Exploring the Route to 1,3,5‐Triazine‐2,4,6‐triisocyanate (C 6 N 6 O 3 ), a Hydrogen‐Free Molecular Precursor for Polymeric C–N–(O) Materials

Abstract

We report on the synthesis of 1,3,5-triazine-2,4,6-triisocyanate [C3N3(NCO)3]. The hydrogen-free compound, a new molecular precursor for the realisation of C–N–(O) networks (e.g., C2N2O, C3N4), has been obtained in a five-step reaction sequence with well-defined intermediates. Starting from cyanoformiate, triethyl 1,3,5-triazine-2,4,6-tricarboxylate (C12O6N3H15) was prepared and identified by means of spectroscopic and thermal methods. In addition, the crystal structures of two modifications were solved [high temperature (HT): P63/m (no. 176), a = 11.07(2) Å, c = 6.83(4) Å, γ = 120°, V = 725.80(2) Å3, Z = 2; low temperature (LT): P21/n (no. 14), a = 21.75(2) Å, b = 6.54(5) Å, c = 21.81(0) Å, β = 119.81(0)°, V = 2693.98(6) Å3, Z = 8]. Hydrolysis of the ester under alkaline conditions (KOH) yielded the corresponding salt of 1,3,5-triazine-2,4,6-tricarboxylic acid, C3N3(COO)3K3·2H2O [P (no. 2), a = 6.95(0) Å, b = 17.45(8) Å, c = 17.54(1) Å, α = 119.76(0)°, β = 92.04(0)°, γ = 93.92(0) °Å; V = 1837.63(0) Å3, Z = 6]. The dried salt was converted into 1,3,5-triazine-2,4,6-tricarbonyl trichloride [C3N3(COCl)3] by reaction with POCl3. The trichloride was also studied by means of single-crystal structure analysis [P21/c (no. 14), a = 9.73(6) Å, b = 11.21(1) Å, c = 17.03(1) Å, β = 91.91(1)°, V = 1857.87(23) Å3, Z = 8]. Further reaction of the molecular acid chloride with AgN3 gave the acyl azide, which was converted in situ into the product 1,3,5-triazine-2,4,6-triisocyanate by means of a thermally induced Curtius rearrangement. The final product was studied by thermal and spectroscopic methods. Owing to the high chemical reactivity of the isocyanate groups, oligomerisation/polymerisation of the molecular compound occurred immediately at room temperature, thus forming amorphous uretdione-type coordination compounds. The uretdione-based network has been converted into the corresponding molecular carbamates (R–NH–CO–OR) by reaction with ethanol [P63/m (no. 176), a = 14.873(12) Å, c = 6.605(7) Å, γ = 120°, Z = 2, V = 1265(2) Å3] and 2-propanol [P43 (no. 78), a = 13.526(8) Å, c = 12.956(2) Å, Z = 4, V = 2370.5(3) Å3]. The potential of this new precursor in the field of carbon nitrides as well as carbon oxynitrides is discussed briefly.