The Family of Ferrocene-Stabilized Silylium Ions: Synthesis, 29Si NMR Characterization, Lewis Acidity, Substituent Scrambling, and Quantum-Chemical Analyses

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 (SiNMR)-Si-29 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)Hand Si-29 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 (SiNMR)-Si-29 chemical shifts revealed a linear relationship between (Si-29) values and the FeSi 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 (SiNMR)-Si-29 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 FeSi interaction.