Brand new bonding skills make boron more like carbon

BORON has been hiding secret skills but its cover is finally blown.

In a tightly sealed flask in a German lab sits an emerald-green crystal that is the first stable compound with a triple chemical bond between two boron atoms. Such bonds were previously reserved for an elite club of atoms, including carbon, which sits next to boron in the periodic table.

In another first, boron atoms have linked up with each other in chains. Carbon's tendency to do so is the basis for organic chemistry, making possible DNA, proteins, alcohols and plastics. Synthetic, or alien, life based on boron remains far-fetched - boron still can't link to other life-related compounds, for one thing. But the feats pave the way for boron-based polymers, and other structures previously undreamed of. "Certainly this will go into the textbooks," says Holger Braunschweig of the University of W├╝rzburg in Germany.

Until now largely obscure, boron occupies a special spot in the periodic table. On one side are metals like beryllium, which give away their outermost electrons to form ionic bonds. On the other side are the non-metals carbon and nitrogen, which prefer sharing electrons in covalent bonds.

A single covalent bond is two electrons shared between two atoms. Carbon, with four outer electrons, and nitrogen with five, form triple covalent bonds consisting of six shared electrons, or three pairs. Boron has three outer electrons, so in principle should be capable of this too, but it has remained aloof. In 2002, Mingfei Zhou and colleagues at Fudan University in Shanghai, China, managed to make a boron triple bond - but only at 8 degrees above absolute zero.

Boron can hold up to eight outer electrons: a pair in each of four slots. In atomic boron, one of these slots is completely empty and the other three are half-full, with one electron apiece. To make triple-bonded boron that could survive at room temperature, Braunschweig and his colleagues filled all four slots. They filled up the empty slot by bonding each boron atom to a molecule that donated two electrons. Each boron atom then completed the filling of its slots by pairing up with another boron atom and pooling its original three electrons (see diagram). In the resulting compound, each boron atom has a full suite of eight outer electrons, making it stable (Science, DOI: 10.1126/science.1221138).

Braunschweig and his team have already begun investigating the reactivity of the novel compound. "It turns out that it shows a rich chemistry," he says.

Triple-bonding is not the only way the researchers got boron to mimic its superstar neighbour, carbon, though. They also coaxed boron atoms into forming a chain. Previous attempts to do this with boron failed and resulted in messy clusters. "It is extremely difficult to form chains of boron atoms," says Braunschweig. The secret was to attach the boron atoms to an iron scaffold, which allowed up to four to form a chain (Nature Chemistry, DOI: 10.1038/nchem.1379).

Next, Braunschweig and his team hope to free this boron chain from its scaffold and increase the chain length to form the boron equivalent of polyethylene, a common plastic. "The material, if it could be made, would have completely different physical behaviour from an ordinary carbon chain," Braunschweig says.

A world of previously forbidden chemistry beckons. "They did not simply find a new flower or a new plant," says Gernot Frenking of the University of Marburg in Germany, who was not involved in the work. "They opened the door to a whole new garden."