Diazine cation radicals

With azines the situation is varied. In the radical cations of pyridine and diazines the semi-occupied orbital is largely confined to the nn orbital(s) (see Scheme 2, structure 2), while the radical cation is of the n type with monoazanaphthalenes, -phenanthrenes and -anthracenes. The situation might change with substitution. As an example, alkylpyridine radical cations are of the n type, like the parent compound, whereas for the 2,5-dimethyl, 2-chloro, and 2-bromo derivatives the structure is of the n type [13]. Likewise, with benzo[c]cinnoline the parent compound and its alkyl derivatives give an n radical cation, but with some dimethoxy derivatives a n structure is found [14] and a switch from n to 7t structure occurs also in passing from 1,2,4,5-tetrazine to its 3,6-diamino derivatives [15]. 

Data on the radical cations are much more definitive because the parent is well established as a fragment formed by the photoionization of benzene and its isomers, and its hetero derivatives such as pyridine and the three diazines. (This ion, its formation and thermochemistry, is reviewed in Ref. 76.) Unequivocally, the most stable C4H4 isomer is the radical cation of methylenecyclopropene. This last species is perhaps more explicitly and sensibly described as the -CH 2 derivative of cyclopropenium cation and so renamed (cyclopropenio)methyl radical (15). From these studies, the heat of formation of this... 

Other methods for the generation of cation-radicals should also be mentioned. Mesoionic 5,5 -azinobis(l,3-diphenyltetrazole) and related mesoionic compounds give cation-radicals on oxidation by lead tetraacetate. The reaction proceeds in the presence of sodium tetrafluoroborate. The tetra-fluoroborate cation-radical salt is stable and can be purified by column chromatography on silica gel and stored in air for several months without appreciable decomposition (Araki et al. 1999). The cation-radicals of A,A -dimethyldiazines are produced by usual oxidation of the parent diazine... 

Only pyrazine among the monocyclic diazines forms well characterized JV,N -dialkylated cation-radicals Ahn and Johnson have presented detailed studies of line-shape variation in the ESR spectra of 1,4-dimethyl and 1,4-diethyl-l,4-dihydropyraziniumyl radicals.324,325 Although diquatemary salts of all three simple diazines have been prepared, it was found that only the dialky Iated pyrazine underwent spontaneous reduction to the cation-radical in alcoholic solution.326... 

The cation-radicals of the three diazine dioxides discussed are not persistent. Quinoxaline N,N -dioxide cation-radical and 108 were observable only at low temperatures. Phenazine Af,AT-dioxide cation-radical is observable at room temperature nevertheless, it dimerizes.359 The product of dimerization has been shown to be 110, and the rates of dimerization to form 110 and the corresponding product from benzo [6]phenazine JV,JV -dioxide have been determined.360 362 The ESR spectrum of the cation-radical of 110 has also been assigned.363... 

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