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 (2, 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 2 to release Mes2B• radicals that are captured by an NHC or diverted to organoboron derivatives. By contrast, ammonia and ethylamine engage 2 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.