A Mechanistic Study of the Influence of Proton Transfer Processes on the Behavior of Thiol/Disulfide Redox Couples

Abstract

The mechanism of the oxidation of 2-mercapto-5-methyl-1,3,4-thiadiazole (McMT) to its disulfide dimer and its subsequent reduction has been examined with a combined approach employing experimental data and digital simulation. In order to elucidate the influence of proton transfers on these redox processes, special attention has been paid to the influence of various bases, including triethylamine, pyridine, 3-chloropyridine, lutidine and 2,6-di-tert-butylpyridine, and proton donors, including methanesulfonic acid and trifluoromethanesulfonic acid, on both the oxidation and reduction reactions. On the basis of detailed comparisons of the experimental data with simulations of several mechanistic models, it is found that proton transfer pathways have a pronounced influence on both the oxidative and reductive pathways. In particular, McMT oxidation is facilitated by a rapid bimolecular proton transfer from McMT to weak bases such as Py that produces McMT-, the thiolate form, which is then oxidized. There is no such facilitation in the presence of the sterically hindered base 2,6-di-tert-butylpyridine, suggesting that the facilitation occurs through the formation of a discrete hydrogen-bonded complex. The overall kinetic scheme by which these redox processes proceed both in the presence and absence of proton transfer agents is discussed, especially with regard to the potential use of a related dithiolate compound as a cathode material in Li secondary batteries.