Indenyl cation antiaromaticity

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. 

We have not yet prepared the unsubstituted indenylidene fluorene dication 17 but the NICSO) (18) values demonstrate that the fluorenyl cation is still more antiaromatic than the indenyl cation, see Table III. 

Evaluation of Antiaromaticity for Fluorenyl and Indenyl Cations Revisited... 

To perform a similar evaluation for the indenyl cation, 18, we must examine the bis-indenylidene dication, 19, and the appropriate data for both systems is given in Table IV. While 19 is more antiaromatic than 18 by all measures, the dependence of the measure of aromaticity and presumably antiaromaticity on ring size requires that the values for 19 be divided by 2. When examined in this manner, the indenyl cation 18 is more antiaromatic than 19, in contrast to the behavior of the fluorenyl system in 7 and 3. 

The antiaromaticity of indenyl cations was examined in the dications of indenylidene fluorenes. Contrary to expectation, the fluorenyl cation was more antiaromatic than the indenyl cation in these systems. The sum of NICS(l)zz values was linearly related to global measures of antiaromaticity such as A, suggesting that they could be used to evaluate antiaromaticity in a local sense, for individual rings. 

Oxidation of 3-/7-substituted-phenyl-indenylidenefluorenes with SbFs in SO2CIF gave 14 for all substituents except OCH3 which failed to oxidize, presumably because complexation with SbFs resulted in a species whose dication was too unstable to form. The dication for 14, Z=H, is shown in Figure 2. The fluorenyl, indenyl, and phenyl protons are labeled and demonstrate conclusively that the greatest paratropic shift is found for protons on the fluorenyl system. Thus the fluorenyl cation in 14 appears to be more antiaromatic than the indenyl cation. 

A numerical scale of calculated values for antiaromaticity was derived from values of the HOMO— LUMO gap for different cations for which a value of 0 indicates no antiaromatic destabilization and with a maximum value of 1.0 for the cyclopentadienyl cation. On this scale of antiaromaticity, the value for the indenyl cation is 0.47 and that for the 9-fluorenyl cation is 0.33. ... 

The cyclopentadienyl and indenyl cations are unambiguously antiaromatic, but the fluorenyl cation 28 is more problematic and for this reason has attracted a great deal of attention. Initial solvolytic studies of 46 indicated that fluorenyl cations 47 are difficult to form (eq 21), and the H NMR spectrum of a substituted fluorenyl cation suggested a diminu-... 

The previous sections have demonstrated the antiaromaticity of a variety of indenyl and fluorenyl cations and dications but there has been no attempt to evaluate the degree of antiaromaticity. That is, are these species very antiaromatic or not particularly antiaromatic at all We need a context for the numbers and the corresponding aromatic dianions provides that context. 

Cations structurally related to benzhydryl are anthracenyl (7)75 87 and fluorenyl (8).69 There has been some dispute as to whether or not the flve-membered ring of the fluorenyl cation is antiaromatic. Clearly the antiaromatic character is less than for the indenyl or cyclopentadienyl cations, but current opinion favors antiaromaticity also for the fluorenyl cation.90 92 This is supported by the large difference in pAR from the anthracenyl cation, (although an additional reason for this difference will be noted later in the chapter, p. 61). Again, the brackets indicate correction of a measurement from TFE H20 to water. 

The calculated excitation energy of the longest wavelength absorption of the cyclopentadienyl cation increases upon annelation going to the indenyl and fluorenyl cations, and this is proposed as a characteristic of antiaromatic systems. ... 

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