Electrochemical and electron spin resonance studies of selected benzazolo[3,2-a]quinolinium salts

The electrochemical reduction of the chloride or perchlorate salts of benzazolo[3,2-a]quinolinium ion and several of its analogues is reported. The compounds studied are the perchlorate salt of 3-nitrobenzothiazolo-and 3-nitro-9-methoxybenzothiazolo[3,2-a]quinolinium, and the chloride salts of 7-ethyl-, 3-nitro-7-methyl-, 3-nitro-7-ethyl-, 3-nitro-7-isopropyl-, 3-nitro-7-butyl- and 3-nitro-7-benzylbenzimidazolo[3,2-a]-quinolinium, respectively. Cyclic voltammetry of the corresponding 3-nitrobenzothiazolo[3,2-a]quinolinium derivatives in N,N-dimethylformamide shows an irreversible peak potential at -0.6 and a quasi-reversible peak at -(1.2–1.3) volts, respectively, relative to the standard calomel electrode. In contrast, the corresponding 3-nitrobenzimidazolo[3,2-a]quinolinium derivatives show, in general, reversible peaks at near -0.8 and -(1.2–1.4) volts, respectively. Upon electrolytic reduction, only the nitro-substituted derivatives produced observable electron paramagnetic resonance electron spin resonance spectra. This observation is explained in terms of the stabilization of the radicals produced by the nitro group. Theoretical MM+/AM1/UHF calculations support the idea that the larger nitrogen splitting is caused by N-12 and the minor splittings by N-7 in the benzimidazolo[3,2-a]quinolinium ion series.