Stabilization of the [C₂N₅]^7- Anion in Recoverable High-Pressure Eu₄Fe_0.864(6)(C₂N₅)₂ Pyronitridocarbonate

Synthesis at extreme conditions enables access to nitrogen-rich carbon-nitrogen anions that cannot be obtained at ambient conditions. Here, through a direct reaction between Eu(N₃)₂ and EuC₂ with Fe in a laser-heated diamond anvil cell (DAC) at 50(3) GPa, we synthesized the first inorganic hydrogen-free pyronitridocarbonate, Eu₄Fe_x(C₂N₅)₂, x = 0.864(6), featuring novel highly charged [C₂N₅]^7- anions, along with the first stoichiometric oxygen-free rare-earth metal guanidinate Eu₅(CN₃)₃. The crystal structures of both compounds were determined via synchrotron single-crystal X-ray diffraction (SCXRD) and fully corroborated by density functional theory (DFT) calculations. Eu₄Fe_x(C₂N₅)₂ was found to be recoverable at pressures close to ambient. Keeping the sample at ambient conditions for 1 day leads to splitting of half of the [C₂N₅]^7- units in Eu₄Fe_x(C₂N₅)₂ into the guanidinate [CN₃]^5- and carbodiimide [CN₂]^2- anions. The statistical analysis of the multigrain SCXRD data and DFT-based electronic structure analysis well defined the chemical nature of the bonding in [C₂N₅]^7- and [CN₃]^5- anions. This study provides a clear synthetic pathway to a new class of inorganic nitridocarbonates.