Some tetrahydroborate derivatives of aluminium: crystal structures of dimethylaluminium tetrahydroborate and the α and β phases of aluminium tris(tetrahydroborate) at low temperature

Abstract

The structures of several aluminium tetrahydroborates have been investigated, primarily by X-ray diffraction of single crystals at low temperatures. The structure of dimethylaluminium tetrahydroborate has been shown to consist of helical polymeric chains in which Me ₂ Al and BH ₄ units alternate. The BH ₄ groups exhibit bidentate ligation with respect to each of the adjacent metal atoms in a manner reminiscent of [MeZnBH ₄ ] and Be(BH ₄ ) ₂ . Solid aluminium tris(tetrahydroborate) exists in two phases with a transition temperature in the range 180–195 K. Each phase is made up of discrete Al(BH ₄ ) ₃ units in which the angle between AlB ₂ and Al(µ-H) ₂ B planes is close to 90° and which conform within the limits of experimental error to D _3h symmetry; in the α phase these molecular units are disposed around a 2 ₁ crystallographic screw axis.