The anti-aromatic dianion and aromatic tetraanion of [18]annulene.

π-Conjugated macrocycles behave differently from analogous linear chains because their electronic wavefunctions resemble a quantum particle on a ring, leading to aromaticity or anti-aromaticity. [18]Annulene, (CH), is the archetypal non-benzenoid aromatic hydrocarbon. Molecules with circuits of 4n + 2 π electrons, such as [18]annulene (n = 4), are aromatic, with enhanced stability and diatropic ring currents (magnetic shielding inside the ring), whereas those with 4n π electrons, such as the dianion of [18]annulene, are expected to be anti-aromatic and exhibit the opposite behaviour. Here we use H NMR spectroscopy to re-evaluate the structure of the [18]annulene dianion. We also show that it can be reduced further to an aromatic tetraanion, which has the same shape as the dianion. The crystal structure of the tetraanion lithium salt confirms its geometry and reveals a metallocene-like sandwich, with five Li cations intercalated between two [18]annulene tetraanions. We also report a heteroleptic sandwich, with [18]annulene and corannulene tetraanion decks.