Synthesis and Characterization of Tetravalent Actinide Complexes with Nitrogen Donor Ligands

In contrast to the dominant trivalent state for the lanthanide series (Ln(III)), a wide variety of oxidation states (from II to VII) of actinides (An) makes their chemistry intricate but attractive. Especially the early An thorium (Th), uranium (U), neptunium (Np) and plutonium (Pu) form highly charged cations with the oxidation state of four (An4+), which are of particularly interest for the coordination chemistry due to their strong interaction with ligands. Furthermore, the tetravalent oxidation state of these An is also preferred under reductive conditions that could potentially occur in the environment. Hence, the understanding of the interaction mechanisms between tetravalent An (An(IV)) and naturally occurring ligands is of crucial importance particularly for the safety assessment of nuclear waste repository.
The overall aim of our investigations lies in the comprehensive characterization of An(IV) complexes with ligands bearing soft donor atoms, such as nitrogen (N), both in the solid state and in solution. The present study focuses particularly on the interaction of An(IV) with N-donor ligands of amidinate and guanidinate type, which could be considered as a simplified model of naturally occurring N-donor organic compounds.

Recently, the Ln(III) complexes with the chiral benzamidine, (S,S)-N,N-Bis-(1-phenylethyl)-benzamidine ((S)-HPEBA, Fig.1), have been successfully synthesized by the group of Prof. Roesky1,2. Our study is inspired by these precedent studies and has succeeded to obtain the first chiral benzamidinate complexes of An(IV) [An((S)-PEBA)3Cl] (An= Th, U and Np) as well as that of Ce(IV), a chemical analog of An(IV).

The structure of the synthesized complexes was determined by single-crystal X-ray diffraction (SC-XRD), revealing that the An(IV) center is coordinated by three chiral benzamidinates and one chloride in a monocapped distorted octahedral coordination geometry.

The isostructurality of the obtained An(IV) complexes enables a direct comparison of the binding situation across the series. Quantum chemical calculations indicate that the bonding between An(IV) and the N atoms in the ligand strengthens by comparing Th(IV) to U(IV) due to the contribution of 5f-electrons to the chemical bonding.
The complexes were also characterized in solution with NMR spectroscopy. Due to the electronic interactions between the paramagnetic metal center and the ligand, significant NMR chemical shifts are observed, which can be further correlated with the distance between the metal center and the measured nuclei as well as the angle towards the principal axis of the molecule. The Th(IV) complex serves as a diamagnetic reference to subtract non-paramagnetic contributions from the observed chemical shift and to calculate the hyperfine shifts. Hence, the structure information on the paramagnetic complexes in solution can be acquired by the paramagnetic NMR spectroscopy.

ACKNOWLEDGEMENTS

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

REFERENCES

1. P. BENNDORF, C. PREUẞ, P W. ROESKY: “Synthesis of Enantiomeric Pure Lithium and Potassium Benzamidinate Complexes” J. Organomet. Chem., 696, 1150 (2011).
2. P. BENNDORF, J. KRATSCH, L. HARTENSTEIN, C. PREUẞ, P W. ROESKY: “Chiral Benzamidinate Ligands in Rare-Earth-Metal Coordination Chemistry” Chem. Eur. J., 18, 14454 (2012).