The Dianion and Tetraanion of [18]Annulene

[18]Annulene, (CH)18, is one of the iconic molecules of organic chemistry. Sondheimer’s investigation of this compound in the 1960s provided a compelling endorsement for molecular orbital theory by showing that Hückel’s rule extends to molecules substantially larger than benzene. 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 are antiaromatic and exhibit the opposite behaviour. In 1973, Oth, Woo and Sondheimer reported that [18]annulene can be reduced to an antiaromatic dianion (20 π-electrons). Here we show that their published structural assignment of this dianion was incorrect, and that [18]annulene can also be reduced to a stable aromatic tetraanion (24 π-electrons). The 1H NMR spectra of the dianion and tetraanion confirm that they are antiaromatic and aromatic, respectively, and indicate that [18]annulene adopts a C2v conformation in both reduced states that contrasts the virtual D6h symmetry of the neutral ring. The crystal structure of the tetraanion lithium salt confirms this geometry and reveals a metallocene-like sandwich, with five Li+ cations intercalated between two [18]annulene tetraanions. We also report a heteroleptic sandwich, with both [18]annulene and corannulene tetraanion decks.