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-Li2), and we unveiled the hyperconjugative antiaromatic character of its metal-free dianion (12–). However, the influence of countercations on the electronic structure of 12– remains unclear. In this study, disodium and dipotassium salts (1-Na2 and 1-K2) were synthesized to investigate the effects of metal cations on the properties, including solid-state structures and hyperconjugative antiaromaticity. In the solid state, 1-Na2 and 1-K2 adopt dimeric and polymeric structures, respectively, in contrast to the monomeric nature of 1-Li2, and the silepin rings in 1-Na2 and 1-K2 are much less twisted than that in 1-Li2. The 1H 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. NICSzz calculations and ACID plots revealed that the antiaromatic character of dibenzosilepinide is strongly modulated by the nature of the countercations.