Aryl Nitrenium and Oxenium Ions with Unusual High-Spin π,π* Ground States: Exploiting (Anti)Aromaticity

Nitrenium and oxenium ions are important reactive intermediates in synthetic and biological processes, and their ground electronic configurations are of great interest due to having distinct reactivities and properties. In general, the closed-shell singlet state of these intermediates usually react as electrophiles, while reactions of the triplet states of these ions react like typical diradicals (e.g., H atom abstractions). Nonsubstituted phenyl nitrenium ions (Ph-NH+) and phenyl oxenium ions (Ph-O+) have closed-shell singlet ground states with large singlet–triplet gaps resulting from a strong break in the degeneracy of the p orbitals on the formal nitrenium/oxenium center. Remarkably, we find computationally (CBS-QB3 and G4MP2) that azulenyl nitrenium and oxenium ions can have triplet ground states depending upon the attachment position on the azulene core. For instance, CBS-QB3 predicts that 1-azulenyl nitrenium ion and 1-azulenyl oxenium ion are singlet ground-state species with considerable singlet–...