Isolation of a triplet benzene dianion

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.