Distorted 40-boron atom fullerene detected mixed with quasiplanar isomer.
The first experimental evidence for a boron fullerene has been produced by researchers in the US and China. Unlike the football shaped C60 structure of buckminsterfullerene, the boron structure has very different symmetry, with a box-like shape containing both hexagonal and heptagonal holes. The researchers now hope that their findings will enable other boron fullerenes to be produced.
Boron cannot form a direct B60 analogue of buckminsterfullerene because it has only three electrons in its outer shell, so after forming three bonds it has no free electrons remaining to form the delocalised π-network essential to the stability of C60. In 2007, however, Boris Yakobson and colleagues at Rice University, US, proposed that this electron deficiency could be overcome by inserting an extra boron atom into the centre of each hexagon, forming B80. And in June, Chinese scientists calculated that a the boron fullerene B38 would be stable.
The stability of these hollow-cage structures has subsequently been challenged, but now Lai-Sheng Wang at Brown University, US, and colleagues have used two different electronic structure algorithms to calculate the most stable possible structure of B40. Both programs indicate that, by a considerable margin, the most stable isomer is a distorted fullerene with a hexagonal hole on the top and bottom and four heptagonal holes around the waist. They have christened this structure borospherene.