| Abstract
| - The dications 6, 7, and 8 and dianions 9, 10, and 11 of the bistricyclic aromatic enes bifluorenylidene(1), 1,1‘-biphenalenylidene (2), and 9-(9H-fluoren-9-ylidene)-1H-phenalene (4), as well as monocations12a and 13a and monoanions 14a and 15a of phenalene (3) and fluorene (5), were subjected to a systematicDFT and ab initio study. B3LYP and MP2 methods were employed to estimate the relative aromaticity/antiaromaticity of these ions, using energetic, magnetic, and structural criteria. The couplings of monoions12a−15a to give the respective diions 6−11 result in a similar destabilization in both the fluorene andphenalene series. The interactions between the C13H8 units in diions 6−11 are weak and are not expectedto result in a significant loss of aromaticity/gain of antiaromaticity, as compared with the respectivemonoions. The antiaromaticity of bifluorenylidene dication (6), relative to that of two fluorenyl cations(12a), is only slightly enhanced as compared with the aromaticity of biphenalenylidene dication ((E)-7))relative to that of two phenalenyl cations (13a). In particular, the homodesmotic reaction 6 + 2·13a =(E)-7 + 2·12a is only slightly exothermic, ΔETot = −6.0 kJ/mol. The energetic effect of the analogousreaction involving anions 9 + 2·15a = (E)-10 + 2·14a is even smaller, ΔETot = −3.4 kJ/mol.Bifluorenylidene dianion (9) and 1,1‘-biphenalenylidene dianion ((E)-10) are aromatic, but the employedcriteria disagree about their relative aromaticity. The electronic and structural properties of heteromerousdication 8 and dianion 11 lie between those of the homomerous diions. Thus, dications 6−8 and dianions9−11 form a continuum of aromaticity/antiaromaticity.
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