The first experimental evidence for a boron buckyball has been obtained by chemists in the US and China. This cluster of 40 boron atoms – coined borospherene – has a hollow, cage-like structure than resembles that of the well-known carbon buckyballs.
C60 was the first spherical fullerene to be synthesized, an effort for which the chemists who made it were awarded the 1996 Nobel Prize in Chemistry. The possibility of analogues comprised exclusively of boron atoms – a neighbor of carbon on the periodic table – has been widely explored but no evidence has been found before now.
“Since the discovery of the C60 buckyball, scientists have wondered if similar cage molecules can be made from other elements. Even though a few have been observed with heavy elements, such as gold or tin, borospherene represents the structure closest to buckyball mainly because of the strong B-B bond,” explains Lai-Sheng Wang, Brown University, Rhode Island, who led this project.
Because boron has one less electron than carbon it cannot form a stable B60 structure. There has been much speculation concerning how many atoms may form a stable boron buckyball, and earlier work by Wang had suggested that 40 might be the magic number.
The work described in Nature Chemistry [Zhai H.-J., Nature Chemistry (2014) doi: 10.1038/nchem.1999] involved a combination of computer modelling and experimental work. The theoretical team members first modelled more than 10,000 possible arrangements of 40 boron atoms bonded to each other and estimated their electron binding energy spectrums. This measure of how strongly a molecule holds on to its electrons acts as a unique electronic fingerprint for that molecule.
The experimental team then zapped chucks of pure boron with a laser, creating a vapor of boron atoms. A helium jet was used to freeze the vapor into clusters of atoms. Clusters of 40 boron atoms were then isolated by weight. The team then used a second laser to knock an electron out of a cluster. The speed at which this electron travelled away from the cluster was measured and used to calculate the cluster’s electron binding energy spectrum. This electronic fingerprint was then matched to one of the 10,000 predicted structures: a buckyball.
This buckyball, like C60, has unprecedented delocalized bonding over the cage structure but unlike C60 it is not completely spherical. “The cage surface consists of triangles, 6-membered rings, and 7-membered rings,’ explains Wang. “Thus, the surface of B40 is less smooth than that of C60.”
Before any practical applications of B40 can be explored a method to make it in bulk form would need to be discovered, explains Wang. Meanwhile his team intends to study the various properties of borospherene and investigate whether there is a whole family of borospherenes, like the fullerene family.