Fluorenylidene and Indenylidene Dications: Insights about Antiaromaticity

Fluorenylidene dications, such as the dications of p- and m- substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through 1 H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, Λ, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic.