Formation of Melamium Adducts by Pyrolysis of Thiourea or Melamine/NH4Cl Mixtures

Pyrolysis of prominent precursor compounds for the synthesis of carbon nitride type materials (e.g., melamine, thiourea) have been studied in detail. Molecular adducts containing monoprotonated melamium C6N11H10+ and melaminium HC3N3(NH2)(3+) ions, respectively, have been identified as intermediates. The adduct C6N11H10Cl center dot 0.5NH(4)Cl was obtained by the reaction of melamine C3N3(NH2)(3) with NH4Cl at 450 degrees C. During the pyrolysis of thiourea, guanidinium thiocyanate was initially formed and subsequently the melamium thiocyanate melamine adduct C6N11H10SCN center dot 2C(3)N(3)(NH2)(3) was isolated at 300 degrees C. A second melaminium thiocyanate melamine adduct with the formula HC3N3(NH2)(3)SCN center dot 2C(3)N(3)(NH2)(3) represents an intermediary reaction product that is best accessible at low pressures. The crystal structures of the compounds were solved by single-crystal XRD. Unequivocal proton localization at the C6N11H10+ ion was established. A typical intramolecular and interannular hydrogen bridge and other characteristic hydrogen-bonding motifs were identified. Additionally, the adducts were investigated by solid-state NMR spectroscopy. Our study provides detailed insight into the thermal condensation of thiourea by identifying and characterizing key intermediates involved in the condensation process leading to carbon nitride type materials. Furthermore, factors promoting the formation of melamium adduct phases over melem are discussed.