M(BH₃NH₂BH₂NH₂BH₃) – the missing link in the mechanism of the thermal decomposition of light alkali metal amidoboranes

Abstract

We report a novel family of hydrogen-rich materials - alkali metal di(amidoborane)borohydrides, M(BH3NH2BH2NH2BH3). The title compounds are related to metal amidoboranes (amidotrihydroborates) but have higher gravimetric H content. Li salt contains 15.1 wt% H and discharges very pure H2 gas. Differences in thermal stability between amidoboranes and respective oligoamidoboranes explain the release of the ammonia impurity (along with H2) during the thermal decomposition of light alkali amidoboranes, LiNH2BH3, NaNH2BH3 and NaLi(NH2BH3)2, and confirm the mechanism of the side decomposition reaction.

Mono- and Bimetalic Amidoboranes

“…Later, this hypothesis was substantiated by Fijalkowski et al [87]. The intermolecular step responsible for both ammonia evolution and initiation of the polymerization of the solid residue was finally suggested to correspond to trimerisation of amidoborate anions with the concomitant formation of BH 3 NH 2 BH 2 NH 2 BH 3´a nions and ammonia molecules [87]. The latter hypothesis finds strong support from the X-ray, Raman and FTIR studies [87].…”

“…Fijalkowski et al [23] further proposed an intermolecular transformation of two amidoborate anions forming hypothetical product containing BH3NHNaBH3 − anions and Na[NH3] + cations, which could easily evolve weakly coordinated ammonia [23,24]. Later, this hypothesis was substantiated by Fijalkowski et al [87]. The intermolecular step responsible for both ammonia evolution and initiation of the polymerization of the solid residue was finally suggested to correspond to trimerisation of amidoborate anions with the concomitant formation of BH3NH2BH2NH2BH3 − anions and ammonia molecules [87].…”

“…A number of papers were published reporting alkali metal amidoboranes as sources of pure hydrogen, e.g., LiAB [18,19,86], NaAB [18,86] and KAB [25]. On the other hand, several other groups reported for these materials the desorption of hydrogen contaminated with ammonia: α-LiAB [20,25,39,89,90], β-LiAB [20] and NaAB [23,24,87,88,98]. Some amidoborane salts (KAB [25,89], Mg(AB) 2 [29], Al(AB) 3 [35], and LiAl(AB) 4 [35,40]) were reported to desorb pure hydrogen and, currently, there are no contradictory reports.…”

“…The intermolecular step responsible for both ammonia evolution and initiation of the polymerization of the solid residue was finally suggested to correspond to trimerisation of amidoborate anions with the concomitant formation of BH 3 NH 2 BH 2 NH 2 BH 3´a nions and ammonia molecules [87]. The latter hypothesis finds strong support from the X-ray, Raman and FTIR studies [87]. One can find signals from LiBH 3 NH 2 BH 2 NH 2 BH 3 and NaBH 3 NH 2 BH 2 NH 2 BH 3 in the X-ray powder patterns of, e.g., α-LiAB [18], β-LiAB [20], NaAB [21], LiNa(AB) 2 [39] and LiAB¨AB [55], synthesized in recent years.…”

“…LiBH3NH2BH2NH2BH3 and NaBH3NH2BH2NH2BH3 were earlier observed by Evans [100] and Ryan [21] but at that time their role as intermediates of the thermal decomposition of LiAB and NaAB had not yet been understood. Selected mechanism discussed above are presented in Figure 7 [19,23,32,86,87] [22] reporting NaAB as source of borazine-free hydrogen (Left. Reproduced from reference [22] with permission from The Royal Society of Chemistry.…”