For decades, chemists have relied on boron-bound nitrenes as fleeting intermediates in synthesis, but no one has been able to directly “see” one. These molecules, also called triplet borylnitrenes, contain a highly reactive, neutral nitrogen atom that features two unpaired electrons with parallel spins. The borylnitrenes rearrange almost instantly, disappearing before their structure can be captured. Now, researchers have designed a precursor that generates a free borylnitrene and traps it in a crystal long enough to reveal its structure in an X-ray “snapshot” (J. Am. Chem. Soc. 2026, DOI: 10.1021/jacs.6c02601).
“Understanding triplet borylnitrenes provides a foundation for gaining control over one of the most reactive nitrogen-containing intermediates in chemistry,” one of the study’s authors, Zhongshu Li of Sun Yat-Sen University, says in an email.
Nitrenes are generally so reactive that before this work, only two arylnitrenes had been isolated: one protected from degradation by bulky molecular shields, and the other stabilized through electronic effects. Borylnitrenes are even more reactive than arylnitrenes, and they can be spectroscopically detected only at temperatures below 90 K. “By revealing their structure and electronic nature,” Li says, “chemists can move from ‘assuming’ how these intermediates behave to actually knowing how they are built and how they react.”
To trap the borylnitrene, Li and collaborators used a specially engineered azide embedded in a rigid boron-containing framework that can be activated with light at low temperature. Irradiation converted the azide into a borylnitrene trapped inside the crystal lattice, where X-ray diffraction revealed a shortened B–N bond consistent with a triplet intermediate.
“One of the most striking observations was the clear structural change that occurred upon nitrene formation,” Li says. “We saw a distinct contraction of the boron–nitrogen bond. This was an important moment because it provided direct structural evidence that the electronic structure had fundamentally changed.”
The bond length fell between the typical lengths of a single bond and a double bond, suggesting an unusual bonding arrangement. Further analysis showed that an unpaired electron on nitrogen is partially delocalized into the surrounding framework, helping stabilize the triplet state.
“By stabilizing and observing a free borylnitrene, we were able to show that it behaves like a classic reactive nitrene,” Li says. The borylnitrene intermediate can pull hydrogen atoms off of other molecules, insert itself into C–H and B–C bonds, coordinate to Lewis bases, and undergo cycloaddition reactions relevant to synthetic chemistry.
