The Tautomeric Forms of Cyameluric Acid Derivatives

The tautomerism of cyameluric acid C 6 N 7 O 3 H 3 ( 1 a ), 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 ( 1 a ): C 6 N 7 O 3 [Sn(C 4 H 9 ) 3 ] 3 ( 3 a ), C 6 N 7 O 3 [Sn(C 2 H 5 ) 3 ] 3 ( 3 b ), and C 6 N 7 O 3 [Si(CH 3 ) 3 ] 3 ( 4 ). In order to investigate the structure of 1 a the mono‐ and dipotassium cyamelurate hydrates K(C 6 N 7 O 3 H 2 ) ⋅ 2 H 2 O ( 5 ) and K 2 (C 6 N 7 O 3 H) ⋅ 1 H 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 1 a is the triketo 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 C 6 N 7 O 3 [Si(CH 3 ) 3 ] 3 ( 4 ) and the solvate C 6 N 7 O 3 [Sn(C 2 H 5 ) 3 ] 3 ⋅ C 2 H 4 Cl 2 ( 3 b ′) show that the former is derived from the symmetric trihydroxy form of 1 a , while 3 b ′ crystallizes as a chain‐like polymer, which contains the tin atoms as multifunctional building blocks, that is, bridging pentacoordinated Et 3 SnO 2 and Et 3 SnON units as well as non‐bridging four‐coordinated Et 3 SnN units. The cyameluric nucleus is part of the polymeric chains of C 6 N 7 O 3 [Sn(C 2 H 5 ) 3 ] 3 ⋅ C 2 H 4 Cl 2 ( 3 b ′), by the action of both tautomeric forms of cyameluric acid, the amide and the ester form.