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

Abstract

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-x)Cl(x) (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-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](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.