Borozenes: Benzene-Like Planar Aromatic Boron Clusters.

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.