Crystal structure of dieuropium(II) dinitridoborate bromide, Eu 2 [BN 2 ]Br Peter Höhn *,I , Yurii Prots I , Ilkin Kokal I,II and Mehmet Somer II I Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany II KoH University, Chemistry Department, 34450 Sariyer-Istanbul, Turkey Received January 30, 2009, accepted and available on-line August 5, 2009; CSD no. 409982 Abstract BBrEu 2 N 2 , trigonal, R3 m (no. 166), a = 4.0728(3) Å, c = 26.589(3) Å, V = 382.0 Å 3 , Z = 3, R gt (F) = 0.029, wR ref (F 2 ) = 0.072, T = 293 K. Source of material Eu 2 [BN 2 ]Br was synthesized from the mixture of EuN, h-BN and EuBr 2 in the molar ratio 2 : 1 : 1 : 1. The finely ground reaction mixture transferred into a niobium ampoule, arc-welded and sealed in an evacuated silica tube was heated up to 1023 K within 8 h. After 24 h of annealing, the reaction ampoule was cooled down to room temperature during 8 h. Eu 2 [BN 2 ]Br forms orange to pale yellow plate-like crystals, which are very sensitive to air and moisture. Therefore, all manipulations were performed under inert conditions in a glove box. For the single crystal investigation the specimens were encapsulated in glass capillaries. Experimental details The structure refinement with full occupancy factors for all atoms resulted in R 1 = 0.036 and wR 2 = 0.085 (Tables 2, 3). While the equivalent atomic displacement parameters (ADP) for Eu and Br atoms showed reasonable values of about 0.013(1) and 0.016(1). Å 2 , respectively, those for the boron position were unre- alistically small, indicating the presence of stronger scattering power in this site. The nitrogen atom also revealed a slightly in- creased ADP value of 0.015(2) Å 2 indicating a smaller contribu- tion. Upon releasing of ADPs and site occupancies for both mentioned positions one obtained R1 = 0.032; wR 2 = 0.074 with ADPs of 0.010(4) Å 2 and 0.013(2) Å 2 and occupancies of 1.69(2) and 0.90(3) for B and N, respectively. The results are compatible with a partial mutual substitution of [N-B-N] 3- dumbells and the spherical Br - units since both anions are possessing practically identical cationic environments (see crystal structure descrip- tion). In the separate cycles of the refinement, the B position was con- strained to be mixed occupied by boron and bromine atoms (Ta- bles 4, 5). Utilizing the constrained values, the degree of disorder was estimated to be 5(2) %. Attempts to apply the same constraint for the Br position failed due to the large difference between the electron contribution of the heavy bromine and the light boron at- oms in this site. Thus, we were not able to adapt completely the structure model describing the mutual replacement of Br - and [BN 2 ] 3- units to our experimental data. However, the yellow color of the compound indicates that europium is still in its Eu 2+ state and the total composition of the compound has still to be re- garded as Eu 2 [BN 2 ]Br. Therefore, this model explains all struc- tural pecularities, charge distribution, magnetic properties etc. in a satisfactory way. For the structure description we employed the ideal occupancy factors for Br, B and N sites, as required by the ideal chemical composition Eu 2 [BN 2 ]Br. Discussion The structure of the title compound is isotypic to Sr 2 [O-Cu-O]Br [1] and can be interpreted as a filled variant of the anti-NaCrS 2 type of structure (S 2 CrNa) [2] with Eu, Br and B at S, Cr and Na positions, respectively. The structure is characterized by layers of edge-sharing [N-B-N]@Eu 6 and [Br]@Eu 6 trigonal antiprisms (in other words - distorted octahedra), which are alternately stacked along [001]. The bond lengths for the strictly linear [BN 2 ] 3- anions are d(B—N) = 1.354(8) Å, corresponding well with those observed for the other known isostoichiometric dinitrido-borate halides [3-5]. As for the hitherto known nitridoborate halides, the two differently shaped anions (rod-like [BN 2 ] 3- and spherical Br - in the crystal structure of Eu 2 [BN 2 ]Br should be spatially well seperated from each other. The crystal structure refinement indicates, however, that a mutual replace- ment of the two moieties is possible. The disorder (ca. 5 %) ap- pears to be reasonable since both units are embedded in the same cationic matrix built up by the condensed Eu 6 trigonal antiprisms with the edge length within the triangle bases of 4.073 Å and mar- ginal differences of the antiprism heights: 4.381 Å ([BN 2 ] 3- ) and 4.481 Å (Br - ). Z. Kristallogr. NCS 224 (2009) 379-380 / DOI 10.1524/ncrs.2009.0166 379 © by Oldenbourg Wissenschaftsverlag, München _____________ * Correspondence author (e-mail: hoehn@cpfs.mpg.de)