Preparation of standard sodium carbonate solution 13
3.2.5 Preparation of standard nitric acid solution 13
3.2.6 Preparation of silver nitrate solution 14
3.2.7 Preparation of salts solutions 14
3.2.8 Preparation of sodium persulphate stock solution 14
3.2.9 Preparation of sodium hypochlorite stock solution 14
3.2.10 Stoichiometric studies 14
3.2.11 Kinetic measurement 15
3.2.12 Effect of acid and catalyst on the reaction rates 16
3.2.13 Effect of changes in ionic strength 16
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3.2.14 Effect of change in dielectric constant on the reaction rates 17
3.2.15 Effect of temperature on the reaction rates 17
3.2.16 Effect of added ions on the reaction rates 18
3.2.17 Spectroscopic test for the presence of intermediate complex 18
3.2.18 Free radical test 18
3.2.19 Products analysis 18
CHAPTER FOUR
4.0 RESULTS 19
4.1 Stoichiometric Studies 19
4.2 Order and Rate Constants of the Reactions 19
4.3 Effect of Acid and Catalyst on the Reaction
Rates 19
4.4 Effect of Changes in Ionic Strength and
Dielectric Constant on the Reaction Rates 19
4.5 Effect of Temperature on the Reaction Rates 46
4.6 Effect of Added Ions on the Reaction Rates 46
4.7 Spectroscopic Test 46
4.8 Free Radical Test 46
4.9 Michaelis – Menten Plot 46
4.10 Product Analysis 66
CHAPTER FIVE
5.0 DISCUSSIONS 71
5.1 Oxidation of [Co(II)EDTA]2- and [Co(II)HEDTAOH2]-
by Persulphate Ion 71
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5.2 Oxidation of [Co(II)EDTA]2- and [Co(II)HEDTAOH2]-
by Hypochlorite Ion 77
5.3 Comparison of the [𝐂𝐨(𝐈𝐈)𝐄𝐃𝐓𝐀]𝟐−−𝐒𝟐𝐎𝟖𝟐− with
[𝐂𝐨(𝐈𝐈)𝐄𝐃𝐓𝐀]𝟐−−𝐂𝐥𝐎− reaction 80
5.4 Comparison of the [𝐂𝐨(𝐈𝐈)𝐇𝐄𝐃𝐓𝐀𝑶𝑯𝟐]−−𝐒𝟐𝐎𝟖𝟐− with
[𝐂𝐨(𝐈𝐈)𝐇𝐄𝐃𝐓𝐀𝑶𝑯𝟐]−−𝐂𝐥𝐎− reaction 81
5.5 Comparison of the [𝐂𝐨(𝐈𝐈)𝐄𝐃𝐓𝐀]𝟐−−𝐒𝟐𝐎𝟖𝟐−
with [𝐂𝐨(𝐈𝐈)𝐇𝐄𝐃𝐓𝐀𝑶𝑯𝟐]−−𝐒𝟐𝐎𝟖𝟐− reaction 81
5.6 Comparison of the [𝐂𝐨(𝐈𝐈)𝐄𝐃𝐓𝐀]𝟐−−𝐂𝐥𝐎−with
[𝐂𝐨(𝐈𝐈)𝐇𝐄𝐃𝐓𝐀𝑶𝑯𝟐]−−𝐂𝐥𝐎− reaction 82
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION 83
6.1 Summary and Conclusion 83
6.2 Recommendation 84
REFERENCES 85
CHAPTER ONE
1.0 INTRODUCTION
Electron transfer (ET) reaction is simply the transfer of electrons between two species like ions, molecules, biological systems etc. The reaction usually involves rearrangement and transfer of outermost shell electrons of reacting species in order to gain stability. Thus, oxidation is the loss of electrons while reduction is the gain of electrons (Wong et al., 2002). Many reactions in inorganic and biological systems involve transfer of electron, thus, electron transfer reaction plays a key role to various physical and biological systems (Onu et al., 2008, 2009, 2015 and 2016; François and Jamal, 2016; Idris et al., 2015; Ilbert and Bonnefoy, 2013; Xie et al., 2012; Xiang-Rong and Xiang-Zhong 2010; Naik et al., 2007 and 2010).
Both Adenosine triphosphate (ATP) and dioxygen radical/hydrogen peroxide (O2-./H2O2) are generated in living cell by electron transfer. ATP is the product of oxidative phosphorylation whereas O2-. is generated by singlet electron reduction of dioxygen, O2 which is reduced by superoxide dismutase, SOD to H2O2 (Mailloux, 2015). Importantly, the controlled production of O2-./H2O2 is required for intrinsic mitochondria signaling (e.g. Modulation of mitochondria processes) and communication with the rest of the cell. This can be checked by understanding the effect of various parametres on the reaction. Moreover, the damaging effect of oxygen towards anaerobic organism is associated with its free radical properties (Ilbert and Bonnefoy, 2013). One of the promising areas that explains the effect of various parametres on this electron transfer reaction is chemical kinetics.
Chemical kinetics also known as reaction kinetics is the area of Chemistry concerned with reaction rates, factors affecting the rates and sequence of steps (mechanistic pathways) by which reactions occur (McMurry and Fay, 2008). The two well established general mechanisms of electron transfer reactions are the outer-sphere and the inner-sphere mechanisms (Cox, 2004). In
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the outer-sphere mechanism, no substitution of the ligands into the inner coordination spheres of either reactant is needed for electron transfer to take place (Housecroft and Sharpe, 2008). On the other hand, the inner-sphere mechanism involves distortion of the inner-coordination sphere of reactants with the formation of a bridged activated intermediate prior to electron transfer (Jagannadham, 2012).
1.1 Statement of Research Problem Aminopolycarboxylic acids that have lost acidic protons form strong complexes with metal ions (Anderegg et al., 2005) which makes them useful complexone in a wide variety of chemical, environmental and medical applications (Onu et al., 2008, 2009, 2015 and 2016; Naik et al., 2007 and 2010; Bugaje, 2006 and Mansour, 2003). Metal-aminocarboxylate complexes are used to study structure, stability, magnetic properties and non-covalent interactions, as well as play an important role in metalloenzyme catalysed reactions (Vuckovic et al., 2011). Also, transition metal containing aminocarboxylate ligands are generally accepted as simple models for some biological systems involving transition metals (Swaroop et al., 1991). Typical examples of these complexes are [𝐶𝑜(𝐼𝐼)𝐸𝐷𝑇𝐴]2− and [𝐶𝑜(𝐼𝐼)𝐻𝐸𝐷𝑇𝐴𝑂𝐻2]−. The biological importance of cobalt is well documented (Michihiko and Sakayu, 1999; David et al., 1999 and Chang et al., 2010). Cobalt is an active centre for co-enzymes called cobalamins (also known as vitamin B12). These are group of biologically active cobalt-containing compounds (corrinoids) (Miller et al., 2005 and Arslan et al., 2013). Cobalamin plays a specific role in amino acid metabolism, normal functioning of the brain and nervous system via the synthesis of myelin as well as an essential factor in DNA synthesis for chromosomal replication and division (Miller et al., 2005; Schoonover et al., 2004; Reynolds, 2006 and Arslan et al., 2013). However, deficiency of cobalt in human leads to pernicious anemia and possible lethal disorder (Michihiko and Sakayu, 1999). These can be treated
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by the use of synthesised cobalamins, hydroxo-cobalamin or cyano-cobalamin (Carmel, 2008 and Andres et al., 2009 and 2010), hence, the need to study their reactions. Moreover, Despite the applications of these important complexes, there is a paucity of information on kinetics of electron transfer reaction of aminocarboxylatocobalt(II) with oxyanions such as persulphate ion, S2O82- and hypochlorite ion, ClO- though, other researchers (Onu et al., 2008, 2009, 2015 and 2016; Naik et al., 2007 and 2010; Bugaje, 2006 and Mansour, 2003) have carried out the study using different oxidants. In fact S2O82- ion even though a good oxidant ( E = +2.01 V) but it is slow to reduction even in the presence of strongly reducing agents due to strong O-O bond which need to be broken in the redox process (Burgess, 1999). In addition, though ClO- ion is the strongest oxidizing agent of the chlorine oxyanions (Mohammed et al., 2010) but its reactions with these complexes have not been reported. In view of the aforementioned important applications of cobalt-aminocarboxylate complexes coupled with desire to gain further insight into the reaction of the complexes with these oxyanions (S2O82- and ClO-), this study is therefore embarked upon to generate kinetic data for the reactions as well as to establish its mechanisms. 1.2 Justification of the Research The study of oxidation of aminocarboxylate complexes by oxy-anions is of potential interest to understanding the mechanism of oxygen transport in biological systems (Onu, 2010). Interestingly, this study will be used as simple models to understanding or to mimic the biochemical pathways of some metabolic processes which involve the use of co-enzymes. Moreover, the study will be significant to the inorganic mechanistic community as the kinetic data generated from this research will complement the much needed kinetic information that will deepen the ever growing field of inorganic reaction mechanism. Also, the research is
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expected to contribute to knowledge particularly in the area of inorganic chemistry, biochemistry and science in general towards better understanding and more efficient utilization of the reactions of these complexes.
1.3 Aim and Objectives
1.3.1 Aim The aim of this work is to study the kinetics of oxidation of aminocarboxylatocobaltate(II) complexes (hereafter, [Co(II)EDTA]2- and [Co(II)HEDTA(OH2)]-) each by persulphate and hypochlorite ions and establish the mechanism of the reactions.
1.3.2 Objectives The aim will be achieved through the following objectives: a. preparation of the complexes b. determination of the stoichiometry of the reactions; c. determination of the kinetic and non-kinetic parametres for the reactions and d. generating the thermodynamic parametres for the reactions
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