5,6‐Bis(methylthio)‐4,7‐diethylbenzo‐[1,2,3]‐trithiole [MBT] was oxidized with two equivalents of SbCl5 to produce a dication, MBT(2+)ċ2SbCl −6, as a stable, dark‐brown solid. MBT(2+) was unexpectedly silent for 1H‐NMR in CD3CN, whereas it was active for ESR, suggesting that MBT(2+) is a triplet‐state dication MBT(2+)‐T. Meanwhile, treatment of 5‐ methylsulfinyl‐6‐methylthio‐4,7‐diethylbenzo[1,2,3]‐ trithiole [MBTMO] with D2SO4 produced MBT(2+), whose 1H‐NMR gave no signals, whereas the solution is active for ESR. These results imply that MBT(2+) prepared from MBTMO is a triplet‐state dication, and a singlet‐state dication, MBT(2+)‐S, initially generated by acidification of MBTMO, isomerized to the triplet‐state dication, MBT(2+)‐T. Since MBT(2+)‐T is active for ESR at room temperature, two molecules of MBT(2+)‐T should form a spin pair in the solution with a sufficient distance between the two radical centers. The structures of MBT(2+)‐S and MBT(2+)‐T were optimized with the DFT method at the B3LYP6‐31G** level. The total energy difference between them was calculated to be 7.90 kcal/mol; MBT(2+)‐T was shown to be more stable than MBT(2+)‐S. A treatment of MBTMO with SbCl5 gave a 1:1 complex. The structure of the complex was determined with X‐ray crystallography, which showed that the complex is the corresponding sulfonium salt, MBTMOċSbCl5. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:111–222, 2005; Published online in Wiley InterScience (http://www.interscience.wiley.com). DOI 10.1002/hc.20078
