Aromaticity with a Twist:  Möbius [4<i>n</i>]Annulenes

Castro, Claire; Cm, Isborn; Karney, William L.; Mauksch, Michael; P, von Ragué Schleyer

doi:10.1021/ol026610g

Cited by 94 publications

(101 citation statements)

References 27 publications

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“…While the Möbius rings of parent [16]annulene are at higher energies with respect to 10a and 10b [257], the stabilization imposed by the belt reverses the energy ordering placing two Möbius rings, of C 1 and C 2 symmetry, at the lowest and nearly equal energy [255]. The synthesized Möbius ring lies ∼ 2 kcal/mol above and has the structure anti − tZcZcZt shown in Figure 43 with C 2 symmetry [255].…”

Section: [16]annulenementioning

confidence: 95%

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

Gellini

Salvi

2010

Symmetry

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The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]-and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.

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Section: [16]annulenementioning

confidence: 95%

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

Gellini

Salvi

2010

Symmetry

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“…Although Hückel aromatic compounds with 4n þ 2 mobile electrons are common, examples of Möbius 4n electrons aromaticity have proven to be much more difficult to realize and to establish [4][5][6][7][8][9][10][11][12][13] . Indeed, the reported Möbius aromatic compounds are very limited and almost all have twisted topologies [14][15][16][17][18] . For instance, although Heilbronner pointed out in 1964 that rings with 20 atoms or more might adopt Möbius geometries 'without any apparent angle or steric repulsion strain' in his prediction 4 , Mauksch et al in 1998 discovered computationally a twisted much smaller ring system, the cyclononatetraenyl cation, which is a Möbius aromatic annulene 5 .…”

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confidence: 99%

Planar Möbius aromatic pentalenes incorporating 16 and 18 valence electron osmiums

et al. 2014

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Aromaticity, a highly stabilizing feature of molecules with delocalized electrons in closed circuits, is generally restricted to 'Hückel' systems with 4n þ 2 mobile electrons. Although the Möbius concept extends the principle of aromaticity to 4n mobile electron species, the rare known examples have complex, twisted topologies whose extension is unlikely. Here we report the realization of osmapentalenes, the first planar Möbius aromatic complexes with 16 and 18 valence electron transition metals. The Möbius aromaticity of these osmapentalenes, documented by X-ray structural, magnetic and theoretical analyses, demonstrates the basis of the aromaticity of the parent osmapentalynes. All these osmapentalenes are formed by both electrophilic and nucleophilic reactions of the in-plane p component of the same carbyne carbon, illustrating ambiphilic carbyne reactivity, which is seldom observed in transition metal chemistry. Our results widen the scope of Möbius aromaticity dramatically and open prospects for the generalization of planar Möbius aromatic chemistry.

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“…2) [4], but in all reported examples, experimental or computergenerated [5][6][7][8][9][10][11][12], the object turns out to have fixed C 2 geometry. Why do molecular Mö bius strips lock into C 2 symmetry?…”

Section: Introductionmentioning

confidence: 93%