Fluorenylidene dications antiaromaticity

In Chapter 11 (the only chapter in the volume that focuses on persistent oxidation dications), N. S. Mills discusses the fluorenylidene dications and their antiaromatic character. 

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, 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. 

Is the behavior of fluorenylidene dications, such as 3 or 5 unusual Comparison of the average chemical shifts of fluorenyl cations 7 with the analogous diphenylmethyl cations shows minimal antiaromaticity in the monocations, with paratropic shifts of 0.5 ppm or less (29). The dramatic para-... 

Fluorenylidene dications 8 have been shown to be more antiaromatic than fluorenyl monocations 7. When the substituent R on 8 has electron-withdrawing... 

In the majority of the systems examined, the magnetic and energetic criteria used gave similar results, suggesting that either is an effective measure of antiaromaticity in fluorenylidene dications. 

The results from studies on fluorenylidene dications such as 8 demonstrate clearly that species of this type are antiaromatic. However, extension of these studies to indenylidene dications such as 13 has important advantages. The... 

Mills, N. S. Levy, A. Plummer, B. F. Antiaromaticity in fluorenylidene dications. Experimental and theoretical evidence for the relationship between the HOMO/LUMO gap and antiaromaticity, J. Org. Chem. 2004, 69, 6623-6633. 

Fluorenylidene and Indenylidene Dications Insights about Antiaromaticity... 

We have demonstrated that oxidation of fluorenylidenes and indenylidenes results in the formation of dications with appreciable antiaromaticity. The antiaromaticity of these species was examined for a series of dications from... 

The 9-fluorenyl cation, being a 4 -electron system, should be antiaromatic, as has been generally believed. However, Richard and coworkers provided experimental and theoretical data suggesting that this cation is not antiaromatic.Recent preparation of several fluorenyl dications, on the other hand, does prove that the fluorenyl system is antiaromatic in the dicationic systems. Mills and coworkers have generated the fluorenyl dications (115-120) by SbFs-oxidation of the tetraben-zofulvalene derivatives containing fluorenylidene and —five- and seven-membered... 

Levy, A. Rakowitz, A. Mills, N. S. Dications of fluorenylidenes. The effect of substituent electronegativity and position on the antiaromaticity of substituted tetrabenzo[5.5]fulvalene dications, 7. Org. Chem. 2003,68,3990-3998. 

---