A concentric planar doubly π-aroMatic B19- 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 B-19(-) cluster, which shows chemical bonding reminiscent of that in [10]annulene (C10H10) and [6]circulene (C24H12). Photoelecton spectroscopy reveals a relatively simple spectrum for B-19(-) is a nearly circular planar structure with a central B-6 pentagonal unit bonded to an outer B-13 ring. Chemical bonding analyses reveal that the B-19(-) cluster possesses a unique double pi-aromaticity in tow concentric pi-systems, with two pi-electrons delocalized over the central pentagonal B-6 unit another ten pi-electrons responsible for the pi-bonding between the central pentagonal unit and the outer ring. Such peculiar chemical bonding does not exist in organic compound; it can only be found in atomic clusters.