Frustrated Lewis Pairs - Quantum Chemical Investigation of Structures, Reactions and Mechanisms

Frustrated Lewis Pairs (FLPs) consist of a Lewis acid and a Lewis base which are  prevented from forming a classical adduct by their large substituents. The electric eld between both partners allows to polarise or even split bonds leading to a plethora of possible reactions, as the following scheme shows: The reactions marked with a * in the above scheme will be presented in this thesis. After a short overview of the quantum chemical methods used in this thesis several studies are presented - some purely theoretical, but most of them cooperations between theoretical and experimental chemistry. Two methodological studies concentrate on the applicability of DFT for the calculation of hydrogen splitting in the electric eld generated by an FLP and give a tutorial on how to generate accurate thermochemical results.Spectroscopic properties like IR-shifts are discussed with the help of the example of H2-activation. To the author's knowledge, this is the first systematic study of the accuracyof DFT for the calculation of H2-IR-shifts. Further studies on the activation of H2 present a series of FLP-systems which activate H2 already at extremely low temperatures of down to -60°C and discuss the relation between their reactivity, stability and basicity.The following chapter concentrates on the activation of other gaseous substrates like thegreenhouse gases CO2 and SO2, which can be turned into highly useful building blocks, as well as the activation of NO with an intramolecular FLP, which leads to a new family of persistent N-oxyl-radicals with a higher reactivity than their famous relative TEMPO. The quantum chemical calculations presented here were able to confirm the structure, explain the high reactivity of the product and also helped to estimate if the reactivity of the radical can be tuned by exchanging the substituents on the bridging C2-unit of theFLP. A similar reaction behaviour can be observed for the two substrates CO and R-CN when reacted with intramolecular FLPs. Here the presented calculations could explain why t-Bu-isonitriles form a different addition product than n-Bu-isonitriles. The last chapter of this thesis focusses on the activation of carbon-carbon double and triple bonds, which are conjugated with each other or with a carbonyl group. Activation reactions of diynes, enynes and ynones reveal that the 1,4-addition to intramolecular FLPs occurs under kinetic reaction control while for intermolecular FLPs it happens under thermodynamic reaction control. The corresponding 1,2-addition products are rarely found in cases with especially small substituents on the substrates. In this context it was also possible to transfer hydrogen to a C-C-triple bond. The results of the conformational study of the reaction products could later be confirmed by NMR-spectroscopic results. In the last study two non-conjugated C-C triple bonds could be activated via a complex reaction mechanism including the formation of an intermediate FLP. Some of the intermediate structures were not accessible spectroscopically and could only confirmed by the presented calculations.