Synthesis of inorganic heptazine-based materials

This dissertation describes research on the synthesis and characterization of extended heptazine–based, graphite–like carbon nitride materials (CN_x), as well as molecular heptazine (C₆N₇) derivatives. Spurred on by recent triazine to heptazine conversion studies, a structural examination was performed on an amorphous nitrogen–rich carbon nitride material formed via the rapid and exothermic self-propagating decomposition of a triazine (C₃N₃) precursor, trichloromelamine (TCM). The thermally stable and insoluble CN_xH_y product was determined to be composed of heptazine repeat units. This conclusion was supported by ¹³C solid state NMR and isolation of molecular heptazine anions after base hydrolysis (structural deconstruction) of the CN_xH_y material.

Modifications to the decomposition of TCM were explored. Introduction of a solid template (NaCl or silica) led to morphological changes in the TCM–CN_x product, observed by scanning electron microscopy. It was found that the sodium salts, NaBr and NaN₃, led to chloride exchange with TCM. The use of mixtures of NH₄Cl and NaN₃ also showed changes in the morphology of the material, while leading to slight changes in the IR spectra. A series of reactions between NaBH₄ and TCM yield novel thermally stable boron carbon nitride (BCN) materials. Reactions between TCM and Li₂C₂ or aromatic organic solids led to CN_x materials with increased carbon contents.

Crystalline metal–heptazine precipitates were generated by cation exchange reaction with the base hydrolysis product of TCM–CN_x, potassium cyamelurate. A structure solution was attempted for the crystalline copper cyamelurate salt, KCu[C₆N₇O₃]·4H₂O. Neutral molecular heptazines were also synthesized; these species included 2,5,8–tribromo–s–heptazine (TBH), 2,5,8–triphenyl–s–heptazine (TPH), 2,5,8–tris(diisopropylamino)–s–heptazine (TAmH), and 2–bis(trimethylsilyl)amido–5,8–dichloroheptazine (DCAH). These materials were sublimable and showed interesting optical absorption and emission properties. A polymeric heptazine material was synthesized by thermal decomposition of DCAH.

Several attempts were made to synthesize polymeric materials from heptazine precursors. Extended solids with C₆N₈ and C₉N₇ stoichiometry were made through solid state metathesis reactions between trichloroheptazine and either lithium nitride or lithium carbide. Powder X–ray diffraction indicated that salt formation was occurring during these reactions and products had the desired stoichiometry by elemental analysis. It was generally observed that CN_x materials containing excess carbon displayed increased thermal stability when compared to pure CN_x.