Inorganic salt-assisted assembly of anionic π-conjugated rings enabling 7Li NMR-based evaluation of antiaromaticity

Abstract

The ¹H NMR-based estimation of antiaromaticity in anionic molecules is challenging because of the difficulty in separately evaluating NMR shielding effects due to paratropic ring currents and negative charges. Herein we propose a novel approach for the ⁷Li NMR-based evaluation of antiaromaticity enabled by inorganic salt-assisted clusterization, which is serendipitously found during our studies on hyperconjugative antiaromaticity. Reduction of a dibenzo[b,f]silepin (1) with lithium afforded the dilithium salt [(Li⁺)₂(thf)₅][1²⁻] (2), which is expected to have antiaromatic character with a pseudo-16π-electron system involving hyperconjugation between the anionic π-clouds and the σ*(Si–Me) orbitals, although the evaluation of its antiaromaticity by NMR was difficult. Compound 2 reacted with 0.2 equivalents of O₂ gas to form a trimeric cluster [(Li⁺)(solv.)_n][(1²⁻)₃(Li₉O₂⁵⁺)] (3), which can be understood as a supramolecular complex composed of three molecules of [Li⁺]₂[1²⁻] and two Li₂O salts. X-ray diffraction analysis revealed that the [Li₉O₂]⁵⁺ core is surrounded by three dibenzosilepinyl dianions (1²⁻), with multiple Li–π coordinations. The trimeric structure is maintained in a toluene solution according to the ¹H and ⁷Li{¹H} NMR spectra, and of particular interest are the significant downfield shifts of ⁷Li{¹H} NMR signals of the Li₉O₂ core (δ(⁷Li) = 6.3, 4.4). These explicit downfield shifts are reasonably explained by the paratropic ring currents of the dianionic dibenzosilepin rings, which was supported by theoretical studies.