| Abstract
| - Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a−g) results in theformation of fluorenylidene dications that are shown to be antiaromatic through calculation of thenucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system.There is a strong linear correlation between the redox potential for the dication and both thecalculated NICS and σm. Redox potentials for formation of dications of analogously substitutedtetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentialsfor these dications are less positive than for formation of the dications of 3a−g and for dications ofp-subtituted diphenylmethylidenefluorenes, 2a−g. The greater instability of dications of 2a−g and3a−g compared to the reference system implies their antiaromaticity, which is supported by thepositive NICS values. The redox potentials for formation of the dications of meta-substituteddiphenylmethylidenes (3a−g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a−g), indicating their greater thermodynamic instability. TheNICS values for dications of 3a−g are more antiaromatic than for dications of 2a−g, which isconsistent with their greater instability of the dications of 3a−g. Although the substituteddiphenylmethyl systems are not able to interact with the fluorenyl system through resonancebecause of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationicsystem. Two models have been suggested for this interaction, σ to p donation and the ability of thecharge on the substituted ring system to affect delocalization. Examination of bond lengths showsvery limited variation, which argues against σ to p donation in these systems. A strong correlationbetween NICS and σ constants suggests that factors that affect the magnitude of the charge onthe benzylic (α) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenylcation. Calculated atomic charges on carbons 1−8 and 10−13 show an increase in positive charge,and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly withNICS, supporting the model in which the amount of delocalization of charge is related to theantiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized byextensive delocalization of electron density.
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