Effects of Countercations in Dianionic Dibenzosilepinyl Salts on the Structure and Hyperconjugative Antiaromaticity

We previously reported the synthesis of dilithium salts of dibenzo[ b, f ]silepinyl dianions ( 1-Li 2 ), and we unveiled the hyperconjugative antiaromatic character of its metal-free dianion ( 1 2– ). However, the influence of countercations on the electronic structure of 1 2– remains unclear. In this study, disodium and dipotassium salts ( 1-Na 2 and 1-K 2 ) were synthesized to investigate the effects of metal cations on the properties, including solid-state structures and hyperconjugative antiaromaticity. In the solid state, 1-Na 2 and 1-K 2 adopt dimeric and polymeric structures, respectively, in contrast to the monomeric nature of 1-Li 2, and the silepin rings in 1-Na 2 and 1-K 2 are much less twisted than that in 1-Li 2 . The 1 H NMR signals of the peripheral protons of the dibenzosilepin skeleton shifted to lower frequencies, and absorption spectra recorded in THF exhibit bathochromic shifts as one descends the group 1 metals, underpinning the impact of the cations on the electronic structures. NICS zz calculations and ACID plots revealed that the antiaromatic character of dibenzosilepinide is strongly modulated by the nature of the countercations.