Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The Chinese University of Hong Kong , Shatin, New Territories, Hong Kong, People's Republic of China.
Acc Chem Res. 2014 May 20;47(5):1623-33. doi: 10.1021/ar500091h. Epub 2014 Apr 29.
Carboranes are a class of polyhedral boron hydride clusters in which one or more of the BH vertices are replaced by CH units. Their chemistry has been dominated by 12-vertex carboranes for over half a century. In contrast, knowledge regarding supercarboranes (carboranes with more than 12 vertices) had been limited merely to possible cage geometries predicted by theoretical work before 2003. Only in recent years has significant progress been made in synthesizing supercarboranes. Such a breakthrough relied on the use of Carbon-Atoms-Adjacent (CAd) nido-carborane dianions or arachno-carborane tetraanions as starting materials. In this Account, we describe our work on constructing and elucidating the chemistry of supercarboranes. Earlier attempted insertions of the formal BR unit into Carbon-Atoms-Apart (CAp) 12-vertex nido-7,9-C2B10H12 did not produce the desired 13-vertex carboranes. Such failure is often attributed to the extraordinary stability of the B12 icosahedron (the "icosahedral barrier"). However, the difference in reducing power between CAp and CAd 12-vertex nido-carborane dianions had been overlooked. Our results have shown that CAd nido-carborane dianions are weaker reducing agents than the CAp isomers, allowing a capitation to prevail over a redox reactivity. This finding provides an entry point to the synthesis of supercarboranes and a series of 13- and 14-vertex closo-carboranes have been prepared and structurally characterized. They share some chemical properties with those of 12-vertex carboranes; on the other hand, they have their own unique characteristics. For example, a 13- vertex closo-carborane can undergo single electron reduction to give a stable carborane radical anion with [2n + 3] framework electrons, which can accept one additional electron to form a 13-vertex CAd nido-carborane dianion. 13-Vertex closo-carborane can also react with various nucleophiles to afford the cage carbon and/or boron extrusion products closo-CB11(-), nido-CB10(-), closo-CB10(-), and closo-C2B10, depending on the nature of the nucleophiles. Studies of supercarboranes remain a relative young research area, particularly in comparison to the rich literature of icosahedral carboranes with 12-vertices. Other supercarboranes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further these efforts.
碳硼烷是一类多面体硼氢簇合物，其中一个或多个 BH 顶点被 CH 单元取代。它们的化学性质在半个多世纪以来一直由 12 个顶点的碳硼烷主导。相比之下，在 2003 年之前，关于超碳硼烷（具有超过 12 个顶点的碳硼烷）的知识仅仅局限于理论工作预测的可能笼状几何结构。直到最近几年，在合成超碳硼烷方面才取得了重大进展。这一突破依赖于使用碳原子相邻（CAd）nido-碳硼烷二负离子或蛛碳硼烷四负离子作为起始材料。在本报告中，我们描述了我们在构建和阐明超碳硼烷化学方面的工作。早期尝试将BR单元插入到碳原子分离（CAp）12 个顶点 nido-7,9-C2B10H12中，并没有产生预期的 13 个顶点碳硼烷。这种失败通常归因于 B12 二十面体（“二十面体障碍”）的非凡稳定性。然而，CAp 和 CAd 12 个顶点 nido-碳硼烷二负离子之间还原能力的差异被忽视了。我们的结果表明，CAd nido-碳硼烷二负离子是比 CAp 异构体较弱的还原剂，允许加帽取代氧化还原反应。这一发现为超碳硼烷的合成提供了一个切入点，一系列 13 个和 14 个顶点的闭碳硼烷已经被制备和结构表征。它们与 12 个顶点碳硼烷具有一些共同的化学性质；另一方面，它们也有自己独特的特点。例如，一个 13 个顶点的闭碳硼烷可以经历单电子还原，形成具有[2n+3]框架电子的稳定碳硼烷自由基阴离子，它可以接受一个额外的电子，形成 13 个顶点的 CAd nido-碳硼烷二负离子。13 个顶点的闭碳硼烷还可以与各种亲核试剂反应，生成笼状碳和/或硼挤出产物 closo-CB11(-)、nido-CB10(-)、closo-CB10(-)和 closo-C2B10，具体取决于亲核试剂的性质。超碳硼烷的研究仍然是一个相对年轻的研究领域，与具有 12 个顶点的富勒烯碳硼烷的丰富文献相比尤其如此。随着该领域的发展，预计还会制备和结构表征其他超碳硼烷，这里详述的结果将进一步推动这方面的工作。
