The tautomeric forms of cyameluric acid derivatives

The tautomerism of cyameluric acid C6N7O3H3 (1a), cyamelurates and other heptazine derivatives has recently been studied by several theoretical investigations. In this experimental study we prepared stannyl and silyl derivatives of cyameluric acid (1a): C6N7O3[Sn(C4H9)(3)](3) (3a), C6N7O3[Sn(C2H5)(3)](3) (3b), and C6N7O3[Si(CH3)(3)](3) (4). In order to investigate the structure of la the mono- and dipotassium cyamelurate hydrates K(C6N7O3H2)-center dot 2H(2)O (5) and K-2(C6N7O3H)center dot 1H(2)O (6) were synthesized by UV/Vis-controlled titration of a potassium cyamelurate solution with aqueous hydrochloric acid. Compounds 3-6 were characterized by FTIR and solid-state NMR spectroscopy as well as simultaneous thermal analysis (TGA, DTA). The single crystal X-ray structures of the salts 5 and 6 show that the hydrogen atoms in both anions are localized on the peripheral nitrogen atoms. This indicates-in combination with the solid-state NMR studies-that the most stable tautomer of solid 1a is the trike-to form with C-3/h symmetry. However, derivatives of both the hydroxyl and the amido tautomers may be formed depending on the substituent atoms: The spectroscopic data and single crystal structures of compounds C6N7O3[Si- (CH3)(3)](3) (4) and the solvate C6N7O3[Sn(C2H5)(3)](3)center dot C2H4Cl2 (3b') show that the former is derived from the symmetric trihydroxy form of la, while 3b' crystallizes as a chain-like polymer, which contains the tin atoms as multifunctional building blocks, that is, bridging pentacoordinated Et3SnO2 and Et3SnON units as well as non-bridging four-coordinated Et3SnN units. The cyameluric nucleus is part of the polymeric chains of C6N7O3[Sn(C2H5)(3)](3)center dot C2H4Cl2 (3b'), by the action of both tautomeric forms of cyameluric acid, the amide and the ester form.