Characterization of Isostructural An(IV) Complexes with Hetero-donor Imine Ligands

The coordination chemistry of actinides (An) serves as fundamental knowledge for chemical engineering and environmental science related to the nuclear industry.[1] However, as compared with other transition metals, the basic chemistry of An is far less explored. The chemistry of An is complicated by, e.g., various possible oxidation states ranging from II to VII for the early An. One possible approach to understand the chemical nature of the An series is the comparison of isostructural compounds containing different actinides with the same oxidation state.[2,3,4] With this approach, the relative changes observed among the An series could allow us to gain insight into their unique chemical nature, such as electronic properties originating from their f-electron orbitals. One major question remaining in the field of An chemistry is the degree of “covalency” across the An series.[5] In order to study the “covalency” across the An series, one would require to perform a systematic study on a wide series of An, including transuranium (TRU) elements. Nonetheless, precedent studies covering TRU elements are rather scarce. This background motivates us to perform the current study focusing on a systematic comparison of the isostructural An complexes (Th, U and Np).
In this study we investigate the coordination chemistry of tetravalent actinides (An(IV)), which is dominant particularly under anoxic environmental conditions.[1] Synthesis of their compounds and the experiments should be conducted under inert and water-free atmosphere. The ligands used in this study are a hetero-donor imine ligand of salen and its derivatives (Fig. 1). These ligands have a capability to coordinate to metal ions tetradentately and exhibit both the hard- (oxygen) and medium-donor (nitrogen) characters, which could be a simple analog of natural occurring organic molecules. The eightfold coordination, which is often preferred for An complexes, can be readily achieved with these ligands by coordination of two ligand molecules. Salen and its derivatives have also been employed as a framework for catalytic and extraction agents.[6,7]