First Series of Tetravalent Thorium-, Uranium- and Neptunium-Amidinate Complexes

Actinides (An) can possess a variety of different oxidation states, which typically range from +III to +VI for the early actinides Th-Cm. They have unique electronic properties originating from the 5f-orbitals, what makes their coordination chemistry a fascinating area of research for both, the nuclear engineering but also for fundamental chemistry. Thorium (Th), uranium (U), neptunium (Np) and plutonium (Pu) can form highly charged cations with the oxidation state of four (An4+), which is the dominant one under reductive conditions. Furthermore, An(IV) are of particular interest for the coordination chemistry because of their strong interaction with ligands.
Hence, the overall aim of our investigations is a deep understanding of the interaction mechanisms between tetravalent An (An(IV)) and ligands bearing soft donor atoms, such as nitrogen (N). Thus, we focused on the synthesis and characterization of a series of An(IV) complexes with the N-donor ligand N,N’-Diisopropylbenzamidine (iPr2BA) both in solution and in solid state.
The structures of the synthesised complex series were determined by single-crystal X-ray diffraction (SC-XRD), showing the An(IV) coordinated by three iPr2BA molecules and one chloro ligand in a monocapped octahedral coordination geometry. This is the very first example of an An(IV) complex series including Np(IV) as a transuranium element with an amidinate ligand. The isostructural complexes allow a direct comparison of the binding situation of the An(IV) across the series. Quantum chemical calculation strongly supported the experimental results to to further study the electronic structure of the complexes.
NMR-spectroscopic investigations of the dissolved complexes in toluene-d8 showed significant chemical shifts due to considerable effects of the paramagnetic metal centres U(IV) and Np(IV) compared to the diamagnetic reference [Th(iPr2BA)3Cl].