Well‐Defined Molecular Magnesium Hydride Clusters: Relationship between Size and Hydrogen‐Elimination Temperature

A new tetranuclear magnesium hydride cluster, [{ NN ‐(MgH)₂}₂], which was based on a N? N‐coupled bis‐β‐diketiminate ligand ( NN ^2?), was obtained from the reaction of [{ NN ‐(MgnBu)₂}₂] with PhSiH₃. Its crystal structure reveals an almost‐tetrahedral arrangement of Mg atoms and two different sets of hydride ions, which give rise to a coupling in the NMR spectrum (J=8.5 Hz). To shed light on the relationship between the cluster size and H₂ release, the thermal decomposition of [{ NN ‐(MgH)₂}₂] and two closely related systems that were based on similar ligands, that is, an octanuclear magnesium hydride cluster and a dimeric magnesium hydride species, have been investigated in detail. A lowering of the H₂‐desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH₂)_n model systems. Deuterium‐labeling studies further demonstrate that the released H₂ solely originates from the oxidative coupling of two hydride ligands and not from other hydrogen sources, such as the β‐diketiminate ligands. Analysis of the DFT‐computed electron density in [{ NN ‐(MgH)₂}₂] reveals a counterintuitive interaction between two formally closed‐shell H^? ligands that are separated by 3.106 Å. This weak interaction could play an important role in H₂ desorption. Although the molecular product after H₂ release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low‐valence tetranuclear Mg(I) cluster [( NN ‐Mg₂)₂], predict a structure with two almost‐parallel, localized Mg? Mg bonds. As in a previously reported β‐diketiminate Mg^I dimer, the Mg? Mg bond is not characterized by a bond critical point, but instead displays a local maximum of electron density midway between the atoms, that is, a non‐nuclear attractor (NNA). Interestingly, both of the NNAs in [( NN ‐Mg₂)₂] are connected through a bond path that suggests that there is bonding between all four Mg^I atoms.     

新型储氢材料及其热力学与动力学问题

氢能作为一种理想的二次能源受到了国内外科研工作者的广泛关注,研制可以在室温和较低压力下方便、安全、高效地储存氢能的材料是氢能发展的瓶颈.到目前为止,固态储氢材料以能量密度高及安全性好等优势被认为极具应用前景,其中以轻质元素构成的氢化物(包括硼氢化物/铝氢化物(可用通式A(MH4)n表示,其中A是碱金属(Li,Na,K)或碱土金属(Be,Mg,Ca);M是硼或铝;n=1～4)、氨基氢化物(如LiNH2等))、氨硼烷(NH3BH3)、金属有机骨架材料(MOFs)是新型储氢材料研究领域的热点,本文将着重就目前这几类储氢材料的研究当中所涉及到的一些热力学及动力学问题进行总结探讨.     

Discrete Magnesium Hydride Aggregates: A Cationic Mg13H18 Cluster Stabilized by NNNN‐Type Macrocycles

Large magnesium hydride aggregates [Mg₁₃(Me₃TACD)₆(μ₂‐H₁₂)(μ₃‐H₆)][A]₂ ((Me₃TACD)H=1,4,7‐trimethyl‐1,4,7,10‐tetraazacyclododecane; A=AlEt₄, AlnBu₄, B{3,5‐(CF₃)₂C₆H₃}₄) were synthesized stepwise from alkyl complexes [Mg₂(Me₃TACD)R₃] (R=Et, nBu) and phenylsilane in the presence of additional Mg^II ions. The central magnesium atom is octahedrally coordinated by six hydrides as in solid α‐MgH₂ of the rutile type. Further coordination to six magnesium atoms leads to a substructure of seven edge‐sharing octahedra as found in the hexagonal layer of brucite (Mg(OH)₂). Upon protonolysis in the presence of 1,2‐dimethoxyethane (DME), this cluster was degraded into a tetranuclear dication [Mg₂(Me₃TACD)(μ‐H)₂(DME)]₂[A]₂.     

镁基储氢合金的表面处理和表面催化

表面处理作为储氢合金性能改善的有效手段，近年来得到了很好的发展和应用，本文简要介绍了镁基储氢合金表面处理的主要方法及其对合金性能的影响。     

Influence of surface contaminations on the hydrogen storage behaviour of metal hydride alloys

Hydrogen storage in metal hydrides is a promising alternative to common storage methods. The surface of a metal hydride plays an important part in the absorption of hydrogen, since important partial reaction steps take place here. The development of surface contaminations and their influence on hydrogen absorption is examined by means of absorption experiments and surface analysis, using X-ray photoelectron spectroscopy (XPS), thermal desorption mass spectrometry (TDMS) and secondary neutral mass spectrometry (SNMS), in this work. All investigations were carried out on a modern AB₂ metal hydride alloy, namely Ti_0.96Zr_0.04Mn_1.43V_0.45Fe_0.08. Surface analysis (SNMS, XPS) shows that long-term air storage (several months) leads to oxide layers about 15 nm thick, with complete oxidation of all main alloy components. By means of in situ oxygen exposure at room temperature and XPS analysis, it can be shown that an oxygen dose of about 100 Langmuirs produces an oxide layer comparable to that after air storage. Manganese enrichment (segregation) is also clearly observed and is theoretically described here. This oxide layer hinders hydrogen absorption, so an activation procedure is necessary in order to use the full capacity of the metal hydride. This procedure consists of heating (T = 120° C) in vacuum and hydrogen flushing at pressures like p = 18 bar. During the activation process the alloy is pulverized to particles of ∼20 μm through lattice stretches. It is shown that this pulverization of the metal hydride (creating clean surface) during hydrogen flushing is essential for complete activation of the material. Re-activation of powder contaminated by small doses of air (p ≈ 0.1 bar) does not lead to full absorption capacity. In ultrahigh vacuum, hydrogen is only taken up by the alloy after sputtering of the surface (which is done in order to remove oxide layers from it), thus creating adsorption sites for the hydrogen. This is shown by TDMS measurements with and without sputtering and oxygen exposure. Figure Investigated metal hydride before and after activation     

1,4,7,10-四氮杂环十二烷:有前途的储氢媒介

运用密度泛函理论(DFT)的B3LYP方法对常见的大环胺类化合物1,4,7,10-四氮杂环十二烷(cyclen)进行结构优化;进而分析了其前线分子轨道和自然键轨道布居(NBO),并确定了吸附的活性点.通过在cyclen的活性点周围放置H2,研究了其储氢性能.结果表明,1,4,7,10-四氮杂环十二烷是一种很有前途的储氢...     

Molecular Magnesium Hydrides

Solid magnesium hydride [MgH₂]_∞ has been pursued as a potential hydrogen‐storage material. Organic chemists were rather interested in soluble magnesium hydride reagents from mid‐20th century. It was only in the last two decades that molecular magnesium hydride chemistry received a major boost from organometallic chemists with a series of structurally well‐characterized examples that continues to build a whole new class of compounds. More than 40 such species have been isolated, ranging from mononuclear terminal hydrides to large hydride clusters with more than 10 magnesium atoms. They provide not only insights into the structure and bonding of Mg?H motifs, but also serve as models for hydrogen‐storage materials. Some of them are also recognized to participate in catalytic transformations, such as hydroelementation. Herein, an overview of these molecular magnesium hydrides is given, focusing on their synthesis and structural characterization.     

Functionalized boranes for hydrogen storage

Using density functional theory, the generalized gradient approximation for the exchange‐correlation potential and Møller–Plesset perturbation theory we study the hydrogen uptake of Li‐ and Mg‐doped boranes. Specifically, we calculate the structures and binding energies of hydrogen molecules sequentially attached to LiB₆H₇, LiB₁₂H₁₃, Li₂B₆H₆, Li₂B₁₂H_12, MgB₆H₆, and MgB₁₂H₁₂. Up to three H₂ molecules can be bound quasi‐molecularly to each of the metal cations with binding energies per H₂ molecule ranging between 0.07 eV and 0.27 eV. The corresponding gravimetric densities lie in the range of 3.49 to 12 wt %, not counting the H atoms bound chemically to the B atoms.     

The first steps of the Li‐B‐H cluster formation

We have theoretically investigated the formation of Li:B:H clusters from standard reactants in gas phase. Molecular electronic structure computations as well as Born‐Oppenheimer molecular dynamics have been carried out to investigate the early stages of the formation of Li:B:H clusters. We describe the thermochemical properties of the different possible reactions and the first stable structures that could initiate the growing process. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

MgH_2储氢热力学研究进展

近年来,材料和能源领域中高能量密度车载储氢材料的研究和开发吸引了世界各国科技工作者的广泛兴趣.MgH2作为一种相对廉价的固体储氢材料,其理论储氢量高达7.6 wt%,且循环吸放氢性能较好,业已成为储氢材料领域的研究热点.本文着重从热力学的角度,对MgH2储氢材料的近期研究进展,特别是其储氢热力学性能的改进,包括纳米化、复合、催化、限域以及理论计算等方面进行简要综述,旨在明确当今MgH2作为潜在可应用储氢材料的研究重点和未来发展趋势.     

Binding of molecular magnesium hydrides to a zirconocene-enyne template

An enyne-zirconium complex stabilizes molecular magnesium hydride (MgH(2) ) and even a molecular hydride, nC(4) H(9) MgH. These systems feature magnesium olefin π?complexation.