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 B₁₉⁻ 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⁻ 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⁻ cluster possesses a unique double π-aromaticity in two concentric π-systems, with two π-electrons delocalized over the central pentagonal B₆ unit and another ten π-electrons responsible for the π-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.