Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius [4n]Annulene Aromaticity

Abstract

The first experimental example of a [4n]annulene derivative with one Mobius twist, 1, was synthesized recently [Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426, 819] and was purported to possess aromatic character. However, critical analysis of the published crystallographic data indicates that the Mobius [16]annulene core of 1 shows large bond alternation (Deltar up to 0.157 A). Delocalization in this core is inhibited by large dihedral angles, which hinders effective pi overlap. This conclusion is supported by computational results (B3LYP/6-311+G) on 1 and several less benzannelated derivatives, based on geometric (Deltar, Deltar(m), Julg A, HOMA) and magnetic (NICS, magnetic susceptibility exaltation) criteria of aromaticity. That benzannelation results in bond localization in the [16]annulene core is shown by additional computations on benzannelated derivatives of other Mobius aromatic species. Additionally, the aromatic stabilization energy (ASE) of 1 has been reinvestigated using two different procedures. Evaluation of uncorrected ISE(II) values of just the polyene bridge portion of 1 and its Huckel counterpart suggests that stabilization of 1 relative to its Huckel isomer is confined to the polyene bridge and is not due to a delocalized pi circuit. Furthermore, application of s-cis/s-trans corrections lowers the ISE(II) value of 1 from 4.0 kcal/mol to 0.6 kcal/mol, suggesting that 1 is nonaromatic.

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…The synthesized Möbius ring lies ∼ 2 kcal/mol above and has the structure anti − tZcZcZt shown in Figure 43 with C 2 symmetry [255]. It has been argued [258] about aromaticity of this system, on the basis of the bond alternation resulting from crystal data [255] and of large dihedral angles inhibiting the overlap between π orbitals. In contrast, the principal component analysis performed on the 153 most stable structures of [16]annulenes and the 25 most stable Möbius stabilized [16]annulenes indicates that the compound of Figure 43 belongs to the cluster of aromatic Möbius [16]annulene systems [256].…”

The Porphyrin Twist: Hückel and Möbius Aromaticity

“…Thus, benzoannelation in these systems helps to stabilize the twist but does not diminish the aromatic character significantly, [5] which contradicts other purely theoretical work. [6] Interestingly, a detailed analysis of figure-eight-shaped expanded porphyrins was presented in a recent review by Herges dealing with the design of Möbius molecules.…”

Solvent‐Dependent Aromatic versus Antiaromatic Conformational Switching in meso‐(Heptakis)pentafluorophenyl [32]Heptaphyrin

“…[15] In 2003, Herges and his co-workers reported a [16]annulene molecule as the first example of stable Mçbius aromatic molecule. [16] Even though these results were confronted with skeptical debates about aromatic character of that molecule, [17] this work boosted scientific interests towards Mçbius aromatic molecules. Along this line, Latos-Grażyń ski with his co-workers and our research group independently reported the first examples of Mçbius aromatic expanded porphyrins: di-p-benzi [28]hexaphyrin [12] and metal complexes of meso-aryl-substituted hexaphyrin, heptaphyrin, and octaphyrins, [13] respectively.…”