Synthesis and Structure of Borazinyl-Substituted Small-Molecule and High Polymeric Phosphazenes: Ceramic Precursors

Mixed-substituent small-molecule cyclic and high molecular weight polymeric (borazinylamino)phosphazenes have been prepared by reactions between aminophosphazenes and chloroborazines as well as between chlorophosphazenes and aminoborazines. A single-crystal X-ray diffraction study of a small-molecule model compound, gem-bis(borazinylamino)cyclotriphosphazene, N₃P₃(NMe₂)₄ (NHB₃N₃Me₅)₂ (10), is reported: triclinic, space group P1, a = 9.424 (2) Å, b = 13.841 (1) Å, c = 14.570 (4) Å, α = 79.40 (1)°, β = 83.44 (2)°, γ = 73.99 (1)°, V = 1791.5 ų, Z = 2, D_c = 1.187 g cm^-3, Mo Kα radiation, λ = 0.71073 Å, µ = 0.196 mm^-1, R = 0.066. The exocyclic P-N and B-N bond lengths of the borazinylamino substituents indicate significant derealization of the nitrogen atom lone pair, with preference into the borazinyl ring rather than the phosphazene ring. Pyrolysis studies of the mixed-substituent (borazinylamino)phosphazene polymers indicate that these polymers are potential precursors to new ceramic materials, with up to 57% ceramic yield when heated to 1000 °C under nitrogen. Continued pyrolysis at 1300 °C for 10 h leads to the production of crystalline hexagonal boron nitride.