Bent and twisted: the electronic structure of 2-azapropenylium ions obtained by guanidine oxidation

New bis-2-azapropenylium ions are obtained by oxidation of guanidino-substituted aromatic compounds. The dications 1(2+) (1 = 1,4-bis-tetramethylguanidinobenzene) and 2(2+) (2 = 1,4-bis-guanidinobenzene) exhibit unexpected bent-twisted structures, that differ from the known 2-azapropenylium ion structures (linear-orthogonal 2-azaallenium ion or bent-planar 2-azaallylium ion), and resemble the structures of carbodicarbenes. Their electronic structure was analysed by a combination of experiments and quantum chemical calculations. The results indicate that the bent-planar structure is disfavoured for steric reasons, while the allene-type linear-orthogonal structure is also disfavoured due to the electron-donating groups that support the bent form. For these reasons a bent-twisted structure is adopted, in which the nitrogen atom is engaged with two different orbitals in pi-bonding with the carbon pi-system and in a weaker pi-interaction with the CN2 group of the guanidino system.