Synthesis, crystal structure and vibrational spectra of Mg-2[BN2]Br: a new magnesium nitridoborate halide with discrete [N-B-N](3-) groups

Abstract

Mg-2[BN2]Br was prepared from stoichiometric mixtures of the respective binary components Mg3N2, MgBr2 and BN in sealed niobium ampoules placed in evacuated silica tubes at 1473 K. The monoclinic crystal structure (C2/c (no. 15); a = 9.1509(3) angstrom, b = 6.6209(3) angstrom, c = 6.4756(2) angstrom, beta = 109.353(3)degrees and Z = 4) was determined from powder data. The crystal structure is characterized by cationic 3(2).4.3.4 nets of magnesium and anionic hexagonal closed packed 3(6) nets of six-connected nodes occupied by [N-B-N](3-) units and Br anions. Both types of planar nets alternate along the [100] direction. The packing of such incoherent nets results in a monoclinic symmetry, which is a compromise for a structure built up from rod-like [N-B-N](3-) units and spherical bromine and magnesium atoms. Bond length and bond angle for the [N-B-N](3-) unit are d(B-N) = 1.354(2) angstrom and angle(N-B-N) = 177.1(9)degrees, respectively. Vibrational spectra were analyzed based on the D-infinity h symmetry of the relevant [N-B-N](3-) moieties, considering the site symmetry splitting. The results are discussed in context with those of the known members of the magnesium dinitridoborate halide series, Mg-2[BN2]Cl and Mg-8[BN2](5)I.