Boron Wrangled Into Stable Triple Bond, Four-Atom Chain

Chemists seeking strange new structures from the main group of elements have bagged two long-sought species with boron at their cores. Researchers in Germany report the synthesis of the world’s first boron-boron triple bond that’s stable at ambient temperatures (Science, DOI: 10.1126/science.1221138) as well as a four-boron chain held together by sp² bonds (Nat. Chem., DOI: 10.1038/nchem.1379).

“Due to its inherent electron deficiency, boron is a very particular element, which behaves completely differently from its neighbor carbon,” says Holger Braunschweig, the University of Würzburg chemist who spearheaded the efforts to create these new species. “We show for the first time that under the right conditions boron can adopt two typical bonding patterns of carbon. Presumably, this will have major implications for future work on boron-containing molecules and, in particular, boron-containing materials,” he says.

The key to getting boron to behave more like carbon, Braunschweig says, was to stabilize the atom with the appropriate groups. In the case of boryne, the researchers used two bulky N-heterocyclic carbenes. For the tetraboron chain, they appropriated an iron atom.

The synthesis of a stable boryne “shows, once again, the unique capabilities of the determined synthetic chemist,” comments Gregory H. Robinson, a chemist at the University of Georgia who specializes in making exotic triple bonds. The new species, he says, “shows that main-group chemistry is alive and well.”

Next, Braunschweig’s team plans to see how the B≤B compound behaves in reactions with small, unreactive molecules that are usually activated only by transition metals. “Expectations are high that we will reveal highly unusual reactivity patterns here,” Braunschweig says.

As for the boron chain, he hopes to create even longer metal-stabilized boron oligomers. “Such boron chains are supposed to have highly interesting electronic properties and could, in the long term, establish a new class of molecular boron-containing materials.”