Syntheses, crystal structures, magnetic properties and vibrational spectra of nitridoborate-halide compounds Sr-2[BN2]Br and Eu-2[BN2]X (X = Br, I) with isolated [BN2](3-) units

Abstract

The title compounds Sr-2[BN2]Br (1), Eu-2[BN2]Br (2) and Eu-2[BN2]I (3) were obtained from reactions of mixtures of Sr-3[BN2](2) and SrBr2 (1) and the binaries EuN, h-BN and EuX2 (X = Br, I) (2, 3), respectively. The crystal structure of Sr-2[BN2]Br was solved from X-ray powder diffraction data and those of the europium compounds from X-ray single crystal data. Sr-2[BN2]Br and Eu-2[BN2]Br are isotypic crystallizing in the rhombohedral space group R (3) over barm (No. 166, Pearson code: hR18; Z = 3; a = 4.11692(2) angstrom, c = 26.4611(2) angstrom (1); a = 4.0728(3) angstrom, c = 26.589(3) angstrom (2)). The crystal structures are built up by layers of condensed edge-sharing [B-N-B]@Eu-6 and [Br]@Eu-6 trigonal antiprisms, which are alternately stacked along [001] Eu-2[BN2]I - isotypic to Sr-2[BN2]I - crystallizes in the monoclinic space group P2(1)/m (No. 11, Pearson code: mP24; Z = 4; a = 10.2548(6) angstrom, b = 4.1587(3) angstrom, c = 13.1234(9) angstrom, beta = 91.215(4)degrees). The crystal structure is characterized by slightly puckered layers formed by condensed edge-sharing I@Eu-6 octahedra which are separated by isolated [BN2](3-) units. The bond lengths for the strictly linear [BN2](3-) anions in (1) and (2) are d(B-N) = 1.351(4) angstrom and 1.356(8) angstrom, respectively. In Eu-2[BN2]I two crystallograhically distinct [BN2](3-) anions are present with d(B1-N) = 1.32(4) angstrom, 1.37(4)angstrom and d(B2-N) = 1.30(4) angstrom, 1.34(4) angstrom, respectively. Their bond angles vary slightly: angle(N-B1-N) = 179(3)degrees and angle(N-B2-N) = 177(3)degrees. The magnetic susceptibility data of the europium compounds (2) and (3) indicate that the Eu ions are divalent with 4f(7) configuration. Vibrational spectra were measured and interpreted based on the D infinity(h) symmetry of the discrete linear [N-B-N](3-) moieties, considering the site symmetry reduction and the presence of two distinct [BN2](3-) groups in (3).