Department of Chemistry, University College London, Gower St, London, WC1E 6BT, United Kingdom.
Phys Chem Chem Phys. 2011 May 7;13(17):7649-59. doi: 10.1039/c0cp02213f. Epub 2011 Feb 18.
Electronic structure calculations have been used to determine and compare the thermodynamics of H(2) release from ammonia borane (NH(3)BH(3)), lithium amidoborane (LiNH(2)BH(3)), and sodium amidoborane (NaNH(2)BH(3)). Using two types of exchange correlation functional we show that in the gas-phase the metal amidoboranes have much higher energies of complexation than ammonia borane, meaning that for the former compounds the B-N bond does not break upon dehydrogenation. Thermodynamically however, both the binding energy for H(2) release and the activation energy for dehydrogenation are much lower for NH(3)BH(3) than for the metal amidoboranes, in contrast to experimental results. We reconcile this by also investigating the effects of dimer complexation (2×NH(3)BH(3), 2×LiNH(2)BH(3)) on the dehydrogenation properties. As previously described in the literature the minimum energy pathway for H(2) release from the 2×NH(3)BH(3) complex involves the formation of a diammoniate of diborane complex (BH(4)NH(3)BH(2)NH(3)). A new mechanism is found for dehydrogenation from the 2×LiNH(2)BH(3) dimer that involves the formation of an analogous dibroane complex (BH(4)LiNH(2)BH(2)LiNH(2)), intriguingly it is lower in energy than the original dimer (by 0.13 eV at ambient temperatures). Additionally, this pathway allows almost thermoneutral release of H(2) from the lithium amidoboranes at room temperature, and has an activation barrier that is lower in energy than for ammonia borane, in contrast to other theoretical research. The transition state for single and dimer lithium amidoborane demonstrates that the light metal atom plays a significant role in acting as a carrier for hydrogen transport during the dehydrogenation process via the formation of a Li-H complex. We posit that it is this mechanism which is responsible, in condensed molecular systems, for the improved dehydrogenation thermodynamics of metal amidoboranes.
已使用电子结构计算来确定和比较氨硼烷（NH3BH3）、锂酰胺硼烷（LiNH2BH3）和钠酰胺硼烷（NaNH2BH3）中 H2 的释放热力学。我们使用两种类型的交换相关泛函表明，在气相中，金属酰胺硼烷的络合能远高于氨硼烷，这意味着对于前一种化合物，B-N 键在脱氢过程中不会断裂。然而，从热力学角度来看，NH3BH3 中 H2 释放的结合能和脱氢的活化能都远低于金属酰胺硼烷，这与实验结果相反。我们通过研究二聚体络合（2×NH3BH3、2×LiNH2BH3）对脱氢性质的影响来调和这一点。如文献中所述，从 2×NH3BH3 络合物中释放 H2 的最低能量途径涉及二硼烷络合物[BH4-][NH3BH2NH3]+的形成。在从 2×LiNH2BH3 二聚体脱氢中发现了一种新的机制，该机制涉及类似的二硼烷络合物[BH4-][LiNH2BH2LiNH2]+的形成，有趣的是，它的能量比原始二聚体低（在环境温度下低 0.13 eV）。此外，该途径允许室温下几乎热中性地从锂酰胺硼烷中释放 H2，其活化能比氨硼烷低，这与其他理论研究相反。单和二聚体锂酰胺硼烷的过渡态表明，轻金属原子在脱氢过程中通过形成 Li-H 络合物作为氢载体，在充当氢载体方面起着重要作用。我们假设，正是这种机制导致金属酰胺硼烷在凝聚态分子体系中脱氢热力学得到改善。
