Baird’s rule predicts that molecules with 4n π electrons should be aromatic in the triplet state, but the realization of simple ring systems with such an electronic ground state has been stymied by these molecules’ tendency to distort into structures bearing a large singlet–triplet gap. Here, we show that the elusive benzene diradical dianion can be stabilized through creation of a binucleating ligand that enforces a tightly constrained inverse sandwich structure and direct magnetic exchange coupling. Specifically, we report the compounds [K(18-crown-6)(THF)₂]₂[M₂(BzN₆-Mes)] (M = Y, Gd; BzN₆-Mes = 1,3,5-tris[2′,6′-(N-mesityl)dimethanamino-4′-tert-butylphenyl]benzene), which feature a trigonal ligand that binds one trivalent metal ion on each face of a central benzene dianion. Antiferromagnetic exchange in the Gd³⁺ compound preferentially stabilizes the triplet state such that it becomes the molecular ground state. Single-crystal X-ray diffraction data and nucleus-independent chemical shift calculations support aromaticity, in agreement with Baird’s rule.

Baird 规则预测具有 4 n π电子的分子在三重态下应该是芳香族的，但是具有这种电子基态的简单环系统的实现一直受到这些分子扭曲成具有大单重态-三重态间隙结构的倾向的阻碍. 在这里，我们表明难以捉摸的苯二价二价阴离子可以通过创建双核配体来稳定，该配体强制执行严格约束的反三明治结构和直接磁交换耦合。具体来说，我们报告了化合物 [K(18-crown-6)(THF) ₂ ] ₂ [M ₂ (BzN ₆ -Mes)] (M = Y, Gd; BzN ₆ -Mes = 1,3,5-tris [2',6'-( N -亚甲基)二甲氨基-4'-tert -butylphenyl]benzene），它的特点是在中心苯二价阴离子的每个面上结合一个三价金属离子的三方配体。^{Gd 3+}化合物中的反铁磁交换优先稳定三重态，使其成为分子基态。单晶 X 射线衍射数据和与核无关的化学位移计算支持芳香性，符合贝尔德规则。