Stabilization of Fully Deprotonated Melaminate Anions (C 3 N 6 ) 6– in M 3 (C 3 N 6 ) (M = Cd, Ca)

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Abstract
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.
Funding Information
  • European Commission (101077963)
  • Johanna Quandt Stiftung
  • Hessisches Ministerium f??r Wissenschaft und Kunst (Loewe Starting Professorship (New Materials))
  • German research council (DFG) (BY101/2-1)
  • German research council (DFG) (BY112/2-1)
  • Adolph Christ Foundation

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