Scientists have obtained the first direct X-ray structure of a free triplet borylnitrene trapped inside a single crystal. Nitrenes form rapidly and vanish almost immediately by rearranging through 1,2-migration, making them hard to examine despite their value in synthesis. A boron-linked form was predicted decades ago yet always rearranged before its structure could be recorded.
In the new work, a specially designed precursor containing an acenaphthene unit released the target species when illuminated at 370 nm. Cooling the crystal to 100 K prevented further movement, allowing the molecule to remain in place long enough for measurement. X-ray data showed the boron-nitrogen distance shortened from 1.437 Å to 1.398 Å, consistent with partial double-bond character. Spectroscopic results confirmed a triplet ground state, and the crystal changed color from red to blue.
Further tests in solution demonstrated typical nitrene reactivity, including hydrogen abstraction and insertion into C–H and B–C bonds, plus coordination to Lewis bases and cycloadditions. The study appeared in the Journal of the American Chemical Society.
