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)₁₈, 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 ¹H 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.

π-共轭大环化合物的行为与类似的线性链不同，因为它们的电子波函数类似于环上的量子粒子，导致芳香性或反芳香性。[18]轮烯，(CH) ₁₈，是典型的非苯环芳香烃。具有 4 n + 2 π 电子电路的分子 ，例如 [18]轮烯 ( n  = 4)，是芳香族的，具有增强的稳定性和变异性环电流（环内部的磁屏蔽），而具有 4 n π 电子的分子，例如作为[18]轮烯的二价阴离子，预计具有反芳香性并表现出相反的行为。在这里，我们使用¹ H NMR 光谱重新评估[18]轮烯二价阴离子的结构。我们还表明，它可以进一步还原为芳香族四阴离子，其形状与二价阴离子相同。四阴离子锂盐的晶体结构证实了其几何形状，并显示出类似茂金属的三明治结构，其中五个Li ⁺阳离子插入两个[18]轮烯四阴离子之间。我们还报告了一种杂配三明治，具有[18]轮烯和corannulene四阴离子甲板。