B22- and B23-: All-Boron Analogues of Anthracene and Phenanthrene

Clusters of boron atoms exhibit intriguing size-dependent structures and chemical bonding that are different from bulk boron and may lead to new boron-based nanostructures. We report a combined photoelectron spectroscopic and ab initio study of the 22- and 23-atom boron clusters. The joint experimental and theoretical investigation shows that B-22(-) and B-23(-) possess quasi-planar and planar structures, respectively. The quasi-planar B-22(-) consists of fourteen peripheral atoms and eight interior atoms in a slightly buckled triangular lattice. Chemical bonding analyses of the closed-shell B-22(2-) species reveal seven delocalized pi orbitals, which are similar to those in anthracene. B-23(-) is a perfectly planar and heart-shaped cluster with a pentagonal cavity and a pi-bonding pattern similar to that in phenanthrene. Thus, B-22(-) and B-23(-), the largest negatively charged boron clusters that have been characterized experimentally to date, can be viewed as all-boron analogues of anthracene and phenanthrene, respectively. The current work shows not only that boron clusters are planar at very large sizes but also that they continue to yield surprises and novel chemical bonding analogous to specific polycyclic aromatic hydrocarbons.