Flat boron has recently emerged as a fascinating concept in cluster science. Here we present computational evidence for the quasi-planar all-boron aromatic B₃₆ (C_6v, ¹A₁) and B₃₆ ⁻ (C_2v, ²A₁) clusters, established as the global-minimum structures on the basis of Stochastic Surface Walking (SSW) searches. The energetics for low-lying isomeric structures are evaluated using the validated density-functional method at the PBE0/6-311+G* level. Our global-minimum structures are in line with a recent report (Z. A. Piazza et al., Nat. Commun., 2014, 5, 3113). These structures consist of two-dimensional close-packing boron with a perfect hexagonal hole at the center, which may serve as molecular models for the monolayer boron α sheet. Chemical bonding analysis indicates that B₃₆ and B₃₆ ⁻ are all-boron analogues of coronene (C₂₄H₁₂), featuring concentric dual π aromaticity with an inner π sextet and an outer π sextet. The hydrogenated B₃₆H₆ (C_6v, ¹A₁) model cluster shows similar bonding properties, which possesses concentric triple aromaticity with inner π, outer π, and outer σ sextets.

Boron clusters have been found to exhibit a variety of interesting electronic, structural, and bonding properties. Of particular interest are the recent discoveries of the 2D hexagonal B₃₆ ^−/0 which led to the concept of borophenes and the 3D fullerene-like B₄₀ ^−/0 which marked the onset of borospherene chemistry. Here, we present a joint photoelectron spectroscopic and first-principles study of B₃₇ ⁻ and B₃₈ ⁻, which are in the transition size range between the 2D borophene-type clusters and the 3D borospherenes. These two clusters are found to possess highly stable 2D global-minimum structures consisting of a double-hexagonal vacancy. Detailed bonding analyses reveal that both B₃₇ ⁻ and B₃₈ ⁻ are all-boron analogues of coronene (C₂₄H₁₂) with a unique delocalized π system, featuring dual π aromaticity. These clusters with double hexagonal vacancies can be viewed as molecular motifs for the χ3-borophene which is the most stable form of borophenes recently synthesized on an Ag(111) substrate.

Photoelectron spectroscopy and theoretical calculations show that the global minimum of the B₄₈⁻cluster has a bilayer structure, suggesting the viability of bilayer borophenes.

We present the viability of an endohedral C _s Ca@B ₃₈ which contains a C _s B ₃₈ ²⁻ borospherene cage composed of interwoven boron double chains with a σ + π double delocalization bonding pattern, extending the B _n ^q ( q = n − 40) borospherene family from n = 39–42 to n = 38.