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This Thesis describes the synthesis and structural analysis of bio-inspired iron and manganese complexes used for the catalytic oxidation of olefin substrates. The development of catalytic systems for oxidation chemistry that are based on first row transition metals and that apply a green oxidant like hydrogen peroxide is an ongoing research topic in the fields of homogeneous catalysis, bioinorganic chemistry, and green chemistry. For the inspiration in the design of such systems, biological oxidation reactions that involve a metallo-enzyme are used as the starting point. The initial design of the ligand used in the catalytic system is based on the active site structure of one particular or of a family of metallo-enzymes. Further catalyst development and optimization is then carried out by ligand variation and reaction parameter optimization. This bio-inspired approach in catalyst design may on the one hand lead to discrete structural analogues of the active site of a metallo-enzyme (structural modeling) and may on the other hand lead to the development of efficient transition metal catalysts that are of use to organic synthesis (functional modeling). The first part of this thesis is dedicated to a study on manganese complexes derived from mixed N,O ligands, while the second part focuses on iron complexes based on related, yet different mixed N,O ligands. The interest in the use of mixed N,O ligands in the development of bio-inspired oxidation catalysts stems from the structure and activity of a class of mono-nuclear non-heme iron enzymes that feature a so-called 2-His-1-carboxylate facial triad in their active site. In this triad, the iron center is coordinated in a facial manner by only three endogenous donor ligands, i.e. two histidines and one glutamate or aspartate. This particular structural feature leads to a uniquely diverse reactivity of enzymes that contain this active site. Efforts to model the structural and reactivity aspects of the facial triad enzymes have mainly focused on the use of all-nitrogen ligands and have to a much lesser extent made use of mixed N,O ligand donor sets. During the research described in this thesis a library of bio-inspired iron and manganese complexes was synthesized and structurally analyzed. The metal complexes derived from the mixed N,O ligands are good structural and functional mimic of their biological counterparts. The complexes were all tested in catalytic oxidation reactions of olefin substrates, showing a diversity of activities. The information obtained by studying these synthesized ligands and their complexes can be used as a lead for the further development of bio-inspired iron and manganese complexes for the oxidation of olefins and other substrates.