The family of ferrocene-stabilized silylium ions: synthesis, 29Si NMR characterization, Lewis acidity, substituent scrambling, and quantum-chemical analyses.
The family of ferrocene-stabilized silylium ions: synthesis, 29Si NMR characterization, Lewis acidity, substituent scrambling, and quantum-chemical analyses.
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DOI：
10.1002/chem.201302885
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发表时间：
2013-12
期刊：
影响因子：
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通讯作者：
Kristine Muether;Peter Hrobárik;Veronika Hrobáriková;M. Kaupp;M. Oestreich
中科院分区：
文献类型：
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作者：
Kristine Muether;Peter Hrobárik;Veronika Hrobáriková;M. Kaupp;M. Oestreich
本研究的目的是深入了解二茂铁稳定的硅鎓离子中硅原子上取代基的独特成键情况，并了解决定这些强刘易斯酸的（29）Si NMR化学位移和亲电性的结构参数。为此，二茂铁稳定的硅阳离子家族的10个新成员，制备了从硅烷与三苯甲基阳离子的氢化物提取反应，并通过多核（1）H和（29）Si NMR光谱进行表征。仔细观察NMR光谱发现，额外的次要信号组不是杂质，而是取代模式不同于最初形成的阳离子的甲硅烷基离子。这些信号的仔细分配提供了实验证据，空间位阻较小的甲硅烷基离子能够与未反应的硅烷前体交换取代基。密度泛函理论的计算提供了机制的洞察，取代基转移，其中迁移基团之间交换的两个硅片段在一个协调的过程中，涉及二茂铁桥接的中间体。此外，对（29）Si NMR化学位移的量子化学分析表明，δ（29）Si值与Si阳离子子集的Fe···Si距离之间存在线性关系。的电子定位功能和电子定位性指标分析表明，铁，硅，和C '（ipso）原子之间的三个中心的两个电子键合吸引子，清楚地区分相应的碳正离子和硼烷的硅阳离子。（29）Si NMR化学位移和刘易斯酸性之间的相关性，根据氟离子亲和力进行评估，仅见于硅离子的子集，有时具有非直观的趋势，表明空间效应和电子效应对Fe···Si相互作用程度的复杂相互作用。
The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene-stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters that determine the (29)Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene-stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and characterized by multinuclear (1)H and (29)Si NMR spectroscopy. A closer look at the NMR spectra revealed that additional minor sets of signals were not impurities but silylium ions with substitution patterns different from that of the initially formed cation. Careful assignment of these signals furnished experimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanistic insight into that substituent transfer in which the migrating group is exchanged between two silicon fragments in a concerted process involving a ferrocene-bridged intermediate. Moreover, the quantum-chemical analysis of the (29)Si NMR chemical shifts revealed a linear relationship between δ((29)Si) values and the Fe···Si distance for subsets of silicon cations. An electron localization function and electron localizability indicator analysis shows a three-center two-electron bonding attractor between the iron, silicon, and C'(ipso) atoms, clearly distinguishing the silicon cations from the corresponding carbenium ions and boranes. Correlations between (29)Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non-intuitive trends, indicating a complicated interplay of steric and electronic effects on the degree of the Fe···Si interaction.