Synthetic and density functional theory studies of dioxygen activating non-heme iron model complexes
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A long standing global scientific challenge has been the activation of O2 at a single metal center, and use of the subsequent metal-based oxidant for a variety of difficult chemical transformations. Towards this end, computational and synthetic methods have been utilized in an approach to develop model compounds capable of this type of chemistry, and to better understand the electronic and mechanistic properties of the observed catalytic reactivity. We have developed a first generation catalyst that has been shown to be fully functional in utilizing α-keto acids for the catalytic activation of O2 and oxidation of organic substrates in a highly conserved manner. This reactivity takes place at room temperature and standard pressure, and resembles the type of chemistry performed by mononuclear non-heme enzymes, which inspired the design of the catalyst. However, these solution-phase reactions do not benefit from the controlled environment provided by a protein active site, and solution studies and DFT simulations demonstrate an isomeric family of reactive species that ultimately deactivate via a dimerization pathway. A second generation catalyst, which incorporates ligand aromatic functionality, has been developed. This complex has been shown to catalytically oxide methanol to formaldehyde in the presence of α-ketoglutarate using O2. The aromatic group provides a synthetic platform, allowing a variety of substituents geared toward increasing complex solubility and the tuning of the redox properties of the metal center. Additionally, the ligand has been functionalized to allow for the immobilization of the catalyst using an azido-functionalized solid support, by means of 'click' chemistry. A procedure for the immobilization of the catalyst has been developed that sets the stage for the preparation of a material that will diminish dimerization and inactivation. Additional insights into potential reaction pathways of the first generation catalyst have been obtained from DFT studies. These simulations have provided energetic comparisons of proposed intermediates and set the stage for future computational and spectroscopic studies. This synergistic approach will not only allow for detailed electronic and mechanistic descriptions of the intimate mechanism, but will be used in the development of next generation catalysts that that can be tuned for desired reactivity properties.