nido-five-vertex clusters:: In and out of boron chemistry
nido-five-vertex clusters:: In and out of boron chemistry
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DOI：
10.1002/anie.200301648
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发表时间：
2003-01-01
影响因子：
16.6
通讯作者：
Bertrand, G
中科院分区：
文献类型：
--
作者：
Canac, Y;Bertrand, G
在硼烷化学的早期，nido-pentaborane B5H9 (1a) 被称为“稳定的五硼烷”，而 arachno-B5H11 被称为“不稳定的五硼烷”。 [1, 2] 由于 BH 顶点与 CH 单元以及裸露的 P 片段是等瓣的，因此可以预期各种 carba-、phospha-、phosphacarba-nido-pentaborane 或甚至全碳或全磷类似物也很容易获得。[3]事实上，第一个碳硼烷笼，即 1, 2-C2B3H7 (1b)，与 B5H9 等电子且等结构，早在 1970 年就由 Grimes 和同事发现。 [4]该化合物是通过 B4H10 与乙炔的反应以低产率 (10%) 制备的（方案 1）。化合物 1b 在高达 508°C 的气相中保持稳定，但在室温下在液相中数分钟内发生不可逆聚合。 1b 的 1, 2-二碳-硝基-戊硼烷结构最初是根据 IR、11B 和 1H NMR 光谱以及质谱数据确定的。 1b 的微波研究结果虽然在 1972 年被宣布为私人通讯，[5] 最终于 1988 年发表[6a]，并于 1998 年报告了气相电子衍射数据和从头计算限制的旋转常数的综合分析。[6b] 直到 2002 年，第二个也是唯一一个已知的具有 Nido-5 顶点几何形状的杂硼烷被报道，即磷碳酰氨基戊硼烷 1c（方案 2）。[7]值得注意的是，1c 的合成路线与 1b 的路线非常相似，产率为 15%（方案 1）。 Greatrex 等人代替了炔烃。他们使用了磷炔，并且还在708℃的气相中进行了反应。与二碳硼烷1b一样，磷碳硼烷1c仅在气相中稳定，并且在室温下在液态下分解。巢结构已通过多核核磁共振波谱和质谱确定。优化的几何形状是在 MP2/6-31G* 级别上计算的。在离开硼化学之前，值得一提的是，最近通过扭曲的菱形四硼烷的硼氢化反应制备了第一个具有方锥体结构的中性闭硼烷，化合物2（方案3）。 [8]该化合物似乎具有热稳定性（mp 1138C，分解），并且已通过单晶 X 射线衍射研究进行了充分表征。如上所述，原则上没有理由将巢五顶点结构限制于含硼化合物。事实上，在 1972 年，Stohrer 和 Hoffmann [9] 提出，反芳香族 [C5H5]+ 离子 3 (R= H) 不会保持平面环戊二烯基结构，而是重新排列成如 1d 所示的方形锥体几何结构（方案 4），这将是独特的稳定结构。尽管最近的计算表明这种说法是不正确的，[10]人们应该承认迄今为止，除了具有几个强给电子取代基的衍生物外，[11][C5R5]+
In the early days of borane chemistry the nido-pentaborane B5H9 (1a) was called the “stable pentaborane” whereas arachno-B5H11 was called the “unstable pentaborane”.[1, 2] Since a BH vertex is isolobal with a CH unit as well as a naked P fragment, one could have expected that a variety of carba-, phospha-, phosphacarba-nido-pentaboranes or even all-carbon or all-phosphorus analogues would be readily obtained.[3] Indeed, the first carborane cage, the 1, 2-C2B3H7 (1b), isoelectronic and isostructural with B5H9 was discovered by Grimes and co-workers as early as 1970.[4] This compound was prepared in low yield (10%) from the reaction of B4H10 with acetylene (Scheme 1). Compound 1b was stable in the gas phase up to 508C, but irreversibly polymerizes within minutes in the liquid phase at room temperature. The 1, 2-dicarba-nido-pentaborane structure of 1b was originally assigned from IR, 11B and 1H NMR spectroscopy, and mass spectrometry data. The results of a microwave study of 1b, although announced as a private communication in 1972,[5] were finally published in 1988 [6a] and in 1998 a combined analysis of gas-phase electron-diffraction data and rotation constants restrained by ab initio calculations was reported.[6b] It was only in 2002 that the second and only other known heteroborane with a nido-five-vertex geometry was reported, namely the phosphacarba-nido-pentaborane 1c (Scheme 2).[7] Of note is that the synthesis of 1c, in 15% yield, was by a route very similar to that which affords 1b (Scheme 1). Instead of an alkyne, Greatrex et al. used a phosphaalkyne, and they also performed the reaction in the gas phase at 708C. Like dicarbaborane 1b, the phosphacarbaborane 1c is only stable in the gas phase and decomposes in the liquid state at room temperature. The nido-structure has been assigned from multinuclear NMR spectroscopy and mass spectrometry. The optimized geometry has been calculated at the MP2/6-31G* level. Before leaving boron chemistry, it is worth mentioning that the first neutral closo-borane featuring a square-pyramidal structure, compound 2, has recently been prepared by hydroboration of a distorted diamond-shaped tetraborane (Scheme 3).[8] This compound appears to be thermally quite stable (mp 1138C, decomposition) and has been fully characterized including a single-crystal X-ray diffraction study. As mentioned above, there is, in principle, no reason to restrict the nidofive-vertex structure to boron-containing compounds. Indeed in 1972 Stohrer and Hoffmann [9] suggested that the antiaromatic [C5H5]+ ion 3 (R= H) does not maintain the planar cyclopentadienyl structure, but rearranges to a square-pyramidal geometry as in 1d (Scheme 4), which would be the unique stable structure. Although recent calculations have shown that this statement was not correct,[10] one should admit that to date, with the exception of derivatives with several strongly electron-donating substituents,[11][C5R5]+