A Crystalline Monomeric Diboryl Diazene

Azo compounds (R─N═N─R') are a foundational class of nitrogen-containing molecules. In contrast to the bench-stability of many azo derivatives, monomeric diboryl diazenes (R₂BN═NBR₂) have long eluded isolation and were presumed to exist only as polymeric materials or fleeting intermediates. Here, we report the synthesis, characterization, and reactivity of an isolable monomeric diboryl diazene, Mes₂BN═NBMes₂ (<b>2</b>, Mes = mesityl), featuring a nearly linear, cumulenic B═N═N═B core. Lewis basic substrates, including N-heterocyclic carbene (NHC), quinones, and fluorenone, induce N₂ extrusion from <b>2</b> to release Mes₂B• radicals that are captured by an NHC or diverted to organoboron derivatives. By contrast, ammonia and ethylamine engage <b>2</b> via B,N cooperative N-H bond activation, affording aminoborane and transient diazene N₂H₂ without radical formation. Collectively, these results establish diboryl diazenes as versatile main group platforms that enable controllable access to boryl radicals and provide insights into N₂ reduction chemistry.