Wang Lai-Sheng
Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.
Acc Chem Res. 2024 Aug 20;57(16):2428-2436. doi: 10.1021/acs.accounts.4c00380. Epub 2024 Aug 3.
ConspectusWith three valence electrons and four valence orbitals, boron (2s2p) is an electron-deficient element, resulting in interesting chemical bonding and structures in both borane molecules and bulk boron materials. The electron deficiency leads to electron sharing and delocalization in borane compounds and bulk boron allotropes, characterized by polyhedral cages, in particular, the ubiquitous B icosahedral cage. During the past two decades, the structures and bonding of size-selected boron clusters have been elucidated via combined photoelectron spectroscopy and theoretical investigations. Unlike bulk boron materials, finite boron clusters have been found to possess 2D structures consisting of B triangles, dotted with tetragonal, pentagonal, or hexagonal holes. The discovery of the planar B cluster with a central hexagonal hole provided the first experimental evidence for the viability of 2D boron nanostructures (borophene), which have been synthesized on inert substrates. The B, B, and B clusters were among the first few boron clusters to be investigated by joint photoelectron spectroscopy and theoretical calculations, and they were all found to possess 2D structures with a central B atom inside a B ring. Recently, the B (), B (), and B () series of closed-shell species were shown to possess similar π bonding akin to that in the CH, CH, and CH series, respectively, and the name "borozene" was coined to highlight their analogy to the classical aromatic hydrocarbon molecules.Among the borozenes, the B species is unique for its high stability originating from both its double aromaticity and the fact that the B ring has the perfect size to host a central B atom. The B borozene has been realized experimentally in a variety of MB and MB complexes. In particular, the B borozene has been observed to stabilize the rare valence-I oxidation state of lanthanides in LnB complexes, as well as a Cu species in CuB. The B ring in B is too small to host a B atom, resulting in a slight out-of-plane distortion. Interestingly, the bowl-shaped B borozene is perfect for coordination to a metal atom, leading to the observation of a series of highly stable MB borozene complexes. On the other hand, the B ring is slightly too large to host the central B atom, such that a low-lying and low-symmetry isomer also exists for B. Even though most 2D boron clusters are aromatic, the B, B, and B borozenes are special because of their high symmetries and their analogy to the series of CH, CH, and CH prototypical aromatic compounds. This Account discusses recent experimental and theoretical advances on the investigations of various borozene complexes. It is expected that many new borozene compounds can be designed and may be eventually synthesized.
概述
硼（2s²2p）有三个价电子和四个价轨道，是一种缺电子元素，这使得硼烷分子和块状硼材料中产生了有趣的化学键和结构。这种电子缺乏导致硼烷化合物和块状硼同素异形体中的电子共享和离域，其特征是多面体笼，特别是普遍存在的二十面体硼笼。在过去二十年中，通过光电子能谱和理论研究相结合的方法，已阐明了尺寸选择的硼簇的结构和键合。与块状硼材料不同，已发现有限的硼簇具有由硼三角形组成的二维结构，上面点缀着四边形、五边形或六边形的孔。具有中心六边形孔的平面硼簇的发现为二维硼纳米结构（硼烯）的可行性提供了首个实验证据，这种结构已在惰性衬底上合成。B₃、B₄和B₅簇是最早通过联合光电子能谱和理论计算进行研究的几个硼簇，它们都被发现具有二维结构，中心有一个硼原子位于硼环内。最近，已表明B₆⁻、B₇⁻和B₈⁻系列的闭壳层物种分别具有与CH₄、C₅H₅⁻和C₆H₆系列类似的π键合，并且创造了“硼苯”这个名称来突出它们与经典芳烃分子的相似性。
在硼苯中，B₆⁻物种因其高稳定性而独特，这源于其双重芳香性以及硼环具有容纳中心硼原子的完美尺寸这一事实。B₆⁻硼苯已在多种MB₆和MB₆⁻配合物中通过实验实现。特别是，已观察到B₆⁻硼苯能稳定LnB₆⁻配合物中镧系元素罕见的 +I 氧化态，以及CuB₆⁻中的一种铜物种。B₆⁻中的硼环太小而无法容纳一个硼原子，导致轻微的面外畸变。有趣的是，碗状的B₇⁻硼苯非常适合与金属原子配位，从而观察到一系列高度稳定的MB₇⁻硼苯配合物。另一方面，B₈⁻的硼环稍大而无法容纳中心硼原子，因此B₈⁻也存在一个低能量和低对称性的异构体。尽管大多数二维硼簇具有芳香性，但B₆⁻、B₇⁻和B₈⁻硼苯因其高对称性以及与CH₄、C₅H₅⁻和C₆H₆系列典型芳烃化合物的相似性而特殊。本综述讨论了各种硼苯配合物研究的最新实验和理论进展。预计可以设计出许多新的硼苯化合物，并最终可能合成出来。
