Melem (2,5,8-Triamino-tri-s-triazine), an Important Intermediate during Condensation of Melamine Rings to Graphitic Carbon Nitride: Synthesis, Structure Determination by X-ray Powder Diffractometry, Solid-State NMR, and Theoretical Studies

Abstract

Single-phase melem (2,5,8-triamino-tri-s-triazine) C₆N₇(NH₂)₃ was obtained as a crystalline powder by thermal treatment of different less condensed C−N−H compounds (e.g., melamine C₃N₃(NH₂)₃, dicyandiamide H₄C₂N₄, ammonium dicyanamide NH₄[N(CN)₂], or cyanamide H₂CN₂, respectively) at temperatures up to 450 °C in sealed glass ampules. The crystal structure was determined ab initio by X-ray powder diffractometry (Cu Kα₁: P2₁/c (No. 14), a = 739.92(1) pm, b = 865.28(3) pm, c = 1338.16(4) pm, β = 99.912(2)°, and Z = 4). In the solid, melem consists of nearly planar C₆N₇(NH₂)₃ molecules which are arranged into parallel layers with an interplanar distance of 327 pm. Detailed ¹³C and ¹⁵N MAS NMR investigations were performed. The presence of the triamino form instead of other possible tautomers was confirmed by a CPPI (cross-polarization combined with polarization inversion) experiment. Furthermore, the compound was characterized using mass spectrometry, vibrational (IR, Raman), and photoluminescence spectroscopy. The structural and vibrational properties of molecular melem were theoretically studied on both the B3LYP and the MP2 level. A structural optimization in the extended state was performed employing density functional methods utilizing LDA and GGA. A good agreement was found between the observed and calculated structural parameters and also for the vibrational frequencies of melem. According to temperature-dependent X-ray powder diffractometry investigations above 560 °C, melem transforms into a graphite-like C−N material.