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Crystal Structure and Thermal Solid-State Reactivity of Ammonium Cyanoureate NH4[H2NC(=O)NCN]

Abstract The ammonium salt of cyanourea NH4[H2NC(=O)NCN] has been synthesised via an acid-base route from the parent acid and characterized by single-crystal and powder X-ray diffraction,NMR and vibrational spectroscopy, mass spectrometry as well as thermal analysis. The molecular salt (P21/c, a = 388.95(8), b = 1121.0(2), c = 1096.4(2) pm, β = 92.57(3)◦, V = 477.5(2)・106 pm3, Z = 4, T = 140 K) which may formally be derived from the related compound ammonium dicyanamide NH4[N(CN)2] by addition of one molecule water, consists of isolated ammonium and cyanoureate ions which are assembled via H・ ・ ・N and H・ ・ ・O hydrogen bonds, forming a three-dimensional arrangement. At elevated temperatures, ammonium cyanoureate undergoes a complex transformation affording the formation of urea and cyanoguanylurea H2NC(=O)NHC(NH2)=NCN or the isomeric ammeline (C3N3)(NH2)2(OH) as the main products, depending on the reaction conditions. The transformation is accompanied by consecutive reactions such as proton transfer and the dis- and reassembly of molecular fragments, yielding a macroscopic segregation of the reaction products. The conversion represents yet another example of a complex reaction proceeding in the solid-state.