A uranium complex offers new C-N bonds

Carbon–nitrogen (C-N) bonds are among the most abundant bonds in organic chemistry and biochemistry. They are extensively found in a host of chemical “backbones” that can be used to build complex molecules, e.g. pharmaceuticals, and chemicals used in agriculture. Given the importance of C-N bonds, chemists are always looking for new and economical ways of making them. Particularly desirable are methods that use cheap and abundant carbon sources such as carbon dioxide for C-N bond formation. EPFL scientists have now used a uranium-based compound to build C-N bonds from carbon dioxide or carbon disulfide. The breakthrough has made the cover of the prestigious journal Angewandte Chemie.

Research into C-N bond formation has seen a surge of new compounds in recent years. Molecular nitrides of transition metals and f-elements, e.g. uranium, have been identified as attractive candidate for promoting C-N bond formation reactions. However, nitride compounds of f-elements are still rare and their reactivity is poorly understood.

The lab of Marinella Mazzanti at EPFL has now discovered a molecule that can be used to build new C-N compounds. The molecule is a nitride-bridged di-uranium(IV) complex supported by siloxide ligands. “The high reactivity of the nitride-bridged di-uranium(IV) complex with electrophiles contrasts with the often encountered inert character of nitride-bridged transition metal complexes,” says Mazzanti. “This highlights the potential of uranium nitrides to promote C-N formation reactions.”

The di-uranium complex is stable in solution for several months at −40°C but decomposes at higher temperatures. With an overnight thermolysis at 80°C, the researchers found that the complex transforms into a new solid-state crystal compound, where a new C-N bond is formed.

Mazzanti’s group explored the properties of the di-uranium nitride compound, which proved to react, in mild conditions, with both carbon dioxide and carbon disulfide to form new molecules containing a C-N bond. Depending on the amount of carbon dioxide, the reaction produced different molecules. “The reaction results in C-N bond formation that can produce cyanate and thiocyanate species,” says Mazzanti. “But it also results in the unprecedented construction of a dicarbamate product that has never been obtained though conventional methods before.” This is of great importance, since carbon dioxide is a cheap and abundant carbon source.

The breakthrough opens avenues for the long-sought synthesis of new organic molecules from metal nitrides, using carbon dioxide as an abundant and cheap feedstock. “Such novel reactivity of uranium nitrides will certainly inspire new approaches to stoichiometric and catalytic C-N bond formation,” says Mazzanti. Finally, the work advances our understanding of uranium nitride reactivity, which is of great interest because of as they are considered to be safer, stronger, denser, more thermally conductive nuclear fuels, with a higher melting temperature than uranium oxide, the most common nuclear fuel. As such, they can be used in generation IV nuclear reactors, as well as fast-neutron nuclear test reactors.

This work was funded by the Swiss National Science Foundation and EPFL.

Reference

Falcone M, Chatelain L, Mazzanti M. Nucleophilic reactivity of a nitride-bridged di-uranium(IV) complex: CO₂ and CS₂ functionalization.Angewandte Chemie XX February 2016. DOI: 10.1002/anie.201600158