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首页> 外文期刊>Chemistry: A European journal >Ammonia–Borane Dehydrogenation by Means of an Unexpected Pentacoordinate Boron Species: Insights from Density Functional and Molecular Dynamics Studies
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Ammonia–Borane Dehydrogenation by Means of an Unexpected Pentacoordinate Boron Species: Insights from Density Functional and Molecular Dynamics Studies

机译：通过意外的五配位硼物种实现氨-硼烷脱氢：密度泛函和分子动力学研究的真知灼见

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摘要

The catalytic release of dihydrogen (H_2) from ammonia– borane (NH_3BH_3, AB) and related amine–borane compounds has received a lot of attention in recent years. These reactions have potential applications in hydrogen storage and in the synthesis of inorganic polymers. However, achieving sustainability for hydrogen storage through this route is still burdened with several challenges. Recent advances in recovering AB from dehydrogenated fuel have raised hopes within the hydrogen-storage community. AB, by its own virtue, is a fascinating molecule; AB belongs to a rare class of molecules that possess oppositely charged hydrogen atoms on adjacent atoms. This attribute can be exploited to facilitate catalytic H_2 release from AB and from similar hydrogenated boron nitride materials. In fact, several elegant transition-metal catalysts have been devised to catalyse the dehydrogenation of AB under moderate-to-ambient conditions. Following these experiments, many theoretical studies have been carried out to elucidate the mechanistic details of this intriguing phenomenon. The current mechanistic understanding is that the dehydrogenation of AB propagates either through the concerted loss of a proton (from the N atom of AB) and loss of a hydride anion (from the B atom of AB) or through stepwise N-H activation (proton loss from AB) followed by B-H activation (hydride loss from AB) or the loss of hydrogen atoms in the reverse sequence. Baker and co-workers made a significant breakthrough by showing that H_2 release from AB can be triggered using non-metal agents, such as organic acids (for example, triflic acid) and electrondeficient boron-based Lewis acids. Their preliminary assumption was that the acid-triggered dehydrogenation of AB commences through hydride loss from AB. Notably, the current consensus is that AB dehydrocoupling only propagates through classical intermediates. On the contrary, our theoretical investigations into hydrogen release from AB (with a triflic acid solution in diglyme), reveals another possibility, in which AB dehydrocoupling propagates through a non-classical pentacoordinate boron-containing species, NH_3BH_4~+. This species is an isolobal analogue of the hypervalent compound, BH_5. Over several decades BH_5 has generated a significant amount of interest among theoreticians and experimentalists. Olah and co-workers were the first to propose the existence of a BH_5 species to explain the formation of copious amounts of H_2, along with HD and D_2, in the reaction of NaBH_4 with D_2SO_4. This proposal was followed by several theoretical studies by the research groups of Kutzelnigg, Bartlett and Schaefer, studies that were not only focussed on the structural determination of this BH_5 species, but also were directed towards understanding the origin of isotope scrambling in such species. Olah and co-workers have theoretically predicted the optimised geometries of similar hypervalent boron containing species, including NH_3BH_4~+. However, to date there are no theoretical studies that have provided in-depth insights regarding the molecular events leading to the formation of such a non-classical pentacoordinate species in a reaction medium. Herein, by using a combined quantum chemical and Car–Parrinello molecular dynamics (MD) study we provide the details of AB dehydrogenation in the presence of explicit solvent molecules. This study shows, for the first time, two important molecular events: formation of a pentacoordinate boron species upon protonation of a B-H bond of AB and the subsequent loss of a H_2 molecule from the non-classical intermediate. In fact, this is the first ab initio simulation of the release of H_2 from AB that emulates experimental AB dehydrocoupling.
机译：近年来，氨-硼烷（NH_3BH_3，AB）和相关的胺-硼烷化合物的催化释放二氢（H_2）受到了广泛关注。这些反应在储氢和无机聚合物的合成中具有潜在的应用。然而，通过这种途径实现储氢的可持续性仍然面临若干挑战。从脱氢燃料中回收AB的最新进展在储氢界引起了希望。 AB本身就是一个迷人的分子。 AB属于稀有分子，在相邻原子上具有带相反电荷的氢原子。可以利用该属性来促进从AB和类似的氢化氮化硼材料中催化H_2的释放。实际上，已经设计出几种优雅的过渡金属催化剂，以在中等至环境条件下催化AB的脱氢。经过这些实验，已经进行了许多理论研究，以阐明这种有趣现象的机理细节。目前的机理理解是，AB的脱氢通过质子的协同损失（来自AB的N原子）和氢化物阴离子（来自AB的B原子）或逐步的NH活化（质子从AB），然后进行BH活化（AB中的氢化物损失）或反向顺序中氢原子的损失。贝克和他的同事取得了重大突破，表明可以使用非金属试剂（例如有机酸（例如三氟甲磺酸）和缺电子的硼基路易斯酸）触发从AB中释放H_2。他们的初步假设是AB的酸引发的脱氢反应是从AB的氢化物损失开始的。值得注意的是，目前的共识是AB脱氢偶联仅通过经典中间体传播。相反，我们对AB中的氢释放进行的理论研究（在二甘醇二甲酸酯中有三氟甲磺酸溶液）揭示了另一种可能性，其中AB脱氢偶联通过非经典的五配位含硼物质NH_3BH_4〜+传播。该物种是高价化合物BH_5的等距类似物。几十年来，BH_5引起了理论家和实验家的极大兴趣。 Olah和他的同事最先提出了BH_5物种的存在，以解释NaBH_4与D_2SO_4反应中大量H_2以及HD和D_2的形成。这项建议之后，库特泽尔尼格（Gutzelnigg），巴特利特（Bartlett）和舍弗（Schaefer）研究小组进行了一些理论研究，这些研究不仅着重于BH_5物种的结构测定，而且还着眼于了解此类物种中同位素加扰的起源。 Olah及其同事从理论上预测了类似高价含硼物种（包括NH_3BH_4〜+）的最佳几何形状。然而，迄今为止，还没有理论研究对导致在反应介质中形成这种非经典五配位物种的分子事件提供了深入的见解。在此，通过结合使用量子化学和Car-Parrinello分子动力学（MD）研究，我们提供了在存在明确溶剂分子的情况下AB脱氢的详细信息。这项研究首次显示了两个重要的分子事件：AB的B-H键质子化形成五配位硼物种，以及随后从非经典中间体中损失H_2分子。实际上，这是从AB释放H_2的第一个从头开始模拟，它模拟了实验性AB脱氢偶联。

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