Formation and scission of the sulfur–sulfur bond: a new approach to reactions between sulfur/polysulfide ions and thiolate ions/disulfides in N,N-dimethylacetamide

The reactivity of sulfur towards a series of thiolate ions RS–[R = phenyl (1), 4-methylphenyl (2), benzyl (3) ethyl (4), propyl (5), butyl (6), sec-butyl (7), tert-butyl (8)] has been studied by spectroelectrochemistry in N,N-dimethylacetamide. Apparently, sulfur reacts in two parallel ways: (i) oxidation of RS– leading to RS2R and S3˙– ions; (ii) preponderant S-nucleophilic process yielding stable RSx– ions (x= 2–5). RSx– species, whose spectrophotometric characteristics have been determined, are successively obtained in the course of sulfur addition with R = alkyl, whereas equilibria between arylpolysulfide ions were observed. At the junction of these two parallel pathways, the slow key equilibrium 2RS4–⇌ RS2R + 2S3˙– has been investigated by addition of RS2R R = 1–8 to S3˙– solutions. In fact, our study is consistent with at first, a monoelectronic transfer between RS–(or RS2–) ions and the very reactive S2 molecules in equilibrium with S8. The fast and competing couplings of the radicals RS˙(or RS2˙), S2˙–(or S3˙–) agree with the simultaneous formation of RS2R, RSx– and polysulfide ions. More generally, the S2/S2˙– redox system is believed to be involved in thiophilic reactions of a number of anions such as RS– towards sulfur rather than the initial opening of the cyclic S8 form.