Potassium zinc borohydrides containing triangular [Zn(BH4)3]-- and tetrahedral [Zn(BH4)xCl4-x]2-- anions
Résumé
Three novel potassium-zinc borohydrides/chlorides are described. KZn(BH4)3 and K2Zn(BH4)xCl4-x form in ball-milled KBH4:ZnCl2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K3Zn(BH4)xCl5-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(BH4)3 contains an anionic complex [Zn(BH4)3]− with D3 (32) symmetry, located inside a rhombohedron K8. KZn(BH4)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 MZn2(BH4)5 (M = Li, Na) and NaZn(BH4)3. The decomposition temperature is much lower than for KBH4. Monoclinic K2Zn(BH4)xCl4-x contains a tetrahedral complex anion [Zn(BH4)xCl4-x]2- located inside an Edshammar polyhedron (pentacapped trigonal prism) K11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K2ZnCl4 (structure type K2SO4). K2Zn(BH4)xCl4-x releases BH4 starting from 395 K, forming Zn and KBH4. As the reaction proceeds and x decreases, the monoclinic distortion of K2Zn(BH4)xCl4-x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K2ZnCl4. Tetragonal K3Zn(BH4)xCl5-x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs3CoCl5 structure type possibly with the space group P42/ncm. K3Zn(BH4)xCl5-x decomposes nearly at the same temperature as KZn(BH4)3, i.e., at 400 K, with the formation of K2Zn(BH4)xCl4-x and KBH4, indicating that the compound is an adduct of the two latter compounds.