Characterization of Tri- and Tetravalent Actinide Complexes with Amidinate Ligands

Understanding the physical and chemical properties of radionuclides interacting with organic ligands is fundamental for a reliable evaluation of the migration behaviour of radionuclides in the environment. In this context, our group has been conducting research with a particular emphasis on the complexation of early actinides with relatively soft N-donor ligands, and recently, the tetravalent actinide complexes with the chiral benzamidinate ((S)-PEBA) have been successfully synthesized. The present study is inspired by these precedent studies to synthesize a new series of amidinate compounds with An(III) and An(IV) to provide and expand a comprehensive understanding of the electronic properties of actinide complexes.

In this study, we succeeded to obtain a series of tetravalent actinide amidinate chloro complexes [AnClx(amid)y] (An= Th, U, and Np; amid= iPr2BA, (S)-PEBA, and Cy2TA). The crystal structures of the benzamidinate complexes were determined by single-crystal XRD, all showing three amidinates and one halide ligand coordinated to the actinide metal center in a mono-capped distorted octahedral coordination geometry. In the case of the Cy2TA ligand with a sterically bulky tert-butyl substituent, only the complex with a metal-to-ligand ratio of 1:2 [UCl2(Cy2TA)2] was obtained. The paramagnetic effects of these actinide complexes were investigated extensively in solution with NMR spectroscopy. Furthermore, the reduction of An(IV) to An(III) afforded the corresponding homoleptic amidinate complexes [An(amid)3] (An= U and Np; amid= iPr2BA and (S)-PEBA), allowing the comparison of structural and chemical bonding situations with isostructural Ln(III) complexes via paramagnetic NMR studies.