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Over the past two decades, the high hydrogen content and favorable dehydrogenation conditions of multi-metallic amidoboranes have gained significant attention for their potential in hydrogen storage. Among them, Al-based complex hydrides have shown promise because of their high polarizing power, light weight, and abundant natural presence. In this work, we successfully synthesized two novel tetrahedrally coordinated Al-based amidoboranes, namely, Li[Al(BH3NHCH2CH2NHBH3)2] and Na(THF)[Al(BH3NHCH2CH2NHBH3)2], using BH3NH2CH2CH2NH2BH3 (EDAB) as a precursor. The structure of Na(THF)[Al(BH3NHCH2CH2NHBH3)2] was determined through modeling based on synchrotron powder X-ray diffraction. Additionally, the formation of the Al-N bond in Li[Al(BH3NHCH2CH2NHBH3)2] and Na(THF)[Al(BH3NHCH2CH2NHBH3)2] was confirmed with IR spectra. Na(THF)[Al(BH3NHCH2CH2NHBH3)2] is more stable in air than Li[Al(BH3NHCH2CH2NHBH3)2]. Importantly, thermal gravimetric analysis and mass spectroscopic characterization confirmed that both compounds release hydrogen without the presence of ammonia, diborane, or ethylenediamine. Our work represents the first example of Al-based amidoboranes with chelation coordination geometry, which provides an essential foundation for understanding the relationship of complex multi-metallic amidoboranes in terms of synthesis, structure, and properties.

A novel species of metal amidoborane ammoniate, [Al(NH2BH3)63−][Al(NH3)63+] has been successfully synthesized in up to 95% via the one-step reaction of AlH3·OEt2 with liquid NH3BH3·nNH3 (n = 1~6) at 0 °C. This solution based reaction method provides an alternative pathway to the traditional mechano-chemical ball milling methods, avoiding possible decomposition. MAS 27Al NMR spectroscopy confirms the formulation of the compound as an Al(NH2BH3)63− complex anion and an Al(NH3)63+ cation. Initial dehydrogenation studies of this aluminum based M-N-B-H compound demonstrate that hydrogen is released at temperatures as low as 65 °C, totaling ~8.6 equivalents of H2 (10.3 wt %) upon heating to 105 °C. This method of synthesis offers a promising route towards the large scale production of metal amidoborane ammoniate moieties.

Lithium amidoborane (LiAB) is known as an efficient hydrogen storage material. The dehydrogenation reaction of LiAB was studied employing temperature-programmed desorption methods at varying temperature and H2 pressure. As the dehydrogenation products are in amorphous form, the XRD technique is not useful for their identification. The two-step decomposition temperatures (74 and 118 °C) were found to hardly change in the 1–80 bar pressure range. This is related either to kinetic effects or to thermal dependence of the reaction enthalpy. Further, the possible joint decomposition of LiNH2BH3 with LiBH4 or MgH2 was investigated. Indeed LiBH4 proved to destabilize LiAB, producing a 10 °C decrease of the first-step decomposition temperature, whereas no significant effect was observed by the addition of MgH2. The 5LiNH2BH3 + LiBH4 assemblage shows improved hydrogen storage properties with respect to pure lithium amidoborane.

Double metal amidoborans are considered to the most promising candidates for metal amidoborans. In this paper, the crystal structures, electronic properties, chemical bonds, hydrogen removal energies, and HOMO-LUMO of NaAB, NaLiAB, and NaMgAB have been studied. The GGA corrected density functional theory have been employed in the first principles calculations. Due to the alkali and alkali earth metals coexisting, the crystal structures of these compounds change significantly and the B–H, N–H and B-N bond lengths shorten. Moreover, the band structures and density of states of NaAB, NaLiAB, and NaMgAB were calculated. The charge density distributions and bond populations are used to understand the nature of bonding. The hydrogen removal energy states removing H(B) and H(N) from NaLiAB more easily than NaAB and NaMgAB. In addition, the frontier molecular orbital reveals that the intermolecular and intramolecular dehydrogenation of NaLiAB and NaMgAB may concur. The calculated HOMO-LUMO energy gaps suggest that the chemical reactivity is: NaLiAB> NaAB> NaMgAB.

In this review, we present an overview on metal amidoboranes, which have recently been considered as hydrogen storage materials for fueling of the low temperature fuel cells. We focus on amidoborane salts containing only metal cations and amidoborate anions. During the last decades, 19 new compounds from this group were described in the literature. We provide a summary of various physical and chemical properties of amidoborane compounds reported up to date.