Experimentelle und computerchemische Untersuchungen von Ruthenium-Benzoat-Komplexen in der molekularen Katalyse

• Experimental and computational investigations of ruthenium-benzoate-complexes in molecular catalysis

Rohmann, Kai Christian; Leitner, Walter (Thesis advisor); Okuda, Jun (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen, 2016

Abstract

The dissertation faces the question, if modern computational methods are capable of acting as a predictive tool for the optimization and the design of homogeneous catalysts. To answer this question, different homogeneously catalyzed transformations have been investigated in combined studies of experimental work and density functional theory(DFT)-calculations.In the first chapter, an elementary homogeneously catalyzed reaction, namely the hydrogenation of olefins, was selected to classify the the reliability of DFT-calculations in a benchmark-study with regard to the reproduction of energy spans in homogeneous catalysis. For this purpose, two new ruthenium-Xantphos-benzoate-complexes (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) were synthesized, characterized and used as catalyst precursors. It could be shown, that lots of the tested density functionals are able to reproduce energy spans for transition-metal-catalyzed-olefin- hydrogenations with a high reliability and deviations of not more than 1-3 kcal mol-1. Concerning reliability and calculation time, the functionals MN12-L, M06-L and B97-D3BJ showed the best performance.In the second chapter, these results were applied to investigate the reaction of carbon dioxide (CO2) with hydrogen (H2) to formic acid both computationally and experimentally. Therefore, a new ruthenium-Acriphos-benzoate-complex (Acriphos = 4,5-bis(diphenylphosphino)acridine) was synthesized, characterized and used as a catalyst precursor. The Acriphos-complex turned out to be a highly active and productive catalyst for the hydrogenation of CO2 to formic acid in presence of bases. In subsequent studies it could be shown, that the complex also catalyzes the hydrogenation to formic acid in absence of bases, when the reaction medium DMSO/H2O is used. At higher reaction temperatures compared to the formic acid synthesis, the complex is also able to catalyze the hydrogenation of CO2 to methanol.The last chapter of the dissertation deals with the carboxylation of arene-C-H-bonds with CO2. The ruthenium-Acriphos-benzoate-complex, which was already used in the second chapter, was applied as a potential catalyst. It turned out, that the carboxylation of arenes with the complex does not seem to be feasible under the tested reaction conditions. The reason is probably, that the activation barrier is in a range where the experimental feasibility is not possible with reaction conditions that are common for homogeneously catalyzed reactions. In summary, the dissertation shows, that with modern DFT-methods, reliable predictions for the activity of molecular catalysts can be made. The reproduction of activation barriers is possibile with a high reliability, which could be shown using the example of transition-metal-catalyzed-hydrogenations. This may open the opportunity for an in-silico-catalyst design in the future.