Borozenes: Benzene-Like Planar Aromatic Boron Clusters.

ConspectusWith three valence electrons and four valence orbitals, boron (2s²2p¹) 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 B₁₂ 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 B₃ triangles, dotted with tetragonal, pentagonal, or hexagonal holes. The discovery of the planar B₃₆ 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 B₇^-, B₈^-, and B₉^- 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_n ring. Recently, the B₇^3- (C_6v), B₈^2- (D_7h), and B₉^- (D_8h) series of closed-shell species were shown to possess similar π bonding akin to that in the C₅H₅^-, C₆H₆, and C₇H₇⁺ series, respectively, and the name "borozene" was coined to highlight their analogy to the classical aromatic hydrocarbon molecules.Among the borozenes, the D_7h B₈^2- species is unique for its high stability originating from both its double aromaticity and the fact that the B₇ ring has the perfect size to host a central B atom. The B₈^2- borozene has been realized experimentally in a variety of MB₈ and M₂B₈ complexes. In particular, the B₈^2- borozene has been observed to stabilize the rare valence-I oxidation state of lanthanides in LnB₈^- complexes, as well as a Cu₂⁺ species in Cu₂B₈^-. The B₆ ring in B₇^3- is too small to host a B atom, resulting in a slight out-of-plane distortion. Interestingly, the bowl-shaped B₇ borozene is perfect for coordination to a metal atom, leading to the observation of a series of highly stable MB₇ borozene complexes. On the other hand, the B₈ ring is slightly too large to host the central B atom, such that a low-lying and low-symmetry isomer also exists for B₉^-. Even though most 2D boron clusters are aromatic, the B₇^3-, B₈^2-, and B₉^- borozenes are special because of their high symmetries and their analogy to the series of C₅H₅^-, C₆H₆, and C₇H₇⁺ 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.