Synthesis of transition metal complexes for catalytic application in green energy storage

This thesis describes the preparation of various PYE- and triazolylidene-based ligands, their coordination to transition metals and the application of those systems in challenging catalysis. The Introduction outlines the general role of transitionmetal-catalysis in relation to sustainable energy storage and fuel production. Tuning of homogeneous catalysts via specific ligand design is discussed. Concepts of cooperative ligands, such as bifunctionality and redox non-innoncence, as well as electronically flexible ligands are introduced. A new ruthenium complex containing a pyridylidene amine-based NNN ligand is explored. It was applied as a catalyst precursor for formic acid dehydrogenation where it showed high activity (TOF ~10,000 h-1) even in the absence of basic additives. Mechanistic investigations using correlation of UV-vis and NMR spectroscopic changes with gas evolution profiles indicate rapid and reversible protonation of the central nitrogen of the NNN ligand as key step of catalyst activation, followed by an associative step for formic acid dehydrogenation. Based on a Ruthenium(II)-arene complex, featuring a tridentate PYE-Amide-Quinoline ligand, a series of derivative complexes, which were systematically modified, were explored in formic acid dehydrogenation. The choice of ancillary arene was shown to be vital for catalyst longevity under catalytic conditions. Furthermore, the essential role of the centrally coordinated amide was demonstrated through substantially lower catalytic activity after protonation or methylation of this position. The introduction of an electron withdrawing CF3 group on the ligand backbone almost tripled the maximum TOF to 27000 h-1 making it the most active ruthenium complex to date in the absence of additives. An iridium triazolylidene complex was heterogenized and applied in water oxidation. Rational ligand design allowed postmodification of the complex and integration into a self-supporting polymer. Preliminary catalytic runs showed equal activity of unsupported and heterogenized complex. Analysis of the aqueous solution showed leaching of iridium into the reaction media, which was attributed to instability of the polymeric backbone. A phenoxy-substituted PYE ruthenium complex was investigated. In the solid state and in non-coordination solvents (CD2Cl2) the complex is present as a phenolate-bridged dimer but suitable ancillary ligands were shown to coordinate to the ruthenium centers and lead to formation of monomeric complexes. The redox behaviour of the complex was investigated by spectroelectrochemical techniques which showed two reversible redox events with distinct color changes. EPR analysis indicates that the first oxidation event is mainly localized on the ligand.

2023

dissertation

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