Substituentenvermittelte Reaktivitätsmodulation von aminosubstituierten Silizium(IV)-Verbindungen

Abstract

The topic of this work is the synthesis and properties of silanes with amino substituents and their corresponding silylium ions. In the first part of this thesis, the first aminosilylium ion was synthesized by hydride abstraction of (Me2N)3SiH. The dictionic ((Me2N)3Si+)2 could be fully characterized in solution and in the solid state. The compound can be described as the dimeric form of the silaguanidinium. Different factors that influence the dimerization process could be determined by suitable theoretical methods. In comparison to silylium ions with aliphatic substituents, the aminosilylium ion shows higher stability and compatibility with common organic solvents. Nevertheless, the dication is able to perform electrophilic, aromatic Friedel-Crafts silylation reactions with electron-rich, aromatic compounds or to initiate hydrodefluorination reactions. In the presence of acetonitrile, an insertion of the nitrile in one of the Si-N-bonds was achieved and the structure was characterized in the solid state. The synthesis of more sterically demanding, monomeric aminosilylium ions and the challenges around this are described in detail. In the second part of this thesis, the influence of aromatic triamido (NNN3−) and amidodiphenolato (ONO3−) substituents of the resulting silanes was investigated. The enforced geometrical strain resulted in a higher levels of Lewis acidity and thus dimerization of the silicon compounds. By installing substituents with different sterical and electronical properties around silicon, it was possible to influence the dimerization process on purpose. The species could be grouped in monomeric, structural-reversible and dimeric compounds. The high Lewis acidity resulting from the pre-organisation of the substituent was evident from the deviation from tetrahedral geometry and the planarization of the compounds in the solid state structures. Different factors which contribute to the dimerization process were determined by theoretical methods and their influence was explained in detail. For effective dimerization, the dispersion interactions of the substituents, low deformation energy at silicon, electrostatic attraction between silicon and nitrogen and a special σ-bond resonance phenomenon were identified as the main factors. The obtained results suggest unique stabilising effects and can be applied for more general conclusions about aggregation of amphiphilic species and Lewis acidity at silicon. The reactivity of the compounds towards different substrates was investigated with help of a model compound. In the third part, a silylium ylidene-like valence isomer will be synthesised in due course. With help of the redox-active substituent, a silylium ylidene compound was obtained via charge redistribution. The combination of the Lewis basic lone pair and the highly electrophilic silicon cation resulted in extraordinary reactivity. Although it was not possible to isolate the silylium ylidene compound, different adducts of the silylium ion with donor molecules could be obtained and its exceptionally high Lewis acidity could be proven by the Gutmann-Beckett method. By investigating secondary reaction products, the silylium ion showed a strong fluoride ion affinity in presence of SbF6− and a particular reactivity towards the weakly coordinating anion B(C6F5)4−.