Radical cations and anions of benzo[c]cinnolines: an electron spin resonance study

Abstract

The radical cations of benzo[c]cinnoline and its four symmetrical dimethyl derivatives have been prepared by exposure of the parent compounds as dilute solutions in CFCl₂ to ⁶⁰Co γ-rays at 77 K. The e.s.r. spectra were all characterised by large hyperfine coupling to two equivalent nitrogen atoms. Analysis of the data gave ca. 9% 2s and 44% 2p character on each nitrogen, thereby establishing a σ-structure. Clear deviation from axial symmetry shows that the orbital axes are tilted with respect to, each other, an estimate of the angle between the two principle directions being ca. 40°. This accords reasonably well with that derived from the p:s ratio of 4.9. The estimated total spin-density on nitrogen is 1.06, showing that the SOMO is strongly localised. Similar treatment of the 2,9- and 3,8-dimethoxy derivatives gave a broad unresolved singlet. This establishes that the SOMO has switched from the σ-(N)-orbital to a π-orbital. Clearly this has very low spin-density on the two nitrogen atoms. We were not able to detect any of the radical cations in the liquid phase. Exposure of dilute solutions of all six compounds as dilute solutions in CD₃OD gave parallel features characteristic of two equivalent ¹⁴N nuclei. The perpendicular splittings were close to zero, thus confirming that the SOMO is π, the estimated spin-density on the two nitrogen atoms being ca. 34% each. These anions were also prepared in liquid dimethoxyethane and hexamethylphosphoric triamide, and their e.s.r. and ENDOR spectra were recorded. Relative signs of the ¹H coupling constants were obtained using general triple resonance. This has led to full assignments in all cases. Our assignment for the anion of the parent compound differs from that previously proposed.