On Tautomerism and Amphoterism: An In‐Depth Structural and Physicochemical Characterization of Ammeline and Some of Its Salts

Ammeline is a simple, readily available, molecular compound, which has been known for nearly 200 years. Despite that, no proper structural characterization of ammeline has been conducted so far. For this reason, the prevalent tautomeric form of ammeline in the solid remained unknown to this date. In the course of this study, its crystal structure was finally established by single‐crystal X‐ray diffraction. In this structure, ammeline is exclusively found as its 4,6‐diamino‐1,3,5‐triazin‐2(1H)‐one tautomer and adopts layered structure with an exceptionally high hydrogen bond density. Ammeline shows an interesting amphoteric behavior. Therefore, the synthesis and structural characterization of some of its salts were carried out to investigate the influence of the protonation degree on its molecular structure. In particular, the crystal structure of silver ammelinate monohydrate was solved as the first reported structure containing deprotonated ammeline. Moreover, the crystal structures of three different modifications of ammelinium perchlorate were elucidated and the transformation conditions between them were studied. Lastly, the crystal structure of ammelinediium diperchlorate monohydrate, containing unprecedented doubly protonated ammeline, was determined. The products’ thermal behavior was studied by differential thermal analysis and thermogravimetric analysis. The perchlorate salts were additionally examined for their potential as insensitive high‐energy‐density materials. The crystal structure of ammeline was solved for the first time nearly 200 years after its discovery by Justus von Liebig. Moreover, several salts of ammeline were synthesized and structurally characterized to study its amphoteric behavior and the influence of the protonation degree on its molecular structure in the solid. Finally, the potential of ammeline as part of insensitive high‐energy‐density materials was examined.