ROLE OF ELLMAN’S REAGENT FOR THE OXIDATION OF PYRIDINE N-OXIDE BY A SUPEROXIDE LIGAND IN A CoIII- COMPLEX, IN AQUEOUS ACETATE BUFFER MEDIUM: A KINETICS AND MECHANISTIC STUDIES

Abstract

In aqueous acetate buffer media (pH=4.0-5.2), reaction between the superoxo complex, [(dien)(en)CoIII(O2)CoIII (en) (dien)]5+ (1) and pyridine N-oxide (C5H5NO) proceeds through a complex kinetic pathway and the change of the absorbance of 1 with time also follows a complex pattern. Only in the presence of Ellman’s reagent (5,5´-dithio-bis-(2- nitrobenzoic acid), DTNB), the reaction becomes quantitative and each mole of 1 is reduced by one mole of pyridine Noxide (C5H5NO) to produce the corresponding hydroperoxo complex, [(dien)(en)CoIII (HO2)CoIII(en)(dien)]5+ (2), following proton coupled electron transfer (PCET) path. In presence of DTNB, with large excess of pyridine N-oxide (C5H5NO) over [1], the reaction follows first-order kinetics and the observed rate constant, ko increase linearly with increase in [C5H5NO] but decreases with increase in [H+]. It is also noticed that the reaction rate significantly decreases with increasing proportion of D2O replacing H2O in the solvent. Therefore, an H-atom transfer (HAT) from the reducing species to the bridging superoxide in 1 seems reasonable at the rate determining step. Indeed the present work confesses the forthcoming involvement of Ellman’s reagent as a radical scavenger.