Electronic structures and dehydrogenation properties of bimetallic amidoboranes

Bimetallic amidoboranes (MM′AB, M and M′ denote different metals, respectively) are capable of improving the efficiency of dehydrogenation process and purity of released hydrogen compared to monometallic amidoboranes. In order to elucidate the reason behind this improvement, we perform a solid state Density Functional Theory (DFT) study on several bimetallic amidoboranes recently synthesized, NaMg(NH₂BH₃)₃ (SMAB), Na₂Mg(NH₂BH₃)₄ (DSMAB), Na[Li(NH₂BH₃)₂] (SLAB), and K₂Mg(NH₂BH₃)₄ (DKMAB). Whereas the crystal structure SMAB has not been obtained yet (the other three have been obtained experimentally), we apply the Monte Carlo method to simulate its crystal structure. Then the optimized crystal structures and electronic structures of MM′ABs have been determined for further studies of their thermodynamic properties and the initial dehydrogenation mechanisms. Two metals in MM'AB play totally different roles to improve the appearance of dehydrogenation. Moreover, the first dihydrogen bond of N-H^δ+ ... ^-δH-B prefers being formed between two neighboring [NH₂BH₃] groups rather than in the same [NH₂BH₃] molecule. The molecular dynamic study of the hydrogen diffusion at room temperature shows that H(B) atoms have faster speed than that of H(N) atoms caused by the bond strength and electronic structures. Generally, the diffusion rate of hydrogen is as the sequence of SMAB > SLAB > DKMAB > DSMAB.