Chong Marina, Matsuo Motoaki, Orimo Shin-ichi, Autrey Tom, Jensen Craig M
†Department of Chemistry, University of Hawaii, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States.
‡Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.
Inorg Chem. 2015 Apr 20;54(8):4120-5. doi: 10.1021/acs.inorgchem.5b00373. Epub 2015 Mar 27.
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 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.
通过将硼烷·四氢呋喃添加到镁/汞齐中来制备二硼氢化镁·2四氢呋喃（四氢呋喃 = 四氢呋喃）。在5兆帕氢气压力下，将二硼氢化镁·2四氢呋喃和氢化镁按1:2摩尔比的混合物加热至200°C并保持2小时，几乎可定量转化为硼氢化镁。在5兆帕氢气压力下测量的该反应的差示扫描量热曲线显示，该化合物在约65°C处有一个初始吸热特征，对应于其相变，随后是一个宽的放热特征，在130°C达到最大值，这对应于二硼氢化镁氢化为硼氢化镁。在没有氢化镁的情况下，在5兆帕氢气压力下将二硼氢化镁·2四氢呋喃加热至200°C，主要生成十二硼化镁，以及大量的十硼化镁和硼氢化镁。在5兆帕氢气压力下，将二硼氢化镁·2四氢呋喃和氢化锂按1:4摩尔比的混合物在130°C氢化，除了生成硼氢根离子外，还生成十二硼根离子(2-)，而二硼氢化镁·2四氢呋喃和氢化钠按1:4摩尔比反应则生成硼氢根离子和一种新的硼烷，可能是七硼根离子(-)。含氢化钠的混合物在130°C氢化主要生成这种替代硼烷，这表明它是三硼烷两步转化为硼氢根离子过程中的中间体。由硼氢化镁低温脱氢得到的无溶剂三硼烷二硼氢化镁，也能生成硼氢化镁，但需要更高的温度和压力才能实现二硼氢化镁的完全转化。这些结果表明，三硼烷的可逆转化取决于金属氢化物的稳定性。金属氢化物越稳定，即氢化锂>氢化钠>氢化镁，“再生”效率越低。
