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.