Syntheses, crystal structures, magnetic properties and vibrational spectra of nitridoborate-halide compounds Sr_2[BN_2]Br and Eu_2[BN_2]X (X = Br, I) with isolated [BN_2]_3-units

摘要

The title compounds Sr_2[BN_2]Br (1),Eu_2[BN_2]Br (2) and Eu_2[BN_2]I (3) were obtained from reactions of mixtures of Sr_3[BN_2]_2 and SrBr_2 (1) and the binaries EuN, h-BN and EuX_2 (X = Br, I) (2, 3), respectively. The crystal structure of Sr_2[BN_2]Br was solved from X-ray powder diffraction data and those of the europium compounds from X-ray single crystal data. Sr_2[BN_2]Br and Eu_2[BN_2]Br are isotypic crystallizing in the rhombohedral space group R3m (No. 166, Pearson code: hR18; Z = 3; a = 4.11692(2) A, c = 26.4611(2) A (1); a = 4.0728(3) A, c = 26.589(3) A (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[BN_2]I – isotypic to Sr_2[BN_2]I – crystallizes in the monoclinic space group P21/m (No. 11, Pearson code: mP24; Z = 4; a = 10.2548(6) A, b = 4.1587(3) A, c = 13.1234(9) A, b = 91.215(4)). The crystal structure is characterized by slightly puckered layers formed by condensed edge-sharing I@Eu_6 octahedra which are separated by isolated [BN_2]_3 units. The bond lengths for the strictly linear [BN_2]_3 anions in (1) and (2) are d(B–N) = 1.351(4)A and 1.356(8)A, respectively. In Eu_2[BN_2]I two crystallograhically distinct [BN_2]-3 anions are present with d(B1––N)=1.32(4)A, 1.37(4)A and d(B_2–N)=1.30(4)A, 1.34(4) A, respectively. Their bond angles vary slightly: ff(N–B1–N) = 179(3) and ff(N–B2–N) = 177(3). 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 D1h symmetry of the discrete linear[N–B–N]3 moieties, considering the site symmetry reduction and the presence of two distinct [BN_2]_3 groups in (3).