In dilithium diborane dianion, bulky Eind groups (grey) protect the delicate boron–boron bond (blue).

In dilithium diborane dianion, bulky Eind groups (grey) protect the delicate boron–boron bond (blue).* © Xvazquez

At the RIKEN Advanced Science Institute, formation of a boron–boron covalent bond under laboratory conditions has opened up a new corner of chemistry.

Electrons are the glue that holds chemical compounds together. The metalloid element boron is electron-deficient, so its compounds often exhibit unusual bonding behaviour and rarely form simple structures. Now, for the first time ever, the element can be forced into more conventional behaviour using a new method developed in Japan.

The compound created features two boron atoms held together by a shared pair of electrons: a simple covalent bond. For other elements—carbon, for example—this kind of bonding is typical, but electron-poor boron tends to prefer a more complex arrangement. For example, in the compound diborane (B2H6), two boron atoms are ‘bridged’ by hydrogen atoms, and each boron–hydrogen–boron bond shares a single pair of electrons across three atoms rather than the usual two.

Theory predicts that pumping extra electrons into a compound such as diborane will cause the boron–hydrogen–boron structure to break down and form a boron–boron single bond instead. Until recently, however, any attempt to make and isolate such a structure had failed, resulting only in single boron species.

Researchers at RIKEN suspected that previous attempts probably succeeded in generating the boron–boron single bond, but failed to protect that structure from quickly falling apart through further reaction. They adopted a new strategy, starting with the compound borane, in which the boron atoms have bulky side-groups known as Eind groups stuck to them. Using these bulky Eind groups, they were able to stabilise the new bond, prevent further breakdown and successfully isolate the desired compound.

The next step will be to explore the boron-boron bond’s chemistry and reactivity. It has already proved to be relatively stable: if protected from air and moisture, the compound can be stored for months at ambient temperature. It can also be converted into a three-membered ring, in which a bridging hydrogen atom is the third member, forming a molecule with potentially useful properties.

“We think that the hydrogen-bridged boron–boron bond has a double-bond character,” says Tsukasa Matsuo, one of the principal investigators.

“We would like to explore the new reaction chemistry of multiply bonded boron species.”

* Reprinted with permission from Shoji et al. Copyright 2011 American Chemical Society

For further information contact:

Dr Tsukasa Matsuo
RIKEN Advanced Science Institute, Japan