9-Aminofluorenium Cations: A New Group of anti- Aromatic Molecules

The present study is related to our theoretical in­ vestigations [1—3] of diamino derivatives of the antiaromatic cyclopentadienyl cation (CPC). The theoretical results were confirmed with the brilliant synthesis of 6-amino-l,4-dimethyl-5,7-diphenyll,2,3,4-tetrahydrocyclopentapyrazin-6-ylium-tetrafluoroborate, carried out by Gompper et al. [4] and of some other stable cyclopentadienyl salts [5]. The studies of Gompper [6] demonstrated also the possi­ bility for the CPC derivatives to serve as building blocks-“bausteine” [6] for new materials. The aim of this work, which has the character of a first preliminary report, is to show that the proto­ nated forms of the anils of fluorenone have antiaromatic character. The first theoretical considerations for earring out these investigations are the results from the ab initio studies of system 1 (applying STO-3G basis set with geometry optimization). They show (see diagrams below) that from the alternative structures: classical l a and a/;«-aromatic l b the a/?r/-aromatic structure corresponds to the real system: