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.