Boron helps to gently couple dinitrogen

Dinitrogen tends to be a loner. Extreme conditions, such as intense radiation in the ionosphere, are needed to coerce two or more N₂ molecules to form chains. Various pharmaceuticals and explosives made by humans contain three- and four-atom nitrogen chains. To make these compounds, chemists have to use an indirect route. They first split dinitrogen through a high-temperature and high-pressure industrial process to produce ammonia and amines. Then they stitch together those N₁ compounds into the nitrogen chains.

A new study describes a direct and gentle way to make compounds with nitrogen chains using dinitrogen. Marc-André Légaré and Holger Braunschweig of Julius-Maximilians University Würzburg and coworkers report that an organoboron compound can stitch together two N₂ molecules under near-ambient conditions to form a complex in which an N₄ chain bridges two boron moieties (Science 2019, DOI: 10.1126/science.aav9593).

The study follows work published last year by the Würzburg group in which they succeeded in binding a single dinitrogen molecule between organoboron ligands. This general area of research, known as nitrogen fixation, seeks ways to use highly abundant atmospheric nitrogen in chemical synthesis.

The team made the new nitrogen-chain complex by first synthesizing a dihaloorganoborane precursor via standard methods and then reducing it with a solution of KC8 in the presence of dinitrogen at roughly 4 bar (four times atmospheric pressure) and -30 °C. The group determined the product’s structure using X-ray diffraction and various spectroscopy methods. By teaming up with theoreticians at Goethe University Frankfurt, the chemists studied the molecule’s unusual bonding. They found that the borylene moieties bind dinitrogen similar to how transition metals do—by forming end-on N₂-bridging complexes—yet the boron compound’s reactivity is like that of main-group elements.

Légaré says the group is now working to incorporate the nitrogen chains into organic molecules for use in synthesizing pharmaceutical compounds.

“The beauty of this study is the simplicity of the transformation, in which one reduced boron species can lead to spontaneous coupling of N₂ under fairly mild conditions,” says Frédéric-Georges Fontaine, a catalysis specialist at Laval University.

Fontaine remarks that chemists might expect such a transformation to be mediated by a transition metal complex or alkaline earth species, but this one is done by a fairly simple boron molecule. He describes the work as an important step in the trend to develop metal-free transformations. This trend has shown that main group elements, used under the right set of conditions, can replace conventional catalysts, which can be costly and challenging to prepare. “The prospect of eventually including N2 as a reagent in metal-free catalysis is really exciting,” he adds.