Formation and characterization of melam, melam hydrate, and a melam-melem adduct

摘要

Until recently, melam, [C_3N_3(NH_2) _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 °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_3N_3(NH_2) _2]_2NH×2 H_2O (space group P2 _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.