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Abstract

The use of silicon compounds in challenging molecular transformations usually requires their activation by more reactive species, or a preceding transformation in reactive cationic or low-valent states. Only in the second half of the past decade, bis(perhalocatecholato)silanes (1X, X = F, Cl, Br) were reported as the first silane Lewis superacids – incorporating silicon in a neutral form and its natural oxidation state. Still, this burgeoning substance class suffers from drawbacks attributed to required donorcoordination, self-aggregation, poor solubility, or labile substituents. The present contribution describes strategies toward a second-generation of neutral silicon Lewis superacids exhibiting improved properties and an enhanced reactivity. First, a heuristic structure-effect relation between steric modification of the ligand and the final composition of the silicon species is derived by means of suitable model systems. The found relation is applied in the rational design of electron-withdrawing ligands, ultimately resulting in the new representatives bis(tetra(trifluoromethyl)catecholato)silane (1CF3) and bis(nonafluoro-N-phenyl-ortho-amidophenolato)silane (2). Both show an increased reactivity in comparison to their structural predecessors 1X, with no indication on the detrimental self-aggregation. Computations and experiments underlined that 1CF3 ranks among the strongest neutral Lewis acids currently accessible in the condensed phase. It thus enabled catalytic transformations that have never been mediated by a neutral silane. In cooperative action with 1,2,2,6,6-pentamethylpiperidine (pmp), the enhanced properties of 2 allowed the isolation of a hydridosilicate directly synthesized from H2 for the first time. This singularity provoked a first examination on the role of a tetrahedrally coordinated Lewis acid in the H2 cleavage as hallmark FLP reaction. Moreover, spontaneous, FLP type C–H silylations with 2/pmp leading to anionic silicates are described, which are reversible upon addition of a silaphilic donor. The thermodynamic stability of the silicates incited the assignment of an activation attribute as merely context dependent. Tangentially, a protocol for a catalytic C−C bond formation between N-heterocycles and acrylonitrile was derived. Overall, this work documents the guided evolution of the advancement of neutral, silicon Lewis superacids, now with an extended reactivity portfolio including more challenging bond activations. The here presented findings contribute to the fundamental understanding of the molecular chemistry of silicon – the second most abundant element in the earth’s crust.