Frustrated Lewis-Pairs with Silicon, Germanium and Tin as Lewis-acidic Functions

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The concept of frustrated Lewis acid base pairs (FLP) is based on the interaction between Lewis acids and bases, which are prevented from the formation of a classical adduct. The resulting unquenched reaction potential can be used to bind small molecules, cleave or activate them, e.g. for the heterolytic cleavage of hydrogen or for the activation of carbon dioxide and other greenhouse gases. In this way novel reactions and catalytic processes have become possible. Most of the known FLP chemistry is based on the elements boron and phosphorus, i.e. boranes and phosphanes as acids and bases. Other elements [acids: Al, Zn, P (V), Si+, bases: N, O] are much less explored. With the presentation of a first uncharged intramolecular silicon-phosphorus FLP, (C2F5)3SiCH2P(t-Bu)2, we were recently able to apply our long-standing expertise in the field of geminal donor-acceptor compounds successfully to the modern developments of FLP chemistry. The molecular chemistry of silicon is easily accessible because of its immense industrial importance; we now want to make use of this advantage for FLP chemistry.For this purpose, this project aims at generating novel intramolecular FLP systems of the type (RF)3SiCH2P(R)2, with RF representing electronegative (e. g. fluorinated) alkyl or aryl groups (z. B. C6F5, C5NF4, etc.) and R representing alkyl or amine functions (t-Bu, CH(SiMe3)2, NR'2, etc.). The length and type of the chain between the Si and P atoms will be varied. In addition to silicon germanium and tin will be used to offer softer binding partners in terms of the HSAB concept for the activation of small molecules that are thus bound more weakly and hence in terms of catalytic processes more easily removable. The concept to develop neutral Si-FLP systems will be transferred in a parallel subproject to intermolecular systems with separate acid and base molecules. The acids are compounds such as (RF)4Si, and the bases also include super basic systems such as phosphatranes. The resulting FLP systems will be tested for their activity of binding, activating small molecules and for their catalytic ability (e.g. hydrogenation of unsaturated substrates). Experiments with para-hydrogen will contribute the mechanistic insights. Advanced structural investigations include gas phase electron diffraction measurements for free molecule geometries and experimental charge density studies to shed more light on the bonding situation in more detail.