Stabilization of the [C2N5]7–Anion in Recoverable High-Pressure Eu4Fe0.864(6)(C2N5)2Pyronitridocarbonate

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, Eu4Fex(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. Eu4Fex(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 Eu4Fex(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.