Crystalline Radical Anion of a Diboratriazole and Its Conversion to a Neutral Radical Driven by a Carbene

One-electron reduction of diboratriazole 1 with potassium graphite (KC₈) generates the radical anion 1^•-•K⁺, which undergoes a salt (KCl) elimination reaction upon addition of an N-heterocyclic carbene (NHC) to afford the neutral diboratriazole radical 3. An X-ray diffraction analysis, electron paramagnetic resonance spectroscopy, and computational studies revealed that an unpaired electron in radical species 1^•-•K⁺ and 3 is delocalized over the π-system of the B₂N₃ and carbene rings. Reversible oxidation of 3 gives rise to a diboratriazole cation 4 featuring a 6π aromatic character. Moreover, treating 1^•-•K⁺ with a half equivalent of a bis(NHC) produces a biradical species 5, in which there is little interaction between two radical moieties separated by the bis(NHC) linker, suggesting the dis-biradical property. 5 undergoes stepwise and reversible two-electron oxidation, establishing three formal oxidation states.