Kinetics and mechanisms of the reactions of the electrogenerated tetramethoxythianthrene cation radical and dication with methanol and pyridine derivatives in acetonitrile

Abstract

In order to clarify the differences in reactivity between cation radicals and dications, the kinetics and mechanisms were analysed in detail for the reactions of the electrogenerated 2,3,7,8-tetramethoxythianthrene cation radical (TMT˙⁺) and of the dication (TMT²⁺) with methanol and pyridine derivatives by using a pulse-electrolysis stopped-flow method. From the rate law determined for each reaction, the reaction mechanisms were elucidated, though they are different depending on the couples in the reactions. The reactions of TMT˙⁺ proceeded by multi-step reactions including the electron transfer of another TMT˙⁺ to generate TMT before the rate-determining step (rds). In contrast, in the reactions of TMT²⁺, it was found that the rds was the first or the second step of the reaction; i.e. the unstable cationic intermediate seems to be formed directly, not by the electron transfer. In addition to the excessive charge present in the dicationic state, a mechanism in which TMT²⁺ is easily accessible to the rds also seems to explain the high reactivity of TMT²⁺. The differences in the reaction mechanism depending on the nucleophiles were also supported by observing the substituent effects of pyridine derivatives on the reactions.