Kinetics and mechanism of oxidation of thiourea by a bridging superoxide in the presence of Ellman’s reagent

Abstract

In aqueous acetate buffer, reaction between the superoxo complex [(dien)(en)CoIII(O2)CoIII(en)(dien)]5+ (1) and thiourea (Tu, SC(NH2)2) proceeds through a complex kinetic pathway and the change of the absorbance of 1 with time follows a sigmoidal trace. Only in the presence of Ellman’s reagent (5,5′-dithio-bis-(2-nitrobenzoic acid), DTNB), the reaction becomes quantitative and Tu reduces the bound superoxo group (––) of 1 to the corresponding peroxo group (––) in [(dien)(en)CoIII(O2)CoIII(en)(dien)]4+ (2). In the presence of DTNB, with large excess of Tu over [1], the reaction follows first-order kinetics and the observed rate constant, ko, increases linearly with the increase in [1], [TTu] ([TTu] is the analytical concentration of Tu), [H+], and the media ionic strength (I). The presence of an intercept in the plot of ko versus [H+] suggests that both tautomeric forms of thiourea, i.e. TuZ and its protonated form TuZH+, are kinetically reactive reductants. The calculated rate constant values for k1 (~102) and k2 (~104) indicate that TuZH+ plays the leading role in the redox reaction. The present work also reveals the prospective involvement of Ellman’s reagent as a radical scavenger.