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

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 Fddd, a = 9.7405(3), b = 12.4500(4), and c = 14.6975(4) angstrom) contains a substantially distorted tetrahedral [Al(BH4)(4)](-) anion, where the borohydride groups are coordinated to aluminum atoms via edges. The eta(2)-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 dia-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 degrees 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 degrees C) and Na-Al (90 degrees C) borohydrides suggests that the larger alkali metal cations (weaker Pearson acids) stabilize the weak Pearson base, [Al(BH4)(4)](-).