Hydromelonic acid

Hydromelonic acid, is an elusive chemical compound with formula C9H3N13 or (HNCN)3(C6N7), whose molecule would consist of a heptazine H3(C6N7) molecule, with three cyanamido groups H–N=C=N– or N≡C–NH– substituted for the hydrogen atoms.

The compound had not been properly isolated as of 2010, due to its tendency to polymerize. However, removal of three protons yields the melonate (formerly hydromelonate) anion (NCN)3(C6N7)3−, whose salts are stable and have been known since the 19th century. Removal of only two protons yields the divalent hydrogenmelonate anion H(NCN)3(C6N7)2−.

History

In 1834 Justus von Liebig described the compounds that he named melamine, melam, and melon. In 1835 Leopold Gmelin prepared a potassium salt (later identified as K3[(NCN)3(C6N7)]) by heating potassium ferrocyanide with sulfur); recognizing their connection to the compounds described by Liebig, he named the salt "hydromelonate" and the corresponding acid "hydromelonic". In the following years Liebig prepared the same salt by other methods, such as by fusing potassium thiocyanate with antimony trichloride, and eventually determined the formula C9N13H3 for the acid.

The correct structure for the cation was published in 1937 by Linus Pauling and J. H. Sturdivant.

Starting in the 1970s, melonates have attracted new interest, motivated by research in cubic carbon nitride c-C3N4. Salts of many cations have been synthesized and studied, including mixed-cation salts and calcium hydrogenmelonate Ca[HC9N13]·7H2O. The structure of potassium melonate pentahydrate K3C9N13·5H2O was elucidated only in 2005.