Radical cations and anions of 1,2,4,5-tetrazines: an electron spin resonance and cyclic voltammetric study

Abstract

Exposure to ⁶⁰Co γ-rays of dilute solutions of a range of s-tetrazines in fluorotrichloromethane at 77 K gave the corresponding radical cations, identified by their e.s.r. spectra. Two distinct types of spectra were obtained, one showing large hyperfine coupling to four equivalent ¹⁴N nuclei is assigned to n(σ)-radical cations, and the other, only found for –NR₂ substituents, with strong coupling to two nitrogen nuclei, is assigned to π-radical cations. The former include the parent s-tetrazine and the 3,6-dimethyl, -diphenyl, -dichloro, -dimethoxy, -bis(methylthio), and di(aziridin-1-yl) derivatives. The latter were also prepared in fluid solution, and showed reversible behaviour in their cyclic voltammograms, in contrast with the former group. The corresponding radical anions have been studied in both fluid and solid solution, all derivatives showing major hyperfine coupling to the four ring ¹⁴N nuclei. Total spin densities have been estimated for both types of radical cations and for the radical anions, and in all cases the total is in the 1.05–1.2 range. This deviation from unity is interpreted in terms of spin polarisation giving rise to considerable negative spin densities.