Melamine–melem adduct phases: Investigating the thermal condensation of melamine

By studying the thermal condensation of melamine, we have identified three solid molecular adducts consisting of melamine C3N3(NH2)3 and melem C6N7(NH2)3 in differing molar ratios. We solved the crystal structure of 2 C3N3(NH2)3⋅C6N7(NH2)3 (1; C2/c; a=21.526(4), b=12.595(3), c=6.8483(14) Å; β=94.80(3)°; Z=4; V=1850.2(7) Å3), C3N3(NH2)3⋅C6N7(NH2)3 (2; Pcca; a=7.3280(2), b=7.4842(2), c=24.9167(8) Å; Z=4; V=1366.54(7) Å3), and C3N3(NH2)3⋅3 C6N7(NH2)3 (3; C2/c; a=14.370(3), b=25.809(5), c=8.1560(16) Å; β=94.62(3)°; Z=4; V=3015.0(10) Å3) by using single-crystal XRD. All syntheses were carried out in sealed glass ampoules starting from melamine. By variation of the reaction conditions in terms of temperature, pressure, and the presence of ammonia-binding metals (europium) we gained a detailed insight into the occurrence of the three adduct phases during the thermal condensation process of melamine leading to melem. A rational bulk synthesis allowed us to realize adduct phases as well as phase separation into melamine and melem under equilibrium conditions. A solid-state NMR spectroscopic investigation of adduct 1 was conducted.