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 1oC. 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
d Fe2adi
given as
a bH
dt
[Fe2adi] [reductant] [ H ] for Fe2adi–LSH, Fe2adi –
GSH, and Fe2adi – BTSH systems where a and b are the intercept and slope
respectively,
d Fe 2 adi
dt
a bH
2 [Fe adi] [USH]2 for Fe adi–USH, while the rate
equation, was for Fe2adi –BSH system is
d Fe 2 adi
dt
| 2 2 |
b [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 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-1 and -30195Jk-1 mol-1 for 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/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.
1.0 INTRODUCTION
CHAPTER ONE
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 porphyrin7,27,28 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 Reaction1 Rates
The first step in kinetic analysis of a given reaction is to ascertain the
stoichiometry of the reaction and to identify any side reaction. The fundamental data of chemical kinetics are the concentrations of the reactants and products at different times after a reaction has been initiated.1 The rates of most chemical reactions are sensitive to the temperature aid. In conventional experiments, the temperature of the reaction mixture must be held constant throughout the course of the reaction.
The method employed in monitoring the rate of a reaction depends on the concentration of the species involved and on how fast the concentrations change. Reactions may take seconds, minutes or hours before they can reach equilibrium. The techniques used to monitor the change in concentration are as follows:
1.2.1 Conventional Method (Slow Technique)
Conventional methods involve the measurement of the concentration or any physical property of one or more of the reactants or products as a function of time. For instance, in some reactions absorbance of any of the reactants or products could be measured and related directly to the concentration.
In kinetic analysis, the composition of the system is examined while the reaction is in progress by either withdrawing a small sample or the bulk and the reactants are mixed as they flow together in a reaction container. At different level in the observation tube, the mixtures are examined at different time of mixing and by doing so, the rate of the reaction is obtained.
The conventional method is difficult for rapid reactions due to the fact that: (i) The time it takes to mix reactants or to bring them to a specified temperature
may be significant in comparison with the half life of the reactants.
(ii) Also, the time that it takes to make measurement of concentration is significant compared with the half life.
1.2.2 Monitoring of the Rates of Fast Reactions
The rates of fast reactions can be monitored effectively by the following methods:
(i) Flow Techniques:
Flow techniques were developed in an effort to monitor the rates of a very fast reactions at the shortest possible time.3 Different flow techniques exist depending on the treatment given to the reaction after mixing. They include continuous flow technique, quenched flow method and stopped flow technique.
In continuous flow technique, the reaction solution is allowed to flow along an observation tube where the changes in the reaction mixture is monitored at different points along the tube or at a fixed point in the tube.
Quenched flow method involves quenching a reaction in progress after it has been allowed to proceed for a certain period of time. In this way, a reaction mixture which has reaction time scale on the order of milliseconds can be studied with ease. Once the reaction has been quenched, the mixtures comprising the concentration of reactants, intermediates and products can be measured by chromatographic (slow technique) or spectroscopic method.
In stopped flow technique, the reaction mixture is put to the reaction cuvette, where the reactants are brought into a complete contact in less than 10-3 second.1 The technique allows for the study of reactions that take place on the time scale of millisecond. This technique is efficient in monitoring many biochemical reactions like the enzymatic action of some proteins. Spectroscopic
method is used effectively in this technique.
(ii) Relaxation Method: (Temperature Jump Method)
Relaxation method is used to analyze a very fast reaction. 1,2 When an electric spark is passed through the solution, the spark causes a very large, but brief rise in temperature. This upsets the solution in equilibrium such that it relaxes to another equilibrium state. In this way the concentration of the solution can be measured spectrophotometrically. This is popularly known as temperature jump method.
(iii) Resonance Techniques:
Rates of reaction could be monitored by using nuclear magnetic resonance technique 1. Resonance absorption line is related to the t of the
nucleus in a given energy state. If the life-time of these states is shortened by a chemical interaction, it results into line broadening. 1H n.m.r line broadening has been used to measure the rate of change of various mono and bidentate nitrogen and oxygen donor ligands coordinated to Mn(II),Fe(II), Co(II),Ni(II) and Cu(II).
(iv) Flash Photolysis
This technique can measure rates of reactions that are extremely fast. In this case, a very short but intense flash of light passes through the mixture. After a brief period of time, another flash of light passes through the mixture. The molecules produced in the reaction absorb light from the second flash.3 By taking a photograph, the spectrum of the molecules can be recorded and the intensity of the lines in the spectrum gives a measure of the concentrations of the molecules.
If the time interval between the first and second flashes is changed, the intensity of the lines changes. In this way, a series of experiments allow the way the concentration of the molecules changes with time to be found. An example is the light induced dissociation of chlorine gas. Other methods of monitoring rates of reactions are titrations, colour changes, volume changes, and pressure changes.
1.3 The Theories of Reaction Rate
The general goal of theoretical chemical kinetics is to rationalize many of the empirical (or observed) facts of chemical kinetics in terms of molecular properties. Prominent among these facts are the effects of concentration and temperature on reaction rates. Indeed, the ultimate goal of theoretical chemical kinetics is the calculation of the rate of any reaction from a knowledge of the fundamental properties of the reacting molecules, namely, their masses, diameters, moments of inertia, vibrational frequencies, binding energies etc. The main theories describing the rates of reaction are highlighted below.
1.3.1 Arrhenius Theory
Arrhenius theory states that the rates of a chemical reaction always increases with increase in temperature to a marked extent. It has been observed that as a rule, the specific rate constant of a homogeneous reaction is usually increased by a factor of about two or three for every 1 degree rise in temperature.9,38 An expression relating rate constant with temperature was
derived by Arrhenius in 1889. According to him,
k = Ae –
Ea
k Ae RT
————————————————————— 1.10
Where k is rate constant
A is called pre-exponential factor or frequency factor. Ea is the activation energy
R is the universal gas constant
A and Ea are collectively known as the Arrhenius parameters.
1.3.2 The Collision Theory of Reaction Rate
This theory makes the basic assumption that for a chemical reaction to occur, particles must collide. 9,38 In the reaction
A + B AB ……………………………………………………………(1.11)
The particles A, be the molecules, ions or atoms must collide with particles B. In collision, chemical bonds in atoms and electrons are always rearranged and as a result, new species are produced. According to the collision theory, the rate of any step in a reaction is directly proportional to,
(i) The number of collisions per second between the reacting particles involved in that step and
(ii) The fraction of these collisions that are effective
Actually, not all collisions lead to reaction, otherwise every bimolecular reaction occurring at the same temperature and concentration would occur at the same rate. Besides, since the frequency of binary collision is proportional to
This material content is developed to serve as a GUIDE for students to conduct academic research
KINETICS AND MECHANISMS OF THE REDOX REACTIONS OF µ-ADIPATO-DI (N,N/ BIS(SALICYLIDENE) ETHYLENEDIAMINATOIRON (III) [(FE-SALEN)2ADI] WITH SOME THIOLS
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