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

The lowest energy structures for Bn 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 sp2-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 π-electrons for B6 and B7+ similar to the cyclopropenyl cation and six π-electrons for B82- and B9- similar to benzene. Related chemical bonding topology analyses for low-energy oval B10 and B11- structures as well as for larger planar boron triangle networks with 12, 13, and 14 boron atoms suggest six π-electrons in such structures. Planar networks of boron triangles having 16-19 boron atoms are shown to be systems with 10 π-electrons similar to naphthalene. Similarly, low-energy planar B22 and B23- structures are shown to be 4 π-electron systems 1analogous to linear anthracene and angular phenanthrene, respectively. Intermediate B15- and B21- systems are shown to be systems with 4k rather than 4k + 2 π-electrons with 8 and 12 π-electrons, respectively. Structures based on planar networks of boron triangles are strongly energetically disfavored for B20 relative to a nonplanar decagonal antiprism structure with ideal D10d symmetry.