Planar Networks of Boron Triangles: Analogies to Benzene and Other Planar Aromatic Hydrocarbons

The lowest energy structures for B-n species (n = 6 to 23 except for 20) observed experimentally in the gas phase with a mass spectrometer are planar networks of boron triangles. Such networks are considered to consist of trigonal planar sp(2)-hybridized boron atoms having perpendicular p orbitals similar to the carbon atoms in benzene and other planar aromatic hydrocarbons. Electron bookkeeping for reasonable chemical bonding topologies of wheel-like structures such as B@Bn-1 (n = 6-9) leads to two pi-electrons for B-6 and B-7(+) similar to the cyclopropenyl cation and six pi-electrons for B-8(2-) and B-9(-) similar to benzene. Related chemical bonding topology analyses for low-energy oval B-10 and B-11 - structures as well as for larger planar boron triangle networks with 12, 13, and 14 boron atoms suggest six pi-electrons in such structures. Planar networks of boron triangles having 16-19 boron atoms are shown to be systems with 10 pi-electrons similar to naphthalene. Similarly, low-energy planar B-22 and B-23(-) structures are shown to be 4 pi-electron systems 1analogous to linear anthracene and angular phenanthrene, respectively. Intermediate B-15(-) and B-21(-) systems are shown to be systems with 4k rather than 4k + 2 pi-electrons with 8 and 12 pi-electrons, respectively. Structures based on planar networks of boron triangles are strongly energetically disfavored for B-20 relative to a nonplanar decagonal antiprism structure with ideal D-10d symmetry.