Synthesis, Crystal Structure, Symmetry Relationships, and Electronic Structure of Bismuth Carbodiimide Bi(NCN) and Its Ammonia Adduct Bi(NCN)·NH.

Bi(NCN), the first binary pnictogen carbodiimide, and its ammonia derivative Bi(NCN)·NH have been prepared via nonaqueous liquid-state low-temperature ammonolysis. The crystal structure of Bi(NCN)·NH in space group solved via single-crystal X-ray diffraction corresponds to a two-dimensional-like motif with layers of NCN alternating with honeycomb-like layers of edge-sharing distorted BiN octahedra, half of which are also coordinated by molecular ammonia occupying the octahedral holes. By contrast, Bi(NCN) adopts a higher-symmetric 2/ structure with a single Bi position and stronger distortion but empty octahedral voids. In both cases, Bi and its 6s lone pair are well mirrored by antibonding Bi-N interactions below the Fermi level. Density functional theory calculations reveal an exothermic reaction for the intercalation of NH into Bi(NCN), consistent with the preferential formation of Bi(NCN)·NH in the presence of ammonia. A Bärnighausen tree shows both compounds to be hettotypic derivatives of the 3̅ M(NCN) corundum structure that express highly distorted hexagonal-close-packed layers of NCN in order to accommodate the aspherical Bi cations. Although Bi(NCN) does not resemble the isovalent BiSe in forming two-dimensional layers and a topological insulator, theory suggests a driving force for the spontaneous formation of BiSe/Bi(NCN) sandwiches and a conducting surface state arising within the uppermost Bi(NCN) layer.