Kinetic and mechanistic studies of the reactions of 2-mercaptoethanol and thioglycolic acid with a Co(III)-bound superoxide complex

Abstract

In acid media ([H+] = 0.01–0.06 M), 2-mercaptoethanol (HSCH2CH2OH, abbreviated as MERCAP) and thioglycolic acid (HSCH2COOH, abbreviated as TGA) reduce the superoxo complex [(en)(dien)CoIII(O2)CoIII(en)(dien)]5+ (1) to the corresponding peroxo complex [(en)(dien)CoIII(μ-O2)CoIII(en)(dien)]4+ (2). The observed rate (ko), although proportional to both [MERCAP] and [TGA], is higher for TGA than for MERCAP, which is contrary to the expected trend on the basis of standard reduction potential (–0.14 versus –0.26 V, respectively). Moreover, ko values decrease with increasing media ionic strength (I) and [H+] and thus, thiolate anions are supposed to be the reactive forms of the reductants. Under the experimental conditions, the concentrations of such reductants (–SCH2CH2OH and HOOCCH2S–, respectively) from MERCAP (pKa = 9.7) and TGA (pKa1, pKa2 = 3.53, 10.10) are very small. However, for TGA, the tautomerization between HSCH2COO– and –SCH2COOH (pKi = 7.0) plays a significant role in increasing the concentration of HOOCCH2S–. The rates of both of the reactions are limited by the rate of solvent diffusion and this fact is also supported by the relatively low activation energy (Ea), for the reactions. The Ea values for the reactions with MERCAP and TGA are close enough (29.6 ± 1.3 and 27.0 ± 0.4 kJ M−1 respectively) to suggest a common reaction mechanism for both.