Mechanistic Investigation on the Formation and Dehydrogenation of Calcium Amidoborane Ammoniate

Abstract

Possessing high H2 capacities and interesting dehydrogenation behavior, metal amidoborane ammoniates were prepared by reacting Ca(NH2)2, MgNH, and LiNH2 with ammonia borane to form Ca(NH2BH3)2⋅2 NH3, Mg(NH2BH3)2⋅NH3, and Li(NH2BH3)2⋅NH3 (LiAB⋅NH3). Insight into the mechanisms of amidoborane ammoniate formation and dehydrogenation was obtained by using isotopic labeling techniques. Selective 15N and 2H labeling showed that the formation of the ammoniate occurs via the transfer of one H(N) from ammonia borane to the [NH2]− unit in Ca(NH2)2 giving rise to NH3 and [NH2BH3]−. Supported by theoretical calculations, it is suggested that the improved dehydrogenation properties of metal amidoborane ammoniates compared to metal amidoboranes are a result of the participation of a strong dihydrogen bond between the NH3 molecule and [NH2BH3]−. Our study elucidates the reaction pathway involved in the synthesis and dehydrogenation of Ca(NH2BH3)2⋅2 NH3, and clarifies our understanding of the role of NH3, that is, it is not only involved in stabilizing the structure, but also in improving the dehydrogenation properties of metal amidoboranes.

Brønsted and Lewis Base Behavior of Sodium Amidotrihydridoborate (NaNH2BH3)

“…[5b] The mechanism of this reaction was previously suggested to involve direct substitution of NH 3 in NH 3 BH 3 by NH 2 – or metathesis of Na in NaNH 2 with H of NH 3 in NH 3 BH 3 . [5b] It may be reasonable that the stronger Lewis base, that is, NH 2 – , attacks H δ + on nitrogen in NH 3 BH 3 to give rise to the metal amidoborate, which was confirmed by a 15 N labeling experiment. [8c]…”

“…[5b] It may be reasonable that the stronger Lewis base, that is, NH 2 – , attacks H δ + on nitrogen in NH 3 BH 3 to give rise to the metal amidoborate, which was confirmed by a 15 N labeling experiment. [8c]…”

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

“…The process was shown to occur through protonation of the apical hydride ligand to produce an η 2 -H 2 intermediate, with subsequent HH* shift of the anion prior to deprotonation . Hydrogen–deuterium exchange and an equilibrium isotope effect appear to be responsible for the observation of various deuterated amine groups ([ND 2 BH 3 ] − or [NHDBH 3 ] − ) in the amido moiety of dihydrogen-bonded Ca­(NH 2 BH 3 ) 2 ·2NH 3 . In contrast, possible H/D exchange within DHB fragments was not taken into account despite “this ubiquitous interaction ... evident in the structures” of partially deuterated alkali-metal amidoboranes MND 2 BH 3 .…”

“…290 Hydrogen− deuterium exchange and an equilibrium isotope effect appear to be responsible for the observation of various deuterated amine groups ([ND 2 BH 3 ] − or [NHDBH 3 ] − ) in the amido moiety of dihydrogen-bonded Ca(NH 2 BH 3 ) 2 •2NH 3 . 302 In contrast, possible H/D exchange within DHB fragments was not taken into account despite "this ubiquitous interaction ... evident in the structures" of partially deuterated alkali-metal amidoboranes MND 2 BH 3 . 303 Instead, the authors suggest that evolution of H 2 instead of HD occurs through homopolar BH δ− ••• δ− HB bonds.…”

Lithium amidoborane hydrazinates: synthesis, structure and hydrogen storage properties

“…In the above mentioned ammoniates, the combination of H d+ in NH 3 and H dÀ in BH 3 is one of the driving forces for dehydrogenation. 27 Thus, it is of great interest to probe other H d+ containing ligands to balance H d+ and H dÀ in amidoboranes and improve the dehydrogenation properties.…”