Simple Route to Metal cyclo-N₅^– Salt: High-Pressure Synthesis of CuN₅

A naked cyclo-N5– salt has yet to be recovered to ambient conditions, precluding its application as a high-energy density material for explosive or propulsion applications. Here, we suggest a simple route for the synthesis of a metal cyclo-N5– salt via compressing CuN6. Using first-principles calculations with structural search, we predict a CuN5 compound with a cyclo-N5– anion that is energetically favorite in the pressure range of 50–100 GPa. At ambient conditions, CuN5 comprises alternant-connected cyclo-N5– anions and Cu+ ions, forming a zigzag chain. The copper cyclo-N5– salt is thermodynamically stable with a 2.5 eV band gap at ambient conditions. Further analysis of electronic properties reveals that the Cu atoms not only contribute electrons to change the bonding state of N5 rings but also use empty outer-shell orbitals to accommodate lone pair electrons of N atoms, forming coordinate bonds to stabilize the system. This expands the known cyclo-N5– salt and indicates a simple route to its synthesis.