Ammine-Stabilized Transition-Metal Borohydrides of Iron, Cobalt, and Chromium: Synthesis and Characterization

Iron and cobalt borohydrides stabilized
by ammonia (NH3), [Fe­(NH3)6]­(BH4)2 and [Co­(NH3)6]­(BH4)2, were synthesized along with a solid solution,
[Co­(NH3)6]­(BH4)2–xClx (x ∼ 1), and a bimetallic compound, [Fe­(NH3)6]­(Li2(BH4)4). The compounds
were prepared by new low-temperature, solvent-based synthesis methods,
using dimethyl sulfide or liquid NH3, which allow for the
removal of inert metal halides. The crystal structures were determined
from synchrotron radiation powder X-ray diffraction (SR-PXD) data.
[M­(NH3)6]­(BH4)2 (M = Fe,
Co) and [Co­(NH3)6]­(BH4)2–xClx crystallize in the
cubic crystal system, where the transition metals are octahedrally
coordinated by NH3. Polymeric chains of lithium coordinated
by four bridging BH4– anions are found
in [Fe­(NH3)6]­(Li2(BH4)4). The new compounds have high hydrogen densities of ∼14
wt % H2 and ∼140 g H2/L and release
a mixture of hydrogen and NH3 gas at low temperatures, T < 80 °C. The decomposition mechanisms of the prepared
compounds along with the composites [Fe­(NH3)6]­(BH4)2·nNH3BH3 (n = 2, 4, 6) were studied by thermal
analysis and in situ SR-PXD.