Crystallographic Capture of a Free Triplet Borylnitrene

Borylnitrenes are key intermediates in chemical synthesis, but their propensity for 1,2-migration has precluded direct structural characterization. Here, we report the generation and in crystallo X-ray capture of a free triplet borylnitrene. This was achieved through the design of a 1,3,2-diazaborolyl azide precursor, where the incorporation of a long-wavelength absorber allows the photolysis of the azide unit in the solid state at low temperatures. Crystallographic analysis of the photolyzed crystal reveals a distinct contraction of the B–N bond, which combined with electron paramagnetic resonance (EPR) and UV–vis spectroscopies, coupled to theoretical calculations, established a triplet ground state. This borylnitrene exhibits characteristic reactivities of free nitrenes, including hydrogen-atom abstraction, C–H and C–B bond insertion, Lewis base coordination, and [2 + 1] cycloaddition.