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

By reaction of sodium electride or NaC2H with the anhydrous sodium salt of propiolic acid, Na(OOC−C≡C−H), in liquid ammonia crystalline powders of Na2C3O2 were obtained. The structure analysis based on synchrotron powder diffraction data revealed that Na2C3O2 crystallizes in a monoclinic unit cell (I2/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 Na2C3O2 was formed. No such by‐product was observed in the reaction with NaC2H, which turned out to be a milder metalation route. However, the product of the latter reaction is less crystalline. [ABSTRACT FROM AUTHOR]