Crystalline nitrogen chain radical anions

Long-chain nitrogen ions and radicals ([N_n]^x+/[N_n]^x-, n > 3) are naturally occurring under the intense radiative conditions of the Earth's ionosphere and those of other planetary bodies. However, the strong thermodynamic driving force to lose N₂ renders these types of molecule extremely reactive under ambient conditions such that they can typically be studied only under extreme conditions, for example, at ultrahigh pressures (10 GPa to >200 GPa). Here we report the isolation of a series of five molecules featuring metal unsupported {N₄}^•- units under ambient conditions, with one derivative demonstrating remarkable multi-week long persistence in the solid state. Spectroscopic, crystallographic and computational studies provide insight into the bonding across the {N₄}^•- chain. Reactivity studies reveal that the chain can cleave into N1 and N3 fragments, and can act as a source of nitrene radical anions, an observation that such molecules could act as storable nitrogen group transfer reagents.