Simple Route to Metal cyclo-N-5(-) Salt: High-Pressure Synthesis of CuN5

A naked cyclo-N-5(-) 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-N-5(-) salt via compressing CuN6. Using first-principles calculations with structural search, we predict a CuN5 compound with a cyclo-N-5(-) anion that is energetically favorite in the pressure range of 50-100 GPa. At ambient conditions, CuN5 comprises alternant-connected cyclo-N-5(-) anions and Cu+ ions, forming a zigzag chain. The copper cyclo-N-5(-) 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 N-5 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-N-5(-) salt and indicates a simple route to its synthesis.