Borylenes nab nitrogen

Boron has mastered masquerading as a metal. In recent years, compounds with the little p-block element have managed to latch onto small molecules, such as hydrogen and carbon monoxide, that chemists previously thought only metals could bind and split. Now, boron has gotten hold of the most stubborn small molecule, N₂.

Researchers led by Holger Braunschweig of Julius Maximilian University Würzburg managed the feat by using a borylene—the boron version of a carbene—which features a monovalent boron, a lone pair of electrons, and an empty orbital. “The borylene mimics a very electron-rich transition metal,” Braunschweig explains. It donates its electron density to activate the stable N≡N bond (Science 2018, DOI: 10.1126/science.aaq1684).

“Nitrogen fixation by small molecules is among the most difficult tasks in chemistry,” says Suning Wang, an expert in organoboron chemistry at Ontario’s Queen’s University who was not involved in the research. The discovery, she says, “provides a new paradigm in nitrogen fixation and activation chemistry.”

Although the finding might someday help improve large-scale nitrogen-fixing reactions, such as the ammonia-making Haber-Bosch process, that goal, Braunschweig says, is a long way off. At the moment, the reaction, which requires two borylenes for every N₂ molecule caught, isn’t catalytic.

“For years, low-valent main-group element derivatives were regarded as laboratory curiosities, only of interest for a restricted scientific community,” says Guy Bertrand, a University of California, San Diego, professor who also studies borylenes. “The fixation of N₂ at a low coordinate boron center is a wonderful demonstration of the usefulness of fundamental research.”

Next, Braunschweig and colleagues plan to study the reaction’s mechanism. “Only when we know exactly how the compounds are being formed will we be able to optimize the conditions to make the reaction work better,” he says.