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

Iron and cobalt borohydrides stabilized by ammonia (NH3), Fe(NH3)62 and Co(NH3)62, were synthesized along with a solid solution, Co(NH3)6(2-x)Cl(x) (x ∼ 1), and a bimetallic compound, Fe(NH3)6. 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)62 (M = Fe, Co) and Co(NH3)6(2-x)Cl(x) 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. 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)62·nNH3BH3 (n = 2, 4, 6) were studied by thermal analysis and in situ SR-PXD.