BaB₂Se₆ and Ba₂B₄Se₁₃: THE FIRST ALKALINE EARTH PERSELENOBORATES WITH POLYMERIC LAYERED STRUCTURES

Arno Lindemann, Joachim Kuchinke, Olaf Conrad und Bernt Krebs

Anorganisch-Chemisches Institut der Westfälischen Wilhelms-Universität Münster, Wilhelm-Klemm-Str.8, 48149 Münster, Germany
kuchink@nwz.uni-muenster.de

Keywords: perselenoborates, polymeric layered structures, boron, selenium

A wide variety of ternary and quaternary thio- and perthioborates was synthesized and characterized over the last 35 years [1]. In contrast there is only a small number of selenium-boron compounds. The thallium selenoborate Tl₃BSe₃ [2], containing planar BSe₃^3--anions, the caesium hexaselenoborato-closo-dodecaborate Cs₈B₁₂(BSe₃)₆ [3], featuring persubstituted icosahedral boronclusters and bidentate BSe₃-ligands, and a few more alkali metal and thallium perselenoborates of composition MBSe₃, M₂B₂Se₇ and M₃B₃Se₁₀ are known today [4,5].

BaB2Se6	Ba2B4Se13

The preparation of the first alkaline earth perselenoborates BaB₂Se₆ and Ba₂B₄Se₁₃ from high temperature solid state reactions is reported. Starting from barium selenide, amorphous boron and selenium in appropriate molar ratios the compounds were synthesized in evacuated, carbon-coated quartz glass ampoules at 750C. BaB₂Se₆ crystallizes in the orthorhombic space group Cmca and the lattice parameters a = 11.326(2) A, b = 7.659(2) A and c = 10.315(2) A. The structure consists of infinite, twodimensional anions of stoichiometry [B₂Se₆^2-]_inf parallel to the a-b-plane. The voids are filled with the barium cations. The anions of BaB₂Se₆ contain novel 4-membered B₂Se₂-rings, connected via four diselenide bridges. The boron-selenium distances for tetraedrally coordinated boron are 2.074(4) A to ring-sharing selenium and 2.037(4) A to the bridging diselenium units.

Ba₂B₄Se₁₃ is the second compound of the Ba/B/Se­system that could be characterized by single crystal X-ray structure determination. Just as the above structure it consists of connected BSe₄-tetrahedra.

Ba₂B₄Se₁₃ belongs to the monoclinic space group P2₁/c with lattice parameters a = 12.792(4) A, b = 11.568(4) A, c = 12.879(4) A and b= 103.18(3)^o. The compound contains polymeric anions [B₄Se₁₃^4-]_inf parallel to the a-c-plane. They contain spirocyclically connected B₂Se₃- and B₂Se₄-rings, forming a novel chain type. These chains are connected by diselenide bridges to form the layer. The average B-Se-distance of 2.051 A agrees well with other alkaline perselenoborates.

1. O. Conrad, C. Jansen, B. Krebs, Angew. Chem. (submitted)
2. B. Krebs, W. Hamann, J.Less-Common Met. 137 (1988) 143
3. J. Küper, O. Conrad, B. Krebs, Angew. Chem. ,109 (1997) 1995, Angew. Chem. Int. Ed. Engl. 36 (1997) 1903
4. A. Hammerschmidt, J. Küper, L. Stork, B. Krebs, Z. Anorg.Allg. Chem. 620 (1994) 1898;
5. B. Krebs, W. Hamann, Acta Crystallogr. A40 (1984) C217; Ph. D. theses J. Küper (1996) and C. Jansen (1997), University of Münster