Bioinspired copper and nonheme iron models for oxygen activation: unprecedented reactivities

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The oxidation of hydrocarbons is one of the most important processes in the chemical industry, and it is often related to highly toxic reagents that generate a big amount of undesired residues. In sharp contrast, oxygenase enzymes found in natural systems catalyze the oxidation of hydrocarbons in a regio- and stereoselective manner using O2 as sacrificial oxidant. These enzymes, that often contain iron and/or copper in their active site, are the source of inspiration for the development of novel methodologies to perform oxidative transformations in a more environmentally friendly way, using benign oxidants such as O2 or H2O2. In this line, bioinorganic chemistry constitutes a discipline in which chemistry, biology and even physics join with the aim of studying both the structure and reactivity of enzymes. One of the key issues to further develop more effective bioinspired catalysts is learning the basis of their performance. This doctoral dissertation is focused in the development of copper and iron-based synthetic model systems for oxygen activation. On the first place, we have developed a novel unsymmetric ligand inspired in the unsymmetric active site of some copper-based enzymes. This ligand allows the synthesis of various heterodimetallic complexes using copper(I) and a complementary metal that react with oxygen at low temperatures giving raise to the formation of different copper-dioxygen adducts. Also in the topic of copper-based synthetic models, we have developed novel reactivity for a high valent copper(III) compound generated after reaction of the corresponding copper(I) complex and O2 at cryogenic temperatures. We have discovered that this compound is capable of ortho-hydroxylating-defluorinating aromatic carbon-fluorine bonds, which is a very challenging reaction owing to the strong bond dissociation energy associated with this bond. In the second half of the thesis we have turned our interest towards the development of an iron-based synthetic model. Interestingly, when the ferrous complex reacts with peracids at low temperatures it generates a metastable species that is competent for the oxidation of alkanes and olefins. The spectroscopic characterization of this compound points towards a high valent oxoiron compound that has been postulated along the catalytic cycle of the oxidations mediated by nonheme iron systems even though direct detection of such species remained elusive. Thus, the results obtained during this thesis are of great relevance, because for the first time the compound responsible for the oxidation event has been trapped and kinetically interrogated. Finally, the same ferrous precursor reacts with hydrogen peroxide to form a metastable iron(III) species that is an sluggish oxidant but that it is biologically relevant in the catalytic cycle of iron oxygenases. Interestingly, the addition of acid to this compound promotes the formation of a strong oxidizing species that shows great selectivity in hydrocarbon oxidation. This reactivity pattern is reminiscent to the heterolytic O-O lysis in heme systems to form high valent oxoiron compounds ​
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