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New Light on an Old Story: Formation of Melam during Thermal Condensation of Melamine

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Abstract

We report on the existence and formation of the carbon nitride precursor melam (H2N)2(C3N3)NH(C3N3)(NH2)2, thereby clarifying one of the last unresolved issues posed by the complex thermal condensation of melamine C3N3(NH2)3. Experimental proof is put forward that melam is a direct condensation product of melamine, but can be detected only in small amounts under special reaction conditions owing to its rapid transformation into melem. The coexistence of melamine and melem during thermal condensation yields two adduct phases with distinct compositions [C3N3(NH2)3]2[C6N7(NH2)3] and [C3N3(NH2)3][C6N7(NH2)3]2. They may be considered as co-crystallizates of melamine and melem and can be isolated as intermediates between 590 and 650 K prior to the formation of single-phase melem C6N7(NH2)3. Melam (C2/c, a=1811.0(4), b=1086.7(2), c=1398.4(3) pm, β=96.31(3)°, V=2735.3(9)×106 pm3, T=130 K) adopts a ditriazinylamine-type structure with a twisted conformation about the bridging NH moiety and transforms into melem around 640 K. Two compounds deriving from melam have been synthesized by solution and solid-state reactions. The salt melamium diperchlorate C6N11H11(ClO4)2⋅2 H2O (C2/c, a=1747.8(4), b=1148.2(2), c=993.6(2) pm, β=118.79(3)°, V=1747.4(6)×106 pm3, T=130 K) crystallizes as a dihydrate and exhibits a doubly protonated, planar melam core. In the neutral complex Zn[C6N11H9]Cl2 (P21/c, a=743.00(15), b=2233.2(5), c=762.5(2) pm, β=99.86(3)°, V=1246.5(4)×106 pm3, T=200 K), melam acts as a symmetrically bent bidentate ligand, which is coordinated to the Lewis acid Zn-site through two ring nitrogen atoms.

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