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

Until recently, melam, [C 3 N 3 (NH 2 ) 2 ] 2 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 3 N 3 (NH 2 ) 2 ] 2 NH ⋅ 2 H 2 O (space group P 2 1 / c ; a =676.84(2), b =1220.28(4), c =1394.24(4) pm; β =98.372(2)°; V =1139.28(6)×10 6  pm 3 ; 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.