Single-phase melem (2,5,8-triamino-tri-s-triazine) C6N7(NH2)3 was obtained as a crystalline powder by thermal treatment of different less condensed C-N-H compounds (e.g., melamine C3N3(NH2)3, dicyandiamide H4C2N4, ammonium dicyanamide NH4[N(CN)2], or cyanamide H2CN2, respectively) at temperatures up to 450°C in sealed glass ampules. The crystal structure was determined ab initio by X-ray powder diffractometry (Cu KR1: P21/c (No. 14), a ) 739.92(1) pm, b ) 865.28(3) pm, c ) 1338.16(4) pm, ) 99.912(2)°, and Z ) 4). In the solid, melem consists of nearly planar C6N7(NH2)3 molecules which are arranged into parallel layers with an interplanar distance of 327 pm. Detailed 13 C and 15 N MAS NMR investigations were performed. The presence of the triamino form instead of other possible tautomers was confirmed by a CPPI (cross-polarization combined with polarization inversion) experiment. Furthermore, the compound was characterized using mass spectrometry, vibrational (IR, Raman), and photoluminescence spectroscopy. The structural and vibrational properties of molecular melem were theoretically studied on both the B3LYP and the MP2 level. A structural optimization in the extended state was performed employing density functional methods utilizing LDA and GGA. A good agreement was found between the observed and calculated structural parameters and also for the vibrational frequencies of melem. According to temperature-dependent X-ray powder diffractometry investigations above 560°C, melem transforms into a graphite-like C-N material.Melamine (2,4,6-triamino-s-triazine), C 3 N 3 (NH 2 ) 3 1a, represents an important starting material for several industrial applications, for example, the syntheses of melamine-formaldehyde resins or of fireproof materials. 1,2 Furthermore, it is used for the architecture of supramolecular structures, for example, assemblies built up by cyanuric acid C 3 N 3 (OH) 3 1b or melamine derivatives. [3][4][5] In the past few years, another interest arose in melamine as well as in other compounds, C 3 N 3 X 3 1, containing s-triazine (cyanuric) rings C 3 N 3 , for example, cyanuric chloride C 3 N 3 Cl 3 1c [6][7][8][9][10] or cyanuric azide C 3 N 3 (N 3 ) 3 1d. 11 These are considered to be suitable molecular precursor compounds for the synthesis of graphitic forms of carbon nitride (g-C 3 N 4 ). 6,[12][13][14] In most of the postulated structures of g-C 3 N 4 , s-triazine ring systems are linked through trigonal N atoms forming extended 2D sheets 2 (Scheme 1).Recently, another possible building block for g-C 3 N 4 was taken into account (Scheme 2): tri-s-triazine rings C 6 N 7 which are cross-linked by trigonal N atoms 3. [15][16][17] The possible condensation of three s-triazine rings and the existence of a "cyameluric nucleus" C 6 N 7 4 was first postulated by Pauling † Ludwig-Maximilians-Universität München.

Sodium dicyanamide NaC2N3 was found to undergo two phase transitions. According to thermal analysis and temperature-dependent X-ray powder diffractometry, the transition of alpha-NaC2N3 (1a) to beta-NaC2N3 (1b) occurs at 33 degrees C and is displacive. 1a crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 647.7(1), b = 1494.8(3), c = 357.25(7) pm, beta = 93.496(1) degrees, and Z = 4. The structure was solved from powder diffraction data (Cu Kalpha1, T = 22 degrees C) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.072, Rp = 0.053, and RF = 0.074. 1b crystallizes in the orthorhombic system, space group Pbnm (no. 62), with a = 650.15(5), b = 1495.1(2), c = 360.50(3) pm, and Z = 4. The structure was refined by the Rietveld method using the atomic coordinates of 1a as starting values (Mo Kalpha1, T = 150 degrees C). The final agreement factors were wRp = 0.044, Rp = 0.034, RF = 0.140. The crystal structures of both polymorphs contain sheets of Na+ and N(CN)2- ions which are in la nearly and in 1b exactly coplanar. Above 340 degrees C, 1b trimerizes in the solid to Na3C6N9 (2). 2 crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 1104.82(1), b = 2338.06(3), c = 351.616(3) pm, beta = 97.9132(9)degrees, and Z = 4. The structure was solved from synchrotron powder diffraction data (lambda = 59.733 pm) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.080, Rp = 0.059, and RF = 0.080. The compound contains Na+ and the planar tricyanomelaminate C6N9(3-). The phase transition from 1b to 2 is reconstructive. It occurs in the solid-state without involvement of other phases or intermediates. The crystal structures of 1b and 2 indicate that there is no preorientation of the N(CN)2- in the solid before their trimerization to C6N9(3-).

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