Synthesis, Stability, and Spectroscopic and Electronic Properties of Three Benzocyclohepta[a]azulenylium Ions1

Novel π-systems, three benzocyclohepta[a]azulenylium ions, 7a−c, are synthesized, and their stability and properties have been characterized in terms of the position of the benzo-annulation and compared with those of the parent cyclohepta[a]azulenylium ion 4. Benzocyclohepta[6,7-a]azulenylium ion (7a) (pKR+ = 7.3, Ered = −0.567 V vs Ag/Ag+) and benzocyclohepta[6,5-a]azulenylium ion (7b) (pKR+ = 5.1, Ered = −0.482 V vs Ag/Ag+), which are annulated with benzene on the position having a high bond order of 4, are not appreciably destabilized compared with cyclohepta[a]azulenylium ion (4) (pKR+ = 7.3, Ered = −0.458 V vs Ag/Ag+). On the other hand, benzocyclohepta[7,6-a]azulenylium ion (7c) (pKR+ = 1.6, Ered = −0.197 V vs Ag/Ag+) is considerably destabilized, probably due to enhanced contribution of the quinoid structure of the benzene ring, which is annulated on the position having a low bond order of the cyclohepta[a]azulenylium ion moiety. Furthermore, the cations 7a and 7b are more stable than 12,13-dihydro...