Unmasking the elusive 1,4-diazabutatrienes: the stabilizing role of the N-substituents.

The geometrical and electronic properties of a representative set of diversely-substituted 1,4-diazabutatrienes are analyzed by theoretical and statistical methods. The influence of the substituents on the stabilization of these exotic azacumulenes has been estimated through a homodesmotic reaction and compared with related heterocumulenes. The 1,4-diazabutatrienes are stabilized by π-donor or σ-acceptor substituents and, in some cases, by the combination of one donor with one acceptor substituents at both N termini, a fact that might be associated with the ideally linear geometry of the heterocumulenic core for keeping the optimal orbital overlapping between its atoms.