Selective Reversible Hydrogenation of Mg(B3H8)2/MgH2 to Mg(BH4)2: Pathway to Reversible Borane-Based Hydrogen Storage?

Mg­(B3H8)2·2THF (THF = tetrahydrofuran) was prepared by the addition of BH3·THF to Mg/Hg amalgam. Heating a 1:2 molar mixture of Mg­(B3H8)2·2THF and MgH2 to 200 °C under 5 MPa H2 for 2 h leads to nearly quantitative conversion to Mg­(BH4)2. The differential scanning calorimetry profile of the reaction measured under 5 MPa H2 shows an initial endothermic feature at ∼65 °C for a phase change of the compound followed by a broad exothermic feature that reaches a maximum at 130 °C corresponding to the hydrogenation of Mg­(B3H8)2 to Mg­(BH4)2. Heating Mg­(B3H8)2·2THF to 200 °C under 5 MPa H2 pressure in the absence of MgH2 gives predominantly MgB12H12 as well as significant amounts of MgB10H10 and Mg­(BH4)2. Hydrogenation of a mixture of Mg­(B3H8)2·2THF and LiH in a 1:4 molar ratio at 130 °C under 5 MPa H2 yields [B12H12]2– in addition to [BH4]−, while a 1:4 molar ratio of Mg­(B3H8)2·2THF and NaH yields [BH4]− and a new borane, likely [B2H7]−. Hydrogenation of the NaH-containing mixture at 130 °C gives primarily the alternative borane, indicating it is an intermediate in the two-step conversion of the triborane to [BH4]−. The solvent-free triborane Mg­(B3H8)2, derived from the low-temperature dehydrogenation of Mg­(BH4)2, also produces Mg­(BH4)2, but higher temperature and pressure is required to effect the complete transformation of the Mg­(B3H8)2. These results show that the reversible transformation of the triborane depends on the stability of the metal hydride. The more stable the metal hydride, that is, LiH > NaH > MgH2, the lower is the “regeneration” efficiency.