The Tautomeric Forms of Cyameluric Acid Derivatives

Abstract

The tautomerism of cyameluric acid C6N7O3H3 (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): C6N7O3[Sn(C4H9)3]3 (3 a), C6N7O3[Sn(C2H5)3]3 (3 b), and C6N7O3[Si(CH3)3]3 (4). In order to investigate the structure of 1 a the mono- and dipotassium cyamelurate hydrates K(C6N7O3H2)2 H2O (5) and K2(C6N7O3H)1 H2O (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 C3h 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]3C2H4Cl2 (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 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]3C2H4Cl2 (3 b'), by the action of both tautomeric forms of cyameluric acid, the amide and the ester form.

Modulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides

“…In the Heptazine-based model, the 13 C MAS NMR spectra are expected to present two signal groups. The first one at 164-169 ppm is characteristic for CN 2 (NH 2 ) carbon atom ; the second one at 155-157 ppm is relative to CN 3 groups [43,[45][46]. As ever mentioned, in the case of an ideal stoichiometric hexagonal graphitic C 3 N 4 material, the 13 C MAS NMR spectrum is expected to present a single signal around 150 ppm (CN 3 group).…”

“…The problem is that usual characterizations such as IR spectroscopy can not help to make any distinctions between both models [45]. In this way, NMR spectra have been widely recorded [35,37,43,[45][46] or predicted via ab initio calculations [47]. In the Heptazine-based model, the 13 C MAS NMR spectra are expected to present two signal groups.…”

Formation of a Hydrogen‐Bonded Heptazine Framework by Self‐Assembly of Melem into a Hexagonal Channel Structure

“…Furthermore, the identification of s-heptazine as a building block in the polymers melon and poly(heptazine imide) [13,14] and in extended CN x polymers, [15] as well as quantum chemical calculations that suggest s-heptazine as likely building block of graphitic C 3 N 4 [16,17] have contributed to increased interest in s-heptazines. Numerous substituted s-heptazines, for example, cyameluric chloride C 6 N 7 Cl 3 , [16,18] cyameluric azide C 6 N 7 (N 3 ) 3 , [19] melonates (C 6 N 7 A C H T U N G T R E N N U N G (NCN) 3 ) 3À , [20,21] and several organic derivatives [22,23] have been synthesized and investigated in detail. The structures of melem [24,25] and cyameluric acid [26,27] as well as of numerous salts [22,28,29,30] have also been solved recently.…”

The Nature of Nitrogen‐Modified Titanium Dioxide Photocatalysts Active in Visible Light

“…As it is known that the amino groups in melem can be replaced by OH through nucleophilic attack of hydroxide, [17] TiO 2 -N,C/melamine was treated with sodium hydroxide solution under reflux. The expected cyamelurate and ammonia were obtained (Scheme 2).…”