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Synthesis, Crystal Structure, Thermal Decomposition, and 11B MAS NMR Characterization of Mg(BH4)2(NH3BH3)2

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journal contribution
posted on 12.06.2014, 00:00 by Lars H. Jepsen, Voraksmy Ban, Kasper T. Møller, Young-Su Lee, Young Whan Cho, Flemming Besenbacher, Yaroslav Filinchuk, Jørgen Skibsted, Torben R. Jensen
A metal borohydride–ammonia borane complex, Mg­(BH4)2(NH3BH3)2 was synthesized via a solid-state reaction between Mg­(BH4)2 and NH3BH3. Different mechanochemical reaction mechanisms are observed, since Mg­(BH4)2(NH3BH3)2 is obtained from α-Mg­(BH4)2, whereas a mixture of Mg­(BH4)2(NH3BH3)2, NH3BH3, and amorphous Mg­(BH4)2 is obtained from γ-Mg­(BH4)2. The crystal structure of Mg­(BH4)2(NH3BH3)2 has been determined by powder X-ray diffraction and optimized by first-principles calculations. The borohydride groups act as terminal ligands, and molecular complexes are linked via strong dihydrogen bonds (<2.0 Å), which may contribute to the high melting point of Mg­(BH4)2(NH3BH3)2 found to be ∼48 °C in contrast to those for other molecular metal borohydrides. Precise values for the 11B quadrupole coupling parameters and isotropic chemical shifts are reported for the two NH3BH3 sites and two BH4 sites in Mg­(BH4)2(NH3BH3)2 from 11B MAS NMR spectra of the central and satellite transitions and MQMAS NMR. The 11B quadrupole coupling parameters agree excellently with the electric field gradients for the 11B sites from the DFT calculations and suggest that a more detailed structural model is obtained by DFT optimization, which allows evaluation of the dihydrogen bonding scheme.