Cyclo-pnicta-triazanes: biradicaloids or zwitterions?

Bulky triazenides were utilized in the reaction with pnictogen(III) chlorides (ECl3, E = P, As, Sb, and Bi) yielding triaza-pnicta-butene analogues of the type R-N═N-N(R)-ECl2 (R = Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl). Subsequent reduction of these dichloro-species afforded unprecedented four-membered group 15 heterocycles [E(μ-NTer)2N] bearing one dicoordinate pnictogen and nitrogen atom. These N3E rings represent species at the borderline between singlet biradicaloids and zwitterions with diminished reactivity compared to known singlet biradicaloids. The N3P compound could be identified, while N3As and N3Sb species as well as the precursor compounds were fully characterized.