Stabilization of Fully Deprotonated Melaminate Anions (C₃N₆)^6- in M₃(C₃N₆) (M = Cd, Ca)

Hydrogen-free melaminate salts M₃(C₃N₆) (M = Cd, Ca) were synthesized in laser-heated diamond anvil cells at 34-48 GPa and 2000-2500 K. Cd₃(C₃N₆) was synthesized via a direct reaction between the elements, while Ca₃(C₃N₆) was obtained following a rational chemical design approach from calcium carbodiimide, Ca(NCN), which served as a single-source precursor. Both compounds contain the fully deprotonated melaminate anion (C₃N₆)^6-, representing a fundamental milestone in nitridocarbonate chemistry. The crystal structures of M₃(C₃N₆) were solved and refined using synchrotron single-crystal X-ray diffraction data and were fully corroborated by density functional theory calculations. Cd₃(C₃N₆) crystallizes in the acentric R3c and Ca₃(C₃N₆) in the centrosymmetric R3̅c space groups, and both compounds are recoverable to ambient conditions. Extending this design principle, our calculations indicate that Zn and Pb melaminates are thermodynamically accessible under similar conditions, highlighting the general stability of hydrogen-free nitridocarbonates of selected divalent metals.