Credit: Marc-André Légaré/Julius-Maximilians University Würzburg

Dinitrogen tends to be a loner. Extreme conditions, such as intense radiation in the ionosphere, are needed to coerce two or more N 2 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 N 1 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 N 2 molecules under near-ambient conditions to form a complex in which an N 4 chain bridges two boron moieties (Science 2019, DOI: 10.1126/science.aav9593).

The prospect of eventually including N 2 as a reagent in metal-free catalysis is really exciting. Frédéric-Georges Fontaine, a catalysis specialist at Laval University

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 N 2 -bridging complexes—yet the boron compound’s reactivity is like that of main-group elements.

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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 N 2 under fairly mild conditions,” says Frédéric-Georges Fontaine, a catalysis specialist at Laval University.