Mn(III)acetate-Induced Electron Transfer in Pentaammine Cobalt(III)Complexes of α-Amino Acids in Micellar Medium

A micellar catalyzed oxidation of pentaamminecobalt(III) complexes of α-amino acids by Mn(III)acetate in acidic medium yielding nearly 100% Co(II) and about 100% carbonyl compounds are ultimate products. The unbound amino acids yield about 100% of carbonyl compound in presence of micelles. The effect of variation of sulphuric acid has been carried. The decrease in UV-Visible absorbance at λ=502 nm for Co(III) complex corresponds to nearly 100% of the initial absorbance. In spite of the stoichiometry of Mn(III) to unbound ligand is 2:1, the ratio of Mn(III) to cobalt(III) complex is 1:1 accounting for about 100% reduction at the cobalt(III) centre. The kinetic and stoichiometric results have been accounted. A suitable mechanism consistent with the experimental findings has been proposed by involving a radical cation intermediate.


Introduction
The electron transfer in pentaamminecobalt(III)complexes of amino acids with Mn(III) has been already reported [1][2][3][4][5][6][7] .The extent of Mn(III) oxidation of pentaammine-cobalt(III) complexes of α-amino acids with NaLs and CTAB micelles as important to study the rate of decomposition of the complexes.Since micellar catalysis is an area of tremendous interest due to its occurrence in reaction important to biochemical, biotechnology, and industrial processes.It as the diagnostic tool to find out the Fraction proceeding by synchronous cleavages of N-H and C-C bonds in micellar medium [8][9][10][11][12] .The surfactants used in the present work are NaLS 13 and CTAB 14 .The surfactants are purified by adopting earlier procedure [15][16][17] .

Experimental
Carbonatopentaamminecobalt(III)nitrate was prepared by dissolving 58g of powdered ammonium carbonato in 60 mL of water and 100 mL of concentration aqueous ammonia, adding a solution of 30 g of cobalt(II) nitrate.6 hydrate in 40 mL of water and then bubbling air very slowly through the mixture (20 bubbles/min) for 20 days.The solution was cooled to 0⁰ and 600 mL of methanol was added slowly with stirring.The preparation was kept at 0⁰ for 3 days, and the precipitated carbonato nitrate was filtered off.This was purified by dissolving in twice its weight of water, adding LiCl (1 g of LiCl / 2 g of complex), filtering and then slowly adding an equal volume of methanol.The solution was kept 0⁰ for 10 h and the crystalline complex was filtered off and dried in vacuum.

Pentaaminecobalt(III)Complexes of -Amino Acids
10mmol of the acid and 5 mmol of LiOH (or) NaOH were added to 20 mL of absolute methanol and to the mixture was added 400mg of finely ground carbonatopentaamminecobalt(III)nitrate.The mixture was refluxed for 2hr with frequent shaking.The preparation was cooled to under ice and 1ml of conc.HclO 4 was added, after which the preparation was kept at 0⁰ for an additional 30 min.the precipitate, if any was filtered off and washed with ether.The mother liquor was shaken with 150 mL of ether, generally precipitation an additional portion of the desired complex.

Manganese (III)acetate (dehydrate)
30.3 g anhydrous Mn(OAc) 2 in 440 g glacial acetic acid was refluxed for 2 h.6.82 g KMnO 4 was added and the solution further refluxed for 45 min.The solution was allowed to cool while 75 mL water was added drop wise.After 3 days standing, the precipitate was filtered off.

Kinetic Method
All the glass apparatus were made of Pyrex glass and the stoppers were well ground.The loss of solvent was tested in standard flask and in reaction bottles and found to be negligible.Burettes, pipettes, and standard flasks were standardized by usual procedure.The temperature was controlled by an electrically operated thermostat (Concord Instruments (P) Ltd).It was provided with sufficient thermal lagging, suitable heaters.Stirrer and with proper cooling arrangements for continuous work.The temperature was maintained to ±0.2⁰C using Jackson thermometer worked in conjunction with Sunvic electronic relay.A sensitive thermometer reading to one tenths of a degree was used to measure the temperature accurately.The bath liquid water was covered with a layer of thermo cool bits to minimized beat loss due to radiation and also water loss due to evaporation.

Rate Measurements
The standard solution prepared was thermostated for nearly three hours at the required temperature.The desired amounts of solutions were pipette out into a 1 cm cell.The total volume of a reading mixture in the spectrophotometric cell was kept as 2.5 mL for performing the kinetic run.(Systronic) Digital Spectrophotometer fitted with recording and thermostating arrangements was used to follow the rate of a reaction.Rate of Mn(III)acetate reaction with Co(III) complexes were estimated from the observed decrease in absorbance at 502 nm the λ max For -O-bound Co(III) monomeric complexes.Ionic strength was maintained by addition of suitable quantities of sulphuric acid.The rate of disappearance of Mn(III)acetate was followed and the estimated rates compared with the above values.For the reaction involving unbound ligands.
For all the kinetic experiments, conversions were followed at least for four half lives and the specific rates from successive half-life values agreed with in ±7% and the average values did not differ from those obtained from a plot of logarithmic change in absorbance against time.The pseudo-first order rate constants calculated using integrated rate equations, k 1 = 2.303/t log A 0-A ∞ /A t -A ∞ .Are expressed in sec -1 the values reported are averages of a least two runs and the temperature was kept 28 ± 0.2 o C during the entire series of experiments in sec -1 .

Results and Discussion
The kinetics of Mn(III)acetate oxidation of Cobalt(III) bound Glycine, Alanine, and Valine as been studied in 100% AcOH at temperature of 28± 0.2 o C. The analytical work involved gross estimation of Mn(III)acetate all rate constants are calculated using total Mn(III)acetate determined.The reactions are carried out under Pseudo-first-order conditions with Mn(III)acetate in excess with performed in a spectrophotometer.

Stiocheometric Studies
The stiochemetric studies for the Mn(III)acetate oxidation of pentaamminecobalt(III) complexes of α-amino acids are free ligands were carried out with the oxidant in excess.The [H + ] and ionic strength were maintained as in the corresponding rate measurements.The temperature was maintained at 28±0.2 o C.After 100 h when the reactions was nearing completion, the concentration of unreacted Mn(III)acetate was determined both iodometrically and spectrophotometrically from the change in absorbance measured at 350 mm.In the Mn(III)acetate induced electron transfer in pentaamminecobalt(III) complexes of α-amino acids, from the decrease in absorbance measured for the cobalt(III) complex the amount of cobalt(III) reduced was calculated.This value was then compared with the amount of cobalt (II).

Mechanism
The reaction between Mn(III)acetate oxidation of Co(III) complex of α-amino acids exhibits total second-order kinetics first order with respect to each reactant.The rate of reaction carries with first power of AcOH concentration.Decrease in absorbance at 502nm corresponding to the reduction of Co(III) bound complex.The ligation of Carboxylic acid of α-amino acids by Co(III) changes the order with respect to Co(III) complex to unity.No possibility of binuclear complex formation between Mn(III)acetate and Co(III) complex.One electron transfer to Mn(III)acetate may occur by an outer-sphere path in the slow step as shown by Scheme 1.

Scheme 1 4+log(a-x)
(III)acetate] final was estimated iodometrically after 100 hours at 28±0.2⁰C and ∆[[Mn(III)acetate] was calculated., b Suitable Blank reactions were done and correction for the hydrolysis of the Co(III)  complexes was applied.

Table 1 .
Stochiometric data in the Mn(III)acetate oxidation of Co(III) bound Glycine, Alanine, and Valine.

Table 2 .
Stochiometric data in the Mn(III)acetate reaction with unbound α-amino acids.

Table 3 .
First order rate constants for Mn(III)acetate of Co(III)complexes of α-amino acids in NaLS at 28±0.2

Table 4 .
First order rate constants for Mn(III)acetate of Co(III) complexes of α-amino acids in CTAB at 28±0.2 Dependence of rate on Mn(III)acetate concentration in micellar bound ligand.The rate of oxidation of Glycanto, Alanato and Valinato cobalt(III) complexes depends on Mn(III)acetate concentration.