Activation of molecular oxygen by molybdenum complexes

Molecular oxygen (O2 ) represents the ecologically most benign and from economical points of view the most attractive oxidation agent. However, direct reaction of organic substrates with O 2 requires a spin state conversion, which can be enabled by metal complexes. Thus, aerobic oxidation reactions catalyzed by non-precious and non- toxic metals are of fundamental interest. This provides ample motivation for investigating reactions with direct participation of molecular oxygen in oxidation reactions. While most of inexpensive, abundant metals belong to the first row transition metals, 2nd row molybdenum represents an exceptions as it is far less expensive than precious metals. However, molybdenum complexes that are capable of activating molecular oxygen are scarce. In the project, we will focus on the activation of molecular oxygen by molybdenum complexes. One major goal is the synthesis of peroxo complexes by controlled reaction of molybdenum precursors with O2 . It is intended to systematically analyze structure-function correlations in the O2 activation process in molybdenum complexes and subsequently to study the oxygen atom transfer to various substrates. These efforts include the investigation of the influence of intra- and intermolecular protonation and of electron transfer reactions.

Oxygen is vital for us humans. However, it is also consumed with every fire. In both cases, chemical reactions, so-called oxidations, take place; with oxygen as an oxidation aid. Since it is ecologically safe and cheap, it would be ideally suited to oxidize chemical compounds in general. Oxygen, however, often reacts uncontrollably, which is why suitable catalysts are needed. Due to the chemical structure of oxygen, metal-containing catalysts are required. These metals should also be environmentally safe and cheap. Molybdenum is such a metal because it is a vital trace element and is also non-toxic in large quantities. In this project, fundamental insights were gained on how the molecule oxygen reacts with molybdenum compounds. In the course of this project, many molybdenum compounds were established with different groups directly bound to the molybdenum center. Thanks to the investigations, the exact properties of the metal center and the groups bound to it are now known in order to react with oxygen in a controlled manner. It turns out that very stable molybdenum oxygen compounds are formed. For these molybdenum compounds to act as oxidation catalysts, you must be able to release the oxygen back to a substance to be oxidized. It turned out that the transfer of oxygen is possible with far fewer chemical compounds than expected. In order to facilitate the transfer of oxygen, various possibilities have been tested, which have led to highly interesting compounds. It has been shown that the oxygen on the molybdenum forms a bond with a specific electron-poor molecule, but not particularly good, as the chemical environment prevents this. The molybdenum oxygen compound and the electron-poor molecule are so-called frustrated, resulting in a chemically highly reactive situation. Such frustrated pairs are extremely effective catalysts for a number of reactions. This project provides basic information on metal oxide containing, frustrated pairs that have so far been little studied. The aspects of molybdenum oxygen chemistry examined in this project have greatly expanded our knowledge of this important sub-area and have been published in renowned journals.