研究室工作進展 Jun. 11th, 2019

發表于 2019-06-11

Tetralithio Metalla-aromatics with Two Independent Perpendicular

Dilithio Aromatic Rings Spiro-fused by One Manganese Atom

Yongliang Zhang, Junnian Wei, Miaomiao Zhu, Yue Chi, Wen-Xiong Zhang,

Shengfa Ye,* and Zhenfeng Xi*

Angew. Chem. Int. Ed. 2019, 58, DOI: 10.1002/anie.201904681.(Research Article)

(Dr. Shengfa Ye: Max-Planck Institute for Coal Research, Germany)

Since the concept of aromaticity represents one of the most fundamental and important principles in chemistry, the search for unprecedented and exciting aromatic systems, therefore, continues to drive research in this area. The chemistry of metalla-aromatics continues to be a unique and fascinating topic both theoretically and experimentally, largely because of their unusual chemical bonding and synthetic challenges.

In this work, we realized a class of unprecedented aromatic structures Type II: metalla-aromatics with two independent and perpendicular aromatic rings spiro-fused by a transition-metal spiro atom, of which their corresponding organic analogues are impossible. Tetralithio spiro manganacycles Type II are readily synthesized from dilithio reagents and MnCl₂ in the presence of lithium. Its aromaticity is supported by experimental measurements (X-ray structural analysis, NMR) and theoretical analyses (NICS, ACID, MOs). The spiro atom Mn uses its 3d_xz and 3d_xy orbitals to form the two perpendicular manganacycles, which are two independent 6π aromatic systems. Theoretical analyses reveal that the Li cations play an indispensable role in governing their geometric and electronic structures and hence their aromaticity. Therefore, this work contributes not only to enrich the concept of aromaticity, but also to deepen the understanding of the fundamental chemical bonding.

亮點介紹

合成未知的芳香性體系

芳香性是自然科學中最基礎最重要的概念之一，而芳香性體系在化學、材料科學及生命科學等領域發揮着重要作用。因此，合成新的芳香性體系是發現新功能的重要源泉，具有重要意義。螺芳香性可以定義為螺原子參與形成的芳香體系，前所未有。

螺環化合物[碳為螺原子]是有機化學中常見的環狀化合物。碳原子的成鍵本質決定了有機螺環化合物不可能具有芳香性。但是，如果把碳螺原子換成過渡金屬，則構成金屬雜螺環化合物[金屬離子為螺原子]。由于金屬的軌道特性，金屬雜螺芳香性化合物就成為可能。本研究室已經發現雙锂試劑的共轭丁二烯骨架能夠同兩個碳負離子産生“協同效應”，使其LUMO軌道能級相對于1,3-丁二烯顯著降低，從而可以接受合适的過渡金屬的d電子并離域至其π*軌道(魏俊年等，Angew. Chem. Int. Ed. 2015, 54, 5999; 2015, 54, 9986; J. Am. Chem. Soc. 2016, 138, 60.)。2017年，張永亮等同學利用雙锂試劑的這一獨特性質并結合過渡金屬配位化學，首次成功合成了螺原子為Pt, Pd, Rh的金屬雜螺芳香化合物（張永亮等，J. Am. Chem. Soc. 2017, 139, 5039）。在該類芳香性體系中（上圖I），過渡金屬螺原子利用其一個p_z軌道中的兩個電子與周邊八個碳原子的八個電子構成一個10π電子芳香性結構，每一個五元環都是平面的，而兩個五元螺環也趨向于相互平行；但是，在本工作中（上圖II），張永亮等同學合成了第二類螺芳香性化合物，該類螺芳香性化合物與第一類顯著不同，其中螺原子Mn利用其3d_xy和3d_xz軌道分别與左右的兩個四碳共轭部分構成了兩個獨立的五元6π電子平面芳香性結構，而兩個五元螺環是相互垂直的。

那麼，第三類螺芳香性化合物将以什麼樣的結構和成鍵模式問世呢？這些螺芳香性化合物又将具有什麼樣的反應化學和功能呢？我們拭目以待！