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

The 1 H 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 7 Li 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 O 2 gas to form a trimeric cluster [(Li + )(solv.) n ][(12-) 3 (Li 9 O 2 5+ )] (3), which can be understood as a supramolecular complex composed of three molecules of [Li + ] 2 [12-] and two Li 2 O salts. X-ray diffraction analysis revealed that the [Li 9 O 2 ] 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 1 H and 7 Li{ 1 H} NMR spectra, and of particular interest are the significant downfield shifts of 7 Li{ 1 H} NMR signals of the Li 9 O 2 core ( δ ( 7 Li) = 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.