A concentric planar doubly <i>π</i>-aromatic B₁₉<super>−</super> cluster

Atomic clusters often show unique, size-dependent properties and have become a fertile ground for the discovery of novel molecular structures and chemical bonding. Here we report an investigation of the B19 2 cluster, which shows chemical bonding reminiscent of that in [10]annulene (C10H10) and [6]circulene (C24H12). Photoelectron spectroscopy reveals a relatively simple spectrum for B19 , with a high electron-binding energy. Theoretical calculations show that the global minimum of B19 2 is a nearly circular planar structure with a central B6 pentagonal unit bonded to an outer B13 ring. Chemical bonding analyses reveal that the B19 2 cluster possesses a unique double p-aromaticity in two concentric p-systems, with two p-electrons delocalized over the central pentagonal B6 unit and another ten p-electrons responsible for the p-bonding between the central pentagonal unit and the outer ring. Such peculiar chemical bonding does not exist in organic compounds; it can only be found in atomic clusters.