Development of organometallic pincer complexes and their application in the conversion of organic substrates
Erken, Christina; Leitner, Walter (Thesis advisor); Liauw, Marcel (Thesis advisor)
Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2021
A grand challenge for the industry is the development of sustainable manufactured processes through catalysis and Green Chemistry. Catalysis is the key technology for ecological and economic value creation. It enables the chemical efficient transformation of renewable resources (carbon-based feedstocks) and energy (hydrogen sources) and the production of a broad diversity of materials (base and fine chemicals, pharmaceuticals). In addition to the replacement of fossil resources with renewable materials, the atom economic procedure involves traditional concepts of process efficiency and the focus on chemical yields. A molecularly defined complex is capable of catalysing a chemical reaction through the decrease of the activation energy and the increase of the reaction rate without being consumed. A homogeneous catalyst is beneficial due to the presence of the catalyst and reactant in the same phase, providing a high interaction, higher activities and selectivities, better control of operating conditions as well as the use of mild conditions. The ideal homogeneous catalyst has an optimum in efficiency, selectivity and modularity. Organometallic homogeneous complexes based on noble transition metals are known for their broad applicability in the conversion of organic substrates. In the present thesis, the synthesis and characterisation, as well as the catalytic behaviour, of homogeneous PCP, PNP pincer and PP complexes are reported. The Pd(II)-Anthraphos complexes are investigated in the intermolecular hydroamination of aromatic alkynes with aromatic amines. The cationic complexes lead exclusively to the Markovnikov imine product in most of the cases, enabling good to excellent yields for a broad range of amines and alkynes by using 0.18 mol% catalyst loading, at 90 °C for 4 h under neat conditions. Furthermore, the hydrogenation of esters or carboxylic acids is investigated under the application of the Ir(III)-Anthraphos complexes. These are capable of reducing the acetic acid to the corresponding alcohols at high temperatures (up to over 200 °C) and the esters at moderate temperatures (100 °C) in a green solvent (cyclohexane). In the hydrogenation of ultimately carbon dioxide (CO2), the tested Mn(I)-MACHO complex achieves the reduction to ethyl formate under the assistance of Lewis additives and in a solvent mixture of tetrahydrofuran (THF) and ethanol (EtOH). Finally, the Mn(I)-pincer complex is reported to enable the reduction of a broad range of carboxylic acids using pinacolborane as a reducing agent. The reaction proceeds under relatively mild reaction conditions (115 °C), low catalyst loadings (0.2 mol%) and runs under solvent-less conditions. Mechanistic studies imply that metal-ligand cooperation facilitates substrate activation.