Dications of 3-Phenyl-indenylidene Dibenzo[ a.d ]cycloheptene: The Role of Charge in the Antiaromaticity of Cationic Systems

Dications of 9-(3-phenyl-1H-inden-1-ylidene)-5H-dibenzo[a,d]cycloheptene, 52+, were prepared by oxidation with SbF5 in SO2ClF, and their magnetic behavior was compared to dications of 9-(3-phenyl-1H-inden-1-ylidene)-9H-fluorene, 22+. The good correlation between the experimental 1H NMR shifts for the dications that were oxidized cleanly and the chemical shifts calculated by the GAIO method supported the use of the nucleus independent chemical shifts, NICS, to evaluate the antiaromaticity of the indenyl systems of 22+/52+ and their unsubstituted parent compounds, 62+ and 72+, as well as the antiaromaticity of the fluorenyl system of 22+/72+ and the aromaticity of the dibenzotropylium system of 52+/62+. Antiaromaticity was shown to be directly related to the amount of charge in the antiaromatic systems, with the antiaromatic systems more responsive to changes in the calculated NBO charge than the aromatic systems. The antiaromaticity was also shown to be directly related to the amount of delocalization in the ring system. The aromaticity of the dibenzotropylium system was much less responsive to changes in the amount of charge in the tropylium system, because the aromatic system was much more completely delocalized. Thus, antiaromatic species are more sensitive probes of delocalization than aromatic ones