Dications of fluorenylidenes. Use of magnetic properties to evaluate the antiaromaticity of the dication of tetrabenzo[5.5]fulvalene and substituted fluorenyl cations

The NICS values of the dication of tetrabenzo[5.5]fulvalene (1) show substantial antiaromaticity. Substituted fluorenyl cations possess antiaromatic five-membered rings but the calculated antiaromaticity (NICS) of the six-membered rings depends on the calculational level. Through calculation of magnetic susceptibility exaltation (Lambda), 1 is antiaromatic while fluorenyl cations are not. The paratropic shift seen in the H-1 NMR spectra of 1 and the substituted fluorenyl cations is linearly related to NICS (Six-membered ring) and to Lambda. NICS values suggest that electron delocalization in the fluorenyl cations occurs to maintain the aromaticity of the benzene subunits and to localize the positive charge in the five-membered ring. In contrast, electron delocalization in 1 results in delocalization of a positive charge throughout each fluorenyl system.