Inorganic salt-assisted assembly of anionic π-conjugated rings enabling 7Li NMR-based evaluation of antiaromaticity
Inorganic salt-assisted assembly of anionic π-conjugated rings enabling 7Li NMR-based evaluation of antiaromaticity
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无机盐辅助阴离子 π 共轭环组装，实现基于 7Li NMR 的反芳香性评估
DOI：
10.1039/d2dt02649j
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发表时间：
2022
影响因子：
4
通讯作者：
Takuya Kuwabara
中科院分区：
文献类型：
--
作者：
Shotaro Ito;Youichi Ishii;Takuya Kuwabara
基于 1H NMR 的阴离子分子反芳香性估计具有挑战性，因为很难单独评估由顺向环电流和负电荷引起的 NMR 屏蔽效应。在此，我们提出了一种通过无机盐辅助聚类实现基于 7Li NMR 的反芳香性评估的新方法，这是我们在超共轭反芳香性研究中偶然发现的。用锂还原二苯并[b,f]西乐平 (1) 得到二锂盐 [(Li+)2(thf)5][12−] (2)，预计其具有反芳香特性，具有涉及阴离子 π 云和 σ*(Si–Me) 轨道之间超共轭的伪 16π 电子系统，尽管通过 NMR 评估其反芳香性很困难。化合物2与0.2当量的O2气体反应形成三聚簇[(Li+)(solv.)n][(12−)3(Li9O25+)](3)，可以理解为由三个分子[Li+]2[12−]和两个Li2O盐组成的超分子复合物。 X 射线衍射分析表明，[Li9O2]5+ 核心被三个二苯并硅杂二价阴离子 (12−) 包围，具有多个 Li-π 配位。根据 1H 和 7Li{1H} NMR 谱，三聚结构保持在甲苯溶液中，特别令人感兴趣的是 Li9O2 核的 7Li{1H} NMR 信号的显着低场位移 (δ(7Li) = 6.3, 4.4)。这些明显的低场位移可以通过双阴离子二苯并硅杂环的对位环电流合理地解释，并得到理论研究的支持。
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.