Trimerization of alkali dicyanamides M[N(CN)2] and formation of tricyanomelaminates M3[C6N9] (M = K, Rb) in the melt:: Crystal structure determination of three polymorphs of K[N(CN)2], two of Rb[N(CN)2], and one of K3[C6N9] and Rb3[C6N9] from X-ray powder diffractometry

The alkali dicyanamides M[N(CN)(2)] (M=K, Rb) were synthesized through ion exchange, and the corresponding tricyanomelaminates M-3[C6N9] were obtained by heating the respective dicyanamides. The thermal behavior of the dicyanamides and their reaction to form the tricyanomelaminates were investigated by temperature-dependent X-ray powder diffractometry and thermoanalytical measurements. Potassium dicyanamide K[N(CN)(2)] was found to undergo four phase transitions: At 136 degreesC the low-temperature modification alpha -K[N(CN)(2)] transforms to beta -K[N(CN)(2)], and at 187 degrees C the latter transforms to the high-temperature modification gamma -K[N(CN)(2)], which melts at 232 degreesC. Above 310 degreesC the dicyanamide ions [N(CN)(2)](-) trimerize and the resulting tricyanomelaminate K-3[C6N9] solidifies. Two modifications of rubidium dicyanamide have been identified: Even at - 25 degreesC, the a form slowly transforms to beta -Rb[N(CN)(2)] within weeks. Rb[N(CN)(2)] has a melting point of 190 degreesC. Above 260 degreesC the dicyanamide ions [N(CN)(2)] of the rubidium salt trimerize in the melt and the tricyanomelaminate Rb-3[C6N9] solidifies. The crystal structures of all phases were determined by powder diffraction methods and were refined by the Rietveld method. alpha -K[N(CN)(2)] (Pbcm, a = 836.52(1), b = 646.90(1), c = 721.27(1) pm, Z = 4), gamma -K[N(CN)(2)] (Pnma, a = 855.40(3), b = 387.80(1), c = 1252.73(4) pm, Z = 4), and Rb[N(CN)(2)] (C2/c, a = 1381.56(2), b = 1000.02(1), c = 1443.28(2) pm, beta = 116.8963(6)degrees, Z=16) represent new structure types. The crystal structure of beta -K[N(CN)(2)] (P2(1)/n, a = 726.92(1), b = 1596.34(2), c = 387.037(5) pm, beta = 111.8782(6)degrees, Z = 4) is similar but not isotypic to the structure of alpha -Na[N(CN)(2)]. alpha -Rb[N(CN)(2)] (Pbcm, a = 856.09(1), b = 661.711(7), c = 765.067(9) pm, Z = 4) is isotypic with alpha -K[N(CN)(2)]. The alkali dicyanamides contain the bent planar anion [N(CN)(2)]- of approximate symmetry C-2v (average bond lengths: C-N-bridge 133, C-N-term 113 pm; average angles N-C-N 170 degrees, C-N-C 120 degrees). K-3[C6N9] (P2(1)/c, a = 373.82(1), b = 1192.48(5), c = 2,500.4(1)pm, beta = 101.406(3)degrees, Z=4) and Rb-3[C6N9] (P2(1)/c, a=389.93(2), b=1226.06(6), c=2547.5(1)pm, beta = 98.741(5)'degrees, Z = 4) are isotypic and they contain the planar cyclic anion [C6N9](3-) Although structurally related, Na-3[C6N9] is not isotypic with the tricyanomelaminates M-3[C6N9] (M = K, Rb).