Planar B₃₈^- and B₃₇^- clusters with a double-hexagonal vacancy: molecular motifs for borophenes

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.