Aktivierung kleiner Moleküle mit maßgeschneiderten Rhodium- und Rutheniumkomplexen : experimentelle und computerchemische Untersuchungen (2013)

The activation of small molecules represents a central field of research in homogeneous catalysis. One of the major challenges of catalyst development addresses the utilization of these molecules in an atom economic fashion to avoid waste production and to follow the principles of green chemistry. The utilization of CO2 as a sustainable C1 building block in chemical value chains is a very attractive transformation due to its abundance, non-toxicity and low-cost, but represents a challenging small molecule to activate. Atom economic processes for the valorization of CO2 require a suitable reducing agent, of which H2 offers great potential to reach this goal. Since CO2 as well H2 are very stable molecules, effective catalyst structures have to be developed to facilitate the simultaneous activation of both compounds. In this thesis, different concepts for small molecule activation are presented, with an emphasis on the development of novel atom economic transformations of CO2 and hydrogen transfer reactions. A main theme in this thesis was the application of reactive hydride ligands to achieve the activation of inert small molecules. The hydrocarboxylation of olefins with CO2 and H2 to produce carboxylic acids is an atom economic way of CO2 utilization that has not been previously realized experimentally. The addition of CO2 and H2 to olefins to achieve an overall hydrocarboxylation reaction can proceed via two different routes, an indirect and a direct one. For the indirect hydrocarboxylation the gas mixture of CO2 and H2 is converted to CO and H2O in a reverse water gas shift reaction. The resulting CO and H2O are then used in situ in combination with olefins to synthesize the desired carboxylic acids in a hydroxycarbonylation reaction. This concept was applied in Chapter I of this thesis, in which a systematic modification of the reaction components and an optimization of the reaction conditions made it possible to develop a catalyst system that facilitates the hydrocarboxylation of olefins and alcohols as substrates. In contrast, the direct hydrocarboxylation proceeds without the prior conversion of CO2 to CO, in which CO2 insertion is the crucial elementary step of this pathway. This approach was pursued in Chapter II of this thesis. In the study 38 rhodium pincer complexes were analyzed as potential catalyst structures with computational methods owing to their capability to insert CO2 into Rh-C bonds. The results of this investigation were applied in calculations of complete catalytic cycles for the hydrocarboxylation as well as the most likely occurring site reactions for promising candidates. By doing so, it was possible to identify a functional catalyst lead structure for the selective conversion of olefins, H2 and CO2. While the first two chapters focus on the utilization of CO2 as a raw material, the third and the fourth chapters of the thesis cover different aspects of hydrogen transfer reactions that are relevant for rational catalyst design. In Chapter III a new concept of bifunctionality is presented that has been realized aiming to applications in homogeneous catalysis. The combination of ruthenium hydride complexes with flexible Lewis acid functionalized ligands provided both a hydride donor and a Lewis-base acceptor within one complex, in which cooperative effects and dynamical behavior were obtained. To examine characteristic reactivities and interactions of the hydride ligand with the Lewis acid residue, different derivatives with varying Lewis acid strength were synthesized. Furthermore, the pendant Lewis acid moiety can operate as anchor for Lewis basic substrates in this system. Thereby, the anchored substrates are activated and coordinated in close proximity to the reactive hydride ligand at the same time. A novel type of proton/hydride bifunctionality was observed for the alcohol adducts of these complexes. Bifunctionality is also the central topic in various transfer hydrogenation pathways that have been investigated in Chapter IV of this thesis with another ruthenium complex acting as a catalyst. This specific pincer complex contains both classical and non-classical hydride ligands and for this reason is able to catalyze the transfer hydrogenation of ketones without further addition of base additives. To elucidate the underlying reaction mechanism a detailed DFT study was performed, in which nine complementary reaction pathways were analyzed. By comparison of experimentally determined activation barriers and computationally derived energies it was possible to identify an outer sphere mechanism as the operating reaction pathways of this catalytic transfer hydrogenation.