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

Area of Science:

• Solid-state chemistry
• Crystallography
• Spectroscopy

Background:

• The synthesis and characterization of novel chemical structures are fundamental to advancing materials science.
• The propiolic acid anion is known, but its incorporation into a stable, crystalline Y-shaped structure has remained elusive.
• Understanding the reactivity and stability of such unique anions is crucial for potential applications.

Purpose of the Study:

• To synthesize and characterize a crystalline compound containing the unprecedented Y-shaped ^-C≡C-COO^- anion.
• To elucidate the crystal structure and bonding of the newly formed compound, Na₂C₃O₂.
• To compare different synthetic routes for obtaining this novel material.

Main Methods:

• Synthesis in liquid ammonia using sodium electride or NaC₂H with the sodium salt of propiolic acid.
• Structure analysis utilizing synchrotron powder diffraction data.
• Spectroscopic investigations including IR/Raman and solid-state NMR.
• Density Functional Theory (DFT)-based ab initio calculations for spectral assignment.

Main Results:

• Crystalline sodium dicyanopropiolate (Na₂C₃O₂) was successfully obtained.
• Synchrotron powder diffraction revealed a monoclinic unit cell (I2/a, Z=4) containing the unique Y-shaped ^-C≡C-COO^- anion.
• Spectroscopic and computational methods confirmed the proposed structure and bonding of the anion.
• Reaction with NaC₂H provided a milder route with no by-products, albeit yielding less crystalline material.

Conclusions:

• The synthesis and structural confirmation of the Y-shaped ^-C≡C-COO^- anion in Na₂C₃O₂ represent a significant discovery in inorganic and materials chemistry.
• The study demonstrates the feasibility of accessing unprecedented anionic structures through careful control of synthetic conditions.
• NaC₂H offers a milder alternative to sodium electride for the synthesis of Na₂C₃O₂, with implications for future research on related compounds.