Uranium-mediated activation of small, inert molecules

The activation of small, inert molecules is still one of the major challenges of chemistry. This problem is not limited to exotic substances but includes such simple molecules as dinitrogen and carbon monoxide. The conversion of nitrogen to ammonia via the Haber-Bosch process is the most important access to nitrogen- containing chemicals. A large amount of these are used for the production of fertilizers and thus is of significant importance for food production. The Fischer-Tropsch process, which employs carbon monoxide, is a major source of hydrocarbons beside fossil resources. In view of the limited natural resources this process is of increasing importance. The mentioned procedures are not only connected by their central relevance to industrial chemistry but also share the problem of employing very inert substrates. Molecular nitrogen and carbon monoxide are among the molecules with the strongest bonds known. Despite the fact that transitions metal catalysts are used, these processes work at very high temperatures and pressures to overcome the exceptionally high bonding energies. This goes along with a high demand of energy which is both an economical and ecological problem. Hence, great efforts are made to develop alternative catalytic systems. In this context the reaction behaviour of uranium compounds could be used to improve the situation. The closer investigation of the chemical nature of this element was promoted initially by insights into the special properties of the rare earth elements. The chemistry of uranium, in addition to that of lanthanides, also showed complementary features compared to that of transition metals. For a long time the investigations of uranium focused on the development of methods to enrich fissable isotopes to primarily produce fuels for nuclear power supply. A side effect of this is the availability of highly depleted uranium. This circumstance proved to have a promoting effect on the investigation of the chemistry of this element. The presented research project aims at the development of two similar catalytic systems. A room temperature version of the Fischer-Tropsch process shall be developed, which is based on the reductive dimerization of carbon monoxide by uranium compounds. Starting from known stoichiometric synthetic cycles the variation of the different aspects of the reactions shall be varied to optimize the system and to investigate possibilities to form more complex organic compounds. Further the uranium-mediated borylation of aromatic compounds shall be translated from a stoichiometric reaction to a catalytic process. The affinity of uranium to aromatic compounds like benzene allows conversion under mild conditions while tolerating various functional groups. This way an alternative method for the formation of a class of organic compounds could be developed which is of central importance not only for organic chemistry.

A major challenge in modern chemistry is to transform environmentally dangerous waste into materials that we can reuse to make new products. Current processes to produce hydrocarbon feedstock from carbon oxides such as the Fischer-Tropsch process require high temperatures and pressures resulting in a high demand of energy. Non-fissile uranium is a by-product of nuclear energy production and its chemistry renders it a potent candidate to help solving problems to produce artificial hydrocarbon fuels from pollutants such as carbon monoxide and carbon dioxide. In order to develop low temperature and pressure alternatives we need to improve our knowledge and understanding of the related chemistry. This project contributed to the progress in this field following an approach inspired by nature. Similar to natural systems, where often two or more metals collaborate to perform a difficult chemical transformation, the collaborative interaction of uranium with transition metals was investigated in the context of small molecule activation. The systematic combination of the actinide with the metals of group 9, 10, and 11 provided insight into bonding interaction between d- and f-block elements. Under unprecedented preservation of the uranium-transition metal bond throughout manipulation of the ligand sphere of the actinide, multimetallic compounds were prepared that were found to be highly active towards small molecule activation.