From Triazines to Heptazines: Novel Nonmetal Tricyanomelaminates as Precursors for Graphitic Carbon Nitride Materials

Abstract

The first non-metal tricyanomelaminates have been synthesized via metathesis reactions and characterized by means of single-crystal X-ray diffraction and vibrational and solid-state NMR spectroscopy. The crystal structures of [NH 4 ] 2 [C 6 N 9 H] ( 1 ) ( P 2 1 / c, a = 1060.8(2) pm, b = 1146.2(2) pm, c = 913.32(18) pm, β = 112.36(3)°, V = 1027.0(4) × 10 6 pm 3 ), [C(NH 2 ) 3 ] 3 [C 6 N 9 ]·2 H 2 O ( 2 ) ( P 2 1 2 1 2 1, a = 762.12(15) pm, b = 1333.6(3) pm, c = 1856.6(4) pm, V = 1887.0(7) × 10 6 pm 3 ) and [C 3 N 6 H 7 ] 2 [C 6 N 9 H]·2.5 H 2 O ( 3 ) ( P 1̄, a = 1029.5(2) pm, b = 1120.3(2) pm, c = 1120.7(2) pm, α = 104.22(3)°, β = 112.74(3)°, γ = 104.62(3)°, V = 1064.8(4) × 10 6 pm 3 ) are composed of singly protonated ( 1 and 3 ) or nonprotonated ( 2 ) tricyanomelaminate ions, which, together with the respective counterions, form two-dimensional, layered structures ( 1 and 3 ) or a quasi three-dimensional network ( 2 ). Particular emphasis has been placed on the elucidation of the thermal reactivity of the three molecular salts by means of thermal analysis and vibrational and NMR spectroscopy, as well as temperature-dependent X-ray powder diffraction. The title compounds were found to be promising candidates as molecular CN x precursors for the synthesis of graphitic carbon nitride materials. Upon being heated, ammonium and guanidinium tricyanomelaminate uniformly pass the crystalline, heptazine (C 6 N 7 )-based intermediate melem (C 6 N 7 (NH 2 ) 3 ), which decomposes and forms a semi-amorphous CN x H y material with a pronounced layered structure. Identical pyrolysis products are obtained for the melaminium salt, a classical triazine (C 3 N 3 )-based CN x precursor, after passing an intermediate, possibly cross-linked phase at low temperatures. Preliminary solid-state NMR investigations of the final products best conform to heptazine-based structure models for g-C 3 N 4 that have commonly been rather disregarded in favor of triazine-based ones.