Bonding Trends in Tetravalent Actinide (Th - Pu) Complex Series

Actinides play an important role in chemical engineering and environmental science related to the nuclear industry or nuclear waste repositories.1 One of the major tools to obtain a profound knowledge about actinide (An) binding is their coordination chemistry. However, the understanding of complexation properties of the actinides is lacking behind those of the d- or 4f-elements. Characteristic of the actinides is their huge variety of possible oxidation states, typically ranging from +II to +VII for early An. A suitable approach to explore fundamental physico-chemical properties of the actinides is to study series of isostructural An compounds in which the An is in the same oxidation state.2 Therefore our investigations are directed towards the synthesis of actinide complexes with the f-element in the oxidation state IV, the dominant oxidation state particularly under anoxic environmental conditions. Observed changes in e.g., the binding situation or magnetic effects along such a series may deliver insight into the elements’ unique electronic properties mainly originating from the f-electrons. One important question in the field of An chemistry is the degree of “covalency” in compounds across the An series,3 which may be addressed by systematic studies on series of An compounds, including transuranium (TRU) elements.
In these studies, we investigate the coordination chemistry of tetravalent actinides (An(IV)), using organic mono- or dianionic ligands with O- and mixed O/N-donor atoms of the acetylacetonate and salen-type.4 The An complexes are typically synthesized via salt metathesis reactions under strict exclusion of moisture and air. Single crystal X-ray diffraction analysis provides insight into isostructural complex series, which were achieved in each case. The resulting compounds were further analysed by NMR, IR, UV-vis-NIR, and EPR spectroscopy and cyclic voltammetry. These results are used as a basis to further analyse bonding trends along the actinide series by means of quantum chemical calculations.
From the results, trendlines along the actinides An = Th, U, Np and Pu in various complex series were obtained, which shed some light in the ongoing debate of covalency in actinide bonding.

References
[1] L. S. Natrajan, A. N. Swineburn, M. B. Andrews, S. Randall, S. L. Heath, Coord. Chem. Rev. 2014, 266-267, 171-193.
[2] M. B. Jones, A. J. Gaunt, J. C. Gordon, N. Kaltsoyannis, Chem. Sci. 2013, 4, 1189-1203.
[3] M. P. Kelley, J. Su, M. Urban, M. Luckey, E. R. Batista, P. Yang, J. C. Shafer, J. Am. Chem. Soc. 2017, 139, 9901-9908.
[4] T. Radoske, J. März, M. Patzschke, P. Kaden, O. Walter, M. Schmidt, T. Stumpf, Chem. Eur. J. 2020, 26, 16853-16859.

Acknowledgement
This study was supported by the German Federal Ministry of Education and Research (BMBF) funding under the project No. 02NUK046B (FENABIUM).