Indenyl cations

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. 

Our investigations of derivatives of 13 began with the oxidation of 3-phenylindenylidenefluorenes to give 14 (57). The indenylidenefluorene system would allow us to examine indenyl cations in the context of fluorenyl cations. In... 

We were concerned that we had overcompensated for the instability of the indenyl cation with the inclusion of the stabilizing phenyl substituent. That is, did the presence of the phenyl substituent cause a change in the resonance hybrid giving an enhanced contribution of 15 over 16 ... 

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. 

The principal fragment ion in the mass spectra of benzo[6]selenophene (69JCS(B)97i) and benzo[Z>]tellurophene (70JHC219) results from loss of the heteroatom, and is in fact the base peak in the latter case. Both 2- and 3-methylbenzoselenophene appear to rearrange to a benzoselenopyrylium species prior to loss of selenium, thereby forming the indenyl cation (Scheme 2), which is the most abundant ion (75OMS1084). The loss of the heteroatom is... 

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. 

The cleavage of a-phenylindenes to form indenylic cations is also described Conversions of this type are used to generate anthracenium ions from 9-phenyl-9,10--dihydroanthracene and its derivatives (see Sect. II). 

The indenyl cation 27 has not been directly observed by NMR, but the H NMR spectra of ions substituted in the five-membered ring were interpreted as indicating charge was delocalized to all three carbons, and this was suggested to indicate that the ion did not have allylic character. 

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-... 

Cobalt tricarbonyl complexes of substituted cyclopentadienyl and indenyl cations (48a,b) and of fluorenyl cation (48c) have been studied by variable temperature NMR and X-ray structures obtained of mixed-metal FeCo(CO)6 complexes of substituted... 

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