Synthesis and Stabilization of a Benzene Dianion with a Triplet Ground State and Baird Aromaticity

The triplet benzene dianion is predicted to be aromatic based on Baird’s rule. However, it has remained elusive due to the Jahn–Teller distortion. Herein, we report isolation of a benzene dianion with a triplet ground state in an inverse-sandwich europium benzene complex. Combined experimental and theoretical studies unveil that the strong antiferromagnetic coupling between the benzene dianion and 4f⁷ Eu²⁺ ions is pivotal to the stabilization of the triplet state of the benzene dianion with Baird aromaticity. In addition, a comparison with analogous ytterbium benzene and europium para-xylene complexes further reveals that the spin state of the benzene dianion depends on the spin–spin interaction and local symmetry. These results accomplish the isolation of the long-sought triplet benzene dianion and illustrate the effectiveness of spin modulation using lanthanide ions.