Mg2[BN2]Cl and Mg8[BN2]5I: novel magnesium nitridoborate halides - syntheses, crystal structures, and vibrational spectra

Abstract

The title compounds have been synthesized at 1473 K from stoichiometric mixtures of the binary components Mg3N2, MgX2 (X = Cl, I) and BN in arc-welded steel ampoules encapsulated in evacuated silica tubes. Mg-2[BN2]Cl (1) and Mg-8[BN2](5)I (2) crystallize in the orthorhombic space groups Pbca (no. 61) and Imma (no. 74), respectively, with a = 6.6139(8) Angstrom, b = 9.766(1) Angstrom, c = 10.600(1) Angstrom, Z = 8 for 1 and a = 13.535(3) Angstrom, b = 9.350(2) Angstrom, c = 11.194(2) Angstrom, Z = 4 for 2. The crystal structures are characterized mainly by Mg-6 trigonal prisms which are condensed to 3D frameworks in different ways. Part of the trigonal prisms are centered by the [N-B-N](3-) anions and other voids in the framework by the X- anions. The magnesium environment around Cl- is a very distorted monocapped trigonal prism (CN = 6+1) and that of I- is a bicapped heptagonal prism (CN = 14+2). The bond lengths and bond angles for the relevant [BN2](3-) anions are d(B-N) = 1.330 - 1.338 Angstrom, angleN-B-N = 175.8degrees in 1 and d(B-N) = 1.330 - 1.339 Angstrom, angleN-B-N = 176.8degrees - 178.0degrees in 2. The vibrational spectra of the title compounds have been recorded and interpreted based on the D-infinityh symmetry of the relevant [N-B-N](3-) groups considering the site symmetry splitting.