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) 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 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 °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]−.