Synthesis of LiBC ₄ N ₄ , BC ₃ N ₃ , and Related C−N Compounds of Boron:  New Precursors to Light Element Ceramics

The synthesis of a novel framework cyanide with composition LiBC 4 N 4 ( 1 ) is described. X-ray powder diffraction as well as 7 Li, 11 B, and 13 C NMR of 1 reveal an ordered structure which consists of two interpenetrating diamond-like networks of BC 4 and LiN 4 tetrahedra linked by C−N bonds. The structure is simple cubic, symmetry P 43̄ m, 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 NMe 3 and SiMe 3 CN, and a single-crystal X-ray structure of B(CN) 3 NMe 3 ( 3 ) has been obtained. The novel solid-state cyanide BC 3 N 3 ( 2 ) is prepared from the thermal elimination of NCSiMe 3 from B(CN) 3 NCSiMe 3 ( 4 ). In route to 4, compounds B(CN) 2 (SMe)NCSiMe 3 ( 5 ) and B(CN) 2 (SMe)NMe 3 ( 6 ) have also been isolated. A single-crystal X-ray structure of 6 reveals that the compound crystallizes in the monoclinic space group P 2 1 / 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 Si 3 N 4, diamond, and graphite.