Aromatic rings have been created with the twisted topology of a Möbius band.

After 70 years, chemists in Germany have finally discovered a way around the Hückel rule (all aromatic rings must have 4n+2 pi electrons) and created the first stable aromatic compound with 4n delocalised electrons. Dariush Ajami and Rainer Herges at the University of Kiel based their synthesis on an idea of theoretician Edgar Heilbronner, who suggested in 1964 that the anti-aromatic properties of large rings with 4n mobile electrons could be fixed if the ring was forced to adopt the twisted topology of a MÖbius band. (This mathematical curiosity is a closed loop incorporating a 180° twist, which as a result has only one surface and only one edge.)

Numerous theoretical calculations have dealt with the possibility of MÖbius-shaped aromatics, but in the reality of the test tube they remained elusive for decades. The main question was, how can one introduce a 180 degree twist without upsetting the pi resonance of the twisted ring? Herges had been grappling with that question for years, when two pieces of cardboard gave him the crucial inspiration: 'I built a half-pipe and connected its ends with the ring,' he says. 'When you do that you immediately recognise that the MÖbius band is less strained than a normal ring.' Translated into chemistry, this meant that his graduate student, Dariush Ajami had to cut open a normal planar ring structure (cyclooctatetraene, COT), and insert the half-pipe compound which has the π orbitals arranged radially instead of parallel to the symmetry axis (tetradehydro-dianthracene). The resulting hybrid structure should retain a C₂ symmetry axis which bisects this half-pipe.

The researchers first performed calculations on the prospective MÖbius-stabilised [16]annulene, and found that the product should be stable and accessible. However, when Ajami started the synthetic work, it turned out that the planned merger of two 8-membered rings didn't proceed in the desired way (COT sensitised the formation of a triplet state in TDDA, which decayed by ring opening rather than ring metathesis). Instead, he had to resort to a more strained reactand, consisting of three cyclobutan rings.

This compound yielded a mixture of products including the desired C₂ symmetrical twisted ring, which the authors characterised and found to be truly (though not perfectly) aromatic ( Nature, 2003, 426, 819).

Herges says, 'we are currently trying to synthesize MÖbius annulenes which are more strongly aromatic, with almost complete bondlength equalisation'. The twisted benzene, so to speak. If you found Hückel theory a little bit confusing, don't even try to imagine the molecular orbitals lined up on a plane with only one surface.

'These compounds,' says Herges with a touch of understatement, 'should have interesting magneto-optical properties.'

Michael Gross