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

<jats:title>Abstract</jats:title><jats:p>Until recently, melam, [C<jats:sub>3</jats:sub>N<jats:sub>3</jats:sub>(NH<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>]<jats:sub>2</jats:sub>NH, 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 °C and 0.2 MPa ammonia yielded melam in large amounts, prolonged treatment under these conditions (9 days) led to the formation of a melam–melem adduct, thus enabling the first insight into the condensation process of melam into melem. The hydrothermal treatment of melam at 300 °C (24 h) yields melam hydrate, [C<jats:sub>3</jats:sub>N<jats:sub>3</jats:sub>(NH<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>]<jats:sub>2</jats:sub>NH<jats:bold>⋅</jats:bold>2 H<jats:sub>2</jats:sub>O (space group <jats:italic>P</jats:italic>2<jats:sub>1</jats:sub>/<jats:italic>c</jats:italic>; <jats:italic>a</jats:italic>=676.84(2), <jats:italic>b</jats:italic>=1220.28(4), <jats:italic>c</jats:italic>=1394.24(4) pm; <jats:italic>β</jats:italic>=98.372(2)°; <jats:italic>V</jats:italic>=1139.28(6)×10<jats:sup>6</jats:sup> pm<jats:sup>3</jats:sup>; <jats:italic>Z</jats:italic>=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.</jats:p>