Na₂C₃O₂: The First Crystalline Compound with the Elusive ⁻C≡C−COO⁻ Anion

By reaction of sodium electride or NaC 2 H with the anhydrous sodium salt of propiolic acid, Na(OOC−C≡C−H), in liquid ammonia crystalline powders of Na 2 C 3 O 2 were obtained. The structure analysis based on synchrotron powder diffraction data revealed that Na 2 C 3 O 2 crystallizes in a monoclinic unit cell ( I 2/ a , Z =4) exhibiting the elusive Y‐shaped − C≡C−COO − anion, which is unprecedented in a crystalline compound up to now. IR/Raman and solid‐state NMR spectroscopic investigations with assignments supported by DFT‐based ab initio calculations confirm this finding. Reaction with sodium electride led to a higher crystallinity of the product, but additionally a by‐product apparently due to decomposition and polymerization of Na 2 C 3 O 2 was formed. No such by‐product was observed in the reaction with NaC 2 H, which turned out to be a milder metalation route. However, the product of the latter reaction is less crystalline.