Structure Elucidation of a Melam-Melem Adduct by a Combined Approach of Synchrotron X-ray Diffraction and DFT Calculations.

Melam-melem (1:1), an adduct compound that can be obtained from dicyandiamide in autoclave reactions at 450 °C and elevated ammonia pressure, had previously been described based on mass spectrometry and NMR spectroscopy, but only incompletely characterized. The crystal structure of this compound has now been elucidated by means of synchrotron microfocus diffraction and subsequent quantum-chemical structure optimization applying DFT methods. The structure was refined in triclinic space group P 1 ‾ based on X-ray data. Cell parameters of a=4.56(2), b=19.34(8), c=21.58(11) Å, α=73.34(11)°, ß=89.1(2)°, and γ=88.4(2)° were experimentally obtained. The resulting cell volumes agree with the DFT optimized value to within 7 %. Molecular units in the structure form stacks that are interconnected by a vast array of hydrogen bridge interactions. Remarkably large melam dihedral angles of 48.4° were found that allow melam to interact with melem molecules from different stack layers, thus forming a 3D network. π-stacking interactions appear to play no major role in this structure.

Triazine-based carbon nitrides for visible-light-driven hydrogen evolution.

A new dimension: The doping of amorphous poly(triazine imide) (PTI) through ionothermal copolymerization of dicyandiamide with 4-amino-2,6-dihydroxypyrimidine (4AP) results in triazine-based carbon nitrides with increased photoactivity for water splitting compared to crystalline poly(triazine imide) (PTI/Li(+)Cl(-), see picture) and melon-type carbon nitrides. This family of carbon nitride semiconductors has potential as low-cost, environmentally clean photocatalysts for solar fuel production.

Formation and characterization of melam, melam hydrate, and a melam-melem adduct.

Until recently, melam, [C(3)N(3)(NH(2))(2)](2)NH, has been regarded as a short-lived intermediate in the condensation process of melamine that is only detectable under special reaction conditions owing to its high reactivity. A new synthetic approach has allowed a closer look at the formation and condensation behavior of melam by using elevated ammonia pressure in autoclaves. Whereas the thermal treatment of dicyandiamide at 450 °C and 0.2 MPa ammonia yielded melam in large amounts, prolonged treatment under these conditions (9 days) led to the formation of a melam-melem adduct, thus enabling the first insight into the condensation process of melam into melem. The hydrothermal treatment of melam at 300 °C (24 h) yields melam hydrate, [C(3)N(3)(NH(2))(2)](2)NH⋅2 H(2)O (space group P2(1)/c; a=676.84(2), b=1220.28(4), c=1394.24(4) pm; ß=98.372(2)°; V=1139.28(6)×10(6)  pm(3); Z=4), which crystallizes as a layered structure that is composed of almost-planar melam molecules, thereby forming ellipsoidal rosette-like motifs. The resulting voids are filled with four water molecules, thus forming a dense network of hydrogen bonds.

Poly(triazine imide) with intercalation of lithium and chloride ions [(C3N3)2(NH(x)Li(1-x))3⋠LiCl]: a crystalline 2D carbon nitride network.

Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li(+)Cl(-)) was synthesized by temperature-induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well-known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li(+)Cl(-) resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li(+)Cl(-) a structure solution from both powder X-ray and electron diffraction patterns using direct methods was possible; this yielded a triazine-based structure model, in contrast to the proposed fully condensed heptazine-based structure that has been reported recently. Further information from solid-state NMR and FTIR spectroscopy as well as high-resolution TEM investigations was used for Rietveld refinement with a goodness-of-fit (χ(2)) of 5.035 and wRp=0.05937. PTI/Li(+)Cl(-) (P6(3)cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide-bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li(+) and Cl(-) ions. The presence of salt ions in the nanocrystallites as well as the existence of sp(2)-hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy-loss spectroscopy (EELS) measurements. Solid-state NMR spectroscopy investigations using (15)N-labeled PTI/Li(+)Cl(-) proved the absence of heptazine building blocks and NH(2) groups and corroborated the highly condensed, triazine-based structure model.