Radical ions of [2.2](2,5)pyrazinophanes: an electron paramagnetic resonance study

Abstract

In the pseudo-geminal(1a) and pseudo-ortho[2.2](2,5)pyrazinophane (2a) the methylene hydrogens oriented syn towards the aza function can be selectively replaced by deuterium, to give 1b and 2b. The corresponding radical anions were generated in 1,2-dimethoxyethane by reaction of the parent compounds with alkali metal mirror, and their EPR and ENDOR spectra were recorded. General triple resonance, providing the relative signs of the ¹H coupling constants, and selective deuteriation of the syn methylene hydrogens led to full assignment in all cases. As anticipated, the magnitudes of the nitrogen and ring proton coupling constants are about one half the magnitudes of those of the corresponding 2,5-dimethylpyrazine radical anion 3˙^–. For the two sets of methylene protons, considerably different coupling constants were found; 1a˙^–: a(H_syn)=+2.54, a(H_anti)=+0.24 G; 2a˙^–: a(H_syn)=+1.60, a(H_anti)=+0.30 G. The radical cations of 1a and 2a were prepared by exposure of the parent compounds, as dilute solutions in CFCl₃, to ⁶⁰Co γ-rays at 77 K. The results establish that these are π-radicals rather than σ-radicals despite the fact that the dimethylated monomers (pyrazine radical cations) are clearly σ-species. The results for the pseudo-ortho-derivative cation show that there are two sets of inequivalent CH₂ protons.