Florian S. Tschernuth

The germanium-centred Lewis superacid Ge(pin^F)₂ (1) was isolated as acetonitrile mono-adduct 1·MeCN and thoroughly characterized by methods including X-ray crystallography and quantum chemical calculations.

The emerging field of Lewis acidic silanes demonstrates the versability of molecular silicon compounds for catalytic applications. Nevertheless, when compared to the multifunctional boron Lewis acid B(C₆F₅)₃, silicon derivatives still lack in terms of reactivity. In this regard, we demonstrate the installation of perfluorotolyl groups (Tol^F) on neutral silicon atoms to obtain the respective tetra‐ and trisubstituted silanes Si(Tol^F)₄ and HSi(Tol^F)₃. These compounds were fully characterized including SC‐XRD analysis but unexpectedly showed no significant Lewis acidity. By using strongly electron‐withdrawing perfluorocresolato groups (OTol^F) the tetrasubstituted silane Si(OTol^F)₄ was obtained, bearing an 8 % increased Δδ(³¹P) shift when applying the Gutmann‐Beckett method, compared to literature‐known Si(OPh^F)₄. Ultimately the heteroleptic Si(Ph^F)₂pin^F was successfully synthesized and fully characterized including SC‐XRD analysis, introducing a highly Lewis acidic silicon atom holding two silicon‐carbon bonds.

The exceptional catalytic activity of a neutral Si(IV) Lewis superacid was highlighted on the degradation of oligo‐ and polyethers via ring closing metathesis. Model reaction experiments with diglyme and 1,5‐dimethoxypentane outlined a significantly increased reactivity surpassing comparable Lewis acid catalyst presented in the literature. The high observed activity was explained by the kinetic effect of the bulky perfluorinated substituents, blocking a second coordination site that is responsible for catalyst deactivation by substrate chelation. Further experiments with aliphatic oligoethers outlined a gradual activity loss of the catalyst due to mechanistic disparities favoring side‐reactions when more than one internal product unit is present. The extraordinary affinity for the cleavage of etheric C−O bonds was ultimately demonstrated by the reaction with diethylether, producing a pentavalent silicate species that was fully characterized including SC‐XRD analysis.

Despite the earth abundance and easy availability of silicon, only few examples of isolable neutral silicon centered Lewis superacids are precedent in the literature. To approach the general drawbacks of limited solubility and unselective deactivation pathways, we introduce a Lewis superacid, based on perfluorinated pinacol substituents. The compound is easily synthesized on a gram‐scale as the corresponding acetonitrile mono‐adduct 1⋅(MeCN) and was fully characterized, including single crystal X‐ray diffraction analysis (SC‐XRD) and state‐of‐the‐art computations. Lewis acidity investigations by the Gutmann‐Beckett method and fluoride abstraction experiments indicate a Lewis superacidic nature. The challenging Si−F bond activation of Et₃SiF is realized and promising catalytic properties are demonstrated, consolidating the potential applicability of silicon centered Lewis acids in synthetic catalysis.

Obwohl Silicium ein einfach zugängliches und häufig vorkommendes Element der Erdkruste ist, gibt es bisher nur wenige Beispiele isolierbarer Lewis‐Supersäuren auf der Basis von neutralen Siliciumverbindungen. Um Löslichkeitsproblemen und unselektiven Deaktivierungsmechanismen entgegenzuwirken, stellen wir in dieser Arbeit eine Lewis‐Supersäure mit perfluorierten Pinakolat‐Substituenten vor. Die Titelverbindung wird auf einfachstem Weg im Gramm‐Maßstab als das Acetonitril‐Monoaddukt 1⋅(MeCN) synthetisiert und inklusive modernster Berechnungen vollständig charakterisiert. Untersuchungen der Lewis‐Azidität mittels Gutmann‐Beckett‐Methode und weiterführende Experimente zur Fluorid‐Abstraktion bestätigen den Charakter einer Lewis‐Supersäure. Die Aktivierung der Si−F‐Bindung von Et₃SiF sowie vielversprechende Katalyse‐Experimente demonstrieren das hohe Potenzial Silicium‐basierter Lewis‐Säuren in der katalytischen Synthese.