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.