Formation and Characterization of Melam, Melam Hydrate, and a Melam-Melem Adduct

Until recently, melam, [C3N3(NH2)2]2NH, has been regarded as a short-lived intermediate in the condensation process of melamine that is only detectable under special reaction conditions owing to its high reactivity. A new synthetic approach has allowed a closer look at the formation and condensation behavior of melam by using elevated ammonia pressure in autoclaves. Whereas the thermal treatment of dicyandiamide at 450 degrees C and 0.2MPa ammonia yielded melam in large amounts, prolonged treatment under these conditions (9days) led to the formation of a melammelem adduct, thus enabling the first insight into the condensation process of melam into melem. The hydrothermal treatment of melam at 300 degrees C (24h) yields melam hydrate, [C3N3(NH2)2]2NH2H2O (space group P21/c; a=676.84(2), b=1220.28(4), c=1394.24(4)pm; =98.372(2)degrees; V=1139.28(6)x106pm3; Z=4), which crystallizes as a layered structure that is composed of almost-planar melam molecules, thereby forming ellipsoidal rosette-like motifs. The resulting voids are filled with four water molecules, thus forming a dense network of hydrogen bonds.