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 B-36(-/0) which led to the concept of borophenes and the 3D fullerene-like B-40(-/0) which marked the onset of borospherene chemistry. Here, we present a joint photoelectron spectroscopic and first-principles study of B-37(-) and B-38(-), 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 B-37(-) and B-38(-) are allboron analogues of coronene (C24H12) with a unique delocalized pi system, featuring dual pi aromaticity. These clusters with double hexagonal vacancies can be viewed as molecular motifs for the x(3)-borophene which is the most stable form of borophenes recently synthesized on an Ag(111) substrate.