Capturing a Tetratomic Nitrogen Ring cyclo-N4 inside an Aza[ 82 ]fullerene

Abstract Synthesis of polymeric nitrogen compounds is a formidable task due to the proneness of nitrogen to the formation of N ≡ N triple bond, one of the strongest chemical bonds known. Here, we report an arc-discharge approach to successfully stabilize the elusive four-membered nitrogen ring (cyclo-N4) in an unprecedented endohedral metallofullerene Dy2N4@C81N (Dy-I). Its molecular structure has been unambiguously determined by X-ray crystallography to show a covalently bonded cyclo-N4 plane bridging two dysprosium ions inside an aza[82]fullerene cage, highlighting the stabilization of cyclo-N4 as a concurrent result of fullerene encapsulation and metal coordination. Our computational results further reveal a six-center-one-electron (6c-1e) bond delocalized over the inverse-sandwich Dy-N4-Dy cluster. This chemical peculiarity stems from the diffuse radical character of the highly anionic cyclo-N43− ligand, which is confirmed by electron paramagnetic resonance (EPR) spectrum of Y2N4@C81N (Y-I).