Coordination Chemistry of Tri- and Tetravalent Actinides with Benzamidinate Ligands

Because of their excellent properties of stabilizing transition metal complexes in various oxidation states, amidinate ligands have received considerable attention in the field of coordination chemistry over the last decades as versatile soft N-donor ligands. There have been a number of studies on transition metal amidinate complexes including lanthanides and some early actinides. Still, these studies are mainly limited to thorium (Th) and high valent (V, VI) uranyl complexes. Recently, the tetravalent actinide complexes with the chiral benzamidinate ((S)-PEBA) have been successfully synthesized in our group. The present study is inspired by these precedent studies to synthesize a new series of benzamidinate compounds with An(III) and An(IV) to provide and expand a comprehensive understanding of the electronic properties of actinide compounds.

In this study, we succeeded to obtain a series of tetravalent actinide tris-benzamidinate chloro complexes [AnCl(amid)3] (An= Th, U, and Np; amid= iPr2BA and (S)-PEBA). The crystal structures of the model actinide complexes were determined by single-crystal XRD, showing three benzamidinates and one halide ligand coordinated to the actinide metal center in a mono-capped distorted octahedral coordination geometry. We also synthesized additional halide complex series (F, Br) by halogen exchange reactions on these chloro complexes to investigate the conformational stability of the complex. 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 benzamidinate 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.