Chemistry:Dithiolium salt

1,2- and 1,3-dithiolium cations.

Dithiolium salts are compounds of the formula [(RC)₃S₂]⁺X⁻ (R = H, alkyl, aryl, etc.). These salts consist of a planar organic cation with a variety of anions such as halides. The five-membered ring cations are observed in either of two isomers, 1,2- and 1,3-dithiolium cations. These cations differ with respect to the relative positions of the pair of sulfur atoms. Both isomers feature a planar ring, which is aromatic owing to the presence of 6π electrons. For example, the 1,2-ditholium ring can be represented as an allyl cation of the three carbons, with each sulfur atom donating one of its lone pairs of electrons to give a total of three pairs.

Structure of diphenyl-1,2-dithiolium cation (as its bisulfate salt). Selected distances: S-S = 2.019(5); S-C=173(1); C_ring-C_ring = 1.38(1) Å.

Preparation, occurrence, reactions

1,2-Dithiolium cations have been prepared from 1,3-diketones by treatment with H₂S and oxidants such as bromine.

(RCO)
2CH
2 + 2 H
2S + Br
2 → [(RC)
3S
2]⁺Br⁻ + 2 H
2O + HBr

1,3-Dithiolium cations are often prepared by alkylation of the corresponding unsaturated dithio- or trithiocarbonates:

(RC)
2S
2CE + R'+
→ [(RC)
2S
2CER']+

The analogous reaction of electrophiles with 1,2-dithiole-2-ones affords 1,2-dithiolium cations.