engleski

How ammonia borane travels toward amidoboranes? The case of several single- and bimetallic amidoboranes

For the first time, Raman spectroscopy was applied for in-situ monitoring of uninterrupted mechanochemical preparations of several single- and bimetallic amidoboranes from ammonia borane and corresponding hydrides. This approach allowed a real-time observation of key intermediate phases and a straightforward follow-up of the reaction course. Detailed analysis of time-dependent spectra revealed a two-step mechanism through ANH2BH3•NH3BH3 (A = Li, Na) adducts as key intermediate phases which further reacted with remaining hydride phase, giving NaLi(NH2BH3)2 or A2E(NH2BH3)4 (E = Mg, Ca) as final products. The intermediates partially take a competitive pathway toward the oligomeric A(BH3NH2BH2NH2BH3) phases. The crystal structure of the novel bimetallic amidoborane Li2Mg(NH2BH3)4 was solved from high-resolution powder diffraction data and showed an analogous metal coordination as in Na2Mg(NH2BH3)4, but a significantly different crystal packing. In order to check the hypothesis of unique alkaline metal-containing intermediate adduct, mechanochemical preparation of sodium amidoborane, NaNH2BH3, was thoroughly investigated. Although described in several publications, without any significant consideration of the optimal synthesis conditions. We report a series of ball-milling nNH3BH3 + NaH (n = 1, 2, 3) reactions that should result in sodium amidoborane. By doing the reactions in the same milling jars and using the same balls, the influence of milling time and interventions during the milling process (i.e. degassing of milling jar) were investigated. Additionally, the preparation of the starting chemicals was also taken into consideration. These considerations shed a new light into the mechanistic details of synthesis of sodium amidoborane. Namely, it occurs in at least two steps, with production of the NaAB•AB adduct as a crucial intermediate that, together with starting chemicals other than AB determines further reaction pathway toward the final product. Sodium amidoborane by itself suffers further rearrangement to long-chain product. The main message of this study is that the reaction conditions highly influence the reactivity and must be very carefully controlled for successful synthesis of sodium amidoborane. Thermal dehydrogenation of the obtained materials will be also discussed.

Amidoborane ; mechanochemistry ; ammonia borane ; hydrogen storage

nije evidentirano