Synthesis of LiBC4N4, BC3N3, and Related C−N Compounds of Boron:  New Precursors to Light Element Ceramics

The synthesis of a novel framework cyanide with composition LiBC4N4 (1) is described. X-ray powder diffraction as well as 7Li, 11B, and 13C NMR of 1 reveal an ordered structure which consists of two interpenetrating diamond-like networks of BC4 and LiN4 tetrahedra linked by C−N bonds. The structure is simple cubic, symmetry P43̄m, a = 5476(3) Å. In solution the presence of the B(CN)4 anion has been established for the first time by spectroscopic methods. Molecular B(CN)3 is isolated for the first time as a Lewis acid−base complex with bases such as NMe3 and SiMe3CN, and a single-crystal X-ray structure of B(CN)3NMe3 (3) has been obtained. The novel solid-state cyanide BC3N3 (2) is prepared from the thermal elimination of NCSiMe3 from B(CN)3NCSiMe3 (4). In route to 4, compounds B(CN)2(SMe)NCSiMe3 (5) and B(CN)2(SMe)NMe3 (6) have also been isolated. A single-crystal X-ray structure of 6 reveals that the compound crystallizes in the monoclinic space group P21/n. The new crystalline solids 2 and 1 (isoelectronic to C) are promising precursors for high-pressure and high-temperature syntheses of novel phases with structures related to Si3N4, diamond, and graphite.