Crystalline intermediates during polycondensation reactions in the C-N-Cl system - The paddle-wheel molecule N(C₆N₇Cl ₂)₃

Solid state metathesis reactions between cyanuric chloride and C-N-H or alkali metal-(B-)C-N compounds, respectively, were carried out in the temperature range between 150°C to 500°C, studying intermediate stages of reactions and targeting the formation of carbon nitride materials by elimination of HCl or alkali metal chlorides. Although cyanuric chloride was reacted with quite a number of different reaction partners such as melamine, cyanamide, lithium nitride, lithium or sodium carbodiimide, lithium nitridoborate or sodium dicyandiamide, always the same intermediate compounds appeared in the reactions mixtures. Colorless, needle-shaped crystals of the tertiary amine N(C₃N₃Cl₂)₃ (1) were obtained at temperatures around 200-250°C. Temperatures as high as 400°C yielded yellow, plate-like crystals of the heptazine compound C ₆N₇Cl₃ (2). At even higher temperatures, the reaction products were of poorer crystallinity, but evidence of the formation of another crystalline intermediate was given by X-ray powder diffraction and electron diffraction experiments. This third intermediate is assumed to be a tertiary amine, quite similar to 1, however, having heptazine ligands instead of triazine ligands and is assigned with the formula N(C₆N ₇Cl₂)₃ (3). Theoretical calculations were performed for the structures and the vibrational spectra of 1 and 3. Theoretical calculations and a structure refinement based of Xray powder diffraction data yielded a plausible structural model for compound 3.