The Structure of 2‐Azaallenium Cations

According to an X ‐ray structural analysis, the 2‐azaallenium hexachloroantimonate 23 crystallizes with the topology of an allene (geometry 1a ) and not of a 2‐azaallyl cation ( 1b ). Ab initio molecular orbital calculations (3–21 G basis set) and semiempirical MNDO calculations for the unsubstituted 2‐azaallenium cation and for some mono‐ and disubstituted derivatives ( 24 ) confirm the experimental results: In many cases the allenium geometry 24a is more stable than the 2‐azaallylium form 24b. Electron‐releasing substituents reduce the energy difference between 24b and 24a. With two amino substituents the allylium geometry 24b is energetically preferred to the allenium form 24a. In general, substituted 2‐azaallenium salts are sterically flexible around the central nitrogen atom. Compound 23 was synthesized by a new method.