The First Halide-Free Bimetallic Aluminum Borohydride: Synthesis, Structure, Stability, and Decomposition Pathway

Abstract

Interaction of solid KBH₄ with liquid Al­(BH₄)₃ at room temperature yields a solid bimetallic borohydride KAl­(BH₄)₄. According to the synchrotron X-ray powder diffraction, its crystal structure (space group <i>Fddd, a</i> = 9.7405(3), <i>b</i> = 12.4500(4), and <i>c</i> = 14.6975(4) Å) contains a substantially distorted tetrahedral [Al­(BH₄)₄]⁻ anion, where the borohydride groups are coordinated to aluminum atoms via edges. The η²-coordination of BH₄^– is confirmed by the infrared and Raman spectroscopies. The title compound is the first aluminum-based borohydride complex not stabilized by halide anions or by bulky organic cations. It is not isostructural to bimetallic chlorides, where more regular tetrahedral AlCl₄^– anions are present. Instead, it is isomorphic to the LT phase of TbAsO₄ and can be also viewed as consisting of two interpenetrated <i>dia</i>-type nets where BH₄ ligand is bridging Al and K cations. Variable temperature X-ray powder diffraction, TGA, DSC, and TGA-MS data reveal a single step of decomposition at 160 °C, with an evolution of hydrogen and some amount of diborane. Aluminum borohydride is not released in significant amounts; however, some crystalline KBH₄ forms upon decomposition. The higher decomposition temperature than in chloride-substituted Li–Al (70 °C) and Na–Al (90 °C) borohydrides suggests that the larger alkali metal cations (weaker Pearson acids) stabilize the weak Pearson base, [Al­(BH₄)₄]⁻