Stable biradical in the benzimidazoline series, containing two cation-radical fragments with strong exchange interactions
A large number of uncharged stable biradicals are known [i]; there have been practically no reports, however, of biradicals containing two cation-radical fragments [bi(cationradicals)]. The only reported examples of radicals of this type have been polymeric viologens containing two or more unpaired electrons, in which the cation-radical functional groups are composed of nonaromatic y,y-dipyridylidene units [2]~ The exchange interaction between the unpaired electrons in this class of radicals is negligible; thus, their EPR spectra do not differ from the spectra of the corresponding viologen monoradicals. Viologenic biand poly(cation-radicals) are formed upon reduction of polypyridinium salts [2]. It is assumed that two-electron oxidation of compounds of the type R-X-R, in which the R groups are more readily ionized than the bridging group X, should provide a more general method for the preparation of hi(cation-radicals). We succeeded in carrying out this type of reaction in the case of 1,4-di(l,2,3-trimethylbenzimidazolinyl-2)benzene (I), * which contains two benzimidazoline functional groups linked by a nonconjugating p-phenylene bridge.