Potassium zinc borohydrides containing triangular [Zn(BH4)3]- and tetrahedral [Zn(BH4)xCl4-]2- anions

Three novel potassium-zinc borohydrides/chlorides are described. KZn(BH 4) 3 and K 2Zn(BH 4) xCl 4-x form in ball-milled KBH 4:ZnCl 2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K 3Zn(BH 4) xCl 5-x forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH 4) 3 contains an anionic complex [Zn(BH 4) 3] - with D 3 (32) symmetry, located inside a rhombohedron K 8. KZn(BH 4) 3 contains 8.1 wt % of hydrogen and decomposes at ∼385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn 2(BH 4) 5 (M = Li, Na) and NaZn(BH 4) 3. The decomposition temperature is much lower than for KBH 4. Monoclinic K 2Zn(BH 4) xCl 4-x contains a tetrahedral complex anion [Zn(BH 4) xCl 4-x] 2- located inside an Edshammar polyhedron (pentacapped trigonal prism) K 11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K 2ZnCl 4 (structure type K 2SO 4). K 2Zn(BH 4) xCl 4-x releases BH 4 starting from 395 K, forming Zn and KBH 4. As the reaction proceeds and x decreases, the monoclinic distortion of K 2Zn(BH 4) xCl 4-x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K 2ZnCl 4. Tetragonal K 3Zn(BH 4) xCl 5-x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs 3CoCl 5 structure type possibly with the space group P4 2/ncm. K 3Zn(BH 4) xCl 5-x decomposes nearly at the same temperature as KZn(BH 4) 3, i.e., at ∼400 K, with the formation of K 2Zn(BH 4) xCl 4-x and KBH 4, indicating that the compound is an adduct of the two latter compounds. © 2011 American Chemical Society.