Synthesis, crystal structure determination from X-ray powder diffractometry and vibrational spectroscopy of the tricyanomelaminate monohydrates M 3[C 6N 9]·H 2O (M=K, Rb)

Abstract

The alkali tricyanomelaminate monohydrates M 3[C 6N 9]·H 2O with M=K, Rb were synthesized by aqueous ion exchange starting from anhydrous Na 3[C 6N 9] and KCl or RbCl, respectively. The crystal structure of Rb 3[C 6N 9]·H 2O was solved on the basis of synchrotron X-ray powder diffraction data and refined by the Rietveld method ( P1, Z=1, a=393.17(1), b=826.48(2), c=1036.28(3) pm, α=70.798(1), β=81.872(2), γ=87.504(1)°, wR p=0.016, R p=0.011, R F=0.043). K 3[C 6N 9]·H 2O was found to be isotypic and its crystal structure was refined by the Rietveld method on the basis of conventional X-ray powder diffraction data ( a=372.91(2), b=809.20(5), c=1012.22(5) pm, α=71.209(2), β=80.524(3), γ=86.653(3)°, wR p=0.084, R p=0.064, R F=0.052). Both compounds contain M + ions, cyclic tricyanomelaminate ions [C 6N 9] 3− and H 2O molecules. The salts were characterized by vibrational spectroscopy (FTIR and Raman). According to thermogravimetric investigations the monohydrates lose their crystal water between 80–180 °C (K 3[C 6N 9]·H 2O) and 60–200 °C (Rb 3[C 6N 9]·H 2O), respectively, yielding the respective anhydrous tricyanomelaminates M 3[C 6N 9] (M=K, Rb).