Evidence for a Single Electron Shift in a Lewis Acid-Base Reaction
Evidence for a Single Electron Shift in a Lewis Acid-Base Reaction
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DOI：
10.1021/jacs.8b09214
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发表时间：
2018-11-14
影响因子：
15
通讯作者：
Mueller, Thomas
中科院分区：
文献类型：
--
作者：
Dong, Zhaowen;Cramer, Hanna H.;Mueller, Thomas
报道了一种亲核的半茂烯基锗烯与三-五氟苯硼烷(B-(C6F5)(3))之间的路易斯酸碱反应，以几乎定量的收率得到了常规的硼锗键产物。该反应异常缓慢，在反应过程中，可通过EPR和紫外-可见光谱检测到自由基中间体。这表明反应是由一个电子转移步骤引起的。本文所涉及的锗自由基阳离子是由三烷基阳离子或强硅基路易斯酸氧化锗二甲苯独立合成的。对一种含自由基阳离子的全氟四芳基硼酸盐进行了全面表征，包括XRD分析。其结构特征和DFT计算结果表明，自由基阳离子是由配体到铪原子的氧化还原诱导电子转移（RIET）形成的以铪（III）为中心的自由基。这种价异构化减缓了自由基的耦合，从而形成极性路易斯酸碱产物。根据最近的发现，自由基对是在受挫的路易斯对中形成的，简要地讨论了这一观察的含义。
The Lewis acid base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B-(C6F5)(3)) to give the conventional B Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV-vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid-base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs.