Trimerization of Alkali Dicyanamides M[N(CN) ₂ ] and Formation of Tricyanomelaminates M ₃ [C ₆ N ₉ ] (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 K ₃ [C ₆

Abstract

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).

The Sodium (Iso)Cyanurates Nax[H3–xC3N3O3]·yH2O (x = 1–3, y = 0, 1): A Key‐Series for Understanding the Crystal Chemistry of Metal (Iso)Cyanurates

“…In the latter cases, there is significant mass left even at temperatures as high as 1200 °C (pure NaNCO should have completely evaporated), similar to what has been found for rubidium and cesium isocyanurates . As it is well known for gaseous molecular compounds and cyanate melts,, more complex condensation reactions might happen, which might lead to carbonitridic species of higher thermal stability. The elucidation of possible reaction pathways and identification of the products is still subject of our current research and will be treated in a subsequent study.…”

Ionic Liquids Containing the Triply Negatively Charged Tricyanomelaminate Anion and a B(C6F5)3 Adduct Anion

“…[MMIm] 3 [tcmel] : This compound crystallizes in the triclinic space group P ${\bar 1}$ with two formula units per cell (cf. Na 3 [tcmel] and K 3 [tcmel], which crystallize in monoclinic space group P 2 1 / c ) 4e. The asymmetric unit consists of one [MMIm] 3 [tcmel] unit with three MMIm + ions and one tcmel anion [C 3 N 6 (CN) 3 ] 3− .…”

“…Local C s symmetry for the C 3 N 3 (NCN) 3 fragment is observed instead of possible C 3 h symmetry. Bond lengths and angles of the melaminate anion are in the expected range and have been discussed on numerous occasions 4e. 47…”

Rb3[C6N7(NCN)3]·3H2O and Cs3[C6N7(NCN)3]·3H2O – Synthesis, Crystal Structure and Thermal Behavior of Two Novel Alkali Melonates

“…Table 2). The M-O distances (283-371 pm (1) and 297-359 pm (2)) and the M-N distances (299-362 pm (1) and 317-360 pm (2)) are in accordance with the ionic radii sums [50] and are comparable to the values observed in other solids for the coordination of Rb + and Cs + by crystal water [40,[51][52][53][54] and C-N ligands [40,52,[55][56][57].…”