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 M-3[C6N7S3], M=Na, K. These compounds were characterised by FTIR, Raman, solution C-13 and H-1 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 pK(a) 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 Na-3[C6N7S3] center dot 10 H2O and K-3[C6N7S3] center dot 6 H2O 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 monoprotonated anions exist in the thione form.