Heteroaromatic cations, sulfur-stabilized

The unsubstituted TTE 16 is nonaromatic, in the Hiickel sense. Oxidation to the cation radical and dication occurs sequentially and reversibly at relatively low potentials ( 1/2 = 0.37 V and 1/2 = 0.67 V vs. SCE in 147t-electron system. In contrast to the neutral TTF, both the cation radical and dication are aromatic as a result of the brt-electron heteroaromaticity of the 1,3-dithiolium cation. The radical cation and dication can be isolated as stable crystalline compounds due to the effective resonance stabilization of the aromatic dithiolium and, to a minor extent, the polarizable sulfur atoms <1996SR1, 1997SL1211, 1999PS99, 2001AGE1372>. 

Russell and co-workers have obtained cation-radicals in the 1,2-dithiete system. Such radicals are cyclic, conjugated within the heterocycle, and possess (4 -P 1) electrons, with = 1 they are consequently heteroaromatic witliin the definition given in the introduction to Part I. 3,4-Dimethyl-l,2-dithiete cation radical 89 (R = R = Me) was obtained by treatment of acetoin in sulfuric acid with sulfide ion. The radical is persistent at ambient temperature and unaffected by oxygen. This evident stability, and its formation from open-chain precursors to the exclusion of acyclic possibilities such as 90, whose oxygen equivalent exists, implies that aromatic stabilization of 89 and similar radicals is a matter of fact and not merely definition. 

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