Thermal Decomposition Behavior of Hydrated Magnesium Dodecahydrododecaborates

MgB₁₂H₁₂ is an intermediate in the hydrogen desorption and sorption processes of magnesium borohydride, which is an important candidate material for hydrogen storage. It is thus highly desirable to synthesize anhydrous MgB₁₂H₁₂ in order to study its properties and its role in the hydrogenation and dehydrogenation of magnesium borohydride. Contrary to the literature claim, we find that anhydrous MgB₁₂H₁₂ cannot be obtained from simple thermal decomposition of Mg(H₂O)₆B₁₂H₁₂·6H₂O (<b>1</b>) which has different thermal decomposition behavior from that of most hydrated alkali and alkaline earth salts of dodecahydrododecaborates. Thermal decomposition of <b>1</b> involves both dehydration and dehydrogenation processes in three steps, resulting in the formation of complexes Mg(H₂O)₆B₁₂H₁₂ (<b>2</b>), Mg(H₂O)₃B₁₂H₁₂ (<b>3</b>), and Mg(μ-OH)_<i>x</i>B₁₂H_{12−<i>x</i>} (<b>4</b>) that were characterized by XRD, IR, and ¹¹B NMR. Dehydrogenation was also confirmed by both the generation of hydrogen observed in TPD-MS spectra and the formation of polyhydroxylated complexes

Formation Mechanisms, Structure, Solution Behavior, and Reactivity of Aminodiborane

A facile synthesis of cyclic aminodiborane (NH₂B₂H₅, ADB) from ammonia borane (NH₃·BH₃, AB) and THF·BH₃ has made it possible to determine its important characteristics. Ammonia diborane (NH₃BH₂(μ-H)­BH₃, AaDB) and aminoborane (NH₂BH₂, AoB) were identified as key intermediates in the formation of ADB. Elimination of molecular hydrogen occurred from an ion pair, [H₂B­(NH₃) (THF)]⁺[BH₄]⁻. Protic-hydridic hydrogen scrambling was proved on the basis of analysis of the molecular hydrogen products, ADB and other reagents through ²H NMR and MS, and it was proposed that the scrambling occurred as the ion pair reversibly formed a BH₅-like intermediate, [(THF)­BH₂NH₂]­(η²-H₂)­BH₃. Loss of molecular hydrogen from the ion pair led to the formation of AoB, most of which was trapped by BH₃ to form ADB with a small amount oligomerizing to (NH₂BH₂)_<i>n</i>. Theoretical calculations showed the thermodynamic feasibility of the proposed intermediates and the activation processes. The structure of the ADB·THF complex was found from X-ray single crystal analysis to be a three-dimensional array of zigzag chains of ADB and THF, maintained by hydrogen and dihydrogen bonding. Room temperature exchange of terminal and bridge hydrogens in ADB was observed in THF solution, while such exchange was not observed in diethyl ether or toluene. Both experimental and theoretical results confirm that the B–H–B bridge in ADB is stronger than that in diborane (B₂H₆, DB). The B–H–B bridge is opened when ADB and NaH react to form sodium aminodiboronate, Na­[NH₂(BH₃)₂]. The structure of the sodium salt as its 18-crown-6 ether adduct was determined by X-ray single crystal analysis