ABSTRACT
Copper(II) catalysed reduction of aminocarboxylatocobaltate(III) ion (hereafter referred to as [CoEDTA]–) and aminocarboxylatocobalt(III) complex (hereafter referred to as [Co(HEDTA)OH2]) by L-ascorbic acid (hereafter referred to as H2A) and Hydrazine (hereafter referred to as N2H4) have been spectrophotometrically studied in aqueous acidic medium. Stoichiometric studies gave a mole ratio of 2:1 for [CoEDTA]– : H2A, [CoEDTA]– : N2H4, and [Co(HEDTA)OH2] : N2H4 systems, but 1:1 for the [Co(HEDTA)OH2] : H2A system respectively. A first order with respect to the concentrations of each reactant was obtained for the [CoEDTA]– – H2A, [CoEDTA]– – N2H4, and [Co(HEDTA)OH2] – N2H4 systems. For the [Co(HEDTA)OH2] – H2A system, a first order with respect to [Co(HEDTA)OH2] and a half order with respect to [H2A] was obtained. Further kinetic studies on the effect of [H+] and [Cu2+] gave result which conforms to the following rate equations:
a = 3.47 × 10-4 s-1 , b = 4.9 s-1, c = 18.25 s-1, d = 4.47 s-1/2 and e = 1.8 × 102 s-1.
Negative salt effects were observed for [Co(EDTA)]– – H2A and [Co(EDTA)]– – N2H4 systems while neutral salt effect were obtained for [Co(HEDTA)OH2] – H2A and [Co(HEDTA)OH2]– N2H4 systems respectively. The test for the effect of added ions showed catalysis for
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[Co(EDTA)]– – H2A, [Co(HEDTA)OH2] – H2A and [Co(HEDTA)OH2] – N2H4 systems, and no effect on [Co(EDTA)]– – N2H4 reaction. Results from the thermodynamic measurements indicated that ΔH‡ = 72.75 kJ mol-1 and ΔS‡ 43.65 JK–1 mol–1 for [Co(EDTA)]– – H2A system, ΔH‡ = 31.46 kJ mol-1 and ΔS‡ = –175.58 JK–1 mol–1 for [Co(EDTA)]– – N2H4 system, ΔH‡ = 85.21 kJ mol-1 and ΔS‡ = 7.02 JK–1 mol–1 for [Co(HEDTA)OH2] – H2A system, ΔH‡ = 82.89 kJ mol-1 and ΔS‡ = 17.13 JK-1 mol-1 for [Co(HEDTA)OH2] – N2H4 system respectively. The zero intercept obtained from the Michaelis – Menten plots, enhanced rate from added ions, and the absence of spectroscopically determinable intermediates led to the proposal that the [Co(EDTA)]– – H2A and [Co(HEDTA)OH2] – N2H4 reactions occurred via the outer – sphere mechanism. However, the non – zero intercept on the Michaelis – Menten plot, the presence of a spectroscopically determinable intermediate, negative values of the entropy of activation and non – catalysis by added ions led to the suggestion that the [Co(EDTA)]– – N2H4 reaction occurred via the inner –sphere mechanism. Both the outer – sphere and inner – sphere mechanisms have been proposed for the [Co(HEDTA)OH2] – H2A reaction due to non –zero intercept on the Michaelis – Menten plot, catalysis by added ions and positive values of activation entropy.
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TABLE OF CONTENTS
Cover page i Fly leaf ii Title page iii Declaration iv Certification v Acknowledgement vi Dedication vii Table of Contents viii List of Tables xiii List of Figures xv Abbreviations xix Abstract xx CHAPTER ONE 1 1.0 INTRODUCTION 1 1.1 Statement of Research Problem 2 1.2 Justification of the Study 3 1.3 Aim of the Study 3 1.4 Objectives of the Study 4
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CHAPTER TWO 5 2.0 LITERATURE REVIEW 5 2.1 Ascorbic Acid 5 2.1.1 Reactions of ascorbic acid 6 2.2 Hydrazine Monohydrate 6 2.2.1 Reactions of hydrazine monohydrate 7 2.3 Cobalt(III) Complexes 8 2.3.1 Reactions of cobalt(III) complexes 8 2.3.2 Cobalt(III) complexes of aminocarboxylates 10 CHAPTER THREE 13 3.0 MATERIALS AND METHODS 13 3.1 Materials 13 3.1.1 Preparation of cobalt(III) complexes 14 3.1.1.1 Preparation of [Co(EDTA)]– solution 14 3.1.1.2 Preparation of [Co(HEDTA)OH2] solution 14 3.1.2 Preparation of stock L-ascorbic acid solution 15 3.1.3 Preparation of stock hydrazine solution 15
3.1.4 Preparation of standard perchloric acid solution 15
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3.1.5 Preparation of standard sulphuric acid solution 15 3.1.6 Preparation of stock copper sulphate solution 16 3.1.7 Preparation of stock sodium perchlorate solution 16 3.1.8 Preparation of stock sodium sulphate solution 16 3.1.9 Preparation of sodium carbonate solution 16 3.1.10 Preparation of salt solutions 17 3.2 Methods 17 3.2.1 Stoichiometric studies 17 3.2.2 Kinetic measurements 18 3.2.3 Effect of [H+] on the reaction rate 19 3.2.4 Effect of catalyst on the rate of the reaction 20 3.2.5 Effect of ionic strength on the reaction rate 21 3.2.6 Effect of dielectric constant of reaction medium on the rate of the reaction 21 3.2.7 Effect of change in temperature on the reaction rate 22 3.2.8 Effect of added ions on the reaction rate 22 3.2.9 Test for intermediate complex 23 3.2.10 Test for free radical 23
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3.2.11 Product analysis 23 3.2.11.1Qualitative methods 23 3.2.11.2 Spectrophotometric methods 24 CHAPTER FOUR 25 4.0 RESULTS 25 4.1 Stoichiometry 25 4.2 Determination of Order of the Reactions with Respect to the Reactants 30 4.3 Effect of Hydrogen Ion Concentration on the Rate of the Reactions 43 4.4 Effect of Catalyst Concentration on the Rate of the Reactions 46 4.5 Effect of Ionic Strength of the Reaction Medium on the Rate of the Reaction 55 4.6 Effect of Dielectric Constants of the Reaction Medium on the Rate of the Reaction 58 4.7 Effect of Added Anions on the Rate of the Reaction 58 4.8 Effect of Added Cations on the Rate of the Reaction 75 4.9 Effect of Change in Temperature on the Rate of Reaction 75 4.10 Test for Intermediate 88 4.10.1 Test for free radical 88 4.10.2 Michaelis – Menten plot 88 4.10.3 Spectrophotometric test 88
4.11 Product Analysis 97
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4.11.1 Qualitative methods 97 4.11.2 Spectrophotometric methods 97 CHAPTER FIVE 98 5.0 DISCUSSION 98 5.1 [Co(EDTA)]– – H2A System 98 5.2 [Co(EDTA)]– – N2H4 System 102 5.3 [CoIII(HEDTA)OH2] – H2A System 106 5.4 [CoIII(HEDTA)OH2] – N2H4 System 110 5.5 Comparative Analysis of the Cobalt(III) Reactions Studied 113 5.5.1 [Co(EDTA)]– – H2A and [Co(EDTA)]– – N2H4 sytems 113 5.5.2 [Co(HEDTA)OH2] – H2A and [Co(HEDTA)OH2] – N2H4 systems 113 5.5.3 [Co(EDTA)]– – H2A and [Co(HEDTA)OH2] – H2A systems 114 5.5.4 [Co(EDTA)]– – N2H4 and [Co(HEDTA)OH2] – N2H4 systems 114 CHAPTER SIX 115 6.0 SUMMARY, CONCLUSION AND RECOMMENDATION 115 6.1 Summary 115 6.2 Conclusion 116 6.3 Recommendation 117 REFERENCES 118
CHAPTER ONE
1.0 INTRODUCTION
Many electron transfer reactions occur naturally, and they have vast applications in modern chemistry, thus making the kinetic study in this area ever promising. In various kinetic studies, the rate of the reaction is often monitored by changing experimental conditions such as temperature, concentrations and pressure (Asperger, 2003). These changes in experimental conditions give good information that sheds light on the mechanism of such reactions and these mechanisms are generally classified into the outer – sphere and inner – sphere mechanisms (Twigg, 1983; Miessler et al., 2014). Of the many mechanisms obtained from kinetic studies, those involving metal complexes are of great interest, due to the wide applications of such metal complexes (Davies, 1982; Bazhko, 2009; Gamenera et al., 2013; Joseph et al., 2013; Jiewei et al., 2014). Many researchers investigated reduction of these metal complexes with various reductants in order to gain kinetic data, of the various reductants used; L-ascorbic acid and hydrazine have proven to be very potent. Works exist on the reduction of these metal complexes by L-ascorbic acid (Laurence and Ellis, 1972; Kustin and Toppen, 1973; Pelizzetti et al., 1978; Lappin, 1981; Brzyska and Krol, 1988; El-Zaru and Hodali, 1990; Davis, 1992; Leal et al., 1993; Saha et al.,1995; Abdur-Rashid, 1996) and hydrazine (Carl et al., 1975; Sadagopa et al., 1975; Micheal et al., 1978; David, 1984; Sultan et al., 1985; Patapati et al., 1986; Zagal et al., 1986; Max et al, 1998; 1999; Rupa et al., 1999; Larsen et al., 2001; Jhimli et al., 2004; Zheng et al., 2005; Mshelia et al., 2010; Ravi and Brian, 2012; Cantillo et al., 2013; Ghanbari et al., 2013; Streszewski et al., 2014; Shirin et al., 2016). In fact, there is a growing interest on the redox activity of these reductants with Co(III) complexes (Mondal and Banerjee, 2009; Majumdar, 2010; Gain et al., 2011; Sadhana et al., 2014; 2015), due to the inert nature of some Co(III)
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complexes towards redox reactions (Davis, 1992; Hemzeh, 2001) which in turn provides a promising field for researchers to explore the use of catalysts in enhancing reaction rates (Luty-Blocho et al., 2013; Singh et al., 2014). 1.1 Statement of Research Problem Co(III) aminocarboxylato complexes have vast industrial and pharmaceutical applications and are important antimicrobial agents. Their antimicrobial activities have been attributed to series of redox reactions involving the metal complexes with electron sensitive substrates accessible within intracellular bio – molecules of these microbes, which often leads to oxidative stress (Liu, 2002; Joseph et al., 2013). Furthermore, these complexes are useful in agricultural industries, as they have been used for top dressing in Co – deficient pastures. These complexes when mixed in little amount with fertilizers can be assimilated by Co – deficient plants, which is subsequently involved in important redox activities to facilitate effective plant growth (Sherrell, 1990; Sekhon, 2003). Co(III) aminocarboxylato complexes can also be useful and can serve as models for vitamin B12 (cobalamine). The Co – N bonds found in both Co(III) aminocarboxylato complexes as well as cobalamine, and the fact that the central Co(III) metal ion in cobalamine is often reduced to Co(II) during bioactive processes makes Co(III) aminocarboxylato complexes useful models for studying redox processes as it relays to cobalamine.
It is of interest to note however that even though there exist vast applications of these complexes which are in turn related to their redox activaties, the kinetic data on the redox reactions of these cobalt(III) aminocarboxylato complexes are scanty. The paucity of information as to their redox reactions have been linked to the slow rate and sometimes kinetically unfavourable nature of their redox reactions (Hamzeh, 2001; Sadhana et al., 2015). Therefore,
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kinetic studies of the reaction of these complexes with ascorbic acid and hydrazine which involves the use of catalyst is embarked upon to get full insight into the mechanism that underlie the reaction. 1.2 Justification of the Research It is established that Co(III) aminocarboxylato complexes have vast uses which are based on their redox properties. In spite of this, kinetic data on their redox reactions are scanty, there is therefore the need to study the kinetics of their reaction and consequently establish the mechanisms of these reactions. This will go a long way in enhancing their activities by effectively altering their reaction conditions and hence, optimising their usefulness. Moreover, the data generated from this study would satisfy the need to understand the redox processes involving Co(III) complexes in biological systems. In addition, unveiling the mechanism behind the redox processes of these stable cobalt(III) complexes hold great importance in the kinetic community, this is partly because the complexes are widely common. As inorganic reaction mechanism forms a basic curriculum of inorganic chemistry in various graduate and undergraduate courses, the findings in this study would therefore come in handy in bridging the ambiguity encountered while addressing the kinetics and mechanisms of Co(III) aminocarboxylato complexes. 1.3 Aim of the Study The aim of this work is to study catalysed electron transfer reactions of [Co(EDTA)]– and [Co(HEDTA)OH2] complexes with L-ascorbic acid and hydrazine monohydrate respectively.
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1.4 Objectives of the Study The stated aim would be achieved via the following objectives: a. To determine the stoichiometry and analyse the products of the reactions. b. To carry out kinetic and temperature dependent studies. c. To propose plausible mechanism for the reactions.
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