Kinetics and Mechanism of Oxidation of Hexamethylenediaminetetraacetatocobaltate(II) Complex by Periodate Ion in Aqueous Medium

The kinetics and mechanism of oxidation of [CoII HDTA]4- (Where HDTA=Hexamethylenediamine tetraacetic acid} by periodate ion has been studied in aqueous acidic medium. The reactions has been investigated spectrophotometrically at λmax = 580 nm under pseudo- first -order condition by taking large excess of oxidant [IO4] at pH = 4.0±0.02, I = 0.1 M (CH3COONa + NaNO3).and temperature = 30± 0.1 °C The electron transfer reaction between [CoII HDTA] 4- and [IO4-] obeys inner sphere reaction pathway through the formation of long-lived intermediate complex which finally get converted into a corresponding [CoIIIHDTA] 3- complex as final reaction product. The experimental observations have shown that the reaction obey first- order dependence in [CoII HDTA] 4--. The variation of pseudo-first-order rate constants (kobs) with[IO4-], keeping other reaction variables fixed at constant value was found to obey the rate law: kobs=a[IO4-]2/b+c[IO4-], which is consistent with a three step mechanistic scheme. The values of kobs are almost constant with increasing pH, which can be attributed to the reaction of deprotonated form of [CoIII HDTA] 4- complex only, in the entire pH region. Eyring’s equation has been used to calculate the thermal or activation parameters and found to be, ΔH# = 28.69 kJ mole–1; ΔS# = – 481.13 J K-1 mole–1 respectively and support the proposed mechanistic scheme.


Introduction
Sodium meta periodate ] IO [ 4 − has been used for the oxidation of vast variety of inorganic [1][2][3][4] and organic [5][6] substrates. The periodate oxidation of [Co II L(H 2 O)] 2-n (L= EDTA, HEDTA, NTA, DTPA, PDTA and HPDTA) complexes have been the subject of interest for several investigators in the past and recently 7 . The detailed kinetic and spectroscopic studies on periodate oxidation of these complexes suggests that the electron transfer takes place through the formation of inner sphere complexes of type [LCo II -OIO 3 ] 1-n and [LCo II -(OIO 3 ) 2 ] -n which eventually get converted slowly producing [Co III L(H 2 O)] 3-n as final reaction product. The periodate oxidation of V(IV) 2 , Fe(II) 4 and Cr(III) 8  The oxidation of cobalt(II) complexes by ] IO [ 4 − appears to proceed by two parallel pathways, the first obeying first-order kinetics in each reactant and the other first order in cobalt(II) complexe and second order in oxidant concentration. The pathway which is first order in each reactant concentration, almost certainly proceed via an inner sphere electron transfer process and lends further support to the proposed mechanistic scheme for the reaction at hand.
In the present study, an attempt have been made to verify the consistency in kinetics and mechanism of oxidation of [Co II L(H 2 O)] 2-n (L=Hexamethylenediamine tetraacetic acid (HDTA)) by periodate ion in acidic buffer medium to assess the reactivity of periodate ion with relatively more stable complex of Co(II) with HDTA.

Experimental
Double distilled water was used to prepare all the solutions throughout the present study. Since polyaminocarboxylates are sparingly soluble in water, accordingly a small amount of dil. sodium hydroxide was added to prepare stock solution of ligand viz. HDTA (Aldrich) by weighing its calculated amounts. Fresh solution of sodium meta periodate (S.d Fine-Chem Limited) was prepared and wrapped with aluminium foil to avoid photochemical decomposition 11 . Sodium acetate (Merck) and sodium nitrate (Sigma) were prepared by weighing their calculated amounts. The standard solution of acetic acid was prepared by titrating it against sodium hydroxide using phenolphthalein as an indicator.
The progress of reaction was monitored spectrophotometrically using a single beam visible spectrophotometer DIGI-110 (SISCO, India) equipped with thermostated cell compartment. A Shimadzu double beam spectrophotometer model UV-240 was used for recording UV-visible spectrum of reactants and products as well as the repetitive spectral scans of the reaction mixture as a function of time under specified experimental conditions. The pH of the reaction mixtures were checked on a Toshniwal digital pH meter. The pH meter was standardized regularly using standard BDH buffers of pH = 4.0 and 7.0.

Kinetic measurements
The solution of [Co II HDTA (H 2 O)] 2was prepared by dissolving 10% excess of ligand viz. HDTA over cobalt nitrate solution, in order to ensure the complete complexation. The excess ligand also reduces the possibility to catalyse the periodate ion oxidations by any trace metal ions such as Cu(II) 12 and Mn(II) 13 present as impurities. The Co(II) complexes were prepared immediately before each kinetic run to avoid the possibility of any air oxidation of Co(II) complexes [14][15][16] . The kinetics of oxidation of [Co II L(H 2 O)] 2-n complex was studied in pH range 3.0-5.0 where no significant air oxidation of cobalt(II) complex was noticed.
The temperature of reactants were maintained at 30±0.1°C by hanging them for at least half an hour in a thermostat and subsequently mixed in a sequence [Co II L(H 2 O)] 2-n , buffer and sodium meta-periodate. The reaction mixture was shaken properly and transferred immediately to a cuvette of spectrophotometer having 10 mm path length. The temperature of the cuvette compartment too was maintained at 30±0.1°C by a circulatory water arrangement from thermostat. The reaction was found to proceed in forward direction with an increase in absorbance at 580 nm, where

Results and Discussion
The eq.(1) is in good conformity with the equation of a straight line. The values of k 2 and k 3 can be obtained from the intercepts and the slopes of the linear plots given in Figure 2. The values of k 2 and k 3 are found to be 2.66×10 -2 M -1 sec -1 , 6.6×10 -2 M -2 sec -1 for It is interesting to note that the value of k 2 obtained from the intercepts of the plots given in Figure 2 is in excellent agreement with the slopes (k f ) calculated from the plots of Figure 1. The mathematical basis of eq.(1) will be presented on the basis of proposed mechanistic scheme for the reactions at hand (vide supra).  Table 3. The values of k f are found to be almost constant with increasing ionic strength of medium. This constancy in values of k f or k obs with ionic strength may be either due to k 2 or k 3 or both (vide supra). The influence of temperature on oxidation of both [Co II HDTA(H 2 O)] 2by periodate ion have been studied in the temperature range T = 298-323 o K. Activation parameters have been calculated by plotting ln(k f /T) versus (1/T) using Eyring's equation (2).

The effect of complex variation on periodate oxidation of [Co II HDTA(H 2 O)] 2-
Where all the terms have their usual meanings. The values of evaluated activation parameters are compiled in Table 4. The negative value of entropy of activation (∆S # ) suggests that reaction proceed via on associative (S N ²) pathway through an inner sphere electron transfer. The    (11) Comparison of eq.(7) and eq. (11) gives: The graph plotted between k obs versus ] IO [ 4 − shown in Figure 1 and Figure 2 have a slope which is found to be equal to k 1 k 1 et = k f . Hence, the experimental rate law for first pathway [eq.  (5) and (6) of proposed mechanism are to be considered simultaneously to be rate determining steps and the rate law is given by eq. (13).
Now on comparing eq.(14) with eq. (7), we get From eq. (14) and eq. (9), we get: The theoretical rate law for the second path of reaction given by eq.(16) is in conformity with the empirical experimental rate law given by eq.(1) where k 2 = k 1 et k 1 and k 3 = k 1 k 2 k 2 et . It was earlier proposed that the oxidations by periodate ion generally proceed via an inner sphere mechanism 1,3-5 . Hence, the oxidation process involving second order dependence in  The pseudo-first-order rate constant (k obs ) and forward rate constant (k f ) are found to be invariant with increasing ionic strength of the medium ( Table 3). The constancy in values of k obs and k f with increasing ionic strength is due to k 2 . Eqs. (5) and (6) (5) and (6) and there is no second ionic species reacting in these steps. Therefore, the values of k obs and k f are found to be fairly constant. The values of activation parameters for the system show that the reaction is probably exothermic in nature due to formation of [LCo II -OIO 3 ] 1-n in each case. We could not make any inference on the entry of second periodate ion in absence of any experimental data on activation parameters for second path of reaction. However, it is expected that this path will have relatively higher values of enthalpy of activation in each case and will be endothermic in nature.