Theoretical Notions of Aromaticity and Antiaromaticity: Phenalenyl Ions versus Fluorenyl Ions

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 monocations 12a and 13a and monoanions 14a and 15a of phenalene (3) and fluorene (5), were subjected to a systematic DFT 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 monoions 12a-15a to give the respective diions 6-11 result in a similar destabilization in both the fluorene and phenalene series. The interactions between the C13H8 units in diions 6-11 are weak and are not expected to result in a significant loss of aromaticity/gain of antiaromaticity, as compared with the respective monoions. 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, DeltaE(Tot) = -6.0 kJ/mol. The energetic effect of the analogous reaction involving anions 9 + 2.15a = (E)-10 + 2.14a is even smaller, DeltaE(Tot) = -3.4 kJ/mol. Bifluorenylidene dianion (9) and 1,1'-biphenalenylidene dianion ((E)-10) are aromatic, but the employed criteria disagree about their relative aromaticity. The electronic and structural properties of heteromerous dication 8 and dianion 11 lie between those of the homomerous diions. Thus, dications 6-8 and dianions 9-11 form a continuum of aromaticity/antiaromaticity.

The Fluorenyl Cation

“…[1][2][3][4][5][6] In 1980, Olah and Schleyer et al described unsuccessful attempts to isolate 1 in superacidic media: "We were not able to generate the parent 9-fluorenyl cation 1-H by slowly adding SO 2 or SO 2 ClF solutions of possible precursors… at À120 8C. The resulting solutions immediately became dark and unidentifiable polymeric materials were formed".…”

Aromatic changes in isoelectronic derivatives of phenalenyl radicals by central carbon replacement

“…Furthermore, the NICS 1 value of PLY 0 (À5.8 ppm) is also consistent with that previously (À6.2 ppm) found by Pogodin. 57 In the neutral compounds, substitution of the central carbon by tetrel atoms results in less negative NICS values. This is also true for icosagen substitutions and for pnictogen atoms (only for NICS 0, þ1 and þ2).…”

“…56 In addition, cationic derivatives of phenalenyl have been studied and the aromatic behaviour of such systems has also been described. 57 The problem of aromaticity in radical species is particularly interesting. 15,58e62 Mandado et al studied the differences in aromaticity in monocyclic radicals and reported that it is necessary to take into account the a and b electron-densities.…”

Structures and aromaticity of the planar B2XY (X=N, P and Y=O, S, Se) and Al2MN (M=C, Si, Ge and N=S, Se) clusters

“…Despite the fact that various dispute has been getting along with the development of this concept, certainly, we must admit that aromaticity has been playing an important role in interpreting molecular properties for many compounds [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The substantial energy stabilization, bond length equalization, unusual reactivity, and characteristic spectroscopic features as well as distinctive magnetic properties are some important aspects with which researchers depict this special concept.…”