# Development of molecular transition-metal catalysts for the reverse water-gas shift reaction and the selective transformation of carbon dioxide and hydrogen to formic acid esters and methanol

• Entwicklung molekularer Übergangs-Metall Katalysatoren für die Reverse Wasser-Gas Shift Reaktion und die selektive Umwandlung von Kohlenstoffdioxid und Wasserstoff zu Ameisensäureestern und Methanol

The present dissertation focuses on the combined utilization of carbon dioxid and molecular hydrogen catalyzed by molecular transition metal complexes. Chapter 1 gives a general overview on the challenges associated with the substrates molecular hydrogen and carbon dioxide. Furthermore, the academic and industrial interests and the current state of the art in homogeneous catalysis are described. Chapter 2 centers on the reduction of carbon dioxide with molecular hydrogen via the formation of formato intermediate species, leading to the synthesis of valuable, liquid C1-building blocks such as methyl formate (MF), dimethoxy methane (DMM) and methanol (MeOH). Chapters 2.1 and 2.2 describe the state of the art and development of versatile [Ru(E-triphos $^{deriv}$.)(tmm)] (E = C, N, Si, P) complexes, identified as highly active in carbon dioxide hydrogenation to C1-building blocks. The complexes were applied in catalysis in the upcoming chapters 2.3, 2.4 and 2.5, addressing the production of MF, DMM and MeOH from carbon dioxide, respectively. For the selective synthesis of MF, a novel Ru complex based on a sterically demanding and electron donating ligand was explored. Novel Triphos-analogue Ru catalysts for both transformations of carbon dioxide and hydrogen to either DMM or MeOH could be developed. In chapter 2.6, the first transfer hydrogenation of carbon dioxide to MeOH, utilizing alcohols as liquid hydrogen carrier, is illustrated. The catalytic performance of selected [Ru(E-triphos $^{deriv}$.)(tmm)] catalysts is evaluated and correlated to structural parameters. In chapter 2.7, the obtained results on Ru catalyzed carbon dioxide hydrogenation are compared and put into context. Furthermore, the generation of a dicarbonyl complex was found to be a unique deactivation pathway. Several approaches for a reactivation were investigated and a new route to convert the resting state in a more productive form was established. Chapter 3 portrays the carbon dioxide reduction via the formation of hydroxycarbonyl complexes, resulting in the formation of carbon monoxide and water in the reverse water-gas shift reaction (rWGSR). For the efficient, low temperature rWGSR, two catalyst systems, comprising molecular Pd- and Ni-pincer complexes, could be developed and were explored in detail in sections 3.2 and 3.3. The key step leading to high catalytic productivity could be identified and mechanistically correlated to a novel metal-ligand cooperation.