Irran E, Jürgens B, Schnick W
Department Chemie, Ludwig-Maximilians-Universität, München, Germany.
Chemistry. 2001 Dec 17;7(24):5372-81. doi: 10.1002/1521-3765(20011217)7:24<5372::aid-chem5372>3.0.co;2-#.
The alkali dicyanamides M[N(CN)2] (M=K, Rb) were synthesized through ion exchange, and the corresponding tricyanomelaminates M3[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 degrees C the low-temperature modification alpha-K[N(CN)2] transforms to beta-K[N(CN)2], and at 187degrees C the latter transforms to the high-temperature modification gamma-K[N(CN)2], which melts at 232 degrees C. Above 310 degrees C the dicyanamide ions [N(CN)2]- trimerize and the resulting tricyanomelaminate K3[C6N9] solidifies. Two modifications of rubidium dicyanamide have been identified: Even at -25 degrees C, the a form slowly transforms to beta-Rb[N(CN)2] within weeks. Rb[N(CN)2] has a melting point of 190 degrees C. Above 260 degrees C the dicyanamide ions [N(CN)2]- of the rubidium salt trimerize in the melt and the tricyanomelaminate Rb3[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 = 46.90(1), c =7 21.27(1) pm, Z = 4), gamma-K[N(CN)2] (Pnma, a = 855.40(3), b = 387.80(1), 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, 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, 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 C2, (average bond lengths: C-N(bridge) 133, C-N(term) 113 pm; average angles N-C-N 170 degrees, C-N-C 120 degrees). K3[C6N9] (P2(1/c), a = 373.82(1), b = 1192.48(5), c = 2500.4(1) pm, beta = 101.406(3) degrees, Z = 4) and Rb,[C6N9] (P2(1/c), a = 389.93(2), b = 1226.06(6), c = 2547.5(1) pm, 98.741(5) degrees, Z=4) are isotypic and they contain the planar cyclic anion [C6N9]3-. Although structurally related, Na3[C6N9] is not isotypic with the tricyanomelaminates M3[C6N9] (M = K, Rb).
通过离子交换合成了碱金属双氰胺盐M[N(CN)₂]（M = K、Rb），并通过加热相应的双氰胺盐得到了三聚氰胺氰酸盐M₃[C₆N₉]。采用变温X射线粉末衍射和热分析测量手段研究了双氰胺盐的热行为及其形成三聚氰胺氰酸盐的反应。发现氰胺化钾K[N(CN)₂]经历了四个相变：在136℃时，低温变体α-K[N(CN)₂]转变为β-K[N(CN)₂]，在187℃时，后者转变为高温变体γ-K[N(CN)₂]，γ-K[N(CN)₂]在232℃时熔化。在310℃以上，双氰胺根离子[N(CN)₂]⁻三聚，生成的三聚氰胺氰酸盐K₃[C₆N₉]固化。已鉴定出铷双氰胺盐的两种变体：即使在-25℃时，α型也会在数周内缓慢转变为β-Rb[N(CN)₂]。Rb[N(CN)₂]的熔点为190℃。在260℃以上，铷盐的双氰胺根离子[N(CN)₂]⁻在熔体中三聚，三聚氰胺氰酸盐Rb₃[C₆N₉]固化。所有相的晶体结构均通过粉末衍射方法测定，并采用Rietveld方法进行精修。α-K[N(CN)₂]（Pbcm，a = 836.52(1)，b = 46.90(1)，c = 721.27(1) pm，Z = 4）、γ-K[N(CN)₂]（Pnma，a = 855.40(3)，b = 387.80(1)，c = 1252.73(4) pm，Z = 4）和Rb[N(CN)₂]（C2/c，a = 1381.56(2)，b = 1000.02(1)，c = 1443.28(2) pm，β = 116.8963(6)°，Z = 16）代表新的结构类型。β-K[N(CN)₂]（P2(1/n)，a = 726.92(1)，b = 1596.34(2)，c = 387.037(5) pm，β = 111.8782(6)°，Z = 4）的晶体结构与α-Na[N(CN)₂]的结构相似但不同型。α-Rb[N(CN)₂]（Pbcm，a = 856.09(1)，b = 661.711(7)，c = 765.067(9) pm，Z = 4）与α-K[N(CN)₂]同型。碱金属双氰胺盐含有近似C₂对称性的平面弯曲阴离子[N(CN)₂]⁻（平均键长：C-N(桥) 133，C-N(端) 113 pm；平均角度N-C-N 170°；C-N-C 120°）。K₃[C₆N₉]（P2(1/c)，a = 373.82(1)，b = 1192.48(5)，c = 2500.4(1) pm，β = 101.406(3)°，Z = 4）和Rb₃[C₆N₉]（P2(1/c)，a = 389.93(2)，b = 1226.06(6)，c = 2547.5(1) pm，β = 98.741(5)°，Z = 4）同型，它们含有平面环状阴离子[C₆N₉]³⁻。尽管结构相关，但Na₃[C₆N₉]与三聚氰胺氰酸盐M₃[C₆N₉]（M = K、Rb）不同型。
