Potassium Zinc Borohydrides Containing Triangular [Zn(BH<sub>4</sub>)<sub>3</sub>]<sup>−</sup> and Tetrahedral [Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>4–<i>x</i></sub>]<sup>2–</sup> Anions
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Abstract
Three novel potassium–zinc borohydrides/chlorides are described. KZn(BH<sub>4</sub>)<sub>3</sub> and K<sub>2</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>4–<i>x</i></sub> form in ball-milled KBH<sub>4</sub>:ZnCl<sub>2</sub> mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K<sub>3</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>5–<i>x</i></sub> 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<sub>4</sub>)<sub>3</sub> contains an anionic complex [Zn(BH<sub>4</sub>)<sub>3</sub>]<sup>−</sup> with <i>D</i><sub>3</sub> (32) symmetry, located inside a rhombohedron K<sub>8</sub>. KZn(BH<sub>4</sub>)<sub>3</sub> 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<sub>2</sub>(BH<sub>4</sub>)<sub>5</sub> (M = Li, Na) and NaZn(BH<sub>4</sub>)<sub>3</sub>. The decomposition temperature is much lower than for KBH<sub>4</sub>. Monoclinic K<sub>2</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>4–<i>x</i></sub> contains a tetrahedral complex anion [Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>4–<i>x</i></sub>]<sup>2</sup><sup>–</sup> located inside an Edshammar polyhedron (pentacapped trigonal prism) K<sub>11</sub>. The compound is a monoclinically distorted variant of the paraelectric orthorhombic <i>ht</i>-phase of K<sub>2</sub>ZnCl<sub>4</sub> (structure type K<sub>2</sub>SO<sub>4</sub>). K<sub>2</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>4–<i>x</i></sub> releases BH<sub>4</sub> starting from 395 K, forming Zn and KBH<sub>4</sub>. As the reaction proceeds and <i>x</i> decreases, the monoclinic distortion of K<sub>2</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>4–<i>x</i></sub> diminishes and the structure transforms at 445 K into the orthorhombic <i>ht</i>-phase of K<sub>2</sub>ZnCl<sub>4</sub>. Tetragonal K<sub>3</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>5–<i>x</i></sub> is a substitutional and deformation variant of the tetragonal (<i>I</i>4/<i>mcm</i>) Cs<sub>3</sub>CoCl<sub>5</sub> structure type possibly with the space group <i>P</i>4<sub>2</sub>/<i>ncm</i>. K<sub>3</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>5–<i>x</i></sub> decomposes nearly at the same temperature as KZn(BH<sub>4</sub>)<sub>3</sub>, i.e., at ∼400 K, with the formation of K<sub>2</sub>Zn(BH<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>4–<i>x</i></sub> and KBH<sub>4</sub>, indicating that the compound is an adduct of the two latter compounds
