Reduction of Borazines Mediated by Low-Valent Chromium Species
Reduction of Borazines Mediated by Low-Valent Chromium Species
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DOI：
10.1002/anie.201206668
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发表时间：
2012-01-01
影响因子：
16.6
通讯作者：
Szymczak, Nathaniel K.
中科院分区：
文献类型：
--
作者：
Carter, Tyler J.;Kampf, Jeff W.;Szymczak, Nathaniel K.
在过去的十年中，人们已经做出了大量的努力来开发高容量的H2存储材料。[1]低分子量BN材料，例如氨-硼烷（NH3 BH 3; AB），由于其对于化学H2储存的特别高的重量密度（19.6wt%）而受到特别关注。[2]虽然BN材料的脱氢以提供H2已被高度研究[3]，但废BN燃料的低能量再生受到的关注较少[4]，并且仍然是在BN化合物可用作储氢材料之前需要解决的令人垂涎的目标。然而，如实验和计算研究所揭示的，用H2直接氢化废BN燃料受到显著的热力学约束[5，6]，从而限制了再氢化的努力。虽然最近报道了通过替代途径再生废AB燃料的重大进展，[4c，d]这些程序使用高毒性和高能量的还原剂（肼和烷基锡），这对储氢系统的净能量平衡产生了负面影响。因此，允许以高于其相关联的热力学屏障的最小能量成本进行再生的方法是特别感兴趣的。具体地，用衍生自H2的还原剂或离散的H+/乙烯当量氢化废AB燃料是高度期望的。然而，缺乏解决这些系统中还原途径的实验研究，因此这种再生策略的机制是未知的或不明确的。因此，在设计/优化用于再生废AB燃料的催化剂之前，需要详细了解还原当量如何转移到富含BN的分子。在此，我们报告了一个逐步的金属介导的环硼氮烷还原策略，使用的质子供体结合的模拟和氢化物模拟模型的关键反应，这些物种可以促进废AB燃料，使用六甲基环硼氮烷作为原型基板。[7]我们证明了逐步金属介导的B= N键的还原是可实现的，说明了废BN燃料再生的关键步骤（方案1）。由于氢化物等价物可以使用H2和/或还原的金属碎片的质子化产生，这些反应证明了原则上可以通过低能途径进行的还原策略的可行性，这仍然是BN储氢材料再生的挑战。低价铬片段{Cr（CO）3}作为能够结合环硼氮烷并随后介导环的逐步还原的平台。之前对类似芳烃-{Cr（CO）3}加合物的研究表明，氢化物和质子等价物的顺序加成得到环己二烯加合物;[8]我们假设当芳烃被环硼氮取代时，类似的反应性是可能的。选择烷基取代的环硼氮烷作为废AB燃料的合适模型底物，因为质子和烷基B= N键还原的热力学要求相似。[9]此外，烷基环硼氮烷对BN交联不太敏感，并且还表现出低挥发性，这允许更容易处理。选择配合物η6-[（Me 6 B3 N3）Cr（CO）3]（1）进行初始研究，因为它是一种稳定的、易于合成的配合物，与η6-[（H6 B3 N3）Cr（CO）3]密切相关，而η6-[（H6 B3 N3）Cr（CO）3]以前没有被分离为纯物质。虽然之前描述了1，[10]但没有报道结构表征。因此，在对先前报道的方法进行修改后，在69.
The past decade has seen significant effort focused on developing high-capacity H2 storage materials.[1] Low-molecular-weight BN materials, such as ammonia–borane (NH3BH3; AB) have received particular attention because of their exceptionally high gravimetric density (19.6 wt%) for chemical H2 storage.[2] While the dehydrogenation of BN materials to afford H2 has been highly studied,[3] the lowenergy regeneration of spent BN fuels has received less attention [4] and remains a coveted goal that needs to be addressed before BN compounds can be utilized as hydrogen storage materials. However, the direct hydrogenation of spent BN fuels with H2 suffers from significant thermodynamic constraints, as revealed by experimental and computational studies,[5, 6] thus limiting rehydrogenation efforts. Although significant advances were recently reported for the regeneration of spent AB fuels by alternative pathways,[4c, d] these procedures utilize highly toxic and highenergy reductants (hydrazine and alkyl tin hydrides), which negatively impact the net energy balance for the hydrogen storage system. Accordingly, methods that permit regeneration with minimal energy cost above their associated thermodynamic barrier are of particular interest. Specifically, the hydrogenation of spent AB fuels with a reductant derived from H2, or discrete H+/eÀ equivalents, is highly desirable. However, experimental studies to address reductive pathways in these systems are lacking, and thus the mechanism for such regeneration strategies is unknown or ill-defined. Therefore, a detailed picture of how reducing equivalents are transferred to BN-rich molecules is required prior to designing/optimizing catalysts for the regeneration of spent AB fuels. Herein, we report a stepwise metal-mediated borazine reduction strategy that uses hydrides and hydride mimics in conjunction with proton donors to model key reactions that these species could promote on spent AB fuels, using hexamethylborazine as an archetypal substrate.[7] We demonstrate that stepwise metal-mediated reduction of B= N bonds is achievable, illustrating a key step of spent BN fuel regeneration (Scheme 1). Because hydride equivalents can be generated using H2 and/or protonation of reduced metal fragments, these reactions demonstrate the feasibility of a reduction strategy that could in principle proceed through low-energy pathways, which remains a challenge for the regeneration of BN hydrogen storage materials. The low-valent chromium fragment {Cr (CO) 3} was targeted as a platform capable of binding borazine and subsequently mediating the stepwise reduction of the ring. Prior studies on analogous arene–{Cr (CO) 3} adducts showed that the sequential addition of hydride and proton equivalents afforded cyclohexadiene adducts;[8] we hypothesized that similar reactivity would be possible when arenes were replaced with borazines. Alkyl-substituted borazines were selected as suitable model substrates for spent AB fuels because the thermodynamic requirements for protio and alkyl B= N bond reduction are similar.[9] Furthermore, alkyl borazines are less susceptible to BN cross-linking, and also exhibit low volatility, which allow for greater ease of handling. The complex η6-[(Me6B3N3) Cr (CO) 3](1) was selected for initial studies because it is a stable, synthetically tractable complex that is closely related to η6-[(H6B3N3) Cr (CO) 3], which has not been previously isolated as a pure material. Although 1 was previously described,[10] no structural characterization was reported. Accordingly, following a modification of the previously reported procedure,[10b] η6-[(Me6B3N3) Cr (CO) 3] was prepared in 69 …