Synthesis and Characterization of U(IV) Imidazol-2-ylidene Complexes

In the field of actinide coordination chemistry, it is assumed that ligands bearing soft donor atoms, according to Pearson’s hard-soft-acid-base concept, such as sulphur, phosphorous or carbon lead to stable complexes. Furthermore, due to the expected strong orbital overlap between the metal and ligand, the formed complexes would provide us a deep insight into the electronic situation of the actinides. However, the majority of published actinide compounds still focusses on complexes with hard donor atoms such as oxygen.
A few examples of actinide-carbene complexes reported in the literature emphasise the remarkable strong σ donor properties of the carbon donor ligands, making the complexes e.g. excellent catalysts in organic synthesis1. Of particular interest are N-heterocyclic carbenes (NHCs) based on an imidazole-2-ylidene backbone, also known as “Arduengo carbenes”. For instance, the stability and electronic properties of these ligands can be easily tuned by synthetic introduction of suitable substituents at the nitrogen atoms.
The aim of this study is the synthesis of tetravalent actinide (An(IV)) complexes with soft-donor carbene ligands according to Figure 1 and the characterisation of the formed complexes in solution by NMR-, IR- and UV-vis spectroscopy as well as the solid-state characterisation with the help of single crystal X-ray diffraction. The acquired experimental results are further supported by quantum chemical calculations to further study the electronic structure of the complexes.