Synthesis of Thiocyameluric Acid C6N7S3H3, its Reaction to Alkali Metal Thiocyamelurates and Organic Tris(dithio)cyamelurates

Thiocyameluric acid C6N7S3H3, the tri-thio analogue of cyameluric acid, is a key compound for the synthesis of new s-heptazine (tri-s-triazine) derivatives. Here, two different routes for the synthesis of thiocyameluric acid and its reaction to tris(aryldithio)- and tris(alkyldithio)cyamelurates C6N7(SSR)3 are reported as well as transformation to alkali metal thiocyamelurates M3[C6N7S3], M=Na, K. These compounds were characterised by FTIR, Raman, solution 13C and 1H NMR spectroscopies, thermal gravimetric analysis (TGA) and elemental analysis. The three (de)protonation steps of thiocyameluric acid were investigated by acid-base titration followed via UV/Vis absorption spectroscopy. While it was not possible to determine the three pKa values, it could be postulated that the acid strength probably increases in the following order: cyanuric acid (C3N3O3H3) < thiocyanuric acid (C3N3S3H3) < cyameluric acid (C6N7O3H3) < thiocyameluric acid (C6N7S3H3). Single crystals of Na3 [C6N7S3]·10H2O and K3[C6N7S3]·6H2O were obtained and the structures analyzed by single crystal X-ray diffraction. Additionally, quantum chemical calculations were performed to get insights into the electronic structure of thiocyameluric acid and to clarify the thiol-thione tautomerism. Based on a comparison of calculated and measured vibrational spectra it can be concluded that thiocyameluric acid and the di- and mono-protonated anions exist in the thione form.