The debut of all-boron fullerene

Boron is the fifth element and next to carbon in the periodic table of the elements, but boron has been researched less than its neighbor carbon. Borax sourced from a natural boronrich mine in Yamdrok Lake, Tibet, China, has been used as a fusion agent to make glass from ancient times. Elemental boron, however, was elusive until 1808 when it was prepared from electrolysis of melted boron trioxide or by reducing boric acid with potassium. Structures of elemental boron include an icosahedral (Ih) B12 motif connected in an ordered manner, as exemplified by α-rhombohedral boron (Fig. 1a). The dangling bonds in each Ih-B12 unit can be saturated by hydrogen to produce borane species. In stark contrast to the ubiquitous Ih-B12 unit in elemental boron allotropes and boranes, a different B12 all-boron clusterwith aC3v-symmetric planar structure that resembles a solid disk with three boron atoms in the center surrounded by a peripheral ring of boron atoms has also been observed (Fig. 1b) [1]. With six delocalized π electrons that exhibit ‘disk delocalization’, the planar B12 cluster is analogous to benzene with its wellknown cyclic delocalization. Bn clusters adopt 2D planar or quasi-planar structures even up to n = 36 [2]. Because of their diverse set of structural and bonding characteristics, scientists believe that boron and carbon form a set of complementary chemical systems and have long been fascinated with whether or not structures such as cage-like all-boron clusters are as prevalent as all-carbon fullerenes. A series of all-boron cage configurations stabilized by three-center Figure 1. The structures of all-boron clusters (B12 and B40). (a) Icosahedral B12 motifs bound by threecenter two-electronσ bonds (indicated by dashed lines) to form a B12 layer inα-rhombohedral boron; (b) Top and side views of a C3v-symmetric B12 cluster; (c) Top and side views of Cs-symmetric B40 with two adjacent hexagonal holes; (d) Top and side views of a neutral B40 cluster with two hexagonal and four heptagonal holes. This figure was redrawn according to the coordinates of a B40 cluster provided in the supplementary information of [5].